New Method for Designing an Optimum Distributed Cooling System for Effluent Thermal Treatment

Transcription

1 Int. J. Environ. Res., 3(): , Spring 009 ISSN: Ne Metod for Designing n Optimum Distributed ooling System for Effluent erml retment Atei, A. 1*, Pnjesi, M. H., Grie, M. 3, nd ouni, N. 1 Deprtment of Energy Engineering, Grdute Scool of te Environment nd Energy, Science nd Reserc Brnc of Islmic Azd University, ern, Irn Deprtment of emicl Engineering, University of ern, ern, Irn 3 Deprtment of Mecnicl Engineering, K. N. oosi University of ecnology, ern, Irn Received 17 My 008; Revised 15 Dec 008; Accepted 5 Dec 008 ABSRA: emperture restrictions on queous effluents dictte tt strems it temperture iger tn te permitted level needed to pss troug cooling systems to reduce te effluent temperture before discrge. In tis study, by considering te grouping design rules bsed on pinc tecnology, n optimum design for distributed effluent cooling system, s been developed. A counter-flo et cooling toer, it mecnicl ir drft, is lso ssumed s n effluent terml tretment fcility in predicting te exit ter nd ir conditions of te toer in te system. In tis ne design metod, n optimum inlet flo rte to cooling toer s been cieved by exploring te fesible region. Also, te evportion loss effect, flexible design vribles, nd pysicl properties ve been incorported in trgeting te optiml conditions for te cooling toer. A cse study is presented to illustrte te design metodology nd te optimiztion model of cooling systems. Key ords: Grouping Design Rules, Pinc ecnology, rgeting, Wet ooling oer INRODUION Wile cemicl, pysicl or biologicl tretment processes cn be used for controlling te cemicl pollution problems of effluents (Srprstzde et l., 007), te terml tretment system is required for effluent temperture reduction problems en te temperture of te effluent strems is too ig to be discrged directly to te receiving ter. e terml tretment of effluents in processing industries is most often crried in centrl cooling fcility (Kim et l., 001). Hoever, becuse of te inefficient performnce of suc cooling systems, n lterntive policy, suc s distributed cooling system, sould be considered. e design of n effluent terml tretment sould be bsed on sustinble development to improve te qulity of umn life nd keep te nturl environment clen. Sustinble development provides meork for te integrtion of *orresponding utor E-mil: -tei@otmil.com ecosystems it industril ctivities nd systemtic ys to ennce process efficiency (Atei, 008). Becuse discrge regultions ve driven up effluent terml tretment costs (Mteti et l., 008), process integrtion tecniques ve empsized te reduction of te temperture of te effluent nd te design of cost-efficient temperture reduction systems s mens of pollution prevention. Pinc nlysis is te tool most commonly used for integrtion purposes. is tecnology is bsed on defining te trgeting before generting te design, nd exploits conceptul understnding. Vrious systemtic metods bsed on pinc nlysis ve plyed key roles in sving energy nd ter in process design (Linnoff nd Smit, 1994; Smit, 005). is tecnology s been used to design distributed effluent temperture reduction systems. 155

2 Atei, A. et l. Kim, Svulescu nd Smit (001) estblised design metodology for distributed systems for effluent cooling. In te Kim, Svulescu nd Smit Design Metod (KSSDM), ic is bsed on te grouping design rules, minimum inlet effluent flo rte to te cooling toer is cieved it mtemticl progrmming. Severl ssumptions re mde in te KSSDM, but tese ssumptions mke te design inccurte. Some of tese ssumptions re (Atei, 008): 1.Fixed evportive loss t constnt et rejection;.fixed cooling toer exit temperture ile incresing in effluent flo rte to ccount effect of evportion loss; 3.Fixed pysicl properties; It s been noted tt te mount of evportion loss is dependent on te cnges of te temperture nd flo rte of inlet conditions nd constnt et rejection vlue does not necessrily ensure fixed evportion rte (Pnjesi nd Atei, 