draught

7/17/2019 Draught

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Performance Analysis of Draught Systems

Proper combustion requires sufficient Breathing…..

P M V Subbarao

Associate Professor

Mechanical Engineering Department

II Delhi

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Draft !equire" to Establish Air #lo$

Air in

#lue as out

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%atural Draft

&chimney

atmgas

AB

pA ' pref (∆ p

)* z T

dz

R

pg dp

−=

Zref

∫ −

+= A

ref

Z

Z air air

ref A z T

dz

R

pg p p

)* ∫

−+=

B

ref

Z

Z gas gas

ref B z T

dz

R

pg p p

)*

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%atural Draft

• %atural Draft across the furnace+

• ∆ pnat ' pA , pB

The difference in pressure will drive the exhaust.

•Natural draft establishes the furnace breathing by

– Continuous exhalation of flue gas

– Continuous inhalation of fresh air.

• The amount of flow is limited by the strength of the draft.

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Mechanical *Artificial)Draft - In"uce" Draft

&chimney

atm

gasA

B

pB ' pfan+s

pA ' patm ( ρatm g &chimney

B

Essential $hen %atural Draft cannot generate require" amount of breathing

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Mechanical *Artificial)Draft - #orce" Draft

&chimney

atmgas

A

B

pB ' patm ( ρgas g &chimney

pA ' pfan

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Mechanical *Artifical)Draft - Balance" Draft

&chimney

atm

gas

A B

pB ' pfan+s

pA ' pfan.b

B

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!esistance to Air / 0as #lo$ hrough Steam

0enerator System

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(1e

21e

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Platen Super heater

!eheater

#inal Super heater

3S&

Economi4er

#urnace

5in" Bo6

7oal Pul1eri4er

# D #an

Secon"ary air "uct

P A #an

PA "uct

AP&

#lue gas "uct

7oal bun8er

7oal fee"er

Boiler "rum

9:; M5 P<5E! P3A% S0

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Pressure drop in Air and Gas uct !ystems

"ernoulli e#uation $ pressure drop across a flow passage

9

9

9

9

9

9

:

9

:

:

:

99 ρ ρ ρ

P gZ

u P

m

W gZ

u P ∆+++=

∆

∆+++

Paud L f pdl 9

9 ρ =∆%rictional resistance along flow path&

$here f ' coefficient of friction

L ' length of the "uct+ m

d dl ' equi1alent "iameter of the "uct+ m

ρ ' "ensity of air or gas calculate" at the mean gas temperature+ 8g=m>

u ' cross section a1erage 1elocity of air or gas in the "uct+ m=sec

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'#uivalent diameter for rectangular duct is given as

( )

( ) 9?.;

@9?.;

>.:ba

abd

dl +

=

9

.;

!e

C.?

.>

log

;@9?.;

+

=

dl d

k

f

where a and b are sides of the duct( mm.

The coefficient of friction for flow through tubes can be approximated as shown below(

for )*** + ,e+-*

(

-*/0

+ 123ddl4+*.*-

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5inor 6osses

Calculation of 6ocal pressure drops&

where 7p 8 local pressure drop

9 8 local resistance factor(

ρ 8 density of air or gas at the position of the pressure drop calculated( 2g3m

:

u 8 velocity of air through the fittings m3s.

Pau

K pl 9

9 ρ

=∆

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Pressure drop across a burner

pa

9 8 -.) for tangential burner

:.* for swirl burner

9

9u K P ρ

=∆

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Pressure drop across heating surfaces

Pressure drop across tube bundles&

;nline arrangement& 9 8 n 9*

<here n 8 number of tube rows along the flow direction

9* 8 loss coefficient for one row of tubes

9* depends on =- 8 s-3d( => 8 s>3d ( ? 8 1s- / d 4

<here s- is lateral pitch @ s> is longitudinal pitch

;f =- +8 => &

9* 8 -.)> 1=- $-4 $ *.)

? $*.>

,e $*.>

;f =- => &

9* 8 *.:> 1=- $-4 $ *.)

