Fans InPower Plants

7/30/2019 Fans InPower Plants

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Fans in Power Stations

PMI


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Power Plant Schematic

Chapter 1: Basic ConceptsESOE 505221 Fluid Mechanics 2


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Need of Fans

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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Fan Operation

Fans cause pressure increase by:

Centrifugal force created by rotation of thecolumn of air trapped between two bladesKinetic energy is supplied to the air through

Chapter 1: Basic ConceptsESOE 505221 Fluid Mechanics 4 4

e mpe erTotal pressure = velocity head + staticpressure


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Selection Considerations

Quantity of air to be moved per unit time

Estimated system resistance andexpected variationsAmount of noise ermitted


Space available for fanEconomic implications


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SYSTEM RESISTANCE

When a gas is forced through a duct

system, a loss in pressure occurs. Thisloss in pressure is called systemresistance.


System resistance is composed of twocomponents:Friction losses andDynamic losses.


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Variation of System Resistance with Flow



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Draft Losses


Total losses

p

Percent Boiler Rating

Furnace, SH & RH Losses

Economizer Losses

Ducts & dampers losses


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Fan Classification

Designated as dynamic pumpsCentrifugal

Further classification by entry and exit of fluid/gas throughimpeller

4 aerodynamic classifications:


Axial flowRadial flow : referred to as centrifugal fanCross flow

Mixed flow


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Centrifugal Fans



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Components of Centrifugal Fan



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Centrifugal Fan Operation

Fans cause a pressure increase through two methods Centrifugal force is created by the rotation of the column of airtrapped between two blades. Kinetic energy is supplied to the air through the impeller Total pressure = velocity head + static pressure

Blades are airfoil-type, backward-curved, forwardcurved, or radial


Airfoil-types are complex and expensive but very efficient;

theyre used for large systems where the cost is justified.


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Wheel Blade Types and Rotation


Applications where the fans will handle clean air or clean gas, the highlyefficient backward inclined airfoils are the preferred design. In anapplication where the fan is subject to erosion due to heavy dustloading, a straight radial type fan provides erosion resistance, but at theexpense of efficiency.


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Backward-Curved Fan Blades

Have a self-limiting power characteristic,so if sized correctly the motor wontoverheat or burn out even if conditionschange.High efficiency and stable operationmake this blade type popular.


the right of the peak pressure flow rate toachieve both high efficiency and a stableflow rate.This type of fan operates stably because

the pressure difference provided by thefan drops if the flow rate goes up. If theopposite were true, increased anincreased flow rate would cause increasefan power, which is unstable.


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Section of a Centrifugal Fan



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Radial BladesSimilar performance to abackward-curved except that itseasier to overheat because as flowrate goes up, so does power.

Easier to maintain, so its used indirty situations (easy to cleanstraight blades, and they dontcollect as much)


a es are s ronger an o ertypes.Used primarily in industrial systemsin a corrosive or erosiveenvironment, such as materialhandling of airborne particulate orwhere high static pressure isrequired.


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Airfoil-blade centrifugal fan



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Forward-Curved Blades

Have problems with instabilitybecause a specified pressure

rise can fit three different flowrates.Burnout can also be a


increases with flow rate.Quieter than other fans; usedfor most furnace blowers

Usually limited to cleanservice applications


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Axial Flow Fans

Common types: propeller,tubeaxial, vaneaxialTubeaxial: impeller is inside a tubeto guide airflow and improveperformanceVaneaxial: like a tubeaxial except


the impeller are used to reduceswirl and improve performanceUsed to deliver large flow rates butsmall increase in pressure

Examples include fans used forventilation without ductwork, mobileroom fans, and fans used to coolcomputers


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Two-Stage Axial Fan Assembly



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Axial-flow fan for induced-draft service



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System Pressure Effects

Fan curves are typically given in terms of total pressure vs.volumetric flow rateA typical fan running at a fixed speed can provide a greatervolumetric flow rate for systems with smaller total pressure drops (ifwere to the right of the peak in the fan curve).Total pressure loss=static pressure loss+dynamic pressure loss


If exit and inlet area of a duct are about the same, the dynamicpressure loss (or gain) may be minimal.


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Effect of Blade Type on Erosion

Resistance and Efficiency



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Fan Curves

Manufacturer will provide a fan curve for each fanThe fan curves predict the pressure-flow rate

performance of each fan.Choose a fan that gives you the volumetric flow rate youneed for your system pressure drop.


Choose a fan that has its peak efficiency at or near youroperating point.Sometimes will provide data in a table rather than in agraph.


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Centrifugal Fan Performance 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

=

31 1

2 2

kW N

kW N

=

Varying the RPM by 10%

decreases or increases air delivery by 10%.


decreases or increases the static pressure by 19%.


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 Laws

Law 1 relates to effect of changing size, speed, or density onvolume flow, pressure, and power levelLaw 2 relates to effect of changing size, pressure, or density onvolume flow rate, speed, and powerLaw 3 shows effect of changing size, volume flow, or density onspeed, pressure, and power


e aws on y app y to aero ynam ca y s m ar ans at t e samepoint of rating on the performance curve.


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Fan Applications in Power Plant



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Scheme of Air and Gas Path

Chapter 1: Basic ConceptsESOE 505221 Fluid Mechanics 2913 June 2011 PMI Revision 00 29


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Draught System Pressure Variation

Pressure drop calculation in air & gas path and itscomparison with design value.Assessment of ID and FD fan power as a function of

furnace pressure.Assessment of effective kinetic rate coefficient as afunction of furnace pressure.


