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.
