First refrigeration systems
The first known method of artificial refrigeration was demonstrated by William Cullen at the University of Glasgow in Scotland in 1748. Cullen used a pump to create a partial vacuum over a container of ethyl ether, which then boiled, absorbing heat from the surrounding air. The experiment even created a small amount of ice, but had no practical application at that time.
In 1805, American inventor Oliver Evans designed but never built a refrigeration system based on the Vapor-compression refrigeration cycle rather than chemical solutions or volatile liquids such as ethyl ether.
In 1820, the British scientist Michael Faraday liquified ammonia and other gases by using high pressures and low temperatures.
An American living in Great Britain, Jacob Perkins, obtained the first patent for a vapor-compression refrigeration system in 1834. Perkins built a prototype system and it actually worked, although it did not succeed commercially.[3]
In 1842, an American physician, John Gorrie, designed the first system for refrigerating water to produce ice. He also conceived the idea of using his refrigeration system to cool the air for comfort in homes and hospitals (i.e., air-conditioning). His system compressed air, then partially cooled the hot compressed air with water before allowing it to expand while doing part of the work required to drive the air compressor. That isentropic expansion cooled the air to a temperature low enough to freeze water and produce ice, or to flow "through a pipe for effecting refrigeration otherwise" as stated in his patent granted by the U.S. Patent Office in 1851.[4] Gorrie built a working prototype, but his system was a commercial failure.
Alexander Twining began experimenting with vapor-compression refrigeration in 1848 and obtained patents in 1850 and 1853. He is credited with having initiated commercial refrigeration in the United States by 1856.
Soon after that, James Harrison, born in Scotland and subsequently emigrated to Australia, introduced commercial vapor-compression refrigeration to breweries and meat packing houses. By 1861, a dozen of his systems were in operation.
The first 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. Due to the toxicity of ammonia, such systems were not developed for use in homes, but were used to manufacture ice for sale. The consumer public at that time still used the ice box with ice brought in from commercial suppliers, many of whom were still harvesting ice and storing it in an icehouse.
Thaddeus Lowe, an American balloonist from the Civil War, had experimented over the years with the properties of gases. One of his mainstay enterprises was the high-volume production of hydrogen gas. He also held several patents on ice making machines. His "Compression Ice Machine" would revolutionize the cold storage industry. In 1869 he and other investors purchased an old steamship onto which they loaded one of Lowe’s refrigeration units and began shipping fresh fruit from New York to the Gulf Coast area, and fresh meat from Galveston, Texas back to New York. Because of Lowe’s lack of knowledge about shipping, the business was a costly failure, and it was difficult for the public to get used to the idea of being able to consume meat that had been so long out of the packing house.
Domestic refrigeration appeared about 1910. 1918, Kelvinator made the first refrigerator for the American market.[citation needed] General Electric introduced some of the first hermetic units in 1928, called the Monitor Top.[citation needed]
Widespread commercial use
By the 1870s breweries had become the largest users of commercial refrigeration units though some still relied on harvested ice. Though the ice-harvesting industry had grown immensely by the turn of the 20th century, pollution and sewage had begun to creep into natural ice making it a problem in the metropolitan suburbs. Eventually breweries began to complain of tainted ice. This raised demand for more modern and consumer-ready refrigeration and ice-making machines. In 1895 German engineer Carl von Linde set up a large-scale process for the production of liquid air and eventually liquid oxygen for use in safe household refrigerators.
Refrigerated railroad cars were introduced in the 1840s for the short-run transportation of dairy products. In 1867 J.B. Sutherland of Detroit, Michigan patented the refrigerator car designed with ice tanks at either end of the car and ventilator flaps near the floor which would create a gravity draft of cold air through the car.
By 1900 the meat packing houses of Chicago had adopted ammonia-cycle commercial refrigeration. By 1914 almost every location used artificial refrigeration. The big meat packers, Armour, Swift, and Wilson, had purchased the most expensive units which they installed on train cars and in branch houses and storage facilities in the more remote distribution areas.
It was not until the middle of the 20th century that refrigeration units were designed for installation on tractor-trailer rigs (trucks). Refrigerated trucks are used to transport perishable goods, such as frozen foods, fruit and vegetables, and temperature-sensitive chemicals. Most modern refrigerators keep the temperature between -40 and +20 °C and have a maximum payload of around 24 000 kg. gross weight (in Europe)
Home and consumer use
With the invention of synthetic refrigerants like Freon, safer refrigerators were possible for home and consumer use. Freon is referred to as a CFC (Chlorofluorocarbon), halocarbon, or haloalkane.
