How should thermal-oil heating be specified for industrial heating elements?

For thermal-oil service, specify from the permissible film temperature rather than a catalog rating. First use circulation flow, oil viscosity, inlet/outlet temperatures and the oil's allowable film temperature to calculate the required surface load, then derive watt density, power, tube diameter, sheath material and immersed length. Excessive watt density drives the sheath well above oil temperature, accelerating coking and oil degradation. Match protection rating, immersed length and termination to the installation, avoiding dry firing and local stagnation, and provide independent temperature control and over-temperature protection with setting margin. Verify by measured flow, heat-up rate and sheath temperature under agreed operating conditions, and keep operating data on file.