Fan laws: The infinity laws for fans are used in HVAC to express the relationship between variables involved in fan performance and power. They are the basic proportional relationships between:
They are frequently used to estimate air moving device requirements. Additionally the fan laws can be used to calculate the performance of air movers in different sizes and speeds.
The volume of air moved by a fan changes in direct proportion to the change in speed of the fan impeller and can be calculated as follows-
V2 = (u2/U1) x V1
This is a useful relationship as it allows us to use changes in volume when calculating the other fan laws.
A fan produces 1000m3/h at an impeller speed of 2000rpm, what is the resulting airflow if the speed is reduced to 1000rpm?
V2 = (1000/2000) x 1000 = 500m3/h
The pressure developed by the fan and the pressure drop in a system varies with the square of the change of speed of the impeller or volume passing through the system.
P2 = (U2/U1)2 x P1
A fan produces 200Pa at an impeller speed of 1000rpm, what is the change in pressure if the impeller speed was increased to 2000rpm?
P2 = (2000/1000)2 x 200
P2 = 800Pa
The power required to move air through the fan or system varies with the cube of the change in volume and is calculated as follows:
W2 = (V2/V1)3 x W1
Interestingly the noise generated by the fan is also a power relationship and again varies with the cube of the change in air volume being handled.
Fan laws are essentially about impellers and what happens to their characteristics when they undergo changes in rotational speed, air density, or are scaled in size. They also help with the understanding of ventilation systems and the relationship between air volume flow rate and system total pressure.
For more information contact one of our technical engineers on 01782 349 430 or visit our technical pages for more information on fan selection material.