Developed in the late 1920s, Freon is much less toxic than some of the refrigerants used earlier (i.e., ammonia, methyl chloride and sulfur dioxide) and Freon-12 has a boiling point of -22 °F (-30 °C). The intent was to provide refrigeration units for home use without the use of toxic refrigerants. At the same time the units needed to be made smaller which meant using refrigerants that could do more work with fewer parts in less space. A refrigerant like Freon answered that need.
The Freon patents were initially held by the automotive industry who used it for auto air-conditioning, but the product was far too useful to limit to automotive use. By 1930 Freon was available on the open market.
As of 1989, Freon was banned via the Montreal Protocol due to the negative effects it has on the ozone layer.
Current applications of refrigeration
Probably the most widely-used current applications of refrigeration are for the air-conditioning of private homes and public buildings, and the refrigeration of foodstuffs in homes, restaurants and large storage warehouses. The use of refrigerators in our kitchens for the storage of fruits and vegetables has allowed us to add fresh salads to our diets year round, and to store fish and meats safely for long periods.
In commerce and manufacturing, there are many uses for refrigeration. Refrigeration is used to liquify gases like oxygen, nitrogen, propane and methane for example. In compressed air purification, it is used to condense water vapour from compressed air to reduce its moisture content. In oil refineries, chemical plants, and petrochemical plants, refrigeration is used to maintain certain processes at their required low temperatures (for example, in the alkylation of butenes and butane to produce a high octane gasoline component). Metal workers use refrigeration to temper steel and cutlery. In transporting temperature-sensitive foodstuffs and other materials by trucks, trains, airplanes and sea-going vessels, refrigeration is a necessity.
Dairy products are constantly in need of refrigeration, and it was only discovered in the past few decades that eggs needed to be refrigerated during shipment rather than waiting to be refrigerated after arrival at the grocery store. Meats, poultry and fish all must be kept in climate-controlled environments before being sold. Refrigeration also helps keep fruits and vegetables edible longer.
Methods of refrigeration
Methods of refrigeration can be classified as non-cyclic, cyclic and thermoelectric.
Non-cyclic refrigeration
In these methods, refrigeration can be accomplished by melting ice or by subliming dry ice. These methods are used for small-scale refrigeration such as in laboratories and workshops, or in portable coolers.
Ice owes its effectiveness as a cooling agent to its constant melting point of 0 °C (32 °F). In order to melt, ice must absorb 333.1 kJ/kg (143.3 Btu/lb) of heat. Foodstuffs maintained at this temperature or slightly above have an increased storage life. Solid carbon dioxide, known as dry ice, is used also as a refrigerant. Having no liquid phase at normal atmospheric pressure, it sublimes directly from the solid to vapor phase at a temperature of -78.5 °C (-109.3 °F). Dry ice is effective for maintaining products at low temperatures during the period of sublimation.
Cyclic refrigeration
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 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 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. 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. Insulation is used to reduce the work and energy required to achieve and maintain a lower temperature in the cooled space. The operating principle of the refrigeration cycle was described mathematically by Sadi Carnot in 1824 as a heat engine.
The most common types of refrigeration systems use the reverse-Rankine vapor-compression refrigeration cycle although absorption heat pumps are used in a minority of applications. It is possible to build a refrigeration system which does not contain a refrigerant, and therefore does not operate a refrigeration cycle — the most common form being thermoelectric cooling used in some portable coolers.
Cyclic refrigeration can be classified as:
Vapour cycle, and
Gas cycle
Vapour cycle refrigeration can further be classified as:
Vapour compression refrigeration
Gas absorption refrigeration
Vapour-compression cycle
(See Vapour-compression refrigeration for more complete technical details)
The vapour-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems. Figure 1 provides a schematic diagram of the components of a typical vapour-compression refrigeration system.
Vapour compression refrigerationThe thermodynamics of the cycle can be analyzed . In this cycle, a circulating refrigerant such as Freon enters the compressor as a vapour. From point 1 to point 2, the vapour is compressed at constant entropy and exits the compressor superheated. From point 2 to point 3 and on to point 4, the superheated vapour travels through the condenser which first cools and removes the superheat and then condenses the vapour into a liquid by removing additional heat at constant pressure and temperature. Between points 4 and 5, the liquid refrigerant goes through the 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.
