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{{Redirect|VCRS|videocassette recorders|VCRs}}▼
{{Short description|Refrigeration process}}
▲{{Redirect|VCRS|videocassette recorders|VCRs}}
[[File:refrigeration PV diagram.svg|thumb|300px|A
'''Vapour-compression refrigeration''' or '''vapor-compression refrigeration system''' ('''VCRS'''),<ref>{{cite book|author=Y. V. C. Rao|title=An Introduction to Thermodynamics|edition=2nd|publisher=Universities Press|year=2003|isbn=978-81-7371-461-0|url=https://universitiespress.com/details?id=9788173714610}}</ref> in which the [[refrigerant]] undergoes [[phase transition|phase change]]s, is one of the many [[refrigeration cycle]]s and is the most widely used method for [[air
Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere. A device that performs this function may also be called an [[air conditioning|air conditioner]], [[refrigerator]], [[air source heat pump]], [[geothermal heat pump]], or chiller ([[heat pump and refrigeration cycle|heat pump]]).
==Description
[[File:Refrigeration.png|frame|right|Figure 1: Vapor compression refrigeration]]
Vapor-compression uses a circulating liquid [[refrigerant]] as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Figure 1 depicts a typical, single-stage vapor-compression system. All such systems have four components: a [[gas compressor|compressor]], a [[Condenser (heat transfer)|condenser]], a metering device or [[thermal expansion valve]] (also called a [[throttle]] valve), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic state known as a [[boiling point#Saturation temperature and pressure|saturated vapor]]<ref>Saturated vapors and saturated liquids are vapors and liquids at their [[saturation temperature]] and [[saturation pressure]]. A superheated vapor is at a temperature higher than the saturation temperature corresponding to its pressure.</ref> and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be [[condensation|condensed]] with either cooling water or cooling air flowing across the coil or tubes.
The superheated vapor then passes through the [[Condenser (heat transfer)|condenser]]. This is where heat is transferred from the circulating refrigerant
The condensed liquid refrigerant, in the thermodynamic state known as a [[boiling point#Saturation temperature and pressure|saturated liquid]], is next routed through an [[Thermal expansion valve|expansion valve]] where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic [[flash evaporation]] of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated.
The cold refrigerant liquid and vapor mixture is then routed through the coil or tubes in the evaporator.
To complete the [[refrigeration cycle]], the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor. Over time, the evaporator may collect ice or water from ambient [[humidity]]. The ice is melted through [[auto-defrost|defrosting]]. The water from the melted ice or the evaporator then drips into a drip pan, and the water is carried away by gravity or
===Refrigerants===
{{See also|List of refrigerants}}
The [[working fluid selection|selection]] of [[working fluid]] has a significant impact on the performance of the refrigeration cycles and as such it plays a key role when it comes to designing or simply choosing an ideal machine for a certain task. One of the most widespread refrigerants is "[[haloalkane|Freon]]". Freon is a trade name for a family of [[haloalkane]]
Unfortunately, chlorine- and fluorine-bearing refrigerants reach the upper atmosphere when they escape. In the [[stratosphere]], substances like [[Chlorofluorocarbon|CFCs]] and [[HCFC]]s break up due to [[UV]] radiation, releasing their chlorine
Newer refrigerants
More benign refrigerants are currently the subject of research, such as [[supercritical fluid|supercritical]] [[carbon dioxide]], known as [[R-744]].<ref>[http://www.r744.com/knowledge/faq r744.com – Everything R744] {{Webarchive|url=https://web.archive.org/web/20170724071330/http://www.r744.com/knowledge/faq |date=2017-07-24 }}, The Natural Refrigerant R744 (CO)<sub>2</sub>, 2006–2012</ref> These have similar efficiencies{{Citation needed|date=September 2009}} compared to existing CFC- and HFC-based compounds, and have many orders of magnitude lower global warming potential. General industry and governing body push are toward more GWP-friendly refrigerants. In industrial settings [[ammonia]], as well as gasses like [[ethylene]], [[propane]], [[iso-butane]] and other [[hydrocarbons]] are commonly used (and have their own R-x customary numbers), depending on required temperatures and pressures. Many of these gases are
==Thermodynamic analysis of the system==
[[File:RefrigerationTS.png|frame|right|Figure 2: Temperature–Entropy diagram]]
The [[thermodynamics]] of the vapor compression cycle can be analyzed on a temperature versus [[entropy]] diagram as depicted in Figure 2. At point 1 in the diagram, the circulating refrigerant enters the [[gas compressor|compressor]] as a low-temperature, low-pressure saturated vapor. From point 1 to point 2, the vapor is [[isentropic process|isentropically]] compressed (compressed at constant entropy) and exits the
From point 2 to point 3, the vapor travels through part of the condenser which removes the heat by cooling the vapor. Between point 3 and point 4, the vapor travels through the remainder of the condenser and is condensed into a high-temperature, high-pressure subcooled liquid. Subcool is the amount of sensible heat removed from the liquid below its maximum saturation. The condensation process occurs at essentially constant pressure.
