Steam chest pressure.

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David Powell
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Joined: Thu Mar 13, 2003 10:38 pm
Location: Pickering Ontario Canada.

Steam chest pressure.

Post by David Powell »

In trying to analyse and improve the performance of our 4 inch scale Clayton steam wagon I added a steam chest pressure gauge and the results have surprised me. The motionworks turn over, out of gear, with 5 lbs or so on the gauge, will move the wagon slowly in high gear with 10 lbs on the clock and the highest steady pressure I see is 40 lbs at about 8 mph in third notch in high gear, all without the semi trailer.
Have other folk fitted gauges to models , what sort of results have you had ? Our wagon blows off at 80 lbs.I rather wonder is the throughput of the Superheater insufficient?
Regards David Powell.
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Bill Shields
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Re: Steam chest pressure.

Post by Bill Shields »

Very similar on model locos.

Even at 100 psi a chest pressure of 35 psi is common running for most of my locos. The only time I see boiler pressure there is when I have the valve gear centered and the throttle open.

Bob Thomas did extensive testing in Nellie -> a 1" loco.. and documented it. Perhaps he will share it here.
Too many things going on to bother listing them.
Wolfgang
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Re: Steam chest pressure.

Post by Wolfgang »

When choosing/designing an engine, it is best to use 50% of the available boiler pressure for the engine power calculations. For a model engine perhaps as low as 33% is even more appropriate. I think that 50% is recommended in the model engineering literature.

If the steam chest pressures are required to be closer to the boiler pressure (for more power), then the flow areas of steam pipes, and cylinder steam ports, would need to be enlarged considerably. The high speed steam locomotives in Britain and Germany had enormous steam pipes and cylinder passages.

Fortunately on model steam trains, where the wheels will spin even with modest steam passages, this type of power gain is usually not useful.

For a steam driven pump or generator, more power is always useful. Provided, of course, that the boiler is able to supply the needed quantity at the requisite pressure.
david griner
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Re: Steam chest pressure.

Post by david griner »

Hello,

On my engine, the running steam chest pressure is right around 22 psi in the third notch up. As the load increases, the pressure gradually increases, but have not exceeded 45 psi. Just thought this might add to the information.

The tree gauges show are from left to right...............Brake pressure................steam chest pressure.................exhaust pressure.

Take care,

Dave
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cbrew
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Re: Steam chest pressure.

Post by cbrew »

i have been running chest gauge for a while.
it is very useful when pushing the limits of the locomotive. on dry steel rail, I can normally put 90 psi in the chest and not slip the drivers.
on dry aluminum rail, I have seen 135 on he gauge right after starting a heavy with the pressure dropping after the train is moving. when i running hard, I am watching that gauge more then i am watching the boiler pressure gauge.
If it is not live steam. its not worth it.
Berkman
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Re: Steam chest pressure.

Post by Berkman »

Do you also have an exhaust pressure gauge?

The combination of cylinder/exhaust pressure gauges could be used to fine tune valve gear settings and exhaust nozzle
B&OBob
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Re: Steam chest pressure.

Post by B&OBob »

Obsrvations of 20+ years ago mentioned by Bill were based on steam chest pressure measurements with a cab-mounted gauge temporarily connected to the top of one steam chest. The locomotive was a 1" scale 2-4-0, 80 PSI boiler pressure.

Starting on level grade, pressure immediately shot up to boiler pressure on the first few strokes but quickly receded to 20 psi as the train (of myself and three empty cars) slowly accelerated. Pressure then dropped to zero at the bottom of a long down grade, and would have gone engative if not for the snifting (drifting) valve. Through a tunnel and then a long up-grade at good speed with throttle wide open, pressure rose to only 40 psi. As cutoff was inceasesed from 75% full-gear to 40% while the throttle was simultaneously opened to maintain speed, pressure rose only to about 60% of boiler pressure.

One should not be mislead by supposing throttling steam to the cylinders is a wasteful loss of energy. To the contrary, throttling steam actually increases superheat, or will tend to dry "wet" steam! It has to; the Second Law of Thermodynamics states that heat energy can be neither created nor destroyed, so If pressure drops through a throttle, the temperature of output steam must increase to maintain the same total energy.

