[Note: Part 1 of this article presents the basics of air compressors. Part 2 presents the classification and 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 to generate the force needed to perform some useful task. It must generate sufficient compressed air at the required pressure and flow rate. It may be noted that compressed air is susceptible to contamination.
Pressure Development
In the figure above, 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.
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Terms and Definitions
Free Air: It is the air in 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.
The flow rate of Air
The flow rate of air with respect to 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 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 as the actual delivery volume [Free Air Delivery (FAD)] or the standard delivery volume.
Free Air Delivery (FAD):
It is the volume of compressed air a compressor delivers 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 the standard atmospheric inlet conditions. It is expressed in terms of lpm (std) or cfm (std).
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