2- Thermal Performances Investigation of a Wet Cooling Tower -2 | Physical Sciences

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performance of a wet cooling tower
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  Thermal performances investigation of a wet cooling tower M. Lemouari  a , M. Boumaza  b,* , I.M. Mujtaba  c a Department of Processes Engineering, Faculty of Sciences and Engineering Sciences, University of Bejaia, Algeria b Department of Chemical Engineering, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia c School of Engineering, Design and Technology, University of Bradford, Bradford, UK  Received 14 February 2006; accepted 29 August 2006Available online 18 October 2006 Abstract This paper presents an experimental investigation of the thermal performances of a forced draft counter flow wet cooling tower filledwith an ‘‘VGA’’ (Vertical Grid Apparatus) type packing. The packing is 0.42 m high and consists of four (04) galvanised sheets having azigzag form, between which are disposed three (03) metallic vertical grids in parallel with a cross sectional test area of 0.0222 m 2 (0.15 m  ·  0.148 m). This study investigates the effect of the air and water flow rates on the cooling water range as well as the tower char-acteristic, for different inlet water temperatures. Two operating regimes were observed during the air water contact, a pellicular regime(PR) and a bubble and dispersion regime (BDR). These two regimes can determine the best way to promote the heat transfer. The BDRregime seems to be more efficient than the pellicular regime, as it enables to cool larger water flow rates. The comparison between theobtained results and those found in the literature for other types of packing indicates that this type possesses very interesting thermalperformances.   2006 Elsevier Ltd. All rights reserved. Keywords:  Wet; Cooling; Tower; Packing; Flow; Thermal; Characteristics 1. Introduction Usually industrial processes produce large quantities of heat which must be permanently removed in order to main-tain standard operating parameters. Cooling towers filledwith packing are widely used to dissipate large heat loadsfrom these processes, such as power generation units,chemical and petrochemical plants and refrigeration andair-conditioning systems, to the atmosphere. Their princi-ple is based on heat and mass transfer using direct contactbetween ambient air and hot water through some types of packing. Indeed, the type of packing used in cooling towerhas an important role in the tower as it controls the heatand mass transfer processes between water and air. Severalresearchers have investigated this subject through experi-mental analysis of the heat and mass transfer processes inthese equipments.Simpson and Sherwood [1] studied the performances of forced draft cooling towers with a 1.05 m packing heightconsisted of wood slats. Kelly and Swenson [2] studied theheat transfer and pressure drop characteristics of splashgrid type cooling tower packing. The authors correlatedthe tower characteristic with the water/air mass flow ratioand mentioned that the factors affecting the value of thetower characteristic were found to be the water-to-air ratio,the packed height, the deck geometry and, to a very smallextent, the hot water temperature. They also mentioned thatthe tower characteristic at a given water-to-air ratio wasfound to be independent of wet bulb temperature and airloading, within the limits of air loading used in commercialcooling towers. Barile et al. [3] studied the performances of a turbulent bed cooling tower. They [3] correlated the towercharacteristic with the water/air mass flow ratio.El-Dessouky [4] studied the thermal and hydraulic per-formances of a three-phase fluidized bed cooling tower. 1359-4311/$ - see front matter    2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.applthermaleng.2006.08.014 * Corresponding author. Tel.: +966 14679151; fax: +966 14678770. E-mail address:  boumaza_m@hotmail.com (M. Boumaza). www.elsevier.com/locate/apthermeng Applied Thermal Engineering 27 (2007) 902–909  He used spongy rubber balls 12.7 mm in diameter and witha density of 375 kg/m 3 as a packing, and developed a cor-relation between the tower characteristic, hot water inlettemperature, static bed height, and the water/air mass fluxratio. Bedekar et al. [5] studied experimentally the perfor-mance of a counter flow packed bed mechanical coolingtower, using a film type packing. Their results were pre-sented in terms of tower characteristics, water outlet tem-perature and efficiency as functions of the water to airflow rate ratio,  L / G  . They concluded