Quantcast Chapter 2. Positive Displacement Compressors

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1 DESCRIPTION.  Reciprocating air compressors are manufactured in a variety
of shapes, sizes, and capacities. Single-stage machines draw air from the
atmosphere and discharge it into the receiver or storage tank.  Two-stage
compressors (figure 2-l) bring the air up to intermediate pressure in one
cylinder and to final pressure in a second cylinder.  Where two or more stages
are employed, the unit is defined as a multistage air compressor. Multistage
compressors produce higher discharge pressures. Stationary air compressors
are usually water-cooled, with the exception of small units that are
air-cooled.  Portable units are also usually air-cooled. Air-cooled
compressors utilize finned cylinders to increase the radiating area.
Compressor drives include electric motors, steam reciprocating engines, steam
turbines, or internal combustion engines. Drives may be direct connected,
connected through reduction gears, or belt connected. Operating and
maintenance instructions for electric motors, internal combustion engines,
steam engines, and steam turbine drives are contained in NAVFAC MO-205,
Central Heating and Steam Electric Generating Plants.
1.1 High-Pressure Systems.  Although high-pressure air compressors can
compress air to pressures of approximately 100,000 pounds-force per square
inch gauge (psig), in this manual discussion of high-pressure systems is
limited to the 400- to 6,000-psig range.  Multistage reciprocating compressors
are commonly used for this service.  Depending upon the discharge pressure,
the compressor will have from two to five stages of compression, intercoolers
between stages, and an aftercooler.  Smaller compressors may be air-cooled or
a combination of air- and water-cooled while larger compressors are normally
Power for larger compressors is usually provided by electric
motors, although in some installations the compressors may be powered by
diesel or steam engines.  In smaller compressor applications, gasoline engine
drives may be provided.  Power is normally transmitted from the power source
to the compressor through a direct drive or V-belts. Steam engines are
usually integral with the compressor.  Typical applications for high-pressure
air are:
Testing and operating catapults
Testing and launching missiles
Torpedo workshops
Wind tunnels
Ammunition depots
2.1 Explosive Hazards.  Although compressed air at low or medium pressures is
dangerous if carelessly handled, the dangers associated with high-pressure
systems are of much greater consequence.  Serious explosions, complete
destruction of facilities, and heavy loss of life have been attributed to
unsafe practices involving high-pressure compressed air systems. A serious


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