Allen,

When engineering a forced air cooling system, there are two first order variables placed into the equation: the heat generated by the device and the maximum temperature rise allowed.

The required fan CFM at sea level can be estimated as:

CFM = 3.16 * Watts of Heat Generated / ΔT

where ΔT is the maximum desired temperature rise and CFM is the cubic feet per minute of the fan

Note that this is only an estimate. If the equipment is to be operated at higher elevation levels for example, then the airflow must be increased as cooling is not determined by airflow but by the mass of air. Thus the reduced air density at elevation requires an increased CFM.

Then the system impedance must be estimated or measured. To measure the system impedance, a manometer is typically used while the required CFM is flowing. A simple example of this is:

This can be thought of as "back pressure" on the fan. A particular fan can only produce a given CFM under a given system impedance. Thus the engineer must consult the charts of the fan manufacturer to select an appropriate fan for the job. In high system impedance situations, placing fans in series (push / pull) is often more effective than a single higher CFM fan. In low impedance systems, parallel fans are also a design option. The number of fans can also be affected by system reliability goals.

The reduction of system impedance can be quite intuitive (e.g. straighter paths, larger exit vents, etc.) and can also become quite esoteric (shaped inlet and outlet ports, heat stacking, etc.). By constructing a simple manometer, the hobbyist can experiment with techniques to lower the system impedance and directly confirm the effectiveness. Any meaningful reduction would allow the fan speed to be reduced or a lower CFM (quieter) fan to be used (see formulas below and note about pressurized systems).

Since a well designed product will consider the altitude variations of where the product might be used, it is often possible to reduce the CFM or speed of the fan when the product will be consistently operated closer to sea level. There are some relationships that can help estimate the effects of such changes:

1.) CFM is directly related to speed ratio. Cut the speed in half and the CFM drops to one half.

2.) Pressure is directly related to the square of the speed ratio.

3.) Noise reduction of the fan is related to speed by approximately: ΔdB = 50 * log

_{10}(New speed / original speed)

So if you drop the fan speed in half, the CFM drops by 50%, the noise drops by 15 dB, and the temperature rise will double - assuming constant heat generation inside the device.

Some designs, particularly those with closely spaced cooling fins, require a minimum system impedance in order to properly cool the device. If the system impedance is too low, the air currents tend to flow around the closely spaced fins and thereby not adequately cool the device. By pressurizing the enclosure, the density of the air is increased thereby increasing the cooling capacity at the expense of a (typically) noisier fan.

- Glenn W9IQ