Double Barreled Boiler from Stan Bray Literature

[2E26]

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.

Jon

[2E26]

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.

Jon

[Jasonb]

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.

[MJM460]

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 Stan’s 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.

MJM460

[2E26]

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.

Jon

[2E26]

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.

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

[GWRdriver]

[snip] . . .  I can't rely on the metal parts to stay where I put them.  - Jon

You can, if you use a few little tricks.  For instance, for holding the water legs in place, instead of prick-popping the holes you could wrap a loop or two of fine copper wire around each leg.  If you are neat about it and haven't left a few stray hairs standing up the wire will disappear in the solder joint.  Even if there are some stray hairs they can be nipped off and all will be well.

[Mike R]

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.

Again - 1.5mm will likely be too thin for most boilers above 2.5" diameter as an end plate or any flat surface. 


The code part 3 I mentioned is actually a bit thin on construction details, but if you're planning on building larger boilers that stray into the mainstream model boiler size and pressures (over 2.5" diameter and 60 to 100psi operating), then the code part 1 is for copper boilers and does have more guidelines such as what I have attached.  As you can see it starts at 1.6mm material thickness.  The other thing I barely mentioned was stays - the thinner the material, the more stays are required (there's a table for that too but I won't copy it here).



Material thickness, material type(strength), staying/support, shape (round vs flat) and design pressure (and temperature) all come together in a boiler design.
Change any one of those items and it affects all the others.


Also, my comments aren't about it holding together while fabricating - stays and flanges can help for that but that usually isn't why they're included in a design - its about being safe while operating.  The guidelines are there to put in place practices that ensure a large margin of safety.  Anything less and YMMV w.r.t safety.  It may be fine, but maybe not...

[2E26]

My boiler barrels are 1.125 inch in outer diameter.

As far as the end plates I don't know what I'm going to do yet, but it's likely I won't get to building this boiler until October. I'm going on extended business travel soon.

I'm aware that flanging the end plates and using stays are for increasing strength while operating. However, using stays that clamp the end plates down while silver soldering will not hurt, as I don't want anything moving while I have a torch pointed at it.

I'm going to reference the KN Harris book on model boilers before I do anything else. I'm following the calculations on that book. I also need to state that this boiler will likely run up to 15-20 PSI total, with a safety valve that prevents it from getting higher than that.

Jon

[uuu]

I quite like to flux up a joint, then lay solder wire down, and heat until the wire flashes into the joint.  Instead of trying to poke the solder at the joint.   It's a reliable way of gauging the temperature and avoids overheating, especially close to previous joints.  And you know how much solder has gone in, so you can expect proper penetration.  A flanged joint, particularly if you leave a small upstand on the barrel, gives you somewhere to lay the solder so it won't fall off while heating.

Just saying.

Wilf

[crueby]

I quite like to flux up a joint, then lay solder wire down, and heat until the wire flashes into the joint.  Instead of trying to poke the solder at the joint.   It's a reliable way of gauging the temperature and avoids overheating, especially close to previous joints.  And you know how much solder has gone in, so you can expect proper penetration.  A flanged joint, particularly if you leave a small upstand on the barrel, gives you somewhere to lay the solder so it won't fall off while heating.

Just saying.

Wilf

I do the same as Wilf described. For some joints, where there is a deep face to join, I'll wait till it flashes and flows in, then poke in some more to ensure it has enough to flow all the way through.

[2E26]

For the engine parts I've soft soldered, I do the same thing.

Jon

[2E26]

My silver solder came in today. It's Harris safety-silv 45, 1/16" wire. I also got some Sparex 2, which will go well for pickling the parts before soldering.

Next I need my flux (arrives next week) and some bronze bar to make bushings. Once that happens, I'll evaluate my plans for the end plates on this boiler. In addition to the resources already provided, I have a good book by Alex Weiss that contains boiler calculations. His boilers are much larger than the boilers I intend to make.

Jon