The practical limits of forced-air cooling of electronic equipment

Cooling with forced air is analyzed from a general perspective. It is demonstrated that there are physical processes that limit the maximum dissipation of heat from a frame or shelf of electronic equipment. These constraints are shown to arise from the limitations on allowable acoustic noise. Some results from fluid mechanics are used to construct a pressure-flow model of heat removal. Two problems of determining maximum heat dissipation are then described. In the first, the maximum dissipation in a fan-cooled shelf is derived subject to an acoustic noise constraint. In the second problem, a shelf of circuit cards is examined for which the pressure drop is already given. For this case the maximum dissipation is derived and shown to be proportional to the square-root of pressure and to the top area of the shelf. It is noted that both of these results are essential in designing new system and in specifying high-dissipation elements such as power supplies