Barhoumi, Messaoud

Bellagi, Ahmed

Ezzine, Nizar Ben

Mejbri, Khalifa

English

Purdue E-Pubs

Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering2004Second Law Study of Ammonia-Water DoubleEffect Absorption ChillerNizar Ben EzzineUnité de Recherche Thermique et Thermodynamique des Procédés IndustrielsMessaoud BarhoumiUnité de Recherche Thermique et Thermodynamique des Procédés IndustrielsKhalifa MejbriUnité de Recherche Thermique et Thermodynamique des Procédés IndustrielsAhmed BellagiUnité de Recherche Thermique et Thermodynamique des Procédés IndustrielsFollow this and additional works at: http://docs.lib.purdue.edu/iraccThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlEzzine, Nizar Ben; Barhoumi, Messaoud; Mejbri, Khalifa; and Bellagi, Ahmed, "Second Law Study of Ammonia-Water Double EffectAbsorption Chiller" (2004). International Refrigeration and Air Conditioning Conference. Paper 673.http://docs.lib.purdue.edu/iracc/673 R107, Page 1   International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004  SECOND LAW STUDY OF AMMONIA-WATER DOUBLE EFFECT ABSORPTION CHILLER   Nizar BEN EZZINE1*, Messaoud BARHOUMI, Khalifa MEJBRI, Ahmed BELLAGI2  Unité de Recherche Thermique et Thermodynamique des Procédés Industriels Département Energétique, Ecole Nationale d’Ingénieurs de Monastir Ibn El Jazzar 5019 Monastir, Tunisie 1 n_benezzine@yahoo.fr 2 a.bellagi@enim.rnu.tn      ABSTRACT  A second law analysis of an ammonia -water double-effect double-generator absorption chiller is performed. The irreversibility in each component in the chiller is quantified and the contribution of each component to the overall system efficiency determined. Computer simulation was carried out in order to determine the stream properties and heat and work exchanged with the surroundings [1]. Simulation results where then used to analyse the entropy generation and irreversibility (or exergy destruction) of each component. The results indicate that in the absorber, the solution heat exchangers, and the condenser the greatest irreversibilities occur. In these equipements may lay the most improvement potential of the cycle efficiency.    1.  INTRODUCTION  The common way to specify the energy efficiency of an absorption chiller is to provide the coefficient of performance COP , based on the first law of thermodynamics. This parameter, however, makes no reference to the best possible performance and gives no information regarding where the irreversibilities in the chiller occur. Also, it cannot be used to determine the contribution of each component of the chiller to the overall efficiency. The second law analysis addresses the energy and the entropy balances for the system. It can be used to determine the entropy generation and the irreversibility (or exergy destruction) in each component. Since the COP reflects the amount of destruction of the available energy in the system, a second law analysis gives information on the potential of improvement in each component of the machine [2, 4, 5].              R107, Page 2   International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004  2. CYCLE DESCRIPTION  Figure 1 shows schematically a double effect-double generator configuration. Such a machine is composed essentially of two condensers, an evaporator, two steam generators each one provided with a boiler and distillation column, an absorber, four expansion valves, two pumps and an adjustable three-way valve. The steam (3) flowing from the evaporator to the absorber, is absorbed by the weak solution (9). The absorption heat is rejected towards the environment (cooling water or air) which also receives the energy released by the first condenser.  The strong solution (4) is distributed, by an adjustable three-way valve, between the two stages of the machine comprising each a generator, a condenser and a solution heat exchanger.  A fraction α  of the strong solution (4) is sent to the first generator. The separation of the refrigerant is performed at two temperature levels : the energy needed in the first boiler is supplied by the second condenser (Cond2). Large temperature differences between heat exchanging streams are so avoided. The first condenser (Cond1) receives the throtted condensate from the second condenser and the refrigerant vapor from the distillation column over the first boiler.  The strong solution (14) supplying the second generator is a cold source for the deparation column over the 2nd boiler and determines thus the purity limit of refrigerant (23) leaving the last stage of this column. For a rational use of the energy supplied, three counter-current heat exchangers (HEX1, HEX2, HEX3) are used.       