Your on the right track with the copper tube. I actually tried the coil and then went with a 90 o ell soldered on each corner to maximize the exposed surface to the ice. I also tried a small and med. sized heat exchanger (rad.) submerged in ice. The copper coil won, hands down! I was able to purchase a very thin copper tubing, the same type they use on fin tube radiation on heating systems. That improved the transfer of the cold icy salt solution to the circlating water. It wasn't major (about 4 o) but I was looking for max effect. The use of rubber tubing (3/4") between the ice chest and the original system served as an insulator and kept the water cold until it reached it's target. The transfer surface is the key to how quickly you can chill your water. You have a 35' coil of 5/8 OD copper. The circumference of the tube is the transfer surface. So.... 5/8 x 3.14 = 1.962" of circumference x 35' (or 420") = 824 sq. in. of transfer surface. That will be what you compare your other ideas to. Where are you mounting your cooler? What type connections?
