Thermodynamics

Very low temperatures can be achieved by operating two ormore vapor-compression systems in series, called cascading.The COP of a refrigeration system also increases as a resultof cascading. Another way of improving the performance of avapor-compression refrigeration system is by using multistagecompression with regenerative cooling. A refrigeratorwith a single compressor can provide refrigeration at severaltemperatures by throttling the refrigerant in stages. Thevapor-compression refrigeration cycle can also be used to liquefygases after some modifications.The power cycles can be used as refrigeration cycles bysimply reversing them. Of these, the reversed Brayton cycle,which is also known as the gas refrigeration cycle, is usedto cool aircraft and to obtain very low (cryogenic) temperaturesafter it is modified with regeneration. The work outputof the turbine can be used to reduce the work input requirementsto the compressor. Thus the COP of a gas refrigerationcycle isCOP absorption q Lw net,inq Lw comp,in w turb,outChapter 11 | 637Another form of refrigeration that becomes economicallyattractive when there is a source of inexpensive thermal energyat a temperature of 100 to 200°C is absorption refrigeration,where the refrigerant is absorbed by a transport medium andcompressed in liquid form. The most widely used absorptionrefrigeration system is the ammonia–water system, whereammonia serves as the refrigerant and water as the transportmedium. The work input to the pump is usually very small,and the COP of absorption refrigeration systems is defined asCOP absorption Desired outputRequired input Q L Q LQ gen W pump,in Q genThe maximum COP an absorption refrigeration system canhave is determined by assuming totally reversible conditions,which yieldsCOP rev,absorption h th,rev COP R,rev a1 T 0ba bT s T 0 T Lwhere T 0 , T L , and T s are the thermodynamic temperatures ofthe environment, the refrigerated space, and the heat source,respectively.T LREFERENCES AND SUGGESTED READINGS1. ASHRAE, Handbook of Fundamentals. Atlanta: AmericanSociety of Heating, Refrigerating, and Air-ConditioningEngineers, 1985.2. Heat Pump Systems—A Technology Review. OECDReport, Paris, 1982.3. B. Nagengast. “A Historical Look at CFC Refrigerants.”ASHRAE Journal 30, no. 11 (November 1988), pp. 37–39.4. W. F. Stoecker. “Growing Opportunities for AmmoniaRefrigeration.” Proceedings of the Meeting of theInternational Institute of Ammonia Refrigeration, Austin,Texas, 1989.5. W. F. Stoecker and J. W. Jones. Refrigeration and AirConditioning. 2nd ed. New York: McGraw-Hill, 1982.6. K. Wark and D. E. Richards. Thermodynamics. 6th ed.New York: McGraw-Hill, 1999.PROBLEMS*The Reversed Carnot Cycle11–1C Why is the reversed Carnot cycle executed within thesaturation dome not a realistic model for refrigeration cycles?*Problems designated by a “C” are concept questions, and studentsare encouraged to answer them all. Problems designated by an “E”are in English units, and the SI users can ignore them. Problemswith a CD-EES icon are solved using EES, and complete solutionstogether with parametric studies are included on the enclosed DVD.Problems with a computer-EES icon are comprehensive in nature,and are intended to be solved with a computer, preferably using theEES software that accompanies this text.11–2 A steady-flow Carnot refrigeration cycle uses refrigerant-134aas the working fluid. The refrigerant changes fromsaturated vapor to saturated liquid at 30°C in the condenseras it rejects heat. The evaporator pressure is 160 kPa. Showthe cycle on a T-s diagram relative to saturation lines, anddetermine (a) the coefficient of performance, (b) the amountof heat absorbed from the refrigerated space, and (c) the network input. Answers: (a) 5.64, (b) 147 kJ/kg, (c) 26.1 kJ/kg11–3E Refrigerant-134a enters the condenser of a steadyflowCarnot refrigerator as a saturated vapor at 90 psia, and itleaves with a quality of 0.05. The heat absorption from therefrigerated space takes place at a pressure of 30 psia. Show