Fan Laws - Rules for changes to fan characteristics

By applying the following laws it is possible to determine the change in performance of a fan unit due to:

1) Change in speed:

A) Volume (V) varies directly proportional as the fan speed (N): V2/V1 = N2/N1

B) Pressure (P) varies as the square of the fan speed (N): P2/P1 = (N2/N1)²

C) Power absorbed (K) varies as the cube of the fan speed (N): K2/K1 = (N2/N1)³

The above laws apply to fans connected to a system operating against a fixed resistance and handling air or gas at a constant density.

2) Change in diameter:

A) Volume varies as the cube of the diameter (D): V2/V1 = D2/D1

B) Pressure varies as the square of the diameter (D): P2/P1 = (D2/D1)²

C) Power absorbed varies as the diameter to the power five: K2/K1 = (D2/D1)^5

These laws relate to geometrically similar fans only.

3) Change in density:

A) Volume remains unchanged.

B) Pressure varies directly as density (I): P2/P1 = I2/I1

C) Power absorbed varies directly as density (I). K2/K1 = I2/I1

These fan laws apply to a fan working at a fixed speed and connected to a system in which the gas density has been changed.