Author Topic: Talking Thermodynamics  (Read 93535 times)

Offline MJM460

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Re: Talking Thermodynamics
« Reply #15 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
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Offline Flyboy Jim

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Re: Talking Thermodynamics
« Reply #16 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
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #17 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.

Offline paul gough

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Re: Talking Thermodynamics
« Reply #18 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.

Offline Steamer5

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Re: Talking Thermodynamics
« Reply #19 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
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #20 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.

Offline Jasonb

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Re: Talking Thermodynamics
« Reply #21 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.

Offline Zephyrin

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Re: Talking Thermodynamics
« Reply #22 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.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #23 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
« Last Edit: May 13, 2017, 01:29:12 PM by MJM460 »
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #24 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.

Offline Flyboy Jim

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Re: Talking Thermodynamics
« Reply #25 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
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Offline Steamer5

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Re: Talking Thermodynamics
« Reply #26 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
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #27 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.

Offline 10KPete

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Re: Talking Thermodynamics
« Reply #28 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
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Offline Steamer5

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Re: Talking Thermodynamics
« Reply #29 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
Get excited and make something!