Inlet filter CFM rating required?

by Kevin
(Des Moines, WA.)

What CFM rating must an inlet filter posses to allow for a compressor output of 7 cfm @ 150psi?

Since I believe that as air gets compressed it will have lower volume therefore 7 cfm at 1 atmosphere is a lot less air (by molecule/second count) than 7 cfm at 150 psi.

I would like a 7 CFM output at 150psi and am wondering how much volume of air my inlet filter needs to pass at 1 atmosphere in order to limit the pressure drop across the inlet filter and allow for approximately 7 cfm @ 150psi to be produced by the pump.

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Jan 13, 2019
You're welcome.
by: Doug in

I know what you mean about terminology.

Glad my answer made sense to you.

Have fun!

Jan 13, 2019
Thank you!
by: Kevin

Thank you for the great answer! I learned a lot from your excellent explanation.

Based on on your answer it seems as if maybe the quoted flow ratings, for the commonly available inlet filters, I'm seeing, must be an indication of what can can reasonably be expected to be a supportable output cfm at typical working shop pressures, not the volume of air being taken in.

While I did wonder if that might be the case when I made my posted inquiry here, not being an expert in the terminology, I was struggling to ask about that in a coherent manner.

Thanks again, very helpful.

Jan 11, 2019
by: Doug in

The key is what's an atmosphere.

1 atmosphere ~ 14.7 psi ~ 1 bar

150 psi*v/t ~ 10.2 bar*v/t

10.2 bar x 7*v/t ~ 71.4 bar*v/t

v/t cancels- so filter passes 71.4 cfm @ 1 bar for 7 cfm @ 150 psi out.

The sticky part is you're never going to get to zero drop, due to inertia and restrictions in the pump itself.

On the other hand, pretty much any filter you use will likely easily pass way more than that with minimal drop.

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