Now showing 1 - 2 of 2
  • Publication
    Evaluation of defrost options for secondary coolants in multi-temperature indirect transport refrigeration part I: experimental results
    (The International Institute of Refrigeration, 2008) ; ; ;
    This 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.
      461
  • Publication
    Evaluation of Defrost Options for Secondary Coolants in Secondary Loop Multi-temperature Transport Refrigiration Systems - Mathematical Modelling & Sensitivity Analysis
    This 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.
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