Industrial fans like induced draft(ID), forced draft (FD), primary air(PA) and secondary air(SA) fans play a key role in providing critical support to the various process activities and maintaining the efficiency of the factory operations.
Draught fans are a type of industrial fans and are typically heavy duty centrifugal fans. Draught fans are an essential and irreplaceable component of the industry. They are divided in two types – Forced Draught or Forced Draft (FD Fan) and Induced Draught or Induced Draft (ID Fan). The major difference between the two is that the FD fan pushes the outside air into the heating system whereas the ID fan pulls out flue gases from the system and releases them into the atmosphere. Both forced draft and induced draft fan operate in such a way that it balances the air system in the factory.
A typical arrangement of FD fan uses inlet and outlet dampers to maintain and control the air pressure needed inside the system. These fans have a wheel at the center that is hung on the shaft along with filter, silencer, rain hood, and inlet boxes. The most frequently used centrifugal fans as FD fans are airfoil and backward curved blade centrifugal fans. The FD fan builds positive pressure as it helps push the air in the systems. They are widely used in industries of oven, furnaces, boilers and also in dust collection equipment. FD fans are easier to maintain and have cleaner operating conditions as compared to ID fans.
An induced draft fan is usually located at the outlet between the dust collector and the chimney. All gaseous combustion products or flue gases are exhausted from the machines such as boilers or furnaces and released into the outer atmosphere. The ID fans withdraw the air from the systems, therefore they produce the negative pressures in the systems. They are more susceptible to corrosion and erosion problems as ID fans handle hot flue gases. The most frequently used centrifugal fans as ID fans are radial and backward inclined blade centrifugal fans.
The Primary Air (PA) fan and Secondary Air (SA) fan are also the types of industrial fans. Both primary and secondary air have a crucial role in fuel combustion and must be provided in different ratios according to the fuel quantity. Primary air is the basic amount of air required for complete combustion of fuel and it depends on the required quantity and composition of fuel. The fuel is fed either manually or through the screw feeder into the furnace and primary air is provided from the bottom through the Air box. A primary air fan helps in segregating the fuel particles so that each fuel particle could come in direct contact with the air which allows efficient combustion of fuel.
Secondary air is supplied to compensate for the primary air that went unreacted. Generally, it is said to be extra air that must be supplied for complete combustion of fuel. A secondary air fan supplies secondary air through the nozzles installed on the walls of the furnace. Steam boilers with external furnaces are mostly provided with both PA and SA fans. A forced draft fan which is designed for a particular boiler must be able to supply a sufficient quantity of air which is the sum of both primary and secondary air.
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