THEORETICAL ANALYSIS ON THE PREDICTION OF PERFORMANCE COEFFICIENT OF TWO-STAGE CASCADE REFRIGERATION SYSTEM USING VARIOUS ALTERNATIVE REFRIGERANTS
This paper presents a comparison of the refrigerants CFC-12, HCFC-22, CFC-502, and their alternatives, such as HFC-134a, HFC-152a, HFC-404A, HFC-407C, HC-290, HC-600a, R717 (ammonia), and three mixtures composed of HFC-134a, HFC-152a, HC-600a, and HC-290. A theoretical performance study on a cascade refrigeration system was performed using two refrigeration cycles connected through a heat exchanger in the middle working as an evaporator for the high-pressure cycle and a condenser for the low-pressure cycle. The condensing temperatures varied between 40 and 60 °C, the evaporating temperatures varied between -50 and -10 C, and the heat exchanger temperature was kept constant at 1 °C. The refrigerants in both cycles are assumed to be same at first. The alternative refrigerants of HFC-152a and R717 had higher coefficients of performance (COPs) than other refrigerants and also low refrigerant charge rates for all operating conditions in the case study, according to the results of the theoretical analysis. The effects of the investigated refrigerants on the depletion of the ozone layer, increase in global warming, and flammable and toxic characteristics were considered and alternative refrigerant mixtures that may be suggested were determined. It was found that refrigerant blends of HC290/HC600a (55/45 by wt%), as a nonazeotropic mixture, and HFC-152a/HFC-134a (14/86 by wt%) and HFC-134a/HC600a (82/18 by wt%), as azeotropic mixtures, gave lower performance coefficients (COPs) and required lower refrigerant charge rates than their base pure refrigerants in the analysis. The separate usage of the refrigerants R717 and HFC-152a for the highand low-pressure sections of the cascade system, respectively, were found to be the best combination in terms of the analysis for the determination of coefficients of performance (COP) and refrigerant charge rates. The results of this study are of technological importance for efficient design when the industrial systems are assigned to utilize various alternative refrigerants.
Experimental investigation of R-134a and R-600a refrigerant blend in domestic vapour compression refrigeration system
The refrigeration industry today needs refrigerants with global warming potential (GWP) ≤ 150 according to the European Union protocol, alongside other properties, which no single refrigerant has met as at today. This study focused on blending of existing refrigerants towards obtaining blends with GWP ≤ 150. It investigated the performance of blends of two existing refrigerant, R-134a and R-600a, blended together at ratios of 11%/89%, 7.5%/92.5% and 3.6%/96.4% to form blends K, L and M respectively. The blends were to have, according to the blending ratios, estimated GWPs ≤ 150 using Ali’s model. The blends flammability was tested and the blends were then charged into domestic vapour compression systems, where their operating temperatures and pressures were obtained every 5 minutes during the experimentation. The flammability test shows they are flammable while results obtained at ambient temperatures (37oC, 32oC) under same operating conditions indicated evaporator temperatures (7.7 o C, -3.1 o C, -3.7 o C, -4.7 o C, -5.3 o C) ; (6.2 o C, -3 o C, -4 o C, -4.8 o C, -6.5 o C) for R-134a, K, L, M and R-600a respectively. Also the analysis of the results gave corresponding average Coefficient of Performance (COP) of (0.8389, 1.0708, 1.0898, 1.1181, and 1.1373) and (0.8283, 1.0923, 1.1254, 1.1579 and 1.2159) for the aforementioned respective refrigerants at ambient temperatures of 37 o C and 32 o C, respectively. Thus, the blends can replace R-134a without changing pipe, cooling method and lubricants and still obtain higher COP, though mildly flammable−having slow burning velocity and low heat of combustion. That blend K offers significant improvement in flammability compared to R-600a alongside higher COP to R-134a, makes it the best alternative of the blends. Keywords: Coefficient of Performance (COP), Vapour Compression Refrigeration System, Flammability, Global warming Potential (GWP), Refrigerant, Refrigerant blend
The Italian Renaissance: Culture and Society in Italy
In this newly revised edition of his widely acclaimed work, Peter Burke presents a social and cultural history of the Italian Renaissance. He discusses the social and political institutions that existed in Italy during the fifteenth and sixteenth centuries, and he analyzes the ways of thinking and seeing that characterized this period of extraordinary artistic creativity. Developing a distinctive approach, the author is concerned not only with the finished works of Michelangelo, Raphael, Leonardo da Vinci, and others, but also with the social background, patterns of recruitment, and means of subsistence of this "cultural elite." He thus makes a major contribution both to our understanding of the Italian Renaissance and to our comprehension of the complex relations between culture and society. An excellent social history of the lives and culture of the artists and artisans which made it possible for the arts to flourish.