Editing Refrigeration (section)

===Cyclic refrigeration===
{{Main|Heat pump and refrigeration cycle}}
This consists of a refrigeration cycle, where heat is removed from a low-temperature space or source and rejected to a high-temperature sink with the help of external work, and its inverse, the [[thermodynamic power cycle]]. In the power cycle, heat is supplied from a high-temperature source to the engine, part of the heat being used to produce work and the rest being rejected to a low-temperature sink. This satisfies the [[second law of thermodynamics]].

A ''refrigeration cycle'' describes the changes that take place in the refrigerant as it alternately absorbs and rejects heat as it circulates through a [[refrigerator]]. It is also applied to heating, ventilation, and air conditioning [[HVACR]] work, when describing the "process" of refrigerant flow through an HVACR unit, whether it is a packaged or split system.

Heat naturally flows from hot to cold. [[Mechanical work|Work]] is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. [[Thermal insulation|Insulation]] is used to reduce the work and [[energy]] needed to achieve and maintain a lower temperature in the cooled space. The operating principle of the refrigeration cycle was described mathematically by [[Nicolas Léonard Sadi Carnot|Sadi Carnot]] in 1824 as a [[Carnot heat engine|heat engine]].

The most common types of refrigeration systems use the reverse-Rankine [[vapor-compression refrigeration]] cycle, although [[absorption heat pump]]s are used in a minority of applications.

Cyclic refrigeration can be classified as:
#Vapor cycle, and
#Gas cycle

Vapor cycle refrigeration can further be classified as:
#[[Vapor-compression refrigeration]]
#Sorption Refrigeration
##[[Absorption refrigerator|Vapor-absorption refrigeration]]
##[[Adsorption refrigeration]]

====Vapor-compression cycle====
{{See also|Vapor-compression refrigeration}}
[[File:Refrigeration.png|frame|right|Figure 1: Vapor compression refrigeration]]
[[File:RefrigerationTS.png|frame|right|Figure 2: Temperature–Entropy diagram]]
The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and [[Industrial refrigerator|industrial refrigeration]] systems. Figure 1 provides a schematic diagram of the components of a typical vapor-compression refrigeration system.

The [[thermodynamics]] of the cycle can be analyzed on a diagram<ref>[http://web.me.unr.edu/me372/Spring2001/Vapor%20Compression%20Refrigeration%20Cycles.pdf The Ideal Vapor-Compression Cycle] {{webarchive|url=https://web.archive.org/web/20070226113352/http://web.me.unr.edu/me372/Spring2001/Vapor%20Compression%20Refrigeration%20Cycles.pdf |date=2007-02-26}}</ref> as shown in Figure 2. In this cycle, a circulating refrigerant such as a low boiling hydrocarbon or [[hydrofluorocarbons]] enters the [[gas compressor|compressor]] as a vapour. From point 1 to point 2, the vapor is compressed at constant [[entropy]] and exits the compressor as a vapor at a higher temperature, but still below the [[vapor pressure]] at that temperature. From point 2 to point 3 and on to point 4, the vapor travels through the [[condenser (heat transfer)|condenser]] which cools the vapour until it starts condensing, and then condenses the vapor into a liquid by removing additional heat at constant pressure and temperature. Between points 4 and 5, the liquid refrigerant goes through the [[thermal expansion valve|expansion valve]] (also called a throttle valve) where its pressure abruptly decreases, causing [[flash evaporation]] and auto-refrigeration of, typically, less than half of the liquid.

That results in a mixture of liquid and vapour at a lower temperature and pressure as shown at point 5. The cold liquid-vapor mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air (from the space being refrigerated) being blown by a fan across the evaporator coil or tubes. The resulting refrigerant vapour returns to the compressor inlet at point 1 to complete the thermodynamic cycle.

The above discussion is based on the ideal vapour-compression refrigeration cycle, and does not take into account real-world effects like frictional pressure drop in the system, slight [[thermodynamic reversibility|thermodynamic irreversibility]] during the compression of the refrigerant vapor, or [[ideal gas|non-ideal gas]] behavior, if any. Vapor compression refrigerators can be arranged in two stages in [[cascade refrigeration]] systems, with the second stage cooling the condenser of the first stage. This can be used for achieving very low temperatures.

