Paper No. : 04 Paper Title: Unit Operations in Food Processing Module-08: Principles of Heat Exchangers

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1 Paper N. : 04 Paper tle: Unt Operatns n Fd Pressng Mdule-08: Prnples f Heat Eangers Heat eangers are deves tat faltate te eange f eat between tw fluds tat are at dfferent temperatures wtut te fluds mng nt ntat. Heat s transferred frm te t flud t te ld flud. Heat eangers are lassfed ardng t eat transfer press, type f nstrutn and flw arrangement as fllws.. Heat ransfer Press. Dret ntat eat eangers ese lasses f eangers d nt ave a pysal barrer between te t and te ld fluds. Dret mng f te fluds takes plae w results n smultaneus eat and mass transfer. s setup s nly emplyed wen te mng f fluds des nt pse prblems, r s atually reured. A gd eample f ts type f eat eanger wuld be a lng twer.. Regeneratve Eangers Heat s alternately stred and remved frm a eat surfae n a regeneratve eanger. Called te matr f te generatr, te eat transfer surfae s mved nt and ut f te fed t and ld flud streams r, te ld and te t urrents s swted nt and ut f t as te ase may be. ese tw types are alled rtary and fed regeneratve matr eangers respetvely..3 Reuperatrs It s ne f te mst vtal types f eat eanger. ere s a lear demaratng pysal barrer between te t and te ld fluds all alng ter flw. It s perfet fr ases were ne annt affrd t m te fluds. Evapratrs f an e plant and autmble radatrs are eamples f ts type f eanger.. Desgn And Cnstrutn. Sell and eat tube eangers If larger flws are nvlved, a sell and tube eangers s used, w s te mst mprtant type f eanger n use n te press ndustres. In tese eangers te flws are ntnuus. Many tubes n parallel are used were ne flud flws nsde tese tubes. e tubes arranged n bundle, are enlsed n a sell and te ter flud flws utsde te tubes n te sell sde. Baffles are used t dret te flw f te sell-sde flud and t supprt te tubes. W lud suld be n sell sde f a sell and tube HE and wy? Even tug tere are n strt ndtns n ts te fllwng pnts are taken nt nsderatn nrmally.. e rrsve flud sall pass trug te tube sde as te replaement f tubes s easer and eaper tan sell sde. e t, azardus flud sall pass trug tube sde. Beause n ase f any leakage t wn t get epsed t atmspere.

2 . Plate and frame eat eanger s eanger nssts f a seres f tn plates nrmal t te dretn f flw f te fluds. e plates prvde a large surfae area fr eat eange and are, at sme plaes, mre nvenent tan te sell and eat tube eanger prmarly beause f ts unue sape. s as a majr advantage ver a nventnal eat eanger n tat te fluds are epsed t a mu larger surfae area beause te fluds spread ut ver te plates. e g eat transfer effeny fr su a small pysal sze as nreased te dmest t water (DHW flwrate f mbnatn blers. e small plate eat eanger as made a great mpat n dmest eatng and t-water. Larger mmeral versns use gaskets between te plates, smaller versn tend t be brazed..3 Spral eat eangers s eanger s frmed by spralng tw parallel flat seets t frm a spral nsstng f tw flw annels. e fluds flw trug tese annels and beause f te sape, get a lt f surfae area fr eat transfer. e fluds generally flw n a unter urrent fasn n a spral eat eanger..4 Duble-ppe Heat Eanger It nssts f tw nentr tubes wt ne flud flwng n te nner tube and te ter flud flwng n te annular spae between te tw ppes(see f:.e fluds an be n nurrent r unterurrent flw. e eangers an be made frm a par f sngle lengts f ppe wt fttngs at te ends r frm a number f pars nternneted n seres. ese an be used fr small flw rates..5 Crss flw Heat Eanger e fluds mve n rss flw (perpendular t ea ter wt ne flud flwng n te tubes and te ter flud flwng ver te tubes n te transverse dretn. e flud nsde te tubes s nsdered t be unmed sne t s med t an mve abut freely between te tubes and tere wll be a tendeny fr te gas temperature t eualze n te dretn f nrmal t te flw. Fr te unmed flud nsde te tubes tere wll be a temperature gradent bt parallel and nrmal t te dretn f flw. ere are tw nfguratns f rss flw eangers:. Fnned wt bt fluds unmed. Unfnned wt ne flud med and te ter unmed 3. Flw arrangements Heat eangers are lassfed by ter flw arrangements. ere are tree bas types f eat eangers Parallel Flw Cunter Current Flw 3 Crss Flw