Monitoring steam chest pressure is only one way to increase locomotive performance and interest in operating conditions. On a visit to England abbout the time my observations were made, I had the pleasure of driving a 3/4" scale LNER K4 that had a water manometer in its cab connected to the smokebox. It provided a very sensitive indication of S.B. pressure that was closely related to quite subtle changes in locomotive performance. Another potential area for investigation is a variable blast nozzle that would permit generation of high draft to facilitate rapid recovery of a green fire, but enabling redustion of draft for maintaining vigorous runing without liftin safety valves.

We don't have to turn our locomotives into rolling laboratories, but they have a lot to tell us about how they feel. We only have to listen!

B&OBob
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rudd
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Re: Steam chest pressure.

Post by rudd »

Bob,

I am not sure I understand your middle paragraph. You mention superheat, which may be the part I am missing.
The steam tables tell me that saturated steam can only exist at certain relationships between pressure and temperature. Steam at 100 psi is going to be at 328psi. Steam at 50 psi is going to be about 280 degrees. I'd think that the steam looses heat by expanding to fill a larger space. This was how compounding was calculated.
Then there is Boyle's Law - "The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system"

This explains why compressed air feels cold when it comes out of the nozzle on the end of a hose.

What am I missing?
B&OBob
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Re: Steam chest pressure.

Post by B&OBob »

It's been 72 years since I studied thermodynamics, so I have to watch my step here. What you say about saturated steam at various static pressures is correct. but those are steady state values for two different circumstances not involving the flow of steam. However, when saturated steam is extracted from a boiler by flow through a throttle, its condition on the low pressure side of the throttle is rather different than at the throttle's input depending on how much work is extracted from the steam. The quality (superheat) of the steam will actually increase due to the effect of enthalpy and this is where my memory sputters out.

I do recall that the same principles are involved with a calorimiter, a device commonly used to measure steam quality, where steam is throttled into a well-insulated chamber and temperatures before and after the throttle indicate the amount of superheat. Beyond that, will someone who can explain enthalpy and what happens with throttled steam please step ofrward?

B&OBob
David Powell
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Location: Pickering Ontario Canada.

Re: Steam chest pressure.

Post by David Powell »

After wondering for a while and driving the wagon many times on my test runs near home I decided to add a fourth coil to the superheater arrangements.
It took some shoehorning to get the coil in between and around the three existing coils.
It is a bit shorter than the others but has the advantage of passing twice directly beneath the base of the chimney.
So I now have four parallel coils of 1/4 inch od copper tube of lengths varying from the original 2 6 footers, one about 5 feet and this last one about 4 feet long. They feed two cylinders 2 inch bore 3 inch stroke. Adding the fourth coil has given the performance much more life.
A brief test in the garden showed that steam chest pressure would rise 20 lbs or more higher in the same circumstances than previously and I can now climb the 25% or so grade in top gear with steam chest pressure only 10 lbs or so less than boiler pressure.
Hope this is of interest. More details to follow as I do more testing. Regards David Powell.
B&OBob
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Re: Steam chest pressure.

Post by B&OBob »

Hi David --

Your suspicion about the efficacy of the original superheater coils seems to have been correct:
I rather wonder is the throughput of the Superheater insufficient?
The additional coil, a somewhat marginal increase in supeheater area, resulted in a remarkable increase in steam chest pressure, possibly by improving the steam quality from "almost dry" to superheated. That appears to be verified by the Clayton's equally impressive improvement in road performance.

Looking forward to further reports!

B&OBob
David Powell
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Joined: Thu Mar 13, 2003 10:38 pm
Location: Pickering Ontario Canada.

Re: Steam chest pressure.

Post by David Powell »

I do not think the steam is any hotter, but certainly there is more of it getting to the cylinders.I will do more testing with the thermometer when I have a reliable helper available. In round figures with just 3 coils running out of gear the superheater arrangement gave 100 degrees F of superheat .Regards David Powell.
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