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2 Boilers

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2.1 Introduction to Boiler 

• It is an enclosed Pressure

Vessel

• Heat generated byCombustion of Fuel istransferred to water to

 become steam

• Process: !a"oration

• #team !olume increases to1$%&& times from water and "roduces tremendous force

• Care is must to a!oide'"losion.

What is a boiler?

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Boiler Specification

• Boiler Make & Year :XYZ & 2003

• MCRMa!i"#" Contin#o#s Ratin$% :0'() * & +00oC%

• Rate, Workin$ (ress#re :0-./ k$c"2$%

• '1pe of Boiler : 3 (ass *ire t#be

• *#el *ire, : *#el il• )eatin$ s#rface : M2

()at is F*+ 1&&,

C -

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2.2 Boiler #ystems

*l#e $as s1ste"

Water treat"ent s1ste"

*ee, ater +45

Con,ensate s1ste"

Stea" S1ste"

Blo ,on s1ste"

*#el s#ppl1 s1ste"

+ir S#ppl1 s1ste"

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2-3 Boiler '1pes an, Classifications

• Fire in tube or Hot gas through

tubes and boiler feed water in shellside

• Fire ubes submerged in water 

+pplication• Used for small steam capacities

( upto 25T/hr and 175!g/cm2

Merits

• /ow Ca"ital Cost and fuelfficient 023

• +cce"ts wide * loadfluctuations

• Pac4aged Boiler 

Fire ube Boiler 

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Boiler '1pes an, Classifications

(ater ube Boiler • (ater flow t)roug) tubes

• (ater ubes surrounded by)ot gas

+pplication

• 5sed for Power Plants

• #team ca"acities range from6.78 12& t9)r 

Characteristics

• Hig) Ca"ital Cost

• 5sed for )ig) "ressure )ig)ca"acity steam boiler 

• emands more controls

• Calls for !ery stringent water

;uality

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• Pac4age boilers aregenerally of s)ell ty"e

wit) fire tube design

"ore number ofpasses#so more heat

transfer /arge number of small

diameter tubes leading togood con!ecti!e )eattransfer.

Hig)er t)ermal efficiency

(acka$e, Boiler

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Chain 6rate or 'ra7elin$ 6rate Stoker Boiler Coal is fed on

one end of amo!ing steel

c)ain grate

+s) dro"s off atend

Coal gratecontrols rate ofcoal feed intofurnace bycontrolling t)e

t)ic4ness of t)efuel bed.

Coal must beuniform in si

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Sprea,er Stoker BoilerUses both suspension

and grate burning

$oal fed continuousl%o&er burning coalbed

$oal fines burn insuspension andlarger coal piecesburn on grate

'ood fleibilit% to

meet changing loadre)uirements

Preferred o!er ot)erty"e of sto4ers inindustrial a""lication

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(#l7eri8e, *#el Boiler

 'an$ential firin$

Coal is "ul!erised to a fine "owder$ so t)at less t)an 2 is =>&&

microns$ and ?&8?7 is below ?7 microns.Coal is blown wit) "art of t)e combustion air into t)e boiler "lantt)roug) a series of burner no

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+,7anta$es

• Its ability to burn all ran4s of coal from ant)racitic

to lignitic$ and it "ermits combination firing 0i.e.$

can use coal$ oil and gas in same burner3. Because

of t)ese ad!antages$ t)ere is wides"read use of "ul!eri

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*l#i,i8e, be, Co"b#stion *BC% boiler

Furt)er$ increase in

!elocity gi!es rise to

 bubble formation$ !igorous

turbulence and ra"idmi'ing and the be, is sai,

to be fl#i,i8e,- 

Coal is fed continuously into a )ot air agitatedrefractory sand bed$  t)ecoal will burn ra"idly andt)e be, attains a #nifor"

te"perat#re

istributed air is "assed u"ward t)roug) a bed of solid "articles

)e "articles are undisturbed at low !elocity.+s air !elocity is increased$

a stage is reac)ed w)en t)e particles are s#spen,e, in the air

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Fluidised Bed Combustion

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*l#i,i8e,9be, boiler Cont,--%

+,7anta$es :

• Hig)er rates of )eat transfer between combustion "rocessand boiler tubes 0t)us re,#ce, f#rnace area an, si8e

re#ire,%;

• combustion tem"erature 7&oC is lower t)an in a

con!entional furnace. )e lower furnace tem"eraturesmeans re,#ce, 4

! pro,#ction.

