CFM equivalency

[Gregg Feldstone]

Does anyone know the formula for converting CFM@100psi to CFM@90psi and visa verse ? Someone on another thread said to subtract or add 33%. A compressor manufacturer said that's way too much but did not tell me a formula.

[Jeff Booth]

Are you doing the compressor for a short burst or continuous? The short burst is easy because of the pressure regulation, with the continuous the pressure output varies with the flowrate.

For the compressor - You need to know about your load. I would use the notion that volume flow tracks with the square of the pressure - that would cover orifice losses and turbulent losses. If it were nothing but piping then you would have to decide if the flow in the pipe was laminar or turbulent because pressure losses resulting from laminar flow track linearly with the flow. Unless you really have a lot of piping to worry about then skip adding that, else get out your Moody chart and apply your pipe diameter and length to that as well.

To use the square law, take the given flow at 100 psi - divide the 100psi by the square the given flow to determine a constant. Then flow at 90 psi = the square root of 90psi/constant

Constant = 100psi/(CFMat100)^2

CFMat90 (CFM@90) = (90PSI/Constant)^0.5

If you have all the data points and you want to PM me, I would help you with it.

Jeff