Performance Analysis of Draught Systems

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Performance Analysis of Draught Systems
P M V Subbarao Associate Professor Mechanical
Engineering Department IIT Delhi
Proper combustion requires sufficient Breathing..
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Draft Required to Establish Air Flow
Flue as out
Air in
3
Natural Draft
Zref
pA pref Dp
Hchimney
Tgas
Tatm
B
A
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Natural Draft

• Natural Draft across the furnace,
• Dpnat 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 Induced Draft
Essential when Natural Draft cannot generate
required amount of breathing
Hchimney
pB pfan,s
pA patm ratm g Hchimney
Tatm
B
B
A
Tgas
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Mechanical (Artificial)Draft Forced Draft
Hchimney
pB patm rgas g Hchimney
pA pfan
Tgas
Tatm
B
A
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Mechanical (Artifical)Draft Balanced Draft
Hchimney
pB pfan,s
pA pfan.b
B
Tatm
A
B
Tgas
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Resistance to Air Gas Flow Through Steam
Generator System
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ve
-ve
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210 MW POWER PLANT SG
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Pressure drop in Air and Gas Duct Systems

• Bernoulli equation pressure drop across a flow
  passage
•  

 
Frictional resistance along flow path
where f coefficient of friction L
length of the duct, m ddl equivalent
diameter of the duct, m ? density of
air or gas calculated at the mean gas
temperature, kg/m3 u cross section
average velocity of air or gas in the duct, m/sec
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Equivalent diameter for rectangular duct is given
as
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 5000 lt Relt108, 10-6lt (k/ddl)lt0.01
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Minor Losses Calculation of Local pressure
drops   where ?p local pressure
drop K local resistance factor, r density
of air or gas at the position of the pressure
drop calculated, kg/m3 u velocity of air
through the fittings m/s.
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Pressure drop across a burner
pa K 1.5 for tangential burner 3.0
for swirl burner
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Pressure drop across heating surfaces
Pressure drop across tube bundles     
Inline arrangement K n K0 Where n number
of tube rows along the flow direction K0 loss
coefficient for one row of tubes K0 depends on
s1 s1/d, s2 s2/d , F (s1 - d )
Where s1 is lateral pitch s2 is
longitudinal pitch If s1 lt s2 K0 1.52
(s1 1) 0.5 F 0.2 Re 0.2
If s1 gt s2 K0 0.32 (s1 1) 0.5 (F
0.9) 0.2 Re 0.2/F    
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Staggered Arrangement The loss coefficient is
obtained as K K0 (n1) Where K0 is the
coefficient of frictional resistance of one row
of tubes
K0 depends on s1 s1/d, F (s1 - d ) / (s2l
- d ) Where s2 l is the diagonal tube pitch
given by s2 l v ( 0.25 s12 s22)
and K0 can be written as, K0 Cs
Re-0.27 Cs is design parameter of the staggered
banks
S1
S2
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  For 0.17 lt F lt 1.7 and s1 gt 2.0, Cs
3.2 If s1 lt 2.0,then Cs given as Cs 3.2
(4.6 2.7 F)(2 - s1) For F 1.7 5.2,
Cs 0.44(F1)2
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•  
• Cross-Flow over Finned Tubes
• Inline arrangement
• 
  
• for round fins
• where s1l (pitch of fin, Pf / diamter
  of tube, d)
• s2ll (height of fin, hf / diamter of tube,
  d)
• Re ( u pf / ?)
• For square fine with 0.33

