COPs OF R718 IN COMPARISION WITH OTHER MODERN REFRIGERANTS

Water as a refrigerant (R718) is compared with other modern refrigerants (R717, R290 R134a, R12, R22, and R152a) regarding refrigeration capacity and COP (coefficient of performance). A computer program simulating a theoretical vapor compression refrigeration cycle was developed to calculate COPs, compression ratios, and discharge temperatures of the refrigerants from the compressor. The effects of temperature lift, which is the temperature difference between condenser and evaporator, and polytropic efficiency are also investigated. It is shown that for evaporator temperatures above 20 ̊C and small temperature lifts (5K), R718 gives the highest COP, assuming exactly the same cycle parameters. For medium temperature lifts (20...25K), this evaporator temperature is above 35 ̊C, whereas for even greater temperature lifts it decreases again. Furthermore this evaporator temperature at which R718 gives a greater COP than the other refrigerants decreases with increasing values of polytropic efficiency INTRODUCTION Water as a refrigerant is one of the oldest refrigerants being used for refrigeration applications down to freezing because of its easy availability and excellent thermodynamics and chemical properties. Beside these advantages, there are technical challenges that result from its high specific volume at low temperatures, necessary high pressure ratios across the compressor, and resulting high compressor outlet temperature. These challenges have been overcome by designing and manufacturing special compressors for water vapor compression applications. Water vapor compression applications have been classified according to the compressor type used in the refrigeration cycle, which include single and multistage centrifugal, multistage axial, Roots, liquid ring, cycloid, and jet/ejector compressors Wight et al. [1]. Most of the studies in the literature have been based on the applications in which centrifugal compressors have been used Madsboll et al.[2]; Elovic and Holmes [3]; Madsboll and Minds [4]; Albring [5]; Albring and Heinrich [6]; Koren and Ophir [7]; Albring and Heinrich [8]; Müller [9]. Madsboll et al.[2] have presented the applications based on the production of vacuum ice and water chiller where the swept volume is very large (50 m/s 360 m/s) in which centrifugal compressors are used to meet the requirements. A centrifugal compressor having very large volume flow rate and high pressure ratios with very thin and light radial blades has been designed and developed to produce ice slurries Elovic and Holmes [3]. Madsboll and Minds [4] have designed a new cooling system in which a centrifugal compressor is used to compress water vapor addressing environmental concerns, especially the Greenhouse effect. They also have described the results of computer simulation comparing water with other refrigerants. Centrifugal compressors having a suction volume 0.5 m/s and 5 m/s have been used in a refrigeration system that was designed for demonstration and experimental purposes Albring [5]. Centrifugal compressors have also been studied in a twostage heat pump application and were also classified with respect to the pressure ratio across the compressor Albring and Heinrich [6] Koren and Ophir [7] have given information about the application of water vapor technology for commercial applications such as ice machines and chillers in which centrifugal compressors have been used to compress the water vapor. Design specifications for the centrifugal compressors that are employed in water chiller application have been presented Albring and Heinrich [8] A design algorithm for an advanced centrifugal compressor has also been created by Müller [9]. There have been a few studies about axial compressors and their applications Paul [10-11]. Claiming advantages over centrifugal compressors like smaller size, insensitivity to liquid droplet erosion and the possibility of building several stages on a single shaft, axial compressors have been projected for vapor compression refrigeration applications ranging from 150 kW to 3000 kW Paul [10]. An example study of applying a six-stage axial compressor has been presented by Paul [11]. CMES-04 Proceedings of the First Cappadocia International Mechanical Engineering