A refrigerator (often called a "fridge" for short) is a cooling appliance comprising a thermally insulated compartment and a mechanism to transfer heat from it to the external environment, cooling the contents to a temperature below ambient. Refrigerators are extensively used to store foods that deteriorate at ambient temperatures; spoilage from bacterial growth and other processes is much slower at low temperatures. A refrigerator maintains a temperature a few degrees above the freezing point of water, while a similar device, a freezer, maintains a temperature below the freezing point of water. The refrigerator is a relatively modern invention among kitchen appliances. It replaced the common icebox, which had been used for almost a century and a half prior, and is sometimes still called by the original name icebox.
Freezers keep their contents frozen. They are used both in households and for commercial use. Most freezers operate at around -18 °C (0 °F). Domestic freezers can be included as a compartment in a refrigerator, sharing the same mechanism or with a separate mechanism, or can be stand-alone units. Domestic freezers are generally either 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. They used toxic ammonia gas systems, 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 nontoxic, nonflammable 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 UK.
Design
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 both (dual compartment). 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 R134a enters the compressor as a low-pressure vapor at its boiling point. The vapor is compressed and exits the compressor as a superheated high-pressure 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. Before the refrigerant leaves the condenser it will have been subcooled below the boiling point. The subcooled liquid refrigerant passes through the metering device, 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 vapor, now slightly superheated, returns to the compressor inlet to complete the thermodynamic cycle.
An absorption refrigerator works differently from a compressor refrigerator, using a source of heat, and typically runs more quietly. 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 in complexes such as hospitals and universities. These large systems are used to chill a brine solution that is circulated through the building.
Features
The inside of a common US home refrigerator
Newer refrigerators may include features such as the following:
- Frost-free refrigeration.
- A power failure warning, which alerts 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 that can be moved around 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 liters (about 2 ft3) 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 build-up of frost required periodic thawing of the units to maintain their efficiency. Advances in frost-free refrigeration introduced in the 1950s eliminated the thawing task. Also, early units featured freezer compartments located within the larger refrigerator that were accessed by opening the refrigerator door first and then the smaller internal freezer door. Units featuring entirely separate freezer compartments were introduced in the early 1960s and became 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 the concern arose because of the freon coolant, which damages the ozone layer. 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, instead of freon.)
Disposal of discarded refrigerators is regulated. Many regulations mandate the removal of the doors: Children playing hide-and-seek have been asphyxiated while hiding inside a discarded refrigerator. This was particularly true for 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.
Types of Domestic Refrigerators
Domestic refrigerators and freezers for food storage are made in a range of sizes. Amongst 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 true freezer may have a small compartment to make ice. 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 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 invariably powered by electricity; absorption units can be designed to be powered by any heat source. Electricity, oil, gas (natural gas or propane), and dual power gas/electricity units are available (these are typically found in RV’s).
Refrigeration units for commercial and industrial applications can be made in any size, shape, and style to fit the customer's needs.
Energy Efficiency
A frost-free 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 20 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% less energy than models made before 1993. The most energy efficient unit made in the US is designed to run on 12 or 110 volts, and consumes about 1/2 kWh per day. But even ordinary units are quite efficient; some smaller units use little more than 1kWh per day. Larger units, especially those with large freezers and icemakers, may use as much as 4 kwh per day.
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 or land on which to raise livestock, 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 all 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. The bulk purchase also provides cost savings. Ice cream, a popular commodity of the 20th century, previously coudl only be enjoyed by traveling to where the product was made fresh and eating it on the spot. Now it is a practically ubiquitous 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
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 liters or cubic feet. Typically the volume of a combined fridge-freezer is split to 100 L (3.53 ft3) for the freezer and 140 L (4.94 ft3) 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) is best for the freezer. Some refrigerators require a certain external temperature (60 °F) 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 rating system to grade freezers.
- * : min temperature = -6°C. Maximum storage time for frozen food is 1 week.
- ** : min temperature = -12°C. Maximum storage time for frozen food is 1 month.
- *** : min temperature = -18°C. Maximum storage time for frozen food is 3 months.
- *(***) : min 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 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 increases in temperature.
