R744와 R717을 사용한 캐스케이드 냉동시스템의 성능 및 엑서지 특성 = Performance and Exergy Characteristics of a Cascade Refrigeration System with R744 and R717

국문 초록 (Abstract)

본 논문에서는 규제 프레온 냉매의 대체 물질인 자연냉매를 작동유체로 사용하는 R744-R717용 캐스케이드 냉동시스템에서 운전조건의 변화에 따른 성능과 엑서지의 특성을 분석한 것이다. 논문에서 고려한 작동변수로는 응축온도, 증발온도, 과열도, 과냉각도, 압축효율, 캐스케이드 열교환기 온도차 등이다. 이에 대한 주요 결과를 요약하면 다음과 같다.

증발온도 및 과냉각도의 증가에 따라 캐스케이드 냉동시스템의 성능계수(COPₜ)와 엑서지 효율 ηₓ(COPratio)은 상승하였으나 응축온도 및 과열도가 증가할수록 성능계수와 엑서지 효율은 저하되는 결과를 나타내었다.

그리고 캐스케이드 증발온도가 증가할수록 R717용 고온사이클의 성능계수는 증가하는 반면에, R744용 저온사이클의 성능계수는 감소한다. 또한, 증발온도와 압축효율 감소 시 전체 냉동사이클의 성능계수와 효율의 변화가 커서 안정적이지 못하다. 특히, 압축효율 감소 시 성능계수와 엑서지 효율이 크게 감소하는데, 이는 장기간 캐스케이드 냉동시스템으로 운전할 때 단점이 될 수 있다.

따라서 이러한 이유로 전체 캐스케이드 냉동사이클의 성능계수와 엑서지 효율은 거의 일정하다. 또한 캐스케이드 증발온도가 증가할수록 R717용 응축기와 압축기의 엑서지 손실이 가장 큰 것을 알 수 있다. 따라서 R744와 R717용 캐스케이드 냉동시스템의 성능계수(COPₜ)와 엑서지 효율(ηₓ) 향상을 위해서는 R717용 응축기와 압축기의 엑서지 손실을 줄여야만 한다.

반면, R744-R717용 캐스케이드 냉동시스템은 자연냉매를 사용하여 친환경적이며, R717 고온사이클과 R744 저온사이클에 사용되는 냉매의 적절한 선택에 의해서 다양한 온도대에서 고효율 저온 냉동시스템을 구성할 수 있다.

다국어 초록 (Multilingual Abstract)

In this paper, we analyzed the performance and exergy characteristics according to changes in operating conditions in a cascade refrigeration system for R744-R717, which uses natural refrigerants as working fluids as alternatives to regulated freon refrigerants.

The operating variables considered in this paper include condensation temperature, evaporation temperature, degree of superheating, degree of subcooling, compression efficiency, cascade heat exchanger temperature difference, and exergy efficiency.

The main results are summarized as follows: As the evaporation temperature and degree of subcooling increased, the coefficient of performance (COPₜ) and exergy efficiency (ηₓ) of the cascade refrigeration system increased, but as the condensation temperature and degree of superheating increased, COPₜ and ηₓ decreased.

As the cascade evaporation temperature increases, the coefficient of performance of the high-temperature cycle for R717 (COPₕ) increases, while the coefficient of performance of the low-temperature cycle for R744 (COPₗ) decreases.

In addition, when the evaporation temperature and compression efficiency decrease, COPₜ and ηₓ of the entire cascade refrigeration system change significantly and are therefore unstable. In particular, when the compression efficiency decreases, COPₜ and ηₓ decrease significantly, which can be a disadvantage when operating a cascade refrigeration system for a long period of time.

Therefore, for this reason, COPₜ and ηₓ of the entire cascade refrigeration cycle are almost constant. Additionally, it can be seen that as the cascade evaporation temperature increases, the exergy loss of the condenser and compressor for R717 is the largest.

Therefore, in order to improve COPₜ and ηₓ of the cascade refrigeration system for R744 and R717, the exergy loss of the condenser and compressor for R717 must be reduced.

On the other hand, the cascade refrigeration system for R744-R717 is environmentally friendly as it uses natural refrigerants, and can be configured as a high-efficiency low-temperature refrigeration system in various temperature ranges by appropriately selecting the refrigerants used in the R717 high-temperature cycle and the R744 low-temperature cycle.

목차 (Table of Contents)

• Ⅰ. 서 론 ·············································································································1
• 1. 연구의 목적 ·································································································1
• 2. 연구의 방법 및 범위 ·················································································2
• 3. 논문의 구성 ·································································································5
• Ⅱ. 성능 분석 ····································································································7
• Ⅲ. 결과 및 고찰 ····························································································18
• 1. 엑서지 분석 ·······························································································18
• 2. 성능 특성 ···································································································20
• 3. 성능 결과 ···································································································44
• Ⅳ. 결 론 ···········································································································48
• 참고문헌 ·········································································································50
• Abstract ··········································································································53