A fan is normally required to convey a certain quantity that can be expressed in volume or in weight per unit of time and a cetain pressure normally expressed in mm H2O or Pascal, necessary to overcome the pressure drop that occurs the circuits, where this fluid can circulate.

   To perform the task required of it, the fan – whatever type it may be – must transmit a certain quantity of energy to the fluid that passes through it, energy that it receives, in turn, from the electric control motor.Of course the two energies are not the same otherwise fan efficiency would be 100%. The mechanical energy the motor supplies the fan with always higher than the energy the fan transmits to the conveyed fluid. So fan efficiency is obtained from the ratio between the first and second energy.