The performance of a fan in terms of pressure, volume flow
and power absorbed depends on a number of factors, the most
obvious of which are:-
1. The design and type of fan,
2. The point of operation on the volume flow/pressure characteristic,
3. The size of fan,
4. The speed of rotation of impeller,
5. The condition of the air or gas passing through the fan.
It is customary for a manufacturer to make a range of fans
of varying size to single design, thus producing a series
of geometrically similar fans (homologous series). It is
convenient to be able to compute the performance of each
fan from the minimum test data. The pressure/volume flow
relationship it is not generally capable of being expressed
as a simple mathematical function. However, by considering
any single point of operation on the characteristic curve
(for example, the point at which the fan efficiency is a
maximum), it is possible to drive some simple relationships,
generally known as the fan laws.
For all fan laws: hTa = hTb and (point of rating)a = (point
of rating)b
