Gary

Sorry, but you are mistaken. Most compressors are rated in delivered
flow. Centrifugal units are frequently rated in inlet cfm. Somewhere
in this thread I gave the example of a compressor rated at 10 cfm and
a very high pressure. Specifially I will use a high pressure unit
that existed at Mare Island Naval Shipyard in Vallejo CA, It produced
15 CFM at 4500 psi, but it's inlet capacity was about 500 cfm at 14.7
psi. Think of it this way. If you have an air tool that requires 4.5
cfm at 90 psi, what size compressor do you need to run it? Remember
it needs 4.5 cfm at 90 psi. Convert 4.5 cfm at 90 psi and 100% rh to
cfm at inlet conditions and I think you will find you have a much
larger number.
Gary H
On Fri, 26 Dec 2003 01:41:01 -0500, Gary Coffman
wrote:

On Wed, 24 Dec 2003 19:50:30 -0600, Richard J Kinch wrote:
jim rozen writes:
Cubic feet per minute is only a time
rate of volume.

No, no, no, no, no. CFM in compressors refers to the flow rate OF THE
INPUT AIR AT ATMOSPHERIC PRESSURE.

That's right. When you see a compressor rated at say 10 CFM @ 90 PSI,
that means 10 CFM of *inlet* air compressed to 90 PSI. It does NOT mean
10 cubic feet of 90 PSI air. This should make sense when you notice that
a compressor may have several fairly similar CFM ratings given at several
different pressures. That's because the only differences are due to
differences in the pumping *efficiency* of the compressor at different
delivery pressures. It is still inhaling roughly the same amount of air per
stroke at the same strokes per minute, so it still has roughly the same
CFM rating whether that's given at a delivery pressure of 45 PSI, 90 PSI,
or 140 PSI. It just takes longer to initially pump up the tank to the higher
delivery pressures.

Gary