Air & Draft
system
9 April 2015
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Presentation Plan
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Various auxiliary equipments in a boiler
Need for Draft System
Primary and secondary air system in Boiler and their fans
Other fans in a boiler
Basic of fans: Fan Types and selection
Fan laws and factors affecting fan performance
Performance curves
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Arrangement of Boiler Auxiliaries
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Need of Draft System
Combustion Air
Boiler
Flue Gases
• Air needed for combustion
• Flue are needed to be evacuated
• Losses due to flow need to be overcome
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How to select a fan
• Air or Gas flow -Kg/hr
• Density (function of temperature and pressure)
• System resistance (losses)
Major Fans in Boiler Draft System
• PA Fans
• FD Fans
• ID Fans
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PRIMARY AIR/MILL SEAL AIR
SYSTEM
• Ambient air is drawn into the primary air ducting
by two 50% duty, motor driven axial reaction
fans.
• Air discharging from each fan is divided into two
parts, one passes first through a air pre-heater
then through a gate into the P.A bus duct. The
second goes to the cold air duct. The mix of both
is used to carry the pulverized coal to the boiler.
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Components of PA Fan
• Suction Bend With volume Measurement
instruments
• Fan housing with Guide Vanes
• Main Bearings (Anti Friction)
• Rotor with impeller with adjustable blade with
pitch control
• Guide vane housing with guide vanes
• Diffuser with pressure measurement instruments
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SECONDARY AIR SYSTEM
• Ambient air is drawn into the secondary air system by two
50% duty, motor driven axial reaction forced draft fans with
variable pitch control.
• Air discharging from each fan passes first through a air
preheated then through a isolating damper into the
secondary air bust duct.
• The cross over duct extends around to each side of the
boiler furnace to form two secondary air to burner ducts.
• At the sides of the furnace, the ducts split to supply air to
two corners. Then split again to supply air to each of
nineteen burner/air nozzle elevations in the burner box.
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Burner Box Dampers
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Coal/Air Dampers
Secondary Air Dampers
Oil/Secondary Air Dampers
Bottom Tier Secondary Air Damper
Over Fire Damper
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Primary and Secondary air System
in the Boiler
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Axial Fans FD Application
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Induced Draft System
• There are three induced draught fans per boiler,
two operating and one standby
• In 500 MW fans are single-stage, double-inlet
centrifugal fans. Principal fan elements of the fan
are:
• Housing
• Inlet dampers
• Rotor with Bearing
• Shaft Seal
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ID Fan
• The rotor consists of shaft and assembled impeller and runs in
two sleeve bearings that are arranged outside of the housing.
• The impeller consists of a centre disc and two cover discs that are
reinforced by forged rings. The bent blades are welded into
position between the impeller discs.
• The blades are protected by screwed - on wear plates.
• The shaft is of hollow design. The fan shaft has been rated so that
max. operating speed is below the critical speed. Impeller and
shaft are connected by means of a flange. This screwing is
protected by wear plates.
• The fan housing is sealed by means of two-part labyrinth seals.
• Bearings are lubricated with oil.
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Other Fans in The System
• Ignitor Air Fan: Provide combustion air to the ignitors.
Take suction air from the atmosphere and supplies air to
the ignitor wind box.
• Scanner Air Fan: Supplies cooling air to flame scanners.
Normally there are two fans taking suction from FD Fan
discharge duct.
• Mill Seal air fan: Seal air fans provide air for the sealing
of Mill bearing. Suction is from cold Primary air and
pressure is boosted up to maintain the differential
pressure
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AIR AND FLUE GAS PATH TYPICAL 500 MW BOILER
SCAPH-A
PRIMARY
PAPH-A
TO MILLS
PA FAN -A
ECO BYPASS
TO ESP
AA205
SCAPH-A
SECOND
SAPH-A
AEROFOIL
AA203
AA202
FD FAN -B
HOT PA HDR
AA205
AA201
SAPH-B
AA204
AEROFOIL
COLD SEC. AIR
AA203
AA202
SCAPH-B
SECOND
SA HDR
HOT
ECONOMISER-3
ECONOMISER -2
ECONOMISER -1
HORIZONTAL S.H
RE HEATER
PLATEN S.H
COMBUSTION
CHAMBER
DIVISION
PANELETTE S.H
AA201
AA204
ECO BYPASS
FD FAN-A
TO ESP
BIPLANE DAMPER
DIVERTER DAMPER
PAPH-B
HOT PRIMARY AIR
HOT SEC. AIR
FLUE GAS
PNEUMATICALLY O/P
KNIFE EDGE GATE
MOTOR O/P LOUVER
DAMPER
SCAPH-B
PRIMARY
COLD PRIM AIR
TO MILLS
PA FAN -A
PNEUMATICALLY O/P
LOUVER DAMPER
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O/P GATE
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Scheme of Air and Gas Path
