Watt density is one of the most critical factors affecting element life. Watt density, expressed in watts per square inch of heater surface area, determines the heater operating temperature for a given set of conditions. The sheath temperature of an electric heating element should be limited to provide a reasonable heater life and to avoid possible damage of the medium being heated. We can estimate the sheath temperature or select the appropriate watt density based on the charts and tables provided.
Watt density is calculated as follows:Watt Density = element wattage/3.14 x element diameter (inches) x heated length (inches)
An example is: An 8 kW flange heater has three 0.475” diameter elements with a “B” dimension of 47 inches and a 2 inch cold end. The watt density is: 0.475 x 3.14 x (47 in. – 2 in.) x 3 (# of elements) x 2 (u-bend) = 403 in2 8,000 Watts ÷ 403 in2 = 20 W/in2
Also critically important, the sheath material must
be suitable for the corrosive conditions and the extreme temperatures of the application. Copper sheathed elements are typically used for low temperature applications such as heating water and some aqueous solutions. Steel is generally used with oil heating applications. Stainless steel and INCOLOY® are used when heating corrosive solutions or high-temperature gas/air. Table I and II list the maximum recommended operating temperatures for common sheath materials.
Watt Density and Sheath Material Selection
Selecting the proper watt density and the proper sheath material is critical to heater life and, if applicable, fluid integrity. If the watt density is too high, the fluid may carbonize, break down chemically, or the elements may burn out. If the watt density is too low the heater price will be high. The greatest heater life will come from the lowest watt density practical for the application.
In general, watt density is determined by three factors:
1) maximum outlet temperature
2) type of fluid heated and
3) fluid flow rate
Sheath material depends on the type of fluid and maximum temperature. If the sheath material selection is incorrect, it will corrode, destroying the element.
Heating Air and Other Gases
Charts A through C give maximum sheath temperatures for a range of watt densities and velocities. This data is based upon air at atmospheric pressure and a 75°F inlet air temperature. For higher inlet temperatures, the sheath temperature will increase accordingly. Contaminants in the air, while normally not affecting the sheath temperature, may require a more highly corrosion-resistant sheath and/or fin material. For heating compressed air and other gases, consult the factory or your local Heatrex representative for watt density and sheath recommendations.