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Analysis of crystallization risk in double effect absorption refrigeration systems
Applied Thermal Engineering 31 (2011) 1712e1717
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Analysis of crystallization risk in double effect absorption refrigeration systems L. Garousi Farshi a, *, S.M. Seyed Mahmoudi a, *, M.A. Rosen b a b
Faculty of Mechanical Engineering, University of Tabriz, Tabriz, East Azarbaijan, Iran Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2010 Accepted 10 February 2011 Available online 1 March 2011
Absorption refrigeration systems are an alternative to vapor compression ones in cooling and refrigeration applications. In comparison with single effect absorption units, double effect systems have improved performance. Also, they are more available commercially than the other multi effect absorption cycles. An important challenge in the operation of such systems is the possibility of crystallization within them. This is especially true in developing air-cooled absorption systems, which are attractive because cooling tower and associated installation and maintenance issues can be avoided. Therefore, distinguishing the working conditions that may cause crystallization can be useful in the design and control of these systems. In this paper a computational model has been developed to study and compare the effects of operating parameters on crystallization phenomena in three classes of double effect lithium bromideewater absorption refrigeration systems (series, parallel and reverse parallel) with identical refrigeration capacities. It is shown that the range of operating conditions without crystallization risks in the parallel and the reverse parallel configurations is wider than those of the series flow system. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Crystallization Double effect absorption Chiller Refrigeration Cooling
1. Introduction Producing cold by means of absorption refrigeration systems has advantages, as these systems are thermally activated and do not require high input shaft power. Also, the ability to use low grade heat, such as that obtained from solar thermal systems, fuel cells, residual heat, geothermal energy, etc., can provide good reasons for using absorption machines. Other benefits of absorption refrigeration include being independent of electric grids, potentially having less CO2 emissions, causing no ozone layer depletion, and using natural refrigerants, and these contribute to increasing the attractiveness of these machines. However, absorption refrigeration systems suffer from having low efficiencies and a limitation on the temperature of utilized heat source. These two points have motivated the development of advanced absorption machines [1], including different configurations of double effect systems. The most common working fluid combinations used in absorption refrigeration systems are lithium bromideewater (LiBreH2O) and ammoniaewater (NH3eH2O). The NH3eH2O absorption system is more complicated than the LiBreH2O system,
* Corresponding authors. Tel.: þ98 411 3393056; fax: þ98 411 3354153. E-mail addresses: [email protected] (L. Garousi Farshi), s_mahmoudi@ tabrizu.ac.ir (S.M. Seyed Mahmoudi), [email protected] (M.A. Rosen). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.02.013
since it needs a rectifying column that assures no water vapors enter the evaporator; otherwise the evaporator could no longer operate at the pressures and temperatures usually required in ammoniaewater chillers. LiBreH2O absorption systems however have higher COPs than NH3eH2O systems and are commercially available as absorption chillers for air conditioning applications [2]. It must be noted that LiBr is a salt, and in its solid state it has a crystalline structure. When LiBr is dissolved in water, there is a specific minimum solution temperature for any given salt concentration. The salt begins to leave the solution and crystallize below this minimum temperature. Crystallization must be avoided in the design and operation of LiBreH2O absorption systems because the formation of slush in the piping network forms a solid, blocking the flow quickly. If this occurs, the concentrated solution temperature needs to be raised significantly above its saturation point so that the salt crystals dissolve within a reasonable time, freeing the machine. Restoring absorber operation after crystallization is very labor intensive and time consuming [3]. The double effect absorption refrigeration cycle was introduced in 1956e1958 [4]. The earliest theoretical study of a double effect absorption system was provided for various working fluids by Kaushik and Chandra [5] and by Garimella and Christensen [6]. Some studies have concentrated on energy analysis [7e12], and others have focused on exergy analysis of these systems [13e18]. In none of these works, however, is the location in which crystallization is most likely to occur specified and reports on the effect of
L. Garousi Farshi et al. / Applied Thermal Engineering 31 (2011) 1712e1717
Nomenclature
1713
Greek symbols heat exchanger effectiveness
h COP D h _ m P Pdrop1 Pdrop2 Q_ Q_ LPG;out T _ W X
coefficient of performance distribution ratio specific enthalpy, kJ/kg K mass flow rate, kg/s pressure, kPa pressure drop between evaporator and absorber, Pa pressure drop between low pressure generator and condenser, Pa heat transfer rate, kW energy supply rate from external source to LPG, kW temperature, K work transfer rate, kW LiBr concentration
operating conditions on solution crystallization in absorption systems are lacking. Florides et al. [19] and Liao and Radermacher [3] identify several causes of crystallization and approaches to avoid it. In their study, only single effect systems are considered. Notwithstanding the numerous applications of double effect absorption systems in the refrigeration and air conditioning industry and differences in operations between single and double effect systems, little is known about the latter regarding crystallization. In particular, more attention is needed concerning the comparison of crystallization limits in different configurations of double effect systems. The present article aims to redress this lack of knowledge via a methodical and comparative investigation of the risk of crystallization in these systems, considering a broad range of component temperatures, effectiveness of solution heat exchangers, and pressure drops between evaporator and absorber and also between the LPG and condenser. Furthermore, as low grade waste heat can be supplied to the LPG when available, the effect of this heat addition on the possibility of crystallization is investigated. 2. Description of double effect systems and causes of crystallization Depending on the solution flow, double effect absorption chillers can be classified as series, parallel and reverse parallel systems, according to ASHRAE [20]. Fig. 1a shows a schematic diagram of a series flow double effect LiBreH2O absorption refrigeration system. It involves three pressure levels, i.e. high, medium and low. The high pressure generator (HPG) functions at high pressure and high temperature, while the low pressure generator (LPG) and condenser operate at medium pressure, and the evaporator and absorber work at low pressures. The strong solution (in which the refrigerant concentration is high) leaving the absorber is pumped to the HPG through two solution heat exchangers. A high temperature heat source is utilized in the HPG to generate water vapor from the strong solution. The weak solution leaving the HPG enters the LPG via solution expansion valve (EV4). In the LPG the water vapor as refrigerant coming from the HPG is condensed due to the low temperature of the weak solution and its latent heat is utilized in generating water vapor from the weak solution. In addition to this latent heat, low grade energy may be supplied to the LPG to produce more vapor. The weak solution, therefore, becomes weaker and is delivered to the
