brazing heat exchanger

[negativentropy]

So I'm trying to manufacture a heat exchanger for a prototype - hot gas inside the tubes, water on the outside of the tubes. Ive got the parts all made and fitted. Now how to braze it...?

My plan is to buy jewelers silver solder paste (melting point 1200 F), have everything in position, apply solder paste to each tube standing a bit proud of the manifold and then apply heat with acetylene. If that doesnt work Ill try it in a kiln and do one side at a time (bury the other side in sand)...

Is this a reasonable way to do this, any tips, other ideas... Of course every joint has to be air tight...

 

[davidh]

i'd try a kiln with very controllable heat and large enough to put the complete part in. you might look for a school or university with a large kiln. . . interesting looking project. . .

 

[T Bredehoft]

I made one of these 30 years ago, wound up TIG welding the end of each tube. I had tried silver solder (55%) and could not keep the adjacent tubes from contracting after they were soldered. Looks like a fire tube boiler to me.

Bye the way, that's a great picture, the graph paper, the caliper for scale, it all works.

 

[Ulma Doctor]

So I'm trying to manufacture a heat exchanger for a prototype - hot gas inside the tubes, water on the outside of the tubes. Ive got the parts all made and fitted. Now how to braze it...?

My plan is to buy jewelers silver solder paste (melting point 1200 F), have everything in position, apply solder paste to each tube standing a bit proud of the manifold and then apply heat with acetylene. If that doesnt work Ill try it in a kiln and do one side at a time (bury the other side in sand)...

Is this a reasonable way to do this, any tips, other ideas... Of course every joint has to be air tight...

I like the Acetylene heat plan.
you can control the heat a little better and don't have to heat the entire assembly.
my 2 cents,
i'd consider brazing up one end completely with the solder paste and allowing it to cool for a bit before going back to brazing up the other side.
neat looking project!
good luck!
mike)

 

[chips&more]

I know your concern. You put all that work into it and then the brazing part can make or break it. Most importantly, it must be clean. I would glass bead it. Have not tried the paste you suggested. I would use a flux and cut up metered pieces of low temp brazing rod placed evenly about the project. I would not use an acetylene flame, it has too much carbon in it and could contaminate your attempt. If the project is not too big a heat sink, maybe a Mapp gas torch? Our two torches. Remember, the hard solider/braze flows towards the heat. Remove the heat as soon as the braze wicks into everything. The faster and more evenly heated the part is the better your results. It must braze on your first attempt. Trying to reheat it again almost never works. It ain’t easy, I would practice on a few first before you go for it...Good Luck, Dave

 

[negativentropy]

Ill post on the results - itll probably be a couple weeks for supplies to arrive. If I can get this itll be the last major manufacturing hurdle in this project.

 

[Southtowns27]

Wow that thing is tiny... I'm impressed. Is there any way to roll the tubes into the tube sheet? Might help with the seal and help hold them in place. As far as soldering or brazing them in, it might be worth your while to find a high powered electric soldering iron. I'm sure there's something out there that would get hot enough for your application. You might be able to customize the shape of the tip so you could heat just one tube at a time.