1? $ *.B4 $*.>

,e $*.>3?

Pau

K p bt 9

9 ρ

=∆

S:

S9

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!taggered Arrangement

The loss coefficient is obtained as

9 8 9* 1n-4

<here 9* is the coefficient of frictional resistance of one row of tubes

9* depends on =- 8 s-3d( ? 8 1s- / d 4 3 1s>

l / d 4

<here s> l is the diagonal tube pitch given by

s>l 8 D 1 *.>) s-

> s>

>4

and 9* can be written as(

9* 8 Cs ,e/*.>E

Cs is design parameter of the staggered ban2s

S:

S9

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%or *.-E +8 ? +8 -.E and =- 8 >.*( Cs 8 :.>

;f =- + >.*(then Cs given as

Cs 8 :.> 1F.0 $ >.E ?41> / =-4

%or ? 8 -.E $ ).>( Cs 8 *.FF1?-4>

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Cross/%low over %inned Tubes

;nline arrangement

for round fins

where =-l 8 1pitch of fin( Pf 3 diamter of tube( d4

=>ll 8 1height of fin( hf 3 diamter of tube( d4

,e 8 1 u pf 3 4

%or s#uare fine with 8 *.::

( )[ ] :9.;

:

9C.: !e:

@;@.;*C@C.;DB.9 −−−+=

σ

σ σ n

K f ll

f l

[ ] :9.;

:

!e:B?.9D;.: −−+=σ

σ n K f ll

f ll

σ

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-4 !taggered Arrangement

for round fins s-8s>8d>hf

for round fins s-8s>8>d

[ ] 9C.;B9.;

!e;.9 −−

= f l n K σ

[ ] C?.;

9C.;B9.;

!eB.9 −

−−

= f ll f l n K σ σ

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Gas side pressure drop in finned/tube economiHers

Pau

K pmc9

9 ρ

=∆

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Pressure drop in tubular air heaters

where Δpmc is the pressure drop in the tube

9in and 9out are local resistance factors at inlet and outlet

Pau

K K p p out incmmc9

)*9

ρ ++∆=∆

Pressure drop through rotary air heater

Corrugated plate/corrugated setting plate

,e 8 >. x -*:

f 8 *.E ,e/*.>)

,e + >. x -*:

f 8 ).E ,e/*.)

Pau

d

L f p

dl

mc9

9 ρ

=∆

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Corrugated plate/ plane setting plate

,e 8 -.F x -*: f 8 *.0 ,e

/*.>)

,e + -.F x -*:

f 8 :: ,e/*.

Plane plate/ plane setting plate

,e 8 -.F x -*: f 8 *. :: ,e

/*.>)

,e + -.F x -*:

f 8 B*3 ,e

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Pressure "rop in "ucts oining air heater an" "ust collector

The volume flow rate of gases at the induced draft fan is determined by (

where Igf 8 volume flow rate of gases at the exit of the duct( m:

3s

Tg 8 temperature of flue gas leaving the duct(*

C

∆α 8 lea2age air ratio behind the air heater

( )

+∆+= 9B>

9B>; g g f g T V V BV α

" 8 fuel firing rate( 2g3s

where αe is the excess air ratio in the flue gas at the duct exit

; is the col" air temperature+;

7

α α

α α

∆+

∆+=

e

g e

f g

T T T

;

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Pressure drop through convective section

5ass conservation for unchanged density(

u A 8 u- A- 8 u> A>

local pressure loss( ∆p- 8

total loss( ∆p 8 ∆p- ∆p> JJJJJJJ.∆pn 8 12-- 2>

> JJJJ.. 2n

n4

where 2-- 8 2- 1A3A-4

>( 2

-> 8 2> 1A3A-4

>

999

9:

:

9

:

9

:

9

:::

u K

u

A

A K

u K p ===∆

9

9u

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Ash 7ollectors

• #ollo$ing able is use" to estimate the pressure "rop in Ash

collectors.

