CHAPTER11 - cen84959_ch11.qxd 4:48 PM Page 607 Chapter 11...

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Chapter 11 REFRIGERATION CYCLES | 607 A major application area of thermodynamics is refrigera- tion , which is the transfer of heat from a lower temper- ature region to a higher temperature one. Devices that produce refrigeration are called refrigerators, and the cycles on which they operate are called refrigeration cycles . The most frequently used refrigeration cycle is the vapor-compression refrigeration cycle in which the refrigerant is vaporized and condensed alternately and is compressed in the vapor phase. Another well-known refrigeration cycle is the gas refrigeration cycle in which the refrigerant remains in the gaseous phase throughout. Other refrigeration cycles discussed in this chapter are cascade refrigeration , where more than one refrigeration cycle is used; absorption refrigeration , where the refrigerant is dissolved in a liquid before it is compressed; and, as a Topic of Special Interest, thermoelectric refrigeration, where refrigera- tion is produced by the passage of electric current through two dissimilar materials. Objectives The objectives of Chapter 11 are to: Introduce the concepts of refrigerators and heat pumps and the measure of their performance. Analyze the ideal vapor-compression refrigeration cycle. Analyze the actual vapor-compression refrigeration cycle. Review the factors involved in selecting the right refrigerant for an application. Discuss the operation of refrigeration and heat pump systems. Evaluate the performance of innovative vapor-compression refrigeration systems. Analyze gas refrigeration systems. Introduce the concepts of absorption-refrigeration systems. Review the concepts of thermoelectric power generation and refrigeration.
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11–1 REFRIGERATORS AND HEAT PUMPS We all know from experience that heat flows in the direction of decreasing temperature, that is, from high-temperature regions to low-temperature ones. This heat-transfer process occurs in nature without requiring any devices. The reverse process, however, cannot occur by itself. The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators. Refrigerators are cyclic devices, and the working fluids used in the refrig- eration cycles are called refrigerants. A refrigerator is shown schematically in Fig. 11–1 a . Here Q L is the magnitude of the heat removed from the refrig- erated space at temperature T L ,Q H is the magnitude of the heat rejected to the warm space at temperature T H , and W net,in is the net work input to the refrigerator. As discussed in Chap. 6, Q L and Q H represent magnitudes and thus are positive quantities. Another device that transfers heat from a low-temperature medium to a high-temperature one is the heat pump. Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only. The objec- tive of a refrigerator is to maintain the refrigerated space at a low tempera- ture by removing heat from it. Discharging this heat to a higher-temperature medium is merely a necessary part of the operation, not the purpose. The
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  • Spring '09
  • Y
  • Heat Pump, COP, Vapor-Compression Refrigeration Cycle, refrigeration cycle

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