Model Engine Maker

General Category => Chatterbox => Topic started by: MJM460 on May 11, 2017, 12:40:10 PM

Title: Talking Thermodynamics
Post by: MJM460 on May 11, 2017, 12:40:10 PM
I suggested starting this thread in response to a little side track on Chris's Lombard Hauler thread.  It seemed to spark some interest so here goes.  I am sure there are others with some knowledge of thermodynamics, so feel free to jump in to the discussion, especially if you disagree.  The thoughts here are offered in the spirit of sharing knowledge, rather than wasting it, and enhancing our understanding of the marvellous machines we make.

First I will try and address the questions already raised while they are fresh, then I will post again the information in Chris's thread to keep it all together.  Then we will see where it leads.

Flyboy Jim, pressure does work on the gas in a piston when the piston moves.  There are two basic cases.  In a cylinder with the inlet valve open, with adequate inlet piping etc so that the pressure remains constant.  Then work (W) = Pressure (P) times volume change (v).  Note that it does not matter whether the gas is air or steam or any other gas.  A pressure of 15 psi exerts the same force on the piston what ever gas is involved.

The second case is a closed piston, i.e. The inlet and exhaust ports are both closed.  Then when the piston moves, the volume increases, (expansion) and this results in a falling pressure.  The formula for work done is more complex and involves the inlet pressure, the pressure ratio and a factor which for an ideal gas and perfect engine turns out to involve the specific heat of the gas.  This factor is slightly different for air  and steam.  The maths is definitely not mental arithmetic, but easy enough with a scientific calculator or a spreadsheet such as excel.  However the result is only a 2% difference which is not significant compared with other factors such as friction and heat gain or loss.  We all know a steam cylinder loses heat, lagging is applied to reduce this heat loss which reduces the steam pressure faster than expansion alone. 

I mention the possibility of heat gain because when the gas expands, it's temperature falls.  Steam is still hot compared with the atmosphere, but if air starts at atmospheric temperature and then gets cooler, then the cylinder gains heat, and the pressure falls less quickly than due to expansion alone.  I have seem the exhaust pipe of a small industrial turbine ice up when the turbine was run on air instead of steam.

RonGinger, the expansion of water to steam is indeed around 1700:1 however that expansion occurs in the boiler.  We are talking about expansion of the gas in the cylinder, and only from the point where the inlet valve closes, for example around 50% of stroke, depending on the notching of the valve gear, to the point where the exhaust valve opens, ideally this occurs very close to the bottom of the stroke, and used for the example, but may be quite different in a real engine.  It turns out that the actual cut off point makes little difference.  My 2:1 is based on expansion in the cylinder from about 50% stroke to bottom dead centre. 

Jo, I cannot argue with your observation.  In my experience, when observation appears contradictory to theory, it usually means that there are more than one thing happening, and the one with the biggest influence may not be the suggested theory.  It is then important to look more closely at what is going on.  For example, the heat gain or loss mentioned above, or thermal expansion at steam temperatures might be changing clearances.  Could you please tell us more about the differences you see?  The point is often mentioned and there will be a lot of learning in that for us all.

Paul, your question is the big one that brings us to the purpose of this thread, how do they really work?  This post is already too long, I will start another.

MJM460
Title: Re: Talking Thermodynamics
Post by: Jo on May 11, 2017, 12:54:42 PM
Lost me  :noidea:, lets see if I can explain things in one sentence:

When gas expands it gets colder - which chills the container (cylinder) so when using air at room temperature to run an engine the result is that the engine will be chilled and things will get tighter/harder to turn over, if you use steam it will warm the engine and things will expand, making it easier to turn over the engine.

So if you want to wear out your steam engine fast - run it on compressed air   ::).

Jo
Title: Re: Talking Thermodynamics
Post by: Jasonb on May 11, 2017, 01:12:11 PM
Jo, won't the piston also see a similar temperature change to the cylinder and the clearances stay the same assuming similar materials for both.
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on May 11, 2017, 01:19:42 PM

When gas expands it gets colder - which chills the container (cylinder) so when using air at room temperature to run an engine the result is that the engine will be chilled and things will get tighter/harder to turn over

So if you want to wear out your steam engine fast - run it on compressed air   ::).

Jo

Surely though by using air it won't be at room temperature as it heats up during compression, the subsequent heat lost after expansion in the cylinder is simply returning things back to room temperature isn't it ?

I'm no expert but I'd expect the cylinder to remain at room temperature (albeit with other factors like friction affecting the result)

Title: Re: Talking Thermodynamics
Post by: Jo on May 11, 2017, 01:53:33 PM
Ok I tried to make it too simple  :facepalm2: back to the theory....

Steam contains far more energy than air at the same pressure, this pressure is a symptom of the available heat energy in the operating gas. Were the gas is admitted into a cylinder through the whole of the stoke there would be very little difference in performance between steam or air (or even water). As soon as you cut off the supply stroke the gas expands while delivering work, so it draws on its heat content to provide the energy to do that work.

When compressed air expands the only heat it can acquire comes from a fall in its own temperature (which causes a reduction in specific volume) and from the cylinder walls. But when steam expands it can draw all the energy from within, and that reservoir of energy is considerable as it also includes the latent heat of evaporation of the water and if the spent steam is still a gas when it exits the engine this unused energy will have also warmed the engine.

So what does this mean in practise?

If you put your hand on a steam engine running on air you will notice that all of the engine will have over time cooled down, i.e. bearings, fits, covers/cylinders, piston etc. this will all have reduced clearances, in this situation lubrication is critical. If you put your hand on a Steam engine you will find it quickly heats up, this means the clearances are increasing, lubrication is still important but not as critical as running on air.

For this reason if you intend on running an engine on air rather than steam you alter the clearances (and the valve timing  ::)) to take it into account.

Jo
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on May 11, 2017, 02:11:25 PM
Here is a link to the H.K. Porter book "Modern Compressed Air Locomotives" it has a comparison of air and steam locomotives.

The storage tanks held high pressure air much higher than a steam locomotive. A reducing valve was used to reduce the pressure to 150 psi.

https://books.google.ca/books?id=uIslAAAAMAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

Dan
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on May 11, 2017, 02:13:18 PM
Ok I tried to make it too simple  :facepalm2: back to the theory....


When compressed air expands the only heat it can acquire comes from a fall in its own temperature (which causes a reduction in specific volume) and from the cylinder walls.

Yes but the point I'm making is that by compressing air in the first place it becomes higher than room temperature,, once it expands (and therefore cools) within the cylinder it is returning back to room temperature but not lower.

I'd argue the energy you are using is obtained by first compressing the air (and heating it) then releasing it in the engine cylinder to expand back to atmospheric pressure and return to room temperature.



Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 11, 2017, 02:14:19 PM
Although we are always looking for efficiency in our running costs, what about using Mercury instead of water air !!! The Americans did it apparently !!!
Title: Re: Talking Thermodynamics
Post by: Jasonb on May 11, 2017, 02:21:05 PM
Although an engine run on air will cool slightly the difference is generally far less than the heat gain of one run on steam. If we take ambient air temp as 20deg C a steam engine couled get upto over 100deg C during running depending on teh steam pressure and themp but I've yet to see one run on air get below freezing so the amount of contraction is minimal in practice and really not worth worrying about.

Also the bearings are usually quite a way from the cylinder so they will get even less of a temp change
Title: Re: Talking Thermodynamics
Post by: Jo on May 11, 2017, 02:36:53 PM
Although an engine run on air will cool slightly the difference is generally far less than the heat gain of one run on steam. If we take ambient air temp as 20deg C a steam engine couled get upto over 100deg C during running depending on teh steam pressure and themp but I've yet to see one run on air get below freezing so the amount of contraction is minimal in practice and really not worth worrying about.

Also the bearings are usually quite a way from the cylinder so they will get even less of a temp change

Hepolite recommended a difference of 0.05mm per 25mm ( 0.002" per inch ::) ) of bore difference for the gap on their rings between the two applications.

If you have ever seen someone run a double or triple expansion engine on air for a few hours you will notice that the LP cylinder covers begin to ice up. Which is why last year the guy at the GMES show was running his Triple on an electric motor  ;)

Jo
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on May 11, 2017, 02:52:55 PM
Jo,
The way Porter solved the problem of 2 stage air motors was to have an atmospheric reheater. The cold air from the HP cylinder was sent through a long set of tubes so the atmosphere could reheat the air before the LP cylinder.

Dan
Title: Re: Talking Thermodynamics
Post by: Jasonb on May 11, 2017, 03:13:26 PM

Hepolite recommended a difference of 0.05mm per 25mm ( 0.002" per inch ::) ) of bore difference for the gap on their rings between the two applications.Jo

So thats 0.0006" per inch on diameter ( 0.002 / 3.142). So taking what I said above for say an 80degree heat gain there is 0.0005" increase when running on steam but only 0.0001" decrease when running on air if there were a 20deg drop per 1" diameter.

I also assume that Hepolite refer to engines doing work where the air pressure will need to be as high as the steam pressure so we would be looking at 115psi steam  to get our engine upto 100deg C but as we tend to play with our engines when running on air and I can easily run mine on 5psi or less there will be far less cooling at those low air pressures. maybe 10%.

So I'm of the opinion that 0.00001" change of diameter on a 1" bore piston/cylinder combination  or 0.000005" clearance is just  not worth worrying about in the practical word.

J

PS Have you noticed that all my "steam" engines run alloy pistons so if anything the piston/cylinder clerance will go up on mine when cooled not down ;)
Title: Re: Talking Thermodynamics
Post by: paul gough on May 11, 2017, 09:53:39 PM
With this discussion we need to be very clear as to what we are addressing in making a point and responding to it. It is very easy to digress, be misinterpreted, and to confound theoretical explication by imprecise language. Also we should be aware that sometimes theory and practise don't obviously match the circumstance. As an example; for the purposes of argument, lets assume a steam engine and a compressed air engine were in fact the same, the steam engine would still be a superior mechanism, (moving a mass from A to B), due to being 'self sustaining', utilising all the stored energy within the fuel being burnt and transferred to the steam, whereas the air engine has only the tanks capacity so somewhat limited. However a coal burning locomotive might not be first choice in a gunpowder factory and the air engine superior, for obvious reasons. So we have to make it clear what we are trying to get across. From what I gather, the original purpose of the investigation seems to be aimed at revealing what is precisely  going on in the cylinder when a gas of equal qualities acts on a piston. I should like to have this verified, so I don't get confused. I am an old man with a feeble mind and have to be careful before opening my mouth in ventures such as this. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: Maryak on May 12, 2017, 12:37:48 AM
Adiabatic - Air Compressor theory, heat and its associated energy are removed. With the heat removed the gas has little expansion available to do work.

Isothermal - Steam Engine theory, heat and its associated energy are retained. With the heat retained expansion is available to do work.

Running on air a reciprocating engine benefits from cylinder oil lubrication.

Running on saturated steam a reciprocating engine is happy with the water in the steam as the cylinder lubricant. Boilers do not like mineral oils.

Simplified but I hope you get my drift.

Regards
Bob
Title: Re: Talking Thermodynamics
Post by: Flyboy Jim on May 12, 2017, 03:16:12 AM
I see some real potential for this thread. To add to what Paul said.............hopefully the knowledgeable folks will keep in mind that a lot of us are pretty uninformed when it comes to dealing with Thermodynamics.........so it's okay to talk down to us.  :)

Jim
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 12, 2017, 01:21:42 PM
Wow!  That is an amazing response.  Thank you to everyone who has responded and all those who were interested enough to read on the topic.  Talk about letting the genie out of the bottle.  I do hope we can continue the thread and not disappoint.

First,  Flyboy Jim, I hope that I don't talk down to anybody.  It is a strength of this forum and a tribute to all its members that it is really minimal.  If a point is not clear, please say so, this will keep the thread relevant and grounded in reality.  If I have lost Jo on the first go, then I have a long way to go in my explanations and need all the help I can get.

Some clarifications on some of the posts.  When air is compressed, it heats.   After the compressor, air tends to cool in the air tank and piping after the compressor.  In addition, if there is a pressure regulator, there will be cooling due to expansion after the regulator.  I for one find the air hose is not noticeably warm when I run an engine on low pressure though the compressor discharge is quite hot.  But it is ok to consider the air as initially warmer than atmospheric, and the exhaust temperature is then above or below atmospheric depending on the amount of work done and consequent cooling effect.

Please don't compare an air and steam locomotive yet, that can come later if you want, as the differences are all about the processes involved in delivering gas at pressure to the piston face.

An adiabatic process is one that occurs without heat flow to or from external sources, it does not mean without heat!  Isothermal process means constant temperature process.  As doing work involves cooling, heat must be added from an external source throughout the process to keep the temperature constant.

These engines are all heat engines.  To understand why the air engine is a heat engine we must identify the absolute zero of temperature, the point at which molecular motion stops.  In the metric system, the scale is called Kelvin, and zero is -273.15 deg C.  In the imperial system the scale is called Rankine and the same zero is -459.67 deg F.  So even in Siberia, air is relatively warm on the absolute temperature scale.

Paul has it nailed when he notes that the original purpose of this thread was to understand what is going on inside the cylinder, the point where the heat of the gas molecules interacts with mechanical components to do work.  At least I suggest that is the starting point.  We can then work back to explore the thermodynamics of the other parts of the system if the interest continues.  Everything else is there only to turn the chemical energy in the fuel to energetic motion of molecules and to deliver those molecules to the face of the piston with as little loss as possible.

 Jo has kindly passed on her observations on running engines on air.  I am relieved that the engines get noticeably cooler.  I was concerned that when running lightly loaded, the heat due to friction might mask the effect, as there is less temperature change when less work is done.

I definitely agree that a big factor in the difference between running on air and steam comes down to thermal expansion and changes in clearances.  It is not a simple exercise to determine the change in clearances at different temperatures.  If everything is at a uniform temperature, there is no change in clearance.  However the inside of the cylinder is in close contact with steam, and approaches steam temperature, while the outside is in contact with the atmosphere, and much cooler.  We reduce the temperature difference by lagging but the cooler outside expands less and constrains the expansion of the inside through internal stresses in the metal.  The piston is also in close contact with the steam, but has a heat loss path through the piston rod.  You can see the problem, which effect has the larger influence?  I will go with the practical observation.  If the engine loosens up on steam and tightens on air then that is the answer.

Obviously our displacement lubricators do not work with air, so other provisions have to be made.

I also agree that there is a big difference in the energy content of air and steam.  However, the real issue here is whether that energy difference makes a difference to the conversion of heat to work in the engine.  Our starting point was air and steam at the same pressure.

So back to what happens in the cylinder.  No matter how loud we sing the song that "heat is work and work is heat", (was it Flanders and Swan?), it is still amazing that our little engines can turn heat into work.  It is not so long ago that no one could even imagine that the heat we feel streaming from the sun by day and from our fires by night can be harnessed to replace horses, let alone drive mighty locomotives across continents, ships across oceans and even aeroplanes through the skies.  And all this happens at the face of the piston where the energy in the random motion of tiny molecules interacts with the face of the piston.   Surely that is worth trying to understand.

I hope that clears a few issues, or at least puts them on hold for a while.  This post is more than long enough so next time I will try and get back to what happens at the piston face, and the difference between engine output on air compared with steam.  Thank you for your interest.

MJM460
Title: Re: Talking Thermodynamics
Post by: Flyboy Jim on May 12, 2017, 03:21:10 PM
Good info. As someone who is at the bottom of the "understanding chain" I'm on board so far. I appreciate you taking this one small step at a time.

I wasn't sure what a displacement lubricator was so looked it up: https://en.wikipedia.org/wiki/Automatic_Lubricator

It's easy to understand the idea of steam or air providing the pressure to move the piston, but I'd never thought of it in terms of heat. It's been over 50 years since my college physics class and I'm thinking I may have forgotten a few things.  :Doh:

Jim
Title: Re: Talking Thermodynamics
Post by: paul gough on May 12, 2017, 11:18:10 PM
MJM, Please take the position as Tutor for Thermodynamics 101, your explanations are adequate and soothing to the minds of people who otherwise might have been scorched by even hearing the term uttered. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: paul gough on May 13, 2017, 12:59:54 AM
Practical question/proposition: Jo has mentioned the 'chilling' effect to cylinders and an acquaintance who resorted to an electric motor to run his compound engine due to icing up. From our discussions so far it would seem that a simple expedient of introducing some heat to the air supply prior to its entry to the engine might help overcome this problem and at least theoretically enhance the power of the engine. I have in mind something simple; a length of copper tube, perhaps with a twisted strip of thin sheet metal inserted  to prevent laminar flow and enhance the surface area for the air to impinge upon. Under this section of pipe a couple of small 'tea candles' could be situated thus superheating, (in a minor way), the air supply. Of course more sophisticated arrangements could be developed from this, but for a quick solution for an exhibitor such a contrivance might suffice. Comments eagerly sought. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: Steamer5 on May 13, 2017, 01:57:27 AM
Hi Paul,
 Both my current & last job did this for dropping natural gas pressure to prevent hydrate formation, (for those that don't know hydrates are nasty little blighters, basically methane gas molecules are small enough to fit inside water molecules & when they turn to ice are more stable than ice itself, they are also hard to get ride of) last job we used a steam heated exchanger to heat the gas prior to dropping the pressure, current job use an electric "superheater" to do the same thing.
So pre heating the air prior to using it in an engine I think would work quite well. Probably not require in a single cylinder but a triple where the pressure is progressively dropped would help out.
Mind you another  other option could be to use drier air, then maybe the ice would be kept to the outside.

Like others I'm finding this thread very informative.

Cheers Kerrin
Title: Re: Talking Thermodynamics
Post by: paul gough on May 13, 2017, 04:58:46 AM
Thanks for the comment Kerrin, I have only ever used the stuff that burns you to run engines and was 'speculating in isolation'. Nice to know the idea has some application but also learning that there is only a need to apply it to multi-expansion engines. I understand methane hydrates have a very nasty potential to impact on our atmosphere if the vast quantity of them in the frigid parts of the Northern hemisphere ever go through a phase change. I now have a curiosity itch, is there any role in more advanced pneumatic power applications for heating the air and to what degree, but this is an extra curricular activity, we have enough in this thread with thermodynamics and its asides. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: Jasonb on May 13, 2017, 07:40:31 AM
I think the issue of running the tripple on air is that the compressed air really only works 100% on the HP cylinder and by the time it has found its way to the LP cylinder it is actually being drawn through the engine due to the large volume displaced which expands (cools) it far more than you would get with a simple single or double high. I'm assuming he was running it as a compound and not using the modified pupework that acted a bit like a simpling valve.
Title: Re: Talking Thermodynamics
Post by: Zephyrin on May 13, 2017, 10:33:08 AM
Little compressed air engine for model airplane were common before diesel or glow engines, they become ice cold in 1 min or so and frozen in 2 (for those able to run that long!), the same holds true for CO2 engine, based on the same principle...

obiously a steam engine that run on compressed air requires a mechanical lubricator, and not the condensing type of oiler, and also the appropriate oil.

I personaly spend a lot of energy (heat ?) to run my engine with steam...just because its so more appealing, more efficient, much softer and far less mechanical ratling noise, compressed air is second-best.
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 13, 2017, 01:19:17 PM
A time for humble pie.

Thank you to Paul and Jim for your kind comments. 

Preheating the air will raise the outlet temperature and potentially prevent ice formation.  The ratio of absolute temperatures will be the same for the same amount of work, so an inlet temperature of say 20 deg will result in less than 20 deg rise in the exhaust temperature.

It sounds like Kerrin has worked in a gas plant somewhere.  North or South Island? Hydrates are a real problem there, but are not likely in our models.  However we can get ice inside our models if the air is  not sufficiently dry.  Fortunately it melts when things warm up.  Ice on the outside only indicates internal temperature below zero C.

I started this thread with a view to creating a "build log" of a knowledge base of thermodynamics as it applies to understanding, designing and running our engines.  I hope that this will add depth to our enjoyment of our hobby. 

I am not claiming to be an expert, which is just as well as you will soon see.  I am hoping to learn as well as to contribute.  So first some humble pie!  I started by trying to understand the oft stated assertions that steam produces so much more work than air, and that expansion (expansive) work is so wonderful, but there is never an explanation.

Turned out to be not such a simple problem, particularly the expansive work part.

Certainly while the inlet valve is open there is agreement that steam, air or even water makes no difference, though it is better that we keep to compressible gases.  However, once the inlet valve closes, until the exhaust valve opens, the situation is quite different.  I also want to get back to the third stage of the power stroke, when the exhaust valve is open.  For air adiabatic expansion can be calculated using the ideal gas law.  For steam, this assumption is not accurate enough to be useful hence the variation of pressure with volume is not known, and the work output cannot be calculated.

Thanks to Jo for her comment on the energy content of steam that forced me to put aside the reversible process and ideal gas assumptions and to take a detailed look at the process for steam.

It has been a long day of heavy reading, detailed maths and all those abra-ca-dabras, enthalpy, entropy irreversibility, along with the first and second laws of thermodynamics.  I suspect that those who would be interested in the detail are quite capable of doing it themselves, and I don't want to turn off the others, so here is the short version.

For the expansion phase, the inlet and exhaust valves are both closed.  The cylinder pressure reduces as the piston continues to move.  Thus the work done in this part of the stroke is less than it would be if the inlet valve remained open.  However fuel consumption is efficiently reduced if the maximum power is not required.  The issue is that the way the pressure varies as the volume increases is not known for steam, so the work output cannot be easily calculated.

Fortunately the work done can be calculated using steam tables and the first and second laws.  The summary is that a given volume of steam does around four times the work done by the same volume of air.  So the old guys were right, though only for the expansion phase, I just never understood why.

There are offsetting factors that reduce the difference.  First, if our lagging is not perfect, the heat loss causes additional steam condensation hence further reducing the pressure and reducing the work output.  Second, the same processes that produce the extra work during expansion probably mean more loss during the exhaust stroke.  I am a bit cautious on that one as I have not yet done the maths.  Third, it is becoming apparent to me that the practicalities of our most common valve gears mean that the expansion phase is probably not a big part of the total work output, at least for a single cylinder engine, thus lessening the impact.

We need someone to document construction of a test stand and measuring the actual work output of an engine and comparing the results for air and steam.  Practical observation is the best way to determine issues where several complex processes occur at the same time.

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on May 13, 2017, 11:38:48 PM
 MJM you state;" The summary is that a given volume of steam does around four times the work done by the same volume of air. "

As the above quote can be taken as a general statement of outcomes. I would like to be sure, if I repeat this statement, that I cannot be challenged because of some point I am unaware of. Therefore does the statement hold in a broad range of circumstances or are there  constraints and caveats. Sorry if I appear pedantic but I really want to KNOW. I find thermodynamics and its associate, fluid dynamics, fascinating phenomena, so appreciate the lengths and effort you are putting in to get information across in an intelligible form. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: Flyboy Jim on May 14, 2017, 03:07:57 AM
I'm finding out that in order to really understand what's going on with these engines I needed to go back and re-learn the basics. I googled the topic of "Thermodynamics for Dummies" but what I found was way too complicated for this dummy.  :Doh: Then I found a site called Physics4Kids: http://www.physics4kids.com/files/thermo_intro.html Now this is more like it!  :whoohoo: I think this will work. For a while there I was afraid I might have to google "Thermodynamics for Kindergartners"!  :facepalm2:

Jim
Title: Re: Talking Thermodynamics
Post by: Steamer5 on May 14, 2017, 04:58:10 AM
Hi MJM,
 Yes you are right current job is in a gas plant, North Island. Currently there are none in the South.
Previous job in petrochemical. Can't recall an issue with hydrates in the gas heater, but we were always on the the watch for issues. Learnt about them since & they are right little beggars given half a chance!

This thread is getting better & better! Jims link should make some fun reading!

Not totally on track, as we are discussing steam piston engines, but if we throw  steam turbines into the mix then they get some 30% of there power by taking the steam pressure down to a reasonable vacuum, makes one think what would happen if you could do that on a piston engine.....

Cheers Kerrin
Title: Re: Talking Thermodynamics
Post by: paul gough on May 14, 2017, 05:22:46 AM
Kerrin, Air Pumps (vacuum pumps) on condensers in ships and land installations with reciprocators did this to various degrees, pretty much essential with triple and quadruple expansion if you want to get something out of the low pressure cylinders and balance their output with the high and intermediate cylinders. Regards Paul.
Title: Re: Talking Thermodynamics
Post by: 10KPete on May 14, 2017, 05:49:16 AM
I'm not sure this will provide any illumination, but here is a document I've learned a lot from. The link is to my DropBox.

https://www.dropbox.com/s/sc9zcjtsbc6i28g/Edwards%20Wet%20Air%20pump%20copy.pdf?dl=0

Pete
Title: Re: Talking Thermodynamics
Post by: Steamer5 on May 14, 2017, 06:29:05 AM
Hi Paul,
 Thanks for the info! The grey cell got a jog & I seem to remember reading some thing about that in the dim distant past! Was think more along the lines of a loco.......they probably tried it somewhere....oh yeah think I read of it in Africa were water is in short supply!
We used ejectors for the big turbines, one was an extraction / condensing turbine.....it gulped up 160 tonnes / hr of 100 bar steam, after 7 blades, spat out, about 150 tonnes/hr  24 bar steam, & took the rest to vacuum.

Cheers Kerrin
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 14, 2017, 12:20:11 PM
Ideal Gas Laws

Great to see the continuing interests in this topic.  Thank you to all those who have contributed, as well as those who are just following along.

I will address some of the comments later, but first, thank you Paul for highlighting the point that there may be qualifications I should have included in my summary.  So no, better not too quote the initial result without qualification.  The result is really only valid for the conditions assumed for the calculation.  First it is for the expansion part of the stroke only, when both inlet and exhaust valves are closed.  I assumed a start pressure for each calculation of approximately 15 psi as suggested in an earlier thread.  I have worked in metric units so I have made small adjustments and used the values for the nearest value listed in my steam tables to minimise interpolation.  I have also assumed an adiabatic process, with everything up or down to  a steady temperature and no heat gain or loss to the atmosphere.

For air I assumed 17 deg C, on reflection 27 might have been a better choice.  Again to use table entries directly.  Obviously, steam at 17 deg C would be fully condensed, so I used 150 deg C.  This temperature would be reasonable if steam was generated in a boiler at 50 psi, and throttled to 15 psi then some heat loss in the delivery pipe.  Not identical conditions, but attempting to be realistic.  After expansion (2:1), steam tables indicate the steam will be condensing at about 95 deg C.

It is this proximity to condensing that is the reason for a large part of the departure from ideal gas laws, and when condensing starts the ideal gas law does not apply at all.  The heat released in condensing causes the pressure to be much higher than predicted by the ideal gas laws, and consequently, the extra work.  I must admit to never having thought of it in this detail before.  Thank you Jo for pointing me in this direction.  I look forward to learning what other surprises you have in store for us.  I have learned heaps.

You might ask what if I had assumed hotter steam so that it did not condense.  I believe as you get further away from the condensing area, the behaviour gets closer to the ideal gas law, but I expect the departure would be quite significant until the temperature gets quite high, dangerous and rather impractical for our purpose.  I have not done further calculations as they are rather tedious without a specialised computer program, even with a calculator.  I am trying to keep to the issues that potentially have practical application in our model building, and have not spent time in purely theoretical explorations.

The big deal about ideal gas laws?  I probably did not make this clear enough.  When we try to calculate the work done by an expanding fluid, the only things we really know are the starting pressure, temperature, and density of the fluid, and we carry the calculation to the end point at which we know only the volume and fluid composition.  Note we do not know what the pressure and temperature will be.  Early work on this problem (Boyle and Charles are two names that come to mind) gave rise to the ideal gas law which was found to be closely approximated by many gases.  Without this gas law, we do not know how the pressure varies throughout the expansion and cannot calculate the work done.. 

When the ideal gas law does not apply, we basically have to rely on empirical data, such as that contained in steam tables which contain the results of huge amounts of experimental work.  For air there are tables of standard integrals which account for the much smaller but real departure from ideal gas.  Both steam tables and standard integrals for air were used in my calculation, however we should keep in mind that the number only applies for the assumed start conditions. 

Let me try if this summary stands up: While the inlet valve is open enough that the pressure is essentially constant, there is no significant difference in air or steam or carbon dioxide or other gases, however, if our valve gear has both inlet and exhaust valves closed so the work is done by gas expansion, then steam will indeed produce more work than air at the same pressure.  We can finally agree with the old timers on that one.

Kerrin, I had a short assignment at Kapuni some years ago.  Beautiful country in the shadow of Mt Teranaki.  And black sand on the beaches.  Please keep the references to useful sources coming in.  We will talk about condensers and other equipment a bit later, but next time I plan to try and make progress where we started, on the cylinder.

MJM460
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on May 14, 2017, 02:36:17 PM
James Watt invented the steam engine and most writers believe he also invented the steam engine indicator. The second invention is not as widely known because he kept that one secret to keep ahead of the competition.

With a steam engine indicator, we have a PV curve for the full stroke of any piston engine. I have taken indicator cards on a 900mm bore slow speed diesel engine.

The reason I linked this thread to the Porter air locomotives is that it has PV diagrams for air engines. There are lots of sources of PV diagrams for steam engines but they are a lot harder to find for air engines.

I for one wish I could look at one of the historic diagrams and be able to say that was good or that was bad, but my study of the diagrams has not progressed beyond getting Peabody's book on the subject. (Cecil Peabody was the head of Marine Engineering at MIT)

Here is an article on the invention of the steam engine indicator and it has a list of sources on the last page.
http://www.farmcollector.com/steam-traction/story-steam-engine-indicator?pageid=1#PageContent1

Dan
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on May 14, 2017, 06:41:10 PM
Here is Figure 4: Idealized indicator diagram, from the article linked in the last post.
Illustration by Bruce E. Babcock

(http://www.7-8ths.info/gallery/6/213-140517103607.jpeg)

Dan
Title: Re: Talking Thermodynamics
Post by: derekwarner on May 15, 2017, 03:12:31 AM
I agree understanding what is happening with/within our model steam plants is essential

To achieve this an inexpensive digital laser pyrometer + a small digital [load cell] weight scale and a laser tachometer ...all for approx. $60.00 is inexpensive...otherwise all we can say is my fingers are burnt :cussing:.....& a few pressure gauges installed within the plant + your wrist watch

The specification for my gas tank is 105gm capacity
The net weight is 398gm
With the gas canister @ 26 degrees C, I can achieve a gross weight of 501gm......or 103gm of gas transferred  :ThumbsUp:

The gas discharge pressure from the tank during this example test was 40 PSI, however the gas tank temperature after the fill was ~~13 degrees C
The known volume of water 600ml after 6 minutes boiled and achieved 135 degrees C at the boiler discharge isolation valve [outer casing shell]
This same 135 degrees C indicates as 45 PSI [~~3 Bar] which is the boiler relief valve set point
[from tables, the same 45 PSI actually requires 143.7xx degrees C]

So from here, engine running times/speeds, exhaust steam temperatures from the engine.....to the de-oiler, and the resultant volume/weight of exhaust water can be measured

I am sure many of our members completed Applied Heat I, II & III in theoretical and steam practical work tests during our training in earlier days  :old: , and remembering the principals are important, however from an aspect of these engineering marvels...it is the understanding what is happening within our model steam plant is the real question

Derek

PS 1......Mixed units of Measurement .....a point of explanation is whilst my original training was under the Imperial system, my later and chosen system is SI......however to this day in Australia, the majority of model steam gauges [Miniature Steam Gauges UK] are marketed in Imperial PSI

When monitoring gas pressure, the indication of 40 PSI as a whole number in a visual report, stating that this pressure as an assumed 2.72 Bar from a gauge that has an accuracy of +/- 5% on FSD and reported as such is not appropriate   :disagree:

PS 2......steam table reference added 16/5
 
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 15, 2017, 09:46:48 AM
Indicator Diagrams

Thanks to Dan and Derek for interesting and informative posts.  I will follow up on testing along the lines of Derek's post when we look at the whole plant.  In the meantime, please collect lots of data in preparation.

Dan's post of an indicator diagram is right on cue and right on topic, so I will use that to round out this initial discussion.  For those unfamiliar with this diagram I will try and talk you through it as an indicator diagram provides a pictorial representation of what actually happens in the engine as opposed to prediction by theory, that necessary empirical data.

So we are all on the same page I will assume we are talking about the top face of the piston in a vertical engine, cylinder above the crankshaft.  The indicator diagram is a graphical representation of the variation of pressure within the engine throughout the cycle.  Like Dan, I am not able to tell whether a real diagram is good or bad, but I will try and talk us through what it tells us. 

The horizontal scale is the volume of the cylinder.  The right hand side is bottom dead centre, and the left side is top dead centre.  The part of the scale to the left of the diagram to the zero on the volume scale is the clearance volume when the piston is at top dead centre. 

The vertical axis is the pressure measurement.  The cycle progresses in the direction of the arrows on the loop on the inside of the actual diagram, clockwise.  When the inlet valve opens, the pressure rapidly rises to the steam supply pressure.  The top boundary of the diagram is the power stroke.  The pressure is roughly constant while the piston moves down with the inlet valve still open.  For this part of the cycle, work done by the gas is simply pressure times volume change, regardless of the gas used.

Note where the inlet valve closes.  After this is the expansion period.  With the inlet valve closed, our inlet pressure gauge no longer tells us anything useful about the pressure in the cylinder.  The curve then shows how the pressure might fall as the expansion proceeds.  Again the work done by the gas is the pressure times volume change, but as the pressure is changing, we have to break up the volume change into bits so small that we can consider the pressure constant during each small volume change.  We then add up all work done by the gas during each bit of volume change for the total.  This process is called integration.  Simple when it is said quickly, but we don't actually know the pressure at any point on the curve once the inlet valve is closed, so we can't do the sums.  The shape of the curve as mentioned in earlier posts, depends on the gas composition.  The indicator diagram actually measures the pressure, so solves the problem.

The indicator diagram is produced by an instrument that gets the piston position from the cross head, and has a pressure tapping into the cylinder, and prints out the graph.  This diagram was described as idealised, meaning the instrument was imaginary, and the diagram is to show the concepts.  In Dan's case, the instrument was real and actually measured the pressure throughout the stroke.  The instrument is delicate, expensive and not many of us have access to one.  For various practical reasons, they are only available for slow speed machines.  I know of ones that were made for 400 rpm machines but I do not know the current state of the technology.

The diagram shown is typical of the pressure path for an ideal gas, and the shape is also similar for steam, as enough indicator diagrams have been made on real machines for us to know the general shape.  If we had a diagram for air drawn on top of the steam diagram, we would find that the diagram for steam shows higher pressure than the air diagram throughout the stroke, so that the area under the steam curve is greater than the area under the air curve, meaning more work done by the gas.

What can we learn from the diagram?  First, we can see the work done during expansion is always less than if the inlet valve remained open.  Second, we use less steam and fuel if we have some expansion.  Third, while steam does more work during expansion than air, how big a proportion of the total work for the stroke depends on just where the valve closes, later closing, less proportional difference.  Finally, we need to look at the exhaust stroke before we can understand the lower boundary of the curve.

Looking at the curves, while the calculated end pressure for steam was about 15% higher than the end pressure for air, it is hard to conceive a reasonable pressure curve for steam that would support a large difference in work done, but no doubt that steam would produce more work than air during expansion.  It is likely that the adiabatic process assumption, no heat gain or loss, is responsible for reducing the difference. There is little doubt that the steam engine looses heat to the atmosphere.  This reduces the pressure and hence the work done by steam.  So yes, steam produces more work than air, but the size of the difference is not yet determined.

MJM460
Title: Re: Talking Thermodynamics
Post by: Flyboy Jim on May 15, 2017, 02:59:43 PM
Good explanation.

Looking at the diagram begs the question: If the inlet stayed open longer, keeping the pressure in the cylinder higher for longer, wouldn't there be more work (power) produced during the stroke. I'm sure it has something to do with reaching a "point of diminishing returns", but not sure where that would be.

Jim
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on May 15, 2017, 04:12:51 PM
Jim,
The term used for the point in the stroke that the valve gear closes the inlet valve in a steam engine is known as cut off.

This is expressed as a percent of piston stroke, so 100% cut off means that the valve stays open the full length of the stroke and is the max power for a given cylinder.

Looking at the diagram in post 32 by eye the cut off point is less that 50% of the stroke it is about 30% of the stroke which gives good economy.

Steam locomotives have a cut off of around 85% for full gear. The dotted line in figure 4 shows the effect of hooking up or notching back the valve gear. The cut off is chosen by the designer for the type of work the steam engine will be doing.

Further reading:
https://en.wikipedia.org/wiki/Cutoff_(steam_engine)

Dan
Title: Re: Talking Thermodynamics
Post by: PStechPaul on May 15, 2017, 08:48:14 PM
This is an interesting discussion, and I'm following to some small extent. I wonder how much effect there is based on the sinusoidal motion of the piston due it being connected to a flywheel, and the amount of torque applied based on angular position. There would seem to be some considerable "torque ripple" due to the changing pressure on the piston and its contribution to rotary torque of the flywheel through the length of the stroke. Also perhaps some sort of proportional inlet valve could adjust the volume of steam entering the cylinder to correspond to the position of the piston, and the volume of the chamber.
Title: Re: Talking Thermodynamics
Post by: Flyboy Jim on May 16, 2017, 03:08:05 AM
Jim,
The term used for the point in the stroke that the valve gear closes the inlet valve in a steam engine is known as cut off.

This is expressed as a percent of piston stroke, so 100% cut off means that the valve stays open the full length of the stroke and is the max power for a given cylinder.

Looking at the diagram in post 32 by eye the cut off point is less that 50% of the stroke it is about 30% of the stroke which gives good economy.

Steam locomotives have a cut off of around 85% for full gear. The dotted line in figure 4 shows the effect of hooking up or notching back the valve gear. The cut off is chosen by the designer for the type of work the steam engine will be doing.

Further reading:
https://en.wikipedia.org/wiki/Cutoff_(steam_engine)

Dan

Thanks Dan. I've heard the term "cutoff" before but didn't know exactly what it was. That helped a couple more pieces fall into place.

Jim
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 16, 2017, 12:46:07 PM
The Exhaust Stroke

Thank you for more great contributions, and to everyone else who is sticking with it.  Jim, there are not really diminishing returns, keeping the valve open to the end of the stroke results in the maximum work for a given size of cylinder.  This is not the same as maximum work from a given quantity of steam.  Cutting off early saves steam, so reduces fuel costs, when you don't need the maximum output.  Think of a locomotive going over a hill.  Going up, maximum effort is required.  Then, much less is required when going down the other side, so fuel can be saved by cutting off early.

Glad to have you with us, Paul.  There is a very large torque fluctuation through out each revolution, it even goes negative for a single cylinder engine.  We will be exploring that in the near future.

Thanks Dan for your help in explaining the indicator diagram, it helps me keep these posts a little shorter.

To continue our look at the cylinder, we now need to look at the exhaust stroke.  When the exhaust valve opens, the release point on the diagram, there is no more expansion, and the pressure in the cylinder goes into pushing steam into the exhaust system, where the remaining energy is essentially lost.  At least as far as this cylinder is concerned.

At the bottom dead centre (still thinking in terms of a vertical engine) another very significant change occurs, the piston starts to move upwards.  Why is this so important?  It is because in calculating work done, the two quantities, force and displacement, are vectors.  This means that these quantities have both magnitude and direction.  The cylinder constrains the piston to move only up and down, but opposite directions have opposite sign.  We unconsciously defined down as positive when we multiplied pressure times area to give force then by the piston displacement to give work done by the gas which we assumed was positive.  On the return stroke, the force on the piston is still down, but the piston is moving upwards which must then be negative.  So the work done by the gas in the exhaust stroke is negative.  Alternatively, we can consider that the piston is now doing work on the gas, in pushing the steam out of the cylinder.  This is shown on the diagram as the lower boundary of the diagram until the exhaust valve closes.  Continuing movement (with the inlet valve still closed) results in compression of the remaining gas shown by the upward curve on the left hand boundary of the diagram.  In a well timed engine, the compression achieves close to steam inlet pressure just as the inlet valve opens and the cycle repeats.

So we now see that the work done by the gas during the complete cycle is the work done during the down stroke minus the work done by the piston during the upwards or exhaust stroke.

I might appear to have laboured this point a little, but is really is where "the rubber hits the road" so to speak, in the amazing process of converting the heat from the fuel, to energy in the form of vibration of gas molecules and then to mechanical work.

Next we will look at the piston.  Thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 16, 2017, 02:29:03 PM
Using the planimeter to calculate horsepower and stuff.......
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on May 16, 2017, 07:46:19 PM
I have only used a planimeter a few times in an engineering lab years ago. It is used to trace the indicator card lines in a complete loop. Knowing the spring constant on the steam engine indicator and the stroke reduction the work done can be calculated. This is known as indicated horsepower.

Peabody spends a bit of time describing the types of error that is involved with using an indicator. These include inertia of the moving parts and friction in the stroke reduction gear and a few other types of errors.

The modern way to do this is with a pressure transducer which will eliminate most of the types of error associated with a steam engine indicator.

The expansion curve for steam is very similar to a rectangular hyperbola. This is sometimes drawn between the point of cutoff and the release point as a standard reference line. It is also used in design to approximate the PV (pressure volume) curve and draw a design best case indicator card which can be used with a planimeter to calculate the indicated horsepower.

http://mathworld.wolfram.com/RectangularHyperbola.html

Dan
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 17, 2017, 12:26:35 PM
The other side of the piston.

Planimeters, those were the days.  I cannot help but marvel at the ability and skills of our fore fathers in designing and making those things.  I got to use one in my Dynamics of Machines subject, but not since.  And attended the 50 year anniversary last year.

So far I have tried to be very consistent in referring to the "work done by the gas" on the piston, specifically on the top face of a vertical engine.

If we consider the sides of the piston, the gas forces are always balanced by the equal and opposite force diametrically opposite.  There can be mechanical forces on the piston sides but these do not concern us at the moment.  However we cannot ignore what happens on the lower side.

There are two arrangements to consider, the first being a double acting cylinder as on the first steam locomotives we ever saw.  Most real working engines are also double acting for reasons that will become obvious if it is not self evident.  The lower side of the piston is in an enclosed cylinder, much as the upper side.  The main obvious difference from the upper side is the piston rod and the provision for it to exit the cylinder through the gland.  The lower face of the piston experiences gas forces, apart from the centre section which is occupied by the piston rod.  The piston rod does not escape gas forces, it experiences atmospheric pressure on its lower end.

The gas force on the lower face is directed upwards, and follows the same cycle as the the force on the upper face, but out of phase, so the lower face is in the exhaust phase while the upper face is experiencing the inlet phase.  Clearly there are two working strokes in each elevation.

When the pressure in the top of the cylinder is highest, the net force is down and the piston moves down, and vice versa.  The net work on the piston is available at the piston rod to overcome friction within the engine and to do the useful external work of the engine.

The situation in a single acting engine is slightly different.  The bottom of the cylinder is open to atmospheric pressure.  When gas is admitted to the top of the cylinder, the piston moves down for the power stroke.  Work is done by the piston on the air in the cylinder to expel it from the cylinder. 

As single acting engines normally exhaust to atmospheric pressure, the cylinder pressure during the exhaust must of necessity be greater than atmospheric pressure.  The force exerted by atmospheric pressure is less than the force from the gas above. Hence the net force on the piston is down, motion is upwards, so work by the gas above the piston is negative.  That is, the piston is doing work on the gas.

There is only one power stroke per revolution for a single acting engine.  During the exhaust stroke, the net output (and torque) is negative, and the engine slows down.  A flywheel stores energy by speeding up during the power stroke, and returns this energy as it slows down on the exhaust stroke.

I might appear to have laboured the point a bit, but without this understanding of how gas pressure converts energy to work, it can be quite difficult to understand why a Stirling engine, which appears to be single acting, under some circumstances is actually double acting.  (A topic for much later!)

Next time, how is all this relevant?

Thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on May 17, 2017, 10:46:40 PM
Well MJM460...as of this morning we see.... "Topic: Talking Thermodynamics  (Read 1143 times)"

This is always a good indicator [no pun intended] in understanding how many members are reading this thread so from the numbers I suggest you have a good repeat audience  :happyreader: ...yes viewing and reading the work

Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 18, 2017, 02:01:22 PM
Yesterday was just one of those days!

Thanks Derek, yes, it is great to see so much interest.  I have always thought we were a bit too reluctant to get really technical, let alone the maths.  Our aero modeller colleagues include quite a bit of aerodynamics in their literature, so I don't know why we have to apologise before including a bit of thermodynamics.  It is my intention to show how relevant it all is to our engine making.

But yesterday?  It was not a good day.  If you found yesterday's post a bit hard to follow, please don't despair, it did not do much for me either when I read again today.

Just as in building we sometimes don't get a part up to our standard and have to consign it to the scrap bin, in trying to build a knowledge base, the same thing happens.  I assume that is what the "modify" button is for?  I will try it in the next few days and let everyone know when it is ready to read again.  I have at least given thought to how I should have approached it.

But that is not the complete reason for my heading.  Earlier in the day, I was working on the brown stuff, when my jigsaw went bang, and all the lights went out.  The machine was not even warm, and I did not see any smoke get out.  So I reset the breakers, restored the lights and tried to start and again bang and no lights.  It was only about 25 years old, but clearly a trip to the tool shop was required.  The man did not miss a beat.  What a pity you did not bring it in last Friday, he said.  It might have still been on warranty!  I was not quick enough to point out it was not broken then.  It was really disappointing.  Despite using power tools since primary school, that is the first I have ever broken.  But the new one is really nice. 

To top it all off I found that my belt sander was so old that the correct size belts do not seem to be available.  (Much older than the jigsaw, it was my Dad's.)  Looks like more tool shopping required.  What a blow!  I never saw what others see I retail therapy, then one day I happened on a really good tool store.  Now I get it.

I have resolved some internet technologies this week, so I will prepare a diagram to illustrate where we are up to, as I should now be able to communicate with the scanner.  But it has been a long day, and it is late, I will be very lucky to to get any thinking time tomorrow so it may be Saturday.

Thank you for all the interest.

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on May 18, 2017, 05:59:05 PM
All accomplished craftsmen proceed at a measured pace, and pause to consider the next step in a complex sequence. Putting together a concise but intelligible series of explanations is no mean feat. Also, a person never knows their subject completely until they have had to explain it to someone who doesn't understand it, an enlightenment that every instructor discovers not too far into their career.  Quality not quantity! Don't feel pressured to produce every day. I and others appreciate your efforts. Regards Paul Gough. 
Title: Re: Talking Thermodynamics
Post by: derekwarner on May 18, 2017, 11:34:35 PM
Many years ago, the Director General of Education in NSW was a chap by the name of Sir Harold Whyndam.....

His claim to fame in education was the introduction  :slap: of the Whyndam Scheme....I just happened to be in the first year of such Gini-Pig changes

The Scheme bought revolutionary changes ...yes changes like Applied Heat teachers being allowed to use their last years papers and continue with Imperial units of measure ....or those with an enquiring mind could apply the new SI system [no slide rules or calculators.....just Trig Tables]

I was working 3 shifts at the time...so on afternoon and night shift I attended college in daylight hours and studied Applied Heat in Imperial......however when on day shift, my Applied Heat teacher  in the evening [being a Combustion Engineer from the Steel Industry] decided SI was the way to go....after all he studied SI in Europe years earlier

The consequence of this is that I failed Applied Heat that year, and repeated the subject in SI the following year

Derek
Title: Re: Talking Thermodynamics
Post by: Flyboy Jim on May 19, 2017, 03:11:44 AM
Yesterday was just one of those days!

Thanks Derek, yes, it is great to see so much interest.  I have always thought we were a bit too reluctant to get really technical, let alone the maths.  Our aero modeller colleagues include quite a bit of aerodynamics in their literature, so I don't know why we have to apologise before including a bit of thermodynamics.  It is my intention to show how relevant it all is to our engine making.

But yesterday?  It was not a good day.  If you found yesterday's post a bit hard to follow, please don't despair, it did not do much for me either when I read again today.

Just as in building we sometimes don't get a part up to our standard and have to consign it to the scrap bin, in trying to build a knowledge base, the same thing happens.  I assume that is what the "modify" button is for?  I will try it in the next few days and let everyone know when it is ready to read again.  I have at least given thought to how I should have approached it.

But that is not the complete reason for my heading.  Earlier in the day, I was working on the brown stuff, when my jigsaw went bang, and all the lights went out.  The machine was not even warm, and I did not see any smoke get out.  So I reset the breakers, restored the lights and tried to start and again bang and no lights.  It was only about 25 years old, but clearly a trip to the tool shop was required.  The man did not miss a beat.  What a pity you did not bring it in last Friday, he said.  It might have still been on warranty!  I was not quick enough to point out it was not broken then.  It was really disappointing.  Despite using power tools since primary school, that is the first I have ever broken.  But the new one is really nice. 

To top it all off I found that my belt sander was so old that the correct size belts do not seem to be available.  (Much older than the jigsaw, it was my Dad's.)  Looks like more tool shopping required.  What a blow!  I never saw what others see I retail therapy, then one day I happened on a really good tool store.  Now I get it.

I have resolved some internet technologies this week, so I will prepare a diagram to illustrate where we are up to, as I should now be able to communicate with the scanner.  But it has been a long day, and it is late, I will be very lucky to to get any thinking time tomorrow so it may be Saturday.

Thank you for all the interest.

MJM460

Well....... speaking as someone at the bottom of the "knowledge chain", I pretty much got the drift of what you were saying, so all wasn't lost as far as I'm concerned.

Looking forward to more. As was said by another..........don't feel like you have to push yourself.

Bummer about your jigsaw and belt sander. I've got a few of those kinds of tools myself.

Jim

PS: Do you go by a name other than MJM360? I may have missed it.
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 19, 2017, 01:54:59 PM
The other side of the piston - Take 2

Thanks Paul for your encouragement.  Every day is not viable in the long term, there will be slower periods so thank you for your understanding.  But I would like to get past this one while it is fresh if I can.

Derek, sorry about your trials with the subject.  Such arbitrary changes don't make it easy.  I also studied in imperial units.   Even have my copy of 7 figure tables still.  Then, just as we went metric with all the advertising to help us convert, I went to work in Canada, near Brian's territory.  No metric conversion there.  When I came home, the conversion was essentially complete, and I had to work in metric units cold turkey.  Needless to say, I am still not alone in going out to buy 2.4 m of 4 x 2!  I now prefer to do my calculations in metric units and I hope you will see why when I get to talking about units of measurement.  But I know many forum members are more familiar with imperial units and I will try and remember to convert key numbers.  In the mean time, if the answer does not look right when using imperial units, just alternately multiply and divide by g until it does!  Or something like that I think.

Glad you got something from that last post Jim, I still hope to improve it for you.  Yes sentimentally sad about the tools, but it is an opportunity to go shopping, and the economy needs me.  I am a probably unnecessarily shy about using my name, even paranoid perhaps, but if we ever get the chance to meet, or you need a pm, I will make sure you have my name.

So here goes on the lower side of the piston, still thinking in terms of a vertical engine with the cylinder on top.  To make the explanation totally consistent and complementary with the top, I need to continue to talk in terms of the work done by the gas.  I have mentioned that we unconsciously defined down as the positive direction in the description of the top side power stroke, and we have to stay with that definition for the lower side. 

Why the significance of direction?  Remember that force and displacement quantities are vectors, so have magnitude and direction.  On the lower side of the piston, the force direction is up, so it is  negative.  On the power stroke, the displacement is also up and again negative.  Work is force times displacement, and when we multiply two negative quantities, we get a positive for the work done by the gas.  This is expected, because I would not classify work as a vector, I suggest there is no such thing as negative work.  When our calculations indicate negative work by the gas, it simply means the piston is doing work on the gas, gather than the other way around.

So we have the power strokes on the top and bottom of the piston both do work on the piston.  The work by gas on the top acts to push the piston down, while the work by the gas on the bottom pushes it up.  If our valve gear is timed correctly, the two strokes occur alternately, so we get two power strokes per revolution.  More output without making the engine a whole lot larger.

For a double acting engine, the processes follow the same sequence of admission, cut off, release and compression as the top side, just half a rev later.  Or, if you prefer it, 180 degrees out of phase.  There is one minor difference however.  The gas pressure is not applied to the whole area of the piston, as the area occupied by the piston rod has atmospheric pressure, not gas pressure.  This means the force and consequently the work done is slightly less.  Not much less for an engine with a small diameter rod in relation to the cylinder bore, essentially a low pressure engine, which describes all of our models.  However on a larger, very high pressure machine, the rod size can be very significant.

For a single acting engine, it might be harder to see how all this works.  This post is more than long enough, so I will continue that next time.


MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 20, 2017, 12:32:52 PM
The lower side of a single acting cylinder

I hope my last post made sense to everyone, and that it was enough improvement on my earlier attempt on the lower side of the piston.

I want to continue this time by applying the same reasoning to a single acting engine, such as the little oscillators made by several companies.  Many of us had one of these when we were very young, or at least saw them in our local toy shop.

The top side of the piston in a single acting engine acts the same as the top side of a double acting engine, but how these process work on the lower side of a single acting engine may seem a little more obscure.

The gas pressure under the piston on these engines is near enough to constant atmospheric pressure throughout both the power and exhaust strokes.

Now, in thermodynamics we have to think in terms of absolute units, where atmospheric pressure is close to the defined standard atmosphere of 101.325 kPa, or 14.696 psi, varying slightly as the succession of weather patterns pass over us.  Please don't get upset about the .325 kPa or 0.096 psi, it can't be read on a normal pressure gauge, or a slide rule.  Even the third significant figure is probably not significant in practical terms. 

There is no such thing as negative pressure on the absolute scale.  Pressure is caused by the impact of many molecules on the face of the piston, no pressure implies no molecules, as in outer space for example.  There is no mechanism for the molecules to pull the piston towards themselves.

On our single acting cylinder, atmospheric air pressure does work on the underside of the piston on the up stroke, and the piston does work on the air on the down stroke, just as for the gas on our double acting cylinder.  The problem is that the gas (atmospheric air) pressure does not vary significantly, so the work done by air on the up stroke is no larger than the work the piston must do on the down stroke.  If you are really being pedantic, it is a little bit smaller.  There is no excess to contribute to the engine output.  In fact we need a source of energy to do the work necessary for the exhaust stroke of the top side.  We will eventually see how the flywheel supplies this energy.  At least we do not need valve gear for the lower side of the piston of a single acting engine.

There we have it, I hope a little clearer this time.  Next time, a little consolidation and summary of the key points before we move further away from the piston faces.

Thanks for dropping in

MJM460
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on May 20, 2017, 04:07:15 PM
The lower side of a single acting cylinder

On our single acting cylinder, atmospheric air pressure does work on the underside of the piston on the up stroke, and the piston does work on the air on the down stroke,

I think the first half of that statement is only true for an engine with a vacuum in the upper section on the up stroke like a Newcomen engine or a flame gulper engine.

The bottom side of the piston is acting like a very low pressure air compressor. The upstroke is the suction stroke for the atmosphere, it is not pushing the piston the flywheel is making the piston move.

In any case this is a very very small amount of work and I do not even remember it even being mentioned in any class.

The large low speed two-stroke diesel ships I worked used the underside of the piston as an air pump at low speeds because that is where the turbocharger does not supply enough boost pressure. In this case the amount of work the underside of the piston is doing is large enough to be considered.

Dan
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 21, 2017, 02:31:48 PM
Looking at the whole piston

Hi Dan, thanks for a great introduction to this next post.  You will not be alone in your thinking, just a bit ahead of me.  I hope that what follows will clarify my meaning.

So far, I have been looking at the upper and lower face of the piston separately.  This approach is of interest because the point at which the energetic vibrations of the gas molecules act at a surface to do mechanical work is the surface of the piston.  The gas molecules colliding with the walls of the cylinder are seen as pressure, which over an area produces a force.

On the sides of the cylinder, no movement results from the force, so no work is done. 
(W=F x d, F x 0=0).  Similarly for the heads of the cylinder.  However the piston is not fixed relative to the rest of the cylinder, and it moves as a result of the collisions. The distance is no longer zero, so this is where the work is done.  This is where heat is harnessed to do mechanical work.

We can ignore the sides of the piston, as the gas forces are in equilibrium, no movement at right angles to the cylinder axis, so again no work by the gas.  (There may be mechanical forces but that is another discussion.)  Of course, a piston has both top and bottom surface, and as we have seen the force on each varies throughout each revolution, the direction of the force on the two sides is opposite, and of course the direction of movement alternates between up and down.

The piston moves in the direction of the larger force.  So we now need to look at the force balance on the piston.  This involves understanding the exact stage of the cycle on each side of the piston at each instant, so we can understand how the resulting force on the piston changes with time.

Now the job of the valve gear is to coordinate the movement of the valve with that of the piston so it all happens in an orderly manner and the engine is able to run continuously.

If we look at the double acting cylinder first, starting just as the piston approaches the top dead centre.  Here, the valve opens the gas supply to the top of the piston, and we have a downwards force, somewhere near the maximum for the cycle.

At the about the same time, the valve opens the lower side of the piston to the exhaust system.  The point of release.  The remaining pressure under the piston is rapidly depressurised into the exhaust The piston moving down overcomes the remaining pressure under the the piston, which is near the minimum for the cycle, and pushes the remaining steam out of the cylinder during the down movement.  The force on the top side is enough to not only enable the piston to push the exhaust out, but with excess force available to do external work.  Remember the absolute pressure cannot be negative, the pressure can only be less than that on the other side.  So the piston moves in response to the difference between the pressure on the top and the pressure on the bottom.

(For those familiar with the formula Horse Power = P x L x A x N/ 33000, it is important to understand that P is actually the difference in pressure between the two sides of the piston, but I will get to this in the next few posts.)

As the piston moves down, the valve closes the gas inlet, cutoff point, and expansion starts, then finally opens the exhaust port, or release point all on top of the piston.  During this same downwards movement, the valve has opened the exhaust then near the bottom dead centre, then later closes it to start compression.

I think it would be helpful to try and construct a pressure - volume diagram which shows both sides, aligned so we can see the relationship between the force on each side.  I will try and have a diagram with my next post when I will again try and show what the above means for a single acting cylinder.

Thanks everyone for following

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 22, 2017, 12:03:34 PM
Another clarification.

I don't think my last post really clarified much, and probably did not answer Dan's question.  Another part in the bin!  So bear with me while I try again to summarise the important points in only three or four short paragraphs.

Thermodynamics discussions are best based on absolute values of pressure.  Zero pressure is full vacuum, very difficult to achieve.  Atmospheric pressure is about 101.3 kPa. Our usual gauges read zero at this point.

The gas pressure on top of the cylinder is different from that on the lower side.  Each of these pressures varies through a similar series of processes but displaced by half a revolution.  By analysing the top and bottom separately we can see how the resulting force on the piston varies throughout the revolution.

For a single acting engine with atmospheric exhaust, the work on the lower side of the piston is as I described earlier, but because the pressure does not vary greatly throughout the cycle, there is no contribution to the engine output.  (This would not be the case if the exhaust system was lower than atmospheric pressure due to a condenser.  But let's leave this until later.)

I hope this helps clarify the approach I am taking. 

Still working on a diagram.  The conventional presentations do not easily show both sides of the piston at the same time.  I am working on it.

Next time I will summarise the key points so far and how these points can help us build a better engine.

Thanks for following

MJM460
Title: Re: Talking Thermodynamics
Post by: Maryak on May 23, 2017, 01:37:42 AM
  The conventional presentations do not easily show both sides of the piston at the same time.

By that do you mean simultaneously? I ask because in full size double acting engines using a Dobie McGuiness Indicator both strokes are recorded on one card then calculated to give the mean cylinder IHP.
Of course one stroke is recorded then the cocks switched to record the other stroke immediately after. This is a finicky and fickle operation which usually takes more than one attempt to get an acceptable card of the cylinder.

(http://i389.photobucket.com/albums/oo340/Maryak/MEP_0001_zpssndtylny.jpg)
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 23, 2017, 02:24:51 PM
The key message so far.

Thank you Maryak for a really great example of an indicator diagram showing top and bottom faces of a double acting cylinder.  It appears to be very realistic.  Did you by any chance take it yourself?  It certainly shows some interesting departures from the "ideal" diagrams we often see in text books, and many details which I would like to come back to later.  I had thought that presentation would not illustrate what I wanted, but now, on looking at it carefully, I think I will use it after all in due course.

My previous posts had many words and probably did not help as much as I would have liked.  They say if you are in a hole, stop digging.  I do not want to get bogged down in pure theory, as my interest is more into the practical areas where thermodynamics helps us understand our engines and how to make them better.

So, let's summarise the key messages to date, so we do not get lost in some of the inevitable side tracks of the discussion.

Our little engines are heat engines, engines which harness heat energy to do mechanical work.  The conversion interface is the face of the piston, where the random motion of molecules of gas is experienced as pressure.  The pressure on the piston causes it to move, thereby doing mechanical work.

We saw that the resulting force on the piston is the vector sum of the force on on the power stroke side of the piston and the other face which is exhaust pressure (or atmosphere for a single acting engine.)

Work is done when a force moves through a distance.  Force is pressure times area, the distance moved is the stroke.  If we rearrange this, we see that work equals pressure times swept volume.

So the potential work output of the engine depends only on the differential pressure on the piston and the swept volume.  No gas properties, no temperature, just differential pressure and the volume swept by the piston.

Obviously, the swept volume is determined by the basic size of the engine.  Clearly a larger engine should be able to do more work, no surprise there.

For a given size of engine then, the only thing we can or need do is deliver our chosen gas to the face of the piston at maximum pressure, while minimising the pressure on the exhaust side.

It might see surprising that it can be reduced to this one parameter.  Every part of an engine and its associated equipment is directed to that end.  However, the pressure at the piston varies greatly throughout the cycle, and as you may have seen in our discussion of air vs. steam, it is not always easy to predict the pressure at any point, especially during the expansion which occurs when the inlet valve closes.

Then, of course, having converted heat to mechanical work, we must then transmit that work to our chosen load with as little loss as possible.  As we all know , there are many interesting ways to loose some of our output to friction, and I hope to explore some of these as the discussion proceeds.

Next time I hope a brief look at converting the piston movement to a rotational output.

Thank for following along

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 23, 2017, 02:47:59 PM
Hi Just a quick question ...When the Mallard Achieved the 126 MPH record what would be the temperature difference between the inlet steam and the exhaust steam ??.............Also how fast are the molecules moving in relation to the linear speed of the piston ??...........!
Willbert......
Title: Re: Talking Thermodynamics
Post by: Maryak on May 23, 2017, 10:08:35 PM

Thank you Maryak for a really great example of an indicator diagram showing top and bottom faces of a double acting cylinder.  It appears to be very realistic.  Did you by any chance take it yourself?


Your welcome. No it is not one taken by me, (I'm a little more clumsy with the stylus). It is from Southerns Verbal Notes and Sketches.

Regards
Bob
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 24, 2017, 12:31:22 PM
Velocity of molecules.

Hi steam boat willy, I am not sure about the temperature difference for the Mallard.  My grandson saw the Mallard in a museum and brought home an HO model only a month ago!  Temperature difference depends on the inlet temperature and degree of expansion, which would not be much on a record attempt when max power and speed are more important than efficiency.  After release, the steam is basically throttled by the exhaust valve to the exhaust system, and throttling involves a only a small temperature drop, so I would guess a relatively small temperature drop overall.

The speed of molecular motion is the obvious question when the collision of such tiny particles with the piston is responsible for so much pressure.  I am sure many others are also wondering.  My ancient text book, by Jolly, published in 1961 (it was nearly new when I bought it!), has a calculation for oxygen at 0 C based in ideal gas.  I substituted the relevant figures for steam at 150 C, and the found an RMS velocity of 765 m/s.  It is random motion with a wide range, but the book says RMS velocity is about 10% higher than the average (velocity distribution is not sinusoidal).  The energy in this vibration is proportional to the speed squared.

For an idea of piston speeds, my little oscillator piston has a stroke of 16 mm, and runs about 2000 rpm on a digital non contact tachometer.  This means the piston travels 64 m a minute or 1.06 m/s.  My unreliable memory recalls client standards for compressors to require less than 5 m/s.  As the limits are based on ring wear, I assume engines are similar.  The motion is roughly sinusoidal, to the maximum speed is 2 times the average and RMS speed is 0.707 times the maximum.  So a very large number of collisions of tiny particles at around 760 m/s with the piston which is 0 m/s at top and bottom dead centres up to a max of 2 m/s for my oscillator, or 10 m/s for my industrial machines.  Eighteen grams of steam at atmospheric conditions has around 6.03 x 10^23 molecules, (text about 5 bar pressure deleted with apologies).  Many, many random collisions, so many it feels like a constant pressure.

I hope that gives everyone an idea of the numbers involved, a worthwhile diversion from what I intended.  There is always another day for that.

Maryak, I know what you mean about the difficulty of getting such a good looking diagram from the instrument.  However, from that source, it is clearly well informed about a real indicator diagram, unlike most I have seen, which are constructed more to show an ideal engine cycle, and do not reflect a real machine.  It is a great example and I am sure I will be referring back to it.  Thank you.

I think that is enough for today, so back to rotational output next time.

MJM460
Title: Re: Talking Thermodynamics
Post by: Maryak on May 25, 2017, 12:18:36 AM
The book I referred to, (Vol 1), contains many indicator diagrams with various results due to +tve -tve pressure, improper valve settings, ring wear etc. It may be worth a trip to a library or steam preservation society.

Regards
Bob
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 26, 2017, 12:01:23 PM
Producing rotation.

Not much progress yesterday.  Out to dinner with friends - pan fried salmon and steamed vegetables, all home cooked.  Delicious.

But I did notice a small error in my reply on the number of molecules.  Went from mass to volume too quickly, I have edited the offending bit out.  As our engines have nothing like 18 gm. in the cylinder, I made a rough calculation of how many would be in the swept volume of my little oscillator.  At atmospheric pressure it is about 10^20 molecules.  At 5 bar, it would be 5 times that.  Even on this small scale, a huge number of collisions.

The first engines were limited to pumping type applications which could directly use the reciprocating action of the piston.  The crank and conrod seems obvious now, but it was a major breakthrough at the time.  Patent squabbles led to the development of the scotch crank, and various clever geared mechanisms, we all know the story.  But today, patents have expired and the crank and conrod is practically universal, so I will stay with that.

The geometry of the mechanism means that the torque is zero at top and bottom dead centre, and reaches a maximum at about 90 degrees, which is about double the average.  Like a sine curve, but slightly modified by the con rod geometry.  And this assuming the pressure is constant throughout the stroke.  Any reduction in pressure through the stroke results in a reduction in torque from the constant pressure calculation.  The point is that the torque output is far from uniform.  Even worse on a single acting engine when the torque is negative on the exhaust stroke.  This fluctuation is a consequence of the geometry, and cannot be eliminated by pressure variations.

Now most of us have heard of Newtons laws and the formula F=ma.  (Force equals mass times acceleration) which applies to linear motion.  Less well know is the rotational equivalent, Torque equals Moment of Inertia times angular acceleration.  Both of these are now expressed more elegantly in two of the conservation laws of physics.  The fluctuating torque results in a fluctuating rotational speed which would be unsatisfactory in most applications, if indeed the mechanism could be persuaded to pass through the zero points top and bottom.

In order to even out the speed fluctuation, and get through the zeros, a flywheel is mounted on the shaft.  The shape of a flywheel is designed to provide maximum moment of inertia, and hence minimise the change of rotational speed caused by the changing torque.  So the conrod and crank combine with the flywheel to produce an acceptably smooth rotation.  What is acceptably smooth?  Well, for an example, the standards for my large industrial reciprocating compressors required the speed fluctuation to be limited to +/-7% within each revolution.

Perhaps more on flywheels next time?

Thanks for following.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 27, 2017, 01:18:24 PM
Flywheels - a little diversion into physics.

Last time I started on flywheels.  The flywheel accelerates in response to a torque, meaning it spins at an increasing angular velocity, and in so doing stores energy.  When the torque produced by the engine is less than that absorbed by the load, the flywheel returns this energy by slowing down.  This enables a single acting engine to run, and reduces the speed fluctuations of a double acting engine that otherwise result from the normal production of torque.  The physics of this is described by the law of conservation of angular momentum.

We have an intuitive feeling for momentum in linear systems, that "something" which makes a body in motion continue in a straight line unless acted on by an external force (Newtons Law).  It can be quantified as the product of mass and velocity.  It has the units kg.m/s.  (In imperial units ft. lb(mass)/s.)  Velocity is a vector, and when we multiply by mass to get momentum the resulting momentum quantity is also a vector (with the same direction as the velocity).

In spinning objects, such as spinning tops, gyroscopes, bicycle wheels, and ice skaters, the analogous quantity is angular momentum.  It is quantified by multiplying the moment of inertia by the rotational speed.

But what is moment of inertia?  First it must be defined in relation to an axis of rotation.  Then, for a point mass, it is mass times (distance from the axis)^2.  It has units of kg.m^2.  So moment of inertia is proportional to mass, but the distance term means that it is also dependant on the distribution of mass, or shape.  The squared part means that shape is much more important than mass.  We can see that the typical flywheel, which has a heavy rim plus minimal spokes to keep it centred on the axis of rotation, and a small hub to fix it on the shaft, should have a large moment of inertia relative to its mass.  A solid disk would have much more mass (and bearing load), but only slightly larger moment of inertia than a spoked flywheel.

Next, we need to know the rotational speed.  No great mystery in that, but note that the units of angular rotation are radians, not degrees, grad, or even revolutions.   A revolution of 360 degrees is two times Pi radians.  Rotations have no dimensions, but are vectors and have direction.  Hence rotational speed is measured in radians/s, and has the units just "per second".

Now we can multiply moment of inertia (kg.m^2) multiply by angular velocity (per second) to get units of angular momentum as kg.m^2/s.  This is the quantity which tends to average out speed fluctuations.  Like linear momentum it is a vector.  The direction is conventionally defined as coinciding with the axis of rotation.  If you make the typical "thumbs up" sign with your right hand, when your fingers follow the direction of rim rotation, your thumb shows the positive direction.  (This could require a "thumbs down", so don't be insulted, it could be just someone trying to work out the direction of the angular momentum of a spinning top!)

Two interesting points to close on.  Angular momentum and torque are both vectors.  When torques and angular momentum interact, we can predict the resulting vector mathematically by performing a vector product multiplication.  The result has a direction which is determined by a right hand rule.  This vector multiplication predicts all the movements we see when a gyroscope reacts to gravity, or we hold a spinning bicycle wheel by its axle and try to move the axle in various ways.

Second, linear momentum and angular momentum are quite independent quantities with independent conservation laws, and an object can have both at the same time.  (There is a third conservation law which can also be useful to us, but back to that later.)

I hope this answers many common questions about flywheels.  Next time I will try and return to getting more out of our engines.

Thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 30, 2017, 12:31:47 PM
Flywheels (2)

Thinking about my last post, I realised that it might be good to put some numbers to a real flywheel before moving on, just to give a sense of proportion.  I calculated the mass of the flywheel on my little oscillator, nothing complex.  It is machined from a 2 inch bar, cleaned up to 50 mm diameter. 15 mm in the axial direction.  I left the hub and rim full length, and machined a recess in each side to leave a web 6 mm thick.  Nothing fancy, you guys are inspiring me to at least drill some lightening holes or even curved spokes.  But this was a few years ago.

I believe the material is cast iron.  I have used cgs units for such a small flywheel to avoid having too many decimal places in the numbers, but for any other calculations I need to do it in ISO metric or mks system.  I was interested in a few key results.

The moment of inertia turned out to be 537 gm.cm^2.  This is 80% of the moment of inertia for a solid disk, and 436 gm.cm^2 (or 83% ) of this is due to the rim which was 15 mm wide and 6 mm in the radial direction.

What if I had made it of steel?  Well the density of steel is about 7% greater than cast iron, and both mass and moment of inertia increase by 7%.

Brass would make more difference and you might like the appearance or rust resistance.  Brass density is about 17% greater than cast iron, and gives about 17% higher moment of inertia.

What if we want to save some weight?  Perhaps Jim is designing his steam powered plane.  The density of aluminium is only 38% that of cast iron.  So if we use the same design, mass and moment inertia will be 38%.  If the original flywheel was just enough, that would not do.  Can we make a lighter flywheel from aluminium if we modify the design?  The clue is in the R^2 term in moment of inertia.  If we increase the diameter to just 66 mm diameter compared with the original 50 mm) we will have an aluminium flywheel with the same moment of inertia as the original cast iron, but only 60% of the mass.

Now as a practical matter, I found the calculation was quite sensitive to even 0.1 mm change in the overall diameter, and I think we would all agree that flywheel dimensions are not that critical, clearly most are "adequate" rather than bare minimum.  If we want our engine to fly it would be worth experimenting to find the minimum required moment of inertia using cheap steel discs.  Then we could make a lighter flywheel to that moment of inertia by increasing the diameter and using aluminium.

I hope that is of interest.

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on June 08, 2017, 12:37:58 PM
More on producing rotation

Sorry to have been a bit silent for a while.  Family and other activities always need due attention.

I have also decided that this thread needs pictures, surprising that no one has mentioned it, but I think I have overcome some technical challenges, so here goes.

Figure 1 has an overall sketch engine of the simple vertical engine I am thinking of in my terminology and arrangement.  You can see why I was the engineer, not the draughtsman. 

I have included a sketch of the piston showing the important gas pressures on the top and bottom faces.  Remember I am using absolute pressures and I am assuming for the moment an atmospheric exhaust system.  We will discuss condensing later.  Hence the exhaust pressure is a little above atmospheric pressure to push the exhaust gas out of the cylinder.  You can see the relationship between the pressure on the piston faces and the resulting force on the piston.  The exact pressure is not important, and is varying from moment to moment anyway.

Figure 1 also includes sketches showing the forces on the wrist pin, and the triangle of forces which shows the relative size of the forces.  The horizontal force is not due to gas on the sides of the piston, but a result of the angle of the conrod. 

Finally, I have shown the torque calculation.  The force is the conrod load, and the moment arm (a) which causes the torque is shown in red.  Torque varies from zero to a maximum on the power strokes, and for a single acting engine is actually negative during the exhaust stroke.

Figure 2 shows the gas pressure, force triangle and torque for three important cases.

Detail (a) is for the power producing down stroke.  Piston force is downward.  Note that the conrod is in compression.  I have shown the rotation as anticlockwise.  This is the normal trigonometry convention, and means that your calculator gives the correct result for the parts of the circle where trigonometric ratios are negative.  Only important if you put these calculations into a spreadsheet, then increase the crank angle in increments, and have the computer recalculate for each angle through a full revolution.

Detail (b) is for the upstroke of a double acting engine.  Note how the moment arm is shown, and that the piston force is now upward and the conrod is now in tension.

Detail (c) is for the upstroke of a single acting engine.  As there is no steam pressure under the piston, the flywheel provides the necessary energy to push the piston upwards, and expel the exhaust.  Note that the conrod stays in compression for the upstroke of a single acting engine, compared with load reversal for a double acting engine.  This factor is an important consideration in the lubrication of the wrist pin and crank pin journals.  If you are familiar with larger full size engines, the pistons are much heavier in relation to the gas forces, and momentum considerations modify this simple analysis to some extent.

I know I have been labouring these points a bit, but this is the process by which the pressure energy in our gas produces force on the piston, which produces mechanical work when the piston moves in the direction of the gas force.  The force on the piston loads the conrod which gives torque at the output shaft. 

This is the process by which energy in the gas is changed to mechanical work.  This is how our engines really work!

Thank for looking in, more next time.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 08, 2017, 12:39:36 PM
I had no luck attaching two figures, so here is the second

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 11, 2017, 02:11:48 PM
Completing the P-V diagram

I have included a idealised sketch P-V diagram, fig. 3, to illustrate the discussion.  Note I have assumed the pressure during admission to be constant, and I have rounded the corners to acknowledge the point that real valves take a finite time and rotation to open or close.

We have discussed the top boundary of the P-V diagram, shown in blue, admission and expansion of the power stroke, and also the lower boundary, the exhaust stroke, also in blue, but to describe the complete cycle so our engine can run continuously, we need to connect up the ends with the lines shown in red.

At the end of the power stroke, the cylinder pressure is still well above the exhaust pressure when the exhaust valve opens.  The remaining pressure expands into the exhaust system with that familiar chuff or beat of the steam locomotive in heavy load.  Of course when the valve gear is notched up to give earlier cut off, the beat is less distinct as expansion from an earlier cut off may be close to exhaust pressure when the exhaust valve opens.  From the thermodynamics point of view, the significance of this process, called the release, is that all the remaining energy in the steam goes out the exhaust and plays no further part in doing useful work.  When efficiency or water conservation is important, of course we can capture some of this heat in a feed water heater, and return it to the system to use in a later cycle.

At the end of the exhaust stroke, we have to get the cylinder contents back up to the inlet admission pressure.  There are two alternative processes possible, and in practice we probably get a little of both.  In the diagram, I have shown the exhaust valve closing a little before the bottom dead centre, resulting in compression of the steam remaining in the cylinder.  The energy for this compression of course comes from the flywheel, and so subtracts from the net output of the cycle.  Is there an alternative?

The alternative is to close the exhaust at bottom dead centre, and open the inlet valve with the cylinder pressure still at exhaust pressure, so that new steam expands into the cylinder and flows until the pressure equalises at the admission pressure.  We get a little more work output from the cycle, at the expense of using a bit more steam compared with the cycle in which we use compression.

So what is the difference?

The difference comes down to efficiency.  The energy in the steam which expands into the cylinder while it is still at low pressure is downgraded in terms of its ability to produce mechanical work.  It all goes into heat in the exhaust stream.  So there is extra steam consumption but no extra work output.  On the other hand, the energy which goes into compression remains mostly able to be converted back into work.  There are losses, but not total loss.  So a little less work output per cycle, but a steam saving which is greater than the work lost.

There is another more practical benefit to having some compression at the end of the exhaust cycle.  This is often described as cushioning, and is related to the need to stop then reverse the kinetic energy of the moving piston.  It may be even more directly related to softening any noise from slack in crank pins and wrist pins.  I don't know the full explanation, but the observation will have an explanation, whether I understand it or not.

Next time I will look at the more realistic P-V diagram that Maryak provided and how this differs from the idealised ones I have referred to so far.

Thanks for following along

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 12, 2017, 12:59:10 PM
Introducing some reality.

Not so many comments these days, I hope I have not turned everyone off after the initial enthusiastic response.  I think we may now be beyond the tedious parts and things should move a bit quicker.

By now, it should be obvious that to get the most power out of our engine, we have to get the highest possible pressure to the face of the piston, and at the same time, the lowest possible exhaust pressure, so that we have the greatest differential pressure difference on the piston.

We have assumed an idealised picture of the p-v diagram.  But what happens in practice?

Maryak has kindly provided a p-v diagram (reply#53, page 4 of this thread) from a text book written for marine engineers, about 1916 I think.  It shows the diagram for both the top and the bottom of the piston.  As the textbook was intended to instruct, I assume it shows features, both good and bad which indicate the health of the cylinder.  So what can we see?  Let's look first at the diagram for the top, starting with admission at the top right, moving anticlockwise around the diagram.

We can see the admission process is not really constant pressure, however a drop from around 110 to 92 is probably a reasonable approximation as we will later see.  Then we appear to have cut-off and expansion, and the diagram again looks reasonable, ending with release, when the exhaust valve opens before bottom dead centre, so the cylinder pressure is right down at exhaust pressure when the return or exhaust stroke begins.  Finally we see compression to perhaps 60% of the inlet pressure.  The vertical section at top dead centre to the start of the down stroke probably implies steam flow in to raise the pressure up to the inlet pressure.  To those of you who have experience with a real indicator diagram, does that sound like a reasonable interpretation.

If we then look at the bottom trace, we have a different picture.  The admission starts at a pressure slightly higher pressure than for the top.  Then the expansion section requires some imagination to see, rather the pressure falls quicker than it did on the top diagram down to release.  Then a very flat exhaust pressure until the exhaust valve closes in time to give compression.  The exhaust valve seems to be quite good, as compression is a little better than for the top.  No doubt the student was asked to explain all the departures from the expected trace.  I leave you to think about what faults could cause the pressure trace illustrated, a bit of a detective logic puzzle.

We can see that the engineer did not use a planimeter, but a used simple arithmetic averaging process to find the indicated mean effective (differential) pressure in the cylinder and indicated power.  We can see the top has a higher mean effective pressure, 870, and the area of the diagram shows there are power losses on the underside of the piston which has an average of only 820, presumably psi.

Next time we will start to look at the issues which get in the way of achieving maximum differential pressure across the piston.

Thanks for dropping in.

MJM460
Title: Re: Talking Thermodynamics
Post by: Steamer5 on June 12, 2017, 01:24:21 PM
Hi MJM,
 Still following & enjoying the read & explanations & learning stuff!

Cheers Kerrin
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 13, 2017, 01:37:42 PM
Hi Kerrin,

Thanks for your reply, I am glad you are still finding it worthwhile.  I will assume you speak for many.

I have been doing lots of calculations today and it's getting late but I will have more tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 14, 2017, 02:10:52 PM
Supply and exhaust pressure

It is time to ask what pressure we might have at the piston face.  It is obviously less than boiler pressure, but how much less?

On its way from the boiler to the cylinder, the steam must go through pipes, possibly a throttle valve, the steam chest, valves and the cylinder steam passages.  These items all involve pressure loss.  The losses are caused by wall friction in pipes, acceleration losses in entering the pipe end, and more losses when the steam expands into a larger chamber at the end of the pipe.  How can we get an idea of the magnitude of these losses?

If we look at pipe friction, there is a formula known as Darcy's formula which is used in industry to calculate pipe friction.  If we look at my little oscillator again, I have measured the steam flow by measuring the weight of water poured into the boiler before the run, and subtracting the amount I draw out with a syringe at the end.  I also note the time between the engine starting and stopping for the run.  It seems to be about 0.73 kg/hr.  Not much, but I am using a small Meths burner similar to that supplied by Mamod and other small engine manufacturers.  From this, Darcy's formula gives about 1.45 kPa per meter in a 5/32" tube (2.55 mm inside diameter).  As my supply pipe including a superheater is only about 500 mm, I think we can ignore this source of loss.  Alternatively compensate by running the boiler about 1 kPa higher.  Those of you who are familiar with metric units of pressure will appreciate that you would need a very accurate pressure gauge!  Clearly not worth doing a lot more maths to evaluate this more closely, but if you have any doubt, increasing the tube size to 3/16" reduces this loss to about 0.4 kPa/m.

The other losses occur each time the flow path changes cross sectional area.  As the mass flow is the same all the way along the path, changes of flow area involve changes of velocity.  Each time the velocity changes there is a loss of energy. 

To keep this post to a more reasonable length, I will look in more detail at these other losses, next time.

Thanks for dropping in

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 15, 2017, 01:47:32 PM
Losses along the flow path

Last time I looked at friction losses in the steam line, this time the losses due to velocity changes along the path from the boiler to the cylinder.

We all know about a convergent - divergent Venturi and have probably heard of Bernoulli, and the law which says the pressure energy can basically be converted to velocity energy and vice versa.  However there are some very important qualifications, perhaps not so well known.  First the reduction in area at the entrance must be smooth, not too quick, and the entry must be well rounded.  Not exactly what we see at the boiler outlet or other points in the steam path where the flow area reduces.  Similarly, the exit from a venturi must be very gentle, diverging at about 15 degrees or less with the entry and exit very well rounded to minimise any generation of turbulence.  You see these shapes if you look closely at injectors used on locomotives.  They have to be very carefully made and appear to be a bit of a black art.  Again, nothing like the pipe entrance to the steam chest.  Each of the sudden area reductions or expansions involves an energy loss due to turbulence which dissipates energy in the form of heat, and reduces the ability of the fluid to do work by reducing the pressure.  On a well insulated pipe, (you do insulate your pipes, don't you?) conservation of energy applies and no work is done, but the energy simply goes through to the exhaust as a higher temperature.  So how do we evaluate these losses?

Essentially the losses are proportional to the velocity squared.  So we need to know the velocity in our steam pipe.  In metric units, the energy due to velocity is just v^2/2, to give N.m/kg, but as we conventionally use kJ, kilojoules as the unit of energy we need to further divide by 1000 to give kJ/kg.  Pressure energy can be found by dividing the pressure in Pascals by the density to give N.m/kg.  Because a pascal is such a small unit, (atmospheric pressure is about 101000 Pa) we generally use kPa, so dividing by density then gives kJ/kg which is directly comparable with the velocity energy.  Then by a little mathematical manipulation, we can show a pressure drop for a change of velocity.

I don't propose to provide all the maths though it is simple enough, as the result is enough to identify some conclusions.  For an exhaust system without condensing, the pressure is close to atmospheric, and steam density about 1.7 kg/m^3.    We can then calculate the pressure loss for a change in velocity.

I tried to insert a table, but it did not come across, but basically for velocity of 10, 30, 100 and 200 m/s, the corresponding velocity pressure is 0.085, 0.765, 8.5 and 34 kPa

Each time our steam enters a pipe, we loose pressure equivalent to the change in velocity and again when it exits into a larger flow area such as the steam chest.  Some extra losses around the valve, and through the valve, entry to the steam passages and again on exit into the cylinder.  Say at least six times the velocity pressure.  Clearly providing the valves open quickly to at least
 area of the steam pipe, these losses would indicate a velocity of around 30 m/s might be acceptable, but losses increase very rapidly above that.

I calculated the velocity for my little oscillator, and found 15 m/s for 5/32" tube, 8 m/s for 3/16" and 5 m/s for 1/4 " tube for the steam supply and about double that for the exhaust.

 I was expecting worse,  but these figures certainly seem to support using 5/32" tube as is typical for supply to these small engines, with a size larger, say 3/16" for the exhaust.  Of course the flow is doubled for a double acting engine, and my little gas burner provides about twice the heat of my Meths burners, so larger tubes are a good idea.  Personally I use 3/16" tube for steam supply and 1/4" exhaust but I have only built small engines so far.  For larger engines it is a matter of calculating the velocity for your steam supply and exhaust, using density from the steam tables for the pressure you intend, and keep the openings in your fittings and steam passages close as practical to or larger than the steam pipe internal diameter.  Ideally aim for less than 20 m/s.

The item I have skipped through quickly is the valves, so I will look at this in more detail next time.

Thanks for dropping in

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on June 16, 2017, 04:52:37 AM
Back on May the 15th of May...I offered

'I agree understanding what is happening with/within our model steam plants is essential' and spoke of using an inexpensive digital laser pyrometer to help achieve this

So all the lagged lines and temperatures seemed understandable or as expected until I got to the de-oiler  :Mad: ......

This proprietary built de-oiler unit steam inlet is a single 5/32" OD copper tube.......[disregard the second 5/32" inlet installed  for the blowdown of the displacement lubricator] .....the discharge from the de-oiler again a single 5/32" OD copper tube]

I adapted further 5/32" x 0.014 wall brass tubing interconnected with K&S telescoping brass 1/8" x 0.014 wall internal joiners and a lot of bends ....all the way to the chimney connection to atmosphere

The end result [pressure drop caused by my tube work ID and unintentional internal orifice plates :Doh: ] was causing the steam discharge to condense in the de-oiler as opposed to exit the chimney as steam.....

Attempting some calculations, leads me to understand that my engine steam discharge line sizes are also too small  :facepalm:

So the first modification is to increase the discharge tube size from the de-oiler top plate to the chimney top to 1/4" OD x 0.014" wall without any internal reduction influence....and then trial & measure for the reduction in condensate to water to steam ratio

Dependent on that trial, a second possibility is to increase the actual engine steam discharge tube line sizes and a new manufactured squareish box type de-oiler with similar 1/4" full flow inlet/s

[I have previously confirmed that lagged steam tube to the engine provide a ~~ 3 degree C + gradient over non-lagged tubes] 

Thanks MJM460.......a great thought provoking thread :ThumbsUp:

Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 16, 2017, 01:40:24 PM
Thanks Derek for your feedback and for telling us about your adventures.  It is sad to find after all the work to make a neat installation as you have, that it does not work the way you expected due to something no one ever talks about.  It is the sort of issue that prompted me to start this thread.  We need to talk about thermodynamics and other obscure engineering subjects to avoid some of the issues you have found.

You did not say what size engine, but what I can see in the photo suggests it may be two cylinder with cranks at 90 degrees which suggests double acting.  Unless you are into real watchmaking, I have no doubt that 1/8" is too small anywhere in the exhaust steam system.  Probably OK for a water outlet, but then surface tension can trip you up in those sizes.

Instead of building a new separator, how about taking the exhaust from that "filler" plug, is it 1/4" or perhaps 5/16"?  I assume it is actually for emptying, so a removable section of the exhaust would give you access after a day's run.

Of course the practicalities of connecting the exhaust to an engine cylinder often preclude using a reasonable size pipe.  But a fitting which screws into the block on one end can be larger at the other.  The loss from one small passage right at the engine is probably acceptable.  Then continue in 1/4" or, for a bigger engine, perhaps 5/16" to the separator.  And even bigger if your model permits it from the separator to the stack.  Again, you can get away with smaller entry to the separator especially if it is tangential.  The velocity will give a swirl which helps separate the condensate.  Perhaps a topic for another time.

I remember your point about the radiation temperature instrument, and they certainly have their uses, especially for temperature difference comparisons.  I will put a little post on temperature measuring a little later.  (It is on my list.)

I probably put the emphasis on the wrong part of the maths in my last post and glossed over the useful bit.  You can calculate the pressure loss for each change in velocity using

Pressure loss = density x velocity squared / 2000

Pressure will be in kPa when you use m/s for velocity, and kg / m^3 for density.  The two is a constant in the energy equation what ever units are chosen, while the divide by 1000 converts pressure in Pascals to kPa. 

You need to devise a test run generally involving weights to determine your approximate flow rate.  For those less familiar with steam tables, they do not appear to contain density, but instead have columns for specific volume which is 1/density.  With flow rate, and density, you can easily calculate velocity for each tube size.  Not mental arithmetic, for me anyway, but computers are magnificent for such tasks.

I know the Bernoulli equation implies that this velocity pressure is converted back to static pressure  when the velocity reduces, but in practice this only happens in a well made  venturi, and you can estimate that you loose a velocity pressure at each entrance and each exit for your tubes.

I hope this lets you get things working well without too much trial and error.  Look for a velocity less than 20 m/s.

Back to valves next time.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 16, 2017, 02:08:15 PM
still following along .....with the help of my scientific dictionary !!...interesting info here.....in the old days engines only had two pipes inlet and exhaust. this made model engines look less unclutered with sleek lines and uninterrupted views. later on with all the thermodynamic knowledge they began to become more of a plumbers nightmare !!....A comparison can be made with an old Morriss minor A series engine and the modern car engine ....lots of room in the engine compartment then  and now there is hardly any room to get a spanner in .......
.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 16, 2017, 02:19:36 PM
Also can we talk about sound    ?  My morris Minor is very loud compared to a Rolls Royce so is some of the energy converted to sound...............?
Title: Re: Talking Thermodynamics
Post by: derekwarner on June 16, 2017, 11:24:26 PM
OK thanks MJM460....

I am awaiting the supply of 1/4" female copper long radius bends for the new exhaust trunking exiting the de-olier........[1/4" OD x 0.014" wall full flow]

Will report back on physical results with temperatures...however the final result  = volume of water consumed : to exhaust steam condensed & retained as water in the de-oiler......

Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 17, 2017, 12:05:21 PM
Symmetry in valve events?

Thanks for the comments steam guy, sorry you need the dictionary, I was hoping to keep this understandable, so that the thermodynamics is accessible to all.  You will not be alone, so if you tell me some of the words you needed to look up, I will try and explain what I mean a bit more clearly.  As with all dictionaries, they tell you the word definition, but often do not help much with understanding the sentence.  And for some forum members, English is not the language of choice, and I want it to be accessible to everyone.

I know what you mean about the plumbing nightmare under the bonnet these days, I used to be able to sit on the mudguard with both feet comfortably down beside the engine while I worked on my old Holden, but not any more.  So much extra equipment crammed in, mostly more about pollution control, equipment such as air conditioning and modern engine control systems than pure thermodynamics, though I notice my Subaru keeps the engine air separate from the cooling air through the radiator, which is pure thermodynamic reasoning.

Yes, some of the energy is converted into noise, but a very small part of it.  It does not require much energy to make a lot of noise.  Most losses are directly or indirectly turned into heat.  I can talk a little about noise if you like, but if I could make your Morris sound like a Rolls, I would be a lot wealthier, and would be able to afford more machines (and castings!). It's on the list.

Derek, I am not quite with you on your aim with the "water retained to water discharged as steam".  Do you want to condense and collect more?  Or do you want collect minimum, just enough to collect the oil and send more water up the stack as steam after removing the oil?  These are very different issues to minimising the back pressure for power output.

Back to the valves.  Some of you may be feeling there must be more resistance in our inlet and outlet piping, especially as we are so often advised to have a free flowing exhaust.  The issue comes down to two areas.  For the inlet piping, it is velocity just as I have described, and then valve opening .  Especially for larger engines, it is important to have sufficiently large piping to keep the velocity low.  The valve opening requirements however are very different for inlet and exhaust.  Surprisingly so.  There is no symmetry there at all.

Most of our valve linkages whether a simple eccentric, Stevensons reversing hear, Hackworth, Joy or one of the many others (the W one used commonly on steam locomotives is too hard to spell), all produce a simple harmonic, or sinusoidal motion to the valves.  They move slowly at the extremes, full open, at each end, and fastest at middle of their stroke, which is of course about 90 degrees different from the piston.  This does mean they move fastest when the piston is moving slowly at the top and bottom dead centre, but they still take a finite time and number of degrees of rotation to open or close fully.

For the inlet valve things are nowhere near as bad as you might expect.   When the valve is opening, the piston is at top dead centre, minimum cylinder volume, and starts to move slowly down.  It turns out that the valve opening roughly matches the rate at which the cylinder volume is increasing (due to the piston moving down) so that the velocity in the valve port stays reasonable.  By reasonable, I mean the velocity required to admit enough steam to maintain the pressure in the cylinder stays within the maximum velocity I have suggested, and the valve opening does not cause any undue restriction.

This is especially true if the exhaust closing at the end of the preceding exhaust stroke provides a good compression before the inlet valve opens.  But even if it does not, the cylinder is at minimum volume and not much steam is required to get up to full pressure.  So it all works as expected.

The exhaust valve is very different!  Can you see why?  But this post is long enough already, so next time the exhaust valve operation.

Thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on June 17, 2017, 03:15:38 PM
As I see it - the exhaust valve opens slowly when the biggest volume of used steam must escape ..... (closing the exhaust works nicely).
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 17, 2017, 05:37:30 PM
Talking about exhaust's  I have always wondered why my exhaust valves are smaller than the inlet valves ??....is it the same with diesel engines ? and is it why they burn out every few years ........
Title: Re: Talking Thermodynamics
Post by: Zephyrin on June 17, 2017, 06:26:20 PM
IMHO, the best way to visualise the position of the steam valve during the cycle is the oval or elliptical diagram:
The oval diagram is in the CW rotation.
The steam valve is plotted (ordinate) in function of piston position in abscissa, normalised here in % of the stroke.
The green line corresponds to the opening or closing of steam exhaust.
Yellow lines are the positions of the external edge of the steam port, the crankshaft side is the upper line, and back side is the lower yellow line.
When the steam valve crosses a yellow line, steam port opens or closes. I.e. on upper yellow line, which corresponds to the crankshaft side of the cylinder, the admission opens near BDC, 0%, cut off being near 60% of the piston stroke.
One can easily appreciate the fast opening of the steam admission near dead center.
One also sees that the steam distribution on both sides of the cylinder is not symmetrical, owing to rod length.


Title: Re: Talking Thermodynamics
Post by: MJM460 on June 18, 2017, 01:49:41 PM
Exhaust Valve issues

Thank you for the oval diagram Zephyrin, such diagrams can be really useful in visualising valve event timing.  I will come back to that in a near future post.  Can I assume that what you refer to as the steam chest might be what I usually call the valve?  I usually think of the steam chest as the stationary part held to the cylinder block with studs.

Steam guy willy,  thanks for your post and pictures.  At first it might appear to contradict what I have said about exhaust valve opening, but of course, engine design also involves compromise.  Clearly the valves are about the largest that could fit the space.  The designer has to consider whether both should be the same size, or make one larger.  In internal combustion engines, the power output from a given engine is limited by the amount of air that can be delivered into the cylinder, despite losses in the air cleaner and carburettor.  Fuel is much easier to get enough.  So the designer has made the inlet valve a bit bigger to reduce the resistance to air entry.  For the exhaust, there is extra pressure available to get the exhaust gas out , so a little more resistance is perhaps less of a problem, even though bigger exhaust valves would be desirable.  They burn out because they pass very hot gases from (possibly) incomplete combustion at high velocity and including any unburnt carbon.  So very hot and abrasive.  Surprising they last as long as they do.  In contrast, a steam engine has the boiler pressure available to help get the steam into the cylinder.  The exhaust is the steam engine problem.

Thanks for your reply Admiral_dk, you have hit the nail squarely on the head.  At the end of the power stroke, the cylinder is full of steam that is at significant pressure, particularly if there was late cutoff, and the volume of the cylinder at this point is at maximum.  Of course, we can help a bit by early release, the torque due to the force on the piston is quite low within 10 to 15 degrees of top and bottom dead centre.  Can we make some estimate of how much of a problem we have?

I do not want to try and make an exact calculation, but if we assume early cutoff at 15 deg before the centre, and that we want the pressure to be essentially dissipated by 15 deg after, we have say, 30 degrees of rotation to get the pressure down.  At 2000 rpm, which is 33.3 revs per second or 0.03 seconds per revolution, or 0.0025 seconds to reduce the steam pressure to that of the exhaust system.  We then have to make an estimate of how much steam is to be discharged.  If we assume the pressure in the cylinder is 300 kPa(abs) and it expands to atmospheric pressure On release to exhaust, then 1922 mm^3 (including clearance volume) of my little oscillator would expand to 4545 mm^3.  Thus 2623mm^3 of "extra volume" has to be discharged in 0.0025 seconds.

Next we assume the valve is open to the full area of a 3/16" tube in the exhaust, 8.76mm^2, which requires a velocity of 120 m/s.  Applying v^2/2000 with an average density about 1 kg/m^3 gives a back pressure of 7 kPa.

While a pretty rough calculation, this does not seem too bad a result.  However as the valve is opening from closed for the first 15 degree, the average area might be considerably less than I have assumed.  My slide valve engines have an exhaust passage 7 mm wide.  So this must be open by 1.25 mm to give the assumed 8.76 mm^2.  As the total valve travel is only 5 mm, 1.25 mm within 15 degrees is probably optimistic.   If the area is only 4.4 mm^2, the pack pressure would be 4 times, so 28 kPa, and starting to impose serious negative torque on our engine output.  These calculations are very rough, but do serve to give an idea of the range of back pressure imposed by inadequate exhaust passage sizes.  I think enough to show that 5/32" is too small for this engine exhaust, 3/16" dubious and 1/4" better.  This compares with possibly 5/32" for steam supply, and 3/16" not really too large.

Next time, I will summarise the key points relating to valve events and the P-V diagram, and start to look at how our valve behaviour compares with the ideal.

MJM460
Title: Re: Talking Thermodynamics
Post by: Zephyrin on June 18, 2017, 03:04:44 PM
Of course, sorry, I did a confusion between valve and chest...I've corrected the post !
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 19, 2017, 02:33:28 PM
Thanks Zephyrin,  No problem, I have it almost worked out, but I wanted to check that we were talking about the same direction.

I have spent my time today cutting and colouring, Cutout Aided Design, otherwise known as CAD, trying to work out how best to describe the sequence.  I must admit to a severe case of eyes glazing over when trying to work it out, and have previously given up after setting the valve to just start opening as the crankshaft turns over top dead centre, and, on finding the engine ran ok, letting the exhaust look after itself.  The valve dimensions must have been good enough.

However for this thread, I want to match all the valve events to the P-V diagram to see how well it matches the ideal.  The oval diagram seems to be helping, I hope to have it sorted by tomorrow or perhaps Wednesday.  Have some babysitting commitments for the grandchildren tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 19, 2017, 03:41:52 PM
Thanks for the info ...Actually the exhaust valve failed during a long journey and i was able to get there and and back on 3 cylinders..about 120 miles !! after this happened...A brief summarisation of thermodynamics might be " Everything is a compromise " when it comes down to building engines !!!! ;D ;D   :popcorn:
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 20, 2017, 02:43:23 PM
Hi steam guy willy,

I once lost a valve in of a V-8 while on a much longer trip, I was in the Smokey Mountains, and home at the time was in Canada.  No question of driving on.   Getting it fixed became a highlight of our holiday due to meeting some great people.  But I can really feel for you losing one cylinder out of a small four.

Got diverted onto thinking about controlling propane on Brian's thread today, so not quite ready to continue those slide valves, but am coming to terms with Zephyrins oval diagram, so a little progress, and l hope to be ready tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on June 20, 2017, 04:55:42 PM
Have you considered Dockstader's valve gear programs? One of the outputs is the sine diagram.
http://www.billp.org/Dockstader/ValveGear.html

It would be nice to see some of the formulas you are using, a lot of pipe formulas are on the web in calculator form try Engineers Edge or Engineers Toolbox.

http://www.engineersedge.com/
http://www.engineeringtoolbox.com/

Dan
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 21, 2017, 11:36:19 AM

Real valves

Thanks for those suggestions Dan, I am glad to see that you are still following and your input always welcome.  I will put those topics on my list.  I am planning on a post on units of measurements and you might be surprised to see how much can be done with little more than the definitions.  But also a little Physics for the three conservation laws I have used and also the laws of thermodynamics. It is on the list, so I will get to it.

Casting our minds back to the P-V diagram, the key events in each cycle are the opening and closing of the inlet valve, followed by the opening and closing of the exhaust valve.  Remember that these events occur on the top of the piston, and on a double acting engine, they also occur on the lower side, but half a turn later so that the piston is first pushed down, then pushed up. 

When we set the valves on a new engine, we take off the steam chest cover, and we can see when the steam port is about to open, set this to happen at piston top dead centre, then we can see where on the crank rotation it closes.  We can also see the maximum opening at each end and so adjust the valve position on the rod so both ends open about equally.  But what of the exhaust?

Most books draw a cylinder cross section through the ports with the valve at mid travel, and show lap, and the relation of the exhaust cavity to the edges of the exhaust ports.  But unless more drawings are provided, I for one, find it difficult to keep track of the exhaust events as the valve is shifted only in my imagination.  So, does the exhaust open and close at the right points as Required for the valve events in the P-V diagram?  How much cut off do we get? And do release and compression occur as we want?

Of course with modern computer programs, we can model the valve movement and watch it on the screen as the crank shaft rotates.  But such programs are relatively recent, and quite expensive if we don't have access through our workplace.  For those of us a bit technology challenged is there a simple solution?

To solve this problem, I have resorted to Cutout Aided Design (CAD) shown in the first attachment.  The valve is the blue part that is a separate cut out that I can move back and forward over the outline of the cylinder ports to see when the ports open and close.  It is actually also useful to check the design for features such as how valve sealing surfaces and the bars between ports relate to provide clear port opening.

Many geometric constructions were developed when people only had hand calculations, and these were converted to allow computer calculations even before modern models appeared.  My text books have four variations and Zephyrin has shown us a fifth which is not in any of my books.  No doubt there are others.

Now that I have studied it carefully, I am with Zephyrin, in feeling that his oval diagram is probably the easiest to use.  Certainly the easiest I have seen so far.  It shows quite clearly the effect of lead and lap for both the inlet and exhaust events in a simple construction, at least simple with the benefit of a spreadsheet to do the repetitive calculations.

The oval diagram is produced by making a graph of valve position against piston position, which gives the oval outline.  The points around this oval correspond to the edge of the steam valve that determines valve opening, proceeding clockwise.  The same points can also be considered the edge of the exhaust cavity that determines exhaust port opening.  The steam lap is shown by the red line, and the valve starts to open when the inlet edge of the valve crosses this line at the piston dead centre.  The port finally  closes again when the oval crosses this red line on the way back to the centre position.  Similarly in the lower half of the diagram for the other end of the cylinder.

My first attempt is shown in the second attachment.  I have saved a little time by assuming a scotch crank design, but with a few more columns in my spreadsheet, and a little more trigonometry, the effect of the conrod can easily be included.

I think this item turned out way too long, so I have split it here and will continue to work through the diagram next time. 

Thanks for reading,

MJM460
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on June 21, 2017, 02:43:18 PM
Many geometric constructions were developed when people only had hand calculations, and these were converted to allow computer calculations even before modern models appeared.  My text books have four variations and Zephyrin has shown us a fifth which is not in any of my books.  No doubt there are others.

MJ,
I have about 50 textbooks on steam valve gears, could you say which ones you are using? You said four valve gear graphical constructions are in the books, off the top of my head I can think of Zeuner, Reuleaux, and Bilgram and the oval diagram.

Zeuner was the first one and I have his book but it is really a lot of Greek in the form of complex algebra. The way Zeuner made his first diagrams was very interesting. He made a cutaway steam engine and attached a drafting board spinning at crank speed. A pencil attached to the valve to gave the trace lines that we now call a Zeuner diagram.

I know how to construct a Reuleaux diagram and it would be my choice if the valve is not symmetrical at the center of the exhaust port.

My favorite valve diagram is the Bilgram, for me, it is much simpler to see what happens when the variables of the valve are changed.

If you post the valve dimensions needed, I can construct the diagrams I just mentioned.

The historical designers of steam engines used the drafting board like we use calculators and computers. I have studied the graphical methods used to set out Stephenson's valve gear and my graphical solution matched exactly the Excel program in Don Ashton's book. This was very nice because the answers are not in the back of the book and it was nice to know I got it correct.

Dan
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on June 21, 2017, 07:30:10 PM
MJ,
I failed to mention that one of Dockstader's programs is a Zeuner diagram. If you capture a screen shot of the Zeuner valve diagram of the valve you are discussing, I can show a step by step how to draw a Reuleaux and a Bilgram diagram.

Dan
Title: Re: Talking Thermodynamics
Post by: Zephyrin on June 21, 2017, 11:36:21 PM
Hi
Each of the Charlie Dockstader's programs also contains the oval diagram, which can be modified dynamically, amazing, you move a cursor to change a dimension and see the effect on the diagram. In these programs, PV diagram are simply a model, without thermodynamics in them, hence of limited use.
I spend hours (and night too) with it while drawing the plans of my little locos...
But the Dockstader's package is not fully compatible with the recent windows version.

Z.
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 22, 2017, 12:15:20 AM
Hi Dan and Zephyrin,

You have three out of the four in my text book (by Thomas Bevan), the fourth was called a rectangular diagram.  I also have some of the well known hobby books collected over many years.  But I defer to your expertise on these matters.

My motivation in writing this thread was the realisation over many years of hobby reading that the thermodynamics, that was basic to my every day work, gave me clear answers to many of the questions raised, but never answered in any of the hobby magazines or books I have read.   Or worse, answered wrongly.  The same questions keep coming.  And even simple maths is avoided like poison. 

I hoped that I could build a knowledge base of theory that would help us all understand our models better and remove some of the mystery.  I am not an academic and having people like you looking over my shoulder and joining the conversation is really helpful, just as in building an engine.  And I feel that any forum is best seen as a conversation.

My purpose in looking at valve diagrams was simply to show that the valve events on the P-V diagram actually relate to real valve events in our models, so informing what we are trying to achieve with valve setting.  Most of my machines were electric motor driven so I never had much call to go beyond the eye glazing stage of valve diagrams.  I will be more comfortable when I get back to thermodynamics and other stuff that I can understand.

You both clearly have the expertise in this area.  May I suggest that you start a separate thread (or threads) on valve gear and valve linkages plus any other areas that you think would help.  I would really like to understand those valves in the A and G beam engines that have two valve rods and presumably a two part valve.  If you could eventually get to that .......we would be really cooking.  (I will talk about the gas part.)

If we can get three or four such threads going it might even justify a separate sub board to keep the theory topics together.  After all, aero modellers would not think of trying to advance their hobby without discussing aerodynamics.  We could clearly do with some internal combustion engine topics as well.  A little theory helps in most endeavours.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 22, 2017, 12:10:08 PM
The Oval Diagram

Back to continuing to work through the oval diagram which was attached to my earlier post.

For an engine with no exhaust lap, the edge of the exhaust port is the horizontal line at the valve mid or zero position.  Exhaust lap would show as two horizontal lines, positioned above and below the centre point by the amount of exhaust lap.  We can see the exhaust valve closes a bit before piston top dead centre to give some compression, and opens before bottom dead centre for release.  All as expected. 

The angle of advance for this diagram had to be selected as 30 deg, to make the lap required match my measured valve equal to the valve displacement at piston dead centre.  This suggests my measured lap is really too big and I need to modify the valve ends.  On the other hand, for a simple mill engine, the diagram shows early cut off to give some expansion, and the exhaust valve closes to give some compression so it could be quite efficient for an engine with no reversing gear.

If you want to draw this diagram for your engine, start with the crank angles, then calculate the eccentric rotation as the crank rotation plus 90 degrees plus the advance angle.  It is also helpful to remember that the trigonometric ratios assume that an angle increases anticlockwise.  After that, simple trigonometry, a formula to increase the crank angle by some set amount for each row, and copy your formulae down until you reach one revolution, and use the graphing function of your spreadsheet.  However Dan and Zephryrin have provided links to web sites where the maths is already done, and use of these facilities is a good way to understand your valve setting.

In summary I am now satisfied that the traditional recommendation of no exhaust lap is a very good place to start, and in addition I now have a logical method to analyse the effect of changes in lap on either inlet or exhaust side.  But I still go back to my CAD method to check the width of the bars between the ports, and to make sure that the passages are clear and open.  No more glazed eyes.

So much for a slide valve, it all looks good when analysed according to our theory.  But what about my little oscillator?  I will look at that next time.

Thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on June 22, 2017, 07:08:13 PM
MJ,
I found the rectangular diagram, it is also known as the harmonic diagram and the sine diagram. It is very similar to the oval diagram only the valve motion and the piston motion are separate curves and not combined as in the oval diagram.

The other three diagrams Zeuner, Reuleaux, and Bilgram, are for a different purpose entirely. They are all graphical constructions used to calculate the relationship between the geometry of the common D slide valve. They all give the same answer. The reason to use one over the other is really personal preference. The slide valve variable that these three diagrams are used to calculate are: valve travel, angle of advance, steam lap, exhaust lap, lead, cutoff, release, compression, and admission.

Obviously to start a slide valve design at least 4 of the above variables have to be known. The Zeuner diagram program by Dockstader has slider inputs for valve travel, cutoff, lead, and exhaust lap. The rest of the variables are calculated and displayed as changes are made to the inputs. I find this a very cumbersome because I have a lot of Shay locomotive data
that includes the valve travel and the angle of advance. I also know the lead for Shay locomotives is 1/16". This information with a glance at the drawings for the steam lap and I can construct a Bilgram diagram. It would be simple to show how to draw a Bilgram diagram for the valve in this thread.

I was a marine engineer and I have a love of vertical steam engines with Stephenson reverse gear. The first time I saw a Shay I saw a marine engine that got lost in the woods and I had to know more. The study of valve design and Stephenson gear took me many years and quite a few times I thought I would never figure out the secrets of how to design Shay valve gear. Finally understanding Bilgram only led to a much harder challenge of understanding Stephenson reverse.

You mentioned not many textbooks cover the thermodynamics of steam engines, well steam engines were fairly well developed while thermodynamics was still in its infancy. I believe it was the need to understand the theory of steam engines that was the driving force behind the discipline of thermodynamics. The earliest book in my collection of steam engine design books that covers thermodynamics similar to what I had in college is The Steam-Engine Theory and Practice 1905 by William Ripper.

The Wright brothers flight at Kitty Hawk would not have been possible without the wind tunnel data they generated before the successful airplane design. They aerodynamic data they generated was the best in the world in 1902.
https://wright.nasa.gov/airplane/results.html

Dan
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 22, 2017, 09:03:48 PM
A new question.......does it take the same amount of coal/heat to boil water in an enclosed boiler at the top of a mountain as it does at sea level ? If the boiler is filled at sea level and then taken up a mountain, any difference ?? etc etc etc.......So can you to test a boiler at any altitude ??
Title: Re: Talking Thermodynamics
Post by: Jo on June 22, 2017, 10:01:32 PM
Willy you need to define the question better: the boiling point of the water will change as a function of altitude as does the combustion efficiency of the fuel. To provide the same volume of steam at altitude you need a bigger boiler or you would need to compress the air going into the combustion chamber.

Jo
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 23, 2017, 01:55:53 AM
Willy you need to define the question better: the boiling point of the water will change as a function of altitude as does the combustion efficiency of the fuel. To provide the same volume of steam at altitude you need a bigger boiler or you would need to compress the air going into the combustion chamber.

Jo
 
Thanks Jo , I am a complete novice when it comes to thermodynamics, but being an autodidact i like to find out about things. I do make and say completely wrong statements and then get told and informed about what is actually correct and then i learn stuff !! I am part of a few clubs and things and am always being advised as to what is actually going on ! !! However i still only know what i know. Sorry about this lengthy diatribe but i am a gemini so i have an excuse !!!! I like your work with all your models and wish i could be  more prolific with what i do......... :)
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 23, 2017, 05:26:38 AM
Hi Dan,  I thought you might be a marine engineer.  I have the greatest respect for your profession.  When at sea, the safety of the ship and the lives of all who sail on her are dependent on your ability to keep the engine running.  In the middle of a vast ocean, there is no one nearby to call on for help.  My activities were essentially land based, and steam power meant a turbine, so I never had need to come to terms with valve gear.

I suspect that you and I together are in a small (elite?) group of people who, when asked if we have read the classics, cheerfully reply, "Of course", thinking they mean classical thermodynamics.  After all, is there anything else you would read?   I did not mean that there were no text books on thermodynamics, just that it is glaringly absent from the modelling press.

So sincerely, please consider a thread about your adventures in valve gear, from the Stevenson's to the Shay.  If I have difficulty understanding them, I am sure there will be many others.  I for one will certainly be following along.

Now, Willy and Jo,  between you, you have mentioned in two (now three) short posts, at least six fundamental questions, the very questions which I suspect are behind nearly all of the confusion in our  understanding of steam engines and the boilers which fire them. 

I am very glad you asked, as I think you have collectively eloquently put your fingers on the big issues which most were afraid to mention.  I will start with the issue of boiling of water and get back to coal and combustion efficiency a bit later.

The basic issues of boiling water at elevation are dependant on whether you are talking about water in an open saucepan or kettle (even with a lid), or a closed space such as a boiler.  If you are  a mountaineer, or even live in Denver or Mexico City, you will observe that water boils at less than 100 deg C (212 deg F), and this might cause problems making a cup of tea or boiling an egg, the typical early school science examples.  To understand this we need to understand a few basics.

First, if we have both vapour (steam) and liquid water in a closed container, the pressure of the water vapour is dependant on the temperature alone.  If things are not happening too fast, they are considered to be in equilibrium, the pressure is uniquely determined by the temperature, and can be looked up in any steam tables.  It is called the vapour pressure.  Hold that thought for a moment.

If we could look at a scale where we could see the molecules, we would see the molecules in the gas moving fast in all directions.  They are a relatively long way apart, and each barely affected by the others except when they collide.   Some hit the vessel wall and bounce back into the fray.  But some hit the surface of the water.  Of these, some will lose enough energy in the collisions with water molecules that they are unable to escape, and stay in the liquid. 

In the water, molecules are also in random motion, but a lot slower and the molecules are closer together.  This close, there are attraction forces that keep the molecules in close proximity so that water stays together.  Some molecules hit the walls and bounce back.  Some reach the liquid surface, but are unable to escape the attractive forces of the closely spaced water molecules.  But  of these, a few with higher than average energy actually escape and join the gas.

If the number entering is the same as the number leaving the liquid, we call this equilibrium.  If there are more molecules leaving, this is not equilibrium, the water must be warmer than the equilibrium temperature, and extra molecules in the gas make their presence felt as extra pressure.  With more pressure, more molecules will return to the liquid and a new equilibrium is soon reached at a higher temperature.

If we heat an equilibrium mixture of water and vapour, some of the water will evaporate, but the temperature will not change until all the water has turned to steam.  The heat that goes into evaporating the water without increasing its temperature is called latent heat, and provides the energy necessary to get the molecules up to a velocity which allows them to escape the attractive forces at liquid spacing.

Boiling occurs when we put in heat fast enough that the water starts expanding into steam below the water  surface.  The huge change of volume causes the vigorous bubbling we call boiling when the suddenly large bubbles of steam quickly rise to the surface.

Now all of this is always true as I have described.  But notice I have only talked about a closed container containing only water.  No mention of anything else.  The confusing bit, the part that explains your confusion arises when, in addition to water there is something else also present, in particular if the something else is air due to the loose lid on our kettle.  The presence of air actually changes nothing but our perception, but that is the root cause of our problem.

That is enough to absorb in one sitting, don't be surprised if it takes more than one reading.  Next time I will explain what happens when our container also contains air.  And that will explain the conundrum of elevation.

Thanks for bearing with me

MJM460
Title: Re: Talking Thermodynamics
Post by: jadge on June 23, 2017, 07:49:55 AM
The book "The Steam Engine and Other Heat Engines" by J Alfred Ewing covers steam engines, including turbines, from both a pragmatic and thermodynamic viewpoint, aimed at undergraduate level. First published in 1894 I have a paperback copy of the 4th edition, published in 1926. The paperback copy wa published in 2013 by Cambridge University Press, who also published the original editions. It is what I use as a basis for understanding the thermodynamics of steam engines.

Andrew
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on June 23, 2017, 05:09:47 PM
MJ and Andrew,
The book I mentioned by Ripper is the 4th edition. The main reason I mentioned it is the first edition of the book printed in 1899 was the first text to include a temperature-entropy chart. The introduction of the temperature-entropy chart was by Macfarlane Gray who read his paper at the meeting if the Institution of Mechanical Engineers in Paris July 1889.

As to starting a thread on Stephenson valve gear, I have one on the web already, unfortunately, it is behind a privacy screen on 7-8ths.info which is a 7/8" scale model RR forum. The good news is I have not built the Shay engine I did the study for and I will cover the topic here when I get to the engine.

I was on vacation for a few weeks and did not read some of this very carefully and I was not near my library. You determined that the usual steam pipe was too small for your design. The problem is the assumption of 2000 rpm. This is a very fast speed for a double acting engine. Most double acting steam engines are much slower. Locomotives run about 300 rpm and marine engines vary from about 120 rpm to around 750 rpm. The reason a double acting engine does not make a good choice for high rpm's is the piston pushes and pulls the con rod, if the bearings are not kept tight there will be a knock as the force changes direction. A single acting engine is much more suitable for high rpm's because the con rod force is always in the same direction. To get the same power out of a single acting engine compared to a double acting engine of the same bore and stroke, the single acting engine has to turn at twice the speed.

Dan
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 24, 2017, 01:06:51 PM
Effects of air in the kettle

Hi Dan, I am really looking forward to following your Shay build, together with your valve gear analysis.  I hope it is not to early to start preparing popcorn, or at least planting the corn seed. 

You are probably right about 2000 rpm being too fast for running my little engine.  This would lead to needing a higher volume of steam, and requiring larger steam pipes.  It was a measured speed, using a non- contacting digital tachometer.  However, the engine was running unloaded, and I would expect it to run much slower under load.  It was also a short stroke oscillating engine which may affect the desirable running speed as the piston speed was quite moderate.  I agree with you about the possibility knocking at the crank pin due to rod load reversal, but the main purpose of the post was to show a rational basis for selecting pipe sizes.  I have noticed people here have a fair idea of the speed they want to run their engines and as you suggest, it is generally much slower.  I hope eventually to build a suitable dynamometer, probably based on a generator to allow me to measure power output, and I will repeat all my tests.

Back to boiling water at elevation-

Last time, I described boiling in a closed vessel with only water, partly liquid, partly vapour in the vessel.  So what happens in our kettle when we also have air at the liquid surface?   I suggested that it changed only our perception, but on reflection, it might have been better to say it changes everything.  Essentially, the pressure is now fixed at the local atmospheric pressure, and as boiling temperature is a function of pressure alone, it varies with the local atmospheric pressure.

Suppose we are boiling our kettle for a cup of tea at the top of a mountain.  We have been told there might be a problem, so we have brought a thermometer, which we put in the kettle and find it starts boiling at 95 deg C.  In this case, the water surface is in contact with the atmosphere through the spout and also the little vent hole you will notice in the lid.  Even the little gaps due to the fit of the lid help, so the pressure at the liquid surface is tied directly to the local atmospheric pressure.

 We look in our handy pocket extract of the steam tables, and find the equilibrium vapour pressure at 95 deg C is only 84.55 kPa.  This means the water vapour pressure at the water surface is only 84.55 kPa, and as the vigorous boiling tends to displace any air from the immediate surface, there is no gas mixture at the liquid surface. Any excess pressure in the kettle is eliminated by leakage to atmosphere, carrying with it any air that was initially in the kettle.  Further away from the surface, we have a mixture of air and water, and we don't know how much of each.  Well away from our kettle, we would find the air pressure measured by our barometer, had we brought an absolute pressure barometer such as a Mercury column, is only 84.55 kPa(absolute) compared with an average around 101.3 at sea level, and it matches the pressure in the steam tables that corresponds with our boiling temperature. This corresponds to very roughly 1500 metres.  If we were on Mt Everest, the boiling pressure would be nearer 50 kPa, and the temperature only a little above 80 deg C.  That might make tea making quite problematic.

The variation pressure (and hence in boiling temperature) with altitude varies in a complex manner depending on the atmospheric temperature variation and the amount of water in the air due to humidity, in addition to  elevation, and not necessary to know for our current discussion.  Google will find you some good information on this if you are interested.  I picked 95 deg as a point available in my steam tables, just for an example.  In fact even in your kitchen at sea level, it is difficult to get water to boil at exactly 100 deg C, due to the variation in atmospheric pressure as the weather patterns pass by, so if you are calibrating your thermometer, you need to know the atmospheric pressure at your location and make adjustments.  Again not necessary for our current purpose.

So I answer to the original question, in an open kettle, the water boiling point depends on the atmospheric pressure which depends on altitude, but in our closed boiler with only water present and not in contact with the atmosphere, the elevation does not affect the absolute pressure at which water boils.

One of the issues with boiling water in an open kettle is that boiling is a very vigorous action that is the very opposite of the reversible process necessary to keep the liquid and vapour in equilibrium.  The rapid motion as the vapour bubbles rise to the surface sweeps away any air, so the vapour near the surface is essentially all water vapour.  In an open vessel, the pressure is fixed to atmospheric pressure.  This very departure from equilibrium introduces an asymmetry between boiling and condensing.  When we get to discuss condensers, you will see a very different and perhaps surprising effect of even an small amount of air.

I hope that helps to answer some of the questions.  Next time I will discuss how these concepts apply to boiler testing.

Thanks for following along.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 24, 2017, 02:26:03 PM
Thermometers....? If it is of the mercury type in a glass tube, is the 15Lb atmospheric pressure sort of squeezing the tube slightly.? at altitude is this squeezing less so the temperature would appear to read less ?? If the boiler is at altitude would this reduced pressure allow the tube to expand slightly ? Would the boudon tube in the pressure gauge also expand slightly thereby giving a different reading than at sea level ? or does everything cancel each other out ? Sorry to be so pedantic about this but as a novice it would be good to know how things actually behave !!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 24, 2017, 02:35:20 PM
In one of my electrically heated boilers there is a low water safety device that switches off the power. This is in the form of an insulated probe (PTFE) . When the water level drops below the end of the probe it switches off the 250Volts using a separate 9Volt battery circuit. I am surprised that as the boiler is full of steam, this wet steam does not complete the circuit. !! Is this going off at a completely separate tangent or is there some relevance to the topic in hand ??..............
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 24, 2017, 02:40:17 PM
I have noticed that when you buy a new barometer from a shop, they actually calibrate it for you, Here in Norwich the local shop that sells them is 8 meters above sea level !!!..........Also as we know there is a standard  Meter ,Yard Kilogram etc etc so is there a standard i Bar , 15Lbs  somewhere ??or is this being a bit silly !!!!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 25, 2017, 12:59:59 PM
Hi Willy, you are in good form tonight, but no, they are not silly questions, and your aim of wanting to understand exactly how things work is why I am writing this thread.  It's just that you are asking questions faster than I can answer.  After all, for both of us, this is our hobby, not our full time occupation.

You are quite right observing that the glass thermometer is subject to external pressure and the inside is closed off from the outside.  So it is squeezed a little less at altitude.  But I emphasise the LITTLE less.  One atmosphere is very small compared with the strength if the thermometer tube under external pressure, so it does not have much effect.  I have not had cause previously to look up a stress strain curve for glass, but I would hazard a guess (and you know engineers are not comfortable with guesses), that the effect on diameter is so small that you would not be able to measure it with your standard workshop tools, and I am trying to stay practical.  If you are really worried, you could use instead a thermocouple, properly calibrated at sea level of course, for your temperature measurement.  I am sure any pressure effect could then be safely ignored.

You know when we only had slide rules for calculation, we had an advantage.  You could only read at best three significant figures, unless you had a monster slide rule, but there was a real limit to the accuracy you could obtain.  Surveyors used seven figure log tables, but for most engineering purposes three significant figures are enough.  Any more is only useful so you can get an exact balance of calculated results.  We used to dismiss small errors as slide rule errors, but there were times when this hid a real error due to an incorrect calculation.

Thermodynamic calculations often involve subtracting two large numbers for a small result.  It would be good to be able to measure temperature to better than 0.1 deg C, but that requires laboratory quality instruments that are not affordable for most, and not really required.  I am confident the error in your thermometer from 1 atmosphere change in pressure would be much less than 0.1 deg C.

So you are right in your understanding of what happens, but I believe the magnitude is small enough not to matter.

Your electric boiler level probe does not use a mechanical contact which opens or closes, but a conductivity probe which measures a change in resistance (which results in a change in a small current in a circuit,) so the range of possible readings is continuous or analogue, not binary.  It then uses an amplifier to lift the currents to a useful level, perhaps to operate a relay, which opens to shut off the mains voltage.  So it depends on the difference in conductivity between steam and water.  Again I do not know, or have immediate access to the data, but you could use your volt meter to measure the resistance of your probe first in cold water, then as you heat the boiler to boiling, then proceed to low level so you measure the steam resistance.

You need to be very safety conscious when you do this test.  Just disconnect your level controller from the probe for the entire test and do not open up the circuit with the mains power connections.  You are the safety switch, so watch the level gauge and switch off as soon as you make the necessary readings.  Also work assuming there could be a fault which allows your probe to touch the mains voltage in the heater element (even though it is most unlikely).  Use a 600 V insulated meter rated for mains use and proceed as you would if everything was at 240 V.  Don't take any risks, again this is only your hobby.  If you are not sure, don't do it, just take my word for it or research the operation of your probe from other sources.  I expect you don't need any of these warnings, but neither of us know who else is reading this, and what their state of understanding or the condition of their equipment might be.

The topic is only limited by what I am prepared to try and answer, based on my 40 years in an industry where this understanding was as basic as arithmetic to an accountant.  I am trying to pass a little of it on to where it might be helpful.  It gives you a fair bit of latitude on topics.

Definitely not being silly about units.  I had intended to get to it anyway, but now you have asked, this is how it works at a practical level.

Pressure is force per unit area, so it's measurement is based on force and area.

Force is defined by Newtons equation, F equals mass times acceleration.  Usually written F= ma

It is the unit which most distinguishes Imperial and ISO metric units, and which causes the most confusion in all calculations involving force.  Metric for a few lines, then I will explain that further.

Mass is measured in kilograms and we have a standard bar of some expensive exotic alloy for that.

Acceleration is defined as change of velocity per second, velocity and acceleration require only length and time for measurement, and we have standards for those.

Now a very powerful analysis technique for these problems is called dimensional analysis.  Don't run away, it simply relies on a principal that in a rational equation, each side of the equals sign has to have the same units.  In addition, each term that is added or subtracted must have the same units.

In plain language, you cannot add apples and pears, and apples cannot equal oranges.  Sounds simple but that principle is behind the equations for force and the scaling of wave generation when ships move through water and many other complex problems.

So if we define F=Ma, we are saying the units of F are the same as the units of m times a.

Let's do it.  I will use square brackets to mean "the dimensions of", just read it that way, then

[F] = [m] times [a]. Then continue with [m] = kg,  and [a] = m/s^2. Read s^2 as seconds squared or
s raised to the power of 2.

So [F] = kg times meters divided by seconds squared.  Or more conveniently [F] = kg.m/s^2

The metric unit of force was given the name Newton and the symbol N, so 1 Newton = 1 kg.m^2.

Not so hard was it?  You only need mass length and time for these measurements.

You might say what about kilogram force?  Now, Wash your mouth out, that is not an ISO unit.  It is a hangover from the same issue as that curse of students everywhere, the pound force.  There was a time when people did not understand the difference between mass and force (has much changed?). Force was given the units of pound, the same as the unit of mass, and both combine in the term weight.  You can see the confusion.  People have tried to extract themselves from this by introducing either the small unit of force, the poundal, or the large unit of mass, the slug.  Personally I don't like either of them.  Alternatively they introduce a constant g, being the same value as the agreed standard acceleration due to gravity, but when to include it asks the student.  So metric for me.  The constant is nearly always one.  But I understand the issues of converting workshop tooling and material dimensions.

Now we have units of force, kg.m/s^2, we can move back to pressure.

Pressure is force per unit area.  Area is length squared, and we have a standard for length.  We have a standard for mass, and one for time, so we standards for all the dimensions we need to measure pressure.  There is no need for a standard bar.

I hope that answers the questions at least sufficient for the moment, it's getting late.

Happy reading

MJM460

Ps forgot the bourdon tube, oh well, next time.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 25, 2017, 01:31:59 PM
Thanks for this, getting a bit clearer.......you mention the Poundal and also the Slug  never heard of that, but, hopefully getting rid of all the slugs on my allotment will be a weight off my mind !! Between the 9 volt battery connection circuit and the 250 volt mains circuit there is a light sensitive IC so it should be quite safe exploring relative resistances with my 1960's AVO or would a modern digital instrument be better ?........also as you are in the antipodes does the water go down the plug hole anticlockwise...the correolis effect ? Or is this another urban myth ?
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 26, 2017, 08:36:48 AM
A little more on pressure measurement.

Hi Willy, I am glad this is making things a little clearer, but sounds like it needs more work.

I realised, after sleeping on it, that I had stopped a bit early on the pressure standard, so a little more to complete the trail from the mass, length and time standards to the units for pressure.

The unit of pressure, Newton per square meter, or N/m^2, was given the name Pascal.  Now this is a very small unit, so we usually use kiloPascal, or kPa, and you will realise that 1 kPa = 1000 Pa.

To help you appreciate just how small, atmospheric pressure, around 14.7 psi is 101.3 kPa, or
101300 Pa.  Mind you for a candle powered Stirling engine, the Pascal would be a very convenient unit.

I should also correct my statement about the pound force.  I found an appropriate reference and there was a standard pound force, defined as the force exerted by a one pound mass due to gravity.  It was complex to use, to eliminate the effect of the air displaced by the mass and so on.  When it was applied to Newtons law, which basically says F is proportional to mass times acceleration, it was found that the proportionality constant could not be 1, but had to have a value equal to the gravitational acceleration.  So the equation in imperial units had to become F = ma/g, and that constant, g, has plagued students ever since.  As the value of g varies at different points on the earth and with elevation, a standard value, agreed at 32.174, is used for the value of the constant.

The principals of dimensional analysis can not be avoided, so the constant also had to have units to make the equation dimensionally correct.  A little maths and you will see the units must be
 lbm.ft/(lbf.s^2).  Quite an awkward mouthful, but necessary to maintain the distinction between force and mass.  In addition, it violates two desirable features of a good standard unit.  First, in a fundamental equation such as F is proportional to m times a, the preferred constant is 1, and second if a constant is required, it should be dimensionless, meaning it has no units.  However the use of g with the applicable units was necessary to accommodate the traditional use of pound as a unit for weight.  Slugs are best fed with snail bait, or fed to birds, but not both.

You can see the beauty of the ISO system.  In Newtons equation, the constant is unity, and it is dimensionless, the equation becomes F = ma and with this equation we can easily deduce the units for force without an additional reference standard.  The resulting unit of Force, the kg.m/s^2, is given the name Newton.  When we apply this to pressure we get Newton per square meter.

I had intended to talk about your question about the bourdon tube.  We had better get M. Bourdon's name spelt correctly or we will have Marv chiding us, ever so gently of course.

You might have noticed in another thread, that one of our forum members is actually making his own pressure gauges, and started with a finite element analysis of the curved tube that forms the basis of the instrument.   Oh, to have had access to that software earlier in my career!  Even access now.  It is a thread well worth following.  I thought it was a bit too much like watch making for my eyes and fingers, but on closer reading, I find it involves more watch breaking. 

I can't do that finite element analysis, but I do have a good feeling for piping.  So I will put on my piping engineers hat, and see how we go.  Now, the Bourdon tube is very like a pipe if you recognise that it is nearly all bend and not much straight.  Also, it has a closed end, and is flattened  a bit, so that its cross section is oval rather than round, but it is still just a pipe and a pipe is something I understand.  So what do we get from this.  First a pipe is designed to hold pressure, and the strength required is determined by the difference between the inside and outside pressure and the diameter. If the inside pressure is higher, the stress-strain relationship for the material means it increases in cross sectional area.  If the higher pressure is outside, it decreases. 

There is also longitudinal stretching under pressure.  When the pipe is bent, the longitudinal dimension change has a very interesting effect.  The bend tends to open up a bit.  The cross section also tends to flatten a bit towards an oval shape.  If it has already been flattened a bit, the change in curvature will be even  greater.  It might surprise you that this happens, and can be easily measured, even on a 12" diameter pipe with 1/2 " thick steel walls, if you apply enough pressure.  Of course the little Bourdon tube is thinner and smaller and the straightening is a lot more noticeable.  The closed end is linked by levers and a gear to the needle so the movement is even more visible.  The scale is marked zero at the needle location when the tube is open to the atmosphere (same pressure inside and outside).  It can move either way from this zero point depending on whether the pressure being measured is greater or less than atmospheric, and the scale is calibrated accordingly. 

The important point in regard to your question, is that the straightening of the tube is due to the stress in the metal, and the radii of the  longitudinal bend and the cross section al radius, not dependant on volume contained in the tube.  Remember, the tube is open to the space whose pressure you are measuring, so any change in volume on results in a very small movement of fluid into or out of the tube.  Any volume effect is already reflected in the scale calibration.

Next time I hope to get back to the questions remaining from you and Jo's earlier comments.

Thank you for helping make this more of a conversation, with your questions.

MJM460

PS - your Avo meter will be fine.  I assumed an optoisolator in your circuit, but you have to disconnect your circuit to make an accurate resistance measurement.  Plug hole circulation is urban myth, Coriolis is real, but plumbing detail has more effect.  If you take notice you should also see different plug holes, sometimes CW, sometimes CCW rotation.  You also have Coriolis in the northern hemisphere!
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 27, 2017, 01:38:16 PM
Some more diverse issues

Hi Willy,  I have gone back and checked my list of your questions and still have a few from post #92 and Jo's post #93.  I will try and cover those, then hopefully I can get crack to valve events on my little oscillator.

You asked about testing at altitude.  I think it has been indirectly answered, but to be clear, testing is about the strength of the boiler to contain pressure, and not about the boiling point of water.  This means we are only interested in the difference between the inside and outside pressures.  It does not matter whether you test it up the mountain or at the depth of the ocean, so long as you measure the difference between the inside and outside pressures.  We have covered the pressure gauge so you know it measures the difference between the local atmospheric pressure and the measured pressure, so it does just what we want.  You can confidently test up the mountain.

You asked about coal and heat to boil water.  The heat released when burning coal, or any other fuel for that matter, is proportional to the mass of coal. You will find the value called calorific value expressed as kJ/kg of coal.  I haven't spoken about joule as a unit, a bit later, I will.  However, a Joule (J) is a measure of energy equivalent to a Newton meter of mechanical work.  We cannot convert all the energy onto work, or even most of it, but we can use the same units to measure it.  In imperial units, the calorific value of your fuel would be expressed in Btu/lb, a British Thermal Unit being a measure of heat energy.

The calorific value of the coal is not dependant on the pressure, so you need the same mass of coal to get the same heat when you burn it up the mountain.

Now, burning coal requires a certain mass of oxygen to burn a given mass of coal, in the same way as any other fuel.  At sea level, the density of air is well known and depends on temperature.  From density, expressed in kg/m^3, we can calculate the volume of a kg of air and hence the volume required by a mass of coal.  In practice, it will not burn very completely, unless we provide excess air to ensure that every molecule of carbon in the coal has a good chance to meet an available oxygen molecule.  A bit like you need more boys than girls at a dance to ensure that every girl has a good chance of meeting a suitable boy.  About 15% or 20% excess air might be a good place to start.  Too much means heat energy is lost up the stack in the oxygen that was not used and the accompanying nitrogen.

Up the mountain two things happen as you know.  The pressure is lower than at sea level, and so normally is the temperature.  The density depends on both, so both have to be taken into account. The end result is a lower density than sea level, so a greater volume of air is required to carry the same mass of oxygen required for our coal.  Greater volume involves more resistance to flow, so you would need a tall stack, or a very good blower to get enough draft to draw this extra air through the coal bed.  Alternatively you could use a forced draft, perhaps a fan, probably not a compressor, to push the air volume through the bed.  Of course if you are on Mt Everest, you would probably have oxygen bottles in your kit.  If you had enough extra over what you need to get back down, you could get the required mass of oxygen from a much smaller volume of air.  Assuming of course that it is enriched oxygen in your bottles, not just compressed air.  I assume it would be.

I am not a combustion specialist so I do not know how the altered volume of air in the bed would affect combustion.  It could be good or bad, I really don't know, so the combustion efficiency I cannot answer.  I hope someone else can join our conversation to help us both understand that one.  Also, I think there are factors both increasing and reducing heat transfer.  Again I am not sure which one predominates to advise on whether you would need more or less heat transfer area.

You asked about whether it matters if you fill the boiler at sea level (and, I assume tighten the filler connection) then take it up, or fill it at the top.  This is a much more interesting question than I thought when you first asked it. 

Had to attend the grandchildren's school concert tonight.  It is quite late, so this and your last question will have to wait until tomorrow.  Wonderful to see these young people developing their talents.  There will be good musicians for us to listen to well into the future.

I hope this is filling some of the gaps and we are building a firm foundation for the rest of our knowledge base.

Thanks for following along

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 27, 2017, 04:02:58 PM
A small point of interest ..the local boiler inspector has said the the regulations state that when testing a boiler for leaks the water must be between 7 and 21 degrees centigrade. this is when the boiler is pumped full of water to check for weeps.  still following along ..interesting stuff ......
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 28, 2017, 01:13:22 PM
Our mountain top experience.

Hi Willy,  glad you are still following along.  I really appreciate your insightful questions that prevent me from glossing over details.  I don't know which regulations your inspector is talking about, or what is behind them, other than the fact that pressure will change with temperature, and so a steady temperature is necessary for constant pressure.  In fact, at least a tiny bubble of air is helpful in your boiler when testing, and almost impossible to avoid, otherwise it would be hard to keep the pressure steady through a test.  I do know that in industry, I have been involved in pressure testing in locations where, if we had to wait until the temperature was below 21 C, we would have had to wait until the next ice age.  Well, perhaps at night we would get there, at least in winter, but not for long enough for a big test, and it is very difficult to see any leak at night.  I have also been involved with tests where we would have had to wait until spring if we wanted to test above 7 C, we had to use antifreeze for the test fluid.  In fact, I had to purchase a whole train load, 7 full tanker cars, of antifreeze, and if my memory serves, we still had to get a top up.  I suspect there was a shortage of antifreeze in Canada that year.   Not so far from Brian's territory.  No more war stories, but best to comply with the inspectors requirements.

Back to your question about the difference between filling at sea level or at elevation for our mountain top operation.

Let's first think about the basic difference between the two cases.  The significant issues are the  pressure and temperature.  At sea level, let's assume the pressure is 100 kPa, we have a low pressure system passing over, and the temperature is 30 deg C.  (It is hot at the foot of the mountain).  We fill the boiler to the proper level, and seal the filler plug.  Our steam tables tell us the vapour pressure of the water at 30 C is 4.246 kPa, and of course this is absolute.  The total pressure is 100 kPa, atmospheric pressure is by definition absolute, so the air pressure in our boiler is just under 96 kPa (by subtraction).  We take it up the mountain, the water and trapped air do not know or care what the pressure is outside.  We use an absolute pressure gauge to measure the pressure inside the boiler instead of our standard bourdon tube type, and yes such things exist, so please accept it for now.  What pressure do we expect see on our gauge at the mountain top? 

We might be surprised to find our absolute pressure gauge, when we reach the mountain top, reads only a little below 89 kPa.  Of course our gauge cannot tell whether the water vapour or the air pressure or both have changed, we need our steam tables and some calculations to help with that.  We only know that reading is the total pressure in our sealed boiler.

Our thermometer tells us that the temperature has fallen to 5 deg C.  Just as well it is not below 0!  We do not want freezing to complicate things.  The steam tables tell us that at 5 deg C, the water pressure will be 0.872 kPa.  Near enough to 1 kPa, unless you are Mr. Keenan or Mr. Keyes.  The air mass in the boiler has not changed, and the volume of water has changed but by an insignificant amount.  However the fixed mass of air in the boiler has cooled down along with the water. 

We know that the pressure of a gas at constant volume is proportional to the absolute temperature.  The absolute temperature is temperature in deg C plus 273.  So at sea level, the absolute temperature was 273+30=303 K.  (That is K for Kelvin, and conventionally used without any degree sign.). Up the mountain, the absolute temperature is 273+5=278 K. 

Now we can calculate the air pressure in our boiler as 96*278/303=88 kPa.  The total pressure must therefore be 88 from air plus a touch under 1 for the water, or 89 kPa, just as read on our gauge.  It may even be a little less, as there is still plenty of space between those water molecules, and some of the air molecules which hit the water surface will not bounce back, but will stay in the water.  We find the solubility of air in water is higher at lower temperature. 

Next time we will light the fire, and heat our boiler slowly while we watch what is happening.

Thanks for following along

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 29, 2017, 12:05:50 PM
Following on from my last post.

As we heat the boiler, (it is well insulated, so no miscellaneous heat losses,) we see the pressure and temperature rising.  There is no problem here, as our safety valve, properly set at sea level, looks only at the difference between the internal and external pressure, which is exactly what is required to protect the boiler from over pressure.  We can be confident that it will lift to protect the boiler if required.  (No ice remember.)  It does not matter if it is a spring type, or weight and lever.  (Please ignore the buoyancy effects of the weight in the rarified air if you have a weight type, they will be tiny.)

When it all gets up to 30 deg C, the temperature we had at sea level when we sealed the boiler, we find the pressure is back to 100 kPa, just as it was down there.  The air pressure has increased, and so, independently, has the water vapour pressure.  If we could see in sufficiently clearly, we might see some small bubbles caused by that extra air that dissolved being driven out.

Next we pause at 95 C, the temperature that our water boiled in our earlier saucepan experiment.  The absolute pressure is now about 202 kPa.  This means we have 202 - 84.55 = 117 kPa difference between inside and outside the boiler.  No problem so long as our safety valve setting is above 117 kPa.

Similar drill, at 95 C the water vapour pressure is 84.55 kPa.  The air pressure that was 96 kPa way back when we sealed the boiler at seal level and 30 C is now 96 x (273+95)/(273+30) = 116 kPa.  So the total is 84.55 + 116, say 202 and remembering that there will be even less air dissolved in the water at this temperature so our calculation might be a little low.

Is it boiling this time?  If we put a screw driver blade to the boiler and the handle against our ear, (being careful not to singe our hair,) it certainly sounds quiet.  If we could see inside sufficiently clearly, we would see some more of those small air bubbles as the air that was dissolved, even at 30 C, is driven out, but no boiling.  Something is different here from when we boiled the kettle with a vented lid.

In case you are worried, we can continue heating until the safety valve lifts without any problems,  good idea to test it while we are watching closely in case it is stuck, and we see the pressure continues to rise, due to both the air increasing in temperature, and due to the water vapour pressure increasing with water  temperature.  Boiling will only start when the safety valve lifts, or we open the stop valve to our engine.  So we have time to think about what is going on.

Remember the description of boiling from our saucepan experiment.  Boiling occurs when the vapour pressure of the water exceeds the total pressure at the liquid surface.  (That should be a bit clearer description of boiling than I used before, all this close looking has helped me clarify things in my own mind, particularly the role of the air in the boiler.)  In our vented kettle, the pressure is fixed by the vent to atmosphere, and the water vapour pressure soon exceeds that when heated.  Any vapour generated expands and results in a large volume of steam, which issues from the vent, or spout whistle, and the air is inevitably entrained in the flow, so is soon lost to the kettle.  So the pressure does not increase.  The turbulent boiling is not anything like equilibrium.

In our sealed boiler, conditions are close enough to equilibrium.  As the water evaporates to keep the vapour in equilibrium with its liquid at the surface, the air pressure, is not only still there, it is also rising with temperature.  This is where Dalton's law of partial pressures comes in.  It says that in a mixture of gasses, each acts on its own, as though the others were not there.  And this is close enough to what happens until very high pressures.  Essentially, while the air is still there, the water vapour pressure cannot exceed the pressure at the surface, and even with out the air, it can only equal, not exceed the surface pressure.  Water cannot undergo that huge expansion into vapour which causes the turbulence we call boiling, the pressure just continues to rise until something changes, preferably nothing more than the safety valve lifting, or the engine throttle opening.

As soon as some mass escapes the fixed volume of our boiler, the pressure must decrease a little.  In a fixed volume, the pressure is proportional to mass.  Some extra water will evaporate to replace the lost steam, but so long as the total pressure is above that equilibrium vapour pressure for the water temperature, no dramatic boiling happens.  Remember we are proceeding slowly here, to trying to stay close enough to equilibrium.  As soon as the pressure drops below the vapour pressure, some water will evaporate which tends to maintain the pressure, but we keep letting some out.   The remaining air is not very important, and some of it is inevitably entrained in the escaping steam, but is not replaced, so soon we are only dealing with water alone, and steam tables tell us the relationship between temperature and pressure, which is now totally water vapour pressure.

A bit of extra heat, and the vapour pressure will exceed the pressure at the surface, and boiling commences.  It is not very dramatic, as we know from every time we start one of our boilers with air and water sealed inside, the same processes happen.  The only difference is the precise starting conditions.  The water which evaporates, absorbs the latent heat from the remaining liquid, cooling it, so only a very limited mass can evaporate, limited by the heat coming in from our fire.

I do seem to use a lot of words in answering these questions.  I hope it is making sense.  But I will wait until tomorrow to talk more about equilibrium, and the other half of this double banger question.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 30, 2017, 12:52:12 PM
Equilibrium

The issue of equilibrium has been mentioned often, but perhaps its meaning and importance not clear.  Not all processes occur in equilibrium.  As I write this, I am also responsible for supervising the making of soup for lunch.  In the pot, in addition to uninteresting fillers like pumpkin and carrot, etc, there is some water in the bottom, placed there to help conduct the heat so things don't burn, and also some ice, from frozen stock added to help flavour, The impurities in the stock mean I don't really know it's melting temperature, but close enough for our purpose.

But food!  That should provoke some interest.  I notice that the water has begun to boil, (the lid lifts as necessary to prevent any pressure increase) so there is steam (water vapour), water and ice all in the pot, at the same time, along with some unknown quantity of air.  Now that interests all thermodynamicists, as we all know it is only possible to have water, ice and water vapour at the same time in equilibrium at a specific water vapour pressure and temperature, called the triple point, about 0.01 deg C and 0.6113 kPa, (if that number of decimal places is consistent with the term "about").  Now my thermometer tells me the temperature is hovering around 99 C, (you do monitor your soup temperature don't you?) and my steam tables tell me therefore, the vapour pressure should be close to 100 kPa, well above that triple point pressure.  Of course, there is also air, as my pot is not sealed.  This all  tells me that the system is not in equilibrium.  Equilibrium requires no turbulence and also that the temperature be uniform throughout, which it cannot be with ice, water and water vapour all at the same time, except at that one special temperature and pressure.  Any two, but not the whole three.  And equilibrium is a necessary condition if we are comparing our process with those ideal processes such as adiabatic, isothermal, and similar.  If we are nowhere near equilibrium, our actual results may be very different from our calculations.

But mmmm! That soup smells good.  To make the soup a bit quicker, I started with the stove on high (setting was 9) and watched carefully while everything heated up and the water started boiling.  Then I turned the stove down to only 2, which was enough to maintain the temperature just simmering, with a little excess heat to just evaporate a small amount of steam to cook and soften the vegetables which were not submerged in the water.  No wild boiling or drama, just a wisp of steam from the lid vent, probably not enough to reliably remove all the air, just enough to limit the steam lifting the lid to an amount I could accept.  This keeps the temperature relatively uniform, rather than burning the vegetables where they touch the pot.  Very efficient use of steam to carry a lot of heat from the bottom of the pot to where it is required for the cooking, unlike our small engines which convert only a tiny portion of the heat to the work that we require.  Of course, the rest of the heat is mostly still contained in the exhaust, and could very well be used to cook the vegetables, if you don't mind a bit of oil flavouring.  This is called combined heat and power, or sometimes cogeneration.  And of course some of the heat is lost to the atmosphere through our cladding.

So after a little thermodynamics lesson in the soup, it's  time for a break for lunch.  Next time, I will think about what happens in Willy's alternative scenario, carrying our boiler empty, and filling it at the top of the mountain from a handy stream.

I hope everyone is still with me,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 01, 2017, 01:39:48 PM
I had hoped to continue today, but my post is not quite ready, so I hope to continue tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 02, 2017, 10:20:31 AM
We fill our boiler at the top of the mountain.

A bad day yesterday.  A beautiful page of writing, full of calculations, all carefully checked, but in the end it did not clearly answer the question.  The knowledge base equivalent of the beautifully machined part that does not fit.  Cutting a second time did not help, and it was all consigned to the bin.  So here goes with a second attempt.

I believe I have covered the first part of Willy's question, what if we fill (and seal) our boiler at sea level, then carry it up the mountain to operate, even if with a few too many calculations.  Now the other part, is it different if we carry the boiler up empty and fill it from a convenient stream at the top? 

We begin by looking at our starting conditions for this case.  It is a way back, so I will remind you that at the top of the mountain, the atmospheric pressure was only 84.55 kPa, remembering that atmospheric pressure is always expressed in absolute pressure units, whether it be measured as 14.7 psi or 101.3 kPa, or some other value as in this case.  Also  we found that water in our vented kettle boiled at 95 deg C.   More recently, we decided the air temperature at the mountain top was only 5 deg C, definitely a bit chilly unless you are in Barrie in early spring, when any plus temperature means it is time to put away the scarves and heavy jackets.

When we fill our boiler, both the water and air will be at 5 deg C, and we assume the metal boiler shell, no longer in our pack, has also cooled to 5 deg C.  The absolute pressure will be 84.55 kPa as before, then we install and tighten the plug and connect our absolute pressure gauge.  At 5 deg C, the water vapour pressure is 0.872 kPa as before, so the air pressure in our boiler is 84.55-0.87= 83.68 kPa.

Now you might have noticed that I have jumped around a bit on the number of significant figures and accuracy.  A barometer is in fact an absolute pressure gauge and is calibrated in hectopascals, (10^2 Pa, or hPa, it seems Mr. Apple does not know they exist!)  Standard atmospheric pressure is 1013 hPa, and that fourth significant figure is quite significant in determining the weather we will experience as the highs and lows of our weather pattern pass over.

You will also notice that I have not bothered to look at humidity.  Despite its "couldn't care less" attitude to air molecules, water is very inclusive about water molecules.  They are all treated equally, whether they arrived with the air as humidity, or they evaporated from the water.  The vapour pressure of water is only dependant on the total number of molecules in the space, which in turns depends on the temperature of the liquid at the surface.

Our operating conditions are dictated by the boiler design which is determined by the difference between inside and outside pressures.  Now, our standard pressure gauge measures just this very difference.  So up the mountain, we still operate to the gauge pressure, not the absolute pressure.  The same gauge pressure at the top of the mountain, where the atmospheric pressure is lower than at sea level, means the absolute pressure in the boiler is lower, and hence the boiling temperature of the water (once all the air is expelled) will be lower.

The other difference will be the starting conditions when we are up the mountain.  At sea level the air density is a little more, so there is a little more mass of air in the boiler which will add to the water vapour pressure so changing the total pressure in the boiler until it has all gone out mixed with steam production.  Also, air, water and the boiler mass will be sea level temperature of 30 deg C.

At the mountain top, the air density is less, and the temperature is less.

Because of the lower density, if we fill the boiler at the mountain top, we will trap a smaller mass of air when we insert the plug.  This will always exert a lower partial pressure contribution to the total pressure than the greater mass we trapped at sea level.  However, once we reach our operating pressure and allow a little steam to escape, the air will be mixed with the steam and soon gone.  Then both cases are the same.  Boiling commences as soon as the vapour pressure of the liquid exceeds the total pressure at the liquid surface.

The temperature effect is unchanged between the two cases, as we have already noted the boiler and its contents will be at the same temperature whether we fill at sea level or at the top. So our starting conditions at the top of the mountain are 5 deg C for both cases and slightly less air partial pressure for the case when we fill at the top.

I hope that answers that question, so next time on to the issue of heat.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 03, 2017, 07:09:30 AM
A question of heat

Hi Willy,  now for the final question to the list I made from your earlier posts, the question of heat.

 It was only one word in your original question, I almost missed it.  I am assuming the implied question was, "Is there a difference in the heat required when we operate the boiler up the mountain compared with sea level?"

I will work with the operating conditions we have already chosen, 275 kPa(g) as normal operating pressure.  This means a slightly lower absolute pressure on the mountain than at sea level, but the boiler has been designed for a difference between inside and outside pressure, not an absolute pressure.  Our standard pressure gauge provides the right measure of operating pressure, both on the plain and on the mountain.

We can determine the required heat input from the starting conditions and our operating conditions, using the law of conservation of energy and the steam tables.  Conservation of energy is the generalised form of the first law of thermodynamics.  I am only considering the boiler, no superheater for simplicity.  It will adequately illustrate the principle. 

Heating the water from our starting conditions until it reaches our operating pressure is a constant volume process in our sealed boiler.  A feature of this process is that no external work is done.

Careful application of the first law of thermodynamics reveals that for a constant volume process, the heat input required is equal to the change in internal energy.  Now the steam tables conveniently tabulate values of internal energy over the range we are interested in, so perhaps I had better briefly explain a bit more about steam tables.  You might have a copy in a thermodynamics text book, or you can purchase them as a separate booklet.  Or you can, as usual, ask Google.

Steam tables have two basic sections.  The first is just a solid block of figures that apply to the two phase range of conditions, that is when you have both liquid and vapour in equilibrium, and is the section used for boiling and condensing conditions.  Even this is divided into two parts.  One part has the temperature in the first column, and corresponding equilibrium pressure in the second.  The other part has pressure in the first column and the corresponding equilibrium temperature in the second.  Use the one that best suits your particular conditions, in particular whether you know the pressure, and want to know the temperature, or do you know the temperature.

The other part of the steam tables looks like a number of separate small tables.  This section covers the superheat range.  More about that section when we talk about boilers.

We will use the first section, called the saturated steam table, as we have both liquid and vapour in our boiler.  After pressure and temperature, the  next two columns show the specific volume of liquid and the specific volume of saturated vapour.  Specific volume is simply the reciprocal of density, and I assume the reason for the convention is because when calculations were done by hand, specific volume involved less division, so was easier to deal with.  Specific volume is the volume in cubic meters occupied by 1 kg of fluid.  You can see the specific volume of the liquid is very close to 0.001 m^3 or 1 litre per kg as we all know, while as vapour, it occupies a much larger volume and you can see the expansion ratio in these two columns.

Then we have normally have three columns for internal energy.  The first, Uf, is for water just about to boil, referred to as saturated liquid.  The third, Ug, is for saturated vapour, vapour when the last of the liquid has just evaporated, before superheat starts.  The second column, Ufg, is the difference between the two, which saves a step in many calculations, again for the days when calculations were done by hand.

At sea level, we sealed our boiler at 30 deg C which is our starting condition.  We could look up the internal energy of water at 30 deg C,  and again at our operating pressure, but this is where things get complicated!  Some of the operating pressure is due to the remaining air, so the water is not yet up to operating temperature.  It will not be until we let the air out, and we will also let out steam in the process.  Once we let some steam out, we no longer have a constant volume process, so internal energy no longer tells us what we need to know.

It all makes a precise calculation quite difficult. So I will try and answer your question a different way that I hope will tell you what you want to know.

I think that is enough for one session, so back to this question again tomorrow.

I hope it is all making sense so far.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 04, 2017, 11:48:18 AM
Continuing with the question of how much heat is required to boil water.

My first application of conservation of energy to this problem led to a further difficulties when my initial assumption of a constant volume process fell down.

Let's start again, this time looking only at the water.  Starting at 30 deg C, we gently heat our boiler until the pressure reaches our operating pressure of 275 kPa(g).  Now we let out a little steam air mixture to control the pressure at 275 kPa(g).  We no longer have a constant volume process, as our steam is expanding into our outlet pipe.  If we hold it all at our selected operating pressure, we have changed to a constant pressure process.  We can no longer rely on the internal energy to solve our problem, we have to go back to the first law of thermodynamics and apply it to a constant pressure process. 

This time our text book introduces the concept of enthalpy, one of your magic words that so far I have managed to avoid.  The first law analysis tells us that for a constant pressure process, the heat input equals the change in enthalpy!  Again, in most thermodynamics texts if you want to look at exactly how that is determined from conservation of energy.

So I am going to have to try and explain enthalpy in one easy lesson.  That will test me, but let's have a go.

Many of the thermodynamic problems associated with both gas and liquid are solved by applying those basics laws of physics, conservation of energy, and conservation of momentum.  I have referred to those previously.  We start with the three basic properties that we can measure, pressure, temperature and specific volume (or density if you prefer), then apply conservation of energy and/or conservation of momentum to find the solution.  When we do this, some combinations of the measured properties come up often.  Specifically, the combination U + p x v.  Internal energy plus pressure times specific volume.  It turns out that this combination behaves very like another property, but we have no gauge to measure it directly.  Steam can have the same value for U+pv for the same mass when evaluated at different pressure or temperature.  Some processes can proceed along a path on a pressure-temperature diagram, along which the value of U+pv is the same all the way.  Just as we can proceed along a path where the pressure, or the temperature (but not both) are the same all the way.  Hence this combination of U+pv behaves like a property, and is given the name enthalpy, and the symbol, h.  Enthalpy, like internal energy, has the units of kJ/kg and looks rather like a particular sort of energy.  It occurs so often, and is so useful that the next three columns of the steam tables list the value of enthalpy in a similar format to the columns for internal energy.  In case you are wondering, it occurs frequently in processes involving work at an external boundary, such as in our engines.  I hope that is sufficient for the moment.

So back to the question.  If we control the pressure in the boiler as the steam escapes, it becomes a constant pressure process which we can analyse using the enthalpy columns of the steam tables.  To find out how much heat is required to heat our water from the cusp of boiling until all the liquid is evaporated into steam while we let out some steam and air to hold the pressure constant, we can simply look up the change of enthalpy.  Furthermore, it turns out that we can use change of enthalpy right back to our starting point, even though the first part of our heating is really constant volume.  This conveniently avoids the issue of just how we changed from constant volume to constant pressure.

If we continue just looking at the water, we can now start at 30 deg C, and look up the enthalpy for water, hf = 125.79 kJ/kg.  We can then look at the enthalpy of the saturated vapour at our operating absolute pressure of 375 kPa, (275 kPa(g)) and find hg = 2735.6 kJ/kg.  Then the heat input required by the water is 2735.6 - 125.79 = 2609.8 kJ/kg.

Now if we go to the mountain top we do the same analysis with two differences.  First the starting temperature, and hence the enthalpy of the water, is lower at only 5 deg C, and second our absolute pressure at 275 kPa(g) is only 359.55 kPa absolute compared with 375 kPa at the foot of the mountain.

Now starting at 5 deg C, we can see the enthalpy of the saturated liquid is just 20.98 kJ/kg.   Our operating pressure of 359.55, say 360, is close enough to the saturation pressure for 140 deg C.  You can see hg for the steam released from the boiler will be 2733.9 kJ/kg.  When we insert the figures and do the subtraction, we see that we need 2733.9 - 20.98 = 2712.9 to boil our water on the mountain.

Comparing the figures, we see that we need 103.1 kJ/kg extra to boil our water at the mountain top.  That is interesting, but how did it come about?

If we look closely at the figures, we see that we needed 104.8 extra to heat the water from 5 deg C to our mountain top boiling point of 140 deg C, while the enthalpy of the steam at the lower absolute pressure on the mountain is only 1.7 less than it was at the higher absolute pressure on the plain.

My conclusion is therefore, that we will need more heat to produce our steam at the mountain top, and the reason is almost entirely due to the lower starting temperature.  The slightly lower absolute pressure at our operating gauge pressure makes a difference of only 1 part in about 2600, which is well beyond the accuracy of most of our experiments, while the extra heat to get from 5 to 30 degrees, (104.8 kJ/kg) is about 4% of the heat required to boil from 30 deg C.

A small boiler like we might use for these experiments might contain about 1 kg of copper.  This would require 42 kJ/kg to heat from 30 to 140 deg and about 52  to heat from 5 to 140, thus adding about 12 kJ/kg to our extra heat requirement, and we need to allow a little for the lagging.  The other losses will be somewhere near proportional to the heat required by the water.  On this basis the total heat required on the mountain would be not much more than 5% more than on the plain.

The differences we have found may not seem very much. They could have been made to stand out a bit better had I continued assuming you might want to climb Mt Everest.  But I am trying to stay practical, and let's face it, it's crowded up there these days, unlike when Sir Edmond Hillary did the climb.  It is unlikely that the crowd would be willing to hang around, stamping their feet to keep warm, while you carefully measure your boiler temperature!

Next time, back to my little oscillator and its exhaust port, I hope it will have something for everyone.

Thanks for hanging in there,

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on July 04, 2017, 01:01:02 PM
.....'Next time, back to my little oscillator and its exhaust port, I hope it will have something for everyone'

Looking forward to the exhaust lesson MJM.....this is where I am currently chasing my temperatures OK :cussing: ...however without clear resolution  :headscratch:

Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 05, 2017, 01:30:14 PM
Returning to talking about engines.
 
Hi Derek, glad to have you still looking in.  In your previous posts, you were recognising that your exhaust piping was too small and were awaiting parts, which I assume have now arrived.  I assume that "still chasing temperatures" means you might be trying to condense the steam.  Or are you just trying to collect the oil?   Or are you trying to achieve vacuum conditions for your exhaust?  Please let me know which.  First, two posts on oscillating engines, then I will go on to exhaust.  I am sure that through that discussion, together we will be able to solve the problem.

 Those questions and comments by Willy and Jo, were deceptive in their depth, and quite a challenge to answer clearly.  I hope that I have done them justice.  Some of the issues will be discussed further when we come to boilers, and will also help our discussion of condensers.  They have given me the prompt to deal with some basic issues before I go too far.  Thank you both, and thank you to the others who have contributed by asking questions or commenting so far.

Back in post #90, I had been talking about valves and their part in conducting the sequence of processes that enable our engines to run in a continuous manner.  I had looked at the requirements for the valve opening to match the volume swept by the piston.  It turns out that the inlet valve opening matches quite closely the opening required, based on looking at required velocity in the port to maintain pressure as the piston goes down from top dead centre.

The real issue comes with the exhaust valve opening.  While the piston is slowing in its decent, the piston is full of steam which has done its work and must be expelled in the exhaust stroke and this is the most demanding event from the capacity point of view.  The slide valve is not too bad, with wide ports with sides parallel to the edge of the valve plate, so opening occurs quite quickly, and allows rapid exhaust of the pressure in the cylinder.  But looking so closely does show the advantage of those wide straight sided ports typical of a slide valve engine, in case you are tempted to just drill round ports.  A piston valve has a similar characteristic, with the ports wrapping around the valve.

That brings me back to my little oscillating engine.  Most of us have built, or contemplated building at least one of those, or at least seen them in action in toy engines like the Mamod range and others, excellent models which gave many of us our first hands on experience of steam.  I still have the one so much enjoyed by my brother and I.  Of course in those days, real trains were hauled by those magnificent steam locomotives, with the associated sound and billowing steam.  And we got coal specs in our eye when we put our heads out of the windows, which actually opened.

In an oscillating engine, also known as a wobbler, the valve port is formed by the overlapping portion of the drilled ports, one in the cylinder and one in the port block on the engine stand.  They are generally located so the openings do not overlap at all at top and bottom dead centre.  As the engine rotates, the cylinder port starts to overlap the port in the block on the stand to start admitting steam.  A typical design has them finally fully overlapping at the maximum cylinder movement, about 80 deg of rotation, after which they start closing again.  There is some variation, sometimes planned and sometimes due to accuracy of drilling in the required location.  There is only a momentary complete circular opening.  For most of the revolution, the opening is only the overlapping section of the drilled cylinder and steam ports.  In case the words are not clear, I have attached two sketches.  One shows the general port layout, the other, four steps in the port opening, the open section filled in red.  Once maximum opening is reached, they progressively close until bottom dead centre.  The exhaust port opens in a similar manner on the upstroke.  With a little trigonometry and some circle geometry in a spreadsheet, we can calculate the actual opening at each point of the engine revolution.

It should be obvious however, even without calculation, that just when the engine needs a large exhaust opening to quickly exhaust the steam, the port is nearly closed, and only slowly opens as the exhaust stroke progresses.  Consequently, the piston starts its return stroke for the exhaust with significant pressure in the cylinder, and the port is not fully open until the piston is over half way back to the top.

We have already seen that pressure in the cylinder on the exhaust stroke actually works against the piston movement, subtracting from the work done in the power stroke and reducing the output of the engine.  In a single acting engine, the work necessary to drive the exhaust stroke comes from the flywheel, and in the process, the engine slows during each exhaust stroke.  Despite this, the engines do run, and quite well, though they do not handle much load.  If we could allow the cylinder to exhaust freely, the engine would be considerably more capable.

Next time, I will look at two possible solutions that have been proposed at various times to overcome this problem.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 05, 2017, 02:10:20 PM
Hi, still following along........one of my favourite sayings is : the adiabatic enthalpy : principal  but i don't know if you can use the two words together !! It does however impress lesser mortals !! One question on a more practical note is ..Where is the best place to introduce the feed water into the boiler ??
Or is this a topic all in itself ? :happyreader:  also rather then a round hole for the Wobbler port would it be beneficial to have an oblong hole so as to introduce cut off  ?? this is also a whole new subject possibly !!
Title: Re: Talking Thermodynamics
Post by: Maryak on July 06, 2017, 12:14:38 AM
.Where is the best place to introduce the feed water into the boiler ??

In full size practice, feed water is introduced via internal feed pipes which evenly distribute the water along the length of the steam drum in watertube boilers. In in firetube boilers there is a distribution box fitted in the middle of the shell which acts as a form of feed heater. The idea is to reduce the effect of the difference in temperatures between the boiler water temperature and the feed water temperature.

Regards
Bob
Title: Re: Talking Thermodynamics
Post by: derekwarner on July 06, 2017, 01:34:01 AM
....."Or are you trying to achieve vacuum conditions for your exhaust?  Please let me know which"

MJM.....explanatory PM sent so as not to discolour  :toilet_claw: or cloud your thread......

Looking forward to the next instalment & reading with interest  ....Derek   :DrinkPint:
Title: Re: Talking Thermodynamics
Post by: Zephyrin on July 06, 2017, 07:51:42 AM
Quote
the port is nearly closed, and only slowly opens as the exhaust stroke progresses.  Consequently, the piston starts its return stroke for the exhaust with significant pressure in the cylinder, and the port is not fully open until the piston is over half way back to the top.
as opening and closure takes time, to get widely open port when needed, we need advance in the steam distribution, but a simple wobbler cannot cope to that.
And there no doubt that straight edges are preferable for the steam ports, as the rate of increase of the opened area is much higher than with circular holes for an identical valve displacement.
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 06, 2017, 12:31:47 PM
Exhaust ports for oscillating engines

Hi Willy, Bob and Zephryn, glad to hear from you all again.  Also received a PM from Derek for me to think about. 

Now, adiabatic enthalpy!  That is a mouthful.  You probably can construct a sentence with the two words together, but without a little more context, they are perhaps not as complete or clear in their meaning as you might like.  I suspect those who nod in wise agreement did not really understand what you said, just as you implied.

To get away from the thermodynamics, a student of English might class "adiabatic" as an adverb, while "enthalpy" is a noun, so you can see the problem.

Enthalpy is a property, just like pressure and temperature (though we have to calculate its value, as we don't have a direct reading gauge), perhaps refer back to Post #113 for more detail.  It's value does not depend on the process involved in getting the fluid, in this case steam, to that condition, just final temperature, pressure and density.

On the other hand, Adiabatic is the description of a specific ideal process, meaning a process that occurs without heat transfer.  Expansion in a well insulated cylinder is a process that closely approximates adiabatic, as is throttling through a valve or nozzle.  An adiabatic process can be analysed using basic thermodynamics, and the work output can be predicted.  Unfortunately most real processes, such as the expansion of steam after admission is cut off, cannot be analysed, so we can't predict the amount of power our engine will produce.  Not very useful if your brief is to design an engine for the Queen Mary.  But we can compare the power produced by a real engine on test with an ideal adiabatic engine, and define an efficiency accordingly.  Just one of many definitions of efficiency, but more about efficiency another time.

So your favourite term needs to be used in the context of an enthalpy change.  An adiabatic process involves a change of enthalpy from which we can calculate the work produced.  A real engine involves a different enthalpy change from that which occurs in a true adiabatic engine, always less, due to thermodynamic losses classed as irreversibility.  Now you are prepared for the one who asks for further explanation, and you can safely refer to "adiabatic enthalpy change" as opposed to real engine enthalpy change.

Bob has very clearly answered your question about feed water entry.  We don't need quite such a complex arrangement in a model boiler, but the principle is still to minimise the temperature difference at the entry point.   Always below the surface to minimise splashing and carryover, and you could extend your inlet pipe a little to get the worst temperature gradient further from the shell.  You could think about other ways of reducing the temperature difference in preparation for a future topic.

Your final question, and Zephryn's comment about whether an oval or straight sided port would help the oscillating engine, very neatly introduces what I was thinking of today.

K. N. Harris, in his excellent book on engines, looked at this problem and suggested modifying the ports to have straight sides.  Many of us have a copy of this book, but I have attached a sketch to show what he described.  It always struck me as a great theoretical solution to the problem, but I did not have the practical knowledge to be able to implement it.  I made a test block and tried with fine files, and tried the traditional " chippies" cold chisel to get the required port shape, both without success, probably due to inadequate skill level in both cases.

Florian might be able to comment on whether his excellent thread on making a broach could be adapted to produce the required shape.  I suspect the real problem for broaching in this application is that the required holes are very shallow.  The cylinder port for example, the broach could only protrude one cylinder diameter before it hit the wall on the far side.  In the base port block, holes do not go right through, and I am not sure if there is space to drill and tap the back face and insert a plug after broaching.

Now, having a mill and rotary table, and having learned many new skills by following the experts on this forum, I have also come to wonder, as you have, about making straight sided oval ports.  I even have a 2.5 mm milling cutter, and could buy a 2.0 mm one, though the bigger ports are fine on my 12 mm bore model.  I suspect that this is what you mean, but again, I have attached a sketch to show what I propose.  Now that I have sketched out, it looks very like Mr Harris' design.  Should work.  As Zephryn says, still not a perfect solution, but surely better than round ports.   Later this year I may get a chance to give it a try.  But not with my present schedule.

Mr. Harris also presents another solution to the exhaust opening conundrum.  This one may be even more effective, and is certainly more interesting from the thermodynamics point of view.  It was even the subject of a patent in the early days of steam, so on to that next time.

Thanks for following,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 06, 2017, 02:37:12 PM
Hi, more interesting stuff...i am thinking about a wobbler engine with triangular shaped ports with a 90 degrees offset wobbler part operating at the back of the port plates to achieve a cut off sequence.....!! or would this be clueless rather than corliss ?? !!!! might produce a drawing  !! Also i seem to remember an article in Model  Engineer some time ago with someone doing some experiments along these lines, the person might have been from Australia if i recall..Thanks for all this info,  it will help when i actually meet someone in person that also knows all about T.....
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 07, 2017, 07:15:04 AM
An alternative solution for oscillating engines.

Hi Willy, that second plate looks like an interesting concept to explore, it would be good to have some way of adjusting valve events.  Definitely not clueless.  You might need an eccentric to drive the plate to allow you to extend the shaft through the two fixed parts.  Also to allow timing adjustments.  I am not sure what the triangular port is about.  To me, ports are a source of irreversible loss, they all  turn good enthalpy into wasted heat in the exhaust.  Since we must have them, they should be large as practical, and always fully open or closed, with opening/closing times minimised.  And timing is the important thing.  But perhaps there is something I have not thought of.  Well worth sketching out and exploring the valve events.  There is also that very interesting design with the slide valve on the side of the cylinder, that has been the subject of some recent build logs. 

The background article in your picture is also  interesting, but I think it is not very helpful to talk about scaling nature.  We are building real engines, just small in size.  So some things might not work quite the way they do in full size, such as viscosity and surface tension.  However, mostly these are more obvious in ships for displacement and wave making, and aeroplanes  for wing lift and drag.  Almost certainly a few factors in making a miniature violin as well if we want the notes to be in the audible range.  It is more useful to think about which particular bit of nature you are trying to scale, than lumping all nature into one catchall term.

To round out my discussion of valve opening, and for oscillating engines in particular, I would like to apply some thermodynamics to the engine Mr Harris called a Uniflow engine.  It was not really an original idea, there was an early patent on the principle that I uncovered in a simple Google search, though it did not seem to have made its owners any great fortune.

The idea was to place an additional exhaust port low down in the cylinder, so that it was only uncovered when the piston was close to the bottom of its travel.  The idea was that the steam, instead of having to reverse its travel to be exhausted at the top, could continue moving down and out, thus avoiding the force on the piston necessary reduce the momentum of the steam and then reverse its direction.  He concludes by inviting his readers to try it, and see the extra output from the engine.  I would certainly encourage anyone interested to give it a try by adding the Uniflow port to their next engine.  And certainly a class of young people about to embark on engineering studies by building a small oscillating engine.  It will help introduce them to some basic thermodynamic principles of heat engines.  I hate to admit that I have not yet done it myself, but I have no doubt about the claimed increase in performance being real.

Observations usually are real, and when they depart from theory, my observation is that it is usually the wrong theory being applied, or often a different theory, also applicable, has more influence on the outcome.  The published explanation in the Uniflow patent appropriately describes momentum, but then alarm bells ring and I find myself skeptical about the explanation for the function of the port.  And I do not wish to be in any way disparaging about Mr Harris' effort, he has certainly made a great contribution to our hobby with his books on engines and boilers.

First, we have to deal with the fact that it is the momentum of the molecules being changed at the piston face which causes the force on the piston that drives the engine.  The crank mechanism means that the piston velocity is reduced to zero at bottom dead centre, so it matters not whether it then accelerates up, down or probably sideways to exit the port.  Of course the torque is minimal near the bottom of the stroke, so loss of torque due to opening the port is not much.

Then we note that the overall downward velocity of the steam is a very small nett velocity, superimposed on the quite high velocity of the molecules in otherwise random directions.  Remember way back, I looked up the typical molecular velocity of about 500 m/s, while the average piston speed is only about 5 m/s.  So the momentum due to the average downward velocity of only 1% of the momentum of the typical molecule, surely does not make that much difference.

I suspect that it is only when we appreciate the difficulty of opening the exhaust port enough to exhaust the pressure at the beginning of the exhaust stroke, that we see the more likely reason for the observed performance improvement.  It provides additional exhaust port area just when it is most needed.  In any case, on a single acting engine, drilling the port, but keeping it separate from the top exhaust, with some provision to block and thus disable it, would surely be an interesting experiment.  Not so easy on a double acting engine.  The piston length would have to be carefully proportioned for the port to work on both up and down strokes.  It would also be more difficult to keep it separate from the other exhaust ports so the engine could be reversed.  Still good food for thought, and a potentially interesting experiment.

That is not all that can be said about thermodynamics in relation to engines, however I believe that clears the decks enough to move on tomorrow to exhaust systems, which should help Derek solve his current issues.  (Derek, I have replied to your message). And I will return to some other engine topics as the need arises. 

I hope you have found this explanation of how an engine changes heat contained in the random motion of tiny molecules, to mechanical work, interesting and informative.

Thanks for dropping in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 08, 2017, 11:26:26 AM
Exhaust systems.

Thinking still about a vertical engine, first single acting then adding any additional comments on the difference for double acting, lets now have a look at the exhaust system.

 Keep in mind that the work output of the engine is always reduced by the back pressure on the piston during the exhaust stroke.  It is obvious then, that we can increase the output of the engine by reducing the pressure in the exhaust system.  Now this approach is limited in extent, as we can only at best get to full vacuum, which gives only 101 kPa (14.7 psi) additional difference in pressure across the piston.  In practice, we are unlikely to achieve anywhere near this.  However, in most cases our models are operating at quite low pressure, so even minimal vacuum is a significant proportion.

There are four basic exhaust concepts, so let's look at each in turn.

First, there is the simple engine outlet to atmosphere.  This is seen on some of the beginner engines with a simple piston valve and no obvious way to collect the exhaust into a simple pipe.  Also on simple Mamod engines.  Sometimes, the exhaust is directed up the boiler stack.

This system is fine for simple engines and for engines we only intend to run on air.  It's simplicity is the big advantage.  The exhaust pressure is fixed at atmospheric at the pipe outlet, and the back pressure at the piston depends on how adequately the piping and port is sized.  However, the back pressure on the piston during the exhaust stroke will always be something above atmospheric.

As soon as we add a lubricator, whether using air or steam, the problems begin.  We will have a thin film of oil everywhere near the engine, even in our hair, if we run for very long.  If we are running a boat, a tiny film of oil on the water surface scatters light in a way that makes it very obvious, and that is quite rightly frowned upon.  We start thinking about adding an oil collector to our exhaust system, the second approach to exhaust system design.

The temperature at which oil boils is much higher than the exhaust temperature of a steam engine.  Even at 100 deg C, the vapour pressure of oil is very low, and can reasonably be ignored.  Most of the oil ends up as very tiny droplets, which are entrained in the exhaust, not even enough of them to make a visible fog.  Alternatively, they end up as part of an emulsion with the fine droplets of water in the exhaust which are what we actually see when the exhaust hits the air.  Even the tiny water droplets in a fog do have mass and density, enough to make it possible to separate them from the  un-condensed portion of the water vapour.  They do not settle rapidly due to the air viscosity (you can look up Stokes Law for more information on this). However, if the exhaust stream has some velocity, and we make it go though some rapid changes of direction, we can separate the droplets from our exhaust, and essentially all the oil with them.

The first picture below shows two separators I have built.  The one on the right works very well.  When the engine first starts, the cold metal condenses some of the steam, and water droplets run out the little gooseneck drain pipe pretty freely.  As the metal heats up, the condensing slows, and there is clear vapour out the top which only becomes visible some distance from the stack as the steam cools in the air and condenses to form some fog.  I collect the water from the drain in a little tin, and after a run, the tin contains a few ml. of oily water emulsion.  Not much compared with around 200 ml of water turned to steam in the boiler.  I have not been able to usefully compare the oil collected with how much is lost from the lubricator.  This would be interesting, but somewhat difficult due to small quantities.  Clearly some oil remains smeared on the cylinder walls and piston, and ends up in the drip tray under the engine, and some is clearly carried away, but the problem seems pretty well solved.

The separator on the left of the first picture is still a work in progress and I still do not consider it successful.  More on that later.  First how does a separator work?  Let's look more closely at the one on the right.  The horizontal steam inlet is not on the centreline of the vertical cylinder, but enters tangentially.  As a result, the steam entering is directed against the cylindrical wall and forced to follow round a circular path.  Centrifugal force results in heavier particles, our oily water fog, hitting the walls, and running down to be collected at the bottom.  Dry vapour, being less dense tends to stay closer to the centre.  Now, the vertical outlet extends down inside the cylinder to below the inlet.  Thus, the vapour whirling around in the cylinder has to make its way downward, then inward before it can exit vertically up the outlet stack.  More sudden changes of direction.  The heavier droplets tend to continue in straight paths due to their momentum.  The second picture shows the outlet removed, and you can see the internal extension.  No complex science to it.  Guided by the principle, I made it out of scrap tubing left over from a household plumbing job.  In industry, these cyclonic separators are carefully designed and velocities determined to remove a calculated percentage of particles above a certain size.  But the simple approach seems to work adequately for my purpose.

I will talk next time about the design on the left in these pictures.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 09, 2017, 12:06:23 PM
Another simple separator.

 Last time I included a description and pictures of two simple exhaust separators I have made.  Apologies for the typo in the last line yesterday, I have now corrected it.  I noted that the one on the left is still a work in progress, as I am not happy with its performance.  I was trying to achieve a very low profile to go under my paddle engine style joy valve engine posted in my engine showcase thread (currently on page 3 of Engine Showcase).  Very low cylinders are an advantage for stability in a boat, but make collecting exhaust oil difficult as the exhaust should slope down to the collector. 

Initially the larger diameter section of the vertical part was not included.  The inlet was near the top of one end of the horizontal cylinder.  The drain outlet was low on the other end.  Inside, both pies were extended to a point near the opposite end.  The idea was the engine exhaust has to turn 180 deg at the end of the internal pipe and return to near the inlet end before turning 90 deg to exit via the vertical stack.  Seemed worth a try, but the tube I had was really too small for my level of ability.  Despite several tries, the tubes seem to interfere on the inside and did not end up where required.  I did eventually discover the reason, but I decided a larger diameter cylinder would be a better solution.  I then tried making the separator part in the vertical section that you can see in the photos, back to the whirling design, and keeping the horizontal cylinder as a simple collection pot.  The inlet height was just adequate, but I could have made the whole thing vertical.  Both arrangements of this one still carry over oily water and splutter out the stack, so clearly not satisfactory.  The horizontal tube is only 19 mm o.d., I need to get some 25/25.4 mm tube and try again, but I will be lucky to get back to it this year.

The simple exhaust separator as described does quite a good job of removing oil and incidental condensate, but does not really do any useful condensing of the exhaust steam.  In fact the momentum changes in all those changes of direction actually increase the back pressure on the engine.

The next level of exhaust system development is to include a condenser.  A ship crossing the ocean might want to collect and reuse the water for example, or you may just wish to collect all the water from your exhaust to eliminate any possibility of escaping oil.  Unfortunately, a simple condenser will still not give you any vacuum to increase your engine output.  In addition, the collected water needs good oil removal treatment before it is suitable for the boiler.  Those with experience of full size steam plant will know the issue for achieving vacuum is air, but let's look first at simple condensing as a half way step for those with less experience.

In order to design a condenser, we need to know how much heat has to be rejected to condense the water.  Again we apply the first law of thermodynamics or conservation of energy.  A condenser is basically constant pressure, and there is a continuous flow of steam in and water out.  For this case, our text book tells us that the heat transferred is equal to the change in enthalpy, just as in the boiler.  The main difference is we must add heat to the water in the boiler, while we have to take heat away in the condenser.

Of course we must again begin with the starting conditions at the condenser inlet.  We can measure the exhaust temperature, and we know the pressure is fixed by the outlet to the atmosphere.  It is quite likely that the exhaust temperature, once we get steady running conditions, will be very close to  99 or100 deg C, depending on the atmospheric pressure.  However, we will normally have wet steam, that is steam that is partially condensing, and that is where our problems begin.

  If we have achieved some superheat in our boiler, more on that later, the engine exhaust will probably be in the range of 95% dry, meaning about 5% of the steam is condensed, but 95% of the latent heat is still to be removed.  We can look up the steam tables and work out the enthalpy for 95% dryness, but for our present purpose, assuming dry saturated steam will be close enough.  The enthalpy of dry saturated steam at 100 kPa and 99.6 deg C is 2676 kJ/kg, while saturated water is 419 kJ/kg.  So we have to reject 2258 kJ/kg, usually to cooling water.  Now this is a similar magnitude to the heat input to the boiler.  Please don't worry that the figures are not exactly the same as our earlier boiler example, the main difference is the engine performance, which we have not looked at, and that assumption of dry steam.  There is also a further difference of 292 kJ/kg, nearly 13%, depending on whether we just want to condense at 100 deg, or continue to cool it to say 30 deg C.

Let's have a quick look at the implications of that.  Think about the quantity of heat rejected at the condenser, compared with the boiler heat input.  It means that most of the heat from our fuel goes into evaporating water so it can go up the stack.  That is the prime reason for the very low overall thermal efficiency of a steam plant.  Even the best full size plants with every known heat recovery trick in the book, were only approaching 30% last time I looked.  I am not sure if power station scale plant running supercritical surpass that these days, or by how much, if they do.  If you have read the reports of the locomotive efficiency competitions run by some clubs, you will know the best 5" gauge locomotives are only around 5%.  There would seem to some opportunity and incentive to look at practical heat recovery methods.  A good starting point for next time.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 10, 2017, 10:46:42 AM
Waste heat recovery?

We noted last time that most of the heat from the fuel goes into evaporating water that goes out with the exhaust, this is the largest loss or energy wastage in any steam plant.  Close behind, as next largest loss, is the flue gas that goes to atmosphere directly.  Then we have friction, which also ends up as heat, and other relatively small losses.  The large amounts of heat in the flue gas and the engine exhaust surely provide some opportunity for recovery to help generating steam.  We will talk about the boiler later, but can we reuse any of the heat in the exhaust steam?

We know it's temperature is relatively low, around 100 deg.  A fundamental law of heat transfer is that heat will only flow from a higher temperature to a lower temperature.  So exhaust heat is only useful for heating objects up to 100 deg.  In fact, practical limitations on heat transfer area mean that we can possibly get to around 80 or 90 deg, though more likely lower.  In full size practice, steam plant can be integrated with district heating systems to directly replace burning of fuel for heating.  On a model, it's a bit over the top to heat the captains cabin.  Feed water could be pre-heated.  Fuel needs vapourising, a process that absorbs heat that otherwise comes from the fuel.  And exhaust heat can be rejected to cooling water.  In practice, we have plenty of heat in the exhaust, but it's low temperature limits the places it can be usefully recovered.

In the early days, condensing was by direct cold water injection, a process still useful in some situations. And some forum members are interested in historical engines which used direct injection condensing.  The condensed steam and injected water all end up at the same temperature.  How do we analyse this case?  It's not constant volume or constant pressure, but we can still look up enthalpy of the steam and water before and after mixing, and assume it all ends up at the same temperature.

Let's assume the steam is saturated vapour at 100 deg, hg=2676 kJ/kg and we want to add enough water to just condense it, and have it all end up at saturated liquid at 100 deg.  If our water starts at say 15 deg, then hf = 62.99 kJ/kg.  When it is all saturated water at 100 deg, hr = 419 kJ/kg.  The steam has to loose 2676-419=2257 kJ/kg, (it's listed in the tables as hfg, no need for sums this time), while each kg of water gains 419-63= 356 kJ.  Division on your calculator 2257/356=6.3 kg of water for each kg of steam.  To put this into perspective, the tables tell us dry saturated steam at 100 C occupies 1.67 m^3/kg, while 6.3 kg of water occupies only 0.0063 m^3, or just 6.3 litres, so a very small amount by volume.  If we want it all to end at a lower temperature, we can use a higher water flow.  We can look up the saturated water enthalpy at the temperature we choose, and recalculate the water flow requirement.

Apart from direct injection, condensing exhaust steam requires the use of a heat exchanger.  Now understanding a heat exchanger requires a basic understanding of heat transfer.  So let's look at that next time.

MJM460
 
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 10, 2017, 11:24:43 AM
hi, still following along... a useful unit of waste heat at 100 degrees could be cups of tea/coffee per person per hour perhaps..!!! ;D actually i am taking in this info seriously and often wonder about the ideal steam engine configuration . As we all know no energy is lost or created, it has to end up somewhere. Also when calculating E does the digging up the coal, oil,gas and transportation  come into the equation ?
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 10, 2017, 12:57:21 PM
Hi Willy,  glad to find that you are still following along.  And once again you put your finger on right on the important points and the source of much confusion.  And you continue to keep me on track, it is much appreciated, thank you.

I will give some thought to how best to answer  for tomorrow. 

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 11, 2017, 12:44:47 PM
Hi Willy,

Thanks again for the question on units, an omission I had better fix before proceeding.

I have been using kJ as the unit of energy, usually in the context of kJ/kg, or kiloJoules/kilogram.  If you are more familiar with imperial units, you might want to compare this with the BTU (British Thermal Unit.  For all practical purposes, one BTU = 1 kJ/kg.  It is not exact, the exact figure is 1.05505585262 kJ.  Now considering that the BTU depends on the pound mass, while the kJ depends on the kg mass, and that these two mass standards are provided by different relatively arbitrary size lumps of exotic alloy in a measurement laboratory somewhere, there was never going to be a convenient simple conversion factor.  Given the source standards, it is surprising that it is anywhere near so close.  You can see from this that a kJ/kg relates to a Btu/lb by a factor that reflects the ratio of kg to lb.  The factor is 1 Btu/lb = 2.326 kJ/kg.

But in terms of the new world rational standard units of cups of tea, we need some simple calculations.  Obviously you would want to use the heat required to heat the water from 15 deg C to 100 deg C, and of course we have to define the standard cup.

The attached picture shows three cups from our cupboard.  The formal garden party model has a capacity of 150 ml, or 0.15 kg of water.  The centre on 250 ml, and the coffee mug on the right 320 ml.  I am advised by an impeccable source, that the standard metric cup, as used in cooking is 250 ml.  Adopting this standard gives a very convenient conversion factor of 4 cups equals one kg.  None of that 12 significant figure nonsense.  And a good size for coffee as well.

So the direct injection condenser I talked about last time required 6.3 kg of water at 15 deg C for each kg of steam at 100 deg C.  So we multiply by 4, and we find a kg of steam will heat the water for 4 x 6.3 = 25.2 cups of tea, or more realistically 25 cups plus a small top up for the first finished.

Then the context changed, to how many cups per hour?  Once we introduce time, we are no longer just talking about energy, we are talking about rate of energy transfer, better known as power.  I will leave that until a later day, but need to expand on the difference.

In talking about kJ/kg, or cups of tea per kg of steam, we don't know or care about how long it takes, (unless we are next in line for a cup of tea of course).  A big engine could use a kilogram of steam in a few seconds, while my little oscillator will take several hours, they can both make the same number of cups, from each kg of steam, it will just be a very slow tea party with the little oscillator, not a Sarah Palin event at all.  Oops, no more politics!

Digging coal, transport of coal and crude oil and processing all require energy as your question implies.  Most crude oil produced so far comes out of the ground at high pressure, but this is changing as available oil is used, and more pumping is required these days.  I don't have the detailed information to do the calculations, but if you want to know how much of the worlds energy reserve is used to run the car, you would have to add the energy to transport the oil, process it in a refinery, transport it to the gas station, pump it out of the underground tank into your car.  In principal, the cost of all this energy input is included in the cost you pay when you fill up.  And the heat from all this activity is lost to the atmosphere from where part is radiated to the cold of outer space.  The calculation of how much is radiated is complex and is best left to the experts.

Finally, the ideal configuration for a steam plant.  I really don't think there is one ideal.  It is a matter of understanding the energy balance for your steam plant, and knowing exactly where all the heat is going.  Then you can look at applications which could usefully use the heat before it finally is exhausted to atmosphere thus replacing fuel that would otherwise be consumed.

Power stations operate at very  high pressure which gives higher efficiency.  They use feed water heating, air pre-heating, fuel pre-heating, reheat circuits, and every other possible means to raise efficiency.  Power stations in colder climates use district heating schemes to use exhaust heat before it is lost to the atmosphere.  I have worked in industrial plants where power is produced by turbines with exhaust pressure above atmospheric pressure, so the condensing temperature was high enough to drive a process heating operation which otherwise would have required more fuel.  This scheme, often called cogeneration, produces power at a the thermal efficiency of around 80%, based on the extra fuel consumed for power plus process compared with the minimum for the process heating alone.  So it is a matter of knowing the potential, and looking at the opportunities.  Unfortunately, such opportunities do not always exist, or the quantities do not match sufficiently well, or the temperatures just do not allow the necessary heat transfer.

I hope that resolves the question on units of energy, and provides a little useful background for many interesting discussions.

Back to heat transfer next time.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 12, 2017, 01:13:24 PM
My next step in exhaust system exploration is to add a condenser.  So far in our discussion of energy, we have only looked at steam properties.  These properties are purely a function of temperature, pressure and volume, and can be looked up in the steam tables.  Properties do not depend on previous history, or how the steam came to be at those conditions.  One place where there is difficulty with this approach is when an engine exhaust consists of wet steam.  I would prefer to leave how the exhaust steam condition can be estimated, and proceed on the basis of condensing saturated dry steam.  It turns out that this is normally not too bad an estimate, and besides, wet steam is a mixture, perhaps a fog of saturated water in droplets and dry steam.  We only have to condense the steam, and it is perhaps 90 - 95% of the total exhaust.

If we have hot steam and cool water in proximity, whether separated by perhaps the copper wall of a tube, or closely mixed like our direct injection process, heat will flow from the hot steam to the colder water until it is all at the same temperature, then no further heat will flow.  (Known as the zeroth law of thermodynamics.)  However with our guests waiting for their tea, we need to make sure that the necessary heat transfer occurs in an acceptable time.

A condenser has to be able to transfer heat at the rate necessary to condense all the exhaust steam.  So now we not only need to know the steam properties, listed in units of kJ/kg in the steam tables, we need to know how many kg/hr are being produced.

I have three miniature boilers, and the test measurements I have made are all under 0.6 kg/hr using methylated spirits fuel.  They are quite small boilers for small engines.  A slightly larger boiler for a twin cylinder engine might evaporate 1 kg/hr.  This figure is obviously easy to scale up or down for any boiler.

We have already found that to condense 1 kg of dry steam at 100 deg C involves heat transfer of 2257 kJ, and we want to do this in an hour.  In terms of the new national standard, teacups 25.2 per hour.  We now know how many guests we can invite to the party.  And in ISO Metric units,  where the time unit is seconds, we need a heat exchanger rated for 2257 kJ/hr = 0.627 KJ/s. If we remember that 1 kJ = 1000 J, and 1 J/s = 1 Watt which is the unit for power.  So the heating power of our exchanger will be 627 W, or 0.627 kW which is a bit more than half of a common 1 kW one bar electric radiator.  This conversion of units of mechanical power, J/s, to units of heat, and electrical power with the constant equal to 1 in each case, is another plus for the ISO metric system.

To design a heat exchanger for this rating requires some understanding of heat transfer, so we had better look at that next time.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 12, 2017, 02:11:01 PM
Hi, so much info here ....leading to so many more questions !!  Is the weight of the steam proportional to the pressure of the steam  ? Possibly yes. If you have a vertical pipe full of water the pressure at the bottom sends a jet quite far out however at the top of the pipe it will just dribble out !!!???   Also we now don't use Pounds /inches anymore but are the formulas still the same as in MKS units  ?? Looking at one of my books i am waiting for the 37th edition that might have been brought up to date !! Also iv you have an enclosed cylinder full of steam and then at the same temperature you squeeze the piston into it ...what will happen ??.....( my brain is starting to hurt now, sorry) !!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 12, 2017, 02:20:15 PM
Hi, Also in an infernal combustion engine the inlet/exhaust ports are different sizes and the cams can be set differently . so .could a steam engine have four ports  2 for the inlet cycle and two (larger)? for the exhaust cycle ? My brain is really starting to hurt now.!! Is it because i am an autodidact ??................
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on July 12, 2017, 04:36:26 PM
so .could a steam engine have four ports  2 for the inlet cycle and two (larger)? for the exhaust cycle ?

This sounds a lot like Corliss valve gear, see:
https://en.wikipedia.org/wiki/Corliss_steam_engine

Dan
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 12, 2017, 05:11:16 PM
Actually ,yes you are right ...i was thinking about a slide valve engine though !! I have never really looked at corliss steam engine dynamics ,only heard the annoying tic, click,clunk, etc etc... Ok thats dealt with !!  Ok  so steam pressure works in all directions yes? so if you convert a certain weight of water into steam and introduce it into a closed container, will the steam working in all directions cancel out the extra weight until it condenses and becomes water at the bottom of the container ??? or is weight and gravity not compatible and is that why rockets are not powered by steam engines ??  Sorry i am not trying to be flippant just curious and asking the same questions that Watts  pupils might have asked all those years ago !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 13, 2017, 01:21:43 PM
I had intended to talk about heat transfer this time but Willy has more questions of the type that prompted me to start this thread.  They are not frivolous questions, Willy,  I suspect that you have very eloquently put into words what others are also wondering.  So I will look at your questions before continuing with heat transfer.

Now after my last post, I had better add a small point before you ask.  One horse power is equal to 746 watts, so 1 kW = 1.34 HP.

So is weight proportional to pressure?  Back to basics for this one.  Weight is the force due to the action of gravity on a mass, so weight is proportional to mass, but requires a value for g, roughly 9.8 in metric units, or 32 in Imperial units unless you really need to be super accurate. 

If you know the pressure, volume and temperature, then the steam tables will tell you the specific volume, reciprocal of density, so you can work out the mass and hence the force due to gravity.  So pressure is a factor determining the force, but it is only one of three factors involved.

Now your column of water.  So far, in each of the problems involving the first law of thermodynamics, or conservation of energy, I have assumed there is no change of elevation.  In each case there should be a term for change of elevation, however the pressure change is also easily derived from density considerations.  The formula is P = density x g x height.  It is a case where the gravitational constant is required in an SI calculation.  You can check the formula is dimensionally correct by looking at the dimensions on each side of the equation.

[P] = [F] / [A] = [m x acceleration] / [area] = kg. m / s^2 / m^2 = kg / (m.s^2)

Similarly, for the [density x g x height] = (kg / m^3) x (m/s^2) x (m) = kg.m.m/(m^3.s^2)

So [density x g x h] = kg/(m.s^2) = [P]

This analysis does not prove the formula, that comes mostly from definitions of the various factors, but it shows that the dimensions of acceleration are required, which gives the clue to the student that g is required, a very powerful cheat if you like.

That might seem a bit off the track, but it shows that height and density combine to make pressure.  In your water column, the difference in the pressure at the hole near the top and the pressure at the hole near the bottom is proportional to the density and the vertical height between the holes.

For a water column, the density is 1000 kg/ m^3, while for steam, lets assume atmospheric pressure and at 100 deg C, the steam tables tell us the specific volume is 1.67 m^3/kg, so the density is about 0.6 kg/m^3.  It is clear the difference in pressure for every meter in height difference will be nearly 2000:1.  You can see why the difference in height for steam is not very significant for steam in a model.

The energy equation also should include a velocity term, like height difference, the difference in velocity at the inlet and outlet is usually very small for a model, and so omitted.  But when a more complete energy equation is used, there is also a velocity term.  If the equation is written for your column experiment, we should include both terms, and we see the pressure due to height can be directly related to the velocity of discharge at each hole.  As you note, the velocity from the lower hole is much higher than the velocity from the top hole.  With steam there would still be a difference, but the difference would be considerably less, and for practical purposes we would normally ignore it.

That is pretty heavy going, I hope I have provided at least a little clarity. It is also getting late here, so I will look at the mks system, and your steam cylinder and isothermal compression next time.

Thank you also to Dan for answering the valve question, I will add a few comments, but Dan is the expert on valve gear, so I am delighted to see his contribution. Preferably, "talking" should involve more conversation and participants, and be less like a lecture.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 13, 2017, 03:11:53 PM
Thanks for that.......also in my book..There are tables that mention the Latitude at Dublin.... Is this significant ?? Do thermodynamic tables alter at the equator  from the poles ??
Title: Re: Talking Thermodynamics
Post by: Maryak on July 13, 2017, 09:19:24 PM
Hi, Also in an infernal combustion engine the inlet/exhaust ports are different sizes and the cams can be set differently . so .could a steam engine have four ports  2 for the inlet cycle and two (larger)? for the exhaust cycle ? My brain is really starting to hurt now.!! Is it because i am an autodidact ??................

Some steam engines were fitted with double ported slide valves.


(http://i389.photobucket.com/albums/oo340/Maryak/dpsv_zps67t8yoii.jpg)
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 14, 2017, 02:22:31 PM
Continuing on Willy's questions from post #130 -

The next question was whether the mks system we may have been used to is the same as SI.  This was a very contentious issue at the time of adoption of SI metric, as it seemed to some that everyone was being made to change so no one was disadvantaged, and no one had a head start.  However this was a spurious argument, put up by people who did not understand physics.

The fundamental huge leap forward inherent in the SI metric system is the definition of the unit of force by application of Newton's first law and the associated equation, F= ma.  This step provides a clearly defined unit of force (the Newton) and removes the necessity to use the arbitrary constant equal in magnitude of the acceleration due to gravity, and instead allowing the constant to be one, and removing the confusion around force and mass.  This is a major contribution to a rational set of units.  Previously all systems defined force in terms of the effect of gravity on a mass, giving rise to the imperial unit of pound force, the mks system with its kgf, cgs with gmf, and probably others.  So mks system is a sort of inferial metric system using sensible size units for large industrial equipment (the chemical engineer's test tubes capacity measured in tons) and cgs is good for laboratory size equipment where grams and centimetres are more convenient.  The systems arose from historical understanding of mass and force.  Newton's law is now recognised as a fundamental law of physics, and now more eloquently expressed as the law of conservation of momentum, which applies to bodies moving in a straight line.  A very similar universal law which is totally analogous, expressed as the law of conservation of angular momentum, applies to torque, moment of inertia and acceleration of angular rotation.  Using this basic law of physics to define force results in a very rational set of units, and clearly defines the relationship between mass and force in a universal way, it applies equally on the moon, on Mars or anywhere else in the universe.  So please remember these legacy unit systems only for the purpose of working out conversion factors, and then continue in SI metric.

The question on steam enclosed in a cylinder with one end a piston is very interesting and the answer might not be what you would expect.  When you move the piston to reduce the volume, the steam is compressed.  If you have adiabatic compression, that is no heat flow in or out, the pressure increases, along with the temperature.  So the properties of the steam change toward superheated, and yes, you can compress steam.  The work input adds more than enough energy to avoid condensation.  However you asked what if you remove heat as you compress so the temperature does not rise.  Now for a permanent gas, such as air, isothermal compression means less work to do the compression.  However with steam, removing heat means the properties change toward condensation.  I have not yet found an example to rely on, to say what happens if you try isothermal compression on steam, but I would tackle the problem this way.  Using steam tables, if you use the superheat section, and look up the properties at a temperature like 150 deg C, starting from say 0.01 MPa, you will see that as you move to the next pressure and reduce the volume (your cylinder contains a constant mass, so reducing volume means reducing the specific volume), while the pressure increases, the saturation temperature also increases, so by about 0.48 MPa, the saturation temperature exceeds your 150 deg C, so steam will start condensing.  It seems likely that if you start at the saturation temperature, then the reduction in volume will cause some condensation immediately.  But if you do not remove all the heat added by the work you do in moving the piston, you can keep compressing and avoid condensing.

Your text book is up to date for its time, and much of the maths is still current, though the units are more difficult to deal with than SI.  The main trick to following the text is to keep track of which assumptions are current at any point in the text.  The pages you have shown talk about isothermal and adiabatic processes, and calculate the work done as the process proceeds.  It was already understood that you had to know the process to calculate the work done.  Modern terminology is that work done is a process dependent quantity, unlike enthalpy and enthalpy, which purely depend on the measured pressure, temperature, and specific volume, not the process followed to get there.  Personally, I find a modern text much easier to read, and it is definitely worth getting an SI version of the steam tables.

I am not sure about the Dixon table.  It looks like an early version of a steam table.  I note that the pressure is measured in inches of mercury, which is of course dependent on the local value of gravity.  Now the gravitational acceleration changes from the equator to the poles, and also with height above sea level.  Hence the use of the defined latitude (of Dublin in this case).  The last column looks like enthalpy with a reference temperature of 32 deg F, but I am not sure what the H and L columns are.  Modern steam tables use absolute pressure, and there is no dependence on latitude.

I hope that adds a bit more clarity, and a couple more blocks to our knowledge base.  I will check on what I have missed from your post for next time.

Thanks for following along

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on July 15, 2017, 10:35:31 AM
Hi Willy, a few more answers for you in the hope of dealing adequately with the outstanding issues.

While I basically leave valves to Dan and Maryak, I was going to add a couple of comments.  If I remember history correctly, the first engines were controlled by a boy on the taps, opening and closing them as required.  Obviously a real impediment to high speed, continuous operation.  Then some lever systems were developed to let the boy off, but still pretty clumsy.  Jobs sacrificed to technology even then!  So the slide valve was a huge step forward.  Then, I suspect that, just as increases in boiler pressure had to wait for pressure vessel technology to advance, I suspect that valve design had to wait for advances in machining technology, and even metallurgy.  The slide valve worked well, but there was a lot of development in the linkages which drive the valve to allow early cut off and reversing.  That valve of Maryaks would be a real trick to machine, perhaps requiring soldering some layers together.  Would be interesting to understand that one better.

Now post #133.  Some real confusion there.  I will try and make things a little clearer.  First introducing steam into a closed container.  You did not say if the container already had air in it, and you did not make any comment on heat transfer during the operation.  Now steam is normally hotter than atmospheric pressure, unless the steam itself is at quite low pressure.  So you can expect heat to flow from high temperature to low temperature, unless you make specific provisions.  You might for example insulate the vessel very well so heat transfer is prevented.  Let's start with that.  We assume no heat transfer in or out.  You introduced the steam to the container, rather than evaporated steam in the container.  So not a constant volume process, and no mention of moving pistons for external work.  So let's just assume some steam in the container, and the steam definitely has mass.  Steam will fill the entire space.  Pressure works in all directions, so pressure is experienced on the walls, but they do not move, so no work done.  As you imply, the steam "has weight", but weight is simply the result of gravity acting on a mass, there is no conflict there.  But I expect that you are alluding to the normal assumption of equal pressure in all directions, so what is weight doing?  Basically, just as we saw in your water column experiment, weight means that the steam pressure is higher at the bottom of the container than at the top.  Let's see if we can calculate the pressure difference.  Basically, that conservation of energy equation gives us the necessary clue.  We saw before that the pressure energy due the height if the fluid, in this case steam, is calculated as P= density x g x h.

If our steam is at 100 kPa (absolute), we have looked up the specific volume (tells us density) for that before.  I think the density was about 0.6 kg/m^3.  The value of g in SI units is near enough to 9.8 m/s^s, and as we are mostly interested in models, let's assume the height of our container is just 1 metre.  It is then easy to scale for other heights.  So P = 0.6 x 9.8 x 1 = 5.9, so what are the units?  Using our dimensional analysis, we see we have kg/(m.s^2) which is the dimensions of pressure in Newton/m^2.  So we have nearly 6 Newton's per square meter which has the name Pascal. A Pascal is a very small unit.  We have 1000 Pa = 1 kPa.  And standard atmospheric pressure is about 101.3 kPa.  So 6 Pa is 6 / 100000 times 1 atmosphere.  Or 6/100000 x 14.7 psi or 0.0009 psi.  You can see why it is normally considered not important.  The more so in most of our model sizes?

Condensing requires the steam to lose heat.  So with our assumption of no heat transfer, and no work done, no condensing.

Oh, and in case you were assuming some air in the container to which the steam was added.  So we can continue to assume no heat transfer, the air and the container and the air have to be at steam temperature, otherwise irreversible mixing occurs until the temperature is uniform.  Then Dalton's law of partial pressures tells us that the air and steam each fill the space as if the other was not there.  So it makes no difference to the steam, however, the total pressure in the container is the sum of the pressure due the air plus the pressure to the steam.  Each called a partial pressure in this case.  Dalton's law is close enough until quite high pressure.

And a water powered rocket?  Some one has no doubt tried it.  However rockets are about thrust to mass ratio, which of course gives acceleration.  You would need a large mass of water, plus fuel to heat it, then most of the heat goes into evaporating the steam before there is energy to accelerate the steam out of the rocket nozzles at high velocity.  And the latent heat is lost with escaping steam.  Conservation of momentum then tells us the resulting propulsive force.  Modern rockets whether solid fuel or liquid fuel usually employ chemical reactions to release much higher amounts of energy, and give much higher thrust to mass ratio, so make a more efficient rocket.  However, you might have seen those soda pop rockets where the pop bottle part full of water is pumped up with air, inverted so the neck is at the bottom, and fitted with a nozzle with a means of sudden release.  They go up in a spectacular manner, but the next continent is not in any danger.  No doubt something similar could be done with steam, but with a high probability of scolding the rocketeer, definitely not recommended.

So not silly questions, but questions which highlight the significant consequences of physics and thermodynamics.  I hope I have been able to provide at least a little clarity.  More building blocks in place.

Thanks to all who are looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: Stuart on July 15, 2017, 11:17:32 AM
By ek my brain hurts in a good way

All this has reminded me of when I worked in HVAC and because we used chillers 4 500 ton units ( with now banned R11 ) I had to do a refrigeration theory course amongst other at Manchester Uni.
All the talk of  e’s ( sorry my dyslexia is bad today) made me think how much I learned in the past but now do not use ,it’s been over 22 years since I was in work but we used the SI system for the HVAC calcs

Thank for taking the time to explain the behaviour of gases

A add on topic could be the explanation as to why it takes so much energy input/extraction to change the state of , gas ,fluid or solid into the next state

Stuart
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 16, 2017, 12:31:32 PM
Thanks for dropping in, Stuart, I'm glad the brain hurt is in a good way, and I appreciate the thanks.  You will understand my granddaughter who recently told her mother that "Grandpa really is hard work!"

Those 500 ton chillers should be able to keep a lot of beer cold.  I never had much to do with any of the freons, went straight to the hard stuff, like propane, propylene and ethylene.  They are very good refrigerants but there is an issue of flammability that makes them unsuitable for most uses.  Same principal though.  Refrigeration is great for getting to understand the principals, and always useful.

The issue of learning all this good stuff, then forgetting overtime due to not using it often enough, I think is pretty normal.  I don't think any theory will help you find and cure a leak, or install a unit in insufficient space, and when you have the pressure of jobs to complete there is not much time for contemplation.  But over the years I have come to realise that my career was a little unusual, in that I frequently needed to use the theory, all through my career.  And so many questions are asked in the model engineering community that are not really answered, so I was prompted to have a go.  Vixens comment on 4000 strong in Hugh Currin's thread struck a note though.  I know we have a few student members of the forum, and I would be delighted if I could help them in getting started too.  This is intended to be for everyone interested, not just the old hands. 

Now your question on the amount of energy required to cause a change of state, solid to liquid or liquid to gas.  This is an interesting question which leads into some of the fundamental ideas in physics, atoms molecules and the forces between these tiny building blocks of the universe.  Just what forces hold the separate atoms together whether in a fluid state such as a liquid, or in a solid state where the strength can literally be like steel?

In a gas, the individual molecules are moving rapidly in all directions and they are a big distance apart relative to their size, apart from the occasional collision.  Conservation of momentum applies to each molecule, so they travel in a straight line until a force makes them change, either collision with another molecule, or the wall of the vessel which contains them.  So gas molecules always fill the space, and they are so far apart relative to their size that they each act independently.  But there are forces between them none the less.  These are gravitational forces, not gravity in the sense of gravity causing weight on earth, though essentially the same force in action.  Basically, any two masses are attracted to wards each other by a gravitational force which is proportional to the product of their masses and inversely proportional to the distance between them.  Symbolically F is proportional to m1 x m2 / d^2.  And there is a universal gravitational constant which completes this equation.  You can look it up if you like.  Now gravity is classed by physicists as a weak force.  The gravity that holds us on earth does not feel weak, but then compared with an atom, the masses are large, and the distance is about the same magnitude as the size of the objects, and if we have a rocket with enough energy, it can escape.  If m1 is the earth, the mass is obviously large, and m2 our body mass (even without being political about it) is also very large compared with atoms.  You can see why gravity varies from place to place, the distance between centre of mass of the earth and the centre of our body mass changes of we climb a mountain, descend to the bottom of the sea, or travel towards the poles from the equator, ( the earth is not a perfect sphere but technically an oblate spheroid, sort of like a slightly squashed ball.

But in molecular dimensions, the masses are extremely small, and the distances large compared with the size of the molecules.  Also gravity acts over very large distances, just getting weaker as the distance increases.  It is the Suns gravitational force which holds Jupiter and Pluto in their orbits, not to mention Earth.  So the gravitational forces exist between molecules in a gas, but are very small, and are classed as weak forces.  The energy in the fast moving molecules allows them to very easily escape the attractive forces between them.

Now this may not seem relevant to your question, but it is necessary to have an understood starting point.  That's probably more than enough for one session.  I will have a go at moving from this starting point to understanding latent heat of liquids next time.

Thanks for dropping in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 16, 2017, 01:37:25 PM
more fascinating info and a whole new set of questions............. so in an enclosed vessel half full of air and water at sea level the water sinks to the bottom. if this vessel were taken to outer space how would the water know where to go ??  How does gravity get through the walls of the vessel ??What is the temperature of outer space and would the water be frozen ?   Just wondering really...........
Title: Re: Talking Thermodynamics
Post by: Stuart on July 16, 2017, 07:34:42 PM
Thanks MJM

Ever whished you had not asked :lolb:

Now my brain is even more tasked

Yes the chillers were pretty big all to keep some IBM  main frames cool long time ago they needed chilled water to cool them
‘Bluechip “ was at the same complex looking after bit on the computor floor

The most interesting session I did a Manchester uni was on vibration analysis of machines using fast Fournier transformation but at that time the resultant graphs had to be manually interpretated hence the course , it main use was to predict a machine failure before it failed , but in practice some times it beat you the failure   :censored:

Thanks again I will follow with interest
Title: Re: Talking Thermodynamics
Post by: Jo on July 16, 2017, 08:56:28 PM
Some where you lost me: Please send beer  :noidea: I feel the need for experimentation   :naughty:

Jo
Title: Re: Talking Thermodynamics
Post by: crueby on July 16, 2017, 10:30:56 PM
more fascinating info and a whole new set of questions............. so in an enclosed vessel half full of air and water at sea level the water sinks to the bottom. if this vessel were taken to outer space how would the water know where to go ??  How does gravity get through the walls of the vessel ??What is the temperature of outer space and would the water be frozen ?   Just wondering really...........

Well, I remember watching astronauts (different from us wingnuts) up in space playing with water - when squeezed out of the bottle into the air, the surface tension kept it in a sphere bobbling and wobbling about. So, that container would likely have a blob of water floating around in it. Depending on the surface of the inside of the tank, anyway... I think.... Jo is right, we need some beer to help the thought experiment get forgotten!!
  :cheers:
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 17, 2017, 12:12:31 PM
I had intended to continue to discussing latent heat for condensation and freezing, but not before I acknowledge today's contributions, they are all thought provoking and very welcome. 

Willy, I think Chris has answered your question about water in space.  Many thanks Chris.  To analyse that case, the energy equation would need to include a term for energy in surface tension.  Gravity is described by physicists as a field which acts through space, and difficult to detect unless you are working on those gravity wave detectors, apart from its interaction with mass.  A bit like magnetic and electric fields, though those are more obviously affected by solid materials.  When you find the complete explanation, you should check that your suit is good enough for the ceremony. 

Empty space does not have a measurable temperature.  Temperature results in matter as a result of the balance between energy in and energy out.  So analysis of the water temperature would depend on how the space ship external temperature is transferred to things inside.  The side of the craft facing the sun would be quite hot, no clouds to shield it, while the other side I expect would be cold, as it effectively radiates to absolute zero. Presumably there are heat insulating layers and clever tricks to make sure the inside surface temperature are acceptable to the astronauts.  We will eventually look at means of heat transfer either in the condenser topic or when we get to boilers.  Outside the space ship the total vacuum means all the water would evaporate and the resulting vapour would attempt to fill space.  Before long it would all be gone, unless the gravity of a nearby planet or other space body captured it.

Stuart, you are welcome, it's good to have you on board, and please don't go away, there is more.  I have set the scene, but still have not answered your question on latent heat.  That fast Fourier transformation technique (FFT) is interesting to apply to a frequency spectrum, but I suspect that there is something futile about trying to predict random events.  However the analysis can help us see them coming, if we can interpret the signs.  Basically outside my expertise, I will leave any more details to the maintenance experts, and I suspect that few if any of us have the equipment necessary to apply this to our models.

Sorry Jo, Ade has not yet included the "send beer" button on the forum.  I suspect he is waiting for Bill Gates to get his act together on incorporating a missing sub routine from Windows, so please don't overwhelm him with requests.  But I can provide a little more information for those who have not come across the tons unit in refrigeration.  Will that help?

A ton of refrigeration is the heat necessary to produce a ton of ice at 32 deg F, starting from water  at 32 deg F.  Please correct me if I am wrong, Stuart.  Obviously another infernal unit, but four times 500 ton units should be able to cool an awful lot of beer, especially if you don't actually want to freeze it.  Though I understand that you guys don't actually chill your beer, but serve it tepid.  Strange!  By the way, great to see you back fondling castings, we don't want them getting lonely, and best wishes for your upcoming post op assessment.  We all hope the reports will be good.

Now let's try and make a little progress on that latent heat question.  I described the molecules in a gas state last time.  When heat is lost from the gas, the molecules slow down.  We sense it as cooling.  As a gas cools, the molecular speeds slow, all relatively predictable and uniform. 

But as well as the weak force there are also forces classed as strong forces. (I should possibly be saying weak forces, not sure for the moment if there are other forces also classed as weak forces.  Perhaps we have a physicist reading who can chip in.)  Now the strong forces are very strong, even beside the force of gravity that holds the universe together.  But the strong forces act over a much smaller distance.  It is one of the strong forces, or perhaps several that, when the energy of the molecules gets sufficiently low, the strong attractive force captures any that come close but do not have sufficient energy to escape, and holds the atoms closer together.  This is when liquid droplets start to form.  They still have a large range of movement relative to their size, and they all still move in a random manner, though loosely connected way that we recognise as liquid, by those strong forces.  In this state, the random movement allows the boundary to be flexible and to conform to the shape of their container. Gravity over the whole mass causes liquid to rest in the bottom of a container, or a river to run down hill.
 
We observe that  heat travels from higher temperature to a lower temperature.  In molecular terms, the faster ones keep loosing more than they gain from their collisions with the slower ones while the slow ones are kept moving by the energy they gain from the faster ones. So there is no further temperature change with cooling until all the molecules join the liquid.  Then the liquid can continue to cool.  So the latent heat is the heat that must be lost by the gas molecules before they can no longer escape that close range where the strong attractive forces take hold.  Similarly, to evaporate the liquid, energy must be supplied to give the molecules in the liquid enough energy to escape the strong forces.  The ones that escape are the higher energy ones, so their escape cools the liquid.  Again, they all have to evaporate before the temperature can rise.  I hope I have explained it well enough to explain the constant temperature part of the observation.

 As the liquid molecules are cooled, they continue to get slower.  There comes a point where the free movement within the liquid becomes limited by the close proximity of the surrounding molecules, and the molecular motion becomes more of a vibration.  Molecules tend to jiggle until they drop into a matrix where they still all fit, and the constraints to movement by the surrounding molecules causes the mass to take on the properties of a solid.  But the molecules are still moving within that matrix.  They do not actually stick together.  If they get too close, they actually repel with enough force to resist the attractive force.  As solidification involves losing enough heat that the molecules drop into this regular pattern characteristic of a solid, melting involves adding enough energy for all the molecules to "pop out" or escape that matrix.

I am sorry if my terminology is a bit woolly here, but I am not a physicist, this is just a mechanical engineers understanding of the mechanism, but I hope that is sufficient to answer the satisfy curiosity for now.

Thank you to everyone who is following along, I hope not too much brain pain.

MJM460
Title: Re: Talking Thermodynamics
Post by: Stuart on July 17, 2017, 01:57:32 PM
sounds about right ,

note the chillers where at the bank about 22 years ago mainframe du dads are now better

we only chilled down to 42  deg F ( they were USA machines ) and the controls were also made by Jonhsons ) we did get them to change to a better system

the analysis was used to spot trends and to diagnose actual faults e.g. a peak at twice mains frequency would indicate a motor over load

still looking in  :)
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 18, 2017, 01:14:50 PM
Hi Stuart, clearly you would not want to actually freeze the water in your computer cooling application.  This highlights the point that a ton of refrigeration is a measure of the amount of heat removal capacity in terms of the amount if ice that could be produced, not necessarily the amount that is produced.  Obviously a historical unit from the early days when the main use of industrial refrigeration was to produce ice for food preservation.  There were no domestic refrigerators, the ice man came around with a horse drawn trailer and deliver ice to each household, closely followed by the milk man.  I am sure that I am not the only one who can remember the family ice chest.

To finish off your question on latent heat, it is interesting that as the solid cools further, there can be further phase changes which X-Ray diffraction reveals as different crystal structures.

We are perhaps getting into deeper theory than we need to understand and build an engine, but if would like to read more about it I would suggest a little book called Six Easy Pieces, a small collection from the Richard Feynman lectures in physics. It has been very helpful to me in developing my understanding.  His Nobel prize should be enough authority for us to rely on.  And a very interesting, readable and readily available little book.  If you get hooked, move on to Six Not So Easy pieces, which are, well, not so easy, but very interesting and worth having a go at.

A shorter post this time, I need to get the average down.  Tomorrow, I will get back to heat transfer and condensers.

Thanks for dropping in.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 19, 2017, 12:31:50 PM
The heat transfer equation

To design our condenser, we need to look at the factors which are relevant to heat transfer.  The primary factors are the temperature difference, heat transfer is faster with a higher temperature difference, area available for heat transfer, a direct measure of the necessary physical size of the exchanger.  It turns out that these factors are related in a deceptively simple looking equation.  The equation that applies to all heat transfer problems is written as follows:-

Q = U x A x (delta T).

In the equation, delta is the Greek capital letter usually written like an isosceles triangle with the base flat.  However I haven't conquered the technique on my iPad, so I will abbreviate delta T to dT, it means temperature difference.

Q is the heat transfer rate in Watts (or J/s)

U is called the overall heat transfer coefficient.  The units are w/(m^2.K)

A is the heat transfer area in m^2

Delta T is the temperature difference driving the heat transfer.  Technically the units are the absolute temperature units, Kelvin, but as it is normally in the context of a temperature difference, so you can also use deg C.  However radiation heat transfer calculations, for example, must use K.

Looks like a simple equation, and it would be if any of the terms were as simple as they appear.   The equation does tell us that the heat transfer rate is dependant on the heat transfer area, the the temperature difference, and a heat transfer coefficient.  Time to examine each of these in more detail.

The simplest to understand is the heat transfer area, A.  Measured as you would expect in square meters.  In model sizes we will mostly measure in mm, so we have to deal with a lot of zeros.  One square meter = 1,000,000 square mm, which can be conveniently written as 10E6.  You might read this as 10 exponential 6, or 10 to the power 6.  When dividing is required, you can instead multiply by 10E-6, ten to the power -6.  I believe excel allows this, and has engineering and scientific notation, unfortunately not Numbers for iPad.  I tend to use 10^6 (10 raised to the power of 6) for this reason.

The other wrinkle with area, particularly when tubes or pipes are involved, is that the area based on the outside diameter is larger than the area based on the inside diameter, so you need to keep track of which is the relevant area.

The temperature difference in dT is a bit more complicated.  In our condenser, the steam side of the heat exchanger is essentially constant temperature while the latent heat is transferred.  However if we are cooling with water, the water temperature is rising as is moved through the exchanger and takes up heat.  Similar considerations if the heat transfer is between two liquids except that then both fluid temperatures are changing.  It is necessary in these conditions to use a log mean temperature difference.  It involves the temperature difference on each end as well as the natural logarithm of the ratio of the inlet and outlet temperature differences.  I can write it out if anyone is really interested, but I don't think it will be useful for most readers.

The final factor, U is the really difficult one, a look at that next time.

MJM460
Title: Re: Talking Thermodynamics
Post by: Kim on July 19, 2017, 03:37:22 PM
The temperature difference in dT is a bit more complicated.  In our condenser, the steam side of the heat exchanger is essentially constant temperature while the latent heat is transferred.  However if we are cooling with water, the water temperature is rising as is moved through the exchanger and takes up heat.  Similar considerations if the heat transfer is between two liquids except that then both fluid temperatures are changing.  It is necessary in these conditions to use a log mean temperature difference.  It involves the temperature difference on each end as well as the natural logarithm of the ratio of the inlet and outlet temperature differences.  I can write it out if anyone is really interested, but I don't think it will be useful for most readers.

So, why would we use natural Log of the ratio rather than just the average of the delta temperatures between the inlet and outlet?

I'm finding this thread quite interesting. Thanks MJM!
Kim
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 20, 2017, 01:00:33 PM
hi ,still following along but as i am more hands on than brain on and i find the all this a bit heavy going, especially as i was brought up on yards feet and inches.!! I am getting the jist of things but am more into the practical applications.........when i have to drink a hot cup of tea in a hurryi always put 3 teaspoons in it much to the amusement of the Baristas !! i then try to explain using words like  Adiabatic Enthalpy etc etc etc.....also were men like Watt, Woolfe, and Corliss really educated in thermodynamics or were they just practical engineers ??
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 20, 2017, 01:25:40 PM
The heat transfer coefficient.

Hi Kim, thanks for following along, I am glad you are finding it interesting.  Cooling problems proceed at a rate proportional to the temperature difference, so it proceeds most quickly when the temperature difference is greatest, and as the heat transfer proceeds the temperature difference reduces so the heat transfer slows.  An arithmetic average only gives the right answer when the process proceeds at an even rate.  When the maths is done in detail, and I have to admit I find it a bit too hard, it comes to integration of a 1/x term, which results in the natural log term, which properly accounts for the change in rate as the temperature difference decreases.  The classic example is the cooling of a cup of coffee.  Cooling proceeds at a rate proportional to the temperature difference between the coffee and the air.  Not totally true with evaporation at a free surface, but not a bad approximation for a good cappacinno with a thick layer of good insulating foam on the surface.  The temperature initially drops very quickly but then the temperature change becomes slower and slower.   The cooling follows an exponential function which also involves that natural log function.  Of course Willy's teaspoons absorb a bit of heat to start the process even quicker, but then with the lower temperature further cooling is slower.  His tea is always a little cooler than if he had not used the teaspoons.  Thanks for that example Willy.  No need for anything too technical, just specific heat of the metal and temperature difference.  But if you really want to get technical, stainless steel ones will work slower, but will have a better cooling effect.  But you don't want hot ones from the dishwasher.  I believe those gentlemen were mostly practical engineers who achieved remarkable results with very little theory and rather crude instruments to guide them.  The theory was only in its infancy at the time.  I will leave it to the historians to say just what order some of the significant events happened, but your historical text will give you some ideas of the status of the technology.  Sorry about the units, I am trying to translate a few key numbers, but please remind me if more would be helpful, or particular ones I miss.

Returning to the discussion on the heat transfer coefficient.

The heat transfer equation would be quite easy to use if U was a constant which we could look up in a book for our particular application.  In some situations it comes pretty close, for example conduction in a solid, where U = conductivity/ thickness.  We can look up the conductivity of the material we are interested in and insert it in the equation.  For example, the conductivity of pure aluminium is 236 W/m.K, while copper is 399.  Alloys tend to lower the conductivity, a few percent of copper in aluminium reduces the conductivity to 164 W/m.K, while brass is 111 W/m.K and bronze is only 26 W/m.K.  A good insulating material such as cork would be 0.042 W/m.K.  When we do the dimensional analysis, we see that the thickness has to be in metres. 

It is worth putting some typical thicknesses for copper and brass, for example say 1 mm ((0.001 m) wall thickness, to see the difference in heat transfer over a square meter for each degree of temperature difference.  Similarly say 2 mm (0.002 m) of cork.  These figures can be helpful in indicating differences in materials for some applications.  The high conductivity means that heat is transferred with not much temperature drop, especially across a thin metal wall. 

However to measure the metal wall temperature introduces difficulties.  If you just place a thermocouple on the metal surface, there will be a contact resistance that will affect the reading, and it is difficult to get really good contact.  In addition, the side of the thermocouple not contacting the metal is in contact with the surrounding air and the wiring to the meter.  The thermocouple will read something in between the metal temperature and the air temperature.

Sometimes an electrical analogy is used to analyse heat transfer problems.  The voltage  difference is the analogy of temperature, and the current the analogy of heat flow.  Resistance is the reciprocal of conductivity which is the analogy of heat conductivity.  Now to measure a voltage accurately, we all know we must have a meter which has very high resistance so it draws minimal current.  Not a problem with digital meters, but a real accuracy issue in the days of analogue meters.  In our temperature measurement the accuracy is reduced by the heat transfer from the metal to the thermocouple and on to the air.  If we cover the thermocouple with some insulating material, we reduce the heat flow, and the thermocouple will then be very close to the metal temperature. 

If we want to measure the steam temperature inside our pipe for example, we can put a thermocouple junction on the pipe and hold it firmly in place with some insulating material, a wrap or two of silicon tape is excellent for the purpose.  Some extra insulating material over the thermocouple wires is also a good idea.  Insulating tube can be purchased from an electronics component supplier.  Almost every reasonable digital voltmeter these days comes with a thermocouple as well as its voltage probes, and is well worth buying if you don't have one.

You might want to use an infrared non contact instrument.  These instruments are not affected by the heat loss but unfortunately are influenced by the radiation characteristic of the surface, known as emissivity.  This varies with surface colour, surface finish and material.  Ok for measuring change of temperature over time or different locations on an essentially identical surface, but not good for small differences, or especially the difference between the temperature of different materials.  Even the difference between shiny and tarnished surfaces will introduce a significant measurement error.  A thermocouple is a better option, though they are useful in some specific cases.

Binding the thermocouple to your steam pipe is not ideal, especially if you want to measure several different points by relocating one thermocouple.  In industry, a device called a thermowell is used.  This is basically a solid rod, with an external thread and screwed into a fitting incorporated into the pipe or even pressure vessel so most of the rod is inside the pressure shell. It is drilled with a blind hole so the thermocouple can be inserted from the outside without causing leakage.  The pipe can be properly insulated and the insulation left in place.  The thermocouple can be either permanently fitted for continuous measurement, or a portable unit used for occasional measurement.  I have made miniature thermowells like the one in the attached photo.  It is a short one for use in an elbow fitting in piping.  I use a long one as the filler plug in my boilers, so the thermocouple is closer to the liquid level.  You can see how it is fitted in the pictures of my engines in the gallery post.  This is probably the best way to measure temperature in the boiler, (which allows you to use steam tables to determine the pressure more accurately than a miniature pressure gauge).  Also probably the most practical way to measure superheater outlet, engine inlet and engine exhaust temperatures.  I make them out of brass not bronze for better conductivity, but probably should try one in copper, or a silver soldered fabrication with a copper blind tube or drilled rod, and bronze screwed part.

Next time a little about heat transfer coefficients in a condenser.

Thanks for dropping in.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 20, 2017, 02:57:28 PM
Hi, Another practical question i have is what is the ideal shape  (with no pecuniary limitations) are the cooling fins on air head IC engines ? Parallel, tapered outwards, tapered inwards, Perforated etc etc. Also if the exhaust pipe on a steam engine has more surface area for the same weight of metal is that better, also you mention the difference between brass and bronze as conductors of heat ? what is this as a percentage ? We are advised to use bronze in boilers so as to prevent dezincification !....more good stuff to stir up the grey matter. I have also heard that a matt black surface will absorb heat better ,But, it will also give off heat better ? That is why i have painted my house radiators black, much to the chargrin of the council inspector of my council house !!!
Title: Re: Talking Thermodynamics
Post by: Stuart on July 20, 2017, 03:14:52 PM
Only if the paint is very thin , the fillers that are in the paint could nullify the benefit.

The main thing to take up on is as MJM said temp difference matters , get the rads hot is you need to get the room warmed up .

Or install fan coil units then the water can be hotter due to the coil being enclosed so safer to people.

Or you can do the job properly and fit a optimiser controller , work out the heat loss though the building structure then control the heating/cooling from an outside temp sensor in a Stevenson’s screen

The joys of thermo dynamics  :lolb: :lolb:
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 20, 2017, 03:22:05 PM
Hi. Also when i made the electrically heated steam boilers, i made the brass holders for the cartridge heaters with a row of grooves to get quicker and more heat transfer. Would this have been correct thinking or would there have been no advantage ?? Also in the pics are the Pressure switch and the low water gauge that i mentioned in a previous post ! Thanks Stuart for info.......i shall get onto the council about that !!!
Title: Re: Talking Thermodynamics
Post by: Kim on July 21, 2017, 03:29:02 AM
Cooling problems proceed at a rate proportional to the temperature difference, so it proceeds most quickly when the temperature difference is greatest, and as the heat transfer proceeds the temperature difference reduces so the heat transfer slows.  An arithmetic average only gives the right answer when the process proceeds at an even rate.  When the maths is done in detail, and I have to admit I find it a bit too hard, it comes to integration of a 1/x term, which results in the natural log term, which properly accounts for the change in rate as the temperature difference decreases.  The classic example is the cooling of a cup of coffee.  Cooling proceeds at a rate proportional to the temperature difference between the coffee and the air.  Not totally true with evaporation at a free surface, but not a bad approximation for a good cappacinno with a thick layer of good insulating foam on the surface.  The temperature initially drops very quickly but then the temperature change becomes slower and slower.   The cooling follows an exponential function which also involves that natural log function. 

Thank you for the very clear explanation.  It makes perfect sense when you think about it that way!
Kim
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 21, 2017, 01:41:41 PM
Well, quite a few interesting questions there.  I will try and address each one in turn, though some will be more easily answered after the post I had prepared.  So I may defer some until tomorrow, particularly your question on fins, Willy.  The answer will make much more sense after I make some explanation of why we have fins.  But the steam exhaust pipe, I am wondering where you are coming from there.  The exhaust pipe is not basically intended as providing condensing area, although there is no harm in taking every bit of area we can get.  So long as the pipe is sloping down, that is, so the water droplets do not fall back into the engine, or sit around causing corrosion when the engine is not running.  Essentially the flow in the exhaust is sort of steady, I know it is a stream of pulses, but the pipe finds an average temperature.  The weight of metal and its specific heat do tend to stabilise things a bit there, but I am not sure it is any real advantage.  If anything, extra metal will condense more steam when the engine first starts until it gets to temperature, then if the pipe is insulated it makes no difference.  If we are wanting to loose heat from the pipe, less metal so it increases in temperature a bit at each pulse means slightly higher mean temperature difference, but I think it would be a very small difference, at least for practical tube thicknesses.  Perhaps overthinking the issue a little.

The difference between brass and bronze, and I will add stainless steel for your tea coolers, right on topic with that one.  For straight conduction issues, conductivity, k in W/(m.K) is the relevant property.  I have not found how to incorporate a table in a post, so please make yourself a table with these figures.  I have also included the specific heat, Cp in J/(kg.K), which I will come back to.
Copper, k= 399 W/(m.K), Cp=383J/(kg.K)
brass, k=111 W/(m.K), Cp=385 J/(kg.K)
Bronze k=26 W/(m.K), Cp=343 J/(kg.K)
Stainless steel, k=14.4 W/(m.K), Cp=461 J/(kg.K)

You can see there is a large difference between brass and bronze.  Copper has much higher conductivity again.  I feel that it is better not to talk about percentage, as that facilitates overlooking the contact resistance, I discussed yesterday, when thinking about the heat transfer.  In comparison, Bronze is not a great conductor at all, and stainless steel, (SS), is even worse.  Dezincification is a form of corrosion where the zinc content of brass is dissolved out, leaving behind a much weaker material which cannot be relied on to safely contain pressure.  There are many contradictory stories surrounding this issue, and I am not enough of a metallurgist to be able to explain the issue more clearly.  However, if you get dezincification of a boiler bush, you would have to condemn the boiler, or at least do major repairs to replace the bushes.  So the codes require us to make them bronze.  There is not so much emphasis on the plug which screws into the bush.  I guess this is because it is easily removed and inspected, but please don't quote me in a discussion with the boiler inspector.  The safe approach is to make any pressure containing component in bronze. 

Let's consider the impact of this on two quite different components.  First, my little thermowell.  When screwed into the pipe or boiler, this component is surrounded by the fluid being measured, and there is no intentional heat flow unless there is a temperature change.  With no heat flow, there is no temperature drop, so the conductivity does not matter.  When the temperature changes, some heat has to flow until the new stable temperature is reached.  Conductivity will affect the time taken to reach the new temperature, so the material has an influence on the speed of response of the instrument.  Temperature instruments are always slow to respond, but this makes it worse.  Perhaps I should use bronze after all.

Now the bush for your heater element.  Obviously the bush silver soldered into the boiler shell should be bronze.  Now, the well for your heating element is removable, so probably could be made of brass.   However I suspect it is not often inspected, but then, any leakage might be contained within your wiring conduit, perhaps!  Probably better to use bronze.  Heat transfer through the wall thickness of bronze will mean a temperature drop, so your element has to reach a higher temperature to drive the heat through the sleeve into the water.  Here you are wanting really good heat transfer, so your element within the tube does not have to get so hot to transfer enough heat.  In this application, bronze is not an ideal solution.  This leads me to wonder if we could make a bronze part to screw on to the the boiler, with a copper tube silver soldered in to contain the element, and a bronze plug silver soldered in the end.  Even better conductivity, and no dezincification.

I am assuming that your cartridge heater comes with the element insulated and sealed in a sheath which you have to introduce into a pressure tight component, much like a longer version of my thermowell with the hole drilled to accept the heater sheath, and flanged onto the boiler bush.  I addition you have machined grooves on the outside of your pressure tight sleeve.  I am going to leave my comments on these grooves until after I discuss convection, as I hope it will make more sense then.

Then, your household radiators.  First we need to clarify the terminology.  I know that "radiator" is the normal terminology, but it is probably actually a convection heater.  Especially if the heat comes from circulating hot water.  A common arrangement is a thermostat which controls the water flow so when the temperature is too high, the water flow is reduced.  I am not sure if the controlled temperature is the room temperature or the water temperature, though I am inclined to guess the water temperature.  So either a fairly flat panel, or a more complex column arrangement.  This is a typical heat transfer problem where transfer is by all three of conduction, convection and radiation.  Usually however, one of these modes predominates and the others relatively unimportant.  The term radiator suggests the main mode is radiation.  However radiation depends on difference between the fourth power of the absolute temperature of the heater surface and room objects.  I suggest for a hot water system, this is not a very important contributor, but most heat is transferred by heating the air in contact with the metal surface.  The warm air then rises and circulates the air in the room, thus distributing the heat.  This is called convection.  Conduction is only important if objects are in contact with the metal surface, not usually recommended, though sitting on it is very comforting when you first come in from the cold.

I am totally in agreement with Stuart on the paint, especially if the heater really is a radiator.  A very thin black paint can in principle help increase the emissivity of a radiating surface, but getting this right is tricky.   You would need to do some careful experimenting, as the wrong paint does not work so well.  It also, as you say, increases the absorption.  However, for primarily convection heating, while the metal surface detail is important, paint is more likely an insulating layer which means the temperature difference between the water and the room will be greater.  Surface roughness is probably helpful.

Thanks for joining in again, Stuart.  Good to have you on board.  It would be interesting to hear more about your idea for an optimiser for household climate control.  These days with Arduino microprocessors and the means of programming them so readily available a fully automated household climate control system should be an easy project.  However the logic for the optimisation needs to be developed.

I am glad the explanation was clear Kim, any time it is not, please ask again.  Building blocks in a knowledge base are only useful if everyone one can understand them.

My intent was to continue to look at heat transfer coefficients, but I think this post is long enough.  Better to continue next time.  Not sure I have completely answered all those questions yet either.

Thanks for following along

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 21, 2017, 03:33:36 PM
Thanks for the info ...cool.... i hope i am not hijacking these posts !! just a thought but no questions this time.! this pic is the exhaust arrangement of a small double acting steam engine in the local museum. As you can see the exhaust pipe goes strait up ?!!!! However it was taken apart and reassembled, so they may have got it the wrong way round as it is bolted on. I did make a model of this and used the same configureation . I may have to write a comment in the visitors book !!they also got other parts in the wrong places as well !!! Also the cartridge heaters are designed to fit into reamed holes, and make contact when they heat up.
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 22, 2017, 01:07:15 PM
Hi Willy, I am glad that you are finding the information interesting, and don't worry about hijacking the thread.  It is all about building a knowledge base that is helpful to model engines.  I always have the choice to defer a reply until later.  I am just as happy to look at radiation now as after convection, so I chose to look your heater questions when you asked.  Mostly, I find your questions right on topic, and they help keep me grounded in just what it would be useful to cover, so they are very helpful, thank you. 

That is a beautiful model you have made of an interesting museum model.  Just a bit more about the exhaust, I was thinking particularly in reference to an exhaust pipe connecting the engine to a condenser.  Points in piping which collect liquid cause all sorts of problems, hence my comments about making the piping self draining.  Of course, if there is no condenser, the exhaust steam still has to go somewhere.  In an industrial situation, a horizontal hot exhaust is quite dangerous, downwards not much better, and upwards is probably the best option.  Of course, weather conditions determine whether the steam mixes with air and is carried away, or condenses into a fine fog that is carried away, or condenses into drops which rain down on everything in the area.  On my models, I run the exhaust down to a separator/oil separator, where any water (mostly only on starting) is collected and drained, while the dry steam is discharged straight up.  I suspect that in the period of the model, there were no such niceties, or possibly no cylinder lubrication.  They may have just relied on condensate for lubrication.  So the vertical exhaust on the the museum example and your model is most likely quite ok.

The requirement for a reamed hole for your electric elements is interesting.  Can you ream a blind hole?  By the way an electric element is easier to deal with in experimental work.  You know the power dissipation of the element based on supply voltage and element resistance, and the element just gets hotter until all the heat is lost to the water.  So good contact, and a high conductivity sheath both help to limit the maximum temperature of the element.  Do you also use a smear of heat conducting grease to help even more?  I am not sure if there is anything available that would not cause other problems, such as mess or corrosion.

Yesterday, I mentioned that heat transferred by radiation was proportional to the difference in the fourth power of temperatures.  Mathematically Q = U x A x (T1^4 - T2^4).  The temperatures have to be absolute, K or Rankine if you must.  Radiant heat travels in straight lines like light, but not evenly in all directions.  Think of a car headlight compared with a room light with a translucent diffuser. So A and U have to be modified by a view factor which relates how much of the radiated heat is actually received by the receiving surface, also by directional factors and so on.  But if the water in your heater is say 65 C, the heat transferred by radiation will not be much compared with the heat from a red hot glowing element.  You can get a good idea of whether the main mechanism is radiation or convection by holding your hand facing the heater but say a metre away at the height of the middle of the heater and feeling the warmth.  Then hold your hand above the heater, again about a metre distant but directly above the heater palm facing downward, and feeling the warm air rising.  Which one gives you the most heat?  On the other hand if you have a fireplace, or campfire, or perhaps a one or two bar electric radiator, and hold your hand facing the fire, a metre will be too close for comfort due to radiant heat.

Now let's return to our condenser.  We usually have a metal wall, usually tube, with the condensing steam on one side and perhaps water on the other.  This is when things get difficult and we have to look again at types of heat transfer.

Most of us learnt in school science that heat is transferred by conduction, convection and radiation.  We then usually talk about conduction, possibly mention radiation as heat transfer from the sun to earth through space, and rarely go into convection in any depth.  Now this is not entirely without reason.  Convection is quite complicated.  And radiation not much different in complexity as we saw when looking at your household radiators.  My text book on heat transfer is significantly bigger and heavier going than my book on thermodynamics.  And much of this is due to study of convection.  I don't want to go too deeply into heat transfer, but it is necessary to just peek in and try and understand the main issues.

With convection we find that we have to look at three resistances to heat transfer which are effectively in series, to use an electrical analogy.  We are talking about conductivity which is the reciprocal of resistance.   We have the steam side of the tube, the metal tube wall, and the water side.  Now we can look up the thermal conductivity of steam at 100 deg C, it is about 0.025 W/m.K, and the conductivity of water at say 25 deg C, about 0.6 W/m.K.  Now this is not looking good for our condenser, steam is about as good an insulator as cork and water not much better.  But this is where convection comes in.  The copper tube is a good conductor, and will have only a small temperature drop from one side to the other. 

Now let's look at the water side of the tube, and conduction of heat from the tube to the water.  The water temperature is measured well away from the tube, and if the water is not flowing, perhaps trapped in a porous foam, the temperature will increase as you move the measuring point closer to the tube with a linear temperature gradient as heat flows by conduction, and will match the metal temperature very close to the wall.  However, if the water is free to move, everything changes.  The water against the wall expands as it get hotter, and the hot water rises through the more dense cooler water, and is replaced by cooler water.  This means that the temperature gradient near the wall is very steep, much steeper than if there was no water movement and only conduction.  It is  this high temperature difference across the layer very close to the wall which means much higher heat transfer than the conductivity would imply.  In addition, water has a high specific heat, so the moving water can carry away a lot of heat.  So the water side will carry away heat very well.  If we then use a pump to force the water past the tube the heat transfer will be further increased.  So the heat transfer on the water side is not only proportional to the temperature difference but also to the water velocity, whether it is a forced (pumped) flow, or natural convection flow.

The steam side has a quite different situation.  The steam cooling against the tube condenses and forms a film in the tube.  Of course gravity makes the film flow down and off, and you can see this is quite complicated to analyse.  Just to make it interesting, water is one of few substances which in this situation can form droplets which drop off the tube.  The resulting turbulence further increases the heat transfer.  I assume that now days with computers the maths is doable and coefficients can be calculated.  For most of my working life, approximate coefficients were assumed based on industry experience, and there are books with suggested figures for various situations.  In the end it should be clear that to calculate coefficients from theory is probably not practical for most of us.  I am sorry if this is too heavy going.

Let's just take away the summary that the convection coefficient for a condenser is determined by three individual coefficients, the steam side, the tube wall and the water side.  Usually one of these provides most of the resistance to heat transfer and so determines the overall heat transfer rate.

Next time I will talk about a few things we can learn from this basic understanding of heat transfer.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 22, 2017, 03:33:30 PM
Hi, I have had another look at the Bridewell exhaust pipe and there is a drain cock arrangement at the bottom of it, see pic.and there was a hydrostatic lubricator in the steam line. Also you get two types of basic reamers The Hand type have a longer taper and the morse taper type has a very short taper, about 3-5% and these are to be used on the lathe specifically for blind holes .....And Bronze is approx 88% copper  12%tin   and brass is 65% Cu and 35% Zn...so yes bronze is a much better conductor.
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 23, 2017, 01:23:39 PM
Went for a bike ride this morning.  Disturbed a mob of kangaroos that were grazing by the path.  They all bounded away in that typical flowing motion and were gone.  Not the everyday experience even for for most of us in this country.  All too quick for any chance of a picture unfortunately.

Willy, I am not surprised that there is a drain cock at the bottom of the vertical exhaust of your prototype engine.  There is a lot of condensate to get rid of when the engine starts, and a little more when it is shutdown, the two occasions when it is good to be able to drain condensate.  If you have ever looked at a petroleum refinery or petrochemical plant and seen that mess of piping everywhere, you may be interested to know that every one of those pipes has a drain valve at every low point where liquid would collect, and a vent valve at every high point where air would be trapped.  A little known fact, I believe real rather than alternative fact, is that the equipment count with the best correlation to the total plant cost is the number of high and low point drains and vents.  Unfortunately, when you finally know the number, you know the cost anyway, so not very useful for budgeting.

Thank you for the information about reamers, I have often wondered which I should buy.  One of those puzzles that we all have to deal with as beginners in this hobby.

Now conductivity.  I am not sure whether you are expecting intuitively that more copper is the same as better conductivity, or if you are using a very different text book to mine.  Pure metals such as copper, aluminium, have relatively high conductivity.  Tin and zinc much lower. The conductivity of pure copper 399, and aluminium 236, while zinc is only 121 and tin 67, all in W/(m.K) from the same book.  However, when the elements are melted together to make an alloy, the conductivity is normally less than either of the components.  So brass is 111, which is less than either copper or zinc.  Similarly, bronze is only 26, so again less than copper or tin.  You could also look at Duralumin, an aluminium copper alloy with only 5% copper is only 164, and aluminium bronze, also a copper aluminium alloy but this time 95% copper, 5% aluminium is only 83.  I don't have the facilities to test these, and do not have the opportunity at the moment to check other sources, but I would suggest that while any one of these could be a misprint in one book, the for whole lot to be misprints, all the same direction, I suggest would be unusual.   Interestingly, Carbon in iron to make steel seems to increase conductivity, and chrome or nickel individually in iron have a very different result to the combination in stainless steel.  Definitely an area where intuition does not help much. 

In summary, I suggest brass is actually the better conductor, while bronze is only better than stainless steel.

Getting back to our topic of heat transfer and condensers, I hope you can  see why I am not proposing that we try and calculate a heat transfer coefficient for a new condenser.  However we can do it the other way, if we have a condenser, and we can measure how much steam it is condensing, and the inlet and outlet temperatures, we can calculate an approximate overall coefficient.   If we have a condenser, we can calculate an overall heat transfer coefficient from its performance, and this would be a reasonable guide to a similar condenser with more or less area.

We can also apply the knowledge to other problems such as when to add fins to our surface.  On an internal combustion engine, with air cooling, we can identify a film coefficient for inside the cylinder, conducting heat to the metal cylinder wall, then the conductivity of the metal wall, then the metal to the air surrounding the engine.  In this case the combustion gases are very hot, and provide a large temperature difference to drive heat transfer into the metal, and a smooth cylinder wall is an obvious necessity, so the main variables we can play with are the metal composition and its thickness.  When we look at the transfer coefficient from the metal to the surrounding air we find this is the lowest of the three and basically controls the overall heat transfer.  We can increase the coefficient by forcing a cooling air flow instead of relying on natural convection, and we can increase the metal surface area by adding fins.  Of course adding fins is not quite as simple as we would like.  Basically heat has to travel to the parts of the fin which provide the extra area.  So conductivity along the fin becomes more important.  In travelling to the end of the fin there is a temperature drop which reduces the temperature difference to the air, so the extremities of the fin are less significant as the fin becomes deeper.  In the end, a very deep fin has no heat transfer advantage over one of the "just right" depth.  The "just right" depth depends on the the ratio of the surface coefficient and the conduction coefficient, so again no simple answer, but cast iron fins would have a different ideal depth to aluminium ones.  But you can see some clues to the question on fin shape.  A fin that is thickest further from the cylinder wall is not only tricky to make, but has more metal where it is less useful.  On the other hand, one that is thicker nearer the cylinder is actually more effective than a constant thickness fin, but you will notice that I am summarising the answer rather than trying to include all the maths.

This logic applies generally to problems involving transfer to air.   Transfer from condensing steam to a copper tube wall is very good, conduction through a copper tube wall good while transfer to air from the metal is much lower.  So we see the extended surface approach on our car radiators for example.  Some indication of when fins might be a good idea is provided is given by industrial steam condensers.  A normal water cooled condenser for an engine or steam turbine exhaust is normally built using plain tubes, in very large numbers, I might add.  It is quite unusual to use air cooling for a condenser, partly because water is normally cooler than air and easily achieves a lower condensing temperature, and hence pressure.  However, I do have experience of one quite large air cooled steam condenser.  I can assure you it was very large and had very large fins on the outside of the tubes in addition to large fans to force a high air flow.  So air side film coefficients something between water and air cooling is the indicator for adding fins.

You might also be interested to know a little more about your use of teaspoons to cool your tea.  In addition to just absorbing heat to warm the spoon, thus cooling the tea, the handles of the spoons also act as extended surface area, in addition to the surfaces of the cup, so the spoon handles are fins on your tea cup which increase the cooling rate.  I am serious. The specific example is in my text book!  Also, I assume you sit the cup on two pencils, or more spoons, so air can circulate underneath instead of the bottom being insulated by the saucer.  I think it's time to get out the stop watches and thermometers and do some serious thermodynamics over a few cups of tea.

Next time I will have a look at your electric boiler sheaths for the heating elements.

Thanks for following along

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 23, 2017, 02:36:45 PM
Hi MJM,  Wow.OMG. My intuitive expectations are so wildly incorrect i feel i need to go back to school, However as a Baby Boomer we did not really do Science or technology lessons.!! Interesting about conductivity (thermal not electrical) or there might be some correlation, I do not have the text books to hand to look up specific tables. When you have an amalgam of Tin /Lead as in solder the melting point is less than either of the two metals, so i assume this is correct with brass and bronze ? I have always wondered why cast iron melts at about 1100 degrees and steel about 2000  ? I use german or Nickel silver (copper/Nickel) quite a lot in my models so what is the value for this ?  Interesting info about the tea cup cooling, and i will now have to explain why i am now taking 5 teaspoons (M'Lud)!! instead of 3 !! Interesting about the fins on IC engines...On my 1920's Scootamotor the cylinder was steel with lots of very thin fins turned up out the thickness on the billet,.I notice that fins seem to be evenly placed around cylinders rather than longer at the back where there is less and higher temp air flow or are they made from an asthetically pleasing rather than a thermodynamically correct logic.? On Condensers ,The tubes in the boilers are steel where we want maximum heat transfer ,but, incondensers the tubes are invariably brass ,where we also want maximum heat transfer !! This is to do with cost considerations i suppose.....in some of the GWR locomotives the inner fire boxes were made of pure copper !!! .Thanks for all this info, i have always said that if you have all the correct Info about anything, you can make the
correct decisions about how you proceed using that info.
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 24, 2017, 11:51:40 AM
Hi Willy,  intuition is often helpful in forming a hypothesis as a basis for further inquiry.  Like you, I am quite surprised when I come across examples which are not in agreement with intuition.  However, intuition still helped you formulate the question, and when I looked carefully at a few examples of data, together we found useful information.  Data leads us to modify the initial hypothesis to something like "when metals are melted together in an alloy, the thermal conductivity is less than either of the pure metals".  When I then looked at more complex alloys, like steel, or alloys of more than two metals, we have to modify the hypothesis a bit more to be even less positive.  Something like "often less than any of the components" or "may be less".  Not so definitive, but useful none the less.  Then on thinking further, steel may have other trace or significant quantities of other metals, but it also has carbon, which is of course not a metal.  And we know the properties of steel are very sensitive to even very small amounts of carbon.  I believe there is a fairly good correlation between thermal and electrical conductivity, but again, there are some exceptions.  You may have one in your electric heater cartridges where there are conflicting requirements for good electrical insulation and good thermal conductivity.

I don't think it is necessary to go back to school at our age, we already have a life time of knowledge and experience that schools are trying to prepare kids for.  Life is not long enough to learn everything.  We are better to concentrate of filling in the gaps in our knowledge in areas that interest us, a privilege of our age.  Of course we may choose to do this by private reading, discussions or even by taking a class.  I am also a baby boomer.  In high school we had to choose between science or humanities, and each stream only did one light subject from the other side.  The stuff we are talking about here was only introduced at tertiary level, but most of the real learning came from a lifetime of needing to apply these principles in my every day work.

The melting points of alloys vary over a wide range, depending on composition and these are usually described in a phase diagram.  You will also find there are true eutectic alloys that have a simple single melting point, and many other alloy compositions that melt over a temperature range.  Your tin lead solder example is one of these, where the eutectic alloy heats up then suddenly melts, while other (generally cheaper) alloys with less exact composition, start to soften and gradually melt as they get hotter.  The best source for these phase diagrams is a metallurgy text book, which you may be able to find in your local library, particularly if it has a good technical section.  Definitely not my strong subject, and unfortunately my text is not accessible for now.

The shape of the fins on your scooter engine will be determined by a lot of things, not simply thermodynamics.  In fact thermodynamics (of the engine performance) probably would prefer less heat loss for higher engine efficiency.  The main reason for the fins is so nothing melts, and the necessary cooling aims to do it uniformly so the cylinder stays straight.  Also fins might be shorter lower on the cylinder, as the piston limits the time that hot gases are near the bottom end of the cylinder, so there is less heat to be taken from there.  Again well beyond my knowledge.

When you refer to steel tubes in a boiler, I assume you are talking about steel boilers.  Perhaps locomotives, but ships and stationary engines also.  Again, heat transfer is desirable but even in a boiler, not the primary consideration.  The major thing is strength, particularly at temperature.  Copper and its alloys are not strong enough, particularly at high temperatures.  Copper is chosen for model boilers as it is easily worked by people such as us, and readily joined by soldering or brazing.  But copper strength at only 200 deg C is only half that at room temperature.  Better make sure you have plenty of water in the boiler, especially with a good hot coal fire.  Steel is stronger than copper at all temperatures and has useful strength to quite high temperatures.  But steel working (as opposed to machining) and joining by welding to maintain full strength even at temperature, is not for the average hobbyists.  Of course steel is subject to rusting, much to our dismay when we find it.  While copper is very much less so.  Some special stainless steels, known as duplex stainless steels, have been developed to resist corrosion as well as have high strength at temperature.  In fact there are groups working on making model boilers from duplex stainless, but you can safely assume that these people are real expert welders in their working life.  They are not average hobbyists, or even just good welders.  So many more factors than pure thermodynamics are involved in choosing the best material for an application.  It is necessary to have not only correct information, but also to the extent practical, complete information.

 Very few real life problems are single dimensional, most have many factors, and very few real problems are binary.  Despite computer logic, the answer is not often limited to yes or no, black or white.  Rather the answer is mostly maybe, or some shade of grey.  And rarely do we have really complete information.

Today's adventure in the long paddock, a horseman rode out onto the highway, and started walking in small circles in my lane.  Obviously a sign to stop.  Out came about 100 head of cattle, being driven across the road by two drovers on horse back.  They had put out signs to advise the traffic coming towards us, but I think they forgot our direction.  To complete the picture, we came up behind a truck with a rather superfluous "Wide Load" sign on the back.  The road is one lane each way, yes it is a main highway, each lane about 4 meters wide.  The truck was carrying a bulldozer with a cab about 6 meters high, and a blade at least 6 meters wide, protruding well over the centre line.  Even the pilot vehicles for wide loads coming towards us, usually quite pushy about claiming their side of the road and more, moved over for that one.  We had a very sedate peaceful drive behind him for about 50 km before he pulled off into a rest area.  Obviously mining in the area as well as cattle.

I was going to continue the heat transfer discussion by looking at Willy's electric heating elements, but I seem to have been side tracked.  Oh well, life is more than pure thermodynamics.  That will be tomorrow.

Thanks for dropping in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on July 25, 2017, 11:33:25 AM
In post 128 you state, "It is a matter of understanding the energy balance for your engine, knowing exactly where all the heat is going."

Well, this and all the discussion lately has raised a question I have never been able to answer satisfactorily. I would like to optimise the design and material choice for small gauge 1 locomotives with inside cylinders, fixed cut off with dimensions of the order of 8X12 mm, (to a little larger), that operate with methylated spirit fuel, single firing rate and throttle setting, hence mostly more or less stable cylinder conditions. I previously had two locos like this, twin cylindered Aster Lions, (Titfield Thunderbolts). One has a brass block cylinder and the other brass tubes silver soldered into end plates, neither had cylinder lagging. The latter engine seemed to be very slightly superior in run time/distance and a little less condensate out of the chimney, but I had not done any measuring unfortunately and have now donated this engine to a mate some thousands of kilometres to the South so can't do any analysis now.

For future engines of similar type and size I am wondering if moving to bronze cylinders built up from two tubes with end plates silver soldered together and insulated with Kaowool lagging would achieve any superior results than the use of a bronze block for the inside cylinders. I have some intuitive and experiential notions but am not competent enough to work out the thermodynamics of it all. Given that modern loco cylinders were relatively thin walled steel castings I am presuming models with big lumps of heat absorbing and transferring material like solid block brass fabrication or castings are potentially working against us. But to what degree?

So, I beg your indulgence in pondering this issue, and are able to address my specific enquiries by showing us some thermodynamics that might resolve the issue. Thanks for your labours to date, if it be any consolation some of us labour just as intensively trying to absorb your energy output. Regards, Paul Gough. P.S. Operating pressure in my loco is 50 to 60 psi so assume cyl. pressure to be somewhere around 50 or a bit more.
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 25, 2017, 01:23:28 PM
Heat transfer example - electric boiler heater elements.

Hi Paul, I was just about to post this when I saw your reply.  More very interesting questions.  I will give some thought to those, and will definitely come back with my ideas.  I suspect it will continue to involve taking a little bite at a time and weaving these bites between bits on condensers, but I am heading towards boilers, so a very timely query.  I am sorry it is such heavy going, any suggestions on how to make it easier to follow would be welcome.  Even requests for more explanation on the hardest bits.  Then I can try to come up with some examples that will help.  Always a balance between too many words and not enough explanation.  By the way, is tropical Queensland anywhere near Rocky?

While this subject is perhaps a little out of order, I was talking about condensers after all, but it is the last outstanding part of Willy's question, and it is as good an example as any to round out the heat transfer discussion.

Let's look at the complete heat transfer path from the heating element to the water.  Electrical energy is used to heat a coil of wire. The wire has significant resistance, and ohms law gives us the current as V/R.  I believe the elements are connected to alternating current power supply from the mains, not sure whether it is direct to the mains or perhaps through a transformer.  The wire is probably formed into a coil to fit enough length into a compact space, so may have some inductance.  With AC, inductance causes a phase lag so the current is not in phase with the current, but the heating effect is only due to the resistance.  Electrical power is calculated in Watts as V x I.  Alternatively, a little more manipulation of the equation can give power directly from the voltage and resistance as V^2/R.  Looking at it this way is useful in understanding the impact of reduced voltage on the power output.  You can see that this formula has no components relating to temperature, or thermal conductivity.  But a heat balance on the element tells us that the temperature will get high enough to reject all the heat input from the electrical input.  We might think that ideally, we would put the element directly onto the water.  This would result in the lowest wire temperature, but there would be a real danger of someone getting electrocuted, particularly with mains power (220 - 240 V in UK, Europe and Australia for our US readers).  There is also quite a high probability that some of the electrical current might take an alternative path through the water, producing hydrogen and oxygen.  You don't want any sparks around that combination.  And there is no need, the wire has very high temperature tolerance and is unlikely to melt.  It is safer to surround the wire with an insulating material.  In industry, and even domestic hot water services,  a temperature measuring element is also incorporated into the bundle within the sheath.   This element is to protect the wire from excessive temperature by isolating the power supply when some high temperature limit is reached.  I don't know what specific material is used for the insulation, it may be some sort of ceramic or a magnesium compound.  Willy, do you know what it is?  As I have previously mentioned, there are conflicting requirements for its selection.  It needs to be a good electrical insulator, but ideally have good thermal conductivity.  In the end, there must be no compromise on the electrical insulation properties.  Then the whole lot is placed in a metal sheath so it can be safely sealed up.  The sheath is almost certainly kept thin, perhaps for better heat transfer, but it is probably not rated for significant external pressure.  I assume the manufacturer has some specification for the maximum temperature this sheath should be allowed to achieve.

 In order to use it in a pressure boiler, the user has to produce a second sheath designed to withstand external pressure, and also designed so it can be installed in the boiler without causing leakage.  Willy has produced a brass sheath similar to the thermowells I make for temperature measurement, which he has grooved to increase the surface area for heat transfer to the water. 

If we assume that steam is to be raised at 350 kPa(g), 450 kPa(a), it's temperature will be about 148 deg C.  The heat path from the manufacturers metal sheath to the water involves a contact thermal resistance between the sheath and Willy's boiler fitting, the conductivity of this fitting and finally the film resistance inherent in the convection transfer to the water.  I was interested to learn that the heater elements are designed to be fitted in a reamed hole.  This implies very close contact, possibly even slight interference that compresses the manufacturers sheath, even if just a little to ensure very good contact.  Certainly an air gap would be a real problem, and must be avoided to the extent possible.  I don't know if any heat conductive grease is used, similar to that used in assembling heat sinks on electronic components where there is the same problem.  Willy can choose the material from which to make his well.  As we have already noted brass has better conductivity than bronze which might also be chosen.  The better conductivity of brass means that the element will not get so hot.  Stainless steel could also be used, however stainless steel has even lower conductivity,

Finally, the question of fins.  Now adding fins adds surface area which you would think would always help.  However this is another of those cases where intuition does not always give us the right answer.  So how do we answer the question for this particular case.  The maths for determining the effectiveness of fins is complex.  You will find it in a text book on heat transfer if you want to look in more detail, perhaps in the local library.  My quick summary is that there is a dimensionless ratio called the Biot number, my book uses the symbol Bi, which tells us when fins are likely to be helpful.  The Biot number is defined by Bi = h x d/ k, where h is the convection film coefficient, and k is the conductivity of the fin material, and d is half the fin thickness (please don't ask why the half, it's in the book).  Fins are worth using when Bi is very small, meaning much less than 1, while they actually are counterproductive when Bi is much greater than 1.  As an indication, convection transfer to a gas generally gives a very small Bi due to a very low film coefficient on the air side.  Heating water is on the borderline, while boiling water has a very high film coefficient, especially if the temperature difference is enough to cause vigorous boiling, so fins are counterproductive.  The border between fins helping and not helping with brass fins is somewhere in the region between just gently heating water and vigorous steam production.  Unfortunately I do not know whether your fins are useful or not, so the issue becomes one to devise an experiment which will tell us whether fins help or not.  That sounds like a good spot to pause until tomorrow.

A bit more adventure in the long paddock today.  Came around a bend on the highway, only to find a small herd of cattle, perhaps about 20, standing on the road.  We stopped.  Beautiful healthy looking animals.  Collecting fresh steak for dinner had some appeal, but it tends to do a lot of damage to the car.  So we just tooted.  Half started moving on, the others moved back on the road.  No drovers in sight, so we waited.  Eventually they moved off and we went slowly past.  I reckon somebody left the gate open!

Tomorrow I will try and devise an experiment that will help Willy decide whether or not to continue machining his fins.  Has anyone got any suggestions?

MJM460
Title: Re: Talking Thermodynamics
Post by: Kim on July 25, 2017, 02:39:07 PM
The Biot number is defined by Bi = h x d/ k, where h is the convection film coefficient, and k is the conductivity of the fin material, and d is half the fin thickness (please don't ask why the half, it's in the book).

Intuitively, it seems that the 1/2 the fin thickness makes sense because the maximum path for the heat transfer would only be 1/2 the thickness of the fin.  The heat can conduct to the outside on either side of the fin, it doesn't have to travel all the way through the fin.

Might be wrong, but this logic makes sense to me :)
Kim
Title: Re: Talking Thermodynamics
Post by: paul gough on July 25, 2017, 02:49:00 PM
Thanks very much for offering to provide some ideas which might resolve the question at issue. Please don't feel obliged to provide explanations that labour the point, it is my responsibility to work at understanding what is presented then to seek further assistance if needed and as this is not paid work for you please engage with my enquiry at your leisure and when you think it appropriate to the discussion. My comment about 'labouring just as intensively' was not meant to be literally the case, more a bad metaphor to indicate I/we are paying attention, working at keeping up with the prodigious information flow, and attempting to assimilate it as best I/we can, I guess  something akin to diligent students.   

As a Far Nth. Queenslander,I don't regard Rockhampton as being in the tropics, just on the edge of it, a bit of a conceit I admit. I reside in the hills behind Cairns. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: jadge on July 25, 2017, 03:21:16 PM
I had a most interesting chat with steam_guy_willy a couple of years back at the Forncett Model Engineers day about his electric boiler. I was interested as I wanted a simple boiler to provide steam for injector testing.

The heating elements used are obtainable from commercial distributors such as RS in the UK. They are basically resistance heating elements running on 240VAC, at least the ones I was interested in. Although they contain a helical wound element I doubt that the inductance will have any impact at 50Hz. One could get an estimate of inductance from the Wheeler formula, but even if it was 1mH that's only about 0.3ohm at 50Hz. If the heater is 500W then the DC resistance is on the order of 115ohms when hot.

The internal insulating material is magnesium oxide; average thermal conductivity but high dielectric strength. The outer case is usually sealed stainless steel.

The heating element depends upon a good heatsink to work. If you have an element about the size of your finger dissipating 500W and relying primarily on convection in air it's going to get mighty hot, mighty quick! Hence the need for a reamed hole and thermal grease. The manufacturer of the elements sell a high temperature grease, but rather stupidly RS do not stock it. Since I need the boiler to run at 170psi (~190°C) that rules out most of the general heatsink compounds. In high power electronics any sort of air gap, or even air pockets, is death to heat transfer, and thus to the semiconductor device.

As an aside I decided not to follow the steam_guy_willy design on the electronics side. For £18 I got a PID controller, thermocouple and solid state relay from Ebay. So it should be simple to implement a closed loop control system, based on temperature, and thus indirectly steam pressure. Mind you it might be prudent to add a proper safety valve, and possibly a water level detector.

The main thing I know about thermodynamics is that, despite doing a course in it at university, I still don't really understand entropy.  :noidea:

Andrew
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 25, 2017, 03:22:24 PM
Hi MJM, These are the RS Components  Cartridge heaters that i used in this boiler. They are the 500 watt 100mm x10mm ...250volt ones. the boiler dimensions are 3" x6". The boiler takes about 7 mins to get to 2 bar (30 psi) , this is to get over the attention span problems of the youth today !!!. I do not use any sealant just push them in slowly as the air needs to come out. Interestingly when i demonstrated it at Beeleigh mill because it was outside they had a large extension cable un wound and it took about half an hour to heat up !! I think the inductance of the coil had something to do with it. This boiler was featured in the Engineering in Miniature magazine a few years ago......No Questions at the mo ,so you can catch up !
.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 25, 2017, 03:32:26 PM
Hi Just seen the above comment that overlapped mine. when i talked to RS they did not mention the high temp grease so i did not use it.!! I used the control system designed by a friend that used a 9 volt battery for the low water safety circuit to trigger the 250 volt circuit, incidentally you do need a fairlly good battery ,as although the led's light up the circuit will not function properly !!
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on July 25, 2017, 09:01:53 PM
The first thing that springs to mind when considering a high temperature insulator that is a good thermal conductor, is aluminium oxide as the material is used inside certain electronic components as such. The thermal conductivity of a few um is almost the same as aluminium and it will withstand thousand volts or more (depending on layer thickness). Aluminium oxide is extremely strong / tough, but still it can be damaged and then the insulation properties are gone, so ....  :noidea:

Best wishes

Per
Title: Re: Talking Thermodynamics
Post by: jadge on July 25, 2017, 10:14:50 PM
Some power semiconductors, especially RF power devices, used beryllium oxide as an electrical insulator, but also with a very high thermal conductivity. There were dire warnings about not cutting open such devices and creating dust, as BeO is carcinogenic.

Andrew
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 26, 2017, 01:28:45 PM
Hi,  Entropy !!  Is this the elephant in the room ?? James Clark Maxwell certainly thought so........!! Found this reference in quite a good book that should be on every model engineers shelf? !!!!!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 26, 2017, 01:50:34 PM
Thank you everyone for the helpful comments and contributions.  I will try and do justice to each of you, however it is getting very late due to an unexpected water leakage problem we discovered this afternoon and that had to be attended to first.  Have just about everything dry, and back in place and will attend to repairing the fault tomorrow.

Kim, I believe that you are on the right track with your suggestion of the two sides of the fin meaning there will be a two somewhere, alternatively a half will have to appear in a number of other places.  I think it is just a matter of convenience which approach is used.  On the issue of conduction path, remember that heat has to flow from where the fin attaches to the cylinder along the the complete length of the fin to the tip, not just through the fin thickness.  This is why there are diminishing returns on making fins ever longer.

Paul, FNQ makes sense, thank you.  Unfortunately turning left at Rocky, but if you ever visit the Deep South, please do yell out.  I am trying to find that balance between too many words and too superficial.  So please do say something any time you need a bit more explanation.  Life does not have to be that hard.

Thanks jadge for the extra information on the heaters.  I totally agree with you that the inductance would not significantly change anything.  Also, only the resistance is responsible for the heat, and we only measure resistance with a normal multimeter.  Good information on the MgO and the stainless steel sheath thank you.  Interesting that the manufacturer does sell high temperature grease.  However, I also wonder if there is any problem inserting a close fitting sheath in a reamed hole with a coat of grease, could the air get out?  Or would it make a very effective pneumatic ram?  The PID controller is a great solution for precise pressure control.  I think Willy's system is a simple low level protection,  and he expects to use all the heat, so temperature control is not really an issue.  And I would always recommend having the safety valve.  And low level protection is also a good idea.  While pressure inferred from temperature measurement is probably more accurate than little pressure gauges, when the system is first heated there is still air in the boiler, so the pressure can be higher than you would expect based on the temperature, until the air is lost with the first steam production.  Remember the mountain top experiments?

Willy, thanks for the element data.  Looks like a good range available for many boiler sizes and capacities.  If you are using those little rectangular 9 Volt batteries, they will not last very long with an LED and a relay.  The relay should be energised to make contact in the 240 V circuit, so that in case of a fault the power is isolated.  The ones I am thinking of would not last very long as they have very little energy capacity.  I would recommend a 9 V plug pack with a reasonable current capacity.  After all you already have 240 V power for your heater.  Alternatively a 12 V motorcycle battery with the appropriate regulator to supply your 9 V would have enough capacity to last a reasonable time.  An unusually long boiler heat up time at your display venue could be due to excessive voltage drop in the long extension lead, especially if there is a poor contact somewhere along the way.  Remember the power is V^2/R, so a small under voltage does significantly impact on your heater power.  I do not advocate exposing mains power terminals to measure the voltage unless you are an electrician, but you could check each plug and even the power switch with one of those infra red temperature devices.  They are excellent in such applications.  A hot spot indicates a poor contact, and may indicate a faulty cord or switch.

Thanks also to jadge and Admiral_dk for suggestions of further materials with suitable combinations of thermal and electrical properties.  Clearly there are many options.  Obviously the carcinogen one would be well avoided, but I suspect that enclosed in a stainless steel sheath it would be well enough protected from damage in normal use.  I suspect to find a material that could be bent or deformed without consequence would be more of a challenge.  I hope most users would understand that the element should be treated with reasonable care.  Above all it would be interesting to know which insulating material the manufacturer has selected.

Now recognising the importance of a proper heat sink to the element, and the need to avoid impediments to thermal conduction, we are back to Willy's conundrum of whether to machine the grooves in the outside of his pressure boiler insert.  In most situations, we could measure all the inlet and outlet temperatures on two samples and easily deduce which is most effective.  But in this specific electric heater case, the element temperature varies to ensure rejection of all the heat, and we can't measure that temperature.  With the heat input to the boiler always equal to the element power rating, there is no expected variation in time to heat, whether the outer surfaces has fins or not.

I would suggest that the mathematics to calculate the difference is certainly too hard for me, and I am not sure if the heat transfer coefficient can be calculated with sufficient accuracy to give a clear answer.  I suggest the most definitive way to answer the question is to make two element carriers, one with grooves and one without.  And conduct an experiment.  But what experiment?

I have implied that the element resistance is constant, however resistance of a metal is temperature dependent, and does change as the temperature rises.  I seem to recall that the resistance would increase as the temperature rises.  Increased resistance means reduced current.  So in principal, if we had an ammeter in the circuit, we might expect to see a different current in each case, and the lower current implies lower temperature, hence better heat transfer.  But I am not sure how much the temperature changes, and whether the measurements would be sensitive enough.  Also I am definitely not advocating putting your multimeter in the mains circuit to measure the current.  Perhaps one of those clamp meters or a Hall effect current detector could be used if it is sensitive enough.

Another possibility is to make the two element carriers say 12 mm longer than required, assuming the boiler shell is long enough to accommodate the longer element mounting.  A thermocouple could then be tucked in after the element is inserted, and perhaps a little kaowool or similar fibre  insulation pushed into hold everything secure.  The thermocouple would then measure a temperature which might be close to the element sheath temperature, and more importantly, might vary in the same way as the element temperature.  Again the lower temperature implies better heat transfer to the water, and hence gives our answer.  I think I would find this method the most practical, and probably most likely to provide a definitive answer.  But I would have to try it to find out.  What do you think?  Is there another method that might resolve the issue?

I hope the plumbing issues will be resolved tomorrow and that at last I can not only address that condenser, but also think a bit about Paul's locomotives.

Thanks for following along

MJM460

PS Willy I saw your post just as I was about to post today's essay.  Looks like I will have to have a go at entropy sooner rather than later.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 26, 2017, 02:23:57 PM
Thanks for the latest info.......... New question  ........would the boiler produce steam quicker if the filler cap is left loose to allow the air to escape and not be compressed by the expansion of the heating up water ??or could you increase the pressure in the boiler first with a bike pump ???.... and ........if there is a device to vibrate the boiler at a certain frequency would this also speed up steam production ?? please only answer these questions at your leisure !! These cartridge heaters were never meant to be used in my model boiler i suspect, but used primarily to heat up large chunks of metal used in industry where one could have an exit hole to release the air. Also the grease would enable the item to be removed easily if it needed changing.
Title: Re: Talking Thermodynamics
Post by: paul gough on July 26, 2017, 04:10:50 PM
I am sitting here at 1am pondering the interaction of the 'dancing' molecules of steam, (a gas), interacting with the 'buzzing' somewhat constrained molecules of the piston surface, (a solid). Now, one can easily comprehend the affect of the force(s) of the steam molecules on the piston as analogous with the force(s) of a ball thrown against a wall or some such. But is our mechanical or engineering description/analogy all there is to it and sufficient??? After all most volume of the piston is in fact the space between the atoms, this leads me to think that some sort of other interaction(s) is/are occurring rather than just our day to day and somewhat 'gross' explanation. Are we in fact talking of molecular/atomic forces interacting and if so there would presumably be losses involved in any energy transfer between them, are these significant or relevant??? I don't want to side track this thread into the dense and dark regions of physics or expect any technical explanation, however I would appreciate having a laymans handle on the phenomena and if there is any connection or relevance to our discussions on thermodynamics. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 26, 2017, 08:22:03 PM
Hi, Paul, I an see what you are getting at and are some metals that may be a bit elastic or have large spaces between the molecular structure be better at transferring the energy in the steam propellant. err ,um, i think what i am saying are some (things) better than others when used as a piston in a steam engine ???
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 26, 2017, 08:24:29 PM
Hi MJM, Saw this and thought of you !!! I am a member of the Newcomen Society and they have in there magazines quite a lot of info on Australian engines...........
Title: Re: Talking Thermodynamics
Post by: paul gough on July 27, 2017, 12:42:06 AM
Hi Willy, The physical properties of metals at the molecular level and their various capacities to exchange energy is not something I would claim any understanding to, so whether an 'elastic' metal had different behaviours to an inelastic one I have not a clue, to me it would be like trying to speculate on what happens in a black hole. We can only hope there is a higher authority amongst us who can deliver enlightenment.

I also found the Newcomen society a valuable storehouse of very interesting information, I spent many weeks reading a large proportion of their articles going back to Volume 1 of the transactions, (Journal), while I was researching disk engines some time ago,  they are available to members via the web. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 27, 2017, 01:12:47 PM
Well at least the plumbing issue is resolved and fixed.  No amount of theory will predict that the tube had not been pushed far enough into a quick-lock fitting.  But something made me look very carefully into the fitting, make some measurements and stick a little bit of tape on the tube to mark the correct insertion.  I had just not used enough muscle.  I had to get that out of the way before discussing entropy.

I have avoided mentioning entropy up until now but Willy, your post last night suggests it is on your mind, so let's have a go.  The page you scanned from your book made a quite suitable introduction. 

You may remember that I described enthalpy as a property that only depended on pressure temperature pressure and specific volume, all things we can measure.  It did not depend on the previous history of how the substance came to have that particular pressure, temperature and specific volume.  That sentence is basically the definition of a property.  I noted that enthalpy is calculated from the pressure temperature and specific volume, we don't have an enthalpy meter.  But the calculation of enthalpy comes into many significant problems and thermodynamics (particularly problems involving heat and work, so obviously interesting to anyone designing and operating heat engines).  It is so useful that it is convenient to have the value tabulated in steam tables, and the tables for other fluids such as refrigerants.

Entropy is another such calculated property, given the symbol S.  No such thing as an entropy meter, it is the outcome of a calculation.  I think the main reason that entropy is so mysterious is that it is not a simple calculation involving things we can measure, but it requires a bit more complex calculation that has a surprising applicability.  The calculation involves summing the quotient of heat input divided by temperature as a substance undergoes an ideal reversible process, which goes in such small steps that the temperature can be considered constant within each step.  In symbols it is written
dS = dQ / T at each step, and leads to the result that e change of entropy with the process (S2-S1) is found by integration  of dQ/T through the process.  Now none of us want to have to do integral maths to understand our engines, but we don't have to.  The unexpected result is that this change in entropy, even though it is calculated by considering an ideal reversible process, is exactly the same, whether the process was reversible, or ideal, or was not reversible.  The reversible process allows the change to be calculated, but then the result also applies to any real irreversible process as well.  So we can leave it to a few boffins to calculate the values  and include them in our steam tables, and tables for other fluids.

Now apart from the interesting unexpected nature of this property, why do we care?  It comes down to the second law of thermodynamics.  If we first note that the first law calculates the heat exchanged in a process based purely on conservation of energy, but gives us no idea of whether the process can actually occur.  For example we can calculate the heat lost by your tea to your teaspoons when you plunge them in to the hot brew.  The first law tells is that it is the same amount as is gained by the spoons.  But we can also calculate how much heat would be gained by the coffee if it became hotter by cooling the teaspoons.  Now we know that can't happen.  The second law of thermodynamics tells us it can't happen, but does not really quantify why it can't happen.  This is where entropy comes in.  It turns out that the only processes that can happen (without heat or work input) are processes that result in an increase of entropy.  This is relatively easily extended to mean that the entropy of the universe is increasing.  Philosophers ponder if there is a limit, if so what happens when the limit is reached, or is there a mechanism somewhere in the universe that reverses the increase of entropy.  But I find all that way too esoteric.  I would rather leave such discussions to someone who cares.

However, accepting the concept of entropy, and having it tabulated in steam tables is very useful.  So let me illustrate by showing how it helps us understand a steam engine.  Let's assume we have a boiler, and the steam outlet pipe then loops back through the fire box a couple of times as a superheater, then on to the engine.  We can measure the temperature and pressure at the engine inlet. We would all like to know how much work our engine can produce.  We need to know the exhaust pressure, but we don't know the exhaust temperature.  We know the amount of work is given by the change of enthalpy, but we need to evaluate that change.  Now we remember that ideally steam expansion in an engine is an adiabatic process, meaning no heat transfer in or out.  And it turns out that an ideal or adiabatic process also means no change in entropy.  So if we look up the entropy of our steam at the engine inlet, it will have the same value at the exhaust of our adiabatic engine.  With a bit of interpolation of the steam tables, we can find the temperature, and enthalpy of the exhaust steam, and also the exhaust steam quality or dryness if the exhaust is wet steam.  Now we can easily calculate the change in enthalpy by subtraction.

Using entropy, just using the tabulated values in the steam tables, has allowed us to calculate the power output of an ideal engine.  The second law of thermodynamics tells us that any real engine will produce less power than an ideal or adiabatic one.  This leads to a definition of adiabatic efficiency, but that can come another time.

Now your other comments, letting the air out when your element is inserted.  Your explanation seems likely.  It would take some ingenuity, but it should be possible to devise a way to solder a return bend on the end of your reamed tube, and use say 3 mm tube vent back to the mounting flange. This would allow you to use the grease.  But your other suggestions to reduce the time to raise steam?  I am tempted to stick my neck out and suggest the only way to raise steam quicker is to put in more heat, either use a bigger element, or put in two or three elements.  You see your heater power rating is the amount of heat generated by the element per unit time.  A 500 Watt element means 500 Joules/second.  We have previously looked at how to calculate how much heat is required to heat water from cold, say 15 deg C to saturation temperature, and we can look up the specific heat of copper and calculate the heat required to increase the copper temperature from 15 to our operating temperature.  You have an advantage over fired boilers, you know how much heat is produced and you can insulate it well to limit loss to the atmosphere, though we should make an estimate of the heat that will be absorbed by the insulation.  So you only need to weigh the empty boiler, and the quantity of water you fill , and you can calculate the time required.  Similarly, once you start steam production, you can calculate the heat required per kg of steam, and so you can easily determine how much steam you can make with 500 watts, as no more heat is absorbed by the copper or insulation once steady temperature is reached.  All your effort to improve the heat transfer coefficient only reduces the temperature the element must reach to transfer the rated heat.  It does not affect the time to heat or the the amount of steam you can raise.  So long as you insulate the boiler well!  I will hold the air questions until I get to condensers, and try and address Pauls comments tomorrow. 

Paul, you have obviously understood well my explanation of how heat is changed to work in our engines. I will address your questions on this next time.

Oh and thanks for the notice about the Mildura conference.  Mildura is about 600 km from Melbourne, quite a solid drive.  Unfortunately I have too many other commitments in October, but it's good to see these events being held "locally".

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 28, 2017, 11:37:39 AM
I hope my discussion on entropy yesterday was enough for our purposes, as it did not allow me to continue looking at yesterday's other questions.  So today, Paul's question on the molecular interaction that produces force on the piston.  Paul, you have quite properly observed that while I described the action of the gas molecules in detail, I treated the piston more superficially as though it was a uniform solid.  The only thing I can say in defence is that there are good precedents for this approach.  However, it is not necessary to simplify, as treating the metal piston as jiggling molecules is quite valid, it just adds another layer of detail, and does not change the original conclusion.  It is all about scale of the motion and particle mass.  You will remember that gas molecules move at high velocity in all directions.  The velocity on average is enough that the molecules easily escape the close range attractive forces any time they come into collision with another.  I remember looking up the mean free path and it's very large compared with the size of the molecules, but unfortunately don't have access to the book at the moment.

On the other hand, the metal molecules have much less energy.  Obviously not still in the vapour state, or even liquid.  Molecules are still moving but the energy is so low that they are well in the grip of the short range attraction forces.  Their energy and hence velocity is so low that they have  dropped into a regular close packed pattern that can actually be identified by X-ray diffraction techniques and in a larger scale, in the crystal structure.  The molecules are still moving in this array, they cannot "clump together" as when the molecules get very close together there are repulsive forces.  But there is not much room between the jiggling molecules of a metal that gas molecules can penetrate and get lost in the metal.  Not much room, but not no room.  There are some gaps or faults in the regular structure that make grain boundaries in all but very carefully produced single crystal structures.  A few gas molecules sometimes get trapped in these gaps, particularly small molecules like hydrogen, and this can cause problems in welding the affected metals.  But solubility of gases in metals is much less than in liquids for example.  In addition most practical piston metal atoms are much heavier than gas molecules.

So what happens when a gas molecule collides with the metal surface?  First it approaches a nearly solid wall of vibrating metal atoms, that are moving in a tightly packed array.  Even if a gas molecule penetrates the first layer, I am guessing it does not often get through the second without colliding with one.  Of course the motion is all random so the gas molecule could collide with one moving towards it, or one moving away, or like billiard balls it could hit at an angle.  The metal atoms are on average all moving in one direction that we identify as the piston movement.  But at each collision, conservation of momentum applies as a basic law of physics.  It is the physical law behind Newton's law about bodies continuing to move unless acted on by an external force.  Unlike energy, there is no equivalent of energy conversion with momentum.  Conservation of momentum can be applied separately in each of three perpendicular directions, though only components in the direction of piston movement is of interest in production of work.  The law of conservation of energy still applies, but the sum of the energies of all the particles will be less after the collision, some does the work to accelerate the metal atom and some is converted to heat.  So conversation of momentum is easier to apply in this case.

It is worth noting that in addition to conservation of momentum, which should probably be called linear momentum, there is an analogous law of conservation of angular momentum.  This applies to the spin motion of a particle, again it applies to spin around three perpendicular axes.  The formula for angular momentum are very similar to those for linear momentum, just angular velocity and torque instead of velocity and force.  And moment of inertia, instead of mass.

So the gas particle bounces back into the gas space as it still has too much energy to be captured by the close range attractive forces, and the space between gas molecules is enough for it to be hardly noticed.  But what about the metal particle?  Well it bounces off on the opposite direction, back into the metal.  Much lower velocity as it is so much heavier than the gas molecule.  Also it is in a close packed array so as soon as it tries to move beyond it own little space, it collides with another molecule, which collides with another and so on right through the metal, and can be seen in movement at the other side millions or is it billions of atoms away.  But the close range attraction prevents the last particle popping out the other side.  So the solid piston acts like, well, like a solid.  The net force due to all this change of momentum on the top and bottom face of the piston is carried through to the crank pin where it produces torque as I have already described.

I have probably still glossed over a few details but I hope that is enough to illustrate that the molecular motion model is still valid when you include the molecular model of the metal piston.

I think the next question was about how the structure of the molecules affects the collisions, compared with solid billiard balls.  Of course solid billiard balls are a concept that most if not all of us are familiar with, even if some have invested more time in study of the billiard table than others.  Atoms, however, at a first level of detail, consist of a positively charged massive nucleus, surrounded by a cloud of negatively charged orbiting electrons, each with relatively little mass.  And mostly empty space between.  So what does a collision look like at this scale?  Well I am not a physicist, so I am not sure that I can give a definitive model.  However I would think that the strong close range repulsion forces, possibly something to do with the positively charged nuclei, might mean that the collision involves elastic forces strong enough to look like reaction of elastic solid bodies.  But it would need some detailed study of molecular physics to be sure.  Perhaps some holiday reading to look out for.  Of course it is even more complex when, instead of single atoms, we have molecules.  For example oxygen and nitrogen both involve molecules consisting of two atoms, while water has one oxygen and two hydrogen atoms, hence H2O.  These combinations mean we have atoms that are not at all like a spherical ball.  Their asymmetrical form would have some influence on the energy contained in the spin around each of the three perpendicular axes, and also the momentum change when they collide.  And of course the molecules implies another energy change in a chemical reaction which involves molecular chemistry.  Again well out of my field.  Personally I am happy to stick with the simpler assumptions, I think they give enough information for my purpose.  But I recognise that as a gap in my knowledge.

There is one other issue in the molecular model.  A characteristic of the random motion of molecules is that the magnitude of the velocity as well as the direction is random.  This means that there are large numbers of molecules with velocity well above and others well below the average.  But is is reasonable to discuss on the basis of an effective average.  Studying at this level of detail is however a whole new field. 

The other outstanding question was about the elastic properties of metals and how these affect our selection of a piston material.  I wanted to start this one from the point that all materials deform under stress.  In simple language they compress or stretch like a spring.  Just some deform less than others.  Also some are quite linear and steel is in this category, some less linear.  When the material is deformed an elastic material returns to original form when the force is removed.  There is usually a limit beyond which the deformation becomes plastic.  That is the material does not return full to the original shape.   Selection of material involves not only consideration of strength, but also temperature resistance, chemical resistance, wear resistance, friction and so on, and also consideration of manufacture such as machinability.  Availability and machinability are probably among the first priority for model engines.  All the commonly used piston materials have slightly different but adequate modulus of elasticity and strength, although care over specific choice of alloy is necessary if aluminium is required for higher temperature applications, due to both strength and thermal expansion.  Basically the differences in modulus of elasticity is not very important in the conversion of energy to work.  Thermal conductivity is important for heat transfer applications, and also when thermal insulation is required, but again not in pistons.

I hope that deals with some of the philosophical issues, next time back to those engine questions from Paul and perhaps Willy's air in the boiler issues.

