Influence of the mass flow ratio water-air on the volumetric mass transfer coefficient in a cooling tower

Transcription

1 International Journal of CemTec Researc CODEN (UA): IJCRGG, IN: , IN(Online): Vol.11 No.1, pp , 18 Influence of te mass flow ratio water-air on te volumetric mass transfer coefficient in a cooling tower uis Obregón Quiñones 1 *, Jorge Duarte Forero, Guillermo Valencia Ocoa 1 Researc Group on ustainable Cemical and Biocemical Processes, Universidad del Atlántico, Carrera 3 No 8 49, Puerto Colombia, Colombia. Efficient Energ Management Researc Group, Universidad del Atlántico Carrera 3 No 8 49, Puerto Colombia, Colombia. Abstract: A laborator scale mecanical draft cooling tower wit acrlic film tpe packing was studied. Te performance of te tower was assessed at different mass flow ratios of water to air/g, maintaining constant te water and air inlet temperatures and te packing material. It was found tat te relation of te volumetric mass transfer coefficient ka versus te volumetric eat transfer coefficient ais not affected b te operating conditions. Te small differences obtained are due to te experimental error. For all te value of te ratio /G in te range of ka/ a [3,5] te assumption of ka/ a= cannot be considered and te relation as te following expression k a a.34. However, for ka/ a>5 te assumption of -m= is valid, and te value of kais approximatel 8 Kg/.m 3. Kewords : Cooling tower, efficienc, tower packing, volumetric mass transfer coefficient. 1. Introduction Cooling towers are important refrigeration sstems used in man different kinds of industries around te world. Teir advantage consists in te direct contact gas-liquid tat cause a better eat transfer rate 1. Tis advantage is improved wit te use of packing towers tat increase te contact area gas-liquid,3. Man different geometries and arrangementsof te packing ave been created to enance te performance of te cooling towers 4. Ever packing as an impact in te volumetric mass transfer coefficient wic is te most important parameter used to model te beavior of te towers. Tere are oter parameters tat affect te volumetric mass transfer coefficientsuc as te inlet temperatures of te fluids. Tere as been lots of researc focusing on enancing te performance of te cooling towers studing te parameters tat ave te igest influence on eat and mass transfer. It was done a stud of te effect of te tower eigt to base diameter ratio in te performance of a cooling tower 5 b means of CFD simulations. Muangnoi et al., 14, studied te relationsip of water-jet cooling tower approac temperature, eat rejection load, wet-bulb temperature, water to air ratio, tower spra zone eigt, droplet diameter, discarge droplet velocit and air velocit in a cooling tower to uis Obregón Quiñones et al /International Journal of CemTec Researc, 18,11(1): DOI= ttp://dx.doi.org/1.9/ijctr

2 uis Obregón Quiñones et al /International Journal of CemTec Researc, 18,11(1): improve sstem operation 6.Mirabdola et al., made an experimental investigation of te termal performance caracteristics of a direct contact counter flow wet cooling tower filled wit rotational splas tpe packing 7. owe studied te mass transfer coefficients as a function of /G 8 wile Kloppers and Kröger 9 studied te same parameter as a function of mass liquid rate and gas rate separatel. In tis work, a laborator scale mecanical draft cooling tower wit acrlic film tpe packing was studied. Te packing was sized according to te design parameters found in te work of Moiuddin AKM et al., Te performance of te tower was evaluated at different mass flow ratios of water to air /G, maintaining constant te water and air inlet temperatures and te packing material. It was found tat te relation /G as a significative effect on te outlet air temperature and in te efficienc of te tower. No effect was found in te volumetric mass transfer coefficient.. Metodolog Te experiments were performed in a cooling tower were te ot liquid water at 48 o C enters at te top of te tower wile te cool air enters at te bottom of te tower at a dr bulb temperature of 6. o C and wet bulb temperature of.4 o C. Figure 1 sows a sceme of te sstem wit te main parameters used to make te mass and energ balance. G, T G, H Top T = Mass flux of liquid water, kg/ m G, T G, H T G Mass flux of dr air, kg/ m H= Humidit ratio of air, Kg water/ Kg dr air = Entalp of air, KJ/Kg dr air G, T G1, H Air Bottom T 1 1 iquid water Figure 1.ketc of te cooling tower Figure 1 sows tat dr air enters in te bottom of te tower and as it rises, its umidit increases because of te evaporation caused b te contact air-water. Tis contact is increased b te use of film tpe packing tower making te liquid water to decrease te temperature.te total eigt of te tower was 1.41 m and contain tree decks film tpe inside. Te space from deck to deck was.35 m and te transversal area of te column was.15x.15m.te material of te film tpe packing was acrlic. Te sape of te film tpe packing is sown in figure. It was used a digital anemometer to determine te velocit of te air, a rotameter to measure te flow rate of te liquid water, and a set of temperature sensors to measure te temperature of te air as well as its psical properties.

