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Sci-Tech Dictionary:

refrigeration

(ri′frij·ə′rā·shən)

(mechanical engineering) The cooling of a space or substance below the environmental temperature.


 
 
Sci-Tech Encyclopedia: Refrigeration

The cooling of a space or substance below the environmental temperature. Mechanical refrigeration is primarily an application of thermodynamics wherein the cooling medium, or refrigerant, goes through a cycle so that it can be recovered for reuse. The commonly used basic cycles, in order of importance, are vapor-compression, absorption, steam-jet or steam-ejector, and air. Each cycle operates between two pressure levels, and all except the air cycle use a two-phase working medium which alternates cyclically between the liquid and vapor phases.

The term “refrigeration” is used to signify cooling below the environmental temperature to lower than about 150 K (−190°F; −123°C). The term “cryogenics” is used to signify cooling to temperatures lower than 150 K. See also Cryogenics.

Vapor-compression cycle

The vapor-compression cycle consists of an evaporator in which the liquid refrigerant boils at low temperature to produce cooling, a compressor to raise the pressure and temperature of the gaseous refrigerant, a condenser in which the refrigerant discharges its heat to the environment, usually a receiver for storing the liquid condensed in the condenser, and an expansion valve through which the liquid expands from the high-pressure level in the condenser to the low-pressure level in the evaporator. This cycle may also be used for heating if the useful energy is taken off at the condenser level instead of at the evaporator level. See also Heat pump.

Absorption cycle

The absorption cycle accomplishes compression by using a secondary fluid to absorb the refrigerant gas, which leaves the evaporator at low temperature and pressure. Heat is applied, by means such as steam or gas flame, to distill the refrigerant at high temperature and pressure. The most-used refrigerant in the basic cycle is ammonia; the secondary fluid is then water. This system is used for the lower temperatures. Another system is lithium bromide-water, where the water is used as the refrigerant. This is used for higher temperatures. Due to corrosion, special inhibitors must be used in the lithium bromide-water system. The condenser, receiver, expansion valve, and evaporator are essentially the same as in any vapor-compression cycle. The compressor is replaced by an absorber, generator, pump, heat exchanger, and controlling-pressure reducing valve.

Steam-jet cycle

The steam-jet cycle uses water as the refrigerant. High-velocity steam jets provide a high vacuum in the evaporator, causing the water to boil at low temperature and at the same time compressing the flashed vapor up to the condenser pressure level. Its use is limited to air conditioning and other applications for temperatures above 32°F (0°C).

Air cycle

The air cycle, used primarily in airplane air conditioning, differs from the other cycles in that the working fluid, air, remains as a gas throughout the cycle. Air coolers replace the condenser, and the useful cooling effect is obtained by a refrigerator instead of by an evaporator. A compressor is used, but the expansion valve is replaced by an expansion engine or turbine which recovers the work of expansion. Systems may be open or closed. In the closed system, the refrigerant air is completely contained within the piping and components, and is continuously reused. In the open system, the refrigerator is replaced by the space to be cooled, the refrigerant air being expanded directly into the space rather than through a cooling coil.

Refrigerants

The working fluid in a two-phase refrigeration cycle is called a refrigerant. A useful way to classify refrigerants is to divide them into primary and secondary. Primary refrigerants are those fluids (pure substances, azeotropic mixtures which behave physically as a single pure compound, and zeotropes which have temperature glides in the condenser and evaporator) used to directly achieve the cooling effect in cycles where they alternately absorb and reject heat. Secondary refrigerants are heat transfer or heat carrier fluids. See also Air conditioning; Air cooling; Automotive climate control; Cold storage; Cooling tower; Marine refrigeration; Refrigerator.


 

Components of a refrigerator. A compressor pressurizes the refrigerant gas, heating it and forcing …
(click to enlarge)
Components of a refrigerator. A compressor pressurizes the refrigerant gas, heating it and forcing … (credit: © Merriam-Webster Inc.)
Process of removing heat from an enclosed space or from a substance in order to lower the temperature. In industrialized nations and prosperous regions in the developing world, refrigeration is used chiefly to store foodstuffs at low temperatures, thus inhibiting the destructive action of bacteria, yeasts, and molds. Many perishable products can be frozen, permitting them to be kept for months and even years with little loss in nutrition or flavour or change in appearance. See also air-conditioning; cooling system; heat exchanger.

For more information on refrigeration, visit Britannica.com.

