Although there are a number of different types of fans they fall into one of two basic categories; axial or centrifugal; which are also referred to as radial fans. In most cases people are very aware of radial fans as this is what they have in their home, they can be desk or stand fans as well as wall mounted exhaust fans; even the fan on your car is a radial design as is the cooling fan for your computer.
Axial fans blow air in the same direction as the fan shaft whereas a centrifugal air blower draws air in and discharges it perpendicular to the fan shaft. Although both designs move air they have very different characteristics and uses. The axial fan has a series of blades similar to a propeller; these types of fans can move huge volumes of air but at relatively low pressure. Centrifugal fans on the other hand have curved blades and are used because the design allows the fan to develop high pressure which makes it ideal for certain air pollution control systems and other industrial applications.
A centrifugal air blower is made up of a fan wheel, housing and a drive mechanism and at times dampers for both the inlet and outlet. The fan wheel is the key to the function, the wheel is made from a series of blades which are mounted to a central hub; this wheel assembly rides on a fan shaft which passes through the housing. With an axial fan the air enters the fan from the back and exits straight through at the front air for a centrifugal fan enters the housing from the side and is discharged from the housing at 90 degrees to the direction of entry.
The wheel must be driven by a motor; the type of drive can be direct, with belts or variable speed. The type of drive design is that which determines the speed that the fan wheel rotates. In the case of a direct drive the wheel will mimic the speed of the motor and is fixed. When the speed of the fan wheel wants to be slower than that of the motor it is usually done with two pulleys of different diameters and drive belts. The difference in pulley sizes determines the ratio of input RPMs to output speed.
Because a centrifugal air blower can produce high air pressures it is used in such diverse applications as grain driers to greenhouse ventilation to HVAC applications.