Air & Draft system

Presentation PlanVarious auxiliary equipments in a boilerNeed for Draft SystemPrimary and secondary air system in Boiler and their fansOther fans in a boilerBasic of fans: Fan Types and selectionFan laws and factors affecting fan performancePerformance curves

Arrangement of Boiler Auxiliaries

Need of Draft System

Boiler

Air needed for combustion Flue are needed to be evacuated Losses due to flow need to be overcome

How to select a fanAir or Gas flow -Kg/hrDensity (function of temperature and pressure)System, resistance (losses) Major Fans in Boiler Draft SystemPA FansFD FansID Fans

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.

Components of PA FanSuction Bend With volume Measurement instrumentsFan housing with Guide VanesMain Bearings (Anti Friction)Rotor with impeller with adjustable blade with pitch controlGuide vane housing with guide vanesDiffuser with pressure measurement instruments

SECONDARY AIR SYSTEMAmbient 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.

Burner Box DampersCoal/Air DampersSecondary Air DampersOil/Secondary Air DampersBottom Tier Secondary Air DamperOver Fire Damper

Primary and Secondary air System in the Boiler

Axial Fans FD Application

Induced Draft SystemThere 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 BearingShaft Seal

ID FanThe 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.

Other Fans in The SystemIgnitor 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

PAPH-ASAPH-ASAPH-BPAPH-B FD FAN -BFD FAN-APA FAN -APA FAN -ASCAPH-B PRIMARYSCAPH-B SECONDSCAPH-A PRIMARYSCAPH-A SECONDTO MILLSTO MILLSHOT PA HDRHOT SA HDRECO BYPASSECO BYPASSECONOMISER-3ECONOMISER -2ECONOMISER -1HORIZONTAL S.HRE HEATERPLATEN S.HDIVISION PANELETTE S.HCOMBUSTION CHAMBERAEROFOILAEROFOILCOLD SEC. AIRCOLD PRIM AIRHOT PRIMARY AIRHOT SEC. AIRFLUE GASPNEUMATICALLY O/P KNIFE EDGE GATEMOTOR O/P LOUVER DAMPERPNEUMATICALLY O/P LOUVER DAMPERMOTOR O/P GATEBIPLANE DAMPERDIVERTER DAMPERAA201AA202AA203AA204AA205AA201AA202AA203AA205AA204AIR AND FLUE GAS PATH TYPICAL 500 MW BOILERTO ESPTO ESP

Scheme of Air and Gas Path

ID Fan at Location

Variation of Pressure in Boiler

Chart1

-10

200

183

143

100

-5

-5

-6

-30

-42

-77

-195

-238

29

Furnace Pressure At various Points in Boiler

Points in Boiler

Furnace Pressure

Furnace Pressure At various Points in Boiler

Chart2

-10

200

183

143

100

-5

-5

-6

-30

-42

-77

-195

-238

29

Furnace Pressure

Points in Boiler

Furnace Pressure

Furnace Pressure At Various Points in Boiler

Sheet1

Primary Air

PA Fan Inletmmwc-13-13-13

PA Fan Outletmmwc804804747

Airheater Inletmmwc788788731

Airheater Outletmmwc752752700

Mill Inletmmwc625625576

Mill Outletmmwc355355326

Secondary Air

1FD Fan Inletmmwc-12-12-101

2FD Fan Outletmmwc2272272002

3Airheater Inletmmwc2062061833

4Airheater Outletmmwc1551551434

5Windbox Pressuremmwc1001001005

6Furnacemmwc-5-5-56

7Superheater Platen Inletmmwc-5-5-57

8Reheater Inletmmwc-6-6-68

9LTSH Inletmmwc-35-35-309

10Economiser Inletmmwc-50-50-4210

11Airheater Inletmmwc-85-85-7711

12E.P. Inletmmwc-215-215-19512

13I.D. Fan Inletmmwc-276-276-23813

14I.D. Fan Outletmmwc33332914

Sheet2

Sheet3

Basics on Fans

Difference between fans, blowers and compressorsAs 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 :

Typical centrifugal fan operation

Fan typesCentrifugal fanAxial fan

Centrifugal Fan: TypesPaddle Blade (Radial blade)Forward Curved (Multi vane)Backward Curved

Axial Flow Fan: Types

Tube Axial

Vane Axial

Propeller

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

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

System characteristic curve

System curve

Fan Laws

Flow Speed

Pressure (Speed)2

Power (Speed)3

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 pressureFan Static Pressure r SP = SP (Fan outlet) SP (Fan inlet)SP (Fan outlet) Static pressure at fan outlet, mm WCSP (Fan inlet) Static pressure at fan inlet, mm WCr SP = 0.05 (-10) = 10.05 mm W.C.

Speed vs Power

Flow control

Damper - Most PopularVariable Speed DriveRecirculationDamper VFCVFDIdealPowerFlow257550100100755025

Impact of speed reduction

Use of VSD: Boiler ID fan case study

Use of VSD: Boiler ID fan case study (contd.)

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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.

But different applications call for different kinds of fans to be used.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.

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 methodsRecirculation: 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 fansChanging 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.