Thanks for following,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on July 28, 2017, 01:27:21 PM
Thank you very much for this extended trip to the interface where the molecules interact. It has cleared some of the fog and settled my thinking as I am now comfortable that we have treated steam and steel (our piston) as equals, molecularly speaking. Your explanations have given me a pretty precise 'picture' of the actions of molecules on the piston from a macro and microscopic perspective. Now I need to get comfortable with all the thermal or heat energy issues. This I think will be very energetic mental exercise and hope my brain does not pass through some super critical phase and end up suffering runaway entropy!

Looking forward to seeing your thoughts on the model cylinder materials/construction. Regards, Paul Gough. P.S. I just noticed the average hit rate to this thread, (approx. 6000/90 days= 66). This would seem to me to be very satisfying to have a packed classroom every lecture!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 28, 2017, 03:09:11 PM
Hi..........I have just had an eureka moment and now realise that the study of Thermodynamics was to determine how long a steam engine engineer could have for a tea break !!!However this was before they invented stainless steel teaspoons !!!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 29, 2017, 12:29:48 PM
Hi Paul, I am glad the explanation on gas collisions with the metal surface made things clearer for you.  You know, brain meltdown depends not on energy, but on rate of energy movement, or power.  Energy is measured in Joules, J, while power is joules per second, J/s, also named Watts. So to avoid the brain meltdown, it is a case of pacing yourself. I think that was your words very early in the piece?  However the main thing is that we make it interesting and preferably also useful.

Now you were asking about cylinder arrangements for your little locomotives, so let's discuss the cylinder after all that talk about pistons.  In principal, the cylinder is only there to complete the enclosure around the piston so the steam is constrained and the high pressure can act on the piston.  Of course, when the pressure causes a force on the piston, there is an equal and opposite force on the head of the cylinder.  The magnitude of the force is P x A, and the direction perpendicular to the cylinder head.  Remember to use consistent units, so pressure is Newton/square metre, or Pascals, and area is in square metres.  Then force is in Newton's.  This force has not been mentioned so far as the cylinder head on a stationary engine is by definition stationary, meaning that it does not move in response to the force, and hence does no work.  Remember work = Force x distance.  If distance = 0, then work is zero.  So what happens to the force?  The force is transferred to the frame through the cylinder mounting fastenings, and is resisted by the main bearings.  And of course the big end bearing is providing an opposite force on the main bearings, so the whole lot is in equilibrium.

This is a very important point in the engine design.  The primary strength design of the engine is based on the magnitude of this force.  The cylinder head bolts must carry the load in tension, the cylinder mounting bolts must carry the load in shear, or there may be a shear key to transfer the load to the frame.  Similarly for the main bearing mounting.  On the piston side, the force is carried through the piston rod, and with some allowance for the angulation, through the con rod.  It is a major factor in the bearing design.  There must also be an allowance for inertia loads, and bending moments, when things are not in line, but the basic rod load or cylinder head load is the basic design load.  My compressor experience tells me that major manufacturers design a model series around nominal steps in rod load capacity and design the frame and motion parts from that, rather than start from scratch every time.

Thermodynamics assumes that expansion in an engine is a nominally adiabatic process, meaning no heat transfer in or out.  This assumption is used almost entirely because it makes mathematical analysis possible, not because it represents reality.  The performance of a real engine is determined on a test stand and compared with the ideal adiabatic engine.  So what is the effect of the heat transfer on engine performance?  Now heat input hardly ever happens so let's deal with that first.  I have not been able to quickly find an example with the first law equation for this example, and I am not going to try and derive it.  However basically it says that Heat input = change in internal energy plus the work done.   We don't know how much of the heat goes into internal energy and how much goes into work output.  So let's put together a few things.  The analysis of an ideal adiabatic process assumes the process proceeds through a series of very small steps.  And the first law applies to each of these steps.  The work output is pressure times area times the distance the piston moves in each tiny increment.  So the heat probably goes mostly into internal energy, but that will be reflected in the pressure for the next increment.  So the pressure does not drop as much as you would expect in that increment with the consequence it is higher for the next increment, which means some more work produced.  I don't know how much more, but directionally, heat input to the cylinder during expansion will increase the engine output.

In a real steam engine, the more likely position is that heat is lost from the cylinder.  We know the outside of the cylinder is hot, so it will loose heat to the air, and that heat comes from the steam.  Applying the same logic we can see that in this case, the heat loss reduces the engine work output.

Now to think about how we can use this information for your engines.  First, we can at least reduce the heat loss by adding insulation, or cladding to the cylinder.  This is done in many of the model builds on this forum, reflecting the fact that this was indeed full size practice.  Relatively simple step that adds to appearance and directionally improves the engine output by reducing losses. 

In the early days, when engines operated at very low pressure, some went a step further and tried adding a steam jacket around the cylinder.  This obviously adds a level of complexity, but the question becomes where should the steam come from?  You might be tempted to say what about recovering heat from the exhaust steam by using some of it in the jacket.  Now remembering that heat moves from a high temperature to a lower temperature, and as the exhaust temperature is lower than the average temperature in the cylinder, it will not actually provide any heat input.  However, it means the temperature difference driving the heat loss is the difference between the cylinder temperature and exhaust temperature, say 100 deg C, compared with say 20 deg C atmospheric temperature.  So there would be less heat loss, even though no actual heat gain.  If we use some of the engine supply steam in the jacket, we now have a small heat input to the engine.  However I feel that on balance of probability, that steam might produce more work if used in the engine.

I don't know how much extra power is produced by reducing the heat loss.  If we think about Willy's electric heater of 500 watts, similar in magnitude to my methylated spirits burners which produce about 600 watts, you engine burner might be similar.  I suspect the engine output from such a boiler might be around 2 to 5 watts.  If we made a dynamometer and did an engine test, we might expect to identify a difference of say 5 to 10%.  A full size test stand only achieves around 1% accuracy, so I doubt we would do better.  I don't know if the difference would be measurable, but race winning performance is certainly achieved by tiny differences, so it is probably important if we want to achieve the best we can.  At the very worst it will only help us avoid one source of burned fingers.

But what about the question of a solid block compared with a light weight fabrication?  Before we try and answer that, there is another temperature variation in the cylinder wall when the engine is running that we should be aware of.  At the beginning of the power stroke, hot steam from the boiler superheater is admitted to the cylinder.  This will tend to lose some heat to the cylinder, causing some loss in efficiency as we have discussed.  Then as the piston moves down, and the inlet valve closes, the steam starts expanding.  In expanding without heat input, the steam cools, so that it is close to exhaust temperature at the bottom of the stroke.  Then the exhaust steam is pushed out at the top of the cylinder cooling it.  So in each cycle, the cylinder is repeatedly heated and cooled, clearly a heat loss that leads it loss of efficiency.  It also causes a little variation in thermal expansion, which may in time lead to sealing problems with the  head gasket, just conjecture, I don't know if the movement is enough to cause such problems.  This temperature cycling occurs under the insulation, whether insulation is present or not.  It is probably the clue to the answer on whether to use a lightweight fabrication.  A more massive block provides a thermal inertia which would tend to stabilise the temperature a bit which may be helpful.  And a bit of mass generally helps with traction in a small engine, so I would probably lean towards a solid block (with insulation around it, rather than a very lightweight fabrication.  That also appeals to my skill level with soldering as I currently need to borrow a big enough burner, preferably with operator attached, for anything of significant size.  It think it would take some experimenting and very careful observation to determine if there is a real difference in performance.  Of course a solid block will absorb more heat on startup and hence produce more condensate initially, but once everything is warmed up and running it would not make any difference to condensate in the exhaust.  I suspect the choice will probably be determined by ease of manufacture and availability of material, rather than thermodynamic considerations. 

Similarly with material choices.  Compatibility between the piston and cylinder materials is probably the main consideration.  Thermal expansion needs to be considered.  I suspect an aluminium piston might seize in a cast iron cylinder, while a cast iron piston in an aluminium cylinder might leak excessively.  So it is worth thinking about whether the clearances in your engine will increase or decrease when it heats up.

Thanks for pointing out the viewing numbers.  I hope it means that there is interest in the topic, and assume that others will join in at the appropriate time if they would like to add something, or clarify something.  I am not a teacher or lecturer, and you can probably tell from some posts that I was not any loss to that profession, I am just trying to pass on some things I learned in my career in the hope that it will be helpful.

Next time Willy's questions about air in the boiler.  By the way Willy, we are all waiting for your posts of the cooling curves you get with and without the teaspoons in your tea.

Thanks to everyone for reading,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 29, 2017, 01:10:03 PM
Thanks for this latests info,.....as the cylinder block expands should the holes in the frames be slightly elongated away from the middle bolts so there is no shearing stresses on them ? or even worse could it distort the frames, especially if it is a solid block??can one work out the actual length increase for a freezing cold cylinder block, (early in the morning) to when it is working at 250 LBS/s   16 Bar later on in the day half way to Scotland from london ? All these forces are ones that we don't usually think about.!! They say that you can suspend a London bus from a 1/4" bolt!! but i am not sure. I shall endeavour to do that experiment soon...promise...I have just found some graph paper in a skip ! so that has prompted me !! Ok I have a   K.type thermocouple ? and i might use it on the Foreignhight scale as it will be more sensitive !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 30, 2017, 12:22:37 PM
Hi Willy,  you are quite right to point out that I stopped the discussion on cylinder construction a bit early, and expansion and how to live with it should have been included.  I quite deliberately say how to live with it, rather than control it because we basically have nearly no choice.  If you heat steel, it expands.  If you try and restrain this expansion, you will encounter tremendous forces as you would have to apply the force necessary to compress the steel back to the size it was before heating.  We can calculate the expansion, and the expansion due to forces, just as you suggest.  You can look up some properties of metals, and you will find a yield strength, an ultimate strength, a Young's Modulus and a coefficient of thermal expansion.  You may also find Poison's Ratio.  Let's look at each in turn.  The yield strength is the stress limit beyond which there will be some plastic definition.  The ultimate strength is the stress beyond which the material can be expected to break.  If you take a piece of steel strip, say 2 mm thigh by 25 mm wide and gently bend it with your fingers.  If you bend it just a little, it will spring back straight.  You may be able to bend it a little more and still have it spring back.  But there comes a point where it no longer springs back all the way to straight.  Without going into the stress analysis, in bending, the stress is not uniform as it might be if you just tried to stretch the steel lengthwise, but the point where the strip no longer springs back to straight, is the point where the highest stressed areas undergoes some plastic deformation, or permanent deformation.

Young's modulus is the property that relates the load to the deformation in the linear or elastic range.  This is the one you use to calculate the stretching or compression due to the pressure load, or other mechanical loads.  Nearly all mechanical design aims to keep the stresses within the electric limit.  Now there is considerable variation in the strength of even the best materials.  And it is quite difficult to apply a load so it is totally uniform.  In practice, quite generous safety factors are normally applied, so that there is minimal chance of the elastic limit being exceeded.

The coefficient of thermal expansion is the one you use to calculate how much the material expands as it is heated.  (Or contracts when it is cooled.). The figure for steel, from memory, i.e. not very reliable, is about 6.3 x 10^-6 in/in per degree Fahrenheit.  Funny tricks the memory plays.  But you will recognise that in/in is dimensionless, just a reminder really, so the units are really just per degree F.  But the expansion is measured in millionths of an inch in each inch of length of the component for each degree temperature change. My access to books and technology is limited at the moment, I don't have the right book with me, so apologies for being approximate.  The figures will vary for other materials.  I think aluminium is much more, brass and other copper alloys in between. 

Thermal expansion is interesting.  Things expand from a geometrical centre in all directions, whether the material is there or not.  So an engine cylinder expands just as much as a solid block.  The bore expands, not contracts.  So if the piston is the same material, and the same temperature, the clearance will also expand in proportion.  And this expansion, so long as the temperature is uniform, causes no stress in the material.  Well if not from the expansion, where do thermal stresses come from?  There are two causes for thermal stresses.  First if the temperature is not uniform.  If the temperature is not uniform, the expansion is different in different parts, but they are joined together.  So what ever stress is necessary to deform the parts so they are still joined, will appear.  Either that, or something will break.  If heating is slow, and particularly if things are insulated, the temperature can be kept uniform within reasonable limits.

The second source of thermal stresses is when different materials are in the same component.  If you fabricate a cylinder from say a brass tube and attached flanges made from a material with a lower coefficient of expansion, the flanges will tend to compress the ends of the cylinder to a slightly smaller diameter.  Of course with Paul's very small cylinders, the magnitude of this difference might be much smaller than reasonable construction tolerances, but it is worth knowing what is happening.  So the whole fabrication is best made from the one material.  Of course you may wonder about the silver solder.  When everything cools down, the solder may experience some local stresses.  Similar to the distortion of an improperly supported welded joint when it cools.  But even if there is a little plastic deformation on cooling, this is not a disaster, the stress is relieved by the plastic deformation.  Disaster is only when things are so severe that the joint cracks.  There are many build logs on this forum that illustrate that a well made silver soldered fabrication is normally quite sound.  So no reason to avoid fabricated components.

Willy, your second question was about how to support the cylinder on the frame to avoid expansion issues.  We are all familiar with the well known vertical engines.  The cylinder is supported on top of the cross head guide.  The cylinder expands upwards from the mounting face.  The close contact of the cylinder with the mounting flange means they are practically the same temperature, so both the cylinder and the head are expanding together.  If the cylinder and the cross head guide are both cast iron, they even expand the same amount.  The cross head guide and its mounting bolts are all stretched vertically by the rod load forces, but as we have observed they are usually made strong enough to resist the forces.

When the cylinder is mounted without the symmetry of the vertical engine, for example my mill engine which you can see in the engine gallery, the cylinder is mounted on its side on a steel base plate.  The centre line of the cylinder is well above the plate, as are the main bearings.  So the forces on the cylinder and main bearings are not in line with the base plate.  This of course results in some bending forces on the base plate.  Now you can see that the base plate is quite solid and easily resists the bending forces from this small engine.  There was a recent excellent build log by one of our members is Germany, my apologies to him that I can't remember his name for the moment, but look for the In Line Engine.  The cylinder is mounted on its centre line, and the symmetrical frame on each side properly balances all the loads.  It also has an interesting valve linkage and governor, but I believe the "In Line" designation refers mainly to the main rod load being in line with the centreline of the symmetrical frame.  It was beautiful workmanship.

Paul's locomotive had cylinders mounted I think between the locomotive frames.  Each frame is relatively flexible alone, but the cross bracing between frames is intended to stiffen the frames and the symmetrical nature of the frame and cylinder block configuration properly resists the bending loads as well as the tension.

I think that leaves the issue of expansion of the cylinder where it is attached to a frame on the side.  It is time to defer that one until tomorrow.

Looking forward to seeing those cooling curves Willy.  You look well equipped so far.  Degree F does give you smaller steps which may help give smoother curves.  Remember to control or at least record all the significant variables, ambient temperature, cup design, mass of tea.  May be better to do it with hot or boiling water instead of sacrificing a perfectly good cup of tea, though it is in a good cause.  The i Pad timer is very good for such experiments, it will record several times requiring only one touch for each time, so by a single touch say every 5 degrees, you will have an accurately recorded time to use in your graph.  You may want to make a thermowell by folding a drinking straw, and inserting the thermocouple in one side instead of placing it directly in your tea.  A K type thermocouple is a standard device for which the metals of the two wires are defined by standards, as is the voltage for each temperature.  Fortunately the calculation from voltage to temperature is included internally in most multimeters these days.  It should not need calibration, but it never hurts to check it at 32 F and 212 F.  Easier said than done though.  The boiling temperature is affected by atmospheric pressure, so not easy to achieve the 212,  the ice point is a bit easier as in principal you only need ice, and water in an open vessel, hence also water vapour to give 0.01 C = 32.018 F, near enough to 32, but do have plenty of ice.

Thanks everyone for following

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 30, 2017, 05:14:56 PM
Hi, I have done a preliminary graph and it is quite interesting asa there is a quick drop in temp, then it stays the same for a few mins then falls gradually 1 degree a minuet. it looks like the heat rushes into the cup and spoons then finds it has nowhere else to go so comes back and thinks about it then decides to make the best of a bad job and gives off the rest of the heat in a begrudgingly manner ??!!! Are there technical terms for some of these adjectives ?!!. What i did was to pour the coffee this time in to the cup with the the teaspoons and temp node already in it. In the cafe the temp of the water to make tea is actually 80 degrees rather than 100 c Next i will try it with equal dimensions bars of copper, brass, bronze, aluminium and SS. and see how this performs. In all my text books there is no mention at all of differential expansion problems, and looking at a huge Triple expansion engine that would be in the Titanic, one wonders how much the engine would grow !!?
Title: Re: Talking Thermodynamics
Post by: paul gough on July 30, 2017, 11:30:21 PM
Thanks again for the comprehensive reply on the cylinders, and the extension to 'expansion' prompted by Willys questions, thanks Wily.  I was particularly taken by your sentence, "Things expand from a geometrical centre in all directions, whether the material is there or not." Conjuring an image of a cylinder in my mind as I read this extraordinarily succinct and illuminating sentence led to an immediate grasp of the phenomena. A situation where previously my intellectual myopia had glossed over the origin, geometrical centre, because I was only thinking in terms of the bore getting bigger or the cylinder body growing as more or less separate things. Oh, if only all engineering explanations were so eloquent!!!

I have a fantastical question that keeps penetrating my mind and which I can't reconcile because I am confounded by internal energy and the spectre of irreversibility/reversibility etc. The Scenario: (1) A normally working cylinder and piston arrangement, but the cylinder body, including all covers, piston and rod etc. are somehow heated to and maintained at the temperature of the inlet steam, (say 150C); (2) All these elements heated to a high degree and maintained, (say 5x inlet temp).  (3) Would case (1) be somewhat equivalent to all the components being made of a perfect insulating material? Thus, are there any substantial changes to how our steam (heat) engine works, (or doesn't work), with these three situations???? Many thanks for your patience and effort in dealing with my confusions. Regards Paul Gough.   
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 31, 2017, 01:13:55 PM
Willy's tea cooling experiment.

I was going to talk about thermal expansion but I would first like to look at Willy's tea cooling experimental results.  Willy, your graph is very well done and gives a lot of information about what is going on.  Let's look at three distinct phases, first that quick initial drop, then the steady temperature then the continuing cooling.  Your explanation is not bad so let's look at the technicalities.  Then try and deduce what it means.

That initial quick cooling is due to the teaspoons absorbing heat as they absorb heat from the coffee.  Remember stainless steel has poor thermal conductivity so it is not instantaneous.  We would expect the temperature drop to be greater for additional teaspoons.  But we have some  complicating factors.  First, the cup will also absorb some heat from the coffee.  We can get a good idea of how much heat the cup absorbs by looking at the curve for zero teaspoons.  There is a gap you have not told us about, the coffee starts at 80 deg C, which is 176 deg F, but your graph starts at 150 F.  Is that perhaps due to some air cooling of the coffee on the way from the machine to the cup?  Then I notice the 3 spoon experiment started at a lower temperature.

We can analyse what is happening in this way.  We have three components in the experiment, the cup, the coffee and the teaspoons.  Each brings an initial quota of heat to the experiment.  We could use absolute zero for the starting temperature, but the properties are not exactly constant over the necessary temperature range.  It is easier and a bit more accurate to use 0 , F or C depending on which we are working with.  The initial quota of heat for each is calculated as
Mass x specific heat x temperature - ref temperature, which we have nominated as zero, you can now see why.  When the coffee hits the cup and teaspoons, all three end up at the same temperature.  So we need only to calculate a temperature, t at which all three have the same temperature, but the total heat is still equal to the sum of the three initial quotas.  So you can see we need to know the initial temperature of the cup, the teaspoons and the hot water.  We don't know much about the cup, and it will end up hotter on the inside against the coffee, than the outside which is being cooled by the air.  However the zero teaspoons experiment probably tells us how much heat the cup absorbs, if we know it's initial temperature.

The initial starting point is probably not very important if we have the first temperature reading, and note that this is probably not at zero time for the complete curve.  However to keep the teaspoons and cup effect separate, it may be better to let the cup cool the coffee, then add the teaspoons as soon as the initial drop is complete, say as soon as you have two consecutive readings that show the levelling off, so you see the quick drop when the spoons are added separately from the effect of the cup.

Then we have that roughly constant temperature period.  We have to be careful in our explanation of this phase to be sure that we do not violate known laws of thermodynamics, in particular that heat cannot flow from one item at one temperature to any other place which is at a higher temperature.  Heat can only flow from hot to cold.  However, plunging the cold teaspoons into a hot cup of coffee is not equilibrium at every tiny increment through the change.  It is not a reversible process, but it also does not happen slowly is a series of tiny equilibrium steps.  It is likely that the coffee in the cup is initially over cooled in the immediate vicinity of the spoons, leaving it a bit cooler than the coffee in the more distant parts of the cup.  Then, the heat flows from the warmer coffee to the cooler until it is all at the same temperature as convection currents redistribute the heat in the cup.  The temperature measurement may also be affected by the location of the thermocouple, relative to the teaspoons.  It really requires a bit more experimentation.  The initial purpose of the teaspoons was to cool the coffee to drinking temperature, in which case we might try stirring the coffee to maximise heat transfer coefficients to the cup and air at the surface and keep the temperature more uniform.  Worth a try.  May have to try a few times at home with the kettle, to save some coffee cost, while determining the optimum experimental method.

Now the third stage.  Let's just look at the two that start at the same temperature.  It certainly looks linear, but if you continued until almost at air temperature, (which we should record), it would show a curve, as the heat loss slows due to the decreasing temperature difference.  You can see the curve in the zero teaspoons curve.  But notice that the curves with and without teaspoons cross over after a while.  Can we explain that?

Let me make a suggestion.  When the teaspoons are plunged into the coffee, the heat is not lost, some of it is just stored in the spoons.  It is not lost until it goes to the air either through the walls of the cup, or through evaporation at the surface.  But the lower temperature due to the spoons reduces the temperature difference driving the cooling, so the coffee cools slower, but the same total amount of heat has eventually to be transferred to the air.  The cup with no spoons initially cools faster so the heat is actually lost to the air.  So when the slow one gets down to the point where it is the same temperature as the one with the spoons, the two at that moment cool at the same rate, but the one with the spoons still has more heat remaining in the coffee plus spoons so it's temperature drops slower.  So the spoons not only initially cool the drink to drinking temperature more quickly, they actually keep it near that temperature for longer.  Bonus points to anyone who saw that coming, I certainly did not.  It also casts some doubt on my text book which suggested that the spoons added cooling surface like fins to help cool it faster.  Either it's a very small effect, or the conductivity of the stainless steel puts the handles in that category where fins actually are counterproductive.

I think everyone will now be waiting for the next experimental results.

Thank you Paul for your kind words, I am glad that my explanation was helpful.  I hope that others have found it equally helpful.  Many people are confused by this point, but one time I had to analyse a large vertical pressure vessel with heavy parts suspended inside from a nozzle in the middle of the top head.  It was during my work on this that the penny dropped. 

Enough for today, current intention is to continue on control of thermal expansion and the cylinder heating questions tomorrow.

Thanks to everyone following,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 01, 2017, 11:50:06 AM
Thanks for this synopsis and i shall rethink the next experiments as there are so many variables to consider I may start with boiling water at home rather than  coffee in the cafe as i was getting some funny looks !! Also trying to get the waitresses to come over strait away with the coffee was a bit difficult . ! was going to transpose the 3 teaspoon graph onto the starting point for the others but that would have made the graph a bit over loaded!! so , we shall do some more work over more controlled conditions.........
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 01, 2017, 01:16:39 PM
Thermal expansion issues.

Hi Willy, We are obviously thinking about this at about exactly the same time.  I should have mentioned yesterday that the end temperature would not be much affected by the order you do things in those first few seconds, however by carefully doing things in a suitable order, you can see the separate effects, and thus isolate the effect of the teaspoons from the other things such as air cooling during pouring, and heating the cup.  Of course I didn't think of the time for the waitress to deliver.  Boiling water at home allows you to control much more of this.  By the way, if you have some silver plated brass teaspoons, these have better conductivity than stainless steel so the handles might make more effective fins, would be interesting to see.  The specific heat of brass is 385 J/kg.C compared with 461 for stainless steel so you would need about 20% more mass of spoons, about 6 brass spoons compared with 5 SS ones of the same mass.  Though you could stay with the same number of spoons and see a smaller initial temperature drop.

Continuing our discussion of thermal expansion issues, let's look at a few examples.  Willy you mentioned the engines on the Titanic.

Now I have no idea of what size these engines are, or what temperature they operate at, so I will make a few wild guesses to use for calculating representative numbers.  Thermal expansion is fully proportional to the length being heated, or cooled, so if the real engines were double the height I have guessed, then the expansion will be double.  Perhaps someone will be able to join in with the actual engine dimensions.  Let's assume an engine 6 metres high from the crankshaft centre line to the top cylinder head, consisting of a cylinder 1 metre high and support structure 5 m high.  In the typical model marine engine the cylinder is mounted on a table at the top of columns.  A larger ship might have a more substantial support incorporating the cross head guide.  Further let's assume the engine was built at 15 degrees C and the steam inlet temperature is say 250 deg C.

Thermal expansion is determined by the length of the component, the temperature change and the coefficient of thermal expansion of the material.  If the engines were made of cast iron, the coefficient of thermal expansion is 13.5 by 10^-6/deg C (or 7.5 by 10^-6/deg F).

Let's assume the cylinder is well insulated and operates at the steam inlet temperature of 250 deg C.  As I have previously mentioned, steam cools during expansion so the average temperature would be a little lower, but then the steam inlet temperature may be a bit higher.  These assumptions contribute to inaccuracy, but the calculation is still useful.

The cylinder then expands by d = 13.5 x 10^-6 x 1 x (250-15) = 0.003 m or 3 mm.  This may not seem like much on an engine of that size, however the clearance between the piston and the cylinder head at top dead centre may not be much larger than that.

If we look at the cylinder support, the metal at the top is in close contact with the cylinder so let's assume 250 degrees C.  However at the bottom end, at crankshaft level, it is probably nearer engine room temperature, say 40 deg C.  We can assume that thermal expansion is linear so the expansion can be calculated by assuming it is all at the average temperature, (250+ 40)/2 = 145.

Now calculate the expansion d = 13.5 x 10^-6 x 5 x (145-15) = 0.0088 m or nearly 9 mm.

So we can estimate that the distance between the crankshaft and the top cylinder head is 9+3= 12 mm.  You can see it would be pretty small on a model engine.  It is important to understand that, so long as we allow that expansion to go unrestrained, there is no force or stress involved.  It would take very high forces to restrain the engine to its original dimensions.

So far I have assumed that the whole cylinder block is at one temperature, the steam inlet temperature.  However, the question was about a triple expansion engine, and the temperature for the the low pressure cylinder is quite different from the high pressure cylinder.

For the lp cylinder, assuming a condensing engine, the cylinder temperature might be nearer 40 deg C.  The temperature difference is then 40-15= 25 deg C, instead of 250-15=235 in the high pressure cylinder.  With a temperature difference of approximately 1/10, so the expansion of the cylinder would be 0.3 mm instead of 3 mm.  More importantly, the stand at the lp end has a thermal expansion very close to zero, compared with 9mm at the hp end.

Even with the necessary approximations, I think you can see the problem.  If the three cylinders are machined in one casting, there will be significant internal stresses in the casting as one end expands under the inlet steam temperature, while the other end undergoes minimal expansion at the lower operating temperature.

Remember at the start of the discussion on thermal expansion I said we cannot control this expansion, we can only learn to live with it.  I suspect that in this case, the engineer might set up the engine cold with the lp end a little high, so that when the engine is hot, both ends are at the same height.  He would then have very careful heat up procedures to avoid any problems during warm up.  Now we have a few marine engineers on the forum, perhaps they could chip in and comment on the thermal expansion issues they see, and what they actually do.  I don't have any practical experience with real ship engines so would appreciate any help that is offered.

There is one area that we can control with regard to the thermal expansion.  I mentioned earlier that materials expand in all directions away from a geometric centre.  However nothing says that geometric centre has to be stationary.  We can choose which part of the engine is fixed stationary, and let the expansion go from there.  The geometric centre moves away from that fixed point, and the rest of the material expands away from the altered position of the geometric centre.  On a ship, the crankshaft would be fixed in line with the propellor shaft, and the engine allowed to grow upwards from there.  The remaining problem to be solved is that the piping to the engine must be designed to be flexible enough to allow for the movement of the cylinder flanges.

I hope this simple approach to expansion sheds some light on the issue.  These days with finite element methods we can do a much more exact analysis of exactly what happens, however that might be considered a bit over the top for our model building purposes.

We should have a quick look at how expansion effects a horizontal mill engine before leaving this topic.  Also a few questions from earlier posts still to be tidied up, then back to condensers.

Thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 01, 2017, 03:21:55 PM
Having looked at Utube ,the engine are described as 4 cylinder triple expansion engines ??. Also all the cylinders are separate !! so they knew about thermal expansion. I do not know what the other cylinder did in the engine ...any ideas? Will look further into this and here a few pics off the web
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on August 01, 2017, 03:54:06 PM
Willy, it was common to use 2 cylinders for the low pressure expansion to keep the cylinders smaller. If you notice the violet line it is the exhaust and it is connected to both LP cylinders on the ends.

Dan
Title: Re: Talking Thermodynamics
Post by: Ye-Ole Steam Dude on August 01, 2017, 05:03:32 PM
Having looked at Utube ,the engine are described as 4 cylinder triple expansion engines ??. Also all the cylinders are separate !! so they knew about thermal expansion. I do not know what the other cylinder did in the engine ...any ideas? Will look further into this and here a few pics off the web

It appears looking at the picture from right to left, the fourth cylinder somehow helped “balance” out the crankshaft. Again looking right to left, cylinder #1 is at 0-degrees, cylinder #2 is in retard at 120-degrees ( assuming clockwise rotation), cylinder #3 is at 0-degrees, and cylinder #4 is in retard by 240-degrees. Just my observation and would like to find out.
Title: Re: Talking Thermodynamics
Post by: Maryak on August 01, 2017, 11:23:21 PM
Triple Expansion Steam Engines

1. Cylinder Volumes:
HP = 1.0
IP = 2.6
LP = 7.0

To keep the LP Cylinder a manageable diameter it was split into 2 cylinders at 3.5 x HP volume giving a 4 cylinder triple expansion engine normally arranged from front to back LP,HP,IP,LP.

With this arrangement balance was achieved by setting the cranks at:
Between Forward LP and HP 1660
Between Forward LP and IP 2700
Between Forward and Rear LPs 700
In addition suitable counterweights were added to the crankshaft.

With Titanic and Olympus, not shown is a turbine which utilised the LP exhaust to drive the centre propeller. For manoeuvering the exhaust was switched over from this turbine and sent directly to the condenser. This made emergency reversing/stopping difficult as until this exhaust was removed, the turbine continued to move the centre propeller ahead. Transfer was accomplished by some big hand operated valves!

HTH
Regards
Bob
Title: Re: Talking Thermodynamics
Post by: Ye-Ole Steam Dude on August 02, 2017, 01:38:16 AM
Thanks Bob for giving us this information.

Thomas
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 02, 2017, 02:15:30 AM
Hi all thanks for the info ,it all makes sense actually. I don't know why the HP cylinder looks different with no cylinder head bolts though. there is a wonderfull model made by someone in Germany on the web Utube that is really superb,
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 02, 2017, 11:41:21 AM
More on thermal expansion and triple expansion engines.

Thanks Willy for those great pictures, they make the arrangement very clear.  Thanks Dan for coming in.  From the pictures we can see that not only are the cylinders separate as you have noted, but also the supports, rather like three separate engines with a common crankshaft.  Because the lp cylinders each expand the same amount, the pipe can be attached to both as you have observed.  Thanks also to Thomas for the observations on the angles.  Maryak, you must have worked on those engines, thank you for all the detail.  The turbine makes great sense as it is so much more suitable than reciprocating engines for the very large volume of low pressure steam before it is condensed.

I think balancing is probably quite complex for these engines.  There is not only the the normal balancing of a reciprocating masses, and the inertia forces for such large pistons, also balancing the power strokes and also the steam distribution.  Perhaps you will be able to explain it to us all.  I can see that the steam exhaust for the hp cylinder must be taken in by the ip cylinder, but I quickly get lost extending it to the lp stage, particularly when the two cylinders are not in line or even at 180 degrees to each other.

In terms of the thermodynamics, this is a great example of the problem with trying to extract that expansive power of the steam, the volume gets huge.  Adding a turbine is a great way to expand a large volume of steam to the lowest possible pressure, which is of course limited by the temperature of the sea water available for condensing.  Even the course into northern waters would have contributed to better efficiency due to cooler water available for condensing.  Or would have done, if it was not for a small miscalculation of the risk associated with icebergs.  Obviously the advantage of the turbine was enough to justify the cost of the complexity, telling us something about the economics of shipping.  Pity the accountants never had to swing those valves, even once would probably justify the installation of power operators for that job.

A great introduction to my intended return to the topic of condensing, but I would like to know more about that valve timing and influence of the volume of the steam chests and crossover piping for that triple expansion engine.

Apologies for the walls of text.  I know this thread needs pictures, so just a little update on the adventures in the long paddock.  The long paddock seems to have been joined by the long rail.  The train in the picture was as best I could measure it with the car speedo, around 1800 metres long, about 1.1 miles.  It was stationary, took 70 seconds to pass it at 92 kph.  How about one of you people who like repetitive work modelling this one?  Simple enough, only four electric locomotives, two at the front, one in the middle and one at the tail end, and a few identical cars in between.  Might have to scale down the voltage for your club track though.  The signs only said high voltage.  I don't know what the locomotive power rating is, but my compressors with several megawatt size motors were 11000 V.  Perhaps the trains accept higher currents.  Interesting that each car was marked with direction of travel, I presume something to do with the unloading method, but also implies that it is turned around by a large loop, or perhaps the direction is only relevant to the fully loaded cars.

Thanks for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: Ye-Ole Steam Dude on August 02, 2017, 01:19:12 PM
Triple Expansion Steam Engines

1. Cylinder Volumes:
HP = 1.0
IP = 2.6
LP = 7.0

To keep the LP Cylinder a manageable diameter it was split into 2 cylinders at 3.5 x HP volume giving a 4 cylinder triple expansion engine normally arranged from front to back LP,HP,IP,LP.

With this arrangement balance was achieved by setting the cranks at:
Between Forward LP and HP 1660
Between Forward LP and IP 2700
Between Forward and Rear LPs 700
In addition suitable counterweights were added to the crankshaft.

With Titanic and Olympus, not shown is a turbine which utilised the LP exhaust to drive the centre propeller. For manoeuvering the exhaust was switched over from this turbine and sent directly to the condenser. This made emergency reversing/stopping difficult as until this exhaust was removed, the turbine continued to move the centre propeller ahead. Transfer was accomplished by some big hand operated valves!

HTH
Regards
Bob

G' day Maryak,

I am a bit confused with order of arrangement for each cylinder, in the attached first photo Cylinder No.-1 is not at the same timing as Cylinder No.-4. Your numbers show each LP ( 1 and 4 ) are each at 70 degrees. In the photo, No.-4 appears to be close to 160 degrees ( or so ) opposite from No.-1. It looks like No.1 and No.3 are the same. Am I looking at this incorrectly?

Thank you,
Thomas
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 03, 2017, 12:59:22 PM
More on Titanic engines

Hi Thomas, I can see what you mean about the angles, though I think I have interpreted them slightly differently from what you have described.  The penny dropped when I read Maryak's post, that the angles are only nominally about balance, but the odd angles come from the need to progressively expand the steam a bit in the hp cylinder, then transfer it to the ip cylinder and expand it a bit more, then finally transfer it to the lp cylinders where we not only expand it a bit more, but also provide as steady a flow as possible to the turbine which is a continuous steady flow machine.  There are compromises along the way, especially if the cut off varies as speed is reduced to cruising speed from maximum.  I will leave it to Maryak to add a bit more explanation, I have attached a little sketch showing my interpretation of his figures.  You will note I have drawn it with the hp cylinder at top dead centre.  If you think about the hp cylinder as the engine rotates, you can see that steam admission to cut off follows much the same sort of pulsation says a single cylinder engine as you would expect, with steam into the top of the cylinder for the first half rotation, and into the bottom of the cylinder for the second half.

Then if you look at the lp cylinders, the rear lp which I have labelled RLP, is exhausting at max volume, while the FLP so just starting to exhaust at minimum flow.  The combination gives a fairly steady flow to the turbine for the first half revolution.  Similarly the bottom end of the lp cylinders gives reasonable steady flow for the second half.  The angular difference between them seems to give quite good flow for the whole revolution, another reason apart from sheer size for having two lp cylinders.

So it remains to see if the ip cylinder properly accepts exhaust from the hp cylinder and exhausts it to the lp cylinders in turn.  Its a bit mind bending, but if you turn the drawing around, or redraw it so the ip cylinder is at top dead centre, and follow the similar thought process, I think you will find it works.  The complication is when the cut off varies, when the process might be a bit less smooth, and the volume of the steam crossover piping and steam chests become important.  I am hoping that Maryak will shed some light on that as well.

I am suspicious that the artist might have taken some liberties for artistic presentation, not expecting anyone to analyse it in this much detail, or maybe the artists angles will also work, I just don't know.

But my area is thermodynamics and I look for what thermodynamics tells us that makes it worthwhile building such a complex arrangement.  Why not just a three cylinder triple to provide more power than a simple single or twin cylinder engine?

Basically there are two ways that expansion to a low pressure (which can only be achieved by condensing) provides more power output from the engine.  The first we have covered earlier, the force on the piston is due to the difference in pressure between the top side and the bottom side of the piston.  More differential pressure gives more work output.  Even on a simple oscillating engine.  But then, if the valve gear can cut off admission, the steam trapped in the cylinder is able to do more work by continuing to expand to a lower pressure, to provide even more work output, providing this lower pressure can be exhausted to a lower pressure in the condenser.  Theory might let us expand to a very low pressure, and get even more work out, but as you can see in the Titanic engine example, the volume becomes huge.  Now the arrangement with the turbine following the low pressure stage is very clever, because a turbine is easily able to handle a much larger volume of steam, and expand it to a lower pressure in a reasonably sized machine.  With two large triple expansion engines exhausting to a turbine, the scale is clearly such that economics support installing the turbine.  Otherwise they would not have done it.

Of course, condensing also adds further complexity.  We need a heat exchanger.  We have already see that the heat rejected in condensing is close to the same as the heat added in the boiler for evaporation.  The heat transfer equation is the same, Q=U x A x dT.

Now the heat transfer coefficient for condensing is not as high as for boiling, but it is still quite high.   The temperature difference is the real problem.  Even in winter the water temperature is always above zero, and for most of us, at the club pond, it will be quite a bit higher.  Winter is building time, rather than sailing time.  As the water takes up heat, it's temperature rises.  It can't get above the steam inlet temperature, and realistically, even in full scale industrial condensers, it will at best get to a maximum 10 - 20 degrees below the steam temperature, and that requires a lot of tubes.  You can see it if you find a ships condenser picture.  We can probably anticipate an LMTD less than 50 degrees.  Compare that with the firing temperature in a boiler.  I really don't know the fire temperature, but I am guessing that the temperature difference is more than 200 degrees, remembering that the flue gasses are much lower than the fire temperature at the chimney end of the boiler.  So with lower heat transfer coefficient and much lower temperature difference, we need considerably more heat transfer area in the condenser than the boiler.  However, providing we use enough tubes, we can condense the steam.  And this is done on ships so that water can be reused, instead of using the considerably more expensive process of desalination of sea water for boiler feed.

Now while it is possible, I don't advocate trying to calculate the area needed for a condenser.  I would use a published design, scale it to my estimated engine steam consumption, preferably by testing, then make it as large as I can accommodate in the model.  Then I suspect my time would be more enjoyably spent building a new condenser, with the size adjusted as necessary to condense all the steam if it turns out to be insufficient.  There is actually no real problem in making the condenser too large.  You just get a nearer approach temperature, or even sub cool the water a bit.  So long as it is not too heavy, or takes too much space in your model, it will be ok.

By building a condenser, you have the possibility of increasing your engine power output by increasing the differential pressure on the piston or pistons.  Unfortunately there is another problem we have to deal with, and that will be the topic for next time, or when we deal with some of the interesting little side tracks on our project.

Thanks for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: Ye-Ole Steam Dude on August 03, 2017, 01:20:42 PM
Hello MJM460,

I appreciate your explanation and drawing, this design is so interesting and I wish that I knew more about the complete process. This kind of work is very impressive when I think about the time and era it was conceived, when you factor in what tools and knowledge was available to the designers.

Enjoying following this thread and thanks again,
Thomas
Title: Re: Talking Thermodynamics
Post by: Ye-Ole Steam Dude on August 03, 2017, 04:55:33 PM
I found this information and drawing on the internet and it is pretty straight forward in explaining the cylinder arrangement. Hope this will help.

Titanic’s 4-cylinder reciprocating engines were balanced on what was called the Yarrow, Schlick, and Tweedy system. The crank throws were not arranged at 90-degree intervals as one might assume. Instead, vibration was reduced by adjustment of the relative crank angles and crank sequence being used. Beginning with the HP cylinder piston at top dead center (TDC), the crank sequence and angles of the engines were: HP at TDC, then a 106° rotation for the IP to TDC, then a 100° rotation for the forward LP to TDC, then a 54° rotation for the aft LP to TDC, then a 100° rotation for the HP to return to TDC. This is the link to the website: http://www.titanicology.com/Titanica/TitanicsPrimeMover.htm
 

This crank arrangement is shown in the diagram below.

 

Title: Re: Talking Thermodynamics
Post by: MJM460 on August 04, 2017, 12:27:03 PM
Hi Thomas, I am glad that you are enjoying the thread, it is all about sharing knowledge, so it's great to have some interest.

That was a great find on the titanic engines.  Balance is not so easily achieved when pistons are different sizes, but I think the sequencing of valve events also has an influence on the angles between cranks.  It also appears that slightly different angles might be selected, depending on just what the designer is trying to achieve.  I am still fascinated that the two low pressure pistons with the angular displacement between the cranks are able to produce a steady enough flow to the turbine, instead of the normal zero flow at top and bottom dead centre, I had not thought of that.  But I will let Bob or others tell us more about triple expansion engines, while I return to the condenser topic.

I mentioned last time that there was more to a condenser than just removing heat from the steam.  The problem is air.  So first where does the air come from?  Two sources, each in fact quite small.  There is always some dissolved air in water.  In industry, this air is first boiled out in a vessel called a deaerator.  This is followed by a chemical oxygen scavenger.  With steel boilers air removal is particularly important as oxygen causes corrosion in the warm wet environment in the boiler.  But you can safely assume there will be some air in your model boiler feed water.  The second source comes in as soon as you are able to maintain some vacuum.  There will be some air in leakage.  The quantity is small, but it accumulates in a condenser so its presence soon becomes important.

If we remember Willy's mountain top experiences, we talked about some air in the boiler when it was sealed up.  The air and water vapour in the vapour space act independently, so the total pressure is the sum of the air pressure plus the water vapour pressure.  We had to heat the air, as well as heat the water.  Fortunately, in the boiler, the air is entrained in the steam production, so it does not accumulate and is soon near enough to zero.

In a condenser, air is a non-condensable and unless we make special provision, there is no path out, so it accumulates.  And in accumulating, the partial pressure of air increases, and hence the total pressure in the condenser increases.  Even though the steam condenses at quite a low pressure due to the cooling water temperature, the piston sees the total pressure during the exhaust stroke, so there is no advantage in the low water condensing temperature.

You can see where this is leading, prototype engines with condensers also have an air pump.  The job of the air pump is to remove the air down to the low pressure of the condensing steam, so the condenser truly does provide the low exhaust back pressure we are looking for.

In many of the prototype machines often selected for modelling, the air pump is some sort of diaphragm pump with sufficient displacement to deal with a volume of low pressure air, remembering that some of the low pressure steam will also enter the air pump with the air.  The pump then has to compress the air and steam mixture to above atmospheric pressure so it can be discharged to the exhaust stack.  An air pump is in fact an air compressor, much like a bike pump, and clearance volume is critical, unlike the water pump for which clearance volume is not so important. 

You will also recognise that the condensed water is also at low pressure compared with atmospheric pressure, so normally a water pump is used to remove the condensate.  There are even some quite ingenious designs where the air and water pumps are combined.  When you remember the purpose, and what the design is trying to achieve, these designs become a little easier to understand.

In summary, adding condensing to an engine, increases the power output due to the reduced back pressure on the piston.   We achieve this advantage even on simple engines without early cutoff and expansion.

We can condense at atmospheric pressure simply to recover the water, but there is no real power advantage to this over a simple atmospheric exhaust.  To achieve the advantage of low back pressure due to condensing, we need a condenser, a condensate pump and an air pump.

The low temperature difference inherent in condensing means we need a large heat transfer area, more than required in the boiler that produces the steam, usually provided by means of a large number of tubes.

I believe I have some questions from Willy and Paul which have not been addressed, so I will have a go at those next time.

Thanks for reading,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 04, 2017, 02:38:53 PM
Hi, more explanations all good ...I was wondering ,if i may enquire about the jet condensing arrangement on the Woolf compound engine i am making. The cylinders are double acting but the air pump is single acting? and i was thinking that when the exhaust steam is condensing and forming a vacuum ,is this vacuum impeding the opening of the flap valves to the air pump at various places in the beam engine cycle ? Here are a few pics and drawings of the Beeleigh Mill engine, I made the drawings showing the use of hippopotamus hide valves as used in some engines before they took the air pump apart and discovered ordinary metal to metal flap valves. Just a thought and i may be wrong , however the force produced by the Newcommen atmospheric engines was quite considerable, and could this force be calculate into horses power as they used to say.....Also the jet condenser valve is always open rather than connected to the valve events ?
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on August 04, 2017, 02:56:59 PM
MJM, I have to admit that I struggled with thermodynamics in school. You mentioned the engine book by K.N. Harris but I think his boiler book is more likely far better known. I think a good topic for this thread would to explain how to size a boiler to an engine using the boiler book as a starting point.

K.N. Harris was an engineer and had a lot of practical knowledge.

Dan
Title: Re: Talking Thermodynamics
Post by: Maryak on August 05, 2017, 01:14:09 AM
G' day Maryak,

I am a bit confused with order of arrangement for each cylinder, in the attached first photo Cylinder No.-1 is not at the same timing as Cylinder No.-4. Your numbers show each LP ( 1 and 4 ) are each at 70 degrees. In the photo, No.-4 appears to be close to 160 degrees ( or so ) opposite from No.-1. It looks like No.1 and No.3 are the same. Am I looking at this incorrectly?

Thank you,
Thomas

Hi Thomas, sorry to be late in responding below is a diagram which I hope explains the differences

(http://i389.photobucket.com/albums/oo340/Maryak/Drawing1_zpsv7jziyha.jpg)

Regards
Bob
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 05, 2017, 12:35:47 PM
Out on the plains, the brolgas are dancing

Hi Willy, Just in case I have created some confusion, I should make a small clarification on jet condensing.  I mentioned that steam industrial steam plants sometimes use a jet ejector instead of an air pump.  Much the same principle as the injectors used for boiler feed water on some locomotives.  It literally uses a steam jet and Venturi arrangement to create a low pressure so that air flows in, at which point an diverging nozzle increases the pressure enough to discharge the air to the atmosphere.  This is quite different to the jet condenser on your engine, which uses a spray of cool water directly injected into the exhaust steam in order to condense the steam.  No tubes involved.  Saves soldering in all those tubes.  Basically the latent heat in the steam heats the water by direct contact with the water spray.  The end result is dependent on the amount of water sprayed in.  You could use just enough to condense the steam and the whole mass of steam plus water ends up as saturated water at the steam condensing pressure, or you can add extra water and sub cool the whole mixture to some extent.

The actual condensing pressure depends on your air pump.  The air pump removes the mixture of air plus water vapour from the condenser space, thus lowering the pressure in that space.  It then compresses the air and vapour to a pressure high enough to discharge to the atmosphere.  In your engine, the air pump handles both the air/vapour mixture and the condensed water.  While it is handling both, it is strictly a compressor.  The difference is basically in the ratio of swept volume to clearance volume.  In a typical feed pump, and also on a dedicated condensate pump, the swept volume or displacement of the piston is relatively small, while the clearance volume, basically dependant on the valve chamber arrangement does not really matter and can be relatively large.  As soon as the piston starts moving towards the valve chamber, a water filled cylinder rapidly increases in pressure until the discharge valve opens and the liquid is discharged into the outlet pipe, in the case of a boiler feed pump, right up to boiler pressure.  Even a quite small bubble of air in the water pump causes a real problem.  In the presence of a small bubble, the bubble must be reduced in volume, or compressed, until the pressure is sufficient to open the discharge valve.  If the clearance volume is significant, the bubble is compressed in the clearance space but not enough to open the discharge valve.  When the piston starts moving down again, the bubble just expands to its original volume and there is no discharge flow.  In your pump, the displacement is quite large.  When the air pump piston moves up, it reduces the pressure so that all the water flows past the first flap (or ball) valve, and the stroke is such that a volume of air and water vapour also passes the first valve.  When the piston then starts moving down, the pressure is increased so that the valve in the piston opens and air and water flows to the top side of the piston.  In fact you have a two stage compressor, and on the next upstroke, not only does more air and water flow past the lower valve, the water on top of the piston is lifted, and the air above the piston compressed until the top valve opens and water and air are discharged into the top chamber.  The gland in the top plate prevents air leaking back past the pump rod and increasing the volume that has to be discharged.  When the piston is at its lowest point, the remaining volume is quite small so sufficient of the vapour is compressed to a high enough pressure to flow through the valve vent holes.  (Don't make them too small!). Not important that air pump is single acting while the engine is double acting.  Some variation or fluctuation in the condenser pressure will not matter.  Compressibility of the air, vapour will smoothe the pulsations considerably.

By the way, vacuum is a useful concept in conjunction with a pressure gauge that measures only the difference between the measured pressure and atmospheric pressure.  However, there is no such thing as negative pressure.  Zero pressure is the vacuum of deep outer space, or in your equipment, but only if you a have a really, I mean super really good vacuum pump.  Atmospheric pressure is about 14.7 psi or 101.3 kPa, just over 1 bar.  The pressure in your jet compressor, is unlikely to ever get to zero but easily somewhere in the range 10 to 14 psi, and at best possibly even lower.  The discharge of your pump must be something above 14.7 psi in order to discharge to atmosphere.  Remember the molecular model of gases.  Pressure comes from the change of momentum when the gas molecules hit a surface and bounce off.  There is no negative pressure.

So your valves will open when the pressure on the underside is higher than the pressure on the top side, so the force is available to open the valve.  When the higher pressure is on top, the valve leather or plate moves down onto the support place and closes off the vent holes, so the water and air cannot flow back.  To understand how they work, just look at the direction of pressure difference, and don't worry about vacuum.  When your engine starts, the condenser part will be full of air.  So long as the air pump removes more than is introduced by the feed water plus the inevitable air inleakage, the pressure will, fall to something near the saturation pressure of the condensed steam after a short run time and give you quite a good vacuum to increase your engine output.

The horses power of the engine can be calculated, but in addition to the force, it requires the stroke length and number of strokes per minute to complete the calculation.  I will explain that another time, as I have a few units not addressed so far.

Hi Dan, thanks for looking in.  I intend to go on to boilers when we get past condensing, and I will include your suggestion of looking at analysing Mr Harris' boiler capacity method as you suggest.  I have the book and will try and make some notes in the mean time.

Hi Maryak, thanks for that explanation.  I wonder if optimising the flow for that turbine might be one factor in the choice of crank angles for the Titanic, while for more typical engine arrangements, exhaust pulsations would not be so important so other factors such as balance can be given more importance.

A wild bird, voluntarily in the park stands about 5 ft high and has around 6 ft wingspan!

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 06, 2017, 12:30:05 PM
Hi Willy, I have held over some of your questions from earlier posts, so I think it is time to talk about them.

In post #174, you asked about some ways of raising steam a bit quicker.  I think the first step is to think about how much energy is required to raise steam, then where the heat goes, and then what is the effect of the procedures that you propose.

Your boiler is a great example, as you can insulate it very well, and then assume that all the energy from your electric element goes into the boiler and its contents.  So the heat goes into the copper of the boiler, the water you put in the boiler, and the air that is in the boiler when you tighten the plug.

I did some rough calculations assuming the empty boiler has a mass of 1 kg and holds 0.5 kg of water.  It depends on all the dimensions, but I guessed the mass of air at around 0.01 kg, but it's probably quite a bit less.  I also assumed the whole lot starts at 15 deg C.

To get the whole lot to 100 deg, but without making steam, remember that constant volume process at the top of the mountain?  The copper will absorb 33 kJ, the water about 178 kJ, and the air about 0.08 kJ.  You can see the water takes much more heat than the copper, while the air is absorbing a negligible proportion of the heat.  Your 500 watt heating element provides 500 J/sec, or 0.5 kJ/s, so requires about 7 min to get everything to 100 deg C.  Obviously plus or minus a bit depending on the actual mass of your boiler and water.  The element is surrounded by the water, and air is only heated via the water and top of the boiler shell, so does not really have any effect on  heat transfer.  Of course, if you leave the plug loose, air will escape as the heating progresses, but more importantly, some water vapour will escape with it.  Now, heating 1 kg of water from 15 to 100 deg C requires 356 kJ, but evaporating this 1 kg to steam requires 2676 kJ.  Remember I assumed the boiler only contained 0.5 kg.  You can see it takes much more heat to evaporate water than simply to heat it.  The escaping steam will take away with it much more heat than the energy absorbed by the tiny mass of air.  Clearly not the way to go.  Please also remember that while the water expands on heating, and so does compress the air, the volume change is tiny.  You can use the steam tables to find the difference in the specific volume of water at each temperature.  You will see the compression is negligible.  Similarly the steam does not compress the air.  The steam and air molecules occupy the space essentially independently, and the measured gauge pressure is the sum of the separate pressure of each component.  The air pressure does increase, but due to its increase in temperature, and the energy is accounted for in calculating the heat absorbed by air.

Now those calculations were based on raising the temperature to 100 deg C.  At this temperature, the water vapour pressure will be 101 kPa, or I atmosphere.  The air will be about 130 kPa, giving a total of 230 kPa(absolute) or 130 gauge pressure.  We could start releasing the steam air mixture, however the pressure will rapidly drop as the air escapes.  Not easy to see, as we expect the pressure to drop when some steam is let out to the engine.  And at 100 deg C we will then only produce steam at atmospheric pressure, not real useful unless we have a condenser.  But we should also be aware that while air and steam are both present in the boiler, the pressure gauge will show a higher pressure than you would expect from the steam tables.  If necessary, the safety valve will release a little, or if you open the steam valve, the engine will start but rapidly slow down and possibly stop.  About another 4 minutes will raise the temperature of water plus boiler to 150 deg, so somewhere in that 4 minutes, you will start getting enough steam to run your engine, especially if it is unloaded.

The experiment is not difficult to try, so it is worth checking the time to raise steam with the plug loose, and comparing the time to raise steam with it tight from the same temperature.  The time consuming bit is waiting for the boiler to cool between trials.  Possibly only practical to do one or two tests each day.  A brief break in machining time on separate days is probably more efficient than waiting for it all to cool.

You could of course compress the air in the boiler with a bike pump.  This would give an initial pressure reading that might imply you could open the steam valve.  If you have a suitable fitting, try it.  However, my guess would be that the engine will run for a few seconds while the air pressure reduces, and you then have to wait for the heat input to get the water up to temperature.  I would not recommend spending time making the fitting unless you want it for other purposes.

It would be interesting to know the actual mass of your boiler, and the quantity of water you use to fill and you normal heat up time to compare with the calculation.  But the more important question is whether you can make the boiler produce steam a bit quicker.  You can see now where the heat goes, and how much is required.  Your vibration idea, I assume is with the thought of increasing the heat transfer.  I think I may have already answered this one, the calculation did not need to know the heat transfer coefficient, only copper specific heat, mass of water and copper, and steam tables.  You heating element only provides so much energy, 500 watts, and just gets as hot as it needs to, to achieve adequate heat transfer.  I suspect the heating element insulation is the main limit, but if there is any effect of vibrating the boiler during heating, it is only to reduce the temperature of the element.

So is there anything that can be done to speed up the process?  Well, first we could reduce the heat required, by starting at say 80 degrees instead of 15 deg.  This is relatively easy if you fill the boiler from a boiling kettle instead of cold water.  You might go a step further by pouring in some boiling water, letting it sit for a few minutes to heat the copper, then tipping it out and refilling with water from the freshly boiled kettle.  However all of this takes time, so you would probably not achieve much difference in time from just filling with hot water in a single step, may even be slower.

You could increase the heating element power output by using a higher voltage.  Power output is just as sensitive to an increase in voltage as to a low voltage, so with a variable transformer, you could increase the voltage and shorten the heat up time.  I do not recommend this.  It would work, but you would run a real risk of exceeding the temperature at which your element burns out.  Better to stay within the rating.  However, it's worth making sure that your power cords have the current carrying capacity, so that you are not working on low voltage.

The next trick involves modifying the boiler, or making a new one, to use an element with a higher rating, or even two elements (which you connect in parallel) so you put in the required energy in a shorter time.  Probably the best solution, though a long power cord will need an even heavier rating.

I think that leaves only some outstanding questions from Paul that have not been addressed.  I will have a go at those next time.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 06, 2017, 01:37:59 PM
Thanks for that, so we use copper as it is easier to work with and join together with silver solder, and also it is traditional and not too prone to deterioration. Could one however line the inside with a stainless steel sheath the ends as well as the tube, or even plate it with something ,cadmium or nickel ?? Just thinking about this !! Also in my boiler there are two 500 watt elements actually. I was thinking with the vibration that this would help the release of the bubbles a bit quicker? So, more questions .......and will they ever end !! I do find all this quite fascinating actually and of course this is why with modern engines of all sorts there are so many additional bits and pieces that fill up the engine compartments and spaces around the engines.!!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 06, 2017, 04:37:46 PM
This is the video of this boiler from a few years ago    https://www.youtube.com/watch?v=63c9KR0bqb8  
Title: Re: Talking Thermodynamics
Post by: Maryak on August 06, 2017, 11:03:49 PM
Hi Maryak, thanks for that explanation.  I wonder if optimising the flow for that turbine might be one factor in the choice of crank angles for the Titanic, while for more typical engine arrangements, exhaust pulsations would not be so important so other factors such as balance can be given more importance.

MJM460

I think that may well be the case. I suspect that at cruising speed the LP inlet and exhaust pressures would be higher than without the turbine.  Based on a total HP of 51000 and the difference in propeller sizes I estimate the turbine would be producing some 13000 HP of the total.

Regards Bob
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 07, 2017, 02:01:35 PM
I have been intending for some time to address Paul's questions from post #208, so this time I will start with those.

So what about the effect of jacketing the cylinder, and even supplying the jacket with much higher temperature steam.  To understand what the jacket heating will do its best to first understand the basic engine cycle and what happens to the cycle if we add or remove heat during the cycle.

Generally, the steam engine is analysed as four processes which are repeated every revolution.  First, adiabatic expansion, then constant volume cooling, third, adiabatic compression and finally constant volume heat input.  This is referred to as an ideal cycle, and the thing most ideal about it is that it can be easily analysed.  Now any real engine is not very much like this, but the ideal cycle does provide a useful basis against which we can compare the performance of our real engines.

Now if we look at the effect of unintended heat input or loss around the cycle, we can see that during the expansion process, we initially approach constant pressure process which requires heat input.  The heat supplied by the incoming steam far outweighs any heat input or loss through the cylinder walls, which can reasonably be ignored.  Then after cutoff, heat input or loss causes the pressure to fall slower or faster than we would expect based on adiabatic expansion.  Heat input means the pressure falls less quickly, so more work output is achieved, so this is beneficial, just difficult to analyse.  Obviously heat loss means faster pressure drop and less work output.

But let's continue around the cycle.  At the end of expansion, we release the remaining steam and associated heat into the exhaust.  It is not entirely constant volume, but continues into the exhaust stroke.  While you are exhausting steam, any heat input is increasing its pressure so is undesirable.   

Next the ideal cycle has adiabatic compression.  In real engines, this occurs only after the exhaust valve closes, but again heat input means that this process occurs at a higher temperature, and this requires more work input, and so again is a reduction of work output.

Finally we have near constant heat input as the steam valve opens and admits more steam.

I don't know if I have painted this clearly enough, but the summary is that during the downstroke, any heat input is an advantage, during the return stroke, heat input reduces the cycle output.  As it is not practical to switch the heating on and off each cycle, any advantage during expansion will be reduced but probably not eliminated on the return stroke.  Similarly, any heat loss is probably more significant on the expansion cycle than the exhaust so there is a net loss.

It is very hard to quantify these effects.  Certainly, the extent is limited by the small surface area available for heat transfer.  The example of full size practice which seems to be to apply lagging suggests there is a slight advantage to insulation, but not enough to justify the complexity of installing and supplying steam to the jacket.  An even higher temperature would give more temperature difference, but again the extent is limited by area, and it would probably be more effective to apply the heat in the boiler or superheater so the supply steam is hotter.  Then the heat is only supplied to the engine when it is needed.  But if a jacket is supplied with steam at about the inlet temperature, this comes close to the same thing as perfect insulation for the enclosed cylinder.  Obviously the jacket itself then needs lagging.

I hope that is enough to satisfy your curiosity for now.

Now on to Willy's question about the stainless steel cladding of the boiler.  First Willy, I have to ask why you would want to?  I know you can silver solder stainless, but I am not sure of the strength of the resulting job.  If you can join it by welding, stainless steel gives a boiler which has higher strength at high temperature than copper, but it is very susceptible to cracking if there are even tiny amounts of chlorides in the water.  These come in even good potable water, and are very hard to eliminate.  Hence the incentive to try and use duplex stainless steels.  And the thermal conductivity is less than copper, so it's main advantage is high temperature strength.  Copper is better for heat transfer.  I am not sure of the effect of plating on silver soldered joints, but I am sure I have seen model boilers that were externally plated for appearance.  I suspect it even has an advantage in lower emissivity that copper that has lost its shine, so less radiant heat loss.  However a simpler approach is insulation.

Steam bubbles detach very readily, and lead to very high heat transfer coefficients for boiling, so I suspect there is no real advantage in vibrating the boiler.  As I mentioned earlier, the element insulation is probably the main determinant of heat transfer rate, and the consequent element temperature necessary to dissipate the heat.

Having 2 by 500 watt elements means twice the heat input, therefore half the heat up time, and twice the steam production.  It is still necessary to know the mass of copper, and the mass of water to calculate the time more precisely.  It would be interesting to know how long your boiler does need to raise steam.

Thanks Maryak, I think that you are right about those lp cylinder pressures with and without the turbine.  The total pressure range tends to distribute itself over the number of stages, and the turbine is effectively an extra stage when it is on line.  But it depends on valve events and steam chest volumes.  Do you have any more information about that aspect of the triple expansion engine?

Thanks everyone for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 07, 2017, 03:20:11 PM
Hi thanks for the comments and questions ,I shall endeavour to measure the actually size and water volume and times to steam up soon. Talking about steam jackets the engine i am making ,Woolf Compound by Wentworth and sons is a steam jacketed engine that has a bit later sister engine that is still working. The Ramm Brewery engine is steam jacketed as well but does have wooden cladding although not around the steam chest parts of the engine. In the Beeleigh engine there is no cladding and also no indication of how it may have been attached, ie no threaded bolt holes to secure it. This system of jacketing on this double acting engine means the cylinders have no provision for steam/ condensate removal !! so one assumes that the engine block is first brought up to temperature with the valves in midway closed position, then once the engine block is fully hot the condensate is drained off by a drain cock at the bottom of the cylinder block. the flywheel is then barred into the starting position and the cylinder steam valve opened. This enormous mass and area of cast iron must have been responsible for lots of conduction and radiation of heat continuously during its working cycles !! here are some pics of the two engines, The Beeleigh engine hose burnt down in 1875 so the wooden cladding may have rotted away over the last 140 years
Title: Re: Talking Thermodynamics
Post by: Maryak on August 08, 2017, 01:16:09 AM
Thanks Maryak, I think that you are right about those lp cylinder pressures with and without the turbine.  The total pressure range tends to distribute itself over the number of stages, and the turbine is effectively an extra stage when it is on line.  But it depends on valve events and steam chest volumes.  Do you have any more information about that aspect of the triple expansion engine?

MJM460

Sorry, I can't pin that down. The information I have been able to find gives conflicting results as to power. i.e. one source suggests 48000 HP with 16000 provided by each of the 3 engines another says total HP 51000. There is information regarding the speeds of the triples at 76 rpm and the turbine at 9psi inlet and 165 rpm. Some of this seems counter intuitive when comparing propeller locations diameters and speeds.

Based on my experience with air pumps in tip top condition a condenser vacuum of 26" Hg is the best I have seen in cold seawater at economical cruising speed.

To give a 9 psi LP exhaust I estimate LP inlet at 24 psi. This suggests that with the turbine out of the loop LP exhaust would now be around 20" Hg and LP inlet around 9 psi, all of this is assuming no linking in or out i.e. line in line valve settings, (which with a standard way shaft is always the case when going astern).

The condensers would now be "pulling" steam through the engines and the 4th stage expansion would be lost. I suspect CW pumps would also need to be operated at higher rpm to cope with the additional heat arriving in the condensers.
Title: Re: Talking Thermodynamics
Post by: paul gough on August 08, 2017, 01:46:33 AM
Thanks MJM for considering and running through what is going on with a 'heated cylinder'. I am pleased you did not think it an altogether flippant enquiry. I have been interested in whether attempts to keep cylinders 'warm' in a steam engine is/was of any significant value in practice, especially for models. Thus trying to understand what might go on in an extreme situation can deepen understanding. As I am primarily interested in locomotives it is, (usually), to these I address my investigations. All this line of inquiry was prompted by noting that the full size Lion locomotive, (Titfield Thunderbolt), had the top of the inside cylinders forming the base of the smokebox so the steam chest at least received some heat from the flue gases, but maybe the cylinders also got a bit warmer on a longer run?? As the Lion would, I presume, have been a coke burner like most locos, (railway not colliery lines), before 1850, there might not have been too much soot/ash build up to create an insulating layer on top to impede heat transfer. So I wondered if any noticeable benefit might have occurred beyond the obvious hopeful reduction in condensation formation due to the locos maximum boiler operating pressure of 50 p.s.i. I also notice on my drawing of Stephensons Patentee loco it had the cylinders and steam chest, except for cyl. covers, totally enclosed in the smokebox, presumably it would have achieved a greater heating effect on the cylinders than Lion. Regards Paul Gough.

 
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 08, 2017, 01:33:46 PM
Hi Willy, interesting that those engines had steam jacketing.  I suspect the boiler pressure was not high, and probably minimal if any superheat, so minimising losses was probably even more important than for a modern engine.  I suspect that the tapped opening you have arrowed is the condensate drain for the jacket.  Today it would have a steam trap on it, but I don't know if they had been invented then.  The engineer probably had to regularly drain the jacket by opening a valve briefly.  The engine could be cleared of condensate by barring it over when it had heated well, before opening the steam valve.  It is great that you have two contemporary engines to help you work out the authentic arrangement after so much time has passed since they were new.

Hi Maryak, I wonder if the 165 rpm is actually the turbine speed or is it the propeller speed?  Modern turbines would certainly be much faster for good efficiency, and use a gear to give the slower required propellor speed.  Certainly the engines would have been designed to be optimum when going forward with the turbine in operation.  Then operation in reverse without the turbine would be an off design condition, where lower efficiency would be acceptable.  I also assume you would not often if ever need full power in reverse.  When the lp cylinder is connected directly to the condenser, it would of course have higher than normal power due to the higher differential pressure,  but I suspect there would be some ripple effect through the ip and hp cylinders which might each operate under slightly modified pressures.  Usually a multi stage engine design aims to have roughly equal pressure ratio in each stage, and equal pressure ratio gives roughly equal power, so your figure of 13,000 HP for the turbine out of a total of 51,000 looks about right.  It would mean about 19,000 HP for each of the triple expansion engines.  I guess further explanation will have to wait until we have more information.

A vacuum of 26 inches of mercury, means an absolute pressure of only 13 kPa.  I assume that 9 psi is a gauge pressure, so absolute pressure of 163 kPa.  This means a pressure ratio of 12.3:1 for the turbine, enough for two extra reciprocating stages, but they would be impractically large.  However, a turbine can handle the volume of steam easily in a moderate size machine, even if with lower efficiency.  A really ingenious arrangement for good fuel efficiency.

No problems answering questions, Paul.  I think this thread probably more useful when it addresses the questions people are wondering about, rather than some subject no one is interested in.  It is interesting that heating or cooling of the cylinder is an asymmetric problem.  We know that heat loss is detrimental, but we can reduce heat loss until it is hardly significant by insulation, which has low conductivity.  We can even reduce heat loss to zero by steam jacketing, so the cylinder has no temperature difference to drive heat transfer.  I presume we would also lag the jacket to minimise the steam requirements. Interesting that insulation works by addressing the heat transfer coefficient, while jacketing addresses the temperature difference.

However, it is more difficult to drive this further by adding heat to the cylinder to increase the work out.  We can use high temperature in the jacket, though we probably don't have a steam source at higher temperature than the engine supply.  The size of the cylinder severely limits the area available for heat transfer, so it is difficult to achieve much heat input.  Compare the geometry and temperature difference with that of the boiler to get an idea of the relative magnitudes.  If we had a higher temperature steam available, we would probably be better to supply it to the engine directly, rather than try and drive it through the cylinder wall.  Even a high temperature source such as the smoke box is limited by the film coefficient of the gas to metal, so a high temperature heat source such as the smoke box probably does not achieve much more than a standard jacket.  However it has a great advantage in simplicity, no moving parts or extra connections, and in fact difficult to see how it could be avoided on a typical locomotive.

Now I have introduced power and efficiency this time, so next time. I had better define those terms and their units of measurement.

Thanks for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 09, 2017, 01:24:47 PM
For the dinosaurs amongst us

Came across some dinosaur foot prints near the road today.  Well not really near, more like 110 km off the road really, a little detour from the long paddock, but interesting to see a moment in time captured by nature, the time of a stampede.  And not found unaided of course.  The palaeontologists told us where they were, and how they solved the puzzle, and how this moment is still preserved for the future.

Very early in this thread, I looked at the SI metric system and the definitions of force and work.  I have avoided introducing other terms as long as I could, because in the explanation of how engines work, I wanted to concentrate on what work is, and it's definition and how any engine turns heat into work.  No fundamental difference between big and small.  However it was necessary to talk about power in the discussions on the engines of the Titanic, so I had better define it.

If you want to lift a mass of 1 kg to a height of 10 metres, you need a force of 1 kg x 9.8 m/s^2  = 9.8 N.  The distance is 10 metres, so the work required is 98 N.m.  Now any size engine can, in principal, do this.  My tiny single acting Mamod engine an with sufficient gearing and given enough time, can do the job, though I admit that it would have to be very good quality gearing with near perfect bearings so that friction did not require more work than the lifting task.  On the other hand a full size ships engine would not even notice if this task was added to its normal load, and would do it in the blink of an eye.  While the amount of work done is the same in each case, there is clearly a difference in the engines.  This difference is power.  So what is power?

Power is defined as the rate of doing work.  Alternatively, it is the amount of work done per unit of time.  The unit for measurement of power is the Watt, which is defined as one Newton.metre per second.  You can see a Watt is the power required to do one Newton.metre of work in one second.

The power developed by engines we are familiar with varies over quite a wide range.  The little Mamod engine I have already mentioned would be measured in thousandths of a watt, or milliwatts.  Similarly, many low temperature difference Stirling engines would be measured in milliwatts.  For most of my life in making model boats, 10 W was considered almost unattainable, certainly for mere mortals.  How things have changed with the advent of better batteries and brushless motors.  A typical sewing machine motor, or a mini-lathe might be 150 W.  My lathe motor is about 1 thousand watts, or one kilowatt, or kW.  Our cars might be a bit less than 200 kW, while formula one, I should know, but it escapes me for a moment, come in Jo, can you help us here?

The compressors used in automotive service centres for tyre inflation are around 5 kW, while the ones I am familiar with from my working career range from about 600 kW to 20 MW, 20 million watts, 20 megawatts.  I am sure that you can all think of others, larger and smaller.

Now we sometimes see an engine described as "powerful".  We need some understanding of what this might mean.  We have on this forum, an amazing thread by strictlybusiness on a 0.9 cu. in. engine developing 7.2 HP.  Amazing for a small engine, for a model aeroplane or boat, but not very impressive for a Formula one race car.  Unless you mean 7.2 extra HP, of course, when in that context it would probably be considered a major breakthrough.  None of which takes anything away from the incredible feat of getting so much power out of such a tiny engine.  Similarly, a formula one engine would be no use in a model in the usual size range.  So, is there a measurement which allows us to compare our model engines despite the differences size?

I suggest that when we say an engine is powerful, we actually imply "powerful for its size", so we need a meaningful measure of size to convey the idea with any precision.  We could use some representative length, but the commonly used measure is the mass of the engine, which in the SI metric system would be in kilograms.  If we describe an engine as producing so many watts per kilogram, we would have a measure, generally referred to as power to weight ratio, that is meaningful for a wide range of engine sizes.   Power to weight ratio has dimensions (W/kg in the metric system), so the numbers will depend on the units you are using, you need a conversion factor if you want to compare W/kg with HP/lbm for example which would be the equivalent in imperial units.  Power to weight ratio is an important consideration in comparing engines for a specific application.  For an aeroplane, for example you need a high power to weight ratio, but can afford to check and maintain it often.  A steam engine with its boiler, and condenser is at a severe disadvantage for this application.   For a national grid electric power generation, weight is just not an issue, you put much more priority on reliability and low fuel and maintenance costs.  The turbines used in a typical national grid power station were never meant to fly or move in any manner at all, but they reliably operate 24/7/365.

So to get back to Willy's question on the horses power of his engine, first we had better look a bit closer at the commonly used units for power.  I have already mentioned the metric unit of power is the Watt.  Interesting that this watt is exactly comparable with the electrical unit of Watts, calculated by multiplying volts by amps (by power factor if the voltage is AC).  I never have quite understood  how the definitions of volts and amps could come up with such an elegant result.  Or is the definition of the unit of resistance (ohm) the secret?  In imperial units, power is measured in Ft.lbf/second.  However we normally use the unit horsepower, first defined by James Watt, I believe.  The horsepower is defined as 550 ft.lbf/sec or 33,000 ft.lbf/min. *  I have heard that James Watt must have had a very large horse, as the figure is apparently a bit high for normal horses, however, it gave a basis for comparing the power of his steam engines with the contemporary technology, which at that time was horses.  Fascinating that Willy's engine name plate specifies the power in horses power, but the concept of the engine being able to match the power of a number of horses makes sense for his advertising purposes.  It obviously later became modified to horsepower. 

We can now see that to calculate the number of horses power of the engine, we need to know the pressure in the cylinder and the piston diameter so we can calculate the force in lbf exerted by the piston rod.  We then need to know the stroke length in feet, so we know how much work is done by each stroke.  Finally we need to know the number of strokes per minute, or hour that the engine completes.  So we multiply force by stroke length by strokes per minute and divide by 33000 * to get horses power.  And because electrical watts are watts in both imperial and metric systems, we all know that one horse power is equal to seven hundred and forty six watts.  (1 HP= 746 W, or 0.746 kW)

Next time I will,talk about efficiency, which surprisingly is a little more complex, as there are many definitions in common use.

I know the thread needs pictures, but unfortunately dinosaur footprints do not photograph very well. However, Mr Google is quite good at finding dinosaur stampede photographs that are way better than I was able to take.

Thanks everyone for reading

MJM460

* corrections 10/8/17 -  1 HP = 550 ft.lbm/sec
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 09, 2017, 02:24:31 PM
Hi MJM, I think they increased the Horses power rating by adding more weight to the flywheel and increasing the steam pressure ? presumably this would work. ?? On my BMW motor bike the R69 engine of 600 cc is rated at 35 horsepower., but the R60 engine of 600cc is only rated at 25 horse power !! I think the only difference is the compression ratio is higher for the R69 engine ??!! so does compression ratio come into the equation ?? also the R69 engine can increase the rpm by 1000 to actually go up to 100MPH. With marine and areo engines one can feather the propellers to give more speed/power so is this part of the equation as well ??  Sorry i am using the question key such a lot perhaps i could remove it !!!
Title: Re: Talking Thermodynamics
Post by: paul gough on August 09, 2017, 10:09:25 PM
Horsepower??? For the novice, a confounding species with too many breeds to choose from! Eg., Brake H.P, Indicated H.P., Shaft H.P., Boiler H.P., Drawbar H.P., etc, etc. Maybe a quick explanation why the need for various sorts is in order. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 10, 2017, 01:45:44 PM
A little more horsepower?

Oops, one horsepower = 550 ft.lbm/sec or 33000 ft.lb/min, and no one picked me up!  I have corrected yesterday's post, I hope no one has wasted too much time worrying about that one.  It would have been a quite puny horse.  My apologies if I have led you astray by this one.

Thanks Willy and Paul for your questions, please don't remove the question key, either of you.  Your questions prompt me on areas where people need a bit more clarification, and I am sure that you are not alone with these questions.

On the power of those two ancient engines, the increase in steam pressure does increase the force on the piston rod, hence the work done on each stroke.  Increased work per stroke at the same number of strokes per minute means more power.  So increasing the steam pressure would have increased the power output.  Depending on the load, it may even increase the number of strokes per minute.

The flywheel however does not produce any power.  It simply stores energy when the engine tends to speed up, and returns power when the engine tends to slow down.  Remember when I looked at the torque produced by the force in the piston rod?  It rises to a peak, and falls to zero twice every revolution.  The engine accelerates under the higher than average torque, and slows when the torque is below average.  Extra weight in the flywheel is a disadvantage because it increases the bearing loads and hence losses, but it is a necessary side effect of adding extra moment of inertia.  I would assume they added the weight to the rim where it has the most beneficial effect.  However, increasing steam pressure increases that peak of torque twice each revolution, while zero is still zero, so increasing the power by increasing steam pressure increases the speed fluctuation of the engine, not usually desirable.  It is also possible that the speed fluctuation before the modifications was already higher than desirable, thus increasing the necessity for increasing the flywheel inertia when the steam pressure was increased.

On your modern BMW engines, I assume twin cylinder, those are beautiful machines.  I don't know the specific details of the engine, but in principle you are quite right, increasing the compression ratio does contribute to increased power output.  It effectively increases the mean effective pressure, so increases the torque produced by the engine.  However there are many other aspects to tuning up, or hotting up an engine.  Some of the things that can be done include grinding the top of pistons so each has the same compression, and grinding the bottom of the piston skirts to make the two the same weight.  I have it on good authority that checking the pistons at top dead centre and making them within 0.001" of each other and grinding the bottom so the weights are within 0.1 gm makes a significant difference for a street machine, but not even close to good enough for the race track.  Paying this level of attention to bearing clearances, and every other aspect of the engine construction will significantly increase its power with otherwise standard components, and will increase the maximum rpm.  Modification to cam profiles, valve spring rates, port polishing and even different air cleaners can all make a difference, especially if you have access to a dynamometer to help you see the differences.  Not really necessary to do anything heroic to make quite a difference.  However things which increase torque are the most desirable, as torque gives acceleration, while higher top speed only loses your licence, unless you confine yourself to the track of course. 

Feathering an aeroplane propellor does not change the power output of the engine, rather it changes the load.  This allows the engine to run at the optimum rpm for the immediate task.  When climbing, maximum power is required and a certain pitch will allow the engine to run at the right rpm to provide maximum power.  Slowing down and descending requires less power, but purely reducing rpm is not necessarily good for fuel efficiency.  A lesser pitch will allow the engine to run at the optimum rpm with much lower fuel consumption.  Perhaps flyboy Jim (or we may have other pilots on the forum), can elaborate on this one. I hope that makes it a bit clearer.

Paul, thanks for pointing out the different ways in which power is measured.  I have simply provided the definition, 1 Newton.metre per second = 1 watt, however everyone wants to claim that their engine is more powerful than the competition.  There is kudos in having the highest power engine, size does matter, it seems.  The engine maker does not want to have his figures compromised by what is added after the engine is sold, while the end user only wants to know if there is enough power available to drive his generator, propellor or compressor, or just make his car or bike go faster.  So everyone measures the power in the way that best suits the vested interest.  Let's have a look at some of the common terms, and how they reflect different measurement locations.  Remember above all they nearly all use the same definition of one horse power = 550 ft.lb/sec, or 1 watt = 1 Newton.metre/sec.  Of course horse power is a specific  imperial unit, in SI terms, is is usual to just refer to power.

First the one exception, the boiler horsepower.  It is actually defined in terms of raising a defined quantity of steam at 0 psig and 212 deg F, from water at zero psig and 212 deg F.  It is approximately 33475 Btu/hr or 9811 watts.  This rate of energy transfer is about(?) 13.1547 times the equivalent of the mechanical horse power definition.  It is not a measure of mechanical power, but intended to be an indication of the size engine the boiler would be suitable for.  A nice lead in to our discussion on efficiency, as it implies a steam plant efficiency of about 7.6%.  So hold that idea of efficiency, but otherwise it is not really a useful concept.  Better to define a boiler capacity in terms of rate of steam production at a specified pressure and temperature, and match it to the engine specifications.

For a given engine, the indicated horsepower can be expected to be the highest.  It is calculated from the area of the indicator diagram which we looked at earlier in this thread.  It gives a measure of the power provided by the action of steam on the piston.  It does not allow for friction in the rings or bearings, or the power required by the feed water or lubrication pumps, or condensate pump,or air pump.  So a nice big figure, also useful when looking at and optimising valve events, but does not help much in assessing the size of generator the engine could drive, or the power available to drive the ship or aeroplane propellor.  Again, some of the marine engineers might like to come in and explain a little more about how they use these diagrams.

Brake horse power and shaft horse power are very similar.  I am never sure whether there is actually any difference in the numbers.  Brake horse power is the term used in the engine is tested on a brake, an artificial load used for the specific purpose of measuring the engine output.  Shaft horse power is I believe the same number, but the term is used to refer to the power available at the output shaft to drive the end users load.  In each case there is room for some ambiguity, and in specifying an engine it is preferable to separately list the power consumed by engine accessories, such as oil pumps, cooling water pumps, and radiator fans and specifically identify the available power after allowances for all necessary accessories.  There is some justification for the engine makers desire to not include all these loads.  In many industrial applications there are good reasons for driving these accessories with electric motors, thus leaving the entire engine output available to drive the intended load.  And of course, they are all measured with the engine new and clean.  Usually, power output gradually declines for ever afterwards, depending on the quality of maintenance.

Drawbar power is useful for prime mover applications and might apply to a traction engine as well as a locomotive.  A typical traction engine requires a considerable amount of power just to move itself.  A prospective purchaser who wanted it to pull logs out of the forest, or a plough or a trailer wants to know how much power is available for this purpose.  It would indicate the drawbar pull and towing speed.  The engine efficiency competitions run by some clubs for locomotives actually measure draw bar power and compare this with fuel consumption to calculate efficiency, another neat introduction to that topic.  If the engine is used as a stationary power supply, say for a saw mill, the power available would be much higher than the drawbar power.

That probably enough for now, does anyone have any other definitions of power output they have come across?

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 10, 2017, 02:05:44 PM
Hi MJM, thanks for the reply there is lots more to increasing Horse power than i thought......also i can give you another definition of power output from this little booklet i have in my possession   4 Man Power !!!!! thats a new one on me !! however it is in print so must be true !!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 10, 2017, 02:22:03 PM
Hi MJM ,just looked up Bisschof gas engines  So can be run by any boy or girl !! the beginning of girl power then !! Also engines available in  ! man and a half power !! Brilliant stuff.........
Title: Re: Talking Thermodynamics
Post by: simplyloco on August 10, 2017, 02:42:31 PM
Many many years ago, aspiring race mechanics like me were not only matching pistons in our Austin Mini's, we were balancing up the combustion chambers by grinding them out and measuring each volume by introducing water with a pipette through a tiny v cut in the edge of a piece of perspex placed over each chamber in turn. I'm not sure it went any faster but it felt good! Before that I skimmed the heads of my Ariel Arrow 2 cycle bike and fitted padding in the crankcase: now that DID go better!
John
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on August 10, 2017, 03:02:44 PM
MJM,
Some nameplates have a continuous and max or peak HP rating. If you google horsepower litigation you will find the current legal wrangling. There are cases that go back to the time of James Watt. The invention of the steam engine indicator was kept secret for many years and is first mentioned in a court case about engine horsepower. You can not always take nameplate or catalog data as true information.

Dan
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 11, 2017, 01:00:25 PM
Horse power or man power

Great examples of different ways of rating engines.  It shows ingenuity to the fore in the days before standard measurements, and manufacturers tried various ways to convince potential customers that theirs was the one to buy.  But of course, there was no standard for power measurement, so all those terms could not be precisely compared, until James Watt defined the horsepower.

Interesting that boys and girls were treated as equal in that add, Willy, pity it all seems to have gone down hill from there.

Hi simply loco, great to have you on board.  I think smoother running resulting from better balancing of weight, compression ratio, and fuel mixture always results in an engine being able to run faster, as there are losses associated with rough running.  And vibration is a reasonable criteria for calling it the maximum power.  But the tuning tricks you learned with those activities have a long and honourable tradition.  And very rewarding to my way of thinking, what ever directions your career took later.  Unfortunately my time was spent in other areas, so I never got to learn some of those things. 

Hi Dan, modern engines still have a published maximum or peak rating, as well as a continuos rating.   Suppose there is always something that limits the maximum power output from an engine in an absolute way, but most engine ratings are determined by the criteria you impose.  A formula 1 race engine is expected to run at the maximum power possible for a bit over two hours then the mechanics get to restore it for next time.  Even then, I understand they will get up to tricks like disconnecting oil or water pumps for a little extra power on the road for those critical final timing laps.  They know just how far they can push it without damage.  We expect our personal cars to last much longer between servicing, and expect the servicing costs to be more moderate.  So continuous operation might be limited by some critical temperatures, perhaps exhaust, water or oil temperature.  If you exceed this limit, you will shorten the life of the engine, but you can exceed it for a short time, occasionally, with minimal effect on reliability.  And it's not totally unreasonable, even in your car you want to be able to put your foot down to get to the top of a hill, then are happy to coast down the other side while the engine cools a little.  Car engines are rated nearer peak power, which cannot be used continuously, while my industrial machines had to run at 100% load continuously.

I think all those legal problems can be traced back to someone seeking advantage.  The manufacturer exaggerates his claims, while the purchaser wants to pay the least even when it means buying a smaller engine, so in a way they collude with each other, and it often ends with tears.

I guess catalogue and nameplate rating are at best a reasonable average for similar engines, but even when you specify a test run and certified test results, the results really only apply to the specific conditions of the test.  And of course the test is at best only a few hours, and no real indication of how long it could run at those conditions.  Most of my clients in industry expected the engine on the test stand to demonstrate at least 10% more power than that required by the driven machine, which was also demonstrated on the test stand.  But they then expected the equipment to run 24/7 for three years or more.  Not so hard for a centrifugal machine where there is really only one moving rotor, and hydrodynamic bearings mean nor rubbing or wearing parts, even seals are separated by a gas film.  But quite a feat for a large reciprocating compressor.   They weren't portable by any reasonable assessment.

No new topic today, I will be entering tin can and string territory for communication for the next week or ten days on the long paddock, so I don't expect to be able to make regular posts.  I will check in when a freak of the atmosphere, together with adequate solar power enable it.  I believe it usually involves standing on the left foot with your right elbow in your left ear while you hold the phone high over your head pointing in the precise direction of the nearest tower which is beyond the horizon.  Or something like that.

So thanks for following along, keep thinking about thermodynamics of your engines and the regular discussion will return in about two or three weeks, rather than try to continue through odd unpredictable times in that period.

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on August 28, 2017, 07:24:12 AM
MJM...

We know you are out in the back blocks of OZ somewhere :shrug: ...& probably with little or poor e-mail conductivity  ......

However just wondering....should we send out a search party?   :LittleAngel:

Derek

PS....I am in Adelaide for a few weeks...& called into a Hotel called the Thirsty Camel.... I called into the bar...yelled out your name but no one answered so guessed you were not there :drinking-41:
Title: Re: Talking Thermodynamics
Post by: Stuart on August 28, 2017, 08:21:12 AM
The reference brings a smile to my face

I have posted this before, but I did armature and stator winding as part of my apprentiship

Ok a job came across from the fitters name plate said 5 up so I looked in my book for Royce 5hp not in there but a 25 hp was so of we go take all the details , they were the same as the 25hp so I called over my mentor to check , his reply was yes they are all the same in that frame size they are all 25hp but if it 3as ordered as a 5hp the factory just stamped the nameplate as such

So it’s a case of caveat emptor

Keep up the lessons always a good read
Title: Re: Talking Thermodynamics
Post by: Steam Haulage on August 28, 2017, 09:01:51 AM
First the one exception, the boiler horsepower.  It is actually defined in terms of raising a defined quantity of steam at 0 psig and 212 deg F, from water at zero psig and 212 deg F.  ? ?

I don't understand.

Jerry :old:
Title: Re: Talking Thermodynamics
Post by: Stuart on August 28, 2017, 11:18:11 AM
Jerry
That’s the energy required to change the state eg. From liquid to a gas (steam) without raising the temp.
It works the other way as well when the said gas is converted back to a liquid it gives up that energy.

The same goes to convert liquid to a solid you remove the energy

In simple terms that’s why your fridge gets hot at the back and cold inside , note you do not cool a fridge you just transfer the heat from inside to out side


Here is a couple of horse power to ponder BHP ( brake horse power  ) measured with a dyno on the output shaft note this is not the input power

Then we have nHP.  Notional hp it’s a steam equivalent form the days of yore

Just remembered some more

The old time RAC hp rating for cars this was not the hp (power ) but the engine size. Eg 12hp was a 1200cc engine

Then you had the hp of a stationary/portable  steam engine this how many good horses it took to move it to its place of work again back in time

But for this old person  :old: ( yes I do look like that inc stick ) 1hp = 746 Watts period

Soory to muddy the waters
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 28, 2017, 01:03:40 PM
On the outer Barcoo-

Back in real internet territory with regular capacity, so it is obviously time for a bit more thermodynamics!

Thanks everyone for the continuing interest.  Derek, I haven't seen any camels, thought you might have guessed a bit closer.  I am holding off the explanation until the story is complete, but I will get there.  Thanks Stewart for both your contributions.  You may be interested to know that modern outboard engines are rated in a similar way to your 5.  If you order the lower power you get a lower price, but I believe the only difference is the jet in the carburettor.  Welcome aboard, Jerry, Stewart has explained that the definition is only about latent heat, it does not even include the heat necessary to raise the water temperature to the boiling point.  It was a definition introduced in about 1876 and refined by 1884 to give buyers an idea of what size engine the boiler was suitable for.  A little more on that below.  Stuart, I suspect your definition of the horse power of those stationary engines is a little tongue in cheek, but it reminds us of how many tones of fire breathing monster was necessary to replace a horse, and the numbers were possibly similar.  I guess wood was cheaper and more readily available than hay.  I also prefer to confine my use of the term horse power to the precise technical definition of 746 watts, or even better 746 J/s, remembering that one J is one Newton metre, so directly related to the rate of doing work.

Last time we were talking about power, and units for its measurement.  And we introduced the term efficiency as the topic for this time.  Efficiency is a term which, in addition to many common uses has a specific technical meaning.  Well, it should have a specific meaning, but in the way of those wanting to sell products, even the technical meaning gets modified so there are several variations.

The technical definition I prefer is more specifically called the overall thermal efficiency.  This refers to the output power of the plant or machine as a fraction, or more commonly, a percentage of the energy in.  For a steam plant for example, we can measure the power output of the engine, or at least we can in full size practice with a well equipped test stand, and we can measure the fuel consumption, and use the fuel calorific value the calculate the energy content of the fuel.  Then simply divide the engine power output by the energy in the fuel, using consistent units of course.  We can then multiply by 100 if we want to express this fraction as a percentage.

Thinking back to the old definition of a boiler horse power, which was the horse power of the engine the boiler might be expected to drive.  One boiler horse power was defined as the energy input required to evaporate 34.5 lb of water at 212 F to steam, equal to 9811 watts.  Now if 9811 watts input is required for each horsepower of engine output, that is each 746 watts of engine output, we can divide 746 by 9811 and multiply by 100 to get an assumed plant efficiency of 7.6%.  Not very impressive really, but such was the technology of the time.  We must remember that in the way of the legal profession, this boiler horse power was not a guaranteed engine output, simply and expected output from some other manufacturers engine, and almost certainly defined the boiler operation and cleanliness.

What does 7.6% mean in terms of fuel consumption?  Well, we need to know the calorific value of the fuel.   Various standards and Google pages give the calorific value of coal as about 25.46 MJ/kg, or 25,460 kJ/kg.  The energy input of 9811 watts equals 9811 J/sec equals 35,319,000 J/hr or 35,319 kJ/hr.  Finally a simple division 35319/25460 yields 1.38 kg of coal per hour for each horse power.

I don't know how that compares with Sabino or modern coal fired merchant ships, but as the definition was finally agreed in 1884, I suggest it is nearer James Watts engines than the modern counterparts.  Does anyone have any figures for the performance of modern coal fired engines in terms of coal consumption and horsepower?

Next time I will look at what efficiency means for our little oscillating engine.

Now it's hard to think of a suitable picture, for this thread, but in the effort to make sure that we don't all turn into thermodynamics nerds, I am going to suggest a little culture, just to balance the technical content.  How about a little poetry?  If you went to the right school you will remember it, but a couple of verses should jog some memories.

On the outer Barcoo,
Where the churches are few,
And men of religion are scanty.
On a road never crossed,
'cept by folk who are lost,
one Michael Magee had a shanty.

Now this Mike was the dad
of a ten year old lad,
plump, healthy and stoutly conditioned
............

Thanks for looking in again,

MJM460
Title: Re: Talking Thermodynamics
Post by: Stuart on August 28, 2017, 02:37:16 PM
Yes I have suspected that the definition of the how many horses etc is suspect but I bet it was on someone’s catalog sales pitch at some time , but in the real world it would have given the farmer a idea how big that fire belching monster was

Stuart
Title: Re: Talking Thermodynamics
Post by: Maryak on August 29, 2017, 01:54:42 AM
James Watt decided that is was a load of 550lbf that a "good" horse could raise 1 ft in 1 sec. It was also acknowledged that it was deliberately set high!!

For me it is interesting how our measurement systems relate to the world  around those who defined them at a particular point in history. The only one which is universal or perhaps earthieversal is time and at the risk of inciting a riot it's one hell of a stretch to confer on it anything relating to increments of 10's.

 
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 29, 2017, 12:09:54 PM
More on efficiency

Hi Stuart, I think the industry was still talking about water pumping for mines when the term horsepower was coined as a unit of power, but when portable engines first came in, the ratings were not very high.  I suspect to set up a pair of ploughing engines required enough horses to ensure that they would continue to receive their hay for the foreseeable future.

Hi Maryak, it's good to have you still looking in, your contributions are always appreciated.  I did understand that James Watt's definition of a horse power implied a very strong horse.  I did not realise that it was deliberately so.  Quite the opposite to the normal sales hype which tends to exaggerate the engine output.  However it would tend to attract happy repeat customers who would be delighted to find that their 4 HP engine could actually do what they previously required five to do, and keep it up all day without needing a rest, even when the tubes were a bit sooted up.

I suspect that the earthieverse standard for time probably would be either a day or a year, though the day at least does vary a bit.  Twelve months makes sense in terms of dividing it into four seasons as in moderate climates or two for the tropics.  It can also be divided into three or even six, but the moon phases occurring roughly 13 times per year mucks up the logic a bit.  As you say, trying for ten months in a year, days in a month, or hours in a day does not seem to result in a nice logical system.  It does seem necessary to accommodate the one to three hundred and sixty five and a quarter ratio of orbits around the sun to rotations of the earth, which is not easy with base 10.  But we could have ten "hours" per day, each of 100 "minutes", again each of 100 "seconds" by adjusting the length of the second.  It would then involve a different strange number of vibrations of those caesium atoms.

I understand that the second as a unit of time was selected by an ancient civilisation, who used a base 60 number system and hence the 60 seconds to a minute, 60 minutes to an hour.  Base 60 has the advantage of having many factors so you can divide it by 2, 3, 4, 5 and 6, and also 10, 12, 15, 20 and 30 without needing fractions.  They did pretty well to measure seconds, though I seem to remember from physics that a pendulum with about a metre length has a period of 1 second.  But they were working all this out before the metre was defined.  Perhaps they had a unit of length that corresponded with a one second pendulum.  We can certainly measure time pretty accurately these days with the device on our wrist, but the time differences in Formula 1 and Olympic events make a second seem pretty large.  Though I wouldn't be ashamed to come second to Ian Thorpe by one hundredth of a second or so.  Yet no one can remember who came second except the one who did it.  And those atomic vibrations would still be necessary so we can measure the difference in time a signal takes to reach us from different satellites and calculate our position on the earths surface within 5 metres.

I still have to learn not to be too quick to announce the next topic.  A little while after posting, I usually remember other aspects that I had intended to cover.  I believe that in some cultures it is called a stairways moment.  So our little engine will have to wait a bit longer, at least until I am off the stairway.

Last time I referred back to the old definition of boiler horsepower which implied a thermal efficiency of 7.6%, and noted that it was not very impressive.  So what do modern engines achieve?  Last time I looked, it was only the very best large scale plants that were just nudging 50%, and that required all the subtle tricks of heat recovery, together with high pressures and temperature.  No doubt they are a little higher now, but I suspect not much.  Most are much nearer 30%.

At the other end of the scale we have the locomotive efficiency competitions reported quite regularly In Model Engineer Magazine.  To understand the reported results, and how they relate to overall thermal efficiency, we have to note that the rankings are based on drawbar power.  That is, they have a carriage towed by the engine that measures the drawbar pull and the speed over the measured run.  So we should qualify the results as drawbar efficiency.  It is not really fair to compare drawbar efficiency with efficiency based on a stationary engine output horsepower.  It obviously is measured after all the work to drive the locomotive along the track is used, including moving the mass of the boiler, and friction introduced by the bogey cars etc.  Of course it is not an unreasonable measure for a mobile engine.  It does not matter how much power is produced by the actual engine parts, if most of it is used just driving itself and its boiler along the track.  We are definitely more interested in how much load it can pull.  I don't have access to my magazine collection at the moment, but from memory, the best 5 inch gauge locomotives are around 5%, and the smaller gauges nearer 1%.  Perhaps someone has a report handy and can confirm or update these figures.  I think those figures for a 5 in gauge loco are quite impressive and it is obvious that friction leads to proportionally greater losses in smaller engines.

If only somewhere between one and five percent of the heat available from the fuel is converted into work, what happens to the rest?  Is it really lost?  What about that basic law of physics, conservation of energy?

The first law of thermodynamics says that heat energy can be converted to work, but the second basically says not only can we not get more energy out than we put in, we will always loose some in the process.  Perhaps more accurately, we will always get less work out than the energy we put in.  Conservation of energy says we do not actually lose energy, it just goes somewhere other than getting converted into work. 

That is probably enough for one session.  Next time I will look at where the energy goes.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 30, 2017, 01:56:27 PM
Conservation of energy -

Last time, I started looking at efficiency and how we cannot convert all the energy from fuel into work.  The old definition of a boiler horsepower implies an efficiency of only 7.6%.  The law of conservation of energy says the remaining heat is not lost, it just is not converted to work.  This raises the obvious question of what happens to the rest, 92.4% of the energy from fuel that is not converted to work.  Let's start looking at where it all goes.

If we start by looking at the boiler.  Fuel needs oxygen to burn, and we normally obtain the oxygen from air.  As a part of the combustion process, carbon in the fuel is oxidised to carbon dioxide.  Hydrogen is oxidised to H2O.  Any sulphur in the fuel on oxidised to sulphur dioxide, SO2.   Air is about 80% nitrogen.  In very high temperature processes, there are even oxides of nitrogen produced, but for the most part, nitrogen just goes along for the ride, absorbing heat from combustion, and carrying quite a bit of heat up the stack.  The oxidation reactions all release heat, so the combustion products are hot.  This heat crosses the heat transfer surfaces to boil the water in the boiler.  But all the heat cannot be transferred, heat is only transferred from a hotter substance to a cooler one.  So in the end, after the maximum possible heat transfer, the whole mass of combustion products is hotter than the steam we are producing, and it goes up the stack carrying all that heat with it.  Obviously a significant heat loss from our process.  Now this would be bad enough if we only used enough air to exactly supply the requirement for combustion.  Unfortunately, it is very difficult to mix the fuel and air sufficiently well to achieve this, so even very sophisticated burners require excess air, just to achieve complete combustion.  It is not efficient to allow unturned fuel to go up the stack.  But perhaps more importantly, insufficient air means the burning only partly proceeds.  Rather than getting CO2 plus unburned fuel, we find carbon burns to carbon monoxide.  While CO2 cannot sustain breathing, it is in fact produced in our bodies during breathing, and so long as there is sufficient oxygen available, it is not particularly harmful.  However, carbon monoxide is quite toxic in even small concentrations.  This is why it is necessary to have good ventilation if we have combustion indoors.  Preferably a chimney over the fire place, as there is usually a tiny amount of CO formed in any combustion, and we cannot afford to allow its concentration to accumulate.

You may suggest leaving out the nitrogen and just burning fuel in oxygen to reduce the heat lost by the nitrogen being heated on the way through.  Separating the nitrogen is not practical for two reasons.  First, it takes a huge amount of energy to separate them, so that would not help our efficiency.   Of course in a model, we could cheat a bit, and buy in some liquid oxygen, and not count the energy used "by others" to separate it from air.  That brings up the second reason.  It would be very dangerous.  Even a slight increase in the oxygen content of the atmosphere causes combustible stuff to burn much more vigorously, and makes even stuff we normally consider non combustible burn quite well.  Given a high enough oxygen concentration, even iron, steel and concrete will burn.   I have been told that if there is a fire in an oxygen plant, there is nothing to clean up afterwards, even the concrete foundations are destroyed.  Fortunately, I don't have direct experience of this.  I think we had better just accept that nitrogen in the air will reduce our efficiency.  Perhaps 20 to 30% of the energy in the fuel is lost up the stack in a typical boiler.  Even more in a hand stoked coal fired boiler, as the practicalities of stoking mean that there is usually even more excess air than found in say a gas fired boiler.

Staying with the boiler for the moment, apart from the stack gases, the other obvious source of heat loss is the loss from the furnace or boiler casing.  If we hold our hand near the boiler we can feel the radiant heat.  If we hold our hand above the boiler, we can feel the hot air rising due to the convection losses.  This is one source of heat loss we can do something about.  If we insulate the furnace casing well, we can significantly reduce these losses.  It is even easier in a marine fire tube boiler, where the combustion is completely contained within the boiler.  We can insulate the shell quite well with as much thickness as the installation will allow and really minimise these external losses.

A slightly shorter post this time, but a good place to pause.  Next time, I will follow through the steam cycle and look at where energy is lost around the engine.  There should be some items there that we can attend to, to increase our engine efficiency.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 30, 2017, 02:47:55 PM
Hi.Good to see you back.....talking about time.......Using a year would be interesting as a leap year would through out the calculations !!  Also on efficiency a couple of pics and text from the Engineer in 1881 using the superheat to heat up the exhaust from the HP to LP cylinders......
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 31, 2017, 12:08:55 PM
Hi Willy,  while I like the getting away from it all, it is also good to have communication, so good to be back and glad to have you on board again.  I guess the main disadvantage of adopting a year as a time standard is that approximate quarter day that gives rise to the leap years.  Any standard quantity has to have an agreed constant value.  Perhaps the second or the caesium atom vibration is the best after all.

Interesting to see that the reheat cycle was used so long ago, and to see just how it was implemented in those days.  The reheat cycle is used in modern power stations, where the exhaust from a high pressure turbine is piped back to the boiler, and through a secondary superheater coil, before returning to the intermediate pressure turbine.  My old textbook even shows the intermediate turbine exhaust through another reheat coil in the boiler before returning to the lp turbine.  The heat input to the reheat circuit increases the energy available for the next pressure level and so increases the power station output and efficiency without requiring more fuel.

Last time I was looking at where the heat goes in a boiler, this time let's look at the losses in the engine.  We have already looked at how the work is generated during the power stroke of the engine, but where are the "losses" in the engine?  Remember, the law of conservation of energy says the energy is not lost, it just does not get converted to work.  There are basically two quite different loss mechanisms.  First there are the processes that result in heat passing through the engine without being converted to work.  Then there are processes that use the work already created within the engine before there is any excess at the output shaft.

Heat loss from the cylinder, is heat from the steam that does not contribute to the engine output.  We have talked about reducing this loss by cylinder cladding, or even jacketing.  Cladding, being totally simple and passive is probably the most useful approach.  Jacketing is more complex, and also consumes steam that just may be more effectively used inside the cylinder.

Leakage past the piston is another area that steam is consumed without being effectively converted to work.  We can work on the piston to cylinder fit, but generally piston rings seem to be the most commonly applied design to reduce blow-by without introducing too much friction.  I will leave it to those with more expertise to describe how best to make effective rings.  If, like me, you are not up to making good rings, you can do much worse than just machine the groves as though you were going to insert rings.  Sometimes a soft packing is inserted, but the ring grooves are helpful even without it.  The flow past the piston is caused by the pressure difference between the power stroke side and the exhaust side of the piston.  This pressure difference accelerates the steam flow through the gap.  When grooves are machined in the piston, the flow expands into the groove, generally slowing but with lots of turbulence and a minimum of pressure recovery normally seen in a well designed Venturi.  More energy is consumed in accelerating the flow again into the next section of the gap.  The expansion losses occur again at the next groove.  The result of all the lost energy in each groove and the energy to reaccelerate the flow means the resistance to flow past the piston is much higher than for a plain piston, so the flow is much less.  It is called a labyrinth piston.

In case you think this would not work, I can assure you that it is used in labyrinth piston compressors.  These machines are used when the machines must be completely oil free.  The piston is supported by the cross head and piston rod so that it does not touch the cylinder walls.  Helped by a vertical configuration.  Even hydrogen can be compressed to quite high pressure s with these machines, though they do have a very large number of grooves.  Search for labyrinth piston compressors, and perhaps the Sulzer site as a manufacturer of these machines.  If you can't make good piston rings, it can't hurt to include two or three ring grooves anyway.

In a similar way, rod packing leakage reduces the output of the under side of the piston.  We usually insert a soft packing to minimise this leakage.  In a similar way, if you make some packing ring grooves they reduce the loss a little, even without the packing, which can introduce a lot of friction in a small engine if too tight.

It is worth looking at all these sources of steam blow by, but in the end they are not the major component of that 92% we are looking for.

The other main loss in the engine occurs after the energy is successfully converted to work, the work is used within the engine, so not available for doing external work.

I expect others will think of more areas where energy is lost before it is converted to work, otherwise I will look at the work lost within the engine.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on August 31, 2017, 01:08:32 PM
Thanks MJM for re-awakening my brain regarding labyrinth pistons, I shall chase them further on the web. I am currently pondering methods of eliminating friction from my small twin cylinder Gauge 1 Lion loco, 3/8 D pistons and contemplating the viability of replacing the slide valves with outside admission piston valves as the load on each of the tiny slide valves at 60 psi is 2.2 kilos and on a surface area, (underside of valve), of 0.06  sq. inches. Glad the discussion is rolling on again. Trust you enjoyed the dry country down South and got to see the bright lights of Thargomindah. Regards Paul Gough. 
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 31, 2017, 09:09:22 PM
Hi is there optimum shapes for these labyrinth grooves and do they get smaller towards the middle ? There is quite a lot of info in my old books from the 1830's on, and it is surprising the amount of knowledge that was available all those years ago to engine builders .However the pecuniary considerations tended to outweigh the actual finished engines that were produced. What would be the best wood be for cylinder lagging btw and are there tables the same as for metals regarding conduction etc etc ?
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 31, 2017, 11:54:18 PM
I have just read through your posts now and was wondering how efficient is the human body ?? in Asia where the POW men ate a lot of rice and not much else ,they still managed to do a hard days work? Vegans also manage to become successful athletes without any meat etc so how does one man power compare to 1 horsepower ? so are food calories equatable to coal calories ?  just wondering !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 01, 2017, 01:58:55 PM
More on efficiency -

Hi Paul, good to have you back.  Unfortunately I missed Thargomindah, passed by a bit north of there.  Was probably closer some years ago when I did some work in SA.  Wife says we had to bypass it as the coffee might not be as good as at Gregory.  With your little valves, I think the normal force is 60 x 0.06 = 3.6 lbf or about 1.6 kgf for each valve, so 3.2 kgf for two valves, it seems quite large in proportion to such a small engine, but it is a well known disadvantage of slide valves.  But the friction force which is at right angles to the normal force is smaller.  Steel on steel has a friction coefficient of about 0.3, but a well lubricated bronze surface should be significantly less.  Search for friction coefficient for various surfaces.  Multiply the normal force by the friction coefficient to get the force necessary to move the valves.  The point will be whether you can make piston valves in such a small size which seal well enough to be more efficient than the more simple slide valve.  I would also ask if inside admission piston valve would be better balanced for forces, involve less sealing issues with the gland packing and less rod end force influence.  I never got to see a real labyrinth piston up close, but the principle is sound, so I always put a groove or three in my small pistons on the grounds that it should be better than plain pistons.  It should be possible to make a spare piston for an engine and test both ways, but I feel the difference would be masked by the inevitable difference in diameter that I would almost certainly produce.

Hi Willy, my Heat transfer text book has a table of thermal properties including conductivity for metals and a separate table for insulating materials.  Pine, fir and spruce are all listed as 0.15 W/(m.K), while oak is listed as 0.19, and cork 0.042.  This suggests that any timber will do, if you compare with figures of 80 to 110 for copper and brass.  Just use something that will take a nice finish if the wood will be visible, but use cork if it will be covered by a metal sheet, and make it as thick as aesthetics will allow.  You are quite right to notice that the human body is in another league for efficiency, but it is still subject to the same laws of thermodynamics.  Not all the energy in the food is converted into work.  Some goes out as heat in breath temperature, breath humidity, or perspiration, or conduction to air and quite a bit is used just to breath, think and circulate blood, and digest the food.  Meat is a good source of protein necessary to build muscle, (vegans must get this from other foods), and is also good for sustained energy release.  But energy is more quickly released from carbohydrate foods such as rice.  And the body is incredibly good at conserving energy particularly in time of drought, no doubt why food restriction dieting is so difficult, our body just says why, we might be hungry tomorrow?  And then closes down to conserve as much energy as possible until more food is available.  But those POWs also paid a terrible price in terms of weight loss, far beyond what was compatible with good health.  I have a conversion Ap on my iPad that says one food calorie is equivalent to 1000 calories.  It makes sense in terms of the published calorie values of foods and the energy we produce.  My heat transfer book has quite comprehensive conversion tables but does not include food calories.  It does however consistently include in brackets (thermochemical) after calories and Btu in each description of energy units in the table, which is perhaps a pointed reference to the "nonstandard" energy unit used in those other disciplines.

I don't have a reference to a standard manpower, though I know it varies in a wide range over the population.  I was in Fremantle when the Americas cup was sailed there, (for work, really) and a university professor and his team attempted to answer the question by building a machine with a similar action to the coffee grinder winches on the boats.  They carted the machine around various pubs and challenged willing guys as to who could produce the most power.  The machine successfully gathered data until the races were over and the real winch grinders joined in.  The machine had to be rebuilt several times before it was strong enough to absorb the power of those guys.  They just blew it apart at every attempt.  Similarly, cyclists are known to have a good power to weight ratio, not the power of winch grinders, but not as heavy and so were usually favoured in the early experiments on man powered flight.  But I don't know the actual figures of power produced.  Of course, I am in another league, and don't expect to be called up as a winch grinder or for man powered flight anytime soon.

Regarding the labyrinth groves, as far as I know they are simply square groves like empty piston ring grooves.  The idea is that there is no Venturi effect with the associated pressure recovery when the gas slows and expands into the groove, the velocity energy is mostly lost in turbulence, which of course heats the gas a little, increasing the volume that has to accelerate onto the next space.  The energy lost adds up with each groove, so reduces the bypass flow.  Besides those labyrinth piston compressors, the similar labyrinth configuration is used on the shaft of steam turbines, gas turbines and on centrifugal and axial compressors, however for a really effective seal modern manufacturing techniques allow dry, non-contacting mechanical seals with really low leakage, and low power loss, so the labyrinth seals tend to be more of a first stage, especially for flammable or hazardous gases.

Didn't make much progress with the losses in engines today, but a little discussion around these interesting questions is helpful, so I really appreciate the questions.  Perhaps tomorrow.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on September 01, 2017, 05:33:44 PM
Hope I'm not deluding myself on the figures I presented: Valve dimensions, 0.457 x 0.185 = 0.084 sq. ins. x 60 psi = 5.04 lbs.
Underside of valve (or load bearing surface): 0.084 minus area of steam passage (0.285 x 0.091 = 0.026), thus 0.084-0.026 = 0.058, rounded off to 0.06 sq. ins. Thus approx. 5lbs or 2.2kg load on 0.06 sq. ins. If I'm wrong, please help, my old brain can't see where.

Don't know that I could get inside admission type into the space available, the outside admission type seems just possible, maybe, subject to achieving satisfactory sealing. I am thinking of one of the engineered 'plastics', perhaps a graphite embedded teflon or nylon or some such for the piston heads and valves. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on September 01, 2017, 08:12:08 PM
Human Power - I kind of thought I knew, but ..... ended Googling it and according to Wikipedia https://en.wikipedia.org/wiki/Human_power (https://en.wikipedia.org/wiki/Human_power)

Quote
A trained cyclist can produce about 400 watts of mechanical power for an hour or more, but adults of good average fitness average between 50 and 150 watts for an hour of vigorous exercise. A healthy well-fed laborer over the course of an 8-hour work shift can sustain an average output of about 75 watts.

Quite a range - from not very much to rather impressive.
Title: Re: Talking Thermodynamics
Post by: Maryak on September 01, 2017, 10:45:27 PM


Last time I was looking at where the heat goes in a boiler, this time let's look at the losses in the engine. 

Heat losses Marine Steam Plant

(http://i389.photobucket.com/albums/oo340/Maryak/Heatloss.jpg)

Sometimes a picture is helpful.

Regards Bob
Title: Re: Talking Thermodynamics
Post by: derekwarner on September 02, 2017, 01:02:41 AM
Whilst a little simplistic on labyrinth grooves [for model steam engine piston valves] , I posted the following on August 6/2017 on Mammod Steam Models site
____________________________________________________________

To take this further here is some light reading

https://www.google.com.au/url?sa=... FQjCNEkXGWRYQuZJVuLfNb_X_LrJiyyrw

However for some further understanding we go back to Gas Laws..

 P1 V1 = P2 V2...

 P1 = steam pressure of the system
 V1 = the annular volume in the piston valve spool
 V2 = the annular clearance volume between the valve spool and the valve body

So we have P1 & V1 doing their work  with each stroke created by the eccentric movement

Then the same P1 travels along the minute [a ~~0.001"] annular clearance path V2, then the steam literally falls into the larger volume of the groove

This pressure drop then creates a reduced value of P2 in the groove........

Since we are talking relatively low steam pressures here, the value of P2 may well approach that of atmospheric pressure [1 Bar] and hence the body of steam in V2 will partially condense and simply shuttle back & forth as a lubricant and being contained by atmospheric pressure

The reason this steam does not spray out is that the new pressure P2  is in ~~ balance with atmosphere 

Of course, a little condensate will migrate out along the valve spool .....thus maintaining the Gas Law balance

Full sized higher pressure steam turbine engines utilize similar multi Labyrinth sealing arrangements
______________________________________________________________________________________________

Derek
Title: Re: Talking Thermodynamics
Post by: paul gough on September 02, 2017, 06:18:02 AM
Derek, I keep getting an invalid URL notice if I try to go to your 'light reading' site. Could you check there is no typos etc. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: derekwarner on September 02, 2017, 07:27:09 AM
Sorry Paul........here is the full extension thread link....I don't understand why copy & paste did not function???....

As you can see, the actual link at the very end ...FQjCN is the same leadin  ........Derek

http://redirect.viglink.com/?format=go&jsonp=vglnk_150433320089612&key=190cd6dc27c66f8edf4cd0d943583f3b&libId=j72x4w9101000abj000DAb8zr06z2&loc=http%3A%2F%2Fmodelsteam.myfreeforum.org%2Fabout97187.html&v=1&out=https%3A%2F%2Fwww.google.com.au%2Furl%3Fsa%3Dt%26rct%3Dj%26q%3D%26esrc%3Ds%26source%3Dweb%26cd%3D1%26cad%3Drja%26uact%3D8%26ved%3D0ahUKEwi-pam4ocHVAhXGG5QKHfMdCBoQFggmMAA%26url%3Dhttps%253A%252F%252Fen.wikipedia.org%252Fwiki%252FLabyrinth_seal%26usg%3DAFQjCNEkXGWRYQuZJVuLfNb_X_LrJiyyrw&ref=http%3A%2F%2Fmodelsteam.myfreeforum.org%2Fforum7.php&title=The%20Unofficial%20Mamod%20and%20Other%20Steam%20Forum%20%3A%3A%20Piston%20Valves&txt=https%3A%2F%2Fwww.google.com.au%2Furl%3Fsa%3D...FQjCNEkXGWRYQuZJVuLfNb_X_LrJiyyrw
Title: Re: Talking Thermodynamics
Post by: Steam Haulage on September 02, 2017, 09:53:03 AM
Hi All,

Labyrinth seals
Please note my liberal use of ‘may’ and ‘perhaps’ in the following.

May I suggest that the concept of the labyrinth seal goes back further than we might appreciate. Perhaps even before Jerónimo de Ayanz y Beaumont, Thomas Savery, James Watt et al were conceived.

It may be that the idea goes back into the mists of time in the joinery trade of our forefathers.

I have seen many examples of grooves in timber door frames. Before the adoption on a large scale of plastic as a construction material almost every frame would have a semi-circular groove cut into the hinge, lock and top of the frame. Apprentice joiners were instructed by the journey man they worked under that this was to allow winds to enter any gap and then reverse its direction and so prevent the draught from entering the inside of the building.

When I was a lad this groove was in common use, and visible in every house in which my family and relations lived, as well as in every door-frame at the school, built in 1909, which I attended in the fifties.

Steam Haulage

Title: Re: Talking Thermodynamics
Post by: MJM460 on September 02, 2017, 02:24:52 PM
Hi Paul, now I see where you are coming from on the area and force on your slide valve.  I don't see myself as in any position to arbitrate on right and wrong, but I would tackle the problem slightly differently.  I will leave it to you to decide on whether it is helpful.  The slide valve has two sides, just like the piston.  The steam chest side is easy, steam chest pressure over the area of the valve gives your 5.1 lbf forcing the valve against the valve face.  The other side of the valve is more complex.  The pressure in the valve cavity is a bit higher than exhaust pressure, higher by the pressure required to drive the exhaust flow through the port to the exhaust system, so we can estimate the net force on the valve over the exhaust cavity area as a bit above exhaust pressure.    However the sealing faces of the valve are more complex.  I suspect that there is a thin film of steam and oil separating the faces however slightly, and the pressure in this film varies perhaps roughly linearly between the valve chest pressure on the outer perimeter, and the valve cavity pressure around its periphery.  On this basis, I would estimate the average pressure over the sealing face is about half way between steam chest pressure and valve cavity pressure, tending to lift the valve, so tending to partially balance the pressure on the steam chest side.  Then, as I mentioned yesterday, the valve rod does not have to resist the normal force, but only the friction force, so now we have to estimate a suitable friction coefficient.  I hope this helps.

Yesterday, I mentioned inside admission, because the steam pressure forces on the piston valve are better balanced in the inside admission arrangement and the rod sealing and unbalanced rod forces are determined only by exhaust pressure.  However the challenge of making piston valves with sealing rings on such a small engine would be totally beyond me.  I suspect that you would be better to stay with the simplicity of slide valves, and if necessary, increase the piston diameter slightly to provide the extra force.  Avoiding the issue, I know.

Thanks for joining in Admiral_dk, glad to have you on board.  I think you have answered yesterday's question on human power.  Now that you have put the figures in, I seem to remember being on the tread mill for a stress test after a heart attack many years ago, closely monitored by a cardiologist and all the instrumentation, and being told that the load, which was provided by a small generator was about equal to a 60 watt globe, but fortunately I only had to keep it up for about 15 minutes.  But definitely at the low end as you might expect in the circumstances.  For an 8 hour day, I suspect your 75 watts is about a reasonably good effort when it has to be kept up all week.

Hi Maryak, thanks for another of your excellent diagrams.  It is very helpful in showing all the losses in proportion.  Interesting to note that the final figure driving the ship is only 6%, but I suspect in model sizes, the mechanical losses are a bigger proportion.  It is one of those wrinkles in the maths, that if we say halved the mechanical losses, it would not make much difference to the overall efficiency, but all the reduced loss would appear as extra output power so would have a big percentage effect on the out put power.  That is what is behind my intent to look at some of the losses that we can do something about.  At the end of the day however, as your diagram shows, the major losses are in the heat contained in the exhaust, followed by the heat carried up the stack.

Hi Derek, it is good to see someone else having a go at explaining some of the many little puzzles we come up against in every design.  If we can nut out the theory, it will definitely help inform out experimentation.  As with Paul's problem, I would take a slightly different approach, and would start with a diagram showing where conditions are known, specifically the top and bottom edges of the piston.  This pressure drop forces the flow past the piston.  Each time the flow encounters a groove, it slows down, but with this geometry, no pressure recovery.  There is then a small pressure drop to supply the energy to reaccelerate the flow into the next section of the annulus, followed by a steady pressure loss towards the next flow.  The pressure cannot get below exhaust pressure, but is always a little lower when there are grooves, so restricting the leakage flow a little.  I hope that helps your thinking a little.

Hi steam haulage, good to have you aboard also.  A truly interesting little piece of history around those grooves.  I expect the journeyman learned the explanation when he was the apprentice and so on back to time immemorial.  I suspect neither he nor his apprentice or their forbears really understood the explanation.  I suspect that we can prove using the law of conservation of energy that the wind cannot be turned back with enough energy to flow back on itself, however that does not mean there is not a labyrinth effect there that means the groves might reduce the draft a bit, and that would always be welcome.  I hope that I have continued your most appropriate use of perhaps and may.

Once again, time and words have run away, so perhaps next time for looking at what we can do on our engines.

Thanks for reading along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 02, 2017, 03:09:41 PM
Hi I have now spent all day looking at doors in Medieval city of Norwich and have not found any of these grooves yet !!! In the previous posts there is mention of grid iron valves these were used as there was less movement in the valve so contributing to greater efficiency ,however this is from the 1881 book...so would this be correct. A local engine builder uses PTFE  'rings' on his piston valves and found that the diameter should be slightly less than normal rings due to the expansion of the PTFE. lots of good info coming on this site. keep it coming !! Also this was the first i have heard of gridiron valves in the last 70 years !! How would the efficiency compare with a diesel engine in a ship ?? or a steam turbine ?
Title: Re: Talking Thermodynamics
Post by: derekwarner on September 02, 2017, 04:05:21 PM
Gents......my earlier posting in Mamod Models from August the 6th, was in answer to a question asked by another member as to what the two fine grooves on a model steam engine piston valve spool by way of the gland area could be?

I simply offered a thought and basic explanation of the pressure drop that occurs when a series of labyrinth grooves are used in fluid sealing applications

As you have noted MJM, the pressure reductions contained/constrained by a series of labyrinth grooves cannot exceed 'exhaust pressure', so again considering the model steam engine application, and from my recent trials of excessive making of condensate  :mischief: one could expect exhaust steam pressure [within the system] to be at the physical exhaust point be approaching atmosphere

In this low pressure [say 2 to 3 Bar] application, the resulting steam/condensate [as liquid] trapped in the labyrinth grooves will tend to act as a lubricant and buffer against physical displacement outside of the gland area

As mentioned, on my return to NSW, I will conduct the series of tests with the larger flow capacity exhaust system.... Derek

Derek

   
Title: Re: Talking Thermodynamics
Post by: Maryak on September 02, 2017, 11:10:21 PM
Hi All,

In full size turbines the labyrinth glands on the turbines are balanced to atmosphere with gland steam.

Often LP turbine admission is at the centre, (to balance axial thrust), with steam flowing out to the ends hence the glands are not much above condenser pressure. Without sealing steam being supplied to the glands condenser pressure would tend to rise towards atmospheric pressure causing a drop in plant efficiency.

Basically all the glands in the turbine set are interconnected at their inner and outer pockets and the gland steam system is balanced to maintain minimal leakage of gland steam from the the outer pockets. The fine adjustment valve shown below is in modern plant connected to a controller which automatically maintains the system in balance over varying load/speed ranges.....stop, ahead and astern

(http://i389.photobucket.com/albums/oo340/Maryak/GSsteam_zpsj9amcicy.jpg)

Regards Bob
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 03, 2017, 12:36:20 PM
Hi Willy, I wonder if the door detailing, being passed down from master to apprentice, became a regional thing.  I am not sure just what a grid valve looks like, however if the design enabled a shorter stroke, it would reduce the work done by the valve rod.  Remember the definition of work, force times distance.  Even with the same normal force on the valve and the same friction coefficient, half the stroke means half the work dine by the rod and half the parasitic power lost to the engine output from this cause.  A possible answer to Pauls dilemma is to reduce the valve travel, though there are practical limits on such small models.  Valve rod forces have always been known to be high however.  The early joy valve locomotives were I believe known for breaking the con rod due to the side force from the valve linkage.  Or perhaps it was just poor design.  My text book has worked examples for the efficiency of steam plants with 2 and 4 MPa operating pressure, turbine drives and 10 kPa exhaust pressure, quite good vacuum.  2 MPa gave about 30%, 4 MPa gave 35%.  Interestingly, the reheat example gave less than 1% more, but is favoured in steam plants because it gives much drier steam at the low pressure turbine exhaust, so less blade erosion by the condensate droplets.  I think reciprocating machines are a bit higher efficiency than turbines, but they are limited to much lower speeds and power outputs.  My somewhat unreliable memory recalls large industrial Diesel engines could achieve about 30% when I was using them for compressor drives.  In a simple open cycle gas turbine, about a third of the energy goes out the exhaust as tonnes of hot gas, a third is consumed by the compressor at the air inlet, and a third is available as output power, very roughly.  Again this is from using them as compressor drives.  As jet aeroplane drives, there is no mechanical output, apart from that consumed by the compressor on the front end and the fuel pumps, the power inherent in that mass of gas exhausting at the back end provides the thrust. 

We do not get this performance from our steam models due to the low boiler  pressure and normally atmospheric exhaust.  I am not advocating that we try and emulate these pressures, let alone the much higher pressure used in modern industry.  A typical petrochemical plant has steam systems operating at 10 MPa, while utility scale power plants are built to supercritical pressures, that is above 22 MPa.  Steam at any pressure is dangerous, and these very high pressures are extremely so.  Fuel costs for a model are very reasonable as the running time is generally low.  We do not need to go to those extremes in the pursuit of fuel efficiency, but it is interesting to work out just what we can do to increase the power output within the limits of our operating conditions.

Hi Derek, my little Mamod engine is a single acting oscillating engine.  I am not familiar with the design of piston valves for such small engines.  I am sure the small size makes them quite different from the common designs for the larger locomotives.  I really can't picture the grooves in the gland area that you are talking about.

Hi Maryak, gland systems can get quite complex as shown by your diagram.  But I am sure that conditions in the boiler room were unpleasant enough without extra steam leakage from shaft seals.  At the end of the day however, you have reminded us that labyrinth seals still leak, just a bit less than a plain shaft or cylinder.  A well made mechanical seal of appropriate material is still the far better solution.  I am sure that perhaps carbon rings from that material used for Stirling engine power Pistons could be made for this size, but until my skills develop, I turn the grooves as they should be better than plain pistons.

Back when we were all sidetracked by the mention of labyrinths, I had talked about some of the ways heat is lost before it is turned into work, so next I think it worth talking about some of the ways we lose work output by consuming it in the engine before it gets to the output shaft.  It always seems a bit unfair when there are so many limits to how much of our energy can be converted to work, that we then lose a significant fraction of the work we do produce to friction within the engine.  The labyrinth discussion was really prompted by the desire to reduce friction between the piston and cylinder without losing too much work due to steam bypassing the piston.  Piston rings are intended to reduce the flow, but at a cost of causing friction.  Making good rings is a challenge for a beginner, for me, anyway.  Of course a packing can also be used with a bit of judgement on how much can be added without making it too tight.  Some people advocate using o-rings, and whatever works is helpful, though it was not what O rings were designed for.  My guess is that they are initially a touch tight, and some rubber wears away to leave the clearance "just right", when they are better on a small model than any poor fitting alternative.  The concept of an abradable seal is actually used in full size compressors as an alternative to machining the seal to perfectly fit each shaft, so there is even full size precedent, even if it is in shaft seals.  Cylinder lubrication helps reduce friction, and Derek has already mentioned that the moisture in wet steam helps lubrication.  That is why it becomes more critical to use the right oil if we have a lot of superheat, so dryer steam at the exhaust with less condensate.

Then, in no particular order, we can look at bearings, main bearings, big end, and small end.  The cross head guides, and all the pivots and slides of the valve mechanism, or port face and pivot on an oscillator, all involve friction, which takes some of our valuable work and turns it into heat.  I am not suggesting that this friction can be eliminated, but attention to alignment, clearances and lubrication all leave more work available at the output shaft.  In the extreme, some modern compressors have magnetic bearings where the electronic controls vary the current to electromagnets so the magnetic forces to keep the shaft cantered in the bearings despite fluid load and unbalance of a high speed machine.  Incredibly low loss, but the electronics are mind bending, and I doubt they will be practical for model engines in the near future.  I don't know if anyone is experimenting with improved bearing design or lubrication for models.

That's enough for today, I still want to summarise the effect of the exhaust system, though the detail has probably been well enough covered.

Thanks for dropping in

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 03, 2017, 01:10:49 PM
 Hi MJM, this is the pic of the gridiron valve......It does seem to have a lot of metal to metal contact, so although it has a shorter stroke does this cancel out the friction losses? also in the pic the copper expansion joints ?? thanks for the input .........
Title: Re: Talking Thermodynamics
Post by: paul gough on September 03, 2017, 04:57:27 PM
Just quickly, part of my reasoning for outside admission piston valves is to have very short exhaust passages straight out of the top of the block and into the blast pipe which is co-incident with its centre, an initial attempt in concert with reducing the piston clearances to about ten thou to reduce exhaust and compression volumes. The block cannot be enlarged nor can boiler out put be increased without an extra wick on the metho burner, whilst this has been done previously it is forcing the tiny boiler, so larger dia. cylinders with larger steam demand is out. There is no room to move, so to speak with the Lion, it is about as small as one can go in Gauge One and still have a loco that is not a pain to operate. Putting twin inside cylinders and an eccentric driven water pump off the main axle underneath was quite a task, so I am left with trying to come up with ways of improving performance within the existing dimensional constraints. The 6 1/2 lbs of force on the 3/8" piston working against the 5 lbs load on the valve, plus any other imposts does not give this little engine much in the way of reserve or very good slow running. Labrinth piston heads and piston valves are an exploration into possibilities and I had considered combining these with the use of engineered plastics, if that is the correct term, for these components if an appropriate grade can be got. I understand Aster used Rulon for their piston rings in their piston valve engines, I understand it is a teflon with powdered mica filler, but is a bit soft for a piston head. So not only is the physical design of the engine important but trying to find alternatives to traditional materials is part of the game in situations where things are approaching functional limits. Anybody who has any ideas or experience with teflon, nylon, or like derivatives for use as pistons with steam is very much encouraged to assist. I had not considered carbon and know nothing of its characteristics for use with steam, again any info. welcome. Thanks MJM for reminding me that reducing the travel of the valve is also a valuable thing to chase if achievable. Sorry about this not so quick post. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 04, 2017, 01:33:30 PM
Hi Willy, sorry that I missed the significance of the grid iron valve drawing in your earlier post.  If I understand the system, there are separate inlet and exhaust valves, so presumably two cams and linkages.  But I can see how it reduces the stroke of each valve.  Friction is interesting.  The conventional model of friction is independent of area, just depends on the total normal force.  More area means less pressure on the surface but the total friction is assumed to be the same.  With the grid plates on edge like the drawing, even the weight is hardly a big contributor.  It the steam force pressing the valve on the face is F, then the friction force to move the valve along the face is friction coefficient times F.  It does not depend on area.  As I have mentioned to Paul, the friction coefficient for dry steel on steel is generally given as 0.3, so the friction force is only 0.3 times F.  For a very smooth, well lubricated surface, the friction coefficient might be 0.1 or less.  Now friction is hard to measure, because it tends to have a stick-slip motion.  It takes a bit bigger force to start the movement, but then a smaller force is enough to keep it moving.  So the coefficient is at best an approximation, but it does allow design to proceed.  It is possible that the area has a small secondary influence, but it is usually ignored.  Sometimes the friction coefficient is quoted as both a static friction value, the value to get movement started, and dynamic value, the value once movement begins.

Hi Paul, now I can see what you are doing.  Thank you for your clear description.  Sounds like a great project.  I can only admire your watch making skills.  Will you be posting a build log or at least some pictures?  It looks like a project where, if you can use a millimetre better than others, you have made a major breakthrough.
Remember though, the valve rod does not have to work against the 5 lbs.  The rod force will be the friction coefficient times that force, and with a little oil or even condensate lubrication, together with a nicely smooth surface, the rod force can be expected to be nearer 0.5 lbs.  It may be worth rigging up a little test rig to get a rough measurement.  You can't simulate the temperature, or pressure on the sealing face of the valve, bit you will get an idea.  Regarding sealing rings, I can imagine that you have very little room.  Generally in the applications I am more familiar with, pure teflon is considered too soft, with too low a melting point.  It softens and spreads rather than take the load.  A filled teflon, is normally used for higher load bearing properties.  Graphite or other fill materials give different properties, but availability tends to override for hobby applications, so some experimenting with the available materials in a small stationary engine rather than risk damaging your amazing locomotive.  Carbon is quite fragile and so would require a two piece piston (or ring)  construction if that is practical in your design, but is used in pump and compressor seals to quite high temperatures, so may be worth a try.   At least the experiments could be carried out with much less effort in a separate single cylinder stationary engine, allowing more trials of less mainstream ideas.

The last area of loss that I wanted to list was, as Maryaks recent heat energy diagram showed (see post #242), the heat that goes straight through to the exhaust as latent heat of the exhaust stream.  We have to condense all of the exhaust totally to water before we can pump it back to boiler pressure, or simply reject it to the atmosphere, either way, the heat is not converted to work.  That diagram nominated a figure of 70% of the fuel energy.  Regardless of the accuracy of this exact figure, it shows that it is a major factor.  Can we do anything about it?  It is low temperature heat, so not easy to use elsewhere.  Perhaps as a boiler feed water heater if we have a continuous boiler feed pump.  Perhaps a little in preheating our fuel, or even the incoming air, or for avoiding the pressure loss in butane, but the majority is unavoidable loss.  The giant cooling towers attached to modern power stations attest to the limited alternative uses for this heat apart from some very specific circumstances.  I have for example worked in plants where the turbines exhaust to a low pressure steam system, around 100 kPa(g), sacrificing some work output, but using the exhaust heat for process heating purposes.  These processes tend to be relatively constant, only minimally dependant on weather conditions, but if the exhaust steam was not available, fuel would have to be burned anyway to supply the process needs.  When you do the calculations on how much extra fuel you have to burn to drive the turbine, you get the turbine power at an efficiency of around 80%.  Much higher than can be achieved by any isolated power plant, no matter how sophisticated.  But it is really only an accounting exercise, the process requirements reduce the fuel consumption attributed to the turbine power requirement, but do not actually increase the conversion of energy in the fuel to work. I am sure the process people attribute a very poor efficiency to the turbines and take credit for using the "waste" heat to reduce the fuel costs attributed to their process.

I think that covers most of the directions energy goes other than conversion to work, so I would like to look at the thermodynamic limits on how much of the fuel energy can possibly be converted to work in an ideal power plant, which leads to different measures of efficiency.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 04, 2017, 02:15:34 PM
Hi just wondering about waste heat and thermocouples ?? at least it should power a transistor radio for the fireman to listen to the latest action from Headingly, !!! is there any info about this anywhere ? It could even illuminate pressure gauges and things on dark nights on locomotives !! I can remember in the Practical Wireless Mag from the fifties a design of a thermocouple connected to a Hooka for people to listen to the radio in India,!!
Title: Re: Talking Thermodynamics
Post by: paul gough on September 04, 2017, 06:48:15 PM
Thanks for your comments MJM. Funny you should mention utilising a stationary engine for test purpose as only last week when discussing my 'headaches' with another I mentioned how I might be driven to getting a twin horizontal stationary engine so as to test two alternative materials at the same time and measure their relative performance.. This little project is likely to run into years, as there are other challenging things that please me to tackle. 'Adequate' lubrication rather than the general flood of oil that is often a messy characteristic of these little engines  and multiple orifice blast nozzle so as to eliminate back pressure, just to mention two. One of my 'madnesses' is to develop a mechanical lubricator for Gauge One size locos. Desktop size engineering is somewhat challenging and should keep my old brain from rusting up even if the body fails and manufacturing things becomes beyond me.

 Your exploration of things theoretical presents aspects for consideration that are often unknown or overlooked and I am very pleased to be able to read your words and also the responses and questions they provoke. This thread is a unique opportunity to begin to understand what is happening  and progress the ideas of miniature mechanicians. Thank you. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 05, 2017, 01:06:14 PM
A theoretical limit-

Hi Willy, that is a good idea.  It is called the Peltier, or Seebeck effect.  Both are reversible in the sense that if you pass a current through a pair of junctions in one direction, one gets warmer and the other cooler.  Reverse the current and the hot and cold sides are reversed.  Reverse the heat flow, supply the heat instead of electrical energy, and you get a current.  Small units are readily available as drink coolers/heaters in camping and car accessory shops.  Discreet units are sometimes available in electronics outlets, and certainly on line which can be heated to get a current.  They are not very efficient but we will come back to that.  Thermocouples only give about 5 mV for 100 deg C temperature difference, or is it microvolt?  Either way you need a lot of junctions.  But some combinations of semiconductors also work, and much more efficiently.  I found a reference which implied about 6% energy conversion to electricity.  Does not sound impressive, but if 70% of our burner energy goes to the condenser, and we convert 6% of that to electricity, that makes 4.2% of the fuel turned to electricity.  For a small model that is much more than the engine output, so more than doubles the output of our plant.  Worth a few experiments perhaps.  Of course we do have to carry the heat away from the cool plate, or it soon approaches the exhaust temperature and electricity stops.  Still need a river or a cooling tower, or perhaps a good heat sink and use some of the power to drive a fan so air carries away the heat.  Try looking for Peltier or Seebeck, or Peltier-Seebeck effect in your preferred search engine.  Similarly if you have one of those three way camping fridges which have a low power absorption refrigeration cycle, it may be possible to replace the heating element with your exhaust steam as a heat source, and keep the beer cool.  Not sure if it could be scaled up to full size plant.  In industry, refrigeration plants using absorption cycle are sometimes used when there is a large source of low grade heat together with the need for refrigeration.  But to be economical, they are normally huge.

Hi Paul, thank you for your kind words of encouragement they are much appreciated, as are all the questions which keep the thread rolling along.

My thinking on a small test engine was to avoid the work and inevitable damage involved in disassembling a locomotive to do many tests.  I thought a single cylinder mill engine style would enable easy manufacture and installation of the relevant test components, so speed up the learning process.  The valve gear can be a simple eccentric for piston ring and piston valve tests, or as complex as you like to test valve gear variations.  It is a test instrument, not a historical model, so optimise it for easy changing of components.

Engineers wanting to sell their engines were not really impressed with low efficiency, and wanted to claim higher figures.  Sadie Carnot was looking at how high the efficiency of an engine could be.  I think it had previously been concluded that 100% was not possible, it was determined by the maximum and minimum temperature of the cycle, but what was possible?  He came up with a theoretical ideal (even though not very practical) cycle which he was able to show would have the maximum possible efficiency, which he showed could be calculated as 1 - Tl/Th.  This is a fraction which can be multiplied by 100 to give a percentage.  The temperatures are expressed in absolute terms so K or R , depending on your preference.  Tl is the low temperature at which heat is rejected, or condenser temperature.  Th is the maximum temperature at which heat is supplied, the boiler pressure for saturated steam, or the superheater outlet temperature. 

We could calculate the efficiency of a Carnot cycle engine operating between our cycle temperature limits.  That 4 MPa boiler we looked at earlier, had a superheater outlet temperature of   400 C or 673 K.  The condenser temperature for 10 kPa would be 46 C or 319 K.

We can calculate the efficiency of a Carnot cycle between these temperatures as 1 - 319/673 =  0.53 or 53%.  Now remember the thermal efficiency of the actual cycle was 35%, If 53% is the thermodynamic limit of what can be achieved, we can perhaps say our cycle has 35/53 = 66% of the Carnot efficiency.  Sounds a lot better than 35%, doesn't it?  We could do a similar calculation for our model.  But it also raises the question, why not build our plant to operate on the Carnot cycle?

I will try and look at that next time

Thanks for your interest and encouragement

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 06, 2017, 01:07:32 PM
After yesterday's post, I must apologise to M. Carnot for spelling his name wrongly.  Nicholas Leonard Sadi Carnot lived from 1796 to 1832, and published his work around 1824.  A salient reminder of how our life expectancy has changed since then.  If a Carnot cycle engine has the highest possible efficiency for an engine working between specified temperature limits, why not make an engine working on the Carnot cycle?  The truth is that it is not a very practical cycle.  It produces very little work and it would be difficult to make one that would overcome its own friction even.  It's only real use is to quantify the maximum possible work that can be obtained from a given temperature difference against which you can compare the performance of a real engine.  But all is not totally lost, there is actually one real engine that has the same maximum efficiency as a Carnot engine working between the same temperatures.  It is the Stirling cycle, which many forum members enjoy.  It has disadvantages in terms of power for its size unless you have a pressurised engine with sophisticated seals, but it is inherently a very efficient cycle, especially if the the regenerator is included.  It's efficiency is illustrated by the engines which produce measurable power from a tea light candle in a well known competition.  Try that with a Rankine cycle! 

A Rankine cycle, while still described in terms of ideal processes, is a much more practical cycle as history has demonstrated.

There are several reasons why a Rankine, or normal steam cycle cannot match the efficiency of a Carnot cycle engine.  The boiler heat is not all transferred at a single maximum temperature.  Most is transferred at the boiler temperature then the temperature raised to the maximum which is only reached at the superheater outlet.  In fact, the heat transfer starts at the outlet of the feed water pump or feedwater heater, which further reduces the average temperature at which heat transfer takes place.  Then we force the heat transfer by a large temperature difference to achieve the required heat transfer from a small area.  This forcing produces vigorous bubbling and circulation in the boiler, which improves the heat transfer coefficient, but absorbs energy which cannot then be converted to work.  So it is nothing like an ideal reversible heat transfer process.

I don't think there is much more needs to be said about the Carnot cycle, however when it comes to engines, there are other ways to compare efficiency which I will introduce next time.

Thanks for dropping in

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 07, 2017, 12:43:20 PM
Engine efficiency -

Last time I talked about Carnot efficiency.  While the cycle is impractical in terms of designing an engine, its strength is in indicating a theoretical upper limit to the power that can be obtained from any given cycle and its maximum and minimum temperatures.  I should have noted in passing that you cannot use a temperature lower than the surrounding atmosphere, unless you cheat by ignoring the energy necessary to produce that lower temperature.   Hence the usual minimum temperature is usually around 15 - 25 deg C, although you would have an obvious advantage if your engine is in Northern Canada, Siberia or even wintering down in Antartica.  Satellites in outer space have an even bigger theoretical advantage from this point of view. Of course for steam cycle engines, the freezing point of water provides another limitation on the minimum temperatures.  However for most practical applications, the lower temperature is pretty much fixed by our location on earth, and to get higher efficiency, we must go to ever higher steam temperatures.  A quick calculation shows why we need supercritical boilers to achieve anything near 50%

However engine manufacturers wanting to sell their product don't want their performance figures reduced by cycle conditions outside their engine.  So they like to advertise their machine efficiency by comparing with what an ideal machine could do based on the same inlet and exhaust conditions.  Now a suitable process for engine comparison is the adiabatic cycle.  Remember, no heat transfer in or out during expansion.  We can calculate the efficiency of an ideal adiabatic machine based on the actual inlet and exhaust conditions.  We then conduct a performance test for the real machine to determine its power output.  I have witnessed many compressor tests where the process is similar.  They go to great lengths to reproduce the specified guarantee conditions, though in the case of compressors, there are recognised procedures for producing equivalent conditions from which the performance at the real conditions can accurately be calculated.  This is done where for example the gas being compressed is flammable and it would be difficult to conduct the test safely on the test stand.  The the efficiency of the machine is the actual test result divided by the output of that ideal adiabatic machine and multiplied by 100 to express it as a percentage.  Real steam turbines are in the region of 80%, perhaps higher, especially for very large machines, but reciprocating machines, I don't have real information.  But our models?  I suspect friction is a bigger proportion of the potential output power in a model compared with a full size engine, so our figures are certainly much less.  This is often called adiabatic efficiency.

Remember that this approach required a test of the actual machine.  To determine the adiabatic efficiency of our model, we need to do a performance test.  This requires a load which we can suitably instrument to measure speed and torque, from which we can calculate the power.  We also need to measure the steam inlet temperature and pressure, and the exhaust pressure, and preferably exhaust temperature.  I generally run my engines unloaded.  The entire load is the internal friction in the engine.  I still have to make a suitable load to conduct a power measurement on a loaded engine.  Measuring rpm is easy with a digital non-contact tachometer which is readily available at electronics stores.  It may also be a suitable project for Picaxe or Arduino microprocessors for the electronics enthusiasts among us.  I think the best way to measure the torque is to use a little digital scale, which these days are available with a resolution of 0.1 gm.  But I still have to design a suitable machine.  The issue is easily understood from the earlier discussion on the torque produced by a reciprocating engine, it fluctuates.  In a single cylinder engine, from zero to double the average, twice each revolution.  I don't really believe the text book drag brakes can really give a meaningful reading without considerable damping.  I am thinking in terms of perhaps a model ship propellor in a container suspended on bearings so we measure the torque on the container rather than balance the whole boiler plant on a scale.  A little DC motor used as a generator may be even better, though it would still have torque fluctuations.  Perhaps it can be calibrated so the current and voltage output can be measured and power calculated.  Any suggestions for a practical small brake are more than welcome.

Next time a closer look at the adiabatic power output from typical model operating conditions.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on September 07, 2017, 10:53:39 PM
Looks like you'll have to move from a classroom to a lecture hall soon! 9096 reads for 120 days, 75/day,(up from 66), thats a lot of bums on seats. Thermodynamics and associated goings on can't be as daunting or esoteric as might be thought. Beyond doubt now that this thread is providing succour to an enthusiastic and curious crowd. Bravo! Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 08, 2017, 01:08:44 PM
A short one today -

Hi Paul, thanks for those encouraging words.  Have we really been going 120 days?  Time really flies when you are having fun.  I do hope that everyone is enjoying the ride, and learning something as we go.  I know I am.

That may seem like a strange statement, but when you are used to having all the test results from a well instrumented test stand, and have real information from the machine designers, it is all pretty straightforward.  When you attempt to see what you can deduce about a model from some limited home instruments, it can be a different story.

I have available results from a few runs of my engines, but I have accumulated instruments as I went along, finding out what I need as I go, as we all do with our tooling.  So inevitably many of the test results are incomplete, however I do have some that allow me to deduce a surprising amount.  Grand father duties have kept me quite busy so I have been a bit slow doing the calculations.  But I will keep working at it.  I need to fill out a few extra lines in my steam table with some intermediate values.  Then with a few results from a simple model test, I will show you what can be done in the next day or so.  I can also see quite a test program as my next project.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on September 09, 2017, 10:59:22 AM
An engine test

Hi everyone, calculations all sorted out.  On a model test, the actual work output is quite small as you would expect, but to calculate it requires subtraction of two much larger numbers.  Consequently the large ones must be as accurate as possible, and no short cuts or back of the envelope estimates allowed.  Regardless of accuracy, which is another issue for later, I now have a set of calculations using numbers from a test of my own engine and they all make sense.  So first, what did the test involve, what calculations are involved, then most importantly, what do they mean?

The engine for this test is my first slide valve engine, all to my own design including the boiler, though obviously the concept can hardly be claimed to be original.  It is shown in the attached picture, and there is another view in the engine show case from soon after I joined the forum.  It is 12 mm bore and 20 mm stroke, and double acting.

The boiler is methylated spirits fired, it is about 2 in diameter (a standard copper tube size), 200 mm long, with four water tubes underneath.  It has a sheet metal furnace casing made from tin plate cut from a large coffee tin as a trial run for my sheet metal skill development.  Obviously looks very rusty now, so not worthy of showing on this forum, which is why it is covered in the pictures.  A new furnace casing from stainless steel or brass is on the list, and it will be fitted with some insulation.  The steam outlet (3/16 in tubing), winds back into the combustion space where there are two full turns around the space before it exits the casing.  I fill the cold boiler through a funnel, and insert a plug which protrudes 25 mm into the boiler and carries a thermocouple for boiling temperature measurement.  From the temperature, I get the steam pressure from the steam tables.  With no feed pump, run times are limited, but I can calculate steam rates by weighing the water fill, and the water I extract with a syringe when it is all cooled down.  The fuel burns out before the water level gets too low.

The steam pipe connection the boiler to the engine is insulated with a silicone tape, and could be improved.  It has a displacement lubricator and a thermowell for a thermocouple on the engine inlet.  The exhaust outlet also has a thermowell so I can measure the steam inlet and exhaust temperatures.  It then has an oil/condensate separator with a drain that I have described previously, and a vertical exhaust stack.

The burner is perhaps described as a semi vapourising type, something along the lines of a Trangia camp stove burner, a bit more vigorous than wicks, but needs development to get a bit more heat.  I would like to build one of the so called silent types, but apparently everyone knows how to do that so they are never described in articles or books that I have read, and I still don't know how.  The burner tank holds about 50 ml of Meths, and I calculate the quantities by weighing before and after a run which is just over 20 minutes from light up to extinguished.

I use the kitchen digital scale (resolution 1 gm.) for weighing water and fuel, a multimeter or two with thermocouples for temperature and the most recent addition, an electronic temperature meter with no voltmeter functions, but it has two thermocouple inlets which I can read either one at a time or as a difference.  Even to 0.1 degree, if I could get the rest of the procedure sufficiently repeatable.  I also have a non-contacting infrared thermometer which is good for comparative readings around the place, and a non-contacting digital tachometer for engine rpm.  The missing item is a means of measuring torque.  However that is not required for the test I am describing.  The engine is just free running without any external load.

I will come back to accuracy another time, just using the readings as they come so far, to see how the whole setup works.  Though I have compared the thermocouple readings at the ice point and boiling point and they are looking pretty good.  Readings are generally passing the "looks reasonable" test, and some consistency checks, so I think quite adequate for illustrative purposes.  Better accuracy can come later, but a few more repeat runs to ensure the results are repeatable is probably the main requirement.

To conduct a test, I fill the boiler with a weighed quantity of water, and fill the fuel tank which I also weigh.  Tighten the plug, set up the thermocouple instruments and record the time when I light up.  The boiler temperature takes about 6 min to get to 100 C.  No cylinder drain valve on this one, so I gently turn the engine over by hand to blow out the condensate which heats the cylinder while the boiler temperature rises.  Within about a minute, the engine runs up quite nicely and shortly after the boiler settles at about 116 C which the steam tables tell me corresponds to 0.175 MPa absolute, or about 75kPa gauge.  For the metrically challenged, this is about 10.9 psig.  At this pressure the engine runs at about 1100 rpm.

Nothing unusual in any of that, except perhaps for the instruments.  But it is probably worthwhile describing it so you can see the areas where compromise and approximations are necessary.

My notes say I filled the boiler with 200 g of water and extracted 58 g after it cooled, so 142 g evaporated.  Similarly 42 g of fuel used.

From light up to engine running was 6 min 40 seconds and I extinguished the flame after a further 8 min of engine run time.

I read the boiler temperature, engine inlet and exhaust temperatures and engine rpm often enough to judge typical figures, and runs seem pretty steady.  So I have extracted the following -

Boiler temperature 116 C.
Engine inlet 138 C
Engine exhaust 104 C
RPM 1100

It doesn't look like much but you may be surprised to find out what it can tell us.  Next time I will use these figures to get a picture of the engine thermodynamic performance.

Thanks for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 10, 2017, 11:31:57 AM
Some calculations -

The test was actually done some time ago.  Now you have an idea what was involved, and we have some results, so time to set out some calculations. 

Now don't be too intimidated by that, they are little more than simple arithmetic.  A bit tedious to do by hand, but when I started, I had no computer, and in fact no calculator.  And no, I am not that ancient.  I had a slide rule for multiplication, which together with a pencil and paper is enough, but I suggest trying that way these days is wasting time that could be better used making swarf.  In fact, the calculations are quite repetitive, so the best way to do them is in a spreadsheet.  Once we work out the formula once, it is copied, then pasted into each place it is needed.  The only trick is what calculations to do, and finding meaning in the result, and that is what this post is about.  Well perhaps this one and one or two more.

We will use the first and second laws of thermodynamics, and here is where we find the usefulness of those calculated properties, enthalpy and entropy that we discussed earlier.  With the measured results, specifically temperature, we can look up the other properties in the steam tables.  The tables list properties on a " per kg" basis, or per lb if you are using imperial tables.  We then multiply those values by the steam and fuel rates of our engine.  So let's work those out first.

The fuel burned was 42 g in 14 min 40 seconds, this is an average of 2.86 g/min or 0.0477 g/s.

The heating value of the methylated spirits is 26000 kJ/kg.  This is the lower heating value as it assumes the water produced as a product of combustion is not condensed, but goes up the stack to atmosphere as steam.  As an aside, it allows for commercial meths, in this country anyway, being 5% water, and this water must also be turned into steam, and also the vaporisation of liquid meths, which only burns as a vapour.  So we can multiply the heating value by the consumption rate to get the energy input into our boiler.  Now for model purposes, kg and kJ are very big units and all the calculations are cluttered by too many zeros.  So note that 26000 kJ/kg is the same as 26000 J/g.  Then 0.0477 g/s x 26000 J/g = 1240 J/s or 1240 watts is our heat input from the fuel. 

Now, I usually assume that this average is also the uniform rate of fuel consumption, a bit rough, but I have checked by stopping when steam pressure is achieved, extinguishing the flame and reweighing the burner.  It seems about right based on that point anyway.  More complex to design an experiment to give more detail.

We can also estimate the steam production rate.  Remember that steam production started after 6 min 40 seconds.  The heat during this time goes into heating the water, the boiler shell and the furnace.  We could calculate the heat required for the water and the boiler shell separately, but it is not of major interest, but once these items get to steam temperature, they don't absorb more heat, so we can ignore them and assume the remaining heat goes into steam production, and all the steam produced is produced in that 8 minutes of engine running.

This does not all go through the engine as some is condensed in heating the piping and cylinder, but it left the boiler as steam, so it is still steam production.

We had 142 g of steam produced in 8 min, so if we assume  an even rate we have 17.75 g/min or 0.296 g/s.  Given the assumptions, I probably should have called it 0.3.  Certainly three significant figures is not justified, but it helps with making calculations consistant.

With these basic measurements that we can all do, we already have a rate of fuel consumption, boiler heat input and steam production.

The next step involves looking at steam tables to determine the enthalpy and entropy of the steam.  From these we can then work out the boiler efficiency from how much heat from the fuel ended up as energy in the form of steam, and we can calculate the work an ideal engine would produce, and even how much work our engine produced, though that requires further explanation later.

The steam tables I have do involve quite large steps, so they do not have the precise temperatures or pressures that my boiler is operating at.  Steam properties are also published in graphical form, a common one using temperature and entropy as the axes.  Another uses enthalpy and entropy, and some even use pressure and specific volume.  They are hard to use with any accuracy, but they show the general trend of the information.  A close look at any of these will show that most properties appear as curves, with the only straight lines appearing in the two phase region where liquid and vapour both exist in equilibrium.  Fortunately, when finding the value of properties between the ones tabulated in the tables, it is normally considered accurate enough to assume the properties are linear between any two consecutive listed values.  This means we can use a simple linear method to construct a table with the additional values we need.

The boiler temperature of 116 deg is actually listed in the pressure tables, (well actually 116.06, but surely close enough) where the equilibrium pressure is 0.175 MPa.  Yes the tables uses MPa instead of kPa, however it is easy to multiply the small numbers we need by 1000, so no real problem, and no interpolation needed.  We can directly see the enthalpy, (and entropy of we needed it) of both the saturated water (hf), and the saturated steam (hg) at this temperature and pressure.

The pressures are absolute, so 0.175 MPa is 175 kPa absolute, say 75 kPa(gauge).  For the metrically challenged, about 10.9 psig.  The boiler is good for 700 kPag, and in any case has a properly set safety valve which I ease with the pliers before starting, so no problems there.

So my boiler temperature tells me the steam pressure in the boiler is 75 kPag, and the air, which causes additional pressure is soon expelled as soon as I let a bit of steam out of the boiler.  Still a gauge would be nice to have.

Next we have the engine inlet temperature of 138 deg C.  This is after the superheater, (actually even after the steam pipe and lubricator) and so we have at least 22 deg C of superheat for those who thought our super heaters were little more than driers.  You will see that it is enough to be useful.

My superheated steam tables unfortunately do not include 0.175 MPa, nor do they contain 138 deg C.  As well, they are in deg C not K, as 138 deg C is 600 K, and that might have been included.  You might be luckier when you do a test.  Worth finding tables listed each way if you come across them.  This means we have to interpolate the superheat tables, first to get a table for 0.175 MPa, then again to get values for 138 deg C.  Not very easy to work to a desirable superheater outlet temperature, as it would require adjusting the length, and even then would probably not achieve the same result at another boiler load or firing rate.

This might all be a bit tedious for those that already know how to do it, but I want to bring along those that have never used a formula in a spreadsheet as well, so please bear with me.  But enough words this time and interpolation next time

Thanks for looking in patiently.

MJM460
 
Title: Re: Talking Thermodynamics
Post by: paul gough on September 11, 2017, 02:13:44 AM
A typo that might confuse a novice from paragraph on heat input from fuel. "So note that 26000 kJ/kJ is the same as 26000 J/g." Regards Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 11, 2017, 04:44:17 AM
Thanks, Paul.  Well spotted. 

I read it all at least three times, but things still slip through.  Often Apple makes very puzzling corrections, but that one was surely all my own work.   Assistance in finding these things is always welcome.

It is now corrected.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 11, 2017, 11:37:35 AM
I notice the exhaust pipe is also lagged ,Is that so you can collect the condensate from the bottom of the chimney ??
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 11, 2017, 12:37:51 PM
Interpolation

Hi Willy, welcome back.  I hope the show went well.  Two reasons for the exhaust pipe insulation.  First, it reduces the risk of burning my fingers, it is still over 100 C.  Second, quite the opposite of collecting the condensate, it reduces condensation in the exhaust pipe, so the vent steam is more dry, and so less hot  droplets raining down.  The dry steam soon mixes with the air and is dissipated before it condenses.  Remember that condensation involves rejection of heat, the opposite of evaporation in the boiler.  On the other hand, the oil does drop out in the separator and seems to mostly end up in the drain tin.

Following on from yesterday, we need to do three interpolations of the steam tables to get values for the missing temperature at the missing pressure, but it's not too hard with a spreadsheet.

I start by putting the missing temperature into the nearest two pressure tables in the superheat section of the steam tables, that is 0.1 and 0.2 MPa.  So let's start with the 0.1 MPa table.  In my table the nearest temperature below 138 is 100, and the nearest one above is 150 deg C

I set up a little table like this in my spreadsheet-

T1,  v1,  u1,  h1,  s1
Tx,  vx,  ux,   hx,  sx
T2,  v2,  u2,  h2,  s2

Please read the 1, 2, and x as subscripts, and obviously T1 is 100, v1, u1, h1 and s1 are the values at 100, Tx is 138 and the x subscripts refer to the values at 138 which so far are unknown, and the 2 subscripts refer to the values at 200.  Now, I transcribe the known values listed in the tables into just this small table.

Then I construct a formula for finding ux.  (I will get back to vx shortly)

ux = u1 + (Tx- T1)/(T2- T1) x (u2 - u1)

I type this formula into the cell for ux in the spreadsheet and when it is complete, I exit the cell.

Remember in a spreadsheet, a formula starts with an "=" sign.  You select the cells you want to use with the mouse and the spreadsheet will insert the cell reference rather than the number within the cell.  Use all the signs, including brackets, but no spaces.  In Excell, Open Office, and others you exit the cell by pressing enter.  In Numbers, on an iPad with a touch screen, you touch the green tick.  And the answer appears in the cell as the number you are looking for.  Obviously it should be a bit above 1/3 of the way between the values for u1 and u2 as a rough rationality check.

If you are not familiar with the procedure it may seem a bit tedious, but now comes the powerful bit.  You can copy that formula and paste it into as many places as you wish.  But there is more, as they say on TV.  The spreadsheet shows the cell references for the cells you select, perhaps B3 etc., but it remembers them as relative positions, eg cell above, cell below, cell two to the left etc. from the formula location.  This means you can copy it to another place, providing all the cells you want are in the same relative locations.  But you can change this by using absolute references.  This is indicated in Excell by a $ sign.  You can do three things.  With one $ sign $B3 means always column B, but the same row relative to where our formula is located.  B$3 means always row 3 but the same column relative to the formula cell location.  Then with two $ signs, $B$3 means always cell B3, regardless of where on the spreadsheet the formula is located.

In this case, the temperatures are always in the same column regardless of whether we are calculating v, u, h or s, so the cells in the formula which refer to a T1, Tx or T2 need to be edited with $ signs to fix the column, for example $B3.

I can now copy my formula for ux and paste it into the cells for vx, hx, and sx, and job done.

You may have noticed that v is smaller at T2, while all the others are getting bigger.  The same formula is used, the required negative sign is already there in the term (v2-v1) as v2 is the smaller value, so no problem at all.

Now the same process to insert a 138 deg row in the 0.2 MPa table.  Then I set up a third table consisting of the two 138 deg rows just found, and with the pressures 0.1, the required 0.175 and 0.2 in the first column instead of temperatures.  With the pressures in this location, the formula remains the same, so I can copy and paste the same formula into this third table, and I now have the table I need for 138 deg C and 0.175 MPa.

I hope that if you have been reluctant to use formulas in a spreadsheet, you might now feel encouraged to try.  That first one may well have taken longer than a pencil, paper and a calculator, but in this exercise I copied that formula, then pasted it 15 times with 15 mouse clicks and job done.  All of a sudden the initial time was well spent.  (The extra four were for that 104 deg exhaust temp.). Don't hesitate to ask a question if this is not clear.

So here is what we have so far -

Boiler 116 deg, water and vapour together means 2 phase region, so use saturated steam pressure table which gives P = 0.175, vf = 0.001057, vg = 1.1593, hf = 486.99, hg = 2700.6, sf = 1.4849, sg = 7.1717

Engine inlet 138 deg superheated, pressure assume equal boiler pressure = 0.175 MPa, interpolation gives v = 1.167, h = 2745.91, s = 7.3018.

Engine exhaust 104 deg, p = 0.1 MPa ( actually atmospheric, assumed 100 kPa), v= 1.72, h = 2684.2, s = 7.380

Units, Pressure MPa, (= 1000 kPa), specific volume, v, m3/kg, enthalpy, h, kJ/kg (= J/g), entropy, s, kJ/kg.K (= J/g.K).  K is for Kelvin, the absolute temperature unit for metric units.

Well, that is the tedious part, we have the figures for both steam rates and steam properties, but what do they mean, and what can we learn from them?

Next time, I will apply the first and second laws of thermodynamics to the boiler and to the engine for some interesting and even surprising results.

All eyes open for any typos that need correcting please,

Thanks for looking in.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 12, 2017, 12:18:32 PM
Calculated Steam properties for test and Boiler performance -

Another small correction to the post about two days ago, the total fuel burning time was actually only 14 min 40 seconds, so the fuel burned was 0.0477 g/s and the heat released 1240 watts.  I have now corrected that post.  Another case of old eyes dropping a line, or something like that.  Please let me know if you see any others.

All of those figures we gathered in the last post can be a bit daunting and hard to picture, so it will be helpful to portray them graphically.  All our readings were actually temperature, so we will use that for the vertical axis.  The second law will have us looking at entropy, and it will be useful to have that as the second (horizontal) axis.  This Temperature - Entropy diagram is quite commonly used to portray a plant cycle.  Pressure, specific volume and enthalpy can then all be shown on the diagram, but better not add too many or it will get very cluttered.  You can also illustrate your cycle on pressure - volume or even enthalpy - entropy if that makes things clearer to you.

I have hand drawn the diagram attached, but tried to keep everything in the correct relative positions.  The bell shaped curve is the boundary of the two phase area where water can exist as liquid and vapour at the same time.  It is bounded by a horizontal straight line at the bottom below which water will be solid.  To the left of the curve, water is liquid, and to the right, vapour.  Above the top of the curve, called the critical temperature, which has only one pressure, the critical pressure, water changes from clearly liquid on the left, to clearly vapour on the right with no clearly distinguishable boundary between, unlike the distinct liquid, vapour two phase region that we are so familiar with.  I should point out that the peak of the bell curve is not correctly shown in proportion, it should be at 22.09 MPa and 374.14 C, the critical pressure and temperature for steam.

I have drawn the constant pressure lines at 0.175 MPa and also for 0.1 MPa to illustrate our steam plant process.  The 0.175 line should extend to the left and down to room temperature, but that part is only important before we start generating steam, the heat up period, which was about 6 min 40 seconds.  So point 1 is the saturated water at vapour pressure for 116 deg C.  Point 2 is the saturated vapour condition, steam 116 deg C just at the point superheating starts.  Point 3 is actually the engine inlet, but I am assuming close enough to the superheater outlet, as the line between is insulated.  Point 4 is the engine outlet temperature measured in the test.  We cannot easily calculate this condition, it can only come from a test.  In full size, the manufacturers have some pretty good computer programs, so they can predict the performance for guarantee purposes, but in the end, those programs contain an efficiency which comes from a large number of previous test runs of similar engines.

So the obvious question, how have I arrived at points 4 and 5?  Let's look at the thermodynamic analysis of this process.  For the boiler, the first law says Q = h3 - h1.  That is, the heat transferred into the boiler during steam production is equal to the difference in enthalpy between the superheater outlet and the saturated liquid point.  I will come back and put some figures in that shortly.  Then for the engine, where we want to determine the work output of an ideal adiabatic engine, the second law of thermodynamics says s5 = s3 for an ideal adiabatic engine.   An adiabatic engine is used as our standard for comparison because no real engine can produce more power than an ideal adiabatic engine.  I know the exhaust pressure of both the ideal engine and our model, it is open to atmosphere, so it is atmospheric pressure.  In a perfect world I would have measured it with a calibrated barometer, but I didn't, don't have one, so I am assuming it was 100 kPa (absolute) or 0.1 MPa.  Two independent properties are enough to define all the properties of steam using the steam tables. 

Note that point 5 is shown inside the two phase region.  The value of entropy from point 3 at 0.175 MPa, when applied to the 0.1 MPa line in the steam tables confirms that to be the case.  In that area, temperature and pressure are not independent, as the is only one possible pressure for each pressure where liquid and vapour are in equilibrium.  However, temperature and entropy are independent so are sufficient to completely define the steam properties.

On the engine exhaust, I only measured the temperature.  However, the pressure is the same as at point 5, at atmospheric pressure, i.e. 0.1 MPa, where the saturation temperature is known at only 99.6 C, so our measured temperature of 104 deg means our actual exhaust is superheated.  In that area, pressure and temperature are independent, so sufficient to completely define the steam at the exhaust.  You will notice on the diagram that my engine exhaust is shown with higher entropy than it had at point 3.  The second law says that no engine exhaust entropy will be less than the inlet, and only for an ideal reversible engine, will it be equal.  For all real engines, the exhaust entropy will be higher than at the engine inlet.

For the purpose of calculating engine performance, I need to calculate the enthalpy at points 4 and 5.  The first law of thermodynamics says for the ideal engine, W = h3 - h5, and for my real engine, W = h3 - h4.  That is the work output equals the difference in enthalpy between the inlet and the exhaust.  We can perhaps accept that for the ideal engine by assuming no friction, but our real engine requires further explanation.  We will get to that later.

Now we have all the information we need, and have defined our method, so let's put in some figures.

For the boiler, the heat input h3 - h1 = 2745.91 - 486.99 = 2258.9 kJ/kg ( or J/g).

My steam production rate was 0.296 g/s, so heat in the steam = 2258.9 x 0.296 = 668 J/s = 668 Watts.  I can compare that with the heat from the fuel burned, which was 1240 watts.  I can conclude that 668 J/s of that heat was transferred into the steam, the rest was lost partly up the stack, and partly through losses from the boiler casing.  That means a boiler efficiency of 54%.  I think perhaps acceptable for a simple boiler.  I was hoping for a bit more.  Perhaps I will be able to improve it with some casing insulation, and perhaps some experimenting with air flow.  In another post, I will look at the heat transfer surface area and see if I can calculate a steam production per unit of surface area to compare with what K. N. Harris suggests.  I suspect it will be lower than his as I think he assumes coal firing, which I would expect to produce a much higher temperature than my small Meths burner.

Surprising how long it takes to describe some of these things.  I hope it is sufficiently clear to illustrate the procedure, and provide enough guidance for anyone who would like to do such a test on their own engine.  Please ask if anything is unclear.  Now I think it's time to take a break, and calculate the performance of that ideal engine next time.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on September 12, 2017, 10:06:23 PM
Now this is really interesting stuff!!! These numbers start to give me a ball park notion of what might be going on in my tiny Lion boiler and give me more inspiration to clamber up the learning curve. Unfortunately my computer skill ,(I don't have any), doesn't include spread sheets,  so I'll have to fall back on the old laborious ways to work things out when I master the methods. I remember many many decades ago when I was involved in Herpetology and running around sticking quick acting thermometers up tiger snake cloacas trying to determine their preferred body temperature we were lent some thermocouples and a millivolt meter from the uni. Less intrusive for the animals than a glass tube and made the job a little easier. These thermocouples were hand made by the techy at the uni., I think they were silver and something and was wondering if you knew of the metals used in the ones suitable for our steam temperature ranges. It might be possible for model engineers to make them?? Looking forward to the results on the engine. What a great thing it would be if a whole bunch of people rigged up their models and took some measurements and analysed them, maybe some interesting things might result from a broad investigation on our small sizes. Especially things like the effectiveness of lagging, its minimum thickness etc. etc. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: derekwarner on September 13, 2017, 01:30:49 AM
Morning Paul......without digressing too far from the thread, I have found my $15.00 digital laser pyrometer absolutely invaluable in understanding temperatures in my steam plant...[with simple understanding of actuals]...[without these, all I knew was that things were hot :Mad:]

My boiler steam discharge valve presents as 135 degrees C, however this is an external reading spotted on the body of the valve - from the steam tables, the boiler at 3 Bar  is producing steam at ~~142 degrees C....so everything is relative in comparison]

I can trace the temperature to the lubricator body, the steam regulator, the steam inlet fitting to the engine....the exhaust and all the way back to the de-oiler] - monitoring gas temperature is interesting......including lagging of the gas line

One simple point I can confirm [before & after test] is that insulating the steam tube from the boiler to the engine, provides steam that is ~~3.?? degrees C degrees hotter that in the uninsulated state....[and this is over approx. 220 mm long run of 1/8" OD copper tube]

My interest in this area was that I was producing too much water condensate in the de-oiler and needed to understand what was happening. To this end I have increased the exhaust tubing from a combination of 1/8" and 5/32" to 1/4" x 0.014 full flow K&S brass

[The insulated tubes with the X's were the 1/8" & 5/32" exhaust tubes to and from the de-oiler which are now redundant]


Whilst we are thinking seriously about efficiencies, a $15.00 set of digital scales [the type available were 7kg max]  are essential for confirming the volume/weight of gas burnt......[my gas tank has a published volume of 105gm......I have managed to get 103gm inserted at 26 degrees C]

Apologies MJM for deflecting out of line.... :happyreader: ....Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 13, 2017, 06:01:38 AM
Hi Paul, thanks for your encouraging words, and for being kind enough not to mention that it was not one of my best efforts.  Sometimes a day does not go quite right, even when building a knowledge base.  I am determined to do better on the engine performance.  But first your thermocouples.  Obtaining these is much easier these days.  The go-to combination in industry is Copper-Constantin for all except quite special temperature ranges.  I believe the junction is electrically welded, but no need to worry.  Nearly all reasonable digital multimeters have one supplied.  The temperature- voltage characteristic is well known, even laid down in international standards.  It is a bit non linear, but that is all built into the multimeter.  Probably less expensive for the whole combination than trying to buy a length of the wire in hobby quantities plus plugs to fit your meter.  I included a picture of one of mine earlier, and Willy included a picture of his which had a nice stainless steel sheathed probe.  I have to experiment to see how the probe goes with my thermowells, but it would be great for stack gas, as well as for Willy's coffee.  I even checked mine at ice point and boiling point to see if it was really correctly calibrated.  That is probably over the top, but all the ones I have tested have come up spot on, give or take the unavoidable experimental error involved.  (Actual atmospheric pressure for boiling point, plus the difficulties of getting a true equilibrium ice/ water mixture for the ice point.  It takes lots of ice, only enough water to let you put the thermocouple in.)  Tempted to include a "war story" about purchasing small quantities of thermocouple wire, but I believe this is strongly discouraged.  If you don't have s temperature scale on your meter, it is worth considering upgrading your multimeter, Christmas is not far away.  Avoid the cheapest.  If you look at the wire into the plug, some are very flimsy.  However if your multimeter has the temperature scale but no probe, Jaycar and Altronics both sell both types separately at a reasonable price.

Back to the ideal adiabatic engine performance - see if I can make it clearer this time!

Refer back to my little sketch.  You will remember that I measure the boiler inside temperature where steam is saturated and both liquid and vapour exist together, so the steam tables gave me the pressure of 0.175 MPa (absolute).  I assumed that pressure still at the engine inlet, as the pipe is short, and no throttle valve.  I measured the engine inlet temperature, 138 deg C.  It is above saturation temperature, so steam there is obviously superheated, so temperature and pressure are independent, and two independent properties are necessary and sufficient to determine all the other properties.  I interpolated the steam tables to get the remaining steam properties, particularly specific volume, enthalpy and entropy.

Paul, you will not be the only one not used to spreadsheets, so try following the procedure from post #267, one step at a time, to put in that first formula for interpolation, and let me know what point you get into trouble.  Spreadsheet formulae are so useful for so many purposes that I don't want to leave anyone behind on that one, even if you don't want to go much further.  If you have a computer with Office on it, try Excel, or you might download Open Office, (it's free).  On an iPad, or Mac, worth buying Numbers, but there are many others.  Lotus and Multiplan were two very early ones, they are all sufficiently similar until you try much more advanced functions.  I am sure that you will not regret the effort.

The engine exhaust is more tricky.  We know the pressure is atmospheric, I assumed 100 kPa, but the steam could still be either wet or superheated.  For my real engine, I measured the exhaust temperature as 104 deg C.  Now the steam tables tell me that for 100 kPa, or 0.1 MPa, the equilibrium temperature is 99.6 deg, so our exhaust is superheated.  Hence pressure and temperature are independent and hence sufficient.  Another row of interpolation of the superheat tables and I have my exhaust steam properties.

Real engine exhaust steam by interpolation -
T = 104, P = 0.1, v= 1.72, h = 2684.22, s = 7.380.


It's worth noting that if you don't have a superheater, or only an ineffective one, your exhaust steam will probably be wet steam, and we are stuck, but superheated exhaust is more informative.

Before we look at what that means, let's look at the ideal reversible adiabatic engine performance.  Again, the exhaust pressure is 0.1 MPa, but where are we on the curve?  This is where that property, entropy comes in.  The second law of thermodynamics says for an ideal adiabatic engine operating between points 3 and 5 (on my sketch), s3 = s5.  We already have s3 = 3.018 from interpolation of the steam table for the engine inlet, so s5 = 3.018.  Two independent properties, P and s, so the steam is completely defined, but how do we find those other properties?

First, look at the saturated steam table row for 0.1 MPa, and check the entropy columns.  You will see that our ideal engine exhaust entropy of 3.018 lies between the dry vapour value, sg, and the saturated liquid value, sf, which means that the steam is wet.  For we steam we can calculate a quality factor, or dryness factor.  Quality is another of the properties, but it only exists in the region between saturated liquid and dry saturated vapour.  My text book gives it the symbol x, and you can think of it as the mass fraction of the steam in the boiler which is vapour, the rest being liquid.  For our ideal adiabatic engine exhaust the dryness fraction, x is calculated as follows _

x = (3.018 - sf)/(sg - sf). We substitute the values from the steam table and then

x = (3.018 - 1.3026)/(7.3594 - 1.3026) = 0.99 or quite close to saturation.

Now we use the dryness fraction to calculate the other properties by simple linear proportion.  We really only need h5 so let's calculate it.  We look up hf= 417.46 and hg= 2675.5, then

 h5 = hf + 0.99 x (hg - hf) = 417.46 + 0.99 x (2675.5 - 417.46) = 2652.92

If you are using paper and calculator, you will notice that between hf and hg columns there is one headed hfg.  The definition is  hfg = hg - hf to save you one subtraction.  At one time, none of us had computers, or even calculators.

Now, still talking about our ideal adiabatic engine, we apply the first law of thermodynamics which tell us that the work done on the piston, W, by each kg of steam is W = h3 - h5 = 2745.91 - 2652.92 = 92.99, say 93.  The units used by the tables are kJ/kg, or J/g.

We multiply this by our steam flow rate, 0.296 g/s to get 27.53 J/s or Watts.

That is the work done by an ideal adiabatic engine with my steam conditions and flow rate.  No engine can exceed that, and any real engine will produce less!  Not very impressive, only 2.2% of the heat released by burning the fuel, but it cannot be exceeded, or even matched by any real engine.  No point beating myself about the head because I can't get 30%.

Now to the really interesting one, the exhaust of my real engine.  What can that tell me?

That will be our topic for next time.  This one has been a bit long, but where to split it?

I hope that everyone is still with me,

Thanks for following along.

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on September 13, 2017, 06:29:55 AM
Thanks Derek for those excellent photos of your beautiful engine and boiler.  Your post appeared while I was inserting mine.

No apologies needed, you are right on topic.  I believe I have at last a clue as to where your condensate comes from.  I am attending my granddaughters choir performance this evening, hence my early post.  I gather from the pictures that you might not have a superheater, so after the next post, I will have a go at calculating the exhaust steam properties, starting from saturated steam instead of superheated.  It might reveal something interesting.

I also have an infrared thermometer. It is very useful, but as you are aware, it has inherent errors so is best for comparative measurements.  The outside of a tube is always cooler than the steam inside unless the tube is well insulated, but then you can't use infra red.  However, the difference between ends of a tube is probably more than close enough to the difference in temperatures inside.  Interesting that the insulation makes 3 degrees difference.  I suspect that 1/8 is very small and a significant restriction in that engine, close to being a laminar flow orifice, like a domestic fridge.  You don't want to fill your piping with temperature elbows like I have done in your boat, but on the bench for test purposes, they can be very instructive.  However, replacing the plug in your boiler with a thermowell allows use of a thermocouple to check the gauge pressure reading any time, even in the boat, by just poking in the thermocouple, so I would think may always be worthwhile.

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on September 14, 2017, 12:51:54 AM
Thankyou Derek and MJM for the comments regarding thermocouples/multimeters and digital thermometers, things have certainly advanced in this area since the early seventies when I was getting preferred temperature ranges of reptiles and I will look around to see what is available.

MJM, I am interested to know what constitutes a good thermowell. I take it these are used where the temperature probe cannot be inserted into the medium for direct measurement through a gland nut arrangement. Obviously one wants to get the closest to the internal temperature as possible, so does one manufacture the orifice of the well to a push fit for the probe dia. and make the profile at the bottom the same profile as the end of the probe to get contact here also? Or alternatively, does one use a clearance around the probe and some proprietary filler that sets to gain the appropriate heat transfers. The design of a good thermo-well would seem to me to be an absolute necessity to avoid introducing errors and ending up with spurious results. Are there any other considerations or do's and don'ts regarding the acquisition of data and trip ups with instrumentation etc.

Derek your temps regarding insulation are interesting, however it would be helpful to know the material used and the thicknesses. From my experience in full size stationary steam facilities pipe lagging is quite thick, as a rough guide it was as large in diameter or often larger than the size of the flanges on the pipes and nearly always had sheet metal cowling around it. Now dragging from the depths of memory, (1960s), regarding steam locos, I think the asbestos lagging rope used around the steam pipes on NSWGR locos, eg. to the air pump, was about 1/2 inch, so the dia would be roughly twice that of the pipe. I wonder if lagging on models is adequate, have you done any tests on a pipe or a vessel with a set thickness then increased it by, x2,x3,x4 thickness to see what the optimal thickness of a particular insulating material is?
Title: Re: Talking Thermodynamics
Post by: derekwarner on September 14, 2017, 01:38:41 AM
Paul......

The detail of the model lagging for the 1/8" OD copper tube from the boiler steam isolation stop vavle to the engine is as follows

3.175 diameter tube.....add a 1.78 diameter section Viton o-ring [to act as a diameter guide] at each end of the spool, wrap 1.0 diameter cotton string around the spool [superglued at ends & occasionally at bends] , paste a wet premixed cellulose [Bunnings type] Polyfiller material into the string and to exceed the major diameter of the O-Ring...[this may take 3 or 4 coats of Polyfiller & sanding to achieve the oversize diameter]

After sand back to the uniform diameter, two coats of enamel primer, one coat of gloss enamel paint....so the insulation OD for this tube is ~~ 8mm diameter, or a nominal 2 mm wall thickness.

I have bent & set ~~ all tube spools to include 90 degree bends as this lagging material has very little resistance to bending and stress cracking will appear through the enamel top coat at bends if care is not taken...[this also requires the spool fittings have sufficient clearance when being dissembled so as not to require any moment of bending]

On area of concern is the non insulated fittings in the line of components [lubricator, regulator and the entry fittings to the engine itself].....my Scottish steam regulator is a 20mm bronze cube ...[I have lagged external faces of the regulator with timber planks] but it is clearly a source of heat loss...[image below]....these screwed of flanged fittings in the lagged steam line provide an excellent point for the temperature comparisons....being as such, provide totally repeatable set points 

For exhaust lines I have used the similar insulation relationship which is governed by the 1.78 section O-Rings for the correspondingly larger diameter tubes etc

I have not conducted any varying diameter lagging test comparisons, suffice to say I can place thumb & forefinger around the steam line directly below/from the boiler isolation valve [135 degrees C]....and the lagging OD is hot, but not sufficient to burn skin

Sheet metal sheathing over lagging on full sized steam applications is I suggest only to provide resistance to any form of mechanical abrasion, as wet pasted or preformed asbestos and latter synthetic insulation material is very soft

Lagging in full sized applications includes covering all flange & connection points or and inline component parts as these are potentially a source of gross heat loss

Derek



Title: Re: Talking Thermodynamics
Post by: paul gough on September 14, 2017, 07:55:13 AM
 Derek, thanks for the detailed reply. My experiences in four full size plants has seen most of the pipe flanges exposed, i.e. high temp hot water heating systems for a uni. campus, fruit juice processor and food manufacture, only the drug manufacturer lagged most of the flanging. All these were below 200 psi though and accept that these plants were not chasing efficiencies as one of their primary goals. Interestingly the uni underground distribution system of quite a few kilometres, (about eight from memory), had eight and six inch piping with no insulation, the ground was the insulation! This was sometimes helpful in winter, a leak could be found by going for a long walk following the route of the piping, a telltale wisp of water vapour or a relatively warm wet area somewhere near the spot would indicate the leak. My experience with small, other less 'organised' plants has demonstrated insulation is often a very neglected area or of no concern to the operators. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: Zephyrin on September 14, 2017, 08:33:31 AM
some friends at my club use a ribbon of plaster for orthopaedic contention to hold a layer of insulating stuff around pipes, (just like on a broken leg !), a paint layer or a teflon band keeps away the moisture.
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on September 14, 2017, 12:03:05 PM
Here in Denmark we have real central heating - ei. Aarhus where I work has 300,000 people living and the heating is provided from two plants for the whole city + some villages are ; one being the electricity plant and the other the waste incinerator.

"Water" leaving the incinerator plant is between 200 and 250 degree C and around 80-90 degree C when it arrives to the buildings some 10-100Km. down the line. I don't know if there is any heat exchangers along the way.

The pipes are insulated so that the outside diameter is tre times the metal pipe in the middle - local pipes are around 250mm. ~10" and the ones leaving the plants are 2-3 times that diameter.

Pipe integrity is tested continuesly with the help of two copper wires running parallel in the insulation for the whole length of the piping. An electric pulse is transmitted out through the wires and the reflection is measured. If the resulting reflection arrives back at the right time and has the correct shape, all is well and a new pulse is transmitted. Any dammage to the insulation results in an earlier reflection and the time from transmission to reception gives the exact distance to the dammage - so they know where to dig even though there might not be a visual leak at the site.

The pipes where not insulated in my childhood and that was also the reason the rusted and broke - plus no snow or ice on the surface over them - but even back then we never had the heat missing for more than a few hours and after the insulated ones I have newer experienced any breakdowns.

EDIT : I really should make a point about the fact that the heat would have been wasted / is a waste product from the primary function at the two plants + it is a nice in the winter where we are around -10 degree C.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 14, 2017, 01:40:57 PM
Hi Paul, I don't think the thermowell design is over critical, the main thing is to provide a way of inserting the element so it is surrounded by steam without compromising the pressure containment.  No problems with liquids creeping through the wires and their insulation etc.  The main issue with contact resistance is that it slows the response of the thermowell which adds to all the other issues that make temperature measurement inherently slow response.  Special quick response designs are available, but not really needed for our application.  The first approach is usually to put a few drops of glycerine in the thermowell to improve the heat transfer, but very messy in a model where we probably do not really intend a permanent installation.  Slow response alone does not affect accuracy, as in principle, there should be no continuous heat loss once the thermocouple is heated.  So the next issue as you imply, is conduction away from the fitting where it is in the atmosphere and conduction along the wires.  Some insulation around the insertion point in the piping is probably more important on our models as the insertion fitting is closer and more bulky relative to the thermowell depth.  In industry, vortex shedding induced vibration leading to breakage is more of a problem than minor heat losses.  I make my models along the lines of the industrial ones which are basically machined from a bar, and internally drilled for the thermocouple element.  Small ones usually screwed into the appropriate pipe fitting, but larger ones are flanged.  The sheathed thermocouples could be inserted through a gland, but I think the simplicity and reliability of a thermowell is a more practical solution.

You have prompted a few ideas with your insulation comment.  Very interesting to see the different approaches in different environments.  In hydrocarbon industries, insulation falls into two basic categories, hot and cold.  Cold insulation has to be right, and cover complete, otherwise ice breaks the insulation off.  Hot insulation is applied for safety reasons, to reduce the surface temperature so people do not get burned.  And of course also for heat conservation.  It is generally a compromise between what can be maintained with normal practice, and cost.  The cheapest part, and the most result in terms of reduced heat loss is the straight pipe sections, usually with special formed sections for elbows.  Much more fiddling required to cover flanges and valves etc. which are really a relatively small part of the total heat loss.  However there are generally also higher quality specifications which are applied when heat conservation is more important.  Generally we used to use magnesia, but Calcium silicate, foam glass and fibre glass are increasingly used, unfortunately.  I say unfortunately because they wick up any oil spills then become flammable.  Installations are almost universally outdoor, so metal sheeting is applied for weather protection, and also for preventing mechanical damage, which in plant sizes includes people walking on the piping.  Obviously some different considerations from those of well protected indoor installations.  Some experimentation on different thicknesses would be interesting, but might be best carried out on a test rig with a steam pipe say 1 metre long, so there is some chance of measuring the reasonably small differences.  The first layer makes the most difference, then diminishing returns.

Thanks Derek, for describing your methods in detail.  You certainly achieve a good looking result.  As you say, avoiding damaging the insulation after installation can be a challenge.  Looks very realistic for what I have seen in ships engine rooms.  My silicone tape is not so elegant, but it is flexible, and I can always add another layer.

Hi Zephyrin, I have also heard of plaster of Paris being used in this way, I guess the source of the materials depends on who you know.  But that plaster soaked gauze is a very convenient way of applying the paste and then reinforcing the set material.

Admiral _dk, thanks for that description of your district heating plants.  Don't have them here, 37 C today, and Spring has barely started.  Also, a very interesting method for detecting insulation faults.  It is a real problem for such extensive systems, and these days, even that low temperature heat is valuable.

I wanted to get back to the calculations on that exhaust steam, but it is already late, so next time.  I am back in the long paddock, having turned my head towards home the short way.  The Buildup has started, should have left for home a week ago.  Today's temperature was accompanied by quite high humidity, which persisted through the night.  Seven thousand two hundred kilometres so far, but only 4400 home from here now.  And should be able to post most days.  Please understand if I miss a day, as there are some gaps in the service.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 16, 2017, 10:35:54 AM
Real Engine output -

Hi everyone, Internet gaps appeared quicker than I anticipated, so glad I had mentioned it.  Usually available each night, but the place we found last night only had the wrong carrier.  Along the long paddock, there are occasional reflecting antenna dishes with a little pad at the focus.  If you put your phone there, you get Internet and or phone reception, but not even a meter away.  Ok for email and messages, and stuff that can be read off line, but not much help for reading an active forum.  I will try and stop next time I see one with enough warning, and get a photo.  Possibly three or fours days time, as we should be good for the next two nights.

Last time went into acknowledging very interesting and relevant comments that people had posted, so tonight back to the test run data.  You might remember that I had calculated the power output of  an ideal adiabatic engine, using the second law to determine the exhaust entropy, which defined the steam condition so allowed calculation of the enthalpy.  The first law then allowed us to calculate the output of that ideal engine as 27.5 watts.  Next, we note that we have a measured exhaust condition for the real test engine, it was superheated to 104 deg C at atmospheric pressure, so we could find the exhaust enthalpy directly by interpolating the superheated steam tables as 2684.2 kJ/kg or J/g.

Now the first law says the work produced by our real engine is h3 - h4 = 2745.91 - 2684.22 = 61.7 kg/kg (J/g).  We multiply this by our steam rate, 0.296 g/s also W = 61.7 x 0.296 = 18.3 J/s.

Not much compared with 668 watts in our steam, only 2.7 % efficiency, based on heat in the steam, even less based on fuel energy.  But it is 66% of the output of that ideal engine, which was only 27.5 watts, and we know that cannot be exceeded or even equaled by any real engine.  So, not so shabby after all.  If I was an engine manufacturer, I would try and use an efficiency defined by comparison with an ideal engine in my sales pitch!  I think both the actual numbers, and the fact that they can be calculated at all is the most interesting thing about the whole exercise of seeing what can be found out by applying basic instrumentation and the laws of thermodynamics to a simple test run. 

Of course you are wondering how we can calculate a power output when the engine was running uncoupled, surely not producing any output at all.  It is quite correct that the engine was not producing any shaft output power, but the observation helps our understanding of what these calculations really mean.  And another illustration of different definitions of power.

The power output calculated from the first law, (h3 - h4) x steam flow rate, gives the work done by the steam on the piston in the power strokes.  Remember way back, how heat is converted to work?  Portions of this work are then used inside the engine, before we get anything to the output shaft.  Some goes into pushing the exhaust on the other side of the piston out through the ports, even before there is a net differential pressure and hence net force on the piston.  Some of the force on the piston goes into overcoming the friction of the piston in the cylinder and the remainder produces the torque necessary to overcome friction in the bearings, and pins.  Some steam, it may be quite a bit in my case, may even bypass the piston by flowing between the piston and cylinder, thus producing no work.  I only have one or two labyrinth grooves, and this will not be nearly enough, especially if the piston clearance is too great.  However, wherever the work is used, it is not available at the output shaft, so I have to get more power out of the engine to have any output at all.  I suspect it would be a bit optimistic to hope to double the power produced by the steam without increasing the losses so that we had another18 Watts available to drive something, but so many of the engines on this forum do produce enough useful work to overcome more than engine friction, that I am sure I can get something out of it.  Obviously the next project has to be a brake of some kind to provide a predictable load for the engine for more tests.

Before I do more testing, as that must be on hold for the moment, is there anything else we can learn about where this power is consumed?  Now I am not a politician, I have posed the question without being sure how I will get an answer, let alone what troubles I will get myself into in the process.  But there are a couple of formulae that relate mean effective pressure and torque to the power of an engine.  I will use those to try a few calculations and see if they yield anything interesting for next time.

Thanks for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 16, 2017, 03:34:16 PM
Hi, I have just bought the two volume set of the John Farey  "A treatise on the steam engine"  from 1827 and what a treat that is !! there are 90 pages on MR Woolf alone !!!!! A quick question on his compound engines .........................as the HP cylinder is  supplied with full steam with no cut off, how much work is done by the LP cylinder that is moving anyway as it is coupled directly with the HP side ????...Answers on a very large postcard please !!!!!Also does the water level in the boiler have any significant detrimental effects on efficiency ?? Thanks for all this info from all our subscribers .
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 17, 2017, 02:17:21 PM
Hi Willy, the second question first.  Heat transfer is determined by three resistances in series, the film resistance for transfer from the hot gases to the copper, the conductivity of the copper, then the film resistance from the copper to the fluid on the inside of the boiler.  Obviously we would the first two should be unaffected by the water level, however the internal convection coefficient is very different for steam compared with boiling water.  Boiling water has a very low film resistance, or high conductivity, partly due to the specific heat of water and partly due to the agitation by steam bubbles which continually carry away the heated fluid so it is replaced by cooler water.  This effectively increases the temperature gradient, and hence gives much better heat transfer.  Transfer to steam is much less effective, partly the lower specific heat of the steam and partly because there is no vigorous mixing due to the bubbling, just normal convection.  Usually when calculating the surface area of a small boiler, only the area below the water level is counted, and heat transfer to the steam ignored.  When the water level is low, the heat transfer will be less, so losses to the stack more, and efficiency lower.  However, all of this is for a fired boiler.  For your electric boiler, this does not apply, as the element should always be fully submerged in liquid, while the boiler shell should be well insulated so there is minimal heat loss.

The second question is a little more difficult.  I believe that what happens in the compound engine is that the volume of the exhaust side of the hp cylinder is getting smaller during the exhaust stroke, while the volume of the inlet side of the lp cylinder is getting larger, but at a greater rate due to the larger diameter.  So the total volume contained in the exhaust side of the hp plus the inlet side of the lp is getting larger or expanding.  At the hp piston face, work is being done on the gas, the energy coming from the steam on the inlet side, while at the lp piston face, the steam is doing work on the lp piston.  As always, the work is the pressure times the change in volume, but calculating the actual amount of work is complex as the pressure is changing as well as the volume.  The work input by the hp piston means the process has heat input at the same time as work is being done so it cannot be compared with an ideal adiabatic process.  With the geometry and timing all known, I assume the process could be analysed, but it is beyond me.  Is that about the right sized postcard?

Regarding the additional calculations on my little engine test, I have tried to calculate an equivalent mean effective pressure, and an equivalent torque from the usual formula, without getting any sensible answers.  I think I am making a mathematical mistake somewhere, and just need some time to check it all.  Just not sure what at the moment, really puzzling.  May need to look at some other topics for and come back to that one, unless someone has noticed the problem or had more success with the numbers.  At least I have had some success with comparing a model engine with the thermodynamic model even with the simplest of test set ups.

Crossed the Tropic of Capricorn late today, back in the temperate zone, cooler days and cool nights (making sleeping easier) only four days from the tropical heat and humidity on the coast.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on September 17, 2017, 04:36:05 PM
Thank you Willy for asking these questions and a very great thank you to MJM for the lucid explication of heat transfer in the boiler, it caused me to have an "Of course, what a fool am I" moment. Despite knowing of the various boundary resistances and even more if you add a soot layer on the fire side and a scale layer on the water side I had completely missed grasping the differential in heat transfer between steam and water when considering the circumstances pertaining to the poor steaming of a particular design Gauge 1 loco, a 12inch gauge loco that often defied the efforts of those who fired her and the behaviour of a vertical boiler when it had lower water levels. This one element left out of my thinking has finally explained a conundrum of many decades for the vertical and the 12" loco and illuminates perfectly part of the poor performance of the G1 loco. It is one thing to 'know' a number of facts but missing one critical one or not putting them all together can leave one sorely bereft of satisfying explanations for the behaviour of things! I owe you a beer, VB I presume. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 18, 2017, 02:43:50 AM
>Hi, MJM, I will be firing up my boiler later on today BST and could you give me all the measurements and volumes and temperatures that i need to record. Will it matter how full the boiler is ,or do you just need the volume of water. The pressure gauge is not very accurate but could i slide my temp gauge probe under the lagging. any suggestions would be welcome. It will be coupled up to my engine so that will be fun . !!
Title: Re: Talking Thermodynamics
Post by: 10KPete on September 18, 2017, 04:24:00 AM
Geez, Willy, that's a beautiful engine.....

Pete
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 18, 2017, 11:00:57 AM
Hi Paul, I am glad that I could help explain those issues, at least a little bit, it is the main purpose of this thread.  No need for a beer, but if you are ever down south, let me know and we can perhaps share a meal or at least a coffee.  Remember we have the national steam museum there, and it has some amazing historical engines actually running.  With some better understanding it will perhaps be easier to work out a solution to the problem.  With the locomotive of course, like the electric boiler, the answer only applies to boilers which are fired at least partly around the outside shell.  Obviously if it is fired through a centre flue, the flue and any return tubes must always be covered.  Not sure if you have return tubes on those little boilers.  But if you have a film of calcium salts on the inside, that will also do it.  May just need a good clean out, perhaps with vinegar.  We recently passed through a place where the water had a distinct sulphurous smell.  But it quickly faded away, the locals say the sulphur disappears but H2S does that to your nostrils anyway, and the water seemed quite good.  But we noticed that all the scale, which had accumulated from all the places where the water filters up from the ground through limestone, fell off the kettle in only two days there!

Willy, I hope I am in time, we are 10 hours ahead here, or perhaps 9.5.  It is 7 pm here as I write.  I am with Pete, that is a beautiful engine, and great to see it complete after following your build.  Will you be running the new one, or just the mill engine for today?  For the boiler performance, if you can slip the thermocouple element in under the insulation, it will give quite a good idea of the steam temperature inside.  From this we can look up the pressure in the steam tables to compare with your pressure gauge.  As it will be saturated steam, not superheated, we  can also look up the properties of the steam directly, and calculate the steam flow assuming the element rating is accurate.  Of course it may not be accurate but we can roughly check it.

If you can weigh the water into the boiler, and at the end of the run, weigh the water that you drain out when it is all cool, you can determine how much steam was generated.  If you note the time that steaming starts, and again when you switch off the power, you then know how much energy was input during steam generation.  There should be some agreement between this and the output based on element rating.  Differences will arise because before your engine runs, steam will be condensed in heating your pipes and cylinder etc. so it is difficult to determine exactly when steaming starts.  There is also an error due to the heat loss from the ends and through the insulation, but keep this to a minimum, even if it involves a little extra temporary insulation.  I don't know if you know the mass of your boiler, which I think is copper (?) and is needed if you want to analyse the heat up period, but it is not worth stripping off the insulation for it.

For the engine, it is a bit more complex.  If you have a throttle valve, you cannot assume the engine steam inlet pressure is the same as the boiler pressure.  Also you need a means of measuring the temperature at the engine inlet and exhaust.  Probably some special pipe fittings for test purposes to accommodate these measurements and delete the throttle, so not for today's run.  Some sort of tachometer is also desirable.  Measurements on that engine driving the little generator will be particularly interesting.  Voltage and current are easy, but of course we don't know the efficiency of the generator, so probably better to measure speed and torque.  But by then you will have built the test stand I need, so again, not today's project.

So I suggest today concentrate on the boiler, and make a up a few fittings when you have a gap between projects for future testing.  I am looking forward to seeing the boiler test results.

Last night I drove myself to distraction trying to make sense of the formulas for torque and mean effective pressure, and not getting answers that were consistent.  I think it is a matter of some repeat test runs, to ensure I have consistent result, and check those formulas when at last I have access to my test books again to be sure I understand the assumptions inherent in the formulas.

MJM460
Title: Re: Talking Thermodynamics
Post by: Steam Haulage on September 18, 2017, 12:31:24 PM
Hi,
In post 282 you talk of 'film resistance'. Could you provide your definition and a give me a good source of background reading on this topic -please?
Steam Haulage
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 18, 2017, 03:10:32 PM
Hi, the first test is to show the efficiency of the lagging. there is a 1/16" lagging cloth bought from the model engineers propriety show and some not very close fitting mahogany planks. The temp gauge prong is close to the boiler shell after scraping away the felt and hollowing out the plank to make a tight fit. The boiler is 16 gauge copper and is not lagged at the ends. you can see the two 500 Watt elements. The amount of water put in is 600 mL about half full. I shall endeavour to measure the length and diameter etc and i shall time the element switch  on ...to the safety vale blow off and note what the pressure gauge reads, also the rise in temp from ambient to valve lift. with your tables it should be possible to get some sort of efficiency reading !! With the new engine the inlet exhaust temp may be quite close as it is a steam jacketed cylinder

 and the exhaust comes from the bottom of the casting to the condenser. As this is all one huge lump of metal and the jacket is filled with steam first and drained at the bottom until steam escapes there may be not much difference. The engine was tested under steam before it was all taken apart to be painted ,and all was well as can be seen from a previous post on the Beeleigh mill posts.However i had to take it all apart ,make all the correct bearing parts etc etc for it and lost the timing and porting lengths a bit so may
 
have to make a few adjustments before it will work again. Thanks for taking the trouble to help with this and i shall film the results as we go along and hope there is not too much "film resistance" to confound the results !!!. Incedently in Blighty when we film stuff ,we take ' Footage'
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 18, 2017, 03:19:43 PM
And am getting in a muddle with putting the text on ,,,so to complete the last bit, in Metric countries do they take "meterage" ?? just wondering !!!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 19, 2017, 01:24:24 AM
so, i have had the boiler in steam....the length is 180mm, diameter is 75mm and the thickness is 1.7mm. the overall thickness of shell insulation and wood cladding is 10mm. The temperature started at 16 degrees centigrade and the graph shows minuets and degrees .after a slow pick up the graph was in a strait line till the safety valve blew off at 118 degrees. the valve was screwed down to give a temp of 143 degrees with the pressure gauge showing 50 Lbs square inch.I have included the graph to show the readings. I connected the boiler to the new engine but it was reluctant to run properly so i will need to make some adjustments here and there.So what can we deduce from these figures ??
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 19, 2017, 11:19:22 AM
An electric boiler test -

Hi Steam Haulage, glad to have you on board.  Film coefficient is a term associated with convection heat transfer.  The introductory texts usually talk about heat transfer from a heated plate to a fluid, liquid or gas.  The primary variables are area and temperature difference and arranged as though there was a constant heat transfer factor, so Q = (coefficient) x A x (T2 - T1)
The coefficient is near enough to a constant for conduction, but not really constant, and not a simple relationship in convection.  For a fluid heated by a plate, the coefficient is given the symbol h.  The issue is that the heat transfer alters the properties of the fluid, many of which are temperature dependent.  So the heating affects the flow, and the flow addicts the practical temperature difference.  All this goes on in an area close to the plate, overlapping and very like the boundary layer in flow situations.  If you are looking for more information, it will be in any text book on heat transfer, often titled Engineering Heat Transfer, or something similar.  These books are usually quite separate from thermodynamics texts.  My book is too old to be a useful recommendation, though much newer than some of Willy's reference books.  Spend an hour in a good technical library and choose one that seems readable before you buy, they are usually pretty heavy in every sense of the word.  Or if you know any current or recent engineering students they will know which ones are current and readable.

Hi Willy, that is a great start on the testing.  I will try and summarise what I saw in it, without requiring too large a post card.

First, a little interpolation of the steam tables gives the pressure at 118 deg C as equivalent to 12.6 psig .  If the temperature reading is a bit low, 120 deg C would give about 14.3 psig, both assuming a slightly low atmospheric pressure system at 100 kPa or14.5 psi.  No superheater, so we only need the saturated steam tables, and temperature and pressure are not independent.

With the higher safety valve setting, 143 deg C gives 42.6 psig compared with the pressure gauge 50 psig.  If the temperature was actually 145 deg C, the pressure would be 45.7, say 46 psig, so I suggest your gauge reads a bit low, but not bad for such a small gauge.  The needle even looks a bit lower than 50 in the photo.  I don't know how much error there would be in your readings, but more insulation wrapped around or even just close to the tip of the probe would help reduce the error.

With the temperature, we not only know the pressure, but the steam tables tell us the specific volume, internal energy, enthalpy and entropy at both saturated liquid and dry saturated vapour condition.  (Points 1 and 2 on the sketch for my tests).  The latent heat, or enthalpy change from 1 to 2 is 2737 - 602 = 2135 kJ//kg.

The heater of 1000 Watts is 1000 J/s so takes 2135 seconds to evaporate 1 kg of water.  We don't have the actual experimental results to compare with this, but there is more we can do.

You filled with 600 ml of water or 0.6 kg.  Water at 16 deg has an enthalpy of 67 kJ/kg.  So to heat it to point 1, you need 602 - 67 = 535 kJ/kg so 535000 x 0.6 = 321000 kJ to heat to 148 deg.

Copper has a density of about 8933 kg/m3 and specific heat of about 383 J/kg.K.  From your dimensions and the density of copper, I estimate your boiler mass to be about 0.75 kg.  So to heat the copper 16 to 148 deg, 127 deg, you need 0.75 x 535 x 127 = 36000 J.  You can see this is much smaller than the heat required for water.  The insulation will need even less, and I have ignored that.  Total heat to get from 16 to 148 = 357000J.  Your heating element should do this in 357 seconds, say 6 minutes, assuming no heat loss.  Of course the boiler ends are not insulated, and the insulation is pretty thin, so I suggest you can feel some heat coming off the insulation.  It looks like your boiler took about 9 min, so this gives you an idea of the heat loss.  Very roughly about 67% efficiency, though I have ignored the lower heat loss at lower temperature.  But as a first estimate I would say perhaps only 67% heat into steam during steaming.  On this basis, 670 J/s into the steam which requires h2 - h1, 2737 - 602 = 2135 kJ/kg, so 3187 seconds per kg of steam, or 53 minutes per kg, so about 0.314 gram/sec.  It will be interesting to see what you get.

Perhaps not as high efficiency as I would have expected, but you could try some temporary insulation of the ends, perhaps a layer of felt then whatever else you have on hand.  And wrap a folded towel around the cylindrical section.  Unlike a fired boiler you can use all sorts of insulation while you carefully monitor the temperature.  Just for a test, then perhaps layers of cork on top of your insulation, then a few extra wood strips to complete the larger circle to finish it off to compliment your beautiful engine.

Now I hope you can see how the figures are used, you can see which ones are most important to improve the accuracy.  And just how much can be learned from a simple test rig.

Oh, by the way, metric countries have put all the film in museums and archives, and just shoot video.

Should have internet tomorrow, but you never know for sure 'til you try.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 19, 2017, 09:21:50 PM
Hi MJM thanks for the figures and explanation. First  16 to 148 degrees  127 degrees .....should that be132  degrees ??  The boiler is not actually 100% steam tight as there are quite a few whiffs of steam escaping here and there !! Also the wooden lagging is not very close fitting and yes it is hot to the touch. I am wondering about temperature being able to accurately define pressure ?? as, if you have an empty boiler that is being heated ,it will still show up on the thermometer. ? .also if you open the steam valve and release all the pressure, the boiler is still very hot which will show on the reading. If you do heat up an empty boiler will it show a pressure  positive or negative as if the outside expands will it register a slight vacuum ? it is quite cool in this world of exponential curves and logarithms and stuff  that the time pressure curve is an actuall straight line !! Talking about Willys books ,i found this in John Fareys 1827 tome....However i am not actually qualified enough to agree or disagree !! I suppose the reading would have been more accurate if the thermometer was inside the boiler.? The Beeleigh engine has no lagging on it and there are no places on the cylinder block that has bolt hole attachments
, I shall look at the Ramm brewery sister engine to see if that is lagged, Thanks for all this info , really interesting.
Willbert........It looks like the Rams engine is half lagged actually.
Title: Re: Talking Thermodynamics
Post by: crueby on September 19, 2017, 09:34:06 PM
Willy - I love the line under the table in the picture you just showed - "Observe this table is entirely erroneous"

 :shrug:
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 20, 2017, 02:14:34 AM
hi Chris,  yes brilliant i love it too , you won't see that in a modern book, It was probably said by Dr Dionosor Lardner the well known vicar that also disagreed with I.K.Brunnel and other engineers of that period !!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 20, 2017, 12:02:15 PM
Hi Willy, when I saw your post I thought that was the inevitable consequence of balancing a calculator and an iPad on my knee, while sitting under a shady tree in bright sunshine, 1000 km from nowhere in particular.  Still, such errors are easy to make and it was in the first line of my notes.  So I reread your post and found it actually said 143 deg C, not 148.  Still, I am not above that kind of mistake and I am glad that you are checking on me.  It probably will be me next time, so let's both keep checking.   Not to worry, I put an extra line of data from the tables in my little spreadsheet, copied the formula to a few more squares, and followed through the calculations.  I was surprised how little difference it made, the difference is only a small percentage of the approximately 2000 kJ/kg latent heat so makes a small percentage difference to the answers.  The biggest difference is actually in the calculated pressure.  For 148 deg C, the pressure gauge should read 51 psig.  Again in case the temperature is reading low, 150 deg C would give 54.5 psig.  However following through the remaining calculations, enthalpy of saturated water is 623, dry saturated steam is 2744 and enthalpy to make steam is 2121 kJ/kg, always seem strange it is lower at higher pressure but that is the shape of the bell curve that outlines the two phase region, and quite small differences.  So your heaters potentially evaporate about 0.47 g/ sec, 28.3 g/min, the difference only showing in the third significant figure.  It was worth doing the calculations twice just to see the sensitivity to the variables.

Heat for water is now 556 kJ/kg, so 333600 J for 600 g of water, 37917 g for the copper total 371517 J so 371 seconds or 6.2 min.  In the right direction, but clearly not the explanation for the lower efficiency.  My first guess is still insulation.  Try the temporary fix first, just to check, then think about how to do a job more worthy of your beautiful engine.  Other significant quantities where accuracy is important are the mass of water added to the empty boiler, volume is difficult to get that accurate, if you can get access to a digital kitchen scale that reads to the nearest g, with a switch to give oz.  The other big one is the actual power input of your heating element.  Not so easy unless you have a power monitor as many shops sell for checking household appliances.  But surely your kitchen needs a new accurate scale, so you can get accurate weights doesn't it?  For the cooking of course!

The other issue is that escaping steam.  To evaporate a kg of steam requires about 2000kJ, where as to heat a kg of water by one degree requires only 4.2 kJ, so a little steam escaping is carrying away a disproportionate amount of heat and not delivering it to the engine.  Besides which it can scold your fingers, so worth fixing.

Regarding your additional questions, if you have water and steam in a closed container in equilibrium, the temperature and water vapour pressure are directly related and can be read from the steam tables.  Of course, if you also have air in the boiler, the air pressure adds to the water vapour pressure.  But you said the safety valve listed at about 118 degrees before you screwed it down a bit, so much of the air will have been swept out with the steam at that point.  Of course some air could still be there, so when you first get to the new set pressure, there could still be some air, and your gauge could read high.  If the final temperature was 143, and still some air, the gauge reading of 50 could be correct, but at 148, and the gauge reading 50 it is reading a little low compared with 51 expected.  After a bit more steam escapes, the huge volume of steam compared with the initial air volume means air is soon gone.  In summary, for steam vapour and liquid in equilibrium, temperature definitely defines the water vapour pressure.  Other gases in the space, such as air, act independently in addition to the water vapour, and the gauge and your safety valve both read/respond to the total of all the partial pressures.

If you suddenly release the pressure, the system will not be in equilibrium for a short time.  The water will cool as some evaporates to make a bit more steam.  The copper will cool by giving heat to the water.  Soon the pressure is atmospheric, the water is in equilibrium at atmospheric pressure so 100 deg C, the copper is also at 100 deg C, steam production essentially stops and the thermocouple also soon reaches/responds to that temperature.  Now the whole lot is back in equilibrium, the temperature is 100 deg C and the steam tables tell us the pressure is 101.4 kPa or 14.7 psi absolute.

If you heat the empty boiler, presumably air at atmospheric pressure and plug tight, not empty as in full vacuum, you have no water, so the steam tables are not relevant.  As the air heats, it  behaves roughly like an ideal gas, and the pressure rises in proportion to the absolute temperature, which is 473 + T, so quite slowly.  However the convection heat transfer is quite poor compared with water or steam, and the air has to carry the heat to the boiler shell, so it is unlikely to be in equilibrium.  With such poor heat transfer to cool the element, it will overheat and burn out, so not a recommended experiment.  One for theory only.

Your graph has the initial curve while the temperature gradient within the element sheath and water film are established, which involves some heat storage, then with uniform heat input and near constant specific heat of the water and copper, temperature is a linear function of heat input, but yes, interesting that it's linear, compared with all the curves involved in cooling of your coffee.

Lagging of an engine reduces the heat loss, which improves the engine efficiency, but heating just further reduces this loss.  The area is really too small to make a significant heat input.  I think I have discussed this in response to one of Paul's questions.

I hope that answers the current questions, they are good questions and helpful for directing my explanations, so keep them coming.  Remember that the principle behind using temperature to determine pressure involves two assumptions.  Water as both  liquid and vapour present at the same time in equilibrium or very close to it, and no other gases in the vapour space, both of which are sufficiently accurate soon after steam flow starts and the temperature stabilises.

By the way, I checked a couple of the figures in that extract from the book, and have to agree with the editor who added the comment.  The pressures have small errors at any temperature and the volume expansion is way out.  Like you and Chris, I like the honesty.  I suspect a modern editor may have quoted the figures for historical accuracy of the text, but recognised that they did not have good quality data at that time, so don't use them for calculations.  But a fun observation.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 21, 2017, 01:36:05 PM
Hi Willy, a little post script on yesterday's post.  I hope I did not leave you with the impression that the final temperature does not make much difference.  You can see it makes a small difference to the boiler, affecting the time to raise steam and the amount of steam raised.  For these issues, the percentage change with a small temperature difference is quite small.  However, when we get to looking at the engine, we will take that approximately 2000 kJ/kg in the steam and subtract a quite similar number for the exhaust steam to get quite a small difference available for the engine to do work.  Then a small difference in the roughly 2000 for steam will make a much larger percentage difference to the potential work output.  I will look at that next time.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 21, 2017, 03:00:21 PM
Hi MJM I have now had my 'Fitters' hat on and the engine is running freely. I connected it to an air compressor working at about 40 psi as this was easier to investigate the tight spots !! i actually needed to lift the main bearings about 3/32'' as the pistons were fouling the cylinder heads !! this would be  about an inch of packing with the full size engine.!! I think it will be quite difficult to measure the engine temperatures as the exhaust disappears below the engine into the condenser/airpump block that should be filled with water but is not and is under the engine surrounded by the box. Here is a video of the engine running with air, it is a bit more spectacular with steam of course ,what with all those not quite steam tight glands and things.

Beeleigh mill model beam engine.
Not a valid vimeo URL Anyone
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 22, 2017, 01:10:00 PM
Hi Willy,  thank you for putting up those videos of your engine running on air and steam.  It is really great to see them running after watching the care and skill you have applied to build it with historical accuracy.  It is a real work of art.

I would not worry too much about not measuring the exhaust temperature.  I don't think it is the most important part of building such an engine.  If you really want to, your thermocouple will fit in quite a small hole and on the condenser side, could have a simple o-ring gland if there was a place for a plug that would ensure it measured the steam exhaust temperature, and not the condensing water temperature.   The exhaust pressure can also be measured if you can make an unobtrusive tapping point for a plastic u-tube manometer, but I would not spoil the appearance of a beautiful engine for it.  We don't have contractural requirements to demonstrate on our models, and we can learn all we need to know by setting up any of our other engines, particularly one that is more easily fitted with the test points.  I assume the condensing is achieved by direct injection of water, as I don't remember any tubes in the condenser, which I assume is mainly the air pump and condensate pump with a water injection nozzle.

It is worth doing what you have done with the boiler, because knowing how much steam you are making, and how much heat you are loosing due to insulation does inform your efforts to improve heat up time and steam production, and gives you an idea of the limits before you just need a bigger or more elements if you need more steam.  A spare boiler plug that acts as a thermowell and accepts your thermocouple for temperature measurement is not a big deal and easily replaced with a normal plug for display purposes would give slightly more accurate measurements.  And you have learned a lot of thermodynamics in the process.

If I heard you correctly in the video, the engine actually ran with a boiler temperature of 116 deg C.  Tomorrow I may be a bit quiet due to a family visit, but in the next couple of days I will calculate the potential output of an ideal adiabatic engine at each of the three temperatures you have mentioned, just to see the difference between 116, 143 and 148 deg C on the potential performance of the engine.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 22, 2017, 03:22:35 PM
Hi, more questions ......if you want to produce more steam to run an engine continuously would it be better to have less water in the electric boiler (no need to cover the firebox with H2O) or will the higher level make no difference ? I was surprised at how long the boiler took to cool down as well and If i have a spare couple of hours i could draw up a graph for that as well !! To get rid of the air in the boiler would it make sense to leave the steam valve open until it reaches 100 degrees  (boiling) and then close it ? will it take longer to reach operating pressure to achieve this ?  Also my boiler is at 3.8672493 meters above mean sea level if this is relevant !!!! Thanks for all the additional info.....I have some really shiny thin shim plate stuff that i could put round the wooden lagged boiler, and would this be better ? it would stop draughts taking away any convected heat perhaps ? Do you have any info on Wiredrawing that one reads about btw?
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 23, 2017, 12:23:36 AM
Also will the actual mains voltage have an effect on the input wattage available at the element it is rated at 250 volts but if only 230 v will this be relevant ,and are there tables available ? I don't know the actually voltage at the element and how much may be missing in the control Cct, however i could find out ,but that will mean taking things apart !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 23, 2017, 02:44:46 PM
Hi Willy, only a short time available tonight, so last question first.  I will try and get to the others tomorrow.  With AC, ohms law still applies though you must use impedance instead of resistance to be correct.  Fortunately your heater is almost certainly a pure resistive load so the values of resistance and impedance are the same.  Again the voltage we quote for AC, whether it be 230 or 250 volts is an RMS voltage, not the peak of the alternating voltage which is significantly higher, and the RMS voltage happens to have the same effect on a heating element as a DC voltage of that value.  So you need to use two formulae.  Ohms law V= I x R bit in the form I =  V/R.  Then power in watts, W = V x I = V^2/R.  You can calculate R = 250 ^2/ W, ie from rated voltage and rated power.  Then power at 230 V = 230^2/R. Because the voltage is squared, the effect of low voltage will be marked.

You also need to be aware that the resistance normally varies with the element temperature, usually increases with temperature.  I don't know of your element is rated based on the element hot or cold, but quite possibly cold at a defined temperature of perhaps 25 deg C. Because they don't know what temperature you will run at.  You can measure the resistance of the element cold and see if this agrees with your calculated figure from the rating.  You may be able to get a close hot reading if when your boiler is operating for some time and hot, switch off, and quickly unplug and measure.  It will cool a bit before you get a reading, but will give you an idea of the change.  Better would be if you are equipped with an AC current meter that clips on around the wires and measure the current hot and cold.  But don't fool with 230 V AC unless you really have the right equipment.  You can't measure resistance with power connected.

Finally, even with a rating of 1000 watts, unless you have a test certificate you still have to regard that as approximate.  I would hope it is within 10% of that, so in the range 900 to 1100, but I don't know what accuracy standard it is made to.  In the end measurements are necessary for accurate data, and even then the instruments should be calibrated.  Outside a formal lab environment there will be a measurement error in practice.

I will try and answer the other questions tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 24, 2017, 01:02:09 AM
hi, thanks for that the and here is some info about the Cartridge heaters. I did not use the sealing compound though , also some pics of the cartridge heaters with their housings, if they need to be taken into account. They are a pretty complex shape though to calculate the mass/volume !! Looking forward to more info ...and i hope i have not hilacked this thread too much .In the pictures are the electrical safety valve and the water level switch. this switch device is used with a 9 volt battery Cct ,to trigger the on/off mains Cct so as to be safe from electric shocks !! the insulation is PTFE.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 24, 2017, 06:01:27 AM
More on electric boilers

Hi Willy, back to your questions in post #298.  First, you are not hijacking the thread, it is to answer questions like this that I am writing the thread, and I suspect there are many others with similar questions, and I hope they are finding it helpful and will join in when, between you and I, we miss something, or it is just still not clear to them.  And I hope also if they notice something I have got wrong.  Mistakes are just too easy in some of this stuff, I can only do my best, and correct things when they are noticed.

Does it matter whether you fill the boiler or not?  When you raise steam in an electric boiler, or any boiler for that matter, you perform two different processes in sequence.  First you heat up the whole system to your steaming temperature, then you evaporate liquid to produce steam at constant temperature.  When all the water is gone, the system temperature will start increasing if the energy input continues.  However, if you do continue with a dry boiler, or even a partially covered element or firebox, you will soon find some component reaches a mechanical strength limit at the higher than design temperature, something will melt.  On your case the element will probably burn out.  In a fired boiler, most likely the soldered joint will soften and release pressure before the copper softens enough to start yielding, as will be evidenced by bulging which will not spring back when the whole mess cools down.  It all depends on which limit is reached first in a particular boiler.

To understand the answer to your question, first consider each of those two basic processes.  While you are heating up, higher water level means more water to heat, so it will take longer for the constant energy input of your heater to get it all up to temperature.  Most of the energy input is stored in the water, copper and insulation, and a portion is lost to atmosphere as we have already seen.  This lost portion is initially low while all the temperatures are low, and increases with increasing temperature.

Once it is all up to steaming temperature, further heat input does not increase the temperature, it mostly goes into evaporating water into steam, although the heat loss to atmosphere continues, now at a constant rate due to the constant temperature.  How much steam is now found by an energy balance, and how much energy to evaporate a kg (or lbm) of steam.  We get this from the steam tables and it does not depend on how much water is available at saturated condition for the temperature, containing hf kJ/kg as enthalpy.  If you have less water during the steaming process, you will run out quicker, but it makes no difference to the steam production rate.  Steam production is completely determined by the energy input, the heat loss, and hg - hf for the water at that temperature.  And steam pressure is determined completely determined by the temperature, or vice versa.  And this pressure will be the pressure in the boiler once a little steam production has carried away the air that was in the boiler when it was filled.  Obviously less air if the boiler was more full.  But you cannot fill it completely as the steam needs some space to separate from the liquid, otherwise a lot of water will be carried over with the first steam.  Experiment is probably the best way to determine the maximum fill level before water carryover becomes too much of a problem.

The minimum water level must cover the element.  It does not matter whether it is a fired boiler or electric, the heating surface will get too hot if it is not covered by liquid as the heat transfer coefficient for vapour is very low compared with water, especially vigorously boiling water.  You could consider a little lower level and use a feed pump to maintain the level.  However, the engine will do work driving the pump before there is any available to do anything else, and the water entering the boiler will be at less than steaming temperature, so requires more heat to get to that dry saturated condition of steam.  You can only partly offset this by some boiler preheating the water (using exhaust steam for example).  You can't use an electric jug because the pump will not handle near boiling water due to its valve pressure drop and acceleration losses.

Will leaving the boiler open until at 100 deg help?  Remember the mountain top experiment?
  The pressure of the water vapour at 16 deg C is only 1.8 kPa, so 2% of the total boiler pressure at atmospheric pressure, and a nice vacuum of you can condense at that temperature.  The rest of the pressure is due to air.  Heating the air to 116 deg C increases the pressure in proportion to the absolute temperatures, so (273+116)/(273+ 16) = 1.34.  So the air at approximately 100 kPa in the cold boiler increases to 134 kPa in the sealed boiler when hot.  This is an increase in gauge pressure of 34 kPa or about 5 psi.  If this, plus the water vapour vapour pressure exceeds your safety valve setting, it will lift.  The air that escapes with that steam is not replaced, while the steam is quickly replaced by more evaporation so the air is soon gone, and the air heat content is negligible compared with the steam, so no big difference in the heat required to get to that point.  If you leave the boiler open while heating, yes the air escapes, with little saving in heat requirement, but steam escapes with it, taking away the latent heat.  So it would be slower to get to steam raising.  The air will help drive your engine, so not a significant problem.  Seal the boiler cold and start heating.

Sheet metal around the boiler.  You normally have sheet metal cladding around industrial insulation.  Yes, it reduces convection from within the insulation, but conducts heat pretty well so it gains heat from the loss through the insulation, and in turn looses that heat to the air with only minimal reduction in overall loss.  It is put there, not to increase heat conservation, but to provide weather protection and protection from mechanical damage.  Note that wet insulation is a very poor insulator, as the water has higher conductivity than insulation and higher specific heat so it takes more heat to get up to the temperature profile.  So add your metal sheet, but only to hide and protect the insulation you put under it.  But wood looks even better, even if it is brown stuff.

Wire drawing - I see this term used in two ways and I don't see it as a specific technical term.  I believe it is normally used to determine the erosion of steel by high velocity wet steam in a situation such as a gate valve, which is only designed to be fully open or shut, is used for throttling.  The pressure energy becomes high velocity at the small gap of a nearly closed valve, and if the steam is wet, it will, sooner rather than later, gouge a bigger path and the valve will no longer seal properly when shut.  A gate valve is best followed by a globe valve which is made for throttling, or at least only used in throttling mode as a controlled opening rather than sudden opening.  However, I have also seen the term used to describe reducing pressure during flow.  To me that is the cause rather than the result.

A point about accuracy - your elevation, quoted to 7 decimal places of a meter implies that the last tenth of a micron is known to plus or minus less than 5 digits.  Measuring anything to this accuracy is a real challenge for even the very best micrometers, and I challenge any surveyor to measure elevation to that precision.  I thought you might have converted inches and feet with a calculator that gave you that many figures, but one inch is exactly 25.40 millimetres by definition, and dividing by 25.4 gives about 12 feet 8 1/4 inches, but not exactly, so I don't know how you arrived at the precise figure.  For its effect on the absolute pressure, the nearest 10 metres is probably more than accurate enough.  The weather bureau publishes atmospheric pressures all reduced to sea level, and I suggest that 3 metres is close enough to just look at the weather bureau latest observation, and assume it applies without correction to your location.  If you live in Denver, you would definitely have to calculate a correction.

Getting to be a long post, but I think that brings us to today's question.  I better get back to being sociable, might get back later, otherwise tomorrow.

I hope all that is helpful

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 24, 2017, 02:03:59 PM
Hi, Ok thanks for all those replies and observations, it helps out when trying to be too clever !! I have just thought about using boilers with steam engines and your word 'Vacuum' suddenly reminded me that when using a boiler one must be very careful to allow air to enter when the engine was stopped and the boiler shut done to stop the boiler imploding when it cools down !!
Title: Re: Talking Thermodynamics
Post by: paul gough on September 25, 2017, 01:08:22 AM
Re wire-drawing, MJM is no doubt correct regarding the erosion by condensate in a seat or some such as being what should be the primary meaning for the term. However it has common usage in discussing steam circuits in locomotives, certainly in publications from Great Britain. It is normal, (or should be), in a publication to use "wire drawing of the steam" when first entering into any discussion and then shorten it to wire-drawing unless the context is already known. As MJM said, it is a pressure drop across an apparatus, eg., the drop in pressure between the entry of a superheater element or set of elements due to friction and their outlet, any steam circuit, or even can be used to discuss a loss associated with constricted ports, either their size or the opening at a specific point of the valves travel over them. In very general terms it is a lack of sufficient cross section area of the passage be it a pipe, orifice plate, valve, superheater element etc. This at least is my understanding from my reading over the years and hope it helps illuminate the phenomena. Regards Paul Gough. PS, I understand that the first or top portion of an indicator diagram where it slopes slightly can indicate wire-drawing to an experienced eye, maybe our marine engineers that are on board could expand on this.
Title: Re: Talking Thermodynamics
Post by: paul gough on September 25, 2017, 08:09:29 AM
Before someone jumps on me for lack of clarity in my PS of the preceding post. I am not referring to the normal slight down slope of the upper portion of the diagram that is due to the increase in cylinder volume from piston travel. I am trying to covey there is a departure in the trace from this expected and normal line that a discerning and experienced eye can infer wire-drawing. Unfortunately I am not able to elaborate further as I have no experience in taking extensive numbers of indicator diagrams and interpreting them, it might be that marine engineers who served on steamships might know of it and shed further light on this, I don't think there would be any steam loco designers or engineers from loco testing stations still extant let alone reading this thread to provide a clear explanation. I mention it as one example of a defect that could be seen on a diagram. Perhaps some other examples of deficiencies that appear on diagrams might be discussed as well. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 25, 2017, 11:42:26 AM
Home, sweet home -

Some of you have noticed that I have been travelling recently.  Just arrived home this afternoon, and it is always good to arrive safely home after a road trip of 12240 km, average fuel consumption 11.2 l/100 km and circumnavigating roughly a half of our continent.  Hooked up our little caravan to the Subaru Outback on July 13, and went, well, outback.  I think Hugh calls it Snow birding, we call it becoming grey nomads.  People from the southern states driving north to escape the winter and find some sunshine make for crowded roads and parks, well sometimes you might see at least 10 other cars in a day.  I suppose we all go at about the same speed, but one time we had to stop for around 5 minutes for roadworks, no one came up to wait behind us.   Then they mysteriously appear, one by one at the road houses and campgrounds, for overnight stops.  But is is a great way to get a real feel for the country, and to experience blue skies and clear starry nights in a way that is not possible in the city.  And to see some wild life in its natural habitat.  We were a little later than most in turning south to put the sun on our backs for the long road home, so perhaps that is why the road seemed less busy than normal.

Thanks Paul for some clarification of wire drawing.  It is consistent with some of my reading and it would seem to be a term more commonly used in the pressure drop sense in the marine industry.  In my industry, it was a term usually muttered while looking at the cutaway in the hardened satellite of a gate valve no longer sealing shut.  Cut like a water jet.  Like you, I would like to see someone start a thread on the indicator diagrams, they are only rarely used in my industry, then only on compressors, as the drivers are all turbines or electric motors.  So I have no experience at all on taking or interpreting them.  However perhaps it is a bit outside the scope of this forum, as I suspect that none of us have available an indicator device suitable for use on our models.

Thanks Willy for those pictures of your boiler and the screen shots of the data sheets.  It seems that the elements may actually be rated for 230 V, so the calculation would then indicate the higher power output at 250 V.  I also note that the tolerance on power is +5%, -10%, but the question of whether this applies to a hot or cold element is not answered, but I would assume cold.  So probably lower output when hot.  The grease is intended as a heat transfer compound rather than a lubricant.  I think we have discussed this before.  I do notice that your boiler seems to have flat ends without stays, though I notice the bushings have not been soldered in the picture, so you may have added stays later in the construction process.  Of course you now have the boiler pressure tested and steaming, so if it is now dimensionally stable, I guess that is practical evidence of its strength.  However, I keep wondering if your pressure housings for the elements could be extended right through and fixed at the other end, just like a large diameter hollow stay, you could then use grease on the sheaths without air displacement issues. 

I am not sure what external pressure the boiler could stand.  The effect of internal pressure being below atmospheric pressure, the condition commonly referred to as vacuum, is that the shell has the high pressure on the outside, that is why I refer to external pressure.  The commonly quoted formula for shell thickness is technically referred to as the thin shell formula, and it's derivation rests on the assumption that the material is in tension to contain the higher pressure on the inside.  Under external pressure, the shell is in compression, the failure mode is the collapse you refer to, and the strength very different, usually much less.  The boiler is relatively short, so the flat ends provide considerable stiffening against collapse, but as you say, it is good to open a path for air before the boiler cools down and avoid the issue.  And if this allows some extra steam to escape, it may speed the cooling.  You have given me the boiler dimensions, so when I get everything unpacked and put away, I will calculate the strength under external pressure for those dimensions.

I think that brings us up to date on the previous questions, so time for a good nights sleep.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 26, 2017, 12:37:37 PM
Boiler design considerations -

Yesterday I mentioned the thin shell formula often quoted for boiler shells, and noted that this formula applies for internal pressure of a cylindrical shell only.  It does not apply for external pressure such as the loading on the centre fire tube of a marine boiler.  It also does not apply to flat ends, or other shapes such as locomotive fire boxes.  It is obvious really, the diameter of curvature of a flat plate is infinite, so the thin shell formula gives an infinite thickness for any pressure.  But blank flanges for piping are flat, and not infinite thickness, there must be another formula.  And of course there is.  Blank flanges are quite thick when compared with the wall thickness of the circular  pipe, and we would not want to make our flat ends so thick.  The answer is usually found in the provision of stays which support the plate at intervals that leave the unsupported section strong enough in a reasonable thickness.  The designers of boilers carefully follow the rules for spacing of the stays.  I don't intend to branch into boiler design, but it is probably worth looking at some of the considerations involved, so it is clear why building to a published design supported by the appropriate calculations is the best course for a builder not experienced in pressure vessel design.  Boilers are even more complex to design because the surfaces subject to the combustion gases require additional calculations to determine the appropriate metal temperatures and design strength.  Allowance has to be made for the reduction in strength of the material at higher temperatures.  Hence there are separate codes for boilers and for unfired pressure vessels.

When a cylindrical shell has the high pressure on the outside, such as the marine boiler fire tube already mentioned, the failure mode is quite different from when the higher pressure is on the inside.  If subject to to great a pressure, the tube collapses, or squashes.  The collapse pressure is quite sensitive to any departures from true circular form such as dents, unlike the internal pressure case.  The formula are again quite complex, and the Australian Miniature Boiler Code for example refers to the pressure vessel code for these calculations.  Needless to say, the thickness required to resist buckling under external pressure is much greater than required for internal pressure.  This is normally reflected in the specified thickness for the design.  Again, specifying the thickness is best left to the experts.  Calculations for external pressure are included in the AS/NZ, BS and ASME pressure vessel codes.

Now Willy mentioned the possibility of collapse of his boiler due to the low pressure if it is allowed to cool to atmospheric temperature.  This is a real possibility for industrial steam containing vessels, and now days the full scale pressure vessels that I am familiar with are designed for full vacuum as well as for the required internal pressure.  The relatively small diameter of Willy's boiler, combined with the fact that the shell is not fired, so the metal temperature is well controlled, means that the design check is relatively easy.  The internal design pressure is probably the overriding consideration, especially as the maximum possible vacuum is only a little over 100 kPa or 14.7 psi.  A quick first pass on the dimensions indicates it is probably quite strong enough to resist collapse, however I would still recommend a formal check by the boiler designer, as it is never wise to commit to the results of an unchecked calculation.  The best procedure is always to admit air while the vessel cools, at least until you have the formal calculation.  Collapse under vacuum used to be demonstrated in every junior science program using a square thin walled metal can.  I presume it still is, the result is quite spectacular but not particularly dangerous as the can is usually surrounded by the sink when the cold water is splashed over it to condense the steam.  And of course steam is not escaping as the pressure is reducing, dramatically in this case, but it is a lot of work to rebuild a boiler that collapses.  Again the failure mode is much different from failure under internal pressure test.

Another case where the thin wall, formula does not give all the answers is when you put a hole in the shell for inlets, outlets level glasses and so on.  We all know the normal design solution is a bush soldered into the shell.  The dimensions of the bushes are carefully specified in the model boiler codes.  These bushes are in fact designed to properly compensate for the missing part of the shell.  So long as you don't cut down on the amount of metal, you can use the standard bush designs with confidence.  Even make them a bit bigger, but no smaller.  The problem comes when you want a larger opening, perhaps for a steam turret.  The opening has to be properly reinforced and the design supported by appropriate calculations.  This even applies to tee fittings in tubing and piping, where the metal thickness is carefully controlled so the cylindrical tube or pipe can safely contain the internal pressure.  In a forged pipe fitting, the metal thickness is so carefully controlled that it is not immediately obvious that the extra thickness is there.  But it is.

So a boiler has many components which all contribute to pressure containment, and only the circular shell with internal pressure is described by that thin shell formula.  Even then, allowance has to be made for corrosion, and fabrication techniques.  Even welded joints are not considered as strong as the base metal unless they are proven by x-ray examination.  Tennessee Whiskey is well familiar with these inspections and his certifications are testament to his skill with welding equipment.  For most of us a satisfactory silver soldered joint in a copper boiler is easier to achieve, and the code requirements for the joint design are intended to allow for the expected fabrication methods, when supported by knowledgable inspection.

If this is of any interest, perhaps a little on testing tomorrow.

Thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on September 26, 2017, 02:08:19 PM
Re -thinking about efficiencies in our system as a measure of heat in, (boiler fuel) to work done, (engine output) it is known that higher pressures and superheat give better efficiencies even in models. Logically it then should be the case that pre-heating the feed water by means of exhaust steam or exhaust gases would do likewise. But to what extent??

The question arises for us modellers and particularly for rather small models is there any point in feed water heating? Is something primitive like running a feed water line through the firebox, (I'm talking metho or small gas fired boilers here), or some similar arrangement going to be practically worth it and enhance performance, eg. save fuel or reduce the firing rate. If it is a worthy theoretical proposition then we need to ask/find out what minimum temperature increase is needed to make a feed water heater a worthwhile modification. Is there any way to work this out, other than the obvious build one and try it method? Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 26, 2017, 03:10:07 PM
Hi MJM , I was talking to the club members today about the cooling of model boilers in locomotive and one chap said that as the boiler cooled it actually sucked new feedwter in through the clacks. so there was no need for a 'snifting' valve. Also where is the best place to have the inlet water valve ? Taking into account if the valve fails ....under  the water level or above it?
Willbert.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 27, 2017, 11:10:39 AM
More on feedwater heating -

Hi Paul, consideration of feedwater heating is an area where perhaps logic lets us down.  But let's work through two, perhaps three situations.  First without a feed pump, as in my models so far, and I think Willy's electric boiler.  Preheating in this case means boiling the jug, and filling the boiler from the jug before the plug is tightened.  Now you can see what happens.  With the water at say 80 deg by the time the plug is in and the burner lit, the enthalpy of the water is 334 kJ/kg instead of 63 kJ/kg at 15 deg C.  If our chosen pressure is say 0.198 MPa (approx 2 bar, or 30 psi) the enthalpy of saturated water is 503.7 kJ/kg then that heating phase before steaming begins has to contribute (503.7-334)/(503.7-63) or close to 40 % of the heat required to heat cold water to the steaming point.  Clearly reduces startup time, and either saves fuel or extends the run time from a fixed amount of fuel, with the attendant risk of low water level.  However, once that steaming point is reached, the heat required to produce steam is entirely determined by that chosen pressure, and the heat available from the burner, the actual steam rate is not affected by the preheating.

If the steam plant has a feed pump, then some cold water is continually added to the boiler, presumably the quantity adjusted to match the steam rate so the level remains close to constant.  This means the average temperature of the water in the boiler near the entry point, is a little below the steaming temperature.  Mixing quickly brings the feedwater up to steaming temperature, but at the expense of the quantity of steam produced.  Conservation of energy and the saturated steam pressure-temperature relationship both still apply, and the energy to heat the feedwater must come from somewhere.  Now you can see that if you have a feed water heater using exhaust steam energy which is otherwise lost to the process, less energy is needed to heat the incoming water so some of the lost steam production is restored.  As the steam production in the end has to be balanced against consumption, and assuming the load and speed is the same for both cases, the fuel consumption is slightly reduced in response to the preheating.  When you are shovelling coal in a small locomotive, it is probably hard to tell, but the theory depends only on conservation of energy which is a fundamental law of physics and always applies.  With a small Meths burner, there is probably no adjustment, so a little more steam is produced and the locomotive possibly sees slightly higher pressure and goes a little faster.  If you use a radio to throttle the steam and maintain constant speed, the boiler pressure will increase a little, as will the stack temperature and a new equilibrium is reached.  Of course the differences might be masked by the other variables, however, it is still useful to understand the theory.

Now that third case, what if you have a hand pump?  First, I suggest that the hand pump will be an intermittent operation, you are unlikely to sit there steadily working the pump continuously.  Two feedwater heater designs are possible.  You could top up the feed water tank from that electric jug, useful things those, and if all else fails you can make a cup of tea.  Probably even worth insulating the tank.  If you plan on this one, it is best to have a raised tank, so you always have a positive head on the pump suction valve, otherwise you may get a vapour lock in your pump.  But that will reduce the additional heat needed to heat the makeup water.  Alternatively you could use some exhaust heat, either by a coil in the tank, again it needs to be raised, or you could even make a little heat exchanger and pump the water through this.  A little more difficult to control, probably more practical with an engine driven pump.  The water in the exchanger will get quite close to exhaust temperature while there is no flow, then the warmer water will be moved into the boiler when pump operation resumes.

In summary, using exhaust steam to preheat the feedwater does in principal reduce fuel consumption or increase steam production, however it has no effect on the engine inlet or exhaust, unless of course it imposes a significant back pressure on the exhaust.

There is a limit as to how much exhaust heat can be recovered.  You will remember that heat only flows from a high temperature to a lower temperature.  The exhaust temperature will be at about 100 deg if you do not have a superheater, or a bit above if you do.  The boiler temperature will be something above the exhaust temperature, and we have looked at a few cases earlier, say 116 - 120 degrees.  But then we actually need a temperature difference to drive the flow, so we can't get the feedwater temperature to the exhaust temperature of 100 degrees.  And the heat required to heat the water to that temperature is a very small portion of the total heat in the exhaust steam.

Remember the equation for heat transfer, Q = U x A x (T2 - T1). To get a given heat transfer, as the temperature difference reduces, the required area increases.  Practical area in most large industrial installations rarely achieve less than 10 deg approach, and in a small model feed water heater, almost certainly a very much bigger gap. 

So there you can see the effect of feedwater heating, you know what you are trying to achieve, but as U is notoriously difficult to predict, a little test rig would be desirable to see if you get a worth while extra run length with a practical heater size.

Another heat source for feedwater heating that could potentially give a higher boiler inlet temperature is the flue gas.  The main issue is to ensure that feedwater heating only gets heat that is left after the boiler and superheater have used all that is possible.  Otherwise the feed water heating is at the expense of steam production, and I suspect that would be counterproductive.  And at the end of the day, you don't want to cool the flue gas to the point where the water component starts to condense, as that nearly always leads to corrosion problems.

Willy, the feed check valve admitting water if the pressure falls below atmospheric is a good point.  Atmospheric pressure drives the water in the direction that lifts the ball and permits flow.  If you have a feed pump.  Of course the boiler could end up over full as the cool water lowers the temperature and hence vapour pressure of the water.  The only air admitted to the boiler would be the amount dissolved in the cold water.

We discussed the location for the feedwater inlet back in post #117 when Maryak provided a very interesting picture of full size practice.  It is on page 8 of 21 of the thread (based on my forum settings).  The recommendation was not too close to the shell, and normally near but below the surface.  The second part of your question, taking into account the possibility of check valve failure,  I am sure you are thinking of whether water or steam could escape from the boiler.  There are a few things to consider.  First, if you have a feed pump, you actually have three check valves in a row.  As any one will prevent back flow, all three have to be in bad shape, and any leakage should be quite small.  I suspect an issue with the feed pump might be noticed before there was too much problem, but perhaps others have some relevant experience they can contribute.  Note that when water escapes through the leaking valves it is at boiler temperature, so superheated water when the pressure is reduced.  This means a portion will flash to steam until the excess heat is absorbed, and this will tend to spit hot water around.  Definitely not desirable.

Next time I suggest looking at the potential work output from your engine, and the differences in potential work output for the three temperatures.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 27, 2017, 01:24:09 PM
Hi, May i ask about flash steam plants with regards to feed pumps.....I don't know much about them and have always wondered how you introduce water into the coil when the pressure in the coil is very high to drive the engine ? are you trying to pump fresh water into the coils above the pressure already there. and does this require a lot of energy that reduces the efficiency of the plant? Of course with an injector the water does have to be cold.! Also is there an actual temperature difference between the steam and the water level in a boiler? If the temp is the same is it steam or water ? or have i missed something here. Are there any pictures of unclad boilers that show the temperatures with the different colours as are used with buildings.? I do have a handdraulic feed pump on my boiler, but the sight glass does not work any more as i think it is scaled up.!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 27, 2017, 01:32:47 PM
steamguy
435 views

3

0

SHARE

https://www.youtube.com/watch?v=63c9KR0bqb8
 
steamguywilly3
Published on Jul 12, 2010
SUBSCRIBE 7
 SUBSCRIBE SUBSCRIBED UNSUBSCRIBE
electrically heated steam engine


Here is a video talking about the steam plant.....This is on Utube if you can find it .
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 27, 2017, 08:10:07 PM
Hi ,MJM Here is a vid of my steam crane also electrically heated........and a quick disappearing trick at the end !!..I will be doing another steam up of the boiler with some Rockwool. does this need to be fairly loose or quite tightly packed ?? I would imagine a bit loose to allow the air to do its part. I shall also hold it in place withe some plywood. The contemporary thinking from the club boiler inspector is to have the flanged end plates showing outwards so one can see the depth of them, however this will give more area of copper exposed to the outside to conduct the heat into the air. !https://youtu.be/2EkjRslFgSM
https://www.youtube.com/watch?v=2EkjRslFgSM
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 28, 2017, 01:15:20 PM
Hi Willy, more very interesting and relevant questions.  I hope that I can clarify a few points for you, so here goes.

Flash steam plants.  First fluids always flow from high pressure to low pressure, you need a pump or compressor to make them flow the other way, so yes, the feed pump must provide enough pressure to exceed the highest pressure in the coils, and that occurs right at the pump piston face.  The pressure is already lower on the coil side of the discharge valve and continues to drop through the coil to the engine.  The pump does indeed use power from the engine, but this is a small portion of the power developed by the engine.  Of course in our models friction is probably a greater loss in proportion to the work done than in a full size plant, but providing the packing and piston ring are not too tight it is still a reasonably small portion.  This is because the volume change of the water is very small with pressure increase, which means that work for compression is also very small.

You mentioned injectors, there is a lot of heat added to the water that enters the injector, and if it gets up to the point where it starts to vapourise too soon, the injector passages choke up and it stops working.  With a pump, the suction stroke of the pump lowers the pressure for the incoming water, and again if due to inlet temperature, the water vapourises under the lower pressure, the pump becomes vapour locked.  However there is more latitude than with an injector.

Water and steam are nominally at the same temperature.  I say nominally, because the boiler is not normally in true equilibrium while heating is going on.  The heating surface is at a higher temperature than the water, otherwise heat would not flow.  The water starts to vapourise, but it's pressure is slightly higher than at the surface due to the water depth, very small depth in a model boiler, and the expansion to steam means the density is very low so it is rapidly displaced to the surface.  If you turn off you element when the boiler is up to pressure, boiling will quickly cease, and condition will get quite close to equilibrium, especially with good insulation.  When all the water in the boiler is in equilibrium, the liquid and vapour are at the same temperature. 

Liquid and vapour are two separate phases, which under certain conditions can exist at the same time for most fluids.  Our boilers have only water in them once the air which was initially in the boiler is displaced.  The gas bottle used by many has perhaps propane, a chemical substance classed as a hydrocarbon, which is highly flammable and a useful fuel.  Propane also has a range of conditions where liquid and vapour exist at the same time, just the pressure and corresponding temperature are different from those for water.  Similarly, most things we might normally consider gases due to their nature at atmospheric temperature, such as butane, air, carbon dioxide and even natural gas all can exist as liquid at some temperature, very low in all those cases, and have a two phase region where liquid and vapour exist in equilibrium at the same time.

So liquid and vapour (or gas) are terms which refer to the phase state of a substance.  However, water is a substance whose molecules contain two hydrogen atoms and one oxygen atom chemically bonded together.  The substance water can exist as solid, liquid or vapour, and we are all familiar with all three of these phases.  There are quite a range of conditions over which any two of those phases can exist in equilibrium at the same time, however all three can only exist at the same time and in equilibrium at one specific temperature and pressure known as the triple point.

By the way, even in your cup of tea, liquid water and vapour are approximately in equilibrium, and of course there is also air at the surface.  When the water is hotter than the equilibrium temperature for the water vapour pressure in the air, some of it evaporates, and under many conditions you can see this as a steam cloud rising from the cup.  If you blow the steam away, more liquid will evaporate to replace it, and this evaporation takes heat out of the tea.  If the tea is cooler than the equilibrium temperature for the vapour pressure in the air, some of the vapour will condense as it does on any cool surface.

In the boiler, there is liquid water and water vapour at the same time, and when in equilibrium they are at the same temperature and pressure.  However liquid water has a much higher density than water vapour, so gravity and surface tension combine to place all the liquid in a continuous phase at the bottom of the boiler, while the vapour fills all the vapour space, and there is a clear visible boundary between them.  Vigorous boiling is not true equilibrium, but the equilibrium temperature is considered the best estimate of the average temperature of the whole.

I have not seen any thermal pictures showing the temperature difference in a boiler, but I am sure that they will exist.  I don't know what temperature range is normal, certainly it will depend on the energy density which determines the necessary temperature difference for heat flow.  One of my catalogues has an instrument for viewing the temperature colours, but it is around $1000 so I will not be picking one up any time soon.

Thank you for those two videos, really excellent productions.  Clearly your safety valve works well and switching off the power reduces the steam production very quickly.  I love that crane.  It just needs to be nearer the edge of the table so it can raise and lower things from the floor.  Is the control valve just a disk valve to reverse the directions?

If you are adding rock wool, it should be packed as tight as practical.  Air doing its thing means transferring heat by convection, so you want to limit the air movement.   But rockwool is quite a suitable insulating material.  For a more permanent job it needs a metal or preferably wood cladding for appearance and mechanical protection.  Not sure if it is as bad as glass fibre, but worth taking the normal precautions for loose fine fibres when you are working with it.  Saves the itchy fingers at least.

I will come back to those flanged end plates tomorrow as it is getting late.  I suspect the above will raise more questions, but the post is long enough so I will finish off and return to the topic if there is any clarification required.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 29, 2017, 01:10:34 AM
Hi thanks for all this info , and i'm glad i don't have to pay the full rate for these consultancy fees !!! . I have noticed when using propane bottles in the summer that you do get a frost line forming at the level of the liquid  when using the gas at full tilt ! I shall do another boiler test with the extra insulation. These videos were done 8 and 10 years ago so are not of the quality and ease that we can do today on my Apple computer !! .
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 29, 2017, 11:55:15 AM
On boiling propane -

Hi Willy, just to complete yesterday's question on the way the ends are placed.  The inspector has to take into considerations the issue that get presented and I am sure that after a while the most common and troublesome problems are well known.  I would expect that the inspectors talk to each other and compare notes about the design that are people are more likely to present correctly.  If I understand the issue addressed in the code, it is to verify that the silver solder has flowed right through the joint as well as that the flange is the correct length.  I guess with those 5.5 mm cameras mentioned in another thread, inspecting the penetration on the inside is a bit easier, but the measurement is not.  Has to be measured before soldering.  I don't really know.  However the issue of heat loss is easy to control as with the flange facing outwards it is pretty easy to apply as much insulation as you need to minimise the heat loss.  So I would talk to the inspector about the design, present all the prepared parts before soldering, and go along with the instructions given, and insulate the end result (after the hydrostatic and steaming tests are approved).  Your outside cladding then determines the visible appearance of the boiler.

The propane in your gas bottle behaves very like the water in your boiler, but the equilibrium pressure and temperature are just in a different range.  At atmospheric pressure, propane boils at minus 42 deg C.  At 20 degrees, it's equilibrium pressure is about 700 kPa(g), or 100 psig.  When you open your fuel valve to burn some gas, the pressure drops as has leaves the bottle.  This means the gas and liquid are no longer in equilibrium.  The vapour pressure of the liquid is then higher than the gas pressure.  Some liquid has to be boiled off to maintain the equilibrium.  Now we don't generally put a flame under the gas bottle, so we only have the heat available in the liquid and from the atmospheric air.  The temperature difference is initially zero, so no heat transfer from air and the heat comes from the liquid which rapidly cools.  Now with a large enough bottle surface area and pleasant temperatures like we enjoy here, say 20 degrees or more, and a low fuel off take, you may soon get enough heat transfer to maintain the new low pressure.  But the liquid will be cooler than it was when the gas valve was closed.  Turn up the burner a bit, and the liquid temperature soon falls below the dew point of the atmospheric air, and moisture condenses on the outside of the bottle.  If the propane pressure falls to around 350 kPa(g), the temperature will be around zero and any further drop in pressure soon causes that moisture to freeze, as you have seen.  Much more likely in your climate where you spend much more time below 20 than above.  However in Canada, and many US states a different story, and low ambient temperatures can be insufficient to maintain the gas burner pressure requirement.  The propane itself does not freeze at these temperatures.

With butane, the pressures are quite a bit lower, so in cool climates, some exhaust steam heat, or even just conduction through a common base plate with the boiler is used to maintain the pressure in the small gas bottles generally used in models.  I can give you the butane pressures if you need them.

It's not really about consulting fees, it is a pleasure and a privilege to contribute something back to the forum where I learn so much, and my machining skills have a long way to go.  It is just useful information for many aspects of our modelling hobby, not well known within the model making community, but was basic to my work throughout my career.  It is not secret information, just basic thermodynamics.  I am delighted that you are finding it interesting, and many others obviously keep coming back despite my often wordy style.

Thanks for looking in

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 30, 2017, 01:22:20 AM
hi <Thanks for that... I was looking at a refrigeration van today and when the chap opens the door a whole lot of white swirling mist appeared. Can you/ explain what i was actually seeing it looked a bit like steam but obviously not ? !! Thanks for the info about the rock wool and i shall pack it as tight as possible............Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 30, 2017, 12:47:06 PM
Hi Willy, that type of question is always a little tricky to answer because there are so many contradictory intuitive thoughts about the process involved.  The way I work it out is like this.  The air in the refrigerated van is cool compared with the atmospheric air outside.  Hence when the door is opened, and the cooler, more dense air flows out and is warmed when it mixes with the outside air.  In the process the warm air is cooled.  Now if it happens to be fairly high humidity, it can be cooled below the dew point temperature, the temperature at which the humidity becomes 100%, and if cooled further, excess moisture must condense.  The moisture is well mixed with the air so condenses in tiny droplets.  These appear as a mist, like a fog or a cloud.  So what you saw was a fog of moisture condensed out of the outside air by the cool air from the van.  The tiny droplets are so small that viscous effects in the air prevent them from quickly falling into a continuous liquid phase, and they remain suspended for quite a while.  Eventually the overwhelming quantity of the warmer outside air evaporates the droplets and so the fog does not spread far.

It is often said that warm air can hold more moisture than cold air, which is a reasonable enough observation.  However it can also be looked at in terms of partial pressure.  You can look at the steam tables and see that at low temperature, the equilibrium pressure is lower.  If the water vapour partial pressure in the air reaches that equilibrium pressure, it cannot increase further and any excess results in some condensation.  This point in an air mixture is described as 100% relative humidity.  If there is less moisture in the air, so the partial pressure of the water is less than the equilibrium pressure for that temperature, we use the term humidity, or relative humidity, which is defined as the percentage of the equilibrium pressure of the moisture in air.

When you look at this way, you can easily see that if you have air with a certain moisture content, and you then cool it, then the same absolute moisture content becomes closer to the equilibrium pressure at the lower temperature.  So the humidity becomes higher and eventually if you continue cooling the air, it reaches equilibrium pressure for the new air temperature, and condensation starts as you saw at the back of the van.

I hope that dispels the mystery of the fog at the van, which is the same process as the formation of  fog on a chilly morning, or of a cloud high in the sky, and of the visible steam from a kettle or from your engine exhaust.  Water vapour itself does not reflect light and is quite invisible.

Just a short post tonight, a short night as we start daylight savings, so must put the clocks forward, after a big family day today.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Kim on September 30, 2017, 05:26:00 PM
OK, so now it's my turn to ask a question, that came out of your answer to Willy here.

What exactly IS steam?  Steam is water in gaseous form, right?  We just happened to give it a special name. In fact we gave water is so special, it got 3 names, one for each form (Ice, water, steam - where as most things only get one name and you have to specify form "Liquid Natural Gas" for example).  But my question: how is 'steam' different than water suspended in air?

At standard atmospheric pressure, the water has to be at 100C to become steam.  So, I can see that the water vapor hovering around the door of the refrigeration truck couldn't be steam, because it is clearly not 100C.   But then what makes water vapor different from steam? Is it just the amount of energy contained in the individual water molecule?  Is that the only difference?  Is Humidity NOT water in gaseous form?  What is humidity then?

Thank you for all the interesting discussion MJM.  I did take a term of Thermodynamics in school, and I'm following most of what your saying, but clearly, making that head learning mesh with the real world is challenging me.

Kim
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 30, 2017, 07:37:10 PM
Hi Kim ,good questions there and would it be possible to answer  using words of two syllables or less ?!! And to add to this ....if you had water in a sealed tank and forced air at high pressure into it what would happen ??
Title: Re: Talking Thermodynamics
Post by: paul gough on September 30, 2017, 11:14:19 PM
Taking a step back to horsepowers and specifically boiler horsepower. Re-reading 'Perfecting the American Steam Locomotive' by J Parker Lamb, in Chapter 3, 'The Physics of Steam Power, p. 43, he gives the equation; boiler horse power (maximum) = 1/6 grate area (sq ft) X boiler pressure (psi). He states this is an approximate value and an empirical formula. I presume it was used as a comparative figure by designers, maybe a guide to match boiler capacity to maximum demand of certain size cylinders and a means of estimating fuel consumption. Were there any other uses for the resultant? Was it just a designers 'rule of thumb'? Anyone have any more on this??? Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 01, 2017, 11:51:19 AM
More on humidity -

Hi Kim, always glad to have more questions.  I think you are right on the mark pointing out that water is so special.  After all it is a basic necessity for life, at least life as we know it.  Only three words for water says something about the climate where you live.  I believe that the Eskimos have around thirty names for snow.  As a sometimes skier, I can describe about six, but thirty is amazing.  I suspect that is the heart of the dilemma.  We knew about water, ice and steam long before we knew anything about thermodynamics, so we had words for the different forms we knew, long before we had the need for more specific terms, or even knowledge to explain our observations.

To get to the basics, first the term water, defines the chemical compound consisting of two hydrogen atoms and one oxygen in each molecule, what ever its form.  But the term obviously overlaps common usage for the liquid.  We know it exists in three forms, usually known as phases, solid, liquid and gas, as do many other compounds.  And we tend to use the word steam for the gas phase, but perhaps more particularly when we can see it, like near the spout of a boiling kettle.  And sometimes we use the word vapour.  But it is not so well understood that the gas phase does not reflect light and cannot be seen, like very close the the spout of the kettle, and the mist we see a small distance from the spout is in fact fine droplets of condensed liquid.  Same as a morning fog, sea fog or even thin cloud.

I had to resort to the dictionary to check some of the normal usage definitions.  It says steam is water in the form of a gas or vapour, or water changed to this form by boiling, and extensively used for mechanical power or for heating purposes.  Also the mist which forms when when gas or vapour from boiling water condenses in air.  It also defines steam point as the equilibrium temperature of liquid and vapour phases of water at 101.315 kPa which is equal to 100 degrees C.

So it seems that you are correct in associating the word steam with boiling, and in answer to your question about the difference between steam and the water suspended in air as fog etc. I suggest it is tied to the method of generation of that form in common usage, but the first clause in the dictionary definition just says water in the form of gas or vapour, followed by the word or.

Applying thermodynamics and our knowledge of how the equilibrium pressure of the liquid and gas phases of water change with temperature, I suggest that there is no physical difference.

That leads to the question of humidity.  I suggest this is our everyday description of the moisture content normally found in air.  Normally, moisture content of air, which is proportional to the partial pressure of water vapour in the air, is not at the equilibrium pressure for the prevailing atmospheric temperature, but always less.  The weather bureau records relative humidity which is the percentage of the equilibrium pressure at that temperature.  This means that the actual moisture content of the air varies with temperature, when at the same relative humidity.    As I mentioned yesterday, the steam tables tell us the equilibrium pressure for each temperature.  The relative humidity reading tells us the proportion of that equilibrium pressure which is actually present.  It is then clear that if air with a certain partial pressure of water vapour is cooled, then eventually a temperature is reached where that partial pressure does equal the equilibrium pressure, so condensation must begin if cooling continues.

I have every sympathy for your feeling of challenge in applying the basic thermodynamics you learned.  I went into a career where it was a basic tool used every day, but while I realised that the knowledge had the answer to everyday questions, some of Willy's questions have really got me thinking carefully to make sure I had an answer that was properly supported by the theory.  I still check very carefully for relevant examples in my textbook before I post on some of the more obscure ones.

Hi Willy, I think I failed on the request for words of no more than two syllables, but I hope the longer words are generally well enough known that you won't need a dictionary to read it.  I should have mentioned yesterday, that when that condensation begins, the heat released helps warm the cooler air so the condensation soon stops, hence the limited extent of the mist.  But the new question, you have a boiler for example with water as a liquid, plus some vapour, at the equilibrium pressure for the temperature once the system has settled and all the temperatures are equal, plus some air to make a total pressure of atmospheric pressure prior to sealing the boiler.  If you now use a compressor to add more air, you increase the partial pressure of the air, but not the water vapour.  If you are extremely pedantic, you have to make sure the air is cooled to atmospheric temperature before it enters the boiler, otherwise it will just need more time to be at true equilibrium.

Some of the air will dissolve in the liquid phase, a quite small amount, and the rest just adds to the total pressure in the boiler.  You didn't say how high was high.  You boiler probably is designed for something around 100 psig when cold, or 700 kPag.  At this sort of pressure, the air in the vapour space and the water vapour act near enough to independently, as described by Dalton's law of partial pressures.  If you have a suitable vessel and suitable compressor, the situation is probably about the same at 500 psig.  You could continue to 5000 psig.  I had to specify a compressor for that once, and you don't want to go that far, but somewhere around that stage, perhaps lower or even higher, the atoms in the vapour space do get sufficiently crowded to affect each other and you can no longer simply add the partial pressure.  I am not very familiar with working in that range, and I am not sure just where it starts to be important.  Then, when you release the pressure, the amount dissolved can no longer be accommodated, and the excess dissolved air bubbles out like an opened can of soft drink.

Hi Paul, I can see that boiler horse power is still fascinating you.   As you have said, the term and also the formula are just empirical guidelines, and the formula is not dimensionally consistent unless that constant, 1/6, has the right units.  It is not just a pure number.  However there is some sense to the form of the formula, as grate area is probably a good indicator of the amount of coal that can be burned in a given time, and hence energy input per unit of time. And we know that pressure is necessary for the steam to do work.  So it makes sense that if you make a graph of potential engine brake horsepower against grate area times pressure, you might get some sort of correlation.  Especially in the days before super heaters.  Not necessarily linear, though a straight line can always be drawn as an approximation.  But whether the 1/6 factor is appropriate I really don't know.   To avoid variation due to the engine efficiency, you could use the output of that ideal adiabatic engine to make the results a bit more consistent.  Probably no help to you in designing your little locomotive boilers, but a bit of interesting history.

Thanks to everyone for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on October 01, 2017, 11:30:14 PM
Thanks for your comments on the boiler horsepower issue. I have always been fascinated by the goings on in boilers, particularly loco boilers with their high steaming rates and the relatively tight space they have to occupy. I am hoping that at some point our thermo discussions might flow toward and into the 'what is happening' from fire hole door to stack top. Almost none of us are combustion or chemical engineers but some grasp of underlying principles regarding what is required for a successful boiler and the combustion that goes on in it might expand our understanding and avoid misconceptions and advance design.

 One specific question I have and have not been able to find any satisfactory answer to is; do the boundary layers scale down, with steam, air, products of combustion remain more ore less a constant dimension from full size to model dimensions and is there any significant change in this 'dimensioning' when the fluid is relatively static, eg. in a vessel, or when moving at some velocity, such as steam in a pipe that is vented or flue gases in fire tubes, (leaving aside here 'disruption' such as by steam bubbles in the water adjacent to a heating surface)? Finally, if there is dimensional variability, what parameters might impact, eg. pressure, temperature, velocity, the proportion of the vessel eg. does the boundary layer in a 5mm  copper steam pipe differ from one  50mm in dia. or a 12 mm fire tube compared to a 50mm one? Also does the material matter, by this I mean is a higher conductance metal like copper have different boundary layer formation than that adjacent to a steel tube? Hope all this is not too headache inducing! Regards, Paul Gough.






Title: Re: Talking Thermodynamics
Post by: MJM460 on October 02, 2017, 01:57:28 PM
Boundary layers and scaling -

Hi Paul, obviously you are talking about scaling of size, and not that boiler scale that grows in our boilers if the water contains various impurities.  And you allude to the fact that the gas composition and properties are the same in our models as in full size, and how that might affect our models.

Most of the research work in fluid mechanics is done on scale models and much thought goes into how to address this very problem.  We don't have many,if any at all, cases where we can find a suitable fluid to use in a model that has scale density and viscosity in particular.  Dimensional analysis is one of the techniques used to uncover various combinations of properties that are dimensionless so experimental results can apply to all sizes.  For example the length of a pipe divided by its diameter is dimensionless, so an experiment can be done with a certain size and length of pipe and the results are found to apply to another pipe diameter if the length is such that the ratio of length to diameter is the same.  One example is the development of a velocity profile in a pipe.  At the entrance to the pipe, the flow velocity is roughly uniform across the circular cross section of the pipe.  However, the fluid velocity in contact with the pipe wall tends to be zero.  In low flow situations, the flow proceeds along the pipe, viscosity means the stationary layer at the wall slows the layer immediately inside and so on until a roughly parabolic profile develops where the velocity on the centre line is about twice the average velocity calculated from the flow through the pipe and the cross sectional area.  This profile develops fully at a length where the L/D ratio is in the range of 60, though it varies with the actual velocity.  At higher flow rates, the flow becomes turbulent, the profile is a bit more of a flat topped parabola, which is fully developed at L/D in the range 25 - 40.  Unfortunately when applied to the diameters in our models, the lengths are equal or perhaps longer than our model, so we generally operate in the range of the developing profile.  Another of those dimensionless groups, given the name Reynolds number after the one who identified it, involves viscosity, density, velocity and diameter in a dimensionless combination.  Knowing those properties do form a dimensionless combination you can work it out yourself with a little trial and error.  Then, while you cannot find corresponding "scale fluids", if you run your experiment at equal Reynolds numbers of your model and full size, many results will correlate nicely.  So Reynolds number effectively replaces velocity and compensates for viscosity, diameter and density as well.  Of course with a ship model, there is wave making to consider, but there another dimensionless group called the Froude number, will allow you to calculate a speed for your model that will make the same wave pattern as the full size ship at its appropriate speed.  But fundamentally the flow in a model boiler tube of a certain l/d can be accurately compared with the flow in a full size boiler tube of the same l/d.  One of the two must be determined by experiment, then the other can be calculated.  Of course in fluid mechanics experiments, every effort is taken to stick to the issue of flow and avoid other energy transfer.  Mostly because the properties of viscosity and even density are quite temperature sensitive.  It is hard enough to understand what is happening in the boundary between the stationary wall and the bulk fluid with steady properties, without having to deal with changing viscosity and density as well.  And the reverse applies, the velocity affects the effective temperature profile.  If the velocity is high, warmed fluid is carried on quickly and replaced by cooler fluid so the temperature gradient is effectively changed, and that changes the viscosity which changes the velocity profile.  Not to hard to get a headache in that area, and probably not very useful to go much further. 

The take away points are first, the velocity in a pipe is zero at the wall and increases towards the centre to give a roughly parabolic velocity profile, perhaps a bit flattened in the centre, and that large and small sizes can be compared provided that size and velocity at the comparison points have certain dimensionless combinations of properties controlled to be equal in the sizes being compared.

The other point you alluded to is the effect of the velocity of a fluid on the heat transfer rate.  It is worth noting that velocity has a huge impact, mostly because of its effect on the temperature gradient near the wall, and this temperature gradient plus conductivity of the fluid are the main factors influencing the film coefficient which determines the overall heat transfer rate.  It is sufficiently important that there are two distinct types of convection heat transfer recognised.  First there is natural convection, where the velocity is determined only by the change in fluid density due to the heat transfer.  Second, there is forced convection, where the fluid is given additional velocity, perhaps by a fan or a pump.

You could look at the coffee cooling experiment, perhaps tea so we don't yet have to talk about the effect of froth on the surface.  If you just let the cup sit, the tea is warmer than the air, so liquid and vapour are not in equilibrium at the surface.  Some tea evaporates to cool the air and increase the vapour pressure at the surface towards equilibrium.  However, this also warms the air near the surface, which decreases in density, and warm air rises to be replaced by more cool air.  This is called evaporative cooling, and occurs in a cooling tower, a coolgardie safe, or an evaporative air conditioner.  Similarly, there is heat transfer through the sides of the cup as we know because the cup feels hot, so it does loose heat to the air, but slower than the surface as China is not a very good conductor.  Never the less, it is an example of natural convection.  Eventually the tea cools to a drinkable temperature and we normally avoid continuing the experiment until the tea is at air temperature.  That would not only be wasteful of the tea, but also of or time, as the nearer the tea gets to air temperature, the slower the heat transfer goes, proportional to temperature difference, remember?

Now if you blow on the surface of the tea, you remove that extra vapour, so the surface is no nearer equilibrium and the evaporation and evaporative cooling rate continue at a higher rate.  Similarly blowing cool air on the sides of the cup carries away the warmed air, replacing it with cooler air, so the effective temperature gradient is greater and the cooling proceeds faster, unless of course you are a politician!  That is called forced convection.  The increase in cooling rate is not linear with velocity, so the complexity continues if you are trying to analyse the situation.  But a fan would supply more air at a higher velocity than blowing, so does further increase the cooling rate, and shortens the time to cool.  Even so, as the temperature difference between the cup and the air decreases, the heat transfer rate slows.

The flow and boundary layer problems are discussed in any engineering fluid mechanics text book, they are pretty heavy reading but worth a look in a library.  The heat transfer is described in any engineering heat transfer text book, again very heavy reading, but definitely the place to go if you want to explore the issues further.

I hope that helps.  I think back to wet steam next time, but eventually we will get to the boiler, though I am limited in not being a combustion engineer, so any contributions will be welcome.  Oh, and I will talk about the effect of copper or steel properties on the heat transfer.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 02, 2017, 02:53:19 PM
Hi, interesting stuff again ....LBSC used to say that you cannot scale nature , so interesting to note that they do make models that then work when scaled up. On reading about combustion in boilers it was noted that air intake above the fire grate helped wth the compleat combustion of the gasses above the fire. also when burning wood in an open fire you never use a grate ,(this is what my mum use to say when we had open fireplaces in the house) !! I do have a book somewhere that talks about combustion produced by Charringtons a coal supplier but i don't know where that is at the moment...........
Title: Re: Talking Thermodynamics
Post by: paul gough on October 02, 2017, 05:15:31 PM
Hi Willy, Air supply above the grate goes back a very long way, talking locos here, many of the large modern engines on the U.S. had air jets along the sides of fire boxes, most I think induced by steam, some compressed air, a few things brought about their re-application. Things had reached the limits inside a conventional firebox/combustion chamber due to enormous grate sizes of the big semi-articulated locos (Mallets) firebox volumes weren't sufficient for combustion, nor was turbulence or air fuel ratio of certain types of fuel, usually coal. The history of 'overtire jets' as they are commonly called was an interesting line of research I did a few years ago and it leads back to very early locos in Britain. There were penalties for 'smoke nuisance' and all sorts of things were tried, including 'air tubes', at this time they were at the front of the firebox, just tubes secured through the outer and inner wrappers of the firebox, later more tubes in rows and different positions some with steam jets were tried out. About the time when locomotives transitioned to burning coal rather than coke it was found that larger firebox volumes and the invention of the 'blower' to induce more draught, especially when stationary, overcame the 'smoke problem' sufficiently for the extra boiler making of 'tubes' and maintenance to override any gain, but of course the Super Power era of locos in the U.S. caused things to go full circle so to speak.

Hi MJM, thanks for the detailed comments. They go some way in explaining conditions and to the improvement in steaming of a couple of boilers, loco type, that I 'played with'. The gas fired one had a boiler excessively long and the coal fired one had fire tubes too small, thus amounting to the same thing. Introducing thin strips of metal plate twisted into a low amplitude spiral the same width as the tube dia. brought about better steaming in both boilers. The gas fired one transitioned to thin stainless strips as a permanent fix but the 12" gauge coal burner was a failure in that the strips became impossible to remove to clean the tubes due to being cemented in by the soot or burnt out. We deduced that more turbulence and slower velocity of the gases contributed to better heat transfer. Seems it wasn't far wrong.

One question please: Does the friction in small steam supply pipes to an engine you spoke of previously, 1/8" Dia I think, come from friction between the 'stationary' boundary layer and the main body of fluid or from turbulence caused by the high velocities induced by the small pipe, if both, is one dominant?? I'm  not clear on this.  Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 03, 2017, 10:45:32 AM
More on scaling models -

Hi Willy,  we always had open fires when we were young, some had a grate, some not.  A good fire can be made in either.  I just checked with my wife who is a real expert fire maker, a result of her farming background, and she also has no preference for grate or no grate.  Air over the top is helpful in making sure combustion is complete, you don't want the possibility of carbon monoxide entering the room as it is very toxic, unlike carbon dioxide.  There are a few other nasties in the partially burnt fuel, depending on just what you are burning, so a good idea to introduce some excess air.   But you also need air into the seat of the fire to support combustion.  A fire needs to be compact enough to generate a high enough temperature to exceed the required temperature for combustion of wood, and needs enough air for combustion but not enough to cool the fire and extinguish it.  In burner technology, these two air sources are called primary air and secondary air.  I always thought the grate was useful in small fireplaces to help prevent burning wood rolling out, not so necessary in a larger one where it is easier to build the fire so it is quite stable.  There will be something behind your mothers advice, there usually is, but I am not sure that I understand the real reason.  I wonder if anyone else has more detail.

Hi Paul, Thanks for the interesting information on secondary air in locomotives.  I probably signed off a bit prematurely last night, not time wise of course, but there is probably some value in following the topic a little further.x

The Reynolds number is density x velocity x diameter / viscosity.  The units for density, velocity and diameter are well known, though perhaps I should emphasise the unit for diameter is metres, not mm.  The units for viscosity are Newton seconds per metre.  You can easily check that it is dimensionless.  That formula emphasises the place of density and viscosity.  Tables often list kinematic viscosity instead of absolute viscosity, but kinematic viscosity = viscosity / density, so it can easily be used in the formula as that ratio is directly in the formula.  Kinematic viscosity has the units L^2/T.  If you have a larger tube and a smaller one, and you operate both at the same Reynolds number for equal flow patterns, viscosity and density will be the same for both, so you can see there is an inverse relationship between velocity and diameter between a smaller and a larger tube.  Now  dimensional analysis tells you that you need a quantity with the dimensions length, so a representative size.  It does not tell you what that length should be.   You might be aware that our aeronautical colleagues use the wing cord in calculating Reynolds number,  not the span, while in piping, using diameter as the representative size yields better correlations than using length when the experiments are analysed.

Another relevant dimensionless number is the ratio of cross sectional area to surface area of a tube.  Cross sectional area is important to the flow rate through the tube for a given pressure drop,  while the surface area is important to heat transfer.  For problems involving heat transfer and flow, you can't get the same conditions in both tubes for both flow and heat transfer, but some investigation might lead to an optimum diameter, where larger does not have enough surface area, and smaller does not allow enough flow.  And of course you need enough pressure drop to drive the flow.  So just finding a dimensionless ratio does not mean you can use it to scale the design.  This example illustrates that our models are small boilers, not scaled down ones.  However when trying to model a prototype, we do use a constant length ratio for as much as possible so that the overall proportion and appearance is preserved.

Those spirals are an interesting technique.  I am not sure whether they just provide extra surface area to absorb heat then transfer it to the tube wall by conduction where it contacts, or whether they modify the flow pattern so improving convection heat transfer.  Or more likely it is a complex combination of both possibly plus other factors.

The definition of friction factor uses only the wall shear stress and V^2/2g.  There is another term, apparent shear stress, which varies across the radius of the pipe.  The variation is due to the effect of turbulence and momentum transfer, but the momentum transfer requires a transverse component of velocity.  Obviously must be zero at the wall, but increases as you move into the bulk flow where the flow is turbulent.  The apparent shear stress appears to be linear between the wall and the centre, so near the wall viscous effects predominate, then moving inwards, the two become equal, then momentum transfer and turbulent effects predominate.  I will be interested to see how you use this, but again there is more detail in any engineering fluid mechanics text.  The maths gets pretty heavy quickly and I am definitely less comfortable in this area.  I hope that is sufficient for now.

Still planning to return to wet steam next time,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on October 03, 2017, 12:32:08 PM
Again, thanks for the leads into the goings on in pipes, I really hope I have not caused any of this threads devotees to run away because of my fluid dynamics enquiries. I think I have enough grasp of things now to know what to look for when I get the chance to pursue some combustion and fluid flow tomes. I'll have to take a trip down to J.C.U. library, I think they teach some of the M.Eng. undergrad program in Cairns, so maybe I'll find a few books there. For the moment I'll retreat to my cell, meditate on your discourses and take a vow of silence, lest our followers become restive at my abstruse questions. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: Steam Haulage on October 03, 2017, 12:39:42 PM
Hi MJM,

Could you please expand the definitions which you use in your 4th Paragraph regarding viscosity and density. I am especially interested in how you measure these values and the practical units of measurement in the various forms of apparatus used. I presume your work depends on treating water and steam/air mixtures as fluids.

Jerry
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 03, 2017, 03:07:33 PM
Hi Paul interesting about the induced air into the fire box .....would this air be better hot or cold and would it help to be dry or moist ? i think i have heard somewhere that IC engines run better in the early morning when there is more moisture in the air ??   
Hi MJM I think my mum said that the wood would reflect the heat back and forth to each other to stop it burning too quickly, but keep the radiant heat from escaping,also the grate would make it burn to quickly as well , This may have been because she would fall asleep after supper in front of the fire and when she woke up after snoring a lot she could rake the fire a bit and get a nice hot blaze going !! she also had a large metal plate to put against the fire place to increase the draft . this was after she almost burnt the house down when she used the daily paper across the fireplace that caught fire by the suction going up the chimney and floating around the room.Before i leave my mum....she used to fall asleep reading a book ,but say she never did, until we took the book away and turned it upside down !!!She would then wake up and glare at the culprit and say Time for bed !!!.......
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 04, 2017, 12:11:08 AM
Hi MJM I have redone the boiler test after insulating the end and also some of the underneath. The results seem to be quite different though First of all it would not get above 126 C without the safety valve blowing off this time and i cannot screw down the safety valve any more .It may be that the probe did not go in as far this time by about 1/4" due to the end piece of wood. Also the temp/time curve is slightly non linear. and the temp difference per minute varies quite na lot from 16 degrees in the middle to 9 degrees at beginning and end !!  So something strange going on here to think about ....the clock and thermometer are the same so it must be the boiler.... the leaks seem to be a little bit worse as well.... any way...your turn now to come up with some explanation if possible please !........Thanks.....The latest graph is the red line btw
Title: Re: Talking Thermodynamics
Post by: paul gough on October 04, 2017, 12:33:23 AM
Well, so much for my vow of silence! Hi Willy I hope the following will help.

 Regarding 'Air Tubes' see attached photos for only a couple of historical examples. First photo from the loco 'Liver', L&M Rly, 1837. Three grates, bottom two for coal and top one for coke, but note the air tubes through the front of the firebox supplying supplementary secondary air to grate 'b', (middle). The two fire hole doors also had 'perforations' to allow main supply of secondary air in above the grate, as is generally normal for most locos. This is the first example of 'air tubes' I have been able to find.

Photo 2, shows D. K. Clarks "Steam Induced Air Currents" as applied to an Eastern Counties Rly. loco, 1858. He claims it, " is quite successful in preventing smoke, making a bright fire, keeping up steam, and working economically. By inclining downwards the jets of steam, the air may be thrown at any desired inclination upon the fuel and amongst the smoke."

As you can see secondary air delivered through tubes goes back to the very first decade of railways and the steam jet type to the 1850s. These are only two examples and it was just a part of the rapid evolution of firebox design for locos happening around the middle of the 19th century.

Now addressing your question; as far as locos are concerned, I doubt that there would be any significant difference in practical terms. Remember steam locos operated outside in hot dry sunny conditions in deserts and frigid foggy or sleet driven nights atop mountains, sometimes even encountering them on the same trip or shift. I doubt that even in the sophisticated 'Super Power' era of steam locos, temp. and humidity differences in secondary, or for that matter primary air, would have concerned designers. However drivers or firemen who were especially sensitive to their engines and the various conditions under which they ran them may well have 'sensed' a difference on a sunny summer day and a snowy night, maybe would have been able to quantify it very roughly in noticing less shovels of coal on a long continuous grade, but I suspect they would have been the only ones who could. Having said this I should think the modern large stationary plant people might have different views on the matter, as they can do many things not possible on locos, but I have not looked into this area.

I too 'sensed' better running from my motor bikes when I rode, (half a century ago!), considerable distances when it was cold and foggy, especially at night, but I never tried to quantify any difference, let alone accurately do so, the bikes just seemed to 'go' better. Obviously cold air is denser and a bit more oxygen but I was riding mostly at moderate elevations also, 600-1000 metres, so less oxygen too. As to what happens to H2O in the combustion chamber of a bike I know not, perhaps it breaks down to H & O and assists. I suppose the F1 car racers might know, SCO might have an insight. When I was in the Diesel Traction Section in the N.S.W. railways it was a non issue and no-one from the design office ever mentioned it either, but diesel loco operators in Sth. and Nth. America whose locos have to traverse very high altitudes as well as work in incredible temperature/humidity ranges might have investigated it. Regards, Paul Gough.



Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 04, 2017, 11:56:45 AM
Just a quick message I think the leak at the back may have wetted the rock wool and so caused a heat loss there but am not sure !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 04, 2017, 12:20:31 PM
Viscosity and a boiler test -

Hi Paul, your questions are always welcome, I don't mind a little excursion into fluid mechanics, there are plenty of fluids flowing through pipes in a refinery, but perhaps I can leave you to start a thread about historical locomotives.  Of course real life examples are always helpful in a discussion like this, so not a very clear line, more a wide grey area.

Hi steam haulage, welcome aboard.  Viscosity is that characteristic which makes oil or honey flow more slowly than water if the jar is tipped.  In a plain journal bearing, the resistance to rotation or shear force per unit area of the bearing surface is proportional to the velocity gradient across the oil film thickness, h.  Mathematically, F/(pixDxL) is proportional to V/h, where V is the journal surface velocity.  The proportionality factor turns out to be a constant for a large group of fluids which are called Newtonian Fluids.  Of course the constant does change with temperature.  The constant is given the name viscosity, or more accurately, absolute viscosity.  (Non-Newtonian fluids are those where the factor is not a constant, but varies with the shear rate.) So absolute viscosity is a direct measure of the shear force in a fluid when it flows.   In many situations, the combination viscosity/density occurs, and this term is given the name kinematic viscosity. 

Viscosity is measured by a Saybolt viscometer, which is a strictly standardised cup like device with a short vertical tube in the bottom, all immersed in a temperature controlled oil bath.  The number of seconds it takes for 60 cm^3 of the fluid to flow out through the tube is used in an empirical formula which actually gives a value for the kinematic viscosity.  From this, absolute viscosity is found by multiplying by density.  You may have heard of Saybolt seconds.  My heat transfer book has tables of fluid properties, and it lists kinematic viscosity and density, leaving you to do the multiplication if you need absolute viscosity.  I am not sure if that answers your question, but please ask if you want more or something a little different.

Hi Willy, I am glad you mentioned the newspaper trick, I have used it often in my youth.  It is not supposed to catch fire, but if it does, it is being strongly pulled up the chimney, so I am not sure how it flew into the room.  Falling asleep during a fireside read has a long and honourable tradition, as has stirring the ashes to get the fire going again the next morning.  But I suspect that your mother is right, not if you use a grate, as it will allow extra air so the fuel is more likely to completely burn out.  But the radiant heat is what you need if you are sitting across the room, instead of standing close with your back to the fire.

Now a new boiler test, so back to the real topic at hand.  Great job on that boiler end.  It can be tidied up when you are happy with the overall result.  That safety valve is a bit of a puzzle, it allowed a higher pressure last time.  Of course, there is an inconsistency with the pressure gauge readings in your picture, which suggests there is still air in the boiler, contrary to my understanding that it is lost pretty quickly once the safety valve lifts.  Is it worth continuing the run a little longer to see if the pressure settles back?  Last time I understood that the safety valve lifted early, then was screwed down to the final pressure setting, so I assumed the air was gone.  But let's look at what else your graph shows.

First the little curve at the beginning.  As with last time, I suggest this is due to having to first heat the element then the encapsulating insulation and sheath before water starts to heat.  Then it is fairly linear until you reach 100 degrees, when the water vapour pressure finally exceeds the atmospheric pressure of the air and water vapour that was in the boiler when it was sealed.  Before this point, notice that each temperature is reached just a little earlier than last time, a small effect but almost certainly the result of your end insulation, or the moisture from that leak. Unfortunately the other end is still bare, and the area of the shell which has thin insulation is much greater than the area of the end, so the effect is not yet dramatic.   Then once you reach 100 degrees, the pressure continues to rise, but the temperature is a bit slower to rise for two reasons.  First it takes more energy to evaporate the water that is evaporating at a higher rate, and of course, that is the point where your leakage starts to be evident, and of course the leakage increases as the pressure rises.  (Actually a guess, but perhaps you can confirm.)

As your electric boiler has constant energy input, the energy input is the same now at about 125 deg C as it was previously at 143 (or 148, I am a bit unsure which).  I approximated to 125 because it explicitly occurs in my steam tables so a bit easier to use.  We calculated the heat required to get the boiler and water up to steaming temperature last time,  if we look just at the water, to get from 125 to say 143, we only need 80 J/g, but to evaporate steam at 125 degrees requires 2188.5 J/g, so a leak of about 0.46 g/s absorbs all the available energy, so prevents the temperature and pressure from rising.  The method is a bit rough, but I suggest it shows that a leak has a significant impact on the maximum pressure that can be reached.  But it is a leak of the whole engine consumption.  That would be quite a leak so probably not the whole answer.  And remember we showed last time a significant heat loss and the small area of insulation has not reduced that much.  So possibly only 60% of the heat becomes steam.  Possibly still some issue with that safety valve.  Needs some careful observation and more thought.  Of course if the engine was running, there is most of the steam, and a much smaller leak is all that is necessary to prevent the boiler reaching the original higher temperature.

I suggest the next step is some gaskets or thread sealant to eliminate the leaks, and put some insulation around the cylindrical shell, and even a bit behind the sight glass and around the other fittings.  Try and add half an inch or more over the whole boiler with just a little hole so you can see the gauge.  But you can see that some simple calculations can give some valuable insight into what is going on.  Even when there are a few assumptions, there is a pointer to the direction for further experiment.

I will still keep trying for wet steam next time, and that moisture in the engine inlet.

Thanks everyone for looking in.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 05, 2017, 01:21:21 PM
Wet Steam -

I have been trying to get to this topic for a while in preparation for talking about engine exhaust and engine power output.

I have often referred to water in a boiler, where there are normally two distinct phases.  The liquid, which has a defined volume fills the lower part of the boiler, then is bounded by a free surface, and a gas phase which occupies the remaining space.  In a closed boiler, without significant temperature gradients, the liquid phase and the gas phase are in temperature equilibrium, and the pressure of water in the gas phase is determined by the temperature, and the values are listed in the steam tables.  If the water vapour pressure is lower than equilibrium pressure, some of the liquid will evaporate until the pressure is equal to the equilibrium pressure.  In the process the liquid will loose energy to the gas, and the temperature will fall.  Of course this means the water, liquid and gas, is a little cooler than the atmosphere, so heat will flow in, and so on.  Eventually the liquid and vapour and the atmosphere will all be at the same temperature and the gas phase will be at the equilibrium pressure for that temperature.  Now for a given volume of gas phase, the mass of water is directionally proportional to the absolute pressure. 

Let's assume the boiler initially containing only dry air, has a capacity of about 1.5 litres, and we introduce 1 kg of water, which occupies about 1 litre.  About one litre of the air initially in the boiler will be displaced back to the atmosphere, and at the stage the plug is inserted and tightened.  The air contributes to the pressure in the boiler but has no effect on the water vapour which will evaporate from the liquid until that equilibrium pressure is reached.  Let's assume the temperature is 15 deg C.  At this temperature the vapour pressure is only 1.7 kPa.  And let's further just for simplicity assume that we achieved the whole process so far in a manner that means the total mass of water in the boiler including vapour and liquid is 1 kg.  Now the specific volume of dry saturated water vapour is 77.93 m^3/kg.  So the 0.5 litre vapour space contains only 0.0065 g, and the rest of the water remains in the liquid.

We can see that the total water in the boiler is 99.97% liquid and the dryness fraction or quality of total is near enough to zero.  But this is made up of a liquid phase which is 100 % liquid with enthalpy of 419.04 kJ/kg, plus a distinct vapour phase which is essentially 100% gas phase with enthalpy of 2676.1 kJ/kg.

If we now allow this dry steam to flow to an engine, and look at the exhaust, we will generally see a healthy vapour cloud, a cloud of steam which is starting to condense, so it also contains a portion in a liquid phase (as finely divided droplets) dispersed in a stream of dry steam.  This is termed wet steam.  Like the boiler contents it is part liquid phase and part gas phase.  If the temperature could be kept constant, the fine vapour droplets would be expected to eventually settle out and form a distinct liquid phase.  In order to determine how much enthalpy change occurred over the engine, we have to determine the properties of that exhaust steam.  And the enthalpy change is the energy extracted from the steam by the engine.  The enthalpy of that wet steam is somewhere between hf and hg at that pressure.

For an atmospheric exhaust, we know the pressure.  Let's assume the weather pattern has our local pressure at that standard pressure of 101.35 kPa.  We can measure the temperature, but as we did not have a superheater, it will probably be very close to 100 deg C, depending on insulation and heat losses to atmosphere.  However the steam tables tell us that temperature,  it is not independent of the pressure, so we need another independent property of the steam to help us determine the properties of the wet exhaust steam, and specifically the enthalpy.  This one has been a bit heavy going but it will be clearer where I am going next time.

Next time, I will talk about how we use the concept of an ideal adiabatic engine and that property, entropy, to work out at least a limiting condition, that provides an estimate of the maximum change of enthalpy that could have occurred.

