ASHRAE AB-10-014-2010 The Optimal Match of Streams for Maximum Heat Transfer from a Gas Fired Absorption Refrigeration Unit.pdf

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1、2010 ASHRAE 483ABSTRACT In this study, a 5 tons of refrigeration (60,000 BTU h-1) commercial absorption refrigeration unit was characterized and instrumented, and a simplified thermal and exergetic anal-ysis of the system was performed, aiming the optimization of external operating parameters for ma

2、ximum thermodynamic performance. The first and second law of thermodynamics were used to evaluate the energy (first law) and the exergy (second law) efficiencies of the system. The experimental results showed the existence of a double maximum for the ther-mal and exergetic efficiencies for the optim

3、ized unit with respect to the water mass flow rates of the cold and hot sides of the absorption refrigerator. Maximum variations of 30% and 44% in the first and second law efficiencies, respectively, were observed according to the mass flow rate range used, which stresses the importance of the optim

4、a found for maxi-mum thermodynamic performance, and therefore minimum energy consumption in actual engineering applications.INTRODUCTIONIn the last few decades, due to the increasing level of pollution worldwide and the cost of energy, the search for maximum exploitation of the available energy has

5、lead to the development and use of cogeneration or trigeneration systems. Heating, ventilation, air-conditioning, and refrigeration systems (HVAC-R) play a major role in modern society energy consumption. These systems are mostly based on the vapor compression cycle, due to high efficiency, but the

6、vapor compression cycle needs work input, and high energy consumption is still observed, therefore research efforts have been made to develop intelligent refrigeration systems in order to reduce energy consumption (Vargas and Parise 1995; Buze-lin et al. 2005). Hence, alternative HVAC-R systems have

7、 been the subject of much recent scientific research. Among these systems, absorption refrigeration is receiving great attention since it may produce energy, heat and cold, using, as energy source, waste heat from industrial processes or, for instance, exhaust gases in automobiles (Temir and Bilge 2

8、004).The major companies working on this area focus on large capacity absorption systems, i.e. above 100 TR. However, since most refrigeration and air-cooling units are of small capacity and operate based on vapor compression cycle systems, there is still a vast field in which absorption systems cou

9、ld be employed.An absorption system also allows the direct use of primary energy, particularly solar energy and natural gas, for refrigeration purposes (Ezzine et al. 2004). Although this system is less costly and simpler than vapor compression systems, its comparatively low coefficient of performan

10、ce has limited its use to few and specific applications. Nevertheless, the absorption refrigeration system may reach a refrigeration capacity higher than that of a vapor compression system when energy sources such as waste (residual) heat from industrial processes, gas or vapor turbines, sunlight or

11、 biomass are used instead of electricity (Adewusi and Zubair 2004).The performance of absorption systems is dependent on an adequate choice of the refrigerant/sorbent working pair, and ammonia-water has been receiving great attention since these fluids do not contribute to the greenhouse effect (Bru

12、no et al. 1999; Lazzarin et al. 1996).The technical literature is rich in publications on the absorption refrigeration field. Particularly, Abreu (1999) and The Optimal Match of Streams for Maximum Heat Transfer from a Gas Fired Absorption Refrigeration UnitM.V.A. Pereira, PE J.V.C. Vargas, PhD S.C.

13、 Amico, PhDJ.A.R. Parise, PhD R.S. Matos J.C. Ordonez, PhDMember ASHRAEJ.V.C. Vargas and R.S. Matos are professors and M.V.A. Pereira is a graduate student in the Departamento de Engenharia Mecanica, Univer-sidade Federal do Parana, Curitiba/PR, Brazil. S.C. Amico is a professor in the Departamento

14、de Engenharia de Materials, Universidade Federal do Rio Grande do Sul, Porto Alegre/RS, Brazil. J.A.R. Parise is a professor in the Departamento de Engenharia Mecanica, Pontificia Universidade Catolica do Rio de Janeiro, Rio de Janeiro/RJ, Brazil. J.C. Ordonez is a professor in the Center for Advanc

15、ed Power Systems and Department of Mechnical Engineering, Florida State University, Tallahassee, FL. AB-10-0142010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Addi

16、tional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.484 ASHRAE TransactionsVillela and Silveira (2005) used as heat source for absorption systems, the combustion of liquid petroleum gas (LPG) and biogas, respect

