Cascade and Secondary Coolant Supermarket Refrigeration Systems: Modelling and New Frost Correlations

Abstract

Nowadays traditional (direct expansion) supermarket refrigeration systems are mostly employed in supermarket establishments for refrigerating food products and beverages in the store. However, the installations of long piping system, fittings and joints in traditional systems are causing substantial refrigerant losses. The refrigerant losses in turn bring about cost and high environmental damage in terms of ozone layer depletion and global warming potential. Additionally, defrosting of air-coils is one of the most energy consuming processes in supermarket refrigeration systems due to susceptibility of the air-coils to moisture. Hence, the frost forming on air-coils as a result of moisture transfer should be removed to keep display cabinets under the required temperature. Various studies, though limited in scope, have been conducted both numerically and experimentally by several researchers in order to provide efficient and environmentally friendly supermarket refrigeration technologies. Empirical and mathematical expressions have also been continuously developed to quantify frost characteristics on flat plates and round tubes thereby determining the appropriate defrost periods. Cascade and secondary coolant refrigeration systems are the potential candidates to replace traditional ones due to the fact that the former can work on natural refrigerants and the latter essentially eliminates long connecting lines and environmentally damaging refrigerants. Development of empirical correlations for frost characteristics on real heat exchangers could also provide accurate prediction of defrost periods thereby eliminating unnecessary waste of energy and deterioration of food products. The current study, therefore, presents (a) mathematical expressions for carbon dioxide-ammonia (R744-R717) cascade refrigeration system; (b) mathematical expressions for frost property and air pressure drop; and (c) a numerical model for medium-temperature secondary coolant system incorporating the new frost property correlations. The thermodynamic analysis of the cascade refrigeration system is useful for the supermarket refrigeration industry to optimize the design and operating parameters of the system. The development of the frost property correlations from experiments on a lab-scale flat-finned-tube heat exchanger is also useful for the supermarket refrigeration industry for better prediction and control of defrost periods and duration for medium temperature air-coils. The medium-temperature secondary coolant model adopted the most appropriate heat transfer, mass transfer and pressure drop correlations obtained from the open literature. The system components such as air-coil, plate heat exchangers, distribution lines, coolant pump and compressor were modeled independently. Each component model was validated and linked to form a complete overall medium-temperature secondary coolant refrigeration system model. The experimental results showed that COP of the Monopropylene glycol/water based medium temperature secondary coolant refrigeration system could be 1.33, whereas the simulated results on a typical supermarket showed that COP of the system could be as high as 1.75. The fundamental difference between the existing secondary coolant models and the current one is that frost characteristics have been incorporated in the air-coil model. In addition to this, complete independent models have been developed for plate heat exchangers based on their respective applications. Hence, the main advantage of the current medium-temperature secondary coolant refrigeration system model is that defrost periods and time span required to defrost frosted air-coils can be accurately determined to achieve energy savings and prevent product deterioration in the display cabinets. The plate heat exchanger models can also enable one to reasonably determine pressure drops thereby leading to an appropriate coolant pump selection. Finally, a step-by-step exercise of the application of the secondary coolant model, which has been included in this project, can be used to completely design, select, evaluate and install such systems or retrofit the existing traditional direct expansion refrigeration systems in supermarkets.