I hope everyone is still following

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 05, 2017, 01:40:42 PM
Hi.....If after filling the boiler to correct level we suck out all the air with a vacuum pump will the steam be raised in a shorter time ? and i have been fixing the leaks using Fibre washes rather than copper. also the boiler takes about 2.5 hours to get back to ambient temp !!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 05, 2017, 11:08:27 PM
Hi just a quick question ....Woolf talks about Steam becoming rarified ?? and i have fixed the leeks and the safety valve blows at 50 Lbs /square" whilst showing a temp of 136 degrees......but back to topic !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 06, 2017, 12:25:08 PM
Hi Willy, I am glad that the fibre washers fixed the leaks, we want to keep all the steam for the engine.  From the photo, that safety valve is blowing down well, surely all the air was gone when you took the pressure.  The equilibrium pressure for 136 degrees is only 32 psig, so the gauge looks a bit high at this point.  Perhaps some of the earlier readings were taken while there was still air in the boiler, an important point to observe carefully as the boiler heats, together with just when the safety valve lifts or you turn over the engine a few revs.  Remember the steam tables use absolute pressure, so you have to subtract atmospheric pressure from the tabulated value to get the gauge pressure.

Evacuating the air in principal would help, but when you look at the numbers, not enough to be worth the effort.  The specific heat of air at constant volume, before the safety valve lifts is only 0.72 kJ/kJ.K, and the density is only 1.2 kg/m^3, so half a litre of air at atmospheric pressure when you tighten the plug has only 0.5 g.  Hence it requires about 0.35 J/deg C.  To heat it by about 130 deg to 136, takes about 47 J.  You heater puts in 1000 J/s, so you would save less than 0.05 seconds.  I don't think you could evacuate the boiler in that time.  You can rest assured that the heat goes mostly into the water, plus the copper, plus heat leakage due to limited insulation.  Once the boiler is up to temperature, the copper takes no more as it stays at constant temperature.

The time to cool down is the limit to whether you can do more than one experiment each day.  Of course you don't need to let it cool down to refill the boiler for another run, if it is below about 90, you can do it with care and once below about 60 the plug will feel hot but not normally burn your fingers.  Of course with better insulation, it will take even longer to cool.

Your questions nicely introduced the first question I wanted to address, the air content does not take much heat.  However, until discharged it does affect the total pressure read on your gauge.  Initially the air pressure was 101.3 - 1.7, the 1.7 being the water vapour pressure, say 99.6 kPa (absolute) and when this is heated to 100 deg C from 15, it's pressure increases to 99.6 x 373/288 = 128 kPa.  The total pressure in the boiler at 100 C is 128 from air plus 101.3 from the steam, so 230 kPa (abs) or 130 kPag.  About 18 psig.  If we turn the engine a few times as soon as the pressure gets up a bit, the air will be lost reasonably quickly, while the water will evaporate to replace the lost vapour.  If you keep this process in mind as you approach steaming temperature, you will see how the air loss affect the deviation between the pressure gauge and the steam table value for your temperature.

Every thermodynamics text book has the example of some liquid in a closed volume, presumably after impurities such as air are evacuated, and applying heat until the liquid is all evaporated.  If you felt we were proceeding into the fog, you were right, but the fog did not seem to clear in front of us.  But with a nights sleep the fog has cleared somewhat.  Clearly, the textbook writers are not very practical, and do not ever apply some figures to their example.  As we saw yesterday, the pressure rises with very little of the total mass evaporating, due to the huge expansion when liquid evaporates to steam in a confined volume.  When this happens accidentally in an uncontrolled manner, such as adding water to very hot oil or metal, it is called a BLEVE, boiling liquid expanding vapour explosion, and it is well known for destroying large pressure vessels.  Our experiment assumed controlled heat addition at a slow rate.  With so little water evaporating to increase the pressure, we were never getting to the point I hoped for, where the term wet steam would make sense.  We had 1 kg of water in a 0.0015 m^3 boiler, a typical practical model starting point.  To evaporate all the water per the text book experiment, we need to get to a specific volume of about 0.0015 m^3/kg.  Now when we look for that specific volume in the saturated steam table, the table runs out at 0.003, at the critical pressure, when the temperature is 374 deg C.  Above that the water is supercritical and we are not going there.  In fact a copper boiler is designed on a copper temperature around 200 deg C.and we will exceed this at about 1554 kPa, but our boilers are not even designed for anything like that.  So we are never getting to the point usually claimed, that the pressure and temperature stay constant until all the water is evaporated, then both start to rise.  In order to demonstrate this within reasonable pressure range, our 1.5 litre boiler must start with significantly less than 1 g of water, barely enough to form a droplet, and certainly not enough to cover the heating element, and we all know that is only going to end in tears.   Another approach is needed if we are to understand wet steam.  Essentially the total mass in the boiler is always very near saturated liquid, while it is in the form of 99.9% liquid and less than 0.1% vapour.

Once we open the stop valve slowly, the steam that escapes is very nearly dry saturated vapour.  Of course many will know that if the boiler is too full, and we draw off a lot of steam, the resulting vigorous boiling will mean that some liquid is carried over with the steam.  Full size boilers have separators that separate the water from the outlet steam, but not very easy in a model.  A well designed steam dome would probably do nearly as well.

The area where understanding wet steam is most useful is in looking at the engine exhaust.  We will get to that eventually, but first, lets gradually open the stop valve and look at what happens.  A good place however to stop, so next time.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 06, 2017, 03:21:53 PM
Hi MJM , thanks for this ,I have noticed that the pressure at 100 C was quite high ...18 Lbs" by your calculation ,unlike my little pressure gauge  and found this out the hard way by assuming that when water boils in an open vessel there is no pressure !! So, When i opened the filler cap just below 100C there was a violent erruption of hot steam /water that drenched me somewhat. So question ...at what temperature is the pressure at 1Lb above atmospheric ? and how much is this due to just the water expanding, as presumably the steam has not been generated at this point ? or is the steam so saturated that it does not give the appearance of steam. If i had a very accurate pressure gauge that would be quite good at seeing what is happening inside the boiler. But do you need to make an adjustment with the air being partially compressed in the Boudon tube. Yes the boiler had blown off a few times before i took the photo as i was trying to take a photo at the exact point the safety valve lifted without drenching the camera !! The humidity level in my house has now increased quite a lot too !!!And i am enjoying a monsoon /sauna experience !! If the temp probe was inside the boiler it might give a more accurate reading as well.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 07, 2017, 01:14:21 AM
Hi MJM ..i have now done the next boiler test and we now have a strait line GREEN ..time/temp  again like the first one BLACK. I think that the RED line graph was curving because the leaks were actually getting worse/better.... The readjusted valve blew off at 135 C to release the steam/air and the next time it blew off was slightly higher 138 C. the time was also shorter to achieve this result ,so more in keeping with your first thoughts of about 8 minuets !! I was talking to a friend about my sauna/monsoon event and he told me that water does expand about 4% at 100 C or something. I was thinking that the temp/pressure tables may vary with the amount of water that is actually in the boiler to start off with ?.....this is quite fun and i always wanted to be a scientist so i could bash things and squash things and even blow things up !!! always under strict control of course. My boiler is now about 8 years old so it would be interesting to look at the scale build up as the sight glass is not functioning any more !!  good to hear about more analysis with the latest test.....If there was no heat loss with 100% insulation how long would it take to steam up to this temp/pressure or is that not possible to calculate ?? i will have to get more rockwool do any further tests of course.
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 07, 2017, 12:44:29 PM
Another boiler test -

Hi Willy, that is a very encouraging test, and a great idea to plot it on the same graph as the earlier ones to aid comparison.  I will get to that shortly.  But your first post first.

When you boil an open kettle, the pressure is held at atmospheric pressure, so zero gauge pressure, by the open path to atmosphere.  When you first tighten the plug in the boiler, it is at atmospheric pressure and temperature (unless you used that electric jug), the vapour space is mostly air at the prevailing humidity.  There will be a small amount of evaporation in the boiler, depending on the difference between the moisture in the atmosphere, and the vapour pressure at that temperature.  But eventually if we assume the boiler settles out at say 15 deg C, the saturation vapour pressure is only 1.7 kPa (absolute).  If the humidity is say 50%. Then the moisture content of the air is 0.5 x 1.7 or 0.85 kPa, and if the boiler is allowed to truly settle the extra 0.85 will evaporate, cooling the water, so more heat comes in, and slowly it will get there.  But we are playing with 0.8 kPa out of a total of 101.3 and the rest, over 99%, is air.

When you start heating the sealed boiler, you can't ignore the air, it will be close to water temperature, and when you get to 100 degrees, the air is also at 100 degrees.  And it will have increased in pressure from the original, near enough to 100 kPa, in proportion to the increase in absolute temperature rise, so 100 x (273 + 100 )/ (273 + 15 ) = 129.5 kPa.  I think I rounded it to 130 kPa last time.  At the same time, the water vapour pressure has increased to 101.3 kPa at 100 deg C.  The two apply independently so the total pressure you have in the boiler, and would be reading on an accurate pressure gauge, would be 230 kPa (absolute) or 130 kPa gauge.  This is about 18 psig.  Not a good idea to release the pressure by unscrewing the plug!

But let's look at what else is going on inside the boiler.  The steam tables tell us that the specific volume at 100 deg C is 1.673 m^3/kg, so assuming we still have very close to half a litre of vapour space, the vapour space now contains about 0.8 g of water, so about 0.08 % of the water volume has evaporated.  We are probably justified in ignoring that.  Your friend is right, you can see the expansion of water in the tables by looking at the vf columns, the specific volume of the liquid.  It's about 4.4%.  I guess that means the vapour volume is about 8% less than our initial 0.5 litres.  I certainly did overlook that, and it means that the air is compressed a little more.  But I think the sauna was caused by another point.  Boiling only starts if the water vapour pressure is above your actual atmospheric pressure when you released the plug.  Your temperature meter has a degree of inaccuracy.  If you did not check the  calibration at 0 and 100, the spec sheet that came with it will tell you the limits, but with any digital instrument, you can safely assume that the last digit is still uncertain by at least +/- 1.  That means the temperature could have been 99 or 101.  Also the tables tell you the absolute pressure, your gauge reads difference between the atmosphere and the boiler.  Depending on the weather, a low pressure system for example, your absolute pressure could have been perhaps 99.5 kPa without being extreme.  The result of these two factors is that the vapour pressure of the water could have been a little bit above atmospheric pressure.  While the plug was tight, evaporation matched the temperature rise, and boiling was suppressed by the additional pressure of the air trapped in the boiler.  When you released the plug, the excess pressure was released, and if the vapour pressure was above the actual atmospheric pressure, boiling is no longer suppressed and the huge expansion of water as it turns to steam so the reason for your boiler eruption.

I hope that you weren't scolded, but please, if you want to release the pressure in the boiler after you have started heating, use the regulator, and release it through the engine, or a whistle that you provide for the purpose.

The steam tables list the volume and energy values on a unit mass basis, that is 1kg for the metric tables or 1 lbm for imperial tables.  Obviously the pressure and temperature are not affected by the mass.  When you need to use the total energy, enthalpy or entropy, you multiply the value in the tables by the mass in your system.  I hope that is a bit clearer, but please continue to ask if there is something still not clear.

Now for that new test.  What a difference sealing those leaks made.  They seem to have been confirmed as the culprit for those curves.  And now you can more easily see the difference your insulation made.  Looks like about 1 min 15 sec shorter time to heat up.

I quickly ran the calculations again for 135 degrees assuming 16 degree atmospheric temperature, (I hope you did record the starting temperature).  I assumed 600 ml of water again, which of course it was completely emptied after the last test.  And again 750 g of copper in the boiler from my estimate last time.  And still assuming the heater is 1000 watts, though that data sheet implied that any error would make it on the low side.  This is the data needed for the calculation.  I can easily correct any figures that are not correct, the beauty of using a spreadsheet.  Heat required for the water is 300.3 kJ and the 34.2 kJ for the copper.  So again ignoring the heat stored in the insulation which will be much lower than the copper, it looks like 335 seconds or 5.6 min to heat up if there were no losses.  The actual time was 7 min, so the losses were only 20% of the heat input.  Alternatively, 80% of the heat went into the copper and water.  There is room for a little improvement with more complete and thicker insulation, but you can see only about 1 1/2 min time  savings before steaming starts, and that is still assuming the heater is a full 1000 watts.  Perhaps someone could lend you one of those energy metering devices sold for checking appliance consumption so you could check it.

I continued on to calculate how much steam production you might expect.  Once steaming begins, temperature is constant so no more storage in the water or copper.  We look up hg for the steam at 135 deg and subtract the enthalpy of the water hg.  Better still, look up hfg, which is the result of the subtraction and is the enthalpy to evaporate a kg of steam from water at the equilibrium temperature.  Divide the heat input, 1 kJ/s by the enthalpy needed for evaporation of 1 kJ to get the number of kg per second evaporated.  It is pretty small but works out to about 22.1 g/min with the current heat losses, compared with about 28 g/min if there were no losses.  It will be interesting to run the engine for a significant time without uncovering the element of course, and see what it uses.  Does your temperature control cycle the element on and off?  Or does it run continually?  Looking forward to the next step.

Well, in analysing the behaviour of the boiler when the plug was removed, I covered what I had hoped for, so two in one this evening.

Thanks for following along

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 08, 2017, 10:51:08 AM
Theory and observation -

Yesterday I tried to analyse the processes which followed Willy's unscrewing the boiler plug when the temperature gauge showed 100 deg C.  I came up with two factors contributing to the ensuing eruption, i.e. the air pressure which is also in the boiler from the original fill, and a small discrepancy in the actual temperature and the display on a digital instrument.  A bit more sleeping on it, and I have some additional factors to add.

First, when that air, now at 100 deg C escapes, it carries with it a significant amount of steam.  That means the total pressure on in the vapour space is reduced, but the steam partial pressure is also reduced, so that again, the equilibrium vapour pressure of the liquid is above the partial pressure in the vapour space.  That difference also triggers boiling a small mass of water, but resulting in a huge volume of steam.   I suspect that difference in vapour pressure would result even if all the air was gone due to the engine running or even safety valve lifting when the pressure was achieved.    A lot depends on the exact sequence and timing of events.  In the absence of an engine or a whistle to control the steam release, I would leave it till a much lower temperature, then use a socket and extension to release the plug with my hands well clear.

The whole exercise does show the difficulties of applying a little bit of theory.  There is always the possibility of other theories also applying, and sometimes the theory we did not think of is more important in the particular circumstance.  So it is important not only to learn the theory, but also to proceed cautiously until the process is understood.  Steam is hot and dangerous, and experiments with banging and squashing and blowing things up always need to be carried out with caution and due consideration of a safe vantage point.

You do not need to make an adjustment for air in the bourdon tube.  In principal, the pressure reading is affected by both the height of the gauge above or below the point where the pressure is measured and also by phase changes in the connecting tube.  Even if you have a very long horizontal tube, there is no effect.  However, some steam will condense in the tube which will act like a U-tube manometer.  The leg of the manometer with the bourdon tube will have trapped air which is at the same pressure as the water compressing it.  If there is a big change in elevation between the gauge and the boiler, the density of the water and the height of water in the tube will make a difference.  But on a model, the difference is practically only an inch or so.  And a few inches of water are not significant compared with the pressure you are measuring.  If you have 10 metres of water column, it makes a difference equal to 1 atmosphere, say 14.7 psi or 101 kPa.

Just exactly when the steam is generated is worth looking at a little closer.  The first thing is that when the temperature has evened out, so all is at the same temperature, then the water vapour pressure in the vapour space is always equal to the equilibrium vapour pressure listed in the steam tables, and that vapour pressure is in addition to the the partial pressure of any other substance, such as air, in the space.

I suspect the question is really about the difference between evaporation, which we would normally expect to happen slowly, and vigorous boiling.  Slow evaporation takes place at the surface relatively quietly.  If there is no heat source, this evaporation absorbs heat from the liquid and cools it, for example when you blow the vapour away to cool your coffee.  Under this slow evaporation there is no significant departure from equilibrium.  The pressure in the liquid varies with depth due to the liquid density, and is higher below the liquid surface.  So evaporation below the surface level is suppressed.  If heat is applied at a high rate, the temperature near the heating element will be higher than the bulk temperature and next to the element, and can become higher than the equilibrium temperature of the bulk of the water.  So some of the water then flashes into vapour, undergoing that huge expansion into bubbles that rise vigorously to the surface.

It the water is in an open vessel, the air pressure adds to the vapour pressure at the surface.  This additional pressure at the surface suppresses boiling, but the evaporation still occurs so that very near the surface the equilibrium vapour pressure is achieved.  While this equilibrium is less than atmospheric pressure, that is the temperature is less than 100 deg C, there is still air, which increases the total pressure at the surface and boiling is suppressed.  However once the water is at or nearly 100, the equilibrium vapour pressure is about atmospheric pressure, and a significant temperature gradient provided by the heating element soon means the vapour pressure exceeds the local total pressure and boiling begins.  It is that unrestrained expansion of liquid to vapour that causes the vigorous boiling, and the point it starts depends on both the bulk temperature and the heat transfer rate (due to it causing a temperature gradient.)

If the water is in a sealed boiler, still with that initial air, like your boiler, then the evaporating water and the air as the liquid heats can no longer escape and the total pressure increases due to heating the air and increase of water vapour pressure with temperature, so boiling continues to be suppressed.  However, if you then release the pressure, the air and significant steam escapes, the bulk liquid temperature exceeds the equilibrium temperature for that new pressure and unrestrained evaporation quickly proceeds, the rapid huge expansion pushing water out with it.  Preferably the escape is controlled by slowly opening your regulator and the escaping steam, together with the air that remains happily drives the engine until all the air is gone then the steam continues on.

But in the unrestrained expansion, ensuing clouds of steam consist of a mixture of dry vapour and entrained water and very fine droplets which appear as a cloud or mist or even hot rain.  The evaporation absorbs heat from the liquid, cooling it until something close to equilibrium is again attained, though by that time the eruption is complete.

It's a little long winded, but I hope that in the end it does clarify things a little.

Thanks to everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 08, 2017, 06:23:40 PM
Hi Thanks for all these explanations, its a lot clearer now. When i did the time/temp graph i also wrote down the differences between each minute and found them to be slightly different ...especially after 100C so is there a valid explanation of why this should be so ?? The graph did sort of make a strait line ! I will try and sort out the boiler soon but only need a handful of rockwool so will keep a look out in the local skips. The temp controls on the panel of the boiler circuitry allows a sort of stoker control of the "fuel" by pulsing the electric to the elements . This is so one can regulate the steam production with the steam consumption required by the engine. this stops the safety valve blowing off all the time !! I have sent a photo showing the graph details ....The pencil is the minuets and the red numbers follow the red line and the black numbers are for the latest green line...I am running out of questions gradually, so will await your new post,! this has been an interesting journey and i am sure there is lots more to thermodynamics than what we think we know and assume !! There engine is not connected to the boiler at the moment as i was using it to demonstrate the Beeleigh model and was wondering what we can actually learn from running the small "Persious" engine from the boiler ?
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 09, 2017, 12:57:36 PM
Hi Willy, I am glad that things are little clearer now, I have also enjoyed the journey, I hope there is more to come.  I certainly have a few more points I was going to cover, that I hope will interest you.  You have tested the weaknesses in my understanding and I have learned a lot from working out the answers.  It has really consolidated my understanding in areas that I thought I already understood.

With regard to the insulation, rockwell is usually best, but as you have no flame, fibreglass will also do.  However it is a bit prone to breaking off fine fibres which itch the skin, so best to wear gloves.  Those disposable ones will do.  And worth using one of those paper painting masks as well.  No point in breathing in small fibres if you can avoid it.  Then wrap it with a cloth or foil cladding.

On/off control for the boiler is great for controlling the boiler pressure, but makes it hard to tell how long the element is on once steady conditions are maintained.  However in answer to your question about an engine test, I suspect it is not worth the effort at this stage, as I expect it is an unloaded test.  If you are interested, I will at some stage talk about the measurements needed to measure real power output.  However I will also write a few posts about the unloaded tests I have done on mine, and will include the differences with respect to yours. 

Tabulating those differences is a very clever way of identifying the departure from a straight line on a graph that looks pretty straight, even by my favourite method of holding my eye close to the paper level and looking along the line.  But let's look at what is going on and see if we can identify what is behind that small departure from linear.  I suggest the fact of the two 15's in a row, shows your observation of a curve is real and not just a random fluctuation due to the timing of the gauge updates.  This randomness is unavoidable with a digital instrument.  It may explain the 18 and the first 15, and may be reduced by an instrument reading in 0.1 increments.  However I think the second 15 is a definitive indication of a change in slope.

I will suggest some things that are assumed to be constant in expecting a straight line.  We are assuming constant ambient air temperature, constant convection heat transfer coefficient to the air outside the boiler, constant heat input from the heating element, and constant specific heats for the water and for the copper, constant mass of copper and water and constant temperature gradient from the element to the outside air. 

You might have a feeling for whether the air temperature was changing during that 9 minutes, but I suspect not changing much.  I have tried to find more information on the specific heat of copper and while all the tables list it, usually at 20 degrees, only a constant value seems to be given.  Perhaps I don't have the right book, but in the range from ambient to the melting point of copper, 100 degrees is not much and I suspect any change in this range is not significant.  The heating element could increase in resistance as the temperature rises.  Unfortunately, not listed on the data sheet.  We would need to do some specific measurements.  If it changes, I would expect the change to be about uniform over the whole range, rather than only start at 100 deg.

The specific heat of water is easily checked from the steam tables.  Just check the change in enthalpy between 90 and 95 deg, and divide this by 5 to get delta h per degree.  Now do this for 15 and 20 degrees.  It looks like 4.194 kJ/kg.K at 20 deg and 4.208 kJ/kg.K at 95, so it does increase and in the right direction, but only by 0.003%.  Not sure that we would see it, but could be a factor.  I did not check it at an intermediate temperature to see if it was a linear change, or did indeed start nearer 90 deg.  The convection heat transfer coefficient from the wood cladding to the air is hardest to quantify.  Instinctively, should I say dangerously, I would expect that as the test proceeds, the air moves increasingly rapidly, so might transfer heat slightly better, so might not be exactly proportional to the temperature difference, but might increase the heat loss slightly with temperature.  But it would be quite difficult to quantify this, so I am reluctant to identify this as the cause.  Then we have assumed constant mass of copper and water.  I have no doubt about the copper, but a tiny wisp of steam from a recalcitrant gasket or even the safety valve means the mass of water could be decreasing, but more importantly it takes a lot of latent heat with it, so it could show in the measurements.  I would try a piece of thin paper, taped to the end of a ruler to wave around as a sensitive test for a steam leak, not your index finger!  The safety valve is certainly a factor that we would expect to be zero until the pressure starts to approach its set pressure, then even the best safety valves are likely to leak the slightest wisp, and we are clearly not looking for something big.

Perhaps the most likely cause of the slight curvature is a part from the specific heat of water, and a little from the merest wisp of a leak.  Definitely not a definitive answer.  Some suggestions to ponder, but overall, I suggest that overall the test shows that for revel of accuracy of the available instrumentation, the assumption of linear is not too bad.  And it has provided valuable information on the heat loss, the value of insulation, where the heat goes during the heat up period, and the potential rate of steam production from your boiler.  A lot of information from a relatively simple test with readily available instruments.  And lots of other leading along the way.  So thank you for all your help.

That is enough for another day.  Thanks everyone for taking interest.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 10, 2017, 12:36:31 AM
High MJM ,thanks for this explanation and yes there were a few wisps of steam actually. Would it be possible to add accurate  boiler pressure values  to this graph, and what temperature reading would the pressure be at atmospheric to open the filler cap safely ? Perhaps the wood needs to heat up gradually before it starts to radiate the heat ?as it is insulated partly from the copper ? On the side of the boiler is a neon lamp that is connected to the elements and when the potentiometer is full on this lamp is fully on. When the pot is turned down the lamp will flash on and off in time with the pulses of  the current, the lower the pot the slower the flashes, so when it is full on the elements are getting the full currant.

Title: Re: Talking Thermodynamics
Post by: MJM460 on October 10, 2017, 12:16:00 PM
Boiler pressure and air -

Hi Willy,  I am sad to say that we can't plot the pressure directly against temperature to get a really accurate figure.   The pressure predicted from temperature has to be interpreted with an understanding of the effect of the air that is in the boiler when you first seal the plug.  Earlier in this thread, I have said a couple of times that once you start steam production, the air is soon gone.  You have prompted me to actually do the detailed calculations, and I am starting to realise that the air might take more time to become negligible than I thought.  So let's look at what really happens to the air.  First it is important to realise that the air and water vapour both fill the space independently.  So while the water vapour pressure is accurately found from the steam tables, any air remaining in the boiler adds a further component to the pressure seen by the gauge.  We have already seen that at 100 deg C, the air pressure is 130 kPa and the water vapour pressure is 100 kPa, so a total of 230 kPa.  Keep in mind that these are absolute pressures, so we subtract the atmospheric pressure, say 100 kPa, to get 130 kPa gauge or 18 psig, an 18 psi difference between the gauge and the steam tables.  If we continue to say 150 deg C, the calculation gives a gauge pressure of 75 psig compared with the water vapour pressure contribution of 54.3 psig.  The error is now just over 20 psig.  Both cases so far assume all the original air still in the boiler.

Now the air and water vapour in the vapour are fairly well mixed.  I suspect the steam being generated at the surface might mean a bit lower concentration of air near the surface, but the rapid movement of the molecules means and and water are probably reasonably well mixed further from the liquid surface.  When a little vapour escapes through the engine or the safety valve, the mass of each in the escaping steam is proportional to the partial pressures.  The lost steam is quickly replaced while the heat is applied, but the air is not.  But the air lost in the next time period is proportional to the remaining amount.  After a little has escaped with the steam, the remaining partial pressure is reduced, so the proportion of air in the escaping steam is reduced.  The first half, in a certain time, then half of the remaining amount takes a similar time.  Like the temperature of your coffee in the cooling experiment, which approaches the ambient temperature ever more slowly, the air loss happens increasingly slowly and in principal, while it gets close enough for practical purposes, it does take a significant time.  As the air is lost the discrepancy between the gauge reading and the water vapour pressure obtained from the temperature reduces, but the gauge pressure should always be a little higher than the prediction.  Fortunately the safety valve works on gauge pressure, so always protects us.  Bit if we are relying on the temperature for our pressure , we need around 20 psig safety margin between the design pressure and the intended operating pressure.  If the  safety valve is set a bit closer than that, it will lift before our intended operating pressure, the escaping steam takes a portion of the air with it, thus reducing the air pressure and reducing the difference between the steam table value and the actual gauge pressure.  The engine also runs on total pressure and does not care if it is water vapour or air, and I find the engine starts to run quite soon after 100 deg C is reached, but the air at this stage means the actual gauge pressure is a bit higher than I have assumed.

A further factor in all of this is that as the steam is used, the water level reduces, so the remaining vapour space is larger.  The steam needed to fill this volume evaporates from the water, but there is no more air, so the air pressure is further reduced as the level falls, and the error due to the air content is further reduced.  So I think that up to 150 deg C, the error is a maximum of about 20 psi which should be noticeable on the gauge, but the error reduces as steam production goes on, and the reduction in error should be noticeable.  After a few runs, you should have a good idea of how long it takes to get down to an acceptable error, then you will always have a good idea whether your pressure gauge calibration is still correct.

I must admit the error is a bit greater than I was expecting, and the time for the air to effectively be gone is certainly longer than I thought.  It looks like I had better add pressure gauges to my two small boilers which currently only have temperature measurement, plus a carefully checked safety valve.

Then, what temperature can you safely remove the plug?   In industry, you can open piping or equipment only when you have proved the pressure is zero by opening a vent valve.  Even then, sometimes the vent valve is blocked by rust or dirt, and there is still pressure inside.  That is another place where Murphy interferes with the best laid plans.  On our models if I wanted to be sure, I would add a whistle, though a vent valve would also do.  Personally I normally let it cool down overnight.  In order to refill for another run as soon as possible, I suggest even with a gauge, zero is really hard to confirm.  It clearly has to be less than 100 deg C, sixty degrees is normally considered a safe temperature to touch, though if you do not have tough skin it will still feel pretty hot.  Then I would leave the regulator open and help the engine turn over while any pressure was obvious.  At 60 deg C, the water vapour pressure is about 20 kPa absolute, so needs a lot of air to get to 100 kPa absolute or close to zero gauge.  With experience and suitable precautions against getting scolded, you may find a little higher Ok.  If it is really desirable to to get started sooner, probably the safest course is to use a hand pump to refill the boiler to the appropriate level gauge indication.  Of course, once you pump in a bit of cold water, the temperature will soon fall, so you can remove the plug if necessary to check the level.

You have mentioned that your level gauge is not very reliable.  I believe small level gauges are notorious, and the vigorous boiling of water makes the level bounce around.  Also, when boiling is occurring, part of the vapour is below the water level, and sudden changes of pressure change the effective density of the liquid phase which then upsets the level gauge as it does in a large boiler.  When the boiler is cooling for refilling, the density should be more steady, but you still have to deal with surface tension issues and the meniscus which both change the apparent level.  It may help to take the gauge apart and clean the paths into the boiler in case they are salted up, but these are practical issues and I don't have a lot of experience with gauges in model sizes.  Industrial level gauges have much larger passages and different problems.  Your colleagues in the club should be able to make more useful suggestions.

That neon light sounds like a nifty way to tell if the boiler is lightly or heavily loaded.  I like that feature, but it makes it hard to determine how much time the element is on, and hence how much heat is actually being generated.  I assume it is fully on during the heat up phase, a bit of flashing as you near the safety valve setting would be an additional cause of a curve in the heat up curve, however, I suspect that that wisp of steam is a sufficient explanation of a very slight curvature.

I hope that clarifies the issues sufficiently,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 10, 2017, 03:34:45 PM
Hi MJM, Thanks for your further info, and leaving things to cool down provides a good excuse to go to a cafe and drink coffee etc etc . I have done a test with my thermometer and putting it in a boiling kettle only shows a reading of 93 C actually so %7 out, i have yet to get some ice but that will be interesting soon.! So all these calculations in fact are quite hypothetical, and need actual laboratory conditions to get an accurate reading and the formulae should be more like  Air pressure (at x humidity, at x height above sea level ,at x wind speed  ,at x etc etc etc)  X the rest of a formula should bring in all the variables necessary but do need to be known.....

Willbert....
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 11, 2017, 12:36:57 PM
More on boiler pressure and air-

Hi Willy, I am surprised that the thermocouple is so far out, but I am not yet convinced.  To get an accurate temperature reference at boiling water temperature of 100, you need to get as close as possible to equilibrium, just like in your boiler.  So you need a lid on the kettle, just tilted enough to admit the probe, and perhaps stack a few face washers around to minimise the steam escape, and after a while enough air is gone and you will get much closer to 100.  Careful with those face washers, the steam will make them hot.  If you get 98 per 99, I would accept your meter as reading correctly.  But also check the barometric pressure.  Either your calibrated barometer, or check the weather bureau data for your area.  Standard atmospheric pressure is 101.35 kPa absolute, or 1013.5 hPa, which is the water vapour pressure at 100 deg C.  If there is a low pressure system passing over, 1000hPa will give 99.63.  You can see this in the steam tables.  It takes a bit of interpolation for other pressures.  Similarly for zero degrees, you need water liquid and solid ice in equilibrium.  So lots of ice, minimal water, insulate your jug with a towel, probe deep into the water through the slightly tilted lid, along with a stirring stick so you can keep it well mixed, and some more face washers to minimise heat gain.  It is harder in practice to exclude the air from the zero point than the boiling point as there is not enough vapour generated to displace air.  You would need a vacuum pump to get that triple point condition.  But again if you can get within 1 or 2 degrees of the liquid-solid equilibrium, I would accept the metre as more accurate than the calibration check process.

Now before you dismiss all this as hypothetical, please bear with me while I have a go at a more precise explanation.  A nights sleep, a powerful analysis tool, enabled me to come up with a simpler and more precise calculation of the air pressure contribution to the total pressure over time.  Looking at the boiler in isolation and trying to calculate the loss of air was not very easy, so I took the easy way out and gave only the maximum error, which occurs as you first heat up, and decided that I really did not know how long it took to loose the air.  But if we expand the control volume, to use text book terminology, to include the engine, we have a situation which is much simpler to analyse.  Now it does not make any difference to the boiler whether the engine is a conventional reciprocating engine which takes many strokes per minute, or if it is a very large piston which just accepts all the steam for your total run time in one very long stroke.  Nor does it matter to the boiler what the engine does with the steam next, just as long at it does not gain or lose heat while it takes in all this steam.  Now I have assumed that your run time ends when your 600 ml of water is reduced to 300, in order to stop while the element is still covered.  So you evaporate 300 ml, or 300 grams of water, at a constant temperature of 135 deg.  Now the initial 1.7 grams of water to give saturation vapour pressure at the 15 degrees starting temperature is not very significant compared with the total of 300 grams evaporated.  I assumed an initial vapour space of 0.5 litres.  Now when 300 grams of water evaporated at 135 deg C it makes 175 litres of vapour.  But the initial charge of air at atmospheric pressure is only 0.556 grams.  While the water evaporates into the vapour space as the total volume expands into that very big cylinder, so the mass of water in the vapour increases by evaporation of the liquid, the air mass is constant and in the end, we have expanded the initial air from 0.5 litres to 175 litres.  We had initially calculated that when the temperature reaches 135 deg, the absolute air pressure is 130 kPa.  The expansion of the air volume, remember gases always fill the whole space, the ideal gas law tells us the pressure of air at the larger volume.  Because we are assuming a constant temperature before we start the expansion, the ideal gas law is basically P1 x V1 = P2 x V2.  The condition in the boiler at any time is just a known sample of that total end volume.

I basically calculated the air pressure, total pressure and gauge pressure 10 times for the volume expanding 0.5 l to 175 l.  If the run lasts about 20 minutes, that is roughly every 2 minutes.  Now if that model is clear and accepted, the resulting gauge pressures are the answer to the problem. 

You will remember that at 135 the pressure from water vapour is 31.3 psig.  With the typical small gauge on a model boiler, with scale marks at 10 or 20 psi intervals, it would be pretty hard to read that more precisely than calling it 30.  The calculation gave 31.87 psig after just the first 30 ml was evaporated.  And 31.5 after 90 ml was evaporated, so a very rapid initial fall which then only slowly trends towards the long term value.  Now if the air remains always well mixed as assumed by the assumptions, it is never all gone.  The pressure after evaporating 300 ml was 31.40, only 0.1 psi high, but in practice it was close enough after about 4 to 6 minutes.  Not just the first blast from the safety valve, but never the less, quite quickly after the engine starts to run.  The error is really only significant during that initial heat up to safety valve set pressure that the maximum error is observed.  And that is readily calculated from the initial vapour space volume. 

While the initial humidity would normally be less than 100%, after a short time in the sealed boiler  it will be made up by evaporation from the liquid, but will make less than1% difference to the pressure contribution of the air, so I think can be ignored. 

I think this is a more rigorous approach to the calculation, and I hope it makes sense.  Like a metal part, a knowledge building block sometimes takes more than one attempt.  The first analysis that comes to mind is not necessarily satisfactory, and sometimes needs some additional work. 

The clear learning that comes from the whole conversation is the influence of the initial air in the boiler.  For pressure testing a calibrated gauge is required, and I am sure that a boiler inspector would insist on a calibrated gauge for the initial safety valve setting.  However, a model gauge calibration and safety valve setting can be checked quite accurately towards the end of a run.  And used with understanding of the influence of air, the temperature gauge is quite adequate for normal monitoring of your boiler operation.

In addition we have learned that attempting to prove zero gauge pressure by subtraction of atmospheric pressure from an absolute pressure of a similar magnitude, inferred by the temperature reading is subject to errors that make it unreliable.  Better to be safe by waiting longer for cooling, or preferably, use a whistle.

I hope that is a more satisfactory ending point than yesterday, thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 11, 2017, 02:31:33 PM
HiMJM, thanks for this  ,more info get my head around !! I have been testing the Thermocouple in melting ice and i think the battery may need changing as it should show 32 on the farenhieght  scale and 0 on the centigrade scale !! Also an account from the 1827 Farey book about removal of the pressure suddenly from the boiler ....very colourful language !! Here are a few pics of the readings ......Also he mentions  'Sensible heat" again !
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 12, 2017, 12:41:32 PM
Hi Willy,  I think you will find your battery ok, those instruments usually have a low battery indicator, but your open glass with one or two ice blocks just won't cut it as a zero degree reference.  Make a tray of ice blocks in the freezer compartment, many little blocks are best as there is more contact with the water.  Put your glass of water, half full, in the fridge while the ice blocks freeze, (perhaps overnight?). Then fill the glass with as many ice blocks as will fit, fold a towel to the height of your glass and wrap it around, better still leave the towel in the fridge as well, mix the ice and water really well and cover the top.  After a while to allow it to get to equilibrium it will get a lot closer to zero.  I would suggest that with the open glass which may have started at room temperature, the 4 degree reading is possibly quite accurate.  Similar care is needed to get close to equilibrium temperature at the boiling point.

Sensible heat is reflected in a temperature change you can sense with a thermometer, or even your finger if the temperature is appropriate.  You know by now that freezing or melting of ice and evaporation or condensing of water both involve quite large amounts of heat.  However both processes occur at constant temperature, and you cannot sense the heat input with a temperature instrument.  You would actually have to measure the volume of the solid, or weigh it, and measure the volume or mass of liquid, and from this infer the heat change from steam table data.  But it's a wonderful article from your book.  The language makes it hard to read, and demonstrates the difficulty of describing things if you do not have the mathematical tools, quantitative data, and even those more intangible properties of enthalpy and entropy.  A bit like Pythagorus describing his theory for right angled triangles.

I think with yesterday's calculation, I can now provide your requested graph. You will remember yesterday I said that the total pressure approached the equilibrium value quite quickly, and today I did a few more calculations in that first 10% of the run! and I am really back to my original contention that the air is soon effectively gone.  If the total run is 20 minutes, the pressure will be virtually true steam pressure in about 20 seconds.  A short safety valve blast or enough steam to warm up the engine.  This should be evident by comparing the gauge readings with the temperature readings.  So you should first see a pressure around 20 psi high, which should drop back quickly once the safety valve lifts, especially then if you release a little steam to warm up the engine cylinder.  I have two nice graphs, one for the heat up period, and one for the run time.  I am having technology troubles I getting a picture within the size limit, but I should get there tomorrow.

Just a short post tonight, I need the pictures to make it all a bit clearer,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 12, 2017, 01:28:33 PM
 Hi MJM , So Sensible heat is not the obverse of silly heat !!! I have wondered about this for some time as all the old books talk about Sensible heat !! Also ice does funny things when cooling/heating as i have been led to believe . I wonder when the words Enthalpy and Adiabatic came into being ?  Like Mr Watt and Mr Farraday,  was there a Mr Enthalpy and a Mr Adiabatic ? !!!sorry just being a bit silly there  But it would make for interesting formulas IE   1 Cholmondly-Smythe divided by 1 Monatgue-Fitzpatric = 1 Dionosius-Ericson.........!!Also from your rethinking on these posts ,if you did pull a vacuum in the boiler would it behave differently ?? or should one try that out ? .Using an electrical element would be a good way of getting uniform readings from the heat input point of view....unless you tried to do it just after the World cup Football game when everybody switched on their own kettles !! Sorry about these ramblings but i have always thought out side the box !! Good to here about your continued interest with my own small contributions

Willy
Title: Re: Talking Thermodynamics
Post by: paul gough on October 12, 2017, 10:53:59 PM
Hi Willy,  I note you have included a Dionysius in your formula. Caution! If this is of the type 'Lardner' be aware that it may contain a lot of hot air, causing unstable results. When placed under scrutiny it yields surprising behaviour, including 'criminal conversation'! An introduction to its behaviour can be had here;<https://en.wikipedia.org/wiki/Dionysius_Lardner> Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 13, 2017, 01:00:06 AM
Hi Paul ,found this in one of his books.........says it all really !!! will look your link up...........

willy.
Title: Re: Talking Thermodynamics
Post by: paul gough on October 13, 2017, 08:04:48 AM
Hi Willy, glad you get a laugh from Dr. Lardner, he is something of an engineering/science Sir Humphry from Yes Minister, a consummate obfuscator, and it seems, at times an incompetent, if not fraudulent, self promoter among other more nefarious activities. When looking at historical sources and it includes engineering historical writings, one has to be as alert to mythology as much as one would when considering the practicality of Icarus's flying equipment or the veracity of the events occurrence. All sources need to be measured against other evidence from the era to determine which is credible, (for the time), and which is to be ignored/avoided. I had two of his books and threw them in the bin. Apologies to MJM for this diversion. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 13, 2017, 08:07:26 AM
Hi Willy and Paul, that Lardner character seems to have been quite controversial.  Good to hear from you again Paul.  Willy, the page you included is pretty hard to read, without the previous page anyway.  Print is clear enough, but the language is heavy going.  I am really not sure what he is getting at.  Obviously no friend of Mr Wolfe though.  Looking now at Pauls second post, that probably means I was not far off the mark!  Personally I prefer to go to the more modern books, they tend to be easier to read as well as including more up to date ideas.  Nevertheless, a bit of humour is always appreciated, especially in a thread like this.  And your thinking outside the square, seems always to find the gap in my description that I need to fill in, so I hope everyone else is finding them as helpful as I am.  I don't know where the terms enthalpy and entropy came from.  But just as well those other characters didn't intermarry and combine hyphenated names!

Found the limitation of the Numbers spreadsheet on iPad today.  I was having trouble getting the file size of my graphs down, and of course Apple is quite sure you don't need to know file size, so it takes some trickery.  Finally decided to send it all to the desk top machine, which then would not open a Numbers spreadsheet properly.  In the end, got the figures across, re constructed the graphs, printed on a better scanner and so on.  Numbers is really great for reviewing a spreadsheet, but is more limited when it comes to building one, particularly on graphs.  However, all done now, so back on subject.

Attached are the two graphs which show the results of the calculations relevant to boiler pressure and temperature readings, I hope pretty much exactly what you wanted.  The heat up one shows the relationship of the total gauge pressure, seen on the pressure gauge to the equilibrium water vapour pressure.  The lines slowly diverge due to the increasing contribution of the air pressure as the boiler heats up.  The air pressure increase is a linear function of absolute temperature.  It has a very tiny dependence on the starting temperature, due to the very low water vapour pressure at ambient temperatures, but you can see what happens.  Unfortunately, if you let some air out by lifting the safety valve or a vent early, you would change the mass of air and need a new total pressure curve a bit lower than the one I have shown.

Once you start releasing steam, whether to the atmosphere via a safety valve, or through an engine, the Run Time graph is the one that applies.  You can see that the air contribution reduces very quickly in 1 - 2 % of the total run time to near enough to zero, considering what can be read on a small gauge.  That would be less than 20 seconds of a twenty minute run.  But the error does not totally disappear, even after 300 ml of water has been evaporated.  It still contributes 0.1 psi to the gauge pressure and that is still reducing but at a very slow rate.  You would have to evaporate as much water again to half that error.  The graph is not very dependent on the steaming temperature, but is determined mainly by the initial quantity of air in the vapour space, which was assumed to be 0.5 litres.

So, while I would reiterate the point that you always need a safety valve, and a gauge is good check that the valve is working at the correct pressure, your temperature gauge is probably much easier to read accurately for general run purposes than a tiny gauge, so long as it is used with some understanding.  After only a few minutes of engine running, the gauge and the vapour pressure from your temperature reading should match.

In principal, you could reduce the mass of air in the boiler with a vacuum pump, and this would reduce the associated error during heat up, as well as that long tail on the run time graph, so long as you have a tight shut off regulator that stays shut under vacuum.  I only have a direct line to the engine, and low boiler pressure would lift the slide valve and draw in air.  The oscillating engine should be ok though.  However I am not convinced it is worth the effort.  The graphs are easier to use than I was expecting.  So here they are.

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on October 13, 2017, 08:42:32 AM
MJM, Willy, This air/vapour, temp/pressure exercise has indeed been an interesting phenomena to have explored and explained. I have only ever considered the negative 'oxygen' aspect of air in the boiler and where practicable raised steam with a relatively 'high' water level and brought it down appropriately as it heats and expands all the while having an open vent from the steam space so that over the course of steam raising there is an evacuation of air with vapour and also the vent gives a good indicator of that point when you just start to get pressure, often a gauge does not. With a loco boiler this vent is often, for convenience, via the steamway cock and blow through cock on the gauge glass or the blower in the smokebox. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 13, 2017, 12:54:32 PM
Hi Paul, your note is a perfect addition, not out of context at all.  You see it is another example of where theory, no matter how complete, is not all that must be considered for any particular case. 

I assume that you are talking about  steel boilers, where of course a lot of effort goes into feed water treatment to remove oxygen down to minute levels in order to minimise corrosion throughout the system.  Even quite basic steam plants generally have a feed water de-aerator and a chemical treatment plan that includes a chemical oxygen scavenger.  There is no point in doing this if the oxygen scavenging system is then loaded up with oxygen introduced as part of the start up procedure.  And your procedure is a good way to eliminate that initial air by sweeping it out with steam during the warm up.  In addition, in a full size steel boiler, it is normally important to heat up slowly to minimise temperature differences which introduce thermal stresses that can be quite undesirable.

If Willy followed your procedure, he could also drive the air out of the boiler during startup, and then the temperature measurement would give a more accurate boiler pressure almost from the start, as there would not be air to add to the water vapour pressure.  However, he has a copper boiler where the corrosion issue are not very significant.  I am not sure how much oxygen is involved in scale formation, but on model run times, it is generally easily handled with a descaling procedure.  His electric heater arrangement produces very little in the way of uneven expansion as the sheath is free to expand and the water then carries heat to the shell quite evenly.  In addition, you will remember that Willy wanted to raise steam more quickly with the limited heat input from his heating element.  In that case, any early steam loss absorbs a disproportionate amount of energy due to the latent heat it carries away, and so increases the time until steam is up to pressure for running the engine.  He is also dealing with the issues of trying to read pressure accurately on a tiny model pressure gauge.  In the hobby world, accurate temperature measurement is easier these days due to readily available thermocouple instruments.

So in one case corrosion is the biggest concern, thermal expansion stresses undesirable,  accurate pressure gauges readily available, startup time not really important, it is very short as a proportion of the normal operating time of a full size machine, and these factors determine the development of the startup procedure.  Though the ships captain probably doesn't see it that way.

In the other case, corrosion not really an issue, difficult to measure pressure with small gauges, desire to reduce startup time.  In this case, understanding how the thermodynamics affects pressure inferred from temperature measurement, and starting the boiler completely sealed to minimise heat loss through vented steam may become an appropriate procedure.  However, to get rid of air through steam venting when the pressure is fixed by the open vent, then to seal and raise pressure could be in the long term, the best procedure.  It allows a little more time for examining the intricate details of the engine while the boiler heats up.  Once the engine is running, the details move too fast.

I am glad you are finding it an interesting aspect to explore,

Thanks for following along

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on October 13, 2017, 05:55:49 PM
I guess I should have prefaced my comments by indicating I was principally addressing steel boilers from larger 'model' sizes i.e. 12 inch gauge locos to full size. Though our codes generally demand steel from anything over 8 inch dia. barrels, which is smallish, and would I presume capture a lot of 71/4 gauge models. I thought the 'scavenging' of  air via venting during steam up might have been useful knowledge/technique to somebody especially if they don't have full size experience or are isolated enthusiasts, like me, who don't have the luxury of contact with like minded peers. I agree with all your commentary. A few of years ago I visited Mr. Jaycar, (electronics parts shop), when on a trip down south looking for temp. probes and a digital readout. At the time what was on offer was a too cumbersome to incorporate into my tiny G1 Lion loco. However there was a reasonably smallish probe/digital readout combo for temp. that one could reconfigure the connections so as to calibrate it for pressure, thus achieving a digital pressure readout/gauge. The instructions with the little screen had a wiring diagram showing the particular connections that achieved any numeral or combination of numerals on the display, so a bit of patience and diligence with a small soldering iron was all it would have taken to "make" an electronic pressure gauge from the temp. probe & readout unit and which would probably be suitable for a stationary set up, e.g. Willy's. However I'm not up to date with what might be available now, maybe micro sized digital pressure sensing units can be had readily?? Hope there is something useful in all my babble. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 14, 2017, 12:50:00 AM
Hi MJM ,thanks for that ,i have been looking for a temp/preasure table on the web but could not find any. Do these graphs depend on the ratio of air (compressible ) to water (non compressible) ? Interesting about the steel boiler water treatment ! I notice that the heat up graph is non linear but this may be because the time part is missing ? Would it be possible to make a 3D model including a time element?.....
Willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on October 14, 2017, 02:34:01 AM
Willy.....this may help. :happyreader:.....Imperial ,Si........any combination and in either direction.....Derek


http://www.tlv.com/global/TI/calculator/steam-table-temperature.html
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 14, 2017, 04:16:58 AM
Engine Performance and testing -

Hi Paul, making measurements on your gauge 1 engines brings a whole new level of small scale to miniature instruments. If you have a suitable plug to replace with a thermowell, you could insert the thermocouple from a cheap multimeter into the thermowell to measure temperature while the engine was at rest.  Then just pull it out when you are ready to send the engine on its way.  Alternatively, you could hold the thermocouple tip against the boiler shell with an insulating pad of felt so it is not cooled by the air, and probably get close enough.  Experiments on a stationary boiler big enough to accommodate both would give you a good idea of the accuracy of this method.   I expect that you don't ride on the tender, so you don't really need to install the instrument on board.   The real size limitations are the batteries, unless you were prepared to use hearing aid batteries, and the screen, which clearly has to be large enough to be read.

Hi Willy, this link is to an open course on Thermodynamics and it includes a copy of the steam tables that can be downloaded as an Excel spreadsheet and read by most spreadsheet programs.
https://www.ohio.edu/mechanical/thermo/index.html
The course gets heavy quickly but you may find some interesting sections. 

The graphs don't use the volume of the water space, only the vapour space, and preferably the initial temperature.  Time is not relevant to the heat up graph, the non-linear part is just the water vapour pressure-temperature curve. 

Hi Derek, they tiv site is interesting, but I find a full set of tables is even more useful.  They hard to find on line though.    Hope you are now home safely from your travels.  The thread is getting close to your exhaust issue.

Willy recently asked if there was any point in an engine test on his horizontal engine.  I guess it applies equally to the Woolf Compound Mill engine.  So let's talk about what is required for an engine test, and what we can learn from a simple test.

Ideally for a complete engine test we need to measure the inlet pressure and temperature, the exhaust pressure, and if our boiler has a superheater, the exhaust pressure and temperature, more on that later.  We need to ensure the steam flow, and engine rpm, and finally we need to measure the engine output torque.  You see, we cannot predict the actual power that will be developed by a real engine, we do have to measure it.  This applies even to full size engines, though a full size engine manufacturer has access to many previous test results of similar engines, from which a pretty good estimate can be made for the engine efficiency.  However, if you as the customer want extra assurance that you will get the promised power from your new engine, you can specify, and pay for a full test on your engine after it is built.  Alternatively, you might accept the prediction and accept a lower cost simpler test just to prove that it runs ok without mechanical or steam flow problems, quite like our first runs on air.  With a known engine efficiency, or more specifically adiabatic efficiency, you can calculate the power which would be developed by an ideal engine, and from the efficiency, you can calculate the output of your real engine from the given steam conditions.

As model builders, we don't have access to many previous tests at all, let alone tests of similar engines to the one we are building, so we don't have a knowledge base of reliable efficiency data.  We are left with doing our own tests, so we need those instruments.  The temperatures are relatively easy, we have already discussed those, the more difficult ones are the engine inlet pressure, and the torque. 

Even that inlet pressure is easy if we are prepared to accept any pressure losses in our piping and regulator as part of the engine inefficiency, and we know the boiler pressure pretty well, at least if you have been following this thread.  But measuring torque requires more thought.  Most of the trouble with torque measurement is that our engines do not produce a uniform torque. For one cylinder, it goes from maximum to zero twice each revolution.  A two cylinder engine with cranks at 90 deg means four pulses per revolution, but at least the peaks are spaced so there are no zeros, but it fluctuates considerably nevertheless.  I know every dynamics text book describes a friction brake on the flywheel, and a good flywheel should keep the speed fluctuations to around 7%.  That should help, but I am not convinced.  I know I had to do a test with such a brake in my student days, and I really can't remember how much the reading fluctuated.  But we got pretty good at estimating the average or mid position of a vibrating needle, so that's what we had to do.  Now with electric integration and averaging it should be different, but there's no fun in that!  So I am thinking about a torque measurement which somehow averages the torque.  But, is there anything we can do without torque measurement?

I suggest that is a good place to stop and continue next time.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on October 15, 2017, 04:36:46 AM
I am sure there would be good fishing in the Ohio River where the cooling water from that  :Mad: 2.6M kW steam power station is located

Yes MJM...back in Wollongong however have not yet completed the exhaust temperature tests....[awaiting the insulation lagging to be completed over the new 1/4" OD exhaust line tubing]

Another question :headscratch: ...forward thinking...is 'to lag or not to lag' our gas tubing supply.......from the disposable canister to the refillable tank is fixed, however from the refillable tank to the burner jet is subject to considerable temperature change.....& occasional icing

I plan to install a gas isolation valve before the actual gas jet, then relocate the gas pressure gauge location to the discharge side of the  regulator. This way will then allow me to confirm the reduced gas pressure at the jet...........and also by isolating the boiler pilot pressure to the regulator, the actual gas tank pressure can be confirmed....

Derek 
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 15, 2017, 11:19:00 AM
A simple engine test -

Good to have you back Derek.  I suspect they are not allowed to discharge cooling water into the river in Ohio, those very large concrete structures, the nearest one is issuing clouds of steam, I believe are natural draft cooling towers, and the white plume is the evaporating cooling water re-condensing in the cooler atmosphere.  A blow down flow is needed on a cooling water system, just like a continuously operating boiler and that is probably being treated and has time to cool in those low circular pools before discharge.  It looks like one tower is shut down, although there are designs which reheat the air above the coils to eliminate the plume.  I am not sure what the steam discharge from the stack is about, and that plume makes it hard to see the second stack, or precisely which is discharging that wisp of bluish smoke.  Perhaps some of our colleagues can come in with some more explanations. 

With your gas piping, remember the lpg in the container, whether the disposal or your refillable tank, boils just like water with latent heat and the two phase region, but at lower temperatures.  Your burner needs the gas phase, but the "l" in lpg is short for liquified.  The container has adequate mass in liquid form but  the liquid must evaporate into vapour for your burner.  As you don't have a heater on the container, the energy for the latent heat comes from the liquid, so it cools until heat in from the atmosphere is equal to that required for evaporation.  And at the lower temperature the vapour pressure is lower, so your burner sees ever lower pressure until the heat balance is achieved.  To get more heat in from the atmosphere, it is better not to insulate the pipe or even the bottle.  The ice on the outside of the pipe means the internal gas temperature is so low that the outside surface of the pipe is below zero, and moisture from humidity in the air first condenses then freezes on the pipe.  Removing that insulation will allow more heat in from the atmosphere, higher gas temperature, though it will still be cool, and most important, more pressure to the burner.  The regulator is probably running wide open, due to the low bottle pressure, so increasing the temperature in the bottle will help get your regulator back in control.

Back to those simple engine tests - You may remember I did a simple test on my mill engine some time ago.  Next job on the list is to repeat that a few times to ensure that I have consistent measurements.  I did measure the exhaust temperature, and came up with an efficiency for my engine.  Now I could do that test because my boiler, a fired boiler, though only Meths for fuel, has a superheater coil.  And it is quite effective.  With a boiler temperature of around 116 deg C, the superheater outlet was 138 C.  At that time, I showed the steam conditions on a temperature-entropy diagram and used the second law of thermodynamics to calculate the power output of an ideal adiabatic engine.  Yes that is repetition (tautology?), but deliberate for emphasis on the 'ideal' part of adiabatic.  I have repeated the diagram below for convenience.  The boiler outlet is point 2 and the superheater outlet is point 3.  The second law says the exhaust entropy of such an engine is equal to the inlet steam entropy, so the engine expansion shows as a vertical line on that diagram, the line from 3 to 5.  Entropy and pressure are enough to define all the exhaust steam properties.  You can see on the diagram, the adiabatic exhaust, point 5, will be only just wet steam, 99% dryness factor (by calculation).  Now my measured exhaust temperature was 104 C, meaning the real engine exhaust was superheated at atmospheric pressure, point 4 on the diagram where the remaining enthalpy is more than the enthalpy of saturated steam.  This basically means my engine had extracted less power from the steam than the ideal engine, and the ratio is the real adiabatic engine efficiency.  I don't have a torque measurement to calculate shaft power, but I know how much energy was extracted from the steam, before the inevitable friction losses and so on.

Now Willy's engine is in a different situation.  The engine itself is quite similar to mine to a casual observer, it is just better made.  But his boiler is electrically heated, and has no superheater.  In fact not easy way to add a superheater.  So the engine is receiving saturated steam, steam from point 2 on the diagram.  The ideal engine process is a vertical line down to exhaust pressure as before.  You can see from the earlier diagram that without the superheater, the exhaust steam is much more wet, or has a much lower dryness factor.  It looks quite likely that even a real engine exhaust would be in that wet, two phase region at atmospheric temperature.

Now the earlier diagram is not to scale.  It shows the two phase region as a bell shaped curve, very similar to any text book you care to pick up.  And gives the form of the curve in an easy to remember picture.  I did take care to ensure that point 5 is on the correct side of the saturated vapour line.  Also the constant pressure lines, 1-2-3 and the atmospheric pressure line through 5 and 4 are also the correct shape.  However, just to be sure that I am not being mislead by the out of scale part, I have re drawn the diagram to scale, and included it as the second attachment.  I am not the first to do this, in fact they are fairly easy to find as a complete T - S diagram for steam, covered with extra lines for every other property.  But the tiny part relevant to our model is so small it is difficult to pick up values with sufficient accuracy.  So the second  picture below is drawn to scale from the properties in the steam tables, and you can see that it is no fancy drafting job. 

We have talked about several operating pressures for Willy's boiler, I am still not sure if it was 143 or 148, and there was 135 and also 118 mentioned somewhere.  So I have plotted the constant temperature-pressure line for the boiler for 148, 135 and 116 so we can see the effect on engine performance over this range.  I have kept the temperature axis to scale to cover the exhaust at 100 deg C up to 150 C to cover over the 148.  I could not complete the bell curve on the paper at a reasonable scale, the top is at 374.14 C where the pressure is over 22,000 kPa, known as the critical point.  It is quite important in thermodynamics, so has been measured quite accurately.  Above this temperature or pressure, there is no distinct liquid phase.  But we don't need it for our calculations.  Similarly I have shown a second break in the curve which is bounded at the bottom by a straight line at zero degrees, where water freezes.  This does not have to be measured, it is a defined point on the temperature curve, as is 100 deg, the boiling point of water at 101.35 kPa and a few other points.  That is why we can use zero and 100 to check the accuracy of our temperature meters.

The enthalpy and entropy for dry saturated steam are both in the steam tables for 100 degrees, but the other temperatures needed some interpolation, a spreadsheet comes in handy for that.  Now using that second law condition, the adiabatic engine exhaust entropy is equal to the inlet entropy, I drew in the constant entropy lines which are vertical on this diagram, and calculated the dryness of the exhaust in each case.  You can see from the diagram they are very much to the dry end of the line, all above 90%, but it is better to do the calculations to get good figures for comparison.

That is enough to take in for one session, don't forget about the gas question either, so I will leave you with the diagrams to look at, and continue next time.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 15, 2017, 02:42:25 PM
Hi Mom thanks for this , I was thinking about making a superheater for this boiler but it would have to be an external one retrofitted and the electric control system would be necessary to avoid burning the element out.!! I was wondering how one might measure the exhaust pressure without restricting it as it just goes strait to atmosphere or is it calculated from the temp ?. I have just bought 2  (analogue) ? thermometers (griffen and george) and notice that the scales are different lengths so perhaps they are calibrated separately ? When using a superheater....the temp goes up and also the pressure ...,,but as it is an enclosed system is the pressure also in the boiler itself as well and how does this muddle the figures for the boiler temp/pressure etc ?? perhaps this is easily explained with the correct Maths ?!!! I may rig up the engine back to the boiler and it does have a small load on it withthw dynamo and the magnet driven cycle speedo, and perhaps this may be useful in any calculations......The magnet is placed in the periphery of the cast iron fly wheel. Looks like more fun on the way............The thermometers go up to 360 C but do they need insulating as they are about 12" long so doing superheating tests will be possible !!
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on October 15, 2017, 09:32:33 PM
Hi,

I've kind of followed this thread for a while now and learnt something along the way although I'd be the first to admit as a layman some of the technical info is way above my head but I'd like to ask a few questions of the experts.

As I understand it water expands 1600x when turning to steam at atmospheric pressure, if we assume atmospheric pressure is 15 psi am I right in saying that a boiler that can convert 1 cc of water to steam per min can theoretically run a 1 cc engine at 1600rpm using 15 psi (assuming we ignore heat loss, inefficiencies etc and the engine is single acting.)

If we take a small boiler like the GLR, how much water can something like that realistically convert to steam per min ?





Title: Re: Talking Thermodynamics
Post by: MJM460 on October 16, 2017, 08:27:42 AM
Superheaters and steam volumes -

Hi Willy, your superheater design concept is spot on.  It might be well worth making that central sheath for the element hollow right through and soldered into both ends so you can put the element in, with some heat transfer grease and put a thermocouple in from the other end for your over temperature protection.

 However a few points to be aware of.  Heat transfer is not nearly as good to steam as it is to boiling water.  To put numbers on it, my heat transfer text book gives figures of 30 - 300 kJ/ m^2.K for superheated steam, and 3000 to 60000 for boiling water.  The wide range in each case makes them difficult to use in design, but you can see the difference in magnitude.  So heat transfer is limited by the steam side film coefficient.  You need lots of area and this is where fins come in.  The higher the fins the better, within reason.  Of course flow is longitudinal, so you need longitudinal fins.  Plenty of build logs present ideas on how to machine these.  You also need a lower heat intensity element, that is more surface area per watt of rating.

 It is not all bad though.  If we look at say 400 kPa absolute, 143.6 deg C, a touch over 40 psig, the enthalpy of the dry steam leaving the boiler is 2738.6, while superheated steam at the same pressure and say 200 deg C enthalpy is 2860.5 kJ/kg.  Only an extra 122 kJ/kg compared with the 2133.8 kJ/kg which the boiler needed to evaporate the water at 143 deg C to dry steam at the same temperature.  You can see that the superheater element only needs to be 50 or 60 watts to nicely compliment your boiler and achieve plenty of superheat.  But it will be harder to transfer that heat to the steam, and the element will tend to run hotter.  So a low intensity element, access for heat transfer grease and a thermocouple for your temperature protection, longitudinal fins as high as possible, and plenty of insulation.  You will need a condensate drain to aid startup which will require a bit of thought so as not to overheat anything.

Those thermometers may be individually properly calibrated for a small variation in the diameter of the capillary that carries the Mercury column up to the appropriate scale mark.  I don't know much about the manufacturing process.  I believe there is an error in the readings due to heat transfer along the glass, and it can be calculated, but it's lost in the ancient mists of school science.  Alternatively it might be possible to reduce it to an acceptable level by insulating the part of the stem below your expected reading with some felt or silicone tubing.

In the superheater, the pressure does not increase.  If anything, there is a small pressure loss due to pipe friction as the steam flows along.  Assume boiler pressure and use a generous size tube between the boiler and superheater.  That line 1-2-3 on the first diagram yesterday is a constant pressure line, the pressure is no longer a straight horizontal line outside the two phase region. 

Using a bike speedo for rpm should be ok so long as you use the wheel size set in the device to calculate rpm from the wheel circumference and speed.  The Dynamo will provide a nice little load.  Unfortunately the V and I measurements do not give us engine power as we do not know the electrical efficiency of the Dynamo.  That is why we need torque measurement.  But V x I does give a sensitive indication of load changes.  Just be aware that efficiency might also be changing.  We should be able to see a slower run speed, higher steady boiler pressure and lower steam rate with the load on, to compare with the free running unloaded engine.  We should then be able to see and  compare the power output of our adiabatic engine which will be an indicator of the actual load.  The exhaust will most likely still be in the wet region so temperature measurement is not useful.  But it  should be a worthwhile run in every way.

Hi Gas Mantle, great to have you on board.  The purpose of this thread is to try and make that technical information understood and accessible to all, so all questions welcome.  The expansion as steam changes from liquid to vapour is best found by using the specific two volume columns in the steam tables.  The precise figure for standard atmosphere is 1602 times.  At 15 psig, say 30 psi absolute, or 206.8 kPa.  Compared with the steam at atmospheric pressure the steam at 15 psig is compressed and so a smaller volume.  I will round that pressure out to 200 kPa (if you look at the tables you will see why), and see that the specific volume of the liquid vf, is 0.001061 m^3/kg, while the specific volume of the dry saturated steam, vg, is 0.8857 m^3/kg.  A simple division, using a calculator gives an expansion ratio for 15 psig of 835.  So a perfect 1cc single acting engine should run 835 revs for each cc of water evaporated at 200 kPa.  Or 1 cc per minute evaporated should give 835 rpm.  I have found it hard to match the figures in practice.  Valve leakage, piston leakage and any valve overlap all increase the steam consumption, or reduce revs, while early cut off should decrease steam consumption.  I am still working on just which it is, or possible all the above on my engines.  But it is a valid starting point. 

The fundamental limit to how much steam a boiler can raise is the amount of energy in the fuel you  burn.  Weigh your fuel container (with burner attached is ok) on the most accurate digital scale you have access to, before and after a timed run and calculate the mass of fuel burned.  Doesn't matter that boiler first heats up, then generates steam, as the burner fuel consumption should be pretty constant with time what ever.  Then look up the fuel calorific value, I can probably find a figure if you tell me what fuel you use, and multiply the two to get the energy from the fuel.  Then it depends on the boiler how much of this is turned to steam.  The gas from the stack is hot, so that tells you that energy is being lost, and it can only be cooled to a temperature something above the steam temperature in the very best boilers, so some loss from that source is inevitable.  There is also radiation from the boiler casing.  So the next experiment is to see how much steam you are getting. 

You will have seen from the calculations on Willy's boiler that it is helpful if you can weigh the bare, empty boiler, otherwise calculate it from the density of copper and the boiler dimensions and thickness.  Weigh a suitable amount of water into the boiler, light up and run for a suitable time, and carefully time from light up to first steam, and on to flame out.  Finally carefully drain and collect all the remaining water from the boiler.  The water evaporated is obtained by subtraction, and the time for that evaporation is from the first steam to the end. 

So you can now work out how much heat to raise steam, and how much heat was used to raise a known quantity of steam.  From that you have your boiler efficiency, which you can use with reasonable confidence for future runs of that boiler.  If the boiler has too little heat transfer area, the stack gases will be hotter and more heat lost.  If it has plenty of area, the stack gas will not be as hot, and if necessary you can give the burner more pressure or use a larger burner.  I don't know the particular boiler you mentioned, but testing the burner and calculating the heat transfer area will give you a good idea of how it would perform compared with other similar boilers.  I hope that helps, but don't hesitate to ask for any clarification you require.  Others will probably be wondering the same thing.

I think that is enough for a session, so I will try and return to those t-s diagrams and what we can learn from them next time.

Thanks for reading,

MJM460
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on October 16, 2017, 12:40:31 PM
Thanks  :)

I seem to remember reading somewhere that small copper boilers like the sort of thing hobby engineers use should be able to evaporate 1cu" of water per min for every 400sq" of surface area in contact will the water.

Does that sound about right as a very rough guide? I can't remember where I read it and may have got the figures wrong.
Title: Re: Talking Thermodynamics
Post by: paul gough on October 16, 2017, 12:53:38 PM
Re lagging and heat losses. Reading in, 'The Efficient Use of Steam', by Oliver Lyle, 1948, the author concludes for steel pipes 3" dia or below with internal temps. of 300 degrees F., (which would work out for us at about 50 p.s.i.g.), a lagging thickness, (85% magnesia or asbestos), of 1" is sufficient, his chart shows approx. lagging surface temp. of 107 F. in still air 70 F. A lot of our model boilers are made of copper so I assume heat loss potential higher than a steel pipe. Author quotes heat loss for a bare 6" steel pipe in still air with internal temp. of 300 F. as 646 Btu/sq. ft./hr. and 98 Btu loss for 1" thick lagging. A common dia. for 5" and 71/4 gauge locos.

Question; (1) Are the insulating properties of our much used Kaowool sheet insulation comparable to the above materials?

               (2) 1" thick insulation on a 3" boiler is pretty thick by most builders standards. Are we underestimating the thickness we should
                     apply to our boilers when there is room to do so?
                                                                                            Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 17, 2017, 12:00:35 AM
Hi MJM ,thanks for the suggestions about the superheater design. I was going to use a 500 Watt element i have in stock but if i did use this what would happen ? in a Locomotive the superheater goes back into the firebox where the fire is at its hottest !! so do we not need to do this ?
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 17, 2017, 11:21:02 AM
Boiler potential, insulation and superheater design -

Hi Gas Mantle, that rule of thumb for boiler capacity has a very similar form to that in the K. N. Harris book on boilers.  It is not without basis, but has some big implied assumptions that really need to be stated whenever it is quoted.  Dan was asking about it some time ago and I have been waiting for an opportunity to get back to it.  For a start, look at the dimensions of the terms.  A valid equation has first to have equal dimensions on each side of the = sign.  You cannot add or multiply apples and oranges.  So that constant, 400, has dimensions to make that side of the equation dimensionally the same as the other side.  Undesirable, but no real problem, it just means that the correct units must be used, cu.in/min on one side and square inches on the other side, or a bit of work to calculate the equivalent constant for use in other systems.

   If we go back to the basic heat transfer equation (it was discussed back with condensing,) Q=U x A x delta T.  The simple form does hide some complexity.  First Q has units of Watts, but so long as consistent units are used both sides, you can equally well use Btu/hr of you prefer, and you use consistent units for the other quantities.  The delta T is log mean temperature difference, and should strictly be written delta T lm with the lm as subscripts meaning "log mean to base e".  For SI, the units are deg C, and it is a temperature difference so you can use C or K (or F or R).  A is straightforward, m^2 for SI, (or ft^2).  U is more complex, but it's units are W/m^2.hr.C, (Btu/ft^2.hr.F) and unfortunately it is not a simple constant, nor is it easy to predict a value.  Whole books on that subject are heavier in every way than thermodynamics books.  They get there in the end, but I don't intend to go there. 

If we compare the two formulae, we can see the similarities.  A cubic inch of water has a fixed mass, particularly of the temperature is specified, and that mass takes a known amount of energy to evaporate it, again "known" means using the steam tables and requires the pressure or temperature to be known.  So cubic inches can be regarded as a proxy for heat input to the water.  Now in most model boilers, neither the delta T, nor the heat transfer coefficient is known, so it is not unreasonable to combine the two into one factor.  But you can't easily determine what this factor will be.  For example, for a coal fired locomotive the temperature in the firebox will be pretty high.  A very different temperature would apply for a gas burner, or a methylated spirits burner, so any figure determined by a boiler test would really only apply to the same method of firing.  Then the log mean temperature difference, LMTD, varies with the arrangement of area and gas flow through the boiler, so would strictly only apply to similar boilers.   But more than that, both the LMTD and the maximum possible heat transfer are limited by the burner size.  Conservation of energy is a fundamental law of physics and it means in this case you cannot transfer more heat into the water than is released in the firebox.  Basically if you have a small fire in a big boiler, the stack gases will be cool to only a tiny bit above the steam temperature quite quickly, and the rest of the boiler area just gets a tiny bit closer.  On the other hand, if you put a huge burner into a small boiler, you will definitely get more steam than from the small burner, but the flue gases will arrive at the stack before they have lost all the heat that can be transferred at the steam temperature if the area was bigger, and so will go up the stack at higher temperature, wasting some the energy in the fuel.  You can see the problems, however, I would guess that formula may have been arrived at based on coal fired locomotive boilers.  Similar successful boilers might well be consistent enough in proportions of fire box and heat transfer area to come close to following this simple formula.  I don't have a better idea, so as long as the formula is used as a rule of thumb, with understanding of its limitations, it can serve as a starting point for design.  It could be useful for scaling up an existing boiler which you could test, to a larger scale similar boiler with a similar arrangement.  If you were starting a new locomotive design, you could run a few calculations in a spreadsheet for some of the past entries in the various locomotive efficiency competitions, to get an idea of the applicability of the formula.  Then add data from any tests you are able to do on your own models.

Hi Paul, generally industry places a high value on the immediate cost of materials and labour costs to apply insulation, and undervalues the long term cost of the energy losses which are usually incurred "by others", so insulation thicknesses are generally less than desirable for heat conservation.  You can also see it in domestic fridges, where the sales hype is the illogical assertion of more internal volume for storage of your food within the smallest possible outside dimensions.  The salesman does not place any value of the insulation hidden between the outside shell and the liner.  And the average plastic Esky has no insulation in the roughly 1/2 inch gap between the outside shell and liner.  And simply, yes, we put too little insulation on our models for heat conservation purposes.  But note that qualification, in our models we are trying to optimise size of the boiler, heat transfer area and appearance, and the fuel cost is not high for the typical number of operating hours, while appearance is paramount.  So we have valid reasons for using minimal thickness.  It is usually simpler to achieve performance by pouring in more heat.  But whenever we can apply some insulation, it reduces heat up times and gives more steam for a given fuel burning rate, and reduces the incidence of burnt fingers, and if it is practical, more is better.

K values for some suitable insulating materials in Watts/m.K - cork 0.04, glass fibre 0.035, kapok 0.035, plaster 0.814, polystyrene 0.157, softwoods 0.15, oak 0.19, wool 0.038.  I also have 50% magnesia 2.68.  Asbestos is 0.113, so not only dangerous, but not the best for insulation value.  It was used because it has fire resistance at really high temperatures.  All compared with copper 83 and steel 43.  Steel will have a higher temperature gradient than copper, but with insulation restricting the heat flow, both temperature gradients will be small enough to be insignificant.

OK, Willy, what will happen when you use a 1000 watt element in your superheater?  It's easy to apply a few numbers and this will require us to look at the superheat section of the steam tables. This section looks complex, but it is actually a quite simple separate table for each pressure.  Each little table starts at the saturation temperature, steam is not superheated below that, then jumps into a sensible series and has a single column for each of specific volume, internal energy, enthalpy and entropy.  So let's look at the steam from your boiler at 400 kPa abs, about 42 psig, that we used last time as the superheater input.  You will remember the enthalpy of that dry saturated steam leaving the boiler was 2738.6 KJ/kg.  And if you look back at the saturated steam section of the tables, you will see that your 1000 Watt element put 2133 KJ/kg into each kg of steam to evaporate it from water at that temperature.  Now remember your electric heater delivers its rated power output by increasing its own temperature until the heat transfer is enough.  In your superheater, your 1000 watt element will just get hotter until there is 1000 watts of heat transferred.  So you will add 2133 kJ/kg to the steam from your boiler.  The superheater outlet enthalpy will be 2738.6 + 2133 = 4871 kJ/kg.  Now we look up that enthalpy in the little superheat table for 400 kPa, and it is there.  (You can see that enthalpy is turning out to be a very useful concept).  Look across to the temperature column (it's on the left side of the page, but applies right across that line on the page).  Superheated steam with an enthalpy of 4871 KJ/kg has a temperature in the range of 1000 to 1100 deg C, you need to interpolate to get a precise figure.  I am assuming the heat loss is no greater than that from the boiler, so I assume some good insulation.  So what would that temperature mean?  Well the code design for copper boilers generally does not give a strength for temperatures above 200 or 250 deg C, so your boiler inspector would probably require a steel boiler, among other things if he knew the predicted discharge temperature.  Yes, the superheater is classed as a pressure vessel and requires testing and approval.  (Strictly, piping also).  If the copper temperature goes above 200, the strength continues to reduce, until copper melts at about 1350 deg C, so I would expect it to be pretty soft at over 1000 C and the shell would probably bulge at some point.  Not like a balloon, but a sizeable bulge, then as the boiler continues to maintain the pressure, it would split!  However silver solder melts at a somewhat lower temperature, so the ends or bushings might blow out first.  Of course the heater element has to be a bit above the steam temperature, so it might burn out before we get to this temperature.  It's a bit of a race to see which fails first.  But there are no winners in that race, whichever outcome, it will end in tears.  Better hope it's the element, 'cos steam that hot steam is very dangerous.  Just as well this is a hypothetical experiment, it's not a wise one to carry out with real equipment, however small.  Better stick with the recommendation last time of 50 to 60 watts, then the outlet temperature will only get to about 200 deg, so long as you maintain the steam flow.  I hope that makes the design process a bit clearer.  So you still need a high temperature cutout, and it should operate if there is high temperature or if the boiler power is cut.  That is an inclusive OR, either one or both should cut the superheater power.

Of course you may ask about using a controller to reduce the operating time of the heater.  It's possible with PWM of the power supply or other fancy digital controls.   The issues are reliability and heat transfer element intensity.  It is very hard to adequately cool the element when your fluid is dry steam instead of boiling water.   And all the while, you are a single failure of one of the internal electronic components away from 1000 degrees.  This is not intended to be a damper on the idea.  It is quite practical to build an electrically heated superheater as you suggest, they are used in industry.  You just have to understand the heat transfer and thermodynamics to develop a safe and realistic design.  If the elements of the lower rating are not available you could use say 100 Watt with a controller, or a transformer to reduce the voltage.

The last point, you are correct in that if the heating was from say a heat transfer oil which changes temperature as it looses heat, you would arrange the flows to be countercurrent, however the element is nominally a constant temperature device.  Think one of those one bar electric radiators, it's the same temperature all the way along as the heat is produced evenly in each metre of wire.  In the superheater, the steam temperature rises as the steam flows along the element, so the element temperature has to change in response to maintain the heat transfer all the way along.  If you connect to the other end, the hotter end of the element will always the outlet end.

I hope you find that interesting and helpful in you consideration of an electric superheater.

Thanks to everyone for looking in,

MJM460


Title: Re: Talking Thermodynamics
Post by: MJM460 on October 18, 2017, 11:42:30 AM
Back to Adiabatic engine discussion -

Those little diversions have given us an extra interesting steam condition to add to our exploration of the performance of an adiabatic engine.  You might remember that a few posts back, (post #363)
that I drew the relevant part of the T - S diagram to scale and showed three of the boiler temperatures we had been looking at for Willy's boiler.  (The second diagram)  The horizontal lines labelled 118, 135 and 143 (all deg C), show the boiler process for evaporation of water at those temperatures, while the one labelled 100 (deg C) is the condensing process for steam at standard atmospheric pressure.  The one labelled 15 shows condensation (or boiling) at the atmospheric temperature of 15 deg C.

There is a table to the right of the diagram which shows the saturation pressure in kPa (abs) for those temperatures and the entropy at the wet and dry ends of the lines, sf and sg.

 If we supply steam at one of those conditions to an adiabatic engine, then the second law of thermodynamics says that for this ideal engine, the exhaust entropy will be equal to the inlet entropy.  (Any real engine exhaust will have higher entropy.)

This means that the ideal engine expansion is that vertical line from the dry saturated steam leaving the boiler to the exhaust steam line.  The lines for the three conditions are very close together, so I have only drawn two.  You can see they all arrive at a wet steam condition.  In the wet steam or two phase area, temperature and pressure are not independent, but we now have the entropy at the exhaust condition, and this is enough to completely define the steam properties.  You see even entropy eventually comes in useful.  It is a property of the diagram that in the wet steam, the mass fractions of liquid and vapour are proportional to the values between the liquid and vapour lines for example, sf and sg, and as we now know those values, that fraction can be calculated.  Then the same fraction applies to all other properties, in particular, enthalpy.  So we can now calculate the enthalpy at the exhaust steam condition.

I have calculated those values and summarised the results in the table at the bottom of the diagram.  And the last column is the difference in the engine steam inlet enthalpy and the exhaust enthalpy, which is the work extracted from the steam by the adiabatic engine.   These are all on a per kg basis, but if you multiply by the steam rate, you have the ideal engine power output.    Remember, these are for an ideal engine, and the output of any real engine will be much less, but they are a maximum limit, and show the difference in potential output for different steam conditions.  You can see that the answer from subtracting two large quantities, the enthalpy of the supply steam and enthalpy of the exhaust steam which are a similar magnitude, is only about 10 % of either of those quantities, so it is necessary to try and carry the calculations through with the same accuracy as the tables to minimise the accumulation of errors.

You can see that as you would expect that the engine provides more output from the higher pressure steam, but you might be surprised at how much more compared with the extra heat input to the steam.  For example the ideal engine output is nearly doubled between 118 and 135 deg C inlet temperatures for about 1% extra energy input.  You can see why industry aims for ever higher temperatures and pressures.  I'm not suggesting that the model engine output is doubled by this higher inlet temperature, but there is no doubt that the potential power output is significantly increased.

It is also interesting to note that at higher temperatures, higher up that bell curve, the energy output to evaporate the steam, hfg, reduces, the increased energy contained in the higher pressure steam is all put in during the boiler heat up phase of our simple boiler, and once up to temperature the boiler can produce a small amount more steam, perhaps further increasing the engine output for our burner capacity.   Of course with continuous feedwater injection, the heat up and evaporation process happen at the same time, so for a given burner or electric element size, there is overall less steam produced.  Might be worth fiddling a bit more with the calculations to try and extract more understanding.

Now Willy has proposed a superheater.  To save a little effort in interpolation, I worked the example  at 143 deg C where the boiler pressure would be 400 kPa.  A pity it is not on the diagram, but the lines were getting to close together.  However a close look at the figures on the diagram suggests the 400 kPa steam inlet, would give exhaust steam about 0.925 dryness fraction and an engine output around 220.

When we add the superheater, the 50 watt one of course, to give 200 degrees C the steam enthalpy is 2860.5, and the entropy, from the superheat table, is 7.1706, compared with the 6.8959* (*note this figure has been corrected after the original post) of saturated steam, but still less than saturated steam at the exhaust temperature of 100 (7.3594). From this we can calculate exhaust dryness = (7.1706-1.3026)/(7.3594-1.3026) = 0.9688, clearly much more dry with the superheater, but still in the wet region.  We can then use this figure to calculate the exhaust enthalpy = 0.9688 x (2675.5-417.46)+417.46 = 2605

Finally steam inlet enthalpy - exhaust enthalpy = 2860.0 - 2605 = 255.5 KJ/kg.  This is a lot more than from the saturated steam but only little higher than if the boiler was operated at 148 deg,  possibly not enough to be worth while.  Perhaps we should make the superheater a little bigger to give say 250 deg C.  This would give us dry steam exhaust but more importantly  would show a real benefit  for the superheater compared with the boiler a little higher pressure.  I repeated the calculations for 250 deg C superheater outlet temperature, it requires 105 watt element, and increases the adiabatic engine output to 280 KJ/kg.  This looks like it would be more worthwhile.  The adiabatic engine exhaust would be very close to 104 deg C.  But worth looking at the boiler temperature required to achieve this output without superheat.

By the way, if a 100 - 110 W element is not available, remember the power is proportional to V^2.  So if you use a 240 - 110 V step down transformer, a 500 watt element rated at 240 V will produce about 105 watts at 110 V.

Another interesting point is that the specific volume of the superheated steam is higher, the superheater increases the volume of the steam.  If the engine can't run faster to use the extra volume, the system pressure right back to the boiler will rise unless the heat input is cut back.  So for a free running engine, whether loaded or not there will be a new balance of steam pressure and flow to use all the energy put in.  Remember the superheater cannot produce more pressure than the boiler.

You can see there is value in doing some calculations before you start cutting metal in order to reduce the amount of error in trial and error.  We still have to look at what all this means for a real engine.

Thanks for stopping by,

MJM460

Title: Re: Talking Thermodynamics
Post by: Gas_mantle on October 18, 2017, 07:57:49 PM
Thanks MJM,

As a general principle do small model boilers generate more steam when coal fired as opposed to gas ?
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 19, 2017, 02:39:54 AM
Hi MJM thanks for this new info.....I did hesitate when trying to make a superheater  as i thought there might be problems with this design !! I was wondering if the superheater pipe was inside  the boiler heater sheath would this work ,a bit like a locomotive superheater that is actually in the fire box.As this sheath is at the same temp as the water it is boiling it is not at a much higher temp??
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 19, 2017, 11:10:02 AM
Superheaters and real engines -

Hi Gas Mantle, I don't have direct experience with coal fired models, but I am sure there are many others on this forum who will be able to answer your question.  However a few observations can be made that might be relevant to understanding the issues.  First, with coal, you need a critical mass of hot coals to sustain combustion, just putting a match to coal will burn a little coal while it is there, but the fire does not continue when the match is removed.  You need a bit more concentrated heat  to make a coal fire which is self sustaining.  In addition you need an adequate draft to draw enough the air through the coal bed, and of course space for a big mass of coal and lots of draft are both in short supply in a small model.  You can do quite a bit with the coal particle size, but I suspect there is a minimum practical size for a coal fire, and I don't know how much coal the smallest practical fire could burn in a given time, or at what rate coal could be burned in a small model.  With gas, the particle size is molecular, and so long as you have a steady supply and the appropriate air/fuel ratio, the tiniest flame will continue.  At the other end of the scale, even in a small model, the available gas pressure means you can burn a relatively large mass of fuel with ease, and the gas pressure provides energy to achieve the required draft.   On the other hand, a coal fire has a very high transfer rate for radiation heat transfer.  The predominance of energy in that red and infra red region gives very high radiation coefficients.  The energy distribution in a clean gas flame is biased more towards the blue and beyond, which results in a lower radiant transfer coefficient.  You can feel this difference by holding your hand an appropriate distance from the flame.  So a coal fire can transfer much of its heat by radiation, where the high temperature helps more, leaving less to transfer by convection.  A gas fire does not transfer as much by radiation, so generally needs more area for convection heat transfer.  Some people insert a wire coil in the gas path to collect heat by convection from the hot flame.  The wire in turn glows red, and radiates more efficiently to the tube surface.  I don't know how well this works in practice, but it has a sound basis.

So the question is more complex than it might seem, and the answer really relies on experiment.  The theory should help understanding of the experimental results, but ultimately you need to talk with people who have done the experiments.  If you have a suitable small boiler, experimenting with each fuel, including coal size, bed depth, blower arrangements and different gas burners to see which will evaporate the most water in comparable arrangement would be the best way.  I seem to remember that Florian's Cochrane boiler is set up to allow both fuels, but it is probably not a small as you are thinking.  Comparison between different boiler arrangements is more difficult.  If the fuel rate releases the same amount of energy for two cases, they should be able to produce the same amount of steam, but coal will probably give better results with a generous firebox for radiation transfer, while gas will probably need more area in the flues for convection.  I will be interested to find out what you are able to learn.

Hi Willy, I am having trouble imagining how you would put the superheater tube in the sheath with your electric element.  Remember, you element just gets as hot as it needs to in order to loose all the heat generated, and relies on good cooling of the sheath to limit the temperature rise so the element does not melt before it rejects enough heat.  For a boiler, you slide the sheath into a close fitting tube which is part of the pressure containment, preferably with some heat transfer grease to aid transfer to keep the element cool.  You can't make it loose enough to accommodate a superheater tube, as the element cooling would not be adequate in a sheath with enough excess space.  I suppose you could use a solid rod of brass or copper, and drill two or even three holes length wise, one a close fit on your element and the other one or two fitted at each end to connect to the steam pipe.  I am not sure if you would get a high enough temperature in the steam tube to be worthwhile.  Heat only flows from hot to cold, and you need enough temperature gradient to transfer enough heat.  Remember the example with superheat to only 200 degrees compared with a higher boiler pressure.  It would be difficult to control the heat distribution to achieve adequate superheating, and at the same time adequately cool the heating element.  Your separate device concept is not only easier to analyse, it is more flexible in operating conditions, and I suspect it would be much more successful.  But an interesting idea to ponder if the specification sheet for the element allows a high enough temperature.

In a fired boiler, you are correct that the conductivity of copper means the copper is quite close to the water temperature, but still higher, as heat is transferring from copper to the water, but the flue gas is at much higher temperature to transfer all this heat via convection to the copper.  I suspect though that while the superheater tube passes through the flue tube, most superheating might occur in the part exposed to radiant heat in the firebox.  This could also be why many writers question the effectiveness of these superheaters.  You can see the arrangement is quite different from the one in my small boiler, where the steam tube is run into the fire box, two turns around the firebox, then straight out through the furnace wall from where it is insulated until it gets to the lubricator and engine inlet.

Last time, I looked at the performance of an adiabatic engine based on a few steam temperatures from Willy's boiler test, and I suspect typical of what many of us achieve.  The exhaust steam was generally in the wet region, however even the wettest exhaust steam was over 90% dry steam.  So, even in an ideal engine less than 10% of the steam arrives in the exhaust as liquid.  I don't think this is enough to explain the troubles Derek is having with condensate, but let's look at a what happens in a real engine.

Unfortunately, after being so useful in our consideration of the adiabatic engine, the second law of thermodynamics goes all wishy-washy for a real engine.  It just says the entropy of the exhaust will be greater than the entropy of the inlet steam.  No clue at all about how much more.  This means we do not know enough about the exhaust steam to do the calculations we did for the adiabatic engine.  Following on from the lack of information about entropy, the implication of exhaust entropy being higher for a real engine than an adiabatic one is that the exhaust enthalpy of a real engine will be higher than for the ideal engine, meaning that less enthalpy is converted to work in the real engine, but we don't know how much less.     In fact, it is necessary to do an engine test in order to find out how much power any real engine develops.

The simple engine test I did on my engine gave an exhaust temperature of 104 deg C.  Now this is above the saturation temperature for atmospheric exhaust pressure which means the exhaust is superheated.  Only just, but we would not expect any condensate in the engine.  Now the constant pressure line in the superheated steam region is no longer a constant temperature, and the temperature and pressure are now enough with a little interpolation to determine all the properties of steam including the enthalpy, so we can calculate the work extracted from the steam.  When the calculations are carried out, my engine extracted about 66% of the enthalpy extracted by the ideal engine.  This figure is defined as the adiabatic efficiency of the engine, and you can see why an engine manufacturer might prefer to quote this instead of the thermal efficiency.  But at least it tends to be independent of the steam conditions over a reasonable range, so there is some justification.

You will have noticed that I have been very careful not to call the work extracted from the steam the engine output.  We would hope that it is indicative of the shaft power output, but it is worth looking in a bit more detail at just what this figure represents.  A great topic for another time, but there is a little more to learn about exhaust steam first.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 19, 2017, 06:12:55 PM
Hi MJM, first, i was going to incorporate it into the heater sheath if i made a new boiler...and if it only requires 50 watts could i make it so the saturated steam pipe only goes in and out of the sheath for  1/10 of the length. this would require some cunning metal work !! Secondly. would it be possible to calculate the temperatures in the different places on the drawing when the boiler is at full steam production  A to G ??
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 20, 2017, 01:08:16 PM
Superheater design continued -

Hi Willy, so I was not too far out on your intention for incorporating the superheater in the boiler, but first the temperature questions.  Easy ones first.

H, atmospheric temperature, I am assuming 15 or 16 deg, i.e. your summer, not mine!

B, C and D, water and steam in the boiler, reasonable to assume all three are equal, and equal to the saturation temperature for the pressure, especially once the air is mostly gone.  The heat transfer within the boiler is largely facilitated by the mass transfer associated with vigorous boiling, which does not look much like the benign situation with a clear liquid interface like you have drawn.  The huge volume expansion when a little bit of water flashes into vapour and the associated huge density difference mean it is very turbulent, vapour quickly rises and is replaced by more water.  This is why the boiling heat transfer coefficient is so high, and the temperature is reasonably uniform throughout.

E, steam in the outlet pipe to the engine, same as B, C and D above, providing you have some insulation wrapped around it to minimise heat loss before the engine.

Calculation of F, the outside temperature of copper is a fairly straightforward variation of a problem illustrated in most heat transfer texts  In reality, it is easier because earlier we did calculate the total heat loss from your boiler from your test run.  If we proportion this heat loss to the area of the ends and the cylindrical part of the shell or put extra insulation on the ends and ignore the ends, we can use the standard formula to calculate the corresponding outside temperature.  The formula is a bit more complicated than for conduction through a flat plate because as the heat travels outward the area is increasing due to the increased radius.  It is getting late here and I have an early morning coming, so I will give you the formula, and you can easily work it out.

The basic formula is q = 2 x Pi x k x L x (T1-T2) / ln (R2/R1)

Here, q is the heat loss in Watts which in your case we already know, k is the conductivity of copper, use 399 W/m.K, which is actually for pure copper, and small amounts of alloy reduce it markedly, worth trying how much difference it would make if it was only 200, and brass is only 111.  T1 is your steam temperature, T2 is the outside temperature you are looking for.  L is the length of the boiler in m, R1 and R2 are the inside and outside radii of the shell, also on m.  The function ln means log to base e, or natural logarithm, and is one of the standard functions provided in any spreadsheet program or scientific calculator.

So you can manipulate the formula to find T2, using a calculator, or just use trial and error in a spreadsheet to find the temperature that gives the right answer.  That is the joy of a spreadsheet, but that formula is not difficult to manipulate to find T2 directly.  You might also know that if R1=R2, R2/R1=1, and ln(1) = 0, so T2-T1 =0 as you would expect, and it is still very small when the difference between R1 and R2 is small, as in a thin boiler shell.  But that ln term starts making a difference for a thick shell.

Having found T2 you can apply the same formula to the insulation, using k = about 0.035, to find the temperature difference across the insulation as q is the same for both insulation and shell.  This figure will be a little smaller than the difference between the copper temperature and the atmospheric temperature as there is a film resistance between the insulation surface and the air, and a contact resistance between the copper and the insulation.  There are standard formulae which enable us to calculate these film resistances now your test run has given us a heat loss based on the heat up phase, (as I have mentioned before they are hard to predict purely on theory).   You have already done the experimental determination, we just have to find our way through the calculations, which I will have a go at tomorrow.

Still have to tackle the temperatures A and B at the heater, and then look closer at your superheater design.  I need to get a couple of sketches into small files for that, so it won't happen tonight.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 20, 2017, 05:11:01 PM
Hi MJM , thanks for this ....ok .....a thermodynamic conundrum.....If the element is rated at 500 watts and it is 100 mm long does this workout at 5 watts per mm  ?  the total heat given off is  X degrees but at each mm is the heat given off only X ÷ 500 degrees ?  the heat given off is uniform from every square mm so perhaps not ?? Am i being a bit silly here or is this a valid question......and it  relates to only getting a 50 watts input for the superheater input from the existing 100 mm long element. !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 21, 2017, 10:32:04 AM
Hi Willy, not silly, but a little bit of wholly thinking there.  Remember to be consistent with the units.  You can add apples and oranges to make fruit salad, but you can't multiply them, and you never get pears.  If you stick to consistent units, the issues will be easier to understand.  The total heat from the element is 500 watts, the name given to Joules/sec.  Degrees is not part of it.  Then heat transfer comes in, a bit like flow of energy, and the flow requires a temperature difference to drive it, just like flow of electrical energy requires volts.  You can even use current as an analogy for the energy flow.  Strictly, physicists don't like to call it flow, but it's a bit like flow for most of us.  Your heater generates an amount of energy, or power, which you can calculate as V x I, or V^2/R, by conversion of electrical energy and it will be uniform along the length of the wire, just as the resistance is uniform along the length of the wire.  We don't know how the wire is arranged inside the insulation in the sheath, but reasonable to assume it is distributed uniformly, probably in a coil, possibly even on a ceramic former, we just don't know.  If you are picky, the spec sheet you posted a while back says the is a short non heating bit on each end within the 100 mm, so probably only about 85 mm, but as some heat will flow axially to the ends of the sheath 100 mm is near enough for our purposes.

Now all the heat generated by the electrical energy must escape.  If the heat flow out of the element is less than that generated, the extra heat will be stored in the form of higher temperature of the element.  So the element temperature goes up, and eventually the temperature difference between the wire and the water is enough to drive heat transfer equal the heat generated, and we at last get a stable temperature for the wire.  The temperatures are determined, not by the heater energy input, but by the heat balance, and the law of conservation of energy.

Now the heat must flow through the insulation within the sheath, the spec sheet says it is magnesia, which has a conductivity of about 2.68 W/m.K, and we don't know the thickness, then through the manufacturers sheath which I think is stainless steel, conductivity 14.4 W/m.K, again thickness unknown, then through the contact resistance between the stainless steel and your brass component, on through your pressure containing sheath, brass, conductivity 111 W/m.K, you know the thickness of that, and finally through the boiling liquid film into the water, which is very well stirred up, so a uniform temperature except for a very thin film very close to the the surface of your sheath.  All those thermal resistances need a temperature difference to drive the heat transfer.

We can actually make an estimate of the film coefficient using the general equations and your boiler test results.  I can go through it if you wish, but for the moment I will just tell you that from your boiler test, it is in the range of 5300 W/m^2.K, (which is very modest for boiling water, for which it can range from 3,000 to 60,000) based on the outside area of your pressure containing part, and this means the outside surface temperature of the brass will be in the range 10 to 100 deg hotter than the water, I suspect more likely less than 50.  The temperature gradient through the brass will depend on the thickness, but it is of the order of 3 degrees.  The temperature of the wire looks like it will be 30 - 200 degrees above the stainless steel sheath temperature depending on the thickness of the magnesia, possibly only 0.5 to 1 mm thick in your 10 mm diameter heater.

When you look at all those components, clearly your high conductivity brass is the least of the problem, and with the high film coefficient to the water, no fins are necessary.  Not sure about that contact resistance, but the spec sheet says you need H7 fit, and a design that would accommodate that heat transfer grease would be a good idea.  Clearly the highest resistance is the magnesia insulation, which by the way, allows enough current leakage to trip an earth leakage relay, so you need a good earth so that your equipment cannot build up a voltage, and I don't know what you have to do about the earth leakage protection.

The whole arrangement so far is all symmetrical and radially uniform around the centre line of the element, and it is easily analysed with simple one dimensional techniques only slightly modified.  You can make a simple electrical analogy with temperatures and thermal resistance analogous to voltages and electrical resistance.  With constant temperature water and constant heat generation along the length, it is all very uniform. 

 When you add your superheater tube, all that changes.  If you make a cross section through your pressure containment, you now have three specified temperatures, the element, which must be the highest temperature if heat is to flow the right way, and the superheater must be well above the water temperature as we saw in the engine performance discussion.  You now have three paths instead of just one, you have heater to superheater, heater to water, and superheater to the water.  The geometry of these paths is quite irregular, even though the high conductivity brass is the main heat conductor.  It can be solved, but requires finite element techniques.  Even then, it is not simple as it is a three dimensional problem.  As the steam temperature rises in the superheater, the temperature distribution has to change, so that the heater is still the highest temperature, and so that enough heat is transferred into the superheater despite much lower film coefficients.  This means there is no constant temperature along the length of the element.  Unfortunately I don't have a suitable programme.  I know we have some students on the forum and I hope they are reading.  They may be able to help.  These days students seem to use finite element like we used a slide rule, and it is good they spend their time that way, it is a very powerful tool once you know how to drive it, and enables solving of problems that can never be tackled with a slide rule, a calculator, or even a spreadsheet.

Generally, I would expect the placement of the superheater would be quite critical, it has to be close enough to the heater to get enough heat and force the heater temperature high enough, while leaving an adequate heat path direct to the water so the element cools enough.  In addition,  the rising superheater temperature would mean there are longitudinal heat flow as well as radial so not at all easy to analyse.  Any required adjustment after a test run has to be changed by changing the geometry, so not an easy exercise, but is your only available variable to control the heat flows.  Your separate superheater behaviour is much more predictable, and more controllable as you can adjust the superheater energy input independently of the boiler energy input.  I know I am a bit conservative, but in the oil industry they do not like the crash and burn approach, and even on model scale, that hot steam is very dangerous if you do not totally contain it.

I hope that helps to clarify a few more issues, and perhaps gives a slightly clearer picture of what happens inside your boiler, and how the element transfers energy to the water.

Thanks for reading,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 21, 2017, 02:08:57 PM
Hi MJM ,once again thanks and i was a bit confused with the "Degrees is not part of it" ! . so i wondering how much heat(temperature) you can get from a 500Watt heater given enough seconds to do it ? In my boiler if there was no safety valve ,and the boiler was strong enough, what is the maximum temp/ pressure that one could attain ? and would a time/temp graph of this show a continuous line to infinity ? Also i can see now that an entirely separate superheater would be required as in a Loco firebox, I am now feeling a bleet sheepish with my wooly thinking !!!,And this is why we need people like you to help with our knowledge and prevent unnecessary contraptions from being built !! there have been quite a few expensive constructions in the past that have been failures, as depicted in The Engineer over the previous centuries .
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 22, 2017, 12:19:54 PM
Heat and temperature -

Hi Willy, I hope my terminology did not offend you.  And the "wooly" term refers as much to my explanation as anything else as it was obviously not sufficiently clear.  So thank you for you kind words.  We are very fortunate to have access to information and tools that Watt and his colleagues could hardy dream of.  We can calculate a likely result and narrow down to those experiments with a reasonable chance of success.  They mostly had only trial and plenty of errors, and really amazing intuition. 

I was also trying to emphasise that all those words about dimensional analysis are not just esoteric waffle, dimensional analysis is a very practical approach to every day problems.  If you check the dimensions of each part of your problem, and they are the same, then you are likely to have all the elements of the problem and that is a good start.  Especially when working in SI units, when there are very few constants which have units associated them.  Your question, as usual, puts your finger right on the area that is still not adequately explained.  So let's try again.

The electrical power input from your heating elements can be calculated from the electrical formula, P= V x I, and the answer is in watts, or J/sec.  The big advantage of SI units is that mechanical power is also measured in J/sec, and they really are equal.  You only need talk about J/s, you don't have to qualify it as to whether is is mechanical or electrical joules.  For historical reasons, the unit J/s is given the name Watt.  Nothing in your boiler is able to provide any "push back" to change the electrical energy flow, no matter what temperature it reaches.  That is why I said temperature has nothing to do with it.

Now the fundamental law of physics comes into the picture, conservation of energy.  Energy does not disappear, it can always be accounted for, even if sometimes it is hard to know where to look.  But in your boiler with the electric heating element, we know the energy input, and the total energy output, or heat losses, plus any energy stored by increase in temperature of the materials in the system is always equal to the input.  When the heat losses equals the heat input, there is no more storage, so we have a steady temperature.  Now you will remember that I divided your operation into two phases, first a heat up process then steam production.  In the heat up phase, there is no steam production, there is some heat loss to the air from the shell which you can detect by holding your hand close, and all the rest of the heat input is stored in the water, the water vapour in equilibrium with the liquid, the copper shell, and even a little in the insulation.  And we did a calculation of how much energy was required for the storage.  We can calculate this using the mass of the materials, specific heat of copper, and the steam tables for the water.  Probably should have allowed a little for the heat stored in the element which also got hotter.  Then I assumed that the difference between the heat required for the observed heating of the materials and the input from the element was lost to the atmosphere.  It seemed like a lot and I suggested some insulation which would reduce the losses.  It did not reduce it as much as I expected, and I mentioned that we only know the rated output of the element, but did not have facilities to measure it, a potential source of error.  We will get back to that.

Once the boiler is up to your selected pressure, and you open the steam regulator, it runs at essentially constant temperature, so the losses continue at a constant rate.  We should have measured the steam consumption to check the losses at this temperature, because they are not constant during the heat up period, but vary with the temperature.  Now we have constant temperature, no more storage, so there are losses to atmosphere, and heat carried out with the steam.  We can measure the steam production, the tables tell us the energy carried out in the steam, and calculate the heat losses by subtraction from the energy input.

The point is that the energy equation is a complete explanation of the energy flow, and while the balance between storage, atmospheric losses and steam production changes with temperature, you only use the temperature as a measure of the amount of energy stored, and the properties of the steam, but nothing to do with the total amount of energy input.  And all because of your electric heating element which is ideal for demonstrating these things for that reason.

Now if we look at the heat transfer, the process which describes the flow of heat we do need temperature.  The equation is basically q = U x A x temperature difference.

Now let's look at the dimensions, q is J/s.  U has the dimensions of W/m^2.K, and of course W is the same as J/s.  A is the area in square metres, or m^2, and temperature difference, whether in C or K, has the dimensions of temperature or K.

The right hand side of the equation then has the units of J/(s. m^2 .K) times m^2 times K.  You can see the metres and temperature units "cancel" out, leaving only J/s, the same as the left hand side of the equation.  So we are off to a good start in understanding heat transfer, as our units are dimensionally consistent.

So on to look at your question about the ultimate temperature and/ or pressure.  I am glad that you are aware that the boiler could bust, this must remain a purely hypothetical thought experiment.  I am pretty confident that the boiler will fail before the heating element, not a good result.  You definitely need that safety valve and a high temperature switch, both properly set.

First you insulate the boiler, right over the top, with say 3 inches of good insulation such as rock wool tightly packed, or any one of several others with a similar conductivity, around 0.04 W/m.K, so the heat loss is minimal.  And then we start the heat up process.  Initially nearly all the heat goes into the water, the copper and a little into the insulation.  And as the temperature rises, the heat loss to the atmosphere increases.  We don't set a limit when we open the regulator, we want to see how high it goes, at least before bedtime!

Now that law of conservation of energy comes in.  The energy input is constant.  If there are no losses, all the energy goes into storage, and so the temperature just rises, as the energy is stored as sensible heat (higher temperature) plus all the ways energy is stored in steam.  It is not possible to eliminate all losses, some heat will be lost to the atmosphere, and as the temperature rises this heat loss will increase, it will increase until it equals the heat input, or until something breaks.

Now I did a few calculations with increasing insulation thickness from practically none and calculated the temperature difference required to dissipate the full 1000 J/s of the heater input.  It takes very little insulation to get over 1000 deg C, way above any safe temperature for your silver soldered copper boiler.  Something will definitely break, and no safe way of doing the experiment.

So in summary, energy input is not related to temperature in your electric boiler.  Temperature is a property of steam that we can measure, and if we have two independent such properties, the steam tables can tell us all the properties of the steam.  Temperature is also the measure of sensible heat stored in the copper, and other materials as energy is stored in them.  But temperature is not a quantity that has to be defined to determine energy input.

A fired boiler has a fundamental difference which sets it apart from your electric heated boiler.  The energy input comes from the mass of fuel burned and the fuel properties, but to release the chemical energy contained in the fuel, you have to burn it which requires oxygen, which usually only comes in a mixture with four times as much nitrogen which absorbs heat but makes no contribution to the amount of heat, and the combustion products have to be allowed to escape for the fire to continue.  They take a considerable amount of heat with them, and can if necessary take it all.  And it always takes away all the heat that is not transferred into the boiler water and copper.  It is never negligible.

I hope that makes it a little clearer.  But I noticed something interesting in the process of formulating an answer.  I will send you a pm after I have posted this, please let me know if you don't receive it.

Thanks to all for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 23, 2017, 12:18:30 PM
Unfortunately I have not much to add today.  I spent the available time reviewing where we are up to, and doing some calculations in preparation for the next step.

I hope to have something useful to discuss tomorrow.

Apologies to all the regular readers.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 24, 2017, 02:27:54 AM
Hi MJM i have checked out the currant going into the boiler and it looks like just over 4 amps  "Virtual" ?  so this may help with the calculations !
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 24, 2017, 12:01:40 PM
On Electric heating elements -

Hi Willy, that is a beautiful meter, and I am guessing it is at least as good as your Avometer.  With the zero clearly correct when power is off, I would say it reads pretty close to 4.0 amps.  I am guessing that "virtual amps" is the terminology of the time for what we now call RMS amps.  That stands for root mean square, and without diverging too deeply into the maths, it gives us the effective average of a sine wave.  For a pure resistive load with AC voltage we can apply ohms law that we are familiar with for DC, if we use RMS volts and amps, not the maximum voltage of the sine wave.  So it could be described as the effective volts and current that give us the same heating effect as a DC voltage of that value.  Perhaps "virtually the same as as a DC current of that value"? 

So let's look at what all that can tell us about the electric heater.  It is rated at 240 volts and 500 watts.  If we use the definition of power, power = volts times amps, we can say rated amps = power divided by volts, or 500/240 = 2.08 amps. With two elements in parallel, a total of 4.16 amps.  I am guessing you would see that 0.16 easily on that meter scale.  We can also use ohms law to calculate the expected resistance of the element, based on 240 volts rating with rated current of 2.08 amps, R = V/ I = 240/2.08 = 115 Ohms.


On the other hand, with the measured current of 4 amps, and assuming the voltage to be the rated voltage, 240 V, we get power = V x I = 240 x 4 = 960 watts, or two times 480 watts, compared with the specification 500 watts.  This time, using ohms law, we find R = V/R = 240 / 2 = 120 ohms for each element.

Already, we can see an inconsistency in the data.  The manufacturer has to be reasonably careful about the published ratings, so I would expect that data to be accurate, with the understanding that it is probably assuming all at a standard temperature of 20 deg C.  These ratings are at the specified voltage of 240 V mentioned on the data sheet.  The discrepancy in calculated resistance values would be expected if the actual voltage was less than the rated voltage.  Now I want an alternative to measuring the voltage, as mains voltage is too dangerous for unqualified people to mess with, meaning most of us, including me.  However, with an element removed from the boiler, and no power connected, we can safely measure the resistance of the element.  Now the actual resistance should be the same in both cases.  We have assumed the voltage is the same as the rated voltage, 240 V.  Now many sources I have seen mention 220 V as the normal European voltage, and there is some tolerance allowed to the electricity authorities, so it is reasonable to rate equipment for 240 V, so long as it can also operate properly at the minimum allowable supply and at the expected normal voltage.  We can assume the resistance is the same, whether connected to 240 V or passing 4 amps, so ohms law tells us the voltage must be different between the two cases.  If we measure the resistance of an element, we can calculate the voltage at 4 amps, then with voltage and current we can calculate the actual power output at our actual voltage.

Now if we do a little algebra on Ohms law, V= I x R, and on the definition of electrical power, P = V x I, we can derive P = I^2 x R, or P = V^2 / R.  Note P = power in Watts or J/s, I is in amps and V in Volts.

That second form is particularly interesting as it tells us that the power is proportional to voltage squared, which means a small difference in voltage has a big effect on power.  If the actual voltage is nearer 220 V, it would make a significant difference to the heater output.  If the resistance is 115 ohms, with 220 V, the power output would be only 420 watts each element.

There is a little more we need to understand about a resistive heater.  I have mentioned a few times that resistance increases with temperature, but I didn't have the relevant data for the wire nominated on the heater spec sheet, chrome-nickel resistive wire.  Well, I have found it.  That hyper physics site from gsu is a really great resource.  Should have thought of it earlier as I use it often.  The paid hyper physics Ap on the Ap store is money well spent, but it often turns up in response to a normal web search.  So I assumed the resistance of the element might increase by say 6 ohms when up to temperature for generating steam.  It turns out that the increase in temperature to cause this change from the specified 20 degrees would result in an element temperature of 158 degrees.  Now this is a little above our steam temperature, but probably not enough to give the required heat flow.  Hence it is likely that 6 ohms rise is a low estimate.  If we look at that voltage form of the power equation, P = V^2/R, we can see that as the increase in temperature causes the resistance to increase, the power output decreases.  This tells us that in our particular application, the temperature increase of the wire in the element and any amount of reduced voltage compared with the rated 240 V, both cause the power output of the element to be below the rated power, and that will affect our heat balance.

We need to return now to that heat balance to see how this all fits together, so that will be the topic for another post.  I hope this is all making sense so far.

Thanks for reading,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 25, 2017, 03:02:20 AM
Hi MJM ,i have tested another element not connected up and it reads 112.9 ohms so your calculation is pretty close !!Thanks for all the latest info and the theoretical and the actual figures depend on so many other factors that we  are not usually aware of .!! and the temp when i took the reading was 19 C....
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 25, 2017, 12:16:27 PM
Electric heaters continued -

Hi Willy, measured resistances provide hugely useful additional data for out investigation, thank you.  I think that makes two you have measured now, one was 109, and this one say 113.  This gives us some idea of the variation, and it is probably within reasonable manufacturing tolerance. 

Last time, I used the measured current, and assuming the rated voltage, calculated the resistance.  But now with that measured resistance we can manipulate ohms law again to  V = I x R.  Let's use our measured value of current, divided by two for each of two parallel elements, so 2 amps each element.  In addition let's use the average of the two resistances you have measured, or 111 ohms.  We put both of those into ohms law, so V = I x R = 2 x 111 = 222 Volts.

Now with better estimate of the voltage, we can use the power definition P = V x I = 222 x 2 = 444 watts.  Now this is quite a bit below the rated value of 500 watts for each element, and that will increase our heat up time.  The data sheet is still correct in specifying 500 watts, because it also specifies the voltage as 240.  When we use the element, we have to use the data sheet specifications and in addition apply the actual voltage we have in our location and equipment in order to understand the heat we will get from the element.

We also need to be aware of the fact that we are unlikely to have just picked out two elements one above the average and the other exactly the same amount below the average of all the elements that have been made.  In the vague shadows of my memory there were lectures on statistics, and the average called an estimate of the average based on a sample of two, and there was a method of calculating a standard deviation and with it, the likely spread.  But this thread is about thermodynamics, not statistics, so I won't go there, apart from mentioning that the difference between those two measured values gives us an idea of the variation between different elements.  To get any more accurate values, we have to measure the actual ones we intend to use.

Then there is that question of the increase in resistance with temperature.  I finally found the value of temperature coefficients resistance and the formula to calculate the resistance at different temperatures.  For Nichrome resistor wire the coefficient is 0.0004 per deg C. 

If we measure the resistance R0 at any temperature T0, then the resistance R at another temperature, T, is found using the following formula:

R = R0 x (1 + 0.0004 x (T - T0))

If we apply this to your element with R0 measured at 19, and we assume the heater wire starts at 17, and in order to transfer its output to the water it gets say 50 degrees above the water, we can calculate the resistance at each of your measured temperature points up to 135 deg C where the element would be 135 + 50 = 185.  Over this temperature range the element which measured our assumed average for the elements of 111 at 19 degrees, ranges from approx.110.9 at 17 to 117 at 185 degrees, and the power output of the element ranges from 440 watts at the start to 412 watts when it is at 185 , generating steam at 135 degrees with a constant voltage of 220 V.  Now this only 82% of the output we all assumed based on the data sheet.  It still requires that estimate of the temperature difference between the wire and the water, but it is enough to give you the idea.

Now I used a spreadsheet for all those calculations, which, because of the repetitive nature, saved quite a bit of time.  I was able to develop the formula in one cell or row, then simply copy and paste the formulae and the computer did the rest.  Actually the iPad did it.  Just a matter of learning how to use those absolute and relative references.  Then, as extra data was discovered, a simple substitution, and the spreadsheet programme updated everything in an instant.

I think that just about covers the performance of electric heaters, but if I have missed anything please ask, and I will try and cover it before returning to the calculation of those temperatures you asked for a few days back.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on October 25, 2017, 08:09:36 PM
Just as a little info - EU standardized the mains voltage to 230V +10/-6% one phase and 400V +10/-6% three phase more than 20 year ago (instead of the 220V / 240V in different member countries).

In practicality I actually measure between 227V and 231V with a very precise True-RMS Fluke meter at any given day over the last 22 years - and I often do as part of fault analyse on all the gear I've repaired over the years .....

Hmm - just checked WiKi to confirm the date 1995 - and discovered that there are places in the UK (a few) that still uses some very old power stations that supply 250V (still within the +10% limit) and some more that supply 240V .... Here in Denmark we adjusted our power stations from 220V to 230 within a few months after the new (back then) regulation was activated.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 25, 2017, 11:58:53 PM
Hi all, i have just checked my mains voltage and it is 240.3 volts.......!Thanks for more info on the boiler heating front ...Due to to all the disparate parameters that can occur with all the info it is quite a formidable task to actually come up with  exact figures. ! I suppose that is why when Rolls Royce is asked about power and performance , the reply is "adequate" !!
Title: Re: Talking Thermodynamics
Post by: crueby on October 26, 2017, 12:07:55 AM
Hi all, i have just checked my mains voltage and it is 240.3 volts.......!Thanks for more info on the boiler heating front ...Due to to all the disparate parameters that can occur with all the info it is quite a formidable task to actually come up with  exact figures. ! I suppose that is why when Rolls Royce is asked about power and performance , the reply is "adequate" !!

When the Rolls Royce test driver comes back from the track all wide-eyed and quietly saying 'wow' over and over, do they translate that to 'adequate' for the brochure?!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 26, 2017, 02:14:55 AM
Hi Chris , possibly ...It is the british stiff upper lip and "reserve ' that made us what we are/were.!  Also MJM does the specific gravity and the hard/softness of the water have any bearing on the steam ability in the boiler ??  Should i assemble for the next boiler test Hydrometer,Barometer,Ammeter, Voltmeter, temperature gauge ,pressure gauge , measuring jug etc etc etc
Title: Re: Talking Thermodynamics
Post by: Stuart on October 26, 2017, 11:32:46 AM
Steam Guy Willy

That’s why the rpm indicator in a modern roller is not as you would think.

At the bottom it says 100%power aviaiable and at what would be red line it says 0% avialable

Yes it’s calibrate  100% to 0%

But by ekk does it look odd

And no I do not have a roller but a humble BMW
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 26, 2017, 01:22:38 PM
Measurement accuracy and insulation-

Hi Admiral dk, thanks the more definitive answer on what the supply voltage should be.  It looks like you have a pretty solid power supply.  So 230 V +10%/-6% means max 253, min 218.  And presumably that is defined at the substation, so in the home could we see small voltage drops depending on power drawn in the street and elsewhere in the house? 

Hi Willy,  well the measured voltage does not match any of the calculated values.  This suggests that the resistance of the heaters in the boiler may be a little different from the one you measured.  It shows the difficulty of getting really accurate data.  However it is more about understanding the sources of error and their relative importance than pinning down the exact numbers.

With power = V^2/R, you can see for example that 230 volts instead of 240, would make a 9% difference to the output of our heater.  Similarly, a 2 ohm change in resistance makes about a 2% difference to the power.

  Salt dissolved in water does change its freezing point as many will know, and I expect it also changes the boiling point a little.  But more importantly, it is likely to cause foaming and carryover, and it stays in the liquid phase when water evaporates, and eventually builds up to cause scale, so distilled water is preferred if the domestic supply is not too good.  The rest of the data required is about accuracy, I believe digital temperature instruments are pretty good, pressure gauges not so good and not easy to check if you don't have access to a dead weight tester, barometer desirable, and you can check it against weather bureau observations or just use the weather bureau observations for your area, a jug is usually not a very accurate instrument for measuring water, better to use a digital scale and the specific volume from the steam tables.  Similarly for fuel quantities.  Time is now very easy to measure sufficiently accurately, unlike the problems our predecessors had with measuring time.  But basically you only really need to measure the quantities that appear in the calculations.  Probably the most important is that the readings have to be at the appropriate time.  The voltage, current and temperature at the actual time of the test are probably most important and a good voltmeter and ammeter permanently connected inside the control system would be best.  These along with carefully weighed water fill and a once off careful measure of the boiler mass.  But we can get enough understanding  of the thermodynamics by doing the possible, and leaving the search for perfection to the laboratories that develop the steam tables and other data we rely on.  We can just keep an eye on the effect of errors in our data.

Hi Chris, glad to have you along.  I don't think there is a "awe" factor.  They just cruise quietly around the track, shave an "adequate" margin of the lap record, the passenger in the back asks when the test is going to start, because he will need to hang on to his champagne, the driver blames that D*** clock, how was he expected to concentrate on driving with that ticking!

I think the present measurements are sufficient to indicate that the power input is a little lower than anticipated, though it does depend on the voltage and current at the time of the test, and assumes the instruments are of acceptable accuracy.

I wanted to go back a step to the questions you asked in your post #375, in the particular the outside temperature of the insulation.  Now I have been prevaricating a little on returning to this while I get some calculations which survive the "looks reasonable test". More on that later.  I think I gave figures for most of the temperatures you asked but not the outside of the insulation.

Now barring a measurement, perhaps with one of those infra red instruments, but please don't buy one for the purpose, I will measure mine next time I have an opportunity.  So far still settling in from travels, and shop time has been used for domestic projects.  But industrial insulation is designed to give an outside surface temperature less than 60 or 65 degrees depending on the particular company standards.  About the limit of safe to touch with bare skin.  If we do a simple calculation, by ignoring contact resistance between the insulation and the boiler shell, and assume the steam temperature for the inside temperature of the insulation is the same as the steam temperature, and the outside temperature is about 60 deg C, we can use the cylindrical insulation equation to estimate the heat loss from the boiler.  We know the outside diameter of the boiler, so we can use our proposed insulation thickness to determine the outside diameter of the insulation.  As the insulation thickness only appears in the form of the ratio of outside diameter to inside diameter, the units do not matter as long as we use the same for inside and outside.  Similarly it does not matter if we use radius (which comes from the maths involved in derivation of the equation) or the diameter.  We know the length of the boiler which must be in meters for the equation.  We have an assumed temperature difference, 135 - 60 = 75 deg C.  The final factor, k for conductivity of the insulation is available in heat transfer text books and material data sheets for most insulating materials.  For clean dry fibreglass wool firmly packed, my particular book gives a value of 0.05 KJ/m.K

I will remind you of the formula, there is no need to memorise all these, just file them so they can be retrieved, or in my case, remember where to find them in a text book.

q= 2 x Pi x k x L x (T1 - T2)/ln(R2/R1)

q is in J/s.  It will be a heat loss if the inside temperature T1 is greater than the outside temperature, T2, or a gain for a cold pipe.  The term ln is the natural logarithm, or logarithm to base e, and is a standard function in most spreadsheets and also on a reasonable scientific calculator.

For 10 mm of insulation, this equation gives a heat loss of 17 J/s.  For 6 mm, we get 27 J/s.

I think Willy wrapped a bit more thickness than this so should have had less loss through the insulation, which would result in a lower outside temperature, but one end was not insulated.

We need to ask ourselves whether this looks like a reasonable value for a boiler only 180 mm long.  In my former work place, pipe lengths were reconnect in hundreds of meters, not all, but the longer ones did not get any thicker insulation.  As the heat loss is directly proportional to the length, 17 J/s from 180 mm means 17 x 100/0.18 = 9444 J/s, or 9.4 kW per 100 meters.  If you have driven past or seen photos of a refinery or gas plant, you will appreciate there are a lot of pipes, many insulated, and this would represent a very significant operating cost.  It is not surprising that the insulation is more likely to be 25 mm.  However for our boiler, the length means it is not so important, and we are likely to opt for less thickness for appearance reasons.  We might like to limit it to about 6 mm, say 3 mm of cork and 3 mm of timber planks say for a model boat, probably less is practical for a locomotive.  So for the moment we will assume something in the range of 6 to 10 mm, and see what that means.

Next time I would like to go back to Willy's test runs, and see how that estimate for the heat loss compares with the experimental results.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 26, 2017, 01:27:30 PM
Hi Stuart,

Great to have you on board, your post came in while I was trying to wind up those clockwork Internet, it's very slow tonight,but got there in the end.

MJM460
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on October 26, 2017, 01:52:08 PM
Hi, I'm still following along and wanted to chip in with a question if that's ok.

Is there a simple formula for assessing the necessary diameter/bore of steam lines between an engine and boiler if the pressure and volume of steam is known ?

At the moment I rely on looking at other similar models and using similar but I'm sure there must be a more scientific method of ensuring my steam lines have adequate capacity.

Thanks
Peter
Title: Re: Talking Thermodynamics
Post by: crueby on October 26, 2017, 04:02:41 PM
Hi, I'm still following along and wanted to chip in with a question if that's ok.

Is there a simple formula for assessing the necessary diameter/bore of steam lines between an engine and boiler if the pressure and volume of steam is known ?

At the moment I rely on looking at other similar models and using similar but I'm sure there must be a more scientific method of ensuring my steam lines have adequate capacity.

Thanks
Peter
Thats a good question, and by extension is there a way to calculate the bore of the steam passages to the cylinder for a given bore/stroke? I've built ones that ran freely but did not generate much power, and I have wondered if the passages I drilled were too small.

Reading along, this is good stuff!
Title: Re: Talking Thermodynamics
Post by: Stuart on October 26, 2017, 04:35:35 PM
Don’t fully understand myself but the passages were big on my class 4 tank the piston was approx 1.5 inches the steam passage was .25 inches wide and spaned approx 120 degrees

Here is some info but don’t ask as I don’t fully understand

https://commons.m.wikimedia.org/wiki/File:Indicator_diagram_steam_admission.svg

Stuart
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 27, 2017, 02:51:05 AM
Hi MJM thanks for the info , good to see other people joining in with this thread , And i was a bit worried that we had hi-jacked it with all the talk about RR stuff,  There is info and talk about 'wire drawing' with small steam passages, but don't really understand that exactly ! good practical questions coming up.............
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 27, 2017, 11:08:18 AM
Pipe friction and valve passages -

Like Willy, I think it is great to see questions from more participants.

Hi Gas Mantle, your questions are very welcome.  No need to hesitate.  But a double benefit to your question.  I have been wondering for a while if I should try and compile an index of topics, and whether this could be made to stick with the main thread.  I guess that last point is easily covered by updating the index regularly.  But then I thought of that search button which I have avoided using.  I have found it pretty unsatisfactory on many commercial sites.  I knew I had said something on the topic before, so after a fruitless time flicking through the old posts, I tried the search button.  I put in "pipe friction" and only nine posts came up.  Consistent with my normal lack of success, the ones I wanted were the last two, but with only nine, that is not a problem.  They were all in the thread I had open when I put in the search.  So a really useful search function.  The general web search results I want on other subjects have been found as far down as page 23, generally after a pile of rubbish that bears no relevance to the query, and yes, I know I am probably the only person in the world who has ever persisted that far.  But search works really well on our site, another indication of the great site Ade and the admins have created, and are still creating, so a big thank you Ade and everyone else involved.

Anyway, don't be embarrassed that the topic was discussed before, I agree entirely that it is too hard to scan through nearly 400 posts to find anything.  That index may still be a good idea. 

The answer to your question basically depends on understanding the effects of pipe friction losses.  Pressure losses are proportional to the velocity squared, with a minor effect of the wall surface roughness.  Copper tubes are all pretty smooth, valve passages possibly less so, depending on how they have been made.  When the steam is forced to pass through a passage with a small cross sectional area, it's velocity has to increase, which takes energy.  There is also some extra loss at bends, especially sharp bends which cause extra internal turbulence.  And when the flow area expands again, to a larger pipe or into the cylinder, unless the transition is very carefully formed, the velocity energy tends to be lost in turbulence.  So the simplest criteria for pipe sizing comes down to velocity.  The criteria I am familiar with in industry are based on pipe lengths measured in hundreds of meters.  For a typical model application with tube lengths being nearer 100 mm, the industrial criteria tend to give trivial results.  I had a bit of a look at the energy involved in velocity, termed velocity head, can be calculated in KJ/kg, as V^2/2.  After some discussion of the issues, and looking at my own engines which seem to be satisfactory, I suggested a criteria of keeping velocity less that 20 m/sec.  It is not a very rigid limit, and needs to be confirmed by more data from other people's models.  But when it comes to comparing sizes for different size models, if the pipe sizes are modified so that the velocity is similar, if one is satisfactory, the other should also be.  Those earlier posts were on 14th and 15th of June, so should not be too hard to find, or try the search button, I think you will be pleasantly surprised.

The thing that is hard to assess is how much loss is acceptable.  However when you when you look at those adiabatic engine calculations in recent thread, any loss of pressure has a big effect on the available enthalpy for work output, especially in a real engine.  It seems out of proportion to its magnitude compared with boiler pressure, and exhaust resistance is as important as inlet pressure.  The piston has to push the exhaust out before there is any net force on the piston to drive the crank.  Perhaps I should revisit that aspect when we get back back to that topic.

There is also a formula, the D'Arcy formula, which I may have also mentioned in an earlier post, but it is really more useful for long pipelines.  To use the velocity criteria, use the specific volume column of the steam tables for the density.  I don't have any regulators on my engines so far, so I tend to assume the pressure is the same as boiler pressure.  If a regulator has a full open flow area similar to the pipe flow area, it will not introduce big velocity changes and the error assuming boiler pressure it probably acceptable.  You also need the temperature if you have a superheater.  And of course velocity and density mean you need to estimate the mass or volume flow.  As a reasonable estimate, use the operating pressure, with volume from engine rpm and piston displacement.  Though I have to admit this gives a quite low estimate for my engines when compared with the actual water evaporated from the boiler tests.  I am still trying to fit in tracking that down.  I think that in addition to that gap year someone mentioned recently, I need a sabbatical, about every third year, would be even better.

Any way, try a few calculations and come back with what you find, or more questions if it is not clear.

Hi Chris, good to see you back again.  How you fit in the time, I cannot imagine.  With valve passages, cross sectional area  and velocity are still the criteria, however they must be big enough for the steam volume at exhaust pressure.  This makes them bigger than otherwise necessary for inlet steam, but when the same passage is used for both, you have to size based on the larger requirement.  If you compare the steam inlet pipe flow area and exhaust flow area, and use the appropriate pressures you will see if there is a big discrepancy.  I think K.N. Harris and Martin Evans both have some criteria in their books based on fraction of the piston face area, if you need more reading.

With regard to free running and power, you can calculate the potential power from the enthalpy of the inlet and exhaust steam, but we have still to discuss efficiencies and losses in a real engine.  To get more torque, the engine needs the pressure through to the piston face, and at the same time, minimal back pressure on the exhaust.  If the passages are a bit small the exhaust back pressure may limit the differential pressure on the piston, which of course limits the force on the piston rod and the torque.  You also need sufficient energy input at the boiler to make the volume of steam required for the piston displacement and rpm without loosing boiler pressure.  Then large enough passages to deliver that pressure to the piston, and a large enough exhaust passage to let the exhaust flow freely while requiring minimal back pressure.  So a little more testing and size information required.  And of course if you are taking more power for your load, the flywheel needs enough moment of inertia to store the energy necessary to average out that torque, which as you know goes through the maximum and zero twice per revolution for each cylinder.  Obviously 90 degree crank displacement helps reduce the fluctuations.

Hi Stuart, thanks for coming in with that example.  It sounds like quite a generous passage area, but those locomotives are normally well proven as satisfactory runners under load, so a good indicator of suitable passage area for the piston size and rpm.  The flow area is the passage cross sectional area at right angles to the flow.  I am not sure if this can be calculated from the information you have given, though the necessary information is probably on the drawings.

Hi Willy, I don't think you have hijacked the thread, you will have gathered that it is guided more by the questions than any pre decided "syllabus".  Some questions which appear simple on the surface require covering a lot of material and a plain yes or no is not a satisfactory answer.  I feel it is important to discuss the topics people ask about.  Like lectures in your past, it is no good moving on until the basics are understood, and I sometimes need a few tries.  And yes I seem to remember wire drawing being raised in an earlier post.  Wire drawing or energy loss due to changes in velocity, seem to be similar topic, but I prefer talking about velocity and velocity head or pressure loss.

I hope I have provided helpful information in response to the questions, or at least a starting point for further discussion.  However I think the heat balance topic will have to wait until another time.

Thanks everyone for dropping in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Stuart on October 27, 2017, 11:32:15 AM
The size of the port in the cylinder has to be sized to get the spent gas (steam ) out , and by ref to the steam diagrams will be at a lower pressure and temperature that the entry steam

So question is are the ports sized for the egress or the entrance of the gas ?

I do know that the steam feed from the super heaters CSA is much smaller than the port size , and that the blast nozzle is smaller than the feed pipe in this loco model it’s 0.312 inches , that would be sized to increase the velocity to improve the blast up the petticoat pipe to get the fire going better , Evan in model form the coal fire is incandescent well past yellow after a good pull

But again more questions that answeres , guess we should time travel and ask the likes of Churchward for their reasons
Title: Re: Talking Thermodynamics
Post by: crueby on October 27, 2017, 01:49:12 PM
MJM - good point about the exhaust steam needing to get back out the same passages, I had been thinking mainly about the input of steam. If I get the chance I really want to pull the cylinders on the Lombard model and up the diameter of the passages between the valve ports and the cylinders to see how it changes the power. The cylinders are a .625 bore x .810 stroke, and the passages were about .100" (forgot to note exactly which drill I used). The passage from the exhaust port to the exhaust pipe is much larger, but I had forgotten that the exhaust had to come through that smaller passage first. It seems to run better on air than steam, and the expanded volume of the exhaust could explain a lot of that.

Great idea about the index, not sure how to make that work either.

 :popcorn:
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 27, 2017, 02:19:00 PM
Hi MJM et al....will a hydrostatic lubricator help with the steam flow and how much oil ends up in the exhaust passages and into the condenser ? On most models there are really neat quite tight bends which look really nice !! however would larger slower bends be 'better' for the steam flow ? With restricted flow, ports and piping are we discovering Giffards contribution to boiler practice and did he work out his "magic box' from scientific analysis or just plain observation from leaky pipes and stuff?? Would be interesting to Know that !!. On most backheads and pipework in a full size
 locomotive all the copper pips are rather untidy looking large copper pipes !

Willy.
Title: Re: Talking Thermodynamics
Post by: Stuart on October 27, 2017, 03:52:52 PM
Willy

food for thought if you have ever used steam injectors for boiler water feed they work much better with slow bends in the pipe work they tend to pick up better

oops MJM there is another set of questions how do steam injectors work I know its to do with the restriction and pressure changes in an injector cone the bit that confuses me is how the water jumps the gap with enough energy to overcome the static pressure in the boiler shell


Title: Re: Talking Thermodynamics
Post by: MJM460 on October 28, 2017, 01:11:50 PM
Port sizes and pressure losses -

Hi Stuart, ports have to be sized for the exhaust flow, but even that is not as straightforward as it looks, as the flow in the exhaust is highly impulsive and this gives much greater losses than a steady flow.  I will come back to that.  As you say, the blast pipe has a smaller diameter than the ports and piping to get a good draft effect, and all of this is significantly larger diameter than the inlet piping.  It seems that your locomotive has generous port sizes and I suspect that is true of all successful locomotive designs.

Hi Chris, I have tried several times to come up with meaningful velocity criteria for pipe and port sizing, but the normal calculations never seem to fully explain the observed effects.  I suspect the reason is that the usual calculations assume the flow is constant at the average rate.  This is perhaps a reasonable assumption for the inlet, as when the flow into the cylinder stops at top and bottom dead centres, the flow in the inlet pipe tends to continue to build the steam chest pressure up towards boiler pressure.  A little bit of steam chest volume is probably helpful in smoothing the flow.  For the outer end of the cylinder, the piston is at top/outer dead centre, and starts moving slowly per the sinusoidal like motion, while the valve opens so the steam flow can more or less keep up.    Near the bottom of the stroke, the valve is closing slowly again, and as the piston is slowing to zero, the valve opening and the pipe sizes are still more or less appropriate.  But the exhaust is quite different in nature.  When the exhaust valve starts to open, still a sinusoidal like motion from the eccentrics, the cylinder is at quite high pressure.  Not up to boiler pressure, but not expanded much unless there is quite early cutoff.  Now a full cylinder of steam has to depressurise through the opening exhaust valve to the atmospheric pressure of the exhaust, so it expands considerably in volume and you need over double the cylinder volume to flow through the partly open exhaust valve opening, cylinder ports and exhaust passage.  About a full cylinder volume has to pass through the passages before the pressure gets down to exhaust pressure and that is before the piston starts pushing the remaining steam out.  And of course the remaining pressure in the cylinder during the exhaust stroke is opposing the work done by the steam on the other side of the piston.  This requires a much higher velocity than the average if the exhaust is to flow freely, and of course the developed back pressure is proportional to the velocity squared.  Consequently the exhaust pressure losses are much more significant than the inlet losses.  It is probably also an advantage if the valve timing allows a slightly early release, as there is more power lost by high exhaust pressure than by a slightly lower pressure near the end of the stroke where torque is nearly zero anyway.  There was more detail on this in the posts back on 18 June and the following days.  (That search button really does work well!)  Overall, looking closely at the release and early exhaust flow pulses is more productive in explaining engine performance than pressure losses based on average flow.

Hi Willy, the lubricator, whether displacement or pumped, is about lubrication of the cylinder wall to minimise scoring by the piston.  Unfortunately it does not help steam flow much, probably the opposite if anything.  Most of the oil appears in the exhaust and even the simple separators I described earlier collect a fair bit of it.  However I suspect some is also carried out with the exhaust steam as droplets so small they cannot be seen.  But in a closed shed, there is usually a bit of oil grime that builds up over time if given a chance.  I have not managed to measure the amount lost from the lubricator to compare with the amount collected to get an idea of what proportion is actually collected.  I am not familiar with Giffards work.

Many of the pipes on the back head of a locomotive have no flow, so sizing is affected by different considerations.  The blowdown is normally no flow, as is the whistle, pressure gauge and whistle.and a small connection is adequate.  Whistles and water inlet which sometimes have flow should not share any part of the passage with the level gauge, as they will cause pressure variations which upset the level readings.  The level gauge also tends to be affected by surface tension in small sizes.  Most of the untidy bit, I think is due to the difficulty of fitting everything in.  Long radius bends do cause less loss than shorter radius, but any smooth bend is better than the sharp corners at intersecting drilled passages.

Stuart, injectors are a whole new level of velocity effects.  I will try and get back to those in a few days.  Certainly they are counterintuitive in that they are able take steam from the boiler, use it to pick up cold water, and return the more water to the boiler pressure than the amount of steam consumed.  I am not sure that I can totally explain them, but I have a little information which may help in understanding them.  Of course making them is an art form of the darker kind.

Thanks everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Stuart on October 28, 2017, 09:35:27 PM
Thanks

Yes it’s a black art , you have to have the sun at the correct position ,hold your tongue at the correct angle , and hope

Yes I have made a few to known good sizes but they are tricky blighters

The loco in question has been sold for a good sum of gelt ,the boiler is the one in my sig on test at 200 psi for SWP of 100 psi the super heaters were many as full size practice with half of them over the fire , and yes there were two fusuable plugs installed

Looking forward to you thinking on injectors ,it’s ok to make them but it’s nice to understand them

As you know there are two basic types lifting and non lifting , not to mention the alligator mouth type
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 29, 2017, 12:15:09 PM
Injector thermodynamics step 1 -

Hi Stuart, it's not too hard to stop me on the intricacies of injectors.  I will leave it to you to explain the different types.  I will try and explain some of the thermodynamics, however, I am not at all certain thatI will get to a complete explanation.  I will just proceed one step at a time and see where it leads, even if that is a dead end.  No problems with that.  If the sun position is important, I will be in trouble as the sun goes around the sky to the north of us here in the Southern Hemisphere, it will never be right unless it can be compensated for buy holding your tongue the opposite way.

At the core of an injector, is a carefully crafted nozzle.  Now these nozzles are very interesting devices.  Near the entrance we can use the Bernoulli equation to calculate the increase in velocity as the flow passage decreases in area.  However, Bernoulli's equation has been derived for incompressible fluids.  It works quite well for low velocities, but as the velocity increases, the equation needs a little modification.  It turns out that the modification involves the speed of sound, or more precisely, the ratio of the velocity to the speed of sound, a ratio that we know as the Mach number.  For velocities up to about half the speed of sound in the fluid, the errors in ignoring compressibility are less than about 6%.  As the nozzle flow area reduces, the velocity increases, and the pressure decreases, until the point where the velocity is equal to the speed of sound.  In order to increase the velocity further, that is to a velocity greater than the speed of sound, the flow area has to increase.  Totally counter intuitive, that one!  In this diverging section of the nozzle, after the throat where the area is a minimum, the nozzle diverges, and the velocity continues to increase, and the pressure continues to drop. 

As the speed of sound is so important, we had better learn how to determine the speed of sound.  Now we are somewhat familiar with this concept in air, but did you know that the speed of sound varies for different gases, and is also dependent on the temperature?  The maths to derive it gets heavy, but the formula for the speed of sound comes out as follows:

Speed of sound, c = square root of (k x R x T) metres/sec.

In this equation, which assumes the gas is close enough to following the ideal gas relationships, k is the ratio of specific heats.  Another term which probably raises more questions than it answers, but it varies with the particular gas and is close enough to 1.4 for air, or 1.3 for steam over a wide range of temperatures.  R is called the gas constant.  It is associated with another basic constant of physics, the universal gas constant which equals 8.3144, and does have units associated with it..  The gas constant for any gas is the universal gas constant divided by the molecular weight.  So for steam, R = 8.3144/ 18.015 = 0.46152.  I will not go into the units for that one unless someone is particularly interested.  Finally, T is the absolute temperature in deg K = deg C + 273.15.

So to calculate the speed of sound, we need the steam temperature.  If we assume the absolute pressure is 600 kPa (about 75 psig), and just saturated steam from the boiler so no superheat, the temperature will be 158.85 deg C, or 432 deg K.  Then we can calculate the speed of sound as sqrt(1.3 x 8.3144/18.015 x 432 x 1000) = 514.4 m/s.  The factor 1000 needs some explanation, the gas constant is in kJ/kg.K, but the formula requires Joules.  I can refine this figure if you tell me the  actual steam conditions for your injector steam supply.

Now there is another interesting thing about reaching the speed of sound, the mathematics also tells us the pressure ratio, based on the inlet pressure.  The pressure at Mach 1 will be 0.545 x the inlet pressure.  Now for 600 kPa inlet pressure, 600 x 0.545 = 327 kPa.  That is still well above atmospheric pressure at which cold water is supplied.  But the pressure at some point in the injector must obviously be less than atmospheric pressure for the water to flow in.  So we need further pressure reduction. 

That is enough for one session.  I need to look up a few things for the next step so I will come back to this in a day or two, but I will try and continue the other topics we have been discussing until I have the next steps.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: Stuart on October 29, 2017, 12:31:49 PM
Thanks

I think I will have read that a few time to ingest the info contained , brain 🧠 gets a bit bit at 70

As to the injector types in my experience in model sizes there is as I have stated lifting and non lifting , in each there is vertical and horizontal

Now in the full size there are many with some very odd designers offerings the jaw type had a pair of jaws that controlled the action .

Have a look at this YT chanel

https://m.youtube.com/channel/UCBdj-vOveiEFWe3vnGoJUag

Ok it’s not models but Dave explains / shows the workings of some very odd injectors to feed his boiler and feed water container
Title: Re: Talking Thermodynamics
Post by: Stuart on October 29, 2017, 01:56:48 PM
For the masochist ones

Here is a good book on how to make working model injectors

http://www.teepublishing.co.uk/books/operation-valve-gears-injectors/miniature-injectors-inside-and-out/

The last set I made was a scale outline with the injector body slid in the scale shell sealed with very small section o rings

They did work but not after much knashing of teeth


Title: Re: Talking Thermodynamics
Post by: crueby on October 29, 2017, 02:16:17 PM
The Maine logging museum put up an article about steam injectors like the ones used on their Lombard log hauler, here is the link:

http://www.maineforestandloggingmuseum.org/wp-content/uploads/2015/02/Steam-Injectors.pdf

and the text about it from their page:

"We are fortunate to have the original Hancock Inspirators on our Lombard. The above link has some fascinating information about our inspirators from a book  titled “Basic Steam Locomotive Maintenance”. This book was published about 100 years ago to train locomotive engineers. There is a special section on our Hancock inspirators with diagrams, theory, operating principals, and troubleshooting. This is the best information on our inspirators we have found. Have also attached a brief thermodynamic calculation at the end showing how we need a steam jet speed approaching 120 mph to develop 200 psi of injection pressure."
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 29, 2017, 11:05:45 PM
hi  All is it better to use superheated steam for an injector ?   also does the ambient humidity come into the steam tables ?

Willy
Title: Re: Talking Thermodynamics
Post by: Stuart on October 30, 2017, 07:00:13 AM
Willy

I have never seen super heated steam used for injectors model or full size, the steam comes from the backhead .

The superheaters are after the regulator so it’s of no use to fill the boiler when stationary


Stuart
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 30, 2017, 11:29:52 AM
Injector inlet nozzle continued -

Hi Stuart, I hope yesterday's post is starting to make some sense, the primary thing is to understand that that when the velocity gets more than about half of the speed of sound then the analysis needs to take into account the compressible nature of the fluid.  Also that the speed of sound is easily calculated from the gas molecular weight, absolute temperature and the ratio of specific heats.  Then we can proceed a bit further today.  I have also found the book on my bookshelves by Ted Crawford and published by the Australian Model Engineering Magazine.  It is very good on the basic principals and even more on construction.  But like most sources, avoids direct discussion of the thermodynamics.  I am trying to cross this barrier so there is enough understanding of the thermodynamics to inform practical experiments even if the practice is in the end a bit of a dark art.

Hi Chris, that is probably as good as anything as an introduction to injectors, by the way iBooks does not seem to like page 93 of the extract, though it is fine directly on the web.  The article gives a good idea of the basic processes.

Hi Willy, Stuart has basically answered on the practice.  I think it will become clear why saturated steam is used when I get to the mixing nozzle which follows the inlet nozzle.

The steam tables properties of water apply to all water including atmospheric water which we describe as humidity.  All water except when it is chemically bound into crystals or other material.  Remember, water vapour in air is just another component, and all the components behave independently of the other gases that form atmospheric air.  Atmospheric pressure is the total of the partial pressure of all the individual gases that form the atmosphere.  It is at the same temperature as the air, and the saturation properties are found in the steam tables for that temperature.  However, the vapour pressure is normally less than the saturation pressure.  The humidity is defined by the actual water vapour pressure in the atmosphere divided by the saturation pressure at that temperature and expressed as a percentage.  When the temperature drops, the saturation pressure (in the tables) drops but the vapour pressure in the atmosphere remains the same.  Eventually the temperature drops to the point where the actual water vapour pressure equals the saturation pressure at that temperature.  This is called the dew point and is obvious because any further temperature drop causes water to condense on various surfaces, or higher up on dust particles in the atmosphere where we see it as clouds, and eventually rain, or snow.  So atmospheric humidity adds a little water to your boiler at the point where you seal the plug.  At that point the total water content of the boiler is described by the tables whether it's source was your jug or the atmospheric humidity.

Back to the injector inlet nozzle.  When the steam flows through the inlet nozzle, the velocity increases as the flow area decreases, and the velocity energy comes from transformation of the pressure energy per Bernoulli's theorem.  As the velocity increases, compressible flow equations have to be used, and there is a critical pressure ratio where the velocity equals the velocity of sound.  For steam considered as an ideal gas, this pressure ratio is about 0.545, for saturated steam as in an injector the ratio is nearer 0.577.  For 600 kPa steam, remember it is absolute pressure, 0.577 x 600 = 346 kPa.  Now this is too high to allow water to flow in as we require in an injector.  But the compressible flow has a remarkable property that is not intuitive, not to me anyway.  If the pressure is to reduce further and velocity to further increase, the flow area must increase, so we need a divergent nozzle to go further.  As we need a pressure around 100 kPa at the end of the inlet nozzle, we clearly need a nozzle which first converges to a minimum throat area, then diverges. 

Now the pressure downstream of this throat is interesting.  There are two solutions to the equations, one which gives a higher velocity, and the other giving a lower velocity.  The lower velocity solution means the exit pressure of the divergent section will be higher than at the throat and does not result in supersonic flow.  The other solution gives supersonic flow and a specific outlet pressure that satisfies all the conditions.  This pressure depends on the flow area at the outlet end of the divergent section of the nozzle, and there is only one pressure for any given outlet flow area.  Now I looked at the nozzle dimensions for a working model injector in Crawford's book, and found the exit area is 3.16 times the throat area.  For our 600 kPa steam (still based on that ideal gas behaviour) that exit area ratio results in a pressure of about 27 kPa, quite a bit lower than we need.  When the actual pressure at the outlet of the nozzle is higher , like the 100 kPa we need, there will be a sonic shock wave to balance all the conditions, and that shock will occur part way through the divergent part of the nozzle.

In the spirit of proceeding one step at a time, I think it a good idea to stop there, and continue next time as there are still three points to understand about that inlet nozzle.

Thanks for following along,

MJM460

Title: Re: Talking Thermodynamics
Post by: Stuart on October 30, 2017, 12:22:14 PM
My brain hurts with all the new info

Pity I am not ‘five alive ‘ from the film we’re the robot said do not disassemble Stephany

You will have to give me a day or two to digest all this info

But a big thank you for your time to educate the great unwashed

Stuart
Title: Re: Talking Thermodynamics
Post by: crueby on October 30, 2017, 02:39:42 PM
My brain hurts with all the new info

Pity I am not ‘five alive ‘ from the film we’re the robot said do not disassemble Stephany

You will have to give me a day or two to digest all this info

But a big thank you for your time to educate the great unwashed

Stuart
I hope you drive better than that robot. That was a funny movie!
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 31, 2017, 11:42:36 AM
More on Injector Inlet Nozzles -

Hi Stuart,  sorry about the headaches, perhaps a little consolidation will work like Aspro.

The analysis of the inlet nozzle uses three basic equation, conservation of energy, the definition of entropy and conservation of mass.  Now conservation of mass is the dubious one, everyone thought it was a fundamental law until Einstein came along with his famous equation, E = m x c^2, which relates energy, E, and mass, m and the speed of light, c.  This effectively means that mass is affected by velocity.  Conservation of energy still stands as a fundamental law of physics, but conservation of mass is now understood as only an approximation.  Now the correction is fortunately very small.  Unless you are being too realistic in your models of space rocketry, or perhaps too realistic in your model of a nuclear powers submarine, (careful, Chris,).  Or it could be that you are building a small cyclotron in your basement.  That probably lets most of us off.  However for completeness, the correction involves the term (v/c)^2, where v is the velocity of your mass and c is the speed of light.  The speed of light is so large that this correction is much smaller than the error in our very best available means of measuring mass for normal velocities.  So for all practical purposes, we can ignore the correction and assume conservation of mass without detectable error.  Assuming conservation of mass leads to the very useful continuity equation that you may have seen mentioned.

Conservation of energy is the fundamental law of physics that leads to the first law of thermodynamics.  You might notice that each time I use the first law, I usually include the clause, "for this case", and you might wonder why it looks a little different each time.  Basically, an equation for conservation of energy should include a term for every conceivable type of energy.  Not just heat, but velocity, elevation, electrical energy, magnetic fields chemical energy and so on.  However in most practical problems, most of these terms can be assumed to be zero.  Changes in elevation for example, especially on a model scale, usually are insignificant compared with heat.  On the other hand, if you are building a hydro electric power station, changes in elevation are very important.  The point is that the behaviour of steam as it flows through the nozzle is explained by the normal laws of thermodynamics.

For flow through a nozzle, generally there is no significant heat transfer in or out, so we can say Q = 0, and there is no external work done so we can say W = 0, and similarly changes in elevation are insignificant especially of the nozzle centre line is horizontal.  So the first law, or conservation of energy leads to Bernoulli's theorem relating pressure and velocity for incompressible fluids such as water, or gases at low velocity.  For compressible fluids, some fancy maths around these same basic laws, leads to the speed of sound, and the importance of the ratio of gas velocity to speed of sound in calculations for compressible fluids.

The pressure drop from steam pressure at the injector inlet to atmospheric pressure so that water can flow in is more than enough to cause the velocity of the steam to exceed the speed of sound.  So it is a good idea to have a basic idea of the behaviour of gases at supersonic speed.

Now the maths is not particularly hard, the issue is just which sums to do.  I think answering that is beyond what any of us want or need, even to make an injector, though it could be useful if you really want to design one from scratch.  And I am not sure I am up to leading us all through that.  So just a review of the features of flow in a nozzle which produces supersonic velocity.

In the inlet side of the nozzle, as the flow area decreases, a converging shape, the velocity increases and the pressure decreases.  When the velocity reaches sonic velocity, the gas can go no faster unless the flow area increases, a diverging passage.  Not very intuitive as I have mentioned.  Sonic velocity occurs when the pressure drops to about 0.55 times the upstream pressure.  The smallest cross section is called the throat, and the velocity at the throat is sonic velocity, and the flow area at the throat determines the flow through the nozzle.  Analysis of a converging -diverging nozzle is based on assuming an ideal gas, and adiabatic flow.

Now, moving to the next interesting point about a supersonic nozzle, once sonic velocity is reached at the throat, the downstream pressure no longer determines the flow, the flow is entirely determined by the speed of sound and the flow area at the throat.  Finally, adiabatic flow only leads to two possible outlet pressures for a given shape of the diverging part of the nozzle, and only one of these is supersonic, which is of course the one we are interested in.

If the downstream pressure is different from the adiabatic solution, there will be a shockwave at the nozzle outlet, where the pressure and temperature suddenly increase, the velocity decreases to subsonic, and subsonic behaviour continues into the downstream flow passages.  I tried the maths for our assumed 600 kPa saturated steam and found there is a standing shock at the exit which reduces the velocity and increases the temperature to the nozzle downstream pressure of 100 kPa (remembering that all the pressures and temperatures are absolute for this analysis).

The first law equations allow the enthalpy to be predicted based on the assumed ideal gas and adiabatic expansion.  The predicted enthalpy indicates that the steam exiting the nozzle is quite wet, around 70% dry, after the shock.  So now time to think about the effects of real expansion instead of adiabatic, also to think about that saturated steam which does not precisely follow the behaviour of an ideal gas.

I suspect that is more than enough to either take in or to gloss over, don't worry too much about which.  The bits you understand will be helpful but I will try and continue tomorrow in a way that will not matter too much if the above is a bit hazy.

Thanks for following along,

MJM460

Title: Re: Talking Thermodynamics
Post by: crueby on October 31, 2017, 12:18:55 PM
Quote
Unless you are being too realistic in your models of space rocketry, or perhaps too realistic in your model of a nuclear powers submarine, (careful, Chris,).


Don't worry, I am always careful when I run my nuke sub!!   :Lol:

Title: Re: Talking Thermodynamics
Post by: Stuart on October 31, 2017, 12:39:51 PM
Thanks for more info , it’s sinking in slowly after a couple of rereads

As to the asprin doubt that would do any good as I am on 8 paracetamol and 8 codine on a good day but can go to 16 codine , yep these are from the doc .

Normally it’s cruthes to get about ,but at the extra codine workshop is a no no

Long time ago when I was at Manchester uni studying vibration analysis of rotating machines using fast fourniour /sp ( I am a bit dyslexic) transformation to produce the info required . We were taken down in the store rooms and the had the original apparatus to demonstrate laminar flow in a liquid and it still worked fine it looked a bit grim but demonstrated the effect very well .

Keep up the drip feed your chunks are just right size
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 31, 2017, 03:19:31 PM
Hi MJM,  So Mr Giffard must have been really clever to know all this !! Somewhere in my brain is an account i read of getting sparks and electricity from steam nozzles that are open to atmosphere but don't know if this is relevant or interesting for this thread ??!!!!  Also Jo..when i put messages on the messages page ,i get the answers but can't find my actual message that i sent  ? can you help with this please ?? Thanks
Willy.....
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 01, 2017, 12:20:12 PM
Injector inlet nozzle continued -

Hi Chris, so long as you are able to control that conversion of mass to energy!  I guess you have it patented.

Hi Stuart, that much codeine is serious stuff.  I am sorry to hear that you do not enjoy better health.  That flow apparatus would have been interesting, but no laminar flow in an injector nozzle.   Fast Fourier transforms.  Really interesting how they work on real data to give the frequency analysts.  Definitely not intuitive.  I need to keep to pretty small chunks on injectors.  I sat through the lectures but missed something early on so it did not make as much sense as it should have, and it was 50 years ago.   It is heavy going, wading through the appropriate part of the text book after so long, it's even only reached in the last chapter of the book.  It is starting to make sense to me, but the proof is whether I am making the description clear enough to everyone else.

Hi Willy, I am not sure when Giffard's book was written.  Crawford mentions one written in 1910, but says injectors were well developed 50 years before then.  I can only admire those pioneers who made these things work without the benefit of even poor lectures, no accurate steam tables and machinery that was not as accurate as we all have in our workshops.  It is interesting that you mention sparks, it is what I meant yesterday when I said a complete energy equation has to include terms for every form of energy, electrical, magnetic and so on.  (Please don't ask me what the term would be, I will leave that to the electrical engineers on the forum!). Sparks are also a hazard in a hydrocarbon plant when steam issues from a high pressure hose.  Also in diesel fuel transfers at high flow, or Av. Gas where earth straps are needed for vehicle filling.  It comes from electrons being stripped off metal atoms as a result of the high velocity flow, and builds up high voltages which discharge causing the Sparks.  Filters are a particular problem.  Can even happen in a large tank with high flow of a fluid with low enough conductivity to not self discharge.  But it takes energy, and is another factor contributing to the unaccounted for "losses".

Continuing with the injector inlet nozzle, calculation of the velocities, pressures and temperatures for ideal gases requires the gas tables, a standard table, a bit like the steam tables but describing compressible gas flow.  I only have an extract of these tables for air, so I would have to calculate my own for steam.  It's a much simpler job than steam tables, but more time than I want to spend at the moment.  So I have done some calculations to get the flavour, but will just describe the results rather than give potentially misleading figures.

So far I have been discussing the nozzle in terms of ideal gas, which of course saturated steam is really not.  However the first law, continuity and the entropy definition all still apply, so we calculate velocity from the change in enthalpy, and assuming constant entropy.  It is understood that for saturated steam, the critical pressure ratio that produces sonic velocity is 0.577, compared with the 0.545 for an ideal gas.  So the throat pressure is not quite as low as for isentropic expansion of saturated steam.   Using the assumption of no change in entropy, with the pressure from the critical pressure ratio, we can use the steam tables to get the temperature and enthalpy at the throat.  Then, while the maths is a bit obscure, it is also possible to calculate the downstream steam conditions.  I expect there is still a shock at the outlet and subsonic steam flow after the nozzle.  The exiting steam is quite wet, and while there are some delayed condensation effects due to the velocity not allowing equilibrium to be reached, I expect that the condensation occurs quite suddenly with the shock.  The end result is a jet of wet steam with the gas phase and the condensate droplets subsonic but travelling a very high velocity and at very low pressure.  All this can be demonstrated using the steam tables for the steam properties and the normal shock equations for compressible flow.

The discussion so far is still assuming adiabatic expansion, also called isentropic expansion, or ideal expansion.  But the second law of thermodynamics says that for all real processes, the entropy will increase.  Essentially meaning there will be losses.  The energy used to create those sparks for example, and also friction.  This means that the change in enthalpy will be less than predicted by assuming ideal adiabatic expansion.  Nozzles of a reasonable size, as would be encountered in full size practice are quite efficient and the enthalpy drop can be 90% or more of the ideal expansion, and this results in velocities approaching 95% of that predicted.  However as usual, friction is more significant in small models and Crawford suggests the velocity might be only 80%.  I don't think he has data for this, but makes this assumption when calculating flows and pressures for his model injectors, so it is probably near enough for our purposes.

The pressure at the outlet of the steam nozzle is determined by the water inlet, and is quite capable of lifting as necessary.  Once water reaches the immediate exit area of the steam nozzle, the pressure depends on the height being lifted and the resistance to water flow, but will be quite close to atmospheric pressure once water is flowing.  The sonic nozzle flow is not affected by the exit pressure, so long as it is below that critical pressure at the nozzle throat.  The pressure just inside the nozzle exit is determined by the isentropic flow equations (which give a very low pressure) but probably a little higher for real flow conditions.  The sonic shock wave at the nozzle exit then brings this pressure up to the external pressure determined by the water inlet.  Water flows in, driven by the difference between atmospheric pressure at the water tank surface, height differences and friction as the water flows.

I hope that makes sense, and is not too much for one session.  Obviously not enough to design the injector, but I am aiming to go far enough to understand what is happening in the nozzle, and perhaps giving some direction to help direct any experimentation.

Thanks for dropping in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Stuart on November 01, 2017, 12:35:23 PM
still reading  :cartwheel:  and slowly understanding

yes I have to be careful with those meds no drinking  :DrinkPint: at all been on them now for over ten years. thank you for your comment


Stuart
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 01, 2017, 12:39:43 PM
Hi MJM, Here is the page about Henfi Giffard...........1852  !!
Willy.
Title: Re: Talking Thermodynamics
Post by: Zephyrin on November 01, 2017, 11:03:17 PM
Hi,
The patent for the injector by Henri Giffard dates from 1858, and the first locomotive equipped with an injector dates from 1859, the success was immediate, and the fortune of Giffard made, and entirely given to university libraries!

Nice to see the reference to Henri Giffard; a very talented engineer and a fearless experimenter.
 a few years ago, I did a study on the patent of the steam engine that powered his dirigible airship of 1852, to make a model of this aircraft...I was very impressed by the lecture of this patent, more a general dissertation on the feasibility of a steered navigation into the air...
  I did a model of the steam engine and boiler (1/10th scale), but the blimp (not my part of this project) was not done according to the Giffard model, the project was changed and my engine never took of, alas...
This model engine, made according to the words and musics of Giffard, mainly calculations in facts,  works perfectly.

[youtube1]https://www.youtube.com/watch?v=7la96uyP0Yo[/youtube1]
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 02, 2017, 02:06:37 AM
Hi Zephyrin,  lovely to see this, thanks and also lots more info on these injectors....cool..
Willy.
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 02, 2017, 12:29:59 PM
The injector mixing cone -

Hi Zephyrin, thank you for that extra interesting information about Giffard, and for the video of your beautiful model of his engine.  This thread definitely needs a video or two, so that is more than enough of a connection with the topic.  Also really interesting strobe effects with the video of that propellor.  I wonder if you can use that to estimate rpm.

Last time I was looking at the expansion of saturated steam and the steam cone outlet.  The book I prefer to use really left the subject a bit early for the purposes of this discussion.  It almost felt like mid sentence.  So I dug out the book I used in my university days.  Obviously a lot closer to the age of steam, and it has a quite  clear section on expansion of saturated steam.  The high velocity of steam throughout the nozzle means the steam does not have time to condense in accordance with the normal equilibrium pressure, even though that normally happens very quickly, and continues to flow in a supersaturated or sub cooled state just as though it was superheated.  This means the normal compressible flow equations can be applied with reasonable accuracy.  Then at the exit shock wave, the steam condenses along with a jump in pressure and entropy across the shock.  Across the shock, conservation of energy and mass still apply, but not constant entropy so the adiabatic flow assumption must be abandoned.  It is also difficult to calculate the exact velocity after the shock, as the exact position in the diverging part of the nozzle varies with the difference between the pressure in the downstream passage and the isentropic pressure for the flow area compared with the throat.  It appears likely that there is a juggling act between the ideal exit area for the supply pressure and the pressure at the water inlet for the designer and experimenter.  The bigger the pressure rise across the shock, the lower the Mach number and hence velocity after the shock, but the lower the potential minimum pressure for the incoming water.  This may have an influence on how much lift the injector can provide, as well as the range of inlet pressure for satisfactory operation.

The important point however is that conservation of energy and conservation of mass both apply across the shock.  This means that at least in principal the steam conditions after the shock can be determined. 

The steam nozzle exit is of course the point where water enters.  Now water flows in, driven by the difference between atmospheric pressure and the pressure at the steam nozzle exit after allowing for friction losses in the flow passage, so very close to, or a little below 100 kPa.  The first law equation requires us to add the mass flow times the enthalpy plus kinetic energy terms for each of the two incoming streams with the total equal to the total mass times the enthalpy plus kinetic energy of the total flow into the next section of the injector.  The whole secret to injector design is to make sure the resulting mixture enthalpy means the whole lot is in liquid phase.

The first law equation simply adds the enthalpy and kinetic energy terms but gives no guidance on the division between enthalpy and kinetic energy in the resulting mixture.  This is where another fundamental law of physics comes in, the law of conservation of momentum.  Now conservation of momentum is the principal behind Newton's second law.  You have heard of that before, even if you did not know about conservation of momentum.  The quantity that makes those masses continue in a state of uniform motion is called momentum, and its magnitude is equal to the mass times the velocity.  So increasing mass or increasing velocity has the same effect on momentum.  Momentum is a vector so has direction, and can be applied independently on three perpendicular axes.  Conservation of momentum governs collisions of billiard balls, cars and trains, and steam condensate with water droplets.  It is totally analogous to the angular momentum we discussed in relation to flywheels.  You can add the product of the steam mass times its velocity, and the water mass times its inlet velocity, and the total equals the product of the total mass times the mixture velocity. 

So in that busy region between the steam cone and the mixing cone entrance, steam in droplets from the condensation in the exit shock at very high velocity, collide with droplets of the incoming water which is being atomised in the high shear area where it meets the steam, and collisions between the condensate and the water result in the droplets combining and travelling on at a velocity determined by conservation of momentum, which is much slower than the steam velocity.  Now remember that energy equation which still applies.  Kinetic energy is mass times velocity squared, so that collision results in the steam giving a significant portion of its energy to the incoming water, and in a properly proportioned injector, all the steam is condensed.  At some point in the mixing cone, all the steam is condensed and there is solid water in the converging passage of the mixing cone, in particular, the steam should be all condensed before it reaches the side vent to the underside of the ball valve of the turret.  The water stream is well below sonic, which in liquid is well above any practical velocity.  In a converging nozzle, the pressure decreases and the velocity increases so that a high velocity stream of water in liquid phase exits the mixing cone and flows directly into the into the inlet flare of the outlet or delivery cone.

That is probably enough for another session.  Before we leave the mixing cone, I need to look at that side vent and the ball valve, and perhaps a little more about that water inlet passage, but that can be next time.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: Stuart on November 02, 2017, 12:43:52 PM
Thanks that’s going to need a couple of re reads

Stuart
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 03, 2017, 11:14:44 AM
Mixing cone continued -

Hi Stuart, take your time reading, I hope it is eventually helpful and informative.  I will keep going in order to try and keep all the posts on the topic together, but don't hesitate to post a question when you get to it, it won't surprise me if my explanations sometimes need a bit of clarification.

Last time I was talking about the mixing cone.  I did not tackle the side vent to the ball turret or the drain connection, as it seemed better to leave those for a separate post.

All the discussion so far has been assuming an injector already working in a steady flow condition.  Now it is obvious that despite the injector taking steam from the boiler, and returning the steam to the boiler along with water picked up along the way, it must drop to a bit below atmospheric pressure in the middle to allow that water to flow in without the help of a pump.  Just to add emphasis to this point for the sceptics, there is that drain connection to the atmosphere which proves beyond doubt that the pressure is only at atmospheric pressure at that point. 

You might also be wondering how the process starts in the first place.  You see, once the steam is flowing in the steam nozzle, the pressure falls with subsonic flow in the contracting section and continues to fall with accompanying supersonic flow in the diverging section.  However if the pressure is the same throughout, there is no flow, so no pressure drop.  If we just open the steam valve without any outlet to the atmosphere or other low enough pressure source, the pressure would be the same throughout before any flow condition was established, and flow stops.  That is where the turret and the drain come in.  They establish the initial pressure drop greater than the critical pressure drop which causes sonic velocity flow at the throat, and once that sonic flow occurs, the expanding cone is the right shape to cause the flow to continue accelerating and the pressure to further fall to the nozzle outlet and converting as much as possible of the energy in the steam to kinetic energy.  At the outlet nozzle, the pressure is less than atmospheric, so the water flows in, mixes with the steam and the energy balance means all the steam condenses, providing it was not too hot in the first place.  It all happens very quickly, and I don't even know if that initial flow starting mechanism means there is a short discharge of visible steam from the drain connection, or if it happens too quickly to see.  But that initial passage open to atmosphere is essential to start the process. 

I don't know if the ball valve passage also comes into it at this stage, or just the drain passage.  As the steam takes some time to condense, it is possible that both form part of the outlet path for the steam.  At the end of the mixing cone, there is a high velocity jet of solid water, and this water is proven beyond doubt to be at atmospheric pressure, by the presence of that drain connection located between the end of the mixing cone and the delivery cone entrance. 

Now unless everything is proportioned and controlled perfectly, it is possible that there is more steam than the amount necessary to pick up all the water.  The volume of even a small quantity of steam is so great that it would block the flow through the nozzle.  But the side vent under the ball valve conveniently lets any excess steam escape.  Almost certainly some energy effects to be explained there, but they defeat me for the moment.  The outlet from the turret goes back past the outlet end of the nozzle and out to atmosphere possibly signalling that the steam supply should be cut back a little.

On the other hand, if the steam picks up more water than the delivery cone can handle, the excess water also has an escape path to the drain at the entrance to the delivery cone.

That seems like a suitable place to stop for today.  Tomorrow the delivery cone.

Thanks for dropping in,

MJM460

Title: Re: Talking Thermodynamics
Post by: Stuart on November 03, 2017, 12:42:01 PM
Thanks for the brain fodder update , yes it all is falling into place . They are indeed a simple but at the same time a complicated bit of kit

But I have a question for you what produces the distinctive sound when they pickup and operate

Eg. The gulp at pickup and the singing sound when they are operating ( sound like a kettle before the boil)

Thanks for your time

Title: Re: Talking Thermodynamics
Post by: MJM460 on November 04, 2017, 11:16:26 AM
The injector delivery cone -

Hi Stuart, glad it is falling into place.  It is usually helpful to understand the theory, even though it can still be another matter entirely to design and manufacture a physical item with appropriate tolerances to make it all work.  And injectors are definitely an illustration of this point. The actual range of conditions where everything falls into place are very narrow, particularly in our very small scale.

The equations show there is a sonic shock at or just inside the outlet of the steam cone when the injector is in operation.  This is similar to the sonic boom caused by supersonic aircraft.  The sound of the injector running is almost certainly due to that.  Similarly, when the injector is first started, any initial air is carried away by the steam to the drain line, then as the water approaches, droplets are atomised and finally the system drops into full condensation.  When an initial gas or two phase flow collapses into full condensation in full size, it is accompanied by loud noises and violent shaking forces, quite frightening if you are under it when it happens in a large pipe, and that initial gulp you hear with an injector is again almost certainly the small scale equivalent of that sound.  It is the bubble collapse due to condensation that does it, it is usually much less severe if you are just displacing air with water when condensation is not a major factor.

I think we are now up to the outlet cone, at last.  Perhaps the most spectacular but also the simplest bit to explain, even if it is a bit counter intuitive.  The outlet cone has a rounded inlet, a bit like the mouth of a trumpet, rather than a sharp edge.  The rounded inlet blends into the throat which is the smallest diameter of the cone, then the throat blends smoothly into a diverging, or expanding cone shape, and finally a slightly rounded enlargement at the outlet end. 

For the outlet cone of a horizontal injector, the change of elevation is zero, there is no mechanical work output, we assume there is no significant heat transfer, so the first law reduces to:
(Specific volume, v) times (P2 - P1) + (V2^2 - V1^2)/2 = 0

This is the often quoted Bernoulli equation.  It basically says the sum of pressure energy changes plus the kinetic energy (velocity) changes is zero.  In other words, pressure can be exchanged for velocity, and most importantly for the outlet cone, velocity can be exchanged for pressure.

We can also apply the continuity equation, or conservation of mass, basically density times flow area times velocity is a constant.  When we apply this to a converging conical passage, the velocity increases as the passage area reduces, but equally, if we apply it to a diverging flow, and further, if the density is constant, as it is for a liquid, then as the passage flow area increases the velocity decreases.  And the Bernoulli equation, which is just a different statement of conservation of energy, tells us the pressure will increase.  But I think the extent to which this applies in the outlet or delivery cone probably surprises everyone when they first encounter it.

The fluid in the outlet cone is that jet of liquid from the mixing cone.  The very low to negligible compressibility of the liquid is the reason for the difference in behaviour between the steam cone and the outlet cone in which you will remember, the flow continues to accelerate and the pressure continues to drop in the diverging section of the cone.

Now to put some numbers in the Bernoulli equation we need to remember that the pressure units must be in N/m^2, or Pascals, while we conventionally use kPa for pressure.  1 kPa = 1000 Pa, so we have to multiply the pressures by 1000, then the dimensions are all uniform as required.

Continuing the assumption of the boiler pressure of 600 kPa, and remembering atmospheric pressure is approximately 100 kPa, and assuming a delivery velocity of 10 m/s for a reasonable size outlet tube, I found the inlet velocity has to exceed only 31 m/s for the outlet pressure to exceed the boiler 600 kPa.  These figures are just to show how the formula works, Crawford suggests that about 16 % more velocity is necessary to overcome the losses in a real cone.

So our high velocity steam from the inlet, (remember it is subsonic after the shock but still very high velocity), has to combine with the right quantity of water in the mixing nozzle to give at least 31 m/s for the liquid jet flowing straight at the delivery cone trumpet mouth.  The rounded entry gathers in any tendency of the stream to spread, so that stream passes though the small diameter throat into the expanding cone. 

Now fluid mechanics demonstrates that there are limitations to the shape of the cone for the flow to  follow the expanding cone wall and give the full pressure increase.  My memory is that the included angle of the cone must be less than about 15 degrees.  Otherwise the flow separates from the wall,    The energy is dissipated in turbulence and the pressure does not increase as anticipated.  It is interesting to note that Crawford in his book recommends less than 13 degrees, so this seems consistent with what I had previously been taught.  He also points out that with a very long cone, the friction forces become significant, so he also recommends a minimum angle of about 6 degrees. 

Now there many gaps and clearances that affect the flow of water and steam and the working pressure range of any given design, and I have not attempted to explain any of these.  In reality, I suspect many of them are arrived at by careful experimentation rather than theoretical analysis.  Probably the most critical shapes are those transitions between the nozzle throats and the expanding section which must take the form of a very smooth curve.  Similarly, the end of the steam cone.  I have not discussed the outside shape of the steam cone, where the water flows, but this edge needs to have a quite sharp edge to minimise any turbulence as the water flows past.  However I hope I have given a useful idea of how thermodynamics apply to injectors and help us understand what is going on inside.  We can then use the understanding to help guide the manufacture, trouble shooting when things do not go right and perhaps even help with the experimentation for a new design.  Though I suggest that is not a good place to start if you have not built previously to existing successful designs.

That is a good place to finish this topic, and return to our discussion of pipe diameters for steam and exhaust lines.  And check our application of Bernoulli's equation to those sudden changes of size.  However, if there are questions when the last few posts have been digested, please just ask them.  As I have said before, I am unlikely to have explained it all so clearly that there are no questions at all.

Thanks for following along,

MJM460

Title: Re: Talking Thermodynamics
Post by: Stuart on November 04, 2017, 12:43:17 PM
Thanks for your time in this very well explained subject , looking at my actual data for construction , I am pleases to say they do agree with your explanation , when I made them Eyes are a bit dim for the sizes needed for say a 10oz injector , but the thing was I made two and one worked and one did not ,they were both made to the same size as far as I could measure

Stuart
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 05, 2017, 11:18:52 AM
Injector issues -

Hi Stuart, 10 oz. per min is a seriously small injector.  Would you care to tell us all the throat sizes to give us an idea of the actual sizes you are dealing with?  I know what you mean about the eyesight.  I have often wondered if one of those USB microscopes would be useful for small figures and small parts.  I don't know if anyone has tried one.  To make two injectors and have one not work would be extremely frustrating, but shows how finicky they can be.  You did not say anything about what the problem was.  But I feel congratulations are in order for the one that works.  That is really well done.

I was going to move on, but after my comments about how understanding the thermodynamics can help in a practical way, I think I had better make a couple of suggestions about just what thermodynamics can tell us when things don't work so well.  I don't expect to uncover anything you have not already done, but it is worth trying to tie the theory and practice together.

Going through the thermodynamics as we have done in the last few days has certainly lifted my understanding of these fascinating devices.  The first thing I notice is that the ratio of steam flow to water flow is quite critical to the energy balance.  To much water and there is not enough energy to get up to boiler pressure in the delivery cone, while too little, and the steam does not all condense and the the uncondensed steam volume chokes the nozzles.  The flow of steam is totally defined by the throat area of the steam cone, the inlet pressure and the occurrence of sonic velocity in the throat.    The water is less obvious, it is controlled by the flow area in the annulus between the outside diameter of the steam cone exit end and the diameter of the inlet to the mixing cone at the point where the steam cone sits.  In such a small injector, there are some very tricky tolerances to maintain in both length of tapered cones and diameter.  To make two really identical would call for very high precision.

I assume they were both made with the same reamers, so the angles should be the same, with the potential variance mainly in the depth stop settings, however, we know the action of the injector requires both the steam jet and the water jet to expand radially as the flow proceeds through the expanding section of the two nozzles.  This radial expansion requires forces in the radial direction, and the available force is quite small.  So it is essential that the passage from the throat blends in a smooth curve to the expanding section, not a sharp angle at the intersection.  If there is a sharp break, the flow can separate from the walls of the expanding section, and the expanding part of the nozzle just dissipates the energy in turbulence instead of being converted into pressure.  Perhaps this is more fluid dynamics than thermodynamics but I am not picky about such territorial borders.  Similarly, the end of that steam cone needs to be quite sharp so the water and steam both flow smoothly off, and are not tripped into turbulence by a square blunt end.

The transition from the inlet of the cone to the throat should ideally also be a smooth blended curve, but it is not as critical as the junction to the expanding portion.  If the converging side of the throat involves an a abrupt transition, it just effectively makes the throat a little smaller, but I think less likely to make the whole device breakdown.

Apart from these issues, there are obviously many close tolerance dimensions that must all be within limits and it is well beyond my skill level to make those measurements.  The techniques look like they would have more in common with Rogers amazing work on those small injectors.  Words fail me as I follow his meticulous workmanship.

So please let us know what you have tried and what you have found.  There is plenty of learning in these things even when they are not as successful as we would like.  And mostly, remember the success of the one that worked.

Looks like return to piping will have to be deferred again,

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Stuart on November 05, 2017, 12:12:51 PM
the one that did work well it would not perform so into the bin with it


sizes are from DAG Browns book



Title: Re: Talking Thermodynamics
Post by: MJM460 on November 06, 2017, 11:50:16 AM
A bit of a recap on piping -

Hi Stuart, at least when it is in the bin, you don't need to waste time looking for where the elves hid it.  And the deck is clear for the next project.  Thanks for that drawing, it gives us all a good idea of the actual dimensions of a model injector and the amazing work you have done to get one working.    I think I will now have to buy the book.  I hope in the end I answered your initial question of how the water stream jumps across that gap.

I really did not expect to get so far on injectors, but it is amazing how much you can do by setting out the basic equations, and proceeding one step at a time.  In hindsight, I would have been better to defer that question until a more logical break.  On the other hand, that close look at the injector nozzles, involved a close look at the relevant forms of the energy equation, in particular Bernoulli's equation which applies to incompressible flow and then the differences when the flow involves a compressible fluid.  The injector is a dramatic demonstration of those equations, and I hope that everyone noticed the dramatic increase in pressure on the water flow slowing from a relatively modest 30+ m/s to only 10 m/s.

The obvious question, if that slowing can produce such an amazing pressure increase, why does it matter if we have a short reduction in diameter followed by an expansion back to a similar or larger diameter in our steam lines?  Won't the pressure just be restored?

Let's look a bit closer at what happens.  First, that injector delivery nozzle has a very carefully manufactured smooth transition to a gradually expanding flow passage.  That gradual expansion allows the the internal pressure to provide the energy for the radial velocity involved in this expansion, as pressure in a fluid acts in all directions.  This gradual expansion does of course involve length, and you can see this in the cross section of the nozzle.

When we have a sudden expansion in flow area, the pressure forces are not sufficient to make the flow follow the wall shape.  The flow mostly stays in a small diameter jet in the centre, and the energy is relatively quickly dissipated in turbulence and heat.  No pressure recovery, the pressure is all lost.

Similarly, when we have a sudden size reduction without any attempt to smooth the inlet into a bell shape or similar, the flow is forced to flow radially inwards, and is not able to immediately return to parallel flow at the new diameter and so actually flows through an effective passage diameter smaller than the actual reduced size.  And the energy required to suddenly impose that radial inflow has to come from somewhere.  When viscosity and shear force are taken into account, the losses are certainly more than anticipated based on the ideal flow energy equation.

The other big difference between our steam flow and the injector cones is in the density.

The steam tables give us specific volume, which is the reciprocal of density.  So you can easily look up the specific volume of the liquid water and vapour at the relevant pressure.  From this, we find the density of water is close to 1000 kg/m^3, and does not very much with pressure over the range we are interested in.  On the other hand, steam specific volume varies with significantly with pressure.  So we have a density of 0.59 kg/m^3 at 100 kPa and 3.17 kg/m^3 at 600 kPa.  All absolute pressures of course.

Using density in this case gives a more intuitive feeling for in the energy equation. 

P2 - P1 = density x  (V2^2 - V1^2)/2 / 1000. (The divisor of 1000 on the right hand side is because the equation requires pressure to be in N/m^2 or Pascals, while we conventionally use kPa for pressure to keep with more convenient sized numbers).

You can see that a density of 1000 instead of 3 makes a huge difference to the pressure change.

A factor which is specifically excluded when the energy equation is applied is the pulsating flow associated with our reciprocating engines.  Particularly in the cylinder port passages, the flow is zero at each end of each stroke, and is double the average flow at around mid stroke.  The energy needed to accelerate the flow through this range is not included in the energy equation which is based on steady state, steady flow.  The acceleration losses are also proportional to density, so become really significant in a liquid pump.  These acceleration losses are less prominent for steam flow outside the engine, as there is some averaging.  For example the steam supply can continue to flow while rebuilding the pressure in the steam chest when the cylinder valves are closed.  It is further evened out for multi cylinder engines where the pulse spacing precludes the zeros.

I previously recommended 20 m/s for the steam lines.  I have to admit this is more of a gut feel than solid science.  I am coming to the view that something higher does not result in big losses, so if your model will look better with a size smaller tubing it is probably OK, especially if you are happy with the performance.  I would be interested to see what sort of average velocities have been calculated for different models, and whether anyone has tried different tube sizes and have found any limitations.  Certainly small losses are not very important if you are going to use a regulator to throttle the pressure right down for slow running on display.  More important if you are wanting your engine to drive a heavy load at full throttle.

Industrial guidelines allow much higher velocities however as I have previously mentioned, lines are much longer, and friction is more important, normally flow is steady and overall pressure losses are still quite low.  Changes of size are made with forged fittings that have a relatively smooth transition of diameters, and except for reciprocating pumps flows are steady.  I should add reciprocating compressors also.  There, there is always a large volume pressure vessel on the inlet and on the discharge side of the compressor, as pulsation dampeners in order to keep those pulsating forces to acceptable levels.  Even then, it is important to properly design those pipes to avoid vibrations.

A bit unsatisfactory, in that I cannot give a clear recommendation for velocity limits.  I am probably very conservative for supply and exhaust lines, much less so for the actual cylinder port passages, where that pulsating flow cannot be avoided.  But it is very clear that the criteria for comparing pipe sizes is the average velocity, with knowledge that pulsating flow has greater losses than steady flow.  The required pipe sizes for a model can be successfully sized based on the average velocity in a successful model.

Probably should look at whether compressible flow is significant in our steam tubing before I leave the topic.  And as always, questions are always welcome, they are always excellent pointers to the areas I have not been clear.

I have been looking back to round up some previous unfinished topics, and need to return to Willy's boiler heat balance, and also back to engine efficiency which have both been left incomplete.   First, I will return to that heat balance, as I believe Gas Mantle might be showing us a new boiler in the near future, and I hope he will give us some information on how it goes before I return to engines.

Thanks for reading,

MJM460
Title: Re: Talking Thermodynamics
Post by: Stuart on November 06, 2017, 03:46:30 PM
Thanks

I posted a link to that book in post 406

He goes into great detail on how to and a brief  why they work , very detailed work from Dag , he even describes a test rig to prove your work , as well as how to sharpen a no.80 drill bit

The last one made my 🤯. A bit but a couple of re reads while Linda watches her soaps on the goggle box will sort it out

Thanks again for your time in putting together all this info

Stuart
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 07, 2017, 11:28:56 AM
Hi Stuart,

Thanks for the pointer to that link.  Sharpening a number 80 drill, about 0.34 mm, that would test the eyesight to say the least.  Overall your success is a very good advertisement for the book.

When looking at the injector steam nozzle, I made reference to a velocity of 0.5 times the speed of sound as the point where we need to consider compressible flow effects.  Also, that a pressure ratio of approximately 0.55 is the pressure ratio that gives the speed of sound at the throat of the nozzle.  The energy equation tells us that the pressure drop is proportional to V^2.  A little algebra and we can see that a pressure ratio of about 0.74 is where we need to start needing to use the compressible flow equations.

Now if we are running our boiler at say 450 kPa (approximately 50 psig) but throttle the steam to say 200 kPa (15 psig) to run the engine at a nice display speed, you can see we easily passed the 0.75 ratio which would give us 333 kPa so we need to use the compressible flow equations, but we are throttling to a pressure ratio of 0.44, which is more than enough pressure ratio to accelerate the flow to the speed of sound.  We have seen that the pressure drop in the piping is not very high, so most of the pressure drop occurs in the throttle valve.  The internal flow passages of the typical throttle valve are nothing like the carefully designed inlet cone of the injector, and the actual minimum area of the flow path is not easily defined, that pressure ratio means there is sonic flow at the narrowest point.  There are two implications of this.  First, the flow rate is no longer controlled by the down stream pressure, but by the effective flow area at the throat.  Second, there will be a shock wave where the flow exits that narrow point where the pressure jumps back to the pressure at the engine inlet.  That minimum effective flow area at the throat may not equate to any particular measurable area as the flow tends to squeeze into a slightly smaller diameter with a sudden contraction but the throttle opening does directly control the steam mass flow at that point.

Another situation where things depart from the simple flow concept is if we use a long very small diameter tube, usually called a capillary.  You might have seen one if you have ever looked behind a small refrigerator.  When the diameter is very small, the pressure drop limits to velocity to the point where the normal turbulent flow no longer prevails, and the flow is laminar.  Reynolds number is less than about 2000 for those interested.  In the refrigerator, this is called a laminar flow orifice, and is used instead of a throttle valve to drop the refrigerant pressure from the condenser pressure to the evaporator pressure where the low pressure liquid boils at a low temperature due to the heat flowing in from the storage compartment.  Yes, that will take a second read.  I don't believe there is any sonic shock in this case, as at any point, the pressure just upstream is always only slightly higher than any slightly down stream point.  The main advantage is probably less noise.  A conventional control valve is used in large systems and they can be very noisy.  I am not sure that we have a use for this effect in our steam piping, but it illustrates that part of the requirement to achieve sonic velocity is that the flow area must change in a specific manner if very high velocities are to be obtained, particularly downstream of the throat.  Just an interesting little side track.

Next time, back to boilers.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 08, 2017, 12:46:24 PM
Back to boilers again -

No wonder it is hard to reach a conclusion on those boiler tests that Willy conducted, I keep allowing myself to be distracted, so the discussion so far has been spread out a bit.

Back in post #343, Willy presented the results on two tests of his boiler, one with minimal insulation and one with extra rockwool wrapped around to reduce the heat losses.  Those tests are very interesting as with the electrically heated boiler all the heat flows in and out can in principal be measured with relatively simple instrumentation, thus making it an excellent test platform.

So I have, in the background been exploring the implication of those results.

You will remember that the two tests simply cover the heat up stage where the room temperature water is brought up to the equilibrium temperature and pressure ready to start steam production.  Continuing through steam production will have to wait for another time.  Willy's initial query was on how to shorten the heat up time, so let's see how the results help our understanding.

From the plot of temperature with time as the boiler heats up, and the element rating, the heat input can be calculated.  Using the mass of water used to fill the boiler, the atmospheric temperature and the steam tables, the energy stored in the water can be calculated, and from the dimensions of the boiler, together with standard data on the density and specific heat of copper, the energy stored in the copper can also be calculated. 

The first law of thermodynamics, also known as the law of conservation of energy, says energy is neither created or destroyed, so the balance of energy in minus energy out equals energy stored.

From the basic calculations on the heater we have the energy input.  The calculations for the copper and water are the main storage.  There is also heat stored in the insulation, however this is tiny compared with the water and copper.  And there are heat losses to the atmosphere.  So the obvious assumption is that the losses to atmosphere are equal to the difference between energy input and the recognised storage.

Well how did it all work out?  Willy did an excellent job of recording all the data.  The boiler initially had a 1/16 insulating layer under wooden plank cladding around the cylindrical shell, but nothing on the ends.  The second test was conducted with a good thickness of rockwool wrapped around, though unfortunately still not on the ends.  At this stage I expected that the the heat loss was equal to the heat loss not accounted for, and while the figure was nearly 33 %, which seemed quite high, it should be significantly reduced by the insulation.  With a reduction in heat loss of that magnitude, I would expect a significant reduction in heat up time.  Unfortunately the difference when the test was run was only about 1 minute in a heat up time still around ten minutes compared with the calculated reduction of three or four minutes.  It looked like time to investigate the potential errors in the results.

The biggest figure in the calculation is the heat input which is about three times the missing quantity.  This means any error in the heat input calculation is multiplied three times in the loss calculation.  So the first thing was to have a closer look at those heater elements.  I had calculated the heat input from the manufacturers data sheet for the elements, 500 watts.  However the rating was assuming 240 V.  It seemed worth first investigating the potential error in assuming the 500 watts applied in the boiler.  From these two figures the resistance of the element was calculated, should have been about 115 Ohms, but when Willy measured a spare element it was 113 ohms, which was still within the stated accuracy, but has an effect on the heat output.  Of course a digital meter is only plus or minus one digit at best so there is also some uncertainty in the measurement.  Then the data sheet said the rating was for the element at 20 deg C, and the wire is a Ni-Chrome alloy.  Resistance generally increases with temperature and the coefficient is available from various sources, it is 0.0004 per degree C.  I had all the data in a spreadsheet so I added some columns to take the resistance at 20 degrees, and assuming the element was 50 degrees above the water temperature (so the heat can transfer) I was able to add a simple estimate of the actual resistance as the boiler heated.  It made a difference of 8 ohms over the temperature range and that is enough to reduce the output of the two elements by about 30 watts between cold and hot.  Of course the lower initial resistance than rated increases the heater output so the opposite direction.

It is also worth looking at the next largest figure, the energy stored in the water.  This is approximately ten times the energy stored in the copper, making it the next most important source of errors.  The main measurement is the mass of water, 600 grams.  If this was measured on a digital kitchen scale it should be pretty good, but less so if a volume measuring jug was used.  A difference of 30 grams makes about 35 watts difference.  In addition, the temperature measurement is also crucial, and it takes only 1 degree to make a difference of 40 watts to the energy storage.

The possible errors were adding up but not easy to remove, so I resorted to trying the convection calculations for the heat loss. You will remember I presented the equation, not too complicated if you know the convection transfer coefficient, but the convection coefficient is not easy to determine.  I looked at the range of example problems in the text book and found that a value of 5 Watts/m^2.K might give an upper estimate of the convection heat loss.  Using this with an assumption of 4 mm for the insulation gave a value of only 28 watts for the convection loss to the air.  The unaccounted for heat in the initial calculation was 300 watts without the rockwool and about 210 with rockwool, assuming 113 ohm elements and 235 V at the element.  Clearly, heat loss to the air did not explain the number, the data needs a little refining if the heat balance is to come out.  In practice, we have to keep in context that we are not trying to carry out laboratory quality measurements.  So what do we get from the experiment?

First, it looks very likely that the 500 watt rating for the heater has to be modified by measurement off each element at a known temperature, and preferably our heater controller should include a voltage display of the voltage at the element terminals.  Then, it is clear we need to be as accurate as possible with weighing the water, which involves removing water from previous runs, so we know as accurately as possible the mass of water in the boiler.  Not worth stripping off the cladding at this stage, but in a new boiler, recording the actual copper weight is a good idea, but really a much smaller influence.  Certainly worth making an effort to insulate the ends, and for best results include as much insulation thickness as practical.  It does look like 4mm does not allow big heat losses and 8 mm would reduce the loss by about half again, but the 10 minutes heating time is looking like it is about what should be expected.  The measurement errors seem to be much more important than the loss to atmosphere.

Another way of checking the boiler performance is to continue the test until the water level is low but before the element cuts off on low level, and measure the water remaining in the boiler using a syringe to remove it.  There is no further storage in this stage of the run, but of course it has its own sources of error.  It is worth looking at these next time.

Thanks for following along.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 08, 2017, 02:49:57 PM
Hi MJM thanks for all this, I have more rock wool so i will do some more cladding ,could you advise o how thick it might need to be....is there an optimum thickness before the extra benefits of the insulation is lost by the conduction of itself ? I think you said that it needs to be tightly packed. Also the measuring probe has a small section of the diameter touching the boiler and the greater part touching the wooden batten which is not in close proximity to its neighbouring battens. Also when i filled the boiler some water spilled over the filling hole to drench the insulation and boiler and as it heated up this was evaporating and taking heat away...hence whips of steam coming from the boiler ! The wood is Mahogany and i remember seeing some tables for the insulation property of materials somewhere? including brick and stone ! When i do the next test i shall empty the boiler by turning it upside down and using a funnel to fill it and also weigh the water rather than using a meniscus inaccurate jug !! Presumably less water would boil quicker ? or does the more air cancel this out ??  Is the Answers/questions graph inversely proportional or parabolic ? btw !!! Just thought ..if the two elements are connected in parrallel does this change the overall resistance presented to the voltage Cct ? and the connections also might have their own resistances. And how would one accurately calculate the heat availability of coal/coke/wood Etc and all these data tests where done a long time ago before that part of the equations could be measured accurately ???
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 08, 2017, 03:52:57 PM
Hi MJM et al....Three pages from this 1897 book about wire drawing and ice/water/steam contemporaneous wisdom !
Title: Re: Talking Thermodynamics
Post by: crueby on November 08, 2017, 06:29:05 PM
Nice!

Went and looked, that textbook is available online (free of charge) at:

https://archive.org/details/atextbookonstea00unkngoog

it was scanned through the Google book project.
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 09, 2017, 08:57:19 AM
Back to Boilers -

Hi Willy, glad to have you back asking questions, I hope you enjoyed that little diversion into injectors.

With the rockwool, the important thing is to put some on the ends as well as the cylindrical shell.  Packed tight is ideal, but just do the best you can to attach it.  You can't have it too thick, though it is a case of diminishing returns.  If a layer is enough to cut the heat loss in half, it needs nearly four times thickness to halve it again, so optimum is about cost of the insulation vs. value of the fuel saved.  In this case, the small quantity of insulation was, I hope, rescued from somewhere at minimum cost, and the electricity savings on a ten minute heat up time, or even half an hours steam production, will be difficult to even see on your electricity bill.  But more insulation always reduces the heat loss except for the first very thin layer on a cylindrical surface, not the flat ends, where the increase in area for heat transfer actually exceeds the insulating effect so the heat loss at first increases.  However I did a few trial calculations and it looks like that effect is mainly seen at much smaller diameter than your boiler with very thin insulation.  I suggest if you can get somewhere near an inch over the cylinder and the ends, the heat loss will be negligible, and I am thinking in terms of perhaps 10 mm max for a more permanent solution when we get a better idea of the relative magnitude of the losses.  I think for a model, the optimum is a balance between performance and appearance, rather than cost of the heat lost.

If the probe is touching the shell it should be good, the insulating quality of the timber means there is minimal conduction away from the probe to affect the reading.  With no heat conduction, the temperatures are equal.  Especially with more rockwool over the wood.  The idea is to do something temporary to minimise heat loss to the atmosphere for a test run.  Accurate water quantity is important as the stored heat is approximately 90% in the water and 10% in the copper, with the insulation heat storage and heat loss really negligible.  If you can weigh the water, and then use a funnel to get it all into the boiler, it will improve the accuracy of the results.

Now spilled water is a double whammy.  First, some of the measured water is not in the boiler, but more importantly, the evaporation from the wet insulation increases the losses in the early period.  This can actually be seen in your results in the lower temperature rises in the first minutes.  I had assumed it might be storage in the brass of your pressure sheath before the water received much heat, but spilled water is a more likely explanation.

Less water in the boiler will heat quicker, because it needs less heat to get it up to temperature, but it will also mean less time with steam production.  Surely a longer engine run time would be better than a saving in heat up time.  More air is not really the issue, the specific heat at constant volume for air is about 720 J/kg.K, so the heat stored in half a litre of air starting at atmospheric pressure (about 0.5 g) is tiny compared with the heat absorbed by the water.

Parallel connected heater elements each get the same voltage and then act independently, and the power outputs add.  Of course the voltage source sees the parallel resistance but that has the same effect as adding the two currents.  Try playing around a bit with ohms law and power = V x I.  However any connection resistances reduce the voltage to the elements, as does any resistance in your power controller.  So the voltage really needs to be measured at the final connections to the elements.  The other issue is temperature, even Prof Jamieson says you should read temperatures to 0.1 deg C.  I don't know how he did it in those days.  But it might be worth trying to note the time when the temperature changes, as accurately as possible, perhaps using a stopwatch function on a watch or a tablet, so you can estimate on the basis of timing, intervals less than full degrees.  One of my meters actually has a 0.1 deg option, I think I had better start using it, but I would not be buying a special one.

A very different subject, the heat availability from fuels.  This is determined experimentally using an apparatus called a calorimeter.  Basically it  consists of a flask surrounded by a volume of water, and the whole lot is well insulated.  A small quantity of fuel is put in the flask with enough oxygen for complete combustion, and ignited electrically, these days anyway.  Prof. Jamieson talks about throwing in a lighted fuse and quickly inserting the stopper.  The calorific value of the fuel is then calculated from the temperature rise of the water, after it has all reached a uniform temperature, with allowances for the effects of the heat absorbed by the apparatus.  The data is continually being updated by people interested in such things, so a modern data source should have pretty accurate values.  Two values are normally quoted, the higher one is with all the combustion products cooled so water produced from the hydrogen in the fuel has condensed, and the lower one, when the water is not condensed, so it's latent heat remains in the flue gases.  A hundred years ago, the values were almost certainly less accurate than the data we have today.

That's a great extract from Professor Jamieson's book.  I thought it looked familiar, but on a closer look at the title, mine is a different book, see the photo below.  My edition was given to me by a work colleague when I bought my lathe.  It is a 12th edition from 1899, but the earlier edition prefaces were dated 1896.  I don't look at it often enough for its historical interest, but for daily use, I prefer a modern SI version as the theory has developed a little since then, and now much easier to understand.  I actually use my sons text book, he is in the power industry and doesn't have much immediate need for it.  It is SI, has useful data tables and has a very clear treatment of application of the first and second laws in all the interesting practical situations.  But it's a bit frightening to find that little bundle we brought home it seems not so long ago, is fast approaching his fiftieth!

Hi Chris, great to have you following still.  That Google project looks like a great resource.  The book should be well out of copy-write by now, but I bet the Professor would be delighted to know that people are still interested in his works, 120 years later.  He would also be interested to see the current status of his prediction that it was not physically possible to go above about 450 deg F, or to produce steels or engines to use it, and anyway, the pressures used by Watt and his peers were much more efficient!  But the book has a great drawing of Florian's little Cochrane boiler.

Perhaps a bit late to start a new topic now, but at least we are back on track with boilers.  I hope that I have at least adequately addressed the current questions.

Thank you all for reading,

MJM460

Title: Re: Talking Thermodynamics
Post by: Maryak on November 09, 2017, 08:34:34 PM
A very different subject, the heat availability from fuels.  This is determined experimentally using an apparatus called a calorimeter.  Basically it  consists of a flask surrounded by a volume of water, and the whole lot is well insulated.  A small quantity of fuel is put in the flask with enough oxygen for complete combustion, and ignited electrically, these days anyway.  Prof. Jamieson talks about throwing in a lighted fuse and quickly inserting the stopper.  The calorific value of the fuel is then calculated from the temperature rise of the water, after it has all reached a uniform temperature, with allowances for the effects of the heat absorbed by the apparatus.  The data is continually being updated by people interested in such things, so a modern data source should have pretty accurate values.  Two values are normally quoted, the higher one is with all the combustion products cooled so water produced from the hydrogen in the fuel has condensed, and the lower one, when the water is not condensed, so it's latent heat remains in the flue gases.  A hundred years ago, the values were almost certainly less accurate than the data we have today.

MJM460

Very true, the problem is that unless you know the source of the fuel and that each subsequent supply is from the same source and batch, ( a bit like dye lots for wool when knitting a jumper, different dye lot, slightly different colour), then it is not possible to accurately state the calorific value of the fuel. That is why most fuels calorific values are given as a range.

By custom the basic calorific value for solid and liquid fuels is the gross calorific value at constant volume and for gaseous fuels it is the gross calorific value at constant pressure. The word ‘gross’ here signifies that the water formed and liberated during combustion is in the liquid phase. The values given are approximate because many of the substances listed are not well defined.

e.g. diesel fuel is a term for fuels coming off the distillation tower within the range of the take off tray temperature and are a mix of the condensed gases within that range. Each time a new batch of crude is fed into the tower the results vary.

Whenever we refuelled we used a pensky martin apparatus to determine the flash point of the fuel received and hence the preheat temperature required for combustion. Some times higher sometimes lower but never optimum because mixing with the remains of the previous batch was unavoidable.

Regards Bob
Title: Re: Talking Thermodynamics
Post by: crueby on November 09, 2017, 08:42:03 PM
Quote
Very true, the problem is that unless you know the source of the fuel and that each subsequent supply is from the same source and batch,

True even for wood. At the fall run of the Lombard Hauler up in Maine, in the middle of the day they got into a batch of firewood from another source (different type of tree and different seasoning time) and immediately they noticed the pressure going up faster and the safety valve popped off a couple times till they cut back on how often they added more wood to the fire.
Title: Re: Talking Thermodynamics
Post by: Maryak on November 10, 2017, 05:19:27 AM
Quote
Very true, the problem is that unless you know the source of the fuel and that each subsequent supply is from the same source and batch,

True even for wood. At the fall run of the Lombard Hauler up in Maine, in the middle of the day they got into a batch of firewood from another source (different type of tree and different seasoning time) and immediately they noticed the pressure going up faster and the safety valve popped off a couple times till they cut back on how often they added more wood to the fire.

I was a volunteer engineer on a Murray River Paddle steamer a few years ago, (PS Marion), She was a wood burner. Too make our schedule we sometimes had to make the old girl roll up the carpet, especially going upriver. During those times we had two deckkies and the fireman flat out to keep a head of steam. The wood came from the shire councils tree lopping and clearing so it was often at best damp and new with plenty of sap which clarted up the firetubes. However its saving grace was the price............FREE. This meant finding a bit of sandy riverbank so we could heave a bucket or two of sand in the furnace to scour the tubes. We sent the workboat ahead for sand finding.

Marion would get through between 1 to 1.5 tons of wood per hour when pushed and around 0.5 tons per hour cruising.

I have to say that hand firing any boiler with solid fuel is not my idea of a fun time.

Regards Bob
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 10, 2017, 10:51:42 AM
Time for a cuppa-

Hi Bob, good to hear from you again.  Good point about the variability of fuels.  Coal is well known for this, but any fuel which is not a definable mixture will have variations in carbon to hydrogen ratio, variations in other compounds such as sulphur, which also contribute to the heat produced, and water, which because of its large latent heat also has a big influence.  I guess it would have been more accurate to say we know know more accurately the variability.  In the context of Willy's question, I wonder if Prof. Jamieson's method was accurate enough to be able to put figures on what every boiler fireman knows about this variability.  Diesel from a modern Refinery is not only a mixture from the primary crude oil fractionation, but is a added to by various process units that reform other fractions, not in themselves so saleable as fuel as they come, into fractions compatible with the diesel specification.  Even more so these days when there is less demand for the heavy fuel oils traditionally used in ships and and based boilers. Not being a chemical engineer, or a combustion specialist, I don't know much about the effects of those mixtures on the heating value.  But the properties are tightly controlled to make them acceptable for engine fuels.  And obviously many other properties such as flash point, viscosity and ash content also are very important to good clean combustion, and the boiler attendants need to know how to deal with them.

Would have been interesting to be on the Paddle steamer.  But I have not heard the term "roll up the carpet" in that context.

Hi Chris, I guess wood and coal are the two common fuels with the biggest variability, though probably all those biological based fuels have similar variability.  Wood density and moisture content varies greatly with the location of the tree, season, sap content and so on.  Water can be both free water absorbed in the structure from the air, or bound in water which has a loose chemical bond.  Wood workers have to allow for seasonal variation on dimensions as humidity changes throughout the year, and that is properly seasoned or kiln dried wood.  And I am told that brown coal is a compound designed specifically to absorb water.

I wanted to continue yesterday's topic with reference to the drawing in the book extract that Willy posted, showing a nice clear boundary between the liquid and vapour phases, gradually dropping as the liquid is vaporised.  There are similar pictures in many texts.  Of course, it is consistent with the thermodynamics concept of a reversible process that proceeds in infinitely small steps so that it can be easily reversed, a reversible process.  But have you ever considered what it looks like in a real boiler?

Recently, our electric kettle burned out.  Not very dramatic, it just refused to boil more water.  Now lack of water for tea or coffee in our house has something of the importance of a national emergency, so it was off to buy a new jug.  My wife was attracted by a new range of clear Pyrex clear glass sided jugs, and I readily agreed.  I had a very good use for that transparent jug.  And it not only made a cup of tea, but also provided a good supply of hot water which could be used to fill a boiler to reduce the heat up time.

The picture below shows the boiling in full swing, immediately before the auto cut off.  You can see it is anything but that quiet peaceful pool of the drawn pictures.  I put the ruler alongside to show the scale.  The still water level of the hot water once the bubbling stopped was 93 mm.  The rapid expansion of water when turning into steam, that specific volume change of around 1600 times results in buoyant bubbles which rise quickly to the surface, pushing liquid water ahead of it until it bursts free and separates in the vapour space above the average liquid surface.  The jug rating is 2400 watts compared with 1000 watts for the elements in Willy's boiler, however, the surface area of the bottom metal heating panel is about 132 cm^2, so a heating intensity of 18 watts/cm^2.  Willy's pressure sheath has an area around the element of about 63 cm^2, a heat intensity of 15.8 Watts/cm^2, based on the element rating.  We have seen that in place, Willy's element was actually nearer 900 Watts, but it is likely that similar considerations apply to the jug element.  You can see that the the heat intensity is quite similar, so the larger jug area for the larger element means the boiling action is probably not too different.  You can see in the picture the vigorous turbulence which keeps the water very well mixed, and rapidly moves vapour away from the heating surface, the reason for very high film coefficients for boiling heat transfer.  The good mixing also means that the temperature is very close to the equilibrium temperature.

Notice that the sides of the jug form a vertical cylinder so there is a relatively large surface area for the bubbles to disengage from the liquid.  In a horizontal cylindrical boiler, the nominal surface area for vapour vapour disengagement is much smaller than for the jug, or for a vertical boiler for that matter, unless the level is not much above the centreline.  With this degree of turbulence, it is easy to see why, if our boiler is overfilled, liquid water is carried over with the steam, and this can result in very wet steam at the engine outlet.  In a full sized boiler there are often liquid separators installed in the vapour space of the boiler so that water is returned below the surface, and dry steam goes out to the superheater.  These separators are not very practical in a model.  Some model and even full size designs have a steam dome, and the steam is forced to reverse direction on the way to the outlet, and this helps separate out the water.  Another possible solution would be to make an external separator which has a bottom outlet to return liquid to the boiler, though I am not sure if anyone has tried this.

The jug of course is open to atmosphere so the pressure is known.  In a boiler, I have seen articles where people raise steam at say 50 psig, and then throttle it to run an engine at quite low speed.  This has the effect of reducing the steam specific volume, and hence the size of the bubbles.  Steam at 100 kPa is 1.694 m^3/kg, while at 450 kPa,  approx 50 psig, it is 0.414, or about a quarter of the volume at atmospheric pressure.  This will surely reduce the turbulence and carryover, so is almost certainly the reason for doing it.  With a fired boiler this is at a cost of some efficiency, but carryover is a much bigger problem.  With my Meths fired boilers, the heat input is relatively low, and I just start with a level that does not cause too much problem.  A classic case where practical considerations are more important than the theoretical efficiency.


I hope that contributes to a better understanding of just what goes on inside the boiler.

Thanks for following along,

MJM460

I am having trouble posting so will try without the picture.
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 10, 2017, 11:01:15 AM
Sorry, I am having no luck with the picture, I will try creating a new reduced size version tomorrow, bit I need to abandon the effort for tonight.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 10, 2017, 03:34:14 PM
Hi Bob , wow thats a lot of wood  and is that a short ton  a long ton  a metric tonne etc etc ?!! anyway this works out at about 1 Kg every 6 seconds !!!
 Hi MGM more useful infant good comments about this intriguing subject. It would be interesting to measure the resistance of the elements at different temperatures ? But would be quite difficult at home . Does the distance increase because as the metal expands there is more space between the molecules of the metal ??

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 10, 2017, 07:38:23 PM
I have further reduced the picture size, let's see if this helps.

MJM460
Title: Re: Talking Thermodynamics
Post by: Maryak on November 10, 2017, 09:59:59 PM
Hi Guys,

Willy, long tons. Being an English trained Marine Engineer prior to the introduction of the SI system. Fuel and water carried in British ships was measured in tons and fuel consumption expressed in tons per hour, with water consumption being imperial gallons per hour.

The SI system was envisaged as the cure all for the conglomeration of measurement systems accumulated around the world. Sadly, nowhere has it fully replaced the systems used prior to its development. Those who have taken it up have all managed to tinker around the edges in one way or another.

Rolling up the carpet is a term used by merchant ships and refers to the bow wave generated at full speed ahead.

Regards Bob
Title: Re: Talking Thermodynamics
Post by: steamer on November 11, 2017, 12:13:54 AM
Quote
Very true, the problem is that unless you know the source of the fuel and that each subsequent supply is from the same source and batch,

True even for wood. At the fall run of the Lombard Hauler up in Maine, in the middle of the day they got into a batch of firewood from another source (different type of tree and different seasoning time) and immediately they noticed the pressure going up faster and the safety valve popped off a couple times till they cut back on how often they added more wood to the fire.


I have to say that hand firing any boiler with solid fuel is not my idea of a fun time.

Regards Bob

Amen to that!    Hand firing Sabino when the generator was running was WORK!

Dave
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 11, 2017, 11:29:34 AM
Hi Willy,  You are quite right to appreciate that it would be difficult to measure the resistance at temperature at home.  You would need a precise meter for the resistance, and a precise  temperature controlled oven.  However, if you have current measurement for each element as part of your controller, and voltage measurement at the element terminals, also on your controller panel, you can measure the actual power drawn, and calculate the resistance, at least as accurately as your meters are calibrated.  I am sure you would clearly see the change as the temperature rose.  And the resistance measured this way, and compared with the calculated change, might allow you to improve my estimate of the temperature rise between the water and the element.

I suspect that your second question was actually how does resistance increase.  I will come back to that in a moment.

Hi Maryak, thanks for that explanation, the bow wave makes sense now you say it.  As you know, the basic problem with the measurement systems before SI were mostly to do with the definitions of force and mass, where the many systems defined both of these with reference to separate arbitrary standards.  But from the basic laws of physics, force and mass are precisely linked by Newton's laws, specifically Force = mass x acceleration.  I remember when SI was adopted, the cynics said the new system was invented by politicians because they could not agree which country would get the obvious benefit of having their system adopted.  The truth was that all the previous metric and imperial systems had the same deficiency.  But having a rational system of units greatly benefits the whole future of physics.  Of course it is not so simple to make the change.  A new steam table can easily be published, and all the basic physical constants can all be recalculated, published and the benefits soon flow. 

However, as everyone on this forum knows, in other fields, it is not so simple.  Many of our tools are a fixed physical size, and most can not simply be adjusted to the new standards.  And they are very expensive to replace.  We would complain about having to throw out everything, but this cost flows into every area across the world.  And fixed tooling not only affects our machining operation, but also the operation of the mills which produce the materials we use.   So the practicality is that we will continue to use our pre-existing tooling, and the factories will continue to supply the same material sizes, more for extruded sections than rolled, but wire, pipes, hex sections, not to mention threaded products.  So there is really no simple economical way to make the change. 

I have started putting together my tooling relatively late, so chose to try and stick to mainly metric.  But it is not a complete solution.  I use copper tubes, and brass sections and buy some threaded products that are not available in metric, so have been obliged to include in my collection the appropriate items.  The 5C collets were an expensive decision for example.  I don't expect to see a complete change in my life time, if it will ever occur.  I know you know all this, but it never hurts to remind ourselves of just what the SI system is about, and why we continue to work with a bit of everything.

Hi Steamer, I bet the change to oil firing was not entirely unwelcome.

Now back to Willy's question about electrical resistance and why it increases with temperature.  I had to trawl back to some first year physics that lay almost forgotten in the far recesses of my mind to find a suitable reference and just make sure I was not mis-remembering.  The theory is something like this.  The crystal structure of metallic conductive materials is basically a regular lattice of atomic nuclei surrounded by their electrons, but they tend to be the atoms with only one or two electrons in the outer shell.  Now these outer electrons are so far away from their nuclei, at least in atomic scale, that they are only very loosely bound to the atom, and they tend to float in high speed random motion, much like a gas cloud, right through the whole structure.  The number of electrons and protons is always equal, another of those basic laws, so the resulting charge is always neutral.  Of course the electrons keep colliding with the heavy nuclei, or rather their electric fields interact, creating the effect of collisions, without ever going really close.  Now, when an electric field is imposed by applying a voltage to the ends of the wire, a slow speed drift is added on top of all that random motion.  My Physics book says for "normal" field strengths, that slow drift is only a few cm. per second.  Of course, no specific electron has to travel the length of the wire at that speed, when an extra electron is injected at one end by the applied voltage, the disturbance to the fields propagates at the speed of light, yes really, and a different one is ejected at the other end, so the net charge of the wire is always zero.  Now the interesting thing is that slow velocity adds to the energy of the electrons, so they give a little more energy to the nuclei in the collisions, which means the base crystal warms.  That is how the heat is generated in your electric elements.

From this, you can easily imagine that if you heat the wire, the protons will gain extra energy, so will vibrate faster, and will exchange more energy with the electrons in the collisions.  Because the nucleus is so massive compared with the electron, the electron always looses in these collisions, and this appears to us as increased resistance to our electrical current.

By the way, that slow drift of electrons is actually the opposite direction to the conventional direction ascribed to electric current.  They did not know the mechanism when the convention was adopted, only had two choices, and as Murphy was around even then, they chose the wrong one.

For most of the calculations I describe, the maths is quite simple, and I am quite happy to lay it out so you can follow, if anyone feels it would help them.  Not always so with the derivation of the formula of course, but the results are easy to use, especially with the power of a spreadsheet.  I am sure the author of the particular text on this one, thinks his readers could extend his example of heating due to electric current flow to this issue as well, but he was not as clear as I would need, so please don't ask on this one.  Of course I can refer you to the text if you are interested.

That looks like a good place to stop for another day,

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on November 11, 2017, 08:26:54 PM
The resistance goes up with the temperature in all metals and most other materials (as memory goes) , so you need a very special alloys if you want to get anywhere near constant resistance vs. temp. .... so your question Willy, really should be how did they manage to get a nearly constant resistance at different temperatures, as this has cost a fortune to develop / discover  ;)
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 12, 2017, 02:17:16 AM
Hi MJM  i like the photo of your kettle but to the uninitiated it might look like it has ice cubes in it  innit !! I have been reading Jameson and also have an  1897  Twelth edition  and this is much enlarged and almost twice as thick as my other book ! Also on page 72a he says ....Now ,let 1Lb of steam at a temp of .........    I am trying to imagine a Lb of steam ,and how would you measure it and contain it ??...........Hi Admiral,  If you heat up a wire to a temp that it suddenly melts away then the resistance becomes infinity !!!However just before it melts apart what is the resistance then ?? you don't have to actually answer this as it is a bit hyperthetical ..perhaps.!! I do find this thread increasingly informative and fascinating.btw.....Also there are lots of linear scratch marks on the inch side of your ruler...??
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 12, 2017, 02:23:36 AM
HI MJM ...when i enlarged the photo all the scratch marks disappeared ?!!!!!!Also in your book it looks like it was given away with compliments ..so no problem with reselling borrowing or copying then ?
Title: Re: Talking Thermodynamics
Post by: Steam Haulage on November 12, 2017, 10:12:20 AM
Hi MJM et al.

I have been intrigued by this thread from the very beginning. Being currently forced to relax and take it easy  I have found time to think more clearly about the work which is being done by the contributors.

As someone who has built up a healthy scepticism about theory unsupported by practical experiment I am pleased you are undertaking some work to support your assertions. On many occasions I have worked to attempt to confirm academic conclusions at the lab. bench. As is often the case Madame Curie's experience has been replicated. I might add that I have worked in some of the best commercial labs available and with persons much more qualified than myself. Some times the simple question 'Have you considered all the variables?' can be an eye opener.

However returning to practical application has anybody here looked at studied the papers read at the I Mech E by George Jackson Churchward? He carefully worked to produce some of the best free steaming boilers available.

Jerry
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 12, 2017, 11:27:38 AM
Hi Admiral DK, I am not sure of the mechanism in the negative coefficient materials.  Almost certainly much expensive research, probably supported by some high level physics theory somewhere along the line.  I wonder if they are semiconductors or something.  At least the mechanism for the normal positive coefficient was well enough understood long enough ago to be included in my 1961 text book, so I was able to look it up to check my memory.  It's not really necessary to have a constant resistance in the heating elements, the purpose of the calculations was simply to see if the change in resistance would explain any significant amount of the apparent heat losses in the boiler test.  It certainly helped, but not enough, so there are obviously other discrepancies to be tracked down as well.

Hi Willy, I am glad you liked the picture, I will get back to that in a moment.  To help you picture 1 lb of steam, for any pressure and temperature, the steam tables will tell you the volume.  A bit harder if you have wet steam, then you also need to know the dryness or another property.  It will depend on how the experiment is proposed.  How did they measure it for the tables?  Well they need a well insulated apparatus, so steam does not condense while they are measuring.  One way of achieving this is to submerge it all in a water bath, with the temperature controlled to the required value, then there is no heat transfer to or from the apparatus.  Possibly a piston and cylinder device so the pressure can be controlled to the required value.  Personally I just leave it to the authors of the steam tables, and have a copy handy.  I find that more convenient than an online copy.  Though that spreadsheet from Ohio that I referred you to recently might come in useful, especially when I need some values in a spreadsheet.

Glad you are finding the thread informative and fascinating, in the end, that is what it is all about.

Hi Steam Haulage, glad to have you following along.  I found your reply just as I was about to post, the the rest was already written before I added this.  The theory is not new to me, it was a basic tool for my job as a mechanical engineer in the oil industry.  The theory works very well there, with calibrated instruments, full sized turbo machines and sophisticated computer programs, but I have long been interested to know how far I could apply this well known theory to our model engines.  When it appears that I am feeling my way along, it is because I am exploring how to apply the theory as I go along. I don't actually have all the answers before I start.  Sometimes this shows!  Churchward and others are the pioneers who laid the foundations for both the science and the  practice, and one can only admire the intellect and ability they applied to their work, and what they achieved without the benefit of all the information and calculating power we now take for granted.  His papers would be most interesting to read, but there is not time for everything.  So my models have thermowells for temperature measurement, and my multimeters have thermocouples as well as voltage probes.  I have a non contact tachometer, and need  a suitable pressure gauge or two and to build a suitable torque measuring device.  It's on the list.  Oh, and a digital scale from the kitchen, and a watch for timing.  But these simple tests provide practical figures to use in the calculations to keep them well grounded.  I tried on my own to just do the calculations, but it did not seem satisfying, but I am really enjoying sharing my journey through the theory, and it is even more interesting and useful when it is guided by the questions readers ask.  They make a huge contribution to the direction of the thread, to my enjoyment, and I hope to theirs.  The questions have taken me in interesting directions I never would have thought of.  And they are all very welcome.  Just a little background to help you see where I am coming from.

Now that picture Willy commented on.  There are no scratches on the ruler, nor in the picture on my iPad that I used as the attached file for my post.  Nor is it on the forum post when I open it on my iPad.  But when I look back to the computer where I used photoshop to reduce the file size, some copies also have that scratch like appearance.  I am sure it is not your computer, I wonder if it is some interference pattern between the lines on the ruler and the compacting algorithm, somewhere between the jpg algorithm and the file size reduction procedure.  Perhaps even the ruler or camera were not quite vertical.  The ice cube appearance is interesting.  The camera shutter speed was 1/60 sec, possibly too slow to freeze the motion, perhaps I should try the flash.  I suspect the ice cube effect is a result of the bubble boundary movement while the shutter was open.  An illustration of how the experimental method can interfere with the experimental results.  I will try a few more pictures.

I hoped the picture would give an idea of the extreme turbulence in the boiler when it is generating steam.  The heating plate is at 63 mm on the ruler so the still water depth is about 30 mm.  You can see liquid rising over 25 mm from the surface, pushed up by the rising vapour.  As it falls back, more rises.  When I look through from opposite the handle, the bubbles appear to rise up each side and collapse back in the middle.  No surprise that the liquid and vapour are well mixed and can be assumed close enough to equilibrium temperature.

Now if you imagine this in your horizontal cylindrical boiler, it's not hard to see why you do not want to start the boiler with the level too high.  Our engines do not run too well on liquid, and the steam needs space to allow the liquid and vapour to properly separate, or disengage.

When we first open steam to the engine, we can expect some condensation as the initial steam heats up the mass of material in the engine cylinder and steam tubing.  If you weigh the cylinder, preferably during the build, you don't want to disassemble a completed engine for this purpose, you can calculate how much heat is required to heat the cylinder to steam temperature.  The steam tables will allow you to calculate how much steam has to condense to provide that much heat.  When the engine is already assembled, very normal if you did not have this idea in mind before the engine was complete, you can still make a reasonable estimate of the cylinder mass from the dimensions or as a proportion of the total engine weight if that is more practical.  You have a fair idea how little it really is if you collect the condensate from the cylinder drains and exhaust.  With a simple superheater in a fired boiler, the condensate normally stops as you would expect, quite quickly,  and the engine continues to run on dry steam.  Of course, if you have no superheater, even dry saturated steam from the boiler, when it is expanded to atmospheric pressure in an adiabatic engine becomes a wet exhaust steam, as we saw back in post #363 (on page 25 of 31 with my forum settings).  Perhaps it would be a good time to return to that point, and see what it means for a real engine.  How much liquid could we expect from the expansion of dry saturated steam to wet exhaust in a real engine?  That will require a whole post, at least.  So I will have a go at starting that next time.

Thanks for your interest,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 13, 2017, 01:43:14 AM
Hi MJM do you get 1 Lb of steam from boiling dry 1 Lb of water ? as i am still a bit confused about this ?? Thanks.....
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 13, 2017, 06:24:34 AM
Conservation of Mass? -

Hi Willy, definitely yes, a pound of water, when completely boiled produces a pound of steam.  That is the implication of the principal of conservation of mass.  I should strictly be saying pound mass, or lbm. to clearly differentiate between mass and force in the lb.-ft-second unit system, but we are not talking about forces here, only mass, so I hope no confusion.

The confusion is common enough, when we add sugar to our tea, does either the volume or the mass stay constant?  The change in either quantity is not enough to clearly see.  But in turning liquid to vapour, the volume expansion is clear to all, so it is a better example to illustrate the principal.  It is important to remember that volume is definitely not conserved in most processes, it may increase or decrease in any particular case, while, for all practical purposes, mass is conserved in any process.

  Early scientists had conservation of mass as a fundamental principal, but then along came Einstein, and all that had to go out the window.    He was a real spoilsport to the science community of the day, but it did explain some very puzzling results being observed in experiments at the time.  The problem is that famous equation, E = m x c^2, where E is energy, m is mass and c is the speed of light.  Everyone has seen that equation, it is the first one turned to when a film director wants to imply that there is some science going on, but what does it actually mean?  Putting it very simply it means mass can be turned into energy, and energy into mass.  Now the speed of light is a very large number.  And c^2 much larger again, so in a process where this equation applies, it involves a very large amount of energy for a very small amount of mass.  A deceptively simple looking theory, but controlling such processes is another matter entirely.  I know you like banging and crashing, but we would all rather you did not play around with that one.

There is a bit more to the theory, and it tells us that when objects move at any speed, you should make a correction to the mass dependent on the velocity, so the total energy remains constant, because energy really is conserved, so you have to correct the mass in accordance with its velocity.  Similarly, momentum is conserved, and again requires you to make that same correction to the resting mass.  And time comes into the mix somewhere.

In practice, we don't need to do this unless we are dealing with very high velocities.  For practical low velocities, say less than a few times the speed of sound, yes, the speed of sound is a slow dawdle compared with the speed of light, so for any velocity you are likely to deal with in your engine making, the correction is way smaller than anything you can practically measure.  I have looked out the formula for you, so you can calculate the change in your mass for a few speeds you might consider travelling.  The formula is

m = m0/sqrt(1-v^2/c^2)

In case the equation is a bit obtuse to some, I can put it in words -  mass equals mass at zero velocity divided by the square root of (1 minus v squared divided by c squared).

You will know the speed of light of course, about 3 x 10^5 km/sec, or 186,000 miles per second, and you must use the same units for v and c, then that ratio has no units and works with any units for mass, mass will always be in the same units as the rest mass, m0.

You will soon see that the increase for reasonable velocities, is very small.  Go ahead and try it, a good calculator or a spreadsheet can handle the numbers.  But I think any further treatment of the theory of special relativity should probably belong in a different thread, with a different author, and probably a different forum, along with general relativity.  But that much is enough to understand what I mean when I say conservation of mass is no longer considered a fundamental law of physics, but it is a useful enough approximation to help us solve problems.  You will have noticed that the analysis of most thermodynamic problems relies on the basic laws, conservation of energy, conservation of momentum, and conservation of angular momentum, and usually utilises the approximation of conservation of mass to lead to a useful solution.  Definitely worth knowing the limitations of this approach, so you have confidence in the results.  There are further conservation laws that apply instead of conservation of mass, I can look them up of anyone would like to know them.

Hmm!  Doesn't leave much room for a big post on exhaust steam, and besides, the theory of relativity might be enough to think about for a day or two, so I will give myself an early night and talk about exhaust steam tomorrow.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 14, 2017, 07:13:21 AM
Engine exhaust steam-

Obviously everyone so still thinking about Special Relativity, certainly a direction I did not anticipate when I started this thread.  I hope it was adequate to convince you that conservation of mass is a very good assumption.  Or perhaps everyone is anxious for me to get back to exhaust steam.

I need to refer you back to diagrams I posted in #268 and #363.  My word they are a long way back, shows how long I have been trying to get back to the subject.  For convenience I have attached them again below, providing of course the meet the new size limit.  If they are not there, it will be because they were just to big, and I will reduce them further, but it may be tomorrow.

First diagram was posted to show the adiabatic expansion of steam from three pressure Willy mentioned in his electric boiler tests, so no superheater.  Perhaps the clearest to see is if I use the higher pressure example.  So we were considering the adiabatic expansion of dry steam from the boiler to atmospheric pressure, which I assumed was the standard atmospheric pressure of 101.3 kPa.  The second law of thermodynamics says for adiabatic expansion, the change in entropy is zero.  So for the exhaust steam we have the pressure and entropy, which are independent conditions so they are enough to completely define the exhaust steam.  The results are noted below the graph on the diagram.  Then knowing the properties of the exhaust steam, at least when we do the necessary interpolation of the wet steam table, the work done by the steam during the expansion can be calculated using the first law of thermodynamics, which says the work done is equal to the change in enthalpy between the inlet and exhaust.  Again the answer is noted on the diagram, 254 KJ/kg.  Remember all this is for an ideal engine.  Now the second law of thermodynamics says the change of enthalpy for a real engine is greater than zero.  Unfortunately, it does not say how much greater.  So the exhaust condition of a real engine has higher entropy than the ideal engine, but the answer has to be determined by experiment.  An adiabatic efficiency has been defined as the change in enthalpy for the real engine divided by the change in enthalpy of the adiabatic engine.  You cannot use entropy change for this purpose as the change in entropy for an adiabatic engine is zero, so the calculation would involve dividing by zero, an operation which is not defined in mathematics.  If you try it on your calculator or in a spreadsheet you will always get an error message.  But enthalpy works for the purpose.  I have a vague idea that for a full size real engine, the adiabatic efficiency is found to be about 80%.  Not easy to know what it would be for a model engine, but smaller scale usually results in lower efficiency for various reasons, so more because I am lazy than any real justification, I will assume an appropriate answer is around 75%.  That means the enthalpy change, or work done in a real engine would be 75% the work done in the adiabatic engine, 0.75 x 254 = 191 KJ/kg.  Now you can see I am taking a leaf from Robert Hornby's book, that 75 % figure not only looks reasonable, but it results in the same outlet enthalpy and steam conditions as resulted from ideal expansion of 135 degree steam.  Not coincidence, or some subtle theoretical reason, just saves me the trouble or recalculating the figures.  So it is a completely reasonable assumption for our purpose.

So, when the steam expands from 462 kPa to 100 with an efficiency of 75%, the exhaust enthalpy is 2535 KJ/kg, and the work done by the steam is 191 KJ/kg.  But with those figures we can also calculate the exhaust dryness as 93.8 say 94%, somewhat drier than the adiabatic exhaust, but still 6% of the steam mass is contained in the liquid phase mist of that exhaust vapour.

It is difficult to know what that 6% would look like in terms of a water phase in the exhaust or would it be a mist.  It may the answer to Derek's conundrum of where does all the condensate come from in his engine.  A simple boiler test should give an idea of the boiler steam production.  Some collection of the condensate and measuring the quantity in a given time, would give a clue.  Remember that second law of thermodynamics, it says the change of entropy will always be equal(for an ideal engine) or greater than zero (for all real engines).  It cannot be less.  This means the entropy of the exhaust cannot be less than that that from the ideal engine or 92%.  So an expansion of steam cannot result in less than 92% dryness. 

Now you might wonder if heat loss from the cylinder could cause extra condensation, as that is part of the departure from ideal.  Certainly, when the engine is cold, and steam is first admitted, there is a lot of condensation during the warm up time.  So continued heat loss would contribute to the exhaust steam wetness.  Again we have to go back to those earlier heat loss calculations.   While the cylinder block is heating, heat transfer is only dependent on the steam condensing film coefficient on the inside cylinder surface.  Once the cylinder is up to temperature, further heat loss has to involve convection to the surrounding air.  We have seen how little this is over the whole boiler shell area, and even less when some cladding is applied.  The heat loss to air from the relatively small area of the outside of the cylinder will be very small.  The latent heat of steam is high enough, that the mass of steam condensed by heat loss to air will be tiny and very difficult to pick up experimentally.   The first step to explore the possibilities might be to eliminate the possibility of carryover by doing a short run, starting from a relatively low level, to see if the condensate stops after the initial warmup.

I asked earlier if anyone has applied or seen applied a steam separator to their boiler outlet.  It is interesting to notice that Thomas has built a steam separator on his square boiler.  I don't know if you are reading this Thomas, I don't expect that your separator, (you have called it a steam trap, but that is a matter of terminology), was installed as a result of this thread, so I expect you are thinking along the same lines for your electric boiler which also does not have a superheater, and you know that the engine expansion results in wet steam.  But also, it may reflect your previous experience with carryover from your electric boilers.

I hope that is all clear enough, if so, perhaps I should look at what happens if we start at at a lower pressure, perhaps more typical of an unloaded run at a show.

Thanks for reading,

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on November 15, 2017, 08:46:22 AM
Must have been having a bit of a brain fade yesterday, in two places even.  My comments about a steam separator were of course talking about a separator on the boiler outlet, or engine steam inlet, while the majority of the post was about engine exhaust.  Obviously the two ideas must be kept separate.  If there is a reason for a separator on the boiler outlet, it has to be about liquid carryover, and is a model scale approach to the installation of steam separators in the steam drum of a full size boiler.  I think they were even shown in one of Maryaks early posts.  It is not about wet exhaust steam.  My apologies for that, I should have focussed on the engine exhaust steam.

Second, I included two photos yesterday, then missed referring to the second one.  The reason it was included is because it included an engine expansion line on the T-s diagram for a real engine, that line from point 3 to point 4.  You will see it has a curve towards higher entropy rather than the vertical, constant entropy line of the adiabatic engine.  The scale of the first diagram makes it difficult to show this curve clearly, but it has the same form as the one shown in the second  diagram.  The higher entropy of the real engine exhaust steam means that the real engine exhaust steam enthalpy when it reaches the exhaust pressure is higher than it would be for an adiabatic engine.  The first law says the work done is equal to the change in enthalpy, so the lower the exhaust enthalpy, the more difference from the engine inlet, so the more work done by the steam on the piston.  The higher exhaust enthalpy from a real engine means the steam does less work in the real engine than in the adiabatic engine.

I hope that post makes more sense now.  So let's continue with that second drawing, at least it is on topic.  That second drawing yesterday was drawn to illustrate the process and test results for my own boiler, Meths fired with a superheater tube wound around inside the furnace.  I introduced this in post #268 back on page 18 of the current 31, and did all the calculations then, so I won't repeat them here.  But I would like to point out the differences between the electric boiler with no superheater and the fired boiler at quite similar pressure but with a superheater.  You can see on the second diagram, the expansion of the adiabatic engine point 3 to point 5, with the superheater outlet steam as engine inlet.  It meets the exhaust pressure very close to the saturated vapour line.  You have to look at constant entropy to say which side of the line it really is.  Now that superheater outlet has an entropy of 7.3018 KJ/kg.  If we look in the steam table entry for 101.3 kPa, sg = 7.3549 which is more than the adiabatic exhaust entropy, so the exhaust is wet steam.  Interpolation of that data for 101.3 gives us dryness = (7.3018 - 1.3069)/(7.3549 - 1.3069) = 0.9912. 

To give you an idea of the effect of atmospheric pressure, previously I did the calculation for 100 kPa which has a boiling point of 99.63 C instead of 101.3 kPa with a boiling point of 100 C.  The answer was 0.9905.  You can see it is not a major influence.  If I do not have a barometer reading, I tend to assume the value which gives the most convenient figure for calculations, that is whether it is more convenient to have a temperature of 100 degrees, or a pressure of 100 kPa, as the difference is not significant compared with other measurement errors.  We are trying to gain understanding, not resolve a legal dispute.  The point is that starting with that superheater outlet temperature, the adiabatic engine exhaust is wet steam but only just, with a dryness of about 0.99.

However, when I measure the outlet temperature for the real engine, I find 104 degrees at point 4.  The sloping extension of the horizontal line in the wet region is the correct form for a constant pressure line in the superheat region.  This is above the saturation temperature for atmospheric pressure exhaust.  So I had to go to the superheat table for that exhaust steam.  Definitely advantageous to assume 100 kPa here, to minimise effort in interpolation for a trivial difference to the answer.  Temperature and pressure in the superheated range are independent, and sufficient to completely define the steam properties.  Once again, back in mid September, I did the calculations and found the adiabatic efficiency was 72%.    Not so far from that 75% I assumed yesterday.  I also did the calculations again for 105 degrees, to see the effect of a one degree temperature error.  The digital thermometer only displays whole degrees, so potentially at some point, as little as 0.1 degrees would make it switch to the next digit, so the reading must always be considered to have an uncertainty of +/- 1 digit.

Assuming the exhaust temperature was actually 105 C gave an adiabatic efficiency of 64%, clearly 103 degrees would have pushed it the other way, so certainly that 75% I had assumed yesterday is in the right range, and we should not place too much emphasis on the actual figure over that range.  Clearly my test result was not far from the full size figure I remembered of 80% if the measured temperature had been 103 C, and remembering my memory is not totally reliable.   Perhaps I need to add that cladding to the cylinder now, and see if it makes a difference.  The test work list is mounting, will I ever get back to making an engine?  But I am enjoying the imposed discipline of actually doing the calculations with real numbers, and not just knowing how to do it.  So I hope you are all enjoying the reading, and I hope learning something of interest and even useful towards understanding your engines better.

Next time I would like to look at what those test results mean.  Can we now calculate the power output of our engine? Or what does it actually mean?

Thanks for following along

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 16, 2017, 02:18:10 AM
hi MJM, yet more relevant info  ...great... I have just used my boiler to raise steam for an engine and i have also put lots of rock wool under the boiler in the metal stand which brings the boiler bottom 1.75" above the wooden board. I was surprised to see that the safety valve lifted at 139 C this time rather than 135 c when i put the graph up on a previous post ?? would this be because of the insulation or perhaps the safety valve sticking ?? I did not measure the time however as this was just to raise steam...so i am sure you have an explanation for this ?......Thanks,You can see on the graph the green line where i have put an arrow to show where the safety valve was blowing off
Title: Re: Talking Thermodynamics
Post by: derekwarner on November 16, 2017, 04:48:08 AM
Hullo Willy...I have been following along [& a few contributions] with this thread since day 1

From a mechanical point, the additional insulation to the boiler shell should not have any differing level/point to the actual pressure the relief valve functions

Most model steam relief valves are relatively in-expensive simple direct acting in their design and function...variation in actual lifting pressure could be relative to the surface finish on the passive coils on the ends of the spring, the accuracy in seating of the ball in the spring and against the sealing surface within the valve and each of these could be summed as hysteresis and so could cause a perceived variation in the actual lifting pressure

Another aspect to consider is the additional insulation would provide a more gradual or uniform steam pressure increase relative to the heat input, so this should actually provide a more progressive pressure increase within the boiler acting upon the relief valve

Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 16, 2017, 11:22:15 AM
Hi Willy, good to have you back again.  Along with the extra rockwool under the boiler, did you put some on the ends?  Derek has very well explained some of the reasons your safety valve might lift at a different pressure.  At one time, boiler safety valves were fitted with easing gear, a lever fitted so so the valve stem could be lifted as part of the operating procedure to ensure they were not stuck.  Not sure if they still are, as there are problems with them resealing tightly.  However on my models I normally lift the valve and drop it back on the seat a few times before starting up to ensure it is not corroded up since the last time.  I suspect this might also help reduce the uncertainty of the set pressure, but there will always be some variation for the reasons Derek has described.  I understand why you did not record all the times again, you put a lot of effort into that last time.  Certainly put me to shame, so I will now have to do it, at least for a time or two.  However, I do normally record the time that I light the burner and the times steaming starts and finishes.  If you do this much, you will see clearly of your insulation changes have had a useful effect.  Then from the start and end of steam production, if you also measure the remaining water in the boiler after it has cooled down, you can also calculate the rate of steam production, which will help confirm the heat input and losses, apart from being interesting for looking at the engine in more detail.  In view of our discovery of a slight non-linearity in the first few minutes, it might be worth recording the temperature at about 2 minutes. Considering our earlier discussion, even better might be to record the time as closely as possible immediately the temperature reaches 40 degrees or some other suitable temperature, and another time as it reaches 130 degrees, so you have a more precise time for a given number of whole degrees as an alternative to reading temperatures to 0.1. I can tell you from experience that even with a meter with the facility, it is quite hard to consistently measure a rising temperature to 0.1 deg.

Hi Derek, good to have you checking in again.  I have not heard from you on your condensate issue since we both returned from our travels.  Have you looked at any further?  Or are you, like me, still trying to cut back the backlog of jobs that just wait for your return?  Does the current discussion about exhaust steam point to a possible answer?  Or do you have much more condensate?

Thanks for your very complete answer to Willy's question.  The detail of reasons for the variability is quite interesting.   I hope others will come in with other factors they are aware of, particularly the ones relevant to our model safety valves.  My full size experience is much more with machinery than boilers, and anyway, during commissioning, everything is new and clean and freshly calibrated, so I don't have much to contribute on that.  We have different issues to deal with.

 Before we look at what the calculated work from the steam expansion means, it is worth doing the calculations for the lowest boiler pressure from Willy's test.  A boiler temperature of 118 degrees has a saturation pressure of 187 kPa.  This is about 12.7 psig, so in the range you might use to run a model unloaded at a show.  The steam tables give us enthalpy of 2703.4, entropy of 7.1511.

Expanded to 101.3 kPa, we have hf = 419.04, hfg=2676.1, sf = 1.3069, sg = 7.3549 all as the previous examples.  In an adiabatic engine, the entropy is the same as the supply to the engine, 7.1511.  We can immediately see that for an adiabatic expansion, se = 7.1511, so the exhaust is wet steam.  We can calculate the exhaust steam dryness, 0.966, the enthalpy, 2600 and the enthalpy change 104 KJ/kg for the adiabatic engine.

If we assume that for a real engine, the adiabatic efficiency is again 75%, so we can compare it with the previous example.  Then we calculate the real engine enthalpy change = 0.75 x 104 = 76kJ/kg.  From this, the real exhaust enthalpy is 2676.1- 76 = 2627.4.  Again we have two independent properties, pressure and enthalpy, so we can calculate all the properties of the steam.

Checking that 101.3 kPa data, we can calculate the exhaust dryness 0.978, and even the entropy to see how much it increased if you like.  But even at this low boiler pressure, the exhaust is still wet, though at 98%, this means that only 2% of the steam flow is condensed in the engine, but is this anymore than a vapour mist?  Unfortunately in the tests from a boiler without a superheater, because the exhaust steam from our real engine is wet, we do not have enough information to calculate all the properties of the steam, as temperature and pressure are not independent in the wet range, in particular we cannot calculate the enthalpy and hence the work out from the real engine, but in any case, it cannot be more than from an adiabatic engine.

The boiler test results we have looked at so far have illustrated how we can calculate the work done in adiabatic expansion, which is a real upper limit to the work that can be obtained from any real engine operating between the same boiler and exhaust pressure.

We have seen that for a real engine, an experimental result is needed, but then we can calculate the work from the real expansion, and that it is always less than that from a real engine.  A reasonable guess for a model is about 75%, but we need many more modellers to test their own engines to make a more accurate estimate of typical values.  So far I have data from only one test, so the standard deviation could be quite large.  I will eventually do more tests which will help, but similar independent tests by other modellers is better again.

Then we looked at engine potential performance, starting with dry saturated steam from a boiler and found over a range of pressures the engine exhaust was always in the wet range, though well above 90% dry for 50 psig and higher nearer 99% for lower pressures.  With a superheater, from a similar pressure, the exhaust steam was found on test to be slightly superheated still, and hence gives dry exhaust.  The temperature and pressure are then independent, we can calculate all the steam properties.

So what does all this calculation mean for our engine power output?  You will have noticed that even though we appear to have calculated our engine power output, I have always refrained from calling it that.  Sometimes I have called it the work done by the steam on the piston face, and I think this is the most accurate description.  Now of course there is a long way from work on the piston face to shaft output.

I think that is a good place to take a break.  Tomorrow I will talk about what we can learn from those simple boiler tests.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on November 16, 2017, 01:40:28 PM
Certainly progressing MJM....just slowly......

The revised 1/4" steam exhaust trunking spools have been manufactured, lagged & painted [last coat today] & loosely assembled  :hammerbash: to show the differences

I will confirm condensate outcome at the earliest time....[as I also want to understand the expected differences]

Derek
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 16, 2017, 02:46:26 PM
Hi MJM and Derek, thanks for that explanation , I thought it might have been the safety valve sticking at first but it did continue to blow off at 139 c a few times later. Also i have managed to find the  "bible" Tome by Dalby all 740 pages with lots of info and drawings and tables !!! enjoy !! This also shows the steam trap mentioned elsewhere attached to the steam chest. There is also a table with the steam velocity !!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 16, 2017, 09:49:19 PM
Hi MJM, I have now completed a new boiler test with lots of insulation and was surprised that there was not much difference in the time taken to reach the same temp/ preasures in fact the temp was only 1 degree centigrade higher !! all the other parameters were all the same however and followed exactly the same temperature differences . This either shows the instruments behaved the same or they are not wildly inaccurate !! so we can say that your comments about 4 times the insulation is needed for any appreciable difference in insulation is correct . The blowoff temp also went back to 135 !! although the temp continued to rise with the valve leaking slightly, so over to you ........The starting temp was 18 degrees this time and i measured the weight of water in my jug and at the 600ml graduation the weight was 589 grams ....
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 17, 2017, 02:42:17 AM
HI MGM, It is now about 2.30AM and just checked the temp of the boiler and it is 43 C    the boiler was switched off about 9.10 PM so all that insulation is doing some good !!..............I am about to go to bed but may have to get up in a bit as the 'waterworks' dictate so will keep you posted !!!  Now 3 AM ...40 C,,,, So 1 degree every 10 mins...should be back to 18 c about 6.40 am......!!
Title: Re: Talking Thermodynamics
Post by: derekwarner on November 17, 2017, 04:42:31 AM
Willy......from the steam tables.....we see........

135 degrees C = 30.72 PSI
139 degrees C = 36.26 PSI

So the 4 degrees C increase represents a ~~ a 3% increase in temperature, however the same temperature increase provides an ~~ 18% increase in pressure

Your comment that the valve later returned to the lower lifting point suggests that the valve is faulty :zap:...so understanding this, it should really be stripped & checked

Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 17, 2017, 12:12:59 PM
Hi Derek, thanks for the update, I will await with interest the outcome of further developments.  That pipe insulation looks really great

Hi Willy, more great experimental work.  Back to that in a moment.  Looks like the safety valve is a little erratic.  I wonder if it is a rough spot on the stem or something.  Though I don't have enough records to really know how repeatable the lift pressure can be expected to achieve.  Over speed trips on turbines were a bit like that.  Quite a range was allowed under the test codes, but we had to rerun the test until we had three non trending results.  Sometimes that took a while.  Don't know if the safety valve codes are similar. 

That is another interesting book.  Most books of previous generations had complex graphs like that, as there were only hand calculations, perhaps with the aid of a slide rule, so equations were not much help.  Now with the calculating power of a spreadsheet, the graphs are unnecessary, and an equation, even if it is quite complex, is much more convenient, and calculations easier to check or update for different scenarios.  However it is worth remembering the work of these authors, who did their work developing and using these graphs with limited accurate data.  These days we do indeed stand on the shoulders of giants as someone once said.

Hi again Derek, I am not a great fan of using percentages in relation to temperature and pressure.  Using a percentage implies not only that there is a linear relationship, but also that the linear relationship passes through zero.  For temperature, of course, zero means absolute zero, not zero centigrade.  Similarly, for pressure, absolute pressures needs to be used, not gauge pressure.  A simple test is to extend your example back to very low values.  For example is that temperature comment still valid between say 1 and 5 degrees, the same four degree difference.  On the other hand, when you are talking temperature difference, four degrees difference will usually mean twice as much as two degrees difference, for example in a heat transfer equation, whether you use C, F, R or K to measure the difference.

Now I think that Willy's boiler experiment warrants more examination before we get back to that engine output, so let's give it a try.  First, the answer to the issue of accuracy of temperature measurement is much simpler than I first thought.  If we look at the temperature difference for one minute, we take two readings each at best +/- 1 degree, and subtract them, so possibly at least two degrees error, a large portion of the 15 degree difference being measured.  However, if we ignore the first two minutes where things were clearly not linear, and the last readings where it is likely the safety valve was possibly leaking a little, we have a five minute period with a temperature rise of 82 degrees, but still only that two degrees error.  Much less significant in 82.  So we get around 16.4 degrees per minute average over that period.  This is probably a good basis for checking heat accumulation calculations and heat input.  But also shows your temperature meter is pretty good, so you can rely on it.

Now let's look at those results.  First, the latest run achieved 121 deg from 18 in 7 minutes, while the previous one achieved 120 deg from 17 also in seven minutes, the same temperature rise in the same time.  It looks like the water spilt was perhaps not so important after all, unless you spilt exactly the same amount again.  Then 587g from your 600 ml measure is very good accuracy.  Similarly there is no doubt about your insulation now, and surprisingly little difference between the last two results, unless the ends were insulated for both.  As you can see , once you insulate the boiler, it takes a lot extra to make a huge difference.

Now that cooling experiment, I am delighted that you included that.  I know what you mean about the water works, it catches up with many of us if we live long enough, but might as well get the extra benefit of the cooling readings.  This experiment is particularly interesting as it does not involve steam generation losses, and does not involve the heater with any associated errors.  The process is called Newton's Law cooling.  If you google it, and you will find a maths site from ubc in Canada, near the top.  Don't be put off by the word advanced in the title.  But it leads us through the calculations for a bowl of soup which are easy to use just by substituting your figures.  Basically it means the temperature variation with time is an exponential function, and yes, that is the correct description for this example.  The temperature change in each minute is dependent on the temperature difference.  So it reduces as the boiler cools.  It really assumes the ambient temperature is constant, so I assumed the temperature was 18 all night.  Perhaps not too bad if your house has some insulation, possibly some heating if it is cold, and perhaps a mild night.  But the result is useful even if the air cools a bit more overnight, it means the heat loss will initially be a bit more bit not important.  I can set out the calculations if you like, but for the moment, I calculated a temperature loss of 27 Joule/s at the start when the boiler was still close to 138 degrees.  Obviously much less at the end when the boiler was only 43 C.  Now these figures can be used as a very good estimate of the heat loss from the same boiler temperature during the heating phase.  Not only that, but they agree very well with the heat loss calculations I did earlier, when I wanted to check what those unaccounted for differences might be caused by.  Now the heat stored in the water plus copper is about 733 J/s, so the heat loss by two different methods is about 3%.  However at the start of the experiment, when the temperature is still low, the loss by both methods is much less.  Proportional to the temperature difference remember?  With the boiler at 43 degrees instead of 135, the temperature difference is only 25 instead of 117, so the heat loss would be around 5 Joules/s, so quite negligible.

Of course I actually needed to know if you did any significant steam production and hence reduced the water content of the boiler.  I assumed you stopped after the safety valve lifted, but if you ran the engine, the cooling experiment needs to know the mass of remaining water as there is less remaining stored heat to loose by cooling.  However, that only reduces the calculated heat lost by cooling so does not change the conclusion that the heat loss, once you added insulation, is insignificant.  At this stage, there is more than enough data.  I am very much of the opinion that the only way to really shorten your boiler heat up time would be to add extra heating elements, perhaps a longer boiler with two each end.  Or fill it with hot water from the jug.  I think the only thing you have not done is to generate some steam and determine how much water your boiler evaporates in a given time.  After all, this is the purpose of the boiler, and should give us a good idea of the size of engine it could drive.

That is enough for today, perhaps back to engines tomorrow.

Thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 17, 2017, 03:26:06 PM
Hi MJM and Derek, yes the safety valve is about 8 years old so will need looking at....Thanks MJM for all the additional info however more questions.....First when the boiler is heated with an element that is linear ? the heat rise is linear !  However when it is cooled with a linear ambient temp the graph is as you say exponential ???? also i took more readings which showed my comment about the cooling was wrong !!This measuring was possibly quite an eyeopener for the early  experiementors and i wonder who came up first with the term exponential or was there a Mr/Herr/Mrs  Exponential that was about in the past ......Also when i got up about 12 i noticed the temp of the probe in the boiler was 19 C that i thought was a bit high,,,so i removed it and the temp shown went down to 16 C......I then replaced it and it returned to 19 c ?? i then removed it and it eventually went down to 13 C .!! here are some photos that show this  !!   So any ideas about this ??? Thanks for your time spent with all these queries and it is that question/answer graph coming into its own again !! More photos in the next post ....
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 17, 2017, 03:42:15 PM
Hi...More photos and graphs........Also does an exponential graph ever reach infinity or zero ? When you have a room at ambient temp i think that everything in the room is the same temp, if you touch a bit of steel it feels cold because you fingers feel it as cold ,but if you touch say amber it feels less cold because you fin get takes more/less temp from it  ok...so does a temp probe act in the same way ? and does this explain/confound why the temp rises /falls when the probe is placed next to the copper.(cold) and placed on the  plastic covered wooden bench 9Not so cold, feeling) is this still thermodynamics or quantum mechanics that we are getting into !!??
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 18, 2017, 08:58:01 AM
Hi Willy, I probably should have included the equation for that Newtonian cooling we were discussing yesterday, so here it is.

T(t) = T(a) + (T(0) - T(a)) x e^(-k x t)

As usual I should use subscripts, but on the iPad, it's too hard.  Read it like this :-

The temperature at time t, T(t) equals the ambient temperature, T(a) plus the difference between the initial temperature, T(0), and T(a), or (T(0)-T(a)), raised to the power of (that ^ symbol), minus k  x t.

Now this might need a bit more explanation.  That k is a time constant (not thermal conductivity) and t is the elapsed time in the units you choose, seconds, minutes, hours or whatever is convenient.  The units are not important, but will affect the value of k in your calculation, as it has units which must be based on the same time units.  Also, raising a number to a negative power gives the reciprocal of raising it to a positive power.  The symbol, e, is a well known standard mathematical constant, and is the base for natural logarithms, but that is another subject.  Your calculator has it, as does any spreadsheet program.

So, using any two coincident time and temperature readings from your cooling readings, you can determine k with a little simple algebra.  I used minutes and calculated k = 0.00496, using the times for 138 degrees and 43 degrees.  I spent a bit of time looking at the value of k in terms of the insulation conductivity, the convection coefficient, density, surface area and so on, but decided it was more complex than it was worth.  Unfortunately k is not totally a constant but to see how much it varies you would need a few more readings in the first 100 degrees of cooling to compare with the value I obtained and also to calculate more values from the last bit of cooling, a great job for a spreadsheet.  The formula is a mathematical equation which closely enough follows the same curve as the actual cooling, and does indeed predict infinite time for the boiler to reach ambient temperature.

However, in practice two things happen that mean the temperature is reached sooner.  First, even the equation soon predicts that the temperature difference is no longer large enough to be detectable.  You can calculate how long to get to 19 degrees, the smallest temperature you can measure with your instrument above the ambient of 18.  Second, and equally important is that particularly when you are talking about cooling times of several hours, the ambient temperature is likely to change, thus moving the cheese so to speak.  However the time constant, k is useful in indicating how quickly the cooling is likely to occur, the very low value in this case is caused by the insulation and means a long time to cool.  A high value would be found with no insulation and indicate a shorter time to reach equilibrium temperature.

When your steel and amber are in a room long enough to reach room temperature, as you say, they will all be at the same temperature as the room and almost everything else it the room, however your finger is not.  Your blood supply is trying to keep the finger up to about 37, though it often does not quite make it in cold weather as we all know.  So when you touch anything at 20 degrees, there is heat flow to the object controlled by the contact resistance of your finger, and the conductivity of the object.  Your nerves are close to the skin, again every day experience, so your finger can feel itself being cooled by the steel which of course conducts very well.  When you touch the amber, your finger is not cooled so much as the amber does not conduct so well.  The situation is controlled by the steady state heat flow from your finger which is receiving heat from your blood, to the object you touch.

The thermocouple is quite different as there is no heat source.  Heat only flows from the hot object it is placed against until the thermocouple reaches the temperature of the object it is touching.  So that time constant does determine the time it takes the thermocouple to reach the temperature of the object, when the zeroth law of thermodynamics says there is no further heat flow. 

There is a new one for you.  It really is called that.  While it was only clearly put into words after the first and second laws, logically it is necessary to have this one before you can properly deduce the others.  And it cannot be deduced from other laws.  Definitely still thermodynamics.  If you put some insulation outside the thermocouple so there is minimal loss to the outside air, and use it to hold the junction, or the sheath in the case of your instrument, in close contact with the object you want to measure, you will get both better accuracy and a quicker response to temperature changes.  Oh, and wrap the wires (or the part of the sheath, in contact with the air with some insulating material to minimise heat flow along the wires.  This dependence on conduction to make the temperatures equal is the reason temperature measurement always has a slower response than pressure.

It looks like consideration of engines will have to wait for another day,

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 18, 2017, 01:25:48 PM
MJM, thanks for this explanation and the thermometer is now reading the same both in and out of the boiler !! However i was surprised that it took 15-17 hours to reach equilibrium !! is this a record ? So let us continue with engines now.!!.....
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 18, 2017, 01:39:52 PM
MJM, thanks for this explanation and the thermometer is now reading the same both in and out of the boiler !! However i was surprised that it took 15-17 hours to reach equilibrium !! is this a record ? So let us continue with engines now.!!.....
[/quote

Hi the next time i have 15-18 hours spare i may do the same experiment again and record the first 5 hours , and the remaining 5 hours to complete the graph !! So...with locomotives that stopped running at 11 Pm and then brought back into service the next morning to start running again say about 5 hours ,the boilers must still have been quite warm. And is there any data for this available ?
Title: Re: Talking Thermodynamics
Post by: Maryak on November 19, 2017, 01:42:59 AM
Every steamship I have served in has hand easing gear for the safety valves. If the ship is sinking then releasing boiler pressure lessens the chance of an explosion when the flooding water reaches the boiler(s). i.e. it's part of the abandon ship routine.

Regards Bob
Title: Re: Talking Thermodynamics
Post by: crueby on November 19, 2017, 01:50:21 AM
Every steamship I have served in has hand easing gear for the safety valves. If the ship is sinking then releasing boiler pressure lessens the chance of an explosion when the flooding water reaches the boiler(s). i.e. it's part of the abandon ship routine.

Regards Bob
Reminds me of an old ad I saw for the Sabino when it was being run privately before the museum acquired it. It had a line that: your ticket would be refunded if the ship sank!
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 19, 2017, 09:29:18 AM
Hi Willy, I would be the last to suggest that anybody should sit looking at the boiler cooling for 15 hours.  Like watching the kettle boil, only not as exciting, especially with a metal kettle.   Time would be better spent smelling the roses, contemplating the mysteries of the universe, or having a snooze even.  No real imperative to repeat the experiment, which has already yielded sufficiently accurate excellent data, though if two or three readings happened between switch off before dinner time and bed time, the experiment would be complete, repeatability demonstrated and the data more than good enough for the purpose.  But you have already admirably and quite accurately demonstrated that the heat loss during heat up, even at the high temperature end, is only around 25 J/s compared with around 700 J/s stored in the water and copper, and is a fairly convincing demonstration that there is not much heat lost from the insulated boiler.   I am sure we can conclude that energy missing in the heat balance is just not being provided by the heating elements.

Now the more important next step is to remove that temporary insulation, and work out what you want to do as the permanent job, something more in keeping with the beautiful work you do on the engines.  Then, some convenient occasion, a few readings as the boiler cools by around 50 to 80 degrees would demonstrate the effectiveness of the final effort.  From your photos, I would suggest some thinking about how to reduce the heat loss from the ends, where there seems to have been no insulation at the start of this work.  And I would even leave thicker insulation on the shell to your next boiler, rather than disturb that nice brown cladding.  You have done a great job of some real science there, and enabled us all to learn from your work.  And I feel that you have also made this thread better for your participation and insightful questions.

Your boiler is very well insulated, the only aid to cooling is the steam pipe coming out, like a small cooling fin, a fired boiler has a cooling air flow due to stack draft unless the airflow is deliberately closed off, perhaps for a quicker startup the next day.  Also I don't know how much of the locomotive boiler outline is actually insulated.

Hi Maryak, I hope that was not the only time they were used.  In any case they would need regular testing.  You would not want to find it rusted up when the big day came.  More seriously, especially on steel boilers, the thermal stresses due to the sudden asymmetrical cooling might actually crack those early steels before tougher steels and impact testing were as widespread as today.  In which case, better to have minimum pressure.  So the procedure makes sense.

Hi Chris, did they also offer to dry your clothing?

Ok, so back to engines.

It is worth giving some thought to what that work calculated from the experimentally observed supply and exhaust steam actually means.  The steam tables are considered an accurate model of real steam behaviour down to the level of accuracy implied by the four significant figures tabulated.  You will appreciate by now that has taken some very precise experimental work by dedicated researchers.  Also, the first and second laws of thermodynamics, and that zeroth law, are well established fundamental laws of physics that allow us to calculate those useful extra properties of enthalpy and entropy.  The point is that that calculated work from the observed temperatures and pressures is real, within the limits of accuracy of the instruments used.  I have done the calculations to the limits of the tables, as it makes them easier to check, but have no illusions about the accuracy of my simple test setup.  So without putting too fine a point on the meaning of the last significant figures, let's accept that it is a reliable measure of some quantity of real work done, by real steam in a real engine, and look a bit closer at what it means.

Before there is any of the work available at the shaft, the steam has to overcome the pressure on the exhaust side of the piston, which is pushing the opposite direction.  Also it has to overcome piston friction against the cylinder wall and friction in the rod seal.  Then the  cross head guides, the two conrod end bearings, eccentric strap friction, another pin, valve rod packing and the friction of the valve on the valve face.  So many more unknowns, and when I list them, I am amazed that in a small engine running on such low pressures, even with no extra shaft load, that there is enough work from the steam to run at all.

I know that some have doubts about that exhaust pressure.  But I am coming at this from first principles, applying the relevant laws of thermodynamics.  The pressures are absolute pressures, as this is the relevant pressure when the work done is calculated using the fundamental formula for work.  Work = Force times distance.  In the cylinder, Force on the piston = pressure times area.  So work by the steam is pressure times area times distance, the distance being the length of stroke.  That work is done by the steam.  You then have to start again to calculate the work done by the exhaust steam on the other side of the piston.  And due to the force directions, the net work on the piston requires subtracting the exhaust side pressure from the power side pressure. 

Of course, if you have a single acting engine of any kind, or if you have atmospheric exhaust pressure, you can take a mathematical short cut and use gauge pressure on the power side, then ignore the exhaust side.  It is the same as assuming exactly atmospheric pressure for the exhaust, but you risk missing any effect of the time it takes for the exhaust side of the piston to reduce to atmospheric pressure after the preceding power stroke.  You also potentially hide the understanding of what is going on.  This is probably acceptable for a single acting engine, but as we have seen, this can lead to significant oversights in a steam engine for the exhaust stroke of the cycle, though it is still a suitable calculation for a double acting engine when calculating power through indicator diagrams, as the exhaust pressure is then properly accounted for.

All the sources of friction I have mentioned, and there are sure to be some I have missed, meaning that we still don't know the shaft power output of the engine.  We still have to do an engine test to find what power is available at the shaft.  However the theoretical analysis does give us a real figure for the maximum power output that can be obtained from a real engine under any observed  steam conditions.  If you go on to calculate the overall thermal efficiency, is a discouragingly small figure, but when you know just how much of the steam energy must be carried away in the exhaust steam, it is not surprising.  I hope you can see that there is some justification, when talking about an engine, to talk about adiabatic efficiency.  An engine test which actually measures torque and rpm is still not going to give a very high figure, but it will be a lot more encouraging to compare it with an adiabatic process than the overall thermal figure.

Well, can we make any estimate of all those mechanical losses?  I suppose that we could devise a complex range of experiments and tests to quantify some of those losses.  But not many of us, including me, will be sufficiently motivated.  However, I think there is one thing we can do. 

When we run the engine unloaded, all the work done by the steam is used in overcoming all the friction.  If it runs at constant speed, then there is no excess power at the shaft under those conditions of steam and exhaust pressure and temperature.  It there was any excess, Newton's laws say that the engine will accelerate.   So by measuring the engine rpm, we can say the friction at that time is equal to the work done, and we can make an assumption that that is representative of the friction resistance at that speed.

Next time, let's look at that a bit further, to see of there is anything else we can infer from that suggestion.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 19, 2017, 02:00:54 PM

Reminds me of an old ad I saw for the Sabino when it was being run privately before the museum acquired it. It had a line that: your ticket would be refunded if the ship sank!

Hi Chris , yes, but was there some small print that said   "only the distance not travelled on the ship will be refunded " !!!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 19, 2017, 02:12:19 PM
Hi MJM, Thanks for your latest observations .I have now removed the clumsy insulation and may do another cooling graph showing the initial cooling figures without it. Are there reverse steam tables available showing the cooling cycles ? Also are there available port sizes available for different sorts of engines, Condensing /Compound/ triple expansion,  etc etc ? Also would needle roller bearings help with efficiency that are available in comparable sizes with sintered bronze bushes ? .....I have just given my temp gauge to an 11 year old boy to stick into anything and everything he can find, except his younger sister ,of course !! the new generation of thermodynamic engineers is on the way ?!!!...........also.. what temp does copper /steel/ brass have to be to not feel cold to the touch ??..
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 20, 2017, 06:24:55 AM
Hi Willy, the cooling graph with only the original insulation will be most instructive to compare with the values obtained for your temporary insulation.  Looking forward to the result, if you can get that temperature instrument back.  Ah well, a second one is always useful!  Good idea to give it to him.  The boss' grandson came in to work for secondary school work experience, and I was surprised to find that he not only had never had technical Lego or Meccano, but he also didn't have a voltmeter.  It was a bit cheeky of me, but I called in the boss and told him I had solved his Christmas gift problem, the boy needs a voltmeter, after all the boss was also the chief instrument engineer, and some technical Lego.  Unfortunately, Meccano is not as readily available these days.  But how will STEM education progress if kids don't have voltmeters and other real technical toys.  Unfortunately the same answers from all the kids coming in for a week to see if they would like to choose an engineering career!

Don't need reverse steam tables.  Steam tables list steam properties at various pressures and temperatures.  The characteristic of a property is that it just depends on the measurable states at the time of measurement, and does not depend on how the substance got to that condition.  Like  your height above sea level.  It does not matter whether you came up hill or down to get to your location, nor whether you came from East or West.  When you stand at that little surveyors mark in the pavement near your home, you are always at the same height above sea level.  So if you have steam at 100 kPa and 103 degrees C, you don't have to know how it came to be that way, you just look in the tables to find that it is superheated, the specific volume, internal energy enthalpy and even entropy.  It does not matter if it is engine exhaust, or if it got there by being heated.  In fact the tables are even more reliable than your height above sea level, as even an earthquake, continental shift or climate change cannot change the values.

Port sizes for engines are just about velocity as we have already discussed, and the shape of the transition between different flow areas.  The flow area, gives the velocity from the steam flow and volume which you need to estimate or predict on some way for each port.  Smooth transitions reduce energy losses.  Of course the shape of the cross section will need to be made suitable for the valve shape and movement, but overall you are aiming for that compromise between pressure losses and excessive volumetric clearance.  In most cases, you make the passage as large as other considerations allow.

For any material not to feel cool, it has to be the same temperature as your skin so there is no heat flow.  Obviously your skin temperature is not quite at blood temperature so there will always be some heat flow until objects are at 37 degrees.  But as the objects get close to this, the object will obviously feel warm your skin, so will feel warm until there is equilibrium from the object through to your blood vessels.  I am not quite sure just exactly what point this would be.  And I guess there is a small heat flow that will not produce a temperature difference you can feel, so there is probably a small range where it feels neither warm nor cool.  Time to put a temperature controlled heating element in a block of steel and set that boy to work as your laboratory assistant.

Needle roller bearings, right on topic with that one, I think I have answered it below, let me know if more information is needed.

So far, we have seen that we can show real work being obtained from a real engine, and that in a simple unloaded test run, all that work is used in overcoming friction.  No problem for an exhibition, it is easy to apply a bit more pressure if necessary to make the engine go the speed we would like, and many previous threads show that we all appreciate that running on lower pressure is a good thing, and a compliment to the quality of our work.

Is it possible to use this unloaded test run data as an estimate for the engine friction load for other load conditions?  At the limits of my comfort zone here, entering a new area of application for all that has gone before together with a few other basic principles.  So let's have a look at a few questions that might help us.

Does this friction vary with speed or steam conditions?  Generally, friction load is roughly proportional to the load causing the friction.  So for example the force pushing the the valve against the port face varies with pressure.  This means the valve friction load varies with steam pressure.  The size of the valve has to be sufficient to cover the ports as necessary for proper operation, but any excess area obviously increases the force and therefore the friction at the port face.  Various valve designs have been tried to balance this load and some are almost certainly applicable in model sizes, especially for larger models.

What about all those bearings?  Willy has mentioned needle roller bearings.  Ball or roller bearings both replace sliding friction friction with rolling friction.  We all know that reduces friction, that is why  the wheel was such a great invention.  I think we can take a lead from Professor Senft's work on his LTD Stirling engines, that ball bearings make a worthwhile reduction to the friction when you literally only have candle power (or less) to run your engine.  But these bearings also bring complications.  Heavier loads require lubrication, and that brings a need for dust seals, seals add friction, so a little care is required with the details if we are to benefit.  But almost certainly worthwhile for a model engine as opposed to a model of a historical prototype.  Ball bearings involve a close approximation to a point contact, and this limits the load capacity, as the deformation at the contact point introduces fatigue which limits the life of the bearings, and this becomes significant at higher loads.  Rollers have closer to a line contact, so are able to support much higher load for a given life before the surface fails and the bearing becomes noisy and friction increases.  But with due attention to the details, ball or roller almost certainly offer an area for friction reduction, which of course leaves more of the work done by the steam to be available for shaft work.  Have a look at Beson and Rayman's trials with flash steam for an example.

I don't know much about sintered bronze bearings, but I assume the big advantage is more consistent lubrication by oil seeping through the pores of the sintered structure.  And good lubrication certainly reduces friction. Oh course poor lubrication would do the opposite, neither dry running, or in-appropriate lubrication by say a heavy grease would help one of those LTD engines run well.

Next time I will look at the cross head loads, another interesting area.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 20, 2017, 03:03:47 PM
Hi MJM  thanks for all this ,but i do feel a bit like that annoying kid at the front desk in the class that stops the other kids learning by rote and keeping the teacher on his toes/knees instead of keeping to the allotted time in the curriculum  !! would a bit of lubricated steam leakage help lessen friction ? or is this a further case of diminishing returns !!so only one question for today/night....Thanks

Willy....
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 21, 2017, 10:21:36 AM
Hi Willy, please don't let up on the questions, I have never been one for rote learning.  All those hours we all spent reciting times tables were a real waste, but when you think about it, the teachers had very little in the way of resources, calculators were only a very dim gleam in someone's eye, and no one dreamed that our primary school kids would be walking around with more calculating power in their hands than the Apollo team had to send Neil Armstrong to the moon.  And without those things, there was very little in the way of other tools for the job.  Not much point starting on a slide rule even until you get a bit past the adding and multiplication tables.  Fortunately they can now skip the rote learning, use a calculator, and start learning how to use it and how to use and check their spreadsheets.  At least I hope that is what they are doing.  If not, surely they soon will be.

So it is no use me rabbiting on about some esoteric issue, if no one has understood the first point.  And many others will have the same questions you re asking.  So keep them coming.  It keeps my feet on the ground.  If I can explain a few basic principals to those who are interested, I will have made a good contribution in return for some of what I have learned from all the others generous enough to post their efforts in such an instructive manner.  I don't have a syllabus or timetable to meet.

The displacement lubricators we put in the steam lines just put that little smear of oil into the cylinder that helps reduce friction against the cylinder walls, the valve and port face and the rod packing, similarly the oil we put on the bearings and slides, and it is all very necessary.  Running on air, I know industrial tools have an oiler in line, but I don't know how well they work when running an engine on air at much lower flow rates and pressures.  My full size reciprocating compressors actually inject oil through the cylinder wall to lubricate the rings.  A short post tonight, extended family duties suddenly call.  Expect to be back tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 22, 2017, 09:38:05 AM
Continuing on looking at the various friction loads on our engines. 

I should have mentioned when talking about those bearings, that the friction torque is a function of pressure on the piston, though using ball or roller bearings reduces this significantly from what would be expected from a plain circular bush.  I specifically say circular bush, as that is what we normally use when we drill or ream a hole for a bearing.  With a little oil, the pin can tend to ride on the oil film to reduce the torque more than you might expect from metal on metal friction considerations alone.  Some industrial bearings achieve even more load carrying capacity by using a non circular bush, a lamellar bearing I think it is called, a bit lemon shaped.  Oil builds a wedge between the shaft and the bush and the shaft rides like a hydro planing tyre on the oil film.  Tilting pad bearings have even higher load capacity, as the bush is divided into three or five pads each mounted so it can tilt slightly to the optimum angle for the oil film load carrying capacity.  Of course these bearings have a copious supply of oil supplied at pressure to ensure adequate lubrication.  Very heavy shafts even have high pressure jacking oil to lift the shaft off the bearing for startup.  And a very recent development in turbo machinery is the active magnetic bearing, where the shaft is suspended magnetically so there is no friction even from a thin oil film.  A bit like those desk top pen stands where the pen is suspended magnetically. Sophisticated electronics actively measure the shaft position throughout each revolution and vary the magnetic field continually to keep the shaft on centre.  I doubt that this technology will be available for modellers in the near future, though with the speed of development in electronics, who knows.  Not really in keeping with a traditional steam engine though.

Cross head loads are due to the sideways component of the conrod load when the con rod is at any angle other than in line with the piston rod.  Like the bearing and valve face loads, it is proportional to the force, which in turn is proportional to the operating pressure.  Not a steady load, it varies from zero to a positive and negative maximum each revolution as the conrod and crank angles change.

As we think about all these different friction sources, they are pretty much all proportional to the steam pressure in the valve chest or on the difference in pressure between the upper and lower piston faces.

The power absorbed by that friction for linear motion is force times velocity.  For rotary motion, it is torque times rotational speed.  For rotational speed, the appropriate units are radians per second, which means that we have to multiply our normal units of rpm by 2 times Pi, and then divide by 60.  So power = 2 x Pi x N x T/60.

We had better look at the velocity component of those friction forces.  Do the forces and torques vary with speed?  The normal friction model taught is that the magnitude of the friction force (in the direction resisting motion) is a constant times the normal force.  The constant is called the friction factor, and there are tables of typical values in many books.  However a more sophisticated model includes a stick-slip component when motion first starts.  Then the model talks of static friction which has to be overcome before movement starts, followed by dynamic friction which is a little lower while the motion occurs.  For the intermittent motion occurring in a reciprocating engine, obviously both come into play each stroke.  But I believe that the dynamic component does not vary greatly with speed.

Based on that very simple analysis, I suggest that we can assume that the friction force and torque are constant with speed, but increase in proportion to the absolute steam pressure.  The power, of course is directly proportional to speed, so power is proportional to pressure and speed.  If this assumption is reasonable, we can possibly use the simple unloaded engine test with the steam temperature measurement to make an estimate of the friction load, and when combined with a simple test of steam consumption, perhaps make some estimate of the power potentially available at higher pressure.  I don't know if it is of any practical value, as we don't usually have any better idea of the power required by the load, whether it be a generator or propellor, or a miniature sawmill.  But it's an interesting thought.  It does allow us to make some estimate of the division between engine friction load and the driven load, which may be of some interest.

Definitely at the boundaries of my comfort zone with regard to the application of this basic friction model to our model world, so time to harness our collective knowledge and understanding, to see if we can take this any further. 

Obviously a load test with torque and speed measurement gives the only definitive answer to our engine power output, however a careful analysis of what we know can help us understand the most productive aspects to work on to improve the power output of our engine, especially one just about good enough, but a bit borderline.  We may be able to improve an otherwise satisfactory model instead of having to start again.

My intention is to do a few more tests on my engines to confirm the reliability of the data, and I have made a start on gathering materials to make a simple engine brake for torque measurement.  But until I have something to report, I will move on.  If anyone else tries some tests, please tell us about your results.

Well it has taken quite a while to get to this point, but there at last.  Obviously a great point for more questions, then I will see where I can apply some thermodynamics to boilers and fuels.

Thanks for following along,

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on November 23, 2017, 11:21:38 AM
Making a start on Boilers-

Well, I don't know whether no questions means it is all clear as crystal, or clear as mud.  Or perhaps everyone would rather talk about boilers.  So I will continue with boilers for the moment.

It is hard to know quite where to start, but thermodynamics is one obvious place for this thread.  And the current hot question is consideration of scale factors for boilers.  Then there are  considerations related to fuels,  I am not a combustion engineer, so coal will require some thought, and I don't have may real expertise on the finer points of burners.  My job was more concerned with making sure those hydrocarbons did not burn.  But there are some ideas for a start and we can see where the questions lead from there.

So first some thermodynamics.  The first law, which is basically conservation of energy is the primary consideration for a boiler, and as the boiler limits are the solid shell, there is no shaft work.  Work done by the steam flowing out is taken care of by consideration of enthalpy.  No real change of elevation, or velocity, although the turbulence inside the boiler is obvious.  Really that leaves us with heat in and heat out, and the convenient way to look at that is termed a heat balance.

The heat input comes from the fuel, so in principal, to make more steam, we have to burn more fuel, and ultimately the fuel input is the basic boiler control.  You might be surprised that I do not say regulator.  I will come back to that, but first let's look at the overall heat balance.  And it is always of interest to have a guide to how big a boiler must be to generate a given quantity of steam.

The heat out flow falls into three basic areas, perhaps four if you include a feed pump.  Obviously, steam out, then, for a fired boiler, the combustion gases.  Then there are losses to the atmosphere from the boiler shell, and finally, if you have a feed pump, you are increasing the mass of water in the boiler, and as as the fresh water entering is cooler than the water at steam temperature in the boiler, some heat is absorbed by this water in coming up to temperature.

For an electric powered boiler, like Willy's electric boiler, and the one Thomas has nearly completed, there are no flue gases, and in principal the heat input is accurately known by the power consumption.  However, as we have seen, that electric power input preferably needs measurement by suitable instruments, as inferring power input by nominal power supply voltages, heater specification sheets or even measured resistance values, still leaves plenty of areas for the actual power input to vary from our assumptions.  Please note that I don't recommend allowing access to mains power for voltage and current measurements, unless you have appropriate professional qualifications, but I am am wondering if those power meters we can now buy to check the power consumption of our domestic appliances would be sufficiently accurate.  They only require the normal plugging into a power point.  No need to visit all that again, for the moment anyway

For a fired boiler, the heat from the fuel has to heat the air necessary for combustion, and measuring this air is not so easy in model sizes with the simple equipment most of us have available.  Measuring the flue gas temperature is about the limit for me, and I assume for most of us.  There is probably a way of measuring air flow, but I have not pursued this far, and generally just assume that the heat not accounted for in other ways is contained in the flue gas.  Some of the heat from the fuel will be lost by incomplete combustion, either on the form of unburned fuel, or partially burned fuel.  We can come back to look at that in more detail. 

We have looked at heat loss to the atmosphere and insulation in terms of Willy's electric boiler, and those considerations apply equally to a marine type fired boiler with an internal firebox and flue tubes.  However, for other boiler types, heat can be lost directly by the flue gasses through the furnace wall.  So external insulation is still desirable, but in this case, it is necessary to ensure that the insulation chosen is not combustible.

The heat stored in the copper is a little more complex for a fired heater.  The actual copper shell  temperature will be higher for a fired boiler, as a temperature gradient is necessary to transfer the heat from the flue gases through the copper to the water inside.  Consequently, the heat stored in the copper will be higher than for the electric boiler, at least for the outer shell which is the majority of the mass of copper.  More akin to trying to estimate the temperature of the wire in that heating element and its containment sheath.  This higher temperature also has implications for the strength of the copper, and the design of the pressure containment.

At last we get to the water in the boiler.  The heat stored in the water is similar, whether the boiler is electrically heated of fuel fired.  Similarly for the air in the boiler when it is first sealed.  So there is at least something we have already covered.  All those up and down the mountain considerations might have seemed a bit obtuse, but they were an interesting way to look at the effect of the initial air in the boiler.  As well as quantifying that old science lesson about boiling eggs on the mountain top.

Finally fresh feedwater, whether we have a hand pump or a power driven pump, or even an injector.  The cold water must be heated to the steam equilibrium temperature, and we can look at how much heat this requires.

Well, that is a quick overview of the topic of boilers.  I am thinking of starting on some basic sizing and scaling issues, but I am happy to start where readers interests lie.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 23, 2017, 01:57:19 PM
Hi MJM,  Ok some new questions.....we all know about  solids, liquids and gasses of substances and we use water in our boilers that have these states. However would it be possible to use other liquids in our boilers ? We know about the mercury power station in Shernactady in the USA in the 30's but could one use another liquid such as oil, mineral plant based etc ?  Also could one use a 'blown' boiler mechanism on a locomotive  ?!! ie as the loco whizzes along at 90Kph a tube comes back from the front of the train to force air/oxygen onto the fire ? And has this ever been attempted ?  I have resisted the temptation for more questions to let other people join in the fun  btw !!
 Willy...........
Title: Re: Talking Thermodynamics
Post by: paul gough on November 23, 2017, 09:06:14 PM
Hi Willy, Quite a few liquid media used in boilers. Naptha engines & boilers were quite popular in U.S. river craft and I think there was also an alcohol one. Of course, in modern times liquid salt is used as a heat transfer medium in solar thermal plants. I seem to have misplaced my old tome on early steam boat engines, so am going by memory here. No doubt some of the steam boat enthusiasts here would know more. All I can fathom from the thermodynamic side of things is that the low boiling point of naphtha,(liquid hydrocarbon similar to petrol as far as I know), was probably less powerful/efficient than steam. But my knowledge is very limited and await the Masters, (MJM), words on this. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: derekwarner on November 23, 2017, 09:25:27 PM
Well, without being patronising MJM, you have to date provided some thought provoking scenarios, questions and answers in the subject ...all supported by empirical evidence in a way for members with lesser understanding what is happening with heat, steam & temperatures in and around our miniature boilers, to attempt to grasp and more importantly understand

Not so much at school or college, however more in later life when involved in some form of specialist engineering review, I often wondered what background does this or that person have to be able to provide input to assist the group out of their mire

Many weeks ago you declared you were not from an academic background and now I don't want to start a guessing game, but telling us that ..... your "job was more concerned with making sure those hydrocarbons did not burn" ....has me a little stumped   :shrug:

I could happily go on reading this thread  :happyreader: for years............Derek
Title: Re: Talking Thermodynamics
Post by: Steam Haulage on November 24, 2017, 08:35:42 AM
Steam Guy Willy asked about other fluids.

In my work as a chemist we often needed to keep the heat source remote from the reaction vessel. Although superheated steam might appear to be a workable solution it does not allow use  at the sort of temperatures necessary to make a reaction 'go'. Temperatures from 280deg C and above could be needed.  Nor does it allow for rapid heat transfer at the start and end of the process It can be important to raise the temperature of the reaction vessel rapidly at the start and to cool rapidly at the end to stop the reaction. Our reaction vessels ranged from 500 litres to 6 tonnes reactant capacity, we blanketed them with Nitrogen to avoid oxidation during the process.

For this purpose Dowtherm A or similar fluids are often in use. You can find details of this sort of product at DOW Chemicals website see https://www.dow.com/en-us/heat-transfer/products (https://www.dow.com/en-us/heat-transfer/products)

I was most intrigued when I first heard of these.
Jerry
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 24, 2017, 12:32:30 PM
Hi Willy, your questions are always welcome.  I remember covering Mercury way back in my student years.  I think it was used as a topping cycle, on top of a steam cycle, and it's condenser was at a good temperature to raise steam for the lower part of the cycle, but I would have to look up more of the details.  This arrangement uses a higher maximum temperature, without losing density in the mercury cycle, and maintaining the low temperature of steam condensing, thus increasing the Carnot cycle efficiency, or ideal efficiency limit for the temperature conditions.  But mercury is very toxic, and the temperatures used were very high, so not really suitable for model uses.  Other materials have also been tried, a never ending search for some advantage is the driving force of development.

Blowing the locomotive boiler is not a very good idea, and I doubt that it has been tried, though some of the models in Bensen and Rayman's book on flash steam did try and have this effect working in their favour rather than against them.  The reason it is not so good, is that it results in a positive pressure, above atmospheric, in the firebox.  When you open the door to feed more coal, you would be met by a furious blast coming out.  It is always preferred to have a slight negative gauge pressure in the fire box, so any leakage is inwards, much safer, and the air leakage is easily compensated for in the secondary air regulation.  Even gas and oil furnaces have peep holes so the operator can check the flame, and the appearance of the tubes, so the negative pressure, achieved by the stack draft, or even fan induced is an important safety factor.  Forced draft is also used in conjunction with induced draft, but with the fans balanced so the firebox pressure is still slightly negative.  With short stacks like locomotive boilers, the natural draft is assisted by a blast tube using exhaust steam to help induce more air into the stack without the danger at the firebox end.

Hi Paul, you are correct about naphtha being a slightly higher boiling point fluid than petrol, but very similar in nature.  The higher boiling point means it vaporises less when stored in an atmospheric pressure tank.  But I need a process engineer to join in to give much more detail than that.  I don't have a ready reference to the the naphtha equivalent of the steam tables.  They possibly involve lower latent heats, for vaporisation, so less heat carried away in the exhaust, but I don't know about the change of enthalpy with pressure for producing work in an engine.  But the main issue is flammability.  There will always be leakage, even if only rarely associated with maintenance or breakdowns, there is always oxygen, and as one of the great safety gurus said, ignition sources come as a free bonus.  Even when you do everything to eliminate electrical sparks with special wiring standards.  The prospect of burning a boat to the waterline far out at sea would be enough to put me off the idea.  Or a speeding locomotive engulfed in a ball of flame.  So in the end, safety trumps efficiency and power to weight ratio.  Thank you for your kind words.  Not really a master though, just a mechanical engineer who happened to work in an area where this stuff was really well understood by everyone around me.

Hi Derek, you are correct, in that I started in petrochemicals, an ethylene plant, which is a really great place to start as the chemistry is not too difficult.  Anything with more than five carbon atoms in the molecule was lumped into the general "C5+", and was generally a small fraction of a percent.  After working in an operating plant for about four years, I joined a team in Canada to build a really large one, though to be clear, my role was everything except the core ethylene unit, but it was a small, close knit team.  Then returned to a less specialised contractor where I worked on gas plants, off shore platform processes, refineries and a few other things.  And found a niche for myself specialising in large compressors and other associated machinery.  And the associated piping, pressure vessels, pumps, so a wide ranging, interesting career.  So I generally describe it more generically as the hydrocarbon processing industry.  But surrounded by people with similar or often much greater knowledge, it never seemed so special at the time.  With having hydrocarbons measured perhaps in hundreds of tons, at high temperature, and  pressure, you can see why it is important that it does not catch fire.  But lest I sound like I am writing a resume, I prefer to just make what I write stand on its own, with logic all can follow, or look up, it should not, and does not need to rely on any mysterious special knowledge.  I don't see much "mire" among the members of this forum, the skills demonstrated in every build are truly amazing.  But if I can provide some clarity on some of those questions that puzzle, then I hope that is an adequate contribution in thanks for all that I learn from everyone else.  But thank you also for your kind words.  I am glad you are enjoying the thread, and I hope we will eventually understand and solve your condensate issues.  That will be a real indication of its worth.

Hi Steam Haulage, that sounds like interesting chemistry you were doing, much larger than the usual idea of a test tube.  Those heat transfer fluids like Dowtherm and others come into their own when the temperature-pressure combinations with steam are not suitable is some way.  As you say it's not always immediately obvious why, without looking at a lot more detail.  They have the advantage of not being considered flammable, though you can break them down and make a lot of carbon of you get them too hot, especially in contact with air, which is better excluded.  They have a very low vapour pressure so better for transferring heat than doing work in an engine.

It looks like we are back to the areas of interest, I was going to start into boilers with scaling issues, but a combination of thunderstorms at my grandsons' cricket match, which was very even and went way overtime, together with a very long delay to a train with my granddaughter and her whole school class on board, returning from their school camp, has run away with the evening.  Fortunately nothing worse than a long slow evening dealing with it all unfolding, but little time left, so I will continue tomorrow.

Thanks for your interest,

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on November 24, 2017, 07:19:11 PM
Thanks MJM....it always helps when we know the teacher is not just one page ahead in the same text book  :happyreader:

The group in their mire was certainly not this group, but the fellows clambering for an understanding of the overheat at the Port Kembla steel works in 1995.....[as per my PM]...where we sought the assistance of a person trained in this same transfer of thermodynamic energy

Derek
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 25, 2017, 01:36:55 AM
Hi MJM, thanks for your reply..i have heard of naphtha but thought it was something you put in clothes cupboards to stop moths eating them ...I looked in my book on chemistry and was confused to find the sentence about Moles !! Perhaps i need to start right from the beginning !! Interesting info on these posts as usual....Some of this info was known in 1880 as well!!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 25, 2017, 10:54:27 AM
Hi Derek, I don't think this thread would be possible if I was only a page ahead.  At the very least, everyone would have to be looking at the same book.  Very clearly some are not.  But I like the analogy.  Some topics I have a good understanding due to my daily use of the concepts throughout my career, but other areas, I am the equivalent of only one page ahead, so I do try and use appropriate terminology to indicate the degree of confidence.  At work I was surrounded by others in the same field all with different fields of speciality.  The conversations over lunch were always interesting.

It is always surprising to find how wide a range of problems can be made a little clearer by simply applying that first law, or conservation of energy, and looking at all the heat sources, storage and losses, and doing some simple temperature measurements and calculations.  I have not had much to do with steel works, beyond sourcing a compressor for the furnace blowing, but we didn't get the job, so that was it.

Hi Willy, I think you might mean naphthalene, a white waxy feeling solid usually in flakes or blocks, while naphtha is a liquid quite similar to gasoline.  Neither one (gasoline or naphtha) is a pure substance, but rather they are both mixtures of substances described by their boiling point range and average molecular weight and vapour pressure.  There is some overlap in the actual range of components in each.  To understand the equations on that page you attached, you will have to understand why they talk about moles.  Not cute little animals, but a quantity proportional to the molecular weight of the atoms.  Unless otherwise qualified it usually means one gram for each number of the molecular weight.  However lb.mol and kg.mol or kmol are also used when imperial or SI units are being used.  It is used to determine the quantities of the reactants in a chemical equation.  So in
    2H2 + O2 = 2H2O,
two g of hydrogen plus 32 g of oxygen react to make 36 g of water, with the release of a significant heat.   The term is also used in many thermodynamic calculations, though often in the form of mass divided by molecular weight explicitly instead of using mols. 

If you really want to start at the beginning, I would recommend a newer book.  The science has developed a bit since then, and in Derek's terms, the pioneers were only a few pages ahead of you.  Better to start with the current state of the art.  Which is not to put down the amazing work they did, the current state of the art has developed on the shoulders of those early pioneers.  It is just easier for modern authors to describe when they already have the end of the story, or at least the next volume anyway.  I have noticed the difference between my student text books, and those of my son, a single generation later.  But very interesting to see the state of the art at the time the prototypes we follow were being developed.  Modern knowledge of the subject only makes their work more amazing.

Back to boilers.  I suggested a start on boilers with some considerations of scaling.  Let's look at the steam shovel Chris is making.  I think he is actually drawing using full size dimensions, and he will scale it down later, a beautiful advantage of computer modelling.  My industry did full scale chemical plants that way.  So possibly still thinking about scale.  But let's assume for example, that he chooses 1/12 scale or 1 inch:1 foot.  I think you occasionally drop in, Chris, so please feel free to chip in with any corrections to my assumptions.  Your comment in your build thread prompted my line of thought.

All the linear dimensions are easy, divide the full scale dimension by 12 to get the model size.  Then the scale factors start to compound.  Areas on the model will be 1/12 x 1/12 , or (1/12)^2 or 1/144 of full size.  And volumes, 1/12 x 1/12 x 1/12 or (1/12)^3, making 1/1728 of the volume.  Well at least the mass rapidly becomes manageable, but what are the implications for the boiler?

Our primary interest is whether a scale boiler will produce enough steam for the model.  So let's start by looking at the engine size and the effect on steam consumption and power output.  Well, we have already seen that the steam volume required is equal to the swept volume of the engine times rotational speed.  The swept volume is piston area times stroke, times 2 for double acting.  That simplified calculation omits the effect of the piston rod, but close enough for our purpose.  So the scale factor for piston area times stroke is 1/144 x 1/12, the volume factor of 1/1728.  We still have to estimate the rpm, however if we have an operating pressure and temperature, the steam tables will tell us the mass of steam we must generate.  We can also look at the torque - the maximum torque is proportional to the force on the piston and the crank throw, so that volume factor again, if we adjust it for operating pressure.  However, the scale engine has only to lift scale masses through scale dimensions, so it's looking possible.  Of course in a model, friction tends to be proportionally higher than in full scale, and that is a wild card.  Let's leave aside consideration of scale pressure, or rpm for the moment, and look again at the boiler. 

I think the most obvious requirement is to have a scale outline.  Otherwise, the boiler would not fit the allocated space in the model.  So with length, or diameter and height, to scale, the volume will be 1/1728, so in keeping with the steam quantity, and the necessary volume of water.  But will it generate the required amount of steam?  Well, the stack will be scale height, 1/12, so the draft should help us with the air flow.  But the tubes?  I am guessing a vertical boiler with flue tubes for air flow and heat transfer.  I think I saw a part boiler outline in one of Chris' earlier posts.  Probably gas fired if the gas bottle can be hidden somewhere, it would be hard to stoke a coal fire unless Chris is really planning going overboard on his carved figures, and make them work too.  It will eventually be revealed.  So will those tubes allow the necessary hot air flow and transfer the necessary heat to the water with that scale draft?

