Refrigeration - Wikiparency

# Refrigeration **Refrigeration** is the cooling of a space, a substance or a system to a temperature below that of its surroundings, and holding it there. Because [[Heat|heat]] flows on its own only from hot to cold, keeping a cold space cold means moving heat the wrong way, and the [[Second_law_of_thermodynamics|second law of thermodynamics]] requires that something be spent to do it: shaft work in a compressor, heat at a high temperature in an absorption chiller, electrical work in a thermoelectric module, or a consumable such as melting ice. Two families divide the subject. Non-cyclic refrigeration consumes something — ice, dry ice, liquid nitrogen — and cools until the supply runs out. Cyclic refrigeration circulates a working fluid, the [[Refrigerant|refrigerant]], around a closed loop in which it absorbs heat at low temperature and rejects more heat at high temperature, and runs as long as it is driven. The vapour-compression loop, a compressor, a condenser, an expansion device and an evaporator, is by far the most common; it is the [[Heat_engine|heat engine]] run backwards, and the same thermodynamics that caps an engine's efficiency caps a refrigerator's performance. What the cycle is graded on is the coefficient of performance, the cooling obtained per unit of work paid: `COP_R = q_L/w`. It is routinely greater than one, because the machine moves heat rather than creating cold, and it is bounded by the reversible value `T_L/(T_H − T_L)` set by the two temperatures alone.[^yan] The framework microsim *Refrigeration: the vapor-compression cycle on the pressure-enthalpy diagram* draws the loop on a pressure–enthalpy chart: the reader moves the evaporator and condenser temperatures, watches the rectangle-with-a-slope change shape against the saturation dome, and reads the cooling produced, the work paid, the COP and the Carnot bar above it. ## History The history of refrigeration divides cleanly at the point where cold stopped being harvested and started being manufactured. For most of it, cooling meant moving a consumable: snow and ice from where winter made them to where they were wanted, stored in insulated cellars against the summer. The machine age replaced the commodity with a process, and within a century the process had rearranged agriculture, shipping, cities and diet. ### Ice harvesting Natural ice was cut from lakes and rivers in winter, hauled to [[Ice_house_(building)|ice houses]], packed in sawdust and shipped to cities and to the tropics, an [[Ice_trade|ice trade]] that ran on sawdust and sailing ships. The physics is worth stating because it explains both the scale of the trade and its limits: melting ice absorbs its [[Enthalpy_of_fusion|latent heat of fusion]], about 334 kJ per kilogram, at a fixed 0 °C, so a 25 kg block removes roughly 8.4 MJ from a store as it melts and does so at exactly one temperature, with no way to go lower and no way to hold food below freezing. Salt mixed with ice depresses the melting point and buys a colder bath, which is how ice cream was made, but the supply is still a commodity that has to be cut, carried and replaced — and one that fails in a warm winter. ### Refrigeration research The closed vapour-compression cycle was conceived by the American inventor [[Oliver_Evans|Oliver Evans]] in 1805, though he never built a working machine.[^asme-perkins] [[Jacob_Perkins|Jacob Perkins]] turned the idea into hardware: his 1834 patent, "Apparatus and means for producing ice, and in cooling fluids," is recognised as "the first working device to use a system of vapor-compression for refrigeration, and was a closed-cycle" machine "that could operate continuously," and it was constructed and demonstrated in 1835 by John Hague.[^asme-perkins] Every household [[Refrigerator|refrigerator]] since is a descendant of that loop. The theoretical reason such machines were worth improving came from the same decades that produced [[Thermodynamics|thermodynamics]]. A refrigerator is graded not on how much work it consumes but on the ratio of heat moved to work spent, and that ratio has a ceiling fixed by the reservoir temperatures rather than by the engineer's skill.