Steam engines at altitude

You do wonder if the fire would not burn as hot as at sea level...

I know from cooking at high altitude that it takes longer because water boils at a lower temperature.  It may be that steam, once made, still has the same expansive power when reaching the cylinders.  Creating the steam may be another issue.

Great question. I'll bet that Rich Melvin can enlighten us.  If anyone knows anything about applied physics its Rich.  I do know that internal combustion and external combustion engines work in different ways.  Automobiles engines and steam engines are a good examples  to compare.

What do yo say Rich?

Earl     

I operate a fire pumper steamer at 8000 ft. In Colorado.  Water boils at a much lower temperature, about 195f, and there's less oxygen for the material being burned to use so less btu per hour being produced in the firebox.  I've noticed it takes longer to get steam up but the energy in the boiler of the steam at a given pressure should be the same.  We don't push the steamer hard so I've never experienced reduced performance since we are only operating at a small percentage of capacity for display.  I would guess that a locomotive designed for elevation would need a bigger firebox to produce the same power as one designed to operate at sea level.  The effect of colder weather would certainly be a factor in the winter, that's one of the reasons MR went electric in Montana and Washington.

@Dougklink posted:

The effect of colder weather would certainly be a factor in the winter, that's one of the reasons MR went electric in Montana and Washington.

Interest observation.  I do know that colder air is better for internal combustion engines, that's clearly demonstrated with flying anything with a piston engine.  Humidity also enters into the picture, dry air is denser than moist air.  On a hot and humid day, all those factors as well as the air density conspire to make takeoff runs significantly longer than on a cold and dry winter day.  Also, once you're airborne, rate of climb is much better on that cold and dry day.

@Dougklink posted:

I would guess that a locomotive designed for elevation would need a bigger firebox to produce the same power as one designed to operate at sea level. 

I doubt that there has ever been a steam locomotive in the US, or maybe anywhere, "designed for elevation."

An interesting discussion of steam on Peruvian railways at 14,000 ft can be read in this thread.  In short, a firebox has plenty of air feeding combustion and altitude has a negligible impact.  In contrast, internal combustion engines are constrained by the cu. in. of the cylinder during the intake stroke.  Density affects in the amount of oxygen available to burn in a closed cylinder.

Yeah in an internal combustion engine colder temps mean a more dense air charge, it is why they put coolers on turbos and superchargers. They compress the air charge, but also heat it up at the same time,which negates some of the boost.

I tend to agree that at high altitude the fire would heat up more slowly,will burn less intensely, which could lengthen fire up times. The other thing would be that where water is in tubes on the engine, if the engine goes cold in very cold climate, did they keep a fire up to keep water from freezing? Or could they drain it? ( Not really altitude related obviously w cold weather).

The steam itself would not be a problem, in that boilers operate under pressure and the steam is super heated ,way above 212 ( no different than a pressure cooker).

The colder temp in theory could make a steam engine more efficient in cold weather applying the thermo efficiency formula egg= t2-t1/t2,where in a heat engine t2 is temp at full value, t1 is starting. Given full efficiency is t1= absolute 0 ( 0k,-273C), it likely isn't a lot. For example, the efficiency at 15C ( 60f) versus -20C ( 0F) ,which is 35C drop, would likely drop T1 maybe a couple of degrees C at most I would guess ( and that is all it is,a guess..never looked at an external combustion engine like that, and I could easily be wrong, physics is a long time ago).

@Tracker John posted:

An interesting discussion of steam on Peruvian railways at 14,000 ft can be read in this thread.  In short, a firebox has plenty of air feeding combustion and altitude has a negligible impact.  In contrast, internal combustion engines are constrained by the cu. in. of the cylinder during the intake stroke.  Density affects in the amount of oxygen available to burn in a closed cylinder.

That was a great thread and it pretty much answers my question, that even the less O2 available with the firebox doesn't really alter anything. If they can operate at 14k feet ( they consider 10k feet the useful limit of breathable air) , then that answers my question.