3 uis Obregón Quiñones et al /International Journal of CemTec Researc, 18,11(1): Figure.Dimensions (in mm) of te film tpe packing designed according to te parameters given b A. K. M. Moiuddin and K. Kant, It was used te relation /G wit te following values [.44; 1.11; 1.88] obtained varing onl te mass flux of liquid water. Wit te given inlet temperatures of te air, it was obtained te inlet entalp, 1=65.81 KJ/Kg, and te relative umidit of7.34%. Te outlet relative umidit of te air obtained was 98%. As can be seen, te air gets out almost saturated wit water indicating tat te tower as a good eat and mass transfer. Te efficiencies of te tower wit tese operating conditions were[5%, 4%, 67.%]. Te air entalp at te top was obtained using te Eq. 1 wic is te energ balance of te cooling tower considering no loss of eat to te surroundings. C T -T +G = C T -T +G (1) werec is te specific eat of te liquid wic is considered constant in te range of temperature of tis sstem.te subscript 1 refers to te bottom of te column. Considering negligible te amount of water evaporated it is obtained Eq. 1 1 G C T T () Tis equation represents te operating line of te cooling tower. Wit Eq. and te saturation line airwater, it is plotted figure 3. Te outlet dr bulb temperature of te air T G was obtained wit te Mickle metod. Figure 3.Caracteristic operating line of te sstem

4 , KJ/Kg, KJ/Kg, KJ/Kg uis Obregón Quiñones et al /International Journal of CemTec Researc, 18,11(1): Figure 3 elps to understand if te sstem is working adequatel and if te air used is getting saturated or not. Te building of te tower was made considering te Merkel equation. Z G ka 1 d - i (3) were ka is te mass transfer coefficient of te sstem. Te subscript 1 refers to te interface water-air. 3. Results Figures 4 sows te saturation curve of te air wit te operating line of te cooling tower for te relation /G= [.44; 1.11; 1.88]. As can be seen, tere are tree different operating lines, figure 4a, b and c tat correspond to te given ratios of/g. Te slopes of te operating line increase as te ratio/g increases. (a) aturation line =.15T -.7T Range: T(6 o C-34 o C) aturation line = 1.834e.7T (b) aturation line aturation line = 1.834e.7T =.15T -.7T Range: T(6 o C-34 o C) = 1.85T T G =3.7 o C (c) aturation line =.15T -.7T Range: T(6 o C-34 o C) = 4.63T T G =3.8 o C aturation line = 1.834e.7T = 7.87T T G =33.3 o C Figure 4. Beavior of te operating line as a function of /G using acrlic film tpe packing a) /G=.44, b) /G=1.11, c) /G=1.88 Figure 5a sows tat te outlet air temperature increases as /G increases. It appens because tere is a iger amount of liquid in contact wit te gas causing a iger eat transfer rate resulting in an increase in te temperature of te air. Tis relation is not linear and as a tendenc to te inlet temperature of te liquid. Figure 5b sows tat te efficienc of te column decreases as /G increases because tere is a iger amount of water to cool using te same amount of air. It causes a decrease in te capacit of cooling of te tower. It as a perbolic beavior wic tends to zero.

5 , KJ/Kg Efficienc uis Obregón Quiñones et al /International Journal of CemTec Researc, 18,11(1): TG T =48 o C TG 1 =6. o C A=.15x.15m Acrlic film tpe packing (a) /G T =48 o C TG 1 =6. o C A=.15x.15m Acrlic film tpe packing (b).5 /G Figure 5. Effect of te ratio /G on te a) outlet air temperature and b) efficienc of te cooling tower using acrlic film tpe packing. Figure 6 sows te beavior of te operating line wen te ratio of te volumetric mass transfer coefficient to te volumetric eat transfer coefficient canges to wen /G=1.88. As can be seen, even for a large cange in te value of -m, te outlet air temperature remains te same. It means tat tere is a direct relation between te k a and a. aturation line =.15T -.7T Range: T(6 o C-34 o C) aturation line = 1.834e.7T T G =33.3 o C = 7.87T Figure 6. Beavior of te operating line wen -m= at /G=1.88 As mentioned before, tere is a relation between te k a and a. Tis relation is sown in figure 7 for values of te ratio /G= [.44; 1.11; 1.88]. It can be seen tat tere is no effect on te operating conditions in te relation k avs. a. For a large range of -mfor an value of /G, te relation kavs-m remain constant. Te small difference is due to te fact tat te are experimental data. Tat is te corresponding error. Considering tat te lines obtained practicall do not suffer variation, te relation kavs-m remains constant. However, if it is canged te packing tower, te area, or te eigt of te tower, tis relation canges.

