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Heat ~ Systems Vo l. 7, No. 2, pp. 151-15 7, 1987. 0890-4332/87 $3.00 +0.0 0Printed in Great Britain. Pergamon Journals Ltd

BOILER FUEL SAVINGS BY HEAT RECOVERY ANDREDUCED STANDBY LOSSES

B. M. GraBsD epar tm en t o f F ue l and E ne rgy , U n ive r s i t y o f L eeds , L eeds , L S 2 9JT , U . K .

(Receired I August 1986)A lm rac t - -T h i s pape r i s conce rned w i th an i nves t i ga t i on i n to t he fue l s av ings t ha t can be ach i eved i n afue l- f ir ed package bo i l e r by t he use o f an econom ise r w h ich i nco rpora t e s a f lue dam per , l i nked v i aa con t ro l l e r , t o t he bu rne r . T he fue l s av ings a r e accom pl i shed by tw o m eans .

I . By di rect heat recovery f rom the f lue gases leaving the boi ler and2 . B y a r educ t i on i n bo re r d r augh t and coo l ing l o s ses by c los ing t he f l ue dam per w h ich i sac t i va t ed on burne r shu t dow n . T hese l o s ses occur du r ing no rm a l bu rne r cyc l i ng and s t andbyp e r i o d s .E xper im en ta l hea t r ecove ry da t a w ere ob t a ined f rom a ho t gas bo i l e r s im ula t ion t e s t r ig , and show nto be depend ent o n the exhaust tempe rature , excess a i r level and boi ler s ize . S tandby saving s werep red i c t ed f rom a bo i l e r coo l i ng m ode l , based on m easured bo i l e r coo l i ng r a t e s and d rau gh t cond i t i ons .D e t a i ls o f t he coo l i ng m ode l a r e g iven i n t he pape r . T he s t andby sav ings w ere found t o depend on bo i l e rope ra t i ng t im e , t he num ber o f bu rne r cyc l e s du r ing bo i l e r ope ra t i on and t he bo i l e r coo l i ng r a t e t im econs t an t , bu rne r pu rge t im e and f lue gas dam per de l ay t im e . O vera l l t he fue l s av ings t ha t can be ach i evedw ere found t o be i n t he r ange 6 -16% depend ing on t he ope ra t i ng cond i t i ons p reva i l i ng , and add i t i ona ltes ts on boi lers conf i rmed these f indings .

NOMENCLATUREC V fuel calor i f ic valuef f r ac t i on o f o r i g ina l fue l s avedm bo i l e r fue l consum pt ionn burne r cyc l e s /hourN num b er o f bour s /day bo i l e r ope ra t e sq bo i l e r coo l i ng r a t eq ( t ) bo i l e r coo l i ng r a t e a s a func t i on o f t im eQ to t a l hea t s avedt t imeT s t andby t im er/ boi ler efficiencyr bo i l e r coo l i ng t im e cons t an tSubscripts1 , 2 w i thou t , w i th f lue dam p er o r econom ise rs , , s2 s teady s ta te wi tho ut , wi th f lue dam perc , d , p , b cycle , dam per delay, purge, f i r ing (t ime)e , d , t heat recovery, dam per , to ta l ( fue l savings)r, sb bo i l e r runn ing , s t and ing-byt to ta l /day

