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Heat Recover)" Sys tems Vol. 4, No. 2, pp. 77-85, 1984 0198-7593/84 S3.00 + .00Printed in Great Britain. Pergamon Press Ltd

H E A T E X C H A N G E R S F O R S E C O N D A R Y H E A T R E C O V E R YF R O M G L A S S P L A N T S

R . L . WEBBD e p a r t m e n t o f M e c h a n i c a l E n g i n e e r i n g , P e n n s y l v a n i a S t a t e U n i v e r s i t y , P e n n s y l v a n i a , U .S .A .

n . M A R C H I O R IB a b c o c k & W i l c o x , A l l i a n c e , O h i o , U .S .A .

R. E. DURBINU .S . M i l i t a r y A c a d e m y , W e s t P o i n t , N e w Y o r k , U .S .A .

Y-J . WANGO i l T e c h n o l o g y , I n c . , H o u s t o n , T e x a s , U .S .A .

a n dA . K . K U L K A R N I

D e p a r t m e n t o f M e c h a n i c a l E n g i n e e r i n g , P e n n s y l v a n i a S t a t e U n i v e r s i t y , P e n n s y l v a n i a , U .S .A .

A b s t r a c t - - T h i s p a p e r a d d r e s s e s h e a t r e c o v e r y f r o m t h e e x h a u s t g a s e s l e a v i n g t h e b r i c k c h e c k e r sr e g e n e r a t o r o f a s o d a l i m e g l a s s f u r n a c e . T h e m o s t d e s i r a b l e u s e s f o r t h e r e c o v e r e d h e a t a r e e l e c t r i c i t yg e n e r a t i o n a n d b a t c h p r e h e a t . A n i n d u s t r y s u r v e y i n d i c a te d t h a t h e at . e x c h a n g e r f o u l i n g is a p r i m a r y f a c t o rt h a t i n h i b i t s u s e o f s u c h h e a t r e c o v e r y . F o r e l e c tr i c it y g e n e r a t io n , o n e w o u l d u s e e i t h e r a R a n k i n e o r aB r a y t o n c y cl e. T h e f o r m e r w o u l d u s e a f i n n e d - t u b e h e a t e x c h a n g e r , a n d t h e l a t t e r w o u l d u s e a p l a t e - a n d -f i ne x c h a n g e r . T h i s p a p e r d i s c u s s e s t h e f o u l i n g , c o r r o s i o n a n d c l e a n a b i l i ty o f s u c h h e a t e x c h a n g e r s i n t h e g l a ssf u r n a c e e x h a u s t . T h e c o m p o s i t i o n o f t h e g a s e s e n t e r i n g t h e e x c h a n g e r i s d e s c r i b e d a n d t h e e x p e c t e d f o u l i n gm e c h a n i s m s a r e d i s c u ss e d . I t i s c o n c l u d e d t h a t t h e s e e x c h a n g e r s s h o u l d b e c l e a n a b l e a n d n o t s u b j e c t t oe x c e s s i v e c o r r o s i o n , p r o v i d e d t h e y o p e r a t e a b o v e t h e a c i d d e w p o i n t . T h e u s e o f M g O w o u l d f u r t h e re n h a n c e t h e c l e a n a b i l i t y o f t h e f o u l i n g d e p o s i t. T h e f l u i d i z e d -b e d h e a t e x c h a n g e r i s a n i n t e r e s t i n g c o n c e p tf o r b a t c h p r e h e a t . I f t h e c o n c e p t c a n b e s u c c e ss f u ll y e m p l o y e d , i t w o u l d b e a " s e l f c le a n i n g " h e a te x c h a n g e r .

I N T R O D U C T I O NThis paper r e su l t ed f rom a Depar tmen t o f Energy sponso red su rvey o f t he g l ass i ndus t ryconcern ing second ary hea t r ecovery f rom g lass f u rnace gases [1 ]. Ku lka rn i e t a l . [2] characterizedthe exhaus t gas compos i t i on and the fou l ing po ten t i a l i n t he p r imary hea t r ecovery "b r i ckchecker s" r egenera to r .The g l ass i ndus t ry c onsum es 3 x l08 MB tu o f ene rgy pe r yea r , 809 /0 o f wh ich i s na tu r a l gas . Ava lved pe r iod ic f l ow r egenera to r i s p r esen t ly used to r ecover hea t f r om the me l t ing t ank exhaus tgases. Th i s r egenera to r cons i s t s o f an a r r ay o f b r icks s t acked in an "o pen check erw ork" pa t t e rn ,th rough wh ich the exhau s t gases and com bus t ion a i r a r e a l te rna t e ly passed . The cyc le i s changeda t app rox imate ly 20 min in t e rva l s . The exhaus t gas i n l e t t empera tu r e i s app rox imate ly 143C andthe ou t l e t t emp era tu r e r anges f rom 815 to 430C, depend ing on the nu mb er o f passes i n the "b r i ckchecker s" r egenera to r . Typ ica l ly used b r i ck checker s des igns have gas ou t l e t t empera tu r es o f540-650C, wh ich y i e ld a i r p r ehea t t empera tu r es approx imate ly 1100C, and opera t e a t a t he rmaleffectiveness of 75-80~o.

Al thoug h the r e is cons ide r ab le po ten t i a l f o r add i t i ona l hea t r ecovery , g la ss man ufac tu r e r s do n o te m p l o y s e c o n d a r y h e a t r e c o v e ry e q u i p m e n t d o w n s t r e a m f r o m t h e c h e c k e rs r e g e n e ra t o r. T h e k e ypurpose o f t h i s s tudy was to i den t i f y uses f o r add i t i ona l r ecovered hea t , e s t ab l i sh the needed hea texchanger t ypes and iden t i fy ope ra t ing conce rns tha t a f f ec t t he v i ab i li t y o f such seco ndary hea tr ecovery exchanger s . O ur indus t ry su rvey [1 ] show ed tha t co nce rn over po ten t i a l gas - s ide fou l ingi s t he key ba r r i e r t ha t p r esen t ly inh ib i ts u se o f secondary hea t r ecovery equ ipmen t .H.~s *2 A 77

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7 8 R . L . W E B B C I e l ~ .

