Controlling Shell and Tube Heat Exchangers [1]

8/7/2019 Controlling Shell and Tube Heat Exchangers [1]

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IA,eM: TG' BASI:CS

C o n t r o l l i n gUr s

A l l I I I l b o l v e 5 h : e n , u m d l u b eE~,l i ro ' l !ge:rss i i .p l~! : > q p i ' [ l i i n e l l , I b ,W 'I : I :~ e rD ~r l l d i g , e rb e I a n

c b a

11II1,rodUCi I, io :n,Sh""U and tube heat exchangers arean10rlg th~ PJ.lore oo r u f u . s i ngp i eG . e iS ofeq]u.iprnerrt fur the proeess conhoJellgine~r. The prim:.iJp]e of operation'issimple ~nollgh~. ' 1wo- fluids af d : i f c .fel'-ent temperatures are brol !J Jgh t ]n:w

the :inlet of t l l te ether, Th.e J]eat 0011"tents, m e aU IlP ZY e:: i i !cl~angedfrom one .t 1 ; u r i ( ) j . ' f u , o , the (ither mel vke versa Noe l l . e r gy isd!d .ed o r reoo~dLS'lnoo the b,eat de l l l ( J J nd s - v f 1 . 1 ' 1 1 3 1pDO( le ss:a :t ti e hot CO l [l sr ont , Rnd theJliea.t 0011JOOi l ta f thefawo : l l i ; l i d . > S ii s not

e l e s e ( O l l 1 t ; , . , I i I : : t b ut a r,g prel.le.1l!tedfrom.!Jlru:Jng bya ph~'sk:al ban::ie:a;:,JI'hete:m,perst!l,l.r~ o f t.h~ two ' f l iL l l ,d .s \ r i l ltend to qJuali'lll!l_ By al.ranglngootHl !J ta re~;m"a l i t nom ],t t s po s s i .b 1 . e< fo [ t~,g temp41'


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Where Do WeMeCiS11Jre? 'At the fundamonta.llevel, there :isonly one variable thatcan b e COII'~i'OnL\ld. th e amount o fheat being exchanged. In pl'act.]cals i tu a t i on s itis not p o s s i b l e to mea-sure lu~a~flu", It is,always t1l(~tern-perature of one Uuid or tbe otherwhieh is being raessured l ll jld 001'[-t l"olLled. It is not possible to controlbo t - h since the heat a d d e d from one]s ta ke n f 'iu rn . tile ( , It - hm ' " Th e l ' e fo r , e ,tlile first con s ide , r a t i on is to sp~cifytbe place at wh i c h the t empen r t . 1 ! l I " eis 00be kept oOllStant Thisis usual ly withiu ap ie ,ce ofe qu ip r n e n t s o In e w h e -r edownstream of the outleto f o n e o fth e f lu id s _A s s um -ing th~re is fiot much tern-perature change a,long thepip"mg, the measurementm a y he anywhere b e t w e e nthe outlet itself and the-po-jnt of interest, perhapsat the base ofa distillationto we r _ In c a s e s Wh~ l l ' ( l themeasurement is beingm ad e d o w ns tre am o f 'a b y-p a s s va lve , the fmoth,eldownstreera, the bet-tel't .h e m i x in g will b e , a nd the,

eoaatant ,either., theheat eiXchangetrmustbe de 's i gned Po, r th('lworst case and mustb e o on tJ :- o U e d to m ikei'~O] ;1 < 9 ra te , a ,t the par-ticularrate l'\equircd.bJ~th~p ro ee ss a t e ~ re -[ ')'mumel1! in tune, Thfll.hea exchanger i t s 1 2 1 fis not constan t. Itscharacteristic changeswi t : l l time, Th e m o s tcom.mon change is areductio in th-e heatt['ansfeir rate due'. tofo ul in g o f'th e s ur fa oe s_Ex.challg~rs are initially ,oversized to al-low for the foulingwhich graduaUybuilds up during useuntil the e ; . :cnange l l ' is1110 Jonger capahle ofp e r f o i rn . i n g it.s duty.Once it hag, beeneleaned, it is againove r s iz e d .

h e ll a n d l u b l e hUI ,1l e l o b a n g ,e r s D i ll e 8 d l O n g im e O J O I O , G O i D I l'u : s in r gI J l : e c e s , I II " o Q u l p m e O I . lo l rt h . e p r o l c e s s I } ; o n t l i o l .eill'gl'n88r~

m o r e r9'p.res1e l l t


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/!.,m'on,g this ~l'O:~i:on (Jf a l tem.a : :~~W5. s om e 1Eu s t . ti e : b e :E t e r . th 3 J ! l1 J . o t t. ~~s" The 'j )' re . J :e :n -e t l cho iioo . d ep e :n t l s , as~ Jwa .y s , .fiiltJ!i.e paLtJ iCW !3! l' a i tmrf J ioo"Th~re 'are a nmmlb'er 6 varieties.tlf the bas:ic~hen anutuIJe exchr t l l ge : ri l ia t ca .n . beron,troUed a lo ng s :irn .ih l! ']ines. Plate ex.t:hangm~s O'I'ISlst of! ; !h i n s h e e t s o,feb11iil1;ga.ted. m c i:a .t T heQO'lTU~tj~[!~ . fi"e fl)\I:In~d'0 0 prod.tlG~p~ssa~s 00 i l 1at: t h ~ tw.o f lmi ld:spassi I D ! (ippos.it.(l (ijl">actioHS


