Thermodynamic Modeling and Optimization of Air Handling Units

Size: px

Start display at page:

Download "Thermodynamic Modeling and Optimization of Air Handling Units"

Reynold McCoy
6 years ago
Views:

1 Prceedings f Clim 2007 WellBeing Indrs Thermdynmic Mdeling nd Optimiztin f ir Hndling Units Mhmmd Hssn Sidi 1 nd Dvud Mhbbi 2 1 Shrif University f Technlgy, Schl f Mechnicl Engineering, Tehrn, IRN 2 Shrif University f Technlgy, Schl f Mechnicl Engineering, Tehrn, IRN Crrespnding emil: Smn@shrif.edu SUMMRY ir hndling unit (HU) is defined s self-cntined unit tht the cnditins f ir vry while pssing thrugh it nd rech t the desired temperture nd humidity. T perfrm vritins in wether cnditins vrius prcesses such s heting, cling, humidifictin, dehumidifictin nd mixing re pplied. In this reserch thermdynmic mdeling nd mthemticl ptimiztin f ir hndling units pprching minimum energy cnsumptin is chieved. The bjective functin fr ptimiztin is pressure drp f ir crssing cil per cling nd heting ld f the system. This functin cmprises ll therml nd gemetricl prmeters f the cils such s cil surfce re, number f rws, fin spcing nd ir side pressure drp f the cil. The bjective functin is minimized using Lgrnge multipliers methd. The ptimiztin results re cmpsed f minimum pressure drp, ptimum re, ptimum number f rws nd fin spcing. The effects f vrying the cling nd heting ld, fin efficiency nd the surfce re f the cil n fn pwer cnsumptin re investigted s well. INTRODUCTION The mjr utilities in residentil, cmmercil nd industril buildings re electricl distributin netwrks, ir cnditining systems, stem systems nd cmpressed ir systems. The utilities must perte t the highest pssible verll efficiency fr minimiztin f the building s energy requirement. Generlly, the energy cnsumptin is minimum when the surces such s electricl energy perfectly trck the lds. These lds might be cling r heting lds, which re directly ffecting the humn cmfrt cnditins. When there is mismtch between the surce nd ld, energy lsses will be higher [1]. One f these utilities is ir hndling unit in which there re few studies but the energy efficiency but it. n HU is the primry equipment in n ir system f centrl hydrnic system. It hndles nd cnditins the ir nd distributes it t vrius cnditined spces. HUs utilize vrius types f equipments, rrnged in specific rder, s tht spce cnditins cn be mintined. HUs my cnsist f supply fn nd cil sectin with chilled wter r direct expnsin cil, prehet r rehet cil, heting cil sectin, filter sectin, mixing bx, r cmbintin f mixing bx nd filter [2,3]. In HUs ne f the mjr cmpnents in energy cnsumptin f system re cils. The pressure drp f ir pssing the system depends n therml nd gemetricl prmeters f cil. In this reserch heting cils re mdeled by lg men temperture difference nd cling nd dehumidifying cils re mdeled by lg men enthlpy difference. Since then their ptimiztin prceeds. ttempts shw tht the investigtin f HUs hs been perfrmed cnsidering nly ne cmpnent f the HU. Rbinsn [4] investigted experimentlly dmper cntrl chrcteristics nd mixing bx effectiveness f ir hndling units. The results frm tests indicted tht the mixing effectiveness f the mixing bx ws functin f the dmper psitin. Liu et l. [5] develped n ir filter pressure lss mdel fr fn energy clcultin in ir hndling units nd shwed t be highly cnsistent with vilble experimentl dt. Rulet et l. [6] investigted rel het recvery with ir hndling units. Their reserch ddressed rel energy recvery with ir hndling units frm thereticl pint f view nd presented results f mesurements n 13 units. Snye nd mlekmhmmdi [7] develped new methd f therml nd ecnmicl ptimum design f ir cnditining units with vpr cmpressin refrigertin system. The bjective functin fr ptimiztin ws ttl cst per unit cling ld f the system including cpitl investment fr cmpnents s well s the required electricity cst.

