Anyone that’s been near an air compressor is familiar with the term CFM.  It’s quite simple and is an acronym for “cubic feet per minute”.  This is a flow measurement based on volume, not weight.

Any cubic foot of air occupies the same space but will have different weights depending on:

Temperature

Humidity

Absolute Pressure

 

Why is this important?

It’s important to understand to properly match your plant load requirements to your air compressor purchase.  Different manufacturers display their compressor flow data with various terminology.

ICFM – Inlet Cubic Feet per Minute

ICFM is a measurement of the air flow prior to any component of the compression equipment such as an inlet filter (which will cause a drop in pressure)

SCFM – Standard Cubic Feet per Minute

SCFM is a measurement of air flow at an industry standard condition.  That specific condition is normally  stated in the U.S. as 14.696 PSIA (pounds per square inch – absolute), 60 degree F (520 degree R) and 0% relative humidity (RH).

ACFM – Actual Cubic Feet per Minute

ACFM is the actual cubic feet of air that is being delivered from the compression equipment.  If the exact conditions at the compressor location are equal to the SCFM standard conditions then the ACFM would equal the SCFM.  However, this almost never happens!

FAD – Free Air Delivered

FAD is the actual quantity of compressed air converted back to the inlet conditions of the compressor.

 

The 2 most typical stated flow rates normally seen are ICFM and SCFM.  As long as all the performance data is kept in one type of calculation then comparison of various compressors should work.

However, to assure you are getting a compressor that will actually meet the plant requirements I always ask the client to specify the worst possible operating conditions so compressor performance can be calculated at that point.

If you select a compressor based on SCFM alone then you will not get the stated flow when the summer conditions are 98 degree F ambient and the cooling water is 110 degree F and humidity is 52%.

The flow rates of air compressors is a widely changing variable and many white papers have been written on the subject so end users truly understand what performance they will receive from a specific unit.

We are available to present an entire session on this topic if you would like to completely understand air compressor flow rates.  As a bonus for our readers that are in the engineering discipline, PDH credits can be received for this seminar.