공학박사 학위논문 중앙집중 물냉각기에 직렬-직렬 대향류 방식을 채택한 공기조화시스템의 성능 해석과 최적화 Overall Optimization and Performance Analysis of an Air Conditioning System by adopting Series-Series Counter-flow to Central Water Chillers 울산대학교 대학원 기계공학과 Nguyen Minh Phu Doctor of Philosophy Overall Optimization and Performance Analysis of an Air Conditioning System by adopting Series-Series Counter-flow to Central Water Chillers The Graduate School of the University of Ulsan Department of Mechanical Engineering Nguyen Minh Phu Overall Optimization and Performance Analysis of an Air Conditioning System by adopting Series-Series Counter-flow to Central Water Chillers 지 도 교 수 이근식 이 논문을 공학박사 학위 논문으로 제출함 2012 년 5 월 울산대학교 대학원 기계공학과 Nguyen Minh Phu Overall Optimization and Performance Analysis of an Air Conditioning System by adopting Series-Series Counter-flow to Central Water Chillers Supervisor: Prof. Geun Sik Lee A Dissertation Submitted to the Graduate School of the University of Ulsan In partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy by Nguyen Minh Phu Department of Mechanical Engineering University of Ulsan, Republic of Korea May, 2012 Nguyen Minh Phu 의 공학박사 학위 논문을 인준함 심 사 위원장 김무현 인 심 사 위 원 이근식 인 심 사 위 원 원성필 인 심 사 위 원 이목인 인 심 사 위 원 김무근 인 울산대학교 대학원 2012 년 5 월 Overall Optimization and Performance Analysis of an Air Conditioning System by adopting Series-Series Counter-flow to Central Water Chillers This certifies that the dissertation of Nguyen Minh Phu is approved. Committee Chairman Prof. MOO HYUN KIM Committee Member Prof.
GEUN SIK LEE Committee Member Prof. SUNG PIL WON Committee Member Prof. MOK IN LEE Committee Member Prof. MOO GEUN KIM Department of Mechanical Engineering University of Ulsan, Republic of Korea May, 2012 ACKNOWLEDGEMENTS I would like to express my heartfelt gratitude to my advisor, Prof.-Ing Geun Sik Lee, who not only gives me priceless guidance in scientific work but also supports me the financial help for three and half years studying in the University of Ulsan.
I would like to thank professors in the committee for their suggestions and comments throughout the research. Sincerely thank to University of Ulsan that grants me the scholarship and opportunity for studying in Korea. I also express my gratitude to professors and staff members of the Department of Mechanical Engineering for their kind help. Special thank to Korea and Korean people for their civilization and friendship.
I would like to thank my family members, who encourage and suffer a separation during the period I am away from home. Specially thank to all members of the Entire Energy Harmony laboratory who support me a lot in my research work. I’d like to thank the Union of Vietnamese Students at University of Ulsan for spiritual help and consulting abroad living experience, and close friends in Vietnam who always look forward to my returning our motherland. Ulsan, Republic of Korea.
May, 2012 NGUYEN MINH PHU CONTENTS Abstract. iii List of figures. vi List of tables .2 Problem statement, necessity, objectives and thesis layout. Analysis and design of a water chiller using commercial enhanced tubes .1 Heat transfer model .2 Water pressure drop model .3 Results and discussion .4 Comparison to commercial software.
Effect of water temperature and water flow rate on wet cooling tower .2 Merkel’s theory and the empirical correlations .3 Formulation of psychrometric properties of moist air .4 Solution procedure and validation .5 Results and discussion. Experimental and modeling studies of a dehumidifying cooling coil .2 Enthalpy potential method .3 Experimental set-up .5 Results and discussion. Overall optimization and exergy analysis of the air-conditioning system with series-series counter-flow chillers .2 Description of the central air conditioning system and solution procedure .3 Results and discussion. Discussion, conclusions, and future work .1 Discussion and conclusions.
105 List of publications. 114 -ii- Overall Optimization and Performance Analysis of an Air Conditioning System by adopting Series-Series Counter-flow to Central Water Chillers By Nguyen Minh Phu Department of Mechanical Engineering Graduate School, University of Ulsan ABSTRACT When central water chillers in air conditioning are arranged in series-series counter-flow (SSCF), compressor lift of each chiller decreases in comparison with that of water chillers in parallel. That means that the compressor power of the chillers in SSCF is lower than that of chillers in parallel. However, the water pressure drop of the SSCF chillers increases, i.
increase in power of water pumps, because the water flow rates of SSCF chillers are the flow rates of the whole system. This disadvantage will be solved by increasing the temperature difference of water flow through an evaporator and condenser, but the water flow rates will decrease. Under the conditions of high temperature differences and low water flow rates, cooling coil and cooling tower may be affected. Hence, in this thesis, we consider overall air conditioning system including SSCF water chillers, cooling coil and cooling tower.
Those components are modeled and simulated in viewpoints of the first and second laws of thermodynamics. Model formulations of transfers and pressure drops are performed in the thesis, namely as follows. The water chiller model is based on the logarithmic mean temperature difference (LMTD) method. Refrigerant R134a condenses and evaporates on the tube bundles of condenser and evaporator of the chiller; whereas, chilled water and condenser water pass inside tubes.
The tubes are commercial enhanced tubes so that heat transfer areas can be diminished and the problem can be reached reality. Refrigerant pressure drop is neglected. But water pressure -iii- drop is considered. Results of the water chiller model are verified by the commercial software HTRI Xchanger suite.
