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Parametric Analysis of Different Binary Mixtures in Solution Heat Exchanger of Absorption Refrigeration System.

INTRODUCTION

Absorption refrigeration (ABR) systems can be operated with low-grade heat sources such as solar energy or industrial waste heat, with significant benefits for the environment. Furthermore, unlike vapor compression refrigeration systems, which employ organic refrigerants that may have detrimental environmental effects, ABR systems use mainly water-LiBr and ammonia-water fluids, which exhibit better environmental performance. On top of this, additional benefits compared to vapour compression systems include the lower electric power consumption (around 5% of the capacity), the utilization of waste heat or renewable energy sources (solar energy, biomass combustion, etc.) and reduced noise and vibration due to absence of heavy compressor (Abed et al. 2017; Banu and Sudharsan 2018). Several different working fluid pairs have been tested in ABR systems (Jian et al. 2012; Banu and Sudharsan 2018; Papadopoulos et al. 2019) and water-LiBr and ammonia water being the most popular (Kang et al. 2000).

ABR systems comprise of different component such as absorber, solution pump, generator, solution heat exchanger, refrigerant heat exchanger, condenser, and evaporator. The heat added in the generator is rejected from the condenser and the absorber to the atmosphere, introduces irreversibilities in the system as useful heat being practically wasted. Park et al. (2004) concluded that the performance of the ABR can be improved by eliminating the irreversibilites using internal heat recovery and reducing the sensible heat load of the system. Such a recovery of heat helps in enhancing system's overall performance (Kaushik and Bharadwaj 1982; Sun 1998; Kang et al. 2000). Generally, the refrigerant heat exchanger (RHE) and solution heat exchangers (SHE) are used for internal heat recovery in ABR (Sozen 2001; Kayankli and Kilic 2007; Du et al. 2014; Abed et al. 2015; Chen et al. 2017). Among the two, the SHE is more important than the RHE in terms of heat recovery. The coefficient of performance (COP) of ABR increases by 40% when effectiveness of the SHE is increased from zero to one, whereas the RHE has a negligible COP improvement of only 2.8%, under same increase in effectiveness (Kaynaki and Kilic 2007). The SHE enables significant improvement of the ABR efficiency when the absorber and generator temperature difference is more than 40 [degress]C (Rivera et al. 1994). From the open literature it is observed that heat transfer analysis of the SHE are associated mainly withdifferent geometries and for very few refrigerant/absorbent pairs. Table 1 summarizes different geometries of the SHE used in ABR systems.

Generally, SHE are of plate or shell-and-tube type, depending on the scale of the ABR (large scale-shell and tube heat exchanger; small scale-plate heat exchanger) (Jeong et al. 2009; Chen et al. 2018). From Table 1, it is observed that heat transfer and pressure drop characteristics across shell and tube heat exchangers used in ABR have not been previously reported. In the current paper, different combinations of binary mixtures such as water-LiCl and water-LiI in addition to water-LiBr are tested and analyzed for shell and tube type SHE. The detailed simulation is supported by the Aspen Plus platform. The effect of concentration of absorbent, hot and cold side inlet temperature on heat transfer coefficient and pressure drop are investigated.

MODEL DEVELOPMENT AND VALIDATION

The current simulation has been carried out using ASPEN plus platform. This particular software is selected as it is equipped with a variety of fluid data banks (Al-Malah (2017), making it suitable for quantitative modeling of various industrial applications (Darwish et al. 2008; Somers et al. 2011; Querol et al. 2011; Pei et al. 2013; Mansouri et al. 2015; Kaushal and Tyagi 2017; Wang et al. 2017; Moreno et al. 2018; Unlu and Hilmioglu 2019). The absorbents considered in the current analysis exhibit dissociation in mixtures with water as the refrigerant, hence an ELECNRTL property method is selected to obtain desired thermophysical properties (Somers 2009; Wang et al. 2017). This method is suitable for aqueous and mixed solvent system and having capability to handle very low and very high concentration. However, vapor phase properties are discribbed accurately upto medium pressure range. In current study, the absorbents exhibits electrolyte characteristics and mixture does not undergose through phase change at considered range of operating parameters. The geometrical model of a simple shell and tube heat exchanger is build by exploring the EDR (Exchanger Design and Rating) browser of the software. The detailed information of the geometrical parameters used in this work are available in Yehia et al. (2014, 2016). A counter flow arrangement is considered with hot solution flowing through the shell side while cold solution flows through the tube side. The simulation results of the shell and tube heat exchanger are validated with previous studies of (Bell 1981; Ozden and Tari 2010; Yehia et al. 2014, 2016) and show good agreement with them. The comparison of heat transfer coefficient and pressure drop across the shell and tube heat exchanger are presented in Figure 1. The predicted shell side heat transfer coefficient and pressure drop shows an average absolute error up to 14.3% compared to previous studies with similar geometry. The various parameters considered for the parametric analysis performed in current study are presented in Table 2.