[^yan] Nineteenth-century machines ran far below it: by the time [[Carl_von_Linde|Carl von Linde]] took up the problem at Munich's Polytechnic School, where he had been professor of theoretical machine research since 1868, existing cooling machines reached roughly a fifth of the theoretical maximum, and his first prototype doubled that.[^tum-linde] ### Commercial use The first commercial applications were the ones where the product was itself the point. [[John_Gorrie|John Gorrie]], a physician in the American South, patented an ice machine that made ice "by absorbing its heat of liquefaction with expanding air" — U.S. Patent 8080 of May 6, 1851, the first patent for a mechanical refrigerating or ice-making machine issued by the U.S. Patent Office.[^si-gorrie] Linde's first industrial machine went to a brewery, the Dreher works in Trieste, in 1875, and proved durable enough that industrial demand followed quickly; he was ennobled in 1897.[^tum-linde] [[Brewing|Brewing]], [[Meat_packing_industry|meat packing]] and ice making were the three trades that paid for the technology before anyone thought of putting it in a kitchen, and each did so for a different reason: a brewery needs a steady low temperature it cannot get from a cellar in summer, a packing house needs to hold a perishable product long enough to sell it, and an ice plant sells the cold itself. Between them they carried the capital cost of a machine that was, at first, large, dangerous and expensive to run. ### Home and consumer use Domestic cooling began as a box with a block of ice in it and a drip tray beneath, and stayed that way until a refrigeration machine could be made small, quiet and safe enough for a kitchen. The early sealed household units used sulfur dioxide and methyl formate, which were efficient and poisonous; the arrival of the [[Chlorofluorocarbon|chlorofluorocarbons]] made the domestic refrigerator ordinary, and their consequences for the [[Ozone_layer|ozone layer]] made it a regulated appliance three generations later. The appliance that resulted is the most widely owned heat engine in the world, and almost nobody who owns one thinks of it as a machine at all. ## Impact on settlement patterns in the United States of America Refrigerated transport changed where food could be grown relative to where it could be sold, and with it where people could live. A perishable crop is only worth growing within the distance it can survive; extend that distance and the map of agricultural regions is redrawn, because land far from the eastern markets becomes worth farming. The [[Refrigerator_car|refrigerated rail car]] did that first. Carried on the expanding railroad network, it let meat and produce move from producing regions to distant urban markets, and it was the [[Meat_packing_industry|meat-packing industry]] that used it first and hardest — railroads were initially reluctant to adopt cars that would undercut the live-cattle traffic they already carried. Growers followed. California became a major shipper of fruit, and other regions specialised in crops they could now sell a continent away. The same technology, in the form of [[Air_conditioning|air conditioning]] as much as food refrigeration, supported the dispersed twentieth-century metropolitan growth sometimes called the galactic city: a pattern of low-density settlement made possible by highway transport, cooled buildings and a food system that could supply a city from farms hundreds or thousands of miles away. Refrigeration is not the cause of that pattern, but it is one of the conditions without which it could not have taken the form it did. ## Impact on agriculture and food production Once a perishable product can be moved and stored, the economics of producing it change. Production concentrates where the land and climate suit the crop rather than where the customers are, and farms grow larger and more specialised because the market they sell into is national rather than local. The clearest single demonstration came from the other side of the world. On February 15, 1882, the sailing ship *Dunedin* left Port Chalmers in New Zealand with about 5,000 mutton and lamb carcasses in a mechanically refrigerated hold, arriving in London in late May; it was New Zealand's first successful shipment of frozen meat to Britain, and it opened the frozen meat and dairy trade that became "the cornerstone of New Zealand's 20th-century economy."