Primary Air (PA) fans deliver the required fuel from the pulverizers to the furnace to meet generation demand. Force Draft (FD) and Induced Draft (ID) fans are used to control air and combustion gas flow through the boiler. In the case when both FD and ID fans are utilized, the system is referred to as balanced draft. The most common variant, balanced draft systems operate slightly below atmospheric pressure to ensure safe operation and the removal of flue gas from the furnace.
The key to success of efficient combustion is accurately matching the proper air to fuel ratio entering the furnace. Proper stoichiometric conditions allow plants to maximize fuel usage, and at the same time minimize fugitive emissions at the source of combustion. In systems with large centrifugal fans, flow control is obtained through automation of Inlet Dampers or variable inlet vanes with damper drives. In a balanced draft system, optimized combustion also needs to be obtained with a constant furnace pressure during all load changes. Optimal tuning of both the combustion control and furnace draft loops requires state of the art and fast acting, high resolution, repeatable damper drives.