Induction Coil Design

3 turns minimum, ¼" clearance.

Our coils are 3 turns with minimal clearance but they are water cooled.

Although this is not my field, I would think the diameter of the tubing, wall thickness, and tubing material would enter into the equation.

Without more information, I would suggest a simple approach: Increase water flow to prevent excessive temperature in the induction coil.

the water has little bearing on the temperature, it seves as a source for electron flow. the amount of heat is a function of time.

the part being heated must not touch the coill

Is that true? With our older induction heater (looked like it belonged in the Science Museum), the only time we had coil failure was when something interrupted the water supply.

without a water supply the energy will be focused on the empty coil and burn it.

was your heater a LEPEL by any chance?

I don't know what make it was & it has now been scrapped. It was a unit about 7ft high & 3x4ft base area with some very large valves inside. The replacement unit is a box of about 15" cubed.