This is where things get complex.  The flue gas will be similar molecular weight and composition to the real machine.  It will have similar density, specific heat, viscosity, and thermal conductivity.  Flow through tubes is not too bad to understand.  Friction factor, draft and velocity related by a known equation, which involves density tube wall roughness, viscosity, and the well known Reynolds number.  However heat transfer introduces further complexity.  As the flue gas progresses through the tubes, it looses heat to the tube wall and becomes cooler.  Temperature affects density and viscosity, and those both affect velocity due to the draft, and the velocity affects heat transfer.  Which affects the temperature and so on.  Even the advanced texts introduce more of those dimensionless parameters, parameters which make Reynolds number seem very elementary to use, and resort to empiracle factors.  I don't have the expertise to go too far into this, I am nearer only one page ahead in the book on that, but in a nut shell, this is where we must stop trying to make a scale boiler, and instead recognise that we are building a real boiler, albeit a rather small one.

Let's call a break there, already a lot to take in.  We will see if we can progress a bit further tomorrow.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: crueby on November 25, 2017, 01:34:00 PM
I was not planning on full animitronic figures for the stoker, but I can try!! Closest I'm getting to that is to do wire-driven systems for the head and flippers on the sea-turtle RC submarine...
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 25, 2017, 11:53:17 PM
Hi MJM, In the old days of live steam loco building with coal..(battle of the boilers) LBSC designed these boilers that worked tolerable well with fewer outsized tubes and superheater flues using stock material ,etc etc . So Could there be any considerable improvement by making very small/ large changes to the published designs. ?? Thinking about the I.M.L.E.C  here in blighty that takes place every year.!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 26, 2017, 11:41:14 AM
Hi Chris, yes, I thought a working fireman might be over the top, glad to see you are following.  I hope you do not mind me mentioning your boiler question as an example for this part of the thread

Hi Willy, I believe there have been a few articles where people have written up experiments on experimental work on boiler design, but the main mentions of them I have found refer to very old magazines, not accessible to many of us, and certainly not accessible for me.  I suspect that people like LBSC came up with reasonable designs that could be relied upon after trial and error, with plenty of errors early in their model making career.  And we benefit from their experience which they have been kind enough to share.  Many good models have been built by following their designs.  But I don't expect that the designs are necessarily optimum.  It's a lot of work to build a series of boilers with slight design variations, especially after you have an adequate performance.  Those IMLEC competitions are really interesting.  I have not seen the latest ones, but have old magazines with reports on some earlier ones.  One article included the calculations they do, and they looked pretty good to me.  Of course they measure overall thermal efficiency based on speed, drawbar pull and coal burned.  This is a very severe test of a locomotive as all the friction in the locomotive cylinders, valve gear and axle bearings, feed water pumps, even the tender, all count against your measured efficiency.  Just to get on the board is a significant modelling achievement.  Unfortunately for our purposes, the results I have seen do not separate boiler efficiency from engine performance, so hard to draw conclusions on specific design features.  When you do a steam test on your boiler, and we put it beside my Meths fired boiler tests, we will not be the first to do so, but we will be discovering and expanding information that is not readily available from other sources.  But always willing to include any work that others have done as well.  Quite a few data points are really required for any general conclusion.

Before concluding yesterday, I should have looked at scaling of flywheel inertia, and expanded a little on the step from the engine steam requirement, to work output, heat input and fuel requirement.  So let's have a little look at those.

Way back we calculated the moment of inertia for flywheels of different diameter and material.  The formula for a disc is I = 1/2 x M x R^2.  If we look just at a thin rim, I = M x R^2.  Mass in the rim is much more effective, you will appreciate that the mass of the disk is much heavier than the mass of the rim for the same moment of inertia, so increases are best made in the rim section and its diameter to minimise weight increase.  As the mass is proportional to a length ^3, the inertia for a small flywheel is 1/1728 x 1/144, so way smaller than the linear scale factor or even the volumetric scale factor.  However it is not immediately clear whether the flywheel will need modification or not.  Inertia depends not just on mass, but the mass distribution.  The purpose of the flywheel is to store energy during the peak torque parts of the revolution, and feed it back in the low torque parts of the revolution, top and bottom dead centres.  It stores energy by accelerating or increasing rotational speed, and feeds it back by slowing down.  The energy stored in the flywheel is proportional to the rotational speed squared, so the lack of moment of inertia can be made up for by an increase in speed fluctuation throughout the revolution.  Additionally, it can be shown that if the rotational speed (rpm) of the model is higher than the full size machine, there is more energy stored and the percentage speed change required will be less.  So while it depends on the operating speed of the model compared with the full size, it is worth being aware that some experimenting with the flywheel inertia may be required.  I will leave it to those with more experience of running scale engines to say whether scale flywheels are adequate, and how operating speeds compare with full size speeds.

Yesterday, I did look at engine size, and work output of the scale engine, but perhaps did not make it clear that give or take some adjustment for pressure and engine efficiency, producing 1/1728 of the work, requires 1/1728 of the fuel and strictly requires 1/1728 of the air.  I am a bit wary of being adamant , as I am not sure that our combustion air is as well controlled in a model as in full scale, and we may have extra excess air.  This would mean a greater volume of flue gas, and a lower flue gas temperature, which of course all affects our heat flow.  But as a reasonable assumption, it means we need that 1/1728 fraction of steam to be produced, as I assumed yesterday.

Basically we are at the stage of looking at how to design a real boiler, which involves real flue gas properties, similar flue gas temperatures to our prototypes, real steam properties, but just in very small size.  When we reduce the boiler shell size, it reduced the volume but only in proportion to steam flows, but when those flue gases have to flow through very small tubes, the ratios of heat transfer area and friction pressure drops do not remain in proportion.

Designing the full size boilers was not my area, and I don't know if the correlations they use really extend down to model sizes.  But even in those full size boilers, the data they use is based on experimental data.  They just have plenty of data available, it's even all rolled into computer programs which tends to disguise the fact that it's basis is in the end experimental data.

So where does that leave us in designing our boiler.  I really don't know of any way to come up with a straight theoretical design.  So the next best thing is to look at some successful model or small size boiler designs which are known to work well enough, and build that heat transfer surface into our scale size outside shell.

It has been mentioned before that KN Harris in his book on Model Boilers gave some figures to use as a basis for predicting the amount of steam we might get from a model with a given heat transfer area.  Next time, I will look at his figures, and compare them with our understanding of heat transfer.  He was a very practical man, and I have no doubt about the figures as a guide.  However, if we can understand how they relate to the thermodynamics, we might better understand where they come from and their limitations and hence get better guidance from them.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 27, 2017, 11:11:29 AM
Not surprising that there are few questions on yesterday's post, it is never satisfactory to get to a point where the theory does not really provide an answer, at least an answer we can all understand.  In fact there are only a very few convection problems that can be completely solved by theory alone, and generally some experimental data is required.

So we move on to looking at KN Harris' guideline for steam that can be expected from a boiler design.  He suggests a figure expressed in cubic inches of water evaporated per minute per 100 sq in of heating surface .  Now that is surely a most inferial of imperial units!  Actually, on closer examination, it is not really such a silly unit, as he suggests figures ranging from 0.6 to 2 1/2 cubic inches per minute which cover a wide range of boiler designs, and are very convenient size numbers if all your calculations are being done by hand, as is likely at the time the book was written.  So let's look to see if there is any justification for these units in thermodynamics.

Earlier, when looking at condensing, I looked at the basic equation for heat transfer in a heat exchanger.  Q = U x A x delta T.  If you are not familiar with the delta terminology, it basically is a short term for temperature difference, usually written as the Greek letter/symbol.

This equation describes the basic relationship between the critical variables in any heat transfer problem.  A is the heat transfer area, and is exactly what you would expect, and measured in square meters.  Delta T is a little more complex, sometimes it is a logarithmic mean temperature difference, which is a form of effective average used in heat exchangers where the temperature difference varies along the feat transfer surface, which is biased towards the end of the exchanger with the higher temperature difference.  But basically it means the heat transfer is proportional to the temperature difference, however that difference is measured.  However, in a fired boiler, it is not very easy to measure the temperature in the firebox, where the highest temperature difference exists, so most heat transfer per unit area.

The complex factor is the heat transfer coefficient, U.  It is only a simple term with readily available data for simple conduction, for example through a flat metal slab.  For convection, it is quite complex. 

So if we look again at that equation, and combine the two complex factors into one combined constant, let's call it K.

Then the equation becomes simply Q = K x A.

The right hand side definitely looks like Mr Harris's equation, so does cubic inches per minute per 100 sq inches give us a measure of heat transfer?  And how much heat is required for 1 cubic inch per minute.

We can easily convert 100 sq in to square meters.  And if we use the density of water to be 1000 kg per cubic meter, a little maths converts the units to 15.24 kg per hour per square metre.  Now we can refer to the steam tables.  Of course, we need to select an operating pressure.  It seems that sometimes 50 psig is intended, and other times 100 psig.  I decided to work our the heat required for both, assuming just the heat necessary to evaporate saturated water to dry saturated steam.

It might surprise you to learn that it requires a little less heat to generate steam at 100 psig than it does at 50.  It's about the difference in the heat necessary to raise the water temperature to the higher saturation temperature.  Have a look at in the steam tables.  It's tabulated directly as hfg.  However at 50 psig or about 450 kPa the heat required is 2120.7 kJ/kg, while at 100 psig, about 800 kPa it is 2048 kJ/kg.  For evaporation of 15.24 kg/hr, it is the same as about 8.8 kW for the lower pressure and 8.5 kW for the higher pressure, remembering that these figures are for 1 cubic inch of water per minute.  The difference is probably less than the range of error in the basic figures in this context.

The remaining thing to look at is the temperature part of the heat transfer equation.  Basically, rolling the temperature into that constant K and having one value, or just a narrow range of values, for a boiler design means we are assuming the effective temperature difference is the same for all boilers of the same type.  It can still be different for a different boiler with a different value of K, but  generally I would expect that while the effective temperature difference might be different for different fuels, the same fuel, with the same burner type might give the same temperature in any boiler type.

At this stage, it is clear that the units suggested by Mr Harris are a reasonable simplification, and I believe he was a rather practical man who would have had some data from boilers he knew to base the figures upon, so I expect the figures given are sound.  However it is also clear that we need to read very carefully the description of the boiler design in conjunction with the rating, as I would expect that boilers would have to be compared on the basis of the same fuel as well as the same general boiler arrangement.  Generally he is clear that some would be better fired with gas or liquid fuel, while others are suitable for coal firing.  Similarly he says some would be very capable of operating at 100 psig while others are described as suitable for lower operating pressure.  So long as you are comparing on the basis of the same fuel and operating pressure as was assumed for the quoted figure, the figures in the book are a reasonable starting point.

Next time, I will have a look at what else we could do to decide if a given boiler arrangement would provide the amount of steam we need for a given engine.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 27, 2017, 02:32:53 PM
Hi MJM, more info to ruminate on ....I have been informed that the Cochran boiler at the Forncett museum takes about 8 days to cool down completely ,but it is well insulated !! however i don't know if this is still full of water or if it is blown down to empty it  ?? so is it better to let it cool down slowly or quickly when empty ...am thinking about thermal expansion /contraction here and what advice for loco's ?! I allways leave the water in the boiler on mine as it does not have a blowdown valve. Also i have been reading up about the Norwich heat pump installed in this BLDG just 300 Yds from my by house !! Apparently the brass pipes are still embedded in the river...also a graph showing Dewpoint/relative humidity /saturated air/weight ratio Moisture/air/grains per Ib  and dry bulb temp.......!! All this from a 1949 book...
Title: Re: Talking Thermodynamics
Post by: crueby on November 27, 2017, 07:19:40 PM
Willy, you have quite a book collection!

For the model boilers, I always like to cool them with the throttle open so as not to draw a vacuum as it cools the vapors, not sure if that is needed but it seems like a good idea. Once it cools enough to handle and the pressure is low enough I'll usually take the water fill plug out too, then drain it when all is cool.
MJM - good question for you, is the vacuum it will make when cooling enough to require doing this on a model?
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 28, 2017, 11:57:37 AM
Hi Willy,  that Cochrane boiler is an interesting one.  I assume you followed Florian's wonderful build.  Eight days certainly indicates good insulation.  I suspect it might be full of water, or nearly so to require that long.  It is helpful if you normally want to start up again quickly, however a real pain if you want to enter the boiler for maintenance.  In the oil industry, I seem to remember a guide line of around 100 degrees per hour for heating or cooling to prevent thermal stress problems, but they generally involve relatively simple vessels unlike that Cochrane or the average locomotive style.  I suspect the forum members who have operated real engines have a better handle on their guidelines for full size boilers in marine or locomotive use.  I will come back to blow down shortly.

That heat pump is really interesting.  Of course a heat pump is simply a refrigeration cycle (run in the normal direction by the way, despite the sales hype about reverse cycle,) but where you are interested in the heat rejected from the condenser, rather than the heat absorbed by the evaporator.  So that building, instead of burning coal for heating, used the fact that the river is warmer on the bottom, and because it is still running, has more heat content than ice and they can get heat by cooling the river a little with that submerged evaporator, and reject it into the building at the higher condenser temperature.  The power supplied to the motor also ends up mostly as heat, but in total the building only pays for the motor power, but also gets additional heat from the river.  If the pipes are buried below the river it is even better as you don't have to go far before the earths surface before you find a relatively stable reasonably warm temperature.  A great early example which is still paying off the original investment.

The psychometric chart is interesting.  No wonder they need a chart with those units, which they must have used to teach Mr Harris.  Grains per pound, really!  I hope you have a grains unit option in your calculators.  But to use units like that, or any units really, in those days before calculators, those charts were really necessary.  And they had advantages.  To get reading more accurate than perhaps two or three significant figures is not usually possible, which keeps your feet on the ground with regard to the accuracy of the data.  This is a factor is easily overlooked when even a cheap calculator will give us seven figures if the calculation does not come out to an even number before then.  Let alone using double precision on a 64 bit computer.  The other advantage of the chart is that you can relatively easily see how those variables are changing as conditions change.  But there is so much on that chart, it's a bit mind bending trying to work it out.  So the emphasis is on the relatively part.

The chart is very useful to weather forecasters who are interested in using their wet bulb thermometers to determine humidity and dew point.  These days it is easier to just use the steam tables, and the weather forecasters basically have the elegant sections built into their computer programs.  They now use SI units, though with their own little idiosyncrasies, specifically hectopascal for pressure.  At least it is obviously based on the Pascal, or Newton per square meter.  The only other bit you need is that relative humidity is the percentage of the saturation vapour pressure for water at the air temperature.

Hi Chris, I certainly agree with you about Willy's book collection, it includes some very interesting titles.  The maximum vacuum produced on cooling of a sealed boiler can be seen in the steam tables.  If there is no air, the total absolute pressure in the boiler will be the equilibrium water vapour  pressure at the final cold temperature of the boiler.  However for normal room temperatures the vapour pressure is sufficiently close to zero that for practical purposes it is better to assume absolute zero pressure (or -14.7 psi) rather than worry about the likely actual minimum.  I have been involved with vessels and piping in Ontario, Canada, where the daily winter temperatures often sat around -20 F from memory, or the more impressive, though harder to remember -33 C.  The difference from zero pressure is not enough to affect vessel design, even at 15 C, where the equilibrium vapour pressure of water is 1.7 kPa, while standard atmospheric pressure is 101.3 kPa.  To design a vessel for external pressure  is more complex, as you have to calculate the pressure at which buckling of the tube occurs, a catastrophic failure a bit like squashing the tube flat.  If our boilers were designed for low pressure and made from the minimum thickness required by the usual thin shell formula, they would certainly be in danger.  For a boiler designed for an engine expected to do a bit of real work, so say 75 to 100 psig, and then made to the next larger tube wall thickness anyway, most designs will stand more than 15 psig external pressure.  The answer for the ends is not so clear, as they are basically reinforced flat plates.  Some designs are good for either direction so are ok, but I have not extensively checked this out.  When I design my boilers, I certainly check for external pressure equal to full vacuum, and I would hope that these days, most designers do.  I don't know what has been done in the past, and it is always safer to use a procedure that does not cause a vacuum unless you know for sure that the design has been checked.

The problem with admitting air to prevent that vacuum is that with air comes oxygen, so it is better to avoid admitting air, particularly with steel boilers.  It is not so important for copper boilers where corrosion is not the same problem.  I know of one boiler where the operating procedure solves both these issues by keeping the feed pump running, with the bypass on the level valve open, so that as the boiler cools down, more water is admitted, so that is is finally cold and full of water.  Obviously the startup procedure has to take this into account and let the water out down to the normal level under the first wisp of steam pressure.  However for copper boilers, I think there is no problem with your procedure.  Except for the issue of blowdown.

I would recommend a good blowdown procedure, even for our model boilers.  All the water we use to fill our boilers contains some salts.  Distilled water has a very low concentration, but normal drinking water varies from place to place, and may range from needing some extra flavour, through to hardly drinkable unless you were born to it.  When water evaporates to make steam, essentially all the salt is left behind.  Then we top up with more water, with its attendant quota of salt, boil away the water while the salt stays behind as before.  Soon the salt concentration in the boiler is much higher than in the feed water.  Unfortunately most salts have a solubility limit, above which the excess salt precipitates as a solid.  Especially calcium salts, whose precipitate is barely soluble in water so the solids build up in the boiler and I suspect we have all heard at least one horror tale of the result.

To some extent, emptying the boiler when it is finally cold removes all the remaining salt from the boiler, except for those calcium salts.  Unfortunately their solubility is low enough that they will precipitate as the boiler cools.  And emptying the boiler only removes the liquid with it's still dissolved salt.  You probably won't even see it for the first few runs, but it builds up a little at a time, and once precipitated, is notoriously hard to redissolve.  You can see it in your household kettle, where the purpose is not steam, but water, so is nearly all blowdown into your teapot.  The remaining salts still manage to build up.  You can already see it on the heating element of that new glass kettle I bought recently, and we have a particularly good water supply.  Many people have their favourite recipes for cleaning the kettle, the iron and other devices, but the best is to minimise the requirement by removing a good quantity of the water while it is still hot.  In the full size boiler, it is done under the final low pressure to force it out , but if you only have a plug, you might scold your fingers.  It really needs a little blowdown valve, like the ones used for cylinder blowdown, to get most of the water out before the salts start precipitating, with a tube to take the water to a suitable container.

I mentioned a full size boiler, but if the boiler is intended to run 24/7 for long periods, it is not sufficient to blowdown on shutdown.  The operating procedure usually calls for a mixture of a small continuous blowdown, supplemented by the occasional intermittent blowdown.  Of course they often have the benefit of some laboratory equipment to test the boiler water and guide the decision on how much to blow down, and when some extra is needed.  The actual amounts will depend very much on the quality of the feed water treatment.

A bit of a diversion tonight, though very much on boilers.  Will try and get back on track next time.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 28, 2017, 03:26:13 PM
Hi MJM , More interesting info and talking about the Cochran Boiler i don't quite understand the text that talks about elongation of 20 % in 8 inches ?? does it means the tubes expand to 9.6 inches ?? also the heat pump was only used for about 3 years as it was used for experimental purposes  to see the savings. There was also difficulty getting the parts for it just after the war. !! I can provide more info if you would like and just using the electric for the pump gave a savings of  .8 d per therm without thermal storage  to 2.3d per therm with thermal storage !!!  d = pennies in old money btw. Of course one of the advantages of the heat pump is that you can use it to cool the BLDG in high summer !!?.....
Title: Re: Talking Thermodynamics
Post by: crueby on November 28, 2017, 06:45:34 PM
I think that means the metal elongates that much when under test at 20 to 30 tons, before failing.
Title: Re: Talking Thermodynamics
Post by: paul gough on November 28, 2017, 10:56:21 PM
Hi Willy, MJM, regarding boilers and steam raising times or rates, it is somewhat variable. It depends on the type of boiler, whether there are 'delicate' structures in the firebox such as brick arches, the quantity or mass of the refractory such as firebrick walls etc.  'Tight' boilers, e.g. locomotive types which have riveted construction and a myriad of very rigid firebox stays need to be treated carefully, fractured stays and fractures in the firebox plating especially around the foundation ring are costly failures. Also excessive expansion will cause leaking tubes, a pain in the butt. At times full size practice got a 'cold' loco into steam in 3-4 hours, but this is fast, six or a bit longer would be better. It is also important to note that full size practice can not always be best to follow. Full sized locomotives were generally in steam for something like a week and their boilers were regularly rebuilt every 3-4 years if working all the time, actual time depends on how hard they worked and water quality, not many modellers would want to rebuild this often. Secondly, it is also important that from cold to a bit beyond boiling point, say 5 p.s.i. is when things need to go gently, from here everything should be 'hot' and expansion not so much of a problem, things can speed up a bit. Obviously model copper boilers can withstand heat up times much faster than larger steel ones and their mass is insignificant compared to big ones, but any steel boiler should be treated gently. With my 12" gauge loco, 14'' Dia barrel, I usually allowed 4 hours to 5 p.s.i. and it had a Briggs coil firebox, the other locos with standards loco type fireboxes never less time and we took a little more care while things were 'cold'. As to blowing down, usually we let things cool down naturally after dropping the fire and blew the boilers down to empty from about 5 p.s.i. if we were going to store the boiler dry for an extended time, the residual heat aided in drying the boiler out. If the loco was to be steamed again within say a month or less then it was generally stored wet. In this case it is filled till the injector can't get any more in, the injector overflow will run continuously, or the injector will cut out due to low pressure when things get near full and the boiler would be topped up through a plug when cold. Important to have the boiler absolutely full of treated water to exclude air and thus oxygen, an enemy of wet steel. Hope this gives you a bit of an idea on how things go. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: derekwarner on November 29, 2017, 01:24:12 AM
Thanks Paul for the scenario on larger 12" Gauge rail engines raising steam etc

My only experience [even as Secretary of the ILS] has been in watching [asking & listening to my questions] of 5" gauge engines in their steam raising and shut down procedures which differs somewhat over your larger gauge engine

Many of my colleagues even with a vast  :old: working experience with 5" gauge copper boilers, have little experience with smaller copper boilers for the model marine steam ...say around the 4" diameter vertical of horizontal 500 ml capacity gas fired boilers

These model marine boilers are often shut down when a low water level is reached & I had not previously considered the best way to shut down the boiler until the next steam-up which could be 1 week or longer away

My only consideration to date has been the installation of an anti vacuum valve on the boiler top to stop steam oil being drawn backwards into the boiler

I understand the only path for this steam oil is via the steam stop valve which should be closed after shutdown, so the anti vacuum valve is a double insurance

So MJM & followers, from this could I open the question of recommended shut down procedures and any additional storage points for such model gas fired boilers 

Derek

 
Title: Re: Talking Thermodynamics
Post by: paul gough on November 29, 2017, 02:20:16 AM
Hi Derek, It is a long time since I was a member of the group that has the 12" gauge railway but still get down to see whats going on and sometimes have a play with the engines. For some years now I have been fiddling at the edges of Gauge One, a couple of Aster Lions, and am changing a few things on them slowly as I get the time.

I am not very experienced directly in the ways of the in-between gauges,i.e. between Gauge One and 12" and bigger, but have been something of an occasional observer since an apprentice, back then visiting Sydney Live Steam Loco Soc. on weekends in the sixties. I am interested to know why your steamboat boilers might need a differing approach to shut down from others? Is there some reason why it is not practicable to blow them down when there is a couple of pounds left and store them 'dry', then fill them to an appropriate level and fire them up again when desired? Looking forward to learning about this as I have access to more than a kilometre long stretch of still water, most of the time, and have often thought of a steam boat project after seeing a couple of the builds on this site. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 29, 2017, 11:51:18 AM
Sorry everyone, a huge day today, with an early start, over 30 deg, ending with a four hour drive into the bright setting sun.  A long post is not happening tonight.

I will leave it to the many who have much more experience thanI on these normal operating procedures to continue the conversation.  I hope to be back tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 30, 2017, 11:01:38 AM
Able at last to catch a breath for long enough to look at the posts from two days ago. 

Hi Willy, that is a very interesting article on the Cochrane boiler.  Now we know the origin of the name.  I had assumed it was after the designer.  Perhaps it still is. 

The heat pump is just a more generic name for the refrigeration cycle, and whether we call it a heat pump or a refrigerator purely depends on whether we are more interested in the heat rejected at the condenser or the heat absorbed (from our cold space) by the evaporator.  We tend to call it reverse cycle when we have the option to do either.  However this does not involve a simple "flow the other direction" sort of meaning.  It means the equipment includes valving to exchange the position in the circuit of the outside heat exchanger, in this case, the pipes under the river, with the indoor heat exchanger.  So that the indoor exchanger can either absorb heat to cool the building or reject heat to warm the building.  You may notice if you have one of these in your home, that if you change the function from heating to cooling or vice versa, it sits for quite a few minutes with sounds of gas depressuring and liquid gurgling as the pressures settle out to uniform throughout, before it starts in the alternative mode of operation.  Not sure of it is a system of three way valves, or just solenoid operated block valves that get operated in the appropriate sequence.  They are usually hidden inside the enclosures.

I was brought up on £sd, we only changed to $ and c around 1966.  So perhaps you could say I am bilingual.  But on a world wide forum, it probably did look strange to many.

Hi Chris, thanks for the answer on elongation.  I hope you don't mind if I add a little detail.  But as you implied in another post, I am not good at brief! 

Because elongation is quoted as a percentage, the eight inches does not matter, but it tells you the length of the sample used by the test procedure, and while it implies that the 8" tube would expand as you said, it is not intended to work quite that way.  To provide a little background, specifications for materials include both chemical composition and strength requirements.  The chemical composition alone does not guarantee a particular strength, as the strength is determined not only by the composition, but also by the grain structure, which is influenced by cooling rates, subsequent heat treatment and the degree of hot and cold working in forming the completed product.  The basic test is the tensile test, where a defined size of test piece is stretched in a machine until it breaks.  During the stretching, the change in length of a carefully specified part of the specimen is recorded, and a graph made of the stress, tensile load divided by the cross sectional area, and the strain, change in length per unit length of the specimen.  When tension is initially applied, an elastic material behaviour gives a stress-strain graph which is a straight line like a spring.  If the load is reduced, the graph retraces its path, at least pretty nearly exactly. 

At a certain point, called the yield point, the material suddenly gives a little, and there is a little stretch without any significant increase in load, a short roughly horizontal part of the curve, then the load further increases the strain continues to increase, but no longer linear.  This part of the curve is called plastic deformation, and when the load is relaxed, the material length goes down but does not retrace the original path, some of that stretch is permanent, behaviour more like plasticine. 

If the load is increased further the plastic deformation continues, until the sample breaks.  The load immediately prior to breaking divided by the original cross sectional area, is called the ultimate strength, and can be quite a bit above the yield strength.  And the elongation immediately before break is recorded as a percentage elongation.  If you look closely at the broken sample, you will see that it has necked in around the break, but the strength is calculated based on the original cross section.

Now a high elongation figure implies a very ductile material.  This is very important because it has a major impact on just how the material fails.  Ductility means a lot of stretch before failure, the change of shape of the structure after yielding here in many cases redistributes the load and reduces the stress, so the failure is less likely to progress to rupture.  In that Cochrane boiler, the tubes are under external pressure (flue gas on the inside, water on the outside.  If the strength of the tubes is exceeded, the tube will fail by collapsing inwards, and will look like it has been pinched between rollers.  With a very ductile material, it will go completely flat without splitting along the sharp crease, and soon the tube is supported by its own opposite side and the failure progresses no further.  Obviously also good if you are going to expand the tubes to seal them.    For the boiler plates, you will notice that the firebox is also subject to external pressure.    Harder to say just how far they would fail, but preferably they should actually split enough to leak, so extinguish the flame (steam is quite good at that) but the leak should not progress into a full blown split.  The ductility allows the stresses to redistribute around the end of a split to stop it "running".

You will note they specified even higher ductility for the plates exposed to flame, where the higher temperatures means lower yield strength, and more likely to fail first.  The figures for the other plates imply a high quality steel with very uniform properties, and even if they fail, perhaps due to corrosion, the failure is more likely to be of a leak before burst nature.

Hi Paul, thanks for all that information on warm up procedures.  You are quite right in pointing out that the complex shape of many boilers, and locomotive type in particular which means that local stresses due to temperature gradients are more critical.  Thermal expansion alone does not cause thermal stresses if the temperature is uniform, the stresses come from temperature gradients and the resulting non uniform expansion.  Our chemical plant vessels are simple shapes even if the bolted in internals make them look more complex, so they tend to heat uniformly.  The important caution on heating rate in a chemical plant is to go slowly near 100 deg when heating oil and such, as if there is any little pocket of water, suddenly engulfed by hot oil, it instantly evaporates to a large volume, and unless the vessel is very large, the pressure necessarily has to rise, and there have been cases where this has burst the vessel.  Given a special name, this event is called a BLEVE, or boiling liquid, expanding vapour explosion.  Not good to be nearby.  But otherwise the simple vessel shapes tend to heat up quite uniformly.

By the way, surely you can hear that stretch of water calling out to you, why are you waiting?

Hi Derek, thanks for that additional information.  Oil is certainly undesirable in boilers as it reduces the film coefficients on heating surfaces, reducing steam capacity and causing hot spots.  Do you have a pumped oil system that causes the oil to by injected as the pressure falls?  I am not sure it would happen with a displacement lubricator.

More interesting questions leading interesting directions as usual.  Perhaps back on track tomorrow?

Thanks for following along

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 30, 2017, 02:24:39 PM
Hi MJM , Old books are cool !! and you allways get a bit more info from each although they talk about the same things ! One thing i have noticed in my green house is that when the water in the copper pipe irrigation system freezes and splits the pipe , the split is allways at the top of the pipe ?? any ideas about this ? also when it splits the pipe how does it decide where to do it ? so back to topic !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 01, 2017, 10:33:05 AM
Hi Willy, that pipe freezing question is another in interesting one.  First, we know that water expands when freezing.  The minimum specific volume is at about 4 deg C.  That s why the ice floats on a pond or in your drink. We also know that ice feels quite solid, and when subject to pressure it does not change much in volume.  In fact, if you try and restrain that expansion, it develops a very large pressure, and so breaks containers, pipes etc.  And again, we know that if you let water freeze around something, say a rod suspended partly in the water, the ice grips the rod quite securely, you can't easily pull it out of the ice.  The expansion easily breaks pipes and glass bottles, as the ice does not simply slide along or around the pipe as it freezes. If it freezes in a part filled bottle, it swells nearer the centre, rather than slip up the walls, and generally breaks the bottle rather than slip along.  You people who live in colder climates can correct me here if you disagree with any of these observations.  The pipe breaks when the applied stress is greater than the strength, with the exact location possibly determined by the weakest point in a slightly non-uniform material, however, why always on the top?  I don't really know.  We would expect the any weak points in different pipes to be randomly located, so if that was the explanation, it would not always be in the same location.  In a pipe, the first freezing will tend to be around the top due to that density difference.   Perhaps the explanation is in the way the ice grips the material, and once the remaining water is trapped, the expansion of the ice increases the pressure which will be very uniform in the liquid filled area, but that gripping the material means non-uniform stresses in the iced up area.  The split probably indicates the highest stressed area.  It would be interesting to measure carefully a split pipe and see if you can detect any yielding of the tube, or even see any stretch marks in the surface.  I will be interested to see if others have an alternative explanation. 

Of course, the big thing is to understand how to avoid the pipe splitting.  The best way is probably to have all the pipe below the house and buried deep.  Another method is to drain it, though this requires very careful installation to ensure that it does drain completely, as any remaining water pocket can still split the pipe.  You probably don't want to use antifreeze, as it's not good for the garden in spring.  Many gardens in Canada have fantastic ice sculptures I winter.  They keep the pipes flowing, and the sculpture builds itself where the water sprays out of the end.  This seemed more effective than I would have thought in the temperatures.  No doubt it is not fool proof.  A pumped circuit with some heat source might be better, unless you have plenty water to waste.  Electric heat tracing preferably with some insulation also works, especially on a blind end to an unused tap that cannot be buried deep enough or located under the house.   But I wonder if the irrigation system would be sufficiently elastic at the temperature, to not split if it was constructed from plastic hose?  But most plastics are quite brittle at low temperature, but perhaps the is a plastic that is still elastic at these temperatures.

Any way, back to boilers.  I was looking at trying to understand whether a scale boiler would produce enough steam to run the scale model engine.  Now, if the engine is a display model, and basically running unloaded, or perhaps minimally loaded, it does not need much steam and probably reasonable to assume our scale boiler will be adequate if we put a reasonable heat transfer arrangement inside the scale outline.  By reasonable, I mean tube diameters similar to those in boilers we know to work well, with a burner suitable for the firebox dimensions.

However, if we want the engine to really work, we need a bit more assurance that our boiler can produce enough volume of steam at sufficient pressure to provide the required torque. 

I have already stated that it is not really practical to make a calculation of the expected steam production from theory alone for any heat transfer arrangement.  In the absence of a theoretical solution, we need some experimental data, performance data for similar successful boiler arrangements.  In fact, at this stage, we only have some theoretical scaling factors for the engine itself.  It looks like it should be good enough to do the work required on model scale, though we don't know how the load scales to model size.  For Chris's steam shovel, we know the scale factors for the weight of the bucket, perhaps even with a shovel full of sand or suitable size gravel, and also for the height the scoop must be lifted, and this all scales to give us confidence that the engine would be OK.  The wild card is friction, where especially in a complex mechanism, there may be a disproportionally higher friction load to be overcome.  Where the load is almost entirely friction, for example a track drive, or  steering, it is more difficult to know how the load on the model engine compares with the full size.

Of course, Chris is in a great position here.  Having just completed his amazing Lombard log hauler, which clearly worked so well, he can say with some confidence that the scale engine is adequate.  And already having that completed model at hand, some basic measurements of fuel consumption of his gas burner, and also of the steam production, along with the now known required operating pressure, he is also in a position to make an estimate of the steam consumption of the engine, which he can then compare with a theoretical calculation.  The exercise when conducted in isolation might seem a bit theoretical, but in the context of embarking on the Marion Shovel, those simple tests on the Lombard will provide an very good idea of how well the theoretical calculation compares with the model operation under load.  The engine speed can be calculated from the speed over a measured distance and the known gear ratios, if a suitable tachometer is not available to measure the engine rpm directly.

But the Lombard performance also gives a very suitable estimate of the steam production from the scale boiler, as constructed with a centre flue type of tube arrangement.  He could calculate a "Harris boiler factor", though personally, I would suggest using more rational units.  Units which are consistent with easily measured weight of water per unit of time, and the steam table units for heat, and consistent units for area would minimise the need for conversion factors in calculating and using the factors.  So for example, I would use SI units.  This means kg for water weights, Watts for heat units and square meters for area.  Of course, it is reasonable to look at using grams and square cm to avoid having too many zeros in all the numbers.  Alternatively use engineering or scientific notation.  Seconds should be used for time to be consistent with the normal heat flow units of Watts, or J/s.  Again minutes might be considered.  Deg C for temperature and kPa for pressure.

For imperial units, lbm or pound mass for water and steam quantities, BTU for heat units and sq ft for area.  After all a sq ft is only 144 sq in, so using 100 sq in as the area unit seems to require unnecessary conversions.  The steam tables would then be using deg F and psi.

Which ever units are preferred, each data point requires recording of the operating pressure, burner type and heating area arrangement in addition to the constant for steam production.

Next time I will consider the small vertical boiler outline for the Steam Shovel.

Thanks for following along.

MJM460

Title: Re: Talking Thermodynamics
Post by: crueby on December 01, 2017, 02:07:12 PM
Hi MJM,


The engines on my Lombard model are actually a little off scale, I made the pistons a bit smaller to reduce the steam need, and also give more room for the head bolts. The boiler is quite different, being a single large firetube vs the dozens of smaller original ones, and the model extended the boiler main tube back through the firebox area. So, it is not close to a true scale boiler except for outside dimensions.


The boiler in the steam shovel is another horizontal one, real one has 115 2" firetubes and a 10' long 5' diameter water shell, with a 5' long and wide firebox, and a 1-1/2' long smokebox. I really doubt a true scale version would fire well, the firetubes would be only 1/8" , seems pretty small for good flow, isn't it?
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 02, 2017, 09:59:15 AM
Hi Chris, thank you for coming in with that information on both the Lombard and the Marion.  I had forgotten that you had adjusted the engine cylinder sizes, though you have confirmed my recollection of the boiler.

As I said yesterday, the Lombard is really fully loaded in the same way as the full size machine, though your videos all show running on the flat, where the full size might have had to go up steep slopes.  But we will all forgive you for that, as you would need to ensure the bumps were only scale size if you get off the flat surfaces.  However the big question is, are you happy that the engine with the slightly smaller piston diameter is sufficiently powerful.  Of course your operating pressure, running speed and gear ratios also affect the matching of the engine to the load.  But those are sensible adjustments that allow you to match the engine to the load.  But overall, are you happy with the performance?  And do you expect to reduce the cylinder diameters on the Marion for similar reasons to the Lombard?  No calculations of scale factors can match your observation of the resulting engines for being sure to account for all the complex factors involved.

I am glad you corrected me on the Marion Boiler.  I had an impression it was vertical, but now you mention it there was some early modelling that did show the front of the boiler.  This means that the boiler of the Lombard is again a perfect example on which to base your expectations for the new model.  You mentioned that you were worried about it being a bit small.  I suspect that with engine free wheeling or at least assisted by the weight on the downward travel, means that the engine is really only working half time, so the boiler can afford to loose a bit of ground, slowing up towards the top of its travel, and catch up when less steam is required on the down.  The steering would also be an intermittent steam load, but of course the drive would be a more continuous load when being used.  Fortunately the travel speed is probably not over critical. 

I totally agree that those very small tubes would be likely to prevent good gas flow for the flue gases, the wall friction will be larger in proportion to the gas flow and available draft, and this is not likely to lead to good steam raising performance.  I am certainly a believer in your approach of a suitable working arrangement inside a scale outline.  It is a very small size boiler we are making, and have to look at the physics of real flue gas and heat transfer.  I presume that you will again use a centre flue type of arrangement, and a similar gas burner, even if a different size.  So the Lombard model is a really good indicator of the likely performance, like the Harris factor, with known operating conditions.  But I would suggest Watts and square metres, or Btu/hr and sq ft as the units.  The heat transfer area ratio would be a good scale factor for steam production for the Marion and other similar centre flue boilers, such as those used in marine models.

I am not so sure whether we could then use the same scale factor for a vertical boiler.  Before we go too far down the track with assumption on a vertical, we really need some test data from a small size vertical.  I am hoping that Gas Mantle will do some tests and come in with data.  As he also has the option of coal firing, comparison gas and coal fired test results would be really informative.  Also any comparison of simple verticals with a single centre flue with the multi tubular type, each correlated to heating surface area, would be most instructive.

I have spent a great deal of time perusing the builds on this forum from long before I joined, and there are many amazing builds recorded here.  It is a real treasure trove for every one interested in model engine building.  Many of the threads include boilers.  If we can all, over time, do some simple test runs, we will have a real reference to help predict the likely performance of layouts people are proposing to build, far more useful than any theoretical predictions.

A shorter post tonight, I don't have much more to say about performance prediction for boilers.   Tomorrow I will look at what goes on in the boiler, then perhaps move on to the behaviour of gas and even liquid fuels, as there seem to be many questions on these in the wider modelling press.

Thanks for following along

MJM460


Title: Re: Talking Thermodynamics
Post by: crueby on December 02, 2017, 02:15:43 PM
The lombard model runs fairly well, but I suspect that the ports in the cylinder are too small to get good flow, especially on the exhaust. Next time I am bored and want some fiddly work I want to pull the cylinders and drill out the passages. It runs well no load on air, on steam I think it is getting to much back pressure.
The burner is still a work in progress too, not enough heat from it to keep pressure up. I had a larger capacity burner made, with more holes and the next size up nozzle, but they neglected to increase the size of the air intake holes and it won't burn properly. Again, need to disassemble those parts to redo. Can you tell I hate taking things apart and reassembling them over and over?! Especially when having to reseal joints. Probably will do that in the spring, too cold outside for pleasant steaming anyway.
I am not planning on cutting down the size on the pistons for the shovel, want to keep max power, and I think this one will be demonstrated on air mostly, and won't be needing to travel long distances like the guage 1 locos do.
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on December 02, 2017, 02:58:27 PM

I am not so sure whether we could then use the same scale factor for a vertical boiler.  Before we go too far down the track with assumption on a vertical, we really need some test data from a small size vertical.  I am hoping that Gas Mantle will do some tests and come in with data.  As he also has the option of coal firing, comparison gas and coal fired test results would be really informative.  Also any comparison of simple verticals with a single centre flue with the multi tubular type, each correlated to heating surface area, would be most instructive.


Hi,

I've just bought a larger ram hand pump for the feed water and a proper firing shovel to make controlling of the boiler easier, I've also managed to get hold of a 3.5" dia ceramic gas burner for when I can't be bothered with coal.

The pump has larger size fittings so I've ordered larger pipework and some more solder, once I get the pump fitted up (in a few days) I'll do a few tests - I wanted to post a video of my Grasshopper engine running on steam  :)

I'll get some camping gas on Monday and see how that performs too :-)

Peter.

Title: Re: Talking Thermodynamics
Post by: paul gough on December 02, 2017, 10:52:19 PM
In case somebody wants to know the general proportions of a satisfactory very small boiler and engine on a loco, the following sizing and performance might assist. The boiler is from a Gauge 1 Aster lion, the barrel is 35mm OD, 33mm ID and 110mm long between the front and rear tubeplates, 3 fire tubes 1/4 ID. with boiler pressure of 60psi. Water volume at 70% full is approximately 60ml, the external firebox has two methylated spirit, (alcohol), burners with wick tubes 3/8 Dia. The engine differs from the original single cylinder, 10x14mm, in that it is now twin double acting cylinders 3/8x3/4. The boiler performance is satisfactory and the safety valve will 'feather' for considerable periods even with a maximum load, for this track, of seven four wheeled wagons, three of which were filled with fine gravel and whilst it slowed on the 4m long 1:80 grade, it had no difficulty with it, all of this at a sedate speed something approximating a normal walking pace on the flat areas. Commencing with a full boiler and tender,  (with the feed pump by-pass shut continuously), tender holds 55ml of metho and 55ml water. The train ran continuously until fuel or water ran out, can't remember which and covered about 81/2 laps of the 80 metre oval track, so over 600 metres. Unfortunately I did not note the length of time for the run. In general, the best proportions for fire tubes, horizontal, seems to be 1:20 maximum, i.e. ID to length. The Lion is about 1:18. With the initial twin cylinder conversion, despite the swept volumes of the single and twin cyls. being approximately equal an experiment with 3 wick tubes in the firebox was tried, but it was found to be excessive and unnecessary. At some time in the future when I get all the adjustments I want to do completed I will attempt to carry out some accurate quantitative testing. The slide valve port sizes are 1.5mm for inlet and 2mm for exhaust. Hope a bit of this provides someone with a ball park guide for their needs. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 03, 2017, 11:00:39 AM
Hi Chris, I know what you mean about not taking a nicely sealed and operating engine apart.  I would also be reluctant.  So this is where this thread can demonstrate whether it is useful or not.  All that theory is only of academic interest of it can't help us solve practical problems.  So here is what I would make of your observations.

There are potentially two separate problems, the engine and the burner.  But we should also look at the boiler.  If I understand your description correctly, the engine runs ok on air, I think you even showed it running across the floor with an air hose trailing behind in one of your videos, so looks good running loaded as well as unloaded.  Now, I know I have been very shy about revisiting that original question from my first few posts, hit an area where I was not enough pages ahead on the very first topic.  I have been hesitant to just sit down and do the work, but I am still reasonably confident that within the margin of reasonable pressure adjustments, if it runs well on air, it should also run well on steam.  So while you feel that the exhaust passages could be larger, and will make them larger on the Marion engines, it is probably not the real problem. 

The effect of small passages is that part of the work done by the steam on the power stroke is used just to push the exhaust out from the other side of the piston.  If we were short of power, larger passages will leave a bit more of the work done by the steam available for shaft work.  So it would give you more power out of the same engine and steam flow.  However, if the engine power is adequate, more power might give you a bit more speed if you want it.  It does provide more torque from the engine, so should allow the engine to run on a bit lower pressure.  However there are always more than one approach to solving a problem, and more pressure can be provided by more heat into the boiler. 

In this case you mention that the burner seems unable to maintain the boiler pressure.  Compare with with Pauls little locomotive, running at full load with the safety valve simmering.  So your work on the burner seems to be the first priority.  You mentioned that you have a burner with a larger orifice, so that will give more fuel flow.  So you are in the right area, working on the air flow to burn it cleanly.  One other point you might look at is the temperature of your gas tank.  Pretty easy to see if the walls are wet with condensation.  Inside the gas tank, your fuel is a boiling liquid, behaving much like the water in our boiler.  At lower temperature, the equilibrium vapour pressure is lower, so less fuel flow through the burner nozzle.  This could be the issue if the burner starts off great, giving plenty of heat but then fades as the condensation, water from the atmosphere, or even ice forms on the tank.  If the tank is just cool, but the flame is not fading noticeably, then the larger nozzle is required for more flow.  Otherwise you may just need a little extra heat into the tank, and we can discuss ways of providing that.

I am not a burner expert, and don't have a good picture of your burner in my mind, but we have some real experts in that area on the forum, and with a picture of the burner and some basic air hole and nozzle sizes, they may be able to come in and help you.

I mentioned that we should also look at the boiler.  Not that any changes would be easy if practical at all, but it would tell us whether to use a similar boiler heating surface layout for the Marion, so worth a look.  You see, not being able to maintain pressure means not enough heat into the boiler, as you have correctly assumed.  However, there are two possible causes, the obvious one is not enough heat from the burner, but the second is not enough heat transfer area in the boiler.  If the heat transfer area is not adequate, the heat from the burner will just go up the stack in the form of hotter flue gas temperature.  I don't push the testing to far, but the flue gas from my little gas fired centreflue boiler is only warm to the hand waved over the stack.  I need to use the sheathed thermocouple to measure it down in the stack, I don't put my hand too close there, as the gas must be hotter than the steam, so at some point before too much cold air mixes with the flue gas, it has to be hot enough to burn me.  Clearly our boiler data would benefit from a flue gas temperature measurement, to tell us what is normal for a well performing boiler.  However, if we can test that flue gas temperature and it is high compared with other boilers, we can assume that it will be worth putting a bit more area, perhaps closer spacing of the cross tubes, or even a larger flue tube for the burner, relative to the boiler size for your next boiler.  If the flue gas is pretty typical of a good boiler performance, then it is back to the burner.  If it is very hot, more area next time, but more practical to compensate by a bigger burner and getting the flue gas hotter for an existing boiler.  Might need a bigger gas tank.

If all else fails on the burner front, then it may in the end be necessary to enlarge the engine exhaust passages to get sufficient shaft power from lower steam pressure.

I hope that provides some helpful thought on what to try, but thank you for telling us your thoughts, your Lombard is a perfect example for this discussion, and we all want to see it run as best it possibly can.  But nothing can take away from your achievement in getting it to run, just as it is.

Hi Gas Mantle, I hope you have your thermocouples ready and are carefully measuring water and fuel quantities.  The performance of your boiler will be most interesting as an example of a good vertical multi-tube boiler.  In view of the above, it would be really great if you have one of those thermocouples in a stainless steel sheath like the illustrations of the one Willy has, so you can measure the flue gas temperature.  With the feed pump, you need to measure the water initially in the boiler plus the amount you pump out of the feed tank for the total water evaporated.  Looking forward to results on gas and coal.

Hi Paul, thank you for those observations on your locomotive.  Obviously a very good performer, so very suitable as a basis for projecting to other sizes of boilers with similar configuration.  I can't quite picture your firebox and how the burner wicks are located, but it obviously works.  So some simple time measurements, for both fuel and water, with an intermediate time for heat up to steaming temperature would be great.  And again, if you are able to measure the stack temperature it would be really great.  Water consumption is probably best measured by measuring all the water you put in the tender and the boiler, then subtracting all you can extract with a syringe after the run.  I assume that running out of water actually means low level rather than literally running out, on the other hand I don't expect those Meths wicks could melt silver solder, so I could be wrong about your practice.  I can look out good data on the heating value of Meths for you, but do you have any indication of the pressure?  Actually I think you said the safety valve was simmering so if we assume the set pressure is similar when hot to what it is cold, you know the pressure quite accurately, at the expense of knowing how much steam reaches the engine.

Good to know the tube length to diameter ratio.  I don't know if 20:1 is a recommended maximum or suggested optimum.  Length obviously determines pressure drop, or resistance to flow, so for a fixed draft length determines flue gas flow.  Extra length also gives extra residence time for the gas to transfer heat.  Remember the units for heat transfer coefficient are J/m^2.s.K, so more area and more time both mean more heat transfer.  Have you calculated the total heat transfer area?

I guess that original single cylinder engine is a practical working cylinder size rather than a scale size, a totally appropriate approach for such a small scale, but you might like to comment on this aspect, and how you arrived at cylinder sizes.

Quite a long post without even getting to adding to the subject, but way more valuable direction for discussion.

Thanks to Chris, Gas Mantle and Paul for their descriptions of their models, the thread cannot be complete without those contributions, and thanks to everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: crueby on December 03, 2017, 02:50:56 PM
Some great food for thought. With the burner issue, it is hard (well, impossible) for it to hold higher pressure up for long when the engine is going, so it is running the engine at a lower pressure than I can with air. Sounds like the first and easiest thing to tackle!
The tests of the drive across the floor was before the boiler was made, so there was less weight to move, by about half, which means lots less friction from the track parts, but it also runs decently on the roller display base on air. It still doesn't have the power I would expect given the gearing, that is why I suspected the port passages. Might be a combination of the two  but sounds like the burner is the best place to start.
Thanks!
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on December 03, 2017, 03:44:00 PM
Hi

I wasn't really intending going as far as thermocouples etc, all I want is a boiler with enough guts to power a decent sized model engine at a healthy speed and be able to maintain pressure.

I've done a few rough calculations using KN Harris's book and if my maths is correct I reckon my boiler should use about 9lb of water (just under a gallon) per hour at 75 psi.   That to me sounds a lot but I know with the smaller hand pump I was using it was very thirsty to the point that on my own I'd struggle to fire and control the water for a full hour.
Title: Re: Talking Thermodynamics
Post by: paul gough on December 03, 2017, 10:15:07 PM
Please note, I made an error in my last post regarding the boiler tube proportion, it is 1:18 not 1:12. I apologise for this, I'm afraid a feeble old mind gets feebler. I have modified the last posting.

Below is a photo to show the wick arrangement and the external dry firebox.

The proportions I mentioned are not definitive, and are figures that are ball park and have proven them selves to builders. A loco is a balance of quite a number of things if it is to operate successfully. For example, any one of the following may cause a loco to be less than satisfactory;  grate proportion, firebox volume, tube number or proportions, primary and secondary air, chimney proportions, blast orifice, blast pipe height and of course the valve port proportions and the steam circuit dimensioning and consequent back pressure and there are more. Plus of course the fuel type will determine some things, e.g. a 'clean' fuel will sometimes allow the use of smaller tubes than coal. All my understanding comes with a caveat, it applies to a Stephenson boiler arrangement on a model loco, I have had no experience with the gas fired ones that are open at the front end and don't rely on draught, in fact I have no experience with gas firing in models, only in full size boilers. Regards, Paul Gough. PS, the wick arrangement fits up in the firebox, you can see the little tab at the bottom front of the firebox which holds the front of the horizontal feed tube and the support bracket at the rear where the tube is clamped.
Title: Re: Talking Thermodynamics
Post by: paul gough on December 04, 2017, 01:31:29 AM
Sorry MJM, I did not address your last question. With a model locomotive or similar machine the primary determinant of power or cylinder capacity is the size of the boiler or its ability to generate and deliver steam, no different than full size. In a model keeping to scale dimensioning, (boiler), often introduces constraints to steam production, and scale size cylinders could well 'eat' more steam than the model boiler could reasonably generate. So, cylinder sizes are generally not scale size, they are better sized to the boilers capacity. Another caveat applies here, many model loco designs and thus their boiler/cylinder arrangements never reach their full output potential for various reasons, e.g. a flat track, drivers wants an easy experience, track is too short or curves too sharp and does not allow full throttle usage and control by cut off etc. etc. So it is possible for an apparently satisfactory loco not to be so when a strenuous continuous effort is demanded. A big failing of many a model in many scales is lack of sufficient adhesive weight.

With my little engine the twin cylinder sizes were based on the swept volume of the single cylinder original as this had proved reasonable for the boilers capacity. I have a bias towards slower, longer stroke designs if possible for any particular application. So, the current twin cyl. size is a beginning point in a long road of experimentation. I expect they could be enlarged a little and the two 3/8 wicks, maybe with a little adjustment, would still provide adequate steam from the boiler. I have yet to get around to trying a multi orifice blast nozzle and various 'front end' variations. At the moment I am looking at cylinder head design and piston clearances as I perceive improvement can be had in this area. This tiny engine offers a myriad of experimental options for chasing optimal performance, in my case performance means extracting the maximum output or load hauling capacity from the loco.  I like the challenge of exploring what might be done to improve things so focus towards the design issues or exploring inadequacies. This is a slow, and to many people tedious, process of do one thing and one thing only, then run things again. Seemingly, an endless path if there are a reasonable number of modifications to try out, especially if some of the ideas are misplaced forcing one to return to the start. You said some time back that I would do well to get a little stationary engine and experiment with that and it would obviate pulling down a loco all the time. Yes this is so, but every time I dismantle the loco I think of something else that could be attended to and there would be no variations in operating conditions other than load. Wet rails, gradients, windy cold conditions, adhesion factors etc. etc. that come into play with a loco adding another layer of complexity and interest. Regards Paul.
Title: Re: Talking Thermodynamics
Post by: paul gough on December 04, 2017, 05:55:40 AM
Chris, From what I can understand your boiler is not a true stephenson type, in that it does not require a vacuum in the smoke box. Your poker burner type appears to be similar to the type that used to be fitted to Gauge 1 locos and requires an opening in the smokebox sufficient to allow the gases to escape. The gas fired G1 locos, English style Pacifics, of my friend have approximately a half inch square hole in the bottom of the smokebox with the hollow saddle being the means of conducting the gases away, sort of upside down chimney so to speak. You may have to experiment to find the appropriate size. If you get better performance with smokebox door open a bit then this might confirm things. As to the whine of the burner I am told by my friend this can be considerably improved by adjusting the burner up or down the flue, small increments only, until the most satisfactory sound occurs. It is also necessary to balance your primary air as close as possible for the burners needs. Thats about all I can contribute I'm afraid, my gas experience is small and based on a few observations only, I limit myself to metho, (alcohol). Hope there something useful here. Regards Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 04, 2017, 10:34:54 AM
Hi Chris, it always goes against the grain to suggest burning more fuel instead of first improving efficiency.  But I used to have a boss that always said we have to move forward from where we are now, not from where we would prefer to be.  At the end of the day, a burner well matched to the boiler and able to keep the safety simmering is what we are aiming for.  It can always be turned down a bit when less power is required.  Or when you have time between projects to do some more tweaking the engine.  I am sure that I am not the only one torn between wanting to see that Lombard at its best, and wanting to see the progress on the Marion.

Hi Gas Mantle, keeping my fingers crossed that you will manage to weigh enough water and fuel to demonstrate that 9 lb per hour.  That would be a really good performance.  I can always keep hoping for some temperature measurements when you have an engine or two under power.

Hi Paul, 1:18 is quite close to the 1:20.  I am sure it is not that precise a requirement.  Unfortunately it is a lot of work to systematically explore higher and ratios to learn the reality.  That is why I am hoping we can correlate the experience of many, and so answer some of these questions.  First you need data, then analysis becomes possible.  But I would not be worried about your memory yet!

Thanks for that firebox photo.  That is quite a high setting for a small loco.

I am interested in your comments on draft.  Of course there must always be a pressure difference to drive flow of the flue gasses through the tubes, whether or not the smoke box is actually slightly below atmospheric, and whether or not the draft is assisted by an exhaust blower.  It is possible that with a gas burner, the fuel pressure driving fuel through the nozzle provides the kinetic energy necessary to drive the flue gas through the tubes.   Leaving the smoke box door open might reduce the back pressure due to the stack.

I am sure that many will find your observations on burner adjustments helpful.

Your engine experiments are also interesting.  It is always a juggle over whether the power train is limited by the boiler or the engine.  Ideally, all the limits are reached at the same time.  You can always throttle back the fuel or use the regulator to limit the steam flow to the engine when less power is required.

Your comment about engines not needing to run at full power at all times applies equally to full size machines.  How often do you drive the car with the foot to the floor?  It is always necessary to have strategies for lower load, if only to enable start up and shutdown.  My full size compressor practice was always to have a control system that was operable and stable at all loads from zero to 100%.

I certainly admire your patience to proceed step by step as you have described, it is surely the best way to build knowledge and understanding.  I must admit to finding it very difficult to resist just tweaking something else as well.  Of course this is also very time consuming so I hope that by adding a little theory, we can avoid some of the trials that are really not likely to work.

I am another believer in Meths as a safe fuel, but I am finding that burning enough in a small boiler is more of a challenge than I was expecting.  Do you have a design for one of those silent type vapourising burners?

Once more, lots to think about from Chris, Gas Mantle and Paul.  It is contributions like these that add the real value to this thread.

Thanks to all for reading,

MJM460

Title: Re: Talking Thermodynamics
Post by: paul gough on December 04, 2017, 12:01:12 PM
Hi MJM, I like the experience of exploration, it is my main motivator, once a machine is 'adequate' it holds little interest for me. Others would term it as the journey being important not the destination. As to the length of time involved in the experimental process, this is of no relevance to me.

Yes, the wicks are high set, again as original but not off the investigation list, another area to be played with.

As to vaporising metho burners, I have no experience or suggestions relating to them. Anything I am likely to encounter could be adequately fired with multiple wicks or slightly bigger ones. The larger American Gauge 1 models have up to 6 I believe. If I ever got one of these I would probably convert it to coal fired. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: crueby on December 04, 2017, 01:12:38 PM
Chris, From what I can understand your boiler is not a true stephenson type, in that it does not require a vacuum in the smoke box. Your poker burner type appears to be similar to the type that used to be fitted to Gauge 1 locos and requires an opening in the smokebox sufficient to allow the gases to escape. The gas fired G1 locos, English style Pacifics, of my friend have approximately a half inch square hole in the bottom of the smokebox with the hollow saddle being the means of conducting the gases away, sort of upside down chimney so to speak. You may have to experiment to find the appropriate size. If you get better performance with smokebox door open a bit then this might confirm things. As to the whine of the burner I am told by my friend this can be considerably improved by adjusting the burner up or down the flue, small increments only, until the most satisfactory sound occurs. It is also necessary to balance your primary air as close as possible for the burners needs. Thats about all I can contribute I'm afraid, my gas experience is small and based on a few observations only, I limit myself to metho, (alcohol). Hope there something useful here. Regards Paul.
I don't understand what you mean about the opening the smokebox. The firetube ends at the smokebox, and the gasses exit through the smokestack. What other opening is needed? Maybe it's that I don't know what a Stephenson type boiler is? Can you elaborate on that?


The opening I was referring to that needs to be larger with the larger burner nozzle is the side openings in the poker burner next to the nozzle tip for the air to be drawn into, to give the proper mix with the butane so it can burn at the exit holes in the poker. This burner and boiler setup is just like most butane gas G1 locos, just scaled up. The firetube has a series of cross tubes to increase the surface area as well. With the first burner setup the gasses at the stack were barely warm, and it took a long time to get up to pressure, and it could not maintain it in use. The second burner did better, and the gasses out the stack were warmer, but nowhere near what my G1 locos can put out. I am hoping that the larger nozzle and poker setup will get it to the point where it can maintain pressure at something less than full opening of the gas, the way you can adjust the burner flow on the G1 to keep it at full pressure.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 04, 2017, 05:11:56 PM
Hi MJM ,...I have been thinking about injectors...and how much steam do they use to put water into the boiler ? and can this be measured as an efficiency ?   sort of, weight of steam used.to weight of water injected ?? Just an academic question really !!When i have observed injectors at the local track there is quite a lot of dribbling water at the overflow. I have re-red the posts from page 29 on injectors but did not see any mention of the Q's here. 
 
Title: Re: Talking Thermodynamics
Post by: paul gough on December 04, 2017, 08:31:14 PM
Hi Chris, If there is an improvement in steaming or burner performance by opening the smokebox door a bit then it might indicate something akin to a back pressure or restriction in flow of the gases, acting a bit like a damper, in this case I am assuming the stack is insufficient to provide an unrestricted flow due to its  relative small cross section area. A Stephenson loco type boiler absolutely requires a vacuum created by the engines exhaust blast, (or in a few rare condensing engines an exhaust fan), to draw the fire and products of combustion into the smokebox where they are engaged by the blast and forcefully ejected. From my observation some of these poker burner types operate with no vacuum as they have the smokebox 'open' to the atmosphere at two points. One being the stack out of which the engines steam exhaust goes and another at the bottom of the smokebox, on the models I have seen this second opening  is arranged connected to atmosphere through the smokebox saddle. If your burner operates satisfactorily without the smokebox door being sealed then I think you might have a 'flow' issue. A loco type boiler would not steam at all with anything but the minutest opening to atmosphere if at all. If you intend to operate with the stack as the only exit point for gases then you would have to ensure there was no restriction to flow but also not an excessive draw. The gas is to some extent forcing a flow and the blast is inducing another, these would have to be balanced to achieve a satisfactory result. Any form of balanced draught is more sensitive than just the induced draught from the blast. As I said I am not a gas person and have very limited understanding of the exact requirements for the best results, so, would not comment on specific points like burner design. I am trying to fathom your system from first principles, so to speak, in an attempt to offer a possibility that might have been overlooked. Many of the gas locos I have seen have a porous refractory 'grate' through which the gas flows and as the fire is above but proximal to it and heats it to a red heat adding a radiant component, just like some of the old gas heaters in homes.  These types are near to silent in operation. Trust this improves what I was trying to convey. Regards Paul.
Title: Re: Talking Thermodynamics
Post by: crueby on December 04, 2017, 08:41:55 PM
Hi Chris, If there is an improvement in steaming or burner performance by opening the smokebox door a bit then it might indicate something akin to a back pressure or restriction in flow of the gases, acting a bit like a damper, in this case I am assuming the stack is insufficient to provide an unrestricted flow due to its  relative small cross section area. A Stephenson loco type boiler absolutely requires a vacuum created by the engines exhaust blast, (or in a few rare condensing engines an exhaust fan), to draw the fire and products of combustion into the smokebox where they are engaged by the blast and forcefully ejected. From my observation some of these poker burner types operate with no vacuum as they have the smokebox 'open' to the atmosphere at two points. One being the stack out of which the engines steam exhaust goes and another at the bottom of the smokebox, on the models I have seen this second opening  is arranged connected to atmosphere through the smokebox saddle. If your burner operates satisfactorily without the smokebox door being sealed then I think you might have a 'flow' issue. A loco type boiler would not steam at all with anything but the minutest opening to atmosphere if at all. If you intend to operate with the stack as the only exit point for gases then you would have to ensure there was no restriction to flow but also not an excessive draw. The gas is to some extent forcing a flow and the blast is inducing another, these would have to be balanced to achieve a satisfactory result. Any form of balanced draught is more sensitive than just the induced draught from the blast. As I said I am not a gas person and have very limited understanding of the exact requirements for the best results, so, would not comment on specific points like burner design. I am trying to fathom your system from first principles, so to speak, in an attempt to offer a possibility that might have been overlooked. Many of the gas locos I have seen have a porous refractory 'grate' through which the gas flows and as the fire is above but proximal to it and heats it to a red heat adding a radiant component, just like some of the old gas heaters in homes.  These types are near to silent in operation. Trust this improves what I was trying to convey. Regards Paul.
Now I follow - thanks!

On the Lombard, it seems to make no difference whether the smokebox door is open or not. There IS a small opening at the bottom, where the exhaust pipe from the engines comes in, that opening was made larger than the pipe to allow easier fitting, but it is probably helping the draft as you mention. I had made the inside diameter of the stack as large as possible, seems to have been enough. The force of the gas coming in from the burner gets the flow going no problem on these, where on a coal fired boiler they usually run a fan above the stack to get things started. On the Shay model, Kozo included a blower pipe, controlled from a valve in the cab, to run steam up the stack to aid in draft as needed, but again that was for a coal fired version.

Thanks!!
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on December 04, 2017, 08:42:39 PM
On a similar subject I have some copper tubing and solder on order, once that arrives I intend to make a fitting to route the engine exhaust steam up the chimney of my vertical boiler. I think it will make little difference to performance when running on gas but I hope on coal it will improve performance  :)

At the moment if I turn on the steam blower just very slightly giving a gentle draught up the chimney the fire burns far hotter (although burns a lot more coal  :( ), I'm hoping the engine exhaust will have a similar effect.
Title: Re: Talking Thermodynamics
Post by: crueby on December 04, 2017, 08:47:59 PM
Hi MJM ,...I have been thinking about injectors...and how much steam do they use to put water into the boiler ? and can this be measured as an efficiency ?   sort of, weight of steam used.to weight of water injected ?? Just an academic question really !!When i have observed injectors at the local track there is quite a lot of dribbling water at the overflow. I have re-red the posts from page 29 on injectors but did not see any mention of the Q's here.
Willy, some good info on injectors here:
http://www.maineforestandloggingmuseum.org/wp-content/uploads/2015/02/Steam-Injectors.pdf

It does not include tables of efficiency though. There would always be some drainage from the injector during use, but much more makes it into the boiler. Seems like they could always collect the runoff and dump it back into the storage tank if there was a lot.

Another question about them - some locos use feed pumps, some use injectors - what makes the decision, and do they ever use both? Okay, thats two questions!
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on December 04, 2017, 08:53:11 PM
This guy has quite a few videos on boilers etc, he mentions in this video he is very shortly going to make another video showing the performance of an injector on a Stuart 504 boiler.

It may be worth keeping an eye on :-

https://www.youtube.com/watch?v=d4Yt0tJx384
Title: Re: Talking Thermodynamics
Post by: paul gough on December 04, 2017, 11:58:07 PM
Chris, Injectors are for direct feed, pumps offer less quantity and only work when a loco is moving in the case of axle pumps. This is why very old locos before the advent of injectors had to run up and down the track if they were standing for any time. Later types were driven by other means and independent of loco movement, these later types were often used in conjunction with a feed water heater usually with exhaust steam. Very modern engines even had exhaust steam still having some superheat so as to ensure dry steam and sufficient heat for auxiliaries like feedwaterheaters. On very small locos, e.g Gauge 1, injectors would be difficult to configure and probably somewhat cantankerous in operation, though maybe not impossible, so axle feed pumps are pretty much the sole feed mechanism, other than some manual feed arrangement. Injectors have always been regarded as something of a dark art when it comes to making them, but probably really only a thorough understanding of them combined with extreme care and exactitude in machining and assembly. A good clean injector and properly adjusted should not have much water, if any, discharging from the overflow. Regards Paul.
Title: Re: Talking Thermodynamics
Post by: crueby on December 05, 2017, 12:33:42 AM
On the full size Shay my model is based on (Kozo New Shay, 1920s) there was a feed pump on the left side, steam driven not axle driven. Same setup on the ones at Cass I rode on. Any idea why they would use a steam pump vs injectors then? Curious as to the tradeoff.
Title: Re: Talking Thermodynamics
Post by: paul gough on December 05, 2017, 02:09:48 AM
Hi again Chris, As far as I was aware Shays had lifting injectors and pumps on the side were for 'taking water' from trackside streams or reservoirs to refill the locos tanks/tender. I should think they were probably used for other purposes as well such as fighting fires along the track. I'm not a Shay expert but I bet there would be someone on this forum who could give a detailed answer covering all the different Shays and whether boiler feed pumps were ever used. Regards Paul.
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on December 05, 2017, 02:25:25 AM
Chris,
I have looked at a lot of Shay records and I have never seen a steam pump listed as original equipment. They always had one or two injectors. The device Paul is talking about is known as the siphon. It is a low pressure injector to add water to the tender tank from a pond or stream.

I have seen several photos with a duplex steam pump on the left running board and these were added to make sure there was a way to add water to the boiler. Shays mostly operated remotely in the woods and not all the crews trusted injectors. The crews also favored steam brakes for the locomotive over air brakes because once you run out of air you are in real trouble on a grade. There was always steam for the steam brakes.

Dan
Title: Re: Talking Thermodynamics
Post by: crueby on December 05, 2017, 02:31:01 AM
Hi again Chris, As far as I was aware Shays had lifting injectors and pumps on the side were for 'taking water' from trackside streams or reservoirs to refill the locos tanks/tender. I should think they were probably used for other purposes as well such as fighting fires along the track. I'm not a Shay expert but I bet there would be someone on this forum who could give a detailed answer covering all the different Shays and whether boiler feed pumps were ever used. Regards Paul.
Interesting how many combinations there were. On the Lombard they had a pair of injectors to feed the boiler, and a third one to pump water into the saddle tank from streams. At Cass, I could hear the side pump going while it was sitting on the siding waiting to go, no water supply there, so it had to be using it to replenish the boiler. Given all the different manufacturers and models, there was probably every combination used at one time or another.
Title: Re: Talking Thermodynamics
Post by: 10KPete on December 05, 2017, 04:17:36 AM
And sometimes the water quality was such that strainers couldn't handle the volume of detritus that would plug an injector. A pump has less of a problem with junk.

Injectors need clean water.

Pete
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 05, 2017, 06:17:05 AM
Hi Paul, you are right on target with time not being the issue, research cannot be done to time or budget.  Pity our politicians do not understand that.  I will be very interested to follow your investigation of those wicks, they are very suitable for these tiny boilers and the small engines I tend to prefer.  If they will do one of your locomotives, surely they will do a small boat.

Hi Chris, adjusting those gas nozzles and air openings seems to be quite critical for clean combustion.  My little centre flue boiler from Marine Steam has a whole page instruction on how to adjust the gas orifice position, and that is after they have done all the work to get the sizes right.  Not to mention matching it to the ceramic burner and the boiler flue.  Not sure about the differences (other than appearance) between the ceramic burners and the poker ones you use.  Your description of the time to heat up, and the stack temperature seems to indicate that the boiler heating surface is adequate, just needs more heat from the boiler.  The hotter stack from the G1 suggests they opt for more heat to maximise the steam from a limited boiler size.  On a larger model, with room for more heating surface, a lower stack temperature from the same burner  means you are getting more steam from the same heat input.  But if you need more steam, the principle holds.  If you want to see this carried to the extreme, check out those flash hydros in Benson and Rayman's book, three pressure fed petrol blow lamps firing one relatively short steam coil.  The boiler flue gas outlet would surely singe your eyebrows.  But there are times when efficiency is not the issue.

Hi Willy, those injectors are really fascinating, and a great illustration of the meaning and use of the energy equation.  The energy for the work done in forcing water into a boiler compared with the energy contained in the steam used would give a true measure of the efficiency.   I suspect it is limited by the energy equations and not easy to optimise, just getting a model injector working is enough.  But they seem to move a lot of water quickly, and don't require the engine to be running as would the alternative of a shaft or axle pump.  You can actually calculate the steam flow as accurately as you know the throat diameter in the steam nozzle and the upstream pressure, and most seem to be rated on water rate.  I have not calculated a figure, just laziness I guess.

Hi Paul, I am another not familiar with the Stephenson name in that context, another interesting historical detail coming out.  But every boiler needs a pressure difference to drive the flow through the tubes.  With coal firing, you need enough draft to allow atmospheric pressure to drive the air through the coal bed.  This almost certainly involves more resistance than those wicks, which also need adequate air.  So a coal bed almost certainly requires more active draft production, steam blown, exhaust blown or one of those induction fans to get it all started.

With a gas burner, the gas vapour pressure in the fuel container means the gas is delivered at high velocity through the burner orifice, which gives energy to transport the flue gas, just like those injectors.

Come to think of it that gas pressure is probably enough to cause sonic flow in the orifice, but as the outlet is generally square edged, or just a little chamfer, sonic velocity is the upper limit.  Perhaps we need to try a 13 deg expanding exit to further increase the velocity, though that would require a very tiny tapered reamer!

I hope that adds a little more to what you were saying in slightly different words.  Once you know where the energy is coming from, to force the flue gas flow in a particular boiler setting, it becomes more obvious how to control it. 

It sounds like your colleagues using gas firing use square ceramic burners, similar the the round one in my centre flue boiler.

Hi again Chris, I am glad that Paul was able to explain the smoke door issue.  If leaving it open a little seems to make no difference, it suggests that the energy mainly comes from the gas supply, and the stack is large enough not to provide undue restriction.  So it is back to the size of your boiler.  Presumably either a mixing problem or an air/fuel ratio problem.  Mixing is about the location of the air holes relative to the fuel jet, while the air fuel ratio is about size of the air holes.  Some burners also require separate primary and secondary air flow.  I am really hoping that one of the burner experts on the forum will join in with more information on this one.

Hi Gas Mantle, there have been magazine articles on blower designs, obviously an area worth a little investigation.  Start simple as you have suggested, and experiment with the nozzle diameter, which would be different for a steam blower to an exhaust blower.  Location and distance from the stack seem to be critical parameters to induce maximum flow.  Don't be sad about all that extra coal being burned, that is where the heat comes from to generate your steam.  It is a sign of success.   I can see your next project will be a star wheel gated feed chute, to save on the shovelling.  That video could be interesting as you say.  There is nothing like a video to help understanding of just what it all looks like in operation.

Hi again Chris, that site has some really good information on full size injectors.  I expect the efficiency is actually determined by the energy equation and the fine detail of the configuration.  But the adjustments on those full size injectors might enable a little tweaking, not so practical in model sizes.  You are allowed all the questions you want.  If there are too many in one day, I might have to make a list and answer over a few days, but I can do that.

I seem to remember someone mentioning that regulations require two independent sources of feed water, it is really important not to run out.  I think an axle pump would actually use less energy, so great when the vehicle is running, but no help when stationary, for a red light or perhaps picking up passengers.  If you get low level due to a pump failure, or from a sudden change in operating conditions, you want water NOW!  And don't worry about the efficiency.  Just get shovelling.

That's about enough for another day, thank you everyone for contributing, and also for just dropping in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 05, 2017, 06:23:52 AM
Oops!  I even updated the thread immediately before posting but did not get the usual warning.  I wonder if I was just on the wrong page, as they came in over a long enough time period, so I should have seen them.

My apologies to those who I have not acknowledged, I just did not see them, so I will respond tomorrow.  Thanks for all those contributions.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 06, 2017, 12:31:16 PM
Thank you Paul, Chris, Dan and Pete for that additional information on the sort of feedwater makeup systems provides on various prototypes.  I am sorry that I did not notice your contributions  when I posted yesterday.  I think that I was on the previous page, and while I refreshed it before posting, I did not get the warning, and did not notice an extra page appearing.  I will keep a better look out for that in future.

In summary, different prototypes had different combinations of injectors, axle pumps and steam driven pumps, and I was interested to learn, even an injector like device designed to lift water out of a creek or dam into the tender, and even for fire fighting was mentioned.  Obviously a mixture of devices increases security of feed water supply, by having a suitable device which will work when the vehicle was stationary, as well as axle driven pumps which very efficiently pump water roughly in proportion to consumption when the vehicle is moving.  Interesting that even the crew's degree of trust  and area of operation influenced what equipment was provided.  Great background for us all.

When I was talking about Chris' boiler on the Lombard, I suggested checking the stack temperature as a means of indicating whether the boiler heat transfer area was adequate for the burner.  I mentioned it more in passing as the reason for concentration on the boiler, but possibly should have made the basis for this simple observation more clear.

A boiler with a centre flue in particular is a relatively simple heat exchanger, where the boiling water  in the boiler operates at constant temperature, while the flue gas starts at the firebox temperature, where the heat released by fuel combustion is taken by the incoming air and combustion products, and cools down as it travels through the flue, transferring heat to the water.  At the Firefox end, the  temperature difference between the flue gas and the boiling water is very large, so the heat transfer equation, q = U x A x delta T gives a high transfer rate because the local temperature difference driving heat transfer at that end is very large.  At the smoke box end, the flue gas is cooler due to the heat transferred to the water on the way through, hence the temperature difference at this end is much smaller.  So as the flue gas proceeds through the flue, not only does it get cooler, but the heat transfer rate becomes slower.  It is very like Willy's coffee cooling experiment.  The flue gas never quite gets to the water temperature, no matter how much area we provide.  The overall average heat transfer is that logarithmic mean rather than just a simple average, and as we previously discussed, the answer is biased towards the higher temperature difference end, where a larger portion of the heat transfer happens.  This logarithmic mean temperature difference, or LMTD, is calculated with the formula
  LMTD = (inlet end delta T - outlet end delta T)/ln(inlet end delta T)/(outlet end delta T)
That "ln" term is short for natural logarithm, or logarithm with a base e, and is available on any scientific calculator or spreadsheet function.

However, if you imagine a very long flue, and just assume that the draft is sufficient, then most of the heat is transferred in the first part of the flue, but as the flue gas moves towards the stack end, less and less heat is transferred, and the temperature change is minimal.  This is when the flue gas will feel barely warm.  Remember that it is above steam temperature until it starts mixing with outside air so don't push your luck with putting your finger down the stack.  A stainless steel sheathed thermocouple is a much better idea.

Now if you consider a shorter flue, the flue does not have enough area for the temperature to get quite so low, so is warmer when it exits the flue and the stack.  On those little gauge 1 locomotives, it actually feels quite hot.  Heat is being lost up the stack.  Without burning any more fuel, you can generate more steam if you could have more area.

On the other hand, Chris observed that on the Lombard, the stack gas was quite cool.  So clearly the heat transfer area was adequate to get the flue gas to an area where the diminishing returns make further area ineffective.  And there is scope to generate more steam by using a bigger burner.

If on the other hand, Chris found that the Lombard boiler stack temperature was hotter than any he had experienced on his other models, we could conclude that the burner is already generating about as much steam as the heat transfer area will achieve.  In that case, we need to look back at that engine to see if enlarging the ports gives enough extra shaft power from the available steam, or do we need to somehow get more heat transfer area into that scale outline boiler.  At the very least, there would be an indication of whether the scale Marion boiler might contain enough area for its scale engines.

So a simple observation, waving our hand over the stack tells us whether the boiler and burner are well matched.  If the stack gas is relatively cool, then by enlarging the burner to burn more fuel, we can force the boiler a bit, and get more steam, which means the pressure can be better maintained with the same engine load.

At the end of the day, modifying the boiler is a big job that most of us would be reluctant to start.  So it is quite comforting that the heating surface in the scale outline Lombard boiler looks like it possibly has capacity to generate more steam with just a bigger burner.   However it is worth looking at the details and see of the boiler for the next model could accommodate more heat transfer area.  And "more" is relative to the boiler size, hence the value in a test to derive a figure such as Harris has used for the type of boiler and burner.

I hope that clarified the basis for that stack temperature test, and hence added to our understanding of our boiler performance.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: crueby on December 06, 2017, 12:35:35 PM
Great clarification - thanks!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 06, 2017, 02:17:19 PM
Hi MJM,....With a coal fire you need air above the fire to compleat combustion  ..available through the grate and fire door. so do you also need a similar arrangement with meths or gas burners ??
Willy
Title: Re: Talking Thermodynamics
Post by: crueby on December 06, 2017, 02:35:10 PM
Hi MJM,....With a coal fire you need air above the fire to compleat combustion  ..available through the grate and fire door. so do you also need a similar arrangement with meths or gas burners ??
Willy
Thats something I was wondering about with the butane jet in the poker burner style - the commercial G1 locos don't have a seperate air intake aside from the one in the side of the burner next to the gas nozzle, but they don't seal around the burner to the flue well either, not sure if they depend on that gap or if it is just cheaper not to seal it. Don't know about the ceramic style ones, though the wick type would definitely need an air supply.
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 07, 2017, 11:07:28 AM
Hi Willy, I would like someone with more experience to comment on your statement about needing air above the coal bed.  But if you remember your mother's fire lighting trick with the news paper, there are two things she is achieving.  First, by reducing the air flow, the temperature of the flue gas is higher, so less density which means more draft to draw in more air, more velocity, hence the roar.  But also, she has the gap at the bottom of the fireplace, not the top.  So with the newspaper in place the air is directed into the fuel she has set for the fire, just a bit easier and less likely to lead to hyper-ventilation than blowing to get the fire going.  But I have always understood that the stoking door should normally be closed, and you get enough air to cool the fire and destroy the draft drawing the air through the coal bed if you leave it open too long.

All fuels require adequate oxygen for combustion, whether liquid, gas or solid.  And most have a quite narrow range of air fuel ratio which will actually make a flammable mixture.  Liquid fuels generally have to be vapourised as it is only the vapour that will adequately mix with air and burn.  It is certainly true for Meths, and candle wax, and I suspect this is also true of coal, even though the vapour is only very close to the surface, and not obvious.  If the air is restricted too much, there may not be enough to adequately mix and burn completely.  There might be some unburned fuel, or some that is partly burned to carbon monoxide, the source of the danger in confined spaces.  So to get good mixing, the air is often admitted in two stages, primary air which gives good mixing and allows ignition to occur, and secondary air which admits additional air to ensure complete combustion, which actually requires excess air over the exact chemically correct (stoichiometric) amount.

There are also velocity effects, the flame has a characteristic flame velocity.  The flame will sit where, if it moved closer to the fuel source the gas velocity would carry it away, while if it was further away, the flame easily propagates back to the burner.  That does not look like a very good description.  Look at a soldering burner.  The gas velocity in the jet is way above the flame propagation velocity, so the flame cannot move back into the jet or worse.  The jet has enough metal to actually cool the gas below ignition temperature if the flame enters the jet when you turn it very low or off.  In the main burner tube, the primary air plus fuel velocity is enough to carry the flame out to the end of the tube, but the velocity is not enough for the flame to lift off further unless you have way too big a jet.  And more air comes in at that point.  But I don't know enough to suggest how much primary or secondary air is required.

With a Meths burner, using a wick, the wetted surface of the wick encourages vapourization, and it is the vapour that mixes with air and burns.  The flame is generally a bit softer, and the division of primary and secondary air is not so obvious.  But the yellow flame and occasional tiny spark from a wick or a candle indicates incomplete combustion.  If there is space above the flame, there is enough air drawn in to burn the bright yellow glowing carbon particles.  But if the wick is too close to the boiler, or to a metal spoon, the gas is cooled before it burns, and the black unburnt carbon is deposited on the boiler or spoon.

There are also vapourising Meths burners, where the burner, and primary air holes are not unlike a similar size gas burner.  I even have a stove like this for camping as it is a very safe fuel.  And of course there are those petrol and kerosene blow lamps with their vapourising coils and fierce flames.

As an aside, solid rocket fuels contain the oxygen in one of the compounds used to make the fuel.  So once you have enough heat to start vapourising the fuel, the vapour has the oxygen and fuel intimately mixed, and you know the result.  You can burn a lot of fuel in a very short time when the oxygen is that well mixed without the air flow considerations getting in the way.  And minimal nitrogen to dilute and slow the process.

Hi Chris, I am glad the clarification was helpful.  Always difficult to identify the boundary between too many words and an incomplete explanation.  I will keep trying.

I assume your poker style burner is like those ones described on the Southern Steam Trains website.  They get over the problem of low radiant heat  from a clear or faint blue flame by allowing the flame to heat a SS mesh to a red or orange colour, which then is a better radiator than a pale blue or invisible flame.  If I understand his explanation, he uses much less gas with this arrangement than an open soldering type of burner, and still gets enough steam.  When you look closely at the picture, the air holes seem relatively large, compared with the ones on my ceramic burner anyway, so I assume all the air mixes there and the lower velocities allow adequate mixing for complete combustion.  Then the gap around the burner admitting a bit more air is probably acceptable but not necessary. 

When I look at my ceramic burner from Miniature Steam Models, the primary air holes appear  smaller than on those poker burners, and there are some secondary air holes around the outside but behind the ceramic insert.  The shroud then is a close sliding fit over the end of the boiler flue, not air tight, but not fully open.

I would expect the relative sizes of the primary and secondary air holes has been the subject of considerable experimentation.  The position of the gas jet relative to the primary air holes is critical and the burner comes with instructions on how to adjust it for a flame outside the boiler that becomes optimum when the burner is fitted over the boiler flue tube.  I suspect that is about induction of the right air quantity when installed in the boiler, where the draft obviously has an effect on the air flow.

Someone familiar with the design of full size burners might have access to computer programs that gave suitable answers for the starting point for a tiny burner, but I expect a successful design would still take considerable experimentation.  I for one am happy to pay for the results of all that work, so I can use my time on other projects.

If there is excess air, the total flue gas will have a lower temperature, which decreases heat transfer, but if there is enough draft, there will be more velocity, and this tends to increase the heat transfer coefficient.  So there is almost certainly a happy sweet spot between minimum air with maximum temperature, and more air at a lower mixed flue gas temperature.

I hope that adds a little to the understanding of burners, but you can see that my knowledge is more in the area of fluid flows, pressures and velocities, and boiling of fluids than the details of combustion, so there is plenty of room for those with more experience in the area to come in and help out.

Thanks for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on December 07, 2017, 12:22:15 PM
This short video may be of interest to those wanting to know about coal firing.

It's about firing steam locos when the UK had a 'proper' railway but it is informative on the subject of coal firing in general.

https://www.youtube.com/watch?v=NHo860Q66Gw
Title: Re: Talking Thermodynamics
Post by: crueby on December 07, 2017, 02:22:12 PM
Getting the balance right on the air hole in the poker burner is why some include a sliding outer bit of tube, so the hole can be partially blocked to tune it. The little butane soldering torch I have for small work has that too.
Title: Re: Talking Thermodynamics
Post by: Kim on December 07, 2017, 02:45:06 PM
Sorry for the rudimentary question here, but people keep talking about a "poker burner."  Can someone give a quick explanation of what that is?

Thanks,
Kim
Title: Re: Talking Thermodynamics
Post by: crueby on December 07, 2017, 03:09:46 PM
Sure thing - the poker burners are a small butane fired burner, has a nozzle at the back end with a couple of air intake holes next to it, than the other end has a few rows of small holes where the mixed gasses come out and get burned. That end sits inside the firetube in a small boiler - this style is used in most of the comercially available G1 locomotives - simple, no moving parts, works fairly well. Sometimes the holes are replaced with cross slots, sometimes covered in metal mesh to give a more radiant effect and cut down on the 'howl' that they typically have. I've attached a picture that I found, shows one lit, but not really burning evenly. They usually burn better when they are in the firetube. Also a pic from Roundhouse's website showing the system.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 07, 2017, 03:39:58 PM
Hi MJM et al , more good info here and interesting that the fire doors were not completely closed allowing air above the fire with an explanation of how the fire burns etc etc.........
Title: Re: Talking Thermodynamics
Post by: Kim on December 07, 2017, 06:18:37 PM
Thanks Chris, seems simple enough.  But why are they called a "poke" burner? Cause you poke holes in a tube to make the burner?  I kept reading it Porker burner the first several times I saw it and thought it was what Cletus uses to do his Barbecuing! :)

Kim
Title: Re: Talking Thermodynamics
Post by: crueby on December 07, 2017, 06:31:16 PM
Thanks Chris, seems simple enough.  But why are they called a "poke" burner? Cause you poke holes in a tube to make the burner?  I kept reading it Pork burner the first several times I saw it and thought it was what Cletus uses to do his Barbecuing! :)

Kim
:ROFL:


I believe its called a poker burner since it pokes into the firetube, but thats a guess. Yet another case of the English language being strange. We could do worse, call it a straight-line-ring-burner...   :atcomputer:   like whi is a fly a fly and an ant isn't a crawl? ... But, that could be a whole other thread!

Title: Re: Talking Thermodynamics
Post by: Gas_mantle on December 07, 2017, 06:39:54 PM
I assume the term comes from the similarity with a domestic coal fire poker :-

http://www.retonthenet.co.uk/antique-vintage-ornate-brass-handled-coal-or-log-fire-poker-fireplace-hearth-tools-1920s-1930s-fireside-5891-p.asp

Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 07, 2017, 11:10:32 PM


I believe its called a poker burner since it pokes into the firetube, but thats a guess. Yet another case of the English language being strange. We could do worse, call it a straight-line-ring-burner...   :atcomputer:   like whi is a fly a fly and an ant isn't a crawl? ... But, that could be a whole other thread!


[/quote]

Hi Chris, ok yes ,l am allways threading on ants thats why they don't crawl any more !!  Oh , Right ..i see what you did there !!!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 08, 2017, 10:49:48 AM
Hi Gas Mantle, that was a great video on coal burning, thank you, very informative.

Hi Chris, those sliding sleeves are a clever way to make the air flow easily adjustable.  There still seems to be a real skill in working out the correct adjustment, especially for a boiler where you need to adjust the flame outside the boiler for how it needs to be when in position.  Thank you for the description of the poker burner.  Interesting that you mentioned noise levels.  That Southern Steam Trains site that I found also mentioned noise, but also that the poker burner with the radiant mesh needed a lot less gas to provide sufficient steam raising.  He put the lower noise level down to the consequent lower gas flow.  I wonder if the silent spirit burners I have seen mentioned are similar.

Hi Kim, I also had to look up the poker burner.  I am sure I read somewhere that early modellers of small gauge locomotives used a bar of cast iron which they heated with a blow lamp, or in a fireplace then put it into the firebox to generate steam.  I can't imagine that it gave a long run, and I am not sure now where I read about it.

Hi Willy, you were obviously ahead of me on that secondary air for the coal burning.  I wonder what they would make of a modern power station boiler which fires pulverised coal.  But I guess the principle is still similar.  The issues with a gas burner are less obvious.  But even the gas turbines that power aircraft have primary then secondary air introduction, and sometimes even tertiary air, but that is entirely another subject.  Got to be something to do with making a flammable mixture for combustion then adding the necessary excess air, but I really don't know any more than that.

Thanks to everyone for inserting a bit of fun on the name of the burner.  This thread needs a little humour otherwise it will tend to get seriously boring.

I don't really have a lot to add to the burner conversation, but people with more knowledge on the subject are most welcome to come and join in.

I don't know if all that helps you Chris, with your larger burner for the Lombard.  I don't know if you are planning a larger jet, and adjusting the air holes, or a new larger burner, but we are all keen to hear how you get on.

A bit of a shorter post tonight.  Any suggestions for where to next?

Thanks to everyone for dropping in and especially to those contributing,

MJM460
Title: Re: Talking Thermodynamics
Post by: crueby on December 08, 2017, 12:44:23 PM
I do have a larger jet, nbr 8 vs the original nbr 5, got to make up a new burner tube with the larger air intake holes and room for the sliding sleeve. Will update when that happens...
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 09, 2017, 11:41:03 AM
Hi Chris, do you know the size of those jets?  I will check butane vapour pressures tomorrow, and can make a reasonable estimate of the change in flow and heat input from the sizes if you have them.  Even if not, the increase in flow with the necessary additional area should give much better capacity to maintain the steam pressure for you.  Certainly the quickest fix from the current position.

We have a house full of visitors for the next three days, so while I will check in and keep up with any posts, I will need to keep my input brief until they are gone.

Thanks to all for looking in.

MJM460
Title: Re: Talking Thermodynamics
Post by: crueby on December 09, 2017, 01:25:08 PM
Hi Chris, do you know the size of those jets?  I will check butane vapour pressures tomorrow, and can make a reasonable estimate of the change in flow and heat input from the sizes if you have them.  Even if not, the increase in flow with the necessary additional area should give much better capacity to maintain the steam pressure for you.  Certainly the quickest fix from the current position.

We have a house full of visitors for the next three days, so while I will check in and keep up with any posts, I will need to keep my input brief until they are gone.

Thanks to all for looking in.

MJM460
The listings I have seen show nbr 5 is .2mm and nbr 8 is .25mm diameter. The air holes in the side of the burner for nbr 5 is .25", am guessing that the hole for the nbr 8 must go up by at least the same proportions as the areas of the nozzle hole, if not more since the gas is a pressured flow and the air is not.
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 10, 2017, 09:49:13 AM
Hi Chris,

Butane is usually a mixture of normal butane and isobutane.  I will talk about what the terms mean later in the week.  Isobutane has a bit higher vapour pressure than normal butane, so the vapour pressure depends on how much of each.  But a 50% mixture would be about 450 kPa at 40 deg C.  Even pure normal butane would be about 377 kPa, while pure isobutane would be 528 kPa at 40 C, but the purification requires a lot of energy, and I suspect you have about 30% of one, I am just not sure which for the moment.

The camping gas we get here in throwaway containers also has a good percentage of propane which boosts the pressure considerably.  I don't know what you have. 

However I would estimate that the fuel tank would initially have high enough vapour pressure to give sonic velocity in the jet if you have reasonable temperatures.  If you basically have a butane mix, as you boil off gas, it will cool and probably the pressure will be too low for sonic.  If you have some propane, it will stay sonic.  Because the nozzle does not have the diverging outlet, you never get above sonic velocity.

The important thing is that for constant presssure on the boiler side and near sonic velocity in the jet, mass flow proportional to upstream pressure and jet area is a good estimate.  Because of the uptstream pressure component, the flow drops off as the pressure falls, and it will drop more when you boil off more gas with the bigger jet.  The momentum (mass times velocity) of the gas jet gives energy to induce the air flow, and increasing the air holes in the same proportion by area would, I think, give you a similar air/fuel ratio, and nearly 50% more flow.  Thus 50% more heat.  Should be a great place to start on seeing if you can maintain the pressure.  If you are providing the adjustable sleeve, it may be worth making the holes a bit bigger as you can reduce the air flow if required, but the adjusting the sleeve also shifts the position of the air intake relative to the gas jet, so may also have an effect on combustion.

Still keeping posts short for a few days, but I hope that is enough to keep you thinking while you scale those  Marion drawings.

Thanks for dropping in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on December 10, 2017, 07:26:16 PM
Quote
But even the gas turbines that power aircraft have primary then secondary air introduction, and sometimes even tertiary air, but that is entirely another subject.

The extra air inlets in an aircraft gas turbine has everything to do with cooling - as they create a boundary layer between the very hot combustion gasses and the metal of the engine. Getting the more than 1700 degree centigrade combustion gasses separated from the metal is vital for engine life  :o

The first two (or more) turbine wheels has hollow blades that are air cooled from the inside + holes for letting some of the cooling air out to form yet another boundary layer around the blades.

A certain percentage of the incoming air is use for cooling the engine parts at different locations. The first is use to cool the last compressor stages, then maybe 40% is feed to the combustion chamber for power production. The next is use to cool the outside of the chamber and enters as secondary air as cooling (some might be used to burn too). Now some is used to cool the turbine housing and stator blades.

There is more than one airflow in a modern engine :
One going through the inner parts and out through the last rotating compressor blades and out through the first rotating turbine blades.
Next layer goes through the power section and is burned.
Third layer is used to cool stator + turbine blades and combustion chamber + turbine chamber.
All modern Jet engines has a bypass fourth layer created from the big fan in the inlet - this is the main thrust and also cools the outside of the engine (not the part we see outside) - you will not see the fourth layer in a gas turbine for helicopters for instance.

I probably shouldn't have answered this - but I could not help myself  :-[
Title: Re: Talking Thermodynamics
Post by: crueby on December 10, 2017, 07:56:03 PM
The extra air inlets in an aircraft gas turbine has everything to do with cooling - as they create a boundary layer between the very hot combustion gasses and the metal of the engine. Getting the more than 1700 degree centigrade combustion gasses separated from the metal is vital for engine life  :o

The first two (or more) turbine wheels has hollow blades that are air cooled from the inside + holes for letting some of the cooling air out to form yet another boundary layer around the blades.

A certain percentage of the incoming air is use for cooling the engine parts at different locations. The first is use to cool the last compressor stages, then maybe 40% is feed to the combustion chamber for power production. The next is use to cool the outside of the chamber and enters as secondary air as cooling (some might be used to burn too). Now some is used to cool the turbine housing and stator blades.

There is more than one airflow in a modern engine :
One going through the inner parts and out through the last rotating compressor blades and out through the first rotating turbine blades.
Next layer goes through the power section and is burned.
Third layer is used to cool stator + turbine blades and combustion chamber + turbine chamber.
All modern Jet engines has a bypass fourth layer created from the big fan in the inlet - this is the main thrust and also cools the outside of the engine (not the part we see outside) - you will not see the fourth layer in a gas turbine for helicopters for instance.
Wow - thats a very different kind of 'jet' - I was referring to the tiny gas nozzle jet in the poker burner. Sounds like you have some experience with the big kind - built any working models of jet turbines?
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on December 11, 2017, 11:17:23 AM
Sorry - I got carried away with MJM460's comment about aircraft turbines  :-[

No - I'm just interested in engines  ;D in my childhood it was steam engines, in my preteens it became IC engines and this interest has never stopped and like you I like to research my subject ;D
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 11, 2017, 12:44:56 PM
Hi Admiral DK, those gas turbines are truly fascinating and high tech machines with blades cast as single crystal high temperature alloys with all those cooling passages, and still the performance limit is the turbine inlet temperature, so as not to melt them, or simply abrade the blades so they become less efficient.  And all that just to produce a lot of high velocity air.  But thanks for putting it in, there will be quite a few interested.

In aircraft the turbine just turns the compressor that gets air into the engine, and the exhaust air then provides the thrust to power the aircraft. Though as you say, some have that extra stage to provide even more thrust, by providing even more air flow.  In my compressor applications, the hot air drives a further turbine called a power turbine, often on a separate free turning shaft, which turns the process compressor, or in power generation applications, a generator.  About a third of the power produced drives the front compressor just to get air into the machine, a third is mechanical power output, and a third hot exhaust gas.

It was probably a mistake for me to mention them, but I was getting at the path for the air involved in combustion.  In the combustion chamber, it is divided into primary air mixed with fuel for combustion and secondary air to ensure complete combustion, just the same as any other combustion chamber.  Well, actually they operate at much higher pressure and flow rate, but otherwise similar.  Then all those other streams further increase the mass flow through the machine in order to provide thrust, or power turbine drive and to ensure they don't melt anything on the way through.  Power output is about mass flow, but that is a whole other topic.  Sorry for the distraction.

In the discussion of Chris' poker burners, we were only discussing the actual combustion air with the necessary air fuel ratio.  Butane will only burn in fuel to air ratio of about 2 to 10% fuel.  Too rich, or too lean and it will not ignite.  So those air holes we have been talking about have to be the right size to meter the air in at an appropriate rate for the fuel supplied through the gas jet.  Alternatively, they can be a little big and have that sliding sleeve partially cover them for adjustment.

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on December 12, 2017, 11:28:52 AM
A little Chemistry-

A few days ago, when talking about Chris's poker burner for butane, I mentioned normal butane and iso butane, and the difference in vapour pressures.  It might be useful to talk a bit about what these are.  As both are chemical compounds classed as hydrocarbons, which means the molecules are predominately combinations of hydrogen and carbon atoms.   Hydrogen has a molecular weight of 1, and also has one point where it can bond to another atom to make a compound.  Carbon has a molecular weight of 12, and has four points where it can bond to other atoms.  So let's look at what happens.

The simplest hydrocarbon is methane, which has one carbon atom, with one hydrogen atom bonded to each of its sites, a total of four hydrogen atoms in each molecule.  It is symbolically represented as CH4, and is the predominant component of natural gas, whether supplied as pipeline gas or liquified as liquified natural gas or LNG.  When it burns, the carbon joins with two oxygen atoms, providing there is plenty of oxygen available, to form Carbon dioxide, or CO2.  However, if there is insufficient oxygen, it can also form Carbon monoxide, CO, which is extremely toxic to life.  You can get some CO, even if the quantity of oxygen is sufficient, but with so little excess that it is difficult for every carbon atom to find the necessary two oxygen atoms.

The four hydrogen atoms join with one oxygen molecule (which has two oxygen atoms, so O2) to make two water molecules, water being H2O.

The combination of either hydrogen or carbon with oxygen is called oxidation, and releases a lot of heat, and the flue gases contain CO2 and water in vapour form.  In addition, the oxygen normally comes as air, which is eighty percent nitrogen.  At the simplest level nitrogen goes along for the ride, but absorbs some of the heat produced to so that all flue gas components exit at the same temperature.  Thus the nitrogen limits the maximum temperature that is achieved, but it also adds to the amount of heat that is lost up the stack after the heat transfer area has allowed as much heat as possible to the water in the boiler.  If that temperature is still above 100 C, the water exits as vapour and we get the lower calorific value.  If we are able to use the heat at a lower temperature so the water condenses, then we also benefit from the latent heat as the water condenses, so we get the higher calorific value of the fuel.

Now, methane is not very useful for our hobby.  It liquifies at about -161 C, and as a gas, it has such low density, that we can't carry a worthwhile amount as compressed gas on a model at reasonable pressure.  At 40 C, to keep methane liquid, you need a pressure of roughly 35,000 kPa.  So why have I spent so much time on this?  Now you will see the delightful simplicity of dealing with the hydrocarbon series that includes methane but also, ethane, propane, butane pentane and so on.  You know have the knowledge for a basic understanding of all of them.  One more step, then let's see how it works.

You will remember that I mentioned that a carbon atom has four points that can bond to another atom.  In methane each of those bonds to a hydrogen.  But there are other possibilities.  What if one of those points bonds to another carbon atom.  Each carbon has now bonded to the other carbon atom at one of its four points, but it has three more.  A hydrogen can bond to each of the three points on each carbon, and we have a molecule with two carbons and six hydrogen atoms, C2H6, which is called ethane.  Usually present to some extent in natural gas, but the industry tries to separate ethane as a chemical feedstock.  With a boiling point of -88 C at atmospheric pressure, or around 6,000 kPa at 40 C, it is still not much use to us as a fuel for our models, so let's continue.

If we now have three carbon molecules, one joins to another carbon at each of two sites, while the other two carbons join to that first one, the middle one on opposite sides, and to three hydrogen so with the remaining three sites.  Of course the middle one has only two more sites so only joins to two hydrogens.  We can represent this as CH3-CH2-CH3, alternatively C3H8, which we call propane.  For representing on paper in those symbolic representations, it is convenient to think of those four carbon bonds as being located at 90 degrees, and so the picture forms of a straight chain of Carbon atoms with all the remaining sites occupied by hydrogen atoms.  In reality, the sites are located around a sphere, in three dimensions, so it is a bit of a wriggly chain, but close enough.  I am not trying to turn you all into molecular physicists.  And that chain with three carbons is called propane, and generically simplified to C3, at least when the context is clear that we are talking about these simple hydrocarbon chains.  Now propane we know as the gas in our bar-b-cue bottles.  It's boiling point at atmospheric pressure is about -42 C, and vapour pressure at 40 C is 1341 kPa.  When it burns, the carbon combines with oxygen to form CO2, while the hydrogens form water.  But three carbons to each 8 hydrogens gives a bigger proportion of CO2 to H2O than burning one carbon for each four hydrogens.  So methane is a bit less carbon intensive than ethane.

I hope it is obvious that there is a pattern here.  The next step is four carbons, it now has two centre atoms which then only have two sites for hydrogen.  So it is represented symbolically as
CH3-CH2-CH2-CH3, or more concisely, C4H10.  Now with four carbon atoms there are two possible arrangements, that depend on whether the second middle carbon is still arranged in a roughly straight line, or if it joins to adjacent sites to make a little branch.  You might think of it as a bit like a propane, with a carbon(with its three hydrogens) attached to the centre of the three instead of a single hydrogen.   This requires a three dimensional symbol rather than a straight line representation, but still has four carbons and 10 hydrogens.  This is known as an isomer.  There are other structures also known as isomers, it is not  a total definition, but is enough for our purposes.  The "straight line" version is known as normal butane, or n-butane, while the branched version is called iso- butane or sometimes i-butane.  The butane fuel we use for our gas fired boilers is usually a mixture of normal butane and iso butane.  Burning butane again produces CO2 and H2O, and a little more carbon intensive again.

I suspect that is enough to take in at one go, so next time, I will discuss some properties of butane that interest us when we use it to fire our boilers.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on December 13, 2017, 06:20:09 AM
Glad we are getting to the nitty gritty of combustion, so thanks for breaching this subject. Flame temperatures, in air, for propane, butane and methylated spirits are all very roughly 1900 degrees Celsius, so I expect for our uses, model boiler burners, we can treat them as equal on this point. What I would like to know is; are there other parameters we need to consider when trying to determine the most appropriate fuel for our requirements for a given boiler design? One of which might be, greatest heat output per unit volume of fuel burnt and hopefully greatest volume of steam generated for that volume of fuel. But, I understand gas velocities are an issue in heat transference, I am assuming/guessing gas velocities in a poker burner are higher than a ceramic bed type and a metho wick burner, so do we derive higher heat transfers and thus steam generation from a poker than a ceramic bed or a metho wick, if so what would be the degree of difference if we tried all three burners in the same boiler, assuming one to be a suitable design for all three types. I have seen figures for energy released burning ethanol as 30kJ/g, propane 50kJ/g and butane 49kJ/g. Does this necessarily mean we are going to have to burn an equivalent proportion more metho, (ethanol), despite same flame temperatures to get the same quantity of heat output and thus steam production. Hope these questions are conveyed meaningfully, took me some time to frame them, I'm afraid an old mind finds it just as difficult to ask a sensible question as to answer one!!! Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 13, 2017, 10:36:11 AM
Hi Paul, great to see you back again.  Don't worry too much about that old mind, you have very clearly put some very difficult questions.  Those figures you have quoted are all close enough to the standard data, though I will comment a little more on methylated spirits further down the trail.  This series is background which I feel is necessary to enable to sensibly describe how our gas fuels behave in our gas tanks.  Then I will try and look at combustion, so I am glad you are finding the topic interesting.  It is pushing my limits, but I think I will be able to help you move further forward on your questions.  I am doing a little extra reading and will try your questions after I have discussed the pressure temperature characteristics of these fuels.

Just a little more chemistry, then we can get back to butane and other camping gas fuels commonly used.  for our purpose.  In the meantime, look on the containers for your fuel for the composition details, you already know some of the names to look for, and let me know what you are using, in case we are offered different mixtures in different parts of the world.

So back to that pattern.  We have already covered molecules from the very low molecular weight (also called light hydrocarbons) methane, or C1, (with its attendant hydrogens of course), C2, C3 and C4.  You can see the pattern, and it continues to pentane, C5, hexane, C6, heptane, or C7, but it's then octane, a name you have definitely heard, and so on.  The series are called paraffins, you might have heard this name in relation to the wax your grandmother used to seal jars of home made jam, maybe you still do the same.

You might have noticed another pattern.  As the number of carbons, or the molecular weight increases, the substance becomes easier to liquefy.  It condenses at a higher temperature, and the vapour pressure of the liquid, represented by that figure of 40 C, becomes lower.  Once we get to C5 and above, they are liquid at normal atmospheric temperatures, though the early ones still evaporate quite quickly in a tank without a lid.  The very heavy ones start looking like a waxy solid, I am not sure just where that starts.  The C5 through to around C10, again I am not sure exactly the upper limit, can be found in normal automotive fuels, then you go through distillate, heavy fuel oil and right up to tar.  Most of these are rarely separated into pure components, but are generally simply separated to a narrow boiling point range of components.

Of course, once you get to C5, there are more than 2 isomers, and by C6, the angles between the bonding points are such that the chain can go right around to make a ring, the characteristic of the aromatic hydrocarbons, no longer paraffins.  In fact even four carbons can form a ring, called naphthene, again not a paraffin.

The other possibility you might be wondering about is whether those carbons with four bonding sites could join to each other with more than one of the points.  The answer is yes, and double bonds, or even triple bonds are quite normal.  Well, in my world anyway.  Even C2 can form with a double bond, C2H4, called ethylene, the basic constituent of polyethylene.  And even a triple bond, C2H2 which you all know as acetylene.  These are called unsaturated hydrocarbons, as they do not have all the possible bonding points taken up with hydrogen.  You don't want either of these in your fuel tank as a rule.

Enough chemistry, we are model engine makers, not chemical engineers, so let's go back to butane, and the properties that govern how it behaves in our basic gas fired steam plants.  Now the two isomers of butane have quite similar properties, though not identical.  The gas containers here often have a mixture of both isomers of butane so we had better have a look at both.  The calorific value of each is very similar, the very small difference being due to a different energy of formation for each of the isomers.  With the same number of carbons and hydrogens to burn in the same reaction with oxygen this is not very surprising.  The big difference is in the vapour pressure.

Normal butane boils at -0.49 C at atmospheric pressure, and has a vapour pressure of 377 kPa at 40 C.  Iso-butane boils at -11.81 C at atmospheric pressure and has a vapour pressure at 40 C of 528 kPa.  All the pressures in this context are absolute pressures.  Atmospheric pressure has no relevance as these flammable gases must be kept separate from the atmosphere, except where you want it it burn.  You can divide kPa by 7 (or 6.89 if you want to be more accurate) to get psi, or just divide by 100 to get bar, depending on your preferred pressure units.

When you design a fuel tank, you have to design for the difference between atmospheric pressure on the outside and the vapour pressure in the inside.  And you need the vapour pressure for the highest temperature your tank is likely to be exposed to, perhaps out in the direct sunlight.  Here that would be at least 65 C, so much higher pressure than I have quoted for 40 C.  I prefer to buy a commercial fuel tank for safety sake, and I recommend you do the same.

The pure substances, whether normal or iso, behave in a very similar manner to water, just at different pressures and temperatures, and of course different latent heat, enthalpy and entropy values.  I will scan the chart and attach it tomorrow, it is publicly available information, but perhaps a bit harder to find than some.  Other substances with similar behaviour are all the refrigerants, the chlorinated hydrocarbons, ammonia and so on.  Depending on the size of your plant, and the temperatures you want, all these lighter paraffin series hydrocarbons are even very good refrigerants, just not so commonly used, as leakage and sparks must be absolutely avoided.  Sorry, another side track, so back to topic.

The other thing we need to know about the commercially supplied fuels is that they are not normally supplied as pure substances, but as a mixture.  I have three containers in my garage, one, as sold for those very common single or double burner portable stoves which use disposable containers, does not even specify the composition beyond liquified butane.  I suspect it is actually a mixture of normal and iso, as purification of just one isomer is more expensive.  It also claims to have an EN417 connection, but it has no threads.  I didn't know if that is really a legitimate variation, but it would not work with my fitting for transferring the fuel to the gas tank.

The others clearly label the contents, one has 44% n-butane, 29% iso-butane and 27% propane.
While the other has 75% isobutane and 25% propane.  Fortunately, we don't need to know the exact composition, though it is helpful for working out the calorific value with accuracy.  Again, fortunately, there is only 2% difference in the calorific value of propane and butane, as Paul has mentioned, and a much smaller difference between normal and iso-butane. 

While we really don't need to know the precise composition, it is useful to understand in a qualitative sort of way the general effect of the difference in composition.  And first we need to know how the components behave when you have different components mixed, as in those cans.

I have discussed before mixtures of gases, and how the total pressure is the sum of the partial pressures of each of the components.  In mixtures of gases, each component acts independently, as though the others were just not there.  But in this case, we have both gas and liquid, and the vapour pressure comes into it some how.  And in this case, the vapour pressure of the mixture ends up being somewhere between the individual components, a sort of average weighted for the composition.  So if you had say 90% n-butane and 10% propane, the mixture would be close to the vapour pressure of n-butane, but that propane will lift the vapour pressure a bit.  Similarly if it was 90% propane, the vapour pressure would be lowered a bit by the n-butane. 

You will note I have been cautious about implying any exact relationship here.  The really interesting thing here is that the vapour and liquid in equilibrium will have different compositions.  There will be a higher concentration of the lighter or higher vapour pressure component in the gas phase, and a lower concentration in the liquid, while the lower vapour pressure component will have a higher concentration in the liquid phase.  That difference is the basis of the process for separation, or fractionation, whether you are separating propane from butane, or alcohol from water.  Of course, that also means the lighter component tends to be used slightly faster than the heavier component, so the gas composition changes as the gas burns.  That's enough chemical engineering for our needs. 

I keep promising to get back to how the fuel behaves in our fuel tanks, but if you can understand the last two posts, it will be pretty easy to explain.  As this is already a long post, let's continue tomorrow.  I hope it is making sense so far.

Thanks for following,

MJM460



Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 14, 2017, 03:11:13 AM
Hi MJM ....talking about CO manufacture ..when i cook my tea on my gas stove  (town) my C0 meter starts off at 0 and after 4 mins goes up to 17 PPM . after about 18 mins it has reduced slowly to 0 again although the gas is still alight although at a lower setting. Also a bit more info from this old book about injectors...using two of them sometimes. Also on a lighter note we talk about hydrocarbons ...C3 ,C4 , C5,  i was wondering what the number is given to the flatulant gas Pantain that i often produce ??!!!!
Title: Re: Talking Thermodynamics
Post by: Noitoen on December 14, 2017, 08:54:53 AM
C0 meters also respond to CO2.
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 14, 2017, 09:06:15 AM
Hi Willy, great to have you back again.  Interesting that by using injectors in series, they could use some of the exhaust steam energy to reduce the amount of boiler pressure steam the main injector used to make up the feedwater.  Not sure it would be very practical in a model.  If you remember the sizes of the nozzles when we were looking at injectors, and now consider making them even smaller.  I suspect you would be in an area where very different drilling and reaming equipment would be required.

Hydrogen Sulphide is a well known gas for producing rotten egg type odours.  However, there is a whole class of sulphur compounds known as mercaptans which are added to otherwise odourless flammable gases to warn of their presence due to leakage.  Hydrogen sulphide itself is extremely toxic.  I am not sure it the mercaptans are similarly toxic, but probably the source of your observations.

The CO monitor in the kitchen is an interesting experiment.  I hope that stove has a good chimney over it.  I suspect what is happening is that when you first light the stove, the air in the chimney has uniform temperature so there is no, or at least minimal draft.  When you light the stove, producing CO2, water, and obviously also some CO, it spreads through the kitchen and is picked up by your monitor.  But the air in the chimney is also heated and the updraft starts.  When it is well established, it starts ventilating the room and sweeps out most or all of the accumulated CO.  Only surmising, of course.  You would need to monitor air currents and so on to be sure.  I am not sure if it is possible to adjust the air flow to the burner, to get more secondary air, or if the reaction rates for the combustion reactions mean some CO is just inevitable, perhaps due to cooling of the combustion gases when they get close to the kettle.

Hi Noitoen, welcome aboard.  That is an interesting comment.  I wonder what that means in these times when atmospheric readings in the cleanest areas on earth have reached 400 ppm.  Willy's reading was only 17 ppm.  Is the background level of CO2 taken into account by the calibration do you think?

Yesterday I was looking at the behaviour of a two component gas, and found that the mixture of gases has a vapour pressure in between the vapour pressure of each of the pure substances.  Now at the simplest level, but adequate for our purpose, we can think of the mixture as behaving a bit like a substance with that intermediate vapour pressure. 

Our fuel tank starts off at ambient temperature and the corresponding vapour pressure.  When we open the outlet gas valve and light the burner, we are drawing off some of the gas, which tends to reduce the vapour phase pressure.  To restore equilibrium pressure, some of the liquid evaporates.  This involves latent heat, and so requires heat input.  In the absence of heat input, the heat comes from the sensible heat of the liquid, and it gets cooler.  The temperature difference then drives heat input from the atmosphere, but the temperature difference is made evident by condensation of humidity from the atmosphere.  However, at the lower temperature, the vapour pressure is lower, and if it is a chilly day at the lake, the temperature and hence the pressure may fall so low that the burner does not get enough gas to generate the required amount of steam.  This observation is reported often enough in the modelling press, but I hope that you can now see that the explanation is simple enough.

There is a very important difference between our two component mixture and a simple pure substance.  If we have a pure substance, whether it be propane, butane or even water,  the liquid and vapour have the same composition.  However with a two component mixture, we find that the liquid and vapour each have a different composition.  There vapour will have more of the higher vapour pressure component than the whole mixture, while the liquid will have a higher concentration of the lower vapour pressure component.  As we draw off the vapour, we are burning more of the lighter component, so the composition of the remaining fuel slowly changes as we draw off gas for our burner.  This means the vapour pressure of the remaining mixture is always reducing.  Now the extent of this, and a detailed analysis of how the pressure changes with fuel consumption is beyond me.  We need a chemical engineer to join in and help out here. 

The simple message is that the liquified gas in the container is a boiling liquid in equilibrium with its vapour pressure, just as steam in a boiler.  The pressure and corresponding temperature is different for different substances, but they all behave in a similar manner.   As we draw as we draw off gas to burn, the latent heat must be supplied to continue the evaporation process.  This involves reducing the temperature of the liquid until the incoming heat from the atmosphere is sufficient to maintain the pressure. 

When our fuel is a mixture of components, the vapour pressure is somewhere between the vapour pressure of each component.  The vapour and liquid have slightly different composition, with more of the higher vapour pressure component in the vapour phase.  As we draw off gas from the mixture, the composition of the remaining liquid slowly changes, due to the concentration of the lower vapour pressure component, so that by the time the last drop of liquid remains, the vapour pressure at a given temperature is lower than it was at that same temperature at the start.

It is worth thinking about what this means to the gas in the container that we use to refill our steam plant gas tank.  We turn the container upside down, and transfer liquid to our gas tank.  As the liquid transfers, some must evaporate to maintain the pressure in the vapour space.  Again, the heat to evaporate the liquid comes from the sensible heat of the remaining liquid.  And again, the liquid and vapour are slightly different composition, but this time of course, the higher vapour pressure component stays in the can.  So each time we fill our gas tank, the average composition in the can increases in the percentage of the higher vapour pressure component.  This looks like it certainly maintains if not actually increasing the vapour pressure in the can if it is compared at the same temperature after filling steam plant fuel tank.

I hope that helps you understand the behaviour of the liquified petroleum gas or LPG in your fuel tank.  If the fall in gas pressure makes it difficult to maintain steam pressure, it might be worth exploring ways to gently heat the tank. Perhaps the tray under the boiler can be extended under the gas tank.  It may be worth experimenting with running some exhaust steam near the gas tank, but go gently, you don't want the tank heated much above about 40 C. 

Certainly, it is worth reading the composition data for the gas container you intend to buy.  In winter, you would choose, if you have the option, the one with more propane will have higher vapour pressure, similarly, more iso butane will result in a higher pressure than one with more normal butane.  In summer, you might choose a different composition, especially in a jot climate.  Yesterday it was forecast to reach 37 C, bit in my carport, under a roof and on the wall facing South, it was over 38, and the infra red thermometer showed readings up to 61 on the sandstone paving slabs in the back yard.  Not a good day to leave the gas tank out in the sun.

I am not sure that there is much more needs to be said about gas tanks and gas pressure, but feel free to ask if there is something I have overlooked.

Tomorrow, I will look at combustion issues, starting with those questions Paul wrote in yesterday.  We can see where the trail goes from there.

Thanks for dropping in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 14, 2017, 01:03:33 PM
Hi MJM an interesting experiment might be to attach a thermometer to the small propane/butane gas tank to see how much the temperature varies with usage !  and so another gauge to install on the loco backhead with appropriate control cocks to warm or cool it ??!! One could also use the cooling effect of the gas tank to cool the water going into the injector on hot days ??!!
Title: Re: Talking Thermodynamics
Post by: Kim on December 14, 2017, 06:15:10 PM
Hi MJM,
I've been enjoying this discussion and finding it quite fascinating.  Brings back memories of the minimal chemistry I took in college!

I've got a question though about how you're saying that each compound in the mixture will have its own boiling point, which acts to bring the overall boiling point somewhere between that of the various compounds.  This makes sense to me in your description.  But when I think about adding antifreeze to water in your radiator, you want the compound to freeze at a lower temperature than just the water.  But if they each act independently, does that mean the water will still freeze at 32, and the antifreeze will freeze at a lower temp?  That can't be quite right, can it?

Kim
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 14, 2017, 07:53:39 PM
Hi Kim, Unfortunately or otherwise there are allways exceptions that make the rule  ?!!

I've got a question though about how you're saying that each compound in the mixture will have its own boiling point, which acts to bring the overall boiling point somewhere between that of the various compounds.  This makes sense to me in your description.  But when I think about adding antifreeze to water in your radiator, you want the compound to freeze at a lower temperature than just the water.  But if they each act independently, does that mean the water will still freeze at 32, and the antifreeze will freeze at a lower temp?  That can't be quite right, can it?

Kim
[/quote]
Title: Re: Talking Thermodynamics
Post by: Noitoen on December 14, 2017, 08:55:25 PM
About the CO sensor reading CO2, I speak from experience at work. We have a refrigerator recycling machine which shreds  the already decontaminated fridges. At first we remove the refrigerant gas and motor and then the rest is shreded in a controlled atmosphere.  We are obliged to recover the expansion gas of the insulation foam and since this gas now a days is mainly pentane, the shredder is flooded with nitrogen to minimise the risk of explosion. To control the atmosphere there is an array of sensors that measures the oxigen, pentane and Co. In case there's a fire high CO reading is an early warning and is controlled by nitrogen flooding. The atmosphere inside th shredder is circulated in a cryogenic system that freezes all "freezeable" gases and the cleened nitrogen rich atmosphere is returned to the shredding chamber. At the end of the shift and after a defrost cycle, the gas is recovered in high pressure cylinders.

On certain refrigerators the expansion gas is Co2 and when we shred to many in a row, the high concentration triggers the CO alarm. We know for sure that it's not a fire issue, it's just a false alarm.
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on December 14, 2017, 09:13:13 PM
I'm not exactly an gas sensor expert, but I do know that there are many different types and while some only are sensitive to one kind of molecule (gas), others are to sensitive to two or more. What kind is used in a piece of equipment depends on specifications and price.

This mean that Noitoen is quite right in his experience, while it is (almost) safe to assume (Ass-U-Me   >:D ) that Willy's sensor only measures CO ....
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 15, 2017, 11:42:30 AM
I am glad to see that people are finding the thread interesting.  It might have seemed a long way around, but I always feel it is more satisfying to understand the "why" rather than just be given another rule to follow.  Rules rarely always apply exactly as remembered. 

Hi Willy, on a large enough model to have the space, temperature measurement and a steam valve controlled by the operator, or better still an automatic temperature controller would be the way to go.  I tend to think in terms of perhaps a model boat, with no one on board, so something simpler and inherently safe is required.  On a really large system, you might actually draw liquid from the tank, and evaporate it using exhaust steam in a separate heat exchanger.  But perhaps not the simplest scheme for a small model, or for the inexperienced.  A full size boat, if not coal fired would be better oil fired than gas, as boats are holes in the water, purpose designed to accumulate heavy gases until they reach an ignition point.

Hi Kim, glad you are enjoying the thread and were reminded of your college chemistry.  The great thing about starting work in an ethylene plant is that you get to really understand that basic level, but don't have to get lost in the heavy detail of the rest of the chemical world. 

I hope that I have not led anyone astray by reminding us of the earlier statements about mixtures of gases, where the components act independently.  When we have two phase mixtures, meaning that there is condensed liquid and vapour in equilibrium, as in this discussion, the behaviour of the two components is not independent, and the pressure/temperature of the mixture is in between the temperature and pressure that each component would be if in separate containers.

My wife tells me I should not speak on the negative case, as people often overlook the "don't".  When I sent my kids to get my coffee, I always said "don't forget the sugar."  They always did, and I think the game became whether they could remember to forget the sugar.  Probably still sugar in the bottom of the washing machine from their pockets when they forgot.

However, we are now talking about simple mixtures of a family or series of hydrocarbons that mix quite freely.  What ever the concentration, the mixture boils over a distinct temperature range, and the vapour and liquid compositions are each different from the average for the whole container.

I am glad you asked about that antifreeze in water, because that is an example of a mixture used by many forum readers in their cars in winter.  Possibly even in the water hoppers of their engines.  In this case, the mixture behaves a little differently.  At a concentration of 65% glycol, it freezes at a single temperature of around -60, a bit like a eutectic alloy in metals, though I understand not easy to measure accurately for some reason.  But also, like metal solutions, either side of that specific concentration, the solution freezes over a range of temperature.  If you have excess water, the solution starts to form ice crystals at a higher temperature as water concentration increases, to the limit of 0 C for pure water, but is still not completely frozen until -60 C.  It forms a sort of slushy mixture at temperatures in between.  On the other hand, if you have excess glycol, the point at which freezing starts, increases as the glycol concentration increases, up to the freezing point of pure glycol, or -12 C, by separation of flakes of solid glycol.  But note the freezing point of that mixture is way lower than either glycol or water alone.  That slushy mixture provides burst protection for your pipes, providing there is room for a significant expansion as the solid ice crystals form as they can move around.  But if you want freeze protection, that is, no ice crystals to abrade seals and so on for pumping at low temperature, the higher temperature is the criteria, and you need the 65% mixture for lowest temperature protection. 

The mixture also provides a small increase in boiling temperature.  This is not linear with concentration, but a modest lift in boiling point until quite a rich mixture, then rapid rise in boiling point with further increase of concentration up to the boiling point of pure glycol.

Hi Noitoen, thanks for telling us about that recycling operation. It gives a whole extra meaning to the old phrase about whole of life costs, when a plant like that is needed to dispose of the old equipment.  Perhaps they need to use air to expand those foams.  I could specify a suitable compressor for them.  So the monitor response in that case might be due to overwhelming the sensor with CO2, compared with the more normal air with background levels of CO2.  I have also heard reports that they also respond to hydrogen from battery charging, also an environment different from that intended by the manufacturer.  I guess your plant was evacuated a few times before the cause was discovered.

Hi Admiral D K, depending on the detection principal, I guess most detectors would respond to some degree to a range of gases and it depends on what the manufacturer can do to identify a response to a particular component by the signal analysis software.  I suspect that in normal air composition, including the normal range of background CO2, it is more sensitive to the specified gas, otherwise it would be no use as a warning device.  I guess that in the domestic situation Willy has described, it would be foolish to ignore the alarm until the cause was completely understood.  Something definitely worth questioning the manufacturer on, especially these days when it can be quite easy to contact them.  Good to see that the standard definition for 'assume' is understood in your country too.

Hi again Willy, I hope those points have clarified the point about the apparent anomaly in behaviour, not an exception to a rule, but just requiring a better understanding of how it all works.  Admittedly still a bit of homework required before we all completely understand what is happening with the CO monitor.

Thanks for the quite searching questions, I hope that my replies have clarified the issues adequately.

I think that is long enough that I should not start a new topic at this point, but perhaps we can get back to Pauls questions on combustion tomorrow.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on December 15, 2017, 12:21:40 PM
Glad we are getting into a little detail with fuels. Some time ago I pondered the prospect of experimenting with hydrogen peroxide as an accelerator or enhancer for combustion with metho, (meths, ethanol), via its release of oxygen. As it is compatible, with ethanol and water, I conclude that it should freely mix with metho and enhance its performance as a generator of heat. I am specifically concerned with its performance using a wick type burner supplied with fuel by the standard 'chicken feed' system as commonly used on metho fired gauge one model locos, but may be applicable in other scenarios. As there may be some risk when experimenting with such things it is wise to PROCEED WITH CAUTION, research first before striking a match! This article might be a suitable starter for those who are interested, <http://www.odec.ca/projects/2007/park7l2/index.html>. MJM have you any knowledge or comment regarding the above application? I look forward to your reply on my previous enquiries. I have to say, thank you, again, for the time you must be putting in satisfying all your inquisitors. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 16, 2017, 01:24:39 AM
Hi MJM just a quick question ........wondering about thermal coal ??!!
Title: Re: Talking Thermodynamics
Post by: crueby on December 16, 2017, 01:37:45 AM
Hi MJM just a quick question ........wondering about thermal coal ??!!
Anything like thermal underwear??
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 16, 2017, 11:38:01 AM
Hi Paul, it has taken a while, but good to be talking about combustion and your questions from a few days ago.  Talking about accelerants is probably not where I would choose to begin, but why not?  I looked up that web site, a secondary science project from Canada, if I have understood correctly.  It's good to see young people working on science projects, they are the scientists and engineers of the future.  Very hard to comment on the project.  I would assume the supervisors main job was to ensure the students did not come to any harm, and not to contribute to the project.    Perhaps help with choice of equipment and so on.  It's not a measure of the supervisors' knowledge.

So let's just look at whether some peroxide will help your locomotive performance.  In principle, adding an oxidiser as an accelerant provides oxygen for combustion without the nitrogen which slows the reaction, basically by getting in the way of the fuel and oxygen getting together. And by absorbing some of the heat produced, it limits the maximum temperature reached in the reaction.  But it does not add to the heat produced, which comes from the reactions between oxygen and the hydrogen and carbon in the fuel. 

The students state that peroxide decomposes into oxygen and water in an exothermic reaction, that is, a reaction that releases heat.  I have not checked that, but have no reason not to believe it.  But the quantity of heat is key, as that is the only source of extra energy.  It would be interesting to know how much heat is released per unit of peroxide decomposed.  I do know that hydrogen peroxide is highly unstable.   The students overcame this issue by using a solution in water.  The concentration available in the local pharmacy is quite low, but adequate for disinfectant and similar household purposes.  I think they used a more concentrated solution, which is probably available from suitable sources.  But it still comes with plenty of water.

The student experiment then involved carefully measuring the temperature in a beaker above the flame as a measure of the heat produced by the flame.   They did this very carefully and they documented it well.

When those students progress in the years ahead and start studying thermodynamics, they will see what was missing from their experiment.  You have been following this thread from the start, so I hope that you can also see the issue.  Otherwise I have some gaps to go back and fill.

So I suggest we go back to the basics, and look at your earlier questions, then return to see if the accelerant would be useful.  Thank you for recognising the time these posts require, but it really is a privilege to share the information with people who are interested.  And it is something I can give in return for all I learn from other members who post on this forum.

Hi Willy, coal is not a pure substance, but is dug out of the ground, where it was laid by the processes of nature in times long past.  It always comes with various earth based impurities, and also a variation in the basic carbon content, density, heating value etc.  It ranges from the soft, wet brown coal we have here, which the metallurgical engineers describe as something made by our creator to sop up water, through to the hard black coals.  The miners separate out what they dig up, and divide it into various quality levels, which generally attract differing prices in the market place. 

Thermal coals are the quality generally selected for burning to produce heat for steam, electricity etc, generally the lower price ones, as opposed to the metallurgical coals which are selected for steel making and other high value processes.  At work, I had the experts down the corridor, but I did not get involved so much in those projects.

Hi Chris, so not thermal underwear.  The best of that comes from other natural sources, particularly merino wool.  There is a very good brand manufactured in New Zealand, and we produce a lot of good merino wool here.  I hope that is not considered advertising, but the farmers can do with some support.  The main other source for thermal underwear is the the fleece of the Wild Orlon, a strange beast of US origin, that is born from parents coming from an ethylene plant.  Also natural, in that the ethylene comes from oil and gas wells, albeit with a little help from chemical engineers as midwives.  Or is it Vets?  But more generally described as 'guaranteed to contain no natural ingredients'.

So Paul, you mentioned flame temperatures that were quite similar over a range of fuels.  I had not seen it stated so clearly before, but it was there when I checked, in plain view in my text book.  Perhaps I never opened that page.  The figure was a little higher than you quoted, but it was clearly stated as for stoichiometric combustion.  That word just means the exact theoretical air fuel ratio.  In practice, excess air is required to ensure that all the fuel atoms actually meet an oxygen atom while the temperature is enough to initiate the reaction.  A bit like you need more guys than gals at a dance to ensure that every girl is able to find a partner.  Of course that means some of the blokes miss out, but some one has to.

Generally 15 - 20% excess air is required to ensure complete combustion.  This means that the reaction not only has to heat the combustion products to temperature, and the nitrogen that accompanies the oxygen, but also all that extra air has to be heated to the same resulting temperature, and that limits the temperature reached.  It is possible that allowance for excess air explains the difference in the temperature in my text and the figure you have quoted.

So what are the other parameters?  Well temperature is an result of the heat contained in the flue gases, but is not the measure of the heat.  Each component of the flue gas has a property called specific heat, which is a measure of the heat (in Joules) required to raise the temperature of a kilogram of the substance by one degree kelvin.  A difference of one degree Kelvin being the same as a difference of one degree centigrade.  And of course you can use Btu, lbm. and degree F if you prefer.   Immediately, you can see that you need a gas composition, mass of flue gas, (which is equal to the mass of fuel plus mass of air), specific heat of each component, and the temperature rise from atmospheric temperature to that flame temperature.

Now there are two values of specific heat, depending on whether your reaction is at constant volume, so called Cv, or constant pressure, Cp.  Fortunately you only need to know one of these, as the difference between them involves only the universal gas constant and the molecular weight of the substance.  And while it is called a constant, it actually varies with temperature, so you need the appropriate value for the temperature range you are referring to.  However, there is a simple short cut through all this, as the heating value of the fuel is readily available data for common fuels, and the ones you quoted agree with the data I have.  The value for methylated spirits needs a little further explanation, but I will come back to that later.

 The units of that heating value are Joules per Kg, or more conveniently MJ/kg, as none of us like keeping track of too many zeros in our calculations.  Or KJ/g for the quantities in your small locomotives. 

So that flame temperature gives a figure we need for heat transfer calculations, and the mass of fuel burned per second tells us how much heat we have available.

Several more questions to answer before we have dealt with your post (#557, 12 Dec), so I will continue those tomorrow.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on December 17, 2017, 06:36:25 AM
MJM, Thanks for your advice regarding my alcohol addiction thus far. As you say any given amount of fuel has to heat all the gases in the air supporting combustion and this was one of my many 'maybe thoughts' regarding the use of H2O2 as a provider of oxygen, it might be able to reduce primary air volume, but like most of my thinking in this area it is conjectural as I have little knowledge of fuel chemistry or the delicate art of combustion engineering and am only able to apply, mostly forgotten, schoolboy chemistry. I was hoping that I might draw in some model rocketeers or maybe someone who has tinkered with a scale version of the torpedo motors that use H2O2 for the oxygen supply who may have lifted some fog from my thinking and pointed out pitfalls and baseline requirements, e.g. such as whether a stabiliser or catalyst might be required for my uses. I could probably go on and on, but don't want to drag discussion off on too tangential a trajectory, after all this website is primarily dedicated to model engine making and fear too much chemistry might induce a lot of yawning, or worse! turn people off this thread. So I'm happy with a brief indulgence but of course very grateful  if it pleases you to go into some depth. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: Steam Haulage on December 17, 2017, 09:25:15 AM
What part do azeotropic mixtures play?
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 17, 2017, 12:20:43 PM
Continuing on combustion -

Hi Paul, I hope you mean addiction to alcohol as a locomotive fuel, I will try and go al little further yet, as I am also a great believer in its suitability.  I started the thread after noticing that chemistry and thermodynamics seem to be actively avoided in the usual sources, and hoped that there would be some interest in an introduction to the concepts that help us further our hobby.  I think the number of reads probably speaks for itself, but I hope people will drag me back if I go to far into the esoteric, rather than drift away.

Hi Steam Haulage, I was hoping no one would ask.  An azeotrope is a mixture that boils with a constant composition, and despite a lot of heavy reading on the subject, was unable to be sure if it applied the ethanol water mixtures or not.  It might do, as you cannot purify ethanol beyond 95% by simple distillation.  I am reasonably sure that it does not apply to the mixtures of ethane, propane butane and pentane, the simple paraffin series hydrocarbons, and also the olefins and diolefins.  With those, I am well familiar with separation to high purity ethylene and ethane, or propylene and propane for example.  But even further in left field, you cannot separate normal water from heavy water by simple distillation either.

Quite a few concepts introduced last time.  I hope it was not too confusing, and you were able to keep track of whether 'it' was nitrogen, as intended in one sentence, or oxygen as was intended in the very next sentence.  Comes of writing at the end of a long day.

I suspect that it is possible to calculate that flame temperature from the energy balance, though it requires data for the specific heat of the reactants over a wide temperature range, while I only have data for  one relatively low temperature.  It is not so surprising that propane and butane have very similar heating values, as they all and made up of carbon and hydrogen, with very similar carbon to hydrogen ratios. 

Ethanol has one oxygen in the chemical makeup of the molecule, which can be written symbolically as CH3-CH2-OH and the lower heating value is possibly explained partly by the lack of that extra hydrogen, but it is also because one of the hydrogens has already combined with oxygen in that hydroxyl (-OH) group attached to one of the carbons, so we get no heat from that reaction either.   The oxygen which has sort of taken the place of a hydrogen atom does not add to the energy released during combustion, but actually reduces the heat released compared with the release if had not been there, as in say ethane which also has 2 carbon atoms and 6 hydrogens.  That is certainly an oversimplified explanation, but adequate for our purposes.  Clearly the effect of this oxygen is anything but an accelerant, so does not tell us much about that peroxide experiment.

There is another factor which, in practice, further reduces the heating value of ethanol in the normal form of methylated spirits.  If you look carefully at the container, you will see somewhere the composition.  It is normally 95% ethanol and 5% water.  There is a perfectly valid reason for this, in that ethanol is normally obtained from fermentation processes (I don't know why I am telling you this!) which produce a solution of ethanol in water.  The ethanol is purified by a distillation process, but it is only possible to achieve 95% purification by this process.  A result of those vapour pressures and liquid concentrations when a two phase mixture of substances is boiled, that we spoke about earlier.  There is also another compound added to methylated spirits to make it undrinkable, well to most of us anyway.  When we then take that 95% ethanol mixture, and burn it, we end up having to evaporate that water, which means we have to supply that latent heat, and which goes along for the temperature ride up to the flame temperature.  It does not add to the energy produced, but simply dilutes the flue gases and reduces the maximum temperature reached from the reaction.  It does not reduce the energy produced either, except perhaps by getting in the way, similar to nitrogen, so may result in a bit less complete combustion.  You can see the result of this in the sticky black soot deposited on your boiler if the flames are allowed to lick the boiler.

So at this stage, I hope I have drawn a clear enough distinction between temperature and heat release, and introduced some of the concepts that make methylated spirits a different fuel to burn.  We can come back to that later, but first let's look at how that distinction between heat and temperature helps us understand the process of boiling water in our boilers.

The heat transfer area in the boiler has to enable the transfer of energy from the combustion products to the water in order to generate steam.  The rate at which the heat can be transferred is determined by the temperature difference between the gases and the water at steam pressure and temperature, and in addition by the heat transfer film coefficient.  Then, as heat is transferred from the gases, the gas temperature is reduced, so the temperature difference is reduced and the rate of energy transfer per unit of area is reduced as the gases travel over the heating surface.  This is where that log mean temperature difference comes in.  The simple one to understand intuitively is the marine centre flue boiler, a description I would also apply to Chris's Lombard model.  The hot gas from the burner starts at one end and travels along the flue, loosing heat to the water, and consequently falling in temperature as it travels along to the smoke box end.  Clearly the biggest temperature difference is at the burner end.  The first, say one third of the flue length transfers the biggest part of the heat due to the larger temperature difference.  The next third starts with a much lower temperature, so looses heat at a much lower rate, and similarly for the final third, where the heat transfer rate is lower so the temperature change is not so much.  Clearly there is diminishing returns on adding length to the boiler in the hope of cooling the flue gases further to raise more steam.

There is also limited usefulness in raising that flame temperature, say by using an accelerant, or perhaps using some oxygen enriched air.  Yes it would increase that initial heat transfer coefficient, but at very high temperatures, the water side boiling coefficient is affected, I believe it is something along the lines of the whole area becoming vapour on the metal surface, instead of boiling liquid, and this dramatically reduces the heat transfer rate.  Just where on the temperature scale, I am not sure, quite possibly a little benefit before you get to that point, but just not linear, ad infinitum.  On the other hand, burning more fuel so there is more heat and a greater volume of flue gas does two things.  It increases the velocity of the flue gases, which tends to increase the heat transfer rate a little, but the greater heat content of the flue gas means that the temperature at the end of that first third of the length is higher, hence the temperature difference at the start of the next third would be higher, and so more heat transferred there, and so on to the end of the boiler.  At the smoke box, with the same length of boiler, the flue gas is still at a higher temperature than it would have been with less fuel burned, so more heat is lost up the stack.

Lower overall boiler efficiency, but more steam generated in the same boiler heat transfer area.  That is the point we were discussing a few days ago about waving the hand over the smoke stack to see if the boiler could handle more heat.  The actual flue gas in the smoke box cannot be cooler than the steam in the boiler, so is quite hot enough to burn your hand.  But as the flue gas exits the stack, it mixes with air, and is cooled in a relatively short distance.  So start with your hand high, and cautiously bring it down, and get out of there before it feels to uncomfortable.  Experience with several different boilers will soon show you the difference between boilers by how close to the stack is comfortable.

So if the flame temperature is about the same for many fuels and if we don't really want to increase it too much anyway, is there anything we can do to increase the steam production from our model?  And how do we decide on the best fuel for our model?

This has been pretty heavy going, I might need to answer a few questions on the topic, otherwise I will continue on those last two questions tomorrow.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 17, 2017, 11:15:50 PM
Hi MJM ,when i made violins i used industrial meths for the spirit varnish as it had
 no colour . The meths you buy in the tool shops has a blue dye in it to make it not so attractive to the people that drink it!!! is there a difference between the two ?Willy......
Title: Re: Talking Thermodynamics
Post by: Steamer5 on December 18, 2017, 08:22:22 AM
Hi Willy,
 No difference except the blue stuff added & the stuff to make you sick!

Now having a dad in his 9 decade has some advantages....I know how to take the blue stuff out of meths, I hasten to say NOT TO DRINK.....but best not to put how here.........

Cheers Kerrin
Title: Re: Talking Thermodynamics
Post by: paul gough on December 18, 2017, 10:31:56 AM
Hi MJM, My alcoholic tendencies pertain only to its use as a fuel in burners. I am following your words carefully and will refrain from further questions until you have covered those already put. You might be interested in a slightly more advanced discussion of ethanol and H2O2 in a paper from the 21st Brazilian Congress of Mechanical Engineering, 2011, at the following site; <http://www.abcm.org.br/anais/cobem/2011/PDF/197101.pdf>. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 18, 2017, 10:41:03 AM
Hi Willy, here they put blue dye in kerosene and Meths is normally clear.  I doubt that it makes much difference to to the properties as a fuel.  Possibly responsible for the smell some people complain about.  We use it as our main cooking fuel on a small boat, much as hikers use those Trangia outfits, and don't notice any particular smell.

Hi Kerrin, thanks for joining in.  Taking the blue stuff out might overcome the reported odour problems.

Hi Steam Haulage, are you able to add anything about azeotropes.  I thought I understood it, but I usually check these things before including them in the post unless I am really confident.  When it came to it, I could not find a satisfactory definition that made it clear whether that water alcohol mixture is an azeotrope.  That is why I did not mention them earlier.  Can you confirm the term applies to that mixture of water and ethanol?

Paul, I hope I have made some progress on your questions in this post.  I will look up that site later, we are obviously all writing at the same real time.  You can see that the heat necessary to get any amount of steam your boiler can produce, can generally be obtained from almost any fuel so long as you have a burner that will burn it at the required rate.  You are quite right, to observe that you have to burn proportionately more ethanol than butane to produce the same amount of steam.  It is important to be careful with the units.  Each of the fuels has a different density, ethanol density of 790 kg/m^3, compared with butane about 570 (depending on the ratio of normal to ISO butane), helps partly overcome that difference.  It is probably better to stay with mass measurement for comparative purposes.

Of course, sometimes you will see heating values based on vapour volume.  The reason is that one mole of any substance occupies the same volume.  So volume measurements are proportional to the number of moles of the substances.  But only if you are using vapour volumes.  You should not mix vapour and liquid volumes when calculating air fuel ratios and so on.

So the choice of fuel is based on convenience, safety, availability and the practicality of the burner in the particular boiler setting.  For your small locomotives, I suspect there are practical difficulties associated with coal, that some will see as a challenge and persist for the sake of following prototype, but probably not the simplest choice until the models are larger.

Gas requires a pressure vessel for containment and has safety issues in terms of the consequence of leakage.  However it burns cleanly, and suitable burners are commercially available in a range of  sizes.  Whether to use propane or butane will depend on availability in your area, which in turn probably depends to some extent on climate.

Methylated spirits, the common name for ethanol based fuel, has a huge safety advantage for model uses.  You can put out Meths fires with water.  Alcohol is soluble in water in all proportions, the alcohol is absorbed into the water and cooled below its vapour pressure, so the flame is cooled and snuffed out.  It is the vapour that burns, not the liquid.  It is also relatively cheap and readily available.  But as you know, it can can be a bit tricky to burn.  Ok, a wick will burn a certain amount, and a bigger wick or more wicks will burn more Meths for more heat.  Those Trangia burners start with a pool of liquid which you light at the surface, and the arrangement starts vapourising some of the fuel, while the pool surface fire is extinguished. 

I have seen comments about the air requirement, the theoretical air for ethyl alcohol s 14.3 to one by volume, compared with 23.8 for propane and 31.0 for butane for stoichiometric combustion.  And then there is the heat absorbed to evaporate the water content.  But you have done a lot of work in this area and I would be very interested to learn more about it if you would write it up, either here or in a separate thread to help us all get a bit further on understanding this fuel.

The stove I mentioned that we use on our boat, is very effective.  It does totally evaporate the fuel into a burner much like a gas burner in a domestic stove, but with the straight liquid tube over the burner to evaporate the fuel.  The tank is slightly elevated, so it has about 75 mm of liquid as pressure to drive the flow through the needle valve regulator.  I have attempted to copy this without much success, but need to get back to that experiment.  However that is all getting off the track of your carefully thought out questions.

You mentioned the difference between different burner configurations, such as the poker type, ceramic, or wicks.  When heat transfer is discussed we nearly always jump straight to convection heat transfer.  However, for our boilers, particularly at the burner end, I expect that radiation is also a significant factor. 

Radiation is a little different from conduction and convection.  It travels in straight lines, not dependent on the air or gas in the way, or at least only minimally so.  But it requires line of sight.  A large part of the maths related to radiation is about calculation of this view factor.  Radiation heat transfer is proportional to the fourth power of the absolute temperature, so that flame temperature is quite important in this case.  And the radiation intensity is dependent on the frequency of the radiation and most heat transfer is in the visible to infra red region of the spectrum.  If you can't see it, or feel the heat on your hand outside the hot gas stream, then radiation is not very important.  The frequency is dependent on the temperature.  For the conventional locomotive boiler, that firebox has a very effective view factor for the coal fire on the grate.  Now the heat transfer area also radiates back, but because of its lower temperature the net heat transfer proceeds from the hot coal to the cooler plate.  Now that is a very brief summary of the key points for radiative heat transfer, but it might be enough to guide some useful experiments.  Once the combustion products leave that firebox and enter the flue tubes, radiation is a much less important factor, as gas tends to be a poor radiator.  In the flue tubes, convection is the predominant heat transfer mechanism, and this is where gas velocities have an influence on the gas side film coefficient.

A coal fire is a very good radiator, providing it can see the receiving surface, and it provides plenty of hot gas for convection transfer in the tubes.  Gas fires however, with a much less visible and predominantly blue flame, are not such good radiators.  This is where that ceramic grate comes in.  The ceramic is heated to red hot which radiates to the heat transfer area.  Some people advocate some stainless steel mesh placed so it is heated to glowing by the flame, and then radiates to the boiler tube.  The idea is to increase the total heat transfer by harnessing some radiation transfer.  I have no experience to say how effective this is.  I hope others who have tried this scheme can tell us how well it works.

Remember the importance of that line of sight.  When ethanol is burned on a wick, located like that picture you provided a few posts back, it appears to be outside the boiler, with only a shroud to guide the flue gases into the tubes.  This shroud will get hot, but if I understand the arrangement correctly, it has limited ability to transfer that heat to the boiler water, so it appears that the radiant heat is largely wasted.  Not easy to analyse in detail, as the shroud is able to transfer some heat to  the flue gases by convection, so it is not all lost.  But possibly worth thinking about boiler arrangements that have some sort of water wall around and over the wicks.  My little boilers have superheater coils around the firebox.  Nothing fancy, about two turns around the inside of the casing.  They are not in the flame like the main pot boiler but receive heat by radiation.  They superheat the steam from about 110 C in the boiler to 138 C so clearly absorb significant heat.  I had not thought of it quite this way before, but it possibly points to the way to arrange the water space around the flame to take advantage of the radiated heat.

Now that poker burner.  I have not seen one in action close up.  But it does look like it would provide significant radiant heat over its length.  So making the flue tube large enough to accommodate the poker burner is probably a worthwhile idea, as the tube is submerged in the boiler.  The combustion chamber end of the tube has to be large enough to allow the burner to function properly.  The combustion gases then continue through the rest of the tube, where cross tubes or in small sizes possibly even solid cross rods can collect extra heat by convection.

 Unfortunately my theoretical knowledge is not enough to quantify the different arrangements or to determine the optimum layout.  We are back to being reliant on experiment.  And of course the experiment is only really useful if we can arrive at a suitable method to organise and analyse the data.  But I hope those few thoughts on the subject will help with understanding, and help guide productive experiment.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: Noitoen on December 18, 2017, 11:15:00 AM
Hi Willy,
 No difference except the blue stuff added & the stuff to make you sick!

Now having a dad in his 9 decade has some advantages....I know how to take the blue stuff out of meths, I hasten to say NOT TO DRINK.....but best not to put how here.........

Cheers Kerrin

In South Africa, they used a loaf of dried bread to filter out the blue stuff for drinking purposes :LittleDevil:
Title: Re: Talking Thermodynamics
Post by: paul gough on December 19, 2017, 07:33:46 AM
Hi MJM, Your discussion and insights into the goings on concerning fuel, combustion and associated boiler issues has provided much to ponder and look into. One of the useful things also about all these exchanges is that many, if not all, the parameters that one has to consider are brought together in a couple of pages of text rather than randomly scattered throughout myriad publications etc. This should make it easier and quicker for anyone researching to get a thorough grip on things.

Coal firing in Gauge One is not unheard of for locos of middling to larger sizes, a number of the locos from Aster had a coal firing option as do a few from Accucraft, quite a number of youtube videos show methods of firing etc. Quite a number of self builds also exist and a few with 'wet' fireboxes but the ones I have seen are just conversions from meths dry fireboxes. With regard to my very small model, the wick flame does impinge on the back end of the underside of the boiler barrel and the dry firebox or shroud has ceramic sheet insulation around the inside so does not, if at all, contribute to heat transfer to the boiler. It is only 1mm thick sheet but I intend doubling this if not tripling its thickness, more changes, more experiments, more time. This one little loco and all the things I think of to try out both with boiler/burner/fuel  and the mechanism will probably more than occupy me up to the end of my tenure on the planet. With the wicks, it is important to establish a good blue flame, which involves establishing the best wick length, the number or how tightly packed the strands are in the wick tube, the height the wick sits above the end of the tube and ensuring proper balanced fuel delivery to each wick, how high the wicks sit inside the firebox, the size of the opening at the base of the firebox too big and you suck in too much cold air, also the cross section area of any narrowing of the flame/gas pathway from wicks to fire tubes, not too small but not too big, also the flame length should be determined by setting up burners in the open or outside the fire box in still conditions and measuring it, there should be enough length of flame getting to the tubes in my experience. Some restriction to air flow in experiments can be had by placing metal fly screen or other fine mesh at the bottom of the firebox or in the flame/gas path to observe results. Some have experimented with small holes drilled around wick tubes near their ends, but the only person I know who did this found no significant changes, but as far as I am aware it was a one off try and no extensive tests were performed. As to what can be done on such small model locos, it is generally restricted by the dimensional limitations inherent with any particular example being modelled as well as the general smallness of gauge one models, obviously I have made a rod for my back trying to advance performance with one of the smallest of models, bigger U.S. types would make things considerably easier, however I just happen to like pre 1850 locos. Flame behaviour is observed using a small inspection mirror from an auto parts supplier, I prefer the more rectangular ones rather than circular types. At some point I intend to make a glass walled firebox, a glass 'spy hole' to observe the flame entering the fire tubes and a glass fronted smokebox which will allow direct observation, even if only for a short time due to possible 'sooting'. An easy experiment is to try various wick materials, eg. stainless scouring pad, steel wool or tightly rolled fine mesh etc., etc. As too superheating, I agree it is useful, especially in attempting to eliminate condensation and maintaining higher cylinder body temps, but lubrication needs to be assured when going to much above wet steam temps especially for the slide valves, obviously metal on metal, whereas the pistons are frequently supported by Viton or Teflon rings so not quite so critical. For the moment thats about all I can bring to mind on general principles, no doubt I haven't covered everything, if I remember anything important I'll put it up. If I ever get to setting up a stationary boiler test bed for small engines I think I would go for  a 'colonial', nice and simple but allows various number or sizes of wicks for metho, other fuels, and burner types to be easily installed under the barrel for comparative tests. Once again, thank you for sharing what is in your brain and the results of 'reading up', knowledge is a precious thing and to get it as freely as we do from this thread and website shows considerable generosity. Best wishes for the Christmas, Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 19, 2017, 11:50:47 AM
Some pictures at last!

Hi Noitoen, simple technology the best perhaps?  Unfortunately I suspect the bread is only acting as a filter and removing the dye particles.  Remember how they used to demonstrate Brownian motion in junior science?  But a filter does not remove solutions of other compounds from the mixture   So I would assume the poison they add to discourage drinking is still there.  It is also added to our Meths, which like Willy's industrial brew, is colourless, so the poison does not in itself add the colour.  Of course, if the dye is responsible for the odour when burnt, or perhaps some residue deposits on the burner or boiler, it may still be worth removing the dye, even for burning as a fuel.

Hi Paul, thank you for writing up such a great description of your experiments.  So many things to try before you even think of starting on gas burners.  Some of those experiments I guess contribute quite small differences.  How are you assessing the results?  Keeping a good record of what you have done must be a challenge.

While confining your experiments to a small engine will have its limitations, it's much more enjoyable to keep to a prototype you like, and by confining yourself to one platform, you must be getting a really good idea of how things all work together.

My reference to my superheater may have been confusing.  I was only intending to point out that there is useful radiant heat available at the sides of the flame, and presumably over the flame, as demonstrated by the heat collected by my superheater, and due to the T^4 effect, possibly worth harnessing instead of relying solely on convection.  By collecting some radiant heat and the remainder by convection, there may be an overall increase in efficiency.  It is quite hard to predict how much however, because radiant heat that is not collected almost certainly at least partly leaves more available heat in the flue gas for convection.  But radiant heat reaching the dry firebox wall probably also adds to the heat loss from the boiler.  Obviously more experiment required. 

So far I have not appeared to have lubrication issues with the superheat I am achieving, and using just a displacement lubricator.  But those viton and teflon rings will be getting near their temperature limits anyway, so may not tolerate much superheat.  However, with graphited packing?  Oh, and slide valves.  Not necessarily metal on metal.  Almost certainly an oily film of steam or condensate between the surfaces of even the most smooth surfaces unless they are tightly bolted together.  The principle of working out the balance force on the valve usually assumes an approximately linear gradient between the pressure at the edges where the pressure is definitely known.  For example in the exhaust cavity and around the outside of the valve.  That is what I was working on when commenting about the asymmetry of the valve possibly tilting the valve in Chris' Marion Shovel engines.

The glass smokebox idea sounds interesting.  Yes, it will probably need cleaning for each run, but you may not need to use it often to learn what it can tell you.  If you can just make it interchangeable with the normal metal one.

Thanks for your kind words on the thread.  I am delighted that you are finding it interesting.  But please remember that the comments and questions by you and Willy and so many others are giving me wonderful pointers as to just where the knowledge that I gained in my very different work environment can be useful in helping us understand our models.  Together we have gone way further than I imagined when I started this thread.  The sharing is definitely a two way street.

I have attached a photo of the burner from the metho stove I mentioned last time.  The mirror underneath was not as helpful as I had hoped.  The liquid from the fuel tank comes in at the left, and the needle valve is in the tube on the right, with the seat near the left hand end of that hex bar.  The length of the tube through the burner is all that it takes to evaporate the fuel, the little plate on the left hand side increases the heat transfer to the tube, perhaps most importantly while the stove is being preheated for lighting.  The jet is in the side of the little vertical pin on the bottom of the hex bar.  When the needle valve is opened, the vapour stream induces air as it crosses the gap to the opening in the side of the burner.  To light it, about a teaspoon of liquid is allowed to dribble out of the jet into the bottom tray, then the needle shut.  You light the liquid surface with a match or empty spark lighter, and open the needle valve again just before the flame goes out, and you now get vapour to the burner.  The last of the flame lights the main burner.   Boils a kettle quite well, perhaps a bit slower than a propane gas stove, more fierce than a wick flame and works well for us.

I also promised a few days ago to attach the diagram of butane properties.  Attached is the pressure enthalpy diagram for iso-butane.  Normal butane is very similar, just the pressure at any temperature is a bit lower.  I have attached the same diagram for water so you can see the form is very similar, just the numbers differ for two quite different substances.  The diagram I was previously using for steam, when discussing engines and work output, was a temperature entropy diagram.  Again a similar form with that two phase area very obvious, but a slightly different shape.  So I have also included the T-S diagram for water for comparison.  Unfortunately I do not have a T-S diagram for butane.  These are copied from the Gas Processors Association handbook.  I hope it is acceptable to use that extract for illustrative  purposes, in a discussion such as this.  Unfortunately I had to fold the i-butane diagram to fit my scanner.  The sloped lines that lost their label are the specific volume lines.  You can see the diagrams are more use to help understanding how the other properties change with a change in any selected property, than they are for picking off accurate values over a small range.  Always better to use tables if you have them.  But the point of including them is to show that very different substances behave in a similar manner in the two phase region.  If you understand the temperature-pressure relationship for boiling water in your boiler, then you also understand the behaviour of the liquified gas in your fuel tank.  You don't have to start learning from the beginning for gas.  Or for the refrigerant in your refrigerator or your air conditioner.

A little shorter post this evening to make up for a few recent longer ones.

Thanks to everyone following.  If you are packing up to travel to friends and family for Christmas, safe travels and best wishes,

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on December 20, 2017, 12:04:15 PM
Hi Paul, I looked up that Brazillian site on peroxide in rocket fuel.  Quite interesting.  They were looking at very high temperatures and pressures as you might expect in rocket propulsion.  You clearly would not want those pressures and temperatures in the firebox of your locomotives.  I also looked up some other properties of hydrogen peroxide, and can see why the supervisors of those students were pretty careful to use low concentration solutions.  At these levels, it appears that the heat from decomposition of peroxide is less that the heat required to evaporate the associated water.  This would seem pretty safe, but a bit pointless if you need more heat to evaporate the water than you gain from the decomposition.  The decomposed peroxide, consisting of water and oxygen reduces the amount of air required for combustion of the ethanol, so perhaps gives a higher peak temperature, but at the cost of less available heat due to the energy contained in that water vapour.  I say perhaps gives a higher temperature because it is possible that the water contained in the flue gas absorbs nearly as much heat as the nitrogen that comes if you simply use air.  Remember that adding more oxygen in this way does not in itself produce extra heat from burning the fuel.  So while there is a contribution of heat from the peroxide decomposition, a safe dilution absorbs even more heat, simply to evaporate the water that comes with it.

It appears that at much higher temperatures, the reaction is highly unstable.  Read 'verging on explosive', if not actually explosive.  As far as I can see, it would be very hard to control the reaction safely, and I don't have the necessary knowledge to find a safe way through the various parameters, so don't have much to offer in this area.

I don't know if you have found other accelerants that might be more controllable, but unless there is actually more heat output from the accelerant, it looks unlikely to be useful while staying with safe parameters.  It might be more productive to work on burner design to burn Meths at a greater rate in air for more heat, and experiment with the heat transfer area arrangement to make most steam from this.

I wanted to take a photo of the burner I have been using for my model, that might be promising in this direction, but time ran away today, and I could not get it done.  I will keep that on the list, for later.

I think you implied that you had more questions and I suspect we had better turn our attention to those.

If you do further work on those accelerants, I think we would all be pleased to hear how it went.  I am sure that I don't have the last words on that one.

A short post tonight.  I have spent too much time trying to find out about peroxide, and on some other calculations I want eventually to get back to.

I hope the Christmas preparations are going smoothly for everyone, and wish you safe travels if you are travelling to relatives or friends for your celebrations.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 20, 2017, 08:41:17 PM
hi MJM  I was wondering what the graph would look like if the squares were linear rather than logarithmic ?? is there a reason for this on the middle graph ??  Lots of really good info to take on here as usual  ....Thanks and a merry winterval and a preposterous new year !!!........
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 20, 2017, 08:43:15 PM
Sorry ,that should be prosperous !!!!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 20, 2017, 09:06:55 PM
Hi MJM saw this inThe Engineer in Graces Guide....
Title: Re: Talking Thermodynamics
Post by: Steamer5 on December 21, 2017, 06:48:17 AM
Hi MJM,
 My grandmother used to use peroxide on any cut that we got......funny that we only ever got one cut! We learnt fast.
Oh by the way once poured on the cut, we also took off like a rocket!

Sorry for the diversion, but the mention of peroxide reminded me of its other uses

Cheers Kerrin
Title: Re: Talking Thermodynamics
Post by: paul gough on December 21, 2017, 07:03:47 AM
MJM, Thanks for the time put in on H2O2, it does seem to have issues with its use, but I thought it might just have been a possible with a micro jet/nozzle perhaps using silver as a catalyst in a separate chamber before the burner, as this appears to be one of the choices of model rocketeers, but I am now at the edge of my knowledge and need to go much further in reading up before trying to make any flames. It is really a bit of an outlier of an idea I fully admit and any application was really intended for a larger stationary boiler than the tiny loco, I hope I did not waste your time. About the only question I have for the moment is, have you a notion of what the lowest temperature for flue gases would be worthwhile in trying to utilise for steam generation in our small boilers. Regards Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 21, 2017, 12:21:55 PM
Hi Willy, the logarithmic axis compresses the vertical scale to show the information for a large range of pressures on a reasonable size piece of paper.  Remember when I included that T-S diagram to scale, I had to leave gaps in the temperature scale to show enough detail in the interesting areas.  Using the log axis does compress the vertical height of the diagram, but no point moves right or left.  The path of a vertical constant enthalpy line on that P-H diagram passes through exactly the same states as it does on a linear scale.  Though you would need a roll of Christmas wrapping paper to draw that diagram on a reasonable linear scale.

Very interesting concept for a fire less engine.  A little hard to read, it comes up a bit blurry, so probably needs a few more pixels, while not exceeding the permitted limit of course.  I think the diagrams are showing data for four trips on a real route.  So looks very promising apart from the nasty solutions which have to be contained at pressure and temperature.  But the thermodynamics are not very clearly documented.  Without data on the amount of heat being produced it is hard to know whether it was really a practical concept.  It's would be very interesting to have the thermochemical data.  Also with relatively small temperature differences, a lot of area is required for heat transfer, though liquid film coefficients are higher than for gas. 

Hi Kerrin, peroxide was also my fathers favourite antiseptic.  As you say, it did sting, but my memory is that it was not much different from the straight Detol my mother preferred.  However the faint scars on my hands are all from more recent work by dermatologists, not from the antiseptics of my childhood. I think it was about a 3% solution, and from what I can see, the heat from dissociation is not enough to evaporate the water for this concentration.  I think that is why that school project did not show anything spectacular.  Great to know you are still following.

Hi Paul, no worries about the peroxide, it was an interesting line of research.  I am just sorry that I was not able to contribute much.  I believe the issue is still whether the peroxide is actually able to contribute a positive amount of energy, rather than have it all absorbed in evaporating water.  It looks like somewhere around 65% solution creates enough heat to increase the concentration of hydrogen peroxide, but whether you can still control the reaction with equipment practical on a small locomotive, I don't know.  The chemistry is a bit outside the area I am familiar with. 

However, if you can harness the principal to burn fuel at twice the rate of your wicks, it might still be worthwhile.  For your larger stationary boiler, it may be more productive to add an engine driven fuel pump to force ethanol through an evaporator coil, might be an easier way to burn fuel at a greater rate in a conventional burner.

I don't know if your question about temperature is in relation to Willy's fire less engine, or if you have some other idea in mind.  The issue of lower temperature difference is just how much extra area is required.  That heat transfer equation Q = U x A x delta T still applies, using of course the log mean temperature difference (LMTD).  If you reduce the delta T you can calculate the extra area you need.

I want to get out and burn some fuel and use my sheathed thermocouple to get a better idea of the smoke box temperature in my boiler.  Perhaps over the holiday period.  If you are thinking of just more area in your present boiler, the inlet end temperature is probably something well less than that adiabatic flame temperature, due to the excess air.  Perhaps half the temperature rise above the ambient air temperature, but let's assume 800 C for the moment.  And the smoke box temperature might be around 200 degrees.  (Otherwise it would not transfer much heat to the superheaters used in larger locomotives.). And of course the water side temperature is the steam equilibrium temperature for the pressure from the steam tables.  Say 140 C.  That stays constant for the whole calculation.

Then LMTD = (800-140) - (200-140)/ln((800-140)/(200-140)) = 250

First it is worth giving a bit more thought to refining that 800 and 200 figures, they are just wild guesses to illustrate the calculation.  Then change the 200 to say 150, a ten degree difference at the outlet end, and recalculate the LMTD.  You can then easily calculate the the extra area required for the same Q, assuming the same U.  You will quickly see that most of the heat is transferred  in the first part of the boiler where the local temperature difference is larger.  Alternatively if you are thinking of a different heat source, you would change the 800 to the figure you are thinking of.

Of course U is in reality not a constant, and will change in this type of example, but if you assume a constant, it will give you a good idea of the effect of temperature difference on the area required.  And of course the calculation also works the other way.  You could increase the outlet temperature difference and see the change in area, or the change in heat transferred with the same area.  In practice you would make the outlet temperature higher by burning more fuel.  Or you could calculate the effect of varying the steam pressure, hence temperature, the 140 in the example above.

I don't know if that answers your question.  I am happy to try again if I have misunderstood your meaning.

For the next few days, I will still be looking in each day (too much goes on on this forum to miss a day) and will continue to reply on any questions.  But perhaps you would like to have a think about what new topics you would like to look at in the New Year. 

Until then best wishes for all the joys of Christmas and a safe and happy new year to everyone. 

(By the way Willy, I am not familiar with that winterval, but we are expecting a cool Christmas here, not much over 30 I believe.  Centigrade of course!)

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Steam Haulage on December 21, 2017, 08:12:00 PM
Hi MJM,

I'll respond more fully after Christmas, meanwhile I can confirm that water and the other lower alcohols do not form azeotropic mixtures. In a former life we were desperately trying to convert from a 70/30 toluene/isopropanol solvent blend to use a primarily water borne system. Water has the advantage of great infinity with the substrate we were using.

After Christmas a little more explanation and some more mixtures.

Merry Christmas and A Happy New Year.
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 22, 2017, 11:54:53 AM
Hi Steam Haulage, thank you for that, I will look forward to learning more about azeotropes when your time allows.  I was looking for the word that applied to that maximum ethanol concentration that you can get by distillation to separate the water.  Azeotrope came to mind, but to be sure I looked it up.  It was clearly not the word I wanted so I am still on the lookout if you know the one I need as well.

That little diversion into the interesting area of using peroxide as an accelerant diverted the train of thought.  I am glad we followed it, and will be interested if more comes out, but perhaps it is time to get back to the area of how best to arrange the heat transfer area for maximum steam production in our scaled down boiler outline, or any miniature boiler for that matter.  All the boilers we build count as miniature boilers, whether the outline is a scaled down model of a full size prototype or not.

As I mentioned before, the issue for scaling down is that the gas properties governing gas flow, heat transfer and combustion do not scale down.  Consequently we have to try and understand how these things work in small scale.

I am not sure that it is productive to try and deduce the design totally from first principals.  At least, not with my knowledge level.  If someone has the experience to make even a little progress in this area, please come in. All contributions to our knowledge base are welcome.

Because real flue gas has real viscosity and density, in our miniature boiler the flue tubes are working in a different flow regime, the useful parameter in research into this area is the Reynolds number. Can't go to far down that track without determining velocities and so on, and it will probably not aid our understanding much.

As the flue gas travels along a tube, the radial velocity profile across the tube changes from essentially uniform at the entrance to a parabolic profile some distance down the tube as the wall friction and shear stress slow the fluid nearer the wall.

The temperature profile in the radial direction also develops in much the same way.  And this is where it gets complex.  The temperature profile affects the viscosity, and the viscosity affects the flow profile which in turn affects heat transfer from the wall to the fluid in the centre. Which in turn affects temperature and so on.

The consequence is that the temperature profile, the heat transfer coefficient and the velocity profile are all changing in the first part of the the length of the tube, which may be most of the length in a small boiler. In the end, most predictions are based on experimental data, and only a very few cases can be completely solved by theoretical analysis.  In an ideal world, we would have available data for the range of tube sizes we use with minimal need for interpolation.   But making a range of boilers, with a range of flue tube diameters and lengths and test firing them is enough to put off most experimenters. 

The next best thing is to collect data from boilers that have been made and try to make some sense of the data.

Tube diameter and length are obvious parameters.   Also steam production and fuel consumption.  Area per tube is easily calculated from the diameter and length, and the number of tubes is easily normalised by dividing the steam production by the number of tubes.  Unfortunately it takes quite an effort to measure steam production, and even fuel consumption.  However, it is almost certainly worth noting how much water and fuel have to be added to refill after a run, and noting the times from light up to steam production and end of run.

At the simplest level, for a given boiler length, a larger diameter flue tube gives more area, and so providing it can be fitted so there is sufficient water capacity above the tube, larger would be preferred.  With a burner such as Chris's poker burner, that is really the only option, unless the shell and burner sizes allow two tubes and two burners.  With wicks such as Pauls locomotive, a single large tube leaves a large volume of water below the top of the tube, where it cannot be used for steaming capacity.  Though it does provide a surge capacity by evaporating if the load changes.    However it is worth doing the maths to see if two smaller tubes would give more area, or three.  However, even if the maths says there is more area, the smaller diameter means the flow area for flue gas may be smaller, in which case there is more resistance to flow, and it is necessary for the flue gas to flow freely to allow more inlet air for continuing combustion.  There is likely to be an optimum diameter for any tube length, where there is most heat transfer before the flow resistance starts adversely affecting combustion.  This is what is behind the length to diameter guidelines provided in several books, so they are a good place to start.

I would assume that the available successful designs are already in the range of what is practical without necessarily being optimum.  A bit like those injector designs.  So the likely way forward likely involves first careful analysis of the performance of existing designs, the looking to see if there is much variation in size of the tubes.  If different successful models have different tube sizes, then there is likely to be scope for perhaps a small change of tube size, if it results in getting more heat transfer area.  It would be more risky to try a different tube size if all the model designs you can find have the same tube diameter.

All of that is thinking in terms of heat transfer by convection.  It is also worth putting some thought into some radiation heat transfer.  It is a normal factor in the conventional locomotive boiler with wet walls, but requires a bit more creativity to take advantage of radiation heat transfer in the small locomotive with a wick type burner.  But worth the experiment if it can be done in addition to the normal and proven flue tubes.  Has anyone tried a solid stay in an area too small for an additional tube, but with the firebox end of the stay projecting back into the firebox to collect heat by radiation and transfer it along the stay by conduction, and into the water, as an example?

Not much help to an individual constructor wanting to optimise the heating area of a new boiler.  Most useful is analysis of performance of similar boilers in a club setting, and this is probably the most promising area for furthering the science of our boiler design.  Or perhaps the data is already available. 

Have a very Merry Christmas and a Happy and Safe New Year,

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 22, 2017, 03:12:41 PM
Hi MJM ,lots to think about there....When we make our boilers we use standard available stock components . However that is because they are cheap. ! Anything to do with engineering that is curved, tapered  or compounded shapes is costly !! If it was possible to make tapered tubes with varying thicknesses would this help with heat transfer ?? just a thought !.........Samson Fox made boilers with corrugated fireboxes i think and the chap that designed these made a lot of money and built the Royal college of music ,next to the the Royal Albert Hall in London (england) with the proceeds i seem to recall ? This may need confirmation however !!

One wishes that one could scale down scalding btw !!......As GB is heading for total PCishness one has to refer to this time of year as Winterval so as not to offend any body !!
Willy
Title: Re: Talking Thermodynamics
Post by: paul gough on December 23, 2017, 08:01:16 AM
Hi Wily, Don't know what 'scalding btw' is, but idiotic bureaucratic affectations like 'winterval' deserve a resounding 'Bah Humbug!!!!' Have a good Christmas. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: paul gough on December 23, 2017, 08:12:53 AM
Hi MJM, my interest in a rough figure for lowest worthwhile flue gas temperature was due to my interest in thinking about multiple pass fire tube boilers, in my case, a colonial, but useful to know for a scotch marine or 'economic' multi pass as well. Regards, Paul Gough.


Title: Re: Talking Thermodynamics
Post by: MJM460 on December 23, 2017, 11:46:56 AM
I am with Paul on that Humbug.  Puts the lie to the 'correct' part of PC, besides the season is summer. But better stay off the politics. 

Tried to celebrate Christmas as normal this evening, a bit early, but spreading it out helps with the different directions everyone is pulled by the expanding family.  But with heavy hearts this year.  For the first time, one of our generation is missing.  Still pretty raw.  I know we are indeed a fortunate generation to get this far without having to deal with it earlier, but the knowledge does not help much when the time comes.  Will  have to cheer up for the grandkids Monday.

Three great issues on topic, from Paul and Willy, I will have a go at those in the next few days, but hard to put my heart into it tonight.

Instead, I will wish you all peace and joy for the Christmas season, especially to those also dealing with loss of loved ones, and safe travels for the New Year.  A time to appreciate all those around you.

Thanks to everyone for so much interest in this thread, I hope to hear from more of you next year.  I am sure there are more questions lurking in the background, or even needing a clearer explanation on topics already attempted.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 23, 2017, 01:51:43 PM
HI MJM ,   btw is a modern thing and means  By the way , btw....Sorry to hear about your loss and wish all the best for you in the new year...
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 24, 2017, 11:04:04 AM
Hi Willy, a little more thermodynamics before Christmas.  That article on the corrugated flue tubes is quite interesting.  It always adds balance and interest to hear a little of the history alongside the theory.  The thing about internal flue tubes is that they are actually under external pressure.  That is, the pressure on the outside of the tube (Steam pressure), is higher than the pressure on the inside (flue gas at atmospheric pressure).  In this circumstance, the likely failure mode is termed elastic instability.  That is a fancy way of saying that if the tube starts to deform under pressure, the deformation increases the stress causing increased deformation, and this rapidly leads to a tube collapse, just like it was squashed in the vice.  That corrugated tube has higher resistance to that form of collapse.  In addition, the corrugated tube is more flexible in axial expansion and contraction. 

In a centre flue boiler, the flue tube has a higher temperature that the shell and hence expands more, in particular lengthwise.  This means the tube is trying to push the ends apart, while the shell is trying to hold them together, thus causing additional thermal stresses in the shell, tube and the ends.  Also not good for additional return tubes if they are only expanded in.  With a steel boiler, the modulus of elasticity is very high, effectively the similar to a spring rate for a coiled spring, if you think of the shell as a very stiff spring.  The corrugated tube is flexible in the axial direction and does not provide nearly as much longitudinal force on the shell.  Of course in a wet back type, the corrugated tube has insufficient axial stiffness to resist the pressure forces, and needs extra axial support.  I don't know if the corrugated tube is used in both types, but I am sure that your books will tell us.

In a copper boiler, I believe you will find the flue tube is relatively softer, and undergoes a little plastic compression which reduces the stress, so the corrugated form is not necessary.

You also mentioned the possibility of conical tubes, and whether this would be any advantage.  First, let's think about what would happen with a tapered tube.  If we are talking about a flue tube, the gas travelling along a cylindrical tube looses heat through the tube wall to the water, so drops in temperature, meaning the specific volume will reduce, and hence the velocity reduces.  If the tube is tapered, so that the diameter reduces toward the cooler end, this would tend to maintain the velocity, at least to some extent.  Lower velocity would mean less pressure drop, so the available stack draft will be able to draw more air.  Lower velocity might also have a tendency to reduce the heat transfer coefficient, but quite difficult to quantify.  With the tapered tube, the higher average velocity means more friction pressure drop, so the available stack draft draws less air.  Similarly there will be a small effect on the heat transfer coefficient.

Alternatively, if you are thinking of tapered cross tubes in the flue, presumably they would be orientated with the larger diameter at the higher end, which might be helpful for the rapidly expanding volume as water becomes steam in these tubes.  Once again there might be a small effect on flow and heat transfer.

Assuming the tapered tubes are relatively easily produced, I would be inclined to assume that the beneficial effect if any would be small, and it would be more economical to make up for any difference in performance by providing additional area in the form of additional cylindrical tubes.

I suspect shorter cross tubes would be relatively easily produced, and are mentioned in some of the less common boiler configurations.  I am not sure if they are really any advantage.  Again, I would tend to just add a few more tubes if possible in the required boiler outline.

Hi Paul, you were asking about the lowest worthwhile temperature difference.  For a boiler, the flue gas enthalpy can be approximated using the standard tables of ideal gas integrals for air.  In particular these tables allow us to easily estimate the enthalpy change for a temperature difference in the flue gas.  I looked it up at a high temperature, 1080 K (807 C), and 480 K (207 C), and in both cases it is very close to 1 KJ/kg.K, just a little higher, 1.15 at 1080.  So 10 degrees temperature change gives us the same amount of heat at both the firebox end and the smokebox end of the boiler.  However, the cost at each end is very different.  Still assuming the firebox flue gas temperature of 800 C, and a steam temperature of 140 degrees the local temperature difference at the firebox end is 660 C.  At the smokebox end, again using the previous assumptions, the flue gas temperature was assumed to be 200 C, so the difference is only 60 degrees.  If we put these temperature differences in the heat transfer equation, q= U x A x delta T, then we see that the area required with only 60 degrees temperature difference is 11 times the required heat transfer area at the firebox end with 660 degrees difference.  Basically, as the temperature gets lower, more area is required to transfer the same amount of heat.  The old idea of diminishing returns.  In practice, I suspect you put the maximum practical area in the required or selected boiler outline, and accept the resulting steam production.  Alternatively, you modify the burner to pile in more heat, which will give more steam, even though there will also be more losses up the stack, so lower efficiency.

Obviously in a different situation, refrigeration, the effort required to increase the heat flow requires a compressor.  This makes the heat effectively more valuable, not to mention that the very high temperatures available from combustion are not available.  Consequently, much lower temperature differences are economical.

I hope that adequately answers the question.

I plan on looking briefly in for a read tomorrow, but taking a night off from posting, so just time to wish everyone joy and peace for Christmas.  I look forward to coming back to continue the discussion.

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on December 26, 2017, 10:13:20 AM
Having a quiet afternoon today after all the excitement with the grandchildren yesterday.  Spent a pleasant hour going back over some of the old subjects.  My word, we have covered a lot of ground.  Not very sure where to go from here.

Willy's question about the corrugated fire tubes sort of lead into a discussion of boiler design considerations.  We have already covered that one, but I found I had promised to calculate the allowable external pressure for Willy's boiler, so see if it would hold against full vacuum, but had never gone back there. 

Will not be posting tomorrow, but then I had better get out my thinking cap and do those external pressure calculations. 

I am sure that will not be the only question I left open and did not return to, so if there is something that interests you that I missed or glossed over, please give me a reminder.   Also happy to go back over anything that perhaps needs a bit more explanation.  And as always, happy to have new questions.

Thanks for looking in.  I hope that you are all enjoying a few quieter days before New Year,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 26, 2017, 02:00:35 PM
Hi MJM ,Ok what i would really like to be explained in words of two syllables or less is why in triple expansion engines the volume of the cylinders get bigger ?? Most explanations just use big words that the Proffesors ( people that profess to know) Sorry , make up from greek or latin ,That the hoi  palloy are also not aquatinted with !!!!! This concept of 'expansion' goes against the thought summation that many people might have !! If a quantity of steam goes from a small cylinder into a large cylinder ,doesn't lots of the presure get "lost" (a two syllable word) when it enters this larger volume ??If the cylinders were the same size what would the actual results be and show ,and who decided that the larger cylinders would work satisfactorily ,and how did they work this out ? perhaps Mr Woolf could help there. This is actually a serious question and not a flippant enquiry at all ....Thanking you in advance...
Willy
Title: Re: Talking Thermodynamics
Post by: crueby on December 26, 2017, 02:09:49 PM
Hi MJM ,Ok what i would really like to be explained in words of two syllables or less is why in triple expansion engines the volume of the cylinders get bigger ?? Most explanations just use big words that the Proffesors ( people that profess to know) Sorry , make up from greek or latin ,That the hoi  palloy are also not aquatinted with !!!!! This concept of 'expansion' goes against the thought summation that many people might have !! If a quantity of steam goes from a small cylinder into a large cylinder ,doesn't lots of the presure get "lost" (a two syllable word) when it enters this larger volume ?? This is actually a serious question and not a flippant enquiry at all ....Thanking you in advance...
Willy

Ooh! A chance to see if I understand this one properly (and find out how far off I am!).

My understanding is that the steam goes into the high pressure cylinder (the small one), and part way down the piston travel the valve is closed. The steam continues to expand, pushing the piston down the rest of the way. At the bottom of the travel, the pressure has already dropped due to the work it has done to move the piston from part-way down to all the way down, and it is now occupying a larger space.
Then, the exhaust valve allows the steam to move to the lower pressure cylinder, and part way down the travel of that piston the valve closes, and the steam pushes that piston down the rest of the way, expanding to fill that new even larger volume. The second cylinder is larger than the first, so that even at the lower pressure per square inch, it is still doing an equivalent amount of work since the second piston has more square inches to press against.
If there is a third cylinder, the same applies there, it needs to be larger to get the work out of the new even lower pressure steam.

How'd I do? No math formulas, but is that concept correct??
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on December 26, 2017, 02:46:51 PM
Ok what i would really like to be explained in words of two syllables or less is why in triple expansion engines the volume of the cylinders get bigger ??

Willy,
The reason the cylinders get bigger is the amount of work done by each cylinder is by design equal or as close to equal as possible. As you stated the pressure goes down so the size of the piston has to get larger to make the work equal.

Dan
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 27, 2017, 02:50:35 AM
Hi Chris and Dan,  quite easy to say ...However the next cylinder is larger and so is the bore and the end cap, also the piston is heavier and has more mass. As the cylinder dimensions are larger the pressure acts against all the surfaces , not just the piston face. and also the piston is moving anyway. !! So it would be interesting to have a really technical explanation ,but in layman's terms. Also the steam pressure may be the same as full-size engine but the scale dimensions are much smaller. Or am i just being pedantic or something.? So using the same pressure how does one go about designing a small engine. Thanks for the explanations and it will be interesting to hear what other people say  :headscratch: :noidea: Also what would happen if all the cylinders were the same diameter ????? .. I have just had a brainwave .....as the travel is still the same does the diameter somehow compensate for that ????
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on December 27, 2017, 03:33:54 AM
I've kind of thought about compound engines and how they work etc, I can see that as the steam performs work and expands it needs progressively larger cylinders.

What I did ponder though is that as the steam travels further through the engine it must be losing heat (energy?) so is there scope for having 3 cylinder compound where the central high pressure cylinder feeds 2 low pressure cylinders at either side ? If the 2 secondary cylinders are the same size as each other aren't we reducing the travel of the steam but still having a 3 cylinder compound ?
Title: Re: Talking Thermodynamics
Post by: crueby on December 27, 2017, 03:49:10 AM
I've kind of thought about compound engines and how they work etc, I can see that as the steam performs work and expands it needs progressively larger cylinders.

What I did ponder though is that as the steam travels further through the engine it must be losing heat (energy?) so is there scope for having 3 cylinder compound where the central high pressure cylinder feeds 2 low pressure cylinders at either side ? If the 2 secondary cylinders are the same size as each other aren't we reducing the travel of the steam but still having a 3 cylinder compound ?
You would have to cut the volumes of the 2 secondary cylinders in half from the normal ones since you only have a fixed amount of steam from the primary cylinder. I don't recall where I saw it, but someone did build what you describe. There is also the combination that the Titanic used, with the tertiary stage being a turbine rather than a cylinder, but it only was functional at higher speeds on the cylinders. As I recall, that turbine drove the center prop, the outer ones driven by the compound engines. Very clever stuff!
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on December 27, 2017, 04:02:24 AM
You would have to cut the volumes of the 2 secondary cylinders in half from the normal ones since you only have a fixed amount of steam from the primary cylinder. I don't recall where I saw it, but someone did build what you describe. There is also the combination that the Titanic used, with the tertiary stage being a turbine rather than a cylinder, but it only was functional at higher speeds on the cylinders. As I recall, that turbine drove the center prop, the outer ones driven by the compound engines. Very clever stuff!

Chris, I take your point about the secondary cylinder size and they'd need to be assessed mathematically but the idea of having 2 equal sized secondaries must have some merit - or am I missing something  :headscratch:

As for the Titanic, I could be wrong but I think the turbine driving the centre prop also drove the generators to supply the ships electricity.

You've got me thinking now, I need to read up on the Titanic again  :)

Edit to add :-

I've had a bit of a look at the Titanic arrangement, it appears it had 4cyl compounds consisting of high pressure, intermediate pressure and 2 low pressure cylinders followed by the turbine  :o

http://www.titanicology.com/Titanica/TitanicsPrimeMover.htm
Title: Re: Talking Thermodynamics
Post by: 10KPete on December 27, 2017, 07:40:05 AM
Yes, the Titanic power plant was four 3 cylinder compounds ( 1 high, 2 low ) the exhaust driving center-line turbines. My reading strongly suggested that the turbines were "ON" all the time, ie: when the compounds were going, the turbines were going. If I recall, the literature said that this was designed to significantly reduce the amount of coal burned. And, from what little data is available from the voyage, that economy was looking very real.

Great stuff!!
Pete
Title: Re: Talking Thermodynamics
Post by: 10KPete on December 27, 2017, 07:44:22 AM
You would have to cut the volumes of the 2 secondary cylinders in half from the normal ones since you only have a fixed amount of steam from the primary cylinder. I don't recall where I saw it, but someone did build what you describe. There is also the combination that the Titanic used, with the tertiary stage being a turbine rather than a cylinder, but it only was functional at higher speeds on the cylinders. As I recall, that turbine drove the center prop, the outer ones driven by the compound engines. Very clever stuff!

Chris, I take your point about the secondary cylinder size and they'd need to be assessed mathematically but the idea of having 2 equal sized secondaries must have some merit - or am I missing something  :headscratch:

As for the Titanic, I could be wrong but I think the turbine driving the centre prop also drove the generators to supply the ships electricity.

You've got me thinking now, I need to read up on the Titanic again  :)

Edit to add :-

I've had a bit of a look at the Titanic arrangement, it appears it had 4cyl compounds consisting of high pressure, intermediate pressure and 2 low pressure cylinders followed by the turbine  :o

http://www.titanicology.com/Titanica/TitanicsPrimeMover.htm

Big dynamos were driven by turbines directly. I think there were 4 mains and a couple of aux. dynos.

Pete
Title: Re: Talking Thermodynamics
Post by: paul gough on December 27, 2017, 09:34:45 AM
I was under the impression Titanics propulsion was two, four cylinder triple expansion engines, ie. Two engines: HP, IP, LP x 2. with a low pressure turbine for the centre screw. Paul Gough
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on December 27, 2017, 09:59:29 AM
I was under the impression Titanics propulsion was two, four cylinder triple expansion engines, ie. Two engines: HP, IP, LP x 2. with a low pressure turbine for the centre screw. Paul Gough

Paul, that's how I understand it to be. Have a look at the link I posted a few posts earlier.

I think the centre screw served only for added maneuverability at low speed in docks etc but otherwise the turbine supplied electricity. (I'm not certain but that's how I thought it was  :headscratch: ).
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 27, 2017, 10:53:29 AM
Hi everyone, I am glad to see that you were all thinking about thermodynamics while I was out this afternoon with my daughter from Darwin and her little family, at the performance of Disney's version of Aladdin.  What a great show for all ages. 

Hi Willy, that is a great question, especially with Ramon's compound engine build going on.  I wonder if we can get him to explain the steam paths and valve events for his wonderful build.  It was also one of the topics suggested way back at the start of this thread.  Needless to say, the cylinder size is based on the requirements of physics, and not an arbitrary choice.

Hi Chris, glad to have you along.  I think a pretty good explanation of what is going on.  It is worth remembering that for conventional valves driven by eccentrics or radial valve gear, the valve motion is approximately sinusoidal, so does not shut off so sharply, as we have discussed before.  I believe the Corliss comes closer, but I have yet to have a good look at those drawings of the MEM Corliss to get a better feeling for it.  But basically I am sure that the concept is correct as you have described it.

Hi Dan, thanks for coming in again.  It is interesting that the design of multistage engines is usually based on equal amount of work in each stage.  Basically it turns out, if my memory is correct, that equal work in each stage also means equal pressure ratio, or volume ratio in each stage.  It has a slight efficiency advantage, and in addition leads to more even torque and better balance.  Same applies in compressors, and also in multistage turbo machines, both compressors and turbines.  The pressures in the stages make an equal volume ratio, not equal volume change or pressure difference to make equal work.

Hi Willy, clearly Chris and Dan have not convinced you, but you do have a remarkable ability to put your finger clearly on the precise problem that is worrying you.  It is always instructive to go back to first principles, and work it through.  I will look at Gas Mantle and Chris further inputs first then come back to that.

Hi Gas Mantle, you have mentioned heat and energy changes through the engine.  To the extent that enthalpy is a property relating to energy in general, it is definitely a part of the equation.  However, our ideal engine is analysed by comparison to an adiabatic process.  That is a process with no heat transfer in or out.  However in a real engine, there are heat flows which lead to energy losses, and a lower efficiency than the ideal engine.  While we have been talking about multi-cylinder engines so far in this topic, strictly we are referring to double or triple stage expansion.  The number of cylinders is, in principle, not relevant.

The difference can be illustrated by describing two ways the cylinders can operate in a two cylinder engine.  Commonly in models, both cylinders receive the full boiler pressure, but the cranks are displaced 180 degrees for single acting, or 90 degrees for double acting, to even out the torque fluctuations and achieve better balance.  Often called a tandem configuration I think. 

Alternatively the HP steam can be supplied to one cylinder, and the steam expanded part of the way to exhaust pressure, then transferred to the other cylinder where it is further expanded to the eventual exhaust pressure.  This is called double expansion or two stage expansion.  The same principle can be expanded to triple expansion, or even quadruple expansion.  Of course at the lower exhaust pressure, particularly when there is a condenser, the volume of the low pressure becomes so large that the piston becomes impractically large for balance.  So often, two smaller pistons are used for the large volume, low pressure stage instead of one large one.  It enables some evening out of the torque fluctuations by different crank angles, as well as better balance of the reciprocating forces.  This would result in for example a three cylinder double expansion engine.  I believe quadruple expansion was used at the heyday of steam when boiler pressures became high enough and condensers were sufficiently effective.  In principle you could go to five or six, but the lp steam volumes become impractically large and the difference in efficiency probably not sufficient to justify the extra complexity.  The double expansion uses half the steam quantity, but due to the second expansion, produces perhaps 75% of the work output.

Hi again Chris, those Titanic engines are a great example of the extreme volume of that low pressure steam, that they could run a turbine, which handles much higher volumes than a reciprocating engine.  If you search for Titanic in this thread, we talked about them some time ago, and there were some great contributions showing the configuration, crank angles and operation of the turbine at sea, while disconnecting it for reverse.

Lots more posts while I have been writing.  Thanks Pete and Paul for adding to the discussion.  Again the Titanic configuration can be found by searching this thread.

So Willy, with the basic arrangement of the engines covered and the difference between double expansion and tandem engines defined, let's go back to the basics on how engines turn the heat in steam into mechanical work.  In a simple single acting single cylinder, there is only one moving surface, the face of the piston.  The fundamental definition of the quantity of work is given by the basic equation W=F x S, work (Newton.metres) = Force (Newton's) x distance (s in metres).  The distance moved, s, is critical to the definition. If the distance is zero, the work is zero.  If you prefer imperial units, we probably should rewrite the equation as the units of work are then ft.lbf.  So W (ft.lbf) = s (feet) x F (lbf).  Note that for imperial units, we have to make that confusing distinction between pounds force and pounds mass.  In a cylinder, the force = P x A.  Force equals pressure times area in which ever units you prefer.  The distance, s, that the force moves through is equal to the length of stroke, L.

Force and distance are both vectors, which means they need both a magnitude and a direction to define them.  The work is positive shaft output work if both force and distance are in the same direction.  If the movement is in the opposite direction to the force, then work is negative.  That means energy has to be extracted from the flywheel to keep it all moving.  Hence we saw earlier that during the admission stroke the force is large due to the high pressure steam, and in the same direction as the movement, so the work output is positive.  During the exhaust stroke, the piston movement is in the opposite direction to the steam force on the piston face, so work is negative.  Fortunately the force is much lower due to the lower exhaust pressure, so the negative work during the exhaust stroke is much less than the positive work during admission, and we have an engine which produces positive shaft output work.

But now consider a double expansion engine.  First, steam is admitted and expands into the HP cylinder from the boiler, doing work on the piston while the pressure is maintained by the heat into the boiler.  Then the inlet valve closes and the steam is expanded to a lower pressure in the HP cylinder, doing more work during the expansion.

Then the release occurs, when the exhaust valve is opened.  But instead of expanding into the exhaust system, it expands into the transfer pipe and the inlet valve to the lp cylinder is opened.  So now, instead of having a simple cylinder with only one moving piston face, we now have a more complex 'cylinder' with a piston at each end, and the transfer pipe in the middle.  So when we apply the energy equation, still assuming adiabatic expansion, we have two moving surfaces, so the work output equals the work being done in the HP cylinder during its exhaust stroke plus the work being done in the lp cylinder on its inlet stroke.  If we ignore the small losses in the transfer passage and valve ports (remember we had to make them large for minimum losses), then the pressure is the same in both cylinders.  Now in the hp cylinder, in the exhaust stroke of its cycle, the pressure and piston movement are in opposite directions, so the work output is negative, the same as for any other exhaust stroke.  While in the lp  cylinder, the force and movement are in the same direction, so the work is positive.  Usually both cylinders have the same stroke, so let's look at the options.

But first let's look again at that formula for work. We have W = F x s, or F x L as the force moves through a distance s, which is the length of stroke, L, and we have F = P x A.  If we combine these we have W = P x A x L, and A x L you might remember as the change in volume during the stroke.  Pressure, remember is also a vector which has direction, as does V.  So if the volume is expanding as rotation progresses, the net work is positive during this transfer part of the stroke.  On the other hand, if the volume is decreasing, it is compression, the net work is negative, and energy would have to be extracted from the flywheel. 

The third option is that the volume is unchanging as the transfer proceeds.  If the volume is not changing, the work is zero.  Well, slightly negative, as there are losses despite our best efforts, but approximately zero.  The best engine performance is achieved if there is positive work output during the transfer, which surely requires the lp cylinder to have larger volume, by larger diameter, or perhaps two cylinders.  When the lp cylinder inlet valve closes, lp expansion starts until the release phase when the lp exhaust valve opens.  So there is work output during the hp admission phase, the hp expansion, the transfer and lp admission, and finally the lp or second expansion phase.  Finally the lp exhausts to the exhaust system.

It would be a bit tedious to type it out in detail, and no less so to try and read it, but obviously the process could be extended exactly the same way to a third and even a fourth stage.

I hope that makes it all a little clearer.  It is a basic explanation, and I am sure there are variations which require a little more thought, but it should make the principle clear.

Looking forward to more discussion tomorrow,

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on December 27, 2017, 11:07:11 AM
I was thrown by Petes reference to four 3 cyl. Compounds, so proffered the correct description as I understood it. I see your description is in accord with mine. I am no expert on compounding, but fundamentally a central HP with an LP on either side would be workable. This was tried on at least one loco but most usually with two outside HP cyls. and one LP between the frames. The LP usually had a 'starting' valve that allowed boiler steam into the receiver for a short period on starting. The only problem with one HP cyl. is you may find starting difficult if the HP piston stops on dead centre, this is why two HP cyls and one LP are more common, at least from what I have seen. Volumes of both the receiver and LP cyl. need to be appropriate as does any passageway and porting, also in models condensation needs to be adequately dealt with. A three cyl. compound should be smoother or better balanced than a twin too. I had a lovely time a few years ago when on a river steamboat which was powered by a three cyl. compound engine from a naval pinnace, it ran like a Swiss watch! Regards Paul.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 27, 2017, 02:30:32 PM
Hi MJM, et al, That now seems a lot clearer and shows how clever Mr Woolf and others were with their thinking. I see you mention the Flywheel in your texts and with a stationary engine and a beam engine a large flywheel is part of the mechanism......With a beam engine however the pistons move up and down together rather than a triple  where the cranks are at 120 degrees. So with this configuration the valves and porting must be different (accumulators ) spring to mind?. So a new question has triggered my brain cells ......with an IC engine ,were there any two stage engines built or even thought about ?? and would this actually work ??or is that another silly concept !!  Talking about doubles ...you can make mine a triple tipple if we ever meet up !!!  :naughty:
Willy...
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on December 27, 2017, 03:04:24 PM
You would have to cut the volumes of the 2 secondary cylinders in half from the normal ones since you only have a fixed amount of steam from the primary cylinder. I don't recall where I saw it, but someone did build what you describe. There is also the combination that the Titanic used, with the tertiary stage being a turbine rather than a cylinder, but it only was functional at higher speeds on the cylinders. As I recall, that turbine drove the center prop, the outer ones driven by the compound engines. Very clever stuff!

Chris, I take your point about the secondary cylinder size and they'd need to be assessed mathematically but the idea of having 2 equal sized secondaries must have some merit - or am I missing something  :headscratch:

As for the Titanic, I could be wrong but I think the turbine driving the centre prop also drove the generators to supply the ships electricity.

You've got me thinking now, I need to read up on the Titanic again  :)

Edit to add :-

I've had a bit of a look at the Titanic arrangement, it appears it had 4cyl compounds consisting of high pressure, intermediate pressure and 2 low pressure cylinders followed by the turbine  :o

http://www.titanicology.com/Titanica/TitanicsPrimeMover.htm

Big dynamos were driven by turbines directly. I think there were 4 mains and a couple of aux. dynos.

Pete


The word compound used to describe a reciprocating steam engine is generally reserved for a double expansion engine. Anything above that is a triple expansion or a quadruple expansion or even the rare quintuple expansion.

The four main generators were 400Kw and driven by Allen vertical three-crank compound engines. These were located in a watertight compartment aft of the turbine room. There were also two 30Kw units on the Salon deck level.

This might seem like a lot of power but compared to a container ship I was on we had three 1Mw diesel generators. The Titanic used electric power for ventilation fans and electric lights. The axillary pumps all around the engine room were steam driven which was the common practice of the day.

The lights stayed on until the last because the ship started sinking from the bow and the main generators were all the way aft at the lowest level. When the engineers abandoned their posts they were too far below decks to reach the surface.

Dan
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on December 27, 2017, 03:16:58 PM
A bit more on the Allen three-crank compounds. They produced 580 Hp at 325 rpm using 185 psi steam pressure. The HP cylinder was 17" in diameter and the two LP cylinders were 20" in diameter all with a stroke of 13".

Dan
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on December 27, 2017, 04:00:10 PM
I think the centre screw served only for added maneuverability at low speed in docks etc but otherwise the turbine supplied electricity. (I'm not certain but that's how I thought it was  :headscratch: ).

The turbine did not have a reverse and was not used during maneuvering it was only used at sea speed.

Dan
Title: Re: Talking Thermodynamics
Post by: asm109 on December 27, 2017, 06:27:21 PM
?. So a new question has triggered my brain cells ......with an IC engine ,were there any two stage engines built or even thought about ?? and would this actually work ??or is that another silly concept !!

Actually, yes this has been done.  the exhaust from the cylinder is expanded through a turbine.  The power from the turbine is usually used to run a compressor to pump more air into the motor, but it could have been geared down and coupled to the output.

You know this system as a turbo charged car engine.
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 28, 2017, 11:38:08 AM
Hi Paul, I suspect the differences in requirements of locomotives from the requirements of stationary or marine engines determine a slightly different approach.  I am interested in your mention of a receiver, I assume as part of the transfer pipe.  I would be interested in any more information you have on that.  I assume it is necessary in order to minimise the compression losses when the hp exhaust does not correspond with lp inlet timing.  That steam valve to the lp manifold is a well recognised answer to the problem of self starting of compound engines at top or bottom dead centres.  It is probably more than just at the centres, as cut off is generally somewhat before actual dead centre, even at full valve travel, so it is more than just a point in the rotation.  I assume the other option to start an engine with a single hp cylinder is to use barring gear.  Not really practical with a locomotive, but would normally be an option for a stationary or even a marine engine, either instead of or in addition to the steam valve to supply steam directly to the lp cylinder, ignoring slip eccentrics of course.

So I would guess you would never find compounding on a shunting loco, where most of the power is actually required for acceleration, and frequent stops and reversing is normal, so maximum pressure for maximum part of the stroke is the requirement.  On the other hand, for mainline locos, once moving, it continues for the longer trip and there would be some incentive to look at the improved efficiency, so less coal and water consumption of the compound engine. 

Hi Willy,  I am glad that the issues around compounding are starting to look a bit clearer.  With beam engines, the flywheel is still required to get past the dead centres, but the engines I assume are normally double acting.  In this case, when the top of the hp cylinder reaches the bottom of its stroke, and it's exhaust valve opens, the bottom of the lp cylinder is just about to open its inlet valve for admission to it's upward power stroke, so there is really no conflict in having the actual pistons moving in the same direction if the top of the hp cylinder connects to the bottom of the lp cylinder.  Of course you will be much more aware than I, of whether this is the way your engines are connected.  If connected the other way, then that receiver would be required to hold the steam from the hp exhaust until it was required by the lp cylinder.  But I am also interested to know if receivers were used in other configurations to allow for different valve timing.

I will come back to your question on i.c. engines shortly.

Hi again Dan, thanks for that information on the generators.  Thank you for confirming that those turbines we were talking about were for forward propulsion only, and for the other interesting details.

Hi asm109, welcome to the forum.  It is good to see a new member joining in, and I am delighted to have you reading this thread.  Turbo chargers are of course well known by now in automotive and also larger industrial engines.  I assume also pretty common in trucks and i.c. marine engines.  Reciprocating engines are basically limited in power by the air they can induce, hence usually less than complete combustion, and using the exhaust energy to drive a blower for more air flow is a good way to increase power output without adding the extra weight of a larger engine.  Adding more fuel is the easy part.  But it is interesting to think about why i.c. engines resort to turbo expanders for the exhaust instead of using double expansion like a steam engine, so this leads back to Willy's question.  Also, please consider introducing yourself on the introduction board so we all know a little more about you and your interests.  You can be sure of a warm welcome from the whole forum.

There is a fundamental difference between steam and i.c. engines that determines that use of a turbo expander for exhaust gas instead of double (or triple) expansion.  In the steam engine, the high pressure steam raised in the boiler is admitted through a significant part of the stroke before the inlet valve closes.  At this stage the piston has moved through around half of its stroke or even more.  But let's assume half for easy calculation.  The remainder of the stroke is all that is available for further expansion of the steam, so at best it can expand to about twice its volume.  Now steam expansion can be approximately described by a polytropic process with the polytropic exponent, n = about 1.3.  There is another multi syllable word that I will come back to as I don't believe I have used it before (tomorrow).  Basically it means the expansion is described by the equation
 P1 x V1^n = P2 x V2^n, where n is roughly 1.3.

For V2 = V1 x 2, the equation tells us that P2 = 0.4 x P1, so if our inlet pressure is 400 kPa, it is expanded to about 160 kPa or about 60 kPa(g).  There is still room for more expansion, particularly if we have a condenser, though the advantage is more obvious if we have a much higher inlet pressure.  You can see that a decision to go to compound arrangement involves consideration of both the inlet pressure and whether or not there is a condenser.

But now let's look at an i.c. engine.  Where does the pressure come from.  Well first we have that compression stroke.  Quite a range of compression ratios used but anything up to perhaps 15:1 for a diesel.  Much lower for some of the hit and miss engines.  Perhaps someone can help out with a better idea of the practical range.  One of my early cars had 9:1, yes, it was a gasoline engine, so that gives 9 times atmospheric pressure, but that is only the start of it!  That is before ignition.  When ignition occurs, the pressure rises really fast.  I don't really know how high, but all that energy in the fuel is released as heat, resulting in a rapid rise of temperature and pressure, and essentially all before the piston has moved very far down.  As the piston moves down, there is scope for perhaps 8:1 expansion before the exhaust valve or port opens.  Again that is just a guess and someone might be able to help out with better figures, but even then the pressure is still very high, and plenty of scope for more expansion to produce more work output.  But it is hardly worth the effort of introducing a second cylinder for further expansion as we cannot repeat that sudden rise in pressure that we get on that initial ignition.  Large volume of exhaust, high pressure, that is where an expansion turbine comes into its own.  A turbine can handle a large volume and produce power at very high rpm compared with a reciprocating machine which has severe volume limitations.  Now that combination just exactly matches the general requirements of a turbo compressor, which can be used to increase the inlet air pressure to get more mass of air into the cylinder of the engine, so it can burn more fuel and hence produce more power.  Much more than could be achieved by simply using the expander for shaft work to drive a generator or even add to the power at the wheels.

So it is about volumes and pressures and the characteristics of reciprocating engines compared with turbines that drive the design decisions.  And you can see when we go back to the Titanic machinery, where the exhaust steam from the reciprocating engines becomes too large for further reciprocating cylinders for an additional stage of expansion, but comes into the range where a turbine can be used.  And the extra power of the turbine provides a very useful reduction of coal consumption for that trans Atlantic passage.  Well, perhaps not that passage, but it would have done if the journey had gone the full distance.

So a few loose ends in terms of the range of compression ratios for i.c. engines, and the pressure obtained on ignition and so on, but I hope that sheds some light on the differences driving the design of i.c. and steam engines in the search for greater power output or efficiency.

Thanks to everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 29, 2017, 12:04:16 PM
Polytropic?

Despite Willy's request for no more than two syllable words, I had to introduce a new word to yesterday's post. 

In order to calculate the work done when steam is expanded in an engine, or even just the pressure that results from the volume change in the engine, we need to know the equation that links pressure and volume during the expansion.  A simple assumption is that steam follows the ideal gas equation, and often that is good enough, and I tried to use that assumption very early in this thread.  Fortunately I was called out on that, and still have to go back and revisit the topic I was discussing at the time.  It is a good enough assumption in many circumstances, providing that the fluid is not too close to condensing.  For steam, the best model is the steam tables, and that is the model I have mostly used through the thread.  However, when saturated, or only slightly saturated steam is expanded, the steam tables require a lot of interpolation to get any reasonable idea of the pressure as the volume expands, and the maths, while essentially simple, tends to hide the basic picture of what is going on.

The ideal gas equation provides a relationship between pressure and volume for those gases that follow it closely.  For an ideal gas the equation is P1 x V1^k = P2 x V2^k, and having an equation such as this allows us to predict the work output from and adiabatic process.

Furthermore, for an ideal gas, that exponent, k turns out to be the ratio of specific heats, and the value is readily available for a wide range of gases.  Definitely not intuitive as to why the ratio of specific heats should be relevant.

For gases that do not follow the ideal law, it is found that many do follow a very similar behaviour, if we substitute n for k, where n is a constant, but it is just not equal to the ratio of specific heats.  This process is called a polytropic process.  The value of n has to be found experimentally.  Steam is one of the gases that can be approximated as a polytropic process.  We cannot use n = 1, basically Boyles law, as the temperature is changing while external work is done.  Yesterday I assumed we were expanding the steam from 400 kPa to double its volume after the inlet valve closed.  If n was equal to one, we would assume the pressure dropped to half, or 200 kPa, remembering that we are talking absolute pressures.  If we assume the ideal gas equation, for steam, then k=1.4.  The final pressure would then be 152 kPa.  If we use n= 1.3, we get 160 kPa.  That value of 1.3 is approximately the polytropic exponent for steam

You can see there is not a huge difference.  Very important for prediction of the work output of an engine, but not so important in this case.  However, it makes sense to understand what techniques are available for gases that do not exactly follow the ideal gas law.  And the polytropic process is a useful description of the behaviour of a wide range of real gases, and allows us to predict the work output from expansion of such gases.

You might ask why not just use the steam tables, or similar tables for the gas you are dealing with?   Indeed, that is the best approach when the tables are available.  However there are a very wide range of gas mixtures for which tables are not available, and the polytropic process is useful concept for the design of compressors or expanders for these gas mixtures.

I don't want to get too bogged down on this, it is getting to far from practical modelling applications.  At the same time, I wanted to indicate that P1 x V1 is not a constant for adiabatic expansion, so the pressure ratio cannot be simply obtained from the simple volume ratio.

I hope that little diversion has made the discussion on compound expansion a little more complete.

Pretty heavy going, so I will call that enough for tonight,

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 29, 2017, 02:56:42 PM
Hi MJM ,Ok at last we have found the flaw in our thinking and perhaps this thread should have been entitled as ....Thermononintuitivedynamics  !! Sorry ..only joking !!!However good to see that it is exceptions that make rules, And i am sure that god was quietly smirking when Boyles and Coles (Coleslaw ?)) were developing their theories ?!! but then came down to earth with a bump when Einstien actually saw through it all and came up with Mc2 theory...!!! err umm sorry ,i am being a bit obtuse again and just thinking outside the box, again. A real question this time ...I have heard that racing car engines do a bit better early in the morning when the dew content of the air is higher ?? Is this because the water content changes to steam that will give a higher pressure in the cylinder ?? Perhaps i have redeemed myself with that one? Thanks for the new word and i shall use that with gay abandon next time i am at the model engineers meeting !! Oops there i go again......
Title: Re: Talking Thermodynamics
Post by: crueby on December 29, 2017, 03:59:17 PM
Hi MJM ,Ok at last we have found the flaw in our thinking and perhaps this thread should have been entitled as ....Thermononintuitivedynamics  !! Sorry ..only joking !!!However good to see that it is exceptions that make rules, And i am sure that god was quietly smirking when Boyles and Coles (Coleslaw ?)) were developing their theories ?!! but then came down to earth with a bump when Einstien actually saw through it all and came up with Mc2 theory...!!! err umm sorry ,i am being a bit obtuse again and just thinking outside the box, again. A real question this time ...I have heard that racing car engines do a bit better early in the morning when the dew content of the air is higher ?? Is this because the water content changes to steam that will give a higher pressure in the cylinder ?? Perhaps i have redeemed myself with that one? Thanks for the new word and i shall use that with gay abandon next time i am at the model engineers meeting !! Oops there i go again......
Uh oh, have we drifted into the realm of Wile E Coyote physics?! :Lol:
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 30, 2017, 12:01:57 PM
Early morning engines-

Hi Willy, not sure what you mean by flaw, it was a perfectly reasonable question, I hope now with a perfectly logical answer.  Perhaps Mr. Charles is the guy you were trying to think of, it began with C anyway, he came up with the volume relationship for temperature changes similar to Boyles law for pressures.  Close enough over a limited range of gas compositions and conditions, these are the laws of gas behaviour we were all taught in early science lessons.  But a bit of humour is always welcome. It could be a very dry thread with no odd bits of humour.  Unfortunately my sense of humour must be a bit off, as no one seems to get it when I include a lighter comment.  Oh well, I can only try.  But it is just as well you and Chris are also in there helping me along.

Hi Chris, perhaps you are thinking of the banging and crashing that Willy likes to see in science experiments.  W. E. Coyote seems to be at the forefront of discovering new principals in physics complete with the banging and crashing.

Regarding the cars I think it is a common enough observation that cars run better in the cool of the morning.  And it is not because it is too hot for cars to be out in the midday sun, as it is for mad dogs and E.......  Of course the common claim is that the moisture content of the air is higher at those times, hence the large number of inventions over the years that are supposed to improve car power or fuel consumption by injecting a little water in the inlet manifold.  Despite the claims, the only application that I am aware of that provides any advantage is in gas turbines, where water is sprayed into the intake manifold for a bit extra power output.  (But only where there is high atmospheric temperature and plenty of water!)  The basic explanation is that evaporation of the water in the inlet cools the air, and hence increases the air mass flow through the turbine.  Air for combustion is not an issue in gas turbines, but more mass flow through the turbine does result in a bit more power output.  If adding water to car engines really helped with power output, surely it would be standard in formula one, but I don't believe they do it.

It is a pity that all those inventors of devices to add to i.c. engines, did not study a little more thermodynamics.  The water is no advantage to the reciprocating engine, in fact as far as I can understand, it is quite the opposite.  The higher specific heat, Cv, of steam compared with air means that water in the combustion gases absorbs more heat, and hence limits the maximum temperature and hence pressure obtained when the fuel is burned, so actually reduces the mean effective pressure and power output.  I am not far enough ahead on that one to be confident in leading you through the calculations, but it is in the section on the Otto cycle in any modern Thermodynamics text book.

But it is interesting that so many focus on the observation of moisture content of the air (which is quite difficult to measure) in trying to explain the early morning performance of their car engines.  If they actually took out a wet bulb thermometer and looked up the psychometric chart they might even be surprised at the answer.  It's just a thought, but we can easily check that one using our steam tables.  I hope you haven't lost them already.  If there is a dense fog, there is more moisture content, though it takes extra energy to evaporate the moisture at the conditions in an engine, but if we are just considering humidity, we can use the steam tables.  At 10 deg C the equilibrium vapour pressure of water is 1.227 kPa, while at 40 deg C, the vapour pressure of water is 7.384 kPa.  So if we compare moisture content at the same humidity, there is about six times more moisture in the 40 degree air!  In fact to get the moisture content as low as even 100% humidity air at 10 deg, the humidity would have to be below 17% humidity (1.227/7.384) and that is uncomfortably low.  Makes big sparks when you take off your shirt.

The other observation that could be made regarding early damp mornings, is that the temperature is lower, and this is so easily measured with reasonable accuracy.  And this is where Mr Charles' law comes in.  It is close enough for this purpose.  Charles' law can be simply stated as saying the density of air is inversely proportional to the absolute temperature.  If you continue to compare a 10 deg C morning, 283 K, with a warm sunny 40 deg C day, 313 K, then the air density in the morning is 313/283 times or just over 10% higher than on the warmer day.  In case you think I am choosing an unrealistic temperature range for a day, just because those temperatures are normal enough here, even comparing say 5 deg C, 278 K, and 25 deg C, 298 K, then the density is still 298/278 = 7 % higher.  (Remember inversely proportional to temperature.) You would get the same percentage changes in density, if you chose to use deg F with R for absolute temperature.

Now our car engines are limited in their power output by the air mass that can be induced into the cylinder to burn the fuel.  More air mass has more oxygen and so can burn more fuel, or even just burn it more completely, and this results in more power output.  I think you will find that this is the real explanation for the better performance on cool mornings.  But perhaps that (lower) moisture content of the early morning air is also part of the answer.  I suspect that others with more expertise in this area will come in and add to (and I hope support) these thoughts.  As I have mentioned previously car engines are not really my area. 

Let's give that a little time to sink in, and see what tomorrow brings.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 30, 2017, 01:57:24 PM
Hi MJM ,I was not expecting quite that amount of explanation !! but it is all relevant as part of this thread of course..... And what is the difference between a dry /wet bulb thermometer ? Also just an aside with IC engines ....in the past i have had a lot of trouble trying to start engines that are very reluctant to show any life !! And i used to think that if i was having so much difficulty with modern ones ,how did they manage to get the original first ones to work !!! Also thanks for the explanation of Chris's comment as i did not quite understand what he was getting at ,so the the joke fell on me !!!
And that should have been the Flaw in "my" thinking methinks...............
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on December 30, 2017, 03:09:35 PM
One of the readings I took for the log book on diesel ships was the wet bulb temperature. It is simply a thermometer with cloth wick over the bulb and water in a small reservoir. On deck they would use a sling psychrometer to measure the wet bulb temperature. It is similar but on a lanyard so it can be swung in a circle. See:
https://scool.larc.nasa.gov/psychrometer.html

Dan
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 31, 2017, 10:04:24 AM
More Cartoon Physics -

Thanks Willy for putting some fun into the thread, and along with it so many insightful questions.

Hi Dan, I guess there is not room to swing a cat in the engine room, let alone a sling psychrometer, but that is of course the traditional instrument for measuring humidity and air moisture content, with of course the psychometric chart.  The modern electronic devices so cheap I guess there are not so many genuine sling psychrometers in use these days.  I have never tried the two side by side to see how they compare.

Continuing on a lighter theme today, it's supposed to be party night.  Of course some of us will watch the kiddies fireworks in about 2 minutes ago as I write this, and then off to bed.

It is interesting that Chris inserted Wile E Coyote into yesterday's post.  In live theatre, it does not matter if the actors understand physics or not, they will obey the laws anyway.  But in a cartoon, the laws of physics only apply if the cartoonists remember to put them in.  So Wile can run past the edge of the cliff, and he only falls if the cartoonists knows what should happen and draws it in.

They learned very early that if they took that a step further and deliberately suspend the laws of physics, then show the character realising that something is amiss, then, "whoops!"

So cartoonists actually need to understand the laws of physics, In particular Newton's laws, or the modern expression as laws of conservation of energy, momentum and angular momentum, and some of the funniest humour is when they play with suspension of these laws.

I actually have a collection of the early Disney cartoons, Steam Boat Willy and so on, on those modern media, you know, the shiny round ones, careful to put them the right way up and don't scratch them.  One includes an interview with one of the animators who talks about some of the ways they learned to do this.

I wonder if there would be more interest in science if physics teachers introduced the Coyote to teach the basic laws?

Don't tell your grandkids about this.  When you tell them they have had enough screen time, time to turn it off, you will be met with delighted cries of "but we are doing our physics homework!"

Have a happy New Year everyone, make sure you get home safe, and best wishes for the whole year ahead.

And a special thanks to all who have been following along in this exploration of the thermodynamics around our hobby.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 31, 2017, 01:38:55 PM
Hi MJM , a happy new year to you and i will have to look up Wile e Coyote !! Also   how do you bump start a helicopter ???     

push it off a cliff !!!............Talk to you next year

Willy...............
Title: Re: Talking Thermodynamics
Post by: crueby on December 31, 2017, 01:47:17 PM
Hi MJM , a happy new year to you and i will have to look up Wile e Coyote !! Also   how do you bump start a helicopter ???     

push it off a cliff !!!............Talk to you next year

Willy...............
Love the helicopter tip!!


But, you never saw the Roadrunner/Coyote cartoons? What a wasted youth!  :Lol:
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 31, 2017, 06:07:42 PM
Hi Chris, I have now  brilliant cool ....would make good health and safety videos !!! Lots of 'fails' here as you can see on modern vids of people doing silly crazy things !!!
Title: Re: Talking Thermodynamics
Post by: paul gough on January 01, 2018, 02:27:38 AM
MJM, In reply to your request for information relating to receivers I am afraid in my downsized library I no longer have much on the subject as I gave up on it some time ago as being too problematic, as much of the literature was either too dense or the outcomes conflicted or contradictory to provide any clear or simple conclusions. In addition locomotive models are rarely in a position to operate for any period with full or large regulator openings and cutoffs of say roughly 50% which would approximate full size practices, at least of figures from France I have seen mentioned. Loco compounding is most assuredly an area open to investigation by by an enterprising and inquisitive modeller, but time and patience being necessary, as there is much literature to digest and experimentation to endure. H. Holcroft in discussion on paper No. 528, 'Some Questions about the Steam Locomotive', (J. of the Institute of Loco. Engineers, 1953), succinctly states the case when talking of the complexity of issues and the resultant compromises facing a loco designer, even more so when entering the area of compounding. "Some extraordinary anomalies arose from the diversity. On the French railways they found compound engines with superheat which were capable of exerting a horsepower several times that of any existing British locomotive and, what is more, a 20% saving in fuel was claimed for it.  Yet in Germany conditions were such that they could not find sufficient saving to justify the additional expenditure on capital and maintenance costs of a compound" and in Britain he goes on to say the limiting loading gauge pushed things towards simple multi cylinder arrangements. Addressing your particular enquiry about receivers, I am dredging the depths of memory so anything presented is partial in the extreme, very, very roughly as an example the proportions that come to mind are 1:1.4 for the receiver and 1:1.7 for the low pressure cylinder relative to the HP. The volume of the receiver is important, it will have to be adequate, (pressure and volume of steam), when the loco is operating under various throttle and cut off positions but also close attention has to be payed to the porting of the cylinders especially throttling of the inlet ports at small cut offs at high speed, also back pressures from the blast pipe are critical and probably only established through operation and experiment. Also, in full size power distribution became problematic and divided drive was applied, but it could bring its own compromises, a short and steep angled connecting rod for the internal low pressure cylinder(s). If you have a strong interest in Loco compounding might I suggest the book by John Van Riemsdijk, 'Compound Locomotives' as a good starter and covers the historical development of these engines, it is relatively cheap second hand, maybe US.$ 20-50 from Abebooks or some such, there are many others of various vintage and depth discussing the issues. If you are particularly interested I still have two copies of papers presented to the Institution of Locomotive Engineers, 1. An Investigation Into The Cylinder Losses In A Compound Locomotive, by E.L. Diamond, 1927; 2. Compound Locomotives Of The P.L.M., by R. G. E. Vallantin, 1931. If you desire photo copies send me a PM with an address and I will endeavour to send them on to you when I return home next month. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: paul gough on January 01, 2018, 03:06:07 AM
Rereading my previous I might have painted a discouraging picture regarding modelling a compound loco. This was not my intention. I don't think there would be too much difficulty in doing so from an established design or even a full sized prototype. I gave up because I could not develop a fuller understanding of how and why things were so nor develop any consistent standards to follow from a design perspective. I came to the conclusion that one would have to be very precise about what one was trying to do/achieve and adhere to these and develop the best compromise that was possible, probably through a lot of experimentation. Back then I did not have an MJM to assist with derailments to   my train of thought! Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 01, 2018, 10:58:29 AM
Hi Willy, I did not provide a very complete complete discussion of you post #621, last night, I was sitting here trying to watch the New Years entertainment leading up to the early fireworks show.  In the end, I finally posted after the fireworks started.  I trust that Dan's explanation of the wet and dry bulb thermometers was adequate.  The sling type is used because it can give a reasonably predictable air velocity and hence evaporative heat transfer coefficient when the standard wet bulb  thermometer is swung around on the end of the standard length sling, with a reasonable tolerance on the required rpm.  I can't remember the precise procedure, but the wet and dry bulb readings and the psychometric chart enable measurement, or at least a reasonable estimate of the humidity and moisture content of the air to be determined.

I totally agree with you in our admiration of those early pioneers, and how they managed to get those early engines running.  Men of great intellect and intuition and I assume lots of patience and many failures.  (And very few if any women working in the field if our history books are any indication, though that may be because the men wrote the books.)

I hope I did not bore you with the long reply on the early morning engine performance.  I have just heard the story so often and decided to see if I could really explain it.  I hope the puzzle is now  explained well enough.

Now missing out on the road runner in your youth is one thing, but you will appreciate it even more now.  But what did you watch when you went to the Saturday afternoon pictures?  Surely Disney did not have a total monopoly on the cartoon humour.

Hi Chris, thanks again for reminding us of the road runner cartoons, they were really very funny.  Perhaps I had also better remember the helicopter tip.  But first I have to get a helicopter and learn to fly it!

Hi Paul, whenI started talking about compound engines, I was not thinking locomotives, and your initial post on the topic reminded me that not all engines are stationary or marine.  With all the magnificent scale models that railroad people construct, the one thing truly difficult to model is the mile.  Even at 3.5 mm per foot, one mile requires 18.5 meters of track, and not many layouts could even accommodate that, let alone a real cross continental journey.  I guess for most of that first mile if not more, most main line locomotives would still be accelerating up to speed, so not really ready to notch up the valve gear, let alone benefit from compounding.  It would surely only be the largest of full scale locomotives that would consider compounding.  More use in US or here.  Or on the trans Siberian, than most British lines I would guess, other than perhaps a long distance north-south express.  The inertia of a full train is huge compared with any marine or stationary application.  To get started you want maximum torque, and that requires maximum pressure on the largest possible piston area.  However, it is only more laps around the loop.

Additionally I believe you work in a fairly small scale, so this would make it quite difficult to make a compound engine with power output to even equal a standard twin layout, let alone show any real advantage.  You see, while we are often reminded of the 'expansive power of steam', and the great efficiency it promises, the large volume of steam that has to be accommodated in the lp cylinders as it expands is generally not mentioned.  And these large cylinders have to be accommodated between or perhaps outside the frames.  This is where efficiency meets power to weight ratio.

The receivers are a interesting concept.  I have heard them mentioned before, but did not have any real information about them.  The concept sort of helps with any mismatch of the hp exhaust opening to the lp inlet opening, but the pressure swings in the receiver and heat losses from the shell also add losses.  A bit of a mind bending exercise to work out how the valve timing and receiver pressure interact.  Unless of course the piping takes the steam back to the firebox for reheating.  However, more power to those keen modellers who faithfully follow the prototype layout. 

Fortunately we now have a better understanding of the theory than those early pioneers, so we should be able to use this to guide our experimentation at least roughly in the right direction.

I am really hoping that Willy will come in before we move on, with more detail on the steam flow paths in that beautiful Woolf Compound mill engine, or someone else who has built and run a double or triple expansion engine, to help us all understand how they are set up, and how they have performed.

I do hope everyone had a pleasant New Years Day and not too many headaches.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 02, 2018, 12:36:54 AM
https://www.youtube.com/watch?v=edMVvwoOD2k

Hi MJM ,Saw this about receivers on the web....   I shall try and and get some info about the Woolf compound with pics soon Got this book about valve gears and events and it is full of good info , but also tells you that you need to compromise with the actually settings quite a lot!!! Talking about Pauls thread on Compounds and efficiency ,one would be more inclined to chat to the firemen that had to shovel the coal !! as well as the M.I.C.E.  experts when drawing conclusions !!..talk to you soon .........
Title: Re: Talking Thermodynamics
Post by: crueby on January 02, 2018, 01:26:39 AM
That video imay be good in how it shows the piping, I don't know about that part, but the motion at the end was showing a 180 offset, not a 90.
Title: Re: Talking Thermodynamics
Post by: paul gough on January 02, 2018, 05:11:05 AM
A quick last post before I'm off for about six weeks. Regarding notching up, drivers often start notching up after a few revolutions, especially on passenger trains, though not exclusively. It is very much a save steam and the fireman's back and get the loco into as close to stable demand situation as soon as possible, all contingent on load, grade, railhead conditions and engine condition. Fireman don't enjoy the company of drivers who thrash their engines,ie. run with excessively long cut offs. The complexity of compounding for the most part was its downfall on locos, they were, after all, hugely expensive apparatus to maintain. J. Parker Lamb, 'Perfecting The American Steam Locomotive' P.73, mentions the end for compound rigid frame locos in the U.S. came at the start of WW1 and quotes David P. Morgan, "The bullet that slew it (the compound) was a gadget called the superheater. At one stroke it gave simple engines compound economies without compound maintenance costs." and had the advantage they could be fitted to existing engines. Compound articulated locos lingered for some time more. You are quite correct in thinking it 'mind bending' regarding valve events, when you think of the French compounds running at 4 to 6 revs a second on an express and the driver managing outputs through two independent sets of valve gear it is a marvel, just think of the time any particular port is open! Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 02, 2018, 11:59:24 AM
Hi Willy, thanks for posting that video.  Unfortunately my internet is limited at the moment, so I can't  play it, but I will come back to it later, and watch it when I can.

That is an interesting looking book.  A lot of effort was put into studying valve action at the time, there is plenty to learn from them.

I hope that I have not offended you with my comments on efficiency and I see that Paul is thinking on similar lines with his post on life on the plate.  I certainly did not intend to imply that I had any definitive conclusions on the issue.  Like your book says, it's all about compromise.  The theory helps us understand what is going on, so helps guide our experimentation, but in the end, it all has to be proven and the efficiency determined by experiment and testing.  The success of the engines actually built and the experience of the firemen, soon tells which were the best performers.  And in the end, that is determined by the best compromise between the competing issues, thermodynamic efficiency being only one factor, and not necessarily the most important.  Drivability, maintainability, physical size, even cost all come into it, and I am sure other things as well.   

M.I.C.E?  Now I have heard of the institutes of Chemical engineers, and Civil engineers, but which one are you thinking of?

Hi Chris, as I mentioned above, I have not been able to watch the video, so I can't really comment on the 180 vs 90 degree crank angles in the video, and how they fit into the illustration of the place of the receiver in the scheme of things.

Hi Paul, safe travels.  We look forward to hearing all about it on your return.  And thank you for that interesting background on the practicalities of driving those locomotives.  It makes sense when you put it that way, that maximum acceleration might not be the best strategy once the train is moving.  Also a very interesting comment on the superheater killing compounding, as I don't see them as mutually exclusive.   I will look forward to reading about it on your return.  I will send my address in a pm nearer your return time.  I am sure the drivers on those engines would be relying on the tried and tested scientific method of "feel" rather than worrying about valve events while driving.  After all, once the engine has been built, the valve events will follow in turn.  The driver has to determine the best way to operate the engine, of course in conjunction with the fireman.  And it is all affected by the track with its curves and gradients, the load, and so on.

Thank you for all the contributions, and to those who are looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 02, 2018, 03:22:43 PM
Hi MJM, So here is my understanding of the steam paths for this engine.....When i first discovered this engine in late 2014 it was as it was when the mill burnt down in 1875 !! They had lifted the cylinder covers but the rest of the engine was all inside the casting so i had to work out what was happening from Intuition  !!. Fortunately my intuition was in top gear !  So....the engine, a Mr Woolf compound with jet condenser has both cylinders connected together from the beam and the valves also connected together from the single eccentric. I had to guess a bit from the outside shape of the casting what went on inside. The cylinders have a steam jacket surrounding them that heats up both cylinders prior to starting the engine. There are no relief valves on the cylinders but a drain cock at the bottom of the larger LP side . The exhaust steam goes directly from the HP to the LP steam chest and then goes down the outside of the casting to the Jet condenser that is operated by a lever once the engine has started. The cylinder sizes are HP 270 mm and LP. 440 mm....I don't seem to have the piston travel dimensions to hand for some reason....  will come back to that. The engine is now being restored and they have started to take it apart so a few pics of that later or you can see them on the  Beeleigh mill construction site on the MEM threads. The engine is rated at 12 Horses power and is original as built in 1830 circa. hope this is of help ....Also M.I.C.E.    Member of the institutionalised contraption engineers ? !!!And no you have not offended me at all ,Just pleased that there is someone out there with all the info to hand...That can put me right !!!
willy
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 02, 2018, 05:48:21 PM
Hi Paul, just wondering if part of the equation for efficiency had a component of  "the wrong type of snow " and the "wrong sort of leaves on the line" as this sort of thing has disastrous effects on our British locomotives !!! On British locos the inside cylinder Compounds had the eccentric sheaves put on the crankshafts with keyways...so they had to be in exactly the right place (with all the compromises) to work correctly, with no adjustments possible afterwards. so, did they make full size loco mockups to experiment with first ?? Good info here  and a Ib of footplate experience is worth a page full of formulas ,perhaps.
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 03, 2018, 12:13:53 PM
Hi Willy, finally managed to view the video.  I don't like to be critical of people's best efforts especially when they appear to be struggling with language.  But I did not find it very helpful, and many little inaccuracies combined with a poor animation, made it hard to follow.  I think I would go back to Mr Woolf's books.

You have done an amazing job of working out how that engine worked under the casting.  The cylinders are straightforward, they go up and down in phase, the stroke of the LP being longer than the hp due to the differences in the position they are connected to the main beam.  So if one side of the hp cylinder exhausts to the other side of the lp, then steam could be admitted to the hp, then expanded due to the volume changes as it is transferred to the lp, and finally exhausted from the lp.  It is all up to the valve arrangement, and the linkage to the eccentric.

If the valve has no lap, and set at 90 degrees with no lead, I can see it all working with the valves also moving in phase, half way down as the pistons pass top dead centre.  Half way up as the Pistons pass through bottom dead centre.  I am not sure I can get my head around how it would work with valve lap plus lead with the eccentric advanced the suitable amount.  Would it add a little expansion phase to each cylinder?  And how well would the transfer process work?  I think I will hope that someone with CAD modelling skills can whip up a simple (but accurate) model to work this one out in very slow motion.  But again the practical bit is the important part, how did you set your valve positions and the eccentric position on your model?  Did you include any lap?

M.I.C.E, hmmm, I wonder where we all join up?  Glad I did not offend, I am just trying to apply thermodynamics to explain some aspects of how our engines work.  I don't claim any special ownership of the information.  In the chemical engineering circles, it is readily enough available and well understood, and spills over quite naturally into the mechanical aspects of chemical process plants where I spent most of my career.

It might be worth having a look at what thermodynamics tell us about compounding and, now that Paul has introduced it, superheaters, and the reasons it might have killed compound engine advances.  Mind you, I think compounding might have continued for marine and stationary engines, until they were displaced by turbines.  In my experience, these days, steam turbines are available and commonly used as an alternative to electric motors for pump drives when the diversity of power supply is required for reliability.  For example, power station boiler feed pumps are often duplicated, with one electric drive, especially for startup, and the other steam driven so feed water is not interrupted due to a simple power fault.  In process plants, turbines down to under 50 kW are used, though I can't remember the lower limit of the power we considered practical.

It seems a long way back now that I introduced the T - S diagram to show how the thermodynamic properties change in the cylinder of an engine.  I will attach it below again for reference, as it is not very convenient to have to keep referring back a long way.  First let's refresh our memories of what this diagram shows.  The line 1 - 2 is the constant temperature, constant pressure boiling in the boiler.  The sloped curve, 2 - 3, is the constant pressure temperature increase in the superheater.  The vertical line,  3 - 5, or constant entropy, is the ideal adiabatic expansion of the superheated steam to the exhaust pressure.  For adiabatic expansion, the entropy is constant through this expansion, and the change in enthalpy is due to the work done by the steam in the cylinder.  Adiabatic expansion is an ideal process which cannot be achieved in a real engine, but the work done by the steam is the maximum that could be done by any engine, and is the standard by which we assess the adiabatic efficiency of a real engine.  The line 3 - 4 represents the expansion between the same pressure limits in a real engine, it ends at the same pressure but a higher temperature than the ideal engine.  (Remember that line 5 - 4, is the constant pressure line for the exhaust pressure.)

To progress our examination of compound engines, we need to refine that diagram, and show the compound more accurately than implied by the simple line 3 - 4.  But first let's look at the simple superheater cycle.  Without superheat, the engine expansion is a vertical line from point 2.  To understand why the superheater is so important we need to look at the overall thermal efficiency of the cycle without superheat and with superheat.

Now the concept probably sounds a bit complex, so I will break it down to about three steps.  It will also help me get to bed at a reasonable time.  So tomorrow, I will show you how the thermal efficiency of the cycle changes with superheat.  Then I will try and show the differences between that ideal superheat cycle and a more real one.  Finally, I will try and show the compound engine on that diagram and the corresponding thermal efficiency.

Let's think about that much for the moment and I will continue tomorrow.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 04, 2018, 01:06:43 AM
Hi MJM , yes i think you are right about the graphics on some of these "helpful" videos...as they are produced on  CAD  some shapes and dimensions are not actually correct.  I have checked my drawings and there is 1/4" lap on the valves as far as i can tell. There is 1/8" exhaust lap on the HP side but no Exhaust lap on the LP side i had to measure the ports whilst inside the valve chests which made it a bit difficult. The piston travel on the LP side is 40 " but i don't know the width of the piston and the HP piston rod is 18" towards the beam bearing from the LP side. The beam end centres are 123".  I hope this is useful and this engine was designed and built almost 200 years ago !! I don't know exactly how much eccentric lead there is as they have only just managed to turn the engine over.
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 04, 2018, 11:25:50 AM
Hi Willy, I was really intending to ask," how did you set up the valve laps and eccentric position on your model?"  Did you make the valves to exact length, and set the eccentric at 90 degrees, or did you make your valves with a little bit of lap and advance the eccentric accordingly? 

Obviously the beam angular motion is determined by the crank on the flywheel shaft and the distance its conrod  connection point is from the centre pivot.  The arc lengths traced by the connecting rod connection points for the hp and lp cylinders are determined by their distance from the same centre pivot, so clearly a bit more for the lp cylinder. 

Yesterday, I started to break down the steps in understanding why Paul's pioneer would have said the invention of the superheater killed the development of compound engines.

I don't know what pressure those early locomotives would have run at, but just as an example, I assumed 700 kPa (g) or 800 kPa absolute, very close to 100 psig.  Probably more realistic for a model with a heavy load, but it will do for the purpose.  So if we assume that line from 1 to 2 in yesterday's drawing is 800 kPa, for which the steam tables tell us the temperature is 170.4 deg C, we can now look up the enthalpy and entropy of the dry saturated steam, (point 2) and the corresponding properties for the saturated liquid, (point 1 on that drawing.)

If we further assume that the water in the tender which is pumped into the boiler is at 20 degrees C, we can look up the enthalpy of that water and calculate the increase in enthalpy from cool water to dry saturated steam.  It works out to be 2685.1 KJ/kg.  That heat plus the amount that goes up the stack has to obtained by burning coal, which the fireman has to shovel into the firebox.

With no superheater, that dry saturated steam is expanded in the engine to the exhaust pressure of 100 kPa, atmospheric pressure.  Now, just as we worked out in a previous post on an engine test, we can calculate the work that could be obtained from an ideal adiabatic expansion over that pressure range.  Remember the process was to use the second law of thermodynamics, which says for an ideal adiabatic expansion there is no change in entropy.  That entropy is very useful in looking at the performance of engines, and you can see why I had to introduce it quite early in the thread.  When you know the entropy of the exhaust steam, you can calculate the enthalpy, and hence the enthalpy difference which goes into work during the expansion.  The entropy means that the exhaust steam is wet steam, with dryness of 87%, which means the exhaust enthalpy is 2393.9, and the change in enthalpy is 375.2 KJ/kg.

From that we can calculate the overall thermal efficiency of an ideal adiabatic engine as 375.2/2685.1  = 14%

It is important to remember that this is the best performance that can be obtained from an ideal engine, and the performance of a real engine will be much less.  In fact we could guess a boiler efficiency of about 70%, meaning the rest of the energy is lost up the stack, and we could estimate the efficiency of a real engine of perhaps 75% of an adiabatic engine, giving not much over 52% of the adiabatic efficiency for a real engine and boiler.  However all the figures can be assumed as roughly proportional in this type of calculation.

The interesting result of these calculations comes when we continue through to the same calculations for a boiler with a superheater.

When the boiler has a superheater, the superheater is represented by that section from 2 to 3 on yesterday's drawing.  That is the form of the constant pressure line, once you start superheating.

I assumed a superheater outlet temperature of 350 deg C.  Again, I don't know what figure would be more typical, but that will do for our purpose.  To heat the dry saturated steam at point 2 to point 3, an additional 401.6 KJ/kg has to be added to the steam.

When the superheated steam is expanded in our ideal adiabatic engine, the exhaust at point 5. Is also slightly superheated, in this case to 109.4 deg or 9.4 degrees of superheat.  Again, the entropy does not change in our ideal engine so the exhaust enthalpy can be calculated and hence the change in enthalpy during the expansion, or work produced.  When the calculations are completed, the work output from an ideal adiabatic expansion is 466 KJ/kg.  A quick review shows that we put 14% more energy into the steam but obtained 24% more work out put.

If we assume the engine runs on the same route, with the same load, we would expect the work required to be the same, so if we get more work out of quantity of steam we actually need significantly less steam when a superheater is included, and that means shovelling less coal.

Again I have kept emphasising that the calculations are based on an ideal engine, one that the laws of thermodynamics show cannot be exceed by any real engine.  And in fact a real engine will always have losses that mean it produces less work output than that ideal engine.  But it seems reasonable to assume that the losses might be roughly proportional and we would get a similar result if we were able to compare real engines.

Now that is probably enough headache producing exercise for another night, so tomorrow, I will try and see if the laws of thermodynamics can show us the difference in performance between a simple single expansion engine and a double expansion or compound engine.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 04, 2018, 10:35:45 PM
Hi MJM , When i took the measurements of the engine sometime ago the engine was unable to turn and the valve rods and components took quite a few twists and turns via several bell cranks before they reached the valves them selves. This photo shows the crank and eccentric in line, sort of.... when i come to timing the model it was done more by dead reckoning than actual measurement. So i did get the engine to run as can be seen from the videos...The way i did it was to make a sheave with an extended boss to take a grub screw, once the engine was running with the sheave in the correct position this boss was removed and a slot cut to line up with the crankshaft.
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 05, 2018, 10:58:33 AM
Hi Willy, with the eccentric approximately in line with the cranks, you can see with the horizontal eccentric strap, the valve would be in mid travel, while the flywheel end of the beam is at top of its travel, hence the pistons at the bottom of their travel.  So basically very close to the normal 90 degrees for a vertical or horizontal engine.  Of course assuming that the eccentric strap is horizontal when the valve is at its extreme positions.  But your tuning the final position, once again is the practical approach.  I like the idea of making the eccentric easy to adjust then modifying it after the best position is found, I am noting that for future reference.  Simple approach to an otherwise frustrating problem.  I trust also that if there was a receiver of reasonable volume, you would have found it.  So it looks like the hp exhaust directly opens to the lp inlet, so the expansion which began in the top of the hp cylinder continues as the underside of the lp cylinder receives the steam into an enlarging volume, and hence contributes output work in the lp cylinder, before being exhausted to the exhaust condenser.  The same process occurs between the steam admitted to the underside of the hp piston, which is then further expanded on the top of the lp piston.  Relatively simple to see, now you have done all the work, but a great effort to get so much information from the machine in its state at the time.

Now I have been trying to work through the differences between simple expansion and compound engines from the thermodynamics point of view.  I have been looking at the engine performance from the view of the adiabatic expansion model.  The reason for this is that the laws of thermodynamics tell us that adiabatic expansion gives the greatest possible work output from the steam at the given conditions.  Any real engine will give a lower work output.  So the first thing is to have a clear understanding of the model.  Basically, the model assumes the mass of steam at boiler pressure, expanded to the exhaust conditions with no heat transfer. 

How does this compare with a real engine?  Well, first, steam is admitted to the cylinder while the inlet valve is open.  During this process, work is done on the piston, the expansion inherent in the work production includes the whole volume of steam back to the boiler, where of course the pressure is maintained by the heat input, but it is only the work and heat transfer in the cylinder that interests us. 

When the valves are shut, the expansion continues in the cylinder, but of course the piston has already travelled through part of its stroke, so there is only part of the stroke remaining, not anywhere near enough to expand the steam to exhaust pressure.  So what happens next.

The thing is that both simple expansion engines and compound engines are different attempts to approximate that ideal adiabatic expansion in a real machine.  They are not fundamentally different processes, just different approximations to the same process.  They are both trying to efficiently extract the work potentially available in the steam.

Yesterday I worked through that adiabatic process for an engine operating at 800 kPa inlet pressure, remember this is absolute pressure, around 100 psig.  With an atmospheric exhaust system.  Before the superheater was invented, saturated steam was expanded and gave a wet steam exhaust. 

The volume of the steam at exhaust pressure is just about 7 times the volume of the inlet steam at 800 kPa.  Way too high for our simple cylinder to accommodate after the inlet valve has closed.

If we assume cutoff at about 50% of piston travel, simply for easy calculation, the volume doubles from cut off to the end of stroke.  From the steam tables we can read that the pressure would be about 380 kPa, not exactly, that requires more interpolation of the tables than I have done for this exercise, but I think close enough for the purpose.  (About 40 psig).  Still well above the 100 kPa atmospheric exhaust.

This is where we see the difference between the simple expansion engine and the compound.

In a simple engine, the exhaust valve is opened, and the remaining pressure lost into the exhaust system.  In a compound engine, the steam is exhausted into the lp cylinder, with or without a receiver in between.  Let's follow the example of Willy's engine, and ignore the receiver for the moment.  The lp cylinder with its larger diameter and longer stroke opens up more volume than is being exhausted by the hp cylinder, so there is a net work output in the lp cylinder as the steam continues to expand to the full volume of the lp cylinder, before finally being exhausted to the exhaust system at 100 kPa.

So the issue comes down to how much work is produced in each engine, and preferably which real engine design produces more work from a given quantity of steam.  In other words, which is the most efficient.

I suspect that as good a way to look at the difference between these two ideas is to draw an indicator diagram for each, or probably more accurately, a P - V diagram for each engine cycle.

I have put some thought into how best to draw this, and am not quite there yet.  I hope tomorrow, but it is predicted to be over 40 deg C here, and no air conditioning.  So I am not sure how much I will achieve.  Might be a very short post, but I will at least check in.

Until then, thanks again for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 05, 2018, 03:39:15 PM
Hi MJM,  + 40 in Aussie and - 40 in USA  so is the climate in balance ?? So when this engine was made they were still experimenting with the technology. On a later Triple they had 3 pressure gauges presumably to record the 3 cylinder pressures ? On the Beeleigh there are holes drilled through the cylinder caps and it is likely that they had a pressure gauge attached to show what was happening.? These holes also had a double valved oil reservoir to introduce oil into the cylinders at periodic points.   Also is it possible to design an engine that has zero pressure  Atmospheric at the end of the LP stroke ? or is it advisable to have some back pressure as in IC practice ? There is a sister engine at the Ramm brewery that is still running that has a few modifications to it.....Also is there a graph /table that shows the introduction of technical terms over the last 300 years ?? Some more homework for you i'm afraid !!! The Ramm engines have also been insulated with Mahogany slats !
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 07, 2018, 12:02:49 PM
Hi Willy, looks like the average is zero.  I suppose that is the case somewhere.  Three pressure gauges almost certainly for the three valve chests or cylinder inlet pressure.  These pressures are not discretionary, but inevitable from the relative cylinder volumes.  As mentioned earlier, the best efficiency is obtained from equal pressure ratio in each stage.  If the second stage cylinder is too big for this, it will accept more steam, and hence lower the interstage pressure.  If it is too small, it will not accept all the steam from the hp cylinder unless the pressure rises a bit.  So the eventually settled pressure is determined by the relative cylinder volumes.  Similarly for the third cylinder.  If the interstate pressures start to drift, it may indicate that valves are passing, altering the capacity of the cylinder, so those gauges would be very useful to the engineer.

Fluids only flow from a higher pressure to a lower pressure.  If the pressure is uniform everywhere, there will be no flow.  So when the exhaust valve opens, the pressure in the cylinder has to be higher than outside to make the steam flow through the valve.  If the pressure is too low in the cylinder, steam from the exhaust will flow back in until the pressures equalise, then the piston trying to 'squeeze' the exhaust out will cause the pressure in the cylinder to rise until the necessary flow occurs.  The pressure at the start of the exhaust is not discretionary, but necessary due to fluid dynamics.

If you want the cylinder to have zero gauge pressure at the end of the stroke, say 100 kPa absolute, then you would need a condenser to lower the exhaust system pressure enough for the exhaust steam to flow out through the valves.  Then of course you need a condensate pump and an air pump, so you might as well make them all sufficient to make the vacuum a bit more useful.

As for the homework, surely if such a table or graph exists, surely it will be hidden in your library somewhere.  It is certainly not in mine, unless of course it is in that book by Professor Jamison, but I have not noticed it.  But it is the only historical book that I have.

By the way, sorry about last night.  Not only the end of a very hot day, but I must have chosen the the time that Ade was doing the maintenance.   It has to be done, and it is sure to be the wrong time for someone on a world wide forum.  It has given me time to improve my posts little, I hope.

Forty two degrees they say here.  You might wonder why we don't get acclimatised to it.  You start to if you get more than two weeks in a row, though you don't really get fully acclimatised to those temperatures.  However, Friday was about mid twenties, as was today.  You don't get acclimatised to a one day scorcher.  That is our weather pattern.

I have been looking in detail at compound expansion.  The T-S diagram that I use to determine the work done by an ideal adiabatic engine does not easily show the difference between a compound expansion and a simple expansion engine.  I tried a simplified picture of an indicator diagram.  You know the type produced by a special instrument as part of engine monitoring.

In a real indicator diagram, the corners are all rounded due to the throttling effects during the finite opening and closing time of the valves as we have previously discussed.  In idealised diagrams often produced in text books or other articles, the corners are more square, just to show the principles clearly.  The area in a plot of pressure vs. volume is the work done by the cylinder and can be extended to the power by the engine.  The rounded corners all reduce the actual work done, but square corners make the area of an idealised diagram easier to calculate.  I have even avoided including the compression phase, which again reduces the calculated work done if they are included.  But the simplification allows easy calculation and comparison of the two cycles, so is appropriate for the present purpose.

In the attached hand drawn diagram, I have included two cycles.  On the left is the simple cycle, exhausting to atmospheric exhaust.  The area with the cross hatching represents the work done through the whole cycle.  For the calculations I used the pressures from a few days ago, and drew the diagram for an hp cylinder volume of 1 m^3.  So the pressure times volume is a measure of the work out, proportionally smaller for a smaller volume engine.

On the right is the compound cycle.  The pressure ratio being similar for each stage gives 283 kPa for the interstate pressure.  I have taken the liberty of using 280 kPa in case I need to go to the steam tables, to minimise the amount of interpolation required.  I have assumed the volume of the lp cylinder is 1.7 times the volume of the hp cylinder.

In both cases I have assumed the initial cut off at 50%, so the volume ratio during the expansion is 1:2.  The steam tables are not very convenient, but are the accurate model to determine the resulting pressure at the end of the expansion.  It turns out that the 800 kPa steam expanded to double the volume gives a pressure of about 380 kPa.  Again, I used 375 kPa so as to have the figures without interpolation.  But it still requires a trial and error method.

You can see that the calculated result of all this simplification, about 20% more work output from the compound engine.  Not a figure we can totally rely on for accuracy, but almost certainly in correct proportion.  I have overlooked the effect of the volume of the transfer pipe and valve losses and so on.  But surprisingly similar to the extra work output from the superheat cycle I calculated a few days ago.  Obviously a lot simpler than resorting to compound engine and so much simpler to maintain.  Just as that pioneer said.

The part I do not yet understand is just why we do not use both superheat and compounding.  That would seem to return the advantage to compounding.  Though compounding still leaves us with the problem of starting at any point in the rotation.  I wonder if anyone else can offer some clues.  It is a pity that Paul is away at the moment, he may have some answers.

By the way, the comparison of the hp stage of the compound cycle with the simple cycle shows very clearly the effect of exhaust pressure.  For the simple cycle, with the exhaust pressure assumed atmospheric, the negative work from the exhaust cycle is that area under the the horizontal line at 100 kPa.  For the hp cylinder of the compound cycle, the exhaust cycle negative work is the area under the line at 280 kPa.

For both cycles the positive work from the power stroke is the area below the top line horizontal at 800 kPa for the first fifty percent of the stroke, then the curved line representing the pressure during the expansion, right down to the zero line.  The cross hatched area is the resulting positive work output of the cycle.  The steam is expanded to double the volume it occupied at 800 kPa.  It only expands to 380 kPa, regardless of the exhaust pressure.  The pressure is then throttled through the exhaust valves to only a little above the exhaust pressure.  This is all easy to see in the simplified diagram.  If we round the corners to more accurately show a real engine, it tends to obscure these principals, but the process is still as I have described.  The corner rounding reduces the positive work from the power stroke, and increases the negative work from the exhaust stroke, so reducing the net work output.

You can see the difference exhausting at 280 kPa instead of 100 kPa.  You can extend this to a condensing cycle where the exhaust pressure is significantly below 100 kPa.  There is advantage in power output to a condensing cycle, even when there is no further expansion, due to the reduced negative work of the exhaust cycle.

You will also notice that I assumed an early cut off.  I don't know if the valves can actually be arranged to achieve this.  However compound expansion engines are often fitted with Stevenson's valve gear so I assume the cut off is achieved in a similar manner to a simple engine.  Just for interest sake, I did the calculations assuming constant pressure for the whole cycle, another variation of the simplification, but the results were in similar proportion.  I expect this would also apply to in between points of cut off.

I hope this has clarified the compound cycle and how it is different from a simple expansion.  I will await the resulting comments and questions with interest.

Thanks for following along.

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on January 07, 2018, 12:08:42 PM
Whoops!  That picture is upside down.  Not sure how I did that.  I hope you can still follow it.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 07, 2018, 12:42:47 PM
Hi MJM Ok Done
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 07, 2018, 09:19:37 PM
Thanks, Willy.  I hope the diagram makes more sense now with the text now.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 08, 2018, 11:31:18 AM
Thanks again, Willy, for turning that diagram around.  It is much appreciated.

I hope that no comments means the post was perfectly clear, though I am not convinced.  But I thought there were a couple of points that could do with some further explanation.

First the units of work.  I used kPa for pressure and m^3 for volume.  Using Pascals of course does obscure the fact that a Pascal is the same as a Newton per sq m.  So kPa is the same as kN/m^2.  When this is multiplied by the volume in m^3, the units for work are kN.m.  If I had used a more realistic cylinder swept volume of one litre, then the units of work would be N.m, much more suitable for a model.  This calculation of the work output of the cycle is the same as the calculation of work from an indicator diagram, though the more complex shape of a real engine diagram means that the area must be calculated using either graphical methods, or a planimeter to measure the area.

Another thing you may have noticed was the increase in volume of the steam during the expansion.  A volume of steam equal to half the hp cylinder volume, occupies 1.7 times the volume of the hp cylinder by the time it has been expanded in the lp cylinder, so an expansion of 3.4:1.  If the  engine is equipped with a condenser system to extract even more work, the volume keeps expanding, so that a very low pressure cylinder has to be very large.  We can get more work out of the steam by expanding it to a lower pressure, but at the expense of requiring very large cylinders.  Alternatively you can use other methods of handling the large volume, such as those turbines on Titanic.  All a delicate balancing act between power output, efficiency, and cost.

The calculations have been based on a double expansion, with a pressure ratio of 2.8 across each stage.  If the same 800 kPa steam is expanded in a triple expansion engine, the ideal pressure ratio over each stage would be the cube root of the total pressure ratio (800/100 = 8). The cube root of 8 is 2, (2 x 2 x 2 = 8) so the pressure ratio over each stage would be 2.  The cylinder volumes would then have to be sized to give interstage pressures of 200 kPa and 400 kPa for the supply steam at 800 kPa and exhaust of 100 kPa.  The cutoff would have to be limited so the first stage expansion did not result in a pressure lower than 400 kPa, in fact it has to be a bit higher to enable the exhaust steam from the hp cylinder to flow to the next stage.  Similarly for the second and third stages.  Once again we see that those cylinder volumes and interstage pressures are not really arbitrary, but determined by the steam inlet and exhaust pressures Anne the number of stages.

It was mentioned earlier that some engine designs use a receiver between stages.  When the exhaust of one cylinder passes into the fixed volume of a receiver, the pressure rises, while when the steam passes from the receiver to the next stage, the pressure falls.  Like any other interstage pressure, the receiver quickly settles to a pressure level where the mass of steam from the higher pressure stage is exactly equal to the mass of steam taken by the lower pressure stage.   If the receiver consists solely of the transfer pipe, it would be necessary to have the inlet and exhaust valves of the cylinders right in time, otherwise very high pressure fluctuations would occur in that transfer pipe.  While if the receiver is quite large, pressure fluctuations would be much smaller.  I have to conclude that the purpose of the receiver is to accommodate differences in valve timing which I assume would be inevitable once the driver starts to notch up the valve gear to reduce steam consumption as the train gets moving.  Working out the valve timing for variable cut off by hand calculation feels a bit too hard these days when computer modelling techniques are available, so I will give that a miss.

I am not sure that there is much more I can say about compound expansion.  I hope that little introduction has been both interesting and useful.

Any ideas on what you would like to look at next?

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 08, 2018, 03:28:14 PM
https://www.youtube.com/watch?v=2EkjRslFgSM

Hi MJM ,  So how did early designers decide on the dimensions  of the cylinders for Compounds and later Triples ?  And here is an electrically heated steam crane i made some time ago... i could do a heat up graph for this soon if it still works ?!!! Thanks for all the info you have provided and I was wondering how much Fluid dynamics and thermodynamics overlap. Also my thermometer is quite inaccurate as when i tried to take my temp it only registers 92/93  rather than 98.4 !!! it did mention on the blurb that it was only about 3/5% accurate  !! but how much this would vary at higher temperatures is unknown ?!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 09, 2018, 10:35:08 AM
Hi Willy, I am not sure of the time line, perhaps your library has some answers on that.  So I don't know if understanding the theory lead to development of compound expansion designs, or if the theory followed later.  I have previously spoken in praise of the intuition of those early pioneers, who achieved so much well in advance of the developing theory.  They may well have been led by intuition to build a compound  engine in the search for more efficiency.  In this case, they may not have had much idea how to size the cylinders, it could even be affected by the inaccuracies in those tables you have pointed out before.  They may have started with something as simple as reconnecting the steam to a two cylinder engine to see if there was advantage in expanding the steam twice.  You see, the interstage pressure will find its own level, depending on the relative cylinder volumes as I have explained in the last few posts.  So you can build an engine with any relative cylinder sizes and it will work, it is just that this may not be the most efficient cylinder sizing.  Finding the optimum interstage pressure level and relative cylinder sizes for maximum efficiency and power output may have come later.  Another case where combining history with theory makes the whole study more interesting.

I love that little crane.  Is that based on one in a book, or your own design from a historical prototype?  If you want to do a heat up curve, I will help you through the calculations.  It will be interesting to see how it compares with the horizontal boiler.  If you have a boiler drawing, it will help with calculation of the mass of copper.  If you can measure the resistance of the element, that will reduce another source of error.  I assume the insulation is just those wood strips, or do you also have an insulation layer?  But surely this does not mean you have run out of questions!

Fluid mechanics and Thermodynamics.  I feel that subject titles are an indication of the central emphasis.  However, at the boundaries, it is difficult to draw clear lines.  For example, in convection heat transfer, the temperature gradient is determined by the fluid flow, and the velocity boundary layer.  But the flow is influenced by the fluid properties of density and viscosity, which both change with temperature.  So the temperature gradient affects the flow, and the flow affects the temperature gradient, you can't separate the two.  Even if we confine the heat transfer to solid conduction, well solids are only liquids whose temperature has fallen below the freezing point.  Though some solids break down chemically before they melt if you try and heat them.  So there, and also in dealing with combustion heat sources, you rapidly get mixed up with chemistry.  This thread could not be written without a language subject, and geography affects ambient conditions, history determines how much knowledge has been accumulated for you to build on, and explains things like development of motor cars, pneumatic tyres and so on.  So no clear boundaries.  And you can't run a hydrocarbon processing plant without an understanding of fluid mechanics, so don't feel that questions have to be limited to one area.  As always, I will say if I don't know.

Thermometers.  Quite a range of subjects today.  Hmmm, if your body temperature is only 92/93, perhaps you should see the doctor, if it is not too late.  I assume you put the thermometer under your tongue for a reasonable time in the normal manner.  Some authorities say that putting it in the other place is more accurate, but don't break it.  Under arm is often used for babies, but I suspect it is a less accurate method.

The thermometer accuracy depends not only on the temperature at the bulb, but the stem temperature also has an influence, and for best accuracy, there is a stem correction that has to be taken into account.  I have not done this since high school, but Mr G should be able to tell you how to do it.  It is also determined by the accuracy of the capillary.  If the diameter is not accurate to the intended value when the scale was engraved, the reading will be a constant percentage out, and you could make a calibration curve by calibrating at only two different temperatures.  The ice point and boiling points are good points to start as temperatures you can reproduce at home.  However, if the thermometer is poorly made, so the diameter of the capillary varies along the stem, then the accuracy will vary with temperature and you would need to calibrate by comparing with a more accurate thermometer at several temperatures over the range you want to use.  It might be possible to check the consistency of diameter with a low power microscope with a scale, perhaps one of those computer ones that are readily available these days.  By the way, I hope we are not talking about that good quality thermometer with the certificate you showed us previously.  In that case, it is important to very carefully calibrate before you complain. 

While the ice point and boiling point are accurate reference temperatures, it is sometimes difficult to get your temperature instrument to show them accurately.  The boiling point does depend on atmospheric pressure so you need to check the atmospheric pressure.   Again, Mr G will find the official bom reading in your area for you.  The ice point is even harder.  You need an insulated container full of small ice blocks or even crushed ice made from very pure water and minimum water, all well stirred (not shaken).  I am not sure how much the freezing temperature of potable water varies, but impurities change it and if your water is particularly hard, it may vary a little.  The stem correction may be another factor.  Body temperature is a little harder to measure with the same accuracy. 

As always, accurate readings are simple in concept, but sometimes more difficult in practice, you can only do your best with the procedure, and try a few repeat measurements to get an idea of variability.  Essentially you have to minimise any temperature differences throughout the fluid you are measuring, and minimise any heat gain or loss to the stem, and give time for everything to reach uniform temperature.  But the readings at ice point and boiling point give a very good idea of the accuracy of your thermometer.  I hope that helps.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 10, 2018, 11:59:44 AM
I have two small Meths fired boilers and I keep promising myself that I will conduct some heat up and cool down tests on them.  The excellent job that Willy did for his electric boiler put me to shame, and inspired me to finally get a round tuit.  Everyone needs at least one of those.  So in the holiday period after Christmas, I finally did some test runs. 

It was a bit of an adventure, I suspect it always is, when a model has sat for a while and not fired up.  I have been setting up a spreadsheet to see if I can do similar calculations to those I did on Willy's boiler.  Obviously the big difference with a fired boiler is that part of the heat is lost in the flue gas, but I want to see how much of the heat from the fuel burned can be accounted for in the water and the copper shell. 

The first one is a simple pot boiler from 1 1/2 inch copper tube with torispherical ends.  It sits on a stainless steel firebox, and is open at the top, similar to the little Mamod units.  Not much use for insulation on these, as the flame licks pretty much the whole boiler apart from the ends.  Perhaps I could insulate those.  The steam outlet is taken from a banjo type fitting under the safety valve, and curls back under the boiler and around the firebox before out to the engine.  The engine is actually driving a small generator, which has not yet had a load connected.    The generator is a small 280 size brushed DC motor.  When the engine runs at about 1000 rpm, the motor produces an open circuit voltage of about 1.3 V.  The short circuit current read about 0.3 amps before the engine stalled, clearly needs a bit more steam pressure to keep it running.  I think I should design a load to draw about 0.15 amps and try again.  The drive belt is a simple rubber band, and the pulley sizes mean that the motor is turned at about 3000 rpm.  Still very slow compared with its no load speed as a motor, or even it's loaded speed, hence the low voltage.

I will try attaching a photo, I think I have it small enough to attach.  Otherwise I will have to try again tomorrow.  Stealing time during a beach holiday to do the calculations, and only part done at the moment.  So instead of writing, I will continue with the calculations, and hope to have some results to publish tomorrow.  So just a short post tonight.

Thanks for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 11, 2018, 01:10:17 AM
hi MJM, that looks quite an interesting engine , is the doubled back steam pipe used as sort of superheater ? there is quite a lot of brass on the fittings  that would make a good radiator ! there are also quite a few odd parts attached that seem to have nothing connected to them  ? is this all original and has it evolved over time via different makers? Is it also floating in a pool?  I checked my new thermometer with some older mercury thermometers graduated in degrees Celsius and there is 1 degree different.
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 11, 2018, 11:35:03 AM
Hi Willy, yes the steam outlet doubles back into the furnace as a superheater.  Made from a 450 mm length of tube, a bit over a full turn around inside the furnace casing.  Heats steam from 110 deg (143 kPa absolute) to 127 C so quite effective for a small model.  Perhaps I should make a new one from a 1000 mm length to see how it affects overall performance.  You are quite right to observe that all those brass fittings probably loose a lot of heat from such a small model.  I have often thought I should try wrapping them with that silicon tape.  But first a few more runs to establish a base line performance, so I will be able to see how much difference it makes.

Everything is connected and almost steam tight as shown.  Almost, as I found some of those fitting nuts were too long, so would not tighten quite enough.  They have now been touched up with a file and a milling cutter and I think I have a new run proving them totally tight.  I think I must have mixed them up before assembly, and they are obviously not interchangeable.

A few points that might be confusing you:-

The filler plug is also a thermowell that I have shown previously in pictures.  The solid brass plug protrudes about an inch into the boiler, but the hole you can see is is only drilled 7/8" deep for the thermocouple, (not fitted in the picture), so it does not penetrate into the steam space.  Drilled 3 mm diameter, so all my thermocouples fit.  The safety valve is on top of a banjo fitting for the steam outlet.  The safety valve has the straight through passage unobstructed, the steam outlet is the side branch.

The 90 degree elbows are combined with thermowells for the engine inlet and outlet thermocouples.  The horizontal bend on the inlet means only a very short thermowell, the exhaust one allows a longer thermowell, so probably gives more accurate reading.

You can also see the displacement lubricator in the background, with the oil plug on top and the white hand wheel. 

The engine is actually my second engine, double acting, with flanged cylinder heads and a screwed gland, built with the same frame dimensions and the same 12.0 mm bore as the first which was single acting, see my avatar.  With identical frame dimensions, it is interchangeable in the steam plant, but obviously a bit more power output if I can keep up the steam pressure at the higher flow.

The two little threaded holes in the top of the frame are the ends of the vertical steam drillings and are plugged with 3 mm grub screws.  Probably rusted in by now.  The little screwed fitting on the side is a warm up valve.  When opened, it connects the inlet and outlet steam drillings, allowing condensate to be blown through to the condensate separator during startup, with a little steam flow to warm the frame.  Then I close it while the engine runs.  I am still learning to make small hand wheels, I need to revisit that one.  The piece of white tape on the flywheel rim is the reflective tape for my optical tachometer.

The Meths burner sitting beside the boiler is also reflected in the stainless furnace casing, where you can see most of the fuel tank.  Unfortunately the filler plug with a tiny vent is just out of sight in the photo.  The join in the fuel line was an attempt to make the tanks interchangeable for different burners, but I would be better to get more confident at bending up and silver soldering the tanks, and just make a new one for each burner.  The "wick" in the centre part is actually cut from a porous water filter element, not sure if it is clay or ceramic.  There are little tongues of blue flame from the holes in the two side sections, the whole thing being an attempt at a semi vapourising burner.  Another area for further development.  The steam separator chimney exhausts essentially dry steam, while some oily condensate discharges from the gooseneck in the base of the separator.  I collect the condensate with an empty tin that fits under the outlet.  I have previously described the separator.

The boiler is my own design.  I turned the plugs to form the torispherical ends, a form that has just two radii, but approximates the semi elliptical form, and much easier to make.  Fully code compliant, it avoids the need for staying of a flat end.  But next boiler I make will have a central stay to make it easier to hold the ends in place during soldering.  But I will still use the torispherical form.  Had a bit of help with the soldering, but at least I made all the parts, and did the pressure test.

The "wood" base has a channel cut in under the furnace box to allow the burner to be placed lower.  It was initially too close to the boiler.  It is easily packed up, to allow experimentation with the burner height.

It is all "floating" on a glass topped table in our garden.  The glass surface is textured, and now you mention it, it does look rather like water. 

I know it is not up the the normal standard for this forum, but it was my first effort.  Definitely more of a test bed than a piece of art, but I can only get better with all the tips I pick up reading the amazing builds from you and so many other forum members.

I hope that you will now see my first attempt in slightly better light.

Always a pain when the thermometers don't all give the same consistent reading.  But you can allow for a known error.  Next test is to determine which one(s) are correct.

I think that is enough for one post.  I should have some of my calculations ready for tomorrow.  Beach weather is expected to include large hail stones!  So perhaps some indoor time.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 11, 2018, 02:42:53 PM
Hi MJM , Ok all is explained now and very clear , and ready to go with perhaps lots of areas for improvement with lagging to show how effective this can be !! Does all the steam escape from the exhaust 'Chimney' or can you collect all the exhaust water with the little overflow curved pipe to show how much is used ? It looks like a well thought out arrangement to get interesting results. You mention the white tape for a tachometer .... on my engine i inserted a magnet on the fly wheel and used a cyclometer to get a reading of miles /hour.!! these can be recalibrated for wheel diameter to show "With cunning calibration" RPM  i think.? I have also drawn in the thermowell places on you photo !! please forgive me if this is an unsolicited addition ...but it is quite easy to do this on my apple pooter !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 12, 2018, 10:22:59 AM
Hi Willy, I am glad that my little steam plant is clearer now.  Not as beautiful as your models, but works ok and is set up to do some exploration in thermodynamics.  I am pleased with it as my first attempt.

It would be quite hard to apply some insulation, except perhaps for the boiler ends, and some of the steam piping, as the flame must be allowed to heat the vessel shell.  You can see the similarity with the little Mamod style, but perhaps a top to the furnace casing and a chimney would help reduce the losses to the atmosphere.

The exhaust steam mostly escapes from the separator chimney.  I say mostly because when the boiler first starts steaming, there is a dribble of condensate from the gooseneck, but this soon stops when the piping, engine and separator are all warmed up.  The water that is collected is quite oily, demonstrating that the lubricator works, at least at some stage.  Over what period is another area worthy of some more experimentation.  It's simple enough in principal, but making sensible measurements of such small quantities is not so easy.

The optical tachometer is quite good for an instantaneous reading of rpm as it does not add any load to the engine, and it shows how much an ungoverned engine speed varies over a run.  Your cyclometer is a good idea.  I find it hard to believe that it would add a measurable load, though it is in fact a tiny generator.  But with some cunning calculation, it can be calibrated to measure total revolutions.  For rpm, you would have to time sections of the run, and calculate the average rpm over each time period.  More importantly, the total number of revs an be multiplied be the engine displacement to measure the total steam through the engine.  The collected condensate and the water remaining in the boiler afterwards allow separation of the initial water fill into steam through the engine, steam used for heating up the engine and piping, and that remaining in the boiler.  So both instruments are actually quite useful.

Thanks for marking up the photo, a good way to ask a very clear question.  There are actually only three thermowells, the filler plug, engine inlet and engine exhaust.  The fourth point you have indicated is the adjusting nut on the safety valve.  It is where the stem would protrude if it was long enough.  My skills at turning a thin spindle let me down on that first attempt, so it is not as long as it should be.  It would be better longer so it could be more easily lifted with a pair of pliers to prove that it was not stuck.  I have to remove the valve occasionally with this stem length.  It is an area I could now improve by making a new stem, as I am doing better on those these days.

I learned from doing the calculations I did on your electric boiler the impact of the time between actual full degree temperature changes on the thermometer, and the exact timing of the readings.  So I adopted the method I suggested for you to try in future, I used the iPad stop watch to record a lap time for each reading.  I also found that I had a temperature meter which has a resolution of a tenth of a degree.  Resolution is not the same as accuracy, and I am not pretending that it is that accurate.  But as the same instrument has two thermocouples that read the same when placed close together, I suspect that temperature differences and changes are quite consistent.  More importantly, by watching the tenths increase, I get a good warning of when the whole degree will turn up, so the time when I tapped the iPad could be more consistent.  The iPad records with a resolution of 0.01 seconds, but my reactions are not that good.  I transferred the readings to my log book after the run, but rounded them to the nearest whole second. 

Apart from a few random speed measurements and periodic monitoring of the steam temperature in the boiler and at the engine inlet, I only recorded the time from the engine starting, and the time it stopped.  Boiler pressure and superheater temperature stayed quite constant during most of the run.  I then started recording the cool down time starting from 82 degrees in the boiler.  When the engine stopped, and I removed the burner, the boiler cooled to 82 quite quickly, so no readings for the initial bit.

The tiny boiler heats up quite quickly, so it felt like it was all action.  Much easier with full size equipment where temperatures take a lot longer to rise.  I copied all the readings into a spreadsheet, and calculated the time differences with a formula copied down the column, to eliminate silly mathematical errors.  The spreadsheet was then used to draw the graphs for heat up time and for cool down time.  I have attached a picture of the graphs for heat up time and for the cool down time.

You can see that the method of taking the readings resulted in much more even curves.  I was quite pleased.  Initially, it looked like an irregularity had crept in, but when I checked carefully I had made an error in transcription of one of the readings.  I corrected the error, and the results shown in the graphs are exactly as I recorded them. 

You can see towards the end of the heat up, a fall off in the rate of temperature rise, much as with your electric boiler.  My observation was that there were a few steam leaks in the piping that I spoke of earlier, that most likely explains that one.  I have fixed those now, I hope, and I have two more runs, well one anyway, but that is another story.

This post is getting long enough, so I will have a closer look at those graphs and what they tell tomorrow.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 12, 2018, 02:52:00 PM
HI MJM, intersting observations there ...and the cooling graph only goes down to 30 degrees ,  so is that the ambient temperature where you are at the moment ? It is also very much more linear than the heat up curve. Also only 36 minuets rather than 18 hours !!  On the heat up part it is also quite quick 8 minuets . So the next job is to work out how much energy is required to get to this temperature. also a good analogy between our boilers would be your 'Haystack' and my 'lancashire' type of boiler.I would like to see what sort of thermocouples you actually use as well with a picture ? also if you have more thermocouples you could place them at different points above and around the boiler ... Also you could use some of the shiny stainless steel to reflect back some of the heat with a shroud, until it heats up and radiates even more heat. I was with a friend recently with an old cast iron open fireplace and was quite amazed at how hot the whole fireplace surround was and giving off radiated heat.!! So We have radiation ,conduction, convection and also reflection of heat happening here..Also the accountants in these engine establishments must have been horrified at the amount they had to pay for fuel !! Also saw this about flues...
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 13, 2018, 01:02:59 AM
HI MJM , still thinking about fuel for boilers and remembered that some of those early meths powered 2.5" gauge locos were still able to do passenger hauling on garden tracks
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 13, 2018, 11:50:15 AM
Hi Willy, yes, I only followed the cooling temperature down to 30 degrees.  The ambient was about 21, and you can see the times were getting longer and the need to keep watching the temperature so closely was testing my concentration.  Obviously the uninsulated boiler cools much faster than your insulated electric boiler.  As would any boiler with a flue for firing, even a centre flue type with insulation on the outer shell.  I suspect the initial cooling down to 50 or even 60 is probably the most informative.

Only 8 minutes to heat up mostly reflects the fact that there is only 130 g of water in the tiny boiler, just a simple pot with no water tubes.  With enough fuel being burned it, can heat up quite quickly.  But that should show up in the energy calculations which obviously come next.  Well, next after your other questions.

So is the simple pot boiler called a haystack?  I have heard the term before but not sure which type of boiler arrangement was being described.  I will also be interested to see the performance of your Lancaster boiler, the more different boilers we can compare, the better. 

I have attached a picture of one of my thermocouples.  You can see it is used with a fairly ordinary multimeter for the readout.  The thermocouple is just that little dot of weld metal joining the two wires at the end of the blue heat shrink.  It is the joining of the two special wire compositions that results in the voltage output when the junction is heated.  It is known as a type 'k', for which the specification ties down the copper and constantan wire, and the voltage - temperature characteristic is also defined in the standard.  Unfortunately very low voltage and not linear with temperature, so possible, but not easy to make your own instrument,  I think the photo might also have been in a post around 11 July 2017, or shortly after.

A few comments about radiant heat.  Those reflections are light being reflected in the same way that radiant heat is reflected.  The only difference between radiant heat and light is the wavelength.  When radiant heat falls on a surface, there are three possibilities, it is absorbed, it is reflected, or it is  transmitted through.  Most surfaces there is some combination.  It is obvious for most surfaces other than glass and similar, that transmission is zero.  Your black cast iron fireplace, mostly absorbs the heat, the dull black surface is the clue, while my shiny stainless steel mostly reflects the heat, just as it reflects light.

When your cast iron fireplace surround receives radiant heat, it absorbs most of it and a tiny portion is reflected.  The absorbed heat makes the iron hot, and as the temperature rises, it in turn radiates heat, proportional to its absolute temperature raised to the fourth power, and with a range of wavelengths, just like light, and most heat at a wavelength which also depends on the temperature.  Eventually it gets hot enough that the amount of radiated heat is equal to the amount of heat received, and the temperature stops rising.  At this stage it is quite hot as you have observed.

When radiant heat falls on the shiny stainless steel surface, most is reflected, however a tiny proportion is absorbed, and obviously none is transmitted.  The absorbed heat means that it gets hot, and starts radiating, both back to the heat source, and also to the atmosphere from the outside.  And eventually the radiant heat loss equals the heat received and the temperature stabilises.  However much less heat is absorbed than if it was a black iron casing, so the temperature stabilises at a lower temperature.  Still hot enough to burn your finger though.

The spiral corrugations are interesting.  When external pressure pushes the ends apart, it tends to try and untwist, but that is resisted by torsional stresses in the outer vessel shell.  Clearly stronger axially than the conventional circumferential corrugations, but I am not sure how much pressure it would support without additional longitudinal stays.  I just don't know.

Heat from Methylated spirits is just as good as heat from anything else, though the flame temperature may be lower than that of gas or coal.  However the main issue is burning it at a great enough rate to provide the steam you need.  It is all about the burner, plus of course sufficient heat transfer area.  In those 2 1/2 gauge locos, I suspect they would have some form of vaporising burner, and I would be most interested to see the design.

The starting point for the energy calculations on my little boiler is the quantity of fuel burned.  I see from my notes that I poured proximate lay 32 ml of liquid with a mass of 25 g.  I suspect that I really need a scale with a resolution of 0.1 g, as the whole gram introduces too much error, about 4%.  It is probably OK, but does not leave much room for other errors to accumulate.  I could use more Meths, but then I would need a bigger boiler, so as not to run it dry before the flame died.

Now I have always used 26,000 kJ/kg as the lower heating value for Meths.  I know in our recent discussion, ethanol was quoted as 29,000.  I don't know if this is just different sources, or whether the difference is due to the effect of the 5% water normally present in meths.  The water has to be evaporated and heated to flue gas temperature, and I suspect this may explain the difference.  I will try some calculations later.  But using 26,000 for the moment, 25 g means 650 kJ of heat released.

The burner was alight for a total of 10 min 42 seconds, of which 7 min 21 sec was the heat up time.  That means it was alight for 1062 seconds.  We can multiply 650 x 1000 to get J/s, then 650 x 1000/1062 gives 612 Joules/s, or 612 watts as the average heat rate during the whole run.

Now just as the electric element changed resistance as it heated up and hence changed the rate of energy input, the burner does not really burn at a very uniform rate.  My burner tends to start off a bit slow, rather like a simple wick, then there comes a point where some vapour is produced and the blue tongues of flame appear at the row holes down each side of the burner.  This seems to increase the heat output.  This is an additional complication on top of the obvious very significant heat loss with the flue gas.  Also, when the burner is first lit, the boiler and its water is cold, so there is a bigger temperature difference between the flue gas and boiler shell at the start than later when the boiler is up to steam temperature.

This is where I copied Willy's idea of measuring the temperature and time during heat up, it means the boiler heat input can be calculated every 5 degrees interval during the heat up, and I believe the graph of the heat up time energy input will show the increase in heat output, but that will have to wait until tomorrow, when I will try and lead you through the method I use to do the calculations.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on January 13, 2018, 02:25:12 PM
So is the simple pot boiler called a haystack?  I have heard the term before but not sure which type of boiler arrangement was being described. 

MJM, here is a thread about haystack boilers:

http://www.modelenginemaker.com/index.php/topic,6900.msg142585.html#msg14258

Cheers Dan
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 13, 2018, 05:33:59 PM
Hi MJM, so ,some info about Haystack boilers has appeared on a new thread , and here are some pics of the Lancashire type of boiler.....I was also wondering where fair comes between ?! on the insulation chart ? Also does lit meths give off more heat if the ambient temperature is warmer. Also with thermocouples it says to calibrate them use ice at atmospheric pressure .....if you have a container with ice in it and compress the air will it melt at a higher temp ? Does compressed ice in a glacier behave in a different way ? can you compress ice ?
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 14, 2018, 11:31:35 AM
Hi Dan, thanks for that article.  It suggests that my properly code designed horizontal cylindrical boiler is not a haystack type, which appears to be characterised by a flat bottom, and no attempt at pressure design, at least as we understand it.  The lead top would be like a giant fusible plug.  Easy to be critical now, with our knowledge of the theory they still had to develop.  The truth is they did not know any better.

Hi Willy, I think Dan has answered the question about the haystack boiler design, but it was an interesting little diversion into history.  Obviously by the time your Lancaster design came along, they had a better idea of pressure vessel design, I can even see external reinforcing rings on those fire tubes.  They also clearly understood the importance of surface area for heat transfer to extract the maximum amount heat from the flue gas.  It will be great to see a test run on that one as well.

Very hard to put an accurate number on 'fair'.  The other problem is the table does not include values for thermal conductivity, specific heat and density.  I must admit to being surprised at the high temperatures suggested for TFE, perhaps of one of those things needing reliable citations.

Burning meths, or any other fuel for that matter, releases a fixed amount of energy per kg of fuel, based on its chemistry, and is not dependent on the initial air temperature.  However, that heat has to be absorbed by the combustion gases, which increases their temperature by an amount which depends on the density and specific heat.  If the air starts at a higher temperature, it reaches a higher temperature after combustion, from the same amount of energy released by the fuel.  The higher temperature of the combustion gases means more heat transferred to the water.  Hence the installation of air pre heaters on large boilers.  If the air is preheated using flue gas entering the stack, it reduces the fuel required, so increases the boiler efficiency.

For calibration of your thermocouple, it is important that you do not just use ice, but you need an ice-water mixture.  Ice and liquid water together behave much like liquid and vapour in equilibrium in that, at constant pressure, the temperature is constant.  At atmospheric pressure, the temperature for ice and liquid water together in equilibrium is zero deg C, and this is quite accurate enough to calibrate your thermometer.  If you can reduce the pressure enough, to just 0.6113 kPa, the triple point for water where solid, liquid and vapour all exist together in equilibrium is at 0.01 degrees C.  Ice can not exist in equilibrium above 0.01 C.  As the pressure is increased, water is one of those strange substances where it's freezing temperature gets lower, but as it is still only 0 at atmospheric pressure, you need a lot of pressure to get much measurable change. 

So ice and water together, well mixed until the temperature is uniform, when the ice starts melting, the temperature stays at 0 until all the ice is melted.  Of course it is a little tricky to get the temperature really uniform, a well insulated vessel helps.

Ice on its own can be much colder.  Just ask any of our members who live at high latitudes.  If you have ice at say -20 C, and gently warm it, it sublimes directly to vapour without ever melting to water.  So ice alone from your fridge, probably around -15 C for proper food preservation, is not a suitable reference.  You have to add some water and stir it until the ice starts melting and the temperature is constant.  A thermocouple probably only needs one reference temperature, though it is better to also check the readout instrument at a second temperature, and as I have mentioned before, the boiling point at atmospheric pressure is also suitable.  You do need to allow for the actual atmospheric pressure, 1000 hPa, 99.6 C, while 1013 hPa gives 100 C.  Again it is tricky to achieve a close to equilibrium temperature as you have to maintain the heat input to keep the liquid boiling.

I don't think that ice in a glacier is any different, though it has a lot of air mixed in when it starts as snow, then I guess the compression by more falling on top squeezes the water into the gaps and a lot of the air is lost, though some remains trapped for scientists in the future to measure the composition.

The next step in analysing my test run is to calculate the energy taken up by the boiler, specifically the copper and the water.  Now the copper is easy.  We assume the copper is the same temperature as the water, though in practice, is is a bit higher so that heat is transferred through the copper to the water.  The energy taken up by the water is in the steam tables as enthalpy of the saturated liquid, hf.  If we look up the hf for each recorded temperature, we can calculate the enthalpy difference.

My preferred method is to use a spreadsheet, as there is a lot of repetitious looking up the steam tables, and it all becomes quite difficult to check the calculations if done by hand.  However, spreadsheets have a function VLOOKUP, which is used to look up a value in a vertical column of a table or array of values.  So if I list all the temperatures I need, and use the steam tables to find hf for each temperature, the VLOOKUP function can be used to lookup the values of hf with a simple formula.

The attached picture is the top left corner of my spreadsheet.  You can see the table of enthalpy values in the first two columns.  It extends from cell A4 to B61, and if we make these absolute references by adding the $ signs ($A$4:$B$61), it can be referred to by a formula in a cell any where on the sheet.  It is relatively easy to recheck the numbers in this table, then it remains correct every time it is used. 

The next important columns you can see include the time and temperature recordings.  Then I used a formula to look up the enthalpy, hf for each temperature.  The formula looks like this:
=VLOOKUP(temperature, $A$4:$B$61, 2, exact match)

The '2' is the column of the table with the value we want for the temperature in column 1.  The 'exact match' is used to accept only that, most spreadsheets have an alternative term that allows the nearest match.  In this case it is better to get an error if the exact temperature is not listed, rather than just the nearest temperature listed. 

This might seem a lot of work, easier to look up a value in the book of steam tables, but then the magic begins.  If we copy that cell with the formula, then paste it right down the column, the computer looks up all the other values.  We can even copy the same formula and paste it to use it again for the cooling curves.

I have gone through that in tedious detail for those who may not have used that type of function before.  I hope it is helpful and encourages a few more to try.  And to expand the range of formulae you use in the spreadsheets you use.

I will continue with the energy analysis of the boiler test next time.

Thanks for following along,

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 14, 2018, 05:02:47 PM
Hi MJM, thanks for more useful info and answering those questions, and  what happens to ice at absolute zero ? will it contract and split into small pieces? And why does your hand 'stick' to frozen steel pipes ??
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 15, 2018, 12:12:45 PM
Hi Willy, I don't know how much you and other forum members use the formulas available in the typical spreadsheet, so it is hard to know if it is helpful to describe the process, or if everyone knows all that stuff.  I guess there will be some at every stage along the way, so I hope that I can help some progress a little further.  But I am happy to continue answering any questions that are posted, with the proviso that I will say so if I don't know the answer.

It is interesting that with all the space in my text book on the behaviour of water, it does not clearly answer your question about cooling the ice, but I will explain the situation as I understand it.  While water expands when it changes from liquid to solid, I expect that the solid ice contracts like everything else if you further cool it.  I base this on the fact that molecules are always in motion, even in solids, and vibrate about a central point.  As the substance looses heat, the vibration slows, and with it the amplitude of that vibration.  The overall effect is that the molecules effectively take a bit less space and the solid contracts.  There is always plenty of space between the molecules, (relative to the size of the molecules) so they can always get a bit closer together.  I have read that even at absolute zero, this vibration never really becomes zero, though I am not sure how that works.  However the nearer you get to zero, the harder it is to get any temperature gradient, so there are minimal temperature differences to cause thermal stresses that might split the ice into pieces, so I suggest that it just gets cold with nothing exciting to look at.

When your finger touches very cold steel, it rapidly looses heat to the steel until the temperature at the surface of your skin is 0 C or below.  Any moisture on your skin would freeze, again my assumption, I assume there is normally enough moisture to freeze and stick the skin to the steel.  Ice on warm steel is slippery due to the contact area being lubricated by a thin film of liquid or even vapour, but I don't know much about the mechanism for sticking, but my observation is that ice sticks to the metal or plastic tray in the freezer in the kitchen, so I assume that your finger sticks in a similar manner.

A bit more analysis of my test run.  Yesterday I showed how I use a VLOOKUP function to find steam properties in a table.  Those figures are generally on a 'per kg' basis, so that has to be multiplied by the mass of water in the boiler to get the amount of heat absorbed.  The rest of the calculations are mainly simple multiplication or addition, so I will not go into detail unless anyone needs it.  I have recorded the measured water mass in a row above the main calculations.  As long as I use an absolute reference, i.e.use those $ signs, then any formula can pick up that value from anywhere in the spreadsheet.  I also include data for the mass and specific heat of copper in the boiler, and an assumed temperature difference between the copper and the water to enable heat transfer.  This is a significant difference from an electric boiler, where there is minimum heat transfer through the insulated shell so it is reasonable to assume no temperature difference.

I generally find that many of the values in the first row are initial conditions, like ambient temperature, and it is the second row where the calculations begin.  So on the spreadsheet I attached yesterday, the second row has the calculations for changes in the various properties through to the heat stored in each temperature interval.  Once this row is completed, the whole row can be copied, and pasted down the rest of the table and the whole calculation is complete.  Finally, the spreadsheet is able to plot a graph of any selected properties.  The chart type should normally be a scatter graph in spreadsheet terminology, plotting a different variable on each axis seems a foreign concept to spreadsheet developers, who seem to favour pie charts or columns for comparison.

I have attached a picture of the graphical output today.  Ignore the cooling curve for the moment.  The heat up curve has a few interesting features, and it is worth considering what these features might mean.  While the temperature-time graph appears to make a very smooth line, the energy calculations seem to amplify those little time variations between the temperature steps.  On the temperature -time curve, the points seem to vary very slightly each side of the smooth line much like they do on a hand plot.  But the energy curve seems to amplify these variations.  I have carefully checked the calculations, and there may yet be an error I have not found, but it does seem to agree with the recorded time steps and if anything, it is the smooth line in the temperature-time graph that is the anomaly.

Looking at the stored energy graph for the heat up period, my current interpretation is that the first time interval might be in error if I did not start the timer quick enough after I lit the burner, less than ten seconds longer, hardy visible on the temperature-time curve, would make the first step fall into the same line as the next few.  I will get a better idea when I repeat the test a few times.

Then at the 4 minute mark, the rate of heating seems to increase.  This is about the point where those blue tongues of flame from the burner become apparent.  I am thinking that might explain a higher heating rate.  Again, a few repeat runs will tell if this continues to happen.

Finally, the heat up rate falls right off.  This seems to start at about 100 C, when steaming would start in an open boiler, and sure enough this is also where those steam leaks I mentioned appeared.  Obviously heat is lost with that steam that I have not accounted for.  I fixed the steam leaks, fingers crossed, so that should line out on future tests.

One more very interesting point that turned up.  I calculated the heat rate from the burner assuming that the fuel consumption was uniform with time.  I don't think this is totally true, but I don't yet have the means to measure the rate at intermediate times.  So I calculated  the average heat release per second for the whole run based on Meths LCV 26,000 KJ/kg, and the measured 25 g of fuel poured into the burner, and 17 minutes 41 seconds from light up to flame out, as 615 watts.  I used this to calculate the heat input during each time interval compared with the heat stored in the copper and water.  Again the marvels of using a spreadsheet.  I would never have spent the time to do that by hand.  It looks like about 40% of the available heat is taken up by the copper and the water.  The rest is lost in the hot combustion gases leaving the boiler.

That is enough to think about for one day, I will talk about the steaming period next time.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 15, 2018, 02:44:57 PM
Hi MJM , Interesting stuff going on with the ice... I am not trying to test your knowledge but asking for a fuller practical knowledge of what is actually happening !!... To get an idea of the meths consumption you could have the meths burner sitting on some sort of high resolution weight scales !! (just to add to the confusion)  On my graph where the heat rate falls off ,The red curve, this was also due to the steam escaping from leaking glands, and after tightening them up the line then returned to being linear again (The green line).   good to see where these experiments are heading.....Also I don't quite understand the   Heat up -energy stored graph  ??
Willy.
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 16, 2018, 09:57:16 AM
Hi Willy, so long as my explanations make sense.  The important thing is to understand the physics behind the explanation, not just take a grab bag of miscellaneous facts to memorise.  Please let me know any time I need to have another try.  That atomic theory of matter explains a multitude of otherwise puzzling observations.

With the Meths burner, I have been thinking of boring holes through the burner channel of the base so the burner could be supported on columns off a digital scale underneath.  First I need a scale with 0.1 g resolution.  I don't think the current 1 g resolution is good enough.

Your graph with the effect of steam leakage so clear, is what prompted me to record multiple readings through the heat up.  There is so much more information that can be teased out with more data.

