Determining System Impedance
Determining the actual airflow produced by a fan mounted in an enclosure is much more difficult than calculating the airflow required. Obstructions in the airflow path cause static pressure within the enclosure, referred to as system impedance (expressed in static pressure as a function of flow in CFM). To maximize airflow, any resistance should be minimized, except for baffles that may be necessary to direct the airflow.

A typical system impedance curve for electronic equipment follows what is called the "square law". This law states that static pressure changes as a square function of changes in the CFM. The chart to the right displays typical impedance curves. Static pressure through complex systems cannot be easily arrived at by calculation.

The experimental method of finding airflow through an enclosure is very accurate, but costly, time consuming and cumbersome. In practice, empirical methods are normally used to estimate airflow resistance. Experience shows that:

• An empty enclosure usually reduces airflow by 5 to 20%.
• A densely packed enclosure reduces airflow by 60% or more.
• Most enclosures have a static pressure between 0.05 and 0.15 in H2O.

Once you know the volume of air and the static pressure of the system to be cooled, you can determine the fan specifications for your product.