Formulas For Fan Applications

 

 

How to Use the Fan Laws for Performance Changes

 

There are two reasons why a fan’s performance may need to be changed:

• The system or area requires additional airflow (CFM) .

• The actual system static pressure (SP) is different from the design value.

When these situations occur, it is important to understand how they can affect the fan’s performance.

The effect on the fan’s performance can be shown by using the Fan Laws, shown below

 

         

 

           

Subscript 1 = existing conditions

Subscript 2 = new conditions

 

SYSTEM CURVE EQUATION

 

The following formula is used to find other points on the system line when SP1 and CFM1 are known. Most, but not

all, systems follow this relationship.

 

 

IMPORTANT NOTE:

 

The new RPM should be checked to make sure it does not exceed the maximum allowable RPM for the fan that is installed.