Steam Expansion

Suppose I fill a chamber (1.7446 cu. ft.) on top of a piston with, say 250 PSIG steam (1.7446 cu. ft. per pound) then shut off the steam inlet valve and let the pressure move the piston full stroke to a "expose" a total cylinder volume of 32.7174 cu. ft. (the volume of steam at 5 inches Hg vacuum).

a number of things. efficiency of your system just to name one.

And remember if the volume inside your cylinder goes into vacuum, the steam at just below 212 degrees F will not change phase to a liquid. At 28" Hg vacuum it will remain as steam to as low as about 120 degree F.

Check out a WASP program for that to find out what 5 Hg is.

p911

I am fully aware of the nature of steam, and especially in a vacuum condition.

My question is "what will happen?"

If the steam stays non-vacuum, and just gets cooler, then the efficiency will by based on the HP formula of "(pressure X area X distance traveled in the stroke (in feet) X number of strokes per minute X number of active pistons) / 33000 pound-feet per minute" will use the AVERAGE of the initial pressure (250 PSIG) minus the final pressure (0 PSIG) or 125 PSIG as the pressure for the full stroke. Nothing here affects efficiency, just the total amount of power I can get out of the engine the contains the piston.

Which goes back to the question- What will happen?

how such condensate is normally drained above the cylinder? For safety, has it to be horizontal cylinder for quick draining?

I agree that it is likely that, given the conditions I proposed, a vacuum will likely be drawn. Still ot sure whether any condensate would be created, due to vacuum conditioning likely reflashing any condensate back into vacuum steam.

That is why I posted this thread- to see if my "fears" would play out.

SO- FOR THE REST OF THIS DISCUSSION- LET US ASSUME THAT THE TOTAL VOLUME OF THE CYLINDER IS NO MORE THAN THE VOLUME OF ONE POUND OF STEAM AT ZERO PSIG (28.8879 Cu Ft) - or perhaps just slightly less than that value so the steam cools and drops pressure due to expansion, but stays steam under non-negative pressure.

Does everyone agree that the vapor will stay as steam, and that the exhaust gas will be about 212F, 0 PSIG?

Google "adiabatic expansion".

Having trouble getting past this part: "250 PSIG steam (1.7446 cu. ft. per pound) then shut off the steam inlet valve and let the pressure move the piston full stroke"

Either the piston is locked in place or it takes 250 PSIG to move it. Other wise the piston would begin to move at some point below 250PSIG, proir to valve shut-off. Maybe it does and maybe this doesn't matter.

Once the valve is closed, how can any "work" occur? How much force is required to move the piston?

Are you working on a steam driven supertanker? Otherwise, how will you fit an engine with a single cylinder displacement of 28.8879 cu. ft. into your flivver?

This is over my head.

I have read up a bit on steam and will try to comment, but beware that I'm not all that familiar.

As I understand it, the theoretical upper limit on Rankine steam cycle efficiency depends on the two temperatures (evaporating and condensing). For 565°C steam and 30°C condensing, this is 63% (higher temperatures can't be handled by the usual materials). The best cycles so far obtained commercially are around 42%, so there would seem to be room for improvement.

That said, I don't understand how something like 4% of "normal" flow (even at 100% efficiency) could equal 100% of "normal" flow at 42% efficiency. Steam has only so much energy in it, even under perfect conditions.

The idea of expanding in stages, rather than all in one fell swoop, probably has advantages (similar in reverse to those of multistage refrigerant compression). These advantages will be partially offset by the increasing size of lower pressure expansion devices (engines/turbines) that will be needed to handle the same mass flow.

All in all, I suspect that any gains will be incremental, at some cost in complexity.

I have heard only a little about "uniflow" or "crossflow" or stratification, usually without diagrams that specify the anticipated PH/TS conditions at each step.

Another thing I'm unsure of is whether the arithmetical average of pressure is correct throughout the piston stroke; instead it might be the logarithmic average, similar to LMTD in heat transfer calculations.

In reply to Item 9 that won't be deemed "off topic"-

Although several folks found your thread off-topic, I believe that it was a real-world case of exactly what I THOUGHT might happen, and agreed that I needed to use adequate steam volume and/or small enough cylinder displacement to avoid what happened to you before you "knew better".

Thanks for the input. You have confirmed what "the numbers" have predicted before I built a larger version of your toy to confirm the same results.