Subscript II Abs Con e eva HPG HTHE i LPG LTHE max p
second law absorber condenser exit evaporator high pressure generator high temperature heat exchanger inlet or each component low pressure generator low temperature heat exchanger maximum pump
absorber through the low temperature heat exchanger (LTHE) and solution expansion valve (EV3). Water vapor generated from both the high and low pressure generators passes to the condenser and heats the cooling stream as it condenses. Then the liquid refrigerant is delivered to the evaporator through an expansion valve (EV1) where its pressure is reduced to that of the evaporator. The cold vapor from the evaporator is dissolved in the weak solution from the LPG through the LTHE, thereby rejecting its heat of absorption in the absorber to the cooling stream. The path of refrigerant from condenser to absorber is the same for all the three systems. Fig. 1b shows the parallel flow system. As shown in this figure, the strong solution leaving the absorber is pumped to the LTHE after which it is divided into two streams. One stream flows to the HPG through a high temperature heat exchanger (HTHE) and the other passes to the LPG via EV4. A high temperature heat source is used to provide heat to the HPG for water vapor generation from the strong solution. In the LPG the refrigerant vapor from the HPG is condensed and its latent heat is utilized to evaporate the water content of the strong solution. The weak solution leaving the HPG mixes with the weak solution from LPG at a mixing point (P1). The combined weak solution flows to the absorber via the LTHE and EV3. The reverse parallel system is shown in Fig. 1c. As that figure indicates, the strong solution leaving the absorber is pumped to the LPG through the LTHE. The weak solution from the low pressure generator is divided into two streams at point P1. One stream is pumped to the high pressure generator through the HTHE and the other passes to point P2 where it mixes with the weak solution from the HPG. The mixture from this point flows to the absorber. In the LPG the vapor received from the HPG is condensed and its latent heat is utilized in generating water vapor from the strong solution. In this system, as for the others, the possibility exists of making use of an external low grade heat source in the LPG. The use of two solution pumps in this system is advantageous as it provides improved control of the performance of the system at part load conditions. Considering practical conditions for the above mentioned absorption systems, the lithium bromide concentration in water at some points in the cycles is high and close to the crystallization border, for instance point A in Fig. 2, which indicates the relation between temperature, pressure and concentration of lithium bromide in a LiBreH2O solution. When the lithium bromide concentration increases and/or the solution temperature decreases,
1714
L. Garousi Farshi et al. / Applied Thermal Engineering 31 (2011) 1712e1717
Fig. 1. Double effect absorption refrigeration systems: (a) Series flow, (b) Parallel flow, (c) Reverse parallel flow.
point A moves toward point B and crystallization may occur, interrupting machine operation. The actual location within the chiller at which crystallization may occur is determined by the mechanical structure of the system; however, crystallization is most likely to take place in the weak solution entering the absorber, as this is the point of highest concentration and lowest temperature, i.e., point 17 in Fig. 1aec. 3. Simulation and analysis of systems Several assumptions are made in the simulations and analyses of the absorption refrigeration systems, as summarized in Table 1. The systems produce chilled water in the evaporator, and the HPG is driven by a pressurized hot water stream. Also, the system rejects heat to cooling water at the condenser and the absorber. When there exists an external low grade heat source for the LPG, it is also assumed in the form of a hot water stream. The outlet and
inlet temperatures of these streams are presented in Table 2, along with the reference conditions for components used in the simulations and the ranges of their variations are. All of these conditions are fixed except where stated otherwise. In this analysis, the thermo-physical properties of the working fluids are expressed as analytic functions. A new set of computationally efficient formulations for thermodynamic properties of LiBreH2O solution developed by Patek and Klomfar [21] are used in this work. The equations for the thermal properties of steam are obtained from correlations provided by Spencer [22]. In order to analyze the absorption systems, principles of mass and energy conservation are used. The general equations of these principles are specified below: Mass conservation:
X
_l ¼ m
X
_e m
(1)
L. Garousi Farshi et al. / Applied Thermal Engineering 31 (2011) 1712e1717
1715
Table 2 Reference operating parameters and their ranges of variation and inlet/outlet temperatures of cooling or heating streams.
Fig. 2. Property chart for LiBr/H2O solution with crystallization curve.
X
_ l Xi ¼ m
X
_ e Xe m
(2)
Energy conservation:
X
Q_
X
_ ¼ W
X
_ e he m
X
_ l hi m
(3)
Reference conditions
Variation range
Inlet and outlet temperatures of cooling and heating streams
Tcon ¼ Tabs ¼ 310 K (37 C)
305e315 K
Teva ¼ 277 K (4 C)
277e283 K
THPG ¼ 400 K (128 C)
380e480 K
hLTHE ¼ 70% hHTHE ¼ 70% hpumps ¼ 95%
Inlet temperature of cooling water ¼ Tcon 8 Outlet temperature of cooling water ¼ Tcon 3 Inlet temperature of chilled water ¼ Teva þ 8 Outlet temperature of chilled water ¼ Teva þ 3 Inlet temperature of hot water ¼ THPG þ 18 Outlet temperature of hot water ¼ THPG þ 10
50e90% 50e90%
Pdrop1 ¼ 0 (pressure drop between evaporator and absorber) Pdrop2 ¼ 0 (pressure drop between LPG and condenser) Q_ LPG;out ¼ 0 (low grade h eat supplied at LPG)
0e100 Pa
0e500 Pa
0e300 kW
These equations are applied to each component of the systems. The distribution ratio D for parallel and reverse parallel systems respectively can be expressed as follows (see Fig. 1b and c):
_ 6c =m _4 D ¼ m
5. Results and discussion
(4a)
and
_ 15b =m _ 15 D ¼ m
Inlet temperature of hot water ¼ TLPG þ 18 Outlet temperature of hot water ¼ TLPG þ 10
(4b)
