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.