# Let's talk steam

A conversation with a friend today, brought up the question of how well a steam engine runs on compressed air: That answer is, "It will run on air
but not very well compared to performance on steam".
My question is this: Does anyone have a simple compressed air/steam "rule of thumb" ?
Now, I know this gets into all sorts of complex thermodynamic calculations. For example, the Brake HP of any engine is a direct function of pressure. Pressure, however, in order to fit into conventional formulae must be given in Mean Effective Pressure (MEP). Enter hairy thermo-math here. MEP would be a sort of integral (mean) pressure in any heat engine. The type of engine, amount of moisture in the steam, percentage of cut-off, insulation of cylinder walls, size of passages including valve openings, on and on, etc., etc., to nauseam, all enter into MEP. The old timers, at least those mentioned in "Modern Locomotive Construction" circa 1892 (sold by Lindsay) commonly used 90 psi as the MEP of a representative locomotive of the time. So much for the math. Don't send me any formulae for calculating MEP - I've got that. I'm looking for shortcuts, here, thank you.
What I'd like to see is a comparison of the HP output of a steam engine running on a given amount of input (boiler) pressure compared to the HP output of the same engine running on the same amount of input compressed air pressure.
Analyze this from the standpoint of engine performance only, neglecting boiler HP or compressor HP.
Bob Swinney
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To be fair I think you need to figure in some kind of flow rate as well in this, ie 50 psi air at 10 cfm, vs 50 psi steam at such and such a boiler water feed rate.
Jim
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Jim sez:
"To be fair I think you need to figure in some kind of flow rate as well in this, ie 50 psi air at 10 cfm, vs 50 psi steam at such and such a boiler water feed rate."
Each component is considered to enter the engine through the normal design passages and at a volume of flow consistent with what the engine can take. The boiler and the air compressor are adequate to "keep up" with the demand of the engine. We want to know how the output of the engine compares with the same amount of input pressure from steam as from air.
Bob Swinney
Robert Swinney says...

air
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I have measured the output of my 2 cylinder model engine on both steam and air at the same pressure using a generator as a load. Measurements at 100 PSI and load adjusted via excitation to give the same RPM. Results are steam 5 to 10 % greater than with air. I attribute this to the oil viscosity change with temperature since the same oil and oil feed rate was used for both tests. Cylinders were noticeably chilled when running on air. Steam was superheated to 500 to 600 degree F area.
Joe Hanulec
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Joe sez:
"I have measured the output of my 2 cylinder model engine on both steam and

air.
Joe, Good information. Do you know if the engine was running "valves wide open" or if there was a cut off point such as 50% of stroke or etc.; and if the same cutoff point was existent in both tests?
Bob Swinney

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To make things somewhat simplistic when superheated water turns to steam and is allowed to escape it will expand something like 1600 times(this is why a dry crown sheet is deadly) its volume as water. When you compress air say too 150 psi that is only 10 times the general atmophieric pressure. Compressed air quickly looses its power when allowed to expand in the cylinder where as steam keeps pushing as it expands. We are currently building a 12" guage steam locomotive and have it running on compressed air at the present time. It will run nicely down to about 25psi, but of course this is wheels up and no load. Our hope is to have at least a running boiler pressure of 150psi if not alittle higher.
tim
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On 10 Sep 2004 23:08:59 GMT, snipped-for-privacy@aol.com (TSJABS) wrote:

That's true, but that happens in the boiler where the *liquid* water volume turns to a *gas* (steam) volume. Once that happens, the flow of gas (steam or compressed air) into the engine is determined by the pressure (same for both), the valve aperture (same for both), and the valve opening duration (same for both). So the fact that the volume of water expands 1600 times when it changes from liquid to vapor is irrelevant for comparing engine performance.

The same is true for an equal volume of non-condensing steam at 150 PSI. 150 PSI is 150 PSI.

Both continue to push equally until pressure falls to atmospheric.
Now if both start out at the same temperature, then both will reach atmospheric pressure after the same amount of expansion. But we know that the steam is at a temperature of at least 681 R while the compressed air is at tank temperature, which for a big enough air tank is close enough to room temperature to use that number, ie 469 R. And we know that PV=nRT.
So, if the engine expansion ratio is large enough to allow both gases to expand to atmospheric pressure, the advantage for steam is crudely the ratio of the working gas temperatures. 681/469 = 1.45
Of course if the expansion ratio is less than 10 to 1 for an engine working with a 150 PSI input pressure, there won't be any observable advantage for steam over compressed air, since the expansion ratio will only be enough to expand the compressed air to atmospheric, and not enough more to take advantage of the higher temperature of the steam.
Gary
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Gary sez: " Both continue to push equally until pressure falls to atmospheric."
They do not continue to push equally. The heat energy contained in steam equates to more "push" if both steam and air are at the same pressure at cutoff. Air pressure degrades rapidly after cutoff. Steam pressure after cutoff falls more slowly because of contained heat. Perhaps my original question would have been more relevant if I had said, "Oh, BTW, the steam is superheated as it enters the cylinder".
Bob Swinney
(TSJABS) wrote:

steam and

a dry

volume
steam
working
enough
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Bob,
The cutoff was the same for both air and steam, my recollection is 75 %.
Joe
"> Joe,

