How should flange dimensions be specified for commercial heating elements?

A commercial heating element flange cannot be chosen by nominal size alone. Specify outside diameter, bolt count and circle, bolt size, sealing face, insertion length and the actual vessel opening, then check these against sheath diameter, material, watt density, ingress protection and control method. A nominal size is not enough for purchasing: ask the supplier for a tolerance-rated drawing showing hole positions and gasket details, or supply the original part for measurement, and confirm removal clearance for frequent cleaning. After fitting, verify voltage, current, insulation resistance, heat-up behaviour and seal contact to prevent excessive sheath temperature or corrosion from high watt density.