Call number: LD2668 .T4 1959 W43

Call number: LD2668 .T4 1966 K29 / Master of Science

This thesis describes the work undertaken over a 3 year period under a Department of the Environment, Transport and the Regions (DETR) Partners in Technology programme in collaboration with four industrial partners. The aim of the work was to investigate the two-phase flow heat transfer and fluid dynamic performance of the non-ozone depleting refrigerants and lubricating oil mixtures in a horizontal tube. A single tube test facility was designed and built to determine local heat transfer coefficients and overall pressure drop for evaporation and condensation. A circulating pump system pressurised through an accumulator charged with nitrogen was selected to enable accurate setting of the evaporation and condensation conditions. The system was validated using R22. Tests were undertaken for R404A, R407C, Isceon 59 with a Polyol-ester lubricating oil. The experimental data consist of two-phase local heat transfer coefficients and overall pressure drops inside a smooth copper tube. Numerous data were obtained for evaporation and condensation, and constituted a large database of two-phase heat transfer coefficients. The experimental results were compared with several existing correlations traditionally used in heat exchanger design, to assess their suitability with the new refrigerants. As a result of these comparisons offering a poor agreement, a model characterising the variation of the local heat transfer coefficient was developed. The evaporation model consists in a modification of the Kattan et al. model (1998a, b & c). The new model predicts the experimental results well, with a standard deviation of 6.1% for refrigerant R407C and Isceon 59. The effects of lubricating oils on two-phase flow heat transfer are also discussed qualitatively. This work provides a design tool when dealing with the non-ozone depleting refrigerants, and recommendations are made on the use of several correlations.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

A thesis submitted to the Faculty of Engineering, University of the Witwatersrand, Johat.nesburq, in fulfilment of the requirements for the degree of Doctor of Philosophy Johannesburg, 1998 / This thesis contributes to accurate, practicable techniques of ascertaining and assessing site performance of large refrigerating machines chilling water 'for cooling deep South African mines. It applies to all vapourcompression machines cooling fluids in steady, continuous processes. To assess whether a water chilling machine is performing satisfactorily, both its actual performance, and the corresponding normal or optimal performance of which it is capable, must be ascertained. Both requirements r esent difficulties on site. in particular, the traditional "heat balance" method of verifying the apparent performance obtained from measurements in the water circuits does not prove that such performance is accurate. The calibration of typical site instrumentation is not assured, so an "acceptable" heat imbalance may conceal large but similar errors - which thus also balance out - in the apparent constituents of the heat balance. Three methods of independently ascertaining actual performance, so verifying apparent performance, are presented, The first is an enhanced method, applicable to custom-built machines as well as conventional ones, of ascertaining the efficiency of the actual refrigerating process from measurements in the refrigerant circuit. This detects errors concealed in an "acceptable" heat balance. Where some refrigerant-circuit measurements are unavailable, an inexact version of this method still indicates the relative likelihood of the apparent performance being acceptably accurate. The third method, where these two are inadequate, is ascertaining actual performance using available measurements and fundamental machine modelling. Such modelling is also the most versatile method of predicting corresponding normal or optimal performance. A computer program simulating complete mine water chilling installations is used here. Actual performance can then be meaningfully assessed and appropriate remedial action justified, as shown in seven case studies. An outcorr.e for conventional water chillinq machines with a centrifugal compressor is that keeping heat exchangers clean may prejudice efficiency under part-duties lf a machine has been designed for optimum efficiency at full duty. An alternative control philosophy of maximising the machine load may then yield better performance. If these techniques are included ill an automated system of fault diagnosis, they will be of most use to burdened mine staff, who are generally not refrigeration experts.