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200

~ C om b In e d f lo w

16 0U lO

50402 .0

o ,2 5 5.0

neither ca se do we want to i n t e r r up ttI le altai f l ow , If w e 'cv1 sh to miDj , m :i zepressure drop J a buttrnf;iy valveis the likeliest choice, H ow e v e r , evena wide open buttedly has, some pres-SC1IU'edI'OP, It m a y b e i ; 'l" eJ ar erth enthat 'of the heat exchangel.' itself~I'hisfI),eansUUlt even w' l l en the valvei.s wide open only half 'the flow crh ~5 ; S .win b y p a s s the e xC :h an ~ ;r , 'T oa~ompligh a ) ;" reare l ' degree (If by-pass, arestriction must he'p]ared ont h e nowthwl~gh the e x e h a ng~l'. ' l l . ' l " l ! erestriotion should be adjustablesines o(mditiffJlS change and w e donot want more restriction than nee-,e~sa.ry,The easiest way to do this iswith a hand 'valve. Since these valvesare often in rela.tivdy inacceseibleplaces, l ' e ;mo te , a,c tu~: ! . '~O"l "S I J 1 l l a yhe.added, Once that is done itbecomesan obvious matter to arrange auto-:m~~tk:;contrels 6(1 that once the l Jy-pass is fully open. the restrictionvalve starts ~,oclose.and vice versa.This is. of eourse, 3! split l"3JIl,ge.The valve positioners, or TIPs , arecalibrated so that 0 50% signa]opens the outlet and 50 lO()% sig-r U u c lo s es t he bypass valve. Wi t h Urisarrangen'n.enL, at leest one of theva lve s ' 5 fully o p en a ft.all times a n dthe effective C'l 1". '3JF1gez from _100%to 200% of that of a single. valve. Itshould be noted that by alTangingfol" miolm.um p r ' e S S U 1 : 1 e drop W ' B m u s t

accept that tliepresS,lwe(E1:1QP,andconseq[u,OOJ! l t~Y ' fu e .now , \ i n n vary ast h e \ 'a J 'l 'J il po , s : . i . t ' 5 ! ( i I r ; l _ ~ d } ID i g e . ~~otea l s othat tbe,re mnst b e , either a single IfP tubed to hath valves, 01' two liPs(or positil)Ders) .must be wired in se-ries, Eitbm' way, special care mustbe taken dur:i:ng ,cQl1sh'lll.ctioD andmamtenanne,'l 'h e , ex !'l m . p le shO' lo ' ln in Figum 2sh ow s th e Fa i l: O I)eO valve 0C llo t ' ! . t .he'Que in the bypa!;"s.Let us assumeH u t t the p '1 ' o ,c . es s stream. is beingheated, Th,e failure modes ofthe tw ovalves is such th.flta signal failuret . f , l either, 0 < 1 ' both, valves will I'I!!:$ultin less heat. beillg dcl . iLer ,e(~to , tileprocess stream. Failure also meansth a t t1 he m . ed iu m W:iUIlO't b e coo, led"T he o p po sit e f a . i1 m : e re sp o ns e is e;3S-i.~yarranged, Iti s a m atte I ' o f c h o ice .Once tha choicehas been made, theoonko] aetiea of th,~ eontroller he-

H S QU ne l 'M08nillgllo iSS 1 08 ,D o m B I t o COBIJo l l l ea ' D e l SS I l e l D p e n r m r e billU J ro W i l l l IU e K h !o rU le ,ln [e t , o rl i e o 'U U o l lo l ' [ h e ' p rO ,C I SSf l U l :d . .

44 INS-itiUf,!HoITATLON &. CON.ROL JOuR A~. JUl'f - AI JGUST 2001

OOT Ii JeSa ma t te r ! of ~ lle d .l !l ct ;i .o ]l: A :s s um e th a t 'tb e p ro ce s s s t r e amQuu,et Istee ho,t.That is" it isa : b o , Y e t h e , set-point.'. Th en 'U liedtlNi.at.ion O\ f th e c o n tr o l -ler is I)Os~tive. ' /i . .o ;s um e , tn!'} C lo n tx ol le ,]' a e uo n i s,po~ i1 l . iv ,e . T b Is ' p : r c d U ~ 1 i : l 8 a r ' r e i ; n , gv ,o h r e o u tp u t s ig n .a ) thl;l!t will tendto open the Qutlet and close thebyp3 ! s s .

'. I tw O 'l .d d raise th e l iemperaol t .u l"(!of the :process steam. WJ.'Iong!The. l r iu : u a t i o , T ) t s gHtingwQnse._, Tbe CQn . t t o n ~ ' 'must be clmfig-ured to be reve;rse acting.'. N ow a rising Qu t le t t e m p e r ar u ze\Ii':ill e a . 1 ! J . E ! e a fumng valve :s.ignal I t: 'will open t he b :} rp a ss a nd c lo s ethe outlet... This win l owe : r : the, t empe : r


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80

-~~ 20. . . . . .d)(L

e .f )l lc ul at io n b l ocks to 1)11:1-vide opposite eon e r e l ac-tion for tile tw o va lve s isvalid, with one dJiffe:renee, The s]gllal ia notdoubl ~d audonlv oneb~oekis needed t o l 'e 'V e r s et . he signalSome comraenrs Onth ro o -w ! ;I .Y '! (! dve sa re inorder, F:igul!'e 4 shows. atypi.cal eharacneriatic.Th e flo w 00][ nne side de-Cl:l%l~eSgradually ,a.~ thenow tIhrf,l luglf the o t h e rincreases. I ( i} l1l1JLy thenow thrlQugh the inIetport is'emlstant througfr-milt the entire stmkingrange oftne val.ve. Somet il r,e e "w a ,y v al ve .'5 b a v e aiather hn , " g egal) b e tw e e nt he t w o e xi t P ( j. l .: tsw h e reboth sides have morethan 50.% flow. Suchv al ve s s h0u ld '0 0 avo~dedas th~ result in s lo rp rpy eontrol be,'canse the kmp gain near the, m i a ! p oin t j.s 0 0 0 sm a ll , Th e. ca m e cmul i d . 'e l ra t io n a ; -P l lI ie s to t h e tW (J .- va l ve a r -rangement, The two val yeS shouldbe chosen to h a ve fr ur ly Iinear cha r -aeteristies sothat the combi:ned. flowis approotimately constant. Itmay be

