Been trolling the forums for quite some time now and figured I would add some information to the pot....
Ive been building a 3 tier setup for a few weeks now, and will post more about the rest later, but for now- here are a few pics of the heat exchanger I made from a 1 gal $8 stainless steel jar I found at walmart and a coil of 1/2" copper tube.
It has a 2kW 120v heating element (il post info on how its controlled when I have a finished design... still work in progress) inside about 6 coils of tube. 1/2" 90 degree copper street elbows on each end, terminated with 1/2" FNPT copper adapters.
The tube was bent around a 20oz paintball CO2 cylinder by hand using a neat trick i picked up... If you fill the copper with regulartable salt before you try and bend it, it will prevent it from kinking (to some extent). You could use sand, but that is abbrasive to the copper and might not come out completely. Salt will disolve in water when you flush it out and wont scratch copper. Works if you dont have a pipe bender.
Silver solder was used on all joints (no welds!). The trick is flux flux and more flux. 100% water tight. 8" long heating element is screwed into a 1.25"x1" reducing bushing with two 1.25" lock washers (one inside and one outside on the busing) for structural support. Not the prettiest joint on the bottom, but it holds water.