4. Validation of the simulation A computer program has been developed using Engineering Equation Solver (EES) software [23] to carry out the thermodynamic analysis for each of the three configurations of double effect absorption refrigeration systems. In the programs the new set of formulations for LiBreH2O solution properties is used as internal functions instead of data from the EES database. The formulations are more accurate and provide properties at comparatively wider ranges of temperature and concentration. In order to validate the present model, the obtained results have been compared with those reported by Gomri [14] for a series flow double effect system. A good agreement is seen between the two works as presented in Table 3. Table 1 Assumptions used in simulation and analysis of systems. Simulations and analyses are performed under steady conditions Conditions of refrigerant (water) at the exits of condenser and evaporator are saturated Solution is at equilibrium condition at the exits of absorber, HPG, LPG and at the corresponding device temperatures Pressure losses due to friction in the heat exchangers and the connecting piping are negligible, except between the evaporator and the absorber and between the LPG and the condenser; the effect of these pressure losses is assessed Heat exchange between the systems and the surroundings, other than that prescribed by heat transfer at the generator, evaporator, condenser and the absorber, is assumed negligible A temperature difference of 5 K is assumed for heat transfer at the LPG Simulations and analyses are carried out for a constant refrigeration capacity in all the systems, where Q_ eva ¼ 300 kW
To study the possibility of crystallization occurrence in the absorber entrance in the three cycles, a parametric study was carried out to assess the effect of parameters such as temperature and pressure at different points in the cycles and also that of heat exchanger effectiveness and heat supply to the LPG on the temperature and concentration of point 17. The results are shown in
Table 3 Comparison of results from present work with those of Gomri [14]. Point
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
_ (kg/s) m
T (K)
X (% LiBr)
Gomri
Present work
Gomri
Present work
308 277 277 308 308 335.49 379.81 403 349.09 349.09 403 355.46 308 353 353 321.67 321.67 421 413 300 305 285 280 300 305
308 277 277 308 308 335.49 379.8 403 356.09 356.09 403 355.45 308 353 353 321.67 321.67 421 413 300 305 285 280 300 305
0.127 0.127 0.127 1.737 1.737 1.737 1.737 1.671 1.671 1.671 0.065 0.065 0.065 0.062 1.610 1.610 1.610 7.340 7.340 7.990 7.990 14.285 14.285 18.409 18.409
0.127 0.127 0.127 1.735 1.735 1.735 1.735 1.670 1.670 1.670 0.065 0.065 0.065 0.062 1.608 1.608 1.608 7.330 7.330 7.993 7.993 14.321 14.321 18.415 18.415
Gomri
h (kJ/kg K) Present work
55.869 55.869 55.869 55.869 58.056 58.056 58.056
55.880 55.880 55.880 55.880 58.070 58.070 58.070
60.278 60.278 60.278
60.294 60.294 60.294
Present work 146.59 146.59 2507.87 87.67 87.67 143.14 235.43 288.40 192.49 192.49 2740.53 345.21 345.21 2649.57 195.84 135.98 135.98 623.68 589.24 113.12 134.04 50.36 29.42 113.12 134.04
L. Garousi Farshi et al. / Applied Thermal Engineering 31 (2011) 1712e1717 338
1716
420
455
420
328
465 465 425
420
450
450 323
400
460
series(T_con=T_abs=305K)
425
series(T_con=T_abs=310K) series(T_con=T_abs=315K)
400
425
Parallel(T_con=T_abs=305K)
400
313
318
Temperature ( K)
333
480
Parallel(T_con=T_abs=310K) Parallel(T_con=T_abs=315K)
445 445
380 380
Reverse parallel(T_con=T_abs=305K) Reverse parallel(T_con=T_abs=310K) Reverse parallel(T_con=T_abs=315K)
385 T HPG=380K T_HPG=380K
0.54
0.56
0.58
Poly. (Crystallization)
0.6
0.62
0.64
0.66
0.68
0.7
0.72
0.74
X ( Lithium Bromide concentration)
Fig. 3. Variation in temperature and concentration of solution entering the absorber (point 17) for several high pressure generator and condenser temperatures.
This may seem to be in contradiction with what happens in practice, but it must be noted that in practice we usually deal with an existing system in which, when the absorber temperature is lowered, the absorption potential and consequently the mass flow rate of the solution exiting the absorber increases while the rate of heat input to the generator stays almost constant. Therefore, the solution flow enters the absorber with a lower concentration and temperature implying a lower possibility of crystallization at this point [3]. In the present work, however, the design of a new system is dealt with and the generator temperature is constant (variable input heat); therefore, lowering the absorber temperature will result in an increase of concentration and a decrease of temperature of the solution entering the absorber, increasing the possibility of crystallization. This point is also mentioned as a caution of possible crystallization in the literature [3,19]. In the parallel and reverse parallel cycles it is observed that for the studied condition there is a weak possibility of crystallization and this possibility in the reverse parallel cycle is greater than that for the parallel system. The sudden variations in concentrations and temperatures of point 17 in the parallel and reverse parallel cases are caused by the criteria used in selecting the optimum distribution ratio. For each of the double effect systems the relation among the properties at the absorber entrance, THPG and Teva, is shown in Fig. 4. The effect of THPG is similar to that described by Fig. 3. As shown in Fig. 4, the possibility of crystallization with increasing Teva is greater for the series flow cycle (see the position of
326
series( T_HPG=420K) Parallel( T_HPG=420K) Reverse parallel( T_HPG=420K) Poly. (Crystallization)
324
325
Temperature ( K)
327
328
Figs. 3e6. In each figure the crystallization line is drawn based on data obtained from ASHRAE [24]. Note that, in the case of parallel and reverse parallel systems, the optimum values of D for maximum COP are selected for calculations in every operating condition. As a sample, the optimum values of D in many operating conditions are provided in Table 4. For optimization purposes EES has many choices. If there is one degree of freedom, EES will minimize/maximize the selected variable using either the Quadratic Approximation method or Golden Section search. The former is usually faster, but the latter is more reliable. So the latter was chosen for calculation and after calcu_ were verified to be positive. lating the optimum D, values of m Fig. 3 shows the variation of the concentration of the weak solution entering the absorber (point 17 in the three configurations) with its temperature at different values of THPG and Tcon (¼ Tabs). It is shown that in the double effect series flow cycle for all the examined values of Tcon ¼ Tabs an increase in THPG increases the temperature and concentration of LiBr at the entrance of the absorber. It is noted however that at some regions in the figure there are slight decreases in T17 after the growth. In each operating condition, there is a maximum value for THPG beyond which crystallization occurs. This HPG temperature increases with an increase in the value of Tcon (¼ Tabs) indicating that the cycle can utilize higher temperature heat sources (see the position of THPG ¼ 425 K, for example). However, as Tcon (¼ Tabs) decreases the possibility of crystallization increases.
Fig. 5. Variation in temperature and concentration of solution entering the absorber (point 17) with effectiveness of high and low temperature heat exchangers.
0.6
0.62
0.64
0.66
0.68
X ( Lithium Bromide concentration)
Fig. 4. Variation in temperature and concentration of solution entering the absorber (point 17) for several high pressure generator and evaporator temperatures.
Fig. 6. Variation in temperature and concentration of solution entering the absorber (point 17) with external heat supplied to low pressure generator.