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ID Fan at Location
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Variation of Pressure in Boiler
Furnace Pressure
Furnace Pressure At Various Points in Boiler
250
1
200
2
150
3
100
4
50
5
0
1
2
3
4
5
6
7
8
9
10
11
12
13
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Airheater Outlet
Windbox Pressure
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Superheater Platen Inlet
9
-150
10
-200
11
-250
12
-300
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Airheater Inlet
Furnace
8
Points in Boiler
FD Fan Outlet
6
-50
-100
FD Fan Inlet
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Reheater Inlet
LTSH Inlet
Economiser Inlet
Airheater Inlet
E.P. Inlet
I.D. Fan Inlet
I.D. Fan Outlet
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Basics on Fans
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Difference between fans, blowers
and compressors
As per ASME the specific pressure, i.e, the ratio of the discharge
pressure over the suction pressure is used for defining the fans,
blowers and compressors as highlighted below :
Equipment
Fans
Blowers
Compressors
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Specific Ratio
Pressure rise (mm wcl)
Up to 1.11
1136
1.11 to 1.20
1136 – 2066
more than 1.20
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Typical centrifugal fan
operation
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Fan types
Centrifugal fan
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Axial fan
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Centrifugal Fan: Types
Paddle Blade
(Radial blade)
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Forward Curved
(Multi vane)
Backward Curved
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Axial Flow Fan: Types
Tube Axial
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Vane Axial
Propeller
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Fan Types and Efficiencies
Peak Efficiency
Range
Airfoil, backwardly curved/inclined 79-83
Modified radial
72-79
Redial
69-75
Pressure blower
58-68
Forwardly curved
60-65
Axial fan
vanaxial
78-85
Tubeaxial
67-72
Propeller
45-50
Centrifugal Fans
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Axial-flow Fans
Type
Characteristics
Centrifugal Fans
Typical
Applications
Type
Characteristics
Typical
Applications
Prope
ller
Low pressure, high
flow, low efficiency,
peak efficiency close
to point of free air
delivery
Aircirculation,
ventilation,
exhaust
Radial
High pressure,
medium flow,
efficiency close to
tube-axial fans, power
increases continuously
Various industrial
applications, suitable
for dust laden, moist
air/gases
Tubeaxial
Medium pressure,
high flow, higher
efficiency than
propeller type, dip in
pressure-flow curve
before peak pressure
point.
HVAC,
drying ovens,
exhaust
systems
Forwardcurved
blades
Medium pressure,
high flow, dip in
pressure curve,
efficiency higher than
radial fans, power
rises continuously
Low pressure
HVAC, packaged
units, suitable for
clean and dust laden
air / gases
Vaneaxial
High pressure,
medium flow, dip in
pressure-flow curve,
use of guide vanes
improves efficiency
High pressure Backward
applications
curved
including
blades
HVAC
systems,
exhausts
High pressure, high
flow, high efficiency,
power reduces as flow
increases beyond
point of highest
efficiency
HVAC, various
industrial
applications, forced
draft fans, etc.
Same as backward
curved type, highest
efficiency
Same as backward
curved, but for clean
air applications
Airfoil type
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System characteristic
curve
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System curve
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Fan Laws
Flow ? Speed
Pressure ? (Speed)2
Q1 N 1

Q2 N 2
SP1  N 1 


SP 2  N 2 
2
Varying the RPM by 10% Varying the RPM by 10%
decreases or increases air decreases or increases the
delivery by 10%.
static pressure by 19%.
Power ? (Speed)3
kW 1  N 1 


kW 2  N 2 
3
Varying the RPM by 10%
decreases or increases the
power requirement by
27%.
Where Q – flow, SP – Static Pressure, kW – Power and N – speed (RPM)
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Fan static pressure
Fan Static Pressure r SP = SP (Fan outlet) – SP (Fan inlet)
SP (Fan outlet) – Static pressure at fan outlet, mm WC
SP (Fan inlet) – Static pressure at fan inlet, mm WC
r SP = 0.05 – (-10)
= 10.05 mm W.C.
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Speed vs Power
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% Speed
% Power
100
100
90
73
80
51
70
34
60
22
50
13
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Flow control
Damper - Most Popular
Variable Speed Drive
Recirculation
Damper
100
Power
VFC
75
50
VFD
25
Ideal
25
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Flow
75
100
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Impact of speed reduction
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Use of VSD: Boiler ID fan
case study
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Use of VSD: Boiler ID fan
case study (contd.)
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THANK YOU
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