Temperature–Entropy results in a mixture of liquid and vapour at a lower temperature and pressure as shown at point 5. The cold liquid-vapour 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 irreversiblity during the compression of the refrigerant vapour, or non-ideal gas behavior (if any).
More information about the design and performance of vapour-compression refrigeration systems is available in the classic "Perry's Chemical Engineers' Handbook".[7]
Vapour absorption cycle
(See gas absorption refrigerator for more details)
In the early years of the twentieth century, the vapour absorption cycle using water-ammonia systems was popular and widely used but, after the development of the vapour compression cycle, it lost much of its importance because of its low coefficient of performance (about one fifth of that of the vapour compression cycle). Nowadays, the vapour absorption cycle is used only where waste heat is available or where heat is derived from solar collectors.
The absorption cycle is similar to the compression cycle, except for the method of raising the pressure of the refrigerant vapour. 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 vapour from the high-pressure liquid. Some work is required by the liquid pump but, for a given quantity of refrigerant, it is much smaller that that needed by the compressor in the vapour compression cycle. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) and water (absorbent), and water (refrigerant) and lithium bromide (absorbent).
Gas cycle
When the working fluid is a gas that is compressed and expanded but doesn't 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 exchangers 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 will require a large mass flow rate and would be bulky.
Because of their lower efficiency and larger bulk, air cycle coolers are not often used nowadays in terrestrial cooling devices. The air cycle machine is very common, however, on gas turbine-powered 'jet' aircraft because compressed air is readily available from the engines' compressor sections. These jet aircrafts' cooling and ventilation units also serve the purpose of pressurizing the aircraft.
Thermoelectric refrigeration
Thermoelectric cooling uses the Peltier effect to create a heat flux between the junction of two different types of materials. This effect is commonly used in camping and portable coolers and for cooling electronic components and small instruments.
Magnetic refrigeration
Magnetic refrigeration, or adiabatic demagnetization, is a cooling technology based on the magnetocaloric effect, an intrinsic property of magnetic solids. The refrigerant is often a paramagnetic salt, such as cerium magnesium nitrate. The active magnetic dipoles in this case are those of the electron shells of the paramagnetic atoms.
A strong magnetic field is applied to the refrigerant, forcing its various magnetic dipoles to align and putting these degrees of freedom of the refrigerant into a state of lowered entropy. A heat sink then absorbs the heat released by the refrigerant due to its loss of entropy. Thermal contact with the heat sink is then broken so that the system is insulated, and the magnetic field is switched off. This increases the heat capacity of the refrigerant, thus decreasing its temperature below the temperature of the heat sink.
Because few materials exhibit the required properties at room temperature, applications have so far been limited to cryogenics and research.
Other methods
Other methods of refrigeration include the Air cycle machine used in aircraft; the Vortex tube used for spot cooling, when compressed air is available; and Thermoacoustic refrigeration using sound waves in a pressurised gas to drive heat transfer and heat exchange.
Unit of refrigeration
Domestic and commercial refrigerators may be rated in kJ/s, or Btu/h of cooling. Commercial refrigerators are mostly rated in tons of refrigeration. One ton of refrigeration capacity can freeze one short ton of water at 0 °C (32 °F) in 24 hours. Based on that:
Latent heat of ice (i.e., heat of fusion) ≈ 144 Btu / lb (or 334.5 kJ/kg)
One short ton = 2000 lb
Heat to be extracted = 2000 * 144 = 288000 Btu / 24 hours = 12000 Btu/hour = 200 Btu / Minute
1 ton refrigeration = 200 Btu / minute = 3.517 kJ/s = 3.517 kilowatts[8]
A much less common definition is: 1 tonne of refrigeration is the rate of heat removal required to freeze a metric ton (i.e., 1000 kg) of water at 0 °C in 24 hours. Based on the heat of fusion being 334.5 kJ/kg, 1 tonne of refrigeration = 13,938 kJ/h = 3.872 kW. As can be seen, 1 tonne of refrigeration is 10% larger than 1 ton of refrigeration.
Most residential air conditioning units range in capacity from about 1 to 5 tons of refrigeration.
