The function of PA & SA Blowers and Fans in Steam Boiler Thermal Power Plants
There are 3 basic things are needed for the combustion of any fuel to take place,
- Air (Oxygen to specific)
- and heat
In fact, some of the air which has been provided for combustion goes unreacted with the fuel.
As the quantity of fuel that is fed at once and variation in the sizes of fuel particles, some of the extra air must be provided to compensate for an air that is unreacted.
Primary Air Fan ( PA Fan)
Primary air is generally the basic amount of air required for complete combustion of fuel and it depends upon the composition and quantity of fuel required by the boiler. Different fuels will require a different amount of primary air.
Primary air is generally provided from the bottom through the Airbox and the fuel is fed either manually or through the screw feeder into the furnace.
Primary air fans also help in segregating the fuel particles so that each fuel particle could come in direct contact with the air resulting in efficient combustion of fuel particles.
Secondary Air Fans
The secondary air, on the other hand, is provided to make up for the primary air that went unreacted. It is generally said to be an extra air that must be provided for making up the combustion of fuel.
Secondary air is generally provided through the nozzles installed on the walls of the furnace.
A Forced Draft Fan designed for a particular boiler must be able to provide a sufficient quantity of air which is the sum of both Primary and Secondary air.
The air before getting into the boiler is made to pass through the Air Preheater (APH) where the inlet air to the boiler gets heated by the outgoing fume and then this preheated air helps in getting rid of the moisture content of the fuel and further increases the boiler efficiency.
Providing air at different stages generally reduces the running out of temperature and will supply more convective heat when single stage combustion will increase the combustion temperature and would provide a lot of radiative heat transfer as compared to convective heat transfer.