How should watts per meter be specified for industrial heating elements?

Specify watts per meter from allowable surface load, not from a catalog figure. Calculate the heated area from sheath diameter and effective length, then check it against medium, flow rate and sheath material. Still air, moving air, water and oil tolerate very different values, and lower flow or poor heat removal lowers the limit. Sheath diameter, element material, protection rating and temperature control all cap the limit, and terminal temperature must be checked separately to avoid end overheating. Ask the supplier for the design basis, then verify current, sheath temperature and insulation resistance on a sample or first article under real operating conditions before freezing the drawing, tolerances and installation clearance.