Journal of Cemial Engineering and Materials Siene Vol. 2(2), pp. 21-27, February 2011 Available online at ttp://www.aademijournals.org/jems ISSN-2141-6605 2011 Aademi Journals Full Lengt Resear Paper Te estimation of te ooling tower eigt by modeling te water and air ontat situation in ooling tower falling film Mortaza Golizade* and Mosen Momayyeza Department of Cemial Engineering, Islami Azad University-Aar Bran, Aar, I.R Iran. Aepted 1 Deember 2010 Deterioration of te filling material in traditional ooling towers is of serious onern. In tis study, ooling towers of Arvand and Boualisia Petroemial plants in te Sout of Iran are used as te filling material. Te size of te ooling towers and outlet onditions are measured in a real situation and ompared wit eat and mass transfer orrelations by modeling te towers and ten alulating te tower eigts. An experimental study to model te eat and mass balane equations and teir relationsip by tower eigt is onduted. Te previous orrelations found in te literature did not predit te relationsip between tese equations and tower eigt for te tested towers. A new formula by supplying te numerial metods was developed, and new variables defined. Tis orrelation an predit te eigt of tower witin an error of ±10%. Te developed orrelation is used along wit teoretial modeling to predit te ooling tower outlet onditions witin an error of ±5%. Key words: Cooling tower, modeling, mass transfer oeffiient, tower eigt. INTRODUCTION Cooling towers are one of te most widely equient units used in ooling systems, wi also onsist of a network of eat exangers in losed iruit tat onsume water only to make up for te inerent losses in te proess. Te termal performane of ooling towers as vital importane in te operation of a proess. Beause of teir relevane in te proessing industry, tere are many works in te literature tat address ooling water systems, Moreover, speifi aspets are studied, namely design of ooling towers, ontrol and operation of towers; modeling and simulation of te termal performane and mass transfer on te eigt of te tower. Te igly integrated features of ooling water systems (a single tower usually supplies multiple users) produe strong interations among te ydrauli and termal and mass proess variables. For instane, te overall point of te pump, wi results from its arateristis and on te entire ooling system. In addition, te reyle water flow rate depends on te *Corresponding autor. E-mail: morteza_g2000@yaoo.om. Tel: +98 426 2225551. Fax: +98 426 2225551. operating distribution of rates in te parallel branes is also a funtion of teir resistane to flow tat is determined from pipe diameters, equivalent lengts and adjustment of te valves in ea of te pipeline segments (Soylemez, 2004; Kloppers and Kroger, 2005). Te operation of te system is even more omplex at te termal and mass levels. In ea eat exanger, a given eat load must be removed from proess requirements. Furtermore, te inlet and outlet temperatures of te ooling water in te proess eat exangers must be witin ranges tat are ompatible wit te apaity of te ooling tower, as te total termal load tat is removed from te proess units (troug te eat exangers) must be removed from te system in te ooling tower (Tomas and Houston, 1959a; Lowe and Cristie, 1962; Tomas and Houston, 1959b). On te oter and, te transferred mass of te water in te ooling tower to air anges te outlet temperatures and flow rates of air. Te outlet water temperature at te ooling tower is determined by its performane inluding air veloity, rate, and temperature and transferred water mass from ot water to te air affeting te eigt of te ooling tower (Lebrun and Silva, 2002; Pannkoke, 1996; Badran, 2003).
