Refrigeration is a process in which work is done to move heat from one location to another. The work of heat transport is traditionally driven by mechanical work, but can also be driven by heat, magnetism, electricity, laser, or other means. Refrigeration has many applications, including, but not limited to: household refrigerators, industrial freezers, cryogenics, and air conditioning. Heat pumps may use the heat output of the refrigeration process, and also may be designed to be reversible, but are otherwise similar to refrigeration units.
The use of ice to refrigerate and thus preserve food goes back to prehistoric times. Through the ages, the seasonal harvesting of snow and ice was a regular practice of most of the ancient cultures: Chinese, Greeks, Romans, Persians. Ice and snow were stored in caves or dugouts lined with straw or other insulating materials. The Persians stored ice in a pit called a yakhchal. Rationing of the ice allowed the preservation of foods over the warm periods. This practice worked well through the centuries, with icehouses remaining in use into the 20th 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 and cakes.
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 to ship ice long distances, 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 1756. Cullen used a pump to create a partial vacuum over a container of diethyl 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 1758, Benjamin Franklin and John Hadley, professor of chemistry at Cambridge University, conducted an experiment to explore the principle of evaporation as a means to rapidly cool an object. Franklin and Hadley confirmed evaporation of highly volatile liquids, such as alcohol and ether, could be used to drive down the temperature of an object past the freezing point of water. They conducted their experiment with the bulb of a mercury thermometer as their object and with a bellows used to "quicken" the evaporation; they lowered the temperature of the thermometer bulb down to 7 °F (−14 °C), while the ambient temperature was 65 °F (18 °C). Franklin noted that soon after they passed the freezing point of water (32 °F), a thin film of ice formed on the surface of the thermometer's bulb and that the ice mass was about a quarter inch thick when they stopped the experiment upon reaching 7 °F (−14 °C). Franklin concluded, "From this experiment, one may see the possibility of freezing a man to death on a warm summer's day".
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.
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 that worked although it did not succeed commercially.
In 1842, an American physician, John Gorrie, designed the first system to refrigerate 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). It was also thought at the time that cooling the rooms would prevent the spread of disease (particularly malaria) by preventing the formation of "bad air". His system used a steam engine to compress air, then partly cooled the hot compressed air with water before allowing it to expand while doing part of the work needed 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. 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.
Meanwhile in Australia, James Harrison began operation of a mechanical ice-making machine in 1851 on the banks of the Barwon River at Rocky Point in Geelong, Victoria. His first commercial ice-making machine followed in 1854, and his patent for an ether liquid-vapour compression refrigeration system was granted in 1855. Harrison introduced commercial vapour-compression refrigeration to breweries and meat packing houses, and by 1861, a dozen of his systems were in operation.
Australian, Argentine, and American concerns experimented with refrigerated shipping in the mid-1870s; the first commercial success came 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. J & E Hall of Dartford, England outfitted the 'SS Selembria' with a vapor compression system to bring 30,000 carcasses of mutton from the Falkland Islands in 1886.
Carl Paul Gottfried Linde, ennobled in 1897 as Ritter von Linde, was a German engineer who developed refrigeration and gas separation technologies. Linde started in the design of locomotive engines, and then cut his teeth in refrigeration developing large refrigerated tanks for the production of lager in the 1870s. In 1890, he became a lecturer at the Technische Hochschule in Munich. A few years later, he became a full professor and set up a laboratory where he worked on developing new refrigeration cycles. In 1892, an order from the Guinness Brewery in Dublin for a carbon dioxide liquefaction plant drove Linde's research into the area of low temperature refrigeration, and in 1894 he started work on a process for the liquefaction of air. In 1895, Linde first achieved success, and filed for patent protection of his process (not approved in the United States until 1903). In 1901, Linde began work on a technique to obtain pure oxygen and nitrogen based on the fractional distillation of liquefied air. By 1910, coworkers (including Linde's son Friedrich) had developed the Linde double-column process, variants of which are still in common use today.
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. The Servel company built gas powered, absorption refrigerators in Evansville IN from 1927 through 1956. In the United States at that time the consumer public still used ice boxes with ice brought in from commercial suppliers, many of whom were still harvesting ice and storing it in icehouses.
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, other investors and he 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.
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 the demand for more modern and consumer-ready refrigeration and ice-making machines.
Refrigerated railroad cars were introduced in the US in the 1840s for short-run transport 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. That same year in San Antonio, Texas, a French immigrant named Andrew Muhl built an ice-making machine to help service the expanding beef industry before moving it to Waco in 1871. In 1873, the patent for this machine was contracted by the Columbus Iron Works, a company acquired by the W. C. Bradley Co., which went on to produce the world's first commercial ice-makers.
