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and Heaters

MI Barrel and Nozzle

Applications and Technical Data

Calculating Watt Density

Watt density is the amount of wattage per square inch of heated area. To determine watt density, divide the total wattage by the heated area.

Watt Density = ^Total ^WattsHeated Area

To apply this equation we must define the term "heated area."

to the type of heater being considered.

2.To use the formula below, insert the no heat factors, diameter and width (in inches).

Heated Area = (3.14 x Diameter - No-Heat Factor) x Width

Heated area is the total contact surface of the heater less areas of no heat that are found around terminals, mounting holes, etc.

Heated Area =

Total Contact Area - No-Heat Area

To calculate the heated area:

1. Locate the no-heat factor from the chart below that corresponds

W ^Q


	Type	Factor
	1 pc. lead unit Type B, C,
	H,E or90°B	1.37
	1 pc. post terminal	1.60
	2 pc. expandable post term	3.18
	2 pc. expandable lead unit	3.00

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Maximum Allowable Watt Density

The following derating factors apply to the Maximum Allowable Watt Density chart, which are shown in both inch base and metric for your convenience. Please review these factors and the chart to determine the correct watt density curve for your application.

For two-piece units used in vertical applications, refer to Clamping Matrix Application Guide, page 16.

For applications where insulating blankets are used, multiply W/in²(W/cm2) by 0.75.

Derating Factors:

• For units over two inches

(51 mm) in width, multiply watt density by 0.8.

• In applications where unusual operating conditions are present, such as irregular mounting surfaces, contact the Watlow factory in St. Louis, Missouri,

for watt density limitations.

Maximum Allowable Watt Density

Watt Density-W/cm²

520

□

Allowable watt density for 1/2" wide

MI band heaters:

• Moderate cycling

• Heaters properly clamped

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Watt Density-W/in²