Because d i f fe ren t hea t exchan ger t ypes may have d i f fe ren t fou l ing cha rac te r i s ti c s , i t i s impo r t an tto e s tab l i sh t he cand ida t e hea t exchanger t ype s t ha t m ay be used fo r secondary hea t recovery . Th i swi l l depend on the se l ec t ed use fo r t he recovered hea t .

H E A T R E C O V E R Y U S E VS H E A T E X C H A N G E R T Y P EUses for the recovered heat

Four po t en t i a l u ses fo r add i t i ona l recovered hea t were i den t i f i ed . These a re : (1 ) add i t i ona l a i rp rehea t ; (2) s t eam f o r p rocess u se o r space hea t ing ; (3) e l ect r ic i t y genera t i on and (4) p rehea t o f t heraw m ate r i a l s (ba t ch ) fed t o t he me l t i ng t ank . There i s no t su f f i c ien t need fo r s t eam to j u s t i fy t ha tposs ib i l i t y . Because t he p resen t b r i ck checkers regenera to rs ope ra t e a t re l a t i ve ly h igh the rmale f fect iveness , a g rea t a mo un t o f hea t t rans fe r su r face w ou ld be requ i red to ob t a in a s igni f ican tinc remen t o f add i t i ona l a i r p rehea t . A l so , a i r l eakage in to t he exhaus t gases a t the regenera to rbu t t e r f l y va lve wou ld have a nega t ive e f fec t on such s econ dary a i r p rehea t . The m os t p rom is inguses fo r add i t i ona l h ea t recove ry a re e l ec tr i c it y genera t i on (cogenera t ion ) a nd ba t ch p rehea t . Bo tho f these uses have a re l a t ive ly l ow hea t s i nk t empera tu re , so a cons ide rab ly sm a l le r NT U i s requ i redfo r a g iven inc remen ta l hea t recovery than wou ld be requ i red fo r t he add i t i ona l a i r p rehea t op t ion .T h e i n le t h e a t s in k t e m p e r a t u r e s f o r c o g e n e r a t io n a n d b a t c h p r e h e a t a r e a p p r o x i m a t e l y 2 6 0 C a n d30C, respect ively .Tab le 1 p resen t s a m a t r ix o f t he poss ib l e u ses fo r add i t i ona l recovered hea t v s cand ida t e hea texchangers . The remain ing d i scuss ion wil l be l im i t ed t o hea t re covery fo r cogen era t ion and ba t chprehea t .Cogeneration

In a cogenera t ion sys t em, t he ho t exhaus t gases p rov ide t he hea t i npu t t o a power genera t i oncyc l e . The tu rb ine d r ives a genera to r , wh ich p rov ides e l ec t r i c i t y fo r p l an t u se . Two bas i c t ypes o fp o w e r c y cl e s m a y b e c o n s i d e r ed : ( I ) a R a n k i n e c y c l e u s in g s t e a m o r a n o r g a n i c w o r k i n g f lu id o r(2) a Bra y ton (gas t u rb ine ) cyc l e , wh ich uses a i r a s t he w ork ing f l uid . These cyc l es w i ll u se d i f feren tt y p e s o f h e a t r e c o v e r y e x c h an g e r s . T h e R a n k i n e c y c l e n o r m a l l y e m p l o y s a b a r e o r f i n n ed t u b e b a n k ,w i t h t h e e x h a u s t g a s e s i n c ro s s f lo w o v e r t h e o u t e r t u b e s u r fa c e s . T h e B r a y t o n c y c le p r o p o s e d b yAiR esea rc h [9 ] u ses a p l a t e -and- f in exchanger ; t h i s hea t exch anger c ons t ru c t ion i s i l lu s t ra t ed by F ig .1. The en v i s ioned Bra y ton cyc l e u ses the exhau s t gases t o h ea t com pressed in le t a ir , wh ich i s t hene x p a n d e d t h r o u g h t h e t u r b in e t o a p p r o x i m a t e l y a t m o s p h e r i c p r e s s u re . T h e h o t t u r b i n e e x h a u s t a i ri s duc t ed to t he checkers regenera to r a s p rehea t ed a i r . Hna t [3 ] p rov ides an exce l l en t desc r ip t i onof t hese hea t recovery cyc l es and p rov ides ana lys i s o f t he ir sys t em pe r fo rma nce .

The f i nned- tube and the p l a t e -and- f in exchangers a re w ide ly used in commerc i a l p rac t i ce ,a l t h o u g h t h e y h a v e n o t b e e n a p p l i e d t o h e a t r e c o v e r y f r o m g l a s s fu r n a c e s. G l a s s in d u s t r y p e r s o nn e la re ve ry conce rned abou t t he i r fou l ing po ten t i a l .A f lu id i zed -bed hea t exchanger m ay be cons ide red as an a l t e rna t ive t o t he " ' ind i rec t con tac t "e x c h a n g e r s d i s c u s s e d a b o v e . T h e m e d i u m t o b e h e a t e d w o u l d p a s s t h r o u g h h o r i z o n t a l t u b e simmersed in t he f l u id i zed bed . Graded g l ass cu l l e t may be an accep tab l e bed ma te r i a l . The ho te x h a u s t g a s e s w o u l d e n t e r a t t h e b o t t o m o f t h e b e d , v i a a d i s t r ib u t o r p l a t e , a n d " f lu i d iz e " t h e b e dpar t ic l e s . The sco ur ing ac t ion o f the pa r t i c l e s w il l p reven t t he fo rm at ion o f a fou l an t dep os i t onthe hea t exchanger t ubes . The Na2SO 4 pa r t i cu l a t e s wo u ld be en t ra ined f rom the bed : Such a concep tw o u l d b e " s e l f - c le a n i n g " h e a t e x c h a n g e r . H o w e v e r , a p e n a l ty m u s t b e p a i d , in t e r m s o f f a n p o w e rrequ i red to f l u id i ze t he bed . To da t e , t h i s concep t has no t been t e s t ed .

