FUNDAMENTALS & DESIGN

Transcription

1 Fundatin f Tecnical Educatin Cllege f Tecnical/ Basa 4 Lectues FUNDAMENTALS & DESIGN OF HEAT EXCHANGER 1. Classificatin f Heat Excanges 2. Calculatins f Heat Excange 3. Heat Tansfe Applicatins 4. Cnstuctin f Sell-And-Tube Heat Excanges Lectue M. Amjed Amed Ali

2 Heat Excange Cllege f Tecnical HEAT EXCHANGER 1 PREFACE A Heat Excange: eat enegy is tansfeed fm ne bdy fluid steam t ante. eat tansfe equatins ae applied t calculate tis tansfe f enegy s as t cay it ut efficiently and unde cntlled cnditins. Te equipment ges unde many names, suc as biles, pasteuizes, jacketed pans, feezes, ai eates, ckes, vens and s n. Te pupse f te eat excange is : 1. T eat cl a steam flweing fm equipment t ante. 2. T vapize a liquid steam 3. T cndensate a vap steam. 2 CLASSIFICATION OF HEAT EXCHANGERS: 2.1. TYPES OF APPLICATION a. Biles and Steam Geneats b. Cndenses c. Radiats d. Evapats e. Cling twes (diect cntact) f. Regeneats (peidic eat flw t and cld fluid altenately ccupy te space f te eat excange) g. Recupeates (cntinuus eat flw t and cld fluid ae sepaated by a wall sell and tube eat excanges) 2.2 FLUID FLOW ARRANGEMENT a) C-cuent paallel flw : Te fluids can flw in te same diectin tug te equipment b) Cuntecuent flw: Te fluids can flw in te ppsite diectins tug te equipment c) Css flw: tey can flw at igt angles t eac te. M. Amjed Amed 2

3 Heat Excange Cllege f Tecnical Fig.(1) Duble Pipe Heat Excange 2.3 Mixed and Unmixed fluid: a. Bt fluids unmixed b. One fluid mixed ante unmixed c. Bt fluids mixed Fig.(2) Mixed and Unmixed fluid 3 CONTINUOUS-FLOW HEAT EXCHANGERS It is vey ften cnvenient t use eat excanges in wic ne bt f te mateials tat ae excanging eat ae fluids, One f te fluids is usually passed tug pipes tubes, and te te fluid steam is passed und acss tese. Mst actual eat excanges f tis type ave a mixed flw patten, but it is ften pssible t teat tem fm te pint f view f te pedminant flw patten. 3.1 DOUBLE-PIPE HEAT EXCHANGER A duble-pipe eat excange is cnstucted fm tw pipes, ne inside te te. Fist fluid flws inside te inne pipe wile te secnd fluid flws in te annula space between te tw pipes. T btain lage eat excange aea, seveal pipes ae aanged side-by-side and fittings ae attaced t allw te fluids t cntact te pipes in seies. Fig.(3) Duble-Pipe Heat excange 3.2 SHELL-AND-TUBE HEAT EXCHANGER If vey lage eat excange aeas ae equied. Swn belw is a bundle f small-diamete tubes wic ae aanged paallel t eac te and eside inside a muc lage-diamete tube called te "sell", muc like stands f uncked Spagetti cme in a tube-saped cntaine. Te tubes ae all maniflded tgete at eite end s 1-1 Heat Excange tat te "tube fluid" entes te left side and is distibuted equally amng all te tubes. At te igt side, te fluid exits fm eac tube, is mixed tgete in a secnd manifld, ten leaves as a single steam. Te secnd fluid flws in te space in between te utside f tubes. Baffle plates inside te sell fce te sell fluid t flw acss te tubes epeatedly as te fluid mves alng te lengt f te sell. 1-2 Heat Excange Half f te tubes ave flw fm left t igt wile te M. Amjed Amed 3

4 Heat Excange Cllege f Tecnical te alf ave flw in te ppsite diectin. 1 pass f sell-1 pass f tubes 1 pass f sell-2 pass f tubes 2 pass f sell-4 pass f tubes Fig.(4) Diffeent type f sell and tube Heat excange M. Amjed Amed 4

