Refrigeration And Air Conditioning By Pl Ballaney Pdf ^HOT^ Free Download
Refrigeration And Air Conditioning By Pl Ballaney Pdf Free Download
brief history of refrigeration the first working refrigerators were based on ammonia (ammonium carbide) that was used since the early 18th century. the ammonia was compressed and then cooled to become a liquid. the liquid ammonia was then distributed into a large number of tubes where it passed over a hot surface. this process caused a net heat transfer from the tubes to the ammonia. thus, the tubes were cooled, and the ammonia became a gas.
refrigerants are chemicals that absorb and release heat in order to maintain a desired temperature. almost all of the refrigerants used in refrigeration systems today are non-ozone depletable chemicals. the most common refrigerant used in commercial refrigeration is r-12 (tetrafluoroethane). this refrigerant is environmentally safe, non-toxic and non-flammable. the refrigerant is also capable of being handled safely by most people. r-12 has been used in commercial refrigeration since the early 1930s.
safety and health concerns have become a major issue in the refrigeration industry. a major concern is the migration of r-12 from the refrigerant system into the surrounding atmosphere. the chemical properties of r-12 make it very difficult to contain in a refrigeration system. r-12 also has a very low boiling point of approximately -65 oc. any refrigeration system with r-12 must include an expansion valve. this is because the refrigerant gas is a superheated gas at the point of initial refrigeration and the expansion valve reduces the pressure of the refrigerant and refrigeration and allows for the refrigerant to change to a liquid at a lower temperature and pressure.
There are three refrigeration terms – namely, supercooling, ordinary subcooling and ordinary cooling. All three refrigeration processes are briefly discussed to begin with. In supercooling, heat is removed from the refrigerant in the vapor form and is condensed into liquid refrigerant after decreasing the temperature down to its saturated vapor pressure, i.e., when temperature of the evaporating refrigerant is 0oC.
Such cooling is the cheapest and most widely practiced kind of refrigeration. The application of supercooling is primarily in storage of foodstuffs and perishable items etc. which require cooling immediately after packing. When packaged foods are taken out of the refrigerator and transported to the place of consumption, the effect of supercooling may have to be withdrawn by the addition of water to make the food shelf cool enough to prevent spoilage.
Ordinary subcooling is achieved by circulating the heat exchange medium through the refrigerant before it is returned to the evaporator. This kind of refrigeration is considerably more expensive than supercooling because of the more complex system required to ensure that the cooling is achieved in the required efficient manner. At present, this type of refrigeration is limited mainly to use in special applications in which the additional cost is not a factor, such as in the domestic refrigerator.
Refrigeration based on ordinary subcooling of the refrigerant is known as ordinary refrigeration. In ordinary refrigeration, heat is removed from the refrigerant to make the temperature of the refrigerant lower than that of the saturated liquid at the given pressure. This is done by the phase change of the refrigerant from liquid to vapor, thereby lowering the temperature of the refrigerant and thus the temperature of the heat exchange medium. The reverse of this process is known as cooling.
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