FD Fans

CFW supplies FD fans in various configurations together with all the necessary ancillary equipment. We can also do retrofits of existing installations, supply custom fans and undertake various kinds of after-sales service, such as repairs.

Forced draught (FD) fans are found in a range of modern industries, but the term is most frequently used for boiler fans. These fans provide positive pressure, and in balanced draught boilers, they are usually the component regulating system airflow. They are also used without ID fans in pressurized furnaces. However, while pressurized furnaces operate with lower energy costs, this arrangement can result in a build-up of ash and coal particles outside the pressure boundary of the boiler, resulting in problems of maintenance, safety and environmental protection.

The FD fan supplies secondary air from the wind box to the boiler through an air preheater (APH) for generally better efficiency. This secondary air improves the combustion process by controlling the flame temperature and shape, as well as helping to mix the fuel and air better. The result is that a larger proportion of the material in the system undergoes complete combustion.

Usually, dampers are used to manage the amount of air that the FD fan puts into the system. These may be outlet or inlet dampers. However, dampers impose a significant penalty on fan efficiency as they restrict airflow without reducing the energy expenditure of the fan. An alternative is to use a Variable Speed Drive so that the fan impeller speed can be altered. The trade-off is that VSDs have significantly higher capital costs.

The fans are often used in an arrangement with an integral inlet box and a centrally supported wheel between bearings. A variety of centrifugal fans have been used in boilers, although variable-pitch axial fans are also found (in pulverized coal boilers, for example).

It is important that the fans should be properly arranged and fitted with the necessary accessories to realize the performance for which the fan was specified. Considering the entire system is necessary, such that installation and adjustments involving the supply of air is best undertaken by air technology professionals. Refitting boilers to provide for balanced draught operation is one example.