5 Heat Excange Cllege f Tecnical 4 TYPES OF HEATING AND COOLING CURVES: T in T ut T c ut T c in T in T in T c ut T ut T c ut T ut T c in T c in (c) C >>C c a cndensing vap (d) C <<C c an evapating liquid Desupeeating Subcling T in Cndensing T ut T in Cndensing T ut T c ut Evapating T c in T c ut Heating T c in (e) Ht and cld fluid Canging pase (f) Ht fluid Canging pase T in Cling T ut T in Patial Cndensing T c ut Evapating Supeeating Heating T c in T c ut Heating T ut T c in (g) cld fluid Canging pase () cndensable and nncndensable cmpnents Fig(11) Tempeatue distibutin f a diffeent cunte cuent eat excange. M. Amjed Amed 5

6 Heat Excange Cllege f Tecnical 4 HEAT TRANSFER CALCULATIONS Te pimay bjective in te temal design f eat excanges is t detemine te necessay suface aea equied t tansfe eat at a given ate f given fluid tempeatues and flw ates. te fundamental eat tansfe elatin q UA T (1) te veall eat tansfe cefficient U is pptinal t te ecipcal f te sum f te temal esistances. F te cmmn cnfiguatins wic we sall encunte; cylindical wall: 1 U (2) 1 1 ln k i i i 1 Ui i 1 i (3) 1 ln k i i UAU A U i A i wee subscipts i and epesent te inside and utside sufaces f te wall, espectively, te veall eat tansfe cefficient and te suface aea must be cmpatible, i.e., q U A T U i A i T (1) Table(1) gives appximate values f U f sme cmmnly encunteed fluids. Te wide ange f values cited esults: 1. A divesity f eat excange mateials (f diffeent k value) 2. A flw cnditins (influencing te film cefficients, ), 3. Gemetic cnfiguatin. M. Amjed Amed 6

7 Heat Excange Cllege f Tecnical M. Amjed Amed 7 Table(1) Oveall Heat Tansfe Cefficient Fuling Resistance Te pefmance f eat excanges depends upn te eat tansfe sufaces being clean and uncected. Suld suface depsits be pesent, temal esistance inceases, esulting in deceased pefmance. Tis added esistance is usually accunted f by a fuling fact (Fuling Resistance, R f ) wic must be included alng wit te temal esistances wen calculating te veall eat tansfe cefficient. i i fi i i f R k R U 1 ln 1 1 (4) i fi i i f i i i R k R U 1 ln 1 1 (5) Typical values f R, (m 2.K/W) ange fm f clean vaps t f t ive wate. 1. Mateial depsits n t sufaces

8 Heat Excange Cllege f Tecnical 2. Rust impuities 3. Stng effect wen biling ccus R f R fi Clean Afte sme time Fig.(5) fuling esistance in duble pipe eat excange 5 LOG-MEAN TEMPERATURE DIFFERENCE(LMTD METHOD) 5.1 DOUBLE-PIPE HEAT EXCHANGER A paallel-flw flat-plate excange, wse tempeatue pfiles ae swn in Fig.(2) We sall assume tat: 1. U is cnstant 2. eat excange takes place nly between te tw fluids 3. te tempeatues f bt fluids ae cnstant ve a given css-sectin 4. te specific eats f te fluids ae cnstant T 1 T 2 da Fig.(6) Te T 1 and T 2 expessins in paallel-flw and cunte-flw eat excange M. Amjed Amed 8

9 Heat Excange Cllege f Tecnical te eat tansfe between te t and cld fluids f a diffeential lengt dx is dq UdA(T -T c ) (6) since da is te pduct f lengt dx and a cnstant widt. Te enegy gained by te cld fluid is equal t tat given up by te t fluid, dq m c C c dt c -m C dt (7) wee m is te mass flw ate and C is te specific eat. Slving f te tempeatue diffeentials fm equatin (7), dt c dq m C c c dt dq m C Taking Tei diffeence, we get; 1 1 dt dtc dq( m C m C ) (8) Eliminating dq between (6) and (8) yields wic integates t give c 1 1 d ( T Tc ) UA( T Tc )( ) (9) m C m C 2 d( T Tc ) 1 UA( ( T T ) m C c 1 m C 1 c c c ) c c (10) wee te T tems ae as swn in Fig. (5). And Fm an enegy balance n eac fluid, and substitutin f tese expessins int (10) gives, in tems f te diffeences in end tempeatues, Upn cmpaing tis esult wit eq.(1), we see tat T2 T1 T ln( T2 / T1 ) T lm (11) M. Amjed Amed 9