Ice harvesting
The use of ice to refrigerate and thus preserve food goes back to prehistoric times.[1][2] Through the ages, the seasonal harvesting of snow and ice was a regular practice of most of the ancient cultures: Chinese, Hebrews, Greeks, Romans, Persians. Ice and snow were stored in caves or dugouts lined with straw or other insulating materials. The Persians stored ice in pits called Yakhchals. Rationing of the ice allowed the preservation of foods over the hot periods. This practice worked well down through the centuries, with icehouses remaining in use into the twentieth century.
In the 16th century, the discovery of chemical refrigeration was one of the first steps toward artificial means of refrigeration. Sodium nitrate or potassium nitrate, when added to water, lowered the water temperature and created a sort of refrigeration bath for cooling substances. In France, cold drinks and liqueurs were produced by spinning long-necked bottles in water with dissolved saltpeter
During the first half of the 19th century, ice harvesting had become big business in America. New Englander Frederic Tudor, who became known as the "Ice King", worked on developing better insulation products for the long distance shipment of ice, especially to the tropics
Refrigeration is the process of removing heat from an enclosed space, or from a substance, and rejecting it elsewhere for the primary purpose of lowering the temperature of the enclosed space or substance and then maintaining that lower temperature. To satisfy the Second Law of Thermodynamics, mechanical work must be performed to accomplish this.
The intermediate stage in the history of cooling foods was to add chemicals like sodium nitrate or potassium nitrate to water causing the temperature to fall. Cooling wine via this method was recorded in 1550, as were the words "to refrigerate”.
Cooling drinks came into vogue by 1600 in France. Instead of cooling water at night, people rotated long-necked bottles in water in which saltpeter had been dissolved. This solution could be used to produce very low temperatures and to make ice. By the end of the 17th century, iced liquors and frozen juices were popular in French society.
The first known artificial refrigeration was demonstrated by William Cullen at the University of Glasgow in 1748. Cullen let ethyl ether boil into a partial vacuum; he did not, however, use the result to any practical purpose.
Ice was first shipped commercially out of Canal Street in New York City to Charleston, South Carolina in 1799. Unfortunately, there was not much ice left when the shipment arrived. New Englanders Frederick Tudor and Nathaniel Wyeth saw the potential for the ice business and revolutionized the industry through their efforts in the first half of the 1800s. Tudor, who became known as the “Ice King”, focused on shipping ice to tropical climates. He experimented with insulating materials and built icehouses that decreased melting losses from 66 percent to less than 8 percent. Wyeth devised a method of quickly and cheaply cutting uniform blocks of ice that transformed the ice industry, making it possible to speed handling techniques in storage, transportation and distribution with less waste.
In 1805, an American inventor, Oliver Evans, designed the first refrigeration machine that used vapor instead of liquid. Evans never constructed his machine, but one similar to it was built by an American physician, John Gorrie.
In 1842, the American physician John Gorrie, to cool sickrooms in a Florida hospital, designed and built an air-cooling apparatus for treating yellow-fever patients. His basic principle--that of compressing a gas, cooling it by sending it through radiating coils, and then expanding it to lower the temperature further--is the one most often used in refrigerators today. Giving up his medical practice to engage in time-consuming experimentation with ice making, he was granted the first U.S. patent for mechanical refrigeration in 1851.
Commercial refrigeration is believed to have been initiated by an American businessperson, Alexander C. Twinning, in 1856. Shortly afterward, an Australian, James Harrison, examined the refrigerators used by Gorrie and Twinning and introduced vapor-compression refrigeration to the brewing and meatpacking industries.
Ferdinand Carré of France developed a somewhat more complex system in 1859. Unlike earlier compression-compression machines, which used air as a coolant, Carré's equipment contained rapidly expanding ammonia. (Ammonia liquefies at a much lower temperature than water and is thus able to absorb more heat.) Carré's refrigerators were widely used, and vapor compression refrigeration became, and still is, the most widely used method of cooling. However, the cost, size, and complexity of refrigeration systems of the time, coupled with the toxicity of their ammonia coolants, prevented the general use of mechanical refrigerators in the home. Most households used iceboxes that were supplied almost daily with blocks of ice from a local refrigeration plant.
