Electronic Design

Estimating External Surface Case Temperature Rise

In military equipment packaging, it is often desirable to house electronic devices in closed cases for protection against dust, humidity, and low pressure. The initial analysis of anticipated case heat dissipation, required during the packaging study, can be tedious and lengthy, but this nomogram provides a useful preliminary estimate.

Only the ambient altitude and unit surface power dissipation density need to be known to determine the expected surface temperature rise over ambient. The as-sumptions and approximations required to simplify the relationships include: the case is closed and unlouvered; air is the surrounding medium; and free convection and radiation take place only from the case surface.

The procedure for use of the nomogram is as follows:

  1. Determine the amount of heat (in watts) to be dissipated by the case. For a closed unit with no special external cooling provisions, this would be the total power input.
  2. Determine surface power density in watts per square inch by di-viding the heat dissipated by the total exposed surface dissipation area (including ribs, etc.)
  3. Enter the nomogram with this value and the appropriate altitude and read the resultant temperature rise of unit case surface over the ambient. (Electronic Design, Oct. 26, 1960, p. 131)

This nomogram, by J.R. Baum of Motorola Military Electronics Div., Scottsdale, Ariz., exemplifies the design aids that were regularly sent to the magazine in the 1950's and 1960's. I suppose all of these nomograms ended up in some handbook. Are engineers still creating new ones?

See associated figure

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