22 J. Cem. Eng. Mater. Si. Figure 1. Arvand petroemial plant ooling tower semati. Table 1. Inlet water ondition for te supplied ooling towers. Cooling water flow rate(m 3 /) 12,000 16,000 Density water@45 C(kg/m 3 ) 991 991 Hot water temperature (T1)( C) 45 45 Site atmosperi pressure (bar) 1 1 Site elevation(m) 3.2 3.2 Dry bulb temperature of inlet air( C) 50 50 Wet bulb temperature of inlet air ( C) 31 31 Relative umidity of inlet air(at 48 C) 30% 30% Evaporation losses 1.9% of irulation water 1.9% of irulation water Drift (wind age) losses 1.9% of irulation water 1.9% of irulation water Drift (Wind age) losses 0.005% of irulation water 0.005% of irulation water No. of ells 3 4 In a ooling tower, te water surfae is extended by filling, wi presents a film surfae or reates droplets. Te air flow may be ross flow or ounter flow and aused by meanial means, onvetion urrents or by natural wind. In meanial draft towers, air is moved by one or more meanially driven fans to provide a onstant air flow. Te funtion of te fill is to inrease te available surfae in te tower, eiter by spreading te liquid over a greater surfae or by retarding te rate of fall of te droplet surfae troug te apparatus. Te fill sould be strong, ligt and deterioration resistant. In tis study, real ooling towers from Arvand and Boualisina petroemial plants in te Sout of Iran were used as te filling material (Sirok et al., 2003; Kairouani et al., 2004; Kloppers and Kr oger, 2005). EXPERIMENTAL Te tested ooling tower is an indued draft ounter flow type from Arvand and Boualisina petroemial plants. Te semati diagram of te Arvandan tower is sown in Figure 1. Some of te applied ooling tower data are written as follows: Inlet water onditions All information is sown in Table 1. Main dimension of one ell of ooling tower All dimensions are given in Table 2. Condition of outlet air Wit assuming tat te air outlet temperature is te same as te temperature of ot water (113 ºF) and te ondition of te air outlet is saturated, all oter parameters were alulated and presented in Table 3. Furtermore, te alulation proedure for Arvand petroemial plant ooling tower is written as: Pv1 = Vapor pressure = 9.6 Kpa (at T1 = 45ºC) =1.4 psi (at T1 = 113ºF) W1 = Absolute umidity = 18/29 29 W1 = 0.622 1.4/(14.5-1.4) W1 = Absolute umidity=0.07 Pv¹ P-Pv1 lb H 2O g1 = Saturated vapor entalpy (at T1 = 45 C= 113 F) = 2582.43 kj/kg = 1110. 25 (BtU/lb of water) 1 = 0.24 (T1 - T0) + W1 1=0.24 (113-32) + 0.07 (1110.25) = 97.16 Btu Outlet water onditions For bot ooling towers, te outlet temperature was 35 C. Condition of inlet air Te data are written in Table 4 and also te alulation for tis part
Golizade and Momayyeza 23 Table 2. Main dimension of one ell of ooling tower. Wetted area per ell(m 2 ) 242 242 Inner ell size(1 w) [ m m] 14.4 16.8 14.4 16.8 Cooling tower size (1 w) [m m] 44.2 18 60 18 Top of basin urb 0.0 0.0 Dept of basin slab 3 m 3 m Heigt of air inlet 5 m 5 m Water distribution eigt 8.35 m 8.35 m Table 3. Condition of outlet air Pv 1 (at T 1 = 45 C)(kpa) 9.6 9.6 W 1( lb H 2O) 0.07 0.07 g 1(at T 1 = 45 C)( BTU for lbm of water) 1110.25 1110.25 1(BTU for lbm of dry air) 97.16 97.16 of Arvand tower is written in below: T2 = 31 C = 87.8 F Pv 2 = 4.5 Kpa(at T=31 C) = 0.652 psi (at Tw=87.8 F) W 2 = 0.622 0.652 = 0.03 14.5-0.652 g 2 = 2557.35 kj/kg =1099. 