I had a thought after watching Moonshiners the other day - build a wort chiller like the condenser on a still (the worm). So it would sort of be the reverse of a immersion chiller now - the tubing is submersed in ice water and the hot wort travels through the copper tubing. Standard ICs seem to be pretty inefficient - how much tap water goes through to cool 5 gallons of wort.
Does anybody know why this isn't done? If not, about how many feet of tubing do you think it would take to cool the wort down to pitching temps?
As others have said, this is nothing new. But I will say that when I first started brewing I used this exact same technique, running 5 gallons of wort through my coiled copper chiller which was immersed in my sink with ice water. You have to keep the water moving (like stirring with your hand or something) otherwise the water near the coils warms up enough to become less effective. Also, I had to change my water 2 or 3 times for a 5 gallon batch. It was kind of a PITA.
The really nice thing about an immersion chiller is that you place it in the boil 20 minutes before knock out to sanitize it. And you can be sure the copper is clean and no patina is building up. You can't do this on a CFC. I much prefer an IC set up over a CFC for that convenience.