T a b l e 1 . C a n d i d a t e h e a t e x c h a n g e r s f o r s e c o n d a r y h e a t r e c o v e r y a p p l i c a t io n sU s e d f o r r e c o v e r e d h e a t

H e a t e x c h a n g e r A i r S t e a m B a t c h. . . . . . . t y p e . . . . . . . . . . p r e h e a t _ g e n e r a t io n . . . . _ Co g_ cn er atio n p r e h e a tC h e c k e r s r e g e n . +R o t a r y r e g e n . +F i n n ~ l - t u b e + + +P l a t e - a n d - f i n + +S t a t i c b e d +F l u i d i z e d b e d +

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H e a t e x c h a n g e r s f o r s e c o n d a r y h e a t r e c o v e r y f r o m g l a s s p l a n t s 79

GA SSURFA~

Fig . 1 . C r o s s f l o w p l a t e - a n d - f in h e a t e x c h a n g e r w i t h i l lu s t r a t io n o f t h e " o ff s e t s t r i p - f i n " .

B a t c h p r e h e a tThis may be obtained by pass ing the exhaust gases through a f lu idized bed or a s tatic-bed heat

exchanger. Used here, the hot exhaust gases would f lu idize pel let ized batch materials . The use ofbatch preheat wo uld al leviate the need for other secondary h eat recovery uses. T his use of recoveredheat wo uld yield furnace energy savings o f approxim ately 20~o. Sakhuja and C ole [4] d iscuss aprogram to deve lop th i s concept .

G A S C O M P O S IT I O N L E A V I N G T H E R E G E N E R A T O RA se con dary h eat exchan ger will be supplied w ith the 540--650C gase s that leave the regenerator.

In addit ion to CO2, N2, 02 and H 20 vapor, these gases contain particulates and conde nsible vaporsthat are the cause of foul ing, and may cause corros ion of the heat transfer surface. Tables 2 and3 give the com posit io n o f the part iculates and the contam inant va pors , respect ively , for soda- l imeglass manufacture.

F O U L I N G A N D C O R R O S I O N M E C H A N I S M SKulkarni e t a l . [2] have shown that severe chemical react ions and foul ing exis t within the

regenerator. The react ion of the volat i le e lements , and their subsequent condensat ion andsolidifica tion yield a slag or particulates that solidify a t 884C. Th e fine, solid particulates listedin Table 2 are the main source of foul ing in a heat exchanger downstream from the regenerator.

As the gases l is ted in Table 3 pass through the heat exchanger, two chemical react ions areposs ible that may cause corros ion, and further contribute to surface foul ing.

Table 2 . Comp os i t ion o f part i cula tes l eav ing the regen-erator [5]Concentrat ion (ppm by weight) 20 0Size 90% < 4 ~mC h e m i c al c o m p o s i t io n ( % b y w t )Na2SO4 90 %K2SO4 3oCa SO4 4~SiO 2 , AI203 + Others 3%

T a b l e 3 . C o m p o s i t i o n o f v a p o r c o n t a m i n a n t sleaving the regenerator [5, 6]G a s f i r e d O i l -f i re d

C o m p o s i t i o n p p m ( b y w t ) p p m ( b y w t )S O 2 10-80 up to 27 0SO~ 0-5 5-30N O , 400 400HCI 15 15H F 1.5 1.5

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8 0 R . L . W E I3 1a et aL

A "h o t e nd" co r ros io n reac t ion m ay resu l t f rom the reac t ion o f K2SO4 o r Na2SO4 wi th SO~ [7 ].The reac t ion p roduc ts a re co r ros ive l iqu id py rosu lpha tes (K2S207 o r Na2S207); these py rosu lph a tessol id ify a t 765 an d 75 4F, respect ively . Th e l iquid py rosu lpha tes will tend to e ntra p the Na , SO4par t icu la te s . As the su r face tem pera tu re i s lowered , the depos i t d r ie s to a ha rd sca le , wh ich i s qu i ted i f ficu lt to rem ove by a i r l anc ing . Bu t , i t is wa te r so lub le and ca n be rem oved by w a te r wash ing[8 ] . Re id [7 ] p resen ts da ta , wh ich show tha t SO3 concen t ra t ions above 150 and 2200 ppm a rerequ i red to fo rm K2S207 and Na2S207 , re spect ively . Howev er , AiR esea rch [9 ] conc lud ed tha tNa2S207 was fo rmed wi th 27 .6 ppm SO3 concen t ra t ion (by we igh t ) .A "co ld end" co r ros ion reac t ion wi l l occur i f the H:SO4 fo rmed by reac t ion o f SO3 and H,Ov a p o r s c o n d e n s es . Th e d e w p o i n t o f t h e H 2 S O4 dep end s on the SO3 con cen tra t io n [6]. I t is a lsoposs ib le fo r the cond ensed ac id to reac t wi th Na2SO 4 to fo rm Na2SO 7. An d , i f the wa te r vapor a l socondenses , NaHSO4 wi l l be fo rmed . These condensa tes fo rm a "s t icky" su r face which en t raps the

Na2SO 4 pa r t icu la te s . The re su l t ing depos i t m ay be rem oved by s team o r w a te r wash [6] .The add i t ion o f M gO to the f lue gases wil l ac t to con sum e the excess SO3 fo rm ing MgSO4. Th iswi l l be benef ic ia l fo r bo th the ho t and co ld co r ros ion and fou l ing s i tua t ions . I f the "s t icky"conden sa tes can be p reven ted , the su r face fou l ing p rob le m wi ll be g rea t ly a llev iated . The re su l tan tdepos i t s a re d ry , so f t and can be removed by a i r l anc ing o r s team soo t b lowers [10 ] .

