Refrigeration
This article is licensed under theGNU Free Documentation License.It uses material from the Wikipedia article "Refrigeration" (click for full Wikipedia text) 'Refrigeration ' is defined as 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.
Historical Applications
Ice harvesting
The use of ice to refrigerate and thus preserve food goes back to prehistoric times.
["Refrigeration fundamentals throughout history" ]["Air conditioning and refrigeration chronology" ] Through the ages, the seasonal harvesting of snow and ice was a regular practice of most of the ancient cultures: Chinese, Hebrews, Greeks, Romans. Ice and snow were stored in caves or dugouts lined with straw or other insulating materials. Rationing of the ice allowed the preservation of foods over the hot periods. This practice worked well down through the centuries. In the 16th century, the discovery of chemical refrigeration was one of the first steps toward artificial means of refrigeration. Sodium nitrate or potassium nitrate, when added to water, lowered the water temperature and created a sort of refrigeration bath for cooling substances. In France, cold drinks and liqueurs were produced by spinning long-necked bottles in water with dissolved saltpeter. During the first half of the 19th century, ice harvesting had become big business in America. New England Frederic Tudor, who became known as the "Ice King", worked on developing better Thermal 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 of vaporization 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 brilliant British scientist, Michael Faraday, liquified ammonia and other gases by using high pressures and low temperatures. An American living in Great Britain, Jacob Perkins, obtained the first patent for a vapor-compression refrigeration system in 1834. Perkins built a prototype system and it actually worked, although it did not succeed commercially.
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 gas 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.
["Improved process for the artificial production of ice", U.S. Patent Office, Patent 8080, 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. Soon after that, James Harrison, born in Scotland and subsequently emigrated to Australia, introduced commercial vapor-compression refrigeration to breweries and meat packing houses. By 1861, a dozen of his systems were in operation. The first Absorption refrigeration refrigeration system using gaseous ammonia dissolved in water (referred to as "aqua ammonia") was developed by Ferdinand Carré of France in 1859 and patented in 1860. Due to the toxicity of ammonia, such systems were not developed for use in homes, but were used to manufacture ice for sale. The consumer public at that time still used the ice box with ice brought in from commercial suppliers, many of whom were still harvesting ice and storing it in an Icehouse (building). 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. He established a cold storage business and then took it further. He and other investors purchased an old steamship onto which they loaded Lowe’s carbon dioxide dry ice refrigeration units and began shipping fresh meat from Texas to Louisiana in 1869. Then they 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, but it generated worldwide demand for refrigerated shipping.
Widespread commercial use
By the 1870’s 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 Carl von Linde set up a large-scale process for the production of liquid air and eventually liquid oxygen for use in safe household refrigerators. Refrigerated railroad cars were introduced in the 1840s for the short-run transportation of dairy products. In 1867 J.B. Sutherland of Detroit, Michigan patented the refrigerator car designed with ice tanks at either end of the car and ventilator flaps near the floor which would create a gravity draft of cold air through the car. By 1900 the meat packing houses of Chicago had adopted ammonia-cycle commercial refrigeration. By 1914 almost every location used artificial refrigeration. The big meat packers, Armour, Swift, and Wilson, had purchased the most expensive units which they installed on train cars and in branch houses and storage facilities in the more remote distribution areas. It was not until the middle of the 20th century that refrigeration units were designed for installation on tractor-trailer rigs (trucks). Refrigerated trucks are used to transport perishable goods, such as frozen foods, fruit and vegetables, and temperature-sensitive chemicals. Most modern refrigerators keep the temperature between -40 and +20 °C and have a maximum payload of around 24 000 kg. gross weight (in Europe)
Home and consumer use
With the invention of synthetic refrigerants like Haloalkane, safer refrigerators were possible for home and consumer use. Freon is referred to as a CFC (chlorofluorocarbon), halocarbon, or haloalkane. Developed in the late 1920’s, Freon is much less toxic than some of the refrigerants used earlier (i.e., ammonia, methyl chloride and sulfur dioxide) and Freon-12 has a boiling point of -22 °F (-30 °C). The intent was to provide refrigeration units for home use without the use of toxic refrigerants. At the same time the units needed to be made smaller which meant using refrigerants that could do more work with fewer parts in less space. A refrigerant like Freon answered that need. The Freon patents were initially held by the automotive industry who used it for auto air-conditioning, but the product was far too useful to limit to automotive use. By 1930 Freon was available on the open market. Currently, Freon is banned due to the negative effects it has on the ozone layer.
