Rating of Air Compressors and Air Equipment

There is no universal standard for rating air compressors, air equipment and tools. Common terms are:

• CFM
• ICFM
• ACFM
• FAD
• ANR
• SCFM
• nl/min

CFM

• CFM (Cubic Feet per Minute) is the imperial method of describing the volume flow rate of compressed air. It must be defined further to take account of pressure, temperature and relative humidity - see below.

ICFM

• ICFM (Inlet CFM) rating is used to measure air flow in CFM (ft³/min) as it enters the air compressor intake.

ACFM

• ACFM (Actual CFM) rating is used to measure air flow in CFM at some reference point at local conditions. This is the actual volume flow rate in the pipework after the compressor.

FAD

• FAD (Free Air Delivery) (f.a.d) is the actual quantity of compressed air converted back to the inlet conditions of the compressor. The units for FAD are CFM in the imperial system and l/min in the SI system. The units are in general measured according the ambient inlet standard conditions ISO 1217: Ambient temperature = 20 ^oC, Ambient pressure = 1 bar abs, Relative humidity = 0%, Cooling water/air = 20 ^oC and Effective working pressure at discharge valve = 7 bar abs.
• 1 m³/min (f.a.d) = 1000 liter/min (f.a.d) = 1000 dm³/min (f.a.d) = 16.7 l/s (f.a.d) = 16.7 dm³/s (f.a.d) = 35.26 ft³/min (f.a.d)

ANR

• ANR (Atmosphere Normale de Reference) is quantity of air at conditions 1.01325 bar absolute, 20 ^oC and 65% RH (Relative Humidity).

SCFM

• SCFM (Standard CFM) is the flow in CFM measured at some reference point but converted back to standard air conditions (Standard Reference Atmosphere) 14.696 psia, 60 ^oF.

nl/min

• nl/min is the flow in l/min measured at some reference point but converted to standard air conditions 1.01325 bar absolute, 0 ^oC and 0% RH (Relative Humidity).

ISO 1217

• standard reference ambient conditions - temperature 20 ^oC, pressure 1 bar abs, relative humidity 0%, cooling air/water 20 ^oC, and working pressure at outlet 7 bar absolute.    

Example - Rating a Compressor

A typical rating of a compressor may look like this