Air Compressor - determining CFM


Old 12-28-07, 07:45 AM
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Air Compressor - determining CFM

All air tools are appear to be rated at xx CFM @ xx PSI. It's easy enough to regulate the PSI on a compressor with the turn of a knob, but how do I know if I'm actually getting the required CFM?
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Old 12-28-07, 08:08 AM
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Hey Noc7
There should be a label on the compressor, or in the manual, or at the manufacturers site, that lists what the compressor is capable of. You can't regulate the CFM btw, it will put out as much as it can based on the draw of the tool.

ex: if your compressor is rated 6.0 CFM at 90PSI, you can run a max of tools that total 6.0cfm at 90psi

Comp ( 6.0 cfm @90psi)
Sander( 2.0 [email protected]) + Grinder(4.0 [email protected]) =6.0 cfm

These numbers are just for illustration, they are not accurate.

I've found that you realistically need at least double the tools draw to run them on a continuous basis for things like sanders, griders, impact tools. Nailguns and staplers and the like use a much lower volume of air (except roofing guns, those guys bang out some nails!!!)

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Old 12-28-07, 08:24 AM
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Red face

In the absense of the info on your compressor or to determine if your compressor is no longer up to its capacity you can guage the output by operating a particular tool.

You can allow the compressor to build up pressure and shut off.
You then operate the tool continuously and see what the air supply does.
If the compressor runs continuously but maintains whatever supplied presure the tool requires it then at least can deliver the rated capacity to the tool.

Gunguy45 is right about needing a compressor much larger than the tools capacity however.
If only removing one tire with an imapct with an undersized compressor you likely will be ok.
If however you run the tool for longer periods the heat generated by the compressor will cause moisture problems, not to mention wear on the compressor.

Last edited by GregH; 12-28-07 at 07:32 PM. Reason: Remove that thumbs down icon that should not have been there!
Old 12-28-07, 09:09 AM
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Specs of mine:

4.5 scfm @ 40 psi; 3.7 scfm @ 90 psi
Running HP of 1.3
Pump RPM: 3450
8-gallon with a maximum psi of 125

I was playing around with a HVLP gun that is rated at 6-7 CFM at 50-60 PSI, set the compressor to 50 PSI and even though the compressor was running, it was keeping up to 50 PSI. I don't plan to do much painting, but I do plan to spray popcorn/stipple a few times with a hopper and do some minor bodywork touchups with the HVLP. These are the only two continuous uses I can think of.

So to sum it up, try to base yourself on the numbers given by the manufacturer + the numbers of the tool(s) you're going to use, and then just watch to see if the compressor can keep up the PSI?
Old 12-28-07, 11:24 AM
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I've done a lot of work with my little 11 gal 1hp compressor before I was able to get a nice 60 gal 6hp. A too small air compressor will still operate most tools. The problem is, it won't keep up. Sanders and paint guns use the most cfm. I have used a sander for a long period with less cfm than was needed = the sander runs at a slower speed. With a paint gun, reduced cfm can mean the paint won't atomize correctly = not so good paint job if you can paint a little, let the compressor recover, then spray a little more, you may be able to get by with an undersized compressor.

Most small compressors are only capable of powering 1 tool at a time.

As mentioned already, an undersized air compressor will have to work extra hard to keep up = more wear and tear on the air pump and motor. On the other side of the spectrum - I once painted a wrecker using the guy's 120 gal 2 stage air compressor. I was on my 2nd or 3rd cup of paint when the compressor finally came on
Old 12-28-07, 07:37 PM
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Sounds like justification for a bigger compressor!
Old 12-28-07, 07:40 PM
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Originally Posted by mitch17 View Post
Sounds like justification for a bigger compressor!
Just got this one for Christmas, trying to see what limits I can push it to and what I can expect. I'll probably spray a few base boards and some stipple in the next couple of years during odd jobs, but thats probably it for more continuous use(and that won't even be steady continuous as I'll be constantly stopping shooting both). I was impressed that it was able to hold 50 psi for the HVLP.
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