 
Architecture: refrigeration

The process by which heat is absorbed from a body or substance by expansion or vaporization of a refrigerant, lowering its body temperature and maintaining the temperature below its surroundings.


 
US History Encyclopedia: Refrigeration

The preservation of winter ice, principally for cooling drinks in summer, was practiced from antiquity, when ice was stored in insulated caves. Ice cooling was so popular in America that, within a decade of the Revolution, the United States led the world in both the production and consumption of ice. As the use of ice was extended to the preservation of fish, meat, and dairy products, ice production became a major industry in the northern states, and ice was shipped not only to the southern states but to the West Indies and South America. By 1880 the demand in New York City exceeded the capacity of 160 large commercial icehouses on the Hudson River. This industry reached its peak in 1886, when 25 million pounds were "harvested" by cutting lake and river ice with horse-drawn saws. The advent of mechanical refrigeration in the nineteenth century began a slow decline in the ice trade. The natural ice industry had expired by 1920.

A method of making ice artificially, lowering the temperature of water by accelerating its evaporation, had long been practiced in India, where water in porous clay dishes laid on straw evaporated at night so rapidly as to cause ice to form on the surface of the remaining water. This method endured until as late as 1871, when a U.S. patent utilized this principle for a refrigerator that depended on the evaporation of water from the porous lining of the food compartment. But previous scientific discoveries had suggested other, more efficient methods of cooling.

Several types of refrigeration machines were developed in the nineteenth century. All depended on the absorption of heat by expanding gases, which had been a subject of scientific research in the previous century. It had been observed that the release of humid compressed air was accompanied by a stream of pellets of ice and snow. This phenomenon, the cooling effect of expanding air, led to the development of the Gorrie ice machine in the 1840s. John Gorrie, a physician in Apalachicola, Florida, sought ways to relieve the suffering of malaria victims in the southern summer climate. Gorrie's machine injected water into a cylinder in which air was compressed by a steam engine. This cooled the air. The air expanded in contact with coils containing brine, which was then used to freeze water. Gorrie's machine saw scarce use (although at least one was built—in England), but his 1851 patent, the first in the United States for an ice machine, served as a model for others, including the first large machines that in the 1880s began to compete with lakes and rivers as a source of ice.

More important were "absorption" machines, based on the observation, made by Sir John Leslie in 1810, that concentrated sulfuric acid, which absorbs water, could accelerate the evaporation of water in a dish to such a degree as to freeze the remaining water. This method of artificial ice production was patented in England in 1834. Scientists further increased the efficiency level by using ammonia as the evaporating fluid and water as the absorbing fluid. Ferdinand Carré developed the first important example in France in 1858. In this machine, Carré connected by tube a vessel containing a solution of ammonia in water to a second vessel. When the former heated and the latter cooled (by immersing it in cold water), the ammonia evaporated from the first vessel and condensed in the second. Heating was then terminated and the ammonia allowed to reevaporate, producing a refrigerating effect on the surface of the second (ammonia-containing) chamber. Such a "machine" was no automatic refrigerator, but it was inexpensive and simple and well suited for use in isolated areas. One of them, the Crosley "Icyball," was manufactured in large numbers in the United States in the 1930s. Supplied with a handle, the dumbbell-shaped apparatus had to occasionally be set on a kerosene burner, where the "hot ball" was warmed and then moved to the "ice box," where the "cold ball" exercised its cooling effect (the hot ball being allowed to hang outside).

The vapor compression system would soon replace all of these early designs. In a vapor compression machine, a volatile fluid is circulated while being alternately condensed (with the evolution of heat) and evaporated (with the absorption of heat). This is the principle of the modern refrigeration machine. Oliver Evans, an ingenious American mechanic, had proposed in 1805 to use ether in such a machine, and in 1834 Jacob Perkins, an American living in London, actually built one, using a volatile "ether" obtained by distilling rubber. Perkins built only one machine, but improved versions—based in large part on the patents of Alexander C. Twining of New Haven, Connecticut—were developed and actually manufactured in the 1850s.

