산업용 냉동기의 온도제어 특성에 관한 이론 및 실험적 연구

Abstract

As the advances in technology have improved, a refrigeration system is essentially utilized for making not only human life more comfortable but also production processes in the high-tech industries more productive. A refrigerator can be applied in a wide range of fields depending on the purpose of its usage, especially, a refrigerator commonly used in the industry is customized by different equipment and systems for an industry. Machine tools such as milling machine, a shaper , and etc, are used to products which have high precision and accuracy known as the precision machine. Since machining technologies have been highly advanced, the heat generated by extremely high cutting speed and feed rate in machine tools leads to thermal deformation of the machine tools structure and reducing the precision and reliability of machine tools. Therefore, a refrigerator system has become an indispensable part to cool down a ball screw and high speed rotating components easily affected by the generated heat in machine tools. Since the amount of the heat produced from the main shaft of a machine tool sharply varies under the operational conditions, a highly accuracy temperature control is required to optimally cool and adjust the temperature of cooling oil flowing through the main shaft of a machine tool. Most existing industrial refrigeration systems still use an on-off control method which maintains a constant speed control of a chiller system. However, this control method does not properly respond to variable loads and causes the increase in the power consumption and an inaccurate temperature control of a refrigeration system due to repeatedly the start and stop operation. Consequently, the continuous research is essential to accurately adjust the temperature of cooling oil in order to reduce the power consumption and increase the precision and reliability of machine tools. In the field of machine tools, an oil or water refrigeration system is generally used to minimize the expansion and contraction of a machine tools structure resulted from the high speed machining. Currently, most industrial refrigeration systems widely used in the industry use an on-off controller of an compressor in a chiller in order to get a desired temperature of cooling oil. This controller is originally designed to operate keeping within ±1℃ of the temperature difference between a secondary fluid such as oil or water and its set-point. In reality, its temperature difference can vary ranging between ±3～7℃ under variable load conditions, which causes the decrease in the quality of products from machine tools. Thus, two alternative methods are regarded as a noticeable improvement to replace an on-off control method: Hot-gas bypass control method and Inverter control method. A Hot-gas bypass control method is to manage the temperature with the gas at high temperature and pressure from a compressor outlet. An inverter control method is to adjust the speed of compressor based on variable loads so as to control the temperature. In this study, the experiments on an industrial refrigeration system using several Hot-gas bypass control methods are carried out to compare and analyze performance characteristics with regard to ambient temperature conditions, temperature variations at outlet of evaporator and the pressure drop in a heat exchanger. Furthermore, once an optimal method of Hot-gas bypass control methods is selected through the numerical results of the performance, more detailed characteristics of an industrial refrigeration systems are studied with experiments on diverse conditions. Next, the experiment apparatus is designed to test an inverter control method applied in an industrial refrigeration system and the system operating characteristics under diverse conditions are investigated compared with a Hot-gas bypass control method. Last, an on-off control, Hot-gas bypass control, and inverter control methods are respectively applied in the same industrial refrigeration system. An availability of an industrial refrigeration system is determined in accordance with its purpose and objective with improving the learning with the discrete characteristics of an industrial refrigeration system operated by three different control methods through the analysis of the results from experiments.