Now showing 1 - 5 of 5
- PublicationEvaluation of Defrost Options for Secondary Coolants in Secondary Loop Multi-temperature Transport Refrigiration Systems - Mathematical Modelling & Sensitivity AnalysisThis paper describes a mathematical model of the defrost process for a finned-tube air chiller, utilised as a heat exchanger in asecondary loop multi-temperature transport refrigeration system, where an antifreeze mixture is deployed as a sensible secondary working fluid. Two defrost modes are modeled: an electric mode which effects defrost by localised resistance heating of the chiller secondary working fluid, and a hot gas primary circuit mode that indirectly heats the secondary working fluid by means of a primary to secondary heat exchanger. The model, which was implemented using the Engineering Equation Solver (EES), is based on a finite difference approach to analyse the heat transfer from the secondary working fluid, through a single finned heat exchanger section, to the frost. An iterative scheme is used to integrate for the overall heat exchanger, taking into account temperature glide associated with the secondary working fluid. The overall heat exchanger model is incorporated within a system defrost model, which allows the entire defrost process to be modeled. The model was validated for the standard United Nations Agreement on Transportation of Perishable Produce (ATP) for cold room set-points of 0C, -10C and -20C, by comparison with experimental data from a full scale laboratory based test programme. The validated model is used to carry out defrost sensitivity studies which examine defrost behavior for a range of performance parameters.
- PublicationPerformance evaluation of an economised indirect multi-temperature transport refrigeration systemDirect expansion (DX) refrigeration technology is almost exclusively used in multi-temperature transport refrigeration systems. Multi-temperature systems use up to three evaporators, requiring large refrigerant charges and system pressure control to operate over a wide range of set-point conditions. Despite incremental design improvements over the past decade, environmental and control issues continue to arise with DX systems. Deployment of indirect refrigeration systems (IDX) offers an alternative approach to address these issues. Indirect systems can however suffer from performance penalties, where reduced cooling capacity and COP occur under certain operating conditions. One strategy, aimed at offsetting the disadvantage of reduced refrigeration capacity, is to incorporate an economiser circuit into the primary cycle of the IDX system. Economiser cycles can enhance the refrigeration effect of the primary refrigerant in the primary to secondary heat exchanger of the indirect system. In this paper, for a multi-temperature transport refrigeration system, the performance of an optimised economised indirect system is compared with a non-economised indirect system and a contemporary direct expansion system. All tests for the economised IDX system were carried out using optimised refrigerant mass flowrate injection ratios, which were established for different operating boundary conditions. Tests were carried out to ATP standard for a Class C multi-compartment vehicle for a range of set point temperatures from -20ºC to +10ºC.
- PublicationInfluence of mass-flow injection ratio on an economised indirect multi-temperature transport refrigeration systemRefrigerant leakage associated with multi-temperature direct expansion (DX) systems in transport refrigeration applications has lead to increased interest in alternative refrigeration concepts. One alternative design approach aimed at reducing refrigerant charge and simplifying system control, involves the use of an indirect (IDX) refrigeration circuit. Recent investigations, concerned with the deployment of indirect systems for supermarket applications have shown that penalties in cooling capacity and COP can exist under certain operating conditions. These performance deficiencies are attributed to a number of factors including: additional power requirements of secondary circulation pumps, temperature glide associated with the secondary working fluid, and increased compressor pressure lift due to the additional primary to secondary heat exchanger. One design strategy, aimed at offsetting the disadvantage of reduced refrigeration capacity is to incorporate an economiser circuit into the primary cycle of the IDX system. Economiser cycles can enhance the refrigeration effect of the primary refrigerant in the primary to secondary heat exchanger of the indirect system. The net effect can result in increased refrigeration capacity, if appropriate mass flowrates of refrigerant are expanded through the economiser circuit. However, to date, there is little evidence of the use of an economiser cycle in more complex refrigeration systems, such as multi temperature transport systems, which are subject to significant variation in boundary conditions and operate frequently under transient conditions. This paper examines for a multi-temperature indirect transport refrigeration system, the influence of injection ratio on system performance under ATP conditions as well as the sensitivity of system parameters to different injection ratios. It is found that system capacity and COP can be modulated by regulation of injection ratio. An optimum injection ratio exists which allows maximisation of system capacity and COP respectively. This optimum injection ratio appears to be dependent on set-point and ambient conditions.
- PublicationEvaluation of defrost options for secondary coolants in multi-temperature indirect transport refrigeration part I: experimental resultsThis paper examines defrost performance issues associated with a finned-tube air chiller, utilised as a heat exchanger in an indirect multi-temperature transport refrigeration system, where a glycol antifreeze mixture is deployed as a secondary working fluid. Two approaches to defrost are examined: a direct electric defrost mode which effects defrost by localised resistance heating of the secondary working fluid; and a hot gas primary circuit, that indirectly heats the secondary working fluid by means of a primary to secondary heat exchanger. Investigation into the different defrost modes were carried out for chamber set point conditions of -20, -12, 0oC, for a number of defrost rated inputs between 0.5 and 3.9 kW. For a unit mass of frost, the duration of defrost was found to be inversely proportional to defrost energy input, however this relation was found to be non-linear. Defrost efficiency was found to decrease with defrost energy input, whereas the required specific total energy for defrost was found to increase with defrost energy input.
- PublicationA real time control strategy for optimisation of an economised indirect multi-temperature transport refrigeration systemThis paper describes an approach for control of an economiser cycle based on the use of economiser pressure as theprimary control parameter. In the study, the economiser cycle was used to optimise a multi-temperature indirect (IDX) transport refrigeration system, where hydronic secondary loops were utilised. In transport refrigeration applications, IDX systemscan offer the potential to address a number of important environmental and control issues associated with direct expansion (DX) systems. IDX systems may also give rise to reduced capacity and COP through increased compressor pressure ratios associated with the hydronic secondary circuit and power requirements of the liquid secondary pumps. One approach by which this issue can be addressed is through use of aneconomiser cycle, which provides a mechanism for performance enhancement by augmenting the refrigeration effect of the primary refrigerant, in the primary to secondary heat exchanger of these systems. Previous work ascertained that by control of the mass-flow injection ratio, an economiser cycle can be used to optimise indirect multi-temperature systems for a wide range ofdiverse operating conditions. This method of control necessitates mass-flow instrumentation which is impracticable for field applications. An alternative method of control described here, is based on a more easily measured economiser pressure, thereby eliminating the requirement of mass-flow instrumentation.