The optimum high pressure for CO2 transcritical refrigeration systems with internal heat exchangers

The system performance of a CO₂ refrigeration system is greatly affected by the compressor discharge pressure. An internal heat exchanger (IHX) with high effectiveness is an important factor to achieve high system performance. The expression traditionally used to describe the heat exchange effectiveness is not suitable for CO₂ systems. As a result a practical effectiveness expression for IHX, based on enthalpy difference, has been derived and is reported in this paper. Detailed analysis on the relationship between the optimum high pressure P_k,opt and other systematic parameters was performed. Evaporating temperature has little influence on P_k,opt; and IHX can minimize the sensitivity of the system to the refrigerant quality x at the evaporator outlet. Based on simulation data, a correlation of P_k,opt was developed that predicts the simulation values with a deviation of less than 3.6% in the whole range and 0.94% when the evaporating temperature t₁=5.3 °C. The results reported in this paper can be used in optimum control and performance evaluation of the whole system.