heat exchangers_chemical engineering magazine

8/19/2019 HEAT EXCHANGERS_Chemical Engineering Magazine

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Processing & Handling :: Thermal & Energy ManagementMay 1, 2014

Unlocking the Secrets of Plate-and-Frame HeatExchangers

An understanding of the design, sizing, specification and installation of plate-and- frame heat exchangers is necessary to evaluate vendors’ proposed designs

Grittaya Srinaphasawadi, Process Equipment Technology and Wiroon Tanthapanichakoon, SCG

Chemicals

Plate-and-frame heat exchangers, or plate heat exchangers (PHEs), are a type of compactheat exchanger. PHEs hae !een "#dely $sed #n the food #nd$str#es, ma#nly !eca$se theyeas#ly meet health and san#tat#on re%$#rements d$e to the#r s#mple d#sassem!ly for clean#ng.&hey hae recently !ecome more common #n the chem#cal process #nd$str#es ('P) forcerta#n appl#cat#ons and operat#onal cond#t#ons. enerally, a PHE #s des#gned and s$ppl#ed !yendors excl$s#ely, ma*#ng them a proer!#al +!lac* !ox for eng#neers. Ho"eer, #t #s a poor pract#ce to rely only on endors, "ho, at t#mes, may not f$lly $nderstand act$al fl$#d propert#esor process re%$#rements at a spec#f#c s#te. &h#s can lead to des#gn m#sta*es or e%$#pment#nade%$ac#es.

&h#s art#cle coers the pract#cal des#gn aspects of PHEs, and expla#ns ho" to des#gn a PHE$s#ng a generally accepted method der#ed from heat-exchanger des#gn f$ndamentals. &h#smethod can !e appl#ed to the prel#m#nary des#gn of a PHE and can also !e $sed to re#e"endors proposed e%$#pment for #ts s$#ta!#l#ty for a re%$#red ser#ce.

Nomenclatre

Ach 'ross-sect#onal flo" area of one channel, m2

Ae &otal effect#e heat-transfer area, m2

A1 Plate tr$e s$rface area, m2

A1 p Plate planar proected area, m2

b Mean channel flo" gap, m

β 'heron angle, deg

C h Heat-transfer correlat#on coeff#c#ent

C p /pec#f#c heat capac#ty, *g-

De Hydra$l#c d#ameter, m

D p Port d#ameter, m

3 p Press$re drop, Pa

f ann#ng fr#ct#on factor

F LMTD correct#on F -factor


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F m o$l#ng marg#n, 5

h 'onect#e heat-transfer coeff#c#ent, 6m2-

k l$#d thermal cond$ct##ty, 6m-

k w Plate thermal cond$ct##ty, 6m-

K p Emp#r#cal coeff#c#ent of the ann#ng fr#ct#on factor for each type of cheron plate and7eynolds n$m!er

Lc 'ompressed-plate pac* length meas$red !et"een the t"o head plates, m

Lhoriz Hor#8ontal port-to-port center d#stance, m

L p 9ert#cal d#stance !et"een top port !ottom edge to !ottom port top edge, m

Lvert Effect#e flo" length !et"een the ert#cal ports, m

Lw Effect#e channel "#dth (!et"een gas*et grooes), m

LMTD :og-mean temperat$re d#fference, ;'

mch Mass flo"rate thro$gh each flo" channel, *gs

µ l$#d #scos#ty,


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T " Hot s#de #nlet temperat$re, ;'

T # Hot s#de o$tlet temperat$re, ;'

t " 'old s#de #nlet temperat$re, ;'

t # 'old s#de o$tlet temperat$re, ;'

$ @erall heat-transfer coeff#c#ent, 6m2-

% ch l$#d eloc#ty #n a flo" channel, ms

% p l$#d eloc#ty thro$gh the port, ms

& Mass flo"rate thro$gh e#ther hot s#de or cold s#de, *gs

S!erscri!ts

m Po"er exponent for 7eynolds n$m!er effect on press$re drop (for calc$lat#on of ann#ngfr#ct#on factor f )

