• Journal of the Korea Academia-Industrial cooperation Society
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• v.15 no.2
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• pp.625-631
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• 2014

In order to obtain a low evaporation temperature ranging from $-30^{\circ}C{\sim}-50^{\circ}C$, a cascade refrigeration system and two-stage compression one-stage expansion refrigeration system is required. However, the research results of performance comparison of these refrigeration system are very scarce. This paper were compared the performance characteristics of R744-R404A cascade refrigeration system and R404A two-stage compression refrigeration system. The COP of R404A two-stage compression refrigeration system is about 36~57% greater than that of R744-R404A cascade refrigeration system in the range of evaporation temperature of $-30^{\circ}C{\sim}-50^{\circ}C$. But R404A two-stage compression refrigeration system is unstable because COP is significantly changed when evaporating temperature and compressor efficiency decreased. In particular, when compressor efficiency decreased, COP is significantly decreased. In this case, not efficient for long-term use. Whereas R744-R404A cascade refrigeration system using natural refrigerants. Therefore, it is environmentally friendly. And this system is high-efficiency refrigeration system. The reason it can be configured by selecting the suitable refrigerant at high-temperature side and low-temperature side. From the above results, select the appropriate low temperature refrigeration system by considering the environmental and performance aspects.

• Journal of the Korean Institute of Gas
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• v.16 no.3
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• pp.42-47
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• 2012

In this paper, alternative natural refrigerant R290(Propane), R600(Butane), R717(Ammonia), R1270(Propylene) for freon refrigerant R22 were used working fluids for 2-stage compression and 1-stage expansion refrigeration system. The operating parameters considered in this study included evaporation temperature, condensation temperature, subcooling degree, superheating degree, mass flow rate ratio of inter-cooler. The main results were summarized as follows : The COP of 2-stage compression and 1-stage expansion refrigeration system increases with the increasing subcooling degree and mass flow rate ration of inter-cooler, but decreases with the increasing evaporating temperature, condensing temperature and superheating degree. Therefore, subcooling degree, mass flow rate ratio of inter-cooler of 2-stage compression and 1-stage expansion refrigeration system using natural refrigerants have an effect on COP of this system. The COP of natural refrigerants was higher than the COP of freon R22 in this study, so points to be considered are the security, the attached facilities for natural refrigerants than COP.

• Proceedings of the KSME Conference
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• 2005.11a
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• pp.106.1-106.1
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• 2005

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.1
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• pp.17-24
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• 2012

There has been little study on the performance characteristics of the effective heat pump systems in the cooling mode using the two-stage compression cycles have hardly performed. In this study, the performance of the two-stage compression $CO_2$ cycle with an FI (flash intercooler) and the FGB (flash gas bypass) was investigated by using a theoretical method. The performance analysis was carried out with aby varyingiation (the indoor temperature, outdoor temperature, and 1st- and 2nd-stage EEV openings. As of a result, the coefficients of performance (COPs) of the Bbasic, FI, and FGB cycles were decreased by 28.5%, 22.1%, and 24.5%, respectively, for various outdoor temperature conditions. In addition to, the performance variation of the two-stage compression cycle was smaller than that of the single-stage compression cycle. The performance of the FI and FGB cycles was improved by 13.5%, and 6.9%, respectively, when the 1st-stage EEV opening was increased from 32% to 48%, and by 0.9%, and 2.6%, respectively, when the 1st- andthe 2nd-stage EEV opening was increased from 32% to 48%, andwas increased from 42% to 58%, respectively. The FI cycle showed anthe most improved performance for any given operating conditions.

• The Magazine of the Society of Air-Conditioning and Refrigerating Engineers of Korea
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• v.38 no.2
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• pp.14-17
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• 2009

• Korea Journal of Geothermal Energy
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• v.4 no.1
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• pp.45-50
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• 2008

• The Magazine of the Society of Air-Conditioning and Refrigerating Engineers of Korea
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• v.31 no.7
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• pp.45-52
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• 2002

1991년에 Maryland 대학에 Dr. Reinhard Rader-macher에 의해 환경에너지공학연구소(CEEE)가 설립되었다. 이 연구소는 환경 및 경제적인 관점에서 에너지 변환 시스템을 개발하는데 선두적인 역할들 수행해왔다. 환경 에너지 공학 연구소는 산업체, 정부,및 연구소에서 지원 받는 컨소시엄 형태의 연구 센터이다. 대체 냉매, $CO_2$초월임계 사이클에 관한 연구를 1993년에 시작한 이래, 현재 세계적으로 40여 개의 회사가 지원을 하고 있다. 2단 압축 $CO_2$ 사이클 최적화, 초월 임계 사이클에서의 오일에 따른 열전달 영향, 초월$CO_2$임계 시스템에서의 오일 정체, $CO_2$압축기 모델링, 자동차에서의 $CO_2$기후 조절 시스템, $CO_2$냉매를 이용한 에어컨, $CO_2$저온 냉동 시스템 등에 관한 연구를 수행하고 있다. CEEE는 향후 연구로 구성요소 및 시스템 최적화, 효율향상, 시스템 적용확대에 관한 연구를 할 예정이다. 센터는 보고서, 컨소시엄 미팅. 워크샵, 교과목 개설, 당문 연구자 초청들을 통해 산업계 및 기술을 전달하고 있다. 본 고에서는 환경에너지 공학 연구소에서 $CO_2$ 초월임계 사이클에 초점을 맞추어 연구소의 연구활동을 기술한다.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.18 no.8
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• pp.641-648
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• 2006

In this study, a two-stage phase-separate cycle was investigated analytically to improve the performance of the $CO_2$ system in the cooling mode. The simulation results were verified with the measured data. The predictions using the simulation model were consistent with the measured data within ${\pm}20%$ deviations. The performance of the modified $CO_2$ system with the two-stage phase-separated cycle was analyzed with the variations of outdoor temperature and EEV opening. The cooling COP decreased with the increase of compressor frequency. The highest COP was 2.7 at compressor frequencies of 30 Hz and 30 Hz for the first and second compressors, respectively. In addition, the cooling COP increased by 9.3% with an application of optimum control of the first and second-stage EEV openings.

• Journal of IKEEE
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• v.2 no.2 s.3
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• pp.233-238
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• 1998

In this paper, the implementation of dual rate ADPCM using G.726 16Kbps and 40Kbps speech codec algorithm is handled. For small signals, the low rate 16Kbps coding algorithm shows almost the same SNR as the high rate 40Kbps coding algorithm , while the high rate 40Kbps coding algorithm shows the higher SNR than the low rate 16Kbps coding algorithm fur large signal. To obtain the good trade-off between the data rate and synthesized speech quality, we applied low rate 16Kbps for the small signal and high rate 40Kbps for the large signal. Various threshold values determining the rate are applied for good trade-off between data rate and speech quality. The simulation result shows the good speech quality at a low rate comparing with 16Kbps & 40Kbps.

• Proceedings of the SAREK Conference
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• 2006.06a
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• pp.501-505
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• 2006

A preliminary performance test of a 30RT 2-stage screw heat pump was carried out in order to develop a high performance large-scale unutilized energy source heat pump, which will be used in district heating and cooling. Two issues on the system control were investigated in this study, A stable 2-stage heating operation is guaranteed only if the load-side water inlet temperature is over a certain value, to where the 1-stage heating operation should be done first from a cold start. An oil shortage problem in low stage compressor, which depends on the degree of suction superheat, was solved by the proper oil level control scheme.