CHAPTER 1 INTRODUCTION

Transcription

1 CHAPTER 1 INTRODUCTION 1.1 CFC REFRIGERANTS Since the 1930s, chlorofluorocarbons (CFCs) have been widely used as foam blowing agents, aerosols and especially refrigerants due to their pre-eminent properties such as stability, non-toxicity, nonflammability and thermodynamic properties. In particular, R12 has been predominantly used for small refrigeration units including domestic refrigerator/freezers. Refrigerant R12 has been the most dominant refrigerant for refrigeration industry. However, they also have harmful effect on the earth s protective ozone layer [1]. So, they are being regulated internationally by Montreal Protocol since 1989 [2]. Later, it was also proved that CFCs also contributed significantly to the global warming problem. The global warming potential of R12 is considered to be 8500 times that of CO2 over 100 years [3]. Greenhouse gas emissions led to the Kyoto Protocol in It was thus decided that by 2010 [4], producing and using of CFCs should be prohibited completely all over the world. In consequence, a lot of research has been done to find the suitable eco-friendly replacement of CFCs. 1.2 INITIAL ALTERNATIVES TO R12 REFRIGERANT At the advent of the Montreal protocol the following refrigerants were chosen for the alternative to R12.

2 1.2.1 HCFC Refrigerant The initial alternatives included some hydrochlorofluorocarbons, or HCFCs, but they are likely to be phased out internationally around 2040, because their ozone depletion potentials and global warming potentials are in relative high levels though less than those of CFCs [5]. By that time compounds such as HFCs (hydrofluorocarbons), which are benign to the ozone layer, are likely to have replaced HCFCs. As a result, it has become an urgent issue to search and develop CFC and HCFC alternatives. While the presence of single component refrigerants reduces the performance possibilities, the solution appears to lie in using the synthetic mixtures. The search for alternate refrigerants as substitutes for R134a in various refrigeration systems has been an important area of research in the RAC sector. As per the Montreal Protocol, developed countries have already phased out R12 and developing countries like India have to do the same before the end of None of the alternative refrigerants can be used in the R12 based appliances without making system modifications or technology change [6]. In developing countries the growth of RAC sector has picked up momentum only in the last decade and hence the immediate change of technology may cause setbacks to the RAC sector [7] R134a Refrigerant For the past decade, R12 has been replaced with R134a in refrigerators and automobile air conditioners. At present in India more than 80% of the refrigerators are working with R134a [8]. R134a

3 posseses favourable characteristics such as zero ODP, nonflammability, stability and similar vapour pressure to that of R12. Earlier studies indicate that the energy efficiency of R134a obtained in actual refrigerators was lower than that of R12 [9] Hydrocarbon Refrigerant Hydrofluorocarbons, such as R134a, have almost zero ozone depletion potential, as they do not contain chlorine atoms in their chemical structure. Similar to R12, they are safe, non inflammable and have similar vapour pressures [10]. However, they have lower energy efficiency and are more expensive than R12. They also have a low negative environmental effect of global warming potential [11]. The concern against the increase of global warming has been the prior issue of study in the present century. Thus, in 1997 the Kyoto protocol was agreed by many countries there by calling for the reduction in emissions of greenhouse gases including HFCs. The GWP of R134a is 1300 which is considerably high but lower than R12 [12]. Naturally occurring substances such as water, carbon dioxide, ammonia and hydrocarbons are believed to be environmentally safe refrigerants. Now in India CFCs phase out was successfully implemented by replacing R12 with R134a, but it has to be controlled due to relatively high GWP. So, interest towards environmentally safe refrigerants is growing. At the same time the performance of the refrigerants and their flammability are other crucial factors that have to be taken into account while selecting the refrigerants. Furthermore, it is desirable that the designed refrigerants, replace the current

4 refrigerants without any major change in the system equipment. A trade-off point between all these factors has been considered while proposing the mixtures in the present work. The aim of the present investigation is to find substitutes for R134a refrigerators. Using hydrocarbons is an environmentally sound alternative to CFCs/HFCs [13]. HCs as a refrigerant have been used since the beginning of 20 th century. The development of the CFCs in the 1930s placed the HC technology in the background. Hydrocarbon technology has come to the forefront. Most of the natural refrigerants are also considerably cheaper than their synthetic alternatives [14]. The general conclusion is that there is no ideal refrigerant today. Natural refrigerant should be chosen whenever possible for the sake of environment protection. In fact, hydrocarbons are known to have such advantages as low cost, availability, compatibility with the conventional mineral oils as well as PAG and POE [15]. However, their use has been held up in other developed countries mainly due to their high flammability. They offer other advantages of being very economical and easily available in large amounts. They are environmental friendly with zero ozone depletion potential, and they do not cause the greenhouse warming effect [16]. The major limitation is that they are highly flammable substances and must be handled with caution. Also, blends of some refrigerants can be considered as substitutes or alternatives to existing refrigerants. There are an increasing number of scientists and engineers, including environmentalists who believe that an alternative solution,

