INVESTIGATION OF PERFORMANCE DEGRADATION OF EVAPORATOR FOR LOW TEMPERATURE REFRIGERATION APPLICATIONS
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An evaporator test stand has been designed, sized, constructed, calibrated and operated to investigate the evaporator performance degradation for low temperature refrigeration applications. Measurements have been taken of air- and refrigerant-side temperatures, refrigerant-side pressures, air- and refrigerant-side differential pressures, air- and refrigerant-side mass flow rate and the power consumption of the electric defrost heater. The system is designed to work as a commercial refrigeration system for low temperature applications including freezing and defrost cycles. A manual and automatic defrost control has been designed and tested. The calculated values are the air- and refrigerant-side capacities. The tests have shown that the system is able to accomplish all desired test conditions. The performance of the evaporator was evaluated continuously during testing. Its performance degraded constantly by accumulation of frost during each frosting cycle, which resulted in the loss of superheat towards the end of each cycle. The defrost operation could not restore the full capacity of the evaporator. The defrost cycle consists out of two phases, melting the ice and re-cooling the coil. The first phase melts all the ice of the coil but the drainage of the resulting water is incomplete. The re-cooling phase freezes the amount of water residue on the evaporator coil, which causes an accelerated frost formation in the following freezing cycle. All over this behavior causes an accelerated reduction in evaporator capacity and therefore earlier superheat loss. It has been investigated for two air flow rates. The comparison of cycles with different air flow rates showed that with an increased air flow rate the frost density increases and the free flow area of the air passage in the evaporator decreases more slowly.