wrote:
On Jun 18, 7:25 pm, GeorgeB wrote:

Based on the units with bigger motors, the HP is a marketing game, not
real. Industry assumes ... ASSUMES ... about 4 cfm (to 100 psi) per
horsepower. Small units will be less efficient ...maybe 3 cfm. Large
(50 hp+) units will be a little better, perhaps 4.4 or so.

It is often missed that capacity is INLET air.

This issue has been beaten to death on the rec.crafts.metalworking
discussion group. Everyone knows that Sears compressors are wildly
overrated. I was in the transportation air brake industry for years,
and me and my guys rebuilt about 12,000 compressors in that time and
tested every one of them on a dyno. We found that, as you have said,
that one HP will pump around 4 CFM. We had the cutout at 120 psi, but
of course, as also mentioned, there's nowhere near 4 CFM being
delivered at 120 psi. The 4 CFM is free air, at atmospheric pressure.
A really good compressor has as little volume as absolutely
possible when the piston is at TDC. This is to drive out as much of
the compressed air as possible; any air left in the cylinder at TDC
will expand as the piston travels downward again and so the intake
valves won't open until the cylinder pressure drops below atmospheric
pressure. A cheap compressor might have so much unswept volume that,
at the higher pressures, the intakes don't open until the piston is
halfway down. Not efficient at all. Unswept volume includes that
between the piston and head, whatever cavities the intake and
discharge valves may have, and so forth.
So the CFM rating is a zero discharge pressure, and it will
drop, depending on the efficiency and overall design of the
compressor, to considerably less as the tank pressure rises. You can't
take cylinder area and multiply it by stroke and RPM to get a reliable
CFM figure, but I think that's what the retailers do. You will be
disappointed if you have a 4 CFM spray gun and expect the 4 CFM
compressor to keep up with it. The spray gun requires 4 CFM at around
40 or 60 psi, the compressor is rated at zero.

Dan


Dan,

While I agree with almost all of what you have said, and you are right
on the mark, I have seen, and often, compressors that are rated at a set
pressure, ie... indicating a set displacement (in cfm) AT a pressure of
xx psi. Curtis and other old line commercial compressors are often rated
this way, cfm @ psi.

George