2 Prceedings f Clim 2007 WellBeing Indrs THERMODYNMIC MODELING 1. Heting cil mdeling The ttl het trnsfer rte frm heting cil is: q = UΔ Tm, (1) where U is verll het trnsfer cefficient, is ttl utside surfce re f the cil nd ΔTm is lg men temperture difference. Cnsidering the tube mteril t be cpper hving high therml cnductivity, s the therml resistnce f cpper tubes is ignred nd the verll het trnsfer cefficient bsed n the utside surfce re f the cil U cn be clculted s U = 1 1 η h h h ( + η ) i i F, (2) where h i is inner surfce het trnsfer cefficient, h is uter surfce het trnsfer cefficient, inner surfce re f the cil, F is ttl surfce re f fins nd η is fin efficiency. McQuistn [8,9] develped the crreltin between Chiltn-Clburn j fctrs nd prmeter JP fr dry cils which is s fllws: i is j s h = = + Gc. p, 0.4 JP = ReD p GD. ReD = μ 23 Pr JP, (3) In equtin (3), tube nd μ is fluid viscsity. p is ttl utside surfce re f tubes, G is mss velcity, D is utside dimeter f G = ρ. V, (4) min where ρ is fluid density nd Vmin is ir velcity t minimum flw re. Fr ht nd chilled wter t turbulent flw inside the tubes, the inner surfce het trnsfer cefficient h i, cn be clculted by Dittus-Belter [10,11] equtin: hd i i k w 0.023Re Pr n 0.8 =, (5) In equtin (5), Di is inside dimeter f tube nd kw is therml cnductivity f wter. When the tube wll temperturet t > T bulk, n = 0.4 nd whent t < T bulk, n=0.3. Here Tbulk indictes the bulky wter temperture beynd the bundry lyer.

3 Prceedings f Clim 2007 WellBeing Indrs 2. Cling nd dehumidifying (wet) cil mdeling In cling nd dehumidifying cil, bth het nd mss trnsfer ccurred. With dehumidifictin, the ir side surfce is wetted with liquid wter. Since wter vpr trnsfer des nt depend n temperture difference lne, it fllws tht the methd generlly used in the nlysis f dry ir side surfce des nt suffice. In this pper, mdeling prcedure fr wet cling cils will be presented by dpting the methd f Threlkeld [12] which bses the nlysis n enthlpy ptentil. The ttl het het trnsfer rte fr cling nd dehumidifying cil is: q= U Δ h, (6) w, m where Uw, is verll het trnsfer cefficient fr wet cil nd Δhm is lg men enthlpy difference. If the therml resistnce f cpper tubes is ignred, the verll het trnsfer cefficient bsed n the utside surfce re f the wet cil U, cn be clculted s w U w, = 1 b b b R + + h h h ( 1 η ) ( + η ) wm, w wm, i i, w F w, w, (7) In Eq. (7) the cmbined wet surfce cefficient h w, must be clculted by Eq. (8). In equtin (7), ηw is wet fin efficiency. The quntity b is the slpe f Eq. (9) which represents sturtin enthlpy f ir s functin f temperture ver smll temperture rnge. h w, 1 = c b h + y k ( ) p, wm, w w, (8) The quntity b R shuld be evluted t the men wter temperture inside tubes t R while the quntity b wm, shuld be evluted t the men wter film surfce temperture. h = + bt., (9) s where hs is enthlpy f sturted mist ir nd t s is temperture f sturted mist ir. In Eq. (8) the term yw kw is usully smll, s tht n estimte f wter film thickness is nt criticl [13]. The cnvectin het trnsfer cefficient h in Eq. (8) cn be clculted by Eq. (3). The inner surfce het trnsfer cefficient h i is determined by Eq. (5). s

