boiler draught mechanical engineering 4th sem

boiler draught

1. BOI L ER DRAU G HT

2. D E FINITI O N Boiler dr a ug h t may b e defined a s the s m all d i f ference between the pressure o f outsi d e a i r and that o f gas e s within a furnace o r chimney a t the grate l e v e l, w h ich causes flow o f a i r / hot fl u e gases to take p l ace throu g h bo i l e r . Th e draught i s nec e ssary to f o rce a i r thr o ugh the fuel bed / grate to a i d i n proper combustion o f fu e l an d to remove the products o f co m bustion i . e . flue gases to the a t mosphere a f ter they have g i ven the i r hea t to w a ter b e i n g evaporated i n b o i l e r .

3. Draught a lso provides vel o c ity t o flue g ases an d so i n creases the hea t t r ansfer c o -e f ficie n t i n the bo i l e r . Thu s dra u ght i s ess e ntial l y req u ired i n a boiler and can b e produced b y a number of metho d s.

4. N A T U R A L D R A U GHT Natural draught i s obta i ned nat u ra l ly b y the use of a chimne y . Ch i m n ey i s a con i c a l shape vertic a l tub u l a r steel or masonry o r concre t e s t ructu r e having a l a rge he i ght. The flue g ases after t r ansferri n g their hea t i n the boiler are guided b y chimney to a considera b le he i ght i n the atmosph e r e . A s chimney ha s a l a r g e hei g ht an d i s the on l y outl e t f r om bo i l e r , i t rema i ns fil l ed b y ho t flue gas e s.

5. These exha u st gases, howe v er have g iven th e ir hea t to water i n the boile r , are still h o tter t h a n the bo i l e r room a i r . Due to this, the e x ha ust gas e s are lighter i n weight than outside a i r an d so l i f t s u p naturally i n the ch i m n ey and fina l ly esca p e out i n the atmosphere f r om top o f the chimne y . A s the flue gas e s l i ft u p i n the ch i mne y , more gases f r om fuel be d fl o w to w ards c h imney to t a ke their p l ac e . D u r ing th i s process, boiler an d d o the they first flow throu g h the j o b o f hea t i n g o f w a ter and produce steam.

6. A s flue gas e s flow throu g h boiler an d then to chimne y , fresh a ir f r o m outsi d e naturally e nters the boiler and help i n bu r ning o f fuel an d pr o duction of ho t flue gas e s. So, i n this w a y , a sma l l pressure d i f ference is naturally created between the b a se o f chimney and the a i r i n let po i nt o f boiler becau s e o f de n sity d i f ference betw e e n ho t flue gas e s inside the chimney an d f r esh col d er a i r outsi d e . T h is press u re d i f ference i s ca l led n a tural draught, bec a use i t i s produced natural l y .

7. A R TIFICIAL DR A UGHT When the draught i s produced b y some e x ter na l agency i . e . mechan i c al fan/blower o r b y s t eam jet itself, i t i s cal l ed artificial draugh t . In mod e rn commercial bo i l e r s, more v alue of draught i s req u ir e d to i n crease t h e hea t t r ansfer c o - e f ficie n t an d he n ce the ther m al e f ficienc y . S o , a r tificial draught i s mu s t to u s e to overcome the flow resistance o f fe r ed b y l a rge flue passa g e s .

8. FORC E D D R A U GHT It i s a pos i tive pressure draugh t . Th e fan i s i n st a lled a t the base o f the boiler before grate whi c h forces the outside air throu g h fu e l bed, furnace and a i r pre-he a ter and then fl u e gases throu g h flue passa g e, econ o mizer etc. Th e enclo s ure for the furnace has to b e ve r y tightly sealed so t h at gases f r om the furnace d o n ot l e ak ou t i n the bo i l e r hous e .

9. IND U C E D D R A U GHT In this sys t em a fan o r b l ower i s l o c ated a t o r near the base o f the chi m ney w h ich creates a partial vacuum i n the furnace an d flue passa g e . Thus the a i r an d fl u e gas e s a r e drawn throu g h t h e boiler du e to co m para t iv e ly h i g her pressure of outsi d e a i r . It i s convenient to produce induc e d draught and like i n forced dra u ght, an y type o f fan/blower may b e use d .

10. BAL A N C ED D R AUGHT It i s a combin a tion o f forced an d i n duc e d draught. Forced d r aught fan overcomes t h e resista n ce i n air pre-he a ter an d gra t e . Induc e d d raught fan overcom e s draught l o sses through b o ile r , ec o n o miz e r and co n n e ct i ng flue passa g es etc. Dep e n d i n g o n the type o f fuel burnt an d type of boile r , the f a n o r b l ower used m a y b e o f an y type as radi a l o r a x i a l et c .

