  In the design of a thermal system — and in the design of an electric heating element watt density is one of the most important considerations. The ability of the heater to dissipate heat from the resistance element to the heater sheath and then from the sheath to the process is determined by watt density.

Too high a watt density can result in:

• Failure of the heater.
• Damage to the material being heated.
• Damage to the equipment or other components.

Low watt density.

• Heaters do not produce as much heat per square inch as high-density elements.
• But heaters have a longer lifespan.
• They are not as noisy as high-density elements during heating cycles

How to Determine Watt Density

Watt density is the rated wattage per unit of the heated surface area, most commonly expressed in W/in2 or W/cm2. Each electric heater product has unique watt density characteristics and can range from less than 5 to more than 300 W/in2 (less than 1 to more than 46 W/cm2).

The term “Watt density” refers to the heat flow rate or surface loading. It is the number of Watts per square inch of heated surface area.

For calculation purposes, stock cartridge heaters have a 1/4″ unheated length at each end.
Thus, for a 3/4″ diameter  x 10″ HL heater rated 1200 Watts, the Watt density calculation would be as follows:

Watt Density = Element Wattage
( 3.14 x Element Diameter x Heated Length )

Where:
W=wattage = 1200 W
Π = pi (3.14)
D= diameter = 0.75 inch
HL = Heated Length = 9.5 inch

Watt Density =  _____1200 ____
( 3.14 x .75 x 9.5 )

= 53.64 W/in

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