I N T R O D U C T I O NI n o r d e r t o i n c r e a s e t h e e f f i c i e n c y o f b o i l e r s a n d r e d u c e f u e l c o n s u m p t i o n , e c o n o m i s e r s a r ef r e q u e n tl y f it te d o n t o b o i l e r e x h a u s t s t o r e c o v e r a d d i t io n a l h e a t f r o m t h e c o m b u s t i o n p r o d u c t s , a n dl o w e r b o i l e r e x h a u s t t e m p e r a t u r e s . A c o n v e n t i o n a l e c o n o m i s e r c a n t h e r e f o r e b e c o n s i d e r e d t o b ea hea t ex cha ng er , i n w h i ch t h e hea t reco v ered i s t ra ns ferred t o t he bo i l er re t urn w a t er o r f eedw a t er .T h e l i m i ta t i on o n t h e h e a t r e c o v er e d i s g o v e r n e d b y t h e d e w - p o i n t o f th e c o m b u s t i o n p r o d u c t s ,a n d t h e t y p ic a l h e a t r e c o v e ry i s e q u i v a l e n t t o a 3 - 5 % f u e l s a v in g . I n m a n y c a s e s , b e c a u s e o f t h el o w p r e s s u r e d r o p c o n s t r a i n t s o f t h e e c o n o m i s e r d e s i g n , t h e e c o n o m i s e r h a s t o i n c o r p o r a t e e x t e n d e ds u r f a ce s s o t h a t i t i s n o t e x c e s s iv e l y l a rg e . T h e l a tt e r c a n l e a d t o t h e e c o n o m i s e r b e c o m i n g e x p e n s i v ew i t h e x c e s si v e l y l o n g p a c k - b a c k p e r i o d s . I t h a s t h e r e fo r e b e c o m e n e c e s s a r y t o d e v e l o p e c o n o m i s e r st ha t g i v e enha nced f ue l s a v i ng s , i n o rder f o r t hem t o be eco no m i ca l l y v i a b l e , w i t h rea l i s t i cp a c k - b a c k p e r i o d s . T h e o b j e c t i v e o f th i s s t u d y w a s t o a s s e s s t h e p e r f o r m a n c e o f a n e c o n o m i s e r

151

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1 5 2 B . M . G IB B S

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I F L u e Oos inlet2 z n s u m t ~3. Tu~e tx~xlte system4. Flue Oos r , omdo ry ~uOe5. Woter inter6. Wot~ ex i t7 By-oOS$ domoer8. Ftm Om domoer9 / i 0 . D o m ~ r sett ln 9 motors

Fig. i . T h e R o n d r a E c o m a t .

w h i c h w a s d e s i g n e d t o g i v e n e n h a n c e d f u e l s a v i n g s b y r e d u c i n g t h e b o i l e r s t a n d b y l o s s e s , d u e t ot h e i n c l u s i o n o f a m o t o r i s e d d a m p e r i n t h e e c o n o m i s e r . T h i s p a p e r p r e s e n t s t h e r e s u l t s o f t h i si nv es t i g a t i o n .

D E S C R I P T I O N A N D O P E R A T I O N O F E C O N O M I S E RT h e e c o n o m i s e r i s m a n u f a c t u r e d b y T u b o r E n g i n e e r i n g o f S w i t z e r l a n d , a n d c o m e s u n d e r t h e

t r a d e n a m e , R o n d r a - E c o m a t .T h e co no m i s er , F i g . 1 , co n s i s t s o f a cy li ndr i ca l bo d y w i t h a cen tra l f lue g a s t ube a nd w i t h t w oh e a d e r s a t e a c h e n d . A r o u n d t h e c en t r a l f lu e t u b e i s a b u n d l e o f s m a l le r h e a t e x c h a n g e r t u b e s w h i c h

a re l o ca t ed i n a w a t er - co o l ed cy l i ndr i ca l s he l l t ha t i s bo unded by t he i nner f l ue t ube a nd o ut er s he l lca s i ng . T he t ubes a re w e l ded i n t o po s i t i o n be t w een t w o end p l a t e s . A da m per i s f i t t ed i n t o t he c o n o m i s e r g a s e x h a u s t w h i c h i s d e s i g n e d t o r e d u c e d r a u g h t l o s s e s w h e n t h e b o i l e r i s o n s t a n d b y

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Boiler fuel savings 153a nd t he burn e r i s no t f i ri ng . A se c ond d a m pe r i s f i tt e d i n t o t he c e n t ra l f l ue tube w hi c h d i ve r t s t heh o t c o m b u s t i o n p r o d u c t s t h r o u g h t h e s m a l l e r h e a t t r a n s f e r t u b e s c a u s i n g h e a t e x c h a n g e t o t a k ep l a c e t o t h e w a t e r f l o w i n g t h r o u g h t h e e c o n o m i s e r s h e l l . B o t h d a m p e r s a r e m o t o r o p e r a t e d a n dl i nke d t o t he burne r v i a a c on t ro l l e r .

