# PSI to CFM

If I have an unlimited air supply with 800 PSI and fed through a 1' pipe how do I calculate the CFM

## Answers and Replies

I believe thie thread provides the answer you are looking for:

https://www.physicsforums.com/showthread.php?t=129961

In particular post #3:
Assuming no change in height, the pressure would be equal to 1/2 the density * (velocity of air) squared, or velocity = sqrt (2*Pressure/density) and flow rate equals velocity times cross sectional area, so cfm = area of pipe * sqrt (2*Pressure/density)

russ_watters
Mentor
800 psi is way above the choke pressure for air, so no form of Bernoulli's equation will apply (that version is only good up to a few hundred fpm) and the max velocity is the speed of sound.

However, these questions almost always gloss over the issue of the outlet pressure: is this really 800psi air being released to atmosphere (an extremely unusual scenario) or is it being transported through a pipe to a regulator to use at 100psi? It is much more likely the actual pressure drop is only a few psi to deliver the high pressure air to a load...

...so we need more information about what this system is doing. Where is the air going?

If you are talking about air flowing only through a straight smooth (no friction) pipe then it would go a little something like this.

We can assume that 800psi is well above choked pressure as russ_waters has said. This means your air can only go as fast as the speed of sound.

speed of sound = sqrt(kRT) in this case I will use 1115 feet/sec.

We already know the diameter of the pipe so it becomes an easy problem.

CFM = pi()*(diameter/2)^2*(speed of sound)*(60sec/min)

CFM = 52500