2 july 20151 air & draft system
TRANSCRIPT
April 19, 2023 1
Air & Draft system
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April 19, 2023 2
Presentation Plan• 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
April 19, 2023 3
Arrangement of Boiler Auxiliaries
April 19, 2023 4
Need of Draft System
BoilerCombustion Air Flue Gases
• Air needed for combustion
• Flue are needed to be evacuated
• Losses due to flow need to be overcome
April 19, 2023 5
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
April 19, 2023 6
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.
April 19, 2023 7
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
April 19, 2023 8
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.
April 19, 2023 9
Burner Box Dampers
• Coal/Air Dampers• Secondary Air Dampers• Oil/Secondary Air Dampers• Bottom Tier Secondary Air Damper• Over Fire Damper
April 19, 2023 10
Primary and Secondary air System in the Boiler
April 19, 2023 11
Axial Fans FD Application
April 19, 2023 12
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.
April 19, 2023 14
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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PA
PH-A
SA
PH-A
SA
PH-B
PA
PH-B
FD FAN -B
FD FAN-A
PA FAN -A
PA FAN -A
SCA
PH-B
PR
IMA
RY
SCA
PH-B
SE
CO
ND
SCA
PH-A
PR
IMA
RY
SCA
PH-A
SE
CO
ND
TO MILLS
TO MILLS
HO
T P
A H
DR
HO
T
S
A H
DR
ECO BYPASS
ECO BYPASS
EC
ON
OM
ISE
R-3
EC
ON
OM
ISE
R -
2
EC
ON
OM
ISE
R -
1
HO
RIZ
ON
TA
L S
.H
RE
HE
AT
ER
PL
AT
EN
S.H
DIV
ISIO
N
PA
NE
LE
TT
E S
.H
COMBUSTION CHAMBER
AEROFOIL
AEROFOIL
COLD SEC. AIR
COLD PRIM AIR
HOT PRIMARY AIR
HOT SEC. AIR
FLUE GAS
PNEUMATICALLY O/P KNIFE EDGE GATE
MOTOR O/P LOUVER DAMPER
PNEUMATICALLY O/P LOUVER DAMPER
MOTOR O/P GATE
BIPLANE DAMPER
DIVERTER DAMPER
AA201
AA202AA203
AA204
AA205
AA201
AA202AA203AA205
AA204
AIR AND FLUE GAS PATH TYPICAL 500 MW BOILER
TO ESP
TO ESP
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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 BoilerFurnace Pressure At Various Points in Boiler
-300
-250
-200
-150
-100
-50
0
50
100
150
200
250
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Points in Boiler
Furn
ace
Pre
ssur
e
1 FD Fan Inlet
2FD Fan Outlet
3Airheater Inlet
4Airheater Outlet
5Windbox Pressure
6 Furnace
7 Superheater Platen Inlet
8Reheater Inlet
9 LTSH Inlet
10 Economiser Inlet
11Airheater Inlet
12 E.P. Inlet
13 I.D. Fan Inlet
14I.D. Fan Outlet
April 19, 2023 19
Basics on Fans
April 19, 2023 20
Difference between fans, blowers and compressors
Equipment Specific Ratio Pressure rise (mm wcl)
Fans Up to 1.11 1136
Blowers 1.11 to 1.20 1136 – 2066
Compressors more than 1.20 -
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 :
April 19, 2023 21
Typical centrifugal fan operation
April 19, 2023 22
Fan types
Centrifugal fan Axial fan
April 19, 2023 23
Centrifugal Fan: Types
Paddle Blade (Radial blade)
Forward Curved (Multi vane)
Backward Curved
April 19, 2023 24
Axial Flow Fan: Types
Tube Axial
Vane Axial
Propeller
April 19, 2023 25
Fan Types and Efficiencies
Centrifugal Fans 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
April 19, 2023 26
Axial-flow Fans Centrifugal Fans
Type Characteristics Typical
Applications Type Characteristics
Typical Applications
Propeller
Low pressure, high flow, low efficiency, peak efficiency close to point of free air delivery
Air-circulation, 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
Tube-axial
Medium pressure, high flow, higher efficiency than propeller type, dip in pressure-flow curve before peak pressure point.
HVAC, drying ovens, exhaust systems
Forward-curved 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
Vane-axial
High pressure, medium flow, dip in pressure-flow curve, use of guide vanes improves efficiency
High pressure applications including HVAC systems, exhausts
Backward curved blades
High pressure, high flow, high efficiency, power reduces as flow increases beyond point of highest efficiency
HVAC, various industrial applications, forced draft fans, etc.
Airfoil type Same as backward curved type, highest efficiency
Same as backward curved, but for clean air applications
April 19, 2023 27
System characteristic curve
April 19, 2023 28
April 19, 2023 29
System curve
April 19, 2023 30
Fan Laws
Flow ? Speed Pressure ? (Speed)2 Power ? (Speed)3
1 1
2 2
Q N
Q N
21 1
2 2
SP N
SP N
3
1 1
2 2
kW N
kW N
Varying the RPM by 10% decreases or increases air delivery by 10%.
Varying the RPM by 10% decreases or increases the static pressure by 19%.
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)
April 19, 2023 31
Fan static pressure
Fan Static Pressure 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
SP = 0.05 – (-10) = 10.05 mm W.C.
April 19, 2023 32
Speed vs Power
% Speed % Power
100 100
90 73
80 51
70 34
60 22
50 13
April 19, 2023 33
Flow control
Damper - Most PopularVariable Speed Drive
Recirculation
Damper
VFC
VFD
Ideal
Power
Flow25 7550 100
100
75
50
25
April 19, 2023 34
Impact of speed reduction
April 19, 2023 35
Use of VSD: Boiler ID fan case study
April 19, 2023 36
Use of VSD: Boiler ID fan case study (contd.)
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THANK YOU