The fan curve is graph depicting the various points that the fan can operate. It indicates the amount of CFM the fan will provide at a given total static pressure, which is dependent on the connected ducted system. Fans should be selected to operate at the stable region. The stable region is the area on the fan curve where there is a single flow rate [CFM] value for ever pressure value. In the unstable region, a pressure value can have multiple CFM values, which will cause the fan system to surge. The stable region also has very little change in CFM for large changes in total pressure.
The second curve that works in conjunction with the fan curve is the system resistance curve. This curve is summation of all the friction losses in the ducting system at varying CFM's. Typically, the friction losses are summed up at the design CFM values, then this design point is connected to the 0,0 point by an upward sloping square polynomial curve, as shown below. If for example, the ducting system has a closed damper or dirty filter, this will cause the curve to shift to the left. If a damper is opened or the dirty filter is cleaned then the curve will shift to the right.
Combining the system curve with the selected fan curve, determines the operating point of the fan system, indicated in the figure below in green. Following the vertical line down determines the CFM and the horizontal line from that point indicates the operating total pressure. During system operation as dampers close, the system curve shifts toward the right in red. This movement decreases the amount of CFM delivered by the fan. The opposite occurs as dampers open in the system, the amount of CFM delivered by the fan increases.
It has been shown that the amount of CFM blown by a fan can be changed by shifting the system resistance curve. However, the volumetric flow rate can also be changed by changing the speed of the fan, which shifts the fan curve.