I just finished building my heat exchanger and thought I'd share some photos. Did a test run with 6 gallons of water and was able to raise the temp a little over 2 degrees per minute.
Here is the inside view. I was able to stuff 25' of 1/2" ID tube in there. The tank holds about a gallon and a half. I went with two 120V 1500W elements wired in series and connected @ 240V since I wanted 3000W but couldn't find a single element that was short enough.
Here is how its wired. The green wire leaving the box is to ground the tank since the box is JB welded to it.
Here is the stirrer motor.
Here is the stirrer prop which I made from a piece of aluminum stock. Not pretty, but it seems to work. You can also see a float switch which cuts out the element if the water level gets too low.
Here is a shot of how it fits on my stand, although I still need to permanently bolt it to the frame.