Process Fans Used in Power Plants
In power plants the heat energy is generated by boilers. Boilers use industrial fans for the following reasons:
• to supply air required for combustion
• remove products of combustion
• deliver fuel to the burners
• circulate gases for the better heat transfer
Depending on these four functions, fans in power plants are divided into three main types. They are draft fans (induced draft fans, forced draft fans), primary air fans and gas recirculation fans.
Draft fans in power plants are used to maintain the flow of gases in the boiler. In a balanced draft system, boiler uses both induced draft placed at outlet and a forced draft placed at the inlet.
• Forced draft (FD) fans
> Forced draft fans in boilers are used to supply air which is necessary for fuel combustion. FD fans push the air through combustion air supply system and into the furnace.
> FD fans are more efficient fans in power plants as they operate in a cleaner operating conditions.
> Most common type of fans that are used as FD fans are radial airfoil centrifugal fans or variable pitch axial fans.
• Induced draft (ID) fans
> Induced draft fans are present at the outlet of the boiler system. Induced draft fans create a negative pressure or suction which removes all exhaust gases and flue gases after combustion from the furnace.
> As they handle hot flue gases, they are vulnerable to erosion and corrosion even when used with particulate removal equipment (Electrostatic precipitator ESP)
> To withstand the effects of corrosion and erosion, fans used as ID fans are usually modified radial, forward curved, backward inclined blade centrifugal fans.
Primary air (PA) fans:
• Primary air fans are usually high pressure fans, which are used to supply air for transportation of coal directly from the pulverizer to the furnace.
• Primary fans usually have a cleaner operation as they handle relatively cleaner air. They provide a positive pressure upstream of coal pulverizer. Primary air fan upstream from the pulverizer, pushes coal and air mixture through pulverizer into the furnace.
• Fans used as PA fans are high pressure like airfoil centrifugal fans or a multistage variable axial fans.
Gas recirculation fans:
• Gas recirculation fans in boilers are used to control steam temperature, furnace heat absorption and slagging of heating surfaces.
• Gases from economizer outlet and pre-heater inlet are absorbed by gas recirculation fans and are discharged to either bottom of the furnace for steam temperature control or to any other location in the furnace.
• The operating conditions of gas recirculation fans are very harsh due to heavy loads of dust and extreme temperature excursions.
• Gas recirculation use straight or modified radials or forward curved backwardly inclined centrifugal fans.
Process fans in power plants are important as they provide the necessary air required for combustion and removal of exhaust gases. The main advantage of using industrial fans in power plants is for gas-recirculation and to maintain the required temperature in the furnace.
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