Prediction of Metastable States for Throttling in Pressure Relief Valves for Sustainable CO2 Heat Pump Technologies

Abstract

Abstract. Incorporating carbon dioxide (CO2) refrigerant into the heat pump (HP) technology can reduce climate change effects due to working fluid loss. However, CO2 refrigerants in an HP system may change from a gaseous state to a liquid or solid “snow” during the throttling and pressure-relief valve processes, which may block downstream flow in pipes and cause unsafe operation issues. This paper describes a Soave–Redlich–Kwong (SRK) statistical-thermodynamics-based model to predict CO2 refrigerant metastable state properties (enthalpy, temperature, and volume) exhibited during throttling through a leakage path or pressure relief valve in HP technologies. In addition, the simulation can predict changes in density fluctuations states exhibited during the process that can indicate an impending phase change and can predict exit state conditions for widely varying supercritical initial equilibrium states. Results indicate that when a constant enthalpy throttling line passes the freezing point temperature and approaches the spinodal line, the throttling process can result in sublimation of solid-state CO2 which may cause a blockage. Therefore, this simulation can be used to explore how changing initial state conditions, CO2 pressure ranges, and thermomechanical characteristics may result in the blockage of the pressure relief passage and unsafe conditions for users.