Beginning in the 1840s, refrigerated cars were used to transport milk and butter. By 1860, refrigerated transport was limited to mostly seafood and dairy products. The refrigerated railroad car was patented by J.B. Sutherland of Detroit, Michigan in 1867. He designed an insulated car with ice bunkers in each end. Air came in on the top, passed through the bunkers, and circulated through the car by gravity, controlled by the use of hanging flaps that created differences in air temperature. The first refrigerated car to carry fresh fruit was built in 1867 by Parker Earle of Illinois, who shipped strawberries on the Illinois Central Railroad. Each chest contained 100 pounds of ice and 200 quarts of strawberries. It was not until 1949 that a refrigeration system made its way into the trucking industry by way of a roof-mounted cooling device, patented by Fred Jones.
Brewing was the first activity in the northern states to use mechanical refrigeration extensively, beginning with an absorption machine used by S. Liebmann’s Sons Brewing Company in Brooklyn, New York in 1870. Commercial refrigeration was primarily directed at breweries in the 1870s and by 1891, nearly every brewery was equipped with refrigerating machines.
Natural ice supply became an industry unto itself. More companies entered the business, prices decreased, and refrigeration using ice became more accessible. By 1879, there were 35 commercial ice plants in America, more than 200 a decade later, and 2,000 by 1909. No pond was safe from scraping for ice production, not even Thoreau’s Walden Pond, where 1,000 tons of ice was extracted each day in 1847.
However, as time went on, ice, as a refrigeration agent, became a health problem. Says Bern Nagengast, co-author of Heat and Cold: Mastering the Great Indoors (published by the American Society of Heating, Refrigeration and Air-conditioning Engineers), “Good sources were harder and harder to find. By the 1890’s, natural ice became a problem because of pollution and sewage dumping.” Signs of a problem were first evident in the brewing industry. Soon the meatpacking and dairy industries followed with their complaints. Refrigeration technology provided the solution: ice, mechanically manufactured, giving birth to mechanical refrigeration.
Carl (Paul Gottfried) von Linde in 1895 set up a large-scale plant for the production of liquid air. Six years later he developed a method for separating pure liquid oxygen from liquid air that resulted in widespread industrial conversion to processes utilizing oxygen (e.g., in steel manufacture).
Though meat-packers were slower to adopt refrigeration than the breweries, they ultimately used refrigeration pervasively. By 1914, the machinery installed in almost all American packing plants was the ammonia compression system, which had a refrigeration capacity of well over 90,000 tons/day.
Despite the inherent advantages, refrigeration had its problems. Refrigerants like sulfur dioxide and methyl chloride were causing people to die. Ammonia had an equally serious toxic effect if it leaked. Refrigeration engineers searched for acceptable substitutes until the 1920s, when a number of synthetic refrigerants called halocarbons or CFCs (chlorofluorocarbons) were developed by Frigidaire. The best known of these substances was patented under the brand name of Freon. Chemically, Freon was created by the substitution of two chlorine and two fluorine atoms for the four hydrogen atoms in methane (CH4); the result, dichlorodifluoromethane (CCl2F2), is odorless and is toxic only in extremely large doses.
Though ice, brewing, and meatpacking industries were refrigeration’s major beneficiaries, many other industries found refrigeration a boon to their business.
In metalworking, for instance, mechanically produced cold helped temper cutlery and tools. Iron production got a boost, as refrigeration removed moisture from the air delivered to blast furnaces, increasing production. Textile mills used refrigeration in mercerizing, bleaching, and dyeing. Oil refineries found it essential, as did the manufacturers of paper, drugs, soap, glue, shoe polish, perfume, celluloid, and photographic materials.
Fur and woolen goods storage could beat the moths by using refrigerated warehouses. Refrigeration also helped nurseries and florists, especially to meet seasonal needs since cut flowers could last longer. Moreover, there was the morbid application of preserving human bodies. Hospitality businesses including hotels, restaurants, saloons, and soda fountains, proved to be big markets for ice.
In WWI, refrigeration in ammunition factories provided the required strict control of temperatures and humidity. Allied fighting ships held carbon-dioxide machines to keep ammunition well below temperatures at which high explosives became unstable.
In 1973, Prof. James Lovelock reported finding trace amounts of refrigerant gases in the atmosphere. In 1974, Sherwood Rowland and Mario Molina predicted that chlorofluorocarbon refrigerant gases would reach the high stratosphere and there damage the protective mantle of the oxygen allotrope, ozone. In 1985 the "ozone hole" over the Antarctic had been discovered and by 1990 Rowland and Molina's prediction was proved correct.