008). In te KSSDM, becuse of te evportive loss effect, te inlet effluent flo rte is incresed. Hoever, tis ill ffect te cooling toer size nd performnce, ccordingly, te trgeted vlues suc s cost nd opertionl prmeters cnnot be cieved t te syntesis stge. e temperture of te toer exit ter cnges it vritions in te inlet ter conditions (flo rte nd temperture). erefore, te fixed temperture of te exit ter (fixed pproc vlue) cnnot gurntee minimum cost of cooling or its optimlity (Pnjesi nd Atei, 008). e pysicl properties of te system lso cnge under different design supply conditions. o cieve ccurte results in ter ir systems, it is necessry to consider te governing conditions, en clculting te system properties. Hence, te trgets in KSSDM could not be met in prctice. Moreover, it te KSSDM, te trgets re set regrdless of te model. Hoever, detiled model llos us to simultneously consider te effects of te relted spects of te ole system. Furtermore, metod for designing te et cooling toer to cieve te trgeted totl cost of cooling toer is not ddressed in te KSSDM.In tis study, ne design metodology ic is clled Optimum Design metod of distributed Effluent ooling system (ODE) for costeffective effluent cooling is introduced to overcome te forementioned problems nd te limittions of te KSSDM. In te ODE, by considering te evportive loss effect on te cooling toer supply flo rte, te pinc tecnology s been improved. e presented design metod, ODE, considers te energy implictions of effluent systems. In tis metodology, te trgets re set first nd ten te terml tretment netorks re designed to cieve te trgeted vlues on te bsis of te grouping design rules ic proposed by Kou nd Smit (1997). o cieve tese objectives, ne lgoritm s been developed bsed on compreensive model of distributed cooling system. In tis model, mecnicl drft counterflo et-cooling toer s been ssumed. o cieve optimiztion, computtions nd mtemticl clcultions ere performed it coding in Visul Studio 003, ++. Finlly, tis design metodology s been used on n illustrtive exmple nd te results ve been compred it te conventionl design of te effluent cooling system. MAERIALS & MEHODS Industril processes usully produce lrge quntities of queous ste, ic must be permnently removed to mintin stndrd operting prmeters. e terml tretment of steter is required to solve effluent temperture problems, to meet environmentl criteri. e introduction of cooling system is terefore necessry to reduce effluent tempertures. ere re mny options for cooling to stisfy environmentl regultions. In tis pper, et-cooling toers ill be studied mong te cooling options, becuse tis metod is idely used in te process industries.erml tretment of effluents in te process industries is most often crried in centrl cooling fcility (Fig. 1). In conventionl terml tretment systems (centrlized cooling systems), effluent strems generted om vrious processes nd plnts re collected in common seer before terml tretment. After collecting ll effluent strems nd combining into single effluent, te effluent strem generted is likely to ve loer energy level, becuse te energy level of effluent it iger temperture becomes degrded due to mixing of te effluent strems. erefore, centrl cooling systems need to remove te et om effluent strems it loer temperture nd iger 156

3 Int. J. Environ. Res., 3(): , Spring 009 flo rte reltive to cooling systems plced on te iger temperture effluents before mixing (Kim et l., 001). If cooling systems use cooling toer s cooling fcility, centrl cooling systems lso result in inefficient nd expensive cooling. t is becuse cooling conditions it ig rnge (te temperture difference beteen inlet nd let cooling ter) nd lo flo rte re more efficient tn conditions it lo rnge nd ig flo rte. is results om te cooling mecnisms of cooling toers (Bedekr et l., 1998). As centrlized cooling policy cnnot void te degrdtion cused by mixing effluents it lo tempertures, distributed cooling policy (Fig. 1b). for cooling systems sould be considered. considertion te