Interview questions and answers on Boiler fans


1-What do you mean by fan & and what are the various fans used in power plant Boilers?

Fan is a Mechanical device , used to move volume air, vapour or flue gas at specific static pressure

Fans used are;

1-Induced draft fan (ID fan)

2-Forced draft fan (FD fan)

3-Secondary air fan (SA fan)

4-Cinder reinjection fan (CR fan)

2-What type of fans used in Boiler?

Generally centrifugal radial, forward & backward curved fans

3-What are the function of Boiler fans in Boiler

SL No.

Fan type



ID fan

1-To suck & expel the flue gas from Boiler

2-To maintain negative & balanced draft in furnace


FD fan

1-To provide combustion air for Boiler

2-To maintain balanced draft in Boiler

3-To maintain fluidisation on bed


SA fan/PA fan

1-To carry fuel into the furnace or bed

2-To assist fuel in suspension burning (OFA)

3-To spread the fuel in case of Biomass Boilers


CR fan

To re-inject unburnt fuel from Bank, Economiser & APH back into the furnace


4-Which type of fan is more efficient & why?

Centrifugal backward curved is more efficient as they do not over load & speed controlling is very easy. Fan operates beyond the maximum efficiency point at lower in put power

5-Differentiate between forward & backward curved centrifugal fans

Sl No.

              Backward curved

              Forward curved


Less number of blades

More number of blades


For this type of impeller, flow is produced in a radial direction because the impeller develops static pressure across the longer length of blade.  On the front side of the blade a positive pressure is generated pushing the air outwards and on the reverse side of the blade a negative pressure is generated

For this type of impeller, flow from the impeller is produced in a tangential direction.  The forward curve of the blade imparts kinetic energy to the air requiring a scroll housing to convert the kinetic energy into static pressure


Does not over load the motor at higher flow rates

Overloads the motor at higher flow rate


High pressure

Low pressure


Lower & medium flow rate

Higher flow


Higher efficiency

Lower efficiency

 6-What are the various parts of Boiler fans (centrifugal type)?

  • Parts of Boiler fans
  • Casing
  • Inlet cone
  • Evase
  • Inlet box
  • Impeller blade
  • Impeller shroud
  • Impeller back plate
  • Shaft & coupling
  • Bearings

7-What are the material of composition (MOC) of Fan’s various parts

SL No.

Particular Fan part




Mild steel IS 2062


Inlet cone

& inlet box

Mild steel IS 2062



Mild steel IS 2062


Impeller blade



Impeller shroud



Impeller back plate





 8-What is thickness of fans casing

Casing of ID fans is 8mm & that of FD, SA & PA fans is 6mm

9-What are factors considered for Boiler fans design?

Following factors are considered while designing the Boiler fans

  • Required flow
  • Inlet & outlet static pressure
  • Total pressure
  • Operating & design temperature
  • Density of gas/air
  • Dust load
  • Relative humidity
  • Nose level
  • Fan speed

 10-What type of bearings used for Boiler fans?

Generally, spherical roller bearings for fans having speed up to 1500 TPM

11-What are the various instruments used for fans?

Draft sensor (draft transmitter)

Flow meter (Aerofoil)

Temperature sensors

12-What are the protections given for fans?

Start permissive,

Inlet Damper close

Bearing temperature normal

Bearing vibration normal


High bearing vibrations trip > 7mm/sec

High bearing temperature trip>90 deg C (for journal & rolling contact bearings)

High/low static pressure

Over load

13- Explain the terms static pressure, total pressure and velocity pressure.

Static Pressure is

Resistance to flow

Equal in all directions

Can be Positive or Negative

Independent of air velocity

Measured by pressure tap perpendicular to airflow

Used for fan selection


Total Pressure is


A fluid in motion will exert a Total Pressure on an object in its path.

Total Pressure measured by pressure tap pointed directly into the air stream.

Used to find velocity pressure.

