Friday, 9 April 2010
Thursday, 14 January 2010
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 that 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 while the ambient temperature was 65°F. 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. Franklin concluded, "From this experiment, one may see the possibility of freezing a man to death on a warm summer's day".[4]
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.[5]
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 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.[6] Gorrie built a working prototype, but his system was a commercial failure.
Wednesday, 13 January 2010
process of lowering the temperature and maintaining it in a given space for the purpose of chilling foods, preserving certain substances, or providing an atmosphere conducive to bodily comfort. Storing perishable foods, furs, pharmaceuticals, or other items under refrigeration is commonly known as cold storage. Such refrigeration checks both bacterial growth and adverse chemical reactions that occur in the normal atmosphere.
The use of natural or manufactured ice for refrigeration was widespread until shortly before World War I, when mechanical or electric refrigerators became available. Ice owes its effectiveness as a cooling agent to the fact that it has a constant fusion temperature of 0° C (32° F). In order to melt, ice must absorb heat amounting to 333.1 kJ/kg (143.3 Btu/lb). Melting ice in the presence of a dissolving salt lowers its melting point by several degrees. Foodstuffs maintained at this temperature or slightly above have an increased storage life. Solid 
In mechanical refrigeration, constant cooling is achieved by the circulation of a refrigerant in a closed system, in which it evaporates to a gas and then condenses back again to a liquid in a continuous cycle. If no leakage occurs, the refrigerant lasts indefinitely throughout the entire life of the system. All that is required to maintain cooling is a constant supply of energy, or power, and a method of dissipating waste heat. The two main types of mechanical refrigeration systems used are the compression system, used in domestic units for large cold-storage applications and for most air conditioning; and the absorption system, now employed largely for heat-operated air-conditioning units but formerly also used for heat-operated domestic units.
Compression Systems.
Compression systems employ four elements in the refrigeration cycle: compressor, condenser, expansion valve, and evaporator. In the evaporator the refrigerant is vaporized and heat is absorbed from the material contents or the space being cooled. The vapor next is drawn into a motor-driven compressor and elevated to high
Refrigerants.
For every refrigerant there is a specific boiling, or vaporization, temperature associated with each pressure, so that it is only necessary to control the pressure in the evaporator to obtain a desired temperature. A similar pressure-temperature relationship holds in the condenser. One of the most widely used refrigerants for many years has been dichlorodifluoromethane, known popularly as Refrigerant-12. This synthetic chlorofluorocarbon (CFC) when used as a refrigerant would, for example, vaporize at –6.7° C (20° F) in its evaporator under a pressure of 246.2 kPa (35.7 psi), and after compression to 909.2 kPa (131.9 psi) would condense at 37.8° C (100° F) in the condenser. The resulting condensed liquid would then enter the expansion valve to drop to evaporator pressure and repeat the cycle of absorbing heat at low temperature and low pressure and dissipating heat at the much higher condenser pressure and temperature. In small domestic refrigerators used for food storage, the condenser heat is dissipated into the kitchen or other room housing the refrigerator. With air-conditioning units the condenser heat must be dissipated out of doors or directly into cooling water.
In a domestic refrigeration system the evaporator, called the freezer, is placed in an insulated space. Sometimes this space constitutes the whole refrigerator cabinet. The compressor is often oversized, so that if it ran continuously it would produce progressively lower temperatures. To maintain the interior of the box within the desired temperature range, the motor driving the compressor is controlled by a thermostatic switch.
A frozen-food refrigerator resembles the household refrigerator except that its compressor and motor must be of sufficient size to handle the larger gas volume of the refrigerant at its lower evaporator pressure. For example, to maintain a temperature of –23.3° C (–10° F) an evaporator pressure of 132.3 kPa (19.2 psi) is required with Refrigerant-12.
Absorption System.
A few household units, called gas refrigerators, operate on the absorption principle. In such gas refrigerators a strong solution of ammonia in water is heated by a gas flame in a container called a generator, and the ammonia is driven off as a vapor, which passes into a condenser. Changed to a liquid state in the condenser, the ammonia flows to the evaporator as in the compression system. Instead of the gas being inducted into a compressor on exit from the evaporator, however, the ammonia gas is reabsorbed in the partially cooled, weak solution returning from the generator, to form the strong ammonia solution. This process of reabsorption occurs in a container called the absorber, from which the enriched liquid flows back to the generator to complete the cycle.
Increasing use of absorption refrigeration now occurs in refrigeration units for comfort space cooling, for which purpose refrigerant temperatures of 45° to 50° F (7.2° to 10° C) are suitable. In this temperature range, water can be used as a refrigerant with an aqueous salt solution, usually lithium bromide, as the absorbent material. The very cold boiling water from the evaporator is absorbed in concentrated salt solution. This solution is then pumped into the generator, where, at elevated temperature, the surplus water is boiled off to increase the salt concentration of the solution; this solution, after cooling, recirculates back to the absorber to complete the cycle. The system operates at high vacuum at an evaporator pressure of about 1.0 kPa (0.145 psi); the generator and condenser operate at about 10.0 kPa (1.45 psi). The units are usually direct-fired or use steam generated in a boiler.
