G Venkatarathnam

Many zeotropic refrigerant mixtures are being proposed as alternatives to some CFCs and HCFCs. One of the advantages of zeotropes is the possibility of reduction in entropy generation by matching the temperature glides of refrigerant and heat-transfer fluid in both condenser and evaporator. However, perfect glide matching may not be possible with some zeotropes. Also, pinch points may occur with some zeotropes. In this paper the conditions that cause pinch points to occur are identified. It is shown that there exists certain limits for the temperature glide of the heat-transfer fluid in condensers and evaporators to avoid pinch points. The limits are unique for each mixture and are independent of the heat-transfer fluid. These limits can be used to evaluate quickly the suitability of a zeotropic refrigerant mixture for glide matching. These methods are illustrated with examples. Copyright © 1996 Elsevier Science Ltd and IIR.

Auto refrigerant cascade (ARC) refrigerators operating with zeotropic mixtures provide refrigeration at temperatures less than 173 K (-100 °C) using a single compressor. Different authors have suggested different cascade heat exchangers for ARC refrigerators. There is no study in literature that suggests at what temperature ranges one, two or three cascade heat exchangers are necessary. In this paper the performance of an ARC refrigerator operating in the liquid refrigerant supply mode and operating with optimized hydrocarbon mixtures and different cascade heat exchangers is studied. The optimum number of cascade heat exchangers (stages) to be used for different operating temperatures is suggested. © 2010 Elsevier Ltd. All rights reserved.

There is a worldwide interest in the development of auto refrigerant cascade (ARC) refrigerators operating with refrigerant mixtures. Both flammable and non-flammable refrigerant mixtures can be used in these systems. The performance of an ARC system with optimum nitrogen-hydrocarbon and argon-hydrocarbon mixtures between 90 and 160 K is presented in this paper. © 2009 Elsevier Ltd. All rights reserved.

With the mandate of Montreal Protocol banning ozone depleting substances, and Kyoto Protocol later on curtailing the use of substances which contribute to global warming, conventional refrigerants are to be replaced by environment-friendly working fluids. Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are being substituted by hydrofluorocarbons (HFCs), hydrofluorooelifins (HFOs), and a variety of mixtures. In view of the global warming potential of these newly synthesized refrigerants, the recent trend is to go back to the originally used natural fluids such as ammonia, carbon dioxide, hydrocarbons, water vapour, etc. In this article, various issues related to this changeover of refrigerants being used in vapour compression refrigeration systems are discussed. © 2012 Indian Academy of Sciences.

The performance of a vapor absorption refrigeration system (VARS) operating with ionic liquid [hmim][TF2N] and HFC refrigerants R32, R152a and R125 and HFO refrigerants R1234ze(E) and R1234yz has been analysed. The PC-SAFT equation of state is used to estimate the properties of ionic liquid-refrigerant solutions as well as that of pure refrigerants. The effect of absorber, generator, evaporating and condensing temperatures on the coefficient of performance and circulation ratio has been studied. A COP value of 0.5 and above can be obtained with the combination R32-[hmim][TF2N] in typical air-conditioning applications. An exergy analysis has been carried out to understand the major sources of losses in different components. The results show that the performance of a VARS with [hmim]TF2N] and HFC refrigerants R32 and R152a is better than that with [hmim]TF2N] and HFO refrigerants R1234yf and R1234ze(E). The results of this study show that ionic liquid-low GWP refrigerants R32/R152a combinations are suitable for use in vapor absorption refrigeration systems for air conditioning applications.

Many zeotropic refrigerant mixtures are being proposed as alternatives to CFCs and HFCFs refrigerants that are being phased out. The main advantage in the use of zeotropic mixtures is the possibility of reduction in entropy generation in the condenser and evaporator by matching the glides of refrigerant and heat transfer fluid. Glide matching is difficult with some zeotropic mixtures due to the nonlinear variation of enthalpy with temperature during phase change. Also, there is a distinct possibility of formation of pinch points with some mixtures, resulting in entropy generation. In this paper, the performance of four mixtures of R32, R134a, R125, R143a and R23 are studied as alternatives to R22 and R502 from thermodynamic as well as temperature pinch points of view. The results demonstrate the importance and the need for pinch point analysis in the evaluation of alternative zeotropic refrigerant mixtures.

Single stage refrigeration systems that use refrigerant mixtures can be used as an alternative to conventional cascade refrigeration systems for very low temperature refrigeration applications. The exergy efficiency of a single stage low temperature refrigerator operating with a mixture depends strongly on mixture composition. Few studies are available in open literature on single stage mixed refrigerant systems working with fluorocarbon and hydrocarbon mixtures for very low temperature refrigeration. In this work, the PC SAFT equation of state has been used to thermodynamically model and optimize the mixture composition for single stage refrigerator operating at -80°C, the results of which is presented in this paper.

Capillary tube expansion devices are used extensively in small closed cycle J-T refrigerators operating with refrigerant mixtures due to its low cost and the absence of any moving parts. It is possible for J-T refrigerators operating with mixtures that the velocity of refrigerant mixture at capillary tube outlet reaches a value where it equals the speed of sound at certain conditions. The variation of the speed of sound of nitrogen-hydrocarbon mixtures used in J-T refrigerators has been studied in two phase (vapour-liquid) and three-phase (Vapour-liquid-liquid) region as a function of temperature and pressure in this work. Also the conditions under which choking occurs in practical J-T refrigerators is investigated.

The performance of Joule-Thomson (J-T) refrigerators is strongly influenced by the composition of the refrigerant mixture used. The mixture composition changes if leaks occur, or when a silica-gel drier is used in the system. In this experimental study, we show how the composition changes can be estimated from the cooldown characteristics of J-T refrigerators without using a gas chromatograph. A novel way of representing the cooldown characteristics of J-T refrigerators is presented. The variation of compressor power during the cooldown is also discussed. The results presented here are useful for developers as well as field service engineers in selectively modifying the composition without discarding the entire mixture. © 2010 Elsevier Ltd. All rights reserved.

Matrix heat exchangers are used in a number of applications such as helium liquefiers, sorption refrigerators, and in Kleemenko cryocoolers. In this paper the methods for the optimum sizing of balanced flow and unbalanced flow matrix heat exchangers of rectangular and circular shapes are presented. Using the methods developed, the relative size of matrix heat exchangers of rectangular and circular shapes are compared.