system limittions. e tird stge trgets te optimum cooling toer supply line by exploring te fesible re. e fourt stge is te design of te cooling netork to cieve te trget, bsed on te modified grouping rules nd te finl stge is te design of te cooling toer to cieve te trgeted totl cost of te cooling toer, considering trgeted tempertures nd flo rtes of inlet nd let conditions. Accordingly, te trgeting procedure of te ODE contins te first to tird forementioned stges nd te fourt nd fift stges mke te design procedure of te ODE, see (Fig. ). rgeting Procedure Design Procedure Stge 1 Stge Stge 3 Stge 4 Stge 5 )entrlized cooling system b) Distributed cooling system Fig. 1. entrlized nd distributed effluent cooling systems e ne Optimum Design metod of distributed Effluent ooling system, ic is clled ODE, for cost-efficient effluent temperture reduction s cieved using systemtic pproc. Wit te ODE, te optimum distributed effluent system is designed in five stges. e first stge is te construction of te effluent composite curve. e second stge is te genertion of te fesible region, tking into Fig.. rgeting nd design procedures in te ODE In first stge of te ODE, to construct te effluent composite curve, te environmentl discrge limit for te effluent temperture is specified. e disposble et to be removed by cooling ec effluent strem is ten clculted. e individul profiles re plotted on grp of temperture versus disposble et, s son in (Fig. 3). (Smit, 005). e composite curve is constructed by combining ll te individul profiles into single curve itin te temperture intervls (Fig. 3b) In second stge of te ODE, to generte te fesible region, te upper nd loer limits of te inlet effluent flo rte to cooling toer re specified.fesibility constrints on te inlet mss flo rte to cooling toer is: l u m, in m in, m, in (1) l m, in nd m u, in re te loer nd upper limits of te inlet strem flo rte ic re expressed t exit strem temperture of te cooling toer.ooling toer ter let temperture vries beteen te minimum vlue, considering et bulb nd minimum pproc temperture, nd te mximum vlue, considering environmentl temperture discrge limit: 157

4 Optimum Distributed ooling System ( ) ( ) 1 1 Effluent omposite urve 3 env 3 env,min env Disposble Het () () (3),min = WB + MA Accordingly, upper limit of te inlet strem flo rte expresses te full centrlized conditions (Atei, 008): n u m, in = mi (4) i= 1 Fig. 4. sos fesible boundries for te distributed cooling system. As son in (Fig. 4). te fesible re is te region limited by te full centrlized conditions (mximum ter flo rte), considering te environmentl temperture discrge limit, nd te minimum cooling flo rte, considering te minimum exit strem temperture of te cooling toer. 1 Pinc ( o ) ( o ) env ( o ) Effluent omposite urve Disposble Het (b) Fig. 3. onstruction of te effluent profile composite curve In te tird stge of te ODE, te optimum inlet effluent flo rte to cooling toer sould be trgeted by exploring te fesible re.in distributed cooling systems, te segregtion of te effluents mintins ig driving force for cooling nd lso mximizes te potentil of recovering s muc et s possible om te effluent strems. o investigte te interctions itin te system, distributed terml tretment model, including cooling toer, s been introduced. o cieve tis im, oter system components ve been dded to te cooling toer model. e model s been developed to illustrte te conditions of te exit ter nd te ir om te system for te given design conditions.(fig. 5). sos te sceme for distributed system. o determine te properties of te ir nd ter in te system, relted blnces re set up for te overll control volume, te cooling toer, nd te pcking re. e mss nd energy blnces for te overll system (Fig. 5). re expressed by Equtions 5 nd 6, respectively. m m 1, + m + m + m 3, by pss = + m evp (5) Outmin WB Disposble Het m 1, + m p1 1, by pss + m p, env 3 ( m + m ) + m p env p3 3 evp =, evp (6) Min Flo rte it Evp. Loss Effect Min Flo rte it Evp. Loss Effect Mx.Flo rte Fig. 4. Fesible boundries for te distributed cooling system e mecnism of et nd mss trnsfer beteen te mbient ir nd te ter inside te cooling toer pcking is illustrted in (Fig. 6). 158

