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One problem in induction heating is that energy is lost because the object being heated radiates energy, that energy then heats the coils (which are water cooled) and the coils suck away the energy.

One idea is to insulate the object being heated. For example, a thin-walled cylinder of polished tungsten could be placed between the object being heated and the coil. This will reflect some of the infrared energy being emitted from the object and reduce the heat loss. Another potential material is gold. So, for example, silica plated with the gold might be a possible. The issue with gold is that it is a good infrared reflector, but it melts at 1400C and the object can reach that temperature, so it would be at a borderline temperature for melting. Also, gold is not paramagnetic.

The problem with this idea is that tungsten is paramagnetic so it will absorb some of the energy from the coil.

How can I compute whether using a tungsten cylinder reflector would have a net benefit without doing an actual experiment?

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    $\begingroup$ I don't think tungsten is easy to machine. $\endgroup$ – M. Enns Aug 1 '16 at 16:06
  • $\begingroup$ @M.Enns The cylinder would not be machined. It would sinter cast, then polished or alternatively tungsten-plated silica would be used. $\endgroup$ – Ambrose Swasey Aug 1 '16 at 16:24
  • $\begingroup$ @M.Enns Sintered tungsten can be machined by wire EDM. Very slow. Very expensive. $\endgroup$ – Solomon Slow Aug 1 '16 at 17:49
  • $\begingroup$ @M.Enns Why tungsten plate silica? All you need is a metal coat whose melting point is less than silica. Anyway, the temperature of the silica will rise to the same as the pure tungsten surround as you originally wanted, given equal reflectance. $\endgroup$ – user56903 Aug 1 '16 at 20:12
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    $\begingroup$ A strongly reflecting metal will not get very hot to begin with. You could simply use polished stainless steel or even aluminum (with sufficient cooling from the outside e.g. with nitrogen). $\endgroup$ – CuriousOne Aug 1 '16 at 21:22
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An alternative might be to surround the object with a carbon fiber mat. Conductivity across the fibers is low compared to along them. Let them heat up, and re-radiate. Still needs some experimentation, but it's a lot cheaper and easier than tungsten

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  • $\begingroup$ This would probably be a lot less reflective than tungsten. Also, this does not really answer the question, which is how to do a calculation that would estimate the net benefit, if any. $\endgroup$ – Ambrose Swasey Aug 1 '16 at 16:25
  • $\begingroup$ The use of polished tungsten is totally impractical from an engineering POV. It's the kind of no-expense-spared- solution used by military contractors to squeeze as much money as possible out of a project. You want radiated energy fed back into the target - my solution will give you that. $\endgroup$ – user56903 Aug 1 '16 at 20:08
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After doing more research I found the following table on IR reflectivity:

enter image description here

From this it appears silver is by far the best reflector, rhodium is #2 and platinum is number 3. Silver melts at forging temperatures so it may or may not be able to hold up in the environment. For that reason rhodium may be the best selection.

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