Effects of sub-cooler on cooling performance of variable refrigerant flow air conditioning system

Abstract The contradiction between increasing demand of variable refrigerant flow (VRF) air conditioning system and some technical bottlenecks, such as low energy efficiency and high noise, stimulated great interest in studying related technologies. This work attempted to adopt the sub-cooling technology to improve energy efficiency, decrease noise, and ensure reliable operation. The effects of sub-cooler on the cooling capacity, energy efficiency ratio, sub-cooling degree and discharge super-heated degree were tested and qualitatively analyzed. The results show that the reasonable adjustment of the sub-cooling electronic expansive valve opening can improve the energy efficiency ratio, increase the sub-cooling degree and ensure the safe discharge super-heated degree. The maximum energy efficiency ratio was 3.3%, 3.1% and 3.0% higher than those without the sub-cooler when one, two and four indoors were thermo-on, respectively. And the maximum sub-cooling degree was 25.6 °C, 21.9 °C and 17.6 °C higher than those without the sub-cooler, respectively. The experimental results were analyzed by the pressure-enthalpy diagram and thermodynamic theory. Furthermore, the control model of the sub-cooling electronic expansive valve was built, in which a certain sub-cooling degree was used to control the electronic expansive valve opening, and the discharge super-heated degree was involved as the control restriction condition for the reliable operation. The investigation results provide theoretical guidance for the application of the sub-cooler in variable refrigerant flow air conditioning system.