Evaluation of defrost options for secondary coolants in multi-temperature indirect transport refrigeration part I: experimental results

Abstract

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.