This document discusses fans used in thermal power plants. It describes the main types of fans used - forced draft fans, induced draft fans, primary air fans, and gas recirculation fans. It explains their purposes and characteristics. The key points are: forced draft fans supply air for combustion, induced draft fans exhaust flue gas, primary air fans supply air to coal pulverizers, and gas recirculation fans control steam temperature. Common damage mechanisms for power plant fans are erosion, corrosion, and vibration. Condition monitoring is important to predict failures.

1. FANS IN THERMAL POWER
PLANTS
SHIVAJI CHOUDHURY

2. Fan
 A fan can be considered a mechanical device that
moves a volume of fluid such as air, gas, or vapor
through a pressure driven flow.
 Large capacity fan units typically consist of a bladed,
rotating impeller enclosed in a stationary casing.
 The rotor system causes the motion of the air/gas
and the casing directs the output flow.
 The rotor can be powered through a driver such as a
motor, a turbine driver, or a fluid-driver.

3. Fan Components

4. Types of Fans
 A fan is classified by the
direction of its flow
through the bladed
passages of the impeller.
 A centrifugal fan
moves the air
perpendicular to the
rotational axis of the
impeller .
 an axial fan moves the
air parallel to the
rotational axis of the
impeller.

5. Centrifugal (radial) Fan Blade Types

6. Function of Fans in Boiler
 Supply air required for combustion
 Remove products of combustion
 Deliver fuel to the burners
 Circulate the gases for better heat
transfer

7. Types of Power Plant Fans
There are four main types of fans used
in fossil power plants.
 Forced draft fans,
 Induced draft fans,
 Primary air fans,
 Gas-recirculation fans.

8. Draft Fans
 Draft fans are generally responsible
for maintaining the flow of gases
through the boiler.
 A balanced draft system uses both a
forced draft fan at the inlet of the
system and an induced draft fan at the
outlet of the boiler system.

9. BOILER
Boiler drum
Reheater
Final Super
Platen Super heater
heater LTSH
Economizer
Coal bunker
Wind Box
Secondary
PA duct
air duct
Furnace
Flue gas
APH duct Coal feeder
F D Fan P A Fan
Coal
Pulverizer

10. Forced Draft Fans
 Forced draft fans (FD) supply the air
necessary for fuel combustion by pushing the
air through the combustion air supply system
and into the furnace.
 These fans are typically the most efficient
fans in the power plant because they have
the cleanest operating environment .
 Typically, these fans are centrifugal fans
utilizing radial airfoil blading or variable pitch
axial fans.

11. Induced Draft Fans
 Induced draft fans (ID) are placed at the outlet of the
boiler system and exhaust all gaseous combustion
products, or flue gas, from the boiler by creating a
negative pressure or suction within the furnace.
 These fans handle hot flue gas, they are generally
more susceptible to erosion and corrosion even with
particulate removal equipment (ESP).
 If greater wear resistance is necessary, a modified
radial, forward-curved, or backward-inclined blading
can be used at the expense of efficiency loss.

12. Primary air fans
 Primary air fans (PA) are high pressure fans used to
supply the air for the transportation of coal directly from
the pulverizer to the furnace .
 These fans provide a positive pressure upstream of
the coal pulverizer and handle relatively clean air .
 A PA fan upstream from the pulverizer, or a “cold” PA
fan, pushes the coal/air mixture through the
pulverizer and is most commonly used.
 Cold PA fans typically are airfoil centrifugal fans or
multi-stage axial fans similar to FD fans.

13. Gas Recirculation Fans
 Gas-recirculation fans are used to control steam
temperature, furnace heat absorption, and slagging of
heating surfaces.
 These fans extract gas from the economizer outlet
and the pre-heater inlet and then discharge the gas
either to the bottom of the furnace for steam
temperature control or to various locations in the
furnace.
 The duty cycle of a gas recirculation fan is very
stringent due to heavy dust loads and extreme
temperature excursions.
 Straight or modified radials or forward curved,
backwardly inclined centrifugal wheels are appropriate
for gas-recirculation fans.

14. PA
FAN
TO
ESP
FD
FAN
TO
ESP
PA

15. FLUE
GAS
FROM
APH
ESP ID FAN

16. Draft System -500 MW
SECOND
FURNACE PASS
FD
FAN AHP AHP ESP ID
FAN
+156 -73
mmwcl
-5 -221 +36
mmwcl mmwcl mmwcl mmwcl
AHP ESP
SECOND
Chimney
FD AHP FURNACE
PASS
FAN

17. Axial-Flow Fan

18. Airfoil-Blade Centrifugal Fan

19. Damage Mechanisms
Most common damage mechanisms
associated with power plant fans
failure are :
 Erosion,
 Corrosion,
 Vibration
which are responsible for serious and
costly maintenance.