that the tower perfor-mance decrease with an increase in the  L / G   ratio, howeverthey did not suggest any correlation in their work. Goshay-shi and Missenden [6] also studied experimentally the masstransfer and the pressure drop characteristics of manytypes of corrugated packing, including smooth and roughsurface corrugated packing in atmospheric cooling towers.Their experiments were conducted in a 0.15 m  ·  0.15 mcounter flow sectional test area with 1.60 m packing height.From their experimental data, a correlation between thepacking mass transfer coefficient and the pressure dropwas proposed. Milosavljevic and Heikkila [7] carried outexperimental measurements on two pilot-scale cooling tow-ers in order to analyze the performance of different coolingtower filling materials. They tested seven types of counterflow film type fills and correlated their pressure drop dataas well as the volumetric heat transfer coefficient with thewater and air flow rates. More recently, Kloppers and Kro¨-ger [8] studied the loss coefficient for wet cooling tower fills.They tested trickle, splash and film type fills in a counterflow wet cooling tower with a cross sectional test area of 1.5 m  ·  1.5 m. They [8] proposed a new form of empiricalequation that correlates fill loss coefficient as a functionof the air and water mass flow rates. There exist severalother mathematical models which can correlate heat andmass transfer processes occurring in wet cooling towers,such as the models proposed and discussed by Khanet al. [9] and Kloppers and Kro¨ger [10].The main objective of this study is to investigate thethermal performances of a forced draft counter flow wetcooling tower filled with an ‘‘V.G.A.’’ type packing. Thistype of packing which was first proposed for the masstransfer processes between gas and liquid [11] has not beenused in cooling water systems using direct contact betweenwater and air. Recently, Lemouari [12] and Lemouari andBoumaza [13,14] used this packing in an evaporative cool-ing system to study its thermal and hydraulic perfor-mances. Therefore, this study presents an experimentalinvestigation of the thermal performances of cooling tow-ers filled with the ‘‘V.G.A.’’ type packing. This packingconsists of vertical grids disposed between walls in the formof zigzag. The principle of its performance is as follows: thegas (air) enters by the bottom of the tower and arrives bythe top of that while crossing several times the verticalgrids, whereas the liquid (water) is introduced at the topof the tower and flows along the vertical grids.The obtained results which relate mainly the tower char-acteristic as well as the cooling water range temperature(35   C and 50   C) with the air and water flow rates seemto be in agreement with those shown in the literature. Thissuggests the validation of these results. 2. Experimental apparatus and procedure The experimental apparatus used in this study is illus-trated in Fig. 1. It consists mainly of a cooling tower (1)which represents the main device used in this test, a coldwater basin (2), a storage tank (3) which contains two elec-tric heaters (12), a water pump (4), a flow meter device (5),a by-pass pipe (6), a water distributor (7), a fan (8), an airdistribution chamber (9), a water-drops separator (10), athermostat (11). Auxiliaries items are also used such astemperatures and pressures measuring devices (13), (14)as well as system for the regulation of water levels (15) inthe feed basin. The cooling tower [12] has a parallel formof dimensions 206 mm  ·  148 mm  ·  550 mm, and is madeof Plexiglas. It is filled with the ‘‘VGA’’ type packing hav-ing a cross-sectional area of 150 mm  ·  148 mm, height of 420 mm and consists of four (04) galvanised zigzag formsheets, between which are disposed three (03) metallic ver-tical grids in parallel. The distance between each two gridsis 50 mm (width of the cell). The water distributor [12] ismade of copper tubes of 10/12 and 6/8 mm diameters,respectively. Fine droplets sweeping the width of the zigzagstarting from the top of the tower are introduced throughthis distributor. The considered measurements which weretaken consist of the temperatures increase (dry and wet) of the air at the entry and exit of the tower, as well as the inletand outlet water temperatures.The experimental procedure is as follows: – Initiating the circulation of a water flow, and lightingthe electrical heaters at the same time. Nomenclature a  contact area air–water, m 2 G   air mass flow rate, kg/h L  water mass flow rate, kg/h T  1  inlet water temperature (  C) t 1  inlet air temperature (  C) R  cooling water temperature range (  C) V   volume of the exchange core, m 3 G  0 air mass flux, kg/m 2 h L 0 water mass flux, kg/m 2 h T  2  outlet water temperature (  C) t 2  outlet air temperature (  C) KaV/L  tower characteristic (dimensionless) M. Lemouari et al. / Applied Thermal Engineering 27 (2007) 902–909  903   – As soon as the temperature of feed water exceeds fewdegrees the desired temperature, air is injected byswitching on the fan. – After a few moments, the temperature of waterdecreases and increases again by its initial value (setpoint) corresponding to the measurement values of thedry and wet temperatures of the air at the entry andthe exit of the tower and the inlet and outlet watertemperature. 