Refrigerants and the Environment.
Refrigerant-12 and related CFCs, Refrigerant-11 and Refrigerant-22, are currently the major compounds used in the cooling and insulation systems of home refrigeration units. It has been found, however, that CFCs pose a major threat to the global environment through their role in the destruction of the
See also
For further information on this topic, see the Bibliography, section
Thursday, 17 December 2009
how is refrigerator work
There are two things that need to be known for refrigeration.
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- A gas cools on expansion.
- When you have two things that are different temperatures that touch or are near each other, the hotter surface cools and the colder surface warms up. This is a law of physics called the Second Law of Thermodynamics.
Wednesday, 9 December 2009
bibliography
Thursday, 3 December 2009
No Refrigeration? No Problem!
• Well, sometimes it isn't so much a choice, like when your reefer dies and you're in the middle of paradise - very warm paradise. You may suddenly find you need to survive without the fridge for several weeks while waiting for parts or waiting for someone who can service your system. Don't let lack of refrigeration ruin your cruise.
• Maybe you happily cruise with a well-running fridge but you've realized that you can fit only so much food in your refrigerator - and it isn't enough to last you between your provisioning stops. You need ideas that will extend your provisions for several weeks or months without refrigeration.
• Or perhaps you've chosen to live without refrigeration because of the time and money involved in having a system aboard. To have refrigeration you will need:
• money for a good refrigeration system
• money and time to build and insulate a box or at the very least reinsulate your existing box.
• money for an energy source to run the refrigeration
• money or the expertise to keep it running.
I believe that you don't need to have it ALL before you go cruising. Not everyone will be willing to give up the reefer but if you're on a tight budget and believe - as I do - that it's much better to go cruising NOW than wait until that elusive day when you can afford to have it all - then you might be one of many cruisers who say, "No refrigeration? No problem."
Here are some tips for living without refrigeration.
Wednesday, 2 December 2009
Storage Times For Refrigerated Foods
| Storage Times For Refrigerated Foods | |
|---|---|
| Eggs | |
| Fresh, in shell | 3-5 weeks |
| Raw yolks, whites | 2-4 days |
| Hard-cooked | 1 week |
| Liquid pasteurized eggs, egg substitutes | Unopened, 10 days Opened, 3 days |
| Cooked egg dishes | 3-4 days |
| Mayonnaise, commercial, opened | 2 months |
| Deli and Vacuum-Packed Products | |
| Store-prepared (or homemade) egg, chicken, tuna, ham, and macaroni salads | 3-5 days |
| Pre-stuffed pork, lamb chops, and chicken breasts | 1 day |
| Store-cooked dinners and entrees | 3-4 days |
| Commercial brand vacuum-packed dinners with/USDA seal, unopened | 2 weeks |
| Raw Hamburger, Ground and Stew Meat | |
| Ground beef, turkey, veal, pork, lamb | 1-2 days |
| Stew meats | 1-2 days |
| Ham, Corned Beef | |
| Ham, canned, labeled "Keep Refrigerated" | Unopened, 6-9 months Opened, 3-5 days |
| Ham, fully cooked, whole | 7 days |
| Ham, fully cooked, half | 3-5 days |
| Ham, fully cooked, slices | 3-4 days |
| Corned beef in pouch with pickling juices | 5-7 days |
| Hot Dogs and Luncheon Meats | |
| Hot dogs | Unopened package, 2 weeks Opened package, 1 week |
| Luncheon meats | Unopened package, 2 weeks Opened package, 3-5 days |
| Bacon and Sausage | |
| Bacon | 7 days |
| Sausage, raw from meat or poultry | 1-2 days |
| Smoked breakfast links, patties | 7 days |
| Summer sausage labeled "Keep Refrigerated" | Unopened, 3 months Opened, 3 weeks |
| Hard sausage (such as Pepperoni) | 2-3 weeks |
| Cooked Meat, Poultry, and Fish Leftovers | |
| Pieces and cooked casseroles | 3-4 days |
| Gravy and broth, patties, and nuggets | 1-2 days |
| Soups and Stews | 3-4 days |
| Fresh Meat (Beef, Veal, Lamb, and Pork) | |
| Steaks, chops, roasts | 3-5 days |
| Variety meats (Tongue, kidneys, liver, heart, chitterlings) | 1-2 days |
| Fresh Poultry | |
| Chicken or turkey, whole | 1-2 days |
| Chicken or turkey, parts | 1-2 days |
| Giblets | 1-2 days |
| Fresh Fish and Shellfish | |
| Fresh Fish and Shellfish | 1-2 days |