10 Heat Excange Cllege f Tecnical Tis aveage effective tempeatue diffeence is called te lg-mean tempeatue diffeence (LMTD). It can easily be swn tat te subscipts 1 and 2 may be intecanged witut canging te value f T lm. 5.2 MULTIPASS AND CROSS-FLOW HEAT EXCHANGERS F me cmplex eat excanges, suc as tse invlving multiple tubes, seveal sell passes, css flw, deteminatin f te aveage effective tempeatue diffeence is s difficult tat te usual pactice is t mdify (1) by a cectin fact, giving q UAF T lm (1.3) Cectin facts F f seveal cmmn cnfiguatins ae given in Fig.(6). In tese figues te ntatin (T 1,T 2, t 1, t 2 ) t dente te tempeatues f te tw fluid steams as been intduced, since it is immateial wete te t fluid flws tug te sell te tubes. It is nmally celated as a functin f tw dimensinless tempeatue atis: M. Amjed Amed 10

11 Heat Excange Cllege f Tecnical M. Amjed Amed 11

12 Heat Excange Cllege f Tecnical M. Amjed Amed 12

13 Heat Excange Cllege f Tecnical EXAMPLE 1 In a cunte flw eat excange, wate is being cilled by a sdium clide bine. If te ate f flw f te bine is 1.8 kg/s and tat f te wate is 1.05 kg/s, estimate te tempeatue t wic te wate is cled if te bine entes at -8 C and leaves at 10 C, and if te wate entes te excange at 32 C. If te aea f te eattansfe suface f tis excange is 55 m 2, wat is te veall eat-tansfe cefficient? Take te specific eats t be 3.38 and 4.18 kj/kg C f te bine and te wate espectively. Slutin By eat balance, eat lss in bine eat gain in wate 1.8 x 3.38 x [10 - (-8)] 1.05 x 4.18 x (32 - T w2 ) T w2 7 C. f cunteflw T 1 [32-10] 22 C and T 2 [7 - (-8)] 15 C. T m (22-15)/ln(22/15) 18.3 C. q UA T m Teefe 3.38 x 1.8 x 18 U x 55 x 18.3 U 0.11 kj/m 2 C EXAMPLE 2 Detemine te eat tansfe suface aea and lengt equied f a eat excange cnstucted fm a m OD tube t cl 6.93 kg/s f a 95% etyl alcl slutin (c p 3810 J/kg K) fm 65.6 C t 39.4 C, using 6.30 kg/s f wate available at 10 C (c p 4187 J/kg K). Assume tat te veall cefficient f eat tansfe based n te ute-tube aea is 568 W/m 2 K and cnside eac f te fllwing aangements: (a) Paallel-flw tube and sell (b) Cunteflw tube and sell (c) Cunteflw excange wit 2 sell passes and 72 tube passes, te alcl flwing tug te sell and te wate flwing tug te tubes (d) Css-flw, wit ne tube pass and ne sell pass, sell-side fluid mixed SOLUTION (a) Witing te enegy balance as m c p (T,in - T ut ) m c c pc (T c,ut T c, in ) we btain (6.93)(3810)( ) (6.30)(4187)(T c, ut - 10) T c, ut36.2 C Te ate f eat flw fm te alcl t te wate is q m c p (T,in T,ut ) q (6.93 kg/s)(3810 J/kg K)( )(K) q 691,800 W T2 T1 T lm ln( T2 / T1 ) Fm te eat tansfe suface aea is M. Amjed Amed 13

14 Heat Excange Cllege f Tecnical m OD tube LA/πD 830 m (b) F te cunteflw aangement, LMTD C, because m c c pc m c p. Te equied aea is wic is abut 40% less tan te aea necessay f paallel flw. (c) F te cunteflw aangement, we detemine te apppiate mean tempeatue diffeence by applying te cectin fact fund fm Fig t te mean tempeatue f cunteflw: and te eat capacity ate ati is Fm te cat f Fig. 14, F 0.97 and te eat tansfe aea is Te lengt f te excange f seventy-tw m OD tubes in paallel wuld be (d) F te css-flw aangement Fig.(2)b, te cectin fact is fund fm te cat f Fig t be Te equied suface aea is tus 47.0 m 2, abut 10% lage tan tat f te evesed-cuent excange. M. Amjed Amed 14

