Operational stability of a direct expansion air conditioning system under variable speed operation and its control application

Abstract

In comparison to chilled water­based large­scale central air conditioning (A/C) systems, direct expansion (DX) A/C systems are simpler in configuration, more energy efficient and generally cost less to own and maintain. Therefore, for the last few decades, DX A/C systems have found increased applications in buildings, especially in small to medium­scale buildings. A DX A/C system is made up of a DX refrigeration plant and an air­distribution sub­system. In the DX refrigeration plant, there exists an expansion valve (EV)­evaporator control loop to regulate the refrigerant mass flow rate entering the evaporator in response to the degree of refrigerant superheat (DS) at the evaporator's exit. Fundamentally, a DX A/C system can be classified as a special kind of refrigeration system, and the issue of operational instability often encountered in a refrigeration system would also occur in a DX A/C system. The issue of operational instability in a refrigeration system has been extensively studied. There have been two different views on the possible causes for the operational instability, or hunting, namely, the inherent operational characteristics of an evaporator and the operational characteristics of an EV. While hunting has been widely observed in both thermostatic expansion valve (TEV) and electronic expansion valve (EEV) controlled refrigeration systems, limited numbers of studies on investigating the operational characteristics of an EEV on the operational stability of the EEV­controlled refrigeration systemmay be identified. On the other hand, the wide application of variable­speed (VS) technology has made the continuous control of compressor speed and supply fan speed in a DX A/Csystem more practical, paving the way for achieving simultaneous control of indoor air temperature and relative humidity (RH) using DX A/C systems. Previous extensive studies on the inherent operational characteristics of a VS DX A/C system expressed in terms of the inherent correlation between its output total cooling capacity (TCC) and equipment sensible heat ratio (E SHR) were carried out, with the issue of operational stability being ignored when both compressor speed and supply fan speed were simultaneously varied. However, hunting was actually observed in a VS DX A/C system when it was VS operated for simultaneously controlling indoor air temperature and humidity, resulting in a relatively low operational safety and high energy consumption. Therefore, in this Thesis, a systematic study to investigate the operational stability of an EEV controlled VS DX A/C system under VS operation is reported. The Thesis, first of all, begins with presenting a theoretical and experimental study on the influences of the operational characteristics of a proportional­integral (PI) controlled EEV on the operational stability of the DX A/C system. Using the classical control theory, EEV's PI settings and time constant of EEV's temperature sensor on stability were analyzed. The theoretical analysis results using the classical control theory were further verified experimentally using an EEV­controlled experimental DX A/C system. The study results showed that a larger proportional or integral gain for the PI­controlled EEV would lead to a higher chance for the EEV­evaporator control loop to become unstable, while slowing down the rate of DS signal transfer by increasing EEV's time constant may help mitigate system instability. The study results confirmed that the operational characteristics of an EV in a refrigeration system could impact its operational stability and further suggested an effective approach to mitigate the instability problem encountered in an EEV­controlled refrigeration system by incorporating a first­order transfer function in its EEV­evaporator control loop to slow down the rate of DS signal transfer. Secondly, this Thesis reports a study to investigate the inherent operational characteristics of the VS DX A/C system considering its operational stability at different DS settings and inlet air states, which may be regarded as a follow­up to the previous reported studies on the inherent operational characteristics of the VS DX A/C system when the issues of instability were ignored. Using the experimental VS DX A/C system, the inherent correlations between its TCC and E SHR at different combinations of compressor speed and supply fan speed were studied,and the unstable operating points of speed combinations under different DS settings and inlet air states identified. The experimental results suggested while different DS settings may not significantly influence the inherent correlations between TCC and E SHR, but did impact the operational stability. Furthermore, VS operation and different inlet air states to the DX evaporator also influenced the operational stability. A higher compressor speed or a lower supply fan speed, and a lower inlet air temperatureor RH level would lead to a higher chance to instability. Finally, the development of a new capacity controller that is able to not only simultaneously control indoor air temperature and humidity, but also select an optimized DS setting to properly balance the operational safety and efficiency of the VS DX A/C system is reported in this Thesis. The new capacity controller was developed by adding a control module to a previously developed capacity controller. The core of the control module was an artificial neural network (ANN) based model representing the known relationship between the inherent operational characteristics and operational stability of the VS DX A/C system. The new capacity controller was experimentally tested using the experimental VS DX A/C system. The test results showed that the hunting of the experimental DX A/C system was mitigated and a slight improvement in operational efficiency achieved when using the new capacity controller for simultaneously controlling indoor air temperature and humidity. The study results reported in this Thesis have provided a better understanding of the operational stability of an EEV­controlled DX A/C system under VS operation. Furthermore, the study has also laid a good basis for developing capacity controllers to address the issues of operational instability for VS DX A/C systems. The outputs from this study can help improve the operational safety and energy efficiency of a VS DX A/C system when it is VS operated for achieving a better indoor thermal environment control.