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A compressor is a mechanical device used to increase the pressure of various compressible liquids or gases, the most common of which is air. Compressors are used throughout industry to supply air to workshops or appliances, to power pneumatic tools, paint sprayers and sandblasting equipment, to phase shift refrigerants for air conditioning and refrigeration, to supply natural gas through pipelines, etc. Like pumps, compressors are divided into centrifugal (or dynamic, or kinetic) and volumetric; however, if the pumps are primarily centrifugal pumps, the compressors are usually positive displacement. They range in size from glove boxes that inflate tires to giant piston or turbochargers found at plumbing shops. Positive displacement compressors can be further classified into reciprocating compressors, which are dominated by the reciprocating type, and rotary compressors, such as screw and rotary vane compressors.
In this guide, we will use the terms “compressor” and “air compressor” to refer primarily to air compressors, and in some special cases, we will use the term “compressor” to refer to more specific gases.
Compressors can be characterized in several different ways, but they are usually grouped into categories based on the method of operation they use to produce compressed air or gas. In the following sections, we give an overview and describe common types of compressors. Types covered include:
Due to the nature of compressor design, there is also a market for remanufactured air compressors, and remanufactured air compressors may be an option to purchase new compressors.

Reciprocating compressors or reciprocating compressors rely on the reciprocating movement of one or more pistons to compress gas in a cylinder (or cylinders) and release it through a valve into a high pressure receiving tank. In many cases, the storage tank and the compressor are mounted on a common frame or skid in the form of a so-called package. While the primary use of reciprocating compressors is to provide compressed air as an energy source, pipeline operators also use reciprocating compressors to transport natural gas. Reciprocating compressors are usually selected based on the desired pressure (psi) and flow (scfm). Typical factory air systems provide compressed air in the 90-110 psi range at 30 to 2500 cfm; these bands are usually available through commercial off-the-shelf devices. The ventilation system of the plant can be designed for one unit or for several smaller units spaced throughout the plant.
To achieve higher air pressure than a single-stage compressor can provide, two-stage units can be used. Compressed air entering the second stage usually passes through an intercooler beforehand to remove some of the heat generated in the first stage cycle.
Speaking of heat, many reciprocating compressors are designed to run in a single duty cycle, not in continuous operation. In many cases, this circulation allows heat generated during operation to be dissipated through air-cooled fins.
Piston compressors are oil and oil-free. For some applications requiring the highest quality oil-free air, other designs are more suitable.

Diaphragm compressors are a somewhat specialized reciprocating design that uses concentric shafts mounted on an engine to vibrate a flexible disk that alternately expands and contracts the volume of the compression chamber. Like a diaphragm pump, the drive is isolated from the process fluid by a flexible disk so the lubricant cannot come into contact with any gases. Diaphragm air compressors are relatively small capacity machines suitable for applications that require very clean air, such as those found in many laboratories and medical facilities.
Screw compressors are rotary compressors known for their ability to run at 100% duty cycle, making them ideal for trailer applications such as construction or road construction. Using geared and coupled rotors, these units suck in gas at the drive end, compress it as the rotors form an assembly, the gas moves axially and exits the compressed gas compressor housing through the outlet port at the non-drive end. The operation of screw compressors makes them quieter than reciprocating compressors by reducing vibration. Another advantage of screw compressors over reciprocating ones is the absence of pulsation of the forced air. These units can be oil or water lubricated and can also be designed to provide oil free air. These designs meet critical oil-free maintenance requirements.
Vane compressors are based on a series of vanes mounted in a rotor that move along the inner wall of an eccentric cavity. As the vanes rotate from the suction side of the eccentric chamber to the discharge side, they reduce the volume of the space they span, thereby compressing the gas trapped in that space. The blades slide over the oil film that forms on the walls of the eccentric chamber, providing a seal. Vane compressors cannot provide oil-free air, but they can provide pulsation-free compressed air. Because they use bushings instead of bearings, and because they run relatively slowly compared to screw compressors, they are also resistant to contaminants in the environment. They are relatively quiet, reliable and capable of running at 100% duty cycle. Some sources state that rotary vane compressors have been largely replaced by screw compressors in air compressors. They are used in many airless applications in the oil and gas and other process industries.

