I would like to investigate that, but for me, being able to the manufacture the coils at a reasonable price is also important. I would say:
- No copper, stainless is much more durable. Copper has a higher thermal conductivity, but it will oxidize and turn green and nasty. I have made a copper/silicone and stainless/silicone CFC and the copper one looked nasty after very quickly, while the stainless one is as good as new.
- No wire around the tubing, not easy to make.
- With the diameters and flow rates we're talking about, you don't have to worry about laminar flow. It will be turbulent.
So I'm pretty much set on a 11 mm ID, 12 mm OD tube, with a silicone or similar outer tube, 18 or 20mm OD. Mainly have to determine length.