What happens to the air system performance when fan rotation speed is changed?

Fan is designed to be operated in a defined design point. However, often the real operation point is something else.

If we analyze a fixed system with no change in the aerodynamics or airflow characteristics meaning ductwork with all related devices remains unchanged but only the sped of fan is changed. For this kind of assembly we can define the system laws to estimate the effect on

  1. volume flow capacity
  2. pressure
  3. power consumption

Effect of the density

Fan discharges the same volume of air with each revolution. An important fact is that a fan discharges the same volume of air regardless of gas density. This needs to be taken into the consideration when calculating the needed mass flows of the air systems on varying temperature levels. Density effects on the power consumption of the fan direct proportionally.

Volume flow capacity

Fan speed effect on flow rate (qv) is proportional to the change in fan speed.


Fan speed effect on pressure (p) is proportional to the square of the change in speed.

Power consumption

Change in the power requirement (P) is proportional to the cube of the change in speed.


When changing the air system topology, process conditions or fan parameters the process and operation point of the fan needs to be estimated. Relatively small change can lead to the situation where process cannot be operated any more with the existing machinery.

Kalle Riihimäki

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