3 3. Parallel flw Heat eangers were te fluds mve n te same dretn are referred t as parallel flw. In Parallel flw eat eangers, te utlet temperature f te "ld" flud an never eeed te utlet temperature f te "t" flud. e eanger s perfrmng at ts best wen te utlet temperatures are eual. 3. Cunter flw Eangers were fluds mve n te ppste dretn are referred t as unter flw. Cunter flw eat eangers are nerently mre effent tan parallel flw eat eangers beause tey reate a mre unfrm temperature dfferene between te fluds, ver te entre lengt f te flud pat. Cunter flw eat eangers an allw te "ld" flud t et wt a ger temperature tan te etng "t" flud. e effeny f a unter flw eat eanger s smetmes araterzed by te Lg Mean emperature Dfferene (LMD between te fluds. Lwer values f LMD ndate better eat eanger perfrmane. 3.3 Crss flw In rss flw eangers, te t and ld fluds mve perpendular t ea ter. s s ften a nvenent way t pysally late te nlet and utlet prts n a small pakage, wever, t s less termally effent tan a purely unter flw desgn.many atual eat eangers are a mture f rss flw and unter flw due t spae nstrants tat fre te flw pats t wnd bak and frt. Heat Eanger Analyss Overall Heat ransfer Ceffent LMD Effetveness f eat eangers Overall Heat ransfer Ceffent Under steady peratn, te mass flw rate f ea flud stream flwng trug a eat eanger remans nstant. Regn I: Ht Lud Sld Cnvetn. w A = Heat ransfer Ceffent f t flud(w/(m K = emperature f t flud. = emperature f nner wall. w A = Inner wall area. Regn II: Cndutn arss Wall k. Alm w w r r = emperature f uter wall (K w 3

4 w = emperature f nner wall (K r = Inner radus r = Outer radus A = Lg mean rss setnal area (m /S lm Regn III : Sld Cld Lud Cnvetn w A = Heat ransfer Ceffent f ld flud (W/(m K. = emperature f ld flud. A = Crss setnal area f uter wall (m /s. w w w w. A. r ka. A r lm Addng te abve tree euatns.. A r r k. A lm. A = ( R R R3 Here, R =. A r R = r k. Alm R 3 = U = U =. A A A R Were R = R R R 4

5 U. A. Fulng fatrs and U values In atual prate, eat- transfer surfaes d nt reman lean. Drt, st, sale, and ter depsts frm n ne r bt sdes f te tubes f an eange and n ter eat- transfer surfaes. ese depsts frm addtnal resstanes t te flw f eat and redue te verall eat-transfer effent U. avd r lessen tese fulng prblems emals nbtrs are ften added t mnmze rrsn, salt depstn, and algae grwt. e effet f su depsts and fulng s usually taken are f n desgn by addng a term fr te resstane f te fulng n te nsde and te utsde f te tube. = A d R f R f = A d / R f = Fulng resstane fr te nsde f te tube. / R f = Fulng resstane fr te utsde f te tube. d = Fulng effent fr te nsde f te tube n W/m K d = Fulng effent fr te utsde f te tube n W/m K U A. R = d r r k. A lm A A A A d Lg-Mean emperature Dfferene (LMD Lg-Mean emperature Dfferene s used t relate te ttal eat transfer rate t nlet and utlet flud temperatures. It determnes te temperature drvng fre fr eat transfer n flw systems. e LMD s a lgartm average f te temperature dfferene between te t and ld streams at ea end f te eanger. e larger te LMD, te mre eat s transferred. let Q UA = emperature f ld flud n K. = emperature f t flud IN K. LM LM = Lg-mean temperature dfferene. 5