RESULTS AND DISCUSSION

Effect of concentration

In order to investigate influence of the absorbent concentration, the cold and hot side solution inlet temperature and the cold side flow rate are held constant as 273 K (31.7 F), 383 K (247.7 F) and 0.5 kg/s (1.1 lb/s) respectively. Whereas, the hot side mass flow rate is varied from 0.5 to 2.0 kg/s (1.1 to 4.4 lb/s). As expected, the shell side heat transfer coefficient increases with increase in mass flow rate for all binary mixtures under consideration as shown in Figure 2(a). It is interesting to note that the heat transfer coefficient is susceptible to the concentration of the absorbent present in the mixture. It is observed that with increase in absorbent concentration, the heat transfer coefficient decreases for all three binary mixtures. The percentage reduction in the heat transfer coefficient with respect to a lower absorbent mass fraction of 0.45 is shown in Figure 2(b). The heat transfer coefficient reduces almost linearly with increase in the LiI concentration in mixture, while a non-linear trend observed for the LiCl and LiBr. When the mass fraction of the absorbent increases from 0.45 to 0.75, an average reduction in heat transfer coefficient for water-LiCl, water-LiBr, and water-LiI are 24%, 53% and 58%, respectively. The decrease in heat transfer coefficient can be attributed to the inversely proportional relation with the Prandtl number, which is the ratio of momentum diffusivity to thermal diffusivity. With increase in the absorbent concentration in the mixture, the solution becomes stronger and results into increase in the Prandtl number (Song et al. 2019).

For a given range of mass flow rate, the heat transfer coefficient of the water-LiCl mixture is 19% and 8% lower than that of water-LiBr when the absorbent concentration changes from 0.45 to 0.75, respectively. Unlike water-LiCl, in case of water-LiI it is higher by 14% and 1% for the absorbent concentration changes from 0.45 to 0.75, respectively.

Like heat transfer coefficient, pressure drop is also increases with increase in mass flow rate as shown in Figure 3(a). The pressure drop is also decreases with increase in absorbent concentration. The reduction trend is almost linear for all three mixtures but stiffer in case of water-LiI compared to water-LiCl and water-LiBr as shown in Figure 3(b). The reduction in the pressure drop is 17%, 24% and 28% respectively for water-LiCl, water-LiBr and water-LiI when concentration increases from 0.45 to 0.75. The higher pressure drop penalty is observed for water-LiCl followed by water-LiBr and the lowest is for water-LiI. In comparison to water-LiBr, the pressure drop penalty of water-LiCl is 10% and 21% higher while it is 3% and 8% lower for mass fraction of 0.45 and 0.75, respectively.

Effect of hot and cold side inlet solution temperature

An impact of the hot solution inlet temperature on heat transfer coefficient and pressure drop characteristics is investigated and presented in Figure 4. For this analysis, the cold side solution inlet temperature and mass fraction are held constant at 283 K (49.7 F) and 0.5 kg/s (1.1 lb/s), respectively. An insignificant increase in heat transfer coefficient is observed over the tested range of the hot solution inlet temperature from 373 to 403 K (211.7 to 265.7 F). As the hot solution temperature increases, the heat transfer coefficient increases because of increase in temperature difference between hot- and cold- solution, which drives more heat to transfer across the tube wall. The increment in heat transfer is visible at higher mass flow rate of the hot solution. At any given flow rate, the heat transfer coefficient is higher for water-LiI followed by water-LiBr and water-LiCl. Furthermore, it is observed that the shell side pressure drop almost remains unaffected over the tested temperature range.

The inlet temperature of the cold side also shows negligible influence on both shell side heat transfer coefficient and pressure drop as shown in Figure 5.