Simulation results show that with larger number of tubes than a specified one, the length of the tube can be short if a tube of small diameter is used. It avoids a design of large number of tube and large tube diameter. This is because inner thermal resistance can be lessened with the tube of small diameter. This conclusion reduces in capital cost of heat exchanger.
Tube diameter must be as low as 10.08 mm when number of tubes is more than 630 if the condenser capacity is 3412.4 kW with the common design conditions of cooling water and refrigerant R134a. However, tube of small diameter results in high water pressure drop. Therefore, an optimization can be carried out by trade-off of capital and operating costs. A cooling tower model is derived by using Merkel’s theory in company with the empirical correlations of transfer and air pressure drop from previous studies.
The model optimizes power of the cooling tower fan. The optimization problem is compared with the previous study that used the optimization package DICOPT (DIscrete and Continuous OPTimizer). A prominent result can be deduced. A condition of high temperature of water entering the cooling tower and low water flow does not bring an effective energy consumption of the fan.
With that condition, the air flow rate can be reduced but the length of cooling tower fill has to be longer in order to increase the contact time between water of high temperature and air. This trade-off leads to maximum power consumption of the fan at a certain value of the entering water temperature. That means the water temperature should be higher or lower than this value. From this work, the water temperature is 36oC and doesn’t depend on the thermal duty of the tower.
Only at a rather high water temperature, potential of heat and mass transfer can be strengthened and therefore fan power can be lowered. The cooling coil model is based on the enthalpy potential method and sustainable correlations of McQuiston about continuous plate fin under wet and dry environments. An experimental apparatus is set up to validate the model and carry out further studies. From analysis, it can be deduced that air pressure drop under fully wet and fully dry conditions are well predicted by the correlations of McQuiston.
However, the correlations have not covered the prediction in partially wet condition. Hence, a linear correlation between fully wet and -iv- fully dry zones is proposed to predict the air pressure drop in partially wet condition and proposition is confirmed by the experiment. A combination of the above models to simulate overall air conditioning system is then conducted in this study. The water temperature entering cooling coil, the water temperature leaving cooling tower, condenser water flow rate and number of SSCF chillers are assigned as key parameters.
Results can be seen as follows. The COP (Coefficient of performance) of the entire system can obtain maximum at a certain number of SSCF chillers when SSCF chillers system is operated with the same operating parameters of conventional parallel chillers system and when number of SSCF chillers is more than 4, the air conditioning system with SSCF chillers is ineffective. Trade-offs of power consumptions were pointed out as functions of the water temperature entering cooling coil, the water temperature leaving cooling tower, and the condenser water flow rate in order to see that an overall optimization is necessary. The COP after optimization can be greater up to 26% than that of conventional parallel chillers and the COP reduces as the number of chiller in SSCF increases.
Optimal parameters corresponding to number of SSCF chillers were shown so that they are can set in reality. Exergy analysis was performed with the parameters that make the maximum COP at each number of SSCF chillers. Result shows that the exergy efficiency of SSCF chillers system (83%) is higher than that of parallel chillers system (77%). Irreversibilities of the components in SSCF chillers are lower than those of components in parallel chillers.
But irreversibilities of cooling coil and cooling tower in SSCF chillers are slightly higher and lower than those in parallel chillers, respectively. Key words: Air conditioning, Overall optimization, Heat exchanger, Series-series counter- flow. -v- LIST OF FIGURES Fig. a) Parallel, b) Series-series counter-flow.2 Trends of water temperatures, condensation temperature and evaporation temperature.
a) Parallel chillers, b) SSCF chillers.3 Conceptualization of reduced lift (Groenke and Schwedler, 2002).4 Structure of the present thesis with a central air conditioning system.1 Pressure-enthalpy diagram of the considered water chiller.2 Notations used in the finned-tube.3 Enhanced boiling surface of turbo-B tube (Browne and Bansal, 1999).4 Boiling heat transfer coefficient on Turbo-B tube (Wolverine, Inc.5 Tube length vs. the number of tubes and tube passes.6 Reynolds number vs. the number of tubes and tube passes.7 Water pressure drop vs. the number of tubes and tube passes.8 Allowable tube side ( Pt ) and shell side ( Ps ) pressure drops (Muralikrishna and Shenoy, 2000).9 Tube length vs.
the number of tubes and tube inner diameter.10 Thermal resistances for four cases.11 Reynolds number vs. the number of tubes and tube inner diameter.12 Water pressure drop vs. the number of tubes and tube inner diameter.13 Tube length vs. the number of tubes and tube passes.14 Reynolds number vs.
the number of tubes and tube passes.15 Water pressure drop vs. the number of tubes and tube passes.16 An interface of HTRI software.17 Graphical results from HTRI software. a) HTRI condenser, b) HTRI evaporator.1 A sketch of cooling tower and its notations.2 A differential control volume of a counter-flow cooling tower.3 Equilibrium and operating lines on enthalpy-temperature diagram (Serna-González et al.4 Algorithm for searching the optimal fan power .5 Comparison about a) TAC, b) Me and c) Kfi to examples in previous study (Serna- González et al.6 Merkel number against Tw,in for all the considered Q .7 Dry air flow rate against Tw,in and Q .8 Cross sectional area of fill against Tw,in and Q .9 Fill height against Tw,in for all the considered Q .10 Loss coefficient against Tw,in for all the considered Q .11 Total pressure drop for all the considered Q .12 Fan power against Tw,in and Q .1 Circular fin in fully dry (a), fully wet (b), and partially wet (c) surface conditions (Pirompugd et al.2 Tube and wet fin with notations.3 Photo and the detailed test coil of the present experimental setup.