Refrigeration

Refrigeration is a heat removal process in which the surrounding temperature is reduced in order to preserve food, or in medical treatment it is the lowering of a body’s temperature for therapeutic purposes. Mechanical work is usually the main way of transporting, but it can also be driven by heat, magnetism, electricity, laser, or other means. Refrigeration has many applications, including 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.

Historical applications

Ice harvesting

Using ice to refrigerate in order to preserve food, it’s part of the history. In old times, most of the ancient cultures such as Romans, Greeks, Chinese used to harvest snow and ice in the peak season and then, store it in caves or dugouts lined with straw or other insulating materials. Persians named the dugout  “yakhchal”. Their main purpose was the preservation of foods over the warm periods. This practice worked well through the centuries, proved itself efficient.

Chemical refrigeration was first discovered in the 16th century and and it was an important step 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.
In the 1st half of the 19th century, ice harvesting became big business in America. Known as the “Ice King”, Englander Frederic Tudor 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.

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.

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.

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). His system compressed 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.

Widespread commercial use

Even since 1870s, a major customer of the commercial refrigeration units have been the breweries, although some of them still relied on harvested ice. Though the ice-harvesting industry has proved to be very successful at the end of the 20th century, pollution and sewage had begun to creep into natural ice, making it a problem in the metropolitan suburbs. Lately, the breweries began to complain of the contaminated ice. This raised the demand for more modern and consumer-ready refrigeration and ice-making machines.

Carl von Linde, an engineering professor at the Technological University Munich in Germany, patented an improved method of liquefying gases in 1876. The process that he’s been working on , 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 the 1840s, refrigerated railroad cars were introduced in the US for short-run transport of dairy products. In 1867, J.B. Sutherland of Detroit, Michigan, patented the refrigerator car designed with ice tanks. The same year, this time in San Antonio, Texas, Andrew Muhl, a french immigrant , built an ice-making machine, to help beef industry expand before moving it to Waco in 1871. The patent for this machine was contracted by the Columbus Iron Works in 1873, a company acquired by the W. C. Bradley Co., which went on to produce the world’s first commercial ice-makers.

By 1914, almost every location where meat packing took place used artificial refrigeration. The big meat packers, Armour, Swift, and Wilson, had purchased the most expensive units which they installed on train cars and storage facilities in the more remote distribution areas.

Until the middle of the 20th century that specific refrigeration units weren’t designed to be installed on trucks. Now, these 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

Once the synthetic refrigerants (based mostly on a chlorofluorocarbon (CFC) chemical) were invented, 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 such as ammonia, sulfur dioxide etc. The purpose was to create refrigeration equipment for home use without danger. As this being asked, the CFC refrigerants looked like the answer to the question. In the 1970s, the compounds proved 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.

Methods of refrigeration

Non-cyclic refrigeration

In non-cyclic refrigeration, cooling is accomplished by melting ice or by subliming dry ice.  Such methods are only used for small-scale refrigeration (eg.: in laboratories, 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 efficient for maintaining products at low temperatures during sublimation.

Cyclic refrigeration

In the cyclic process of refrigeration the heat is removed from the low temperature reservoir and is thrown to high temperature reservoir. 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 means 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 HVAC work (Heating, Ventilation, and Air-Conditioning).

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. There are two types of cyclic process of refrigeration: vapor cycle and gas cycle. The vapor cycle is classified into vapor compression cycle and vapor absorption cycle.

Unit of refrigeration

When measuring the capacity of refrigeration, we talk about units of power. If household and commercial refrigerators may be rated in kJ/s, regarding the industrial refrigeration systems, most of the world uses the kilowatt (kW) as the basic unit. Usually, commercial and industrial refrigeration systems in North America are rated in tons of refrigeration (TR- the energy removal rate that will freeze one short ton of water at 0 °C (32 °F) in one day). 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).

An important factor in determining a system’s overall efficiency is the coefficient of performance (CoP), representing the capacity in kW divided by the energy input in kW and it’s not for industrial refrigeration in North America. The owners and manufacturers of these systems typically use performance factor (PF), which is a system’s energy expressed 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.

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