N.R. Murphy Ltd. - Fans - Most Commonly Used Fan Laws

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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