5 Int. J. Environ. Res., 3(): , Spring 009 Effluent1 1 m 1 m evp Effluent m in m,in Effluent3 3 m 3 ir m 4 m,by pss Pcking m, env m, +m,by pss Fig. 5. Sceme of te distributed cooling system f m Evportive Wter +d ώ?+d? ώ m +dm Le = f = α (10) D pd dz=dv/a Wter f -d f -d m -dm. Q rej Air ώ? m Fig. 6. ontrol volume of te toer pcking e energy blnce for te cooling toer is expressed in Eqution 7. m, in p, inin Qevp = m, p, (7) e rejection et troug te ter is given in Eqution 8 (Hollnds, 003). dq rej d dm = m ( z) p + dz (8) dz dz e ter energy blnce in terms of te et nd mss trnsfer coefficients is given by Eqution 9. md f = [ d A ( ) + (9) d A ( ωs ω) fg, ] dz By substituting te Leis fctor, expressed by Eqution 10 (Kloppers nd Kröger, 005), in Eqution 9, te ter energy blnce yields Eqution 11. m d + ( ω ω) s f = A [ Le d fg, ] dz f p ( ) (11) Were is te et trnsfer coefficient of ter (Deng nd n, 003). An mount of ter is evported in te control volume. e ter flo is cooled by temperture decrement becuse of te ltent et of evportion nd becuse of convective et trnsfer. Evportion t te ter surfce cn be ritten s Eqution 1 (Kröger, 004). dq dmevp = mdω (1) = md f fmdω (13) + e trgeting metod of ODE incorportes te evportion effect. e let ter conditions of flo rte nd temperture re ffected by evportion. In oter ords, te exit ter flo rte of te cooling toer is reduced by evportive loss. is ffects te cooling performnce. us, evportive loss forces n increse in te cooling toer supply flo rte, but te environmentl discrge limit is stisfied. erefore, ne boundry for te minimum ter flo rte is set, tking into considertion te evportion loss 159

6 Atei, A. et l. effect. e ne minimum ter flo rte of ODE trgeting s lso son in Fig. 4.e setting ter supply is beteen te ne minimum ter flo rte nd te mximum flo rte. e optimum mount of ter is determined by exploring te fesible region, tking into considertion te minimum totl cost. e constnt vlue of te pproc does not gurntee te minimum cooling cost. onversely, te performnces of te cooling toer nd te distributed system is governed by te cooling toer design vribles: rnge, pproc, ir flo rte, nd ter flo rte (Kn nd Zubir, 001). erefore, to cieve te optimum trget supply, te cooling toer design vribles re considered s rigid vlues. e rnge nd pproc definitions re expressed by Eqution 14 nd Eqution 15, respectively (Kn et l., 004). R = in (14) A = WB (15) e ter entering te cooling toer is given s Eqution 16 (Atei et l., 009). Disposed Het m in = (16), p e mount of evportion depends on te ir flo rte, te umidity of te inlet ir nd te umidity of te cooling toer let ir. e exit ir umidity is relted to te ter temperture nd te trnsfer re of te pcking. erefore, te evported ter loss is not constnt en flexible design vribles re considered.e cnge in te ir umidity rtio long te cooling toer is expressed by Eqution 17 (Quresi nd Zubir, 006). d ω KA = ( ωs ω) (17) dz m e sturted umidity rtio t ter temperture is given by Eqution 18 (Atei et l., 009). s P ω s = 0.6 s (18) ( P P ) e cooling toer crcteristic is given by Eqution 19 (Kröger nd Kloppers, 005). KV = in pd (19) L ( ) s v e operting cost nd te cpitl cost of te cooling toer ve different effects on te overll cost of te distributed cooling system (Prsd, 004). erefore, te problem of trgeting te distributed cooling system becomes n optimiztion problem, to find te optiml cooling line. e totl cost of te cooling toer, s te objective function, is expressed in Eqution 0 (Söylemez, 001). Ai Min = i + (0) Ry A i elece f m Ry Z S 3 K el RyZA fn ρ Ai η fnη motor + A ic At end of tird stge of ODE, te trgeting procedure of te ODE ill be completed. In oter ords, te optimum effluent tempertures nd flo rtes of inlet nd let conditions cn be trgeted to cieve minimum totl cost of cooling system.after compliction of te trgeting procedure, te design