• 동력기계공학회지
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• 제13권6호
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• pp.70-75
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• 2009

In this paper, cycle performance analysis of R744($CO_2$) two-stage compression and one-stage expansion refrigeration system is presented to offer the basic design data for the operating parameters of the system. The operating parameters considered in this study include superheating degree, compressor efficiency, gas cooling pressure, mass flowrate ratio, outlet temperature of gas cooler and evaporating temperature in the carbon dioxide two-stage refrigeration cycle. The main results were summarized as follows : The cooling capacity of two-stage compression and one-stage expansion refrigeration system increases with the increasing superheating degree, compressor efficiency and gas cooling pressure, but decreases with the increasing mass flowrate ratio and evaporating temperature. The compression work of two-stage compression and one-stage expansion refrigeration system increases with the increasing superheating degree, outlet temperature of gas cooler, gas cooling pressure and evaporating temperature, but decreases with the increasing compressor efficiency and mass flowrate ratio. The COP of two-stage compression and one-stage expansion refrigeration system increases with the increasing compressor efficiency, but decreases with the increasing superheating degree, gas cooling pressure, mass flowrate ratio and evaporating temperature. Therefore, superheating degree, compressor efficiency, gas cooling pressure, mass flowrate ratio, outlet temperature of gas cooler and evaporating temperature of R744($CO_2$) two-stage compression and one-stage expansion refrigeration system have an effect on the cooling capacity, compressor work and COP of this system.

• 한국유체기계학회 논문집
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• 제10권2호
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• pp.16-21
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• 2007

Potential advantages of using vapor injection in a two stage rotary compressor for a $CO_2$ heat pump water heater system were addressed in this paper by numerical simulation. Vapor separated from a flash tank in the middle of the expansion process can be used for injection into the second stage suction plenum of the compressor to improve the system performance. Vapor injection increases the intermediate pressure between the two stages, thus increasing the first stage compressor work and reducing that of the second stage. As a whole, however, the compressor input power increases due to injected mass flow rate for the second stage. Computer simulation showed that increment of the cooling capacity by vapor injection exceeded that of the compressor work, thus improving the system performance. COP improvement by vapor injection was calculated to be about 5-14% for normal operating conditions. With vapor injection, a maximum COP was found when the displacement volume of the second stage becomes 90-95% of that of the first stage of the compressor.

• 설비공학논문집
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• 제17권1호
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• pp.1-7
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• 2005

The heat transfer characteristics of an internal heat exchanger for $CO_2$ refrigeration cycle are numerically investigated. The numerical model is verified using the published experimental results for the concentric tube type internal heat exchanger. The Hardy-Cross Method gives very good agreement between the calculation and experimental results on the heat transfer rates and exit temperatures. Also, appropriate combination of heat transfer correlations is found. The operating parameters of the heat exchanger are calculated at transcritical region of $CO_2.$ The heat transfer rate of the counter flow type heat exchanger shows the $32\%$ greater than that of the parallel flow type heat exchanger. The increase of heat exchanger length enhances the heat transfer rate. The thermodynamic characteristics and heat transfer coefficient of $CO_2$ in the internal heat exchanger are estimated.

• 한국자동차공학회논문집
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• 제17권1호
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• pp.146-153
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• 2009

The performance characteristics of heat exchangers which consist of a gas cooler, an evaporator and an internal heat exchanger have been investigated at various operating conditions of $CO_2$ air conditioning system by experiments. The heat exchangers were designed for use in the vehicle $CO_2$ air conditioning system, when considering the characteristics of heat transfer and high pressure as $CO_2$ refrigerant. This paper studied the performance of heat exchangers at various compressor speeds and expansion valve openings, and quantified the heat transfer rates and pressure drops. Heat transfer rates at the gas cooler and the evaporator were 6.9 kW and 5.2 kW, respectively, when the compressor speed was 4000 rpm and refrigerant vapor quality at the evaporator outlet was 0.98. Therefore, this paper carried out that the heat exchangers were analyzed to achieve superior performance for the vehicle transcritical $CO_2$ cycle.

• 설비공학논문집
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• 제20권4호
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• pp.230-237
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• 2008

The objective of this paper is to investigate the performance characteristics of a $CO_2$ cooling and water heating system using a twin-rotary compressor with the compression volume ratio of 0.6. The cooling performances of the $CO_2$ heat pump were measured and analyzed with the variations of charge amount, EEV opening, and compressor frequency. In addition, the performance of the combined system including cooling and water heating was also measured and analyzed by varying inlet temperature of the EEV. As a result, the optimal normalized charge and cooling COP in the cooling mode were 0.307 and 2.06, respectively. The application of the water heating into the $CO_2$ heat pump improved the cooling performance over 78% and decreased the EEV inlet temperature by $8^{\circ}C$, which can increase system reliability.

• Journal of Advanced Marine Engineering and Technology
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• 제33권6호
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• pp.827-834
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• 2009

The cooling heat transfer coefficient of $CO_2$ in a brazing type small diameter tube was investigated experimentally. The main components of the refrigerant loop are a receiver, a $CO_2$ compressor, a mass flow meter, an evaporator and a brazing type small diameter tube as a test section. The mass flux of $CO_2$ is $400{\sim}1600$ [kg/$m^2s$], the mass flowrate of coolant were varied from 0.15 to 0.3 [kg/s], and the cooling pressure of gas cooler were from 8 to 10 [MPa]. The cooling heat transfer coefficients of the brazing type small diameter copper tube is about $4{\sim}11.7%$ higher than that of the conventional type small diameter copper tube. In comparison with test results and existing correlations, correlations failed to predict the cooling heat transfer coefficient of $CO_2$ in a brazing type small diameter copper tube. therefore, it is necessary to develope reliable and accurate predictions determining the cooling heat transfer coefficient of $CO_2$ in a brazing type small diameter copper tube.

• 한국산학기술학회논문지
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• 제11권12호
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• pp.4721-4726
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• 2010

본 논문은 압축기 인버터 주파수 제어에 따른 CO2용 수냉식 열펌프의 성능 특성에 대해서 실험적으로 조사하였다. 실험장치는 압축기, 가스냉각기, 팽창밸브, 증발기, 내부 열교환기, 수액기로 구성된다. 실험장치에 사용된 모든 열교환기는 동관으로 제작된 이중관식 대향류형이다. 가스냉각기와 증발기는 2.4 m인 소시험부 6개와 4개로 각각 구성된다. 실험결과를 요약하면, 우선 증발기와 가스냉각기의 입구온도와 냉매유량이 일정한 조건하에서 압축기 인버터 주파수가 증가할수록 압축비와 토출압력이 증가한다. 또한 인버터 주파수가 증가할수록 난방능력과 압축일량은 증가하는 반면 성능계수는 감소한다. 그리고 증발기 입구 2차유체의 온도가 $15^{\circ}C$에서 $25^{\circ}C$로 증가함에 따라 압축비와 압축일량은 감소하지만 질량유량, 난방능력, 성능계수는 증가한다. 위의 이러한 경향은 종래의 프레온계 냉매 시스템의 성능 변화와 유사하다.