Scroll air compressors use stationary and orbital scrolls which reduce the amount of space between them as the orbital scrolls follow the path of the stationary scrolls. The gas inlet occurs at the outer edges of the vortex, and the compressed gas is released closer to the center. Because the scrolls do not touch, no lubricating oil is required, making the compressor virtually oil-free. However, scroll compressors are somewhat limited in performance because oil is not used to remove the heat of compression as in other designs. They are commonly used in low cost air compressors and home air conditioner compressors.
Rotary compressors are high capacity, low pressure devices that are more properly classified as blowers. To learn more about blowers, download our free Thomas Blower Buying Guide.
Centrifugal compressors rely on high-speed pump-like impellers to speed up the gas to build up pressure. They are mainly used in high volume applications such as commercial refrigeration units over 100 hp. and large process plants where they can reach 20,000 hp. and deliver volumes in the 200,000 cfm range. Centrifugal compressors are almost the same design as centrifugal pumps, and the gas is thrown outward by the action of the rotating impeller, thereby increasing the speed of the gas. The gas expands in the volute of the body, slowing down and increasing the pressure.
Centrifugal compressors have a lower compression ratio than positive displacement compressors, but they can handle larger volumes of gas. Many centrifugal compressors use multiple stages to increase the compression ratio. In these multi-stage compressors, the gas usually passes through an intercooler between stages.

Axial compressors provide the highest air volumes, from 80 to 13 million cubic feet per minute in industrial machines. Jet engines use this type of compressor to produce a wider range of displacements. Compared to centrifugal compressors, axial compressors tend to be multi-stage designs due to their relatively low compression ratio. Like centrifugal units, axial compressors increase pressure by first increasing the velocity of the gas. Axial compressors then slow down the gas through curved stationary vanes, increasing its pressure.
The air compressor can be electric, usually choose a 12 volt DC air compressor or a 24 volt DC air compressor. Compressors are also available for standard AC voltage levels such as 120V, 220V or 440V.
Alternative fuel options include an air compressor powered by an engine running on a combustible fuel source such as gasoline or diesel. In general, electric compressors are ideal where exhaust gas removal is important or where operation is important where the use or absence of flammable fuels is undesirable or important. The noise factor also plays an important role in fuel choice, as electric air compressors are generally quieter than engine driven air compressors.
In addition, some air compressors may be hydraulically driven, which also avoids the use of combustible fuel sources and associated exhaust problems.

When it comes to choosing an air compressor for a general workshop, the choice often comes down to either a reciprocating compressor or a screw compressor. Reciprocating compressors are generally cheaper than screw compressors, require less maintenance, and perform well in dirty operating conditions. However, they are much noisier than screw compressors and are more prone to seepage of oil into the compressed air supply system, a phenomenon known as “carry over”. Since reciprocating compressors generate a lot of heat during operation, they must be sized for their duty cycle – the rule of thumb is 25% off and 75% on. A radial screw compressor can run 100% of the time and is almost preferable. One potential problem with screw compressors, however, is that increasing their power to increase their performance can lead to problems, as they are not particularly well suited to frequent starts and stops. The tight tolerances between the rotors mean that the compressor must be kept at operating temperature to achieve efficient compression. The size requires more attention to the use of air; reciprocating compressor size can be increased without such problems.
A body shop that constantly uses paint air may find that a radial screw compressor has a low carryover rate and would like to run continuously; reciprocating compressors can perform better when the air is used less frequently and is critical to the cleanliness of the air supplied. a repair business that doesn’t care.
Regardless of the type of compressor, compressed air is usually cooled, dried and filtered before being conveyed through the ducts. Plant ventilation specification writers need to select these components based on the size of the system they are designing. In addition, they should consider installing filter regulator lubricators at the point of delivery.

Larger compressors mounted on trailers are usually engine-driven screw compressors. They are designed to run continuously whether air is used or vented.
While scroll compressors dominate low-cost refrigeration and air compressors, they are beginning to make inroads into other markets as well. They are particularly suitable for industrial processes requiring very clean air (class 0) such as pharmaceuticals, food processing, electronics, etc., as well as for cleanrooms, laboratories and medical/dental environments. Manufacturers offer units up to 40 hp that can deliver almost 100 cfm at pressures up to 145 psi. Larger installations often contain multiple scroll compressors as the technology does not scale beyond 3-5 horsepower.
If an application involves the compression of hazardous gases, designers often consider diaphragm or sliding vane compressors, and for very large compressed volumes motorized compressors.
Oil plays an important role in the operation of any compressor as it is used to carry away the heat generated during compression. In many designs, the oil also provides the seal. In reciprocating compressors, oil lubricates the crank and piston pin bearings, as well as the side walls of the cylinders. As in a piston engine, the rings on the piston seal the compression chamber and control the flow of oil into it. In screw compressors, oil is injected into the compressor block to seal the two non-contacting rotors and remove some of the heat generated during compression. Rotary vane compressors use oil to seal the tiny space between the vane tips and the housing bore. Scroll compressors usually do not use oil and are therefore called oil-free compressors, but of course they have a limited capacity. Centrifugal compressors do not introduce oil into the compressed stream, but they are different from their positive displacement counterparts.

