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Title: Control and design for a multi-evaporator air-conditioning (MEAC) system
Authors: Pan, Yan
Degree: Ph.D.
Issue Date: 2012
Abstract: Multi-evaporator air-conditioning (MEAC) systems are widely used in small- and medium-sized buildings, offering many advantages over conventional chilled water based air-conditioning systems in terms of installation convenience, high design flexibility, being easy to maintain and commission, better indoor thermal comfort control and higher energy efficiency. Although a number of capacity control algorithms have been developed based on system simulation, no experimental-based capacity controller developments for MEAC systems may be identified, except where the capacity controller developed for an MEAC system was based on a model obtained through system identification, which cannot be generically applied to other MEAC systems. On the other hand, the pipelines in an MEAC system are usually complicated, causing a large refrigerant pressure drop, which could in turn further lower evaporating temperature, increase condensing temperature, reduce system's operating efficiency and result in refrigerant mal-distribution among evaporators. Furthermore, different designs for an MEAC system would affect its operating performance and total power input, but no previous studies on applying the Constructal Theory to the refrigerant pipework layout design of an MEAC system for minimizing the total power requirement have been reported. Therefore, a comprehensive research program consisting of developing a novel capacity control algorithm, and investigating the effects of large pressure drop along a refrigerant distribution pipework on the operating performance in a dual-evaporator air conditioning (DEAC) system, discovering the best possible pipework layout design of MEAC systems for minimizing the total power input requirement using the Constructal Theory has been carried out and is reported in this thesis. This thesis starts with reporting the development of a novel capacity control algorithm (NCCA) for a DEAC system, a typical MEAC system. The controller imitated On-Off control of a single-evaporator A/C system (SEAC) in each indoor unit of the DEAC system by using variable-speed compressor and electronic expansion valves (EEVs). Experimental tests validated its control accuracy and robustness. However, indoor air temperature controlled using the NCCA may still be subjected to significant fluctuations under certain operating conditions due to the use of temperature dead-band and time delay for compressor start-up, and the interaction among indoor units. An improved novel capacity control algorithm (INCCA) was therefore further developed, and further controllability tests carried out. The test results showed that the INCCA was simple but could effectively restrain the magnitude of temperature variations and avoid the frequent altering in compressor speed.
Secondly, the thesis presents a modeling study on the effects of refrigerant pipeline length on the operational performance of a DEAC system. To facilitate the intended modeling study, a physical-based steady-state mathematical model has been developed. The model contained a sub-module which was specifically devoted to accounting for the influence of refrigerant pipeline length on system operational performance. The model has been validated by comparing its prediction results with the experimental results previously reported by others. Using the model developed, the effects of refrigerant pipeline length on the operating performance of the DEAC system have been studied and are reported, and the layout optimization of a DEAC system was studied for the highest possible operational efficiency. Simulation results indicated that the DEAC system's COP decreased with an increase in the refrigerant pipeline length. The simulation results also suggested that for a DEAC system, its highest COP would be resulted in when the outdoor unit was located equally between the two indoor units and its lowest COP when the outdoor unit was located close to either of the indoor unit. Thirdly, this thesis documents an analytical study of applying the Constructal Theory to discovering the best possible refrigerant pipework layout design in MEAC systems for minimizing the total power requirement. Two approaches that might be used in the study were considered. The first approach was based on the first law of thermodynamics and consisted of changing the configuration of an MEAC system in such a way that the work required per unit of heat removed from an indoor space was reduced. The second approach was based on the second law of thermodynamics, aiming at reducing the irreversibility of the system. It was shown that the use of the two approaches would yield equivalent results. However, the first approach was simpler and more familiar with for people, because it did not require the calculation and discussion of entropy generation. Therefore the first approach was adopted in the study. The study started from a single-room case (SEAC systems), and was extended to a dual-room case (DEAC systems) and finally to a multi-room case (MEAC systems). Two ways to arrange the refrigerant pipework were considered. One was to position the refrigerant pipework inside room(s) and the other outside the room(s). The analytical study results show that for an SEAC system, its heat exchanger (i.e., indoor unit) area was fixed by the cooling load it had to deal with, while for a DEAC system and an MEAC system, their total heat exchanger areas were decided by the total cooling load they had to handle. The heat exchanger area in each room however was influenced by the location of the outdoor unit of the MEAC system when the distance among all rooms was fixed. This suggested that if the cooling load in one room was specified, the operating performance of an MEAC system would be influenced by the design of its pipework. The study results for all the three study cases also indicated that the optimum diameters of refrigerant pipelines were independent of their lengths. This can guide the designer of an MEAC system to calculate an optimum pipeline diameter and find out the total heat exchanger area first, and then to determine the pipelines length and heat exchanger area in each room as a function of the total system cost and outdoor unit position.
Subjects: Air conditioning -- Design and construction.
Evaporative cooling.
Hong Kong Polytechnic University -- Dissertations
Pages: xxii, 190 leaves : ill. ; 30 cm.
Appears in Collections:Thesis

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