useful t o . p r o t the two characteristicson f I I . p ~ , e e e o f graph p a p e r and add!them up to . see jf th e va lve eombine-tion is a;a t iafacoolY.A . close l o o k at . F'i gu .r e 4 ; sb o iW '" :i !that the Cv of the two p!)ds is notequal, Thi s ]s aL rno3 t alway,s the ca s eas the ' V a lvestem interferes 'win!l theflow t:li:rough one of the ports, Thepo:rt\orith the greater C :should be

t, 'heal - vo'P~nto the eat exchanger. TheJlthe flowrestriction caused by tile

100

Op ' p om teAction.1he second!arrangement atb\lmpts tokeep the pressure a.rop, and hencethe flow, as constant as pessibleacross 'llie enth~ ,Srr.Ok]Ilg nmge ofthe va]ves. To a1;:cnmp,lish this,\'v(lwould. like the outlet valveto becgiuto close the moment tba byp,uJ:5he-gins to OPIllIl._. Itwill be fully closedthemoment the bypV!ss isfuUy open.This is an ~oppi},Siteaction" arrange-msnt.A three-way '''flow spUtter" or"d iva r t e r ' val ve i s f i. "qu e : !1 tl yu s e d tocombine tb til funetioJ:ls of the twoli'flIlvesinte one, body. For h~ltte:l"fl.y" < l h ~ e s a n~ech.mieru l in _k r na y b e m -

Tbi~i s n o t a s ,t a nd ; ID 1 ; odt ea -t.m:e,but itcan be ea,sHy3n'lillng'ed wirth a, DCS.H : e n . " e is the, way 1 ;0 d o i t . :Send th600njl,ro~]e.l< out-~ut to two c;ah;.u1atiulillbl-oc:b. The fin;t dlluhl;esthe sig,n:aJ is 0 tha.t therange 0 5( l%I ;J .e l ;QmJ i . ' !S0 100%, Any sign,,,] be-yond 50% is igIlo'redi , 9 , 1 [ ;th e output o ft h,e e a lc ul a-tion block cannot exceed100%, Th.e second blocksebtraets the s ig l l a : l from10 0% a nd th en d oe ble s it.The range 50 HW%then becomes 100 0%.A:I~y signal below 50% isignored as the output ofthe caleelanon block can-not r a n be]ow 0%. Thetwo outputs are then seneto their respective valvesWhich output goes to Fl'g. 4: TImll'e' Way Va/ !fe !Curv~s .whicl-t V3 ! l ve dependsenthe failure mode of the valves. N O 'Wth a t 'I o o thvalves DOW o p e ra ts o n 0 100% $]gna~s.. Son)Jl models afDeSprovid(l f o ~ 'scalin.g and l inear iza t ionillthe OUtput-Dlodui les. U t J r is f e .a . 't " ur eis aveilable, separate caleuleficnblocks are not required.This method of achieving splitrange action has additional advan-ta g es b e sid e s .a llo w in g-the two . v a lv e sto have the same failure mo d e . .Itc o st s a n a d d it io n al : OlllJtput s lo t,a p airof w i r e s awl 3I]'lW but fheadvea-t a g e . is : that t l! le t w o. 'V ' a lves m" e ~ S i e l f -contained" and do not r,equlI'e anyspecial treatment with respect towiring in series, mounting of liIPs orsplit range calibratlon.

o 20 40 60 80

stalled 00join two v8i}vesto one. ae-t u a . tD f i ' s o ' th3i t " ",h e n u ne o p ~ n.s , t l1!eether-closes. It]5 also IH}~$ible~ usetw o se[)1ill.rate va lvo s with ( l i thG '1 :asingle liP, tW!() lIPs in seeies, (W tW(lseparate (lUtput signals, The deserip-tioOnin the previous Sl.'lcti!ollon using'

' I h e s ~ m D I 'e s t 'w a :v 1 0 1 c a m1 0 U i t S U l m i h :8 1 ! OD J l r l o l l BC l l i D na l n a l l l S ~ S~ s 'to ' u a c e a u r u n d i.6 : liD OP~ I ' : omd i em e a S U l e m e O I b J l m eI c o lt t ro l le r ; '1 0 'd i e u a b B , .,a n db a G i k ' l o ' l h o m ! o a s u r e m e n l .