15 Heat Excange Cllege f Tecnical 6 HEAT EXCHANGER EFFECTIVENESS (NTU METHOD) Ante appac intduces a definitin f eat excange effectiveness ε : q ε (13) q max Wee: 0 ε 1 and ε 0 (evapatin & cndensatin) q : Actual eat tansfe q max : maximum pssible eat tansfe is tat wic wuld esult if ne fluid undewent a tempeatue cange equal t te maximum tempeatue diffeence (T i - T ci ) Tis metd uses te effectiveness ε t eliminate te unknwn discage tempeatue and gives a slutin f effectiveness in tems f te knwn paametes (m, C, A, and U). Letting C mc, qc (T i -T ) C c (T c -T ci ) (15) Te maximum pssible eat tansfe ccus wen te fluid f smalle C undeges te maximum tempeatue diffeence available, q max C min ( T i - T ci ) (16) Tis tansfe wuld be attained in a cunteflw excange f infinite aea. Cmbining (14) and (16),we get te basic equatin f detemining te eat tansfe in eat excanges wit unknwn discage tempeatues: q actual ε C mim (T i - T ci ) (17) 6.1 PARALLEL-FLOW HEAT EXCHANGER Cnside te simple paallel-flw eat excange f Fig.(5) unde te same assumptins used in Sectin 5.1 t detemine te lg-mean tempeatue diffeence. Cmbining (13), (14) and (I5), we get tw expessins f effectiveness, C (Ti - T) Cc(Tc - Tci) ε C (T - T ) (18) min i ci Cmin(Ti - Tci) Since eite te t te cld fluid may ave te minimum value f C, tee ae tw pssible values f effectiveness: Ti - T C < Cc evapatin ε (19) T - T Tc - Tci C > Cccndensatin ε c (20) Ti - Tci wee subscipts n ε designate te fluid wic as te minimum C. Retuning t (9), it may be witten in tems f te Cs t give i ci T T T - T i c ci UA C exp 1 C C Fm te enegy balance equatin (15), T c T ci C /C c (T i -T ) (22) Substituting tis elatin in t eq. (21) afte adding and subtacting T i gives c M. Amjed Amed 15

16 Heat Excange Cllege f Tecnical T Wic simplified t: i T Ti-T T -T ( T i ci ci i T T T i i (Tci C T -T T i ci i ci C /C T -T i ) C /Cc (T T -T UA C 1 exp T T C 1 ( C i ) Ti-Tci (1 C /Cc ) Substituting tis elatin in t equatin (19) i ci /C c -T c (T (T i i -T -T )) UA C exp 1 C C ) UA C exp 1 C C ) UA C 1 exp 1 C C c c c c similaity in t eq.(20) we get, (23) Equatins (23) and (24) may bt be expessed as (24) (25) It suld be nted tat (25) cntains nly te veall eat tansfe cefficient, aea, fluid ppeties, and flw ates. Giving te effectiveness f a paallel-flw eat excange in tems f tw dimensinless atis (UA/C min ) and (C min /C max ), a UA/C min, is called te Numbe f Tansfe Units may be cnsideed as a eat excange size fact, NTU UA/C min (26) M. Amjed Amed 16 D

17 Heat Excange Cllege f Tecnical T in T c ut T T in T T c T ut T c ut T c in T c T ut T c in (mc p ) t > (mc p ) cld C min (mc p ) cld T c > T (mc p ) cld > (mc p ) t C min (mc p ) t T > T c T in T c ut A T in T c ut T ut T c in T ut T c in A A q(mc p ) cld (T i -T ci ) q(mc p ) t (T i -T ci ) M. Amjed Amed 17

18 Heat Excange Cllege f Tecnical Table (2) Expessins f te effectiveness f te cnfiguatins wee CC min /C max Nte tat f an evapat cndense C 0, because ne fluid emains at a cnstant tempeatue, making its effective specific eat infinite. M. Amjed Amed 18

