General compressed air equipment description and operation

Compressed air systems can use multi-stage compressors and/or provide multi- compressor installations with or without multiple coarse to fine filters depending on process needs. Following is some of the equipment used in various combinations in compressed air systems. A description of the equipment and its operation is provided below.

1. Air compressor types. Compressors are most often supplied skid mounted as a packaged unit. The assembled 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. Following is a discussion of various types of compressors.
   1. Rotary helical screw air compressors are positive displacement machines. A twin-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 diminishing 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.
   2. 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 18,000 cfm. A blow-off silencer is needed to control noise. Centrifugal compressors require no lubrication in contact with the air stream and therefore provide oil free air.
   3. 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 of the 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 then repeated. These compressors can be specially constructed to operate as oil free. One or more stages can be used. Multiple stages are used with inter-cooling between stages in large applications. Because of their reciprocating mass they pulsate, vibrate, create harmonics in piping systems and, for larger sizes, require special foundations and special noise attenuation.
   4. 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 of the rotor. The volume of air between the housing, eccentrically mounted rotor, and two vanes changes as the rotor rotates, compressing the air. An outlet port is located in the housing where the rotor is closest to it. An inlet port is located just past the outlet port. One or more stages may be used. These compressors are used in systems up to 3000 cfm. They can be oil injected or oil free. One or two stages can be used. They have a low operating cost, no pulsation, are free of vibration, and do not require special foundations.
2. Dryers. Compressed air dryers are most often supplied skid mounted as a packaged unit. The assembled package includes all major components and controls mounted and pre-wired, requiring only that they be anchored to a foundation, hooked up to the compressed air piping system and oil/water separator and connected to power. Following is a discussion of various compressed air dryers.
   1. Refrigerant dryers use a refrigeration cycle to cool the compressed air down to 33°F as a minimum. Below this temperature the condensate will freeze and stop air flow. This type of dryer has a low initial cost, low operating cost and is inherently reliable. It is the most common type of dryer used for plant air, air operated tools, pneumatic instrumentation for HVAC systems and material conveying.
   2. Regenerative dryers using desiccant to dry compressed air, can dry air to minus 150°F. Twin towers of moisture adsorbing desiccant are used. One is in operation while the other is being regenerated with a heat source or compressed air. Initial cost varies from low to high and operating costs vary from moderate to high. These dryers are used where very dry air is needed.
   3. Deliquescent dryers use an absorbent material and can only dry air to a maximum of 20°F below inlet temperature. The moisture and absorbent react and form a liquid which is drained from the dryer. The effluent is corrosive, must be disposed of as a hazardous material and carryover into the system may be possible, causing maintenance problems downstream. These problems along with minimal moisture removed reduce the attractive low initial cost and low to moderate operating cost of the system.