The basic components of today’s modern vapor-compression refrigeration system are a compressor; a condenser; an expansion device, which can be a valve, a capillary tube, an engine, or a turbine; and an evaporator. The gas coolant is first compressed, usually by a piston, and then pushed through a tube into the condenser. In the condenser, the winding tube containing the vapor is passed through either circulating air or a bath of water, which removes some of the heat energy of the compressed gas. The cooled vapor is passed through an expansion device to an area of much lower pressure; as the vapor expands, it draws the energy of its expansion from its surroundings or the medium in contact with it. Evaporators may directly cool a space by letting the vapor come into contact with the area to be chilled, or they may act indirectly--i.e. by cooling a secondary medium such as water. In most domestic refrigerators, the coil containing the evaporator directly contacts the air in the food compartment. At the end of the process, the warmed gas is drawn toward the compressor.
A refrigerator (often called a "fridge" for short) is a cooling appliance for the storage and preservation of perishable food; food kept in a refrigerator lasts longer than that left at room temperature as the cold inhibits bacterial growth. A refrigerator maintains a cold temperature above the freezing point of water. The refrigerator is a relatively modern invention amongst kitchen appliances. It replaced the common icebox which had been a household item for almost a century and a half prior, and is sometimes still called by the original name "icebox".
The word freezer is generally used to describe an appliance that keeps foods frozen. Freezers are common as household units for storing food but are also used in commercial settings. Most freezers operate around -18 °C (0 °F). Domestic freezers can be included as a compartment in a refrigerator or can be standalone units. Domestic freezers are generally upright units, resembling refrigerators, or chests, resembling upright units laid on their backs. Many modern freezers come with an icemaker.
Commercial fridge and freezer units, which go by many other names, were in use for almost 40 years prior to the common home models. The fact that they operated with toxic ammonia gas systems made them unsafe for home use. Practical household refrigerators were introduced in the 1920s and gained wider acceptance in the 1930s as prices fell and non-toxic, nonflammable synthetic refrigerants, such as Freon or R-12 refrigerants were introduced.
How a refrigerator works
Main article: refrigeration
Refrigerators work by the use of heat pumps operating in a refrigeration cycle. An industrial refrigerator is simply a refrigerator used in an industrial setting, usually in a restaurant or supermarket. They may consist of either a cooling compartment only (a larger refrigerator) or a freezing compartment only (a freezer) or contain both. The industry has nicknames for these units as well sometimes referring to them as a “cold box” or a “walk-in.” The dual compartment was introduced commercially by General Electric in 1939.
The vapor compression cycle is used in most household refrigerators. In this cycle, a circulating refrigerant such as freon enters the compressor as a vapor at its boiling point. The vapor is compressed and exits the compressor as a superheated vapor. The superheated vapor travels through part of the condenser which removes the superheat by cooling the vapor. The vapor travels through the remainder of the condenser and is condensed into a liquid at its boiling point. The saturated liquid refrigerant passes through the expansion valve (also called a throttle valve) where its pressure abruptly decreases. The decrease in pressure results in the flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes). The cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator. There a fan circulates room air across the coil or tubes, and the refrigerant is totally vaporized, extracting heat from the air which is then returned to the food compartment. The refrigerant vapour returns to the compressor inlet to complete the thermodynamic cycle.
An absorption refrigerator works differently from a compressor refrigerator, and typically runs more quietly.
Some refrigerators are now divided into four zones to store different types of food:
-18 °C (0 °F) (freezer)
0 °C (32 °F) (meats)
5 °C (40 °F) (refrigerator)
10 °C (50 °F) (vegetables)
The capacity of a refrigerator is measured in either litres or cubic feet (US). Typically the volume of a combined fridge-freezer is split to 100 litres (3.53 cubic feet) for the freezer and 140 litres (4.94 cubic feet) for the refrigerator, although these values are highly variable.
Temperature settings for refrigerator and freezer compartments are often given arbitrary numbers (for example, 1 through 9, warmest to coldest) by manufacturers, but generally 2 to 8 °C (36 to 46 °F) is ideal for the refrigerator compartment and -18 °C (0 °F) for the freezer.
European freezers, and refrigerators with a freezer compartment, have a four star rating to grade freezers.