Velocity Pressure

Cannot be measured directly.

A Pitot tube uses both Static pressure and Total pressure taps.

Used for measuring CFM in a system.

Total Pressure = Static Pressure + Velocity Pressure

17- What is the differential static pressure?

 It is the difference between the static pressure at fan inlet and out let

18- What are the flow control methods employed in fans?

 Controlling inlet guide vanes

Discharge damper control method

 V belt position change on pulley (If fan is stepped pulley driven type)

 Incorporating Variable Frequency Drives (VFD) to fan motors to control the speed

19-Calculate the differential static pressure of ID fan, whose inlet static pressure is -300 mmwc & outlet 0 mmwc

We have,

Differential static pressure = Outlet static pressure-Inlet static pressure = 0-(-300) = 300 mmwc

20- Calculate the differential static pressure of FD fan, whose inlet static pressure is -10 mmwc & outlet 250 mmwc

Differential static pressure = Out let static pressure-Inlet static pressure = 250-10= 240 mmwc

21-What size of FD air duct is required to get flow 70 m3 /sec. at 11 m/sec. speed?

We have,

 Volumetric flow = Area of duct (M2) X Velocity of air (m/sec.)

Area of duct = 70 / 11 = 6.36 M2


22- State the relation between fan’s flow, static pressure, and speed and power consumption.

Following are the some affinity laws related to fans:

A. Fan speed is directly proportional to its flow.

(N1/N2) = (Q1/Q2)

Where N1 & N2 are speed RPM & Q1 & Q2 are flow in M3/hr

B. Static pressure is directly proportional to square of the speed.

(SP1/SP2) = (N1/N2)2

Where, SP1 & SP2 are static pressure at two different speed/RPM

C. Power consumption is directly proportional to cube of speed.

(P1/P2) = (N1/N2)3

Where, P1&P2 are power consumption at speed N1 & N2

23-Calculate the power consumed by an ID fan if its speed reduced from 900 RPM to 750 RPM, take 120 KW power consumption at 900 RPM

As per affinity law

(P1/P2) = (N1/N2)3

(120/P2) = (900/750)3

P2 = 69.44 KW

24-Calculate the anticipated static pressure when FD fan RPM is increased from 1100 RPM to 1175 RPM, static pressure at 1100 RPM is 220 mmwc

As per affinity law

(SP1/SP2) = (N1/N2)2

(220/SP2) = (1100/1175)2

SP2 = 268.3 mmwc

25-Boiler ID fan of flow capacity 55 m3/sec having static inlet pressure & efficiency are  is 300mmwc & 78% respectively, calculate the rated power required to run the fan if motor of efficiency 96% is used to drive the fan.

We have,

Motor rated power = Flow (M3/sec) X Static differential pressure (mmwc) / (102 X Fan static efficiency X motor efficiency)

Motor rated power P = 55 X 300 / (102 X 0.78 X 0.96)

Motor rated power P =216.03 KW

26-A Boiler FD fan of capacity 24m3/sec having total pressure 265 mmwc & Mechanical efficiency 82%, then calculate the fan shaft input power

Fan shaft power = Flow (M3/sec) X Total differential pressure (mmwc) / (102 X Fan mechanical efficiency)

Fan shaft power = 24 X 265 / (102 X 0.82) =76.04 KW

27-Calculate the rated capacity of Boiler FD fans based on following data.

SL No.




