Fans - Most Commonly Used Fan Laws
1. When speed is varied on the same size fan:

Fan air delivery varies directly as the speed

cfm2 = (rpm2/rpm1) x cfm1

Fan pressure varies as the square of the speed

SP2 = (rpm2/rpm1)2 x SP1

Required fan horsepower varies as the cube of the speed

hp2 = (rpm2/rpm1)3 x hp1

2. For variations in air density:
• fan capacity remains constant
• fan pressure varies directly as density
• fan horsepower varies directly as density
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