procedure of te ODE ic contins te fourt nd fift stges sould be considered. In te fourt stge of te ODE, te cooling netork to cieve te trget sould be designed. o cieve tis im, te modified grouping rules ic proposed by Kou nd Smit (1997) cn be pplied. For pplying te grouping rules in te fourt stge of ODE, some modifictions re needed to ccount te effects of evportion loss.becuse te optimum cooling toer supply line in ODE, ic s cieved in te tird stge, does not correspond to te minimum flo rte (becuse of te evportion loss effect), no pinc point is creted it te limiting cooling ter composite curve (Fig. 4). e grouping rules re bsed on te concept of pinc tecnology cnnot be pplied to problems it pinc. e ne minimum cooling toer supply line represents boundry beteen te fesible nd non-fesible opertions. In oter ords, ny composite curve belo te originl one is fesible. erefore, te effluent composite curve must be modified to crete ne pinc point it te desired cooling toer supply line in te fesible region. o cieve tis im, pinc migrtion is pplied on te bsis of temperture 160

7 Int. J. Environ. Res., 3(): , Spring 009 sifting, in ic te effluent composite curve moves long te temperture xis (Kim nd Smit, 001). e migrted pinc temperture cn be clculted it Eqution 1. (Fig. 7). sos te creting of migrted pinc point. * ( ) (1) in Pinc = QPinc + Qotl e effluent strems it strting temperture locted bove te migrted pinc (Group I) pss troug te terml tretment process totlly. e effluent strems locted t te migrted pinc (Group II) re prtilly treted nd prtilly bypssed. e effluent strems locted belo te migrted pinc (Group III) totlly bypss te terml tretment process. in * Pinc env,min WB ( ) Effluent omposite urve Ne Pinc Q Pinc Q otl Fig. 7. reting migrted pinc point Q (kw) After te trget flo rtes for te terml tretment processes for given effluent strems re set, terml tretment netorks re designed to cieve tose trgets it te modified grouping strtegy considering te migrted pinc point. Moreover, te operting crcteristics of te cooling toer re incorported into te ODE nd re combined it te distributed cooling system to identify te optimum design for effluent temperture reduction.in finl stge of ODE, design of te cooling toer is crried troug mtemticl itertionl procedure, using knon design vribles, to cieve te totl cost of cooling toer ic s trgeted in te tird stge of ODE.In counter-flo et cooling toer, te process consists of gs pse (ir) floing uprd nd liquid pse (ter) floing donrds, nd lrge interfce beteen tese to pses. It s noted tt te rte of energy trnsferred om te ter is equl to te rte of energy gined by ir (Eqution ). Q = m ) () (,, in e ir flo rte of te toer cn be cieved troug Eqution 3, it knon ter flo rte ic s clculted by Eqution 16 (Atei et l., 009). m p m = (3) p e cnge in te ir umidity rtio long te cooling toer nd te sturted umidity rtio t ter temperture ere given in Equtions Also te ir nd ter tempertures re given in Equtions 4 nd 5 (Kim nd Smit, 001). d dz d dz d A = ( i ) (4) m p A = ( i ) (5) m p For te ir-ter system, et nd mss trnsfer coefficients re represented s function of ir nd ter flo rtes. e relted coefficients re given in Equtions 6 to 8 (oulson nd Ricrdson, 1996). K A = m m (6) d 1 b1 b c1 A = m m (7) d b c A = m m (8) c e optimum et nd mss trnsfer re cn be clculted by Eqution 9 (Söylemez, 001). 3 3 elece f m Ry Z S[6.5 + Kel + ( A / Afn ) ] Ai, opt = 3 ρ η fnη motor i (9) e optimum cross sectionl re is given by Eqution 30 (Atei et l., 009). Ai, opt Ar, opt = (30) RyZ 161