6 , X,,,, LM ln( / Fr parallel flw,,,,,,,, C-urrent (parallel flw ase,,,, e, d far d lu lu F Ft ld C us egna Et H ae H,, m m Fr unter urrent flw,,,,,,,, Cunter urrent flw ase: 6

7 ,,,,,, m e d d lu far lu F Ft ld C us egna Et H ae H,,, m,,, = emperature dfferene. = emperature dfferene at ne end = emperature dfferene at ter end. = = Inlet temperature f t flud. = = Inlet temperature f ld flud. = = Outlet temperature f t flud. = = Outlet temperature f ld flud. lm,cf > lm,pf lm,cf = Lg mean temperature dfferene fr unter flw. lm,pf = Lg mean temperature dfferene fr parallel flw. If U s same fr parallel and unter flw ten A CF < A PF If bt streams eange eter latent eat r sensble eat, but nt bt, ten 7

8 avg lm ln end end ln end end Fr bt -urrent and unterurrent flw, were: end = emperature dfferene between te tw streams at end f te lumn (K end = emperature dfferene between te tw streams at te end f te lumn (K It makes n dfferene w ends are desgnated r. e Effetveness-NU Metd A eat eanger effetveness, s defned n terms f te atual eat transfer rate,, and te mum eat transfer rate,, tat wuld be btaned wt a eat eanger f nfnte area. C C ( mn, (,,, C( C ( mn,,,, = Mamum eat transfer. = Inlet temperature f t flud. = Inlet temperature f ld flud., = Outlet temperature f t flud (K, = Outlet temperature ld flud (K C =(m p = Mass flw rate f t flud (Kg/s = Heat apaty f t flud (W/K. C =(m p = Mass flw rate f ld flud (Kg/s = Heat apaty f ld flud (W/K. e mum pssble eat transfer rate ( wll be based n I. e mum temperature dfferene drvng fre n te eat eanger ts wuld be te dfferene between te enterng temperatures f te t and ld fluds. II. e flud tat mgt underg ts mum temperature dfferene s te ne avng te mnmum value f m p. e prdut m p s alled te eat apaty rate. e mum eat transfer s m P - mn. e mnmum flud may be eter te t r ld flud dependng n te m P prdut value. If te t flud s te mnmum flud (C < C, we ave = and (m - P C ( C ( C C mn ( ( 8

9 If te ld flud s te mnmum flud (C < C, we ave. =, and (m - C ( C ( C C mn ( ( In eter ase, we ave m P, -, m P mn p Fr te unter-urrent flw ase f ld flud s nsdered t be mnmum flud.e C =C mn mn C mn - ep- NU - C C C mn mn - ep - NU - C C If C =C mn te same result wuld be btaned. Fr -urrent r parallel flw ase C - ep - NU C C mn C mn In te analyss f eat eangers by ts metd, we defne te number f transfer unts, NU, as te rat UA/ m P mn,.e. UA NU m p mn e NU ndates a measure f te sze f te eat eanger. e effetveness s derved belw fr te -urrent and unter urrent duble-ppe eat eanger prblems. s epressn apples fr C mn /C < In te lmt wen C mn /C =, NU NU Fr any eat eanger: e = f(nu,c mn /C 9

10 NU (number f transfer unts desgnates te nn-dmensnal eat transfer sze f te eat eanger: UA NU C mn Steps t alulate te Effetveness-NU Perfrmane Analyss Calulate te apaty rat C r = C mn /C and NU = UA/C mn frm nput data Determne te effetveness frm te apprprate arts r -NU euatns fr te gven eat eanger and spefed flw arrangement. 3 Wen s knwn, alulate te ttal eat transfer rate 4 Calulate te utlet temperature. Szng Analyss Wen te utlet and nlet temperatures are knwn, alulate. Calulate te apaty rat C r = C mn /C 3 Calulate te verall eat transfer effent, U 4 Wen and C and te flw arrangement are knwn, determne NU frm te -NU euatns. Wen NU s knwn, alulate te ttal eat transfer surfae area. Referenes. ransprt Presses and Unt Operatns (3 rd Edtn, C. J. Geankpls, Prente Hall In. Publ., Unt peratns n fd pressng, R.L. Earle and M.D. Earl, NZIFS (In. Publ.,