Critical surface power of high-temperature nonmetallic electrical heaters

The problem of calculation the maximum surface power for high-temperature nonmetallic electric heaters was solved analytically. A mathematical model is proposed that describes the destruction of such heaters due to thermal stresses under critical thermal load. An analytical expressions for the critical surface power for nonmetallic resistive heaters with three main geometries (rod, tube, plate) have been obtained. These expressions consider the mechanical, thermal, and electrical properties of the heater material and its geometric parameters. Quantitative estimation of the critical surface power for graphite heaters in the form of rods and plates for temperatures up to 2500 °C has been carried out. It has been shown that for typical grades of graphite, the critical surface power can reach 1250 W/cm2 for rod heaters at 1000 °C as compared to 1–5 W/cm2 for resistance alloys. A good correlation of the critical surface power obtained on the basis of the proposed model with certain literature data for this parameter is shown. The application of the proposed model of heater destruction as a theoretical basis for a new method for measuring the mechanical and thermal properties of nonmetallic conductive materials at high temperatures is discussed.