Surface Loading for Electric Resistance Heating Element Materials

Surface Loading for Heating Elements
The surface loading on a heating element is one of the most important factors in the design review for a heating element. The element temperature is dependent on how much energy has to be transmitted from the heating element to the surrounding area, the temperature of the surrounding area, and finally the ease of heat transfer from the element to the surrounding area.

The amount of energy to be transmitted and the desired temperature of the area are generally known. The hard part is calculating the heat transfer from the element to the surrounding area. There are three modes of heat transfer, Conduction, Convection, and Radiation. Conduction of energy from an element used in industrial processes is generally relatively low (but can be important in the case of imbedded heaters) and is generally not accounted for. Convection plays a role in ovens operating at relatively low temperatures, up to about (1400°F / 760°C), but the effect is reduced as the temperature increases (with increasing temperature, the air thins and can carry less energy).

That leaves radiation as the primary form. Unfortunately in the formula for radiant heat transfer, there are two variables that are moving targets, emissivity and the shape factor, These variables are difficult to estimate to a level of accuracy needed to properly calculate element temperatures. The emissivity of a heating element varies with the temperature, but also with the type and amount of oxide on the surface. The shape factor can become very complex based on the exact configuration of the heating element (radiating onto itself or ceramic supports, or even the closeness to the furnace wall.

For these reasons guidelines are generally used that give acceptable life to the heating elements. Even by using these guidelines, one must be careful to follow the assumptions made.

In addition to the tables that show element watt loading for different furnace temperatures, there are also tables that show recommended wall loading. These tables are useful in generating the basic furnace design and verifying that the desire amount of energy can be applied. They cannot be used in evaluating a specific element design.