3. Thermal performance of an experimental cooling tower In this study, two different parameters were used indetermining experimentally the thermal performances of the cooling tower, the cooling water range ( R ), which isdefined as the difference between the inlet water and theoutlet water temperature,  R  ¼  T  1    T  2  ð 1 Þ The second parameter is the tower characteristic,  KaV/L ,which is the most widely used in practice to define the ther-mal performances of wet cooling towers, and is defined asEq. (2)  KaV   L  ¼ Z   T  1 T  2 C   Pw d T  H  w    H   ð 2 Þ Eq. (2) is solved numerically to evaluate the tower charac-teristic for different experimental operating conditions. Thefollowing equations were used for the numericalintegration:  H  w  ¼  a e k T  ð 3 Þ H  w  is the Enthalpy of saturated air where  a  and  k  are givenby: a  ¼  18 : 573 ;  k  ¼  0 : 05610 for 14   C 6 T   6 36   C; a  ¼  20 : 231 ;  k  ¼  0 : 05314 for 17   C 6 T   6 44   C; a  ¼  20 : 900 ;  k  ¼  0 : 05200 for 17   C 6 T   6 51   C : 10717T 1 12T 2 31615141364111298t d1 t w1 III5t w2 t d2 Fig. 1. Schematic diagram of the experimental apparatus. (1) The cooling tower filled with the ‘‘V.G.A.’’ type packing, (2) load tank, (3) water basin, (4)water circulation pump, (5) flow meter, (6) by-pass pipe, (7) water distributor, (8) fan, (9) air distribution chamber, (10) drift eliminator, (11) thermostat,(12) heaters, (13) digital temperature indicator, (14) manometer, (15) float valve, (16) make-up tank, (17) connection for orifice differential pressure, (I)zigzag walls, (II) vertical grids.904  M. Lemouari et al. / Applied Thermal Engineering 27 (2007) 902–909   H   ¼  H  1  þ  C   pw ð  L = G  Þð T  1    T  Þ ð 4 Þ where  H  1  is the enthalpy of moist air at the entry of thetower, and is given by the following equation [17]:  H  1  ¼ ð 1 : 005  þ  1 : 884 w Þ t  1 d   þ  2502 : 3 w  ð 5 Þ w  = specific humidity of moist air (kg/kg).Eq. (3) was obtained by approximating the enthalpy of the air in saturation,  H  w  and using the values tabulatedin the literature [15–17]. 4. Results and discussion Two operating regimes were observed during the air andwater contact, through the ‘‘VGA’’ type packing in thecooling tower: – A first regime, called pellicular regime (PR), exists withlow water flow rates. – A second regime, called bubble and dispersion regime(BDR), appears with relatively larger water flow rates,as reported by Lemouari [12]. Therefore, these twohydrodynamic regimes, enable to identify two differentstates of heat transfer phenomena were distinguished,as shown in Figs. 4–7. 4.1. Tower characteristic Figs. 2–4 show the variation of the tower characteristic, KaV/L , with the water/air mass flow ratio,  L / G  , for an inletwater temperature of 35   C, 43   C and 50   C, respectively.The tower characteristic decreases with an increase of   L / G  .This decrease becomes less pronounced as  L / G   increases,for the case of the bubble and dispersion regime (BDR).The effect of   L / G   on the tower characteristic, as explainedby El-Dessouky [4], can be attributed to the decrease in thefraction of water that evaporates per unit of inlet water. Ithas also been observed during the experiments, that an 0.11.010.0L/G0.11.010.0100.0    T  o  w  e  r  c   h  a  r  a  c   t  e  r   i  s   t   i  c ,   K  a   V   /   L Inlet water temperature: 35 ˚CPRRBDReference [15]Reference [6] Fig. 2. Tower characteristic vs. water/air mass flow ratio: experimental results and correlations in [6,15]. 0.11.010.0 L/G 0.11.010.0    T  o  w  e  r  c   h  a  r  a  c   t  e  r   i  s   t   i  c ,   K  a   V   /   L Inlet water temperature: 43˚C PRRBDBarile et al. [3]: d = 19.05 mmBarile et al. [3]: d = 38.10 mm Fig. 3. Tower characteristic vs. water/air mass flow ratio: experimental results and Barile et al’s correlations [3]. M. Lemouari et al. / Applied Thermal Engineering 27 (2007) 902–909  905
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