• In addition$ t)e limestone 0CaC,>3 and dolomite

0AgC,>3 react wit) #,2 to form calcium and magnesium

sulfides$ res"ecti!ely$ solids w)ic) do not esca"e u" t)estac4 )is means t)e "lant can easil1 #se hi$h s#lf#r

coal-

• *#el *le!ibilit1: Aulti fuel firing

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)ere are two met)ods of assessing boiler efficiency. 

%  'he 5irect Metho,: ()ere t)e energy gain of t)e wor4ing

fluid 0water and steam3 is com"ared wit) t)e energy content of t)e

 boiler fuel.

2%  'he

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=!a"ple:'1pe of boiler: Coal fire, Boiler

Heat input data

Qty of coal consumed :1.8 TPH

GCV of coal :3200K.Cal/k

)eat o#tp#t ,ata

! Qty of steam en : 8 TPH

! "team #$/tem#:10 k/cm2%&/1800C

! 'nt(al#y of steam%sat& at 10 k/cm2%& #$essu$e

:))* K.Cal/k! +eed ,ate$ tem#e$atu$e : 8*0 C

! 'nt(al#y of feed ,ate$ : 8* K.Cal/k

Find out the Find efficiency ?

Find out the Evaporation Ratio?

Efficiency Calculation by Direct Method

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-ole$ effcency %η&: Q x (H – h) x !!  (" x #C$) 

(e$e Q Quantty of steam ene$ated #e$ (ou$ %k/($&  H  'nt(al#y of satu$ated steam %kcal/k&  h  'nt(al#y of feed ,ate$ %kcal/k&

  " Quantty of fuel used #e$ (ou$ %k/($&  #C$ G$oss calo$fc alue of t(e fuel %kcal/k&

 

%oiler efficiency ( )&  8 TPH 1000K/T %))*8*& 100  1.8 TPH 1000K/T 3200 

& '!! Evaporation Ratio  8 Tonne of steam/1.8 Ton of coal 

& **

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Boiler Flue gas

Stea" #tp#t

=fficienc1 > 00 @2@3@/@.@A@@%

0by In irect Aet)od3

+ir 

Fuel In"ut$ 1&&

1. ry Flue gas loss

2. H2 loss

>. Aoisture in fuel

6. Aoisture in air 

7. C, loss

?. Fly as) loss

%. #urface loss

. Bottom as) loss

()at are t)e losses t)at occur in a boiler-

Feed water Blow down

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=!a"ple:

)e following are t)e data collected for a ty"ical oil fired boiler.

Find out t)e efficiency of t)e boiler by indirect met)od and Boiler!a"oration ratio.

5ltimate analysis of ,il

C : 6.& H2: 12.&

  #: >.& ,2: 1.&

DCV of ,il : 1&2&& 4cal94g#team Deneration Pressure : ?4g9cm20g38saturated

nt)al"y of steam : %%& 4Cal94g

Feed water tem"erature : %&oC

Percentage of ,'ygen in flue gas: ?  Percentage of C,2  in flue gas: 11

  Flue gas tem"erature 0f 3 : 22& &C

  +mbient tem"erature 0a3 : 2? &C

Humidity of air : &.&1 4g94g of dry air

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Sol#tionStep9: *in, the theoretical air re#ire"ent 

E 4g94g of oil 

>D-A ! /% @ DE3/- ! 2 %F @ /-3. ! 3%G00 4g94g of oil

E16 4g of air94g of oil 

Step92: *in, the H=!cess air s#pplie, 

'cess air su""lied 0+3 E E E 7&

 

1&&93F>7.603G90.>6H3%.11I0 22   S  xO H  xC  x   +−+

1&&21

2

2  xO

O

−1&&

?21

? x

−

Step93: *in, the +ct#al "ass of air s#pplie,+ct#al "ass of air s#pplie, k$ of f#el E 1 = +91&& ' )eoritical +ir 

0++#3 E 1 = 7&91&& ' 16

E 1.7 ' 16

E 21 4g of air94g of oil

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Step9/: =sti"ation of all losses

 

i- 5r1 fl#e $as loss

 