5 Btu lb of water 2 = 0.24 (T1 - T0) + Wg 2 = 0.24(87.8-32) + 0.03(1099.5) = 46.4 4 Btu Te equation for eat balane is given by: Q = G ( air outlet air inlet) G = Mass flow rate of dry air Q = 138.1 MW = 130,893.5 Btu/Se () H G = 130,893.5 = 2578.7 /se (97.16 46.4) Ea ell air flow rate = 2578.7 3 = 859.5(/s) = 389.86 kg/s Air speifi volume @ 31 C = 0.872 m 3 /kg 389.86 kg/s 0.872 m 3 /kg = 340 m 3 /s L = 11,892,000 kg/ = 26,217,340 G = 2578.7(/s) = 9,283,320 (/) L = 26,217,340 = 2.82 (It will be finalized by SPX ompany alulation) G 9,283,320 340=V*14.4*16.8 V=1.4m/s(air veloity) Te power of seleted fan is = 165 Kw Cooling tower design data All information is sown in Table 5 Termal design data of ooling tower Tis data is presented in Table 6. Meanial design data of ooling tower All relevant information is illustrated in Table 7. Cooling tower dimension Cooling tower dimentions are written in Table 8. RESULTS AND DISCUSSION Wen air flow passes a wetted surfae, tere is a transfer of sensible and latent eat. If tere is a differene in temperature between te air and te wetted surfae, eat will be transferred. If tere is a differene in te partial
24 J. Cem. Eng. Mater. Si. Table 4. Condition of inlet air. T ( C ) 2 31 31 Pv 2(kpa) 4.5 4.5 W 2 (lb H 2O) 0.03 0.03 g 2(at T 1 = 45 C)( BTU for lbm of water) 1099.5 1099.5 2(BTU for lbm of dry air) 46.4 46.4 Q(BTU/S) 130,893.5 174,493.1 G( / s) 2578.7 3,437.61 Ea ell air flow rate(() S 859.5 859.5 Air speifi volume (at 31 C)( m 3 /kg) 0.872 0.872 L() 26,217,340 34,956,450 G () 9,283,320 12,375,396 Te power of seleted fan(kw) 165 165 Table 5. Cooling tower design data. Number of ooling towers 1 1 Number of ells per ooling tower 3 4 Type of ooling tower WIC240 WIC240 Working area per ell(m 2 ) 242 242 Arrangement of ooling tower In line In line Number of air inlet sides/ell 2 2 pressure of water vapor in te air and tat of te water, tere will be a mass transfer. Tis transfer of mass auses a termal energy transfer beause if some water evaporates from te water layer, te latent eat of tis vaporized water will be supplied to te air. Te onept of entalpy potential is a very useful one in quantifying te transfer of eat (sensible and latent) in tose proesses and omponents were tere is a diret ontat between te air and water (Kloppers and Kro ger, 2005; Smrekar et al., 2006; Zai and Fu, 2006; Lemouari, 2007). Te expression for transfer of te total eat, dqt troug a differential area, da is expressed by Stoeker and Jones (1985): dq t = da C ( i a ) Te name of entalpy potential originates from te above equation beause te potential for te transfer of te sum (1) of te sensible and latent eats is te differene between te entalpy of te saturated air at te wetted surfae temperature i and te entalpy of te air stream a (Fredman and Saxén, 1995; Al-Nimr, 1998; Tan and Deng, 2003). Te rate of eat removed from te water is equal to te rate gained by te air, so te following expression an be written: dq t = m a.d a = 4.19m w.dt (2) Te eat transfer oeffiient an be alulated by Equating Equations (1) and (2) and rearranging: A C m w out dt = 4.19 in i a However, A = av and V = SZ, so Equation (3) an be written as: (3)