The heat up energy stored graph is a bit tricky, and as always it is difficult to know just what any graph of this sort is showing.  Is it highlighting experimental errors or detail of events?  The x axis is time and the y axis, stored energy, is the sum of the energy taken up by the water, and the energy taken up by the copper as it heats up.  That sum is divided by the time interval to give the heat rate in Joules/s or Watts.  That division seems to amplify the effect of variations in the time taken for each 10 degree interval.

I have carefully checked the calculations, but an error is always possible.  However, if I assume the data and calculations are correct, I then tend to assume a single reading out of line with the others could be the effect of a timing error or similar, while three in a row are probably showing something. 

On the graph I posted yesterday, I suspect the first point is a timing error, or some other startup effect, then comes two groups of three in a row that seem to be regular, and the change in gradient at about 4 minutes might be significant.  The burner seems to work like a conventional wick until about this time, then the tongues of blue flame appear at the holes on the two side rows.  This may reflect a change in the rate of heat release.  Finally the tail off seems consistent with the observed steam leakage, so is most likely due to heat not accounted for in the water or copper.

This is the basis of my explanations yesterday.  It remains to be seen whether these effects are seen in further runs.  I believe I fixed the steam leaks, so that much at least will be interesting to see.

Based on the total fuel burned, and the LCV, the average heat release is 612.1 Watts.  Quite a bit lower than your electric element, but probably appropriate for such a small boiler.  It certainly runs my little single and double acting oscillating engines (wobblers) quite well.

I did two extra runs in the period before my holiday, but on one, I got carried away with the technology, and cleared the iPad stop watch data in preparation for the cool down.  Unfortunately, I had not written down the times.  Whoops!  It was not even necessary, the iPad had plenty of capacity to record all the additional times I wanted without clearing the data between heat up and cool down.  But I got a track of the cool down anyway.  When I get through the analysis of this one, I will get out my notes and see how they compare.

When the boiler got to 110 deg C, I drained all the condensate, and the engine started as soon as I moved it off top dead centre.  It ran for 10 min 21 secs at a speed which varied a bit around 980 rpm.  I extracted 30 g of water from the boiler after it cooled down and as I started with 139 g of water in the boiler I assume that 109 g were evaporated as steam.  Averaged over 621 seconds, that is 0.176 g/s.  The boiler temperature stayed pretty constant at 110, while the superheater outlet/engine inlet varied a few degrees each side of 135, so I took that as the superheater outlet temperature.

Now the saturation pressure for 110 deg is 143 kPa ( so 43 kPa gauge or 6 psig).  The steam tables tell us that hfg, the heat necessary to evaporate water from 110 C to dry steam at 110, is 2230.2 KJ/kg.  It takes a little interpolation of the superheat tables to get the enthalpy change in the superheater, but I calculated it to be 51 KJ/kg.  Add those together and multiply by the steam flow rate and we get 400.4 J/s absorbed making steam. 

Now, remember the burner average heat release was 612 J/s, so 65% of the heat from the fuel was absorbed by the steam. 

Again, remember the heat up period, the heat absorbed was only 40% of the heat released.  Again it is interesting to contemplate the reasons for this difference.  Part of it will almost certainly be explained by the slow burner startup.  If less than average fuel was burned during those first 7 minutes, then more than average fuel was burned during the ten minutes steaming time.  Those differences would bring the percentage absorbed in each phase closer together.  I don't know if it is enough to explain the difference.

As a first step, if we calculate the total heat release from the fuel burned, we get 0.025 x 26,000 = 650 kj.  The total heat absorbed by the steam is 248.7 kJ, and the total by water and copper during heat up is 76.6 kJ.  248.7 + 76.6 = 325.3 kJ which is right on 50 % of the heat released in the burner.

Without a specific measurement to tell the proportion of the heat release during heat up compared with during steaming, it is perhaps best to take this overall figure  of 50% as the best estimate of the efficiency of this little boiler.

That seems like a good place to stop for the day.  I hope it has been of interest.  Tomorrow I will look at the cooling test.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 17, 2018, 01:28:50 AM
Hi MJM, more good info here and this is me with the thermometer reading my alleged temp ....also lots of info on the net about efficiency of the Lancashire boilers,  however some say 65 plus %  and others up  to 85%.. To test the efficiency do you have to let it actually power an engine under load for a period of time?
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 17, 2018, 11:25:37 AM
Hi Willy, that thermometer certainly looks suspect, as you obviously look healthy enough.  I assume you had it under your tongue for the usual time to let it reach equilibrium.  How did it go for zero in a well mixed ice-water mixture?  And again for 100 in boiling water?  The best way to try for boiling point might be to use your nicely insulated electric boiler without the plug, so boiling is determined by atmospheric pressure (don't forget to read the barometer).  You could try the thermocouple in the filler opening for a few minutes, and also compare it with your test position under the insulation.

It seems a bit anomalous that to test a boiler efficiency, it is helpful to run an engine.  The engine tends to meter out the steam production and maintain a steady pressure.  If you do another run with a load on the engine, you will get a higher pressure. 

Obviously, the steam production is the period we are most interested in.  In addition, during steaming, the temperatures are all constant, compared with heat up when the metal and water temperatures are continually changing.  More subtly, the changes to water temperature changes the heat transfer coefficient a little, but there is a major change of heat transfer coefficient on the water side when boiling starts.  Remember the earlier discussion on heat transfer, there is a factor of about 10:1 in heat transfer coefficient when boiling begins.  This means the copper temperature is much closer to the water temperature, which in principal increases the temperature difference to flue gas.  I am not sure how important this is, particularly at the burner end of flue tubes, where the temperature difference is quite high.  However at the stack end, where the temperature difference is expected to be much lower, it may significant, though not so important in this little horizontal pot boiler where the flue gas flows across the boiler.  In addition, the burner output would be expected to be more uniform at a long term rate than during the initial heat up.  Clearly you would not expect the efficiency of this boiler to match those Lancaster boilers with their multiple flue tubes in addition to the outside of the shell.

I know that I said I would look at the cooling curve today, but when I thought a bit more I decided that it would make more sense to put the heat up results from my third run into the spreadsheet, so they can be compared with the first.  Please don't mention the second!

The spreadsheet really comes to the fore now.  No need to look up any more steam table values, the table already there is still easily accessed by exactly the same formula as for the first run.  A small adjustment to the formula to access the location of the basic data for the fuel and water fill and water removed at the end, and the main job is to simply insert the time and temperature data in the same columns of the rows below the previous data.  Then just copy the first row of formulae and paste them down for enough rows.  Then you can go direct to the graph function.

I have moved round the graphs so the ones for the first and third runs are one above the other with the same scales for easy comparison. 

Once again the temperature rise with time seems quite as expected.  Still a little bit of a slow down after 100 degrees, but no where as much as in the first run.  And interestingly, the graph of heat stored per second has much the same form as the first run.  The high rate in the first minute shows clearly again.  By chance, I had two quick readings in that first minute, and they show a level of consistency that indicates that feature means something.  I took all the readings about two weeks ago, and am only now getting to analyse them, so the only difference between the runs is practice, as I did not do any analysis to identify anything I should look for between the runs.

I am starting to feel that initial high reading, this time two readings, are telling us something.  I wonder if the burner initially flares a bit, then settles to its warm up.  I will have to conduct more runs and watch that burner more closely.  Perhaps I can set up the camera to take a photo every 10 seconds or so, to better understand its behaviour.

Then again we have two sections with a slightly different gradient, the gradient change at about 4 1/2 minutes compared with 4 minutes the first time.  Again the appearance of a similar behaviour tends to confirm that this is a significant feature.

Finally, a bit of a tail off after the temperature reaches 100.  Not as much as the first time, but still there.  My observation was that the steam leaks at the pipe fittings had been fixed, however the only shut off valve I am using is the port face of the engine, which I set at top dead centre.  It would not be surprising if that leaked slightly.  Also, at some stage around that point, I blew a bit of steam through that drain bypass on the engine, to clear the initial condensate.  Pity I did not note just when I did it.  But I think you can see, with the small boiler, heat up was pretty quick, and with all those readings it all seemed to be happening at once.  I tend to be a bit slow in getting it all recorded in those circumstances.  But clearly less tail off than the first run, and the temperature of 110 deg C was reached a little sooner than the first run.

Then the engine started.  It ran for 11 min 29 seconds before the flame visibly died, and was extinguished about 10 seconds later.  I will talk about that part tomorrow.

I hope the pictures are clear enough, a hand held iPhone does not give the same sharpness as a scanner, but all I have at the moment.  But the form of the lines is easy to see and compare

Thanks for following along.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 18, 2018, 01:31:09 AM
Hi MJM, i suppose with a naked flame  lots of other things can affect it ,like fido waging his tail....mother in law leaving the front door open....the cat finding a warmer place to  snuggle up to ...!! lots of things that don't ever appear in the formulas. I suppose laboratory situations can nullify extraneous effects like these, costly and quite time consuming. I don't have a fridge at home so may have to do the zero centigrade in the local cafe.. but could do the boiling water at home !! Good progress with the graphs btw....

Title: Re: Talking Thermodynamics
Post by: MJM460 on January 18, 2018, 11:25:31 AM
Hi Willy, I think you are on the right track, thinking about wafting air currents.  Not the mother in law, dog or cat, but the front and back door were open, allowing beautifully gentle breezes to waft through.  I had not thought of it as affecting the burner and heat up, but wait until you see the cooling curves.  I suspect the effect is small compared with the heat release of the burner, but it played havoc with the cooling, where the heat rates are much smaller, so a huge percentage change.

Sorry you don't have a fridge, but if the cafe will provide a glass of icy water and ice blocks with your coffee, that will do.  But every engineer needs a fridge for calibrating thermometers.  It even comes in handy for keeping milk, butter and meat if you can't think of anything else.

Short post tonight.  My calculations for the steam production and overall efficiency did not stand some basic checking, I am skipping a line somewhere on this tiny screen.  The heat up curves are fine, it is the overall efficiency and the steam production energy balance that has the problem.  Oh for a real computer with a keyboard instead of losing half the screen.  Time ran out on me to find the problem and correct it, so tomorrow is checking, and I hope back on track. 

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 18, 2018, 11:49:16 PM
hi MJM ,  ok cool.....here is melting ice and boiling water on my temp gauge , the ambient being 17 degrees .... Also ,about  Meths....if you leave it to evaporate what is left in the container ??....
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 19, 2018, 11:55:36 AM
Hi Willy, there definitely seems something strange about that thermometer.  It looks like about 3 deg low at 100, but 2 deg low at 37.  Then 3 deg high at zero.  You could draw a calibration curve through those three, but I am not sure that it would be right. 

I have searched through some old notes and found my calibration notes from two separate occasions.  I had two different thermocouples and meters, and compared the two both times.  I now have three more thermocouples, and two meters, one of which has inputs for two thermocouples, so it's time I tried again.  However, from the notes I have, the first time I read 99 degrees at boiling, and 1 degree at the ice point.  I should have quit at that point, but I kept wondering if I could eliminate those 1 degree differences.  The next time, I managed to get readings of 1 deg C at the ice point, but unfortunately I could not get so close to boiling the second time and have recordings of 95 and 97 the second time.  I have also noted that I tried switching the thermocouples but got even worse results.  I noted that it took 3 to 5 minutes of careful maintaining the temperature and stirring to get stable readings from an instrument initially at room temperature.  I seem to remember giving up in disgust, with the intention of trying again another day.  I did use an insulated cup for the ice point and improvised a lid with a folded tea towel, in the attempt to minimise any temperature gradients in the ice-water mixture.  I suspect I mainly proved that the experimental technique is important, and I seem to have done better the first time.  Did not think of a photo, as that all happened before I found this forum. 

However, all the attempts help you understand a little better just what is required to get a good reading, and the time it takes to come to a steady reading.  It also gives some credence to those specifications that often come with the meter.  I expect, (or is it hope?) that differences between readings on a specific meter might be more accurate than the absolute values displayed.  And it is also a warning not to place too much faith in the accuracy.

One other point, did you try comparing the thermocouple with that good glass thermometer you have?

Meths is a mixture of ethanol with 5% water.  When it evaporates, the vapour is mostly ethanol but with a small amount of water, and in principal there is nothing left in the container at the end.  However I think you mentioned that your Meths is dyed blue, so there may be traces of the dye staining the container at the end.  I am not sure what happens to the flavouring they put in to discourage drinking, but the quantity may be too small to easily see the residue if it remains.  Our Meths is clear, and I don't notice anything remaining when it evaporates.

It was 42 here today, the second 40+ in a row, and yes we are a metric country, so not great progress today.  Must have been diabolical at the tennis.

However I did manage to check the calculations for heat up and steaming phases of my boiler tests, and corrected the problem that was worrying me yesterday, so I am happy that they are now correct within the limitations of the data accuracy, an important point considering our discussion on calibration. 

The third run, I found slightly higher efficiency for heat transfer during heat up and lower efficiency during steaming than the first run.  As I have mentioned before, I don't have data on how the burner heat release varies throughout the run.  So a figure for the overall test, heat absorbed during heat up and steaming is probably the most useful one.  The first run gave an efficiency of 49%, while the third gave only 45%.  Interesting that the third run for which I had fixed the steam leaks, gave lower efficiency, even though I had the impression that I had much less loss in terms of steam leaks.  An awful shame that I mucked up the second run, a third would be quite useful at this stage.  But it gives some idea of the repeatability of these tests.

I will try and produce the graphs for the cooling tests tomorrow.  The weather forecast is for below 30 tomorrow, so should be more comfortable.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on January 20, 2018, 10:39:33 AM
Quite a big day for us here.  Fortunately not as hot as yesterday, but a big day for us, our fiftieth anniversary.  Celebrated with a lovely meal at a local restaurant.  Not easy for us with dietary requirements.  With a low fat for one, and Fodmap friendly for the other, most places do an abysmal job.  But tonight's meal was just perfect for us both, and really excellent.  Just shows what a real chef can do.

I know there are others who have reached this milestone recently or are fast approaching it, so it's not a unique event in this group, more a matter of joining the club.  Another great thing about being a member of this forum.

In a quiet break, I even managed to get those cooling curves sorted.  You might remember that I did three test firings in my small Meths fired pot boiler, recording the temperatures during heating and cooling in the excellent manner Willy demonstrated earlier. 

With a fired boiler, we don't have that highly predictable heat input of the electric element in an electrically heated boiler.  Instead the boiler heat up is highly influenced by the rate of fuel burning in the burner.  So the heat up and steaming measurements are as much a test of the burner as a test of the boiler.

The cooling test is a little different.  On Willy's electric boiler, the cooling test was a very good demonstration of the effectiveness of the boiler insulation.  This was done by comparing the cooling curves with and without insulation.  However on a fired boiler, we can't insulate the outer shell.  Well, no more than the boiler ends, which are outside the firebox anyway.    And the difference in cooling times between this little uninsulated boiler and Willy's nicely insulated electric boiler shows how much difference the insulation makes. 

Really, I am not yet sure what the cooling tests on a fired boiler actually show, I am still thinking about that.  However, the times and temperatures were recorded, and the calculations done, and I have attached the graphs below.

There are four graphs.  The time temperature graph is exactly as expected and follows the form predicted by Newton's law of cooling.  A very similar form is seen if I construct a graph of the remaining stored heat as cooling proceeds, as you would expect.  Note that between heat up and cooling, the boiler steamed for a period so the stored heat at the start of the cool down is only due to the copper and the remaining water.  I weighed the water I extracted with a syringe after the cooling was complete to determine the remaining water after steam production.  Subtracting this from the initial fill amount also tells the quantity of steam produced.

However, when I calculated that heat loss as Joule per second, and plotted that value, the variations are amplified and the graph is quite erratic.  The fourth graph is a simplified cooling in Joules per second, plotting only one point every five degrees instead of every degree.  This certainly smoothed the curve by averaging things out. But based on every degree, when I put in the connecting lines they look like the stitches from a very poorly adjusted zig-zag sewing machine.

I leave you with those tonight and will look at them more closely tomorrow.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on January 21, 2018, 12:03:46 PM
While I was at it, I put in the time and temperature data for the second and third run that I had recorded when I did those heat up runs.  Copied the formulae from the completed ongoing curve for run 1, and pasted them in to the rows for the additional data.

Then all that remains is to construct the additional graphs, and the three cooling runs can be compared side by side.  So today's picture has runs 2 and 3 to sit beside the ones for run 1 in yesterday's post.  At least, I hope it is attached.  Every time I send one of these pictures to myself, the iPhone offers me a different three file sizes.  The smallest ones appear a bit fuzzy so I have been selecting the largest one that fits in the forum limit.  Unfortunately, having offered me a specific size, when I open the received mail, it tells me a different size.  Very frustrating.

The runs are not quite the same.  It turns out that I only recorded the times at 5 degree intervals.  I think I found recording every one degree a bit too intense.  At that stage, I had not done any of the analysis, so did not know about the variation in the heat rate on a per second basis.

The other difference, for runs 2 and 3, is that I was organised enough to start recording at 100 deg.  Seemed like a good idea at the time, however, as you can see, it seems to have introduced more irregularities, ones that were missed in run 1 when I started at 80 degrees.  More to try and understand.

I have had some time to look at the calculations and the actual numbers.  The burner heat release rate for the two heat up tests were 612 Watts and 590 Watts.  Interesting variation in successive runs for a single burner, but probably within tolerance for 25 g of fuel weighed on a scale with a resolution of whole grams.  Like other tests, it is obvious that repeated runs are desirable to get an idea of the variability of the results.  The curves which rely on these figures seem quite uniform and predictable.

For the cooling tests, the cooling rate starts off at about 20 Watts, and reduce with reducing temperature to about 3 Watts at 30 degrees.  The ambient temperature is 22 deg.  This difference in the absolute size of the heat transfer rate is probably the main reason the cooling curves seem so erratic.

I mentioned earlier, that I had the boiler set up in the doorway to the outdoors, where I was getting very welcome light breezes making the day very pleasant.  I did not think of these same breezes affecting my test results.  But it makes sense that with the boiler at 70-80 degrees, the breezes might affect that cooling rate by a few watts, a big percentage of the nominal 15-20 Watts.  While when the boiler has cooled down, the cooling effect of the breezes might be less significant.  Clearly the next set of tests need to be conducted is still air.  However recording 5 degree intervals does average things out a bit.

In the later two cooling tests, starting at 100 clearly introduced something new.  The expected result is a more rapid cooling between 100 and 95 that we expect between 80 and 75. Yet this was not the result.  Clearly some more thinking is required.  And almost certainly more testing required.

I am still thinking about the practical implications of these cooling curves.  There is not much point in thinking too much about them if there is no practical outcome.  The heat up and steaming tests however, potentially give us a method to predict the steam output of a larger or smaller similar boiler, and also a method to compare the efficiency of different boiler designs.  Clearly a useful outcome.  So tomorrow, I will have a go at calculating a performance parameter.

Thanks for looking in,

MJM460



Title: Re: Talking Thermodynamics
Post by: paul gough on January 22, 2018, 08:47:06 AM
HI MJM, Back early, sent you a P.M. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 22, 2018, 11:15:37 AM
Hi Paul, good to have you back.  I have sent you an email. 

Some time back, when thinking about comparative boiler ratings, I had a look at the parameter K. N. Harris used, cu. in./min/100 sq.in.  I suggested that a logical metric equivalent might be kg/hr.m^2.  So what would be the value of that parameter for the little pot boiler I have been testing? 

The boiler shell is 38 mm o.d. and 150 mm long.  However, only 130 mm is in the firebox exposed to the combustion gases.  The remaining 10 mm of the shell each end and the boiler ends are not exposed to the flames.  In addition, it is usual to assume only half of the shell area, recognising that that is about the average portion of the shell with boiling water on the inside, where the film coefficient is very high compared with the steam space, where the inside coefficient is very small.  On this basis the boiler heating surface area is 0.00776 m^2.

The steam raised was 0.153 g/s.  If we divide by 1000 to get kg, and multiply by 3600 we get 0.55 kg/hr.  Now divide by the heating area in m^2, and our parameter is 71 kg/hr.m^2.

This is quite a convenient figure, neither too large or too small, for comparison with other boilers. 

Of course, you can immediately see the deficiency of this parameter, the steam rate was determined as much by the heat release of the burner as by the boiler design.  Now I call that burner my 50 mm size, as the burner is 50 mm long.  It has a capacity of 25 g of Meths with the square tank, and the heat test runs so far indicate a heat rate of approximately 600 watts.  The test result calculations I have been describing the last few posts indicate that about 50% of this heat can be accounted for in the heat required to heat the boiler and its contents and the heat absorbed by the steam production.   Now I do have a second burner, a 90 mm size, with obviously higher heat output.  At the first opportunity, I will do some runs with that one. 

You can see that this simple performance parameter probably mainly applies to coal fired boilers, where the grate size is reasonably defined in relation to the boiler type and size, and it is probably reasonable to assume a predictable relationship between grate area and heat release from the coal burned.

The next step is to do similar tests on a different boiler.  I have a second boiler, fitted up with a slide valve engine.  It was a busy few days between Christmas and New Year, and I have my readings from that one as well, ready for analysis.  I am getting lots of practice at copying and pasting those formulae in the spreadsheet.  Saves an awful lot of calculations.

The further I go with looking at this little boiler, the more I find myself in Paul's position with those little locos, such a simple plant, yet so much scope for experiment.  But a second boiler will be interesting to compare.

Still thinking about what can be learned from those cooling curves.  Stay tuned.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 23, 2018, 12:44:51 AM
Hi MJM, I don't quite understand the graphs on the right .? could you explain them as they don't seem to follow the cool down at all...Thanks
Title: Re: Talking Thermodynamics
Post by: paul gough on January 23, 2018, 06:10:58 AM
Hi MJM, Had a little bit of time to myself and took the opportunity to quickly look at a couple of your cooling graphs. I for one am interested and pleased you are diligently delving into cooling rates, these may well prove to be valuable in working out a standard method to assess heat losses and insulation performance. This is an area that is often given scant attention as many model operators only care about the engine running to their satisfaction and often having the view that a boiler is a somewhat inconvenient accessory with the only requirement that it makes enough steam. It has been said to me that insulation is irrelevant in gauge 1, this statement by a very experienced builder/operator. I don't hold to this as an absolute, it depends on what you regard as important.

Now a question, looking at run 2 and 3 cool down graphs, reply 671, I note a change in slope of the line from about 60 degrees on both graphs despite one being much more of a straight line than the other. Do you think this is showing something or just an artefact from your method?

When ascertaining fuel or water consumption, are you taking into account the initial or residual wetting of the tank, piping and with fuel the wicks. What is used to wet things or what remains behind inside may be significant with such tiny volumes? To ascertain wetting volume just fill the equipment when absolutely dry with a known volume and then accurately measure the remaining fluid when emptied and subtract. I look forward to getting back in a couple of weeks to start catching up on all I have missed. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 23, 2018, 10:56:25 AM
Hi Paul, great to have you back, even briefly.  I think the cool down test is most interesting for boilers which have the ability to be insulated for at least part of the shell.  Willy's electric boiler is an obvious example, but any boiler mainly fired within a centre flue fire tube, can have insulation on the outer shell.  The usefulness of the test is more obscure for pot boilers with all or most of the shell exposed to the combustion gases. 

As I understand your gauge 1 locomotives, the boiler has an external firebox at the back end, but the water space is heated by the combustion gases passing through a centre flue.  In this case, the outside shell of the boiler is exposed to ambient air, which will obviously involve a heat loss, and could potentially benefit from insulation.  In addition to the shell, the firebox enclosure is another area for heat losses that could be reduced by insulation.  The cool down test might not help much with this, but it might be possible to see some effect in the heat up test, before steaming starts.

The heat loss to the atmosphere, is heat not available to raise steam.  The cooling test gives the opportunity to quantify the heat loss, so it can be compared with the heat release from the burner.  In fact, it should be compared with the heat absorbed in the steam, as it is not lost directly from the flue gas, it is lost after passing through the copper and the water.  If it is a very small proportion, it might be easier to turn up the burner than find space for insulation.  Obviously the limitations of scale appearance are also relevant.  And it is possible that having more water space might be considered more important.  But if you can fit a thin layer of timber or cork, is it worth while?  Cooling tests with and without insulation can quantify the difference.  Unfortunately there is diminishing returns in adding additional insulation.  For example, 2 mm of cork will not save twice as much heat as 1mm.  Cooling test data is probably the easiest way to answer this question also.

On the other hand, if you are trying to get efficiency, for an efficiency competition perhaps, then insulation is a good way to reduce losses, and hence increase efficiency.

I had given some thought to that initial wetting of the burner parts, I weigh the burner when it is clearly empty, and not been used for sometime, so any remaining fuel would have evaporated.  The quantity I use to fill the burner is selected to burn out before the boiler runs dry.  And I usually weigh again after the run to check that the fuel has all gone.  Possibly some of the fuel either evaporates from the hot burner, and certainly some is used in that final burn down between the steam temperature fall off and when the flame extinguishes.

For the water, unfortunately the whole steam plant including that MDF base is above the capacity of my scale.  So for the water, I weigh out the 130 g in a plastic jug and carefully pour it in with a funnel.  When the run is complete and the boiler has cooled down, I extract the remaining water with a tube on a syringe, and weigh the water extracted.  Obviously there may be a bit left that the syringe does not extract.  I think I should possibly pour in a measured amount to the really empty boiler, and immediately empty it with the syringe.  This would give an indication of how much might be left behind.  But in any case it would give me a more repeatable start point.

I have continued to think about those cooling tests on my little fired boiler, and doing some calculations.  I think I am getting something interesting if not entirely sure that it will be useful.  I will be travelling tomorrow, so no calculations, though I will almost certainly check in.  I also had a preliminary look at my second boiler.  So that is still on my list.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 23, 2018, 11:05:05 AM
Hi Paul, sorry, I missed answering your question.  Certainly a great observation.  I had noticed that the initial part of the cool down curve seemed to be showing something else happening.  Certainly in those tests that started at 100.  That slight inflection at about 60 degrees does seem to indicate a transition between different processes.  I am not really sure what it is about.  I am wondering if there is some water still evaporating, providing some heat to counterbalance the losses.  Because there is no regulator, perhaps something to do with air entry.  I need to pay more attention to the engine position during cool down to use the engine ports to better close off the boiler.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 25, 2018, 11:04:46 AM
Analysis of the cooling curve-

The cooling curve has the obvious use in exploring the effectiveness of insulation on those boilers which include insulation on a significant area of the shell.  However the question remains, is there anything else it can tell us, particularly on a fired boiler where there is minimum shell area available for insulation?

If you remember back to when we started looking at heat transfer and heat transfer coefficients, calculations are based on the basic equation, Q = U x A x delta T.  You will also remember that the equation is not as easy to use as it might look, as the coefficient U in particular, is not a constant, and it's value cannot easily be determined from theory except in a few specific cases.

The interesting thing about the cooling experiment is that we know three out of the four terms, Q, A, and the temperature difference.  Thus we can rearrange the equation to read
U = Q/(A x delta T)
The cooling experiment covers a range of known temperature differences.  Using the equation in this form, we can can calculate a value of U over that range.  The delight of using a spreadsheet is that the facility to copy and paste formulae makes easy work of these repetitive calculations.

There are two different approaches we can take to the calculations.  First we can look at the heat loss over each small temperature interval, and calculate the heat transfer coefficient at each temperature level.  Strictly, the temperature difference is changing throughout the interval, so I should really use the log mean temperature difference.  But I took a short cut, and took the average temperature difference.  It's a minor simplification that introduces a very small error when the temperature change is small.  Obviously the relevant temperature difference is the difference between the inside of the boiler and the ambient temperature.

The first cooling test started at 82 degrees, and the ambient temperature was 22.  The next reading was at 81 degrees, and my spreadsheet already had the heat loss between the 82 and 81.  I calculated the temperature difference as (82+81)/2 - 22 or 59.5.  Using the equation above, the heat transfer coefficient was 77.2

It is always a good principal to make sure that the units are consistent.   I converted the heat loss figure to kJ/hr.  Then using delta T in degrees C and heat transfer area in square meters, you can see the units are all consistent.  So the answer was 77.2 kJ/hr.m^2.C for this first time interval.

Then the formulae were copied, and pasted down the column to calculate the result for each interval down to a boiler temperature of 30 where my recording of the temperature and time ceased.

The spreadsheet is then used to very easily plot a graph of the results.  This is in the first attachment.  The x axis is the temperature difference between the boiler and ambient, while the y axis is the heat transfer coefficient.

You can see the graph reflects, or even amplifies the little errors in the experimental results.  It is also possible that the coefficient flows a relationship to a temperature difference of 40 degrees, or a boiler temperature of 62 degrees.  Then it possibly follows a slightly different relationship, a change that Paul noticed in the heat loss curves.  I am not sure of the meaning of this, whether it is significant, or just coincidence within the accuracy of the experimental method.

The equations were just as easily copied into the relevant rows for the second and third cooling tests.  These are shown in the second attachment.  Again, you can see that quote distinct change in behaviour above about 60 degree temperature difference, or boiler temperature of 80 degrees.

It is clear in all three graphs, that the heat transfer coefficient is not constant, but generally increases a small amount with increasing temperature.  It is tempting to extend the trend to higher temperatures to see what else this could tell us.  It is always a danger to extend an interpretation beyond the limits of the experimental data, called extrapolation.  Of we were to extend this to a temperature difference of say 200 or 300 degrees.  It would probably still be below 100.  However, there is always the possibility that those changes that have already been noticed, might influence the actual direction as the temperature difference increases, or something else might come into the picture.  However I will come back to explore that a bit further next time.

You will have noticed the reference lines, labelled "overall".  I mentioned earlier that there were two approaches to analysing the data.  I have already looked at analysing the data incrementally, that is, looking at each increment.  We have seen earlier that his does tend to amplify those little experimental errors.  The other approach is to look at the overall picture presented by the data.

What if we calculate a overall coefficient based on the cumulative heat loss from 82 degrees down to 30 degrees.  The temperature difference is sufficient that we had better calculate a log mean temperature difference, or LMTD.  Remember the method, in words,
LMTD = (Starting temperature difference - end temperature difference)/ln(Starting temperature difference / end temperature difference)
The function "ln" is the natural logarithm, or log to base e, rather than the log to base 10 that you might be more familiar with.

The LMTD for the first test was 25.5.  This compares with the average of 42.9, or the median value of 45.5, reflecting the fact that the time at low temperature intervals heavily influences the total heat transfer.  Then using that LMTD, I calculated the overall heat transfer coefficient of 67.0 degrees.  This is the value for the overall reference line and you can see how this lies a little below the average.

The LMTD is the relevant temperature difference to use when the temperatures are not uniform, whether due to changes with time as in this cooling experiment, or changing with position, for example the cooling of flue gas along a centre flue in a marine boiler.

I thought it worth checking those overall heat transfer coefficients with my heat transfer text book.  I rushed in a bit, choosing kJ/hr as the heat transfer rate units, I should have used Watts, as strictly, the unit of time in the SI system is the second.  Then I would have had units of Watts/m^2.K for the heat transfer coefficient, the units in the brief table in the text book.  Not to worry, to convert we multiply kJ by 1000 to get Joules, and divide by 3600, the number of seconds in an hour.  (Watt is the name given to J/sec in SI.). So if we divide numbers around 70 by 3.6, we get 19, right in the range of 5 to 25 the text book suggests for natural convection in air.  The upper limit of that range, 25 W/m^2.K is equivalent to 90 kJ/hr.m^2.K on my graphs.

Ok, so that was pretty heavy going.  But I am getting an idea of where it might take us.  Please don't hesitate to ask a question if any of this is not very clear, you will almost certainly be speaking for many.  I will explore the idea a bit further tomorrow.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: kvom on January 25, 2018, 05:03:21 PM
I took thermodynamics in college 49 years ago.  The textbook was shorter than this thread.   :Lol: :mischief:
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 26, 2018, 01:59:30 AM
Hi MJM,  still following along but the recent posts are getting a bit difficult to comprehend with all the equations but i am getting the gist of things.....so keep the info coming ....thanks ....
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 26, 2018, 10:29:51 AM
Hi Kvom, great to have you checking in.  I am still a long way short of either of the two books I generally refer to, and the heat transfer book is much bigger again.  Any sort of conversation is bound to be less concise than a typical text book.  However, it is not the length that is important, but whether it is interesting, logical and understandable.  So I hope you are finding something of interest in amongst all the words.  I am always willing to have another go if I have not been sufficiently clear, and questions or comments are always welcome.

Hi Willy, we have all missed you the last few days.  Sorry to hear about your shed.  I hope you have it all back together without too much loss.  Good to see your freelance engine moving again.  I am enjoying following your progress.

I like to include the key formulae so that if people want to do their own calculations, they can see what I have done.  But it is also important that I sufficiently well explain the results and my interpretation.  But of I leave too much out, please prompt me to fill in the gaps.

Last time, I did some analysis of the cooling curves for my little Meths fired pot boiler to see what might turn up.

I found that the heat transfer coefficient is roughly constant as the temperature difference reduced from 60 deg to 10 deg.  With constant heat transfer coefficient, the heat transferred is proportional to the temperature difference, and in the range 20 Watts down to 3 Watts for those temperature differences.  When I compare this with a heat transfer rate of around 400 Watts during steam raising, this clearly indicates that those low temperature differences are not very useful for our steam raising.  I remember some time back, Paul asking about the minimum useful temperature difference.  I think this test gives a more definitive answer to that question. 

In a refrigeration system, temperature differences are quite limited, so a large area is required for sufficient heat transfer, and systems do operate with these very low temperature differences. In our small boilers, where the practical area is limited, much higher temperature differences are required.  Burning more fuel to increase the temperature difference is a good way to increase the steam production rather than a small increase in area.

Are there any other ideas of what else can be learned from the cooling tests on a fired boiler? Or have we exhausted that topic?

Thanks for following along,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 26, 2018, 11:01:11 PM
Hi MJM , The shed is back together again.!! Here is a practical thermodynamic event you might like to talk about....not mine but something i saw at an exhibition!!!  Not a valid vimeo URL    so ..Heat input /loss / horsepower ,,etc etc etc...
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 27, 2018, 10:03:52 AM
Hi Willy, glad the shed as together again.  That video clip is a very interesting pendulum demonstration.  It is hard to tell from a 15 second clip, but I assume the pendulum bob is a magnet, and the bolt in the block of steel allows a fine position adjustment.  I must admit that I had to look up the Curie effect, it describes the way magnetism changes with temperature.  As temperature increases, the magnetisation decreases, or inversely proportional.

I assume it is all carefully set up so that as the magnetic bob approaches the steel bolt, the flame increases the temperature enough to reduce the magnetisation, so the bob is allowed to swing back.  On the back swing it cools and the magnetisation increases.  I imagine it would take quite a bit of experiment to get it all just right. 

It is not an area that I am familiar with, however my thermodynamics text book says work is done when the magnetisation changes, so it is that work that is just enough to overcome the friction and keep the pendulum swinging.

The text book also contains a little paragraph about systems involving work other than pressure, enthalpy, velocity and elevation.  It includes electrical energy, surface tension, strain in a wire, and also magnetic systems, etc.  Each time we analyse a system using the first law of thermodynamics, we should in principle include all these forms of work in order to make a complete statement of the first law.   However, in most practical problems, many of the forms of energy do not change, so they can be ignored.  Changes in velocity and elevation are often negligible, for example.  Similarly surface tension and magnetisation.

In the Curie pendulum, we have a case where velocity and elevation are both changing, there is friction in the pivot and due to air resistance, and the little flame changes the magnetisation to put enough energy into the system to overcome the friction and air resistance.  A delightful reminder that we must include all the relevant forms of energy in our statement of the first law, or conservation of energy.

As regarding efficiency, I imagine the efficiency is incredibly low.  Most of the energy released by the burning fuel will just heat air, and only a very small portion will be turned to work, but please don't ask me to put numbers on it, but the second law says there will be losses.

But it is interesting that magnetic work is used in very low temperature refrigeration systems, systems attempting to reach as near as possible to absolute zero.  But I don't have any experience with those systems.  Nor do I really understand how they work.  But magnetic forces are able to do much more than just keep a pendulum swinging.

A great diversion from some repetitive calculations that will give me a chance to continue the calculations to the point where I can compare my two boilers, and let you know the results.  Might be a couple of days, had a few interruptions today.  It is important to have a life apart from thermodynamics.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 28, 2018, 03:37:36 AM
Hi MJM, I did look it up on the web but as usual you have added more info to make it more interesting and relevant ...
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 28, 2018, 11:21:28 AM
Hi Willy,  keeping it interesting and relevant is the whole idea.  Glad I was able to add something, and that you enjoyed it.

Not much progress here today, another scorcher at 41 deg C.  I took my thermocouple outside at about 8:30 pm this evening, it was still reading 30.  Not expected to go down a lot more overnight.  Just as well we have an air conditioner in the bedroom.  Another hot one tomorrow with a cool change in the afternoon and guests for dinner, so may not have much to report tomorrow either.  However then we have a few cooler days coming.  Thank goodness.  I check in each day so I can start thinking on any questions, so any ideas on what next are welcome.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 29, 2018, 06:42:09 PM
Hi MJM I saw this in the "Engineer" magazine of 1887 and it shows a refrigeration boat using charcoal to insulate it ....So Charcoal is a fuel like wood and it reminded me of one of my brick workshops that had a woodburning stove, anyway after a cold winter i realised that i could insulate it with the wooden pallets rather than burning them ,there by using them to keep the place warm for ever rather than for a few hours, so my wood burning would be more economical !! So is there a figure for the insulation coefficient of crushed charcoal ?   ...here is the accompany  article...
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 30, 2018, 12:04:44 PM
Hi Willy, that sure is an interesting concept.  My text books don't include a figure for the thermal conductivity of charcoal, but has figures around 0.15 to 0.19 for various timbers varieties.  Cork is better at about 0.04, but it is in the range of other good insulation materials.  I had a bit of a search, but most of the information was about methods to increase the conductivity for use in adsorption refrigeration systems, not quite what we want in this case.

There was one reference from Finland, where it was mentioned that charcoal has a lower conductivity than timber, which they were using in a similar way to what you did.  But if there was a figure for charcoal, I missed it.  As I read of your experiments I was reminded of that saying about give a man a fish, similar to give him a piece of wood to burn and he is soon cold again, but show him....

I know that you asked about the insulating properties of charcoal, but the really interesting thing about that article was the refrigeration system, which used air as the refrigerant.  I have a special interest in the topic, as when I was a boy, I spent my holidays in my uncles orchard, picking apples for export to UK.  Lack of refrigerated ships was the limit to suitable varieties that would stand up to the trip.  I don't think apples would tolerate such low temperatures however, so that had to wait for more conventional refrigeration.

In that article, the air was pressurised to about 45 psig, and of course the hold and return pressure must be very close to atmospheric, so an absolute pressure ratio of about 4:1.  Did you notice that  after being cooled in the after cooler to something above sea temperature, the compressed air was cooled by expanding and doing work in the expansion cylinders mounted in tandem with the compression cylinders? 

A simple orifice, as used in a conventional refrigerator which condenses the refrigerant will not work, as expansion by throttling of air only gives a very small temperature drop, determined by the Joule Thompson coefficient.  But with an expander producing external work, the story is quite different.

With a pressure ratio of about 4:1 with air (the ratio of specific heats, Cp/C = 1.4) doing work by expansion gives a temperature ratio of 0.673, which has to be applied to the absolute temperature, and in principle, an ideal adiabatic expansion would give a temperature around -97 F, assuming the air could be cooled to about 27 degrees, but depending on the sea water temperature and how close the air was cooled.  So the claimed temperature, while probably a bit optimistic, was not totally out of range.  Obviously if the air is to be cooled to that temperature, the dew point must be a bit lower again, so the system does not block up with ice.  So the air system would have to be totally closed, and the makeup air (to compensate for any leakage) would have to go through a very effective dryer.

The expansion work would help with the work of compression, but as we now understand the second law of thermodynamics, we know it will never be enough to do all the compression. However it would reduce the power required to drive the compressor.  A bit of complexity, the expansion cylinder(s) and the necessary drier, and you have a very reliable safe system with free refrigerant, which does not require a separate evaporator with its attendant temperature loss.

The article seems to finish a sentence or two early, do you have the rest of the article?

Still doing the maths, with many interruptions, to get the results from the tests on my second boiler.  Should be able to compare results later in the week.

Thanks for dropping in.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 30, 2018, 03:42:51 PM
Hi, MJM,  here is the rest of the article  and i thought it might interest you !! It was in the "Engineer" 1887 .October 14th.... that you can look up in Graces Guide.  I am slowly reading through them all !!! quite slowly.............there may be some follow up articles later ....
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 31, 2018, 10:42:30 AM
Hi Willy,  thank you for posting the rest of that article.  It is quite an interesting machine that would make a great model with its compound expansion engine, air compressor and air expander all on the one base.  An engine with a real load built in.  Would be a great compliment to someone's ice cream machine.  Unfortunately, quite a bit above my current skill level, though I would like to think thatI will eventually get there. 

The valve gear looks interesting.  There would be no need for reversing, but it seems to have two eccentrics to the engine, one of those two part valves perhaps.  Then rotary valves for the air expander, and obviously the after cooler in there somewhere.  The compressor also seems to have driven valves, of some kind.  I don't know why it has that arrangement, but it seems to have a further four eccentrics!

It also gives me new admiration for the research that you and others put into figuring out the design of your historical models.  Unless there are more detailed sketches or drawings available, there would be a huge amount of work to just figure out how it all works. 

The calculations are going slowly.  Not that they are so hard, but the spreadsheet is getting a bit big, and there were a couple of hiccups on the iPad.  I wondered if it is too big, and decided to move it over to the computer.  Unfortunately I have a Windows machine, and don't have Numbers on it.  The iPad numbers spreadsheet does not move so easily.  Probably should have started there in the first place, but the iPad is so conveniently portable.  I am having to recheck all the formula, and redraw all the graphs.  That, and the inevitable interruptions for the medical  appointments that seem to come with advancing wisdom, this week anyway.  Time consuming and exhausting but at least all good results so far.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 31, 2018, 03:39:13 PM
HI MJM, Here is another engine that looks interesting... and is fuelled by coal tar...I have tried to get all the text in a reasonable size to read for you...this is from Graces Guide ...the Engineer 1888....Jan 27th...An early diesel engine ??!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 01, 2018, 11:29:25 AM
Hi Willy, I can see that you are getting full value out of that Graces Guide.  I assume you have found it in the Webb with the normal search?  Not had time to explore that yet.

It is indeed an interesting engine, internal combustion with regeneration.   With tar as a fuel, I imagine there would be a lot of cleaning out required. Interesting also that the write up looks at Carnot efficiency.  Really quite fascinating writing in that magazine, good technical content but very readable.  It will take quite a bit of study to follow the explanation completely.  Really amazing that the inventors were able to develop such a complex engine with the knowledge, data and calculating power available at the time.

It will be quite a challenge to follow the description right through, but unfortunately no time to do it at the moment.  I am having enough trouble making time for the calculations on my own engine tests.  Making progress, but I need to do some checking and construct the graphs.  Sorry not to have enough progress to report this evening.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 02, 2018, 11:34:52 AM
Well I have some initial results from the boiler tests on my second boiler. 

You will remember that the first boiler was just a simple pot type with only the lower part enclosed in the firebox, much like some of the little Mamod models. 

The second boiler is larger, made from 2" copper tube and a little longer, and is enclosed in a full furnace and firebox with a stack for the flue gases.  The furnace is not a work of art.  I made it out of an empty coffee tin from work, as a cheap way to try my skill at the sheet metal work before I started on more expensive material.  I also wanted to try the concept.  It has no external insulation.  I have purchased some manifold insulation from the car parts store, and I am in the process of bending that up to fit on the outside of the furnace.  It can only get better from there. You can see it in the first picture attached.  The boiler is fitted with extended bushes for the filler plug and safety valve and these are used to support the boiler from the top of the casing.   

Also in the photo, you can also see two burners, the 50 mm one I made for the small boiler, and the 90 mm one I made to get more heat input to this boiler.  Both use the same fuel tank.  The thermocouple, the thermowell that accommodates the thermocouple and also acts as a filler plug are all included in the photo.  It was quite warm still when I took the photo at about 8:30 pm!  The boiler is connected to my horizontal slide valve mill engine.  I am obviously still very much a beginner at the machining and engine building, but it works, so a satisfying early effort.

I removed the boiler from the casing for the second photo, so you can see the heating area details.  Instead of being a simple pot type, it has four 1/4 inch diameter longitudinal water tubes to increase the heating area.    The steam is taken from a bush in the centre of the boiler, using a banjo fitting, and the steam pipe, of 3/16 tube passes two full turns around the firebox as a superheater before exiting the firebox to the engine via the lubricator.  The pipe is insulated for much of its length outside the furnace with silicon tape.

I knew from previous runs that it made steam quite quickly, but only when I carefully measured every 10 degrees during heat up did I realise just how quickly.  First trial reached 100 deg C in 3 min 10 sec, and was steaming at 110 deg C in 3 min 30.  I was flat out getting the readings, and I only had to tap the iPad at each temperature, so the times could be written down later.

I tried a second time after it had all cooled down.  Spectacular but not a great success.    First two readings ok then I blinked and it was already over 60, and while I did a double take at that, it was at 100 in 2 min 30 seconds!  I am wondering if I had spilt a little Meths while filling the burner, and it flared up as the burner got going, thus increasing the heat rate for a short time.

It took until 3 min 20 sec to get to 110, but it turned out that the slide valve had not seated and the exhaust separator was blowing steam.  No really useful results from that run, it was too difficult to get the readings with sufficient timing accuracy.  I think the valve might be sticking due to the oil I was using, as it had not played up previously.  I now have some real steam oil which I want to try (after a thorough clean) before another run, and before thinning the nut a little so it lets the valve move a bit easier.

However, I did try a third run, but using the little 50 mm burner for a slower heat up.  I am interested to get steam production figures both ways.  Of course the lower heat input really restricts the steam production, so it's just an experiment.  The quick heat up is not an issue when you just want steam to run your engine.  It is better to have a higher steam rate, or higher pressure for the engine, but I am wondering if the smaller burner results in less proportion of the heat lost up the stack or boiler efficiency.

Basically the bigger burner puts out about 1800 watts compared with about 600 for the smaller burner.  Clearly the extra heat in the firebox affects the burner performance in addition to it just being a larger burner.  The boiler takes about 200 g of water at each fill compared with 130 in the smaller boiler, so the bigger boiler has more heat input per kg of water.  Similarly, the larger boiler has a mass of 770 g compared with 350 g for the smaller one.  So it is not surprising that larger boiler heats up more quickly, and produces more steam.

I have some graphs showing the heat up performance (just have to reduce them to size), and will calculate the performance parameters to compare the two boilers.  Hope to be able to produce these for tomorrow.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 03, 2018, 02:50:18 AM
Hi MJM,  lots of info there and lots of further experiments in the pipe line... would lengthening  or shortening the chimney have any effect on the way the burners perform ? Also has the temp gauge got two separate probes and what make /model is it and are they still available ?...
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 03, 2018, 11:20:52 AM
Hi Willy, the information has been a bit slow developing, you are seeing it in real time.  I made the mistake of trying a new spreadsheet program, Libre Office.  I am sure it will be good when I am more used to it, but it did not like the way the iPad spreadsheet, Numbers, handled graphs or times.  So I had a lot of conversion to do that would not have been necessary if I used it from the beginning.  But the iPad on the knee, while watching the Tele, is too much of a convenience to ignore.

That temperature instrument is a Digitech, model QM1601.  I bought it in Jaycar, and I expect they would still have them if I wanted another.  They are the local store for electronic components and other similar stuff, but I suspect they are mainly or only Aust and NZ.  As they do mail order, it may be worth looking them up.  But it is not a special brand and probably no better or worse than a similar instrument with another label.

Yes, it has two thermocouples, and the facility to read with a resolution of 0.1 deg.  Resolution is not the same as accuracy of course, but it helps me anticipate when the whole number will change, especially when things are happening rapidly.  With two thermocouples, it has the facility to read either one, or it can read the difference in the two temperatures, so quite useful for the current experiments and similar.  Last time I checked it at zero and 100 C it seemed pretty close, but I suppose I should do it again.

I have made some progress with the heating and steaming tests for my second boiler, the one I described yesterday.  I have attached two time-temperature graphs, one with the 90 mm burner I made for this boiler, and just to experiment with heat rate, I have included a run in the same boiler with the 50 mm burner I made for the simple pot boiler.  I have just noticed that I should have used the same scale for the time axis to make the two easier to compare.

You can see many similarities with the previous curves for the smaller boiler.  I think it is clear that the burner is a bit slow to warm up then takes off at a greater heat release rate, then the temperature rise rate falls of as the boiler starts to steam with that sticky slide valve.  Perhaps I need a proper isolation valve.  Then at least I could determine with some certainty when steaming actually starts. 

I had a look at the calculations for the heat absorbed in each temperature interval, but really they were all over the place, and difficult to make any sense of.  Yet the overall result and the temperature rise seemed reasonable.  So I have not tried to produce the graphs, as it does not seem likely that I can provide any reasonable explanation for the behaviour.

I also had a look at the overall parameters, the one we were discussing in relation to K.N. Harris's
Boiler parameter.  So first I calculated the heat transfer area for the second boiler.  As with the little pot boiler, I assumed half the shell area as having water or boiling on the inside and so much more effective than the top part for heat transfer.  Then I calculated the area of those four water tubes.  It is interesting that on small boilers like this, they make a big contribution to the heating area, and in fact nearly double the area compared with the bottom half of the shell.  The ends of the shell are inside the furnace enclosure so I included half their area as well.  So 0.0165 m^2 for the shell, plus 0.0136 m^2 for the four water tubes (1/4 " tube by 170 mm long), or 0.03 m^2 in total.  This compares with only 0.0078 m^2 for the simple boiler.

Well, with nearly four times the heating area, I wanted to compare the steam production.  I was easily able to calculate the average heat output from the burner fuel consumption, about 600 watts from the small 50 mm burner, and 1800 watts from the 90 mm burner.  Clearly the heat output is about more than just length, but then I did not set out to just lengthen the same design, the larger one is a bit wider and has different size and numbers of holes in the two side sections.  It is likely that the larger heat output also causes the burner to run a bit hotter, and hence to vaporise and burn more Meths.  In fact even the small burner, when used in the larger boiler, was nearer 630 watts, so a little more than when in the small boiler.

The steam production from the 90 mm burner in the larger boiler was 0.25 g/s or 0.86 kg/h (remembering that the steaming time was only 5 min).  But this is only about 50% more than from the smaller burner in the smaller boiler, despite three times the heat output.  The heat absorbed in the steam was only 570 watts of the 1800 released by the burner, so the boiler efficiency down to  around 30 %.  Clearly those water tubes did not contribute much to the heat transfer or efficiency.

It is interesting to compare this with the little burner in the simple boiler where the steam production was about 0.17 g/s in both the simple boiler, and in the larger water tube design.  In both cases the efficiency was nearer 60%.  Clearly again, all that extra area did not add much to the heat transfer.

Finally, I calculated that boiler parameter for each of the three cases. 

About 70 kg/hr.m^2 for the simple pot boiler.

Only 20 kg/hr.m^2 for the same 50 mm burner in the larger boiler firebox.

And 30 kg/hr.m^2 for the larger burner.

That is a lot of figures to take in.  With things happening so quickly, I did not get sufficiently accurate or sufficiently many readings to justify analysis in finer detail, though I will still look at the cooling curves.

More important to think about what it all means.  I will think about that for next time, I am starting to see some valuable learning possible, if I can sort these out and follow through with some further experiment.  You see, all the theory will help understand the direction, but will not tell you some of the basic answers, unless you perform a suitable experiment to confirm it.  To really understand things, you need some theory, and the theory needs to be supported by experiment.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 04, 2018, 03:25:53 AM
Hi MJM thanks for the new info and i will be seeing what else comes up with more tweaking ! meanwhile saw this from the Melbourne show....had me scratching my head.......
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 04, 2018, 11:47:03 AM
Hi Willy, the thermo-acoustic engine operates on a novel principle, though I have no idea how it works.  Someone has been very clever in converting from a physical principal to a working engine.  The Melbourne show!  I would really like to go, but I believe it is on one of our big holiday weekends, and I am nearly always out of town.  It would be a great opportunity to meet up with other forum members, I must check up when it is on, and see if it is at last possible to attend.  I am sure it is good to be so busy in retirement, but some times there are distressing time clashes.  Should have taken that gap year, but if I had, I would be up to a sabbatical by now.

I felt that first boiler, at only 38 mm diameter was really too small for anything other than demonstrating a small oscillating engine.  It was more a practice for silver soldering and forming those torispherical ends.   It actually does quite well to power a double acting one.  So I intended, as a next step, to just build a larger diameter, and perhaps a bit longer, of the same design.  However, I was told that the water tubes would make so much difference that I should skip that step and put in the water tubes.  When I had a look at the extra area, compared with the cylindrical shell, I went along with the suggestion, and included those four water tubes.

Now, looking at the details of those test results, I have to ask myself what happened?  The simple pot boiler seems to offer the highest steam production per unit area.  Even if I did not include the water tube area, it would not reach the steam production per unit area of the simple boiler.  With the same burner, I expected that it might heat up a bit quicker, which it indeed did, as the extra area could be expected to absorb more heat by cooling the flue gas to a lower temperature.  In fact, now that I look more closely, it did heat up much quicker, the larger boiler has more copper (770 g compared with 350 g), contains more water, 200 g compared with 130 g, yet heated to steaming temperature of 110 C in close to 5 minutes, instead of 7 min.  Yet this does not seem to be reflected in the steam production figures.  Perhaps I need to recheck those calculations, and explore that difference a bit further.

With the larger burner, I assumed I might get similar efficiency to the pot boiler, or even a bit more, especially with the full furnace enclosure.  But as usual, experiment does not follow the laws of intuition.

What really surprises me is that during steam production, the little pot boiler, without even a full enclosure, has significantly better efficiency than the larger boiler with water tubes.  The quicker heat up time is not matched by higher steam production.  I calculated a very similar efficiency on the early runs, back when the small boiler was first built, and did a much simpler test, so I think the difference is real.  I now need to have a look at the differences between the boilers and think about possible explanations for what is going on, preferably resulting in an understanding of how to improve the boiler design.

A big difference between the boilers is that furnace enclosure.  The little boiler has a simple firebox, but it is made from sheet stainless steel.  It not only has a lower thermal conductivity than the tin plated steel of the bigger boiler, it is still quite reflective, and does not seem to be blackening very much on the inside.  But it does not enclose the top of the boiler, it only extends up to the mid point of the shell.

The tin plate of larger boiler is not blackening badly, but it is not nearly as shiny as the stainless steel.  It also has plenty of air entry opening.  The little burner looks quite lost in the firebox so it is likely that it is getting too much excess air, absorbing a lot of heat, and lowering the temperature difference available for heat transfer.  The air entries were made for the larger burner.  In addition, that large tin plate casing may be absorbing more heat than the stainless, and consequently loosing more heat to the outside, especially for the small 50 mm burner which just sits at one end of the 230 mm long enclosure.  In addition, I placed the burner near the entry opening at the opposite end from the stack, so the flue gases could rise around the boiler, and pass along to the stack end, maximising the contact with the shell.  I wanted to avoid having the combustion gas take the shorter route around the shell to the stack if I placed the burner at the stack end.

With the larger burner, things appear more mysterious.  Possibly the losses from the casing are proportionately more, and in reality, the air holes are just a guess, basically just drilled holes along the full length each side.  I also made provision for some entry around a baffle at the stack end.  Possibly too much air, for even the larger burner.

The other thing I am thinking about is the size of those water tubes.  They are made from 1/4 inch diameter tubing.   I am wondering if perhaps these are too small, perhaps resulting in the water really boiling in the tube and expanding out into the boiler, pop-pop boat fashion, rather than heating strongly but flowing by the density difference in a more even manner.  This might explain the apparent improvement in heat transfer during heat up to get that shorter heat up time, but minimal if any extra steam production.  So making steam, then bubbling the steam through the water in the main shell, rather than promoting vigorous circulation.   

Unfortunately, the only way I can see to check this is to build a new boiler with larger tubes.  Though Ramon has shown us, in his wonderful Wide-awake build, that extensive rebuilding of a boiler is possible, I think it is probably worth building a new boiler.  Not that much extra work.  Still thinking about this.  In the long run, I suspect it will be well worth exploring whether larger diameter water tubes perform in a more satisfactory manner, providing I can source 5/16" tubing from one of our local suppliers.  I am not sure that I could bend 3/8 tubes to a sufficiently small radius.  Does anyone have ideas on the merits of larger diameter tubes? 

In the mean time, I am bending up some engine manifold insulation to make a layer of insulation on the boiler casing, a simple way to check if heat losses from the casing are important.  Horrible stuff to bend, the outer perforated metal layers separate from the insulating sheet when I bend it.  But I now have the roof section bent close enough, and drilled the holes for the bushings and the stack this afternoon.  Fortunately, I already had suitable size hole saws, and they worked quite well.  I will just cut flat pieces for the sides and ends, and pop rivet them all on.  I want quick rather than beautiful, until I source some thin stainless sheet and make a new casing.  Though I have a very different interpretation of quick to Chris and others.  I like some time in the day to sleep and eat, and exercise, etc.

Then, when I have some insulation on the shell, I will experiment with restricting some of those air holes.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 05, 2018, 02:15:57 AM
Hi MJM , I have seen other boilers with those tubes that come street out of the bottom and then curve round and go to the other end in a strait line to enter at an angle into the boiler. I think this might allow the flow to be better . I don't know in which direction  the flow would be ? . i don't know if your fairly strait tubes on your boiler would boil in the middle and try to flow in both directions at the same time ? I cannot quite see exactly how your tubes are arranged so they may be ok ...so more experimentation........
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 05, 2018, 11:55:51 AM
Hi Willy, your sketch is exactly right.   I will take another photo to show it better.  Thank you for posting that.  The water tubes extend 30 mm below the boiler at one end and only 20mm at the other.   I actually placed the burner at the low end, just as you have shown.

I think this was a mistake.  The lower end should act as down comers, that is, the water should descend at that end, flow along the rising portion of the tube as it heats, and finally return to the boiler at the higher end.  This means that gravity, and density gradients all work in the same direction.  But as you have shown, with the heater at the low end, it is heating that end more strongly, so it is expanding and trying to rise in that end.  The sloping middle section is receiving more gentle heat, but the resulting density gradient is actually trying to cause flow to the higher end, the opposite direction.  Not likely to work at its best.  I suspect the strong heat at the burner end wins, but the flow will not be as strong as if the density gradient in the centre section was helping the flow.

If I put the burner at the high end, opposite to your drawing, the steam will be rising at that end due to the lower density, and as the flue gases pass to the other end the cool so the down comers will see the coolest flue gas which is what is needed.

One option would be to put a baffle to shield the down comers from the heat of the burner, and push the burner in a little further, so as to apply more heat to the sloping section.  The other option is to turn around the boiler in the casing.

Fortunately, the boiler is symmetrical with respect to the bush positions, and the steam outlet at the centre has a banjo connection, so can easily be swung around to the other side, and the whole boiler can be put back in the casing with the low end of the water tubes at the stack end where I should have put it in the first place.  So when I get that insulation on the outside complete, I will reassemble it that way.

I also had a good look at the heat up calculations.  They do in fact support the expectation that the tubes should be providing better heat transfer, during the heating up anyway.  I want to recheck the steam calculations, then I will give you the results.  It seems the behaviour during steaming might be different to the heating up behaviour, but first, I want to recheck the calculations so I can give you firm figures.

I have been out to dinner this evening after a busy day of chores and ran out of time, but hope to make some more progress tomorrow.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 06, 2018, 11:38:33 AM
Made some progress today.  A little progress in the shop on insulating that furnace casing.  Yesterday, the  roof pieces were cut and drilled.  Today, I cut the material for the ends and one side and pop riveted them on.  A bit rough, in keeping with the tin can enclosure, but it will enable a trial, and hopefully show some benefit of the insulation.

I also had a fresh look at the spreadsheet.  I found a couple of minor errors and tidied it all up so I am happy with it now.  Time to try and form some conclusions.

The boiler performance parameter that I calculated the other day stood up to checking. I found a few  inconsistent units, so I will list them again.

Pot boiler run 1, steam production 39 W/m^2
            Run 3, steam production 35 W/m^2

Water tube boiler run 1, steam production 19 W/m^2, burner 90 mm
                           Run 2, steam production 20 W/m^2, burner 90 mm
                           run 3, steam production 14 W/m^2, burner 50 mm

Watts/m^2 is my suggested SI alternative to K. N. Harris' boiler parameter of cubic inches per minute per square foot.  Using watts allows calculation of the steam production for any steam conditions, though in reality the steam temperature will have a small effect by changing the temperature difference.  The difference in the figures for different runs that are otherwise similar gives an idea of the repeatability of the tests.  Obviously a larger number of tests is required to get a more reliable number.

Clearly the extra area in the water tube boiler did not result in proportionally more steam production.  Just for fun, I calculated the figures for the larger boiler, based on ignoring the area of the water tubes.  The result for the larger burner were 34 and 38 W/m^2, close enough to the same as the pot boiler with the small burner.  This is what is making me wonder if the tubes are contributing anything to steam production.  With the smaller burner, the figure was 24 W/m^2.  This would seem to indicate there was not enough heat available to increase the steam production in proportion to the area.  It is instead limited by the heat available.

Next, I compared the actual steam production.  The 50 mm burner produced close enough to 0.17 g/s, which ever boiler.  The 90 mm burner produced about 0.26 g/s, more than the 50 mm burner, but not in proportion to the extra heat release from the fuel burned.  The 50 mm burner was about 610 W, while the 90 mm burner released nearly 1800 Watts.  Three times the heat released, but only 50% more steam.

Finally, I looked at the boiler efficiency, that is, the proportion of heat release actually absorbed in heating water(plus copper) or generating steam.

For the potboiler, the heat absorbed during heat up is about 30%, while during steam raising, it was about 60%.  I suspect this difference is due to the higher film coefficient of the boiling liquid.  When this same burner was placed in the larger boiler with the water tubes, the efficiency during heat up was increased to over 60%, while the efficiency during steaming was unchanged, so still about 60%.  The higher efficiency during heat up was evident in the shorter heat up time.  But the extra area did not seem to result in higher efficiency during boiling and no significant increase in steam production.  Definitely some sort of anomaly.

The larger boiler and burner showed only a bit higher efficiency than the little burner in the pot boiler during heat up.  But it really did not show any increase in efficiency during steaming.  Not easy to guess what is happening, but the explanation that seems plausible is that those tubes are not contributing in the expected manner.

I don't know if any of that makes much sense.  I think the next step is to see if that insulation of the casing, and turning around the boiler make any difference.  I doubt that I will have anything new by tomorrow, but perhaps by the weekend. 

I am also planning to build a new boiler with 5/16 water tubes, but that will take longer.  I will keep you posted on the progress.


As promised yesterday, another photo of my boiler with water tubes.  I think this view shows a little better that the water tubes are sloped.  I tried a few other angles, but none were any clearer.

Thanks for looking in

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 06, 2018, 05:13:28 PM
Hi MJM, would it make any difference adjusting the height of the burner ??
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 07, 2018, 10:33:49 AM
Hi Willy, I suspect the height of the burner is quite important.  Too high, and there is not enough space to allow the combustion to proceed to completion before the gases are cooled by heat transfer to the boiler.  Not sure about too low, probably OK so long as the setting does not allow too much additional excess air.  The base really provides an insulated well for the burner which means it will not cause extra heat loss, though just creating. A higher firebox would of course result in extra heat loss to the atmosphere.  But it would raise the whole centre of gravity, and as I would like the boiler in the end to be suitable for a boat, it is not good for stability, a very different problem.

However, to allow experimentation on this, the base for the smaller boiler was made with a trench cut out of the base under the furnace and lined with tin plate to allow the burner to be set lower relative to the boiler for just this purpose.  I can pack it to intermediate heights with strips of wood.  My early simple tests were not sensitive enough to pick up much difference over the available height range.  I dropped that idea after a couple of indecisive tests, but I think the more detailed tests I have been inspired to try by seeing your efforts on your electric boiler, are more  sensitive, and so it is worth doing some more tests.  If it is successful, I could try the same scheme on the larger boiler.  But not before I test it again with the insulation and the down comers at the stack end.  I know that is two things altered, but I am sure that both are necessary, and should have been part of the original design, so overall improvement will be interesting without it being necessary to attribute a specific proportion to each factor.

I have also been thinking about the difference in performance between those two burners.  It seems that the small one is limited by its low heat release rate, while the larger one is limited by the area available for heat transfer (indicated by the low efficiency).  I did not manage to check the stack gas temperatures, it all happened too quickly for that, but I am thinking an intermediate size burner, say 70 mm long might produce more steam, but also achieve higher efficiency.  However, only after the tests with the boiler turned end for end and the insulation on the casing.

Turned around the superheater today so the boiler can be installed the other way around when the insulation is complete.  However it was 38 today and definitely too hot in the shed.  Exercise, shopping and picking up grand kids along with a similar temperature makes it unlikely that I will get much done tomorrow either.  But it is great to have the opportunity to stay close to the kids and grandkids.

Sketch up that new boiler or burner perhaps, if I am lucky.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 08, 2018, 11:38:55 AM
Hi Willy, I just had to go back and have another look at that Hargreaves Thermomotor you posted the other day.   You asked if it was an early Diesel.  As I understand it, the defining feature of the diesel is compression ignition, with fuel injection measured to give some approximation to a constant pressure expansion on the power stroke.  This engine is more like a piston version of the  cycle used in Gas Turbines, with hot tube (brick) ignition, a regenerator and heat exchangers to make it a true regenerative cycle.  Gas turbines have a separate combustion chamber, and are generally categorised as external combustion, where this one has the fuel injected into the power piston where it is ignited by hot surfaces.  However the other similarity with gas turbines is the separate cylinders for compression and expansion.  It has a few auxiliaries for startup and a rather confusing water injection and collection system that absorbs some of the heat of compression (may be a disadvantage to thermal efficiency) but then uses the hot water to cool the power piston, thus reducing loses in that area.  Possibly a zero sum game, but it's also possible that there are also subtle advantages that make it worth doing. It reduces the work required by the compressor, which absorbs a significant portion of the power from a gas turbine.  I really have not looked at that part so closely. 

But the heart of the machine is the air pump for compression (I would call it a compressor), the counter flow heat exchangers which recover exhaust heat to preheat the air prior to combustion, and the expansion piston which does the useful work including driving the compressor.  All up, quite an interesting concept.  It appears to have worked, in that working engines are reported, where as I understand that the more conventional simple reciprocating version of the gas turbine cycle are not considered very practical.  Amazing stuff hidden in that library of yours.

Just a little time in the shed today.  Managed to fix the remaining insulation surface.  A bit untidy, though some angle trim around all the corners would improve it no end.  I assume I could make that with tin plate, which might be more compatible with the metal faces of the insulation I used than aluminium or brass.  Expect to get the boiler reinstalled tomorrow, and the engine slide valve cleaned up, ready for some tests over the weekend.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 09, 2018, 11:25:43 AM
Not much to report today, however, between the inevitable shopping and taking the grandkids to cricket, I managed to refitt the boiler the right way around, shorten a union nut that was too long to seat properly, clean the valve, hopefully reseating properly now, all ready to try another test.  Tomorrow, I hope.  Also a photo.  It was too dark when we got home from the cricket.  The boy did quite well, hit a couple of fours, but brother and sister would have preferred to be home with screens.  It does not look too bad from the its best angle.  I am also thinking about how to fix a thermocouple in the stack to try and get a reliable stack temperature reading.

MJM460



Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 10, 2018, 01:15:09 AM
Hi MJM , Interesting stuff with the  Thermomotor and has our understanding of thermodynamics changed since then or have we learned lots of new stuff? If we built the same engine with modern materials would it be even more efficient ? or was it at a peak of current theory ??.........
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 10, 2018, 11:41:52 AM
Hi Willy, there is more to see in that Thermomotor every time I look at it.  Like many drawings of the time, it does not show the mechanism for the valve actuation.  I guess they were all very secretive, and wanted to hide the critical details from those who would copy their design and sell in competition.  The top cylinder, being a simple air compressor, would work with normal plate valves, with light springs to close them.  But then the power cylinder, the lower one, is an expander and needs valves to be mechanically opened in the same way as a steam engine, though because of the combustion, most of the pressure comes from combustion of fuel, and only enough air is admitted for combustion.  However, it seems to me that still requires active valve opening mechanism.  There seems to be a governor in one of the drawings, but I can't make out how it is linked to the fuel admission valve or air throttle valve.

The heat exchangers come close to true counter current flow, so quite good at heat recovery.  Even the beam is a little unusual to me, in that instead of a solid column, the flimsy looking column seems to rock, and instead of any Watts linkage, the piston end of the beam appears to be guided in a straight line, cross head style.

I don't think our understanding of thermodynamics has changed so much as we have learned lots more stuff.  As I have said before, I am in admiration of these early pioneers who achieved so much with intuition and trial and error.  I suggest lots of error was involved, but matched by their persistence.

I suggested that it was a bit like a gas turbine cycle, though shifting the combustion to inside the expansion cylinder means that most of the pressure comes from combustion, and the engine is not completely reliant on the compressor to provide all the pressure, as it does in the gas turbine.  This means that instead of a large portion of the expander work being used to drive the compressor, leaving only a little, if any, for external work, the high pressure comes from combustion with much less compression work required.  In that respect, it has similarities with the diesel cycle, as you originally suggested, even though it has separate compression and expansion cylinders, and all those heat exchangers to make it a regenerative cycle, in a way I am not sure is possible when compression and expansion occur in the same cylinder.

If we made one today, improvements in materials and pressure vessel technology would almost certainly allow for some more optimisation of operating conditions.  Similarly machining technology and sealing technology allow more accurate construction and less seal friction and leakage.  But I have no idea whether we could make it a huge amount better.  I suggest that the heat exchangers and regenerator would make it heavy and expensive for its power output.  So low fuel costs of our recent past mean there is little economic incentive to go to such lengths in the quest for efficiency.  As fuel costs rise, the economics change and perhaps we will see them reappear.

Well I tried two tests with my modified boiler arrangement.  What a difference!  I don't have any doubt that the water tubes are now working very well.  So much so, that they are producing plenty of steam, well before the water in the main shell reaches 100 degrees.  Clearly things are not in thermal equilibrium.  I tried exchanging the safety valve and filler plug/thermowell locations to put the measurement point over the down comers end, but this made no real difference.  Had trouble with the engine again.  I suspect partly the valve is still a little tight on the nut, and partly because the port face is vertical, Gravity is not helping to seat the valve.  At one point, the valve suddenly seated and the engine took off.  Obviously plenty of steam, and I was only using the small 50 mm burner.  Calculations will come, I hope tomorrow, as will the photographs, which I managed to take, but time ran out on me before I could download and resize them.

I definitely need a regulator valve, so I need to make one of those, and modify the steam pipe to accommodate it.  I think it would be an advantage to be able to retain the pressure high enough to inhibit boiling until the whole boiler capacity is up to temperature.

Similarly, the exhaust separator has not been working too well.  It was gooed up with a very sticky water-oil emulsion.  Clearly I am suffering the penalty for using normal lubricating oil.  I have cleaned it all out, and started using real steam oil, so we will see if that improves things. 

I don't know whether to install a screw in the valve chest cover so I can press the valve onto the seat before I start, or perhaps a light plate spring to hold the valve against the port face.  Any recommendations?

In the spirit of experimentation, not necessarily in gentle equilibrium, though I hope avoiding banging and crashing, I will try one run with the 90 mm burner if time allows, tomorrow or Monday.

Thanks for looking in,

MJM460


Title: Re: Talking Thermodynamics
Post by: MJM460 on February 11, 2018, 10:48:48 AM
Not the progress I hoped for today, but progress none the less.

Decided to try a run with the larger (90 mm) burner to compare with the 50 mm burner before drawing too many hard conclusions.

It was quite an adventure.  I am always surprised at how much I get out of these tests compared with just watching the engine run, even though that is always the first highlight with a new engine.  But knowing the actual temperatures at each location does say so much more about what is happening.  After the two runs with the 50 mm burner, I was not hopeful about the larger one.  Expected it might be more of the same, with the possibility of being a bit on the wild side.  When the temperature rises too rapidly, it is hard to get all the readings recorded before the next one comes up.  But once again, it was not what I expected.

Before starting, I removed the valve chest cover, now I know why I should have noticed that studs are preferred, (but my next engine was made with studs to hold the valve chest).  Carefully oiled the valve with steam oil, rotated the flywheel a few times to move the valve back and forth, and made sure it was pressed to the valve face, hoping the oil film would stick well enough while I replaced the cover, and tightened the cap screws.  I hope I can eventually solve that problem.  Strange it did not arise in earlier runs.  I am assuming a lot to do with lubrication, and gradual buildup of that sticky goo.

I filled the boiler with the normal quantity of water and the burner with Meths.  (I am starting to doubt the repeatability of the kitchen scale, the first sign that the battery is getting a bit flat.  A frustrating characteristic.  I am thinking of a new scale with 0.1 g resolution, and I hope better repeatability, but not today).

Switched on all the meters, set up the iPad as a timer, lit up the burner, and prepared myself for quick action.  At first nothing seemed to happen.  Then the temperature started to rise, just a little, and I could hear bubbles bursting at the surface.  Nothing dramatic, perhaps 3 or 4 a second and the temperature started to rise steadily in a very predictable manner, so I was able to take some sensible readings.  Nervous as it approached 80 deg, but it continued to near enough to 100 before a some steam leak was apparent.  When it reached 110, I flicked the flywheel and away it went.  The non-contact tachometer gave me readings of 1066 rpm.  I checked some temperatures on the outside of the insulation and the stack temperature.  It rose to over 300 degrees, obviously plenty of scope there for a feed water heater for heat recovery, (if I had a feed pump), or perhaps more area for steam generation.  I took readings of 1160 rpm then 1300 when a sudden noise from the engine let me know things had gone pear shaped.  Unfortunately, the crank pin had unscrewed itself, screwed up the run somewhat.  Nothing went well after that, but I adjusted the valve to let steam blow through and eventually the burner ran out of fuel.

Cool down was quite orderly, and I was able to take readings from 95 all the way down to 35, so more good data to look at on lower temperature differential heat transfer.

Very clearly, with the extra heat available, the water tubes worked as intended, and the whole boiler heated up, more evenly.  An interesting difference from heating with the smaller burner, but very clear on demonstrating the importance of which end the down comers are located.

I took some more temperature readings (with the infra red instrument) for the outside of the insulation.  It was still over 100 degrees over the top of the boiler, though in the 50's low on the sides.  Definitely room for improvement with more insulation.  Another experiment to try in the near future.  I will try and find some rockwool as temporary insulation for a future test.  The future test list is growing, I had better write it down while I remember it.

When I had a closer look at the crank disk, I had marked it out to cut off the usual sections to improve the balance a little, but now I remember that I could not resist running the engine first.  Never got back to do it, or to lock-tight the pin.  Have already hacked at the disk with a hacksaw, ready to mount on the mill for a neater job in the next day or so.  Now I know why it is called a hacksaw!

Then family called, so still have to reduce those photos and input the data for the calculations.  Fortunately the same headings and formula I used for previous runs can be copied down again, so it won't take long when I get a chance to put in the data.  Perhaps after the dentist tomorrow.  Only a check up, I hope.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 11, 2018, 11:51:37 PM
HI MJM, yes there is always another thing to do with engines !!! Do you take the ambient temperature when firing the engine up and can you take this into account on the spreadsheet ?. As the water is at a higher temp as well as the meths. And does the meths evaporate at a different amount when warmer or cooler, and will it boil at a different rate to H2O ?  I notice you take the highest temp to 110 degrees ...is there something significant with this temp or is this when the safety valve lifts ? Interesting to note that if something is "wrong" then the gauges will let you know. this is why there are so many sensors all brought together on huge ocean going liners....it beats having to go round the engine  with a stick in your ear !!! Also my mum always said don't hack at it "let the tool do the work'...!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 12, 2018, 11:00:18 AM
Hi Willy, somebody once said "if you have not measured it, you don't really know it", or something like that.  But even an approximate measurement is better than just a guess.  And changes in measurements are more important than the actual values.  Not to be too hard on guesses though.  A good guess can save a lot of time and effort by helping decide what to measure.  Not really practical to measure everything.

I do measure the ambient temperature, though in what I have done so far, the main influence of ambient temperature is the temperature at which it all starts, so how much heat required to get it to rise through the first temperature interval.  It is always interesting to see that the temperature in the boiler and at the engine inlet differ before any heat is applied.  Only a few degrees and I don't really know what it means.  Certainly, the water is nearly always cooler than air temperature, and at that low temperature difference, it takes a really long tome to equalise.  I usually run out of patience and just wait until the readings seem stable.  However, now I am measuring the stack temperature.  This leads to the interesting possibility of estimating the temperature of the hot gases resulting from from combustion and possibly the air flow rate by a further application of that heat balance.  I will come back to that in a future post after I have tried some calculations.

The other main application of the ambient temperature measurement is in highlighting the differences in experimental conditions.  For example, in your electric boiler, the heat loss to the atmosphere is determined by the difference between the water temperature and the air temperature, along with the insulation thickness and properties.  I suspect it is possible to compensate for the difference in the temperature difference in some of the calculations to make them comparable whether you conduct the experiment in high summer or the depths of winter, though I have not carried that through so far.  Extra insulation reduces the loss to atmosphere,  hence reduces the importance of the atmospheric temperature.

Boiling ethanol (Meths) is similar in principal to boiling of water, though the temperatures and latent heats are different.  So, at atmospheric pressure, ethanol boils at 70.3 deg C, and the latent heat to boil it is 840 kJ/kg, compared with 2257 kJ/kg for water.  Working out the calorific value for Meths as a fuel requires allowance for the sensible heat and latent heat to evaporate it.  It looks like this is allowed for in the published data, so you don't have to do it again.  So when the ambient temperature is higher, it takes less heat to raise the fuel to its evaporation temperature, but in practice, this makes very little difference as the heat release of 26,000 kJ/kg is very large compared with the differences in sensible heat requirement, unless you are conducting a very accurately controlled laboratory experiment.

The 110 degrees is a bit arbitrary.  It is only equivalent to about 6 psig, but is enough to run the unloaded engine at 1000 - 1500 rpm, so it is adequate.  Without a regulator valve I have very little choice, but by this stage, the steam generation is enough to mean that the heat up calculation assumption of no work done is no longer valid.  So to is an appropriate point to stop.  With the engine running, the temperature varies up and down a little but when I look closely at the steam tables, it makes little difference to the heat required for each kg of steam.  It does not move far from that 110.  The difference in latent heat is compensated for by the heat up or cool down heat associated with any temperature change.  So I just assume the steam is generated at the average rate at a steady 110 degrees.  A bit rough, but close enough to help our understanding.  Steaming calculations are based on a steam rate that is approximate, but the realistic order of magnitude.  Obviously I had to ignore most of the result on those two runs with the boiler the wrong way around.  When there is obviously steam produced, while the boiler temperature reading is only 85 - 90, you can't place any reliability on the calculations.  The safety valve is set much higher, in line with the boiler design pressure.   I did test it way back, and I should have a record of it somewhere.  I lift the stem to ensure it is not stuck before each run, but without a regulator, I cannot raise enough pressure to lift it.  Another project to add to the list.

Not only ocean liners have all those instruments but also chemical plants and power stations.  Not to mention jet aircraft.  They used to even put a few key instruments in cars.  However, they assumed we were all to dumb to know what they meant, so proved that no one read them by leaving them off.  Another stupid economy perhaps?

Ok, I asked for the comment about the hacksaw.  Actually, the problem was that while I usually have good success with a timber file guide, the guide slipped on the small component so I finished the cut without it.  I should know not to do that by now.  Even a violin apprenticeship would not give me the eye/hand coordination to do that.

However, today I pressed on, set up the rotary table in the mill and had a go at neatening up the cut.  Actually made some swarf as you can see in the attached photos.  Had to reintroduce myself to the mill as it is the first swarf I have made since I started this thread.  The modified crank "disk" is now fitted with the crank pin, and will be good to go tomorrow.  I carefully cleaned the threads of oil and applied some loctite to lock the threads.  Had to move the whole shaft to ensure I did not get loctite in the bearings, so will have to retime the engine tomorrow.  That motivated me to make those studs at long last.

Last picture shows it all set up, almost ready to go.  You can see the simple bent wire clip I made to hold a sheathed thermocouple in a fixed position in the stack.  The insulation does not look too bad from that angle.

Started putting the data into the calculations, and will continue that tomorrow.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: paul gough on February 12, 2018, 11:31:25 AM
Hi MJM, Your adventure with the little boiler is proving to be very interesting, and demonstrates that even this simple steam generator is not just a pot of water with a flame under it, it is a system and each of the systems components have to work effectively. For this to be achieved sufficient understanding of the behaviour of each component and of the whole system needs to be understood if optimal conditions are to prevail. When a teenager I came across this type of boiler in model form and thought I knew how it worked, but did not fully understand the full implications of tube design/arrangement until a little later in the year when I was 'playing' with a friends 6"x8" single cylinder stationary engine and steaming its companion boiler. It was an ex- army cooks field kitchen boiler of essentially the same design with two under tubes and about 15" dia. barrel. But, the long leg or down comer was reversed to what I understood to be the 'normal' position, i.e. normally  rising away from the front of the boiler. Something my mate nor I grasped immediately.
All was revealed when we decided to wash out the boiler which had been standing for a long time and conveniently had a handhole at the front and allowed a good view of the proximal tube openings in the bottom of the barrel from the short riser. These tubes were still completely full of water as they had no provision for a drain. So we 15 year old first year apprentices decided to dry them out with a small fire. That was something of a revelation, it instantly revealed the flow direction with water oozing and then flowing out of the tubes at the front end. We immediately grasped the reason for the 'reversed' tube slope in our 'light bulb' moment. Most of the heat from the fire was always in the front two thirds of the firebox and the radiant heat in the front half, the rear end being a sort of dead spot and so relatively cold. My first lesson in boiler design! Ever since then I have had as much interest in boiler design as in engines, perhaps more so, as there are so many factors to consider. I look forward to seeing more data and conclusions from the tests. It might be worthwhile recording feed water temperature and when you finally settle on a standard method of relevant parameters to run a repeat test on a real Melbourne Winter morning and see what differences this environmental parameter has on result. This might reveal something for us Gauge 1 people who operate in the full range of outside conditions. Willy's question about temperature effects on alcohol has got me to thinking whether there is any significant difference in vaporisation rates and hence wick outputs in hot or cold conditions. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 13, 2018, 11:29:40 AM
Hi Paul, good to have you back again.  I assume no longer limited to the phone screen.

I do always record the ambient temperature, though I have previously not had much direct use of it in the calculations.  However, now I have some insulation, so I can assume most of the unaccounted for heat goes up the stack, and I am measuring the stack temperature, I can calculate  the air temperature rise, so I am exploring how I can use that.  In the simple closed system with no feed pump, the main effect of atmospheric temperature is in determining the starting temperature.  For the simple furnace casing with no insulation, it also effects the heat loss from the casing.  That in turn cools the flue gas a little more, so must reduce the temperature difference available to transfer heat to the boiler.  Important with a small burner, but less so with a larger burner where the area is not sufficient to transfer all the heat anyway.

Further to what I said yesterday on the effect of ambient temperature on the Meths, I have noticed that if I remove the burner as soon as it goes out, so I can weigh for any remaining Meths, the burner part is very hot, too hot to place on the plastic scale pan without an insulating protective layer.  I generally zero the scale with a piece of 20 mm thick timber on the pan, on which I place the burner.  However, the tank end, which stays outside the furnace enclosure, is barely more than slightly warm to touch.  So I conclude that most of the heat in the burner comes from the conditions in the firebox, which are obviously much warmer than the outside air temperature.  Also, while my data book does not have a specific heat for ethanol, but water is a whisker over 4 kJ/kg.  So assuming ethanol is a similar magnitude, even with a 20 degree ambient temperature range, 80 kJ/kg is not important beside the 26000 kJ/kg heat of combustion.  So I suspect the Melbourne ambient temperature range has little noticeable effect, other than the burner might be a bit slower to start when everything around it is colder.

Had quite a successful day today.  I finished those studs.  Must have been putting off the trip to the bolt shop for too long, I could only find 3 of the necessary 4 M3 nuts.  Couldn't even find anything to steal one from, so three studs and a capscrew until I get to the shop.  Still, three studs do hold the steam chest in place when the cover is removed for valve timing adjustment.  I was able to get the engine reassembled, so it again looks just like in the previous pictures.  And reset the valve timing.

Now I wanted to run it, to ensure everything was ok again, but I had really run out of time.

One question nagging at the back of my mind was Willy's question about the importance of the height of the boiler above the burner.  I was starting to wonder if the little 50 mm burner was just getting lost in the larger firebox, and if perhaps the flame was barely high enough to reach the boiler shell.  Could that explain the unexpected steaming performance?

I decided to do just a quick run, minimal measurements, just boiler temperature and stack temperature. Measured water and fuel by the volume marks in the containers, and only timed roughly with my wrist watch.  And I lifted the burner by about 20 mm (the top of the burner was previously about 50 mm below the boiler shell).    I wanted only to do enough to see if the burner height made a significant difference, and of course, hopefully to demonstrate that the engine was again a runner.  I hoped to be able to complete this much in the limited time available.

A remarkably successful test.  When I first lit the burner, the boiler temperature took quite a while to start responding.  I started to hear those tell tale bubble bursting sounds, and finally the temperature was rising.  What a difference.  It continued all the way to 100 then 105 when there was clearly steaming happening, a couple of flicks of the flywheel and the engine was away.  Slowly at first, then as the boiler temperature (pressure) rose a little more to 110 to 115 deg C the engine revs increased to around 1000 rpm.  Not quite as steady as previously, possibly the engine is a bit stiff after the extensive rebuild.  As the burner died away, the temperature slowly dropped, and the engine continued to tick over beautifully down to around 300 rpm.  Slowed over a longer time period than previously.  Not sure what that means.  I noticed the superheater was not having any measurable effect.  (Yes I know, I said minimal instrumentation, but it was still set up).  I wonder if I lifted the burner too far.  I will try another quick test tomorrow with a lower packer, then I will try a full test with the height that seems best.

Also made a bit more progress on the calculations, which I will continue tomorrow.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 13, 2018, 11:57:32 PM
Hi MJM , more interesting stuff going on there and i like the pictures !! If you did use the waste heat for the boiler feed would that reduce the efficiency of the boiler somewhat, or would it be negligible ? perhaps this is measurable as well ?? When the meths burner is alight what shape does the radiant heat take coming off, assuming no draughts. could one see this with an infrared camera ? What is the speed of the radiated heat to atmosphere or is that a silly question?? Are there figures from heating types available for different burners from the makers of commercial and model boilers?  more questions i'm afraid...
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 14, 2018, 12:33:02 PM
Hi Willy,

I am glad you liked the photos.  I will try and lift my game and include a few more pictures, sketches, graphs or something. 

You can see that my machining and set up skills are not up to so many on this forum, but I am enjoying trying and learning.  That very nominal effort at balancing the crank disk made a huge difference.  You can't balance the reciprocating mass of the piston and cross head so easily, but the rotating force from the big end and crank pin also introduces a vibration at right angles to the piston travel.  This part of the vibration is almost totally gone, so that the mass of the baseplate and wooden base is now barely moving.  Clearly that normal shaping of the disk comes very close to what is actually required.  The crank pin is almost at one end of the large wooden base, so introduces a rocking movement, which was previously quite obvious.  The moment of inertia around that horizontal axis is relatively small.  The piston in the horizontal plane has to move the whole mass of the baseplate, engine and boiler, so this results in less response to the forced vibration.  F=m x a is easily transposed to a= F/m, so large mass means small acceleration.

Waste heat is almost by definition heat that would otherwise be rejected to atmosphere, so wasted.  So if you put the feed water heater in the smokebox, or at the base of the stack, you are using heat that would otherwise be wasted, so you improve the overall efficiency with no detrimental effect.  Obviously if you put it in the firebox, with the superheater, you reduce the temperature of the combustion gases which would otherwise be available to generate more steam, so any benefit in higher feed water temperature is offset by reduced steam generation.  Nominally a zero sum game, but the second law says you always loose some.  Similarly, if you recover some heat from the engine exhaust, so long as you don't introduce extra back pressure, the heat recovery is an overall benefit.  In fact you may be able to slightly lower the exhaust back pressure with some condensing, which would be a further benefit, even if limited to a very small difference by the need to remove air from the system.

It would be really interesting to take a picture with an infrared camera.  I used to have a film camera with a special focus mark for infrared, but I never purchased the necessary filter.  I can't remember if special film was also required.  I looked through the settings on my digital camera, and could not find anything.  This evening I tried a photo in the dark garage of a candle as a quick test, but not convinced it showed anything special in the way of an infra red image.  Also searched the Ap store, but everything that looked relevant was just an interface to a special camera.  I am tied up for the next few days, but then I could try taking some photos of the burner say every 30 seconds for five minutes or so so that we can see the flame developing.  That will have to be next week.

Radiated heat, not a silly question, it travels at the speed of light and does not require air in between.  It is electromagnetic radiation, just like light but a longer wavelength.  The amount of heat transmitted from a surface is proportional to the absolute temperature raised to the fourth power, but the cold surface, unless at absolute zero, also radiates back, proportional to the fourth power of its absolute temperature, but at a different wavelength.  So the driving force is the difference in the fourth powers of the absolute temperatures.  The next step in calculating the amount of heat transferred involves absorptivity, emissivity, view factors and so on, so it gets very complicated.

I don't know what data is provided with commercial burners, but if they tell you fuel consumption, you can multiply that by the calorific value of the fuel to get the heat rate.  You use the lower calorific value, as you do not use the heat down to the temperature at which water is condensed, so get no benefit from the latent heat of the water in the flue gases.  If you need data for your fuel, I may be able to provide it.

More questions are always welcome.  I hope I am generally providing satisfactory answers, but I am always willing to have another go if I miss the point of your question.  So please continue asking.

I should have said a little more about burning Meths in the last few posts about the effect of temperature.  One of the reasons that Meths is a good safe fuel for our purpose as it is quite difficult to light.  You cannot easily light vapour at the vapour pressure at normal temperatures, nor can you easily set the liquid alight.  You have to put a match, or electric spark right in a critical zone near the surface, where the fuel air mixture is just right.  This happens quite close to the surface, at a wick, or a free surface.  The heat to continue the vapour production comes from the flame.  In a vaporising burner, you have to supply the heat necessary to evaporate the liquid, and apply the lighter quite close to the vapour jet from the burner, again where the air fuel ratio is just right.   This is not to say that the fire won't flash to the liquid surface if you apply enough heat, and I don't advocate pressing the testing of this too far.  But in a suitable safe area, with a small amount of liquid in a tin lid, you can easily see how close you have to put the match to light the Meths.  If you have enough depth to submerge the match, the liquid will extinguish it, though you will probably light the surface in the process.  If you spill much liquid under a lit Meths stove, it will eventually get enough heat to catch a light, in fact, before the kettle boils, don't ask how I know,   Finally, water is all you need to put out the fire.  The liquid absorbs the water, is cooled to the point where the vapour pressure is too low for continued combustion, and the flame is snuffed out.  This is unlike most liquid fuel fires, where water will spread the fire, so not at all suitable.

Remember, if you must try it, a clear area with no flammable materials, a surface that will contain any liquid spills, and a bucket of water as an extinguisher.  And the fuel bottle capped and put away before getting out the matches.  Remember safety first and always.

I tried the burner at an intermediate height today.  Started to produce steam at 85 degrees again, but after a few seconds, the engine slide valve decided to seal, and the temperature then continued to rise to 100, then on to 110 as normal.  The engine is a little tight after the rebuild and required about 115 degrees to start, but as the burner died down, the engine just slowed but continued to run down to around 105, when it was ticking over beautifully at about 300 rpm.  Then the flame went out with a small pop, and the engine stopped. 

Probably now worth doing a full test at each of those two heights, even at another in between, preferably after I make a stop valve.  Any suggestions for a simple design with an good through passage?  I don't need or even want to regulate, just want to be able to isolate the boiler for safety valve tests and then to control when steam production really starts.

Also continued experimenting with the calculations.  This post is getting awfully long and I have no pictures, so I will tell you more about it tomorrow.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 15, 2018, 11:53:21 AM
I have often thought there must be a way to use a heat balance to calculate the combustion temperature.  The equation is simple enough, heat flow = mass flow times specific heat times temperature difference.  Unfortunately the equation has two unknowns, so is not sufficient to solve the equation.  Similarly, looking at the flue gas side of the boiler, the heat transferred to the boiler = mass flow times specific heat times temperature difference.  Again two variables, in fact, if we assume the flue gas is very like air, it is mostly air after all, it is the same two variables.  Two equations, two unknowns, can be completely solved, providing the equations are really independent.

If we follow the gas path through, air at ambient temperature is heated to a combustion temperature and absorbs the heat released by the combustion of the fuel.  The temperature difference in this case is the difference between atmospheric temperature and the temperature reached as a result of combustion.  Note that this is not the same as the combustion temperature data available from some sources.  That temperature assumes the exact theoretical air quantity.  Excess air will reduce the maximum temperature reached, as part of the heat is absorbed by the extra air.

The combustion gases then pass over the boiler and heat is transferred to the steam and, in the process, the gas is cooled to the stack temperature.  This time the temperature difference is the difference between the temperature reached in combustion and the stack temperature.
 
Thus, the equations use quite different data, so can be considered independent.  I set up the equations in my trusty spreadsheet, and sure enough was able to solve for the mass flow and the combustion temperature.  And as I know the fuel flow rate, I can also calculate the air fuel ratio.

The process is of course rather simplified.  The combustion gases contain a fair amount of air, but also water vapour and carbon dioxide.  And the mass flow is the sum of the air flow and the fuel flow.  As a first approximation, I assumed the mass flows equal.  As the fuel mass flow is only 2-5%  of the calculated air flow, this approximation is reasonable, though it is a source of a small error. 

The formula also includes the specific heat of the gas, and another approximation, I assumed the flue gas has the same specific heat as air.  I need to have a look at the flue gas composition and the specific heat of each of the components, to check the accuracy of this assumption, but I am making the assumption for the moment.  When I have some better data, I will have a closer look at the likely error introduced by these simplifications.

Finally, I assumed that all the heat of combustion was either absorbed in the copper or the water or steam, or lost up the stack.  In other words, I assumed the heat lost by the boiler casing was zero.  My infrared thermometer reads around 100 degrees on the outside of the casing above the burner location and 70 to 80 degrees towards the stack, so the heat loss is not going to be zero, despite there being some insulation.  However, I can find some rockwool and tie it around the casing and reduce the heat loss to near enough to zero.  If I use only the data from during steam production the casing temperature will be essentially constant, so the heat absorbed by the casing can be ignored.

I used representative numbers from the tests so far, though you will remember that the engine lost its crank pin after some minutes of running, so I did not get a complete satisfactory run after turning the boiler around.  I need to get back to the shed, now the engine has been rebuilt, and do a few runs to get some consistent results.  Then I will produce some graphs

In the lack of really good data, I tested the calculations with some representative figures from previous runs.  I expect the numbers are not reliable enough to post, but I was amazed to see that my simple rig actually provided enough information to calculate the air flow and flame temperature.  That will allow me to experiment with blocking up some of the airflow, particularly the part that enters where it probably just dilutes and cools the flue gases, and does not actually help combustion.  The first very rough figures suggest two to three or more times the theoretical air requirement, so some reduction should be possible, and reduced excess air should result in higher gas temperatures and higher heat transfer to the boiler, so more steam.  More exciting work ahead.

I won't be able to achieve a lot until next week, but in the meantime I will still look in every day, so keep up the discussion.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: paul gough on February 15, 2018, 12:29:02 PM
I will be very interested to see some results and conclusions regarding the air requirements. I fiddled about with this aspect by fitting some metal mesh, (flyscreen), to the bottom of the firebox on my loco and found no observable difference to performance. But, this was really only to see if it choked the fire  and in no way could be considered a real test  nor provided any further suggestions. I feel it did appear to lift the flame a little but could not determine if it was due to increased draught, screen induced turbulence, increased air velocity through the screen or maybe some combination. Looking forward to your tests and thoughts. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 16, 2018, 01:39:09 AM
Hi MJM , wondering about the ambient temp ... as heat rises is there a bit of a natural upward draft when it is really warm that can affect heat loss from any of the surfaces. ?? Still trying to absorb all this info btw.!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 16, 2018, 10:22:31 AM
Hi Paul, like you, I have tried fiddling with the air vents to reduce the air flow, but was unable to see any obvious difference.  However, there is no doubt that excess air reduces the maximum temperature of the flue gas, and reduces the efficiency of the boiler of any size.  I was only doing a simple test,  time to produce steam and total time of the run, based on weighed in water and fuel.  The little boiler did not have a totally enclosed furnace, so no stack temperature measurement.  I am hoping that with the more detailed tests with measurements during heat up, and also measuring stack temperature, I might be able to detect a real difference.

All furnaces require some excess air in order to achieve complete combustion, and the common wisdom is that Meths requires plenty of air, implying more excess than other fuels.  I am not sure what observations this is based on, however we will see.

I am also thinking of a series of tests of the burner, starting with say 10 g of Meths, then increasing the amount each run by 10 g each test.  My thinking is that the burner seems to start a bit slowly, then more vigorously after a few minutes.  Finally it tails off until it extinguishes.  I am thinking that a 10 g run includes all these stages.  If I add a further 10 g, I assume the extra fuel will extend that middle vigorous phase, so the difference in time will give a fuel consumption for that phase. A few more tests might converge to a uniform incremental rate.  Can you think of any variations to this that might give me a better idea of how the heat rate varies through a run? 

I did think of weighing the burner at intervals, or by modifying the base plate, continually, but I don't think my scale has adequate resolution or repeatability.  So that method will have to wait for a new scale.

Hi Willy, I hope that you are finding the challenge interesting, and similarly the many other silent readers.  The aim is that the knowledge should both be developed and shared, so I hope many are learning something.

I know the conventional wisdom is that "heat rises".  However, this is really quite mis-leading.  Heat travels from a hot area to a cooler place under the influence of the temperature difference.  Gravity does not influence the heat flow.  Now that is the total picture for heat flow by conduction.  Radiation is even more interesting.  A hot object radiates heat, proportional to the fourth power of the absolute temperature, whether there is anything to receive it or not.   However if there is something receiving the radiant heat, it's temperature increases and it radiates back but proportional to the fourth power of its temperature.  So the resulting heat transfer is proportional to the difference in those fourth powers.

It is in convection that Gravity has an influence, but not on the heat.  When a fluid is heated, it expands, which means it's density decreases.  And of course lower density stuff tends to float on higher density stuff, so the heated air rises.  It is the rising air that then transports the heat.  A bit pedantic, I know, but every now and then you meet a situation where assuming that heat rises gives you the wrong answer.  Certainly with radiation, it can definitely flow downwards, and also with conduction.  If you put a hot brick on an ice block, the heat definitely travels down to the ice, just as surely as it flows horizontally if you place the brick beside the ice block.  And of course it flows up if you put the ice block on top of the brick.

With the boiler in an enclosed furnace, the inside of the furnace wall receives heat by radiation and convection within the firebox.  Heat travels through the wall material, which preferably includes a layer of insulation, raising the temperature of the outside surface.  This surface is in contact with air, so the air is heated, and consequently expands.  Also the viscosity reduces, so we have the interface between thermodynamics and fluid mechanics.  The hot air rises, and is replaced by more of the cooler surrounding air.  This process is called convection.  The rate at which heat is transferred by convection is proportional to the difference in temperature between the furnace wall and the surrounding air, which is of course the ambient temperature.  And if you hold your hand above the furnace wall, you will feel the warmer air rising.  So the ambient temperature influence is its effect on that temperature difference.

So if your atmospheric temperature is low, say 5 deg, the temperature difference will be higher than if the ambient temperature is say 30 deg.  This larger temperature difference means more heat transfer, the furnace wall is cooled more by that cooler air. 

But then, more heat loss means more temperature difference between the inside and outside walls of the furnace.  As a result the outside wall temperature is lower with a lower outside temperature.  Of course the higher heat loss also affects conditions inside the firebox, but that had better wait for another day.

You can draw a temperature profile between the inside and the outside air to illustrate all this.  Our internet has defaulted to clockwork performance, I will be lucky to get text through tonight, so I will wait until the wind changes, before I try and post a drawing.

Thanks for following along,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 16, 2018, 08:35:32 PM
I MJM two questions to ponder over Dose the height above sea level dictate how much oxygen there is available in the air so the fuel is less fuelish  ?  foolish question? !!!  and as the meths evaporates does the cooling effect ,affect the performance of the fuel?
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 17, 2018, 09:28:47 AM
Hi, Willy, the height above sea level affects the air pressure and hence density as we saw in the earlier discussion on filling the boiler at the top of the mountain.  It does not affect the fuel quality, though it does affect the boiling point, just as it does for water.  At lower atmospheric pressure, the vapour pressure will match the atmospheric pressure, where boiling starts, at a bit lower temperature.  Just as with water, the latent heat is a bit higher at the lower temperature, but it takes about the same amount less heat to reach that lower boiling point, so not much overall effect.  It does not affect the calorific value, so the fuel is just as fuelish as at sea level.

However, at the higher elevation, lower air pressure, means that a given volume of air has less mass, so less nitrogen but more importantly, less oxygen.  It would be more accurate to say the air is less airish.  You need a greater volume of air to get the same amount of oxygen for complete combustion, as you need at least the same amount of excess air as at sea level to ensure complete combustion.  May even need a touch more.

I believe that accurate laboratory standard data for calorific value uses a measured mass of vapour  with a measured mass of air ignited in a closed calorimeter submerged in liquid in an insulated container.  This gives a calorific value for ethanol.  Methylated spirits is a liquid with 5% water, the highest purity obtainable by simple distillation.  So to get a true calorific value for methylated spirits, you have to allow for only 95% ethanol, heating the ethanol to boiling point, evaporation of the ethanol, heating and evaporating the water.  This results in a slightly smaller value, and probably explains the differences in value quoted from different sources.  I don't have access to my data book for the moment, but I can look it up Tuesday.  But as always, heat released by burning the fuel is only available for generating steam after you have allowed for the heat used to evaporate the fuel.  And of course, at the lowest flue gas temperature that is useful for generating steam, the water vapour in the flue gas both from that initial 5% and from combustion of the hydrogen content of the fuel is still vapour, so again you cannot take credit for the latent heat of this water vapour which is only released when the water condenses.  Calorific value is quoted as two values the higher calorific value or hcv, and the lower calorific value or lcv.  You need to use the lower value for steam generating purposes.  I hope that makes it all a bit clearer.

Well I must have held my tongue at just the right position yesterday when I pressed post, and the text went through, phew!  So I have sketched that temperature profile I talked about  yesterday.  I have assumed the same temperature inside the furnace, though in fact, when atmospheric temperature is lower, the heat loss is greater, so the flue gas in the furnace is a little cooler.

The outside of the furnace wall finds its own value between the flue gas temperature and the ambient temperature so that the heat transfer through the insulated wall is the same as the heat carried away by convection.  You can see the linear profile through the insulated wall, due to a constant thermal conductivity of the insulating material.  The parabolic profile on the air side is due to the velocity profile of the rising air at the wall surface.  No velocity at the wall so higher temperature by conduction through the air, further from the wall the air rises at an increasing velocity, thus replacing the warmed air with cooler air from below, and lowering the air temperature at that distance from the wall.  The change of air velocity with distance from the wall results in that curved part of the temperature profile.  I will try attaching the picture, and see if it goes.  If it does not, I will try again later, or tomorrow.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 17, 2018, 09:31:45 AM
Success.  First time, it timed out before completion, second try, the picture included twice!

No understanding the Internet.

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on February 17, 2018, 10:16:43 AM
Hi MJM, Interested to see the heat loss diagram from the firebox wall. The ambients are more or less the probable operating range for most G1 locos in OZ so would likely be indicative of conditions for a stationary loco shielded from the wind, thus a baseline or starting point for an investigation. Can you quantify, (even approximately), 1) the distance at which temp. line goes horizontal, 2) the heat losses for the two ambient temps. even as a proportion or % increase in loss.

I think your incremental increase in fuel supply test should provide a reasonable guide, I can't think of anything better. If I ever get my engine back together I will probably pursue some adaptation of your experimental method, and so am pondering the degree of accuracy achievable and necessary to get repeatable and consistent results. I don't want to fork out about $500 for a very accurate scale if I can utilise a syringe to measure fuel and water volumetrically and then convert to grams. I recognise that we are dealing with minute quantities and volumes, thus accuracy is important, but would like your opinion on what % or range of inaccuracy would be acceptable at this scale for all the parameters. Regards, Paul. 
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 17, 2018, 01:26:57 PM
Hi MJM , just put the photo up again !!! Glad to see a slide rule ...with its curser still attached !! Also saw this in a Model Engineer  15th Jan 1901 edition of a chap fromNew Zealand doing practical experiments with boilers !!!  He says raising steam took 3 1/2 mins !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 18, 2018, 11:50:31 AM
Hi Paul, not so easy to answer that lot, but I will see if I can convey the general idea.

I really should have shown that the profile on the inside of the firebox which has a similar curve near the wall, in that the hot gas is cooled in the vicinity of the wall, so the profile curves down as it gets close to a minimum temperature at the wall.  Heat transfer to the wall on the inside is also by convection.  It is not so easy to give a figure for the thickness of the thermal or velocity boundary layers, the velocity starts at zero at the bottom of the wall, while the temperature starts at the wall temperature, and the profile develops as the air rises up the wall.  My sketch represents the profile at some height above the bottom.

Rather than try and define the profile totally, which involves some pretty heavy differential equations, it is usual to talk about an overall coefficient, a number that gives the same total heat transfer at the sum of each little bit of the wall.

I looked at some of the worked examples in my textbook to find some figures that might be typical of our case.  It looks like the boundary layer thickness essentially becomes constant some where in the range of 2 to 12 mm from the wall.  I hope that is close enough for your purpose.

The steel internal box makes very little difference to the profile as it is a good conductor compared with the insulation or the air.  The insulation and the air boundary layers resist heat flow like electrical resistors in series, but we are talking about conductivity, the reciprocal of resistance, so the conductivities involve the reciprocal of the sum of reciprocals, similar to the formula for resistors in parallel.

I assumed the conductivity of the insulation is about 0.05, and it was 3 mm thick.  Typical convection coefficients for similar cases look like something in the range of 2 to 5 J/m^2.K.  I assumed a typical flue gas part way through the boiler as 225, giving a temperature difference of 200 at 25 degrees ambient.  At 5 degrees ambient, the difference is 220 degrees.  On this basis, there is about 12% more heat loss at the lower temperature.

I think when I get through some more analysis of the cooling runs, I might be able to calculate an external heat transfer coefficient for my boiler, we will see later in the week.

Of course all this has been about natural convection.  When your locomotive starts moving, the velocity of the locomotive becomes an air velocity imposed on the system.  This is called forced convection.  The boundary layer is thinner, and the heat transfer higher due to a higher effective temperature difference across the air film.

Thanks for thinking about the incremental fuel experiment, I will give it a try.

My small boiler uses about 25 g of Meths, so 1 g would make an error of about 4%.  However, it uses 130 g of water, so the same 1 g is only about 0.7%.  Generally these percentage errors all simply add together through the process.  However, square or square root functions don't carry through so easily.   When the calculations are done in a spreadsheet, I would just change the fuel quantity or whatever, by the one g, and check the change in the answer, just to be sure.  It only takes a few seconds.  I would also balk at $500 for a scale.  My daughter in law has one from Jaycar that she uses for her hobbies as well as cooking, 2 kg, with a resolution of 0.1 g, that was around $150, but still sizeable cost.  To give you an idea, the scale included a wind shield to assist in getting steady readings.  A syringe might be a suitable alternative, that is much more accurate than the marks on a jug, or even a medicine glass.

Hi Willy, I thought might like the slide rule.  No use at all without the cursor.  I bought that little one for my pocket for when the more usual 10" size was inconvenient to carry.  When I told the younger engineers at work that I did not even have a calculator for the first eight years of my career, the look on their faces said they thought I must have grown up in the Stone Age.

I don't suppose many of the original subscribers to that 1901 magazine are reading these posts.  They are quite interesting.  He was quite the master of understatement!  If I have understood correctly, in one sentence he said he built a locomotive style boiler with about 6" barrel and five 1" tubes.  That must be almost the shortest build log in history.  You can imagine the reaction if that was all someone wrote here.  But then he said he tried larger (than the 1") and smaller, but found the 1" best!  So he built three locomotive style boilers in two sentences?

They are no small boilers either, barely classed as miniature.  As for getting those boats to the pond, you have to admire him.  However he makes a lot of sense in his opinions.  Lots of surface area per unit volume will give quick steaming if you have enough fire.  But the designs are complex, and certainly not shown in all their detail.  He did mention stays being required on one of them, another understatement.  And even admitted that one was not easy to build.  Only one?  But his figure 5 is an interesting concept, giving plenty of area and also enough volume.  But despite having designed a few full size vessels, I would have to give that one a lot of thought.  You have to admire those early guys, no codes to guide them, did not know what would not hold pressure, so they just built them, and presumably built a stronger one if the first effort failed.  And fail they must have done.  That is why we have the codes today.  Interesting articles, thanks for posting them.

Thanks everyone for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: paul gough on February 18, 2018, 10:36:52 PM
HI MJM, Thanks for the quantities, it helps me considerably when constructing a mental picture  during my day dreaming sessions. Following this investigation is teasing out some clearer thinking from the neuronal fog and eventually I'll be more certain about a line of tests suitable for a loco.

Regarding heat loss due to air movement, A reasonable guiding figure for velocity of G1 locos would be to use 1m/sec, (e.g.  Indoor or dead still outside conditions and assumes no contribution from wind). Am wondering if you could have a stab at predicting what further % heat loss there might be using this figure over that already mentioned.

I am inclined to think I should pay more attention to insulating the firebox on my loco!

A shame you have to sweat it out with Melbourne's elevated ambients, do you think it of any consequence to obtain or convert test results to standard temperature i.e. 20C, so making comparisons easy for others or future experiments.

Thanks for the tip, I will have to check 'Jaycar' more thoroughly, a satisfactory scale at $120 is attractive, also useful for biological specimens. I'm on the lookout for a measuring cylinder and a sizeable pipette to check my syringes accuracy, should be able to get some used scientific glassware somewhere at a reasonable price. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: paul gough on February 19, 2018, 08:32:48 AM
Hi again MJM, I was pondering thermocouple placement for exhaust gas temperature and then realised this could be problematic. Most stack temperature measurements that I have had experience with on boilers were really to provide a guide to combustion conditions and an indicator that things were going well, or not, particularly when operators are somewhat remote from the boiler. It is the variance from the observed normal for any given load that alerts you to problems. As the actual temperature could be widely variable depending on placement of the probe in the stack  the obvious question arises, where is the best place for our purposes to place a probe/thermocouple? Is it necessary to compute the gas flow and heat content to determine the average gas temperature one should be getting, then go chase a spot where to drill the hole in the stack to insert the probe? Hope I'm not confounding everyone with unnecessary complications, but I really would like to know how to best place a probe and why. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 19, 2018, 12:42:57 PM
Hi Paul, I will have a look at the convection equations in my text book and see if I can get an idea of the difference with the moving loco.  However that will have to wait until tomorrow.  Drove 300 km today, slow with roadworks and other stops we had to make, and then a meeting this evening.  I don't think the equations will make any sense tonight.

Insulation of the firebox would help reduce the heat losses, if you can fit it within the locomotive outline.  In that size, it is not worth making it ugly to save a tiny amount of fuel.

Melbourne temperatures seem a bit more tiring than in my younger days, though we have a couple of cool spots in the house these days.  At least we do not get the humidity you have to put up with, though some tolerate it better than others.  Do you have the buildup before the wet like Darwin?

I suggest a good way to standardise the results is to calculate the overall heat transfer coefficient, as I suspect it is much less sensitive to the actual ambient temperature.  Then you can use that coefficient with the temperature difference for the conditions you want to compare.  I have not done enough tests over a wide range of conditions to see how important ambient is to a boiler.  Ambient temperature is much more significant to us, as we operate at about 37 so the difference between 35 and 25 is huge, but for a boiler operating at 150 - 300 deg?  However there are a few inevitable errors in the work we can realistically do, so in general, I think comparative results are more important than actual figures.  Melbourne is a bit problematic in that regard though, as we can have 35 one day, and 25 the next, or sometimes later the same day.  But most places probably don't change quite so rapidly, so tests one day can probably be reasonably compared with those done on another, at least in the same season.

I used to get laboratory glassware from Selbys.  They were still around to sell me a chemistry set for one of my kids. It was the real thing, not just salt and sugar like most you can buy these days.  Not sure if they are still around, but your local chemist can probably get some in for you, they should know, or have access to the current suppliers.

The measured stack temperature is probably quite sensitive to thermocouple position.  Partly due to heat loss from the stack, and partly due to the temperature profile across the stack.  The heat loss can be addressed by insulating the stack, so eliminating height as a big factor, but the profile across the stack is a flat topped parabola, due to convection loss from the walls, and due to development of the profile with height.  A very tall stack would have a full parabolic profile, but most model stacks are not tall enough for the full profile to develop.  The velocity is highest in the centre, and the flow is slowed down by the shear forces where it is nearer to the walls to zero at the wall.  That slower layer then slows the next layer in and so on.

My approach is to make a simple clip to hold the thermocouple in a fixed position, so readings can be compared, and to place the sheath (or thermowell if I was using a bare thermocouple) so it extends about 2/3 to 3/4 of the diameter of the stack.  This way, the thermocouple passes through, and hence is influenced by most of the profile.  Then it is reasonable to assume the reading is an average across the profile at that location.  I am assuming the error between the reading made this way and the true average bulk temperature is small enough compared with all the other errors in these simple experiments.   I have not yet drilled a hole for a more permanent mounting, though that will be the best in the long run.  In the mean time, a simple bent wire clip is holding the thermocouple in a fixed position.  I try my best to eliminate obvious errors where practical, then rely on comparative results to be close enough to not be misleading.  I know I am exploring the boiler and engine performance with some pretty deep theory, but I also have to keep in mind the practicalities.  Things have to be kept somewhere near balance.  I am trying to understand the principles so I can improve the steam plants, but have to remember that I am not writing a paper for the national physics laboratory.  I hope that helps with understanding how far to pursue absolute accuracy.

Definitely time for bed, it's a new day tomorrow.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on February 20, 2018, 04:58:21 AM
Hi MJM, Thankyou for the reply to my enquiries, you must have a very resilient brain. If I drove 300 klm's I don't think the contents of my cranium would be capable of serious discourse or considered written replies at the end of the day.

The high humidity can be a little tiresome for some people when temps. are over 30, but it is when the herps., (reptiles), are most likely to be active, so I am quite happy to live with it. There is a pre wet 'build up' in Nov./Dec. but the wets are now paltry compared to what was. We have not had a real wet for years, so bad are wet season failures that central western Queensland has had six consecutive failures of wet season rains and are now in dire circumstances, hardly any cattle left in those areas. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 20, 2018, 10:44:06 AM
Hi Paul, so long as my post made sense.  I actually bought a calibrated eye dropper from our local chemist last week.  Only 1 ml, so a bit small for fuel, but great for filling the displacement lubricator which tends to trap air bubbles. 

I have had a bit more of a look at the chapters on convection today.  The maths is quite intimidating, but when I skim the working out parts (basically mathematical gymnastics that are easier to follow than develop), and look for the conclusions, I am finding some equations that I can actually use.  I will put them in a spreadsheet to allow exploration of some of the variables, and easier checking.  It looks like it might be possible to estimate boundary layer thickness, heat transfer coefficient and heat loss.  Order of magnitude anyway, and more importantly, give helpful guidance on the difference when single variables such as ambient temperature, or velocity are changed to answer your questions.

You mentioned about 1 m/s.  What about size of that firebox?  I have assumed about 50 mm long, but better to use a more realistic figure.  Also the height.  Just thinking about the size of the firebox, which I assume is shaped like a conventional loco boiler, but in that size, dry sides and top?

I learned a few things.  Probably obvious in hindsight.  The analysis assumes only natural convection, forced convection or radiation, but in reality, usually all three are in operation.  In your loco, natural convection leads to rising air flow, while the motion imposes the forced convection with horizontal flow.  And of course your fingers held nearby tell you there is radiation.  All at the same time.  However there are criteria for the relative importance of natural and forced convection.  It's all tied up with Reynolds number, Prandtl number and Nusselt number.  Sounds complex, but when you put in the actual figures, it is not as complex as it first looks.  It looks like in this case, the thermal boundary layer is a little thicker than the velocity boundary layer, but most of the maths is about comparing three potential velocity profiles with experimental results, that are in reality all close enough for our purposes.   I will see if I can make a bit more progress tomorrow.

Also had a good day in the shop today, well, good for me anyway.  One of those stops along the way yesterday was at the bolt shop.   A long way from home, but right by the highway we were travelling.  Only a matter of stopping, whereas the ones closer to home involve 30 min travel each way.  So I now have enough M3 nuts, and the engine has four studs.  I won't have to hide anything next time I take a photo.  Found a bit of stainless steel shim, so am pondering whether I can use this as a spring to lightly hold the valve against the port face.  I can see big advantages in arrangements which have the port face on top, though they need radial valve gear, or extra linkages.  I think I also need a tiny condensate drain for the steam chest.  Along with that steam shut off valve.  All projects to follow, but I can make progress without them in the mean time

I also made up the missing hand wheel for that displacement lubricator.  Could have designed something more compact, but I used some small off cuts of brass and plastic that I had.  And in the process, I reminded myself how to centre on a shaft, drill and tap an M3 hole for the grub screw.  Won't need the pliers for that lubricator now, much better.

Also one of my thermocouples was playing up.  Looked terrific on the table beside the others with all reading the same for room temperature.  But it did not respond when I put it in the thermowell in the steam line.  Seemed to be something in the plug at the meter end of the wire.  Not the plug that has three screws holding it together, that would have been too easy, but one of the ones heat welded.  Unfortunately two of mine are that type.  Attacked it gently (if that is the appropriate word) with the hobby knife, it opened quite easily, but not much to see.  I tightened up the crimps on the terminals, but no consistent result.  Turned out that the inner insulation had been trimmed back to far.  The bare wires were just twisted together and the cover pulled down to conceal it all.  The point those wires crossed became the measuring junction, and it was just measuring the temperature in the plug.  Grrrrr!  I don't know how it worked well for so long.  Now, when I pinch the welded junction (the intended junction) between my fingers, the meter quickly responds, climbing to about 30 C.  You don't get much more when measuring body temperature that way, but it is an easy quick check that the thermocouple is working properly.  The previous calibration checks are not affected by any of this, so now it properly responds, I can use it with confidence again.  Might actually get a full run tomorrow, the first with the boiler turned around, burner raised stack temperature measurement and insulation on the furnace.  It has been a bit frustrating, and a long process, but progress is made one step at a time and let's keep fingers crossed that it is all right this time.  Tentatively tomorrow, but my 11 year old grandson has an electronics project for school, and discovered he left his tools in Darwin.  It is a complex life they lead.  That and possibly helping a friend who recently had a small stroke get to a medical appointment will take precedence.  And replenishing the fridge.

Stay tuned,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on February 21, 2018, 06:44:34 AM
Hi MJM, Your posts are making sense and I am getting an ever clearer picture of what is going on and an appreciation of the subtle and complex goings on with respect to heat and its investigation. I would not be competent to do a lot of the maths nor in fact have the understanding of when and where to apply it appropriately, nevertheless, by apprehending some salient propositions and absorbing the conclusions advanced from your tests, along with your hints and suggestions, I think I can come up with a testing methodology and enhancements to my boiler/loco, some simple, such as modifications, some less so and needing re-manufacture of significant components to gain better performance. Measurement is now so easy with the cheap, (relatively), electronic tools available from the likes of 'Jaycar' it is important to develop skill at using them and interpreting the information. This includes things like siting of probes and understanding just what exactly one is measuring and the implications of it. Stack temperature an example, and for me, the extension of it to the smokebox, a somewhat more complex environment into which to stick a probe looking for something meaningful. Therefore I want to thank you for the considerable time and effort you put in to assisting me with your replies.

My firebox is approximately 35 long x 20 wide mm. and about 25mm high to the bottom of the boiler barrel and has a 12mm overhang past the rear of the barrel where a 20mm. extension upwards delivers gases to the tubes. The sum of all the firebox surfaces comes very close to 35 square centimetres, then there is the 7 sq. cm. of the open base, if that is to be included/added as heat loss area, (??). These surfaces have been insulated on the inside with the thin ceramic fibre sheeting (probably 3mm originally but now about 1 mm.), this internal insulation is very deteriorated and needs replacement. I am of a mind to remove it, run a test without it, replace it and then, lastly, add the insulation to the outside and re-run the test a third time to see if a second layer externally has any value. I'm also of a mind to stick a thermocouple probe into the volume of the firebox behind the rear tube plate as this might be more even and stable than the firebox proper, my thinking is it might help interpret firebox/combustion conditions as well as be indicative of conditions in the tubes. My only concern are does the 'cheap' sheathed K type thermocouples from Jaycar able to take the 1000 C.(??), or more, temperatures.  Also would it be problematic to leave the probe in such temperatures for say, half an hour, or more?

I am happy you state, "I learned a few things." I felt a bit guilty in loading you with hefty enquiries so hope this in some way makes it worthwhile. If you are after some small parts or components for you boiler etc. you might find, <argyleloco.com.au> has something useful. They are the main supplier for all things Gauge 1 in the OZ region, most of the bits are listed under "Accessories" and then choose one of the brandnames, not everything is listed, eg fuel/water tubing, ceramic sheeting, wick material etc., so a phone call might be worthwhile. Michael Ragg is the owner and is located in Olinda up in the Dandenongs. I'll shut up now. Regards Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 21, 2018, 11:07:11 AM
Hi Paul, so long as you are gaining understanding, I am quite happy to continue.  The number of reads is still increasing, so I am assuming many others are reading too, but not saying anything.  I hope I am not too intimidating, it would be nice to hear occasionally what they think, and I am sure there are many other questions on people's minds.

As you say good temperature instruments are quite cheap these days, and the function is built in to most digital voltmeters.  The Jaycar catalogue and the online data gives the temperature limits, and many go to 1000 C.  The bare welded junction I showed in some earlier posts is probably not up to being inserted directly into flue gas, but in a suitable thermowell I would expect it to be fine.

I have one of the sheathed type, similar to the one in some of Willy's pictures.  I use it in the stack.  I keep meaning to do the finger test at various points along it to see how it would go in a thermowell as it seems a bit more rugged than the others. 

I will keep those firebox dimensions in mind as the calculations develop.  Not much progress on them today due to diversions I mentioned yesterday.  I like the idea of insulation inside the firebox.  Performance without detracting from appearance.  Most industrial furnaces are insulated on the inside.  Keeps the metal cooler, so stronger when you are trying to support a tall stack with attendant wind and potential earthquake loads.  An extra layer on the outside raises the metal temperature unnecessarily.  Inside the firebox is pretty rough on insulation.  Gas velocities fatigue fibres, and even erode solid reinforced concrete insulation.  I would suggest replacing what you have with the original 3 mm, and if you want to experiment, try two layers, but both on the inside.

A thermocouple (in a thermowell) in that chamber at the front of the boiler would be interesting to try, so long as it can be put so it is in a gas stream, but not in direct view of the flames, where radiant heat would upset the reading.  But more important, one at the entry to the stack so it gets a roughly uniform outlet gas temperature, as the flame temperature cancels out in the air flow calculations, but the second equation allows calculation of the gas temperature, well enough for our purposes.  I will look up argyle, they must be near miniature steam, who were shifting up there last time I visited.

I mentioned the trials of working on a troublesome thermocouple yesterday.  Did a run today.  Apart from being unusually slow to start, it went quite well and behaved normally once it reached about 30, until just before I quit taking the cooling readings.  I wanted just one more at 45, having a good series down to 50 when all of a sudden the boiler temperature jumped to about 70.  I rattled things around, took the thermocouple out and it immediately started falling to ambient.  Put it back in the boiler filler plug thermowell, same thing.  When I pulled it out again it responded well to the finger pinch test, so I swapped it with another that had been reading engine inlet during the run.  It also read the higher temperature.  It seems to be a valid reading, on two different thermocouples, yet not consistent with the stack temperature which was around 45 and infra red readings all around the outside of the casing.  Can't explain that one at all.  Could not see or feel anything untoward.

More analysis in the next few days, but the superheater does not seem to work well with the small burner in the large boiler, whether the burner is raised or not.  I think I need to go back to the bigger burner.  And I will also try running the 50 mm burner in the open to see if I can get an idea of how uniform the flame is over time.  I suspect it is a bit slow to warm up, then cools off a bit towards the end.  If it burns well outside the boiler, I will experiment with enlarging some of the vapour holes.

On the surface, a perfectly normal run, but amazing how much you see when you sook very closely with a few simple instruments.  Perhaps I am looking too closely.  I need to take a step back and assess where I am up to.

Thanks to everyone looking in, don't be shy about comments or questions.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 21, 2018, 11:45:00 PM
HI MJM, still looking in......Does your meths burner give you the most amount of heat possible as i have seen drawings where the meths is heated and evaporated ,then ignited under pressure into the boiler ?? these are ideas from 1901 though !! also is the short letter about aluminium true and if so when you heat the AL and Antimony to the lowish temperature  to melt ,how do the get to the higher temp ?? or do the two metals freeze on contact ?
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 22, 2018, 11:49:09 AM
Hi Willy, I think the amount of heat per kg of fuel is pretty much fixed by the chemistry.  However different burner designs may burn more fuel per second in the given size of furnace, and hence raise more steam.  More excess air lowers the maximum temperature attained, but more gas at lower temperature is still the same amount of heat.  Pressurised fuel with a vaporiser would force fuel into the burner at a higher rate, so releasing more heat per second, but only the same amount per kg of fuel.  That stove burner I showed some time back vapourises the fuel in the pipe that passes through the burner and boils a kettle quite quickly with no sign of a wick.  The pressure available is only that due to the height of Meths in the fuel tank, perhaps two inches depending on whether it is nearly empty or just filled.  The biggest problem is that if the flame does go out, the fuel continues to flow.  Not good if there are still hot surfaces.  I can increase the quantity of fuel for a longer run (if I can supply enough water) using a chicken feeder arrangement, but I am not sure what arrangement the pressurised ones use, unless it is a variation of the ones that were used in times past for kerosene or petrol.  I keep seeing references to the "silent type", that appear to have some vaporising provision, but I have never seen a design.  Do you know what they look like? 

I still need to experiment more with the size of vapour holes in my burner, and need to try enlargening them further, even to the point of making them too big and having to make a new burner.  But I would like to try one of those silent ones.  They look a bit like those poker burners that Chris mentioned, but with the vaporising tube along the top for liquid fuel.

That is an interesting article about the alloy melting temperature.  Generally in a two phase mixture, even when the phases are liquid and solid, the mixture properties are somewhere between the properties of the components, proportional to the composition.  But some mixtures form a different "compound like" structure, that has properties higher or lower than either of the components.  Such as in this case.  It has implications in some metal refining processes that involve progressively melting a section along a bar so some of the impurities float out to the end.  But that intermediate composition would limit the purity that could be obtained.  It would act a bit like a mixture of that alloy and one of the components, depending on which component was most prevalent in the initial mixture.  I suspect it is similar to distillation of ethanol and water, you can't get past that 95% ethanol mixture, which ever side you start. 

But your article is particularly pertinent for me at the moment.  I recently bought a book of recent science writings, in which one essay was about "Impossible Alloys".  A researcher, thinking about how alloys behave, like the small amounts of carbon in iron to make steel.  Generally the alloys are added in small amounts, and too much makes the material brittle, like a very high carbon steel.  He wondered if you go further and add very high amounts of the alloy material, would you get past this point and further improve the material.  The idea was that more chaotic structure is lower entropy, and hence more stable.  Hence they are called low entropy alloys.  He did some experiments and found alloys that did not follow any of the conventional rules so was stuck with trying random combinations, of which there are too many millions to exhaustively try.  But following the line of research is leading to high melting point stronger alloys for gas turbines and similar exotic materials.  Others have found that the copper in brass and bronze can be replaced by a lower cost mixture of aluminium, zinc, nickel and manganese, to make a stronger more wear resistant alloy which is potentially lower cost.  Unfortunately they did not mention machinability, is it too rude to say writers do not think of such things?  But keep a look out for "Low Entropy Alloys".  Possibly from Ampco Metal in Switzerland.  You might be able to update your engine with low entropy metal for the cross head slides and bearings!

I am continuing the analysis of my recent boiler runs in the background.  Trying to get some interesting results to talk about, without having to post too many equations.  It is going a bit slow, as we have the carpet layers coming Monday.  Carpet through most of the house is as bad as painting, in terms of the preparation and restoring of order after that is required.  Everything off the floor!  In the whole house.

Paul, I had a look at my Jaycar catalogue today, it is time I got a new one.  But it confirmed 1000 - 1200 degrees for the welded junction and its insulation, though I would suggest installing it in a thermowell to isolate it from flame or steam.  However the plastic handle at the end of the steel sheathed model that I used for the stack temperature limits that type to only around 250 deg C.  I was a bit suspicious of that handle, and if you look back to the picture, I fixed it so the plastic was to the side of the stack.  I don't think I exceeded 250 anyway, or at least not by much, and the handle still looks ok.  The industrial ones used in the hydrocarbon and other industries do have a more rugged arrangement with a metal sheath, but then they are used at all temperatures, 24/7, 365 days a year.  And inside they are just similar type k thermocouples.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on February 22, 2018, 11:57:23 AM
Hi Willy, apologies, I missed your question -  I imagine that if you have the two metals, both molten at a temperature somewhere between the 650 of the components and the 1000+ of the alloy, and mixed them, that you would get a solid phase forming, presumably floating or perhaps finely dispersed, a bit like a hot version of a slushy drink.  Just as if you melted the alloy and then cooled the liquid, you would get the solid phase forming, either floating or finely dispersed.  I am not a metallurgist, so have not seen it done, but I can't imagine how else it would happen, but I don't have any reason to doubt the basic facts of the letter.  Perhaps some of those who do their own casting can comment.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 23, 2018, 12:09:28 PM
Still continuing to do the analysis, but slow progress with all the moving books and all other small objects.  My wife said if she ever feels like replacing the carpet again, I should put her to bed with an Asprin until the feeling passes. 

However I did put together the equations for combustion of ethanol in air, and calculated the flue gas composition for exact theoretical air, two times theoretical and ten times theoretical air.  Next step is to estimate the specific heat for these mixtures.  It is interesting to see that while the mass of the fuel plus air equals the mass of flue gas, the volume of flue gas is greater than the volume of fuel (as vapour) plus combustion air if compared at the same temperature.  It is only about 7% for ethanol as fuel! though it is much more for longer chain liquid hydrocarbon fuels.  As the excess air just travels through absorbing heat, so it dilutes the flue gases, so reduces the change in volume.  And of course because it's temperature of the flue gas is higher, the volume of flue gas is even greater.  Worth considering when thinking about the diameter of the stack, but also any other restrictions to the gas flow.

As my first estimate of the air flow from the test run used the specific heat at ambient temperature, I will calculate a better value.  Equations for the variation of specific heat with temperature are available, and I found the table for all the flue gas components is included in an appendix to the thermodynamics text, so this gives the opportunity to calculate a better value, which will improve the accuracy of the calculated temperature and air flow.  Then I can go on to calculate those heat transfer coefficients. 

I also managed a few minutes in the shed, and tried that "pinch test" on the metal sheathed thermocouple.  It responded quite quickly to the last 20 mm being pinched between the fingers.  It also responded when gripped in the next 20 mm, if slightly slower to respond.  When pinched in the third 20 mm, the response was still obvious, but much slower than when the fingers were closer to the end.  This suggests to me that the heat is conducted along the sheath, and so affects the reading.  To minimise this effect, I suggest it will be worth slipping some heat resistant insulation over the portion of the sheath which is outside the temperature you want to measure.  The electronics shop sells a woven glass fibre tube that will work quite well, so I need to pick up a length of that and put it on the part of the sheath which sits outside the stack.  Probably better to use the bare thermocouple inside a thermowell when practical.

Not much more to report.  I hope to make some more progress on the calculations tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 24, 2018, 11:17:29 AM
There is always a danger of loosing direction when you get too caught up in theoretical calculations.  This is not helped as the spreadsheet gets bigger, but you can only see a small part at a time.  It is worth stepping back for a few minutes and taking stock of progress and checking that the direction is still appropriate.  After all the purpose of all this investigation is to use theory to improve our boiler and engine operation.  Despite appearances, I am not into a purely theoretical exercise, I want practical results.

The initial reason for testing a model boiler is to see how much of the energy released by combustion of the fuel actually ends up in the steam.  In Willy's electric boiler, it is easy, as the electrical energy input either ends up in the steam or it is lost through the outer surface of the boiler.  The cooling tests provide a good method of measuring the heat lost.  A measurement of the water lost from the boiler in steam and the time over which actual steam production occurs tells us the rate at which steam is produced, from which we can make some estimate of the engine that the boiler would drive, and a check estimate of the heat losses.  We can reduce the heat loss by adding insulation.

In a fired boiler, it is a little more complicated.  First, heat is necessarily lost in the flue gas, and in addition there is heat lost through the furnace wall.  In my simple boilers the efficiency for steam production is quite low.  It seems worth trying to increase this efficiency before I try and make a bigger burner.  So it is worth trying to understand where the heat goes.

The flue gas obviously carries heat to the atmosphere.  There are two main contributors to the amount of heat in the flue gas.  First, we can't practically cool the flue gas below the steam temperature, and in fact, the outlet temperature has to be enough above the steam temperature to provide a reasonable temperature difference to transfer heat across the Copper to the water. 

We can measure the stack temperature, but we really need to know the gas flow rate and gas specific heat if we are to determine how much heat is in the gas.  So far I have progressed the calculations to the point where I found that we can estimate the air flow of we measure fuel consumption, ambient temperature and stack temperature.  This was a pleasant surprise, it is not always easy to anticipate what can be calculated until you actually try.

Calculating the air flow then revealed a second factor in how much heat is lost in the flue gas.  Burning fuel requires a certain amount of air to provide the necessary oxygen.  And the air comes with nitrogen which simply absorbs some of the heat.  But in practice, to get complete combustion, we actually need excess air, otherwise the dynamics of the combustion reaction tend to mean some of the carbon ends up as carbon monoxide.  This is undesirable, firstly because carbon monoxide is extremely toxic, even in quite small concentrations.  But secondly, burning the carbon monoxide in air to get carbon dioxide releases more heat.  The heat lost in incomplete combustion is a greater penalty than the loss to more air flow through the boiler.  In industry somewhere in the range of 15 to 20% excess air is found necessary in a well designed burner.  I found that I had more than ten times the theoretical air.  Never mind the accuracy of the measurements, that is potentially quite a penalty.

Why is excess air a problem?  Well, when the fuel is burned the heat released is all absorbed by the gaseous products of combustion, and also by the extra unnecessary air.  The temperature reached depends on the mass of air flow.  More excess air does not reduce the heat released in combustion, but it does reduce the maximum temperature reached.  As the flue gas progresses through the boiler, it is the temperature of the gas stream that determines the rate of heat transfer to the boiler.  Lower temperature means less heat transfer, even though the gas stream contains the same amount of heat.  And more mass going up the stack is also more heat loss.

If I can reduce the excess air, while still leaving enough for complete combustion, I will increase the maximum temperature in the firebox, and get more heat transfer to the boiler, so more steam.  Early quick tests by just closing off some of the air intake were inconclusive in terms of actually seeing a difference, however, with the more detailed test, I am hoping to see an increase in maximum temperature reached.  That may even result in a higher stack temperature, (I am not sure if it will actually be higher or lower) but with a smaller flow it will be less heat lost up the stack. So should also be reflected in more steam production.

So that is the first line of research/experimentation.

 I would also like to get an idea of how much heat is lost from the furnace walls.  I have made a crude attempt to reduce this heat loss by applying a layer of suitable insulation.  This gets the wall temperature to a level where I can use a more effective insulation like cork or fibreglass, even rockwool, which will be suitable but might melt against the hot steel casing if I did not have that first layer.  Insulation suitable for the higher temperature is generally less effective as insulation.

But the issue is whether I can estimate how much heat is lost from the furnace wall?  Is it important compared with the heat into the steam or the flue gas?  It's only worth reducing it so far.  So the convection calculations are aimed at estimating the casing heat loss.

These calculations get pretty heavy.  Not sure how far I will get, made some definite progress but I am wondering how much more effort is worth putting into it.  I am certainly clarifying my understanding of convection and will in due course will describe it in a bit more detail.  Perhaps I can see a way to make a very approximate estimate as a start.  I am also starting to think about whether I can do a simple test to make a measurement. 

The cooling test worked very well on the electric boiler which was completely insulated, and yielded very interesting results.  But the fired boiler has to have flue gas.  So I can't easily separate the flue gas losses from the wall losses while firing, but I am wondering if, when the burner is extinguished, I could block the stack with perhaps a cork in the top of the stack, the cooling rate for the boiler would then be essentially due to the heat loss from the walls.  That cooling test might give an answer, accurate enough for the purpose, and a lot quicker.  Besides its more fun to play with fire and steam than sit in front of even the most fascinating spreadsheet!

Food for thought while I shift more furniture tomorrow, then Monday is the big day.  Though then everything has to be put back.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on February 24, 2018, 12:05:42 PM
Hi MJM, It's been excellent weather for frogs, so been out in the field for a few days. Very good to read the review of the work so far, and the projected exploration going forward. Helps to keep the objective in mind. Was interested to read your comment on 'high temp.' insulation being less effective, so will have to carry out a few comparative tests between the ceramic fibre/kaowool sheet and cork of similar thickness. Can you inform me what the max temp. thin cork sheet, say 2 or 3mm might take before becoming damaged. Found one reference giving 140 C. as the high end but don't know if this is the maximum. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 25, 2018, 10:36:46 AM
Hi Paul, I hope you were able to find some interesting frogs.  Do you count them, or try to find new varieties?

I find with a spreadsheet, that once you get out to around 200 rows or columns it is easy to loose your way.  Perhaps a bit before that!  It is often necessary to use a few columns or rows to just copy key earlier ones, so you keep the ones still being used in new calculations close to the viewing screen.  In addition, a little paragraph to summarise where I am up to helps keep me on direction, a bit like a road map.

I don't have any data for the limits for cork, but basically it is a wood product, it is flammable and so I would not put it where it is exposed to the flames.  In fact, that manifold insulation material that I used is barely keeping the entire outside surface below 140C, though certainly most of the surface is below.  Similarly fibreglass has a melting point that makes me cautious about using it in flame contact.  Possibly there are binders that don't help.  There is a big variation in the conductivity of various materials, but generally structurally strong materials have higher conductivity than typical insulation materials.  For example, cork 0.04 W/m^2.K,  similar to other good insulation materials, compared with magnesite (50% MgO) 2.68,  brick, about 0.4, concrete about 0.13.

When I find a material that I am unsure of, I generally test its flammability with a Meths burner, then with the propane torch I use for silver soldering.  If it catches alight, or just starts smouldering, I generally class it as outside/low temperature insulation only.  Probably needs a longer exposure to be very sure, but there is no need to persist with something doubtful in the quantities we need.

Ceramic fibre is used internally in industrial furnaces, especially where the furnace is to be factory fabricated, as a whole or in pieces for field assembly, it is light weight for transport and lifting on site, and does not lead to big distortions in transport.  However, the fibres suffer from fatigue in high gas velocity areas, so tend to need periodic replacement if subject to high gas velocities.  I would think this would take a long time in a model, so with a suitable fixing system it could be quite suitable.  With the insulation on the inside, the outside temperature can possibly be kept low enough for a suitable paint system, as well as retaining a scale profile.

Forced convection turns out to be a bit simpler than natural convection as the velocity is known, and there are exact solutions to the calculation of the velocity profile, and from this, there are methods to calculate a temperature profile.  Much of my text book is devoted to examination of the difference between exact solutions and assumed polynomial velocity distributions.  That and the difference between constant wall temperature an constant heat flux at the wall.  Reality is probably somewhere between these, yet the four solutions differ by less than 20%, so of the real solution is between the values it is probably within 10% of the average of the four.  Not only does that seem adequately close for our purposes, but at the end of the day, most of the examples around heat loss from surfaces at moderate temperature seem to give a heat transfer coefficient of around 3 to 5 W/m^2.K, so I might try using four, and see how important the heat loss is compared with heat absorbed by the steam and in the stack gas.  This will indicate whether I more insulation would be worthwhile, or perhaps the wall loss could be ignored.

With a stationary boiler, the heat transfer from the walls is by natural convection.  More complicated,as the heating of the air near the wall gives rise to the density changes which cause buoyancy effects to drive the air flow, and to viscosity changes which change the flow behaviour of the air.  Different answers for vertical, sloping or horizontal plates.  I am still trying to understand the implication of the equations, and I am looking closely at the range of answers to comparable worked problems, such as the cooling of an insulated oven.  Again, I may be able to find a suitable order of magnitude answer, to compare with the heat flows in the boiler, and thus its importance.  I think the cooling test with the stack plugged might also yield a reasonable estimate of the losses at operating temperatures.  Might be more useful than extensive complex calculations. 

Well, carpets tomorrow.  House is obviously chaos, so daughter and family dropped in for dinner.  Just the day we needed visitors.  But my wife said she had watched my mother cope with such things many times so knew what to do, and did quite well.  What a wonderful woman.  I did pitch in and peel extra vegetables and lift the heavy dishes in and out of the oven, but mostly followed instructions.  I am not much of a cook.

Might be quiet tomorrow, but I am sure I will check in to keep up with progress on all the wonderful builds.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 27, 2018, 01:44:04 AM
Hi MJM  still following along saw some interesting things in my books and on the web  Farienhiet scales and  stem conservation and heat properties of combustible materials ....Also what is 'wood alcohol'....
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 27, 2018, 11:30:58 AM
Hi Willy, more interesting historical information from your archives.  I suppose it could have been decided that the human body temperature was 100 and adjust the water boiling point accordingly (at about 213,) but water boiling at atmospheric pressure is probably a better reference point.  Did the article about that locomotive explain the purpose of the water tank above the boiler, other than holding water, of course?

Hydrocarbon chains with an -OH group are called alcohols, but not rings containing the same group, which are called phenols.  Methyl alcohol, also called methanol has 1 carbon plus 3 hydrogens plus that -OH.  Compare this with methane with one carbon plus 4 hydrogens.  Unfortunately methyl alcohol is quite poisonous, and is added to ethyl alcohol to make it undrinkable in methylated spirits, bit good antifreeze.

Ethyl alcohol is the one in drinks, it has two carbons, 5 hydrogens plus that OH.  Some would say it is also poisonous!  Compare this with ethane.

Three carbons gives propanol in the similar pattern.

Methanol was called wood alcohol in the early days of humanity, as it was generally obtained from a destructive distillation of wood.  These days it is synthesised in a methanol plant which uses natural gas, methane, which is burned with insufficient pure oxygen so carbon monoxide production is maximised, then a catalyst is used to produce the methanol, used as a chemical feedstock.  I can't clearly remember all the reactions, but I believe one of the New Zealand forum members worked in methanol plants at one stage.  He may be able to amplify the details.  I always pull up short at the concept of burning natural gas in pure oxygen, no matter how much short of the theoretical oxygen required; intuitively, that seems unwise.  Definitely something not to try at home!  But it illustrates the point that even with an accelerant, for which pure oxygen is about as good as you can get, if you don't have enough oxygen, you will get incomplete combustion.

Well, I decided I had done enough reading about convection and that it was time to try the calculations.  Convection calculations involve a lot of empirical correlations, and the amount of work to do the experimental work is reduced by correlating using some of those dimensionless groups of variables that I mentioned early in this thread.  Most of us have heard of Reynolds number, if you are interested in boats you might have heard of Froude number (or at least used it in terms of a critical speed in knots equal to square root of the waterline length in feet, not exactly dimensionless, but...).  Well convection involves many more.  Numbers like Prandtl number, Grashof number, Rayleigh number, and even a few more obscure properties of air.  Then correlations are found by plotting experimental results in these terms and determining the equations that best fit.  Even the so called "exact solution" that I found, turned out to be an exact solution to equations that were approximations. 

The resulting equations are quite intimidating, I don't think any of you want me to post them.  I don't think I would have tried them without a spreadsheet.  Truly horrible things to input to a calculator.  However, I was quite surprised to not only get an answer to the calculation of the heat loss, but it even seemed quite reasonable.

I used a rough average of the wall temperatures I measured with the infrared thermometer of about  100 C and an ambient of 20 C with the 50 mm burner.  I measured up the outside surface dimensions of my boiler casing, and looked up all the air properties at the appropriate temperatures, and got an answer of 90 Watts.  This is about 15% of the heat input of that small burner, so it's a significant loss, but intuitively, (that dangerous word again) it seems to be about the right magnitude.  Most loss was from the vertical walls and ends, and the inclined sections between the walls and the roof, and only 10 of the 90 Watts from the flat roof.

The heat transfer coefficients came out as part of the calculations, showing some interesting aspects.  The ends are taller than the side walls and have a lower average heat transfer coefficient.  I think this is because the air heats as it rises, so the temperature difference, and hence contribution to heat transfer of the area located at the top of the walls is less.  6.5 for the walls compared with 6.15 for the taller ends.  The inclined surfaces had a higher coefficient, 8.31 which reflects the much smaller height, but I think the calculation assumes cool air at the bottom, whereas those inclined surfaces will have had warm air from the top of the sides.  As the formula are the same for both, I possibly should have just treated them as one taller surface.  This would give a coefficient similar or a bit lower than the ends, perhaps 6.0.   The flat top had an even higher coefficient, 10.2, but again the cooler incoming air is actually already warmed by the sides and inclined walls.  The small contribution to the heat loss despite the higher coefficient is due to the smaller area of the roof.

Two important comments in the the book, first, the accuracy is not likely to be better than +/-25%, secondly, there will also likely be radiation which may be a significant magnitude.  It just said most analytical approaches assume only one mode.  I suppose I could try to estimate a radiation transfer rate and add the two.  But not right now.  It is clear that the heat loss is probably enough to make additional insulation worthwhile.  If I can get a lower surface temperature, I can change that figure, and also the ambient in the spreadsheet, and even an iPad will redo the whole calculation in a flash.  The difference will be more instructive than the actual figure.

I think the next step is to try that cooling experiment, with the stack plugged, and see if that gives anything similar.  Have been putting the house back together.  The carpet installation looks good, and we are pleased with the colour choice, but putting everything back on its place will take priority over more testing.  Then there is a wooden boat festival next week.  What did Chris say about too many hobbies?

I will also try that forced convection calculation for Pauls locomotive.  A similar problem to the inclined walls on my furnace in that the sides of the firebox of the locomotive are in the air stream already warmed by the boiler in front.  But the boiler has a different wall temperature to that of the firebox.  I will think about that for a bit longer but feeling encouraged.

I hope that is all of interest,

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: paul gough on February 27, 2018, 01:16:47 PM
Hi MJM, Very interested to read most of the loss was from the verticals , sides and ends, and little from the top. This is pretty much describes my little loco firebox as the top is the underside of the boiler barrel and not a loss. Your figure of 15% loss is also noted, notwithstanding the caveat of +/- 25%. 15% seems to be a reasonable proportion and is probably worth chasing any means to reduce it. I find these explorations fascinating and the findings instructive, thank you for the mental toil. I look forward to more revelations!

I'm just about to depart for some more 'frogging', locally. 10pm to 2am has proved to be a pretty good time range around here, but after a week of it my old carcass is starting to flag. Mostly we just note the species found and where but anything pertinent is noted also. Rarely anything out of the usual, but even just raw numbers help in indicating trends in depopulation or disappearance, all too frequently the only circumstances that prevail. We did have a little highlight the other night when we came across a scrub python with a belly bulge, (probably someones chook), the snake was approximately 4 1/2 metres long. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 27, 2018, 10:58:50 PM
Hi MJM, The loco is using the spare steam as a feed water heater during the stationary and less demanding cycles  to heat the water that is held in the large tank. this is then pumped into the boiler using axle pumps rather than using the injectors to push in cold water !!..Interesting info on different fuels ...Peat having a really high flame temp .... but i don't understand the other parameters ! Does radiated heat cause its own 'draft' pushing the convection heat away from the boiler ? or how do these interact ?
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 28, 2018, 12:17:19 PM
Hi Paul, the greater heat loss from the walls of my furnace is mostly due to the greater area.  Remember it encloses the whole boiler.  The horizontal roof is a small portion of the area.  The heat transfer coefficient is actually higher than for the horizontal surfaces, though I suspect it is a high estimate, as the formula assumes it is ambient air coming in at the sides.  In my boiler configuration, it is the warmed air from the sides doing the cooling of the top.  This reduces the effective temperature difference and so reduces the heat transfer rate.

As always, it is important to consider the meaning of a percentage.  The calculated heat loss was 90 watts, that is 15 % of the burner output of 620 watts, more accurately 14.5%.  The +/- 25% is +/- 25% of 90, so +/-22.5W.  So 90 + 22.5 = 112.5 which is 18% of the burner output of 620 watts. Then 90 - 22.5 = 67.5 which is 11%.  So plus or minus 25% of the heat loss means 11 to 18% of the burner output or  +/- 3.5% of the heat output.  So while 25% error looks bad, it is only 3.5% of the heat output.

You boiler is a quite different configuration.  With the boiler on top, or even forming the top of the fire box, obviously no loss there, the heat goes where it is wanted.  Depending on whether the sides of the firebox are inside or outside the frames, they may or may not be subject to that air velocity caused by the motion.  The bottom is the air inlet, with air flowing in, there is no convection out, however you can probably see reflections of the flame on the tracks, so there is a radiant loss.

The front of the firebox is facing the air flow, but located between the frames, behind the cylinders axles etc, above the tracks and below the boiler, so probably minimal air flow, and the air is probably heated by the boiler and the cylinder heat losses.  This heated air is the main flow into the burner which thus receives preheated air, so your locomotive actually uses waste heat to preheat the air, the classic waste heat recovery system!  It may not be very efficient, but every bit helps.

Not sure about the back of the firebox.  Any air flow in this area is preheated by all the sources I have mentioned.  In addition it is probably easier to tuck a bit of insulation on the outside of the box/duct taking gas from the flame to the centre flue.

I know I have mentioned placing insulation inside the firebox.  This is based on assuming the sides of the firebox are visible in a side view of the engine.  However I suspect I do not really have a complete picture of the arrangement.  With the firebox under the boiler, possibly silver soldered to the boiler, then the sides would be an important "fin", conducting absorbed heat to the boiler, and insulation on the outside would be better, if it is hidden between the frames.  I think I need a cross section through the boiler and firebox to better understand the arrangement.  And a side view of the locomotive.

Counting frogs gives an early indication of possible problems.  Without the count, that early warning is missed, so counting them is an important contribution to research.  I am afraid that I removed the hours from midnight through to about 6 am from my clock some time ago.  They no longer exist!

Those pythons can give you quite a start.  Especially if you are from the western district where snakes are all black, brown or tigers and all quite poisonous.  My wife stepped out of our van in the dark one night when it was parked on our daughters front lawn in Katherine, NT, and nearly stood on the front end of one, of which the back end was still hidden in the garden bushes over two meters away.  The scream brought the whole neighbourhood running.  Right in the city!

Hi Willy.  An interesting idea.  I guess the high mounting of the tank provided a bit of extra inlet pressure to the axle pumps, so the water could be heated to a higher temperature before it vapour locked the pump.

The thing about those flame temperatures is that they are all quite close to 2000 deg F.  The small differences are due to the different air fuel ratios required by the different carbon to hydrogen ratios of the fuels.  Twelve kg of carbon requires 32 kg of air for complete combustion, while only 4 kg of hydrogen requires that same 32 kg of oxygen.  And hydrogen releases over 4 times as much heat per kg of fuel in burning to make water compared with carbon burning to make CO2. The carbon to hydrogen ratios only vary a small amount, reflected in the very similar flame temperatures and heating values.  But peat is usually water logged, so I presume the figures are based on dried fuel, without penalty for the heat required for the drying.

Those other columns are the calorific values of the fuels.  As you can see, there is a higher value and a lower value.  The difference is whether the water is condensed from the flue gas, thus releasing the latent heat.  The lower value is applicable to boiler work as it is assumed the water is lost up the stack while still vapour, this carrying away that latent heat.  Again, there is a quite small range of differences between the higher and lower values, reflecting that small difference between the carbon to hydrogen ratios, and hence the amount of water vapour in the flue gas.

In this table, the scientist uses calories as the unit of heat.  Each calory is equal to 4.180 joule.  When I was at high school, that factor was called the mechanical equivalent of heat.  It was experimentally determined by the pioneers, before the clear understanding of the first law of thermodynamics, so there are a few definitions, each giving a very slightly different value.  However, the Joule is defined by reference to the unit of mass and length, so is much more precisely defined, and is the preferred unit for heat measurement.  The calorie was defined as the heat required to raise 1 kg of water by 1 degree C (similar in principal to the definition of the BTU.)

Air is transparent, or nearly so, to radiant heat.  It is essentially unheated by radiation.  Radiated heat from the sun warms the earth which in turn warms the air by convection.  There are small differences in the degree of transparency of different gases, and the transparency also varies slightly with frequency or wavelength of the radiation, leading to the greenhouse effect, but that is a different subject.  For the purposes of our analysis, it is close enough to assume air is transparent to radiant heat.  This means we can calculate the radiant heat loss and the convection loss separately, and simply add the results.  There is no "pushing of the air" or separate draft.

That is a long enough post without my introducing anything new, but very interesting questions.  I hope my answers help with understanding them.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 01, 2018, 11:54:21 PM
 Hi MJM, thanks for the info.... and we are experiencing a severe cold weather event and there is a wind chill factor making it extra cold, I have heard that for every mile an hour of wind your body drops one degree ? is this true and if you are on a motorbike at 100 miles an hour do you get quite cold !! also does this have any effect on steam railway locomotives ? When flame fuels burn the oxygen in the air does the fire then burn the hydrogen content as well and create water ? or is this another silly question ? !!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 02, 2018, 12:08:02 PM
Hi Willy, wind chill definitely cools the body faster than still air, but it is not as simple  as a direct linear relationship with speed.  Basically the wind just increases the heat transfer coefficient between your body and the air.  It accelerates your heat loss, but the wind cannot cool you to a lower temperature than the air.  Of course, if you are sweating, or wet from rain or mist, (or if you have been swimming), the wind can evaporate that moisture, and cool you a bit further.  If the air is humid, the lower limit is the dew point.  It is a bit more complex to calculate the limit if the moisture is from rain etc, but the more moisture in the air, the less can evaporate, so less cooling effect.  If the wind is off the desert on a sunny day, and the actual air temperature is above 37 degrees, it can only cool you by evaporating sweat, otherwise it actually heats you.

Essentially the question is very relevant to the convection discussion.  The wind just increases the convection transfer coefficient.  The simplest treatment of convection is to assume it is like conduction, using the air thickness over which the air reaches that wind speed.  Natural convection has a relatively thick boundary layer, so the temperature gradient is not very high.  Wind effectively makes the boundary layer much thinner, so the temperature gradient, and the corresponding heat loss is much higher.  This higher heat loss will lower your skin temperature, and eventually cause hypothermia.  Feeling cold is a warning to do something about it.  Layers of clothes modify that temperature gradient, but the wind making the air boundary layer thinner, then makes the outside temperature of the clothing lower, thus increasing the heat loss. So you need an extra layer to keep your skin temperature comfortable, normally about 34, compared with your core temperature of 37.  But as our Scandinavian colleagues will remind you, there is no such thing as bad weather, just bad clothing!  So wear an extra layer or two, preferably a wind proof one if you are intending to travel at that speed.  Generally that wind chill factor the weather bureau will announce is a temperature which is estimated to produce about the same heat loss as you will experience in the wind, hence it is an estimate of the temperature it will feel like.

Under normal metabolism, the average human produces about 90 watts.  So comfort is about balancing the heat loss with that heat production.  To much loss and you feel cold, to little and you feel hot.

When the fuel molecules have carbon and hydrogen, so all petroleum fuels, methyl, ethyl and propyl alcohols, which all seem topical at the moment, and all coal, then combustion produces carbon dioxide and water.  As you say the oxygen in air, which is in the form of molecules each of which contains two atoms of oxygen, combine with carbon to make CO2, and hydrogen to make water.  Each 2 hydrogen atoms combine with one oxygen atom to make H2O, or water.  So you need four carbons to combine with one O2 molecule.  That is why we have to decide whether the water in the flue gas will condense or not, and so have a higher and a lower calorific value.  Not a silly question. 

If there is sulphur in the fuel, so some hydrocarbon fuels and many coals, sulphur is burned to form sulphur dioxide, not good at all, which is why refineries are normally modified these days to remove sulphur and produce low sulphur fuels.  And so we all need to pay the extra fuel costs for this additional processing.  If the combustion is really hot, you can even get a small amount of nitrogen combining with oxygen to give various oxides of nitrogen.  So cars these days have relatively low compression ratios, it reduces the production of oxides of nitrogen.  However the majority of the energy release comes from the carbon and hydrogen reactions, the others are more important as pollutants than in energy release.  So not a silly question, I hope the answer makes sense.

Sorry about being absent yesterday.  We arrived at the "cottage" and had no power.  No power means no refrigeration, or limited to about two days, which our batteries can supply.  Our solar panel is enough for lighting, but we need a bigger panel to cover the fridge consumption.  So lots of testing and investigation.  I stop short of sticking the meter probes into live 240 v though, so had to call an electrician.   Turned out the power cord had failed!  Looked OK on the minimum load of a multimeter but significant resistance when tested under load.  So a new cord, and all is well.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on March 03, 2018, 11:36:56 AM
At the wooden boat festival today.  What a wonderful display.  Nearly two hundred wooden boats of all ages and designs.  From the magnificent Lady Nelson, a historical square rigger, to little rowing boats and wood strip sailing canoes.  A really good mix of power, sail and even steam.  All beautifully built or restored and presented with varnish gleaming and paint perfect.  An amazing display for a small town far from the big cities.  I will try and put up some pictures next week when I am again at my computer.

A few days back, I had a go at calculating  the convection heat loss from my boiler, the larger one with the completely enclosed firebox and boiler.  The outside of the casing has a layer of manifold gasket material from the local Autobarn as heat resistant insulation.  I basically just looked up the necessary properties of air at the estimated average wall temperature on the outside of the insulation of 100 degrees C based on several infra red thermometer readings of the casing temperature.  Then inserted these values into the equations presented in my heat transfer text book for natural convection from vertical and horizontal surfaces.  I put all the necessary equations is a spreadsheet, so the calculations are easily checked and easily modified if necessary.  Using an ambient temperature of 20 deg C the answer was 90 watts.  You will remember that the calculation only claimed to be +/-25%, which as a percentage of the total burner heat release of 620 Watts, does not seem to bad.

However, more important than the actual figure is the impact of small changes in some of those variables.  For example, I have been asked about the impact of a variation in ambient temperature.  Using a spreadsheet makes answering these "what if?" questions very easy.  At least it is easy so long as you set up the spreadsheet with a specific cell for each of the relevant variables.  Then you use that cell location in any formulae that use the values, rather than inserting the figures directly.  Then, by changing the number in the cell for the variable, the computer, or even iPad instantly recalculates the whole spreadsheet to give you the new answer.  Not for the whole boiler performance, but just for the convection loss contribution to performance.

The first item I wanted to explore was the effect of how I calculated those inclined sections at the top of the walls.  My first calculation treated these separately.  In case that description is a bit confusing, I have attached again an earlier picture of the boiler prior to lighting up for the last set of tests.  Now that means the calculation assumes the air temperature at the bottom of the inclined section was at ambient temperature.  When I thought about it, the inclined section starts at the top of the vertical walls where the air is already warmed by the vertical section.  Perhaps it would be better, seeing that the equations for an inclined wall are the same as those for a vertical wall, to just add the relevant inclined length to the height of the walls and treat them as one higher wall.
Originally I had calculated 30 watts for the vertical parts of the walls and 14.4 watts for the inclined sections, a total of 44.4 watts.  The calculation for just higher walls gave 38.1 watts, and then of course no additional for the inclined.  So the total is now 38.1 which is 6.3 watts less in the total of 90.

This makes sense, as the the section at the top of the higher wall sees warmer air, so has a lower temperature difference between air and the wall, so less heat loss from that section.  Essentially the take away learning is that if we double the wall height, the total heat loss will be higher, but less than double.

Second, I tried varying the ambient temperature.  The initial figure of 90 W was derived for 20 deg ambient.  When I recalculated for 15 degrees, the loss became 97, while for 25 deg it became 83 W.  This just reflects the difference in temperature between the wall and ambient.

The third exploration was to see the effect of an increase in the measured wall temperature, more accurately stated as an estimated average of the wall temperature readings.  The tests of the insulated casing so far have been done with the small burner, which I have previously concluded is too small for this boiler.  The next series of tests we be done with the larger burner.  Regular readers might remember that this burner seems almost too big.  So a mid size burner is on the project list.  However, with the bigger burner, I would expect the flue gas temperature inside the casing will be significantly higher than with the small burner.  This will likely mean higher temperatures measured on the outside of the furnace insulation.  If the temperature becomes 125, the spreadsheet quickly tells me that the heat loss will rise from 90 to 125 watts.  This reflects the 105 deg temperature difference between the wall temperature and ambient compared with 80 deg ambient used previously.  The tests will have to wait a week or two, but it will be interesting to see the result.

That is probably enough on natural convection until I have some new results.  So tomorrow I will try and explore further forced convection.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 03, 2018, 10:51:06 PM
Hi MJM, Thanks for all these posts explaining everything you do you must have so much Knowledge about thermodynamics tucked away in your grey matter....so a question about model boats ....In my old copies of ME they talk about model yachts as 4th rater or 125th rater and everything in between !! so what does this actually mean ? Also do you know anything about the really cold weather and its effects on the body and bones that can cause discomfort etc
Willy.....
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 04, 2018, 10:52:35 AM
Hi Willy, those two questions are real outliers for thermodynamics, but like many of us I have many hobbies.  Not possible to have too many, but.... 

My collection of model boats magazine goes back to 1964.  Dis-continued the subscription unfortunately sometime in the eighties, but have bought newsstand copies intermittently since when opportunity permits.  Back in the early days, the 10 Rater used to feature prominently among the designs that were reviewed each month, along with Marbleheads and A class.  The Marblehead is similar in size to the 50/500 class in the US.  I always wanted to build one, but other pressures prevailed, though eventually I completed a Starlet design that featured as the Christmas plan.  It still sails well on the very occasional outing. 

In yacht racing, the problem is always to develop a handicap system to make racing as fair as possible between all the different sizes and designs of boats.  The aim was to balance encouragement of design progress and limits to design to minimise unequal advantage.  The rating formulae were a calculation based on various specified measurements, length, beam, draft, and many others, which gave a single number (not necessarily dimensionless) as the result.  This number was called the rating.  Yachts with equal rating were supposed to be able to raced fairly.  I don't know if the same formula was used in all classes, or in the models, or if there were variations.    But I assume each one is based on a similar idea.  And of course classes all add extra requirements, generally in the hope of making racing more equal by prohibiting features seen to provide an unfair advantage.  Similarly, the twelve metre class of Americas Cup fame was a formula based on various measurements, and if the result was twelve, the boat was classed as a twelve metre.  But I am not sure that there is anything about those boats that actually measures 12 metres.  Modern yachting uses many more sophisticated rules, some even based on complex fluid dynamics calculation, yet I don't believe there is any of them that is universally accepted as meaning that all designs of the same rating can start on one line, cover the same course in any weather, and none have a design advantage.  That elusive quest for making the race a pure test of skill, and eliminate the check book race or fancy design variations is not yet over.  Most big races consider the handicap result as the main prize, though first across the line tends to get the publicity.  But many skippers that don't win are quite inclined to blame the handicapper!  It seems that one design is about the best that can be achieved, so equal designs can compete fairly, with different class rules have different approaches to how much latitude is allowed.  A never ending topic around the bar after the race.  Essentially, some weather and wave conditions will favour a light weight design, while other conditions will favour a heavier design.

Effect of cold, almost a medical question, and if you mean extreme cold it might be better to ask some of the Canadian or Scandinavian members about frost bite.  Exposed skin in sufficiently cold weather will freeze, and like fruit in the freezer, the cells are damaged, so quite a severe injury.  I hope your two feet of snow were quickly warmed up, and not left cold for long. 

My closest first hand knowledge is when we took our baby daughter all wrapped up in her pram which I pushed in front of me, outdoor skating in Ontario.  We took her to the doctor when she developed a rash on her cheeks, the only part not totally covered.  He thought it was hilarious that two adults could come into his rooms in a small country town, without any knowledge of frost bite.  We got a lesson on how to protect her, and ourselves, in the surgery.  Fortunately not too serious, though if you know about it, you can still see marks on her now middle aged cheeks.  And of course reading about mountain climbers and those embarking on Antarctic treks, whose fingers and toes seem to suffer a heavy toll.

However, if you mean not quite such extreme cold, look up hypothermia.  Basically your body operates at about 37 deg C and can only tolerate a quite small variation in core temperature.  You know about too hot if you have ever had the flue.  You skin is normally nearer about 34, but you cannot stand much drop in core temperature.  You produce about 90 watts from normal metabolism.  If you loose much more than this, the body cannot compensate and you get cold. 

If you have insufficient clothing, we all know about shivering, but it becomes more dangerous when we get a bit colder, and shivering stops.  All the body processes slow with no good results.  Decision making fails early, so you are less likely to make sensible, potentially life saving decisions if you are too cold.  Someone else has to take charge and get you warmed up.  Preferably by wrapping you in a thermal blanket and sleeping bag so you start warming yourself.  Other heating methods are considered dangerous.  Some medical procedures cool you more than this, but only under intensive, skilled medical supervision.  But even in pleasant water for swimming, you will eventually succumb if you are not wearing an adequate wet suit, and the time you can expect to survive diminishes rapidly with lower water temperatures.  Hikers, skiers, canoeists and sailors all tend to come across the information if they keep up with practicing their sport in the outdoors.

The boat show continued today with a race.  I think I have a picture of the full size version of the model in your picture, and under full sail.  The weather was perfect, and a race with all these classic sailing yachts under full sail was something to see.  The quick and dirty boat building race was a pile of laughs, but admiring all those boats so beautifully presented, occupied us all day.  Beautiful, but I don't think I would want to take on the maintenance, especially of the many that are too big to take home on a trailer and have to be stored afloat.  I assume they have extensive covers.  Pictures as soon as I get back to my computer.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 05, 2018, 10:48:34 AM
Forced convection-

Forced convection is when the air (or other fluid) flow is imposed on the system, independent of the heat transfer.  This tends to cause a much higher temperature gradient between an object and the air, giving high heat transfer coefficients, much higher than for natural convection.  We are most interested in air cooling in the present discussion, though the same considerations apply with water cooling or other fluids.  To simplify the language, I will assume air cooling in the following.  I can follow through with the changes necessary if the fluid is water, or other fluids if it is of interest.

Think of Paul's question about the cooling of the sides of the firebox of his miniature locomotive.  The speed of the locomotive gives the free stream velocity relative to the plate.  I will come back to this example.

Fluid mechanics assumes that when there is forced convection past a flat plate, located parallel to the flow, the layer of air next the plate is stationary relative to the plate.  Then the next layer out experiences a drag force which makes it move in the direction of the air flow.  This layer is affected by the next and so on until the air, which is still quite close to the plate, is essentially the same as the flow more distant, or the free stream velocity.  Generally when the flow reaches 99% of the free stream velocity, it is considered the edge of the air affected by the plate.  This part is called the boundary layer.  Fluid mechanics provides some equations for the thickness of the boundary layer.  It does not matter whether the plate moves through still air, or the plate is stationary and the air moves, the equations are the same.  Think of a miniature observer standing on the plate, the observer cannot tell whether it is the air moving, or the plate.  More realistically, if you put your hand out the window (not in traffic mind you) the force of the air on your hand is the same whether it is due to wind with the car stationary, or due to the car moving.  We used to be obliged to indicate our intentions to turn or stop by hand signals which required the window to be open.  Now it is illegal to have any body part out of the window.  Unfortunately, this is a necessary change and none of us want too much information on the statistics that drove the change.

When there is also heat transfer, let's assume for simplicity of language, the plate is hot, and being cooled by the air stream.  The temperature of the air next to the plate is essentially the same as the plate temperature.  The heat transfer increases the temperature of the next layer, thus reducing its density and viscosity, which both are important to the fluid mechanics for velocity of the air close to the plate.  At some distance from the plate, the air temperature is essentially the same as the free stream air so the concept of a thermal boundary layer seems sensible.  For most fluids the fluid reaches free stream temperature a little closer to the plate than the velocity reaches the free stream velocity.  The heat transfer does not affect the basic fluid mechanics equations, but it does affect the fluid properties.  Thus the fluid properties are evaluated at a temperature mid way between the plate temperature and the free stream temperature, and the normal fluid mechanics equations, which are based on our old friends, conservation of mass, conservation of energy and conservation of momentum, are still totally applicable.  But just as in the natural convection case, we have to use solutions to approximate equations, which sounds to me much like we only have an approximate solution.  But it's has apparently been demonstrated by the necessary practical experiments, to be close enough to be quite useful.

This is pretty heavy stuff, so that will be enough for tonight.  Tomorrow I will try and include some diagrams of that velocity field, and perhaps take it a little further.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on March 05, 2018, 12:51:54 PM
Hi MJM, your step by step explanation of this "...pretty heavy stuff," is much appreciated as it makes things quite clear. I look forward to more analysis and hopefully some general conclusions can be proffered.

I was daydreaming, again, about boilers and materials, or more specifically thicknesses of same. I remember when an apprentice seeing a copper inner firebox for one of the small loco classes in the workshop that had been 'forgotten' (?), and it struck me how thick it was compared to the steel versions then in use. Now I accept steel is stronger, cheaper, thinner etc. etc. but I wonder if there was any heat transfer advantage over the steel. Yes, the copper is a better heat conductor but it is also a lot thicker, I'm guessing from memory, but maybe two or three times thicker. Also, would there be any advantage in the extra thermal mass of a thicker material. Just wondered if there might be any counter-intuitive advantage in making a particular boiler component thicker than the normal general approach of thinnest possible. I'm thinking of course with respect to a steam locomotive, which can't compete with the efficiencies of stationary set ups, and in particular small model all copper loco boilers. Any thoughts on this?? Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on March 05, 2018, 09:38:02 PM
Frostbite - well I heard a radio talk about extreme cold vs. what we got now (a few days ago).
The expert has been working in the Central Antarctica with daytime temperature of -55C and night -70C and he often didn't use gloves if he had to do something that required precision ...!!!
Having students in for tutorial in the deep freezer at the university he works at, is often in T-shirts ..... but being outside in a typical Danish winter at -1C with a high air humidity is extremely cold to him and requires very good clothes  :insane:

So his conclusion is that the relative humidity is the most important factor about how low temperature affect us and how bad.

I'm not sure it made you any wiser - but you asked.
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 06, 2018, 10:06:44 AM
Hi Paul, glad to hear that my descriptions are making things clearer for you.  Please let me know if you need any gaps filled in.

The formula for heat transfer q=U x A x (T1 - T2), in which U is the heat transfer coefficient.  I have quoted this formula before, but it is very informative.   For conduction through steel or copper, U = k/L, where k is the thermal conductivity and L is the thickness of the material (in meters).   The thermal conductivity of copper is 399 W/m.K, while for steel it is 36.  Stainless steel is only 14.    For the same resistance to heat transfer, copper can be 11 times thicker than steel and 28 times the thickness of stainless steel.

The other factor of importance to heat transfer is the specific heat of the material.  Copper is 383 J/kg.K while for steel it is 486 and stainless steel 499.  This means more heat is stored in a kg of steel or stainless than steel, but the smaller thickness means there is actually less stored heat in steel, especially as copper has slightly higher density.  For your small locos, this means a slower heat up time, but once you are at steam temperature, it becomes irrelevant.  Essentially it is important during temperature changes, but not during steady steaming conditions.

The thermal conductivity, specific heat and density are all included in the parameter, thermal diffusivity which is equal to k/d.c where d is the density, c the specific heat, and is included in material properties in heat transfer texts.  For copper the value is 116.6 x 10^-6.  For steel, 0.97 x 10^-6, and for SS, 0.387 x 10^-6.  It is used in unsteady heat transfer equations, and a higher value means that temperature "spreads" more quickly if one surface is heated.  But it is not important for steady state problems.

But an even more important consideration is corrosion.  Corrosion of copper is not much of a problem at the conditions in our boilers.  However for steel it is a a significant consideration.  Full size boilers made from steel require careful feed water treatment to minimise corrosion, and operating procedures to minimise oxygen entry.  You might think of stainless steel, however, SS is prone to an insidious form of corrosion called chloride stress corrosion cracking.  Full size pressure vessel failures have been attributed to minute concentrations of chloride in potable water used for hydro-testing.  You need to use a Duplex stainless, but that introduces real fabrication difficulties for even experienced welders.  It's diabolical stuff to weld.

You mentioned the strength of steel, which allows it to achieve the necessary strength without too much thickness.  This is in addition to its lower cost per kg.  Copper would be quite impractical for a high pressure due to both strength and cost.  But all around, I think copper is still by far the best material for our model boilers.  And it is also the easiest to fabricate.

Hi Admiral DK, I was hoping you would come in on that one, thank you.  You deal with the issue much more continuously than I do.  I have worked outdoors in temperatures ranging from -33 to +55, even experienced -45, though only for a walk in the park.  I definitely prefer the lower temperatures.  Never was in Antarctica, though that would be an appealing adventure, with adequate clothing.  I think the T shirt in the freezer indicates your speaker has pretty good circulation, or perhaps is well acclimatised and active in the freezer, but I agree with him on humid air around zero.  In my experience,  the worst temperature range for comfort is that range from -5 to +5.  It is nearly always moist, and often windy.  The coat that kept me cosy in -33 was just plain inadequate at a windy +5.  And yes, I have worn it in both.  Below about -5, the air is usually quite dry, the sky often sunny and blue, so long as there is no wind, and you are wearing good clothing, it can be quite pleasant.  Obviously, blizzards are another matter.

Wind, of course, increases the convection heat loss, so increases the demands on the clothing.  You definitely need a wind proof outer layer.

I will attempt to get back to forced convection tomorrow,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 07, 2018, 01:10:25 PM
Well I indicated that I would come back to that forced convection.  I have gone over the calculations many times and I can't see any sensible, simple way to present them so I will stick with the key points in the process and the end result.

I have already mentioned that the treatment of forced convection starts, as with most energy problems, with writing out the equations for conservation of energy, momentum and mass, then attempts to solve these for some simple cases.   For a flat plate, like the side of Paul's firebox moving through air, the first step is to obtain the equations for the velocity of the air flow close to the plate.  The air flow meets the front of the plate at the full velocity.  The air immediately against the plate is considered stationary with respect to the plate, and as the air moves along the plate a velocity profile develops between zero at the plate and the free stream velocity some distance away.  Remember, it does not matter if the plate is stationary in an air stream, or the plate moves through still air.

Solving the equations with some very clever maths, and a few judicious assumptions, gives a mathematical description of the velocity field.  The distance from the plate at which the velocity is 99% of the free stream velocity is defined as the edge of this boundary layer.

In this case, the plate is hot and is being cooled by the air.  The air also varies in temperature from equal to the plate temperature to equal the free stream air temperature some distance away.  The temperature normally reaches the free stream temperature at a thickness a bit less than the thickness of the velocity boundary layer.

I have attached a copy of the sketch the text book provides to illustrate this velocity boundary layer, and which also shows the thermal boundary.  It might help make sense of the words.  The scale in the y direction is very different from the x direction.  The boundary is relatively thin.

Our old friend, dimensional analysis is used to come up with some non-dimensional numbers that might be relevant.  However, the dimensional analysis cannot predict the precise relationship.  When experimental work is carried out, and the results graphed in terms of those non-dimensional numbers, some clear relationships appear.  In particular, one such number is called the Prandtl number, after a pioneer in this field.  It is a combination of some properties of the fluid that varies only with temperature.  It appears commonly in these and other calculations so is directly listed in tables of properties of gases and even some liquids, particularly when those properties are included in a heat transfer book.  It turns out that there is a relationship between the thermal boundary layer thickness and the velocity boundary layer thickness that depends only on the Prandtl number.

Further dimensional analysis shows another dimensionless number called the Nusselt number, which is a combination of the heat transfer coeficient, the thermal conductivity and the length of the plate.  And a bit more maths shows there could be a relationship between this Nusselt number, the Prandtl number, and the well known Reynolds number.  Experimental work then reveals the relationship between the three.

Nu = 0.664 x Pr^1/3 x Re^1/2

Now that looks very mysterious until you remember that the Nusselt number contains the heat transfer coefficient along with two other known quantities, the length of the plate and the air conductivity, while the Reynolds number contains the air velocity, length of the plate and the air viscosity.  Hence that one equation relates the heat transfer coefficient to the air velocity.

If you remember Paul's locomotive was moving at about 1 m/s, and the side of the firebox is 35 mm long and 25 mm high.  If we assume the firebox outside surface is 127 deg C (an odd looking choice that gives 400 K as the absolute temperature, so avoids interpolation of the gas property tables) we can use that equation above to calculate the heat transfer coefficient.

I put all the numbers into a spreadsheet and found the heat transfer coefficient was 21 W/m^2.K.

To give you an idea of the sensitivity to velocity, I put 2 m/s into the spreadsheet and the heat transfer coefficient became 29 W/m^2.K, higher but not directly proportional.

Now the area of the firebox side and the temperature difference can be inserted with the heat transfer coefficient Into the normal equation for the heat loss.  Because the area of the firebox side  is very small, the heat loss is only about 0.2 milliwatts.  Small enough to be neglegible.  It is likely that radiated heat exceeds this, and is thus the predominant mode of heat transfer.  It looks like I will have to look at radiation heat transfer after all.

I am not sure it is useful to say too much more about it.  I find the maths quite intimidating, so I doubt that many will be interested in more detail.  However one interesting thing that did came out of the calculations.  I found the heat transfer text book branched out into fluid mechanics.  Because that forced convection analysis was based on the basic equations of energy and motion, the maths also yielded the drag force on the plate.  And by solving the equations together, another no dimensional group appeared that related the heat transfer coefficient to the drag force on the plate.  So it was concluded that if you measured the drag force, you could predict the heat transfer coefficient without having to do heat transfer experiments.  Remember, even those results I have presented require a measurement of the wall temperature, for which I made an assumption.  Unfortunately, it is much easier to measure that wall temperature than it is to measure the drag force to a satisfactory accuracy.

I hope that is of interest, please let me know if you want any more information about the procedure.

Thanks for looking in,

MJM460

 
Title: Re: Talking Thermodynamics
Post by: paul gough on March 07, 2018, 06:53:27 PM
Thanks for the analysis thus far, and look forward to some guidance with radiated heat losses. I have to say I am surprised at the minuscule result. I had in mind something amounting to a few watts, not fractions of a milliwatt!! Thus several orders of magnitude in error. Just shows what difference there can be between ones ideas and the real situation. My only comment is that the surface temp. is probably higher than the one used as the metal has discoloured, but assume this would not make any significant difference to the result. Again, thanks, I find these investigations intriguing. It's 5 am. so off to a swamp to see whats happening, heaps of rain. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 08, 2018, 09:56:56 AM
Whoops!  Paul, you are right in feeling those heat losses were a bit low, your intuition was not too bad after all.  I must admit I also thought they looked a bit low, but the heat transfer coefficient looked about right, and the heat loss formula is so simple once you have the coefficient.  I checked that the formula referred to all the right cells, and that I hadn't dropped a line, even did a quick order of magnitude estimate, a la slide rule, and got the right figures, but of course, not the power of 10.  I assumed that it was probably due to the tiny area.  However it was after midnight, fortunately not rising at 5 am, so I posted it and made a mental note to do a bit more checking this morning.  I finally found it, a divide sign instead of multiply.  That will do it every time, especially when it applies to the temperature difference, a figure very close to 100.  Eyesight must be dimming more than I thought.

Thanks for a better estimate of the wall temperature.  It should obviously be higher than the value for my insulated firebox, and that colouring will be a good clue.  I did the calculations again for 200 deg C wall temperature.

The corrected value for the heat loss for 127 degrees wall temperature, 1 m/s is 1.9 watts, I am sure you will agree a more reasonable answer.  When I increased the velocity to 2 m/s, it was 2.7 watts.

When I increased the wall temperature to 200 degrees C, the heat loss increased to 3.3 watts for 1 m/s, and to 4.6 watts for 2 m/s.

The heat transfer coefficient calculation was not affected by this error, and did not change significantly for the higher temperature.  All the change in heat transfer coefficient was when the velocity was changed.

Remember that even the text books only claim this analysis to predict heat loss within 25%.  Also, it is carried out for one surface in the air stream much like the side of a tank loco.  Apart from the other side, top and back surfaces, I suspect the firebox sides are partly behind the boiler, and may even be inside frames.  Some cross sections and a plan view, even just simple sketches, would help understanding the actual arrangement. 

If you look closely at that velocity profile and temperature profile in the boundary layer illustration that I posted yesterday, you will notice that right at the front of the plate, the boundary layer is very thin, so the temperature gradient between the wall and free stream temperature is very high.  A high temperature gradient means a very high heat transfer rate.  Further along the plate, the boundary layers for velocity and temperature are thicker, so the temperature gradient is less steep.  This is reflected in the calculations which, when looked at in detail, show that the average heat transfer coefficient over the whole plate is actually double the value at the back end of the plate.  Such is the effect of that higher temperature gradient where the boundary layer is thinner.  It is this average for the whole plate that I used. 

Essentially a higher velocity reduces the boundary layer thickness and so increases the heat transfer rate, as the calculations showed.

In those calculations, the properties of air in the boundary layer are taken at the temperature mid way between the wall temperature and the free stream temperature.  The text book simply used the nearest temperature which was tabulated in the tables. When I increased the wall temperature in the calculations, the rounded out mid temperature was the same in each case.  Just to better understand the influence of temperature, I tried using the air properties for the next 50 degree temperature interval in the tables.  It only affected the fourth significant figure in the answer, so the text book simplification of using the nearest temperature for which the values are tabulated, very desirable in terms of avoiding the work necessary to interpolate all the values, seems fully justified for a calculation that is probably only within 25%.

Redoing all those calculations on a slide rule, or even with a calculator, would have been quite tedious.  However, with a spreadsheet, I first corrected the equation with the error, then copied the whole block and pasted it along side, to get a second true copy.  Then it was a simple matter of changing the figures in the cells for velocity or temperature, and the alternative calculation was complete in a flash.  All the work is in the first run through of the calculation, and exploring variations is a very simple exercise on a spreadsheet.

I will be checking in, but not making longer posts tomorrow and over the weekend, as our daughter and her family will be visiting, 6 in a very small space will make for a busy and distracted weekend.

I hope the frogs turned out on cue so there were plenty to count,

Thanks to all for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on March 09, 2018, 01:04:38 AM
Hi MJM, Glad you discovered the error. I am assuming you are saying one side of the firebox is about 3 W. loss. So the total area would be of the order of 10 W. I'll be very interested to see an estimate of the radiant losses. Finally did a reasonably accurate sketch of the firebox, trust it is of use.
 
When I get to rebuilding the loco I will renew the insulation inside the firebox, this may get the wall temp. down to something akin to the lower figure you used. By then I might even have a thermocouple to get a reading. Awful morning at the swamp, endless heavy rain and one of those "buggar all" trips in terms of frogs, but plenty of waterfowl so we spent a couple of hours counting/identifying reptiles with feathers. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 09, 2018, 02:54:04 AM
Hi MJM, we talk about the heating up of the boiler and can calculate the amount of neat needed to produce steam  but can we also calculate the amount of extra heat needed with the insulation that we may use when we construct a boiler...and do they do this in industry ?

willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 09, 2018, 11:17:16 AM
Hi Paul, it is nice to have found the problem.  Always unsatisfactory to feel that a result is not quite right, but be unable to track it down.  The answer now seems to be the right order of magnitude.  So the next thing is to understand how the air flow past the firebox actually looks.  Is that plate in a clear airflow?  I can see it is tucked in under the boiler.  I assume the wheels and engine are also under or just outside the boiler in plan view, and these will "drag" the air along, and also add extra turbulence.  Also the engine cylinders must be sitting in there somewhere.  The heat loss from those cylinders is even preheating the air, some of which will enter the firebox.  I think I mentioned these factors a few days back, but you were travelling, and may have missed some of those posts.  All these factors reduce the overall accuracy of any prediction. Turbulence makes the heat transfer better, but preheating due to cylinder and boiler losses reduce the loss from the firebox by reducing the temperature difference to the air.  And for the front of the firebox, the air is probably travelling close to loco speed, so nearly stationary relative to the front face.  Back to natural convection.

Insulation on the inside would reduce the loss significantly, but if that firebox is soldered to the boiler, it is a bit like a heated fin, so again there are pluses and minuses whether the insulation is inside or outside. 

At the end of the day, it all has to be kept in proportion.  So now we need to know how much fuel is being burned in a given time to see how the burner heat release compares with that initial estimate of the losses.  Similarly, if the water consumption, approximately equal to steam production, is measured, along with the time of actual steam production, we can calculate the heat input to the steam, if we know the pressure, so with the three main factors we can easily see their relative importance.  I usually measure the quantity of water used to fill the boiler, then extract the remainder with a syringe at the end of the run to determine the quantity of steam produced.

Unfortunate about the early morning weather, but I guess expected at this time of year.  However, the feathered reptiles also have to be counted, so not all is lost.  I hope you had a good rain coat!

Hi Willy, insulation is put around the furnace casing to reduce the heat losses so the boiler actually requires less heat, both for heating up and for steam production.  Of course when the boiler is started from cold, some heat is stored in the insulation which has a temperature profile due to the heat conducted through it, but that heat is already lost from the furnace, so I believe it would come from the lost heat, it would not require additional heat from the burner.  However, even insulation has a density, thermal conductivity and specific heat.  So as long as the material is known, and the temperatures of the inside and outside walls calculated or measured, it is possible to calculate how much heat is stored in the insulation.  It is only temporarily stored in the insulation, then finally lost when the boiler cools down.  Initially, when the boiler fires up from cold, the furnace heat transfer is determined by the flue gas temperature, and the initial cold temperature of the furnace side of the insulation.  As the insulation warms, that inside temperature rises, thus reducing the temperature difference from the flue gas, so reduces the rate of loss to the steady state value.  In an industrial boiler, the heat up time is usually a very small part of the total operating time, so not considered important.  At least in the idustry I was in.  However for other industries where boiler operation is more intermittant, heat up time could be a consideration.

In an industrial boiler, I am sure the heat loss through the insulation would be directly or indirectly calculated, in order to determine the most economical thickness.  Obviously this requires data on fuel costs etc.  By indirectly, I mean that the designer may have specifications for a given size and operating conditions, which specify the thickness to use. However, the original writer of the specification would normally do some sort of heat loss and cost calculations to determine the thickness and type of insulation to specify.

We have a very full house here tonight, so just a short post.  I hope that answers the questions.  Back tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on March 10, 2018, 03:57:40 AM
As you say there is probably some shielding of the firebox by the cylinders, wheels and framing etc. and some heat floating around underneath to make precise analysis virtually impossible. Nevertheless a credible approximation for the open side of the firebox is nice to have and given a loco operating outside can be subject to 'chilling' low temperature wind from any direction, often blowing at a greater speed than that of the forward motion of the engine, I am inclined just to extend the result, say 4 W. by  X3 or X4 to give a 'worst case' projection that might be worth trying to mitigate, especially if you are able to contribute any further loss due to radiation to the final figure. These little gauge 1 locos have very little in reserve, anywhere, whether it be heat production, volumes of water or fuel, and of course losses are myriad. Add even a mildly adverse operating circumstance of cold windy conditions and the impact can lead to unsuccessful or problematic running. So, every avenue to an advantage or mitigating a disadvantage, if practicable, is likely to be worthwhile. As far as I'm aware, your analysis and quantifications are the only ones I have seen, and advance our knowledge in what might be actually going on in these tiny model circumstances. Investigations such as these may throw light on something that has not been noticed or ignored or is even counterintuitive to that applying on the larger scale, who knows, no numbers no knowledge. So I look forward to more. Regards, Paul Gough. 
Title: Re: Talking Thermodynamics
Post by: Zephyrin on March 10, 2018, 07:51:45 AM
Hi Paul,
A point I notice in the loco boiler drawing that you show is the dry firebox...not surrounded by water.
As you are rebuilding the loco, why not change that; I agree that this most probably requires a new boiler...
I also struggle with small Gauge 1 loco boilers to make them to produce a maximum of steam while saving alcohol at the same time, and not being to hot to handle at the end of their half hour run; a difficult equation to solve !
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 10, 2018, 10:36:59 AM
Hi Paul, it is always important to understand the assumptions on which any calculations are based.  You will remember from the sketch of the boundary layer development, that the air stream is shown  as the full free stream velocity at the front edge of the plate.  The very thin boundary layer at the front leads to a higher local heat transfer coefficient close to the front of the plate, which reduces along the plate as the boundary layer develops.  This leads to a higher average heat transfer coefficient for the whole plate.  Turbulence and heating prior to the front of the plate almost certainly increases the thickness of the layer, as well as reducing the temperature difference,  so reduces the overall average heat transfer coefficient.  Hence the calculations probably do, as you suggest, provide an upper estimate.  Especially for the front and back plates which are almost certainly nearer natural convection.  But on the other hand, a cold wind in almost any direction almost certainly increases the loss.  It is all about picturing the air flow.

Thank you for you encouraging words about the calculations though, I am glad they are providing useful insights.  I will try looking at radiation more closely when I am back at my bigger desk where I can spread things out a bit.

Hi Zephyrin, thanks for coming in on this one.  I am glad that you are still following along.  Water cooled fire boxes in the style of full size and larger models would help by both increasing the area for heat transfer for steam raising, and also limiting the outside wall temperature to steam temperature.  With a little insulation on the outside it would help enormously to reduce losses.  It would be interesting to know how you approach the issues of doing this in such a small size, especially staying the flat surfaces in metal that is presumably relatively thin, and how you manage to put it together.

Well, the little family had a great day and have moved on to their next gathering.  We went on a ten mile boat trip then after exploring the local market and buying a lamb roast from the farmer's stall, which we immediately put on the bar-b-que, and had it with cake and chocolate from the other speciality stalls as son-in-laws birthday celebration, before sailing back on a perfect sunny day with light winds.  Now we are exhausted, and the peace is wonderful.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on March 10, 2018, 02:20:40 PM
Hi Zephyrin, Many times in my idle moments I have thought about a true loco type boiler in gauge 1, but unless it is a large model or a freelance design accomodating the firebox within the framing for a narrow firebox type could be difficult. So for practical reasons I tend to think it is not viable, (though not impossible), unless one went for a modern engine design with a wide firebox type boiler where it was more or less above the top of the frames as in many of the modern U.S. locomotives and a sufficient number of wicks could be installed. There would be a larger heat output required for a wet firebox loco, two layers of metal and the water to heat up, so one would have to look at an existing model, say one of the successful larger Aster models of U.S. locos, e.g. the Great Northern, S2, 4-8-4 and guesstimate the increased heat demand and whether more wicks could be installed, I think there are 5 in the S2, so getting more in might be difficult if you tried to stick exactly to modelling the prototype. If you are into non main line locos it might also be practical to look at a narrow gauge type, such as the 600mm Fowler engines used in the sugar cane industry. The Fowlers had relatively 'fat' boilers and when built to 7/8ths inch scale give fair sized boilers, but again getting a wet firebox between the frames might be problematic. I have never thought about what might be possible with the newer types of gas firing with permeable ceramic grates where the fire is above it, it might be easier than trying to use methylated spirits and wicks. The other possibility is some adaptation of a Briggs type boiler which has a wet crown sheet but dry firebox sides but with a rectangular coil pipe inside, these are now very common in Australia in larger model gauges and very successful. However, it would be difficult to produce the relatively sharp bends in the coil without getting crush. There have been some models running without the coil and just well insulated dry sides. There is much room for experimentation in G1, but boilers are an area that holds little interest for most builders and there is now another layer of compromise, in that if the boiler is to be operated in anything other than your own shed with nobody else around the legal boiler codes can make innovative designs difficult or impossible to get certified. I have mentioned mostly the difficulties, but hopefully this might save someone some time. For myself, I have now reached the limits imposed by advancing maturity and the impact of disabilities so my construction capacities are mostly over, but I hope curiosity and the challenge spurs someone with more years available to try some innovative designing.  Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 10, 2018, 02:51:48 PM
Hi MJM, Just some observations and practical questions about steam engines..................with an early Newcommen engine , all the heat used to create the steam (3/4 Lbs) was lost in the condensing process ....so could this be a termed a really efficient engine ?? rather than a modern loco where the Heated steam just whizzes through the engine very quickly and just Exhausts ??  !! These Newcommen engines were sometimes used for up to 100 years !!! Also thinking about Mr Farienhiehts thermometer and 0 Degrees being ice and salt ,what is the actual reason for this being very cold ??
Title: Re: Talking Thermodynamics
Post by: Zephyrin on March 11, 2018, 11:26:09 AM
Hi Paul,
The Aster models of club members I have had in hands for fixing parts doesn't have a particularly efficient boiler, simple pot or smithies boiler, in a casing lined with asbestos cardboard; and leaving little room for the hot burning meths from largely dimensioned wicks. It is the intense draught by the exhaust which allows this configuration to run properly.  The  alcohol consumption being large to huge.
As wagon pullers, these locos are generally good, as long as MJM460 is not looking at their efficiency...

As a model builder, I try to build more "efficient" locos, at large, according to common sense; this involves boilers internally fired, with a semi-wet firebox, smoke tubes and small wicks; it is also much more satisfying than doing a plain pot boiler.   
While reading this thread, I understand I have to do some measures in addition to ethanol consumption only to demonstrate that they are more efficient !

Quote
For myself, I have now reached the limits imposed by advancing maturity and the impact of disabilities so my construction capacities are mostly over, but I hope curiosity and the challenge spurs someone with more years available to try some innovative designing.
I got this point, I'm not a young man anymore...
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 11, 2018, 11:55:39 AM
Hi Paul, I was also wondering about the practicality of a water surrounded firebox, but like you was thinking about the design and level of skill required to build it, and not confident of my own skill.  I will be very interested to see where this conversation leads.  I suspect with the boiler equivalent of watch making, it can be done, and due to the small areas which have to be supported, "thick" plates could be used to simplify staying arrangements that are not practical in a much larger model.  For example a firebox top machined from solid as an insert to the boiler to direct the combustion gases into the fire tube, so that more of the wick flame is enclosed by water.

Hi Zephyrn, I am most interested that you already have such a design and would like to learn a little more about what you have done, and what a water cooled firebox would look like in this size.  The main reason I have placed some emphasis on efficiency is that if we have a reasonable estimate of efficiency, and measure how much fuel we burn, we can use the two to get a reasonable estimate of the power output.  Though of course, generally in a model, unlike industry, we don't really know how much power we actually need.  So in the end, it is just about better understanding the science that enables our little models to turn heat to mechanical work.  Reducing losses does increase efficiency, but in this size range, I assume that we are not looking for efficiency for fuel savings sake, but more because it helps us get more power out of limited heating capacity, so helps tip the balance in favour of getting a working model.

And I suspect that many of us are that age where experience exceeds strength, and eyesight and coordination are not what they were.

Hi Willy, I suggest quite the opposite, those Newcomen engines would not be very efficient at all.  Let's quickly review the basics.  Efficiency is the energy out as useful work divided by the energy in from burning fuel.

Let's say the boiler is fed from the river at a balmy 15 deg C, it is heated to boil 3/4 psig, around 5 kPag.  I wish you had chosen a figure closer to 3 psig, which would have been directly in the steam tables, but, just for you, I did the interpolations, and found the heat required to raise that river water to about 102 C and boil it to produce dry steam is 2615 kJ/kg.

Now those engines are referred to as atmospheric engines, as the steam pressure only raises the piston against gravity, and then the water is condensed by spraying in more of that river water.  Atmospheric pressure, aided by gravity, then does work on the piston by pushing it down against the reduced pressure on the condensing side.  (That might take more than one reading!)

More water is pumped from the river into the cylinder, involving more losses in the work required to pump that water), the steam is condensed, the latent heat warms the river water to let's say 80 degrees, which determines the level of vacuum attained, about half an atmosphere or say 7 psia, and the warm water at 80 is discharged back to the river taking the residual heat of 80 degree water with it.

Now, let's compare that with an ideal adiabatic engine.  Steam at 3/4 psig is expanded to 7 psia which condenses at 80 degree C.  Using the second law, entropy is constant through an ideal expansion, otherwise it increases, the ideal engine would do work equal to 127 kJ/kg of steam.  A real engine say 75 % of that so 95.25 kJ/kJ, though more likely those early engines were much worse, is there any chance that you have a figure for steam per horses power?  Or even lb of coal per horse power.  Anyway 95.25 divided by the heat in, 2615 means 3.5% efficiency as an estimate of the best that could be achieved from those steam conditions and not achieved by many of our models.  The only way that will set the world on fire is if spilt fuel sets fire to the factory.  A modern ordinary industrial steam plant will be in the range of 30%.  Perhaps they were considered more efficient compared with non condensing engines of the time which received no input from atmospheric pressure to increase the minuscule output.

Well, if the energy does not get converted to work, where does it go?  Basically, it goes to the river as heat in that 80 degree water.

You might say that surely the vacuum will be lower, but I suspect I have been optimistic.  You see, it is limited by the pressure determined by the final temperature of the water and steam, now condensed, in the cylinder.  To heat river water from 15 to 80 C requires only 271 kJ/kg of water, while to condense steam at 80C after expansion from 102 C requires removal of 2590 kJ/kg of steam, so the spray pump must inject 9.6 kg of water for each kg of steam it condenses.   Well, perhaps river water is cheap, and they use 20 or 30 kg of water for each kg of steam, that would lower the pressure a bit,  but definitely diminishing returns.  And any air leaking in lowers the condensing temperature, but does not lower the pressure any further, unless you have a very good air pump.

Such engines were the foundation of the industrial revolution, but they worked by brute force, not by efficiency.  They could do the work of many men in a relatively small space, so the men were instead employed cutting down trees, or digging coal, and feeding it to the furnace.

I have tried to avoid those equations, which have all been presented before, and just given you the results.  But it is just data from the steam tables as before. Efficiency is about useful work out, compared with the heat input.  I hope this clarifies a little how those engines worked, and even a little more about the efficiency of modern engines.

As for salt in water, I am not sure of the mechanism, only that salt does indeed lower the freezing point of the mixture.  Note that I don't think adding salt actually cools the mixture, just lowers the temperature at which the mixture freezes when it is cooled.  I was always told that 0 F was the lowest temperature he could find (remembering he did not have any refrigeration, and also probably never travelled to a really cold climate, so he assumed, incorrectly as we now know, that this was the coldest possible temperature, therefor could reasonably be termed an absolute zero.  Only 460 deg out!

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on March 11, 2018, 03:23:31 PM
With the risk of side tracking this thread, I will ask a question I've been thinking about just about every time I see a steam cylinder here. I this case it was Ricks Conway build that triggered it.

A lot of our model steam cylinders has a steam passage going from the steam chest to the same end of the cylinder made of two or more parallel holes of small diameter. As this give a much higher flow resistance than a single hole of bigger diameter or an oval one + a much bigger cooling area ...!!!... it really should give a much reduced efficiency and reduced power of the engine .... or is there something that works differently here than in other engine types ?
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 11, 2018, 03:38:55 PM
Hi Admiral, a lot of the Stuart Turner cylinders have cast in steam passages of rectangular section. However they do need a thorough clean out before use. I think the reason most built up cylinders have drilled holes is because it is quick and easy to do so !!
Willy.
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on March 11, 2018, 04:07:18 PM
Admiral, K.N. Harris in his book "Model Stationary and Marine Steam Engines" has nothing good to say about simple drilled steam passages except for what Willy said it is easy.

He gives two methods to make proper passages, one is to have a false port face so the passages can be milled, and the other is to chain drill then fill the holes with brass silver soldered in and chain drill the webs and clean up with a file.

Dan
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on March 11, 2018, 06:14:35 PM
On the subject of steam passages / ports etc, this model loco has had larger cylinders fitted and an unusual port shape to enable things to fit on the existing engine.

The relevant part starts at 2min 30.

https://www.youtube.com/watch?v=eJu9IzNpJd4
Title: Re: Talking Thermodynamics
Post by: paul gough on March 11, 2018, 08:39:57 PM
Hi Zephyrin & MJM, Here is a photo, (hope its clear enough), of an old sketch of ideas for a 'wet' firebox. Never drew it up carefully so dimensions and proportions are only tentative and for memory jogging. I think the angled water tubes where the flame passes through them to get to the fire tubes is potentially a better idea than the vertical straight tubes as there is not much radiant heat with meths wicks, but only building a few boilers would sort out which was the most effective. Obviously the lower water leg(s) is shown without a connecting tube from the barrel, hope this does not cause confusion. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: Zephyrin on March 11, 2018, 10:52:41 PM
HI Admiral_dk
Yes, steam passages have to be enlarged, streamlined and polished, removing elbows and smoothing curves, to facilitate high speed steam flow, in line with the Chapelon's studies.
But on a small model, streamlining the tiny steam passages is not that easy, grilling them the largest we can should do it !

I post a plan of one of my Gauge 1 loco boiler with 3 flue tubes through the barrel and water tubes in the furnace. Only the sides of the alcohol burner are surrounded with water. This boiler is not difficult to do; in 2 brazing sessions, plus a 3thd one for ferrules and fittings.
pictures in this google album:
https://goo.gl/photos/tn1b8kkCZrqG1Z2V9

Z.
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 12, 2018, 10:38:02 AM
Hi Admiral dk, good to have you joining in again.  You are quite right about the multiple small passages.  As others have said, it is mostly about simplicity.  For small engines which are most likely to spend their lives running quietly and unloaded, it is probably a reasonable simplification for a beginner.  After all, if you are going to run with the throttle partly open, it does not really matter if some of the throttling happens in the ports and passages.  However, for an engine expected to work hard, such as passenger hauling locomotives, all these losses are clearly undesirable.

The basic theory is that the friction drag that resists flow comes from the viscous forces which lead to a shear force per unit area at the passage wall which is proportional to the velocity gradient near the wall.  The total drag force is then proportional to velocity gradient and the passage wall area, which in turn is proportional to the diameter for a round passage.  The heat transfer is a little more complex, but as I see it, there is nothing wrong with your theory.  The small engines we build still follow the normal rules.  I don't know what happens if we go much smaller again, to nano scales.  Always dubious to extrapolate too far.

The flow through the passage is increased by differential pressure over the length, and flow is resisted by the drag force of that shear force at the wall.  The total flow is then determined by flow  velocity and the flow area.  The flow area in turn is proportional to the diameter squared.  So, small diameter passages have more wall surface area per unit of mass flow, and hence present more resistance to flow.  Rectangular or oval passages have more flow area compared with the wall area especially in comparison with multiple small diameter holes.

Hi Willy, thanks for coming in.  I find I can remove metal when necessary with a large file, but when dimensions compel me to use a small file, I find it very difficult to remove much metal.  I don't know if there are better files than the needle files I use, but your comment makes total sense in my limited experience.  I can't match the beautiful job you are achieving with your freelance build, it is opening my eyes to the possibilities of bench work with files, but I suspect that I don't have your skilled artists eye.  You clearly use the skills from your previous life to great effect.

Hi Dan, thank you.  Glad you are still following along.  We are clearly all in agreement that the only justification is easy.

Hi Gas Mantle, good to have you coming in again.  I hope you got your boiler working as you wanted and settled that feedwater question.  I don't have enough data at the moment for videos, but I am sure that many will appreciate your post.

Hi Paul.  The sloped tubes are probably longer so present more heat transfer area, but I don't think they count as water cooled walls.  The outer walls are still solid sheet, but the tubes certainly increase the heating area, so if they are easier to fabricate, they are worth trying.  The vertical tubes are rather like the water tube walls of a full size boiler, and probably still need insulation on the outside.  You have to be careful with the dimensions of the ligaments between the tubes where they join into the header boxes.  May not be in the end any easier to fabricate than Zephyrin's design.

Hi Zephyrin, can you tell us more about Chapelon, or where his or her work can be found?  In the mean time I certainly follow your suggestion to drill the largest hole practical in my small engines.  I mostly aim for the full inside diameter of the steam pipe as a minimum.

Thanks for posting the drawing of that boiler.  It is a very interesting design, with quite a few interesting details.  It certainly shows what can be achieved with a small scale boiler.  Is the design publicly available or available for purchase anywhere?

Travelling again tomorrow.  Will see how I feel in the evening, but will definitely check in.  It is great to see so many replies on these topics.  Thank you all for following and especially for contributing.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 13, 2018, 11:15:59 AM
Been a long day today, including the 300 km drive.  Started some reading to help me sort out how to tackle radiation heat transfer.  It's a long time since I did that stuff on any detail, but it starts to look familiar with a little reading as a refresher.  We will see how it goes tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 13, 2018, 07:34:04 PM
Hi MJM, there is a swiss firm called Grobert and Valorbe that specialise in bizarre small files   quite expensive but most of mine came from secondhand stalls and car boot sales  so a file for every eventuality !! Having a good handle is quite important...especially the ergonomic non vegan type !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 14, 2018, 12:06:07 PM
Hi Willy, that is quite a collection of files.  As others have mentioned in previous threads, those non-vegan ones are quite impressive, even apart from the imaginative use of materials at hand, rather than the expensive commercial product.  Do you find that old files from a car boot sale still cut well enough to be useful?  I have always understood that they should not be just thrown in a drawer, and you don't know how the previous owner looked after them.  A blunt file is only metal that needs heat treatment to be useable for engine parts.  (Obviously you don't throw the files in the drawer, but rather, place them carefully?)

Before moving on to radiation calculations, I thought I would have a quick look at the little boiler design that Zephyrin posted along side the Australian Model Boiler codes.  I don't know what design pressure these little locomotives use, but if it is less than 520 kPa it would even fit within the Sub-miniature Boiler Code.  I don't consider this a design check, but just a first step to see if it would be likely to be acceptable for club running.  It seems to fit within the acceptable types (as type F if you have the code) and I think while some minor adjustments to the design would be required, it would be worth talking to the club boiler inspector about.  I suspect the firebox plate thicknesses might have to be increased a little, but I think they would still be workable.  The firebox width might also have to be increased to comply with the ligament requirements on the flue tubes.  It is always interesting to see the differences in detail that occur when a different code is applied.  They are not all equal except for their design intent of providing a safe design and being based on the same basic formulae.

I am wondering if it would be a practical compact design to use in a model boat, so steam could be used in smaller models than most of those I have seen.  Perhaps I have just not seen so many.  But when I sit my boilers, particularly the commercially made centre flue one beside the plans I have a dream of building, the boats all seem too small.  Not sure I could launch them if I scale them up too much.  Of course, more space than you would imagine is always required for the piping and such.  Just as in a full size plant!

Still working on those radiation calculations.  The section in the book starts with saying radiation involves understanding of quantum mechanics and related sciences.  Fortunately it quickly moves on to a simpler approach.  Reading is slow, as we are back to reassembling our home after that carpet laying.  It had to wait while we went to see that classic boat festival.  Which reminds me, I promised some photos.  I have the computer reconnected, so with some luck I will load up a few tomorrow if people are interested.  There were a few steam driven ones, even one model, but most were 1:1 scale wood and sail.

Oh, by the way, many guests to dinner tomorrow, so next post will probably be Friday.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 17, 2018, 11:30:26 AM
Radiation Heat Transfer-

As you know, I have been slogging through the radiation chapters in the heat transfer text book, it's been a long time since I looked at it in quite so much detail.  But it is interesting to review it all again with a view to calculation of a simple cooling problem that is relevant to our modelling.  At last I feel that I am in a position to write a brief summary that might help with a basic understanding.

The book I use starts in a quite intimidating manner, talking about electromagnetic radiation, statistical mechanics, quantum mechanics as well as thermodynamics.  Quite a mouthful.  So why do these terms from modern physics belong in an introductory text on heat transfer?

Electromagnetic radiation because radiant heat is an electromagnetic radiation just like light, radio waves, microwaves, X-Ray's and so on.  It is interesting that some problems in electromagnetic radiation can also be solved by treating it as a stream of particles.  It is not dependant on having any material in between objects to be part of the process.  Think of heat from the sun radiated to earth through the deep vacuum of space.

Quantum physics is the science used to analyse and understand atomic and molecular activities associated with the energy transfer.  The energy is evident in the vibration and rotation of molecules and movement of electrons between shells.

Statistical mechanics is involved because the energy is transported over a range of wavelengths, there is no single wavelength associated with a temperature or material or energy level.

Thermodynamics is the science that deals with the properties of bulk materials, rather than single atoms or molecules.

But in the end, knowing all that won't have any direct application to our attempts to calculate a heat transfer coefficient.  There is no need to be intimidated by all the big words.

Heat is transmitted in a range of the electromagnetic spectrum that includes all the infra red, all the visible light and a small part of ultra violet.  Higher energy is associated with higher energy and shorter wavelength.

Just like in engine cycles, there is the concept of an ideal adiabatic engine, in heat transfer, there is the concept of an ideal "black body".  This concept describes a body that emits or absorbs the maximum amount of energy possible for any temperature or wavelength.  Like the ideal engine, no real object behaves exactly like a black body, and most objects are approximated as a grey body, defined as a body that emits or absorbs some percentage of the maximum possible energy, always less than 100%, but still the same fraction of the black body emissivity for any wavelength.  Unfortunately no real surface even behaves like that definition of a grey body, and real surfaces have emissivity and absorptivity that vary with wavelength.  When this factor is included in analysis of transparent materials such as glass, we have the basis of the greenhouse effect.  And in addition, the intensity of the radiated heat is dependant on the direction you are looking from, it is not equal in strength in all directions from the radiating surface.

You might think your intuitive knowledge of the behaviour of light would help you understanding of radiative heat transfer.  Unfortunately again our intuition let's us down again.  Both black paint and white paint are quite close to black body behaviour in heat transfer.

Early workers in the area worked out from theory that the total heat energy radiated is proportional to the absolute temperature raised to the fourth power.  The constant of proportionality cannot be deduced from theory, but is determined experimentally.  The constant is called the Stefan-Boltzmann constant, and it's value is 5.675 x 10^-8.  We will use it shortly to calculate the heat transfer from Paul's locomotive firebox.  It is obviously a very small number, but then normal temperatures as absolute temperature are quite large (20 deg C is 293 K) so T^4 is a very large number, and it all works out.  The number can be found in tables of physical constants.  You might even have it on your calculator.

Every surface radiates this radiant heat in accordance with that Stefan-Boltzmann's law, so radiative heat transfer occurs when the surface radiates more or less energy than it is receiving from everything else around it.  So the heat transfer equation is q is proportional to T1^4-T2^4.

Now the proportionality constant for heat transfer is not just the Stefan-Boltzmann constant, that number must be multiplied by the area of the surface, the emissivity and a view factor.

The view factor accounts for detailed problems like heat transfer in an oven for example where an object is being heated from some surfaces while other surfaces are passive insulation which looses heat.  Calculation of these view factors becomes quite complex, and takes up a disproportionate amount of the introductory chapter of most heat transfer text books.  For cooling of a small object in a large relatively constant temperature surrounding, the whole lot can be considerably simplified. 

So let's move on from the heavy theory, and look at how it is used to solve the simple cooling problem we started out with.

The most simple calculation method assumes the surface being cooled is a black body.  Let's use the example of Paul's firebox, 35 mm x 25 mm in area at about 200 deg C.  If we assume the surroundings with a direct view of that plate are very large, approximately infinite compared with the plate, and at a uniform 20 deg C, then the view factor becomes 1, and the rate of cooling by radiation is given by the following formula.  (Remember, length is measured in metres.)

Q = 0.035 x 0.025 x 5.675 x 10^-8 x ((200+273)^4- (20+273)^4)  = 1.3 Watts.

If we assume the emissivity of the firebox plate is 0.8, that is, not black, but a bit oxidised rather than shiny polished, and allow for the surroundings to be more of a grey body than the ideal black, say emissivity of 0.92,  the calculation is a bit more complex, but the answer turns out close enough to 0.8 x 1.3 = 1.04.

If you remember back, the forced convection cooling rate for 200 deg C and the locomotive moving at 1 m/s was 3.2 watts, or for 2 m/s 4.6 watts.  So the radiation cooling, which occurs in addition to convection, is about an additional 30%, a significant enough figure to suggest it should not be ignored.  Unlike convection, radiation heat transfer is not affected by the locomotive speed.

I hope that was not too hard, and helps with a basic understanding of radiation heat transfer.  I think you can see why it is often mentioned then forgotten as people move on to conduction and sometimes even convection.

I happy to discuss it further if someone would like some more information.  Otherwise I suspect it is time for a new topic. 

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on March 17, 2018, 01:13:32 PM
Hi MJM, Thank you indeed for the very well communicated summary of radiative losses, and in particular for the worked example pertaining to one side of my little firebox. Having quantifications for losses goes a long way in allowing one to get a feel for what is happening and also allows one to make a considered choice as to whether some action should be taken to reduce it. I am very grateful for all the time you put in chasing the convective and radiative losses as well as the broader guide through the 'thorn thicket' of thermodynamics. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 18, 2018, 11:27:07 AM
Hi Paul, thank you for your encouragement, it is much appreciated.  I never cease to be amazed when you take fourth power of absolute temperatures, quite large numbers, and multiply it by a standard physics constant, and a very small area in square meters, that the answer comes out quite reasonable, even when the dimensions are only a few millimetres.

However, now that we have some estimates of the heat loss from the firebox sides, we really need to fire up that boiler and measure the fuel consumption, so we can see the size of the losses in proportion to the burner output.  This will tell us how much effort to put into reducing those losses.

It is also important to look at where the frames are, to see what shielding is already in place.

I should have mentioned that the radiated heat travels at the speed of light, as do all other electromagnetic waves, but not aeroplanes, as was reported in a recent issue of New Scientist magazine, which reported the effect on a plane doing 1.2 times the speed of light.  I think they meant the speed of sound in their little news item.  A plane travelling at more than the speed of light would be a really big news item, surely worth much more than a short paragraph!

Insulation for radiant heat is interesting.  The sun on a tin roof heats that roof which in turn radiates the heat down into the shed, or house underneath unless there is a good lining.  The air under the iron gets hot and much of that heat is transferred inside.  To insulate the roof from the radiant heat of the sun, a second roof above the first, just separated by an air space that is well ventilated, is very effective.  The outer roof receives the heat from the sun, blocking the direct path to the building roof.  It of course gets hot, but radiates to the inner roof based on its temperature, which is much lower than the sun's temperature, so much less heat transferred.  The air that is heated between the layers flows out the ventilation spaces and is replaced by cooler air.  You can carry this further and put a third layer over the top for even more reduction of the heat transmitted to the building.  Probably diminishing returns on any more than three layers.  This is the same principal as setting a tent fly over the tent, though it is less effective unless the outer surface is highly reflective (as some are).  Part of the issue with a tent is that the material is not totally opaque, so some of the Suns rays are transmitted through the cloth to the inner layer, but it is definitely cooler inside a tent with a fly than in one without. 

Of course, if you are setting up the tent in the snow that seems to be falling in many northern parts at the moment, you have to reverse the thinking a bit.  Remember, the net heat transfer is from the higher temperature to the lower temperature.  And the higher temperature, if you can manage it, is inside the tent.  So you want the shiny material facing in to keep the heat in.  Of course convection is probably the bigger issue.  I have camped in the snow in around -16 from memory (in Ontario) when the scouts needed a few more nights camping experience to qualify to go to the World Jamboree, but I don't think we thought much about heat radiation on that occasion.  We probably put the shiny sides of the fly facing out as you would in summer.  The principal also applies to your picnic Esky (or drink cooler), if you are putting a shiny blanket over it to slow the drinks warming.  Heat is entering from the outside, so you need the shiny side of the blanket facing out.  However, for the emergency heat blanket in your first aid kit, you want to conserve the patient's heat on the inside of the blanket, so shiny side in.  It all depends on which way the heat is travelling.

For your firebox, which is the source of the heat, a second layer perhaps provided by the frame is very close to the heat source.  It heats up with the radiated heat, and radiates back more heat as its temperature rises.  This in effect reduces the temperature difference between the firebox and the surroundings in its view, so reduces the net heat loss.  So either an air gap, or a layer of insulation (opaque to radiated heat) would reduce the heat loss from the firebox.  However, as more heat is lost through convection, the insulation would probably be the best approach.

Roofs, tents, drink coolers, first aid emergency blankets, despite its bad press, it is worth a little effort to understand radiant heat as it has so many applications.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 18, 2018, 02:19:10 PM
Hi MJM, More good stuff here .....I always wondered about the reason for flysheets ..??so nothing to do with fly's then !! Also found out about calculations for model boat 'ratings'  Quite a long formulae.....his day job might have been a thermodynamics engineer !!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 19, 2018, 01:09:28 AM
Hi MJM just a few technical questions........... on my electric boiler the output pipe is 5/32" ,and the inlet pipe to my new engine will be larger , about 1/4"  if i want to connect the two together should i have the connection as close to the boiler as possible or to the engine ?.... when the two pipes are joined is there a significant pressure drop  into the larger pipe ? Also is it possible to work out with all the info on my boiler how large an engine can be run assuming the boiler is kept fed with cold water ? The new engine i am making is double acting with slide valve , running at say 150 RPM. with 1. 1/2" stroke by 3/4' bore......Also could you insulate a boiler using chicken 'Bacofoil' with the shiny side inside and could you use several layers at a suitable distance apart ?
Willy.
Title: Re: Talking Thermodynamics
Post by: Zephyrin on March 19, 2018, 08:21:41 AM
Hi MJM460
The boiler I have shown is for a plain Gauge 1 loco, 1/32 scale. Its a small loco even for its gauge.
The pressure is usually between 1.5 to 4 bar depending on the load, and the water contain is 0.1 litre, an axial pump try to keep this value constant.
I did another boiler on the same design for a larger loco, in this album.
https://goo.gl/photos/dY6fYHntSb5wJUUP9
the boiler is surrounded with a layer of 3mm of cork, and a foil of tinned steel.
After the run the loco is too hot to handle with bare hand, not a surprise, as is the case for all these tiny machines...
And yes, the frame also is hot, but heated mostly by the cylinders between the frame more than by the firebox, owing to a too powerful draught ?

I don't known the code you are referring to...I should say that in our club we don't have a boiler inspector...Yes some incidents may occur, loco catching fire, train derailments and falling on the floor etc.

all my boilers are properly tested and safe, I did 17 locos in this gauge or smaller...
A popular loco boiler drawing is present in "the Project" loco booklet, edited by the Gauge 1 Association, UK, which certainly has the approval of boiler's inspectors...
in this plan, as in a large majority of plan, a single larger flue tube is traversed with inaccessible cross tubes.

For live steamed boat, maybe all the boiler are now butane fired, a much easier solution, owing to the risk of alcohol spilling during the launching of the boat.
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on March 19, 2018, 12:43:11 PM
Hi MJM just a few technical questions........... on my electric boiler the output pipe is 5/32" ,and the inlet pipe to my new engine will be larger , about 1/4"  if i want to connect the two together should i have the connection as close to the boiler as possible or to the engine ?.... when the two pipes are joined is there a significant pressure drop  into the larger pipe ?
Willy.

Hi Willy, I experimented with different sized pipes on my boiler and found that it made little difference so I think the position of the joint with have no noticeable effect. I could be wrong but the way I view it is that when the steam leaves the pipe and enters the steam chest it is effectively entering a pipe with a huge increase in size (albeit a rectangular pipe / chamber).

I'd be interested to hear what the experts say though as I've pondered similar questions in the past.
Title: Re: Talking Thermodynamics
Post by: paul gough on March 19, 2018, 12:44:38 PM
Hi MJM, Could you give some guidance on the addition of a reflective layer to our insulation sheeting. Is it always superior when added than just using the 1, 2, or 3mm ceramic sheeting; does it matter whether it is inward or outward facing; and what materials might be effective that we could easily access, eg. aluminium cooking foil??? Regards, Paul Gough
Title: Re: Talking Thermodynamics
Post by: paul gough on March 19, 2018, 01:14:10 PM
It just stuck me that at or near the 1st anniversary of this thread you will have something near 50,000 reads!!! This shows there is quite 'demand' for knowledge/information on thermodynamics and the associated ramblings by modellers. I wonder if you would be interested in distilling all that has presented thus far and publishing it?? I think it would be pretty attractive to both the publisher and reader of A.M.E. magazine or some such. Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 19, 2018, 01:46:12 PM
Hi Willy, good to see you are still following along.  Tent flys don't help much with flies as they are not enclosed, but generally open at the ends and lower sides to allow plenty of air flow.

Thanks for posting that formula for the model boat rating formula, unfortunately I can't clearly read the second page which has some of the important details.  Despite its length, it is actually quite a simple formula.  It adds length and sail, area which are known to make a boat faster, forcing the designer for each class to find that balance between longer for less wave making resistance, and sail area to provide a higher driving force.  You cannot add length and area, but the square root of area has dimensions of length so can be added.  The 1/3 factor is an attempt to equalise the relative importance of sail area and length from the potential speed point of view, probably arrived at by careful observation of the performance of many existing designs.  The other factors are simple "taxes" on factors that the rule makers want to discourage, to avoid designs with undue advantage.  Basically those girth measurements are smallest for a hull with vertical ends so the waterline length is the same as length overall.  When the yacht has long overhangs and that graceful form of older designs, if the beam at the deck line is is significant, there is the possibility of hull shapes that have a longer waterline length when heeled over under the pressure of wind on the sail.  The rule designers wanted to discourage this factor, so the resulting designs would be narrow at the ends of the waterline so minimal extra length when heeled.  I think the second page has some words about why extra freeboard was also discouraged.  Beam is indirectly implied by that G measurement, as a wider hull can carry more sail area for its length. 

Of course the formula is still not dimensionless and the measurement system has to be part of the definition.  Obviously a boat rated in meters is a much bigger boat than one rated in feet.  Depth was probably not penalised enough if I look at the latest designs in model yachts, which by the way I have read do tend to lead the way for full size practice.  The trend is to very deep keels, as the better righting moment from a deep keel allows more sail area, and the torpedo bulb at the bottom of a deep fin keel does not have as much surface area drag as the traditional long keels the rule designers were no doubt imagining.

Regarding your boiler, I would make the change to the larger pipe diameter so as to have the minimum length of the smaller diameter pipe, as the resistance to flow varies with the velocity squared, so larger diameter same mass flow and steam conditions means lower velocity, less friction loss, but not a huge difference, particularly as you then have a throttle at the inlet.

At the join there will be one velocity head of pressure lost.  When the units are correct it will not be a very large number.  The velocity can in principal be converted back to higher pressure in accordance with the well known Bernoulli formula to reduce the loss, however, the diverging walls for the size change must be less than about 15 degrees, and nearer 12 would be better.  Remember that delivery cone on the injector we looked at a while back.  You could make a proper diverging adaptor, but the pressure gain in this case would not be really worth the effort.  Velocities are so much less than in an injector, and you could probably gain the pressure elsewhere a lot easier.

It is easy to work out the mass of steam from the electric element rating and the steam conditions using the steam tables.  The steam tables also tell us the volume of that steam.

Similarly, we can easily work out the swept volume of the cylinders, even allowing for the piston rod diameter if you wish.  So for a given rpm, the maximum amount of steam consumed by the engine can be calculated.  We don't really know how much the valve timing will reduce the total steam taken by the cylinders.  Nor do we know how much the throttle will lower the pressure at the cylinder when it is partly closed by the governor.  The lower pressure increases the volume of the steam, so reduces the mass flow consumed by the engine.  And of course it is pretty hard to estimate steam losses at valve and piston rod demands and piston ring blow by, or even exhaust valve leakage.  However, that basic calculation of steam mass and volume give a good idea of whether the boiler will do.  And if we have underestimated the volume the engine uses it will go a bit slower, while it will go faster of we have more volume available with sufficient pressure.

It is quite late here now, so I will look at the calculation tomorrow.  By the way, I am following your build and admiring your skill in both the design and bench work with the files.  I did go to the wood working tool shop yesterday.  They had a whole cabinet full of brand new planes of various designs, all those little ones that you would have put to good use in your previous life.  Retail therapy at its best.  Now I now what women are talking about, they just go to the wrong shops.

Hi Zephyrin, thanks for coming in again.  Please be assured that I am not criticising your design in the least, in fact I am admiring both the design and your skill in soldering it together.  I am quite confident that you will have tested it and that it is quite safe.  And of course, no code will protect against spilled fuel, derailments etc.

We have our own code here.  I am not inclined to be too critical of it, as I am aware that it is a balance of the politics of self regulation vs government authorities stepping in, and I am not aware of all the reasons some code requirements were introduced.  However I have heard that some well proven designs do not comply and have been rejected after many years of successful and safe running.  A section for subminiature boilers has been introduced recently, so for interest I looked at how your design would compare.  I don't know what the inspectors would say, but it looks to me like they might call for a modification to the firebox width to meet the requirements for ligaments around the fire tubes.  However, the sub miniature code also relies heavily on hydro testing and steam testing, so it is also possible that it could be totally acceptable based on testing.  Like some other forum members, I am not in a club, and don't really know much about how the rules are actually applied.  My experience is more with full size pressure vessels.  No way of understanding the impact of applying a different code without checking a design against the code, so the exercise was interesting, but I probably should have kept my mouth shut.

I am quite interested to read that gauge 1 book and the designs it includes, however it is down my list for the moment.  But I will have a closer look at the links you have provided, they are quite interesting, thank you.

I guess gas is better contained, hence less likely to be spilled, providing the fuel is transferred to the tank with plenty of ventilation.   My background makes me well aware of the consequences of igniting a gas leak, so I am more inclined to Meths, which is easily extinguished with water.  However, I also have a commercially constructed boiler and gas burner, and will be happy enough to use it when I eventually get a boat built. 

Do you have any information on the steam production and fuel consumption of your little boilers?  It would be interesting to calculate the performance against heating area for comparison with other arrangements.

I notice three more replies arrived while I was typing, thank you Paul and Gas Mantle.  It is after midnight here, so I hope you will understand if I respond on those tomorrow.  Really great to have comments and contributions.

Thanks everyone for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 20, 2018, 10:18:12 AM
Quite a backlog from yesterday, there were even three replies while I was writing.  So let's back up a bit and try and catch up on all the issues this time.

First Willy, you were asking about your boiler and the new engine.  At this stage, you will obviously hook them up and try it, however, the purpose of this thread is to demonstrate that you can make a sensible estimate before you build the boiler, so you have a better chance of a successful result.  If that sounds a bit like hedging my bets, remember that even full size equipment usually needs a performance test to prove that it is meeting the design intent.  Your boiler is an excellent test subject, as the heat from the electric element all goes into the boiler and it contents, apart from those losses to the atmosphere we spent some time discussing.  So let's see what the calculations say. As one who has followed your thread from the start, I am guessing it will not be long before you can test it all out.

First a bit of a reminder of the results we have obtained from your testing.

The electric elements are each nominally 500 watts, but we found as best we could determine from the measurements you were able to make, that the actual heat output was about 430 watts for a variety of reasons when the boiler and its contents were heated up to 135 degrees C.

We also found, from your cooling tests, that the heat loss from the boiler after you added that rockwool insulation was about 150 watts, compared with 310 with the basic timber strip insulation. 
Lets assume the rockwool is still in place.  So 430 - 150 = 280 watts.  Remember, 1 watt is one joule/sec, so the net energy input at 135 deg C so 280 watts or 0.280 kJ/sec.  This is all available for steam production, as the copper was heated to 135 C with the water during heating up, so now raising steam at constant temperature, it absorbs no further heat.  The heat just transfers through.

Now let's look at the steam tables.  We use the section with temperature in the first column, and look at the line for 135 C.  We find the pressure should be 313 kPa which is 212 kPag or about 30 psig, a good check for your pressure gauge.  We also find the specific volume of the steam is 0.5822 m^3/kg and the enthalpy for evaporation (hfg) is 2159.6 kJ/kg.  We divide the heat input per second by the hfg to get 0.00012 kg/s.  Now this is quite a small number and it is tempting to multiply it by 1000 and work in grams, but mistakes are less likely if we don't change the units, and the calculator has no trouble keeping track of the zeros.  We now multiply that mass of steam by the specific volume at those conditions to get m^3/s and by a further 60 to get 0.00453 m^3/min, breaking my own rule to work with rpm!

I am going to keep to the rule on the volumes of your cylinder, but rather than be totally consistent and use revs/s, I am going to carefully stay with revs/min.  So for an engine 1 1/2 inch stroke x 3/4 inch bore the swept volume in m^3 is Pi/4 x (0.01905)^2 x 0.0381 x 2 in each revolution.

That last 2 is for the double acting cylinder. Strictly we should subtract the piston rod cross sectional area, but unless your rod is really huge diameter, it is not a big error.  Let's keep it simple.

A little maths and multiply the volume by your rotational speed of 150 rpm and we get 0.00326 m^3/min.  At first glance we can see that the engine could potentially run at about 210 rpm, which looks promising for a satisfactory performance.

Before we get to excited, we should look at the sources of error in making this calculation.

First, we don't really know at what point your valve cut off will occur, and in any case, the motion of the slide valve driven by eccentrics, even through the Allen reversing gear essentially involves some throttling while the valve opens and closes, so the engine should take in a smaller volume of steam.  Secondly, we do not know just where the exhaust valve will close, and how much recompression of the remaining steam will occur, which could affect the actual steam flow either way, depending on whether the inlet port opens to a cylinder of steam recompressed to above or below the supply pressure.

Third, we do not now about losses around the engine, valve rod packing losses, piston rod packing losses or how much loss due to blow by past the piston, or, dare I mention boiler fittings?

Finally we need to think about the governor action, as the governor throttles the steam inlet to the engine.  With the engine stationary, or at relatively low speed, the weights do not fly out and the throttle valve should be full open.  As the engine accelerates to the set speed, the weights start to fly outwards, and the linkage uses this action to start closing the throttle.  And if the weights, springs and linkages are all suitably in proportion, the governor finds a throttle position that keeps the engine speed approximately steady.  I assume you know all that, but have you thought of how this affects the boiler?

As the throttle closes, the steam flow reduces, but the heating elements are not connected to the throttle, and so the energy input to the boiler continues.  The pressure and temperature start to rise.  With the higher temperature the losses increase, and to make steam at the higher pressure more heat is required.  Either the extra heat required by the steam plus the extra losses is enough to just allow for the reduced steam production, or the heating element control system sees the higher temperature and cuts the power, reducing the heat input to the boiler.  Eventually the system is in balance again, with just the right amount of heat input for the steam the governor allows to the engine.  If all else fails the safety valve should limit the pressure rise.

If you add an engine driven boiler feed pump, the engine will require more steam to supply the power required by the pump, and the boiler will need more heat input to heat the cooler water up to steam temperature.  So the final equilibrium will be at a slightly different point. 

The next step is to complete that engine, connect it to the boiler, and see what speed it runs, but it looks encouraging as a suitable boiler.

I am not familiar with "Bacofoil", I assume it is a form of aluminium foil perhaps with some reinforcing fibre.  Three layers could easily be added using some packing rope or similar flexible material to support them off the boiler.  The spacing is not particularly critical.  And preferably shiny side in, as you said.  If the spacer material minimises air movement out of the gap, it should be quite effective, though most of the foils will conduct heat quite well, and air between the layers will transfer additional heat between the layers by convection.  Hard to prevent this except by evacuating the space between the layers, like a glass or stainless steel thermos flask.  Perhaps better to mount your boiler inside one of those wide mouth thermos flasks, with some suitable flexible spacer/support material!

Remember that the calculations for Paul's firebox showed about  3 Joules lost by convection for each one by radiant heat transfer, so reducing convection is the key.  Also, while foils start quite shiny, so the low absorptivity and high reflectivity increase their effectiveness, but as the surface oxidises, that advantage is rapidly lost.  So for best effect it would be best to use stainless shim.  Lower conductivity and reflectivity maintained better.  Would be a very interesting experiment, particularly of weight was a significant factor.  But it would be easily crushed flat, so not easy to maintain.

Hi Zephyrin, I followed that link to your photo page.  That tank locomotive is truly amazing.  If you are ever building another I think your build log would be very popular.  Absolutely beautiful miniature engineering.  I liked those animations of the valve gear as well.  I am particularly interested in the Joy valve gear, and have built an engine with radial valve gear like the Joy, but with radial links instead of that characteristic curved slider.  I thought it might involve less friction and easier construction.  But you have built the complete slider beautifully. 

I am not surprised the engine frames get hot.  Conduction from the cylinders, radiation from the firebox, mechanical friction in bearings and even churning of the air all contribute to the heating.  That strong draft would certainly help with the cooling, and besides it is a good heat recovery system, using those losses to preheat the combustion air.  I guess the insulation of the boiler is a compromise between external outline and the size of the boiler drum, but I am sure it all helps.  The cork probably blocks all the radiant heat transfer but the tin plate wrapper would look a whole lot better as the outside layer.

Hi Gas Mantle, if we look at the steam flow from Willy's boiler above, the steam velocity will be 14.7 m/s for 5/32 tube, and 4.65 m/s for a 1/4 tube.  The Bernoulli formula can be used to calculate the pressure loss for the velocity change, which turns out to be 167 N/m^2, or 167 Pascal, more usually expressed as 0.167 kPa or 0.024 psi.  Not really surprising that you couldn't tell the difference, it would take a really accurate instrument, if possible at all given the pulsating nature of the flow.  So minimising the loss is correct in theory, but in practice, as you have pointed out, other considerations such as ease of manufacture are almost certainly much more important.

When the steam expands into a large space such as the valve chest, the velocity becomes very low, and the total velocity head is lost.  The velocity head in 1/4 inch tube is nearly zero anyway.

I took it a step further and tried 1/8 inch tube.  I have a couple of short lengths, and the ID was between 2.4 and 2.5 mm, not in accordance with the standard wall thicknesses.  However, the velocity is about 16.6 m/s, so the velocity head is only about 20% more so still not very important.  It certainly justifies the tiny tubes used on the little Mamod and similar engines, but to me it would look too small on a larger engine.

Personally I still use slightly larger tubes than smaller, but I can't really say that smaller will not work.  On the exhaust side, I definitely use the larger size when practical, as any resistance reduces the flow from the exhaust stroke, so increases the exhaust back pressure and reduces the engine power.  There is so little, I don't want to loose any.  But I can see that smaller tubes are necessary on the little locos that Paul and Zephyrin are building.  And they clearly work well.

Hi Paul, I have probably answered your question on radiant heat shielding in the replies above.  Remember that your ceramic fibre probably shields most of the radiant heat which travels in line of sight.  However, fibre is porous so there will be extra convection transfer.  The outer non-porous layer prevents that air flow so is important for heat conservation as well as appearance.  Use the thickness you can accommodate.  The system used by Zephyrin is probably a good guide as to what experience recommends.

I looked up the start date, 11 May last year, so a bit early yet.  I don't dwell on it but it is helpful to my confidence to see that people are still reading, a factor that others have also clearly felt.  But as always, I particularly appreciate the replies by regulars such as you and Willy, but also from all the many others who periodically come in.  So thank you for pointing that out.  I have wondered from the start what the response would be.  I have written in the spirit of developing a knowledge base, building up the application of thermodynamics to our hobby as I gather data and apply the maths that I know from my working life.  I have been surprised by how far it has gone and pleased with the progress.  Though certainly it is a work in progress that would require some editing for publishing.  But I am not against the idea if there is interest from the publishers, and would certainly be prepared to put the effort into tidying up the text.  It is pretty hard to find stuff from early posts despite the excellent search function on the forum, so I am considering celebrating the anniversary by taking some time to assemble an index.

Definitely too long a post, but I hope that I am now up to date with the questions.  I will make it a lighter day tomorrow to allow people to catch up.

Thanks for looking in and for all the responses,

MJM460

Title: Re: Talking Thermodynamics
Post by: Gas_mantle on March 20, 2018, 10:50:40 AM
As someone who is interested in running engines on steam the subject of pipes sizes etc is something I've pondered in the past.

My guess is that ultimately the steam has to negotiate the port in the cylinder and any pipe larger than the port is unneccesary. I accept that different sized pipes will have different thermal losses etc but if we stick to steam flow considerations surely larger pipes achieve nothing ?
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 20, 2018, 11:51:54 PM
Hi MJM , Thanks for all this info, There is a lot more to it than i first imagined. I suppose with a larger pipe as there is more surface area in contact with the air, there is also more heat/cooling transfer ! so if the pipe is not insulated that loss can also be part of the calculation. the length of the 5/32 pipe is about 28" !! this was to reach the connection to the Beeleigh engine that was on quite a large plinth .The Bacofoil that i mentioned is the AL foil that is used to wrap chickens and turkeys in when you roast them. Another question .. when the steam enters this 28" pipe with a valve quite close to the boiler and the valve on the engine closed how does the steam and the air in the pipe interact ?? as the engine block is steam jacketed the steam valve is opened and the drain cock on the cylinders opened and drains the jacket to heat up the cylinders . Once pure steam issues from the drain cock ,this is closed and the valve to the steam chest opened . the governor linkage is then manually lifted and the flywheel given a push. the engine then starts and the governor handle released to do its predetermined duty. this i initial procedure does use up quite a lot of steam initially but the boiler can cope with it . new question ....with a loco there is always steam escaping from the safety valves so is this a waste of  energy ? In my boiler there is a pressure switch that turns off the currant before the safety valve lifts. quite easy with the electrical heating system but not so easy with solid fuel boilers !!  so.. i shall find the rest of the sailing boat article and give you the rest of the formulae. Thanks Gas_mantle for the comment about port size. On this Stuart Turner engine block the ports are cast in and rectangular in shape so the cross sectional are can be calculated and used for the round pipe..... we will see what happens later..
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 21, 2018, 10:53:33 AM
Hi Gas Mantle, I am starting to feel that you are right, and much smaller pipes could be used. 

Certainly energy losses due to change in velocity at size changes directionally lose pressure, but when the calculations are completed with realistic model engine data, the losses, even though they occur over and over, do not add up to much.  The same energy losses occur at changes of direction, and in driving the steam around the valve outline but in total for a the typical length of tube used on a model again is not much.

My initial thoughts on velocity are certainly over conservative, and much higher velocities are definitely acceptable. 

All the discussion so far has been about energy considerations and the effect of velocity changes.  In addition, we need to take friction into account as a source of loss, which is especially significant on long pipelines.  I have looked to the old fluid mechanics book, and carefully checked the calculations.  For drawn tube, which has a very smoothe wall, the friction factor results in about 0.3 kPa for a 300 mm length, and still only about 2 kPa for Willy's 28 inch long tube, so this again does not explain much.

I do keep thinking I am missing something, as there are some inconsistencies.  I am sure that I have read recommendations based on a proportion of piston diameter.  And most models seem to have at least 5/32 tubes.  Also, if you look at scale sizing, I am not sure what a typical full size engine of the type Willy is constructing would be.  Perhaps 4" bore, so 3/4 inch bore is about 1/5 full size.  I don't know what size steam line would have been used, but perhaps around 1 1/2", so proportional diameter around 7.5 mm, say 5/16" tube. 

Certainly exhaust has additional considerations.  First a much higher degree of pulsation means acceleration losses at every exhaust stroke.  And as I mentioned earlier, a little back pressure reduces flow from the exhaust, meaning higher back pressure on the piston and reduced work output.  I suspect this effect increases the importance of those small pressures enormously, but I am rapidly running out of ideas for calculations that would prove it.  I might just have to accept that much smaller pipes would be satisfactory unless more information turns up.  A bit of a surprise after working for forty years with full size piping, which is sized to reduce pressure drop, though the pipes are much longer.

Of course the obvious answer is to try it.  The steam pipe on a simple test setup, (not after the full diorama treatment) is not a very big task. I would be very interested to hear the result if anyone just tries a very small tube, and I think I now have to add to my project list a few experiments on pressure drop.  I think that measuring the pressure drop would require better instruments than I can afford, however if I do something simple, like say setting up my boiler and engine about  a metre apart, and make up two or three steam pipes say 3/16 that I currently use, 5/32 and 1/8 if I can get a suitable length.  I think it is used in refrigeration so it should be available, but the lengths I have at the moment are only about a foot long.  If I get sensible results with a very free exhaust, it would be interesting to continue by trying a smaller exhaust with some length as well.

The advantage of this approach is that the engine will make the flow pulsations realistic, and I can calculate the flow based on water consumption.  I am thinking some plastic tube will either melt or provide suitable insulation.  Don't hold your breath, but I will get there eventually.

Hi Willy, what do you think would be a suitable scale steam line size for your engine?  I have probably exhausted that topic in discussing Gas Mantles comment.  While in principal increasing the size earlier is best, in practice I think I finally have to agree that in practice you are unlikely to see a difference, especially with the governor in operation, so let availability of the tube and appearance dictate.

When there is air in the boiler, and steam is introduced by evaporation over the whole liquid surface area, the system is well mixed and we have liquid and vapour and a phase change and the air affects the boiling temperature.

In your steam pipe, it is very different, you are just mixing two gases, the air does not affect the liquid boiling temperature.  When you first open the boiler valve and steam enters the end of the pipe and the small cross sectional area does not allow good mixing.  There is a tendency for the steam to act a bit like a piston, and compress the air to the same pressure as in the boiler.  However, without a solid piston to clearly separate the two, there is some mixing and the interface will involve a more or less gradual change in concentration from steam at the inlet end of the pipe to air at the other, with most of the change occurring over a short length.  If the pipe is still blocked at the engine end, that random molecular motion we have talked about before will result in mixing over a longer length, but I am not sure how long it would take to be completely mixed.  More likely you open the valve to the jacket and engine, and the flow is initially air but quite soon the entire mass of air has gone and you then have steam from the boiler.  Of course, if the boiler already had air in it from when it was filled, the process will involve the air in the pipe being pressurised by that steam/air mixture.  There will still only be slower mixing with the extra air initially in the pipe, so the engine end will still be mostly air until it is purged out.  As there should be no air being admitted unless there is a lot of air dissolved in the feedwater, the mass of air initially in the boiler is soon purged out and the engine is run on steam.  The initial air in the steam jacket will affect the condensing temperature, which will not be 100 degrees until the air is purged out.  Makes that initial heating a bit gentler I suspect, ideally an air vent as well as the condensate drain valve, but a bit to theoretical to worry about unless you find a problem.

Yes, the steam leaking from the locomotive safety valve wastes energy, as the heat in the steam cannot be recovered and used in the engine, but as you say, the coal fire is not as easy to turn down as your electric element so they don't have much choice.  Another advantage of your electric boiler, the heat input is fully controllable between zero and 100%.

In the talk about pressure drops, I have glossed over the heat transfer issue.  There is the same flow through the pipe, what ever its size.  Clearly, a larger tube has a larger surface area and a longer residence time for heat loss so probably looses more heat.  On the other hand, a smaller tube has higher velocity so higher convection film coefficient on the inside thus increasing the heat transfer.  So pluses and minuses.  I think the main thing is that both require insulation, and if the outside temperature burns if you accidentally touch it, try more insulation thickness.

I thought I was aiming for a shorter post tonight, just as well I was not expecting a long one.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 22, 2018, 02:58:21 AM
Hi MJM , the steam pipe on the Beeleigh engine is 4" outside so a scale pipe would be 1/4' as it is 1/16th scale ....however one can not scale nature  so do the calculations get adjusted with very large and very small thermodynamic installations ?? thanks also for further info....
Willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on March 22, 2018, 10:45:24 AM
Guys....we have some confusion here...steel pipe according to American Standard ANSIB36.10, is measured by nominal bore [NB],

Steel tube is manufactured to an OD dimension

3" NB = 88.9 mm OD...this dimension is constant with the schedule [Pressure Rating or wall thicknesses] hence reducing the ID
This same 3" NB pipe could be schedule Standard [5.49 wall], Extra Strong 7.62 wall] and XX Strong [15.24 wall]

[So in round figures, a 3" NB diameter** pipe could be as little as 60 mm actual bore]

The next size up from this is 4" NB which is 114.3 mm OD

There has been no such product produced as a 4" OD steam pipe

Steam machinery produced at the turn of last Century was invariably produced with the heaviest schedule steam pipe irrespective of the steam pressure

The only reason I offer this is  the great possible variance even before scaling down, hence we really need to understand the scale variance when referenced back to scale sized OD tubing

Derek

** word should not have been used
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 22, 2018, 11:21:00 AM
Hi Willy, the issue of scale is a perplexing one, but it always makes more sense with a real example.  So let's have a look.  Now 1/16 scale means 1/16 full size.  From the appearance point of view, linear dimensions are all scaled by that same scale factor, so the 1/4" tube will look right on your engine, while a 1/8 tube would look ridiculous, regardless of friction losses or lack of them.

Then, your 3/4 bore by 1 1/2" stroke engine corresponds to a 12" bore by 24" stroke full size engine.  We need to know the power rating, operating pressure and running speed of the full size engine to compare with what might happen in the model.  But we can say the piston area of the full size and  model can each be worked out using the formula for the area of a circle, Pi / 4 x d^2.  If you work them both out, you will find the area of the model piston is not 1/16, but 1/256 of the full size.  You need to carry all the decimal places to get the exact answer, or you can take my word for it.  So the steam pressure works on a much smaller area to provide torque relative to the linear scale.

The volume of your model will will be 1/16^3 or 1/4096.  Now the model is probably made of very similar density materials to the full size engine.  So the mass of the model will be 1/4096 of the mass of the full size engine.  This is very convenient, as it will make it a lot easier to carry to an exhibition than if it were 1/16.

When we get to performance, things get more tricky.  First, the model cylinder volume, will be 1/4096 of the real engine cylinder volume, so it will only take in 1/4096 of the volume of steam.  To work out what that means, we need to know about the steam pressure used by the full size engine and the speed it normally ran at.  I expect that you have that data, and probably a good idea what speed you think the model should run at, so rather than my wild guesses, let's wait until you post that data before I look further at performance.

Of course, most of our models tend to be run unloaded, including mine.  This is a pity, because it means they are not doing any of the work they were intended to.  I have made a tentative start with driving a DC motor as a generator, so now I have to add an electrical load.  I have a standard Meccano size end on the shaft of each engine in preparation.  A generator is probably wrong period, unless you build up an open frame machine with visible carbon brushes, so a pump or mine lift, or perhaps a timber saw would be more realistic.  Even an overhead shaft and a full workshop per J.L.   Another whole branch of our fascinating hobby. 

Hang on, I was thinking of your freelance mill engine, but I just read your post again, you said the Beeleigh.  If I remember correctly, that is a compound engine with condensing and condensate/air pump.  That will be a whole new level of test run.  Need a manometer for vacuum, perhaps better a gauge until you have some idea of the vacuum you will achieve, and perhaps a load so you can get reasonable pressure in the hp cylinder.  And of course a method to measure the condensing water flow and temperature rise.  Or am I mistaken again?

However, running unloaded means regardless of the boiler operating pressure, the regulator and governor valve will both be nearly closed, and only minimal pressure is seen by the piston, as you would see if you had a good pressure gauge on the steam chest.  Temperature is no help for pressure once you are out of the boiler.  So that is another thing to remember when assessing the model running conditions.

Scaling of the engine to continue as data is made available.

Hi Derek, good to see you back again.  Did you ever get sorted on that condensate issue?  I must admit that I never even thought of using steel tube in full size, as there are not so many pipe fitting available to suit.  To me, pipe implies to ASTM ANSI Standard B31.3. Or occasionally the similar pipeline codes, and rarely to BS (but mostly only for plumbing) or DIN, but that just reflects my oil industry background.  I guess the one thing for certain is that the Beeleigh engine would not have used ASTM standard piping which is as you say, though the OD standard dimension results in and ID somewhere in the region of the nominal diameter depending on the schedule selected.  However, these calculations are at best very approximate, so to use the nominal diameter results in easier maths, and not large errors in the grand scheme of things.  Obviously for a detailed calculation of the discharge pressure required of a compressor for a long pipeline requires knowing the exact specified diameter and even the manufacturers tolerance.  More likely an early BS piping code might have been used, (I can look out the ID for more recent versions if you wish), or perhaps even manufacturers standard prior to that.  It might be a bit like early thread "standards".  I really don't know the history, my experience only goes back to when I started work in 1967, though the codes were a few years old by the time we got them in the pre-Internet age, as you know.  But which ever way, use of the nominal diameter as the inside diameter gives an idea of the difference between pipe sizes without the complication of many significant figures in the calculation, especially when comparing real and model sizes.  When I calculated the tube velocities for the model sizes, I did use the wall thickness of the tubing I have on my shelf, but the actual numbers still depend on the wall thickness you are using as tubing at least under the AS codes is it is manufactured to a standard OD and several wall thicknesses.  In fact I probably have a mixture of wall thicknesses as when I get to the shop, they rarely have all sizes available, and anyway, I am not sure which would be most suitable anyway.  They are all adequate for the pressures I use, it is about bending them without undue flattening.  So I hope you will forgive this approximation for the sake of simplification.

Finally managed another test run of my engine today.  Basically quite a successful run in terms of data gathered, though the temperature really rocketed up between 40 and 80 deg C, I presume when those water tubes started to produce steam, and I missed a few readings.  Forty degrees in less than a minute caught me by surprise, just didn't even see it happen.  I melted the nice plastic handle on the thermocouple I was using on the stack temperature, should have installed an insulating heat shield on it, but it still seems to work ok.  Also, when the engine got up to 2000 rpm by the non-contact digital tachometer, burner moved around quite a bit due to the vibration, and ended up in the wrong place.  I also had to tighten a safety valve fitting leak, so there was a little steam leak until I found the right size spanner. 

For the cool down, I removed the stack thermocouple and put a block of wood to block the stack air flow.  Then the cooling is just due to heat loss through the insulation.  Of course the temperature inside the furnace is no longer raised by fuel combustion, so I will have to think about how to allow for that.  I have now made a base to allow the burner to be fixed in place for next time.

During the cool down, all proceeded normally until 55 deg and I was waiting for 50, when the temperature suddenly jumped to 65.  This happened once before, so I decided to just continue the cooling until it again reached 55 and continued to take readings while it cooled until I eventually stopped the timer at 39 deg C.  I have swapped over the thermocouples since last time, so it was the same instrument but a different thermocouple to last time it happened.  The instrument takes in two thermocouples.  All the while, the second one read totally rationally.  I have no idea how to explain that.  We will see what shows up, if anything, when I do the calculations.  Any ideas are welcome.

No calculations as yet, but at least I now have the data, so can make a start.

Thanks for looking in,

MJM460

PS corrected ASTM to ANSI, should not have forgotten that one already. 
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 23, 2018, 02:35:22 AM
Hi Derek  , the pipe i think is a cast item as can be seen in the picture with a cast in flange and blobs for the risers during the casting process ..so how thick the walls are i don't know. also it was made about 200 years ago !!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 24, 2018, 10:12:53 AM
Sorry that I missed last night.  I knew I would be late, as we had to deliver a granddaughter to the theatre prior to her show, but it was after midnight when we got home after the show.  And we had to take her brother to the basketball at 08:30.  Both parents were tied up with other activities that have them quite busy, but they don't usually all coincide like that fortunately.  Life was easier when I had to go to work, but not as much fun.

The actors put on a wonderful interpretation of the Shrek musical, plenty of youngsters coming in to keep up the numbers.  Basketball was the grand final.  In the end, they lost in the last 30 seconds, but no disgrace to come runners up , especially in such a close game.  Never more than 6 points between them.  As the sign on the wall says, they are kids(under 10's), it is a game, the coaches are volunteers and the referees human.  And it is not the NBA!  But a wonderful game.  When we arrived, two previous games were both just in the final few minutes.  Both were a draw at the final bell, so went into an extra 5 minutes.  One of them, the draw was achieved in the final 3 seconds.  So a kids league, producing some wonderful games and heaps of fun and skill development for the players.

Willy, do you know what pressure they ran that cast iron pipe at?  I suspect not very high.  These days cast fittings such as valve bodies and so on have to be cast steel.  But then, cast iron was probably as good or better than the boilers.

Not many calculations done today.  Definitely in rest up mode.  Perhaps tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on March 25, 2018, 05:32:36 AM
Willie.....you may be surprised at that cast pipe being steel and not iron...[if you had access to it, one minute with a triangular hand file could determine the material]

In the first year of my apprenticeship [1966], each Saturday [overtime $ :LittleAngel:] my task was the mechanical check of two huge Broom & Wade twin cylinder [HP & LP without intercooler] low speed air compressors.....each flywheel was taller than me.......

The were manufactured in the UK & from memory .....Year Manufacture - 1890 on the name plate

All interconnecting pipework was cast steel...........the complete compressors entablature, casings & pipework all painted gloss light cream colour.....with polished copper lubrication tubing

Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 25, 2018, 09:12:54 AM

Hi Derek,  that's interesting, that cast steel was being used so early.  Do you know when it first came into use?  I don't know the history at all.

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on March 25, 2018, 09:50:42 AM
Sorry all......my words were a little misleading.....'all interconnecting pipework was cast steel' this was true or as intended

'the complete compressors entablature, casings & pipework all painted gloss light cream colour' .......naturally the mass of the entablature was of assumed high quality cast iron, however painted gloss light cream colour

A part of the apprentices tasks was to read the Broom & Wade drawings book.....[text with hand drawn exploded views of great complexity]

This was my first journey into the light hand 'tapping' metallic components with a small hammer

This tapping  :killcomputer: as a maintenance requirement also confirmed the integrity of the cast interconnecting pipespool material as being free of cracking :ThumbsUp:

I am unable to offer comment of cast steel earlier than my 1st experience of the 1890 Broom & Wade compressor name plate

At sometime after the turn of the Century, I believe the original Broom & Wade Engineers, became Broomwade Pty...........

Derek

Title: Re: Talking Thermodynamics
Post by: MJM460 on March 26, 2018, 12:56:34 PM
Hi Derek, it is clear that I am not an expert on metallurgy or history, no surprise there, I have mentioned it before.  I do welcome such comments however, as while they are not thermodynamics, materials science has to advance appropriately to take advantage of the knowledge.  It is no use thermodynamics telling us that higher pressures and temperatures are necessary for increased efficiency if materials science does not at the same time allow us to construct boilers, piping and engines suitable for the higher temperatures and pressures.  Besides, such comments and historical footnotes add interest to a subject that some may otherwise see as somewhat dry.

Not making much progress on the thermodynamic analysis of my recent boiler test runs due to a very hectic home life, but I will get there eventually.

While I am not much on remembering history or metallurgy, I do like topics that involve calculation, especially things involving physics and motion.

I don't know how many others have noticed kvom's thread on the Muncaster Grasshopper engine.  It is going to be another great build.  However, he asked in an early post whether the swinging link was part of a Watts linkage.  I have been interested to notice that the question has not yet been explicitly answered.  It clearly is, but it is not so obvious how it works. 

OK, it is pretty obvious, but the question that attracted me was whether I could demonstrate that  it produced the required vertical linear motion of the top of the piston rod exactly, particularly as the swing link is not fixed on the piston rod centre line as it is on most "conventional" beam engines.  At the same time, it is clear that many successful engines have been built to the design, so how does it work?  Rather than hijack kvom's thread, I thought it might be worth posting the question here.

The Watts motion has at its centre an isosceles triangle, but for this to actually keep the top of the piston rod in a vertical line, the vertex of that triangle must be constrained so that its height above the horizontal line is exactly half of the height of the base of that triangle.  In a conventional beam engine that constraint is provided by the drop link from the intermediate point on the beam.

The link is not necessary on the grasshopper because the beam actually forms not only the top leg of that isosceles triangle, but also constrains the apex of the triangle.  I thought I should be able to calculate the position of a sufficient number of points either prove the design is a perfect watts motion, or alternatively, calculate the probable error.

It turns out that it is a more complex calculation than I anticipated, even for a spreadsheet.  Oh for MathCad or a similar program (and to know how to drive it!).  But I think I have I learned a few things about the Grasshopper design in the process.

If the right hand end of the beam (in the drawings posted) moved in a horizontal straight line the mechanism would be exact with the end of the swing link on the centre line, and the beam end of the link mid way along the beam.  However, there is an error introduced by the rocking column on the right hand end of the beam.  The movement is small, and the error can be reduced by making the column longer.  Muncaster's design has a shorter column than most I have seen.  The error can be further reduced by moving the base of the rocking column towards the cylinder so that it rocks an equal amount each side of the vertical. It also appears that the moving end of the swing link is not at the centre of the beam, but is slightly offset towards the cylinder end.  However, that rocking column moves the end of the beam vertically, by a very small amount admittedly, but it tends to distort that isosceles triangle and pull the top of the piston rod ever so slightly out of line.

It also appears, that the error introduced by the rocking column can be compensated by moving the fixed end of the swing link as Muncaster has done, possibly also involving moving the swing link pin along the beam a little.  I set about to solve the equations by trigonometry, but it's more complicated than it looks.  I tried drawing it out carefully full size, but the various factors are very small and quite difficult to draw sufficiently accurately.  Which of course raises the possibility that the normal tolerances on the linkages and rod packing can actually accommodate the small errors.  I even tried a Meccano model as a 3-D model, but there was too much slack in the pinned joints to be definitive, despite my using bushes at all the critical joints.

I still feel that with more time, I should be able to solve the problem by trigonometry, but it is going to take more time than I have available at the moment.  So I ask, has anyone modelled the engine in a suitable program and been able to prove that Muncaster's design ensures true vertical motion of the top of the piston rod?  Or alternatively worked out just where the fixed end of the swing link should be located to eliminate the error?  Or is there an inevitable small error?

Or perhaps we should just return to Thermodynamics!

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on March 26, 2018, 02:53:36 PM
Hi MJM, A bit damp up here at the moment, 305mm of rain yesterday, but the frogs like it. I was wondering if you could provide a breakdown of the heat input ascribed to the process of generating steam, as percentages. Let us say we can break the process into the following four parts, 1) heat required for water from 'cold' to 100 C.; 2) heat required for phase transition; 3) heat required to reach operating pressure; 4) heat required to reach desired superheat. This would help me grasp the impact of each stage in packing our molecules with energy. If my thinking is bent, please straighten me out. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 27, 2018, 01:17:09 AM
HI MJM, i have always wondered why the pivot point for the pivoting radius rod can be in lots of different positions. some below some above  and some quite far away and some quite close to the beam pivot ??
Title: Re: Talking Thermodynamics
Post by: paul gough on March 27, 2018, 09:54:43 AM
Hi again MJM, A flow on question, sort of connected to the previous is; If one has a boiler being steamed at a given pressure and then it  is steamed at double this pressure, all other things being equal,  does doubling the pressure always require a set percentage more heat to maintain it, or does it vary as pressures rise. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 27, 2018, 01:12:19 PM
Hi Paul, I did hear that you had a shower or two of rain.  Sounds like a lot to most of us, but as the farmers say it takes more than one rainy day to make a wet season.  Still, enough to bring out the frogs for counting.

Thanks for the question, bringing us back to thermodynamics, where I am much more comfortable.  It is a good question, as understanding how much heat is put in to the three stages of producing superheated steam helps us understand many of the issues regarding feed water heating, and why a boiler can not produce as much steam per second when cool water is fed, as opposed to by closed system tests when I measure the steam production "from and at" saturation temperature. 

So the thinking is fine, but before I start quoting figures, let's think about the process we actually follow.  If we fill our boiler with say 20 deg water, the saturation vapour pressure is only 2.34 kPa, that is absolute pressure. But if the air is say 50% humidity, the actual vapour pressure is only 1.17 kPa.  And air has done an amount of work to compress the water to say 100 kPa, or atmospheric pressure.  Now because the change in volume of the water is so small in this compression, even the text books suggest it is close enough to zero.  However, the air really does complicate the calculations.   So let's assume we have previously filled the boiler and run it long enough to expel the air, then allowed it to cool down with the regulator closed so it is really only at 2.34 kPa absolute and no air.  This simplifies the calculations enormously, but if you remember back to Willy's questions about filling the boiler on the mountain, you will remember the rough process.  Of you really want, I can do them again to show the difference, but for tonight, let's just look at the water, and assume the total vapour pressure is only due to water.

Once the boiler is sealed, with the regulator closed, 100 degrees C is no longer a relevant special point.  When heat is applied to the boiler, the pressure will rise, and the first law of thermodynamics says for heating at constant volume, which also means that no work is done during heating, the heat input is equal to the change in enthalpy.  The answer is different if you apply heat with the boiler outlet open as some of the heat goes into the work done by the expanding steam.

The model of steam behaviour for this and all similar problems is the steam tables.  You may have downloaded these when I provided a reference way back.  Again I will look it out again for you if necessary.

The steam tables tell us the enthalpy of saturated water at 20 degrees C is 83.96 kJ/kg.  This is listed in the tables in the column labelled hf.  This is the starting point.  Probably a bit cool for your location, but equally, too warm for many of our fellow forum members.

Let's assume we will produce steam at 500 kPa.  As the tables list absolute pressures, that is equivalent to 400 kPa gauge or about 57 psig.  I have chosen that pressure because it also appears in the superheat tables without complicating the answers by having to interpolate the tables.  At 500 kPa, (or 0.5 MPa, whichever is listed in your copy of the tables) we look up four figures. We find the saturation temperature is a touch under 152 deg C.  This is the temperature at which the water will start to boil at this pressure.  We also look up hf, 640.23 kJ/kg, so the heat added to this point is 640.23 - 83.96 = 556.27 kJ/kg.

The next column, headed hfg, is the heat necessary to evaporate the water at that pressure.  We read 2108.5 kJ/kg as the latent heat.  And the last figure in the column headed hg is the enthalpy of  the steam without any superheat.  We read hg = 2748.7 which is the same as hf + hfg, which the the enthalpy of steam before any superheat is added.

Now we look at the superheat section of the tables.  Basically a collection of small tables, one for each pressure.  In the table for 500 kPa, the saturation temperature is shown in brackets as 152 as before.  We can go down to say 250 C, or approximately 100 degrees of superheat, and across to the column headed h, where we find the enthalpy as 2960.7 kJ/kg.  Now we can subtract the enthalpy of the saturated steam 2960.7 - 2748.7 = 212 kJ/kg which is the heat added by superheater.  This does not look like a lot in the overall scheme, but it makes a big difference to the engine output as I demonstrated earlier in the engine discussion.  I can look up the post number or revisit that again if you prefer.

So now we have the total enthalpy of the superheated steam, and we can subtract the enthalpy of that cold water to see the total added is 2960.7 - 83.96 = 2876.74 kJ/kg, and we can answer your three questions.

Sensible heat to reach boiling point is 556.27 / 2876.74 = 19.3%

Latent heat to boil the steam is 2108.5 / 2876.74 = 73.3%

Heat added to superheat to 250 C is 212 / 2876.74 = 7.4%

Now the second question, what if we double the pressure, yields a surprising result.

So far I have assumed an absolute pressure of 500 kPa.  So double that is 1000 kPa, (or 1 MPa), a convenient figure which is also directly listed in the superheat tables, equivalent to about 128 psig.

Of we assume the same superheater outlet temperature of 250 degrees C, we get the surprising result that h = 2942.6 kPa which is slightly less than the enthalpy of 2960.7 at the lower pressure.  If we left the regulator closed, and just continued heating to the higher pressure, and still controlled the superheater outlet temperature to 250 C.  Because the saturation pressure is 180 C, this is only 70 degrees of superheat, and we don't have to superheat much further to have equal enthalpy to the lower pressure.  I will try and attach a pressure enthalpy diagram for steam, basically the steam tables presented in graphical form, and you will see from the diagram why this result occurs.  In the table, pressure is in MPa, so look at 0.5 MPa and 1 MPa.  Unfortunately, the temperature lines are absolute so 250 is 250 + 273 = 523 which is about half way between the 500 and 550 degree lines.  I think the picture will be clear enough for you to find the appropriate lines.

Perhaps I should mention that while the calculation is a constant volume calculation, it would be necessary to have Willy's electric superheater we discussed at one stage.  It is normally necessary to maintain a flow through a conventional superheater to prevent it becoming too hot, though in a small Meths fired boiler, it is not really necessary, though it is advisable to avoid joins in a copper tube superheater.

Hi Willy, different beam engine designs involve slightly different versions of the Watts linkage, but if we stay with the grasshopper for the purposes of your question, an ideal Watts motion has an isosceles triangle on its side with the piston rod as the base.  The apex of that triangle has to be constrained to move exactly half the distance the top of the piston rod moves.  Then the swinging link is fixed on the vertical centreline of the piston rod, and attached to the midpoint of the beam.  The other end of the beam has to move in a horizontal path at the same elevation as the fixed end of the swinging link. 

In a real engine, with a rocking column at the far end of the beam, so the end of the beam moves in an arc instead of a straight line, that isosceles triangle is "pulled over", just a fraction.  Also you will note the swinging link is not fixed at the beam centre.  It is possible by trigonometry or graphically to find a position displaced from the piston rod centre line where the attachment point on the beam is equidistant from the fixed point at top, middle and bottom of the stroke and so constraining the beam to the correct position at these three points.  So the different positions of the ends of the link are selected to compensate for the arc at the other end of the beam.  The fixed end position might not be at the same elevation as the top of the column at the other end, and some designs seem to have a provision which may be intended to make a small vertical adjustment possible. 

But three points, while they completely define the arc swept by the link, do not necessarily mean that the top of the piston rod moves in a straight line between these end points, it may move in a slight curve.  Some trigonometry is required to check the beam position at intermediate points to see if the linkage is correct or has a small error.  I am still looking at it when I get a chance, perhaps over Easter, as I have not been able to produce a satisfactory calculation so I really don't yet know the answer.  I will let you know if I get a satisfactory answer.  But to be fair, if there is an error in the linkage, it is very small and probably accepted by the inevitable tolerances in the pin linkages and rod packing.  I may have to resort to building an engine to see if I can measure any discrepancy for myself.  I really think the issue is quite academic, and any error is not enough to spoil the operation of the published designs that are well proven, but I have read of others who claim there is a necessary small error, I would like to be able to demonstrate it for myself.  I hope that is enough for now.

Thanks for looking in,

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 28, 2018, 02:08:36 AM
Hi MJM, Thanks for that ...can i ask you about compound,  triple expansion , Quadruple expansion engines ....is there an exact figure for determining the cylinder bores  assuming all the other dimensions are equal ?? also with a beam engine where the inside cylinder is shorter (closer to the beam) is there also a constant for the bore sizes ?? and is this worked out with a formulae ??...thanks...
Willy
Title: Re: Talking Thermodynamics
Post by: paul gough on March 28, 2018, 03:53:02 AM
Thanks MJM for working through and clarifying the proportions of heat input. The chart certainly shows the 'surprising result', my untutored eye has never noticed this before. Demonstrates the power of visuals over lists of numbers.I wonder how many other surprises might be revealed if I was graced with enough knowledge to know where to look and what to look for! Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: paul gough on March 28, 2018, 09:58:32 AM
Seems at least one other person in Nth Queensland has /had an interest in thermodynamics, so thought maybe there is someone on the forum who might like to know of the books existence. Found this book in the Op Shop for $1. Maybe its one of Click Springs old books?? Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 28, 2018, 12:33:17 PM
Hi Willy, that's a bit out of left field, it sounds like you are hatching something! 

If we look at the ideal engine concept, which tells us the maximum amount of work that could be produced from the steam conditions, the thermodynamics tells us we should balance the stages so each stage expands the steam through an equal pressure ratio.  Remembering that the pressures have to be absolute pressures, for a compound engine, we are looking the intermediate stage pressure (Pi),so that P1/Pi = Pi/P2 = sqrt P1/P2, where P1 is the supply pressure and P2 is the exhaust pressure.  To achieve this, we would probably make the ratio of the swept volume of the stages about equal to this same ratio, but it might need a bit of refinement using the specific volume column of the steam tables, probably in practice not a very important factor.  For a triple, the ratio would be the cube root relationship, and four a quad expansion, a one fourth root.

Of course in a real engine, at the start of the expansion in the HP cylinder, we only have the clearance volume at steam pressure when the expansion starts, so the quantity of steam would severely limit the amount of work that could be done by the engine.  We normally leave the inlet valve open for and admission period, then close the inlet valve partway through the stroke and the expansion begins.  The ratio of the volume at cutoff is then expanded to the volume at release, and the pressure ratio for the expansion is determined by that volume ratio.  Then the steam in a compound engine is opened into the second stage.  So the volume of the HP cylinder is then expanded into the increasing volume of the second cylinder during its admission stroke, thus completing the first stage expansion.  When the second stage inlet valve is closed, the expansion continues until the second stage release to the exhaust pressure for a compound, or to the lp cylinder in a triple expansion engine.  Hopefully the pattern is clear to continue to a fourth stage in a four stage expansion.

The situation is complicated by the need to look at the position of the various throws around the crank shaft and the actual valve timing.  Some articles I have read suggest that the valve timing between the cylinders can be made a little more independent by a steam "volume bottle" between the stages, so the HP cylinder exhausts into this volume, while the next stage admits steam from this volume.  It would be particularly important in a reheat cycle where the first stage exhaust is sent back to the boiler for extra heat input, which significantly increases the pipe volume between the stages.

The crank angle and the valve timing give the designer considerable latitude in the best way to control, the expansion and divide the work between the cylinders.  I would need to do a lot more reading to be confident to actually design an engine, but in looking at existing proven designs, I would be looking for that equal volume ratio between the stages.  For equal stroke on each cylinder, it means the same ratio applies to the cross-sectional area so there would be a square root relationship between cylinder diameters on a compound and so on.  And I would be looking closely at the recommended valve timing.

For a beam engine, where the stroke of the cylinders are not equal, the relative length of the strokes would also have to be taken into account in making those equal volume ratios, resulting in a slightly different diameter ratio.  Some of our other members might have more information on this topic.

I am sure the pioneers would have developed rules of thumb for the cylinder diameters, and these rules are probably included in some of your historical books, but I suspect these rules lead to a similar result.  I hope that is enough answer for your purpose, as I don't even have access to my copy of Jamison's book at the moment to see if he says anything helpful on the subject.

Hi Paul, the graphical presentation certainly clarifies the direction of property changes in a way that is difficult to appreciate from the tables.  But the graph is hard to read to sufficient accuracy when the answer to a calculation involves a small difference between two large quantities.  So I find it useful to use the tables to calculate precise answers, but use the diagram alongside to make it clearer just what is happening.  And of course the laws of thermodynamics to tell us which properties are changing in a specific problem.

As an example of something else you might be able to see from the diagram, if we continue to use your example from yesterday, and assume we have kept the regulator closed until 1 MPa but find the load on the engine only requires 0.5 MPa to run at the required speed, so we throttle the steam from 1 MPa to 0.5 MPa.  The first law says that at the throttle valve, there is no heat input or output, and no work done, so the change in enthalpy is determined only by the change in velocity.  This is quite small if the pipe diameter is increased on the low pressure side, so the velocity change is negligible.  So we effectively have constant enthalpy, a vertical line on that diagram.  If you follow that vertical line and see how it compares with the constant temperature lines, you will see that the steam cools just a very little during throttling.

Now remember when we discussed adiabatic expansion, the second law tells us that the work done is the change in enthalpy, and that during the ideal expansion, the entropy is constant.  So, if you follow the slope of the constant entropy line from 1 MPa at the temperature you are considering (either superheated or perhaps just saturated) to the exhaust pressure, the vertical lines show you the change in enthalpy for an ideal engine.  Similarly, if you follow from the conditions at 0.5 MPa, you can see the change to the same exhaust pressure is much less than for the higher pressure.  And of course a real engine does less work in each case, probably quite a similar percentage of the ideal engine enthalpy change.  So you can see that throttling to the lower pressure reduces the potential work output of the engine, as you might expect.

The other thing that you might be able to see from the chart, if you compare the constant entropy expansion from saturated steam and compare it with superheated steam at the same pressure.  You might have to calculate exact values to see in this pressure range, that the extra work obtained from superheated steam is produced at higher efficiency than the work produced from saturated steam.  The reason for this, is that the main reason the efficiency of the steam cycle is so low is the high proportion of latent heat required.  All this latent heat is lost with the exhaust.  The extra heat put into superheating does not involve any extra latent heat, so the resultant extra work from superheated steam is obtained at much higher efficiency than the work from saturated steam at the same pressure.

You might also try following the temperature changes as your water is first boiled (at constant temperature and pressure) then further heated at the same pressure.  The diagram clearly shows how the temperature enthalpy and entropy all change as you add superheat at constant pressure (a horizontal line on the pressure enthalpy diagram).

You will find that for some problems the pressure-enthalpy diagram is most helpful, while for others the temperature-entropy diagram is more useful.  If you look around a bit you will even find an enthalpy-entropy diagram, the one to use is generally the one where you make most use of the vertical and horizontal lines rather than needing the curves.  But they are all presenting the same data from the tables.

I think that book is one of a series on many topics, possibly intended for students or those who find a need to understand more of the topic in their work.

Thanks everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 29, 2018, 12:54:12 PM
Hi Willy,  I was just looking over my post yesterday in response to your problem, and I can see that I did not explain the steps from the volume ratio to the diameter ratio very well.  So let me give you an example.

Let's assume steam supply at 400 kPa and exhaust at 100 kPa, so no condensing.  Remember, absolute pressures so 100 kPa is also atmospheric pressure, just a little lower than standard, for ease of calculation, but realistic in a low pressure weather system.  In imperial units, you might assume exhaust at 14.7 psia, a slight high pressure system and steam supply 58.8 psia, very similar pressures, but because the ratio is the same, supply is four times exhaust, the same relationship between cylinder volumes applies.

For a compound engine, the pressure ratio between the hp and the intermediate pressure should be the square root of the pressure ratio from hp to exhaust, so square root of 4 or 2 and this figure should be used for the cylinder volume ratios.  So V2/V1 = 2, so we can say (d2/d1)^2 = 2 so d2/d1 = 1.414.  The intermediate pressure will be 200 kPa.  If the hp cylinder is 1" diameter, then d2 = 1.414"

For a triple expansion, the pressure ratio should be the cube root (= ratio^1/3) of the overall pressure ratio.  Assuming the same supply and exhaust pressures, the cube root of the pressure ratio, 4^(1/3) = 1.59.  So the lower intermediate pressure = 100 x 1.59 = 159 kPa.

The higher intermediate pressure will be 159 x 1.59 = 253 kPa, and of course 253 x 1.59 = 400.

For our 1" dia hp cylinder, (p2/p1)^1/3 = V2/Vi = (d2/di)^2, so d2/di = (V2/Vi)^(1/3))^1/2 = V2/Vi^(1/6) or 1.26.

For the same 1" hp cylinder intermediate cylinder will have a diameter of 1.26" and the lp cylinder is 1.26 x 1.26 = 1.59" diameter.

In the above calculations I have assumed the stroke of all cylinders is the same.  For your beam engine, the statements about pressure ratio and volume ratio still hold, but the swept volume of the cylinders is a function of d^2x s, so for a compound engine, (p2/p1)^1/2 = V2/V1 = (d2^2 x s2)/(d1^2 x s1).  We can multiply each side by s1/s2 to give V2.s1/V1.s2 = d2^2/d1^2, so d2/d1 = (V2.s1/V1.s2)^1/2.  In other words, we adjust the volume ratio by the ratio of stroke lengths before taking the square root.

Obviously it depends on which cylinder has the shorter stroke.

That's some fairly heavy maths, I hope I got it correct, it has taken a bit of work on dealing with exponents, but I hope it better answers your question for the compound and triple expansion engine.  I hope you can follow this through to a quadruple expansion, the diameter ratio is 1.19).  I will give everyone a chance to absorb that, and if necessary, point out any errors in the maths, and work the example of the beam engine to see the difference it makes whether the hP or lp cylinder has the shorter stroke.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on March 29, 2018, 02:03:50 PM
MJM, just so you know there is a "sup" and "sub" button in the text editor to allow superscript and subscript text which is more work for the author but makes reading formulas easier. All you have to do is select the text to modify and then hit the button.

Cheers Dan

Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 30, 2018, 12:05:40 AM
Hi MJM , I have taken the measurements from the Beeleigh and the cylinder bores are   HP 270 mm and the LP 440 mm the shorter HP cylinders is 83% of the LP cylinder . The exhaust is fed directly to the LP steam chest  So i was wondering how this configures with modern calculations This engine was made about 1830 so how developed were Mr Woolfs understanding of currant thermodynamic laws then ?? On a triple expansion the throws of the cranks are  equal. Is this just for aesthetics or could the throws be different to save metal and weight ??
 just thinking about these things as there is no imminent self designed triple on the way.....
Willy.
.
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 30, 2018, 11:04:33 AM
Hi Dan, good to hear from you again.  Thanks for the tip about subscripts and superscripts, I will try them this time.  On that topic, do you have a preference for "." or "x" for multiply?  And are you happy with the "^" for "raised to the power of"?  I suppose the alternative is the whole exponent in superscript.  Also are you comfortable with the "E" notation, as in 2E6 for 2 x 10^6.  The iPad does not like it but many spreadsheets and scientific calculators offer it as an option.  These conventions can vary between countries, so not always easy to know which an international membership would find easiest.

Hi Willy, with those diameters and stroke of the hp cylinder = 0.83 times the stroke of the lp cylinder, we can put the pressure ratio = volume ratio.  Rather simpler maths than knowing the pressures and calculating the relative diameters.  Of course the stroke ratio comes from the beam dimensions.

Remember the volume of each cylinder is proportional to the diameter squared times the stroke.  I have used the subscript 1 for the first or hp cylinder, and 2 for the second, or lp cylinder.  As a formula we have

Rp= (V2/V1) = (d22 x s2)/(d12x s1) = (d22/d12)x s2/s1

When we substitute those measurements, d1 = 270, d2=  440, we find Rp= 4402 / 2702 x 1 / 0.83 = 3.2

(Wow! That was difficult.  My ipad will not allow me to select the text for some reason, I seem to have to delete and retype.  Not easy.  I have to post to see how I went on the process.  Works better if I retype first then delete!)
 
So I would conclude the engine might have been designed for a pressure ratio of 3.2.  The square root of this is 1.79 so the intermediate pressure would be about 1.79 x exhaust pressure.

If the exhaust pressure was atmospheric pressure, say 100 kPa, then the supply pressure would be 100 x 3.2 = 320 kPa.  This is 220 kPa gauge pressure or about 31 psig.  I would expect the boiler to be a operated at a bit higher pressure, to allow for throttling and piping losses, but does that sound something like the name plate pressure? 

The intermediate pressure would be 1.79 x 100 = 179 kPa.  Again we can calculate gauge pressure of 79 kPa, or about 11 psig.  It would be interesting to know whether Mr Woolf was aiming for that square root relationship, or had some other basis for sizing those cylinders.  (Perhaps it will be in that haul of model engineer magazines.  Great score!) In any case the fluid mechanics is such that the intermediate pressure tends to settle at a level that works with the actual steam pressures cylinder volumes and valve timing.

It also depends on what pressure he expected to achieve in that condenser.  If he expected to achieve say 50 kPa (7.35 psia) then he would only require an inlet pressure of 3.2 x 50 = 160 kPa.  That is only 60 kPag or 8.5 psig, may be a boiler operating at 10 psig, which sounds a bit low.  It is possible that not much vacuum was expected or achieved.

The test of the theory is whether the answers are somewhere close to the real answer, so putting myself on the line here, and I hope I have the maths right.

I would think that Mr Woolf would have been right up on the latest theory at the time, and he would have had a finely tuned intuition as to how it all worked.  However he had to rely on less accurate data than we have available these days.  (What was the footnote on that book you posted a while back?)  He may have also had ideas on valve timing that affected relative cylinder volumes.

In an engine with a conventional crank shaft, the throws are normally equal.  This gives the same piston speed for each cylinder which is an important figure in control of wear rates.  I suspect, but don't really know, that this may also reduce balancing problems.  I don't really know if there is any reason why an engine could not be made with unequal stroke lengths.  Perhaps others may like to comment.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 31, 2018, 11:13:01 AM
Hi Willy, I hope that you are going to let us all down from the suspense by telling us what the real steam conditions were for your engine.  Calculations are ok but only really informative if they are helpful in understanding reality.  I am most interested to know whether Mr Woolf used a similar logic to design his engine for the available steam conditions or had other ideas.

Just as a little side note, I have been following Brian's trials and tribulations with fuel for his Poppin.    He seems to have tried three fuels, methanol, ethanol and propanol.  It is hard to know from the chemistry how each would burn.  The chemistry is simple enough.  Methanol has one carbon, three hydrogens and an OH group.  It is usually used as antifreeze and quite poisonous.  I don't know the composition in the form it is normally sold, pure, or with water or other substances in a mixture.  I have no experience of burning it, but in a chemical plant it needs good fire protection.

Ethanol is the normal drinking alcohol.  Two carbons plus five hydrogens and an OH in the arrangement CH3 - CH2 - OH.  Apart from alcoholic drinks is is commonly sold as methylated spirits, a name that has always confused me as it is chemically ethyl alcohol and normally has five percent water.  It is sometimes called denatured alcohol, and I believe the denaturing agent is methanol, which makes it poisonous, possibly explains the name and sometimes it is coloured blue, both to discourage drinking.  Presumably only a small addition, as it does not appear in the list of ingredients on the label.  It is used as a cleaning agent and a stove fuel.  It is the one I use for my boilers, and also in my stove when I am camping.  My wife uses it as a window cleaner.  In all, we use quite a lot of it.  It seems to burn without much residue, though the mixture sold in these parts is water clear.  Ethanol mixes with water in all proportions, and that gives it a great safety advantage in our use of it for fuel.  Water will extinguish any ethyl alcohol fire, so you only need some water at hand in case any spilled fuel catches fire.  Not quite intuitive, as water should not be used to extinguish other hydrocarbon fires.

Isopropyl alcohol has three carbons and seven hydrogens plus that OH group that makes it an alcohol.  Usually sold as rubbing alcohol and as others have mentioned contains some oil additives  so as not to dry the skin.  I imagine these might be left behind when it is burned, so might foul wicks or fine jets, but again I have no experience of burning it.

Many of these differences have been pointed out by those commenting on Brian's thread.

I see no reason the basic carbon- hydrogen- OH compounds should not be quite similar in the burning characteristics, with the carbon hydrogen ratios causing variations in heat output and the CO2 and water content of the combustion gases.  But without direct experience, I don't know if they actually burn in a similar manner.  I would suspect the main reason for any difference might be the various additives included to make them better for the specific applications they are usually used for.  Another case where theory has to be backed up by practice.

Does anyone else have more information on any differences in the burning of these compounds?  Just something to think about while Willy gets back to us on those steam conditions.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 02, 2018, 04:24:05 AM
HI MJM , here is more of the text about cylinder sizes of compounds ...but nothing definitive though !!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 02, 2018, 12:52:30 PM
Sorry to be absent without notice last night.  We had an international visitor for dinner.  Quite an interesting evening, but too tired after.  Always interesting to get a glimpse of a different culture.

Hi Willy, that is a complex procedure described in your book.  It appears to be a mixture of theoretical ideas and some empirical factors based on experiment.  Interesting that he mentions that square root for compounds, cube root for triples etc. and also talks about valve timing, those pressure ratio factors are recognised these days as giving the best efficiency.  Of course part of the complexity is that the formula seems to be calculating the required size for the hp cylinder for a required power output, not just the relative sizes of the cylinders. 

The point about the cut off for the hp cylinder determining the total power of the engine is clear, as it determines the steam consumption and hence the energy available.

The point about the lp cylinder cutoff is more interesting.  Earlier cutoff of the lp inlet decreases the power of the hp cylinder by increasing the back pressure, clearly.  But increases the power of the lp?  Well, if it increases the back pressure on the hp cylinder, it also means the inlet pressure of the lp is higher when expansion starts and expansion occurs over a greater volume change.   But if the cut off is later, the volume of steam into the lp is increased, but the pressure is lower, and the volume change remaining for further expansion is less.  So higher pressure, smaller volume, greater volume change for expansion, or lower pressure more volume, less volume change for expansion, which produces more power?

Well, as mentioned earlier in the article, the second cylinder admission timing does not affect the total power of the engine, that is determined by the hp inlet timing, but just redistributes the power between the cylinders.  So if the hp cylinder power is reduced, the lp power output must be increased.  So that is the answer, much easier to work it out that way. 

There is also mention of an exhaust pressure of 4 lb. , presumably psi, and clearly absolute.  If Mr Woolf expected such a good vacuum for the exhaust, his cylinder diameters would indicate quite a low boiler pressure.  It will be interesting to apply the formulae in those pages to the engine when you eventually discover the operating pressure (probably similar to the boiler operating pressure) and expected exhaust condition, and see how the power compares with the rating.

So two complex questions intertwined.  What size must the house cylinder be for a given power output?  And what are the proportions of the lp cylinder compared with the hp?  Very interesting that the author starts with the assumption that all the power is developed in the lp cylinder, then uses the overall absolute pressure ratio to size the hp cylinder.  I am not sure that I understand the procedure, but I assume that it worked, and those empirical constants were found by painstaking experiment that revealed or at least indirectly included the necessary efficiency factors.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 03, 2018, 01:58:00 PM
After thinking a little more about last nights topic, I realised that while the most efficient split between stages is equal pressure ratio as I stated before, to set the volume ratio for calculation of the cylinder diameters, I had implied, but not stated directly, that  we could use the equation
 P1 x V1= P2 x V2
which is of course the ideal gas law.  Had I thought a bit longer, I should have remembered that expansion of steam, a real gas, does not follow the ideal gas law, especially when we are talking about expansion of saturated steam.  It gets a little closer if there is a lot of superheat, but for the historical engines in particular, I am sure that we can assume it is saturated steam.  If not the ideal gas law, what should we use?  Obviously, our old friend, the steam tables!

If you look again at a copy of the steam tables, you will see that after the pressure and temperature columns, there are two columns for specific volume, which give the volume of a kg (or pound mass) of steam.  Then follow the energy columns, enthalpy and entropy that we have discussed often.  The steam tables contain our best knowledge of the properties of steam.

So I started again.  It is not nearly so simple, as not all pressures are listed.  If I assumed a supply pressure of 250 kPa, and exhaust 80 kPa, figures in the possible range based on Willy's measured piston dimensions, and took the square root of the pressure ratio, 1.76, as the intermediate pressure, then the intermediate pressure is 250 / 1.76 = 142 kPa.

This would have involved a lot of work interpolating the tables, so to simplify this, I took as the intermediate pressure 150 kPa which is directly tabulated.  This is a pressure ratio of 250 / 150 = 1.67.  The problem is to calculate using steam tables as the model, the volume ratio, and compare this with that value.

This involves first estimating the exhaust steam properties resulting from expansion of steam through that pressure range.  First we use the second law of thermodynamics which says for an ideal adiabatic engine, the entropy of the exhaust is the same as the entropy of the supply steam.  Using this entropy the dryness of the ideal engine exhaust is calculated as 0.97, and hence the enthalpy change can be calculated.  Assuming a real engine adiabatic efficiency, the real engine exhaust conditions can be calculated, (dryness 0.98) and from this comes the volume of exhaust steam and the volume ratio.  My calculation gave the actual volume ratio by steam tables as 1.56.

The volume ratio is proportional to the square of the piston diameter, so the ratio of diameters is the square root of that volume ratio.

So, if we assume the pressure ratio 1.67 is the same as the volume ratio, we get each cylinder diameter should be 1.29 times the previous one.

However, if we use the volume ratio from the steam tables we get the cylinder diameters should be 1.25 times the previous one, and the resulting lp cylinder diameter will be a little smaller than the one resulting from using the ideal gas law.

If we repeat the whole calculation for the expansion from the intermediate pressure to exhaust, I expect we will get a similar ratio, though I have not done the calculation.  Based on the calculations I have done, I suspect that any difference might only affect the third or fourth significant figure, if there is any difference at all.

Now, we have quite accurate steam tables, while Mr Woolf was probably reliant on the ones you posted from the book with the footnote about accuracy, but either way, we should start with diameters based on real volume ratios.  However, at the end of the day the difference is not large and that ratio will just alter the division of work between the cylinders.  The larger lp cylinder, in principal expands the steam a little more.  As even a compound engine does not fully expand the steam to the exhaust pressure, the effect of this is probably only a bit lower pressure in the cylinder immediately before the exhaust valve opens at the release point, and depending on the actual valve timing, might affect the efficiency a little, but the differences will not make the difference between producing adequate power and not.

I hope that gives a little more insight into how the calculations are done, and how the steam tables are used in calculations involving an engine.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 03, 2018, 03:19:35 PM
Hi MJM ,  thanks for all this , i have a book from 1829 about steam engines and i will trawl through it to see what their understanding was then of Thermodynamics,!! Also Will saturated steam ...superhearted steam and compressed air give you the same oomph value  at the same indicated pressure ? Is there a value for oomph btw !! Actually does wet steam just change into dry steam at a certain temp or can you have wet steam at a really high temp ?? Perhaps i should know this already ?? is there a phase change for extra super super heated steam or is that another silly question ??!
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 04, 2018, 12:07:01 PM
Hi Willy, your research into the state of knowledge of thermodynamics at the time your engine was designed are most interesting.  I would be very surprised if someone such as Mr Woolf was not right up to date on the state of knowledge at the day.  He would have been limited by the accuracy of tables as we have mentioned before.  But I am sure his knowledge would have been extensive, and supported by a keen intuition, acute powers of observation and painstaking experiment.

Regarding your question on saturated steam, superheated steam and air at the same pressure, it is the one that tripped me up right at the beginning of this thread and I am quite embarrassed that I have not yet returned to provide the correct answer, and I am going to defer it for a day or two more, but this time I will return to it.  First your other questions.

It is difficult to describe exactly what happens at different pressures as we are all so familiar with a kettle on the stove, or a saucepan when the vapour space above the liquid surface is always at the relatively constant atmospheric pressure, and the composition of the gas in that vapour space is a variable concentration of air and water vapour.

When we say that water boils at 100 degrees, we are actually describing what happens when the equilibrium water vapour pressure of the water is the same as, or exceeds atmospheric pressure.  At this point, the water liquid starts changing to vapour and the resultant rapid volume expansion results in those bubbles we associate with boiling.  As we continue to supply heat to the boiling pot, more of the liquid changes to vapour, but the temperature and vapour pressure remain constant until the last bit of liquid has evaporated.   At this point, the vapour is called dry saturated steam.  Only when this point is reached can the temperature start to increase.  As soon as the temperature starts to rise, it is defined as superheated steam.  As the pot is now dry, the temperature of the pot is no longer cooled by the boiling water, so it also gets very hot.

I have skipped the description before the pot boils dry.  Those bubbles rising rapidly to the surface due to the huge density change tend to carry some liquid into the vapour space with the gas phase, that is the mist we are trying to describe when we use the word vapour instead of gas phase.  Dry saturated steam is when all the liquid in that mist has evaporated, then superheating begins.

Before boiling starts, in principle the water gas phase or vapour phase is in equilibrium with the liquid with the water vapour pressure shown in the steam tables.  In an open pot, or kettle with a leaky lid, atmospheric air mixes with the water vapour and the total pressure of water vapour plus air is constant at atmospheric pressure.  Below 100 degrees, that atmospheric pressure at the surface suppresses any vapour formation below the surface, so we have evaporation at the surface as the liquid absorbs heat, but not that vigorous bubble formation we know as boiling.  Because of that constant atmospheric pressure, the whole process occurs at constant pressure.  The water vapour just displaces some of the air from the kettle.

