This paper examines the performance of a prototype indirect (secondary loop) refrigeration system developed as an alternative design to existing direct expansion multi-temperature systems in transport refrigeration applications. In order to evaluate both systems, a multi-temperature experimental test facility has been designed and built, with the capability to test both systems at standard ATP test conditions. Evaluation is based on side by side testing of the direct and indirect systems for a range of cold room temperature set-point combinations between -20°C and 0°C. The secondary refrigeration system was found to perform satisfactorily over the range of test conditions, with good pull-down characteristics, adequate refrigeration capacity but with lower COP than the direct expansion system. The indirect system exhibited some superior operational characteristics. These included better temperature control, decreased compressor cycling and the elimination of the refrigerant circuit flow mal-distribution often associated with direct expansion systems. Due to presence of an intermediate heat exchanger between the primary and secondary loops, a penalty in capacity is incurred. This can be minimised by careful selection of the secondary refrigerant and by adoption of good operational practices.
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