Golizade and Momayyeza 25 Table 6. Termal design data of ooling tower. Duty(MW) 138.1 184.1 Water flow rate of ooling tower(m 3 /) 12,000 16,000 Water flow rate per ell(m 3 /) 4,000 4,000 Hot water temperature( C) 45 45 Cold water temperature( C) 35 35 Cooling range(k) 10 10 Inlet wet bulb temperature( C) 31 31 Dry bulb temperature( C) 50 50 Average ambient pressure(mbar) 1000 1000 Type of fill Film fill, type FB20 or equal Film fill, type FB20 or equal Evaporation rate in % of water flow (%) 1.9 1.9 Evaporation rate(m 3 /) 228 304 Drift rate in % of water flow(%) >0.005 >0.005 Table 7. Meanial design data of ooling tower. Fan type Axial Axial No. of fans per ell 1 1 Fan diameter/no. of blades(m/ps) 9.9/7 9.9/7 Output power at motor saft(kw) 133 133 Motor size (750/1500 r) (kw) 38.165 38.165 Pumping ead wet setion(mwg) 9.85 9.85 Table 8. Cooling tower dimension. Inner ell size (I W)( m m) 14.4 16.8 14.4 16.8 Cooling tower size (I W)( m m) 44.2 18 60 18 Top of basin urb(m) 0.0 0.0 Dept of basin slab(m) 3.0 3.0 Heigt of air inlet(m) 5.0 5.0 Water distribution eigt(m) 8.35 8.5 Fan dek eigt(m) 11.47 11.47 Heigt of fan stak(m) 2.67 2.67 Total iger of ooling tower(m) 14.34 14.34 Table 9. Te alulated mass oeffiient of te ooling tower and its eigt. a(m 2 ) m w(kg/s) m a(kg air/s) K x V(m/s) Z(m) Data for Arvand tower 242 1111.11 389.86 1200.7 1.4 7.6 Data for Boualisina tower 242 1101 389.86 1197.8 1.5 8.25 az C m w out dt = 4.19 in i a (4) Te relation between te eat and mass transfer oeffiients is expressed by Reynold s analogy (Treybal, 1981):
26 J. Cem. Eng. Mater. Si. K x C = Le 2 3 It is found tat in most ases of air water ontat, te Lewis number Le an be onsidered to be unity as a good approximation (Kern, 1997): C = K x By substituting Equation (6) in Equation (4), te mass transfer oeffiient an be expressed as: K xaz m w out dt = 4.19 in i a Te integration of Equation (7) is solved numerially by dividing te paked eigt into small segments starting from te bottom to te top of te tower wit onsidering te relationsip between air and water flow rates written in Equation 10 (Tan and Deng, 2003; Jin et al., 2007; Giorgia et al., 2009): 0.6781 V = 0.3523. Z (8) In addition, te design of a erami tile ooling tower requires eat and mass transfer orrelations to estimate te tower eigt and to predit te water and air outlet onditions. Several workers ave measured te eat and mass transfer oeffiient in ooling towers. Tomas and Houston developed eat and mass transfer orrelations using a tower of 2 m eigt and 0.3 m 2 ross setion. Tey gave te following relations for te eat and mass transfer oeffiients (Tomas and Houston, 1959): a = x 0.26 0.72 3.0mw ma K a = 0.26 0.72 2.95mw ma (5) (6) (7) (9) (10) In our alulations, firstly we alulated K X from Equation 10 and ten troug Equation 7 and 8 we alulated Z and ompare it wit te real eigt. Te result is written in Table 9. Tis orrelation an predit te eigt of tower witin an error of ±10%. As we an see from te real data, te eigt of waterair ontat for Arvand petroemial plant ooling tower is 8.35 m tat is omparable wit our alulation result (7.6 m) by 10% error. Conlusion A matematial model is used to simulate te effet of any ange in operating onditions of ooling tower, espeially te mass transferring oeffiient, on te termal performane of a ross flow tower. Available data was obtained using a numerial experimental metod wi provides an insigt on te urrent performane of te Arvand and Boualisina petroemial plant towers. It is found tat inreasing te mass transferring oeffiient between water and air, at onstant dry bulb, te eigt of te tower will derease onsiderably. Te evaporation rate is inreased as te dry bulb