Sistemas de refrigeração são sistemas reparáveis. Isto significa que os componentes destes sistemas podem sofrer manutenção várias vezes durante a vida útil do sistema como um todo. Em sistemas de refrigeração o mais conhecido é o que opera com ciclo por compressão de vapor. Neste ciclo, o compressor é o equipamento mais complexo e de maior custo. Dessa forma, deve-se tomar especial cuidado, pois além de problemas como desgaste e falta de lubrificantes, ele está sujeito a sofrer danos decorrentes do funcionamento incorreto ou falho de todos os outros componentes. Dentro dos compressores existentes, o compressor alternativo pode ser comercialmente considerado o cavalo de batalha da refrigeração industrial, dominando a faixa de capacidades inferiores a 300 kW (85 TRs). O compressor alternativo também pode ser utilizado, com vantagens, quando o controle de capacidade se faz necessário, através do procedimento de desativação dos cilindros. Este compressor possui a possibilidade de manutenção no campo de, praticamente, qualquer item construtivo. O termo confiabilidade vem do inglês \"reliability\" que é a probabilidade de um item poder desempenhar sua função requerida, por um intervalo de tempo estabelecido, sob condições definidas de uso. Testes de confiabilidade possibilitam, através de estimativa, com base em estudos de confiabilidade, caracterizar os comportamentos de confiabilidade, da probabilidade de falha e da taxa de falha em relação ao tempo de um componente ou sistema. Uma das ferramentas para permitir avaliar ações que aumentem a confiabilidade é a Análise de Modos e Efeitos de Falha ou FMEA (\"Failure Mode and Effect Analysis\"). Basicamente, esta ferramenta parte da definição das funções dos sistemas e subsistemas analisados, verificando em seguida as falhas que afetam cada função e todos os modos de falha que levam a cada falha. Para cada modo de falha, analisam-se os efeitos e define-se a necessidade de ações para reduzir a possibilidade de ocorrência deste evento, eliminá-lo ou, simplesmente controlar os efeitos indesejáveis. O objetivo deste trabalho é o estudo das principais falhas, modos de falha de compressores semi-herméticos alternativos de sistemas de refrigeração, suas ocorrências e o comportamento destes equipamentos no seu período de garantia, sob a ótica da confiabilidade. / Refrigeration systems are reparable systems. This means that one can perform maintenance actions on them several times during the system life cycle. The most common refrigeration system is the vapor compression cycle. In this cycle, the compressor is the most complex and expensive component. So, the user should take special care because , beside problems like wear and lubricant lack, the compressor can also suffer damages caused by the incorrect work or fault from the others system components. Among the compressor types, the reciprocating compressor can be considered as the most used in the industrial refrigeration, from lowers capacities to up to 300 kW. Also the reciprocating compressor has some advantages, where its cylinders can be disabled for capacity control purposes. One can perform field maintenance of almost every part for this compressor. The word reliability means the probability of some equipment can not perform its requested function for an established time period, under defined operation conditions.. One of the tools to allow actions to improve the reliability is the failure mode and effect analysis (FMEA). This tool is based on the analyzed function and sub function systems definitions seeking faults that affect each function and all fault modes that lead to every fault. For each fault mode, the effects are analyzed and the need of actions to decrease the occurrence of this event is defined, excluding or control this undesirable events. This work objective is to study the most important faults, fault modes of reciprocating semi-hermetic compressors, its occurrences and its behavior during the guarantee period under reliability point of view.

Condensation on banks of tubes is of considerable interest in the power, refrigeration and process industries, where large scale condensers form a significant proportion of plant capital costs. Since the pioneering paper by Nusselt in 1916, numerous investigations, both experimental and theoretical, have made great inroads into the understanding of the important physical factors effecting performance, including effects of vapour shear and condensate inundation on heat-transfer performance. Despite this there are still significant gaps in the knowledge and no single recognised design approach exists for condensers under all conditions. Purely theoretical models have shown some success in modelling condensation on single tubes under high shear regimes, but these have not been successfully extended to full tube banks. The present work begins by drawing together a comprehensive data base of experimental results from those available in the literature. This includes assessing the experimental accuracy of the data and organising it into a consistent format to allow detailed comparison with existing and future models. The resulting data base comprises 13 tube bank geometries, 7 test fluids and over 4000 individual data points. The data base was used to evaluate existing theoretical and empirical models, and highlighted the weaknesses therein. In particular, it was found that empirical approaches were limited to application (ie refrigeration or steam condensers), with development and validation being based on experimental data for single fluids or groups of fluids. When these models were compared to the more comprehensive data base described above their performance was poor. Consequently there is limited confidence in their extension to applications outside those they were developed for. A new empirical based model was then developed. The approach involved identifying relevant dimensionless groups to account for the various physical factors which may affect heat transfer during condensation on a bank of tubes and formulating these into an equation involving a number of initially unknown but empirically obtainable constants. An iterative scheme was then employed to eliminate those groups having little effect on the result while retaining those which proved to be more important. The resulting model predicted the majority of the experimental data base to around 12%. A subsequent parametric study 3 showed the correct dependence of heat transfer coefficient on factors such as vapour velocity and tube row. The thesis concludes with some suggestions for future work.

Thesis (Ph. D.)--School of Mechanical Engineering, Georgia Institute of Technology, 2004. Directed by Sheldon M. Jeter. / Vita. Includes bibliographical references (leaves 195-201).

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