Message text written by Randall
>Pay no attention to the horsepower ratings, they are meaningless.
>Instead, look at cfm at 90 psi. For most air tools, you are going to
>want at least 4-5 cfm at 90 psi. Sand blasters are even higher. And,
>be sure the rating is real cfm, not standard cfm. The standard cfm (or
>scfm) rating at 90 psi is about 7 times the real cfm, so you would need
>28 scfm @ 90 psi to run a 4 cfm air tool. Note that the 90 psi part is
>also important, since even the scfm rating drops with pressure.
SCFM is a way of corolating air flow across a pressure change. When you
take a compressable (like air) and change the pressure from zero PSIG to 90
PSIG (15 PSIA to 105 PSIA) the volume drops by a factor of 7. In
otherwords if you measured the airflow at the intake and the output there
would be 7 CFM at the inlet and 1 CFM at the outlet. The problem with
comparing CFM ratings of different compressors is that the CFM is rated at
different output pressures.
Additionally, the pump capacity diminishes as the pressure goes up so
comparing SCFM ratings of compressors at different pressures is also
problematic. And finding performance curves for the type of compressors
that we hobbyists are likely to consider is highly unlikely.
<snip>
>Look for a shut-off pressure that is at least 120 psi, and a two-stage
>piston-type compressor (not all 2 cyl compressors are 2 stage). IMO the
>'oilless' compressors are also useless for running tools.
Yup. Two stage compressors have a higher volumetric efficiency. The
reasons are probably beyond my Junior level thermodynamics background.
Oilless compressors are just plain obnoxious.
>Randall
Dave
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