L. Garousi Farshi et al. / Applied Thermal Engineering 31 (2011) 1712e1717 Table 4 Optimum values of D for several operating conditions. Parallel configuration
Reverse parallel configuration
Reference conditions with various THPG
Reference conditions with various THPG
THPG (K)
D
THPG (K)
D
400 405 410 415 420 425 430 435 440 445 450
0.51 0.46 0.44 0.43 0.42 0.41 0.40 0.39 0.33 0.3 0.27
400 405 410 415 420 425 430 435 440 445 450
0.75 0.72 0.7 0.68 0.65 0.62 0.59 0.56 0.46 0.4 0.36
THPG ¼ 425 K, for example). In the parallel and reverse parallel cycles it is observed for the studied conditions that, as mentioned earlier, there is a weak possibility of crystallization and this possibility in the reverse parallel cycle is greater than that in for the parallel cycle. It should be noted that in both Figs. 3 and 4 there exist three curves for the series configuration with more or less the same shape. But the curves in Fig. 4, in which the evaporator temperature changes, are very close to each other making it seem as if there is only one curve. However, the positions of points with THPG ¼ 425 K and different Teva shown in the figure explains the difference among the curves. The effects of hHTHE and hLTHE on property changes of point 17 are shown in Fig. 5. It is shown that only in series flow system when a very efficient heat exchanger is used as LTHE, the condition of point 17 approaches the crystallization line, and in this case this matter must be considered carefully. Again in parallel and reverse parallel systems crystallization occurrence is a little possibility. The effect of pressure drops in various positions in the system is also studied and was found to have a negligible effect on the possibility of crystallization in the systems. Fig. 6 shows the effect of supplying heat externally to the LPG. It is clear that in series flow system increasing Q_ LPG;out moves point 17 closer to the crystallization line, but in the parallel and reverse parallel systems the opposite effect is observed. Therefore, when a low grade heat source is available for use in the LPG, in the case of series flow system more caution must be taken to avoid crystallization at the absorber entrance. 6. Conclusions Crystallization is a practical problem typical of waterelithium bromide systems. The possibility of crystallization at the absorber entrance has been assessed here by examining the effect of operating parameters in series, parallel and reverse parallel LiBreH2O double effect absorption refrigeration systems. A computer program using EES software has been developed to simulate the performance of the three systems. In the case of parallel and reverse parallel systems, the optimum distribution ratio for COPmax has been selected for calculations at each operating condition. It is found that, in series flow systems, the possibility of crystallization increases with increasing THPG, Teva and effectiveness of the LTHE
1717
and decreasing Tcon (¼ Tabs). Also, in parallel and reverse parallel cycles, it is determined that, in studied condition, the crystallization possibility is low but in practice the mechanical structure of system and actual operating condition can play very important roles in this phenomenon. Hence, the double effect parallel and reverse parallel flow arrangements are superior in performance to the series flow in terms of crystallization risk. References [1] P. Srikhirin, S. Aphornratana, S. Chungpaibulpatana, A review of absorption refrigeration technologies, Renewable and Sustainable Energy Reviews 5 (2001) 343e372. [2] S. Kalogirou, Recent patents in absorption cooling systems, Recent Patents on Mechanical Engineering 1 (2008) 58e64. [3] X. Liao, R. Radermacher, Absorption chiller crystallization control strategies for integrated cooling heating and power systems, International Journal of Refrigeration 30 (2007) 904e911. [4] G.C. Vliet, M.B. Lawson, R.A. Lithgow, Waterelithium bromide double-effect absorption cooling cycle analysis, ASHRAE Transaction 88 (1982) 811e822. [5] S.C. Kaushik, S. Chandra, Computer modeling and parametric study of a double-effect generation absorption refrigeration cycle, Energy Conversion and Management 25 (1985) 9e14. [6] S. Garimella, R.N. Christensen, Cycle description and performance simulation of a gas-fired hydronically coupled double-effect absorption heat pump system, in: , Proceedings of the ASME: Recent Research in Heat Pump Design, vol. 28, 1992, pp. 7e14. [7] G.P. Xu, Y.Q. Dai, K.W. Tou, C.P. Tso, Theoretical analysis and optimization of a double-effect series-flow-type absorption chiller, Applied Thermal Engineering 16 (1996) 975e987. [8] G.P. Xu, Y.Q. Dai, Theoretical analysis and optimization of a double-effect parallel-flow-type absorption chiller, Applied Thermal Engineering 17 (1997) 157e170. [9] M.B. Arun, M.P. Maiya, S.S. Murthy, Equilibrium low pressure generator temperatures for double-effect series flow absorption refrigeration systems, Applied Thermal Engineering 20 (2000) 227e242. [10] M.B. Arun, M.P. Maiya, S.S. Murthy, Performance comparison of double effect parallel flow and series flow water lithium bromide absorption systems, Applied Thermal Engineering 21 (2001) 1273e1279. [11] Y.L. Liu, R.Z. Wang, Performance prediction of solar/gas driving double effect LiBreH2O absorption system, Renewable Energy 29 (2004) 1677e1695. [12] E. Torrella, D. Sánchez, R. Cabello, J.A. Larumbe, R. Llopis, On-site real-time evaluation of an air-conditioning direct-fired double-effect absorption chiller, Applied Energy 86 (2009) 968e975. [13] T.S. Ravikumar, L. Suganthi, A.S. Anand, Exergy analysis of solar assisted double effect absorption refrigeration system, Renewable Energy 14 (1998) 55e59. [14] R. Gomri, R. Hakimi, Second law analysis of double effect vapor absorption cooler system, Energy Conversion and Management 49 (2008) 3343e3348. [15] R. Gomri, Second law comparison of single effect and double effect vapor absorption refrigeration systems, Energy Conversion and Management 50 (2009) 1279e1287. [16] S.C. Kaushik, A. Arora, Energy and exergy analysis of single effect and series flow double effect waterelithium bromide absorption refrigeration systems, International Journal of Refrigeration 32 (2009) 1247e1258. [17] M.A. Rosen, M.N. Le, I. Dincer, Thermodynamic assessment of an integrated system for cogeneration and district heating and cooling, International Journal of Exergy 1 (2004) 94e110. [18] I. Dincer, M.A. Rosen, Exergy: Energy, Environment and Sustainable Development. Elsevier, Oxford, UK, 2007. [19] G.A. Florides, S.A. Kalogirou, S.A. Tassou, L.C. Wrobel, Design and construction of a LiBreH2O absorption machine, Energy Conversion and Management 44 (2003) 2483e2508. [20] ASHRAE Handbook e Refrigeration. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, 2002. [21] J. Patek, J. Klomfar, Computationally effective formulation of the thermodynamic properties of LiBreH2O solution from 273 to 500 K over full composition range, International Journal of Refrigeration 29 (2006) 566e578. [22] R.C. Spencer, NBS/NRC steam tables, International Journal of Heat and Fluid Flow 6 (1985) 88e89. [23] S.A. Klein, F. Alvarado, Engineering Equation Solver, Version 7.441. F-Chart Software, Middleton, 2005. [24] 2009 ASHRAE Handbook Fundamentals e Refrigeration. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, 2009.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Analysis of crystallization risk in double effect absorption refrigeration systems L. Garousi Farshi a, *, S.M. Seyed Mahmoudi a, *, M.A. Rosen b a b