H E A T E X C H A N G E R F O U L I N G A N D C O R R O S IO NThe p rev ious d i scuss ion has e s tab l ished tha t the fou l ing /co r ros ion cha rac te r i st i c s o f f inned- tubeand p la te -and- f in hea t exchanger types a re o f in te res t . Our g la ss indus t ry su rvey [ l ] showed tha t

the re is ve ry li tt l e u se o f the p ro posed second ary hea t recov ery me thods . D iscuss ions w i the n g i n e e ri n g p e r s o n n el i n d i c a t e d t h a t f o u l in g i s a m a j o r i m p e d i m e n t t o t h e u s e o f s e c o n d a r y r e c o v e ryhea t exchangers . The p rev ious d i scuss ion sugges t s tha t the fou l ing and co r ros ion po ten t ia l fo r as e c o n d a r y r e c o v e r y h e a t e x c h a n g e r i s m u c h l e s s s e v e r e t h a n t h a t o f t h e c h e c k e r s r e g e n e r a t o r .H o w e v e r , t h e u s e o f M g O a d d i t io n , a n d a v o i d a n c e o f " c o l d e n d " c o r r o s i o n a n d f o u l i n g s u g ge s tst h a t t h e f o u l a n t i s r e m o v a b l e b y c o n v e n t i o n a l m e t h o d s .O u r s t u d y s h o w e d t h a t f o u l i n g a n d c o r r o s i o n d a t a e x i s t o n t h e f o l l o w i n g h e a t - e x c h a n g e rg e o m e t r ie s s u pp l ie d w i t h g a se s d o w n s t r e a m f r o m t h e c h e c k e rs r e g e n e ra t o r : ( l ) f i n - a n d - t u b e - - o n l yprop r ie ta ry da ta were fou nd [8 ]; (2) ba re tubes u sed in w a te r - tube a nd f i re - tube bo i le rs [10 ]; (3)p la te - and - f in - -a v e ry tho rou gh s tu dy us ing seve ra l su r face geom et r ie s in a s imula ted g lass fu rnace

exhaust [6, 9].Al th oug h no d a ta ex ist fo r f inned- tube exchan gers in a g la ss fu rnace exhaus t , th i s geom et ry hasbeen wide ly used fo r economize rs in o i l and gas - f i red wa te r - tube bo i le r s , and has been eva lua tedfo r hea t recovery f rom d iese l eng ine exhaus ts [1 l , 12 ] . In these app l ica t ions the gas tem pera tu reen te r ing the hea t exch anger is 230-400C, and the gases con ta in " soo t" pa r t ic le s o f a s izec o m p a r a b l e t o t h e d u s t f r o m g la s s f u r n a c e r e g e n e r a to r s . Co m p a r a b l e f o u l in g m e c h a n i s m s w o u l dbe expec ted in the g lass fu rnace , bo i le r an d d iese l app l ica tions , e .g. pa r t icu la te fou l ing a t the ho tend , and mois tu re condensa t ion , p lus pa r t icu la te fou l ing a t the co ld end .Plate -and - f in fouling~corros ion tes ts

AiR esca rch has m easure d the fou l ing cha rac te r i s t i c s o f the F ig . l p la te -and- f in geome t ry us inga s imula ted g lass fu rnace exhaus t [6 , 9 ] . The geomet r ie s t e s ted a re l i s ted in Tab le 4 , and inc lude118 and 295 f ins m -i p la in f ins , and 197 and 256 f ins m -l offse t s t r ip f ins (Fig . 1). A cross-f lowexchan ger was used wi th the h o t gases on o ne s ide , and a i r on the coo lan t s ide. The h o t gases wereT a b l e 4 . P l a t e - an d - f i n g e o m e t r y p a r a m e t e r s t e s t e d b y A i R e s e a r c h ! 6!

Hea t e x c h a n g e r F i n h e i g h t F i n t h i c k n e s s , F i n d e n s i t y , O l ~ t l e n g t h ,i de n t i f i c a t i on ( r a m) ( r a m) ( f i n s m - ) ) mmF r o n t a l g a sve l oc i t y ( r a s - ) ) *

I 14 0.406 118 Pla in 9.552 14 0.406 256 12.7 6.473 3.7 0.406 295 Pla in 7.324 3.7 0.406 197 4.52 5.865 7 . 4 0 . 406 295 P l a i n 7 . 966 7.4 0.406 197 4.52 6.37* B a s e d o n t o t a l 1 1 4 1 5 2 r a m 2 f r o n t a l a r e a .

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He at exchangers for seco ndary heat recovery from glass plan ts 81c o m p o s e d o f n a tu r a l g a s c o m b u s t i o n p r o d u c t s w i t h d il u ti o n a i r, a n d t h e f o ll o w i n g a d d e dcontaminants (ppm by weight ) : SO2 (200); SO3 (27.6); and HCI (16). The part icula tes were takenf rom an opera t i ng g l ass fu rnace . The i r s ize d i s t r i bu t ion was 11 < d < 75 #m , wh ich i s cons ide rab lyl a rge r t han the d i s t r i bu t ion g iven in Tab le 2 . The fou l ing ra t e s were de f ined by the measu redpressu re d rop . A l l t e s t co res had 114 x 152 mm gas -s ide f ron t a l a re a an d w ere 305 mm deep .Tw o cleaning techn iques w ere tes ted: (1) a t raversing a i r lance consis t ing of a series of smal l holesd r i ll ed a long the ax i s o f a t ube , and (2) an " a i r can non " , wh ich uses a sudden d i scha rge o f a la rgequan t i t y o f h igh p ressu re a i r . The a i r l ance p rov ides a l i ne a r ray o f h igh ve loc i ty a i r j e t s wh ichd i scha rge d i rec t ly on to t he f l ow passages , a t t he ups t ream exchanger face. Each j e t d i scha rges 0 .59kg h -1 throu gh a 1 .14 mm orif ice ." H o t e n d " f o u l i n g t e s t s

