Fan A fan can be considered a mechanical device that moves a volume of fluid such as air, gas, or vapor through a pressure driven flow. Large capacity fan units typically consist of a bladed, rotating impeller enclosed in a stationary casing. The rotor system causes the motion of the air/gas and the casing directs the output flow. The rotor can be powered through a driver such as a motor, a turbine driver, or a fluid-driver.
Types of Fans A fan is classified by the direction of its flow through the bladed passages of the impeller. A centrifugal fan moves the air perpendicular to the rotational axis of the impeller . an axial fan moves the air parallel to the rotational axis of the impeller.
Function of Fans in Boiler Supply air required for combustion Remove products of combustion Deliver fuel to the burners Circulate the gases for better heat transfer
Types of Power Plant FansThere are four main types of fans used in fossil power plants. Forced draft fans, Induced draft fans, Primary air fans, Gas-recirculation fans.
Draft Fans Draft fans are generally responsible for maintaining the flow of gases through the boiler. A balanced draft system uses both a forced draft fan at the inlet of the system and an induced draft fan at the outlet of the boiler system.
BOILER Boiler drum Reheater Final Super Platen Super heater heater LTSH Economizer Coal bunker Wind Box Secondary PA duct air duct Furnace Flue gas APH duct Coal feeder F D Fan P A Fan Coal Pulverizer
Forced Draft Fans Forced draft fans (FD) supply the air necessary for fuel combustion by pushing the air through the combustion air supply system and into the furnace. These fans are typically the most efficient fans in the power plant because they have the cleanest operating environment . Typically, these fans are centrifugal fans utilizing radial airfoil blading or variable pitch axial fans.
Induced Draft Fans Induced draft fans (ID) are placed at the outlet of the boiler system and exhaust all gaseous combustion products, or flue gas, from the boiler by creating a negative pressure or suction within the furnace. These fans handle hot flue gas, they are generally more susceptible to erosion and corrosion even with particulate removal equipment (ESP). If greater wear resistance is necessary, a modified radial, forward-curved, or backward-inclined blading can be used at the expense of efficiency loss.
Primary air fans Primary air fans (PA) are high pressure fans used to supply the air for the transportation of coal directly from the pulverizer to the furnace . These fans provide a positive pressure upstream of the coal pulverizer and handle relatively clean air . A PA fan upstream from the pulverizer, or a “cold” PA fan, pushes the coal/air mixture through the pulverizer and is most commonly used. Cold PA fans typically are airfoil centrifugal fans or multi-stage axial fans similar to FD fans.
Gas Recirculation Fans Gas-recirculation fans are used to control steam temperature, furnace heat absorption, and slagging of heating surfaces. These fans extract gas from the economizer outlet and the pre-heater inlet and then discharge the gas either to the bottom of the furnace for steam temperature control or to various locations in the furnace. The duty cycle of a gas recirculation fan is very stringent due to heavy dust loads and extreme temperature excursions. Straight or modified radials or forward curved, backwardly inclined centrifugal wheels are appropriate for gas-recirculation fans.
Damage Mechanisms Most common damage mechanisms associated with power plant fans failure are : Erosion, Corrosion, Vibration which are responsible for serious and costly maintenance.
Erosion One of the most common damage mechanisms associated with power plant fan failure is erosion, which is responsible for serious and costly maintenance. The rate of erosion depends on suspended particles / fly ash, in the flue gas.
EFFECT OF BLADE TYPE ON EROSIONRESISTANCE AND EFFICIENCYBLADE TYPE TYPICAL MAX STATIC TOLERANCE TO EFFICIENCY ( %) EROSIVE ENVIROMEMTRADIAL 70 HIGHRADIAL TIP 80 MEDIUM TO HIGHBACKWARDLY INCLINEDSOLID 85 MEDIUMAIRFOIL 90 LOW
Resistant to Erosion The rate of erosion experienced by fans used in harsh applications is often controlled by the use of repairable liners, replaceable liners, or renewable coatings. Reducing fan speed and selecting a fan blade type that is more resistant to erosion will slow down the abrasive wall thinning experienced by fan unit surface.
Abrasion-Resistant Impeller Fans that operate in flue gas, such as induced draft fans (IDF) for coal-fired boilers, are required to be resistant to abrasion by ash in the flue gas.
CORROSION The following list is the most common types of corrosion problems found in boiler power plants. • Erosion corrosion • Crevice corrosion • Galvanic corrosion • Pitting • General corrosion (wastage) • Differential Oxygenation • Biological corrosion • Intergranular corrosion
Vibration Possible reasons as to why vibration occurs in fan units are listed below. • Improper balancing • Loose components • Worn/damaged/cracking of fan parts • Improper Lubrication • Improper clearance of moving parts. • Excitation of a resonant frequency • Corrosion, erosion, high/low cycle fatigue effects • Misalignment or bent shaft • Improperly designed or deteriorated foundations • Build-up of material on the rotor
Output Air Flow Control A centrifugal fan utilizing inlet vanes controls the airflow . Most axial fans are operated by variable pitch axial blades, . A fan that is run with a variable speed motor can adjust the speed to control the output flow properties .
CONDITION MONITORING Condition monitoring is the use of advanced technologies to determine equipment condition and, potentially, predict failure. It includes technologies such as the following: • Vibration measurement and analysis • Oil analysis • Nondestructive examination (NDE) • Infrared thermography • Motor current analysis
STARTUP PROCEDURES OF DRAFT FANS The startup procedures, in addition to the controls and interlocks, should follow the requirements of NFPA 85.
FANS -500 MW BOILERFAN F D FAN PA FAN ID FANTYPE AXIAL AXIAL RADIALNO per boiler 2 2 2FLOW ,M3/S 251.6 184.0 574.6PRESS,mmwc 390 1200 467TEMP,DEG C 45 50 150DRIVE MOTOR MOTOR MOTORSPEED,RPM 980 1480 580MOTOR ,KW 1225 2725 3950CONTROL VARIABLE PITCH VARIABLE PITCH INLET DAMPER+ CONTROL CONTROL VFD
FANS -200 MW BOILERFAN F D FAN PA FAN ID FANTYPE AXIAL Reaction RADIAL AXIAL impulseNO per boiler 2 2 2FLOW ,M3/S 105 75 225PRESS, mmwcl 510 1187 356TEMP,DEG C 50 50 136DRIVE MOTOR MOTOR MOTORSPEED,RPM 1480 1480 740MOTOR ,KW 750 1250 1100
Distribution of Aux Power Consumption in a 210 mw Power Plant FANS
Difference between fan andCompressor Fans are similar to compressors; the difference is that fans create a flow of gas whereas compressors increase the pressure of the gas. ASME PTC-11, “Performance Test Code for Fans,” defines a fan as providing a compression ratio of 1.1 or a density change of 7%. ISO 5801 defines the upper limit of fans as a pressure increase of 120 inches Wg (30 kPa).
STANDARDS 1. NFPA 85, Boiler and Combustion System Hazard Code. 2. AMCA 803, Site Performance Test Standard. 3. AMCA 202, Fan Application Manual. 4. AMCA 203, Field Performance Measurements. ASME PTC 11