17、ively, studying the design and performing a thermoeconomic analysis of the analyzed systems. Other stud-ies focused on the exergy analysis of absorption refrigeration systems, including Sedighi et al. (2007), Hasabnis and Bhag-wat (2007), Khaliq and Kumar (2007), Arivazhagan et al. (2006), and Senca

18、n et al. (2005). Simulation and optimization studies have also been published analyzing the absorption refrigeration system in isolation (Vargas et al. 1996; Vargas et al. 2000a; Vargas et al. 2001). However, the exergy analysis and optimization of an absorption refrigerator to produce cool-ing and

19、heating, based on a theoretical-experimental model, could not be found in the open literature.The aim of this work is two-fold: i) to formulate theoret-ically the absorption system heat transfer interactions using a simplified mathematical model for the energy and exergy analysis of an existing LPG

20、(gas fired) driven absorption refrigeration unit, and ii) based on experimental measure-ments, to characterize system pull-down times and to carry out an energetic and exergetic optimization for maximum thermo-dynamic performance of the system, i.e., minimum energy consumption. THEORYAs illustrated

21、in the introduction, many studies have been published in the literature that dealt with simple and complex internal component-wise energetic and exergetic analyses of absorption refrigerator systems. So, it is not within the scope of this study to investigate the absorption cycle itself, but to addr

22、ess how to extract the most from existing absorption units (e.g., single-effect, double-effect, generator-absorber heat exchange-GAX cycle) by investigating the possibility of opti-mally tuning some of their external operating parameters. Such study can be quite challenging, since the thermodynamic

23、fundamentals are far from obvious when the objective is to identify a general optimization opportunity and to pinpoint the candidate parameters to be optimized. For that, simple models that include the basic thermodynamic phenomena of the system to be analyzed are recommended, so that the optimi-zat

24、ion opportunities are visible and expected to be present in actual systems, no matter how complex they may be (Bejan 1988). Therefore, the simplest possible mathematical formu-lation is proposed in this section to be applied together with experimental measurements to achieve the objectives of this s

25、tudy.Energy Analysis of the Absorption System DeliverablesAs the absorption system may simultaneously operate for water cooling and heating, both cold and hot water reservoirs were considered part of the system, each requiring an energy balance. The cooling and heating capacity rates are functions o

26、f the design and operating parameters, as well as the water mass flow rates. For the purpose of modeling the existing absorption unit utilized in this work, that uses shell and tube heat exchangers, with the refrigerant circulating in the inner tubes and the cooled fluid around them, the heat transf

27、er rate actually extracted by the refrigerant in the evaporator from the water/ethylene-glycol mixture in the system cold side ( ) is calculated as follows: (1)where: and are the cold mass flow rate and the average specific heat of the water/ethylene-glycol mixture (75/25 in weight), respectively. T

28、he latter may be calculated by , for and .The temperature variation of the water/ethylene-glycol mixture, , is defined as , where and are, respectively, the outlet and inlet temperature of the mixture at the evaporator. As mentioned earlier in the text, the existing absorption unit utilized in this

29、work uses shell and tube heat exchangers, with the refrigerant/absorbent solution circulating in the inner tubes and water (condenser and absorber) or water/ethylene-glycol mixture (evaporator) around them. Therefore, the heat transfer rate actually captured by the water that circulates in series in

30、 the condenser and absorber in the system hot side, , is calculated by: (2)where: is the hot water mass flow rate, is the temper-ature variation of the water in the system hot side (absorber and condenser), i.e. , and , since in this case the fluid is pure water.In this work, combustion of liquid pe

31、troleum gas (LPG) was used as the heat source and the fuel heat transfer input rate, , was calculated as follows: (3)where: is the fuel mass flow rate and , the fuel lower heating value, which was taken at 25 oC (77 oF), 1 atm (14.7 psi). The performance of the system may be evaluated by considering

32、 the first law (energetic) efficiencies for both systems ( and ), and the herein defined combined system first law (energetic) efficiency ( ) that recognizes QCQCmCcCTC=mCcCcC0.25ceg0.75cw+=cw4.186 kJ kg1K11 BTU lb1R1()=ceg2.391 kJ kg1K10.571 BTU lb1R1()=TCTC TCo,TC,i=TC,oTC,iQHQHmHcHTH=Hm Howle et

33、al. 1992). Two flowmeters, were used to measure flow rates, with a bias limit of 2% and 1% repeatability. The LPG consumption was monitored with a gas volume meter, with a bias limit of 0.016 m3( 0.565 ft3). The water mass flow rate and LPG consump-tion measurements were manually recorded. Figure 2

