How should furnace-door heat loss be specified for industrial heating elements?

Do not size furnace-door heat loss from static furnace-body dissipation alone. Specify it from door-opening frequency, dwell time per opening and furnace temperature: treat each opening as a transient load, first calculate the compensating power needed in the door zone, then convert it into sheath surface load per element so recovery does not overheat the sheath. Match diameter, material and protection rating to that cycling duty, locate elements as close to the door as practical and zone the control to limit interference from adjacent zones. Verify by measured recovery time and sheath temperature rather than nameplate rating alone, and where needed ask the supplier to check surface load for that duty.