Electrically insulating filler for electric heater tubes - using magnesia mixed with glass to give low leakage currents

摘要

The filler is a mixt. of (a) granular MgO; with (b) 0.05-5, pref. 0.02-2, esp. 1-1.5 wt.% glass with low alkali content and glass transformation temp. (T) 650-1000 degrees C. Glass (b) pref. has a specific electric vol. resistance of min. 108 ohm. cm at min. 400 degrees C and pref. above 550 degrees C. Glass (b) pref. contains by wt. 15-25% Al2O3, 45-60% SiO2, 8-28% BaO + CaO + MgO + B2O3, max. 3% ZrO2, and 2% max., esp. 1% max. alkali metal oxide. The mixt. (a, b) may also contain (c) 0.1-2% fused granular cordierite contg. 45-70% SiO2, 15-35% Al2O3, 5% max. CaO, and 10-25% MgO, where 70% max., and esp. 20-50% of the MgO has bee replaced by FeO. Used for surrounding resistance heating wires in tubular heater elements, so leakage currents can be reduced and tubes with smaller dia. used.