Performance evaluation of the variable refrigerant flow (VRF) air-conditioning system subjected to partial and unbalanced thermal loadings

This paper shows the performance evaluation of the variable refrigerant flow (VRF) air-conditioning system subjected to partial and unbalanced thermal (cooling and heating) loadings. The one outdoor unit's and two indoor units' VRF system was used as a test specimen in the controlled outdoor chamber in which temperature and humidity were controlled. The indoor chamber thermal loading was controlled from full to partial thermal loadings and from balanced to unbalanced thermal loadings with respect to the specified rated capacity of the test specimen. The purpose of this study is to determine the real performance of the system when it is operating under actual operating situations. The general results show that the system coefficient of performance much depends on the partial and unbalanced thermal loadings. It is shown that the system maximum coefficient of performance occurred at around 50% of the rated thermal loadings. It is shown that as the load balance ratio decreases, the system coefficient of performance decreases, particularly for the cooling operation, due to the increase of compressor speed to support the refrigerant sub-cooling. In the heating operation, the compressor speed increase is very minimal which results in a very small decrease of the system coefficient of performance as the load balance ratio decreases. Based on this study, the system coefficient of performance becomes much different as the system is operating at partial thermal loadings. Also, during the unbalanced thermal loadings, the system coefficient of performance further decreases, particularly during the cooling operation. With the results of the study and analyses of the system operation, it is important to consider how the VRF system operates in an actual building when estimating the energy consumption of the VRF air-conditioning system to be installed in a new or in a retrofitted building.