refrigerator

A typical refrigerator with its door open

A refrigerator (often called a "fridge" for short) is a cooling appliance comprising a thermally insulated compartment and a heat pump—chemical or mechanical means—to transfer heat from it to the external environment, cooling the contents to a temperature below ambient. Refrigerators are extensively used to store foods which spoil from bacterial growth if not refrigerated. A device described as a "refrigerator" maintains a temperature a few degrees above the freezing point of water; a similar device which maintains a temperature below the freezing point of water is called a "freezer." The refrigerator is a relatively modern invention among kitchen appliances. It replaced the icebox, which had been a common household appliance for almost a century and a half prior. For this reason, a refrigerator is sometimes referred to as an "icebox".

Freezer

Freezers keep their contents frozen. They are used in households and in industry and commerce. Most freezers operate around minus 18 °C (0 °F). Domestic freezers can be included as a separate compartment in a refrigerator, or can be a separate appliance. Domestic freezers are generally upright units resembling refrigerators, or chests resembling upright units laid on their backs. Many upright modern freezers come with an ice dispenser built into their door.

Commercial and domestic refrigerators

Commercial fridge and freezer units, which go by many other names, were in use for almost 40 years prior to the common home models. They used toxic gas systems, which occasionally leaked, making them unsafe for home use. Practical household refrigerators were introduced in 1915 and gained wider acceptance in the United States in the 1930s as prices fell and non-toxic, non-flammable synthetic refrigerants such as Freon or R-12 were introduced. It is notable that while 60% of households in the US owned a refrigerator by the 1930s, it was not until 40 years later, in the 1970s, that the refrigerator achieved a similar level of penetration in the United Kingdom.^[1]

History

A Monitor-style (General Electric format) , more like an icebox with its refrigerating mechanisms on top.

Before the invention of the refrigerator, icehouses were used to provide cool storage for most of the year. Placed near freshwater lakes or packed with snow and ice during the winter, they were once very common. Using natural means to cool foods is still done today. On mountainsides, runoff from melting snow higher up is a convenient way to cool drinks, and during the winter months simply placing milk outside is sufficient to greatly extend its useful life.

In the 11th century, the Persian physicist and chemist, Ibn Sina (Avicenna), invented the refrigerated coil, which condenses aromatic vapours.^[2][3] This was a breakthrough in distillation technology and he made use of it in his steam distillation process, which requires refrigerated tubing, to produce essential oils.^[4]

The first known artificial refrigeration was demonstrated by William Cullen at the University of Glasgow in 1748. Between 1805, when Oliver Evans designed the first refrigeration machine that used vapor instead of liquid, and 1902 when Willis Haviland Carrier demonstrated the first air conditioner, scores of inventors contributed many small advances in cooling machinery. In 1850 or 1851, Dr. John Gorrie demonstrated an ice maker. In 1857, Australian James Harrison introduced vapor-compression refrigeration to the brewing and meat packing 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. The absorption refrigerator was invented by Baltzar von Platen and Carl Munters in 1922, while they were still students at the Royal Institute of Technology in Stockholm, Sweden. It became a worldwide success and was commercialized by Electrolux. Other pioneers included Charles Tellier, David Boyle, and Raoul Pictet. Carl von Linde was the first to patent and make a practical , and compact refrigerator.

At the start of the 20th Century, about half of households in the United States relied on melting ice (in an icebox) to keep food cold, while the remaining half had no cooled storage at all possibly excepting a "root cellar." The ice used for household storage was expensive because ice had to be cut from winter ponds (or mechanically produced), stored centrally until needed, and delivered regularly.

From shortly after 1900 refrigeration of transportation, generally for the carriage of food items long distances, was slowly introduced. The railways in the USA introduced the Refrigerator car on the long haul, over several days, of fruit and vegetables from California to the East. Special freight cars were built with sustantial ice chambers underneath, which were filled with tons of ice at intervals at special "icing stations" along the railroad. Internal ventilation fans driven from the wheels of the car directed cooled air up through ducting among the vehicle contents. Salt was added to the ice in proportion to make the ice melt faster, which by the latent heat principle increased the cooling effect. Meanwhile special refrigerated ships were constructed for long distance movements, such as meat and dairy products from New Zealand to Britain. These ships used the power from the main ship engine, and the refrigerant circulated was brine, a salt and water mixture with a low freezing point.

