Types of Centrifugal Fans Used in Power Plant Boilers

The purpose of boiler in thermal power plants is to covert water into steam with the help of heat energy generated by the combustion of fuels like coal, wood, oil or natural gas. Boiler systems use several types of fans to maintain air flow, recirculate air and remove exhaust gases. Based on the boiler size and air flow requirement different fans are used with varied capabilities.

Predominantly, draft fans play an important role in thermal power plants because they regulate the air pressure inside boiler system. Draft fans are broadly divided into two types – Forced Draft (FD Fan) and Induced Draft (ID Fan). Apart from the draft fans, there are also other basic types of process fans used in power plants. They are primary air fans and flue gas recirculation fans.

Draft fans
The primary difference between a forced draft and induced draft is, FD fan forces outside air into the heating system whereas ID fan draws flue gases from the system out into the atmosphere. Both FD fan & ID fan operate in such a way that it balances the air system in the boiler to make the combustion process efficient.

Forced Draft fan
FD fan or forced draft fan, draws air from the atmosphere and forces it into the furnace through a preheater. These fans are located at the inlet of the boiler to push high pressure fresh air into combustion chamber, where it mixes with the fuel to produce positive pressure. The most common centrifugal fans used as FD fans are backward curved and airfoil centrifugal fans.

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 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.

When compared to ID fans, FD fans are easier to maintain and have cleaner operating conditions.

Induced Draft fan
Induced draft fan is normally located at the outlet between dust collector and chimney.

The fan takes hot flue gases from the boiler through dust collector and delivers it to the chimney into the open atmosphere. ID fan creates 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 when used with Electrostatic precipitators.

The most common type of fans that can be used as ID fans are radial fans and backward inclined blade centrifugal fans.

Other process fans
PA fans
The main function of the Primary air fan or PA fan is to carry fuel to the furnace for combustion.

In power plants PA fans supply air for conveying of the pulverized coal from coal mills to the furnace area. These fans produce a pressure upstream the pulverizer, to push the coal and air mixture through pulverizer into the furnace.

The most common centrifugal fan used as PA fan is backward curved centrifugal fan.

Flue gas recirculation fans
Gas recirculation fans (FGR) are typically used in power plant boilers to reduce the flue gas pollution content in the gases exhausted from thermal combustion processes.

Flue gas re-circulation is a highly effective technique that significantly reduces the Nitrogen oxide (NOx) emissions from burners. Controlling NOx is critical as it the largest contributor to other types of harmful pollutants. NOx emissions are mostly derived from the nitrogen in the combustion air (thermal NOx). These pollutants are produced during combustion process when nitrogen and oxygen react at elevated temperatures. NOx emission levels can be greatly reduced in industrial boilers by recirculating the flue gases from the boiler exhaust duct into the main combustion chamber.

Most common types of gas re-circulation fans are backward curved centrifugal fans.

In power plants, industrial fans play an important role in supplying air necessary for fuel combustion and in the maintenance of furnace temperature. Centrifugal fans of various types and capacities are used as process fans in power plants depending on the system requirement and industrial process.