[^nzhistory] A sheep farmed twelve thousand miles from its buyer had become a tradeable commodity. The last link in the chain was electricity on the farm itself. In the early 1930s roughly nine in ten urban households in the United States had electric power, against about one in ten rural homes, because extending lines into the countryside did not pay; federal [[Rural_electrification|rural electrification]] changed that, and with power came milk coolers, cold rooms and the ability to hold a crop until it could be sold rather than selling it the day it was picked. Refrigeration on the farm today is mostly about arresting bacterial growth and respiration in produce, meat and milk as soon after harvest as possible, because shelf life is decided in the first hours. ## Effects on lifestyle and diet Before refrigeration, an ordinary diet was seasonal and local except for the staples that keep on their own — grain, sugar, dried beans. [[Food_preservation|Preservation]] meant salting, smoking, drying, pickling or fermenting, each of which changes the food. Refrigeration preserves by slowing the chemistry and the microbiology instead of altering the product, so the food that arrives at the table is closer to what left the field. The consequences run in several directions at once. The modern [[Supermarket|supermarket]] is a refrigerated building, and its ability to hold fresh meat, dairy and produce under one roof is what made a single weekly shop possible. The shift away from salt-cured preservation lowered dietary sodium; year-round availability of dairy and fresh meat raised the consumption of both. Cold also made whole categories of food possible that had not existed: frozen vegetables, ice cream as an everyday product rather than a luxury of the ice trade, and chilled ready meals. ## Current applications of refrigeration Refrigeration today is mostly two things: keeping food cold and keeping people comfortable. Air conditioning of homes and public buildings and the [[Cold_chain|cold chain]] for food — domestic refrigerators, retail cabinets, walk-in coolers, cold stores and refrigerated transport — account for the great bulk of installed capacity. Beyond them sit industrial process cooling, the cold required by chemical and pharmaceutical manufacture, ice rinks, ground freezing for civil works, and the [[Cryogenics|cryogenic]] end of the scale where gases are liquefied for storage and transport. The energy involved is now a first-order question for electricity systems. The International Energy Agency reported in 2018 that air conditioners and electric fans "account for about a fifth of the total electricity used in buildings around the world – or 10% of all global electricity consumption today," with 1.6 billion air-conditioning units in service and 5.6 billion projected by 2050.[^iea2018] Because the COP of a real machine sits well below the reversible bound, most of that electricity is paying for irreversibility rather than for heat moved, which is why efficiency standards for cooling equipment are a climate policy and not only a consumer one. The refrigerants themselves are the other policy question. The chlorofluorocarbons that made domestic refrigeration cheap were found to destroy stratospheric ozone and were phased out under the [[Montreal_Protocol|Montreal Protocol]]; their hydrofluorocarbon replacements spare the ozone layer but are potent greenhouse gases, and the Kigali Amendment, adopted in 2016, phases them down in turn — developed countries beginning reductions in 2019, most developing countries freezing use in 2024, with global HFC use expected to fall by 80–85 percent by 2047 and up to 0.5 °C of warming avoided by 2100.