(The)Performance Characteristics of a Refrigeration System with an Accumulator Heat Exchanger

국문 초록 (Abstract)

멀티에어컨 채용에 따른 연장배관의 길이 및 고저차의 증가는 팽창변 입구에서의 플래쉬가스 발생가능성을 증가시키게 된다. 본 논문에서는 냉동시스템에서의 플래쉬가스에 의한 문제점들을 분석하였고, 이러한 문제점들을 줄이기 위한 방안으로 열교환기 내장형 어큐뮬레이터를 도입하였다. 열교환기 내장형 어큐뮬레이터에서의 열전달 메카니즘을 검토하기 위하여 가시화실험을 실시하였으며, 다양한 운전조건, 적용냉매 및 기하학적 변수에 따른 열전달 및 압력강하 성능을 측정 하였다. 열교환기 내장형 어큐뮬레이터가 냉동시스템에 미치는 영향을 평가하였으며, 시스템의 성능을 향상시키기 위한 방안으로 액냉매 분사 시스템을 적용·검토하였다. 팽창변 입구에서의 플래쉬가스 발생은 냉매유량을 현저하게 감소시킬 뿐만 아니라, 냉동시스템의 운전특성을 주기적으로 변화시킴으로써 시스템의 효율 및 신뢰성을 감소시키게 된다. 가시화 실험결과 로부터 열교환기 내장형 어큘뮬레이터 내부에서는 가스 대 액의 열전달이 발생함을 알 수 있었다. 냉매유량의 증가에 따라 열전달율은 증가하였으나 과냉각도차는 감소하였으며, 입구 과냉각도 및 과열도가 감소함에 따라 열전달율 및 과냉각도차 모두 증가하였다. 동일 유량 조건에서 HCFC-22대비, HFC-410A와 HFC-134a의 열전달율은 각각 40% 및 23% 증가하였으나, 과냉각도는 HFC-410A가 9% 감소하였고, HFC-134a는 11% 증가하였다. 또한, 열교환기 내장형 어큐뮬레터의 기하학적 형상이 열전달 및 압력강하 특성에 미치는 영향을 검토하여 설계를 위한 기초 자료를 제공하였다. 상기의 실험결과를 바탕으로 과냉각도 예측을 위한 무차원 상관식을 개발하였으며, 실측결과와의 비교에 의해 정도가 ±15% 이내임을 알 수 있었다. 동일 팽창변 개도조건에서 열교환기 내장형 어큐뮬레이터의 장착은 미장착의 경우보다 냉매유량의 증가뿐만 아니라, 시스템 효율 및 신뢰성의 향상을 가져왔다. 또한, 열교환기 내장형 어큐뮬레이터 장착시에는 최대 냉방능력 및 효율이 미장착의 경우보다 낮은 팽창변 개도에 나타남을 알 수 있었다. 따라서, 멀티에어컨에 열교환기 내장형 어큐뮬레이터를 장착할 경우에는 시스템의 효율 및 신뢰성 향상을 위해서 팽창변 개도에 대한 최적제어가 필요할 것으로 판단된다. 마지막으로 과냉각도 특성을 향상시키기 위한 추가적인 방안으로 열교환기 내장형 어큐뮬레이터 내부로의 냉매액 분사시스템을 도입하였으며, 이러한 시스템의 적용은 과냉각도를 향상시킬 뿐만 아니라 압축기 입구의 과열도를 감소시킴으로써 압축기 토출온도의 감소라는 부가적인 효과를 나타냄을 알 수 있었다.

다국어 초록 (Multilingual Abstract)

The increase of pipe length and height in a multi air-conditioner degrades the performance and reliability of the system due to the flash gas generation at the expansion device inlet. In this study, the problems caused by the flash gas were defined, and the AHX was introduced as a method to reduce the possibility of flash gas generation. To clarify the heat transfer mechanism inside the AHX, a visualization study was conducted at transient and steady state conditions. The performance of the AHX was measured with the variations of operating condition, refrigerant and geometric parameter. The effects of the AHX on a refrigeration system were investigated and compared with those of the non-AHX system for various EEV openings. Finally, the liquid refrigerant injection system was introduced and investigated to improve the system performance.
Due to the flash gas generation at the expansion device inlet, the flow rate was reduced remarkably and the performance of the refrigeration system was fluctuated periodically resulting in the decrease of system efficiency. The surging operation of the refrigeration system also degraded system reliability by increasing the compression ratio and the discharge temperature of the compressor. From the visualization tests, the gas-to-liquid heat transfer mechanism was observed in the AHX at a steady-state condition. The heat transfer rate in the AHX increased with the increment of the flow rate, while the subcooling difference decreased. The reductions of the superheat and subcooling at the AHX inlet increased the heat transfer rate due to the increased temperature difference between the accumulator and IHX. The heat transfer rates of HFC-410A and HFC-134a increased by 40% and 23%, respectively, compared with that of HCFC-22, while the subcooling of HFC-410A decreased by 9% and the subcooling of HFC-134a increased by 11%. The effects of the geometric parameters on the heat transfer and pressure drop characteristics were investigated and a generalized dimensionless correlation was developed to predict the subcooling difference in the AHX with the accuracy of 15%.
The AHX system showed more refrigerant flow rate than the non-AHX system at a constant EEV opening due to higher subcooling, resulting in better performance and reliability. The maximum cooling capacity and COP of the AHX system were observed at lower EEV openings as compared with those for the non-AHX system due to the increased mass flow rate with the AHX installation. Therefore, optimum control of EEV opening is essential to achieve higher system performance and reliability in multi air-conditioners having the AHX. The liquid refrigerant injection system was investigated for the improvement of the subcooling and system performances. The injection system increased the subcooling at the expansion device inlet and decreased the superheat at the compressor inlet resulting in the decrease of the discharge temperature of the compressor.

