Forced Draft (FD) fans purpose is to provide a positive pressure to a system. This basic concept is used in a wide variety of industries but the term FD Fans is most often found in the boiler industry. Fans for boilers force ambient air into the boiler, typically through a preheater to increase overall boiler efficiency. Inlet or outlet dampers are used to control and maintain the system pressure. Typical fan arrangements are 3SI or 3DI which utilize an impeller center hung on the shaft with integral inlet boxes.