To conduct the experiment at any other pressure, we have to enclose the water in a sealed pressure vessel.  Of course, we could take our kettle to the moon, or the international space station, or perhaps even Jupiter to experience different pressure, but I assume you want me to stay practical.

We have previously discussed the issue of that air in the boiler, so this time let's assume we extracted the air with a vacuum pump, or have even just driven it out by raising steam, then shut the isolation valve and let the boiler cool down.  Now, the vapour space will contain only water vapour at the equilibrium pressure for the temperature, assuming that everything is happening slowly enough to ensure equilibrium.  At 15 deg C, the vapour pressure, so the boiler internal pressure, will be quite a good vacuum.

If we now heat that closed boiler, we will see the pressure tending to rise to stay in equilibrium with the liquid as the temperature rises.  If we have a pressure regulator so that the pressure is kept constant by blowing off excess water vapour, the water temperature will stay constant as heat is added at constant pressure until all the liquid has evaporated, at a temperature dependent on the pressure setting of that regulator.

The data for pressure temperature and all the other properties we have spoken about is contained in the steam tables, but is perhaps easier to understand on one of the diagrams.  I have previously posted the Temperature-Entropy diagram, and the Pressure-Enthalpy diagram and there are others.  All are characterised by that line which forms a loop, under which is the two phase region.  If you look closely, the temperature and pressure stay constant within that loop as energy is added.  Specific volume, enthalpy, and entropy change inside that loop as shown by the various sloping lines.  But pressure and temperature are both straight lines of constant temperature and pressure.  So under that loop, we have wet steam in equilibrium with the liquid.  The loop is the line that separates wet steam from liquid water or superheated steam.  If you add heat to superheated steam, it gets more superheated.  If you expand the steam to lower pressure it stays superheated.  You can even compress steam to a higher pressure and it stays still superheated.  The "super" in superheated does not mean anything spectacular, it just means heated above saturation, even if only a fraction of one degree.

At the very high pressure and temperature at the top of the loop there is no distinct phase change between the cooler end where we would call it liquid, and the higher energy side where it is clearly vapour.  There is just a continuous change of density.  Very hard to grasp.

The only other phases changes occur off the bottom of the diagram, where there are phase changes between liquid and solid (ice or snow) and also between solid and vapour.  There are also some more obscure phase changes between some different forms of solid, that I am not so familiar with.  Though I have experienced solid water (snow) changing directly to vapour without becoming water in between.  It is called sublimation.

Once the steam in our boiler is dry, further heat superheats the steam.  Or, if we run the pipe with the steam escaping through that pressure controller through our furnace, then superheating occurs outside the boiler.  The temperature rises quite quickly, and the heat transfer coefficient reduces markedly, so we have to be careful not to melt the boiler.  It no longer takes much heat to increase the temperature, we no longer have to deal with that latent heat.

If we choose to conduct the heating of the boiler at higher temperature, the whole process is much the same, and we have wet steam, saturated steam and liquid, and evaporation at constant temperature and pressure, right up over 370 deg C.  Try looking closely at one of those diagrams, and identifying the lines which define a constant value for some property.  Then follow those lines through the two phase area into the superheated area.

Above 374.14 degrees, there is no longer a distinct liquid phase.  This is called the critical temperature.  The corresponding pressure is 22,090 kPa, called the critical pressure, right at the very top of that loop that encloses the two phase region.  Modern power station boilers can operate above these conditions and are called supercritical boilers.  But I don't recommend a model at these conditions.  A tiny steam leak at those conditions is extremely dangerous and difficult to see until too late.  Up to 374 degrees, you can have wet steam at the appropriate pressure, but the pressure is well above atmospheric pressure.  Your electric boiler at 135 degrees, has liquid and wet steam in equilibrium at 135 degrees, so quite hot wet steam.

So not silly questions, just questions that highlight areas I have not explained well enough.  I hope this makes it a little clearer.  It feels like a lot of words, perhaps I should have stopped at one question for one post.  Please let me know where I have to try again.

Thanks everyone for looking following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on April 04, 2018, 01:29:58 PM
Willy, (hope I am not trespassing on your turf MJM), I was wondering if your 'phase change with super, super heated steam' was another way enquiring about the breakdown of steam to its constituent gases, oxygen and hydrogen. Well out of the model arena but still worth knowing about. The text from a reply to that issue is below. Regards Paul Gough.

2 years ago
Jack Denur
University of North Texas
An appreciable fraction of water will be decomposed into hydrogen and oxygen at a temperature high enough so that the Gibbs free energy change for the decomposition reaction equals zero. At 1 atmosphere pressure this will occur at around 3000K to 4000K. At higher pressures the required temperature will be higher, and at lower pressures the required temperature will be lower, because one mole and hence one volume of water vapor decomposes into 1 1/2 moles and hence 1 1/2 volumes of (hydrogen plus oxygen). So decomposition is favored by high temperature and low pressure and is inhibited by low temperature and high pressure.
The high temperature of steam boilers almost certainly cannot exceed 2000 degrees F which is about 1400K even for short intervals, and probably not 1000K on a sustained basis, as these are typical metallurgical limits. At these temperatures there is not much dissociation of water vapor. See, for example, Thermodynamics by Kenneth Wark, Jr. and Donald E. Richards, 6th edition, Table A-24 on p. 1066. (The material in Sects. 14-5 through14-7 on pp. 762-773 and Tables A-12 through A-15 on pp. 1047-1055 may also be helpful.)
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 05, 2018, 11:55:18 AM
Hi Paul, not breaking in, contributions are very welcome, I would much prefer more discussion rather than question and answer.

First, I hope you did not get to badly hurt by cyclone Iris.  Always seems a bit anomalous that a cyclone should be called Iris.  After all, Iris, the messenger of Zeus is supposed to be one of the more pleasant characters of that lot.  However, I suppose Iris did not treat you as badly as the last one a couple of years ago.

I will comment on your suggestion about dissociation into hydrogen and oxygen.  This involves a chemical change as is not really classed as a phase change.  You started me thinking about what is the definition that separates the two, and I don't really know.  But it is normally more about the changes between solid, liquid and vapour through melting/solidification and evaporation/condensation, without chemical changes.

There is certainly a chapter about such chemical reactions in my thermodynamics book, and plenty about that Gibbs function, but it is not a chapter I am familiar with.  We need a Chemical Engineer to come in on that one.

In most chemical reactions, such as acid plus base giving salt plus water, there is just rearrangement of the reactants, but they are all in the products.

In water dissociation to give hydrogen plus oxygen, the constituent molecules of the products are different from the ones that you started with.  And that Gibbs function tell you you need to contribute a lot of energy, the reaction would much rather go the other way, where hydrogen plus oxygen combine to give out plenty of heat, making hydrogen a very clean fuel, as the combustion products are simply water (or steam as first produced).  But you no longer are talking about steam in dissociation.

However, even in dissociation, all the atoms present at the start are still present after.  Similarly, oil can be dissociated into carbon and hydrogen at sufficiently high temperature (and total lack of oxygen).  This happens in a furnace, and when the gas cools after the dissociation, all compounds of hydrogen and carbon are possible, and do form.  The conditions during cooling determine which compounds are most favoured, and the ones required are later separated out.  The process is generally called cracking.  The heat is such that the water cooled heat exchanger which cools the gases generates 1500 psi steam!  If you call that cool.  Well, I suppose kids today would call it cool.

If you really produce high pressure and temperature, the next step is to split the atom.  Then the original constituent atoms are no longer in the products.  And we all know that releases a huge amount of energy in the process that demolished our simplistic "conservation of mass" law of physics.  Fortunately the loss of mass is so very small that for all our normal modelling purposes, we can assume conservation of mass is close enough for practical purposes.

Other processes which I don't believe are classed as phase changes include precipitation and crystallisation, which both occur at much more moderate conditions.  These are more about solubility of a solid in a fluid.

So certainly another most interesting train of enquiry, but one I don't feel qualified to comment much more on.  Others are most welcome to comment, and many thanks for introducing the question.

I hope things dry out soon, but perhaps you at last have the start of a decent wet, after so many dry seasons.  Should be plenty of frogs to count if they weren't washed away.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 06, 2018, 01:50:47 AM
Hi MJM, et al  more brain food   thanks..Saw this in the engineer mag ...so as the pressure decreases the temp falls .....but if the cylinder does not have time to give up its heat then the pressure should stay the same ...but only intuitively of course !!! Also 750 pages from Dalby full of these diagrams and text and formulae !  unfortunately being three score years and ten  i may not get through it !!!! The Freelance is now a runner on air  ,so when it is closer to being finished  it will be interesting to see it run on steam !  Also the 1829 book that is available as a rather bad facsimile .....
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 06, 2018, 12:27:42 PM
Hi Willy, a truly interesting account of early attempts to analyse engine operation in a more detailed way.  And all done without computers, calculators, or possibly even a slide rule.  I don't know when the slide rule was invented/developed.

As the piston moves down the power stroke, once the admission valve is closed, the volume of the trapped gas increases, so the pressure must fall.  Predicting how much it falls as the volume increases is behind the questions about the ideal gas law, and then real gas correlations such as the steam tables.

The reason the temperature falls is that the gas is doing work on the piston, so energy of the random motion of the gas molecules is being converted to mechanical energy in the form of work.  As the gas looses energy, you must either replace it in the form of heat or the gas must get cooler.  All of this is nothing to do with heat transfer through the cylinder walls.  With steam as our motive fluid, the gas is generally hotter than the cylinder wall, so in addition to the process of converting energy in the gas to work, there is also heat transfer taking place.  If we use air to drive our engines, the air also gets cooler as energy is converted, very cool if the air starts at atmospheric temperature, and the heat flow can be inwards to the gas.

The most efficient engine possible is the adiabatic engine, which you might remember means no heat transfer in or out.  You might approach the no heat transfer condition if you constructed a cylinder out of some sort of perfect insulating material.  But a real engine out of a metallic material absorbs heat from the steam in addition to the heat being lost by conversion to work.  This reduces the engine efficiency.  Those early pioneers were trying to understand how much heat is lost as a step towards understanding how that heat loss  affects engine performance.  Of course the cylinder walls are in contact with an expanding gas with reducing temperature alternated with high temperature incoming steam for the next cycle.

Now that is a complex problem in three dimensional unsteady heat transfer.  The heat transfer book has a chapter for two dimensional unsteady heat transfer.  They then write out the partial differential equations in three dimensions, called the Navier Stokes equations.  When I was a student, that was as far as it went and the lecture ended with an off hand comment about all that remained to be done was to solve those equations.  With a modern computer and a very sophisticated finite element program, these equations can now be solved, probably still with some approximations, but in those days, no way.

The approximations used are a reasonable attempt, and I don't know how accurate the answer.  Sometimes a simple assumption gives you most of the answer, and a huge amount of extra complexity  only gives a little improvement in accuracy.  But it looks like they are assuming the temperature is constant at each level in the cylinder, though I could have skipped through it too quickly.  The issue is complicated by storage of heat in the cylinder walls, so at any point on the wall, the temperature rises as heat is received from the gas then returns some of this heat as the gas further cools.  Lagging the cylinder helps lift the average temperature, and keeping the walls thin reduces the amount of heat stored, but on the other hand, a bit more metal acts a bit like a thermal flywheel, reducing the amount of temperature fluctuation, so helping the walls reach a steady temperature.

And of course, heat travels along the cylinder as well as radially.  But while the problem is very complex, someone has to make a start somewhere, and those guys did an amazing job with the resources at hand.  And each step forward contributed to our knowledge today.

It sounds like you have a lot of interesting reading ahead, but make sure it does not get in the way of making more of your wonderful engines.

Thanks everyone for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 07, 2018, 12:39:28 PM
Not much to say today.  Willy asked the other day about steam vs air, a question I have intended to get back to since my first effort at the very start of this thread.  I have at last made a start.  I am happy with the steam calculation but have to complete the calculations for air.

I have had another 300 km drive today, so all calculations need careful checking.  It is too early yet to say if I have a better answer than before, but more work on it tomorrow will, I hope, produce some results.

With less heavy reading to do, perhaps everyone will have more time to make swarf.

Have a great day,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 07, 2018, 02:47:15 PM
Hi MJM, so when did they know all there was to know about thermodynamics so there was no more improvements to steam engines ??....basically apart from the Corliss configuration nothing has really changed since the 1850's....?
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on April 07, 2018, 06:46:10 PM
Quote
basically apart from the Corliss configuration nothing has really changed since the 1850's....

I'm not sure that I can agree on this unless you will insist that it's only a steam engine if it has pistons - there has been a great increase in efficiency with turbines ....
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 08, 2018, 01:23:09 PM
Hi Willy, you are starting to sound like that director of the US Patents office who was reported to have recommended to the government that the office be closed down because everything that could be invented had been.  Or the one who commented on Alexander Bell's telephone that no thinking man could believe you could send sound down a wire, and in any case, even if you could, why would you want to.

Technology tends to move in steps and stairs.  I don't know what development of reciprocating steam engines followed the Corliss, but I suspect you would need a more recent text book to be sure.  There have certainly been developments in welding and steel making technology that enable operation at higher temperatures and pressures.  But as Admiral DK says, thermodynamics also covers steam turbines, and gas turbines.  I am sure that Sir Frank Whittles invention of the gas turbine was more recent than the 1850's.

I mentioned early in this thread that even the best power station sized plants were just meeting 50% efficiency at the time I retired.  Since then, GE have announced two power plants which exceeded 60%.  They were combined cycle plants, that is gas turbines with steam generators using the exhaust heat to raise steam and drive steam turbines to generate more power.  All up output exceeded the 60%, even if it was only during initial operations with everything new and clean.  I don't know what other conditions had to be met for the test run.  But certainly not possible with the knowledge available in the 1850's.  Sometimes development is something obvious like a new valve gear, other times it is a solution to an equation that is not so visible but opens the door to further explorations.

Hi Admiral DK, good to see you dropping in again.  I am with you entirely, thermodynamics is not limited to reciprocating steam engines.  Apart from steam turbines and gas turbines, I would expect thermodynamics to have played an important part in space exploration, both in the rocket motors and the heat shields.  And also in modern large scale solar.  All developments since the 1850's.

Computers now allow us to solve equations not dreamed of before, so also facilitate the continuing development.  While human beings are thinking, development will continue.

I have made a bit more progress on the calculations for air.  Steam was easy, but the superheated steam case required some interpolation of tables due to a less than ideal assumed steam pressure as the basis, but all done now, so a bit more checking then some results.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on April 08, 2018, 09:20:33 PM
Quote
Hi Admiral DK, good to see you dropping in again.

Oh - I haven't left .... I normally read every post in all forums here every day ...!!!

In the beginning (2013 ?) I only followed the subjects I really found (most-) interesting, but I discovered that even the most uninteresting subjects (in my book) might contain some very interesting tidbits of information and some are just plain fun to read (Jo vs. Jason springs to mind  :stir: ).

I will have to admit that I do not read all post in this thread word for word as some of them are a bit more esoteric on the subject than what tickles my curiosity - and then there are some I can't help comment  8)

Best wishes

Per
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 08, 2018, 11:01:01 PM
Hi MJM, sorry i was being a bit selfish with the 1850 comment !! However this is just my main areas of interest...of course there are modern turbines that have quite a range of systems in built..Delaval etc etc etc It is just the basic concept of cylinder, piston ,crosshead etc through to the steam pipe that has remained the same. Even the brand new Tornado loco is basically just this ?? and how efficient is this engine ?? not taking into account digging the coal and transporting it to the engine. Perhaps there are figures available ??
willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 09, 2018, 12:54:03 PM
Hi Admiral DK, Good to hear that you look in every day.  Like you, I read the latest posts on nearly all threads every day.  I find some (or more accurately, many!) that are clearly way beyond my skill levels, but most are still interesting, and often still have something I can even use.  But I suspect that what we all have in common is an appreciation and admiration of the skill demonstrated in so many builds and an ability to understand and admire the skill necessary to do some of the amazing builds we see here.

Sorry I get a bit esoteric at times, not surprising you skip a bit sometimes.  Don't hesitate to point it out, have a dig at me if you wish, it is the only way I can understand where people are up to.  Of course, sometimes I start out on a calculation because the data is available and the calculations straightforward, but without being sure where it will lead, so like a ship heading out into a fog, just hoping the way forward will clear before we reach the edge of visibility.  Sometimes it surprises me with just what results, sometimes it leads nowhere.  But my basic intent is to keep to areas that help us understand our engines and hopefully sometimes even to improve them. So please let me know if there are some areas you would like me to include, and particularly any you could help with the discussion.  I am sure that I am not the only one with anything to contribute.

Hi Willy, no need to apologise.  I hope you do not mind me having a go at your amazing library.  I am really quite envious.  But I don't think you have ever mentioned a book written even as recently as last century.  But there are too many books to collect them all and I admire your ability to focus on a special area of interest.

I am not sure how you would change the basic concept of a piston in a cylinder with a crank to provide continuous mechanical power output from the heat in a gas.  I suspect the main area of progress on this basic theme is around materials and manufacturing methods.  Followed closely by valve mechanisms and drive methods.  I don't know why we haven't seen much in the way of steam driven Wankel form engines, possibly sealing issues, or computer controlled, electrically operated valves, though possibly it comes down to complex mechanisms with many moving parts being replaced by the simplicity of turbines, basically one perfectly balanced moving part, and a much more favourable power to weight ratio, space to power ratio and capable of producing much higher power in each unit, though the boiler is a downside for transport application.  And of course reciprocating internal combustion engines and gas turbines are the more significant areas of advancement in mechanical power production, especially in transportation and very large scale. 

For smaller applications, and even quite large, electric motors are leading the field.  The power can be produced by one large, very efficient generator plant, with redundancy for reliability, and the power distributed by wiring.  Alternatively, and increasing in importance, power generated by solar or wind power, spread over a wide area to reduce distribution costs.  Of course renewables need not only generating capacity, but also storage for load characteristic matching, but politicians and the media do not seem able to grasp the basics of these things.

I have mentioned that I am getting back to the question of steam vs. air, and having another go at those calculation I started way back.  There is no point in the question if you are not wanting develop some real power, so I have assumed a pressure of 450 kPa and atmospheric exhaust so 100 kPa, about 50 psig.  Should have used 500 kPa so I could use the superheat tables directly, but the interpolation was not too complex.  Obviously steam has to be hot.  At atmospheric temperature and that pressure, the water would be liquid.  So 450 kPa saturated steam is 148 deg C.  I also looked at superheated steam, same pressure but 200 deg C.  I don't believe anyone would be using air at 148 degrees, more likely exiting the air receiver at around 30 deg C.

Now there are a few steps in making the comparison.  First, it is worth calculating how much work  could be produced by a ideal adiabatic engine.  This is the limit to what any engine could achieve, but any real engine will produce significantly less.  (Remember adiabatic means no heat transfer in or out.)  We can only demonstrate how much less the real engine produces by a test run.

Second, it is necessary to understand how the steam is actually used in the engine, and how this differs from that ideal engine.  It is important to remember that while the steam inlet valve is open, heat is being supplied by the incoming fluid.  Work output is equal to pressure times volume change, no difference between steam and air.  I don't believe there was any disagreement on that part.

When the inlet valve closes, the steam trapped in the cylinder starts to expand.  Depending on the cut off intended and how accurately this is achieved by the valve timing, the piston is probably around 30 - 60% of its stroke.  The expansion involves the resultant trapped steam including the clearance volume.  Realistically, in a single expansion engine, does not expand the fluid, whether it is steam or air, to much more than double its volume at cut off.  Then, near the end of the stroke, the exhaust valve opens and the remaining steam is exhausted to exhaust pressure.  Because valves actually open slowly under the action of the eccentrics and valve linkages, it is difficult to be sure exactly what the pressure in a model cylinder would be, but an idea can be obtained from the indicator diagrams that Maryak and others have posted.  But remember it is only expanding fluid that will show any difference between steam and air.

An adiabatic engine calculation assumes the whole volume of the steam is expanded through the full pressure range.

Reciprocating engines are volumetric machines, that is, each revolution of the engine admits a fixed volume of the working fluid, so instead of calculating work on the more usual mass basis, I have used the specific volume to convert the figures to a volumetric basis, so kJ/m3.

So what is the theory?  Saturated steam gave the output of an ideal engine as 618 kJ/m3.

For superheated steam, the figure is 570 kJ/m3.  That will require more explanation later.

For air, the figure is 537 kJ/m3.  I used the standard integral table for air, but the results seem identical with using the ideal gas laws, not very surprising for air.  For steam I used the steam tables which is the recognised best model for steam properties.

If you expand air in an adiabatic engine from 450 kPa to 100 kPa, starting from 30 deg C, the final temperature is -78 C.  A real engine will produce less power, and will have a higher outlet temperature, but if you do real work with your engine on air, you will get a very cold exhaust.  If you are just running unloaded with low pressure, you might have to use thermocouples to detect how much the air is cooling.  I normally only run on steam so I have no experience of running on air other than a simple and quick test on completion of engine assembly to prove I have a runner.  So I have to leave it to others to do some tests to demonstrate the performance of their engines on air.  Perhaps I will make up some fittings and try.

If I assume that the adiabatic efficiency of the engines is the same on air and steam, then we can see that saturated steam has the most "oomph", followed by superheated steam, and not really far behind, air.

Getting to be a long post, so I will pause there, and continue tomorrow.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 09, 2018, 05:07:19 PM
Hi MJM ,thanks for this ..i thought that may have been the case but for different reasons !.....saturated steam has lots of particles of water that act like the shot from a shotgun pushing against the piston ,unlike air that is quite 'soft' ! and also superheated steam that is also absent in :::mass"""!!!Willy ????
PS please correct me if this is an incorrect and silly surmization !!!

Title: Re: Talking Thermodynamics
Post by: MJM460 on April 10, 2018, 01:35:36 PM
Hi Willy, partly right, never silly, but some extra explanations required.  And you already know I am not good at the short answer, so here goes.

Saturated steam is by definition dry, but it is also the boundary boundary between wet steam and superheated steam.  So as soon as you start extracting heat by doing work on the piston, it starts to condense.  But with the piston also moving the pressure is falling, which should limit condensing.  And condensation releases the latent heat, so contributing to maintaining the temperature and pressure.  But condensation reduces the specific volume of the steam so the pressure falls faster than if condensation did not occur.  A bit of a race between falling pressure keeping it dry and loosing heat causing condensation. 

The steam tables tell us the outcome, especially when combined with the second law of thermodynamics.  The second law says that for adiabatic expansion the entropy is constant.  This allows us to calculate the other steam properties using the steam table data, including just how much of the steam condenses.  For a real engine, less heat is converted to work, the entropy increases, and there is less condensation than for an adiabatic engine, but only a test run can tell us exactly how much work the real engine produces.  The quantity of water condensed is quite small, less than 10% for an ideal engine and less again for a real engine! and the velocities in the cylinder are around the same as the piston speed, both for the dry steam remaining and for the droplets.  But it is this condensation which means we cannot use the ideal gas laws for saturated steam, or anything only slightly superheated, we have to use the steam tables as the more accurate model of steam behaviour.  (This is the mistake I made way back in my first attempt at this question.)

When we expand air instead of steam, "soft" is not really a concept that translates to action on the piston.  The piston reacts to pressure.  Pressure over the area of the piston makes a force, and as the piston moves, that force moving through a distance does mechanical work.   Pressure is pressure whether exerted by steam or air.  The density of steam is less than air, but the specific heat of air is less than steam and when you work through all the equations, that ends up meaning you get less work from air.  The higher density of air just reduces the difference.  I did the calculations for air at 27 degrees C, mainly because that is 300 K, and appears directly in the air tables, but also that is realistic for air from the volume tank of a compressor.  Steam of course had to be at the saturation temperature or above so 148 deg C.  I will get back to try the calculations again with 148 for the air temperature, or at least the nearest temperature that appears in the tables to see if that makes much difference, or just makes the exhaust temperature more moderate.  It depends on how much the specific heat varies with temperature in that range, and of course it also varies the density.  No avoiding the equations and specific heat, it is not an intuitive result.

For superheated steam, you are spot on, it is the lower density that means there is less energy per unit volume available for conversion to work.  So that opens the question of why do we bother?  Part of the answer is that our reciprocating engines are volumetric machines.  That means each stroke takes in the same volume.  If we want to use lower density motive fluid, we need to use a larger engine to take in the same mass of the fluid for equal power output.   So if we design an engine for superheated steam instead of saturated steam we would design a larger engine.  So pluses and minus perhaps?  The clincher is that the extra energy to make superheated steam produces more extra work than the extra fuel energy from heat input to the boiler would suggest, so it is more efficient.  Less coal burned to travel a given train journey.  Less coal to cross the Atlantic in the same ship.  If we don't need the extra work output, then the smaller output from the smaller engine uses less coal than just the difference due to the difference in work output would suggest.  So, complex questions to answer, Willy your questions are never silly.

So far I have only discussed a single cylinder engine.  There is a limit to the degree of expansion that is practical.  I suppose with valve gear fully notched up, the steam is cut off very early and there could be a good degree of expansion, with just a short pulse of steam or air admitted each power stroke.  That would mean reduced power output but reduced steam consumption and increased efficiency.  If we have a compound engine, no extra steam is admitted to the lp cylinder, and the steam from the hp cylinder is expanded during the transfer to the lp cylinder.  With a triple we carry this process further with further increase in expansion of the fluid.  I seem to remember people reporting that those very low air temperatures do cause problems when running these engines on air, even though the extreme low temperature predicted by the adiabatic calculation is not reached in a real engine.

We could build a simple twin cylinder engine with equal power output to the triple expansion engine.  It would be much smaller, but would require more fuel.  We trade off engine size and efficiency.

One of the factors reducing the efficiency of a real engine is the heat loss from the cylinder walls.  It is also a departure from the no heat transfer definition of the adiabatic calculation.  You would expect that expanding air, the low temperature would mean there would be a heat gain, which might increase the power output.  The reality is that while the heat gain would be important, the area for heat transfer is very small and there will still be significant cooling due to expansion.  However that heat gain will reduce the cooling somewhat.  Just difficult to separate from other sources of inefficiency which reduce the degree of cooling during expansion.

And just to re-emphasise, the differences between steam, whether saturated or superheated, and air only occur due to the behaviour of the fluids on expansion.  While the admission valve is open, there is continual addition of energy through the incoming fluid, and the engine work output is the same for any fluid at the same inlet pressure.

And in case you are wondering, the reason we don't normally use air to drive a steam engine for producing work is that it takes more work in the compressor to produce the compressed air than the engine produces in expanding the air.  Generally it is easier to store electricity in a battery than air in a pressurised tank to produce power when steam is not practical for some reason.

I hope that all makes sense.

Thanks for following along,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 10, 2018, 08:25:08 PM
Hi MJM, yes that makes sense and may i use your comment about not being silly for my epitaph  ? !!!!including your initials of course !!..I have a new question that i have been saving up though.....would it be possible to make a steam engine more efficient by using the fuel to just heat the cylinder and then inject water into the inlet sides of the valve  thereby dispensing with the boiler completely ? this would sort of be like a diesel cycle !!

Willy
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 11, 2018, 12:02:15 AM
Hi MJM, Also just found a diagram of the Willans engine ! I have not seen this before and also not seen a model of it ...apparantly there is a model in the Science museum, There is an extensive write up in the 1910 Model engineer mag and it looks very complicated and a bit like the Hargreaves type that i posted earlier....So this was an example of the Thermodynamic engineers having a go at getting more power and efficiency from steam
Title: Re: Talking Thermodynamics
Post by: derekwarner on April 11, 2018, 07:21:34 AM
Hullo MJM...and all......I have been following on with each posting.....generally understanding, however sometimes confused  :facepalm:

We see you have chosen 148 degrees C for the point of saturated/dry steam, and from the tables, this equates to 3.5 Bar

This is a point of quandary in that in does not answer for those with steam plants operating at lower pressures

My Saito Y2DR 9cc horizontal twin engine has a recommended WP of 2 Bar, my boiler design is 3 Bar however I intend to trial & run the engine at   somewhat gently & progressively  :hammerbash: higher WP's than recommended

From the tables we see...........

2Bar = 134 degrees C [Saito recommendation]
3Bar = 143 degrees C [my boiler relief setting]
3.5 Bar = 148 degrees C [as used for your calculations]

So again whilst I understand much of the last postings......I cannot necessarily apply these questions/points/& answers to my plant

Derek

Title: Re: Talking Thermodynamics
Post by: MJM460 on April 11, 2018, 12:22:21 PM
Hi Willy, no problems with your epitaph, though I hope and expect it will be a long time before you need it.  Your questions are quite insightful and very clearly identify, in non-technical terms, the point that is puzzling you.  So much better than meaningless "techno-babble", even when they are a bit colourful.  As a result you have made a great contribution to this thread, and in answering your questions, I have often clarified things to myself that were really a bit fuzzy in my mind.  It has been and continues to be great fun.

I reckon I have seen that Willans engine before, though I can't remember where.  The concentric cylinders, with pistons not on the same piston rod, make for an interesting crank and cross head arrangement.  I assume the design was about improving efficiency, but I would need more information on the arrangement to understand how it worked.

Hi Derek, great to hear from you again.  I chose 450 kPa (absolute) as a pressure very close to 50 psig which seemed reasonable for an engine working hard, in the absence of other information.  The actual figure was not important as the purpose was to explore the difference between steam and air at the same pressure.

For your Saito engine at 2 bar gauge, say 300 kPa absolute, and atmospheric exhaust, say 100 kPa, the analysis goes like this.

First we look up the steam tables and pick out the enthalpy of dry saturated steam (2725.3 kJ/kg) and the entropy (6.9919 kJ/kg.K) at 300 kPa.  The corresponding temperature is 133.55 C.

Then we look up the properties of steam at the exhaust pressure.  I have assumed 100 kPa, but the tables also include a line for 101.3 if you prefer it.  We pick out the enthalpy if saturated liquid, hf, and of dry saturated steam hg and, because I am using a calculator, and basically lazy, the difference between the two, which is listed as hfg.  So hf = 417.46, hg = 2675.5 and hfg = 2258.0.

Similarly we look up the corresponding figures for entropy.  We find sf= 1.3026, sg= 7.3594 and sfg = 6.0568.  I suspect you are comfortable with the steam tables, but the detail might help some other readers follow the process.

We now calculate the exhaust conditions for an ideal adiabatic engine with those steam conditions.  The second law of thermodynamics says for an adiabatic process there is no change in entropy.  This means the entropy of the exhaust steam would be 6.9919, the same as the engine supply steam, so now we have two independent properties of the exhaust steam (pressure and entropy) and so can calculate all the others.  That entropy lies between the value for saturated liquid and dry vapour at the exhaust pressure of 100 kPa, so we know it will be wet steam.

We calculate the dryness using the entropy,

Dryness = (sexh - sf) / (sg - sf) = (6.9919 - 1.3026) / 6.0568 = 0.9393

Using the dryness, we calculate the exhaust enthalpy as

 hf + 0.9393 x hfg = 0.417.46 + 0.9393 x 2258 = 2538.4

And the change in enthalpy, or work produced per kg of steam is 2725.3 - 2538.4 = 186.9 kJ/kg

Now all that is for an ideal adiabatic engine.  A real engine will produce an enthalpy change of about 70% (the adiabatic efficiency).  I can't prove this figure from theory, it has to be determined from a test run with careful power output measurement.  Without other information it seems like a reasonable estimate, a model may have lower adiabatic efficiency but I have only rarely seen even a degree or two of superheat in the exhaust of my engines so dry exhaust is a reasonable limit condition.

So 70% adiabatic efficiency means our real engine produces an enthalpy difference of 0.7 x 186.9 = 130.8 kJ/kg.

You can see, the potential work output is only a tiny proportion of the heat used to just evaporate the steam, let alone heat up the water from cold.  So the steam cycle is not high efficiency.  It is limited by the low pressure, and the fact that most of the heat used to evaporate the steam goes out in the exhaust and gets lost to the atmosphere or rejected in the condenser of a full scale plant.  And you can see why engine manufacturers might prefer to talk about adiabatic efficiency.

We can subtract that enthalpy change from the supply steam enthalpy, and find our real engine exhaust enthalpy is more likely to be 2725.3 - 130.8 = 2594.5 which we can see still lies between the enthalpy values for saturated liquid and dry vapour so the exhaust is still wet, and further illustrates that most of the heat from the boiler is rejected in the exhaust. 

We no longer know the entropy, which will have increased, but we can use the enthalpy to calculate the real engine exhaust dryness.

So dryness = (2594.5 - 417.46) / 2258 = 0.964, somewhat dryer than the adiabatic engine exhaust.

This means that just 3.6% of your steam supply appears as condensate in your exhaust.  I doubt if that is enough to explain your condensate issues.

Realistically, there is not enough heat transfer area around the cylinders to produce much more condensate by heat loss once the cylinders have heated up.  And likely even some of the 3.6% will go up your exhaust stack as that vapour mist.

That leaves condensate due to the initial heating of the cylinders, which should soon cease as the cylinders warm up, or carryover due to priming when the boiler is initially at maximum level.

It might be worth trying a run with less than half the normal fill of water, (making sure that it is enough to cover all the heating surfaces) to eliminate priming, or even try firing the boiler with the steam pipe discharging to atmosphere instead of to the engine, so explore the possibility of priming.  Obviously keep an eye on water level and don't run the boiler dry.

Then, if you run the plant out on the bench, perhaps without the separator, just an exhaust line run so you don't get scolded, or make too much mess, and see if the condensate stops after a short time or not.  If it is condensate due to warm up, you will need to instal cylinder drain valves, perhaps like the ones described recently in the Conway thread, or a simple screw down type, or even a plug type if you like a challenge.

I hope that adequately answers your question.  I have raced ahead a bit, based on some of your previous questions, so I hope I have made a reasonable guess at what is behind your question.  Apologies if that issue has been solved long ago.  Would you like me to continue and show the difference if you had a superheater which reached say 150 deg C, which is probably around the limit of what you could achieve in your boiler without going overboard, but importantly, directly in my superheat tables so easy to use.

The procedure for calculating the work output when air is used as the motive fluid is a little different, as the ideal air tables have a very different form from the steam tables, so I will not confuse things by returning straight into the air calculation.  However I will return tomorrow with a calculation based on air at the same temperature as well as the same pressure as the steam in yesterday's example, to see if that explains the difference.  But it is a bit of a theoretical calculation, as we would not normally have access to high temperature air to run our engines.

Thanks for following along,

MJM460

Title: Re: Talking Thermodynamics
Post by: derekwarner on April 12, 2018, 06:35:40 AM
Goodness MJM.....thankyou and that is quite a number of paragraphs and sentences for me to read, re-read and adsorb 

One of the important points noted in non-technical terms is that 'relatively lower' pressures are clearly far less efficient than 'marginally higher' pressures in producing work....[and totally discounting using air as a pressure medium fluid]

Digressing a little, I suspect in 1956, Mr Saito [Senior :old:] in Japan was very conservative in his design calculations data for his steam engines

Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 12, 2018, 01:00:03 PM
Hi Derek,  you are most welcome.  I really only needed your operating pressure to get started.  To give you a chance to take that all in, I will avoid adding too much more tonight.

You are quite right about low pressures being less effective than just a little higher pressures.  With the same calculation procedure as yesterday, we can actually put figures on that.  Now I like round figures to work with, and as a survivor from slide rule days you probably feel much the same.  So I will do the calculation for 400 kPa, (or 300 kPag), just 100 kPa above your rated 200 kPag operating pressure.  As this is equal to your boiler safety valve set pressure, you would not want to operate at this pressure, but would need to keep enough of a margin to ensure the valve was not weeping.  However, the figures do clearly illustrate the point.  I will just summarise rather than go through the complete detail again, it is exactly the same procedure.

We need just two extra figures, hg for 400 kPa (2738.6 kJ/kg), and sg (6.8959 kJ/kg.K).  The saturation temperature will be 143.6 C.  It is worth going back to yesterday's post to compare those values with the figures for 300 kPa.

Now, imagine heating the boiler from cold up to 133.55 C.  The enthalpy of the water is the saturated fluid enthalpy of 561.5 kJ/kg.  Then, instead of evaporating that water to make dry steam at 300 kPa, keep the regulator closed and increase the pressure to 400 kPa before opening the regulator.  The enthalpy of the dry saturated 400 kPa steam is 2738.6 kJ/kg, just 13.3 kJ/kg more than it took to make dry saturated steam at the lower pressure from that same starting point.  But at that condition, the entropy is  actually lower than it was at at 300.

Now, we follow the same procedure to calculate the work produced by an adiabatic engine,  the second law says the entropy for an adiabatic engine is same at inlet and outlet.  From that we calculate the dryness as 0.9235, a little lower than yesterday.  We use that to calculate the enthalpy of the exhaust and subtract this from the enthalpy of the dry saturated inlet steam to get a work output for that adiabatic engine of 235.9 kJ/kg.  Now it is worth looking back to the figure for yesterday.  For that input of just 13.3 kJ/kg, we get an extra 49.0 kJ/kg as work output.  Now that is efficiency!  Even when we look at our real engine, again assuming the same 70% adiabatic efficiency, we get an output of 165.1, compared with 130.8 at 300 kPa.  The exhaust dryness is 0.955, a little more wet than yesterday, that is where the energy for the extra work came from, but still mostly a misty vapour.

As always when I do these calculations, I do check them carefully, however I am always a bit nervous sticking my neck out and posting the results.  But we will get nowhere without some courage.  I have given you all the information necessary to check the result, an exercise which will really help you understand the process.  Even better if you use your own steam tables.  Please let me know the result.

Unfortunately we can't get that sort of result as a total efficiency, but obviously the extra work output at the slightly higher pressure improves the overall result, as you have mentioned.

A short but perhaps heavy post tonight, I will let that sink in before I have a look at the effect of superheating tomorrow.

Oh, by the way, regardless of the conservative design, I don't recommend increasing the safety valve set pressure.  It will be a real pain getting a new design pressure through the boiler inspectors at this stage.  I have had to do this on full size vessels, and the first time it convinced me that for all future designs I would base the design pressure on the actual plate thickness, rather than the minimum required design pressure, so that further raising the design pressure later was not a possibility.

It is also worth remembering that the engine in a ship can only put out the power the propellor can  absorb.  If you increase the steam pressure the propellor will of course turn faster, and produce more thrust, and the power (if the propellor does not cavitate) will mostly go into an unrealistic bow wave.  So it is not necessarily practical to take advantage of the extra output at the higher pressure.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 13, 2018, 01:02:40 PM
Those who have followed yesterday's post may be asking themselves, " If it is so much more  efficient to use steam at 400 kPa than 300 kPa, then, even when we can't use the extra power, should we still raise steam at 400 then throttle back to 300 kPa?"

Once again, we can work through the numbers and see what the thermodynamics says.  The procedure is much the same as the last two problems, the key is simply how the first law of thermodynamics applies this time.  Throttling occurs in a very small space, so there is effectively no heat transfer in or out, and in the throttle, (think a simple orifice plate, or perhaps a throttle valve partly open,) no work is done.  For this case, the first law reduces to no change of enthalpy.  This gives us the necessary two independent properties for the throttle outlet (pressure and enthalpy), from which all the other properties can be calculated.  We can then use these properties as the inlet steam to our adiabatic engine operating at 300 kPa.

The calculations are very similar to the preceding ones, so I won't bore you all by setting them all out again unless someone would like to see it.  The result of throttling dry saturated 400 kPa  steam to 300 kPa is that steam enters the engine slightly superheated, to 140 deg C, so we have to use the superheat table, to find that the adiabatic engine produces just 1.3 kJ/kg more work from that extra 13.3 kJ/kg required to produce the higher pressure steam.  The real engine, still assuming 70% adiabatic efficiency, produces 131.7 kJ/kg or 0.9 kJ/kg more from that higher temperature steam.  The real engine exhaust is still wet steam, but just slightly drier than from the dry saturated 300 kPa steam.
We can see that from the efficiency point of view, there is minimal advantage in generating steam at a higher pressure, then throttling it.  The total heat to evaporate the steam is about 2200 kJ/kg to produce 131 kJ/kg of work, so 0.8 from 13 is slightly better than average, but not nearly as good as just using the higher pressure.  In fact, if the steam is generated at higher temperature, the flue gases will also be discharged at a higher temperature, so increasing the losses, both as heat in the flue gas and as heat loss from the boiler.  In reality, these theoretical differences are almost insignificant compared with the heat input to the steam or even the work produced.

On the other hand, when steam is raised at a higher pressure, the specific volume of the steam is less, (0.46 at 400 kPa, compared with 0.61 cubic meters per kg at 300 kPa) which makes it easier to separate from the liquid, so there may be less carryover and perhaps less tendency for priming, especially when the boiler is at maximum level.  This is a practical issue which may well outweigh any theoretical efficiency considerations.  But, like the efficiency of the engine, it is not really quantifiable by calculations.  However, if you are experiencing difficulty with boiler carryover, increasing the boiler operating pressure and then throttling to the operating pressure you need, may be helpful.

I hope that little diversion has given an idea of the things that can be better understood with a few calculations.

You might have noticed that I am wavering a bit on the pressure terminology.  The preferred SI terminology is to use kPa for absolute pressure, and only qualify it when gauge pressure is meant.  However this is a world wide forum with many people much more familiar with imperial units and psig for pressure, but often just psi, and generally only qualifying it if absolute is intended.  I have tried to stay with kPa as absolute pressure, but to make the mental conversion easy, I have generally tried to add in brackets the gauge pressure in kPa and psi, with the appropriate qualification.  Generally if there is any doubt, absolute pressure is used in all my calculations and steam table references.

Similarly, there is no practical difference between the standard atmospheric pressure of 101.3 kPa and the easier to remember 100 kPa.  As both are listed in the tables, they are equally easy to use until you need to consider superheat, when generally only the 100 kPa table is provided.  As 100 is within the range of atmospheric pressure which occurs due to the normal progression of weather patterns around the planet, occurs directly in the superheated steam tables and is also a nice round figure, it is the value I prefer to use as atmospheric pressure.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 14, 2018, 01:42:07 PM
Now for the last step in the calculations for Derek's engine, the effect of superheating the 300 kPa steam to 150 deg C.  Remember 300 kPa in absolute pressure is 200 kPa on the gauge, about 30 psig, the recommended pressure for his engine.

Now that is not much superheat, but it is realistic for a small boiler, based on the temperatures I have seen from my minimal superheater coil on my Meths fired boilers.  It is not a lot more than obtained from throttling 400 kPa steam per yesterday's calculation.  And, importantly, it appears directly in the superheated steam tables, so requires no interpolation to get the required data.

The calculation is the same as previously so I will just give the results and talk about what they mean.

The enthalpy of the superheated steam is 2761 kJ/kg, which is 35.7 more than saturated steam at that pressure.  The work from an ideal adiabatic engine is calculated as before, based on the second law for an adiabatic engine, so no entropy change.  The exhaust steam would be wet steam despite the superheat at the engine inlet, and would have a dryness fraction of 0.953.  The adiabatic work out would be 190.5 kJ/kg, only 3.6 more than for output from saturated steam.  Certainly not much reward for the effort to add a superheater to the basic boiler.  It is even slightly worse efficiency than the efficiency based on the heat necessary to raise saturated steam, and only 2.3 kJ/kg more than was obtained by throttling 400 kPa saturated steam without the superheater.  So no incentive to add a superheater, at least to this temperature level, if you can achieve a higher temperature the result may be different.

You will remember that when I did the calculations earlier for a somewhat higher pressure, it all worked out as a positive benefit, even if not large.  Really I expected the same again, so I was quite surprised at this result.  Despite careful checking, I have not found any error, but if you decide to try it for yourself, please let me know if you find any errors.  There is nothing like actually trying the calculations to help understanding.  So how does it come about?

If you look back at the pictorial representation of the steam tables (I have attached them again so you don't have to search), and follow the vertical lines of constant entropy on the T-S diagram and constant enthalpy on the P-h diagram.  In the wet region, temperature and pressure are not independent, so you can read higher temperature as higher pressure, and vice versa.

You will see in the T-S diagram, that expanding in an adiabatic engine at constant entropy, the vertical line, always gives an increase in entropy and wet steam at exhaust.  However, when you look at the P- h diagram, the enthalpy of the wet steam increases at higher pressure to a maximum at about 3000 kPa, then starts decreasing with further pressure above that. 

Below 3000 kPa, higher pressure steam always has higher enthalpy, so if you reduce the pressure by throttling, a constant enthalpy process, it becomes slightly superheated.  And of course, all our modelling applications are below this level.  Industrial steam plant of any size, generally works well above this (it's about 420 psig of you are thinking in imperial units). Unless the steam is used mainly for heating and the higher temperatures are not wanted.

Also, remember back to the earlier efficiency discussion, and the Carnot efficiency limit, determined by the ratio of the maximum and minimum absolute temperatures in the cycle.  When we are operating at only 2 bar, exhausting to 100 kPa (133.5 degrees to 99.6) gives us a very low efficiency limit, and efficiency increases more rapidly with increase in temperature in this area.

Unfortunately the graphs I have posted use MPa for pressure, 1 MPa is 1000 kPa.  And the logarithmic scale makes the top of the wet steam area appear closer than it really is.  Similarly the T-s diagram uses absolute temperature, so you need to add 273 to your temperatures in C to find your temperature ion the diagram.

Clearly it is worth doing the calculations for your actual operating conditions before assuming too much from conclusions based on significantly different conditions.  And with 2 bar gauge operating pressure, not unreasonable for a small model, it is clearly not worth adding a superheater.  But by  400 kPa, the benefits are starting to appear, even if still very small in practical terms.

If you want to chase the higher efficiency associated with higher pressure, a good direction to consider would be to make a smaller engine for the same load, so it needed the extra pressure to provide adequate power.  You would calculate a piston diameter which would experience the same force at 400 kPa as the larger one at 300 kPa.

Well, Derek, I hope that has adequately answered your question, and I hope it has been of interest to everyone else.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 15, 2018, 01:31:31 PM
Thinking about the discussion on air powering our models, I had a look in my box of excess tube fittings, spare plugs and so on, and sure enough found an adaptor that allowed me connect the air compressor hose to the engine.  Basically it fitted the boiler instead of my normal thermowell fitting.

I set the compressor pressure control to 30 psig (it is a machine distributed by a major US compressor manufacturer, though almost certainly made in the Far East), and connected the tyre connector to the fitting on the boiler.  When I started the air flow, the engine ran quite nicely, starting at about 900 rpm and soon accelerating to 1220 rpm. 

My engine is fitted with temperature thermowells on the inlet and exhaust, so I monitored the temperatures.  Ambient temperature was 15.6 C, unfortunately the days of 20+ seem to have departed, at least for the moment, if not for the season.

The temperature monitor on the engine inlet was showing 15.9 C.  Side by side the readings of the ambient temperature and the engine inlet thermocouples were the same, but even putting it into the thermowell well before the run, seems to introduce a small variation.  Something to think about on another occasion.  During the engine running, this slowly reduced to 15.4.  Again this is not easy to explain so also back to that later.  Ambient temperature reading did not change.

The exhaust temperature reading started at 18.  That meter does not have 0.1 degree resolution and always seems to read a little higher than the others.  During the runs it reduced to 13 degrees.  Not quickly, probably due to the heat absorbed in cooling the cylinder which is a large lump of bronze.  But eventually a 5 degree reduction from the starting point.  I was a bit worried about cylinder lubrication, as the displacement lubricator does not work with air, so I did not run very long.  The temperature may have dropped a bit lower had I run for longer, but it was moving quite slowly, so perhaps near the minimum.

The pressure on the tyre gauge was about 150 kPag, a bit higher than the steam pressure I usually see based on temperature measurements, but I think we would all agree that apart from the probably inaccurate gauge, the pressure on the gauge before the connector to the tire is not an accurate measurement, pressure at the engine would be much lower.  As the engine speed was about the same to a little lower than when running on steam, I suggest the pressure was around the normal 50 to 75 kPag.

So where was the -78 I talked about earlier?

First, -78 was the adiabatic engine exhaust temperature.  For a real engine with 70% adiabatic efficiency, -55 would be a better estimate, but still way lower than the observed temperature.

Next the pressure ratio.  I assumed 450 kPa, or 350 kPa gauge, exhausting to 100 kPa for the earlier exercise on the basis that there is no need to compare air and steam unless the engine is required to do some real work.

So my little mill engine, running unloaded had, let's say 70 kPa gauge max at the inlet, so 170 kPa  exhausting to 100 kPa.  With a smaller pressure ratio, the exhaust temperature will be much more moderate.  Remember also that the adiabatic engine takes all the steam at inlet pressure and expands it all adiabaticaly to the exhaust pressure.

Rather than calculate a new pressure drop, I took the valve chest cover off and had a close look at the valve timing.  It was my first attempt at a slide valve, and it soon became apparent that I had not provided enough lap.  The outside end of the cylinder opened quite well on top dead centre, and closed just before bottom dead centre, I estimate, with a protractor beside the crank, perhaps 175 degrees.  Now with the sine form of the piston movement, the remaining volume change is minimal.  Almost no expansion at all.  On the inner end, or crankshaft end, it was slightly better, the cut off was about 170 degrees.  Slightly better, but not much better.  So my little mill engine is basically taking in full pressure fluid, whether air or steam, throughout most of the stroke.  This is not very close to the ideal adiabatic expansion.  When the exhaust valve opens, the remaining pressure is basically throttled to exhaust pressure, with no further work production, and perhaps a small temperature reduction.  Surprising to see 5 degree temperature drop, from such a tiny expansion ratio.

I carefully measured the valve and the steam chest, and I think I could make a new valve with a bit more lap.  Then I could reset the eccentric to give the necessary lead, and I should get some more expansion.  Now Chris would have the new valve made and installed by now, but I live in that alternative universe where things move more slowly.  It's on the list, but for now, lets assume I achieved about 40% cutoff, then allowing for the clearance volume in the cylinder and steam passages, say one to two expansion, still less than for that fully loaded up engine. 

I suppose it is possible to set the cut off very early to get a much greater expansion, but the steam inlet flow, and hence the power developed would be much reduced with such a setting.  However, a compound engine could be expected to achieve greater expansion, and hence lower exhaust temperature, with enough steam flow to provide reasonable output.  Alternatively, with Stevenson's valve gear, or Willy's Allen gear, I assume notched back, would give earlier cutoff and more expansion, and would see lower exhaust temperatures.

Clearly life is more complex than being a matter of doing some theoretical calculations and demonstrating the results on a model engine.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 15, 2018, 04:38:22 PM
Hi MJM , interesting info on using compressed air for your engine... is the temperature drop partly because of the draft occurring  and with steam is there a similar cooling effect with this "draft" as in standing next to a slightly open door. ? And rather than making a new valve could you just make the buckle  a rather loose fit in the valve slot to get a similar valve action  as with the lap ? ie if there is a gap it will make the valve move a bit later on the stroke ??!!! also a 1902 engineers pocket book with various tables for different steam engine builders....

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 16, 2018, 02:11:27 PM
Hi Willy, I almost didn't do the run on air, but when I found the necessary fittings sitting with my spares, I though I might as well.  As always, actually doing the experiment, especially with an idea of what you are looking for, usually seems to reveal unexpected detail.  So seeing the relatively small temperature drop, I had a new look at the valve, and looked again at K N Harris' book with new understanding of lap and lead.  Now I need to look at Martin Evans on valve gear as I am sure he mentions how the lead changes, or doesn't, with various valve gears.

I didn't think to take photos yesterday, and it was too late when I was typing the post, so they are attached today.  Yesterday I was running the mill engine.  My own design, first attempt at a slide valve, driven by a simple eccentric.  I was a bit vague about lap and lead at the time, I have thought a lot more about them since.  I was just delighted to achieve a working engine.

Today I had a closer look at the fittings I had, and found I could also connect my air supply to the diagonal engine with a lever operated variation of Joy valve gear, again my own design, with a very similar temperature result, in both forward and reverse.  And even as the gear notched back, a bit of a surprise.  But back to that later.

By draft, I assume you mean around the boiler.  It almost certainly is important, but the boiler is on the inlet side of the engine, so has no effect on the temperature difference across the engine.  The temperature difference across the engine is due to converting energy in the gas into work.  The enthalpy of air is directly proportional to its temperature.  If you extract energy, whether by heat transfer or by converting it to work, the temperature will fall.  The amount of fall when work is done, is dependant on the expansion ratio.  Once the inlet valve closes, further movement of the piston results in expansion and a significant temperature drop.  The late closure of my valve means there is only a very small expansion in this phase of the cycle. 

But what about before the valve closes?  Before the valve closes, work is done as the pressure force acts on the moving piston, but as more air continues to be admitted so the pressure does not appear to drop.  And as air is consumed, more energy is put in by the compressor.  So the heat converted to work is continually replaced by the compressor motor, or when steam is the motive fluid, it's replaced by the heat from combustion of fuel.

When we look at the compressor and the compression process, also an adiabatic process, but the pressure rise means the temperature rises, as you know if you ever put your hand near the compressor head while it is working.   A large industrial machine will have a heat exchanger to cool the air.  In our small shop compressors apart from fins on the compressor, the main cooling surface is the air tank if you have one.  My small compressor as you can see has no tank.  Pressure control is achieved by an adjustable relief valve which is adjusted to vent air at the required pressure.  I then have about 5 metres of air hose.  And of course my little boiler and its superheater  will provide some cooling through the draft that you mention.  But none of that explains what I observed during my test runs.

Ambient temperature in the shop was very steady at 17.9 C.  When I put the thermocouple I was using for the engine inlet, next to the room air device it agreed within 0.2 C.  When I then put it in the thermowell at the engine inlet, it dropped to 15.9 C.  This was before I started the compressor, so it had all had plenty of time to settle at ambient temperature.  Much more difference than the 0.2 degrees when the thermocouples were side by side.  Not sure I totally understand that.  May be something about differences in absorptivity and emissivity to radiant heat that slightly alters the heat balance.  On conduction alone there should be no difference from the air temperature.

When I turned on the compressor, the mystery deepened.  As the engine ran, temperature at the engine inlet dropped.  It seemed to settle out around 14.4, but could have been still dropping slowly.  The head of the compressor was hot, as was the hose connection at the compressor end.  There was definitely heat lost along the hose as the connector on the gauge end was only slightly warm.  The boiler shell and superheater should have been helping the air reach atmospheric temperature, especially with some draft through the furnace, but 14.4 was distinctly cool, when I would have expected it to be still warm from the compression process.  Could have been some heat loss along the hose, and usually some pressure drop through those cheap tyre inflation gauges.  But I have not been able to come up with a convincing explanation for actual cooling.  Even put the thermocouple back beside the room temperature thermocouple and the reading slowly increased back to very close the the ambient temperature.  Very mysterious.  Any suggestions, anyone?

The exhaust temperature decreased, only 1 degree below the engine inlet temperature but definitely cooling in the engine.  Again I removed the valve chest cover, and watched carefully as I turned over the engine.  Again, my radial gear seemed to be cutting off very late in the stroke.  Seemed close enough to 180 degrees admission.  Obviously, I can't see the exhaust cavity of the valve so have no information on that apart from measurements of the valve.  Clearly not enough expansion for any real temperature drop.  As air enters the cylinder, it is replaced by more from the compressor via hose.  I thought there might have been earlier cutoff with the valve travel,notched down, but I was not able to demonstrate this.  I suspect a new valve with more lap may be required but that may require remaking one of the vibrating levers as well to adjust the lead.  I have been thinking about your suggestion of slack in the buckle.  It is getting too late now so I will reply on that  tomorrow.

The Joy valve gear is a radial gear and not easy for me to see how to adjust the valve lead, it is built in to some of the link dimensions, so these links would have to be remade after the lap was carefully measured.  Again, I was just delighted to have a running, reversible engine. 

And one more thing, in case you are wondering why we don't see many full scale application for air motors, that second law of thermodynamics prevents any motor from producing enough power to run the required compressor, you get much less power from the engine the you have to supply to drive the compressor.  Though air has a place in pneumatic tools where the temperature of steam would not be helpful, and in potentially flammable atmospheres where the possibility of sparks must be avoided.

So some interesting results from short runs on air.  I was a bit worried about cylinder lubrication, so did not run for long.  I am not sure what others do, but I suspect I would need to try some sort of pressure lubrication if I am to do much running on air, along with oil suitable for the low temperature.

Photos not posting, may be too big, I will try again then if no success will resize them tomorrow.

I hope you found that interesting, thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 16, 2018, 02:15:23 PM
I tried the photos, one at a time and no success.  Will resize tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 16, 2018, 02:50:38 PM
Hi MJM, Interesting questions to answer...... If you are in a room with everything in the room at ambient temperature....if you touch something metal it feels cold ....if  you touch some thing soft and cuddly it feels warm,   so ......at what actual  temp does the metal have to be to not feel cold , is it somewhere between ambient and body temp ??  when i was talking about 'draft' i was thinking about the rush of air causing its own "draft" inside the pipe ! Does the thermocouple act in the same way as you finger ,feeling the metal as being 'cold' ??

willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 17, 2018, 11:42:51 AM
Hi Willy,

As a furnace draft can be either natural draft, occurring due to the density difference caused by heating of the fluid, or forced draft, driven by a fan, I guess that it is not unreasonable (though not usual) to talk about the draft inside the pipes.  The flow of air (or steam) in the pipe is driven by the compressor or the boiler, and the velocity means that heat transfer is considered forced convection rather than natural convection.  This tends to increase the local temperature gradient near the tube wall, so increases the rate of heat transfer.  As the air compressor discharge is quite hot, in this example the heat transfer is cooling the air.  It would cool quicker in a metal pipe instead of my flexible hose which I assume is mostly rubber, with or without some steel reinforcing mesh.  And certainly, by the time the air reached the connection at the user end of the hose, it was only slightly warm relative to the ambient air temperature.  But further cooling to 14.9 degrees at the engine inlet was a bit of a surprise.

Your comparison of the temperature readings with touching hot or cold surfaces is worth thinking about.  When we touch a metal surface which is say 20 degrees, there is heat transfer, and hence a temperature gradient from our blood supply (about 35 or a touch lower in our fingers) through the flesh and skin to the metal.  Our nerve endings are under our skin, so at an intermediate point on the temperature gradient.  Assuming a large block of metal rather than a thin foil, the heat transfer continues.  The metal,with high conductivity and high specific heat (which determines how much heat is stored in the metal,) the heat transfer continues, but the contact temperature where our skin touches the metal stays close to the bulk metal temperature, the heat transfer continues and the nerve endings can detect the lower temperature.

When we touch something "soft and cuddly", the material is has much lower conductivity than the metal, though similar specific heat.  The lower conductivity means the temperature gradient continues into the bulk material and the temperature in the region of our nerve endings is a little higher so the material feels warmer.  It actually is warmer very close to where we are touching it, heated by transfer from our blood supply.

So the difference in feel is due to the difference in conductivity of the materials and the heat transfer from our blood supply to the object.

So is the situation similar for a thermocouple?  Basically the thermocouple is a welded junction between two different metals that produce a voltage difference in the contact area, and the voltage varies quite predictably with temperature.  You can try it with your digital voltmeter, which will give you something in the range of a few millivolts.

This voltage is measured by a very high impedance circuit, which draws practically no current, perhaps micro amps or less, so the heat generated, volts times amps, is really negligible.

Similarly the mass of a thermocouple is tiny, it is only that little blob where the metals are welded together, mine measure about 1.5 mm in diameter.  If a thermocouple is placed on a flat surface, it sits in that area between the surface and the air, and there is a temperature gradient which affects the thermocouple reading.  However, if the thermocouple is deep in a small diameter hole, or covered with a pad of insulating material, it very quickly arrives at a temperature very close to the material temperature, when there is no further heat transfer.  It takes about 3 - 5 seconds to get within about 0.1 degrees and this time is the reason temperature readings are always quite slow.  However, after this initial period, the the only heat flow to or from the thermocouple is that which occurs along the very thin insulated wires, which are influenced by ambient temperature along most of their length.  I am sure a figure can be calculated, much like the stem correction calculated for an accurate glass thermometer.  But it is small enough that it is usually ignored.

It is hard to believe that my observed temperature difference between the reading with the thermocouple close to the one measuring ambient, when both were about the same, and the temperature reading when I inserted it into the thermowell in the engine inlet piping, which had been sitting with no obvious heating or cooling for most of the day.  Late enough in the day that the night time temperature had gone, and inside my brick garage the temperature reading had been steady for a couple of hours.  No flow, no heat source, long soak time yet nearly two degrees difference in the temperature reading. 

So grasping at straws perhaps, but based on the fact that everything continually looses heat by radiation and at the same time receives heat from everything around it, I am wondering if a difference in emissivity and absorptivity, as on real surfaces, both of these vary with wavelength and direction, and could vary between the brick walls, foil insulated roof, and the brass thermowell and copper piping, which are not polished, but still reasonably shiny.  The third factor in radiation is the reflectivity of a surface, which also varies between materials.  All radiation falling on a solid surface is either absorbed or reflected,  so the difference between brick walls and the metal pipe fittings is a possible explanation. 

There is a law of radiation transfer, Kirchoffs law, which says that when a surface emits the same amount of heat as it absorbs, the emissivity and absorptivity are equal.  The obvious corollary of this is that if they are not equal, the heat emitted is different from the heat absorbed, and the temperature will change until the two are equal.

On the other hand, my textbook has no examples which show an object in a room settling at a different temperature than the ambient temperature in the room, so I am not sure if it is considered obvious, or if I am on the wrong track.  So far, I can't think of an alternative explanation.

Getting back to that valve, let's think about the purpose of lap, being the amount by which the valve dimension exceeds that necessary to just cover the steam ports at its central position.  With no lap, the valve just covers the ports and one or the other is open for the full 180 degrees.  With some lap, the valve has to move off its central position by the amount of lap I order to start opening a steam port.  And it means that the port will be open for less than 180 degrees.  If we then rotate the eccentric, so the port for one direction just starts opening at the dead centre, (lead), then there will be admission starting at the dead centre with shut off occurring before 180 degrees, and expansion for the rest of the stroke.  So expansion requires the port to close before it would with no lap.

If we machine the buckle with extra length,so there is a gap. The crank would be rotated to the dead centre, the valve adjusted to be just about to open.  As the crank rotates the port is opened, to end of the valve travel, when the buckle starts moving back.  However, due to the gap, the valve movement does not start until the buckle has travelled the amount of the gap, so later than with a close fitting gap.  However, early cut off requires the valve to move earlier.  So I believe that providing gap in the buckle, (or a wide slot for the nut), is not an alternative to providing lap.  Does that make sense?  Or have I missed something?

That feels like enough to think about for one day,

Oh, by the way, I believe I have now resized those photos I think, so let's see if I have more success today.

Thanks to all for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 17, 2018, 11:46:05 AM
My Variation of the Joy valve gear with swing links instead of curved slides (in the third photo) is quite difficult to clearly photograph, bit this one may help a little. 

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 18, 2018, 01:43:27 PM
Thinking back to yesterday's dilemma, two unexplained temperature differences, the engine inlet piping compared with atmospheric temperature, then the cooling of the air from the compressor when it reaches the engine inlet. 

I can't add anything to the discussion on radiation heat transfer as a possible explanation of the first, I am by no means sure, but let's think a bit more about that cooling of the inlet air, before it even reached the engine.

First another observation.  Instead of touching various solids, what do we feel if we blow on our finger?  The air definitely feels cool.  So is this due to forced convection cooling of our finger by the air stream?

First try two more experiments.  First with lips pressed together to get a fast flowing air stream, blow on your finger, it feels cool.  Then, with mouth more open, breathe out so the air flow is over your finger.  This time it feels warm.  Not just less cool, but it actually feels warm.  So, is your breath cool or warm?

I suggest the air you breathe out has first filled your lungs, and been in close contact with your blood stream, so I would expect it to be close to blood temperature.  The warm breath is not unexpected.  But if the breath is warm inside the body, why does it feel cool when we purse our lips?

Now remembering that heat only travels from high temperature to low temperature, if the higher velocity air stream feels cooler, it is most likely absorbing heat from your finger, so surely it is actually cooler than your finger.

If you have a thermocouple, try blowing on the thermocouple, first with lips pursed, then with mouth  relaxed open.  The effect of age must be catching up on me, I am sure I used to be able to sustain a breath for much longer than now.  The temperature change, both up and down, seemed too quick to glimpse sensible readings.  My meter has a maximum function so I pressed the "Max" button and the word appeared on the screen, and blew again.  As you would expect, the reading went up and held at the maximum.  Several breaths later, I decided I had found a reliable maximum.  So I reset the meter and tried again.  First noted the room temperature, 20 C.  Sunny day outside.

With my mouth open, the reading reached 34 degrees, seems reasonable for expired air.

When I pursed my lips for the high velocity air stream, the best I could reach was 25 C.

I kept alternating the velocity with quite consistent results.  This time, with the thermocouple, the only heat transfer is the minimal heat required to lift the tiny thermocouple to near the air steam temperature.

Note that the high velocity air, which felt cooler on our finger, registered warmer than room temperature on the thermocouple, but still nearly 9 degrees lower than the reading of the warm breath.

The difference in breath temperatures seems real on the thermocouple, and suggests that while feeling cool on the finger the breath was still 5 degrees above air temperature, so I would expect it to feel warm, but it felt cool.  The high velocity seems to be doing more cooling than you would expect from just considering convection heat transfer.

Apart from heat transfer, the temperature falls if work is done.  But is there any work done when we purse our lips and blow?  If so, where?  Certainly your lungs build pressure, and then contract, so there is pressure causing force, and movement through a distance, but that side is compression and should warm the air, even above the warming by heat transfer in the lungs.

It brought back a memory of a science programme I heard on the radio some time ago.  The speaker explained that the air you breathe out is not entering a vacuum, it has to displace the air already filling the space and push it away.  This is consistent with a statement in the thermodynamics text book that it is not always easy to see if work is being done, or which is the boundary which moves to produce the work.  I was sceptical at the time, but this little experiment seems to support the theory.

The relevance to this thread, is that it returns us to the concept of enthalpy.  You will notice that I always use the enthalpy columns of the steam tables, and don't refer to the internal energy columns.  The definition of enthalpy is internal energy plus pressure times the specific volume.  The pressure times volume term is often called flow work.  It accounts for the work at the system boundary of a system with continuous flow.

When you breathe out with your mouth relaxed open, there is very little resistance to the out breath, so minimal pressure build up.

When you purse your lips, you provide resistance to the outgoing breath which requires your lungs to provide extra pressure to expel the air.  Not much, but it is enough to have a significant effect.

It is the flow work associated with the expansion of that slightly pressurised air that makes the high velocity jet not only feel cooler, but actually is cooler than the air in your lungs.

I hope that helps with understanding of the concept of enthalpy, and also adequately explains just why that high velocity breath out both feels, and is, cooler than the more gentle stream from your partly open mouth.  However, if you have an alternative explanation, please, do tell.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on April 18, 2018, 09:03:44 PM
I'm probably telling you something you know more about than me, but it just hit me, that a freezer has a thin orifice where the gas expands after and that lowers the temperature - could a similar restriction or even turbulence somewhere in your setup have a similar effect ?
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 18, 2018, 11:30:54 PM
Hi MJM , more fascinating stuff..and i guess Robert Stirling was having similar thoughts !!   Also after i washed my hair recently i was using the hair dryer at full blast and it felt hot on top of my head but i could distinctly feel a cold draft lower down at the sides  ? !! i am becoming more and more aware of thermodynamics around the home and when i fill the sink with hot water, instead of adding cold if it is too hot i have a metal tub that i put in the sink and swirl the water around ,and this cools it and saves money on the water rates !! and when i have a bath i use my temp gauge  to adjust it to about 106 fareinhieght. !  Also i was at the car  boot today and there was an inlet manifold gauge scaled in   " Inches of mercury absolute "   , this was reading at 28. he said it came off a Lancaster bomber so i don't know how accurate it was and the needle was about 1/3rd of the way round the scale....so if it was measuring vacuum why was it not calibrated in Lbs square"?? Another experiment for you.....you could suspend two apple with thermocouples attached and blow between them and see what readings you get as they swing towards each other ...or use the compressor, not forgetting to video it of course !!!!
Willy....
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 19, 2018, 01:13:07 PM
Hi Admiral DK.  That is a good question, and it lets me get back to a topic where I am quite comfortable.  However I am always open to learning something new, there are some very knowledgable people on this forum.

The degree of cooling over the pressure drop at an orifice depends on the condition of the fluid at the upstream side of the orifice.  If it is a superheated vapour, well away from the two phase region and also well above the fluid freezing temperature, there is a degree of cooling determined by the Joule Thompson coefficient.  The is coefficient is yet another property of the fluid, and may be positive or negative.  If the vapour is right on the saturation line, it might result in a wet vapour, or a slightly superheated vapour depending on the shape of that p-h curve.

However, if the fluid is either saturated or sub-cooled liquid, as in a refrigeration system, as the pressure drops, some of the liquid evaporates, and as there is usually no source of heat at the orifice, the necessary energy comes from the sensible heat in the liquid, resulting in a large temperature drop.  The portion of the fluid which evaporates flows through the evaporator to the compressor suction, while the liquid is boiled off by heat absorbed in the evaporator, which is at a pressure with a corresponding boiling temperature lower than the contents of the fridge, which in turn are cooled by the loss of heat to the evaporator.

Just how much of the liquid is evaporated can be worked out by remembering that for an orifice or similar throttling process, there is no work done and no heat transfer in or out.  So the first law says the enthalpy does not change.  Then on the pressure-enthalpy diagram, or a property chart similar to the steam tables but for the refrigerant, you can easily see the proportion of the fluid which evaporates.  It varies quite a bit with different refrigerants.

In a large refrigeration system, the pressure drop occurs at a pressure reducing valve and the no heat transfer assumption is pretty good.  In a small domestic refrigerator, the orifice would be very small and liable to blockage.  Instead, a laminar flow orifice is used.  A fancy name for a very long, small bore tube, through which the velocity is small enough to be below the critical Reynolds number, so laminar flow, under the prevailing pressure difference.  You can see it as a long loose coil if you look around the motor compartment of your fridge.  The length of the tube is such that, as the fluid cools, there would be some heat flow in, which results in a small increase in enthalpy, but the advantage of a larger bore (larger than an orifice would be) being less liable to blockage, outweighs the small unintended heat gain.

So getting back to the question, in my mashup air test system, air is clearly a superheated gas, and the Joule Thompson coefficient means it cools a bit as the pressure drops in an orifice.  So this could explain some or all of the temperature drop I observed.  On the other hand, in a refrigeration system, it is liquid being throttled, and the amount of liquid which evaporates at constant enthalpy ensures considerable cooling.

I have had a look for values for the J-T coefficient, and while most Google references seem to come up with all the theory, the ones giving an actual value are hard to find.  It looks like about 0.25  C per bar pressure drop seems in the right order.  My compressor was set at 30 psig, say 2 bar gauge, so around 0.5 degrees could be explained by Joule-Thomson cooling, but not the observed temperature difference.  It can be moor useful cooling when larger pressure drops are available, and used in liquid air plants and similar.

Hi Willy, the air from the hair dryer is quite hot, test it with your thermocouple, so if you are thinning on top, like most of us, you will feel the heat, intensified by the good heat transfer resulting from forced convection.  When this air, which starts with low humidity, evaporates the moisture in your damp hair, the heat necessary to supply the latent heat to evaporate the water comes from the sensible heat in the moisture, which is cooled, and some comes from your head so it even feels cool as well.  When all the moisture is gone, you will feel the heat of the hair dryer. 

All that, so long as the humidity in the room stays low.  The evaporating moisture raises the room humidity.  If you have a lot of damp hair, in a small room, humidity gets high enough to condense on the mirror, and there is no more drying.  I was in a shop one day, before I started thinking too deeply about all this, when the lady ahead of us was complaining that her hair dryer was too small as it would not dry her hair.  Caught by surprise, and none of my business really, I did not think quick enough to ask if the bathroom had a fan, and did she switch it on.  The fan replaces the humid air from the drier with fresh less moisture laden hair, and the drying continues.  The salesman was obviously not into thermodynamics,and duly sold her a drier with a bigger fan and heating element.  Which no doubt did not solve the problem.

When you think of all those applications around the home, it is quite a significant subject in daily life.  Sometimes the effect is quite small, but when you wonder which action to take, it gives a sensible direction to try.

Before aneroid barometers, atmospheric pressure was measured by a mercury in glass manometer.  Still is when accurate readings are required.  But in the days before SI and even calculators, the conversion to psi or bar was pretty awkward, so the pressure was quoted in inches of mercury, or mm in metric countries.  Standard atmospheric pressure is 101.325 kPa, which is  29.92 inches of mercury or 760.002 mm of mercury.  Pressures below standard meant a slight vacuum, but there was confusion as to whether to quote how much below standard as inches of vacuum, or absolute pressure.

Of course, actual atmospheric pressure various as the various weather patterns progress over us, but if the pressure was really only 28 inches, I suggest you might be preparing for a cyclone.  One inch of mercury is equivalent to 0.49 psi, so nearly 2" low.  But check pressure with the weather bureau, to reset the gauge at the correct calibration.

Now, apples, thermocouples, and air jets!  Newton, Bernoulli, and temperature?  Surely this time you are being silly.  Must be time to learn how to insert those poke-poke emojis....  No, I'm still not ready for that!

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 20, 2018, 02:35:03 AM
Hi MJM, Thanks for the explanations...  actually It was the warmest april day for 70 years according to the weather man  don't know about emojis ,but the clues are in the number of exclamation marks !!!!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 20, 2018, 02:04:39 PM
Hi Willy, I assume the warm day was the day you were using the hair drier?  The electric element of the hair drier gets hot enough to dissipate all the electrical power input by heating air, just like your electric boiler elements dissipate all the heat by heating water.  So it will always feel hot on dry skin, whatever the ambient temperature.  But in raising the temperature, the drier lowers humidity, so the hot air stream is able to absorb more moisture from the damp hair.

Sorry, I did not notice the importance of the number of !'s.  Never mind.

A few days back, in post #825, I finally returned to the question of power from air compared with power from steam.  I did the calculations for air at 27 degrees C which seemed a reasonable estimate for the air you might be using at a show.  While the answer was that expansion of air gave less work then expansion of steam on a volumetric basis, it left open the question of whether this difference was simply due to the low temperature of the air compared with the steam.

I was distracted by other issues but tonight, I finally went back and redid the calculations for air at 127 degrees.  Not exactly equal to the steam temperature, but close enough, and as usual, selected because it appears directly in the air tables I am using.  It is also enough above the temperature I used last time to indicate the effect of the initial air temperature.

Interestingly, the difference in answer from expansion of air at 27 C was only 3 kJ/m3 which, in a total of 530 kJ/kg, is about 0.5% different, so not significant.  It is definitely not worth constructing an air heater to increase the work from your engines.  So the answer is still that expansion of air gives about 85% of the work from expansion of steam, but 92% of the work from expansion of superheated steam.

The difference when the air supply was at 127 degrees was that the exhaust would be -13 C instead of -78 C for expansion of 27 C air through the same pressure ratio, while the steam exhaust was wet at 100 kPa, so 99.6 C

All the calculations were on the basis of an ideal adiabatic expansion of steam.  A real engine might give around 70% of the work output of the adiabatic engine, so the corresponding temperatures would be -46 C for expansion of 27 degree air, and +29 C for expansion of the 127 degree air.

And above all, remember that while the inlet valve is open, there is no expansion in the cylinder, the potential work output is the same whether the fluid is air or steam.  The remaining expansion after the inlet valve closes, in a single cylinder engine, is much less than the expansion in an adiabatic engine for the same inlet and exhaust pressures, so the differences in work output and the temperature effect on the exhaust will both be much less pronounced.  It would probably take a compound or even a triple expansion engine to actually achieve the assumed expansion and see the maximum potential cooling of the air.

I do hope that this, at last, answers the question to everyone's satisfaction.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on April 21, 2018, 12:25:30 PM
No comments on yesterday's thoughts, so I thought I would post the formula for calculating the moment of inertia for a flywheel to add to the comments I posted about Brian's new flywheels on his thread about the air motor.

Basically, the important property of a flywheel is not mass, but moment of inertia.  Each element of a flywheel contributes to the moment of inertia by an amount equal to its mass times the distance from the axis of rotation squared.  All such contributions can be added or subtracted to make up the total moment of inertia of a complex part, just the same as you can estimate the mass of a complex component by adding and subtracting smaller elements.

The result of this is that really only a few simple formulae are required to cover most imaginable shapes for a flywheel.

First we need the formula for the moment of inertia of a circular disk.

Moment of inertia, I = 1/2 x M x R2

M = the mass of the disk, measured in kg.  To calculate this, first Calculate its volume, using diameter and thickness Pi x d2 / 4 x t

Diameter thickness and radius are all measured in meters

Then multiply by its density.  Steel is 7800 kg/m3, aluminium 2700 kg/m3,  bronze 8666 kg/m3, cast iron 7270 kg/m3 should cover most requirements.

If you have a simple disk flywheel like some low power Stirling engines, this will be all you need.

If you want a flywheel with a rim and spokes, you can subtract the moment of inertia of a disk equal in diameter to the inner diameter of the rim, then add a small diameter disk the diameter and thickness of the hub.

The spokes can each be approximated to a rod, rotated about its end, with the end at the centre of the flywheel and length equal to the inner radius of the rim.

The formula for this component is I = 1/3 x M x L2

You can ignore the tiny contribution of the end of the rod which is double counted in the hub, or you can reduce the length of the hub by the thickness of the spokes to avoid the double counting.

Sometimes, instead of spokes, I have seen flywheels with a thin disk supporting the rim, with holes bored in that disk for both ornamental and weight reduction reasons.  To allow for these, you first calculate the moment of inertia for the thin disk supporting the rim, then calculate the MoI for the material in the hole about its centre, and make allowance for the fact that it is not rotating about its own axis, but an axis displaced from its centre by a radial distance s, also measured in meters.  Then the formula for the moment of inertia for the metal you remove to make the hole is I = 1/2 x M x R2 + M x s2

Multiply this by the number of holes and subtract the total reduction in moment of inertia due to the holes.

This procedure, calculating the MoI of a component about its centre, then adding M x s^2 to account for the axis of rotation displaced from its centre, is quite general.  For example, the MoI of a rectangular bar rotated about its centre of mass is calculated by I = 1/12 x M x L^2.  If you want to rotate it about its end, you add the impact of moving the axis by L/2, M x (L/2)^2. 

If you do the maths you will see that 1/12 x M x L^2 + M x (L/2)^2 = 1/3 x M x L^2.

Of course, one spoke rotating about its end would result in a rotating out of balance force, but unless you go out of your way to make a flywheel with only one spoke, the out of balance force is balanced by the other spokes for any equally distributed number of spokes.

With these few simple formula, by adding and subtracting the various shapes, you can calculate the moment of inertia of most common flywheel shapes.

Using a spreadsheet, you can easily do some trial and error calculations to arrive at the necessary adjustments to a flywheel dimensions to make one with a different material, or to accommodate different space requirements.

I suggest that model flywheels are rarely designed by a complete theoretical analysis of the torque curve and careful selection of the flywheel moment of inertia to control the engine speed in strict limits as a full size often is.  This probably means that the flywheels are rarely just optimal in terms of dimensions and mass.  More likely, they look about right, and are, like those Rolls Royce engines, adequate.  There is no reason not to try a smaller flywheel, if you have one, or a larger one, than called for by the design, although it's another variable you may not want to play with, if you have a new engine which may be difficult to start.  But if you would like a different size flywheel for a running engine, a few calculations and trying a different flywheel you have access to, might give you a good idea of how much leeway you have to reduce weight.  A good reason to standardise on shaft dimensions where practical for similar size engines.

I hope this little diversion into mechanics is of interest.

MJM460

Edited due to minor errors and omissions which could cause confusion.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 21, 2018, 02:15:28 PM
Hi MJM,  Thanks for the info about flywheels    When i first attempted to start my new freelance engine on air it did not seem to want to operate !  I thought the flywheel was too small ( i used one that i had in store) so to give it more moment of inertia ( aka "welly" ) i attached this large chunk of brass to it.  It then sprang into life as if by magic !!! Intuitively ,one might think that if the  flywheel is balanced then there would be no advantage as one side has to lift against gravity ,whilst the other side is helped by gravity, or ,is that the answer !!!!!!!

Not a valid vimeo URL
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 22, 2018, 11:05:00 AM
Hi Willy, I really like your thinking.  We may not have a box of spare flywheels to try but we all have bars of metal.  Steel is as good as anything, and moment of inertia does not require any particular shape.  Your rectangular bar is as good a shape as any for experimentation, though like a propellor, it can give your fingers a whack of you are not careful.  Gives new meaning to the term flywheel made from bar stock!

If the moment of inertia is not enough, instead of using a heavier bar section, you can simply bolt a short piece at each end for much more inertia in proportion to the extra mass.  But do use through bolts in shear with nuts.  If one comes loose, it will not be good.  Mass at a larger distance from the axis is much more effective than mass near the axis, due to that r2 term.

To calculate the moment of inertia of your bar stock flywheel about its centre of mass is simple enough.

I = 1/12 x M x L2

If you add weights to the ends (while being careful to keep it all in balance), the additional moment of inertia due to each weight equals the mass times radius squared.  The radius is the distance from the centre of the added mass to the axis of rotation.  Strictly you should add the moment of inertia about its own centre as well, but in practice, this is small enough to ignore without significant error.

Perhaps you could do the calculation and let us know the answer.

When you are satisfied you have enough inertia, you can then simply check the flywheel you want to use by calculation.  If it is a bit low, adjust your temporary set up to match, and try it.  Will save spending money on a flywheel that is too small.

Perhaps I should say a bit more about how the "correct" size of flywheel is determined.

When the engine is running, we know for a single cylinder engine that the torque goes through zero and a maximum twice each revolution.  For a single acting engine, it still goes through zero twice, but also a maximum on the power stroke, and a negative maximum on the exhaust stroke each revolution.  A multi-cylinder engine goes through a number of maximums and minimum torques each revolution, though the minimums are still usually positive unless all the cranks are on the zero and 180 degree points.

The load the engine drives also has a torque characteristic that may vary throughout each revolution, like a reciprocating pump or compressor, or may be constant throughout each revolution  like a winch.  When the engine torque is greater than the required torque, the whole lot speeds up, when it is less, it all slows down, and this all happens each revolution.

The flywheel inertia means the flywheel speeds up when there is excess torque, storing rotational kinetic energy in the process, and when the torque is negative, it slows down, returning energy to the system.  The necessary flywheel inertia is that required to keep that speed fluctuation within acceptable limits.  To give you an idea, speed limits of +/-7% within each revolution is considered a quite stringent limit for a 400 rpm system.  (I have been up against this limit on an electrically driven reciprocating compressor, where those fluctuations affected the motor current and were potentially enough to upset the city power system.  Fortunately the engineering was all well done, the flywheel was adequate and the power supply authority happy.)

My non-contacting tachometer only detects the flywheel position once per revolution, so cannot detect the speed variation within the revolution.  The electronics these days is fast enough that if you put the reflectors at multiple points about the flywheel, it is possible to measure the speed multiple times per revolution.  This is done routinely in modern digital governors instead of flyweights.  If you are then able to record these readings and graph them on a suitable time scale, you can easily see the variation.  I have commonly used both 24 and 60 tooth wheels on the shaft.   The digital governor calculates the speed multiple times per revolution even on a high speed turbine, and does not wait for a full revolution to respond to a speed change.  But not so fascinating to watch as your flyweight governor.

The smallest useful moment of inertia for a flywheel has to keep the engine turning, but I suggest that visibly nearly stopping twice each revolution would not meet the expectation, it needs to look reasonably steady even at the lowest speed you want to run, when the flywheel stores the least energy.

How much is too much moment of inertia?  In principle, bigger always gives better speed regulation.   However, bigger usually means more friction in the bearings, and more air friction resistance, so it is definitely too big if the friction forces absorb the entire output of the engine.  Besides, there is usually some weight or dimensional constraint, while our speed regulation requirements are really not very stringent.  So once the engine appears to run at a reasonably steady speed, the smallest and lightest flywheel that looks appropriate is just right, but a bit more never hurts.  Remember that you can make it lighter by making it larger diameter, material density is another factor to play with in balancing performance, diameter and mass.  The D2 term means that a small change in diameter makes a relatively large change in inertia, even with the same mass, so you can slim down a heavy looking rim.

Your comment about gravity not making any difference if the flywheel is balanced is quite correct.  It does not matter if the flywheel is round, or bar shaped, or even propellor shaped, bit must be reasonably balanced.

All this stuff about flywheels and moment of inertia might be easier to understand if you compare it with linear systems.  Most of us have a fair understanding of the place of mass in a linear system, moving through distance, at velocity, force and acceleration.  Associated with linear systems is kinetic energy and linear momentum.  And I hope you have noticed that conservation of energy,  and linear momentum are absolute laws of physics, while conservation of mass is near enough for all our practical purposes, but not absolutely correct.

Rotational systems have exact analogies for all these quantities.  Angle of rotation is the rotational analogy of distance, angular velocity the analogy of linear velocity, torque the analogy of force, rate of change of angular velocity the analogy of acceleration, and moment of inertia the analogy of mass.  Conservation of angular momentum is an absolute law of physics along side conservation of linear momentum, but there is no analogy to conservation of mass.  This is because moment of inertia and hence rotational kinetic energy are not only mass dependant but also dependent on the distribution of that mass.  By moving the mass on a spinning object, you change the moment of inertia, however, as conservation of angular momentum still applies, if the moment of inertia changes, the angular velocity must change to maintain the angular momentum.  The classic example is a skater spinning with arms outstretched, then drawing them in close to the body to spin faster.  This applies to all systems with variable geometry.  With the analogous properties substituted in all of the linear laws of motion, the exact same laws apply to rotational motion.

I hope that helps a little with understanding flywheels and how they work.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on April 22, 2018, 11:53:55 AM
Hullo Willy......I too was a little concerned for your hands or fingers with that rotation a hunk of brass  :facepalm:

I found the text from MJM very interesting as I am now just going thru the mass of inertia in my build

50 diameter x 96gm flywheel on the shaft 4 diameter
18 diameter x 10gm chain pinion on axis on the shaft 4 diameter [not shown]
4.8:1 chain reduction
68 diameter x 120gm flywheel/chain pinion x 6.35 shaft diameter
2 x 25 diameter x 35gm shaft couplings x 6.35 shaft diameter
2 x 130 diameter x 300gm paddle wheels on 6.35 shaft diameter

So my main concern is to ensure there is adequate mechanical security  :killcomputer: of the drive within it's individual components

Derek
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 22, 2018, 01:02:32 PM
Hi MJM, Just a little bit more about flywheels .... this is about a similar engine to the Beeleigh mill  to raise the 'horses power' !! I don't know if the thinking was %100 correct but possibly was as they did know something about this in 1868...So Question,, is it advisable and proper to increase pressure and flywheel weight together ,and are there tables for this ??
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 23, 2018, 01:36:20 PM
Hi Derek, that is a very interesting and compact arrangement.  You have put a lot of work into that.  I like the small chain and wheel.

And you have added another critical variable into the rotational equations for a flywheel, the rotational speed.  With the speed reduced by 1:4.8, the energy stored is proportional to 1/4.82 or 0.04 times the energy at the higher speed.  Is there a reason for putting a flywheel on the low speed shaft?

I presume that the paddle wheels are on that same slow speed shaft.  Their moment of inertia adds to the flywheel on that shaft, and of course they are also the load for the engine.  The torque required by the paddle wheels is likely to be fluctuating as each float dips into the water.  Change in momentum involves a torque, just like change in linear momentum involves a force, the rotational analogy of F = M x a.  The change of flywheel angular momentum with each ripple in the torque from the paddles and from the engine, results in reversing torques, which will test your wheel attachment to the shafts.  I presume that is what you are thinking about with the attachment considerations.  Are you thinking in terms of taper locks, or perhaps keys for reliable attachments?  I will be most interested in seeing further progress.

Remember for rotational systems, it is the moment of inertia, or more correctly, the second moment of mass that is important, it takes the place of mass in a linear system.  For the paddle wheels you need to calculate the moment of inertia of each component, and add them all together.  Fortunately you have many identical except components and you only have to calculate the value for one set and multiply by the number of identical sets.  You can calculate those rings from the formulae I have already calculated, but you may prefer to use the direct formula for a ring, I = M x r2

Hi Willy, your beam engines have hp and lp cylinders, but equivalent to 180 degree crank locations, so two zero torque points each revolution.  The flywheel must store the excess energy by accelerating during the power stroke, and return this energy to the system when the engine torque is less than the load by slowing down.  Increasing the power rating implies increasing the load, perhaps pumping to a greater height, or larger volume, by working at higher speed.  The extra power implies either extra torque from the engine, or higher speed, so the flywheel has extra energy to store each power stroke, and has to return the greater amount of energy to drive the load as the engine torque goes down to zero at the top and bottom dead centres.  Either the engine can be allowed a greater speed fluctuation, or the degree of regulation can be maintained by adding extra moment of inertia to the flywheel.  A band around the outer diameter is a very efficient way of adding moment of inertia as the large radius ensures the maximum increase in moment of inertia for the weight added.  So it makes sense that the two were linked.  Did they have tables?  Or did they perhaps have a rule of thumb for the moment of inertia required for a given power of engine?  I don't know.  These days, a computer is used to calculate the moment of inertia of the whole machinery string, and is combined with a detailed analysis of the torque characteristics of the engine and load, and these figures are used to determine the extra moment of inertia required to ensure the necessary speed regulation is achieved.  I think you mentioned this once before, but I have probably been able to give a more detailed comment this time.  I hope it helps.

I mentioned yesterday that most of us don't have a stock of spare flywheels available to try, however when I looked around, even I had the four shown in the attached picture.  In fact I also have a second very similar to the small one, and even two commercial ones,  one a Mamod and the other a similar model, though those two have different shaft sizes.  All of mine have been drilled and reamed 5 mm diameter, so are interchangeable if they fit on the base.

This afternoon, I put all the dimensions in a spreadsheet and calculated the moment of inertia for each.

The little casting is yet to be machined, but it will be made for the same size shaft as the others.  I don't think that it has had enough fondling to meet the required high standard required on this forum before I start making swarf.  Consequently I had to estimate the finished dimensions.

The calculations are very sensitive to the actual dimensions, (as I found when I tried to match calculated mass to the actual measured weight.  So I assumed the same material density for each which at least makes them all comparable on a dimensional basis.

The smallest is 45 mm diameter, 14 mm wide and weighing 157 g.  The moment of inertia was 20.16E-9 kg.m2.  The E-9, or x 10^-9 is necessary to make the units correct for use in other equations of motion.

The next is 65 mm diameter, 17.4 mm wide and weighs 310 g.  The moment of inertia was 75.53E-9 kg.m2.

The third turned from solid is 75 mm diameter, 16.4 wide and weighs 447 g.  The moment of inertia  was 148.06E-9 kg.m2.

Finally the cast flywheel looks like it will yield a flywheel 58 mm in diameter, 12 mm wide and weighing 129 g with a moment of inertia of 14.95E-9 kg.m2.

The cast one looks anomalous.  Careful examination of all the dimensions shows that the cast flywheel rim is only 11 mm thick in the radial direction, compared with 14 mm on the 45 mm turned wheel.  When I increased the dimension to 14 on both, the larger spoked flywheel had the same moment of inertia with less weight.  That r2 term is so significant that quite small differences have a large impact.

You can see that the larger ones have significantly more moment of inertia than in proportion to their extra mass.  The calculations also show that more than half the moment of inertia comes from the rim, over 70% for the larger ones.  I was a bit timid machining out those webs, but the spokes contribute only 10% of the moment of inertia of the cast example.  The hubs were also relatively insignificant.

I guess I now have some known flywheels to try on my next engine.  But in fact they are all relatively large for my small engines.  (Jason might call them series 12).  Even the smallest is adequate for my single acting oscillator, so they are all more than adequate for the double acting engines, and even more so for a multi-cylinder which might be my next.  Perhaps I should steal Willy's idea, and make a bar stock mock up to see how small I can go on each.  This thread is producing more ideas for projects than I can keep up with.  At least I know I can make more flywheels from that bar I used for the 45 mm one, make them thinner, and perhaps machine out some spokes.

I have been a bit slack not doing those calculations long ago, but you have all prompted me to at last open a spreadsheet, measure my flywheels and put some real figures into the discussion.  I hope you have found it interesting.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 24, 2018, 01:17:03 AM
Hi MJM, would the larger flywheel increase the HP on its own ?   old motorbikes always had large flywheels but i think modern ones do not because they need to accelerate very quickly..is this factor taken into account on a beam engine and is there a slight variation in the radian speed of the flywheel during its revolution, however small?.. I have given you the dimensions of the flywheel and bar if you would like to do the calculation adding your own 'adjustments' . If i wanted to increase the MoI with say an inserted band of lead melted into a channel on the outer part would that work and could you work that out.? say a channel 5mm wide and 10mm deep ? sorry for all these questions ,but ,I think they are relevant !!!! The bar should read 170mm btw
Willy.........
Title: Re: Talking Thermodynamics
Post by: derekwarner on April 24, 2018, 05:32:30 AM
Just back to the chain drive MJM......yes the paddle shaft is the 4.8:1 reduced speed

I had a 48 tooth S/S chain pinion [that looked :facepalm: terrible] and a second 50 diameter bronze spoked flywheel....when I cross checked dimensions it was a perfect marriage  :Love: ......... [Loctite bonded and bolted together with 6 x M2 HH brass bolts]

The S/S roller chain @ 3.1875 mm pitch so @ 1:16 scale equates to ~~ 2" [51 mm] pitch so is reasonably authentic for size of many of the early Australian and European paddlers

It also provides a very robust engagement between the engine crankshaft & the paddle shaft [although an absolute overkill in strength or breaking load]. With the 4.8:1 reduction, I am expecting a relatively smooth paddle shaft motion at say 25 to 75 RPM, with the engine happily ticking over at the higher speeds

I contemplated how to manufacture resilient couplings for the paddle shafts, but ended up purchasing a pair of commercially manufactured couplings with 73 Duro nitrile joining elements [we can imagine the softness of 70 Duro nitrile O-rings] so I am expecting these will provide a little rotary resilience on startup..... [soft start without the bang & clang]

The 50 diameter spoked wheels each have 4 x M3 tapping's, the 10 tooth crank shaft pinion has 2 x M3 tapping's each for M3 S/S HPGS. Finally, the paddle shafts at the wheel hubs are 7.93 mm and the intention is to secure the wheel hubs to the wheel shaft each with an M3 S/S [standard 50:1] taper pin

Derek
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 24, 2018, 11:42:22 AM
Hi MJM just remembered to weigh the items   flywheel is 530 g  and the bar is 540 g....allmost exactly the same !!
willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 24, 2018, 01:51:01 PM
Extra flywheels just seem to pop out of the woodwork, like wire coat hangers. I have two more that I did not think of yesterday.  One on a Stewart vertical that I inherited, and one on an oscillator I made after first year of study, over fifty years ago.  But might need some bushings to put them on my 5mm shafts.

Hi Willy, a flywheel is just an inert lump of metal with no way of adding energy to the system, even if it is spinning around.  So it does not increase the power available at all.  Increasing the power requires increasing the rpm, increasing the torque, or both.  However, when it is spinning, it has kinetic energy it gained from the torque that sent it spinning.  The practicality of life on earth is that the spinning flywheel will have some friction drag due to the bearings and air resistance, but with care we can make both relatively small.  So a good flywheel will spin for a long time if we start it spinning then just let it go.  Just like a toy spinning top.  Let's ignore that friction for the moment.

So what is a good flywheel?  It is one with a large moment of inertia.  Mass is part of the equation, but moment of inertia (I) is the property that is significant in any rotating system.  The flywheel stores the energy it gained as it was spun up to speed.  We can quantify the kinetic energy stored by the equation

KE = 1/2 x I x w^2.

Please read that w as the lower case version of the Greek letter, omega, which seems to be universally used for rotational speed.  And as you correctly implied, it is measured in radians per second.  Radians are dimensionless so the units are really "per second" or seconds^-1.  I have no idea how to get Greek letters into the post.  (Dan, do you know how it's done?)

When the flywheel is spinning, it is subject to the rotational version of all Newton's laws of motion.  When we apply more torque (T) than the minimum to overcome friction, the flywheel the flywheel accelerates.  For straight line or linear motion the equation describing this is F = m x a.  For rotational acceleration, the Greek letter alpha is usually used.  The units are radians per second squared, or sec^-2.  Perhaps I will just write alpha in full.  So the rotational analogy for the well known linear motion is T = I x alpha.  The energy is stored in that higher rotational speed as the system accelerates.

Similarly, if there is more torque required by the load, the flywheel experiences a retarding torque, and slows down.  In this case the torque is negative, so alpha also negative.  The energy from the flywheel continues to drive the load, but the system slows in the process, and if more torque is not applied, the system will soon stop.

Your beam engine, and your single cylinder engine, (assuming both double acting) provide a fluctuating torque.  It is zero twice each revolution, at the top and bottom dead centres.  The variation of torque within each revolution is the top half of two sine waves, one 180 degrees after the other so a continuous series of pulses.  The effective average is the RMS value, similar to a RMS voltage with only two diodes for rectification, sound familiar?  So about 70% of the peak value.  Not exactly a sine wave with slider and crank mechanism, but close enough.

When the engine produces that peak torque, (providing it exceeds the load torque) the flywheel absorbs the extra energy, and stores it by accelerating.  When the engine torque is below the load torque, the flywheel gives back the energy, but also slows down.  This happens continuously within each revolution.  If you have a modern electronic governor, it could use a 60 tooth wheel or even more, and measure the speed 60 times each revolution.  If you recorded these readings, I suggest you would see at least 15 different speeds, each one would occur four times in each revolution, depending on just how the torque zeros lined up with the teeth on the measuring wheel.  The difference between the maximum and average, or minimum and average, within each revolution is that speed variation I have been trying to explain.  It is the variation within each revolution, not the variation over several revolutions.

When your engine power rating was increased, that implies higher torque, from higher pressure steam, or higher speed, perhaps both.  To achieve the same speed regulation within each revolution, the flywheel must store and give back more energy in each revolution.  You can see from the equation for kinetic energy, you only have speed and moment of inertia to play with.  So either you allow more speed variation, or you add more moment of inertia.  In reality, the additional moment of inertia will not be the exact requirement, it can't be unless the engine runs at only one average speed, so increasing the moment of inertia will be accompanied by a change in the speed regulation, depending on whether the extra moment of inertia is more or less than required.

Sorry for all the repetition, I guess the issue is whether I made it sufficiently clear that the speed regulation I was talking about, was the speed variation within each revolution.

Your questions about your brass bar and flywheel are totally relevant, my intention is that we should be able to get practical results from all the calculations.  So I will do the calculations, and also for a bar of steel to give say half the moment of inertia of the brass bar.  This will give you a better idea of how much more you need, so we can check alternative flywheel designs, and get rid of that finger whacker.  I wouldn't start machining that channel yet.  If you replace cast iron with lead you are replacing 7.8 relative density for cast iron with 11.3 for lead, so you only benefit from the difference.  It would be better to think in terms of how you could add a "belt" of bronze or steel to the outside of the rim, or even an extra ring each side of the rim to increase the width, if you don't have room for extra diameter.  First, let's see if we can get a better handle on how much moment of inertia you need.

I had a careful look at the drawings for the required dimensions.  All the required information is there and now you have given me the weights we can compare that with calculated weights.  They probably won't match, but is good when they are near the right figure.  It will be tomorrow though.

Hi Derek, that explains the low speed flywheel.  I probably would have done the same in the circumstances, just to get it all connected up and a test run.  Probably even helpful for test running before those paddle wheels are complete.  However, once you have it all in the boat, that flywheel is only ballast due to the slow speed, if you need ballast, it is better to be able to choose the location.  I would suggest that when you have a spare hour, (like the ones all the rest of us have!) make up a simple flanged wheel to support the chain wheel with less weight.  If you need more moment of inertia, it is better to have it on the high speed shaft.  I like the idea of those flexible couplings.  They make the support of the paddle shaft more difficult, but will reduce any sharp impulsive torques from the ripple load of the paddle wheels.

I am most interested to hear how the speeds turn out, as I definitely have a paddler on my ultimate list.  I am working towards a twin cylinder engine.  Would prefer a centre-flue marine boiler for a boat, so may end up using the bought one as I have not been able to work out a suitable Meths burner design for the centre flue.  Got a long way to go on the skills front though.

Well, time flies when you are tending the bar-b-que, roast lamb for 30 guests for a birthday celebration for a niece this evening.  Time to put the chicken on.  Should get this posted after the guests are gone. 

Well, the party went well.  Noisy and chaotic as you might expect with more than half of them children.  All gone now except the two grand children staying overnight.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 25, 2018, 11:49:44 AM
Hi Willy, I completed those calculations this afternoon.  I have tried to attach a print of the section of the spreadsheet so you can see the basic steps in the calculation.  My cheap printer makes it quite difficult to get a good clear print from a spreadsheet.

Basically I calculated for a full disk for each of the rim sections, and also for the centre part which is missing.  Using negative density, or subtracting the centre is probably the easiest way to calculate the moment of inertia for the rim.

That flywheel has a moment of inertia of 945,000 x 10-9 kg.m2, approximately 3 times the moment of inertia of my 75 mm flywheel, largely due to the larger diameter.  You can see the spokes and hub make an insignificant contribution, while the majority comes from the outer rim.

The brass bar has 40% higher value, 1,337,871 x 10-9 kg.m2

A steel bar 150 mm long x 25 x 12 mm has a value roughly half of this, 663,000 x 10-9 kg.m2 which would make a convenient half size step up from the flywheel alone.  You can see this would make a very simple temporary flywheel to give you an idea of what you need.  The formula for a bar about its centre of gravity is I = 1/12 x m x R2.

The calculated mass was 567 g for the flywheel and 556 g for the bar, a little higher than the measured weights.  The error is probably density differences for the exact alloy your metal is made from, but also the mass is quite dependent on exact measurements.  I expect the discrepancy is within a reasonable tolerance, and more than satisfactory for the purpose.

But while I was doing the calculations, I could not help but wonder why the extra moment of inertia is required.  Your double acting engine goes through zero torque at the top and bottom dead centre, but for a free running engine it really does not take much of a flywheel to get it through that zero to the point where there is again plenty of torque to overcome friction.  I would expect your flywheel is more than adequate for running, and should even give good speed regulation, even under load.

How does the engine behave without the brass bar?  Does it stop at each dead centre, or just always stop at one end?

I am wondering if you have had the valve chest cover off and carefully checked the eccentric settings.  First I wondered if you just had early cut off which gives a bigger range of rotation at zero torque for starting, but once you spin it over by hand to get it started, there is air pressure on the cylinder during expansion after cut off so most of the range of zero torque disappears.  I don't think this is the issue.

However, if the admission is early, probably depending on the exhaust closure point as well, you could get full steam pressure in the cylinder before top dead centre, which would require much more moment of inertia to turn past the top dead centre and keep it running.  If steam reaches full pressure before top dead centre, the flywheel has to supply enough energy to push some of that steam back against boiler pressure until the piston reaches the top dead centre.  Similarly at the bottom dead centre.

I hesitate to tell you how to set the valve gear, and diagnosis is difficult from such a distance, especially with no information on what was happening that you needed the extra flywheel.  The only obvious thing I can think of is early admission so the steam reaches full pressure before the piston reaches top dead centre, but I am confident that you will be able to find the issue.

In any case, I would not suggest modifying that nice looking flywheel yet.

I hope that is helpful,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 25, 2018, 10:46:37 PM
Hi MJM, Thanks for the calculations and the engine is working with its own flywheel now. It was just reluctant to start with. I did some adjustments as well once it got going and it seems fine now !! here is the video.........I was wondering that if the flywheel size makes no difference to increasing the Horsepower did the engine maker need to do this with the Wentworth Engine ?

Not a valid vimeo URL
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 26, 2018, 01:51:22 PM
Hi Willy, I was a bit puzzled, as I thought that you had posted the video of the engine running without the brass bar in your engine build thread.  I assume the brass bar convinced you that you had a working engine, and then you did some adjustments, I assume the eccentric settings, and found the bar was no longer necessary, then made the video.  If you have an hour or so, and a 150 mm long piece of 25 x 12 hot rolled steel, you could make up that steel bar and see how you go with only 60% of your current flywheel moment of inertia.  I really expect it should still run well, even if with a few more tweaks to the eccentric positions.

Remember that your engine has two independent speed regulating mechanisms, or three if you count the steam stop valve.  The flywheel regulates the speed within one revolution only.  Well, it would probably keep it running for three or four revolutions, or even more the engine running free, but only by progressively slowing down until it stops.

The governor regulates the speed by controlling the steam pressure, so actually changes the amount of energy available for conversion to work by adjusting that throttle valve at the steam chest inlet.  The governor should have enough damping, and be slow enough acting, that it only responds to a longer term change in the average speed.  It should not respond to those changes within each revolution, there would be unacceptable wear in the pins of the linkages if it did.  But, if the engine keeps getting faster each revolution, the weights move outwards and the linkage to close the throttle valve a little.  Alternatively, if the engine slows, due to an increase in load, the  governor weights collapse inward, and the linkage opens the throttle to admit more steam.  The spring tension on the linkage determines the speed the engine settles at in the longer term. 

The big trick to governor performance is that it must be slow enough not to respond to those torque fluctuations within each revolution, but fast enough to prevent the engine over-speeding if the load is suddenly lost, for example the belt could come off, or even break on a belt driven load.  Industrial engines have a separate independent speed trip mechanism and a very fast acting throttle valve, called a trip valve, that slams shut if required.

Let's return to the flywheel again for a moment.  The kinetic energy stored is determined by the flywheel moment of inertia and the rotational speed squared.

K. E. = 1/2 x I x (omega)2.

The units of energy are the same as the units work, so the same as the units of torque times rotational angle (T x angle) with the angle measured in radians, which you will remember have no units.  It is also the same as work in a linear system, force times distance.  The key to getting the correct dimensions for force is Newton's equation, F =m x a, so kg.m/s2.

For a rotational system, torque = force times distance, (the distance being the moment arm), so work in both linear and rotational systems has the units of kg.m/s2 x m, or kg.m2/s2 which is the same as the units of K. E. In the equation above.

When the speed of the flywheel slows, the stored energy is reduced.  Conservation of energy says that energy went somewhere, and the answer is that it was returned to the rotating system as work  to help driving the load.  When the torque from the engine is greater than the load, the system speeds up, and the work not absorbed by the load is absorbed by the flywheel, as an increase in speed.  And this cycle continues while the engine runs.

You can see from all of this that the flywheel does not add to the energy of the rotating system, it simply soaks up the excess when it is available, and returns it when there is a deficiency, but both only in very short term.  So the answer to your question on the Wentworth engine is that it was not necessary to modify the flywheel just to keep the engine running, it was included simply to keep the speed fluctuations within each revolution within acceptable limits.  With the increased power available, the load would also have been increased, so more stored energy was required to limit the slowing as the engine passed through the top and bottom dead centres.

I have finally found an Ap that lets me make a quick freehand sketch, as I would with a pencil if we were talking face to face.  They say an Italian needs his hands to talk, I need a pencil!  So let me see if I can include a sketch to illustrate what I have been saying.  Please excuse the shaky freehand experience, the Ap does not seem to include a ruler, let alone a sine function.  I don't know the file size, Apple is quite sure I don't need to know, so refuses to tell me.  If it is too big, I will come back and post them later, when I have access to the computer to resize it.

The first sketch shows the torque in black on the vertical axis and degrees of rotation on the horizontal.  The speed is shown in red on the vertical axis.

The second sketch shows the torque fluctuations for several revolutions, again black.  The red crosses are the rotational speed, measured only once per revolution, for an engine at steady speed.  Measuring speed once per revolution does not see those variations within the revolution.

The blue crosses are the speed, again measured for an engine with steadily increasing speed.  It is the governor which must respond to this steady increase, and shut the throttle plate a little to slow the engine back to the required average speed.  I hope that helps clarify what I have been trying to say.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 26, 2018, 02:50:50 PM
HI MJM, Yes that makes things a lot clearer. The Governor on the Beeleigh engine has a 4 stepped pulley on the crankshaft and a 3 step pulley on the Governor side  so there is a lot of options with the speed control. The engine provides the power for up to 5 corn grinding large stones so quite a variation in the load !
. The starting procedure is to pull the handle on the governor valve down to admit steam and let the engine run up to speed. If the rope breaks then the governor will drop and cut off the steam quickly. here are some pics of the arrangement...The governor valve is inside the main steam valve. This model has been a fascinating engine to make and the restoration of the original is well under way!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 27, 2018, 02:52:17 PM
Hi Willy, it certainly is an interesting engine to be able to look at closely.  You didn't say how the drive is transmitted to the mill-stones, but quite a range, from one or even zero, through to all five, but I assume the speed would be similar, regardless of number of stones being driven.  However, if the engine was powered up, driving all five, and some part of the drive broke, the engine would accelerate quite quickly, and this is what the governor, or these days the separate trip valve, must catch.

You have me confused about the governor though.  When the engine runs, the governor weights fly outwards, and the collar position controls the valve position.  When the engine is running too fast, the weights fly out further, and move the linkage to close the steam valve.  When the engine slows, the weights drop back in and open the valve to restore the set speed.  If the governor drive belt drops off, or breaks, the governor rotates slower and stops, the weights fall inwards, as it would if the engine had slowed, which should move the valve to the full open position.  Not what you need when the governor no longer "knows" the engine speed.

I have given some thought to how to get a feel for how much energy is stored in the flywheel, and what is the impact of the degree of regulation allowed.  I have already mentioned that there are rotational analogies for all the normal laws of motion we learn at high school for linear systems.

I will use your flywheel with a moment of inertia of 935,300E-9 kg.m^2, or 0.0009353 kg.m^2.  Then I will assume you want the engine to run at an average of 500 rpm, and allow a speed variation of +/-10%.  This means the speed varies from 550 rpm maximum to 450 rpm at the slowest point in each revolution.  We have to change these to radians per second for use in the calculations, 57.6 max to 47.1 minimum all while the flywheel rotates 1 revolution, or 2 x Pi radians.

We can use the rotational forms of the equations of motion to calculate the acceleration, and the steady torque necessary to cause this acceleration.  For slowing, acceleration is negative, 0.082 N.m.  We can also calculate the time for one revolution as 0.12 seconds, or an average speed of 500 rpm.  Finally, Power, assuming steady retarding torque is about 4 watts.

Now this does not mean that your engine is producing that power, we have no measurements of what the engine is producing, or how much the speed changes within the revolution.  The flywheel stores energy by speeding up during more than half of the revolution, then returns this stored energy by slowing down during the remainder of the revolution.  It is quite hard to work out what this figure really means as the total energy returned is returned in less than half of the revolution, so the actual load the engine is driving to give this speed variation is probably more than double.  It suggests that this flywheel could be suitable for an engine driving a load of around 10 watts.  That reasoning is pretty obtuse, so don't worry if you can't follow it.  The main conclusion, with a pile of judgement/wild guessing thrown in, is that all the energy figures and rotational speeds are the right order of magnitude, and depending on your steam supply pressure, or probably a bit high, meaning a smaller flywheel would be adequate.

It is interesting to do the same calculations with an average speed of say 1000 rpm.  Double the speed means four times the energy, however, speed regulation based on the same percentage means a wider speed range.  The corresponding "power" is then 34 watts, much higher than I would guess is possible with reasonable steam pressure.  If this is the speed you are aiming for under full load, a considerably smaller flywheel would be adequate.  But you can see a slow rotating beam engine needs a bigger flywheel.

I think the real value in these calculations is not in the actual figures, but in understanding the implications of those equations of motion, and the effect of moment of inertia.  At the end of the day, given that most of us don't have the ability to measure the speed variation within each revolution, the best way to determine the right size for a flywheel is trial and error under the proposed operating conditions.  However, by calculating the moment of inertia, we can compare flywheels, and even use square bars as a temporary flywheel.  We can then select a more conventional wheel with similar moment of inertia.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 28, 2018, 12:49:28 AM
Hi MJM, The engine is in bits at the moment as i am making the new bed to accommodate the dynamo, so i will delay the air test for the moment The governor valve on the Beeleigh engine is cunningly incorporated inside the main steam valve, rather like a butterfly valve. I have made a drawing to show the parts in cross section also some photos.They did manage to break the end off when removing it as well.Is it possible to have too large a flywheel on an engine? and with a boiler supplying steam to an engine is it a good idea to have the boiler producing a higher pressure than the engine requires  as a form of backup to keep the piston moving ? all these questions are possibly of a more practical thinking than a technical mind so i think i should do a lot more reading !!!
Willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on April 28, 2018, 10:48:20 AM
MJM...when you mentioned....'I like the idea of those flexible couplings.  They make the support of the paddle shaft more difficult'

To aide this, the paddle shaft will be supported by six [6] 1/4" ID Grade 304 ZZ shielded ball bearings in miniature plummer blocks

Only the outer [4] will be splashed with water....& hopefully never immersed  :facepalm:

I am enjoying the continued TT thread, however question in my mind that the brass or steel bar mounted on the crank [as opposed to a uniform disk shaped flywheel] would produce horrendous mechanical pulsations x 2 at each revolution that would result in premature bearing/bush failure due to ovality?

Derek

PS1...the mock-up image looks a little like spaghetti or a dogs breakfast of bits on a shaft including 1x resilient coupling...[these couplings will look a little too current Century, :stickpoke:  so plan to cover them on the paddle shaft .....so out of sight]

PS2... Willy....one must wonder how that tapered plug valve spool ever sealed in the valve body.... or is it only after years of non use then,,,, stripdown and de-rusting preservation?
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 28, 2018, 01:25:18 PM
Hi Willy, that is a very clever steam valve design with the governor valve inside the stem.  The combined unit has fewer flanges which are always potential leakage points.  It is a pity it was broken.  I guess someone will work out how to fix it, although if the seal area on the two stems is corroded like the rest of the shaft, it may be preferable to replace one or the other or both.  Or would that be anathema?

Basically, the optimum size for a flywheel is a very flat plateau, and a very large range of flywheels will be satisfactory.  If you have a flywheel that works to your satisfaction, a larger one would do the job with less speed variation within each revolution.  A smaller one would mean the speed would vary more.  As the flywheel gets bigger in moment of inertia, generally the mass will get bigger, and also the diameter.  In most engine plants, mass or diameter, or often both will become a problem, probably before the inevitable air resistance and bearing friction really become significant.  But a bigger flywheel always increases the difficulty of manufacturing, installing and maintaining the plant.

Extra pressure in the boiler does not do much to keep the piston moving through those two zero torque points at top and bottom dead centres.  I suggest that the flywheel is the item which keeps the piston moving through the low torque points in each revolution.  You need to tap into stored energy, or momentum, to carry the engine through those zero torque points.  However, I would expect that extra pressure in the boiler may help the engine respond to sudden load increases, when the governor will quickly open the valve to maintain speed.  But, the extra pressure also has a disadvantage in responding to a sudden load decrease.  I suspect the boiler pressure and also water levels would be operated to optimise boiler operation rather than the engine.  As you say, this is a practical question, and the theory can help predict the effect of those variables, but I suspect that practical boiler operation experience would better inform you of the best way to go.

More reading?  With your library, you certainly have the material.  I don't know about you, but I find that by the time I read all the posts on this forum and spend some time writing this one, I barely have time to keep up with the news, eating and sleeping, and any remaining time seems to get allocated to washing the dishes.  I would have to join Chris's universe to fit in more reading, and he is a bit secretive about how to get there. 

Hi Derek,  glad you are still enjoying the thread.  I like those flexible couplings for the flexibility, and damping they provide.  They look excellent, but I was referring to the difficulty of getting so many solidly mounted bearings lined up.  Sorry I was not more clear.  You are addressing the issue they had on full size paddlers, though your hull will be relatively more stiff so less problematic.  They certainly look ideal for the job.  Ideally you would use only four bearings, with the two couplings supporting a floating section.  As the gear wheel position is determined by your engine layout and would need to be on a solidly supported section, you would need an asymmetrical arrangement, but it is all looking very neat and well thought out.  I am sure you will sort out the issues.

The solid brass bar, drilled and reamed on its centre of mass is perfectly balanced will spin perfectly freely with no pulsations, despite its appearance.  It will stop at any position, just like the round version, but will churn up a bit more air, rather like a badly shaped two blade aeroplane propellor.  If the drill misses the centre line there will be the normal out of balance forces, but no worse than those due to the big end and crank throws.  A small out of balance at suitable orientation can help offset the crankshaft.  The centre of mass is found in the normal way by the intersection of the two corner to corner diagonals drawn on the face to be drilled.  The bar should be straight, and the hole really should be perpendicular to the length of the bar, or you will get a rotating couple, like a two cylinder engine.  You can try it by just drilling a similar shaped rectangular bit of steel and spinning it on a rod of silver steel, use a piece that will need a hole in the centre for its intended use.

I have given a bit more thought to just what those calculations based on the laws of motion are actually telling us.  It is not much use calculating as though the flywheel was returning energy to the system for the full revolution,  because, as we know, the flywheel has to soak up excess energy from the engine for part of the revolution, in order to have energy to return to the system during the rest of the revolution.

The engine torque has a sinusoidal form.  Well, a scotch crank torque characteristic is sinusoidal, a cross head and crank gives a bit more complex form, but a sine form is close enough approximation.  As you know from working with AC electric power, the steady output equivalent to the work done by the sine form is the r.m.s value, which is 1/square root of 2 or 0.707 times the peak value.  If we look up the sine tables (or use trigonometry) and find the angle whose sine is 0.707, we find it is 45 degrees.  So for the first 45 degrees, the engine output is less than the average, while the second 45 degrees, it is greater.  For the next 45 degrees, it is still above the average, but decreasing and for the last 45 degrees of the half revolution it is again less.  Similarly for the remaining half of a full revolution.  So for half the revolution, the engine power is more than average, and for the other half it is less.  For half of the revolution, the flywheel soaks up the excess energy by speeding up, then for the second half, the flywheel returns this energy to the system while slowing down.

If we redo the calculations based on the speed reduction occurring in only half of the revolution, and increasing during the other half, we find the engine supplies the equivalent of 17 watts excess power during the high torque half of the cycle, while the flywheel returns the equivalent of 17 watts during the low torque part of the cycle. 

The steady torque required to slow or accelerate the flywheel within half a revolution is 0.3 N.m.  You could calculate whether your engine is able to produce torque of this order or more by calculating the force on the piston at your supply pressure and multiplying by the crank throw.

Remember these calculations are not based on any engine measurements, but simply the values at which the speed variation equals the assumed 10% with the given moment of inertia.  But I believe it means the flywheel would achieve this performance for an engine with an average power considerably larger than that 17 watts, as the figure is based on a speed variation of only +/-10%.

I suppose it might be possible to estimate the engine power from these calculations, provided we increased the moment of inertia to account for the effect of the engine rotating parts.  But the difficulty of measuring the speed variation within the revolution means it is probably not practical  for most of us.  However, it seems to demonstrate that the flywheel should be adequate for your engine.

One more factor influencing the required size of the flywheel is the load characteristic.  If the load imposes a constant torque throughout each revolution, such as a winch, or loaded conveyor, or your mill stones, the load exceeds the engine output by a considerable amount at some points of the revolution.  If however the load is say a reciprocating pump, the load torque is also zero twice per revolution.  If this characteristic is connected with the zeros at the same orientation, and compared with the engine torque, there will be hardly any deficiency all around, and the flywheel will only have to overcome friction to keep the system moving.

On the other hand, a load which varies with speed such as a centrifugal pump or fan will impose less torque during the slower parts of the revolution and more torque when the speed is faster.  This characteristic will tend to reduce the required flywheel inertia a little.

Well, some quite theoretical calculations, but only reliant on Newton's laws of motion, which in turn rely on the fundamental conservation laws of physics.  I can give more details of the calculations if anyone is interested, but the main purpose in presenting this is to give a better understanding of how the flywheel does its job.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on April 29, 2018, 10:40:53 AM
Thanks MJM....."Ideally you would use only four bearings, with the two couplings supporting a floating section"

Just a point of clarification, this is the case...two [2] plummer blocks are to be the output shaft restraint elements on either side of the engine....or essentially a part of the engine

The paddle wheels will be supported on either side of each wheel...[so these are the four [4] bearings you suggest]...then the resilient coupling on each side of the chain drive shaft ......[remembering this is not the engine crankshaft]

At this stage in time, it is little difficult to setup and photograph the final drive....

I take your point thankyou ....so for the final alignment of the shaft, I don't believe rigidity of the hull will be the issue, however I can see me asking a colleague with the facility of a milling machine table large enough to accept & setup the hull & then take a witness mill cut  :hammerbash: over the four [4] plummer block mounting surfaces 

Derek
__________________________

PS1......can you imagine our Shipwrights 100 years ago on the Murray Riverbanks with 30ft of rubber hose with a glass tube in each end [under the hull of the a Paddler being built] and comparing the level of the water at each bearing support? .....I cannot think of any alternate method!

PS2.....yes, but I still need to close my eyes to accept the brass bar [true & = & on axis] provides that uniform harmonic rotational weight
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 29, 2018, 01:39:46 PM
Hi Derek, I am most interested to understand your drive train and how you are using those couplings, so I will be keenly following your progress.  So, as I understand it, the paddle wheels are each supported by a bearing each side, and the chain wheel is on the floating section between them, supported only by the two flexible couplings.   Are the flexible couplings suitable for the side loading by the chain and that extra flywheel weight?

In addition to the hose and water, I assume that those shipwrights would have used a tight wire stretched across between the outer bearings and measured the centre ones from that.  I once had a boss who was involved in alignment of the machinery in our Snowy Mountains hydro project, (many years earlier) and he often told us how the turbines were all aligned by plumb bob and tight wires.  It would have involved considerable skill.  However, I wonder if your model bearings can be aligned by threading the four bearings on a straight rod just enough under diameter to allow the bearings to slide into place, rather than needing that milling machine.

There is no need to close your eyes imagining that bar.  Perhaps the best way to demonstrate the situation is by carefully marking and drilling a bar at its centroid, a piece of steel say 150 mm long, or even a piece of 4 x 2, so you can suspend it on a piece of rod held horizontal, or even the parallel part of the drill shank.  If you have drilled it at the right place, it will balance on that rod, otherwise balance it up with a touch of the sander.  It will balance with the long dimension vertical or horizontal, or anywhere in between.  If you give it a spin you will feel no rotating force, and it will even demonstrate gyroscopic effects, so long as it is not too long for your arms.  I suggest you just try it.  A more practical example is a two bladed aeroplane propellor, which has to be well balanced on a high speed engine.  Similarly, a conventional ships propellor.

You don't in fact need a regular steel bar, any shape will balance at its centroid, and spin without any rotating force, it is a very interesting feature of the centre of mass and rotational motion.  Even your engineers square has a centroid, and will spin around that.  Of course in this case, the centroid is not on either of the arms, but in between them, so you would need some of Willy's perspex to make a brace between the arms to support a bearing at the centroid.  You may need to break out the old Meccano set to make up that one.

The big advantage of the conventional circular flywheel with spokes and a heavy rim is that it is the optimum shape to provide the largest moment of inertia from a given mass of material.  Alternatively it requires the smallest mass to provide a given moment of inertia.  In addition, it experiences less air resistance than other shapes, due to relatively streamlined shape.  Even with a circular flywheel, the energy used to create the turbulence can produce quite a bit of heat if the guard does not allow sufficient ventilation.  This is a significant issue on large, high speed couplings, despite provisions to fill the space around bolts to minimise turbulence.  (Interesting that that line of thought brought us back to couplings, which have many of the properties of small flywheels, I must still be thinking about them in the back of my mind!)

Obviously a conventional flywheel is more appropriate on our models for many reasons, but it is more difficult to mock up on an experimental basis.  The idea of calculating the moment of inertia of a bar, which can easily be adjusted, and using this to find a suitable moment of inertia for a new engine was simply to provide a method of quickly and cheaply arriving at a suitable flywheel size that can be compared with available flywheel sizes before purchase.  The number does not have to be very precise, the hub and spokes contribute minimal moment of inertia, so just calculate the the moment of inertia of the rim of the available flywheels and compare with the moment of inertia of the bar.  Remember recently, Brian was considering whether to make his flywheels four inch diameter or four and a half from material at hand before he finally purchased two.  I had not thought of using a bar at the time, we can thank Willy for his thinking outside the box, so had not worked it through to suggest it at the time.

A bit shorter post tonight to make up for yesterday,

Thanks for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 30, 2018, 02:45:24 AM
Hi MJM, Thanks for the latest posts and answers to my questions.. I suppose these are the same questions that people have asked in the past and then did lots of experiments to find the answers. !! every time i ask you something you provide the answer which i am grateful for as it would take a lot of reading to find out myself !! the info you provide would be quite difficult to find out from a book as you cannot ask a book questions unfortunately .....I was having a bath tonight and i filled it a bit too full and a bit too hot ,so i was wondering if you have a quantity of water at a certain temperature ,it would take a length of time to cool down. but if you had half the amount of water at the same temp with the same ambient conditions would it take half the time to cool down the same amount ??
The question being if i emptied the water to the correct level would i not have to wait so long for it to reach the more comfortable level..?? Also if you used a bar to get a good performance ,could you use that info to design an adequate flywheel to give the same result and is there a formulae for this ??
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 30, 2018, 11:16:26 AM
Hi Willy, thank you for some feedback on my answers.  As you get to know enough to understand  which topics to look for in text books, and how to use the information, it becomes easier, and quicker.  Hence my reluctance to give a yes or no answer, I feel it is important to understand the why, rather than just learn facts.  Of course long winded answers are no use if they are not understood, so I do hope that my answers are helpful in increasing understanding.

Your bath problem is a typical heat transfer problem which shows some of the complexity in arriving at an exact answer.  The basic approach is to start with the usual equation, that says heat loss is proportional to area, temperature difference and heat transfer coefficient.

Each litre of water, (close enough to 1 kg), has to lose 4.185 Joules of energy to cool one degree.  So if you release half the volume of water down the drain, you have only to wait for half the heat loss required to be lost from the full tub, but that is not the same as half the time.

Cooling is an unsteady process, meaning that the temperature is changing as the process proceeds, so the temperature difference reduces during cooling, which is the reason your coffee cup cooling and your boiler cooling curves have their particular shape.  As you will not want to reduce the temperature all the way to room temperature you will be operating on the steeper part of the curve.

Heat is lost from your bath by a combination of convection from the walls and bottom of the bath tub, and evaporation from the surface.  How much each contributes depends mostly on the tub installation.  If the bath is built in with a wooden frame, some insulation around the tub and perhaps tiles to finish the now straight and regular shape, convection may not be very important due to the low transfer through that construction.   However, if it is an old fashioned galvanised steel tub, on cast iron feet in the centre of the room, convection would be much more important.  The heat lost from the surface by evaporation would be similar regardless of the installation, but would be significantly influenced by any draft which increased air circulation.  So the heat transfer coefficient is not really the same when you lower the level.

Now a bath tub is not usually exactly rectangular, but it is reasonable to assume the vertical sides area would be approximately halved in height, so that halves the heat loss in that area.  Again, the heat transfer area is not the same as you lower the level.  The bottom is not changed.  On the other hand the area for evaporation is not significantly changed, though the surface is a little more sheltered from air movement. 

So you can see the area for heat loss is reduced bit not exactly proportional, and the overall effect depends on the bath installation.  I would be reasonably confident that the remaining water would take less time to cool, but not as little as half.

Finally, it is worth remembering that the heat in that water has been paid for through your utility bill.  If you run the excess water down the drain, that heat is lost with the water.  If you wait for the bath to cool, the heat is retained in the house, and contributes to your comfort.  If you have a thermostat controlling the heating, the excess heat in the water would definitely tend to be off set by a reduction in heat used for house heating. 

If we delve a little deeper into theoretical considerations, cast your mind back to your cup of our coffee and the teaspoons, you could use a large number of teaspoons, or perhaps a lump of cast iron to lower the temperature, but retain the total amount of heat, (providing the bath did not overflow!) but probably not worth the risk of injuring your back or chipping the bath enamel.  Better to lower the level by pumping the excess water to a tub in the lounge room, (obviously using a steam engine driven pump) where it would help heating the house, avoid the overflow risk, and by using a pump and hose, minimise the risk of spoiling the floor by spilling water.  Ok,  getting silly again, but I hope that helps with understanding the concepts of heat compared with temperature, and how the various factors affect heat transfer and heat transfer rates.

With regard to your question about flywheels, the answer is very clearly yes.  I am still admiring your lateral thinking in using a bar for extra inertia to get our engine running.  It did not add to the energy driving the engine, but encouraged you to tweak the timing and eventually get it running without the bar.  Extending that thought to a practical method of determining the required flywheel for a new engine is the reason I have continued discussing that idea.

This is how it works.  By drilling and reaming a rectangular bar to suit your engine shaft right at it's centre of mass, and adding a set screw to hold it in place, you can quickly produce a flywheel of any desired moment of inertia.  Not too much of a chore to produce a larger one if it seems too small, or a smaller one if that seems desirable.

I would suggest a starting point is to sketch a flywheel that "looks about right", and calculate the moment of inertia of the rim.  This is a simple calculation using one formula twice.  First to calculate the moment of inertia of a solid disk the same o.d. as you assumed flywheel, then the same formula a second time for the inertia of a disk the same diameter as the i.d. of your rim and subtract this from the the first answer.  Don't worry about spokes and hub.  The disk formula is
  I = 1/2 x M x R^2.  R is the radius of the rim, or half the diameter. 

For mass, calculate the volume of the disk, and assume a density of about 7200 kg per cubic meter for C.I. or 7800 kg per cubic metre for steel.  Even  use 2800 kg per cubic meter for aluminium.  Use metres for the diameter and width measurements.

Now calculate the size of a rectangular bar of the same moment of inertia.  Use the formula for a bar about its centroid, I = 1/12 x M x L^2.  L is the length of the bar, M is the mass.  Again for the mass, calculate the volume of the bar, and use the appropriate density.  Cut a suitable size bar from whatever you have available, drill and ream at its centre, use a sander or file to balance the bar on your shaft, and weigh it to check the density.  Use the actual weight to update your bar calculation.  Then install it on your engine. 

The flywheel is large enough if the engine runs smoothly to your satisfaction, at the slowest speed you require.  Remember, the slower you want to run, the more moment of inertia you need. 

You can easily increase the moment of inertia of your bar by bolting a short length to each end, using bolts in shear (parallel to the shaft) with nuts or tapped holes, and making sure you keep the assembly in balance.  Each add on mass adds M x R^2 to the inertia, where R is the radius to the centre of mass of the added mass.  Or if necessary you can start again with a longer or wider bar.

If the engine runs very smoothly you may want to try a lighter flywheel, so start again with a shorter bar.  However, as you started with a size for a flywheel that looked right, there is probably no need to go smaller, simply demonstrate that it is adequate.

I hope you can see that this procedure gives a quick and cheap way to determine the moment of inertia you need.  If you intend to drive a load, you may need to try another temporary flywheel to provide more moment of inertia, depending on the load characteristic, if the load device was not available for your initial testing.  I know they I always want to prove I have a runner at the earliest opportunity.

I hope that answers the question in a way that you can use.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 01, 2018, 02:19:16 AM
Hi MJM, just a quick question   (and is the length of the answer always inversely proportional to the question!!!) does anything connected to the flywheel {large gears} ? add to the MoI ?.........
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 01, 2018, 01:22:34 PM
Hi Willy, now that is two questions, surely you don't expect both answers to be short?  Oh well, perhaps I could try, so here goes.

The answers don't have to be long, but you have a real talent for asking complex questions, at least more complex than they look.  But don't give up, simple questions would make for a boring thread.

Yes, every part of the rotating system has its own moment of inertia, and they all add up to contribute to the total.  The crank shaft, balance weights, couplings, the whole lot.  If there are gears in the train with a ratio other than 1:1, allowance has to be made for the speed.  Alternators normally run much slower than a turbine driving them, but the low speed means their moment of inertia has less influence on the high speed turbine, as less energy is stored at low speed.  But a high speed centrifugal compressor driven by a normal electric motor has much more influence relative to its mass and moment of inertia.

I do hope the answers are both short enough, and complete enough, but please ask if there is something I seem to have left out, or if the short answer prompts more questions.  But in the spirit of the questions, I will give everyone's brain a rest by making it a short night.  Might even get to bed a bit earlier myself.

Thanks for looking in,

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 01, 2018, 03:14:57 PM
Hi MJM , I don't mind long answers as everything is relevant !  I was just making an observation !!!! ;D  also i usually make these posts just before i go to bed so am a bit tired and things just pop out of my brain and get typed/triped out randomly !!! So todays question does everything in the motion train have a positive MoI ? as you say  'add up  to' so does that actually mean an increase?
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 02, 2018, 08:35:13 AM
Hi Willy, I may be back to inversely proportional on this one.  I hope I am passing the relevance test.

First, the simple answer is yes, you simply add the moment of inertia for each component to get the total, so long as they all rotate at the same speed.  Just the same as you add the mass of all the components to calculate the total mass in a linear system.

In fact, the moment of inertia of any object is calculated by adding the moment of inertia of each tiny element of the object, down to crystals or even atoms if you must, but the process of addition is "simplified" by the mathematical process of integration.  I am sure you don't want me to go into integral mathematics.  However, in simple words, it is a process of adding the contribution of many small components.

The formula for every component, no matter how small is M x R^2, where R is the distance from the axis of rotation.  The formula for a solid disk I = 1/2 x M x R^2 is simply the result of that integration performed over the whole disk.

For our purposes, rather than perform the integration for a complex shape, we can often use known results for a few simple shapes and add them all up.  The known formulae are normally about the centre of mass (which is always the smallest moment of inertia in that plane of rotation) or some other defined axis of rotation.  For a different parallel axis, you add the moment of inertia about its centre, and the moment of inertia of that mass about the required axis, M x s^2.  Where s is the perpendicular distance between the axis through the centre and the required centre.

If you want to spin your flywheel like a coin on edge, there is a different formula for that axis.

Now that formula always results in a positive answer, there is no negative moment of inertia.  Mass is always positive, and even if your chosen reference point for zero results in a negative R, R^2 is always positive, so M x R^2 is always positive.

However, the easiest way to calculate the moment of inertia for a rim is to imagine you first make a solid disk, then remove the centre.  The moment of inertia of the centre you remove can be subtracted from the larger disk to leave the moment of inertia of the rim.

You continue building up the moment of inertia of a flywheel by adding on the spokes and the hub, or subtracting the moment of inertia of any part you machine away from your initial solid disk.

You would use a similar procedure if you wanted to make a novelty engine with your set square for a flywheel, but including a guard would be a good idea to save fingers.

Note also that that formula does not include g, or any contribution from gravity.  Your flywheel has the same moment of inertia on the moon or Mars or wherever.  In fact it may work a little better on the moon, as there would be little or no air resistance, and the lower gravity means less force on the bearings due to the mass, so less friction.  Give it a spin and it should go for a very long time.

By the way, the discussion since I first looked at this topic has prodded me into going back to my post #849 on 21 April, (on page 57 with my forum settings) on calculating flywheel moment of inertia, and editing it to try and clarify a few points that were badly or incompletely expressed, if not just wrong.  I feel it is better to go back and edit, rather than leave errors stand with corrections inserted many pages later.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on May 02, 2018, 12:16:36 PM
Just a small observation on the comment.....

"if not just wrong.  I feel it is better to go back and edit, rather than leave errors stand with corrections inserted many pages later"

Absolutely......100 years ago we read, studied & learnt by fixed paper text....today with the advantage of live electronic documents, any revision as deemed necessary is to the advantage or the new reader if revised in the original body of the text

This could be simply achieved by inserting the revised text as...eg., jhbs  dvhdv  bsdbhb  Color or Font change

This revision need not reference date or change to the original text, however is a backstop to alert any new reader accessing an older version of the document that may have been stored on an external facility...[zip or disc drive]

Derek
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 02, 2018, 11:50:04 PM
Hi MJM, wondering about the correlation between the flywheel MoI helping the engine go over top dead centre and the revolving load 's MoI trying to stop it ?? or is that incorrect /silly ??? I am thinking this should be a relevant point to consider ?..thanks....Would it be helpful/quicker to use a planimeter to do some of these calculations ?
Willy,
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 03, 2018, 11:43:22 AM
Hi Derek, it is quite a good feature to be able to go back and modify a post when necessary.  Then anyone who jumps in using the search box gets the latest information instead of having to wonder if any corrections were ever posted.  So long as they bookmark the post and do not print or save the text.  Another world from where you and I grew up, with only paper books.  I put a lot of effort into avoiding errors, and correct them when I notice them, however, sometimes the wording could have been clearer when read in the light of a new day.  If you see any other posts which need updates please let me know.

Hi Willy, right on topic and clearly describes the issue you are grappling with, so not silly at all.  Let me try and make it all clearer.  It is not as easy as the short question might make it look.

First it does not matter whether the moment of inertia is part of the engine, the flywheel, or the driven load, it is only the total moment of inertia that matters. 

It is the same in a linear system, one that is more familiar to most of us.  Suppose you are pushing a load on a cart along a smooth level path.  If we assume you have nice ball bearing wheels with negligible friction, only the total load that matters.  It does not matter if the load is a heavy iron cart with a load of a few aluminium blocks, or an aluminium cart with a load of iron, (providing the cart is strong enough!), or even a long plank with a weight on each end.  The equation is F = M x a, or force equals mass times acceleration.  There is no velocity term, velocity does not matter, only acceleration or change of velocity.

You have to push to get the cart moving, because that involves changing the cart's momentum, but once it is moving at your walking pace, you only have to overcome that negligible friction to keep it going.

If you are feeling fit and decide to break into a run, you have to push again to increase the speed and momentum of the cart to your running speed, but then, to keep it going at constant speed, again you only have to overcome that negligible friction.  Do young mum's push those jogging prams in your area?  They make it look effortless, on the flat!   But stay off hills and corners.  We will get back to corners.

You can analyse the system by looking at kinetic energy, 1/2 x mass x velocity squared, this tells you how much work that kinetic energy can do before the system comes to a stop, but it does not tell you much about the time it will take to stop. 

You can also analyse the system by looking at momentum.  Momentum = Mass times velocity.  It is not hard to demonstrate that change of momentum involves force.  In fact change of momentum per unit time equals force.  If you measure the time in seconds that the known mass takes to stop from a known speed, you can calculate the magnitude of the resistance to motion.  Conservation of momentum and conservation of energy provide two independent equations which can be used to analyse a linear dynamic system.

A rotating system is very similar, quite accurately described as analogous.  If you replace mass with Moment of Inertia, and linear velocity with angular velocity, then replace force by torque the same equations apply.  Of course if you use a plank with a mass attached to each end, it is still only the moment of inertia that counts, but in this case the change will be much higher in proportion to the mass of the plank and weights.

So, to try and keep to your specific question, the moment of inertia of the whole system contributes to the angular momentum and kinetic energy, and is important only while the rotational speed is changing.  It stores energy and increases in momentum when there is excess torque available, and returns energy and decreases in momentum when the engine torque is insufficient.  The driven load is trying to slow the whole system all the time, not just at the dead centres, unless the load is also a reciprocating machine. 

The moment of inertia only resists the increase of velocity as it builds angular momentum and rotational kinetic energy when the engine has enough torque to accelerate the system.  But, at the top and bottom dead centres, the system is actually slowing down, so all that the momentum and kinetic energy stored by that moment of inertia is actually applying the positive torque to carry the engine through those dead centres.  The more moment of inertia, the greater the positive torque and less slowing down. 

Now I said earlier that we would get back to corners, but that requires quite a bit more information, so it would be a very long post.  I will save that topic for tomorrow.

You also asked about planimeters.  Integration gives the area under a line in a manner sort of the reverse to differentiation giving the slope of a line.  One way of doing integration would be to plot the curve of the line on graph paper and determine the area under it.  I did learn to use a planimeter, but they were too expensive for me to own, so I generally counted squares on the graph paper.  Where the equation to the line is known, the answers to the integration process are also well known, so the mathematical method would not only be quicker, but more accurate.  A planimeter would come into its own if the equation of the line is not known, say for an indicator diagram, though these days a planimeter is superseded by computer programs which calculate the area enclosed by a line traced out on a digitising pad or perhaps even a touch screen, or better still a photograph.  Or, in absence of a suitable program, just by counting squares.

I hope that answers your question without being too long,

Thanks for following along,

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 04, 2018, 03:05:01 AM
Hi MJM, thanks for the latest posts, a bit busy with the council elections and leafleting and the allotment as it has stopped raining at last...so no questions tonight !! I don't mind long answers actually !! I did ask a question a few posts ago about a steam engine that has the fire heating the cylinder and water being injected, so dispensing with the boiler ?? i have not heard of one ,but would this work ??
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 04, 2018, 12:51:25 PM
Hi Willy, just as well there is no new question, as I was going to continue on the effect on a spinning system of going around corners.

That engine, are you thinking of the Hargreaves engine?  Or a different one.  It actually had the combustion gases entering the cylinder if I remember, so not quite what you asked.  If you think about the heat transfer area necessary to raise enough steam in a boiler, it is obvious that there is not enough area on the outside of a cylinder to raise enough steam.  Was it the Trevanick(?) that had the cylinder inside the boiler.  That would give a suitable amount of area and give an idea of the proportions needed, but could be described as applying the fire to the outside of a (jacketed) cylinder.

Yesterday I said I would return to the effect of going around corners, or turning the axis of a rotating system.

Velocity is a vector, which means it has both magnitude and direction.  So a change of direction is a change of velocity, even when it's magnitude, the speed, stays constant.  Force is also a vector, so also has magnitude and direction.   So to continue moving in the new direction, the force also has to change direction.  Fortunately, the direction of force and velocity is quite obvious.

Now, angular velocity is also a vector and has an associated direction.  Similarly torque is a vector, as is an angle of rotation, and each not only has magnitude, but also has an associated direction.  Of course you will immediately think of rotating in a clockwise or an anticlockwise direction, but those actually are different magnitudes.  If you take clockwise as the positive direction, then anticlockwise is just a negative magnitude, but the direction of the axis of rotation is still the same.  This is not easy to imagine.  The direction of a rotation is defined as the direction of the axis of rotation.  The positive direction is defined by a "right hand rule".  If you wrap your right hand fingers around the shaft in the direction the object is spinning, the positive direction is defined by the direction your thumb is pointing.  Similarly for a torque, the direction is the direction of the axis about which the torque applies.  And again the right hand rule defines the positive direction of a torque.

These definitions are somewhat arbitrary.  Other definitions could be used, so long as the same definition is applied everywhere the direction applies.

If we look then at momentum, momentum equals mass times velocity.  Mass is not a vector and only has magnitude.  But momentum is a vector like the velocity and has the same direction as the velocity.  However, when we look at Newton's law, and conservation of momentum, if we apply a force to something moving the direction of the force and the velocity interact to change the direction of the momentum in the direction of the force.  The equation involves the cosine of the angle between the direction of the force and the velocity, but the result is reasonably intuitive.  It is a two dimensional system which can be solved by arranging the vectors in a triangle.


So how does this work in a rotating system?  Suppose your flywheel rotates on an axis that can change direction.  Think of the front wheel of a bicycle.  The direction of the angular momentum of the wheel is to the left for forward motion.  If you apply a torque to change this direction using the handlebars (assume the stem is close enough to vertical).  Using that right hand rule, the direction of the torque (for turning left) is vertical upwards.  The torque will cause a change of direction of the angular momentum of the wheel.   It becomes a three dimensional problem.  (Another right hand rule is used, involving the thumb, first and second fingers to show the direction of that rotation, but here it gets more complicated, because you have to multiply the torque and angle turned in the right order, to get the right direction.  That requires delving further into vector maths than I intended, so I hope it sufices to say that this process is the mathematics behind precession of a gyroscope.)

The simplest way to visualise and then solve the problem is probably to take the front wheel off your bicycle, hold the ends of the axle with the wheel between your outstretched arms.  Start the wheel turning, then try to change its direction by turning the axle.  Even a slow turning wheel will tell you the answer very quickly.  That is a very minimal description of the procedure, but is probably more than enough vector maths for most of us.

Ok, I got a bit of the subject of yesterday's question, but I mentioned gyroscopic effects in a recent post, so I wanted to touch on the topic somewhere.  But it is about change of momentum!  I hope you found it of interest.

Thanks for looking in,

MJM460

PS Some aspects of this post as originally made did not stand up to re-reading the next day.  So I have removed the unhelpful wording and had another go at discussing these ideas in the next post.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 05, 2018, 12:32:08 PM
I often think of this thread as a build log, as in building a knowledge base.  Well, what is a build log without the odd day where the parts so carefully crafted are consigned to the scrap bin?  Yesterday seems to be one of those days.  It was a bit like engaging the cross feed instead of the longitudinal feed, but was revealed much more like slow motion.  Amazing how the subconscious works while you are asleep.  I hope no one has spent too much time trying to follow the detail of yesterday's post.  Please bear with me while I have another go.

Basically, if you apply a torque to a spinning object there are two possibilities.  If the torque and angular momentum of the object both have the same direction, then the rotational analogy of Newton's law applies and torque equals moment of inertia times angular acceleration.  The object spins faster or slower, depending on whether the torque is positive (accelerating) or negative (retarding) relative to the spinning object.

However, if you apply a torque that is at right angles to the axis of rotation, the speed of rotation of the object does not change, but only its direction.  Newton's law still applies, but must be expressed in vector terms.  The direction of angular acceleration in this case is at right angles to both the torque and angular momentum.  The spinning wheel moves in response to the torque in an unexpected direction. 

There are two ways of doing vector multiplication.  One is usually called a dot product, A . B = scalar result.  For example work (not a vector) is the dot product of two vectors Force and distance.  This product obeys the normal rules, A . B = B . A.  The other way is called a cross product and results in a vector answer.  Unfortunately A x B = - B x A, so the multiplication has to be done in the correct order.

There is a second right hand rule, involving the thumb, forefinger and middle finger, which gives the direction for that angular acceleration in accordance with the rules of vector multiplication.  Torque and angular momentum use the cross product and result in a vector result, angular acceleration.   A simple experiment with a bicycle wheel is probably the easiest way to find the correct direction.  Then, remember whether the torque is the thumb or forefinger, the other is the direction of the angular momentum, while the middle finger then points in the direction of the angular acceleration, so tells you which way the wheel will move.

This is the basis for the unexpected actions of a gyroscope or a spinning bicycle wheel held by the ends of the axle.  You can also feel it with a large portable circular or angle grinder.  The unexpected reaction to movement of spinning wheels adds to the danger of handling them.

If the applied torque is neither parallel or at right angles to the angular momentum, you can use a force or torque triangle to resolve the torque vector into two components, one in the direction of the angular momentum and one at right angles.  You can then apply the effects of each component separately.  The parallel component will change the spin speed, while the component at right angles will change the direction of the spin axis.

I hope that makes yesterday's topic a little clearer.  I have gone back and edited yesterday's post to  improve it a little, and referred to today's post for additional explanation.  It wasn't too bad.  I have removed a doubtful clause and a couple of sentences which did not help convey anything useful.  I hope that will be adequate to convey the best description I can, for a topic that is often seen as mysterious, and to avoid sending anyone the wrong direction.

Thanks for following along,

MJM460


Title: Re: Talking Thermodynamics
Post by: Admiral_dk on May 05, 2018, 08:42:48 PM
You talking about dot products and the hand finger method for the resulting vector, really reminds about how much I have forgotten of my study years  ::)
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 05, 2018, 11:09:44 PM
Hi MJM talking about  'right hand rules'   when i studied electronics we also had a "Flemmings right hand and left hand rules' !! So there must be quite a lot more in different  sciences !!! No questions today just an observation about locomotives using propellers for the drive ??? and Los angeles painting the roads white to stop the black tarmac absorbing the suns heat and giving it back strait away to make everybody feel very hot ??!!!  Vectors and convection ...any correlation ??  Oh dear i seem to have posted a few questions actually !!!!!
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 06, 2018, 12:45:21 PM
Hi Admiral DK, I think most of us are in that category, but it is surprising what will rise again to the surface when suitably prompted.  We are all taught about vectors having magnitude and direction, and we learn from Newton's law, F = m x a.  I often wondered how to apply this to a change of direction but never had time to really sort it out.  Easy for the linear case, but harder to grasp for the rotational analogy.  More recently, thinking about vectors, I made some connections.  Retirement has given me time to assimilate some of the information I gained from education and experience, and make some appropriate connections.  Definitely not there yet, plenty still to keep me going a long time.

Hi Willy, quite a collection of thoughts in different areas today.  My textbooks only have a Fleming's right hand rule.  And it is definitely used in the context of the vector cross product in relation to forces on current carrying conductors in magnetic fields.   The right hand rule goes with the right handed x-y-z coordinate system.  I have not heard of a left hand rule, but that does not mean there isn't one.

Propellor driven locomotives are not exactly intuitive, but some say neither is flying heavier than air aeroplanes, especially those stunt planes that can fly vertically upwards.  A propellor is a thrust producing device.  The momentum of the air is increased by the propellor which applies a force to the air, and the propellor experiences an opposite thrust from the air.  So if the propellor can change the velocity of a great enough mass of air, the thrust is enough to drive a boat, hovercraft, aeroplane or, I guess, even a locomotive.  But you would need to keep well clear of that propellor.  However, thrust is a force, it has direction so is a vector.

Painting roads?  A white reflective coating would reduce the heat absorption by the asphalt, which in return would reduce the convection heating of air.  The reflected radiant heat is not so well absorbed by the air, so some eventually gets back into space.  Even if it is all trapped in the atmosphere, it is better distributed, so you don't get quite such high localised temperatures in the ground level air we occupy.  So it would add to comfort, though it only helps lessen global warming to the extent that some of the reliant heat is reflected back into space.  You might be interested to know that despite us using high temperature formulations for road making here, it actually starts to melt on a sunny day, so perhaps painting it white would help.  But the glare would be horrific, especially early and late in the day.  But I don't believe temperature and heat are classified as vectors, so another topic really, even though heat does transfer in the direction of the temperature gradient.

The interaction of torque and angular momentum produces an angular rotation at right angles to both the torque and the angular momentum suggested by the cross product of the vectors, 

If we use the equation for change of momentum per unit time (=T) and the equation of motion, change of angular velocity equals angular acceleration times time, we can use a bit of algebra to demonstrate that Torque is proportional to the angular velocity times angular acceleration.  In vector terms, the multiplication using the cross product definition seems to give the correct direction of precession providing you multiply omega cross alpha, and not the other way around.  (Remember alpha is the rate of change of angular velocity, or angular acceleration.)

For those interested, using w as omega or angular velocity, and subscripts 2 and 1 for final and initial values.  Then a as alpha for rate of change of angular velocity, T for torque and t for time, I for moment of inertia and use * for multiply to avoid confusion with the two vector operations.  We have the following equations-

 w2 = w1 + a * t

T = I /t * (w2 - w1)

If we substitute the first equation in the second, we find T = I/t w1* a

As w and a are vectors we need to use vector operations, either dot product or cross product.  The physical phenomena seems to require the cross product option.  So T = I/t * w1 x a.  With the operation in this order, the thumb points in the direction of w1 the index finger the direction of the applied torque and middle finger indicates the direction of the torque.  Sorry about the w and a, instead of omega and alpha, it may make more sense if you write it out with a pencil using your preferred notation.

For those shy of a little maths, that last little section can be skipped over.

Thanks for following along.

MJM460



Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 06, 2018, 11:04:23 PM
Hi MJM, I found a rule at the car boot sale today and have learnt a new word that i have never seen before  Intumescent  But i don't know what the compound actually is ??
Perhaps you could en light en us !!!!!!
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 07, 2018, 01:25:00 PM
Hi Willy, I like the general direction of your questions.  (Like vectors they have direction as well as difficulty).  Direction being away from vectors and all that maths!

Intumescent coatings.  Like Admiral DK, I find myself reminded of things that I had forgotten that I once knew.  I have been deep into compressors and the like for so long, I had forgotten all about that stuff.

We used to use intumescent coatings back in the 70's.  It is a fireproofing coating used to provide a degree of protection from extreme heat.  Not a real long term answer, and no where near as good as concrete and other materials applied over steel to increase the time before the steel starts melting, leading to structures collapsing.

Concrete is heavy and not very practical for things that have to be opened such as metal cable trays in overhead pipe racks.  The intumescent coating is more like a thick tarry paint, but when exposed to fire, it reacts and expands to make a short term fire barrier. 

You would not use it instead of concrete, but even quite a small fire can melt a hundred cables in a steel cable tray.  It takes minutes to extinguish the fire and a week or more to get all the cables reconnected and working.  A relatively minor event shuts down half the plant, though a fire in a hydrocarbon processing plant is never considered a minor event.

A layer of that coating is not much heavier than thick paint if you need to get into the cable tray, but if it gives you 20 - 30 minutes before the cables melt, it could be well worthwhile.  I really don't know how well it worked, or if it is still in use.  Sometimes those things seem like a good idea, but turn out to have unanticipated disadvantages.  I also don't know what the compound actually is, but I assume it is something that reacts or breaks down on exposure to heat to make some gas which expands the product to improve its insulating value.

Some one else may be able to come in with more information on how well it works in an actual fire.

Another small topic which I have been intending to return to, is lubrication of steam engines, and the appropriate oil.  My engines are normally run on steam, and I include a displacement lubricator with each.  We have probably all heard or seen the oil company add which starts off with "Oils ain't oils...."  Unfortunately it does not do much to inform us about why their oil might be superior to the opposition's identical product, let alone why we might choose one type of oil over another, even within their own range.  And I always remember one major oil company that threw out my carefully prepared schedule of recommended lubricants (all of their manufacture) for the various equipment items in the plant we were just completing, insisting that it was too expensive to keep all those oils, they were only going to use one, the one they always use, and handled in bulk containers.  I wonder what they told their customers to buy?

However, I started out using standard light engine oil and sometimes machine oil in my lubricators.  Seemed to work ok, in that I had no seizures or other problems attributable to inadequate lubrication.  May not have run long enough for such problems to show up anyway, an oil can might have done the job.  But I started to notice that the liquid I drained from the lubricator, instead of being clear water followed by any remaining oil, was a bit of a grey emulsion.  Furthermore, the emulsion seemed quite stable and did not separate into separate oil and water layers, no matter how long I left it sitting.  It even gummed up the water outlet on the separator, which did not help its efficiency.

Now that emulsion settling and breaking time is an important property to look for in any oil you intend to use.  If you shake up some water with a bit of your oil in a clear container, you want it to separate reasonable quickly.

Running my engines again recently for those recent tests, (not analysed yet by the way) I found the slide valves had stuck, and would not seat to seal when the steam started coming through.  A stop valve probably would have helped by holding the steam in the boiler until there was a bit of pressure and the initial velocity might have had more chance of seating the valve, so I am currently drawing up a stop valve.  But when I took off the steam chest cover, I was greeted by a sticky mess  that took lots of WD40, rags, pipe cleaners etc to remove.  After re-assembly, the engine went as well as ever once again.

I managed to come by some "proper" steam oil, cleaned out my lubricators and used that for the most recent runs.  The first few runs showed quite a change in terms of improved emulsion  separation behaviour, and the last traces of the old oil were gone after a few runs.  Now I drain nearly clear liquid followed by some remaining oil, and the traces of emulsion break quite quickly, leaving nice looking oil floating on clear water.

Ok, now I have to add to my project list more carefully testing those oil behaviours and provide some measurements of times etc.  but for the moment, I am quite happy that the steam oil is definitely an improvement.  After a run, the remainder in the lubricator separates into two distinct layers quickly, and the valve and steam chest appear nicely lubricated instead of gummed up with sticky emulsion.  Clearly different behaviour to normal engine or machine oils.

I don't know much about the additives in different oils, but emulsion breakers are clearly present in the steam oil, perhaps a formulation more suited to the conditions in a steam engine.  Other additives speed air release in systems where there is a lot of turbulence, which leaves the oil full of tiny air bubbles, while others have a detergent action.  So it's worth a little effort to get something suited to your application.

When steam engines are run on air, displacement lubricators don't work, as they rely on condensation of some of the steam to float the oil into the steam line.  There are hydrostatic lubricators which can be used instead, and little pressure pumps.  I would be most interested to hear what people have used and their impressions about the success or otherwise of the systems they have tried.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 08, 2018, 12:59:57 AM
Hi MJM, i do have a can of steam oil...it is called COMPOUND STEEL OIL though ?? it is very gloopy and almost black/green in colour .I have used it with the electrically heated boiler to power different engines and have not noticed any problems with it. It takes a long time to drain and almost doesn't  completely !! Could you find out what the 680  refers to please...

Willy
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on May 08, 2018, 01:51:56 AM
Could you find out what the 680  refers to please...

Willy 680 is the ISO grade. 680 is recommended for higher pressures see:
http://www.lsc-online.com/steam-cylinder-oil/

Dan
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 08, 2018, 12:42:20 PM
Hi Dan, good to hear from you again.  Thanks for coming in with an answer on that.  Do you know if the ISO viscosity grade (vg) is only the viscosity in centistokes at a defined temperature, or are there more conditions or regulations to meet?

In this case, the material data sheet specifies the viscosity as 684 centistokes at 40 deg C, so the 680 looks reasonably obvious, but despite having seven pages of regulatory compliance it does not mention ISO viscosity grade.  I wonder if they simply don't pay the appropriate fees.  Interestingly, they also make a 480 grade, which is actually over 500 Cs at 40, and a 1000 grade, specifically for non-condensing engines, specifically Zee's Stanley Steamer (note Chris has to build a workshop crane and a road truck now, so the Stanley is well down his list!)

Hi Willy, thanks for posting the picture of the can.  I did not know who manufactures this particular oil, but with a name, Mr Google came up with it immediately, with a local manufacturer here, in addition to the one near you, and some in other parts of the world.  With 5 litres on your shelf, it might be worth doing the obligatory search, downloading the safety data sheet from Morris, and you will have on file more ways of saying "nothing to see here" than you will ever need.  More seriously, if you ever feel unwell and wonder why, it will give you the information you need to eliminate your steam oil as a possible cause.   However, it also gives you the viscosity, recommended temperature range and recommended applications, (steam up to 175 psi and over 200 C) in addition to the regulatory safety data.  The high  viscosity is the reason it looks "goopy", but this will reduce at higher temperature in your cylinder, in a manner not specified on the data sheet, but not too much viscous drag on your engine.

I suspect your question is really about what "compounded" means, as the can clearly states it is steam oil.  Probably the spell checker does not believe you, a common problem with technical writing.  Interesting also that the can says "protect from frost".  This is telling us something about the composition without being very specific.  If you read that data sheet, you will find it is a hydrocarbon oil which has been hydro treated and solvent de-waxed before they add fatty compounds including tallow. 

Many petroleum oils come with hydrocarbon based paraffin waxes and similar, which cause the oil to set to a soap-like solid.  The presence of these compounds is indicated by the pour point, and I have seen oils with a pour point well into the ambient temperature range, which makes them difficult to handle.  It is not just an increase in viscosity, but a solidification in the form of a wax crystal structure which traps all the oil over a very small temperature range, rather like freezing.  Quite different from the gradual continuous change of viscosity with temperature.  Definitely looks and feels like a block of soap. 

So they de-wax with a solvent process to remove the paraffin wax, then add tallow and other compounds which improve the properties as a steam cylinder oil.  The hydro treated part means the base oil is treated with hydrogen at high pressure to saturate the double bonds, as in the term saturated hydrocarbon.  Converts ethylene into ethane, propylene into propane and so on.  I don't know if this also breaks the ring compounds.  Some of my compressors were part of a hydro treater plant, but now I am on the spot, I am not 100% sure of all that the process does.  We need input from a chemical engineer for that.

No doubt it is the tallow and other unspecified additives that require the protection from frost, as the de-waxed base oil should only show a viscosity increase at lower temperature.  I would guess that the term "compounded" is used to describe the total of all those things they do to make the oil more suitable for the specific purpose, a bit of a witches brew really.

I have tended to reserve its use for the lubricator and a drop on the piston and valve rods where they come into contact with steam, but the data sheet says it is also good for application on slides and pins by oil can, so it might be good for all those hard to lubricate pins on my diagonal engine.  I guess they all get hot enough to be worth using the special steam oil.  So thanks again for pointing me in the right direction.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: zeeprogrammer on May 08, 2018, 01:12:22 PM
specifically Zee's Stanley Steamer (note Chris has to build a workshop crane and a road truck now, so the Stanley is well down his list!)

I'm flattered that Chris has named his Stanley Steamer after me.  ;D
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on May 08, 2018, 02:15:38 PM
MJM, here is a comparison of the methods used to determine viscosity grades... I did not know that there were so many in existence.
http://www.tribology-abc.com/abc/viscosity.htm

Dan
Title: Re: Talking Thermodynamics
Post by: crueby on May 08, 2018, 02:27:20 PM
specifically Zee's Stanley Steamer (note Chris has to build a workshop crane and a road truck now, so the Stanley is well down his list!)

I'm flattered that Chris has named his Stanley Steamer after me.  ;D
And the Zanley Zeemer also needs special steam oil, already its trouble!   :Lol:
Title: Re: Talking Thermodynamics
Post by: Maryak on May 09, 2018, 12:19:47 AM
The higher viscosity grades, (680+), were normally reserved for use with superheated steam in the likes of Weirs Glissard Valves and are not usually compounded. Compounded normally meaning an emulsifier has been added to the base oil. Good for ensuring the oil gets where it needs to go but making it more difficult to remove from the feed in the hot well before returning to the boiler so better used on non-condensing engines.

Regards Bob
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 09, 2018, 12:02:16 PM
Hi Zee, glad to see you are looking in.  I hope the rest of the thread is also of interest.

Hi Dan, I guess if you are into tribology you could spend a lot of time comparing the different methods.  No doubt they each have advantages and disadvantages.  Unfortunately the data sheet does not specify the method they use, but centistokes at 40 degrees C will probably fit into any system.

Hi Chris, I don't know how you fit in checking so many other threads as well as achieving your amazing progress on the Marion.  I never miss reading a post.  I am learning so much from seeing your methods.

Hi Bob, glad to hear from you again.  I noticed the bit about the emulsifiers in the data sheet.  I have to look again at my observations on the oil drained from my lubricators.  I will try again with a clean glass collector vessel, and see if I can take a good photo.  I don't know if I was fooled by the nice clean oil colour at the surface compared with the grey mucky interface of the previous oils.  I don't know if my observation was mistaken, or if the emulsifiers break down at some point though that does not seem likely.  Perhaps it is some action of the other additives in the other oil.  The data sheet certainly says these oils should not be used if the condensate has to be cleaned for use as boiler feed. 

With no questions from Willy for a day or two, I went back to a list I prepared quite early on, a list of possible topics.  I think we have covered them all except the entry on noise and sound, and that came from Willy.  At the time, I think it was raised in the context of losses that reduce efficiency.  But we have compensated for that omission by covering quite a few extra topics.

I don't really have a lot to say about noise, but even quite loud noises generally involve very tiny numbers when measured in watts.  So while noise is evidence of inefficiency, the numbers don't seem very significant.  What is often more significant is a frequency analysis of noise, as the frequencies present are often important clues as to the actual noise source, just as vibration analysis also yields important clues to the source of the vibration.

This is supposed to be a knowledge base, but it is of not much use if the topics cannot easily be located.  I am going to try a change of pace and spend some time in the next few days, listing topics and post numbers/ dates.  When I have a reasonable listing, I will post the list, it might be possible to make it a "sticky" that stays at the top for easy reference.  While I am doing that, I will still be looking in each day, and will be very glad to look at new questions, or even more information on topics previously covered.  I don't really believe there are no further questions.  So if you have something you have observed and are wondering about, or something I have not explained clearly enough, please post a question.

I also have finally made that thermowell for my gas fired marine boiler, so I will do a test run to calculate the steam raised to add to data about the capacity of various types of boilers.  I don't have a coal fired boiler, so if anyone who has a coal fired boiler it would be great if you could do some steam raising tests and see if we can build up some data to help people decide how big a boiler they need.  That is a topic that has not really been completed, mostly due to lack of data.  I am also preparing some long steam pipes to try if I can do some pressure drop tests.  I will report on these as soon as I get some data.

Thanks to everyone for following along,

MJM460
 

Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 09, 2018, 01:41:37 PM
Hi MJM , I have been quite busy outside with the lovely weather, working at the allotment and socialising...I do have more questions but am always thinking they might be silly or they have been answered before !! so here goes.....If you had a gas in a container that was static and left for a long time  (100 years)  would the individual elements settle out into layers !!!???? Also when you use gas from a bottle it can cause frost to form on the outside . does this inhibit the efficiency and flow to the burner, and if the canister was well insulated would that be better for the burner to be more efficient ?? i don't know what i am trying to say here but it is just an observation !!! I am not spending so much time in the WKSP either ...Also can i still use the 680 oil in the low pressure engines with no adverse problems ??  Also if you did expose the 680 to frost what would happen to it ??   If one knows why and how things happen then one would respond and take heed sensibly !!!

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 10, 2018, 02:10:34 PM
Hi Willy, don't worry about whether you have asked a question before.  If my answer had been really clear, you would assimilate the information and know the answer when the question arose again.  If I have another go at an answer, it will be different words, as I am not into search, copy, past type answers, I prefer to just talk about the subject while providing the information.  And you don't normally ask silly questions. 

A mixture of gases left for a long time?  This is always an interesting question.  Basically, gas molecules do not occupy much space out of the total volume even when they are close to condensing.  And they move around very fast, and experience many collisions with each other and with the walls of the vessel.  The collisions with the walls are the cause of what we experience as pressure, while the collisions ensure exchange of energy so it is well distributed.  But basically, in a vessel, each gas occupies the whole volume as though it was the only one there.  I believe that is generally referred to as Dalton's law of partial pressures.  And in any moderate sized vessel, there is no difference in pressure between the top and the bottom.  Of course gravity has an effect, that is why air stays near our planet, and does not expand out until it is lost in space.  The difference in pressure between the top and bottom of a liquid column is easy to see, and can be calculated by pressure equals density times height times acceleration due to gravity.  The same formula applies for gases, but the density, instead of being 1000 kg/m^3 as it is for water, is nearer 1 kg/m^3 for air, at atmospheric pressure, so the difference in pressure with height is almost insignificant.  However it is real.  So each gas will occupy the whole volume, but it's density in the space will be higher near the bottom than at the top.  As the total pressure is the sum of all the partial pressures, I suspect the heavier components of the mixture become a little more concentrated near the bottom, and the statistics of the collisions mean that the lighter ones become slightly more concentrated nearer the top.  I don't believe they ever settle out into layers, as that would imply minimal velocity in that random motion, but in the long term, especially in a very tall vessel, I would expect to see some evidence of a concentration gradient.  So still all gases found at every height, but the proportion of the heavier ones will be a little higher than average at the bottom and a little lower at the top, while the lighter ones will be slightly more concentrated at the top. 

With the gas bottle, it is important to recognise the the bottles store "liquified gas".  The normal ones are propane or butane or a mixture of the two, stored at a pressure sufficient to keep most of it in liquid form at ambient temperature.  Before you light the burner, the bottle, the vapour and liquid inside the bottle are all near enough to atmospheric temperature.  The pressure in the bottle is the vapour pressure of the liquid gas, in the same manner as steam in your boiler is at the vapour pressure of water at the relevant temperature.  But for liquified petroleum gases the vapour pressure at atmospheric temperature is much higher than the vapour pressure of water at atmospheric temperature.  The boiling point of these liquids is much lower than water at atmospheric pressure.

The burner draws gas from the vapour space at the top of the bottle, so lowering the vapour space pressure.  Some of the liquid evaporates as the vapour pressure of the liquid is now higher than the pressure in the gas space, just like some water evaporates in your boiler when some of the steam is allowed to escape.  The latent heat has to come from somewhere, and unless your gas bottle has a steam coil or similar heat source, the heat can only come from the sensible heat in the remaining liquid.  As a result, the liquid gets cooler, and heat starts to flow in from the atmosphere, due to this temperature difference.  But at the lower temperature, the vapour pressure of the liquid is lower, so there is less pressure to cause flow to your burner, a bit like turning the gas jet down to keep the kettle simmering once it has come to the boil.  You can feel the lower temperature below the liquid level on the outside of the bottle, and if you draw gas fast enough, so that the temperature of the liquid gets below the dew point of the surrounding atmosphere, then moisture from the atmosphere will condense on the outside of the bottle as you have observed. That condensation indicates a lower temperature of the liquid inside the bottle, so lower pressure to the burner.  That assumes the burner is connected directly to the gas bottle.

The flow of gas through the tiny orifice at the burner depends on the pressure immediately upstream of the orifice.  If your fuel is butane, or a mixture with less than about 30% propane! that is probably what you have.  With propane, the vapour pressure is much higher again, and you normally have a regulator.  Now, if you have a regulator in the gas line, the pressure at the burner orifice will be determined by the regulator.  The regulator is in effect a variable orifice, set up to control the pressure downstream of the regulator.  If the pressure at the inlet to the regulator is a bit low, the regulator will open a bit to maintain the downstream pressure, which is also the inlet pressure to the burner orifice.  While the regulator is able to maintain its set pressure, there will be no difference at the burner.  Well, if you are able to look very closely, as the temperature falls, the density will be increasing, so there will be a bit higher mass flow to the burner.  However, once the regulator is fully open, if your gas bottle continues to get colder (which it will do until the heat input from the atmosphere is sufficient to boil the required liquid,) the situation is the same as no regulator.  Lower pressure means lower flow to the burner, so less heat.

A bit of a long winded answer.  I hope it is clear, but it is only when you write it out in full that you see just how many things are going on.

I would definitely continue to use your 680 grade oil for your models.  The Morris data sheet says it is suitable for saturated and low superheated steam up to 175 psi and 230 deg C.  I am mindful of what Bob said regarding using it for higher temperatures in full size practice.  I suspect it is a matter of viscosity range, and balancing lubrication benefits with engine efficiency.

It is important to understand the effect of viscosity.  Basically viscosity determines drag on a lubricated bearing, much as friction determines drag on a dry bearing.  In the simplest terms, while friction at a moving bearing tends to be relatively independent of the sliding speed, viscous drag is proportional to velocity.  And in oil products, viscosity reduces with increasing temperature.  While the oil viscosity is specified at a moderate, standard temperature, in the is case 40 C, at operating temperature, the viscosity is much lower.  So for a high working temperature, you would select oil with higher viscosity (at 40 C), while for a lower working temperature you would select a lower viscosity oil (again at 40 C) to have the right viscosity at a lower working temperature.  Like moment of inertia, there is a considerable range of acceptable viscosity.  In general, with higher viscosity oil, there will be more viscous drag, but the bearing would be able to sustain higher load without metal to metal contact, while with lower viscosity, there would be less drag, and less load bearing capacity.

In a race car, where you want every last watt of power available, or on an ocean liner where fuel is a major operating cost, you would tend to select lower viscosity, and cut it finer on load bearing capacity.  On earth moving machinery, you might go for greater load bearing capacity to handle the unpredictable loads when the shovel hits rocks, and hang the fuel efficiency.  In your model engines, normally run unloaded, or relatively low load, efficiency is not usually a big issue, but you would not want to have to rebuild some of the wearing parts every few runs.  You probably would choose higher viscosity within reason.  So it is not unreasonable for you to continue to use the 680 grade within the manufacturers temperature limits.   Morris have a 1000 grade for higher superheat temperatures, however, Bob, in his marine career, might well choose the lower 680 grade at those operating conditions.  Experience soon pushes industry to a preferred selection within the acceptable range.  However, when you have used all your 680 grade oil, you might choose the 460 grade which would let your engines run a little more freely. 

There is another consideration, that is operating speed.  Because viscous drag is proportional to speed, a high viscosity oil gives good load bearing capacity and only limited drag at low speed.  While if you build a high speed engine for a planing model boat, the drag, with the same oil at similar temperature but at that higher speed would be much higher.  You might then want to go to the 460 grade oil for less drag on the higher speed engine, to leave more of the available power to drive the propellor.

The warning to protect from frost suggests to me that the tallow and other additives in the compounding formulation are prone to freezing.  Without knowing just what they mean, it is hard to tell whether, like ice blocks, you just need to warm them to melt them and stir them well before use in a running engine, or are they like bananas, never quite the same again.  In the mean time, I suspect that acting sensibly means maintain the temperature above freezing in storage.  In a cold climate, that might mean heating in the storage area, or keeping the oil indoors.

I hope that makes a few topics a little clearer, bit please keep asking, they are all interesting and even important questions.

Thanks everyone for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 11, 2018, 01:40:24 AM
Hi MJM, thanks for the info ...the gas question however was asked to enquire if the Carbon  and Hydrogen might separate out !! as most substances are trying to get back to its most stable form ( my car trying its utmost to become iron ore !!!) for example !! and the gas bottle question was about my tea making activities at the allotment ...I boil the kettle with butane and i was wondering if it would be more efficient if the bottle was actually buried under the shed in the soil !! I had one stolen as it was outside the shed and so more about security actually !!...Perhaps i should ask these questions with the actual answer i require !!!  thanks for all these extra observations with your answers as so much more knowledge is actually imparted , also new words... tribology for example ...does this word also equate/relate to 'tribulations' ..as in the phrase rubbing somebody up the wrong way !!??

Willy
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on May 11, 2018, 11:20:34 AM
Some places in the English speaking world gas = gasoline, and that triggered me, before I re-read your question Willy, but ....

Modern gasoline is not the same as what was sold some years ago. The last two years my main road MC has taken many minutes of cranking before firing after the winter break. I discussed this with my local dealer / mechanic and he said that it is funny because they do from time to time get a bike in that has 10-15 year old gasoline in the tank and they start immediately, but take a similar bike that has been standing still for more that two months with modern gasoline and it is almost impossible to start it ...!
My guess is the way modern gasoline is treated - they convert a heavier oil into high octane gasoline and it works nicely as long as it is fresh, but apparently the most flammable part of it evaporates even from inside the pressure lines on the vehicle or changes properties as you suggest :cussing:
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 11, 2018, 12:04:27 PM
Hi Willy, OK, that is a different question.  Yes, amplification of the question to provide a bit of a clue to what you are thinking would help me get to the point a bit quicker.  Not much point in providing lots of irrelevant information, no matter how good the information. 

So not talking about a mixture of gases, but hydrocarbon molecules.   The hydrogen and carbon in hydrocarbon fuels are not separate hydrogen and carbon molecules floating around in the same space, the hydrogen and carbon atoms are very tightly held together in very strict proportions, as discrete molecules, by covalent bonds which are very strong.  Each carbon has four bond sites, while each hydrogen has one.  This gives a huge range of possible combinations.  One carbon can have one hydrogen at each site, so one carbon to four hydrogens makes methane.  But one or more of those sites can join to another carbon, either different ones, or even the same one.  If they join to one or two carbons we get those chains that start with methane, two carbons make ethane, three make propane, four butane and so on, with hydrogen at all the other positions.  Enough variations to make a whole post without even going past four carbons, but if there is a limit, it is much higher.  If a carbon joins to another carbon with two bonds, we get the ...ene series, ethylene, propylene, etc., while if a carbon joins to another with three bonds, we get the acetylene series.  That white paraffin wax your mother used to seal the jar of home made jam is just a very long chain of carbons joined by single bonds, with the other sites occupied by hydrogens.  And those bonds are very strong.  You can bust them apart in a cracking furnace.  (Note, this is not an experiment for the kitchen table!). The gas goes through pipes which form tubes in a furnace.  The high alloy chrome steel tubes are strongly red hot even when your eyes are also experiencing strong sunlight, I don't remember what temperature inside the tubes, but it takes a lot of energy to crack those bonds.  When the gas leaves the furnace and cools, all those separate hydrogens grab what ever bond sites they find, as do the carbons, and all the possible combinations occur in the first second or so.  After fractions of seconds in the furnace, and not much more time to cool, it takes the rest of quite a large petrochemical plant to separate out the ones you want.  Hydrogen and carbon will never settle out of the hydrocarbon molecule by gravity.

Of course, they do like oxygen, so a lot of effort is put into making sure there is no oxygen in those tubes.  And to making sure the tubes don't leak.  But the bonds are strong enough that, in addition to a hydrocarbon and oxygen, you need some energy from an ignition source to break the bonds to give oxygen a chance.  However, your car, being steel will react with oxygen very slowly at atmospheric conditions, especially if there is also a bit of moisture, but the reaction is not so slow if there is a higher oxygen concentration than in atmospheric air and a small spark to start the process.

Regarding your gas bottles, I can't offer much on the security side, but getting the water hot enough for making tea is another mater.  The earth temperature becomes relatively constant not very far down, so most likely warmer than the air above.  You can check the temperature easily enough with your temperature probe at the depth you are prepared to dig.  But the second issue in the equation is the earth conductivity, and whether the earth can transfer enough heat to maintain the pressure.  When the kettle boils, you would need to recover and clean the gas bottle, as the potential for corrosion and bottle leakage is high.

A more reliable procedure might be to stand your gas bottle in a china, glass or even plastic jug.  When the pressure starts to drop enough to affect the burner, tip some of the warmed water into the jug around the bottle, and top up the kettle so it still has enough water for the teapot.  When you have the tea made, you only have to dry the gas bottle.

I don't know for sure where the word tribology comes from, Greek or Latin perhaps?  The ...ology ending is common enough for the sciences, but Tri meaning three does not seem to make sense, unless it refers two surfaces and a lubricant, which are the basis of tribology, or is it trib.. There was a department next door to the engineering school.  I didn't know what went on in there at the time.  The other building I never went inside was the anatomy department, but at least I had some idea of what went on in there, and why I might prefer not to enter!

Seriously, tribology deals with engineering and science of interacting surfaces in relative motion, so friction, bearings and lubrication.  I guess a whole building reflects the importance of understanding friction in every field.  I assume the students experienced plenty of tribulation and did their share of rubbing the wrong way in the most literal sense of the word.

Hi, Admiral DK, I was just about to post when your post appeared.  I am not sure that I have a definitive answer to your conundrum.   I have spent many years of my working life on refinery projects which contribute to those modifications to modern gasoline.  Please let me think about it overnight and I will write something about it tomorrow.  Thanks for posting.  I hope I can come up with something helpful.

Today is a significant day for this thread as it was exactly a year ago that I posted the first entry after asking in another thread if it would be of interest.  I don't know what the number of reads really means, but around 64000 reads in 365 days suggests an average of 175 people a day at least opening the page each day.  I don't spend much time analysing this, but from a modest beginning (though more than enough to encourage me), it has averaged over 400 per day this month.  (We need a bigger theatre Paul!)  I don't know who most of you are, forum members just silently looking in?  Or other visitors?  But I hope you are all finding interesting information, perhaps learning something you didn't know before and are enjoying the read.

I have made about 350 posts, and there have been about 550 by others in response.  Thank you all for responding, and especially to Willy who has provided a stream of interesting questions to start my conversations.   As always, much more satisfactory to feel like I am talking to someone who would like to know, rather than just writing stuff.  However, I am sure Willy won't mind if someone else suggests a topic of interest.  I am quite happy to say so if I don't know anything about the subject, so don't be shy.  And don't be shy to ask again of I miss the point of your question, or even just don't provide the vital step for understanding.  This stuff is all about understanding what goes on, it is not just simply learning facts.

I have a list of projects arising from the discussions, and some boiler tests to complete, so I hope to continue for a while yet.  But if I miss a day here and there, I will probably be out in the shed, collecting some more data, or making swarf for extra fittings I will need, or even at the computer analysing data when the boiler has cooled down and returned to the shelf.  And if you have measured something relevant to the performance of your model, please tell.

Thank you again everyone, whether you are following along, or just looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on May 12, 2018, 12:11:47 PM
Yesterday, Admiral DK's post appeared while I was writing, so I deferred a more complete answer for today. 

Certainly fuels have changed since the early days of oil discovery, but I don't believe there is anything sinister about it.

In the really early days, oil was transported to a refinery where it was separated by simple distillation into fractions of varying boiling point.  No doubt the lightest fractions were lost to the atmosphere during transport and storage, so there would be minimal light components such as ethane and propane.  There would have been a straight gasoline fraction, kerosene, light oils, black oils and tar.  The tars were used in road making, black oil largely went into ships bunkers, many a marine engineer will tell you it was dirty stuff.  But with a few minor variations that is roughly what happened.

Refineries have been modified over the years, initially simply to upgrade the low value fuel products in less demand into higher value gasoline and diesel products, increasingly to find an alternative sales outlet for the reducing bunker demand.  Then more recently to meet more stringent government regulations. 

So lead compounds, initially added to improve octane ratings have been phased out and processing units, called reformers, built to upgrade low octane components to replace the lead.  Catalytic crackers with a fluidised catalyst bed, crack the long chain components in heavier oils and increase the available lighter products.  (The heavier components are a bit easier to crack than the light ones I was talking about yesterday.)  Processing to reduce the sulphur content of the fuel was introduced to reduce air pollution in response to newer regulations.

Those losses to atmosphere are now reduced to nearly zero at every step of the way, and no deliberate venting for many years now.  I have been in teams designing many of those process units.  My responsibility was the compressors, so I know what is done to minimise and even eliminate emissions in that area.  I am not the process expert, but I think that summary will give you the general picture of what has happened.

You can see that gasoline was originally (back when vintage cars were the latest technology, not just 10 or 15 years ago) originally a full range natural product, just as it came out of the ground.  However, now most of the nasties are removed, and less saleable heavy, dirty products reformed into components in the gasoline range.  But gasoline is still a mixture, described in terms of boiling point range, together with initial and final boiling points.  Extra butane is added in winter season to give easier starting of engines, and less in summer when the temperatures make those lighter components less necessary.

I don't know all the details, just the general picture, but it is perhaps not too surprising that the fuel might behave a little differently these days, particularly after long term storage in the uncontrolled conditions in a fuel tank on a vehicle in a shed somewhere.  The other side of these changes is that our cars are no longer spewing out lead compounds or many other nasties.  The refineries are much more tightly controlled and our air is much cleaner for it.   No one is telling me what to say.  I don't even know if "they" would even approve.  But I suggest that the fuel is definitely not inferior, definitely much cleaner and has essentially the same energy content as ever.  It may even be better in some ways.

No doubt the day will come when electric or hydrogen powered vehicles, or who knows what else, will provide our needs, but we don't yet have everything in place for that.  And such change takes time. 

I don't know if that answer is helpful.  It is easy to notice the difficulty of starting, and in these days of conspiracy theories, wonder if we are being cheated.  It takes too many people to design and operate a modern refinery to ever keep the lid on any conspiracy.  I am sure you can rest assured that in return for that loss of starting ability on stale fuel, you have a cleaner, more uniform quality fuel of equal energy content.  The oil industry would probably help their image a bit if they were more open about the changes.  But for most people, so long as the car makers make cars that will run on today's fuel, it's not a big issue, it doesn't last long in the tank, so the conversation should probably give more emphasis to the reduction in air pollution.  It is still continuing.

You can probably remember the days of small high compression engines.  We used to get good performance from quite small engines.  I used to have a 1100 cc engined Renault 10.  More than enough power for most purposes, short of the race track.  Now we have lower compression, but much larger engines, and it has taken many years to get past the fuel efficiency of those high compression engines.  At first we had half the concentration of pollutants, but twice the volume, which I couldn't see as much improvement.  But the nature of the exhaust was changed, fewer oxides of nitrogen are formed in the low compression engines, and that helps our atmosphere in very beneficial ways.    But we are getting there and even surpassing the efficiency those early engines, with much cleaner exhaust.

That is the story as I understand it.  I hope it is essentially accurate.  I am sure we have members of the forum with much more expertise than me on the engine side of things.  We may also have chemical engineers who know more about the changes in refining, so I will leave it to them to correct any misunderstandings I may have, and tell us more about the effects of all the changes in fuel on how they design and tune modern engines.

We can fondly remember the days when those smelly fuels enabled such easy starting no matter how old the fuel, and compare the features of engines that have come about in response.  Be glad the old days are gone.

I hope that helps, thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on May 12, 2018, 08:20:39 PM
Thank very much for you very informative answer - I did admittedly know some of the answers you gave, it was a kind of comment to Willy - but it is always nice go get serious explanation that is easy to digest  :praise2: and the Butane content was new to me but kind of explains it too ....
I read many comments about month old gasoline being useless (most motorcycles needs at least 92 octane) from America (US+ Can) over the last two decades, and always wondered what they talked about as I never had any problems - that is until two years ago for the fuel injected model and some five years for the models with carburettor ....

I wasn't really criticising the product as such, since the bikes runs fine on it when finally started, but it is rather annoying to have starting issues. The really weird thing is that it is only the fuel after the tank that is bad - ie. in the fuel lines, carb etc. - once the old stuff in the lines are used (without the engine starting during cranking), all is good when "fresh" (or not) fuel from the tank reaches the intake ...  :noidea:

Best wishes

Per
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 12, 2018, 11:11:06 PM
Hi MJM , I have an old BMW outfit 1960 and i only ride it Aprill to October  and it then lives outside under a tarp. And when i go to ride it it starts within 3 kicks  with the same petrol in it since the previous year !!...It may be something to do with the really simple carbs. I think the modern ones get clogged up as they are built to high tolerances with muliple jets and things.?Also my bike always starts on the last kick !!! :D :D
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 13, 2018, 12:23:12 PM
Hi Admiral DK, thank you for your kind words.  My whole aim is to contribute to understanding of the basic science behind our engines, so thank you.  I am not sure why there would be a big difference between the fuel in the lines and fuel in the tank.  The daily temperature cycle leads to the tank breathing, so some exchange of the vapour space with the surrounding air.  In this case you might expect to loose slightly more of the lighter components, leaving the heavier components behind.  But that does not seem to be the explanation.  I wonder if it is possible that the lighter components are absorbed into the rubber hose sections of the lines, or whether the copper fuel lines act as a catalyst in some way.  But I really don't know.  It is interesting to think about the different locations, and what might affect the starting behaviour when the fuel reaches the engine.  Perhaps others have some more specific explanation.

Hi Willy, our police here used to ride those bikes (without the side cars) back when they were new, and always put on an amazing display of high speed, precision formation riding at the agricultural show each year.  I used to aim for a trackside seat so they flew past only a few feet away, half going each way and crossing just in front of me.  But you might be onto something there in terms of the simpler carburettors.  I would guess that more sophisticated designs might be more fussy about the fuel composition and properties.  I guess that only riding in the warmer months, when you get the bike out from under the covers, at least the remaining old fuel would have been from the right season, which is also the season where there is less light ends in the mix anyway.  I don't know if that helps.  Almost certainly better than using old summer fuel in winter.

By the way, I hope you are passing on your skills with those files to your young protege.

We were talking the other day about butane and the pressure changing with temperature in the same way that water boiling point pressure varies with temperature.  I have wanted to add some figures to the general description. 

Of course it immediately gets a little more complex.  With one, two or three carbons in the chain, there are not many possibilities for the arrangement of the atoms, and all three are classed as straight chains.  But with four carbons, you can continue the straight chain arrangement, or you can arrange three in that same straight chain, then make a branch by adding the fourth carbon to the middle one.  So you have two variations on butane, called isomers, normal butane, or n-butane, and iso-butane, or i-butane.  You will see both these names on your can of butane for the stove.  This difference in arrangement of the same atoms makes quite a difference to the physical properties, for example the atmospheric pressure boiling point of normal butane is -0.49 deg C, while the atmospheric pressure boiling point of isobutane is -11.81 deg C.  A mixture of the two, as is commonly sold in those disposable containers, has a boiling point somewhere between, say around -5 C, depending on the actual proportions of each in the mixture.  So in the allotment on a cool morning, you have a very small temperature difference to keep the mixture above atmospheric pressure.  At 40 degrees C, normal butane has a vapour pressure of 377 kPa, so 277 kPa above atmospheric pressure, while iso-butane has a vapour pressure of 528 kPa, 428 kPa above atmospheric.  Again the mixture will be somewhere between.  It is not much pressure to get the required flow through those tiny jets, especially if your air temperature is less than 20, and the pressure, hence flow, keeps reducing as the temperature gets lower. 

I don't have a full table like the steam tables for butane, but I have scanned the pressure-enthalpy diagram in the attached picture so you can see the general similarities with the similar diagram for steam I have previously posted.  Unfortunately I have had to fold the diagram, as it is on A3 paper, while my scanner is only A4, but I think you can see the important details.

Propane has much higher vapour pressure at any temperature, so you have a bit more margin to ensure enough flow to your burner.  This is why it is included in many of those slightly more expensive cans used with hiking stoves.  The readily available mix here is about 27% propane with 44% n-butane and 29% i-butane mixture, which is the specified fuel for my little gas fired marine boiler.  Even with the propane content, things get a bit slow in winter.  Now I have the thermowell sorted, I need to get back to further testing of that boiler.  I will need to record the ambient temperature with the results to be comparable with tests conducted in summer.  It will be interesting to compare with the earlier tests of my two spirit fired pot type boilers which have much more heating surface.

Thanks for following along,

MJM460


Title: Re: Talking Thermodynamics
Post by: MJM460 on May 14, 2018, 12:01:20 PM
I recently made up a thermowell to replace the fill plug on my centreflue boiler.  I managed to try out that thermowell on a boiler test run today.

This actually follows on quite nicely from the last few posts, as it is a gas fired boiler.  The gas, as I mentioned yesterday, is a mixture of propane, normal butane and iso-butane.  Such a mixture has a vapour pressure dependent on temperature, just the same as butane or water.  However, there are no tables for such mixtures, and vapour pressures have to be calculated. 
With a mixture of gases, each gas behaves independently of all the others, so long as they are not at extremely high pressure, or very close to condensing conditions, and the pressure in an enclosure is the sum of the individual partial pressures of the gases in the mixture.  When we have a two phase mixture of liquefiable gases, the behaviour is quite different.  The presence of the liquid phase means the vapour pressure of the mixture is dependent on the concentration of each gas in the mixture.  A mixture of two liquified gases has a vapour pressure in between the vapour pressures of the constituents, and the composition of the vapour is slightly richer in the lighter component, while the liquid composition is slightly richer in the heavier component.  In addition, as vapour is released or consumed, because the gas is richer in the lighter component, the composition of the mixture looses a little more of the lighter component, so the composition gradually changes as the gas is consumed.

As I mentioned in response to Willy's question about his tea making adventures, the heat required to evaporate the liquid to replace the consumed gas comes from the remaining liquid, which, as we have all observed, gets cooler.  We see evidence of this in condensation of atmospheric moisture on the outside of the vessel.

As I remember it, my previous trials on this boiler were all conducted in summer.  Yes, the little gas tank became cooler, even a bit of condensation on the outside, but the whole plant worked well.  I could raise pressure, even test the safety valve, and the engine ran at something around 2000 rpm.  Some of you might think that is a bit high, but it will slow down when I connect a load of some kind.

I did run it a week or so ago, so the lubricator has been filled with steam oil, but I did not have the thermowell.  Today is nothing like summer, not to us in this country anyway.  Only 14 C in the shed.  However, I filled the boiler and the gas tank, oiled up the engine, filled the displacement lubricator, fitted all my thermocouples and fired it up.

 I got the temperature up to 110 C, opened the stop valve.  The boiler I purchased included a stop valve, unlike my home built boilers, and the engine ran quite well.  The valve was not gummed up like when I had run it on engine oil in the lubricator.  Started off at about 685 rpm by the digital tachometer.

Interesting to say the least.  If the steam plant was all enclosed in a boat, I am sure it would all warm up and there would be some warmth to help evaporate the liquid gas and keep the pressure up.  But there on the bench in the shed, I had to go and get a coat.  I used the infra red thermometer to check the temperature of the gas tank.  Started at only 10.9 C, obviously not really warmed up after transferring the gas from the container to fill it.  A portent of things to come.  By the time the boiler was up to temperature, the gas tank was down to less than one degree.  And instead of the engine speeding up as the steam pipes warmed up, it barely maintained that initial speed, and in fact soon started slowing.  Thirty minutes later, (the gas should not have lasted that long,) the engine speed was down to 228 rpm, the boiler temperature down to 102, even the stack gas was only 100 C, so I closed the stop valve and called the end of the run.  The gas tank temperature was rising to about 4 degrees, indicating a much slower evaporation rate.  I weighed the gas tank (after drying off the outside) and found there was still 8 of the initial 34 grams in the tank.  I don't have the methods to calculate what the tank pressure would have been, but it was too low to maintain a reasonable steam rate, and would be richer in butane, fortunately minimal change in heating value.

As the burner fuel consumption was obviously reducing continually throughout the run, I don't know what I will be able to make of the other results, however I will make a start on analysis tomorrow.  However, it is clear if anyone wants to run at temperatures around 15 C, some means of providing a little heat to maintain the gas pressure is required.  In a model, enclosing the gas tank next to the boiler may be enough.  Possibly even with a means of opening extra ventilation in warmer weather.  Perhaps a steam jacket fed by the engine exhaust, but obviously with a bypass valve so it could be shut off in warmer weather.  Or perhaps a larger jet, and one of those pressure control valves to effectively control the gas  pressure and hence the fuel flow to the burner.  And a fuel mixture with more propane helps if it is available.

I hope to have some more results to report tomorrow.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 15, 2018, 02:37:29 AM
Hi MJM , interesting things happening here that prompts a few questions... do liquid gasses have different specific gravity's and when you mix them together do they separate out ?  Is the natural gas in our pipes always a gas and can it be liquified. My young friend in the side car got his first steam engine when he was 4 years old !! however he has been living in london since then , but he does get Meccano and things !

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 15, 2018, 01:31:47 PM
Hi Willy, yes, the different liquified gases do indeed have different densities.  The three in my gas mixture are propane, 508 kg/m^3, normal butane, 584 kg/m^3 and iso-butane, 563 kg/m^3.  So all similar and about half the density of water.  They are all fully mixable in each other.  But whether they actually settle out under gravity is an interesting question that often puzzled others besides me at work.  When we got some impurities in a tank, it was hard to mix them well, even though well mixed they would have been within specification.  I don't know if they would significantly settle, given enough time, or if the normal random molecular motion is enough to keep them mixed.

Natural gas is mostly methane, possibly with a small amount of ethane and it can indeed be liquified, as can air which is a mixture of nitrogen and oxygen with a few minor impurities.  In fact liquefaction of these gases, not in the same plant mind you, is the basis of two quite large industries.  You have no doubt heard of liquid air and liquid nitrogen.  When oxygen or nitrogen have to be transported in large quantities, it is usually as liquid.  Natural gas is also stored as a liquid when large quantities are involved.  Liquified natural gas, or LNG is a large industry here and other places, as LNG is shipped to China and other places where it is vapourised into the gas systems as a utility fuel.

However, these are all much lighter gases than propane and butane, so the boiling point at atmospheric pressure is much lower.  Natural gas boils at -161 C and is transported around the world in ships at that temperature.  You will recognise one if you see one in a port or in a picture, as the tanks are spherical, like very large basket balls, sunk into the hull so only the top little bit is visible.  The tank diameter is enough that three or four in a row makes a very large tanker.

Liquid air boils at -194 C which results from the mixture of oxygen, - 183C, and nitrogen, -195C.  If your doctor treats a sunspot with liquid nitrogen, you get a very localised, but very severe case of frostbite.  You can ask how I know if you really must!

The refrigeration units used to liquefy these gases are probably the largest refrigeration systems in the world.  They use turbo expanders to extract energy rather than just throttling.  I always think of my refrigeration systems as very large, but probably a group or two behind those giant plants.  Smaller plants are just not economical, and everyone has to make a profit these days, even if the product is only air.  And natural gas, after being liquified at such low temperature in some of the hottest parts of the world, shipped half way around the world and revapourised, is still sold relatively cheaply, so the volume has to be large to cover cost of the plant and ships plus a profit.

You might wonder why those LNG tanks are spherical.  It is the same principal as those spherical storage tanks you see if you ever drive past a petrochemical plant or a refinery.  They are used for butane and pentane mostly.  The vapour pressure is too high at atmospheric pressure for a normal cylindrical tank with flat or conical roof.  If you calculate the amount of steel for a sphere compared with a horizontal cylindrical pressure vessel, there is always less steel in a sphere.  Once the sphere is large enough, the difference is enough to make the sphere cheaper even allowing for pressing the more complex shape of the plates.  The ones for LNG will be stainless steel, in order to accommodate the low temperature without becoming brittle like ordinary carbon steels.  In addition, they will actually be two spheres, one inside the other, with very dry nitrogen in between along with insulating material, thick enough to prevent frost on the outside.

The ships for LNG have to have refrigeration plants on board.  The systems are not as large as you might expect, as that insulation is pretty good.  Also some of the boil off is used to power the engines, real gas engines you might say, or just big gas engines, depending on your point of view, so the amount to be condensed and returned to the tanks is less.

I am glad to see the young lad off to the right start quite early.  I think I left it too late with my grandchildren, though I have got a great nephew interested in Meccano.  The others would all rather look at a screen or chase a ball - nothing in between.

I hope that answers the right question.  Didn't get much calculation on done today, I hope to have more to show for it tomorrow.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 15, 2018, 11:57:32 PM
Hi MJM, Thanks for the explanation and yes i think i am getting some ideas about Thermodynamics  !! and a new practical question for you ..... I was having afternoon tea/smoothie  and it was quite cold ..so i was thinking about warming it up .and rather than putting hot water in it i thought i could bring it up to ambient temp by putting an ambient temperature knife into it ......I was then thinking that one end is quite thin and the other quite thick. so would the temperature rise quicker or slower in time depending which way the knife was put in ??? I could do an experiment but the cafe doesn't stay open that long !!!!If you drew  graphs at the beginning would they show a slightly different curves ?..thanks
Willy........
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 16, 2018, 12:01:46 PM
Hi Willy, what I can't figure is why you want to warm a smoothie, it's supposed to be cold.  However, I understand that you guys drink your beer warm as well.  Strange!

However, from the thermodynamics point of view, it is a similar problem to the teaspoons in your coffee, or tea, just a different temperature range.  Obviously the temperature gradient is the other way, and heat flows from the knife to the smoothie whereas the heat flowed from the coffee to the spoons in the previous experiment.  But otherwise the sums are very similar.  Heat will flow from the knife to the smoothie, thus warming the drink, and cooling the knife.  It is a little hard to predict the result, as we would need to know the specific heat of the smoothie, and that would depend a lot on how much ice it contains, as well as the precise composition.

However your question is just a little different from the teaspoon one in that the emphasis this time is on the difference in which way around you place the knife.

If you try and balance the knife on your finger at about the point where it just breaks the surface of the drink, I suspect the heavier end would be the handle.  Obviously two different points to check the balance, one for the blade in the drink and the other with the handle submerged. In addition, the surface area of the metal in contact with the drink will be a little different in each case.  So let's consider each case separately. 

If you submerge the blade, you have large surface are in contact with the liquid, with relatively little mass, and therefore stored heat, so the blade will quickly cool to the drink temperature, while transferring only a little heat to the drink.  Most of the heat stored in the knife is in that heavy handle, so must travel along the knife via that small cross sectional area, so some time delay.  All the while, the handle is gaining heat from the atmosphere, as a finned surface, just like the teaspoon handles lost heat in your coffee experiment.  I don't know how much this warms the drink, especially if there is a lot of ice, requiring latent heat before it actually warms.

If we submerge the handle, let's assume that the surface area of flat blade is about the same as the surface area of the elliptical handle, but most of the stored heat is very close to that surface, so this might be expected to transfer to the liquid a little quicker, with only minimum heat to travel the distance down the blade. And the blade exposed to the air would be gaining heat as before.  So I expect the handle submerged might achieve the result a little quicker, but a very similar result in the end.

I suspect it would be difficult to measure the different curves, partly because the difference is probably very small, but also because until that ice melts, the temperature will not rise much.  But in principal, there might be a difference in the shape of the curve between the same end points.  Possibly the temperature stays constant for longer in one case than the other.   The viscosity of a good smoothie means the temperature will not be in equilibrium for the time the change is occurring, so results will depend very much on the location of the thermocouple.  Vigorous stirring will help, partly by keeping the temperature more uniform, and partly by adding extra energy, as the work done during stirring ends up as heat in the liquid.

The other complication is rather like that experiment touching an insulating plastic compared with a block of aluminium.  If the drink contains lots of finely divided ice, it will absorb a lot of heat from your tongue, and so will feel very cold.  As the ice melts, the temperature will not change much, but the liquid will absorb less heat from your tongue, as there is less ice remaining to be melted.  So it will feel less cold, even though it measures the same temperature.

So while I recommend drinking your smoothie cold, particularly on a hot day, there are many lessons in thermodynamics in your little experiment.  It involves concepts of heat (stored in the knife), specific heats,  latent heat, heat transfer, thermal conductivity, work being converted to heat,  conservation of energy and even a little physiology.  Despite the long list, I am not sure if I have included everything.  No wonder it takes a large number of smoothies to keep you cool while you think it all through.  The cafe might not stay open long enough today, but it will open again tomorrow, and they need your custom.

A long day of other essential activities including a late Mother's Day dinner with our daughter and her family, but still made a little progress on those calculations in the remaining hour or less.  So they will have to continue tomorrow.

Thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 17, 2018, 01:06:13 PM
Hmmm!  A thermodynamics lesson in a smoothie seems to have left everyone speechless.  But I also have been wondering, have I mixed up a smoothie and a slushie?  I hope any confusion this caused has not clouded the issue.

The calculations on the centre flue boiler are proving a challenge.  The insulation on my boiler is only one layer of timber slats.  With a temperature of 110 degrees inside the boiler, the wood temperature as measured by the infra red device was 80 degrees.  This probably means quite a bit of external loss, which I will eventually be able to reduce by adding a few layers of cork under the slats.  I have some extra slats, so with some luck, I will be able to add the cork, and add the number of slats to complete the timber again for appearance  But that is another project.  In the mean time, I can estimate the heat loss with temperature from the cooling curve, and add this to the heat stored in the copper and the water, or the heat in the produced steam.

The more complex problem is caused by the fall in gas pressure and hence gas flow, so heat release.  Not to mention that I don't have a vapour pressure curve for the propane-butane mix in the fuel can.  So quite a few estimates, perhaps more accurately described as wild guesses, in order to complete the process.

However, with a first pass at the calculations complete, I find I can produce a cooling curve, and a curve of heat loss at various boiler internal temperatures.  Should be able to post that tomorrow, though the home front schedule is looking full of activities.  I have also made two estimates of an average burner heat release, one for the first part of the test, heat up, and a second for the steam generation part.  Of course, the heat tapering off like that means the steam production also tapers off.  A bit more checking to see if I can make sense of the results after allowing for the heat loss from the boiler shell/cladding before I am convinced that it is useful data to publish.  In hindsight, I should have shut off the burner much earlier, before the gas pressure and steam production tapered off so much.  I may just have to run more tests over different times, which will allow me to make a better estimate of the steam production variation as the gas pressure falls.  Possibly even try a water bath to maintain the gas pressure more even.  It is all much more complex when things vary so much during the test.

So please bear with me while I spend a bit more time on calculations before posting results.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 19, 2018, 12:42:33 AM
Hi MJM, a few more questions ...I have been reading about these underground coal seams that are burning, some for a long time ? so where does the oxygen come from to keep it burning and why dosen't the smoke extinguish it ??  Also the council has been cutting the grass and leaving the cuttings on top...I have collected these in the past and have noticed that once in a heap they get really very hot inside ...can one explain exactly how this occurs ..please   thanks
Willy
Title: Re: Talking Thermodynamics
Post by: crueby on May 19, 2018, 12:45:32 AM
Hi MJM, a few more questions ...I have been reading about these underground coal seams that are burning, some for a long time ? so where does the oxygen come from to keep it burning and why dosen't the smoke extinguish it ??  Also the council has been cutting the grass and leaving the cuttings on top...I have collected these in the past and have noticed that once in a heap they get really very hot inside ...can one explain exactly how this occurs ..please   thanks
Willy
Careful about stacking those cuttings Willy, one of my neighbors did that and the fire department had to put out the fire when it got too hot and caught fire. Decomposition is a warm thing, and the grass is a good insulator.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 19, 2018, 12:58:35 AM
Hi Chris, yes will do that ,also , Our Harry and your

Megan are coming to the allotment tomorrow  !! something to do with royal weeding !!!!
Title: Re: Talking Thermodynamics
Post by: Steamer5 on May 19, 2018, 09:56:23 AM
Hi Willy,
 You are privileged! I got a house full of slightly inebriated girls watch it.....beer helps....might just have to have a wee dram as well!
Right on the compost front, it’s the bacteria that get that temp to climb along with the decomposition. Last place had a couple of large cherry trees that had to have the leaves cleaned up once a week, I used a vac / blower that cut them up when sucked up, by the following weekend the pile was rather toasty!
My understanding is that coal burning underground generates enuff oxygen to via combustion products & what’s released to coninue the process, remember reading about a coal seem in the States that’s been burning under ground for a large number of years.

Cheers Kerrin
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 19, 2018, 12:44:11 PM
Hi Willy, two interesting questions, possibly even related, the pile of grass being not unlike the very earliest stage of the formation of coal.  Your grass clipping example reminds me of the silage that my uncle used to make for the cows on the farm, as an alternative to hay.  The cut grass was put in a pile with vertical iron sheets around it to keep it orderly.  He used to compact it well by driving the tractor back and forth over it.  He would leave a crowbar thrust through the iron, deep into the pile.  He would pull it out each day to check the temperature.  If it burned his hand, it was too hot, or something like that.  If too hot, he would do a bit more compacting with the tractor. 

As the silage was supposed to have better feed value than hay, I assume there was some biological action going on, converting the grass in some way, but I assume oxygen was also important, and compacting left less air in the pile.  If a silage pile ever got totally out of control, it became a pile of ash with no feed value at all.  So I assume a mix of a little combustion with insufficient oxygen, involving oxygen from air trapped between the vegetation, and some biological action, fermenting the grass.  More a case of observation than real research into the detail I am afraid.

Coal seams that have been burning for many and even thousands of years are spread around the world unfortunately.  In total, a significant contribution to atmospheric carbon dioxide.   Coals contain some oxygen bound into the chemical structure, and the softer brown coals, a significant quantity.  However this is bound in the same -OH radical that defines simpler structures as alcohols.  It is not free oxygen that helps start combustion, though there may be some spontaneous decomposition that releases a little.  All my reading suggests that the coal seam fires start from an ignition source, whether lightning, or surface fires that spread over locations where the seam is exposed at the surface.  However, once started the fires continue, probably with oxygen trapped in the earth, slow burning but notoriously difficult to extinguish.  We had one here recently, and it took huge amounts of water flooding to eventually extinguish it.  Earth and coal are both relatively porous substances with many long fine fissures that allow air to percolate deep through.  So once the fire starts, the heat generated together with the available air, all conspire to keep it going.  The smoke contains a fair bit of unburnt carbon but with enough oxygen, there will be no smoke.  In the absence of enough oxygen, some carbon will only go to carbon monoxide, and some carbon will be totally unburned or just the long molecules broken up a bit, creating that sticky tarry stuff characteristic of smoke, and the deposits in the fire tubes of a coal fired boiler.  In the absence of enough oxygen, the hydrogen grabs all the available oxygen first.  Hydrogen in the combustion products definitely is an extreme indication of not enough oxygen, but occurs in the process for producing syngas as a first step in some chemical processes, for example for producing methanol.

Coal is generally considered as having more than 50% carbon.  In brown coal, I assume that is measured after the water is removed by drying, as that stuff is like a sponge.  Coal also contains a considerable amount of hydrogen so is basically a hydrocarbon with much bigger molecules than the simple series we have been looking at.  The molecules take the form of long cross linked structures, each carbon attaching to other carbon and hydrogen atoms. 

Combustion of hydrogen and carbon produces carbon dioxide and water.  But it also contains very small amount sulphur compounds, nitrogen compounds and some others.  Unfortunately these produce the undesirable pollutants that make the combustion process "dirty".  It is hard to object to water as a combustion product, and we all produce and breathe out carbon dioxide, so personally, I would not call it dirty.  It is the amount in the atmosphere, and its contribution to global warming through the greenhouse effect, that is objectionable.  So clean coal means processes to remove those other nasty compounds, either before or after combustion.  Then in principal, coal is similar to any other combustion process.  So what we all desperately need is an alternative energy source that does not involve combustion of carbon. 

I hope that is clear enough and not too political.  There is too much politics and not enough science in most discussion. The politics tends to obstruct any possible progress towards solving the problem.

Thanks Chris and Steamer for coming in, good to have you joining the discussion.  By the way, combustion consumes oxygen.  Some of the oxygen required for combustion might come from the chemical structure, but the combustion products always contain less oxygen than the input to the combustion process.  Any oxygen left over in the combustion process is due to a deficiency of fuel.  But coal does contain some oxygen bound into its chemical structure, which reduces the additional oxygen needed from air to complete combustion of the hydrogen and carbon.

So it seems we are not the only ones in the world where the only available TV program is something about a wedding.  I suppose these days we have on demand services, if we want to watch repeats.    However we will make up for it next week with three days of Brian Cox observing the universe.  I watched it last year, and it was really great.  So patience tonight, and explore the universe next week, as compensation.

I have at last made some progress on the calculations on that boiler test.  I do not mean to imply that they really take so long, it's just that I need an uninterrupted go at it.  Life has not allowed much of that until today, but at last some progress has at last been made.

I think this post is already long enough, so I will summarise my conclusions tomorrow.

Thanks for following along,

MJM460 
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 20, 2018, 12:05:16 PM
Hi MJM, So how does the pile of grass actually ignite ?......
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 20, 2018, 01:36:51 PM
Hi Willy, I have not seen one actually catch fire, but I don't have the long term history that any farmer would have, so I would accept that it could happen.  What I have seen is when you dig out the pile to feed the cows, sometimes there is a little patch of ash deep inside.  Obviously heat generation had proceeded far enough to dry out the grass and then burn it, but probably more of a slow smoulder deep inside the pile (rather than actual flames) when the temperature is high enough and there is enough oxygen in the air that fills the spaces between the blades of grass in the pile.  Any heat generated is trapped in the pile by the insulating properties of the grass around it, so the temperature tends to rise quite high.  In my observation, nearer the surface, there is enough cooling to the atmospheric air so that it does not burst into flame.  However, if the pile is not well controlled and that heating continued without being controlled by further compaction with the tractor, I presume the pile could burst into flames.  At the very least, the patch of ash might be much more extensive, and the feed value of the whole exercise lost.  I would be interested to hear if anyone knows more about these silage piles.

Well, today I managed to get my boiler test analysis to a point where I have something to report.

As I mentioned the other day, testing a gas fired boiler on a cold day (14 C is considered cold around here anyway), really demonstrates the drop in gas pressure as the the temperature drops in response to the heat absorbed by the evaporating gas.  This means the heat input to the boiler is nowhere near steady, so any calculations involving the heat input from the burner are dealing with a continually changing value.  Another feature of this test is that it is the first one where I have had a stop valve so I can keep the boiler closed in until I am ready to start steam production.  My other steam plants simply have the boiler connected to the engine, and rely on finding the point at which the valve ports are closed, assuming they seal tight, which they generally do not.

At this stage, I have some results on the heat input to the boiler, but I have to do a bit more thinking on whether I can draw any useful conclusions beyond the obvious, that the gas tank needs a little more heat input in theses conditions.  But most readers will know that anyway.  I am wondering whether a copper or brass tray under the boiler and gas tank would transfer enough heat to the tank.  Perhaps it will need to extend under the engine and exhaust separator as well.  Or do I need to add fins or a heating coil?

The first attachment shows the data for heating the boiler from 15 degrees to 110 degrees, which was the temperature at which I chose to start steam production.    The steam tables tell us that the equilibrium pressure at 110 degrees is 143 kPa absolute, about 6 psi gauge.  Not high pressure, but more than enough to run an unloaded engine.

The X axis is the time in minutes from light up.  The Y axis on the left hand side applies to the blue and mauve curves which slope up to the right.  The blue curve is the temperature, rising as you might expect.  The mauve curve is the total heat stored in the copper of the boiler and the water it contains, in kJ, again rising with time as you might expect. 

You might remember that the previous heating curves for my Meths fired burners showed the heating rate increasing a little with time, until it became clear that we were producing steam at some rate, so we were no longer accounting for all the heat.  At the same time, when I calculated heat rate per second, it was essentially constant with time.  However, you can see that this time the heating rate decreases with time.

The second Y axis, the scale on the right hand axis, applies to the orange curve.  The units are J/s.  The rate of heat transfer is decreasing with time, consistent with the heat up curve.  This is what you would expect from that decreasing gas pressure.  The thick line is a smooth curve through the data points.  The thin line is a regression line calculated by the computer.  It is of exponential form as you can see from the equation for the line, printed above the left hand side of the chart.  I am not sure if that is the best form of the ones available, but it seems a pretty good fit. 

In case you have not come across a regression line before, it is basically calculated in a way that minimises the sum of the squares of the difference between the line and the data, as opposed to the average deviation.  It is a common definition of best fit for the data.  Despite the irregularities of the curve through the data, it is likely that accurate data would give a smooth curve.  Having an equation allows calculations to use the equation which removes some of the inconsistencies in the data.

For the cooling part of the test, I plugged the funnel with a tissue to minimise the convection cooling through the centre flue, leaving the outside of the boiler as the only cooling surface.    The advantage of this is that I can use the heat loss at each temperature determined from the cooling test to account for the external losses during heat up, and perhaps add this to the heat lost up the stack.  The second attachment is the cooling curve calculated in Joules/sec.  Now I think about it, I should have presented a time temperature and cooling rate temperature as well.   You can see it shows the irregularities that come up each time I calculate these curves.  At this stage, I put it down to the 'per second' calculation exaggerating the inaccuracies of the data.  I am not totally convinced that the temperature readings are entirely consistent.

On the cooling curve, I have also included the regression line.  This time, the exponential curve is the correct form, as cooling follows Newton's law of cooling.  This means I can use the regression line formula to account for the heat loss from the outside surface area, but I am still working on how far I can go with that.

Next step is to calculate how much steam was being generated.  However, this is also dependant on the burner heat release rate, so I may have to do some more experiments with more even gas temperature to draw any useful conclusions.

So that is where I am up to at present, I will try and do a bit more tomorrow.

Thanks for looking in.

MJM460
Title: Re: Talking Thermodynamics
Post by: Steamer5 on May 20, 2018, 03:36:01 PM
Hi MJM,
 Its nothing unusual for hay stacks to catch fire by self generated heat. The usual reason is that the hay was bailed & stacked still partially green & hence with to much moisture content.
Silage on the other hand isn't likely to burn....well I haven't heard of it, but then given that I'm a townie that's not surprising!....silage is cut green & stacked, sometimes allowed to dry a bit & then compressed to get as much air out as possible, then covered with plastic & sealed up to keep the air out until required

Cheers Kerrin
 
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 21, 2018, 02:40:41 PM
Hi Kerrin, I think that defines the difference between silage and hay.  We need a little chemistry, to understand what is happening in silage making.  I suspect there is some biological action/fermentation of green grass involved.  Such reactions result in some heat release, and with the insulating effect of the surrounding pile, the temperature at the middle can get quite high.  Probably just encourages the fermentation if the temperature rise is kept in control by compacting the pile to minimise the available oxygen.  However, if the temperature is allowed to rise too high, there is combustion, and the silage is spoiled.  With limited available oxygen, the amount burned is limited, but it is all loss as far as the farmer is controlled.  So an interesting diversion into what thermodynamics tells us about Willy's observation of his pile of grass clippings.  Chemical engineering thermodynamics even involves explaining the amount of heat produced by that fermentation reaction, but too far out of my field for me to present a complete understanding.

I tried different ways of presenting that cooling data, and it produces pretty graphs, but they don't really tell us anything more that is useful.  They do allow calculating and including the heat loss during heat up, so all except the stack loss is accounted for.  However to proceed to the next step and calculate the boiler steam production capacity or efficiency and air fuel ratio, we need to know the burner heat release.  In this test, the decreasing gas pressure means that the heat release varies over a large range, and information based on averages is too far from reality to tell us very much.  I need to light up the boiler again, and shut off the gas much earlier so that the pressure change is less significant.  I might even see if I can slip a bit of copper sheet under the boiler and gas tank to conduct a bit more heat into the gas bottle to help evaporate that fuel. 

A short post tonight as not too much progress to report.  I will look in each day to continue the discussion on any questions, but will be a bit quiet on the boiler testing until I get some more results.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 22, 2018, 02:04:58 AM
Hi MJM ,  so ,a new word for my vocabulary ..Regression !! ... the curve for the cooling is quite dromederious !! I think you have your own species down under or were they all imported ??  I like the new word but it sounds a bit akward like when Scotland decided to become devolutionied !! I was looking at my water butts on the allotment and they were different colours ...i tested the temperatures with my hand and they seemed slightly different ,so, do the different colours of the rainbow absorb different amounts of heat and are these  equal to the order in the rainbow ??? perhaps i could to an experiment and discover infra red !!!!
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 22, 2018, 01:25:29 PM
Hi Willy, I had to read that three times to work out what you were talking about.  You are quite right about the curve of the data now you mention it.  At this stage I assume it is due to some inaccuracies in my measurements as I would expect it to be a smoother curve.  But perhaps there is something about steam bubbles collapsing, I just don't know.  However, it will be interesting to see if it is repeated in further testing, or varies, and what the data looks like if I combine all the data. 

By the way, we don't have any native dromedaries, they were all imported as transport a long time ago, though many are now wild, whether they escaped or were turned loose.  I understand that due to the isolation, they have remained disease free and may even be an ancient bloodline so I have heard that we now export them back to the original source.

Regression analysis is a statistical technique used to identify relationships between data, or trends.  You might learn about linear regression in a basic maths course, well not really basic of course, which attempts to fit a straight line to data, but most spreadsheets offer more advanced analysis and can determine an equation of a line of best fit using not only linear, but polynomial, logarithmic, exponential, power or moving average.  Some call it a regression line, some call it a trend line.  I selected exponential because that is the form of the equation for Newton's law of cooling, so likely to give a good fit.  But the spreadsheet does all the work, I just took the option to show the line and the equation.  I would not like to have to do it manually.

The water butt question gets to the basics of heat transfer, involving both convection and radiation.  In fact, if the bottom is in contact with the ground, it involves convection as well.  I assume the water butt cools overnight.  In the morning when the sun gets up, the air starts warming and the water butt will receive heat from the air by convection.  It will also receive some heat by radiation, especially if it has an easterly aspect and receives direct solar radiation.

We have looked before at the issues of touching different surfaces to judge temperature, but with different water butts, presumably all the same material, just different paint colours, you might expect to tell the difference with some reliability.  So let's assume the differences you observed are real.  Was there any condensation on the outside?  If so, wind would tend to cause evaporative cooling so one exposed to wind might feel cooler than one more sheltered.

But I think you asked about the different colours, and whether that changes the radiant heat absorbed.  Radiation is a more complex issue than conduction and convection.  Appearance does not necessarily tell you much about absorptivity.  However, radiation falling on a surface is either absorbed, reflected or transmitted.  In this case we can ignore transmission, unless you have a glass water butt.  So partly reflected and partly absorbed.

The colour we see is dependent on the reflected light so the red one reflects more red, while the blue one reflects more blue.  In each case, the rest of the energy, not reflected, is absorbed, so contributes to heating.  The surface then emits energy by radiation to everything around.  The temperature is then the result of all the gains compared with all the heat losses.

The distribution of energy depends on the temperature of the body.  If we look at the Suns energy, peak of energy distribution is in the visible region, whereas for your water butt at less than 300 degrees K the peak is about the middle of the infrared region, which we feel as heat, but can't see.  So the paint colours perhaps alter the amount of reflection in each colour or wavelength, the energy from the sun in the red end of the rainbow does tend to be higher than the amount in the blue end, so the red butt might get hotter than the blue one, (in the same order as the colours of the rainbow) where as at similar temperatures they would both (all) radiate a similar amount.  Or does the red one feel cooler, as it reflects more red which contains more of the Suns energy? It all depends on how much energy is left after the reflection.  Very confusing.  Sticking my neck out there, I don't know which one you found felt warmer.  Black might get hottest, as it reflects least and white reflects all colours so possibly coolest.

However, there is absorption in the atmosphere which affects the energy distribution of the light that reaches the surface.  So a complete study quickly gets complex. 

When you mention infra red, do you mean an infrared non-contact thermometer?  It might give you readings to compare with your hand sensor method.  It would make an interesting comparison.  And you could hold a thermocouple against each surface with a pad of insulating material, or just measure the water temperature inside the container.

In summary, the colours probably affect the reflected proportion of the energy, hence changing the remaining energy to be absorbed.  However, variations in cloud cover, which I omitted to mention, wind exposure and the direction the container is facing are also important, making the outcome difficult to predict.  I would be interested to know what you actually absorbed, and to see whether those observations indicate which mechanism is most important in this case.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 23, 2018, 01:09:12 PM
Hi MJM , Thanks for more interesting info and there is so much more to this than is at first imagined. The reason for the question was to find out if different colours are used in industry to make thermal plants more efficient/stable etc... To get any meaningful data with different colours would need so many different variables to be standardised. The comment about infra red was a sort of joke as to how it was discovered/invented !!!!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 23, 2018, 02:19:33 PM
Hi Willy, I don't know whether there have been internet problems or whether everyone is out enjoying the sunshine.  I have not before seen so many hours with no posts on this forum.  Just arrived home from babysitting, to find your post, nearly 11 hours after the previous post.

You can see why most introductions to heat transfer start by mentioning conduction, convection and radiation, but then start talking about conduction.  Some get to basic considerations in convection, but they rarely get to radiation.

I want to scan the graph of radiated energy energy in each wavelength for bodies of various temperature to illustrate some of the points.  No chance to do this while I was out tonight, so I will do it tomorrow as I think it will help convey some of what happens and make it easier to understand.

We certainly feel here that a black car heats up more than a white one, and I must admit to never having bought a black one.  But I am not sure whether there is much difference between the other colours that I have had.  But what is clear is that the transmitted radiation, that which comes in through the glass does very effectively heat up the inside.  And now we have tinted windows, good tinting really keeps a car from getting nearly so hot.  Before they were allowed, I have burned my hands on the steering wheel, and an uncle came back to his car which was parked in the sun to find the upholstery smouldering and the car full of smoke.  So there is a lot of energy in the Suns short wavelength visible radiation which passes through the clear glass.  The longer wavelength radiation from the car interior does not pass back through the glass, so is trapped inside.

Solar water heater collectors are usually painted black, and I have often wondered if it is just standard paint or something special, but I have never seen a white one.

I will scan that picture tomorrow and the reason for some of these observations will become clear.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 24, 2018, 10:44:58 AM
Radiation Heat Transfer

A little post to add some detail to what I was saying about radiation heat transfer.  A good summary of the subject is provided by the attached data.  I have copied a few pictures from my text book for illustrative and study purposes, but the words are my own.  So please let me know if I have something wrong or just not made it sufficiently clear.

The first picture illustrates the spectrum of electromagnetic radiation.  You can see from the notes on the right hand side of the line that visible light, infra red, X-Ray's and so on are all just variations in wavelength of the same basic phenomena.  The scale is micrometers or thousandths of a millimetre or 10-6 meters.  Keep this in mind when you look at the second picture.  The notes on the left side of the scale say something about the origins of that radiation if you are interested.

The second picture shows the total energy emitted by bodies at various temperatures.  The temperatures are absolute temperatures, so in K.  Zero degrees C is 283 K.  The horizontal scale is in metres, starting at 10-9.  So 10-6 is one micrometer for comparison with the first picture.  The vertical scale is the amount of energy emitted at the particular wavelength, and you can see that both the amount of energy and the wavelength with the most energy vary with the temperature of the body.  And the area under the curve is the total amount of energy from the body at the particular temperature.  The sun's radiation curve is off the top of the drawing, effectively about 5700 K, but of course, what we receive is modified by reflection and absorption by the atmosphere.

All the data in this graph is for a black body, defined as a perfect emitter.  Real surfaces are not perfect emitters so the third picture shows the emissivity of some real surfaces when quite hot.  Unfortunately they did not include brass or brass which we might be silver soldering.  But the emissivity, a number between 0 and 1, is the fraction of the energy emitted by a black body at that temperature and wavelength.  You can see the actual radiant energy from a surface varies in quite an irregular manner with wavelength.  But the biggest factor is still the absolute temperature.

When that radiant heat arrives at a surface, (like the sun on Willy's water butts, or on the earths atmosphere, clouds and all) some is absorbed, some is reflected, and for a transparent or translucent body, some is transmitted through.  Emissivity, absorptivity, and transmissivity are all expressed as fractions between 0 and 1, and the three must add up to 1.  Obviously for timber and metals, in fact anything that is not the slightest bit translucent, the transmissivity is zero, and all the incident energy is either absorbed or reflected.  This brings us to the fourth picture.

The fourth picture shows what happens at some opaque surfaces.  All the incident energy is either absorbed or reflected, so both can be shown on the one graph by simply reversing the scale direction.  So one scale on the left, and the other on the right hand side.  You can see it includes both white and black paint.  Black paint, as you might expect, absorbs all wavelengths equally, and nearly completely.  White paint is the more interesting one.  You can see it reflects most of the light in the visible range, then a second peak at 5 microns, right in the range of those molecular motions indicated on the first picture.  The sloping top  of the main peak with wavelength is probably showing the different reflectivity for the different colours that make up white light.

Now those are the basic characteristics or radiation from a surface, however to understand what happens when an object receives radiant heat from the sun we need a little more data from atmospheric temperature, and what happens when radiant heat falls on a surface.

  The next picture (fifth) is a table of emissivity at ambient temperature and absorptivity to solar radiation.  If we look at the difference between a light coloured surface and a dark coloured surface, you see that light, dark and even black surfaces at atmospheric temperature all emit radiation about equally if they are at the same temperature.  Remember these figures are the fraction of energy emitted compared with an ideal black body.  But the three surfaces are significantly different in their absorptivity to solar energy.  Assuming they are all in the sun at the same angle, and receiving about the same amount of sunlight, the light surface absorbs only 10 - 35% of the energy it receives, the remainder is reflected, so it appears bright from all that reflected light we see with our eyes, while the black surface absorbs most of the incident energy.  It appears dark, as so little light is reflected.  And other dark surfaces are in between.  So the white surface, which absorbs so little of the available sun's energy, would probably feel coolest, while the black surface, which absorbs most of the available energy, should be warmest.  Whew!  That is the same conclusion as I suggested two days ago.  I hope that is also supported by your observations.

The final attachment is for me the most problematic.  It is a table of the transmissivity of glass, with columns for clear glass and different tints.  The table should have an extra line for wavelengths greater than 2000 nanometers, (2 micrometers) listing the transmissivity at some very small number, close enough to zero for these wavelengths.  This seems to me to be critical to the conclusion reached in the problem used to illustrate the use of this table, but only acknowledged in the assumptions made in solving the problem. 

When applying this data to the greenhouse, or car parked in sunlight, we note first that radiation from the surfaces in the greenhouse or the car at a temperature of perhaps 25 - 60 degrees C, 298 to 333 K, is just about entirely in the range greater than 5 micrometers wavelength, while the energy in sunlight has a very significant fraction with wavelength less than 2 micrometers.  So glass lets a high proportion of the sun's energy through, around 88% of visible light and still  67% in the infrared region.  However, as the internal surfaces radiate back, the radiation is long wavelength, so almost none of this energy is transmitted so it is all retained in the car or greenhouse by the glass.

The effect of tinting the glass is that the transparency to the sun's energy is reduced, so less heating overall.  But look at that blue-green tint.  It still lets through 75% of the visible light, but only 22% of the infra red.  So with minimal effect on visibility, only 22% of the infrared is allowed in.  Even allowing for a smaller proportion of the Suns energy being in the infrared region, it must significantly reduce the heat accumulation.  However, whether it is that, or just the overall lower transmissibility of the tinted glass, it certainly works in the high sunlight intensities we experience in this country.  I don't know if the tint on my car is similar to one of those listed or not, but it works.

Unless you want to try analysing the composition or distance of distant stars, or construct an infrared thermometer, you probably don't need to take the maths of radiant heat much further to understand the radiation heat transfer you experience.

I think that all I have to add now is those attachments in the correct order.  I think they are small enough to include all, but if it does not work, I will delete some, and include them in a follow up post.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 24, 2018, 11:34:33 PM
HI MJM, Thanks for all that info...quite strange that black paint and water share the same values...quite a lot to read, learn and inwardly digest !! I am very busy at the moment so not much time to ruminate about thermodynamics !!! However questions are still coming out of the grey matter !!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 26, 2018, 08:13:29 AM
Hi Willy, sorry to be AWOL yesterday.  I am in a house full of visitors so not much time for posting for the next couple of days.  Think of it as time to digest that information on radiation heat transfer, or to think up more questions.

There are many strange, meaning non-intuitive things about radiation heat transfer.  No doubt they make more sense to molecular physisists, but we are only trying to understand model engines.

There is one additional point that I meant to include.  Heat transfer by radiation is still from hot to cold, but unlike conduction and convection, where the relevant temperature difference factor is just the difference in temperature, so T2 - T1, in radiation transfer it is the difference in T4 where T is the absolute temperature.  As 0C is 273K, for most of our practical purposes, T4 is quite a large number.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 27, 2018, 12:44:24 AM
Hi MJM , no worries about missing days !! I am also quite busy.... I have got some acrylic paints to do a simple experiment with the suns heating effects, and just need some suitable containers that this paint will adhere to so we will see what happens !!
Willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on May 27, 2018, 04:01:39 AM
"or car parked in sunlight"

MJM & Willy....I quite liked the 2017 VW Golf Alltrack.......in deep navy blue, and the maroon so compared the internal temperature of both vehicles as compared to a white vehicle of identical build

The 3 vehicles were parked in the same alignment at the local VW distributor

I used my digital lazer pyrometer & checked the dash temperature, the front seat material and the back seat temperature at 12:30 PM......lunch time

According to the B of M, the external ambient in Wollongong was listed as 32 degrees C

The VW sales person was most objectionable in his manner in me querying the identical vehicle temperatures

In round figures, the navy blue  and maroon painted vehicles were 5.5 degrees C warmer than the white vehicle......a black VW Tigwan parked next to the trio of Golfs was 7 degrees C warmer  :Mad: than the white Golf..........

Living in  Australia, I have without exception driven white vehicles............

Derek

 
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 27, 2018, 12:23:52 PM
Hi Willy, I think we all understand busy.  But your questions are always welcome when you get back to thinking about those things.

Hi Derek, I didn't have the infra red temperature instrument last time I went to a car yard (I try and avoid them), so I find your experiment particularly interesting.  Thank you for posting that.  You mentioned the ambient temperature of 32 (I presume a clear sky) but did not mention the internal temperatures you measured. 

I have had two white cars and a couple of darker ones over the years.  White cars have always been said to be cooler, but you don't often get a chance to compare them in really comparable conditions.   Yours are the first really comparable measurements I have seen.   All colours heat up considerably in the sun, so I would also be interested in how much they heated above ambient as well as the differences between different colours.

The relatively small differences you recorded are interesting, in that the colour is making a difference in the expected direction, but not as much as common impressions would suggest. 

However, if you look back at that graph of the distribution of radiation energy with wavelength, you can see that, while there seems to be a difference in reflectivity with wavelength, (or colour in the visible range), most of the radiant energy is outside the visible range, so the small but significant colour difference makes sense for the amount of heat in the visible range reflected from the panels as a proportion of all the radiant heat arriving at the car.

The internal heat accumulation however is more influenced by the combination of the transmissibility of glass with different frequencies, and even more importantly, the difference in the level of energy radiated from the upholstery at perhaps 330 K (57C) and that radiated by the sun at  around 6000 K, even allowing for reflection and absorption in the atmosphere.  Remember, the net heat transfer is due to the difference in T4.   And of course the temperature reached is moderated by convection cooling by the atmospheric air.  Window tinting, and those shiny reflector blankets we prop up inside the windscreen do make a real difference, much more than 5 degrees.  So I think I would buy the colour I liked, along with some reflective blankets for the windscreen.

Thanks for following along,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 27, 2018, 12:57:38 PM
Hi MJM and Derek....  so ,i have done the coffee shop ...next stop  car park !!!! I was eating a square heated piece of quiche in a cafe and  was wondering if The cooling of a fixed mass of metal with 3,4,5,6,7,8,corners would cool down quicker than a round slab ?? thinking about squarish cylinder heads on Motorbikes ???
Willy...........
Title: Re: Talking Thermodynamics
Post by: derekwarner on May 28, 2018, 04:31:20 AM
MJM.....I saw no real point in confirming the individual internal temperatures....only advising that it was a warm summer day in Wollongong and so the individual differences in temperature relative to the external body paint colour

The important points were same vehicle alignment to the road & sun....same vehicle build = same front windscreen angle to the morning sunlight and lunch time overhead sunlight the internals & identical seat material [black synthetic weave material]...also with the Golf build = same floor mats, same door trim, all vehicles were with their doors & windows closed for the morningsame ....same ...same etc

Derek

Title: Re: Talking Thermodynamics
Post by: MJM460 on May 28, 2018, 12:06:15 PM
Hi Willy, it is time all those computer gurus at Apple and Microsoft got their acts together and included a "send package" button in their operating systems, so you could share that quiche with the forum members.  I am sure it would improve the discussion no end!  And we could send chocolates on big days and so on.  It would be quite popular, and might provide the incentive for many who have old, slow computers to update.

When it comes to cooling, I suggest the number of corners is not very relevant, it is mostly about areas.  So the area of the top, area of the base, and total area of the side walls each have a separate heat transfer coefficient for natural convection, and the heat transfer area, area and temperature difference are the factors determining the amount of heat transfer and hence the cooling rate.

When it comes to your motor bike, the cooling fins form an extended surface area.  The increased area however involves heat transfer along the length of the fins, so the temperature in contact with the air is not uniform.  The calculation methods I have seen account for this with an efficiency factor for the fins.  The square fins have a bit more surface area for cooling than uniform circular fins would have, even though the area in the corners would have slightly less efficiency than the area mid way along the sides, due to being a greater distance from the cylinder.  So I would expect the square fins would provide slightly more cooling than round ones with a diameter equal to the distance across the flats of the square, but slightly less than circular fins with diameter equal to the distance across the corners of the squares.

It is hard to know whether the difference is really enough to be important, and hence part of the design intent, or whether the square fins are there purely for appearance.

I did notice your post about the paint colour experiment the other day, sorry I omitted to mention it.  It seems like a really good way to further investigate the effect of the surface temperature on radiation heat transfer from the sun.  I assume you will collect some similar size tins and paint one each colour.  You could use the tins as cookie cutters in a piece of styrofoam for insulating lids.  And leave a small hole in each for your temperature probe.

However, as always, radiation is not the only heat transfer going on.  As the sun heats your tins, the atmospheric air will start some cooling by convection.  But I would expect there would be a difference in temperature after some time in the sun.  Given the small difference Derek's car temperature measurements, the difference might be quite small, but it will be very interesting to find out.  It is only carefully controlled experiments like you are suggesting that really confirm the answers.  Waiting for results with interest.

Hi Derek, how much the car heats up due to the glass effect is a quite different effect on heating the car internal space, so I expect would not change your measured temperature difference with colour.  So well done on thinking of doing those measurements and posting the results.  You have clearly shown that the "white cars are cooler" belief that is so widespread is not totally urban myth, but the effect is not really so great that you would buy a car of a colour you really disliked. 

The other colour understanding that most of us subscribe to, is that black and grey cars are quite difficult to see in the distance on the road in poor light conditions.  They are certainly difficult to see as I have found out once again in driving about 600 km in grey sky's and light rain this last weekend.  But I am not sure that the lighter colours are really any easier to see.  Travelling with headlights on in the day time clearly makes much more difference.  So all around, to keep the car cool when parked in the sun, and to be visible to other drivers on the road, it would seem that while colour makes a difference, reflector blankets and headlights have a much more significant effect on your comfort and safety.  But it still mystifies me why black and grey seem so popular these days.

By the way, which car did you eventually buy?  Or is it still under consideration?  And how much weight did you give to the temperature difference?

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 30, 2018, 12:49:37 PM
Displacement lubricators-

I have been thinking a bit about displacement lubricators, particularly with regard to that steam cylinder oil.  I have seen various comments about how well they work, but nothing really definitive except that they have been used forever, and surely they would have been replaced long ago if engines were showing signs of poor lubrication.  I have thought about whether it is possible to measure how much oil they actually deliver, and how long a given quantity of oil in the lubricator might last.  I am confident that they work ok, as there is a small amount of oil makes it through with  the condensate that drains from the exhaust separator, but it's time to try and quantify it.

It occurred to me that it might be might be worth looking at how much steam a small displacement lubricator might condense.  The recent discussions on radiation heat transfer reminded me of the calculations I did earlier for small locomotive fire boxes.  As with the fire boxes, the lubricator looses heat by convection and radiation.  In addition, the body of the lubricator and oil start at ambient temperature, and are heated by steam on the oil surface, the heat taken by the lubricator  body and oil comes from condensing steam.   As heat is lost by the steam, it condenses, and the water displaces an equal volume of oil into the steam pipe, and hence to the engine.  I include a valve at the lubricator inlet, which I keep closed until the cylinder has warmed up and the engine is running, then I open the valve so that some steam is admitted to the lubricator.  I have attached a picture of one of my lubricators in case my description is unclear.

I measured up one of my lubricators, (they all seem to depart from the drawing dimensions!) and calculated the approximate surface area, and volume.  I assumed the body heated up to about 80 degrees, cool enough to be condensing some steam but hot enough to burn the finger.

Heating the body and oil took slightly more than convection losses for a 10 minute run, about 1000 J for each, about double what was lost by radiation in that 10 minutes.  Ten minutes is about the normal steaming time for my small boilers and in any case can easily be multiplied up for a longer run.  So convection plus radiation plus initial heating of the lubricator body and oil added up to enough heat to produce about 1 ml of condensate, with an additional 0.65 ml each 10 minutes.

So how do these calculations relate to the performance.  First, I opened the lubricator and removed the drain valve at the bottom.  As I suspected, full of sticky emulsion from when I was using engine oil instead of steam cylinder oil.  I have given it plenty of time to drain, and washed it with a good squirt of WD40.  Then I let a bit more WD40 soak overnight in the the reservoir, drained it again and now it looks quite clean.  I left it open overnight to let any remaining WD40 film evaporate.

I normally fill the lubricator with an eye dropper.  It has to be filled slowly as the oil goes slowly past the valve stem so blocks the exiting air.  A point to try and improve in the next design.  It is hard to measure the oil in, as there seems nearly as much oil clinging to the outside of the dropper as inside, but the calculated capacity is about 3 ml.  So the next step is to run the engine again, remember to measure the lubricator body temperature and steaming time, and see how much water will drain out afterwards.

It will be interesting to see if the measured water drained after the run is anywhere near the calculated numbers.  So stay tuned.

Thanks for looking in,

MJM460


Oh dear, the photo is too large to load up.  I will return shortly with it.
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 30, 2018, 12:55:01 PM
You can see the lubricator with the steam valve at the top and drain valve.  Perhaps this time the size will be ok.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 30, 2018, 11:35:45 PM
Hi MJM, Does the lubricator need a drain cap ? i don't have one on mine so should i install one ? Does it eventually fill with water as oil floats on water ?  Also i am drinking hot tea and my glasses have steamed up!!  so why don't your eyeballs steam up ?? another silly question perhaps ??  Nice looking engine btw.......On the colour experiment ,i was thinking of just using brass rod with a hole drilled into it the same size as the probe. I will then just paint the tubes the different colours and have them screws onto a piece of wood. This will be better than using water as the amount may evaporate at different rates and so introduce discrepancies ??  would this wrk  OK ??
Willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on May 31, 2018, 01:06:07 AM
When we think about how these displacement lubricator function, the one point sadly lacking in manufacturers directions, is the preference to fill the lubricator until the oil level is lapping the lubricator internal cross tube with the drilled orifice...this is where the eye droppers MJM mentions ...either underfill or overfill

So fully agree that eye-droppers are best left to eye drop fluid, however it is rather easy to modify standard hypodermic needles to be used to carefully & visually see how much oil is being injected [the modification firstly dispense with the ''sharp'', then inset a 2.0 mm OD brass tube & epoxy cement into the plastic spout of the syringe body .....the stoppers are shortened toothpicks.......[so far {in 12 months?} the oil has not migrated out of the timber] :facepalm2:

The advantage of the quartz glass tubed displacement lubricator is the visual confirmation of condensate to oil remaining or used during a run...[as shown in the second snap]

Some suggest a fixed orifice version wastes too much oil....I understand this, however the most inexpensive commodity in a build is surely the cost of oil...the bottom needle valve drain allows easy pressurised blow down via the tube to the de-olier.....again the visual advantage is clear

The bright light green is ISO 060..lubrication oil [sewing machine oil], the dark olive green is ISO 460 steam oil...so impossible to mix up

Derek

Edit...........

don't be confused, in the second snap, it is the morning sunlight streaming thru the ISO460 pea soup that changes the appearance :Doh: of a lighter oil [the attributes of ..opulence and translucent are probably more appropriate terms]
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 31, 2018, 10:58:47 AM
Hi Willy, whether you have a drain valve at the bottom or not depends largely on how you normally operate your steam engine.  As you suggest, the steam condenses in the lubricator, and the condensed water sinks to the bottom of the reservoir, and in the process lifts the oil, so that some leaks out into the steam pipe.  Eventually, all the oil is gone, and the lubricator is full of water.  If you have a syringe with a small enough tube, you could extract the water through the filler plug when the system has stopped and cooled down.  If you have a feed pump and want to keep the engine running all day, you may need to drain the lubricator from the bottom, depending on how well the shutoff valve works.  With a drain valve at the bottom, as Derek says, the steam pressure can help push all the water out, especially of you want to drain it to the exhaust separator.  I just loosen the screw in valve at the bottom with the filler plug removed, and the water runs out easily enough, then when the oil starts coming through it becomes quite slow, in distinct drops, so it is easy to close the valve and retain the remaining oil for next time.

My lubricator has only one entrance when in operation, a 1 mm diameter hole through to the steam passage.  When I open the valve, which closes off that hole, steam pressurises the lubricator, but there is no flow through, so the steam just condenses in that stationary volume as it loses heat.  That heat loss determines how much steam condenses.  The condensate that forms in the reservoir and sinking to the bottom is the only way the oil can be lifted so it can leak into the steam passage.  I deliberately say "leak" rather than "flow", as it feels like a better description of high viscosity oil through a 1 mm diameter passage, with no real driving pressure.  Of course, the oil viscosity reduces rapidly with temperature, so it is not nearly so viscous where it is in contact with steam, but never-the-less, to describe it as "flow" feels like an exaggeration.

It is also complex to analyse when the system is first started.  I probably overfill the reservoir.  I suppose that when I screw in the plug, a little oil is forced into the steam pipe even with the valve closed, or is there a little air bubble somewhere?  I keep the valve closed until the cylinder has heated and the engine starts, but then the steam can only condense against the surface of the oil in that 1 mm diameter passage.  It is hard to see how too much oil can move, but as there is always a little oil in the condensate that drains from the exhaust separator, clearly some gets through, and it seems to be enough.  I guess that surface tension and capillary effects are important both at the very start, and continuing after a little oil has been used.

Your glasses are quite cool (in every sense of the word!), especially in winter, and compared with the hot steamy vapour rising off your tea as you drink it.  Hence the "steaming up".  And the fogging is because the condensation occurs in tiny droplets which scatter the light.  It is only when those drops coalesce into a continuous film, that it becomes transparent.  However, not only are your eyes close to body temperature, so much warmer than your glasses, but they always have a film of moisture over them to lubricate your eyelids.  Any excess or deficiency is fixed every time you blink, which is generally more often than most people think.  And the film is quite transparent, so no problem to your eyes.

With your colour experiment, I suspect you could do it either way, but you have raised a good point, you would need to carefully measure the same quantity into each tin if you go the water way.  But with the same quantity in each, and styrofoam lids, I don't think evaporation will be much of an issue in the temperature range you will be seeing.  Water has the advantage of better contact with the probe, so might give more consistent measurement.  However if all the holes in the brass are the same depth, and the probe is a reasonable fit, you will soon see if your readings are reasonably consistent.

Hi Derek, I really like that glass tube lubricator.  As you say, you can clearly see how much water you have.  Do you find it stays clear? Or does some emulsion eventually make it hard to see through?  It probably also looses heat more slowly than the copper/brass variety, so further slowing the oil rate, which would help if lubricators generally over supply oil.  Of course, once you blow down the condensate, you have to top up the oil to that ideal level for the lubrication to continue without a break while the condensate is replaced.

Your modified syringes are a great idea for precision oiling and for getting into inaccessible spots.  Eye droppers also work, but mine only hold 1 ml, so not good for larger volumes, like Meths in the fuel tank, though for a top up to get a precise quantity they are ideal.  For the cylinder oil, I find that they probably have at least 0.5 ml on the outside, sticking like honey, so I doubt that they are at all accurate for this.

I never cease to be impressed by the detailed work you do on the piping on your model.  Particularly the neat bend around the lubricator valve stem.

Have you made any more progress on your paddle wheel drive train?

Not much time for an engine test today, besides, I want to install a tray under the boiler and gas tank to see how much that helps the gas pressure.  It was only 12 in the shop today!  I have some sheet brass, so I am inclined to say hang the expense, and use the brass.   I don't see a flood of oil from the exhaust separator, so I tend to agree with you that over oiling is not a significant issue.  Also, the oil that does appear appears gradually throughout the run, I don't see a big blob arriving in a short period.  But comments by other people about lubricators supplying too much or too little oil make me even more interested to see what I can demonstrate, so we all have a little more information when such issues are discussed.

Thanks for looking in,

MJM460



Title: Re: Talking Thermodynamics
Post by: derekwarner on May 31, 2018, 01:21:51 PM
I look at the sequence of events here with a slightly different view

If the lubricator is filled with a heavy Grade Steam/cylinder oil with a virtual ZERO air gap and the engine steam regulator is set to ZERO flow, when the boiler steam stop valve is opened, [the assumed air pocket in the tube to the lubricator], we get a shot of steam which partially pressurises and condenses with the entrapped pocket of air

When the engine steam regulator is opened, this allows a flow of steam....which at the same time provides the same back pressure into the lubricator as is offered by the steam regulator

So all things being equal, a volume of steam admitted into the lubricator [under the uniform steam pressure between the boiler steam stop valve and the steam regulator] condenses and the volume of the condensate displaces the same fluid volume of steam lubrication oil into the steam tube to the steam regulator & hence to power the engine

So ISO 460 steam/cylinder oil at 20 degrees C and atmospheric pressure is yes like pea soup, however when elevated to operating temperatures of say 80 degrees C and 1 Bar...is more like water in it's fluid characteristic's

It is in fact an open loop hydraulic fluid system.... there are no leaks...[only onto the drip tray] with the spent fluid [exhaust steam] going to the de-oiler with the fatty/water content trapped & the mist of clean steam to atmosphere

Derek
 
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 31, 2018, 11:35:47 PM
HI MJM, So, my lubricator gets filled each time i start the boiler up so the oil must be going somewhere ?!! I just assume it works !!  and my eye's do go a bit misty when i watch sad films ..and look at some of the amazing work going on in this forum.  Also saw this in a book from 1836 talking about Naphtha....,apparently the Russians actually drank it !!!!
Willy
PS.... just looked on the web and found this !!!! no trade descriptions act in those days !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 01, 2018, 01:47:59 PM
Hi Derek, I really appreciate your constructive engagement in the topic, it helps the thread enormously.  I don't really think there is much difference in what we are saying.  If I understand correctly, you have two valves, a boiler stop valve, and an engine regulator.  I assume your lubricator is always open the the steam line between the boiler and the engine.

My system is slightly different, you could say less complete.  I also have two valves, and one is the boiler stop valve, but in my case the other is in the top of the lubricator, so when it is shut, there is no contact between the steam and the oil in the lubricator.  My simple steam plants do not have a separate engine regulator.  A governor with a throttle plate will be an interesting challenge for a future project.

I think both systems are quite common, as some of the simple lubricator designs I have seen for small engines simply have the steam line passing completely through the lubricator body, with a small hole in the tube connecting the steam line to the oil space.  However many of the commercial regulators, and various other design drawings have a needle valve separating the lubricator oil space from the steam line.  I have seen it suggested that this needle valve enables regulation of the flow of steam, however, I suspect it is effectively only a stop valve, either open or closed.  Leaving it closed until the engine is running means there is no possibility of "a slug of condensate" entering the lubricator during the warm up, displacing a larger volume of oil when the cylinder drains might be open so the oil might not even contribute to the engine lubrication.  When I open the valve, there is hot, relatively dry steam flowing past in the steam line.  But it may not be a big issue.

Once the engine is running, I open the lubricator valve, and that "shot of steam" pressurises to lubricator to steam pressure.  But assuming the lubricator is cold, or perhaps just warm due to conduction from the steam pipe, the steam starts condensing on the cold metal.  Now we know that steam condenses very quickly on a cold surface, quick enough that condensation on the cylinder walls of a running engine reduces its efficiency, but depending on the actual pressure in the steam line, when the steam condenses the volume of condensate is of the order of 1/1000 of the volume of steam.  New steam keeps coming in to replace the lost volume and condensate increases the volume of liquid in the reservoir, and slowly makes its way to the bottom, so lifting the oil level.

I like your suggestion that the oil should only be filled to the level of the cross bar, leaving an air bubble trapped in the top of the lubricator, and meaning that oil should start to be transferred quite quickly and no problems with seating the filler plug against a liquid full space.

Now I see two mysteries, first the reservoir is a blind enclosure with one tiny hole connecting it to the steam pipe.  With steam flowing in to replace the volume lost by condensing, how does the air get out through the same opening?  Similarly for the oil.  Though I have seen, in close study of pump and compressor seals, that even with a pressure drop by the seal fluid in one direction, gas or liquid does migrate the opposite direction if velocities are not sufficient to keep everything in its place.  And there will be surface tension effects that will help the oil travel, but counterintuitive to have flow in both directions at the same time, especially with a hole only 1mm in diameter.

I don't really know how to calculate the volume flow in that initial period while the steam condenses on the reservoir walls, but once the metal is up to temperature, further heat transfer is limited by convection and radiation to the surrounding environment.  My calculations give an idea of heat transfer by convection and radiation, hence mass flow through that 1 mm passage.  I may even be able to make an estimate of the resulting steam velocity.

Now, I have been out enjoying my granddaughters school play this evening, so I won't try and keep tracking all those zeros tonight.  But I will try it tomorrow, and see if the calculation yields a plausible result.

However, it is clear that the heat loss ensures a continuing controlled condensation in the reservoir that ensures the oil level is continually lifted, so is able to flow out and be carried through to the engine in a controlled manner.

Oh, and the matter of leakage, I totally agree, no leaks, even into the drip tray.  Unfortunately I can't say the same for the engine, I can see some liquid gaskets in my life in the near future.  When I was referring to leakage, I was referring to the oil escaping from the reservoir into the steam line.  The quantity is very small, and to call it flow feels like an exaggeration.  It might be better to call it seepage, or perhaps just call it a very small flow.

More to those little lubricators than meets the eye.

Hi Willy, I agree totally with your logic, if you are able to top up the oil every run, it must be going somewhere and the only way available is to the steam pipe where it is carried on to the engine.  And they definitely seem to work when running on steam, though a different principal is required if you are running on air.  But you did not mention the water.  If you don't empty it, I have to wonder where the water goes, assuming that the steam connection to the lubricator is near the top, similar to mine in the picture.  I know that when mine has cooled down, I remove the plug and open the drain at the bottom, and get quite a bit of water draining out.  I just have not previously made any attempt to measure the quantity.  I would expect that if you don't have a drain valve, you would need to use a syringe to suck it out through the top, or detach the reservoir and tip it out.

I think most of us know of circumstances which make the eyes misty, when there is too much moisture for blinking to adequately remove.  Much more than the quantity which might get past your glasses when you have a hot drink.  But eventually blinking eyelids catch up, perhaps with the help of a handkerchief, and you can see again.

Drinking naphtha?  I certainly wouldn't recommend it.  But poisoning the customers was never a consideration when those snake oil salesmen had a potion to sell.  I believe that in your country, and possibly others, publicans would increase their profits by watering down the beer, then restoring the "kick" by adding arsenic, a totally natural product as my wife would say!  We have a bit of a problem with people sniffing petrol, and over time it really scrambles their brains.  We even have odourless petrol available in some areas to discourage it.  Others choose glue with similar results.  I would expect drinking naphtha would be similarly damaging, with a very high probability of inducing a chemical pneumonia if any gets into the lungs.  It is tragic when people feel so bad, that they have so little respect for the wonders and health of their own brain.  Mind you if some of those remedies involved external application it might sometimes help, though I have no real knowledge to prove it.  However, some of the components are carcinogenic, so unlikely to lead to a happy result in the long term, however it is used.  I would not be in a hurry to try it.  But getting off topic, perhaps we had better return to thermodynamics.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on June 02, 2018, 02:00:42 AM
 'With steam flowing in to replace the volume lost by condensing, how does the air get out through the same opening?  Similarly for the oil'

 :thinking:...following is my understanding of the sequence of events that occur at the steam startup all hidden away within the sealed lubricator ...[although I am always happy and prepared to listen to other thoughts]

The design of [my] displacement lubricator [Winfried Niggle Germany] provides the ~~ 0.7 diameter hole in the steam transfer tube [4 OD x 2 ID] with the hole drilled in the horizontal plane....so as such, when the boiler steam stop valve is opened, each component down stream is pressurised 

With oil prefilled to lap the lower diameter of the steam transfer tube ....the hole is ~~ 1.7 mm above the level of the oil leaving a relatively small volume airspace ......so the steam enters the lubricator bowl via the 0.7 diameter hole, this then continues to condense as bubbles of water that naturally sink to the bottom and carries on until the oil fluid surface meets the 0.7 diameter hole ...

With the oil being virtually incompressible, it overcomes the compressibility of the entrapped 'air and steam' and oil is injected into the steam path to the regulator. This explains how the entrapped air is forced into solution with the oil and is quickly purged as steam oil with air bubbles

So it would appear reasonable to be careful with the oil level especially with cast iron cylinders as under filling the lubricator will then take a longer time period before condensate can displace the oil

With brass cylinders & pistons with the relatively low pressures we are using, a number of knowledgeable modellers suggest the lubricity of LP steam  itself is sufficient for the requirement

I certainly do not discount this, however visualise hardened steel pivot pins between the piston the connecting rods in the internals of my engine......

My understanding of lubricators with needle valves fitted is the valve is an adjustable orifice to adjust or limit the amount of oil that is displaced or thus injected into the steam tube passage. [I have not seen any commercially manufactured lubricators fitted with a valve to limit steam flow]

Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 02, 2018, 01:32:01 PM
Oh, the joys of a sewered society!  Of course we would not want to be without it, but when my neighbour rang the doorbell at 7 am this morning, waking me from a sound sleep, I knew we were in for a bad day.  We had sewage flowing down the side of the house, under the fence to his yard, where the grade diverted it under his house.  He was woken by the smell from his floating heater ducts!

Our area had the sewage installed about 1918 despite material shortages, so not real great.  Our forefathers had the wisdom to know that plumbers have saved many more lives than doctors, and invested in a system with plenty of allowance for flooding rain and the foreseeable population increase.  But our city is now over 4,000,000, an unimaginable number back then.  And modern economists tell us it is cheaper to increase housing density than start a new city.  And that is after they have chewed up all that flood provision.  I don't think they allow for the cost of what follows.

The main sewer blocked near us, and the vagaries of varying land slope and changes of elevation along the street mean our vents are the first low point above the blockage.  So we were greeted by far too much information about what all the neighbours to the west and slightly higher elevation were putting down the sewer.  We had a gusher for over three hours before it was stemmed.   But in the grand scheme of things, after the heat of the moment has passed, I guess that even that is not too bad.  But you don't want the pictures!

We know the procedure well.  We know who to ring, how to prove it was their responsibility, not ours, as if we could ever produce the quantity, no matter how unwell.  It used to happen every five years, now down to two.  The sewer main needs relining, they don't even dig it up these days, but every few years all the contracts are relet, and nobody has any long term memory.  Except us!

So, many phone calls later, showing countless people where everything is, written reports, more phone calls later and so on, we now have the flow going the right direction, our sink will drain, and only the cleanup, by the supply authority of course, to be done tomorrow, and the replacement of the neighbours heating ducts, again!  At least there is no doubt when the problem is this magnitude.  A small issue is much harder as bureaucracy seems always to try and blame the householder, rather than take responsibility.  I assume the same everywhere.

So now I can sit and read a little more thermodynamics for light relief.

Hi Derek, I guess we could toss that back and forth for quite a while, but I am sure you will agree that typing it all gets a bit tedious.  Perhaps over a drink sometime.  But I did manage to carry my calculations a bit further. 

When steam is initially introduced to the lubricator, it condenses against the cooler metal body and oil, heating it all up.  This happens quite quickly but I don't have an estimate for the time this process takes.  However, once the lubricator body is up to a steady temperature, I can estimate the heat loss to the atmosphere from convection and radiation, and this determines the amount of steam that can continue to condense while the engine runs.  With the boiler stop valve fully open (or absent), I assume the steam pipe pressure is close enough to boiler pressure.  Regardless of superheat (though would require a little additional heat loss for condensing) steam is saturated in the top of the lubricator where the condensing is happening, so the steam tables tell me the latent heat, but also the specific volume of the steam.  So using the heat loss calculation, I can calculate the mass and volume of steam entering the lubricator. (It's more like an estimate really, as while the calculations are OK, they rely on a lot of input numbers that can only be approximate).  Then, using the diameter of the passage between the lubricator and the steam pipe, I can calculate the velocity in that 1 mm diameter passage.

Now the biggest issue in all calculations of this type, especially when numbers are very large or very small, is keeping track of the correct order of magnitude.  Perhaps it comes from all those years using a slide rule.  I find the best procedure is to very strictly stick to the fundamental units for the unit system being used.  This is  SI as for all my calculations, and the fundamental units are are kg, meters and seconds.  While the numbers are very small for this particular example, these units are used in the steam tables, so there are no conversions of units required.  Rather than keeping track of large number of zeros, I select scientific format for the numbers with two or three digits, and even an iPad can neatly display the numbers and keep track of the powers of ten.  Unfortunately, the iPad does not offer engineering format, which always has exponents divisible by three, so fits in better with prefixes like mili or micro or nano etc.

So what was the result?  I know most don't like too much calculation, so I will just provide the summary.  After the initial warm up of the metal lubricator body and oil, I calculated a total heat loss by conduction and radiation of 2.4 W.  For steam, I assumed about 300 kPa, as something representative of the likely pressures. The specific volume is about 0.6 m3/kg. And the quantity of steam condensed is about 0.64 grams in ten minutes, quickly converted to 1.2 X 10-6kg/s.

A little maths and the steam velocity in that 1 mm diameter passage is 0.8 m/s.  A 0.7 mm hole would result in a velocity about 30% higher.  Not even difficult, using a spreadsheet.  When I manage some measured temperatures from the next test run, I can update the figures and let the apple recalculate.  Operating pressure ranging from unloaded runs to around 50 psig would make this velocity range from about 0.4 m/s to about 1.6 m/s. 

Now, if my memory is recalling anything about dry gas seals on a pump, the required velocity across the seal faces has to be somewhat higher than 0.8 m/s to prevent liquid migration across the seal faces against the imposed pressure drop.  Counterintuitive, I know, but I had the opportunity, and privilege to talk to experts on this topic in that previous life, and it was quite memorable, even if I can no longer recall the precise figures.   But I am sorry to say that calculating the oil flow rate for that situation is beyond me.

A better approach for oil flow estimates will come from operating the lubricator, timing how long the lubricator is in operating and measuring the water drained at the end of the run.  This is the same as the volume of oil consumed, with the only issue being allowing for the condensed water for that initial metal warming.

I think I have already mentioned that there is more to those displacement lubricators than it first appears.  I hope this little excursion into the detail is interesting.

Thanks for following along.

MJM460
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on June 02, 2018, 03:27:49 PM
Oh, the joys of a sewered society!  Of course we would not want to be without it, but when my neighbour rang the doorbell at 7 am this morning, waking me from a sound sleep, I knew we were in for a bad day.  We had sewage flowing down the side of the house, under the fence to his yard, where the grade diverted it under his house.  He was woken by the smell from his floating heater ducts!


Maybe they should have built a proper pumping station like the Victorians would have done ;-)

http://www.dailymail.co.uk/news/article-3742144/New-museum-opens-inside-Victorian-pumping-station-dubbed-Cistern-Chapel-originally-built-combat-Great-Stink-1858.html
Title: Re: Talking Thermodynamics
Post by: derekwarner on June 03, 2018, 12:17:02 AM
I didn't realise your little city had multiplied itself to the dizzy number of 4M persons  :facepalm:

Well that was certainly a sordid sewerage tail MJM.......not quiet sure why your you neighbour had heating elements near a sewer line?

However only the Scott's would take a disused sewage sludge steam boat tanker and turn it into a Tourist attraction  :embarassed:  it gets even worse...one of the tanks is now a carpenters workshop  :Lol: 

https://www.google.com.au/url?sa=i&source=images&cd=&ved=2ahUKEwiswNPairbbAhUGi7wKHQtAC0QQjhx6BAgBEAM&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FSS_Shieldhall&psig=AOvVaw0uhg2Uf6RTaGHsvwy47KzV&ust=1528066831582648
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 03, 2018, 11:54:49 AM
Hi Gas Mantle, good to hear from you again.  I have been quietly following your progress with that vertical boiler.  I am still hoping that you will eventually try and measure how much steam it produces in a given time.  Probably easier on gas than coal.

They did build a pumping station, just like in Victorian times.  In fact, we are in the state if Victoria, and it was built quite a long time ago, so it was actually Victorian times in many ways.  And it is still operating last I heard.   And I have even been on a tour.  Needless to say, everything you see is pristine clean.  Our problems were not related to pumping problems, but a collector piping blockage.  I believe those baby wipes that you just "throw away thoughtfully" and "wet strength" tissues are high on the list of probable culprits, even in quite large pipes.

Hi Derek, your comment on Melbourne's population prompted me to check the latest figures.  Seems I missed a few folks, the official 2017 figure is 4.8 million.  Accuracy like convection calculations!

The neighbours heating ducts are not really so near the sewer, after all the flow all came from the vents on our property.  But the neighbour is slightly down hill, and the flow ran under the fence, then under his house.  The puzzling bit is why the insurance company did not make sure the ducts were tied up to the floor beams, clear of ground level last time they replaced them.

You laugh at that tourist attraction, but ours is the feature attraction at the children's science museum.  Nothing like letting the kids play there.  But they are learning more real science than they can find in a book shop.

I meant to comment on the water lubrication of brass pistons.  I don't think all those Mamod, and other similar models we had when we were young, had lubricators at all.  At least, mine did not.  And the drop of oil on the port face before a run or even during, probably did not help the piston much.  So moisture from that wet steam was probably all they had.  Of course, some experienced a lot of wear, and I have seen magazine articles by people who have restored them.  But many probably lasted by not being overused.  Somebody may have information on what they do on small  steam launches, where they reuse the water as boiler feed.  They don't really want oil in the boiler, so do they use piston rings that are happy with water lubrication, or just have really good oil separators?

The big clean up was completed today, but took most of the day.  However I have managed to cut a brass plate and put it under the boiler, squeezed between the boiler feet and the base board by the boiler hold down bolts.  The boiler feet are soldered to the boiler shell so probably get quite hot.  I also made a clamp around the gas tank which has an extension tab that bolts down to the boiler tray plate, which I hope will transfer enough heat to the gas tank to keep the pressure up a bit.  Should get to test it this week some time.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 04, 2018, 01:36:24 AM
Hi MJM, I have looked at my displacement lubricator and it is different to others... I took the design from one of my books but cannot find the actual drawing !...... also saw this post on our forum about "growing" cast iron ?? !! also one of the other posts is featuring a triple engine but has quite a few valves to block off the steam input to the individual cylinders.. I think this was to reduce the steam consumption with differing loads. the chap making these valves is doing a great job ,I forget the actual name of the posts , but was wondering how the pistons get lubricated with out the steam entering the cylinders ??
willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on June 04, 2018, 01:58:13 AM
Morning Willy.....must admit, I have not seen a lubricator with this upside down build configuration  :headscratch:

I assume the steel rod entering in @ 9:00 is a valve spindle, and do we see a screw adjustment @ 3:00?

Are there any other tubes soldered to the rear of the lubricator that we cannot see?

Derek
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 04, 2018, 03:25:06 AM
Hi Derek, the valve spindle is the red  rod and the green screw just attaches the assy to the boiler. I made this about ten years ago and I'm sure i copied one from a book !!  I shall endeavour to find it ........There are no other tubes  attached  !!

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 04, 2018, 12:43:30 PM
Hi Willy, that certainly is a different regulator to the few I have seen, but I don't pretend to have seen all variations.  It looks like the rectangular block is a simple right angle valve which is the engine regulator, with or without a separate stop valve on the boiler.  Always hard to guess what goes on inside, but I assume the lubricator body has a tiny passage into the rectangular valve body on the engine side of the valve seat.  I hope that if you can't find the drawing or book, you might find time to look inside and tell us the secret.

When you operate the engine, does it work chicken feeder style?  Or do you screw down that plug occasionally to inject a little oil?  The trouble with the chicken feeder idea is that it would be steam bubbling in in response to the low pressure when some oil leaks out.  And of course the steam would condense, and the condensate would sink to the bottom and be first out, so that does not seem to work as an explanation.  Very puzzling.  But the thought occurs to me that it might work to put a little oil in an engine running on air.  The main requirements would be that the piping ensured that oil would drain into the valve chest, rather than accumulating at a low point, and also, how would you stop the oil flow when the steam is shut off?

Another idea that might work in that configuration, is a vertical tube inside the lubricator, so steam arrives at the top of the reservoir, condenses, sinks, raises the oil level, so that oil runs down the tube into the steam pipe.  It would need some means of draining the condensate, but generally just like the more conventional design.  But then, like the conventional design, it would not work when running on air.

Very interested to find out more.

The thread with the information about cast iron growing with heat treatment is in Brian Rupnow's latest build thread about the twin cylinder oscillating engine, with interesting additional description of the growing cast iron from Ramon.  I have often regretted not knowing a little more about metallurgy so I could understand some of those things.  But there is only time to learn so much in one life time.  I believe Ramon is in the process of preparing a bit more information.  Very interesting, as with so many threads on this forum. 

The triple expansion engine you are referring to would be Maury's build of the Dickson engine.  Another fascinating build, in the kits/castings board.  You can see I read them all.  I know he mentioned single, twin and triple expansion modes.  I am not sure if this is intended as a literal description of how the valves are used, or if that is still to come.  But compound engines often have a means of introducing steam to all cylinders for warm up and startup purposes.  The size of those valves and the associated pipes suggests that steam could be introduced to all three cylinders for more power via simple expansion mode.  Perhaps the cylinders could also be connected in two stage compound mode, for economical running at a different power level.  And of course triple expansion mode.  I hope Maury will eventually tell us more about it, but at the moment, getting the castings out is the main activity.  I guess we will all keep watching keenly.  I suspect we will get through a lot of popcorn before that build is complete. 

But back to your question, I don't think running a cylinder with no steam would work very well.    My best guess is that there is always some steam to all cylinders, but the valves allow various configurations of single, double or triple expansion.  Even modern turbines need "cooling steam" supplied if they are expected to freewheel when the regulator valve is shut.  It seems the engine was built in the early days of electricity, to power AC alternators.  Interestingly, it seems in three phase configuration, but with one alternator on each phase, unlike today's windings which give the three phases from one machine.  However, with AC, the frequency, hence engine speed, has to be constant.  So to get efficient operation at very different electrical loads, you need different torque at the same speed.  Throttling is the simple method of achieving this, but not very efficient over a large range, so I suspect those valves allowed the engine to operate efficiently by selecting different cylinder configurations.  But I await the definitive explanation for those with more knowledge of the engine.

Hi Derek, thanks for joining in.  Like you, I have not seen an inverted design before.  I have suggested some possibilities in my reply to Willy above, but we will both have to wait until we have more information on what is inside those brass components. 

The intent was to run an engine test today and see if I can measure how much condensate the lubricator produces while open to the engine.  I got as far as measuring the lubricator body temperature just after the engine started, but before I opened the valve, then, having dealt with that, I promptly forgot to open the lubricator valve.  Whoops!  Just as well there is sufficient oil when the lubricator is operated correctly and the remaining oil was enough.  So keep the fingers crossed for more multitasking success tomorrow.  The brass plate did keep the gas temperature and hence pressure a bit higher.  I will see how successful when I complete the analysis.

I probably make these engine tests seem a little complex.  That is really self inflicted, as I am interested in obtaining as much information as possible from each run.  It leaves me trying to record four thermocouple instruments, several infra red readings (trying to have the spot at the same place each time), and times from the iPad stopwatch function every few minutes.  Then adding in tacho readings once the engine is running.  I could go back and read the times afterwards, but I find it more reliable to write them in the log to avoid issues with the occasional extra early lap time captures which make the iPad log very confusing later.  And of course operating the cylinder valves while the cylinder warms up.  So I make it complicated for myself, trying to do it all each time, instead of concentrating on one aspect each run.

However, a simple test to determine how much steam the boiler produces in a given run time is a much easier exercise.  Weigh the water into the empty boiler, and again the remaining amount you can extract from the boiler after it has cooled, weigh the fuel, record the time you open the boiler stop valve and the time you end the run.  With these few measurements you can determine the steam production rate.  And a few measurements allow you to calculate the heat transfer area, which is the critical factor in comparing different sized boilers of a similar type.  So don't be afraid to give it a go.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 04, 2018, 09:25:35 PM
Hi MJM , will endeavour to find the drawing ....in the meantime ..how they lubricated things in 1881 !!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 05, 2018, 12:54:28 PM
Hi Willy, I am not sure about all those ingredients, but some remind me of the soap making process.  I wonder if they were trying to make an early version of grease, to get adequate lubrication from a mixture thick enough to stay in place.  Even today, grease has lithium or calcium compounds in different formulations.  But those early versions look like they could have been corrosive, especially if the mixture was not quite as intended.  There is quite a lot of science in grease formulations for different purposes. 

I did another boiler test run this afternoon.  I can see why those early pioneers your books talk about had to spend so much time experimenting and testing.  There is always something that seems to go wrong. 

I think I can confidently say the brass plate was a success.  Without it the gas tank  temperature was down to 2.3 at the end of the run, and the gas flow barely enough to keep the boiler above 102 C, and the gas consumption averaged 0.01 g/s.

With the plate, the gas tank initially cooled down and reached a minimum temperature of 5.5 degrees by the time the boiler reached 100, but as the run continued the gas tank temperature rose to reach 10 C by the end of the run.  This extra temperature was reflected in higher gas consumption.  It was up to 0.014 g/s, so 40% higher.  This was reflected in the engine maintaining its speed, so a boat performance would be improved on cool days.  Of course, in summer, the pressure would be much higher, which the gas tank is designed for, but it might be necessary to have one of those gas pressure regulator/ boiler pressure controllers that restrict the gas pressure to limit the boiler pressure rise, as the gas consumption and steam production would be higher again.  Obviously having the whole plant installed in a boat would retain more heat from the boiler and burner, so the low pressure would not be so much of a limitation as in my cold open garage.

It has been an interesting little experiment to try and quantify the effect of that reducing gas temperature that is so frequently reported.  It also gives a little insight into the skills demonstrated by those experienced modellers that always seem to get their model running well.  But I think I am a long way from getting a steam powered model on the water. 

I just made a simple flat plate with a clamp to increase the heat transfer to the gas tank as in the attached picture.

The lubricator test did not turn out as expected.  I remembered to open the lubricator valve about 30 seconds after the engine started running.  At the end of the run, I shut the lubricator valve while the engine was still running, immediately after closing the boiler stop valve, and left it all closed in during the cooling of the boiler.  When it was all cooled, I opened the filler plug on the lubricator and opened the bottom drain valve to let the condensate to run out.  The lubricator seemed totally empty.  At least all the oil was gone, and while not much appeared in the exhaust, only a few drops of oily emulsion remain in the lubricator.  This was unexpected, as I have often enough drained my lubricators and had about the expected amount of water drain out, followed by a few drops of emulsion, then reasonably clean oil.  I have some ideas of what might have happened, but I will have to watch the lubricator operation more closely until I work out out.  I am starting to really see what those early pioneers were on about.

I probably won't get a chance to do another test tomorrow, but with luck, I might get some analysis of the heating and cooling parts of the test.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 06, 2018, 01:16:51 PM
Just working on the analysis of those latest test runs of the centreflue boiler with the heat plate under the gas tank today.  First look seems promising, and will make some progress tomorrow, though it does not look like I will get much time for it, so only a little progress.  The rest of the world seems to think that other things also have to be done!

I will look in tomorrow, but will only post if I have something more to say.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on June 06, 2018, 02:17:10 PM
Well MJM.........if the wooden slat insulation on the MSM boiler is effective, together with the feet to elevate the boiler from your brass base plate you may not have a great heat transfer to your MSM gas tank

Here I suggest is the great advantage of the $15.00 digital laser pyrometer to use & see what is happening [ie., the temperature of the brass plate?, the actual temperature of the gas tank at the base and the discharge connection? etc]

Are you weighing the gas tank pre & post runs?

We read a great deal [especially from French model steam WEB sites] where they acknowledge their low ambient temperatures and gas transfer issues. Many complex works/words are spoken of gas supply in the liquid phase to the burner.....I read but tend to not consider these with our traditionally higher ambient temperatures

As you note  our early pioneers had their challenges....[we too just follow on]

Derek
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 07, 2018, 01:10:11 AM
Hi MJM , interesting analysis of the plate under the gas tank ... would it be beneficial to lag the upper part of the gas tank  and also the delivery pipe ? also lagging part of the brass plate that is in-between  the gas tank and the boiler ??
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 07, 2018, 12:15:03 PM
Hi Derek, I am glad that you are following along and keeping a watchful eye on my test procedures. 

I certainly feel that the wood strip insulation is not as good as I would like, I should have put a layer of cork under it.  One of those "Dohhh!" moments.  My infra red pyrometer gives a reading of over 100 degrees on the outside of the timber strips, with better insulation, this will be lower and therefore less heat loss through the insulation and by convection to the surrounding air.  I will be adding this in the near future.  With a bit of luck, the planks which are cut around the top fittings will still be ok, and I will be make up for the extra circumference added by the cork, by an extra plank or two at the bottom.  Easy enough to calculate the extra circumference due to the cork.  I am also thinking about how to hold some cladding on the ends, always the most difficult part of insulation of any size vessel.  On full size pressure vessels, we used to have nuts welded on edge as anchors for wire to hold insulation in place.  However, I am not going to solder on to a beautifully finished and tested boiler!

The boiler support feet are actually an advantage, they are brass (or bronze) perhaps castings, but in any case, silver soldered to the boiler shell, so they actually aid, and are probably the main path for heat conduction to the brass plate, which is securely sandwiched by bolts through the feet, the plate and  the wooden base board.

You will be pleased to know that I definitely weigh the gas tank both before and after the run.  Along with using the iPad stop watch function to accurately record the times of lighting and extinguishing the burner, this is the method I use to determine the average gas consumption.  I would like a better quality scale with a resolution of 0.1 g, but these are quite expensive.  However, using the digital kitchen scale, and putting the tank on for a reading then removing it to check that it returns to zero a couple of times, I am getting readings within 1 g, which in reality is probably more than adequate.  My daughter-in-law has one of the better ones, but it needs wind shields around it for consistent readings, even indoors, so there is a limit to what is practical.

Unfortunately the method only gives one overall average flow rate for the whole run.  This is a disadvantage in analysis of flow which starts off high, then slows as the pressure drop, so varies considerably throughout the run.  And I am now finding that with the brass plate, it initially drops, then reaches a minimum and starts to rise again as heat starts coming through that that plate.  However, a change in the overall average is a significant change, even if I don't have any information about how it changes.  I am attempting to make each run a specific different time, so by assuming the pattern is roughly repeating, the difference in amount burned at different times gives an extra clue as to how the rate varies with time.

I have seen similar reports to those you refer to from the northern hemisphere, which is what prompted me to endure the frigid conditions in an unheated Melbourne garage in winter, to see if I could quantify the issue.  And perhaps even quantify some of the methods to overcome the problem.  I have the data for pure propane or butane, but I do not have the ability to calculate the vapour pressure with temperature for a mixture.  Like you, I barely notice the change in our more civilised temperatures at other times of the year, when the pressure is adequate the flow, while still varying, is near enough to constant for most purposes.   The temperature range is such that you need some means of moderating the heat transfer in summer.  You don't really want to be demonstrating the pressure relief on the gas tank in your model with a lighted boiler.  The plate has the advantage that you can slip a sheet of cork under the gas tank or similar, to limit the heat transfer on a warmer day.

Hi Willy, the answer to your questions comes from looking at the temperature differences and the resulting heat flow.  The problem is that evaporating the liquid requires heat, and in the absence of an external source, that heat comes from the remaining liquid which soon becomes cooler than the surrounding air, behaviour that is used to good effect in any refrigerator.  As soon as the liquid temperature falls, heat is able to flow in from the atmosphere, which is the direction we want.  Lagging would close off that source of heat.  It is small, but helpful.  The gas pipe is similar.  The gas comes off at about the liquid temperature, so cooler than atmospheric.  Any heat that leaks in from the atmosphere is heat into the boiler, so while very small, still in the right direction.  So again the conclusion is that lagging the pipe is the wrong direction.

Lagging the plate is a more complex question.  The plate warms by heat conducted from the boiler, as the boiler warms up, so it then also looses heat to the atmosphere.  However, immediately under the boiler, there is also heat supplied by radiation from the boiler.  Whether to insulate there is dependent on the quality of that insulation.  In the end I can use the infra red readings to determine whether I am winning, therefore no insulation or loosing, in which case I would be better to insulate and rely entirely on conduction through the boiler feet.  Closer to the gas tank, I am almost certainly loosing heat from the plate to the air, so I feel I will get a little improvement by including a wood plate on top of the clip holding the tank, both to hold it in closer contact with the base, and also to reduce heat loss to the air.  Similarly, I could attach a layer of cork to the clip around the tank to reduce heat loss to the air.

All in all, a quite involved answer to deceptively simple looking questions.  But all an important contribution to understanding what is going on.

Today turned out even bigger than expected, so not much progress on the analysis, but progress takes time.  I expect to get back to it tomorrow.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on June 07, 2018, 02:14:30 PM
I certainly agree MJM, that your digital scales [load cell] will offer acceptable & repeatable accuracy.......

My refillable [Bix UK manufacture] gas tank has been external wooden strip lagging......for a two fold plan.....

Firstly it will be used in an open decked vessel and exposed directly to the sun in summer and will register 65 degrees C on the endcaps....
[interestingly the disposable the disposable Gasmate cans state they should not be subjected to 50 degrees C]

Secondly, in winter the Gasmate can kept indoors may sit at 18 degrees C, so transferring the gas to the gas tank will with the external lagging help to stabilise & maintain  a similar temperature in the wooden lagged gas tank

Derek
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 08, 2018, 01:35:26 PM
Hi MJM ,  So is there an optimal ambient temp for the gas tank, and will this change during use ?? I managed to put my foot in it at the local club !! A chap came in with an old LBSC type boiler that was soft soldered  in quite a few places and he was asking the boiler inspector about getting a certificate.... the inspector then queried all the leaks and dodgy construction and said it would be very difficult to make it safe ..and quite apologetic about this. I then put my 2 pennyworth bit in and suggested he might use it as a showerhead much to the dismay and mirth from the assembled cognoscenti !!!!! :lolb: :mischief:
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 08, 2018, 02:22:22 PM
Hi Derek, with the kitchen scales, I find that as long as I check consistency as I described, I am happy enough with the accuracy, particularly for the water which depending on the boiler is 100 to 350 grammes of water.  I would like to try for a little better for the fuel, as the numbers I am measuring  are quite small.  So 1 gram on each of the before and after measurements makes a potential 2 gram error in the answer after subtraction of the two figures, a significant error in around 25 gram for a short run.  A longer run would be a little better, but the pressure gets so low under the current conditions if I run for longer.  The main issues for accuracy I find are when the battery gets a bit flat, the readings loose consistency with those simple tests, long before the low battery symbol appears.  The other one is more subtle.  If I do not put the scale on a perfectly flat surface, the readings become inconsistent.  I have found a small piece of laminex covered MDF is fine, but the bench surface is not.  The rough surface seems to affect the accuracy of the scales.

Your wood lagging in summer, to avoid surface temperatures of 65 degrees is a good idea, I do not doubt that you get those temperatures if you have the tank exposed in the sun.  It shows the range of issues that we have to deal with.  Even down here, metal objects left in the sun are way too hot to handle.

The lagging means that the heat lost to evaporate the liquid is not replaced by heat from the surrounding air, so the tank effectively becomes refrigerated, just like an LNG tanker, where the boil off keeps the liquid cold, and the resulting vapour is either used as fuel for the ships engines, or condensed by a refrigeration compressor and returned to the tanks.  Clearly you are running a refrigerated LPG  vessel!  Though only when the burner is operating.

When you refill the tank from the Gasmate container, some liquid is evaporated and escapes during the fill process, so the initial charge to the tank actually ends up a little cooler than the container.  But that Gasmate container looks like it is only butane, presumably a mixture of iso- and normal butane, so the pressure will be even lower than the propane/butane mixture that my burner uses.  If you let me know the mixture you have, I will look out the data.  Some estimating is still required, however, the errors are much less when there is no propane, which has a much higher vapour pressure than the butanes.  My burner uses a mixture of butanes with 30% propane.

It is not a good idea to use a mixture different from what the manufacturer recommends, but with butane, at this time of year you will need even more heat, as the vapour pressure will be quite low.

So thinking about how to cope with summer and winter temperatures, I am recognising that the plate under the boiler is not easy to control apart from the possibility of slipping a layer of insulation under the tank.  And your insulation looks like a good, even necessary idea for summer.  So perhaps for year round use (you don't really plan to leave that wonderful model in the cupboard over winter do you?) a coil around the tank under the insulation, and fed by exhaust steam, possibly taken from the the exhaust separator, and drained to an extra condensate tank in the very bottom of the boat might be the answer.  But don't restrict the engine exhaust by running the whole lot through the coil.  If the pipe has a valve in it somewhere, it can be turned off when not required in summer, when it would just sit under the lagging, or the valve can be opened to get the required heating in winter.  The lagging would then limit the heat loss from the coil to the atmosphere, so still be helpful.

I am sure this idea would require some experimentation to get the right balance for both heating the fuel tank and draining the condensate.  Worth a thought.

Hi Willy, your post appeared just as I was about to post, but fortunately, I did see it in time.  I would suggest not so much an optimum, as a minimum pressure that gets enough gas to the burner to raise an acceptable steam pressure.  I assume a noticeable slowing of a model would be a useful indication that it is time to bring the model back within reach.  And a maximum pressure determined by the fuel tank pressure design.  Within that range, the gas flow will vary with gas temperature.  If the range is acceptable, OK, otherwise, one of those steam pressure control valve that restricts the gas flow/pressure to control steam pressure to a more constant level.

However, for the performance analysis I am trying to do, it would obviously be desirable to have a constant flow, although even an idea of how the flow changes with time would do. Just not so easy in small scale.  Perhaps I should buy and install one of those valves.  For the mixture recommended for my burner, which contains propane as I mentioned above, the pressure is a little higher at any temperature than Derek's butane mixture.  However, I assume the gas jet sizes are different to account for this.  Similarly for the stove you use to make tea at the allotment.

For a racing model, if anyone is contemplating a gas fired flash boiler for one of those hydroplanes, you would want the pressure to stay very high, and you would probably use exhaust heat to keep it high, and design the gas tank for high pressure to suit.  But that approach is not for beginners.

That is a cruel sense of humour you have.  I can imagine the poor guy's feeling.  Soft soldering can probably be re-soldered, then pressure tested if the boiler is well made.  It might then be fine in the shed with a low intensity fire like Meths, though I don't believe soft soldering is ever suitable for coal firing.  However, no matter what, it would be unlikely that the boiler inspector or the club would want to know about it.  It probably should be polished up and consigned to the shelf.  Or a new copper boiler made to similar dimensions if it is part of a nice little steam plant.  It is not really that big a job to make a simple boiler.  I even have a similar age boiler, though at least it is well made, heavy gauge solid copper, and bolted as well as soldered.  I have not fired it up since I was quite young, and I still have the kerosene fuelled pressure stove that was used to fire it.  I would definitely pressure test the boiler before trying it again these days, and then I would probably only use Meths or solid fuel blocks.  Probably won't happen now.  It's more sentimental value, made more than two generations back in grandfathers day.

Still working on the analysis of those boiler test runs.  Managed an extra hour today, results definitely coming.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 10, 2018, 12:16:30 PM
Still watching that lubricator.  It is a long process.  When I first opened the drain after it had cooled down from the run, nothing came out.  I was at least expecting some condensate. 

However, I tend to be a bit slow putting things away, and left the drain open with the measure underneath, and forgot about it for a few days.  When I looked at it today, there was about 3 ml of an oily emulsion in the measure.  When I looked closely, it was starting to separate.  A thin layer of oil on top, with the rest of the emulsion underneath.  I don't know if that emulsion will eventually break to give to distinct layers.  Perhaps I should warm it up a bit, it was only 4 degrees on the workshop this morning.  Even room temperature would help.  So there might be more information coming yet.  That emulsion is quite sticky, still would clearly have some lubricating properties.  Some of those properties that make the compounded oil a good cylinder oil are starting to appear in its behaviour.

I have not made much progress on the analysis for the last two days.  Youngest son was moving house.  I carried a good number of loads up the stairs to the second story, but have had it today.  A quieter day today may see more progress tomorrow.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 11, 2018, 02:33:42 AM
Hi MJM, quite busy at the allotment and also taking advantage of the long summer evenings......Does the mains natural gas start out as a liquid underground and only change to a gas once it leaves the ground ?  also the lubricator came from the design for a steam crane that was published some time ago. This engine is also electrically heated !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 11, 2018, 01:44:48 PM
Hi Willy, 'Tis the season for working on the allotment.  With some good weather, I hope you will have some fresh vegetables to harvest in due course.

Natural gas is gas phase at all pressures under ground, as the temperature keeps it well into the superheated range.  (I have a phase diagram, so I checked my memory before writing that!  The whole two phase region is at very low negative temperature, even at high pressure, while the earth is quite warm apart from the first meter or so in cold winter climates.) Of course, there can be some dissolved in hydrocarbon liquids, but it's vapour pressure tends to make it separate out readily, so it often forms a gas "blanket" above oil in an oil reservoir formation.  The appearance of methane at the surface is usually the first sign that the well is heading for oil.  It used to be known as swamp gas, or marsh gas, but it also appears in cracks and fissures in coal, it is what makes coal mines hazardous, and the reason for the canaries and the safety lamp. 

To transport gas is expensive due to its low density, and thick walled pressure vessels to carry it at high pressure are also not practical.  Instead, large refrigeration plants are built to liquefy methane at around -160 Deg C, so it can be carried in ships in very well insulated tanks at about atmospheric pressure.  The tanks are large enough that they are generally spherical.  I think I have described them before.  It is then stored in similar tanks on shore at the receiving end until it is needed by the gas distribution system, when it is evaporated and warmed to return it to gas phase.  Smaller refrigeration plants keep it liquid while in storage.

Your picture of the crane prompted something in the back of my mind, and sent me perusing my small library. Your model looks quite old, and has almost certainly given some young children a lot of fun and learning.  I wonder if it is the one that Tubal Cane modelled used to inspire his model crane in his "Simple Model Steam Engines" book.  It even has that same combination lubricator and regulator valve.  Though the wheels are missing in the book.  Perhaps he had a similar one.

The drawing shows the lubricator is a simple closed reservoir, with a number 70 drill (about 0.7 mm) into the steam pipe at the bottom.  Which leaves us back pondering how it works.

I would assume there would normally be a small air bubble in the top of the vessel.  When steam starts flowing, the steam pressure will pressurise the reservoir through that small hole, and heat conduction will warm the air, so further increasing its volume so some oil will be forced out.  It is also likely that surface tension effects contribute to some oil entering the steam flow.  Possibly this oil is replaced by some steam.  However, the steam soon condenses to liquid, thus reducing in volume (by that factor of 1000:1), giving back the latent heat, in what is basically a constant pressure process.  The condensate tends to gather at the bottom, though I wonder if it is mostly as an emulsion, like what eventually drained out of my lubricator.  It is an interesting system, which operates in a different way to the more conventional displacement lubricator, but may not be very different in effect.  That compounded cylinder oil, which seems intended to lubricate in the presence of condensate, would come into its own on that design.

I am not sure that I have the operation of this design fully worked out.  I wonder if anyone else has found a more satisfactory explanation of how this type works?

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 11, 2018, 02:09:54 PM
Hi MJM. yes that is the engine that i based mine on ..and a put these wheels on it so it could be moved about on a table to also add interest for a small boy....Tubal Canes model used Meccanno gears but mine were cut out of bought gears. Some of the pictures on google show this lubricator  with a solid cap and others with some sort of screw valve arrangement on the top ?? I haves many books that i forget which ones are the relevent tomes !!
willy
PS...I had another smoothie today that was ice cold...i asked them to put some very hot water in it to warm it up !! but the hot water just stayed on top and would not mix with it ??!!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 12, 2018, 12:57:59 PM
Hi Willy, the similarity between your crane and the one in the book looked like more than coincidence.  The advantage of a small library is that I did not have to look very far to find it.  You are way ahead of me, in that you actually made the model.  I bought the book many years ago, before I bought the lathe, but never got back to building it.  I must admit that I found the number of tiny holes requiring very accurate placement had me a bit intimidated.  Perhaps I should have another look at it.  Using gears other than Meccano, is mentioned in the instructions, as is varying the jib construction.  It looks like you freelanced the boiler firing as well, is that the terminal box and controller for the electric elements?

I wonder if the intermittent operation of the crane is a factor in how that lubricator works.

Hot water has lower density than cold, especially as the smoothie is thickened up with whatever they put in them, which possibly further increased the density, so in principal the hot water could tend to float on top.  The viscosity of the smoothie would also tend to inhibit mixing, which might explain the difference between water in the smoothie and milk added to a cup of tea or coffee.     Do you know what ingredients the smoothie contains?  So while it is not intuitive that the two would not mix, I would assume the heat still had the desired effect by conduction.  Was the hot water poured gently on top?  That would not encourage mixing, but I assume it would mix if stirred with a spoon.  I think we need pictures!  (I don't think I will risk spoiling my iced coffee by adding hot water to see of it does the same.)

The next step in the boiler testing requires insulation to be added to the outside, under the planks, to reduce heat loss.  That will take me a few days at the speed I progress.  To many household projects and social engagements still taking too much time to allow any continuity.  So I will continue to check in each day (too many wonderful threads that I don't want to miss) and will continue the discussion on any questions that are posted, they are always very welcome.  However, I will take a break from trying to add something new until that boiler insulation is complete.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 12, 2018, 02:40:20 PM
Hi MJM,
https://www.youtube.com/watch?v=2EkjRslFgSM

here is a video of the steam crane at work...witha dissapearig trick at the end. The electrics in this engine were analogue  relays and things rather than modern solid state components !!
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 15, 2018, 01:07:00 PM
Hi Willy, thanks for posting that video, wonderful to see it actually running.

Now the boy has grown into a young man, you might be able to do some real engineering tests on the crane with him.

First, how much can it lift at different pressures?  A small bucket with water makes an easily adjusted and calibrated load.  Might need temporary counterweights clamped on so the crane does not overturn.  You will both soon see why building cranes have those big concrete weights.

Second, how does the power vary with pressure?  And how much power is achieved at maximum pressure?  Power is simply calculated if you time how long it takes to lift a known weight through a given distance.  Work is force times distance, power is rate of doing work, or work per unit time, so power is force times distance divided by time.  Might need to lengthen the rope and place the crane near the edge of the table and lift from the floor, so you lift the weight perhaps 0.6 or 0.8 meters for a more practical time period.  Use the stop watch function on the iPad to time the lift, screen time will be an added attraction.

