steamers at altitude

[marknewton]

Now I see why we are at cross purposes. You're talking gauge pressure, whereas our boiler code stipulated the use of gauges that showed absolute pressure.

My first thought is why?

I don't know, and I doubt there's anyone still alive who could answer that question. Suffice to say, it's been that way since I was a 15 year-old apprentice.

...Unless the boiler is being operated at very low pressure above atmospheric, the steam temperature and enthalpy isn't going to vary much between a given gauge pressure at sea level and the same gauge pressure at say 14,000'.

Which is exactly the point I was trying to make earlier.

Mark.

[KCSfan]

I've never known of a boiler pressure gage on a US steam locomotive to read psia - to the best of my knowledge they were all calibrated to indicate boiler pressure in psig. On a cold locomotive they would read 0 psi if properly calibrated whether the engine was sitting in Death Valley or on top of the highest peak in the Rockies.

The highest mainline in the US was at Tennessee Pass where the Rio Grande crossed the Continental Divide at an elevation of just about 10,000 ft. Nominal atmospheric pressure at that altitude is 10.1 psi vs 14.7 at sea level. A steam locomotive whose safety valves were set to pop off at 280 psig might in actuality operate at between 250 and 280 psi depending on the demand for steam, the quality of the coal and the way it was fired. Thus the 3.6 psi difference in atmospheric pressure at the two elevations was virtually inconsequential in terms of affecting the engines thermal performance. It also made no difference whether the engine was superheated or not.

What did matter was the oxygen content of the air which at 10,000 ft is only 69% of what it is at sea level. Thus to get the same number of BTU's out of a given amount of coal it was necessary to supply a proportionately greater volume of combustion air at the higher elevation. I don't know for sure but doubt that locomotives that reguarly ran at the higher altitudes had different exhaust nozzles or grates than their tidewater counterparts. I think it more likely the need for more combustion air was supplied by the blower and the normal grate configuration allowed for the passage of the needed volume of air.

Mark 

[blue streak 1]

Guys: my whole point was at Tennessee pass the atmospheric pressure is approximately 10 psia. Now we all know that water boils at a lower temp in Denver. Having boiled eggs there it takes less heat but more time to cook them. So at Tennessee pass steam will happen at a lower temp and if the fuel is burned at the same efficiency a slight advantage should happen at the lower boiling point.

[selector]

I don't follow...sorry.  The reason water boils at a lower temperature is because its vapour pressure at altitude is less than that at sea level.  At the surface of a volatile liquid there is a vapour pressure.  As pressure on the surface rises, the vapour pressure "loses ground" and is eventually neutralized such that no evaporation takes place at all.  That's in the open.  In a sealed pressure vessel, the vapour pressure is what it is based on the pressure of the gasses on the surface of the water in the vessel, not what the pressure is of the atmopshere outside the vessel.  So the heat required of the firebox is going to be what it takes to get water to sublimate to steam even though it has immense counter pressure of the steam and other gases above the surface in the boiler.  That heat will be precisely what it takes to do it in outer space or at the bottom of the Dead Sea.  It's what happens in the boiler, not outside of it that counts.

[erikem]

At the surface of a volatile liquid there is a vapour pressure.  As pressure on the surface rises, the vapour pressure "loses ground" and is eventually neutralized such that no evaporation takes place at all.

Not quite.  Evaporation will take place as long as the partial pressure of water vapor is less than the vapor pressure no matter what the atmospheric pressure is. There's a term that describes the ratio of the partial pressure of water in air to the vapor pressure of water at that air temperature - relative humidity.

In an enclosed vessel, the partial pressure of water vapor will eventually stabilize at the vapor pressure for surface temperature of water. 

[marknewton]

Guys: my whole point was at Tennessee pass the atmospheric pressure is approximately 10 psia. Now we all know that water boils at a lower temp in Denver. Having boiled eggs there it takes less heat but more time to cook them. So at Tennessee pass steam will happen at a lower temp and if the fuel is burned at the same efficiency a slight advantage should happen at the lower boiling point.

And your whole point is WRONG.

The eggs in Denver are being cooked at in water which is atmospheric pressure, in an open vessel such as a saucepan or pot.

The water in the boiler of a loco at Tennnessee pass is NOT at atmospheric pressure, but at whatever BOILER pressure the loco is operating at, say 200psi. The boiler is an ENCLOSED PRESSURE VESSEL, it is NOT OPEN TO THE ATMOSPHERE. The boiling point IS NOT LOWERED, but RAISED to whatever the boiling temperature is for that pressure.

At 200psi, for a saturated boiler, that temperature is 361.4 degrees F, which is WAY above the boiling temperature for water at 10PSI, is it not?

The boiling point in an ENCLOSED PRESSURE VESSEL is entirely unrelated to the outside air pressure.

Why is this such a difficult concept to understand?

Mark.

[marknewton]

I've never known of a boiler pressure gage on a US steam locomotive to read psia - to the best of my knowledge they were all calibrated to indicate boiler pressure in psig.

Which is why there was some confusion between the various participants. Various boiler codes throughout the world specify things differently than US codes - I should have twigged to this earlier.

But as to your comments re the effect of atmospheric pressure, I couldn't agree more.

Cheers,

Mark.

[KCSfan]

Soo,

I have to take exception with some of your statements. Please understand that my intent is clarification and I certainly don't want anyone to misconstrue my comments as criticism. 

 At high altitudes there is less back pressure on the exhaust stroke so the engine is more efficent.

This would be true only if steam was exhausted directly to the atmosphere without any restriction. However in a locomotive steam is exhausted through a nozzle and back pressure is a function of the size and configuration of this exhaust nozzle and is not affected by atmospheric pressure.

As far as the actual combustion in the firebox the effects of altidude are quite small. . . . A firebox however is a very large area that pulls air constantly through very large openings. There is no shortage of air. 

As I pointed out in a prior reply the oxygen content of atmospheric air at an elevation of 10,000 ft. is only about 70% of what it is at sea level. Thus to get the same number of BTU's out of a given volume of coal at the higher altitude about 30% more combustion air had to be supplied to the firebox.

 

In short steam engines improve their performance as altitude increases (to a point where the air is so rarified that even the massive grate area cannot supply enough, pretty darn high)

In fact steam locomotives were slightly less efficient at higher altitudes though admittedly the difference was so slight it was inconsequential for all practical purposes. There were two principal reasons for this: 1) The additional energy used by the blower to supply the greater volume of air needed for combustion, and 2) greater steam usage by the air pump. At the higher elevations the air pump had to move a greater volume of the less dense air (i.e. operate for longer durations because it was a positive displacement device) to charge and re-charge the air reservoir and train air line.

Mark

 

 

[selector]

Thank-you for your correction, erikem.  I am happy I got it largely right...not bad for an artsy-fartsy type.

-Crandell

[csmith9474]

Somebody should contact the Manitou and Pike's Peak railroad and ask if they have any input in regards to this. They were operating steam there in relatively recent history, so there may be somebody around there that could provide some insight. Those steamers operated at roughly 14,110' (although I don't think they have taken one all the way to the top since they were replaced for normal operations).

[erikem]

Thank-you for your correction, erikem.  I am happy I got it largely right...not bad for an artsy-fartsy type.

-Crandell

You did get it mostly right - while I never worked with boilers and other steam generating hardware, I did take several courses in thermal hydraulics for my Master's degree - so I have academic as opposed to practical knowledge of steam generation.

- Erik