# Need a formula for CFM

## Main Question or Discussion Point

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.

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stewartcs
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

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.

russ_watters
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.

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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.

russ_watters
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?