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Title: Experimental and modeling studies of a direct expansion based air conditioning system having a two-sectioned cooling coil (TS-DXAC) for improved indoor thermal environmental control
Authors: Yang, Liu
Degree: Ph.D.
Issue Date: 2021
Abstract: For buildings located in hot-humid climates, maintaining an appropriate level of indoor air humidity (RHi) is important for improved indoor thermal comfort, better indoor air quality and a higher energy efficiency of air conditioning (A/C) systems. Direct expansion (DX) A/C systems have been widely used to control indoor thermal environments in small- to medium- scale buildings, because they are more energy efficient, require less space to install and cost less in operation and maintenance. However, it is a challenging task to control RHi using a DX A/C system. This is because a single DX A/C system has to simultaneously handle the considerable changes in both sensible space cooling load and latent space cooling load. The commonly used temperature based On-Off control strategy for DX A/C systems can only be used for indoor air temperature (Ti) control, leaving RHi control as a by-product of temperature control. In addition, currently, the design trend of a DX A/C system is to try to reduce its dehumidification capacity for improving its energy performances. Therefore, to meet the requirement of indoor moisture removal, an air-conditioned space is usually overcooled by having a lower Ti setting. Hence, there have been significant research efforts to address the inadequacies in air dehumidification when using DX A/C systems. However, in these efforts, additional dehumidification provisions leading to larger installation spaces, or complex controllers based on variable speed (VS) technology were usually required. Furthermore, for a VS DX A/C system, to output an enhanced dehumidification capacity, a lower supply air flow rate was usually required, leading to a reduced level of thermal comfort because of a lower supply air temperature and a poorer indoor air distribution. Therefore, based on the previous related studies in developing novel A/C systems to provide an enhanced moisture removal capacity, a novel constant speed DX A/C system having a two-sectioned cooling coil (TS-DXAC) has been proposed. The two sections, with one section to mainly deal with the latent cooling load (LCL) and the other sensible cooling load (SCL), are arranged in parallel with their respective matching variable speed supply fans, and the mass flow rates for both refrigerant and air to each section could be adjusted. The total air flow rate of the system can be kept constant, so as not to affect indoor air distribution and occupants' thermal comfort. A research project on developing such a TS-DXAC system using experimental and mathematical modeling approaches has been therefore carried out and the project results are presented in this Thesis. Firstly, the establishment of an experimental TS-DXAC system, which was composed of two sub-systems, i.e., a DX refrigeration sub-system and an air-distribution sub­system, is reported. The experimental TS-DXAC system was placed inside an environmental chamber where air states can be controlled for experimental purposes. All the operating parameters of the experimental TS-DXAC system and the thermal environmental parameters in the chamber can be real-time monitored and recorded using high precision measuring devices. With availability of the experimental TS­DXAC system, its operational characteristics can be experimentally studied, a steady-state mathematical model for the experimental TS-DXAC system to be established also experimentally validated, and finally a control strategy to be developed to operate the experimental TS-DXAC system for improved indoor thermal environment control tested.
Secondly, the Thesis presents an experimental study on the inherent operational characteristics of the experimental TS-DXAC system. The study results showed that the variations in the mass flow rates of both refrigerant and air to the two sections would result in different combinations of the output total cooling capacity (TCC) and equipment sensible heat ratio (E SHR) from the experimental TS-DXAC system, and that the TCC - E SHR relationship was constrained within an irregular area. Furthermore, different inlet air temperatures and humidity levels to the experimental TS-DXAC system would also influence its inherent operational characteristics as reflected by the changes in the positions and shapes of the irregular areas, with the humidity levels influencing more on the operational characteristics. Thirdly, the establishment of a steady-state physical-based mathematical model for the experimental TS-DXAC system is presented. The established model was experimentally validated using the data collected from the experimental TS-DXAC system, with the differences between the experimental and predicted results of being less than 6%. Using the validated TS-DXAC model, a follow-up modeling study was carried out on optimizing the relative sizes of the two sections. The modeling study suggested that a lower surface area ratio for the two sections can lead to a larger variation ranges of both output TCC and E SHR. For example, at the inlet state of 26 °C and 50% relative humidity, when the surface area ratio for the two sections was altered from 1:1 to 1:3, the variation range for output TCC was increased by 33%, and that for E SHR by 51%, which was beneficial to better dehumidification. Finally, the Thesis reports the development of a control strategy to enable the experimental TS-DXAC system to be operated in hot-humid climates. The developed control strategy included two control algorithms, i.e., Algorithm I and Algorithm II, enabling the experimental TS-DXAC system to be operated under two different indoor conditions, i.e., hot-humid and hot-dry, respectively. Controllability tests for the control strategy were carried out using the experimental TS-DXAC system. Two test sets with a total of six tests were organized for the two indoor air conditions. The controllability test results demonstrated that in all the tests, Ti could be directly controlled at the two indoor conditions using either of the two algorithms, while RHi could be directly controlled at a hot-humid indoor condition using Algorithm I. The control strategy enabled the experimental TS-DXAC system to respond to the changes in either indoor settings or space cooling loads by outputting variable sensible and latent cooling capacities.
Subjects: Air conditioning -- Control
Heating -- Control
Humidity -- Control
Hong Kong Polytechnic University -- Dissertations
Pages: xix, 165 pages : color illustrations
Appears in Collections:Thesis

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