2. August 28, 2016 PMI Revision 00 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
3. August 28, 2016 PMI Revision 00 3
Arrangement of Boiler Auxiliaries
4. August 28, 2016 PMI Revision 00 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
5. August 28, 2016 PMI Revision 00 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
6. August 28, 2016 PMI Revision 00 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.
7. August 28, 2016 PMI Revision 00 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
8. August 28, 2016 PMI Revision 00 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.
9. August 28, 2016 PMI Revision 00 9
Burner Box Dampers
• Coal/Air Dampers
• Secondary Air Dampers
• Oil/Secondary Air Dampers
• Bottom Tier Secondary Air Damper
• Over Fire Damper
10. August 28, 2016 PMI Revision 00 10
Primary and Secondary air System
in the Boiler
12. August 28, 2016 PMI Revision 00 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
13. August 28, 2016 PMI Revision 00 13
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.
14. August 28, 2016 PMI Revision 00 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
15. August 28, 2016 PMI Revision 00 15
PAPH-ASAPH-ASAPH-BPAPH-B
FD FAN -B
FD FAN-A
PA FAN -A
PA FAN -A
SCAPH-B
PRIMARY
SCAPH-B
SECOND
SCAPH-A
PRIMARY
SCAPH-A
SECOND
TO MILLS
TO MILLS
HOTPAHDR
HOTSAHDR
ECO BYPASS
ECO BYPASS
ECONOMISER-3
ECONOMISER-2
ECONOMISER-1
HORIZONTALS.H
REHEATER
PLATENS.H
DIVISION
PANELETTES.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
16. August 28, 2016 PMI Revision 00 16
Scheme of Air and Gas Path
20. August 28, 2016 PMI Revision 00 20
Difference between fans,
blowers and compressors
Equipment Specific Ratio Pressure rise (mmWg)
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 :
21. August 28, 2016 PMI Revision 00 21
Typical centrifugal fan
operation
22. August 28, 2016 PMI Revision 00 22
Fan types
Centrifugal fan Axial fan
25. August 28, 2016 PMI Revision 00 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
26. August 28, 2016 PMI Revision 00 26
Axial-flow Fans Centrifugal Fans
Type Characteristics
Typical
Applications
Type Characteristics
Typical
Applications
Prope
ller
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
27. August 28, 2016 PMI Revision 00 27
System characteristic
curve
30. August 28, 2016 PMI Revision 00 30
Fan Laws
Flow ? Speed Pressure ? (Speed)2
Power ? (Speed)3
1 1
2 2
Q N
Q N
=
2
1 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)
31. August 28, 2016 PMI Revision 00 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.
32. August 28, 2016 PMI Revision 00 32
Speed vs Power
% Speed % Power
100 100
90 73
80 51
70 34
60 22
50 13
33. August 28, 2016 PMI Revision 00 33
Flow control
Damper - Most Popular
Variable Speed Drive
Recirculation
Damper
VFC
VFD
Ideal
Power
Flow
25 7550 100
100
75
50
25
34. August 28, 2016 PMI Revision 00 34
Impact of speed
reduction
35. August 28, 2016 PMI Revision 00 35
Use of VSD: Boiler ID
fan case study
36. August 28, 2016 PMI Revision 00 36
Use of VSD: Boiler ID
fan case study (contd.)
Air Inlet: Air enters the inside of the turning impeller wheel.
Impeller Wheel: Imparts energy to the air in the form of motion and pressure. As the wheel turns, air between the blades is moved in the direction of the blade and accelerated outward by centrifugal force.
Shaft: Turned by a motor, coupled either directly to the shaft, or with V-belts and pulleys.
Scroll Housing: Efficiently directs air from the impeller wheel to the fan outlet.
Outlet: Typically connected to a duct which distributes the air to where it is needed.
<number>
<number>
<number>
But different applications call for different kinds of fans to be used.
<number>
The overall static pressure drop across each component of the overall system is related to the square of the airflow rates as illustrated in Figure.
This general relationship between total system static pressure drop and airflow rate is termed the system characteristic curve. For example, if the designer of the system needed 12,000 ACFM to achieve the necessary velocities in the system, he or she would know that the total static pressure drop across the system would be 10 in. W.C. Therefore, the fan would need to generate an airflow of 12,000 ACFM at a fan static pressure rise of at least 10 in. W.C.
<number>
<number>
<number>
The pressure gauge normally used in ducts indicate the static pressure. A simple U-tube manometer can measure the static pressure.
Emphasize the relation between power and speed and explain how small reductions in speed can bring about dramatic reductions in power.
If fans rated for a higher flow rate, but a lesser flow is actually required: then flow reduction is effected by the following methods
Recirculation: Venting the high-pressure air, or recirculating it to the inlet, is often used with positive-displacement blowers. It is sometimes used with fan systems, but is the least efficient method as there is no reduction in the air being moved.
Damper: Restricting the airflow is accomplished with dampers or valves which close off the airflow at the inlet or outlet. Inlet vanes, which swirl the air entering the centrifugal fan or blower, are more efficient than dampers or butterfly valves.
VFC: Variable speed fluid coupling. Motor speed is constant but the fluid coupling speed changes thus changing the speed of fan.
VFD: Variable frequency drive which varies the speed of the motor itself
Changing the blade angle is a method used with some vane-axial fans
Changing the rotational speed is the most efficient. If the volume requirement is constant, it can be achieved by selecting appropriate pulley sizes. If the volume varies with the process, adjustable-speed drives can be used.
