How to Calculate Fan Efficiency

Written by petra wakefield
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How to Calculate Fan Efficiency
(Hemera Technologies/ Images)

Fan efficiency measures how much airflow a fan produces for a given amount of power, as compared to the ideal airflow output. In an idealised fan, there is no pressure drop in the system; but in a real-world fan, the air pressure drops between the input and the blower, causing the amount of air moving out of the fan to be less as well.

Fan efficiency depends upon both the type of fan and the speed at which it operates, according to San Jose State University. Fan efficiency may be calculated mathematically, but manufacturers also provide fan curves, which are a simple tool for estimating efficiency.

Skill level:

Things you need

  • Fan curve

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  1. 1

    Test the fan to determine the brake horsepower (BHP). You may be able to estimate BHP from data given by the manufacturer, but according to George Clifford in the book "Heating, Ventilation and Air Conditioning," you can only determine actual BHP by testing the fan.

  2. 2

    Calculate the fan total pressure by subtracting the total pressure at the outlet from the pressure at the inlet.

  3. 3

    Calculate fan efficiency by multiplying the fan volume in cubic feet per minute (cfm) by the fan total pressure. Divide this number by 6,356 multiplied by the BHP.

  1. 1

    Pinpoint the air volume produced by the fan on the horizontal axis of the graph.

  2. 2

    Follow the vertical line from this point up to the curve marked "TE" for total efficiency.

  3. 3

    Move horizontally to the right vertical axis. The number on this axis gives the total fan efficiency.

Tips and warnings

  • Fan curves can also be used to determine pressure and brake horsepower.

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