목차 (Table of Contents)

• Abstract
• Abstract (In Korean)
• Nomenclature
• Contents
• Chapter 1 Introduction
• 1.1 Background
• 1.2 Literature review
• 1.2.1 Performance of an expansion device on two-phase flow conditions
• 1.2.2 Visualization of refrigerant flow inside an accumulator
• 1.2.3 AHX and its influence on a refrigeration system
• 1.2.4 Alternative refrigerants for air-conditioners
• 1.2.5 Liquid refrigerant injection system
• 1.3 Objectives of this study and the outline
• Chapter 2 Experimental Setup and Methods
• 2.1 Introduction
• 2.2 Overview of experimental setup
• 2.3 Control and measurement system
• 2.3.1 Control methods
• 2.3.2 Control and measurement equipments
• 2.4 Data reduction and measurement reliability
• 2.4.1 Data reduction
• 2.4.2 Measurement reliability
• Chapter 3 Influence of Flash Gas at the Expansion Device Inlet on the Dynamic Characteristics of a Ref- rigeration System
• 3.1 Introduction
• 3.2 Experimental setup and test procedure
• 3.3 Influence on the dynamic characteristics of a refrigeration system
• 3.4 Comparison of the performance according to flash gas ratio
• 3.5 Conclusions
• Chapter 4 Flow Phenomena and Heat Transfer Characteris- tics of Refrigerant-Oil Mixture in AHX
• 4.1 Introduction
• 4.2 Experimental setup and test procedure
• 4.3 Inner flow phenomena and heat transfer characteristics in AHX
• 4.4 Inner flow phenomena and heat transfer characteristics according to EEV opening
• 4.5 Conclusions
• Chapter 5 Heat Transfer and Pressure Drop Characteristics of AHX
• 5.1 Introduction
• 5.2 Experimental setup and procedure
• 5.3 Effects of operating condition
• 5.4 Effects of refrigerant
• 5.5 Effects of geometric parameter
• 5.6 Development of a dimensionless correlation
• 5.7 Conclusions
• Chapter 6 Effects of AHX on the Performance of a Refrige- ration System
• 6.1 Introduction
• 6.2 Experimental setup and test procedure
• 6.3 Test results and discussion
• 6.4 Conclusions
• Chapter 7 Effects of Liquid Refrigerant Injection System on the Performance of an AHX and Refrigeration System
• 7.1 Introduction
• 7.2 Experimental setup and test procedure
• 7.3 Effects of liquid injection on flow rate and subcooling
• 7.4 Effects on the performance of a refrigeration system
• 7.5 Conclusions
• Chapter 8 Concluding Remarks
• 7.1 Conclusions
• 7.2 Future works
• References
• Appendix. Uncertainty analysis
• List of Figures
• Fig. 1.1 Configuration of a refrigeration system with AHX adoption
• Fig. 1.2 (a) Configuration of AHX and (b) P-h diagram
• Fig. 1.3 Objectives, procedures and contents of the study
• Fig. 2.1 Schematic of the experimental setup
• Fig. 2.2 Mechanical components section
• Fig. 2.3 Measurement and control section
• Fig. 2.4 Energy balance in the evaporator
• Fig. 3.1 Schematic of the experimental setup
• Fig. 3.2 Operation of a refrigeration system according to the flash gas ratio
• Fig. 3.3 Variations of subcooling, flow rate, compressor suction and discharge pressures with time at the flash gas ratio of 2.72
• Fig. 3.4 Variations of compression ratio, cooling capacity, compressor power consumption and COP with time at the flash gas ratio of 2.72
• Fig. 3.5 Variations of average and minimum flow rate ratios according to the flash gas ratio
• Fig. 3.6 Variations of suction pressure ratio, discharge pressure ratio and the ratio of compression ratio according to the flash gas ratio
• Fig. 3.7 Variations of maximum compression ratio and discharge temperature according to the flash gas ratio
• Fig. 3.8 Variations of cooling capacity ratio, compressor power consumption ratio and COP ratio according to the flash gas ratio.
• Fig. 3.9 Operating characteristics of a refrigeration system according to the flash gas ratio at the minimum capacity point
• Fig. 4.1 Configuration of the transparent AHX
• Fig. 4.2 Schematic of the experimental setup
• Fig. 4.3 Inner flow phenomena of the refrigerant-oil mixture in the AHX
• Fig. 4.4 Variation of the temperatures at accumulator with time
• Fig. 4.5 Variation of the temperature at the IHX with time
• Fig. 4.6 Variation of the temperature difference at accumulator according to EEV opening
• Fig. 4.7 Variation of the excess temperature according to EEV opening
• Fig. 4.8 Variation of the mixture’s level according to EEV opening
• Fig. 4.9 Variation of the temperature difference between IHX inlet and outlet according to EEV opening
• Fig. 5.1 Schematic of the experimental setup
• Fig. 5.2 Variation of the heat transfer rate according to refrigerant flow rate