Faculty of Mechanical Engineering, University of Tabriz, Tabriz, East Azarbaijan, Iran Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2010 Accepted 10 February 2011 Available online 1 March 2011
Absorption refrigeration systems are an alternative to vapor compression ones in cooling and refrigeration applications. In comparison with single effect absorption units, double effect systems have improved performance. Also, they are more available commercially than the other multi effect absorption cycles. An important challenge in the operation of such systems is the possibility of crystallization within them. This is especially true in developing air-cooled absorption systems, which are attractive because cooling tower and associated installation and maintenance issues can be avoided. Therefore, distinguishing the working conditions that may cause crystallization can be useful in the design and control of these systems. In this paper a computational model has been developed to study and compare the effects of operating parameters on crystallization phenomena in three classes of double effect lithium bromideewater absorption refrigeration systems (series, parallel and reverse parallel) with identical refrigeration capacities. It is shown that the range of operating conditions without crystallization risks in the parallel and the reverse parallel configurations is wider than those of the series flow system. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Crystallization Double effect absorption Chiller Refrigeration Cooling
1. Introduction Producing cold by means of absorption refrigeration systems has advantages, as these systems are thermally activated and do not require high input shaft power. Also, the ability to use low grade heat, such as that obtained from solar thermal systems, fuel cells, residual heat, geothermal energy, etc., can provide good reasons for using absorption machines. Other benefits of absorption refrigeration include being independent of electric grids, potentially having less CO2 emissions, causing no ozone layer depletion, and using natural refrigerants, and these contribute to increasing the attractiveness of these machines. However, absorption refrigeration systems suffer from having low efficiencies and a limitation on the temperature of utilized heat source. These two points have motivated the development of advanced absorption machines [1], including different configurations of double effect systems. The most common working fluid combinations used in absorption refrigeration systems are lithium bromideewater (LiBreH2O) and ammoniaewater (NH3eH2O). The NH3eH2O absorption system is more complicated than the LiBreH2O system,
* Corresponding authors. Tel.: þ98 411 3393056; fax: þ98 411 3354153. E-mail addresses: [email protected] (L. Garousi Farshi), s_mahmoudi@ tabrizu.ac.ir (S.M. Seyed Mahmoudi), [email protected] (M.A. Rosen). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.02.013
since it needs a rectifying column that assures no water vapors enter the evaporator; otherwise the evaporator could no longer operate at the pressures and temperatures usually required in ammoniaewater chillers. LiBreH2O absorption systems however have higher COPs than NH3eH2O systems and are commercially available as absorption chillers for air conditioning applications [2]. It must be noted that LiBr is a salt, and in its solid state it has a crystalline structure. When LiBr is dissolved in water, there is a specific minimum solution temperature for any given salt concentration. The salt begins to leave the solution and crystallize below this minimum temperature. Crystallization must be avoided in the design and operation of LiBreH2O absorption systems because the formation of slush in the piping network forms a solid, blocking the flow quickly. If this occurs, the concentrated solution temperature needs to be raised significantly above its saturation point so that the salt crystals dissolve within a reasonable time, freeing the machine. Restoring absorber operation after crystallization is very labor intensive and time consuming [3]. The double effect absorption refrigeration cycle was introduced in 1956e1958 [4]. The earliest theoretical study of a double effect absorption system was provided for various working fluids by Kaushik and Chandra [5] and by Garimella and Christensen [6]. Some studies have concentrated on energy analysis [7e12], and others have focused on exergy analysis of these systems [13e18]. In none of these works, however, is the location in which crystallization is most likely to occur specified and reports on the effect of
L. Garousi Farshi et al. / Applied Thermal Engineering 31 (2011) 1712e1717
Nomenclature
1713
Greek symbols heat exchanger effectiveness
h COP D h _ m P Pdrop1 Pdrop2 Q_ Q_ LPG;out T _ W X
coefficient of performance distribution ratio specific enthalpy, kJ/kg K mass flow rate, kg/s pressure, kPa pressure drop between evaporator and absorber, Pa pressure drop between low pressure generator and condenser, Pa heat transfer rate, kW energy supply rate from external source to LPG, kW temperature, K work transfer rate, kW LiBr concentration
operating conditions on solution crystallization in absorption systems are lacking. Florides et al. [19] and Liao and Radermacher [3] identify several causes of crystallization and approaches to avoid it. In their study, only single effect systems are considered. Notwithstanding the numerous applications of double effect absorption systems in the refrigeration and air conditioning industry and differences in operations between single and double effect systems, little is known about the latter regarding crystallization. In particular, more attention is needed concerning the comparison of crystallization limits in different configurations of double effect systems. The present article aims to redress this lack of knowledge via a methodical and comparative investigation of the risk of crystallization in these systems, considering a broad range of component temperatures, effectiveness of solution heat exchangers, and pressure drops between evaporator and absorber and also between the LPG and condenser. Furthermore, as low grade waste heat can be supplied to the LPG when available, the effect of this heat addition on the possibility of crystallization is investigated. 2. Description of double effect systems and causes of crystallization Depending on the solution flow, double effect absorption chillers can be classified as series, parallel and reverse parallel systems, according to ASHRAE [20]. Fig. 1a shows a schematic diagram of a series flow double effect LiBreH2O absorption refrigeration system. It involves three pressure levels, i.e. high, medium and low. The high pressure generator (HPG) functions at high pressure and high temperature, while the low pressure generator (LPG) and condenser operate at medium pressure, and the evaporator and absorber work at low pressures. The strong solution (in which the refrigerant concentration is high) leaving the absorber is pumped to the HPG through two solution heat exchangers. A high temperature heat source is utilized in the HPG to generate water vapor from the strong solution. The weak solution leaving the HPG enters the LPG via solution expansion valve (EV4). In the LPG the water vapor as refrigerant coming from the HPG is condensed due to the low temperature of the weak solution and its latent heat is utilized in generating water vapor from the weak solution. In addition to this latent heat, low grade energy may be supplied to the LPG to produce more vapor. The weak solution, therefore, becomes weaker and is delivered to the
