Author Topic: Talking Thermodynamics  (Read 165440 times)

Offline derekwarner

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Re: Talking Thermodynamics
« Reply #1425 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:
« Last Edit: August 21, 2021, 12:08:45 AM by derekwarner »
Derek L Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Offline crueby

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Re: Talking Thermodynamics
« Reply #1426 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.

Offline MJM460

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Water hammer and pulsations
« Reply #1427 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.  Itís 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.  Itís 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 donít 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



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

Offline crueby

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Re: Talking Thermodynamics
« Reply #1428 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

Offline MJM460

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Water hammer and pulsations
« Reply #1429 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 donít 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
The more I learn, the more I find that I still have to learn!

Offline crueby

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Re: Talking Thermodynamics
« Reply #1430 on: August 22, 2021, 02:43:42 PM »
 :ThumbsUp: :cheers:

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1431 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

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1432 on: January 03, 2022, 11:11:40 AM »
Hi Willy, yes, amazing to see it all working.  Itís 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.  Itís 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.

Itís 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 itís an interesting theoretical question.

Glad your internet is up and working again.  Itís 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



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 #1433 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

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1434 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
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 #1435 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


Offline MJM460

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Re: Talking Thermodynamics
« Reply #1436 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




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