Modelling and Optimization of Heat Absorbed in the Evaporator of Low Temperature Cycle of Two-Step Cascade Refrigeration Systems

Abstract

A single-stage refrigeration system has a low Coefficient of Performance (COP), thus it becomes paramount to have cascade refrigeration systems when there is need to have an evaporator temperature that is below -25^oC. Heat absorbed (QE) in the evaporator of low temperature cycle of two-step cascade refrigeration systems is one of the two parameters that determine the COP of the lower temperature cycle of the two-step refrigeration systems which is also a function of the COP of the cascade refrigeration systems. This research aimed at modelling and optimizing QE of the lower temperature cycle of two-step cascade refrigeration systems using eco-friendly refrigerants. Thermodynamic of the cascade refrigeration systems was performed by varying seven operating parameters using refrigerants R-134a in the High Temperature Cycle (HTC) and R-23 in the Low Temperature Cycle (LTC). Heat absorbed in the evaporator of the lower temperature cycle (QE[LTC]) of the refrigeration systems was optimized using Half Factorial Design of Design-Expert 12.0.1. The influence of the evaporating temperature (T_E,HTC), condensing temperature (T_C,HTC), cascade temperature difference (ΔT_CAS,DIFF), evaporating temperature (T_E,LTC), superheating temperature (T_SUP,LTC), sub-cooling temperature (T_SUB,LTC), and refrigerant mass flow rate (ṁ_HTC) was investigated on the values of QE[LTC] of the refrigeration systems. The numerical QE[LTC] value is 14.183 while the measured value is 14.81.The study revealed that all the factors having interaction with TC[HTC] and TE[HTC] have a great influence on the value of QE[LTC].