The earliest demand for refrigeration machines came from breweries, from the Australian meat industry, and from southern states that wanted artificial ice. Only when the operation of such machinery was made reliably automatic could it serve the now familiar purpose of household refrigeration, a step successfully taken by E. J. Copeland of Detroit in 1918. Another important step was the use of less hazardous fluids than the ethers common in commercial refrigerators. Ammonia replaced the ethers in a few machines from the 1860s and became the most common refrigerating fluid by 1900. But ammonia was only slightly less hazardous than ether. The problem was to be solved through a research program remarkable for its basis in theoretical science. In the 1920s, Thomas Midgley Jr., with the support of the General Motors Corporation, studied the relevant characteristics (volatility, toxicity, specific heat, etc.) of a large number of substances. The result was a description of an "ideal" refrigerating fluid, including a prediction of what its chemical composition would be. Midgley then proceeded to synthesize several hitherto unknown substances that his theory indicated would possess these ideal characteristics. In 1930 he announced his success with the compound dichlorodifluoromethane. Under the commercial name Freon 12, it became the most widely used refrigerant.

The most noteworthy subsequent development in the use of refrigeration has been the introduction of frozen foods. It began in 1924 with an apparatus in which prepared fish were carried through a freezing compartment on an endless belt, developed by Clarence Birdseye of Gloucester, Massachusetts. By 1929 Birdseye had modified the apparatus to freeze fresh fruit, and in 1934 frozen foods were introduced commercially. Since World War II, progress in refrigerator design has focused on efficiency. The energy crisis of the 1970s spurred the first state regulatory standard for refrigerator efficiency. A California law passed in 1976 over the objection of appliance manufacturers required that 18-cubic-foot refrigerators sold in the state conform to a minimum efficiency level of 1400 kilowatt-hours (kwh) per year. California's large market share made it relatively easy for the federal government to hold refrigerators to a standard of 900 kwh in 1990 and then 700 kwh in 1993. Advances in refrigerator design have improved the consumption of the average refrigerator by over 60 percent in the last three decades of the twentieth century.

Bibliography

Anderson, Oscar E., Jr. Refrigeration in America: A History of a New Technology and Its Impact. Port Washington, N.Y.: Kennikat Press, 1972.

David, Elizabeth. Harvest of the Cold Months: The Social History of Ice and Ices. New York: Viking, 1995.

 
Columbia Encyclopedia: refrigeration,
process for drawing heat from substances to lower their temperature, often for purposes of preservation. Refrigeration in its modern, portable form also depends on insulating materials that are thin yet effective. Wherever fresh or frozen food must be stored, processed, transported, or sold, refrigeration is indispensable; thus appropriate refrigeration machinery has been developed for trains, ships, factories, and cold-storage plants (used not only for foods but also for fur storage).

See also air conditioning.

Early Methods of Refrigeration

Before the advent of modern refrigeration, perishable foods were kept in cool cellars or in buckets lowered into wells. A device still used in some areas is a room built with porous walls over which water is made to trickle; as the water evaporates the room is cooled. A spring of cold water often determined the site of an American pioneer's home. A springhouse was built over the flowing water, and the cooling fluid was led through troughs in which crocks of butter and cream were placed. In winter, farmers stored ice in icehouses for use in the summer. Similarly, natural ice from commercial icehouses was used in cities until artificial methods of producing ice were initiated in the middle of the 19th cent.

Mechanical Refrigeration Systems

The first patent for mechanical refrigeration was issued (1834) in Great Britain to the American inventor Jacob Perkins. Mechanical refrigeration systems are based on the principle that absorption of heat by a fluid (refrigerant) as it changes from a liquid to a gas lowers the temperature of the objects around it. In the compression system, which is employed in electric home refrigerators and commercial installations, a compressor, controlled by a thermostat, exerts pressure on a vaporized refrigerant, forcing it to pass through a condenser, where it loses heat and liquefies. It then moves through the coils of the refrigeration compartment. There it vaporizes, drawing heat from whatever is in the compartment. The refrigerant then passes back to the compressor, and the cycle is repeated.

Prior to 1996, the refrigerants used in electric refigerators were chlorofluorocarbons (CFCs). However, because of increasing scientific evidence that the CFCs are harmful to the ozone layer of the stratosphere, they were banned by international treaty, the Montreal Protocol, after Jan., 1996. Transitional compounds, such as hydrochlorofluorocarbons (HCFCs), which are less harmful to the ozone layer, are to be used in their place until the year 2020. By that time compounds such as the hydrofluorocarbons (HFCs), which are benign to the ozone layer, are expected to have replaced HCFCs.

