Post on 04-Nov-2015
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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_1124809304.unknown
_1124809157.unknown
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.