34、shows the positioning of all temperature and flow rate readers used for monitoring the system, along with the water and LPG valves, named and , respectively.Pull-Down CharacterizationThe performance of the refrigeration system was evalu-ated using pull-down tests, which consisted of measuring the ti

35、me required for the temperature of the water/ethylene-glycol mixture to fall from a reference (initial) temperature to a desired setpoint band steady state. These tests were carried out using the cooling circuit only, and the water tank initial temperature and the outlet cold temperature setpoint, i

36、.e. at the evaporator outlet, were selected as C and C, respectively. The thermostat controlled electrical resistance that was installed in the 300-litre tank established the water tank initial temperature and the turned off during the entire test, and the refrigerator control system controlled the

37、desired evaporator outlet temperature setpoint band.Energetic and Exergetic AnalysesIn all tests, the absorption refrigeration system was conducted to the established setpoint band steady state detailed in the previous paragraph. The experiments were carried out using various cooling ( = 0.37, 0.50,

38、 0.67 and 0.83 or = 0.81, 1.1, 1.47 and 1.8 ) and heating ( = 0.40, 0.50, 0.67, 0.83, 0.90 or = 0.88, 1.1, 1.47, 1.8 and 2 ) fluids mass flow rates with the objective of investigating the existence of an optimal pair for maximum system first and second law efficiencies within ch,LPGVwVLPG130 110 mCk

39、g s1mClb s1mHkg s1mHlb s1Figure 2 Schematics of the absorption system and positioning of the temperature and flow rate readers.mC,mH()opt2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2).

40、For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.2010 ASHRAE 487those mass flow rates tested ranges. Three experiments were carried out for each set of experimental parameters.Calcu

41、lation of UncertaintiesIn this work, the experimental measurement of tempera-tures, flow rates and LPG volume were used to calculate heat transfer and exergy rates, first and second law efficiencies. For each experimental condition, 3 measurements were taken at steady state. The precision limit ( )

42、for every temperature variation ( ) was estimated as twice the standard deviation of these measurements, with a confidence level of 95%. The related uncertainties were then calculated as follows:(13)where , the bias limit of , was found to be much smaller than , due to the fact that a high precision

43、 thermistor was used (resistance 2250 at 25 oC), with reported bias limits as 0.001 K low as (Dally et al. 1993; Howle et al. 1992).The precision limit of all variables used to determine heat transfer and exergy rates were negligible in comparison with the precision limit of the temperature variatio

44、ns namely, and ). Therefore, the uncertainties associated with the heat transfer and exergy rates were estimated as follows: (14)(15)where: is due to the calculation of entropy variation, being negligible in comparison with , and s represents either H or C, depending on what side of the system is be

45、ing analyzed, i.e., the hot or cold side, respectively.Analysis of Eqs. (13-15) shows that the calculation of uncertainties for all studied parameters is basically dependent on the temperature precision limit . The largest calcu-lated uncertainty in all tests was 0.059 (= 5.9%) and there-fore adopte

46、d for all error bars presented in the results of this work.RESULTS AND DISCUSSIONIn order to find the optimum operating conditions of the absorption refrigeration system, a series of tests was carried out varying the flow rates of the cold and hot systems. On these tests, two constraints of the equi

47、pments were known, the absorption system and the flow meter could only operate on the 0.30-0.90 kg s1(0.66-2 lb s1) and 0.17-0.93 kg s1(0.37-2.05 lb s1) ranges, respectively. Furthermore, although the refrigerator may operate in cooling and heating modes, it prioritizes the cooling mode. For the fir

48、st and second law analyses, the following considerations were used: i. The data were taken at steady-state;ii. The effect of the potential and kinetic energy variations were neglected with respect to the variation of the internal energy of the fluid;iii. For all tests performed, the initial conditio

49、n was defined as and ;iv. The loss and/or gain of heat, pressure and exergy on the cold and hot water pipes were neglected considering the magnitude of the variations on these quantities within the heat exchangers, andv. The LPG consumption ( ) was kept constant and equal to 0.000727 kg s1(0.0016 lb s1). Thus, for a gas density ( ) of 2.5 kg m3(0.155 lb ft3), the LPG vol-umetric flow rate used was LPG= 1.0475 m3 h1 (0.61654 ft3 min1).Pull-Down ExperimentsPull-down tests are interesting to characterize the behavior of any refrigerati