The effect of expander integration on exergy destruction in a dual-evaporator refrigeration system

In this thesis, an exergy analysis was conducted on a vapor compression refrigeration system equipped with a dual-evaporator and an expander-compressor configuration. The system consists of two evaporators operating at low and high temperatures, two compressors, two expansion valves, an expander, an internal heat exchanger, and a mixing chamber. The study thoroughly evaluates key parameters affecting the second-law performance of the system, including condensation temperature, evaporator temperatures, degrees of superheating and subcooling, and the mass flow rate ratio between the evaporators. Moreover, a component-wise exergy destruction analysis was performed for six different refrigerants, and their thermodynamic performances were comparatively assessed. The analyses considered four different mass flow rate scenarios within a mixing ratio range of 0.1–1.5, along with a wide range of temperature variations. The results showed that increasing the condensation temperature significantly increased the total exergy destruction, primarily concentrated in the second compressor and condenser. Conversely, increasing the evaporator temperatures (both low- and high-temperature) led to reduced irreversibilities and improved second-law efficiency. The degree of superheating was found to have a minor impact, causing less than 2% variation in total exergy destruction. Among the refrigerants analysed, R1234yf stood out with the lowest total exergy destruction, while R717 resulted in the highest. However, component-level evaluations revealed that the location of dominant irreversibilities varied depending on the specific refrigerant used.