Between points 5 and 1, the cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator where it is totally vaporized by the warm air (from the space being refrigerated) that a fan circulates across the coil or tubes in the evaporator. The evaporation process occurs at essentially constant temperature. After evaporation is completed, the vapor will start to increase in temperature. The amount of sensible heat added to the vapor above its saturation point, i.e. its [[boiling point]], is called superheat.▼
The resulting
▲Between points 5 and 1, the cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator where it is totally vaporized by the warm air (from the space being refrigerated) that a fan circulates across the coil or tubes in the
▲The resulting refrigerant vapor returns to the [[gas compressor|compressor]] inlet at point 1 to complete the thermodynamic cycle.
The above discussion is based on the ideal vapor-compression refrigeration cycle which does not take into account real world items like frictional pressure drop in the system, slight internal irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).
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===Rotary screw compressors===
[[File:Lysholm screw rotors.jpg|right|thumb|upright=0.5|Lysholm<br />screw compressor]]<!--[[File:Screw_compressors.JPG|thumb|right|Figure 3: Screw Compressors]]-->
{{Main|Rotary screw compressor}}
Rotary screw compressors are also positive displacement compressors. Two meshing screw-rotors rotate in opposite directions, trapping refrigerant vapor, and reducing the volume of the refrigerant along the rotors to the discharge point.
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====Centrifugal Compressor Surge====
Chillers with centrifugal compressors have a 'Centrifugal Compressor Map' that shows the "surge line" and the "choke line." For the same capacity ratings, across a wider span of operating conditions, chillers with the larger diameter lower-speed compressor have a wider 'Centrifugal Compressor Map' and experience surge conditions less than those with the smaller diameter, less expensive, higher-speed compressors. The smaller diameter, higher-speed compressors have a flatter curve.,<ref>[https://tc0802.ashraetcs.org/documents/programs/Seminar%2069%20Houston%20King%2020180627.pdf
As the refrigerant flow rate decreases, some compressors change the gap between the impeller and the volute to maintain the correct velocity to avoid surge conditions.<ref>[https://energy-models.com/centrifugal-chiller-fundamentals
===Scroll compressors===
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In order to lubricate the moving parts of the compressor, oil is added to the refrigerant during installation or commissioning. The type of oil may be mineral or synthetic to suit the compressor type, and also chosen so as not to react with the refrigerant type and other components in the system. In small refrigeration systems the oil is allowed to circulate throughout the whole circuit, but care must be taken to design the pipework and components such that oil can drain back under gravity to the compressor. In larger more distributed systems, especially in retail refrigeration, the oil is normally captured at an oil separator immediately after the compressor, and is in turn re-delivered, by an oil level management system, back to the compressor(s). Oil separators are not 100% efficient so system pipework must still be designed so that oil can drain back by gravity to the oil separator or compressor.
Some newer compressor technologies use [[magnetic bearing]]s or [[air bearing]]s and require no lubrication, for example the [[Danfoss]] Turbocor range of centrifugal compressors. Avoiding the need for oil lubrication and the design requirements and ancillaries associated with it, simplifies the design of the refrigerant system, increases the heat transfer coefficient in evaporators and condensers, eliminates the risk of refrigerant being contaminated with oil, and reduces maintenance requirements.<ref>{{cite web| url=https://download.schneider-electric.com/files?p_Doc_Ref=SPD_VAVR-AE7T7G_EN | title=The Different Types of Cooling Compressors | access-date=2024-01-13}}</ref>
==Control==
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The schematic diagram of a single-stage refrigeration system shown in Figure 1 does not include other equipment items that would be provided in a large commercial or industrial vapor compression refrigeration system, such as:
* A horizontal or vertical [[pressure vessel]], equipped internally with a [[demister (vapor)|demister]], between the evaporator and the compressor inlet to capture and remove any residual, entrained liquid in the refrigerant vapor because liquid may damage the compressor. Such [[
* Large commercial or industrial refrigeration systems may have multiple expansion valves and multiple evaporators in order to refrigerate multiple enclosed spaces or rooms. In such systems, the condensed liquid refrigerant may be routed into a pressure vessel, called a receiver, from which liquid refrigerant is withdrawn and routed through multiple pipelines to the multiple expansion valves and evaporators.
* Filter Dryers, installed before the compressors to catch any moisture or contaminants in the system and thus protect the compressors from internal damage
* Some refrigeration units may have multiple stages which requires the use of multiple compressors in various arrangements.<ref>[https://www.engr.siu.edu/staff1/weston/thermo/Refrigeration/VCRefrigeration.html Vapor-compression refrigeration cycles], Schematic diagrams of multi-stage units, Southern Illinois University Carbondale, 1998-11-30</ref>
In most of the world, the [[cooling capacity]] of refrigeration systems is measured in [[watt]]s. Common residential air conditioning units range in capacity from 3.5 to 18 [[kilowatt]]. In a few countries it is measured in "[[Ton of refrigeration|tons of refrigeration]]", with common residential air conditioning units from about 1 to 5 tons of refrigeration.