8 Optimum Distributed ooling System It is ssumed tt te cooling toer ontl re nd cross-sectionl re ill be pproximtely equl. If te design is for rectngulr cooling toer, te ontl re is given by (Kröger, 004): A A = Z W (31) r o cieve te optimum cooling toer design, n itertive clcultion is required. e computtion procedure is presented in (Fig. 8).e ODE provides y of trgeting te optimum flo rte for terml tretment. It lso provides design guidelines to cieve te trgets in prctice. e ODE is pplied to n illustrtive exmple nd te results re compred it te conventionl design (centrlized design) of te effluent cooling system. RESULS AND DISUSSION e effluent strems dt in (ble 1) s exmined s n illustrtive exmple for optimum design of effluent cooling system, using te proposed design metod (ODE). ble 1. Effluent strem dt Effluent Flo rte (kg/s) emperture ( ) e folloing prmeters ere used for te illustrtive exmple: e electricity cost is 0.1 $/kw. e elimintor crcteristic is m 1. e elimintor iction coefficient is 4.6. e operting period is 8600/yr. e environmentl temperture discrge limit is 30. e et bulb temperture is 0 nd minimum pproc temperture is 5 º. Accordingly, te cooling toer exit temperture vries beteen 5 to 30 for optimiztion purposes. e ODE segregtes strems for terml tretment nd ten combines or bypsses tem if pproprite.in te ODE trgeting procedure, te effluent strem dt nd te environmentl conditions re used to construct te Strt Wter nd Air onditions Assume Z lc. m lc. 1, Z = Z + dz Assume, No Adjust, lc.,,,, lc ε Yes lc. Acr No 1,, ε Yes d lc. i,, dz, Stop Fig. 8. Flocrt of optimum cooling toer design 16

9 Int. J. Environ. Res., 3(): , Spring 009 effluent composite curve nd to define te fesible region. (Fig. 9). sos te effluent composite curve nd te fesible region. We note tt s te cooling flo rte increses, te pproc becomes lrge nd te rnge becomes smll. e optimum cooling toer supply line cn be cieved by exploring te fesible region. e fesible boundry tempertures nd flo rtes re presented in ble. emperture ( ) Effluent omposite urve Mx. Flo rte Min. Flo rte Opt. Supply Line Disposble Het (kw) Fig. 9. e fesible region of te distributed cooling system ble. e fesible boundry ter supply conditions Effluent condition Min. flo rte Mx. flo rte Flo rte (kg/s) Inlet temperture ( ) Outlet temperture ( ) Fig. 10. illustrtes te effect of te ter flo rte on te cpitl nd opertion costs of te distributed cooling system. e results revel tt n increse in te ter flo rte reduces te cooling toer cpitl cost, eres te operting cost increses. erefore, trde-off beteen te cpitl cost nd te energy cost s been introduced. e optimum ter flo rte is cieved by considering tis trde-off. e optimum cooling line, ic is locted beteen te mximum nd minimum flo rtes, cieves te minimum totl cost.(ble 3) sos comprison of te ter supply conditions of ODE nd tose of te conventionl design (centrlized cooling system). ost (k$/yr) O Florte (kg/s) Fig. 10. Overll cost of te distributed cooling system ble 3. Wter supply conditions it conventionl nd ODE design metods Design metod Flo rte (kg/s) Inlet temperture ( ) onventionl Outlet temperture ( ) ODE In te design procedure, te optimum ter supply s been pplied to te effluent netork, tking into considertion te modified grouping design rules. e results indicte tt te migrted pinc point, creted it te ODE metod, is t 3. Accordingly, to cieve te trgeted cooling toer supply flo rte, te effluent 1 nd effluent sould be pssed troug te cooling toer totlly nd te effluent 3 locted t te migrted pinc (Group II) sould be prtilly cooled nd prtilly bypssed. (Fig. 11). sos te optimum effluent netork produced it te ODE on te bsis of te modified grouping design rules.ese results indicte tt, by distributing te cooling system, te effluent strems re prtilly treted termlly by te cooling toer. erefore, te required cooling toer in te distributed system is smller tn tt in te centrlized system. erefore, pplying te ODE s resulted in cost minimiztion reltive to tt of te conventionl (centrlized cooling system) design metod.(ble 4). sos te cost comprison for te cooling toers designed it te conventionl design metod nd ODE. 163