' Po"er exponent for 7eynolds n$m!er effect on conect#e heat-transfer coeff#c#ent

S"scri!ts

c 'old s#de

ch 'hannel

c( 'lean

e Effect#e

f o$led

h Hot s#de

p Port

t &otal

#hat engineers shold kno$

As#ng conect#e heat-transfer pr#nc#ples, a PHE ach#ees heat transfer #a t"o streams offl$#d that are separated !y a sta#nless-steel corr$gated plate, form#ng a small ($s$ally 2BC mm#n s#8e) flo" passage. &h#s small flo" passage, along "#th the geometry of the PHE plates,#nd$ces t$r!$lence, contr#!$t#ng to a ery h#gh conect#e heat-transfer coeff#c#ent D m$chh#gher than that seen #n shell-and-t$!e (/&) heat exchangers.#g$re 1 sho"s the maorcomponents of a PHE.

http://www.che.com/nl/YToyOntpOjA7czo1OiIxMTczMiI7aToxO3M6NDc6InByb2Nlc3NpbmdfYW5kX2hhbmRsaW5nL3RoZXJtYWxfYW5kX2VuZXJneV9tZ210Ijt9/#Figure1http://www.che.com/nl/YToyOntpOjA7czo1OiIxMTczMiI7aToxO3M6NDc6InByb2Nlc3NpbmdfYW5kX2hhbmRsaW5nL3RoZXJtYWxfYW5kX2VuZXJneV9tZ210Ijt9/#Figure1


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#g$re 1.


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/$rface area per $n#t ol$me #s ery h#gh D $s$ally greater than I00 m2m?

E%$#pment #s l#ght"e#ght and compactJ only a small plot space #s re%$#red

Mod#f#cat#ons to meet spec#f#c process re%$#rements can !e ach#eed !y s#mply chang#ngthe n$m!er or form of plates

G "#de range of fl$#ds, #ncl$d#ng ones that are ery #sco$s, can !e processed "#th relat#elyl#ttle expense

&he heat-transfer area can !e eas#ly #ncreased or decreased

6hen lea*s occ$r, contam#nat#on of the process fl$#d #s preented

:ea*s are eas#ly detecta!le

&he a!#l#ty to #nd##d$ally remoe and clean plates decreases ma#ntenance and clean#ngefforts for !oth fl$#d s#desJ /& heat exchangers are more d#ff#c$lt to open, espec#ally on theshells#de

&$r!$lence can !e ach#eed at a relat#ely lo" 7eynolds n$m!er, typ#cally less than C00

:o"er sens#t##ty to #!rat#on than /& heat exchangers

9ery heat-sens#t#e process fl$#ds can !e capa!ly handled

Ho"eer, there are some appl#cat#ons "here /& heat exchangers are a !etter opt#on thanPHEs. /ome of the d#sadantages assoc#ated "#th the $se of PHEs are as follo"sF

f gas*ets deter#orate, espec#ally #n the presence of ha8ardo$s fl$#ds or hydrocar!onm#xt$res, the e%$#pment #s more s$scept#!le to atmospher#c lea*s

G h#gher press$re drop #s re%$#red to #nd$ce t$r!$lent flo" at smaller flo" passages (2BCmm)J at the same press$re drop, a PHE may not pro#de the des#red heat-transferenhancement effect.

