탑차용 냉동시스템에서 증발기의 서리성장에 따른 성능 특성 연구

Abstract

Refrigeration systems such as a general refrigerator, commercial refrigerator, and industrial refrigerator etc. became the necessities of life. The movable commercial refrigerating system is called ‘refrigerator truck’, and it keeps the fresh foods during the transport. Recently the food service industry has been developed due to the improved quality of life. The demands of package service have been increased for frozen foods from on and off line, so the refrigerator truck demands increased. In addition, the groceries are also increased the demand for centralization and localization. For this reason, a study on the refrigeration performance of refrigerator trucks is needed to maintain the high efficiency and reliability of the food transportation for long distance. However, the research is insufficient for the performance of refrigerating system in commercial refrigerator truck. In the refrigeration system, when the surface temperature of cooling coil gets below the dew point temperature and it contacts with vapor of humidity, the porous frost gets to form on the coil during the operation. As the frost growth increase, the rate of air flow through the evaporator decreases leading to a large pressure drop accompanied by a sudden drop in heat exchanger performance. Also, frost in the low temperature condition is developed with the operating time, so it has an effect on the heat exchanger of efficiency degradation owing to heat resistance of frost. Therefore, it is in need to examine that the performance degradation caused by frost growth in the refrigerator truck. However, only few studies have reported on refrigeration performance considering frost growth. Most of the theoretical models have been verified using a domestic refrigerator. However, simulation model of a refrigerator truck hasn't been developed so far. A simulation model is useful to study the performance degradation due to frost growth under different operating conditions for vehicle-mounted units. In this study, an analytical model of a refrigeration system was developed for the R404A refrigerator truck. Besides, the experimental test was carried out in order to verify the developed analytical model. To analyze the performance of refrigerator tuck in the psychrometric chamber, which were compared to the simulation results using the EES(Engineering equation solver) program for refrigeration system of refrigerator truck. As a result, frost thickness was increased with the rise of the indoor air temperature and the compressor rotating speed, but increasing amount of frost was insignificant difference with outdoor air temperature. Furthermore, the optimal refrigeration capacity was decreased by 30% when operating time passed about three hours under basic conditions. In this case of simulation, we predicted the blocking ratio with operating condition to compare the experiment results. The lower indoor temperature, the COP decrement increased with the increasing of blocking ratio, whereas the higher indoor temperature, the decrement of refrigeration capacity increased with frost growth. The variation of indoor temperature, performance curve shows similar trends and a decreasing rate with blocking ratio. The decreasing rate in refrigeration capacity under a high temperature condition was steeper than that under a low outdoor temperature condition with blocking ratio. The refrigeration system of performance is sensitive reaction to high outdoor temperature condition at condenser. In addition, the variation of outdoor air velocity, the system performance was not greatly increased above 4 m/s because a sufficient amount of heat transfers from the refrigerant to air at the condenser. The maximum of the system COP was shown at the compressor speed of 1500 rpm for non-frost and frost condition, simultaneously. The performance under frost condition was more sensitive to the operating condition compared to that under non-frost condition. Through this study, the performance data and optimal control method of system performance in refrigeration system can be provided and it will contribute to developing high efficiency refrigerator system in the movable truck.