[^unep-kigali] ## Methods of refrigeration Every method below moves heat from a cold place to a hot one and pays for the privilege. They differ in what they spend — a consumable, shaft work, heat, an electric current, a magnetic field, a mechanical stress — and therefore in where each is worth using. A useful way to read the list is to ask two questions of each entry: what form of energy is available at the site, and how far below ambient the cold space has to sit. The first decides between a compressor and a heat-driven cycle; the second rules out whole families at once, because a consumable holds one temperature and a vapour cycle is limited by the refrigerant that still boils there. ### Non-cyclic refrigeration Non-cyclic methods cool by consuming a cold substance: melting ice, subliming [[Dry_ice|dry ice]], or evaporating [[Liquid_nitrogen|liquid nitrogen]]. A portable cooler is the simplest case. The advantages are that there are no moving parts, no power supply and no noise; the limits are that the temperature is fixed by the substance — 0 °C for ice, −78 °C for subliming carbon dioxide at atmospheric pressure, −196 °C for boiling nitrogen — and that when the consumable is gone, so is the cooling. Shipping a [[Vaccine|vaccine]] on dry ice and shipping it in a mechanically refrigerated container are the same job solved from opposite ends. ### Cyclic refrigeration A cyclic machine takes heat `q_L` from the cold space, receives work `w`, and rejects `q_H = q_L + w` to the surroundings. It is the [[Thermodynamic_cycle|thermodynamic cycle]] of a power plant run in reverse: instead of taking heat from a hot source and producing work, it takes work and pumps heat up the temperature gradient. Its grade is `COP_R = q_L/w`, and the identical machine judged on the heat it delivers rather than the heat it removes is a [[Heat_pump|heat pump]], with `COP_HP = q_H/w = COP_R + 1` — the same hardware, the same energy flows, a different output counted.[^yan] #### Vapor-compression cycle The vapour-compression loop has four components and four states. The refrigerant leaves the [[Evaporator|evaporator]] as a saturated or slightly superheated vapour at low pressure (1), is raised by the [[Compressor|compressor]] to the condenser pressure and a much higher temperature (2), gives up its heat to the surroundings in the [[Condenser_(heat_transfer)|condenser]] and leaves as a saturated liquid (3), and is throttled back to the evaporator pressure through a [[Thermal_expansion_valve|expansion valve]] or capillary (4), where part of it flashes to vapour and the rest evaporates while absorbing heat from the cold space. On a pressure–[[Enthalpy|enthalpy]] diagram the loop is close to a rectangle: two horizontal lines at the two pressures, a vertical line at the throttle where enthalpy is unchanged, and the sloped compression leg that is the work paid. The figures of merit fall straight out of the four enthalpies: the refrigerating effect is `q_L = h₁ − h₄`, the compressor work is `w = h₂ − h₁`, and `COP_R = (h₁ − h₄)/(h₂ − h₁)`. Two losses are visible on the chart itself. The throttle is irreversible — the vapour fraction `x₄` it produces is refrigerating effect thrown away, which is why the loop's width at the bottom falls short of the dome's. The compressor is not isentropic, so the compression leg leans further right than the ideal and the work is larger than the reversible value. The ceiling is Carnot's. For an evaporator at −10 °C and a condenser at 40 °C, the reversible coefficient of performance is `T_L/(T_H − T_L)` = 263.15/50 = 5.26, and a cycle between the same reservoirs can only approach that [[Carnot_cycle|reversible]] value from below.[^yan] Pull the evaporator lower or push the condenser higher and the bar falls quickly: at −25 °C and 45 °C the same expression gives 248.15/70 = 3.55, which is the thermodynamic reason a freezer costs more to run than a refrigerator and an air conditioner costs more on the hottest day. One caveat travels with every number the microsim prints. Its cycle is computed by the framework's `energy.heatEngine.refrigerationCycle`, but the saturation properties behind the dome come from `energy.refrigerants.sat`, a two-point Clausius–Clapeyron pressure curve with a Watson latent-heat correlation — an ILLUSTRATIVE correlation, not a property table, as the module itself and the sim's sheet both say.[^sim-spec] The shape of the cycle, the direction every quantity moves and the Carnot bound are all honest; a COP read off the chart is a teaching figure and not a machine rating, and a designer sizing real equipment uses tabulated refrigerant properties instead. *Try: pull the evaporator temperature down from −10 °C toward −30 °C with the condenser held at 40 °C — the bottom of the loop stretches left and down, the compression leg grows, the pressure ratio climbs and both the COP and the Carnot bar above it fall together; then raise the condenser to 50 °C and watch the same thing happen from the top.