I will be attempting to make liquid nitrogen from air. Part of the process involves passing air compressed to 200 atm through a coil and rapidly expanding it. The expanded air that does not form liquid from the rapid cooling will pass back over the condensor coils in a counter-current fashion to precool the compressed air. Of course, this countercurrent system will be insulated from the surroundings. My question is this: how long should should I make the countercurrent coil for it to be effective? I know I will transfer more heat with a longer coil, but there has to be a point of diminsishing returns. There are many variables so I think this comes down to experience. Does anyone have any idea? I am planning on something between 8-16 feet for the coil.