5 which has been overlooked, may be provided by using hydrocarbons. These provide the possibility of a zero ODP, together with suitable thermodynamic, physical and chemical properties. It is possible to mix hydrocarbon refrigerants with other alternative refrigerants, such as HFC, to replace R134a in domestic refrigerators [10, 17, 18]. Alternative refrigerants should have stable thermo physical and chemical properties, good miscibility with lubricants and low inflammability. The only limitation of hydrocarbons when compared to other refrigerants is flammability. The reduction in flammability can be achieved by mixing HCs with HFCs [19]. This process reduces the amount of flammable substance and consequently the flammability risk will be reduced. The global warming potential will be atleast two third less when HFCs are used alone. The proposed ternary mixture of HFC/HC used in this study has saturation properties matching with those of R134a. In fact, for the developing countries, meeting all the requirements of various international amendments for environmental protection is quite burdensome. Thus, a change in a major component in refrigeration equipment would be another serious hurdle preventing those countries from adopting energy-efficient and environmentally safe refrigerants. It is quite likely that the first component needs to be changed in adopting a new fluid would be a compressor [6]. For such a case, it would be quite costly for those who implement it to redesign and retool all the manufacturing amenities for new compressors. Thus, in order to adopt environmentally safe refrigerants at a reasonable cost, `drop-in

6 replacement fluids' requiring only minor changes in the system, particularly in the compressor, should be developed from the beginning to be used in the long run. In the present work, a general method of selecting `drop-in fluids' is presented with a main application in visi cooler charged with R134a. To accomplish the goals of the study, a theoretical analysis as well as experimental investigation for the energy consumption and cooling performance is presented. The procedures and data presented in this work will be helpful for the replacement/reduction of ozone depleting/green-house warming refrigerants in the future. The point of contention surrounding the phase out of CFCs is to provide substitutes with optimum benefits and performance. In this work, an experimental study, using hydrofluorocarbon/hydrocarbon (HFC/HC) mixtures with suitable proportions, has been carried out to determine the optimum mixture for replacing R134a in existing visi coolers. Non-azeotropic mixtures have some added advantages over single component and azeotropic refrigerants. The alternatives that are proposed in this report mainly comprise of non-azeotropic mixtures of R134a and hydrocarbons.

7 1.3 MOTIVATION FOR THIS RESEARCH In India more than 80% of the refrigerators are working with R134a, which is the best proposed alternative refrigerant to R12. R134a is a high GWP gas and needs to be controlled as per the Kyoto Protocol. In India some of the refrigerant manufacturers are using HC blend (50%R600a/50%R290) is the drop in replacement for R134a. HC blend is having zero ODP and negligible Global Warming Potential (GWP). The only drawback of HC blend is the flammability. The drawbacks of using R134a and HC blend with respect to GWP and flammability can be overcome by mixing the R134a and HC blend with an appropriate mass fraction. So, that the final mixture leads to decreased GWP due to less mass fraction of R134a, instead of 100% decreased flammability effect due to mixing refrigerant with non flammable refrigerant. With final ternary mixture which is obtained with pure R134a and of flammable these advantages the composition of 25%R134a/37.5%R600a/37.5%R290 will be environment friendly alternative refrigerant. In the present work, investigations have been successfully made to use ternary mixture of R134a/R600a/R290. An improvement in energy efficiency of the new mixture has also been demonstrated in this work.

8 1.4 ORGANIZATION OF THE THESIS The work is presented in six chapters Chapter 1 briefly explains the introduction of the alternative refrigerants and scope for the HFC/HC mixture. Chapter 2 describes literature survey on alternative refrigerants, R12 and R134a refrigerants, need of optimizing the mass of the refrigerant, hydrocarbon refrigerants and zeotropic refrigerant mixtures, design of experiments for optimizing the process parameters. The scope of the present work is explained at the end of this chapter. Chapter 3 explains the classification of refrigerants, desirable properties of refrigerants, Montreal Protocol, Kyoto Protocol, reasons for selecting R134a and HC mixture. Criteria for selection of proposed refrigerant mixtures are discussed. Using REFPROP software the comparison of properties of the selected alternative refrigerant mixtures with R134a and HC mixture, preparation of the ternary mixture is also explained in this chapter. Chapter 4 describes the experimental setup of the present study, instrumentation adapted to carry out the optimization of refrigerant charge and capillary length, pull down test, actual COP, energy consumption of the compressor and refrigeration effect of the considered refrigerants. Experiments performed using Taguchi method based design of experiments (DOE) is presented in detail. Chapter 5 consists of results and discussions regarding the experimental evaluation of optimization of refrigerant charge and capillary length, pull down time, power, refrigeration effect and COP.

9 Comparison between the theoretical and experimental results for COP and compressor power are produced. Optimum conditions obtained from Design of Experiments (DOE) using Taguchi Method is compared with full factorial of experiments. The performance parameters were plotted on graph and discussed. Chapter 6 lists conclusions arrived and the performance parameters have also been discussed. The advantage of the proposed ternary mixture over the conventional is discussed in terms of ODP, GWP, flammability and cost. Optimized values of the length of the capillary and refrigerant charge for the new mixture have been suggested. Scope of future work of the present study has been proposed in this chapter. Appendices consists of 1. Specifications of visi cooler 2. Sample readings of the performance test on a visi cooler 3. List of Publications