[jeff5may]

Rereading the last part of the thread, I realized something: Jake is trying to BUILD an evaporator, not BUY one! Oops, my mistake. In that light, please let me shed some light on that:

The Turbotec HX coils are super-effective for their size. They use thin-wall, spiral-extruded center tubing which not only has more surface area per foot, but also forces turbulent flow at a lower feed rate. Unless you source some similar tubing to fill the interior, the heat transfer will not match the performance per foot of overall length. Not surprisingly, this special-purpose tubing costs more than standard refrigeration tubing.

Also consider the scale of this pump and dump evaporator. It is much larger in capacity than the units you are building now. It will certainly not be as forgiving of mistakes. If you run the refrigerant on the outside of the water, the whole assembly has to withstand much higher pressure than with the refrigerant running through the middle. Also, the refrigerant will be exposed to outdoor temperatures through the exterior wall. Even with super-insulation, some heat will be shed to the great outdoors when it is much colder outside than the pond water. Depending on the insulating factor, this may not hurt your efficiency too much.

With the refrigerant running through the center tubing, only the center tubing has to withstand hundreds of PSI. This makes the whole shell easier and cheaper to assemble. The refrigerant is totally surrounded by water, and in your situation it will always be colder than the pond. If you use some sort of plastic for the shell, the outdoor temperature will not influence the water temperature nearly as much as some sort of metal shell, so for the same level of insulating you get more heat into the refrigerant. If the darn thing freezes up, your plastic shell may take the strain if you have a control mechanism that prevents it from freezing solid. A metal shell is nowhere near as forgiving.