Subscript II Abs Con e eva HPG HTHE i LPG LTHE max p
second law absorber condenser exit evaporator high pressure generator high temperature heat exchanger inlet or each component low pressure generator low temperature heat exchanger maximum pump
absorber through the low temperature heat exchanger (LTHE) and solution expansion valve (EV3). Water vapor generated from both the high and low pressure generators passes to the condenser and heats the cooling stream as it condenses. Then the liquid refrigerant is delivered to the evaporator through an expansion valve (EV1) where its pressure is reduced to that of the evaporator. The cold vapor from the evaporator is dissolved in the weak solution from the LPG through the LTHE, thereby rejecting its heat of absorption in the absorber to the cooling stream. The path of refrigerant from condenser to absorber is the same for all the three systems. Fig. 1b shows the parallel flow system. As shown in this figure, the strong solution leaving the absorber is pumped to the LTHE after which it is divided into two streams. One stream flows to the HPG through a high temperature heat exchanger (HTHE) and the other passes to the LPG via EV4. A high temperature heat source is used to provide heat to the HPG for water vapor generation from the strong solution. In the LPG the refrigerant vapor from the HPG is condensed and its latent heat is utilized to evaporate the water content of the strong solution. The weak solution leaving the HPG mixes with the weak solution from LPG at a mixing point (P1). The combined weak solution flows to the absorber via the LTHE and EV3. The reverse parallel system is shown in Fig. 1c. As that figure indicates, the strong solution leaving the absorber is pumped to the LPG through the LTHE. The weak solution from the low pressure generator is divided into two streams at point P1. One stream is pumped to the high pressure generator through the HTHE and the other passes to point P2 where it mixes with the weak solution from the HPG. The mixture from this point flows to the absorber. In the LPG the vapor received from the HPG is condensed and its latent heat is utilized in generating water vapor from the strong solution. In this system, as for the others, the possibility exists of making use of an external low grade heat source in the LPG. The use of two solution pumps in this system is advantageous as it provides improved control of the performance of the system at part load conditions. Considering practical conditions for the above mentioned absorption systems, the lithium bromide concentration in water at some points in the cycles is high and close to the crystallization border, for instance point A in Fig. 2, which indicates the relation between temperature, pressure and concentration of lithium bromide in a LiBreH2O solution. When the lithium bromide concentration increases and/or the solution temperature decreases,
1714
L. Garousi Farshi et al. / Applied Thermal Engineering 31 (2011) 1712e1717
Fig. 1. Double effect absorption refrigeration systems: (a) Series flow, (b) Parallel flow, (c) Reverse parallel flow.
point A moves toward point B and crystallization may occur, interrupting machine operation. The actual location within the chiller at which crystallization may occur is determined by the mechanical structure of the system; however, crystallization is most likely to take place in the weak solution entering the absorber, as this is the point of highest concentration and lowest temperature, i.e., point 17 in Fig. 1aec. 3. Simulation and analysis of systems Several assumptions are made in the simulations and analyses of the absorption refrigeration systems, as summarized in Table 1. The systems produce chilled water in the evaporator, and the HPG is driven by a pressurized hot water stream. Also, the system rejects heat to cooling water at the condenser and the absorber. When there exists an external low grade heat source for the LPG, it is also assumed in the form of a hot water stream. The outlet and
inlet temperatures of these streams are presented in Table 2, along with the reference conditions for components used in the simulations and the ranges of their variations are. All of these conditions are fixed except where stated otherwise. In this analysis, the thermo-physical properties of the working fluids are expressed as analytic functions. A new set of computationally efficient formulations for thermodynamic properties of LiBreH2O solution developed by Patek and Klomfar [21] are used in this work. The equations for the thermal properties of steam are obtained from correlations provided by Spencer [22]. In order to analyze the absorption systems, principles of mass and energy conservation are used. The general equations of these principles are specified below: Mass conservation:
X
_l ¼ m
X
_e m
(1)
L. Garousi Farshi et al. / Applied Thermal Engineering 31 (2011) 1712e1717
1715
Table 2 Reference operating parameters and their ranges of variation and inlet/outlet temperatures of cooling or heating streams.
Fig. 2. Property chart for LiBr/H2O solution with crystallization curve.
X
_ l Xi ¼ m
X
_ e Xe m
(2)
Energy conservation:
X
Q_
X
_ ¼ W
X
_ e he m
X
_ l hi m
(3)
Reference conditions
Variation range
Inlet and outlet temperatures of cooling and heating streams
Tcon ¼ Tabs ¼ 310 K (37 C)
305e315 K
Teva ¼ 277 K (4 C)
277e283 K
THPG ¼ 400 K (128 C)
380e480 K
hLTHE ¼ 70% hHTHE ¼ 70% hpumps ¼ 95%
Inlet temperature of cooling water ¼ Tcon 8 Outlet temperature of cooling water ¼ Tcon 3 Inlet temperature of chilled water ¼ Teva þ 8 Outlet temperature of chilled water ¼ Teva þ 3 Inlet temperature of hot water ¼ THPG þ 18 Outlet temperature of hot water ¼ THPG þ 10
50e90% 50e90%
Pdrop1 ¼ 0 (pressure drop between evaporator and absorber) Pdrop2 ¼ 0 (pressure drop between LPG and condenser) Q_ LPG;out ¼ 0 (low grade h eat supplied at LPG)
0e100 Pa
0e500 Pa
0e300 kW
These equations are applied to each component of the systems. The distribution ratio D for parallel and reverse parallel systems respectively can be expressed as follows (see Fig. 1b and c):
_ 6c =m _4 D ¼ m
5. Results and discussion
(4a)
and
_ 15b =m _ 15 D ¼ m
Inlet temperature of hot water ¼ TLPG þ 18 Outlet temperature of hot water ¼ TLPG þ 10
(4b)