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Thanks Joe, I'm guessing that would account for the slight amount of difference for steam. Bob Swinney

the
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Bob, you really have answered your own question. HEAT! A steam engine is a heat engine. This is the BIG difference. Air has NO heat to give up. That's it in a nut shell. RichD
On Fri, 10 Sep 2004 13:36:57 -0500, "Robert Swinney"

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RichD sez:

Yeah, but: Compressed air will follow the piston until the point of cutoff. From cutoff until the end of the stroke, the volume of air, trapped in the cylinder can do little more work as the piston moves away and increases volume in the cylinder. For all practical purposes, the air is "dead" at the point of cut off. Contrast this with live steam. Steam at boiler pressure pushes the piston, much the same as air; but at the point of cutoff the steam and cylinder is still hot (it has lost some heat) and is still expanding, doing more work against the piston. Performance after cutoff is one of the fundamental differences between compressed air and steam in a steam engine. I would like to know if there is an easy "rule of thumb" that addresses this and other differences between the performance of compressed air and steam at the same input pressure.
<RichD> wrote in message wrote:

"rule
calculations.
given
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time.
I've
air
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"Robert Swinney" snipped-for-privacy@comcast.net

One of the delights of early/modern steam technology was the indicator--done up with literature and hardwood case--that produced a stylus's readout of engine performance. Perhaps you might use one (or a modern substitute) to compare the steam/air results on a given engine. Frank Morrison.
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Bob,
I don't agree with the statement that the air is dead. At cutoff the pressure in the cylinder with air or steam is still 100PSI (actually somewhat less but lets say 100PSI) now as the piston continues to move and increases the volume the pressure will drop for both steam and air in accordance with gas law pv=nrt. Steam doesn't any magical properties and in fact if not superheated it will begin to condense and not perform as well as air.
Joe

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Joe sez:
"I don't agree with the statement that the air is dead."
My statement was that the air is dead at cutoff for all practical purposes. Air pressure will fall off rapidly as the piston retreats and increases cyl. volume. Steam in a hot cyl. is still expanding and will continue to do work against the piston. This would, I believe, account for the small difference (steam over air) you reported at late cutoff. I wish there was some easy way to estimate these effects.
Bob Swinney

the
at
is
compressed
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Sure thing, PV = nRT
n is the same for both the compressed air and dry steam at the same entry pressure and flow. R is different, but not a whole lot different as long as the steam remains hot enough to be non-condensing in the cylinder. T is very different for the steam and the compressed air. So the ratio of temperatures will give you an approximate ratio of relative performance after cut off.
Gary
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If ratio of temperatures governs relative performance, then hot compressed air would work better than cold compressed air?
In any case, PV=nRT relates to an isothermal (constant temperature) situation. Expansion after cutoff in a steam engine is usually regarded as adiabatic rather than isothermal expansion. In adiabatic expansion the specific heat of the substance is relevant. Specific heat of steam may be quite different than that of air.

pressure
steam remains

the steam

approximate
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On Sat, 11 Sep 2004 10:41:47 +0000 (UTC), "Don Foreman"

Yes.
Indeed it is. But the important number with respect to the work done by Carnot cycle engines is the ratio of specific heats of the particular working fluid. Gases have different specific heats depending on whether the specific heat is measured at constant volume or constant pressure. The ratio of these two values is called gamma. For air it is 1.4. For steam at 150 PSI it is 1.28.
T1 and T2 are still the dominant numbers (T1 is inlet temperature, T2 is outlet temperature, usually assumed to be ambient), but gamma does play a role in the process. Gamma appears as an inverse exponent in the Carnot equations. So the closer to 1 it is, the better. The ratio of gammas for steam and air says that steam should be a 9% better working fluid than air at the same working temperature.
Note that I'm assuming non-condensing operation. If the steam is allowed to condense in the cylinder, then latent heats also have to be considered.
Gary
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I think the difference in gammas tells the tale.
I'm about 4000 miles from my Machery's Handbook just now, but I think change in pressure and volume will dictate outlet temp as fn of input temp so actual inlet temp for a given adiabatic expansion is immaterial -- providing of course that steam does not condense. Outlet temp can be above or below ambient.
wrote:

compressed
as
be
working
specific
these
1.28.
outlet
air says

working
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On Sat, 11 Sep 2004 23:08:43 +0000 (UTC), "Don Foreman"

Outlet temp can't be below ambient unless external work is applied to the piston to expand the gas below ambient pressure. Otherwise, you'd be violating the Carnot limit.
Gary
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