> I was in one tool store, looking at pad sanders with ~15-20 SCFM
> (all at 90 PSI) needs. Then the store down the street showed me a
> couple of facts I don't understand. The first was a sheet of SCFM
> requirements for various air tools, including sanders, all with
> air needs around 4-7 SCFM. The second fact was that the SCFM spec
> for a tool was for 25% use: a 5 SCFM sander would need a 20 CFM
> compressor.
>
> So what is the story about CFM and SCFM and sizing compressors to tools?
Ken, IMO the story about 'consumer' quality (aka cheap) tools and
compressors is that the manufacturers are so wrapped up in 'numbers games',
they flat-out lie about specifications. That's not to say everyone does,
but ...
For one thing, very few tools seem to be rated in scfm. Typically, they
give ratings in cfm, which generally means at whatever the inlet pressure
is. This is not unreasonable, but has the effect of greatly reducing the
reported number. The "Average cfm" sheet your tool store showed you is much
the same thing. Look at it carefully, I'll bet it doesn't say "scfm"
anywhere. "scfm" is a legally defined term, they can be sued for mis-using
it. "cfm" can be defined anyway you like, just like "horsepower" and "fresh
frozen fish".
For example, 1 cfm @ 90psi is roughly 7 scfm (@ 90 psi).
Another problem is that for any given tool type, there is probably a 4:1
range in air consumption for different models and ratings. Any chart that
gives a more limited range has to have an implied limitation on which tools
they are talking about.
OTOH, since scfm numbers are higher, all compressors seem to be rated in
scfm. The slimier makes only specify it at 40 psi (or not at all) to cover
up their lousy efficiency at higher pressures.
So, one solution is to take the cfm rating of the hungriest tool you ever
plan to use, and the line pressure you plan to run. Divide the line
pressure by 15, add 1, and then multiply the tool cfm rating by that number.
That's the scfm you need out of your compressor. Then you can apply the
correction given at the web site you mentioned, as the acfm rating you need
for the compressor at your typical conditions (altitude and humidity).
Then, when you find out what that will cost, decide you can live with having
to stop occasionally to let the compressor catch up <g>
Randall
/// unsubscribe/change address requests to majordomo@autox.team.net or try
/// http://www.team.net/mailman/listinfo
/// Archives at http://www.team.net/archive/shop-talk
|