Page 114 - Root Cause Failure Analysis

Fans, Blowers, and Fluidizers 105
move the increased volume through the system. The mechanical and static efficiencies
are unchanged.

The increased brake horsepower (BHP) required to drive the fan is a very important
point to note. If the Curve #1 fan had been driven by a 2-hp motor. the Curve #2 fan
needs a 3.5-hp motor to meet its volumetric requirement.

Centrifugal-fan selection is based on rating values such as air flow, rpm, air density,
and cost. Table 8-1 is a typical rating table for a centrifugal fan. Table 8-2 provides
air-density ratios.

Installation
Proper fan installation is critical to reliable operation. Suitable foundations, adequate
bearing-support structures, properly sized ductwork, and flow-control devices are the
primary considerations.

Foundations
As with any other rotating machine, fans require a rigid, stable foundation. With the
exception of in-line fans, they must have a concrete footing or pad that is properly
sized to provide a stable footprint and prevent flexing of the rotor-support system.

Bearing-Support Structures
In most cases, with the exception of in-line configurations, fans are supplied with a
vendor-fabricated base. Bases normally consist of fabricated metal stands that support
the motor and fan housing. The problem with most of the fabricated bases is that they
lack the rigidity and stiffness to prevent flexing or distortion of the fan’s rotating ele-
ment. The typical support structure is composed of a relatively light-gauge material
(3/,6 in.) and lacks the cross-bracing or structural stiffeners needed to prevent distor-
tion of the rotor assembly. Because of this limitation, many plants fill the support
structure with concrete or another solid material.

However, this approach does little to correct the problem. When the concrete solidi-
fies, it pulls away from the sides of the support structure. Without direct bonding and
full contact with the walls of the support structure, stiffness is not significantly
improved.

The best solution to this problem is to add cross-braces and structural stiffeners. If
they are properly sized and affixed to the support structure, the stiffness can be
improved and rotor distortion reduced.

Ductwork
Ductwork should be sized to provide minimal friction loss throughout the system.
Bends, junctions with other ductwork, and any change of direction should provide a