Refrigeration

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 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 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. (see pages 12-27 through 12-38)

Vapour absorption cycle

:(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.

Unit of refrigeration

Domestic and commercial refrigerators may be rated in kJ/s, or Btu/hr 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 / lb (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 kilowattsNIST Guide To SI Units

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 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 percent 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

External links

* American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)
* British Institute of Refrigeration
*"Notes on vapor-compression refrigeration", Queens University (Canada)
*"The ideal vapor compression refrigeration cycle", University of Nevada (US)
*The Food Refrigeration and Process Engineering Research Centre
*How Do Absorption Systems Work?
*Scroll down to "Continuous-Cycle Absorption System"
*US Department of Energy: Technology Basics of Absorption Cycles
*"The Refrigeration Cycle", from HowStuffWorks
*Refrigeration World

Refrigeration: Encyclopedia BETA

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