Water will boil at a lower temperature when air pressure is lowered. If the air pressure is lowered enough, water will boil even at room temperature!
What would happen if I built a model freight train with a working steam engine, placed the train in a large sealed chamber, and lowered the air pressure in the chamber. At some point, the water in the boiler would boil simply from the ambient air temperature alone.
Would the little train roll along the tracks forever without another fuel source?
[Homer]
Lisa! Get in here.
In this house, we obey the laws of thermodynamics!
[/Homer]
Where is the energy to condense the water vapor going to come from?
Also, if you lower the pressure in the boiler to the point where the H[sub]2[/sub]O turns gaseous, won’t that eventually raise the pressure in the boiler? Eventually, the pressure will be back to about where you started from, at an apparent equilibrium.
The laws of Thermodynamics are safe. The energy to boil the water (and therefore move the train) is far exceeded by the energy required to reduce the air pressure in the sealed container. No first or second law violations that I can see.
Here’s how it works:
(I could do the math, but choose not to. Trust me, I took a lot of thermo & physics in school.)
Well, let’s think about things here a bit.
We are assuming that the train carries a bit of water, that is uses to “boil off” under low ambient pressure to drive the train.
The first thing that comes to my mind is - how does the boiling water act on a piston? You see, let’s assume that the ambient temperature is 98.24 F (it works well in my Steam Tables). Now, at this temperature the saturation pressure is 0.9 psia - thus, you need to exhaust about 13.7 psi of air from the chamber at sea level. Now, let’s say the water starts to boil - in doing so, it’s volume will increase by a factor of 22,855:1 (at this pressure, the specific volume of saturated water is 0.01612 ft[sup]3[/sup]/lbm, and the specific volume of saturated steam is 368.43 ft[sup]3[/sup]/lbm). Now this volume increase is pretty large, but it acts with an initial force of only 0.9 psia to start. To allow it to have any force on the piston, it seems to me that you would have to increase the temperature of the water, since you would have to be able to allow the steam to enter the piston at some pressure higher than atmospheric (for the system, mind you), or else what is the incentive for the piston to move? Or is the steam boiling going to be able to move the piston, even though in trying to move the piston it’s pressure must increase as it meets the resistance of the piston (talking real pistons here, not frictionless ideal pistons)?
Now, if you had a condensing system that placed the piston’s backside under negative (relative) pressure, maybe. But unless I’m missing something here (and I may very well be) I don’t think this system would work well at all, or generate any appreciable power if it did. Comments anyone?
Well, we do have the big vacuum pump sucking the air out of the chamber. One thing I omitted is how to lower the pressure in the boiler (duh). I also didn’t walk across the room to get my Thermo book with the steam tables (I’m so lazy, sometimes).
This would work a lot better with a turbine, instead of a piston. As the air is sucked out of the system (the sealed chamber), air also moves out of the boiler, through the turbine and into the chamber. This will move the train, some. Eventually we get below 0.9psia inside the boiler, and the water starts to boil. It expands, and moves through the turbine to the chamber. This goes on until A) the water runs out or B)the pressure of the entire system is above 0.9psia, so the water won’t boil anymore. To get it to operate, the pressure either has to start out well below 0.9psia before opening the boiler discharge valve (hmm… another complication) or you have to keep sucking the air out of the chamber, keeping it a little below 0.9psia.
As for the Laws of Thermodynamics:
1st law: Energy is conserved. The energy to move the train comes from the energy expended to suck all of the air out of the chamber.
2nd law: Entropy of the system does not decrease over time. Goes from A) organized system with low pressure air outside boiler/turbine and low pressure water inside boiler to B)more disordered system with water originally in boiler spread throughout system.
Of course this is simplified, but theoretically breaks no laws. And no, it probably wouldn’t work very well, and wouldn’t “generate” any more power than was originally put in to the system by sucking out all of the air (a pretty wasteful process to start out with), even with some of those “frictionless bearings,” and “ignoring air resistance” (things were so much easier in college).
I know quite a bit about Thermo. I’m not questioning anything about either conservation of energy or entropy production and reversibility. I’m wondering out loud if the system would even work at all IRL, and if so to what extent.
