The series fan arrangement is used when higher pressures than a single fan provides are required. Another use case is the requirement for increased safety of fan operation in garage ventilation, where one fan is permanently operated and the other serves as a backup in case of failure of the first fan.
The simplest case is the combination of identical fans that have the same performance characteristics. In the case of speed control of both fans, it is necessary to ensure perfect synchronization of the controllers or to use one speed controller for both fans.
The position of the operating point must always be selected correctly, taking into account the resulting characteristic of both fans in series and any unstable parts of the characteristic. The resistance characteristic of the system must lie in the stable part of the characteristic of both but also one fan in the whole band with an appropriate margin. This will also ensure that the operating point is also in the stable part of the resulting characteristic.
In no case do we recommend the series connection of different fans with different characteristics. It is not recommended to combine different fans with different characteristics, such as the characteristic of an axial fan with an unstable region and a maximum efficiency in the high flow and low pressure regions and the stable characteristic of a radial fan with backward blades and a maximum efficiency in the high pressure and low flow regions. These cases may cause problematic operating conditions that can lead to damage to one of the fans.