CONCLUSION

In this paper, the heat transfer and pressure drop characteristics across shell and tube solution heat exchanger are investigated for binary mixtures namely water-LiBr, water-LiI and water-LiCl. The parameters considered are the shell side absorbent concentration in mixture, inlet temperature of the hot side solution and inlet temperature of the cold side solution. It is observed that concentration has significant impact on heat transfer coefficient and pressure drop. When the mass fraction of the absorbent increases from 0.45 to 0.75, an average reduction in the heat transfer coefficient for water-LiCl, water-LiBr, and water-LiI are 24%, 53% and 58%, respectively. Furthermore, increase in concetration from 0.45 to 0.75 reduces the pressure drop by 17%, 24% and 28% respectively for water-LiCl, water-LiBr and water-LiI. Both, shell side heat transfer coefficient and pressure drop are not significantly affected by inlet temperature of the hot and cold side solution. Among tested mixtures, water-LiI exhibits better heat transfer and lower pressure drop penalty followed by water-LiBr and water-LiCl.

ACKNOWLEDGMENTS

This paper was made possible by an NPRP award [#NPRP10-1215-160030] from the Qatar National Research Fund (a member of The Qatar Foundation).

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Sambhaji T. Kadam, PhD

Ibrahim Hassan, PhD

Aziz Rahman, PhD

Athanasios I. Papadopoulos, PhD

Panos Seferlis, PhD

Sambhaji T. Kadam is a postdoctoral research associate at the Mechanical Engineering Department, Texas A&M University. Ibrahim Hassan is a professor at the Mechanical Engineering Department, Texas A&M University. Aziz Rahman is an assistant professor at the petroleum Engineering Department, Texas A&M University. Athanasios I. Papadopoulos is a professor at the Chemical Process and Energy Resources Institute (CPERI), Centre for Research and Technology Hellas (C ERTH). Panos Seferlis is a professor at the Department of Mechanical Engineering, Aristotle University of Thessaloniki.
Table 1. Summary of different type SHE used in ABR

Author         HE            Working fluid  Remark(s)

Roy and Maiya  NA            R134a-DMAC     * COP of the system is
(2012a, b)                                  influenced by effecivenss
                                            of SHE.
Song et al.    Plate type    Water-LiBr     * Thermal diffusivity of
(2019)                                      the mixture affect heat
                                            transfer performance of
                                            the SHE.
Jeong et al.   Plate type    Water-LiBr     * An elliptical embossing
(2009)                                      perform better compared to
                                            chevron and round embossing.
Vega et al.    Plate type    Water-LiBr     * Heat transfer rate is
(2006)                                      affected by inlet
                                            temperature of hot solution.
Aphornratana   Pipe in pipe  Water-LiBr     * An effectiveness of SHE
and                                         does not have influence on
Sriveerakul                                 solution circulation ratio
(2007)                                      and cooling capacity.
Ramesh et al.  Shell and     Ammonia-water  * They observed that shell
(2017)         helical coil                 side heat Nusselt number is
                                            four times lower than tube
                                            side Nusselt number.
Jawahar and.   Tube in Tube  Ammonia-water  * The effectiveness of the
Saravanan                                   SHE is 80-85% contributing
(2011)                                      internal heat recovery
                                            about 60-70%.
Goyal et al.   Microchannel  Ammonia-water  * An increase in mass flow
(2017)         array                        rate of the strong solution
                                            significantly increases
                                            heat transfer rate.

Table 2. Range of operating parameter

Description                   Unit, SI (I-P)

Binary mixtures               -
Mass fraction of absorbent    -
Shell side mass flow rate     Kg/s (lb/s)
Shell side inlet temperature  K (F)
Shell side inlet pressure     kPa (bar)
Tube side mass flow rate      kg/s(lb/s)
Tube side inlet temperature   K (F)

Description                   Value

Binary mixtures               H2O-LiBr, H2O-LiCl, H2O-LiI
Mass fraction of absorbent    0.45, 0.55, 0.65, 0.75
Shell side mass flow rate     0.5 (1.1), 1 (2.2), 1.5 (3.3), 2 (4.4)
Shell side inlet temperature  373 (211.7), 383 (229.7), 393 (247.7),
                              403 (265.7)
Shell side inlet pressure     200 (2)
Tube side mass flow rate      0.5 (1.1)
Tube side inlet temperature   273 (31.7), 283 (49.7), 293 (67.7)
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Title Annotation:VC-20-C037
Author:Kadam, Sambhaji T.; Hassan, Ibrahim; Rahman, Aziz; Papadopoulos, Athanasios I.; Seferlis, Panos
Publication:ASHRAE Transactions
Date:Jul 1, 2020
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