Might need to max out a bit below the safety valve setting.  By the way, do you need to lift it to make sure it is not stuck?  I was getting a bit worried that it did not lift at full pressure gauge reading.

Nothing wrong with those relays and stuff, especially with a heating element as it is a simple resistive load.  And always useful to actually hearing it switch on and off.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 16, 2018, 12:55:24 AM
Hi MJM, glad you liked it and it was posted quite a while ago when the technology was a bit primitive  !! Also i am glad that i don't have the monopoly on the questions :mischief: :mischief: Yes relays are cool except when in 1964 my trade tester in electronics decided to slip a piece of paper under the contacts !!! it sounded ok with the unmistakable click ...but...nothing happened  !! I have not run this engine for sometime so not sure if it is still ok, also there is not much room on the spindle for any length of 'rope', Still very busy in the allotment at the mo...Ok a question If you have a cubic foot box full of ice level with the top how full will it be when it has all melted ?
Title: Re: Talking Thermodynamics
Post by: derekwarner on June 16, 2018, 03:40:59 AM
Willie....

This...."a cubic foot box full of ice level with the top how full will it be when it has all melted ?" ...is a trick question :shrug:

If the level of the ice is at the top of the box, it [the water]  has already expanded & grown in height level to the top of the box ...so when it melts...the water level must be the lower ...however if the box was full of water, then frozen.....the level of the ice rises above the top of the box & so when it melts the water level assumes the original height in the box

[from 1962 Science at Corrimal High School  :stickpoke:]

Derek
..

Title: Re: Talking Thermodynamics
Post by: MJM460 on June 16, 2018, 01:42:39 PM
Hi Willy, I reckon that crane will still work, or is at least well worth restoring.  But not while you have suitable weather for the allotment.  Rumour has it that you people never have to wait long for a rainy day, so there will be opportunities.  There is not much on the tele these days.  Remove, inspect and do some electrical insulation tests on the heaters and control system, or replace them.  A pressure test on the boiler, ease the safety valve by hand to ensure it is not stuck before you test its lift pressure and adjust if necessary.  It really should lift within the range of the pressure gauge.  Then some work on the engine and gears to get them all moving, touch up the paint and it will be in going order again.  Not much, of you say it quick.  So long as the boiler was originally silver soldered.  I think I would probably not be so keen of it was soft soldered.  Even if it then only joins others on the shelf, it should surely do so as a fully working model.

Regarding the ice question, I see Derek has already answered that one.  But keep the questions coming, they are always interesting and most welcome.

Hi Derek, good to hear from you again.  Thanks for providing an answer to that one.  Always encouraging to have contributions from others.

I wonder of you grew up listening as I did, to Professor Julius Sumner Miller.  I used to set my alarm early so I could listen to him on the radio before I got up in the morning, with his trademark question, "Why is it so?"  Perhaps it is his influence that is responsible for my somewhat long winded answers.  Often the "why" is so much more interesting than the plain facts of the answer. 

Anyone who has ever put an ice block in a drink, or seen ice flows on a river in winter knows that ice floats.  And you don't have to visit Antarctic waters to know that most of the ice block remains below the water surface, only the "tip of the iceberg" is visible above the surrounding water.  Ice floats because it's density is less that that of water at the same temperature.  In other words, water expands on freezing to occupy more volume than the liquid phase from which it forms.

Now we have to turn to the history books to find that a character named Archimedes is credited with discovering that a floating object displaces its weight in water exactly, no more and no less.  Supposedly, he was in the bath, and cried "Eureka, when the principal occurred to him.  If you push the object down, so that it is below the water surface, it displaces more water, so there is an upward force opposing that downward push, and the object will return to the initial floating position as soon as you stop pushing.  Alternatively, if you lift it a bit, the object will no longer displace its weight, and as soon as you stop lifting, the object drops back to its floating position.  This is the reason a boat lifts at every wave to stay afloat, rather than be swamped by each passing wave.  It is quite dramatic to be sitting in a small boat, watching big waves approach you from behind, and always, just in time, the stern rises and the boat is carried safely up and over the wave, only to descend into the trough before the next one, particularly in the dark!  Of course, the dynamic forces involved when waves break in a fierce storm are another matter.  It is also the reason a boat floats upright, despite the centre of gravity being above the water level and centre of buoyancy, but that is a different question.

So it is because the density of ice is lower than that of water, meaning the volume of the ice is more than the volume of liquid formed when it melts, that the liquid level behaves just as you described.

I have been fitting two layers of 3 mm thick cork to the centre-flue boiler to try and reduce the heat loss to the outside.  It seems a pity to have achieved the necessary heat transfer to the water, then to let it be lost through the thin layer of wood strip cladding I had originally fitted.  I actually thought it might have been enough, but my infrared thermometer was indicating an outside surface temperature of the timber of over 95 degrees.  Apart from the heat loss, this is hot enough to burn your skin if you accidentally touch it, so from the safety point of view, more insulation is needed. 

The conduction equation tells us that with the same temperature difference between the boiler and the surrounding air, thicker insulation results in less heat transfer.  When the conduction equation is used in conjunction with the convection equations for heat transfer between the timber surface and the air, we find that the timber surface will have a lower temperature.  So a lower outside surface temperature, and less heat loss.  I hope to be able to demonstrate just how much difference it makes with more test runs when I get the insulation refitted.  Two layers gives 6 mm on the radius, or 12 mm on diameter, and Pi times 12, or 38 mm on the circumference.  I will need about four extra wood strips to complete the cladding over the cork.

Not such a big job, just many interruptions.  Had the whole family to dinner this evening, two of us, our three children with their spouses, and all seven grand children.  It will be some years before we can do this again, as our daughter and her family are returning north to Darwin in about a weeks time.  Nearly 4000 km from here, so don't get to visit often, let alone have everyone together.  A wonderful occasion.

Thanks to all for looking in,

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 17, 2018, 01:56:09 AM
Hi MJM , actually the question about the ice was to ask what the measurement of the reduction in the level of the water once the ice had melted was.? Could this be worked out with a particular formula ? measuring it with a ruler might be a bit difficult as the meniscus would have to be taken into account ! Also why does the volume increase when water freezes and what does the increase consist of ?.......Thanks....

Willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on June 17, 2018, 02:47:50 AM
I seem to remember Julius Sumner Miller  :headscratch: on ABC television every Thursday night @ 7:30 PM...maybe 1st or 2nd year @ highschool so....1962/1963?

In those days...there was only 7, 9 & ABC2.....I was allowed to watch Sumner Miller ...but then sent to my room as The Mavis Bramston Show was on 7

Obviously my parents thought it was in appropriate viewing for me   :lolb:

Who was the other American Uni Lecturer on TV?......always had a cigar butt in his mouth & was a mad keen water speedboat fanatic

edit......had an American type Crewcut....first name Don......?

 
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 17, 2018, 01:16:59 PM
Hi Willy, those are two very good questions.  So not intended as a trick question after all.  Water is one of very few substances which expands as it freezes.  I believe there is one other, some sort of metal alloy, I think used in type setting, though I don't know it's composition.  Most substances contract on freezing.  This is because the molecules slow down as they cool, and the attractive forces between the molecules hold them closer together until they are only vibrating through a very small amplitude, and are effectively quite closely packed in a regular array, rather like marbles in a tin.  Some molecules collapse into a cubic array, some a cubic array but one in the middle of each cube, called body centred cubic, some with with one in the middle of each face of the cube, called face centred cubic.   I hope the names make sense.  The molecules are still moving randomly, but within the confines of those molecular forces, so they stay in a more or less fixed position in that regular array.  And the total volume is smaller than when the molecules are moving further and more freely in liquid phase.

But water freezes into a hexagonal matrix, not flat but three dimensional, but basically the same three axes of symmetry, and the basis for the hexagonal snow flakes.  What's more, instead of fitting really close together, those hexagons each have an empty space in the middle, so the lattice has a lot of empty volume.  Part of it is due to the molecular structure, each molecule having one oxygen atom and two hydrogens.  And the two hydrogens are at approximately 105 degrees apart, but other molecules do not end up sitting to fill that v between the hydrogens.   It is not clear to me just why it does this, that is the field of molecular physics.   It is easy to see how a slightly different arrangement of the same atoms could more completely fill the volume, so the whole space would be more fully occupied.  But it is that empty volume in the middle of each hexagon that is the reason for the extra volume.  It is made up of empty space.

When the ice melts, the molecules increase their movement, and in the process, the atoms do encroach into that space which is empty in the ice matrix, so do a better job of filling all the space, so the volume of the whole contracts.  Very mysterious.

Now, as to how much it expands on freezing, the thermodynamics book I am using at the moment, as well as having property tables for the liquid and vapour regions of the water phase diagram, also has the table for the solid-vapour region of the diagram.  This table has a column for the specific volume of the solid, equivalent to the specific volume of liquid column in the steam tables.

From this we can use the accurate table model for the behaviour of ice.  At the triple point, the only point where you can have liquid vapour and solid in equilibrium, 0.01 deg C, and 0.6113 pPa, the specific volume of ice is 0.0010908 m^3/kg, or a density of 1090.8 kg/m^3.  This compares with 0.001000 for liquid water at those conditions, or a density of 1000 kg/m^3.    (The figure is so exact because that is the definition of the mass of 1 kg.). So water expands by 9.08% when it freezes.

Once frozen, ice then behaves more normally in that it contracts on further cooling   So at -10 C the specific volume is 0.0010891, and at -40 C, the end of the table, it is 0.0010841 m^3/kg.  At least that will cover the temperature range experienced by most forum members, though I am not sure if perhaps Admiral Dk experiences lower temperatures in winter.  And of course any forum members wintering over in northern Canada, Siberia or the Antarctic.

Hi Derek, I must have seen Julius Sumner Miller on the Tele, but I was actually remembering him from radio, again ABC, he had a five minute segment each morning, or was it weekly?  Come to think of it, while TV was introduced in this country in 1956 for the Melbourne Olympics, my family did not have one for five years or so after that.  But late fifties or early sixties.  I don't remember the guy with the cigar, but there have been and still are a few of those characters that present science in an interesting way to the youngsters.  Pity book shops don't follow through with anything just a little more advanced.  They seem to think the budding scientists progress in one invisible step from primary school "safe" experiments to astrophysics, and don't need books in between!  Better stop griping before I incur the wrath.  So our Mavis was suitable for your parents, but not for you!

Thanks to all for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 18, 2018, 01:05:42 AM
Hi MJM, wow that is a pretty comprehensive reply...Thanks...Liquid Vapour !!! that is something to think about!!... So 9.08 % presumably that is a volumetric measurement  so in a container 12" cube how high is the top of the water ?  It is a bit late here and my brain has gone to sleep.. is it about 11" ?  waiting for some rain so i can get in the WKSP.... I was looking in a large tome on Thermodynamics and in the front was a dedication    "to my wife" !! so lucky lady ;D

Willy,
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 18, 2018, 02:19:09 PM
Hi Willy, thanks.  It was another double banger question, but pleased to be able to answer it.  That description of the physical structure if ice and what happens when liquid freezes is well covered in Rochard Feynmann's "Six Easy Pieces".  It is quite easy to read despite him being a Nobel Laureate in Physics.  Apparently he was known to do so much clowning around that people didn't know whether or not to treat him seriously.  Then he won the prize.  Brilliant.

If that ice is in a cube box, and to avoid any tricky implications, assume it has been shaved off level with the top of the cube, then when it melts the height would reduce to about 0.916 times the height of the cube, (1/1.091 times the height) as the sides of the cube are assumed to be unchanged. 

If, however,my out could somehow contain the water with some sort of elastic membrane so it remained a cube, then the length of each side increases by 1.03 times its original length as the liquid freezes.  You can easily check that 1.03 cu ed equals 1.09.

Hmmm, liquid vapour, or solid vapour!  Not very precise terminology for the mischievous mind.  The solid to vapour phase change is called sublimation.  It was taught along with some obscure chemical example when I was at school, an example I have long since forgotten.  But the process really came home to me when I was living in Toronto, when I noticed that while usually the blanket of snow over the city would melt to streams of running water and plenty of mud, sometimes, it just dissapear end overnight, with the roads and pavement stayed quite dry.  Seemed to be not enough heat in the air to explain in terms of melting the ice and then evaporating the water, the ice moved straight from ice to vapour. 

However, properties do not depend on the process, only the end status, so the same amount of energy has to be involved, whether the ice melts to liquid then evaporates, or just sublimes directly to vapour.  And sure enough, the solid-vapour tables show the enthalpy for the change from saturated ice to saturated vapour is about 2800 KJ/kg, so the explanation lies elsewhere.  Still trying to get my head around it, but basically if there is a low enough vapour pressure of water on the air, then as the temperature rises, water changes directly from solid to vapour.  I remember that the air was cold, but can't remember now whether we got a warm breeze that supplied the heat or just what happened. It is quite a long time ago now.  Perhaps someone from northern climates where this process can be observed will come in and help with more explanation.  We don't get much snow in Melbourne, so no opportunities to check the observations.

Hope the garden gets the necessary rain soon for you,

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 21, 2018, 01:15:28 AM
Hi MJM, Just "four easy questions" !!.....when ice is very cold is it less slippery ?? and if you have a quantity of water with a very narrow top opening, does the water evaporate slower than with a very large opening ??  is this useful in various applications ?? and why are questions so easy ??.....Thanks

Willy.....
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 21, 2018, 12:57:21 PM
Hi Willy, good to have you back asking questions again.  Great questions and a fun excuse sometimes just to remind myself of stuff.  Other times, to look up and clarify things I know I should know, but put on the spot, I like to do a bit of checking.  Unfortunately as a student, and usually also at work, you don't have the time to look at things in so much detail, so it is quite rewarding to do it now.  Of course, my work probably involved more of this than for most, but that is where we each collect and contribute different knowledge and experience.

Ice is usually slippery because it is not really very cold, and a little heat from our hand, or from friction is enough to melt some surfaces dot make a thin film of water (say under an ice skate blade or toboggan skid, or snow skis,).  The ice then slides on that film of water which produces minimum friction.  It is called hydrodynamic lubrication, or thin film lubrication.  Of course, if you stop, the thin film quickly freezes, and your sled, or finger becomes stuck to the ice.  If the ice is much colder, the heat from your hand is not enough to melt any ice, and your skin has enough moisture to quickly freeze to the ice.  Not a good idea to test this!  We lived in Canada for a bit over three years and we had to teach the kids not to put their tongues on cold objects.  I feel a bit out of place talking about what winter is like in Canada, but we had a very steep learning curve for some of these things, that most Canadians probably take for granted.  But Toronto seemed to have a lot of negative temperatures when we were there.  But a dry metal object does not slide so well either if the contact pressure and friction are not enough to warm up and melt a thin film of water.  In really cold weather, you have to "kick" your skis forward to start them sliding, and preferably have a film of wax to stop them sticking to the snow.

Evaporation, yes, you are quite right, if your container has a small surface area, it slows evaporation, while a large surface area facilitates evaporation.  Evaporation takes place at the surface, unless you mean in a boiler with a really strong heat input, and continues until the water vapour pressure close above the surface is equal to the equilibrium pressure for that temperature, then it slows down or even stops.  The vapour is easily confined near the surface in vessels with a narrow neck, filled so the only surface is the area of that neck, so evaporation is slow.  On the other hand if you have a very large surface area, not only do you get more evaporation over that large area, but if there is any breeze, it easily sweeps the vapour away, so lowering the vapour pressure, and more has to evaporate to replace it to restore that equilibrium vapour pressure, so evaporation is greatly accelerated.  In a bottle only partly filled, so it has a larger surface area, it is not so obvious, as the vapour is well confined near the surface rather than being  easily carried away by any air currents.

Use of the large area to increase evaporation is useful if you want to use solar radiation to remove the water from salt for salt production, or from brown coal, which has to be dried before burning it results in much excess heat.  Or other similar drying processes.  In fact, if you do your laundry in a washing machine, or a tub, wring it out by hand then leave it all piled up in a basket, it will usually go mouldy before it ever drys.  However, if you hang it on an outside line, or drape it over the furniture so the biggest possible area is exposed to the air, or even tumble it in a tumble drier, the greater surface area exposed to air results in much quicker drying.  Unless of course, the room is closed off, so the humidity rises to the point where evaporation stops.  It is necessary to have a well ventilated area, not just heat alone.

On the other hand, in dryer parts of this country, farmers put floating blankets of plastic balls on the surface of dams (they have to have fences to keep the cows out, and pump the water into a trough for the stock), or people put floating blankets to cover most of the exposed surface of a swimming pool when it is not being used, to reduce evaporation.  It retains the heat in a pool as well, as the evaporation carries away all that latent heat.

So those are just a few examples of that principal being useful.

I think when ever people are both observant and curious, there will always be many opportunities to ask, as Julius Sumner Miller did, "Why is it so?"  And the joy of science studies is in finding the explanation for so many interesting things we see each day.  So yes, the questions come easily, and the answers are often not too difficult if the relevant science is understood.  Of course, sometimes the science is a bit more complex, so the answers don't come so easily.  The science behind most engineering problems is quite well understood these days, which is why we can create such wonderful structures and machines, we just have to include well taught science in schooling, along side the other subjects that are so well represented in bookshops.

And of course not all the science is easy.  Look at that 7.5 hp 30,000 rpm engine in a different thread on this forum, for example.  Even with high powered computers, computational fluid dynamics and so on, there is really complex science behind that and similar designs, and even then some brilliantly informed trial and error.  I have to admit that I would not know where to start, let alone have the skill to make it.  It's really inspiring stuff.

I hope that answers your questions adequately.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 22, 2018, 02:11:21 AM
Hi MJM, thanks for the info....I have seen those covers on swimming pools but just thought they were to keep leaves off the water ..a bit like those wrong sort of leaves that fall on the railway lines here in Blighty !!... so regarding evaporation.. if the water is warming up but the evaporation is slowed down with a small opening is there some sort of complicated dissipation of energy doing something mysterious ??

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 22, 2018, 12:07:16 PM
Hi Willy, they probably help reduce the leaves in the pool, though it is easy to drop them in trying to remove the blankets or the leaves.  I think the main use here is that combination of retaining some solar heat, so the pool is a better temperature for swimming, and reducing evaporation, which reduces the water makeup required, a major consideration in very dry seasons in our climate, and possibly reduce the chemical use as well.  In your climate, the heat conservation is probably the main consideration.

Nothing strange going on, it is just a matter of the heat balance finding an equilibrium.  As the temperature rises due to the solar radiation, the heat losses, which you remember are proportional to temperature differences, all increase.  So the higher temperature increases evaporation from the remaining surface area, and heat loss through the base and sides of the pool also increase, as does the conduction through the blanket to the air above.  Eventually, the total losses equal the total oncoming heat, but it takes a long time, as the rate of temperature rise slows as the temperature approaches the equilibrium temperature.  Then the temperature ceases to rise until something else changes.  But compared with an open free surface for evaporation, the effect of reducing evaporative heat loss with that blanket is to increase the temperature of the water in the pool at which heat balance is achieved.

Perhaps I should have emphasised yesterday that I was primarily thinking of the physical sciences, physics and engineering stuff.  When you start looking at the biological sciences, the science gets very complex quite quickly.  But even so, an understanding of the basic chemistry reactions and processes still makes the next steps a little more understandable.

Thanks for following along,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 27, 2018, 01:15:25 AM
Hi MJM I have been busy with the Organic group at the Royal Norfolk Show this week and it is quite strenuous for me...I had a sorbet with a friend and he said it was something the Arabs developed in the deserts but was not sure how they managed to freeze things there ?? any ideas ? !!

Willy....
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 27, 2018, 12:06:32 PM
Hi Willy, gardening can become quite strenuous in patches, it is not all smelling the roses.  I hope you have good weather and a successful show.

Regarding the sorbet, I am sure they did not have refrigeration in the desert at the time you were thinking about, but inland on a large continent, with a clear sky, things radiate into the absolute zero of outer space, and get pretty cold.  The first time we went to Alice Springs in Central Australia, the van we were towing had pull out canvas tents to cover and enclose the beds which pull out from each end.  The morning we left, we were up a it earlier, and had to break half an inch of ice off the canvas in order to fold it all down for travelling.  Wasn't fun, as we were unprepared and had no gloves. 

They were resourceful people and developed some very clever science.  They would have to be to live in those areas.  Clear skies would not have been a problem, and I am sure that they would have discovered perhaps accidentally, then used the phenomenon more deliberately. They certainly had salt, and certainly would have known how a concentrated salt solution gets even colder than ice before it freezes.   I guess they would have used a concentrated salt solution open to the sky at night, much as we make slushies with those special mugs you put in the refrigerator to cool down, then use it to make the cold drink.  A bit of cool for food preservation and for heat relief would have been really appreciated and they will have learned many ways of storing a bit of cold for use during the day.  There are probably other salts or other compounds which work as well or better, and they would have been of high value.  Or maybe they only developed sorbet when they first had refrigeration.

I have not been totally forgetting about thermodynamics while I have been quieter lately.  I am still working on adding that insulation to the centre flue boiler.  I managed to cut out the required shapes for two layers from an A4 sheet of 3mm cork with nearly none left over at the end.  Of course that increases the diameter by 12 mm, so the circumference by over 38 mm.  So I am looking for suitable wood for a few more planks.  I will also need longer strips of brass for the banding to keep it all in place, as the originals will not extend that far.  I will post it up when I am nearer complete, and then repeat the boiler tests to see how much difference it makes.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 20, 2018, 01:13:18 PM
Well, I have had a bit of a break but have come back to discuss items Willy raised in his Freelance Engine build.  I do not want to hijack that. 

Hi Willy, a good set of drought related questions, but hey, six weeks is not a drought.  Our more recent one here, around 2000, was about 10 years, then as the farmers say, a shower of rain does not break a drought, we had several more years of below average rainfall and now some areas are again in drought.  But if you are used to rain nearly every day, I guess six weeks is a long time.

Let's put aside the sugar question for a while, I will come back to that.

In addition to temperature, a significant determinant of rate of evaporation is humidity.  You will remember that humidity is the water vapour content of the atmosphere compared with the saturation or equilibrium pressure.  You can look up the saturation pressure in the steam tables for any temperature.  Fifty percent humidity means the water vapour pressure in the atmosphere is half of that.

Of course temperature is important.  At freezing temperature, zero degrees C, the saturation pressure is just 0.61 kPa, while at 30 degrees C, the saturation pressure is 4.25 kPa, or seven times as much, and 50 degrees, it is 12.35 kPa.  Each of these figures is the water vapour pressure in air at 100% humidity at that temperature.  Clearly warmer air can carry away more water.

Water at the liquid surface tends to approach the saturation pressure, but the closer it gets, the slower the evaporation, just like the rate of heat loss gets less as the temperatures get closer in heat transfer.

Then, dry air currents, or wind, can carry away the vapour from near the surface so the concentration is lower and evaporation proceeds quickly.  However, if the wind is moist humid air, the vapour pressure near the surface may not be reduced much and evaporation slows.

Even though the soil is not a liquid surface, water evaporates into the air from the soil, at a rate that depends on the humidity in the same way.  So while at low temperature, the amount of moisture in the air is less, and the amount that will evaporate is less, it is the humidity near the surface as determined by air dryness that determines if the moisture keeps evaporating or slows down.

I hope it is obvious that due to the lower water vapour pressure at lower temperature, the whole process is slower at low temperature, and it takes longer to loose a given mass of water.  However, at low temperature, lower humidity air will still increase evaporation rates, while even at higher temperature, high humidity will reduce evaporation rates.

At low temperature, not only are the mass transfer rates lower, but again, if you look at the steam tables, you can see that at 0 degrees, it takes 2501 KJ to evaporate 1 kg of water, while at 30 degrees, it takes 2430 kJ per kg, so slightly less heat to be supplied to evaporate the mass of water.  At 100 deg C it is only 2257, but fortunately we don't get those temperatures in the atmosphere.

As evaporating the water takes heat, and the heat comes from the water and surrounding soil it cools, so reduces the saturation pressure so also reduces the evaporation rate.  The difference between the dew point temperature and the ambient temperature can be considered a driving force for evaporation, but this is really just a different way of looking at the difference in water vapour pressure and saturation pressure, so not an additional effect.

So generally a complex question of heat and mass transfer.  Our bureau of meteorology actually publishes figures for net evaporation rates which tell you whether the evaporation is more or less than rainfall based on monthly averages.  I seem to remember that they even use a dish in a standard enclosure to measure this.  If I can find it quickly, I will attach some figures at the end of the post.

I couldn't find some simple figures to post, but the website has the following definition for evaporation :- "The average evaporation per day as measured by the Class A evaporation pan, for each month and over the year, calculated over the period of record. The term evapotranspiration is sometimes used interchangeably with evaporation, however the two are different. It is more common to use evaporation data when referring to open water surfaces and bare soil, and evapotranspiration when referring to land surfaces with vegetation."

The maps on the site show most of the country has figures in excess of 100 inches per year, while average rainfall in most of these areas is less than 20.  They keep the level in that pan roughly constant by adding water each day, or draining a bit after heavy rain as necessary.  I wonder what the maps look like for your country?

I suspect that impurities in water may affect the saturation pressure slightly, but probably not significant in reasonably clean water.  A bit like salt in water affecting the freezing point.  When the water evaporates, the impurities are left behind, and are detectable as a slight scum on the dish surface, though again, the quantity of impurities in rain water is quite low, so you may have to top up the dish a few times to let the scum build to the point where you can see it.  Just the same as the impurities in your boiler feed water build up, and end up fouling the boiler, even though not usually significant for just one fill.  That is why full size boilers and even cooling towers need some blow down to prevent the continuing buildup.

On the sugar question, yes, the sugar normally stays in the bottom of your cup if it is not well stirred, something I am sure we have all observed.  In addition, the sugar dissolved in liquid has a maximum concentration that varies with temperature, and any excess will not dissolve, and the excess solids usually settle at the bottom due to a density difference.  The reason the dissolved sugar does not mix well is interesting.  Under gravity, everything accelerates towards the centre of the earth at the same rate.  When you drop a feather and a bowling ball from the tower at Pisa, air resistance slows the feather more than the bowling ball, but in the big vacuum chamber at NASA, I have seen films demonstrating them falling at the same rate.  (Yes, I know it was supposed to be a cannon ball, but that does not go well in this PC world!  The physics is the same either way.)

In a liquid, there are many more collisions than in air, and in a collision between a light object and a heavy object, the law of conservation of momentum means the lighter object ends up with higher velocity.  If you follow this through, it tends to mean that the lighter objects are easily bounced upwards, while the heavier ones are not.  This leads to a bias towards a higher concentration of the lighter objects at the top and heavier at the bottom.  I suspect that is the reason for your observation. 

I hope that answers the questions.

Insulating my boiler has had many of the usual life caused interruptions, but it is proceeding.  Like J. L., I found the local stockist had no 1/4 inch wide brass strips.  It was actually only the day before his post.  But fortunately another shop, a bit further away had what I needed.  Eventually I will be able to repeat the tests with the extra insulation.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 21, 2018, 01:13:21 AM
Hi MJM , thanks for that interesting and as thermodynamics depends on microscopic changes of temperatures as well as large changes does this mean that everything comes under the topics of thermodynamics ??
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 21, 2018, 10:57:25 AM
Hi Willy, thermodynamics is about energy transfer so involves most things involving energy, heat or work.   It is hard to avoid in physics and chemistry, and there are many grey areas which overlap into fluid mechanics.  For example, my heat transfer book includes a detailed derivation for the velocity profile in the boundary layer when a fluid flows past a surface.  But thermodynamics is not the much searched for "explanation for everything" even if though that explanation will probably include some energy considerations.

In general subject boundaries are somewhat artificial.  Useful for dividing the whole body of knowledge into manageable chunks, but not all that useful as limits of application. 

Engineering tends to involve a little of everything which means there is always more to learn.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 23, 2018, 12:49:25 AM
Hi MJM, ok cool..I had a look on the Norfolk rainfall website and it suggest that any rain that does fall can evaporate before it reaches the ground !!!  also there is a site in Model Engineer talking about steam production in boilers that is quite interesting... I don't know if you subscribe to the magazine and it is available on line...
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 23, 2018, 12:17:12 PM
Hi Willy, if you look at the BoM radar images, you will often see areas of quite light rain which never actually reach the ground if you are standing under them.  It is not really unexpected.  Air density is higher at ground level and as you go up, the air density and temperature falls.  So, the water vapour is cool enough to condense high up, but as the droplets fall towards earth, they see warmer air and are heated above that condensation point, leaving just quite high humidity.  So not really unusual, and not limited to times of drought.

I do quite often read Model Engineer, but only through casual purchases at the news stand.  I have missed those articles, but it is good to see someone else exploring aspects of thermodynamics and boiler performance.  The Speedy locomotive boiler he is looking at appears to be quite efficient compared with my rather primitive pot boilers.  I agree totally about the small amount of heat taken up by the steam in the superheater.  It would be interesting to know more about the methods he has used to calculate the other losses.  Perhaps they are covered in other articles.

He is discussing a coal fired boiler, for which fuel measurement is more difficult than for a spirit fired burner.  I have concentrated on methylated spirit firing which is a bit easier to measure, and as you have probably noticed, even gas firing is even more difficult to control to steady fitting rate.  I suspect I need a little pressure controller for this.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 24, 2018, 03:53:07 AM
Hi MJM, More on evaporation   If you have an enclosed vessel say half full of water and applied a moderate heat....would the water eventually all evaporate and then fill the space with increased oxygen and hydrogen ?  would this become a flammable mixture that might explode if the temperature became high enough ? or is this another silly question ?  it is 3.50 AM and today it was 32 degrees so sleeping is well nigh impossible !!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 24, 2018, 11:34:26 AM
Hi Willy, not silly questions but perhaps highlighting a misconception which I am sure there will be others who share.  You are not alone.

The fundamental point is that when water evaporates, it is still water molecules, two hydrogen atoms and an oxygen atom tightly bound together by covalent bonds, in each molecule.  It does not break down into hydrogen and oxygen.  Evaporation is not the same as decomposition.

It takes a lot of energy to break that bond.  It is usually done by electrolysis, passing a DC electric current through the water.  You get bubbles of hydrogen at one electrode, and oxygen at the other.  It might be possible to do it with heat, but there is be no doubt that the required heat would never be mistaken for moderate.  More like extreme heat combined with low pressure.

Of course, if you go the electric route, it is best to keep the gases quite separate, as a mixture of hydrogen and oxygen is quite dangerous.  Hydrogen will burn in air over a wide range of concentrations.  And pure oxygen can make a fire with most substances, even things you do not normally consider flammable, like steel and concrete, with a suitable ignition source.

It is not a silly question because these days, in your science shop or electronics outlet, you can buy small size fuel cells which are used in one direction to produce the two gases when an electric current is applied, and the same device when the gases are supplied in the other direction, produces an electric current.  Our local electronics store has them sufficiently efficient to power a small model car as an alternative energy demonstration science kit.  I have been tempted to buy one, and perhaps use it to power a small model boat.  A catalyst in the cell facilitates the reaction so it is more efficient than trying to put the current through pure water.  And if you add salt to make it more conductive, you get some chlorine produced, so not a great idea.  Or you can use a solar panel to supply the current to make the gases, then use the fuel cell to creat electric power to drive a motor, or a hydrogen powered i.c. engine.

If, instead of using a fuel cell, you ignite the hydrogen and oxygen there is a lot of energy released, and it does not take very long!  Also, it does not take much heat to start it off.

So back to the original question about heating a container half filled with water until all the water has evaporated.  I guess when you start, there is air in the other half of the container, just as in your mountain top experiments.  Of course we could assume the vessel has been evacuated and so only contains liquid water and water vapour with the water vapour pressure at the equilibrium vapour pressure for the temperature.  If we try and remove the water vapour, we just boil the liquid water at that temperature.  So the best we could do is boil enough water to displace the air then seal the vessel with just water and water vapour.

When you heat the water some water evaporates and you raise the vapour pressure.  Now the volume of water vapour is around 1000 times the volume of liquid which evaporated to make the vapour.  If the vessel is closed thus constraining the volume, the evaporation will result in a very high pressure, but it will still be water molecules.  In fact, the high pressure will suppress any breakdown of those water molecules into separate hydrogen and oxygen molecules.  (High pressure tends to favour the reaction which results in the fewest molecules).

So a practical experiment involves either a tiny bit of water in a large vessel, or allowing the steam to escape, like your kettle.  That water vapour is not flammable, and in fact you can use a steam lance to extinguish a fire in the right circumstances, the steam basically occupies the space and so excludes oxygen which is necessary for fire to continue.

I hope that makes things clearer for you and all others pondering the same issues.

I know what you mean about sleeping when it is 32 degrees overnight.  In northern parts of this country it happens regularly in the appropriate season.  Definitely don't need heavy blankets!  I certainly need air conditioning, but people who have lived there for long enough to be acclimatised seem to do better.  But I don't believe it is common in your area.  I avoid that time of year, by only travelling there in the dry season.

Thanks everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 24, 2018, 02:26:24 PM
Hi MJM , Ok a lot clearer ..so going on with the questions  ...we talk about hydrocarbons  (methane .Butane ) but with a fuel cell that burns Hydrogen and Oxygen would this be called a Hydrohydrogen   or even a Hydrooxyhydrogen ?? ;D :) :-\.  The sumization with the question was summed up quite succinctly  when you stated that the high-pressure stops the bonds breaking down !!  When water evaporates does the H20 become lighter than the water molecule in the liquid, which may rise into the atmosphere somehow from it. and how high can it elevate ? on the news tonight they suggested closing the curtains during the day to keep the inside of the house cooler !!



willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 25, 2018, 01:31:39 PM
Hi Willy, hydrocarbons is a convenient collective name for a group of chemical compounds in which  each molecule is made up of a number of atoms of hydrogen and carbon.  Hydrogen atoms have only one bond site, while carbon has four.  This means that each carbon atom can join to four other atoms.  The other atoms can be different or even other carbon atoms.  So carbon atoms can join to make long chains, branched chains, or rings.  The chains can even be joined in more than one place, called cross linking, which leads quite solid plastic materials.

Obviously the first one in the hydrocarbon series is methane, with one carbon.  Each of its bonding sites is joined to one hydrogen.  It is the main constituent of natural gas.

If two carbons are joined in one place, we get ethane and each carbon is joined to the other one and to three hydrogens.  Three carbons give propane, four gives butane and so on.  These are the so called straight chain parafinic hydrocarbons, and the series can get very long.  Each extra carbon raises the boiling point through oils to solid waxes.

But after three carbons, things start to get complicated.  The fourth can continue in a straight chain, or it can branch off the side of the chain formed by the first three.  So we get normal butane and ISO butane.  And even at two carbons, those two can join at two places called a double bond, which gives us ethylene, the building block for polyethylene plastic.  And they can join at three places, a triple bond, or acetylene, as we know very unstable, but I am not aware of the possibility of two carbons joining at all four sites.

All of these compounds have many similarities, so it is convenient to have a group name.  However, hydrogen is a single element, not really a member of any group that I am aware of, so there is no need for a group name.  A hydrogen powered engine would be a sufficiently complete description.

When the hydrocarbons burn, they combine with oxygen, and they are all quite good fuels, though the longer chains are perhaps more difficult.  But oxygen is not part of that group name.  Similarly burning implies joining with oxygen with the release of energy, whether the fuel is hydrogen or hydrocarbon.

It is important to understand that the forces that bond the atoms together in a compound are very strong forces that operate only over a very short range.  These forces are much stronger than gravity which tends to be a weak force but it operates over a long range.

These covalent bond forces are much stronger than the inter molecular forces that hold a liquid together.  To give you an idea, think of the energy in the latent heat, required to evaporate water, around 2000 KJ/kg.  Quite a step change from the heat necessary to raise the temperature from say zero, but still small compared with the energy involved in breaking those covalent bonds.  This occurs in chemical plant cracking furnace when ethane for example is heated to around 1000 deg C to break the bonds.  I am not sure how much energy per kg is required, or the exact temperature, but the high alloy steel pipes in the furnace are a very bright red.  Clearly a step change in energy compared with what is required for evaporation.

So back to those escaping water vapour molecules.  First, they are still unchanged molecules of H2O, each having the same mass as when it was in the liquid.  Well, perhaps not quite.  Einstein demonstrated that mass can be converted to energy, with the famous equation E=mc^2.   This means that as the molecule moves with increasing velocity, the mass does decrease, but this effect is minuscule until the velocity gets very close to the speed of light.  Unless you have a synchrotron in your basement (I am not sure if you can buy one on Amazon yet, but who knows?), the effect of this energy-mass conversion is zero for practical purposes. So the vapour molecules are not lighter.

However, the molecules are much further apart than in a liquid.  And the vapour occupies a much larger volume.  Basically it bounces around between the molecules sharing the space.  This motion is random and even at low pressure there are very large numbers of individual molecules.  Close to a liquid surface equilibrium vapour pressure is when there are the same number of molecules in the vapour space hitting and being captured by the liquid as there are escaping from the liquid.  Further from the liquid surface, there tends to be more coming from the liquid surface than there are in the space above coming back.  So the vapour spreads to fill the space, essentially independent of any other molecules, for example oxygen or nitrogen, which also occupy that space.

In a closed vessel, the water vapour essentially spreads through the whole volume, but in the atmosphere, much larger than any practical vessel, the slight increase in the gravitational force nearer the earth, means the water density, or number of atoms per cubic meter tends to be higher nearer the earth.  In the end, I understand that essentially none of the molecules have enough energy to completely escape the pull of gravity, and in effect the earth captures any stray hydrogens which happen to be wandering by in space.  As you go higher, there will be fewer water molecules, just as there are fewer oxygen and nitrogen molecules.  The height of the atmosphere is quite large of course.

Gravity is considered by physicists to be a weak force, not that it seems that way if you fall off a ladder.  But it acts equally on all mass, and does not result in great density differences with height until extreme heights.  But it is insignificant compared with the molecular forces that hold the atoms together in a molecule.

Again Willy, you have a talent for asking innocent looking little questions which lead to so many paths.  But perhaps that has covered the main issues you are thinking about.

The idea of closing the curtains to keep the house cooler in hot weather is a strategy of passive cooling.  The idea is that by closing the curtains, you reduce the heat input from the sun, and confine the warmed air on the inside of the window glass to the immediate area of the glass, where it is hopefully returned to the outside when it is cooler at night.  Of course it is even better if you open them at night when it is cooler outside, to help the heat from inside return to the atmosphere outside.  Best of the lot is to have blinds on the outside of the windows to prevent the Suns radiation entering the house at all.  Again it is even better if you raise the outside blinds at night.  With our plentiful bright sunshine I think these are fairly normal activities in this country.  It makes quite a difference, but you need to take maximum advantage of the night time cooling to obtain best advantage.  We have outside blinds on the west side, and trees on the east, eves and a roll up shade on the north side.  I have to admit we don't often open the west side curtains at night, it is easier to just leave them down for the summer.  Our north side eves shade the full wall height with the high summer sun in summer, and admit the sunshine over the full height of glass doors in winter when the sun is lower in the northern sky.  In spring and autumn the strategy is not so effective, but it helps a little.

In winter of course, we keep the house a little warmer and use less heating by closing the curtains at night and opening them to let the weak winter sun enter during the day.

Always worth employing these passive methods of heating and cooling to at least reduce heating and cooling energy costs.

Thanks for looking in,

MJM460

.
Title: Re: Talking Thermodynamics
Post by: paul gough on July 27, 2018, 03:46:29 AM
Hi MJM, Not been around for a while. Had some issues with the blood pump for a few weeks a while back and after uncoupling from the medical system I decided to spend some time quietly recharging the system. So to dip my toe into the subject again I have been pondering methods I might try in getting some data from gauge 1 locos. One of which I would be pleased if you would comment on. If I embed a thermocouple into a drilled hole in the cylinder body, possibly by extending the depth of one of the cylinder cover bolt holes into the cylinder wall area. 1) do you think this would be a viable, 2) If so would it be better to use a bare thermocouple or a sheathed probe, 3) If bare probe wire, do you think it necessary to use a heat conducting grease or some such in the hole or would the tiny volume make this unnecessary. Hope I am not going over old ground, I cannot remember discussing these things and have yet to get through the 200 odd posts since I disconnected. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 27, 2018, 11:17:50 AM
Hi Paul, it's good to see you back.  I have been worried about you, it seems with good reason.  I hope you are through the worst of it and now on the way to recovery.   But I know it's a long process.

I had my own issues over 25 years ago, though the damage remains.  It is worth finding and continuing a good rehab programme.  I am sure they have developed since I did it, but even then it was well worth while, and many of the things I learned are now just a normal part of my life style.  I have been quite active and well since.

Good to see you still thinking about those locos.  Basically, with thermocouples the decision of sheath vs. no sheath basically comes down to what you are measuring.  If you are measuring a liquid, I think the sheath is the best arrangement to protect the wiring and insulation, and have good thermal contact by immersion of the sheath.

Also in case of very hot gases, such as flue gas, the sheath is able to contain insulation able to withstand the temperature.

The next issue is pressure containment.  The best way to measure a fluid under pressure, such as steam, is to insert the thermocouple into a thermowell, essentially a long plug screwed into a fitting on the pipe or vessel, drilled with a blind hole to accommodate the thermocouple, so it does not compromise the pressure containment.  Actually quite similar to a drilled bolt, providing the outside of the bolt is in good thermal contact with what you want to measure.  And then the sheath is not required.  My thermocouples seem to need a 2,5 mm hole to actually fit, so I imagine your bolts are too small unless you have access to very small thermocouples.  Something in the back of my mind (?) I can't quite recall.  But if the bolt extended into the exhaust passage for example, in principal it should do the job.

The next issue is the position of the thermocouple.  It is important to place the thermocouple where it will measure the temperature you are interested in.  In a cylinder, the temperature varies quite a bit in quite a small space, so I think it would be difficult to be sure just what you were measuring.  So far I have used an extended plug, drilled for the thermocouple as a filler plug to measure steam/water temperature in the boiler, a sheathed thermocouple to measure flue gas, and a rather bulky machined elbow containing a little short thermowell at the engine inlet and exhaust, both as close to the engine as possible, for inlet and exhaust temperatures.  The engine inlet is assumed to be essentially the same as the superheater outlet for practical purposes, though the tubing and lubricator mean there will be a difference.

I have tried using a drop of glycerine as a heat transfer fluid to improve the thermal contact.  I have not found it particularly successful and seem to get variable results, possibly exacerbated by the temperature difference effects I mentioned with cylinders in a very short thermowell.  It seems that it is more useful to clip the thermocouple wire near the thermowell so it does not move around every time I touch the wire or meter.  When I think about it, the full size ones in the plants I am familiar with are quite firmly secured at the head of the thermowell.  In principal the fluid should improve thermal contact to improve speed of response and reduce stem effects so long as you don't completely flood the well, just a drop is enough in our model sizes.  And of course minimal usefulness in a horizontal well!

I think glycerine is suitable, but it's possible I am mistaken and should use something else.  My lack of obvious success with it starts to test my confidence.

I think the first things to work on measuring are the fuel consumption, and the steam temperature.  If you have a superheater, superheater outlet/cylinder inlet.  In your scale, you could make a more compact thermowell by soldering in a length of 1/8 in tube with the end plugged or squashed and silver soldered.  The small diameter will withstand a lot of external pressure.  It does not have to be the bulky screwed fitting you see in my photos.

Don't feel pressured to catch up with the posts you have missed.  They will help you get to sleep for quite a while, the main thing is to enjoy the learning that I hope is contained in them.  I really don't mind repeating if required.  It will eventually become just another part of what you know.

Great to have you back again, 

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on July 28, 2018, 06:47:18 AM
Thanks for the comments MJM. It is likely that being an investigator and possibly minor experimentalist will be the limits of my activity from now. So I try to think up ways to find out what is going on in our little locos, or as near as possible, so as to develop a deeper understanding of their behaviour and maybe discover something of value. All the things you list as important to determine would be part of the investigation, but, I feel there are other things that might be revealed.

 My idea of a drilling into the cylinder block deep enough to locate the thermocouple about midway along the cylinder stroke and at about 180 degrees to the steam chest would, I take it, be a thermo-well of sorts. The temps. would be something of a close approximation, (I would hope), to average internal cylinder temps. Such a measure might be useful in determining time and distance a loco might go, with standardised varying throttle and load before approaching steam chest temp. Provide a guide or comparison for each cylinder. Get an insight into the relationship of superheat temp and cylinder temp. See what transpires with and without cylinder insulation. Attempt to discover any effects linking up, (varying valve travel), may have on cylinder temp. Assess the value of this type of cylinder temp. measurement  against other measures. Utilise anything gained to improve investigation methods further.

All this leads me to ponder some form of 'test stand' I might be able to cobble together and also try to work out how I could apply a variable load while a loco was on the stand. If successful it might convince my friend to let me drill holes and insert wires all over the place in his locos for comparison. He is, at the moment, a bit sceptical about the worth of my inquiries.

One further question, that I cannot find a definitive answer for. Do the common 'needle' type valves used as regulators/throttles in our miniature sizes actually work as a locomotive throttle is supposed to, i.e. a variable orifice volumetric control rather than a pressure varying device?

Thank you for your concern regarding my health, much appreciated. Alas, advanced maturity has a price we all have to pay. Paul Gough.
Title: Re: Talking Thermodynamics
Post by: Zephyrin on July 28, 2018, 10:52:44 AM
IMO, measuring the instant gas temp inside a small cylinder is out of reach for the home workshop, unless the cylinder would be made specialy for that, with a large dead volume to insert a fast and sensitive thermocouple (or maybe 2 or 3)...
But it is certainly easier to monitor the internal pressure, I have ordered a MPX2200ap chip for that purpose, but not yet implemented it, a little bit out of touch with the electronics...

as regards the needle valve, I would say that it is a pressure varying device, but some increase in temp is also expected after the small opening.
but I also wait for the response of MJM...
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 28, 2018, 01:37:39 PM
Hi Paul, regarding your last comment, the good news is that we actually got to the point where these problems appear.  My wife worked helping older frail people with some of the house work.  She always says that based on her observations, growing old is not for sissies.

Drilling a hole in the block is the same basic arrangement as a thermowell, so the extra thermal resistance of a sheath is not necessary.  The cylinder wall temperature varies quite rapidly within each stroke, especially if there is early cut off and some expansion, but even without, the exhaust temperature will rarely be much above 100, yet at the start of the stroke, the incoming steam is nearer your superheater outlet temperature.  Even my little pot boilers have a superheat outlet temperature over 130 C.  It is important to understand the nature of that cylinder temperature.  It varies both in time and location.  It is very difficult to get a location where the temperature is predictable.  As you imply, the slow time constant of most thermocouples will tend to average the reading, but because you can't know exactly the point the thermocouple is contacting, the only really useful information comes from changes in the temperature.  I think this location might be on the side of overthinking the issue, and it would be just as useful if not more so to simply make the best possible measurement of the exhaust temperature, and the inlet temperature.  With these two, you can calculate how much energy is being extracted from the steam, and if you can measure the work done, you can calculate the efficiency.  Then changes in efficiency as the load changes are probably what you are most interested in.  The exhaust temperature will reflect changes in valve cutoff.

Inlet temperature is a bit more problematic, especially when considering changes in inlet throttle valve.  You cannot use the boiler pressure to infer a pressure after the throttle valve so accurate pressure measurement is required.  I have not tested my opinion with a good calibrated gauge, but I am very doubtful that the small gauges we generally see on models are very accurate.  But the simplest thermowell you can design will do the temperature.

For the exhaust the pressure is accurately known if you have a reasonably free path to atmosphere, and again the temperature can be measured with the best thermowell you can devise to most accurately reflect the steam temperature.

For a test rig to apply and measure the load on your engine, I wonder if you can arrange a test rig so the loco driving wheels each rest on two wheels on the test rig, profiled like track cross-section to fit the driving wheels and geared to a flywheel, which can be loaded up with a friction pad on the rim.  The flywheel and gearing can accurately replace the normal linear motion of a locomotive to account for the inertia of the train.  The calculations are relatively simple, not so easy that I can reel them off, but I can help you do them if you go that way.  A digital scale should be accurate enough to measure the friction drag on the pad, and you need to do some electronics for speed measurement.  Again not hard with Picaxe or Arduino.  Should be an interesting exercise to sketch up, then probably not too hard to make up, much of it at a normal desk or tabletop.

Regarding the needle valve, it actually can be said to vary both volume and pressure, but it is not really accurate to say it controls those quantities.

It is better described as a variable restriction orifice.  When the needle is screwed open the area of the flow annulus between the needle and seat increases.  When there is a difference in pressure between the upstream and downstream side, fluid flows through the opening at a rate which is determined by the pressure difference and the open area.  Bernoulli's theorem, which is basically the same as conservation of energy, gives an approximate value for the maximum velocity, but downstream much of this velocity is dissipated in turbulence, and you do not get the pressure recovery Bernoulli would predict on the downstream side.  However, the resultant pressure change means an associated volume change for the mass of fluid.

The smaller the open area at the needle valve the more pressure is lost.  But the pressure loss also depends on the flow.  So if the upstream or downstream pressure changes, the flow will change, as will the the other pressure and the needle valve would require adjustment to compensate if a fixed value is required.

I hope that I am not being too pedantic here, but I would suggest that the identifying feature of a controller is some sort of variable element that automatically responds to a change to restore the desired set value.  Pressure controllers usually have a diaphragm, needle valve and spring set up so a change in the required pressure is compensated for by adjustment of the needle, which is usually connected to the diaphragm.  They can be set up to control upstream, downstream or differential pressure, which ever is required.  More sophisticated controllers might have separate measurement, then an electronic system, these days involving microprocessors or even computers to compute the required response and drive a diaphragm operated control valve as required.

Hi Zephyrin, great to have you on board again.  I totally agree with you that to measure the time varying temperature in a steam engine cylinder would be beyond what most of us have available.  The normal time constant of a thermocouple means that it would give an average at best. 

I am really interested in that pressure measurement chip.  Will use it with Arduino or Picaxe, or are you into programming other controllers?  I am also interested in how you get the steam pressure to the chip.  Most of the ones I have seen seem only suitable for measuring atmospheric air.  I hope that you will keep us all informed on your progress.

Thanks for your comment about the needle valve.  I think the issue is that sometimes a valve is adjusted to control pressure, while if it is the engine regulator, it is used to control speed, and of course, in a piston engine, speed is about volume.  However, more speed means more work, so generally requires more pressure, again pressure and volume related.  So, at the end of the day, I feel the best description for a simple passive device is adjustable orifice.  The word controller is then reserved for a device that automatically responds to a change to restore the set condition.  The governor Brian is documenting is a very basic self contained device, but definitely complies with the definition of a speed controller.

There is a very small temperature change with throttling that varies with the starting pressure.  At the pressures we are usually talking about, it is very small.  The process is basically adiabatic, so no heat input or loss and no work done, and can be calculated with steam tables.  The pressure-enthalpy diagram best shows what happens.  But when we have throttling, measuring that pressure becomes essential.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: Zephyrin on July 29, 2018, 11:07:33 AM
Thanks for your answer MJM, I like the way you simply describe all the events occuring while opening the steam valve ! it's a great opportunity to revise thermodynamics!

As regards the pressure measurement chip, the signal from the chip must be amplified to give a manageable response, that is all I understand, and I have to wait up to the end of the summer for help from a friend for the electronics...
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 29, 2018, 01:42:39 PM
Hi Zephyrin, thank you for your kind words.  You have described exactly what I am trying to do, so I take it that at least sometimes I get there.  Enjoying the learning is the main idea.

My apologies also for mis-spelling your name yesterday, I have gone back and corrected it, I hope I have it right this time.

Yesterday's post was getting very long so I did not want to add more, but I thought that in view of some of the comments about needle valves in several other threads, that a little more explanation today might be a good idea.

Typically a needle valve might be used as a control valve for a gas burner, or as Paul suggested, as a regulator for a small engine.  So let's look first at the gas valve used with a burner.

Initially, the needle valve is closed, assuming that any separate isolating valve on the bottle has been fully opened,  it's upstream pressure is the pressure in the gas bottle.  With the needle valve closed, there is no flow, so no pressure loss between the bottle and the upstream side of the needle valve.

The downstream pressure for the needle valve is atmospheric pressure, as while the burner might have small jets, the path to atmosphere is open and again, with no flow, there is no pressure loss between the burner and the downstream side of of the needle valve.

So the pressure difference across the closed needle valve is the difference between the absolute pressure in the bottle and atmospheric pressure, which is of course the gauge pressure in the bottle.

When we open the needle valve enough to cause a small flow for lighting the burner, the pressure drop across the valve causes a flow.  If our gas is butane at moderate temperature, let's assume the pressure is around 200 kPa again absolute.  The gas velocity at the needle valve is roughly proportional to the square root of the differential pressure.  (We more often see this expressed as the pressure drop is proportional to velocity squared).  This flow causes a pressure drop in the burner jet, and assuming the pressure drop in the tube is negligible, the downstream pressure of the needle valve will be the same as the upstream pressure of the burner jet.  If the flow was higher, the burner jet pressure drop would increase, thus reducing the remaining pressure drop available for the needle valve, so the flow will reduce.  You see, the downstream pressure for the needle valve is determined by an equilibrium pressure drop between the burner jet and the needle valve.

With the burner alight, you now open the needle valve some more, thus increasing the flow area.  With more flow area, and the same pressure difference, the flow increases.  This increased flow increases the pressure drop across the burner jet, which reduces the pressure drop available at the needle valve, so the flow quickly stabilises at a new equilibrium, but a higher flow than with the smaller valve opening.

I did assume a very low gas pressure.  The flow of any compressible gas through an orifice is determined by the pressure difference for small pressure differences up to a point where the resultant velocity equals sonic velocity for the particular gas conditions.  The occurs at a point where the absolute pressure downstream of the orifice is approximately fifty percent of the upstream absolute pressure.  Above that point, the velocity through the orifice remains constant, the flow is determined by the upstream pressure only.  If we are using a propane-butane mix with somewhat higher pressure, we will have sonic velocity, so flow is proportional to the upstream pressure only.  The downstream pressure will again be determined by the burner back pressure, and with propane as fuel and a supply pressure greater than 400 kPa absolute you could have a flow sufficient to produce sonic velocity in the burner jet, so the pressure upstream of the burner results from an equilibrium between the flow through the needle valve, and the pressure necessary to get this same flow through the burner jet.  Any flow which gives more that 200 kPa upstream of the burner orifice will give sonic velocity in the burner orifice, so this is quite common with higher fuel pressure.

We can see that the adjustable nature of the needle valve means it is quite useful to adjust flow by changing the area, but the resulting pressure and flow are both determined by the equilibrium determined by all the elements in the system, not just the needle valve.

The other example which gives a little more insight into the humble needle valve is its use as a regulator for a small locomotive.

When the boiler is up to pressure, let's just open the needle valve a little to let's through some steam to warm the cylinder.  Initially the pressure is not enough to move the train, and the build up of pressure is slowed by condensing of the steam, and leakage out of the cylinder drain cocks.  Pressure builds as the cylinder warms to give a higher condensing pressure and eventually we close the drain cocks.  Now the steam flowing through that needle valve is flowing into a closed space.  More mass in the same volume means the pressure builds until it is sufficient to overcome the resistance of the train and start the piston moving.  Initially, the needle valve pressure difference will probably be sufficient that there is sonic velocity at the seat.  But as the pressure rises in the cylinder, the available pressure drop at the needle valve is reduced and flow is determined by the upstream and down stream pressures.  Initially the piston moves slowly, so this determines very small steam flow, so small pressure drop at the regulator and the piston will see close to boiler pressure.  As the train speed increases, the flow to the piston must increase in line with the engine swept volume at increasing rpm.  Eventually, the volumetric flow is sufficient that the pressure drop across the needle valve is just enough to provide only the necessary pressure on the piston to maintain that engine speed. 

If we want to go faster, as the kids always do, we open the needle valve a little further.  At constant speed, the volumetric flow is the same, but with a larger opening of the valve there is less pressure loss, so higher pressure available downstream at the piston.  This provides more force on the piston, more torque, and the train accelerates some more.  And so on until the regulator is full open.  More speed then requires more boiler pressure, or higher fire, and we go back to opening the gas burner needle valve, or shovelling more coal.

In all cases the volume of the fluid at the lower pressure is greater, or the density lower than upstream.  In one case, the downstream volume is determined by pressure drop through the burner jet due to the gas velocity, in the other the volume downstream is determined by the positive displacement engine capacity and rpm.

That is rather tedious in detail, but I hope that if you have made it to here, it offers a little more insight into the humble needle valve, and how the pressure or flow changes.  I suppose you could call it a control system if you include the human operator making adjustments the the needle valve in response the a pressure gauge reading of engine speed.  Technically, it is an open loop system.  You can increase the flow by adjusting the valve, but there is no feedback to adjust or control the flow in any automatic manner.  The flow can be changed just as readily by changing the upstream pressure, or the resistance or capacity of the downstream system.

I am still intrigued by your pressure measuring chip, I will have to search for a data sheet.  That should be no issue with the number you gave given, I will have a go in the next few days.  I believe there are available operational amplifiers especially intended as instrument amplifiers.  I will be interested to know which one your friend eventually recommends.  Then I guess you can use your preferred micro controller to produce a readout or data logging facility. 

Sorry to inflict another long post on you.  I do seem to like getting down to the nitty gritty detail.  Probably time now though to move on to a new question, unless I have prompted other issues in the above description,

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on August 01, 2018, 10:27:01 AM
Hi MJM, Just read your 2nd. needle valve post after another stint in the hospital and escaping at 4pm today. I'm going to avoid ever going there again!!!

I guess I should have clarified what I meant when referring to a loco throttle being a volume control. For the most part steam chest pressure should be boiler pressure or at least very close to it at maximum cylinder demand/power output, all other conditions are largely irrelevant, the boiler, steam circuit, throttle valve should be adequate to ensure this pressure can be maintained in the chest and the ports, valve events should be capable of delivering adequate steam to the cylinders for maximum output. So when under load a locomotive where possible should have the throttle fully open and power out put governed by valve travel (linking up). Fine adjustments to the throttle and valve travel are made by drivers in accordance with the topography and reference to the steam chest pressure gauge and the exhaust back pressure gauge. This also applied to my twelve inch gauge 0-6-0, maintaining boiler pressure in the steam chest and regulating valve travel for more or less power. I cannot speak for smaller gauges and gauge one is something of a challenge operationally and in actually knowing what is going on. Hence my enquiry regarding needle valves. I am aware it may not be possible to recreate all or any of the conditions that might apply in full size or 12 inch gauge but I think it a reasonable course to ponder and investigate until it is shown definitively to be a waste of time or even detrimental. Thank you for the 2nd post on needle valves it further clarified things.

I would very much like to know if ball valves or similar full flow orifice types behave at all differently particularly when partially opened, say 50% or less. I have thought about replacing  the needle valve throttle/regulator on my gauge one loco with a specimen of this type as an experiment. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 01, 2018, 01:37:35 PM
Hi, Paul, I'm glad you were able to escape, those episodes are not much fun.  Worth working at controlling the causes, but when you need medical help, the hospital is the best place to be.  We are all very glad to have you back.

For maximum power to the wheels, clearly you need a clear open path from the boiler to the valve chest, so there is maximum possible pressure available at the piston face.  The problem is that there are occasions when we don't want the maximum available power.  For example, too much acceleration when starting off might cause wheel slip, too much speed on a down hill stretch with a bend at the bottom will never end happily.  You know the scenarios.  Energy input to the boiler, especially with coal firing, has a very slow time constant, you can't quickly reduce firing to slow down, specially if after that bend is the start of the next uphill grade.  And imagine trying to accelerate a train from stand still by increasing the firing rate when the guard blows the whistle!  So we need a quick and responsive method to match the required quantity of steam to the cylinders and the steam produced by the boiler.

A partly closed regulator, or when necessary totally closed, is an immediate method to reduce steam flow by driver intervention.  As it is an adjustable restriction, it is flexible as to the degree of restriction required.  It is not efficient, but as you say, the aim is to adjust what is available so that eventually the energy produced by the fire is balanced with steam flow to the engine, with minimal inefficient throttling by the regulator.  But this takes time and the train is moving along. Valve gear, by enabling reduction of valve travel, is a more efficient, so more desirable method of adjusting the energy balance, particularly to the extent that it achieves early cut off.  But which ever method is used, it is not without consequences on the boiler and firing.  If the regulator allows less steam flow to the engine, the boiler pressure will tend to rise.  In the absence of other action, the increased pressure means increased temperature so increased heat losses, both through the shell and to the flue gas.  Eventually the safety will lift to achieve the energy balance by "using" the excess steam.  The boiler firing must be reduced to minimise the excess steam production over the requirement.  While temperature and pressure instruments are provided in full size, the practical limitations of the small size of gauge one, (and even considerably larger), mean it is not practical to provide the same level of instrumentation.  Especially when any readings would have to be transmitted to the driver, who is not on board to read simple instruments.

All restrictive regulators act in the same basic manner.  The restricted area means increased velocity and reduced pressure.  Unless the restriction is a carefully shaped Venturi like in an injector, most of this pressure loss is dissipated in turbulence downstream of the restriction, and not recovered, and the overall effect is reduced steam flow, with lower pressure downstream of the restriction.  Not efficient in terms of energy consumption, but quickly achieves the required result, but only as an interim measure to serve until the boiler energy input is adjusted.

The difference between using a needle valve, ball valve, or disk type regulator are essentially in the mechanics.  Operationally, a needle valve offers a predictable progressive opening as the needle is turned, the ball valve initially is progressive, but the opening quickly proceeds to the point where there is no significant throttling, only part way through its quarter turn movement.   A plug valve with oblong openings opens even more quickly, so not very controllable at low flows.  A disk valve can be tailored somewhat by shaping the holes, and a v shaped opening will give better control at small openings than some other shapes.

The mechanical differences between the different types is also significant in longer term use.  Wet steam in particular is quite erosive, and those fine droplets, though hardly visible, are able to cut hardened steel.  A needle valve, just by its shape tends to be least susceptible to this, while a ball valve will soon be damaged to the point where it will no longer shut off.  This means the are normally restricted to applications where they are normally only used full open or shut, with minimal  time throttling.  Disk type regulators seem common in published locomotive designs.  I have no experience with them, but I assume their common use implies that they tend to be satisfactory for the time a model is actually in use.  I would expect you could use which ever type was simplest to fit in your gauge one models, though a ball valve would be a bit twitchy at small throttle openings so a bit harder to control.

I am glad the needle valve explanation was informative.  I think most of the confusion arises from the fact that the restriction actually reduces mass flow by imposing a flow restriction.  The resulting steam conditions downstream of the restriction are really determined by that downstream system, not by the restricting device.  It is reasonable to talk about either the pressure or volumetric flow, depending on the one we actually measure or are most interested in.

Best wishes for a speedy and full recovery so you can get back to the workshop, rail track and the frogs.

Thanks everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 01, 2018, 09:21:44 PM
Hi MJM, interesting concepts there.... you need the same amount of steam to fill the cylinder but it needs to move slower ?? I have always wondered how fast the pistons were moving when the  126 mph world breaking Mallard was running ? also one needs to take into account the momentum of the engine !! and on a level track how far would the engine travel if the steam was suddenly cut off completely ,a simple maths problem that those schoolboys were given at the time i guess !!    In a lead acid battery ,how faster or slower is the evaporation of the electrolyte ?  just  a practical question this time ....
Willy
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on August 02, 2018, 11:36:07 AM
I did a bit of simple schoolboy maths to get a rough idea of how fast the pistons would be moving at 126mph land speed, if I've done it right the figure is a lot lower than I would have expected. I guessed the driving wheels as being about 7ft in dia and the piston stroke as being 3ft - using those figures we get :-)

Wheel circumference 22ft.
1 mile = 5280 ft   5280/22 = 240 wheel revolutions per mile
240 x126 = 30240. Therefore at 126mph the driving wheels make 30240 revolutions per hour.
At 3ft piston stroke we get 3 x 30240 = 90720ft of piston travel in either direction or 181440ft per hour at 126mph.
181440 /5280 = just over 34mph average piston speed.

Admittedly 34mph is an average speed over an estimated 3ft stroke and the speed will be considerably higher at midstroke than at the endstroke but it still is lower than I would have guessed.

I think I've calculated correctly but it is possible I've overlooked something  :headscratch:
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 02, 2018, 12:27:45 PM
Hi Willy, basically to answer your questions we have to consider the locomotive as a system.  If we look at the whole system, which we may represent as a square box (not a black one please, - we do know what goes on inside!), we have a system with fuel input and work out.  Perhaps we should include waste heat out in the exhaust steam, flue gas and other heat losses.  It basically operates in equilibrium, or steady state, when the work out plus the losses exactly consumes the input energy.  If we demand more work output by entering an uphill section of track, or adding more load, without changing the energy input, the train will slow down to use the same amount of power by doing the required work at less speed.  Remember power is work per unit time, measured in watts.  And the fuel input can also be expressed as energy per unit of time, or watts.  And of course we all know the song, heat is work and work is heat.  (We had better be careful here, or Zee might start singing again!)

On a flat track with constant load, if we want to slow down in preparation for arriving at a station, we basically have to reduce the fuel input. 

Of course, as I mentioned yesterday, the time constant associated with reducing fuel input is too long for practical purposes, so we have to do something else for a quick response, and then deal with the consequences of the excess heat input.

If we look inside the box representing the whole system, we see two subsystems with some control gear between, a boiler with its firebox and fuel supply, and an engine which takes in steam, and outputs work at the shaft/wheels, and exhaust steam.

If we look at the boiler, it takes in fuel, outputs steam, and there are losses which take care of the balance.  If we put in more fuel, there will be more steam out.  On the other hand, if we restrict the output steam, with the same fuel input, the boiler pressure will rise, losses in flue gas and convection losses all increase, and if this is not enough to find a new equilibrium, the safety valve will lift.  If we try and take more steam on the outlet side, with the same fuel input, boiler pressure and so temperature will fall and the steam output will settle at the point where the steam plus losses carry away all the input energy.

At the other end of the overall system is the engine.  Steam at the inlet is admitted to the piston where the pressure exerts a force on the piston.  This force acts through the piston rod, conrod, and crank shaft and exerts a torque on the driving wheels which reacts with the track to make a force on the locomotive frame in the appropriate direction.  If this force is sufficient, the locomotive will move, resulting in the piston moving, and as we have previously learned, when a force, like that on the piston, moves through a distance, work is done.  If the force on the piston is more than sufficient to just start the locomotive moving, the excess force where the wheels meet the track will accelerate the locomotive, and once the drawbar slack is taken up, eventually the whole train.

As the train accelerates the piston requires more steam, otherwise the pressure will fall.  Thus the engine accelerates until equilibrium is achieved where the steam required to maintain the pressure at the piston face is just equal to the steam produced by the boiler.  If the boiler is producing more steam than the engine requires at the current speed, the pressure at the piston face will increase, and the engine will accelerate. 

Ultimately the two systems operate at steady conditions when each has found the point where there is equilibrium between input energy and energy consumption.

As we saw yesterday, that control gear between the two systems, let's just assume a throttle valve regulator for the moment restricts the flow of steam, thus the transfer of energy from the boiler to the engine.  When the regulator is adjusted, the flow of steam happens almost immediately, the increase in boiler pressure is a bit slower, so the fireman has some time to react.  The lower steam flow means the steam flowing to the cylinder reduces in pressure, hence lower force on the piston, lower torque at the wheels, and less force to maintain the train velocity so the train slows down.

This brings us to your momentum question.  I have said it before, but can't say it too often, momentum is a very fundamental quantity in physics.  We all have an intuitive idea of what it is, but it can be very precisely measured, and it's effect in a moving system is precisely predictable.

We all know about the law of conservation of energy.  It is a fundamental law of physics, and leads to the first law of thermodynamics.  Not so well known is that conservation of momentum is an equally fundamental law which also always applies.  And it is even easier to use than conservation of energy for many of the problems where conservation of energy is often quoted.  I am thinking of wings, sails and propellors for a start.  So let's look at what momentum means on our moving train as per your question.

Momentum is a property of any moving mass.  It is quantified mathematically as mass times velocity.  SI units come into their own here, there are no arbitrary constants such as "g" required.  Mass in kg times velocity in m/s equals momentum in kg.m/s, ( read this as kilogram meters per second).

The law of conservation of momentum means that a body continues at rest or uniform motion in a straight line unless it is acted on by an external force.  Now, where have we heard that before?  It is Newton's first law of motion. (Or is it the second?  Not really important which order he discovered them.)

Now when a force acts on that moving body for a time, we get a change in momentum.   Change of momentum per unit time equals force.  So if we know the velocity of the train in kg, and measure how many seconds it takes to come to a stop when the steam is suddenly cut off, we can calculate the resistance or total drag on the train.  Of course, this simple calculation requires one extra assumption, that the force is constant.  We know that the air resistance portion of the force is proportional to the velocity squared, but it is also probably quite low compared with the total rolling resistance, unless of course you are thinking of that Mallard at record speed.  It also requires sniffting valves to be fitted to admit air freely, so the engine does not do work creating vacuum or pressure somewhere.  But over a small speed change, where the change in variable resistances can be ignored, the calculation is quite exact.

To calculate the piston speed we need to know the driving wheel diameter and the piston stroke, then it is relatively simple maths to work out how far the piston travels each revolution of the drive wheels and how many revs per unit time the driving wheels rotate at any given speed.

I notice that Gas-Mantle has done the piston speed calculation while I have been writing.  Give or take any minor arithmetic error or inaccuracies in the measurement estimates that should give the correct answer.  The piston speed is a maximum in mid stroke and zero at each dead centre, but the conventional method of specifying piston speed for engines and compressors for industrial machines uses the same simple method, so it is really an average piston speed.  Ring wear is generally roughly determined by piston speed so engineers typically specify a piston speed limit for an acceptable design.  I can't remember the typical figures, but it is a relatively modest figure that can be achieved with long stroke and low rotational speed or short stroke and higher rotational speed.  My compressors were generally classed as low speed, typically about 400 rpm, and typically had about 12 inch stroke.  Oil field compressors were generally described as high speed and were typically 1000-1200 rpm and about six inch stroke.  But the same piston speed limit applied to both.  Of course these criteria apply to industrial machines expected to work 24/7 for long periods without interruption.  I am sure a different criterion would be used for a race engine, or a record attempt, but don't try and run those special engines 24/7 for very long.

Hi Gas-Mantle, thanks for looking in and doing that calculation.  Good to hear from you again here in addition to your other posts.

I suspect any experiment involving momentum and the Mallard would be limited by the available length of straight, level track.  Predicting the total distance requires knowing the total mass, and the total rolling resistance.  A more practical experiment would be to measure time for a relatively small velocity change, and repeat the measurement over a similar speed change but from different starting speeds.  Each measurement enables a drag calculation for that speed range.  Assembling the results on a graph enables construction of a curve showing resistance with speed.  From this curve, the total distance on that ideal never ending straight, level track can be calculated.

On the lead acid battery question, in the old days, batteries had a screw cap with a small vent hole in each cell.  It was important to check battery liquid levels frequently, as it did evaporate.  It could even boil quite vigorously if charged at too high a charging current.  I am sure that you can remember those days.  Then we had sealed maintenance free batteries.  I assume the evaporation resulted in pressure change within the casing.  Severe overcharging could increase the pressure sufficiently to damage the casing.  I don't know if they had any specific over pressure vent.  Perhaps another forum member can comment on that.  Now we have gelled electrolyte batteries and AGM, and as far as I know there is no evaporation problem.  If you are really up to date, lithium Iron is the go!

If you have a battery that is loosing electrolyte, it is probably worth checking the regulator charging rate.  Internal shorts and other cell faults might also be the cause.  These days we do not have those external bars connecting the individual cells so we can't check the voltage of each cell separately.  I assume we can deduce a short circuit by testing the voltage after completion of a suitable charging cycle, and measuring the voltage after a rest time after disconnecting the charger with no load.  It should still be about 12.5 volts minimum to 12.8 or so depending on temperature and whether the battery is fully charged.  A shorted cell would immediately drop this by about 2 volts.  I hope that helps.

Thanks everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on August 02, 2018, 02:43:57 PM
Hi All, Mallards driving wheel dia. is 80 inch and cylinder stroke is 26 inch. At 126mph = 530 rpm or 8.8 revs a second. Piston speed 1060 strokes/min X 2.167ft = 2297 ft/min. Mallards highest horsepower output that I know of was just under 2500 H.P. Now it was going down a slight grade when doing the speed record so would not have been near this figure but consider 8.8 revs per second, the reciprocating masses and whatever power was applied from the cylinders and you can understand why it was damaged doing this run. However, a little more modest speed of 100 mph for the very best modern express locos, on suitable sections of track, was actually timetabled on some of the named passenger expresses in the U.S. and I presume elsewhere. Considering the forces involved at over 8 revs a second with the acceleration and deceleration of the piston and rods all coming to a stop and then reversing direction one can only have the greatest respect for these machines. Consider what it would be like to be the crankpin! Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 03, 2018, 01:00:55 PM
Hi Paul, thank you for the information on the Mallard.  As you can tell, I have not looked too much into its history, or history in general, for that matter.  My grandson brought home an HO gauge electric locomotive, a Triang/Hornby model of it, as his souvenir from the trip his family took to Europe and UK last year, so at least I had heard of it and its famed speed.  When you are thinking of the forces, it is not only the crank pin, but also the cross head pin that has to take all that force.  Of course the whole engine and its frame has to be designed for the forces, and the maximum piston rod force turns out to be the controlling load for the whole frame and moving parts.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 03, 2018, 05:40:18 PM
Hi MJM,  'the fireman needs to act' would this entail putting more coal on the fire to take out some of the heat and then delay a good head of steam to egress the platform again ? lots of interesting info again, cheers.

Willy
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on August 03, 2018, 06:15:40 PM
Willy, this video gives a good introduction in how to fire a steam loco. It's actually a far more skilled job than a lot of people think  ;)

https://www.youtube.com/watch?v=NHo860Q66Gw
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 03, 2018, 11:35:00 PM
hi thanks for that..interesting and informative...I noticed on the film of the Tornado trip ,the fireman wasn't shovelling coal all the time !!....Hi MJM....Also on the news it said that after the heatwave of 52 days when the rain came the water temp dropped 10 degrees and killed all the fish because the oxygen content dramatically decreased !!They then put in fountain pumps to try and get some oxygen back into the water the duck weed also increased dramatically ? I have also heard that here in norfolk they used flint to help break up the clinker in the fire box...it would heat up and explode/shatter !!?
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 04, 2018, 03:07:20 AM
Hi MJM,...a friend has a flat in the attic of a victorian house and she is suffering from the high temps about 32 degrees. She is wondering how to keep it cool and says a fan doesn't make any difference. I have given her the info about closed windows   blankets etc. She says the window  perhaps is not glass but some sort of acrylic. it is a rented flat and there is possibly no insulation in the roof. I was wondering if muslin cloths in buckets of water might cause cooling by evaporation ?  Have you any ideas please....thanks

Willy
Title: Re: Talking Thermodynamics
Post by: crueby on August 04, 2018, 03:22:12 AM
Hi MJM,...a friend has a flat in the attic of a victorian house and she is suffering from the high temps about 32 degrees. She is wondering how to keep it cool and says a fan doesn't make any difference. I have given her the info about closed windows   blankets etc. She says the window  perhaps is not glass but some sort of acrylic. it is a rented flat and there is possibly no insulation in the roof. I was wondering if muslin cloths in buckets of water might cause cooling by evaporation ?  Have you any ideas please....thanks

Willy
Basically the same setup as my upstairs shop, house is a Cape Cod style, with a 45 degree roof so the upstairs rooms only have windows in the end walls. When I bought the place, that space was unfinished, and the heat was awful up there in the summer. Insulation helped a lot, next step was to put one of those whole-house fans in one end window, with that on and the window at the other end open, it draws a tremendous amount of outside air through, cooling the space a lot - the sun on the outside roof and walls heats them up and turns them into a radiator on the inside. Short of air conditioning, it is the only thing that makes the space usable, and does not cost much to buy or run. I keep the drapes closed when the sun is on the windows as well.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 04, 2018, 01:10:28 PM
Hi Willy, I can always rely on you to take an unusual view on what was intended as a simple proposition.  It really tests my understanding and makes it all more interesting as there is normally something in what you say that needs to be understood.  In the context of reducing the fuel input to balance the reduced power output required to travel slower, you need to reduce the rate at which fuel is being burned to reduce the boiler energy input.  Now with gas or oil firing, you can usually reduce the firing rate by closing in on a fuel throttle valve, and get a reasonably quick response, though probably still slower than the driver may require.  With coal, the requirement is still to reduce the rate at which fuel is being burned.  I am not an expert on burning coal, and I will defer to Paul and others on the detail.  Once the coal is on the grate, I assume you can't get it back, so you can only let it die down a bit while perhaps the fireman gets a break in order to slow the fire.  And that is definitely too slow to please the driver.  The issue is more complex than that, with the need to ensure adequate air flow etc.  However, I would expect that while putting in more coal might help in the short term, when it heats up and starts burning, assuming adequate air flow, the rate the coal is burned might be higher if the fire is well managed.  But adding coal as you describe might well be in the fire man's bag of tricks to cope with rapidly changing situations.  The video Gas Mantle has posted might include useful information.  I am on very limited data at the moment, so am unable to watch it until later.

Hi Gas Mantle, thank you for posting that video.  Great to hear from you again.  I will come back and watch it when I am back on full data allowance.  These "pay by the Gb when you exceed the allowance plans" are really aggravating, especially when mostly you have plenty left over.  Unused data allowance should really carryover to be at all fair for the occasional month.  I hope your practice at coal burning with your vertical boiler is benefitting from this discussion.

Now, back to Willy's question about cooling the attic.  Putting blankets or curtains over the window and similar measures help reduce the heat input from the sun, but the heat is still coming in and with the roof not insulated, will not help enough in a long heat wave such as you are experiencing.

A fan works by increasing the evaporation of perspiration, but that same evaporation also increases humidity which reduces comfort by limiting the potential rate of evaporation.  A single person loses about 90 watts from normal life metabolism, so the fan helps the person feel cooler, but is still heating the room.  To improve comfort, you have to add ventilation, with open windows, on opposite sides of the apartment, if practical, and preferably with a fan placed to increase the air flow.  Best if the fan adds to any slight breeze rather than opposes it, so the best direction may change from time to time.  Again, to really work the process, if you know the difference between inside and outside temperatures, you might work the fans really hard at night if the outside air is much cooler, but turn them off during the heat of the day if it is hotter outside.

Your muslin cloth with water and a fan comes into the area of active cooling.  Here you can buy commercial units which optimise the process.  They are called evaporative air conditioners.  My son has one to serve his whole house, and I can assure you, it is very effective.  Much cheaper to run (and buy) than refrigerated air conditioning, as they only have air fans, a water pump and fancy muslin.  Providing you have plentiful water of course.  The cooling is done simply by evaporation of water.  However there are tricks to running them.  You need a window and door open, preferably on opposite sides of the house, and if there is really no breeze, a fan to help the air flow, so there is continuous air exchange with outside.  You also need some continuous water blowdown from the reservoir, as the salts in the water remain, just like in a boiler, so will salt everything up if you don't have blowdown.  If you don't have the necessary ventilation, the air inside gets very humid and uncomfortable, and everything goes mouldy.   And they do not work so well in the tropics with high humidity.

Wet muslin cloth has also long been used here in the outback, in the form of the Coolgardie safe, which was used to keep food cooler before refrigeration became available, and also the hessian water bag that was hung from the frame of the buggy and I can even remember my father using one hanging from the car bumper in the early days on long trips, before cars had air conditioning.  The water was quite cool and refreshing even on the hottest days, so a break to get a drink from the water bag was always welcome.  So if you can arrange the muslin so it wicks up water from a bucket, or keep pouring it on regularly to keep the cloth wet, and a fan blowing over the cloth, you will get enough cooling to be worth trying out.  But you also need that through flow ventilation for air exchange with outside, otherwise the humidity increases to the point of insufficient further evaporation.  And the room is still being heated by you and the fan motor.  Evaporative cooling is self limiting in a closed space, due to that humidity and heat increase.

Best is to see if you can buy one of those portable evaporative coolers.  I have even seen battery operated ones in a camping store, but that was unusual.  If you go this way, make sure you have the required air exchange with the outside.

And you can also buy portable refrigerated air conditioners.  Again, tricks to the operation.  You need to reject the heat to outside.  Some models have only one air hose and actually exhaust the cooled air from the room, so overall are more expensive to run.  The preferred ones have two air hoses, a bit ungainly to install but by rejecting all the heat to air drawn on from the outside, instead of exhausting the cooled air, are more effective and economical to run.  Refrigerated air conditioning is the only exception to the ventilation requirement.  Ventilation is still required for healthy air, but not nearly so much as for simple air exchange or any form of evaporative cooling.

Hi Chris, thanks for joining in.  I have been in the roof space of our house on a hot day, above the insulation, and the roof tiles are really strong radiators.  Attic spaces definitely accumulate heat and  the first step is to increase ventilation as you say, to at least limit the temperature rise in the attic, by keeping it close to the outside temperature.  Roof insulation is very effective in these spaces, but of course not usually an option in a rental situation.  And of course closing window blinds when the sun is on the window.  Anything to reduce the incoming heat, and get rid of any air which is warmed by the remaining heat input.

Oh, and I have missed the comment about the fish!  Fish usually seek deep water where it is cooler, but can't do this in a shallow pond.  If I remember correctly, warm water holds less oxygen than cold, and rain should bring in oxygen saturated water, and disturb the surface to further increase oxygen content.  So I would expect the fish were weakened if not killed by the heat and lack of oxygen, and while they would normally seek the cold water, a sudden 10 degree change will kill them every time, as anyone who has had an aquarium well knows.  They do not tolerate changes of temperature very well.  But the weed will really take off in the heat, especially when the rain brings more oxygen.  Even barnacles and weedy fouling on a boat grow more slowly in the winter cold, and take off when warmer weather comes.

Thanks everyone for following along

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on August 04, 2018, 03:20:09 PM
Hi MJM & Willy, Just a comment regarding firing on locomotives. Firing rates for a class of locomotive,  hauling a particular load with certain characteristics, (Plain bearing 4 wheel stock and speed limited, air braked or not, roller bearing bogie stock etc.), on a particular section of track and the condition of the locomotive were all things known by crews. The were intimately familiar with the locos and the road over which they travelled, they had achieved their positions after decades of experience developed incrementally on various classes of loco and working, so 'knew' what would be needed. Any permanent way restrictions were all known as drivers would be informed of speed restrictions for any sections under repair before leaving the depot.

Oil firing was pretty responsive to fuel supply and coal firing a bit less immediate, adding water via the injector or starting a second one could also affect things. A loco doing proper work would soon 'die' if the inputs did not match demand, and fairly quickly. Firemen and drivers were always anticipating the conditions required ahead and were rarely caught out, and in the days of steam, for the most part they were constantly working quite hard to get the load to its destination. Conditions on preserved railways are usually a 'piece of cake' and even main line tour operations usually have more than enough people crammed into the cabs if the fireman needs help, so are not necessarily accurate guides to 'the good old days'. Hope this gives a sense of how things are/were on a steam loco. Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 05, 2018, 12:08:19 PM
Hi Paul, thanks for the interesting detail on the subtleties of engine firing.  Detail that most of us never appreciate.  Sounds like the voice of experience.

Also sheds some light on the challenges of firing a model at maximum efficiency.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: Gas_mantle on August 05, 2018, 12:23:51 PM
A good signalman could also save the fireman a lot of work  ;)
Title: Re: Talking Thermodynamics
Post by: paul gough on August 05, 2018, 09:41:52 PM
Only too true! A speed check or halt had the potential to turn a portion of the journey, if not all of it, into something of a battle on tightly tabled sections or trains. Clawing back even a minute can create a testing time for the crew.  Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 07, 2018, 12:11:33 AM
Ni MJM , with a  locomotive oil fired boiler is the oil and water consumption closely correlated ? and what is the saving in fuel costs with summer temps of 30 + degrees and winter's of say -20 degrees ??  just wondering !!

willy
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 07, 2018, 12:14:21 AM
Only too true! A speed check or halt had the potential to turn a portion of the journey, if not all of it, into something of a battle on tightly tabled sections or trains. Clawing back even a minute can create a testing time for the crew.  Paul Gough.

Yes ,at 60 mph or 1 mile a minute ..that last mile when you are slowing down for a station will take quite a while ?
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 08, 2018, 01:42:13 PM
Hi Willy, sorry I was absent last night.  Unexpectedly found ourselves at a remote site with no internet service.  Should be ok tomorrow also, but after that, if I am missing a day or so occasionally, you will understand why.  But glad to see that the conversation continued.  We were rewarded last night with a magnificent view of the Milky Way, which in our southern skies can be so brilliant a stripe across the sky that it hides all but the brightest stars.  And again tonight, even clearer.

I will leave Paul to talk about the time and effort to slow down for a station, but I think his comment was more about a signalman showing an unnecessary orange signal, causing an unscheduled slowdown which puts the train a minute behind schedule, then taking a lot of work to catch up.

With regard to the air temperatures, of course the water cannot be at -20, but the air can be.  So let's assume the water is kept at zero instead of +30 by a little feed water heating using exhaust steam, and the steam raised at 1000 kPa, it takes about 4% more energy to heat the water from zero to boil and evaporate to dry saturated steam at 180 degrees C which is the saturation temperature of 1000 kPa steam.  This pressure is just an example, I don't know what pressure a mainline locomotive would operate at.  The figures come simply from reading the steam tables.

However, the air for combustion also has to be heated.  And it can certainly be at -20 in northern parts.  I don't think there are any railways in southern parts that are far enough south to get those temperatures.  Unless Southern Chile and Argentina, perhaps.

I really don't know the typical flue gas outlet temperature for a locomotive, so let's assume for example stack gas at 300 C.  With air entering at 30 and leaving at 300, the heat lost to atmosphere is due to 270 degrees temperature rise.  On the other hand, if the air enters at -20, that temperature rise is 320.  As the specific heat of air is roughly constant, the heat lost to atmosphere is increased by about 20%, and this all has to be supplied by burning more coal.

With 4% extra heat for the water, we can assume 4% extra air required, so the total extra heat is approximately 25%, (1.04 X 1.2).  And of course we should expect more heat losses from the boiler to the air rushing past.  That extra heat all comes from extra coal, so we would expect to need a bit over 25% extra coal, which might require an extra fireman.  A reasonable guess would assume running cost proportional to coal consumption, so about 25% higher cost due to the low temperature. 

I don't know the cost per mile at 30 degrees, as it almost certainly involves more than the straight cost of coal, but whatever the figure at 30 degrees, you could reasonably assume it would increase by about 25% at -20.

I assume you don't really get -20 C in your country, but I can assure you it was common enough in Southern Ontario when we lived in Canada, and almost certainly in Scandinavia, Russia, and northern parts of China.  But I am sure there are other countries too.

So in summary, we need a bit more information on steam conditions, coal and other proportional costs, and coal energy values, then the steam tables tell is the energy requirements, so estimates of cost differences can be relatively simply calculated.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 08, 2018, 11:55:09 PM
HI MJM, more interesting stuff...so overnight and daytime they must have had the means to keep the water above freezing......also the coal at --20 would also need to be heated up to reach combustion point.....Re- friends cooling problem ..it would appear that some expense would be incurred ...so what about in the 3 storey BLDG to leave the letter box open and have the muslin/water contraption next to her slightly ajar door ?? would there be natural convection up the stairs ?? possibly not if heat travels from hot to cold......i have noticed in my ground floor flat when i leave the letterbox open and the sash window open at the top there is a draft...?  Here in blighty they have painted the railway tracks white ,but not all of them, also the chap from the railway said they "stretch" the tracks before they weld them so they can expand/contract.....he was wearing a suit however.......Also they said that they had to shut down one of the Nuclear reactors in Switzerland because the water temp in the lake was too high......so lots of talk in the MSM about the currant high temps...

Willy
Title: Re: Talking Thermodynamics
Post by: paul gough on August 09, 2018, 06:35:56 AM
From memory exhaust gas temps. on a loco are 450F. to 550F. Say 500F. for arguments sake. So your speculation of 300C. is pretty close. Obviously these figures varied somewhat over the course of the journey, but the testing/design staff would have averaged it out unless looking for a particular performance aspect. A caveat to this is, these temps. apply on the older style locos I am familiar with, I probably don't have any test details for modern Post WW1 era locos any more as I have cut down my library to pre 1900 books. However I have in mind there are some data charts in one of my remaining books that may be of interest. I shall look for it tonight and post photos if I find them.

Regarding stopping a train; this is perhaps the most important skill any driver has, again this skill is built up through years of experience, if you ever travelled in a sleeping car you will know what really skilled braking is, a good sleep and not jolted into wakefulness at every stop. Coming over the top of a rise and feeling the mild surge when a couple of thousand tons, especially at speed, becomes the propelling force, serves to remind one of the tremendous forces involved. In the steam days of paramount importance was NOT to make repeated light brake applications. I'm talking air brakes not vacuum. Braking had to be done in a fashion that did not risk depletion of the locomotives capacity to recharged the brake pipe, so goods trains were held in check by bringing their speed right down with one application and then recharging the break pipe while it accelerated again so as to have all the auxiliary reservoirs on each waggon recharged, repeated applications could drain them and then you would have no brakes! If coming down a mountain range,  a number of hand brakes might be set based on the ruling gradient which meant that steam may have to be worked on any light grade or level sections. Coming to a stop at a particular point, a station platform or gently rolling into a loop, was about being at an appropriate speed before making the final stop, all known from experience, knowing the load and what was required ahead as well as the braking characteristics of the rolling stock. Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 09, 2018, 01:40:05 PM
Hi Willy, you are quite right, the coal would also have to be brought up to temperature, and that takes from the energy left for raising steam, so further increases the required amount of coal to be burned.   I don't know if it has to be specifically heated in the tender to stop lumps from freezing together, or if it is dry enough and warm enough for that not to be a problem.  It will soon heat once on the grate, but the heat ultimately comes from burning more coal, unless it can be obtained from waste heat such as exhaust steam or even some flue gas.  It also has to be kept cool enough not to ignite in the tender, so more to it than it looks to the casual observer.  And well out of my field of knowledge.

Returning to the cooling problem, I hope it all made sense, even though installing air conditioners may not be practical.  While heat travels from hot to cold, warm air is less dense so it rises, and definitely will give a draft up through a three storey building so long as you let outside air in at the bottom and out at the top.  Even this natural air ventilation will help limit humidity rise and keep the air more fresh, so will improve comfort.  You can assist the draft by pointing a fan out wards through the upper story window, so long as replacement air is able to flow in downstairs.

Why outwards?  Because the energy consumed by the fan ends up as heat in the air, so if you are trying to cool the room, expel the air that is heated by the fan first.

A separate issue is the muslin.  In the upstairs flat, with a draft over it, it will tend to cool the air as you expect.  But cooler air is more dense, so will tend to oppose the natural convection due to the warmer downstairs air rising.  Still possible to achieve your aim, but even more important to have that fan assist by expelling air from the upstairs window.

Ideally you want that natural convection draft up the stairs, over the wet muslin, through the upstairs living/sitting area and out through the upstairs window, assisted by the fan pointing outwards through the window.  And of course plenty of open windows, doors or letterboxes downstairs to let outside air flow in to replace what the fan blows out.

Painting rails white?  I don't really know why they would do that, unless to reduce the heat input from the sun.  As we saw earlier in this thread the white paint does reduce the heat absorbed by the rails but only for the part of the sun's heat in the visible spectrum.  Still, it all counts.  But I would be concerned about the effect of the paint on wheels slipping during acceleration or braking.  Again, not something I know much about.  Continuously welded track is another area which I don't know much about.  There is quite a bit of science in tying down the track so it does not buckle on expansion when welded into a continuous long length.  A really specialist area that I have not read much about.  It is not much different from long continuously welded pipelines.  Inside a plant we deliberately install extra bends where necessary to take account of expansion and contraction, but that is not a practical solution for railway track.  But stretching the rails takes a lot of force.  A straight track is very stiff in tension or compression. 

The nuclear reactor question might have a simpler explanation.  Unlike the board meeting discussing the nuclear reactor and the bicycle shed, I would rather talk about the nuclear reactor.  Generally they use water for cooling because it has higher specific heat, so requires much smaller heat exchangers than air cooling.  And water usually achieves a lower temperature, partly because of this.  In inland installations, the plant often uses a lake or river as a heat sink for cooling.  The heat taken up by the lake or river causes an increase in water temperature until it is balanced by evaporative cooling from the surface.  This has an adverse effect on the environment, fish life etc. so usually environment authorities specify a maximum allowable discharge temperature from the plant.  If the incoming water is already hotter than normal, it may not be practical to reject enough heat from the plant within the allowable maximum discharge temperature.  I suspect this is the reason the plant had to be shut down.  Alternatively they could invest more money in the larger heat exchangers required for air cooling.

This problem is not unique to nuclear reactors, and conventional power plants can also have operation curtailed by the same constraint.  The nuclear reactor is actually cooled by raising steam, which in turn drives turbines to drive electric power generating machinery.  The turbines need a condenser for the exhaust to maximise the power generated.  The condenser is normally water cooled, but can be air cooled.  Air cooled condensers are common in the Middle East, and I know of one in Australia, though there may be others.  They are a very special beast, but they work reliably even though the minimum condensing temperature, and hence minimum turbine exhaust pressure, is a bit higher than with water cooling.

Hi Paul, I am glad that 300 was a reasonable guess, but more good luck than knowledge I am afraid.  I don't really know locomotive operating pressures either, so my logic was drawing a long bow as they say.  Modern locos might operate at higher pressures, and they would then require higher stack temperatures to transfer the necessary heat to the steam.

I was most interested to read your description of the braking process.  Obviously much more to it than appreciated by most of us.  I could not help thinking about how much greater problem those iron ore trains in the west must pose.  You only have to look at the track and sleepers to feel that those trains are really heavy compared with standard freight trains.  And the train seems to stretch from horizon to horizon.  So the driver and fireman have to manage all of that as well as managing the grate so that it neither goes out, not lifts the safety valve.  And of course does not over fill the boiler or run it dry.

What do you suggest the driver of a small gauge (3 1/2 or 5 inch gauge) could learn from full size practice?

Thanks everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: derekwarner on August 09, 2018, 11:09:51 PM
Digressing a little..& apologies to Paul, MJM & Willy, however just a few weeks ago....in the West of Western Australia, Rio Tinto achieved it's first 28,000 tonne delivery of iron ore by a driverless train set

https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=2ahUKEwiQs8WO_ODcAhVCgbwKHXAMAUIQFjABegQICRAB&url=https%3A%2F%2Fwww.railwaygazette.com%2Fnews%2Ftechnology%2Fsingle-view%2Fview%2Fdriverless-heavy-haul-train-complete-first-iron-ore-delivery.html&usg=AOvVaw1ktGlg84pQjQNB2gwV3K7X
Title: Re: Talking Thermodynamics
Post by: paul gough on August 09, 2018, 11:18:58 PM
I have in mind a figure of around 400C. max. for smokebox temp. on a hard working engine of the older types I am familiar with. I'll try to find something on 'modern' engines. Boiler pressures of modern locos was in the range of 250-300 psi, there were some locos that exceeded this a bit, from memory the 38 class of N.S.W. railways had Australia's highest at 245 P.S.I.

At the time I was in the railroad department of Mt. Newman Mining Co.,(1972), the trains were hauled by three 3600 H.P. Alco locos with 100 cars each of 100 tons capacity. Thus 10,000 tons of ore in each train and at the time 10 trains a day. When seen for the first time they were quite awe inspiring as they were about a mile long, huge by Australian standards for the time. After I left, radio control came in, this allowed two locos to be put in the middle of 50% larger trains for better power distribution but also to relieve the load on the draw gear. Braking back then was by conventional air brakes, but I have heard they use radio controlled braking now, but I know nothing of it. These were all diesel electric hauled trains so no grate or fireman, the crew being a driver and observer. Well, I suppose there was one grate on the locos, it was the crews toilet in the nose of the loco. Due to environmental requirements flushing 'deposits' onto the tracks was not permitted so 'electrically incinerated' toilets were fitted. Trouble was they weren't incinerators they were 'slow bakers' and the odour entering the cab was overwhelming. No-one wanted to open the cab widow, air conditioned cabs, as it was often 40 C. outside. The answer to this when on the move was to go to a trailing unit and relieve oneself or if stationary climb up into the first ore car and contribute something to japanese metallurgy.

Model operation is somewhat different to full size and don't know if there is anything specifically to be learned. Mostly it is about knowing the loco intimately and to do this means familiarity with it under various conditions of operation. The more one knows about how things work, the more one knows what is going on, then combining this with experience produces the 'feel' that good operators have.

Still looking for the tables I referred to previously. Paul Gough.

Title: Re: Talking Thermodynamics
Post by: paul gough on August 09, 2018, 11:28:47 PM
Derek, Only just saw your post, I'll have a look at the article. I'm afraid I am very out of date nowadays, it is sobering to think a lot of my experiences, at the time, related to up to date locos and operation and now it is all well and truely historical and pretty much 'ho hum' by many contemporary standards. Paul Gough.
Title: Re: Talking Thermodynamics
Post by: paul gough on August 10, 2018, 03:04:14 AM
Hi M.J.M., Here are the charts I mentioned previously, see attachments. I hope they are of some interest.
 Loco specs; 4-4-0 express type; cylinders 19x26 inch, valve travel 3 5/8", 1/8" lead, 1" lap at full gear; Driving wheels 7'1"; boiler 4'4" Dia, 11'4" between tube plates, 240 x 1 3/4" O.D. tubes, copper firebox 5'7" x 3'2" at grate, grate area 18.14 sq. ft., firebox heating surface 112.45 sq. ft., tubes 1246.2 sq. ft., total 1358.65 sq. ft., total area through tubes 2.47 sq.ft.; Boiler pressure 175 p.s.i. Regards, Paul Gough
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 12, 2018, 12:52:52 AM
Hi MJM, Just a few observations ... today i had a really cold smoothie and i was thinking about warming it up slightly so i was blowing into it. I noticed that when doing this cold air was being returned into my face ?!!! so was this coldness being driven from the smoothie? When you have hot food and drink you blow on it to cool it down .....so does the same thing happen with ice cold drinks in a reciprocative way, to get back to ambient temperature ??  Just wondering.............Paul , those charts will need some figuring out to draw some conclusions......I use to travel on that line years ago when they had steam engines It would have given something to do to while away the time !!!I was stationed at Arborfield cross when i was an electronics apprentice !!



Willy
Title: Re: Talking Thermodynamics
Post by: paul gough on August 12, 2018, 08:00:47 AM
Hi Willy, The charts can provide various levels of analysis. You don't necessarily have to jump into applying mathematical methods to get something from them. Firstly, they show the kinds of things that were important to know and what was measured under operating conditions and the results/performance then considered against the existing dimensions of the loco. Some things are obvious once you know them, but often it is not until you see figures for various things and start making comparisons and connections that the relationship between them becomes apparent. Have a look at the boiler and engine efficiencies. Why such a difference?

 Some sense of what is happening and what the driver is doing can be got by comparing the vacuum profiles against the speed and line profiles while noting the indicated horsepower trace. Noting  the normal range for the vacuum in the smokebox, what difference between it and the fire hole door, asking are the changes co-incident at each location, does smokebox temperature and vacuum always parallel each other. Just simple comparisons, maybe obvious, but until the numbers are checked, it is only speculation. The numbers help verify ideas, which might then be improved upon.

The book from which these charts are taken is from; 'A Manual of Locomotive Engineering', by W. F. Pettigrew, Charles Griffin & Co., London, 1899. There are a number of later editions, and in the U.K. it should not be too hard to get a copy. I have always considered this book the best I have come across because it explains things comprehensively but easily. Perhaps there one in a library you can check out. 

 In the end, pondering all these things along with some of awareness of the design reasoning/decisions provides insights into what might or might not be  good in model design, always with the caveat, that not all full size ideas or results can be replicated when scaled down. Hope all this is not boring people. I trust M.J.M. can fathom something from the information that is useful. Regards, Paul Gough
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 13, 2018, 01:24:13 AM
hi MJM saw this in the newspaper ...i have not seen this before
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 13, 2018, 12:56:47 PM
Hi everyone, back in civilisation after a few days under the starry skies, with magnificent views of stars, planets and the Milky Way.  Also wonderful surf beach, only one, but 60 km long, magnificent bird life and a four ft. long goanna which was obviously well versed in where there might be food left stored or accidentally dropped.  Got to love the new car (to us), confidently towed the van through deep sand up steep slopes.  Only need a tow out once, but did engage the diff lock a couple of times which got us through.  But my wife says I have to learn to drive more gently, the box of tissues fell off the table, along with two place mats while we came out this morning!

Thanks everyone for keeping up the conversation while I have been thinking of other things.

Hi Derek, not entirely driverless I believe.  I think there are drivers in the control room in Perth, just not on the train.  I wonder if they have changed the electric locomotive that takes over at the end of the line.  It actually had a power cord connected into the mains when I last saw it. 

Hi Paul, those are interesting memories of those times, which do not really seem so long ago to most of us.  We should all  be writing down or recording some of those stories for future generations, we all have them.  And thank you for that interesting information on the locomotive trial runs.

Hi Willy, back to smoothie time again?  When you use a straw to blow through the smoothie, the close contact between the drink and the air enables good heat transfer from the warm breath to the cold drink, leaving the air close to drink temperature when it emerges from the surface.  Just simple heat transfer from a hot material (your breath), to a cooler material (the drink).  When you blow on a hot drink, the cooling is mostly by evaporation at the liquid surface, which is accelerated by the breath carrying away the vapour, which is composed of the higher energy molecules in the liquid, so results in cooling of the liquid.  Also the breath is basically cooler than the hot drink so any heat transfer is from hot to cold, but not very good contact, so only a minor contribution to cooling.  If you blow over a cold drink, there is still cooling by evaporation, but there is less evaporation at the lower temperature, so the cooling is not so effective.  Also the breath is actually warmer than the drink, so the small amount of convection heat transfer that occurs is in the direction of heating the drink.  So in each case, heat travels from the hot material to the cooler material.  In your examples your breath temperature is somewhere between the temperatures of the hot and cold drinks, so the heat flow is the opposite direction in the two cases.

Interesting picture of that "firenado", I also have not seen anything like that.  Obviously some sort of effect of an intense heat source in a cold atmosphere, but it must involve something more, that might be explainable to someone who knows more about conventional tornados.  A coincidental occurrence of such a fire and the atmospheric conditions that cause little "Willy Willies" or "dust devils" I suppose.

Hi Paul, I have certainly been interested to see that real locomotive performance data, and to have your drivers view interpretation of them.  They certainly contain a wealth of information. 

I have been fascinated to see the breakdown of where the heat goes on a real locomotive.  There is so much to learn from these figures, and interesting to consider what we can use to help us understand our models. 

As you say, it is always good to go beyond the simple comparison, and try and understand which is cause and which is effect, and the why of those variations in the various parameters.  Understanding some of the issues will require more understanding of some of the systems, but theory can certainly add some understanding of the why.  But your drivers eye looks at what response is required, when the variation is ok and when something has to be done in response.  Even more importantly, what is to be done.  I will definitely leave that part to you.

With regard to your first question, why are the boiler and engine efficiencies so different, perhaps that is a good one for me to start.

We all know that heat is work and work is heat, and that the first law of thermodynamics allows us to calculate the equivalence.  But transferring between them is not so simple or symmetrical.

It is easy to convert 100% of work to heat, but when we try and change heat to work, we find that we always loose some as heat.  This is stated more emphatically in the second law of thermodynamics.

When we look at how the energy in the random motion of molecules is turned into work, we find it is the linear displacement motion component of that random motion that exerts a force on the piston, and produces work.  But the energy of the molecules is not all linear displacement.  There is energy in spin around each axis of the molecule, as Jo mentioned very early in this thread. The spin results in angular momentum, and that angular momentum has minimal effect on the piston.  Not zero, but not very effective at producing work.  That spin energy tends to stay with the molecule through conservation of angular momentum.  We can see the proportions in the steam tables.  The enthalpy column figure is always slightly higher than the internal energy column.  The difference is approximately the amount of energy that can be made to produce work of we expand the fluid to low enough pressure.

When we apply these concepts to the boiler, we burn fuel to release chemical energy.  The energy released increases the temperature of the combustion gases, and the heat involved is all eventually transferred to the surroundings.  With control of excess air to minimise stack losses, generous heat transfer area to help transfer to the water, and insulation to minimise external losses from the boiler, we can get quite good boiler efficiency, measured as the proportion of heat arriving in the boiler water.  I have seen figures in the low 70% for full size high pressure boilers, in your locomotives and even come close in my model boilers. 

When we look at the engine, the most obvious problem is that of the energy delivered to the engine  in the steam, most of it goes out in the exhaust steam, and only a relatively small proportion is turned to work.  There is still energy that can potentially produce work until the steam is expanded to zero absolute pressure.  However, at this pressure, the volume is so large that expansion to this pressure is quite impractical, so not only is it only possible to turn a part of the incoming energy to work, we can't even practically produce work from all of that small proportion.  The difference in enthalpy values of the inlet and exhaust gives us an idea of the maximum possible efficiency of the engine, but in practice there are further losses due to friction of the moving parts, steam leakage, and heat losses from the repeated heating and cooling of part of the cylinder.  The second law of thermodynamics, in saying that when we transfer heat to work we will loose some, really understates the issue.

I recently saw a report that the newest combined cycle gas turbine power plants from GE had exceeded the previous benchmark a little over fifty percent, but most industrial size plants don't make anywhere near this.  Our little steam plants are nowhere near this level, and the approximately 10% quoted in your tables is representative of real world locomotive performance.   I believe more normal power generation plants achieve around thirty percent.  The efficiency competition results I have seen seem to reach a maximum of around 5% for the best five inch gauge locomotives, and considerably less for smaller gauges.

My own testing so far has all been without load, so quite low pressure.  As there is no work produced above what is consumed by friction, the efficiency is zero.  The adiabatic efficiencies for the steam conditions are also quite low, but should improve when I get to a load test, when higher pressure will be possible with reasonable speed and some work output.

I hope that offers some insight into why the boiler efficiency is so much higher than engine efficiency.  In summary, the boiler is only dealing with heat transfer which can be quite efficient, while the engine is converting heat to work, a process which must always involve losses, which in practice are quite large.

Thanks everyone for following along,

MJM460




Title: Re: Talking Thermodynamics
Post by: MJM460 on September 12, 2018, 02:59:37 AM
Hi Willy, good to be thinking about thermodynamics again after a little break.

 I am home from a 6000 km road trip, a great adventure.  Even included some real four wheel drive activity in soft deep sand, and looking forward to next time, but also glad to be home and able to make a bit more progress on my engines.

So, to continue the discussion about heat rising from your engine thread, we can start with the basics.  Heat travels from high temperature regions to low temperature regions.  The effect of gravity, so up or down is not relevant.  I have said it before, but while "heat rising" is still quoted so often, it is worth repeating.

In fluids, whether liquid or gas phase, the material is free to move under what ever forces are in operation, including gravity.  When the fluid is heated, it expands, so has lower density than cooler fluid surrounding it, and so the lighter fluid "floats" to the surface.  A bit like oil on water, but harder to visualise, as the hot and cold fluid do not appear different, and in any case they are continually mixing at the boundaries, which are hence not at all distinct, unlike oil and water.

Solids also expand on heating so their density reduces, but they are not so mobile that they can move under the minute differences in comparison with gravity, friction etc that are also operating.  So in solids, the question of heat rising tends not to arise.  It is easier to accept that heat travels from a hot surface to a cold surface.  In the absence of external heat source (or sink, as in a nearby colder material) the temperature in a solid soon becomes essentially uniform.  Simply because heat travels from a hot area to a cold area until there is no temperature difference.

You mentioned that you were thinking about phase changes.  Phase changes involve energy flow without temperature change, at least no bulk temperature change of the fluid undergoing the phase change.  But evaporation does involve heat transfer, and the heat has to come from somewhere.

Perhaps that is enough of an introduction to help you put some more words around the specific issue you are thinking of.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 13, 2018, 03:11:37 AM
Hi MJM, good to be back ...yes i think i know the basics of hot air rising and i guess hot water rises as well. I was thinking about molten steel and other metals though .When you silver solder fabrications the solder will flow from the source of heat ,or is this more to do with capillary action. Do the extra hot molecules of steel rise up in a retort or is that silly question ??. Also there is an ice cream freezer in the local cafe that has a clear sliding top. I was enquiring if they could save electricity by putting an insulating blanket over it. the reply was that it was an ultra modern display case that did not need a blanket and if it did it would over heat ??...Our heatwave is over so it is more comfortable working outside.

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 13, 2018, 12:46:12 PM
Hi Willy, yes, air, water and even molten metals.  Mind you there are other process going on in the retort that may interfere with the circulation.

Are you sure silver solder runs away from the source of heat?  I m not an expert on silver soldering, so I will leave it to others to answer.  Certainly there is caplilliary action which is why your silver soldered joint must have a little clearance for the solder to run into, for flat surfaces this is done by centre pop marks which create a little dimple to separate the surfaces.

I think the shop owner has not studied much thermodynamics, but the discussion might be frustrating, I would not bother.   Heat travels from hot to cold, so into the cabinet, probably downwards, but by conduction and convection between the glass top and the surrounding air.  It might be that the cover glass is some sort of super insulator, such as double glazed with nitrogen between (I suspect not, but it's possible, used for window glass in Canada, and other cold places) but that reduces the flow of heat inwards but does not totally stop it.  So the blanket would further reduce the heat inflow.  The refrigeration unit for the cabinet will achieve its set temperature with less running so power consumption will be reduced by the blanket, but it will not overheat.  But if the customers cannot see what is in the cabinet, they will either not buy, which does not suit the shop keeper, or they will open the cabinet while they make their selection, which will allow more heat in, so counteracting the effect of the blanket.  So while you are correct in your conclusion based on thermodynamics, it does not suit the shop keeper who wants the product on display.

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 14, 2018, 02:24:13 AM
HI MJM , I have spoken to the freezer chap and has said that the suppliers said that it shouldn't have a blanket over it as i would interfere with the  cooling system and potentially mess it up !  . I saw this engine at a car boot sale and the chap wanted £350 for it ...so he went home with it ,anyway i couldn't figure out how the meths burner operated with the pipe coming out of the top. Also the lubricator connections seemed a bit strange !!? The engine is a Maxwell Hemmens from the 80's

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 14, 2018, 12:44:20 PM
Hi Willy, the refrigeration unit under the cabinet has to reject all the heat it extracts from the cabinet.

This heat is rejected from the condenser on the back of the cabinet to the surrounding air.  If the air flow around the condenser is obstructed, the condenser temperature will rise, with it the condenser pressure, which will overload the compressor motor.  So it is very important that ventilation around the condenser not be obstructed when any blanket is placed. 

Apart from the observation that a blanket carefully placed over the top of the cabinet might reduce power consumption, it is probably not worth following too far, as with it comes the risk that the blanket slips and blocks the ventilation of the condenser and compressor, so the practical compromise is probably to use the cabinet as supplied, without modification, temporary or otherwise.  A night cover would have to be carefully designed to eliminate this risk, which involves more cost.  All these things are a balance of risk and reward.

I had seen an article on a Meths burner with the tube coming from the top, I can't remember the detail inside, but I thought there was an additional closed tube into the flame that generated the vapour.  But the memory of where I saw the article.  Perhaps someone else knows the principal.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 16, 2018, 12:00:07 AM
Hi MJM, just been looking at the vid of the tiny steam engine ...is there a limit to the speed of engines and does the speed figure in tables sort of backwards ? is there a limit on the speed due to the length weight PSI etc etc  off to Beeleigh Sunday with my engines and will take some photos. the engine will be running with air , but i hope to run it on steam !! Are there limits to the size of engine you can run with a known boiler output ?? ..depending on the type of engine you have...Thanks for the info on the freezer it all makes sense now !!
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 16, 2018, 11:19:12 AM
Hi Willy, glad the freezer makes sense now.  It's the other end of the scale from the systems I worked with, but the principals are exactly the same.

I think I would generally agree with your observation that "the speed limit table runs sort of backwards".  Meaning that in general a small engine can run faster than a big one, or in more precise terminology, the maximum speed is inversely proportional to the size.

When I think about why this is so, I think the issues can be divided into two distinct groups.

First is the forces and stresses that increase with the mass of all the moving parts.  For any given size, increasing speed means greater inertia forces, and higher stresses, so that eventually the weakest component breaks.  This is so important that most modern engines and turbines used in industry have not only a governor, but also a totally independent over-speed trip device.  On steam turbines, this device operates a very fast acting trip valve that cuts off the steam supply and shuts down the turbine.  A high speed machine that over-speeds to destruction is very dangerous.

The other group includes all the factors that limit the speed the machine can reach.  Things like the friction loss in the fuel and air or steam passages, steam supply for a steam engine or turbine, friction in bearings, air resistance to the moving parts, viscous forces in bearings and sliding parts and so on.  Unfortunately these do not always become important enough to limit the speed until speeds already exceed that necessary for something to break.

I guess there are also factors in the grey area between the two groups, such as wear rates.  Generally higher speed means more wear and tear, so a decision has to be made as to how much wear and tear is acceptable.  For many of our steam engines, we like this factor to be minimal, so the engine can run as long as possible, before we have to rebuild parts of it.  For our cars, we like them to run reliably until the next service is due, or longer in case we are a bit late.  For a race engine, maximum power and speed is wanted, and the engine will be fully serviced or rebuilt after each race.  So there is a range of what is acceptable.

So if we return to the question of speed vs. size, small engines like the glow plugs Ramon has been showing, might run 20,000 rpm or so, Lohring's amazing engine much faster again.  Our car engines run at a much more moderate speed.  And a large diesel in a container ship will run quite low rpm at maximum speed.  And those are reciprocating engines.  A little turbine might run 50,000 rpm.  A twenty inch diameter rotor on one of my compressor drives runs about 12,000 rpm, while a large lp turbine on a power station might run only 1500 rpm.

When a linear dimension is doubled, the mass increases eight times and the forces increase with the mass.  If the speed is halved, the forces reduce to quarter, not quite enough, so the bigger engine has to go even slower for similar forces.  Similarly, for the same pressure, if a piston diameter is doubled, the force is four times, again adding to the increase of stress on a larger engine.  But of course we have the famous equation, F= m.a.  Or a= F/m.  From this, we can see that as the mass increases, the acceleration due to a given force is smaller.  Or put the other way, for a given force, a smaller piston accelerates to much higher velocity than a bigger one, so easily reaches a higher speed, even with lower pressures.

The fluid dynamics and other factors which limit the speed attainable are perhaps a bit more complex to analyse, but there is no incentive to design a system to supply fuel to accelerate an engine to the point of destruction, so there are probably some deliberate built in limitations in that area, at least when the engine is fully loaded.  One easy factor to consider is piston speed, usually calculated as the total distance the piston moves in a given time.  This is actually an average speed which is about seventy percent of the maximum speed, but a good indicator of wear rate on rings etc.  Obviously a small piston on a short stroke can go many more rpm to have the same piston speed of a longer stroke engine.  So double the stroke, you have to halve the rpm for the same piston speed and wear rate.

With regard to boiler size, it does not matter if the boiler is too large, apart from weight if it has to be portable, or cost if the boiler is built for the purpose.  The boiler controls, including the safety valve, will look after the boiler even if the engine is drawing no steam.  So if you take your engine along to the Beeleigh museum, and hook it up to a connection on a suitable steam line on the full size boiler, it will be fine.  (Particularly as I am sure it is not highly superheated, or really high pressure, which would each require extra consideration of the connection details.)

There is no one size of boiler that is correct.  Over a reasonably wide range, a larger boiler will allow the engine to do more work, while a smaller one may limit the engine output.

As the boiler gets smaller, it will be limited in the volume of steam it can produce at any given pressure.  The engine will take a certain volume of steam for each stroke, and if this is more than the boiler can provide, the boiler pressure will fall.  The engine will run slower at the lower pressure, so requires less steam, and so on until the boiler is no longer able to supply enough pressure to overcome engine friction at the lowest speed the engine will run.  The limit is when the pressure on the piston does not produce enough force on the piston, hence torque, to accelerate the flywheel to the minimum speed at which the stored energy will carry it past the top and bottom dead centres.  Obviously it has ceased to do any useful work before this.  So a boiler can be too small.  As a guide, it should be able to produce a volume of steam at the required working pressure for the piston displacement at the required engine speed.  It can be a bit smaller if the valve gear provides for earlier cutoff.  The potential steam volume can be calculated once the burner fuel consumption is known, as we have discussed before.

Glad you will run your engine on steam, surely that is required for a "real" steam engine.  However, I can see the convenience of using air for a first test run, quick demonstrations or display days, when insurance and public safety come into play.  But at the end of the day, I think an engine should at least have a test run on steam to be complete.  It is not too hard to build a small boiler you can use in your workshop or garden, so long as you have an adequate size torch.  Or better still, borrow one ( preferably with operator attached).  It is always helpful to have extra hands and even a second torch maintaining the general heat level on a larger job.

I hope that covers all the questions,

Thanks for looking in.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 18, 2018, 12:19:41 AM
Thanks for the very detailed answers MJM,  I have been looking at Flash steam boilers for early hydroplane engines and notice that the copper tube is wound in coils. Would there be any advantage in a back and forth winding rather than a circular arrangement ??. I will need a bit more time through your last explanations. The HP of an engine depends on the speed the engine is running , so with an ic engine that is continually changing speed does the HP change as well ?  Was there a measurement of power before Watt's 33000 Horses were all coupled together ?? :mischief: !!!
Willy
Title: Re: Talking Thermodynamics
Post by: Zephyrin on September 18, 2018, 08:22:37 AM
Nice engine you have Willy, looking as could be overhauled easily...
It is a fast running engine, for driving a boat at a good speed, 8 km/h I would say.
For the oiler, I suppose that the "tube" in the bottom is just holding the oiler on the tube, and not connected with it, otherwise it cannot displace oil, unless this tube is prolonged high in the oiler, up to the oil level.
For the other mystery with the alcohol reservoir, as there is a tap in the alcohol feed tube, this could work by siphoning maybe ?
the alcohol tank may also be filled with cotton wool or another absorbing matter, to damper liquid movements in the tank (it is steam plan for a boat model!), as it was common in old tin plate toys.

Title: Re: Talking Thermodynamics
Post by: MJM460 on September 18, 2018, 10:47:58 AM
Hi Zephryn, good to hear from you again.  Is it possible you are looking at the photos of the engine Willy saw at a car boot sale, but decided the price was too high?  I think you are probably on the right track thinking the burner might work by syphon with the tube extended down to near the bottom.  It would not be able to empty the tank that way, but the tank floor may be above the base of the cylindrical walls.  Interesting to look at the lubricator arrangement also. 

Hi Willy, with power, it helps if you remember some basics.  Power is rate of doing work, or work per unit time.

Work is force times distance, or, for rotating systems, torque times angle turned through.  In fundamental units, angles are measured in radians.  So power is force times distance per second, or force times velocity.  Again, for rotational systems, torque times angle rotated through per second.  We don't normally measure angles using radians, usually revolutions unless we are talking about less than one or two revolutions.    We tend to calibrate tachos in rpm. There are 2 times Pi radians each revolution, so one revolution per minute equals 2 times Pi divided by 60 radians per second.

So, power for a rotational system, such as an engine, is calculated using:-

P = 2 x Pi x T x N divided by 60. Or P=2.pi.N.T/60 where N is rotational speed in rpm.

In SI units, Torque is measured in Newton meters, and power in Newton meters per second.  One Newton meter per second is given the special name Watt.  It is not a coincidence that this is the same unit as given to electrical power, and they are indeed equivalent, though I don't know how the early pioneers in this field managed to align the two.  As I understand it, there was no agreed unit of power before Watt decided his number of ft. lb. / min as the power developed by a good workhorse.  I was always taught that he must have had a very good horse as his "standard" horsepower, but I don't know much about horses.

So determining the power output of a motor requires measurement of both torque and rpm.  The characteristic performance curve for a motor is a graph of torque vs. speed (rpm).  The power at any speed comes from the calculation.  The torque and power both vary with the rpm.  And the maximum power does not correspond with the maximum torque.  There is a separate curve for each throttle setting of an internal combustion engine, while for an electric motor, there is a separate curve for each applied voltage.

When an engine is running, the power developed does indeed vary as the speed varies, and also with throttle setting.

Flash boilers, those hydroplanes were very interesting beasts.  I have had Benson and Rayman's book on experimental flash steam on my shelf for many years, and it appeals as an interesting way of generating steam without the heavy boiler making, though I would be aiming for something a bit more tame than the ones described in the book.  But I often go back and read it again.

The principal is simple enough, the engine driven pump puts water into one end of a coiled tube heated by a blow lamp so that at some point in the coil, or over a short distance, the water flashes into steam, at which point the volume of the water increases close to 1000 fold.  In a confined tube, this increase in volume results in a very large pressure increase.  The operating steam pressure and outlet temperature is determined by the balance between the heat input from the burner, water supplied and the steam consumed by the engine.  I believe they tried measuring the outlet pressure on one of their models and concluded the pressure was frightening!  And there would be a big pressure loss between the pump discharge and the steam outlet due to the small tube diameter.

To start up, I assume you pump in a small amount of water, then light the burner. When some steam is generated, the volume of steam in the confined space of the tube means the pressure would rise pretty quickly, and the engine would be running and continue pumping water.  I can see potential for some wild action until the procedure is all sorted out, but nothing that can't be tamed by a more moderate burner output.

There were some articles in Model Engineer magazine some years back by someone (maybe in France?) who was experimenting with flash steam for a locomotive.  He had some very interesting ideas that would be quite applicable to a model boat or even a stationary engine.  I seem to remember the system had an air tank as an accumulator to smooth the pressure and make the system a bit easier to control.  I wonder if I can still find the articles.

The issue I see with coils is that each turn has a low point for water, not usually a good idea in systems involving a phase change, though the steam generation would move it through pretty quickly, so even a cold spot would not result in water slugs for more than a few coils, so I don't suppose it matters.  I assume you would get a real two phase flow over part of the coil before the water is all turned to steam.  However, flat coils which are basically flat or a continuous slope in one direction might be slightly better, especially in a unit operating at more moderate conditions.  I think the main thing is for the water to enter in the coolest section then move progressively to the hottest section at the outlet.  The simple coil is probably the easiest to form.

Are you thinking of a flash boiler for your new engine?

Thanks everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 19, 2018, 12:02:19 AM
Hi Zephrin, thanks for the update...I suppose one would have to open the tap from the methsnburner and blow into the top to get the siphoning procedure to start and then turn it on just a small amount to feed the burner. It is a single acting engine and i cannot see where the exhaust comes out ?? Also the steam outlet is very low down in the boiler ??

Hi MJM, thanks for more info When i changed the engine in my BMW for a similar CC but higher compression  The HP went up from 28 to 35  also the engine revs increased from about 5000 to 6000  !! this i was a bit confused about to begin with but now it makes sense...also it would get up to 120 mph !! With a flash boiler as it is just a tube you don't kneed at boiler certificate  ,however a loco boiler is a collection of tubes joined together, albeit different shapes !!
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 19, 2018, 12:20:05 PM
Hi Willy, when you changed the engine on your bike, you almost certainly changed more than just the compression ratio.  The increase from 5000 to 6000 rpm would not quite achieve the specified increase in maximum power unless the torque at 6000 rpm is higher than the torque from the old engine at 5000 rpm.  But to get the new engine to be able to spin at 6000 rpm, I would guess that it is probably balanced to tighter tolerances, may have modified cam grind and timing and possibly improved porting and even air cleaner. 

Of course, it does not run continuously at 6000 rpm any more than the original engine ran continuously at 5000 rpm, and neither operates continuously at full throttle.  But in principal, and in practice, power output can be increased either by increasing the operating rpm, or by increasing the torque or some combination of the two.  In fact, increasing torque is probably preferable, gives better acceleration without jeopardising your license so you probably get more opportunity to try it out.  But the main thing is the enjoyment you get from having made the upgrade, and the exhilaration you get when you have an open road to try it out.

Boilers seem to be the topic of the day.  While the flash boiler may just be a tube, the capacity of a long tube might approach the level where the boiler code starts to apply, but we better keep that one quiet.  More importantly the small diameter means the stress is pretty low, and the piping codes are actually more about stresses from supports and thermal expansion considerations.  Even pulsating forces when reciprocating compressors are involved. 

The thing is that the flash boiler is a cylinder under internal pressure, even if it is not a straight cylinder.  In fact, it probably tends to straighten a little under pressure, like a bourdon tube in a pressure gauge.  The locomotive boiler might be a collection of tubes, but the issue of internal vs. external pressure is critical.  The small diameter  of the flash boiler means that pipe ends can be adequate even if flat.  The ends of the locomotive boiler are effectively very large flat plates which need comprehensive staying to withstand pressure.  Then corners around fireboxes and other localised departures from plain cylindrical shapes cause stress concentrations which all require special attention to design.

In case anyone is wondering, the issue with external pressure is that a cylinder will squash flat under external pressure at far lower pressure than the pressure that would burst it when applied on the inside.  The pressure vessel codes include the procedure for checking external pressure capacity.  It is reasonably straightforward, but takes up several pages of the code.  More than just a simple formula.

However, in that flash boiler I was talking about yesterday, while the outer casing looked like a conventional locomotive boiler, inside was the tightly wound long coil of a flash boiler.   Don't know how he got on when stopping at a station!

Thanks everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 20, 2018, 01:20:34 AM
Hi MJM, Yes i changed the engine from a  R60  To a R69  engine so yes there are quite a few differences !! I should have made that a bit clearer !! :-[

Willy
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 23, 2018, 11:42:50 PM
Hi MJM, I was idly sitting in the cafe today in the rain  and looking at Snickers chocolate bars i noticed that the calorie content was 210.... so i was thinking about how many chocolate bars it would take to get a steam engine from  ambient temp up to Scotland from London.? this would of course i include the snacking by driver and fireman !!! We have all the figures available so if one had to make the journey one would have enough time to work it out !!! :mischief: :atcomputer: ;D...any way on a more series note how do the weather fore casters know exactly when it will rain as one can have lots of clouds with no rain as well as some ??

Willy
Title: Re: Talking Thermodynamics
Post by: crueby on September 24, 2018, 02:34:04 AM
We need a gauge calibrated in Snickers per mile...
Title: Re: Talking Thermodynamics
Post by: Jo on September 24, 2018, 09:27:09 AM
We need a gauge calibrated in Snickers per mile...

I thought there are 26.2 miles in a Marathon or as they call them the other side of the pond a "Snicker"  :facepalm:

Jo
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 24, 2018, 12:30:49 PM
Hi Willy, I have always wondered if the food calorie matched the thermal calorie.  Remember that in heat units, one calorie is the heat required to realise one gram of water through one degree C at the standard temperature.  This results in one calorie equal to approximately 4.182 Joule.  Similarly, one BTU is about 1055 J.  I have only quoted approximate conversions, as the exact definitions require too many decimal places.

However, I read recently that there is a factor of two between food calories and thermal calories.  Possibly to allow for the energy involved in digestion of the food, but I don't know how the food calorie is defined, so don't quote me on that.

So if we know the draw bar pull of the locomotive and the distance, we know how much work was done, and work is a form of energy.  But we would need to do some tests to determine the efficiency of energy conversion from the food bars to the draw bar work.  The second law of thermodynamics says some of the energy in the fuel must be lost in the conversion.

Weather forecasting is a bit out of my league, but I understand that the availability of large supercomputers and large quantities of real time data, has allowed meteorologists to develop some amazingly good models to solve the equations of mass and heat transfer for the atmosphere.  But there is some variability, and racing ocean sailors compare the output of several different models and additionally, look out the window in order to decide what route to sail through the weather.  The model we use to check the weather for outdoor activities tells us the wind for every hour over a week, and it is usually close enough for the first day and usually only subject to relatively minor changes for the ones after that.  But the models predict not only wind but also temperature and moisture content, and even the point at which rain occurs.

Hi Chris, your meter should be possible with inputs of drawbar pull and distance, but it would require lots of sampling the Snickers to confirm the conversion efficiency for accurate calibration.  You would not want to delegate that responsibility to the elves!

Hi Jo, yet another definition.  I think it would take more than a snicker to get me through a Marathon, but you may have explained why I never finished a marathon, I thought I could stop after 26!  I obviously never looked at the fine print.

Thanks everyone for following along,

MJM460


Title: Re: Talking Thermodynamics
Post by: MJM460 on September 25, 2018, 12:39:30 PM
While diverting to talk about Snickers, I have not been entirely idle, though you may need a stop motion film to see the progress.  Apart from travelling 6000 km across four states, including towing the van along a sandy track over a sand dune, then leaving the van parked and driving through the deeper soft sand over to the beach  (that's what I call a mobility vehicle), I have been insulating my gas fired centreflue boiler with two layers of thin cork.

Initially the boiler was insulated only with the timber slats that were supplied with it.  To do any meaningful testing I had to make a thermowell to replace the fill plug, so I could measure the temperature in the boiler with a thermocouple.  The first picture shows the thermowell and the original plug.  And of course I did not have the required ME 40 tpi die, so that had to be borrowed first.

I was able to buy a two sheets of 3mm thick cork sheet from the local $2 store (would be great for your clutch plate, Brian, though it did cost a little more than $2).  Some careful arranging of the shape resulted in suitable pieces for two layers on the shell, and enough left for the ends.

The shell was easy, though 6 mm of cork increases the diameter of the shell by 12 mm, and the circumference by Pi times 12 or about 37 mm.  So extra timber strips were required, and of course the brass bands were then not long enough, so timber and brass strips had to be sourced.  However the cylindrical part of the shell was eventually completed.  Definitely not up to exhibition standard, but well enough for a test.  The next pictures show the progress so far.

Paper templates helped prove the layout of disks for the ends.  The actual flat end is a little proud of the ends of the cylinder so an extra ring was cut to fill the gap this created.  And the plaque on the front end of the flue tube required a cut out in the first layer.  The back head was not so easy as I did not want to disturb a well fitted and sealed gauge glass, but some judiciously placed cuts allowed the pieces to be fitted behind the glass.  I looked up the data sheet for the loctite glue I had available, and it still had some strength at something over 100 C so I glued the layers together to help hold them for placement.  Even if the glue lets go it won't matter once the cork is fixed in place. 

But how to hold the circular cork layers on the ends?  I suppose I will have to flange some wide strips or even a disk of thin brass, and catch tabs under the end rings, but I am open to any  suggestions for a better or easier method.  In the mean time, I used some stainless steel picture wire to tie them in place.  Not very tidy, but it will work for a test to see if I gained a worthwhile improvement in performance.  You can see it in the pictures, but I hope to come up with something neater for the long run.

Then finally the big test.  I had run a test with only the original timber, but would have been better to do a couple more to get an idea of consistency.  For all the previous runs of this boiler, it only had the original solid plug, so no way to measure temperature. The ambient temperature for the most recent test was 15 degrees which is a bit warmer than 13 C for the earlier test.  I feel I can probably calculate the effect of this if necessary, but in the event, it did not seem to have caused a noticeable difference.

The end result is interesting.  With the cork insulation, the boiler generated 30% more steam than before.  Allowing for the difference in gas consumption, it would have been 40% more.  This was clearly evident in the engine performance, where it ran at 1900 - 2000 rpm compared with only 700 - 800 rpm for the earlier test.  So a quite pleasing result.  Certainly showed up the deficiency in my engine balancing.

When I calculated the heat loss and produced a graph of heat loss with temperature, the difference is easily seen.  But the importance of this difference is highlighted by the difference losses at steam temperature, compared with the burner output, and so result in the extra steam production.  This graph is in the last attachment.

Apart from the value of the extra insulation on steam production, the other big lesson that comes from this exercise is with regard to boiler design.  It is important to design all the boiler fittings with allowance for the intended thickness of insulation.  Making a neat job around the fittings, and making allowance for removal of the safety valve, filler plug and gauge glass, as well as any extra bushes for feed water or draining was the hardest part of the job.  It is also worth giving some though as to how the end insulation will be held in place.  I still have to work on that.

I hope this is all of some interest,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on September 26, 2018, 10:11:52 AM
Hi MJM, Absolutely Fabulous! Some real numbers to go by and a demonstration of what adequate insulation can do for performance. Is this little boiler one of the 'Miniature Steam' models, (4"), ?  Could you report on how noisy these centre flue ceramic burners are as I have not seen/heard one in operation.  I am wondering if you would be interested in another experiment, ( a bit more work too), and getting some more numbers for us. I have a small quantity of 5mm Kayowool ceramic fibre insulation sheet. Too thick for most of my G1 uses but would be great to see a comparison between a natural product, cork, and the boiler re-done and an identical test done with a synthetic one. If interested please P.M. me with dimensions required for barrel and end plates and a delivery address so I can send it down, add a centimetre or two or so extra for trimming etc.  Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: paul gough on September 26, 2018, 10:35:48 AM
 Hi again MJM, I have found the white exhaust gasket cement in a tube or the high temperature black or red tubed gasket material, both from 'Auto One' shops, to be successful in securing fibre sheet to outside boilers, inside fireboxes of metho burners and on the inside of smokeboxes. Covers for the end of the boiler and its insulation could follow full size as fitted on most package style boilers, a sheet metal dish with a circumferential lip in one or two pieces that fits neatly onto the barrel end. Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 26, 2018, 12:54:59 PM
Hi Paul, I am glad you find the test results interesting.  It seems that not many make the effort to do the measurements, but it's not too hard for just the the boiler.  The boiler is the 3 inch one from Miniature Steam Models. 

You might also be interested in the effect on the outside temperature which I measured with the infrared instrument.  With just the wooden slats, the temperature was typically over 100 C when the boiler was steaming at 110-115 C, which is not very useful.  With the cork under the timber, the temperature at the same location was 55-60.  This at least would not be too dangerous to touch, but suggests that even another layer might be useful.  However, it is already difficult to fit around the fittings, so I think it would be better to just keep that in mind for a future project.  In any case, the losses are getting low enough that it is probably not worth chasing a bit more.  It's getting late now, but I have been meaning to calculate the volume of that steam quantity, and compare it with the engine displacement.  But it is obviously enough for the 12 mm bore by 16 mm stroke double acting engine I have been running.

The burner tends to sing a bit when I first light it, though not loud by any description.  Then it seems to go quiet after a minute or so.  Certainly not like the propane burners of similar diameter that I use for silver soldering.  I have to admit that after that initial light up I am not conscious of hearing it.  But I will take more notice next time I light it up.

My text book gives a thermal conductivity for cork at 20 C of 0.035 W/m.K.  I looked out a few figures for Kaowool, and while they are mostly given for higher temperatures, the trend with temperature suggests it would be about the same at lower temperatures.  So not much to be gained from swapping it over for comparison.  Any difference would be mostly due to any difference in the thickness.  The real difference is in the allowable service temperature, which seems to be around 1300 C for Kaowool.  Cork at that temperature would almost certainly char if not burn.  We used Kaowool on the inside walls of industrial furnaces in the plants I was involved with.  And it is a bit flexible, so we could pre insulate large sections of furnaces and transport them without damage.

It would be easier to apply than cork even, or perhaps especially, on a small model.  However I think I will have to decline your offer.  However, if you want to get rid of some, I would gladly make a contribution for it.  I would try it on the furnace enclosures of my externally fired pot boiler, where the resistance to the higher temperature would be useful.  I don't think cork is really suitable for the higher temperature.  Can you remember where you got it?

I would prefer not to glue the insulation to the ends, it tends to be messy if it ever has to be removed.  Though I will probably reconsider if the boiler gets installed in a boat.  On full size we normally used welded nuts on edge to provide anchors for tie wires to tie the insulation in place until the metal jacket was fitted.  I have tied it on with linen thread then the stainless steel wire for an ugly but serviceable solution for the test, but now I will eventually make something from sheet brass for better looks.  However, I did glue the cork layers together so they did not shift around while I was fixing them.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on September 26, 2018, 02:40:32 PM
Hi MJM, Interesting to hear your view that the kaowool would be approximately equal to cork of comparable thickness. Thanks for the additional information regarding the external spot temperature from the infra-red device. One would intuit this result, but always nice to have some numbers to prove it is actually the case.

The 5mm Kaowool was in fact supplied incorrectly by 'Camden Miniature Steam Services' in the UK. They normally only stock 1mm and 2mm, but their supplier wrongly sent them 5mm, (mislabelled as 2mm),  and it appears no-one checked its thickness before fulfilling my order. Camden was very apologetic and said they will forward 2mm when they receive it. I understand Kaowool is available in Oz but have been told not in these very thin sizes, however, I have not pursued the issue as it was convenient just to order my couple of metres from Camden.  I passed on most of the 5mm to a friend in N.S.W. only keeping a small amount, but again, if you have a use for a small quantity of the 5mm sheet I am happy to send some to you. Regards, Paul Gough. 
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 27, 2018, 01:13:31 PM
Hi Paul,

Intuitively, more insulation would lower the temperature on the outside, but the extent is not easy to guess.  Heat transfer through the insulation is proportional to the temperature difference across the insulation, while heat transfer to the air by convection is proportional to the temperature difference between the surface and the air further away.  Two heat paths in series, so the heat transferred must be the same in each case, and the intermediate temperature results from that heat balance.  But difficult to guess the relative conductivity between the insulation layer and the convection layer.  Experiment tends to be the easiest way, as the convection coefficient is more difficult to estimate as we have seen before.  I know it would be interesting to do the maths and see how it compares, but I tend to be impatient to move on to other projects.  It is a while since I made much swarf, so I will give that more time for a while.  However always willing to try and answer any questions.  I am enjoying the discussion of thermodynamics, and hope it is contributing to all our understanding of our hobby.

Interesting how you came by the 5 mm Kaowool.  I suspect you will eventually use what you have left.  I will see what thickness I can get before I decide the dimensions of the next boiler casing.  I have the 5/16 tubes to use for the water tubes to compare with the current one which has 1/4 inch water tubes.

I am always a little skeptical about temperature readings from the infra red instrument, as they depend on a built in emissivity constant.  I am never sure how closely this applies to the surface I am measuring, especially if I am comparing readings made on different surfaces.  However, for differences between temperature at different times on the same surface, I think they are close enough.  It was certainly pleasing to get that surface temperature below 60 degrees which is around the level usually specified for personnel protection (might be 65?  I haven't checked a specification recently.  Retirement has that effect.)  So now, I am less likely to be burned if I accidentally touch it, or if one of the grand children do.  Though they are pretty safe, as I have not been able to convince any of them to take an interest.  They do bring other joys however.

Have you been running those locomotives lately?

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Zephyrin on September 27, 2018, 02:28:29 PM
Hi MJM,
nice results you provide in this post !
Even if we all feel that insulating our model boiler is useful, it is important that real experiments have been carried out to see how much it matters...
On my little locos, I'm far from the thickness you mention alas, just 3mm cork under the cladding in thin (0.25mm) steel foil, to keep model close to scale dimensions. No miracle, I can't touch the engine at the end of the run, not the 60°C you mention on your swaddled boiler!
With such increase in the thermal efficiency, I clearly can reduces the size of the boiler tube on my next loco to cope with the layer of insulation.
Regards
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 28, 2018, 03:17:57 AM
Hi MJM, More good info.... I was wondering how the boiler is constructed  as the chimney and gas burner look to be quite close in proximity ? Also a question about boilers  when you fire up a boiler you input heat into it at a constant time. if you don't draw off any steam the temperature will rise. so can the heat in the boiler ever get hotter than the flame temperature  or will it release the extra heat in a proportional fashion via conduction radiation convection ??  Would it be possible to connect a system that reduces the heat input with the rise in temperature with our small gas heated boilers or is that quite complicated and so we do it manually ? On the electrically heated boiler it does it by switching off the currant with a pressure switch.....
Willy
Title: Re: Talking Thermodynamics
Post by: paul gough on September 28, 2018, 10:20:27 AM
Hi MJM, I missed taking in all of your comments  regarding temp. of outside cladding ,(26th Sept.). This one in particular, "...but suggests that even another layer might be useful." This then equates to a total of a 9mm thick cork layer of insulation on a 75 mm diameter boiler, quite a lot proportionally and of course any insulation that was inferior to cork would require a greater thickness to achieve comparable results. This I find interesting, as way back in the seventies when we were having our twelve inch gauge locos boilers inspected for their annual tickets I remember discussing minimum insulation thicknesses with the boiler inspector, as our oldest loco had no lagging at all on its belpaire firebox and we were contemplating lagging it with an asbestos plaster type lagging. This loco was a very touchy steamer with an undersize grate and 160 psi pressure, so needed precise firing and consequently any reduction in loss of heat would be useful. The Inspector stated that it would need to be at least 1/2 inch thick, any less and it would likely be a waste of time. Now, your 9mm of cork, presumably a better insulator than our asbestos plaster, is not very far off 1/2 inch. This seems to be roughly in accord with the inspectors opinion on a thickness that would be effective. Therefore, would it be reasonable to assume that most model boilers, even ones that might be considered satisfactorily lagged, are in fact deficient owing to the type material used, or, not having sufficient thickness for a given lagging material??? Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 28, 2018, 01:04:52 PM
Hi Zephyrin, thank you for those comments.  I don't know if anyone else has made similar measurements, perhaps my career was unusual in preparing me with the interest and the theoretical knowledge to want to follow it through.  Of course to be really reliable, I have to do more tests to get an idea of repeatability, and preferably others should also get similar results.  With very small boilers there is a delicate balance of the requirements for heat conservation and the requirements for adequate water capacity.  It is obvious that the thickness of insulation I have used is not practical on a small locomotive where scale appearance is also required.

There is an issue niggling in the back of my mind, it is in both my text books, so I will have to go back to the calculations and see if I can tease out some answers.

Hi Willy, it is a fairly conventional marine type centre flue boiler with cross tubes in the flue, but the flue has a tee piece ear the far end, so that the stack exits through the top of the boiler just before the pressure end plate, instead of via a full diameter smoke box as on most locomotives.

I will come back to your other points tomorrow, as I have been out to dinner with family for my wife's birthday celebration, so am a bit late tonight.  Just remember that heat only travels from high temperature to lower temperature, not the other way.  I will continue from there tomorrow.

Hi Paul, adding more insulation will further reduce the heat losses, but there is definitely diminishing returns.  For the moment I have too many other projects in mind to be motivated to stripping of the wood strips and adding another layer before replacing them.  Really not a huge job, but my progress is very slow due to other activities.  With you and Zephyrin both interested in the similar area, I will after all, go back to the calculations over the next few days and see if I can work out some answers.  I might at least be able to see if there is enough potential for it to be worth while.  And also I want to do a few more runs to check the repeatability of the results.

Thank you especially for the interesting comments, and thank you to all who have looked in,

MJM460
 
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 28, 2018, 09:03:12 PM
Hi MJM, a further bit to add to the heating the boiler...as the cooling of a boiler always takes far longer than the heating up of it why does the addition of the constant heat/time not make the boiler get really really hot very quickly ?? or is this another silly question ??
 Also if you wanted to win one of the IMLEE  engine efficiency  events could you use a 3 1/2" boiler in a 5" gauge loco using up to 5/8" lagging ? Just a thought. Also just a thought about the water gauge that may be counter intuitive  as the steam pressure enters the gauge from the top and is in effect pushing against the liquid in the lower half is the level the same on both sides of the backplate.? i'm thinking that as you cannot compress a liquid the steam pressure will tend to push it back into the boiler ? We know it doesn't but all the text books on fluid dynamics go to great pains to explain how FD actually works in practice !!
Title: Re: Talking Thermodynamics
Post by: paul gough on September 29, 2018, 05:15:50 AM
Hi MJM, I assume your 3 inch boiler is a 4 bar or 60 PSI one therefore 145 C internal temperature. Now if we took your 9mm cork insulation thickness as best practice for this temp/pressure, and knowing there is 20 degree C. increase at 100 PSI (approx. 7 bar), would you consider it worthwhile adding another layer (3mm) of cork to compensate for this 20 degree increase. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 29, 2018, 11:14:31 AM
Hi Willy, back to your questions roughly in order.  Some people talk about a Zeroth law of Thermodynamics - "if there is no heat transfer between to objects in contact, then they are at the same temperature".  And of course the converse is also true.  So, if the boiler could get to the flame temperature, they would be at the same temperature, there would be no further heat transfer, so the boiler could not get any hotter.  But first it takes a very long time to actually get to equal due to the small temperature difference as it gets close.

With your electric boiler, all the heat dissipated in the heating element must go into the boiler, and the element temperature just increases until it transfers all the heat.  Now if we get silly, and insulate an electric boiler with thick layers first of Kaowool, then rock wool, then encase that in polyurethane and so on until we can assume perfect insulation, then the temperature increases without limit.  However in the real world, something will break and so the temperature only increases until something breaks.  Possibly the boiler shell starts expanding like a balloon due to the lower strength of the material at high temperature,  then bursts, or the silver soldering melts, or the electric element fails, by melting like a fuse. 

However, a fired boiler, gas, spirit, coal, wood or whatever is different because you need air for combustion, and you have to vent the combustion products, or the flame is soon extinguished by lack of oxygen.  The air and the combustion gases absorb all of the heat from combustion, and so limit the temperature reached.  Not just the air necessary for combustion, but there is normally excess air required to ensure that complete combustion occurs. If less heat is transferred to the boiler, the combustion products just retain more of the heat, and carry it out up the chimney.  So, if I insulate my centreflue boiler very well, even if I do not allow any steam escape, the boiler will get to quite high temperature and pressure, thus reducing the temperature difference and hence the heat transferred between the combustion gases and the boiler.  With less heat transferred to the boiler, the combustion products remain hotter until eventually they are carrying away all the heat of combustion.  It could potentially get to the point where the copper is no longer strong enough, or the silver soldered joints.  And of course, it is for just this reason we must install a safety valve, set to limit the pressure to some safe level.  As soon as it opens, the escaping steam carries away enough heat to prevent further accumulation of heat or pressure (providing the valve is big enough).

Now with your electric boiler, it is relatively easy to use an electrical signal from a thermocouple to interrupt or modulate the electric power input to prevent the safety valve being needed at times of low steam demand, however for a gas fired boiler, we need to use the boiler pressure to press on a diaphragm and close a needle valve in the gas supply to the burner.  In principle it is simple, and many of the usual books show drawings of how it is done.  However, I have no illusions about the difficulty of tuning of spring stiffness and diaphragm size necessary to get it to work.  And the pipe work to connect it up.  Full size equipment is readily available and I believe such units are available in model sizes.  For me the problems of connecting it up to the gas supply without leakage would be more than enough to dissuade me. 

With spirit firing it is a little easier, though you might need a pilot wick to relight the burner if the fire gets too low, but at least minimal pressure to deal with.  I am not sure how you would do it with coal.

You are right, cooling is slow, but heating is quite quick, depending on the capacity of the burner.  My little burner consumes about 30 g (1 oz) of gas in about twenty minutes, so it is not very big.  It is not that cooling is inherently slower, it is just that it depends entirely on the temperature difference to the air, and of course, this temperature difference reduces as cooling proceeds.  By the time it gets to less than twenty degrees difference, even I run out of patience to watch it cool further.  In fact, a bit earlier than that.  However, for heating the temperature difference results from combustion of air and the boiler, so is greatest when the boiler is first lit up, but more than adequate for much higher heat transfer rate than the cooling process.

A level gauge is connected to the boiler in the steam space at the top and the water space at the bottom so the only pressure difference is due to the density of the water.  It is this that drives water into the bottom of the gauge to nominally the same level as the inside.  I say nominally, because in our model sizes, surface tension effects can cause the level to creep up a bit in the gauge.  We are looking for such small levels that this can be quite significant.  Other less intuitive effects also seem to occur in a vigorously boiling situation, so it is desirable, but not always practical to locate the bottom connection in a quieter part of the boiler.  No insulation on the glass of course, so the heat lost from the glass where the steam is on the inside causes some steam to condense.  This runs to the water filled section, and the tendency to increased height actually drives some of the water back into the boiler space rather than increase the level in the glass.  And more steam enters the glass from the steam space to replace the amount condensed.  The steam pressure in the glass is always the same as in the boiler

For locomotive efficiency, I don't think I would use a smaller boiler, but rather, I would consider sacrificing appearance and add external insulation, with suitable cladding for appearance of course.  But the first step would be to try and work out the sources of inefficiency, and how much heat loss was occurring to see if it was significant in the total.  I would expect that mechanical friction would be a major factor along with firing technique.  Excess air would likely be the biggest source of loss.  It is a very good trick to consistently fire a small coal fire at an efficient air fuel ratio.  I will leave it to others to tell us more about the technique.  For my part, I would start/continue my efforts to improve my engine making skills to get the smoothest running motion works possible.  I have a long way to go.

Hi Paul, I did overlook part of your insulation comment the other day.  On the question of minimum useful thickness of insulation, there is the issue of scale.  Any added insulation will normally be expected to reduce heat loss, but whether this is significant depends on the heat rate of the boiler.  Saving a few joules per second on a full size boiler would not be noticed, but it might reduce the severity of burns if the surface is accidentally touched.  Of course if the surface temperature is too high, the reduction might be a bit academic.  However that same few joules per second on the miniature locomotives that you and Zephyrin run, is a significant portion of the heat input, so quite important.  In full size, I don't believe we ever used less than half an inch thickness but part of that is simply for mechanical handling of the material. 

I don't think I would describe any model that worked as deficient, but it is always interesting to think about ways a model might be improved.  It turns out that very small differences in insulation properties make very little difference overall, once convection from the outside of the material is considered, however they sometimes have very different usefulness at different temperatures.  So cork and Kaowool have very similar insulating properties but you don't want a flame impinging on the cork.  Kaowool will stand very high temperatures on the inside of a large industrial furnace, and not easily broken during building the furnace.  Before the advent of those ceramic fibres, firebrick and refractory cements were used internally, despite relatively poor thermal properties, and other materials used outside the furnace walls to provide additional and more useful reduction of losses.  So those small models, whether insulated with cork or Kaowool would be about equal, to the best of our ability to measure, so long as the flame did not impinge on the cork.  Differences in insulation thickness are more significant so long as the boiler remains large enough to be useful and the appearance acceptable.  As always, many compromises are involved.

To determine the effect of the boiler temperature increase from 145 to 165, I would calculate the temperature difference to the atmosphere, say at 15 degrees, so 130 and 150.  So the heat loss would increase very roughly by 150/130, say 15%, but this is 15% of the already very low heat loss due to the insulation already added, and you would only save a small part of that 15 %.  So there comes a point where the diminishing returns of extra insulation soon make it not worth chasing.  Maybe the next three mm, but probably not the next one.

I think that brings us up to date with the issues raised so far.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 30, 2018, 01:47:46 AM
hi MJM, once again thanks for your extensive reply. another question... we can heat up things very quickly  but is there a way of being able to cool things down very quickly ? perhaps spraying with liquid nitrogen or something similar ? perhaps there is no need to cool things down fast though !!

Wily
Title: Re: Talking Thermodynamics
Post by: derekwarner on September 30, 2018, 04:42:44 AM
Afternoon MJM & all.........still following on with each daily post

Whilst I thought had a pretty good understanding with the ~~3 degrees C steam temperature 'improvement' with hefty insulation of my steam line to the engine, I was a little taken back with MJM's test results  :Director: with the boiler insulation from the past week

Clearly when I placed the single layer of wooden planking to my boiler, my mind did not understand :Doh: the actual importance or need of additional insulation thickness

One point for any future boiler build is to specify all threaded tapping's to have a face protrusion of ~~6 mm  to the boiler shell

I could modify a few 12 point Crows Foot Ring spanners by increasing the set then and also cut out a section.....[as per the image].....this would be the only way of tightening the fittings with the protruding insulation

Derek
Title: Re: Talking Thermodynamics
Post by: 10KPete on September 30, 2018, 04:46:35 AM
It would be a large undertaking but the boiler could be stripped and extensions soldered on...?

Yes, large.

Pete
Title: Re: Talking Thermodynamics
Post by: paul gough on September 30, 2018, 07:18:13 AM
Hi MJM, When I used the term 'deficient' in the previous post I was inferring that the insulation was often, deficient, not the model. Yes, it is very often the case that people want to follow scaled prototypical dimensioning with their creations, but there can be a cost in reduced performance.  To me insulation needs to be thought of volumetrically like other boiler parameters, not as a thin more or less 2 dimensional skin. One of my dreams would be to have two locos exactly the same, but with different boiler diameters, say one with a 4'' (100 mm) dia. barrel with more or less standard model practice lagging, then the second with a boiler barrel of say 3" dia. (75 mm) but with a minimum of say 12-15mm of insulation. Then apply some standardised tests to ascertain if the reduced boiler volume etc. but superior insulation would generate anywhere near the same steaming rates. If the results confirmed that more effective insulation provides significant benefit then it might not be always necessary to try to squeeze in the biggest diameter boiler possible onto the frames. The question is, whether the improvements that you saw with your small gas fired marine/stationary boiler translates to a larger coal fired locomotive models. I think this insulation investigation is creating some real interest and curiosity amongst some modellers and over time might spark some revealing investigations regarding larger models, at least I hope so. Regards Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 30, 2018, 11:50:03 AM
Hi Willy, liquid nitrogen would give a healthy temperature differential so would definitely increase the cooling rate.  In fact, as it would be a boiling liquid against the shell, it would not only provide the increased differential, but would also have a very high film coefficient compared with natural convection by atmospheric air, which would also help. 

Of course it would not be a recommended procedure.  The copper would be OK at the temperature, though there would be a danger of cracking due to temperature differentials causing stress at changes of section such at bushings or end flanges, and I don't know about the ductility of the silver soldered joint at the temperature.  Apart from the very real danger of very severe frost bite if you spill any on yourself.  So in principal yes, it would speed things up, but in practice I agree with your last comment, there is no need to speed up the cooling.

Hi Derek, part of the issue with the small diameter tubes we use as model steam pipes is that the ratio of surface area to volume is high, so heat loss is significant.  I don't know your boiler steam temperature, but the engine can only extract heat down to 100 C unless you have a condenser, so I suspect your three degrees loss avoided would deliver significantly more heat to the engine so increase the engine work output.  If your steam temperature is say 130C at the boiler, 3 degrees would be 10% of the potential work output would be lost.  So the savings are very worthwhile.

However the significant difference with any flue tube boiler is that the lost heat is actually heat from the steam space, having already transferred through the flue tube wall to the water.  So avoiding this loss by insulation does not require any further heat transfer at the flue, it is just straight extra steam available for the engine.  In these small boilers, the heat saved is significant compared with the burner heat release.  But the insulation does make getting at the fittings more difficult, and as you say, it is a very good idea to make the bushes with extended length on the outside to allow for the insulation.  It is difficult to design a spanner for fittings buried deep below the insulation surface, and tube spanners really only work for a filler plug.

Hi Pete, great to have you on board.  Yes a huge job to replace the bushes.  One approach is to make an adaptor bush to screw into the boiler bush with the appropriate thread the outer end to replace the fitting.  Has the disadvantage of an extra joint to seal, though it could be caulked with soft solder if necessary, I assume.  The better approach is to allow for insulation thickness when designing a new boiler.  In fact, that is the approach we used on full size vessels in the oil industry where pressure vessel nozzles were made deliberately longer where insulation was to be fitted.

Hi Paul, the truly definitive demonstration would be your two locomotives as described, but a lot of work unless you were planning on building two anyway.  And then make provision for replacing the less successful boiler.  But for my part, I would be happy to now try the experiment on a 4 inch boiler, and extend the results to other similar size boilers.  It is easier to do the measurements on a typical stationary boiler, though in your scale, the whole locomotive could be placed on the scale before and after to measure the water consumption.  Fuel could be done the same way especially if the fuel tank is easily removed.  Not so practical on a larger coal fired model.  However, I suspect the coal fire puts out a lot more heat than a small spirit or gas burner, so the same heat loss or savings might not be so important.  Remember, the importance of that heat saving is not because it is so large in absolute terms, but because it is large compared with the burner output.

I like your idea of designing the boiler as a whole with the insulation as an integral part if it is going to be applied.  But the other issue with a locomotive where the outside size is constrained by scale considerations, is making sure there is room for adequate water volume, especially when the model is expected to run around the track largely unattended.  So insulation has to be balanced with the possibility of just making a bigger burner.

With the interest in the insulation tests, I can see that I have to proceed with two or three more tests to demonstrate repeatability, one test demonstrates the principal but is no use if it can't be repeated, not to mention considerable embarrassment.  At least the test is quite simple.  The main assumptions are that the burner heat release rate is constant, or at least close enough, and that all the water evaporated happens during the timed steaming time.  So nothing to obtuse.   And I will also dig out the books again and see if I can produce supporting calculations.  The calculations are the best way to explore how it all works on a larger or smaller boiler to minimise unproductive testing.  So I will be busy trying to fit this in to my normal schedule for the next week or more.

Thank you everyone for your interest.  And all the comments are especially appreciated.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 30, 2018, 05:14:15 PM
Hi MJM et al, thinking about a smaller boiler with more insulation....................as there is less water in the boiler available for steam production to run an engine ,, would the continuous topping up with cold water have a detrimental effect on the efficiency of the system ??  Diminishing returns ??!!! Just a thought/

Willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on September 30, 2018, 11:18:00 PM
My actual steam involvement is 5" Gauge 'scale' here in Wollongong....& I spend a great proportion of Public Running Days on the Station ...handing over scale sized buckets of Char....and filling tenders with cold tap water

This invariably is fed by axle pumps or injectors directly into hot boilers

Talking with fellow members, there is no apparent concern   :hammerbash: to preheat the boiler feed water...'as just another shovel of Char will do'

One continuous horizontal U shaped spool would have no joints to leak & could be secured or placed against the boiler shell proper and ultimately under the lagging shell so unseen?

I suspect other 5" & 7" Gauge members up North in Paul's steaming area would have similar thoughts

From this, scale sized model marine boilers would appear to be the winner here if builders 'and manufacturers' were made aware of the benefits of  insulation & the need for longer protruding boiler bushes etc.....

Derek
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 30, 2018, 11:34:49 PM
hi Derek..that sounds like a good idea as a bit more time with the initial steam up but ok after that..!! I had not thought of that actually !!

Willy
Title: Re: Talking Thermodynamics
Post by: paul gough on October 01, 2018, 01:57:39 AM
Hi MJM, Again having no experience with the burner type in your 3" boiler, so, another question. Is this burner designed purely for one output, ie. maximum, or is there a capacity to reduce its output somewhat? If so could you give me a guesstimate of what percentage of maximum it might operate effectively at? My thinking is that two of the 4" boilers from 'Minature Steam' steamed in parallel would be compact and convenient for running a larger 'early design' stationary engine at say 30 psi. 60 rpm. As 'M.S' state the 4" boiler is matched to their high speed twin cylinder model engine, two boilers in parallel should have sufficient capacity to run an engine with a swept volume of twice, (perhaps a bit more), that of one of these engine units, but with lower speeds and reduced pressure applying. Being able to adjust burner output would be necessary for this application.

Hi Derek, Yes, it has been also my observation that the loco modellers don't concern themselves much with trying to evolve more efficient boilers very much. It is easy, as you stated, to just throw in a bit more fuel, or, build bigger engines. This seems to be more the case nowadays where clubs running days are principally for the public and where operational requirements of hauling people seem to have taken over from other more sedate concerns like running extensive tests. As for steamers up here, as far as I am aware there are none near Cairns. I believe there is a 7 1/4 track down in Townsville, but thats too far at 350 klm's for me to drive nowadays. Maybe the marine/stationary modellers like yourself and MJM can help lead the way in convincing others that attempts to improve insulation are a worthwhile exercise rather than just burning more fuel.

Hi Willy, Just as an example, my tiny gauge 1 'Lion' loco boiler has about 80 cc capacity when totally filled, boiler is 35mm Dia by 100mm long with three 8mm fire tubes and generally this means, very roughly, 40 to 60 cc is the volume one would aim at maintaining. The loco has an eccentric pump that is usually on fully, (except for a slowly forming drop at the return to indicate pump is working), when hauling a train and feed water goes into the steam space at the rear end of the boiler without any preheating. The engine when running well under load usually has a feather of steam from the safety valves, so things seem to be well balanced.

Regards to all, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 01, 2018, 02:15:49 PM
Hi Willy, you have cleverly brought us back to the connection between feedwater heating and efficiency that we have looked at before.  Some of the problem is the variety of meanings people apply to "efficiency", which are often more about performing well in some undefined way.  I like to stick with the concept of ratio of work output to input energy, although efficiency compared with some theoretical output, such as adiabatic efficiency is also a recognised technically acceptable definition. 

All the heat necessary to heat the water to the steam raising temperature has to be supplied by the burner, it does not matter whether this is a batch process such as you and I employ when we first fill the boiler with cold water, then heat it up before continuing to evaporate the water at steam temperature, or if it is a continuous process such as Paul's locomotive with an axle driven feed pump trickling in the cold water on a continuous process.  The steam tables tell us that heating the water to boiling point takes about 15% or more of the total heat required to produce steam.  The simplest way to increase efficiency is to reduce the losses to atmosphere.  The two largest sources of loss are the exhaust steam and the flue gas.  If we can find a way to extract more heat from the flue gas, or from the exhaust steam after they have been used to the maximum extent for raising steam or doing work.  One way of using some of this waste heat is to pre-heat the feedwater, so reducing the heat that has to be supplied by the burner.  So providing we are capturing some of the waste heat, and not just taking heat from the boiler, feed water heating will improve efficiency.  The low temperature of exhaust steam limits its usefulness, and the limit for the flue gas is the dew point of water in the flue gas which tends to result in an unacceptable degree of corrosion of boiler and furnace components.

Another way of increasing efficiency which is used in full size practice is to preheat the combustion air usually with flue gas but potentially some heat could be usefully extracted from the exhaust steam.  This directly reduces the amount of heat absorbed in bringing the combustion air up to the temperature exiting the burner.

And of course, as in my current experiments, we can insulate to reduces losses.

It would be nice if the feedwater could be heated by a coil in the reservoir, however, there is a problem with pumps and injectors, in that they do not work well with hot water.  The issue is called Nett Positive Suction Head, or NPSH, which is the pressure in height of water column above the equilibrium vapour pressure (or boiling point) of the water, necessary to push the water into the pump without causing vapour formation.  Effectively it means for model steam plants that the heater has to be at boiler pressure, after the pump, so the pump handles cold water.

Hi Derek, I think the pragmatic view is that "another bucket of char" is probably the best answer.  Improvement of efficiency is more about the satisfaction of doing it well, rather like a good coat of paint, though that at least is visible for all to admire.  In both cases the engine will work perfectly well without it.  Efficiency really only becomes important in full size passenger and cargo services, where the cost of fuel is a significant part of the cost of providing the service.  Or on really miniature engines where a larger burner may not be practical. 

When it comes to designing a feedwater heater, it is important to consider where the heat actually comes from.  To improve efficiency it must truly come from waste heat sources.  If it just comes from the boiler it simply reduces any effects of sudden temperature gradients where it enters the boiler, it does not actually improve efficiency.  Certainly bushes that protrude further from the shell are helpful if you apply insulation, however, I suspect the dreaded cost issue limits any commercial availability.  It is very practical if you are designing or building a new boiler.  Extended bushes would not be a significant change from the boiler integrity point of view.  Otherwise screw in adaptor bushes with a good gasket and a tight joint would fill the gap for an existing boiler.  On thinking further about it, I suspect that solder calking is not really necessary.

Hi Paul, my 3" boiler is just the smaller model of the 4" one from the same source, with the matching burner.  I probably should have bought the larger one, but I was thinking of how big a boat would be needed to accommodate it, and still hope to build one day.  But that is a whole other story.  The burner will certainly operate over quite a range of gas pressure to provide the turndown you require.  There is no fundamental difference between lowering the gas pressure at the burner by cooling of the liquid as the gas is drawn off, or by partially closing the shut off valve.  I normally light the burner as recommended by holding a match to the chimney while I open the gas valve on the gas tank.  I tend to stop opening the valve when the burner lights.  Sometimes I find the stack temperature decreasing, suggesting reduced heat input as the gas bottle cools and the pressure gets too low.  When I then open the valve fully the temperature comes up again.  So I guess I am unintentionally turning the burner down in that early heating period.  Then again as the run continues as the gas liquid continues to cool, it is very clear that the gas flow is not constant, and this is the biggest source of uncertainty in my testing.  It will take a lot more work to eliminate this source of variability.

Boilers will run perfectly well in parallel, so long as you are particularly careful to balance the water levels and steam pressure in each.  The combined steam output will supply a larger engine as you require.  This involves preferably installing a balance line below water level, which is quite separate from the feed water inlets.  My three inch boiler does not have the necessary extra bushes for a balance line plus a water inlet, so if a feed pump is required it would be best to take care that the water inlet is completely symmetrical to each boiler.  The steam lines should also be completely symmetrical, to ensure the pressure remains the same in each boiler.  You would have a single steam valve on the combined outlet line.  These points are required as very small pressure differences in pressure would result in a significant level difference in model size boilers.  And it is most unlikely that the steam generated in each boiler is exactly the same, so water balancing and pressure balancing are both necessary to avoid low or high level in one boiler.

That gauge one boiler sounds like a very well developed and well balanced unit.  You must be very proud of that one.

Thanks everyone for following along,

MJM460
 
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 02, 2018, 02:40:56 AM
Hi MJM , thanks for this and i have another quote to ask people about ....NPSH !! I shall try it out at the club meeting soon !! :mischief: :naughty: .When the exhaust steam comes out of a really fast running engine is it that much cooler ?? or does it get cooler by the time it exits the chimney flue ?

willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 02, 2018, 12:59:53 PM
Hi Willy, the NPSH required at the inlet of a good centrifugal pump with especially smoothed inlet passages to keep cavitation in the pump to an acceptable level is more than a meter.  A reciprocating pump does not have the smooth passage, the valve balls are in the way of the flow, and there is pressure required to accelerate the flow into each inlet stroke, so considerably more is required for a reciprocating pump.  When the water temperature is low enough, atmospheric pressure at the liquid surface does the job, but as the temperature increases, the vapour pressure increases, and this decreases the NPSH available, which is why our feed pumps do not like hot water.  Neither do injectors.  If we wanted to heat the water in the feed tank, the tank would have to be elevated more than would usually be acceptable in a model.  So feed water heaters go on the high pressure side of the pump.

If your friends want to be tricky, they might ask you how to calculate the NPSH available?  So be prepared, and remember it is the total head of liquid at the inlet nozzle after subtracting all friction losses is the inlet line, and most importantly, subtracting the vapour pressure of the fluid.  May be simpler to avoid the topic!

For a fully loaded up engine, it is not the engine running fast that cools the steam, it is the extraction of energy to produce work by the engine that reduces the temperature within the engine.  However, heat loss from the cylinder surface and from the exhaust system also help cool the steam.  And as Derek found, even the steam pipe heat losses contribute to cooling of the steam.  My measurements of the engine exhaust temperature are all very close to 100 degrees, which makes sense as the steam is at atmospheric pressure as it leaves the engine, so has to totally condense before it gets cooler.  However, as soon as some air is entrained in the exhaust steam it can quickly cool further, to the condensing temperature determined by the proportion of water vapour in the air.  (The proportion determines the vapour pressure of the water, hence its condensing temperature.)

For an engine running lightly loaded, heat losses from the cylinder and piping and mixing with atmospheric air are the main processes which soon cool the exhaust to the point where the familiar fog of condensed water is visible.

I hope that makes sense,

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 14, 2018, 12:08:14 AM
Hi MJM , sorry not to have been in touch but have been busy with the Freelance engine ..it is now finished and i have started on a new Eastons & Andersons beam engine , so more challenges and nifty file work to do.! In the meantime i have been insulating my new "kettle"... !!! here are a few pics ..I have been using 3 layers of 1/8" cork and some 1/4" mahogany slats .....so we shall see how this develops !!
Title: Re: Talking Thermodynamics
Post by: derekwarner on October 14, 2018, 03:53:27 AM
Certainly a different thermal experiment Willy   :facepalm:......have you conducted pre insulation trials?.......like timing a given volume of water from ambient....[recorded  temperature] to boiling....and how did you measure or determine boiling point?

Then repeat the identical tests with the recently insulated jug?.......

I think the actual ambient air temperature in your test room [kitchen] would also be needed for the calculation
______________________________

I see the determination of the commencement of the boiling point may be a little bit iffy in the calculation...so looking backwards....you could consider filling the un-insulated jug with a measured volume of water...............then bring and keep the water boiling for say 5 minutes...........pour out & measure the volume of the remaining water

Repeat the same tests  in the same manner with the insulated jug......

Then confirm the test results...

[this would take all of the scientific aberrations  :happyreader: out of the calculation ......you could simply confirm that the jug with the insulation provided a % variation (+/-) in boiling water {as the benefit of insulation} over a non insulated heating vessel]
______________________________

We will just be here  :cheers: awaiting an answer................. Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 14, 2018, 07:42:16 AM
Hi Willy, your freelance engine looked great in the show photos, beautifully finished.  Does the valve gear work over any range of intermediate positions, or is it effectively limited to F-S-R?

When you do that Eastern and Anderson, I hope you are planning to make the complete working valve gear!

On your jug experiment, it might be worth considering the controller part in the bottom, and only insulating the sides of the water containing part.  Otherwise the wiring, switch and shutoff mechanism will get hotter than intended, which may lead to problems.

Hi Derek, you certainly hit the point in talking about controlling all the variables.  I have found that the mass of water is very important, obvious really, but it does not take much variation to confuse the results.  Weighing the water before and after is definitely the best way to go, but still hard to get an accurate small difference between two much larger weighings.

On the issue of determining the point of boiling, I would suggest the thermocouple placed in the spout to measure the water temperature and time to say 95 deg, or a bit higher if it gives consistent results.  Normally you can see the boiling occurring through the glass sides but not through the insulation.  And of course the ambient temperature will have an effect as you say, but not easy to control unless the house has central heating.

I have also been silent lately, but not totally inactive.  I found that with my centreflue boiler, it is not easy to consistently remove the water to the same point each time so I can tell how much was used in steam production, and gives highly variable results.  I need to do some more runs to get either consistency or an idea of the scatter.  But of course the temperature in the shop today would have been about 28, if I had been able to get out there, so another variable compared with my results earlier in the year.

To overcome this, I took the opportunity to purchase a scale that could measure the whole steam plant when I noticed a suitable scale on sale recently,  I should have been less generous with the heavy chipboard base!  The reading also seems to be a little dependent on the positioning of the weight on the scale.  Working on a method to balance the whole unit with the centre of gravity in the same place each time.  Unfortunately, adding water to the boiler shifts the centre of gravity.  But I have a plan, and when family commitments (life, really) allow me some time, I will do more runs and post up a photo and some results.  I am close to the maximum for the scale so the arrangement must not be too heavy.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 15, 2018, 02:36:37 AM
Hi Derek and MJM,  Ok I have looked at the construction of the kettle and i have done a boil test I have timed the test and waited till i could here the water boiling/bubbling. You can see the inner construction of the kettle that has a flat shape where the handle is although the outside is more curved. the kettle does switch off automatically but i do not know the exact temp it will do this !? It has tried to switch off before it is aurally boiling and i think that is because i have carried the insulation around the kettle rather than only covering the actual part that contains the water . the handle and the inner part is where the steam/heat operates the switching off mechanism. I have tried a preliminary test and it boils 1 Litre of water at ambient 20  cent in 3 mins 05 seconds and the kettle is rated at 1Kwatt....the outside only went up to 22 degrees !! so more  testing ... I have not tried the test with no insulation as i was going to do this afterwards and doing 4 tests by removing successive layers of the insulation !!?  This post is a bit longer as i wanted to give all the info. i usually give a short post and this does not give all the info that i should include in the question...so .. this is just an academic exercise really using the electric element to give a really reliable constant heat input. ........
Willy.........
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 15, 2018, 12:09:44 PM
Hi Willy, it sounds like you have it covered.  I suspect that insulating the part below the water container might interfere with the switch off point.  I don't know how those kettles work, whether they are just temperature which would mean a switch point a little below 100 or it might never get there, or some sort of vapour pressure measurement in a bulb deep inside.  However as long as you measure the quantity as accurately as you are able and do several runs under each condition to demonstrate a level of consistency (or the degree of spread) your results will be quite interesting.  Your thermometer might help.

Watching with interest

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 16, 2018, 02:41:42 AM
Hi MJM, Ok so....I thought originally that my kettle with its water at a certain temp with a fixed input heat would boil at the same amount of time  ??   And ..I have done an initial test with the different amounts of insulation  and the time taken to boil and switch itself off was virtually the same. However the temp outside of the kettle increased dramatically with no insulation in the final test. So was the time taken to boil the same because the 1 litre of water with a 1 Kw heater took quite a short time to boil....??  And i was thinking about formulae .. 1X1X1 = 1 however 2x2x2 -= 8  and 3x3x3 = 27 etc or is this intuitive thinking a bit silly ??!!! If 2 litre with 2Kw is heated will it take longer ??  so what is the  verdict with this experiment, and would actuall a longer time , using the steam produced in a closed vessel driving an engine give more pertinent information....... so the conclusion i have is that there would be no saving of electricity to have an insulated kettle ??!!!! :-\....

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 16, 2018, 11:00:25 AM
Hi Willy, quite a lot of information there, so let's go through it one thing at a time.

Kettle plus water at the same temperature with the same heat input should take the same amount of time, provided only that the quantity of water is close enough to the same.

The outside temperature of the insulation or jug decreases with the amount of insulation.  With more insulation, there is more temperature drop through the insulation, so quite as expected.  The method of attaching the thermocouple to the outside is an important factor in the actual readings, but as it was the same in each case, the error is probably about proportional so the direction you have observed is consistent.

1 x 1 x 1 = 1, 2 x 2 etc. the maths is correct for that sum but consider a different sum -

1 / 1 = 1, 2 / 2 = 1, 3 / 3 etc, so the answer depends on which sum you do.  The question is which sum should we have done? (The / symbol meaning divide)

It takes energy to raise the temperature of the water (and the jug).  We can work out how much energy is required for the water by looking up the enthalpy column in the steam tables which give us the energy in a mass of one kilogram.  Using 20 C as the start point (just to be able to find the figures directly listed) and 100 C.  The figures are 83.96 at 20 C and 419.04.  We can subtract these to get 335.08 to heat water from 20 to 100.  The units are KJ/kg, so are for 1 kg, so to find the answer for 2 litres, close enough to 2 kg, we multiply that by 2 so 670 KJ is the amount of heat required.  It is a bit more complicated to work out how much the jug material absorbs, but let's accept that as a constant error, so leave it out.

Now if the jug has a 2 kW element, it gives 2000 W.  One Watt is one Joule/second, so 2 kW supplies 2000 kJ/s, or 2 kJ/s. ( k being used to mean thousand)

So to find the time to heat the water, we want 670 KJ supplied at 2kJ/s, so we divide 670 by 2, or 335 seconds or 5 min 35 seconds.  (Remembering that the jug will absorb some so prolong the time, as will any heat loss from the surface of the jug.)

If you have a smaller jug, which contains 1 litre, but has a 1 kW element, it requires half the energy for the smaller mass, but has half the heat input, so the time will be the same, 5 min 35 sec.

You didn't say what capacity or heating element your jug has, so I have just taken nice easy figures to illustrate.

I have mentioned that it will take extra time due to the heat absorbed by the jug and also due to the heat loss to the atmosphere.  The actual jug should have the same effect on each of your experiments.  However that outside temperature determines the heat loss to the air, which will also prolong the time.  However, the heat loss is proportional to the heat transfer coefficient, and area as well as the temperature.  The area is quite small (in square meters) and the heat transfer for natural convection is also relatively low.  And remember that the heat loss at first, when the outside temperature is very close to the ambient temperature, is almost the same in each case, then slowly increases as the water temperature increases.  But the effect of that heat loss depends on how it compares with the element heat output.  If it is a very small proportion, say around 50 watts, it is only about 2%.  But that same loss will be more significant if you only have a 1 kW jug.  If the measured time is very similar for each of the experiments it suggests that the heat loss is small compared with the element output, and so not detected by the accuracy of your experimental technique.

The other factor in trying to actually predict the answer is the element itself.  You will remember from the discussion of your electric boiler, that the rating is a nominal figure, and the exact heat output depends on the element tolerances and temperature, as well as your exact supply voltage.

Overall an interesting experiment, even if your conclusion is that the manufacturer saving money by not insulating the jug is not costing you a lot of electrical energy, compared with the cost of the insulation. 

You might be interested to find, if you look around a hike equipment store, that hikers gas stoves can be supplied with heat fins that keep the combustion gasses close to the walls of the jug, and increase the heat transfer rate, and insulation around the outside of the fins.  This is quite expensive but saves sufficient gas in boiling the jug to be worth it, not only for the cost of the gas, but because the mass of gas has to be carried.  I guess the fins and insulation both contribute to the savings.

I hope this contributes to understanding the results of the experiment.  I am sure you can see the comparison with your electric boiler.  Just allowing for the different layout and quantities.

Thanks to everyone looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 17, 2018, 02:35:26 AM
Hi MJM, thanks for this, and i suppose that just doing this relatively quick boil up does not give as much info needed for using a boiler as a source of power to drive an engine for an extended period of time. I was wondering if some of the heat used to boil the water actually goes into heating up the insulation which is why the last heat up was a bit less than the others ? Also i think that the radiation / conduction / convection all have different effects with the different amounts of insulation. All in all a worthwhile academic exercise that has some relevance to thinking about everyday energy conservation in the home. !! With an externally fired boiler the insulation would be far more important as you have found with your experiments.

Willy

Title: Re: Talking Thermodynamics
Post by: MJM460 on October 17, 2018, 12:41:57 PM
Hi Willy, I tend to agree that there is not much more that can be gained by further experiments with the kettle in terms of boiler performance, as in a boiler we are more interested in steam production, the heat up time is simply a start up delay, possibly a bit annoying but not really important.  But the experiment with the jug does tell us a lot about heat transfer that is of interest, so don't dismiss it lightly.  Let's summarise it.

First yes, some heat is stored in the cork during heat up.  To understand whether this is important, let's assume 9 mm of cork.  We need the density of cork, about 150 kg/cubic meter, and the specific heat, about 1880 J/kg.C. 

From the density we find that the mass of 9 mm of cork is 0.009 X 150 = 1.35 kg per square meter.  If the kettle area is about 0.2 square meters, that would be 0.27 kg.

Now we use the specific heat to work out how much heat is required to heat that mass of cork by one degree C.  0.27 X 1880 = 508 J/Deg C.

The average temperature of the cork when the kettle boils is approximately 60 deg, so from the stat at ambient temperature until the kettle boils, the heat absorbed by the cork is 508 X 40 = 20304 J, or 20.3 kJ.  This would add about 10 seconds, about 2%, to the time required to come to the boil.   For best accuracy, it should all be taken into account.  The specific heat of cork is relatively high but because the density is low, the actual heat required is quite low.  For comparison, the specific heat of water is 4217 J/kg, but the density is 1000 kg/cu.m.  These figures are all readily available in almost any book on heat transfer, though recent ones are likely to be more accurate than very old texts due to improvement in measurement techniques.

If your jug is glass say 2 mm thick, the density is 2800 kg/kg, the specific heat is 800 J/kg.C.  The similar procedure gives 4480 J / square meter per Deg C, and of course the glass is all heated very close to 100 C or 80 deg rise.  So for 0.2 square meters, takes 71680 J or 71.68 KJ.  Your 2 kW element has to run 35 seconds just to heat the glass.  I don't know how close my guesses are to the actual mass of glass in your jug.  Of course, if your jug is plastic, the density will be much less, but I don't have figures for specific heat.

The other thing your experiment does is gives us some real figures to use in exploring how the theory works.  We have discussed the theory previously, but let's remember the formula and put in some real figures.

I want to recheck my calculations before posting that part, so perhaps tomorrow as it is getting late here.

Thanks to everyone looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 18, 2018, 01:50:43 AM
Hi MJM, the jug is a plastic item and the element is 1KW . I have just got the 2 volumes of this book and i think it was the precursor to the Cassells book. It has a vast amount of info including the 20" of rainfall in Hunstanton  AKA Sunny Hunny !! there are lots of new words in it that i have never herd of before and the original came out about 1890. this edition is about 1918 !! The info therein is mostly in imperious measurements and i don't know if the values for everything has changed after 100 years of further investigations ?? It gives formulas for so many things  Chimney dimensions etc etc. So here are a few pages from the 780 ??
Willy..........
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 18, 2018, 01:52:18 AM
More pics.

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 18, 2018, 11:47:16 AM
Hi Willy, what an interesting book.  You might notice how many of the topics are still the stuff of Thermodynamics and Engineering data books today, and that the laws of thermodynamics were known and being formulated in words even then.

The differences to today are in detail more than substance, and reading the words that reflect the understanding of the time might well be helpful as an intermediate step to understanding the words used today.  The differences seem in the main to be in detail rather than substance.  The information on parallel motion mechanisms was interesting.  Even then it was known that the Watts linkage is only an approximation, understanding that is almost lost today.

I have not checked all the material properties, but I would expect that differences from the values tabulated today would be mostly due to developments in measuring instruments and experimental technique.  So I would expect that today's values are a little more accurate, but probably only enough to give a little more precision rather than a significantly different answer.  And of course if they were using slide rules or manual calculation, they would probably not notice the differences.

Modern computers facilitate doing many more calculations which can sometimes be important.  With manual calculations, and I can assure you even with a slide rule, which would have been a great time saver when it came into general use in science and engineering, the temptation is to minimise the number of calculations and use hand drawn graphs and artistic licence to estimate values in between.

The other thing you will notice with those inferial units is the various constants in the formula.  This comes about from the somewhat arbitrary definition of units of force, and the calorie, which were defined for everyday commercial purposes well before the fundamental relationships between them were understood.  The SI system which I prefer for these calculations has the advantage of being developed later and so the unit of force has been defined in therms of Newton's laws, which results in a rational system of units, where for most formulae, the constant of proportionality is 1, much easier to remember and use in multiplication and division than the ones which crop up in other unit systems, not only ft.-lb.-second, but also metric systems such as centimetre-gram-second, or meter-kg-second.  At least they all use seconds for time.

Have not made any progress on the calculations today.  We are supposed to be retired, sitting around wondering what to do.  Hah!  We have been flat out for a 12 hour day, pausing only for a short lunch and late afternoon break.  Among other things, regrouped the tiles in a bathroom.  So much for nothing to do.  I don't know where all that extra free time is supposed to come from.  So perhaps tomorrow for calculations.

I expect the plastic jug would absorb much less heat than glass, but unfortunately my book does not see to list likely plastics.  It is still worth exploring a little further.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 18, 2018, 11:48:34 PM
Hi MJM , what i really like about this book is that it gives the names of the people that discovered and recorded the actual topics that are explained. as it says on the title page   A good book !!

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 19, 2018, 12:42:02 PM
Hi Willy, I couldn't agree more about the interesting descriptions of what all those early pioneers were discovering.  The names are familiar, many having been given as special names for units or observed phenomenon, but use of the name does not reveal much of what they actually did without further information.

I have done some calculations on the temperature profile of the jug with insulation.  A little more checking required, to easy to make an error in a spreadsheet and have a very accurate looking wrong answer.  Takes careful checking by a fresh mind, so perhaps tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 20, 2018, 11:33:09 AM
Hi Willy, good news at last.  I have been struggling with the analysis of the jug for way too long for a relatively simple concept.  I was getting unlikely results and had clearly made a mistake.  At last I have found it.  A case of systematically checking the formula in each cell until I found it.  So what did I conclude?

Start by remembering the basics - the total heat loss is determined by the resistance of the convection film coefficient between the water and the inside wall of the jug, the conductivity and thickness of your cork insulation, and then the convection film coefficient between the outside of the cork and the air in the room.  The lowest conductivity or highest thermal resistance has the highest effect.  In this case, the water coefficient is so high it is not a big influence on the total resistance.  However the cork and air are close enough in value that both are important.  The same rate of heat loss has to pass through all layers in series.  Unfortunately, we don't know with any accuracy the air side film coefficient.  My text book lists it as somewhere in the range of 5 - 25 W/m^2.K.  Read this as Watts per square meter per deg Kelvin, and of course one degree centigrade is the same magnitude as one Kelvin, so when temperature differences are involved, heat transfer problems for example, you can use either.

The effect of this coefficient is that apart from its effect on the overall rate of heat transfer, it can be directly seen in the temperature difference between the outside of the cork and the room air temperature.  A high film coefficient leads to a small temperature difference across the film, while a low film coefficient leads to a high temperature difference across that film. 

The published figures for the air side film coefficient lead to much higher film temperature differences than those you have observed.  However, if I assume the film coefficient in the range of 100 to 200, the film temperature difference is predicted to be 3 (for 100) or 2 (for 200).  This just seems a bit high compared with the textbook figures.

That outside cork temperature is in practice quite difficult to measure with a thermocouple as the thermocouple is influenced by the air temperature as well as the cork temperature, and the air temperature range occurs over a very small distance.  It is likely that your thermocouple reads low, but it is interesting to try.  An infrared device may be able to measure the temperature differences more accurately as the jug heats, but I haven't tried to compare it.

I would be interested to know if you tried touching the cork with your finger when the jug was hot, and whether the temperature seemed higher than what you were measuring.

Any way, with consideration of a reasonable error range, the calculation yields a heat loss somewhere around 20 Watts, quite small compared with the element rating of 1000 W.  If it is as low as 12, which would require a film coefficient of only 5, the cork surface temperature would be nearer 38 C above ambient, quite a lot higher than the measured value.  A value of 25 for the film coefficient would give a cork surface temperature about 12 C above ambient which would feel warm but definitely not hot.  A coefficient of between 100 and 200 would give a cork surface temperature rise of 2 - 3 C above ambient, which would definitely be in line with your measurement.

If it is of interest, perhaps I could write out the relevant equations and attach them rather than describe the whole procedure.

Just for interest I repeated the calculation for the uninsulated jug.  I don't really have any data on the thickness of the plastic jug walls so I guessed 2 mm, or 0.002 m.  For the plastic, I found a thermal conductivity for plexiglass 0.195 W/m^2.K, so, much less than glass, but high compared with cork.  I am guessing that it might be in the right range for the jug plastic (without any evidence however!)

The issue of what value to use for the air side film coefficient still influences the answers, so I  repeated the same calculations several times to cover the same range, the joy of using a spreadsheet.

The heat loss is is then likely to be in the range 100 to 200 W and the outside wall temperature 35 to 60 degrees above the room temperature, so 55 to 80 C, would definitely feel very hot, and could inflict a burn on your skin.  Now starting to become significant compared to the element, so probably slows that final approach to boiling point.  Of course, when you first switch on the jug, the temperature difference is very low, so the heat loss is also quite low for most of the heating time.

By comparing your readings with the calculation, you can see the influence of the film coefficients and material properties.  You can also see that while the electric power saved, probably would not cover the cost of the cork (in environmental or economic terms), it has real value in reducing the outside temperature of the vessel and so the likelihood hood of burn injuries.

Though of course, most of us learn early to expect the jug to be hot, so probably don't need it.  However when in a laboratory boiling liquids at much higher temperatures, or in some special cases where the occurrence of burns is a problem, including many industrial situations where piping carries hot fluids, the insulation can be useful, and is applied for personal protection.

I guess the last step is to revisit the convection calculations and see if the film coefficient can be estimated a little more closely.  That will have to wait for another day.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 22, 2018, 12:04:46 AM
Hi MJM , thanks for the reply...what i should have done as well was check on the cooling down time of the various states of the insulation breakdown ...that would have been much more interesting ,However that would have taken a lot of time clock watching !!..When i did the cooling down . Also the time cooling down with the boiler it took about 18 hours !! Also i did the kettle timing in reverse with the full insulation in reverse !!! :facepalm:

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 22, 2018, 12:34:40 PM
Hi Willy,

I don't think I would have waited for the cooling time either.  It is not necessary with an electric element as there is no air flow as an extra mechanism for heat loss like there is with a fired boiler.  In that case, the cooling curve allows an approximate measure of the heat loss through the insulation alone.

I am still puzzling over what you mean by reverse?  Obviously not the same as up side down.  The handle facing the other way?  Time backwards? Not usually, though the clock could count down instead of up, though the elapsed time would be the same either way.

But the big learning is about the temperature profile.  I will try and post a picture tomorrow.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 23, 2018, 01:09:55 AM
hi MJM,  By reverse i meant i put all the insulation on first  after cutting it out and making it all fit properly as each layer became larger. I then did the tests starting with all the layers and doing each consecutive test by removing each layer in turn ! Also some of the layers of cork were not completely touching everywhere due to the shape of the kettle, so there were layers of 'air' as well however that should possibly increase the insulation ?.. I can remember wearing an 'airtex' vest years ago but i don't know if they still make them. !


willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 23, 2018, 12:14:50 PM
Hi Willy,

Ok, it really doesn't matter which order you do the experiments, as I am sure you knew.  But you have made an important point about those air gaps.

When two surfaces touch, there is a contact resistance to heat flow, or a local lower heat transfer coefficient.  First there is a temperature drop across the contact faces, and second, the lower conductivity is in the line that all the heat must pass through so further reduces the overall heat transfer.  Air is not a very good conductor, and in your jug experiment, the transfer to air is nearly as important as the transfer through the cork in determining the overall result.  And the transfer that does take place to the air is significantly aided by convection, which keeps replacing warmed air with cooler air, so increases the average temperature difference.

In those little air gaps created by the insulation "fit", there is very limited scope for convection, so the actual conductivity of the air becomes more important as heating of the air reduces the temperature difference driving the heat transfer.  So the air gaps are only helpful in an insulation system.  (They do add a degree of complexity for trying to do a complete calculation, but really only important when you are trying to increase the heat transfer for heating or cooling purposes, so in this case I have ignored them.) The thermal conductivity of air is only 0.026 W/m.K at room temperature.  It increases with temperature, but at 120 C, it is still only 0.033 W/m.K, so lower conductivity than cork at the temperatures we are talking about.  My text book does not list anything better.  This contact resistance is part of the reason it is so hard to measure the outside cork temperature with your thermocouple.  Along with the fact that the thermocouple sheath lies in the thickness of that convection profile, where the temperature is changing rapidly with distance from the cork, so difficult to determine what it is actually measuring.

Unfortunately if you have a bigger air gap, convection is able to start, so increases the conduction.  But in closed cell foams, and those air-tex jackets, (I am not sure of the construction of those), the very popular and incredibly effective down jackets and doonas all work mainly due to the low conductivity and low mobility of the trapped air.

All these problems involving heat transmission through multiple layers are solved using the same equation, q= U x A x (T2 - T1), where U is an overall combined heat transfer coefficient.

For a system involving two convection layers (convection heat transfer coefficient is usually given the symbol h) and a solid material such as your cork, the overall heat transfer coefficient is calculated by combining the individual layer values as shown in the attached extract from the text book, which also show pictorially the temperature profile.  If you have more layers, like the glass wall of the jug, wood outer cladding, or perhaps a second insulating layer of different material, the extra L/k terms are easily added.  L is the thickness and k the thermal conductivity of the layer.

The inside film coefficient for the water is quite high, so really has negligible effect on U, especially once the boiling starts, as the vigorous agitation of the bubbling fluid increases the transfer coefficient to a much higher level again.  But the air side coefficient is just the level that gives it significant importance in our calculation.

So the last step in this little section is to do the calculation of the convection coefficient and see if we can narrow down that a bit.  That will allow us to predict the temperature of the outside of that cork.  I will have a go at that in the next few days.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 24, 2018, 11:40:34 PM
Hi MJM ,thanks for the info........Wondering about water evaporating ... can water evaporate from an enclosed vessel upside down ? I am thinking of a fish bowl that has a lip with a small cavity for some water .... but open at the bottom  Can water only evaporate upwards and what is the lowest temperature to enable evaporations to occur ??  I suppose the answers are available somewhere ..!! I was thinking that if it does evaporate and rises to the top of the bowl does it condense and run back down to the bottom ?
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 25, 2018, 11:51:28 AM
Hi Willy, the nice thing about thermodynamics is that the very small number of laws cover most situations, even if sometimes it is a little obscure how they might operate in a particular situation. 

So the answer to your evaporation question comes back to that basic vapour pressure of the water which of course depends on temperature and is listed in the steam tables.  Water or any fluid really, evaporates when it's vapour pressure is equal or higher than the absolute pressure at the location.  Once the water vapour pressure exceeds the vapour pressure, it evaporates to form a gas phase.  Of course the gas phase has much lower density than the liquid, so under the influence of gravity, the vapour phase tends to separate from the liquid.  However if the pressure is higher than the vapour pressure at the prevailing temperature, the evaporation is suppressed.

Now in a container of liquid, the pressure at the liquid surface is the same as the air pressure, so about 101 kPa, depending on the weather.  At this pressure the equilibrium vapour pressure is reached at 100 deg C.  But below the surface, the local pressure increases due to the density of the water and the depth.  Because the surface pressure is normally the lowest pressure in the liquid, that is where evaporation starts.  In your kettle or in a boiler with a good heat input, the liquid is generally not uniform temperature throughout, and is hotter near the element.  Now as we know, hotter water has a lower density, so starts to rise so mixing up the water and evening out the temperature somewhat.  But it also has a higher vapour pressure, and with sufficient heat input the temperature, hence vapour pressure is high enough that it exceeds the local pressure even allowing for the pressure increase with depth, and so formation of a second phase or evaporation starts making a bubble right there on the element.  The expansion into vapour phase compared with the liquid specific volume means the bubble is quite large compared with the liquid from which it came and rapidly rises under the influence of gravity until it breaks out at the surface.  This is what we call boiling, as opposed to just evaporation.

Now in your fish tank, I am assuming you mean or at least have seen one of those fish tanks where there is a handle like loop filled with water, so fish can swim through the loop even though it is above the free surface at the opening of the tank.  The first thing to realise is that to determine the pressure everywhere in the tank, you start at the only point you know the pressure which is where it meets the atmosphere at that free surface.  As you go deeper in the tank the pressure increases, but equally, as you go above that free surface elevation in the handle, the pressure decreases, as does the temperature at which the vapour pressure reaches the local pressure.  Now in the dimensions of a typical fish tank, the height in meters is quite small, and one meter of water increases the pressure by 9.8 kPa or about 0.1 atmospheres, or 1.5 psi, so three or four inches does not make much difference.  And the vapour pressure increases in a similar manner towards the bottom of the tank.

So the changes in boiling point and vapour pressure above and below that free surface, directionally are lower pressure and boiling point above the level of the free surface, and higher pressure and boiling temperature below the free surface.  But in practical terms, the difference is insignificant.  However, if you warm up the water (preferably remove the fish first) to about 100, vaporisation occurs first at the lowest pressure point, the top of the handle even though there is no contact with air, a vapour bubble will form and gravity keeps it at the highest point.  More vaporisation will cause the bubble to enlarge.  It's expansion displaces the water down until the vapour space breaks through to the free surface.

If the temperature outside the tank is lower than the water temperature, there will be heat lost at the walls, water will condense, and gravity will make the water drops,once they are big enough to coalesce, run down to rejoin the liquid phase.

So evaporation does not require a free surface, though it will cause a very high pressure if there is nowhere for the water to go to make room for a lower pressure bubble.  It is the action of gravity, and the difference in density between the liquid and vapour phases that causes the vapour bubbles to rise.  If the astronauts had a vessel half full of water and heated it in space, beyond the influence of gravity, the bubbles would tend not to "rise", but would stay near where they formed on the element.  And as the heat transfer coefficient to a vapour is much less than to a liquid, the element would get much hotter to dissipate the heat (presumably from an electric element.). Of course it is pretty difficult to get far enough away from everything so the gravity is really zero.  Even beyond the earth's influence, the bubbles would probably tend to 'rise' away from the moon or sun or any other planetary bodies nearby, so to speak, but not so vigorously as in a stronger gravitational field.

As to the lowest temperature that water would evaporate, well it is pretty low.  There is a vapour pressure even over ice.  At about 0.01 deg C there is a point at which liquid vapour and solid can all exist together in equilibrium, called the triple point.  At this point, the vapour pressure is 0.61 kPa, so not very much, but it is the lowest temperature for liquid to exist.  Below that the vapour and solid continue to exist, and the solid can 'evaporate', though the vapour pressure is very low.  So I guess 0.01 degrees is about the lowest temperature at which liquid water can evaporate.

I hope that you can see the problem does not involve any new concepts, just understanding of the implication of the configuration of that fish tank.  And of course, keep separate the effects of phase change  and gravity.

Thanks for following along,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 25, 2018, 11:36:44 PM
Hi MJM. Thanks for the info and i will need to read through it a few times to get more info from it . I should have included a drawing of what i was getting at though and when i get home i shall include one...11 Ok here is the drawing ,it is a small circular fish bowl with a lip that is upside down with some water in the lip . so does the water just evaporate upwards and if so would it condense on the top and run down the sides to go back into the lip ? ...or can the water evaporate SIDEWAYS AND DOWN to escape to atmosphere. If it only evaporates upwards, would the water stay there for ever ??  or is this another unintuitive sillyish question ??!!!

thanks again
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 26, 2018, 12:46:16 PM
Hi Willy, well, not what I was thinking of at all.  Not much room for fish in a bowl placed like that.  But the same physics applies, just a slightly different problem to the one I was thinking of.

I would look at that one in two stages.  First, look just near the surface of the liquid.  In terms of molecular sizes and distance between collisions even a few mm is quite a long way.  The liquid will evaporate at the surface providing only that the humidity of the air in the bowl is not too high, so that the vapour pressure of water vapour in the air is less than the equilibrium vapour pressure at the temperature.  The water will evaporate to make the water vapour pressure near the surface up to the equilibrium vapour pressure.  The random motion of the molecules means that once the molecules are in the vapour space, there is a tendency to move on average into space where the concentration is lower, so they do not stay close to the surface but drift further away throughout the whole space, and the evaporation tends to continue.

Now in an open dish, the slightest air current will tend to move the vapour away from the surface, so more has to evaporate to get to equilibrium, though it tends to spread purely due to the random motion of the molecules anyway.   But under your inverted bowl, movement of air is restricted.  There is still connection with the atmosphere, so the extra water vapour does not increase the pressure in the bowl, some of the air moves out to the atmosphere through the opening, whether large or small and regardless of orientation.  So the pressure does not change.

So we are left with just looking at how gases mix when occupying the same volume.

Water vapour, being a lighter gas than oxygen or nitrogen or most other components of air, tends to rise under the action of gravity.   It is not as simple as a bubble membrane filled with water vapour floating in air, as the vapour molecules of each gas in the mixture are free to move in all directions and tend to each fill the entire space regardless of the other components.  Gravity acts on all the molecules equally, but the lower mass of the water molecules tends to means that after a large number of random collisions, the lighter molecules tend to experience a slow drift upwards. 

This means there is a small tendency for the number air molecules moving out of the bowl at the bottom to be more in proportion to their total numbers and water.  Consequently, the humidity in the bowl will tend to increase and this might eventually inhibit further evaporation.

The heat necessary to evaporate the water comes from the remaining liquid which gets a bit cooler.  This means conditions inside the bowl are a bit cooler than outside so the wall of the bowl is relatively a bit warmer, and their would not cause condensation.  However, if the temperature falls in the room generally, perhaps as evening approaches, the heat loss from inside to outside would result in some condensation once the humidity gets near 100%, and eventually this condensate would run back down into the water.  It is all a question of a delicate heat balance, and which side on the bowl is warmer.  Then heat always travels from the warmer space to the cooler space.  Room temperature is rarely sufficiently constant that you would never get some condensate under some conditions.

Essentially, condensation requires a cooler surface to absorb the latent heat, and humidity sufficiently high that the equilibrium vapour pressure at the prevailing temperature is reached.  If the bowl is sitting on say a stone bench which is still warm from earlier in the day, your could possibly get a situation where the bench supplies heat to evaporate the water which then condenses on the glass wall, giving the heat to the glass and hence to the room air, while the condensed water runs down to join the water at the bottom.  For a while, until all the temperatures become essentially equal, it could look like a continuous process.

I hope that makes it a little clearer.

Thanks everyone for following,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 27, 2018, 04:56:27 PM
Hi MJM , thanks for that...thinking about your comments on condensation ...They say that in  a dessert you can get drink  ing water by placing large stones in hollows to collect the condensate...however the humidity must be really low in the desert. However i suppose this process uses a different way to p produce the moisture ???

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 28, 2018, 07:13:26 AM
Hi Willy, despite many trips to the desert, I have never tried that one.  Though I have heard of the principal being used in desert revegetation programmes.   Watering the young plants is not very practical so they place a rock by each one.  The idea is that the rock cools with the surrounding sand at night, but has a higher specific heat, so stays cooler than the surrounding sand well into the heat of the day.

Any moisture in the sand, perhaps from condensed dew overnight, is also heated during the day, becoming a bit more mobile as vapour, and tends to condense, or at least concentrate in the sand under the rock where it is cooler.  I am not sure if you would get enough to drink, but apparently the plant is able to take advantage of any higher moisture content.  Later in the day, the rock temperature catches up, but the plant has already taken what it can.  Overnight the whole process starts again.  Eventually the plant is big enough that it's own leaves are able to contribute to shading the roots, particularly if it is one of many.

It is difficult to believe it would always have enough effect in all circumstances, as you say the desert gets pretty dry.  But perhaps it does make enough difference in borderline circumstances.  Or it may be more effective than I might think.  And perhaps near the edge of vegetation surrounding a desert, the effect is enough to extend the viable growing area, and so gradually beat back the desert.

But at the end of the day, the principal is the same, molecular motion slows at lower temperature, so vapour is less able to escape from cooler areas, resulting in a slightly higher concentration.  Depending on the concentration reached, water will condense, or perhaps at least concentrate enough for the plant roots to take advantage.  Even the condition of water in soil may be a factor.  If it attaches in some loose chemical way to the molecules in sand, as it does in many compounds, it may not need much increase in concentration to become useful to a thirsty plant.  And may not behave as a simple binary liquid-vapour system.  Perhaps we need a botanist on the forum!  But we are getting away from engines.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 03, 2018, 01:52:34 AM
Hi MJM , I was reading my book and found these pages about Solar Chimneys I have not seen this before and it docent seem very efficient to just get 100 KW.  it would be interesting to find out how many watts you get for the square foot !!

so More calculations i'm afraid !!!

willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 03, 2018, 09:51:49 AM
Hi Willy, that's an interesting project, but a very high chimney.  I have been up one of 100 meters though much smaller diameter.  But 200 m!  You would have a good view from up there.  Not for cyclone or earthquake areas though.

The sun heats the air, which lowers the density of the air.  The hot air rises just like a boiler chimney, or a hot air balloon.  Amazing how much power is generated that way.  Solar panels are rated based on 1000 W/sq.m.  But of course the peak is only reached (or even exceeded in the desert or tropical areas) only for the few peak hours of the day.  The performance they quote as an average is much more important in terms of the total quantity of power generated.  And of course, they really need to be accompanied by storage system so the power generated can be dispatched at the time it is required.  Battery banks for short term issues or pumped hydro system for really large systems.

Driving a turbine with the chimney draft is a great idea, as all the moving parts are accessible from ground level for maintenance.

Other systems being tried around the world use reflectors on a ground level array to concentrate the energy on a heat exchanger to melt salts which in turn are used to generate steam.  The molten salts are also stored in well insulated pressure vessels to spread the time of generation. 

We will get there eventually, probably a mixture of different technologies, to make the most of natural resources such as sun light, wind, tide and underground thermal, in different locations and to get best efficiency in different locations.  And we have to upgrade the grid to cope with many generators spread far apart, with power flowing different directions at different times, rather than most existing grid designs which assume one way distribution from a large central power station.

It all makes for interesting reading and research.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 14, 2018, 11:00:49 PM
Hi MJM, a quick question ...In a steam engine we burn fuel in a boiler to generate steam to drive the engine to create power to do work .......... this fuel can be wood coal gas etc etc ... So in the human body we eat vegetation in the form of veg and this enables the body to function and work also keeping us warm and cool,  So how does the human body change the fuel/food into a functioning  work producing "engine"   ?? This may be more medical than technical, and is it still a thermodynamic Process ??

Willy
Title: Re: Talking Thermodynamics
Post by: crueby on November 14, 2018, 11:15:05 PM
Hi MJM, a quick question ...In a steam engine we burn fuel in a boiler to generate steam to drive the engine to create power to do work .......... this fuel can be wood coal gas etc etc ... So in the human body we eat vegetation in the form of veg and this enables the body to function and work also keeping us warm and cool,  So how does the human body change the fuel/food into a functioning  work producing "engine"   ?? This may be more medical than technical, and is it still a thermodynamic Process ??

Willy
And please, no tech details or pics of your 'boilers' blow-down valve!

Sorry, couldn't resist that one...   :LittleDevil:
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 14, 2018, 11:23:04 PM
Hi Chris , ok i promise !! :cartwheel: :Lol:  ,  if you promise !!
Title: Re: Talking Thermodynamics
Post by: crueby on November 14, 2018, 11:25:00 PM
Hi Chris , ok i promise !! :cartwheel: :Lol:  ,  if you promise !!
Deal!
Hmm, should make Zee agree too....
Title: Re: Talking Thermodynamics
Post by: Zephyrin on November 15, 2018, 08:20:56 AM
Quote
So how does the human body change the fuel/food into a functioning  work producing "engine"   
We are plain combustion engine, like any living organism, we use oxygen to burn carbon compounds (sugars, fat...) and produce carbon dioxide at the end in all the cells of our organism. The respiration brings oxygen to the blood that distribute it in all the cells of the body, removing CO2 in the same time. These combustion processes occur at moderate temperature and in aqueous medium through a long chain of enzymatic reactions, the oxidative metabolism; but they are combustion in the principle.
Energy is stored into the cells through ionic gradients and energy rich chemical bonds, used by the molecular processes underlying life, muscle contraction, conduction of the nervous influx etc...
The yield of these reactions is far better than a steam engine, with billions years of tuning.

Only green plants are able to use light energy directly to incorporate carbon dioxide from the air into organic compounds, in addition to the above processes.
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 15, 2018, 09:52:16 AM
Well said Zephyrin, I can’t add anything to that.  Chemical plants and biological organisms and plants all rely on the same thermodynamics and the other laws of physics to govern their particular variations of basic chemical reactions.

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 16, 2018, 12:07:55 AM
Hi Zephyrin and MJM thanks for the reply..most informative...Next question about evaporation  In hot countries it is possible to keep milk cold by evaporation by placing a piece of muslin over the bottle and having it soaking up water from a dish. however if everything gets to ambient temperature say in a closed room does this still happen ?? or does there have to be external forces at work  ? ie  sunlight ,or a draft for something?? also are there definite parameters to follow to make this work properly and efficiently
Willy.
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 16, 2018, 11:18:31 AM
Hi Willy, in this country, we call it a Coolgardie safe, though that term tends to be used for a whole food cupboard covered in hessian, and kept wet, rather than just a bottle.  The heat absorbed by the evaporation of the water keeps the whole safe closer to the dew point temperature than the dry air temperature and is low enough in low humidity climates to be useful in storing food if refrigeration is not available.  It also keeps the flies off the food.

But like those evaporative air conditioners we talked about previously, the system depends on evaporation of water, so relies upon a continuous supply of fresh low humidity air to carry away the water vapour.  If there is insufficient air flow around the outside of the device, the humidity rises to the point where there is insufficient evaporation for the thing to be effective.  It won’t work for very long in a closed room.

The name is historical, sorry the story eludes me for the moment.  The water bag on the front of the car works on a similar principal.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 16, 2018, 04:37:58 PM
Hi MJM , i sort of came to the same conclusion but wanted to make sure with the maths !!  So do equations with thermodynamics also have a bit about wind speed etc etc ??
Willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on November 17, 2018, 12:16:57 AM
'The water bag on the front of the car works on a similar principal'.............

Goodness Willy...that brings back memories  :old: [maybe 65 years ago] driving from Sydney to Melbourne in an FJ Holden....complete with caravan & twin canvas water bags attached :killcomputer:  to the front bumper either side of the yellow fog light :o

Every now & then, Dad would stop & boil the Billy with water from a bag & with a metho burner stove  for the cup of tea.......more steam involved

From memory...these events only occurred just after a Township where Mum would purchase a little 1/4 pt bottle of fresh milk at a corner store......garages only sold petrol

Derek

Title: Re: Talking Thermodynamics
Post by: MJM460 on November 17, 2018, 10:33:11 AM
Hi Willy, understanding all the things going on with that evaporative cooling involves fluid dynamics as well as thermodynamics and heat transfer all acting together.  The complete calculation may be possible these days with computers running computational fluid dynamics or CFD.  I was taught the equations, though the lesson faded almost as quickly as the sound of the lecturers voice.  Computers able to solve the equations did not exist at the time. 

Without the full computing facility, the problem is approached by assuming the conditions are constant, so ignoring the effect of the increasing air humidity as the evaporation proceeds.  This is near enough if there is a reasonable air flow, so the air is continually being replaced by low humidity air.  I am not even sure how to calculate the the rate at which the water will evaporate for any given air humidity.  As with many such complex problems, the simplest approach is experiment.  Unfortunately I can’t shed much more light on it than that.

Hi Derek, wow! an FJ, probably a new one at that time? A fog light and two water bags, we only had one bag on a second hand old Ford and no fog lamp.  We used the water bag to provide a cool drink on long hot trips.  That water always seemed particularly good.  But it does bring back memories for both of us, and I am sure many others.  I am glad to see that you are still looking in.

Thanks to everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on November 19, 2018, 03:57:04 PM
Hi, MJM  Something festive going on here in the frozen north !! Saw this cool vid of crystals growing in a bubble  !! Some complicated thermodynamics going on here ??

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on November 20, 2018, 10:32:55 AM
Hi Willy, very cool, in every sense of the word. 

I have never seen such a thing, but I have very limited exposure to cold enough conditions, apart from a bit over three years living and working in Canada, and a some time skiing.  However, providing it is not a photoshop job, I suggest it is another example of unsteady state heat transfer phenomena. 

I would expect that the bubble was initially formed from humid air, then the ambient temperature subsequently reduced.  So, what would we expect could be happening, based on our understanding of thermodynamics?  Is it plausible?

First assuming the crystals are ice, some water is freezing to make the crystals, but the bubble looks like it is still liquid, so perhaps it has some soap liquid to make the bubble more long lasting.  That may have lowered its freezing point, so it can remain liquid while the water inside freezes.  However, I am a little uncomfortable with this, as it would be difficult to make sure the soap did not mix with the water inside. 

Or, is freezing a process which causes some separation of a soap/water mixture?  But why, in that case do the crystals form inside and not outside?  There may be an explanation in the difference in humidity (or water vapour partial pressure) inside the bubble compared with outside. 

In some conditions you get snow, and others you get solid ice, and it depends on the path through that vague area at the bottom of the water phase diagram, the solid/liquid boundary.  An area where I am not very familiar, but well understood by those operating snow making machines in the worlds ski fields.  Perhaps others from those colder climates can add something clearer.

I do know the ice can form quite quickly in the right conditions.  I remember serving a cool drink from the cooler in the back of the car to my Boy Scout Troupe on a winter camp experience.  It turned to a slushy in their cups.  (It was around -20F from memory.)  Turned out the “cooler” was actually keeping the drinks warm.  Only an Aussie would not see a problem with serving up cordial after a job well done.  They had just finished pitching their tents ready for the night.  Just as well I had my Newfie associate to make sure I didn’t overlook anything important.  All survived safely.

Anyway a cool video, one that would take some detailed knowledge of thermodynamics to deliberately produce the right conditions, or the simple answer, a lucky coincidence for the camera operator.  Or perhaps everyone who celebrates Christmas in cold climates sees it all the time.  I would like to hear from others to help us understand it more clearly.

Thanks for looking in.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 10, 2019, 02:31:37 AM
Hi MJM,  A prosperous new year to you and I seem to be slowing down somewhat at the moment  I have been reading an 1892 book about electricity and there is a few pages on Thermocouples !! It suggests that Antimony and Bismuth are the best combinations  and I wonder how, in the last 120 years have we progressed further ? also we don't actually here much about thermopiles/couples apart from measuring devices.?  I can remember reading back in the sixties a chap in India listening to his transistor radio whilst smoking a Hookar with the device attached to it ??  Are there any actual electricity producing devices out there driven by heat. ??!!
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 10, 2019, 08:20:28 AM
Hi Willy, A very happy New Year to you as well.  I was suspicious that you had not run out of questions.  Your work on the Easton and Anderson model is more than enough diversion though.  I am an avid follower, and always learning new techniques.

I would suggest there have been considerable advances in use of thermocouple effects.  Apart from the electronics to measure the voltage and display the temperature, and account for the necessary reference junction, many different metal combinations are used.  The “best” in any application depends on temperature range, cost, need for calibration and so on.  The common K type thermocouple uses a chrome alumel junction, and there are standard tables of output voltage with temperature.  (I don’t know what “alumel” is, some sort of nickel alloy, I believe.)  And then semiconductors are also being used, I assume in thermopile applications where many junctions are connected in series in a compact array.

You will find more information if you look up Peltier effect, or Seebeck effect, but even “thermocouple” will yield a wealth of information on the thermoelectric effect.  Possibly way too much!

The effect is used for cooling of industrial instrumentation in desert areas and for cooling computer chips, and there are also thermoelectric heaters and coolers available, even for use as a fridge in your car.  The drink heater/cooler devices are available in stores here.  They use many thermocouples in series to increase the power/heating/cooling output to a useful level.  I believe that the simple reversing the supply polarity changes it from a heater to a cooler.  Not very efficient in terms of current consumption but easy to control, and reliable, due at least in part to being solid state, no moving parts.

The process involves increase of entropy, but I won’t even try to explain that, or even suggest that I could.  But it is unlike many thermodynamic processes in that it is fully reversible, and you can produce current from a temperature difference or produce a temperature difference by supplying the current.

There are reports of watches powered by body heat with this effect, and battery-less radios for remote areas and emergency use.  The most spectacular example of use of the effect to produce electricity is in space craft.  You may have wondered how they use radioactive fuel sources to produce electricity.  I understand that the heat from the radioactive decay is used to heat the hot junction and the cold of outer space for the cold junction is used for a Peltier effect power generator which continues to work for many years long after space craft have travelled out of range of the sun’s heat for PV power generation.

So when you read your historical text, that is the direction it is all heading.  A tribute to the early work of Seebeck, Peltier, Thompson and others.

MJM460
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on January 10, 2019, 11:57:31 AM
Some of these are also used to create electricity today in areas where it do not matter that the hot side stays hot and you have a cooler side too.

Another "funny" use is a fan for a stove in the living room. You place it on the stove / oven and this is the hot side of the element and the fan that get is electricity from the element, blows air over the cold side + the stove, so it distributes the heat in the room.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 11, 2019, 03:19:04 AM
Hi MJM,  thanks for the info.......

willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 14, 2019, 08:39:36 AM
Hi Admiral DK, thanks for joining in once again.  It’s interesting to see the diverse uses for these devices arising once they become available. 

Hi Willy always glad to have a reason to talk about the science and physics that keeps things turning.  There has been a lot of progress since those early experimenters discovered strange effects at the junction of dissimilar metals.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 10, 2019, 07:42:48 PM
Hi MJM, just a quick digestion question from my hospital  bed. i missed a hot meal so they gave me some frozen sandwiches, They were sealed in plastic bags and i was wondering if they would get up to ambient quicker if they were removed from the bags ??? getting better slowly....

Willy
Title: Re: Talking Thermodynamics
Post by: crueby on February 10, 2019, 09:56:23 PM
Hope the date on those was 2019, not 1919....

You must be doing a bit better to be asking thermodynamic questions! Keep that up!
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 11, 2019, 12:28:55 AM
Hi Willy, good to see you thinking about food and thermodynamics again, you must be on the mend.

As usual the question is a bit more complicated than it looks.  The plastic film makes a very thin insulating layer, and adds film coefficients for each side of the plastic.  Then, the air inside the wrap is somewhat restricted for convection.  I assume they are not vacuum packed.

The rough surface of the exposed bread has more surface area than the smooth plastic film, so that would help if unwrapped.  But the generally dry hospital air might make the sandwiches dry out while warming, so taking more time might not be the worst.  If they are like hospital sandwiches here, they don’t need drying out!

All in all, I wouldn’t want to predict the outcome on theoretical grounds, easier to determine by experiment.  You have the time, so perhaps one of your visitors could bring in your thermocouple probe and meter, and you could conduct the experiment, including the taste testing.  I am sure the nurses would let you use a sterile wipe to clean the probe before using it for a food application.  They would probably prefer that to your having someone bring in some saws, files and material for the next bit of your engine.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 11, 2019, 06:43:16 PM
thanks MJM, yes there are a few variables as taste and texture come into it....next time i come into hospital that will be first on the list !!!!   except they wright down everything you bring !!!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 19, 2019, 08:25:00 PM
Hi MJM, I was thinking about a telly programme that i saw 20-25 years ago that was about heating water by using the vibrating water hammer effect.  somebody discovered that the pipes got warm when a water hammer started up. the programme then went on to show in the USA a fire station using electric motor induced water hammers to produce all their hot water, ...Was i dreaming or hallucinating ?and did anything become of it ??   would love to know about the physics behind this !!! and did it come to anything ?

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 20, 2019, 11:37:04 AM
Hi Willy, it sounds like you are feeling more like your normal self.  Keep it up.  That pneumonia can knock you around quite a bit for such a tiny bug.

Water hammer basically comes from the energy that has to be dissipated when you quickly stop a moving mass of some basically no compressible fluid.  You will remember that a moving mass has momentum, or even zero momentum if it is at rest, and it takes energy to change that momentum.

You may not be so familiar with the basic fact that to change the magnitude of momentum requires a force, and the magnitude of the force is calculated from the change of momentum per unit time.  From this calculation it is easy to see that if you quickly stop a moving body you get a large force.  The force necessary to stop a moving car in a very short time is enough to severely damage the car.  If you are unfortunate enough to be in that car, the seat belts have to apply enough force to stop you before you hit the windscreen, and that force is enough to damage you, but if the seat belt stretches just a little, it can double the time it takes to stop you and literally cushion the blow.  But the windscreen stops you very quickly, with the inevitable result.

Similarly if you have a pipe line of water moving at some velocity, and you suddenly stop the flow with a valve or what ever, the momentum of that column of water has to be changed by the valve plate, and as water is nearly not compressible, the whole column stops very quickly, requiring a large force, and that is the cause of the familiar water hammer description of the noise.   The energy of that moving column has to be absorbed, and is eventually dissipated as heat.

The water hammer that I have come across can be quite damaging, so the usual effort is put into preventing it by accumulators with a gas cushion that slow the rate of change, so reducing the maximum force, and preventing the damage.  The sudden compression of that gas cushion will cause it to get hot.  However, just how you would set up a system to continually produce water hammer and absorb the energy directly into heat, I am not sure.  Nor am I sure if this could be done with great efficiency.  I suspect it must involve gravity, as the efficiency of fuel powered devices to keep accelerating the column would likely make the system uneconomical.  I have not come across any such system, but the energy is there for those ingenious enough to work out how to harness it.

Perhaps in a district water supply system, there is a point where an air cushion accumulator experiences frequent enough water pressure surges due the velocity changes as consumption varies through the day, to produce useful heat. 

Perhaps someone else had heard of it and can add something to the discussion.

But we really want to know how to use that energy in an engine to do useful work before it is eventually dissipated as heat.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 21, 2019, 12:31:05 AM
Hi MJM, thanks for that and i'm sure i saw this programme on the TV though...been trying to look it up on the web,...but to no avail....

willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 21, 2019, 10:02:12 PM
Hi Willy, you probably did see it.  It would be interesting to know a bit more about it.

These questions tend to keep the sub conscious ticking over, and the more I think about it, I think gravity has to be involved, along with some consumption pattern that makes it possible to set up a system.  I was thinking of a resonant frequency that made it simple to keep the hammer pulses going, but, while that increases the amplitude, it does not increase the energy available, so is easily damped down if you start absorbing some of the energy as heat or in any other manner.   So the basic requirement is a continuing stream of pulses without expensive energy input.  That could potentially occur in a gravity system.

Resonance requires regular pulses, but resonance is not necessary if there is a continuing stream of pulses, or sudden flow changes, but my imagination has not come up with anything yet.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 27, 2019, 12:50:45 AM
Hi MJM. Just read your post... I missed it last week ,sorry about that.  Would it be possible to type in various significant phrases that might bring something up on the web ??.  still not feeling too hot yet just taking it easy for now.. Thanks

Wily
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 27, 2019, 08:37:00 AM
Hi Willy,

You could try “harnessing water hammer”, or “energy from water hammer”.  Such searches are not my strong point, perhaps someone more practiced will chime in with some suggestions.

One example I have thought of is a water pumping device sometimes used on farms with a running stream.  It basically regularly stops the flow suddenly, and the pressure surge due to the water hammer pushes water through a check valve and actually pumps some water above the stream level into a tank without using a pump.  They are a bit hard to understand, and I can’t remember how the cycle is initiated and maintained, but it might turn up in your search.

Hope you are feeling much better,

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 26, 2019, 02:17:59 AM
Hi MJM,  I have been watching a video about model aeroplane glow plug engines. I notice that to start them a battery is used to cause initial detonation, however when the battery is removed the engine still operates !! i was wondering about the thermodynamics that enables this to happen ...and how big an engine could one build using this technology ??  I am still not 100% and am having to catch up with the allotment and things.

Willy

Title: Re: Talking Thermodynamics
Post by: 10KPete on March 26, 2019, 02:20:17 AM
The little wire coil inside is made hot by the battery. Once the engine starts, the combustion heat keeps the coil glowing.

Pete
Title: Re: Talking Thermodynamics
Post by: simplyloco on March 26, 2019, 09:32:35 AM
Hi Willy,

You could try “harnessing water hammer”, or “energy from water hammer”.  Such searches are not my strong point, perhaps someone more practiced will chime in with some suggestions.

SNIP

MJM460

We used these -not very often!- in my combat engineering days. Very wasteful but they work!
John

https://www.youtube.com/watch?v=4eJLEh5--1w
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 26, 2019, 11:01:16 AM
Hi Pete, thanks for looking in and explaining that.

Hi Willy, sorry to hear that you are not fully back to your previous self, but it is good that you are at least making some progress.  Time on the allotment might be the medicine you need, along with some sunshine.

Basically the heat balance is slightly different when starting a cold engine that it is for a hot engine.  When those glow plug engines are cold there is not enough heat of compression to get to ignition point.  Once they are running, as Pete says there is excess heat available and the compression ratio is enough.  Really the thermodynamics is the same in each case.  The little compression ignition or Diesel engines have a higher compression ratio so they can start on compression heat alone from cold.

On larger size engines, many little marine Diesel engines are fitted with glow plugs and operating these before attempting to start the engine makes them start a lot easier in my experience.  Similarly, my car is a diesel, and I notice the start sequence involves running glow plugs for a short  period before it starts.  All computer controlled of course.  I believe a similar principle is used even for quite large engines.  But these are really beyond my experience.  Perhaps some of the marine engineers on the forum will come in and tell us all more about them.  I think the same might apply to Diesel engines in construction machinery. 

The differences between all these systems basically revolves around how much heat is required to get those first few pops each start.

Hi simplyloco, thanks for looking in.  Yes, those are the pumps used on farms that I was referring to in the earlier post.  Thanks for looking that out and posting. 

Wasteful of course is not a strict technical term.  The second law of thermodynamics means that you cannot use all the energy in the water to pump it all back to the height at which it started, some of that energy is “lost”.  However the water is not lost, that part not pumped continues it’s merry way down hill, and is appreciated by those downstream, and no matter how well they are set up some always continues.  I believe that if well set up, there is not the obvious spillage evident in that video, though I never had much to do with them.  The limitations not only mean the water cannot all be returned to the original height, you probably can’t pump it all to any height using the ram pump, though you can move it all up some of the way simply by containing it in a pipe.  The difference with the ram is that some water must be passed through to establish the velocity that the device suddenly stops so the change of momentum develops the pressure.  It is using the kinetic energy, and can develop enough pressure to raise some of the water above the initial height, thus converting kinetic energy to potential energy, and of course losing some in the process.  Each time the check valve closes, the flow stops, and has to be re-established by a bypass flow after the pressure pulse.  I just can’t remember what makes the check valve flip closed.

A simple pipe down the hill then up the other side is simply using the potential energy of the height, and can raise the water surface up to the original height under conditions of no flow, and all the flow to some smaller height, the difference in height being energy lost in overcoming friction in the pipe.  Obviously you would normally just use the pipe if you have access to the water at enough height, and accept the extra complexity of the ram if you need more pressure that resulting from the height at the point you have access to the water.

In both cases, the energy “loss” is not really lost, it simply is dissipated as heat, and can always be accounted for if you look hard enough.  And it may even be useable to some extent if you have a use for low temperature heat.  Though recovering the heat from friction in the pipe would be a challenge.

Now we just need more information on Willy’s heating system using water hammer.

Good to see the odd question coming through again.

MJM460

Title: Re: Talking Thermodynamics
Post by: Admiral_dk on March 26, 2019, 08:01:52 PM
Quote
a battery is used to cause initial detonation

Are you talking about War equipment or engines Willy  ;)

Hopefully you only get a combustion  ;D

OK, joke aside, the coil 10Kpete mentioned, is made from Platinum (and a few other ingredients) and is a catalyst that "combines Methanol and Oxygen" and while it works much better at high temperature (initiated by the battery), there has been cases where engines has started without being connected to a battery  :zap:
The heat from combustion will keep it working as long as there is fuel and oxygen in the right amounts.
I haven't heard of a limit to going up in size, except for the extreme cost of running your car on that stuff + a complete "overhaul" everytime you shut it of again - hygroscopic effect will ruin the engine otherwise (don't ask how I know). Another reason, is that an ignition system where you can change the timing at will on the fly is much more useful on bigger engines where every single gram doesn't count as much.

I do hope that you soon get better Willy - best wishes

Per
Title: Re: Talking Thermodynamics
Post by: derekwarner on March 27, 2019, 03:27:51 AM
Trust you are progressing Willy...keep up that  :DrinkPint: [medecine] ......

We touched briefly on accumulators and water hammer, however the world of fluid engineering uses hydraulic accumulators for thousands of applications from a convenient precharged storage of energy in weapons missile launchers, to impact adsorbing solutions in the landing gear of jet planes to taking simple pressure harmonics from a fluid thats energy was created via any rotational device [multi element piston pump]

To take this one stage further, a single acting boiler feed make up water pump, using the humble o-ring principal loads the o-ring in a dynamic manner and deforms the soft elastomer to effect the seal in the pump cylinder to reach the boiler relief valve setting......and so yes, we are talking low pressure of say 3 Bar

One factor not well understood is the effect of the collapsing pressure field that sealing elements are subjected to

So our simple rotary pump at 120 pulsations per minute has the o-ring being pressurized every 2 seconds to 3 Bar, then re-subjected to the collapsing pressure field to the partial draw of vacuum as the pump check ball closes [all within milli-seconds frequency]

This is similar to hitting your own head with a hammer each 2 seconds :hammerbash: and expect to walk away without a headache

So the wonderful model steam people from REGNER in Germany have recognized this and produced a miniature model accumulator suitable for direct mounting on the discharge side of a piston pump and suitable for low pressure water applications

The feature here is the accumulator is not filled with a gas precharge, but a Nitrile elastomer ball, which by nature is deformable due to the unique cellular formation of the Nitrile ball itself

So the net total consequence here is that the velocity of the collapsing pressure field is minimized by the discharge of the energy stored within the accumulator........this in theory will save the Nitrite elastomer o-ring from premature failure

I plan to install one such accumulator in the very near future, however am not sure the conventional Pressure Gauges UK model gauge will record & display anything but a flutter each 2 seconds

Derek

Title: Re: Talking Thermodynamics
Post by: MJM460 on March 27, 2019, 11:34:31 AM
Hi Admiral DK, thanks for posting about the catalytic function of the glow plug in those little engines. I for one did not know about that.  I guess while the basic intent of all glow plugs is to provide a little heat to aid in starting a cold engine, tweaks such as catalytic action will greatly enhance the function. 

Do you know if the glow plugs in full size Diesel engines and automotive diesels also have a catalytic component for diesel fuel, or are they simple heaters?

Hi Derek, you are quite right about the great variety of applications for hydraulic accumulators, but in most cases, these are intended to minimise any water hammer induced vibration or damage, with no intention of recovering any energy.  Willy’s original question was about a (fire station?) heating system which used the energy causing the water hammer for building heating, so a slightly different emphasis.  Hence on to ram pumps which use that kinetic energy for pumping some of the water.

Those model accumulators are certainly interesting, the nitrile ball will not require the periodic recharge that is the main issue with most gas cushioned systems.  It will be interesting to see how it performs in your model, and how much effect it has on the pressure gauge.  However, I am not sure if it will help the piston o-ring, as I assume the pump discharge check is in between and would isolate the ring from the accumulator.  And of course the pressure reduction in the cylinder is necessary to the pump function as there will be no intake of water for the next stroke until the cylinder pressure is below the pressure of the water at the inlet.  Worth experimenting with.

I have come across explosive decompression of o-rings when they are depressured even what seems quite slowly, and certainly not requiring frequent decompression.  Very special o-ring materials are required in these services.  But I was dealing with natural gas at 5000 psi!  I am sure it occurs at somewhat lower pressure as well, but I am not sure at what lower pressure it ceases to be a problem.

I wonder if the main reason for o-ring failure when they are used as piston rings is simply wear and tear due to wall friction.  I always understood that o-rings were basically static seals, meant to deform but not ideal for sliding.  I know they are used and with success.  And the catalogues even mention specific o-ring materials for this type of service.    I have at times wondered if they simply wear to fit at a close clearance in higher speed applications, some times called an ablative seal, though I expect they slide quite nicely in slower speed, well lubricated applications.

Thanks for looking in,

MJM460





Title: Re: Talking Thermodynamics
Post by: Admiral_dk on March 27, 2019, 10:25:35 PM
Quote
Do you know if the glow plugs in full size Diesel engines and automotive diesels also have a catalytic component for diesel fuel, or are they simple heaters?

I would be very surprised if they are anything other than "just a heater", but I honestly don't know for sure.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 28, 2019, 02:02:54 AM
Hi MJM  et al , more interesting stuff to think about........... ;D  Just a quick observation when i was cooking my tea    When i lit the gas stove   (Natural gas) I get the nice blue flame....however i decided to use my hoover in the kitchen with the effect on the gas flame to become flickering and orange ?? I don't know why this should happen unless microscopic particles are getting past the filter and interfering with the oxygen levels ?? The before and after  photos  !!!

Willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on March 28, 2019, 05:10:56 AM
Willy....[I hope not too much  :DrinkPint: medecine]........certainly no gas accumulators involved.....I also have a natural gas cooking stove in the kitchen, however couldn't replicate your flame flicker variance with a large hand guided new fangeled design bagless Dyson vacuum cleaner over the kitchen tiles

The exhaust air from the Dyson is advertised as being passed through a series of filter cone jets that increase the velocity of the air, then a swirling action prior to two filter banks....so I suspect  the exhaust air is presented in a different manner [direction & cushion] to your Hoover
machine

So from this, I do understand the significance of the blue flame and also that the orange flicker is usually an indication of the carburizing flame due to lack of oxygen however suspect the volume of air & hence oxygen % is remaining virtually constant, and so the surrounding air to your gas oven top cooking jet is being subjected to turbulence :Mad: & causing the flicker from blue to orange flame? ..

Lest wait & see what MJM thinks........

_____________________________
Sealing elements in Fluid Systems

This is a huge subject in itself  :happyreader: however not really centered around Talking Thermodynamics....suffice to say that o-rings are the least costly dynamic sealing elements available and as such Industry makes great use of their ability/cost in a whole range of applications

Derek 
Title: Re: Talking Thermodynamics
Post by: Jo on March 28, 2019, 07:59:59 AM
So from this, I do understand the significance of the blue flame and also that the orange flicker is usually an indication of the carburizing flame due to lack of oxygen however suspect the volume of air & hence oxygen % is remaining virtually constant, and so the surrounding air to your gas oven top cooking jet is being subjected to turbulence :Mad: & causing the flicker from blue to orange flame? ..

At home I cook on bottled propane gas. Normally the flame is blue but just as the bottle empties and the auto switch over connects to another bottle of gas the flames flicker and go orange due to the pressure drop (there is also a slight smell of propane at the time ::) ). From experience I would question if Willy's gas also suffered from a slight pressure drop..

Jo
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 28, 2019, 12:42:25 PM
Hi Willy, that one is a bit of a puzzle.  The vacuum cleaner does not actually consume any oxygen, well only at the power station producing the electric power perhaps, so not likely to be lower oxygen content of the air in your kitchen.  Assuming this happens every time you run the vacuum cleaner while cooking, and with your natural gas it would be unlikely that a change of gas composition would arrive by coincidence each time you clean.  Does it happen every time or just occasionally?

Your suggestion of fine dust blown out of the vacuum cleaner is certainly a possibility.  There is a whole branch of chemical analysis based on flame colour changes when various substances are passed through a flame.  So silica in dust, or other minerals would cause various colours, or carbon based material might arrive at the flame and glow yellow before finding enough oxygen, could be a reasonable explanation.  But if there is really that much dust kicked up by the cleaner, and considering your recent health history, it might be worth wearing a dust mask while vacuuming, as that dust will also be entering your lungs.

Similarly, as Derek points out, the exhaust from the cleaner could be producing turbulence which conceivably could affect the amount of air drawn into the burner, but a bit of a long shot unless the vacuum cleaner outlet is quite close to the stove.  Perhaps a little more likely if the stove is on a very low setting.

We have an electrostatic air cleaner, and Inhave noticed that there is no extra sparking even when the cleaner is quite close to the air inlet, but our vacuum cleaner also has a fine filter on the air outlet.  It seems to be quite effective.  But we also only have an electric stove in the house, so no hope of observing that here.  Certainly an interesting observation.

Hi Jo, thanks for posting.  That auto changeover sounds like a good idea, nothing worse than running out of gas just after starting to cook a meal in the oven and not noticing.  But I hope you have something that tells you it has switched over so you don’t just empty the second bottle.  As you suggest, the slight pressure drop during the changeover probably means a moment or two of less efficient air induction at the burner.

Derek, I agree totally with your comment on the number of applications of o-rings as sealing elements, even in sliding applications.  The ones I am most familiar with are in mechanical seals for pumps and compressors, where they only deform with no significant sliding, but there are also many applications where sliding seals are performed by o-rings.  And the manufacturers catalogues do recommend the correct formulations for these applications.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 31, 2019, 02:24:42 AM
Hi MJM, thanks for that... I was annealing a piece of brass in the blue flame  and the flame above was going yellow of the same hue as before ? !!! a new question... I was looking at this picture of a boiler burner and it shows the take off pipe of the parrafin canister coming from the top . It also shows a bike pump connector, I suppose a bit like a primus stove. So, what is actually happening in the tank when it is pressurised with air to send a combustable gas to the burner ?? the liquid paraffin is quite heavy and one would think the air should just stay above it ??  Perhaps this is another unintuitive process !!!
Title: Re: Talking Thermodynamics
Post by: Jo on March 31, 2019, 08:47:38 AM
Hi Jo, thanks for posting.  That auto changeover sounds like a good idea, nothing worse than running out of gas just after starting to cook a meal in the oven and not noticing.  But I hope you have something that tells you it has switched over so you don’t just empty the second bottle. 

This is the changeover regulator. When it switches over bottle the internal valve rotates and the arrow in the white handle turns red showing it is pointing at an empty bottle. When you replace the bottle you have to remember to point the arrow at the bottle that was being used  ;) I have a couple of spare full propane bottles that I use for silver soldering kicking around if I forget  ::)

Jo
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 31, 2019, 11:07:26 AM
Hi Willy, Just by coincidence, our household gas heater was being serviced Friday.  The serviceman gave a good blow to move some dust which was difficult to get at with a brush, and the flame took on that yellow appearance for a few moments.  So a strong air jet could explain your stove, but I doubt that your vacuum cleaner was sitting that close to the burner.  So more likely your guess of impurities in the air.  I have not tried brass in the flame but copper usually gives a green tinge in the same manner, but carbon based dust particles, or possibly silica in sandy dust might be the explanation as we discussed before.

I think you will find that the burner operation is easily explained by assuming that the catalogue picture is not a complete engineering drawing.  I am reasonably confident that inside that tank the outlet pipe is extended to the bottom of the tank.

The burner usually has an evaporating coil which picks up the necessary heat to evaporate the liquid.  The control valve then controls the vapour flow to the air mixing and combustion area.

First the tank is filled, then the pump is used to add some air so there is enough fuel pressure to push the liquid over to the burner and to push the vapour through the jet.  You usually have a little tray which you fill with meths and light to evaporate the first fuel until there is enough heat from the burner flame.

Initially you generally fill the tank perhaps half full, and pressurise the remainder with air.  As the fuel is used, the level drops so lowering the air pressure, and to run for a long time, you need to pump in a bit more air to maintain the pressure.  However, some heat conduction along the pipe probably helps a little to raise the vapour pressure of the fuel.

I remember using a primus stove on that principle for cooking when camping, and I also had a Coleman Shellite lamp (it might still be in the cupboard).  And my father had two blow lamps for heating one of those large copper soldering irons for roof spouting work.  Unfortunately they disappeared long ago, before I realised that they would have been very useful for extra heat for silver soldering.  I have even been in country buildings where lights on this principle were the only source of lighting before electricity was connected.  They continued in use in remote areas long after electricity was connected in towns.

I assume the pipe arrangement was so that once you release the pressure, the flame goes out, but also there was minimal likelihood of fuel leakage which might occur if the fuel outlet came directly from the bottom of the tank.

Hi Jo, I have not seen one of those automatic changeover valves, but it sounds like they still need a little attention to ensure the needle is pointing the right way.  And of course the really good part is that you often have a spare big bottle of gas for when a big silver soldering job comes along!

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 31, 2019, 10:44:25 PM
HI MJM, Thanks for the reply, I have found this diagram of a primus stove and yes the tube out to the burner does go down to the bottom of the vessel  so the pump pressure does push just the liquid fuel to the burner. way back in the 50's i lived in a house with no electric and we had Primus stoves and Tilley lamps...when we had guests in the evening that mother didn't want to stay too late i was told to fill the lamps  only half full so when they started to flicker due to low pressure the quests would take the hint and elect to depart !! as soon as they had gone down the road  we would pump them back up and the house would suddenly become a blaze of light again !!!Here
 is a pic........

Willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on April 01, 2019, 01:24:12 AM
Willy...MJM....I remember going gold panning to Hill End near Oberon in mid Western New South Wales with my Dad & Pa & others ...in 1954 ..so I was about 6 years old

No bridge over the Turon River...no electricity, no gas [prior to the availability of Primus gas equipment] cold swim in the river for a bath  :lolb:.......wood stove, tilly lamps [Dad had a 22 rifle to warn :Director: the Foxs away

Am pretty sure the Tilly lamp functioned on vapor pressure of the volatile liquid  [methylated spirit?]

So irrespective of the discharge point of admitted air into the lamp body, it became the pressure vessel..so pressurized air would create a vapor pressure above the top level of the fluid, which was admitted into small holes or oriface in the discharge tube

So the speed or velocity of this rising vapor pressure was enhanced by additional ventuti bringing in air [oxygen] to complete the combustion

Pretty sure, that Miners Safety Lamps work on a near similar principal....so even with today's technology [electronic methaneometers & the like], Miners Safety Lamps are still used in  underground Coal Mines

Derek [these are my words now, not the words of a 6 year old  :shrug: ]
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 01, 2019, 12:39:59 PM
Hi Willy, yes, I still have one of those Primus stoves in my garage.  And Tilley is the other name I remember.  I see Derek also knew of those.

Hi Derek, I can’t remember whether the Tilley was a petrol (shellite) fuel or kerosene.  The kerosene ones usually used meths in the little tray under the burner and evaporator cool for the initial heat up.  I think the Shellite ones had a slightly different procedure. 

I have found that I do still have the Coleman light in the cupboard, I will try and look out how it is lit, perhaps tomorrow.  It definitely uses shellite fuel.  Come to think of it, the power company has notified us that we will have no power tomorrow, so I might even need it if they don’t finish by evening!

The mining safety lights have a configuration that prevents flame spread some how, can’t remember the details.   Somebody might look up Davey safety light, or is it Davy, but it’s getting late here tonight, I was out for dinner.

Thanks to all for looking in.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 18, 2019, 02:42:39 AM
hi MJM ,  Just been thinking about flatulance.... So .... cows and us produce a gas called methane  !!! however is this gas just called methane , and does it also contain traces of butane propane and other flammable components ?? does what is produced depend on what is consumed ?? When i lived near marshes one could stir the dykes and bubbles of gas would appear so is this also a mixture of different gasses ??

Willy

Title: Re: Talking Thermodynamics
Post by: derekwarner on April 18, 2019, 03:42:58 AM
Goodness Willy, with MJM residing in Australia.....and Australia set to have our Federal Elections next [edit] in a few weeks, you may have placed MJM in a rather invidious position as one on the National Parties standing for election has a platform that cows should be banned due to their Methane flatulence  contribution to the Green House effect the World is suffering

1. If that referenced Party were to win Office, the next time one went to McDonalds .......it would be  1/4 Pounder of Kangaroo meat  :facepalm:

2. It appears that Kanga's fart's are low in Methane  :cussing:

3. Cows, or beef cattle are not a Native specie to Australia & should be dispensed with

....Courtesy of WIKI.....
....Beef Cattle in Australia. The First Fleet and On. When first settled, Australia had no native animals suitable for domestication. ... A bull, four cows ....and a bull calf of the Indian Zebu were bought at Cape Town, South Africa.
 
4. So if MJM did bravely respond, his comments could be seen as a Political Statement as which is I understand  would be contrary to the Guidelines,  :happyreader: Rules and Codes  of MEM...or lets wait & see how he treats this heat thermal transfer question  :slap:

Derek

Title: Re: Talking Thermodynamics
Post by: MJM460 on April 18, 2019, 10:49:12 AM
Well, what can I say?  Between politics, climate change and animal liberation, perhaps it would be safer to talk about religion! 

So mindful of all that, the methane from cows is a byproduct of their digestive processes.  I don’t really know enough about the chemistry, but I would be sure there is more than methane involved, including some sulphur compounds and more, but I believe methane has been identified as a significant component, and of interest due to its green house gas contribution.  I don’t know if those same digestive processes have the ability to produce longer chain hydrocarbons, or whether the breakdown of grass and the other things cows eat results in breaking off parts with more than one carbon.  That is a deeper level of chemistry than I ever studied.  I wonder if any other forum members have more knowledge in this area.

Marsh gas, or swamp gas that bubbles up from the swamp as you have observed, Willy, is also largely natural gas or methane which has been produced in the process of decomposition of plant materials in the bottom of the swamp.  Again, I am not sure what longer chain components might be formed in the process.

Both the cows digestion, and the rotting of plant matter are low pressure processes, and in general, low pressure favours products with lower molecular weight.  The formation of natural gas and oil deep underground occurs at very high pressure, generally thousands of psi, with higher temperature than near the surface, and these conditions lead to larger molecules as seen in oil and the lpg gases such as propane and butane.  So that might be a clue.   But also, favouring the heavy components does not mean only heavy components are produced, just that they are produced in much greater proportion then the lighter components.  Similarly, low pressure generally favours lighter molecules, but I would not be surprised if there was a small proportion of heavier components produced even in th elbow pressure reactions.

I hope that helps and that I will not need a helmet to shield me from the responses.

MJM460

Title: Re: Talking Thermodynamics
Post by: crueby on April 18, 2019, 12:15:51 PM
What about the methane production at one end and the hot air at the other end of a politician? Gotta be some high thermal efficiency in that production!  :Lol:
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 18, 2019, 08:31:37 PM
HI MJM, well ...that opened a can of worms .methinks.!!!.or should that be beans !! thanks for the relevant info !! and other historical stuff !!!! In a boat any leaking propane can sink into the bilges and be quite dangerous, so how high can released methane rise into the atmosphere if no wind ??  also is the carbon atom heavier than the 4 hydrogen atoms and in an inclosed space would you get a thin layer of methane with other molecules of gases above and below it ??  thinking about oil retorts here...?

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 19, 2019, 09:40:18 AM
Hi Willy, worms or beans?  Quite a mouthful either way.  Or perhaps not!

Yes, propane molecules are heavier than methane molecules, so methane tends to migrate higher while propane, butane etc tend to accumulate lower down.  Like in the bilge of a boat as you say or a hole in the ground.

But the story is a bit more complicated than that.  They do not form layers in order of density, but rather all move somewhat randomly in all directions, so they all tend to spread and occupy the whole space, largely independently of each other, so in a given volume, the concentration is similar everywhere.

However, added to that random motion is the effect of collisions between molecules.  If you consider just the vertical component of the motion, all have gravity accelerating them towards the centre of the earth, and when two collide, the lighter one bounces off at higher velocity than the heavier one.  So the effect of all those collisions is to cause the lighter ones to go upwards a bit more than down, while the heavier ones tend to go down a bit more than they go up.  So in a very high column of a mixture of gases, the concentration of lighter components will be higher at the top while the concentration of the heavier ones will be higher at the bottom.  But all will be detectable at all heights.

In the boat, butane (MW 58) and propane (MW 44) are much heavier than Oxygen (MW 32) or Nitrogen (MW 28) so will tend to accumulate low down, but there will be an air fuel mixture everywhere just waiting to be found by a spark, and the air circulation tends to be less than outside in free air, especially in the bilges, so a fuel leak can easily build up faster than it is carried away by ventilation, and it eventually reaches a flammable concentration.  And you don’t want to be on that boat when the air fuel mixture finds that spark.  I have seen it close by, fortunately a few hours after, and even more fortunately, some how no one was hurt.

In air, methane (MW 16) is lighter than oxygen and nitrogen, so tends to rise as a result of all those collisions.  The effect of gravity is taking energy from those molecules all the way up, so it gets colder and less dense as fewer molecules have enough energy to keep rising, until it is eventually lost to some other object in the solar system or more likely does more falling back so spreads no further.

Wind tends to add turbulence which mixes it all up a bit, but as turbulence tends to be random, it does not particularly add to the separation which is due to gravity added  to the turbulence.

It’s a bit of a simplified explanation,  and does not explain everything that happens, but it’s close enough  for a basic understanding.

The hydrogen and carbon atoms in each molecule are strongly bound together, so they act as a single molecule, not as separate atoms in a mixture.

And in case you think there is no energy in those collisions, the pressure on the walls of a container of gas under pressure is the result of all those collisions as molecules hit the walls.

I hope that all makes the situation a little clearer.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 19, 2019, 11:37:50 PM
Hi MJM thanks very informative,,,Another question  does it take the same amount of energy to cool things down ,as it does to heat them up ?? conservation says yes but ....... also we can heat things up very quickly but can we cool them down very quickly as well ?? we can use microwaves to heat things up but what are the alternatives with cooling ??

Sorry my questions are so short  and takes a fraction of your time for your answers !!!  Also, Does all those collisions of the molecules on the walls of the containers have a detrimental effect on the containers ??

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 22, 2019, 10:44:19 AM
Hi Willy, I’ve been off line for a couple of days, but have not forgotten your questions.  I think there are four.

If we heat something up from some temperature to a higher temperature, we do get all that heat back in cooling it back to the starting temperature.  Note that this is a different question to whether the heat required to heat something by say 100 deg, do we get the same amount back if we cool it by that same 100 degrees from the same starting temperature?  The amount of heat involved in these cases is usually different because for most materials, the specific heat is not constant but varies with temperature.  So the heat to increase a temperature from 100 to 200 deg is slightly different from the heat required to increase the temperature from 200 to 300 deg.  I hope that makes sense.

How fast we can heat or cool something depends mostly on temperature difference.  If we have a piece of steel at ambient temperature, we could use a blow torch with a flame temperature many hundreds of degrees hotter than the steel, which will heat it quite quickly.  If however we want to cool it quickly from ambient temperature, about the best we could do is to plunge it into liquid nitrogen, but the temperature difference would still only be about 200 degrees.  As the heat transfer is proportional to the temperature difference, cooling would obviously be slower.  Most refrigeration systems we are familiar with have a minimum temperature around -40 C, depending on just which refrigerant is used, so an even smaller temperature difference.

However, if we start at a much higher temperature, say a piece of steel extracted from a blacksmiths forge, cold water would have quite a high temperature difference, and boiling of the water when it hits the hot steel has a very good heat transfer coefficient.  This results in very rapid cooling.

I don’t know of any way to make something to emit microwaves in order to cause cooling.  Microwaves are waves like light and radio waves and infrared heat, so similar to cooling by radiation, which again requires cooler surroundings.

I wonder if you are thinking of processes like peening in your last question.  Many hammer blows on a metal surface cause plastic deformation at the surface which modify the material properties.  However the collisions of gas molecules with the walls of a container are much lower that than the pressure under the hammer used for peening.  So definitely no detrimental effect at any pressure we are likely to encounter.  However, for very high pressures, especially pressures that cycle through a significant range, pressure vessels do have to be designed for fatigue, as a result of the cyclic loading of the vessel walls.  And that pressure is of course the sum total of all those collisions of the gas molecules with the vessel walls, but it is a load on the whole wall thickness, not just a surface effect.  The calculation procedure is covered in a similar manner in the ASME, BS and AS codes.

However, when the gas molecules collide with the molecules of the wall of the container, there is energy exchange resulting from the collisions.  If the gas is hotter than the container wall, the wall will be heated and the gas cooled.  Similarly if the gas is cooler, it will pick up energy from the walls and the container cooled.

I hope that answers the questions.  Thanks everyone for looking in.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 23, 2019, 01:35:32 AM
Hi MJM, thanks for the info ... very informative...
Willy
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 23, 2019, 11:43:44 PM
Hi MJM, I learnt an interesting thing today at the local model engineers club meeting....They have a club loco that has not been used for some time and as one does when confronted with hand wheels you allways have to twiddle them !!! However on this loco none of them would turn and also the regulator seem to be stuck fast. !! on enquiring about this i was informed that when a locomotive has finished running and all the valves and cocks are turned off and the blowdown vale is operated, when the loco cools down the brass cocks and valves shrink slightly and then seize up. I have never heard of this before so was quite surprised at my ignorance about steam engines etc etc.  I was also told that when the boiler was refired every thing would expand and they would become free again.

Willy
Title: Re: Talking Thermodynamics
Post by: Jo on April 24, 2019, 07:01:34 AM
on enquiring about this i was informed that when a locomotive has finished running and all the valves and cocks are turned off and the blowdown vale is operated, when the loco cools down the brass cocks and valves shrink slightly and then seize up.

Which is why it is good practise to open all the valves just after you have blown down and before you put the Loco away. If they are left closed they contract hard shut not only damaging the valve seats but risking that next time, when getting it out for running, someone will try forcing them open  :toilet_claw:

Jo
Title: Re: Talking Thermodynamics
Post by: AVTUR on April 24, 2019, 09:28:24 AM
Sorry to jump in here but I have just seen steam guy willy's first question in Reply #1146 and could not resist answering it. No - Second Law of Thermodynamics.

I know, you have heard it before but in any such operation energy is lost to the outside, radiation, warming up the air due to convection, etc. Conservation of energy (First Law of Thermodynamics) still applies but the losses have to be accounted for.

AVTUR

Title: Re: Talking Thermodynamics
Post by: MJM460 on April 24, 2019, 12:41:49 PM
Hi Avtur,

Good to have you join the discussion, no need to apologise.

I am trying to follow what you are saying.  Are you by any chance referring to post no 1147 for Willy’s question?

 MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 24, 2019, 12:54:54 PM
Hi Willy, I don’t have any experience of running a locomotive, but your comment and the reply do not surprise me.  With full size piping in the oil industry, the normal experience is that when you close or open a valve, when the gate seats, you wind the wheel back to release the pressure on the thread so the nut is in the midst of the backlash.  This does not release the seal, but does make it easier to open the valve when the time comes.

I am not sure if the issue is corrosion of the thread, or thermal expansion/contraction, that jams the valve.

I think Jo has the right answer, they don’t jam if left open.  Thanks Jo.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 25, 2019, 02:22:25 AM
Hi MJM et al, thanks for the extra info...I can't believe i didn't, know this . However i have been working in isolation for most of my life , and that is what is so good about this forum and local clubs closer to home. When i first started making engines and boilers i always used brass with the copper , so there is no way they could get boiler certificates today. I have part built a 3 1/2" County Carlow that i started in 1969  with brass bushes , so that will be just a static model!!

Willy
Title: Re: Talking Thermodynamics
Post by: AVTUR on April 25, 2019, 05:49:46 PM
Good afternoon MJM

My reply was to Willy's first question concerning the amount of energy required to heat something up and then released on cooling down [I will try using quotes]

Hi MJM thanks very informative,,,Another question  does it take the same amount of energy to cool things down ,as it does to heat them up ?? conservation says yes but ....... also we can heat things up very quickly but can we cool them down very quickly as well ?? we can use microwaves to heat things up but what are the alternatives with cooling ??

Sorry my questions are so short  and takes a fraction of your time for your answers !!!  Also, Does all those collisions of the molecules on the walls of the containers have a detrimental effect on the containers ??

Willy

My reply was basically a "knee jerk" reaction by a retired aerothermal engineer.

I should add that I have seen the detrimental effects of high energy molecular collisions on containers far too many times. As MJM says, the container gets hot and if there is enough energy transported by conduction or convection (by moving gas molecules) or radiation it weakens, melts, burns and becomes useless.

I am now going to go away and read all the pages of this thread!

AVTUR

Title: Re: Talking Thermodynamics
Post by: MJM460 on April 26, 2019, 09:31:44 AM
Hi Avtur,

Great to have you on board and I am delighted that you will be reading all those pages,  I hope many of them anyway.

I am sensing that you will not find too much that is new to you.  If you see any posts that need correction or clarification please let me know.  There is so much high quality information on this forum and I dont want the theory side to let down the community.  It will be great to have another contributor to the discussion.  It can be quite difficult to cover all the aspects of a complex question in a manageable post.

As for the effect of very high energy molecules on metals, I suspect that many of the forum members would be most interested in the effect of those molecules in the energy range you have been dealing with, and the special alloys used in that field to extend the operating range and power output.  Definitely way beyond what most have to deal with on the engines we are more familiar with here, which is what I was thinking of in my reply to Willy’s question.  Perhaps you could add a little to my response.

Thanks for joining in.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 08, 2019, 01:29:45 AM
Hi MJM, I have acquired this old 1/16th HP ic engine and it is fitted with a water jacket and a cooling tower . I was wondering exactly how this works. The water level has to be higher than the top pipe connection . As heat rises why dose'nt the hot water just stay at the top of the tower ? as this does work which way does the water travel ? Does the water actually move ,as there is no pump, or is it gust the temperature traveling through the water some how...? or is this another silly prognoses !!!!
Title: Re: Talking Thermodynamics
Post by: 10KPete on May 08, 2019, 03:48:01 AM
Hot water rises pulling cooler water in from the tank. Just like a Ford Model T.

Pete
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 08, 2019, 11:33:40 AM
Hi Willy, no silly prognosis, but the common perception that “heat rises” causes more confusion than clarity.  Heat travels from higher temperature to lower temperature, it does not matter whether that is up or down, gravity has no influence on heat.

We all get taught when quite young that heat is transferred by conduction, convection and radiation.  Conduction and radiation are easily understood as independent of gravity. Convection is broadly divided into forced convection and natural convection.  In forced convection, heat transfer is in the direction of the flow whether up or down.  All the confusion comes from our observation of natural convection.  Basically when a fluid is heated, whether it is gas or liquid, it usually expands, and when a given mass expands, the density becomes less.  Then the low density fluid is displaced upwards by higher density cooler fluid which sinks downwards.  So the fluid flows under the influence of that density gradient.

There is a very interesting anomaly in the general observation that in fact we have all seen.  When water cools from 4 degrees C, instead of contracting and becoming more dense (as it does when cooling but still above 4 deg C), it expands, so get less dense.  So water cooling from 4 deg C rises and the surface water is cooler than the water deep below, and when it finally freezes at O degrees C, the ice is also less dense than the water and so floats.  Ice is not a fluid an might be expected to have a different density than the surrounding water, but the water freezes first where it is coolest, which is at the surface at the surface as we all have seen.

So in your engine, there is one place we know the pressure in the cooling system, that is at the water surface.  If you calculate the increase in pressure as you go down to the bottom of the water tower, you get a somewhat higher pressure than at the surface, the increase being proportional to the density and the vertical height.  Now you now do the calculation in the other side of the circuit, the water tower above the side connection, the copper tube, the cylinder jacket and through the bottom connection to the bottom of the water tower.  The same procedure as the first path, but this time, part of the column is heated in that cylinder jacket and so is lower density than the cooler water in the tower.  The result of that lower density is a lower pressure when you get back to the connection at the bottom of the tower.  That pressure difference drives circulation of the water, up through the cylinder jacket, through the copper tube (so increasing the temperature in the tube and reducing the density of more of that side of the circuit) and back into the top of the tower.  Then it flows down in the tower as it looses heat to the surrounding air.

The tower looses heat to the air, keeping its average temperature lower, while the heat from the cylinder heats the other side of the circuit so maintaining a flow.  This mechanism is called a thermosyphon.  It requires no pumps, just a continuous fluid circuit.  As you mention, the water level has to be above the side connection to the tower near the top. 

If the tower has extended surface area, such as fins or a complete radiator configuration, the tower will stay closer to the outside air temperature, and faster circulation.  The tower on your engine appears to have a good thick layer of paint.  That tends to insulate it just a little, so a very thin layer, just sufficient to prevent corrosion would be preferred.  So a good rub back before freshening the paint, not just another coat on top.

The water does move, it flows around the circuit, carrying and redistributing the heat as it goes. The average temperature around the circuit is something above the temperature of the surrounding air, and the temperature it was at before the engine started.  So the average density is less due to the temperature rise, and the level of the surface rises compared with the level when all is cold.  But the difference in density through the tower compared with the density down through the cylinder jacket is what drives the flow.  And of course, the heat lost by the tower increases the temperature and so lowers the density of the surrounding air.  So this warmed air tends to rise and be replaced by cooler air from the larger surroundings.  Again the rising air carries the heat, which is interpreted as heat rising.  I think that brings us back to where we started!

I hope that clears the fog a little.

Hi Pete, thanks for joining in.  It’s interesting that the radiators in cars have not increased in size as much as the engine power since the model T.  Instead of increasing radiator size and heat transfer area to dissipate the extra heat of our larger engines, the heat transfer coefficient has been increased by moving to forced convection by introducing a water pump.  And of course the air side heat transfer is increased by the higher speed of modern cars which gives more air flow over the radiator.  Its not all proportional of course.  Engine efficiency has improved, radiator design has improved and so on.  But the design of Willy’s engine does have many parallels in other fields.

Thanks everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 08, 2019, 03:13:54 PM
Hi MJM, thanks for your detailed explanation about  Rising Heat...  I really like the way that modern misconceptions and urban myths abound !! The reason i asked about if the water was  is moving was that in a solid medium  (metal) the actual molecules do not move !! So thanks for your time spent replying in such detail
 :praise2:

Willy
Title: Re: Talking Thermodynamics
Post by: AVTUR on May 08, 2019, 06:22:58 PM
Just a little add on. With thermosyphoning the top of the radiator was usually the highest point under a car bonnet. This led to quite powerful cars in the 1930s, such as the SS (forerunner of the Jaguar), having high bonnet lines. Things changed when a water pump and thermostat were used. The engine could run hotter and the radiator did not need the height. This led to controlled engine temperatures which gave other advantages, in addition to efficiency, such as better lubrication.

MJM460 - I still mean to reply to you but at present I am long way from home and on the edge of the known world.
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 10, 2019, 12:58:05 PM
Hi Willy, it is always a pleasure to be able to answer your questions.  You have a talent for seeing things happen around you and wondering why, when most people just take things for granted (or perhaps assume it is magic).  The main thing is so long as I am giving clear explanations.  If not, I need to have another go.

Yes the water actually flows around the circuit.  But the metal molecules also move.  Not travelling like the molecules in a fluid, but jiggling around in the limited space available at their location in the lattice arrangement of the molecules in the structure. 

When heat is applied to a surface of the metal, the amplitude of the jiggling of atoms near that surface  increases, and the collisions with the surrounding atoms increases their jiggling and so on, to transfer the heat through the solid metal.  So long as there is a temperature difference, heat continues to be transferred.  But the atoms do essentially stay in their space in the lattice.

Hi Avtur, I know what you mean about travelling, I am four hours from home at the moment, and in a relatively remote area by the standards of my home state, but the real travel to the edge of the earth for me starts later in the year when we head up to Darwin.  I think it involves dropping off the edge and appearing the other side, or something like that.  Over four thousand km by the shorter route.  That gets boring after the fourth trip, though only a little, and it never looses its appeal.  But the alternative routes are always longer, and we enjoy them too.

Interesting also that you mention that SS model Jag.  Only a month or so back, one of my friends was telling me he had one of those which he had fully restored.  He had started a conversation with one of his business clients, who said it would not be one of those as there were so few around.  The client was somewhat of an expert on them and had the “official” list of all known examples in the world.  Quite a short list.  My friends car was not on it, but was soon proven to be genuine.  I believe it has now been transported from here to US for a show, where it was sold to a collector in The Netherlands, so it is almost home.  A well travelled vehicle.

Thanks for the additional information on the cooling systems.  It is always hard to know when to stop, so always room for additional comments to add to the discussion.

I think we will all be glad to hear your comments when you have the opportunity to read the earlier posts, but it is quite a long thread.  Please let me know if you notice any errors, and I will correct them.  And enjoy your travels.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 20, 2019, 01:46:00 PM
Hi MJM, I have finally got the small IC engine going and it is such a simple machine really just a steam type piston and cylinder    a fuel system that is just basically alcohol burning ...so why did it take such a long time to invent ?? We had all the rules of thermodynamics available to us plus all the materials and technology available for at least a hundred years , and we had the scientists and engineers to hand !!  We also had clockmakers that were capable of precision work as well ... also we are still using 19th century technology in our transport system !!  any way just something to think about really  here is the vid of the engine performing   Not a valid vimeo URL
Willy
Title: Re: Talking Thermodynamics
Post by: Hugh on May 20, 2019, 06:20:54 PM
I think the internal combustion engine is closer to 200+ years old. Wikipedia has a nice detail of the history: https://en.wikipedia.org/wiki/History_of_the_internal_combustion_engine#Prior_to_1860

(An interesting note is that the first real (documented) attempt was by Huygen's; a bit of a hero in scientific circles. The guy contributed to an enormously broad range of subjects. Most significantly, he instigated the current approach of how we think about optics. See: https://en.wikipedia.org/wiki/Christiaan_Huygens)

As for thermodynamics, the key to understanding it is the second law, which really only formulated at around the same time (late 1700s). If you think about it, it's actually pretty impressive how much progress they made on e.g. steam engines etc with such an unclear understanding of the underlying physics.

Hugh

Hi MJM, I have finally got the small IC engine going and it is such a simple machine really just a steam type piston and cylinder    a fuel system that is just basically alcohol burning ...so why did it take such a long time to invent ?? We had all the rules of thermodynamics available to us plus all the materials and technology available for at least a hundred years , and we had the scientists and engineers to hand !!  We also had clockmakers that were capable of precision work as well ... also we are still using 19th century technology in our transport system !!  any way just something to think about really  here is the vid of the engine performing   Not a valid vimeo URL
Willy
Title: Re: Talking Thermodynamics
Post by: AVTUR on May 20, 2019, 08:02:42 PM
Willy

I believe the first successful internal combustion engines were low compression gas engines made in the 1850s. These evolved into the small open crank oil engines made up until the early twentieth century. Here in the UK we have a whole museum dedicated to them at Anson just outside Manchester. The IC engine that we all know and love arrived around 1890. Like the high pressure steam engine by 1900 it has developed to a point where it is difficult to see any further progress being made. Again like the steam engine 120 years ago it is mature, easy and safe technology. A major question is what forces new technologies [this is not a thermodynamics question]?

Hugh

I do not trust Wikipedia as a source of information. Recently I have seen too much rubbish and untruths on the site.
Edited - I realise that I may be being a little too harsh.

MJM

I am back from holiday and have read the first 30 or so pages of this thread. Up to that point it is very steam orientated, not surprisingly, and that is something I know very little about. During my thermodynamics education it was assumed that we never come across a reciprocating steam engine so what steam work we did concerned turbines. I like the way Willy and you have kept the thread going. I will reply to you properly in the next couple of days.

I have a little thermodynamics story that I will tell in the correct place on the site.
Title: Re: Talking Thermodynamics
Post by: Jo on May 20, 2019, 10:06:14 PM
Its not that simple:  ::)

The first notable internal "fire" production engines were based on the Lenoir's 1860 non-compression ignition engine design and less than 500 were built, many under license, by various companies across the world.

The second successful type of production engines were the atmospheric ignition engines designed and made by Otto & Langen from 1867 of which over 2500 were built by 1882.

Otto's four stroke Compression engines went into production in 1876 and the customer demand exceeded their production capability.

Two stroke compression engines went into production by various manufacturers around 1880.

Diesel's compression ignition engine made its production debut in Munich in June 1898....


All of these engine developments were built on engineering capability and technological developments that can be traced back for over a century before they went into production: the compression engines are still with us and they continue to evolve  8)

Jo

P.S.  The jet engine is also a compression ignition engine  :thinking:  :thinking:
Title: Re: Talking Thermodynamics
Post by: Hugh on May 21, 2019, 12:45:25 AM
I do not trust Wikipedia as a source of information. Recently I have seen too much rubbish and untruths on the site.
Edited - I realise that I may be being a little too harsh.

Thanks okay. I also take it with a grain of salt in many cases. In general, however, I find its "core" technical articles to be very detailed and written by genuine experts in that area. For example, the article about my research field was written collaboratively at a major international conference on the subject. As a result, I often point my students there as it saves me many hours of repeated explanations.

Hugh
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 21, 2019, 04:29:27 AM
Thanks to all who have contributed on this one.  What an amazing historical resource is held within this community. 

Hi Willy, I have been silently following your engine projects, you are doing an incredible job on both.  Great to see that engine from the club collection returned to working condition.  I have been a bit quiet due to the distraction of illness in a brother in law, much loved by the whole family.  He finally lost the battle, so we are all gathering around my sister in law.  All some 300 km from home.

I suspect that some of the issue on inventing the ic engine is just getting it all together in the same place on the same day.  Everything from ignition system to suitable fuel.  It all seems so obvious now we know what works.  A similar story with regard to man powered flight, where once all the pieces fitted into place, the resulting aircraft seemingly easily completed the task.

Hi Hugh.  Huygens?  I always associate him with uncertainty of everything, but then again, I am not sure.  Like you, I am continually amazed by how far the pioneers went with so little understanding of the physics.  And so little calculating power, and so little basic data of material properties.  All our progress rests on the shoulders of giants who pioneered the way.

Hi Avtur, I set out to try and answer the question of just how our engines actually use energy to do mechanical work.  The principle is the same ic or steam reciprocating machines, but it was simpler to stick with steam, and include all the material on properties of steam and how these properties relate to the fluid behaviour.  My experience depends more on the thermodynamics than the actual engine.  IC engines have been a very minor part of my working life.

All credit to Willy for his contribution to keeping the thread going with his amazing questions.  I had the easy part.  But thank you for the compliment.  I am looking forward to hearing what you think of the rest of it.  But it is a lot of heavy reading, so don’t spoil the holiday over it.

Thanks Jo, Some really interesting history there.  I assume you are talking about the pulse jet engines.  I remember in my primary school days, some of the local model aircraft enthusiasts first got one.  We could hear it in our back yard, about a mile from the football oval they were flying on.

Thanks to everyone for looking in and contributing,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 22, 2019, 01:45:17 AM
Thanks all for your contributions, perhaps i should do some reading.!!! Its a shame that we have to pay for Graces Guide  browsing now . Oh well back to steam engines now.. I wonder if there are any double acting single piston  IC  engines ??

Willy
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on May 22, 2019, 11:11:02 AM
Hi Willy

Quote
I wonder if there are any double acting single piston  IC  engines ??

I'm guessing that you haven't been following any of the Snow Engine builds on this site ...!
Title: Re: Talking Thermodynamics
Post by: AVTUR on May 22, 2019, 12:08:22 PM
Jo

You are right. The jet engine, I shall call it the gas turbine, cannot really be a compression ignition engine since the heat of compression does not ignite the fuel. However the ideal cycle is the same.

I am not sure whether MJM460 has written about engine cycles but I will have a go. Most of us know how a spark ignition engine, petrol engine, works. This, the Otto Cycle, can be described by a Pressure Volume plot [attachment Otto cycle.JPG] The air fuel mixture is admitted to the cylinder at point 1, bottom dead centre. It is then compressed adiabatically [I know that MJM460 has considered adiabatic processes but all it means is that none of the energy in the process is lost to the outside world. This is a good assumption when the process is rapid as in this case] by the piston and increases in temperature. At top dead centre the mixture is ignited at point 2 and is immediately completely burnt. The pressure of the resulting gas rises to point 3. The hot high pressure gas expands, again adiabatically, pushing the piston back to bottom dead centre and leaves at point 4. The work produced by the cycle is the area between the lines.

This is very much an ideal. However an indicator diagram taken from a four stroke spark ignition engine will look very similar to the ideal cycle for the working strokes.

The low speed compression ignition engine, such as big, slow running ship engines, is described by the Brayton Cycle [Brayton cycle 1.JPG] Here the air is compressed as before but at top dead centre, point 2, heat, that is fuel, is slowly added with combustion continuing as the piston moves away from top dead centre. The pressure of the result gas does not increase. At some point, point 3, the addition of fuel stops and the hot high pressure gas expands adiabatically doing work. I should add for the modern high speed compression ignition engine, such as in your car, the cycle is a mixture of the Otto and Brayton Cycles.

I can describe an ideal conventional gas turbine with exactly the same way with the Brayton Cycle. A given volume of air is compressed adiabatically, heat is added at a constant pressure in a combustion chamber and the resulting gas expanded adiabatically through a turbine which powers the compressor. The surplus energy can then be expanded through a nozzle to give thrust or a turbine to give shaft power.

The obvious difference between the gas turbine and the compression ignition engine is that one is continuous and the other intermittent. This leads to the volume axis of the cycle plot having little meaning. By a neat sleight of hand it can be re-plotted as a Temperature Entropy plot , you just knew the Second Law would be involved [again MJM460 has explained entropy, it is a measure of the unobtainable and thus lost energy in a process. It should always be remembered that entropy can only increase, once the energy is lost you cannot get it back] [attachment Brayton cycle 2.JPG] Again the plot shown is ideal. Do not worry too much about the entropy axis, this becomes important when inefficiencies are taken into account. Again the numbered points correspond to the earlier Pressure Volume plot and to the simple sketch of a gas turbine. [attachment Gas turbine sketch.JPG]
 
This plot simplifies the performance analysis of the engine. You don’t need nasty things like indicator diagrams. All that is required are the thrust or output power, fuel and air flows and the pressure and temperature at the numbered points.

I hope that the above makes some sense. I am quite happy to answer any questions and to take the topic further.

MJM

The above gives away my background. I was a gas turbine combustion engineer for thirty years.

AVTUR
Title: Re: Talking Thermodynamics
Post by: AVTUR on May 22, 2019, 03:57:05 PM
I wonder if there are any double acting single piston  IC  engines ??

Willy

Willy

Have a look at big marine compression ignition engines. They came in many wondrous forms including opposed piston two strokes. Thinking back over 50 years I believe one successful engine, perhaps Danish, had three pistons in one cylinder. I have only seen models of such engines at the South Kensington Science Museum.

AVTUR
Title: Re: Talking Thermodynamics
Post by: Zephyrin on May 22, 2019, 09:23:25 PM
The first Lenoir engines were also double acting, just like steam engine...
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 22, 2019, 11:30:53 PM
hi Avtur, thanks for the info and the diagrams ....very informative..!!


Willy
Title: Re: Talking Thermodynamics
Post by: Jo on May 23, 2019, 07:34:17 AM
The first Lenoir engines were also double acting, just like steam engine...

If you want to make a model of a double acting IC engine then there is the 1895 Mery, castings are were available  :embarassed:

Jo
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 23, 2019, 12:05:54 PM
Hi Avtur,

Thank you for posting those engine cycle diagrams.  I have not really written much on engine cycles as it is not an area that I am too confident to write on.  I would be most pleased if you would be willing to take these to the next level.  Either here, or start a separate thread if you prefer.  I will certainly be following along.

My work was more directly about compressors and pumps, mostly electric driven, but sometimes steam turbines, gas turbines or gas fuelled reciprocating engines.  My main concern was integrating the control scheme for surge control, capacity control and load sharing, rather than the actual nuts and bolts of the machine, along with piping considerations.  So thermodynamics and performance curves, rather than parts diagrams, were my main concern.

I had a feeling about your career from some of your earlier posts, and have been hoping you will come in and add to the thread in areas of your expertise.  It will be really interesting to many of the forum members.  And I will certainly be following along with great interest.

Sorry to have been absent without leave the last few days.  The brother in laws funeral today.  About 300 km from my home and the clan has been gathering over the past week to support my sister in law.  A great tribute to the man when the vintage car club of which he was a founding member turned on one of their biggest rally’s in some time to escort him on his last ride.  And even the children of family friends came from as far as Darwin to be there.  We will all miss him.  So this post is a bit short to do justice to all those who have contributed in the mean time.

A big thank you to all who have looked in and especially to those who have contributed.

MJM460

Title: Re: Talking Thermodynamics
Post by: AVTUR on May 29, 2019, 09:16:53 AM
Good morning

MJM460

Considering your last posting I would like to be a part of this thread and add comments and knowledge. I cannot see myself doing this more than once or twice a week.  I am not sure about taking it to the next level. I don’t see myself writing tutorials since I am likely to wander. Instead I would like to reply to. This keeps things relevant.

My knowledge of steam engines is very limited. I occasionally used steam tables at work but only to solve problems on test rigs. My experience is with liquids and gases, relevant really to model IC engines and gas turbines. However, unless people are interested, I shall try to keep away from the latter since they are a very small area of model engineering.

.........  I assume you are talking about the pulse jet engines.  I remember in my primary school days, some of the local model aircraft enthusiasts first got one.  We could hear it in our back yard, about a mile from the football oval they were flying on.
Comment on pulse jets: As a student I had to do a lab test on one, a truly horrible device. Very noisy, its tailpipe glowed red hot and the thing ran at about 100Hz. One’s body cavity resonated. Thermodynamically, they are very interesting but, unlike a gas turbine, very difficult to analyse.

Willy

When heat is applied to a surface of the metal, the amplitude of the jiggling of atoms near that surface  increases, and the collisions with the surrounding atoms increases their jiggling and so on, to transfer the heat through the solid metal.  So long as there is a temperature difference, heat continues to be transferred.  But the atoms do essentially stay in their space in the lattice.
Going back to MJM460 reply about atoms jiggling in a solid lattice: As heat is added the more they jiggle, as MJM460 said, and the temperature rises. There comes a point where the atoms start smashing the lattice structure and the jiggling does not increase. The heat now being added is used for this destruction. Once the lattice is destroyed the amplitude of the jiggling will increase when more heat is added. However you no longer have a solid but a liquid. The heat added during the destruction is known as the latent heat since it does not produce a temperature rise. For completeness, in such cases the terms atoms and molecules are interchangeable.

AVTUR
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 30, 2019, 08:22:23 AM
Hi Avtur, sorry to be a bit slow to reply, your morning is our late evening, so it is now the following evening.  I will be delighted to have you adding to the conversation. 

I think you might be surprised at how much of your knowledge will help you answer many questions on the forum despite there not being many gas turbine models.  Personally I think when I look at the operating speed of the miniature ones, it is probably a good thing that not too many try and build them.

I know what you mean about two or three days a week.  I guess there was a backlog of questions when I started, but keeping it up for a year did mean I did not make many chips that year.  Come to think of it, this year has not been much better for entirely different reasons.  But a second point of view is always valuable, and especially an extension to an initial answer.  Don’t hesitate to jump in first on a question that interests you, I can always add a bit when my time zone comes around if I have additional thoughts.  It is always difficult to know where to stop when a question opens a lot of doors, I am sure I can be accused of wandering in some of the longer answers.  So some team work will enrich the thread.

I mentioned the pulse jet as possibly being compression ignition, in the context of the question at the time.  I agree with you they are diabolical devices. I have a vague memory that they have a spark plug to get them going, but whether it is a glow element that continues or pure compression ignition, I am not sure. The small industrial gas turbines I have encountered have an igniter, though I have to admit to being no longer sure whether that is a sustained igniter, or if combustion continues under compression heat once the flame is established.  Again, I am sure you can enlighten us.

Thank you also for extending the description on the heating of metals through to melting.  I obviously stopped too soon.

Looking forward to learning more from you as questions arise.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 08, 2019, 12:42:25 PM
Hi MJM, A quick question about evaporation   are there tables about  'natural' evaporation ie the evaporation that takes place without extra external heat applied  ?? just the everyday climatic conditions of temp..humidity  wind speed etc that takes place from puddles  forgotten cups of coffee,  reservoirs,  etc etc ??
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 09, 2019, 09:50:55 AM
Hi Willy, in remote area with limited access.  Look up Adel’s Grove!

I will come back Saturday sometime with some information if no one else would like to give it a try in the mean time.

MJM460

Title: Re: Talking Thermodynamics
Post by: derekwarner on July 09, 2019, 01:16:59 PM
Willy...........not sure what MJM may enlighten, but Mr Google shows that there are pages & pages  :happyreader: of Table Listings on evaporation   :cussing:.....but I see no simple answer to your question......

Best wait for a more professional response

Derek
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 09, 2019, 02:13:23 PM
Hi MJM ,  Wow lovely location ...and no crocodiles looking at the videos !!!!.....
Title: Re: Talking Thermodynamics
Post by: AVTUR on July 09, 2019, 05:26:38 PM
Willy

The rate of evaporation can be calculated. It is the same sum as done when designing air conditioning units. However I have not done any such sums since college over fifty years ago. I will try to come up with an explanation in the next few days. As you say the evaporation from a puddle is more complex.

Before starting to write I have a question for you and MJM - Have you discussed steam tables/charts and the perfect gas equation? Both are required for the sums which are not hard.

AVTUR
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 10, 2019, 12:31:30 AM
Hi MJM and AVTUR... the reason i asked this question was because i filled up the exhaust port on my engine to see if it was leaking....the following morning the level had dropped/evaporated/leaked slightly.  With the electrically heated steam boiler most of the parameters were known , so using steam tables anything missing could be calculated. there was a definite amount of energy going into the boiler ,the water temp and quantity was known ,also the insulation was known , but, can the energy from the ambient temp and climatic conditions be calculated ?? we are given the temp..windspeed..atmospheric pressure, and also the pollen count!! And the jet stream up here in blighty.  So , to cause evaporation what is the number in joules that are available at any moment in time ??
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 10, 2019, 06:23:19 AM
Hi Willy, no salt water crocs, only freshies.  Not too much of an issue if you don’t provoke them.  Yes fabulous location, but did the ominous orange glow show up on the site you looked up?

Only one small hot spot and repeater for all in the area at the moment, so I will write something on evaporation when I get to an area with normal service. 

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 11, 2019, 12:10:40 PM
Well, after three hundred km, about half of which is corrugated surface with loose gravel, and four river crossings, back in the world of internet availability.  We are finding that moving that distance further inland gives us more of a continental climate, so much cooler at night.

Thank you Derek and Avtur for looking in and contributing.  I hope I can justify your confidence Derek.

Avtur, I have discussed steam tables previously, but when questions come up again, I put it down to my explanations being incomplete.  It is perhaps a complex subject to those who have not come across them before, while others of us just need a reminder.   I have not said much about ideal gases and real gases.  Obviously necessary for a more complete analysis of many engine problems, but the simplified description is probably enough to help people understand qualitatively what is going on.  But I would welcome a more complete explanation when you feel it would help understanding.  We have people with a wide range of levels of prior knowledge on the forum, and its good to have a place with information at all levels so everyone can find something new, or even a helpful reminder.

Hi Willy, back to your question.  I take it that the question comes from observing your test of the silver soldered cylinder by filling it with water.  If some water is lost then, the obvious question, is it evaporation? Or is it leakage. And was there a temperature change causing a change in volume of the water?

I think the simple answer is that unless you put in warm water, or filled the cavity when the fabrication was hot from soldering, it is unlikely to be evaporation.  But the question as an interesting one because it involves so many of the topics we have been discussing.

In principle, if everything is at the same temperature, there is no heat transfer.  (The so called zeroth law of thermodynamics.) Evaporation requires the latent heat to be supplied, so how does it happen.

Basically, the temperature of the water is due to the average energy of all the atoms making up the water, but individually the amount of energy in each atom varies over quite a range.  The atoms that escape the liquid surface are the higher energy ones, so they take that energy with them, and leave the liquid a little cooler, as we know when we blow on our hot coffee to speed up the process.  The temperature difference created, makes it possible for more heat to transfer from the air to the liquid.  Removing the water atoms which have escaped into the vapour space by blowing or a fan reduces the number which fall back into the liquid, so establishes a net loss of water atoms in the vicinity of the liquid surface, and so increases the amount of evaporation.

The lower limit of the temperature that can be obtained is known as the dew point, or the temperature at which the water vapour pressure in the air is equal to the equilibrium vapour pressure that we can look up in the steam tables.  The difference between the dew point temperature and the atmospheric temperature is dependent on the humidity of the air.  But I suggest that to calculate the rate at which the water will evaporate is quite difficult, and easier to determine by experiment for a given temperature, physical configuration, air temperature, humidity and air velocity.  Which does not solve your problem of whether there is a leak. 

I would suggest that rather than calculate the amount of evaporation, I would make up an exhaust fitting to accept a plastic tube, fill the cavity with water, plug the end of the tube and mark the water level with a pen.  This should eliminate the possibility of evaporation as equilibrium will soon be reached close to the liquid surface then no further evaporation.  If there is leakage, surely it would appear at one of the other ports which should not be connected?

And don’t forget the third possibility, if the temperature changes, the volume of the water will change, and hence so will the level.

MJM460
Title: Re: Talking Thermodynamics
Post by: AVTUR on July 11, 2019, 05:52:12 PM
Willy and MJM

I have procrastinated too long.

I am a believer in calculations, the simpler the better. They may not give you the right answer but will give you a guide at what is likely. MJM has given an explanation of what is happening but, in most cases, the molecular level is acknowledged and then ignore.

For many following, sums are a great turn off but I will try to explain the steps and the science. I sure there will be questions, please ask them.

EXAMPLE 1. This is a puddle of water with air above it in a totally enclosed insulated box. When this is at equilibrium the air will be holding the maximum amount of steam, as a gas, that it is able to do so. [I am not going to use the word vapour]. This is a humidity of 100% and its value is very dependent on the air temperature. Relative humidity is the ratio of the partial pressure of the steam in the air to the saturated pressure corresponding to the air temperature. Its value can be determined from wet and dry bulb thermometer measurements; however my traditional “barometer” displays it directly.
If the air is not 100% humid it will be able at absorb water from the puddle until it reaches 100%. We have 5lb of water below a volume of 1 ft3 of air at atmospheric pressure in our box both at a temperature of 61°C (334K) [this is absolute temperature and will be required shortly]. Since we have just assembled this box the relative humidity is only 40%, typical for comfortable living. This is not at equilibrium so water will evaporate from the puddle until the air has a humidity of 100%.

Step 1. We need to find out how much steam was originally in the air. Using steam tables find the saturated pressure that corresponds to 334K [since I am an aerothermal engineer I will make no excuses for using K] which is 3.0 lbf/in2 absolute (that is above a vacuum, not above atmospheric pressure). [The significance of the pressure is water will start boiling, that is cease being a liquid, at the corresponding temperature. Remember it is the sub-atmospheric part of the steam tables, the half that no one uses]. Using the perfect gas equation........

[I think the perfect gas equation needs an introduction. It is not as nasty as it sounds. First gases away from very high energies and liquefaction are perfect. The equation is a direct result of our model of molecules bouncing off each other in free space. The equation is
    p = ρ.R.T/m     
        where p = gas pressure in absolute units, ρ = density of the gas, R = universal gas constant (2777 ft2K/s2), T = gas temperature in absolute units, m = molecular weight of the gas.

{Little example, we can use this to calculate the density of air at room temperature. p is 14.7 lbf/in2 (2117 lbf/ft2), T is 288K and m is 29
    Density (ρ) = 2117 x 29/(288 x 2777) = 0.077 lb/ft3}

The gas pressure in the equation can be split to represent each constituent of the gas (known as partial pressure) by mass]

........ the partial pressure of steam in the air, pSTEAM SAT, is the saturated pressure, 3 lbf/in2.

From the relative humidity the initial partial pressure for the steam, pSTEAM
    pSTEAM = pSTEAM SAT x relative humidity = 3.0 x 0.4 = 1.2 lbf/in2

Then mass of steam in air, MSTEAM
    MSTEAM = ρSTEAM x V = pSTEAM.m.V/(R.T)
        where M = mass, V = volume

Assuming the mass of steam in air will be small the molecular weight for air will be used
    MSTEAM = 1.2 x 144 x 18 x 1/(2777 x 334) = 0.00335 lb
        where 18 is the molecular weight of water (and steam!)

And the mass of dry air
    MAIR = (14.7 – 1.2) x 144 x 29 x 1/(2777 x 334) = 0.06078 lb

Step 2. Now to find the amount of steam that can be held by the air. Going back into the gas equation, the mass of steam in the air
    MSTEAM SAT = 3.0 x 144 x 18 x 1/(2777 x 334) = 0.00838 lb

Therefore the mass of water evaporated is
    MSTEAM EVAP =  0.00838 - 0.00335 = 0.00503 lb
        which equates to 0.139 in3 of water.

Comments
1. You may have noticed that
    MSTEAM /MSTEAM SAT = 0.00335/0.00838 = 0.4 which is the relative humidity.

However the mass ratio is not the definition since the above sums contain assumptions which do not compromise the answer. For example the evaporation of the water will lower the overall temperature because of the latent heat of evaporation and the increase in steam partial pressure would increase the pressure above the water.

2. If our model was a thermally insulated box with air being blown through it the equilibrium would never be reached since the steam is removed. After time there would be either no water left or a block of ice because of the evaporative cooling. The new model would need some knowledge of the rate of heat input to get an evaporation rate.

Willy, I realise that this does not answer your question. It is only a start. I am quite happy, if anyone is following, to continue to Example 2 (Air flowing over puddle with heat added).

This has taken quite some time. I should have been riding the bike, motorcycle, today with the lads but I over slept. My second priority, which has gone by the board, was work on the bellcrank engine.

Help! How do I use subscripts and superscripts? I wrote the above in WORD and then copied it across. I lost the paragraph spacing, underlining and subscript and superscript positions.

AVTUR
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 12, 2019, 12:35:37 AM
hi MJM , thanks for the info...i was looking at evaporation tables and it would appear that evaporation can still take place if the ambient temp is lower than the water temp !  I think some of the water may have leaked through the soft rubber 'seal' that was held in place with a clamp....

Hi AVTUR, wow that is quite a lot of info to take in ... so can "steam" actually be quite cool ?? I have a physics expert that can take me through most of this....Presumably steam tables should start at minus 273 Degrees ??  that is quite a novel concept..Thanks for the mass of info and i do need to do some serious brain work ...which at 71 will be quite taxing....

Willy
Title: Re: Talking Thermodynamics
Post by: AVTUR on July 12, 2019, 09:28:11 AM
Willy

Quick reply.

If you connect a glass flask with some warm water in it to a vacuum pump and start pumping out the air the water will begin to boil. We were shown this at school about 58 years ago.

I have to admit I did not want to call gaseous water steam and I was not going to use the term vapour since I feel it is confusing. I started using the term "water as a gas" which is a bit clumsy. Then I realised my college notes used the word steam so I did.

AVTUR
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on July 12, 2019, 11:59:54 AM
Avtur there are two buttons above the posting window labled "sup" and "sub" next to each other. Example :

H2O and V2

Both a first for me on this fora.

I haven't used the funktion in Word since year 1999, but you should be able to save as HTML too, so the format should be keept.

Best wishes

Per
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 12, 2019, 01:23:33 PM
Hi Avtur, thanks so much for coming in with that post.  I know how much time and thought it takes when it comes to writing down the basics.  I also appreciate a view from a different industry, where you clearly use slightly different units, and even a slightly different form of the ideal gas law.  It all helps remind us that our units of measurement are somewhat arbitrary, and the most important point, is that we are consistent in any calculation.  That said, I still go to the timber shop for 2.4 m of 2 x 1!

I think much of the trouble with gas vs vapour is the common usage, with “steam” from a kettle being a visible mixture of water in gas phase and fine liquid phase droplets.  However, throughout my career in the oil industry,  the main process involves continuous boiling and condensing of many different substances and water is only an anomaly because it is not a hydrocarbon, and is a utility rather than a product.

And of course as a mechanical engineer dealing with steam turbines with or without condensers, that first page of the steam tables is quite familiar to me. 

So a broader view of the world is most welcome, thank you.

Oh, and the superscript sand subscripts, I also write in a word processor and copy and paste to the forum.  The forum does not like accepting subscripts or superscripts from my program either.

There is provision in the characters you can find along with the cartoons on the edit page as Admiral dk has noted (thanks, Admiral) but I have found them very awkward to insert into my posts. I don’t know. I have tried it a few times.  About HTML.  I like using the ^ form as in volume of a cube =L^3, but this seem not commonly understood be other forum members, so not very popular.  When used with units, I often just assume the subscript is obvious, as in lb/in2.  Or ignore the subscript notation as in P1 x V1 /T2= P2 x V2/ T2, as again it is obvious.  No easy solution with the available forum editor.

Hi Willy, water can definitely evaporate if it is warmer than the atmosphere around it, and even if it is cooler, just at different rates, and depending mostly on the humidity of the air above the water.

Certainly steam can be quite cool in the right circumstances.  My steam tables go down to 0.01 deg C which is the triple point of water, I.e.the temperature at which water can exist with solid, liquid and gas all in equilibrium.  Water as a vapour can be found in the air above ice, but no liquid below that temperature at normal atmospheric air pressure.  At the triple point the equilibrium vapour pressure is only 0.6 kPa, or about 0.08 psi and is most of that even at -75 C, and I have no information below that.  You need very high pressure to have liquid below 0 deg C.

More commonly, the condensing temperature of a steam engine or turbine, is mostly limited by the temperature of the cooling water or cooling air to something in the range of 30 to 70 deg C.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 03, 2019, 03:32:35 AM
Hi MJM, I have a new question after thinking about steam in cylinders...I don't know if we have covered this , as it is a hypothetical question... I you have a closed insulated cylinder with a tight fitting piston that is full of steam at a certain pressure  and then force down the piston as hard as you can ,..what will happen ??  I was thinking about this when i was pumping up someones tyres !! Presumably  the temperature will rise but what else might happen depending on the speed and force that one might apply....??

Willy
Title: Re: Talking Thermodynamics
Post by: AVTUR on August 03, 2019, 09:15:43 AM
Willy

A nice text book question. If it was air the answer would be simple but since it is steam I am interested in MJM's reply.

AVTUR
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 03, 2019, 02:47:03 PM
Hi Willy, it’s good to have you thinking about thermodynamics again, you must have recovered from the things that have ailed you recently.

As Avtur says, it is a good text book question, but in your usual manner, you have included just a little twist so that the precise example is not in my book.  Or perhaps I am just a bit weary tonight and unable to find it.

But let’s try for a brief answer anyway.  First your definition of the problem includes the cylinder being well insulated.  I will take this to mean that there is no heat transfer in or out.  Second, you said you would push the piston as hard and quickly as you can.  This bit departs from the usual textbook definition, which would require the process to go slowly in tiny steps so that at each stage it could be reversed, usually defined as an ideal reversible process.

