The length of the tube, IMHO is also directly related to the rate of the water flow. If you have 40 ft of tubing that's not going to do you much good if your shooting water out of there at 500 gph. Something like this is far from an exact science, and would require a lot of tweaking to get the output temp close to where you want it.
I've had this sort of setup on my mind a lot lately. I think it would make a wonderful addition to a keg.
