Sometimes it is necessary to install two or more fans "in series" in the same system. If the fan consists of a single device, the device is usually called a multistage fan. Such devices are rarely used in general ventilation and air conditioning systems, but are often used in specialized industrial systems.

In theory, the combined flow-pressure curve of two fans running in series is obtained by adding the fan pressures at the same volume flow rate. In fact, the volume flow rate is somewhat reduced by the increase in air density in the later stages. Performance losses are usually large due to uneven airflow into the second stage fan inlet.

It is recommended that the fan manufacturer review the proposed system design and make some estimates of the performance of the installation.

2. Fan running in parallel

Fans are often installed and operated in parallel in the same system, especially when high air volumes are required. The combined flow-pressure curve in this case is obtained by adding the volume flows of each fan at the same pressure.

If the inlet condition is restricted or the air flow into the inlet is uneven, the total performance of multiple fans will be less than the sum calculated theoretically.

Some fans have a positive slope on the pressure flow curve near the peak pressure point to the left.

If the fan running in parallel is selected in this positive slope range, then the fan will not run smoothly. The closed loop to the left of the peak pressure point is the sum of all volume flows at each pressure. If the system curve intersects the combined flow-pressure curve in the area where the loop is closed, there may be more than one point of operation. This can cause excessive air volume in one of the fans, and if the fans are driven separately, it is possible to overload a motor. This unbalanced airflow state will be capricious, resulting in intermittent load and no load fan.

The aileron controller at the fan outlet or the regulating damper at the inlet and outlet can be used to eliminate unbalanced airflow or to eliminate pulse and reverse operation, as well as damage to the pipe network or drive system.