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| Question: |
| What is watt density and how is it related to heater life? |
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| Answer: |
| Watt density is a measure of the rate of heat being transferred through the surface of the heater. That is, if you were to draw a 1?x1? square on the surface of the heater, how much heat would need to pass through that 1 square inch area. This is called watt density and it is measured as watts/square inch.
Other things being equal, the higher the watt density, the higher the temperature inside the heater. As the temperature inside the heater increases, the materials inside the heater are operating closer to their breaking point resulting in shorter heater life.
Generally, for mica band heaters, for satisfactory heater life, the watt density should be less than 50 watts/square inch for heater diameters less than 3 inches, less that 40 watts / square inch for diameters between 3 and 6 inches and less than 35 watts / square inch for heaters with diameters between 6 and 10 inches.
For cartridge heaters, the watt density should be less than 200 watts / square inch. For ceramic knuckle band heaters, the watt density should be less than 35 watts / square inch.
Also, higher operating temperatures require lower watt density for equivalent temperatures inside the heater and equivalent heater life. |
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