핀관 열교환기의 착상 및 제상 성능 특성에 관한 연구
Year of publication 2005
Title of paper 핀관 열교환기의 착상 및 제상 성능 특성에 관한 연구
Author 김용한
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Abstract As humid air passes over an evaporator coil that is below the freezing point, water condenses onto the evaporator coil surface. Moisture molecules migrate to the frosted surface and the void portion of frost layer leads to frost growth and densification of frost layer. Frost layer acts as an insulation layer between the fins and the air. In addition, the frost blocks airflow path, causing an increase in pressure drop and a decrease in airflow rate. Numerous experimental and theoretical investigations on frosting have been performed and reported, but most researches on frost formation have been focused on simple geometries such as flat plates and round tubes. Due to complex geometry of finned-tube heat exchangers and diversity of operating variables, a further study is required to design optimum heat exchanger, achieving both uniform frost distribution and delay of frost formation. The major objective of this study is to provide experimental data and a model that can be used in the optimal design of an evaporator under frosting conditions. In the present study, an individual row of the heat exchanger coil was tested to simplify analysis of the complicated coil during the frost and defrost processes. After the heat and mass transfer performance of each row was investigated, two pieces of heat exchanger was combined. In this manner, we investigated the effects of the tube space and fin arrangement on the heat transfer performance. From this study, the correlations of heat and mass transfer coefficients and friction factor for the continuous flat plate finned tube and discrete flat plate finned tube were developed as a function of operating conditions and geometric factors of the heat exchanger. Moreover effects of wavy and defrosting coil on the heat transfer performance were investigated. Based on test results, we can find out defrosting coil shows severely bad influence on airflow rate and the effects of wavy fins are reduced with frost growth. Eventually, cooling capacity of the wavy fin coil becomes lower than that of the flat fin coil. In order to obtain stable performance under frosting conditions and recover the reduced heat transfer capacity from frost accumulation, periodic or cyclic defrosting is required to remove accumulated frost. The second major objective of this study is to compare the performance of various defrosting method such as pipe heater, sheath heater and glass heater. In this study, effects of contact resistance and power input of the pipe heater, and installed position of sheath heater and glass heater on defrosting efficiency were examined. As a result, defrosting efficient of pipe heater is improved about 20% by reducing the contact resistance between the pipe heater and fin. In addition, new defrosting concept of the hot air circulation method was introduced. Pre-frosting concept is to capture the moisture molecules in the air before the heat exchanger coil. If pre-frosting devices such as extended cold surface and silica-gel were installed before the evaporator coil, amount of frost accumulation in a finned tube heat exchanger decreases and operating time increases. In this study, the pre-frosting performance of the extended cold surface and silica-gel was tested, but it was not improved by using the pre-frostor. The performance of the heat exchanger itself can be varied when it is applied into the system. Therefore, it is very important to find out the effects of the performance change of a component in the system. In this study, relationship between the decreasing rate of heat exchanger performance and decreasing rate of system performance were investigated. Based on test results, the system performance was varied by 60% of the change in the heat exchanger.
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