• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.27 no.10
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• pp.497-505
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• 2015

The evaporating temperature range required for the low temperature freezing system is from $-50^{\circ}C$ to $-30^{\circ}C$. Since it is difficult to keep the required capacity in a cabinet, it is advantageous to design the system using a cascade refrigeration system. Use of carbon dioxide and ammonia would be advantageous since ammonia is an environment-friendly working fluid and has a high capacity for performance improvement. To investigate the performance characteristics of the R744-R717 cascade refrigeration system, a theoretical model was developed and performance was analyzed according to cascade heat exchanger operating temperature. The optimal cascade R744 condensing temperature was $-5^{\circ}C$, and maximum COP was 1.13 when the temperature difference of the cascade heat exchanger was $5^{\circ}C$. In addition, the total system COP increased by 1.17 when the cascade temperature gap was $3^{\circ}C$ at the middle temperature of $-7.5^{\circ}C$.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.28 no.5
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• pp.202-207
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• 2016

Experimental studies for an indirect R404A-$CO_2$ refrigeration system and a direct R404A refrigeration system were conducted. The configurations of the indirect R404A-$CO_2$ refrigeration system are a R404A refrigeration system as a top cycle and a circulating $CO_2$ system as a bottom cycle. The direct R404A system was modified from indirect R404A-$CO_2$ refrigeration system by removing circuit for $CO_2$ circulation. Various tests for both systems were conducted by changing load side brine temperature from 0 to 5 and $10^{\circ}C$ with cooling brine temperatures for R404A system at 15, 20, or $25^{\circ}C$. The indirect R404A-$CO_2$ refrigeration system showed the highest COP when load side brine temperature was at $10^{\circ}C$ in the evaporator and at cooling brine temperature of $15^{\circ}C$. The COP of 3.04 under that condition was the highest. This indirect R404A-$CO_2$ refrigeration system showed 9.02% higher COP than the direct R404A system that had increased pipeline length of 15 m, which simulated actual installation in a supermarket.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.10 no.10
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• pp.2565-2571
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• 2009

In this paper, cycle performance analysis of cascade refrigeration system using $NH_3-CO_2$(R717-R744) is presented to offer the basic design data for the operating parameters of the system. The operating parameters considered in this study include subcooling and superheating degree and condensing and evaporating temperature in the ammonia(R717) high temperature cycle and the carbon dioxide low temperature cycle. The COP of cascade refrigeration system increases with the increasing superheating degree, but decreases with the increasing subcooling degree. The COP of cascade refrigeration system increases with the increasing condensing temperature, but decreases with the increasing evaporating temperature. Therefore, superheating and subcoolng degree, evaporating and condensing temperature of cascade refrigeration system using $NH_3-CO_2$ have an effect on the COP of this system. A multilinear regression analysis was employed in terms of subcooling, superheating, evaporating, condensing, and cascade heat exchanger temperature difference in order to develop mathematical expressions for maximum COP and an optimum evaporating temperature.