Carl von Linde, an engineering professor at the Technological University Munich in Germany, patented an improved method of liquefying gases in 1876. His new process made possible using gases such as ammonia, sulfur dioxide (SO2) and methyl chloride (CH3Cl) as refrigerants and they were widely used for that purpose until the late 1920s.
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.
By the middle of the 20th century, refrigeration units were designed for installation on 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 based mostly on a chlorofluorocarbon (CFC) chemical, safer refrigerators were possible for home and consumer use. Freon is a trademark of the Dupont Corporation and refers to these CFCs, and later hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC), refrigerants developed in the late 1920s. These refrigerants were considered at the time to be less harmful than the commonly-used refrigerants of the time, including methyl formate, ammonia, methyl chloride, and sulfur dioxide. The intent was to provide refrigeration equipment for home use without danger. These CFC refrigerants answered that need. In the 1970s, though, the compounds were found to be reacting with atmospheric ozone, an important protection against solar ultraviolet radiation, and their use as a refrigerant worldwide was curtailed in the Montreal Protocol of 1987.
Current applications of refrigeration
Probably the most widely used current applications of refrigeration are for air conditioning of private homes and public buildings, and refrigerating foodstuffs in homes, restaurants and large storage warehouses. The use of refrigerators in kitchens for storing fruits and vegetables has allowed adding fresh salads to the modern diet year round, and storing fish and meats safely for long periods.
In commerce and manufacturing, there are many uses for refrigeration. Refrigeration is used to liquify gases - 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 needed low temperatures (for example, in 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 seagoing 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. Before the discovery of refrigeration, many sushi connoisseurs were at risk of contracting diseases. The dangers of unrefrigerated sashimi were 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, thermoelectric and magnetic.
In non-cyclic refrigeration, cooling is accomplished by melting ice or by subliming dry ice (frozen carbon dioxide). 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 melting point of 0 °C (32 °F) at sea level. To melt, ice must absorb 333.55 kJ/kg (about 144 Btu/lb) of heat. Foodstuffs maintained near this temperature have an increased storage life.
Solid carbon dioxide has no liquid phase at normal atmospheric pressure, and sublimes directly from the solid to vapor phase at a temperature of -78.5 °C (-109.3 °F), and is effective for maintaining products at low temperatures during sublimation. Systems such as this where the refrigerant evaporates and is vented to the atmosphere are known as "total loss 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 needed 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.
Cyclic refrigeration can be classified as:
- Vapor cycle, and
- Gas cycle
Vapor cycle refrigeration can further be classified as:
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.
The thermodynamics of the cycle can be analyzed on a diagram 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 as a vapor at a higher temperature, but still below the vapor pressure at that temperature. From point 2 to point 3 and on to point 4, the vapor travels through the condenser which cools the vapor until it starts condensing, 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.
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).
Vapor absorption cycle
In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems was popular and widely used. After the development of the vapor compression cycle, the vapor absorption cycle lost much of its importance because of its low coefficient of performance (about one fifth of that of the vapor compression cycle). Today, the vapor absorption cycle is used mainly where fuel for heating is available but electricity is not, such as in recreational vehicles that carry LP gas. It is also used in industrial environments where plentiful waste heat overcomes its inefficiency.
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 needed 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) with water (absorbent), and water (refrigerant) with lithium bromide (absorbent).
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 needs a large mass flow rate and is bulky.
Because of their lower efficiency and larger bulk, air cycle coolers are not often used nowadays in terrestrial cooling devices. However, the air cycle machine is very common on gas turbine-powered jet aircraft as cooling and ventilation units, because compressed air is readily available from the engines' compressor sections. Such units also serve the purpose of pressurizing the aircraft.
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, 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 needed properties at room temperature, applications have so far been limited to cryogenics and research.
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 pressurized gas to drive heat transfer and heat exchange; steam jet cooling popular in the early 1930s for air conditioning large buildings; thermoelastic cooling using a smart metal alloy stretching and relaxing. Many Stirling cycle heat engines can be run backwards to act as a refrigerator, and therefore these engines have a niche use in cryogenics. In addition there are other types of cryocoolers such as Gifford-McMahon coolers, Joule-Thomson coolers, pulse-tube refrigerators and, for temperatures between 2 mK and 500 mK, dilution refrigerators.