NEW ~nA~IlTII!\iJ:tlWl ce s r mGILU tEVil;LIN~ro~

~.\IOD~ N b . _Gl ' . i I ' l l ' : D 1MEMl ' 1l 1 1l .' 1i ~ U I OOMm i tS I.lfRnc"'-l.HOH I r nH fA l . i 't ' f' E ~';\[m ~ I ! IDA~M ~),I~ Ml\XlI l ' , l IMtp o il m R . . , D I ; !. l, :n ! E~M I) !> iE ~ ~ S , \I'A,~~ A l h" n MeH , -O I l R Y I W slW W I J1 !! P! .~ :u a l ( C 'A l '1 l l A A ~ l o\ ' I Dl I'\It:;C D~ pm O R s s, A N~ ,; ,j.O O ~1 lC "''l 1 'II 'I 'IE~'.~fflf lOCilllttil ! :i !l o 'i TAc r l I :. 'im M; l )) I 1\UM ~ lE fl , u"TUIilSINGLE o m . . 3 . I J O LWOO 1 'I 1P E 1 "' ~! .: (l \M E ~~ iI lS " . ,w; e Mj~ ! I ' iWO SS fl P I ; ! '' ' t i eR-$ . . n l E ~ P . l O C { i l 1 J f l Er. 1'I ir;lMIilOl l lW~1i A"S~Mel"- \ lin (ul)TI'!l tw o' I 'f RA T U H ." C Q I 'I Tl I lO ! -l el l _ I 'I 'IES~U"E ~ A G t M i lACOIlS~, le'IH. [ i i tMORS,

gTN! S'CIEmlHC; 11\11"~ ;,; e 'IO W', ~ 24 . A.:'11""-$11;~t ~Mf9, Mnm (I'll. M.. .. ,m - ~oo ~I,G,I~r..2'(4; j~ r.M~. , ; t 4 ~~1 9 5 S J . ' I ~ 4 ~B ~ J , g c::~;p

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INSnmMENT"'TlGllll OOflHi~Ol JOUR~AL .' .JjJLY - AJ,lGl}ST ~ a ~ . 4 1 5


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exchfllngerwiU help to cancel out thedi . f f e re : oce_Medium Side ThrottlingAvoid using aproeess s]d.e bypassvalve with fluids that are beingh ea te d! a nd h ave atendencytn breakdown or scorch. These include manyfeed praducts and also pe t r o l e umproducts ()j'otherchemieels t b : : 3 J t mayp o ]ym e riz,e o r co ke a t. h ig h te m p eJ !'a -tures, The problem ]s that the (],lilt-.~et.temperature is a blend ofthe hy.pass s:~eam nm;l!the stream tlu"Ough

the process streara at a oonstantt . empe r a tU l " e ' , There is no I " e a s o n tomaintain the flowof oi] in e:x:cessof","hat is needed - it. can be tllnottledW C l o n ' U o i the temperature. In thiscase the valve is placed. on the out-let of the exchanger ..l "he valva is not'expected to handle a ]arge pressuredrop nO r is tight shutoff o f flny par-tleular value, TJle,refure a bntterllyvalve is !Juite acceptable. Further-mo r e its ]OW pressure drop (high Gv)when wide open is an ad vantage.The ,aff~c.b of inlet and ,outlet

@I

D - tin'le d'elay

H aL Oil Medium

0-

Fig. 5: M e ( / J u r n Flow 1 i . J o e S id e Ou lle t T h ro tt .lin {J '.the heater. The peak temperatnm towhicb .9iny part of the stream is ex-posed may oon~iderab]y ex,e-eerlithatof the combined outlet. Over-doneand. half-baked dontaver.age eut!The extreme ease is whenthe ex-changeris 011 fun bypass. The fluidtrapped j l113i(Je the hea t e r wiil l thenbe at the temperature of tb,e heat-ing medium,The: solntion is to CQn i b - o ) the pro-cess temperature by throttling t.heh.eat. e x.c ha :n g e m e d ]1 !l m . Ill.this casethe heat available to the precess ismantpulated ..

The example isa aimpde a.ndrather sbaiglltforwm'ci application ..Hat oil is being supplied to heae ap rce es s s tr e am . It i s d i@s l [' e id 't o keep

heh l two8'H}chl lngel 'S ares l a l C : ' l 1 d l ill ~ s ! e r i B I 5 , l 1 e 'O f \ B o B . B 1 I B o u n B c r e dR a n g l o ti D D a n g o .

46 , l~smU"'ENf"'TION s CiJNTReil JOURNAL .. JUl.. .. AUGuST 2(1);1

thl:ottling' me about the same, sosec-ondary c!O:llsidarabons, come in topbly_ In gener.al it is a good. idea tokieeptihe pressure (In a . hot fluid toreduc~ any chance ofdissolved gasesbubbling out. A va]v!:lon the coolerend may he cheaper and wi l l prob-ablYlast l onge . r . Leaksare less likelyas the.flwd wi l l bemore viseous thanonthe hot sidle. Thus it is best toUll'ottle a heating medium. on. Uleoutlet side,It is rare tha:t a heat exchangercooling theproeess should have ar e a s on not to use by~)aS;8 .control ontbe process side. Al so , itils usually

ll!]]}desir~lble to t :. hr o td e c o o li ng wa-ter sinee it is at ]005,1; mHd.ly corre-sive and ]s, seldom olean, For t J : - - U 5reason it . is 1!1sWfdlypntthrough thetuDes. In order to i m r n "Q~ heat ex-change and also to avoid thG build-u p o f d ep o\~ i:t s a nd fo u.1 :i : l lg ,it is bestto ma i n t a i n Its ve]oc l: i : y . .If ~'tshould:. be necessary tetbI'o,ttIe the cooH.ng medium,eonsideratien must be given to thepos.si.bHity of bailing. (RememberUla : t the coaling medium .fs the onebeing heated~) Assuming that boil-ing is n ot. .n te nd ed , b ut t h < l i . t . th e p os -