Current applications of refrigeration
Probably the most widely-used current applications of refrigeration are for the air-conditioning of private homes and public buildings, and the refrigeration of foodstuffs in homes, restaurants and large storage warehouses. The use of refrigerators in our kitchens for the storage of fruits and vegetables has allowed us to add fresh salads to our diets year round, and to store fish and meats safely for long periods. In commerce and manufacturing, there are many uses for refrigeration. Refrigeration is used to liquify gases like oxygen, nitrogen, propane and methane for example. In oil refineries, chemical manufacturing, 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 rating 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.
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 Sublimation (physics) dry ice. These methods are used for small-scale refrigeration such as in laboratories and workshops, or in portable coolers. Ice owes its effectiveness as a cooling agent to its constant melting point of 0 °C (32 °F). In order to melt, ice must absorb 333.1 kJ/kg (143.3 Btu/lb) of heat. Foodstuffs maintained at this temperature or slightly above have an increased storage life. Solid carbon dioxide, known as dry ice, is used also as a refrigerant. Having no liquid phase at normal atmospheric pressure, it sublimes directly from the solid to vapor phase at a temperature of -78.5 °C (-109.3 °F). Dry ice is effective for maintaining products at low temperatures during the period of sublimation.
Cyclic refrigeration
This consists of a refrigeration cycle and its inverse, the power cycle. In the power cycle, heat is supplied from a high-temperature source to the engine, part of the heat being used to produce work and the rest being rejected to a low-temperature sink. In the refrigeration cycle, heat is removed from a low-temperature space or source and rejected to a high-temperature sink with the help of external work. 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. Heat naturally flows from hot to cold. mechanical 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. Thermal 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 Nicolas Léonard Sadi Carnot in 1824 as a Carnot heat engine. The most common types of refrigeration systems use a cycle based on a phase change heat pump although absorption heat pumps are used in a large minority of applications. It is possible to build a refrigeration system which does not contain a refrigerant, and therefore does not operate a refrigeration cycle — the most common form being thermoelectric cooling used in some portable coolers. Cyclic refrigeration can be classified as: # Vapour cycle, and # Gas cycle Vapour cycle refrigeration can further be classified as: #Vapour compression refrigeration #Gas absorption refrigeration
Vapour-compression cycle
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(See Vapour-compression refrigeration for more complete technical details) The vapour-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems. Figure 1 provides a schematic diagram of the components of a typical vapour-compression refrigeration system. The thermodynamics of the cycle can be analyzed on a diagram
[The Ideal Vapor-Compression Cycle][Scroll down to "The Basic Vapor Compression Cycle and Components"] as shown in Figure 2. In this cycle, a circulating refrigerant such as Freon enters the gas compressor as a vapour. From point 1 to point 2, the vapour is compressed at constant entropy and exits the compressor superheating. From point 2 to point 3 and on to point 4, the superheated vapour travels through the condenser which first cools and removes the superheat and then condenses the vapour into a liquid by removing additional heat at constant pressure and temperature. Between points 4 and 5, the liquid refrigerant goes through the expansion valve (also called a throttle valve) where its pressure abruptly decreases, causing flash evaporation and auto-refrigeration of, typically, less than half of the liquid. That results in a mixture of liquid and vapour at a lower temperature and pressure as shown at point 5. The cold liquid-vapour mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air (from the space being refrigerated) being blown by a fan across the evaporator coil or tubes. The resulting refrigerant vapour returns to the compressor inlet at point 1 to complete the thermodynamic cycle. The above discussion is based on the ideal vapour-compression refrigeration cycle, and does not take into account real-world effects like frictional pressure drop in the system, slight thermodynamic reversibility during the compression of the refrigerant vapour, or ideal gas behavior (if any). More information about the design and performance of vapour-compression refrigeration systems is available in the classic "Perry's Chemical Engineers' Handbook".