*[^sim-spec] #### Absorption cycle An [[Absorption_refrigerator|absorption chiller]] replaces the compressor with a chemical circuit and pays in heat rather than shaft work. The refrigerant evaporates at low pressure as usual, but the vapour is then absorbed into a liquid — ammonia into water, or water vapour into [[Lithium_bromide|lithium bromide]] solution — and the solution is pumped to the high pressure, where a heat-driven generator boils the refrigerant back out and sends it to the condenser. Pumping a liquid costs far less work than compressing a vapour, so the electrical demand is small; the price is a substantial heat input at the generator. That trade is worth making wherever heat is cheap or free: waste heat from an engine or process, solar heat, or a gas flame in a silent absorption refrigerator with no moving parts at all. #### Adsorption cycle The adsorption cycle is the same bargain with a solid in place of the liquid absorbent. Refrigerant vapour is taken up by a porous adsorbent — water on silica gel or [[Zeolite|zeolite]], methanol on activated carbon — and released again when the adsorbent is heated. A basic machine is intermittent: heat drives the refrigerant off and it is condensed; cooling the bed makes it adsorb again, and the refrigerant evaporates and produces the cooling. Two beds operated out of phase make the output continuous. Like absorption, it is chosen when the available energy is heat rather than electricity. #### Gas cycle In a gas cycle the working fluid never condenses. Air is compressed, cooled in a [[Heat_exchanger|heat exchanger]] at the high pressure, then expanded — doing work as it does so, which is what makes it cold — and passed through the space to be cooled. The absence of a phase change means no latent heat is available, so a gas cycle moves less heat per unit of mass flow and its COP is lower than a vapour-compression machine's between the same temperatures. It is used where its other properties win: the [[Air_cycle_machine|air-cycle machine]] in an airliner takes its working fluid from the engine compressors and needs no refrigerant inventory at all, and gas cycles reach [[Cryogenics|cryogenic]] temperatures where no ordinary refrigerant would still be a vapour. ### Thermoelectric refrigeration A thermoelectric module passes a current through junctions of dissimilar semiconductors and moves heat by the [[Peltier_effect|Peltier effect]]. It has no moving parts, no refrigerant and no orientation requirement, and it can be made in any size down to a few millimetres; its COP is low compared with a compressor of the same duty, so it is used where compactness, silence, precise control or reliability matter more than efficiency — instrument cooling, small portable coolers, and temperature stabilisation of detectors and lasers. ### Magnetic refrigeration [[Magnetic_refrigeration|Magnetic refrigeration]] uses the magnetocaloric effect: magnetising a paramagnetic solid aligns its magnetic dipoles and warms it, and demagnetising it adiabatically cools it. Run as a cycle, with the heat of magnetisation rejected while the field is on and the cooling taken while it is off, the process works as a refrigerator with a solid rather than a fluid as the working substance. Adiabatic demagnetisation is a standard laboratory route to very low temperatures, and room-temperature magnetic refrigeration is an active research field because it dispenses with volatile refrigerants entirely. ### Other methods Several further methods occupy niches where their peculiarities are the point: the [[Vortex_tube|vortex tube]], which separates a compressed air stream into hot and cold flows with no moving parts, for spot cooling where compressed air is already available; [[Thermoacoustics|thermoacoustic]] refrigeration, which uses a sound wave in a pressurised gas to pump heat; steam-jet cooling, once used for large buildings; and reversed [[Stirling_engine|Stirling]] machines, which are standard for small cryogenic loads. Elastocaloric cooling exploits a different solid-state effect: a superelastic alloy warms when a stress drives it from the austenitic to the martensitic phase and cools when the stress is released, so a wire cyclically stretched and relaxed can pump heat. Passive systems complete the range, combining [[Passive_daytime_radiative_cooling|radiative cooling]] to the sky, insulation and evaporative cooling to extend the life of stored food without any power input at all. ## Minnesota *This section is specific to Wikitube.