Page 108 - Root Cause Failure Analysis

Fans, Blowers, and Fluidizers 99

rotation of the shaft, which enables the unit to be correctly installed in the duct work.
Figure 8-2 shows the inlet end of a direct-connected, tube-axial fan.

This type of fan should not be used in corrosive or explosive environments since the
motor and bearings cannot be protected. Applications where concentrations of air-
borne abrasives are present should be avoided as well.

Axial fans use three primary types of blades or vanes: backward-curved, forward-
curved, and radial. Each type has specific advantages and disadvantages.

Backward-Curved Blades The backward-curved blade provides the highest effi-
ciency and lowest sound level of all axial-type, centrifugal fan blades. Its advantages
include

Moderate to high volume.
Static pressure range up to approximately 30 in. of water (gauge).
Highest efficiency of any type of fan.
Lowest noise level of any fan for the same pressure and volumetric
requirements.
Self-limiting BHP characteristics. (Motors can be selected to prevent over-
load at any volume and the BHP curve rises to a peak and then declines as
volume increases.)

1

Figure 8-2 Inlet end of a direct-connected tube-axial fan (Higgins and Mobley 1995).