How should flow-dependent heat transfer be specified for industrial heating elements?

Specify flow-dependent heat transfer from the duty, not the catalogue. Using viscosity, specific heat, tube diameter and target temperature rise, calculate the required flow and the resulting Reynolds number, then set the maximum surface load from the convective coefficient. Too low a flow creates a hot boundary layer, scaling and burnout; too high a flow raises pressure drop and erosion. Ask the supplier for design velocity, allowable pressure drop, sheath material, protection rating, mounting geometry and over-temperature control, and verify performance at site flow during acceptance.