In the following graph, the vertical axis represents the heat to be removed and the horizontal axis represents the airflow; both axes are logarithmic.

The sloping lines define the temperature rise in ^o C. To use the graph, find the sloping line that represents the permitted temperature rise. Then, find the point on this line that corresponds to the heat to be removed. The horizontal position of this point shows the airflow required.

Obstructions in the airflow path cause static pressure within the enclosure. To achieve maximum airflow, obstructions should be minimized. However, obstructions in the form of baffles may be necessary to direct the airflow over the components that need cooling.

The left figure  shows the nonlinear relationship between airflow and static pressure for a typical fan.

The left figure and the figure below shows the nonlinear relationship between airflow and static pressure for a typical fan. The performance of a fan in a specific application is determined by the intersection of the System Impedance Curve and the Fan Characteristic Curve. The System Impedance Curve is a property inherent to an individual electronics enclosure. This curve can easily be generated experimentally, by testing the enclosure pressure at various airflow rates.