More information about the design and performance of vapor-compression refrigeration systems is available in the classic ''[[Perry's Chemical Engineers' Handbook]]''.<ref>{{cite book|author1=Perry, R.H. |author2=Green, D.W. |name-list-style=amp |title=Perry's Chemical Engineers' Handbook|edition=6th |publisher=McGraw Hill, Inc.|year=1984|isbn=978-0-07-049479-4|title-link=Perry's Chemical Engineers' Handbook}} (see pp. 12-27 through 12-38)</ref>

====Sorption cycle====
{{unreferenced section|date=February 2020}}

=====Absorption cycle=====
{{Main|Absorption refrigerator}}
In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems or [[lithium bromide|LiBr]]-water was popular and widely used. After the development of the vapor compression cycle, the vapor absorption cycle lost much of its importance because of its low [[coefficient of performance]] (about one fifth of that of the vapor compression cycle). Today, the vapor absorption cycle is used mainly where fuel for heating is available but electricity is not, such as in [[recreational vehicles]] that carry [[liquefied petroleum gas|LP gas]]. It is also used in industrial environments where plentiful waste heat overcomes its inefficiency.

The absorption cycle is similar to the compression cycle, except for the method of raising the pressure of the refrigerant vapor. In the absorption system, the compressor is replaced by an absorber which dissolves the refrigerant in a suitable liquid, a liquid pump which raises the pressure and a generator which, on heat addition, drives off the refrigerant vapor from the high-pressure liquid. Some work is needed by the liquid pump but, for a given quantity of refrigerant, it is much smaller than needed by the compressor in the vapor compression cycle. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) with water (absorbent), and water (refrigerant) with lithium bromide (absorbent).

=====Adsorption cycle=====
{{Main|Adsorption refrigeration}}
The main difference with absorption cycle, is that in adsorption cycle, the refrigerant (adsorbate) could be ammonia, water, [[methanol]], etc., while the adsorbent is a solid, such as [[silica gel]], [[activated carbon]], or [[zeolite]], unlike in the absorption cycle where absorbent is liquid.

The reason adsorption refrigeration technology has been extensively researched in recent 30 years lies in that the operation of an adsorption refrigeration system is often noiseless, non-corrosive and environment friendly.<ref>{{Cite journal|last1=Goyal|first1=Parash|last2=Baredar|first2=Prashant|last3=Mittal|first3=Arvind|last4=Siddiqui|first4=Ameenur. R.|date=2016-01-01|title=Adsorption refrigeration technology – An overview of theory and its solar energy applications|journal=Renewable and Sustainable Energy Reviews|language=en|volume=53|pages=1389–1410|doi=10.1016/j.rser.2015.09.027|bibcode=2016RSERv..53.1389G |issn=1364-0321}}</ref>

====Gas cycle====
{{unreferenced section|date=February 2020}}
When the [[working fluid]] is a gas that is compressed and expanded but does not change phase, the refrigeration cycle is called a ''gas cycle''. [[Air]] is most often this working fluid. As there is no condensation and evaporation intended in a gas cycle, components corresponding to the condenser and evaporator in a vapor compression cycle are the hot and cold gas-to-gas [[heat exchanger]]s in gas cycles.

The gas cycle is less efficient than the vapor compression cycle because the gas cycle works on the reverse [[Brayton cycle]] instead of the reverse [[Rankine cycle]]. As such, the working fluid does not receive and reject heat at constant temperature. In the gas cycle, the refrigeration effect is equal to the product of the specific heat of the gas and the rise in temperature of the gas in the low temperature side. Therefore, for the same cooling load, a gas refrigeration cycle needs a large mass flow rate and is bulky.

Because of their lower efficiency and larger bulk, ''air cycle'' coolers are not often used nowadays in terrestrial cooling devices. However, the [[air cycle machine]] is very common on [[gas turbine]]-powered jet [[aircraft]] as cooling and ventilation units, because compressed air is readily available from the engines' compressor sections. Such units also serve the purpose of pressurizing the aircraft.