Collision of different design concepts in fan-coil unit manufacture and selection

In order to strengthen the research foundation of terminal equipment control of centralized air conditioning chilled water systems, the reasons of common chilled water system small temperature difference operation are explored. By comparing different design thinking and research ideas of fan-coil units under cooling conditions, starting from the basic mechanism cognition, the traditional thinking of small flow rate and large temperature difference is abandoned. Through the theoretical exploration and practical attempts, the ASHRAE manual (2016), chapter 13, section 2.7 “General cooling coil heat transfer characteristics” and chapter 47, section 3 “Typical circulating heating or cooling coil” performance curves are questioned. It is pointed out that the essential difference of heat transfer characteristics between cooling coils, heating coils and dry coils lies in the ability to deal with the latent heat in the air, and that extra caution is needed when using the logarithmic average temperature difference between the air side and the chilled water side for computer-aided design to avoid essential fallacies in the conclusions. It is emphasized that there is a significant time lag in energy transfer during the heat and humidity exchange process, which should be fully considered in the analysis and study. A solution of super design cooling capacity selection and engineering application of non-standard fan-coil units is proposed.