Fans are developed for different applications, which differ in required volume flows, pressure levels and operating conditions (temperature, nature of the air, particles conveyed). In order to meet all these requirements, there are three main types of fans, which can be distinguished by the direction of flow.
- Axial design: axial suction and discharge direction
- Radial design: Axial suction direction and radial discharge direction
- Diagonal design: Axial suction direction and discharge direction at an angle to the axis of rotation of the fan impeller
The characterization of centrifugal fan impellers and the associated performance range can be subdivided via the geometric properties of the impeller. In the case of blade curvature, a primary distinction is made between forward-curved and backward-curved blades. The backward curved blades are curved against the direction of rotation of the fan wheel and the forward curved blades are curved with the direction of rotation, (see Figure 3.4.a). In addition to curved blades, there are also non-curved blades. They usually have lower efficiencies compared to backward curved blades, but have their raison d'être. Radial-end blades without curvature have their application-related advantages in the transport of solids, due to the low tendency to medium adhesion. In addition to these main forms, there are also some mixed forms, which are used less frequently.
Another rough classification of the performance range can be made using the ratio of the intake diameter to the discharge diameter. Here the following applies: the smaller the diameter ratio, the less mass flow is conveyed by the fan impeller. However, a relatively high pressure increase is achieved by the small diameter ratio (see figure 3.4.b).
Finally, a distinction is made according to the design of the fan (see Figure 3.4.c). Fan impellers that have a high conversion rate from kinetic to static energy can be used with a high degree of efficiency in a housing as well as free-blowing. As a rule, these are fan impellers with backward curved blades.
The forward-curved impellers hardly convert any kinetic energy into static energy, so a scroll housing is always recommended here to support the energy conversion. Cross-flow impellers are a special form of forward-curved impellers.