The ope ra t i ng se que nc e o f the da mp e rs i s a s fo l l ows , a nd is i ll us t ra e d sc he m a t i ca l l y i n F i g . 2 .1 . P r i o r t o i gn it ion o f t he burne r , t he fl ue ga s da mp e r a n d t he b ypa s s da m pe r a re se t t o t he ope npos i t i on , a nd t he sys t e m a l l owe d t o purge fo r a p re - se t pe r i od o f t i me .2 . Th e burne r i gn i te s , a nd t he c om bu s t i on p r odu c t s ha ve unre s t r i c te d f l ow t h roug h t he c e n t ra l

f l ow t ube un t i l t he f l ue ga s t e mpe ra t u re i s a c h i e ve d . The ga s bypa ss da mpe r now c l ose s d i ve r t i ngt h e c o m b u s t i o n p r o d u c t s t h r o u g h t h e h e a t e x c h a n g e r t u b e s i n t h e e c o n o m i s e r . T h e b o i l e r w a t e rpa ss i ng t h rough t he e c onomi se r she l l wi l l now be he a t e d by t he c ombus t or p roduc t s l e a v i ng t heboi le r .3 . Whe n t he burne r shu t s o f f , a nd t he bo i l e r i s on s t a ndby , t he f l ue ga s da mpe r a nd t he bypa ssda mpe r a re shu t a f t e r a p re - se t de l a y t i me (whi c h e na b l e s a ny re s i dua l ga se s t o e xha us t ) . Whi l s tt he da mpe rs a re c l ose d t he d ra ught he a t l os se s t h rough t he bo i l e r a re d i mi n i she d , r e su l t i ng i n ac ons i de ra b l e re duc t i on i n bo i l e r s t a ndby he a t l os se s .

M E A S U R E M E N T O F D I R E C T H E A T R E C O V E R Y F R O M T H E E C O N O M I S E RBoiler exhaust gas simulation test rig

I n o r d e r t o b e a b l e to m e a s u r e t h e d i r ec t h e a t r e c o v e r y o f t h e e c o n o m i s e r o v e r a w i d e r a n g e o ft yp i c a l bo i l e r e xha us t c ondi t i ons ( i . e . c ombus t i on p roduc t s f l owra t e s , bo i l e r e xha us t t e mpe ra t u re s ,bo i l e r r e t u rn w a t e r f l ows a nd t e mpe ra t u re s ) , a bo i l e r e xha us t ga s s i mul a t i on t e s t r ig wa s bu i l t. Th i se s se n ti a ll y c ons i s t e d o f a ho t ga s ge ne ra t o r whi c h prov i de d a me t e re d su ppl y o f he a t e d a i r a tt e mpe ra t u re s be t we e n 150 a nd 450C . The ho t ga s ge ne ra t o r wa s c onne c t e d d i re c t l y t o t he i n l e ts i de o f t he e c onom i se r by me a ns o f a 3 m l ong w e l l- i nsu l a te d duc t . Th e e c on om i se r in l e t a nd ou t l e tg a s t e m p e r a t u r e s w e r e m e a s u r e d b y m e a n s o f s u c t io n p y r o m e t e r s w h i c h w e r e l in k e d t o a n e l e c tr o n icd i g i t a l t e mpe ra t u re i nd i c a t o r a nd a c ha r t r e c orde r . The e c onomi se r wa s c onne c t e d t o a me t e re ds u p p l y o f h o t w a t e r , a n d t h e w a t e r s u p p l y t e m p e r a t u r e c o u l d b e v a r ie d b e t w e e n 3 5 a n d 8 0 C . T h ei n l e t a nd ou t l e t e c onomi se r wa t e r t e mpe ra t u re s we re me a sure d by d i re c t i nse r t i on me rc ury i n g l a s st h e r m o m e t e r , c a p a b l e o f an a c c u r a c y o f + 0 . 1 C . I n a d d i t i o n , t h e r m o c o u p l e s w e r e a l so u s e d f o rc o n t i n u o u s w a t e r t e m p e r a t u r e m e a s u r e m e n t s .

