Fan laws, can be used to accurately predict changes (assuming the fan diameter and air density are constant).

Fan law equations


Subscript 1: Describes the existing conditions
Subscript 2: Describes the new conditions

The following example is typical of how the fan laws are applied:

A fan installed in a fixed system is operating at:

  • CFM = 10,000
  • SP = 1.50"
  • BHP = 5.00
  • RPM = 1,000

What RPM is required to move 25% more air (12,500 CFM) through this system?

NOTE: You can view this example as either the installation now desires more air than planned, or the balancing report showed 25% less air than specified.

By rearranging the cfm fan law:


The corresponding static pressure is:


The resulting BHP is:


According to the fan laws, in order to use the original fan, the speed must be increased from 1000 RPM to 1250 RPM, the motor must be changed from a 5 HP to 10 HP.

figure 7Figure 7 illustrates fan curves for both the original and new fan performance.

Important: Check to make sure that the new RPM does not exceed the maximum allowable RPM for the existing fan. Maximum RPMs are shown in fan catalogs. You should consult the fan manufacturer for additional information or if you would like to review the application.

More detailed information on these subjects can be found in both AMCA and ASHRAE publications.