2 Types of Centrifugal Fans Used in Power Plant Boilers
Boiler systems in Thermal Power plants use different types of Centrifugal Fans to supply air required for combustion, deliver fuel to the burners, circulate the gases for better heat transfer and remove exhaust gases. Based on the boiler size and airflow requirement, different types of fans are used with varied capabilities.
Two types of fans used in a boiler power plant are –
1) Draft Fans
2) Other Process Fans
1. DRAFT FANS
Draft fans play an important role in thermal power plants because they are responsible for maintaining the flow of gases through the boiler system.
Two types of Draft fans are used in the power plants, which are – Forced Draft fans and Induced Draft Fans.
Forced Draft Fans:
Forced Draft fans used in a power plant are centrifugal fans, utilizing radial airflow blading or variable pitch axial fans. Forced Draft Fans (FD) supply the air necessary for fuel combustion by pushing air through the combustion air supply system and into furnace. These fans are located at the inlet of boiler to push high pressure fresh air into combustion chamber, where it mixes with the fuel to produce positive pressure.
The key difference between a forced draft and an induced draft fan is, FD fan forces outside air into the heating system whereas an ID fan draws flue gases from the system out into the atmosphere. Both FD fan and ID fan operate in such a manner that it balances the air system in the boiler to make the combustion process efficient.
In Power plants, where coal is used as fuel for combustion, FD fans are used as secondary air fans to regulate proper combustion and maximize the efficiency of the fuel. A typical FD fan arrangement uses inlet and outlet dampers to control and maintain the required air pressure inside the system. These fans have a wheel at the center that is hung on the shaft along with inlet boxes, silencer, filter, and rain hood.
As compared to ID fans, FD fans are more efficient, easier to maintain, and have cleaner operating conditions. The most common Centrifugal Fans used as FD fans are Backward Curved Centrifugal fan and Airfoil centrifugal fans.
Induced Draft Fans:
Induced Draft Fans (ID) are fans that exhaust all gaseous combustion products or flue gases from the boiler, through dust collector and delivers it to the chimney into the open atmosphere. Hence the ID fans are usually located at the outlet of the boiler system (between a dust collector and chimney). ID fan creates a negative pressure or suction to discharge the gases out after combustion from the furnace.
Since ID fans can handle hot flue gases, they have more corrosion and erosion problems even more used with electrostatic precipitators. The most common type of ID fans used in a boiler power plant is Radial Fans and Backward Inclined Blade centrifugal fans.
2. OTHER PROCESS FANS:
Other Process fans used in a boiler power plant are – Primary Air fans and Flue Gas Recirculation Fans
Primary Air Fans:
Primary air fans or PA fans are high-pressure fans, used in the boiler power plants to supply the air for transportation of coal directly from the pulverizer to the furnace. PA fans give positive pressure upstream of the coal pulverizer and handle relatively clean air.
In power plants, PA fans supply air for conveying of the pulverized coal from coal mills to the furnace area. These fans produce pressure upstream of the pulverize, to push the coal and air mixture through the pulverize into the furnace.
Backward Curved Centrifugal Fan is the most common Centrifugal Fan used as a PA fan in a boiler power plant.
Flue gas recirculation fans:
Flue gas recirculation fans are used to reduce the flue Gas Pollution content in the gases exhausted from thermal combustion processes. Flue gas recirculation fans are used in a power plant boiler to control steam temperature, furnace heat absorption, and sagging of the heating surface.
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 controlling steam temperature or to different locations in the furnace.
The most common types of gas re-circulation fans used in a boiler power plant are backward curved centrifugal fans.
As the Backward Curved bladed centrifugal fans are highly useful in the majority of centrifugal fans used in boiler power plants, let us take a brief idea of ‘Backward Curved Blade Centrifugal Fans’.
Backward Curved Blade centrifugal fans:
A Backward curved centrifugal fan is characterized by its cylindrical shape, several large curved blades, and a conical inlet nozzle. It rotates in a clockwise direction. As the fan rotates, a pressure difference is created on the impeller blades. On the leading, convex side of the impeller blade, a positive pressure is created as the rotational motion of the impeller imparts a force in the air. The Impeller Blade pushes the air outwards, exiting in a radial direction.
On the concave side of the impeller blade, a negative pressure is created as the fan rotates, drawing air into the space between the blades. This air is then picked up by the following blade and forced outwards radically in a continuous process. The suction side of the impeller blade draws air from the center of the fan, which results in a directional change of the airflow between the inlet and the exhaust of 90o.
According to Science Direct: Design of impeller blades for intensification on fluid mixing process in a stirred tank
A backward curved centrifugal fan works best when medium pressures and medium volume flows are required. A backward curved fan can be used in a basic mounting design for use in a plenum chamber or incorporated into a scroll housing to direct the exhaust flow.
Non-overloading feature, an ability to install the fan with or without a scroll, and an impeller design that offers the highest static efficiency of all types of fans indicate that a backward curved centrifugal fan is versatile enough to be used across a wide range of installations.
Symbiosis Blower is at the forefront of Design Supply Commissioning of Boiler & Power Plant Centrifugal Fans to Exacting standards and client requirements & expectations. Symbiosis has more than 1500 Installations worldwide and is spreading its footprint to prestigious Infrastructure and development projects in India and abroad.