4. Validation of the simulation A computer program has been developed using Engineering Equation Solver (EES) software [23] to carry out the thermodynamic analysis for each of the three configurations of double effect absorption refrigeration systems. In the programs the new set of formulations for LiBreH2O solution properties is used as internal functions instead of data from the EES database. The formulations are more accurate and provide properties at comparatively wider ranges of temperature and concentration. In order to validate the present model, the obtained results have been compared with those reported by Gomri [14] for a series flow double effect system. A good agreement is seen between the two works as presented in Table 3. Table 1 Assumptions used in simulation and analysis of systems. Simulations and analyses are performed under steady conditions Conditions of refrigerant (water) at the exits of condenser and evaporator are saturated Solution is at equilibrium condition at the exits of absorber, HPG, LPG and at the corresponding device temperatures Pressure losses due to friction in the heat exchangers and the connecting piping are negligible, except between the evaporator and the absorber and between the LPG and the condenser; the effect of these pressure losses is assessed Heat exchange between the systems and the surroundings, other than that prescribed by heat transfer at the generator, evaporator, condenser and the absorber, is assumed negligible A temperature difference of 5 K is assumed for heat transfer at the LPG Simulations and analyses are carried out for a constant refrigeration capacity in all the systems, where Q_ eva ¼ 300 kW
To study the possibility of crystallization occurrence in the absorber entrance in the three cycles, a parametric study was carried out to assess the effect of parameters such as temperature and pressure at different points in the cycles and also that of heat exchanger effectiveness and heat supply to the LPG on the temperature and concentration of point 17. The results are shown in
Table 3 Comparison of results from present work with those of Gomri [14]. Point
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
_ (kg/s) m
T (K)
X (% LiBr)
Gomri
Present work
Gomri
Present work
308 277 277 308 308 335.49 379.81 403 349.09 349.09 403 355.46 308 353 353 321.67 321.67 421 413 300 305 285 280 300 305
308 277 277 308 308 335.49 379.8 403 356.09 356.09 403 355.45 308 353 353 321.67 321.67 421 413 300 305 285 280 300 305
0.127 0.127 0.127 1.737 1.737 1.737 1.737 1.671 1.671 1.671 0.065 0.065 0.065 0.062 1.610 1.610 1.610 7.340 7.340 7.990 7.990 14.285 14.285 18.409 18.409
0.127 0.127 0.127 1.735 1.735 1.735 1.735 1.670 1.670 1.670 0.065 0.065 0.065 0.062 1.608 1.608 1.608 7.330 7.330 7.993 7.993 14.321 14.321 18.415 18.415
Gomri
h (kJ/kg K) Present work
55.869 55.869 55.869 55.869 58.056 58.056 58.056
55.880 55.880 55.880 55.880 58.070 58.070 58.070
60.278 60.278 60.278
60.294 60.294 60.294
Present work 146.59 146.59 2507.87 87.67 87.67 143.14 235.43 288.40 192.49 192.49 2740.53 345.21 345.21 2649.57 195.84 135.98 135.98 623.68 589.24 113.12 134.04 50.36 29.42 113.12 134.04
L. Garousi Farshi et al. / Applied Thermal Engineering 31 (2011) 1712e1717 338
1716
420
455
420
328
465 465 425
420
450
450 323
400
460
series(T_con=T_abs=305K)
425
series(T_con=T_abs=310K) series(T_con=T_abs=315K)
400
425
Parallel(T_con=T_abs=305K)
400
313
318
Temperature ( K)
333
480
Parallel(T_con=T_abs=310K) Parallel(T_con=T_abs=315K)
445 445
380 380
Reverse parallel(T_con=T_abs=305K) Reverse parallel(T_con=T_abs=310K) Reverse parallel(T_con=T_abs=315K)
385 T HPG=380K T_HPG=380K
0.54
0.56
0.58
Poly. (Crystallization)
0.6
0.62
0.64
0.66
0.68
0.7
0.72
0.74
X ( Lithium Bromide concentration)
Fig. 3. Variation in temperature and concentration of solution entering the absorber (point 17) for several high pressure generator and condenser temperatures.
This may seem to be in contradiction with what happens in practice, but it must be noted that in practice we usually deal with an existing system in which, when the absorber temperature is lowered, the absorption potential and consequently the mass flow rate of the solution exiting the absorber increases while the rate of heat input to the generator stays almost constant. Therefore, the solution flow enters the absorber with a lower concentration and temperature implying a lower possibility of crystallization at this point [3]. In the present work, however, the design of a new system is dealt with and the generator temperature is constant (variable input heat); therefore, lowering the absorber temperature will result in an increase of concentration and a decrease of temperature of the solution entering the absorber, increasing the possibility of crystallization. This point is also mentioned as a caution of possible crystallization in the literature [3,19]. In the parallel and reverse parallel cycles it is observed that for the studied condition there is a weak possibility of crystallization and this possibility in the reverse parallel cycle is greater than that for the parallel system. The sudden variations in concentrations and temperatures of point 17 in the parallel and reverse parallel cases are caused by the criteria used in selecting the optimum distribution ratio. For each of the double effect systems the relation among the properties at the absorber entrance, THPG and Teva, is shown in Fig. 4. The effect of THPG is similar to that described by Fig. 3. As shown in Fig. 4, the possibility of crystallization with increasing Teva is greater for the series flow cycle (see the position of
326
series( T_HPG=420K) Parallel( T_HPG=420K) Reverse parallel( T_HPG=420K) Poly. (Crystallization)
324
325
Temperature ( K)
327
328
Figs. 3e6. In each figure the crystallization line is drawn based on data obtained from ASHRAE [24]. Note that, in the case of parallel and reverse parallel systems, the optimum values of D for maximum COP are selected for calculations in every operating condition. As a sample, the optimum values of D in many operating conditions are provided in Table 4. For optimization purposes EES has many choices. If there is one degree of freedom, EES will minimize/maximize the selected variable using either the Quadratic Approximation method or Golden Section search. The former is usually faster, but the latter is more reliable. So the latter was chosen for calculation and after calcu_ were verified to be positive. lating the optimum D, values of m Fig. 3 shows the variation of the concentration of the weak solution entering the absorber (point 17 in the three configurations) with its temperature at different values of THPG and Tcon (¼ Tabs). It is shown that in the double effect series flow cycle for all the examined values of Tcon ¼ Tabs an increase in THPG increases the temperature and concentration of LiBr at the entrance of the absorber. It is noted however that at some regions in the figure there are slight decreases in T17 after the growth. In each operating condition, there is a maximum value for THPG beyond which crystallization occurs. This HPG temperature increases with an increase in the value of Tcon (¼ Tabs) indicating that the cycle can utilize higher temperature heat sources (see the position of THPG ¼ 425 K, for example). However, as Tcon (¼ Tabs) decreases the possibility of crystallization increases.
Fig. 5. Variation in temperature and concentration of solution entering the absorber (point 17) with effectiveness of high and low temperature heat exchangers.
0.6
0.62
0.64
0.66
0.68
X ( Lithium Bromide concentration)
Fig. 4. Variation in temperature and concentration of solution entering the absorber (point 17) for several high pressure generator and evaporator temperatures.
Fig. 6. Variation in temperature and concentration of solution entering the absorber (point 17) with external heat supplied to low pressure generator.
