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v2.1 PS 9 2.006S08 1 MASSACHUSETTS INSTITUTE OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING 2.006 THERMAL FLUIDS II Problem Set 9 DUE: Thursday, April 17, 2008 Problem 1 Condenser Evaporator Compresso W Q e Q c Throttle 1 2 3 4 Most small-scale refrigeration systems use the simple vapor-compression refrigeration cycle shown in the figure above. The use of this cycle is primarily driven by the low-cost and the reliability of machines that use this cycle. There are other refrigeration cycles but in the current economic environment, the first cost of the refrigerator drives the market to the use of the vapor compression cycle for small-scale refrigerators and air conditioners. Prior to 1991, most refrigerators and air conditioners used R-12 (Refrigerant-12, a.k.a. Dichlorodifluoromethane, CF 2 Cl 2 ) as the working fluid. When it was realized that releases of chlorine into the atmosphere (attributed to the venting into the atmosphere of R-12 and similar chlorine-based compounds during the repair and disposal of cooling equipment, the use as a foaming agent in foam insulation, etc.) were depleting the ozone layer, governments agreed to the Montreal Protocol that effectively banned the use of all chlorine-containing refrigerants. The reaction of the “cool” engineering community was to adopt fluorinated refrigerants as the refrigerants of choice. HFC-134a (1,1,1,2-Tetrafluoroethane, CH 2 FCF 3 ,) has become a popular choice for equipment used in the United States. Europe and Asia has also adopted the use of other types of refrigerants such as carbon dioxide and hydrocarbons like isobutane. American companies have generally shunned the use of hydrocarbons (despite the high efficiencies they offer) due to possible legal consequences attributable to the flammability of the working fluid. The issue of global warming has become an imperative in Europe and is becoming an imperative in the United States (although we are behind the Europeans in this cause). It has been recognized that refrigerants released into the atmosphere are very strong global warming agents, for example the release of 1 kg of HFC-134a into the atmosphere has the same effect as releasing ~1200 kg of carbon dioxide into the atmosphere from a global warming viewpoint. Similarly, other fluorinated refrigerants such as HFC-143a, HFC-125, HFC-124 and HFC-123 have Global Warming Potentials (GWP) of 2900, 2500, 430 and 85 kg- CO 2 , respectively. Because of these high global warming potentials, the European Union has been actively discussing banning
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This note was uploaded on 10/06/2011 for the course MECHANICAL 2.006 taught by Professor Blah during the Spring '08 term at MIT.

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