On Mon, 29 Dec 2003 09:56:07 -0800, Grant Erwin wrote:
Gary Coffman wrote:

Yes it makes sense.
Go he http://air.ingersoll-rand.com/CMP/sa...vertlevel3.htm
and then try to explain why the CFM ratings of these compressors don't
markedly change over a 75 to 175 PSI range.

For example, the IR 5 hp dual stage I have is rated at 14.9 CFM @75 PSI,
14.8 CFM @125 PSI, and 14.7 CFM @175 PSI. Same compressor, same
displacement, same speed, and nearly the same CFM over a pressure range
of more than 2:1. If we were to believe they're talking about CFM of
pressurized air, the gas law tells us we would see more than a 2:1 change
in CFM with a 2:1 change in pressure. But we don't. The only way we can
see the numbers they publish is if they're referring to CFM at 1 standard
pressure.

I don't see it that way. If the spec isn't about CFM of pressurized air, then
why would the numbers differ at all for different output pressure? And why
quote CFM into different pressures? Why quote pressures at all?

Volumetric efficiency of a piston pump (or an engine) is a function of the
pressure differential across it (among other things). As the back pressure
increases, volumetric efficiency drops. It doesn't drop a lot in the above
mentioned compressor, but it does drop.

If you look at the specs on my little Chinese portable compressor, you see
it drops a lot more with increasing output pressure. That's partially because
it is a single stage compressor, partially because it turns faster (volumetric
efficiency declines with speed), and partially because the valving isn't as
well designed as the IR compressor, so it doesn't breathe as well.

Gary