Figure 1 : double effect double generator absorption chiller      HEX1 Evap EV1 EV2  Pump1 HEX2 Abs 1stBoiler Mixer Distillation  column Distillation column Pump2 2ndBoiler HEX3 Cond2  Cond1  EV4 Rectifier . . . EV3 . . . . . . . . . . . . . . . . . . . . 1 2 3 4 5 6 7 13 9 8 14 15 16 17 18 19 20 10 11 12 21 22 Coupling QCond2 = QB1 6’ 6’’ 23 24  . EvapQ&  AbsQ&  2BQ&  1CondQ&  Adjustable three-way valve  R107, Page 3   International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004    3. METHODOLOGY OF SIMULATION  The formulation of the simulation model for the chiller proceeds by the following steps:    Ø Determination of  the chiller variance (number degrees of freedom ),  Ø Specification of the fundamental operating conditions :  the cooling capacity, the driving heat source temperature, the useful cold temperature and that of the environment,  Ø Formulation of the mass and energy balances governing the various chiller components.  Ø Characterisation of the heat transfer in the various heat exchangers (pinch method).  To solve the large set of nonlinear equations of the simulation model the program CONLES, available as a FORTRAN 77 code [11], is  used. The fluid thermodynamic properties are calculated in a subroutine incorporated in the program.  4. THERMODYNAMIC ANALYSIS  4.1 Component Analysis  Neglecting the power input to the 2 pumps the governing equations used to evaluate the irreversibility in each component are:  Overall mass balance : ∑∑ =iiee mm &&  (1) Energy balance : ∑∑ −=iiieeeK hmhmQ &&&  (2) Entropy balance : KKiiieeeK,gen TQsmsmS&&&& −−= ∑∑  (3) Irreversibility : K,genSTI && 0=  (4)  Where: KQ&  :     Heat added or removed from component, K, at temperature KT , KT  :      The entropic average temperature at which heat KQ& is exchanged [4, 6], KgenS ,&  : The entropy generation in each component, K, of the chiller,   4.2. Cycle Analysis The coefficient of performance of the chiller is defined as                                                                2BEvapQQCOP =                                                                             (5)         R107, Page 4   International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004    Second law analysis begins by applying the first and second law to the entire system, with only heat crossing the chiller boundaries, as shown in Figure1.  012 =+++ AbsCondBEvap QQQQ &&&&  (6) ∑−=+++KK,genAbsAbs1Cond1Cond2B2BEvapEvap STQTQTQTQ &&&&&   (7)  EvapT , 2BT , 1CondT , and AbsT  the corresponding the entropic average temperatures. The first condenser and the absorber exchange heat with the same heat sink (air). Their entropic average temperature are set equal to the environment temperature at ( 1CondT = AbsT = 0T ). The second boiler exchanges heat at varying temperature. Calculation of the exact entropic average temperature for this desorbing process is complex, and was therefore set to the highest temperature of the weak solution. EvapT  was estimated as the numerical average of the entering and leaving temperatures. The error introduced by these approximations is minimal as shown by Alefeld [4, 6]. When equation 7 is multiplied by 0T  and subtracted from equation 6, the following equation is obtained:  −− −−=∑20020202 BKK,genEvapEvapBBEvapEvapBEvapQS.TTT.TTTT.TTTQQ&&&&   (8)    −− 2020 BBEvapEvapTTT.TTT : is the reversible revCOP  of a thermally -driven refrigerator operating between the three temperature reservoirs at EvapT , 0T  and 2BT . Equation 8 may thus be rewritten as :  ∑−=KK,lostrev COPCOPCOP  (9)  where  −=200BK,genEvapEvapK,lost QS.TTT.TCOP &&  (10)  Equation (10) shows how much the entropy generation of each component, K, of the chiller degrades the reversible revCOP  to the actual COP . Second law efficiency,η , is defined as the ratio of the actual COP  to the maximum (reversible) COP  under the same operating conditions :  revCOPCOP=η   (11)      R107, Page 5   International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004   5. RESULTS  The operating conditions for the cycle are: • Evaporator pressure, maximizing the COP  [1], PE = 3.5 bar, • First condenser and absorber temperature: 312 K, • Evaporator exit temperature: 275 K, • Maximum second generator temperature, T17 = 443 K, • Cooling capacity 17.5 kW at 300-285 K, • Pinchs in heat exchangers: 5 K, • Sub-cooling of absorber and condenser: 3 K.  Computer simulation results presented in [1] are used in this  thermodynamic analysis. Table 1 provides the analysis results.    