6 Ka, Kg/(.m 3 ) Ka, Kg/(.m 3 ) uis Obregón Quiñones et al /International Journal of CemTec Researc, 18,11(1): T =48 o C TG 1 =6. o C A=.15x.15m Acrlic film tpe packing -m=ka/ a /G=1.88 /G=1.11 /G=.44 Figure 7. Beavior of te cooling tower efficienc as a function of /G for different materials of te splas tpe packing. Figure 8 sows te same plot 7 in a sorter range. In tis range te assumption of -m= cannot be taken because te gas pase and te liquid pase ave te similar strengt on te eat resistance. It means tat for a given tower te relation Kavs-m can be obtained wit simple experiments of canging te operating conditions. However, if te psical parameters of te tower canges, te function obtained canges T =48 o C TG 1 =6. o C A=.15x.15m Acrlic film tpe packing m=ka/ a /G=1.88 /G=1.11 /G=.44 Figure 8. Beavior of te cooling tower efficienc as a function of -m for different ratios of /G For all te value of te ratio /G, in te range of -m [3,5] te assumption of -m= cannot be taken because te gas pase and te liquid pase ave te similar strengt on te eat resistance. In tis range,te lines ave te following regression: k a a.34 m=is valid, and te value of ka would be approximatel 8 Kg/.m Conclusions. However, for -m>5 te assumption of - In te present work, it was studied te performance of a cooling tower under different ratios of /Gwit te use of film tpe packing. It was found tat te slopes of te operating line increase as te ratio /G increases causing an increase in te outlet wet dr temperature of teair due to te iger amount of liquid in contact wit te gas causing a iger rate of eat transfer. Te relation between te outlet wet dr air temperature and /Gis not linear and ave a limit to te inlet temperature of te liquid. Tis beavior makes te efficienc of te

7 uis Obregón Quiñones et al /International Journal of CemTec Researc, 18,11(1): column to decrease as /G increases because tere is a iger amount of water to cool using te same amount of air. It causes a decrease in te capacit of cooling of te tower. It as a perbolic beavior wic tends to zero. It was found tat te relation kavs-m is not affected b te operating conditions. For all te value of te ratio /G, in te range of -m [3,5] te assumption of -m= cannot be considered because te gas pase and te liquid pase ave te similar strengt on te eat resistance. However, for -m>5 te assumption of -m=is valid. References 1. Obregon,.G., J.C. Pertuz, and R.A. Dominguez, Performance analsis of a laborator scale cooling tower for different packing materials, water inlet temperature and mass flow ratio water-air.revista Prospectiva, (1): p Garageizi, F., R. Haati, and. Fatemi, Experimental stud on te performance of mecanical cooling tower wit two tpes of film packing.energ Conversion and Management, 7.48(1): p Gosasi, H.R. and J.F. Missenden, Te investigation of cooling tower packing in various arrangements.applied Termal Engineering,.(1): p aali, P., et al., Experimental stud on improving operating conditions of wet cooling towers using various rib numbers of packing.international Journal of Refrigeration, 16.65: p iao, H.T., et al., Influences of eigt to diameter ratios of dr-cooling tower upon termo-flow caracteristics of indirect dr cooling sstem.international Journal of Termal ciences, 15.94: p Muangnoi, T., W. Asvapoositkul, and P. Hungspreugs, Performance caracteristics of a downward spra water-jet cooling tower.applied Termal Engineering, 14.69(1): p Mirabdolaavasani, A., et al., Experimental stud on te termal performance of mecanical cooling tower wit rotational splas tpe packing.energ Conversion and Management, 14.87: p owe, H.J. and D.G. Cristie, Heat transfer and pressure drop data on cooling tower packings, and model studies of te resistance of natural draft towers to airflow, in International Heat Transfer Conference. 1961, New York: American ociet of Mecanical Engineers, 1963.: Colorado. p Kloppers, J.C. and D.G. Kroger, oss coefficient correlation for wet-cooling tower fills.applied Termal Engineering, 3.3(17): p Moiuddin, A.K.M. and K. Kant, Knowledge base for te sstematic design of wet cooling towers. Part II: Fill and oter design parameters.international Journal of Refrigeration, (1): p *****