Once the fan is selected, the fan curve needs to be reviewed. There are two major things to look for. Is the fan stable? And does it allow for adjustment?

A stable operating point can accommodate a moderate variation in static pressure, which not significantly changing the volume. The application determines the volume of air required, while the static pressure is determined by the installation. To accommodate for variances that can occur, careful review of the selected product’s operating point is important. Generally, if the operating point is located in the vertical portion of the fan curve, it is a more stable selection. The fan manufacturer typically determines the maximum system curve based on the optimal range allowing for a margin of error in static pressure.

Some fans can have an unstable area on the fan curve called a stall region. Fans should never be selected in this region because multiple air volumes are possible at one static pressure. The fan is unable to find a stable operating point, so it hunts between the multiple volumes. A fan operating in this stall region can cause excess vibration and stress on all system components, resulting in premature failure. This stall region is typically associated with axial fans.

Once the fan has been checked for stability, determine if the fan allows for adjustment in fan RPM for system variations. A good guideline is to choose a fan that allows for both increases and decreases in RPM for final system balancing.