* The step from refrigerating a building to refrigerating a moving vehicle was taken in [[Minnesota]]. Joseph A. Numero founded the U.S. Thermo Control Company in [[Minneapolis]] in 1938, and Frederick McKinley Jones, who became its vice-president of engineering, designed the automatic units that made it work; the company was renamed Thermo King in 1941, and the Model C, unveiled late that year, was the first cooling unit mounted on the front of a truck body rather than underneath it — a unitary metal body weighing only 700 pounds, with a single-cylinder engine, a reciprocating compressor and a six-volt starter.[^mnopedia] Jones patented more than sixty inventions in his lifetime.[^mnopedia] The patent the pair filed shows what the hard part was. "Air conditioner for vehicles," filed on November 16, 1939 and granted on December 1, 1942, claims a removable unit that circulates, cools and humidifies the air in a vehicle compartment, and — the detail that makes an unattended road unit practical — automatically reverses the refrigerant flow to defrost its own evaporator when ice restricts the airflow.[^jones-patent] A truck-mounted machine cannot be nursed by an engineer the way a warehouse plant can. It has to survive vibration, start itself, and clear its own ice. Thermo King's units went to the U.S. military through the [[World_War_II|Second World War]] and to the food trade afterwards, and the consequence is the [[Cold_chain|cold chain]] described in the sections above: the refrigerated warehouse and the [[Refrigerator_car|refrigerated rail car]] had already decoupled production from consumption at the scale of regions, and the refrigerated [[Truck|truck]] closed the last gap between the rail head and the store. The [[Supermarket|supermarket]], which is a refrigerated building stocked by refrigerated vehicles, followed. ## See also - [[Heat_pump]] - [[Vapor-compression_refrigeration]] - [[Coefficient_of_performance]] - [[Refrigerant]] - [[Air_conditioning]] - [[Heat_engine]] - [[Cryogenics]] - [[Montreal_Protocol]] - [[Heating,_ventilation,_and_air_conditioning]] ## References [^yan]: Yan, Claire Yu (2022). *Introduction to Engineering Thermodynamics*. Victoria, British Columbia: BCcampus (Pressbooks). §6.2 "Refrigerator and heat pump" — `COP_R = Q̇_L/Ẇ_in = Q̇_L/(Q̇_H − Q̇_L)`, `COP_HP = Q̇_H/Ẇ_in`, and the identity `COP_HP = COP_R + 1` — and §6.4 "Carnot cycles" for the reversible bounds `COP_R = T_L/(T_H − T_L)` and `COP_HP = T_H/(T_H − T_L)`. Portal Book 115, coefficient of performance at pp. 243, 255 and 277, cycles and reservoirs at pp. 269–280. CC BY-NC-SA 4.0. https://pressbooks.bccampus.ca/thermo1/ [^asme-perkins]: American Society of Mechanical Engineers. "The Perkins Vapor-Compression Cycle for Refrigeration," ASME Historic Mechanical Engineering Landmark, announced November 13, 2020: Oliver Evans conceived the closed vapour-compression cooling cycle in 1805 but built no working device; Jacob Perkins's 1834 patent "Apparatus and means for producing ice, and in cooling fluids" was "the first working device to use a system of vapor-compression for refrigeration, and was a closed-cycle" machine "that could operate continuously," constructed and demonstrated in 1835 by John Hague. https://www.asme.org/about-asme/engineering-history/landmarks/274-perkins-vapor-compression-cycle-for-refrigeration [^tum-linde]: Technical University of Munich. "Cooling technology" (TUM 150th anniversary series): Carl von Linde held the chair of theoretical machine research at Munich's Polytechnic School from 1868 and began refrigeration experiments around 1870; existing cooling machines of the time reached roughly one-fifth of the theoretical maximum performance and his first prototype doubled that; his refined machine was delivered to the Dreher Brewery in Trieste in 1875, and he received a noble title in 1897. https://www.150.tum.de/en/anniversary-story/cooling-technology/ [^si-gorrie]: Smithsonian Institution, National