In a few exceptional cases, mechanical refrigeration systems had been adapted by the start of the 20th century for use in the homes of the very wealthy, and might be used for cooling both living and food storage areas. One early system was installed at the mansion of Walter Pierce, an oil company executive.^[5]

Marcel Audiffren of France championed the idea of a refrigerating machine for cooling and preserving foods at home. His U.S. patents, issued in 1895 and 1908, were purchased by the American Audiffren Refrigerating Machine Company. Machines based on Audiffren's sulfur dioxide process were manufactured by General Electric in Fort Wayne, Indiana and marketed by the Johns-Manville company. The first unit was sold in 1911. Audiffren machines were expensive, selling for about $1,000 (U.S.) — about twice as much as the cost of an automobile at that time.

General Electric sought to develop refrigerators of its own, and in 1915 the first Guardian unit was assembled in a back yard wash house as a predecessor to the Frigidaire. In 1916 Kelvinator and Servel introduced two units among a field of competing models. This number increased to 200 by 1920. In 1918, Kelvinator had a model with automatic controls.

These home units usually required the installation of the mechanical parts, motor and compressor, in the basement or an adjacent room while the cold box was located in the kitchen. There was a 1922 model that consisted of a wooden cold box, water-cooled compressor, an ice cube tray and a 9 cubic foot compartment for $714. (A 1922 Model-T Ford cost about $450.) In 1923 Frigidaire introduced the first self-contained unit. About this same time porcelain covered metal cabinets began to appear. Ice cube trays were introduced more and more during the 1920s; up to this time freezing was not an auxilliary function of the modern refrigerator.

The first refrigerator to see widespread use was the General Electric "Monitor-Top" refrigerator introduced in 1927. The compressor assembly, which emitted a substantial amount of heat, was placed above the cabinet, and surrounded with a decorative ring. Over 1,000,000 units were produced. As the refrigerating medium, these refrigerators used either sulfur dioxide, which is corrosive to the eyes and may cause loss of vision, painful skin burns and lesions, or methyl formate, which is highly flammable, harmful to the eyes, and toxic if inhaled or ingested. Many of these units are still functional today. These cooling systems cannot legally be recharged with the hazardous original refrigerants if they leak or break down.

Older U.S. refrigerator model, with freezer compartment

The introduction of Freon expanded the refrigerator market during the 1930s. Separate freezers became common during the 1940s, the popular term at the time for the unit was a "deep freeze". But home units of these devices or "appliances" did not go into mass production until after WWII. The 1950s and 1960s saw technical advances like automatic defrosting and automatic ice making. Developments of the 1970s and 80s brought about more efficient refrigerators, even though environmental issues banned the use of very effective (Freon) refrigerants. Early refrigerator models (1916 and on) featured a cold compartment for ice cube trays. Successful processing of fresh vegetables through freezing began in the late 1920s by the Postum Company (the forerunner of General Foods) which had acquired the technology when it bought the rights to Clarence Birdseye's successful fresh freezing methods.

The first successful example of the benefits of frozen foods occurred when General Foods heiress Marjorie Merriweather Post (then wife of Joseph E. Davies, United States Ambassador to the Soviet Union) deployed commercial-grade freezers to Spaso House, the US Embassy in Moscow in advance of the Davies’ arrival. Post, fearful of the food processing safety observed in the USSR, then fully stocked the freezers with products from General Foods' Birdseye unit. The frozen food stores allowed the Davies to lavishly entertain and serve fresh frozen foods that would otherwise be out of season. Upon returning from Moscow, Post (who resumed her maiden name after divorcing Davies) directed General Foods to market frozen product to upscale restaurants.

Introduction of home freezers as separate compartments (larger than necessary just for ice cubes), or as separate units, occurred in the United States in 1940. Frozen foods then began to make the transition from luxury to commonplace.

General technical explanation

A vapor compression cycle is used in most household refrigerators, refrigerator–freezers and freezers. In this cycle, a circulating refrigerant such as R134a enters a compressor as low-pressure vapor at or slightly above room temperature. The vapor is then compressed and exits the compressor as high-pressure superheated vapor. The superheated vapor travels under pressure through coils or tubes comprising "the condenser", which coils or tubes are passively cooled by exposure to room air. (In hot weather, the room or "ambient" air may itself have been cooled by an air conditioner. A cooler ambient temperature demands less work from the refrigerator.) The condenser first removes the superheat from the vapor. The partially-cooled but still hot vapor then travels through the remainder of the condenser. By further loss of its heat, but still being under pressure, it becomes hot liquid. Before the refrigerant leaves the condenser it will have become only slightly warmer than room temperature. This warm liquid refrigerant is pushed by its pressure through a metering or throttling device, also known as an expansion valve (essentially a constriction), just beyond which its pressure abruptly decreases. The sudden decrease in pressure results in explosive-like flash evaporation of a portion (typically about half) of the liquid. (Heat necessary for this flash evaporation is drawn mostly from proximitous still-liquid refrigerant, a phenomenon known as "auto-refrigeration".) The cold and partially vaporized refrigerant continues through (coils or tubes of) the evaporator unit. A fan blows air from the refrigerator or freezer compartment ("box air") across these coils or tubes and the refrigerant completely vaporizes, in the process drawing heat from the box air. This refrigerant's characteristic of vaporizing at low temperature and low pressure (which change of state from liquid to vapor requires heat inflow) is what extracts heat from this normally cool box air, to maintain its coldness. The re-cooled air is then returned to the refrigerator or freezer compartment. It should be understood that cool air in the refrigerator or even the freezer is "hot" relative to colder refrigerant in the evaporator. Refrigerant leaving the evaporator, now fully vaporized and slightly heated, returns to the compressor inlet to continue the cycle.

