Problem 4 (25 points) A Modified Vapor-Compression Cycle A variation of the simple vapor-compression cycle is designed to reduce the compressor work by preheating the refrigerant before it enters the compressor. In the system shown on the next page, a heat exchanger is used to superheat the refrigerant before it enters the compressor. Since the energy to superheat the refrigerant entering the compressor comes from the refrigerant exiting the condenser, this heat exchange process also subcools the refrigerant before it enters the expansion valve. Consider a system like this using R-134a as the refrigerant and operating between 240 kPa and 1000 kPa. The refrigerant leaves the evaporator and condenser as saturated vapor and saturated liquid, respectively, and the compressor is operating isentropically. You may also assume that there are no pressure drops in the condenser, evaporator, or the lines connecting any of the devices. a) Draw this cycle on a T - s diagram. You should have 6 states on your diagram, corresponding to the 6 states on the figure. b) The COP of a refrigeration system can be written as COP = For this cycle, between W in which two states is there Q, and between which two states is there W? in c) Determine the COP of this system when the refrigerant entering the throttling valve is subcooled by 3.48C. d) Consider an adiabatic compressor with the same outlet state and inlet pressure as the compressor used for your analysis in part (c), but for which the inlet refrigerant is a saturated vapor. Determine i. the isentropic efficiency of this compressor, and ii. the ratio of specific work required by this compressor compared to the specific work required by the isentropic compressor of part (a). Compressor www Evaporator Condenser