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Need a formula for CFM

  1. Mar 3, 2010 #1
    I just want the formula that shows how to find for CFM. I have the pipe diameter and the PSI that are available. I'm just using air, nothing special.
     
  2. jcsd
  3. Mar 3, 2010 #2

    stewartcs

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    Science Advisor

    Can you be more specific with what you want to know? I presume you mean how to determine the flow rate of air in cubic feet per minute through a pipe when knowing only the pipe diameter and back pressure?

    CS
     
  4. Mar 3, 2010 #3
    Yes, that's exactly right. I know there's a way to do it. I just haven't been able to work that out.
     
  5. Mar 3, 2010 #4

    russ_watters

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    Staff: Mentor

    The pressure is the static pressure in the pipe? That isn't enough to determine the airflow. You need a differential pressure across an orifice, pressure drop in a venturi or velocity pressure from a pitot tube.

    Once you get some form of velocity pressure (and if you give us more details about your setup, we can tell you how to measure/estimate it), you can use a form of Bernoulli's equation to find the velocity:

    V=4005*sqrt(Vp)
    where
    V= velocity in fpm
    Vp= velocity pressure in inches of water gauge.
     
    Last edited: Mar 3, 2010
  6. Mar 3, 2010 #5
    We're going to have a compressor maintaining a constant pressure of +100psi regulated to 100psi and a 3/8" ID for the pipe at its' most narrow.
     
  7. Mar 3, 2010 #6

    russ_watters

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    Staff: Mentor

    That's static pressure. It doesn't have anything directly to do with velocity (consider an air tank pressurized to 100psi: no airflow at all). What is this pipe supplying? Or is it open ended?
     
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