[ (see pages 12-27 through 12-38)] Vapour absorption cycle
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(See gas absorption refrigerator for more details) In the early years of the twentieth century, the vapour absorption cycle using water-ammonia systems was popular and widely used but, after the development of the vapour compression cycle, it lost much of its importance because of its low coefficient of performance (about one fifth of that of the vapour compression cycle). Nowadays, the vapour absorption cycle is used only where waste heat is available or where heat is derived from solar collectors. The absorption cycle is similar to the compression cycle, except for the method of raising the pressure of the refrigerant vapour. In the absorption system, the compressor is replaced by an absorber which dissolves the refrigerant in a suitable liquid, a liquid pump which raises the pressure and a generator which, on heat addition, drives off the refrigerant vapour from the high-pressure liquid. Some work is required by the liquid pump but, for a given quantity of refrigerant, it is much smaller that that needed by the compressor in the vapour compression cycle. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) and water (absorbent), and water (refrigerant) and lithium bromide (absorbent).
Gas cycle
When the refrigerant is a gas, the refrigerator is said to be working on a gas cycle. As there is no condensation and evaporation in a gas cycle, components corresponding to condenser and evaporator in vapour compression cycle are the cooler and heater in gas cycles. The gas cycle is less efficient than the vapour compression cycle, because the gas cycle works on the Joule cycle, in which gas 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 side. Therefore, for the same load, a gas refrigeration cycle will require a large mass flow and would be bulky. Because of their lower efficiency and larger bulk, gases are not used nowadays as working fluids for refrigerators, except in the case of air conditioning where the air can be used both as a refrigerant and the conditioning medium.
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 paramagnetism salt, such as cerium magnesium nitrate. The active magnetic field 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.
Unit of refrigeration
Domestic and commercial refrigerators may be rated in kilojoule/s, or British thermal unit/h of cooling. Commercial refrigerators are mostly rated in tons of refrigeration. One ton of refrigeration capacity can freeze one short ton of water at 0 °C (32 °F) in 24 hours. Based on that: :Latent heat of ice (i.e., heat of fusion) ≈ 144 Btu / pound (mass) (or 334.5 kJ/kg) :One short ton = 2000 lb :Heat to be extracted = 2000
- 144 = 288000 Btu / 24 hours = 12000 Btu/hour = 200 Btu / Minute :1 ton refrigeration = 200 Btu / minute = 3.517 kJ/s = 3.517 kilowatts
[NIST Guide To SI Units] A much less common definition is: 1 tonne of refrigeration is the rate of heat removal required to freeze a tonne (i.e., 1000 kg) of water at 0 Celsius in 24 hours. Based on the heat of fusion being 334.5 kJ/kg, 1 tonne of refrigeration = 13,938 kJ/h = 3.872 kW. As can be seen, 1 tonne of refrigeration is 10% larger than 1 ton of refrigeration. Most residential air conditioning units range in capacity from about 1 to 5 tons of refrigeration.
References
- Mathur, M.L., Mehta, F.S., Thermal Engineering Vol II
- MSN Encarta Encyclopedia
See also
- Einstein refrigerator
- Freezer
- Heat pump
- HVAC (Heating, Ventilating and Air-conditioning)
- Icebox
- Icyball
- Pot-in-pot refrigerator
- Refrigerant
- Refrigerator
- Refrigerator car
- Refrigeration cycle
- SEER (Seasonal Energy Efficiency Ratio)
- Timeline of low-temperature technology
- Ice cream van
- Vapor-compression refrigeration
- Linde-Hampson liquefaction process
External links
Explain Refrigeration 
ARI CoolNet
KIRBY Refrigeration
Refrigeration Engineers
