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 a wheel center hung on the shaft with integral inlet boxes.