To create an oil-free compressor, manufacturers use different strategies. Reciprocating compressor manufacturers may use a one-piece piston crank with the crankshaft mounted on an eccentric bearing. When these pistons reciprocate within the cylinder, they oscillate within the cylinder. This design eliminates the support of the piston pin on the piston. Reciprocating compressor manufacturers also use various self-lubricating materials in O-rings and cylinder liners. Manufacturers of screw compressors have reduced the gap between the screws, eliminating the need for glands.
However, any of these options come with trade-offs. Increased wear, thermal issues, reduced performance and more frequent maintenance are just some of the disadvantages associated with oil-free air compressors. Obviously, some industries are forced to make such compromises because oil-free air is a must. But if the oil can be filtered or just tolerated, then it makes sense to use a conventional oil compressor.
If you use jackhammers all day long, compressor selection is simple: add up the number of operators who use the compressor, determine the power of their tool, and buy a screw compressor that will fit your needs and last 8 hours on a tank of oil. Of course, it’s not that simple – you may have to take into account the limitations of the environment – but you get the idea.

Things get a little more complicated if you want to supply compressed air to a small shop. Pneumatic tools can be classified according to their purpose: either intermittent action – like a ratchet wrench, or continuous action – like a paint sprayer. Charts are available to help estimate the consumption of various workshop tools. Once these are identified and usage calculated based on average and continuous usage, a rough estimate of the total air compressor power can be determined.
Determine the compressor capacity for the manufacturing plant in the same way. For example, a packaging line may use compressed air to drive cylinders, blowers, etc. Typically, equipment manufacturers specify flow rates for individual machines, but if not, cylinder air flow can be easily obtained by knowing bore diameter, stroke, and cycle rate. each pneumatic block.

Very large manufacturing and processing plants may have equally large compressed air requirements, possibly served by back-up systems. For such operations, always-available air justifies the cost of multiple compressed air systems to avoid costly shutdowns or line shutdowns. Even small operations can benefit from some level of redundancy. When sizing a small air production system, the question to ask yourself is: is it better to use a single compressor (less maintenance, less complexity) or are several smaller compressors (redundant, expandable) more suitable? ?
Compressors suck air from the atmosphere, add heat by compressing it, sometimes add oil to the mixture and, if the air they suck in is not very dry, create a lot of moisture. For some operations, these additional ingredients do not affect the end use and the tool works fine with no performance issues. As the process of pneumatic actuation becomes more complex or more important, more attention is usually paid to improving the quality of the exhaust air.
Compressed air is usually hot, and the first step to reducing that heat is to collect the air in a reservoir. This step not only cools the air, but also allows some of the moisture in the air to condense. Air compressor receiver tanks usually have manual or automatic valves that allow accumulated water to be drained. The passage of air through the aftercooler further removes heat. Refrigerant and sorbent dryers can be added to the air supply line to increase moisture removal. Finally, filters can be installed to remove any entrained lubricant from the supply air, as well as any particulate matter that may be trapped by the inlet filters.

Compressed air is usually dosed to a few drops. With every fall, standard best practice is to install an FRL (Filter, Regulator, Lubricator) that conditions the air according to the needs of the specific tool and allows lubrication to go to any tool that needs it.
When it comes to controlling a reciprocating compressor, there are not many options. The start/stop control is the most common: the compressor feeds a storage tank with upper and lower thresholds. When the lower setpoint limit is reached, the compressor starts and runs until the upper setpoint limit is reached. A variant of this method, called constant speed control, allows the compressor to run for a certain period of time after reaching the upper setpoint, venting air to atmosphere in case the stored air is being used at a higher than normal speed. This process minimizes the number of engine starts during periods of high load. The optional dual control system, normally only available on systems over 10 hp, allows the user to switch between the two control modes.
Screw compressors have more options. In addition to start/stop and constant speed control, screw compressors are available with load/unload control, intake valve modulation, spool valves, automatic dual control, variable speed drives and compressor sequencing for multi-unit applications. The load/unload control uses a discharge side valve and a suction side valve that open and close respectively to reduce flow through the system. (This is a very common system on oil-free screw compressors.) Inlet valve modulation uses proportional control to control the air mass flow of the compressor. Spool valve control effectively shortens the length of the auger by delaying the onset of compression and allowing some intake air to bypass the compression to better meet demand. Automatic dual control switches between start and stop as well as constant speed control according to the required performance. Variable speed drives slow or speed up the rotor by electronically changing the frequency of the AC waveform that turns the electrical machine. Compressor sequencing allows load sharing among multiple compressors, for example by assigning one unit to run continuously to handle baseload and changing the start of two other units to minimize restart losses.

