雪櫃，冷氣機的原理～

Air conditioner
Refrigeration cycle


圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/7/7d/Heatpump.svg/300px-Heatpump.svg.png



圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
A diagram of the refrigeration cycle: 1) condensing coil, 2) expansion valve, 3) evaporator coil, 4) compressor.
In the refrigeration cycle, a heat pump transfers heat from a lower temperature heat source into a higher temperature heat sink. Heat would naturally flow in the opposite direction. This is the most common type of air conditioning. A refrigerator works in much the same way, as it pumps the heat out of the interior into the room in which it stands.
This cycle takes advantage of the universal gas law PV = nRT, where P is pressure, V is volume, R is the universal gas constant, T is temperature, and n is the number of moles of gas (1 mole = 6.022×1023 molecules).
The most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile the compressor is driven by a pulley on the engine's crankshaft, with both using electric motors for air circulation. Since evaporation occurs when heat is absorbed, and condensation occurs when heat is released, air conditioners are designed to use a compressor to cause pressure changes between two compartments, and actively pump a refrigerant around. A refrigerant is pumped into the cooled compartment (the evaporator coil), where the low pressure and low temperature cause the refrigerant to evaporate into a vapor, taking heat with it. In the other compartment (the condenser), the refrigerant vapour is compressed and forced through another heat exchange coil, condensing into a liquid, rejecting the heat previously absorbed from the cooled space.
How a refrigerator works


Main article: refrigeration
Refrigerators work by the use of heat pumps operating in a refrigeration cycle. An industrial refrigerator is simply a refrigerator used in an industrial setting, usually in a restaurant or supermarket. They may consist of either a cooling compartment only (a larger refrigerator) or a freezing compartment only (a freezer) or contain both. The industry has nicknames for these units as well sometimes referring to them as a “cold box” or a “walk-in.” The dual compartment was introduced commercially by General Electric in 1939.
The vapor compression cycle is used in most household refrigerators. In this cycle, a circulating refrigerant such as freon enters the compressor as a vapor at its boiling point. The vapor is compressed and exits the compressor as a superheated vapor. The superheated vapor travels through part of the condenser which removes the superheat by cooling the vapor. The vapor travels through the remainder of the condenser and is condensed into a liquid at its boiling point. The saturated liquid refrigerant passes through the expansion valve where its pressure abruptly decreases. The decrease in pressure results in the flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes). The cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator. There a fan circulates room air across the coil or tubes, and the refrigerant is totally vaporized, extracting heat from the air which is then returned to the food compartment. The refrigerant vapor returns to the compressor inlet to complete the thermodynamic cycle.
An absorption refrigerator works differently from a compressor refrigerator, using a source of heat, and typically runs more quietly.
The Peltier effect uses electricity directly to pump heat; refrigerators using this effect are sometimes used for camping, or where noise is not acceptable. They are totally silent, but less energy-efficient than other methods.
Other alternatives to the vapour-compression cycle but not in current use include thermionic, vortex tube, air cycle, magnetic cooling, Stirling cycle, Malone refrigeration, acoustic cooling, pulse tube and water cycle systems.

雪櫃
揮發性液體具有冷凍的效果，就像酒精在皮膚上揮發之後，皮膚會感到一陣清涼。美國人柏金斯利用這個原理，在一八三四年做了一個實驗，他在一個箱子裏插進很多金屬管，再在管子裏裝進揮發性的液體，又把一碟水放在箱內。結果，水結成冰，這就是最原始的雪櫃了。一八五五年，發明家哈理森設計出第一部工業用雪櫃。不過，起初用作製冷的媒體氨並不好，有刺激性的氣味，後來才改用氟氯碳化物取代。可是，這種化學物質會破壞地球的臭氧層，因此科學家又要研究一種更合用的製冷物質。

冰箱裏的壓縮機定時把熱氣運到冰箱外，散發給周圍的空氣，冰箱裏就能一直保持很低的溫度。


冷氣機
對付炎夏的高溫，最好是躲在清涼的冷氣間，我們之所以能享受這樣舒適的環境,全賴有「冷氣機之父」之稱的開利。一九零二年七月十七日，這個才從康乃爾大學畢業一年的年輕人，在「水牛公司」工作時，發明了冷氣機。冷氣機裏有雪種，雪種可以吸收熱量，變成熱氣體；也可以經由冷凝作用，放出熱量，變成冷液體。因此，藉由雪種的循環作用，就可以不斷地把室內的熱空氣排出室外，送出陣陣涼爽的冷空氣。

因為雪櫃或冷氣機裏面有個壓縮泵, 將雪種加壓, 雪種加壓後就會產生熱量, 當雪種熱量散去後, 然後減壓就會變冷, 原理就好似打邊爐時的罐裝gas咁, 會變冷