selector wrote:Thank-you for your correction, erikem. I am happy I got it largely right...not bad for an artsy-fartsy type. -Crandell
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
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).
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.
SOO-353 wrote: At high altitudes there is less back pressure on the exhaust stroke so the engine is more efficent.
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.
SOO-353 wrote: 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 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.
SOO-353 wrote: 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 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
KCSfan wrote: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.
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.
blue streak 1 wrote: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 wrote: 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.
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.
erikem wrote: marknewton wrote: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?
marknewton wrote: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?
...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'.
The mechanical stress put on a pressure vessel is due to the difference between the internal and external pressure. 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'.
One advantage of running a steam locomotive at altitude is that the absolute back pressure will be less, which could off an improvement in efficiency.
timz wrote:...bring the pressure inside the boiler exactly equal with the air pressure outside (say 14.7 psi outside). If the gauge has been calibrated correctly, what will it read then?
...bring the pressure inside the boiler exactly equal with the air pressure outside (say 14.7 psi outside). If the gauge has been calibrated correctly, what will it read then?
Is the Bourdon tube surrounded by a vacuum? Or by air, at 14.7 psi at sea level?
I looked thru my books and only found a couple of passing references to this matter-- apparently they figured the fact that the pressure gauge reading is the difference between the inside pressure and the outside pressure is too well-known to need stressing.
I think Timz has it right, that the pressure gauge on a steam locomotive is psig, not psia. Pressure gauges that read other than gage pressure are rare. A simple bordoun tube pressure gage read normally reads psig.
http://www.dynisco.com/literature/Application%20Notes/Sanitary/GageAbsPress.pdf
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
marknewton wrote:the gauge will read zero at whatever pressure it was calibrated to off the master gauge.
Cool the boiler all the way down to outside air temperature and open a cylinder **** (see below) or a safety valve, or whatever will bring the pressure inside the boiler exactly equal with the air pressure outside (say 14.7 psi outside). If the gauge has been calibrated correctly, what will it read then?
marknewton wrote:the Bourdon tube that drives it only connects with the steam space inside the boiler, nowhere else. The outside air pressure has no bearing on how it reads.
I looked thru my books and only found a couple of passing references to this matter-- apparently they figured the fact that the pressure gauge reading is the difference between the inside pressure and the outside pressure is too well-known to need stressing. If you have Bruce's book it's on page 151, or if you happen to have one of Baldwin's "Locomotive Data" editions the table of "Properties of Saturated Steam" suggests the same. I'll look around and see what steam tables I can find online.
(Well I'll be darned-- in the first line, it bleeps out that word that rhymes with "lock" but begins with a "c".)
timz wrote: marknewton wrote:The pressure gauge simply shows the pressure inside the boiler, nothing more. It does not compare or give a reading relative to outside air pressure.When we're at sea level (with the outside air pressure at 14.7 psi) and the locomotive pressure gauge reads zero, what is the pressure inside the boiler?
marknewton wrote:The pressure gauge simply shows the pressure inside the boiler, nothing more. It does not compare or give a reading relative to outside air pressure.
When we're at sea level (with the outside air pressure at 14.7 psi) and the locomotive pressure gauge reads zero, what is the pressure inside the boiler?
timz wrote: marknewton wrote:I have to ask, what practical experience do you have on steam locos? None.
marknewton wrote:I have to ask, what practical experience do you have on steam locos?
None.
I have seen a proposal for generating energy on the moon that involves steam and a turbine. Now that is high altitude.
dd
It would seem to me that altidude does not affect the heat energy of the steam if it is in a contained vessel. Thermodynamically a steam engine is a heat engine and they work by flowing heat through a device to harnese it. The energy stored in the water molecules rapid movement is used to push a piston. Steam is a hot expanding gas. The hotter it is the more energy it contains. The cooler the steam is once its out of the engine the more efficent the engine is. At high altitudes there is less back pressure on the exhaust stroke so the engine is more efficent. This is the same principle as the seperate condenser James Watt invented!
