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===Positive displacement=== A positive displacement compressor is a system that compresses the air by the displacement of a mechanical linkage reducing the volume (since the reduction in volume due to a piston in thermodynamics is considered as positive displacement of the piston).{{vague|date=May 2018}} Put another way, a positive displacement compressor is one that operates by drawing in a discrete volume of gas from its inlet then forcing that gas to exit via the compressor's outlet. The increase in the pressure of the gas is due, at least in part, to the compressor pumping it at a mass flow rate which cannot pass through the outlet at the lower pressure and density of the inlet. ====Reciprocating compressors==== {{main|Reciprocating compressor}} [[File:ReciprocatingCompressor.jpg|right|thumb|A motor-driven six-cylinder reciprocating compressor that can operate with two, four or six cylinders.]] '''Reciprocating compressors''' use [[piston]]s driven by a crankshaft. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines.<ref name=Perry /><ref>{{cite book |author1=Bloch, H.P. |author2=Hoefner, J.J. |title=Reciprocating Compressors, Operation and Maintenance |publisher=Gulf Professional Publishing |year=1996 |isbn=0-88415-525-0 }}</ref><ref>[http://www.machinerylubrication.com/article_detail.asp?articleid=775&pagetitle=Reciprocating%20Compressor%20Basics Reciprocating Compressor Basics] {{webarchive|url=https://web.archive.org/web/20090418034216/http://www.machinerylubrication.com/article_detail.asp?articleid=775&pagetitle=Reciprocating%20Compressor%20Basics |date=2009-04-18 }} Adam Davis, Noria Corporation, ''Machinery Lubrication'', July 2005</ref> Small reciprocating compressors from 5 to 30 [[horsepower]] (hp) are commonly seen in automotive applications and are typically for intermittent duty. Larger reciprocating compressors well over {{convert|1000|hp|abbr=on}} are commonly found in large industrial and petroleum applications. [[Discharge pressure]]s can range from low pressure to very high pressure (>18000 psi or 124 MPa). In certain applications, such as air compression, multi-stage double-acting compressors are said to be the most efficient compressors available, and are typically larger, and more costly than comparable rotary units.<ref>{{Cite web|url=https://www.thomasnet.com/articles/machinery-tools-supplies/|archive-url=https://web.archive.org/web/20100428012223/http://www.thomasnet.com/articles/machinery-tools-supplies/industrial-compressed-air-systems|url-status=dead|title=Machinery, Tools & Supplies Articles on ThomasNet|archive-date=April 28, 2010|website=www.thomasnet.com}}</ref> Another type of reciprocating compressor, usually employed in automotive cabin [[air conditioning]] systems,{{citation needed|date=August 2017}} is the swash plate or wobble plate compressor, which uses pistons moved by a swash plate mounted on a shaft (see ''[[axial piston pump]]''). Household, home workshop, and smaller job site compressors are typically reciprocating compressors {{cvt|1.5|hp}} or less with an attached receiver tank. A [[linear compressor]] is a reciprocating compressor with the piston being the rotor of a linear motor. This type of compressor can compress a wide range of gases, including refrigerant, hydrogen, and natural gas. Because of this, it finds use in a wide range of applications in many different industries and can be designed to a wide range of capacities, by varying size, number of cylinders, and cylinder unloading. However, it suffers from higher losses due to clearance volumes, resistance due to discharge and suction valves, weighs more, is difficult to maintain due to having a large number of moving parts, and it has inherent vibration.