PHEs hae l#m#ted $se for gas-to-gas heat-exchanger or !o#l#ng ser#ces "here ol$meexpans#on #s large, !eca$se the re%$#red o$tlet no88le "#ll !e too !#g

as*et mater#als select#on can #mpose constra#nts on operat#ng press$re and temperat$re(/ee &a!le 1 for spec#f#c temperat$re l#m#tat#ons)J conersely, #n /& heat exchangers, thedes#gn temperat$re #s !ased $pon the metall$rgy of the mater#als of constr$ct#on

Process fl$#ds m$st !e clean, non-flamma!le, non-tox#c and nearly free of part#c$late matterJfor d#rty fl$#ds, #nlet stra#ners m$st !e #nstalled


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&G=:E 1. &KP'G: MGLMAM @PE7G&


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#g$re ?. &he gas*et #s compressed $nt#l there #s metal-to-metal contact !et"een adacent plates, form#ng flo" channels on the hotand cold s#des

#g$re 4. &he gas*et #s #nstalled #n the grooe near the edge of each plate

%esign methodology

PHE des#gn rema#ns propr#etary #n nat$re, !eca$se exact des#gn correlat#ons $sed !yendors are often $nd#sclosed. &he PHE conect#e heat-transfer coeff#c#ent, h, can !ecalc$lated $s#ng an #n-t$!e conect#e heat-transfer coeff#c#ent correlat#on, !$t the t$!e #nnerd#ameter m$st !e changed to hydra$l#c d#ameter,De. &h#s des#gn method has !een al#dated"#th commerc#al PHE des#gn and rat#ng soft"are.

&he follo"#ng e%$at#ons $se heat-exchanger f$ndamentals to deelop a methodology fordes#gn#ng a typ#cal PHE. &he nomenclat$re sect#on, as "ell as #g$res ? and 4, expla#n themean#ng of the parameters #n the follo"#ng e%$at#ons. #rst, the process re%$#rements m$st

!e def#ned for the heat d$ty (Q) on the hot and cold s#des of the exchanger, as "ell as the log-mean temperat$re d#fference (:M&>).

Qh & hC p)h( T 1 B T 2) ()

Qc & c C p)c ( t 2 B t 1) (')

(()


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E%$at#ons (4) thro$gh (1?) def#ne geometry for the PHE, !eg#nn#ng "#th the effect#e n$m!erof plates (N e), "h#ch #s t"o less than the total n$m!er of plates, d$e to the a!sence of flo"!et"een the end plate and the coer, sho"n #n E%$at#on (4). E%$at#on (C) g#es thecompressed p#tch ( p), "h#ch #s a f$nct#on of the compressed-plate pac* length meas$red!et"een the t"o head plates (Lc ).

N e N t B 2 ())

p Lc N t (*)

E%$at#ons (N) thro$gh (O) regard channel parameters. &he mean channel flo" gap (b) #s g#en!y E%$at#on (N), the ert#cal port d#stance !y E%$at#on (I) and the effect#e channel "#dth !yE%$at#on ().

b p B t (+)

L p Lvert B D p (,)

L" Lhoriz Q D p ()

&he flo" area of one channel, sho"n #n E%$at#on (O), #s the prod$ct of the channels flo" gapand #ts "#dth.

Ach b R Lw (.)

&he s#ngle-plate tr$e heat-transfer area #s def#ned as the rat#o of the e%$#alent area and then$m!er of effect#e plates. &he total effect#e heat-transfer area of a PHE (or total effect#earea), Ae, depends on plate types and n$m!er of plates, as sho"n #n E%$at#on (10). &heplanar proected area #s g#en #n E%$at#on (11).

A1 Ae * N e (/)

A1p L pR Lw ()

&he s$rface enlargement factor, ϕ, #s the rat#o of plate tr$e s$rface area to plate planarproected area. >ef#ned #n E%$at#on (12), th#s al$e #s normally !et"een 1.20 and 1.2C.

ϕ A1 A1 p (')

&he hydra$l#c d#ameter #s def#ned as fo$r t#mes the channel flo" area d##ded !y the "ettedper#meter. n E%$at#on (1?), #t #s ass$med that b #s m$ch smaller than Lw .

(()


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&he express#ons re%$#red for heat-transfer calc$lat#ons for the des#gn of a PHE are sho"n #nE%$at#ons (14) thro$gh (24). E%$at#ons (14) thro$gh (1N) relate to the channels. E%$at#on (14)g#es the n$m!er of channels per pass, E%$at#on (1C) g#es the mass flo"rate to eachchannel and E%$at#on (1N) def#nes the fl$#d eloc#ty #n a g#en flo" channel.