In the absorption system, widely employed in commercial installations, ammonia is usually used as a refrigerant to cool brine (water containing calcium chloride or sodium chloride) that is then sent through pipes to cool the refrigerated space. The steam-jet system is used where temperatures below 32°F (0°C) are not required; water is used as the refrigerant. Airplanes are cooled or heated through an air cycle system. Research and development is being carried out to apply the Peltier effect (see thermoelectricity) in various practical refrigeration systems.

Preparation of Frozen Foods

An outgrowth of the preservation of foods by refrigeration was the development of a process for preparing frozen foods. Although a number of experimenters contributed to the discovery of a workable process, the name of American inventor Clarence Birdseye is associated with the early successful introduction of the method; one of his chief contributions was his system of freezing perishable foods (packed in individual containers ready for sale) between refrigerated metal plates.

Bibliography

See G. H. Reed, Refrigeration (3d ed. 1974); C. T. Olivo and R. W. Marsh, Principles of Refrigeration (1979).


 
Boating Encyclopedia: Refrigeration

Cooling food and drinks on board: some popular methods
The two most popular methods of cooling drinks and food on small boats are iceboxes and mechanical refrigeration.The first method is simple and cheap, but crude and uncertain. Block ice is heavy and cumbersome, and may not be available everywhere you go.The second method is complicated, expensive, and prone to breakdowns. If you have mechanical refrigeration on board, you will never have to wonder what to do with your spare time.Donald Street, a well-known Caribbean charter skipper and owner of the 45-foot wooden yawl Iolaire, talks from vast experience when he writes in The Ocean Sailing Yacht: “Mechanical refrigeration can break down . . . the original installation is expensive, and mechanical refrigeration is like a chain, the links being the starter motor, the engine, the generator, the water-cooling pumps, the compressor, the condenser, the holding plates, the expansion valves, and so on and on. Failure of a single link renders the whole system inoperable.”

Components of a 12-volt refrigeration unit.
In the same book, Joe Repke, a marine refrigeration engineer, warns: “Anyone intending to do serious cruising must be sure to understand fully the refrigeration system of his boat. He should know the purpose of every component, every valve, and be able to adjust the valves, change the dryers, recharge the system, and so on. Refrigeration experts are few and far between in the middle of the ocean.”Aboard Iolaire, Street depends on ice. “The normal routine in the southern islands is to find a small boy with a dinghy, and give him a dollar along with the money for the ice and couple of ice bags,” he writes. “Then one can sit back and relax, have a couple of rums, and in no time the ice arrives.”His two 100-pound iceboxes chill fresh food and drinks for 6 people for 10 to 12 days. “The great advantage of an icebox is that it is foolproof. One just fills it with ice, and everything gets cold. There is nothing to break down.”Few small boats can carry 200 pounds of ice, of course, so what works for Street is not good for everybody. Eric Hiscock and others have used small absorption refrigerators worked by a constant open kerosene flame, but they can only be used in port—and then with great caution. The manufacturers strongly recommend against their being used on boats.You can have a refrigerator and freezer powered by an electrical motor, but the drain on the batteries is heavy and constant. The sad truth is that refrigeration consumes a lot of energy. Boating author Don Casey estimates that adding 12-volt refrigeration to a normally equipped 35-foot yacht will absorb 100 amp-hours per day in hot climates, and triple the demand on the ship’s batteries. That means running the auxiliary engine of a sailboat for at least 2 hours a day to charge them.The refrigerator can be driven directly from the engine, which eliminates battery problems, but engine-driven systems are expensive—they cost twice or three times as much as 12-volt refrigerators and are more complicated. They are, however, more powerful and, whereas a 12-volt refrigerator cycles on and off all day and night, you need only run an engine-driven compressor once or twice a day to freeze its holdover plate, which is similar to loading the box with ice.Despite its cost and many drawbacks, many cruisers cannot imagine living without refrigeration. “There is no sweeter symphony to my ear than the susurration of wavelets running along the beach of a deserted cove, mixed with the ringing of ice cubes in slender glasses,” writes Don Casey in This Old Boat.The concert may be sweet, but the price of admission is high.See also Iceboxes.

 
Wikipedia: refrigeration

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. The term cooling refers generally to any natural or artificial process by which heat is dissipated. The process of artificially producing extreme cold temperatures is referred to as cryogenics.

Cold is the absence of heat, hence in order to reduce a temperature, one does not "add cold", rather one "removes heat." In order to satisfy the Second Law of Thermodynamics, some form of work must be performed to accomplish this. This work is traditionally done by mechanical work but can also be done by magnetism, laser or other means. However, all refrigeration uses the three basic methods of heat transfer: convection, conduction, or radiation.