Museum of American History. "Gorrie Ice Machine, Patent Model": John Gorrie of New Orleans, Louisiana; U.S. Patent 8080 of May 6, 1851, "the first patent for a mechanical refrigerating or ice-making machine issued by the U.S. Patent Office"; the machine was designed to "convert water into ice artificially by absorbing its heat of liquefaction with expanding air." https://si.edu/object/gorrie-ice-machine-patent-model:nmah_846192 [^nzhistory]: Manatū Taonga — New Zealand Ministry for Culture and Heritage. "First shipment of frozen meat leaves NZ," NZHistory: the *Dunedin* sailed from Port Chalmers on 15 February 1882 with about 5,000 mutton and lamb carcasses and reached London in late May 1882; the shipment "pav[ed] the way for the trade in frozen meat and dairy products that became the cornerstone of New Zealand's 20th-century economy." https://nzhistory.govt.nz/first-shipment-of-frozen-meat-leaves-nz [^iea2018]: International Energy Agency (May 14, 2018). "Air conditioning use emerges as one of the key drivers of global electricity-demand growth," news release accompanying the report *The Future of Cooling*: air conditioners and electric fans "account for about a fifth of the total electricity used in buildings around the world – or 10% of all global electricity consumption today"; 1.6 billion units in use, 5.6 billion projected by 2050. https://www.iea.org/news/air-conditioning-use-emerges-as-one-of-the-key-drivers-of-global-electricity-demand-growth [^unep-kigali]: UNEP Ozone Secretariat. "Kigali Amendment to the Montreal Protocol": adopted in 2016; developed countries began reductions in 2019 and most developing countries were expected to freeze HFC use in 2024; "By 2047, global HFC use is expected to fall by 80–85 percent," and the amendment is "projected to prevent up to 0.5°C of global warming by 2100," with more than 170 countries having ratified. https://ozone.unep.org/kigali-amendment-overview [^mnopedia]: Boehme, Ross. "Thermo King Model C." *MNopedia*, Minnesota Historical Society (first published July 20, 2015; last modified April 15, 2025): Joseph A. Numero founded U.S. Thermo Control in Minneapolis in 1938; the company was renamed Thermo King in 1941; Frederick McKinley Jones served as vice-president of engineering and patented more than sixty inventions; the Model C, unveiled late in 1941, had a "unitary, metal body" weighing 700 pounds with "a Briggs and Stratton single cylinder engine, a Lynch Model Par S-2150 reciprocating compressor, and a six-volt starter," and was the first unit mounted on the front of the vehicle rather than underneath. https://www.mnhs.org/mnopedia/search/index/thing/thermo-king-model-c [^jones-patent]: Numero, Joseph A.; Jones, Frederick M. "Air conditioner for vehicles." U.S. Patent 2,303,857, filed November 16, 1939, granted December 1, 1942; assignee U.S. Thermo Control Company. The claimed unit is removably attachable, circulates, cools and humidifies compartment air, and automatically reverses refrigerant flow to defrost the evaporator when ice restricts the airflow. https://patents.google.com/patent/US2303857A/en [^sim-spec]: Engineering portal pack, sim spec `specs/sims/Refrigeration.json` and its hooks: the cycle is `energy.heatEngine.refrigerationCycle` (states 1–4, `q_L`, `q_H`, `w`, the COP, the Carnot COP, the flash quality `x₄` and the pressure ratio), and the saturation properties come from `energy.refrigerants.sat`, a two-point Clausius–Clapeyron pressure curve with a Watson latent-heat correlation. That correlation is ILLUSTRATIVE and is not a property table; the module states this and the sim's sheet repeats it, and this article repeats it wherever a COP from the chart is quoted. Engineering portal fact sheet, section ENG20, run of September 18, 2026.  **Microsim — three.js (Wikitube framework):** *Refrigeration* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/engineering/Refrigeration.html" data-title="Refrigeration"></div> *Built from `MICROSIM_GUIDE/specs/sims/Refrigeration.json`; part of the [[PORTAL_Engineering|Engineering portal]] spine (section sims and See-also variants).*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Refrigeration) : [Wikitube](https://en.wikitube.io/wiki/Refrigeration) - skeleton pinned to revision 1374333123 (2026-09-18).