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8]bm~y e !K i: s tf l,t h e v :a ~ v ~ ~hQ1!11di bep l a , e e d oil th e o u tle t]n o : rd e r t io :m(tlli.-f u a i : n . preSSUl'(l 0;11th e flu id , lID .( i l J h e l I 'wu rc l ! $ , i n l e t t b l "Q t tl iu ; g i s m re ly u s e do w t h s in g]e ]J lh as e f l l l id ;s ,Cvoss Exidhangal~5Wh e n the heat in a na ] ')lfOOO 5Sstream .]i!l to b e eXic~hllTi !ged!wittt a ;n o ti i er 'p r o-cess stream, th e flow Oil l ne i t l i . e r sidem , '1 IYi nt er fe re w i U lI w h .i le cOT oJ tr ol lm gthe wm perature, An e ~ a .m p]e ~ :swh(!i[l di.stiU


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te;m.ll:~[:a.tl;lIXl-It: has the ad!va!l~t a g f . l of' l - e d : ue i ng h!?l:'.s~powe!' a -sij;r~ COO iH~g G1 ; e ll r aTh~ ]S E e ( lu o o c l.P m l Ividl e v ; o o : y ( l o ' l ) i t i l : ' I ) ) l i oocJil ' l l ique,t i h e J ; ' ! 8 i Jo :eJ~ !n] : im!~C) , .M,F i I T l l i l y , thetn,lnl,(lpwfl i S i n ' J I t h e r [10M. T h i s 1 :9 :~s [ l ee i , ( l l l y ~mlPOl'taJlt in a fim ; th -~ r n e 1 i m . .\l " ~ ,w i l w l" e i. t ma l y Ul)t b e ,[po,'l;siblie~Q~um down the fansSlIi f f ie iantJly in ' i ' l inte r . The sp . i n -1 1 1 1 J < K MflAoo ,sui s t i r U [r i l Ie aireven \~'h~lilthe pi~ch is z~e:l'io.N atm a l d ra ; i t a lo co s m ay . !y1 'ov ic i emore eooling than ]!l : reQlUi 'N,d ,Secondly, the pitch' COtl . t : l o lmechanism ca l l be, a mainte-na n ce h e a d a ch e , Th e Gontm]s:ys-tem angineer ml1S:5 e;'::~lm.kJ.Elth~eq ulplff i.el1 it dt:llJ.wing's to be S 1 1 l ! l : ' > et l " l a l ;Mle meeh an i s a r s , liUJinY e a s yacOOBS forh!lb['ueal]on and. repah~ ..It is a wise idea to. put sepa.I'at;eWi:; all. eaeh fan, Long stii.ngs 'oftubing with m3i[!)Yteesmakeleakdeoo[)t.iona nlghtrn,al'~' Ea.c~~TNl 'e q:u ir es a e p ai a .t e, G 'u .t p ut t1 J ." O "Wt h e e c c n tr e l ~y$ l x lm . a s th e C1Jrrer : t t]QopwiJI not WQi.k ,]F tiher aremom than two in sertes, Notethat if a sfngkWH tl"O Ue r d riye smultiplf! f< ' ln, pitch eontrels, t h e ,P)"000,.5S gain o f t he lo op is I ) . ropor -tional to the unmberof fans in.s s rv i ea If th e ,co n, tw H er i s '~ ~m .e dwit!'J; only half the 1'8]15 I')]llriing,it m a J l' g o u us ta b le wh e n th e ro es tm-e turned on. A can t ro l l e ] ' thatis' tuned with ail falls,rmlningvd ll b e . s lOlf iW if s om e . a r e turnedo f f . It i s ' ! o f c o u r e e . p e s s i b le t o OOn Ug w"e au t :om a t i fc , g ;a i noom : pe n s a -

F ig . tLAen< i l' 1 Cooler LOWtef C:on.tm!s,t h :m wit.hj~ta DCS , ~e l l i b e rto .ch ee k fl)[ d lv i~ l e ' i b y ze ro when~J O fa ns a re :rm :m .m g .)

" V a r~ ~ aJ :~ ,]~ eS p e e d : FU lly lfiiJr'wb]e,lli;nsp~e~:leon . t I"Ol . is h " eMmi r i g

.! swi. 3 1 1 1 1 e n e r g yIIC 1 0 V ~ 1 1a n a l li l! J ,e m e n l &1 1 0 I I t I Y 1 0 S Q c ce !s s I IS I DcO i o k o ! 1 t h l e I h , e ' 3 1 ,r e r C ' D V e ' I ' V w l ~ h o md i s l u l r J J i : n g l l h e pr0geSs .

mere oommoml on ~ecri.al coo]ers.'l fh t, e f il n I D o tm ' S an~O O I ; ~ l ' I , qui . t e;I)UIDU'OUS b ut(l! ot e ixt \i r e,m a lylat:ge. ,HoweVCi., tJ1!~ ( lost or~bee. l t ic t l -Oni 'cs h a s : Q o m ~ d J ( f jv l 1eon-s l. ti !e r a blY i n 11lMIt'lnt year s . 0neway to cu!l tClQslfs1sb) OOooecthO i~Fa ns o f o nA b .. .y to th e s am e s e t o fV..f'D etc-tronieS", On the o t h e rh


