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What is the computation of air flow for an exhaust system?

GBA Editor | Posted in General Questions on

Equipment with two motors with 8″ diameter provision for exhaust system. It is OK if I’m going to use Y connector ( g.i. sheet ) with 10″ diameter and 2-8″ diameter and split them with flexible duct? Existing supply for exhaust system is 12″ diameter.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Dear Anonymous,
    It would be helpful to know more.

    1. What type of equipment?

    2. What distance are you running the ducts?

    3. How many cfm are you moving?

    Duct design is complicated, and is best done by following the Manual D procedure. Some people use rules of thumb, however, although these rules of thumb can get them in trouble.

    In case you have forgotten your 7th grade math, the area of a circle (that is, the cross-section of a round duct) is pi r squared. Therefore, the area of a circle is:
    8 inch diameter = 50.3 square inches
    10 inch diameter = 78.5 square inches
    12 inch diameter = 113.1 square inches

    If you want some rules of thumb, I've heard this one:
    "On a short run a 10" flex duct will carry about 400 cfm. These are guesstimates to close to what they are at short runs:
    8" duct, 200 cfm
    9" duct, 300 cfm
    10" duct, 400 cfm
    12" duct, 600 cfm."

    That rule of thumb came from here:
    http://hvac-talk.com/vbb/showthread.php?t=105192

    The use of this rule of thumb is not recommended without verification.

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