I am working on a home made copper coil heat exchanger (HX) and I thought my learnings might be helpful for others or that others may read this and provide feedback (hopefully before it is too late)
The sizing of the HX is a crucial part of the design. If the HX is too small the water coming from the boiler will leave the HX at a temperature that has not been cooled down enough by the tank which means that the water going back into the boiler will be too warm and the boiler will probably slow down and burn at a less efficient level. If the HX is too large then money is waster on the excess copper.
The most important factor in the sizing of the HX is the heat output rating (BTU/h) of the boiler. STSS claims that their 120 foot long 3/4" copper coil can transfer 26,400 BTU/h with the tank at 120*F. They didn't indicate the boiler output temp used to get this number, so I will assume 180*F. I am planning on using 1/2" tubing instead of 3/4" mainly because it is cheaper.
I can currently get 10' of 1/2" L pipe for $15.35, whereas 10' of 3/4" L pipe is 24.49. If we assume that the ability of a pipe to transfer heat is proportional to the circumference of the pipe (not exactly true but let's assume it is close enough) then the 3/4" tube has 50% better performance than 1/2". However, it costs 60% more, so the 1/2" seems like the better deal. In addition, I know that I can successfully bend the 1/2" tube using electrical conduit bender, and I have a gut feeling that it is better to use go with more coils of 1/2" because the larger number of coils means that i can cover more of the volume of the tank.
So back to sizing... Using the 26,400 BTU/h rating for 120" of 3/4" copper and assuming that heat transfer is proportional to the diameter of the tube, 1/2" tube will transfer 147 BTU/h/ft. Assuming that my boiler is only producing 100k BTU/h (max is 140k according to Tarm), I will need 680 feet
Wow, that seems like too much. One of my assumptions must be wrong.
Somebody on one of the forums said that for a Tarm 40 I would need a total of 400 sq inches of total copper, which works out to 210 feet, which sounds much more reasonable.
8.22 BTU / gal / *F
15*F in 2.2 h, 880gal
The maximum output of the boiler that I am planning on buying is 140k BTU/h