Fans are fabricated to contend with a determined volume flow and pressure loss, usually known as "DESIGN CONDITIONS", calculated for the more extreme working requirements foreseen for the system in which they will be integrated.

Once the fan has been installed and set to work, the actual operation point may differ from what was first envisaged, and variations in volume and pressure will result from corresponding changes in the load or process.

Moreover, most of the fans have to cover, due to application requisites, a wide field of volume and pressure values, in accordance with the different workloads or varying conditions in the plant or installation.
As a consequence of this a means of regulating the fan operation has to be provided in many occasions .