Air Compressor Operation

There are several commonly used standards in system engineering to define air. One definition for a standard cubic foot (scf) of air is the quantity of dry air needed to fill a volume of 1 cf (0.03 m3) at 14.7 psia (l01 kPa) pressure and 60°F (15.55°C) temperature. Another definition is based on air at 36% relative humidity (RH) filling the same volume at the same pressure, but at 68°F (20°C). In both cases, the air density has the value of 0.075 lbm/cf (1.2 kg/m3).

Air services with atmospheric inlet should be specified for 100% RH. The water content must be added to the net dry air requirement of the process. Note that saturated air at 90°F (32.2°C) contains about 3% water vapor by weight.

In the United States, the Compressed Air and Gas Institute (CAGI) has selected as standard conditions 1 bar (14.5 psia or 100 kPa), 20°C (68°F), and 0% RH. The most commonly used units of flow are cf per minute (cfm or ft3/m) or per hour (cfh or ft3/h) and m3 per minute (m3/m) or per hour (m3/h). Power is normally expressed as hp or kW Work of compression may be expressed positively as work output or negatively as work input.

Inlet pressure is generally defined as the absolute total pressure existing at the intake flange of a compressor. Inlet temperature is the initial temperature at the intake flange. Discharge pressure is generally defined as the absolute total pressure at the compressor's discharge flange and is commonly referenced in psig. Discharge temperature is the total temperature at the discharge flange of the compressor.

Typically, compressors are analyzed using ideal gas law with an assumed constant specific heat. A compressibility factor (Z) is used for real gas deviations.

The isentropic work of compression for a real gas is expressed positively as:

Where: 1

W = Work done p = Pressure at inlet

V = Volume at inlet k = Ratio of specific heats r = Compression ratio (p^pj)

Z = Compressibility factor =

and subscripts:

1 = Inlet conditions

2 = Discharge conditions

Compressibility factors become increasingly more critical for analysis of hydrocarbon gases and refrigerants (which generally deviate significantly from the ideal-gas laws) and for non-hydrocarbon gas at very high pressures (i.e., greater than 1,000 psig or 70 bar).

Power (P) requirement is a function of the work done and the adiabatic efficiency of the compressor (nc). It can, therefore, be expressed as:

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