Influence of mass-flow injection ratio on an economised indirect multi-temperature transport refrigeration system

Refrigerant 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.