Indoor environmental control with a direct expansion (DX) air conditioning (A/C) unit in residences in the subtropics

Pao Yue-kong Library Electronic Theses Database

Indoor environmental control with a direct expansion (DX) air conditioning (A/C) unit in residences in the subtropics

 

Author: Li, Zheng
Title: Indoor environmental control with a direct expansion (DX) air conditioning (A/C) unit in residences in the subtropics
Degree: Ph.D.
Year: 2006
Subject: Hong Kong Polytechnic University -- Dissertations
Indoor air pollution -- Prevention
Dwellings -- Air conditioning
Department: Dept. of Building Services Engineering
Pages: xxi, 200 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1957993
URI: http://theses.lib.polyu.edu.hk/handle/200/3075
Abstract: Direct expansion (DX) air conditioning (A/C) units are commonly used in residential buildings in the subtropics. They are normally equipped with single-speed compressors and supply fans, relying on on-off cycling compressors to maintain only indoor air dry-bulb temperature. This results in an uncontrolled equilibrium indoor humidity, leading to a reduced level of occupants' thermal comfort, poor indoor air quality (IAQ), and low energy efficiency. The thesis reports on, first of all, a simulation study on the characteristics of space cooling loads and indoor environmental control in residences in the subtropics, using a building energy simulation program, EnergyPlus. Both the weather conditions and the typical arrangements of high-rise residential blocks in subtropical Hong Kong were used in the simulation study. The simulation results on both the space cooling loads characteristics and the hourly application sensible heat ratio (SHR) in the living/dining room and the master bedroom in a selected west-facing apartment under different operating modes of DX A/C units in the summer design day are presented. The problem of indoor environmental control, based on an on-off cycling compressor to maintain indoor air dry-bulb temperature only, due to the mismatch between the output latent cooling capacity from a DX A/C unit and the space latent cooling load in the two rooms both in the summer design day and during part load conditions was quantitatively investigated. In addition, the influences of indoor furnishings acting as moisture capacitors on indoor relative humidity (RH) levels were also quantitatively studied. Secondly, the thesis presents an experimental study to investigate the inherent operational characteristics of a DX A/C unit at a fixed inlet air state of 24C and 50% RH when the speeds of both its compressor and supply fan were varied. The measured results suggested that varying both compressor and supply fan speeds can lead to varying equipment SHR, or the varying ability to dehumidify of the unit, and may therefore be preferably adopted for indoor environmental control in places subjected to variable latent cooling loads. Generally, lowering supply fan speed was more effective in enhancing the ability to dehumidify than increasing compressor speed. However, varying both speeds would also impact on both the total output cooling capacity and the operating efficiency of a DX A/C unit. Thirdly, this thesis reports on an experimental study on indoor thermal comfort characteristics under the control of a DX A/C unit having variable-speed compressor and supply fan at a fixed space cooling load condition. The results of study suggested that under a given indoor total cooling load with a fixed application SHR, varying both speeds of compressor and supply fan in the DX A/C unit would not only result in different indoor temperatures and RH levels but also impact indoor air velocity and mean radiant temperature (MRT), influencing indoor thermal comfort. Therefore ANSI/ASHRAE Thermal Comfort Standard (55-2004) has been used to evaluate indoor thermal comfort characteristics under the control of a DX A/C unit having variable-speed compressor and supply fan. Experimental results showed that appropriate indoor thermal comfort levels may be attained at different speed combinations, but with varying operating efficiencies. Finally, the thesis presents the development of a novel direct digital control (DDC)-based capacity controller for a DX A/C unit to regulate indoor temperature and RH by simultaneously varying the speeds of both its compressor and supply fan. The capacity controller consisted of a numerical control algorithm (NCA), using a number of real-time measured operating parameters and based on the energy balance between the air side and refrigerant side to determine both speeds. The results of controllability tests of the capacity controller suggested that the controller developed was operational, with an acceptable control accuracy but rooms for improvement with respect to control sensitivity. Therefore attempts to improve control sensitivity were made by incorporating a traditional Proportional-integral (PI) controller using the deviation between the actual RH and its setpoint as a control signal into the DDC-based capacity controller. The results of the controllability tests for the improved controller showed that it can achieve both a control accuracy and a reasonable control sensitivity.

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