ABSOLUTE PRESSURE is the arithmetic sum of gauge and atmospheric pressures. It must be used in all calculations involving the basic gas laws.
ABSOLUTE TEMPERATURE is the temperature of a body referred to the absolute zero, at which point the volume of an ideal gas theoretically becomes zero. On the Fahrenheit scale this is minus 459.67°F; on the Celsius scale it is minus
273.15°C. Engineering values of minus 460°F and minus 273°C are used herein. AFTERCOOLING involves cooling of gas in a heat exchanger following the completion of compression to (1) reduce the temperature, and (2) to liquefy condensable vapors. ALTITUDE is the elevation above sea level. BAROMETRIC PRESSURE is the absolute atmospheric pressure existing at the surface of the earth. It is the weight of a unit column of air above the point of measurement. It varies with altitude and, at any given location, with moisture
content and weather. BRAKE HORSEPOWER is the actual horsepower output to the drive motor. CAPACITY of any compressor is the quantity of gas actually delivered when operating between specified inlet and discharge pressures. For ejectors, capacity is measured in lb/hr. For all other compressor types, capacity is a volume
measured at the conditions of pressure, temperature, gas composition, and moisture content existing at the compressor inlet flange. CLEARANCE in a reciprocating compressor cylinder is that volume contained in one end of the cylinder which is not swept by the movement of the piston. It includes space between piston and head at the end of the compression stroke,
space under the valves, etc., and is expressed as a percentage of the piston displacement per stroke. Clearance may be different for the two ends of a double-acting cylinder. An average is generally used. COMPRESSIBILITY is the factor of a gas or a gas mixture that causes it to differ in volume from that of a perfect gas when each is under the same pressure and temperature conditions. Occasionally it is called deviation. It must be
determined experimentally. SUPER-COMPRESSIBILITY is a term used with various meanings, most frequently the same as compressibility, although this is not assured. A current ASME Power Test Code uses it as a ratio of gas densities rather than volumes. Therefore it
is 1/Z in this case. Super-compressibility should never be used unless its meaning is clarified completely. Compressibility is much to be preferred and is used herein. COMPRESSION EFFICIENCY is the ratio of the theoretical work requirement (using a stated process) to the actual work required to be done on the gas for compression and delivery. Expressed as a percentage, compression efficiency accounts
for leakage and fluid friction losses, and thermodynamic variations from the theoretical process. COMPRESSION RATIO is the ratio of the absolute discharge to the absolute intake pressure. It usually applies to a single stage of compression, but may be applied to a complete multistage compressor as well. | 