temperature inreases and te rate of inrease is almost onstant at different wet bulb temperatures. Te suggested model an predit te tower eigt wit relating te ontat situation between water and air. Nomenlature: a, area of eat and mass transfer (m 2 /m 3 ); Cp, speifi eat (kj/kg K);, entalpy (kj/kg);, eat transfer oeffiient (kw/m 2 K); Kx, mass transfer oeffiient (kw/m 2 K); Le, Lewis number; M, flow rate (kg/s); Q, eat flux (kj/kg); Z, tower eigt (m)x umidity ratio (kg water/kg dry air); Pv, vapor pressure; W, absolute umidity; g, saturated vapor entalpy; G, mass flow rate of dry air; m, superfiial flow rate (mass veloity) (kg/m 2 s); m, flow rate (kg/s); t, ontat time(s); V, air veloity. Subsripts: a, air; I, inlet or interfae; o, outlet; v, vapor; w, water. REFERENCES Al-Nimr MA (1998). Dynami termal beaviour of ooling towers. Energy Convers. Manage., 39: 631 6. Badran AA (2003). Performane of ool towers under various limates in Jordan. Energy Build., 35: 1031 5. Fredman T, Saxén H (1995). Modeling and simulation of a ooling tower. In European simulation multionferene, Prague, p. 66 70. Giorgia F, Cortinovis, José L, Paiva, Ta W, Song, José MP (2009). A systemi approa for optimal ooling tower operation. Energy Conv. Manage., 29(14-15): 3124-3131. Jin GY, Cai WJ, Lu L, Lee EL, Ciang AA. (2007). Simplified modeling of meanial ooling tower for ontrol and optimization of HVAC systems. Energy Convers. Manage., 48: 355 65. Kairouani L, Hassairi M, Tarek Z (2004). Performane of ooling tower in sout of Tunisia. Build. Environ., 39: 351 5. Kern DQ (1997). Proess eat transfer. (New York, MGraw-Hill), (36):2: 149-159. Kloppers JC, Kroger DG (2005). Te Lewis fator and its influene on te performane predition of wet-ooling towers. Int. J. Term. Si., 44: 879 84. Lebrun J, Silva CA (2002). Cooling tower model and experimental validation. ASHRAE Trans., pp. 751 9. Lemouari M, Boumaza M, Mujtaba IM (2007). Termal performanes investigation of a wet ooling tower. Appl. Term. Eng., 27: 902 9. Lowe HJ, Cristie DG, (1962). Heat transfer and pressure drop data on ooling tower pakings, and model studies of te resistane of natural draft towers to air flow. Inst. Me. Eng. (Steam Group) Symposium Heat Trans., 113: 933. Pannkoke T (1996). Cooling tower basis. Heat, Piping, Air Cond., 68: 137 55. Sirok B, Blagojevi B, Novak M, Hoevar M, Jere F (2003). Energy and mass transfer penomena in natural draft ooling towers. Heat. Transf. Eng., 24: 66 75.
Golizade and Momayyeza 27 Smrekar J, Oman J, Sirok B (2006). Improving te effiieny of natural draft ooling towers. Energy Convers. Manage., 47: 1086 100. Soylemez MS (2004). On te optimum performane of fored draft ounter flow ooling towers. Energ. Convers. Manage., 45: 2335 41. Stoeker WF, Jones JW (1985). Refrigeration and air onditioning. (Singapore).1: 74-81. Tan K, Deng SA (2003). Numerial analysis of eat and mass transfer inside a reversibly used water ooling tower. Build Environ., 38: 91 7. Tomas WJ, Houston P (1959a). Simultaneous eat and mass transfer in ooling towers. Brit. Cem. Eng., 160: 217. Tomas WJ, Houston P (1959b). Simultaneous eat and mass transfer in ooling towers. Brit. Cem. Eng., 160: 217. Treybal RE (1981). Mass transfer operations. (New York, MGraw-Hill). Zai Z, Fu S (2006). Improving ooling effiieny of dry-ooling towers under ross-wind onditions by using wind-break metods. Appl. Term. Eng., 26: 1008 17.