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Double Barreled Boiler from Stan Bray Literature

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2E26:

--- Quote from: Jasonb on March 04, 2024, 07:11:24 AM ---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.

--- End quote ---

Maybe. My issue is that the tubes rotate into their holes, so it would be tough to provide upward support to hold them in place without pushing them out of the holes.

I've thought of making a fire proof stay that fits inside the barrel so the ends are kept from going further than I needed.

Mike R:
You should reconsider the material for the end caps and flanging them.  0.7mm is too thin.
AMBSC Code Part 3* indicates:
2.3.10 For boilers with flanged end plates:
         - The minimum end plate thickness for boilers up to 39mm (diameter) is 0.91mm (20SWG)
         ....
2.3.11 Boilers with unflanged plates shall have a minimum plate thickness of 3mm

So even if you flange the material you should be looking for thicker stuff.
There's more in it on staying the plates based on thickness and working pressure.

*For those that don't know the AMBSC is the Australian Miniature Boiler Safety Committee and the "Code Part 3" is specifically geared to "subminiature boilers" - less than 1L, less than 100psi max, and less than 63.5mm outside diameter (2.5" OD).  Its a good reference even if you are not in Australia.  Not perfect but a great place to start to come up with some safety guidelines. I'm not affiliated with them but I find the codes they publish useful.

MJM460:
Hi Jon, it will be interesting to see how the boiler built to the sketch goes.  Please keep us all informed.

It was your curved water tubes that alerted me to the possibility that you were looking at a different book to the one I have.  I actually like them better as they give much more valuable heat transfer area than the short straight ones in the book I have, though they mean the drums will have to be a little higher to give the burner sufficient height, which may be important to the stability of a small model boat.  Perhaps if you gently bend the ends in or out a little, it might help hold them in place for soldering.  Though a wire or flat steel support to ensure they don’t move during heating might also be a good precaution.

I would support Ron’s suggestion of following a suitable code and at least doing the calculations for stayed flat ends to assure yourself that one stay is enough.  I don’t know the answer on that one without doing the calculations.  The simple formula quoted in every book on boilers is for cylindrical shells and internal pressure only.  It does not give the right answer for flat surfaces.  Using the same thickness for the shell and ends is a sure sign that the ends are the weakest link, and that a stay or stays are necessary.

A balance line is a tube connecting the vapour spaces of the two drums.  It allows the pressure to equalise between the two, and ensures that all the water does not end up in one drum, the lower pressure one being the steam outlet, where it will go straight to your engine with undesirable results.  With the pressures equal, gravity can properly control the liquid levels in the two drums.  This boiler is simple in appearance but quite complex in its theory of operation.  No issue if it all works well.  It is quite difficult to predict the flow differences in parallel tubes of any size, let alone such small ones with tiny differences in water level.

MJM460

GWRdriver:
Hi Jon,
That's an interesting design and a nice job so far.  I have a couple of comments . . . . in no particular order,

As small as your barrels are I agree with Mike R, I would try to find some thicker copper (1/16"(min) or 2-3mm) for the heads and pass on flanging.  I would also dispense with the longitudinal stays which IMHO, for your pressures, would be overkill.  However, granted, they would be simple enough to include so have at it.  I would still try to find thicker head material.

Unflanged heads can be accurately seated by turning a very shallow internal shoulder in the barrel ends the depth of which should leave 2mm+/- of wall for a good solder fillet.  I also file nicks in the periphery of unflanged heads in a few places to provide channels for the solder to flow to the insides of the joint.

As for movement in the water legs, I would prick-punch or center-pop, ie, deform the metal around the barrel holes, and that should hold the legs in place well enough for soldering.  Provided the holes are a close enough fit, a couple of pops each should do the trick.

Best of luck,

Harry

2E26:
Thanks. At any rate I have to get some bronze for bushings so I'm not going to start assembly tomorrow. I'm not sure what I'm going to do regarding the end plates yet, my thoughts were that the stay would also hold them in place once heating starts and I can't rely on the metal parts to stay where I put them.

I'll take a look at the ASMBC when I get some time. I'm curious if Stan ever built this boiler, although there is a picture of it in his book. He literally specified to make end plates out of the pipe used for the boiler shell and solder it directly to the pipe.

I've considered buying some 1.5mm sheets do I can make end plates for a couple of larger boilers, so at some point I'll have to get some anyway.

Jon

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