L. Garousi Farshi et al. / Applied Thermal Engineering 31 (2011) 1712e1717 Table 4 Optimum values of D for several operating conditions. Parallel configuration
Reverse parallel configuration
Reference conditions with various THPG
Reference conditions with various THPG
THPG (K)
D
THPG (K)
D
400 405 410 415 420 425 430 435 440 445 450
0.51 0.46 0.44 0.43 0.42 0.41 0.40 0.39 0.33 0.3 0.27
400 405 410 415 420 425 430 435 440 445 450
0.75 0.72 0.7 0.68 0.65 0.62 0.59 0.56 0.46 0.4 0.36
THPG ¼ 425 K, for example). In the parallel and reverse parallel cycles it is observed for the studied conditions that, as mentioned earlier, there is a weak possibility of crystallization and this possibility in the reverse parallel cycle is greater than that in for the parallel cycle. It should be noted that in both Figs. 3 and 4 there exist three curves for the series configuration with more or less the same shape. But the curves in Fig. 4, in which the evaporator temperature changes, are very close to each other making it seem as if there is only one curve. However, the positions of points with THPG ¼ 425 K and different Teva shown in the figure explains the difference among the curves. The effects of hHTHE and hLTHE on property changes of point 17 are shown in Fig. 5. It is shown that only in series flow system when a very efficient heat exchanger is used as LTHE, the condition of point 17 approaches the crystallization line, and in this case this matter must be considered carefully. Again in parallel and reverse parallel systems crystallization occurrence is a little possibility. The effect of pressure drops in various positions in the system is also studied and was found to have a negligible effect on the possibility of crystallization in the systems. Fig. 6 shows the effect of supplying heat externally to the LPG. It is clear that in series flow system increasing Q_ LPG;out moves point 17 closer to the crystallization line, but in the parallel and reverse parallel systems the opposite effect is observed. Therefore, when a low grade heat source is available for use in the LPG, in the case of series flow system more caution must be taken to avoid crystallization at the absorber entrance. 6. Conclusions Crystallization is a practical problem typical of waterelithium bromide systems. The possibility of crystallization at the absorber entrance has been assessed here by examining the effect of operating parameters in series, parallel and reverse parallel LiBreH2O double effect absorption refrigeration systems. A computer program using EES software has been developed to simulate the performance of the three systems. In the case of parallel and reverse parallel systems, the optimum distribution ratio for COPmax has been selected for calculations at each operating condition. It is found that, in series flow systems, the possibility of crystallization increases with increasing THPG, Teva and effectiveness of the LTHE
1717
and decreasing Tcon (¼ Tabs). Also, in parallel and reverse parallel cycles, it is determined that, in studied condition, the crystallization possibility is low but in practice the mechanical structure of system and actual operating condition can play very important roles in this phenomenon. Hence, the double effect parallel and reverse parallel flow arrangements are superior in performance to the series flow in terms of crystallization risk. References [1] P. Srikhirin, S. Aphornratana, S. Chungpaibulpatana, A review of absorption refrigeration technologies, Renewable and Sustainable Energy Reviews 5 (2001) 343e372. [2] S. Kalogirou, Recent patents in absorption cooling systems, Recent Patents on Mechanical Engineering 1 (2008) 58e64. [3] X. Liao, R. Radermacher, Absorption chiller crystallization control strategies for integrated cooling heating and power systems, International Journal of Refrigeration 30 (2007) 904e911. [4] G.C. Vliet, M.B. Lawson, R.A. Lithgow, Waterelithium bromide double-effect absorption cooling cycle analysis, ASHRAE Transaction 88 (1982) 811e822. [5] S.C. Kaushik, S. Chandra, Computer modeling and parametric study of a double-effect generation absorption refrigeration cycle, Energy Conversion and Management 25 (1985) 9e14. [6] S. Garimella, R.N. Christensen, Cycle description and performance simulation of a gas-fired hydronically coupled double-effect absorption heat pump system, in: , Proceedings of the ASME: Recent Research in Heat Pump Design, vol. 28, 1992, pp. 7e14. [7] G.P. Xu, Y.Q. Dai, K.W. Tou, C.P. Tso, Theoretical analysis and optimization of a double-effect series-flow-type absorption chiller, Applied Thermal Engineering 16 (1996) 975e987. [8] G.P. Xu, Y.Q. Dai, Theoretical analysis and optimization of a double-effect parallel-flow-type absorption chiller, Applied Thermal Engineering 17 (1997) 157e170. [9] M.B. Arun, M.P. Maiya, S.S. Murthy, Equilibrium low pressure generator temperatures for double-effect series flow absorption refrigeration systems, Applied Thermal Engineering 20 (2000) 227e242. [10] M.B. Arun, M.P. Maiya, S.S. Murthy, Performance comparison of double effect parallel flow and series flow water lithium bromide absorption systems, Applied Thermal Engineering 21 (2001) 1273e1279. [11] Y.L. Liu, R.Z. Wang, Performance prediction of solar/gas driving double effect LiBreH2O absorption system, Renewable Energy 29 (2004) 1677e1695. [12] E. Torrella, D. Sánchez, R. Cabello, J.A. Larumbe, R. Llopis, On-site real-time evaluation of an air-conditioning direct-fired double-effect absorption chiller, Applied Energy 86 (2009) 968e975. [13] T.S. Ravikumar, L. Suganthi, A.S. Anand, Exergy analysis of solar assisted double effect absorption refrigeration system, Renewable Energy 14 (1998) 55e59. [14] R. Gomri, R. Hakimi, Second law analysis of double effect vapor absorption cooler system, Energy Conversion and Management 49 (2008) 3343e3348. [15] R. Gomri, Second law comparison of single effect and double effect vapor absorption refrigeration systems, Energy Conversion and Management 50 (2009) 1279e1287. [16] S.C. Kaushik, A. Arora, Energy and exergy analysis of single effect and series flow double effect waterelithium bromide absorption refrigeration systems, International Journal of Refrigeration 32 (2009) 1247e1258. [17] M.A. Rosen, M.N. Le, I. Dincer, Thermodynamic assessment of an integrated system for cogeneration and district heating and cooling, International Journal of Exergy 1 (2004) 94e110. [18] I. Dincer, M.A. Rosen, Exergy: Energy, Environment and Sustainable Development. Elsevier, Oxford, UK, 2007. [19] G.A. Florides, S.A. Kalogirou, S.A. Tassou, L.C. Wrobel, Design and construction of a LiBreH2O absorption machine, Energy Conversion and Management 44 (2003) 2483e2508. [20] ASHRAE Handbook e Refrigeration. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, 2002. [21] J. Patek, J. Klomfar, Computationally effective formulation of the thermodynamic properties of LiBreH2O solution from 273 to 500 K over full composition range, International Journal of Refrigeration 29 (2006) 566e578. [22] R.C. Spencer, NBS/NRC steam tables, International Journal of Heat and Fluid Flow 6 (1985) 88e89. [23] S.A. Klein, F. Alvarado, Engineering Equation Solver, Version 7.441. F-Chart Software, Middleton, 2005. [24] 2009 ASHRAE Handbook Fundamentals e Refrigeration. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, 2009.