When sourcing a heating element, engineers have to maneuver several design constraints to obtain the correct element to meet their needs. These constraints are different depending upon the type of element desired. For instance, band heaters often have a minimal inside diameter and width. Tubular heaters have minimum bend radii that are dependent on the cross-sectional size of the tubing. While the specifics vary, one limitation is common to electric heating elements: the design constraint of a maximum permissible power density (watt density).
This value often is measured as the power of the element divided by its total outside heat-emitting surface area. However, in this article, we will be considering the power density on the surface of the resistance wire itself. This value varies based upon many factors, including the type of heater, the heater’s housing materials, the operating environment in which the heater will be used, the certifications the heater must attain and the manufacturer’s capabilities. As you may have guessed, two inter-related goals drive this power limit value: safety and heater life.