Control strategy of compressor and sub-cooler in variable refrigerant flow air conditioning system for high EER and comfortable indoor environment

The contradiction between increasing demand of variable refrigerant flow (VRF) air conditioning system and high energy consumption has stimulated great interest in energy saving without sacrificing the environmental comfort and reliability. This work greatly aimed at achieving enhanced energy conservation, comfortable environment and reliable operation by the coordination control of the compressor and sub-cooler in VRF system. The dynamic control model of the compressor output capacity was developed on the base of the static model, in which the average gas pipe temperature in the indoor units was used to calibrate the dynamic reference pressure control target. The influence of the static and dynamic control model on the cooling performance of VRF system with sub-cooler was comparatively tested and analyzed. The experimental results show that when one and two indoor units were thermo-on, the EERs by the dynamic control model were respectively 1.1-9.4% and 7.9-32.6% higher than those by the static model, and the air outlet temperatures can be controlled at 13.5-15 degrees C, within the suitable range of the air outlet temperature. The investigation results have proved that the control strategy can be considered as an efficient technology to achieve high EER under part load condition, comfortable thermal environment and assure reliable operation of the system.