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Old 09-29-2011, 12:08 PM
Ol' Bob
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Default SCFM vs. CFM

I'm looking at two 2-1/2 hp. Compressors.
The 1st is rated as being capable of providing 5.3 SCFM @ 90psi., to a 10 gal. tank.
The 2nd is rated as being capable of providing 5.4 CFM @ 90 psi.. to a 10 gal. tank.
The tools I will be using all require 5.0 to 5.6 CFM @ 90 psi.
My Question:
Which of these products will come closer to meeting my needs ?
All the discussion I have seen does a pretty good job of skirting the question.
I can't seem to get a conversion factor, so I'm asking a direct question.
For those highly tech-minded persons who may wish to help:
I'm AT sea level, ( 12 Ft. ) and lets assume my shop is at 74deg. f.

Thanks !
Ol' Bob
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Old 09-29-2011, 12:54 PM
JohnS JohnS is offline
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Default Re: SCFM vs. CFM

Quote:
Originally Posted by Ol' Bob View Post
I'm looking at two 2-1/2 hp. Compressors.
The 1st is rated as being capable of providing 5.3 SCFM @ 90psi., to a 10 gal. tank.
The 2nd is rated as being capable of providing 5.4 CFM @ 90 psi.. to a 10 gal. tank.
The tools I will be using all require 5.0 to 5.6 CFM @ 90 psi.
My Question:
Which of these products will come closer to meeting my needs ?
All the discussion I have seen does a pretty good job of skirting the question.
I can't seem to get a conversion factor, so I'm asking a direct question.
For those highly tech-minded persons who may wish to help:
I'm AT sea level, ( 12 Ft. ) and lets assume my shop is at 74deg. f.

Thanks !
Ol' Bob
The performance of a compressor varies somewhat with the inlet air's pressure, temperature, and humidity, so manufacturers measure at "standard" conditions (the "s" in scfm). Different standard conditions prevail in different industries but the air compressor industry uses 14.4 psia, 80 F, 60% RH,
http://www.engineeringtoolbox.com/ra...ors-d_848.html

The first compressor is measured at those conditions. The second compressor is measured either at that (and they forgot the "s") or some unspecified condition that gives it a (slight??) advantage (you can be sure they didn't pick a condition that was disadventageous). The ratings are nearly identical. If the conditions for the one rated in "cfm" aren't too misleading, the performance will be nearly identical. If the tool is run continuously or nearly so, each would approximately support one tool.

If multiple tools are used, it will depend on duty cycle (tools are rarely used continuously)

If the tool is not running, the compressor will "charge" the tank to a higher pressure, and the tank volume will provide some buffer if the compressor can't keep up with the tool's demand. When the pressure "sags" below 90 psi, you may have to stop until the compressor can get the tank back up to pressure. The compressor will have a safety switch to shut off at some pressure (120 psi??)
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