So let’s look at this in two steps.  If we have a well insulated cylinder, there is no heat transfer in or out, it is called an adiabatic process.  And for an adiabatic process we can deduce from the laws of thermodynamics that the process is also isentropic, meaning there is no change of entropy.

As the piston is pushed against the gas, the gas temperature will rise, as will the pressure, as it will with any gas.  You might be thinking that as you increase the pressure, that steam might condense.  But if you look at a temperature entropy diagram for steam, you will see that at the temperature and pressure increases at constant entropy, the steam moves further away from the saturation line, so gets more superheated.  It does not condense, you can compress steam.  It is not often done because it costs more to build and operate the compressor to compress the steam than it does to condense it, pump it to a higher pressure, and evaporate it again. 

Now, you said if you compress the steam as hard (and quickly?) as you can.  The trouble is that this departs from that ideal reversible process, and the change of entropy will not be zero.  This is the second law of thermodynamics.  I have to think quite a bit more about which way this goes, and I hope that Avtur will be able to set us both straight on that one.  If you just meant as hard as you can, meaning to as high a pressure as you can, it could still be slow, close enough to reversible, and the steam will get very hot, and of course, still superheated.

And of course if your insulation is not perfect, and if you loose enough heat during that compression, the steam may condense anyway if the heat loss is sufficient, but it condenses due to the heat loss, not due to the compression.

Compression of steam is fundamentally no different from compressing any other liquifiable gas, such as propane, butane or any refrigerant.  The gas gets hotter as the pressure rises, and more superheated and then needs a condenser to remove the heat to condense it.  The heat loss from a normal compressor is not enough to condense the gas during compression.

Hi Avtur, I am glad you are looking in.  I will be interested in your answer as well.  Steam is obviously not an ideal gas when it is close to the saturation line.  So instead of calculating by the ideal gas law, I would just use the steam tables for an isentropic process.  I suggest that the main difference between steam and the usual refrigerant gases is that the relevant temperatures mean there is usually heat lost from the compressor instead of gained from the atmosphere.

Thanks everyone for looking in.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 03, 2019, 04:24:19 PM
Hi MJM and AVTUR,  thanks for the reply./answer...  perhaps i should just do the experiment !!! A good text book question !!  a bit like a question i once saw where the calculation of the level of the water each side of a ship turning in a circle in still water was required !!! I have always been one of those annoying pupils that sit in the front row that allways has his hand up asking the teacher for a fuller explanation !! especially when i found out that there is always an exception that makes the rule  !  ie  I before E except after C  as in science !!! 

Willy
Title: Re: Talking Thermodynamics
Post by: AVTUR on August 03, 2019, 05:29:55 PM
As I have already said the last time I did any real steam calculations was 50 or so years ago. While I kept my fluid dynamics text book the thermodynamics text book was sold when I left college. If I needed one at work I would borrow my boss's, we all did.

Unlike an ideal gas this is difficult to work out intuitively. Obviously constant entropy is the way to go and then make allowances for losses. At this point I would reach for a steam chart but I do not have one. Therefore I Goggled "steam chart" and found something very different! It was not rude or naughty, just unexpected. The only steam table I have is in Tubal Cain's "Model Engineer's Handbook". It is very condensed (pun not intended) and does not included entropy. Tubal Cain, who was a lecturer at Loughborough University, wisely avoids any mention of entropy in his handbook. As he wrote "As mentioned at the beginning, this has of necessity been a 'skim over the surface': whole books have been written on the subject, one at least running to two volumes".

I am sorry but I think I have copped-out. I will try to find a steam chart.

AVTUR
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 04, 2019, 10:02:44 AM
Hi Avtur,

There are lots of short cut versions of the steam tables on the web, and I have found good ones hard to find.  This link will lead you to some good steam tables in Excel format so very useful.

https://www.ohio.edu/mechanical/thermo/property_tables/index.html

I think you might have to copy and paste that, it does not look like I succeeded in pasting a link unfortunately.  It is provided as part of a standalone course in Thermodynamics by Ohio State University.

My original thermodynamics text is very old, and I have seen it on the shelves of engineers who were older when Iwas young, and it is in Imperial Units.  These days, I refer to a modern one using SI units, much easier to use.  Willy has given me cause to refer to it often in the last year or so.  It was quite instructive, and quite enjoyable.   Nothing like a curly question to test whether you really understand the topic.  He gave me cause to re-read quite a few sections, over and over until I understood it.  But during working years my go to source was some of the various industry data books that had sections with those practical formulae that tend to be used often when dealing with compressors, turbines and hydrocarbon processing.  In the early 80’s, I got hold of an SI version of one that I have nearly worn out over many years.

MJM460

Edit - Now that I have posted it, it does look like I may have succeeded in posting a link.  Now how did I do that?  I won’t test fate by trying again!



Title: Re: Talking Thermodynamics
Post by: AVTUR on August 05, 2019, 12:36:39 PM
MJM

Many thanks for the link. I have downloaded the steam charts and will use them now.

Willy & all

I do not know if MJM has written about steam charts. Please excuse this if he has.

I have attached the two charts. They appear to be in the public domain. If not I will let a moderator tell me off. The charts present the same data in two different forms. The first is the conventional one which is met at college. The second has an emphasis on entropy, using it for the x-axis. This is the one we are going to use.

First, a quick explanation of the chart:
The x-axis is entropy: this is a measure of lost energy according to the second law of thermodynamics. Its actual value is not important but the change in value is very important.
The y-axis is enthalpy: this is the internal energy of the steam. Again change in value is important.
The blue lines are pressure in absolute units. Red lines are temperature (I think they should be in K but I am being pedantic). The green lines are the percentage dryness of the steam (labelled quality), 0% is water and 100% is dry steam (a gas). The black line is important, above this water is a true gas – superheated steam.

Now for Willy’s question. I marked up a chart to show what happens.

We have a well lagged cylinder of steam which we are going to compress. Since no energy will enter or leave the system other that the movement of the piston, it is adiabatic. Also we are going to say that the system is reversible, we can get out what we put in (so much for the Second Law).

I have taken a starting point, Point 1, for convenience on the 0.1 MPa (atmospheric pressure) at an entropy of 7 kJ/kg.K. The chart shows that the enthalpy is 2550 kJ/kg, dryness is 94% and temperature is 100°C.

We now compress the steam to 1 MPa. There is no change in entropy since the process is reversible so we get to Point 2. The steam is superheated and at a temperature of 260°C. Again from the chart, water boils at 180°C at 1 MPa so we have 80°C of superheat. The enthalpy is now 2950 kJ/kg so we have used 400 kJ/kg to compress the steam. If we release the piston we will get all of it back and return to Point 1. In practice if we do this process quickly there will be little time for heat loss. The slower we do it the more heat will be lost.

Apologies, but I am going to take this further. We cannot get away from the Second Law. The adiabatic assumption is reasonable. However reversibility is not, there will be inefficiencies. If we repeat the about with efficiencies of 80% (next chart) we have to use more energy to reach 1 MPa. We need 500 kJ/kg so Point 2 slides up the pressure line and, hopefully to no one’s surprise, entropy has increased. The steam temperature is now 300°C. If we release the piston and allow the pressure to return to 0.1 MPa. the reversible energy released will be 450 kJ/kg (I have not drawn this line on the chart). The actual usable energy will be 360 kJ/kg and we have moved to Point 3, conveniently, dry steam. Again entropy has increased and we have lost140 kJ/kg in the process. Since the system is adiabatic where has this energy gone? It is lost to a myriad of things like friction of the piston against the cylinder (and you won’t get it back).

I hope that the above makes sense!

AVTUR
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 05, 2019, 11:44:10 PM
Hi AVTUR  thanks for this  a bit more clearer now....

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 06, 2019, 12:19:05 PM
Hi Avtur,

Thanks for the additional input.  Glad you were able to download the information.  I assume you found the tables in Excel format as well.

I tend to agree that the temperatures should be absolute, however, as the tables generally assume an arbitrary start temperature of zero at the triple point of water, it probably does not matter too much so long as we all understand when to convert them to absolute for a calculation, and of course, remember to do it. 

Because we all have so clearly in our heads centigrade values for the freezing point and atmospheric pressure boiling point, it is probably helpful to relate things to the references we have in our heads.  273 is a bit of an awkward number to always have to add and subtract, especially if we want to include the extra decimal places.

With that lost work, I suspect that even if we ignore friction of the piston against the cylinder walls, though there will always be some, there is also energy lost in turbulence within the gas in the cylinder.  This cannot be recovered or used, we can only use the work that crosses the boundary of the space we are considering is, I think, the explanation.  As you say, we can’t beat the second law.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 11, 2019, 12:02:37 AM
Hi MJM, thanks for the last post.. I have now been watching a new series of documentaries entries on the telly that is quite interesting . they stated that heat always travels to cold areas ...however if you have a large quantity of very cold icy water and mix it with a very small quantity of warm water the cold water will actually move into the warm water and make it less warm.!!  sorry about being pedantic !! :mischief:  also found  a chart talking about the heat losses in a coal boiler....although i don't quite understand some of the losses/movement of the losses.?.the conclusion being 19% to produce steam !! if this is correct ??...however if the coal is suddenly extinguished/removed the boiler will still produce steam for a while ?? and there will still be some calorific value left in the coal..?? I haven't read the full article so may have got it all wrong !!!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 11, 2019, 12:57:44 PM
Hi MJM , Actually i have got it wrong it is not a boiler !!! :-[ it would appear to be a blast furnace or other ...sorry about that  :-X

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 11, 2019, 01:05:08 PM
Hi Willy, I hope that last post made sense and helped further your understanding.

That sounds like a very interesting program on the BBC.  Programs on basic physics are a bit thin in the programming space, to use the current popular jargon.  I am pleased that they agree that heat moves from hot areas to cold areas.  I think the clue to the conundrum is in that description of icy water.  Remember that hot water is generally less dense than cold water so the hot water tends to rise, while the cooler water tends to sink due to the density difference.  Just as we have previously discussed with air.

But water is strange stuff.  As water is cooled it contracts until it reaches about 4 deg C.  Then it changes its behaviour and actually expands as it cools further.  This is why ice forms on the surface of the pond, while the warmer water flows underneath.  There are only a few other substances which have this same behaviour.

So at moderate temperatures, if you have some cold water and carefully pour some warmer water onto the surface, carefully meaning in a manner so it does not immediately mix up, the warmer water will tend to stay on the surface, and the density difference will slow the mixing.  However, heat will still travel from the warm water to the cooler water.  If you do it the other way and put the cold water carefully onto the surface of the warmer water, the higher density cooler water will tend to sink, and the convection movements resulting will speed the mixing so the temperature evens out more quickly.

However, if that warmer water is only about five or six degrees, so still not swimming temperature, and you put that warm water on top of icy water, the icy water could have lower density than the warm, so the cooler water will rise to the surface, mixing with the warmer water, and the heat of the warm water moves to the cooler water, leaving it cooler.  Because of the unusual density change with temperature, the normal density difference is reversed and the unexpected happens.

You can look up the density/specific volume column of the steam tables and see the range of temperatures over which this happens. 

Also when ice is further cooled, there is a temperature at which it starts to get cooler again.

In summary, heat always moves from a higher temperature body or material to lower temperature material, but less dense fluids, liquid or gas, tend to rise against cooler fluids due the difference in gravitational forces.  Heat moves under a temperature gradient, fluids move under a density gradient.

The type of diagram in your book is a common representation of where the heat goes in a furnace or other equipment.  But I believe that particular diagram has been prepared to show where the heat goes in a furnace used in steel making, so they are making coke, and producer gas, and steam.  I think you will find it is in the section on steel making processes.

A similar diagram can be prepared to show where the heat goes in a normal steam boiler.  I expect you will find it would show about 70% goes into steam, most of the remainder goes up the stack as flue gas and the remainder is lost as heat radiated through the furnace walls.  You can refine the accounting of the losses into all the detail you wish, so you can include heat loss from steam piping, heat into feed water, heat in the ash.  The important thing is that you draw a boundary around the equipment you are including, and only count heat that moves across that boundary.  So if you use some flue gas to preheat incoming air or water, that might not cross the boundary, depending on precisely where you draw the boundary, so would not be counted.

I hope that helps

MJM460

Hi Willy, your extra note arrived while I was typing.  We are now on the same page.  Well done.

Title: Re: Talking Thermodynamics
Post by: AVTUR on August 11, 2019, 03:37:52 PM
Willy and MJM

I have little to add. I watched a programme, one of three, on the BBC a couple of nights ago about temperature which was well put together. I think the presenter was a professor at The Open University.

Many years ago I worked as a research engineer at a steel works, a great and fascinating job which did not last. I did have a little to do with blast furnaces but I would not want to consider their thermodynamics. Obviously the air used in the furnace is pre-heated, generally using a something called a Cowper heater. This is large regenerative heater, a cylinder filled with refractory bricks which absorbs the heat from the furnace gases and then gives it up to the blast air. How the whole system, including the furnace, warms up and cools down is very complex. Once a furnace was started it was not usually shut down until it became dangerous to operate.

I could go on but I am rambling (a bad habit of mine).

AVTUR
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 14, 2019, 11:50:06 PM
Hi MJM, A sort of T question ..when i have a mug of tea and squirt some honey in it ...if i mix it up it tastes of honey...but  if i dont stir it it stays the bottom until i have almost finished it.? Is it something to do with specific gravity or some thing else  also if the honey was the same temp as the tea would it self mix ??? Just a diversion whilst i am staring at the rain...

Willy
Title: Re: Talking Thermodynamics
Post by: AVTUR on August 15, 2019, 03:52:14 AM
Willy

A very nice question! And not a diversion.

AVTUR
Title: Re: Talking Thermodynamics
Post by: Steam Haulage on August 15, 2019, 08:40:39 AM
Hi MJM,

In your recent post about gently pouring various temperature liquids into one container to make 'separate' layers how do you account for Brownian motion?

Still following along although I thought I had deliberately forgotten all this when i retired.

Sorry for the perhaps out of kilter question but in the past I was more concerned with ensuring intimate mixing at various rates so I'm more than rusty.

Jerry
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 15, 2019, 11:22:00 AM
Hi Willy, you have an amazing capacity to draw a physics lesson out of your tea, coffee and slushies. 

Specific gravity, or density is obviously involved, as the honey sits at the bottom, and as it is colder than the tea, convection currents are not helping.

Also, mixing honey into your tea is also resisted by viscosity of the honey, so another starting point is to try and understand viscosity.

Now, as you know, I am an oil man, not an apiarist, so I am sticking my neck out a bit.  Never the less, the chemical compositions of hydrocarbons and sugars have much in common.

Hydrocarbons are molecules consisting of carbon and hydrogen, and the properties of the carbon atom mean that molecules can build up chains which can be extremely long.  We have only talked about the simple ones, but once you have at least six carbons, you can have straight chains, branched chains and even rings.

Sugar molecules are made up of carbon, hydrogen and oxygen.  The molecules look like carbon chains with some of the molecular bond sites taken by hydrogen atoms and some by OH units, if not taken by another carbon, whereas with hydrocarbons all the sites not taken by another carbon are generally taken by hydrogens.

Normal sugar actually has two rings, each of six carbons, joined together, and with the required number of H and OH units.  And these rings can continue to join together to make very long chains, just as in hydrocarbons the chains can get extremely long.

Now with the very simple compounds, even with short chains, the molecules are quite small compared with the space they occupy and the distance they move in the normal random motion due to temperature.  Liquids such as water, propane, butane etc can move relatively freely.  We see that in the low viscosity of these liquids.

But you can readily imagine that as those chains get longer, they start getting tangled up and having to slide over each other.  At a very simplistic level, water is like those “hundreds and thousands” coloured decorations used on cakes and children’s sandwiches (not to mention some adults).  Basically little coloured sugar balls if you use a different name.  While a very long chain tends to be like cooked spaghetti, probably best imagined as relatively short chopped strands rather than really long ones.  A spoon of the little balls spreads everywhere, while the spaghetti lands in a heap.  Don’t carry the analogy too far.  If you drop the spaghetti in a pot of water, it still lands in in a heap, but if you stir it a bit with a spoon, you facilitate the separation of the strands enough to get water between the strands and it can finally spread out to fill the pot.

A bit of an over simplistic analogy I suspect, perhaps someone can suggest a better analogy.  Honey has relatively long chains of those sugar rings, perhaps not as long relatively as spaghetti, but long enough that a single molecule cannot easily get free of the other molecules over its entire length, so it is more reluctant to move like a fluid with smaller molecules, and hence its viscosity.  With the spoon, you separate the honey and give a bit more surface area for the water to mix with the honey.

As Avtur says, we tend to put aside the molecular description of materials early and then forget it, but this is another case where thinking about the molecular structure can be helpful.

Hi Steam Haulage, thank you for looking in.  I hope that you are finding some topics that interest you, and that most basically make sense.

In talking about gently putting the low density fluid on top of the higher density fluid, I was just trying to avoid the vigorous mixing that occurs if you pour one of the fluids in, say from a jug some height above the surface of the other.  Pouring in very gently reduces the mixing caused by density gradient, if the lower density fluid is gently put on top.  In chemistry lessons, long, long ago, we were taught to hold a spoon horizontal near the surface of one fluid and pour the second gently onto the spoon so if flows at low velocity off the edge. 

This principle is used in storage hot water systems.the cold water is introduced at the bottom, while the hot is extracted from the top.  When you have a shower, and the cold water coming into the tank was allowed to mix freely, the hot water would start reducing in temperature quite quickly, and you would continually have to adjust the taps for comfort.  By reducing convection mixing, you get the benefit of higher water temperature for much longer.  There are also baffles around the heating element which also help with the issue.

Brownian motion the visible evidence of another mechanism for mixing, and this occurs as well as convection.  But it is a much smaller range action by our scales of dimensions, so it takes longer to extend its influence throughout a vessel.  So not a contradiction.  So in this small scale molecular motion is not generally very effective in complete mixing, compared with convection, especially if you are in a hurry.   With very high purity products, say the propylene used for polymerisation, around 99.99% purity, a small amount off spec material, can sit in the storage tank for a long time, instead of mixing, despite the molecular motion which might be expected to mix it all up.  It generally comes out as detectable off spec product, even when the quantity is small enough not to put the whole tank off spec if it was properly mixed.  You need energy input via mechanical impellers or jet nozzles on a tank inlet to ensure thorough mixing.

I hope that helps sort things out.

Thanks to everyone looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 16, 2019, 03:12:39 AM
Hi MJM  Et Al.  Thanks for the explanation  I didn't realise that it was that complicated at the molecular level !!  I was thinking the same might be for sugar cubes.?..I will try that tomorrow as it is 03-10 am and i am about to go to bed..... I wonder if Einstien thought about things whilst drinking tea and coffee !!

Willy
Title: Re: Talking Thermodynamics
Post by: paul gough on August 16, 2019, 10:06:11 AM
Hi MJM, Can the rate at which heat flows or is transmitted through a known material of fixed dimension with a fixed heat input at one end, under normal environmental conditions, ever vary? Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 16, 2019, 03:13:00 PM
Hi Willy, the composition of sugars rapidly gets more complex than hydrocarbons, and hydrocarbons get complex enough as the length of the chains increases.  Your normal sugar cubes are basically sucrose, which has two of those ring structures joined together.  Not linked like the chains on Chris’s Marion, but sort of welded together by an oxygen atom which joins the two rings.  So a lumpy molecule but not a really long chain.  Classed as a disaccharide, it is one of the simpler sugars, although there are also monosaccharides which have only one ring, the group which includes fructose.  And the molecules would seem to be relatively regular so that they stack together in a regular crystal.  When you put the sugar cube in a drink, the sugar molecules are quite compatible with water as the H and OH units attached to each carbon tend to attract the H or O atoms of water.  But when packed into a crystal or a sugar cube some stirring helps strip off the outside molecules and allow the water to get at the ones underneath.

Also while six carbons form the basic structure of the ring, which is normally drawn flat, it is actually a distorted ring in three dimensions, but perhaps that is going too deep.

I am sure thinking over a cup of tea or coffee has a long and honourable tradition.  And I believe history records that Archimedes did some of his best thinking in the bath. 

I am not entirely happy with my spaghetti analogy, as strands of spaghetti are quite flexible and bend easily, whereas I am sure that the molecular structures are not so flexible.  But how flexible or rigid a very long chain, I don’t know.  Also, strands of spaghetti are quite slippery, while the molecules with a long chain structure are a bit lumpy, so do not slide too easily.  But the long chain compared with a simple molecule of two or three atoms is the idea I was trying to suggest.

Hi Paul, good to hear from you again.

I am trying to understand the context of your question, as there seems to be two different concepts involved. 

First, conduction generally is proportional to the temperature gradient.  Mathematically, Q = k x A x (T2 - T1)/d, where Q is the heat transferred, k is the conductivity of the material, usually assumed to be a constant, A is the area perpendicular to the heat flow, and d is the distance between the surfaces through which the heat travels. T1 is the temperature of the high temperature side  and T2 the temperature of the low temperature side.

While the material conductivity, k, is generally considered a constant, it can be different at different temperatures, so with a large temperature range, allowance has to be made for this variation.  But the heat input to a system is not part of this equation.

Back to the problem, for any boundary surrounding a heat source, the if heat crossing the boundary is less than the heat input, the temperature inside the boundary will rise, so the temperature difference across the material forming the boundary will increase, so the heat transferred will increase in accordance with the formula.  The temperature of the source  will rise until the heat loss is equal to the heat generated.  Then the temperatures and temperature profiles will stabilise.  During the initial period there will be storage as well as transfer, but when the system becomes stable the heat loss will equal the heat input.  And the heat transferred will increase as the temperature profile develops, until that equilibrium is obtained.

Does that answer the question?

Thanks everyone for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: paul gough on August 17, 2019, 12:48:32 AM
Hi MJM, Knowing what to ask when you don't understand the process sufficiently is quite difficult. I am trying to visualize the 'heat energy flow' through a solid material. Lets say a copper rod of fixed dimension with a heat source at one end. If one could 'observe' what was happening at a single point along the bar, what would one see?? How is the energy transmitted, what enhances or impedes it, can the flow vary, could it be regulated and if it is not self evident in the answers to the previous questions, what causes the different rates of flow with different solid materials. The electric/hydraulic analogy and the bouncing molecules of steam imparting work to the face a piston are all able to be visualized once explained, but I find heat flowing through a solid material difficult to understand precisely. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 17, 2019, 03:40:47 AM
Hi MJM, Just a reply to the honey question... I tried putting a large brown sugar cube in some tea hot  water and the cube sank to the bottom and fell apart...1 Hour later it was still visually the same and did not taste sweet. once it was stirred the colour was dissipated and it tasted sweet . ...thinking about Einstein and Archimedes i wonder how quickly the advancement of science would have progressed if they had actually bathed in tea/coffee !!! :lolb: :lolb:

Thinking about Pauls question ..we know that materials expand when heated...however if a piece of a copper cube were heated between two very solid low thermal imouveable  objects  how would the copper try to expand?? would the molecules become oval on the restricted faces/plane or would the copper expand a larger/different amount on the un restricted faces ??

willy.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 17, 2019, 12:20:13 PM
Hi Paul, just to formulate the question in that way shows a high level understanding of the issue.  Even my trusty Thermodynamics text book dodges the issue of providing a definition of energy, simply stating that it is such a well understood concept that a definition is not really necessary.  It then goes on to define work and heat as energy in transit.  Work crosses a boundary when a force acts through a distance, while heat is energy crossing a boundary due to a temperature difference.  Energy is not a substance that can be identified moving around.  So energy can be stored as potential energy (height), kinetic energy (velocity) or internal energy (which is evident as temperature).  And it can move either as work due to work being done, or as heat due to a temperature difference.  And energy can be changed between the various forms.  It should also be mentioned that energy is not created or destroyed, conservation of energy is one of the fundamental laws of physics.  Mind you, it can sometimes be quite a puzzle to understand where all the energy went, or came from, but if we look hard enough it can be found.

Perhaps not a very good explanation, but that is the nature of the question.  So what does it mean in the context of your question?

If we consider a block of metal at some high temperature, the atoms all have energy, which if it was possible to see them, would be evident in their vigorous motion.  In a solid it is more or less like vibration around a point.  In a cooler block, the atoms are still in motion, but the motion is smaller amplitude or velocity.  The atoms do not all have the same energy, but the temperature is the result of the average. Presumably larger numbers of the atoms are closer to the average, while small numbers depart from average by a larger amount.  Think of the bell curve of a normal distribution.

If one surface of a block is heated to some temperature perhaps by a flame, or by contact with another block already at a high temperature, then the atoms near the hot surface bouncing against the slower moving atoms deeper in the block results in some energy exchange between those atoms, with the higher velocity atoms slowing and the slower moving ones increasing in velocity.  Thus some energy is transferred by the collisions, and while the temperature gradient is maintained the process continues.  If the whole block is at the same temperature, all the atoms have on average, the same energy level, and there is no transfer of energy.

If the material is relatively simple atoms in some sort of crystal array, like most metals, the process of energy transfer proceeds quote rapidly.  Think of silver, copper or iron and so on.  But if the block is a more complex compound, it is much harder to get those molecules vibrating in a pattern that results in predictable collisions.  It still happens, but proceeds more slowly.  Or if the material has a cross linked structure, like a plastic, similar resistance to heat transfer.

So the actual rate of transfer is determined to a large degree by the molecular structure of the material.  And not so easy to regulate in the way would regulate steam or water flow for example.  It is essentially controlled by the material molecular structure and the temperature difference.

Another example where the molecular theory of matter can help our understanding.  I hope it helps answering the question.

Hi Willy, brown sugar and honey are both quite different sugars.  The brown sugar is mostly sucrose, the ordinary table sugar, and a disaccharide, but the last of the molasses has not been washed out in the final crystallisation process in its manufacture.  It also traps some moisture which sort of sticks it together.  So it needs a bit of help by stirring to mix it up well with the water. 

Honey is mostly fructose and glucose, both simple monosaccharides, of which the glucose can chain together into long chains.  But there are over 100 other compounds in honey which contribute to its unique qualities.

Sugar just gets more and more complicated as soon as you look at it all closely.  Definitely well outside my comfort zone.  I prefer the simple things.

Your block of copper is much easier to discuss.

When you heat a metal, it expands in all three directions.  You can look up the coefficient of expansion many text books, or just search the web.  If you constrain this expansion as you have described, you get very high stresses which can be calculated by Hookes law and the modulus of elasticity.  These stresses will soon exceed the yield stress of the copper, and the atoms will be pushed around in the lattice, just as if you had squashed it is a vice, or hit it with a hammer.  Each atom effectively occupies the same volume, but the atoms are pushed around and the lattice distorted.  Similarly, with steel or other metals.  This is the source of thermal expansion stresses.  A major consideration in designing pipes for refineries and gas plants. If a metal object is heated unevenly, the colder parts expand less and effectively constrain the hotter parts which are trying to expand.

I suppose if those pioneers had actually bathed in tea or coffee they would have had scold injuries in places very embarrassing to explain, and it might have held back progress considerably.

MJM460

Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 17, 2019, 01:24:45 PM
Great explanation. I got it| So "cold" is like negative energy, sucking the energy out of the metallic atoms.  Is there a "cold front" as the boundary between agitated atoms and drowsy atoms progresses through the copper? Is that why filling a copper mug with ice, adding vodka and ginger beer will :cheers:  slow the thought processes if ingested?  I have seen this happen!
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 18, 2019, 01:11:45 AM
Hi MJM , thanks for the further info and i suppose the next question is ...what frequencies do the molecules vibrate at ?? also  i possibly won't be doing that experiment  with the tea !!

Willy
Title: Re: Talking Thermodynamics
Post by: paul gough on August 18, 2019, 02:27:30 AM
Hi MJM, Thanks for the detailed explanations. For me these things are best understood if I can convert to a visual representation in my mind and your descriptions have enabled me to 'see' what is going on. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 18, 2019, 12:58:25 PM
Hi Captain Jerry, I am glad that the explanation made sense, sometimes I wonder about my more wordy posts.  It’s good to have you on board.

I find it more logical to think of energy as a positive quantity, as negative motion in random directions as exhibited by the individual atoms does not make sense to me.  Energy is a scalar quantity, not a vector, so has no specific direction.  The flow of energy of course goes in the direction of the temperature gradient, and cannot go in the reverse direction unless you provide an energy input in the form of work.

It progresses across a material in a more gradual manner, not as a step or wave.  To get a feel for this on a very coarse scale, imagine the thickness that block we talked about earlier divided into ten equal parts, and look at the temperature gradient along a line through the block, perpendicular to the surface.  At the start, the temperature is the same all the way along the line, so no temperature gradient, no heat is flowing.  Now we raise the temperature of the surface.  Instantaneously for the first increment of thickness, there is a temperature gradient, so heat flows from the hot surface to the cold surface, but there is still no temperature gradient across the second or subsequent parts of the block.  The temperature gradients tell us that heat is flowing in one side, but not out the other.  The energy equation tells us that the difference between energy in and energy out is stored by the material heating up.

As time progresses, the temperature at the boundary between the first two increments rises due to the heat inflow.  This reduces the temperature gradient across the first increment, but introduces a temperature gradient across the second one.  And so on.  So some heat is stored in the material, some is lost from the other side, and the temperature gradient builds up.  Probably easier to sketch out than describe.

It is possible to apply the appropriate equations, to calculate the time all this takes, but eventually the temperature gradient becomes linear and constant, no further heat is stored.  It can also be done graphically with some calculated parameters for time.

You may realise that what I have described is a simple example of finite element analysis.  We had to do that exact example by hand when I was studying thermodynamics, as no one had a computer capable of doing it at all.  At that time, the UNIVAC computer had a whole 16k of memory for program and calculations, and was fed with punched cards.  And only the geeks got to understand, let alone use that.  Now days, no one, not even students would do it by hand.

And yes, I can see that the ginger beer, vodka and ice in a copper cup would slow the thinking, but which are the critical variables?

Hi Willy, great that my explanation made sense.  But the vibration issue is a bit harder.  For vibration, there must be a restoring force to keep reversing the direction.  I believe that in gases anyway, the molecules travel in straight lines per Newton’s law, until they collide with something, another molecule or a wall of the container, to exert a force which results in them taking off in another direction, like a 3-D billiards game.  I have a physics book which actually calculates the average velocity of the molecules, and the average distance between collisions.  But apart from a little review early in this thread, I have hardly looked at it in around fifty years.

I suspect even in solids, a similar thing happens, though whether the collisions are hard elastic collisions like the billiard balls or more like soft objects where the repulsive forces interact.  I am on shaky ground trying to describe that too closely.  There are vibrations in the system, as we can identify materials by the light they give off, but I will leave it to your theoretical physicist to explain that one.  And more particularly, to tie the two ideas together.

Hi Paul, I think the reason I like engineering and physics is that I can picture how things happen, rather than just learn the result.  My memory never worked well in that area.  Good that the explanation was helpful.

Thanks everyone for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 18, 2019, 08:36:01 PM
When I was In school, a physics classmate of mine could always be counted on to ask the obviously stupid question and after a chorus of groans from the rest of the class, we would often get into some of the most interesting conversations.  By having to rule out Howard's ridiculous hypothesis it was necessary to have a better understanding of the conditions being observed. For example, what happens at temperatures below absolute zero ( -459.67F or 0 Kelvin). A theoretically impossible condition because by definition, atomic motion would cease.


However, a group of German scientist at the University of Munich published their experiment in the Jan 4 issue of The Journal of Science in which they achieved a negative value on the Kelvin scale. Their findings suggest that the temperatures below O Kelvin are equivalent to temperatures above Infinity and that the temperature scale is circular and that the atoms in their experiment were in a state that was hotter than infinity. There are many more interesting possibilities from their finding, including heat engines operating at efficiency greater than 100%.


Why was such an experiment attempted? I am sure that someone like my old buddy Howard asked the question and then followed it up until it couldn't be ignored.  I hope I haven't muddied the water too much it is thinking like that that has led to expanded knowledge.


Jerry

Title: Re: Talking Thermodynamics
Post by: paul gough on August 19, 2019, 02:17:35 AM
Hi MJM, Your refined description of energy flow in reply to Jerry, 'painted' a very clear picture of the process. Thanks for creative writing. Regards, Paul.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 20, 2019, 12:23:15 AM
Hi Jerry, I must admit that I had always thought that molecular motion ceases at zero K and lower temperatures were not possible.  However I found out recently that Richard Feynman says that at 0 K there is still a tiny motion, but still that lower temperatures are not possible.  And who am I to argue with him?  A defeatist attitude I know, but it raises the question what is that residual motion, which I don’t think he answered in the books I have.  I wonder if it is a minor imbalance with the electrons whizzing around, which might give a small motion almost imperceptible unless you are very close to absolute zero.  But that is all conjecture on my part.

That article and experiment sounds interesting.  Thermodynamics as we know it says you need a lower temperature sink in order to cool something, which is surmised as not possible at zero K, so it is intriguing to think about how they did it.  But as we know, all laws of physics are only laws until proven otherwise, it’s just that the ones accepted as laws of physics have been well tested and no exceptions have yet been found.  It will be interesting to see if the the experiments can stand with full peer review and if others can replicate the results of that experiment.

But it all sounds very much like theoretical physics to me, very hard to see how we would use the principle in our model engine making.

Hi Paul, thanks for that.  It’s good to know the explanation makes sense.

Thanks everyone for looking in,

MJM460








Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 20, 2019, 01:20:25 AM
I would not argue with Feynman either, but If I was able to talk to him, I might say "I know its not possible but if you thought it was, how would you go about doing it?"  It is only theoretical physics until it is put to the test.  Extreme temperature experiments may have no relationship to machining as a hobby, but the question certainly does. "How would you go about it?" is a big part of the challenge for me.  I know it can be done with CNC but how would I approach it with limited manual equipment.


By the way, when you talked about doing finite element analysis by hand, I guess you had a slip stick in your hand.  I haven't seen mine in years but I know its here someplace.


Jerry

Title: Re: Talking Thermodynamics
Post by: MJM460 on August 20, 2019, 02:01:21 PM
Hi Captain Jerry, well said on the theoretical physics.  I must admit to thoroughly enjoying Richard Feynman’s lectures, or at least the summaries I have in two books of “easy pieces”.  I have found them quite readable and truly informative.

I still have at least five “slip sticks” that I could lay my hands on.  I get two of them out occasionally to try and keep my hand in, though I would not want to have to do the complex calculations I was once able to do.  That was all I had until I bought my first electronic calculator as a duty free purchase on a flight after my third job, about ten years of my work.  At least I did not have an abacus, but I did have a mechanical calculator that could multiply and divide, or rather that could be used for those calculations.  But a little child dropped it so that was the end of it.  You can’t keep everything of interest forever unfortunately.

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 21, 2019, 02:51:21 AM
Hi MJM,   ..Last night i had a bath and i always run the water to a temp of 105 F... and i keep checking the temp over the 10 minutes it takes to fill.  however the temp of the water by the time it filled it only got to about 101  degrees ?!  when put my foot in it it was
really very hot and when i checked the water again after a few minutes with it switched off it was 112 F !! I then realised that the Thermometer was giving the wrong reading . I think this was because the battery might be a bit low as the device was on continually for the 10 mins ?? is this possible with these type of devices and there is no warning on it to show the battery may be down.!!

Willy
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on August 21, 2019, 12:43:15 PM
Quote
is this possible with these type of devices and there is no warning on it to show the battery may be down.!!

Oh yes, very much so and for several reasons. Cheap sensors give a not very accurate voltage that is compared to a reference voltage and from that the temperature is calculated. So in your case I would say that the Reference Voltage can't be maintained @ low battery voltage => more or less useless readings.
Low voltage indications would drive the price up too .... A quality digital temperature measuring system will set you back hundreds of £.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 21, 2019, 12:50:13 PM
Hi Willy, it is certainly frustrating when a digital instrument does not give reliable answers, there is nothing to see to alert you.

Until recently I have been using electronic calipers.  I have always noticed that when I start getting erratic readings it is a sign that the battery is flat.  Even though the battery warning light is not showing.  After spoiling a few parts I started to check with a micrometer when the measurements are getting close.

I presume that your thermometer could be similar. 

I have recently bought new electronic callipers which I hope will be better, but I have not got to use them much yet.  Keeping my fingers crossed that they will be better.

MJM460

Hmm! I see that Admiral has answered while I was typing, I think we are saying similar things.  Sometimes I am tempted to try those more expensive instruments for my boiler tests, but usually either reneg on the cost, or can’t find something suitable.  But I am an old fashioned shopper.  It’s sure to be available somewhere.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 21, 2019, 08:22:53 PM
Hi MJM a new question a bit more fluidy dynamic...Does pressure  air, steam, water, etc act on a surface at 90 degrees ??   If so if a piston was cone shaped would the pressure be trying to push it sort of sideways at the angle of it .....rather than strait downwards in line with the piston rod ???

this may be a bit silly ?? :-\

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 22, 2019, 12:41:36 PM
Hi Willy, that is really getting back to the topic.  It gets to the basic  understanding of how fluid pressure works in a confined space.

I think it was covered in one of our very first fluid mechanics lectures, over fifty years ago now, the concept that pressure operates equally in all directions.  So for a little sample volume somewhere away from the walls within a confined space, the pressure acts the same in all directions.  Any pressure effect in any direction is exactly cancelled out by the equal pressure in the opposite direction.  Another way of putting it is that pressure is a scalar quantity that has no direction.

However, at the wall of any contained volume, the molecules causing the pressure can only come from one side of the surface.  The pressure acts at right angles to the surface.  Even molecules approaching the wall at an angle, are balanced by the effect of molecules approaching from the other direction.  Not on a one for one basis, but the sum total of the horizontal effect of all the angular collisions is zero, and only the perpendicular component of each collision contributes to the net force.

In your piston example, the pressure on the angled top of the piston acts at right angles to the surface causing a force proportional the surface area, acting at right angles to the surface.  The perpendicular to the surface direction is the direction of the force.  Force has magnitude and direction so force is a vector.

Now, if you remember your vector maths, a vector can be resolved into two components at right angles to each other, using the normal maths of a right angled triangle.  And each of those components is always smaller than the primary vector. 

If we resolve the force on the angled part of the piston as you have drawn it, the horizontal portion on one side of the piston is exactly balanced by the equal force on the other side.  The horizontal forces on the piston are balanced by internal stresses in the piston, meaning that any horizontal section through the crowned part of the piston is in compression.  There is no movement in the horizontal direction, so there is no work done or energy absorbed by the horizontal component of the force.

The vertical component acts in the normal manner to do work on the piston.  While the vertical component of the force at any point is smaller than the force acting on the sloped surface, the area of the sloped surface is larger than that of a flat topped piston.  The result is that the vertical component of the force over the whole piston top is exactly the same as the vertical force on a flat topped piston.

It does not matter if the piston top is conical or domed, or even asymmetric in some form to assist combustion, with calculus, the maths can always be solved and the result is always the same. 
When the vertical component of the force causes the piston to move, work is done by the gas on the piston, and this work is the mechanical work developed in engine.  Some is taken up in bearing friction, some in pushing the spent gases out the exhaust port, some absorbed by water pumps, cooling fans, valve gear etc and with some luck there is even some left to give a useful output.

I hope that answers the question.  Definitely not silly.

MJM460





Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 22, 2019, 02:56:09 PM
Wiley's question was about force, not work,so the short answer is "yes."
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 23, 2019, 02:59:50 AM
Hi MJM, thanks for the explanation...and i suppose if the conical piston were pointed the maths would still hold up !! ?  Does the surface texture also have any effect on the outcome of these forces..ie turbulence that i think was mentioned in a previous post ?

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 23, 2019, 01:30:34 PM
Hi Captain Jerry, I would have to agree with the one word for Willy’s first question, but while correct (for the first question), in my view, it is not at all informative.  Besides the post contained two questions. 

But significantly, a one word answer to a question on a complex matter does not fit with my learning style.  It inevitably leads to additional questions such as how? Or why? Or what are the implications? 

A one word answer also seems to me to reduce the subject matter of the question to a collection random unconnected facts to be learned off by heart, like times tables or spelling.  To me, this type of learning rarely leads to understanding.  I still can’t spell, and I really like having a calculator handy.  So while my style might justifiably be described as unnecessarily wordy, I hope it does help those who hang in there for enough of the subject, to connect the answer to things they already know, and preferably lead to a little more understanding.  Or at least prompt a few thoughts which might lead to more conversation and eventually to increased understanding.

Hi Willy, if the conical piston really comes to a point, it might result in the maths failing if it leads to a divide by zero for example, but the principle still holds, and the net force on the piston is straight along the cylinder.  However the maths mostly involves the projection of the piston crown onto a flat surface, which involves a simple regular shape, so I believe the divide by zero issue does not occur.

It does not matter if the piston crown is some irregular shape, rough or even an inverted crown to make a cavity in the top of the piston.  The total projected area in each direction is still the same, even if there are undercuts.  Though the internal stresses in the piston will be more complex.  And of course the side walls of the piston should preferably be smooth unless the rings are dimensioned to keep the piston from touching the cylinder walls.  The analysis of forces on piston rings and o-rings all relies on the same principle of pressure being the same in all directions.

In more advanced engine design, (another subject out of my league, but others are clearly well into it), the inlet to a cylinder is directed to give a swirl or perhaps a well distributed combustion pattern, or to avoid hot spots etc.  Certainly fluid velocities involved in these ideas will involve friction effects on the piston which are certainly not balanced in the way the hydrostatic pressure is balanced.  My answer is based on static forces due to pressure, and does not account for such dynamic effects.

Mind you, I would expect that the hydrostatic pressure forces, and the inertial forces due to the rotation of the engine and the continuously changing angle of the conrod are each much larger than the friction forces due to gas inlet velocities.  But whatever friction forces there are simply add to the hydrostatic pressure force.

I hope that helps,

MJM460

Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 23, 2019, 07:42:28 PM
I mean no disrespect to you or Willy by offering a one word answer.  Of course it brings up a lot of questions and that is how understanding is developed, not by rote memory. However it is very difficult to anticipate all of the questions and so the discussion of a silly question that you haven't anticipated is often worth having.


Resolving the force vectors on a conical piston face makes it easy to see that the horizontal vectors all cancel out at the axis of the cone, but suppose they don't cancel out. Suppose there is no corresponding opposite force?  What happens if the piston is cylindrical but the top face is the result of a diagonal slice...a flat surface at 45 degrees to the axis of the cylinder? It seems that all of the horizontal vectors are in the same direction.


Question #2. How would you go about making the piston rings?


Jerry
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 24, 2019, 03:15:21 AM
Hi MJM and Jerry,  The thing about delving in deeper with accepted wisdom and established facts is that there is always an exception to make the rule !!!  I do always have more questions with the answers given  to replies but they tend to sort themselves out with deeper thought and sleeping on it ...!!  Keep up with the good work with this fascinating subject...

Willy :) :)
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 24, 2019, 09:05:35 AM
Hi Captain Jerry, I know what you mean about it being difficult to anticipate all the questions that arise out of a particular answer.  Indeed, some of my longer replies have been due to that exact problem.  Often the more detail I put into my reply, the more aspects I see that ought to be covered, but in the end I have to draw a line.  On the other hand, there are times when a one word answer will do.  It depends on the context I suppose.  But your input is always welcome.  I much prefer conversation to one question, one answer.  Most things are more complex than that.

The cross section of the piston crown does not have to be symmetrical for all the horizontal components of force to be balanced out.  It is not easy to picture perhaps, but if a section is more parallel to the axis the area on which the pressure acts is smaller, while a section that is nearer flat as we normally understand the top of a piston involves a larger area spread over the same increment of height.  That is where calculus comes in, it basically divides the whole surface into small increments, each small enough that it can be considered a flat surface to which only one angle applies, then adding up all the horizontal components of the force on each small bit of the area.  It does not matter that your piston crown has a 45 degree slice, it will still be balanced by the components on the other side of the piston, and there is no sideways thrust due to the static pressure.  An inlet stream directed at that surface might provide a sideways thrust, but I believe that force would be very small compared with the pressure forces and the conrod forces on the piston.  Whether it is large enough to cause noticeable wear on one side of the cylinder, I don’t know.

Machining of piston rings is something I still have to learn.  I see Brian sometimes buys ready made ones, but many other threads describe how they are made by people who know far more about the procedure than I.

Hi Willy, sleeping on the question is a good way of using hidden brain resources to sort something out, it is an amazing and very peculiar part of how our brains work.  But as you know, you are always welcome to come back with follow up questions if I have not addressed the aspect that puzzles you, or if in the morning it is still not clear.  But as to the exception to every rule, it usually means the rule is incompletely stated, or perhaps even wrong.

Thanks everyone for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 25, 2019, 04:35:05 AM
Good evening, MJM,


Question #2 about piston rings was mostly facetious but question #1 about a diagonal piston face was very serious and I am afraid that your answer did not resolve it for me.  I have slept on it or to be more precise, I lost sleep over it, and yet it remains a question.  My "Crap-O-Cad" skills are not so good and they don't include color so I have had to resort to full-blown 3D Alibre to help me understand it and to be able to share it here as well.


Alibre' can output PDF files for general sharing so I have included a list of PDF files as attachments.  If you are not able to see these, please let me know and I will see what else I can get.  I can output .STEP files if you have a 3D cad that can read them.


The files show several views of a Green Piston in a transparent Cylinder.  The top of the cylinder is sloped at 45 deg to the axis of the piston and the face of the piston is colored Red.  For the purpose of discussion, the piston is a perfect fit in the cylinder so we can ignore any by-pass pressure acting on the flank of the piston...we are only talking about pressure acting on the Red face of the piston and forces acting on the exposed inner piston wall as well as on the cylinder head are also not part of the question, only those forces acting on the red face of the piston


The second PDF shows a view of the face straight on and of course this is an elliptical shape and the pressure in the cylinder, acting perpendicular to  the face is at a 45 deg angle to the axis of the cylinder.  If this force is resolved into two component forces, there is a vertical force (downward) on each integral location on the face.  There is also a horizontal force acting on each integral location and because it is acting at 45 deg to the surface, it is a horizontal force and is exactly equal to the vertical force.  Unlike an irregular piston surface, all of these resolve force vectors are parallel and in the same direction... there are no off setting (negative) vectors as a result of the pressure.


There is also a view of the piston from the top and the piston appears as a Red circle and the area of this circle that the vertical force vectors act on.  This force is countered by the piston rod and crank and can be measured (or calculated).  There is a bottom view of the piston as well and the bottom face is green which indicates that there is no cylinder pressure acting on this face.


There is a "front view" of the piston and the top surface appears as a Red circle of the same size as the Red circle when viewed from the top and this represents the horizontal forces acting on that face and this force is exactly equal to the vertical force. For clarity, we can say that this force is acting from "front" to "back."


Now, there is a "back view" of the piston and it can be seen that it is all "green", an indication that it is not seeing any cylinder pressure to off set the force from the piston face.  The only thing preventing the piston from being blown out the back wall is contact with the back wall of the cylinder.


There is also a side view showing that there is no left-right or side to side force on the piston.  In this view, the piston is all green.  The view from left and right sides are equivalent.


THEREFOR: It has been shown that for THIS configuration, there is force causing the piston to bear on one side of the cylinder wall and the force is considerable.


Jerry


PS: How's that for wordy?



Title: Re: Talking Thermodynamics
Post by: AVTUR on August 25, 2019, 06:03:01 AM
Jerry (and all)

Good morning

I have been following this but with quite a time delay. Therefore I have not entered the discussion. Also, I know nothing about sugar solutions.

I cannot open your .pdf files. Can you print and then scan them as .jpgs?

AVTUR
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 25, 2019, 11:39:58 AM
Hi Captain Jerry, now I see the issue you are thinking of.  The words did it, I am another having difficulties with the pdf’s, they appear for a second or two then become a blank page.  Very puzzling, as the thumbnail and the file size still seem visible and complete.  I have seen this issue before, but not sure of the cause.  I would suggest printing then scanning either as jpg or PDF, as scans saved in PDF format seem more reliable.  I don’t know if the cad programme PDF format is not quite standard, at least not to Apple requirements.

For the problem as you have described it, I have to agree, the piston would be pushed against the cylinder wall by the unbalanced force.  Fortunately the forces on the cylinder are still balanced, so the engine will not rotate around its crankshaft, or not due to this cause anyway.

 It then it is my turn to ask the ring question.  Do you think it possible to make rings around the cylindrical walls, and following that 45 degree face, to create this situation in a real engine, and if so, why would you want to?

(I should have attributed that question to its source, but I don’t know the name of the person who posed it to Alexander Bell, but we all know the answer!)

However, you are correct, and my answer to Willy’s question does not apply to this configuration.

I am also thinking about real engines in this context.  Could this configuration really be created?  If we have very smooth flat surfaces, and we press them together so as to exclude the air between them, they stick pretty well due to atmospheric pressure on the other side.  Gauge blocks can be wrung together in this way for example.  Blocks wrung together in that way do not slide very easily, so the question arises if we could make a piston to such a perfect fit, would it actually slide up and down as required for the engine to run?  I am not sure that it would be possible to make a piston and cylinder with such a fit all around, but I will leave it to others to answer that.

On the other hand if we assume a tiny gap around the piston, there will be leakage past the piston.  There will be a drop in pressure as the fluid flows down the cylinder wall where the piston wall is present, and I think we can see that the profile on the side with the short wall will be different from the profile on the longer side.  (Draw it out to make it easy to see.). This again leads to the conclusion that again, the force is unbalanced.

However in this case we are dealing with a dynamic pressure change as the fluid flows down the wall, so my answer has to be qualified as a static pressure analysis, with negligible change of elevation.

If we have a conventional well fitted ring located at a point where it can extend right around the piston, there will be gas pressure between the cylinder wall even where the gap is very small.  The horizontal forces above the ring will be balanced across every diameter, whether the wall is vertical or sloped at that point on the circumference.  And the forces below the ring will be balanced by symmetry, whether they are classed as static or dynamic pressures.

Does that adequately address your concerns?

But certainly, this is another case where the left field question leads to further understanding.  I hope this clarification actually clarifies, and that you do not loose anymore sleep.  I don’t want to cause that!  And I hope that between us, we have not added confusion.  But if things are still not clear, please continue the conversation (after some sleep!)

Hi Avtur.  I also had troubles with those pdf’s, but I am not really an expert on sugars.  I did just on spec, try searching for the chemical composition of sugars and found some very good articles among all the usual rubbish.  To open them without any background would have been truly intimidating.  However the similarities with the structure of the hydrocarbons, which I am familiar with, were striking.  It made them relatively easy to read enough to see how it applied to the question at hand, and I hope it allowed me to give a reasonable answer to Willy’s question.  But it certainly did not make me an expert on all things sugary.  Most of us have knowledge in our specific field that can help us understand things in other areas if we can find that initial similarity.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 25, 2019, 03:48:27 PM
Good morning MJM,


So you see that the piston ring question was not totally facetious.  It is relevant only as it is used to define the shape of the expansion chamber and the intersection of the face of the piston with the cylinder walls.  As you point out, if the piston ring is square to the cylinder then all horizontal vectors above it resolve to zero.


The piston ring could be an O-ring but it was the problem of cutting the ring groove parallel with the face that disturbed my sleep.  Some people may be able to solve a problem while sleeping, my sleep engine seems to have a way of creating a problem.  The ring problem is not the ring. It is the sealing of the piston (back to the hobby aspect) but since this is a theoretical experiment, I chose to provide the seal by defining it as a perfect fit.  I suppose an oil film could do it well enough and would also solve the friction. 


The engine rotation problem that you mention could be solved by alternating the orientation of the slope in a multi cylinder engine or it could be put to good use in a radial engine.


To those that tried and failed to open the .pdf files above, I apologize.  I think the problem is that my aging version of Alibre' outputs an obsolete version of .pdf so I have attached jpeg files that should open.


I too, hope that this has not muddied the waters by stirring up the bottom but it should be remembered that no laws were broken or even bent.


Jerry
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 26, 2019, 01:24:59 AM
Hi MJM , well ,i wasn't expecting quite as much discussion as this has generated but it is good that this concept has been explored...it is one of those questions that may seem intuitive but then becomes much more complex ..so thanks to everybody for chipping in   :D :D I did have more questions that would have followed on ,but they seem to have been explored by others...

So thanks to everybody for looking in

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 26, 2019, 12:54:00 PM
Hi Captain Jerry, the extra conversation is most welcome.  Doing the “thought experiment” to simplify a complex problem to a simpler one that can be analysed is often a useful exercise. 

I had certainly not considered the particular case, as I was only thinking of a piston with a crown that was not flat.  So normal rings, when the crown does not have to be symmetrical.

The analysis assuming a pressure profile through the leak path around the piston does indeed support your thought that there would be a sideways force.  Even with an oil film, which will also carry the static pressure.

But I return to the question of whether this would be useful.  The friction due to the side force would presumably be enormous.  The normal symmetrical force distribution minimises this, and with a flat topped piston, or a symmetrical one, and sufficiently close tolerances, an oil free compressor can be built, that relies on grooves without rings to create a Labyrinth to minimise the leakage flow, and relies on the fact that there are no sideways forces on the piston to cause contact with the cylinder walls.

Similarly we can look at a conventional slide valve steam engine.  In larger sizes and with high pressure steam, the out of balance forces on the valve surfaces create such enormous friction that piston valves are the preferred solution in those cases.

So quite a bit of learning involved due to your out of the box thinking, thank you.

Hi Willy, certainly the unexpected leads to more creative thinking.  It is fascinating, the turns which this thread has taken.  Glad that you are still following along.

MJM460




Title: Re: Talking Thermodynamics
Post by: derekwarner on August 26, 2019, 01:20:07 PM
 :old:.............message deleted ..........Derek
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 27, 2019, 01:11:11 AM
Hi MJM, A more practical question now..we have learnt about labyrinth seals ..so are there tables for best practice with cylinder diameter and pressure.?  Say a 20 mm bore and 50 Lbs  square inch..with a 8 mm thick piston... also will the individual grooves vary ??. so number of grooves ,width and depth ?? Another short question !!!  Also with this weeks bath i switched off the temp gauge between  readings .As last time the gauge read a lower reading after 10 mins  and yes it did, it went from 24  to  18 after a while I was wondering if the meter would also read a lower reading from ambient when first switched off after 10 mins ?? !! Really Hot today 33 celsius !! Also with labyrinth grooves ...if you had a really long cylinder with a very long piston could you have enough grooves to actually stop the piston moving ??

willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 27, 2019, 12:26:31 PM
Hi Willy, while conventional piston rings attempt to block off the leakage path between the high pressure above the piston and the lower pressure underneath.  When they are made in one piece, with a small gap, that gap is the major leakage path for a well fitting ring in a smooth cylinder.

Labyrinth pistons do not attempt to block the flow path, but simply to reduce the leakage flow to an acceptable low level.  There is a complete, though small gap between the piston and the cylinder through which gas can flow.  The labyrinth is constructed by grooves in the piston.  When the gas flowing down the wall reaches a groove, it expands into the greater flow area and because it is a sudden expansion, there is little or no pressure recovery, Bernoulli does not apply to sudden changes in the flow passage, there is loss of the kinetic energy of the gas into turbulence.  It is then accelerated into the small gap separating the grooves, which again requires energy which comes from the static pressure.  So at each groove there is more pressure loss for a given flow than if the groove was not there.

The total pressure difference between the piston top and bottom is determined by the operation so all the pressure losses result in a smaller flow. 

To be really useful in a high pressure situation, you need a long piston with many grooves.  However, in my small steam engines, I take the approach that grooves result in less blow by than a smooth piston wall, so can’t do any harm.  In reality my piston is probably too short and too few grooves to make much difference.  But having machined the grooves when the piston was on centre in the lathe, I can come back and add some cotton or graphite packing, which will almost certainly be more useful.

In full size, the principle is used for oxygen compressors by one company that I know of, you can search for labyrinth piston oxygen compressors.  They are expensive beasts so really only used where there is no other viable alternative.  They have to be very accurately made with very small clearances.  I am sure that they would have guides or calculation ,ethos to give the required number of grooves, but they would not be likely to publish that.

I have described the action as though it is all static, but of course the piston is moving in the normal way, and this complicates the flow analysis, but you can see the piston is surrounded by a flowing gas, so I feel that the actual drag on the piston will be quite low.  The drag is mostly due to the viscosity of the gas which is, of course, much lower than the viscosity of any oil film.  I can’t see that it would ever actually stop the piston, more likely it lowers the friction drag a little, but I don’t want to have to try and prove it.  I hope that answers the question.

With regard to your meter, I would be suspicious of the batteries.  I don’t know what sort of power supply it uses, but in my experience, cheap measuring instruments rely on a stable power supply as Roger stated.  If the battery voltage tails off as the battery is used, that will affect the calibration.  New batteries of the best possible quality may help you get more consistent readings.  With button cells, you need the Silver or Lithium chemistry, depending on the voltage your instrument needs, not alkaline batteries.  If it uses a 9V or normal AA or AAA batteries, you need reasonably fresh ones of the best quality you can get.

Hi Captain Jerry, one further thought on your piston with the angled top, when we were talking about potentially being useful for a radial engine, we missed the point that the forces on the cylinder are the same whether from the piston or directly from the gas and always balanced for every case I can think of.  So I think that leaves us with the friction as the most obvious effect of that arrangement.

Hi Derek, good to see that you are still looking in.

Thanks to everyone looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 30, 2019, 01:52:23 AM
Model engine pistons don't have to fit all that well, at least mine don't.  They don't have to do anything but run smoothly so I can appreciate the symphony of motion. Loose fits make smoother movements with less friction and they will run slowly, almost without effort so I can run them without the hissing sound of air bypass and compressor noise.  That is one way to enjoy your time in the shop.


But it leads to a question.  Why don't we run our model engines on higher pressure, say in the range of 50-60 psi?  Or even 80-100 psi? Because with no work to do they would overspeed and tear themselves apart or jump off the bench.  They would need a governor to limit their by reducing avail air pressure or volume and/or both and if we load them up with some kind of resistance like a friction brake, their air leaks and resulting condensation running every where would be embarrassingly obvious. 


To specify a volume of air without also specifying the pressure and temperature has no real meaning. If an engine will run nicely on a given volume of air at 10 psi, why can it not be made to run well on 1/5 of that volume at 50 psi, assuming a constant temperature?


I seem to be spending most of my shop time these days devising methods of eliminating air leakage and bypass and ways to measure the results even if only on a comparative basis.


Jerry





Title: Re: Talking Thermodynamics
Post by: MJM460 on August 30, 2019, 12:42:46 PM
Hi Captain Jerry.  A very interesting question, as it highlights the procedures necessary to analyse what is going on in our engines.

There are many variables in the problem, but not all are independent.  There are only so many degrees of freedom in a particular problem.  Each variable that we specify reduces the number of degrees of freedom remaining.  We can specify some values but not all.  Once the necessary number of variables has been specified so that there are no more degrees of freedom, all the other variables are dependent on those and can be calculated with an appropriate analysis.

In your particular problem, I am assuming you are referring to a reciprocating engine and you are running it on air.  We can look at steam later if you like.

As you say, if the engine runs nicely with 10 psi, and we try and supply it with 50 psi without increasing the load, it will accelerate until either friction absorbs all the extra power, or something breaks.

To answer the question of why we can’t run it on 50 psi at one fifth of the volume of air, let’s look at those variables.  I feel that in you specification of the problem, you have in mind an operating speed of the engine that allows you to see the motion in action.  The volume in the cylinder of a reciprocating machine is fixed, and hence at fixed rpm, volume of air consumed is fixed, it is no longer an independent variable for us to specify.  Volume and pressure are enough to specify the work done by the engine.  The friction is basically fixed by your construction so that it runs freely, so if you increase the pressure, the speed will increase.  If you increase the pressure, you increase the forces on the piston, so the engine speed will increase, thus increasing the air volume used, or you might increase the load to absorb the extra energy being supplied.

Without a governor, and with sufficiently large pipes and valve ports, you might assume that the cylinder pressure is approximately equal to the supply pressure.  However, if you add that governor to throttle the incoming air, you no longer know the pressure in the the cylinder.  If the engine still runs at the same speed as it did with 10 psi, we can assume that the throttle is actually reducing the supply pressure to about 10 psi.  A much smaller volume of 50 psi air is being drawn from the compressor/accumulator, and this volume of 50 psi air is throttled to 10 psi with a consequent expansion in volume to what your engine was using with the 10 psi supply.

If instead of adding a governor, you put a brake on the engine, and increase the supply pressure to maintain the speed, the engine will be doing more work, and as the volume is the same for the same operating speed, more pressure will be required to supply the required energy to drive the extra load.

Or in principal, you could apply a load as you increase the supply pressure, and allow the engine to slow so that at 50 psi, the engine was running at one fifth of the original speed, so using one fifth of the volume from your air supply.

I say in principal, because I am assuming that in order to see the motion in action, you are probably running near the minimum speed the flywheel can maintain.  Unless you have a multi-cylinder double acting engine, it might not actually run that much slower, as the nature of a reciprocating engine is that the torque from each cylinder is developed in a fluctuating manner.   Continuous running is dependent on the flywheel storing enough energy during each pulse to keep the engine running through the low torque angles.

I wonder if that answers your question, or are you thinking of another configuration?

MJM460




Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 30, 2019, 10:04:48 PM
I am of course thinking of a specific configuration but not all that unusual. Trying to make it as simple as possible, lets take time out of the question by looking at only the power stroke of the piston from TDC to BDC.  Lets assume a cylinder bore of 1.414 inches so that the piston surface is 1 sq. in. and a stroke of 5 inches.  That is a very long stroke but it helps with the math.  And of course we need perfect sealing of the piston and of the valves so that there is no by pass.


Now with the piston at TDC, we open the inlet valve to an adequate supply of air at 50 PSI. but nothing happens because I have a firm grip on the flywheel (the load).


Now I manually allow the flywheel to rotate until the piston makes 20% of its stroke. The resulting swept volume is 1 cu.in. and it fills with air at 50 PSI. and the force on the face of the piston is 50 lbs. so to prevent it from breaking my wrist, I slam the inlet valve closed.  But the force on the piston is still 50 pounds so I let the flywheel rotate a bit more so that the piston moves another inch, doubling the swept volume and reducing the pressure by half.  Now the force on the face of the piston is 25 pounds. 


I let the piston travel another 2 inches, again doubling the volume and halving the pressure so that the force on the piston is now down to 12 pounds. Still a considerable force.  During the last possible travel of the piston the volume is increased by 20% and the pressure decreased at the same ratio so it is down  to about 9.6 psi when the exhaust val opens, still way higher than the 5 psi we were comparing it to.


So if I want to run my engine on 50 psi. air, instead of having the governor throttle the pressure to 5 psi, I have the governor linkage to close the inlet quickly at some fraction of the piston stroke and allow the trapped volume of air apply the appropriate force to balance the load.  What I am describing is of course well known as early cut off ala Corliss, Greene.


Have I described this correctly?




Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 31, 2019, 01:41:13 AM
The above comments really needed to be expanded but the dinner bell rung so I had to wind it up, so this a continuation. Read the previous post first.


To continue, if the piston had been allowed to move only 1/2 inch before the inlet valve closed, the pressure in the cylinder after it had moved 1 inch would still be 25 psi. and even if the valve closed after only 1/4 inch of travel, the pressure would still not be reduced to zero after the full 5 inch stroke. And this is with AIR!  So tell me again, why the expansion of a volume of air cannot drive a model engine?


Oh! Temperature!  Air cools as it expands and steam expands as it cools.  What kind of linguistic mumbo jumbo is that! It is actually the other way around. Air expands as it cools and steam cools as it expands.  What's the difference?  Is that why steam engines have to be run in the back yard? 


It must be getting late. I'm amusing myself with half stated arguments.  The TRUTH is that steam looses it's pressure when it is cooled!


Jerry



Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 31, 2019, 02:53:25 AM
Hi Jerry, I think the terms we use have been in place getting on for 300 years .!! the term expansion always confused me until i read in an old book that the elastic nature of steam was a bit like an elastic band pulling the piston down the cylinder !!  also some people say that you don't pull something towards you ...you push it from behind towards you !!  I hope this sheds some light on the matter and it would be much more difficult trying to explain it using German vocabulary !!! with all their multi syllabicular words !!.thinking about the Mallard locomotive that travelled at 126 miles an hour the steam had not much time to cool down whilst passing through the cylinders !!

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 31, 2019, 04:20:49 AM
Hi Captain Jerry, that is a different problem that requires a bit more analysis.  But first, let’s back up a bit. 

(Now with time zone issues, I had written all that follows before I saw your follow up post and Willy’s post.  I think those two follow up posts are on an equally interesting but different track, and what follows still applies to your question.  I will then try later in the day to address the additional issues you have raised.)

There are two basic approaches to analysing a reciprocating machine.  First you can treat it as a highly variable process with inlet strokes and exhaust strokes and highly fluctuating torque and speed, a highly unsteady situation which you can analyse moment to moment.  This approach is necessary to closely look at what happens within the machine.

Alternatively you can notice that all those fluctuations are not random, they are cyclic in nature, everything repeats in a predictable manner, so you can take long term averages, and treat it as a steady flow situation.  Perhaps more precisely a quasi-steady state system.  In this case you look at average values over a complete cycle, and don’t worry too much about the moment to moment fluctuations.  This approach is very satisfactory when you are mainly interested in the situation outside the machine, when the machine is only part of the system, and you are really interested in the locomotive,  ship or refrigeration system in which the machine functions.

Yesterday I used that steady flow method.  There is a well known formula for the power produced by a reciprocating engine,
Power = P x L x A x N / 33000.

Where P is the mean effective pressure difference across the piston through the power stroke, L is the length of stroke, A is the area of the piston, and N is the RPM of the engine.  It is commonly used in calculation of engine power from an indicator diagram.  Because the pressure is a differential pressure, atmospheric pressure is not relevant.

The constant 33,000 is an arbitrary constant to account for the curious mish-mash of units that constitutes the imperial system.  I won’t confuse you with the various historical metric systems (which have the same problem) or the SI system (which does not) at this stage.

If you look at that formula, L x A is the swept volume of the engine, or V.  So we have Power = P x V x N / 33000.  With this version you can easily see that for a given power, if you double the pressure you have to halve the swept volume to produce the same power as you proposed.  So an engine with half the swept volume will require double the pressure deliver the same power at the same speed.  But once you have built a reciprocating engine, the swept volume is no longer a variable.  Note swept volume does not include clearance volume which is more easily made changeable.  You can also see that there is an implied assumption that your air pressure is measured and supplied in a manner that makes the assumed pressure a reasonable estimate of the mean effective pressure, which is not totally unreasonable for a simple engine with the valve open for a substantial portion of the stroke in this context.

Your new statement of the problem can still be treated the same way, but now the actual mean effective pressure is very different from your supply pressure.  Without involving calculus, let’s try and estimate it.

First, let’s continue with your 1 square inch piston area, I will leave you to check the piston diameter.  Then we need to use absolute pressure to correctly estimate the change in pressure with volume.  Let’s approximate atmospheric pressure to 15 psi.  (Instead of 14.7) so your initial pressure is 65 psia.  Now, to calculate the change in pressure when you allow the piston to move, you have to allow not only the volume change due to the piston movement, but also the clearance volume.  Again, let’s opt for easy maths and use 10% clearance volume.  It’s a bit high, but will show you the effect more clearly.  With your one square inch piston and five inch stroke, that makes the clearance volume 0.5 cu. in.  When the piston moves 20% of the stroke, or one inch, we now have a volume of 1.5 cu. in. at 65 psia.

Now this is allowed to expand with the inlet cut off.  At the bottom of the stroke, the volume is 5.5 cu. in.  At this volume, the pressure is
65 x 1.5 / 5.5 = 17.7 psia or 2.7 psig.

To estimate the mean effective pressure over the range 50 psig at the start to 2.7 psig at the end of the stroke, we could take a simple average, about 26 psi.  However the curve of pressure against volume is a hyperbolic shape, and so the mean effective pressure will be something less than this.  We could make a better estimate by calculating the pressure at 50% of stroke, and  calculating an average for the first half and again for the second half, or plot the curve on graph paper and count squares for a better, but still always high estimate.  The repetitive calculations quickly become tedious, so these days we would use a spreadsheet.  Or the integral is probably on your kid’s graphing calculator so you can calculate the area under the curve and use that.  Better to let the kid drive the calculator, I never had a calculator that could do that, and have not learned to drive one, but they are common in secondary school now.  But the answer is probably in the region of 20 to 25 psi.

I think you can see now that your initial 50 psig supply pressure is not the right figure to plug into the P x V calculation.  Of course at the 10 psig supply pressure all the same calculations apply, except that if you cut off at 20% with a 10 psig supply you would expand to less than atmospheric pressure so the pressure difference across the piston will be negative and the piston will probably not travel to the bottom of the stroke without help.

(10 psig = 25 psia.  Expansion from 1.5 cu.in to 5.5 cu. in. by the same calculation as above gives a final pressure 25 x 1.5 / 5.5 = 6.8 psia which is a pretty good vacuum!)

So behind your conundrum is the implied, but perhaps not recognised, requirement to use the mean effective pressure difference in the calculation, along with the effect of that early cut off on that mean effective pressure.

I hope that helps a little.  Back on the additional issues you have raised later.

MJM460



Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 31, 2019, 06:50:34 AM
MJM


I am sorry to say that does not answer my question at all.  The question was why do we run our small engines on 5 psig rather than 50 psig.  and the answer that I expected was that if the speed of the engine is controlled by a throttle type control on top of the steam chest, then the pressure in the cylinder is not at all related to the pressure at the gauge.


In my rush to describe a system before dinner, I incorrectly stated the diameter of the cylinder but after that I was considering the surface area to be 1 sq. in. for simple understanding.  I also chose to ignore what you are calling the clearance volume which I assume to be the space above the piston at TDC because it is arbitrary and only complicates the calculations and because it is possible to conceive of a case where it would be approximately Zero.  Then if I allow the piston to move 1 inch and to hold 1 cu. in. of air at 65 psia, and then close the inlet valve, we will be dealing with the pressure drop that will occur if the piston travels another inch.  I believe that the pressure will drop 50% and be 32.5 psia.  and if the the piston travels another 2 inches, the result will be a volume of 4 cu. in. of air at a pressure of 16.25 psia.  With only another inch of travel available, the volume can increase another 25% of that and at BDC, before the exhaust valve opens, we will have 5 cu. in. of air at 13 psia or -2 psig.  so maybe the exhaust valve should open slightly before BDC.


I chose not to consider average pressure, but it is obviously a positive value, just as I have chosen to ignore the geometry of the crank and the relative torque at the various angles, but it is obvious that an air input pressure of 50 psig. (65 psia) of air with flow cut off at  20% of stroke will be able to turn the crank with a minimal load.  If the load is increased, the cut off point will have to be delayed by some factor and the engine will still turn and if we want the engine to run faster, the cut off point can be delayed further.


I was never expecting the engine to run on 5 psig with an early cut off.  It is also obvious that if an engine with early cut off gear were to be supplied with insufficient air pressure, the governor would be expected to increase the length of time before cut off.


The purpose of my question was to clarify that compressed air does expand and the expansion is sufficient to operate an engine with early cut off gear and to refute the often expressed statement on this and other forums that it is only possible with steam.


I could be wrong as I have mistakenly stated previously.


Jerry
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 31, 2019, 10:53:10 AM
Hi Captain Jerry, I will try and answer your update while you sleep, as I am sure that some of the points are due to the Apple spell checker which seems to be doing a good job of proving its algorithm wrong these days.

I suspect I have already answered some of the points in the first paragraph or two, but I will emphasise that I have never told you or indeed anyone else that you can’t run a model engine on air.  In fact I have gone so far the other way, that I have actually commissioned a full size steam turbine on air.  We could not make the steam connection for the new turbine until the plant was shut down, and the client did not want to shut down the plant until the system had been proven.  Only a small one, from memory less than 50 kW, but it ran so quietly and smoothly on air that three experts witnessing the test thought it was not running until the exhaust piping started to ice up.  When air expands through a turbine, it does get cool!

So on to the next statements.  First it is necessary to understand that expansion describes a path function, that is, knowing that the air is expanding does not tell you how the air will end up unless you know the conditions under which it is expanding.  Probably not a generally understood concept. 

For example, it can be expanding at constant temperature, which requires external heat input.  It can be expanding without heat transfer (adiabatic expansion), it will cool.  it can be expanding at constant pressure.  In this case it will be doing work but the energy expended is being replaced by the necessary fluid input and I suggest it will neither increase or decrease in temperature unless there is also heat transfer.  This one is more tricky, and I might have been wiser to leave it out.  And the expansion could involve more than one of those conditions.  I am sure that you can think of other possibilities, but in each case, the end result will be different and determined by the path.  After the expansion, you know the volume, but you don’t know the pressure.

So, “Air cools as it expands”.  Generally true, unless there is heat input during the expansion.  Try blowing out through tight lips as if blowing out birthday candles, with your finger, you can feel that the air is cool.  If you blow out with your mouth wide open, the air will be warm, close to blood temperature.  Try it!  The cool air for the candle also started at blood temperature before the expansion as it leaves your lips.

“Steam expands as it cools”.  The statement suggests the expansion is the result of the cooling.  I would prefer to say the same as for air, steam cools as it expands.  If you are cooling the steam or air by heat transfer, they both will be reducing in volume not expanding.

With steam there is the additional complication if you are allowing heat transfer, in that the steam can condense in full or in part, which further reduces the specific volume, and the effect may be more or less than the effect of the volume increase.  In fact, even without heat transfer, if the steam expands over an appropriate pressure range while doing work, it will cool enough to partially condense in the cylinder, reducing its specific volume and hence pressure even further, so reducing the work output that might be expected for that degree of expansion, basically by reducing the pressure more than you would expect for the volume change.

Finally, “steam looses its pressure when cooled”.  Well remember that path function again, you have not sufficiently defined the problem.  Imagine an inverted cylinder with a piston making the top end closure.  Assuming the normal perfect seal, no friction.  And a brick or two sitting on top of the piston.  To admit steam, the pressure has to be sufficient to lift the bricks.  Before the piston reaches the top of the cylinder, shut off the steam.  As the cylinder walls transfer the heat from the steam to the atmosphere, the steam reduces in volume as before, but this time the bricks on the piston will maintain the pressure.  We have a case of cooling at constant pressure.

So with statements like expansion, remember to define the path.  Just for interest, work is also a path function.  The end point properties of steam or air in a cylinder doing work depend on the path taken.

You are right about the linguistic mambo jumbo, but I hope I have clarified some of it.

Hi Willy, it is to remove that rubber band analogy that I so often refer back to the molecular theory of matter, and the collisions of those atoms with the vessel or piston walls that is responsible for the pressure.  There is no negative pressure in terms of absolute pressure.  The atoms do not pull the piston, rather, when the pressure is sufficiently low on one side of the piston, the pressure on the other side of the piston is pushing it.  They are always there on each side pushing it, it is simply a matter of pressure difference that determines the direction of the resultant force.  The net force can come from the other side to where you are looking.

Your point about the Mallard cylinders not having much time to loose heat is quite valid, hence so much analysis done on the assumption of adiabatic expansion, i.e. no heat transfer.  But we must always remember that is an approximation to simplify the analysis, and if you can burn your finger on the outside of the cylinder, there is heat transfer.  You would have to run pretty fast to do the experiment.

Hi again Jerry, you are coming back faster than I can answer.  Just as well I have an unusually quiet schedule today.

I think we are getting confused by the terminology here, at least I am.  I am a fairly literal kind of beast.  When I talk about running the engine at a certain pressure, I mean the pressure at the steam chest, which with sufficient size valve ports and passages, is some sort of approximation to the pressure at the piston face while the valves are open.  You are quite right that the supply pressure tells us nothing about the pressure at the engine if we have a manual or governor controlled valve in between.

So if your question is why we run our engines with 5 psi instead of fifty, it is because we make them so large, and apply so little load.  If you want to run at 50 psig with no load other than the inherent friction in the engine, you need to make a much smaller engine.  George B can do it, but most of us seem to find it easier to make larger components, so they are vastly oversized for the work output we demand.

If your question is why we use a 50 psig supply when we only need 5 psig at the engine, the reason depends on whether we are running on air or steam. 

With an air compressor, we tend to like to shut off the compressor to stop the noise.  Then we are running on the energy stored in the accumulator.  And higher pressure means more stored energy so we can run our engine longer before the noise returns.  If like me, your compressor has no accumulator, then it’s a matter of balance between the volume of air supplied by the compressor and the volume of air consumed by your engine.  The compressor throughput actually reduces as the discharge pressure increases.  If your engine is using less air than the compressor provides, the pressure will rise.  In practice it will rise more than we want, so the compressor is equipped with an adjustable relief valve to control the pressure at the compressor, by venting some of the air to the atmosphere.  Our throttle valve if we have one then further reduces the pressure at the engine to control the speed.  Unless we put a pressure gauge on the steam chest (air chest?) we don’t know what the pressure at the engine really is.  I usually set my relief valve to the minimum, barely readable on the gauge, and the engine speed is what it results.  Probably higher than you are trying to achieve.

If you are running on steam, there is a good reason to run the boiler at much higher pressure than we need at the engine.  The volume of the steam produced by a given heat input obviously depends on the pressure.  If the volume is larger, a greater disengagement space is needed above the water level to minimise the water carryover.  If you run the boiler at higher pressure then throttle the steam to the engine, you get less carryover for a given boiler size.  That said, with my little boilers, I don’t have a throttle valve, I have relatively large piping, and with my little meths burners, the steam chest pressure is not much less than the boiler pressure, and providing I take care not to overfill the boiler, I don’t get excessive carryover operating at quite low pressure.

Jumping to your last paragraph, air under pressure is definitely able to expand and drive a model or indeed a full size engine.  Many of the statements of the type you mention may be in relation to compound engines.  Whether full size or model, you need enough pressure at the inlet that the exhaust after expansion is still above the exhaust pressure.  With air, in practical terms, this exhaust pressure must be above atmospheric, and with zero load on the engine and the necessary inlet pressure, it will not run slowly.  And the exhaust temperature will be quite chilly.  With steam, providing you have an air pump and a condenser, you can run with a lower exhaust pressure, but again unless you put an appropriate load on the engine, it will probably run faster than you want.  Even if you use atmospheric steam pressure inlet, you could still exceed your five psig differential pressure with an effective condenser.

If you expand to a pressure lower than the pressure in the exhaust system, air or steam from the exhaust system will rush into the cylinder when the exhaust valve opens.  You could open the exhaust early, and hence balance the cylinder pressure to atmospheric for the remainder of the piston travel.

The energy in the steam or air is proportional to the mass of air or steam admitted in each revolution.  The mass is proportional to pressure with other factors constant, so if you admit your motive fluid at higher pressure, there is more mass, therefore more energy, and under higher pressure, the engine will run faster unless you increase the load.  It is all about energy balance.  You can’t defeat conservation of energy. 

While you are dealing with work and energy and not concerned with torque fluctuations you don’t need to consider the conrod geometry.  You have defined piston travel which is sufficient.  Note that the time for each increment of piston travel and angle of rotation of the crank shaft will be different for each increment.   If you want to delve further into torque and torque fluctuations, then you will need to consider the con rod and crank geometry.

It is difficult to achieve zero clearance volume, but you can get it close enough to ignore if you want. So long as you are aware of the effect this has on the results of your calculation.

There is one more area for confusion in this discussion.  We are actually using that P x V product in two very different ways.  And it is very easy to unconsciously switch without noticing.

The product of P x V appears in the calculation of the power output of the engine.  In that formula, you need to use the mean effective pressure, otherwise you get the wrong answer for the power developed.

When you use the P x V product to calculate the pressure after a volume change, you are assuming the gas behaviour is the same as a theoretical ideal gas.  Then you use the actual absolute pressures and the volumes as you have done.  When you are using air, this is a reasonable assumption.  However there is a trap if you are using steam.  Steam at the pressure and temperatures we are usually involved with in model engines, is very close to that two phase region of the property map.  Under these conditions steam behaviour is not nearly as close to the ideal gas behaviour, and it is best to use the steam tables to determine the pressure from any change resulting from the expansion process.  As we don’t accurately know the path of the expansion, we have to determine the necessary two independent properties from the conditions we do know at the exhaust condition, and use these properties with the steam tables to determine the final pressure.

I hope I have not introduced too many confusing statements.  And I hope that by continuing the conversation, we are increasing understanding of how the engines work.

Thanks to everyone who is still following.  I hope it’s not getting too heavy.

MJM460
Title: Re: Talking Thermodynamics
Post by: Zephyrin on August 31, 2019, 11:05:48 AM
Of course air expands, as steam does, and in my opinion, early cutoff or full gear are functional with air too.
However, the main drawback using compressed air is the excessive cooling (which may freeze the engine) that occurs upon detent as compared to steam, which contains much more "heat"; cooling while steam expands uses the latent heat of evaporation, a large amount of energy gained by the steam during the change of physical state liquid-gas.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 31, 2019, 02:16:39 PM
Hi Zephyrin,  Your comment  (which may freeze the engine) reminded me about my model of the Beeleigh , Woolf compound  steam engine running on air at the forncett steam museum that after about 30 minutes running just decided to stop. It didn't seize up however it just wouldn't run ??!!!.

Hi MJM, thanks for this exhaustive synopsis and could you suggest perhaps a different method of constructing small engines to run on air successfully ,with perhaps different clearances  etc etc . Other peoples engines seem to run OK for the whole day. Was it just because my engine was a compound ??  Perhaps it should have had an oil feed , or even a candle under the engine ??!! (silly).    ?? With refrigerator pumps are there different clearances to cope with the 'coldness' ?? Keep reading through your posts and coming up with questions  ........

Willy
Title: Re: Talking Thermodynamics
Post by: Captain Jerry on August 31, 2019, 03:11:52 PM
Hi, MJM,


I don't mean to draw to fine a line here and I certainly did not interpret anything that you said to mean that an engine would not run on compressed air.  It is also clear that you have real work to do and that I am fully retired.  My only real work is to keep the grass cut and with 8+ acres to maintain that can be a full time job, but it is raining and has been for the last several days so I have time on my hands.


However, we seem to be on different pages.  I am trying to simplify a complicated system with multiple interacting factors to make it possible to understand just one of those factors and I am talking only theoretically. In doing so, I may have failed to complete specified the conditions of the experiment.  I think I said that I was assuming a constant temperature, not because I don't understand that it is a factor but because there is no way to specify temperature changes easily.  I should have said that the cylinder block is a large block of brass with resistance heating precisely regulated to hold the system at a constant 75 deg. F. which is the temp in my shop.


My comments on linguistic mumbo jumbo was to emphasize that it is meaningless to describe a three factor system by describing only two of them.  Such statements are neither true or false, they are incomplete. Almost political in the effort to make a point.


I also hope it is clear that when I mention early cut off, I am talking about a system that is unrelated to an eccentric driven system but mean to describe a governor driven system where inlet events are unrelated to exhaust events and which react to speed/load changes rather than control lever settings.


Clearly, ideal conditions can never be achieved but they provide a  point of reference so that when actual results are not the same as ideal, it is possible to understand the effects of changes to the real system.  Such factors as friction, leaks, valve adjustment, etc., can be better evaluated and methods to measure the other factors can be considered.


I jumped into this discussion without having read all of the preceding posts so I think I will take some time off to allow this thread to resume it's original direction.
Title: Re: Talking Thermodynamics
Post by: AVTUR on August 31, 2019, 06:49:29 PM
MJM, Willy and others

I have been trying to follow Willy's question and replies but the thread is moving very quickly, far too quickly for me.

As I understand it, Willy wants to run a steam engine which has variable cut-off on air. The question is what pressure does he need?

I may have got this wrong but I will try to answer it!

First, forget Boyle's and Charles's laws. They are only of use in school exams. The two equations required are the perfect gas equation [PV = mRT] and, importantly, the equation for expansion and compression [PV^n = constant]. For brevity I will not re-introduce the gas equation. n in the expansion/compression equation equals 1.4 when the process is adiabatic (no heat is lost to the outside world) and the gas is diatomic (the gas molecules have two atoms each). Since the process will not be adiabatic a lower value for n is usually used (something like 1.35). Air is essentially a diatomic gas.

Taking the liberty of making up figures I will try to do a sum
Taking the total volume of cylinder  as 5 in^3 with the air being admitted and expelled from the cylinder during the first and last 20% of the stroke. The piston area is taken as 1 in^2.
The cylinder pressure at the opening of the exhaust will be at atmosphere (this gives the minimum of wasted energy), P2, = 0 lbf/in^2(gauge) = 14.7 lbf/in^2(absolute). The volume of the air in the cylinder will be, V2, = 5 x (1 – 0.2) = 4 in^3.
Considering the expansion of high pressure air to get this, saying n = 1.35, our expansion constant, C, = P2 x V2^n = 14.7 x 4^1.35 = 14.7 x 6.50 = 95.5
At the start of the expansion, when the inlet closes, the volume of air will be, V1, = 5 x 0.2 = 1 in^3. Our expansion equation will give an air supply pressure, P1, = C /V1^n = 95.5/1^1.35 = 95.5/1 =95.5 lbf/in^2(absolute) = 80.8 lbf/in^2 (gauge).

[For curiosity we can calculate the mass of air required and the exhaust air temperature. Taking the gas constant for air, R, as 1151, (trust me) and an air supply temperature, T1, = 15°C = 288 K, the mass of air, m, = P1 x V1 / (R x T1) = 95.5 x 1 / (1151 x 288) = 0.000288 lb. The exhaust air temperature, T2, = P2 x V2 / (m x R) = 14.7 x 4 / (0.000288 x 1151) = 177 K = -96°C.]

The amount of power that will be produced can be calculated by using a “simplified” version of the equation that MJM gave (at Tech we knew it as “no pale ale” – nPALE). We can draw a PV diagram for the device, attached. The area under the thick black lines is the expansion which includes atmospheric pressure. We can get a mean pressure, say 60 lbf/in^2 (my brain rebelled when it was asked to do the calculus) from which we can calculate the mean force on the piston which will (60 - 14.7) x piston area (it is assumed that the atmosphere is acting on the other side of the piston) = 45.3 lbf. The work produced by this expansion is force x distance = force x (cylinder volume / piston area) = 45.3 x 5 / 1 = 226.5 lbf.in  = 18.88 lbf.ft. To get the power we just multiply this work by a speed, dreaming up a figure 60 rev/min. I am saying that the expansion only takes place once a revolution (single acting) so we get 18.88 x 60 = 1133 lbf.ft/min = 0.034 hp.

It should be stressed that the above power is the theoretical produced by the expansion. Quite a bit of it will be used to pump the air out of the cylinder (and perhaps into the cylinder), overcoming friction and other losses. Also the above IS NOT valid for steam.

I hope I have answered the correct question. I am happy to be quizzed.

And I could have been carving some metal. Now my brain hurts.

AVTUR
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 31, 2019, 11:08:00 PM
Hi MJM  et al,  Thinking on the practical side was the Newcomen engine 100% efficient as over the compleat cycle the hot steam returned to water at ambient temp   so gave up all its thermalicity ?    Unlike the Mallard engine that did not really lose its temperature during the engine cycle ?? Can steam at pressure only do work by losing  its temperature

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 01, 2019, 02:15:09 PM
Hi Zephyrin, thanks for joining in.  I suspect the real issue with air is not the temperature drop on expansion, but the fact that it starts at a much lower temperature, usually close to ambient, so the end temperature is below the freezing point of the water which is always present in the air as humidity.   With steam, it starts at a much higher temperature than the usual air supply temperature, so the end temperature is more moderate.  And as you point out, when the expansion continues into the wet region, the steam condenses at least partly, thus giving up some of that latent heat.  This holds the temperature instead of requiring extra cooling to supply the energy for the work done.

Hi Willy, you mentioned that your beautiful Beenleigh engine had stopped and I was hoping that you would tell us more about it.  If it did not seize, I assume it will now turn over ok, and even run if you give it a short burst of air?

I suggest that the problem is mostly due to the fact that it is a compound engine, so involves expansion of air to do work, and gets quite cold in the process.  That little turbine that I ran on air for the client produced frost on the exhaust pipes so quickly that it must have been very cold inside the pipes.   Now compressed air contains all the water from the humidity of the supply air when it leaves the compressor hot.  Usually the compressor has an after cooler to reduce the air temperature to ambient, during which much, though not all of the water drops out in a trap that hopefully has been provided.  But the remaining water is enough to cause problems, and when your engine cools the air I would expect ice would form inside the engine or exhaust, possibly as snow or that rhyme you get when fog freezes, and lodge in some place that locks up the engine.  When the whole engines warms back to ambient temperature the ice melts and with luck is free again and undamaged.

I think lubrication is the other problem with running on air.  With  a simple engine with minimal early cut off, there is minimal cooling, and I understand that they run for considerable periods at your various shows.  Obviously the displacement lubricator used for steam engine do not work, it would be worth talking to other exhibitors about what they do.  Do they squirt in a little oil at the start, does the water in the air lubricate them enough?  Or do they use a low temperature lubricant that does not get too viscous in the engine?  Unfortunately I don’t know the answers, but with all the work you put into that beautiful engine, it is sure worth asking the questions.  It would be great to see it running again.

Certainly different components of engines and compressors operate at different temperatures, so I am sure that manufactures of equipment calculate and allow for different amounts of thermal expansion in components where it might change critical clearances.  The biggest problem occurs when different materials with different coefficients of expansion are used.  If all the materials are the same or similar in coefficient of expansion then the clearances only change when there is a differential temperature between components. On our small engines the differences are surely very small, and if we follow one of our colleagues’ mantra, and have a little clearance, it will never get in the way.  But I believe we have others on the forum with more expertise in this area, perhaps they will come in with more suggestions.

Regarding the Newcomen engine, I could say that it is definitely not 100% efficient because the second law of thermodynamics says that is not possible, but that would be a cop out, or begging the question.  First we should say what we mean by efficiency of the engine.  The definition I prefer is the fraction of the input energy that is converted to work.  Unfortunately a significant portion of the input energy to any engine appears in the exhaust, and in a steam engine, the normally very low efficiency figures are because the exhaust steam is usually mostly still in the vapour phase, so still has all that latent heat.  Sometimes it is easier to look at the losses, which for a 100% efficient engine should be zero.  So look at where energy is lost rather than being converted to work.  If I remember rightly, in that engine, the exhaust is condensed by a water spray.  Then the warm water produced is discharged to the river.   If you do an energy balance you will see that it takes a large quantity of water to condense a small quantity of steam, so there is a lot of heat lost in that discharge stream.  Does that answer the question? Or am I thinking of the wrong engine?

But can steam only do work by losing its temperature?  Certainly some of the energy in steam is is converted to work, so when work is done, some energy is transferred across the boundary of the system, and the fluid in the system then has less energy.  In all the cases I can think of that means it will have a lower temperature.  The energy is not lost, it has just moved outside the system we are looking at, usually to the surrounding atmosphere.

Hi Captain Jerry, I can see that we all need the rain to stop.  But if you could send some here, most of our country would welcome it.  Don’t worry about the real work, I have been retired for ten years now, but as others have found, I have less spare time now than when I was working.  I think it was Dr. Parkinson who had something to say on that matter.  But I would like to get on the same page with you.

Simplifying a complicated system to understand the effect of one factor is a good idea, but it would help to start by identifying or describing the factor you are trying to understand.  I don’t think I have caught on to that yet.  When simplifying the problem, it is a good idea to first identify all the variables involved.  Then it is important to identify which of these variables are independent so you can try and specify them, and which are dependent, meaning they result from the process, and can’t be separately controlled.  Then we can move on to see what simplifying assumptions we should make.  Consider what conditions you know at the start of the process, and what conditions you know at the end, and what process is occurring.  I am guessing that you are talking about an expansion process.  In this case, temperature changes are often the result of the process and if you try and independently alter the temperature, you are almost certainly introduce a second process of heat transfer, so making the issue more complicated.  I would suggest leaving the temperature changes to the thermodynamics.

I would certainly encourage you to engage theory to help guide your experiments.  To do this, I would suggest you first try and define the process you are trying to understand.  Then try and clearly define the theory you intend to use.  Before proceeding further, it is a good idea to do a  check and make sure the theory is sound, accepted physics.  Unfortunately, as you have found, much of the so called theory expounded around the club tea rooms is not so sound, and often fails when put to the test. 

Using an ideal process to provide a point of reference is a good idea, used across all kinds of engines.  The key requirement, is that the ideal process selected can be analysed by calculation.

So to help us all get on the same page, how about starting by defining or at least describing the process that you are trying to understand.  It is even possible that the answer is already known, which will simplify the discussion somewhat.

Hi Avtur, thank you for joining in and especially for providing such a clear and useful description of the ideal gas and the compression /expansion equations.  We all need to print that paragraph and stick it in the front of our preferred reference books.  It will be pinned above my desk for sure.  Polytropic never quite cut it for me.  You were just in time to save me from getting deeper into trouble.

Normally I only put up one post each day, but yesterday I was having a quiet day, and the thread did move on quite fast.  I have had much less spare time today.  I am not surprised that it was a bit hard to follow.  Just to clarify, while Willy has asked most of the questions to date, this one came from Captain Jerry.  You may have noticed his other thread on a quick release valve mechanism, and I suspect the question has arisen out of that.  But I am sure we will get to it soon.  In the mean time I hope you have been able to get back to cutting some metal.

Achieved a milestone today, I backed the car in through our gate and went inside to sit in my chair for a cup of coffee.

What’s so special about that you might be thinking?  Well, I last pulled out of that gate three months ago and have been on a little road trip.  Sixteen thousand, eight hundred km later, I have kept the water on the right, and completed the loop around the country, even if I did cut the odd corner, as time did not allow the coastal road the whole way.  Crossing the Nullarbor is one of those rights of passage.  And now you can see why I have made the odd mention of holes in the net.  It claims to cover over 95% of the population, but the other 5% are spread out over a very large area.  The misconceptions they can create with statistics!

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: Captain Jerry on September 01, 2019, 05:10:43 PM
MJM,


Had I known that you were on such an epic trip, I would not have bothered you with poorly stated questions or examples. If I were to take such a trip, I think I would enjoy it more without the intrusion of electronics and other peoples thoughts. Maybe a very simple GPS for safety. Open windows and paper maps. Did you take a good supply of vegamite? 


Land cruises of that are interesting but my preference is the deep blue water and endless skies.  When I first started cruising, it was primarily magnetic compass and charts.  GPS was available but the readout was position only and sometimes quite slow to resolve.  Communication was short wave.


It won't stop raining here for a few more days and might get much more intense as hurricane Dorian passes. 


Jerry
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 02, 2019, 12:30:05 AM
Hi MJM, more thoughts on heat engines...a steam engine running on compressed air (cold) is still capable of doing useful work..An engine using compressed air turning over a diesel engine for instance  or a turbine that you made ,  however the heat is used in producing the compressed air perhaps ?  unless it is supplied by a water wheel at the bottom of a Dam supplied by rain !! So are these still thermodynamic heat engines or are there different equations for working out the formulae ??  Hope you had a loyely trip.

I have been to Norway , Hong Kong ,Germany, Europe Turkey, Greece,The Med etc and think everybody should travel just to see that other people are basically just like us with the same dreams and aspirations for peace and love !!

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 02, 2019, 12:00:14 PM
Hi Captain Jerry,  your questions are always welcome, I would mostly rather think about how engines work than read a “who done it”, or watch evening TV.  Obviously I don’t have access to my whole library, but I always find space for my favourite reference book and a calculator.  I enjoy keeping up to date with everyone’s projects and even the chatter on the forum, and I also receive our home daily paper on the iPad.  And keep us touch with our well and truly middle age children, and our grand children.  It all keeps our minds engaged.

Neither my wife or I are much into yearning for the good old days, we both like our creature comforts.  It’s good to have the windows shut and a clean filter in the A/C when the dust is blowing and it’s over 30 C.  Navigation is an interesting issue.  We do have a GPS, with lifetime map updates, but it is mainly used to make sure I turn the right way out of the caravan park gate after an overnight stop in an unfamiliar town.  Too easy to get your head turned around by the time you have done three turns around the campground in an unfamiliar town.  And finding your way across a range of hills can occasionally be a challenge.  But the distance to the next turn shown on the GPS is often  500 or more km, as it does not assume you will turn off the highway on to a dirt track.  Electronic tyre pressure monitors are also right up there for importance.  Not to mention engine management.  It’s hardly worth lifting the bonnet (hood?) these days, there is almost nothing you can do without a computer tend the required software.

It is not like the previous generations experienced.  The main routes (don’t really deserve to be called highways) are all sealed, though they are awfully narrow when you have a road train coming towards you.  And all the main centres have the major supermarkets with reasonable prices.  We go off road for short distances, the final bit into some worthwhile camp grounds, and sometimes free camp, as we have solar power, along with refrigeration and water.  So a quite civilised way to see the countryside, and see how people live and use the land.  Did some mine tours and a cotton industry tour, some gorge boat rides and a boat around a major fishing port to learn how the industry operates.  But most days we are just enjoying the different environment and the warmer weather.  It is pretty cold at this time of year in Melbourne which is well south.

Oh, and we have to get back to go sailing.  That might be another interest in common, though we are flat water sailors.

I hope that you are well battened down for the hurricane and are able to avoid the worst of the damage to come through safely.

Hi Willy, heat engines convert energy in the form of the random motion of molecules in to mechanical work.  We sense that motion as temperature.  So yes, engines running on air are definitely heat engines.  Heat is a relative term, it is relative to absolute zero, or -273 C, so there is plenty of heat in atmospheric air.  In fact to produce liquid air or LNG, the lower temperature stages of the necessary refrigeration usually include an expansion turbine doing work by driving a compressor rather than using a throttle valve to reduce the fluid temperature.

(Oh, and that turbine was a purchased item from a major manufacturer.  I don’t have the equipment or knowledge to make that one.  It was driving the lubricating oil pump for the bearings of a very expensive compressor.)

The energy is delivered by the compressor, but a compressor actually loses some energy, it does not produce it.  A compressor can be driven by an internal combustion engine or a steam turbine, but for most of is they are driven by electricity which is generated in a power station.  Apart from coal, there are power stations using nuclear energy, hydro or even wind power.  The energy for hydro power comes from the sun which evaporated water from the seas, which rises to form clouds and falls as rain into the dams in the highlands, as well as falling on the low lands.  So it is heat from the sun.  Wind power also comes from the heat of the sun which causes the air pressure patterns that create the winds that drive the turbines.  And photo voltaic is obviously harnessing the sun’s heat.  So they are all heat engines, if we go far enough in understanding the source.  Each of the devices in the chain merely convert the energy from one form to another, and each involves some losses in the process.  And if you are really into tracing back to the source, coal is energy from the sun trapped a long time ago.

You mentioned a candle half in jest I suspect, when we were talking about the air getting cold, and I missed commenting.  I suggest a candle would not be anywhere near enough, but it you used it to heat the supply air, you could start at a high enough temperature to have an exhaust temperature above freezing, so the idea was not silly.

And with travel, we also find a highlight is speaking to people who live in the remote areas and coming to understand the issues that dictate their lives and attitudes.  As you say, most of us only want to leave in peace.

Thanks for looking in,

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 05, 2019, 02:34:50 AM
Hi MJM , thinking about boiling /evaporating water...H2O  plus everything else in it.....can you use non potable water to cook and drink without any thing else contaminating it ?? Dose the limescale component get carried into the rest of the pipework in an engine , however minuscule ??
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 05, 2019, 08:38:45 AM
Hi Willy, I have seen many signs warning that the water is not fit to drink.  I assume that lawyers are responsible for most of them, otherwise I am not sure how previous generations ever found water to drink.

Of course their immune systems may have been better than ours, and we have to be realistic and appreciate that not many reached the average age of members on this forum.

But my wife grew up on a farm where only tank water was available.  It always tasted good, possibly because of, rather than despite the odd possum or bird that inevitably found their way in through the top opening and drowned!  Eventually the water would start to smell and the tank had to be cleaned out.  Quite a difficult decision on a remote farm in a drought.

More seriously, I suspect that most of the water with such signs was ok to drink, especially if you boil it first, but nobody wants to be sued if you get sick, and nobody has actually tested the water.  It is easier to put up a sign.

When you cook in such water, the cooking process is a step towards sterilising the water, and certainly extends the range of quality that can be consumed.  However, any contaminants in the water that survive the temperature will contaminate the cooked food.  If the water contains inorganic compounds and heavy metals they will be in the cooked food.  And sometimes we add those contaminants deliberately, for example salt or sugar to flavour the food.

When you boil the water, in principle most contaminants stay in the water, but in practice there are always small droplets of liquid water carried over with the steam, despite cyclonic separators and crinkle plate separators that are installed in modern full size boilers.  Even more is carried over from our small boilers without those separators.  Those droplets carry their share of the contaminants in the boiler water, though it is a very small proportion of the total water involved.

In a full size plant, operating 24/7, this carryover eventually results in turbine blades and even piping being fouled by those compounds from the water.  It is interesting that even high speed turbines can become quite badly fouled over time so their performance is affected, without ever getting significantly out of balance.

In our models, I am not sure how many operate enough hours for these deposits to be visible, and the biggest problem tends to be the deposits in the boiler, as most of the contaminants stay behind. 

I will be interested to hear from those who operate their engines for much longer periods to see how long it takes in a model or small craft environment for significant deposits to build up.

MJM460



Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 06, 2019, 02:42:49 AM
HI MJM, If one uses brackish ..even seawater to "steam" vegetables is that still safe......When i was young 60 years ago we had a not very deep tidal well. !! after a few years we actually had a hundred foot well bored ...when the water came out it was sparkling clear !! however  after a few hours it turned a rusty red colour and stained everything red  !!!  so all the sheets and underwear were stained !  we then had to get a filter to remove all the rust.....

Willy
Title: Re: Talking Thermodynamics
Post by: derekwarner on September 06, 2019, 03:42:26 AM
Hi Willy.....when I was young  :old:......[also some 60 years ago] I seem to remember we were taught that rust was the product of oxidation of ferrous [iron or steel] material

So later on in life, also reminded that the rate of rust produced on those iron & steel materials on the floor of the ocean was greatly reduced due to only oxygen available being that en-trained in the water itself

So unless the 100ft deep bore was made with pipes that were internally pre-rusted, I wonder if the rust colour could have been from a mineral deposit in the aquifer?...


Derek
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 06, 2019, 12:38:54 PM
Hi Willy, I should know the answer to that one as I all’s enjoy sailing as an outdoor recreation.  Also, on my recent travels, I saw where salt from the sea is harvested by evaporating the water in shallow ponds in a very low rainfall sunny area.  I know they wash the salt and recrystallise it to remove dirt and such, but am not sure if there is other processing to remove any of the salts other than sodium chloride in the sea.  And I am thinking some of that salt ends up as table salt.

Previous generations used to add lots of salt in cooking, especially potatoes and porridge.  These days, medical advice is that too much salt causes serious health problems so we all reduce our salt intake.  The salt added to the water for cooking does end up in the food.  My wife has taken it so far that she has low sodium levels and has been advised by the doctor to increase her salt intake a little.

I know we can’t drink seawater, but that is because so many of our body processes depend on osmosis, and if you have the concentration on the wrong side of the cell walls, it mucks with our system badly.  Sea water is too salty for us to safely drink.  However it is likely that we could tolerate the salt from cooking with sea water, especially short term in an emergency.  However, with all the plastics and other stuff we dispose of out of sight and out of mind by dumping it in the sea these days, it is less safe than it used to be.

My first thought on the well was that the water might have corroded the pipe brining the water to the surface, as some underground waters can be corrosive, but Derek makes a good point about lack of oxygen, so again I am not sure.  It may have been from mineral deposits.  But then, the ground water in the North wast of Australia is not rusty coloured, yet the ground contains as much iron as anywhere, and digging it up is a huge industry.  But it is so full of calcium, that the dregs of your coffee cup are gritty.  It always pays to leave the last little bit in the cup.

And neither the water there, nor sea water are good in a boiler!  (Just to bring it back to thermodynamics and engines.)

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 14, 2019, 02:48:15 AM
hi MJM,  I am putting the beam engine together and am fitting the piston for the steam test...i have no drawings for the inside of this engine so am wondering if there are strict parameters for the diameter of the piston and the spaces at the ends of the travel ?? are  there different gaps for different types of engines??   

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 14, 2019, 12:56:04 PM
Hi Willy, the spaces at the ends of the cylinder when the piston is at top or bottom dead centres determine the clearance volume.

In principle, the clearance volume has an effect on the efficiency, but I don’t think it is all that important if it is small.  Interested to hear from others on this.  Basically, the steam in the clearance volume is released to exhaust pressure along with the rest of the steam in the cylinder, and has to be replaced with high pressure steam when the inlet valve opens, so there is increased steam consumption, while the steam only does the same amount of work on the piston.

However, in practice, you never want zero clearance or worse, even at maximum dimensional tolerances or tolerable wear on bearings etc.,you don’t want the piston to hit at either end.

On my little engines I aim for about 1 mm each end.  It’s a big proportion for those small engines, but I am not sure that I can reliably achieve less.

However, even on a larger engine 1 mm is probably still achievable without needing to go larger on clearance.  Again it will be interesting to hear what others achieve.

On a large compressor I had purchased for a client, LP cylinders about 30” dia, we were aiming for 1 mm and checked it by inserting a lead wire with the valve removed, and squashing it by barring the machine over.  I think the same considerations would apply for a steam engine.

Obviously for a combustion engine, the clearance determines the compression ratio, so different considerations.

I hope that helps.

MJM460

Title: Re: Talking Thermodynamics
Post by: steamer on September 14, 2019, 02:18:43 PM
Condensation is a bigger problem in small engines than cylinder clearance.     Cylinder clearance on big stuff is handled with compression or Exhaust lap, but on the small stuff...it's really not helpful.

Dave
Title: Re: Talking Thermodynamics
Post by: derekwarner on September 15, 2019, 12:44:48 AM
MJM notes......."checked it by inserting a lead wire".......

In a slightly different cylinder application, we found ''deformable plastic" cord to be a superior medium when attempting to achieve measuring clearances approaching 0.05mm class of fit between a 200 diameter semi spherical rod end ball, and the 200 semi spherical diameter cup in the 580 diameter piston] ……

The piston rod was AS1444 Grade 4140 – Q&T to Condition T 1000 MPa , the piston forging was of identical material including the 1000 MPa Q&T…… I considered that a design of this  H7/g6 > G7/h6 Class, calculated and to be totally achievable when measured in a conventional shaft to hole format, but the Standard did not specify any method of understanding the average deviation of mating spherical surfaces clearance   

After assembly of the rod into the piston and the piston lower plate bolted, the lead wire on disassembly was extremely fragile & self shredded or tore easily during the vertical lifting of the rod from the piston, with subsequent measurement of the lead near impossible

The plastic cord displaced itself however maintained mechanical stability such that it could be accurately measured…

[yes {to bring back reality of :old: dimensions} we were looking for thicknesses of ~~ 0.002”] after the 493kg mass force of the rod weight and the tightening process of the piston bottom end plate had flattened the plastic cord

Derek

Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 15, 2019, 03:07:58 AM
Hi MJM ,Derek and dave ..thanks for the info ..interesting things to think about there...Thanks ..

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 16, 2019, 01:09:07 AM
Hi Dave, good point about the condensate, probably a good reason to have more rather than less clearance on a steam engine.  My compressors didn’t like any liquid either, it turned them into very dangerous beasts.

Hi Derek, it is possible that we also used that plastic cord, I am not sure, now that you mention it.  But we were looking for about 1 mm, so not as difficult as your application.  It’s amazing what can be done by experienced people to get around difficult practical problems such as that.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 28, 2019, 03:52:18 AM
Hi MJM, I have some more questions for you and they are about the Beeleigh Woolf Compound engine that they are currently restoring. They have cleaned up the engine and replaced missing and broken parts and are now running the engine with an electric motor coupled to a car road wheel that operates against the flywheel periphery. The are thinking about running the engine on compressed air and I have a few questions about the viability of this.  The engines cylinders are within the large casting that serves as a steam jacket. there are no drain cocks to the cylinders themselves but just one at the bottom of the steam jacket. To start the engine the steam is let into the steam jacket to heat up the whole engine block right through to the cylinders. The steam is prevented from entering and condensing into the cylinders by having the valves in the midway position so the inlet ports are closed. once the cylinder block is at the operating temperature that is determined by steam rather than water leaving the drain cock at the bottom of the jacket. So a few questions about this. When the engine has stopped there is still steam in the cylinders and ports that will condense when the engine has cooled down. So how will this water escape from the lower parts of the cylinders.?  when you have water in an enclosed space in an engine and the ports will it evaporate after some time so when the engine is restarted the water wont be compressed and shatter the engine ? this may not happen if the engine is once again brought up to temp with steam . But will running the engine on compressed air be a viable option if water has been allowed  to accumulate.  On a locomotive engine the drain cocks are left open to release any water but this engine does not have these drain cocks. ??  just wondering about this and thinking about the thermodynamic actions  of water in enclosed spaces ?? this is quite a lengthy diatribe and I hope it is a valid query . A pic of the cross section of the cylinders arrangement showing the ports at the top of the engine...

willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 28, 2019, 11:52:39 AM
Hi Willy, it’s never a silly question if you don’t know the answer, and it is very important to consider condensate while starting up the engine.  Likewise the condensation of the remaining engine when the engine is shut down.

I can have a go at the theory, but this is another of those areas where nothing beats practical experience, so I hope others will also chip in with their experience of operating reciprocating engines

First the water, the issue is of course that it is practically incompressible, so you have to be careful not to trap it between a moving piston and a stationary head, as stopping the piston suddenly when it hits the water requires a huge pressure force to change the momentum of a heavy piston in such a short time.  And you will get copious quantities of condensate when warming things up. 

In this case, the engine is vertical, so condensate will tend to settle under gravity at the bottom for the underside of the piston and on the piston for the top side.  With the cylinder ports as you have drawn them, if you bar over the engine slowly for a couple of revs, quite a job if you only have a bar and holes in the flywheel, any water at the bottom of the cylinder will be forced up the  passage and will escape to the exhaust, if necessary using the little valve lift that is normally provided for just this reason.  There will be no great pressure generated to accelerate the water if the piston is moving slowly.  Similarly for the top of the piston, when the piston reaches the top, the water will be able to run down the passage to the exhaust port, and again it will lift the valve sufficiently off the face if it is not already partly open to the exhaust.  So, while I think we would all like to see a drain cock at the bottom of the cylinder, the engine has lasted for quite a long time without it, so I suspect it will be ok, especially if it is barred through that first bottom dead centre.  Of course, in a horizontal engine, any water is more likely to be trapped, unless the valves are at the bottom of the cylinder.

Normally you get a lot of condensate when you first admit steam to a cold cylinder.  Using that jacket to preheat the cylinder means the condensate in the jacket is the main quantity to be dealt with, and you said that drains are provided for that.  It is likely that no one ever preheats for long enough to get the cylinder right up to steam temperature, so there will probably be a little condensate formed when steam first enters the cylinder, but the quantity should be manageable especially with a slow start.

When the engine is shutdown, the steam space in the cylinder will still have some steam, although only at atmospheric pressure, or even a bit lower if there is a condenser.  When the engine cools this will of course condense, and with no drains at the bottom, it will remain in the cylinder.  On top of the piston, some condensate will probably remain on the piston and between the piston and cylinder on top of the rings.

Any water remaining in the cylinder will tend to evaporate but only until the vapour pressure reaches equilibrium vapour pressure for water at the temperature of the cylinder.  If some air is blown through the cylinder, it is possible to reduce the humidity and keep evaporating the water until there is insufficient remaining for any condensation.

If I was developing the operating procedure, I would leave the jacket heating on for a while after the engine is shut down, and bar over the engine a few times or blow dry air through to carry out the remaining water vapour.  Then I would be looking for tricks to absorb any remaking water and keep the engine dry until restarted.  Even admitting atmospheric air through the steam line while the engine is kept warm by steam in the jacket and preferably turned over a few times will get it quite dry.  But some of our marine engineers may well have a much better idea.

By the way, that steam jacket would be left open to steam possibly with a steam trap on the outlet, during normal engine operation as condensation on the cylinder walls reduces the efficiency of the engine.  They clearly knew about this at the time that engine was built, and considered the jacket worth the complexity of the cylinder casting.

When the restoration is complete, I assume that there will be no water left in the cylinder, so should not be a problem when first running on air.  In any case any water should be carried out through the cylinder passages to the exhaust as described above.

With running it on air, as a compound engine, the main issue will be having enough load on the engine that the inlet pressure does not result in negative pressure in the lp cylinder at the end of the expansion.  However there may be a simpling valve or similar device to admit high pressure air to the lp cylinder, especially for startup, (including starting on steam) so leaving that open should avoid any problem.

I hope that answers your concerns, but don’t hesitate to ask more questions, or discuss it further if something does not make sense.

Exciting time as the project gets near time to see the engine running after all the work.

MJM460





Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 29, 2019, 12:12:51 AM
Hi MJM, thanks for the reply.    So the water would not be such a problem with a slide valve ..but what about piston valves. ? however locos do have automatic drain cocks.  If you had a sealed tube with some water in it and left it in the Sahara desert with its large chang in day and night temperatures , would the water evaporate and condensate daily  ?  Will using compressed air in the Beeleigh engine actually introduce moisture into the engine ?  On my numerous visits to this engine i have always noticed that it was very. cold . if the use of warmish ambient temperature in the compressor is used will there be condensation building up in the engine ??

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 29, 2019, 11:29:07 AM
Hi Willy, the key to understand what is going on here is to understand how the air behaves, and how the water behaves, then how they each act when there are both in the space at the same time.  It is the same thermodynamics as when we were talking so long ago about filling a boiler at the top or the bottom of the mountain.

Behaviour of Water

The water is simple enough.   If you have a sealed pressure vessel which contains only water, so no air or anything else, and let it sit undisturbed for long enough to reach thermal equilibrium with the surrounding atmosphere, then the pressure in the vessel will be only due the the water and its temperature.  The pressure corresponding to any temperature is listed in the steam tables as we have discussed before.  Remember that reaching thermal equilibrium is just the technical description of everything being at the same temperature.  There are a few ways of achieving this, but I will leave that for another time.  But as you can easily see from the steam tables, higher temperature causes higher pressure, lower temperature, lower pressure.  The pressure gets very close to zero at zero degrees centigrade, and of course the reference point we all know is atmospheric pressure at 100 degrees centigrade.  This means that at all temperatures below 100, the pressure is below atmospheric, but so long as the vessel is designed for an external pressure of 101 kPa (14.7 psig) it will not collapse as that is the maximum external pressure possible in atmospheric air.  Remember that so long as the vessel is totally sealed, the pressure inside is quite independent of the pressure outside.

Behaviour of Air

If you now look at air, we all know that it air is a mixture of gases, the biggest part is nitrogen, then oxygen, then a number of other gases in quite small quantities, normally including a small amount of water.  Let’s just look at dry air.  That is air that does not contain any water.  Perhaps it has been obtained by passing it through an air dryer such as provided in a wood quality air system.  Even those are not perfectly dry, but close enough for our discussion.  Despite being such a complex mixture you can assume that air behaves like a perfect gas, so the absolute pressure is proportional to the absolute temperature.  You don’t have to treat each component separately to be close enough to understanding air behaviour.

So if our vessel is filled with air at atmospheric pressure and temperature and then sealed, when it is heated, the pressure will rise, or if it is cooled the pressure will fall.  Over ambient temperature range, the pressure does not vary a lot because you have to cool it to absolute zero temperature or minus 273 degrees C, and that is quite difficult to achieve.  On the other hand, to increase the pressure of two atmospheres, you have to double the absolute temperature.  If the vessel was sealed at say 20 degrees, or 293 degrees absolute (or Kelvin), so to double the temperature means 586 K or 313 C, not impossible, but careful not to burn your fingers.

Water and Air in the Same Space

So what happens when you have both air and water in the same vessel.  This is simple to achieve,  pour some water into a vessel, leaving it say half liquid water, the remainder is mostly air, and insert the plug.  Now what happens?  Basically, we can look at each component separately.  The temperature will be the same everywhere, while the pressure will be the sum of the pressure due to the air and the pressure due to the water.  First the temperature will even out if the water was not already at air temperature. 

Let’s say it is 20 degrees C and standard atmospheric pressure of 101 kPa.  The liquid water will obviously be in the bottom part of the vessel, the air will still be at atmospheric pressure and temperature.  The vapour space will also contain some water vapour.  At 20 C, the vapour pressure will be 2.4 kPa.  Some of this will be water that was in the air as humidity, while the rest will come from some evaporation of the liquid.  When I look at the humidity here on my little weather station it says 25%.  So if the temperature is 20 C, the water vapour pressure is 25% of that 2.4 listed in the steam stables, or 0.6 kPa.  So some water will have to evaporate and the temperature even out to get to thermal equilibrium, and in the end the water vapour pressure will increase by 2.4 - 0.6 or 1.8 kPa.  The air temperature is unchanged so pressure unchanged to the total pressure in the vessel is increased by 1.8 kPa, and the air humidity in the vessel will be 100%.

If the vessel is now put outside in the desert, then as the temperature rises and falls, some water will evaporate during the day when the metal and internal temperature will exceed 65 degrees, (it will certainly burn your hand if you try and pick it up), and like wise, some will condense overnight when the vessel and its contents cool by radiation to a clear sky.  In these conditions I have had ice forming on the canvas of the caravan that makes folding down in the morning quite difficult.

Application to Air Compression

The final step is to apply this basic thermodynamics to the system where an air compressor takes ambient air and compresses it into a system to drive the engine.  Let’s assume a balmy spring day, about 20 deg C.  I suspect your climate is more humid than here so let’s say 33%.  It could be quite a bit higher if it’s about to rain and the temperature is dropping.  The water vapour pressure in that air will be 33% of 2.4 or 0.8kPa.

Let’s assume that compressor discharges at about 500 kPa (absolute), or about 60 psig, (75 psia).  This is a pressure ratio of 5, so the water vapour pressure will increase five times to 4 kPa.  It is important to recognise that this pressure rise will not cause the water to condense,

The temperature is more complex to estimate, there will be a significant increase due to the work done by the compressor and the compressor efficiency.  I won’t divert to trying to calculate the discharge pressure, but the discharge side of a compressor is generally pretty hot to touch so I am guessing perhaps 70 to 100 deg C.  At 4 kPa, the condensing temperature is about 29 C, so you can see it is well and truly superheated.  But at the discharge of the compressor there should be an after cooler, a heat exchanger, air cooled on a small machine but could be water cooled on a larger machine.  The cooler may or may not be large enough to cool the compressed air below 29, but the additional cooling provided by the air receiver and the distribution piping almost certainly will cool it pretty close to that ambient temperature, and that is where the water will condense.  Ideally the system would include an air dryer to further reduce the water in the system and hence the amount of condensation but these are usually only provided in critical instrument air or breathing air or medical systems.  So water condenses in the distribution system where the temperature reaches 29 or below, leaving the air at 100% humidity at that temperature.  And that condensate will drain into the engine unless the piping is carefully designed so the off take comes off the top of the header, and drains are provided at low points.  And there will be more condensation when everything cools down overnight, so when you start the system you will get some water out at any drain point in the piping.  But while the engine is operating we can assume the air is cooled so some water drops out in the vessel, leaving the air at 100% humidity, or perhaps a little warmer with slightly lower humidity, but basically very humid air.

Air to Power a Steam Engine

Now to the question of what happens when you drive the engine with this air.  We have already talked about that water in the engine remaining from the last operation, so rather than revise that all again, let’s just look at the engine running.

First the hp cylinder.   Near the start of each stroke, the valve position allows air to enter the cylinder and does work by pushing the piston, roughly at constant pressure, and the energy used is supplied by more air entering.  Then the valve reaches the point where it closes off the inlet.  The air in the cylinder continues to do work by pushing in the piston.  As the piston continues to move the air expands, the pressure falls, and significantly, the temperature falls due to the energy converted to work, just the opposite to the heating that occurred when it was compressed.  That is why the running engine feels cold, it really is cold.  The temperature is predicted by the equations for adiabatic expansion.  But as a result of that reduction in pressure and temperature, the air does get to the point where the water vapour that remains in the air will start to condense.  Not good for the cylinder lubrication, but I don’t really know enough about lubrication to suggest an appropriate lubricant.  Some attention needs to be given to what lubrication is necessary.  I suggest the condensate is mostly swept out with the air in the exhaust stroke, it is only when the engine stops that any remaining water stays around to cause corrosion.

The engine is, I believe a compound, so the hp cylinder discharges into the lp cylinder.  The air is now cooler, and is saturated with water (100% humidity), air continues to expand as it transfers from the hp to the location cylinder due to the difference in piston diameter, so it continues to fall in pressure and cool.

Now the complication with a compound engine.  When new, designed to run on steam, it probably exhausted to a condenser at a pressure lower than atmospheric.  But you are not set up to achieve condensing the air, so you will be exhausting to atmospheric pressure unless you have a very good air pump.  If the pressure in the lp cylinder is below atmospheric pressure when the exhaust valve opens, outside air will rush into the lp cylinder when the exhaust valve opens.  This is effectively a reverse pulse to the engine which will make it run rough to say the least.  Of course simpling valves or what ever the engine is set up with for starting will provide a way around the issue by ensuring the air inside the engine is never below atmospheric.  But I don’t know how your engine is set up in that area.

So expansion on the hp cylinder causes the air to cool, continued expansion in the lp cylinder causes it to cool further, hence the engine feels cold when it has been running.  And that cooling causes condensation of the water from the humidity in the atmosphere.

Oh, and the piston valves, I believe that they cannot lift as does a slide valve to release condensate, so those automatic condensate drain valves would seem to be quite essential.  I have even less knowledge of those.  At least I have built and operated model engines with slide valves.

Good to see that you are still thinking about thermodynamics, I hope the above helps clarify some of the issues.  Thanks to all looking in.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 30, 2019, 03:16:01 AM
Hi MJM, thanks for all this info and there is so much more to just Air and Water  than is at first apparent  Things like  'Heavy  water " that is used in Nuclear power stations   I think ??..  Is there a way of actually drying air to prevent condensation ?? This engine does have a condenser and an air pump. The air pump is used to also supply water to the boiler via a pump driven by the beam. Will the condenser  actually help with the efficiency of this engine if supplied with water when run on compressed air ? the condenser is supplied with its own water from the river ? and should they take advantage of this or not have this arrangement working ? I shall pass on your info to the restoration people so they can query it with the compressor suppliers. I will need to read your last post a few more tamest really get to grips with it.   Thanks again for your posts

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 30, 2019, 09:29:13 AM
Hi Willy, the thermodynamics is not so different from what we have previously discussed.  The main difference is in how to apply our understanding of the behaviour of air and of water separately to the case where there are both air and water in the same space.  And it is such a common occurrence that it is worth trying to understand.

I am sorry that it was such a long post, too long, even by my standards.  I will go back tonight and add some headings to more clearly separate the distinct topics.

The effect of the condenser is to lower the pressure at the engine outlet, thus increasing the overall pressure ratio across the engine and increasing the potential work output without increasing fuel consumption.

Unfortunately, as soon as we add a condenser, we have to deal with air in the system.  There is usually some air which was dissolved in the boiler feed water.  Modern high pressure boilers have steam heated deaerators to reduce this dissolved air, and the boiler treatment chemicals used during boiler operation include an oxygen scavenger to collect the air that get through the deaerator.  This is all done to reduce corrosion in the boiler and associated equipment.  Without this water treatment there will be some air in the exhaust.  More importantly, as soon as you include a condenser, and achieve pressures lower than atmospheric pressure, there will be air leakage into the system.  It will leak in through seals around piston rods and valve stems and even flanged joints in piping.

Air in the condenser reduces the partial pressure of water so lowers the temperature at which the water condenses, so the condenser is less able to transfer the necessary heat.  Furthermore, until that air is removed from the condenser, the condenser pressure increases so reducing the engine output.  Thus the air pump is included to compress the air to something above atmospheric pressure so it can be vented to the atmosphere.

The condensed water also has to be pumped so it can be discharged to atmospheric pressure, or even back to the boiler providing it is not contaminated with oil from the engine.  Some air pumps are designed to achieve the air pumping and water pumping in the same device.

It is important to recognise that the low pressure in the condenser is obtained by condensing the water in the closed volume of the condenser.  Cooling water at say 15 deg C cools the exhaust steam to a point where it starts condensing.   At 15 C, water would condense at 1.7 kPa, a pretty decent vacuum, though in practice the limited heat transfer area might only cool the steam to perhaps 25 or 30 C, so the condenser pressure might be 5 or 6 kPa, still not too bad, so long as the air pump is working well.

When you run the engine on air, you will not be surprised to hear that you will not be able to condense the air.  That requires around -180 C, and that requires quite sophisticated refrigeration equipment.  Without that condensing, there will not be any pressure reduction, so I would not suggest pumping the water.  Better to keep the system dry and free of corrosion on the water side.  Of course, the air exits the lp cylinder quite cool, so river water in the condenser tubes would probably heat it a bit, but I am not convinced it would be worth the pumping power consumption.

Clearly the more important issue for running on air is corrosion due to condensed water in the air supply.  Not so much of a problem for a model run occasionally and probably well dried out between runs, even if it is only due to storage in a warm home.  But not so easy to dry that full size museum engine which probably runs quite regularly, but probably also shuts down overnight.  Procedures for long term mothballing would not be appropriate.

I suspect the answer probably lies along the way of driving the engine with that car tyre and admitting warm air for a while to dry everything out prior to shutdown, but I have no experience in that sort of long term procedures. 

Systems which supply dry air employ an air dryer after the accumulator.  So, after the compressor, the after cooler, followed by an accumulator vessel, then a filter/coalescer.  Then a drier.  Commonly it is a refrigerated dryer, which will give a dew point of around -20 to -30 C.  This was not considered good enough for instrument air in a large oil or petrochemical installation and we used molecular sieves, particles of a special resin which absorbs water molecules at high pressure (compressor discharge pressure), then switches over to vent to atmosphere when the water molecules escape the resin at low pressure.  So two pressure vessels, with the appropriate automatic valving to regularly swap between high pressure and low pressure operation.  These systems gave a dew point around -70 C from memory.  A small refrigerated drier might be affordable, depending on the actual engine air consumption, which determines the drier size required, but I am not sure that the larger molecular sieve types would be affordable.  Still, worth an inquiry from suitable suppliers, especially if they could be talked into contributing to the museum by putting a loan system on display.

That’s more than enough for another day, so I won’t start on heavy water.

Thanks to everyone looking in.

MJM460

Title: Re: Talking Thermodynamics
Post by: steamer on October 30, 2019, 12:31:15 PM
Hi Dave, good point about the condensate, probably a good reason to have more rather than less clearance on a steam engine.  My compressors didn’t like any liquid either, it turned them into very dangerous beasts.

Hi Derek, it is possible that we also used that plastic cord, I am not sure, now that you mention it.  But we were looking for about 1 mm, so not as difficult as your application.  It’s amazing what can be done by experienced people to get around difficult practical problems such as that.

MJM460

Models can also benefit from a bit of superheat just to dry it enough that by the time it gets to the cylinder, its saturated steam.  Say 100F worth of superheat is quite beneficial

Dave
Title: Re: Talking Thermodynamics
Post by: steam guy willy on October 30, 2019, 01:53:57 PM
Hi MJM,  Thanks for all this ..there is so much more to these systems than what is initially visible. with our models we just have one pipe from the boiler to the engine ,and one pipe to atmosphere.  On later  full size engines there are so many extra pipes and parts that sort of spoil the visual aesthetics of the complete plant. Like in a modern car ,the whole of the engine compartment is full of 'things' unlike my Morris Minor that just has a carburettor and exhaust pipe !!! The Beeleigh Engine just has the main steam pipe  4"   and the exit pipe 1'5" from the air pump to the boiler feed pump. There is no provision for oil lubrication in the system, just a small two cocked supply valve on the top of the HP cylinder. I suppose any oil floats on top of the water in the air pump.   Not a valid vimeo URL   Here is a video of the air/tea pump operating !!!

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on October 31, 2019, 11:53:58 AM
Hi Willy, as with all things, it is worth knowing the theory to help guide what to expect.  However, it is hard to quantify, in this case, just how much water would remain in the engine after a run, especially as it was not intended to run on air.  So while provision of dryers is possible, and will reduce the water reaching the engine, it is hard to know whether it will be enough, or even if it is necessary.  Warm air while the engine is turned over by that electric motor and car tyre might be all that is required.

With that double valves connection on top of the cylinder, you can open the top one, add a squirt of oil between the valves, then close the top one, and finally open the bottom one.  This allows some oil to enter the engine, even when running.  And I guess some oil would remain in the cylinder when the engine stops.  Probably worth doing again when the engine shuts down.

It would be interesting to hear what others with more experience of running larger engines on air have found.

That water pump has to lift the water, not just the physical height from the hot well, but also against the pressure difference between the condenser and atmospheric pressure.  Of course this won’t be needed when running on air I expect.

The model needs a bigger box for the tea, and a lever so it can be more smoothly operated against the friction of that piston o-ring, but a great model to include in the display.

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 11, 2020, 12:59:32 AM
Hi MJM, a few more questions about boilers.   Is it possible with enough heat to evaporate all of the water in a boiler to steam assuming the boiler is strong enough to contain it without bursting.?   if you have a large boiler with a very small amount of water will the temperature of the steam rise to the same pressure  as dictated by the steam tables ie, do the steam tables only refer to boilers with the correct amount of water in it.??...If the boiler has ,theoretically 100% insulation. is it possible to raise the temperature /pressure to a high amount with say only 101 degrees of heat going into it continually..?

Hoping you are well and ok ,

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 11, 2020, 09:59:53 AM
Hi Willy, good to see you back thinking about thermodynamics again.

Some quite important questions about boilers, let’s see if I can make the issues understandable.

Let’s first assume that you are talking about a boiler which starts off containing water which is partly in liquid form or phase and partly vapour.  We can look at other cases later.

And let’s remove any complications of any air remaining in the boiler after it is filled.  We can do this by heating the boiler until steam is being raised, the pressure is not important, but then open the stop valve so some steam escapes.  This tends to initially be a mixture of steam and air, but after some time, we can assume that essentially all the air has been removed.  We can then close the stop valve, and let the boiler cool down in preparation for the experiment.

First, can all the water in a boiler be evaporated if the boiler is strong enough?  Basically, so long as the boiler is strong enough as you assumed, the answer is yes.  So we should then ask how strong does that boiler have to be?  We can see from the steam tables that the two phase section of the tables stops at 374.14 deg C at which the pressure will be 22090 kPa absolute.  For those who prefer imperial units, that is 705.44 deg F and 3204 psia.  So the boiler does have to be really strong at quite a high temperature.  Our typical copper boilers loose their strength at a much lower temperature than that so could not do it.  Special high temperature alloy steels are required, and yes there are boilers, called super critical boilers which do operate in this region and above.

That temperature and pressure is quoted quite accurately.  It is called the critical point, and is the highest pressure and temperature at which liquid and vapour can exist at the same time.  It is quite important in higher level thermodynamic calculations but not of much interest to model engineers.  And there is room for discussion about just what liquid and vapour means above this point.  There are no distinct different phases.

Now, if the boiler initially contained only vapour, the steam conditions are entirely in the superheat zone.  The increase in pressure does not cause the steam to condense, it is always all vapour.  The upper temperature is limited by the heat source, and certainly property diagrams for steam and steam tables covering superheated steam go much higher.  My superheat tables top out at 60,000 kPa and 1300 deg C.  Not an area for hobby experiments.

If the boiler starts absolutely full of liquid water, the initial heating will increase the pressure much faster, but again I suggest the liquid condition eventually reaches the supercritical zone and becomes what we would probably call vapour without a distinct phase change.  Not a process that I have my experience in, but I hope that is enough to answer the question.

The second question really only involves clarification of the details of the first.  For the purposes of these questions, there is no “proper amount” of water in the boiler.  We either start with two phases or one phase.  If there is only a small amount of water in a large boiler, it would all be boiled into vapour relatively early in the experiment, and after that the vapour in the boiler would be superheated, and the pressure rises more slowly, depending on heat input and density of the steam.  Because heat transfer is not as good as for boiling a liquid, we might get hotter spots on the shell.  The temperature and pressure are now two independent properties, and the two phase section of the tables does not apply, this is where the superheat tables come in.  And with a smaller mass of steam relative to the volume of the boiler, the pressure will always be lower than if we started with a higher mass of water in the same boiler.  The pressure is always dependent on the mass of water in the boiler.  It also is affected by temperature, but temperature and pressure are independent for superheated steam.

The third question is an entirely different issue.  We are now talking about the basic laws of thermodynamics, and no longer talking about the properties of steam. 

Remember, the basic law is that heat travels from a higher temperature area or material to a lower temperature area.  So if your heat source is at 101 degrees, the boiler will never exceed 101 degrees, because once the temperature difference is zero, there is no further heat transfer, regardless of the efficiency of the insulation.



We are both safe and well thank you, and hope your winter is not treating you too badly.  It is all quite surreal here.  The Eastern parts of our state are experiencing the worst bushfires on record, I believe the area is roughly 10 times the long term average, and make no mistake, it is a large area, and the weather is not yet being helpful.  At the same time at home, we are experiencing cool weather, even had to turn the heating back on last week.  The western part of the state is a much larger area, a few smaller fires, but not abnormal, but the whole area is tinder dry from prolonged drought, and the worst months are usually January and February.  We are not out of the woods yet by a long way.  All very hard to comprehend and quite scary.

Thanks everyone for looking in, and thank you to all those thinking about those affected by the fires.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 12, 2020, 01:15:28 AM
Hi MJM , Yes of course what we have learned about heat transfer is correct ,but. was wondering if there is a time element in the equation ?  If  the heat is continually supplied where does the added heat get dissipated if the heating element is in the boiler and the insulation is at 100% efficient.? this may be a purely hypothetical question of course ,but was thinking about the energy that is being used ?? Also, was wondering about how you calculate the Horsepower of a compound steam engine

?

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 12, 2020, 09:30:10 AM
Hi Willy, that is an interesting follow up to the previous question.  The issue lies in the statement of the problem.  Remember that you had already fixed the temperature, at which the heat is supplied at 101 deg, (I assume C, but it does not really matter).  You had also stated perfect insulation, or near enough to, so no heat loss, and now you are adding that energy is continually supplied.  This is too many constraints.

If heat is supplied continuously, then it must go somewhere or accumulate in the form of increasing temperature.  So the question is to look at where it goes. 

Let’s look at the heat source.  You might be thinking of an electric element similar to that in your boiler.  That would make it relatively easy to add lots of insulation to reduce the heat loss to near zero.  Now, if you continue to supply electrical energy to the heating element in the enclosure, the element temperature will rise as we have discussed before.  As there is no heat loss, due to the good insulation, the temperature of the element will rise until something breaks, the element or the insulation melts.  But the temperature will rise. 

To comply with your condition of heat supply at a constant 101 degrees, you would need to interrupt the energy supply to stay within that constraint.  Either the temperature will rise, or the energy supply will be interrupted, or the insulation must fail.

Of course with other heat sources the detail will be different, but the result will be the same.

You might supply the heat using a double boiler in the way that a cook makes a delicate recipe.  The boiler would need to be slightly pressurised to get 101 degrees, as an open saucepan would be closer to 99.  But if no heat flows from the boiling fluid to your insulated box, then if heat continues to be supplied, by a flame for example, the flue gas and losses to atmosphere from the outside of the pot will rise until all the heat from the fuel combustion is lost.

Similarly if you heat is supplied by a hot fluid circulating in a coil, and no heat is transferred to your insulated box, the circulating fluid temperature will not change, and so if energy is continually supplied, the heat loss will simply be somewhere else.  You have to enlarge the boundaries of the system that you examine to see just where.

Switching subjects to calculating the power output of a compound engine, again a very different subject.

If you remember back to the early days of this thread we did talk about different definitions of the  power out put of an engine, and how these might be calculated.

The theoretical power output can be calculated from the steam supply and exhaust conditions using the Carnot temperature difference equation, but more usefully we can use the steam tables to calculate the isentropic or adiabatic power available, again based on the steam supply and exhaust conditions.  The supply conditions are generally known by direct measurement.  If we have two independent properties, the steam tables tell us all the other properties, in particular specific volume (or it’s reciprocal which is density), enthalpy and entropy.  Providing the steam is either dry saturated or superheated, pressure and temperature measurements are sufficient.  If it is wet steam, pressure and temperature are not independent, so another property is required.  At the engine inlet, let’s assume dry saturated or superheated with reliable pressure and temperature measurements, and we can then locate these conditions in the steam tables. 

From the tables, we want to find the enthalpy and entropy in particular, and also the specific volume.

The exhaust steam condition is more interesting.  We can measure the exhaust pressure at the cylinder exhaust connection, and while we can measure the temperature,  the steam is often wet, hence pressure and temperature are not independent, so we need something else.  Fortunately we assumed an ideal adiabatic process, so we know that the process occurs at constant entropy. 

Again we have two independent properties, pressure and entropy, and with some simple (if tedious) interpolation  calculations, we can find all the other properties, in particular the enthalpy.  All these quantities represented by these properties are based on a mass of one kg (or pound mass if you are using imperial units), so apply to any size of engine.  For a particular engine, we need to know the flow rate of steam through the engine on a per second basis, perhaps by steam flow measurement if suitable instruments are available, or a mass balance based on water lost from the boiler, or perhaps estimated using cylinder swept volumes and rpm with the steam specific volume property.

 The work done by that ideal adiabatic process is just equal to the difference in enthalpy, so we just do the simple subtraction.  The units of enthalpy in the steam tables are joules per kg, so the difference between inlet and exhaust enthalpy multiplied by the steam flow in kg/s gives us power in kilojoules/sec, which we also know as Watts.

Now two very important points.  First, no real engine can achieve that ideal work output.  No real process is actually constant entropy, the entropy always increases as expressed by the second law of thermodynamics.  In practical terms this means the actual, change in enthalpy across the engine will be less than the ideal change in enthalpy.  In this calculation we can define an isentropic efficiency as actual change in enthalpy divided by the ideal change in enthalpy.

Ok, a bit of a process so let’s summarise the whole process as the procedure to calculate the work done by the steam on the piston. 

It is perhaps disappointing to realise that a real engine cannot even deliver this amount of work at the output shaft, so we can use it to drive a saw or milling machine or pump or compressor or whatever.  We still need to take into account the energy lost in overcoming friction between piston and cylinder, bearings of all the moving parts, losses in valves and ports and so on.   We can not calculate all of these with any degree of accuracy despite sophisticated computer programmes.  These all reduce the actual engine output. 

In the end we have to do a test.  To find the actual output for the real inlet and exhaust steam conditions.  Engine manufacturers have quite complex test beds on which accurate testing can be carried out, either for a specific engine, or for the manufacturers to use to predict the performance of future engines.

I said two points to summarise.  The second point is that nowhere have I mentioned whether the engine is single acting, double acting, multi cylinder, compound, condensing or otherwise.  I have only used the steam properties at the engine inlet and exhaust, and the steam tables to calculate the maximum potential power output of an ideal engine, and note that to find the efficiency of a real engine must be determined by a test run.  All those different engine configurations are just combinations of different physical components, all of which have the same aim, to produce the maximum amount of useful work from the available energy in the steam.

There is a lot more that could be said, but I hope that is enough to answer your questions, or alternatively to prompt more if I have missed the point you were considering.

Thanks to everyone looking in.

MJM460



Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 13, 2020, 02:21:39 AM
Hi MJM, Thanks for the reply...So we never actually destroy our boilers as we have safety valves and in my boiler a low water sensor and a pressure switch. these safety features prevent any cataclysmic eventualities from happening. And in most boilers there is plenty of escape routes for the heat to dissipate . And I suppose my question was purely hypertheticle., although at the beginning of steam engine and boiler experiments there were many explosions before the theory was worked out.

The usual Horsepower is worked out with the equation     PLAN
                                                                                _________
                                                                                33,000.      but this is for simple single cylinder engines rather than Compound engines. This is of course just the theoretical  calculation rather than the actual physical calculation involving pressure and temperature gauges .  So I was thinking there might be an equation for a compound engine using similar measurements. The reason I ask is because I have the castings for a compound engine with  2 1/2" and 1 3/4" cylinders. here is a pic.  thank you for all your answers and I am happy that you are OK.

Willy
                                                                             
                                                                                 
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 13, 2020, 05:19:41 AM
Hi Willy, well there are still some boiler explosions, so perhaps “hardly ever”.  I don’t know the actual numbers, but equipment fails to operate as intended and mistakes happen.  But a lot of attention and effort goes into making all those safety systems ever more reliable.  Dual safety valves, redundant switches that shut things down, usually with voting systems so critical systems are shutdown if the instruments don’t agree and so on. But we are also now much better at making the equipment strong enough, with steel production techniques and welding procedures so have reduced the number of events.  However we have to remain vigilant and not relax the standards.

That formula for horse power is interesting and like all mathematical formulae it is necessary to understand how it was developed and what assumptions are implied when it is used.  You have used imperial units and that 33000 figure is a constant necessary to account for the vagaries of the units used.  I would have to check whether the dimensions of piston diameter and stroke are in inches or feet, and similarly the units of pressure.  However if we look at the formula carefully, we see pressure multiplied by piston area, which of course gives force.  The pressure is the pressure in the cylinder.  Then force is multiplied by the stroke or distance moved by the piston in one stroke.  Now force moving through distance is mechanical work, so p x L x A is the work done per stroke of the piston.  Then N gives the number of strokes per minute giving power.  As you say, it is for a single acting cylinder.  Multiply by 2 for double acting and by the number of cylinders for a multi cylinder engine.

But, and a very big but at that, the formula implies that the pressure is constant throughout the stroke.  If the pressure varies, the force on the piston varies and the work done for each unit of distance the piston moves will vary, introducing errors into the equation.  Alternatively, by calculating the pressure over each tiny increment of stroke, adding these up and dividing the total by the stroke, we can come up with a mean effective pressure.  In addition, it is not the absolute pressure in the cylinder, but it is the difference in pressure on each side of the piston.  Remember that pressure on the exhaust side of the piston gives a force in the opposite direction to the movement of the piston, and so does negative work.  That is is subtracts from the useful output of the engine.

In a real engine, there are pressure losses in the piping, valves and cylinder passages so to measure the pressure, an indicator was invented which actually measures the pressure in the cylinder, and plots it on a diagram.  A clever device called a planimeter is used to calculate that mean effective pressure using that diagram after an engine test.  The mean effective pressure might be close to the steam chest pressure for a slow running engine with generous valve and passage sizes, providing the valves stay open for most of the stroke.  However, if you have the valves cut off early so you have some steam expansion, the pressure starts reducing for the remainder of the stroke and is nowhere near constant.

Now your compound engine is specifically arranged to take advantage of the work done by expanding the steam instead of just exhausting high pressure steam at the end of the stroke.

So you could use the formula, but you would have to estimate the pressure on each side of the piston through out the stroke.  For the high pressure cylinder this is like a normal double acting engine, though the exhaust pressure will be higher, roughly equal to the lp cylinder inlet.  Then estimate the lp cylinder pressure throughout its stroke.  And of course the exhaust side of the lp piston will experience the final exhaust or preferably condenser pressure.

Then add the results for each side of each piston for each cylinder and multiply by the number of strokes per minute.  A simple enough concept, but working out the valve timing so that the actual cylinder indicator diagram can be estimate does complicate the use of a deceptively simple looking equation.  I think I would go for the adiabatic calculation, and when you have found the engine hidden in that casting set, next project has to be a test stand with a proper brake to measure the power output.  Then you can use the brake output power to calculate the “brake mean effective pressure” which you can use as a basis for estimating what happens in future engines.

It’s a great looking set of castings by the way.  I notice that Jo quickly took interest.  But no pressure to complete the current fabulous project before you start the next one!

Thank you to all looking in.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 14, 2020, 01:12:36 AM
Hi MJM , thanks for the latest info..I have grown up with traction Engines and they were always sold with a power rating of  between 4 and 20 NHP. this being Nominal HP. Actually they were capable of much higher actual HP . I have included a pic and citation for these powerful machines.!!! I was also talking to a friend with a Uni degree in Physics about Thermo D..and he was saying that heat and temperature were actually different, but didn't get an explanation !!

Willy,   
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 14, 2020, 11:44:01 AM
Hi Willy, there were many definitions of horsepower in the early days of steam and as you say, most of them had little relationship to actual engine output.  The NHP one is surprising because it generally gives a figure lower than the actual engine output.  Most of the stake holders had an interest in claiming the highest figure they could justify in order to sell more engines.  On the other hand perhaps they wanted to minimise the figure if that meant less tax had to be paid.  While the tax man almost certainly wanted something easy to measure like bore and stroke, rather than something that required sophisticated testing.  Much the same as the tonnage of ships.

Brake horsepower, the power as measured by a brake, is a more useful measure as it involves testing of the engine, or at least a very similar one.  However all the wildly optimistic claims are still made today.  Quoted brake horse power is usually still qualified as for a new and clean engine, so may not apply after a number of hours of operation.  Also it may or may not include power to run the water pump or cooling fan, alternator, and so on.  So it is still necessary to carefully specify the power requirement of the driven load and require a margin for measurement tolerances and performance reduction over time.

I have to agree with your friend about temperature and heat being different.

Heat is a form of energy that can flow in response to a difference in temperature and can be converted to other forms, in particular in our specific hobby, it can be converted to mechanical work, though it is easier to convert mechanical work to heat. 

Temperature is a property of a substance that we can measure with a thermometer.  Generally if heat is added to an object or a fluid, the temperature will rise, but we cannot determine the amount of heat from temperature alone.  Different materials will experience a different temperature change for the same amount of heat.  We have to know a property of the substance called the specific heat.  This varies greatly for different materials, and is usually determined by experiment.  On the other hand, when water is at its boiling point, you can add or subtract heat and the temperature will not change at all, so temperature tells us nothing about the quantity of heat that is being added.

It is quite difficult to define precisely the difference between heat and temperature because temperature change is closely associated related to heat gain or loss in all our experience.  Most attempts seem to end up going in circles, and my explanation is probably no different.

I don’t know if that helps at all.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 15, 2020, 01:45:35 AM
Hi MJM,  So ..they have now been running the engine on air , This was initiated by doing the pressure test to make sure the whole engine was steam tight.  Being ok the engine ran on air at about  20 PSI. They do have an electric motor to run the engine but it has a non reversing spragg bearing turning the engine with a car road wheel acting the flywheel. I have noticed when I was running the Bressingham engine on air, moisture was forming on the inlet pipe leaving a rust witness on the bolts of the steam chest.

So with the Beeleigh engine there are no drain cocks on the actual cylinders as they are inside the massive steam jacket. This jacket has the 4" steam inlet at the top of the casting and a small steam cock fitting at the bottom of it 1" in diameter.. When the engine is started on steam the steam valve inlet is in the middle closed position so not allowing any steam to enter the HP cylinder.  The steam is then allowed to enter and surround both cylinders allowing them to get up to temperature. this is noted by the condensation coming out of the 1" drain cock. Once actual steam starts to issue from the drain cock the engine is then started by turning the flywheel to open the ports and the engine will start.

 So when running on air the engine being quite cold will condense the warmer to produce moisture. If the engine has the ports closed perhaps after a while water will appear at the drain cock and the temperature of the engine block will rise to ambient temp.?? Once no more water appears then it will be possible to let the engine run on air without water/moistur building up in the bottom of the cylinders ?? will this be the way to go ?? and do you have further thoughts on this.... I have noted your previous comments indwell suggest them to the team. I don't know if they have their own Thermodynamics engineer to advise them about potential problems ? !!!

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on February 15, 2020, 10:26:57 AM
Hi Willy, I must have been tired when I posted last night, I found post no 1283 this morning, where we were discussing running the engine on air, and it looks like you found it too.

I covered what happens in the compressor and piping to the engine inlet, basically the moisture in the air due to humidity is compressed along with the rest of the gas mixture we call air.  In the compressor, the water stays in vapour form so does not condense at the compressor discharge temperature.  However, after the compressor, in the after cooler and accumulator and piping, the air and contained moisture cool, and will generally reach the dew point before it gets back to room temperature, so water condenses in the system.  Hence it is important to have drain points on the accumulator and at the ends of branches in the piping.  You don’t want that water in the piping running down into the engine.  The air is now at 100% humidity at the pressure in the system.

Next we need to consider what happens as this air passes through a throttle valve and the engine.  Because of the condensation in the system there is less moisture in the air than there was in the original inlet air, so depending on the thermodynamic path through which the air passes from high pressure in the piping to atmospheric pressure in the exhaust, it will not necessarily reach its dew point, so moisture will not necessarily condense. 

We can assume adiabatic expansion as the air does work during the expansion, and it will cool in the process.   Part of the expansion will also be throttling so constant enthalpy.  You can see the complexity of the calculation increasing.  By the time the cooler air is exhausted to atmospheric pressure, whether there is any condensation will depend on the final temperature and the initial air humidity.  I am not convinced there will be much condensation, but not keen to try and work through all the calculations by hand.  In the end the answer still relies on many assumptions.

Remember also that when the air in the pipe cools, if the temperature is lower than the atmospheric dew point, there will be condensation on the outside of the piping, but that does not mean condensation inside the pipe.  It just means the inside temperature is lower than the dew point of the outside air. 

It would be very helpful to know the temperature inside the pipe and the atmospheric air humidity and temperature.  But where the engine air finally exhausts to atmosphere, I would expect some sign of moisture if it is condensing.  But if it is merely cool, and all the moisture is on the outside of the pipe, then there is probably not a major worry inside the engine.  In the end it gets back to observation and inspection. 

I think a procedure to warm up the engine with dry air, while you drive it with that car tyre might be adequate.  Along with adequate lubrication while the engine is running.

 Your comment that there was moisture on the outside of the inlet piping needs further thought.  If so, it would indicate that the air inside the pipe was cooler than the atmospheric air dew point.  How did that come about.  Or was it just near the valve chest, where the metal might all be cooled a little by the exhaust.

I hope that helps a little with understanding.  Understanding will always help in interpreting observations.  I really hope that someone else can come in with some experience on this issue.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on February 16, 2020, 01:43:48 AM
Hi MJM , thanks for the info ..I shall talk to the engine crew soon. The pic of the steam chest shows spots of rust around the steel bolts, and there does not seem to be any around the holes that do not hold the steam chest on.? I don't think the moisture has crept through the gasket so is this because the steel reacts differently with brass as far as the thermodynamics is concerned ? of course the brass will not rust and the tube was firmly attached with the jubilee clip so the coldness to create the rust must have come from the ambient atmosphere . the bolts on the other side of the steam chest cover do not seem to be affected ?................................ I have not seen the compressor set up at Beeleigh so don't really know how it is all connected up. I shall make enquires soon.

Willy
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 05, 2020, 11:40:48 PM
Hi MJM, More about Beeleigh Mill , They have have done more runs on compressed air and have firstly just run the engine with the HP cylinder with out the LP slide valve. They found that the engine ran very well but when they replaced the LP valve the engine was not so happy and ran very haphazardly . They put this down to the fact that air is different steam at the same pressure as it does not have the high temperature as the steam,  So the question is that if you used compressed air at the same temp as the steam would it work with the same power and efficiency ?? They have said that the engines performance follows the theory of TH.

Willy

Title: Re: Talking Thermodynamics
Post by: crueby on March 06, 2020, 12:00:04 AM
Willy, hope you don't mind me chiming in, but to 'expand' on your question  I had thought that a compound didn't run well on compressed air since the air didn't continue to expand very long like steam will, that temperature of the air was not the issue?
One other tidbit that I read in one of the books about the Mann steam trucks was that they had a valve that let the driver send steam to the low pressure cylinder directly for extra power on steep hills. Bet it ran a little lumpy, but clever.
Chris
Title: Re: Talking Thermodynamics
Post by: crueby on March 06, 2020, 12:01:17 AM
Oh, and any chance of a video of the big engine running? Love to see the old ones moving.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 06, 2020, 01:39:17 AM
Hi Chris, yes there is a video of it running and it is on a Facebook page. by David Morgan... If you go on to FB you should find it ?? !!! I shall try and get a copy of it if I can >>>??
Willy
Title: Re: Talking Thermodynamics
Post by: crueby on March 06, 2020, 01:50:43 AM
Hi Chris, yes there is a video of it running and it is on a Facebook page. by David Morgan... If you go on to FB you should find it ?? !!! I shall try and get a copy of it if I can >>>??
Willy
I don't have a Facebook account... Do they have it on youtube or something?
Title: Re: Talking Thermodynamics
Post by: crueby on March 06, 2020, 02:01:59 AM
Hi Chris, yes there is a video of it running and it is on a Facebook page. by David Morgan... If you go on to FB you should find it ?? !!! I shall try and get a copy of it if I can >>>??
Willy
Found it! The mill restoration groups web page has pictures and a video link at the bottom of the page:


https://beeleighmill.org.uk/2020/02/17/beeleigh-mill-beam-engine-running/
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 06, 2020, 10:13:19 AM
Hi Willy,

There are really three issues involved in this question.

Putting the difference in perceived performance down to the difference between air and steam, really does not help explain why there might be a difference.

Basically, while the valves are open and steam flowing into the cylinder, assuming that the air and steam pressures are equal, there is no difference.  Of course, there might be a difference between the boiler and the compressor’s ability to maintain the pressure during this part of the stroke.

Once the valve closes, the difference in behaviour has an effect due to the way the pressure changes as the volume increases.  The pressure and volume are related by the equation
P1 x V1^n = P2 x V2^n
If we assume ideal gases then n equals the ration of specific heats which for air is about 1.4, while for steam it is about 1.33. 

Now steam near its saturation point is definitely not an ideal gas.  But doing the calculations based on a pressure of 450 kPa absolute, (about 50 psig) and an expansion of the cylinder volume to twice the volume when the valves closed, clearance volume means this is not the same as half the piston travel, air pressure would become about 161 kPa absolute (about 8.2 psig) while steam would be nearer 170 kPa.  Because of this higher end pressure, the steam would produce a little more work than the steam.  But this is a very rough estimate based on the inaccurate ideal gas assumption.

Because steam behaviour is nothing like an ideal gas.  To get a better answer, we have to use the steam tables.  And more assumptions!  This is not a trivial exercise unless you have access to a suitable computer program, so I will not try and set out the method.  We start with our assumed inlet steam pressure, assuming saturated steam, so no superheat, where we can look up the temperature, specific volume and entropy.  We now need two properties of the exhaust steam (while it is still in the cylinder just as the valve is about to open).  We know the specific volume from our assumption of expansion to twice the original volume.  So the end point specific volume is double the specific volume of the inlet steam.

If we had an accurate exhaust temperature, it would help, but without it, we can try the assumption of ideal adiabatic expansion.  Then we know the entropy after the expansion is the same as the inlet steam.  Then with some interpolation of the tables we find the exhaust will be wet steam, and with some tedious arithmetic estimate the steam as 199 kPa (or about 15 psig).

If we look at the probably adiabatic efficiency we will get a slightly different answer again, but I think you get the picture. 

Realistically, I think this difference is not very significant is because if you adjust the valve cut off a bit, so you hold the inlet pressure for a bit longer when operating on air to get the same power as steam on the original setting.  But there is a difference.

The other issue is the operation of the compound engine.

While I quoted some pressures and the equivalent gauge pressures, in a compound engine, in the transfer pipe from hp exhaust to lp inlet is not connected to atmosphere, and is not influenced by atmospheric pressure.

In my example above the lp inlet would be say 5 to 15 psi which is 20 to 30 psia.  The same expansion to twice the volume as before results in an exhaust pressure which is quite a way below atmospheric pressure.  And that assumed a 50 psig inlet, measured after the throttle valve, say directly on the valve chest.  With an unloaded engine, the inlet pressure may be even lower, and the lp inlet could be less than atmospheric before the expansion.

With an exhaust pressure below atmospheric pressure, when the exhaust valve opens, atmospheric air rushes into the cylinder, just when we really want the steam to rush out.  This produces a short period of reverse torque.  No wonder the engine runs rough.

With the lp valve removed, the hp runs quite well as a single cylinder engine, still lightly loaded though the lp adds a little load, which is probably helpful for smooth operation of the engine.

That valve to let the supply air into the lp inlet I believe is called a simpling valve.  It makes the engine run as a twin cylinder engine instead of compound.  When admitting air to the lp inlet, there is also an effect on the hp cylinder.  Is the hp exhaust bypassed directly to exhaust?  Or does the hp cylinder now find it has that air rushing in when the exhaust valve opens?  Probably even if so, it is less of a problem, a smaller reverse pulse perhaps.  Or is there a corresponding valve which also exhausts the hp to atmosphere.

A recent thread talked about the similar valves on the Dickson which allowed a three cylinder engine to operate in different modes.

The third issue is the effect of the condenser.  The original engine was designed to operate with a condenser.  The condenser reduces the lp exhaust pressure to below atmospheric.  So it may not have needed a big inlet pressure to ensure the lp exhaust was at or above the condenser pressure.  Without the condenser in operation, even steam may be a bit rough if the expansion results in the lp exhaust below atmospheric pressure.  Similarly, hp inlet pressure for air has to be sufficient to get the exhaust pressure after expansion in two cylinders above atmospheric.  This may result in the engine running faster than desired.  Alternatively, it needs to be doing some work to absorb the extra power with the higher inlet pressure.

I suspect it is the issue of expansion to below atmospheric pressure that is causing the issues with the engine running rough, rather than the difference between air and steam.

However, there is almost certainly a cut off point at which steam still exhausts above atmospheric pressure, while air is a little below.

I hope that makes it all a little clearer.

Thanks to everyone looking in.

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on March 06, 2020, 10:16:01 AM
Hi Chris, while temperature is certainly relevant to the energy in the working fluid, whether air or gas, so is the molecular weight.  In addition, there are big differences in the proportion of that energy that is available to be converted to work.  When it comes to the actual engine, it is usually better to just look at the pressure and the indicator diagram.  In the end, it is the pressure on the face of the piston, or more usefully in double acting engines, the difference in pressure between the two sides of the piston, the piston area and the length of the stroke that determines how much work is produced in each stroke. 

Particularly for steam, a lot of the energy input at the boiler goes into the latent heat, heat necessary to evaporate the liquid, and most of this is lost with the exhaust.  There is also energy in the spinning of the molecules about each axis, and even the linear motion of the molecules which are going the wrong direction to push the piston, but does not get converted to work.

Very clever of the designers of that Mann engine to include a valve direct to the lp cylinder.  Would give lots of torque for a hill climb or even just initial acceleration with a heavy load, the move to the more economical compound mode for steady speed or down hill.  I wonder if it was a three way valve or multi port plug valve to change the hp exhaust at the same time.  But basically the same issues as Willy’s compound engine.  It would be interesting to hear more from those who have operated engines with this facility.

Looking forward to see the model.  I hope it will include the truck as well as the engine.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 07, 2020, 02:50:24 AM
Hi MJM & Chris, thanks for the explanation and of course the condenser is part of the process... So are there Vacuum tables ? a bit like steam tables ?  So if you have a quantity of steam  S.cc at a pressure  P kpa and you inject an amount of water W cc at a temperature  T c.  could you work out the new Pressure   +/-  ??   Is this a silly/strange  concept ???  also if you had similar values for air with the same process, would that also be possible ??  Just  wondering about this. Also does the 'dryness' of the air have an effect ? There are a lot of questions here and I may be thinking totally out of the box so sorry about that... I suppose an experiment is required ?

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 07, 2020, 08:15:51 AM
Hi Willy, the steam tables are all in terms of absolute pressure, and they go down to zero.  Well, mine stop at 0.6113 kPa absolute, which is close enough unless you are working in an area where the difference is important.   It corresponds to a temperature of 0.01 degree C, which is the triple point where liquid, vapour and solid can all exist in equilibrium at the same time.  But that is near enough to a full vacuum for most of us.

The tables generally have a first section covering the area from 0 to 100 kPa, or a saturation boiling point then continue with a line at 100 degrees C, and a third section covering the superheat area.  The vacuum area, that from atmospheric pressure down to absolute zero is definitely covered.

Now squirting water into the exhaust to condense the steam is not a new idea, and I suspect your history books will show that it was well known technology long before the tubed condensers we more commonly use today.

Any vacuum exhaust system has two additional components, the air pump and the water pump.  Sometimes these are incorporated into a single combined unit that pumps the water and the air back up to atmospheric pressure. 

Fortunately it needs less power to pump the condensed water from the condenser pressure to atmospheric pressure than the additional power the engine produces due to the lower exhaust pressure.

The enters from atmosphere due to leakage at every joint and seal in the system, as well as some which was dissolved in the original boiler water, and it is the engineers ability to limit the air leakage and maintain the pump that determines the pressure the condenser operates at.  The operating pressure is the total of the partial pressure of the steam at the temperature it is condensing plus the partial pressure of the air in the condenser.

Too many unknowns to solve in a simple manner, so your intuition that some experiment is needed is correct.  We need to know the total pressure of the air plus water vapour, measured with a vacuum gauge or perhaps a mercury manometer for more accuracy.  And we need to know the temperature at which the water is condensing.  Then we need to know about the engine inlet conditions and the power produced by the engine.  Probably easier to just measure the water flow, and inlet and outlet temperature while we are at it.

Obviously, when the engine is designed, many of these things are part of the design, which will be based on an assumed condenser performance.  Then tests will be done during commissioning to prove the contract guaranteed performance of each component.

Finally the engineer has to maintain the plant so the performance is maintained.  By no means an easy job.

At this stage of the life of that plant, I guess very little if any of the original data is available.  But the experiment would be informative.  The main disadvantage of the whole system is that the condensate is mixed with the cooling water and not suitable for returning to the boiler.  The water and the heat it contains is all lost.

I hope that clarifies makes things a little clearer.

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 08, 2020, 02:50:23 AM
Hi MJM, Thanks for the further info that I have shared with David Morgan the chap that writes up the News reports. He has replied and I am sending you this. I have noticed on later engines that there is a drain pipe and cock that drains the condensate from the bottom of the steam chest as well as the cylinders..The water supply for the condenser comes strait from the tidal river that brings the grain to the mill to be turned int flour. He talks about the oil in the condensate and I think as this floats on the water it can be removed by hand from the large reservoir on top of the air pump...(Just thought of that actually !!).Really good of you to go into all the details that you give and share with us.  There is a website that features the engine site.    BMRG. steam engine  Maldon.  This has the video of the engine working with the compressor.  Beeleigh Mill Restoration Group.  The pics of the Beeleigh  Airpump and the one I made for the model. And the drain pipe on the Bressingham engine connected to the steam chest.

Willy

Robert,

Good to hear from you and thanks for getting in touch. Yes, we are very aware of the problem of wet air and did in fact see some water accumulating in the steam chest. The Atlas Copco compressor we were using does not have a drier. We ran the engine for some time using the electric drive to try to dry things out a bit.  We were very choosy on the first day we used the compressor in that it was very cold and dry so was fairly ideal. Since then we have had a couple of trials when it was more humid.  Having completed this testing we probably will not do much more with compressed air unless deciding that it provides a long term solution, which is unlikely.
Your Australian friend’s contribution is also interesting. We are very aware of the role of the condenser indeed the previous millwright was proposing to use very low pressure steam but expected the LP cylinder with an effective condenser to provide the power needed. At this stage we are not certain whether we will commission the cold well since steaming the engine, if at all, will be very infrequent. Over the next few weeks / months we will be deliberating on the preferred option.  Personally, I think steam is some way off because of the complexity of a location for boiler, fuel, water supply and disposal of oily condensate. All good fun!

Best regards

David
Title: Re: Talking Thermodynamics
Post by: MJM460 on March 09, 2020, 12:16:00 AM
Hi Willy, I am sure that Dave and his crew will have a good understanding of these issues, and how the condenser operates.  I can understand his hesitation about operation on steam as that will involve a lot of extra hassle for a team of volunteers, while air operation does show the mechanism in motion.  For most visitors I guess the motion is most fascinating though the steam would add some atmosphere for those who appreciate it.

So that leaves the issues of what supply air pressure to use, and how to deal with the degree of expansion which is built into the valve timing, and also the moisture that will result from condensation of the humidity in the air.  The approach of looking at temperature and humidity before running the engine shows they have a good understanding of the issues.

Driers can be bought as stand alone units separate from the compressor, but as with all similar situations, finance tends to be an issue.  I am sure that they will work it all out with some mix of theory and some experiment to see the extent of any issues.  This is always necessary in the end, as the theoretical calculations always require some assumptions about efficiency, so need experiment, or test runs, to determine the final answers.

I have looked at the website from one of your earlier posts and seen the video of the engine running.  They have done a great job.

I think that anyone who operates an engine for a time, particularly a large one will arrive at the need for drain cocks in many locations.  You should see how many we needed in an oil refinery where stuff is boiling or condensing everywhere!  But you can get away with a lot on a small model where the quantities are small and various tolerances in construction provide a path for the condensate, even if it is turning the engine over by hand until enough condensate is expelled and the engine starts running.  That critical feature of lifting the slide valve off the seat, or pushing the oscillating cylinder off the port face works well.  Piston valves are a different issue if they really seal well, but I have not yet tried one of those.

There will always be some oil in the exhaust condensate from the engine lubrication.  Skimming off the oil that floats to the surface is easy enough, but the oil is pretty finely dispersed by the time it gets through the engine, so while some floats to the surface, some is dissolved, and some is in such fine droplets that they take a very long time to get to the surface.  So skimming off the surface oil does not remove it all.

Of course that does not mean it has not been tried, particularly in the early days.  But all the oil that remains in the condensate will end up fouling the heating surfaces of the boiler so the boiler becomes less efficient and needs more maintenance.  It is more of a problem in higher pressure boilers and I am not sure if there is a pressure below which you can get away with it.

But with river water injected into the condenser, you would be introducing a whole lot of additional impurities, and I am sure you do not have an industrial quality feedwater treatment system to remove them all to an acceptable level.  So I would not suggest reusing the condensate.

But keep thinking about the issues.  Thinking leads to questions, the answers to which add to understanding.  And understanding leads to reducing the necessary experiments to those that are likely to be productive.  I hope that you are finding that you understand these things a little better as we go.

Thanks to everyone looking in, I hope you are finding it interesting reading.

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on March 10, 2020, 03:18:51 AM
Hi MJM Thanks for the post ...and there is actually no mechanism for separating the oil from the condensate and it is pumped strait into the boiler. This does not seem to have had a detrimental effect however. This may be because it is the type called the elephant boiler and this may have been why . Incedently it is the only elephant  boiler that is still in situ with the engine, so very rare.

Willy
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 20, 2020, 02:33:09 AM
Hi MJM, I wast doing  some washing in the bath recently and noticed that after a while these ridges of detritus had occurred ? I left the water in the tub and eventually this pattern appeared..So is this some sort of thermodynamic action or vibration activity ?? and I was wondering how it is formed ..Any ideas and does this have any detrimental effect on the vessel over time ?

thanks willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 20, 2020, 01:37:28 PM
Hi Willy, I am glad to see that your mind is turning back to the great questions of life again.  It shows even better than your thermometer that you are well.

Very hard to diagnose from this distance, and I am not really a water expert.  I would not put it down to any obscure thermodynamics though.

Only you would know if you were washing something very dirty, been crawling around in the allotment perhaps, or if your water from the tap is fresh from the Thames.  Or even just rusty pipes.

You mention leaving it sit for a while before all this appeared, so suspended mud particles could have settled, or if the water is exceptionally hard, meaning it has a high concentration of calcium salts, you could get a bit of precipitation after some cooling and evaporation.  Calcium salts are not very soluble so it is not difficult to reach the solubility limit.

But basically, that is a very long winded way of saying “I have no idea!” 

I hope appropriate to the spirit of your question.

All well here.  Getting a bit more time in the shop, though getting used to doing everything differently takes a huge amount of time.  First zoom meeting today, plus a phone consultation with the doctor.  Life is relatively normal.  But we both miss the company of others.  Communication is so much better than in the time of the Spanish flu, but not the same as normal human interaction, especially with family.  But it gives an idea of the life of my in-laws, a couple with nearest neighbours 3 miles away, the kids away at boarding school, and a road into town that was not for the timid in the cars of the day.  Once a week into town at best, but only in good weather.   The phone line was a party line that all could listen too, and the post mistress always listened, and she would patch you through to the requested number, if anyone was home at the other end. 

Stay well, and consider it “ shelter in place”, just as they advise when the bushfire cuts the exit roads.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 21, 2020, 03:42:40 AM
Hi MJM , the reason I asked you was because I thought you may have had experience with this phenomenon. as you have said you worked on refineries with lots of liquid holding vessels.?? anyway thanks for the message. I am allowed to go out for exercise here in Norwich, England !! and am spending about 5 hours a day there... Glad you are ok .. I do live next to a main rd so it may be lots of traffic causing vibrations ?? 

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 21, 2020, 01:02:10 PM
Hi Willy, over 40 years in refineries and related hydrocarbon processing plants, some in an operating company, so close to operation of these plants, and mostly on design and construct teams.  Always a team effort, none of these things happen without a great team, and the site work and commissioning activities provide a wide range of interesting experiences. 

When it comes to the patterns in your bath, there are a few important factors that are clear.

First, they are the result of having impurities in the water.  As the water drains, there is a rim of moisture in the edge of the bath that remains a little after the level has lowered.  Evaporation from this thin film is a bit quicker than from the general water surface, and leaves the impurities behind, clearly visible on the white enamel.

As the last of the bath drains out, the bath is left a little damp so a bit of a film over the surface as it dries due to evaporation, again leaving that trace behind.  Any disturbance to the surface at around this final stage will cause little waves and then there is often a little variation in gradient.  It could be vibration due to that traffic, especially if the frequencies coincide with the natural frequency of the water surface, and it might explain those patterns.  But might not be significant.  Needs closer observation to see if you can pick anything.  It is surprising how often natural frequencies do coincide, and these coincidences can do incredible damage to steel and concrete structures, from piping to bridges.

Similarly the nature of the impurities is important.  Whether dissolved salts, or dispersion of mud and the like, or most likely a combination, needs closer observation and analysis.  You can see colloidal particles and larger under a microscope, but dissolved salts can’t be seen, so some kind of analysis is required, but the laboratory equipment required is beyond most of us to access.  They would probably be calcium salts which are not very soluble so can form precipitates even with quite low concentrations.  Iron oxides from rusty pipes are also possibly involved.  These sort of deposits tend not to redissolve easily next time you fill the bath so they build up.  And they definitely build up in a boiler if we don’t actively do something about it.  I would not want to use that water in a boiler.

So you can see there is quite a bit of factual information that points to possibilities, but observation, analysis is required to understand what is going on.

But that might give you some clues as to what you might look for.

I grew up on a street where our trams passed our front gate, making a lot of noise which we grew surprisingly used to, a bit like your traffic.  And I have been in houses over underground railways, and they definitely cause vibration.  But I never connected these things with what marks the water leaves in the bath.  But our water supplies are also very good, so not such an obvious problem.  But overall, the best way to avoid those deposits is to wipe out the bath when you have drained it, but before it dries.  This way, your cleaning cloth takes away the remaining solids, and can be washed in the laundry using lots more water.  Simple enough action to reduce the amount of heavy cleaning required.

So plenty of relevant general information is available, but this has to be applied to specific situations to identify any particular case.

I hope that helps.  Or am I missing your real question?

MJM460
Title: Re: Talking Thermodynamics
Post by: scc on April 21, 2020, 09:13:45 PM
Re reply 1299    most compound traction engines have this device, usually called a "snifter". On Burrells it is operated by a spring loaded button. This gives instant HP steam to the LP cylinder with obvious results. It's only used in short bursts to start or save an imminent stall on a hill.             Regards                Terry
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 22, 2020, 02:03:47 AM
Hi MJM ,thanks for the reply and the design of the detritus occurs in nature quite a lot...thinking about zebras for instance !!! and the wave motion on sandy beaches .? I do use the bath water as a 'grey' water supply for plants and washing the car and things and I only empty the bath when I need to so the plug remains in until it is emptied.

Hi Terry, interesting about the 'snifter' valve ..I thought on a locomotive the "snifter/snifting" valve was to stop the engine sucking in the exhaust smoke when the engine was coasting ? I have not heard about this valve before either.!
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 22, 2020, 05:19:35 AM
Hi Terry, it’s great that those threads from long ago are still being read.  I would love to have more of you experience added to the conversation.  Theory takes us so far, but then there has to be practice to demonstrate it.  So please do revive the discussion if you see anything needing correction.

Like Willy I had only heard the term “snifter” in the context of ensuring that locomotives don’t get into trouble when coasting down hill.  So letting the cylinders draw in air when coasting down hill with the throttle closed.

I was more familiar with the “Simpling valve” (Apple does not like the spelling!) to add hp steam to the lp cylinder, or do you use that word in another context?

Willy, I also had in mind ripples in the sand at the waters edge, but you beat me to it.

Makes a lot of sense to use the bath water for gardening, especially in this country, but I thought you had plenty of water.  Or has population density pushed the consumption that far?

MJM460
Title: Re: Talking Thermodynamics
Post by: Hugh on April 22, 2020, 12:10:25 PM
Hi MJM, I wast doing  some washing in the bath recently and noticed that after a while these ridges of detritus had occurred ? I left the water in the tub and eventually this pattern appeared..So is this some sort of thermodynamic action or vibration activity ?? and I was wondering how it is formed ..Any ideas and does this have any detrimental effect on the vessel over time ?
thanks willy

I think it's the result of a harmonic in the way the water (subtly) sloshes around in the bath. In essence, your bathwater is behaving a bit like a guitar string, except instead of a vibration running through the string, you have a waves. Like putting your finger on the fretboard of a guitar, I'm willing to bet that the stripes of dirt would be closer together if your bath was shorter.

Anyway, the waves push the dirt particles in your bath towards the points where the vibrational amplitude is smallest, where they eventually end up when the waves lose steam and die down.

You might imagine that this kind of behaviour could be used as a way to push every small particles around, and you'd be right. This is the foundation for devices for sorting and analysing cells (e.g. when detecting cancer). You can even use it to "levitate" things: https://www.youtube.com/watch?v=0K8zs-KSitc

Hugh
Title: Re: Talking Thermodynamics
Post by: AVTUR on April 22, 2020, 12:35:49 PM
I have tried thinking about this and I agree with Hugh that it is probably a wave effect. However I would have expected the detritus to be washed up at the wave peak since it would have the lowest velocity. Just like a tide line on a beach.

There could be another effect that enhances the collection of detritus and that is aggregation. The particles in the water get caught by those already deposited on the dirty solid surface. If the surface was clean they would easily get washed away. This effect is seen in some obscure chemical reactions.

AVTUR
Title: Re: Talking Thermodynamics
Post by: Hugh on April 22, 2020, 02:15:05 PM
I have tried thinking about this and I agree with Hugh that it is probably a wave effect. However I would have expected the detritus to be washed up at the wave peak since it would have the lowest velocity. Just like a tide line on a beach.

Sort of, but I think the bathtub conundrum is actually a standing wave phenomena in which crud settles in the nodal points where amplitude and velocity are both zero. Perhaps this shows it a bit more clearly: https://www.youtube.com/watch?v=cBZmyG-WqNo

I think a real mind bender is contemplating why the sand under the water at the beach is often rippled.

Hugh
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 23, 2020, 01:09:21 AM
Hi MJM, et al, this is quite interesting and when I made violins I was very interested in the acoustical properties of the timber I was using...but that is a different story. I have spent a few hours looking at those videos so no work tonight !! I suppose the resonance of the chatter when parting off is the closest we will get to this phenomenon and ways of dealing with it ...like sticking some chewing gum on a long boring tool?
willy
Title: Re: Talking Thermodynamics
Post by: AVTUR on April 23, 2020, 09:26:24 AM
Willy

You are taking this thread to exciting places - Spring, Mass, Damper systems. Just as complicated as thermodynamic and fluid flow.

Hugh

I am not sure Willy's detritus is a standing wave.

AVTUR
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 23, 2020, 12:53:44 PM
It’s great to see more people joining the conversation.  I am quite sure that many other forum members have knowledge in all these areas, and mass spring damper systems offer plenty of scope.  They appear in so many interesting areas.  Possibly even another current but unrelated thread that I am following on this forum.

I am with you Hugh on the ripples in the sand.  Some places the sand is quite smooth and some rippled.  In one area I know quite well, those ripples are over a meter peak to trough in a tidal stream.  More commonly they are very close together, which implies a higher frequency than typical waves.  So some harmonic effect between different wave patterns perhaps.

I also think it is hard to be very sure, we have not seen the situation, and we don’t want too much information about anyone’s bath habits.  But a standing wave, if the traffic rumbles happens to force a natural frequency, or alternatively the effect of removing the water a bit at a time, giving a water limit which moves in steps, or as you mention, Avtur, some sort of chemical effect.  There is a water treatment process which absorbs the turbidity particles at one pH and rejects them at a different pH (I just can’t remember which side of the process was acidic and which alkaline).   So these obscure chemical reactions are definitely another possibility.

Willy, I have to present another opinion on whether the acoustic properties of the violin or the lathe tool chatter are different matters.  The maths is the same on all these problems which involve mass, springs and dampers as Avtur suggested.  Even including the chewing gum.  So another interesting discussion from your observations.  I am glad you took the time to put the observation into another of your thought provoking questions.

Some understanding of the behaviour of mass spring damper systems is helpful in so many situations.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 24, 2020, 01:15:57 PM
Ni MJM , It has just occurred to me that I live in a medieval house with a cellar  which is where my workshop is and there is a wooden floor separating them ... and my lathe is actually directly under the bath !! the lathe has a 1425  3/4 HP single phase motor  so the mathematics could possible correlate the waves in the bath !?!?!    there is always a reason for 'things' so this may be the answer ??

Willy
Title: Re: Talking Thermodynamics
Post by: scc on April 24, 2020, 09:29:25 PM
MJM , you are correct to call it a simpling valve, it's correct name.  The uncouth gang that I work with call it the snifter , so that is the term I use.
    Regards   Terry
Title: Re: Talking Thermodynamics
Post by: MJM460 on April 25, 2020, 01:33:31 PM
Hi Terry, thanks for that.  I don’t like to keep using the wrong term, so it is always helpful to have confirmation when these things arise. 

Hi Willy, It all depends on the structure of the building.  If you had something a bit out of balance on the lathe, and the lathe was on a wooden floor with some sort of connection the studs supporting the floor above, there could be some transmission, though if a timber structure, timber has some natural damping.

The lathe would have to be a long way out of balance to cause a forced vibration through the whole structure, and you would surely know it.  But if the speed of the lathe coincided with a natural frequency of the floor above, or the wave frequency of the bath, and some transmission occurred, an amplified resonant frequency could occur.  It seems intuitively to be a bit too much of a coincidence, but I do know from my working life that things only have to coincide with one of the harmonics to get resonance, and often there are so many harmonics that it is hard to get sufficiently far away from them to avoid a degree of resonance.

But you are right, there is always a reason, it just takes careful observation and sometimes sophisticated instrumentation to identify the cause.  But I would not recommend running up stairs to check while you leave the lathe running any time, but especially if it is badly out of balance.  You would need a second person as observer!

MJM460

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 26, 2020, 02:02:26 AM
Hi MJM , ok thanks for that...Lets see if anybody else has any ideas ??  ..... in the meantime  I've been thinking about using microwave energy to use in a steam boiler ??  with modern plastics  and non conductive fittings is this possible and is it used ??  they use induction heating in furnaces so it might be similar ...?  I haven't a clue how one might do it   so just wondering  ?? I have been using my microwave as a means of sterilising my home made compost  before I use it for seedling propagation . I do this so I don't get lots of other seeds  growing in the pots !!...

Willy

Title: Re: Talking Thermodynamics
Post by: MJM460 on April 26, 2020, 01:16:06 PM
Hi Willy, I am also hoping some others have some ideas, so am waiting with interest.

Microwaves can certainly boil water, I do it frequently when heating soup for lunch.  Not that I set out to boil the soup, the jug is better for that, but I notice that by the time the soup is hot, it is often boiling around the edge at the liquid surface.  No problem with a Pyrex glass container, but in the plastic ones, it tends to damage the plastic around the edge of the liquid surface. 

That much energy into a closed pressure vessel will raise the temperature in the same way as any other heat source.  But I am not keen to experiment with making boilers out of plastic!  There may be suitable plastics, but I don’t know of any that have a sufficient safety margins of strength or temperature stability for even relatively low pressure steam.  I am very conservative in that area, silver soldered copper for home construction, and steel only where properly qualified welders, weld procedures and quality control are used.  It is possible to go with less, but I am not advocating it.

Good idea for sterilising soil.  Do you have a specific procedure?  Do you need a cup of water in the microwave in addition to the soil to protect the appliance?  Or is the soil moisture adequate?

Induction stove tops are available.  I don’t know much about the elements, but they seem to require metal pots.  Also they come with warnings about pacemakers.  We do have such a device in the family, so I have no intention of experimenting with them.  Others may have more information on this also.

No social isolation for forum members, and all this “sheltering in place” is giving us all more shop time.   But it is very hard on those who have lost jobs and trying to support a family.  My thoughts and prayers go out to them.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on April 28, 2020, 03:15:15 AM
Hi MJM,  Thanks for the reply.. when I microwave the compost I don't add water to it or in a cup as there is always moisture /condensation inside when I open it ..It does smell a bit though !!  There are a couple of magnets in the microwave construction that are quite easy to remove ,and people do chuck them out when they are broken...

Willy
Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 23, 2020, 11:13:32 PM
Hi MJM.  this is one of the models that my friend Mitch has made for an advertising  company  !!quite cool ..sort of...

https://www.youtube.com/watch?v=0ytOnwTqwF0    

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 24, 2020, 01:25:43 PM
Hi Willy, you seem to like living dangerously.  But if I keep my comments to fair study/ review, and stay away from politics, I hope it will keep us both out of trouble.

It’s a great model that Mitch has made, it will go well with museum visitors.

It very clearly demonstrates the dirty aspects of coal that we can all see, and compares it with the clearly much less dirty gas.  Not totally clean mind you, still some fine particulates, but people with expertise in that area will have to comment on that.  But in summary, the little model is fair and useful as far as it goes.  We can’t discuss bigger issues until we all, or most of us anyway, understand the basics.

I am an engineer, so my area is in the practical application of the science to solve real world problems. Applied science rather than pure science in the research sense.  On that basis, what do I think?

Well, full disclosure, my whole working life was in hydrocarbon processing, but which way did that influence me?  I hope I can present an analysis neutral enough to pass the pub test.

The issue that is rarely included in the context of this model is Carbon Dioxide.  We can’t see it, so it can justifiably be left out of a discussion about “dirty”.  But it is important in climate, and the science is clear on that.  So where does that leave us?  It is a different but closely related discussion.

Coal and gas both release energy by combustion in which hydrogen and carbon are the most significant.  Hydrogen burns with oxygen from the air to give us water, carbon to give us carbon dioxide.  The ratio of hydrogen to carbon is different for each, and gas gets more of its energy from burning hydrogen than does coal, so to that extent it is better, but not by a huge amount.  Gas power stations sometimes work on a combined cycle and can have efficiencies higher than that of a coal fired station, particularly an old coal fired station, so that definitely is in its favour.  Also, a gas powered station can respond quicker to load changes than a coal station.  But in the end, they still produce a lot of Carbon Dioxide and the climate change science can’t just be ignored.

So where does that leave us?  Can we just shut them all down tomorrow?  Well for a start, we have not yet built the alternative, and if you think the present lockdown is causing chaos to the world, just think about shutting down all the coal fired power stations overnight!

We have to remember that the community has a large amount of money tied up in coal stations, regardless of the specific ownership, and I suggest that we really can’t afford to just discard them.  Just like you can’t afford to just send your car to the wreckers, and buy a new one, just because the new one has some nice features that you would really like, and may even be safer, the world community cannot afford to just close down the all old power stations without spreading out the pain.

But again, just like your car, they do wear out, and many of them are already old, and approaching end of life, so they are going to have to be replaced sooner or later.  That makes now a really good time to think about what we want to replace them with.  And like the old car, we can’t just replace them with like for like, as while that feels like it would be easier, that old model went out of production years ago.  The old power stations have served us well to get to where we are today, but the new ones will be much more expensive to build, and in the end, despite all the offers of easy finance, interest free, accessories thrown in, all made as enticements to buy “ before its too late”,  we all pay for them all one way or the other.  And what ever we choose to do, we have to plan early, a national grid size power station cannot be picked up from a big box store, unpacked and switched on preferably after reading the instructions.  In our consideration of currently on line power stations, we probably have to include some that, while construction has not yet started, could already be beyond the point of no return due to design work completion and cancellation costs on other contracts for which preparations are already well advanced.

A more practical approach is to recognise that we will have to replace them sooner or later, and plan carefully how we will go about it.  And the sooner we start to properly plan, the sooner we will understand the required decisions, including what to do when each one becomes no longer economic to run.  We can’t do nothing, they will just break down more and more often until the pain of lights going out convinces us.  Why do I keep going back to thinking of the old car?

If the decision is to go for more renewables, and I think it is feasible, we have to consider what that means in terms of what we need for a stable grid, and how we go about phasing out the old.  (And I don’t underestimate the difficulties of doing that with minimal pain to the community.)

That brings us back to the basic thermodynamics, starting with a very basic law of physics, conservation of energy.  We have discussed that before in this thread.  In power supply terms, it can be written as

Generation - Load = storage.

All power systems have all three, yes, even the current coal based systems, even if we don’t recognise them, particularly the storage, which in different power technologies, can look very different, and behave very differently.   And that is a whole new area, one that I am more interested in discussing if it is of any interest to others.

MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on May 25, 2020, 03:46:53 AM
Hi, MJM. I was just sharing this from Mitch to show what he does for a day job ... he does get time to make very nice models and he has the use of lots of nice machinery.... I was not trying to be political or anything ..indeed my Morris Minor is 55 years old and is petrol and I have just replaced the exhaust valve today as they frequently burn out!!!? One can still buy all the parts and it should see me out...everybody uses the benefits of coal and gas fired power and in a way these are renewable ...if you wait long enough !!! I notice that on the adverts for electric cars they say they are pollution  free whilst driving  !! also people always go for the cheapest option for fuel , whoever they are. 


Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on May 25, 2020, 01:34:20 PM
Hi Willy, quite happy to talk about what is above the ground, or at least only at shallow depth, but deep down is geology.  I will leave that area to them!

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 08, 2020, 02:39:16 AM
Hi MJM, a new  T  question about Microwave ovens... It says that you should not put anything metal in it as it will cause a cataclysmic event ??  If you heat something up like water it takes a certain amount of time ...but if you want to heat up something really solid and dense  perhaps a lump of meat ..the same weight and temp will it use the same amount of time/electricity ??  if you use the timer but with an empty microwave will it use the same amount of electricity as it is not doing any work  !? so the question really is how do microwaves behave differently to an open /enclosed flame or an electric element.  And will a lump of metal drag out extra power from the unit , to harm it ?? , and do you use more/ less electricity given that the same amount of energy is needed ?? incidentally there are a couple of useful magnets inside microwaves ??

willy
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on June 08, 2020, 11:21:30 AM
A certain amount of power are used for the heating of the power tube that creates the microwave radiation, so that remains constant !!!!

Actually what you have is a RADAR Transmitter, where the "Antenna" radiates into the cubicle inside the oven and like any RF Transmiter, it will consume the same amount of power when ON no matter where it ends up. If the Transmitter has a serious antenna problem the power might end up being reflected back to the power stage -> usually result, is a destroyed output stage of the transmitter. This is more or less what can happen if you put enough metal into the oven - but if you have an empty oven or filled with the 'normal' stuff (meat, water, etc.) it behaves like a normal antenna and the power drawn from the wall is constant.
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 08, 2020, 01:32:23 PM
Hi Admiral, I am glad you are on board to answer that one, I am no expert on microwaves.

As always all the heat consumed has to go somewhere, and I guess that at the end of the number of reflections it takes, the energy all goes into heat which escapes from the even.

When radiant energy meets a surface, my understanding is that it is always partly absorbed, partly reflected and partly transmitted through.  In the case of microwaves in the oven I hope it is mostly reflected and absorbed, as we don’t want too much transmitted through the walls of the oven.  I guess the reflections continue until it is all absorbed.  The fan on my microwave ovens continues to blow warm air for quite a while after cooking finishes, with the display saying “cooling”.

But I also have in mind that the problem with putting metals in a microwave, such as an aluminium foil cover on a dish or china with a plated metallic decoration around the rim, is that sparks are induced somehow,  though I am not sure of just how that happens.  I have managed to avoid doing it so far, so it’s only second hand information.

Hi Willy, The question seems to be very similar to the question of a radio transmitter and whether it makes a difference to the transmitter if there is a receiver to pick up the signal.  You might have come across this sort of equipment in your electrical training.  Although in the case of the radio transmitter, the transmitter and all possible receivers are not enclosed in a reflective walled enclosure.

I hope that makes sense and does not cause confusion after the Admirals excellent explanation.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 09, 2020, 02:55:30 AM
Hi MJM  & Admiral. I suppose that all thermodynamic rules are the same in every thing that is happening in our world even if it is not immediately obvious. I have noticed that grass stalks in snow always have a tube of melted snow around them ?  I have heard that when I was in the Army the radar Techs use to heat up their pork pies in the microwave tubes !! So thanks for the info....!!

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on June 09, 2020, 01:01:39 PM
Hi Willy, it all comes down to the most basic laws of physics, conservation of energy, conservation of momentum and conservation of angular momentum.  There are a couple more more obscure ones about conservation f charge, and two laws about some basic sub-atomic particles, but for most of what we do, the sub atomic particles ones lead to the very reasonable assumption of conservation of mass.  So long as you remember that conservation of mass is only an approximation.  Mind you, it’s good enough unless you are getting too realistic about your scale atomic submarine model, or are perhaps building a model Hadron Collider.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on June 11, 2020, 01:09:27 AM
Hi MJM, thanks for that and I thought there was a lot more to this than just 'simple' thermodynamics. also talking about microwave ovens  my toaster machine stoped working because the lever would not stay down !!  so I used a couple of the discarded microwave magnets!!  and now it is working again ......a sort of thinking outside the box remedy !!! these magnets come in pairs and are really easy to remove..

Willy

Title: Re: Talking Thermodynamics
Post by: Admiral_dk on June 11, 2020, 11:08:45 AM
Careful with that solution Willy - we do NOT want you to burn down the house and you have just prevented the safety bit to switch the toaster off when it get too hot !!!!

Per
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 07, 2020, 03:01:35 AM
Hi MJM, just a quick question ...wondering what the central bar of the thermostat is made of ?? I usually saw these apart to use the brass tube ??!!  and  don't worry Per  about the toaster as I count to 100 and then switch it off. I am sure one could do a calculation here ...Colour = time ,,volts. area.. waste heat.. bread density .. etc etc    :lolb: :lolb: :lolb:  ok I know we are in lockdown but I'm sure we have better things to do !!!  also carboot find ...a part Boley staking tool set !!

thanks

Willy.
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 07, 2020, 10:35:32 AM
Hi Willy, great to have you back, I was starting to think that you were missing in action.

I don’t really know the answer to your question.  It depends on just what is in that junction box at the end of the metal tube.

Some have a thermocouple in that outer sheath, so the junction box is simply for convenient connection.

Others have a micro switch and operate by a difference in thermal expansion coefficient between the metal of the outer sheath and the centre rod which would be welded to the sheath at the end dearest from the junction box.  Just what metal is used would depend on the required switch actuation temperature.

I found a table with a few values. 

Brass    18.7
Stainless steel. 17.3
Aluminium   23.9
Carbon steel 10.8
Tungsten 4.3

To calculate the expansion for a change in temperature, divide those values by a million, and multiply by the number of degrees C change.  Then each metre expands or contracts by that rather small number of meters.  If the outer tube is brass then carbon steel or, better still tungsten would give you some differential expansion to operate a switch.  But these are just a few standard materials, I believe there are other alloys which might have a much bigger difference that would be used in that application, but not for building bridges.

It is also possible to amplify that rather small difference using levers to increase the movement.  You can’t stop thermal expansion with any reasonable force, so there is plenty of force available to operate levers and a switch.


MJM460

Title: Re: Talking Thermodynamics
Post by: Admiral_dk on July 07, 2020, 11:56:11 AM
Willy - the safety is there for when you forget or are suddenly otherwise called away.

Weller used another solution in their old soldering irons. The tip had a magnet on the backside, and it lost it's magnatism when it reached a certain temperature. An iron rod 'connected' the magnet to a magnetic switch inside the handle, that switched the heating element On and Off.
Title: Re: Talking Thermodynamics
Post by: derekwarner on July 07, 2020, 01:15:01 PM
mmm... Chromium is put to good use in such thermocouple type devices with it's curious characteristic...... :happyreader:

'Chromium has unique magnetic properties in the sense that chromium is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38 °C, its magnetic ordering changes to paramagnetic'

[I stumbled on this obscure fact when trying to understand the apparent difference in the reported thickness of the electrolytic Nickle deposit [under Chromium] on 380mm diameter hydraulic cylinder piston rods as scientifically measured by a NATA accredited body in Abou Daubi , then the same cylinder rods checked for Nickle depth as measured in Denmark by a DNV registered facility....

Abou Daubi certificates confirmed 42 degrees C, Danish certificates confirmed 3 degrees C  :hammerbash:]

Derek

Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 08, 2020, 02:29:40 AM
Hi MJM, et al ,   I have taken the thermostat apart and the central rod is magnetic there is a makers plate with it...I have not done a spark test yet , or attempted to turn it ..yet ?
 Derek.. interesting about Nickel plating..between steel and chrome  this is the correct way to do it chemically !!! Nowadays on bikes the chrome goes strait on the steel. so in a few years it starts to peel off...after the guarantee ,of course..

willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 08, 2020, 12:52:47 PM
Hi Willy, there you two more possible explanations of the mechanism.  From the picture, I am not sure that it can be identified, but with it in your hand and turning it around, you might be able to work it out.

As for the rod material, your original question, your spark test and a test cut should tell you something useful, but I am no expert on metallurgy.  And worth testing it for corrosion before putting a lot of work into it.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 10, 2020, 02:31:57 AM
Hi MJM, ok...the spark test is a deep red dusty looking a bit like the tipped tool steel but more red than orange   and it turns nicely with curly swarfe it also files well and docent want to go rusty ???? also a pic of the thermo contact breaker...

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 10, 2020, 10:50:53 AM
Hi Willy,

It sounds like that rod will be useful for something.  Can’t beat material that is good to machine and also resists corrosion. You will have to tell me what material the spark test colour is indicating.  You might also rescue some points for an engine, though the spring material might not be good for a high number of cycles.  Perhaps more suitable as a boiler cut off switch.

As to the switch, is there a chance that you have removed a part that allows the movement of that part with the green tape to open and close the contact as it moves?  It certainly looks like a simple device, that would have been quite reliable.

MJM460
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on July 10, 2020, 12:12:15 PM
The picture of the switch imidiately made me think Bi-Metal when I first saw it and these can be adjusted simply with a screw pressing on the Bi-Metal part ....
If this is the case, there should be a "Heat transport system" - so is the yellow rod by any chance copper ?... Certain brass types are ok (not great) in this aplication too ....

Title: Re: Talking Thermodynamics
Post by: MJM460 on July 10, 2020, 12:38:12 PM
Hi Admiral, it’s certainly a puzzle.

In Willy’s first picture, and implied by the original question, the operation seems to depend on a rod within a brass tube, and perhaps differential expansion, but how that connects to the switch is still mysterious.  I am not so familiar with these things.  The oil industry changed to electronic controllers long ago.  I did not have much to do with the early switches that were used in some applications for lower cost, but did not have a good reputation for reliability.  Hence the change to controllers

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 18, 2020, 02:26:50 AM
Hi MJM,  we have just hade quite a few days with the mercury going up to about 37 degrees !! this has set me wondering about cooling fans.......if I am in a sealed room and switch on a fan it will cool me down ? however the fan is just moving the ambient temperature about. there is no heating or cooling attachment so why do if feel colder ?? also if the fan is directed at a piece of metal will this also get colder ??... also I saw this comment in a 1917  Model Engineer magazine about a blackened kettle. I think we have talked about this in a previous post , and as this is true are there any tables to support the science ??

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 18, 2020, 12:53:21 PM
Hi Willy, you won’t be surprised to find that there is not a simple answer to your question about the fan.  It is necessary to think about what is going on.

Your house is gaining energy through the walls and roof by radiation from the sun as well as conduction from the air.  In addition, depending on how the house is ventilated you may be bringing in some of that 37 degree air.  The ground is a little more complicated.  I suspect that you loose a little heat to the ground during the day, but that is an even more complex question. 

And you being in the room contribute a little more heat, at least until the room is up to 37.  The temperature in the room results from the heat balance between the gains and losses.  How hot you feel depends on the difference between your body temperature and the room temperature.  You might guess that as the room approaches 37 degrees you will get quite uncomfortable (and will probably fail the basic COVID screening test!). You do need to be able to loose enough heat to stop your temperature rising further.

When you switch on the fan, you are adding more energy to the room, so it will become a little warmer.  It’s a very small amount in the grand scheme of things, but directionally, you are heating the room.

So why does the fan feel like it makes you cooler?  Well, it’s because of evaporation.  Under those conditions, I am sure that you will be sweating which will make your skin damp.  The air flow from the fan will evaporate some of that sweat, so taking away the latent heat.  That will cool the remaining moisture on your skin, and that does really help you stay a little cooler.  Part of the secret is of course the humidity.  The humidity has to be low, or there is no evaporation, and it is even more uncomfortable.

If the room is fully sealed, the humidity will rise as you sit there in front of the fan, so the fan becomes less effective.  You need low humidity air coming in from outside to maintain the low humidity inside and expel the excess moisture with the exhaust air.

Because the cooling you feel is dependent on moisture evaporation, the block of metal will not be cooled by the fan unless it has a damp surface.  Providing you measure it with a thermometer, just using your finger to test the temperature will fool you.

The kettle question is similarly complex, and involves heat transfer by conduction, convection and radiation, along with issues of emissivity and absorptivity of the surface, so that will have to wait for some other time.

But I hope that I have helped your understanding of the fan question.

 MJM460


Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 19, 2020, 02:17:07 AM
Hi MJM, thanks for this..so yes that makes sense !!  and there are so many variables in these calculations ?!!.  I could do more experiments with the electric boiler as well as the heat input is fixed ?..when I have time  etc etc

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 19, 2020, 12:59:34 PM
Hi Willy, I was thinking about yesterday’s topic, if you have a little spare time in these unusual days.  I know that I just don’t seem to have any spare time, despite all the complaints in the media about having nothing to do. 

The Met bureau used to use a device called a sling psychometer to determine humidity.  It had two thermometers, mounted in a frame so air flowed past the bulbs when the device was swung around.  One had a little pot of water and some cotton cloth wrapped around the bulb, with a tail in the water pot, so that the cotton was kept moist by the wicking action.  The temperatures of the wet bulb and the dry bulb were used with a chart like the one attached, to determine the humidity.  I seem to remember that the device had to be swung at arms length and the number of circles per minute was specified, along with the duration.  The chart is quite difficult to work out how to read, these days it is easier to use an electronic device.

Of course most appartments these days don’t have room to swing a psych, (or was that supposed to be a cat?), even if you had one.  But if you read the room temperature with your thermocouple just in still air, then wrap a strip of rag around the end of the thermocouple, with the end of the rag in a cup of water, then once the rag was nicely wet, I would expect that you would see a difference in temperature depending on whether the fan was running or not, so long as you give the reading time to settle.

 You might find the readings a bit hard to interpret if you read the temperature with the cloth wet, but no fan, as the water might take some time to reach equilibrium with the room temperature. 

MJM460
Title: Re: Talking Thermodynamics
Post by: Admiral_dk on August 19, 2020, 05:14:43 PM
I learned to do these calculations in my youth - a linear equation if memory serves) - but I never used it for anything => completely forgotten til you mention it, or more correctly - I remember the wet and dry bulb bits and the fact that you could use it to calculate the 'missing parameter' in indoor and outdoor climate.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 20, 2020, 03:14:04 AM
HI MJM, thanks for that and i also have seen the wet, and dry bulbs. one of them was circular in shape  but never examined them in detail

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 20, 2020, 12:57:09 PM
Hi Admiral, an equation would certainly be easier than that ASHRAE chart, but I suspect no one does those calculations these days as we all have access to those little electronic devices that just read the humidity directly.  The sensors are even available to combine with microprocessors so you can build your own version.  The ones the met bureau use are probably more expensive though.  I must admit that I have never thought of trying to derive an equation, something to ponder on a winter evening.

Hi Willy, the main thing is the inclusion of two thermometers, one sees the straight air flow for a direct temperature measurement, the dry bulb, and the other surrounded by a cotton wick which is kept damp with water, the wet bulb.  The airflow encourages evaporation of the water which results in a lower temperature.  Much the same in the washroom with those air driers.  The air feels quite cool, but warms to become even hot as your hands dry.  (If you can stand the noise long enough).  In reality, the air reaches full temperature quite quickly, but what you feel is affected by the evaporation.  But as I am sure you know, the best way to dry your hands with one of those things is 10 seconds in the airstream the dry them on your pants.  I assume ladies use their skirts.

MJM460



Title: Re: Talking Thermodynamics
Post by: Admiral_dk on August 20, 2020, 07:13:30 PM
Sorry that I can't help with the equation as I got rid off all the study material that didn't directly cover electronics just a few years after I left ....

The above made me Google "wet and dry bulb formula" and there are the useful stuff as first hit :
http://www.1728.org/relhum.htm (http://www.1728.org/relhum.htm)   and it includes an example on how to use the (3) formulas  :ThumbsUp:
This is great, as it's quite a bit more complicated, than I remembered  :old:

Per
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 21, 2020, 02:34:22 AM
Hi Admiral,...this is quite an amazing formulae and accurate to 14 decimal places !!!! or is there a typo ??


Thanks

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 21, 2020, 12:18:16 PM
Hi Admiral, thanks for those links.  I am really glad to see that formula, as I had a bit of a play with a spread sheet and steam tables, and could not see how to get a linear result unless over a very limited range.  And it seemed that the parameters would have to change depending on the dry bulb you started with.

To see this formula was very encouraging, especially as I tried some curve fitting and the best fit was actually the power law. But I can understand anyone not remembering it, it’s quite a mouthful.

Even though the formula looks a bit daunting, with a modern scientific calculator, or better still, a spreadsheet, it is pretty easy to handle, compared with the days when we depended on a slide rule.

Hi Willy, that “e” factor Is the base for natural logarithms, and if I remember correctly, the number of decimal places to specify it exactly has no end, one of those irrational numbers, I think it is called.  But in the end, the answer is no more accurate than the readings on the thermometers used to read the wet and dry bulb temperatures.  But it never hurts to have a few extra significant figures on the constants, so they do not introduce extra errors.  Nevertheless, 14 places is a bit excessive in any circumstances, but if you put e into the calculator or spreadsheet, the calculator will use the number of places it can handle, but as you probably remember, with a slide rule, three significant figures was about the best that could be done.

I have been puzzling over what the other constants might be, and at this stage, I have no constructive ideas to offer.  I don’t know whether they they are resulting from unit conversions, or other factors.  Perhaps Admiral or one of the other forum members can shed some light on them.  But I think I will just stick with the little electronic display in my workshop.

MJM460



Title: Re: Talking Thermodynamics
Post by: Admiral_dk on August 21, 2020, 12:25:45 PM
Thank you for explaining it for Willy MJM  :ThumbsUp:

I forgot to include the only other useful result from the Google search :

https://en.wikipedia.org/wiki/Wet-bulb_temperature (https://en.wikipedia.org/wiki/Wet-bulb_temperature)

Strangely not using the same formulas - but still a great and a lot more detailed explanation + nice examples from real life .... Notice still NOT Peer Reviewed yet.
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 23, 2020, 02:23:33 PM
Hi Admiral, I hope I did not steal your thunder in my enthusiasm to continue the conversation.

The Wikipedia article is quite interesting.  Not sure that I totally follow it. We have plenty of areas in Australia that experience dry bulb temperatures above 40 degrees, and heat stress is a serious issue in those areas.  Fortunately the humidity is generally quite low, so wet bulb temperatures are considerably below the dry bulb temperatures.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 07, 2020, 11:49:51 PM
Hi MJM,  The show is now over and so much good stuff to look at and study.... I have been pressing some apple juice and bottling it in coke bottles that have this bulbous type of bottom... I have noticed that after a while the bubbles have taken up these shapes that mirror the bottom of the bottle ?? when i put the juice in a flat bottom bottle the bubbles are perfectly flat and even ... so is there some sort of thermodynamics going on here ??

Thanks

Willy
Title: Re: Talking Thermodynamics
Post by: crueby on September 08, 2020, 12:18:14 AM
More like nucleation patterns in the bubble formation. Either those depressions are a rougher surface, or more likely there are particles of apple that are settling in the depressions. Either will give bubbles a friendly place to form.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 08, 2020, 01:04:20 AM
hi Chris, Ok, Interesting hypothesis... I will have to think about this and possibly learn some new words...thanks

willy
Title: Re: Talking Thermodynamics
Post by: crueby on September 08, 2020, 03:12:38 AM
Willy, I learned about such things back when I was a firmware/image-science engineer working on inkjet printers for a decade or so, and was on a team with some really good chemists and fluidics experts. Inkjet printers like you might have on your PC are amazing little machines, with tiny rows of tiny chambers filled with ink, with a tiny little heating element at the back and a tiny little hole at the front (using up a box of the word tiny, I know, but they spit out a 3 to 6 picoliter drop, which is tiny!) . When the heater is pulsed with electricity in the right waveform, it gets hot enough to vaporize some of the water in the ink, causing a little (yes, tiny) bubble to form that fills the chamber, forcing a tiny drop of ink out at very high speed towards the paper. Imagine rows of these firing at 24 to 48 khz, all spaced out at 600 or 1200 chambers per inch. We had a statistician who wanted to do a life test on a print head, and not knowing better set one up, sitting over a beaker, to fire all nozzles of all colors at full speed constantly, so the test would go quickly. It did. The head got so hot it melted the nozzle chambers, the plastic holding the nozzle chip, the plastic carrier, and part of the ink tanks! :Mad:

Anyway, in a 'machine' like that, unintended bubbles in the lines or chambers are a very bad thing, so it was a frequent topic of discussion. I learned that in a saturated solution, bubbles of air, CO2, whatever, want to come out of solution, and 'nucleation sites' are where a lot of it happens - just a pit or imperfection in a smooth surface where the surface tension of the water causes a bubble to stick, grow, release when it gets too big leaving a proto-bubble behind, which starts it all over again. You see it in glasses of anything fizzy - pop, champaign, beer, whatever, where you see a single spot at the bottom generating a never ending stream of bubbles. Now, if your apple juice is 'contaminated' with some yeast especially, it is constantly putting more CO2 into solution, and it wants to get out. Any rough surface, or particle of apple or yeast at the bottom is a perfect spot to form bubbles. In your bottle, the dimples at the bottom will collect those bits and concentrate the bubble formation.
If it gets to a nice alcohol concentration, all you can do is quietly consume the evidence!   :DrinkPint:
Title: Re: Talking Thermodynamics
Post by: paul gough on September 08, 2020, 06:36:52 AM
Hi Chris & Willy, Chris your succinct exposition on the 'micro' workings of a printer was very interesting. Half jokingly, I wondered how we could co-opt the overheating process from pulsed 'sqirters' and adapt or harness such a process to fire Willy's electric boiler.

The description of bubble propagation from imperfect surfaces made me think of the often relatively 'rough as guts' surfaces on our steam, (or pneumatic for some), circuits and the turbulence engendered. I also wondered if these surface 'pits or pores' might not attract and hold minute quantities of condensate which were 'seeding' further condensation, causing even more disturbances to perfect flow. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 08, 2020, 11:01:26 AM
Hi Chris, I think you have nailed it.  I had been trying to think of subtle variations in the airflow around those shapes and the minute temperature changes they might cause, but nothing seemed convincing.

That nucleation process occurs in many areas, but that explanation of the inkjet printer is extremely interesting and certainly moves the science of nucleation to another level.  Thank you. 

Hi Willy, another great question.  Very observant of you.  Or perhaps you make large quantities of cider?

Hi Paul, good to hear from you again.  I hope you are well and that your little locomotives are still running around the track.

MJM460

Title: Re: Talking Thermodynamics
Post by: AVTUR on September 08, 2020, 11:31:18 AM
For a liquid to boil or freeze or a gas to come out of soliution or liquify it needs something to attach to, a nucleation point. I remember as a kid putting asprin in Cola-Cola. More seriously, the water in the atmosphere needs nucleation to produce clouds and rain. Usually dust does the job but man has been known to intervene using suitable crystals (silver nitrate commons to mind). Molten metal need nucleation points to solidify, usually the side of the mould but sometimes a seed crystal is added to grow a single crystal of metal. Further some chemical reactions will only occur on nucleation sites, the production of smoke during combustion being one.

AVTUR
Title: Re: Talking Thermodynamics
Post by: paul gough on September 08, 2020, 12:35:51 PM
Hi MJM, Too much time with oncologists and cardiologists to have done anything over the past year, but they both lost interest in me so suppose I'll be fine. As to the little loco, I am planning on inverting the cylinders, steam chests underneath. This will allow replacing the front driver,( as an 0-4-2), and replacing it with a pony wheel, turning it into a 2-2-2. the cylinder inversion is necessary to accommodate the the lower axle height which is located between the cylinder and valve rod guides/stuffing boxes. Have to try running the cylinders upside down with steam first before I do anything much to see if the valves will seal and lubrication of surfaces are satisfactory. Also might have to put a drain on the steam chest. Very much hope to get a 'Sherline' set up in the near future as these machines seem to be appropriate for my tiny work and if worst comes to worst they'll fit in my coffin and I can take them with me anywhere. Though they'll probably melt where I'm headed. Regards, Paul Gough
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 09, 2020, 12:39:35 AM
Hi All , ok ..interesting .. actually I just drink the apple juice as I am a teetotaller..apart from medicinal brews !! so if the surface of a container was polished microscopically would it slow or even stop the nucleation ??  I am thinking of another experiment that I will report on with this explanation..?!!! Hi Avtur   we use to put liquorice in coke bottles when I was young !!.. I suppose the process does actually give off some heat ? something else I learnt today was  that an explosion occurred in a recycling plant because all the coke bottles still had the lids on when they were being crushed, it was a very hot day and the pressure inside of them increased somewhat and when they got crushed the pressure was released sudenly and destroyed the machine .....

so thanks for the science lesson

Willy
Title: Re: Talking Thermodynamics
Post by: Zephyrin on September 09, 2020, 11:05:00 AM
In a fermenting liquid saturated with CO2, the gas in excess breaks down the intermolecular forces of the liquid to accommodate gas, and forms an interface between the liquid and gaseous phases; this phenomenon is costly in terms of energy, hence the spherical form of the bubbles; which I suppose is the best way to balance the different forces, gas pressure, surface tension...

In a liquid without disturbance for a while, the bubbles rise vertically to the surface, under the effect of Archimedes' thrust, and accumulate as foam precisely above their production area, and release the CO2.
If it was otherwise, or if it sticks to the wall of the glass, it is because a force is exerted on these small bubbles .

I only have experience of Champagne, where one easily observes also that bubbles are growing up while they rise in the glass.

Paul :
I suggest that you put a small spring or a shim elastic bronze on the valve of your loco so that it remains pressed against the port face even upside down...otherwise starting the engine will be impossible if the valve is not pressed firmly against it.
Repairing your locomotive will hopefully be a good remedy against these gloomy thoughts...
Title: Re: Talking Thermodynamics
Post by: paul gough on September 09, 2020, 11:35:39 AM
Hi Zephrin, Thanks for the suggestion regarding support for getting the valve to seat with inverted cylinders. Sorry for sounding gloomy, I was in fact trying to be jocular. One reaches a point where existential threats force you to re-assess things somewhat, and I now take the view that it is best to laugh at adversity and attendant, prescribed, no option outcomes.  Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 09, 2020, 12:06:18 PM
hi All ..thanks.. and I have done the experiment... I bent a piece of studding to a W shape and put it in a square jar with the new apple juice. this was placed in the jar about 12 hours ago and the bubbles have taken up the shape of the  bent steel studding   {sort of}.  so there we have it !!! one could do this to identify your bottles of juice ..say in a shared house ,to mark out your ownership ?!!!

Willy
Title: Re: Talking Thermodynamics
Post by: AVTUR on September 09, 2020, 12:51:50 PM

I only have experience of Champagne, where one easily observes also that bubbles are growing up while they rise in the glass.


The Champagne industry did a lot of research on the development of bubbles when the great wine is poured into a glass. This was in the late 1980s and I would have loved to have been involved. Were you, Zephyrin?

It may not be thermodynamics but it is certainly fluid dynamics.

AVTUR
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 09, 2020, 12:56:45 PM
Hi ..I suppose that anything that changes temp by even   .000000000000001 degree will fall into   Thermodynamics  ?????????????

Just thinking above my pay grade !!

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 09, 2020, 01:58:59 PM
Hi Willy, just by coincidence, there was an article in the newspaper here, I think just last week, where climate scientists somewhere had put a sea water temperature difference of a trillionth of a degree into their climate model.  I read it three times, to be sure I had read it correctly.  It had a major effect on the ebb and flow of the el Nina/el nino pattern that has so much effect on our seasons.  Right up there with the flapping of a butterfly wing in the Amazon.  So clearly thermodynamics, but probably not much help to our understanding of our model engines. 

However, fluid dynamics can tell us a lot of useful stuff so that subject definitely belongs in this thread.

I was thinking of Avtur’s comment about rain seeding with silver compounds.  There were a lot of experiments in this country in the second half of the last century, but I have not seen much about it lately.  I looked up a couple of reliable sources and found that there was the general impression that in the right circumstances, it does produce statistically significant rainfall increases, the right circumstances do not occur very often.  And it requires very significant experimental design and statistical analysis to demonstrate any effect at all, compared with the normal highly variable nature of rainfall.  Then they were talking about perhaps 5%, and made the interesting comment, would that make any difference to Scotland, and would it be any use in Ethiopia?  Apparently the most successful experiments occurred in regions where there was significant air uplift due to mountains to carry the seeded air mass up high into the atmosphere.  And only in a very narrow air temperature range between the limits of “not going to rain anyway”, and “would have rained without seeding”.

So some of these things have a theoretical basis, and do actually occur, but the effect may not be enough to suit our purposes.

Not sure how you would indentify the difference between M and W to tell your glass from mine!  But this question has certainly started some interesting discussions.  Now we need to develop an engine powered by the CO2 generated in cider.

(Perhaps I had better return to Fluid mechanics.)

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on September 09, 2020, 11:48:13 PM
I went looking for some cork sheet, say 1-2 mm thick but only found rubberized cork gasket sheet from auto shops. Can anyone say if this material has insulating properties anywhere close to plain cork. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 10, 2020, 12:54:27 AM
Hi Paul,

I found the thin Cork sheet in one of those $2 variety shops, sold as place mats.  They all seem to be independent, so I guess it depends on what they think they can sell in your area.

I would be concerned about a rubberised material becoming soft and sticky with heat, so might need a bit of an experiment to see if it is satisfactory.  Not sure if the rubber component affects the insulating properties a lot.  Certainly the thermal conductivity of the material is important, but the rubberised material would be a lot better than nothing if you can’t find an alternative.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 10, 2020, 03:47:28 AM
hi MJM ..interesting about cloud seeding ,,but could they just use dust ? rather than silver compounds ..very expensive   also do W 's become M's as you are down under as they say about the antiperdies  ??

Willy
Title: Re: Talking Thermodynamics
Post by: paul gough on September 10, 2020, 07:03:24 AM
Thanks for your thoughts MJM. We have a couple of those "Cheap Charlie" shops up here so I'll check those for the plain cork sheet. I seem to remember the rubberized stuff being rather cheap, and might try it wrapped around a tin and put it in the oven on a low heat and see what happens. Will also try to set a small piece of it alight to see if it is flammable. I had assumed it would have some heat tolerance being for automotive use, but only a presumption on my part.
 I forgot to answer your question about the engines valves in the PM. They are just small rectangular brass slide valves running on the upper surface of the brass cylinder block with drilled porting.

Willy, There would probably be specific reasons for using silver iodide for cloud seeding, but I remember from my uni geography course the size of the particles is critical  as well as the specific atmospheric circumstances that prevail to have any chance of success, which in this country seems to be rare. Oddly enough, when we have large intense bush fires they frequently engender powerful thunder storms often with significant rainfall. I think there might be a close relationship between the chemical and the physical for forming these minute droplets. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 10, 2020, 01:42:36 PM
Hi Willy, The cloud seeding experiments in this country used either silver iodide or dry ice which is the solid phase of carbon dioxide.  I don’t know enough of the chemistry or cloud mechanics to understand why each was chosen, or if other compounds were tried.

Hi Paul, I hope you can find that cork.  With slide valves needing a little room for lift to discharge condensate, I think they will fall away from the valve face when not in operation, if you turn the cylinder over.  But the spring as Zephyrin has suggested should hold them against the face, but still allow a little lift when required.  My mill engine needs the same treatment, as the valve face in the vertical plane sometimes allows the valve to rest far enough off the face to allow the steam or air to pass straight through to the exhaust without pressurising the cylinder.  I am thinking of a little sliver of stainless steel shim that I have somewhere in my “potentially handy bits” box, with a slight bend as a leaf spring.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 11, 2020, 03:54:07 AM
Hi MJM , I have a flat bottomed Petri dish and doing the experiment again ,,,and after 4 hours the bubbles have only started to appear on just one of the arms ?? there may be something different about this arm but will only see it when the ex is done , and the w/m is removed ???

willy
Title: Re: Talking Thermodynamics
Post by: paul gough on September 11, 2020, 11:42:45 AM
Hi All, I carried out some simple kitchen tests on rubberised cork sheet. Branded 'Calibre CS0065, 375 x 400 x 1.6 mm' no country of origin on packet. The tests where carried out in a pot of boiling water on the gas stove and in the electric oven, with a supplementary 'insulation' test using a stainless electric kettle. Make sure the kitchen boss is away, as the odour becomes unpleasant at high temps.
A 30mm dia. 100mm long cylinder of sheet was submerged in boiling water for 15 minutes with no apparent degeneration of the material and no adverse affect after folding it over flat 6 times when it dried.
A 25 x 50 strip placed on oven paper on a tray in a pre-heated oven 75mm above element at 150 C. and soaked for 5 mins. then raised to 150 C. for 5 mins., finally at 250 C. for 5 mins. At 150 C it showed no indications other than slight odour, 200 C. stronger odour with slight darkening, 250 C. strong unpleasant odour and darker, about twice darkness of original but no other sign of degeneration. Folding over 6 times caused cracking to 50% of thickness. Material very slightly stiffer and slightly rougher surface texture.
Pristine material, boiled sheet, oven test sheet all ignited immediately in a butane stove flame.
Insulation test carried out as described above giving single thickness result of 30 seconds and double thickness of 65 seconds.
I think this material may prove resilient, for longer times, up to temperatures of 150 C. Approximately steam temp. at 60 psi or 4 bar.
It may prove superior as an insulator in environments where it is wet or in prolonged damp where other insulating materials degenerate or lose insulating properties. Comparative tests against plain cork are needed but I can't source any thus far. The test material has rather fine cork particles and I have seen another brand with significantly larger particles or granules, whether it is superior or otherwise I don't know. Hope this is of some use. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 11, 2020, 01:24:07 PM
Hi MJM, so 12 hours later the bubbles are forming slowly ...but as there is nolid does the fermentation process take longer ?

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 11, 2020, 01:52:24 PM
Hi Willy, I really do not know much about fermentation -another teetotaller.  I know Ginger beer is usually fermented in a sealed bottle, and the process involves pressure buildup due to the CO2, but I don’t think pressure tight lids are used for all fermentation processes.

I shall watch the continuing experiment with interest.

Hi Paul, it looks like that rubberised material I’ll might be ok for the outside of a boiler where flames are not expected.  I am not sure of the idea behind the water soaking series.  Was this to see if soaking made it easier to bend around the boiler, or just to see what happens if the insulation did accidentally get wet.

 It certainly looks promising for boiler insulation, as I assume 150 degrees would normally be enough.

I bought some insulation intended for engine exhaust manifolds, hoping it would be good for a boiler casing.  I also tried the propane torch test and found that it caught fire without too much persuasion.  It is still working over a tin plate fire box so no direct flame impingement, but I am not sure about its use on an exhaust manifold.  I had previously thought that reports of race car engine fires might have been due to that insulation becoming oil soaked over time, but a simple test showed that the oil was not a necessary component.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on September 11, 2020, 02:33:49 PM
Hi Paul, I was also doing an experiment using cork matting around a kettle ...a bit different to yours though as it was just checking up on the thermal characteristics !! this was on the post  No 1067...Page 71. 72 Also on my Moriss Minor the rocker box gasket is made of a compost cork that withstands the high temp ?!!

Willy
Title: Re: Talking Thermodynamics
Post by: paul gough on September 11, 2020, 06:23:15 PM
Hi Willy, Thanks for your reference. The kitchen is certainly a useful laboratory, if the supervisor grants workshop staff access.

MJM, Boiling the material in a cylindrical form was to test the integrity of the material in a wet condition and at a temperature that would reasonably approximate the conditions under metal boiler cladding if things got wet. I also wanted to see if the material retained its flexibility or retained the tubular shape afterwards and whether it would be suitable gasket material in a hot well. The sheet seems to be flexible enough to use around small pipe. I'm afraid I do not understand what you mean in your question, ".....,or just to see what happens if the insulation did accidentally get red."
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 11, 2020, 11:25:41 PM
Hi Paul, apologies, that was a typo, though I suspect that Apple Intelligence (AI) also played a part.  I cannot see how a typo gets from “wet” to “red”.  I read it through three times before posting, and it still got through.  I will go back and correct it.

As a hot well gasket, I wonder if time will be your enemy with the rubber component.  Will it stick and make the surfaces difficult to part after sitting for some time, especially if left tight?

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on September 12, 2020, 12:59:01 AM
Hi MJM, AI = Automatic Idiocy in these cases.

As you said in your previous post on Sept 10, sticking might be an issue, but  the tests seem to indicate otherwise. In fact there is a very slight 'drying' effect to the surface of the sheet which I put down to 'gassing' of the bonding medium. However, 15 minutes in a pot or oven is not the same as many hours of operation and years tightly pressed against a boiler barrel under metal cladding or secured tightly as a gasket. Time being the issue. Paul Gough.
Title: Re: Talking Thermodynamics
Post by: paul gough on September 16, 2020, 11:33:16 AM
Hi MJM, I am having trouble finding our discussion on small boiler insulation that might guide me for future applications. I did find this article as as something of a guide which is from; radiationbarrier.com/resources:

"Although two objects may be identical, if the surface of one were covered with a material of 90% emissivity, and the surface of the other with a material of 5% emissivity, the result would be a drastic difference in the rate of radiation flow from these two objects. This is demonstrated by comparison of four identical, equally heated iron radiators covered with different materials. Paint one with aluminum paint and another with ordinary enamel. Cover the third with asbestos and the fourth with aluminum foil. Although all have the same temperature, the one covered with aluminum foil would radiate the least (lowest [5%] emissivity). The radiators covered with ordinary paint or asbestos would radiate most because they have the highest emissivity (even higher than the original iron). Painting over the aluminum paint or foil with ordinary paint changes the surface to 90% emissivity."

From this I take it that wrapping our little boilers with aluminium foil is, perhaps, a good idea. But how far does one take this. Should we have just one foil layer and where, eg against the barrel or around the outside of the cork or Kaowool layer. Or, should we have two foil layers one against the barrel and one around the insulation. Finally is there any practical value in repeating this foil/insulation layering. I am thinking here of kitchen aluminium foil. Would you care to comment on this please. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 16, 2020, 02:02:03 PM
Hi Paul,

I suggest that the key to understanding this issue is in understanding the different modes of heat transfer that are occurring.

One of the early lessons on heat transfer is that there are three modes of heat transfer, conduction, convection and radiation.  In effect only two modes, conduction and radiation, as convection is basically conduction involving a fluid.  The effect of the heat on the density of the fluid starts movement due to buoyancy in natural convection or due the the fluid velocity in forced convection, making a huge difference to the transfer rates by changing the temperature gradient near the heat transfer surface, hence it is usually analysed as a separate mode.  Usually, one mode is more important than the others, and the analysis is limited to one mode.  In reality, usually, all three modes are involved, and combined mode problems are quite important in some problems.

The article you have quoted is talking about radiation.  The difficulty in applying this directly to a boiler insulation comes in determining whether most of the boiler heat loss is by radiation, or whether conduction and convection are significant.  In addition, the surface temperature of the material is an important factor.

The figures quoted all assume the surfaces are at the same temperature, and all heat transfer is by radiation.  However, in the boiler case, we are actually interested in loss from the boiler internal contents (steam and water), so the relevant starting temperature is the steam temperature.

Now look at the heat path and the temperature profile from the steam, through the copper shell, somewhat imperfect contact between the shell and the insulating layer, convection from the insulation surface and finally to the bulk ambient temperature of the room and the wall temperature of the building walls for an inside layout, or the sky and surrounding objects if out doors.

Each interface involves a resistance to heat transfer and hence a temperature difference.  So for our boiler, the asbestos - these days we should use ceramic fibre or cork - is heated on the inside surface by that imperfect contact with the shell, and will introduce a resistance to heat transfer through the solid material to the outside surface, so the outside surface will be at a lower temperature than the steam.  The article talks about radiation, so we need to understand that radiation heat transfer is proportional to the fourth power of the absolute temperature.  The absolute temperature bit means we are talking about large numbers, and the fourth power emphasises even small difference.  And the insulating properties of the asbestos means that we are starting at a temperature well below the steam temperature.

On the other hand, aluminium is a good conductor, and even with that imperfect contact with the shell, will be quite close to steam temperature, so the difference in heat loss between aluminium and asbestos will not be as great as first appears by a pure radiation calculation made assuming identical surface temperatures.  And the asbestos surface temperature will be highly dependent on the thickness of the material.  I don’t really know what a detailed comparison would show.

My marine boiler is insulated with cork and timber strips, and the surface temperature is about 50 degrees measured with an infrared temperature monitor, while a foil of aluminium would be much close to steam temperature.  The steam temperature is usually about 120 degreesC.

A layer of foil is a very useful radiation shield.  However it needs to be spaced from the shell to minimise contact, so relying mostly on radiation from the shell combined with convection to heat the foil.  Then the foil will be reach a temperature lower than the boiler shell or the steam.  It is quite reasonable to then support a second layer, again spaced, perhaps by rings or strips cork or timber, so the radiation becomes a two step process and the temperature drops in two steps, each governed by that difference in T^4.  I vaguely remember doing the calculations many years ago, and more layers are useful, but economics becomes a limit, as does the point where convection becomes more significant.  Or resulting outside dimensions for a small model locomotive.

I know what you mean about finding a topic in this thread, as it has a huge number of pages, but I have found that first opening the thread, then using the forum search function, is quite good at turning up relevant posts.  I tried “boiler insulation” just now, and the first two hits were posts from you on this subject, about mid September 2019, so that might help you find the rest of our previous discussion.  There was quite a lot there, as I did find that the discussion refreshed my memory and understanding as I trawled my textbooks for the correct formulae, and data, and applied the various concepts to the topic at hand.  Part of the fun of this thread was the remembering those topics from study and work days, and finding how it could be applied to our models.  I was often quite surprised at how much could be achieved by just applying those first principles as the topics developed.  It would take me quite a while to get back to that level again, as my memory doesn’t hold that level of detail these days, in fact, I don’t think it ever did.  But all that aside, I am quite happy to continue the conversation if it will help.

I hope that helps, or at least prompts some more questions,

MJM460

Title: Re: Talking Thermodynamics
Post by: MJM460 on September 16, 2020, 02:09:31 PM
Whoops, Paul, I should have read those first few posts that turned up more carefully.  However, if you go down the search results to about 24, we were talking about thickness on your boilers. 

More careful selection of the search term might find them more selectively.  Perhaps search for radiation?

MJM460
Title: Re: Talking Thermodynamics
Post by: paul gough on September 16, 2020, 08:59:02 PM
Hi MJM, Thank you for your note regarding doing a search after opening the thread, I had failed to do this, I guess I'll never understand computer operation protocols, even at the most basic level.

I'm sorry for not starting my last paragraph with the context setting phrase, "With respect to radiant losses", this would have saved you a lot of typing. I could not remember or find any conclusions as to whether foil might be advantageous to our small boilers in combination with insulation in a single or multiple sandwich layer(s). Once we insulate our boilers it seems there is no more to do regarding convective or conduction losses, they are what they are, so I wondered if there might be anything to be gained by trying to claw back some radiation losses, even if small. As there is practically no dimensional or economic penalty in adding single or multiple layers of foil, I thought it worth investigating. Thank you for you efforts in explaining things. Regards, Paul Gough.
Title: Re: Talking Thermodynamics
Post by: MJM460 on September 17, 2020, 01:50:28 PM
Hi Paul, the theoretical calculations are quite helpful in letting us know what is going on and what are the critical variables.

Unfortunately, the calculations all depend on general typical data for the various parameters, so in order to get a really definitive answer, we need to do some practical experiments and tests to improve the accuracy of the data to be used in a particular case.

In addition to the difference in T^4 and emissivity that we have already discussed, there are also view factors between the areas being considered. 

Then the convection coefficient also depends on several factors, so after a theoretical exploration to discover the critical factors the best way (and possibly only) way to definitively answer your question is a series of experiments. 

It depends on how accessible your boiler is, to try a couple of different schemes, but in the end, it is the best way.  The different experiments can be compared by plotting cooling curves as the boiler cools. This method is sensitive enough to determine not only which is best, but even the relative difference between different insulation systems.  Sometime differences are small and convenience can outweigh very small differences unless you are trying to win a race.

I find a practical method is to make a replacement filler plug with an internal extension, drilled with a blind hole to accept a thermocouple, as supplied with most digital multimeters these days.  In industry, it is called a thermowell, and provides a way to insert the thermocouple deep into the high temperature zone without breaching pressure containment.  Then a watch to measure the time intervals, a digital kitchen scale to measure the water mass in the boiler and a quiet afternoon activity.  From these curves, the actual heat loss can be calculated for as many variations of the insulation scheme as you have the patience to carry out.  I can help you with the maths if you decide to give it a try.

One thing I meant to mention about radiation is regarding radiation heat transfer through a gas such as air.   Air in mostly transparent to heat, but some heat is absorbed by air and even some reflected back.  And importantly, the degree of absorption is highly dependent on the wavelength of the radiant heat, and is different for different gases..  This is the basis of the often quoted “greenhouse effect”.  So even with good data, a complex calculation by wavelength for each component of the gas is required to provide a thorough calculation.  That is a more specialised area than my general overview of thermodynamics.

And I am totally with you on the computing protocols.  I am sure there are ways to search only for articles which contain all the search terms instead of just any of the terms, but I’m blowed if I can remember the syntax.  I tend to avoid all but simple searches and as few links as possible.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 03, 2020, 03:02:19 AM
Hi MJM ,  A new observation and question..When I boil water in my electric kettle it makes a really loud noise when coming to the boil ...? so is some of the energy that causes this sound 'wasted' and is it measurable  and does it have a value and a name ?? 

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 04, 2020, 11:47:39 AM
Hi Willy, I think we are all familiar with the noise of a kettle boiling, it seems that electric kettles are especially noisy.  We even bought a new one a couple of years ago, because they were claiming low noise technology.  It really was pleasantly quiet compared with the old one, but we have recently noticed that the “quiet” one is becoming increasingly noisy.  In the old days, a kettle on a stove or a billy on a campfire used to “sing” as it neared boiling, then the noise would quieten before being replaced by the bubbling of boiling.  But electric kettles raise it to a whole new level.

Noise we hear is due to minute air pressure fluctuations which propagate through the atmosphere as a wave, and the appropriate parts of our ears respond to and we interpret the fluctuations as sound.  Or at least they used to, but I need the assistance of a hearing aid these days.  Sure I am not the only one.  The pressure fluctuations spread out from the source only reducing slowly as viscosity effects in the air eventually damp out the motion.  Each doubling of distance reduces the noise level by about 6 dB if I remember correctly.

It certainly requires energy to produce noise, so noise production does take a portion of the energy which is not turned to heat in your boiler or kettle, or to work on an engine, so contributes to the losses.  The energy involved in the noise is not destroyed, but those viscous effects mean that it is eventually turned into heat, but in most practical cases, the level of heat involves such a low temperature difference over such a large volume of air that I really doubt if it could be measured. 

I am guessing that the noise is caused by that sudden expansion of liquid into a larger volume of vapour, and the bursting of the bubbles at the surface, affecting the air above the kettle.  Not at all sure about how the singing is produced.  I suspect the electric kettle is so noisy because the heat transfer is at a very high rate a limited area of the kettle, and this heat transfer rate seems to have an effect on the noise production.    You may remember me posting a picture of the water boiling in that new quiet kettle which has glass walls.  The bubbling is certainly very vigorous compared with any kettle I have ever peeked into on the stove, which is sort of consistent with a higher heat transfer rate, and higher noise level for the electric kettle.

I seem to remember that the amount of energy involved in noise is relatively low, I would need to look out the text books and see if I can find an answer, but that’s getting a bit heavy.  Certainly the pressure levels involved are quite low, unless you are standing at the back of a jet aircraft about to take off.

I hope that sheds some light on the issues raised in your question.

MJM460

Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 05, 2020, 02:58:54 AM
Hi MJM , Thanks , yes got me thinking a bit more it and I do remember the clear glass kettle!!  We had an unusual weather event yesterday called 'Thundersnow'.rather like a thunderstorm with lightning but it was during a heavy fall of snow in Scotland in the middle of the night that woke people up ?!!!

Willy
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 29, 2020, 03:13:42 AM
HI MJM, another TH question... if you have a bathroom wit a bath full of water  that has been left for a few days so the water and everything in the bathroom is at ambient temperature.., could i use my metal sheafed thermometer to give the same reading in the air as well as the water ??  if i put my finger in the water it will feel colder "because that is what happens" I did try it with the air at 22 degrees and when i put it in dye water it jumped about between 18-19-20 -21...so will the metal sheath act a bit like ones finger might  ?? As water takes a while to change temp rathertha air that is quicker to respond could this be used to take more accurate readings ??
hope all is will with you and your family in these wearisome times

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 29, 2020, 11:55:03 AM
Hi Willy, I suggest that you can definitely measure the air and water temperatures with your thermocouple meter, and you will get the same answer for each.  Whether that temperature is the same as shown on the room thermostat or some other thermometer depends on how well the respective instruments are calibrated.

But as always, there are a few wrinkles in your question!

First, it is important to understand what happens when you measure a temperature.  The thermometer is really only telling you it’s own temperature.  When the thermometer and an object have been in thermal contact long enough for heat to stop flowing, then the two are at the same temperature, so the thermometer reading of its own temperature is also the temperature of the air or water with which it is in contact. 

When you put the thermometer and the object you are measuring (let’s say the air in the room) in close thermal contact, heat will transfer between the air and the thermometer, remember heat goes from the hotter object to the cooler one, until the two are at the same temperature.  So, if you bring he thermometer into the room from a cooler environment, heat will transfer to the thermometer until it is the same temperature as the air.  It will initially read the its own temperature as determined by where it was.  Then as the temperature changes it’s display will change accordingly.  When the display stops changing, the thermometer is at the same temperature as the air, and only then do you get a valid reading.  If your thermometer starts at 18 and changes over a short time through 19, 20 then finally 21, it says the thermometer came from a cooler room.

Now, when you move the thermometer into close thermal contact with the water, and as you have defined the problem as starting with the air and water at the same temperature, as the thermometer is already at that temperature, there is no temperature difference to drive a temperature change in the thermometer, so it still reads the same temperature, which as already stated is also the water temperature.  Of course, you have to conduct the whole experiment without disturbing the thermal equilibrium by virtue of your body heat, which should be well above 21, otherwise you are in urgent need of medical attention.

This problem is the basis of a rather semantic definition of a law of thermodynamics referred to as the zeroth law.  So named, because logically it must be defined before the other laws make any sense.

When you test the temperature of the air and water using your finger, the situation is very different.  You will know that a thermocouple develops a voltage at the junction of two dissimilar metals, and it is this voltage which is measured at zero current flow, so there is no heat generation in the thermocouple.

Your finger however, providing it is still attached and in good condition, is being warmed by your blood flow and so kept something close to your body temperature.  When your finger is in the air, heat flows from your finger to the air, but the two never reach thermal equilibrium.  The air very close to your finger and your skin will reach a steady temperature which will be somewhere between the bulk air temperature and your blood temperature, but not actually equal to either.  Air thermal conductivity is relatively low, so the skin temperature stays closer to your blood temperature, and because you are used to this situation, it does not feel particularly warm or cool.  It is a different matter if you pop over to visit Brian Rupnow, as the temperature in his neck of the woods is probably well below zero at this time of year, and if you are silly enough to take your gloves off, it really feels cold.  Hence the invention of keyless car entry systems.  But as travel is not permitted at the moment, we had better continue in the bathroom.

Back in your bathroom, if you now dip your finger into the water, as you have observed, it feels cooler.  This is because the thermal conductivity of water is higher than that of air, more heat flows from your finger to the water, the temperature at your skin where the nerve endings reside will be lower and hence the water feels colder even though it is at the same temperature as the air.

You could refine your experiment by storing some blocks of different metal in the bathroom for the experiment.  Say aluminium, brass, mild steel and stainless steel.  You need to dry your finger if you try the water before the metal.  The theory says you should be able to arrange the metals in order of thermal conductivity, based on how cold they feel to your finger, noting that they have all been in the room long enough to have reached thermal equilibrium with the rest of the room.  But I have not tried the experiment, and I don’t know if the test is sensitive enough to tell the differences.  But certainly you can tell the difference between air and water as you have observed.  And the metal blocks will feel cooler than the air.

In summary, yes your thermometer will show the air and water to be the same temperature, but your finger does not give you the right answer.  Your observation when you test the air and water with your finger is totally consistent with the thermodynamics.

We are all well here thank you.  Our state has now achieved 60 days without community transmission, so apart from social distancing requirements, wearing masks, and capacity restrictions on various venues things are feeling more relaxed.  We can travel within the state, and meet in small groups.  The wrist is healing so I am getting a little more time in the workshop, so will soon have some progress to report.

I am glad to see that you are still making progress and thinking about thermodynamics.  I guess the weather is less conducive to tending the allotment these days, but so long as you are well is the main thing.  It has been a different issue for those who lost jobs, or are struggling to supervise on line schooling  for young children while trying to work from home. 

MJM460

Title: Re: Talking Thermodynamics
Post by: AVTUR on December 29, 2020, 12:30:01 PM
A little aside.

We used very fancy thermocouples (Platinum, Platinum/Rhodium) at work to measure very high gas temperatures. The actual measurement was fraught with problems and someone would always ask what are you actually measuring. The answer was always the same same - the temperature of the thermocouple bead.

AVTUR
Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 30, 2020, 03:16:57 AM
thanks  MJM. and  AVTUR for the reply and  trusting the actual readings must be quite stressful especially when things dont seem to add up !! still that is research

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 30, 2020, 10:43:26 AM
Hi Avtur, thanks for commenting.  Those certainly are exotic thermocouples. 

I guess in the situation you were using them, in addition to conduction and convection, you might have radiation between the thermocouple and hotter and cooler components in the gas path.  These would also affect the temperature reached by the thermocouple. 

Willy, the other important point that is highlighted by your question is the time required for the heat transfer to take place.  This is the reason that temperature measurement is inherently slow, so difficult in situations involving rapidly changing temperatures.

MJM460



Title: Re: Talking Thermodynamics
Post by: steam guy willy on December 31, 2020, 02:11:04 AM
Hi MJM ..also I think that when I remove the thermometer from the water ..because the water evaporates it cools it.untill it gets back to ambient, there is so much going on with the TH laws that we are possibly still learning about ? Especially in space where it is very cold /hot but all the parts of the equipment still seems to operate ?!! I think one has to wait for things to settle down to get the true readings as your last paragraph infers ?!!

Thanks and a prosperous and productive new year

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on December 31, 2020, 10:37:16 AM
Hi Willy, you are quite right about there being so much thermodynamics going on.  It is indeed very difficult to frame these questions really completely.

When you take the thermometer out of the water, it is wet, and the water starts to evaporate.  But remember your description of the problem.  The air and water are at the same temperature! 

So why does the water evaporate?  And yes, when it evaporates, it does cool the sheath.

We should tie up one loose end in the problem description, the air humidity was not defined.  I would expect that the air humidity in the room was somewhat below 100%, probably less than 50%, so the water vapour pressure in the air was somewhat below the saturation vapour pressure.  Consequently, some of the water molecules with something above the average energy level will escape the water surface, leaving the remaining water a little cooler. The thermometer will then provide some heat to the now cooler water, and the air will supply heat to return the whole to the original average temperature.  We could carry this on to conclude that the room will end a little cooler, or the system maintaining the room temperature will supply some extra heat.  And of course the bath is also evaporating a little if the water vapour content of the air near the surface is less than the equilibrium vapour pressure for the temperature.  If the air is very still, the humidity builds near the water surface and the evaporation stops.  However air movement near the surface removes those freshly evaporated water molecules and so the process continues further.  But you might disturb this still air by entering the room to conduct the experiment.

The description can go on, but the important points are first that truly reaching thermal equilibrium, when there is no further heat transfer is a slow process, and difficult to achieve.  In practical terms we are normally dealing with close enough rather than exact.

Second, the thermometer is only measuring the temperature of the thermometer, and we need to place the thermometer in close thermal contact with the object or fluid we are measuring, and give them time to reach the same temperature before we take the reading.

Thirdly, trying to answer this question also involves understanding of evaporation of fluids and the associated heat transfer.

Things operating in space involve a whole extra level of thermodynamics, but it is getting very far from my experience.  Also, like the errors in the assumptions of conservation of mass, not likely to be of much practical use to most of us as model engine builders.  So I will use that as an excuse to call the thread back to topic!

So time for me to wish you and yours a very happy and safe new year.  Let’s hope it is a bit easier than this year for all.

Thank you for all the interesting questions that have kept my mind active throughout the year.  I hope I have been able to help you and others who have been reading the thread to understand a little more thermodynamics.

The 9pm fireworks have just happened in Sydney so the kids can go to bed.  The midnight ones will be a little shorter than usual, and people are being encouraged to watch from home.  The Melbourne one have been cancelled to avoid tempting people to gather in large numbers to see the spectacle.  A very different New Year’s Eve for most this year.  Time for us all to imagine alternative ways to enjoy seeing the new year in.

MJM460
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 01, 2021, 12:46:24 PM
Hi Willy, good to have you thinking about thermodynamics again, but I don’t want to hijack your build thread, I will address your question about the bath water temperature here.

Of course it’s not really about engines, but what is a water heater but a poorly designed boiler which they omitted to design for sufficient pressure, and with a thermostat set too low to generate steam?  Of course this latter fault compensates for the first, fortunately, otherwise they would be very dangerous devices.

Humour aside, the same physics applies to water heaters and boilers and every other example of heat and energy transfer, and understanding the thermodynamics helps in so many situations.

So let’s have a look at what factors affect the temperature delivered to the bath.  The thermostat controls the heat input to the system to raise the incoming cold water to the set temperature.  Then there are heat losses from the hot water pipe, which are proportional to the temperature difference between the water and the surrounding air.  The cold water inlet temperature determines how much heat is required to reach the set temperature, but not any losses after the water is heated.

The basic control is the thermostat which controls the heat addition to raise the water in the area of the thermostat to the set temperature.  If the incoming water is cooler, as it is in winter, it will require more heat to get to the set temperature, but in principle it heats to the same temperature.

But winter operation also affects the heat losses.  I suggest that there is likely high heat losses between the heater unit and the bath.  I don’t know about your building standards, but here, in all the systems I have seen the insulation on the hot water lines is pretty minimal at best, and the heat loss from a poorly insulated pipe means lower temperature delivered to your bath, particularly if the heater unit is in an unheated part of the house, and/or part of the piping is outside.  The houses I lived in in Canada, the heater was in the basement which was heated, so not far different between summer and winter, and all the piping was also within the heated walls.  Hence the water delivery temperature was not affected much.  Of course, if the heater or piping had been outdoors, it would have frozen solid and burst!

There is one other factor which can affect the delivered water temperature, and that is the thermostat itself.   Nearly all thermostats depend on a differential temperature.  Part of the actual hardware is cold.  If there was no temperature difference between parts of the thermostat, it would not work very well.  Even the thermocouple in your meter is paired with a cold junction, somewhere in the circuit.  So if the cold junction of the thermostat is outdoors and hence cooler, the thermostat would likely reach its setting at a lower than normal temperature.

In summary however, I would suggest that the primary explanation for the lower water temperature at your bath is the heat loss from the hot water pipe.  If the pipe is accessible, and you are sufficiently keen, you might improve the situation by better insulating the hot water piping.   You would then require less hot water to get a comfortable path, so save a little on the energy cost for heating.

I hope this answers the question.

Thanks everyone for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: crueby on July 01, 2021, 04:59:26 PM
+1 on insulating the pipes - in my house the kitchen is a long run from the water heater, and before I insulated the pipes I had to run the hot water tap for a lot longer before I got hot water in the kitchen sink, the length of pipe from the water heater was acting as a radiator to heat the basement. Also did the shorter runs to the bath just above the heater while I was at it. In the winter, with the basement colder than in summer, the effect was magnified. Also, once you have the tub or sink filled, in winter with very low humidity there would be more evaporation quicker, cooling the water faster, correct?
Title: Re: Talking Thermodynamics
Post by: steam guy willy on July 01, 2021, 06:42:01 PM
Hi MGM and Chris .. I live in a medieval council house .(low rent) and the uninsulated pipe from the combi boiler to the bath is about 7.5 meters in length with a 90 degree bend......The pipe runs under the floor in the cellars and there are  also air bricks under the floor that ventilate the cellars with a through draft.  There is no heating in the cellars so this is why there is so much heat loss. ??
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on July 02, 2021, 10:57:43 AM
Hi Chris, it’s good to know that we are not the only ones where hot water pipes are not well insulated.  I guess there is not much incentive for the builder to spend money on insulation that no one will admire, just to save the owner on heating costs.

Part of the reason it takes so long for the hot water to reach the kitchen is that with, or without insulation, when there is no water flow, the pipe cools down to the basement temperature, and when water flows, it has to first heat the pipe on the way through.  Cooler basement requires more heat to get the metal up to temperature.  Then under constant flow, the heat loss still means the eventual water temperature is a bit lower than in summer.  Heating the pipe may be the biggest problem for the kitchen sink, but a bigger volume required for a bath means the steady heat loss becomes more important.

Humidity is a less obvious but more interesting factor.  Humidity is defined as the vapour pressure of the water in the air as a percentage of the saturation vapour pressure at the prevailing at temperature.  The saturation pressure can be found in the first section of steam tables, those one or two pages that give the saturation pressure for temperatures up to the atmospheric boiling point, 100 C or 212 F.

It is easy to see that the vapour content (which is proportional to the partial pressure) is much less at lower temperature than warmer temperature, so much less vapour in the air, even at the same measured relative humidity. 

Certainly if the humidity of air in the bathroom or kitchen is low, evaporation will be higher and carry away more heat, at least if the ventilation is working.  Otherwise the mirror steams up, indicating humidity near 100%, so no more evaporation.  It certainly contributes to how quickly the bath cools after the tap is turned off.

In summer the humidity might be near what the weather bureau announces for your area.  But if the air conditioning is on, the outside humid air has been cooled, and some water drops out as we know, but the air discharged into the room can be very high humidity.

In winter, the outside air might be quite high humidity at the outside temperature, but when we bring it into the house and heat it up to a more comfortable temperature, unless we add moisture with a humidifier, the humidity at room temperature will be very low.  So as you suggest evaporation will be higher than typical for summer, which will contribute to cooling the water, and steaming up the mirror.

Hi Willy, that well ventilated basement certainly helps cool your bath water in winter as already described.  The 90 degree elbow doesn’t have much effect on heat loss, unless the fitting has a lot more surface area for heat loss than plain pipe, but still not big in the scheme of things.  More effect on the pressure drop due to flow.  Remember the fundamental law is that heat loss is proportional to temperature difference and surface area.  It is hard to estimate how much difference you can make by wrapping the accessible sections of pipe with a suitable insulating material, but it will still take more time for the first hot water to reach the bath, as the pipe will pretty much reach air temperature between baths.  Might be worth a try if most of the pipe is accessible.

Thanks for looking in,

MJM460


Title: Pulsations and water hammer -
Post by: MJM460 on August 18, 2021, 11:03:46 AM
Pulsations and water hammer -

Some time back, when Chris was describing the water headers for the pump section of the Holly engine, he made a comment in passing that perhaps I could explain water hammer in the Talking Thermodynamics thread.  In particular, it was noticed that there were no pulsation chambers on the inlet headers, only on the discharge headers of the Holly engine, and the questions were raised regarding what exactly are they for, and why not on the inlet header.

That was a big challenge, Chris, and I am not sure that I can do it justice, but I decided to give it a try.  After all I had some involvement with pulsation dampeners on pumps in my work, so I should be able to contribute a little.  So here goes.

Pulsations and water hammer are associated with unsteady flow situations, either a significant change in flow, such as suddenly turning off a tap, or repetitive flow changes as produced by a reciprocating pump, the primary interest in the Holly engine thread.

I generally think of pulsations as the pressure changes arising from the inertia forces involved when the velocity of the fluid, water in this case, changes.  Water pipes are generally sized to give velocities less than 10 m/s.  Around 2000 fpm if you prefer those units. 

In comparison, I tend to think of water hammer as being due to the pressure change, originating from the origin of the flow change, travelling as a wave through the fluid at the speed of sound in the fluid, for water about 1200 m/s.  The actual figures vary a bit, but you can see there is a difference of several orders of magnitude.  However, I am not sure that these definitions are universal and in everyday parlance, they seem to be used interchangeably, and I will probably follow suit.  But I am open to more information if someone would be kind enough to contribute.

The important feature of the pressure pulsations in water hammer is that as they travel along a pipe, and at any restriction or change of direction, and even the pipe outlet, reflections occur which travel back through the system.  As the reflections meet the next pressure pulse, they add up.  Moreover if the distances are just right, they can result in a standing wave, and the repetitive pulsations at that stationary point can excite a natural frequency of the pipe work, and you will definitely hear and feel the result.  The same phenomenon in a gas system is the reason for the tone in an organ pipe and all wind instruments, and can break the piping in large industrial gas compression systems if not properly considered in the piping design.    Even in the case of shutting a tap in your home, if those reflections reach a bend where the pipe is not well supported, the pulse can move the pipe with quite a bang.  The solenoid valves on a washing machine can cause a severe case, but a manual tap can be shut more slowly to silence the bang until a few more pipe clips can be added in appropriate places.

The maths of these travelling pressure waves gets very heavy very quickly, and I certainly don’t understand it well enough to try and describe here.  But it’s worth understanding that there are really two different mechanisms involved.

To give an idea of the inertia forces that are involved due to the acceleration of the fluid in the system, the film from the museum that was linked to describe the engine mentioned that at each revolution, the three cylinders combined moved approximately four tonnes of water, this being the combined displaced volume of the three cylinders.  Actually 1.19 m^3 per cylinder, or 3583 kg ( or 3.95 tons) for three cylinders.  It sounds like a lot, but relative to the size of the engine, it is like any of us playing with a half kilo weight, or about 1 lb. relative to our body weight

Four tons is a correct calculation of the displacement of three cylinders but it greatly underestimates  the actual mass of water moved by the engine.  When the flow velocity changes in the course of each piston stroke, all the water in the pipe from the inlet out in the lake has to move.    At the same time, all the water from the pump to the surge tower in the water system also has to move in response to the engine piston movement on the discharge stroke.  For it to be otherwise, the water would have to compress or expand, or a gap in the water column would have to open up and close with the velocity changes.

Actually the water does compress and expand, but only a very little bit, which is the source of those pressure waves which travel at the speed of sound in the water, but not enough to significantly change the volume of water affected by the pump.  Water is not generally considered a compressible fluid for this purpose.

Also, a gap can open up when the pressure is sufficiently low, (when the pressure is below the vapour pressure, water boils to fill the space with vapour), and collapse when the pressure rises above the vapour pressure.  Unfortunately, the collapsing of the gap is very damaging and can damage the pipes in the area where the collapse occurs, or even burst the pipe, so definitely to be avoided.

However, if a gap in the water column is deliberately introduced, in the form of an air filled pulsation chamber, the water column accelerations can be greatly reduced in compressing and expanding the air in the chamber, and so minimise the effect of the long pipes on the inlet and outlet side of the pump.  The important difference is that the air in the pulsation dampeners will not condense, but simply compress and expand in a manner that slows the acceleration of the fluid, and hence reduces the forces on the system.  The air volume undergoes significant volume change as its pressure changes.

Well, that’s more than enough words to introduce the subject of water hammer and pulsations.   To make more progress on understanding pulsations in particular, we need to do some calculations.  So next post I will present the results of some maths that will allow us to quantify the accelerations and forces involved.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: Admiral_dk on August 18, 2021, 11:51:06 AM
Nice explanation - but I see a few errors in your 'constants'.

The speed off sound in water is 1400-1580 m/s. dependent on temperature, salinity and pressure .... not 1200 m/s.

One ton is 1000 Kg. .... (thought the Yanks got that one wrong too with their 907.185 Kg.).

Per
Title: Re: Talking Thermodynamics
Post by: MJM460 on August 18, 2021, 01:28:28 PM
Hi Admiral, thanks for looking in and the kind words.  Regarding the speed of sound in water, I do believe that you are correct.  The figure that I obtained from an old text book on water hammer appears to be a misprint of some kind.  Fortunately the value does not come into my calculations, and the importance is just the comparison of typical velocity of water in pipe, generally less than 10 m/s (in my experience, this is a high figure for general pipe liquid flow) compared with the velocity a pressure change is propagated along the pipe, clearly not much chance of mistaking which one is intended.  So thanks for pointing that out, I have noted the correct value in my text book.

Regarding the ton, the situation is less clear.  The term used in this country is the tonne, or 1000 kg. also called a metric ton.  Quite confusing, as 1000 kg is also referred to as a short ton, or 2204 lb. compared with 2240 lb. also called a long ton.  I should have been precise and stayed with kg, or used tonne, or metric ton.  But then the calculated mass was 3583 kg, so about four tons, but not 4.0 and definitely not 4.00.  I am usually more pedantic about stating “approximately” or “about” when I make such approximations, I must be slipping.  Thank you for helping ensure clarification.  I will try and be more consistent.

MJM460

Title: Re: Talking Thermodynamics
Post by: crueby on August 18, 2021, 02:55:10 PM
Hi MJM,


Great start on the water hammer topic, I'll be following along closely on this one.

EDIT: corrected my wording on the air chambers below.

On the Holly pumping engine, they do have one air chamber on the outlet pipe, where the outlet pipes come together, and none on the inlet side before any of the check valves. On the Allis pumping engine, which I am drawing now from plans of the one in Boston, they put air chambers on every pump inlet and outlet, plus one at the very end of both inlet and outlet pipes. Interesting contrast. Both brand pumps are same era, about the same size, but do have interesting differences in design choices.
The Holly has an inlet pipe down each side of the pump chambers at the bottom, and an outlet down each side at the top, with large banks of check valves between. On the Allis, the one inlet is on one side, one outlet on the other, each with one bank of check valves. Given the 48" pipes, both have a Lot of check valves, total of 1200 and 1400 valves, several inches across each, arranged in beehive towers.


Chris
Title: Re: Talking Thermodynamics
Post by: AVTUR on August 18, 2021, 03:57:01 PM
MJM mentioned that water, or any liquid, is slightly compressible. Really, elastic is a better word since compressible fluids are assumed to be gases.

A water hammer can be used to transmit power similar to hydraulics although I don't think it has actually been used. A Romanian, George Constantinesco, came to London before WW1 and tried to interest the world with this method of power transmission. Unfortunately he said that liquid was compressible, was laughed at and ignored. In the end someone realised what he was saying. His power transmission system came to nothing but the principle was used, with great success, to synchonise a machine gun so that it fired through a rotating propeller.

AVTUR
Title: Re: Talking Thermodynamics
Post by: steam guy willy on August 19, 2021, 03:31:19 AM
Hi All, good see some more content on this thread .. I can remember sometime ago seeing a programme from the USA about using the water hammer to heat water ..? this was filmed in a fire station and this was used with great success to provide them with hot water ??!!! this was possibly about 20 years ago . so does anyone remember this or have any info on it ??!!!

Thanks

Willy
Title: Pulsations and water hammer
Post by: MJM460 on August 19, 2021, 12:08:37 PM
Hi Chris, I am glad that you found the thread. Interesting that the Allis engines have pulsation chambers on both inlet and outlet headers.  I will comment on why that might be in due course.  With such large pumps there is a lot of energy available in closing the check valves.  I believe the large number of small check valves helps overcome the breakages that would be likely if a few larger check valves were used.  When the valves try and stop all that water, large check valves hit the seats with a bang.

Hi Avtur, great to have you looking in.  I agree that a different word might be helpful in emphasising the importance of the difference in volume change with pressure change between gases and liquids.  Old habits die hard, so I will probably continue to talk about high compressibility and very low compress ability, but I will try and include an appropriate adjective to make it clear which meaning I intend.  Our fluids mechanics lecturer told us the story about the machine guns and propellors, but if he described the process, I must have missed it.  Interesting that a power system can be used to harness water hammer.  Of course many farms here have what is called a ram pump, which uses the energy of water descending in a pipe.  At some critical velocity, a check valve shuts, the resulting water hammer causes a pressure pulse which in turn opens another valve to deliver water to an elevated tank.  When the pressure pulse is dissipated, the cycle starts again.

Hi Willy, there is certainly energy in water hammer, and all forms of energy can usually be converted to heat, but how this would be done in a way that results in a useful temperature for water heating eludes me.  Doesn’t mean it can’t be done, just that I don’t know how.

Back to the pulsation dampeners on the Holly engine, and to why they are needed, first we need to know something about the pump plunger motion, and the resulting motion of the water being pumped.

The motion of the Holly engine is definitely a conventional crank and conrod motion.  However, to simplify the maths a little, I have assumed a scotch crank mechanism.  This means the motion of the pistons is simple harmonic motion, or simple sinusoidal motion at the frequency of the shaft rotation.

A conventional crank mechanism has asymmetry introduced by the conrod, and this leads to a motion which is a long series of small modifications to the primary sinusoidal motion, but at ever increasing frequency (2x, 4x, 6x and so on).  If it is of interest, at the end of the topic, I will repeat the calculation including the second harmonic, which is as accurate as most of us will ever need.

The calculations are performed in a spreadsheet.  With the pump represented as in the first photo, the complete mechanism configuration can be described as the crankshaft rotates with one variable, the crank angle as shown.  The mathematical convention requires the rotation direction as shown, in order to get the correct sign of the sine and cosine functions all the way through the revolution. 

The equations for the piston displacement is relatively simple, and velocity and acceleration are obtained by differentiation.  Basically the first line of the spreadsheet sets out those equations for the starting point, then the equations are copied down, with the time incremented each line until a full revolution is achieved.  The time and the rotational speed are used to calculate the angle of the crank shaft which defines the engine geometry, from which displacement, velocity and acceleration can be calculated.

I suspect not many want me to post the maths, but the advantage of the spreadsheet, is that the results can be shown in graphical form.  Also, critical variables such as the rotational speed can be easily changed in a single cell, and the computer then recalculates the whole sheet, and re-plots the graphs.

So let’s look at some results.

The first graph shows the piston position, velocity and acceleration for a single cylinder.  Note that the acceleration numbers are larger than the position and velocity, so these are shown on the right hand axis, whereas the pistons displacement and velocity numbers are similar, allowing them both to be shown on the left hand axis.

The second graph shows just the piston position, but shows each of the three pistons displaced in accordance with the crank positions of the respective cylinders.  It’s not hard to see that the flows produced by the three pistons tend to smooth things out over the revolution when their individual contributions are added.

Of course to understand the water flow, we need to look at the piston velocity and the associated volumetric flow.  And we need to allow for those check valves.  The piston draws water in during the upstroke, the first 180 degrees of crank rotation, then the inlet check valves close and the discharge valves open, so water flows out into the discharge line for the second half of the revolution.

Let’s ignore the discharge line for the moment, and concentrate on the inlet.  The spreadsheet allows for flow only on the upstroke using a simple logic condition.  So looking at a point in the inlet line.

Now the third graph.  You can see the velocity produced in the inlet line, assuming it is the same diameter as the piston, and how the velocity from each piston contributes to the total flow in the inlet line.  I have never before bothered to actually do the calculations, and add the contribution of each piston to show the combined result.  I must admit that I was surprised by six quite similar pulses each revolution.  Intuitively, I did not expect the peak produced by addition when two pistons are contributing to be quite so similar to the pulse produced by each, but there you are.  At any point there is at least one cylinder with closed check valves.  You can see that the result is a surprisingly small range of velocity changes.

But there are now six of these smaller pulses per revolution.  I incorporated these results into graphical form again.  I made a second copy of the graphs, and had a play with the scales.  On one graph, the resultant velocity is plotted at the same scale as the individual piston graphs.  This clearly shows how the overall velocity changes are much more uniform than the individual contributions.  To make the shape of the resultant a little more clear, I put the resultant on the right hand ‘y’ axis, and altered the scales so separate the resultant curves from the individual curves.  If you compare the numbers, you can see they are the same information.  You can decide which presentation is clearer to you.

That’s enough for another post.  Next time we will look at the all important accelerations.

Thanks for looking in.

MJM460
Title: Re: Talking Thermodynamics
Post by: crueby on August 19, 2021, 01:31:47 PM
Very interesting graphs! I would not have expected the combination to look so even, was thinking they would be steeper on one side or something, not expecting the six pulses. Onthe Holly, the pump cylinders are 36" diameter. On the Holly, the pipes down each side are also 36", but the water flow is split between the two sides, and join up at the ends to 48" diameter pipes. On the Allis the pump cylinder is also 36", but with the single-in and single-out piping, the sizes are different than the Holly. The inlet pipes are all 48", and the outlet pipe starts out 36" and then goes to 48" halfway down.
So everyone can see the layouts on these pumps, here are some pictures of the pump levels of the engines, the steam engines up top were removed so the pumps show better. First the Holly, with the piping and check valves split between the two sides. The check valves are in the six towers, between the inlet and outlet pipes, and the air force chambers are only on the upper level on the output side. Also note that the inlet side just dead-ends at the last tower, no air chamber there either, but on the outlet pipe there is one where the two pipes join.

(https://i.postimg.cc/rpcxpHNs/Holly-Piping-Arrangement.jpg)

And here are two views of the Allis pumping level, first the inlet side. On this engine, the check valves are a single layer on each side, at the level of the first joint at the lower end of each tower. On the inlet side, there is a force chamber per pump chamber, with the connection to the inlet pipe between the towers and one more at the far end.

(https://i.postimg.cc/MTpB7j48/Allis-Inlet-Piping-Arrangement.jpg)
and the outlet side, showing how the outlet pipe increases diameter.

(https://i.postimg.cc/vmz9xv0z/Allis-Outlet-Piping-Arrangement.jpg)

One other question about the graphs - they would show the acceleration and velocity at the pipes where the first tower on the inlet side is? At the last tower, the graphs would be a little different, wouldn't they, since by that point there is only one pump to flow to? There would still be pulsations pushing past and oscillating back from the first two towers? Same with the middle position tower, would it be just two of the pulsations?? My head is starting to hurt....!
Title: Re: Talking Thermodynamics
Post by: Lew Hartswick on August 19, 2021, 02:43:26 PM
<One ton is 1000 Kg. .... (thought the Yanks got that one wrong too with their 907.185 Kg.).>

:-)  We really don't give a hoot how may Kg there are in a "ton" . We just know it has 2000 Lb. :-)
    ...lew...  one of those "Yanks". :-)
Title: Re: Talking Thermodynamics
Post by: crueby on August 19, 2021, 03:06:33 PM
<One ton is 1000 Kg. .... (thought the Yanks got that one wrong too with their 907.185 Kg.).>

:-)  We really don't give a hoot how may Kg there are in a "ton" . We just know it has 2000 Lb. :-)
    ...lew...  one of those "Yanks". :-)


Here come the "Inferial" vs Metric comments! 


Chris, another Yank! 
Title: Re: Pulsations and water hammer
Post by: Dan Rowe on August 19, 2021, 04:19:45 PM
With such large pumps there is a lot of energy available in closing the check valves.  I believe the large number of small check valves helps overcome the breakages that would be likely if a few larger check valves were used.  When the valves try and stop all that water, large check valves hit the seats with a bang.

The salt water service pumps on one of the ships I was on had about 18" pipes and is on the lower deck of the engine room. When the pumps were switched over the check went off like a cannon shot that could be quite loud over the regular engine noise three decks away in the control room. One of the ships had a failure of the check valve and the whole class of ships was upgraded to a silent check valve that was designed for submarines the silent service. This solved the problem.

Cheers Dan
Title: Re: Talking Thermodynamics
Post by: crueby on August 19, 2021, 05:06:55 PM
The check valves on these pumps have a hard rubber disc that sits against the hole in the metal plate. The disc is held on a shaft, with a coil spring to hold it closed when there is no pressure difference. Dan, were the ship valves similar, or were they the flapper type?  On my model I used stainless steel ball bearings.
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on August 19, 2021, 05:27:02 PM
Chris, the valve the story was about is a flapper type which is the common way to isolate the standby centrifugal pump when two pumps are used for reliability.

Reciprocating pumps and air compressors use spring loaded valves for check valves and even on a small pump or compressor (small in comparison to the pumps you are modeling) there is usually more than one spring loaded check valve for the suction and discharge. A Worthington steam duplex pump has four valves for both suction and discharge for both water pistons.         

Cheers Dan 
Title: Re: Talking Thermodynamics
Post by: crueby on August 19, 2021, 06:05:07 PM
Chris, the valve the story was about is a flapper type which is the common way to isolate the standby centrifugal pump when two pumps are used for reliability.

Reciprocating pumps and air compressors use spring loaded valves for check valves and even on a small pump or compressor (small in comparison to the pumps you are modeling) there is usually more than one spring loaded check valve for the suction and discharge. A Worthington steam duplex pump has four valves for both suction and discharge for both water pistons.         

Cheers Dan


Good to know, thanks!
Title: Water hammer and pulsations
Post by: MJM460 on August 20, 2021, 01:20:47 PM
Hi Lew, It just illustrates the traps in including colloquial language on an international forum.  When/where I grew up, a ton was 2240 lbs, which divides nicely into 20 hundredweight (cwt) of 112 lbs. But accuracy for most of us was not an issue.  For a lb or two you can carry it, for a cwt, you would need a barrow, and for a ton, you need a truck.  And it could also refer to travelling at above 100 mph, especially on a motor bike in speed restricted zones in London!

Hi Dan, good to have you looking in.  Those silent check valves are also sometimes specified for larger compressors, partly for noise reduction, but more to prevent breakage when they close if there is a bit of pressure behind them.  I guess the noise of those check valves closing would travel pretty well through the steel structure of the ship.

Hi Chris, you have raised some complex issues. 

I thought those vessels you have labelled Force Chambers were an arrangement to house all the check valves.  You need a lot of real estate to fit 1000 check valves.  Do you have a cross section that shows the internal arrangement?  I presume the name came from the drawing, but with details of the internal arrangement, it should be possible to understand their function.

Certainly those changes of section and flow right at the pump cylinders makes things a little complex.  The key to analysis is to simplify the system to something you can analyse, then look at what difference those simplifications make.  If you look at the system from a distance, so the pump is a little blur, and the inlet pipe is a point which sees all the flow.  Because the inlet pipe is larger diameter than the piston, the velocity in the pipe is lower in inverse proportion to the greater diameter, and the acceleration also lower in the same proportion.  The constant (the area ratio) does not affect the differentiation, so the acceleration in the larger pipe is in the same proportion as the velocity.

When you look more closely, as you have suggested, the end of the header sees only the flow from one cylinder, the centre section sees two cylinders.  Also, the two inlet headers of the Holly each see half the flow, but they are smaller diameter than the main inlet from the lake so similar velocity.  But the pressure changes generated by the acceleration and deceleration do not make for big pressure changes in the local area.  Also the mass of water in those bits of pipe close to the engine is actually quite small in the grand scheme of things, as we will see when we get the result of the calculation.  Certainly, the results are not sufficiently accurate to satisfy the Dept of Weights and Measures, but I believe they will certainly be to the nearest ton, given the obvious general agreement on the magnitude of a ton.

Having calculated the velocity of the water pump plunger face (it’s actually the same as the steam piston velocity as the two are directly connected), we need to calculate the accelerations.  That requires the second derivative, obtained by differentiating the velocity equations.  Now that tested the memory cells a bit, so I had to refer to some old maths texts, but fortunately, the resulting equations were also in my old machine dynamics text book Theory of Machines, by Thomas Bevan.  I am sure that some of you will have it on your bookshelf, like mine, unopened for many years.  But it allowed me to check that the maths agreed.  So the equations for acceleration were added to the spreadsheet by adding a few more columns.  Again, only the top row has to be typed in.  It is then copied and pasted down the rest of the rows for one complete revolution. 

Again, the results were put into graphical form, and another surprise.  Six separate accelerations each revolution, as you can see in the first illustration.  The three individual cylinder accelerations are shown in correct phase, and the resultant acceleration shown in green.  The individual piston contributions are shown on the left hand axis, and the combined effect on the right hand side, again with the scale modified to separate the lines for easier understanding.    And each acceleration starts as a positive number, then reduces through zero to a similar negative value, then suddenly reverses to become positive for the next pulse as the influence of the rising piston takes over from the one slowing at the top of its stroke.  This implies a sudden reversal of the pressure and forces in the cylinder and piping.  Obvious when I think about it, I just had not previously carried the calculations through to the graphical result.  When I was studying, we had no computers, not even a calculator, so it would all have to be done on a slide rule.  You can see why the detail was not commonly calculated in detail.  Fewer excuses now that every primary school kid carries around a computer that would have been eagerly taken up by the first astronauts to reach the moon.

Now why all the emphasis on acceleration?  Remember Newton’s laws, and his famous equation,
Force = mass times acceleration, or F = m x a.
The equation is so fundamental to physics, that in the ISO metric measurement system, this equation is used to define the unit of force, unlike previous systems, both imperial and metric, which used the vertical force provided by a standard mass, and hence is reliant on the value of the acceleration due to gravity.  This is very inconvenient, as gravity varies depending on where you are on the face of the earth, let alone in space.

In any case, to make those accelerations happen, a force has to be applied.  Another law formulated by Newton.  We can calculate the accelerations, but in this case, we don’t actually know the mass involved.  Remember, it is not just the volume of water in the cylinder, it is all the water in the inlet pipe from the inlet out in the lake.  Take a look at the second illustration.

I don’t expect that the length of the inlet pipe is shown on the engine drawings, though possibly it’s in some of the other available information.   However, if we make a guess at the depth of the pump inlet below the lake surface, we can calculate the maximum force available to push the water along the inlet pipe to the pump, and hence the maximum mass that can be given the observed acceleration by Newton’s equation.  The second illustration shows the idea.

We need to calculate the available pressure at the plunger face, usually described in terms of head of fluid, and called the net positive suction head, or NPSH.  You may have come across it in terms of the pressure required at the inlet of a pump.  It is the head or pressure necessary at the inlet of the pump after allowance for acceleration, and friction to ensure that cavitation does not occur.  And of course pressure is proportional to head.

The net positive head or suction pressure consists of the pressure at the liquid surface, plus the additional pressure due to the depth of fluid, minus the friction pressure drop in the inlet pipe and importantly, minus the vapour pressure of the fluid.  As we are dealing with water, the vapour pressure can be found in the steam tables for any water temperature.  In ISO units, atmospheric pressure is about 101 kPa, and the vapour pressure at about 20 deg C is about 2 kPa.   By calculating the available pressure at the piston face, we can calculate how much pressure is available for acceleration of the water, we can calculate the force available, and from that we can calculate the maximum mass that can be accelerated.   Remember the pressure at the plunger face cannot be allowed to go below the water vapour pressure, as at that pressure the water will boil and open a steam space which when the pressure rises will collapse, quite violently.

I won’t bore you with the details of the calculations.  The main thing required at this stage is to allow for the lower velocity, and hence the lower acceleration in the inlet line, which is a size or two larger than the pump plungers.  Also, the estimate is a bit on the high side, as I have ignored the pipe friction, and in practice, you need a margin of npsh over the minimum calculated so a little less pressure is available to cause acceleration.

This post is getting too long, so I will stop here while the above sinks in, and I will summarise a few of the results for tomorrow.

Thanks for looking in,

MJM460

Title: Re: Talking Thermodynamics
Post by: crueby on August 20, 2021, 05:37:39 PM


Hi MJM,
Here is a cutaway view of the Holly pump level that should help more:
(https://i.postimg.cc/90NjTP7F/Valve-Cutaway.jpg)
The purple arrow shows the water inlet pipe on the end pump set. The water flows upwards through the first set of check valves into the middle chamber, red arrow. Those things that look like little Daleks are the 'beehives', which are thimble-shaped collections of check valves, 15 check valves per beehive, 7 hives per chamber entrance/exit. The water is drawn into the pump chamber in the center, orange arrow, then on the downstroke pushed back through the check valve chambers on either side and up through the outlet check valve plate, green arrow, and out through the outlet pipe, blue arrow. The orange circle shows the air chamber, the top 2/3 of that is air above the water level. Also note the smaller pipes that connect all the air chambers, they called these 'equalizing pipes' in the plans and they are also mentioned in Holly's patents on the engine.


Closeup view of a beehive(https://i.postimg.cc/vHpFBfDv/Valve-Cage.jpg)
and of a valve
(https://i.postimg.cc/GhBndq32/Valve-Exploded.jpg)
The purple is the rubber valve piece, the green is part of the beehive, that piece is metal. The orange plate and spring above it allow the rubber piece to move up to let the valve open. This is not my design, this is taken right from the builders blueprints.


Title: Re: Talking Thermodynamics
Post by: Dan Rowe on August 20, 2021, 06:08:09 PM
Chris, I spent most of my sea time as a day worker or the maintenance guy. So I was looking for how to get access to the beehive stacks and I see the manhole for the suction side but I can not see how to get the discharge valves to replace parts.

MJM, I read these posts but I usually do not have much to add to the discussion.

Cheers Dan
Title: Re: Talking Thermodynamics
Post by: crueby on August 20, 2021, 07:09:35 PM
Chris, I spent most of my sea time as a day worker or the maintenance guy. So I was looking for how to get access to the beehive stacks and I see the manhole for the suction side but I can not see how to get the discharge valves to replace parts.

MJM, I read these posts but I usually do not have much to add to the discussion.

Cheers Dan
Hi Dan,
Added some more arrows - the red arrows show the manholes above the discharge check valve plates on the side of the towers facing the pumps, the green arrows show the manholes for access to the suction valves, these manholes are between the two plates.
(https://i.postimg.cc/7ZjCNfFJ/Manholes-In-Pumps.jpg)
In looking at the cutaway views, its apparent that the manholes were not cut through the inner walls of the cylinders, just stuck on the outside. Whoops! That would have made for some swearing when the mechanic tried getting in there! Just fixed that....



Title: Re: Talking Thermodynamics
Post by: crueby on August 20, 2021, 07:15:14 PM
Oh, and on the Allis plans, they actually showed the ramp/pully arrangement they used to slide/lift the beehives in/out of position in the chambers through the manholes. The director of the Boston museum said they have some of the spare beehives there in the collection - am going to try and get pictures of them next time I am there.
Title: Re: Talking Thermodynamics
Post by: Dan Rowe on August 20, 2021, 09:25:31 PM
Chris, your drawings are really good and will be very helpful for the general public to understand one of these engines. I think if you made a cross-section across the engine to show a single ram and both the force chambers the water flow would show much better and the manholes for the mechanic will also be clear.

Cheers Dan
Title: Re: Talking Thermodynamics
Post by: crueby on August 20, 2021, 10:39:15 PM
Good idea!
Here is a cross section of the Holly engine from the end on, showing the inlets/outlets either side of the plunger. The red arrows show the incoming water on the upstroke, and the green arrows show the outgoing water on the downstroke.
(https://i.postimg.cc/7ZgRDQgB/Holly-Cross-Section.jpg)

And here is the same view of the Allis engine, which has the incoming water on the left, outgoing water on the right. The chambers on the upper left are connected through pipes between the towers to the incoming water, but there is a plate above the check valves that seperate that chamber from the pressure area. That was a fun one to figure out from the 2D blueprints!

(https://i.postimg.cc/pLNSwtnh/Allis-Cross-Section.jpg)

Title: Re: Talking Thermodynamics
Post by: derekwarner on August 21, 2021, 12:01:00 AM
Still here in the background with a GAG in my mouth  :popcorn: :popcorn: :popcorn:,

From the first day, I had considered these 6 "Force Chambers" with an interconnecting balance pipe, to be high volume, atmospheric pressure charged "Air over Water accumulators"

Would these with the volume to volume ratio not provide silent accumulation transfer and adsorption of pipe hammer?

When I close my eyes, I could imagine a series of rortational  SWOOSH sounds over any banging  :hammerbash:

Derek ....thirsty work this watching  :DrinkPint:
Title: Re: Talking Thermodynamics
Post by: crueby on August 21, 2021, 12:59:06 AM
Still here in the background with a GAG in my mouth  :popcorn: :popcorn: :popcorn: ,

From the first day, I had considered these 6 "Force Chambers" with an interconnecting balance pipe, to be high volume, atmospheric pressure charged "Air over Water accumulators"

Would these with the volume to volume ratio not provide silent accumulation transfer and adsorption of pipe hammer?

When I close my eyes, I could imagine a series of rortational  SWOOSH sounds over any banging  :hammerbash:

Derek ....thirsty work this watching  :DrinkPint:


Now you're making me thirsty! 


The reduction of water hammer is what they are for, same mechanism used on well water pumps in modern houses, just st a whole lot bigger here! Holly's engine patent mentions the chambers as one part of thier system, but its obvious that it is an earlier invention.
Title: Water hammer and pulsations
Post by: MJM460 on August 21, 2021, 12:45:05 PM
Hi Dan, looking in is enough, as long as you will comment if you see a problem.  It’s always helpful to have knowledgeable people looking over my shoulder.  Another vote for those cross-sections to be on display.  They are really excellent.

Hi Derek, no need for a gag, your comments are always welcome.

Hi Chris, thanks for those extra diagrams.  I started out seeing those chambers as pulsation dampeners, but then the Allis arrangement and the force chamber terminology confused me.  The drawing clarifies all.  Definitely pulsation dampeners on the Holly and on the inlet header of the Allis.  That divider plate above the check valves and the large pipe connecting the inlet header and the chambers shows how they work.  The air chamber cannot be on the plunger side of the check valves.  It’s a clever arrangement to get the chamber physically above the check valves, but also on their inlet side.

Getting back to the calculations, graphs provide a more intuitive picture than a large array of figures, but when I look at the spread sheet, the actual maximum velocity for the plunger, and like wise for the inlet pipe if it was 37.5 in, like the plunger, was 1.76 m/s, but in the larger 48 in inlet pipe, this reduces to 1.07.  The respective minimum velocities are 1.52 and 0.93 m/s.  You can see how much less this varies than a single cylinder.

Similarly, the maximum acceleration is 1.84 m/s^2 for the 37.5 in plunger and 1.12 m/s^2 for the 48 inch pipe.

Now assuming that the pump inlet is 5 m below lake level, as in the sketch I included yesterday, the available pressure at the plunger face is 148 kPa (about 21 psi if you prefer).  This pressure implies a force of 105 kN, but over the area of the 48 in inlet line, 173 kN.

The result might surprise you.  The pressure available, over the area of the 48 inch inlet pipe can provide the required acceleration for approximately 150 tons of water in the 48 inch inlet line, which means a maximum line length of about 130 metres.  Should be enough to get out into the lake well clear of the shore line.  If the pumps are deeper than my assumed 5 metres below the lake surface, there is more pressure available, which will overcome friction, or the pipe can be longer.

The other important conclusion is once the maximum velocity is achieved, the decelerations involved in slowing the flow to the minimum velocity are achieved without requiring excessive pressure.  The maximum acceleration pressure at the pump is only about 150 kPa plus the static head, well within the pipe pressure capacity, so no pulsation dampener is required on the inlet.

If Chris has the actual line length, and depth of the pump plungers below the lake surface, I can easily adjust the calculation, as the figures are each in a single cell, referenced where ever it is needed.  But it is clear that the forces necessary to make the water follow the piston do not require high pressure, that would require a pulsation dampener, thus explaining why they are not required on the Holly engine.

Similarly, the Allis designers have included pulsation dampeners on the inlet header.  I don’t have the plunger sizes or operating speeds for the Allison, but based on the calculations for the Holly, you can see that they would be necessary with a longer inlet line or higher operating speed.

There is no need to repeat all this for the discharge side of the pump.  The numbers are the same with signs reversed in line, with the flow out from the pumps.  The difference is the length of the discharge line.  The location of the pump station near the lake means the inlet line length is limited, and hence the mass being accelerated is limited.  However, on the discharge side, it is likely that the piping is considerably longer.  You will have noticed that water systems usually include a high water tower, or an elevated tank high above the surrounding city skyline.  This tank has a free surface, with atmospheric air pressure at the surface.  This water level can absorb pulsations from the pump by fluctuating up and down.  It is the distance from the pump this surface which determines the length of the discharge line.  A likely much higher mass than 150 tons is the reason the pulsation dampener is required on the discharge header. 

The pulsation dampener on the discharge side of the pump is charged with air to something like mid level when the pump is shut down.  This means the pressure in the dampener is equal to the pressure at the pump discharge when it is shutdown.  As the acceleration of the plungers on the discharge side of the pump increases, the extra pressure actually compresses the air in the pulsation dampener.  Some of the displaced volume enters the pulsation dampener instead of trying to accelerate the contents of the entire pipe length.  This has the effect of significantly reducing the accelerations in the line, and so the pipe flow after the pulsation dampener is much more steady than it would be without it, and the remaining forces are insignificant.

Unfortunately, air in the chamber dissolves in water to a small extent, so the air in the pulsation dampener is slowly removed by becoming dissolved in the water.  It is likely that the small bore piping in the drawings is to allow regular topping up the air pressure to keep the level a suitable range.

I hope that clarifies what the pulsation dampener does, and why there are none on the inlet side.  (Or rather just when they would be required.)

 I hope I have also provided a realistic estimate of the actual mass of water which must be accelerated, six pulses every revolution of the engine.  You can see that the four tons of pump displacement is actually quite insignificant, especially in comparison with the mass of the engine.

Thanks for looking in,

MJM460



Title: Re: Talking Thermodynamics
Post by: crueby on August 21, 2021, 01:16:03 PM
Hi MJM,
On the Holly engine, the intake pipes from the lake are quite long, though I don't know the diameter past the wall of the room that the pump engines sit in. There are five engines in a row off a common feed, but they built room for eight in total if needed, so I would imagine the pipe from the lake is quite large. The intake pipe actually goes out to an artificial island a ways out in the lake, where there is a building that houses the intake and pre-filter to keep out fish, scuba divers, loch-ness monsters, etc. Where it comes into the building the 5m depth is a good estimate, it is likely deeper out along the lake but that is not as important as the end elevation.
For the Allis engine, the building is right across the road from the small lake fed by the river, so there the intake is shorter, depth is probably about the same. On that one, the pre-filter was done right at the corner of the building, they had a large well they could raise/lower the screens into. There as well, they were set up for multiple engines and the pipes were large, but I dont have dimensions.
The Holly engines used three pump plungers of 38" diameter and 66" travel, on the Allis the three plungers were 42" diameter, also 66" throw. Each was set up to be able to run in the 12 to 20 rpm range depending on need. The Holly engine had 1260 4-1/2" diameter check valves total (half inlet, half outlet), while the Allis engine has 1512 3-9/16" diameter valves. Some of those diameters were taken up by the radial spokes supporting the center post on the valves. Overall, an amazing amount of water flow.

With the Buffalo engine, the Holly, they were pumping to water towers up to several miles away but on fairly level terrain, and at the Allis engine in Boston they were pumping uphill to towers on the peaks around the area, to supply water to buildings around the city that were on the hills - the pump was called the High Service Station since it supplied the high areas of the city. They were able to do gravity-fed systems from local lakes/rivers for the rest of the city I think. When they retired the Boston pumping station they were taking water from a much higher lake farther out from the city on a long pipeline, so the pumps were no longer required.

Thanks for all the info!!Chris
Title: Water hammer and pulsations
Post by: MJM460 on August 22, 2021, 12:59:09 PM
Hi Chris, interesting that the inlet line lengths are the other way around to what I would have expected.  The Holly with no pulsation dampeners having long inlet line or lines, while the Allis has the pulsation dampeners, but a very short intake system.  That screen well at the corner of the Allis engine building presumably has a free surface, open to atmosphere.  The liquid surface can move up and down in response to the varying flow to the plungers.  That free surface defines the end of the inlet line.  The piping from the lake to that screen well will only experience minimal flow variations.  However, those accelerations and the associated fluctuating forces will still exist, in the short line from the well to the pump inlet, so the Allis designers obviously thought they were worth eliminating.  It may well have made the check valves last longer, but I don’t really know.  Clearly the designers each had their own ideas.

On the other hand, the Holly with the lines out into the lake is where the pulsation dampeners might be expected.

There are many ways of arranging the inlet to so many engines, and the exact arrangement is not very important.  Clearly for eight engines, my simplified sketch of the inlet system is not enough.  The important factor is principle is that it is the whole inlet line to the first free surface that experiences those accelerations.  And minimising those pulsations minimise the forces which can shake machines apart.  As the accelerations are inherent in the motion of reciprocating machines, minimising the mass which must follow those piston movements is the best way to reduce those forces, along with multiple cylinders with cranks spread around the full revolution.

Of course, if the inlet line contains less mass, it just results in the pressure at the plunger face stays higher than the minimum.  In practice, they would not want to cut it too fine, as the bare minimum pressure at the piston would probably result in some cavitation, but that is a whole other topic, getting far from relevance to model engines.

On the discharge side, increased line lengths and the associated greater mass of water potentially leads to much greater acceleration forces, not limited by the low pressure issues of the inlet side.  No easy way to avoid including pulsation dampeners on the discharge side, even on much more modestly sized systems.

I hope I have adequately explained those acceleration losses and the reason for the pulsations dampeners.  Water is so important to us all, and it is always interesting to learn more about the complexity of supplying water to a large city.

I think I have exhausted the topic, but always happy to try and answer questions if required.

Thanks everyone for looking in

MJM460
Title: Re: Talking Thermodynamics
Post by: crueby on August 22, 2021, 02:43:42 PM
 :ThumbsUp: :cheers:
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 03, 2022, 12:18:12 AM
Hi MJM , It was good to see The Holly pumping engine that Chris has built working on air ...!!!!! I was thinking that would there be any advantage in using really hot compressed air as it is a 'heat' engine primarily ?!!  wishing you a prosperouse and productive new year
Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 03, 2022, 11:11:40 AM
Hi Willy, yes, amazing to see it all working.  It’s a huge project to scratch build, and reach this point.

And yes, you would get more power out from the same mass of air at higher temperature, even at the same pressure, but I find the air tables a bit hard to use, in order to find out how much more.  And of course efficiency reduces the amount of extra work you actually get.  I believe there are others on the forum who could do those calculations.  (Hint, hint!)

You can compare it with a steam cycle with reheat.  It’s clearly worth running the steam back to the boiler and running it through an extra coil to raise the temperature.  It would not be done if it was not worthwhile, though the economies of scale on a power station system might make worthwhile something that can never recover its cost or justify the effort on a smaller scale.

It’s hard to get your head around, as obviously the pressure does not increase as the steam or air go through the coils.  But the interstate pressure runs higher when the heating is applied.  There are two effects, the higher interstate pressure reduces the work obtained from the upstream stage but increases the work from the low pressure stage.  The gain from the low pressure stage is more due to the larger piston area.

In addition, with the higher temperature air in the lp cylinder, you get more work out before the pressure drops to the exhaust temperature, assuming the same mass of air.

But whether it would be worth making and firing a heating coil for the air?  I really suspect not.  It would be much easier to just turn up the pressure on the compressor if you need a bit more power from the machine.   But it’s an interesting theoretical question.

Glad your internet is up and working again.  It’s nice to have someone wash all your teaspoons for you, but a bit tedious having to go out every time you want to go on line.

Stay well,

MJM460



Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 04, 2022, 02:51:11 AM
Hi MJM , Thanks so much for the reply, and yes it is quite difficult to get one's head around !!!  It is so easy to ask questions  but quite difficult to workout the answers ?!! I think this is why scientists invent words with lots of syllables  that others can just learn and repeat !!! I suppose one could do an experiment by placing an engine in a large fridge  with the ambient air compressor running it ?? I don't quite know how you would measure the output power unless it was connected to a brake horse power device ?? it sounds like something they could do on the international space station. to while away there time  !!!??   Also would an engine have more power starting cold  with this reducing as the engine heated up ??  So I think this is what people mean about the "need to get out more"...I do have the benefit of the local university of east anglia on my doorstep where I could go to learn about everything that gets into my brain ?!!!..

Thanks  again

Willy
Title: Re: Talking Thermodynamics
Post by: MJM460 on January 04, 2022, 11:30:19 AM
Hi Willy, not sure that the fridge experiment would yield much.  Too many different issues introduced.

When thinking about these thermodynamic problems, we usually try and have only one variable.  So we might add heat with constant volume, or allow to volume to expand, hence doing work, but no heat input.  Much easier to arrive at the relevant equations that way.

But when you heat the air between two stages, you are adding heat to a fixed mass of fluid, air in this case, but also doing work, as the volume is expanding.  This is because the volume expelled from the higher pressure cylinder is less than the volume expanding into the larger diameter lp cylinder.  Hence depending on how much heat you are adding, the pressure increase due to heating might be less than the decrease due to the volume expanding, so the pressure still falls.  Or it might be enough to hold the pressure steady, or it might be enough to more than compensate for the volume change so the pressure becomes higher than it would be without the heat input.  You would be aiming for as much heat input as practical, so as to increase the engine output.  And as always, the actual efficiency of this process has to be confirmed by experiment, always less than the theoretical output from the heat in.

I am not sure what the numbers would be for an engine running on air, but on steam, the work produced by the extra heat input is much greater than if the same amount of heat was used to produce more steam in the boiler, as it does not result in more latent heat lost in the exhaust, which is the main reason the steam cycle efficiency is so low.  The energy used to evaporate the water in the boiler does become extra energy in the steam, but unfortunately it is not convertible to work, it goes out in the exhaust without contributing to the work done by the engine.

I hope that makes the picture a little clearer.

MJM460
Title: Re: Talking Thermodynamics
Post by: steam guy willy on January 05, 2022, 03:23:56 AM
Hi MJM. Thanks and yes a little bit more clearer, but there are still quite a lot of practical things that one needs take into account when doing the calculations !  Thankyou for your explanations  and time spent with my observations and questions .

Cheers

Willy

Title: Re: Talking Thermodynamics
Post by: MJM460 on January 05, 2022, 11:24:05 AM
Hi Willy, you are most welcome.

Your questions are always interesting and thought provoking.  Very far removed from the simple examples used in study courses with everything controlled and uniform.

But the strength of the study approach is that it provides some understanding if you break up the system into smaller blocks where the simple conditions apply.  So in this one, you would look at the ip end, where the piston is doing work on the fluid as the piston pushes the fluid out to the heater.  Then the heater, with fluid in, adding heat in the heater to give new conditions as input to the third section, the lp cylinder, where the fluid does work on the piston.

Then there are all the complications of valve timing which require breaking the system down further.

Plenty to ponder on cold winter evenings.  But here we are into the heat of summer and had a few days around 40 C already.

MJM460




SimplePortal 2.3.5 © 2008-2012, SimplePortal