Centrifugal fans consist of an impeller in a casing having a spirally shaped contour. The air enters the impeller in an axial direction and is discharged at the impeller outer periphery. The air flow moves along the centrifugal direction (or radial direction). Sometimes centrifugal fans are called radial flow type fans. Centrifugal fans are capable of generating relatively high pressures. They are suitable for high pressure applications as compared with axial flow fans. Generally centrifugal fans have three types of blade: forward blade, backward blade and radial blade.