Boiler capacity

100 TPH


Coal consumption

17 TPH


Coal to air ratio



Fan inlet static pressure

5 mmwc


Draft loss in ducts from Fan to APH

15 mmwc


Draft loss in APH

80 mmwc


Draft loss aerofoil flow meter

25 mmwc


Draft loss in ducts from APH to plenum

20 mmwc


Draft loss in Grate

50 mmwc


Static efficiency of the fan



Motor efficiency



Air temperature

28 deg C



Total air required for burning the coal in Boiler = 17 X 6 =102 TPH

Take 20% extra margin on air flow , then air flow = 102 X 120% = 122.4 = 123 TPH

Convert air flow from TPH to m3/sec

Density of air at temperature 28 deg C = 273 X 1.293 / (28+273) = 1.17 kg/m3

Where, 1.293 is density of air at 0 deg C

Air flow = 123000 kg/hr /1.17 =29.2 m3/sec

Based on above flow we can select, two fans of capacity 29.2/2 = 14.6 or 15 m3/sec

Now calculate the total static pressure required to over come the resistance in air flow path

Pressure/draft loss drop = Draft loss in APH inlet ducts + Draft loss in flow element + Draft loss in APH + Draft loss in APH to grate ducts + Draft loss in grate

= 15 + 80 +25 + 20 + 50 = 190 mmwc

Take 20% extra margin on draft

Discharge static pressure = 190 X 120% = 228 = 230 mmwc

Now calculate the motor rated power, P = Flow (M3/sec) X Static differential pressure (mmwc) / (102 X Fan static efficiency X motor efficiency)

P = 15 X (230-5) / (102 X 0.75 X 0.95) = 46.43 KW

Take 15% extra margin on motor capacity

Then, compensated rated motor power = 46.43 X 115% =53.4 KW

Select nearest standard size motor i.e 55 KW

Therefore for boiler of capacity 100 TPH, FD fans capacity 55 KW X 2 are required

28-Why does ID fan trip during Boiler operations?

Reasons are

1-Over load

2-High bearing vibrations

3-High bearing temperature

4-Electrical& Instrumentation related issues like short circuit, mal functions etc

5-Low drum level

6-High Boiler pressure & temperature (if interlocks are provided)

7-Low pressure (more negative pressure) in furnace

29-Why does FD fans trip during Boiler operations?

Reasons are

1-Over load

2-High bearing vibrations

3-High bearing temperature

4-Electrical& Instrumentation related issues like short circuit, mal functions etc

5-High drum level

6-High Boiler pressure & temperature (if interlocks are provided)

7-Low pressure (more negative pressure) in furnace

8-Upon tripping of ID fan

30-What will happen if Boiler ID fan trips (one fan out of two)?

1-Same stream FD fan trips

2-Same stream SA fan trips

3-If pressure drops fuel feeding system trips

4-Boiler pressure becomes low & eventually boiler load reduces

31-What will happen if Boiler FD fan trips (one fan out of two)?

1-Same stream SA fan trips

2-If pressure drops fuel feeding system trips

3-Boiler pressure becomes low & eventually boiler load reduces

32-What are the maintenance activities that you will carryout on Boiler fans

  • Fans cleaning
  • Casing thickness checking
  • Impeller inspection, includes Shroud plate, blade and back plate thickness checking
  • Wear liner inspection and hard facing
  • Inlet cone inspection
  • Drive end non drive end bearings overhauling and grease replacement
  • Shaft run out checking
  • Coupling inspection
  • Shaft ultrasonic continuity test
  • Suction and discharge dampers and pneumatic cylinders overhauling
  • Motors servicing
  • Realignment
  • Manhole doors and inspection doors proper sealing
  • Fan trial and dynamic balancing at site

33-Write down the start of steps of Boiler fans

Start-up checks

  • Ensure all the maintenance activities are carried out on fans and equipments are normalized
  • Ensure all the instruments like bearing temperature sensor, vibration sensor etc are fitted & showing readings in DCS
  • Ensure none of the interlocks are bypassed
  • Ensure Fans suction & discharge dampers are in closed condition
  • Ensure coupling & bearing cooling fan guards are fitted properly
  • Ensure local emergency push button is healthy & in good condition
  • Give clearance to DCS or electrical person to start at 10% initially
  • Check abnormal sound from fan casing, bearing & couplings, if found normal open discharge damper slowly & subsequently suction damper

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