19 Heat Excange Cllege f Tecnical Fig.( 7) M. Amjed Amed 19

20 Heat Excange Cllege f Tecnical Example 4 Fm a pefmance test n a well-baffled single-sell, tw-tube-pass eat excange, te fllwing data ae available: il (c p 2100 J/kg K) in tubulent flw inside te tubes enteed at 340 K. at te ate f 1.00 kg/s and left at 310 K; wate flwing n te sell side enteed at 290 K and left at 300 K. A cange in sevice cnditins equies te cling f a simila il fm an initial tempeatue f 370 K but at tee futs f te flw ate used in te pefmance test. Estimate te utlet tempeatue f te il f te same wate flw ate and inlet tempeatue as befe. Slutin q q c C (T i -T ) C c (T c -T ci ) C c 6300 W/K and te tempeatue ati P is, fm Eq. (8.19), S0.6 R0.33 Fm Fig F 0.94 te veall cnductance is Since te temal esistant n te il side is cntlling, a decease in velcity t 75% f te iginal value will incease (e temal esistance by ugly te velcity ati aised t te 0.8 pwe. Unde te new cnditins, te cnductance, te NTU, and te eat capacity ate ati will teefe be appximately UA (2325)(0.75) W/K NTUUA/C il 1.17 C min C il kg/s)(2100 J/kg K) C max C wate 6300 W/K C min / C max 0.25 fm Fig te effectiveness is equal t Hence T il ut T il in ε T max [0.61( )] K. M. Amjed Amed 20

21 Heat Excange Cllege f Tecnical 7 OTHER HEAT TRANSFER APPLICATIONS 7.1 JACKETED PANS In a jacketed pan, te liquid t be eated is cntained in a vessel, wic may als be pvided wit an agitat t keep te liquid n te mve acss te eattansfe suface, as swn in Fig.3(a). Te suce f eat is cmmnly steam cndensing in te vessel jacket. Pactical cnsideatins f imptance ae: 1. Tee is te minimum f ai wit te steam in te jacket. 2. Te steam is nt supeeated as pat f te suface must ten be used as a desupeeate ve wic lw gas eat-tansfe cefficients apply ate tan ig cndensing cefficients. 3. Steam tapping t emve cndensate and ai is adequate. Sme veall eat tansfe cefficients ae swn in Table 3. Save f biling wate, wic agitates itself, mecanical agitatin is assumed. Wee tee is n agitatin, cefficients may be alved. Table(3) Sme Oveall Heat Tansfe Cefficients In Jacketed Pans Cndensing fluid Heated fluid Pan mateial U (J/m 2 s C) Steam Tin liquid Cast-in 1800 Steam Tick liquid Cast-in 900 Steam Paste Stainless steel 300 Steam Wate, biling Cppe HEATING COILS IMMERSED IN LIQUIDS In sme pcesses, quick eating is equied in te pan, a elical cil may be fitted inside te pan and steam admitted t te cil as swn in Fig.3(b). Tis can give geate eat tansfe ates tan jacketed pans, because tee can be a geate eat tansfe suface and als te eat tansfe cefficients ae ige f cils tan f te pan walls. Steam Cndensate (a) Jacketed pans (b) Heating Cil Fig.(8) Heat excange equipment M. Amjed Amed 21

22 Heat Excange Cllege f Tecnical Example (3) Milk is flwing int a pipe cle and passes tug a tube f 2.5 cm intenal diamete at a ate f 0.4 kg/s. Its initial tempeatue is 49 C and it is wised t cl it t 18 C using a stied bat f cnstant 10 C wate und te pipe. Wat lengt f pipe wuld be equied? Assume an veall cefficient f eat tansfe fm te bat t te milk f 900 J/m 2 s C, and tat te specific eat f milk is 3890 J/kg C. Slutin q m C p (T 1 -T 2 ) q 3890 x 0.4 x (49-18) J/s Als q UA T m T m [(49-10) - (18 10)] / ln[(49-10)1(18-10)] T m 19.6 C. Teefe 48, x A x l9.6 A 2.73 m 2 but A πdl Nw D m. L 2.73/(π x 0.025) 34.8 m Example (4) Steam equied t eat sup in jacketed pan Estimate te steam equiement as yu stat t eat 50 kg f sup in a jacketed pan, if te initial tempeatue f te sup is 18 C and te steam used is at 100 kpa gauge. Te pan as a eating suface f 1 m 2 and te veall eat tansfe cefficient is assumed t be 300 J/m 2 s C. Slutin Fm steam tables, satuatin tempeatue f steam at 100 kpa gauge 120 C and latent eat λ 2202 kj/kg. q UA T 300 x 1 x (120-18) 3.06 x 10 4 J/s Teefe amunt f steam q/λ (3.06 x 10 4 )/(2.202 x 10 6 ) 1.4 x 10-2 kg/s 1.4 x 10-2 x 3.6 x kg/. M. Amjed Amed 22