An absorption refrigerator works differently from a compressor refrigerator, using a source of heat, such as combustion of liquefied petroleum gas, solar thermal energy or an electric heating element. These heat sources are much quieter than the compressor motor in a typical refrigerator. A fan or pump might be the only mechanical moving parts; reliance on convection is considered impractical.

The Peltier effect uses electricity directly to pump heat; refrigerators using this effect are sometimes used for camping, or where noise is not acceptable. They are totally silent, but less energy-efficient than other methods.

Other uses of an absorption refrigerator (or "chiller") would include large systems used in office buildings or complexes such as hospitals and universities. These large systems are used to chill a brine solution that is circulated through the building.

Other alternatives to the vapor-compression cycle but not in current use include thermionic, vortex tube, air cycle, magnetic cooling, Stirling cycle, Malone refrigeration, acoustic cooling, pulse tube and water cycle systems.^[6]

Features

The inside of a common U.S. home refrigerator

Newer refrigerators may include:

• Automatic defrosting;
• 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;
• Cabinet rollers that allow the refrigerator to be easily rolled around for easier cleaning;
• Adjustable shelves and trays which can be repositioned to suit the user;
• 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.

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 frost buildup required periodic thawing ("defrosting") of the units to maintain their efficiency. Advances in automatic defrosting eliminating the thawing task were introduced in the 1950s, but are not universal, due to energy performance and cost. Also, early units featured freezer compartments located within the larger refrigerator, and accessed by opening the refrigerator door, and then the smaller internal 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), which has no ozone layer depleting properties, in place of freon.

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 be pushed open from the inside. This gasket was invented by a man named Herman C. Ells Sr. who didn't want children to lose their lives. However, children can be unwittingly harmed by hiding inside any discarded refrigerator.^[7]

Types of domestic refrigerators

Household refrigerator output in 2000

Domestic refrigerators and freezers for food storage are made in a range of sizes. Among the smallest is a 4 L Peltier fridge advertised as being able to hold 6 cans of beer. A large domestic fridge stands as tall as a person and may be about 1 m wide with a capacity of 600 L. Some models for small households fit under kitchen work surfaces, usually about 86 cm high. Fridges may be combined with freezers, either stacked with fridge or freezer above, below, or side by side. A fridge without a frozen food storage compartment may have a small section just to make ice cubes. Freezers may have drawers to store food in, or they may have no divisions (chest freezers).

Fridges and freezers may be free-standing, or built into a kitchen.

• Compressor refrigerators are by far the most common type; they make a noticeable noise. Absorption or thermo-electric Peltier units are used where quiet running is required; Peltier coolers are used in the smallest refrigerators as they have no bulky mechanism.

• Compressor and Peltier refrigerators are powered by electricity; absorption units can be designed to be powered by any heat source. A noticeable difference between the two types is the absence of refrigerant with the Peltier coolers (these use a different method of cooling). But Peltier coolers use more electricity because they are thermodynamically inefficient.
• Oil, gas (natural gas or propane) and dual power gas/electricity units are also available (typically found in RV's).

• Solar refrigerators and Thermal mass refrigerators are designed to reduce electrical consumption. Solar refrigerators have the added advantage that they do not use refrigerants that are harmful to the environment or flammable. Typical solar designs are absorption refrigerators that use ammonia as the working gas, and employ large mirrors to concentrate sufficient sunlight to reach the temperature required to free gaseous ammonia from the solvent.^[8][9] Most thermal mass refrigerators are designed to use electricity intermittently. As these units are heavily insulated, cooling load is limited primarily to heat introduced by new items to be refrigerated, and ambient air transfer when the unit is open. Very little power is therefore required if opened infrequently. Refrigeration units for commercial and industrial applications can be made in various size, shape or style to fit customer needs.

Energy efficiency

An auto-defrost unit uses a blower fan to keep moisture out of the unit. It also has a heating coil beneath the evaporator that periodically heats the freezer compartment and melts any ice buildup. Some units also have heaters in the side of the door to keep the unit from "weeping." Manual defrost units are available in used-appliance shops or by special order.

