Understanding Air Compressors – Part 1

Bits and Pieces: Fluid Power Fact #4/1

Understanding Air Compressors – Part 1

 [Note: Part 1 of this article presents basics of air compressors. Part 2 presents the classification and the construction details of air compressors. Part 3 presents a brief about the sizing of air compressors.]


A power source in a pneumatic system generates and distributes compressed air to all of its actuators. A compressor converts mechanical energy into pneumatic energy using a compressible medium. The vast majority of pneumatic systems use air as the operating medium. A compressor is designed to take in air at atmospheric pressure and deliver it into a closed system at a higher pressure for generating the force needed to perform some useful task. It must be capable of generating sufficient quantity of compressed air at the required pressure and flow rate. It may be noted that the compressed air is susceptible to the problem of contamination.

Pressure Development

In the figure given below, assume that a compressor delivers 3 m3/min of air to a reservoir having a volume of 2 m3. Using Boyle’s law, the pressure rise in the prime-mover-driven compressor can be determined easily and the values of absolute and gauge pressures with respect to time are given in the Table below.


Terms and Definitions

Free Air: It is the air at the actual conditions, where the compressor is located. As the pressure and temperature vary at different locations and at different times, it follows that this term does not mean air under standard conditions.

Standard Air: The standard inlet conditions of a compressor are specified in the ISO 1217 standard. These conditions are taken as a pressure of 1 bar absolute [14.5 psi absolute], a temperature of 20 0C [68 0F], and a relative humidity of 0%, and a working pressure 7 bar absolute at the outlet.

Flow rate of Air

The flow rate of air in respect of a compressor is the volume of air displaced or delivered per unit of time at the rated speed of the drive shaft and usually under the specified conditions of pressure, temperature, and relative humidity. The flow rate is measured in terms of: (1) Theoretical flow rate (or displacement) and (2) Effective flow rate (or delivery).

Theoretical flow rate (or Displacement Volume)

The theoretical flow rate is the quantity of inlet air that a compressor displaces. For a reciprocating compressor, the theoretical flow rate is the product of the volume of air swept in one revolution of its drive shaft and the number of revolutions per unit of time. It is usually expressed as litres per minute (lpm) or cubic feet per minute (cfm).

Effective flow rate (or Delivery Volume)

The effective flow rate is the quantity of air that a compressor delivers at the specified discharge pressure. The discharge pressure is typically specified at 6 bar [90 psi]. To normalize the effective flow rate, the quantity of the delivered air is usually converted back to the inlet conditions of the compressor. The inlet condition may be taken as the actual atmospheric condition at a given site or the standard atmospheric condition. Accordingly, the effective delivery volume can be expressed in terms of the actual delivery volume [Free Air Delivery (FAD)] or the standard delivery volume.

Free Air Delivery (FAD):

It is the volume of compressed air delivered by a compressor at the specified discharge pressure and is stated in terms of the actual atmospheric inlet conditions. It is expressed in terms of lpm (fad) or cfm (fad).

Standard delivery volume

It is the volume of compressed air delivered by an air compressor at the specified discharge pressure and is normally stated in terms of the standard atmospheric inlet conditions. It is expressed in terms of lpm (std) or cfm (std).




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