4 Prceedings f Clim 2007 WellBeing Indrs OPTIMIZTION The bjective functin fr ptimiztin is pressure drp f ir pssing cil per cling (heting) ld f the system. ir side pressure drp ver finned-tube cil is expressed by [14] 2 Gv 2 i v v m Δ P= ( 1+ σ ) 1 + f, (10) 2 vi min vi where vi is ir specific vlume t the inlet f cil, v is ir specific vlume t the utlet f the cil, vm is men specific vlume nd f is frictin cefficient. In Eq. (10) the prmeters re: min σ =, (11) m& ρv& G = = min σ, (12) where m& is mss flw rte f ir nd V & is ir vlume flw rte. f 0.21 = Re, (13) The rti f free flw (r minimum flw) re t fce re σ is usully 0.54 t 0.6 [9]. The fce re is determined by selecting prper fce velcity f 2.5 t 3 ms. V V = &, (14) Substituting Eqs. (11)- (14), in Eq. (10), Δ P cn be expressed s: V& min v ρvd & v Δ m P = ρ , (15) min vi μ min v i Except nd min, ll ther prmeters f Eq. (15) re knwn fr cil. Therefre ir side pressure drp f cil, Δ P cn be expressed s ( ) Δ P = f, (16), 1 min Ttl utside surfce het trnsfer re f heting nd cling nd dehumidifying cils cn be determined frm Eqs. (1) nd (6) respectively. Substituting inside nd utside surfce het trnsfer cefficients in Eqs. (2) nd (7) nd by using Eqs. (1) nd (6), the ttl utside surfce het trnsfer re cn be expressed s = f2,,, (17) i p F

5 Prceedings f Clim 2007 WellBeing Indrs Ttl utside surfce re f cil s functin f number f rws nd fce re expressed s = F N, (18) s In equtin (18), Fs is cil cre surfce re prmeter nd N is number f rws. Using Tble 1, cil surfce rtis re defined s the fllwing functins in terms f number f rws nd fin spcing. Tble 1 Finned- tube cil cnstructin prmeters [9] Fin Spcing S F (fins/m) p i F F s i s p = S F = S = + F F = S F SF SF F, (19) Substituting Eqs. (17)- (19) in Eq. (16), the bjective functin, Δ P is btined s Δ P = f (, ) 3 N S F, (20) T find the system ptimum design prmeters, the bjective functin (20) is minimized by Lgrnge multipliers methd [15,16]. The bundry cnditins f prblem re number f rws nd fin spcing. Fin spcing is usully expressed in fins per meter, nd it ften vries frm 315 t 591 fins/m (1.6 t 3.1-mm fin spcing) fr cils used in ir cnditining systems. The number f tube rws in heting cils vries frm 1 t 4 rws nd in cling nd dehumidifying cils vries frm 4 t 8 rws [9]. RESULTS 1. Heting cil Figure 1 shws the influence f heting ld n ptimum surfce re nd pressure drp f heting 3 cil in 4200 m hr ir flw rte. Since the cnditins f ir entering the HU cn be different, ptimum design prmeters is determined in three different entering tempertures nmely, 18 C, 7 C nd 5 C. In the ptimiztin prcess f heting cil, the entering wter temperture t cil is lwys 82 C nd leving wter temperture frm it is 71 C. In Fig 1, by incresing the heting ld, the ptimum surfce re f cil increses. If tw different inlet tempertures re cnsidered, t cnstnt heting ld, ptimum surfce re f greter temperture f entering ir is higher thn ptimum surfce re f smller inlet temperture. T justify this result ne cn sy tht by incresing the inlet temperture f ir, lg men temperture difference decreses nd by using Eq. (1) nd cnsidering cnstnt heting ld, ptimum surfce re f cil increses. In this Figure, by