10 Atei, A. et l. 60 º kg/s 5.89 kg/s 45 º kg/s 169 kg/s Pcking 3 º kg/s.34 kg/s kg/s Fig. 11. Optimum effluent netork of te ODE kg/s 30 º ble 4. ost comprison of conventionl design metod nd ODE Design metod Operting cost (k$/yr) pitl cost (k$/yr) otl cost (k$/yr) onventionl ODE e ODE llos te optimum cooling toer design to be cieved. e cooling toer design prmeters for te proposed design metod (ODE) nd te conventionl design (centrlized cooling system) metod re given in (ble 5).It is usully possible to reuse effluent for some usges suc s sing or grdening. Wit te ODE, te evportive loss is reduced by distributing te effluent system. erefore, ODE cieves greter ter conservtion opportunity compred it tt of te conventionl design. In oter ords, ODE provides greter ter reusing opportunities reltive to te conventionl design metod.ble 6 sos te ter conservtion opportunity cieved it te ODE. ble 5. e cooling toer design prmeters for te ODE nd te conventionl design metod Design Q rej W (m) Z (m) A metod (KW) cr (m ) onventionl ODE ble 6. Wter conservtion opportunity it te ODE Design metod Evportive Wter sving loss (kg/s) (%) onventionl ODE % ONLUSION oug qulity of steter is site-specific, te ssessment of effluents usully involves te volume, discrge rte nd concentrtion of pollutnts, te temperture, ph nd te qulity of te receiving ter. e cnge of temperture in surfce /ground ter, by te ddition of steter et, ffects te pysiocemicl nd ydrologicl properties of te ter nd potentilly impcts on overll ecosystem. emperture restrictions on queous effluents dictte tt strems it temperture iger tn te permitted level must pss troug cooling system to reduce te effluent temperture before discrge.it s been used to reduce te temperture of effluents tt te effluents re diluted it regionl ter (river, lke, esturies or costl ter) ner industril sites nd discrged to te environment. But tis prctice is not long-term solution nd is lso restricted by government utorities for ground/surfce ter protection. So te introduction of cooling systems is inevitble for solving effluent temperture reduction problems.effluent temperture reduction cn be ccomplised by simply instlling cooling equipment before discrge. Hoever, tis cn be expensive nd inefficient. As centrlized cooling policy cnnot void te degrdtion cused by mixing effluents it lo tempertures, distributed cooling policy for cooling systems sould be considered. In tis study, ne design metod, te Optimum Design metod of distributed Effluent ooling system (ODE), s been introduced 164

11 Int. J. Environ. Res., 3(): , Spring 009 to cope it problems of terml pollution. In tis metod, te pinc tecnology for effluent temperture reduction s been improved, tking into considertion te system limittions.o cieve tis objective, te effects of evportive loss, flexible design vribles, nd pysicl properties on cooling performnce nd cost ve been considered. Wit tis metod, fesible region limited by te full centrlized conditions nd te minimum cooling flo rte s been explored to cieve te optimum cooling toer supply line. e terml tretment netork s ten been designed it te modified grouping design rules to cieve te trget. Moreover, in te proposed metod, optimum design of et cooling toer s been cieved troug mtemticl model. In te metod introduced ere, ODE, te effluent strems re distributed for cooling purposes. is metod llos te system NOMENLAURE 1,,3, b 1,,3, c 1,,3 interctions to be investigted it ne model of effluent terml tretment. In te proposed model, mecnicl drft counter-flo et cooling toer is ssumed. By distributing te cooling system, te effluent strems re prtilly treted termlly by te cooling toer. erefore, te required cooling toer in te distributed system is smller tn tt required for centrlized system, so pplying te ODE results in minimized totl cost reltive to tt of te conventionl design (centrlized design). After terml tretment troug te cooling toer, te cooled effluents re mixed it te bypssed strems to meet te discrge temperture limittion. If reuse of effluent for some usges re possible, pplying ODE cn be resulted in more ter conservtionl opportunities. Relted coding in Visul Studio 003, ++ s developed to cieve te optimiztion computtions nd mtemticl clcultions. ir ter interfce re per unit volume of toer, m /m 3 fg, entlpy cnge of sturted liquid nd