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Title: Development of a novel direct expansion based standalone enhanced dehumidification air conditioning system for improved year-round indoor humidity control
Authors: Chen, Wenjing
Degree: Ph.D.
Issue Date: 2018
Abstract: In buildings, controlling indoor air humidity at an appropriate level is critically important since this directly affects building occupants' thermal comfort, indoor air quality (IAQ) and the operating efficiency of building A/C systems. In hot and humid climates, such as Hong Kong, air conditioning (A/C) is usually required for up to 7 months from April to October in a year. At different seasons within the 7 months, however, space latent load that directly affects indoor air humidity level can vary significantly. On the other hand, direct expansion (DX) based A/C systems are widely used for indoor environmental control in various buildings in different climates, since they are simpler and more energy efficient, and generally cost less to own and maintain. However, the current trend in designing a conventional DX A/C system is to have a smaller moisture removal capacity in an attempt to boost its energy efficiency ratings. Furthermore, most DX A/C systems are equipped with single-speed compressors and fans, relying on on-off cycling compressor to maintain indoor air temperature only. Therefore, in hot and humid climates, such a DX A/C system, once installed, will have to be operated at different seasonal cooling load situations, and would hence have a hard time in trying to maintain the desired indoor air temperature and humidity at all times, unless supplementary measures to supply variable dehumidification ability are provided. However, these measures are usually complicated and costly, and may only be good during a specific season and become redundant in other seasons. Therefore, a standalone DX based enhanced dehumidification air conditioning (EDAC) system is proposed to provide suitable indoor air humidity control at different seasons in hot and humid climates, without requiring any supplementary measures. The proposed EDAC system has two evaporators and could act as a dehumidifier on the days when less or no additional cooling is required by employing one evaporator as a reheating coil (ADO mode), or as an enhanced dehumidification A/C system by operating the two evaporators together for achieving an improved indoor humidity control (EDAC mode). A research project on the development of such an EDAC system for improved year-round indoor air humidity control in hot and humid climates through experiments and mathematical model has been carried out and the project results are presented in this Thesis.
This Thesis begins with reporting the establishment of a prototype experimental EDAC system consisting of a DX refrigeration plant (refrigerant side) and an air-distribution sub-system (air side). All its major operating parameters can be real-time measured and recorded using high precision measuring devices. The availability of the experimental EDAC system would help facilitate experimentally studying the operational characteristics of the EDAC system, experimentally validating a steady-stated EDAC mathematical model to be developed and developing a year-round control strategy for the EDAC system for improved indoor air humidity control in hot and humid climates. Secondly, the Thesis presents an extensive experimental study on the operational characteristics of the experimental EDAC system at the EDAC mode in terms of the relationship between its output total cooling capacity (TCC) and equipment sensible heat ratio (E SHR), when both the evaporators were operated. The experimental results demonstrated that at a fixed inlet air state, varying refrigerant and air mass flow rates to both evaporators would significantly affect the operational characteristics. The resulted TCC and E SHR relationships were mutually constrained within an irregular area in a TCC-E SHR diagram, thus providing variable dehumidification ability. The experimental results also demonstrated that inlet air temperature and relative humidity would significantly influence the operational characteristics of the EDAC system, resulting in shifted position of, and varied shape of an irregular area of TCC-E SHR relationship in a TCC-E SHR diagram. Thirdly, the development and experimental validation of a physical-based steady-state mathematical model for the experimental EDAC system at the EDAC mode are presented. The model was developed by referring to the previous models for a two-evaporator A/C system. Using the validated EDAC model, a follow-up detailed modeling study was carried out to both demonstrate that the EDAC system was able to provide variable dehumidification ability and to optimize the sizing of the two evaporators used in the EDAC system. Furthermore, the modeling study results indicated that the EDAC system could produce variable dehumidification capacity. The modeling study results also suggested that a lower ratio of surface areas for the two evaporators in an EDAC system was beneficial to enlarging its variation ranges for both total cooling capacity (TCC) and equipment sensible heat ratio (E SHR). Finally, the Thesis presents an experimental study on the development of a year-round control strategy for operating the EDAC system at both the ADO and EDAC mode for achieving improved year-round indoor humidity level in buildings located in hot and humid climates. The experimental results show that in hot and humid climates, the use of the EDAC system and the control strategy was able to achieve an improved year-round indoor humidity control, while still maintaining the required indoor air temperature control at a higher energy efficiency, as compared to the use of a conventional On-Off controlled single evaporator DX A/C system.
Subjects: Hong Kong Polytechnic University -- Dissertations
Air conditioning -- Design and construction
Humidity -- Control
Air conditioning -- China -- Hong Kong
Pages: xx, 165 pages : color illustrations
Appears in Collections:Thesis

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