Enhanced indoor humidity control in a space served by a Direct Expansion (DX) air conditioning (A/C) system

Pao Yue-kong Library Electronic Theses Database

Enhanced indoor humidity control in a space served by a Direct Expansion (DX) air conditioning (A/C) system

 

Author: Xu, Xiangguo
Title: Enhanced indoor humidity control in a space served by a Direct Expansion (DX) air conditioning (A/C) system
Degree: Ph.D.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Air conditioning -- Equipment and supplies.
Indoor air quality -- Mathematical models
Humidity -- Control
Department: Dept. of Building Services Engineering
Pages: xx, 182 leaves : ill. (some col.) ; 31 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2351703
URI: http://theses.lib.polyu.edu.hk/handle/200/5458
Abstract: The use of Direct-Expansion (DX) air conditioning (A/C) systems offers many advantages such as a higher energy efficiency and a lower cost to own and maintain. Therefore, they are found wide applications in buildings, in particular in small-to medium-sized buildings. However, most DX A/C systems are equipped with single-speed compressor and supply fan, relying on On-Off cycling compressor as a low-cost approach to maintaining only indoor air dry-bulb temperature, whereas the indoor air humidity is not controlled directly. In a hot and humid climate like Hong Kong, indoor humidity may remain at a high level in a space served by an On-Off controlled DX A/C System. The situation may become worse when the supply fan in a DX A/C system runs continuously while its compressor is On-Off operated. During an Off-period, the air passing through the system’s cooling coil may cause the re-evaporation of the residual moisture on coil’s finned surface (in the form of tiny water droplets). This leads to indoor humidity to rise and a poor indoor humidity control which can cause discomfort to occupants and degrade indoor air quality, and decrease the energy efficiency of an air conditioning system. This thesis addresses the indoor humidity control using DX A/C systems. It firstly presents a study on condensate retention on a louver-fin-and-tube air cooling and dehumidification coil, which is commonly used in DX A/C systems. Compared to previously related work focusing on the influence of condensate retention on the heat and mass transfer between air and a cooling coil, the study emphasizes the impacts of operating parameters on condensate retention on a cooling coil. A new mathematical model to represent the force balance of retained condensate has been developed. The mass of condensate retained has been measured experimentally under various operating conditions using an existing experimental DX A/C station. The influences of air dry-bulb temperature, moisture content and Reynolds Number on condensate retention are discussed. The model developed relates the mass of condensate retained to condensing rate, and is successful in predicting the trends of condensate retention under normal operating conditions for air cooling applications. Furthermore, by a further detailed discussion of the process of condensate retention and drainage, a new viewpoint on condensate retention and drainage process has been put forward in order to better explain the experimental results and to provide a base for proposing some future related research work.
Secondly, a new model for evaluating the wet fin efficiency of cooling and dehumidification coils has been developed and is reported. The condensate film moving on fin surfaces and its impacts on heat and mass transfer have been taken into account in deriving the governing equation for fin temperature, and consequently the enthalpy change of moving condensate film has been included in the governing equation. The new model has been validated by comparing its predictions with that using the most popular existing McQuiston Model, under the same operating and boundary conditions. The new model developed can replace the existing McQuiston model in evaluating the wet fin efficiency of cooling coils at all different condensing rates. Thirdly, an experimental study on the inherent correlations between the total output cooling capacity and the Equipment Sensible Heat Ratio (SHR) of a DX A/C system is reported. Experiments have been carried out both under different combinations of compressor speed and supply fan speed, and under different inlet air states to the DX evaporator using the experimental DX A/C station. The results obtained would lead to a better understanding of the operating characteristics of a DX A/C system under variable speed operation, so as to better design, operate and control DX A/C systems for improved indoor thermal environmental control. Finally, a new control algorithm to replace the traditional On-Off control algorithm widely used in DX A/C systems is developed. The new control algorithm, which enables both the compressor and the supply air fan in a DX A/C system to operate at high speeds when indoor air dry-bulb temperature setting is not satisfied, and at low speeds otherwise, is termed as the H-L control. Extensive experiments have been carried out also using the experimental DX A/C station, at different operational conditions, under both H-L and On-Off control. The experimental results suggest that the use of the H-L control would result in a better control performance, in terms of an improved indoor humidity level and a higher energy-efficiency for DX A/C systems, when compared to the use of traditional On-Off control. The H-L control is also simpler, and its associated hardware cost is much lower than that of the control algorithms based on variable speed technology. In addition, the use of a High-Low capacity compressor to implement the H-L control has been investigated. A brief description of the experimental procedure of using the High-Low capacity compressor to implement the H-L control is included. Experimental results suggest that a High-Low capacity compressor can replace a variable speed compressor when implementing the H-L control, with satisfactory control performance.

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