Excessive Burn-Out of Cartridge Heaters in Low Drag Heated Body Project.

摘要

This investigation shows that rapid burn-out of cartridge heaters on the low drag body program should not have been caused by line voltage surges, poor thermal contact, or switching of the power measuring device. The thermal design of the heated body appears to be adequate. The accelerated burn-out of the heaters appears to be caused by the cycling of the heater control unit together with the extremely fast-response of the heating elements. Indeed, similar calculations for adjacent zones having CIR 1027 heaters shows heat-up times to be more than twice those of the CIR 10201 heaters and since lower temperatures are used, heater burn-out in these zones has not become a problem. The on-off cycling of the heater control system can be significantly improved by modifying the circuitry to use a time base of 100 half-cycles of the 60 cycle power line instead of 100 full cycles as is presently used. This doubles the on-off rate so that thermal shock to the heating elements is reduced. It is estimated that this modification alone could triple the present life of the heater. Heater selection recommendations in the manufacturer's literature state that a lower watt density for a given application results in longest heater life. Experience gained from previous tests of the heated body shows that lower wattage units can be substituted for all heaters in the most critical heat zones. Suggested substitutes are listed.