CFM for compressed air

Do you know which CFM you should be using?

What is a CFM?

CFM stands for cubic feet per minute. It is the rate of flow of air or gas and is the basic unit of measure of performance for air and gas compressors, blowers and vacuum pumps. It indicates how much air a compressor or blower can move at a given discharge pressure.

Confusion usually develops when an additional letter is added before CFM, i.e. SCFM, ICFM, ACFM. Basically the prefix (S, I, A) indicates the conditions under which the capacity is measured.

SCFM is standard cubic feet per minute. This relates the CFM to a fixed set of standard conditions of atmospheric pressure (14.7 PSIA), ambient or inlet temperature (68 deg F.), and relative humidity (36% RH). Standards expressed here are Compressed Air and Gas Institute (CAGI) standards.

ICFM is inlet cubic feet per minute. This is the CFM at actual inlet conditions at the point of installation. Again, the conditions are specified in terms of actual atmospheric pressure, temperature, and relative humidity.

ACFM stands for actual cubic feet per minute. This is generally measured at the discharge of the compressor, and is the compressed volume, minus any losses through seals, etc. This expression is erroneously used interchangeably with SCFM and ICFM and can be confusing. Air using equipment manufacturers will usually specify the air requirements as SCFM. Rarely will you see requirements specified as ACFM. When we do see this, we question it and probe for more specifics.

SCFM and ICFM can be used interchangeably if the standard conditions and inlet conditions are the same. With positive displacement compressors (rotary screw and reciprocating) and blowers, they will be very close and manufacturers will specify their advertised performance as SCFM. Only in centrifugal compressors and blowers will there be a significant difference.

Positive displacement compressors are designed for a specific discharge pressure, optimizing the performance to secure the maximum flow for a given horsepower. A compressor designed for 100 PSIG will deliver more CFM at a given horsepower than one designed for 125 PSIG. The speed of the air end (pump) is adjusted via gears or v-belts, or by adjusting the effective compression volume of the air end for this optimization.

Customers frequently ask what the capacity of a 125 PSIG compressor will be if they lower the pressure to 100 PSIG. The answer is “not by any significant amount”. The capacity is essentially the same at either pressure since the compressor is designed for one specific point. The converse is also true. A 100 PSIG compressor cannot be increased to 125 PSIG, as it will overload the motor due to the same effective CFM being compressed.

Pye-Barker Supply Co. sales engineers offer their expertise in evaluating your plant application and developing a compressed air system best suited for your needs.

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