D u r i n g t h e te s ts t h e b y p a s s d a m p e r w a s c lo s e d a n d t h e e x h a u s t d a m p e r o p e n e d s o t h a t t h e h o ta i r pa s se d t h rough t he he a t e xc ha nge r t ube s l oc a t e d i n t he she l l . The he a t r e c ove re d i n t hee c o n o m i s e r w a s m e a s u r e d o v e r a w i d e r a n g e o f h o t g a s f l o w r a t e s a n d t e m p e r a t u r e s i n o r d e r t os i mul a t e t he e xha us t c on di t i ons o f a bo i l e r i n t he si ze ra nge 100-200 kW , whi c h wa s t he bo i l e r s iz efor whi c h t he e c onomi se r wa s o r i g i na l l y de s i gne d .Calculation o f the fue l savings fro m the measured heat recovery data

H a v i n g m e a s u r e d t h e a m o u n t o f h e a t t h a t c a n b e r e c o v e r e d b y t h e b o i le r h e a t t r an s f e r s u r f ac e si t i s now poss i b l e t o p re d i c t t he fue l s a v i ngs t ha t c o u l d b e a c h i e ve d i f t he e c onom i se r wa s c oupl e don t o t he ou t l e t o f , a bo i l e r . In o r de r t o c a l c u l a t e t he fue l s a v i ngs , t he fo l l owi ng de t a i ls o f t he bo i l e ra re re qu i re d :

1 . The fue l type , c a l o r if i c va l ue , s t o i c h i ome t r i c a i r a nd wa s t e ga s 6o wra t e s p e r u n i t ma ss o rv o l u m e o f f u e l.2 . The bo i l e r he a t c a pa c i t y e xpre s se d a s t he he a t i n pu t t o t he w a t e r .

3 . The bo i l e r e xha us t ga s t e mpe ra t u re .4 . The ope ra t i ng e xc e ss a i r l eve l o r IC O21 c on t e n t .5 . The re t u rn bo i l e r wa t e r te mp e ra t u re .Calculation procedure. T he a bove bo i l e r da t a e na b l e t he fue l a nd wa s t e ga s f l owra t e s , a nd bo i l e r

e f fi ci e ncy (~m) t o b e c a lc u l a t e d . Th e h e a t r e c ove re d i n t he e c on om i se r c a n t he n b e de t e rm i ne d f romt h e ex p e r i m e n ta l d a t a , u s i n g th e k n o w n w a s t e g a s f lo w r a t e a n d t e m p e r a t u r e ( c o r r e s p o n d i n g t o th es i m u l a te d f l o w o f h o t g a s a t k n o w n t e m p e r a t u r e s i n t h e e x p e ri m e n t) . T h e h e a t r e c o v e r e d is n o wuse d t o o b t a i n t he ne w bo i l e r ef f ic ie nc y , ~2, due t o t he a dd i t i on o f t he e c ono mi se r . F i na l l y t he fue l

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F i g . 4. B o i l e r e ~ c i e n c y d u e t o c o n o m i s c r h e a t e x c h a n g e .

saving, f, can be calculated from the boiler efficiencies, using the relationship= l - , I , / , 1 2 . ( l )

Predicted fuel savings and boiler efficiencies. Figs 3 and 4 show the fuel savings and improvedboiler eflicieneies that would be achieved if the economiser was fitted onto a boiler with a heatoutput of 100-200 kW, firing on a light fuel oil at an excess air level of between 20 and 40%, andan exhaust temperature between 200 to 350C. It can be seen from the calculated fuel savings andboiler effieieneies that:

!. In the normal flue gas temperature range 250/350C, likely to be experienced in practice, fuelsavings between 4 and 8% are achieved depending on the operating excess air, and boilercapacity and flue gas temperature.2. The higher the temperature difference between the return (boiler) water and flue gas, the higheris the fuel saving. Increasing the flue gas temperature from 250 to 350C doubles the heat thatcan be recovered (if the return water is at 55C).

