The Steamboating Forum

I have a 3"+5"x4" compound engine, what size does my hp to lp transfer pipe need to be? The outlet is 3/4"

Sorry for the terminology, not sure what the correct name is.

Terminology is the receiver or receiver pipe.

I'm far from an expert on compounds but I suspect that continuing from the HP exhaust to the LP valve chest with 3/4" would not be too restrictive.

There are issues with the volume of the pipe but I don't think they are likely to be important.

Mike

Since the HP and LP cylinders are not in time with each other, the receiver needs to store steam. You can make this as large as practical; the ideal is prob. to have 2x the displacement of the LP cylinder if this is possible. A minimum is 1 to 1.5 times HP cylinder volume.

Google steam engine compound receiver volume

for interesting reading material.

- Bart

I would need a 2" diameter receiver?

I dunno, Bart. I've never seen a receiver with that much volume. Great, in theory, but there are a whole bunch of small compounds out there running fine with much smaller volumes.

2" dia. is about 3 cu. in. per/inch. His H.P. cylinder is 28 cu. in. so the minimum length would then be 9". Doable but pretty fat looking. With his existing H.P. exhaust port dia. of .75" the pipe would be five feet long. A heck of a condenser!

For least heat loss the surface are should be minimized. With a cylindrical pipe we can approximate a length to diameter ration of 1. That probably wouldn't reach from one cylinder to the other.

So the best solution is (as usual) to figure out what will fit. Measure the length of pipe that reaches and male it the right diameter to get the volume. John mentions 2" which gives him a minimum length of 9".

At any rate, I would insulate the hell out of it. These small engines are just condensers from the git go. Anything that we can do to keep the heat in is well rewarded. I have a friend with a compound locomotive (1/8 scale). He has never seen any indication of pressure in his receiver. The L.P. half of the engine is just along for the ride. I suspect that many of our launch compounds aren't doing that much better.

Mike

Yup... there are practical issues, but having adequate volume is important
since otherwise the exhaust flow from the HP is restricted on initial valve
opening.

side note:
Large scale power plants use reheaters; if you have superheat in the hp steam
you can moderate it very effectively by passing it in a pipe through the receiver;
this also improves overall efficiency.

- Bart

Just ran out and measured the receiver pipe on my Semple "V" it's 1 1/2 in. The inline compound receiver pipe is 1 in in the "Queen" It works well and as I haven't run the Semple I can't say but I suspect one should well insulate it to eliminate condensation