These "accelera ted" foul ing tests (6) used 677C gas in le t temperature , 149C in le t a i r , andinc iden t gas ve loc it i es o f 5 .8 -9 .5 m s -1. The gas ve loc it ie s a re equa l t o t hose t ha t w ou ld b e emp loyedin a Bray ton cyc l e hea t exchanger . A 1600 ppm dus t con cen t ra t i on was used , wh ich i s 8 t imes t heva lue g iven in Tab le 2 . F igu re 2 shows the fou l ing da t a fo r p l a in f i n su r face 3 and o f fse t s t r i p f i nsurface 4 (Tab le 4) . Th e 300 lb in -2 a i r lance, opera t ing a t app rox ima tely 10 h in tervals succe ssful lycleaned the 295 f ins m -~ p la in f in surfac e , b ut wa s ineffect ive for the 197 f ins m -1 offse t s tr ip f in .A iResea rch pe r fo rmed fu r the r exper imen t s w i th t he Tab le 4 , geomet ry 3 , u s ing 220 ppmpar t i cu l a t e l oad ing [9 ] , and inves t i ga ted the e f fec t o f SO3 conce n t ra t i on and me ta l t em pera tu re onthe c l eanab i li t y o f t he depos i t . Dep os i t s on the ba ck face o f t he exchanger were rem oved andchemica l ly ana lyzed fo r ac id i ty (pH ) and Na2S~O7 con ten t . Us ing t i t ra t i on and X-ra y d i f f rac t i onana lysi s , it was conc lud ed tha t t he depos i t s con ta ined s ign if i can t amo un t s o f Na2S207 , bu t noN a H S O 4 . T h e m e t a l t e m p e r a t u r e s, c o r r e s p o n d i n g t o t h e s e d e p o s it l o ca t io n s , w e r e m e a s u r e d w i t hthe rmoc oup les . The re su l ts o f t hese te s t s a re show n in F ig . 3 . Th i s f igu re shows tha t t he de pos i ti n t he examined channe l s w as no t c l eanab le i f t he depos i t pH < 2 .5 ; th i s pH co r res pond s t o 20%Na2S207 . These da t a were used to e s t ab l i sh min imum meta l t empera tu res , fo r wh ich the depos i tpH > 3 , and the channels ar e c leanab le wi th the a i r lance. I t i s no ted tha t [7] indicates that Na2S207shou ld n o t have bee n fo rm ed fo r t hese gases , wh ich con ta ined SO3 concen t ra t i ons o f 27 .6 ppm (wt ) ,or less . This tes t ser ies sho we d tha t th e a i r lance wi l l res tore the p ressu re d ro p of the 7 .5 f ins m -~in p l a in f i n geomet ry t o i t s i n i t i a l l y c l ean va lue . The exchanger p ressu re d rop inc reasedapprox imate ly 4 0~ du r ing the six h pe r iod be tw een c lean ing cyc l es .

A s imi l a r fou l ing behav io u r was obse rved by H ann ake n [8 ] i n t hei r t e s t s o f t he s tuddedf in -and- tube geo me t ry il l u s t ra t ed i n F ig . 5 . The t e s t s were cond uc ted us ing regen era to r ex haus t gasand co o l ing oi l on t he t ube -s ide . The c ha rac t e r o f t he fou l an t was depe nden t on the t ube su r facet empera tu re , a s g iven be low.

1. 465C gas t em pera tu r e w i thou t hea t t rans fe r : t he depos i t cons i s ted o f a ve ry t h in coa t ing o ff ine powder , wh ich was eas i l y removed .

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8 2 R . L . W E nB e t a l .

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( 3 0 0 ) ( 4 0 0 ) ( 5 0 0 ) i 6 0 0 ) ( 7 0 0 )Metal temperature (*C)(F)

F i g . 3 . P l a t e - a n d - f i n c l e a n a b i li t y a s a f u n c t i o n o f S O 3 c o n c e n t r a t i o n ( b y v o l u m e ) a n d m e t a l t e m p e r a t u r e[ 9] . p p m w t = 2 .7 6 p p m v o l .

Ed

O3

(o

Oi I ! I2 0 4 2 6 0 3 t 6 3 7 f( 3 0 0 ) ( 4 0 0 } ( 5 0 0 } ( 6 0 0 } ( 7 0 0 }Minimum temperature C){F)

F i g . 4. R e c o m m e n d e d m i n i m u m m e t a l t e m p e r a t u r e s t o a v o i d t h e h a r d s c al e f o u l in g d e po s i t, b a s e d o np H / > 3 [ 9 ] .

F i g . 5 . S t u d d e d f i n g e o m e t r y o n 2 i n. d i a m e t e r t u b e s t e s t e d b y H a n n a k e n [ 8] .

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Heat exchangers for secondary hea t recovery fro m glass plants 83.

3.4 0 4 - 5 0 0 C g a s t e m p e r a t u r e s , w i t h 2 60 C t u b e t e m p e r a t u r e : a 3 . 2 m m t h ic k d e n s e p o w d e rd e p o s i t. A n a i r l a n c e r e m o v e d " m o s t o f t h e d e p o s i t " .3 4 3 - 4 6 0 C g a s t e m p e r a t u r e , w i t h 1 4 9 C t u b e t e m p e r a t u r e ; t h e t u b e s u r f a c e w a s c o a t e d w i t ha 0 . 1 2 - 0 . 2 5 m m t h i c k h a r d s c a le . O n t o p o f t h is w a s a 6 m m t h i c k l a y e r o f s o f t y e l l o w p o w d e r .T h e a i r la n c e r e m o v e d o n l y th e o u t e r s o f t l a y e r. T h e h a r d s c al e w a s r e m o v a b l e w i th a w a t e rw a s h .

