There are two basic types of fans; centrifugal and axial. In a centrifugal fan the air flow is at right angles to the direction of the fan shaft whereas in an axial fan, the type which is most common in the home, the air travels parallel to the fan shaft. Axial fans can move a great deal of air but centrifugal fans are better where the application specifies high air pressure. This is the primary reason why centrifugal fans are used in air pollution control systems and in a host of industrial situations.
A centrifugal fan consists of the fan wheel which has blades, the fan housing, a drive mechanism and dampers on the inlet and outlet sides. Backward curved fans have blades that are mounted on the central hub and curve backwards, this fan wheel rotates on a central shaft that passes through the fan housing from one side to the other and is supported in bearings. Air is pulled into the center of the fan wheel; the air then turns a full 90 degrees, passing over the blades and exits through the outlet.
There are three common ways to drive the fan wheel; direct drive, belt drive or variable drive. The drive selection is that which determines the rotational speed of the fan wheel and whether or not this speed can be varied. In the case of a direct drive centrifugal fan the motor is coupled directly to the fan shaft, the wheel by necessity turns at the same speed as the motor and is non-variable. When the motor is connected to the drive shaft with one or more belts, the speed of the wheel can be varied by selecting the diameters of the drive and driven pulleys to suit the design speed. It is possible to vary the speed of the fan wheel while in operation through variable speed drives.
Centrifugal fan blades can either be curved forward, radial or backward curved fan blades. Forward curved blades point it the direction of the wheel rotation and the fan is highly efficient. Backward curved fan blades are very efficient, they take minimum energy to operate and they are quite quiet. Radial blades extend straight from the hub and as they are self cleaning this configuration is often used in pollution control systems as they are not as sensitive to the buildup of particulate.
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