Author Topic: Talking Thermodynamics  (Read 136047 times)

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1290 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 donít 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
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1291 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

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1292 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


Offline crueby

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Re: Talking Thermodynamics
« Reply #1293 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

Offline crueby

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Re: Talking Thermodynamics
« Reply #1294 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.

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1295 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

Offline crueby

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Re: Talking Thermodynamics
« Reply #1296 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?

Offline crueby

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Re: Talking Thermodynamics
« Reply #1297 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/

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1298 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 compressorís 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

The more I learn, the more I find that I still have to learn!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1299 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 Willyís 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

The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1300 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

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1301 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


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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1302 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 friendís 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

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1303 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


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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1304 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
« Last Edit: March 10, 2020, 03:23:25 AM by steam guy willy »