High temperature steam has more energy in it than low temperature steam.In fact serious steps are taken to increase pressure to get a higher steam temperature, way beyond atmospheric boiling.This steam is at super heat or super critical heat depending on the specific system.(Can’t remember the temperatures right now)
Steam turbines in power plants have at least three stages where the steam is directed onto the blades, all on the same shaft.Between each stage the steam is dried and there might be some further energy input.
If you look at a steam turbine you will see that the first stage turbine blades are much smaller than the third stage turbine blades but they still provide as much force, if not more than the much larger third stage blades.
This is also true for triple expansion reciprocating steam engines.Here the first stage piston is much smaller than the later stage pistons.
Making steam at reduced pressure means low temperature steam, it simply cannot give up energy it does not have.
In order to have a pressure differencial across the piston there would have to be a closed to atmosphere condensation system at the outlet valve end, you could not vent to atmosphere as steam engines generally do.
I’m trying to imagine double action piston systems with valve operated condensors a bit like the Newcomen pumps used in Cornish tin mines.
To make those condensors work the medium flowing through the heat exchanger pipes would have to be low, low temperature brine probably.This then begs the question of how you get that brine to such a low temperature, this would require a refrigeration unit such as a large chiller or absorption unit.These things are not cheap to run and consume energy in their own right.
I’m also wondering about using differant liquids such as modern refrigerants but in any case the energy consumed in producing the vacuum will outweigh any gains you might make.
From the OP, with my emphasis added:
What part of ‘roll along the tracks forever without another fuel source’ doesn’t rhyme with perpetual motion? Not that the Little Engine That Might won’t scoot along the tracks for a bit. No problem. But forever? If we keep lowering the pressure of the system, we don’t quite have a sealed chamber, and we need a fuel source to do that work, and that is ‘another fuel source.’
Of course, I only have a science minor (and in environmental science at that!) so I shouldn’t really argue with the engineering big guns of the SD. I have no steam tables to look through, nor thermo charts. But I do have thermal underwear, a left-handed steam-shifter, and a yearning to stamp out my own ignorance. What am I missing? Thanks folks!
You guys are SO ready to be Junkyard Wars contestants! Glad I took a thermo class as an elective with my economics degree - I got tired of all the refrigerator repairmen working for my dad talking above my head.
The consensus among four fellow engineers I have discussed this with here (and yes, I actually reserved a room and called a meeting, which shows you my clout 
) is that:
And no, I’m not talking about any god-damned cases that violate the 0th, 1st, 2nd, or 3rd laws of Thermodynamics, thank you.
I chucked my steam tables about 5 minutes after my last thermo exam - and I ain’t never worked on a steam powered airplane, but here goes
Watt’s the problem here?
My conclusion first - I think such a steam train has a marginal chance of running, but only 'till the water ran out, or the chamber pressure equalized, like Tex says. I also think a steam turbine would work better.
My thinking: This is a great question, mavpace - right up there with “how much does a truck full of flying parakeets weigh?”
Anthracite’s thinking (from her first post & e.g.) that you’re just going to take the chamber pressure down just to the point where the water starts to boil. That means the boiler has to be vented to the chamber, and as long as that’s so, you’ll never get any head to pump a piston [biting lip over oral sex joke]. So you got to close off the boiler to get some head - and as soon as you do, the only incentive for boiling (low pressure) is gone. At best you’d get a pathetic chuff out of this thing and you’re done.
But I think Tex is thinking that you’d take the chamber pressure down way lower, giving yourself a chance to have a low enough pressure in the boiler for sustained boiling, and a decent pressure differential w.r.t. the chamber to move a piston (turbine, whatever). Tricky, but it just might work for as long as the pressure difference holds or you still have water to boil.
Nobody thinks this thing will run forever.
Now where’s the energy come from (if this thing runs at all)? Taking Anthracite’s example numbers. Let’s say you pull the chamber down to 0.7 psia and the steam boils off at 0.9 psia. You’d have a petty 0.2 psi to move something with, tops. But what if we take the water from a tea kettle that’s just about to boil at normal conditions? That 211 F water should put off steam in the chamber at nearly 14.7 psia, right? (I’m goin’ out on a limb now - pull me back if you think I’m shakey). Then we’d have a respectable 14 psi to pump with, at least for a while.
