Compressors are mechanical devices used to increase pressure in a variety of compressible fluids, or gases, the most common of these being air. Compressors are used throughout industry to provide shop or instrument air; to power air tools, paint sprayers, and abrasive blast equipment; to phase shift refrigerants for air conditioning and refrigeration; to propel gas through pipelines; etc. As with pumps, compressors are divided into centrifugal (or dynamic or kinetic) and positive-displacement types; but where pumps are predominately represented by centrifugal varieties, compressors are more often of the positive- displacement type. They can range in size from the fits-in-a-glovebox unit that inflates tires to the giant reciprocating or turbocompressor machines found in pipeline service. Positive-displacement compressors can be further broken out into reciprocating types, where the piston style predominates, and rotary types such as the helical screw and rotary vane.
In this guide, we will use both of the terms compressors and air compressors to refer mainly to air compressors, and in a few specialized cases will speak to more specific gases for which compressors are used.
Types of Air Compressor
Compressors may be characterized in several different ways, but are commonly divided into types based on the functional method used to generate the compressed air or gas. In the sections below, we outline and present the common compressor types. The types covered include:
Piston
Diaphragm
Helical Screw
Sliding vane
Scroll
Rotary Lobe
Centrifugal
Axial
Due to the nature of the compressor designs, a market also exists for the rebuilding of air compressors, and reconditioned air compressors may be available as an option over a newly purchased compressor, including special process gas compressors.
Piston Compressors
Piston compressors, or reciprocating compressors, rely on the reciprocating action of one or more pistons to compress gas within a cylinder (or cylinders) and discharge it through valving into high pressure receiving tanks. In many instances, the tank and compressor are mounted in a common frame or skid as a so-called packaged unit. While the major application of piston compressors is providing compressed air as an energy source, piston compressors are also used by pipeline operators for natural gas transmission. Piston compressors are generally selected on the pressure required (psi) and the flow rate (scfm). A typical plant-air system provides compressed air in the 90-110 psi range, with volumes anywhere from 30 to 2500 cfm; these ranges are generally attainable through commercial, off-the-shelf units. Plant-air systems can be sized around a single unit or can be based on multiple smaller units which are spaced throughout the plant.
To achieve higher air pressures than can be provided by a single stage compressor, two-stage units are available. Compressed air entering the second stage normally passes through an intercooler beforehand to eliminate some of the heat generated during the first-stage cycle.
Speaking of heat, many piston compressors are designed to operate within a duty cycle, rather than continuously. Such cycles allow heat generated during the operation to dissipate, in many instances, through air-cooled fins.
Piston compressors are available as both oil-lubricated and oil-free designs. For some applications which require oil-free air of the highest quality, other designs are better suited.
Diaphragm Compressors
A somewhat specialized reciprocating design, the diaphragm compressor uses a motor-mounted concentric that oscillates a flexible disc which alternately expands and contracts the volume of the compression chamber. Much like a diaphragm pump, the drive is sealed from the process fluid by the flexible disc, and thus there is no possibility of lubricant coming into contact with any gas. Diaphragm air compressors with spare parts are relatively low capacity machines that have applications where very clean air is required, as in many laboratory and medical settings.
Helical Screw Compressors
Helical-screw compressors are rotary compressor machines known for their capacity to operate on 100% duty cycle, making them good choices for trailerable applications such as construction or road building. Using geared, meshing male and female rotors, these units pull gas in at the drive end, compress it as the rotors form a cell and the gas travels their length axially, and discharge the compressed gas through a discharge port on the non-drive end of the compressor casing. The rotary screw compressor action makes it quieter than a reciprocating compressor owing to reduced vibration. Another advantage of the screw compressor over piston types is the discharge air is free of pulsations. These units can be oil- or water- lubricated, or they can be designed to make oil-free air. These designs can meet the demands of critical oil-free service.
Sliding Vane Compressors
A sliding-vane compressor relies on a series of vanes, mounted in a rotor, which sweep along the inside wall of an eccentric cavity. The vanes, as they rotate from the suction side to the discharge side of the eccentric cavity, reduce the volume of space they are sweeping past, compressing the gas trapped within the space. The vanes glide along on an oil film which forms on the wall of the eccentric cavity, providing a seal. Sliding-vane compressors cannot be made to provide oil-free air, but they are capable of providing compressed air that is free of pulsations. They are also forgiving of contaminants in their environments owing to the use of bushings rather than bearings and their relatively slow-speed operation compared to screw compressors. They are relatively quiet, reliable, and capable of operating at 100% duty cycles. Some sources claim that rotary vane compressors have been largely overtaken by screw compressors in air-compressor applications. They are used in many non-air applications in the oil and gas and other process industries.
Scroll Compressors
Scroll air compressors use stationary and orbiting spirals which decrease the volume of space between them as the orbiting spirals trace the path of the fixed spirals. Intake of gas occurs at the outer edge of the scrolls and discharge of the compressed gas takes place near the center. Because the scrolls do not contact, no lubricating oil is needed, making the compressor intrinsically oil-free. However, because no oil is used in removing the heat of compression as it is with other designs, capacities for scroll compressors are somewhat limited. They are often used in low-end air compressors and home air-conditioning