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.
A Multi-Substance Add-in for the Analyses of Thermo-fluid Systems using Microsoft Excel
 Abstract — Analyses of thermo-fluid systems require the thermo-physical properties of the working fluids to be repetitively determined at different conditions of pressure and/or temperature. However, relevant computer applications may not be available to many engineering students and practicing engineers particularly in developing countries. This paper presents an add-in that determines these properties for various fluids by using Microsoft Excel. Developed with Microsoft VBA, the add-in provides property functions for ideal gases, saturated and superheated water, saturated and superheated refrigerants for vapour-compression (VC) systems, binary solutions of ammonia-water and water-lithium bromide for vapour-absorption (VA) refrigeration systems, reacting mixtures, and atmospheric air. With its comprehensive library of functions, the add-in enables Microsoft Excel to be used as a modelling platform for conducting parametric and optimisation analyses of advanced power generation and refrigeration cycles, air-conditioning systems, and combustion processes. The present paper highlights the general features of the add-in before focussing on the set of functions that deal with the analyses of VC and VA refrigeration cycles. For the analyses of VC systems, the add-in currently supports ten synthetic and natural refrigerants (R12, R22, R32, R123, R134a, R152a, R717, R290, R600, R600a). The paper verifies the add-in functions for VC analyses by comparing its results with those obtained from relevant published research that used the well recognised REFPROP software. Similarly, the paper verifies the add-in functions for both ammonia-water and water-lithium bromide VA systems. The results obtained for both VC and VA cycles show that the add-in gives reasonably accurate estimations.
Fungal Biotransformations in Pharmaceutical Sciences
The regio- and stereoselective conversion of complex substrates represents one of the major advantage of microbial biotransformation. This chapter covers the biocatalytic capacities of fungi to derivatize bioactive small molecules, both from natural sources and from synthetic compounds. We focus particularly on sterol and taxane biotransformation. We also present examples at the interface of biotransformation and pathway engineering using recombinant organisms, with an emphasis on the production of lovastatin derivatives. Finally, we highlight selected studies on fungal xenometabolism which serves as a model to study the breakdown of drugs in mammals.