Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Energy Vol. zyxwvutsrqponmlkjihgfed 19. No. 6, pp. 707-715, 1994 Copyright @ 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0360-5442/94 $7.00+ 0.00 THERMODYNAMICS OF A VAPOR-COMPRESSION REFRIGERATION CYCLE WITH MECHANICAL SUBCOOLING SYED M. ZUBAIR Mechanical Engineering Department, King Fahd University of Petroleum and Minerals # 1474, Dhahran 31261. Saudi Arabia (Received 30 March 1993; received for publication It November 1993) Abstrati-Refrigeration and air-conditioning systems, when operating under large tem- perature differences between the condenser and evaporator, consume significant amounts of energy. A vapor-compression refrigeration cycle with a mechanical subcooling loop to increase system performance and reduce energy consumption is investigated by using both the first and second laws of thermodynamics. Although the first-law (energy-balance) approach to system analysis shows improvement in the system coefficient of performance (COP) with an increase in the temperature difference between the condenser and evaporator, it fails to locate sources of losses. Identifying and quantifying these sources can be a useful design tool, especially in developing or investigating new, more complex refrigeration cycles. A second-law analysis (in terms of irreversibility) has been carried out for both the simple and the vapor-compression refrigeration cycle with a mechanical subcooling loop. It is found that the performance of the system can be significantly improved by reducing the irreversibilities due to the expansion process. The low- temperature refrigeration system, when operating at the optimum subcooler saturation temperature, may have the following features: (i) 85% reduction in power input; (ii) 65% percent lower irreversibility rate; (iii) 20% reduction in the total refrigerant flow-rate. INTRODUCTION In the vapor-compression refrigeration cycle, the work required in the compression process increases when the discharge pressure is increased or the suction pressure is decreased. These suction and discharge pressures correspond approximately to the evaporating and condensing temperatures of the refrigerant, respectively. The evaporating pressures and temperatures are dictated by the application and the associated heat exchanger. The condensing pressures and temperatures depend on the source of heat rejection. In an air-cooled condensing unit, particularly one used in hot and humid climates (e.g., Saudi Arabia’s coastal cities), where ambient temperatures typically range between 35 and 45”C, refrigeration and air-conditioning systems operate under high temperature differences between the evaporator and condenser. Therefore, the condensing temperatures range from 50 to 7o”C, and produce less than normal refrigerant effect per unit of power consumption. The decrease in cooling effect, under these operating conditions, may cause the compressor to operate for prolonged periods to meet the desired cooling demand. Zubair et al’ have noted that these extreme operating conditions can cause several system-related problems, thereby reducing the life expectancy of the system. The performance of these refrigeration and air-conditioning systems can be significantly improved by further cooling off the liquid refrigerant leaving the condenser coil. Bahel and Zubair* and Zubair3 have stated that an attractive way of subcooling the liquid refrigerant is to add a mechanical subcooling loop in a conventional vapor-compression cycle, as shown in Fig. 1. This add-on system can significantly improve the system performance when operating in locations where the difference between the condensing and evaporating temperatures is large. The majority of the work related to the add-on mechanical subcooling system is proprietary, and there is no systematic thermodynamic analysis of such a system available in the open literature. However, some scattered information on the subject is available in Refs. 4-6 707