Question

HP of fan/blower is proportional to _______, where N is speed.
1. N
2. N²
3. N³
4. √N

Answer 1

Correct Answer - Option 3 : N³

Fan laws:

The fans operate under a predictable set of laws concerning speed, power and pressure. A change in speed (RPM) of any fan will predictably change the pressure rise and power necessary to operate it at the new RPM.

Flow: Flow is directly proportional to the speed.

Q ∝ N

Varying the RPM by 10% decreases or increases air delivery by 10%.

Pressure: Pressure is directly proportional to square of the speed.

SP ∝ N2

Reducing the RPM by 10% decreases the static pressure by 19% and an increase in RPM by 10% increases the static pressure by 21%.

Power: Power requirement is directly proportional to cube of the speed.

P ∝ N3

Reducing the RPM by 10% decreases the power requirement by 27% and an increase in RPM by 10% increases the power requirement by 33%.