3. Increasing the excess air from 20 to 40% results in an increase of about 10% in heat recoveryand fuel savings.4. Within the boiler range 100--200 kW, the 100 kW boiler would save about 35% more fuel thana 200 kW boiler using the same economiser.

ESTIMATION OF BOILER STANDBY SAVINGS DUE TO ECONOMISER FLUEGAS DAMPERThe boiler standby savings can be estimated by an analysis of the burner cycles, and the boiler

cooling behaviour with and without the damper (shown schematically in Fig. 2).Boiler cooling behaviour without flu e damp er installedThe cooling behaviour of a boiler is best deduced from transient cooling tests by allowing theboiler to reach steady state conditions and then cutting the burner off. Several tests on a numberof boilers were carried out and it was established that the transient cooling rate, Fig. 2, could best

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B o i l e r u e l a v i n g s 1 5 5io

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0 0 5 J.O

F i r in ~ t i m e ( % / r e )F i g . 5 . I n f lu e n c e o f b o i l e r o p e r a t i n g t i m e o n s t a n d b y s a v i n g s .

b e a p p r o x i m a t e d b y a n e x p o n e n t ia l , fi rs t o rd e r , e q u a t i o n o f t h e f o r mq ( t ) = q l e - t / " + q , . ( 2 )

T h e t o t a l h e a t l o s t, Q , ( t ) , a t a t i m e t a f t e r t h e b u r n e r h a s s h u t o f f i s o b t a i n e d b y i n t e g r a t i n ge q u a t i o n ( 2 ) , w h i c h y i e l d s

Q l ( t ) = ql~,(I - e - t / ' ' ) + q , , t . (3 )I f t i s g r ea t e r t han 10 t ime cons t an t s t hen

Q , ( t ) ~ q , , t (4 )and the to t a l hea t l o s t w i l l s imp ly be the s t eady s t a t e d r augh t l o sses .B o i l e r co o l i n g b eh a v i o u r w i t h d a m p er i n s t a l l ed

T h e f lu e d a m p e r i s a c ti v a t e d a u t o m a t i c a l l y s o m e t ~ r a i n a f t e r t h e b u r n e r s h u t s d o w n . T h e e f fe c to f t he f lue da m pe r on the t r ans i en t coo l ing be hav io u r is show n in F ig . 2 . Th e overa l l e ff ec t i s t or educe bo th the t r ans i en t and s t eady s t a t e l o sses f rom the bo i l e r . Aga in a ssuming exponen t i a lcoo l ing behav iou r , t he to t a l he a t l oss , Q 2 ( t ) a t t i m e t a f t e r t h e b u r n e r s h u t s o f f c a n b e s h o w n , u s i n gthe same p rocedure a s p r ev ious ly , t o be g iven byQ z ( t ) = [q :n ( ! - e - ' / '' ) + q~,td] + [q2x2(e t ' / '2 - -#~2) + q~ ( t - td)]. (5)

Du r ing a l ong bu rn e r shu t o f f pe r iod , say 10 min o r mo re , t yp ica l ly a t l ow load su ch as n igh t t ime ,the hea t l o sses wi l l be the s t eady d r augh t l o sses wi th the f lue damper ac t iva t ed , i . e .

Q2 (t) "~ q,2t . (6 )O v e r a l l h e a t s a v i n g s d u e t o f l u e d a m p e r

T h i s c o n s i s t s o f t w o c o m p o n e n t s , t h e h e a t s a v i n g d u r i n g n o r m a l b o i l e r o p e r a t i o n w h e n t h eburn e r wi ll be cyc l ing (F ig . 2 ) and the he a t sav ing du r ing bo i l e r o f f pe r iods o r s t andb y pe r iods .