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'. Lol!IlverCQHtvQl: Al!ItA;n:n.a,tio' lo!,l!vel' C ! 1 f i b : Q ' ] 'lla'S similar Pl'o i l : ; l~~lmsa s fan pJtc;hooriJWL ''Fh~J:1i!is @.u.ai tmi . t iQn(l1 p 1 : ' Q I h l 9 l 1 1 l (if,hiYstiql~s~~-The 1iDl,!.V;Q,l"S s~l,cl~Jm 'mXli~;S"I~ootWyfew long and ithMQn~~ d ! ifficn1 t to maintalll s t a b l eC O F f t " l: Q ] as . djr't . ; Ind ! " 'l i' ( la r a C ( l U . n m 1 ! . l ' -kite a'ilel~M:IiIJ,% ~sp~i)!l.f).nyi,QS(llnrd:y 91 d]J! s ty (o' l 'Vi i_] ' "Qrm: !e~l ls~In ~h~at;e,swith stl;i)~gS~MQnflilt . e l m : p e o c a t l J i .! - e s ' W i n g s , iit j S [ p . O s s : i I : i l ! c : !w s ta bil :iS e th e a ir te m p .e il. 'a tl]1 'I eand p' le :vent iditg by oo. l1 troUingi nt er n al r e c: i' re u h 'l ,t ,$ o .i ti l. ;T l lLe ex\~c . lmng ( f is f ii tt ed w iU ' l a due~ lead-iing f r o m the Wp { f U U . e l \ t l o t l rH~bot-wm inlet of the tllMt, ])ampersal!',ep~acedin U'lii~d:~daThd~t 1!Uila ir i nt ake ., A te:rn.pet~frure C.O] [ -t r o :U e r senses U te air '3!buy;e th ef a n " " ,n d c o o t r o .b ~ i tb J ( o p enm g t h eree i reula : t . : i .ol l d u ct 3 Jf ia J.s im J J .~ t a~ueous ly ' d o s i ng tb e ]:ntake_ Nqt:eHla~ an oppositeadim.'fJ. an ' ange-merit, as described abov:e, I:sapprop1r1a.te, F:igu.l'e {)shows a po~sfble al"lal1.ge~~nt using bot~ol1det~ouver' p"$IttI 'c/lrQrl:! thep ro oo s:s a .n d r el1 'im u i aHnn oonb'O~aff the h:i,mmalair Wmpera.tul."e.Adva..r.,d il'riic:bFe ' e d fu l "w , a a ' a !lr~rg'eh e ~ t e x. ch 3i ng 'E jr ts h a;ve b ot .hd e ' 3 J d t i m e a t l J . d c 'o n s :i d e l' ab 1 e i li e l; m a li ne :t :@ ~ , T lt ,s ,s e t w o f a. ct o :~ s c at l, rr ia 'k !eoonri~l a~ffi:eldf, Fee ' ( fGl 'w~rdc : anbe use:li;]]y~wlied itlo~d,d!anges

H o I . O l i M e dh rmt

F~g~7; Feed . F o . l l ! : l ' a f c l Control..a'l~ a p : j. "f Ib ~ . e "j l_'Sines the hBfJ i t ds-UlI .ani9. l iS p.l'OPlJyti9rta.l~o the pwooss:f1IGW r < J ) t i 2 , . o t h . e : r ; th i ng s } }. e j: i1 ; g equa l,

,< 1 f !Q iW l " a < t })m e ' ( tS l ll :t 'em" l ll t can ' b e :use~ t F1 gu~ ~e 7 ' sl lm o , ; rs a ~Y [ ) "i ea lar-r a~ e m e !I :! Jt .. N o te tli~t,t,hr oU:ti: l l l l; o ft l! e ;' Tt J k . .. .. .m :w t ip I i e c f by t Jy e f lo " ll 1> \ 's i g -lVll Th~l is, he.6i1JU!lle the he : a t : l n gm ed hliru H Q m ra"b: : m u~ t :b e . ro ' llghlyp :r op o lC t~ o n. al 0 0 the fe-e.cl.flow. T.~,w o:iks h es t ifthe [il.sw]F~dcharac,ter]s:[ ;ij : : (}ft~ e .. ~ lve is " [ in e IH . ' , . If thee:x.lCh!:HiLge:t]S y : e t Y c large,. it m~ bene . oo s s a ry t.0 ' in 1 :l e xt a ]agot s o m eoch.e : t -fOirmof d ! e 1 a y in .k! th e flo w : s . lg~n.a1~() ~m~yellt];~Fmm . , ~. ef !: i[ !l, g BooiSjJUI1I . apd' C a li ll si n: ta l 'C " .: tH " S ft < sp i k" " to':! tp p e{l F]1 ] t he rem.pe1."a.tllr~..m o retna . t the -( lep ta time-is f]]ver~~I:JEI'O-portlJo'nal W the, fto'Wl."8ire a nd . s rom. .e'1y'pii,caf~val1Ll@ m.l!lst be u rs e d. , B :o m . e. o : r : a n d ! s of DeS have the option at"a'\!'ariab1~.deh!y tim:. "l'his anowS" de--lay ro be, in1>'er~dy pr()pmTIon!] to

Te :m . r$ l e ; tQ ty . r l, ~ ( i ll l , tiB l i 'Z a t i o [ t lA . ; ( 1 o : t lX e ' r. . M W 'f J . 'n e e d Trick"" i u : v o l ~ sop ' t lm i ' t a t ; i on . of tl fved heate.~'. Heati s b e : inQ>5u~ ! pUed J 00 t h 1 3 r l'e-boier o f a(]eretllia:ni 'z!I ' ,as shorwn in Ftgl:l!ffl 8 "J,t is rlUlmred t o - keeJ)ijle tempei l : 'a-t . - 1 ' J J l t l a t t1 le r b Q '~ I D ln'OIH-I tewwe:t een-SCrul l i , T lhe l l e a : t i n , g m . o o iu }l lI i s hot .o l l iw h ich is b e ing heated bya fired.heo\ite:r


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e : tI e ct I s t o m a i n ta i n t i l e , fu rn ac e, a ndt.he hot oil, at the lowed tempera-tu re {:onsist,ent ;widl th e 'h ,e at cle-, r n 8 i :Q ; d o f b l : l , e OOWC[', U\1l01:kll a s [0]-l ow s :. ' A s th e h e a t d em a nd r is e s , th e

