Determine Required Wattage

Every process has a unique wattage requirement to heat that particular load up to temperature or to maintain a particular temperature.

If the required heater wattage is not known, estimate the required wattage using the thermodynamic formulas listed in chapter 16, Engineering. A safety factor of 25% additional wattage is recommended to compensate for unknown variables.

Example

To raise the temperature of an aluminum plate 6" × 12" × 0.5" (3.53 lb.) 200°F (from 70° to 270°F) in 0.5 hours:

Add safety margin: 99 W + 25% = 124 watts

Determine the Heater Size and Watt Density

A flexible heater should use the maximum space available for mounting and heating the process. Factors that affect heater size include the mounting method and watt density.

As a general rule, the following can be applied for silicone rubber heaters:

• Low Heat-Up:  2.5 w/in²
• Average Heat-Up:  5 w/in²
• High Heat-Up:  7.5 w/in² and greater

Continuing the aluminum plate example, determine what size the heater should be:

• Silicone Rubber Heater: 5" × 10" = 50 in²
• Watt Density = 135 watts ÷ 50 in² = 2.7 watts/in²

Since the watt density falls between 2.5 and 5 w/in², the silicone rubber heater selected should work satisfactorily.
Referring to the chart below for a wire wound silicone rubber heater, pressure sensitive adhesive mounting should work well for this application at the required temperature.
If the calculated watt density is too high, a larger heater will lower the required watt density and still produce the same wattage.