Think about a steam turbine in a power station. the large low pressure stage uses steam that is basically 1 psi and by the time its hit the end its pulling a vacuum! This in effect "pulls" more steam through at the high pressure side, again increasing thermal efficency.
Turbocharged engines also follow the same principle- turbos work very well at high altidude because the pressure upstream of the turbo in the manifold is immense and full of hot expanding gases. The lowered atmosheric pressure allows the turbo to spin more freely and use more of the waste exhaust. (superchargers , mechanical and turbo alike have what is known as a "critical altitude" this is the maximum altidude at which the engine will develop its rated sea level power and manifold pressure. The GE B series turbochargers used on WWII aircraft such as the B-17 and B-24 bombers and the P-38 and P-47 fighters had a critical altitude of 25,000 feet. beyond that the engine has to be throttled down to prevent the turbo from overspeeding and being destroyed by centrifugal forces.
As far as the actual combustion in the firebox the effects of altidude are quite small. at high altitude there is not enough air to flow into the cylinders of a reciprocating naturally aspiratied IC engine to burn the fuel. Air is taken in parcel by parcel and if the air is less dence the engine simply cant breath. A firebox however is a very large area that pulls air constantly through very large openings. There is no shortage of air.
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) and IC engines lose their performance respectivly. (unless some means of forced induction is incorperated, even then the critical altitude is lower because of mechanical limitations on turbo RPM)
Oh, oh, oh..! I know this one! It would be whatever pressure the gauge is calibrated to read at its "zero" point. If it is PSIAMSL, it should be very close to the pressure at sea level. If it is some other arbitrary value, then it will read zero when the pressure inside the boiler reaches that point. The pressure in the boiler, practically, will eventually match the outside pressure based on the altitude at the place where the neutralizing takes place. If at sea level, the pressure in the boiler, via leaks here and there, will eventually reach the pressure of sea level. But the gauge may say something entirely different...a non"zero" value depending on its calibration.
I do think the safeties are marginally sensitive to outside pressure at whatever altitude we find ourselves, but it is very small compared to the pressure working on the springs at full working pressure.
Mark, I found this site that does bring back some memories. Although I don't seem to recall the nose on the diesel they show, the rest of it is right on the money.
http://www.kellstransportmuseum.com/Peru/FCCA/FCCA.html
timz wrote:It doesn't make much difference, but--You remember that a given safety-valve setting (and a given reading on the steam gauge) gives a constant difference in pressure between the inside of the boiler and the outside.
It doesn't make much difference, but--
You remember that a given safety-valve setting (and a given reading on the steam gauge) gives a constant difference in pressure between the inside of the boiler and the outside.
timz wrote:So at the top of Everest the absolute pressure inside the boiler is less than it would be at sea level...
So at the top of Everest the absolute pressure inside the boiler is less than it would be at sea level...
selector wrote:Thanks, Mark, I'll try to find an image on the www and see if it flashes a memory or two. It was so long ago, and I certainly would not recall what the diesel was.Nice to have you back "on the job", BTW. -Crandell
Thanks, Mark, I'll try to find an image on the www and see if it flashes a memory or two. It was so long ago, and I certainly would not recall what the diesel was.
Nice to have you back "on the job", BTW.
marknewton wrote:No, no, no - altitude makes no difference. The same amount of heat is required to make steam whether the loco is at sea level or topping over Mt Everest.
You remember that a given safety-valve setting (and a given reading on the steam gauge) gives a constant difference in pressure between the inside of the boiler and the outside. So at the top of Everest the absolute pressure inside the boiler is less than it would be at sea level-- which means saturated steam in the boiler will be slightly cooler, with slightly less energy/enthalpy per pound.
selector wrote:It may well have been an FM, come to think of it.
It may well have been an FM, come to think of it.
blue streak 1 wrote:Yes altitude does make a difference. If the grate area is big enough to burn the fuel at say 90% efficiency less heat is needed to boil water and the steam pressure is the same. That is why feed water heaters were so important. Got the water closer to boiling so heat from boiler did not have to raise temp. So there may be better steaming at altitude.
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