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.
Main parts of a centrifugal fan are :
*Inlet and outlet ducts
The fan wheel consists of a hub with a number of
fan blades attached. The fan blades on the hub can be arranged in three
different ways: forward-curved, backward-curved or radial.
Forward-curved blades, curve in the direction of
the fan wheel’s rotation. These are especially sensitive to particulates and
commonly are only specified for clean-air applications such as air
conditioning.Forward-curved blades provide a low noise level and relatively
small air flow with a high increase in static pressure.
Backward-curved blades, curve against the
direction of the fan wheel’s rotation. Smaller blowers may have
backward-inclined blades, which are straight, not curved. Larger
backward-inclined/-curved blowers have blades whose backward curvatures mimic
that of an airfoil cross section, but both designs provide good operating
efficiency with relatively economical construction techniques. These types of
blowers are designed to handle gas streams with low to moderate particulate
loadings.They can be easily fitted with wear protection but certain blade
curvatures can be prone to solids build-up. Backward curved wheels are often
heavier than corresponding forward-curved equivalents, as they run at higher
speeds and require stronger construction. Backward curved fans can have a high
range of specific speeds but are most often used for medium specific speed
applications—high pressure, medium flow applications.Backward-curved fans are
much more energy efficient than radial blade fans and so, for high power
applications may be a suitable alternative to the lower cost radial bladed fan.
Radial blowers,have wheels whose blades extend
straight out from the centre of the hub. Radial bladed wheels are often used on
particulate-laden gas streams because they are the least sensitive to solid build-up
on the blades, but they are often characterized by greater noise output. High
speeds, low volumes, and high pressures are common with radial blowers[citation
needed], and are often used in vacuum cleaners, pneumatic material conveying
systems, and similar processes.
Flow at the intake and its turning from axial to radial direction causes losses at the intake. Friction and flow separation cause impeller blade losses since there is change in incidence angle.These impeller blade losses are also included in the category.
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