[Pluton140]

Hi, Forum,

I am newbie in TIG welding. Planning soon to weld simple heat exchagers for home heating. The source of heat will be waste oil stove, and itting on top of this will be vessel filled with water, capturing hot gasses in H/E. I have 350 liters SS cylindrical vessel with cone shaped caps, 1.5 meters high. I plan to run thin SS tubing (about 0.5 mm thick, 40 mm dia) through the center and weld them on bottom and top cones. Hot gasses then will directed to the chimney. Essentially, H/E will be part of this vessel. The water pressure will be minimal, about 0.6 bar.

I have basically the question of how to reliably make these tubes part of the vessel: SS weld them by TIG, or braze with say, silicon bronze?

Welding 0.5 mm tubing (304) to 1.8 mm cone (kinda 316) is one option.
But here I need to be careful not to burn tubing through. Welding thin stuff to conical or orthogonal surface seems challenging.

Another option is to braze them. This seems easier and safer, however I am not sure about galvanic corrosion later on.

I also thought to use copper tubing instead if SS. In this case brazing would seems to be the only option.

The water in the heating system will stay the same and rarely be changed, so I think no worry about oxygen corrosion.

I understand this is not exactly question on welding, but rather on design, materials and electrochemistry...but maybe someone has experience and knowlegde on this...

Should I use SS tubing, and use brazing? Or only welding, but then how to weld safely ?

Could someone help me please...

[Poland308]

.6 bar is about 8 psi. Is this an open to atmosphere system? Or is it capped?
Copper won’t hold up well to the flue gas due to carbonic acid being in the condensate of the flue. Any flue gas below 140 F will condense. What temps are you thinking you will be running? Galvanic corrosion can be countered with proper water treatment. SS can be brazed with 45%silver or higher %silver.

[Pluton140]

Yes, Josh, this is an atm. open system. I do not want the flu gas get below about 150 C. But am not sure the T drop across the length of heat exchanger. Since this is a copper 150 cm long...

Do you know any on line calc. to aid on this ?
I will burn waste oil, guess the temp. will be about 600-700 C at input.

What is the water treatment to take care of gavanic corrosion ?
Thank you , Josh

[Poland308]

Your best bet is to talk to a boiler water specialist about water treatment. As far a pipe size and flow rates it depends on design. Too much water flow, or to large of pipe size and you won’t get good heat transfer. You may need to vary the flow rate based on discharge water temp from the leaving side of the heat source. This would help with the condensing flue gas issue. Reality is you would be much safer running the tubes tight to the outside of the burner box and outside of the flue gas stream. Then you could get by with just a check valve and gravity flow would be enough to get heat transfer to the tank. Add a pump and boost the transfer rates. You might want to check out books on basic Hydronics for home heating. The more advanced books will have charts for flow rates, compared to pipe size, and heat transfer rates.