* : max temperature = -6°C. Maximum storage time for frozen food is 1 week
** : max temperature = -12°C. Maximum storage time for frozen food is 1 month
*** : max temperature = -18°C. Maximum storage time for frozen food is 3 months
**** : max temperature = -18°C. Maximum storage time for frozen food is up to 12 months
Although both the three and four star ratings specify the same maximum temperature of -18°C, only a four star freezer is intended to be used for freezing fresh food. Three (or fewer) stars are used for frozen food compartments which are only suitable for storing frozen food; introducing fresh food into such a compartment is likely to result in unacceptable temperature rises.
Types
Freestanding;
Counter/cabinet depth: a refrigerator can be approximately 75 cm (30 inches) deep as opposed to approximately 90 cm (35 inches) deep like a normal refrigerator. This allows the unit to be more flush with surrounding cabinets. A counter depth refrigerator tends to come at a steep price premium despite providing less capacity;
Built under: refrigerators and freezers which can be installed under the counter top. Often known as a Bar Fridge, it is used in personal bars as a means to cool beverages;
In-Column refrigerators or freezers: the appliance is built into a tall cabinet in the kitchen so that it looks like a normal cupboard.
A "Mini fridge" or a "micro-fridge" (with an attached compact microwave oven) for use in a college dormitory or efficiency apartment.
Features
The inside of a common home refrigeratorNewer refrigerators may include:
Automatic defrosting: In any refrigerator, over time, water vapor in the air condenses onto the cooling coils as frost, eventually building up into a thick layer of ice. This ice acts as an insulator, reducing cooling efficiency. In the past, the ice was removed by periodically emptying the refrigerator and turning it off to let the ice melt, perhaps aided by hot water applied by the user (a process know as defrosting). In a refrigerator equipped for frost-free operation, however, a heater and a thermostat are fitted around the cooling coils. The cooling is periodically switched off (with the period varying between every 6 to 24 hours depending on the model) and the heater is turned on until the temperature around the coils slightly exceeds the freezing point of water, after which normal cooling resumes. This melts any frost which has collected around the coils. Melt water drops into a small gulley, through a small pipe which drains into a tray on the top of the compressor from which it is then evaporated into the surrounding air by residual heat generated by the operation of the compressor.[1]
A power failure warning, alerting the user by flashing a temperature display. The maximum temperature reached during the power failure may be displayed, along with information on whether the frozen food has defrosted or may contain harmful bacteria;
Chilled water and ice available from an in-door station, so the door need not be opened;
A Status Indicator to notify the user when it is time to change the water filter;
An in-door ice caddy, which relocates the ice-maker storage to the freezer door and saves approximately 60 litres (about 2 cubic feet) of usable freezer space. It is also removable, and helps to prevent ice-maker clogging;
A cooling zone in the refrigerator door shelves. Air from the freezer section is diverted to the refrigerator door, to better cool milk or juice stored in the door shelf;
An LCD suggesting what types of food should be stored at what temperatures, and the expiration date of the food stored;
Extras unrelated to refrigeration, such as a television set built into a door.
Early freezer units accumulated ice crystals around the freezing units. This was a result of humidity introduced into the units when the doors to the freezer were opened. This build up of frost required periodic thawing of the units to maintain their efficiency. Advances in frost-free refrigeration eliminating the thawing task were introduced in the 1950s. Also, early units featured freezer compartments located within the larger refrigerator, and accessed by opening the refrigerator door, and then the smaller interal freezer door; units featuring entirely separate freezer compartment were introduced in the early 1960s, becoming the industry standard by the middle of that decade.
Later advances included automatic ice units and self compartmentalized freezing units.
An increasingly important environmental concern is the disposal of old refrigerators - initially because of the freon coolant damaging the ozone layer, but as the older generation of refrigerators disappears it is the destruction of CFC-bearing insulation which causes concern. Modern refrigerators usually use a refrigerant called HFC-134a (1,2,2,2-tetrafluoroethane) instead of freon, which has no ozone layer depleting properties.
Disposal of discarded refrigerators is regulated, often mandating the removal of doors: children playing hide-and-seek have been asphyxiated while hiding inside a discarded refrigerator. This was particularly true for the older models that had latching doors. More modern units use a magnetic door gasket to hold the door sealed but can actually be pushed open from the inside. However, children can be unwittingly harmed by hiding inside any discarded refrigerator.[2]