understanding a compound steam engine - knowledgeable person needed :-)

[picclock]

Hi

I understand the explanation of a compound steam engine but I don't understand why this makes the engine more efficient. Hopefully someone with more knowledge can enlighten me.

In a compressed gas engine the idea is to convert the energy stored in the compressed gas to mechanical motion as efficiently as possible. Taking a fixed volume of gas compressed to a given pressure it would seem (at least to me) that all the energy will be extracted when the gas pressure is equal to the external pressure when expanded, in most cases this is 1 atmosphere.

Assuming our fixed volume of gas is compressed to 10 atmospheres and the expansion ratio of the piston/cylinder combination is 10:1 all available energy from the gas will be extracted. I find it difficult to see how this can be improved on.

So what are the advantages of compound steam engine ?. The only thing I can think of in its favour is that most efficient energy extraction occurs when the crankshaft is at 90 degrees to the forces acting upon it. But with good bearings even this advantage would seem to be small.

Wikipedia says :-

In the compound engine, high pressure steam from the boiler expands in a high pressure (HP) cylinder and then enters one or more subsequent lower pressure (LP) cylinders. The complete expansion of the steam now occurs across multiple cylinders and as less expansion now occurs in each cylinder so less heat is lost by the steam in each. This reduces the magnitude of cylinder heating and cooling, increasing the efficiency of the engine.

My problem with this statement is that the temperature loss of the expanding gas is directly proportional to its pressure. The energy contained in the gas is also proportional to its pressure. So what it seems to be saying is that if you extract a small amount of energy the temperature loss is small, so extracting multiple smaller lots is better than 1 large lot.

One could argue that thermal leakage along the cylinder walls may make it slighly less efficient, but a longer cylinder would reduce this anyway. Likewise thermal losses through the piston crown could largely be avoided by the use of an insulated piston.

I'm ignoring the weight and friction loss considerations here because they would seem to make a compound engine very difficult to justify.

If anyone can throw some light on this for me it would be much appreciated.

picclock






 

[PTsideshow]

Don't get carried away, with theory and comparing steam to a gas engine

In the compound engine, high pressure steam from the boiler expands in a high pressure (HP) cylinder and then enters one or more subsequent lower pressure (LP) cylinders. The complete expansion of the steam now occurs across multiple cylinders and as less expansion now occurs in each cylinder so less heat is lost by the steam in each. This reduces the magnitude of cylinder heating and cooling, increasing the efficiency of the engine.

Bottom line and as basic as it gets you are squeezing the most work energy out of the steam by using the heat in it till it is not worth squeezing any more out. Google some old time steam books on line.

Makes no difference Locomotive, stationary, marine or road engine if its a compound cylinders. It all the same whether it dumped out or returned as feed water/ condensate. Get as much heat  work/energy out of it as you can.

No steam engine was ever a model of efficiency. and since the cylinders are hot it runs great after start up.

34 issues on 1st class stationary engineers license, for unlimited pressure or horse power,Boilers, Engines and Turbines  City of Detroit and State of Michigan

[Rob.Wilson]

Hi Picclock


When the steam enters the high pressure cylinder  it expands  and moves the piston to the end of its stroke , but there is still work left in the steam , so then its moved on to the next cylinder , the steam is now at a lower pressure ,, so this cylinder is LARGER in diameter to get the same amount of work from the lower pressure steam  ,,, force = pressure x cross-sectional area   .

In the case of a triple expansion engine the low pressure cylinder is working below 1 bar   

Rob

[cidrontmg]

In the case of a triple expansion engine the low pressure cylinder is working below 1 bar   

Rob

To get some work from the low pressure cylinder, when it is working below 1 bar, necessitates a condenser, so that the cylinder exhaust enters into a partial vacuum. And there the steam finally turns to water. The water from the condenser can be used again to feed the boiler, except it is rather dirty. It carries with it remains of lubricating oil/grease, some rust particles,etc, so it will need filtering and oil separation.
BTW. Running compound engines with compressed air is rather useless. Just about all the work is done in the high pressure cylinder. The lower pressure cylinder(s) are mostly a source of extra friction, and compound engines usually run better on compressed air if you open the LP cylinder drain cocks   

[picclock]

OK I think I get it now.

More energy is extracted by condensing the steam once it reaches low pressure, rather like a newcomen engine. as the steam is condensed and changes state this causes a lower than atmospheric pressure to be produced, adding to the energy output and offsetting the latent heat of vaporisation.  Whilst an extra few psi would not make much difference in our high pressure world, with systems running at 50 psi or less this would be a significant improvement.

At least that makes it make sense to me. I was never really interested in building or running a compound engine, its just somthing thats I've been thinking about it off and on for at least a decade or two (I'm a slow thinker) but could never find an answer that made any sense.  Kinda like an occaisional itch :-)

Many thanks for the info.

picclock