How critical are the coil turns, diameter and spacing...

relative to the diameter of the work piece to be heated?

The short answer is both the diameter and the spacing are extremely critical,  especially if you're trying to get good temperature signatures. There are a lot of processes where we actually don't want the heating to be uniform. We want the heating to be more concentrated in one area than the other. So that's where we play with the:

  • diameter of the copper coils
  • geometry of the copper coil
  • spacing between the copper turns
  • the diameter of the coil
This means the copper tubing is determined based on the power dissipation through that copper coil. If you have to dissipate a lot of power, obviously you're going to go with a larger diameter copper tube because you have to run more water.