11. AD V AN T AGES OF MECHANICAL DRAUGHT It i s more econ o mical an d its control i s eas y . Desir e d value o f draught can b e pro du ced by mechanic a l means w h ich cann o t b e produced by means o f natural draugh t . I t i n creas e s the rate o f comb u stion b y w h ich low grade fuel can a l so b e use d . It reduces the s m oke l e v el and increases the h eat t r ansfer c o -e f ficient o n flue gas e s side thus i n creases the ther m al e f ficie n cy o f bo i l e r . It saves t h e en e rgy a nd the heat o f flue gases can b e best util i zed by i t.

12. DISA D V A N T AGES OF ME C H A NIC A L D R A U GHT Initial costs o f mechan i cal draught system are h i g h . R u nning cost i s a l so h i g h due to requ i r e ment of electricity bu t that i s eas i ly c o mpensat e d b y the savi n gs i n fuel consumptio n . Maintenance cost i s a l so o n h i g h er si d e . Noise l e v el o f boiler i s a l so high d ue to n o isy fan/blo w er etc.

13. ST E AM JET DR A UGHT It i s a ve r y simp l e a nd easy method o f p r od u c ing artificial d r aught w i t h out the need o f a n e l ectric moto r . Steam unde r pressure i s ava i l a b l e i n the bo i l e r . When a small p o rtion o f steam i s p a s s ed through a jet o r nozz l e, pressure en e rgy converts to kinetic en e rgy an d steam comes ou t w i th a h i gh vel o cit y . Th i s h igh velocity s t eam carries, a l ong with it, a l a r g e mass o f a i r o r f l ue gas e s a nd makes i t to flow throu g h bo i l e r .

14. Steam jet i s d i rect e d towards a fi x e d d i rection and carri e s a l l its en e rgy i n ki n etic for m . I t creates s ome vacu u m i n its surroun d ings an d so attra c ts the a i r o f flue gas e s e ither b y carryi n g alo n g w i th i t. Thu s i t h a s the capac i ty to make flow o f the flue gas e s e ither by c arryi n g o r i n ducing towards chimne y .

15. FORC E D ST E AM JET DR A UGHT Steam f r om the bo i ler a fter having been thr o t t l e d to a ga u g e pressure o f 1 .5 to 2 ba r i s suppli e d to the jet o r nozz l es i n stall e d i n ash p i t . Th e steam emerging ou t o f nozz l es with a great velocity drags a i r al o ng the fuel bed , furnac e , flue passa g e an d then to the chimne y . Here ste a m jet i s forcing the air an d flue gas e s to flow through boil e r he n c e i t i s forced steam j e t draugh t .

16. IND U C E D ST E AM J E T DR A UGHT The jet o f s team i s d i verted into smoke bo x or chimne y . Th e kinetic hea d o f t h e steam i s high bu t s ta t ic head i s l o w i . e . i t c r e ates a par t i a l vacu u m wh i c h draws the a i r throu g h the grate, ash p i t, flues and then to motor bo x an d chimne y . Th i s type o f arran g e m ent i s employed i n l o c omotive bo i l e r s . H e re ste a m j e t i s s u cking the f l u e gas e s throu g h bo i l e r , so i t i s Induced Steam Jet Drau g ht.

17. A D V A N T AGES It i s qu i te simp l e an d chea p . It ha s the capa b i l ity o f usi n g l o w grade fuel s . It occup i es very l e ss spac e . Initial cost i s l o w . Mai n tena n ce cost i s l o w . Ex h a ust s t e am from s team en g ine o r turb i ne can be used eas i ly i n Steam Jet Drau g ht.

18. DISA D V A N T AGES  I t can op e rate only when some s t e am is ge n erated. Drau g ht produced i s very l o w .

19. D R A U GHT LOSSES Loss du e t o the f r ictio n a l resist a nce o f fered b y flue ga s passa g e to the flow o f flue gase s . Loss du e to bend s i n ga s fl o w circu i t, which a l s o o f fer flow resistanc e . Loss du e to frict i on hea d i n grate, ec o n o mize r , super he a ter et c . Loss du e to flow resistance o f fered b y chimne y . Loss du e t o imparting some velocity to flue gas e s, which i s required to i n crease h e at tra n sfer i n boil e r an d a l s o to throw away the flue gases f r o m chimne y .