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Performance characteristics and optimization of a hybrid cooler for the cooling of telecommunication equipment
Year of publication 2009
Title of paper Performance characteristics and optimization of a hybrid cooler for the cooling of telecommunication equipment
Author 이선일
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Abstract The objective of this study is to investigate the performance characteristics and optimization of a hybrid cooler combining vapor compression and natural circulation cycles for the cooling of telecommunication equipment. To achieve this purpose, both experimental and numerical study were conducted with consideration of various design parameters and operating conditions. The performance of the hybrid cooler was measured according to the refrigerant charge, outdoor temperature, heat exchanger geometry, and the vertical distance between the condenser and the evaporator. The optimum design conditions for these variables are discussed with respect to the performance of the hybrid cooler in both operating modes. The difference in the optimum refrigerant charge between the two operating modes was solved by installing a liquid receiver. The temperature difference between the indoor and ambient air was introduced as a control parameter for use when changing the operating mode. The simulation model of the hybrid cooler combining vapor compression and natural circulation cycles was developed to analyze the effects of design and operating parameters on the system performance. The vapor compression model was developed based on the ORNL model (Oak Ridge National Laboratory, 1983). The natural circulation cycle was developed by using the sectorial thermal resistance model. The present models for the vapor compression and the natural circulation were validated by using the measured data within relative deviations of 2.3% and 5.5%, respectively. In addition, the developed model was used to investigate effects of ambient temperature and vertical distance on the optimum refrigerant charge for the maximum cooling capacity in each mode and effects of the riser diameter on the optimum vertical distance between the evaporator and the condenser. For the effective operation of the hybrid cooler throughout the year at the optimum refrigerant charge, a modified hybrid cooler which has four operating modes was tested. When the refrigerant charge was fixed, the cooling capacity decreased slowly with the variation of the temperature difference and it decreased dramatically at ΔT=40℃. On the other hand, when the refrigerant charge was controlled by pump down or pump out mode, the cooling capacity increased even though the ambient temperature was extremely low. During the pump down mode, the cooling capacity was not guaranteed because the refrigerant did not flow through the evaporator. Thus, to secure both pump down and cooling effect, a new control algorithm, which Valve 3 repeats on/off cycling at an interval of approximately 15 seconds, was presented. By using the modified hybrid cooler with the liquid receiver module, approximately 29% of energy was saved annually at the target capacity of 1600 W as compared with that for a simple vapor compression cooler.
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박사학위논문