REFRIGERATING ENGINEERING O1=02+E 3.01 The net energy input E,which is required to lift the heat O2 from the lower to the higher temperature,need not necessarily be mechanical work.For example in an absorption refrigeration system heat from a high temperature source is used to operate the cycle, and in an ejector(vacuum)refrigeration system kinetic energy is used. A2.Carnot cycle 3.02 Theoretically,among many other possibilities,we can make use of the reversible Carnot cycle.As Figure 3.02 depicts,the cycle is operated in the anti-clockwise direction between the temperatures T2 and T1.The working fluid executes four reversible processes in the cycle: T E a b Fig.3.02.The Carnot cycle in a temperature-entropy (T-s)diagram. a>b:isothermal expansion at T2 while heat 2 is transferred to the working fluid from the heat source b->c:isentropic compression from T2 to Ti without any heat transfer c->d:isothermal compression at Ti while heat O is transferred from the working fluid to the heat sink d>a:isentropic expansion from Ti to T2 without any heat transfer. The net work input supplied to the working fluid during the cycle is denoted E,and the first law of thermodynamics,Equation 3.01,gives E=O1-O2. A3.Principle of the vapor compression cycle 3.03 In actual applications of refrigeration,cycles operated with mechanical work are predominant.All vapor compression systems function in that way. The working fluid in a vapor compression system is called a refrigerant.In this introductory chapter the discussions are restricted to cycles operating with refrigerants 3:2