"Cold end" fouling testsT h e s e A i R e s e a r c h t e s t s [ 6 ] w e r e c o n d u c t e d u s i n g 5 3 7 C g a s i n l e t , a n d a t 3 8 C a i r i n l e t , w h i c h

a s s u r e d m o i s t u r e c o n d e n s a t i o n . D a t a w e r e o b t a i n e d f o r t h e 2 95 fi ns m - ~p l a i n f in g e o m e t r y o f t a b l e4 u s in g 1 2 f t s -~ a ir v e l o c i t y a n d 2 0 0 p p m d u s t . T w o t e s t s e r ie s w e r e r u n : ( 1 ) w i th w a t e r a n da c i d c o n d e n s a t i o n a n d ( 2 ) a c i d c o n d e n s a t i o n o n l y . I n b o t h t e s t s , a c o n d e n s a t e - p a r t i c l e s l u d g ef o r m e d , w h i c h p l u g g e d t h e f lo w p a s sa g e s . T h e e x c h a n g e r c o r e w a s n o t c l e a n a b l e w i t h e i t h e r th ea i r la n c e o r a i r c a n n o n . T h e g r e a t e s t f o u li n g r a t e o c c u r r e d f o r a c id c o n d e n s a t i o n o n l y . S i gn i fi c a ntc o r r o s i o n w a s o b s e r v e d o n t h e 4 0 9 s t a i n l e s s s t e e l f i n s .

Finned-tube fouling dataF o u l i n g d a t a o n f i n n e d - t u b e s i n g l a s s f u r n a c e e x h a u s t s a r e n o t r e p o r t e d i n t h e l i t e r a t u r e .

P r o p r i e t a r y t e s ts o f t h e F ig . 5 s t u d d e d - f in h a v e b e e n n o t e d .W e h a v e s u g g e s te d t h a t f i n n e d - t u b e s u se d f o r h e a t r e c o v e r y f r o m g a s - t u r b in e a n d d i es e l e x h a u s t s

m a y e x p e r i e n c e si m i la r f o u l i n g m e c h a n i s m s t o t h o s e o f a g la s s f u r n a c e e x h a u s t. R o b e r t s a n dK u b a s c o [ 1 2 ] m e a s u r e d f o u l i n g o n a b a n k o f h e li c al -f in n e ~ t t u b e s i n t h e e x h a u s t o f a g a s tu r b i n e .T h e t u b e s w e r e 1 9.0 5 m m d i a m e t e r w i t h 2 75 f in s m - % a p p r o x i m a t e l y 9 m m f in h e ig h t . T h e t u b e sw e r e e x p o s e d t o 4 5 0 C e x h a u s t g a s e s, a n d t h e fi n t e m p e r a t u r e s w e r e 1 5 0 - 3 15 C . T h e d e p o s i ts w e r e" f l u f f y " a n d e a s il y r e m o v e d b y a i r la n c in g . T h e d e p o s i t t h ic k n e s s i n c r e a s e d a s t h e m e t a l t e m p e r a t u r ew a s d e c r e a s e d . A n a l y s is o f t h e d e p o s i t su g g e s te d a " t w o - s t a g e m e c h a n i s m o f s o o t b u i l d - u p " . T r a c eq u a n t i ti e s o f h y d r o c a r b o n s c o n d e n s e o n t h e s u r fa c e f o r m i n g a t h i n c o a t i n g o f a n a d h e s i v e n a t u r e .P a r t i c u l a te s a r e t r a p p e d b y t h is c o a t in g . A " s e l f - c le a n i n g " m e t h o d w a s e v a l u a t e d , in w h i c h c o o l a n tf lo w t o th e t u b e - s i d e w a s s t o p p e d , a n d t h e h o t g a s es a c t e d t o " b a k e o f f " t h e d e p o s it . T h e" t h r e s h o l d " t e m p e r a t u r e f o r s e lf - c le a n i n g w a s f o u n d t o b e b e t w e e n 3 5 5 a n d 4 2 0 C .

S i l v e st r in i [1 3] m e a s u r e d " c o l d e n d " f o u l i n g a n d c o r r o s i o n o n 3 2 m m d i a m e t e r 1 18 a n d 1 97 f in sm - ~ u b e s i n t h e 2 0 0 - 2 6 0 C e x h a u s t o f a d ie s e l e n g in e . T h e t e s t c o m p a r e d f o u l i n g o n u n c o o l e d t u b e sw i t h t h a t o n c o o l e d t u b e s ( 1 0 4 C w a l l te m p e r a t u r e ) . T h e a c i d c o n d e n s a t i o n t e m p e r a t u r e w a s 1 17 C ." N o m e a s u r a b l e s o o t a c c u m u l a t io n w a s o b s e r v e d o n a n y o f th e u n c o o l e d t u b e s ." H o w e v e r , th ec o o l e d t u b e s e x h i b i t e d s e v e r e f o u l i n g a n d p l u g g i n g .

