FLUID POWER HANDBOOK
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AIR
COMPRESSORS
AIR
compressors supply the compressed air flow for all pneumatic equipment in a system. The compressor adds energy to the air, which is cleaned and conditioned by filters and dryers, then transmitted in piping for use. Compressed air is an energy intensive source of process power, about 7 to 8 units of energy are consumed at the compressor for each unit of mechanical energy produced by a typical compressed-air powered device — and of this, typically 50% of the compressed air is wasted due to leakage and inappropriate use. Most of the energy released by an air compressor is in the form of heat of compression.
When discussing compressed air flows, there are various definitions that relate to compressor capacity: ACFM — actual cubic feet per minute (also called free air delivered, FAD, or inlet cubic feet per minute, ICFM). This is the flow of air taken in by the compressor at site conditions (local atmospheric pressure, temperature and humidity). In general, higher altitudes, temperatures and levels of humidity reduce the capacity of the compressor to produce a given mass of compressed air; therefore, if these conditions exist, a larger compressor must be purchased. CFM — cubic feet per minute. This is the flow of air at a certain point at a certain condition, which must be specified. With regard to sizing air compressors, it is important to understand the wide range of conditions at which the CFM can be stated. SCFM — standard cubic feet per minute. This is the flow of free air measured and converted to a set of standard conditions. The definition of SCFM for air compressor rating purposes (Compressed Air and Gas Institute based on ISO Standard 1217) is the flow of air at 14.5 psig atmospheric pressure, at 68° F and 0% relative humidity. ACFM and SCFM are both measured at atmospheric pressure, not at the pressure the air compressor produces. There are two types of compressors: positive displacement and dynamic (also called centrifugal or axial).
POSITIVE DISPLACEMENT AIR COMPRESSORS Positive displacement compressors take in air and mechanically reduce the space occupied by the air to increase pressure. They can further be divided into rotary and reciprocating types. Rotary compressors are available in sizes from 5 to 600 hp. In rotary screw compressors, filtered air enters the inlet of the air end where male and female rotors unmesh. The air is trapped between the rotors and the air end housing. This space is reduced as the rotors remesh on the opposite side of the air end. Thus, the air is compressed and moved to the discharge port. For lubricated compressors, cooling fluid is injected into the housing, which mixes with the air to seal, lubricate and remove the heat generated by compression. This fluid forms a thin film between the rotors that virtually eliminates metal-tometal contact and wear. The fluid is separated from the compressed air, cooled, filtered and returned to the injection point. The compressed air passes through an after-cooler and water separator to reduce its temperature and water content so it is ready for the air treatment equipment. R TE SY IM AG E C O U
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FLUID POWER WORLD
7 • 2020
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