N cp ( N t B 1)2N pass ())

mch & N cp (*)

% ch ( mchS) Ach( +)

&he 7eynolds


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(X pch)

4f (Lvert N pass De)(S% ch22) ('+)

E%$at#ons (2I) and (2) relate to flo" at the port. E%$at#on (2I) def#nes the fl$#d eloc#tythro$gh the port and E%$at#on (2) def#nes the press$re drop at the port.

% p ( & S)(Y D p24) (',)

(X p p) (1.4 N passS% p22) (')

#nally, "e arr#e at the total press$re drop, sho"n #n E%$at#on (2O), "h#ch #s the s$m of thepress$re drop at the port and the channel press$re drop.

(X pt ) (X pc ) Q (X p p) ('.)

%esign exam!le

&he des#gn method der#ed #n the pre#o$s sect#on "#ll no" !e appl#ed #n an examplescenar#o to conf#rm a endors des#gn. n th#s example, cold "ater #s to !e heated !y a hot o#lstream #n a PHE. As#ng the process spec#f#cat#ons #n &a!le ? and the endors proposeddes#gn #n &a!le 4, "e can $se E%$at#ons (1) thro$gh (2O) to determ#ne #f the proposedexchanger sat#sf#es the des#gn re%$#rements. &a!le C g#es a s$mmary of the calc$lat#onres$lts $s#ng the pre#o$sly der#ed des#gn methodology. t can !e seen from &a!le C that theproposed des#gn #s expected to ach#ee #ts des#red heat transfer and not exceed the allo"a!lepress$re drop. Ho"eer, #t can !e determ#ned from the d#screpanc#es !et"een the proposedand the calc$lated al$es for the oerall heat-transfer coeff#c#ents that the endors des#gn(!ased on a proposed heat-transfer area of 110 m2) #s oers#8ed for th#s ser#ce and can !ef$rther opt#m#8ed to red$ce costs.

&a!le ?. Process /pec#f#cat#ons for a #en PHE

Parameter

Process fl$#d

Mass flo"rate, & , *gs

nlet temperat$re, T ", t ", ;'

@$tlet temperat$re, T # , t # , ;'

Max allo"a!le 3 p, !ars

/pec#f#c heat, Cp, *g-

9#scos#ty, µ, ens#ty, !, *gm?

Prandtl n$m!er, Pr

o$l#ng marg#n, Fm (5)

&a!le 4. 9endors proposed PHE des#gn (!ased on ?04 sta#nless steel)

Parameter

Plate th#c*ness, t , mm

'heron angle, β, deg


11/15

&otal n$m!er of plates, N t

Enlargement factor, ϕ


12/15

,hDe *k+c (1O) 0.

hc (1O) 20

$ c( (20) 1

$ f (22) 10

F

Qc( (2?) N,

Qf (24) C,

Heat transfer res$ltsF $c( and $f are greater than the endors proposed al$es ̂ ood

Qc( and Qf are greater than the re%$#red heat d$ty of 11,NO.4 *6 [ ood.

f h (2C) r

(3 pch)h (2N) 4

% p)h (2I) (1

(3 p p)h (2) 1.

(3 pt )h (2O) 2I

f c (2C) 1.

(3 pch)c (2N) 4

% p)c (2I) (1

(3 p p)c (2) 1.