T h e s e s t u d i e s s h o w a c o n s i s t e n c y w i t h t h e g l a s s e x h a u s t s t u d i e s i n s e v e r a l r e s p e c t s. ( ! ) F o u l i n gis d e p e n d e n t o n t h e w a ll t e m p e r a t u r e a n d t h e s u r f a c e - t o -g a s t e m p e r a t u r e d i ff e r en c e . ( 2 ) I f v a p o r sd o n o t c o n d e n s e , a n e a s i ly r e m o v e d f l u ff y d e p o s i t o c c u r s . I n t h e l i m i t in g c a s e o f z e r o h e a t t r a n s f e r ,t h e p a r t i c u l a t e d e p o s i t i s o f n e g li g i b le th i c k n e s s . ( 3 ) A b o v e t h e a c i d a n d w a t e r v a p o r d e w p o i n t s ,s o m e v a p o r s m a y c o n d e n s e . T h e s e y i e ld a h a r d d e p o s i t o n t h e t u b e s u r f a c e t h a t i s d i ff ic u lt t o r e m o v eb y a i r l a n c i n g . H o w e v e r , i t m a y b e w a s h e d a w a y w i t h w a t e r . ( 4 ) B e l o w t h e a c i d a n d w a t e rd e w p o i n t s , t h e d e p o s i t is s ti c k y a n d c a n n o t b e r e m o v e d b y a i r l an c i n g , a l th o u g h i t m a y b e w a t e rw a s h e d .

Heat transfer with bare tubesR i c h a r d s [ 1 0] d i s c u s se s o p e r a t i o n a l e x p e r i e n c e w i t h th r e e g l a ss p l a n t w a s t e h e a t b o i l e r s in E u r o p e .

T h e g l a ss f u r n a c e s a r e f ir e d w i t h h e a v y o i l ( 0 . 8 - 1 . 8 ~ s u l p h u r ) . A l l b o i l e r s h a v e p l a i n t u b e s , u s eM g O a d d i t i o n a n d a r e s o o t b l o w n o n c e p e r s h i ft . T a b l e 5 g iv e s p a r t i c u l a r s f o r e a c h i n s t a l la t io n .A l l b o i l e r s h a d b e e n i n o p e r a t i o n t w o y e a r s o r l o n g e r a n d s a t i s f a c t o r y o p e r a t i o n i s r e p o r t e d .R i c h a r d s s t a te s t h a t N a _ ,S 20 7 w o u l d b e f o r m e d i f M g O i n je c t i o n w e r e n o t u s e d . T h e M g O c o n s u m e st h e S O 3, a n d n e g a t e s t h e p o s s i b i l it y o f N a 2 S 20 7 f o r m a t i o n . T h e f o u l a n t d e p o s i t s a r e " d r y , f r i a b l e ,a n d c a n b e r e m o v e d b y s o o t b l o w e r s " .

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8 4 R. L. WEB~ et aL

T a b l e 5 . W a s t e h e a t b o i l e r i n s t a l l a ti o n sU n i t 1 U n i t 2 U n i t 3T y p e w a t e r t u b e f ir e t u b e w a t e r t u b e

G a s i n l e t ( C) 460 519 382G a s ou t l e t ( C) 277 320 280T ube - s i de ( C) 293 300 65 .5T ube d i a m e t e r ( nun) 63 ,5 63 .5 31 .8N u m b e r o f t u b e s 9 0 0 1 3 12 1 24S o o t b l o w e r S t e a m S t e a m A i r

F i n n e d - t u b e v s p l a t e - f i n d e s i g n sI t is in t e r es ting to specu la t e on the r e l a t ive fou l ing r a t e s o f these tw o exchanger t ypes , and the i rc leanabi l i ty .The p l a t e - and- f in geom et ry has a con s t an t a r ea f low channe l , and n o s t agna t ion o r sepa ra t ed

f low zones wi th in the f in passage . Bu t , a s t agn a t ion r eg ion ex i st s a t t he f ron t f ace o f t he hea texchanger . One wou ld expec t a r e l a t i ve ly un i fo rm fou l ing depos i t i n t he f in channe l s ( a s wou ldoccur f o r a f i r e - tube bo i l e r des ign ). H ow ever , p lugg ing wi ll occu r o n the f ro n t f ace o f t he exchanger .a s o b s e r v e d in t h e A i R e s e a r c h t e s ts . T h e A i R e s e a r c h r e p o r t s d o n o t d e f in e h o w t h e p r e s s u re d r o pinc r ease was a ppo r t ione d be tw een fou l ing a t t he f ace o f t he exchanger , an d in t he f low passages .Soo t b lowing wi ll a f f ec t f ou lan t r em ova l a s a r e su l t o f h igh shea r s t r e ss a t t he f in su rf ace . There fo r e ,one wo u ld exp ec t t h i s h igh v e loc i ty wi ll be qu i t e e f f ec tive ove r t he en t i r e f i nned su r f ace a r ea .

The f inned tubes exper i ence a s t agna t ion /acce l e r a t ing f low on the u ps t r eam ha l f o f t he tube anda dece l e r a ting f low wi th f low separa t ion on the do wn s t r eam ha l f o f the tube . Bec ause the su r f aceshea r s t r e ss i s g r ea t e r on the f ron t ha l f o f t he tube , one may expec t sma l l e r f ou l ing depos i t s onthe ups t r eam f ace . Th i s i s p r ec i se ly the fou lan t depos i t pa t t e rn obse rved by Kind lman andS i lves tr ini [ 13 ] f o r hea t r e covery f rom d iesel exhaus t w i th ac id conden sa t ion . The fou l in g dep os i tt h i cknesses a t t he f in -t i p f o r t he ups t r e am and d ow ns t r eam f aces a r e show n by F ig . 6 . W hen thesoo tb lower i s ope ra t ed , t he sma l l e s t l oca l su r f ace shea r s t r e sses wi l l occu r on the downs t r eamface - --where the g r ea t es t f ou la n t dep os i t ex i st s . Thus , t he des i r ed c l ean ing shea r s t r e ss d i s t r ibu t ioni s oppos i t e t o t he des i r ed d i s t r i bu t ion .The above obse rva t ions sugges t t ha t t he p l a t e - and- f in des ign wi l l make be t t e r u se o f t he soo tb low er mom en tu m, and wi ll experi ence r e l a tive ly un i fo rm c l ean ing . I f t h i s eva lua t ion i s co r r ec t .i t i s i n t e r es t ing to f u r the r specu la t e on a l t e rna t e f i n - and- tuhe geomet r i e s t ha t w i l l exper i ence thef avorab le c l ean ing cha rac t e r i s t i c s o f t he p l a t e - and- f in des ign . One poss ib i l i t y i s t o employ acoun te r f low exchanger hav ing long i tud ina l ex t e rna l f i n s on the gas - s ide .

