Heat Dissipation in Electrical Enclosures

The accumulation of heat in an

enclosure is potentially damaging

to electrical and electronic devices.

Overheating can shorten the life

expectancy of costly electrical

components or lead to catastrophic

failure. It is therefore important that

system designers are aware of the

temperature implications of their

designs prior to implementation.

 

Enclosure Temperature Rise The

temperature rise illustrated by the curve in the

graph below is the temperature difference

between the air inside the enclosure and the

air outside the enclosure (or ambient air

temperature).This value is described in the

graph as a function of input power in watts per

square foot. In order to predict the

temperature inside the enclosure, the

temperature rise indicated in the graph must

be added to the ambient temperature where

the enclosure is located.

The enclosure temperature rise is not

dependent on the ambient temperature; rather,

the temperature rise for a given enclosure and

heat input are constant. For example, if the

graph indicates a temperature rise of 30° F, the

interior of the enclosure will be 30° F warmer

than the temperature in the surrounding area.

If the temperature in the surrounding area

reaches a maximum of 100° F then the

enclosure interior will reach a maximum

of 130° F.

Since temperatures in an environment often

vary widely, temperatures within enclosures

will also vary. In general, industrial

environments are warmer in the summer than

in the winter.Therefore, when calculating the

warmest enclosure temperature, use the

maximum ambient temperature that is attained

in a given environment.

 

Enclosure Heat Input For any temperature

rise calculation, the heat generated within the

enclosure must be known.This information

can be obtained from the supplier of the

components mounted in the enclosure. Heat

input values are usually given in watts, but may

also appear in BTU/hour. BTU/hour can be

converted to watts by dividing the value by

3.414 (for example, 341 BTU/hour = 100 watts).

It is not possible to approximate the heat input

for a particular application based on enclosure

size. Heat input varies from application to

application for all enclosure sizes.The system

designer must obtain estimates of heat

input from the information that is available.

Safety factors should be considered if any

uncertainty exists.

 

Enclosure Surface Area The physical size

of the enclosure will be the primary factor in

determining its ability to dissipate heat.

The larger the surface area of the enclosure,

the lower the temperature rise due to the heat

generated within it.

To determine the surface area of an enclosure

in square feet, use the following equation:

Surface Area = 2[(A x B) + (A x C) + (B x C)]÷

144 where the enclosure size is AxBxC

This equation includes all six surfaces of the

enclosure. If any surface is not available for

transferring heat (for example, an enclosure

surface mounted against a wall), it should not

be included in the calculation. It is also

noteworthy that enclosure volume cannot be

used in place of surface area

 

Other Enclosure Materials The graph

below applies to enclosures that are gasketed,

non-ventilated, and constructed of painted

steel. Paint color has little effect on enclosure

temperature rise, except when exposed to sun

(see “Outdoor Applications”). Higher

temperature rises can be expected with

aluminum and stainless steel enclosures due to

the poor radiant heat transfer effects of their

metallic finishes.To find the temperature rise of

these enclosures, multiply the results found in

the graph by 1.5. Non-metallic enclosures have

similar heat transfer characteristics to those

constructed of painted steel, so the graph can

be used directly despite the difference in

material.