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Fan CFM Calculator

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berd_2
berd_2 Member Posts: 7
in THE MAIN WALL
How do you figure out how many cfm a squirrel cage fan is producing?

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  • Pete Groenewold
    Pete Groenewold Member Posts: 6
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    Fan Curves

    Blower manufacturers provide "fan curves," which are simply graphs that relate CFM, static pressure, RPM, and horsepower for a given blower (see http://www.delhi-industries.com/Curves/GDDcurves.pdf for examples).

    Although it is sometimes difficult to know who made the blower in your equipment (Lau? Morrison?) I've found that most dimensionally-similar blowers have fairly similar curves even when they're made by different manufacturers. That's handy, because you can print out or download Delhi's curves (for example) and know they'll be +/-5% accurate for other blowers you'll encounter.

    The variables you'll need to know include the width and diameter of the blower wheel, whether it's belt-drive or direct-drive, and whether the housing is standard or tight.

    HTH,

    PG
  • Brad White_2
    Brad White_2 Member Posts: 188
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    Duct Traverse

    Another way, if you do not have the fan curve, is to find a straight section of duct (7x major diameter if possible, and well-sealed) and measure the velocity with a pitot tube and gauge. (Dwyer makes a nice one, electronic, for about $350, but borrow one if you can.) Divide the velocity by the SF cross sectional area of the duct, and you have CFM. Over-simplified, but there it is. If you buy the anemometer (fancy word for the air velocity gauge) you have it forever.
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