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 Low Global Warming Potential Refrigerant Blends in Domestic Vapor Compression Refrigeration System.
The refrigeration industry today needs refrigerants with global warming potential (GWP) ≤ 150 according to the European Union protocol, alongside other guidelines, which no single refrigerant has met as at today. This study focus on blending of existing refrigerants towards obtaining blends with GWP ≤ 150. This paper 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 vapor compression systems, where there 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 (37 o C, 32 o C) 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 gave corresponding average Coefficient of Performance (COP) of 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 an ambient temperature of 32 o C and 37 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. That blend K offers significant improvement in flammability compared to R-600a alongside higher COP to R-134a, making it the best alternative of the blends.
Thermal stability of reconstituted collagen fibrils. Shrinkage characteristics upon in vitro maturation
Thermal stability measured as area shrinkage without tension during heating was determined for membranes of collagen fibrils reconstituted from solutions of highly purified rat skin collagens. Shrinkage in per cent of area at 25 degrees C and shrinkage temperature were quantitated in a standardized way and determined as a function of in vitro maturation time for 11 to 104 days after aggregation for the collagen membranes. Similar to reports on intact rat skin, shrinkage temperature remained constant and shrinkage per cent declined with a rate decreasing with time during maturation. Solubility in water at 80 degrees C for 2 hours was 95-96% and remained unchanged for the maturation time (about 2 months) studied. The decreased shrinkage reflecting a lower degree of collapse is ascribed to an increasing thermal stability of the membranes during maturation. Development of heat-stable bonds in the reconstituted collagen fibrils is taken up to be amenable to this increased stability. Similarity in changes of shrinkage characteristics during in vivo and in vitro maturation indicates that maturation changes in reconstituted collagen fibrils reflect those occurring in intact collagen during in vivo aging.