C O N C L U S I O N S1. I f t he ac id conden sa t ion r eg ime i s avo ided , i t appe a r s t ha t f i nne d , tube an d p l a t e - and- f in

geomet r i e s can be success fu l ly app l i ed to g l a ss secondary hea t r ecovery app l i ca t ions .

2 FLOW x ~ x% .xE ~5 (~

- ~ 05LI. ~ eI I I I l J0 7 14 21 28 35 42 49 56 63 rOTime I days)

F i g . 6 . F o u l i n g t h i c k n e s s n e a r t h e f in t i p o n 3 1 . 7 5 m m d i a m e t e r t u b e s w i t h 1 1 8 fi n s m - ~, 1 5 .9 m m f inh e i g h t [ 1 3 ] .

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He at exchangers for secondary heat recovery from glass plan ts 852 . T h e c o n d i t i o n s f o r w h ic h a l k a l i - p y r o s u l p h a t e s a re f o r m e d a n d c o n d e n s e d a r e n o t c l e a r ly

u n d e r s t o o d . T h e i r f o r m a t i o n a p p a r e n t l y y i e l d s a h a r d s c a le at r e d u c e d w a l l t e m p e r a t u r e s . F i g u r e5 s u g g e st s m i n i m u m m e t a l t e m p e r a t u r e s t h a t w i l l a v o i d t h e h a r d s ca le .

3 . T h e u s e o f M g O a d d i t i o n w i l l a v o i d t h e p y r o s u l p h a t e f o r m a t i o n a n d t h e f o u li n g d e p o s i t w i llb e f l uf fy a n d c l e a n a b l e .

4 . P r e s e n t f o u l i n g d a t a s u g g e s t s t h a t p l a i n f in s s h o u l d b e u s e d .5. T h e p e r m i s s i b l e f i n d e n s i t y w i ll d e p e n d o n t h e d e s i r e d c l e a n i n g f r e q u e n c y . T h e A i R e s e a r c h

p l a t e - a n d - f i n d a t a [6 , 9 ] s u g g e s t t h a t 5 f in s in - 1 w o u l d n o t r e q u i r e c l e a n i n g m o r e t h a n o n c e p e r s h if t.T h e r e i s n o e x p e r i m e n t a l b a s i s to r e c o m m e n d a m a x i m u m f in d e n s it y f o r a f i n n e d - tu b e e x c h a n g e r .

6 . F l u i d i z e d - b e d h e a t r e c o v e r y s y s t e m s o ff e r t h e p o s s i b i l i t y o f s e c o n d a r y h e a t r e c o v e r y , w i t h o u tt h e fo u l i n g p r o b l e m s a s s o c i a t e d w i t h c o n v e n t i o n a l h e a t e x c h a n g e r s .A c k n o w l e d g e m e n t - - T h i s work was performed under Dep ar tment of Energy contract DE-FG07-81IDI2225 with Dr W. H.Thielbahar serving as Technical M onitor .

R E F E R E N C E S1. R. L. Webb and A. K. Kulkarni, Heat exchanger needs for recovering waste heat in the glass making industry, f inalrepor t on DOE Contract DE-FG07-811D12225 (July 1982) .2. A. K. Kulk arni, Y-J. W ang and R. L. Webb, Fou ling and co rrosion in glass furnace regeneration, Int. Conf. on Fo ulingof Heat Exchange Surfaces, Wh ite Haven, P A (31 Octob er-5 November, 1982).3. J. G . Hnat, J. S. Patten and P. R . Sheth, Ran kine and B rayton cycle cogeneration for glass melting, Industrial EnergyConv. Tech. Conf. , Houston, TX (April 1981).4. R. Sakh uja and W . E. Cole, Fluid ized Bed for Glas s Batch Preheating, Ceramic Eng. and Sci. Proc. 41st Conf . on GlassProb lems , pp. 79-86, American Ceramic Soc. (1981).5. Anon., Flue gas characteriz ation of glass furnace, AiResearch Mfg Co., R epo rt 80-16900 (March 1980).6. L. Burgmeier and S. Leung, Hea t exchanger and cleaning system report, AiResearch M fg Co. R epo rt 81-17932 onD.O.E. C on tract EC -77-C-03-1557 (21 August 1981).7. W. T. Reid, Exte rna l Corrosion and Dep os i t s - -B o i lers and Gas Turbines , p. 106. Am erical Elseview, New Yo rk (1981).8. H. Hannaken, PPG Industries, Pittsburgh, PA, pers. comm. (July 1981).9. Ano n., Positive pressure heat exchanger cleaning system test report, AiResearch R epo rt 81-18259 on D.O.E. Co ntractEC-77-C-03-1557 (15 October 1981).

10. B. E. Richards, W aste-H eat Boilers for Fl at Glass Furnaces, Proc . 40 th Conf . on Glass Prob lems , pp. 5 0-58. Universityof I llinois (1979).11. P. K. Engel, R. Silvestrini and R. E. Thompson, Co rrosion an d fouling potentia l in diesel exhausts, ASM E pape r78-W A/FU-5 (1978).12. P. B. Ro berts an d A. J. Kub asco, Combined cycle steam ge nerato r gas-side fouling evaluation: phase I f inal report,solar turbines international report SR79-R-4557-20 (July 1979).13. L. Kin dlm an and R. Silverstr ini, Hea t exchanger fouling an d corrosio n evaluation. AiResearch repo rt 78-1516(2) onD.O.E. Contract DE-AC03-77ET11296 (30 April 1979).

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