FDFan

Duct APH Duct F urnace Duct APHBackpass

ESP IDFan

Chimney D u c

t D u c

t


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Pressure Variation

Furnace Pressure At Various Points in Boiler

50

100

150

200

250

1 FD Fan Inlet

2FD Fan Outlet

3 Airheater Inlet

4 Airheater Outlet

5Windbox Pressure

6 Furnace


-300

-250

-200

-150

-100

-50

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Points in Boiler

F u r n a c e

P r e

s s u r

7Superheater Platen Inlet

8Reheater Inlet

9 LTSH Inlet

10 Economiser Inlet

11 Airheater Inlet

12 E.P. Inlet

13 I.D. Fan Inlet

14I.D. Fan Outlet


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Induced Draft Fans

Move the combustion flue gas through the boiler, air heater, andprecipitator or the baghouse, scrubber, and chimney to theatmospheretypically consume approximately 2% of the gross electrical output.ID fans have the largest design margins of any major equipment in afossil-fueled power plant. typically 15% on flow, 30% on head, and


.Large margins are intended to allow for the following

Uncertainty in determining system requirementsAllowance for wearOperating flexibility

Allowance for pluggage and leakageAir infiltration


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Gas to be handled by ID fan

Theoretical air for combustion

Excess air required at burnerInfiltration- -



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Forced Draft & PA Fans

FD fans provide combustion air for boilersIn pulverized coal-fired boilers, approximately one-third

of the combustion air is PA that is used to transport thepulverized coal to the burners.The PA fan application is similar to the FD fan;


The FD fans for a coal-fired plant consumeapproximately 0.7% of the gross electrical output.The design margins on FD fans are typically smaller thanthe margins on ID fans but still larger than on other major

equipment. Margins of 15% on flow and 30% on head atthe maximum expected ambient temperature arecommon.


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Two-Stage Axial Fan Impeller



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Balanced Draft

The draft to be provided by the fan is determinedby losses through the following boiler

components:FurnaceBoiler and Super-heater


EconomizerAir heaterPrecipitator or Bag-house

DuctworkFlue gas desulfurization system (scrubber)Stack


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Other Fans in The System

Ignitor Air Fan : Provide combustion air to theignitors. Take suction air from the atmosphere andsupplies air to the ignitor wind box.Scanner Air Fan : Supplies cooling air to flamescanners. Normally there are two fans taking suctionfrom FD Fan discharge duct.

Chapter 1: Basic ConceptsESOE 505221 Fluid Mechanics 3713 June 2011 PMI Revision 00 37

Mill Seal air fan : Seal air fans provide air for thesealing of Mill bearing. Suction is from cold Primaryair and pressure is boosted up to maintain thedifferential pressure


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Fan ControlsInlet Vanes

Inlet vanes introduce a swirl to the flow entering a fan.The major disadvantage of inlet vanes is poor efficiency at lower loads.

Inlet vanes are subject to erosion if ash concentrations are high



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Inlet Dampers

Inlet dampers control air flow by introducing aswirl in the flow and pressure drop.Inlet dampers have a low initial cost, are simple,and are not as prone to erosion as inlet vanes.


outside the duct.The biggest disadvantage of inlet dampers istheir low efficiency at low loads.


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Two-Speed Motors

The fan selection with two-speed motors isthe same as with inlet vane control or inletdamper control. The fans for two-speedmotors are often sized so that the fan can


opera e on ow spee a u oa an a normal operating temperature. The highspeed provides the design margin.


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Fluid Drive

Fluid drive is a method of varying the fan speed for flow control.The fan selection is essentially the same as the inlet damperalternative, except that a fluid drive is located between the motorand the fan to control the fan speed.Inlet dampers are typically used in addition to the fluid drive to

increase the speed of response to avoid furnace pressure


The use of the dampers for control during normal operation is typicalbut can be eliminated in most installations.Using speed control with the dampers full open can result in asignificant power savings


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Hydraulic Coupling



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Variable-Speed Motors

Variable-speed motors are directly connected to the fan.The speed of the motor is continuously variable from approximately

10% up to the full speed.Synchronous or induction motors can be used with variable

frequency drives, and the frequency of the power to the motor iscontrolled by an electronic system.


e ncom ng ac power s converte to a usta e vo tage c powerby a thyristor.The adjustable dc power is connected to an inverter, which converts

it to an adjustable ac power output.


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Variable-Pitch Blades (For Axial Fans )

Axial fans can be controlled by varying the blade pitch or speed orby using variable inlet vanes.Either varying the blade pitch or using variable inlet vanes controlsthe flow by operating on the same principle as do variable inletvanes on a centrifugal fan.Varying the blade pitch is more efficient than using variable inlet


.

Variable-pitch blades can provide efficiency as high as that ofvariable-speed control over most of the load range for a lower initialcost.Variable-pitch blades are the most common method of control;

variable inlet vanes are used occasionally, and variable-speedcontrol is rare.


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Variable-Pitch Axial Fan Components



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Bearings

Both centrifugal and axial fans can use eitherball or roller bearings; however, ball and roller

bearings are more common on axial fans.Sliding contact bearings are more common oncentrifu al fans.


Ball and roller bearings consist of four majorcomponents:

Outer race

Inner race Rolling elements Spacer for the rolling elements


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Sleeve Bearing Components


b


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Lubrication Systems

Static lubrication in which each bearing has a fixed supply of oil intheir sumps is very common.This method is simple and very cost-effective; however, it relies onoperator vigilance to detect low oil levels or poor oil quality.Use of temperature sensors to provide remote warning to the controlroom operators, in case of a hot bearing, offers added protection for


.

A gear pump attached to the input shaft of the driver is a secondmethod used to provide lube oil to fan and motor bearings.A third method, the use of fluid drives to supply oil to the bearings,

is also used on some fans.

A fourth method involves the use of a dedicated circulating lube oilsystem.

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Fans InPower Plants