S a v i n g d u r i n g b o i l e r cyc l i n g , Q , . I f t h e b o i le r o p e r a t e s f o r N h / d a y , a n d t h e b u r n e r c y c l e s nt imes /h , t hen the to t a l hea t sav ing , Q , , w i ll be g iven by the d i f f e rence be twee n the to t a l l o sses wi tha n d w i t h o u t t h e f l u e d a m p e r , o rN nQ, = - --~[Q , ( t ) - Q2(t) l . (7)

S t a n d b y s a v i n g s . I f t he bo i l e r i s on s t andb y fo r T h /day , t hen the to t a l h ea t sav ing , Q ,~ , i s g ivenby the express ionQ , , = I Q , ( T ) - Q2(T)I ~ Iq, , + q,21T. (8 )

HR.S. ?/2--D

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156 B.M . GiBeslC

n = 5 cy/hr\ ~ . . . . n 1 5 y /h r

E~ _ " , , , ~ ~, ~ , \ N . 1 2 . ~' , \ " , \% % % % % ~

0 0 5 0F i r i n g time ( t b / t c )

Fig. 6. Effect of burner cycles on standb y savings.

Total heat saved~day, Q , . The t o t a l he a t s a v e d / da y wi ll be t he sum of t he sa v ings ob t a i ne d dur i ngboi l e r c yc l i ng a nd bo i l e r s t a ndby , o r Q, = Q, + Q~. (9)Fuel savings due to damper

I f t he o r i g ina l fue l c onsu mp t i on wa s m~ (ma ss o r vo l um e t r i c f lowra t e ) t he n t he f ra c ti on o f theorigina l fue l saved, fd , ba se d on t he t o t a l he a t s a ve d , Q , , wi l l be g i ve n byQ ,

f ~ = m , CV~------~, (10)Computed fuel savings us ing the boi ler cool ing model

C o m p u t a t i o n s w e r e c a r r ie d o u t f o r a n a s s u m e d b o i l er e ff ic ie n cy o f 8 0 % t im e c o n s t a n t o f I m i n ,burn e r purge t i me o f 0 .5 ra in , a n d da m pe r de l a y t i me o f 0 . 5 ra in . S t e a dy s t a t e c oo l i ng l os se s we ree s t i ma t e d f rom me a sure m e nt s t o b e 1 .5 a nd 0 .5% of the fu ll l oa d i npu t , wi t hou t a nd wi t h fl ueda m pe r re spe c ti ve ly . In it i al c oo l i ng l os s wa s 4 .5 % of t he fu l l l oa d i npu t . Al l c a l c u l a t ions w e rec a r r ie d o u t b y m e a n s o f a c o m p u t e r p r o g r a m m e . T h e c o m p u t e d f u e l s a v in g s a s a f u n c ti o n o fd i me ns i on l e s s f i ri ng t ime (e q u i va l e n t t o t he f ra c t i on o f a c yc l e t ha t t he burn e r f ir e s) a re show n i nF i gs 5 a nd 6 .

The i n f lue nc e o f bo i l e r ope ra t i ng t i me on t he fue l s a v ing i s shown i n F i g . 5 fo r a burne r f ir ing15 t i me s a n hour . The fue l s a v i ngs a s c a n be se e n f rom t he f i gure c a n be subs t a n t i a l de pe ndi ngon t he bo i l e r ope ra t i ng t i me . As e xpe c t e d t he l onge r t he bo i l e r i s on s t a nd-by t he l a rge r a re t hefue l s a v ings . Thu s a bo i l e r ope ra t i ng fo r 12 h , f ir ing t yp i c a l l y fo r a bo ut o ne - t h i rd o f a c yc le ha sa fue l s a v i ng of 5 . 5% c o mp a re d t o 1 .7% for 24 h ope ra t i on . D e c re a s i ng t he num be r o f f ir ing c yc l esp e r h o u r ( n ) i n cr e a se s t h e f u el sa v in g , F i g . 6 , b u t o n l y b y a b o u t 5 - 1 0 % . I f th e b u r n e r s a r emo dul a t i n g t he n t he fue l s a v i ngs a re e ve n gre a t e r t ha n t hose sho wn i n F i g . 6, e. g . i f a burn e r wa sope ra t i ng on ha l f - l oa d , t he fue l s a v i ngs a re doubl e t hose shown i n t he f i gure s .

