Yong Cheol Sa’s research while affiliated with LG Electronics and other places

Naturally, frost forms at fin-tube evaporator surface when refrigerant below 0 °C flows inside the heat exchanger. Frost tends to block the evaporator, hinders efficient heat exchange and degrades the performance of the system. The determination of the start time for defrosting is one of the essential issues that most of air heat pumps are confronted with because it is directly connected to energy consumption. Many types of sensors can be installed for frost detection but they are usually not cost-effective or related logic is not so physically presented. In this study, a novel frost volume calculation method is suggested in order to determine the start time for defrosting in an air-to-air heat pump system. The detailed explanation of the method is presented and verified with experimental data. With this method, the start timing of defrosting can be appropriately judged. At the time of defrosting, the heating capacity decreased from 16.2 to 39.3% and the COP reduced from 12.3 to 26.4% compared with maximum values. The blocking ratio from image processing at the time was between 0.83 and 0.97.

The efficiency and pressure drop in the oil separator of a multi heat pump system were experimentally investigated. R410A and polyvinylether (PVE) oil were used as the refrigerant and lubricant of the system, respectively. The refrigerant mass flow rate and the liquid circulation ratio were varied between 30 and 150 g s⁻¹ and between 1.5% and 4.5%, respectively. Five different oil separators were considered, and the measured efficiencies and pressure drops were analyzed with respect to the height and diameter of the oil separators. The analytical results were used to develop empirical equations for predicting the efficiency and pressure drop of the oil separator. The mean absolute percentage errors between the predicted and measured efficiencies and pressure drops for the oil separators were 0.4% and 11.7%, respectively.

This paper discusses the experimental results of oil retention amount and pressure drop in the gas line of refrigerant. R410A and polyvinylether oil were used as refrigerant and lubricant, respectively. The experiment was conducted under various pipe inner diameters of 14.1, 17.3 and 26.0 mm, and refrigerant mass flux and oil circulation ratio (OCR) were varied from 40 to 250 kgm-2s-1 and 0.5 to 4.0%, respectively. The oil retention amount tends to increase as refrigerant mass flux decreases and OCR increases. The oil retention amount in vertical pipe was higher than that in horizontal pipe. The empirical expressions for predicting the oil retention amount and pressure drop were suggested. The mean absolute percentage errors of each expression for predicting the oil retention amount in horizontal and vertical pipes were 18.1% and 14.1%, respectively, and those for predicting the pressure drop in horizontal and vertical pipes were 22.2% and 20.1%, respectively.