vpor evluted t, kj/kg constnt vlue of mss trnsfer K Mss trnsfer coefficient of ir, m/s coefficient A cooling toer pproc, K toer crcteristic, kg/m 3 s A cr cross section re, m K el elimintor coefficient A fn fn csing re, m L ter flo rte, kg/s A toer ontl re, m Le f Leis fctor A i et nd mss trnsfer re, m m flo rte, kg/s A ic re-independent initil cost, $ n number of effluent strems elec electricity cost, $/kw Q et trnsfer rte, kw initil cost of toer per unit R cooling toer rnge, i volume, $/m 3 specific et of dry ir t Ry elimintor crcteristic, m 1 p constnt pressure, kj/kg p Specific et of ter t constnt S nnul totl opertion time, pressure, kj/kg D diffusion coefficient, m /s temperture, E effluent collection points * migrted pinc temperture, E f economic fctor i temperture of interfce, Entlpy, kj/kg totl cost, $/yr et trnsfer coefficient of ir, kw/m MA minimum pproc, s entlpy of sturted ir t ter temperture, kj/kg V toer volume, m 3 v entlpy of ir ter vpor W cooling toer idt, m mixture, kj/kg d et trnsfer coefficient of ter, kw/m z eigt of control volume, m f entlpy of sturted ter, evluted s ter temperture, kj/kg Z cooling toer eigt, m 165

12 Optimum Distributed ooling System Greek Letters α terml diffusivity, m /s ω umidity of moist ir, kg /kg η Efficiency ω s sturted umidity rtio t ter temperture, kg /kg ρ density, kg/m 3 Subscripts ir n mximum number of strems evp evportion let env environment rej rejection i number of strems opt optimum in inlet ter min minimum WB et bulb Superscripts l loer limit u upper limit REFERENES Atei, A., 008, ombined ter nd terml pinc nlysis in process industries. P.D. tesis, Science nd Reserc Brnc of Islmic Azd University, ern, Irn. Atei, A., M. H. Pnjesi nd M. Grie, 009, A ne lgoritm for optimum design of mecnicl drft et cooling toers. J. Applied. Sci., 9 (3), Bedekr, S.V., Nitirsu, P. nd Seetrmu, K.N., 1998, Experimentl investigtion of te performnce of counter-flo pcked-bed mecnicl cooling toer. Energy, 3, oulson, J. M. nd Ricrdson, J. F., 1996, emicl engineering: Fluid flo, et trnsfer nd mss trnsfer. Oxford, Pergmon Press. Deng, S. nd n, K., 003, A numericl nlysis of et nd mss trnsfer inside reversibly used ter cooling toer. Build. Environ., 38, Hollnds, K.G.., 003, An nlysis of counter-flo spry cooling toer. Int. J. Het Mss rn., 17, Kn, J.R. nd Zubir, S.M., 001, An improved design nd rting nlysis of counter flo et cooling toers. J. Het rn., 13, Kn, J.R., Quresi, B.A. nd Zubir, S.M., 004, A compreensive design nd performnce evlution study of counter flo et cooling toers. Int. J. Reig., 7, Kim, J.K. nd Smit, R., 001, ooling ter system design. em. Eng. Sci., 56, Kim, J.K., Svulescu, L. nd Smit, R., 001, Design of cooling systems for effluent temperture reduction. em. Eng. Sci., 56, Kloppers, J.. nd Kröger, D.G., 005, e Leis fctor nd its influence on te performnce prediction of etcooling toers. Int. J. erm. Sci., 44, Kröger, D.G., 004, Air-cooled et excngers nd cooling toers. Oklom, PennWell orportion. Kuo, W.J. nd Smit, R., 1997, Effluent tretment system design. em. Eng. Sci., 5, Linnoff, B. nd Smit, R., 1994, Pinc nlysis for netork design. HDEH. Mteti, A. H., Munyenyembe, A., Jere, W. nd Nyli, E., 008, Efficiency of oxidtion ponds in steter tretment. Int. J. Environ. Res., (), Pnjesi, M. H. nd A. Atei, 008, Appliction of n environmentlly optimum cooling ter system design to ter nd energy conservtion, Int. J. Environ. Sci. ec., 5 (), Prsd, M., 004, Economic up grdtion nd optiml use of multi-cell cross flo evportive ter cooling toer troug modulr performnce. Appl. erm. Eng., 4, Quresi, B.A. nd Zubsir, S.M., 006, A complete model for et cooling toers it fouling in fills. Appl. erm. Eng., 6, Srprstzde, H., Seedi, M., Neimpoor, F. nd Aminzde, B., 007, Pretretment of Municipl Wsteter by Ennced emicl ogultion. Int. J. Environ. Res., 1(), Smit, R., 005, emicl process design nd integrtion. UK, Jon Wiley nd Sons. Söylemez, M.S., 001, On te optimum sizing of cooling toers. Energ. onvers. Mnge., 4,