6 Prceedings f Clim 2007 WellBeing Indrs incresing the heting ld, ptimum pressure drp f cil increses, since by incresing the heting ld, ptimum surfce re f cil increses nd relevnt with Eq. (10), pressure drp is prprtinl t surfce re f cil. ls t cnstnt heting ld, ptimum pressure drp f greter inlet temperture is higher thn the ptimum pressure drp f smller inlet temperture. This result cn be justified by cnsidering prprtin f pressure drp with surfce re. Figure 1 The influence f heting ld n ptimum surfce re nd pressure drp Figure 2 The influence f heting ld n ptimum number f rws nd fin spcing Figure 2 shws the influence f heting ld n ptimum number f rws nd fin spcing f heting 3 cil in 4200 m hr ir flw rte. In this Figure, t the inlet temperture f 18 C, it is bserved tht by incresing the heting ld, primrily the number f rws increse nd fin spcing remins cnstnt, then the number f rws remin cnstnt nd fin spcing increses. t the inlet temperture f 7 C, by incresing the heting ld, t first number f rws nd fin spcing remin cnstnt nd then bth f them increse. t the inlet temperture f 5 C, by incresing the heting ld, fin spcing decreses since with decresing fin spcing, the number f rws increses, s result f this, cnsequently ptimum surfce re f cil increses. In Figure 2 in sme cses with incresing heting ld, the number f rws r fins remins cnstnt, becuse in tht heting rnge, cil cn hve further het trnsfer withut incresing number f rws r fins. 2. Cling nd dehumidifying cil In Figure 3 the influence f cling ld n ptimum surfce re nd pressure drp f cling nd dehumidifying cil in three different cnditins f ir is shwn. In the ptimiztin prcess f cling nd dehumidifying cil, the entering wter temperture t cil is lwys 7 C nd leving wter temperture frm it is 13 C.

7 Prceedings f Clim 2007 WellBeing Indrs Figure 3 The influence f cling ld n ptimum surfce re nd pressure drp In Figure 3 by incresing the cling ld, ptimum surfce re f cil increses. Optimum pressure drp f pssing ir is prprtinl t utside surfce re f cil nd with incresing the cling ld, it increses. If tw different cnditins f entering ir re cnsidered, t cnstnt cling ld, ptimum surfce re f greter dry bulb temperture f entering ir is smller thn ptimum surfce temperture f smller dry bulb temperture f entering ir. T justify this result, ne cn sy tht by incresing dry bulb temperture f entering ir, lg men enthlpy difference increses nd by using Eq. (6) nd cnsidering cnstnt cling ld, ptimum surfce re f the cil decreses. Figure 4 The influence f cling ld n ptimum number f rws nd fin spcing In Figure 4 by incresing the cling ld my ne f the fllwing cses my ccur nd t ll cses with incresing the cling ld, ptimum surfce re increses.. Number f rws remins cnstnt nd fin spcing increses. b. Bth number f rws nd fin spcing increse. c. Number f rws decrese nd fin spcing increses. d. Number f rws increse nd fin spcing decreses. e. Number f rws increse nd fin spcing remins cnstnt.

8 Prceedings f Clim 2007 WellBeing Indrs 3. Influence f ir flw rte n ptimum surfce re f the cil Figure 5 shw the influence f ir flw rte n ptimum surfce res f heting cil nd wet cil. In bth cils by incresing ir flw rte, cling r heting ld increses with the subsequent f increse f ptimum surfce res f cils. Figure 5 The influence f ir flw rte n ptimum re f heting nd wet cils 4. Influence f fin efficiency n ptimum surfce re f the cil In Figure 6 the influence f fin efficiency n ptimum surfce res f heting cil nd wet cil is shwn. Figure 6 The influence f fin efficiency n ptimum res f heting nd wet cils t cnstnt heting r cling ld, by incresing fin efficiency, ptimum surfce res f heting cil nd wet cil decrese. T justify this result, ne cn sy tht by incresing fin efficiency, verll het trnsfer cefficient increses nd t the cnstnt ld, by using Eqs. (1) nd (6) the ptimum surfce res f cils decrese. In nther wrd, by incresing fin efficiency, heting resistnce f the cil decreses nd it my hve the sme het trnsfer in smller heting ld. DISCUSSION In this pper, methd fr thermdynmic mdeling nd mthemticl ptimiztin f ir hndling units hving cling nd heting cils is prpsed. The design prmeters include therml nd gemetricl prmeters f cling nd heting cils. On the bsis f the previusly discussed results the fllwing cnclusins hve been extrcted. 1. The ptimum cnditins f cils cmpsed f the surfce re, pressure drp f pssing ir, number f rws nd fin spcing in different cnditins f ir nd different heting r cling lds re determined. 2. In heting cils, by incresing the heting ld, the ptimum surfce re f cil increses. In tw different inlet tempertures nd t cnstnt heting ld, ptimum surfce re f greter inlet temperture is higher thn ptimum surfce re f smller inlet temperture.