(3 pt )c (2O) 2C

Press$re drop res$ltsF (3 pt )h and (3 pt )c are less than ? !ars [ ood

0est !ractices

>es#gn and #nstallat#on. n add#t#on to s#8#ng a PHE, #t #s also !enef#c#al for eng#neers to$nderstand some pract#cal cons#derat#ons #n the p$rchas#ng and #nstallat#on of PHEs. Gs$mmary of !est pract#ces for #nstall#ng PHEs #s as follo"sF

&he frame and t#e !olts of a PHE sho$ld !e des#gned to perm#t f$t$re #nstallat#on of at least

205 add#t#onal plates

&he nom#nal th#c*ness of gas*eted plates sho$ld !e a m#n#m$m of 0.C mm to meet des#gncond#t#ons. as*eted plates shall !e #nd##d$ally replacea!le "#tho$t remo#ng any other plate

&he PHE m$st !e des#gned to "#thstand the des#gn press$re of each s#de "hen thepress$re on the other s#de #s atmospher#c press$re, or ac$$m, #f spec#f#ed. &h$s, e#therhydrotest or pne$mat#c press$re tests can !e $nderta*en on only one s#de at a t#me, "#th theother s#de at atmospher#c press$re or ac$$m

G corros#on allo"ance shall !e appl#ed to $nl#ned connect#ons only, and there #s 8erocorros#on allo"ance for plates

as*ets sho$ld !e pos#t#oned #n the grooe aro$nd the heat- transfer s$rface


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as*ets sho$ld !e compressed to ach#ee metal contacts !et"een the plates

Each seal#ng gas*et sho$ld !e one #ntegral p#ece to allo" for !etter press$re res#stance

/pec#fy#ng and p$rchas#ng. >esp#te the dependence on endors to pro#de PHEs, an$nderstand#ng of PHE des#gn data #s cr$c#al to p$rchas#ng eng#neers. or proper PHE des#gn,the p$rchaser sho$ld spec#fy the follo"#ng des#gn data to !e pro#ded !y endorsF

'ompleted datasheets

>eta#ls of the constr$ct#on and assem!ly

Gnt#c#pated l#fe of the gas*ets #n the spec#f#ed ser#ce and #n storage, and spec#al storageg$#del#nes, #f needed to ma#nta#n gas*et l#fe

&he method $sed to s$pport the remoa!le coer

7ecommended spare-parts l#st

#reproof test#ng cert#f#cate for a f#reproof shro$d, #f spec#f#ed

9er#f#cat#on of compat#!#l#ty of the gas*et mater#al and gl$e "#th the spec#f#ed fl$#ds, #ncl$d#ngany spec#f#ed chem#cal clean#ng

Pro"lems $ith 1ery large PHEs

&here are some h#dden str$ct$ral constra#nts that often ca$se pro!lems "#th the $se of PHEs.&hese str$ct$ral #ss$es are espec#ally concern#ng "hen man$fact$rers offer ery largee%$#pment to p$rchasers. &he constra#nts #n %$est#on are the total n$m!er of plates $sed andthe total "e#ght of the plate pac* assem!ly.

n a normal flanged o#nt, the st$d !olts $sed to clamp the o#nt m$st !e properly tor%$ed topro#de s$ff#c#ent clamp#ng force, g##ng the gas*et the ade%$ate seat#ng stress to seal the#nternal press$re. 6#th the re%$#red clamp#ng force, the !olt stress "#ll also !e an accepta!leal$e. 6#th th#s pr#nc#ple, for the same #nternal press$re and the same type and s#8e ofgas*et, the flanged o#nt "#th t"o gas*ets "o$ld need more clamp#ng force than the o#nt "#tha s#ngle gas*et D th#s means that the total n$m!er of !olts "#ll hae to !e #ncreased to res$lt#n that same !olt stress. &he softer gas*et (the one "#th a lo"er gas*et factor) "o$ld normally

need less clamp#ng force than the harder one. 6#th the r$!!er gas*ets typ#cally $sed #nPHEs, #t #s eas#er to pro#de a small clamp#ng force and properly tor%$ed t#e !olts.

