To pump or not to pump that is the question

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James Powell
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Re: To pump or not to pump that is the question

Post by James Powell » Thu Apr 15, 2021 1:48 pm

Because an engine is ~ 3% efficient at extracting the heat from the steam, and a injector is basically 100% at transferring the energy into the water in one form or another. The water going into the boiler- it doesn't matter if it is hot or not (to some extent), but if you can recover heat into the boiler, it increases plant n. That's part of why exhaust steam injectors (in full size) work as well as they do- they recover some more heat energy that otherwise is destined to go up the stack. They work rather better (IMO) than a feed heater fed by a pump, because the pump is going to be less efficient than the injector. (though I am sure that either one are better than cold water pumped in by steam donkey pump).

The big gain is in the form of the 1100 btu/lb that has been put into the steam, that you cannot get back out via an expander. (well, 700 btu for a turbine with a good condenser)

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ChipsAhoy
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Re: To pump or not to pump that is the question

Post by ChipsAhoy » Thu Apr 15, 2021 2:36 pm

Got all that except... where is the wasted energy in the axle pump? Only thing I can see is maybe the drag of the o-ring in the cylinder and the friction off the excentric. If that's it, then I don't see that as an issue.
Where is the pump itself ineffecient?
I do know I would really have my feelings hurt if I were way off down the track somewhere and the axle pump seized up.
Scotty

James Powell
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Re: To pump or not to pump that is the question

Post by James Powell » Thu Apr 15, 2021 5:01 pm

The wasted energy isn't in the pump. It's in getting the energy to run the pump. An injector takes steam with 200+1000+200 btu/lb (so roughly 1400 btu/lb) and puts nearly all that energy back into the boiler.

If you put a US Gallon of water into the boiler, that is 0.06 hp/hr required- regardless of if it comes from the injector or from the engine. In order to generate that with an engine at 100 psi, you have to use something like 1 lb of water through the engine to generate that, or around 1/20th the amount of steam generated to drive the pump. That's because the best you are getting out of the expander is probably 300 BTU/LB extracted energy- so you need to use at least a lb of the 10 lb you are boiling to drive the engine (a lb of saturated steam at 100 psi> 0 PSI can give up to about .12 hp/hr, or 300 btu).

If you put it in with the injector, it will cost you in real terms (for fuel burned) only the .06 hp/hr, or perhaps 0.07-8 due to pumping losses. If you use the expander to drive a pump, the best you are going to do is .4 hp/hr or .5 hp/hr (in BTU) , because it costs you .06 to pump it in, and it comes in cold, so needs the 1100 btu of losses up the stack adding back into the water. It's not something for nothing at all, but where the heat goes. Those 1100 BTU in heat are why an injector doesn't work as a mine pump (for example), because they are not as efficient as a shaft driven pump at converting energy to motion, but they are far more effective if you need hot water too. Even if the injector needs to use .24 hp/hr to "pump" the fluid into the boiler, that is still far less costly than running a donkey engine as a pump- because those losses are in the form of heat into the working fluid, which you are already going to add heat to.

So, it's not that the pump is inefficient- its that the expander (engine) is inefficient. If you didn't realize this before, you know now :). Superheat your steam- it's worth 1 btu/2 F, and you can extract that heat as it is sensible. (a 100 F increase in steam temp at 100 PSI means 50 more BTU/Lb available to extract, which is a 1/7th change...that's why superheaters matter) Use the injectors vs axle pumps where you can. Drive no faster than 55 MPH in the fast lane. (OK, the last might NOT be a good suggestion !)

James

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Bill Shields
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Re: To pump or not to pump that is the question

Post by Bill Shields » Thu Apr 15, 2021 5:20 pm

LVRR2095 wrote:
Thu Apr 15, 2021 12:15 pm
The initial question was whether it was a good idea to REMOVE and already existing and working axle pump.
I still feel that it is not worth the effort to remove it. If you don’t like using it....don’t! It doesn’t hurt anything if you leave it in place. And another thing to consider is the chances are really good that you will not be the last owner of

the locomotive. I have in my collection locomotives built between the 1930’s and up through the 1980’s. All of the original builders are long gone. Live steam locomotives if cared for can last for many decades. A future caretaker of that locomotive may well wish to have that pump. I have a locomotive that formerly had a VanBrocklin twin cylinder pump. Prior to my acquisition of the locomotive it was removed to be “serviced.” Well it was never to be seen again. That locomotive is still a great runner.....it has a tender pump, an axle pump and a temperamental British injector that is totally unreliable. I am really happy that the axle pump is still there!
Keith
Keith
Need a replacement BvB pump? I have 3 new ones looking for homes
Too many things going on to bother listing them.

Berkman
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Re: To pump or not to pump that is the question

Post by Berkman » Thu Apr 15, 2021 6:18 pm

Are the solar ii pumps still made?

wms6
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Re: To pump or not to pump that is the question

Post by wms6 » Mon Apr 19, 2021 4:10 pm

Keep the pump. I've had to resort to the hand pump a couple times even when the engine has a steam water pump, injector and axle pump.
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John Hasler
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Re: To pump or not to pump that is the question

Post by John Hasler » Mon Apr 19, 2021 4:35 pm

You mean hp*hr, not hp/hr.

James Powell
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Re: To pump or not to pump that is the question

Post by James Powell » Tue Apr 20, 2021 10:51 pm

John Hasler wrote:
Mon Apr 19, 2021 4:35 pm
You mean hp*hr, not hp/hr.
Yes, sorry. Having a NASA moment, but mine didn't end with lithographic braking of a space project :)

(most of my schooling was in metric, because it makes more sense, no slugs involved...)

Gra2472
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Re: To pump or not to pump that is the question

Post by Gra2472 » Tue Apr 20, 2021 11:38 pm

As a historian and amateur mechanical engineer, the best part about engineering hobbies, is listening to the real engineers.
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John Hasler
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Re: To pump or not to pump that is the question

Post by John Hasler » Wed Apr 21, 2021 8:26 am

James Powell wrote:
Tue Apr 20, 2021 10:51 pm


(most of my schooling was in metric, because it makes more sense, no slugs involved...)
The slug is British. It is defined as a secondary unit in the US customary system as 14.59 kg but no one ever uses it. The primary unit of mass is the pound.

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Jacob's dad
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Re: To pump or not to pump that is the question

Post by Jacob's dad » Thu Apr 29, 2021 9:41 am

I have an axle pump on my locomotive. I fill the boiler through the pump, so I have never had a problem with the check valves sticking. I fill all of my other steam engines through the pumps also, and that seems to take any stuck checks and free them, especially if they have been in storage for a while.

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