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1)     Staggered Arrangement for
round fins s1s2d2hf for round fins
s1s22d
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Gas side pressure drop in finned-tube economizers
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Pressure drop in tubular air heaters
where ?pmc is the pressure drop in the tube
Kin and Kout are local resistance factors at
inlet and outlet
Pressure drop through rotary air
heater Corrugated
plate-corrugated setting plate Re gt
2.8 x 103 f 0.78 Re-0.25 Re lt
2.8 x 103 f 5.7 Re-0.5
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Corrugated plate- plane setting
plate Re gt 1.4 x 103 f 0.6 Re-0.25 Re lt
1.4 x 103 f 33 Re-0.8 Plane
plate- plane setting plate Re gt 1.4 x 103
f 0. 33 Re-0.25 Re lt 1.4 x 103 f 90/
Re
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Pressure drop in ducts joining air heater and
dust collector The volume flow rate of gases
at the induced draft fan is determined by
, where Vgf volume flow rate of gases at the
exit of the duct, m3/s Tg
temperature of flue gas leaving the duct, 0C
?? leakage air ratio behind the air
heater
B fuel firing rate, kg/s where ?e is the
excess air ratio in the flue gas at the duct exit
T0 is the cold air temperature, 0C
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Pressure drop through convective section Mass
conservation for unchanged density, u A u1 A1
u2 A2 local pressure loss, ?p1 total
loss, ?p ?p1 ?p2 .?pn (k11 k22
.. knn) where k11 k1 (A/A1)2, k12
k2 (A/A1)2
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Ash Collectors

• Following Table is used to estimate the pressure
  drop in Ash collectors.
• Cyclone 15 20 m/s 70 90 500 1000 Pa
• ESP 1 2 m/s 99 100 200 Pa

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Pressure Drop through Stack where
?pst stack pressure drop, Pa f friction
factor Lst height of the chimney, m D
diamter of the chimney , m Kc resistance
factor at the stack outlet ? gas
density in the stack, kg/m3 uc gas
velocity at the chimney outlet, m/s
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Total gas side pressure drop

Pa where ??p1
total pressure drop from the furnace outlet to
the dust collector, Pa
??p2 pressure drop after the dust collector,
Pa ? ash content in
the glue gas, kg/kg pa v
average pressure of the gas, Pa
pg o flue gas density at standard
conditions, kg/Nm3
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The ash fraction of the flue gas calculated as,
where ?f h ratio of fly ash in flue gas
to total ash in the fuel A ash content of
working mass, Vg average volume of gas from
furnace to dust collector calculated from the
average excess air ratio, Nm3/kg of fuel
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The pressure drop from the balance point of the
furnace to the chimney base is ?prest ?pexit
?pgas Dpnd where ?pexit pressure drop up to
the boiler outlet
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Air Pressure Losses
Total losses
Dp
Burner Losses
APH Losses
Ducts dampers losses
Percent Boiler Rating
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Draught Losses
Total losses
Dp
Furnace, SH RH Losses
Economizer Losses
Ducts dampers losses
Percent Boiler Rating
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Modeling of 210 MW Draught System

• Pressure drop calculation in air gas path and
  its comparison with design value.
• Assessment of ID and FD fan power as a function
  of furnace pressure.
• Assessment of effective kinetic rate coefficient
  as a function of furnace pressure.

FD Fan
Back pass
ID Fan
Duct
Duct
APH
Duct
Furnace
Duct
APH
ESP
Chimney
Duct
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Pressure Variation
Duct
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Off Design Pressure Variation
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Draught Control
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Windbox Pressure Control
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Combustion Prediction Control
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Combustion and Draught Control

• The control of combustion in a steam generator is
  extremely critical.
• Maximization of operational efficiency requires
  accurate combustion.
• Fuel consumption rate should exactly match the
  demand for steam.
• The variation of fuel flow rate should be
  executed safely.
• The rate of energy release should occur without
  any risk to the plant, personal or environment.

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The Model for Combustion Control
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Parallel Control of Fuel Air Flow Rate
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Flow Ratio Control Fuel Lead
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Flow Ratio Control Fuel Lead
X
S
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Cross-limited Control System
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Oxygen Trimming of Fuel/air ratio Control
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Combined CO O2 Trimming of Fuel/Air Ratio
Control