◈ What is surface load (w/cm²)? It is the heat (W) generated per unit area with regard to the heat-generating surface, and closely related to the life cycle of the heater. Surface load (W/cm²) = Capacity(W) Heater O.D. (cm) * 3.14 * heater effective length (cm) Example) What is the surface load of a heater with capacity 1kW, effective length 900mm and O.D. 11 mm? ......☞ 1000w / (1.1cm * 3.14 * 90cm) = 3.2 w/cm² Relationship between surface load density and surface temperature in still air