Need a formula for CFM

  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. stewartcs

    stewartcs 2,279
    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. 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. russ_watters

    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. 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. russ_watters

    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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