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A Study on the Cascade Hybrid Cooling/Refrigeration Cycle Equipped With Intercooler and Air-Cooled Condenser in Series

인터쿨러와 공랭식 응축기를 동시에 사용하는 냉방-냉동 겸용 캐스케이드 사이클에 대한 연구

  • Kim, Nae-Hyun (Department of Mechanical Engineering, Incheon National University)
  • Received : 2019.03.25
  • Accepted : 2019.07.05
  • Published : 2019.07.31
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Abstract

Thermodynamic analysis of cascade refrigeration systems has attracted considerable research attention. On the other hand, a system evaluation based on thermodynamic analyses of the individual parts, including the evaporator, condenser, intercooler, expansion valve, etc., has received less attention. In this study, performance analysis was conducted on a cascade refrigeration system, which has an individual cooling and refrigeration evaporator, and equips the intercooler and air-cooled condenser in a series in a lower cycle. The thermo-fluid design was then performed on the major components of the system - upper condenser, lower condenser, cooling evaporator, refrigeration evaporator, intercooler, compressor, electronic expansion valve - of 15 kW refrigeration, and 8 kW cooling capacity using R-410A. A series of simulations were conducted on the designed system. The change in outdoor temperature from 26 C to 38 C resulted in the cooling capacity of the lower evaporator remaining approximately the same, whereas it decreased by 9% at the upper evaporator and by 63% at the intercooler. The COP decreased with increasing outdoor temperature. In addition, the COP of the cycle with the intercooler operation was higher that of the cycle without the intercooler operation. Furthermore, the increase in the upper condenser size by two fold increased the upper evaporator by 4%. On the other hand, the lower evaporator capacity remained the same. The COP of the upper cycle increased with increasing upper condenser size, whereas that of the lower cycle remained almost the same. When the size of the lower condenser was increased 2.8 fold, the intercooler capacity increased by 8%, whereas those of upper and the lower evaporator remained approximately the same. Furthermore, the COP of the lower cycle increased with an increase in the lower condenser. On the other hand, the change of the upper condenser was minimal.

그간 캐스케이드 냉동 시스템에 대해서 열역학적 해석은 다수 수행되었으나 증발기, 응축기, 인터쿨러 등 부품 해석을 통한 시스템 평가는 미진한 상태이다. 본 연구에서는 냉방 및 냉동 열교환기가 별도로 장착되어 있고 하부 사이클에 공랭식 응축기와 인터쿨러가 직렬로 연결되어 있는 캐스케이드 냉동 사이클에 대해 성능 해석을 수행하였다. 우선 증발기, 응축기, 인터쿨러 등 요소부품에 대해 모델링을 수행하고 R-410A를 사용하는 냉방 능력 8 kW, 냉동 능력 15 kW의 캐스케이드 냉동 사이클의 요소 부품의 - 상부 응축기, 하부 응축기, 냉방 증발기, 냉동 증발기, 인터쿨러, 압축기, 전자팽창변 - 설계를 수행하였다. 설계 사양에 대하여 외기 온도를 $26^{\circ}C$에서 $38^{\circ}C$로 변화시키며 해석을 수행한 결과 냉각 열량은 하부 증발기에서는 거의 일정하고 상부 증발기에서는 9% 감소, 인터쿨러에서는 63% 증가하였다. 한편 COP는 외기 온도의 증가에 따라 감소하였다. 인터쿨러가 작동하지 않는 사이클 대비 인터쿨러 사이클이 COP 측면에서 우위를 보였다. 또한 상부 응축기의 크기를 당초 설계치의 2배 증가시키면 하부 증발기 열량은 변함이 없는 반면 상부 증발기 열량은 4% 증가하였다. 한편 상부 응축기의 크기 증가에 따라 상부 사이클의 COP는 증가하는 반면 하부 사이클의 COP는 큰 변화가 없다. 또한 하부 응축기 크기를 2.8배 증가시키면 상하부 증발기의 열량 변화는 거의 없고 인터쿨러의 열량만이 8% 감소하였다. 아울러 하부 사이클의 COP는 응축기의 크기가 증가함에 따라 다소 증가하였으나 상부 사이클의 경우는 그 변화가 미미하였다.

Keywords

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Fig. 1. Schematic drawing of the cascade refrigeration system

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Fig. 2. Schematic drawing showing the control volume of the refrigeration condenser or evaporator

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Fig. 3. Heat transfer coefficient of Park and Hrnjak[8] data predicted by the present model

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Fig. 4. Pressure drops of Park and Hrnjak[8] datapredicted by the present model

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Fig. 5. Heat transfer coefficient of Anowar et al.[12] data predicted by the present model

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Fig. 6. Pressure drops of Anowar et al.[12] data predicted by the present model

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Fig. 7. Schematic drawing showing the control volume of the intercooler

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Fig. 8. Cascade cycle of this study

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Fig. 9. Flow chart of the present cascade system

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Fig. 10. Cooling capacity of the upper and lower cycle at different outdoor temperature

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Fig. 11. Power consumption of the upper and lower cycle at different outdoor temperature

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Fig. 12. COP of the upper and lower cycle at different outdoor temperature

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Fig. 13. Cooling capacity of the upper and lower cycle with the increase of upper condenser size

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Fig. 14. Power consumption of the upper and lower cycle with the increase of upper condenser size

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Fig. 15. COP of the upper and lower cycle with the increase of upper condenser size

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Fig. 16. Cooling capacity of the upper and lower cycle with the increase of the lower condenser size

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Fig. 17. Power consumption of the upper and lower cycle with the increase of the lower condenser size

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Fig. 18. COP of the upper and lower cycle with the increase of the lower condenser size

Table 1. Air and refrigerant temperatures used for the design of the refrigeration components.

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Table 2. Preliminary design specification of the cascade refrigeration system having 15kW and 8 kW cooling capacity at upper and lower cycle

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