How should vacuum heating be specified for industrial heating elements?

Specify vacuum heating from the actual vacuum level, working temperature and residual gas, not from a catalog rating. Since convection nearly vanishes, heat transfer relies on radiation and conduction, so reduce surface load versus air operation to avoid sheath overheating. Choose low-outgassing stainless steel or high-temperature alloy, high-purity MgO insulation, vacuum-tight terminals with adequate creepage, and supports that keep cold ends cool. Ask the supplier for power-temperature curves and outgassing data under vacuum, and fix tube diameter, length, mounting flange, control sensor location and acceptance tests.