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  1. #1
    Join Date
    Nov 2009
    Location
    karachi
    Posts
    174

    calculate cfm in rectangular duct

    Can anyone explain me I want to calculate the cfm in a rectangular duct. what is the procedures and what steps I should follow.

  2. #2
    Join Date
    Dec 2002
    Location
    SouthEast NC ICW & Piedmont Foothills
    Posts
    7,635
    traverse the duct with test holes





    link didn't work
    It`s better to be silent and thought the fool; than speak and remove all doubt.

  3. #3
    Join Date
    Mar 2003
    Posts
    7,326
    area of duct in inches divided by 144 equals square feet. find velocity in feet per minute and multiply to get cfm. or get the right tools.

  4. #4
    Join Date
    Jul 2002
    Posts
    936
    You need to take probably about 20 samples across the duct to finds the flow pressure. Static press-Total press=flow pressure. You can then calculate with your area.

    http://www.dwyer-inst.com.au/Product...troduction.cfm

  5. #5
    Join Date
    Mar 2008
    Location
    Central NH
    Posts
    141

    Quick, but not Cheap

    Flow Hood. Hold it over the opening and read the scale. Problem solved.

  6. #6
    Join Date
    Sep 2010
    Location
    NY
    Posts
    325
    Quote Originally Posted by hedtedjr View Post
    Flow Hood. Hold it over the opening and read the scale. Problem solved.
    Flow hood is the best.... cover and read, cover and read. What was the old fashion way? traverse with a tip that would pick up velocity and somehow it would cancel static to get only velocity and then you get the average of the sum. A few more steps and you have CFM.

    I would like to try a flow hood against an anemometer or a u tube to see how close the readings would be.
    Where is the manual? What does it say?

  7. #7
    Join Date
    Nov 2010
    Location
    Nevada
    Posts
    38
    Spend a few bucks and hire a professional with a flow hood(that has been calibrated recently).

    SG

  8. #8
    Join Date
    Jul 2010
    Location
    Knoxville, TN
    Posts
    14
    A traverse is best and most accurate. But just to get a ball park, you can measure velocity in a couple of places across the duct, take the average, and multiply by the square footage of the duct. Hot wire anemometers are pretty cheap.

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