T O T A L F U E L S A V I N G SPrediction o f fue l savings

In o rde r t o be a b l e t o p re d i c t t he ove ra l l fue l s a v i ngs t ha t woul d be ob t a i ne d by t he i ns t a l l a t i onof t he e c onomi se r t he i n t e ra c t i ng e f fe c t s o f d i re c t he a t r e c ove ry a nd s t a ndby he a t s a v i ngs ha ve t obe c ombi ne d . Th i s s e c t i on shows how t he fue l s a v i ngs fo r t h i s c a se c a n be a s se s se d .

I t wa s shown e a r l i e r t ha t t he fue l s a v i ng , f~ , due t o t he da mpe r c a n be ob t a i ne d f rom e qua t i on(10) p rov i d i ng t he pe r t i ne n t bo i l e r da t a a re know n. T he n e w fue l c onsu mp t i on , m 2 , due t o t hed a m p e r w o u l d t h e r e f o r e b em 2 - - m , ( l - f d ) . (1 1)

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B o i l e r f u e l s a v i n g s 1 5 7The boi le r e ff ic iency however i s increased from r / i to r /2 by di rec t hea t recovery, which resul t s int he fue l c onsumpt i on be i ng fu r t he r r e duc e d t o (c . f . e qua t i on 1 )

t /Z, f f i t r l2~l /?~2. (12)He nc e t he t o t a l fue l s a ve d , f , , i s ob t a i ne d by c ombi n i ng e qua t i ons (11) a nd (12) , whi c h y i e l ds :

/ , = l - ( l - f , ) n , / ~ , . (13)Predicted total ue l savings . In o rd e r t o p re d i c t t he fue l s av i ngs , al l t ha t a re r e qu i re d a re t he b o i l e r

da t a , t he b o i l e r ope ra t i ng t i me a nd t he e s ti ma t e d bur ne r c yc l e s a nd f ir ing t ime . The fue l s a v i ngsc a n t he n be ob t a i ne d f ro m gra ph i c a l p l o t s o f bo i le r ef f ic i enc y (F i g . 3 ) a nd c om pu t e d s t a ndb ysa v ings , whi c h c a n a l so b e p l o t t e d g ra ph i c a l ly (F i gs 5 a nd 6). The fo l l owi ng e xa mpl e i l l us tra t es t hefue l s a v i ngs t ha t c a n be a c h i e ve d fo r a bo i l e r t e s t e d wi t h a n 'Ec oma t ' .

A 200 kW boi l e r wi t h a n o pe ra t i ng e f fi c ie nc y of 79% ope ra t e s fo r 12 h / da y . The e xc e ss a i r l eve lw a s a s s es s e d t o b e 4 0 % , t h e b o i le r e x h a u s t t e m p e r a t u r e 3 0 0 C a n d t h e r e tu r n w a t e r t e m p e r a t u r e55C . Es t i ma t e t he fue l s a v i ngs i f a R o nd ra Ec o ma t wa s f i tt e d , a s sumi ng t he burne r a ve ra ge s 5f i ring cyc les /h and f i res for o ne thi rd of a cyc le .

Fr om Fig. 4 , #2 = 0.835Fr om Fig. 6 , fa ffi 0 .059 a t # = 0.8

t he re fore fd ffi 0.059 X 0.8/0.79 at , = 0.79Eq ua t ion (13) ~ = 1 - (1 - 0 .0597) x 0.79/0.835 ff i 11.04% .

Not e t ha t t h i s fue l s a v i ng i s a bout t wi c e t ha t a c h i e ve d wi t h he a t r e c ove ry a l one . I f t he bo i l e r wa sope ra t i ng fo r 24 h c on t i nuou s l y , t he fuel s a v i ngs a re a bo ut 8 % . In a dd i t i on i f t he burne r wa s o ft he modul a t i ng t ype , t he sa v i ngs a re l i ke l y t o be e ve n gre a t e r s i nc e t he s t a ndby sa v i ngs we rec a l c u l a t e d o n a f u l l - l o a d b a s is .

