Understanding Compressed Air Systems
Compressed air is used in many operations and processes and as a source of energy for heating, ventilating, and air conditioning (HVAC) and process actuators and motors. It may also be supplied for low pressure systems and used for pneumatic control. Air can be compressed in several different ways and supplied at varying pressures and degrees of filtration depending on its use.
A typical compressed air system is capable of producing instrument quality air for pneumatic HVAC controls, tools, conveying systems, and general plant air. The system consists of a single motor driven single stage rotary screw compressor with inlet air filter, a safety relief valve, aftercooler, air/oil separator, air receiver with safety relief valve, prefilter, air dryer, afterfilter, oil/water traps, and oil/water separator.
Compressors are most often supplied skid mounted as a packaged unit.Theassembled package includes all major components, controls, and a sound attenuation enclosure. The unit only requires mounting to a foundation,hook up to system piping, connection to the oil water separator, and power connection.
There are various types of compressors: Rotary helical screw air compressors are positive displacement machines. Atwin-screw compressor consists of accurately matched rotors [one lobe (male) and one helix (female)] that mesh closely when rotating within a close tolerance common housing. One rotor is motor driven while the other is gear driven, turning it in a counter-rotating motion.
The rotors uncover inlet posts at one end allowing air to flow in. As the rotors continue to rotate the air is compressed by the minishing volume between the rotors . At the end of the rotors, ports allow the now compressed air to exit. One or more stages may be used.
These compressors are used in systems up to 3000 cubic feet per minute (cfm). They are usually oil injected to increase sealing, lubricate rotors, and provide cooling. They can also be oil free. One or two stages can be used. They have
a low initial cost, no pulsation, are almost free of vibration, and do not require special foundations.
Centrifugal compressors compress air as it enters the center of a fluted casting, housing a rotating impeller. The impeller imparts kinetic energy to the gas which turns into potential energy as the gas velocity slows, thus increasing pressure. Compression is a continuous process. One or more stages may be used.
Centrifugal compressors are used in large systems up to 15,000 cfm. A blow-off silencer is needed to control noise. Centrifugal compressors require no ubrication in contact with the air stream and therefore provide oil free air.
Reciprocating or positive displacement compressors use a piston in a cylinder to compress air up to a capacity of 6,000 cfm . Air enters the cylinder through a valve when the piston is going down. The valve closes when the piston starts to go up. As the piston approaches the top ofthe cylinder, the air is compressed by the decreasing volume. An exhaust valve opens when the piston is near the top of the cylinder allowing the compressed air to exit. The cycle is them repeated.
Rotary sliding vane compressors use a rotor eccentrically mounted in a cylinder. The rotor has eight or more slots cut along its length .Vanes are placed in the slots. As the rotor rotates, the vanes move out from centrifugal force. As the rotor continues through a rotation, the rotor housing causes the vanes to move back in the slot ofthe rotor.
The volume of air between the housing, eccentrically mounted rotor, and two vanes changes as the rotor rotates, compressing the air.
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