VE l I ve opens furtherI "Vherl~he valve is, opel'l l heyond80 %. th e s et ,p oo nt to th e f u J 1 n a , c . eTemperature OontroUer ]Sraised. A i> the temperature oItbe hot oilrises, the valve dOllOO to !l'Barthe80% value,'II Ali th e h ea t d e._mand. of c h i ; ! r o w e rfalls, the valvecloses below 80%..' AI; th ~ va lv ,e c l o s e s " t h e ' s@ ; tp o i nt' to t l ': lef u rn a c e i s low e r e d 1I ,1nt i]th evalve is onee again at.its 80% tar-

get.In t h i wa y fu e tsUlp: I 'a, l ;m-e o.ft.hehot oil SyG~l is kept atjQ; lowestacceptable value and a m i n i m um ofheat is 10'5t by the fernaee Of tbe. pip.I n g .

Combination Contl:'olSOiluetimes aheat exchanger is US(ldito heat, creool, a flul:t] whose wtalflow ]s beingcontrolled by SOn-iBO:t:ha:I~ pi8:rraflllat(l:!:'. The mos t stmight.fOrward . w a y of con t ro l l ing this is touse a three-way vailw!, or t w o but-oo, rfl ie :s", t o. c on tr o l t he be a t exchang eS in d to us e a no th er valvGW con t r o lth e to ta l f l ow , T h e f lo w { '. Q ntr o ]'\'alvem us t b e o .n t h e e em m on lin e either'up s t r e am Of d own s t . r e am o f the e x-

changer, This 3rt


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ooliaiin th:altthe W : 3 J t : e r dees not boil,w e win p u t theva lve s, o n. th e outletside. Tile valve oontrollin,g the out-let .of full 'IllN:Chaliget' raeeives a 5~.g-n al e qu al1 to h a lf th e s um {If t i le tw oc o n t r oH e ; r o ut pu ts , T li le va lv e ,e o '.l !l t- ro l-l ing t..he bypass reeeives half of thed iffe re no e h etw e en the tw o co ntn ~J Ier outputs, Assuming tha t the fn-stalled eharaeteristic of both valvesis Iineur, the eombinedflow of the:t~ro valves isth en . ,d .e ])cndent entirdyon tile Flow Controller.Th e d i ffCl 'cne ,e betweenth e tw o flew s is dependent011 the Tem p ~ ra tw ~ GM "troller. 111 this Pa!li:iCU]ru's i tu8. ' t i on it is desirabletha:~ both valv"(1~are failopen, If the failure modeof either, or both valvesis faa closed, U1iJr;lsigns ofthe eunnnmg/scalers UY'A and UY-B will have tcbe changed to give theP[1J1pe. l ' result.EquipmentPl'lole.c:liDnThe usual shell and tubeexchanger has no mov:ingparts nor any Inpnt of@}i:-temal energy. Thiel"e arefew machinery protectionissues. Severe corrosion issometimes a problem_ Ifso, eorrcsicn detection de-vices may be installed.These consist of a tbinwire or film of Ulie samemateria] 3: S th.(l!x,changer, The wire lS heldin a h ol Id er that is i rnserw,dth r o ug h a i l J ) i '62 :1e : into th eexchanger. Two electricalcontacts are accessiblefl\)111.1 the outside. Whenthe neaiseanca is rnea-sured, th e e xte n t o f corro-sion call b e detennined di-l : 'Ceily .

1 '} H :~ )r ed e v ie e s a re l " J . ' O tnormally connected Into ad ata lo gg in g n e 'tw o rk. Th eusual practice is tomakethl meaeurements ,villi apontabie monicor on aregular basls, lut.rins:i-,e. .allysafe m o n i tcrs al'e11vailable fo r h aza rd ou s

lecaticns,Aerial coolers require protecticnfrom the e n e : r g y intocwuced by the'llectric me ..tors, Tll.a most seriouslIazal;d! I S a t h F{ lW : I l blade. The result-lug vfbl'atio,n is quite severe and cancaliS'! extenaive damage, A simplest>Jsmic'i'ibrat.ioo. switch mounted. onth(!;$UuctLU'e that holds the towerbearIng of each {an is (ju.te suffi-cient, It works by ha\ring a,smalil wejght held in plaoeby a magnet aga.jnst.thefO ' l1 c e , o faS : ! yr i n g .A "bump"

d i s l o d g e s th e we i g h t fromthe magnet and allows itto open the shuedown O' on tact, The usual meehod of~caHbraho.n" 1: 5 a lightwhack~'i'ii,t'ba hammer. Abutton allows the operater1 ; , 0 reset the ,switch bypushing th e wei,g,ht back


