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Re: [Shop-talk] single vs. dual stage compressors

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Subject: Re: [Shop-talk] single vs. dual stage compressors
From: "Karl Vacek" <>
Date: Thu, 20 Nov 2008 19:58:44 -0600
If that's the rating, then there is no problem.  The size of the tank is 
immaterial (once it's up to pressure) if the free air (SCFM) rating of the 
tool is less than the free air (SCFM) rating of the compressor.  The size of 
the tank only comes into play if you use more air than the compressor can 

Think of it this way - if you piped the exhaust from that die grinder into a 
4 cubic foot plastic bag (that was empty and flattened when you started), 
after one minute the bag would be full of air at about 1 atmosphere of 
pressure, NOT full of air at 90 PSI.

It would take nearly 2 minutes for that compressor to fill a 4 cubic foot 
tank with air at 90 PSI, assuming you weren't using the grinder while you 
also filled the tank.  And that 4 cubic feet of 90 PSI air would fill nearly 
eight 4 cubic foot bags.


>> if the die grinder says its needs 4 cfm @ 90 psi and the
>> compressor says
>> it'll provide 16 cfm @ 90 psi... What's the problem?
> The problem is that the rating of the air compressor is in cubic feet
> _after_ it has been expanded back to atmospheric pressure ... in spite of
> what it says.  Another way to look at it is cubic feet into the intake
> rather than into the tank.
> You can easily see this by looking at how long it takes the compressor to
> fill it's own tank.  For example, a typical 110v compressor might be rated 
> 6
> cfm and have a 20 gallon tank.  But 20 gallons is only about 2.6 cubic 
> feet,
> so if the compressor were really putting out that much air, it would shut
> off in less than 30 seconds.  Instead it takes several minutes to even 
> reach
> 90 psi.
> Randall
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