When choosing any of these control schemes, the idea is to find the best balance between meeting demand and idling costs and the penalty for accelerated equipment wear.
When choosing a compressor mechanism, there are three main parameters that specifiers need to consider, in addition to the many items listed above. These air compressor specifications include:
Although compressors are usually rated in horsepower or kilowatts, these figures do not necessarily indicate the cost of operating the equipment, since it depends on the efficiency of the machine, duty cycle, etc.
Volumetric productivity determines how much air the machine can supply per unit of time. Cubic feet per minute is the most common unit of measurement, although units may vary between manufacturers. Attempts to standardize this measurement, known as scfm, seem to depend on which standard you follow. The Compressed Air and Gas Institute uses the ISO definition for dry air (0% RH) at 14.5 psi. inch and 68°F. Actual cubic feet per minute acfm is another measure of volumetric capacity. It is related to the amount of compressed air supplied at the outlet of the compressor, which is always less than the working volume of the machine due to blowdown losses of the compressor.

Allowable pressure in pounds per square inch depends primarily on the needs of the equipment on which compressed air will work. While many pneumatic tools are designed to operate at normal shop air pressure, special applications such as engine starting require higher pressures. So, for example, when choosing a reciprocating compressor, buyers will find single-stage units delivering pressures up to 135 psi, enough to power everyday tools, but will consider two-stage units for special high-pressure applications.
The power required to drive the compressor will be determined by these volume and pressure ratios. When sizing compressors, specifiers must also consider system losses: piping losses, pressure drops in dryers and filters, etc. The compressor purchaser will also have to decide on a drive such as a motorized belt drive or direct drive gas or diesel fuel, etc.

Compressor manufacturers often publish compressor performance curves so that specifiers can evaluate compressor performance under various operating conditions. This is especially true for centrifugal compressors, which, like centrifugal pumps, can be designed to deliver different volumes and pressures depending on shaft speed and impeller size.
The DOE adopts energy standards for compressors, and some compressor manufacturers publish specifications based on these standards. As more manufacturers publish this data, it should be easier for compressor buyers to classify the energy consumption of comparable compressors.
Compressors find use in a variety of industries and dominate environments familiar to everyday consumers. For example, a portable 12V DC electric air compressor often carried in the glove box or trunk of a car is a common example of a simple version of an air compressor that consumers can use to inflate tires to the correct pressure.

The use of vehicle related air compressors and general vehicle applications include onboard electric air compressors, onboard diesel air compressors, or other onboard air compressors. For example, a truck’s air brake system requires compressed air to operate, so an onboard air compressor is required to charge the brake system. Service vehicles may require onboard air compressors to perform required functions or to ensure that the compressor is mobile and can be deployed to different job sites or locations as needed. For example, a fire engine may include an on-board breathing air compressor capable of filling air tanks to replenish breathing air tanks for firefighters and first responders.

Dental air compressors provide a source of clean compressed air to aid in dental procedures and to power pneumatic dental instruments such as drills or toothbrushes. Selecting the right dental air compressor requires considering several factors, including the required power and pressure.
The use of medical air compressors involves providing a supply of breathing air independent of other gases stored in cylinders and may be used as an option for patients who may be sensitive to oxygen toxicity. Medical breathing air compressors can be portable or fixed systems in a hospital or medical facility. Other uses of a medical air compressor may include supplying air to specialized patient equipment such as compression cuffs where the compressed air is needed to pressurize the patient’s extremities to prevent fluid buildup in the extremities due to impaired heart function.
Laboratory air compressors and air compressors for other specialized industrial applications are used to process and produce specialized gases such as hydrogen, oxygen, argon, helium, nitrogen or gas mixtures (e.g. ammonia compressors) or carbon dioxide, where they can be used in the food industry . and the beverage industry. Helium compressors will supply gas to storage tanks for laboratory purposes such as delicate leak detection, while other gas compressors such as oxygen compressors may be required to store oxygen tanks for use in hospitals and healthcare facilities.