Historical applications

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 Italy, such a solution was used to chill wine.[3]

During the first half of the 19th century, ice harvesting became 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.

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 liquefied 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.[4]

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.[5] 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.

Dunedin, the first commercially successful refrigerated ship.
Enlarge
Dunedin, the first commercially successful refrigerated ship.

Soon after that, James Harrison, who was 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. Australian, Argentinean and American concerns experimented with refrigerated shipping in the mid 1870s, the first commercial success coming when William Soltau Davidson fitted a compression refrigeration unit to the New Zealand vessel Dunedin in 1882, leading to a meat and dairy boom in Australasia and South America.

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. In the United States, 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 mechanical refrigerators became available in the United States around 1911.[6]

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 US 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 or lorries). Refrigerated vehicles 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., methyl formate, 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.

The Montreal Protocol

As of 1989, CFC-based refrigerant was banned via the Montreal Protocol due to the negative effects it has on the ozone layer. The Montreal Protocol was ratified by most CFC producing and consuming nations in Montreal, Quebec, Canada in September 1987. Greenpeace objected to the ratification because the Montreal Protocol instead ratified the use of HFC refrigeration, which are not ozone depleting but are still powerful global warming gases. Searching for an alternative for home use refrigeration, dkk Scharfenstein (Germany) developed a propane-based CFC as well as an HFC-free refrigerator in 1992 with assistance from Greenpeace.[citation needed]

The tenets of the Montreal Protocol were put into effect in the United States via the Clean Air Act legislation in August 1988. The Clean Air Act was further amended in 1990. This was a direct result of a scientific report released in June 1974 by Rowland-Molina[7], detailing how chlorine in CFC and HCFC refrigerants adversely affected the ozone layer. This report prompted the FDA and EPA to ban CFCs as a propellant in 1978 (50% of CFC use at that time was for aerosol can propellant).

  • In January 1992, the EPA required that refrigerant be recovered from all automotive air conditioning systems during system service.
  • In July 1992, the EPA made illegal the venting of CFC and HCFC refrigerants.
  • In June 1993, the EPA required that major leaks in refrigeration systems be fixed within 30 days. A major leak was defined as a leak rate that would equal 35% of the total refrigerant charge of the system (for industrial and commercial refrigerant systems), or 15% of the total refrigerant charge of the system (for all other large refrigerant systems), if that leak were to proceed for an entire year.
  • In July 1993, the EPA instituted the Safe Disposal Requirements, requiring that all refrigerant systems be evacuated prior to retirement or disposal (no matter the size of the system), and putting the onus on the last person in the disposal chain to ensure that the refrigerant was properly captured.
  • In August 1993, the EPA implemented reclamation requirements for refrigerant. If a refrigerant is to change ownership, it must be processed and tested to comply with the American Refrigeration Institute (ARI) standard 700-1993 (now ARI standard 700-1995) requirements for refrigerant purity.
  • In November 1993, the EPA required that all refrigerant recovery equipment meet the standards of ARI 740-1993.
  • In November 1995, the EPA also restricted the venting of HFC refrigerants. These contain no chlorine that can damage the ozone layer (and thus have an ODP (Ozone Depletion Potential) of zero), but still have a high global warming potential.
  • In December 1995, CFC refrigerant importation and production in the US was banned.

It is currently planned to ban all HCFC refrigerant importation and production in the year 2030, although that will likely be accelerated.

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 vapor 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.

One of the most influential uses of refrigeration was in the development of the sushi/sashimi industry in Japan. Prior to the discovery of refrigeration, many sushi connoisseurs suffered great morbidity and mortality from diseases such as hepatitis A, and Diphyllobothriosis, from a common oceanic tapeworm - Diphyllobothrium latum. However the dangers of unrefrigerated sashimi was not brought to light for decades due to the lack of research and healthcare distribution across rural Japan. Around mid-century, the Zojirushi corporation based in Kyoto made breakthroughs in refrigerator designs making refrigerators cheaper and more accessible for restaurant proprietors and the general public.

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 333.55 kJ/kg (approx. 144 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 thermodynamic 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.

Cyclic refrigeration can be classified as:

  1. Vapor cycle, and
  2. Gas cycle

Vapor cycle refrigeration can further be classified as:

  1. Vapor compression refrigeration
  2. Gas absorption refrigeration

Vapor-compression cycle

(See Heat pump and refrigeration cycle and Vapor-compression refrigeration for more details)

The vapor-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 vapor-compression refrigeration system.

