Vapor-compression refrigeration: Difference between revisions

'''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 conditioning|air-conditioning]] of buildings and automobiles. It is also used in domestic and commercial refrigerators[[refrigerator]]s, large-scale warehouses for chilled or frozen storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. [[Oil refinery|Oil refineries]], [[petrochemical]] and [[chemical plant|chemical]] processing plants, and [[natural gas processing]] plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems. [[Cascade refrigeration]] systems may also be implemented using 2two compressors.

==Description of the vapor-compression refrigeration system==

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 or metering device), 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. This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by either the water or the air (whichever may be the case).

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. A fan circulates the warm airAir in the enclosed space circulates across the coil or tubes carryingdue theto coldeither refrigerantthermal liquid[[Natural andconvection|convection]] vaporor mixture. That warm aira [[evaporatesFan (machine)|fan]]. Since the liquidair partis ofwarmer than the cold liquid refrigerant, mixture.heat Atis transferred from the sameair time,to the circulatingrefrigerant, airwhich iscools cooledthe air and thus lowerswarms the temperaturerefrigerant, ofcausing the[[evaporation]], enclosedreturning spaceit to thea desiredgaseous temperaturestate. TheWhile evaporatorliquid isremains wherein the circulating refrigerant absorbsflow, andits removestemperature heatwill not rise above the [[boiling point]] of the refrigerant, which isdepends subsequentlyon rejectedthe pressure in the condenserevaporator. andMost transferredsystems elsewhereare bydesigned to evaporate all of the waterrefrigerant orto airensure usedthat inno liquid is returned to the condensercompressor.

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 by a condensate pump.

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 refrigerantrefrigerants is "[[haloalkane|Freon]]". Freon is a trade name for a family of [[haloalkane]] [[refrigerant]]srefrigerants manufactured by [[DuPont]] and other companies. These refrigerants were commonly used due to their superior stability and safety properties: they were not flammable at room temperature and atmospheric pressure, nor obviously toxic as were the fluids they replaced, such as [[sulfur dioxide]]. Haloalkanes are also an order(s) of magnitude more expensive than petroleum -derived flammable alkanes of similar or better cooling performance.

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 free -radicals. These chlorine free -radicals act as [[catalyst]]s in the breakdown of ozone through chain reactions. One CFC molecule can cause thousands of ozone molecules to break down. This causes severe damage to the [[ozone layer]] that shields the Earth's surface from the Sun's strong UV radiation, and has been shown to lead to increased rates of skin cancer. The chlorine will remain active as a catalyst until and unless it binds with another particle, forming a stable molecule. CFC refrigerants in common but receding usage include [[Trichlorofluoromethane|R-11]] and [[Dichlorodifluoromethane|R-12]].

Newer refrigerants withthat have reduced [[ozone depletion]] effecteffects suchcompared asto CFCs have replaced most CFC use. Examples include [[HCFC]]s (such as [[chlorodifluoromethane|R-22]], used in most homes today) and [[hydrofluorocarbon|HFC]]s (such as [[R-134a]], used in most cars) have replaced most CFC use. HCFCs in turn are being phased out under the [[Montreal Protocol]] and replaced by hydrofluorocarbons (HFCs), thatwhich do not contain [[chlorine]] atoms. Small example of common HFCs in current use: [[R-410A]] (which is itself a blend of other HFCs: [[Difluoromethane|R-32]] and [[R-125]]) ; designed to be a drop-in replacement for [[chlorodifluoromethane|R-22]] in existing installations and [[R-404A]] (blend of HFCs: [[R-125]], [[R-134a]], and [[R-143a]], and was developed as a substitute refrigerant for [[R-502]] and [[chlorodifluoromethane|R-22]]). However, CFCs, HCFCs, and HFCs all have very large [[global warming potential]] (GWP).

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)₂, 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 isare 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 unfortunately flammable, explosive, or toxic; making their use restricted (i.e. well -controlled environmentsenvironment by qualified personnel, or a very small amount of refrigerant used). [[Hydrofluoroolefin|HFO]]s which can be considered to be HFCHFCs with some carbon-carbon bonds being double bounds, do show promise of lowering GWP veryso lowlittle to be of no further concern. In the meantime, various blends of existing refrigerants are used to achieve the required properties and efficiency, at a reasonable cost and lower GWP.

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 [[gas compressor|compressor]] as a [[superheating|superheatedhigh-pressure, high-temperature vapor]]. Superheat is the amount of heat added above the boiling point.

From point 2 to point 3, the vapor travels through part of the condenser which removes the superheatheat by cooling the vapor. Between point 3 and point 4, the vapor travels through the remainder of the condenser and is condensed into a saturatedhigh-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 4 and 5, the saturatedsubcooled liquid refrigerant passes through the expansion valve and undergoes an abrupt decrease of pressure. That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes). The adiabatic flash evaporation process is [[isenthalpic]] (occurs at constant [[enthalpy]]).

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 refrigerantsuperheated vapor returns to the [[gas compressor|compressor]] inlet at point 1 to complete the thermodynamic cycle.

[[File:Lysholm screw rotors.jpg|right|thumb|upright=0.5|Lysholm<br />screw compressor]]<!--[[File:Screw_compressors.JPG|thumb|right|Figure 3: Screw Compressors]]-->

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] Fundamentals of Centrifugal Chillers | Johnson Controls]</ref><ref>[https://www.taylor-engineering.com/wp-content/uploads/2020/04/EDR_DesignGuidelines_CoolToolsChilledWater.pdf] Chilled Water Plant Design Guide | Taylor Engineering | Pages 281]</ref><ref>[https://theengineeringmindset.com/chiller-surge-explained/] Chiller Surge]</ref>

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] Centrifugal Chiller - Fundamentals | McQuay]</ref>

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>

In simple commercial refrigeration systems the compressor is normally controlled by a simple pressure switch, with the expansion performed by a capillary tube or simple thermostatic[[thermal expansion valve]]. In more complex systems, including multiple compressor installations, the use of electronic controls is typical, with adjustable set points to control the pressure at which compressors cut in and cut out, and temperature control by the use of electronic expansion valves.

* 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 [[vapor-liquidvapor–liquid separator]]s are most often referred to as "suction line accumulators". (In other industrial processes, they are called "compressor suction drums" or "knockout pots".)

* 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>

! Refrigeration application !! Short descriptions !! Typical refrigeratorsrefrigerants used

|Electronic cooling||Low-temperature cooling of CMOS circuitry and other components in large computers and servers<ref>Schmidt,{{cite R.R.journal and| Notohardjono, B.D. (2002), [url=https://ieeexplore.ieee.org/document/5388972 "| doi=10.1147/rd.466.0739 | title=High-end server low-temperature cooling"], ''| year=2002 | last1=Schmidt | first1=R. R. | last2=Notohardjono | first2=B. D. | journal=IBM Journal of Research and Development'', Vol.| volume=46, Issue| issue=6, pp.739-751.| pages=739–751 }}</ref> ||[[list of refrigerants|R-134a, R-404A, R-507]]

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 CO₂ 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 CO₂.<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]] (NH₃) 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>

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|authorauthor1=Colin Hempstead| and author2=William E. Worthington (Editors)|title=Encyclopedia of 20th-Century Technology, Volume 2|publisher=Taylor& Francis|date=2005|isbn=1-57958-464-0 |url=https://books.google.com/books?id=0wkIlnNjDWcC}}</ref>

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.

*[[RefrigerationHeat pump and refrigeration cycle]]