refrigeration

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 conditioningair conditioning,
mechanical process for controlling the humidity, temperature, cleanliness, and circulation of air in buildings and rooms. Indoor air is conditioned and regulated to maintain the temperature-humidity ratio that is most comfortable and healthful.
..... Click the link for more information. .

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 chlorofluorocarbonschlorofluorocarbons
(CFCs), organic compounds that contain carbon, chlorine, and fluorine atoms. CFCs are highly effective refrigerants that were developed in response to the pressing need to eliminate toxic and flammable substances, such as sulfur dioxide and ammonia, in
..... Click the link for more information. (CFCs). However, because of increasing scientific evidence that the CFCs are harmful to the ozone layerozone layer
or ozonosphere,
region of the stratosphere containing relatively high concentrations of ozone, located at altitudes of 12–30 mi (19–48 km) above the earth's surface.
..... Click the link for more information. of the stratosphere, they were banned by international treaty, the Montreal ProtocolMontreal Protocol,
officially the Protocol on Substances That Deplete the Ozone Layer, treaty signed on Sept. 16, 1987, at Montreal by 25 nations; 197 nations are now parties to the accord.
..... Click the link for more information. , 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, but because HFCs are greenhouse gases, reductions in their use also have been agreed to.

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&degF; (0&degC;) 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 thermoelectricitythermoelectricity,
direct conversion of heat into electric energy, or vice versa. The term is generally restricted to the irreversible conversion of electricity into heat described by the English physicist James P.
..... Click the link for more information. ) 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).

Refrigeration

the cooling of some medium or object to a temperature below the environmental temperature by removing a certain amount of heat from the medium or object. In industry and engineering, refrigeration is achieved mainly by means of refrigerating machines or freezing mixtures. (See.)

refrigeration

[ri‚frij·ə′rā·shən] (mechanical engineering) The cooling of a space or substance below the environmental temperature.

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 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 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 Air conditioning, Automotive climate control, Cooling tower

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.

