Understanding Watt Density

Watt density (W/cm2) is the heat flux emanating from each square inch of the effective heating area (heated surface) of the element.

The effective heating area is the surface area per linear inch of the heater multiplied by the heated length.

For tubular elements, watt density is determined by the following formulas:

Effective heating area = 3.14 x Dia. x Heated Length

Heater Selection Guidelines
Once the total heat energy requirements have been determined, the selection of the type of electric heater is based on three criteria:

• Maximum Sheath Temperature
• Sheath Material
• Recommended Maximum Watt Density

Maximum Sheath Temperature
The sheath temperature of an electric element should be limited to prevent damage to the heater and provide reasonable life. To a large extent, the maximum sheath temperature of the heating element is determined by the final operating temperature of the process. In direct immersion applications, the sheath temperature will approximate the temperature of the heated media. In clamp-on, air and gas heating applications, the operating sheath temperature can be estimated using factors derived from empirical charts and graphs.

Sheath Material
Element sheath material is selected based on the maximum allowable sheath temperature, the material being heated and corrosion resistance required. Depending on the sheath material and construction, metal-sheathed electric resistance elements will operate satisfactorily at temperatures from less than -300°F (cryogenic) to approximately 1500°F. Copper sheath elements are commonly used for low temperature and direct immersion water heating. Steel is used for oil immersion and strip heater applications.
Stainless steel and INCOLOY® are used for corrosive solutions, high-temperature gas or air heating and cartridge heaters. The table below lists the maximum recommended operating temperatures for common sheath materials: