# Need a formula for CFM

by shellback
Tags: formula
 P: 3 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.
P: 2,284
 Quote by shellback 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.
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
P: 3
 Quote by stewartcs 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
Yes, that's exactly right. I know there's a way to do it. I just haven't been able to work that out.

 Mentor P: 22,297 Need a formula for CFM 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.
P: 3
 Quote by russ_watters 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.
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.
 Mentor P: 22,297 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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