Author Topic: Talking Thermodynamics  (Read 93446 times)

Offline MJM460

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Talking Thermodynamics
« 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
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Offline Jo

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Re: Talking Thermodynamics
« Reply #1 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
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Offline Jasonb

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Re: Talking Thermodynamics
« Reply #2 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.

Offline Gas_mantle

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Re: Talking Thermodynamics
« Reply #3 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)


Offline Jo

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Re: Talking Thermodynamics
« Reply #4 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
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Online Dan Rowe

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Re: Talking Thermodynamics
« Reply #5 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
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Offline Gas_mantle

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Re: Talking Thermodynamics
« Reply #6 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.




Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #7 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 !!!

Offline Jasonb

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Re: Talking Thermodynamics
« Reply #8 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

Offline Jo

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Re: Talking Thermodynamics
« Reply #9 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
« Last Edit: May 11, 2017, 02:42:27 PM by Jo »
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Online Dan Rowe

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Re: Talking Thermodynamics
« Reply #10 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
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Offline Jasonb

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Re: Talking Thermodynamics
« Reply #11 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 ;)
« Last Edit: May 11, 2017, 03:36:31 PM by Jasonb »

Offline paul gough

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Re: Talking Thermodynamics
« Reply #12 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.

Offline Maryak

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Re: Talking Thermodynamics
« Reply #13 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
« Last Edit: May 12, 2017, 12:41:56 AM by Maryak »
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Offline Flyboy Jim

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Re: Talking Thermodynamics
« Reply #14 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
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