Silver solder or not

[cessna]

Hi, I am building a 5" dia vertical boiler, it has twelve fire tubes, the plans give the option of expanding the tube ends or silver soldering them, the tubes are very close to each other, if I solder will I have a problem with melting the one next to the one I am soldering? I would be interested in your opinions regarding the two different methods. I have a 3/8 rod or stay in the center so could go either way.
Thanks
Terry ???

 

[Jasonb]

You would solder all the tubes into the tubeplate in one go so its not an issue. If expanding the tubes the tubeplate will likely need to be thicker.

J

 

[GWRdriver]

Terry,
You don't say what your material is, but I'll assume it's copper. I don't expand tubes in copper as some clearance is needed to insure good capillary flow of the solder into the joints. Expanding will leave you with no clearance, however some means of holding the tubes in place while soldering is needed and I usually rely on interference between the tubes and the holes. This might be a nick or punch mark, or just some difference in roundness of the tubes and hole. The fact that the flues are close together (but not too close I hope) works in your favor. Theoretically the entire end of the boiler is heated to melting temp and held there while the solder is flowed all around the tube joints and into the head flange. It rarely happens in exactly that way in actual practice unless you have a large torch. In most cases you end up heating the entire end only up to just under soldering (red) heat and then one area (or maybe half) at a time will be brought to full heat and the solder flowed in. Because it takes considerably more heat to re-melt silver solder when you get to the boundry of a previously soldered area you are unlikely to re-melt those joints if you use only the heat necessary to make the new joints.

 

[cessna]

Thanks Harry and Jason, the plate is 1/4" thick steel plate, the tubes are 5/8" od. copper.
in the plans they say either solder or expand the tubes for a seal. I think I will give the solder a try.

Terry

 

[GWRdriver]

Terry,
With a 1/4" plate head you could go either way. If I had this job to do, with 5/8" flues in steel, I would expand them. That decision would be based in part upon having occasional difficulty in the past getting silver solder to make a sound bi-metal joint dispite meticulous preparation and cleaning. Also, here in the US steel model boilers with rolled-in copper flues, at least for the larger gauges, has for years been common practice although welded steel flues are becoming an equal choice. Yer picks yer flavour.

 

[Jasonb]

Given those sizes I think I would probably expand the tubes, its going to take a lot of heat to get the solder to flow with a 1/4" steel boiler. Have a look at the thread about teh Le France boiler for how its done.

J