• 7yclone- :? , 9; m=s ; , ;F ?;; , :;;; Pa

• ESP- :, 9 m=s F :;; , 9;; Pa

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Pressure rop through !tac2

where ∆pst 8 stac2 pressure drop( Pa

f 8 friction factor

6st 8 height of the chimney( m

8 diamter of the chimney ( m

9c 8 resistance factor at the stac2 outlet

ρ 8 gas density in the stac2( 2g3m:

uc8 gas velocity at the chimney outlet( m3s

9

9cc

st

syu K

L f p ρ

++=∆

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Total gas side pressure drop

Pa

where Σ∆p- 8 total pressure drop from the furnace outlet to the

dust collector( Pa

Σ∆p> 8 pressure drop after the dust collector( Pa

µ 8 ash content in the glue gas( 2g32g

pa v 8 average pressure of the gas( Pa

pg o 8 flue gas density at standard conditions( 2g3Nm:

[ ]

Σ∆++Σ∆=∆

a!

o g

sy P

p p p :;:>9?9A>.:

):* 9:

ρ µ

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The ash fraction of the flue gas calculated as(

where αf h 8 ratio of fly ash in flue gas to total ash in the fuel

A 8 ash content of wor2ing mass( K

Ig 8 average volume of gas from furnace to dust collector calculated from the average excess air ratio( Nm :32g of

fuel

g o g

" f

V

A

ρ

α µ

:;;

=

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The pressure drop from the balance point of the furnace to the chimney base is

∆prest 8 ∆pexit ∆pgas $∆ pnd

where ∆pexit 8 pressure drop up to the boiler outlet

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otal losses

∆ p

Percent Boiler !ating

Burner 3osses

AP& 3osses

Ducts / "ampers losses

Air Pressure 3osses

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otal losses

∆ p

Percent Boiler !ating

#urnace+ S& / !& 3osses

Economi4er 3osses

Ducts / "ampers losses

Draught 3osses

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#D

#an

Duct AP& Duct #urnace Duct AP&Bac8

pass

ESPID

#an

7himney D u c t

D u c t

Mo"eling of 9:; M5 Draught System

•

Pressure "rop calculation in air / gas path an" itscomparison $ith "esign 1alue.• Assessment of ID an" #D fan po$er as a function of

furnace pressure.• Assessment of effecti1e 8inetic rate coefficient as a

function of furnace pressure.

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Pressure Variation

Duct#D #an Duct S7AP& AP& Duct5in"

Bo6

Boiler AP& ESP ID #an

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<ff Design Pressure Variation

Pressure Variation in Air & Gas Path at Part Load

/>***

/-)**

/-***

/)**

*

)**

-***

-)**

>***

>)**

- > : F ) 0 E B -* -- ->

Path Element

P r e s s u r e

( P a )

Calculated (168 MW) Desin (168 MW)

ID #anESPBoiler AP&5in"

Bo6

DuctAP&DuctS7AP&Duct#D #an

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Draught 7ontrol

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5in"bo6 Pressure 7ontrol

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7ombustion Pre"iction / 7ontrol

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7ombustion an" Draught 7ontrol

• he control of combustion in a steam generator is e6tremely critical.

• Ma6imi4ation of operational efficiency requires accurate combustion.

• #uel consumption rate shoul" e6actly match the "eman" for steam.

• he 1ariation of fuel flo$ rate shoul" be e6ecute" safely.

• he rate of energy release shoul" occur $ithout any ris8 to the plant+

personal or en1ironment.

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he Mo"el for 7ombustion 7ontrol

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Parallel 7ontrol of #uel / Air #lo$ !ate

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#lo$ !atio 7ontrol - #uel 3ea"

#l ! i 7 l # l 3 "

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#lo$ !atio 7ontrol - #uel 3ea"

G

Σ

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7ross2limite" 7ontrol System

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<6ygen rimming of #uel=air ratio 7ontrol

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7ombine" 7< / <9 rimming of #uel=Air !atio 7ontrol

draught

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