Figure 1: Vapor compression refrigeration
Figure 1: Vapor compression refrigeration

The thermodynamics of the cycle can be analyzed on a diagram[8][9] as shown in Figure 2. In this cycle, a circulating refrigerant such as Freon enters the compressor as a vapor. From point 1 to point 2, the vapor is compressed at constant entropy and exits the compressor superheated. From point 2 to point 3 and on to point 4, the superheated vapor travels through the condenser which first cools and removes the superheat and then condenses the vapor 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.

Figure 2: Temperature–Entropy diagram
Figure 2: Temperature–Entropy diagram

That results in a mixture of liquid and vapor at a lower temperature and pressure as shown at point 5. The cold liquid-vapor 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 vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.

The above discussion is based on the ideal vapor-compression refrigeration cycle, and does not take into account real-world effects like frictional pressure drop in the system, slight thermodynamic irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).

More information about the design and performance of vapor-compression refrigeration systems is available in the classic "Perry's Chemical Engineers' Handbook".[10]

Vapor absorption cycle

(See Gas absorption refrigerator for more details)

In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems was popular and widely used but, after the development of the vapor compression cycle, it lost much of its importance because of its low coefficient of performance (about one fifth of that of the vapor compression cycle). Nowadays, the vapor 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 vapor. 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 vapor from the high-pressure liquid. Some work is required by the liquid pump but, for a given quantity of refrigerant, it is much smaller than needed by the compressor in the vapor 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 in the US are mostly rated in tons of refrigeration, but elsewhere in kW. 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) = 333.55 kJ/kg ≈ 144 Btu/lb
One short ton = 2000 lb
Heat extracted = (2000)(144)/24 hr = 288000 Btu/24 hr = 12000 Btu/hr = 200 Btu/min
1 ton refrigeration = 200 Btu/min = 3.517 kJ/s = 3.517 kW[11]

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 333.55 kJ/kg, 1 tonne of refrigeration = 13,898 kJ/h = 3.861 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.

References

  1. ^ "Refrigeration fundamentals throughout history"PDF (72.9 KiB)
  2. ^ "Air conditioning and refrigeration chronology"PDF (265 KiB)
  3. ^ The Advent of Mechanical Refrigeration Alters Daily Life and National Economies Throughout the World. Retrieved on 2007-05-20.
  4. ^ Burstall, Aubrey F. (1965). A History of Mechanical Engineering. The MIT Press. ISBN 0-262-52001-X. 
  5. ^ "Improved process for the artificial production of ice", U.S. Patent Office, Patent 8080, 1851
  6. ^ Modern Marvels. Retrieved on 2007-05-20.
  7. ^ Stratospheric sink for chlorofluoromethanes: chlorine atom-catalysed destruction of ozone, Mario J. Molina & F. S. Rowland, Department of Chemistry, University of California, Irvine, Nature 249, 810 - 812, 28 June 1974.
  8. ^ The Ideal Vapor-Compression Cycle
  9. ^ Scroll down to "The Basic Vapor Compression Cycle and Components"
  10. ^ Perry, R.H. and Green, D.W. (1984). Perry's Chemical Engineers' Handbook, 6th Edition, McGraw Hill, Inc.. ISBN ISBN 0-07-049479-7.  (see pages 12-27 through 12-38)
  11. ^ Guide To SI Units

Additional reading

  • Stoecker and Jones, Refrigeration and Air Conditioning, Tata-McGraw Hill Publishers
  • Mathur, M.L., Mehta, F.S., Thermal Engineering Vol II
  • MSN Encarta Encyclopedia
  • Andrew D. Althouse, Carl H. Turnquist, Alfred F. Bracciano (2003). Modern Refrigeration and Air Conditioning, 18th Edition, Goodheart-Wilcox Publishing. ISBN 1590702808. 
  • Anderson, Oscar Edward (1972). Refrigeration in America: A history of a new technology and its impact. Kennikat Press, 344. ISBN 0804616213. 
  • Shachtman, Tom (2000-12-12). Absolute Zero: And the Conquest of Cold. Mariner Books, 272. ISBN 0618082395. 
  • Woolrich, Willis Raymond (1967). The men who created cold: A history of refrigeration,, [1st ed.], Exposition Press, 212. 

See also

External links


 
 

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