Fan laws
Fans are usually made in ranges of size and speed and if, in a given range, each one is identical in all other respects than size to the others, the fans are said to be “geometrically similar”. Certain laws govern the relative performance of these fans when working at the same point on the pressure-volume characteristic and may be stated briefly as follows:
With constant impeller size.
1. Volume flow varies directly as the speed of rotation.2. Pressure developed varies as (speed of rotation)2
3. b.h.p. absorbed varies as (speed of rotation)3
With constant speed of rotation
4. Volume flow varies as (impeller size)35. Pressure developed varies as (impeller size)2
6. b.h.p. absorbed varies as (impeller size)5
Consequently with varying speed of rotation and impeller size
7. Volume flow varies as (speed of rotation) x (impeller size)38. Pressure developed varies as (speed of rotation)2 x (impeller size)2
9. b.h.p. adsorbed varies as (speed of rotation)3 x (impeller size)5
The main use of these laws is to predict performances of similar fans.
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