ACADs (08-006) Covered

KeywordsHeat tracing, thermal radiation, black body, gray body.

Description

Supporting Material

Radiant Heat Transfer

1.3.6.2 5.3.2.8

Lesson 14Radiant Heat Transfer

DESCRIBE how the following terms relate to radiant

heat transfer:

a. Black body radiationb. Emissivityc. Radiation configuration factor

Thermal Radiation• The transfer of heat by electromagnetic radiation that arises due to the temperature of

a body.

• Most in the infra-red region of the electromagnetic spectrum

• Distinguished from other forms of radiation, such as radio waves, x-rays, or gamma rays.

• Does not need a medium, such as air or metal, to take place.

• Any material that has a temperature above absolute zero gives off some radiant energy.

• Examples:– When a cloud covers the sun, both its heat and light diminish. – The transfer of heat from a fireplace across a room in the line of sight.

Black Body Radiation• A body that emits the maximum amount of heat for its

absolute temperature • Can be expressed by the following equation:

Net flow rate of heat between two black bodies

Emissivity

• A factor by which we multiply the black body heat transfer to take into account that the black body is the ideal case

• Emissivity is a dimensionless number and has a maximum value of 1.0.

• Modified formula for emissivity:

Radiation Configuration FactorRadiative heat transfer rate between two gray bodies can

be calculated by the equation stated below.

Radiation Configuration Factor (cont.)

A simplified version is obtained by combining fa and fe giving the Radiation Configuration Factor (f):

Resistive Electrical Heating

• Electrical Heaters

• Heat Tracing

Electric Heaters

Heat Tracing• Used on mechanical piping systems that carry borated water, caustic soda, and

other solutions• Performs critical process temperature control• Also applied on water piping systems to prevent them from freezing in cold

weather• Application must include

– Identification of requirements– Heater design considerations– Power systems design considerations– Temperature control considerations– Alarm considerations– Finished drawings and documents– Installation of materials– Startup testing– Temperature tests– Maintenance of electric pipe heating systems