• Fig. 5.3 Variations of the subcooling and superheat differences according to the refrigerant flow rate
• Fig. 5.4 Variation of the heat transfer rate according to the inlet superheat and quality
• Fig. 5.5 Variation of the superheat and subcooling differences according to the inlet superheat and quality
• Fig. 5.6 Variation of the heat transfer rate according to the inlet subcooling.
• Fig. 5.7 Variations of the superheat and subcooling differences according to the inlet subcooling
• Fig. 5.8 Variation of the pressure drop in the accumulator
• Fig. 5.9 Variation of the heat transfer rate according to the flow rate
• Fig. 5.10 Variation of the subcooling difference according to the flow rate
• Fig. 5.11 Variation of the heat transfer rate according to the evaporating temperature
• Fig. 5.12 Variation of the subcooling difference according to the evaporating temperature
• Fig. 5.13 Variation of the heat transfer rate according to the condensing temperature
• Fig. 5.14 Variation of the subcooling difference according to the condensing temperature
• Fig. 5.15 Variation of the pressure drop at the accumulator
• Fig. 5.16 Variations of the heat transfer rate and pressure drop according to the accumulator inner volume
• Fig. 5.17 Variations of the heat transfer rate and pressure drop according to the accumulator diameter/height (d/h) ratio
• Fig. 5.18 Variations of the heat transfer rate and pressure drop according to the IHX coil bending diameter
• Fig. 5.19 Variations of the heat transfer rate and pressure drop according to the IHX coil length
• Fig. 5.20 Variations of the heat transfer rate and pressure drop according to the IHX coil diameter
• Fig. 5.21 Effects of the geometric parameters on the performance of the AHX
• Fig. 5.22 Comparison between the measured and predicted subcoolings
• Fig. 5.23 Comparison between the measured and predicted heat transfer rates
• Fig. 6.1 Schematic of the experimental setup
• Fig. 6.2 Variation of refrigerant mass flow rate with EEV opening
• Fig. 6.3 Variation of subcooling with EEV opening
• Fig. 6.4 Variation of superheat with EEV opening
• Fig. 6.5 Variation of compressor discharge and suction pressures with EEV opening
• Fig. 6.6 Variation of power consumption and cooling capacity with EEV opening
• Fig. 6.7 Variation of COP with EEV opening
• Fig. 6.8 Influence of AHX on the refrigeration cycle
• Fig. 7.1 (a) Configuration of the liquid refrigeration injection system with AHX and (b) P-h diagram
• Fig. 7.2 Schematic of the experimental setup
• Fig. 7.3 Variation of the flow rate according to liquid injection ratio
• Fig. 7.4 Variation of heat transfer rate and the subcooling according to liquid injection ratio
• Fig. 7.5 Variation of the compressor pressures and compression ratio according to liquid injection ratio
• Fig. 7.6 Variation of the discharge and suction temperatures of compressor according to liquid injection ratio
• Fig. 7.7 Variation of the cooling capacity, power consumption and COP according to liquid injection ratio
• Fig. 7.8 P-h diagram showing the effects of liquid refrigerant injection
• List of Tables
• Table 2.1 Control parameters and test range
• Table 2.2 Items for temperature measurement and control
• Table 2.3 Items for pressure measurement and control
• Table 2.4 Items for flow rate and power consumption
• Table 2.5 Control items and methods
• Table 2.6 Specification of resistance temperature detector
• Table 2.7 Specification of thermocouple
• Table 2.8 Specification of pressure transducer
• Table 2.9 Specification of differential pressure transducer
• Table 2.10 Specification of mass flow meter
• Table 2.11 Specification of volumetric flow meter
• Table 2.12 Specification of electronic expansion valve
• Table 2.13 Specification of power meter
• Table 2.14 Specification of P.I.D controller
• Table 2.15 Specification of inverter
• Table 2.16 Specification of power regulator
• Table 2.17 Specification of data recorder
• Table 2.18 Comparison of test results for measurement reliability
• Table 2.19 Reproductive test results at superheated condition
• Table 2.20 Reproductive test results at two-phase condition
• Table 5.1 Control parameters and tolerances
• Table 5.2 The experimental parameters and ranges
• Table 5.3 Specifications of the AHX
• Table 5.4 Properties of the refrigerants
• Table 5.5 Experimental conditions
• Table 5.6 Geometric specifications of the baseline AHX
• Table 5.7 Experimental results of the baseline AHX
• Table 5.8 Dimensionless Pi-groups
• Table 7.1 Comparison of the specification of electronic expansion valves
• Table A.1 Summary of uncertainty analysis