9 Prceedings f Clim 2007 WellBeing Indrs 3. In wet cils, by incresing the cling ld, ptimum surfce re f cil increses. In tw different inlet cnditins f ir nd t cnstnt cling ld, ptimum surfce re f greter dry bulb temperture is smller thn ptimum surfce re f smller dry bulb temperture. 4. In bth heting nd wet cils by incresing ir flw rte, ptimum surfce res f cils increse. 5. t cnstnt heting r cling ld, by incresing fin efficiency, ptimum surfce res f bth heting nd wet cils decrese. REFERENCES [1] Siddhrth Bhtt, M., Energy udit Cse Studies II- ir Cnditining (Cling), J. pplied Therml Engineering, Vl. 20, 2000, pp [2] SHRE, Hndbk f Fundmentls, mericn Sciety f Heting, Refrigerting nd ir Cnditining Engineers, tlnt, G, [3] SHRE Hndbk, HVC Systems nd Equipments, mericn Sciety f Heting Refrigerting nd ir Cnditining Engineers, tlnt, G, [4] Rbinsn K.D., Dmper Cntrl Chrcteristics nd Mixing Effectiveness f n ir- Hndling Unit Cmbintin Mixing/Filter Bx, SHRE Trnsctins, Vl. 57, 1998, pp [5] Liu M., Clridge D.E., nd Deng S., n ir Filter Pressure Lss Mdel fr Fn Energy Clcultin in ir Hndling Units, Int. J. Energy Reserch, Vl. 27, 2003, pp [6] Rulet C.., Heidt, F.D, Frdini F., nd Pibiri M.C., Rel Het Recvery with ir Hndling Units, Energy nd Buildings, Vl. 33, 2001, pp [7] Snye S., Mlekmhmmdi H.R., Therml nd Ecnmicl Optimiztin f ir Cnditining Units with Vpr Cmpressin Refrigertin System, J. pplied Therml Engineering, Vl. 24, 2004, pp [8] McQuistn F.C., Prker J.D., Heting, Ventiltin, nd ir Cnditining, 2 nd Ed., Jhn Wiley nd Sns,Inc, [9] Wng S.K., Hndbk f ir Cnditining nd Refrigertin, 2 nd Ed., McGrw-Hill, New Yrk, [10] McQuistn F.C., Het, Mss nd Mmentum Trnsfer Dt fr Five Plte-Fin TubeTrnsfer Surfces, SHRE Trnsctin, Vl. 84, prt 1, 1978, pp [11] McQuistn F.C., Het Mss nd Mmentum Trnsfer in Prllel Plte Dehumidifying Exchnger, SHRE Trnsctin, Vl. 82, prt 2, 1976, pp [12] Threlkeld J.L., Therml Envirnmentl Engineering, Prentice-Hll, New Yrk, [13] Theerkulpisut S., Priprem S., Mdeling Cling Cils, Int.Cmm. Het Mss Trnsfer, Vl. 25, N.1, 1998, pp [14] Kys W.M., Lndn.L., Cmpct Het Exchngers, 3 rd Ed., McGrw-Hill, New Yrk, [15] Stecker W.F., Design f Therml Systems, McGrw-Hill, 1989, pp [16] G.N. Vnderplts, Numericl Optimiztin Techniques fr Engineering Design, McGrw-Hill, 1984.

Physics 102. Final Examination. Spring Semester ( ) P M. Fundamental constants. n = 10P

Physics 102. Final Examination. Spring Semester ( ) P M. Fundamental constants. n = 10P ε µ0 N mp M G T Kuwit University hysics Deprtment hysics 0 Finl Exmintin Spring Semester (0-0) My, 0 Time: 5:00 M :00 M Nme.Student N Sectin N nstructrs: Drs. bdelkrim, frsheh, Dvis, Kkj, Ljk, Mrfi, ichler,