Forced Draught Fans

Forced Draught Fans diameter, slow running, double inlet fans, with airfoil profile blades. Primary air 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 pulverized 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 pressurizing the air in the preheater encourages leakage of the a