So, unlike Tex, I’m thinking that the energy to run anything here has to come from the thermal energy in the water itself. Once that’s gone, either because the water is boiled off, or the chamber pressure rises, you’re going to be all done. I really want to use the word “enthalpy” here, but I think I’d be blowing smoke.
And, no way you’re going to get any decent performance out of this set-up. The energy from a normal steam engine comes from the fire-in-the-belly burning of some real kind of fuel, which we just don’t have here. We’re only moving the little energy that’s in the water and distributing it thru the chamber. I don’t think the energy needed to bring the chamber down has a direct bearing.
I have more to say on this, but dinner’s calling…
OK. So I was lazy in not consulting my steam tables (busted earlier), AND I was lazy for not reading the OP too closely.
My point was: not having anywhere to condense the steam to will not be a major sticking point, as far as the laws of thermodynamics go.
Actually, it could run forever, once it got started, if you use my patented “frictionless parts[sup]TM[/sup]” and my new method of “neglecting drag[sup]TM[/sup]”. Sorry, but I am restricted to selling these only to people who have taken a physics class, but not yet graduated and got yourself a real job.
As for not having to keep lowering the pressure of the system. If we start out with the pressure significantly below the pressure at which H[sub]2[/sub]O will boil at the temperature of the system, before opening the boiler to that pressure by remote control valve, there will be a lot of potential energy stored in that low pressure water. Hell, start out with an “absolute vacuum[sup]TM[/sup]” in the chamber, while you are at it.
casdave
you are talking about the “real world.” That has no place in a theoretical discussion :).
Anthracite
You are truly an unholy dark queen vampire etc.
You made me walk across the room to look in one of my old college texts to remind me about Law #0 and Law #3. I guess I’ve been working in the real world for too long… 0th is just common sense (that’s why they had to go back and add it to the list before the first law), and the 3rd has absolutely zero application if your temperatures are not near absolute zero (at least none that I can think of offhand).
“zeroth” law- If everything is in thermal equalibrium throughout the whole shooting match, nothing will happen. That’s why I had to through in that “isenthalpic valve[sup]TM[/sup]” to keep the boiler out of equilibrium with the system while the chamber is being evacuated.
third law- I hope nothing is approaching absolute zero, or we really will need an absolute vacuum to boil the water.
I want to thank everyone that has posted thus far for their serious and very insightful replies. This question is one that I thought of a long time ago and has been burning me for quite a while. I thought that I might be missing something obvious but now I see that my setup has at least a chance in hell of working. I am not an engineer so I appreciate the expert feedback. Please keep the theoretical analysis coming. I may need to just build the thing to see what happens if all else fails.
Why not just build an old fashioned steam engine. Contrary to my earlier post, I’m out of stock on “frictionless parts[sup]TM[/sup]” right now. Without those, your vacuum train doesn’t have much chance of working. (See all the above posts by kill-joys that want everything to work in the real world). 
You’ve just gotta get a better catalog, TX. Heck, my old AP physics class was full of that stuff.
Contrary to what I implied in my OP, I am not interested in building a perpetual motion generator. That would be a little over ambitious at this point I just wonder if the thing will work at all.
True, but the 3rd law helps to show that absolute zero can serve as the reference point for determining the absolute entropy of a substance. So it has some value at all temperatures.
Such a coincidence - just yesterday I researched the 3rd law again and wrote a mini-piece on it which I e-mailed to pepperlandgirl, so it is still very fresh in my mind.
!!! 
!!!
Ok so this isn’t so much a steam engine per se, but if we have a tank of water and a vacuum chamber, could we make a ‘shotgun’ train? Nozzle out the bottom of the tank, small valve at the top. Open the valve at the top, water begins to boil out creating pressure against the lid, pushing out the nozzle. Have it hit vanes on the axle, and away we go! Ok, so this isn’t so much of a good idea, but the research would be fun.
I wish I had a little more time to post on this. Now I’m thinking that I’ve got some friends over at NASA Marshall who’ve worked on the ISS. I bet they have had this discussion forwards and backwards.
Think about it, take yer old steam locomotive, kick it out the bay of the Space Shuttle into the vacuum of space, and let it chug itself to the moon, Alice!