<ref name="autogenerated1">{{Cite web |title=Ref Doc |url=https://download.schneider-electric.com/files?p_Doc_Ref=SPD_VAVR-AE7T7G_EN |website=download.schneider-electric.com}}</ref> ====Ionic liquid piston compressor==== {{main|Ionic liquid piston compressor}} An [[ionic liquid piston compressor]], ''ionic compressor'' or ''ionic liquid piston pump'' is a [[hydrogen compressor]] based on an [[ionic liquid]] piston instead of a metal piston as in a piston-metal [[diaphragm compressor]]. ====Rotary screw compressors==== [[File:RotaryScrewCompressor.gif|thumb|a Diagram of a rotary screw compressor|alt=www]] {{main|Rotary screw compressor}} '''Rotary screw compressors''' use two meshed rotating positive-displacement [[Helix|helical screws]] to force the gas into a smaller space.<ref name=Perry/><ref>[http://www.blackmer.com/tech-screw.jsp Screw Compressor] {{webarchive|url=https://web.archive.org/web/20080110103341/http://www.blackmer.com/tech-screw.jsp |date=2008-01-10 }} Describes how screw compressors work and include photographs.</ref><ref>[http://www.domnickhunter.com/tech_Centre.asp?chapter=1§ion=3_Screw-Compressors_2_3.htm&getIndex=false Technical Centre] {{webarchive|url=https://web.archive.org/web/20071213211548/http://www.domnickhunter.com/tech_Centre.asp?chapter=1§ion=3_Screw-Compressors_2_3.htm&getIndex=false |date=2007-12-13 }} Discusses oil-flooded screw compressors including a complete system flow diagram</ref> These are usually used for continuous operation in commercial and industrial applications and may be either stationary or portable. Their application can be from {{convert|3|hp|kW}} to over {{convert|1200|hp|kW}} and from low pressure to moderately high pressure (>{{convert|1200|psi|MPa|1|abbr=on|disp=or}}). The classifications of rotary screw compressors vary based on stages, cooling methods, and drive types among others.<ref>{{Cite news|url=http://www.industrialcompressorsolutions.com/articles/how-does-a-rotary-screw-air-compressor-work|title=How Does a Rotary Screw Air Compressor Work?|last=ICS|access-date=2017-08-16|language=en|url-status=live|archive-url=https://web.archive.org/web/20170817001135/http://www.industrialcompressorsolutions.com/articles/how-does-a-rotary-screw-air-compressor-work|archive-date=2017-08-17}}</ref> Rotary screw compressors are commercially produced in Oil Flooded, Water Flooded and Dry type. The efficiency of rotary compressors depends on the air drier,{{clarify|date=May 2014}} and the selection of air drier is always 1.5 times volumetric delivery of the compressor.<ref>{{Cite book|url=https://books.google.com/books?id=1w5yBgAAQBAJ&q=%22The+efficiency+of+rotary+compressors+depends+on+the+air+drier%22&pg=PA643|title=Hydraulic Fracturing Operations: Handbook of Environmental Management Practices|last1=Cheremisinoff|first1=Nicholas P.|last2=Davletshin|first2=Anton|date=2015-01-28|publisher=John Wiley & Sons|isbn=9781119100003|language=en|url-status=live|archive-url=https://web.archive.org/web/20171224222642/https://books.google.com/books?id=1w5yBgAAQBAJ&pg=PA643&lpg=PA643&dq=%22The+efficiency+of+rotary+compressors+depends+on+the+air+drier%22&source=bl&ots=DIZJEjhMaW&sig=coZe1GEy2bN8CTUmSaK8IuA3jSw&hl=en&sa=X&ved=0ahUKEwjV9YOjntzVAhUp7oMKHeIvCLoQ6AEILDAB#v=onepage&q=%22The%20efficiency%20of%20rotary%20compressors%20depends%20on%20the%20air%20drier%22&f=false|archive-date=2017-12-24}}</ref> Designs with a single screw<ref>{{cite web|url=https://www.daikinapplied.uk/features/single-screw-compressor |title=Single Screw Compressor |website=Daikin Applied UK}}</ref> or three screws<ref>{{citation|url=http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2824&context=icec |last=Jacobs |first=John S. |title=Variable Speed Tri-Rotor Screw Compression Technology |date=2006 |publisher=International Compressor Engineering Conference. Paper 1825.}}</ref> instead of two exist. Screw compressors have fewer moving components, larger capacity, less vibration and surging, can operate at variable speeds, and typically have higher efficiency. Small sizes or low rotor speeds are not practical due to inherent leaks caused by clearance between the compression cavities or screws and compressor housing.