A c t u a l u e l s a v in g s . h e a c t u a l f u e l s a v i n g s t h a t h a v e b e e n o b t a i n e d i n p r a c t i c e h a v e b e e n f o u n dt o b e i n t h e a p p r o x i m a t e r a n g e o f 6 - 1 6 % a c c o r d i n g t o t h e b o i l e r a n d o p e r a t i n g c o n d i t i o n sp r ev a il i ng . T h e s e f u e l a v i n g s w e r e d e t e r m i n e d i n l o n g d u r a t i o n t es ts , o v e r i n g p e r i o d s o f t i m e f r o m3 m o n t h s t o a f ul l e a t i n g y e a r . F u e l s a v i n g s w e r e d e d u c e d a f t e r m a k i n g a l l o w a n c e s f o r d i ff e r e nc e si n d e g r e e - d a y s d u r i n g c o m p a r a b l e m o n i t o r i n g p e r i o d s . F u r t h e r d e t a i l s o f t h e s e c a n b e f o u n de l s e w h e r e .

I n s h o r t e r t es ts , c o m p a r i s o n s o f f u e l c o n s u m p t i o n h a v e b e e n m a d e , o n a d a i l y b a s i s , b yc o m p a r i n g p e r f o r m a n c e w i t h a n d w i t h o u t t h e e c o n o m i s c r i n c i r c u i t f o r i d en t ic a l d e g r e e d a y s . T h eb o i l er c y c l e s a n d t e m p e r a t u r e s ( w a t e r , f l u e g a s ) b e i n g r e c o r d e d c o n t i n u o u s l y t h r o u g h o u t t h e t es ts .T h e f u e l s a v i n g s w e r e f o u n d t o b e w i t h i n 1 0 % o f t h o s e p r e d i c t e d b y t h e ' f ue l -s a vi n gs o d e l ' a n di n n e a r l y a ll c a s e s w e r e u n d e r r a t h e r t h a n o v e r - p r e d i c t e d . F u e l s a v i n g s o f a b o u t 6 - 1 4 % w e r eo b t a i n e d i n t h e s e te st s.

C O N C L U S I O N SI . The fue l s a v i ngs t ha t c a n b e a c h i e ve d in a fue l f i re d pa c ka g e bo i l e r by t he use o f e c ono mi se r sc a n b e c ons i de ra b l y e nha nc e d i f t he y i nc orpo ra t e a f l ue da m pe r , t ha t is de si gne d t o re duc e bo i l e r

s t a nd by l os ses . Ove ra l l fue l s a v i ngs i n t he ra nge of 6 -1 6% c oul d b e e xpe c t e d de pe nd i ng on t heb o i l er e x h a u s t c o n d i ti o n s , s t a n d b y t im e a n d c y c le s . U n d e r n o r m a l c o n d i t io n s w h e r e s t a n d b y l o ss e sa re a ppre c i a b l e , t he fue l s a v i ngs c a n be e xpe c t e d t o be a t l e a s t t wi c e t hose f rom c onv e nt i ona l he a tr e c o v e r y e c o n o m / s e r s .

2 . T he R on dra Ec o ma t , b e c a use o f the e nha nc e d fue l sa v i ngs , shou l d be a c os t-e f fe c ti ve me a n so f f u e l s av i ng s , w i th p a c k - b a c k p e r i o d s o f a t l e a s t h a l f t h o s e o f c o n v e n t i o n a l e c o n o m i s e r s .3 . Fue l s a v i ngs by c onv e nt i ona l he a t r e c o ve ry a re l i ke ly t o be i n t he ra ng e o f 4-8/@ de pe n di ngon t he bo i l e r e xha us t t e m pe ra t u re , e xc e ss a i r a nd bo i l e r s iz e. The s t a nd by sa v i ngs de pe n d on t hebo i l e r ope ra t i ng t ime , c oo l i ng ra t e ti me c ons t a n t a nd t he num be r o f burne r c yc le s . Typi c a l s t a ndb yfue l s a v i ngs a re i n t he ra nge 1 .5 t o I0% .4 . He a t r e c ove ry da t a c ombi ne d wi t h t he bo i l e r c oo l i ng mode l c a n p re d i c t fue l s a v i ngs wi t hsuf f i c i e n t a c c ura c y t o e na b l e t he pa yba c k pe r i od a nd fue l s a v i ngs t o be a s se s se d fo r a ny po t e n t i a le c ono mi se r i ns ta l l at i on .