Culver Armature and Motor

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Tools Fan Laws

Fan Laws


A fan operating in a given system moves a definite volume of air through the system and definite SP will result from this flow. If the speed of the fan  is changed, different performance characteristics results. In any single fixed system if the fan speed (RPM) is increased, the volume flow rate (CFM), static pressure (SP), and horsepower (HP) will also increase. CFM is the most commonly changed variable, the example below is based on a change in CFM and illustrates how the new factors can be figured, based on this change.

The Fan Laws are:
  1. New CFM/Old CFM = New speed/Old speed
  2. New SP/Old SP = (New Speed/Old Speed)2
  3. New HP/ Old HP = (New Speed/Old Spees)3


You need to increase the CFM on a belt drive fan from 4500 CFM to 8000 CFM. The fan is currently running at 350 RPM, the original motor is a 3/4 HP, and the original static pressure is 0.75.

1. What is the new fan speed?

  • New speed = Old speed  X  (New CFM/Old CFM)
  • New speed = 350  X  (8000/4500)
  • New speed = 622   (RPM changes directly as CFM changes)

2. What is the new static pressure?

  • New SP = Old SP  X  (New speed/Old speed)2
  • New SP = 0.75  X  (622/350)2
  • New SP = 2.37   (SP changes as the square of the CFM)

3. What is the new HP?

  • New HP = Present HP  X  (New CFM/Present CFM)
  • New HP = .75  X  (8000/4500)3
  • New HP = 4.22   (HP changes as the cube of the CFM)  
Last Updated on Wednesday, 10 June 2009 11:58 


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