<ref name=autogenerated1 /> They depend on fine machining tolerances to avoid high leakage losses and are prone to damage if operated incorrectly or poorly serviced. ====Rotary vane compressors==== [[Image:Rotary vane pump.svg|thumb|300px|Eccentric rotary-vane pump]] {{See also|Rotary vane pump}} '''Rotary vane compressors''' consist of a rotor with a number of blades inserted in radial slots in the rotor. The rotor is mounted offset in a larger housing that is either circular or a more complex shape. As the rotor turns, blades slide in and out of the slots keeping contact with the outer wall of the housing.<ref name=Perry/> Thus, a series of increasing and decreasing volumes is created by the rotating blades. Rotary vane compressors are, with piston compressors one of the oldest of compressor technologies. With suitable port connections, the devices may be either a compressor or a vacuum pump. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Dry vane machines are used at relatively low pressures (e.g., {{convert|2|bar|kPa psi|abbr=on|disp=or}}) for bulk material movement while oil-injected machines have the necessary volumetric efficiency to achieve pressures up to about {{convert|13|bar|kPa psi|abbr=on}} in a single stage. A rotary vane compressor is well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor. Rotary vane compressors can have mechanical efficiencies of about 90%.<ref>{{Cite web|url=http://www.matteicomp.com/|archive-url=https://web.archive.org/web/20100509233715/http://www.matteicomp.com/compressor-news.htm?id=165176063|url-status=dead|title=Rotary Vane Compressors and the Vane Compressor - Compressors for stationary industrial and OEM applications - Mattei|first=Mattei Compressors|last=Inc|archive-date=May 9, 2010|website=www.matteicomp.com}}</ref> ====Rolling piston==== [[File:Rollkolbenverdichter.png|thumb|Rolling piston compressor]] The Rolling piston in a rolling piston style compressor plays the part of a partition between the vane and the rotor.<ref>{{cite web|url=http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1390&context=icec|title=Motion of Rolling Piston in Rotary Compressor|website=Purdue University|archive-url=https://web.archive.org/web/20170816232859/http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1390&context=icec|archive-date=2017-08-16|url-status=live|access-date=2017-08-16}}</ref> Rolling piston forces gas against a stationary vane. Two of these compressors can be mounted on the same shaft to increase capacity and reduce vibration and noise.<ref>{{Cite web|url=https://news.samsung.com/global/what-the-new-samsung-mini-rotary-compressor-means|title=What the New Samsung Mini Rotary Compressor Means|website=news.samsung.com}}</ref> A design without a spring is known as a swing compressor.<ref>{{Cite web|url=https://www.daikin.com/products/ac/lineup/skyair/|title=High Efficiency Compressor to Achieve a High COP {{pipe}} Air Conditioning and Refrigeration {{pipe}} Daikin Global|website=www.daikin.com}}</ref> In refrigeration and air conditioning, this type of compressor is also known as a rotary compressor, with rotary screw compressors being also known simply as screw compressors. It offers higher efficiency than reciprocating compressors due to less losses from the clearance volume between the piston and the compressor casing, it's 40% to 50% smaller and lighter for a given capacity (which can impact material and shipping costs when used in a product), causes less vibration, has fewer components and is more reliable than a reciprocating compressor. But its structure does not allow capacities beyond 5 refrigeration tons, is less reliable than other compressor types, and is less efficient than other compressor types due to losses from the clearance volume.<ref name="autogenerated1"/> ====Scroll compressors==== [[File:Two moving spirals scroll pump.gif|thumb|right|Mechanism of a scroll pump]] {{Main|Scroll compressor}} A '''scroll compressor''', also known as '''scroll pump''' and '''scroll vacuum pump''', uses two interleaved spiral-like vanes to [[pump]] or compress [[fluid]]s such as [[liquids]] and [[gas]]es. The vane geometry may be [[involute]], [[archimedean spiral]], or hybrid curves.<ref>Tischer, J., Utter, R: "Scroll Machine Using Discharge Pressure For Axial Sealing," U.S. Patent 4522575, 1985.