Refrigerators used to consume more energy than any other home appliance, but in the last twenty years great strides have been made to make refrigerators more energy efficient. In the early 1990s a competition was held among the major manufacturers to encourage energy efficiency. Current models that are Energy Star qualified use 50 percent less energy than models made before 1993.^[10] The most energy-efficient unit made in the US is designed to run on 120 or 110 volts, and consumes about half a kilowatt-hour per day.[1] But even ordinary units are quite efficient; some smaller units use less than 0.5 kilowatt-hour per day. Larger units, especially those with large freezers and icemakers, may use as much as 4 kWh per day.

Among the different styles of refrigerators, top-freezer models are more efficient than bottom-freezer models of the same capacity, which are in turn more efficient than side-freezer models. Models with through-the-door ice units are less efficient than those without.^[11] Dr. Tom Chalko in Australia has developed an external thermostat to convert any chest freezer into a chest fridge using only about 0.1kWh per day--the amount of energy used by a 100 watt light bulb in one hour.^[12] A similar device is manufactured by Johnson Controls. ^[13] Scientists at Oxford University have reconstructed a refrigerator invented in 1930 by Albert Einstein in their efforts to replace current technologies with energy efficient green technology. The Einstein refrigerator operates without electricity and uses no moving parts or greenhouse gases.^[14]

Impact on lifestyle

The invention of the refrigerator has allowed the modern family to purchase, store, freeze, prepare and preserve food products in a fresh state for much longer periods of time than was previously possible. For the majority of families without a sizeable garden in which to grow vegetables and raise animals, the advent of the refrigerator along with the modern supermarket led to a vastly more varied diet and improved health resulting from improved nutrition. Dairy products, meats, fish, poultry and vegetables can be kept refrigerated in the same space within the kitchen (although raw meat should be kept separate from other foodstuffs for reasons of hygiene).

The refrigerator allows families to consume more salads, fresh fruits and vegetables during meals without having to own a garden or an orchard. Exotic foodstuffs from far-off countries that have been imported by means of refrigeration can be enjoyed in the home because of the availability of domestic refrigeration.

The luxury of freezing allows households to purchase more foods in bulk that can be eaten at leisure while the bulk purchase provides cost savings (see economies of scale). Ice cream, a popular commodity of the 20th century, was previously only available by traveling long distances to where the product was made fresh and had to be eaten on the spot. Now it is a common food item. Ice on-demand not only adds to the enjoyment of cold drinks, but is useful in first-aid applications, not to mention cold packs that can be kept frozen for picnics or in case of emergency.

Temperature zones and ratings

Commercial for electric refrigerators in Pittsburgh, Pennsylvania, 1926

Some refrigerators are now divided into four zones to store different types of food:

• −18 °C (−0.4 °F) (freezer)
• 0 °C (32 °F) (meats)
• 5 °C (41 °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. Some refrigerators require a certain external temperature (60 °F (16 °C) to run properly. This can be an issue when placing a refrigerator in an unfinished area such as a garage.

European freezers, and refrigerators with a freezer compartment, have a four star rating system to grade freezers.

• *  : min temperature = −6 °C (21.2 °F). Maximum storage time for frozen food is 1 week
• **  : min temperature = −12 °C (10.4 °F). Maximum storage time for frozen food is 1 month
• ***  : min temperature = −18 °C (−0.4 °F). Maximum storage time for frozen food is 3 months
• *(***) : min temperature = −18 °C (−0.4 °F). Maximum storage time for frozen food is up to 12 months

Although both the three and four star ratings specify the same minimum 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.

Non-food use

Refrigerators have many other uses. Examples include laboratories, for storing samples awaiting analysis, and morgues, for storing corpses.

See also

• Absorption refrigerator
• Energy Star
• Ice cream maker
• Magnetic refrigeration
• Pot-in-pot refrigerator
• Refrigerator magnet
• Star rating
• Thermoacoustic refrigeration
• Thermoelectric cooling

Notes and references

External links

Wikimedia Commons has media related to: Domestic refrigerator

Look up refrigerator or freezer in Wiktionary, the free dictionary.

• Refrigeration History
• The History of the Refrigerator and Freezers
• How does a gas-powered fridge actually work (without compressor - using heater to power the heat transfer cycle) Archived Page
• Elert, Glenn. "Refrigerators". The Physics Hypertextbook. http://hypertextbook.com/physics/thermal/refrigerators/. 
• How Refrigerators Work Article by HowStuffWorks
• Refrigerators, Canada Science and Technology Museum
• How Refrigerators Work and What goes wrong with them Article by Apwagner.com