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{| class="wikitable"
|-
! Refrigeration application !! Short descriptions !! Typical
|-
|Domestic refrigeration||Appliances used for keeping food in dwelling units||[[list of refrigerants|R-600a, R-134a, R-22]],
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|Transport refrigeration||Equipment to preserve and store goods, primarily foodstuffs, during transport by road, rail, air, and sea||[[list of refrigerants|R-134a, R-407C, R-410A]]
|-
|Electronic cooling||Low-temperature cooling of CMOS circuitry and other components in large computers and servers<ref>
|-
|Medical refrigeration|| ||[[list of refrigerants|R-134a, R-404A, R-507]]
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Many systems still use [[hydrochlorofluorocarbon|HCFC]] [[refrigerant]]s, which contribute to [[ozone depletion|depletion of the Earth's ozone layer]]. In countries adhering to the [[Montreal Protocol]], HCFCs are due to be phased out and are largely being replaced by ozone-friendly [[hydrofluorocarbon|HFC]]s. However, systems using HFC refrigerants tend to be slightly less efficient than systems using HCFCs. HFCs also have an extremely large [[global warming potential]], because they remain in the atmosphere for many years and trap heat more effectively than [[carbon dioxide]].
With the ultimate phasing out of HCFCs already a certainty, alternative non-[[haloalkane]] refrigerants are gaining popularity. In particular, once-abandoned refrigerants such as [[hydrocarbon]]s ([[butane]] for example) and CO<sub>2</sub> are coming back into more extensive use. For example, [[Coca-Cola]]'s vending machines at the [[2006 FIFA World Cup]] in Germany used refrigeration utilizing CO<sub>2</sub>.<ref>[http://www.thecoca-colacompany.com/citizenship/environmental_report2006.pdf 2006 Environmental Performance, the Coca-Cola Company] {{Webarchive|url=https://web.archive.org/web/20111110131010/http://www.thecoca-colacompany.com/citizenship/environmental_report2006.pdf |date=2011-11-10 }} (scroll down to pdf page 6 of 9 pdf pages).</ref> [[Ammonia]] (NH<sub>3</sub>) is one of the oldest refrigerants, with excellent performance and essentially no pollution problems. However, ammonia has two disadvantages: it is toxic and it is incompatible with copper tubing.<ref>[https://www.osha.gov/SLTC/etools/ammonia_refrigeration/ammonia/index.html Ammonia Refrigeration – Properties of Ammonia], osha.gov, 2011</ref>
==History==
[[File:Gorrie Ice Machine.png|180px|right|thumb|Schematic of Dr. John Gorrie's 1841 mechanical ice machine.]]
In 1805, the American inventor [[Oliver Evans]] described a closed vapor-compression refrigeration cycle for the production of ice by ether under vacuum. Heat would be removed from the environment by recycling vaporized refrigerant, where it would move through a [[gas compressor|compressor]] and [[heating coil|condenser]], and would eventually revert to a liquid form in order to repeat the refrigeration process over again. However, no such refrigeration unit was built by Evans.<ref name=Hempstead>{{cite book|
In 1834, an American expatriate to Great Britain, [[Jacob Perkins]], built the first working vapor-compression refrigeration system in the world.<ref>{{cite book|author=Robert T. Balmer|title=Modern Engineering Thermodynamic|publisher=Academic Press|date=2011|isbn=978-0-12-374996-3|url=https://books.google.com/books?id=VC-RuN6moREC&q=Jacob+Perkins+refrigeration&pg=PA543}}</ref> It was a closed-cycle that could operate continuously, as he described in his patent:
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[[File:AppareilCarré.jpg|thumb|left|[[Ferdinand Carré]]'s ice-making device.]]
The first practical vapor compression refrigeration system was built by [[James Harrison (engineer)|James Harrison]], a British journalist who had emigrated to [[Australia]].<ref>{{Cite web|url=https://museumsvictoria.com.au/scienceworks/whats-on/|title=What's on|website=Scienceworks|date=16 September 2023 }}</ref> His 1856 patent was for a vapor compression system using ether, alcohol or ammonia. He built a mechanical ice-making machine in 1851 on the banks of the Barwon River at Rocky Point in [[Geelong]], [[Victoria (Australia)|Victoria]], and his first commercial ice-making machine followed in 1854. Harrison also introduced commercial vapor-compression refrigeration to breweries and meat packing houses and, by 1861, a dozen of his systems were in operation in Australia and England.
The first [[absorption refrigeration|gas absorption]] refrigeration system using gaseous ammonia dissolved in water (referred to as "aqua ammonia") was developed by [[Ferdinand Carré]] of France in 1859 and patented in 1860. [[Carl von Linde]], an engineering professor at the Technological University Munich in Germany, patented an improved method of liquefying gases in 1876. His new process made possible using gases such as [[ammonia]], [[sulfur dioxide]] {{SO2}}, and [[methyl chloride]] (CH<sub>3</sub>Cl) as refrigerants and they were widely used for that purpose until the late 1920s.
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*[[Refrigerant]]
*[[Refrigeration]]
*[[
*[[Working fluid]]
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