</ref><ref>Caillat, J., Weatherston, R., Bush, J: "Scroll-Type Machine With Axially Compliant Mounting," U.S. Patent 4767293, 1988.</ref><ref>Richardson, Jr., Hubert: "Scroll Compressor With Orbiting Scroll Member Biased By Oil Pressure," U.S. Patent 4875838, 1989.</ref> They operate more smoothly, quietly, and reliably than other types of compressors in the lower volume range. Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid between the scrolls. Due to minimum clearance volume between the fixed scroll and the orbiting scroll, these compressors have a very high [[volumetric efficiency]]. These compressors are extensively used in air conditioning and refrigeration because they are lighter, smaller and have fewer moving parts than reciprocating compressors and they are also more reliable. They are more expensive though, so peltier coolers or rotary and reciprocating compressors may be used in applications where cost is the most important or one of the most important factors to consider when designing a refrigeration or air conditioning system. This type of compressor was used as the [[G-Lader|supercharger]] on Volkswagen G60 and G40 engines in the early 1990s. When compared with reciprocating and rolling piston compressors, scroll compressors are more reliable since they have fewer components and have a simpler structure, are more efficient since they have no clearance volume nor valves, and possess the advantages both of surging less and not vibrating so much. But, when compared with screw and centrifugal compressors, scroll compressors have lower efficiencies and smaller capacities.<ref name="autogenerated1"/> ====Diaphragm compressors==== {{main|Diaphragm compressor}} A '''diaphragm compressor''' (also known as a '''membrane compressor''') is a variant of the conventional reciprocating compressor. The compression of gas occurs by the movement of a flexible membrane, instead of an intake element. The back-and-forth movement of the membrane is driven by a rod and a crankshaft mechanism. Only the membrane and the compressor box come in contact with the gas being compressed.<ref name=Perry/> The degree of flexing and the material constituting the diaphragm affects the maintenance life of the equipment. Generally stiff metal diaphragms may only displace a few cubic centimeters of volume because the metal cannot endure large degrees of flexing without cracking, but the stiffness of a metal diaphragm allows it to pump at high pressures. Rubber or silicone diaphragms are capable of enduring deep pumping strokes of very high flexion, but their low strength limits their use to low-pressure applications, and they need to be replaced as plastic embrittlement occurs. Diaphragm compressors are used for hydrogen and compressed natural gas ([[Compressed natural gas|CNG]]) as well as in a number of other applications. [[File:DiaphragmCompressor.jpg|thumb|right|A three-stage diaphragm compressor]] The photograph on the right depicts a three-stage diaphragm compressor used to compress hydrogen gas to {{convert|6000|psi|MPa|abbr=on}} for use in a prototype [[compressed hydrogen]] and [[compressed natural gas]] (CNG) fueling station built in downtown [[Phoenix, Arizona]] by the [[Arizona Public Service]] company (an electric utilities company). [[Reciprocating compressor]]s were used to compress the [[natural gas]]. The reciprocating natural gas compressor was developed by [[Sertco]].<ref>{{cite journal | author=Eric Slack | title=Sertco | date=Winter 2016 | url=http://www.emi-magazine.com/index.php/featured-content/1067-sertco | publisher=Phoenix Media Corporation | journal=Energy and Mining International | access-date=February 27, 2016 | url-status=live | archive-url=https://web.archive.org/web/20160304170914/http://www.emi-magazine.com/index.php/featured-content/1067-sertco | archive-date=March 4, 2016 }}</ref> The prototype [[alternative fuel]]ing station was built in compliance with all of the prevailing safety, environmental and building codes in Phoenix to demonstrate that such fueling stations could be built in urban areas.
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