Plastics catalog - Watlow - (Version JPG) - Page n° 319 - PDF Catalogue | Technical Documentation

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Power regulator, Cable, Sensor, Tubular heater, Thermocouple wire

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O W

Mica Strip

Applications and Technical Data

Continued

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.

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

Heated Area =

(Length - No-Heat) x Width

Watt Density =

Total Watts

Heated Area

Maximum Allowable Watt Density

The following derating factors are applicable to the Maximum Allowable Watt Density graph. Please review these factors and the graph to determine the maximum watt density for the application.

Derating Factors:

• For heaters mounted less than 25 mm (1 in.) apart on a metal part, derate by 5 percent.

• For heaters mounted less than 51 mm (2 in.) apart and radiating toward each other, derate by

20 percent.

• For termination Types 2 and 5, as well as lead Types C, E and H (see illustrations on page 318 that are less than 51 mm (2 in.) wide, derate as follows: zinc-coated units by 10 percent and stainless steel units by 20 percent.

• For heaters mounted within 76 mm (3 in.) of a reflective surface, derate by 10 percent.

• For heaters mounted 51-150 mm (2-6 in.) apart and radiating toward each other, derate by 10 percent.

• For heaters mounted within 25 mm (1 in.) of a reflective surface, derate by 20 percent.

Application Hints

To maximize the performance of a mica strip heater, ensure:

• Small heaters with 32.3 cm² (5 in²) or less of heated area are 120V~(ac). These heaters can be wired in series for a 240V~(ac) power supply.

• The surface to be heated is clean and smooth, so that heat is transferred efficiently. Even small air gaps can cause hot spotting.

• Terminal post nuts are not overly tightened. Although the posts are securely riveted to the elements, excessive torque could break the connection.

Watt Density—W/cm²

3 4 5

6 7 8

1200

^—r

1—

1—1

1 II 1 III 1 1

- Suspended in air

- Mounted to part using

holes

—

...

Mounted to part using

clamp

s ~

—N

^—

r<ss *4

^—

■

1000

500

800

400

600 \-

300

400 h

200

100

200

10 1520253035

Watt Density—W/in²

319

Plastics catalog - Watlow - #319

Power regulator, Cable, Sensor, Tubular heater, Thermocouple wire