In centrifugal fans, air or other process gaseous fluid, is drawn in by a fan impeller through an inlet, parallel to a rotational axis, and is delivered by the fan impeller in a direction perpendicular to the rotational axis. This air stream delivered by the fan impeller is diverted by a baffle or further processed by a diffuser before entering a volute which collects and directs the air stream towards an outlet. Centrifugal fans can be of a type with single or double intake, i.e. they can draw in air from one side only of the fan impeller or from two opposite sides thereof.
The fan impeller is formed of a bladed rotating wheel consisting of a disk to which blades are secured at their base. In a case of a fan with single intake the blades are arranged only on one side of the disk, whereas in the case of a fan with double intake the blades are arranged on both sides of the disk.
The blades can be arranged in a radial direction or they can be forwardly or backwardly inclined with respect to a rotational direction of the fan impeller. The blades often have a streamlined profile, but in the prior art flat blades, i.e. non-streamlined blades are also known.
Impellers of centrifugal fans can be of a closed or open type according to whether or not a covering element, a so called impeller covering, consisting of a circular frame fastened to a tip of the blades is provided. Fan impellers provided with inclined blades are generally of the closed type in order to increase a bending strength of the blades.
Several approaches have been adopted in the design of centrifugal fans in order to improve their operational characteristics and efficiency. These approaches consist of for, example, providing the inlet and the outlet with well jointed convergent and divergent ducts, respectively and, downstream of the fan impeller, a diffuser formed of fixed blades suitably shaped and directed for guiding an air stream exiting the fan impeller so as to reduce flow losses. For similar purposes, fan impellers are provided with a great number of blades and these sometimes have a curved guide portion at their leading edge.