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T'ha:t maansthe ABMEBailer and Pressure Ves-s e l ( J e r d e , S e ' ct i o r .l l . V I I I ,PL'~S:s.U.r'{! Vess'~]s" PartsUG-125tio ]86111 dealingwi til pressure r . e ] i . e f de-vices. TJlhjs specificationgives Vel,), clear guidelinesconc,eming all aspeets ofpressure reliefrequire-ments and ap'pliea:tion"Fresaure relief nm~tbe provided fm' both theshelland n be s ides, If thesoures o f we:fj:'u;essl.ilI"fl isfrom u,pst~eam. the reliefvalve for- that stream 'isbest p l a ced (in the : inl lQ;t.Otherwise it dues not mr;l~ter n1ucbwbeth.,er it is onthe inlet or ouUet so ~Ol)gas they are ins~d.eany oon-trel 0)' isolation valves. Itisn ot su fficien t to putminimum stops on thevalves < J J S t h e s e arc eOl s : i ~ ya l t e r ed . Even if the SIJCl[ lSare welded in place, thevalve m ay be replaced atsome rutw"e dam and theInodifica~o]1 [O) go t t eu . Ifc a r e fu l anatysis s h o w sthat there are no process,fire, 01 " f~ i lUl ' ! i : oonditionsthat eould possibly re-quire. relief valves, it isstill skunglyadvised to in."stall thermal 1" ( l ] i e f soD .both sides of any e : i : i . c lumge l l 'th,at iscapa~ble of being blocked ..in. It maybe argued that t h e - auid is gas or thotthe prceess is 010t capable of .adding'hei'lt to the I)llocked in excba.ngel' ..This a .1 [ 'g u m ~ n t.o ve r lo e ks t he V < l r io u sunantaeipated , c ond i t i o ns that luayar ise dur i ng test i r . rg and main te -nance. Awol"st case Ilcenarie: Acooler was t,..akencut of serviee andsteam. eleaned, N~)on e had dr"a:il1l'edlth e co ol in g w ai te r w h ie b expanded inthe tubes and ruptured the Joint..".'I'rue, good ma i n t e n a n c e practicewould have prevented t rus incident.But then an NPS % , relief valvewould! have provided a permanentsolution and would have cost a lottess than the damage caused by itsa b s e n c e .AccesSQl j_ 'Y Insn"lU!lil.GllWShJJc,e the purpnse of a . h e 3 1 . t ex-ch


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A IOp,s;i difference eannot be reedfrom two pressure gauges\m.thWO 0psi scales, (Wal" story: Dt.ning com-missioning a cooler ws found tohav e an i nle tp r ea s ur e 'o f 5 ,9 'f 5p s i a n dan outlet prCSlilme0[586 psi. Thesolution to this strange vitibtion 'tlft h e p r e s . ., ;u r e J fl o ,wrclations'hip ",'as wswitch the two gauges.) As wi.th' l ll i ,e r m O l l m J e . o o I " S ,it is p our ,eO O n~ )n1 ytoeliminate PWeSS1IU1e gauges in favomofpreesurs connections only. Justs e eh ow lo ng itl;akes to fin d two p r s s -sure gauges of the right range whenyou need tham,v\~en t ; < ! . 1 : 0 o.~ch,angetaarestaekedin series, they are often connectedf la n g e t o f ia . ng { l. I n t .h : il i c a se , it is n otne ce ssa ry to n .1 lI,veP Is and 'rIs < O I l oo t hma t i n g uo s s l e s . A single set is qu i100sufficient, Unfulb.mately. itmay notbe po s s . : i b l e00put them in an OOEi lyaccessible location. lit may also benceessary 00 mod i f y the n o z z l e s toconform to , pl ' ( l ' ject standards,PCIra ,ell Heal E.. .uha IlgeuAerial coolers may be viewed as anumber of coolers in parallel, A: : > i n g l , e , th e rm om ete r a t th e i ,n l .e t i ssufficient but a separate one at eacho u t l e t to th e h e a d e r is e s s en t i a l .There is abooIutdy no other way toidentify individual plugged or l o u1 edsections V ; r i . t h 0 1 l , 1 t taking the who1eth.lIlg apart,Heat exchangers in parallel donot share the flow equally. Symametrieal 'p'ipmg is like the perflElct ire:at best a pious intention. (Somepeople t l : l i l lk it ]S more lik,e Santa

- .

O l a 1 l H . ) The problem is 'Illlat a n int-I;i~df lo w im b ala nce ca n g ro w . Ifa l'@.. d uC ! t i o nin f l ow causes f o u ll in g wh i c hfurther re s t r i c t s the flow, a p o s it ci v( lfeed-hack loop is set up, which canc ut ( m e e xc ]l O I! IJ g e'l ' entirelyout o f e ir -cU l a t iO f l . :Sonle way must be IO lmd itoforee thefla,ws to balanee, Unfor-

h D S : a u t o r n a l l i c ' 1 1 ' 1 0 1 1 1b f o l i a n l l i l l n , g l r s r i u o t y. n S l a l l r e d , e lcBPt IiIl e l O " B , m e ; I IIC i f i l U c a l . s e : r - u i o e's iu e b a s : '8 . u r n a c o wilhm u l l J l l l e t .u b e p l a s s e l .

tunately, it is me aningless tC) at-t e m p t to eontrol a ny varwble Mth.~out meascrement, and flow mea-sU i "eme . .n t i s expensive, N ot o nly a r e1;11eIns trum ents expenssv e ill'OO"l,"ITflSo f i ns t a1 1 l. 'i d cost a nd m a in oo na no o,the required piping arrangement is.also ,eXlJf,msive.'rhus, automatie flawbala..neing is I'3rely installed exoapt'in extl"emi\'lIy critical sereiee such asa furnace with multiiple t u : b e , PParts UG-l25 W 13:6, Pressure R.eI.ier Device ,

2, Sbm d .e :n ls o f ti1e TIII>u~fl'r Excl . l .< l l1gCl 'Mi3 JRll f :; j: cw . r n l' S Pwroc i a t il o o .

3. API STD 661, Air'(,'o!ll]~diHeat Ex-dUUigru"S for Gen.end :Ref.ine:rySer-vice ' . .-4. A .P I S l 'D 660, Sh.eU and Tube H!i!atExchange's fOT G!~lier.ill RefineryServlee.

INSTnuM~N'~,nG~ ~ CQ:-l imOL J .Q lJR NA L .' !lt~y - AuGuST 200~ 53,

Controlling Shell and Tube Heat Exchangers [1]

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Transcript of Controlling Shell and Tube Heat Exchangers [1]