Unit of refrigeration
The measured capacity of refrigeration is always dimensioned in units of power. Domestic and commercial refrigerators may be rated in kJ/s, or Btu/h of cooling. For commercial and industrial refrigeration systems, most of the world uses the kilowatt (kW) as the basic unit of refrigeration. Typically, commercial and industrial refrigeration systems in North America are rated in tons of refrigeration (TR). Historically, one TR was defined as the energy removal rate that will freeze one short ton of water at 0 °C (32 °F) in one day. This was very important because many early refrigeration systems were in ice houses. The simple unit allowed owners of these early refrigeration systems to measure a day's output of ice against energy consumption, and to compare their plant to one down the street. While ice houses make up a much smaller part of the refrigeration industry than they once did, the unit TR has remained in North America. The unit's value as historically defined was approximately 11,958 Btu/hr (3.505 kW), and has now been conventionally redefined as exactly 12,000 Btu/hr (3.517 kW).
While not truly a unit of capacity, a refrigeration system's coefficient of performance (CoP) is very important in determining a system's overall efficiency. It is defined as refrigeration capacity in kW divided by the energy input in kW. While CoP is a very simple measure of performance, it is typically not used for industrial refrigeration in North America. Owners and manufacturers of these systems typically use performance factor (PF). A system's PF is defined as a system's energy input in horsepower divided by its refrigeration capacity in TR. Both CoP and PF can be applied to either the entire system or to system components. For example, an individual compressor can be rated by comparing the energy needed to run the compressor versus the expected refrigeration capacity based on inlet volume flow rate. It is important to note that both CoP and PF for a refrigeration system are only defined at specific operating conditions, including temperatures and thermal loads. Moving away from the specified operating conditions can dramatically change a system's performance.
- Air conditioning
- Beef ring
- Carnot heat engine
- Cold chain
- Coolgardie safe
- Darcy friction factor formulae
- Einstein refrigerator
- Heat pump
- Heat pump and refrigeration cycle
- Laser cooling
- Pot-in-pot refrigerator
- Pumpable ice technology
- Redundant refrigeration system
- Reefer ship
- Refrigerated container
- Refrigerator car
- Refrigerator truck
- Seasonal energy efficiency ratio (SEER)
- Steam jet cooling
- Vapor-compression refrigeration
- Refrigeration History
- Refriferator and Freezer Safety
- "The Advent of Mechanical Refrigeration Alters Daily Life and National Economies Throughout the World – Science and Its Times: 1800-1899". Retrieved 2007-05-20.
- Arora, Ramesh Chandra. "Mechanical vapour compression refrigeration". Refrigeration and Air Conditioning. New Delhi, India: PHI Learning. p. 3. ISBN 81-203-3915-0.
- Cooling by Evaporation (Letter to John Lining). Benjamin Franklin, London, June 17, 1758
- Burstall, Aubrey F. (1965). A History of Mechanical Engineering. The MIT Press. ISBN 0-262-52001-X.
- "Improved process for the artificial production of ice", U.S. Patent Office, Patent 8080, 1851
- J & E Hall International - History
- "Modern Marvels". Retrieved 2007-05-20.
- Refrigeration, Texas State Historical Association.
- Romance of Ice Plays a Part in History, St. Petersburg Times, June 4, 1927.
- A History of Innovation, W.C. Bradley Company.
- The Ideal Vapor-Compression Cycle
- Scroll down to "The Basic Vapor Compression Cycle and Components"
- Perry, R.H. and Green, D.W. (1984). Perry's Chemical Engineers' Handbook (6th Edition ed.). McGraw Hill, Inc. ISBN ISBN 0-07-049479-7. (see pages 12-27 through 12-38)
- Refrigeration volume, ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA
- 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 ed.). Goodheart-Wilcox Publishing. ISBN 1-59070-280-8.
- Anderson, Oscar Edward (1972). Refrigeration in America: A history of a new technology and its impact. Kennikat Press. p. 344. ISBN 0-8046-1621-3.
- Shachtman, Tom (2000-12-12). Absolute Zero: And the Conquest of Cold. Mariner Books. p. 272. ISBN 0-618-08239-5.
- Woolrich, Willis Raymond (1967). The men who created cold: A history of refrigeration, (1st ed.). Exposition Press. p. 212.
|Wikiversity has learning materials about Underground refrigerated storage room|
- "The Refrigeration Cycle", from HowStuffWorks
- American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)
- International Institute of Refrigeration (IIR)
- "The Refrigeration Cycle: How Central Air Conditioner Works"
- British Institute of Refrigeration
- Refrigeration History
- Scroll down to "Continuous-Cycle Absorption System"
- US Department of Energy: Technology Basics of Absorption Cycles