There are two general classifications of fans: the propeller or axial flow fan and the centrifugal or radial flow fan. In the broadest sense, what sets them apart is how the air passes through the impeller. The propeller or axial flow fan propels the air in an axial direction (Figure 1a) with a swirling tangential motion created by the rotating impeller blades.
In a centrifugal fan the air enters the impeller axially and is accelerated by the blades and discharged radially (Figure 1b). The one exception to this is the tangential/transverse fan where the air enters and discharges radially through the impeller.
The axial flow fan increases the air velocity through rotational or tangential force which produces velocity pressure (VP), kinetic energy, with a very small increase in static pressure (SP), potential energy.
The centrifugal fan induces airflow by the centrifugal force generated in a rotating column of air producing potential energy (SP) and also by the rotational (tangential) velocity imparted to the air as it leaves the tip of the blades producing kinetic energy (VP).
Figure 2 illustrates the components that make up a typical centrifugal fan and covers the common terminology of these components.
Centrifugal fans may be classified into three basic types according to blade configuration:
1. Forward curve
2. Backward inclined
3. Radial or straight blade
Each type has its own application range and limits. Modifications of these basic types include radial tip, mixed flow, and tangential flow.
The tip speed required to produce the required air particle velocity varies substantially with the type of blade used. Figures 3a, 3b, and 3c (Figure 3) show vector diagrams of forces in forward curve, backward curve, and radial blade impellers, respectively. Vector V1 represents the rotational or tangential velocity, and V2 represents the radial velocity of the airflow between the blades with respect to the various blade shapes.
Vector R represents the resultant velocity for each of these blade shapes. Note that R for the forward curve impeller is the largest with the backward inclined impeller the smallest, while the radial blade fan lies somewhere in between. This relationship is best illustrated in Figure 4, which shows a typical tip speed/static pressure relationship for various types of centrifugal fans.
Forward Curve Fans
These fans are sometimes known as “volume,” “squirrel cage,” or “sirocco” blowers. The impeller blades are small and numerous with a pronounced curvature and short chord length. The concave blade curvature faces the direction of rotation. These fans operate at relatively low speeds and pressures (reference Figure 4) which permits light construction of the impeller, shaft, bearings, and housing.
Backward Inclined Fans
These are sometimes called “load limiting” or “non-overloading” fans. The impeller blades are larger and heavier than forward curve blades, usually number from eight to
twelve, and are inclined away from the direction of rotation. They are standardly offered in three blade shapes:
1. Flat single thickness (Figure 6a)
2. Curved single thickness (Figure 6b)
3. Curved airfoil (Figure 6c)
Radial Blade Fans
“Steel plate” or “paddle wheel” are two of the common names for radial blade fans. The impeller blades are generally narrower, deeper and heavier than forward curve and backward inclined blades. A radial blade impeller usually comprises six to twelve equally spaced flat blades extending radially from the center of the hub. These impellers are generally of simple design that lends itself to rugged construction and offers a minimum of ledges, etc., for the accumulation of dust or sticky materials.
There are more variations of the radial blade fans than the forward curve and backward inclined types. Three of the more common impellers are illustrated in Figure 8.