Let's talk steam

Mrs. Harper was unique. When she was teaching basic machines she had an old Crosley engine that she would assemble and crank up in front of the class. When she taught centrifugal motion you had to stay awake or you might get hit by a flying ball. And what she could do with a neon sign transformer was awe inspiring to a 9th grader. :-)

No way she could do that in a class room these days.

Reply to
Glenn Ashmore
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Don sez: "If efficiency is not an issue, running a steam-type engine on compressed

Don, my disclaimer re. thermodynamics set all the issues of efficiency, transport piping, etc., aside. My question was simply about comparing the performance of a steam engine ran on steam to that same engine ran on compressed air *at the same input* pressure.

Bob Swinney

calculations.

Reply to
Robert Swinney

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

Reply to
Robert Swinney

Gary sez: "Sure thing, PV = nRT

Thanks, Gary! Totally agree; the ratio of temperatures is the key. The % stroke, (timing of cutoff) it seems, would play greatly into this. Very hot steam with an early cutoff would have more time to do its thing. The steam "advantage" is enhanced by early cutoff.

*Now if we could only quantify all this - even in the most rudimentary (rule of thumb) terms.*

Bob Swinney

Reply to
Robert Swinney

Don sez: "> Is it possible that some droplets of superheated water enter the cylinder

Careful Don! You are about give away Robert's secret thermonucleur reciprocating engine theory. Oh, alright! So it is a nucleur heated cylinder with water injector. OK?

Bob Swinney

Reply to
Robert Swinney

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

Reply to
Gary Coffman

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.

Reply to
Don Foreman

Thanks to all respondents to my question re. a rule of thumb for performance comparison of steam vs. air at the same input pressure in a steam engine.

After reading all the insightful replies, I am now more convinced than ever, there is no such rule of thumb. Not even close. There are so many variables that guesswork and gospel become intermixed. I broached the question with the disclaimer it not be approached via the rigorous math of thermodynamics. Seemingly, from most responses, there is no other way. Had the question been given more thought it would have been obvious (to me) there is no easy way to quantify such disparities as air and steam - in a steam engine. The very idea borders on sacrilege!

In summary, the excellent responses boiled down to:

Air and steam at the same pressure input to a steam engine, with temperatures of each as they come from respective generators, yield different performance outputs. The amount of difference is not quantifiable without much more general information and a trip through the "thermo-math" jungle. The generalized answer was that "steam is better than air in a steam engine". Most responses reinforced the notion that the point of cutoff plays an important role in any such evaluation. Intuitively, at the same cutoff point (original question criteria) steam performance is "better" than air. It appears that an early cutoff for steam and a very late cutoff for air would tend to place both entities in the best scenario. Those scenarios, however, violate the basic premise of the question.

Again, thanks to all!

Bob Swinney

Reply to
Robert Swinney

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

Reply to
Gary Coffman

Sure it can, if pressure is higher than ambient at ambient temp as in most compressed air systems. Air tools get cold while in use, right? They typically use vanes rather than pistons, but expansion is still going on.

Reply to
Don Foreman

A compressed air system has external mechanical input to create higher than ambient pressure at ambient temperature. A heat engine, such as a steam engine, doesn't.

Gary

Reply to
Gary Coffman

Right. Bob's original question had to do with an expansion engine running on air vs running on steam, both coming from an external source at given pressure without regard to how it got there. The Carnot limit, dealing with efficiency, is irrelevant here.

Reply to
Don Foreman

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