Percentage )eat loss due to dry flue gas E

mE mass of C,2 = mass of #,

2 = mass of J

2 = mass of ,

2

 

" > 2 k$ k$ of oil 

1&&30 x

 fuel of  GCV 

T T  xC  xm a f   p   −

 

 

 

 

  −+++=

1&&

2>316210

1&&

??21

>2

%6&>.&

12

666.& x

 x x xm

1&&

1&2&&

32?22&02>.&21 x

 x x   −E K.16

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+lternati7el1 a si"ple "etho, can be #se, for

,eter"inin$ the ,r1 fl#e $as loss as $i7en belo-

 

Percentage )eat loss due to dry flue gas E

otal mass of flue gas 0m3 E mass of actual air su""lied = mass of

fuel su""lied > 2 @ >22

 

ry flue gas loss E

1&&30 x

 fuel of  GCV 

T T  xC  xm a f   p   −

7?.K1&&1&2&&

32?22&02>.&22

=

− x

 x x

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ii- )eat loss ,#e to e7aporation of ater for"e, ,#e to )2 in f#el

 

E

()ere$ H2 L "ercentage of H

2 in fuel

 

E

E ?.1&

1&&fuelof DCV

 3G80C76'H'K af  "2 x

+

1&&1&2&&

 3G2?8022&&.67H76'12'K x

+

iii- )eat loss ,#e to "oist#re present in air

E 1&&30 x

 fuel of  GCV 

T T  xC  xhumidity x AAS  a f   p   −

 E E &.>22 1&&1&2&&

32?22&067.&&1.&21

 x

 x x x   −

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i7 )eat loss ,#e to ra,iation an, other #nacco#nte, losses

  For a small boiler it is estimated to be 2

Boiler =fficienc1 

i. Heat loss due to dry flue gas : K.16 

ii.  Heat loss due to e!a"oration of water formed due to H2 in fuel : ?.1&

iii.  Heat loss due to moisture "resent in air : &.>22  

i!.  Heat loss due to radiation and ot)er unaccounted loss : 2 

Boiler =fficienc1 E 1&&8 K.16=?.1&=&.>22=2 

> 1&& L 1.7% E 1 0a""3 

!a"oration atio E Heat utili

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2.7 ()y Boiler Blow own -

()en water e!a"orates issol!ed solids gets concentrated

and #olids "reci"itates on tubes. educes t)e )eat transfer rate

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Contin#o#s Blo,on

• + steady and constant dis"atc) of small

stream of concentrated boiler water$ and

re"lacement by steady and constant inflow

of feed water.

•   )is ensures constant # and steam

 "urity.

• )is ty"e of blow down is common in )ig)8

 "ressure boilers.

)e ;uantity of blow down re;uired to control boiler water solids

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)e ;uantity of blow down re;uired to control boiler water solids

concentration is calculated by using t)e following formula:

0Continuous Blow down3

#0#3 in feed water 

>&& ""m

#team > 9)r #03 E&

# 0C3 E>&&& ""m +llowable3

Blow down flow rateE>&&'1&9>&&& E1 :E1 of >$&&& E >& 4g9)r 

Blow down0B3

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2.% Boiler (ater reatment

•

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=!ternal Water 'reat"ent

• (ropose: 'ternal treatment is used to remo!e sus"ended

solids$ dissol!ed solids 0"articularly t)e calcium and

magnesium ions w)ic) are a maMor cause of scale

formation3 and dissol!ed gases 0o'ygen and carbondio'ide3.

• 5ifferent treat"ent (rocess :

 L ion e'c)ange

 L deminerali

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eminerali

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5e9aeration• ()en )eated in boiler systems$ carbon dio'ide

0C,23 and o'ygen 0,23 are released as gases andcombine wit) water 0H2,3 to form carbonic

acid$ 0H2C,>3.

Figure 2.K eaerator 

•In de8aeration$dissol!ed gases$ suc)as o'ygen and carbondio'ide$ are e'"elled

 by "re)eating t)e feedwater before it enterst)e boiler.

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Re7erse

s"osis

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=ner$1 Conser7ation

pport#nities

in Boilers

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- Re,#ce Stack 'e"perat#re

• #tac4 tem"eratures greater t)an 2&&@C

indicates "otential for reco!ery of waste

)eat.•   It also indicate t)e scaling of )eat

transfer9reco!ery e;ui"ment and )ence t)e

urgency of ta4ing an early s)ut down forwater 9 flue side cleaning.