Table 1: Results of the first and the second law analysis for the chiller  First law analysis Second law analysis  Component Q  (kW)     revCOP  COP  KgenS ,&  (W/K) I&  (W) K,radationdegCOP  totalII &&  % η  Evaporator 17.5 0.461 141.982 0.040 3,076 HEX1 2.544 0.436 134.257 0.038 2,909 EV1 0.00 0.181 55.707 0.016 1,207 1stCondenser(Cond1) -18.31 1.915 589.823 0.167 12,78 Absorber -25.925 3.854 1186.95 0.337 25,72 EV2 0.00 0.165 50.971 0.014 1,104 HEX2 30.311 2.591 797.98 0.226 17,288 Mixer 0.00 0.168 51.863 0.015 1,124 HEX3 30.374 2.216 682.587 0.194 14,79 EV3 0.00 0.454 139.929 0.039 3,031 2nd Boiler (B2) 22.9965 1.361 419.27 0.119 9,084 Rectifier 3.535 0.975 300.183 0.085 6,504 Coupling  (Cond2_1st Boiler) 3.514      2.292      0.761 0.208 64.21 0.018 1,391  0.332   Further examining of the irreversibility in each component (figure 2) reveals that the absorber, the solution heat exchangers and the first condenser contribute most to the COP  decrease (70%).  For the ideal process, the reversible COP  is 2,292. Degradations in all the components results in an actual COP  of  0,761. The absorber contribution to the degradation in the machine is the largest, 0.337.    6. CONCLUSION  This study applies the second law analysis to quantify the irreversibility of each component in ammonia -water double effect double generator absorption chiller.  Simulation results indicate that in the absorber, the solution heat exchangers, and the condenser may lay the greatest potential for chiller efficiency improvement.    R107, Page 6   International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004  0200400600800100012001400EvapHEX1EV1COND1ABSORBEREV2HEX2MIXERHEX3EV3B2RECTIFIERCOUPLINGChiller componentIrreversibility (W) Figure 2 : Irreversibility in chiller components.   NOMENCLATURE  COP   h I&  m&    P Q&  genS&   T Greek η  α coefficient of performance enthalpy irreversibility  molar flow rate  pressure  heat transfer  entropy generation  temperature   second law efficiency distribution coefficient (–) (J/mol) (kW) (mol/s) (bar) (kW) (kW/K) (°C, K)   (–) (°C, K) Subscripts Abs B Cond Evap EV Rect HEX  f p  r   absorber boiler condenser evaporator expansion valve rectifier heat exchanger refrigerant weak solution, poor rich, strong solution                 R107, Page 7   International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004  REFERENCES  [1] BEN EZZINE, N., MEJBRI, Kh., BELLAGI, A., July 12-15, 2004, Thermodynamic simulation of ammonia-water double effect absorption chiller,  International Refrigeration and Air Conditioning Conference at Purdue,  [2] Jong Jeong, & al., August2003, Optimum Design Method for a Single Effect Absorption Refrigerator Based on the First and Second Law Analysis”, 21st IIR International congress of refrigeration Washington DC, USA, ICR0441.     [3] Le Goff, P., Laouir, A., 2000, Quelle température de référence choisir dans le calcul de l’exergie , Entropie, n° 29. [4] Sam V. Shelton, & al., Second law study of the Einstein Refrigeration Cycle , G.W.W School of Mechanical Engineering, Georgia Institute of Technology.  [5] Kotas T. J., 1987, The exergy method of thermal plant analysis , Ed. Lavoisier, Paris, (France). [6] Herold, K.E, Radermacher, R., Klein, S. A., 1996, Absorption Chillers and Heat Pumps, CRC Press . [7] Shacham, M., 1986, A Fortran Subroutine for the Numerical Solution of Systems of  Nonlinear Equations, with and without Constraints, Intern. Journal of Numerical Methods in Engineering, 23, p. 1455-1481. [8] Duminil, M., 1992, Systèmes à absorption, à adsorption et thermochimiques en vue de la climatisation , Revue générale de thermique, n°362.  [9] Duminil, M., mars 2000, Exergy et anergie, Revue Générale du Froid, n°1001. [10] Hainsworth W. R., August 1944,  Refrigerants and absorbents, Refrigerating Engineering, vol. 48 [11] Nasri, K., 1997, Frigo pompes à absorption multi étagées de haute performance, Thèse, Institut National Polytechnique de Lorraine.  [12] Les pompes à chaleur à absorption: recherches, développement et perspectives , 1983, PYC édition.                   R107, Page 8   International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004  02004006008001000120014001600EvapHEX1 EV1COND1ABSORBER EV2HEX2MIXERHEX3 EV3 B2RECTIFIERCOUPLINGIrreversibility(W)configuration1"parallel flow" configuration2"serie flow" Figure 2 : Comparison of irreversibility of the chiller components of two configurations.      Figure 7 : Serie flow double effect double generator absorption chiller    ..Evap EV2 Pump1 HEX2 Abs 1stBoiler  Distillation column  Distillation column Pump2 2ndBoiler  HEX3 Cond2  Cond1  Rectifier  ..EV3 ... ...........1 2 3 4 5 6 7 9 8 15 16 17 18 19 20 10 11 12 21 22 Coupling QCond2 = QB1   23 24 EV4 HEX1 EV1 2BQ&  1CondQ&  EvapQ&  AbsQ&  

Second Law Study of Ammonia-Water Double Effect Absorption Chiller

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Second Law Study of Ammonia-Water Double Effect Absorption Chiller

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