> Two stage compressor, minimum 6 to 7 horsepower,
Although the situation has improved somewhat recently, it's still imprudent to
use "horsepower" as an indicator of compressor size. Look at the CFM rating
instead (but see below).
> I wrote in my notes from the list, but can't find in archives anywhere,
> recommendations for an Eagle brand blaster that (I can't read my notes) gives
> 10.5 cfm @ 90PSI? Sound anywhere close to right?
Don't know the brand, but that cfm rating seems a tad on the small size.
Ideally, you'd like to have a compressor that will produce as much air as your
blast rig consumes. But that usually winds up being impractical, because
sandblasters use a *lot* of air. It doesn't help that an air compressor "cfm"
isn't the same as a tool "cfm". Both are rated by cubic feet per minute going
in; but because air gets much smaller when it is compressed, the output of an
air compressor is much smaller than the intake. You can get a pretty good guess
as to how much smaller by taking the ratio of the absolute pressures : roughly
15 psia at the compressor inlet to 105 psia (90 psi gauge plus atmospheric
pressure), or about 7:1. That means you need a compressor rated at 28 cfm (at
90 psi) to keep up with a tool rated 4 cfm @ 90 psi !
Which is where the big tank comes in. A larger tank lets you run an oversize
tool longer, even though the compressor can't keep up.
Two-stage compressors are preferable because they are more efficient at higher
pressures. Most single stage compressors still work fairly well at 90 psi, but
their output drops to almost nothing by 150 psi or so. Two-stage compressors
are generally still quite efficient at 175 psi. (And here again, the higher you
can pump up the tank before starting work, the longer you can work.)
Randall
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