22o C reduction in flue gas tem"erature

increases boiler efficiency by 1

2 *ee, Water (reheatin$ #sin$

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2- *ee, Water (reheatin$ #sin$

=cono"iser• For an older s)ell boiler$

wit) a flue gas e'ittem"erature of 2%&oC$ an

economi.

• Condensingeconomi

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3- Co"b#stion +ir (reheatin$

•  Combustion air "re)eating is an alternati!e

to feedwater )eating.

• In order to im"ro!e t)ermal efficiency by

1$ t)e combustion air tem"erature must be

raised by 2& oC.

/

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/-

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- ontro e!cess a rfor e!ery 1 reduction in e'cess air $&.% rise in efficiency.

)e o"timum e'cess air le!el !aries wit) furnace design$ ty"e of burner$

fuel and "rocess !ariables.. Install oxygen trim system'+BI= 2-. =XC=SS +&

 Jatural gas Hig) "ressure burner 78?

(ood utc) o!er 01&82> t)roug) grates3 andHofft ty"e

2&827

Bagasse +ll furnaces 278>7

Blac4 li;uor eco!ery furnaces for draft and soda8

 "ul"ing "rocesses

>&86&

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6.Blowdown Heat Recovery

• Efficiency Improvement - Up to 2percentage points.

• *lowdown of boilers to reduce thesludge and solid content allows heat

to go down the drain• The amount of blowdown should be

minimi+ed b% following a good watertreatment program, but installing aheat echanger in the blowdown lineallows this waste heat to be used in

preheating ma!eup and feedwater• Heat reco&er% is most suitable for

continuous blowdown operationswhich in turn pro&ides the best watertreatment program

Re,#ction of Scalin$ an,

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- Re,#ction of Scalin$ an,

Soot Iosses

• In oil and coal8fired boilers$ soot buildu" on tubes acts as aninsulator against )eat transfer. +ny suc) de"osits s)ould beremo!ed on a regular basis. le!ated stac4 tem"eratures mayindicate e'cessi!e soot buildu". +lso same result will occur dueto scaling on t)e water side.

• Hig) e'it gas tem"eratures at normal e'cess air indicate "oor)eat transfer "erformance. )is condition can result from agradual build8u" of gas8side or waterside de"osits. (atersidede"osits re;uire a re!iew of water treatment "rocedures and tube

cleaning to remo!e de"osits.• #tac4 tem"erature s)ould be c)ec4ed and recorded regularly asan indicator of soot de"osits. ()en t)e flue gas tem"eraturerises about 2&oC abo!e t)e tem"erature for a newly cleaned

 boiler$ it is time to remo!e t)e soot de"osits

Jariable Spee, Control for *ans

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- Jariable Spee, Control for *ans;

Bloers an, (#"ps

  Denerally$ combustion air control is effected by

t)rottling dam"ers fitted at forced and induced

draft fans. )oug) dam"ers are sim"le means ofcontrol$ t)ey lac4 accuracy$ gi!ing "oor control

c)aracteristics at t)e to" and bottom of t)e

o"erating range.

  If t)e load c)aracteristic of t)e boiler is !ariable$t)e "ossibility of re"lacing t)e dam"ers by a V#

s)ould be e!aluated.

L- =ffect of Boiler Ioa,in$ on =fficienc1

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L- =ffect of Boiler Ioa,in$ on =fficienc1

• +s t)e load falls$ so does t)e !alue of t)e mass flow

rate of t)e flue gases t)roug) t)e tubes. )is reduction

in flow rate for t)e same )eat transfer area$ reduced t)e

e'it flue gas tem"eratures by a small e'tent$ reducingt)e sensible )eat loss.

• Below )alf load$ most combustion a""liances need

more e'cess air to burn t)e fuel com"letely and

increases t)e sensible )eat loss.

• ,"eration of boiler below 27 s)ould be a!oided

• ,"timum efficiency occurs at %787 of full loads

0 B il R l t

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0- Boiler Replace"ent

if the e!istin$ boiler is :

  ,ld and inefficient$ not ca"able of firing c)ea"er

substitution fuel$ o!er or under8si