23 Heat Excange Cllege f Tecnical 7.3 SCRAPED SURFACE HEAT EXCHANGERS Te pcessing industy paticulaly f pducts f ige viscsity, cnsists f a jacketed cylinde wit an intenal cylinde cncentic t te fist and fitted wit scape blades, as illustated in Fig.(9). Te blades tate, causing te fluid t flw tug te annula space between te cylindes wit te ute eat tansfe suface cnstantly scaped. Cefficients f eat tansfe vay wit speeds f tatin but tey ae f te de f J/m 2 s C. Tese macines ae used in te feezing f ice ceam and in te cling f fats duing magaine manufactue. Fig.(9) Heat excange equipment 7.4 PLATE HEAT EXCHANGERS A ppula eat excange f fluids f lw viscsity, is te plate eat excange, wee eating and cling fluids flw tug altenate ttuus passages between vetical plates as illustated in Fig.(10). Te plates ae clamped tgete, sepaated by spacing gaskets, and te eating and cling fluids ae aanged s tat tey flw between altenate plates. Suitable gaskets and cannels cntl te flw and allw paallel cunte cuent flw in any desied numbe f passes. A substantial advantage f tis type f eat excange is tat it ffes a lage tansfe suface tat is eadily accessible f cleaning. Te banks f plates ae aanged s tat tey may be taken apat easily. Oveall eat tansfe cefficients ae f te de f J/m 2 s C. M. Amjed Amed 23

24 Heat Excange Cllege f Tecnical Fig.(10) Plate Heat excange M. Amjed Amed 24

25 Heat Excange Design Cllege Of Tecnical 8 A DESIGN METHODOLOGY 1. Heat Excange Sizing Cse a typical value f U based n te type f sevice, ten detemined te utlet tempeatues based n te pefmance specificatins (numbe f tube, baffle spacing, etc) and te enegy balance Q als calculate eat tansfe aea fm QUAF T lm. 2. Heat Excange Rating is te cmputatinal pcess in wic te inlet flw ate and tempeatues, te fluid ppeties and te eat excange paamete ae taken as input. And te utlet tempeatues and temal duty Q (if te eat excange lengt is specified) n te equied lengt is calculated as utput else pessue dp f eac steam. (U and A ae knwn) 3. Heat Excange Design Is te pcess tat detemine te eat excange specificatins suc as (lengt and diamete f tube, sell tickness, spacing and cut baffle.) Detemine eat tansfe aea based n sizing calculatin Detemine numbe f tube and pass Ceck te velcity if belw acceptable ange, cse suitable numbe f pass. Calculate veall eat tansfe cefficient and estimate te fuling esistance. Ceck U assume -U calculate < Ceck te pessue dp f sell and tube sides, ten detemine te pumping pwe equiements f sell and tubes sides. 1. Ken Metd Ken Metd is used t estimate te size f te eat excange f a given specificatin (Sizing Metd), its esticted t a fixed baffecut (25%) and cannet adequately accunt f baffle-sell and tube-t-baffle leakages Input (knw) T ci T i T c m m c Output (unknwn) T Q d i d 2. Bell Delawae's Metd It's a ating analysis and give me satisfacty pedicatins f te eat tansfe cefficient and pessue dp tan Ken Metd, and it is takes int accunt te effects f leakage and passing. Data f Ken Metd Data f Bell Metd M. Amjed Amed 25

26 Heat Excange Design Cllege Of Tecnical 3. NTU Metd It te eat duty is nt knwn because nly te inlet tempeatues ae given wile te utlet tempeatues ae nt. On te te and te eat excange lengt is fixed and te utlet tempeatues and pessue dps ae t be calculated. Input (knw) T ci T i L m m c Output (unknwn) T T c d i d P tubes P sell Q 4. F-Metd Wen all f te teminal tempeatues ae knwn, tan; Q UAF T lm M. Amjed Amed 26

