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Double Barreled Boiler from Stan Bray Literature

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I'm working on a water tube boiler that will have a very small steam capacity, yet it should be enough to power a small engine. The upper limit for this boiler might be 12mm/0.5 inch engine bore diameter, although that might even be optimistic.

The boiler started as two pieces of 1-inch nominal copper pipe. This means the outer diameter is 1-1/8 inch or 28mm. The end caps are made of the same pipe, cut and rolled into strips. The thickness is something like 0.7mm / 30 thousandths. Needless to say, I won't be setting the safety valve to more than 15-20 PSI, although I'll test the boiler up to 100.

Five water tubes made of 5mm / 3/16" tubing connect the two barrels. The burner will heat the water tubes.

To increase strength, I'm considering making stays out of copper rod that runs the length of the boiler and secures the end plates. That will also aid assembly and silver soldering. The end plates are not dished because the barrel is so small.

I need to get a bronze bar to make some bushings. I plan to make some fill plugs that indicate when the boiler is full.


Here are some more pictures - the end plates and the coil of copper tubing, annealed and coiled around one of the barrels. I used a hacksaw to cut the coil to form the water tubes.

I'd like to find a way to set their length inside the barrels. Some of them liked to slide inside farther than they should.


Could you lay a third piece of tube on top of the other two for the smaller tubes to rest on? Coat it with solvent based correction fluid and then the solder will not stick to it.

Another option may be some stiff wire bent into a "goalpost" shape that will support the small tubes so they don't drop in too far.

Hi Jon, I would not worry too much about those tubes slipping in a bit more one end than the other, in fact I would aim for that, though not more than 10 mm in that barrel size.  The low side, I would aim for 2 - 3 mm only. 

This plan comes from considering how the water in the tubes is replaced when it turns to steam.  Basically there must be a flow through the tube, in this case from one barrel to the other.  That of course means there must be a balance tube or tubes joining the steam spaces of the barrels (not shown in the book I have, may be different in Stanís other books).   If one end of each tube is a bit higher than the other, the circulation will be from the lower end to the higher end.  I would be tempted to make about half of them higher in one side and the other half in the other side, but balance tubes are still required.

The balance line ( or possibly two ) is probably best in the top of the barrels, though it could be in the outlet manifold.  (Though that would involve more friction loss to flow, so requires larger bore tubing and bushes.)

I think it will drive your 12 mm bore engine quite well.  My first boiler was a simple pot type, 42 mm o.d., so about 75% of the heat transfer area of yours before adding the area of those water tubes) and it drives my avatar engine, (12 mm bore) quite well with a simple meths burner.  The boiler pot is 150 mm long.

You also mentioned stays, definitely include a central stay, and generous radii on between the flange and the flat centre.  Rather than test directly to 100 psi, go in steps of say 25 psi, depressure and measure the diameter and especially the length.  Be prepared to stop the rises if there is any sigh of deformation, and use 50% of the pressure reached as your design pressure for steaming, and set your safety valve for that.  Normally run 10-20% below that.  As your boiler will probably never run as high as 100 psi, no point in risking ruining it in an unnecessarily high pressure test.

An interesting project, I looking forward to following your progress.


The water tubes are not straight and level, but curved. If you couldn't tell from the photo the barrels are parallel and the water tubes are little "U" shapes that go between them. The plans specify they stick into the barrels no more than 1-2mm.

I'm not sure what a balance line is, but I'm looking to reproduce the boiler as the plans have drawn. The only connection between the two barrels is the water tubing at the bottom.

The end plates are not flanged. I will flatten them, drill a center hole for a stay, and silver solder them using the stays to keep them in place. I'll want to have a bronze bush on there to install a level plug.

I have no intent to run this thing at 100 PSI or anything near. I'm just using that to ensure it's built solidly enough to give a long service life at about a fifth of that. I will need to come up with a way to hydro test it before use.



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