Ho"eer, #f a large n$m!er of plates #s present, an een h#gher n$m!er of t#e !olts (and alarger total !olt cross-sect#onal area) "o$ld !e re%$#red. G large n$m!er of small t#e !olts "#llg#e a more $n#form gas*et press$re and res$lt #n m$ch !etter seal#ng performance than asmall n$m!er of large !olts ha#ng the same !olt total cross-sect#onal area.

n one case, a man$fact$rer des#gned a PHE "#th C00 plates assem!led #nto one pac*. &h#sma*es #t extremely d#ff#c$lt to hae all C00 gas*ets properly compressed to seal the #nternalpress$re of the process D no matter the tor%$e al$e or n$m!er of t#e !olts, and een tho$ghthe man$fact$rer appl#ed an ample amo$nt of adhes#e to the gas*et to pro#de the seal.

=ear #n m#nd that the total "e#ght of the ent#re plate assem!ly #s hang#ng on the carry#ng !ar.&he h#gher the n$m!er of plates $sed, the hea#er and longer the plate assem!ly #s, #mpos#ngan een greater "e#ght on the !ar, "h#ch acts as a $n#formly d#str#!$ted load !eam. Gccord#ng


14/15

to the !eam theory, the deflect#on of the !eam ar#es l#nearly "#th the load and "#th the c$!eof the !eam length. &h#s ma*es the plate assem!ly more s$scept#!le to lea*age, espec#ally at#ts !ottom. &herefore, the p$rchaser sho$ld ens$re that the PHE #s not too large, and ta*ecaref$l cons#derat#on #n conf#rm#ng the total n$m!er of plates #n the assem!ly. Anderstand#ngthe adantages and d#sadantages of PHEs and /& heat exchangers allo"s for moreeff#c#ent des#gns for heat-exchange systems. Een tho$gh PHE des#gn rema#ns propr#etary to

endors, plant eng#neers sho$ld !e a!le to eal$ate a endors PHE des#gn for correctness$s#ng the methods expla#ned a!oe.

-.ite. b/ Mar/ Pa0e 1ai(e/

2eferences

1. a*ac, /., and others, +Heat ExchangersF /elect#on, 7at#ng, and &hermal >es#gn, 2ndEd#t#on, '7' Press, =oca 7aton, la., March 2002.

2. ays, 6.M. and :ondon, G.:., +'ompact Heat Exchangers, ?rd Ed#t#on, 7epr#nt, r#egerP$!l#sh#ng 'o., 1OO.

?. Hesselgreaes, .E., +'ompact Heat ExchangersF /elect#on, >es#gn, and @perat#on, 1stEd#t#on, Else#er /c#ence :td., 2001.

3thors

4rittaya Srina!hasa$adi #s the manag#ng d#rector of Process E%$#pment &echnology 'o.(CCC Moo 4 &am!ol Ma*am*oo, Gmp$r


15/15

#iroon Tantha!anichakoon #s a lead eng#neer "or*#ng for the process technology center of/' 'hem#cals 'o. (7@' /#te ? &echn#c =$#ld#ng, 2I1 /$*h$m#t 7d., Map &a Ph$t, 7ayong,211C0, &ha#landJ PhoneF QNN-?-O11-240, QNN-?-1II-10)J Ema#lF "#roont_scg.co.th,t"#roon_gma#l.com). He has a$thored seeral #nternat#onal papers and techn#cal art#cles. He#s c$rrently #noled #n process and e%$#pment des#gn, as "ell as !as#c eng#neer#ng for ne"process technolog#es from /' 'hem#cals. He has seeral years of exper#ence as a process

eng#neer "#th an ExxonMo!#l s$!s#d#ary ref#nery #n &ha#land and as a process des#gn andenergy #mproement eng#neer "#th /' 'hem#cals. He o!ta#ned h#s =./.'h.E. andM./.'h.E. degrees from yoto An#ers#ty, apan. He #s a l#censed eng#neer #n &ha#land and asen#or mem!er of G'hE. G M#crosoft Excel spreadsheet deeloped !y the a$thor for theexample pro!lems #s aa#la!le !y re%$est to the a$thors ema#l address.

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