20. Erosion
 One of the most common damage
mechanisms associated with power plant
fan failure is erosion, which is
responsible for serious and costly
maintenance.
 The rate of erosion depends on
suspended particles / fly ash, in the flue
gas.

21. EFFECT OF BLADE TYPE ON EROSION
RESISTANCE AND EFFICIENCY
BLADE TYPE TYPICAL MAX STATIC TOLERANCE TO
EFFICIENCY ( %) EROSIVE
ENVIROMEMT
RADIAL
70 HIGH
RADIAL TIP
80 MEDIUM TO
HIGH
BACKWARDLY INCLINED
SOLID 85 MEDIUM
AIRFOIL
90 LOW

22. Resistant to Erosion
 The rate of erosion experienced by fans used in
harsh applications is often controlled by the use
of repairable liners, replaceable liners, or
renewable coatings.
 Reducing fan speed and selecting a fan blade
type that is more resistant to erosion will slow
down the abrasive wall thinning experienced by
fan unit surface.

23. Abrasion-Resistant Impeller
 Fans that operate
in flue gas, such as
induced draft fans
(IDF) for coal-fired
boilers, are
required to be
resistant to
abrasion by ash in
the flue gas.

24. CORROSION
 The following list is the most common types of
corrosion problems found in boiler power
plants.
 • Erosion corrosion
 • Crevice corrosion
 • Galvanic corrosion
 • Pitting
 • General corrosion (wastage)
 • Differential Oxygenation
 • Biological corrosion
 • Intergranular corrosion

25. Vibration
 Possible reasons as to why vibration
occurs in fan units are listed below.
 • Improper balancing
 • Loose components
 • Worn/damaged/cracking of fan parts
 • Improper Lubrication
 • Improper clearance of moving parts.
 • Excitation of a resonant frequency
 • Corrosion, erosion, high/low cycle fatigue effects
 • Misalignment or bent shaft
 • Improperly designed or deteriorated foundations
 • Build-up of material on the rotor

26. Output Air Flow Control
 A centrifugal fan utilizing inlet vanes
controls the airflow .
 Most axial fans are operated by
variable pitch axial blades, .
 A fan that is run with a variable speed
motor can adjust the speed to control
the output flow properties .

27. Inlet Vanes of Centrifugal Fan
FAN

28. Variable Airfoil Blading of an
Axial-flow Fan

29. CONDITION MONITORING
 Condition monitoring is the use of advanced
technologies to determine equipment
condition and, potentially, predict failure.
 It includes technologies such as the
following:
 • Vibration measurement and analysis
 • Oil analysis
 • Nondestructive examination (NDE)
 • Infrared thermography
 • Motor current analysis

30. STARTUP PROCEDURES OF DRAFT FANS
 The startup procedures, in addition
to the controls and interlocks,
should follow the requirements of
NFPA 85.

31. FANS -500 MW BOILER
FAN F D FAN PA FAN ID FAN
TYPE AXIAL AXIAL RADIAL
NO per boiler 2 2 2
FLOW ,M3/S 251.6 184.0 574.6
PRESS,mmwc 390 1200 467
TEMP,DEG C 45 50 150
DRIVE MOTOR MOTOR MOTOR
SPEED,RPM 980 1480 580
MOTOR ,KW 1225 2725 3950
CONTROL VARIABLE PITCH VARIABLE PITCH INLET DAMPER+
CONTROL CONTROL VFD

32. FANS -200 MW BOILER
FAN F D FAN PA FAN ID FAN
TYPE AXIAL Reaction RADIAL AXIAL impulse
NO per boiler 2 2 2
FLOW ,M3/S 105 75 225
PRESS, mmwcl 510 1187 356
TEMP,DEG C 50 50 136
DRIVE MOTOR MOTOR MOTOR
SPEED,RPM 1480 1480 740
MOTOR ,KW 750 1250 1100

33. ID FAN under erection-500 MW

34. Distribution of Aux Power Consumption in a
210 mw Power Plant
FANS

35. Difference between fan and
Compressor
 Fans are similar to compressors; the
difference is that fans create a flow of gas
whereas compressors increase the pressure
of the gas.
 ASME PTC-11, “Performance Test Code for
Fans,” defines a fan as providing a
compression ratio of 1.1 or a density
change of 7%.
 ISO 5801 defines the upper limit of fans as
a pressure increase of 120 inches Wg (30
kPa).

36. STANDARDS
 1. NFPA 85, Boiler and Combustion
System Hazard Code.
 2. AMCA 803, Site Performance Test
Standard.
 3. AMCA 202, Fan Application Manual.
 4. AMCA 203, Field Performance
Measurements.
 ASME PTC 11

37. THANKING YOU