Tempco Custom Heater Manufacturing

Silicone Rubber Heater Wattage Recommendations

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:

Watts =

3.53 lbs. × (0.24 btu/lb.°F) × 200°F


3.412 btu/watt hr. × 0.5 hrs.

99 watts


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.

Watt Density =

Heater Wattage


Area of Heater

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.

Silicone Rubber Heater Surface Temperature vs. Watt Density

Graph: Surface Temperature vs. Time for Various Watt Densities

The graph shows the relationship between the maximum surface temperature and the watt density of standard silicone rubber heaters. The heater was energized in still air without insulation or a load.

Using this graph the designer can estimate the maximum temperature the heater can reach compared to the watt density of the heater.

Suggested Maximum Watt Density by Heater Type and Mounting Method 

Watt Density Silicone Rubber – Wire Element Silicone Rubber – Foil Element
w/in2 Vulcanized PSA Vulcanized PSA
5 420 to 356°F 350 to 335°F 455 to 419°F 350 to 320°F
  (216 to 180°C) (177 to 168°C) (235 to 215°C) (177 to 160°C)
10 356 to 266°F 335 to 248°F 419 to 383°F 320 to 293°F
  (180 to 130°C) (168 to 120°C) (215 to 195°C) (160 to 145°C)
15 266 to 158°F 248 to 140°F 383 to 347°F 293 to 266°F
  (130 to 70°C) (120 to 60°C) (195 to 175°C) (145 to 130°C)
20 158 to 68°F 140 to 32°F 347 to 311°F 266 to 239°F
  (70 to 20°C) (60 to 0°C) (175 to 155°C) (130 to 115°C)
25 68 to -40°F 32 to -49°F
  (20 to -40°C) (0 to -45°C)
30 311 to 257°F 239 to 185°F
  (155 to 125°C) (115 to 85°C)
35
 
40 257 to 185°F 185 to 104°F
  (125 to 85°C) (85 to 40°C)
50 185 to 50°F 104 to -40°F
  (85 to 10°C) (40 to -40°C)
60 50 to -49°F -40 to -49°F
  (10 to -45°C) (-40 to -45°C)

Note: Use an appropriate Temperature Controller for the application.
View Tempco Temperature Control Products.

 

See Also:

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