Boiler water injection efficiency

[James Powell]

I would disagree with you, pat1027. An injector is close to 100% thermally efficient- the heat entering mostly leaves as heat exiting, with very little losses to what falls out the overflow, and conduction/convection/radiation off the body of the injector. A live steam driven pump is going to be probably 2-3% n at best, because for one, you are throwing away the 970 BTU/Lb of the latent heat of evaporation- which is recaptured in an injector.

It _does_ make a difference, and enough of one to matter with a model. I suspect that an axle driven pump is somewhere in the middle between an injector and a feed pump, but feed pumps that are not operated in a closed feed system are horrible. If you are condensing the steam to recoup the 970 BTU, and using a feed pump because of economizer/feed heater, then it makes sense. If not, forget about it.

The problem that the smaller scales (3/4"), injectors are generally too big to be left on all the time. Exhaust steam injectors are even better from an n prospective, in that they are recouping the above mentioned 970 BTU/lb, post use in the primary expander (engine).

James

[pat1027]

Emerson-Penberthy claims near 100% thermal efficiency for their injectors. I came across one article that goes into length calculating overall efficiency for an injector estimating 85% overall Water weighs 62lbs per cubic foot and steam at 100PSI 3.89. A Keim pump has a steam to water ratio of 1.7:1. I'll assume no lap in the valve to keep it easy. For a pound of steam used by the pump it would add 9.3 pounds of water. Assume the injector has a thermal efficiency of 100%. One of the 9.3lbs of water goes to replace the pound used by the pump or 18% of the water/fuel consumption. On a good day of running I burn 50lbs of coal. Using strictly a reciprocating steam pump I'd burn 59 lbs and not notice the difference. That's a locomotive with plenty of furnace volume. On a smaller locomotive pressed to make a enough steam it could make a difference.

[pat1027]

I'm making the assumption to that the pump and the injector have the same steam consumption for the same amount of water added.

[Bill Shields]

AH....but think about an axle pump's efficiency

if you are running down hill with the throttle closed, gives you all the water free...so it is VERY EFFICIENT

giggle.....

[James Powell]

Remembering that most of what I play with was either very big, or very small, then that brings up that the middle stuff (7.x gauge) that I'm not used to !. So, for 3/4" scale locos, I drive exactly like Bill is suggesting- put the pump on going downhill, let it bypass going uphill. My "big" engines are usually OK with a bit of ham handed driving- the Britannia has a !@#@!@ huge combustion chamber, and will burn black rocks, Caribou is less forgiving about coal choice, but has a decent firebox volume for a 3.5" gauge engine. Where it really starts to matter is as you get smaller- the small steam wagon is rather more difficult, even though it has been converted to propane. It's got about 5"^2 grate area...the same sort of area as our roller had. The traction had a little more- about 7.5^2, and was just manageable on coal. The (2nd) version of the roller had an injector, but it was too big in relation to the boiler to be usable, it would flood the boiler fairly well. (something like 14 oz/min) The traction at that point had a 7 oz/min injector (commercial, about a #00 by the sizes that are sold in the UK), and that was a viable 2nd feed source. Even that was at the margins of workable, it was really a bit on the big size. The engine driven pumps would have been 3/8 by 1/2 or so, single acting. I think the roller was 7/16 when we had the cylinder bored out, but it had started as 3/8".

So, my choice would be an engine driven pump on a small model, followed by an injector, and last a hand waggler (pump). Some of it comes down to driver experience- I drove Britannia with only the axle pump for years- the hand pump works, but wasn't connected, and the injector was very rarely used. The same sort of style for Caribou- I'm waiting on an injector to arrive, but the majority of driving has only been done with the axle pump. The Fowler only has an engine driven pump- no injector fitted, though it should have one. (it's simple- if the pump doesn't work at 5 lb, then dump the fire & fix the problem, then get steam up- I'm yet to have that happen)

James

[Bill Shields]

ALL MONSTER fireboxes compared to my Tich.....which you run with the throttle wide open and control the speed with the water pump....

[Harlock]

I would disagree with you, pat1027. An injector is close to 100% thermally efficient- the heat entering mostly leaves as heat exiting, with very little losses to what falls out the overflow, and conduction/convection/radiation off the body of the injector. A live steam driven pump is going to be probably 2-3% n at best, because for one, you are throwing away the 970 BTU/Lb of the latent heat of evaporation- which is recaptured in an injector.
James

I agree with this reading. On both my locos I noticed when switching to injector-only setups, my steaming ability and fuel efficiency increased greatly. Of course, as Bill Shields says, we are not so concerned with efficiency on our models, but steaming capability yes. There is a net loss with use of any kind of pump vs. an injector. The downhill argument doesn't hold, as even in bypass the pump is using up some energy to pump water in a circle.

If you size the axle pump so that it just maintains level with a little help from an injector now and then, and you never or hardly ever need to use the bypass, that is a good use for the pump. Makes it easier to maintain water level in a boiler for beginners. On free steaming engines there's little reason not to have that kind of setup.

Oversized pump bores are the #1 killer of efficiency in axle pumps.

On Tom Miller's Big Boy, one SS injector was left on all the time, and it just maintained water level on his track pulling his set of cars. They didn't plan it that way, just a bit of luck that it evened out!

[ccvstmr]

Believe someone made a comment a while back in this thread about "reliability". There's a big difference between reliability and efficiency. Injectors, pumps (axle or steam operated) are all subject to design or service/maintenance problems. Axle pumps may be relatively simple devices, but have their own issues (been there, done that). Even an injector is not without at least one moving part...the check valve. With proper care, there's no reason why any of feed water hardware should operate reliably and mostly trouble-free.

Efficiency is another matter. There's a difference between component efficiency and system efficiency. As Harlock pointed out, there is a "net efficiency" to take into consideration. While displacement pumps might have a high efficiency, this is offset by a drop in boiler pressure related to ambient feed water input and the fuel needed to bring that water back up to temperature. Fuel efficient? I think not. The injector provides some assist in this area by using steam. Pressure recovery is usually quick-er. But I will admit...for as inefficient as a steam powered pump might be...it's a hoot to watch and listen to the pump plug 'n chug!

Bottom line, while we try to squeeze as much efficiency out of our steam locomotives...if we were really concerned about efficiency, we would have looked for some other hobby instead of external combustion engines! Happy steaming! Carl B.

[ChipsAhoy]

...... But I will admit...for as inefficient as a steam powered pump might be...it's a hoot to watch and listen to the pump plug 'n chug!
Carl B.

I agree... actually, it's a good motivation to built any of these machines and/or the subsystems.

While the original question was regarding efficiency.... it ain't very efficient if un-reliability causes you to prematurely head back to the barn and blow down. Especially if it happens repeatedly.

I would think (i know i will) anyone could run the discharge of any boiler feed system through a coil in the wasted heat of the smoke box to pre heat. Thus equating or even exceeding the btu savings of an injector. I mean, just how close to operating temp is the water output of the injector going to be? With enough residual time in the smoke box.....

I also have never seen first hand, an injector where, if it wasn't slobbering it was dang sure dribbling. (I've also spent a good bit of steam trying to get it going correctly)

No two ways about it, the duplex pump exhaust pure T steam right out the ports. But it will keep the boiler level up while one sits back and has lunch.

The axle pump always has some kind of "drag" even with the bypass open. Lots of lost motion wearing out parts that are hard to get to and frequently overlooked during routine maintenance.

The hand pump in the tender..... one bicep grows grossly larger than the other.