27 Heat Excange Design Cllege Of Tecnical 9. HEAT EXCHANGER DESIGN BY KERN METHOD Stat T 1, T 2, t 1, Cp, Cp c. 1 q mc p (T in -T ut ) q mλ Calculate: t 2 fm Enegy Balance. 2 T2 Calculate T lm ( T2 T1 )/ ln( ) T Cse Sell and Tube Passes. 3 Fm Cat estimate: S,R Cectin. Fact 4 1 Selectin Ante Type 2 Select te type f flw C-cuent,Cunte-cuent & Css-flw Estimate N. f Bundle Pass. 5 N F < 0.85 Yes Estimate: U Assume fm Table(12.1) Calculate: Aq/U Assume F T. lm 6 A <20 ft 2 20 A 500 ft 2 A >500 ft 2 Select Cil Type Select Duble Pipe H.E. Type Select Sell & Tube H.E. Type 3. 7 Selectin Tube Diamete(d i, d ) 3/8",. 1/2", 1/4" 8 Selectin Tube Lengt (L) 8,10, 12, 16,20 ft Selectin ante d i, d A πd L N. f Tubes N p A π/dl N.... p A/A 9 Velcity fluid inside Tube V... t 10 V t <2 m/s 1 M. Amjed Amed 27

28 Heat Excange Design Cllege Of Tecnical 1 Selectin tubes aangement.. 11 Tiangula Squae Pitc. 12 Sell Diamete D s 8"~120" Baffle type & Space. l B 13, i U calculate 1 R f k 1 ln R fi i i i i U assume U calculate N U assume U calculate < Yes P Tube N P tube <10 Psi Yes Pumping Pwe f Tube.. (P Ptube ) 18 P sell N P sell <3~7.3 Psi Yes Pumping Pwe f Sell.. (P Psell ) 20 Numbe f baffles N b (L/l.. B )-1 21 End M. Amjed Amed 28

29 Heat Excange Design Cllege Of Tecnical Step 7. Cse tube type d i and d Assuming tube lengt (L) (6, 8, 12, 16) ft A πd L Numbe f tube N t A/A Step 12. Calculate te sell diamete D s a. Ds K 2 2 [ πdo N t ( PR) ] CT wee K is te Tube layut cnstant K1 f 90 and 45 K0.87 f 30 and 60 CT is te tube cunt cnstant CT 0.93 ne tube pass CT 0.90 Tw tube pass CT 0.85 Tee tube pass PR is te tube Pitc ati P t /d P t is te tube Pitc Fig(13) Tube pattens b. Bundle Diamete D s D s D b Cleaance D b d (N t /K 1 ) (1/n1) Wee K 1 and n 1 ae cnstant fm Table 12.4 M. Amjed Amed 29

30 Heat Excange Design Cllege Of Tecnical and cleaance between bundle and te sell estimate fm fig. (12.10) Step 13. Baffle type & Space l B l B Ds 5 l B ( )Ds M. Amjed Amed 30

31 Heat Excange Design Cllege Of Tecnical Step 14. Calculate eat tansfe cefficient f inside tube i Nu idi K f j 0.33 Re P µ µ w j : eat tansfe fact fm fig.(12.23) K f Temal cnductivity f te tube side fluid µ w : Viscsity f te fluid at wall tempeatue T w T w 2 Tci Tc Ti 2 2 T M. Amjed Amed 31

32 Heat Excange Design Cllege Of Tecnical Step 14. Calculate eat tansfe cefficient f utside tube 0.36k /3 Re P sell De Wee D e is te equivalent diamete De ( Pt 0.785d ) f squae Pitc d D e ( Pt 0.917d d ) f Tiangula Pitc mde Re Sell As µ A s is te sell css flw aea ( Pt d) D As Pt Step 17. Calculate tube side pessue dp filn p Vc Ptube 4N p ρc di 2 2 fi (1.58 ln(re ) 3.28) s l B 4 2 tube Step 18. Te pumping pwe (P Pt ) f tube side mc Pt P Pt ρ η Wee η p is te pump efficiency η 0.8 p c p M. Amjed Amed 32

33 Heat Excange Design Cllege Of Tecnical M. Amjed Amed 33 Step 19. Calculate sell side pessue dp w B f sell V l L de Ds j P µ µ ρ Wee j f is te factin fact fm fig.(12.30) Step 20. Te pumping pwe (P Psell ) f sell side p sell Psell P m P η ρ Step 21. Calculate te numbe f baffles 1 B b l L N