Maximum Joule heat by tubular susceptor with critical thickness on induction heating

A conductive susceptor has frequently been introduced in an electrical induction furnace to heat materials effectively. In this case, susceptor characteristics have a strong influence on furnace performance, e.g. input power, Joule heat, energy efficiency, etc. This study particularly focused on improving its energy efficiency through an investigation of the Joule heat generated in the susceptor. The Joule heat was obtained numerically by the mutual inductance model, experimentally by a water calorimeter and theoretically by an approximate equation. A thin tubular stainless-steel susceptor (d50 mm x h80 mm, t/δ ∼ 0.1, d the outer diameter, h the height, t the tubular thickness and δ the skin depth) had an extremely high Joule heat value (5.3 x 10-2 W A-2) at 30 kHz alternating current, which was 3.6 times higher than the value generated in a cylindrical susceptor. The tubular thickness where the maximum Joule heat occurs was defined as a critical thickness (tc). For the tubular susceptor with r/δ > 5 where r is the outer radius, the critical thickness was found to be δ2/r by the approximate equation. Furthermore, the maximum Joule heat at the critical thickness depended not on the material properties of the susceptor but only on the geometry of the susceptor. Ultimately, the energy efficiency could be improved up to approximately 84% by the tubular susceptor using the critical thickness.