refrigeration

hypothermia

[hi″po-ther´me-ah] low body temperature; it may be symptomatic of a disease or disorder of the temperature-regulating mechanism of the body, may be due to exposure to cold, or may be induced for certain surgical procedures or as a therapeutic measure. Hypothermia is a nursing diagnosis accepted by the North American Nursing Diagnosis Association, defined as body temperature reduced below the normal range for an individual but not below 35.6°C rectally (36.4°C rectally for the newborn). adj., adj hypother´mal, hypother´�mic.
Emergency treatment for hypothermia includes administration of warm intravenous fluid and use of esophageal rewarming tubes and special rewarming blankets. Resuscitation efforts such as cardiopulmonary resuscitation should continue until the patient is warmed to a normal core temperature; if there is no other change, the patient may be pronounced dead.accidental hypothermia unintended lowering of body heat due to prolonged exposure to cold. Hypothermia is a chilling of the entire body, but the extremities can withstand temperatures as much as 10 to 15°C (20 to 30°F) lower than the torso, where vital organs are located. When the core temperature drops even a few degrees, physiologic changes can lead to fatal cardiac arrhythmias and respiratory failure. Persons most at risk for accidental hypothermia include the very young, the very thin, the very old, the mentally challenged or emotionally unstable, alcohol and drug abusers, and the homeless. Symptoms range from mild shivering and complaints of feeling chilled to loss of consciousness, absence of reflexes, and barely detectable pulse and respirations.Prevention and Treatment. Accidental hypothermia can be avoided by eating high-energy foods, exercising when in the cold, wearing layers of clothing, and covering the head. From one half to two thirds of the body heat is lost through the head. For persons on a fixed or limited income, suggestions for avoiding hypothermia in a cold home must be realistic. Blankets and quilted covers that snap together to form a snug bag are alternatives to turning up the thermostat. A loose knitted cap worn day and night can help reduce loss of body heat. Persons who live alone may need help in finding another individual or agency that can check on them daily when the outside temperatures are very low.
The diagnosis of hypothermia may be missed if a clinical thermometer such as the kind used to measure fever is employed to determine the core temperature of a potential hypothermia victim. These thermometers rarely register temperatures below 34.5°C (94.1°F), while the patient's actual temperature can be as low as 30°C (86°F). Emergency departments should be equipped with special monitoring equipment that gives a true picture of the body temperature.
Once hypothermia is diagnosed, rewarming is indicated. Outside a medical facility the rewarming should be gradual so as to avoid respiratory and cardiac problems associated with rapidly sending cold blood back to the heart. The torso is warmed first by wrapping it in warm blankets or submersion in a tepid bath. Once the core temperature reaches 35°C (95°F), the extremities are warmed.environmental hypothermia accidental hypothermia due to heat loss due to a combination of convection, conduction, and radiation to the surrounding ambient air.induced hypothermia deliberate reduction of the temperature of all or part of the body; sometimes used as an adjunct to anesthesia in surgical procedures involving a limb, and as a protective measure in cardiac and neurologic surgery. The hypothermia may be continued only for the duration of the operation or it may be prolonged for as long as 5 days, depending on the reason for its use. See also treatment" >hypothermia treatment.Local Hypothermia. This is a type of anesthesia" >refrigeration anesthesia restricted to a part of the body, such as a limb. It usually is used to produce surgical anesthesia immediately before amputation. Advantages include minimal risk of shock, lowering of cell metabolism, and elimination of the need for inhalation anesthesia in patients who are poor surgical risks. The part to be anesthetized is packed in ice or wrapped in a special refrigeration unit consisting of coiled tubes. Tourniquets are applied to the limb to inhibit circulation and avoid general chilling of the patient. The limb is chilled for 3 to 5 hours before amputation.General Hypothermia. Generalized lowering of the body temperature decreases the metabolism of tissues and thereby the need for oxygen; it is used in various surgical procedures, especially on the heart. The core temperature is maintained between 26°C and 32°C (78.8°F and 89.6°F).
To induce general hypothermia, the patient is wrapped in a cooling blanket containing coils through which cold water or an antifreeze, or both, are circulated. The fastest method for achieving hypothermia is extracorporeal cooling of the blood; the patient's blood is removed through a cannula inserted in a large vessel, circulated through refrigerated coils and returned via another cannulated vessel.
Rewarming of the patient is accomplished simply by removing cooling blankets and allowing the temperature to rise gradually and naturally. In most cases regular blankets are used to maintain body warmth. External heat in the form of hot water bottles or warm tub baths, if used at all, must be applied with extreme caution to avoid burning the patient.Patient Care. During hypothermia and the rewarming process the patient's temperature, pulse, respiration, and blood pressure must be checked frequently. Special electronic thermometers are often used so that the body temperature can be monitored at all times. In prolonged hypothermia, cardiac irregularities or respiratory difficulties may develop quickly; the patient must be watched constantly for changes in the vital signs, and any changes must be reported immediately. The skin also should be observed for signs of developing pressure ulcers, edema, or marked discoloration.
The patient should be turned at least every 2 hours, with special attention to proper positioning and good body alignment. Decreased secretion of saliva and mouth-breathing demand frequent mouth care. The eyes may need to be irrigated frequently and covered with compresses moistened with physiologic saline solution or artificial tears if the corneal reflex is diminished and eye secretions are reduced.
Intake and output are measured and recorded. An indwelling catheter is inserted prior to induction of hypothermia and is left in place until normal body temperature is established. This is necessary because urinary output is diminished during hypothermia. Fluids are given intravenously and the oral intake of food and liquids is prohibited because of depression of the gag reflex.
Shivering during prolonged hypothermia must be avoided as it tends to elevate the body temperature and increase metabolic needs, thereby defeating the purpose of hypothermia.
During the rewarming process the patient must be observed for signs of increased tendency to bleed and of gastric distention; these are common complications. After the body temperature returns to normal and becomes stabilized, the patient is allowed to progress to a normal diet and physical activities.moderate hypothermia body temperature of 23° to 32°C, resulting from surface cooling.profound hypothermia body temperature of 12° to 20°C.regional hypothermia temperature reduction in a limb or organ resulting from application of external cold or perfusion with a cold solution.symptomatic hypothermia pathologic reduction of body temperature as a result of decreased heat production or increased heat loss. Hypothyroidism, severe blood loss with circulatory failure, and damage to the heat-producing cells of the hypothalamus can lead to decreased heat production. Prolonged exposure to cold, overdosage of antipyretic drugs, such as aspirin, and profuse sweating (diaphoresis) are some causes of increased heat loss and resultant hypothermia.

re·frig·er·a·tion

(rē-frij'ĕr-ā'shŭn), The act of cooling or reducing fever. [L. refrigeratio (see refrigerant)]

re·frig·er·a·tion

(rĕ-frij'ĕr-ā'shŭn) The act of cooling or reducing fever. [L. re-frigero, pp. -atus, pr. p. -ans, to make cold, fr. frigus (frigor-), cold]

refrigeration

re·frig·er·ate

refrigerator

refrigeration

refrigeration,

Early Methods of Refrigeration

Mechanical Refrigeration Systems

Preparation of Frozen Foods

Bibliography

Refrigeration

refrigeration

Refrigeration

Vapor-compression cycle

Absorption cycle

Steam-jet cycle

Air cycle

Refrigerants

refrigeration

refrigeration

hypothermia

re·frig·er·a·tion

re·frig·er·a·tion

refrigeration

Synonyms for refrigeration

noun the process of cooling or freezing (e

Synonyms

Related Words

noun deliberately lowering the body's temperature for therapeutic purposes

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