혼합냉매를 적용한 초저온 냉동기의 냉매충전량 최적화에 관한 연구

Abstract

Semiconductors, representing primary export of South Korea, are set to further expand their industry, reaching an estimated market size of $564 billion by 2024. The growing semiconductor market anticipates an increased demand for ultra-low temperature refrigeration, a crucial element in semiconductor production processes. Ultra-low temperature refrigeration plays a vital role throughout various stages of semiconductor manufacturing. Specifically, it is utilized to dissipate the cutting heat generated during wafer fabrication, a process known for inducing tool wear and contributing to soldering phenomena. Moreover, in the etching process, excess material post-photo processing is removed, generating additional heat. Recognizing the impact of this heat on semiconductor defect rates, refrigeration becomes essential. Additionally, it finds applications in post-processing stages for reliability testing. Beyond semiconductors, there is a rising demand for ultra-low temperature refrigeration in diverse fields, including frozen food preservation, vaccine storage, and the BOG liquefaction process. This study focuses on analyzing the performance characteristics of an ultra-low temperature chiller utilizing the cascade MR-Joule-Thomson cycle. Also, the composition of mixed refrigerant is based on minimum compressor displacement for compactness purposes. The study presents a comprehensive analysis about the refrigeration system performance variations with respect to refrigerant charge quantity within the same composition. First, when more refrigerant was added, the pressure where the mixed refrigerant discharge increased, but the temperature went down. This happened because, in the limited space, there was more refrigerant, making the whole system's pressure go up. The drop in discharge temperature when adding refrigerant could be explained by the suction pressure going up more than the discharge pressure, showing that the compression ratio went down. Additionally, the research showed that adding more refrigerant made cooling capacity big. This is because the amount of refrigerant circulating increased, which happened because of the higher suction pressure when adding more refrigerant, making the refrigerant specific volume less on the suction side. The study also found that adding more refrigerant reduced cooling time needed to reach the target temperature. This happened because, with more refrigerant, there was low quality of intermediate heat exchanger outlet and the evaporator inlet temperature needed for the brine to reach the target temperature went up. But experiments with 14.18% charge level caused the system to shut down unexpectedly because the pressure exceeded 30 bar. So, it is suggested that the optimal charge level is between 13-14%. These findings give useful insights into how refrigerant charging amount affects cryogenic refrigeration systems, helping to figure out the optimal refrigerant charging amount.