Induced Draft fans
The Induced Draught fan draws flue gases out of the boiler, through the air preheater, to maintain furnace conditions at just below atmospheric pressure. Control of furnace pressure is a critical safety consideration.
The Forced Draught fan moves cold air through the air preheater into the boiler. This air provides the oxygen used in the boiler combustion process.
The FD fans have to supply large quantities of low pressure air to the boiler. Process characteristics will tend require large diameter, slow running, double inlet fans, with aerofoil profile blades.
On coal and biomass fired boilers the Primary air fan moves air through the air preheater. The hot air exiting the preheater is mixed with the pulverised fuel and the air flow conveys this fuel/air mix to boiler combustion chamber. Oil fired boilers do not require primary air fans.
Primary air fans often have impellers with backward curved plate or aerofoil bladed designs and as the volume flow requirement is comparatively low they are usually of a single inlet design. To be able to carry the fuel into the boiler the fan pressure requirement is relatively high.
Cold primary air fans operate with relatively clean ambient air.
The use of cold air means that for a given mass flow of air into the boiler, the fan does not need to move as high a volumetric flow as a hot PA fan. This leads to a smaller fan size
The process of pressurising the air in the preheater encourages leakage of the air into the gas stream. This portion of the air is lost into the flue gas exiting the plant, which reduces the boiler efficiency.
Hot Primary Air fans operate with hot air coming out of the air heater.
Using a hot primary air fan has the advantage of reducing the leakage of air into the gas stream within the preheater. As the air is hot, it has a lower density and the fan will have to move a large volume flow to maintain the mass flow required by the boiler combustion process. This means the fan needs to be slightly larger than a cold PA fan
Hot primary air fans also have to be designed to resist abrasion as the hot air will now contain erosive dust particles that have been carried over into the air by the air heater.
The secondary air fan takes air from the same sources as the Primary Air fans, however in this case the air is supplied into the boiler combustion chamber above the grate. This air creates a turbulent condition over the fuel, enhancing combustions and providing an additional oxygen source to help complete the combustion process. (See also Primary Air)
Gas Recirculation fans are not always fitted to a boiler. This fan re-circulates combusted flue gas from the back end of the boiler or economizer through a dust collector and delivers the gas back to the boiler.
The re-circulated gas is introduced either into the immediate vicinity of the initial burning zone of the boiler to control steam temperature or the gas is introduced near to the boiler outlet to assist in the control the furnace gas temperature, in a process known as gas tempering.
Boiler Over Fire Air is used in a 2 stage combustion process to reduce the harmful Nitrogen Oxide (NOx) content in the flue gas exiting the process. A small portion of secondary air is boosted in pressure by the BOFA fans and is introduced into a second combustion zone in the boiler. This assists in the complete combustion of the gases and encourages the formation of Nitrogen (N2) rather than NOx.
BOFA fans require design features to minimise erosion and to enable them to operate at high air temperatures.