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

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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

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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

• 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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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

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Basics on Fans

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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 :

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Typical centrifugal fan operation

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Fan types

Centrifugal fan Axial fan

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Centrifugal Fan: Types

Paddle Blade (Radial blade)

Forward Curved (Multi vane)

Backward Curved

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Axial Flow Fan: Types

Tube Axial

Vane Axial

Propeller

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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

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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

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System characteristic curve

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System curve

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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)

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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.

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Speed vs Power

% Speed % Power

100 100

90 73

80 51

70 34

60 22

50 13

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Flow control

Damper - Most PopularVariable Speed Drive

Recirculation

Damper

VFC

VFD

Ideal

Power

Flow25 7550 100

100

75

50

25

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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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