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Process Parameters 225

pump. This problem can be prevented by using well-maintained inlet strainers or
filters.

10.2 FANS, BLOWERS, AND FLUIDIZERS
Tables 10–4 and 10–5 list the common failure modes for fans, blowers, and fluidiz-
ers. Typical problems with these devices include output below rating, vibration and
noise, and overloaded driver bearings.

10.2.1 Centrifugal Fans
Centrifugal fans are extremely sensitive to variations in either suction or discharge
conditions. In addition to variations in ambient conditions (e.g., temperature, humid-
ity), control variables can have a direct effect on fan performance and reliability.

Most of the problems that limit fan performance and reliability are either directly or
indirectly caused by improper application, installation, operation, or maintenance;
however, the majority is caused by misapplication or poor operating practices. Table
10–4 lists failure modes of centrifugal fans and their causes. Some of the more
common failures are aerodynamic instability, plate-out, speed changes, and lateral
flexibility.

Aerodynamic Instability

Generally, the control range of centrifugal fans is about 15 percent above and 15
percent below its BEP. When fans are operated outside of this range, they tend to
become progressively unstable, which causes the fan’s rotor assembly and shaft to
deflect from their true centerline. This deflection increases the vibration energy of the
fan and accelerates the wear rate of bearings and other drive-train components.

Plate-Out
Dirt, moisture, and other contaminates tend to adhere to the fan’s rotating element.
This buildup, called plate-out, increases the mass of the rotor assembly and decreases
its critical speed, the point where the phenomenon referred to as resonance occurs.
This occurs because the additional mass affects the rotor’s natural frequency. Even if
the fan’s speed does not change, the change in natural frequency may cause its criti-
cal speed (note that machines may have more than one) to coincide with the actual
rotor speed. If this occurs, the fan will resonate, or experience severe vibration, and
may catastrophically fail. The symptoms of plate-out are often confused with those
of mechanical imbalance because both dramatically increase the vibration associated
with the fan’s running speed.

The problem of plate-out can be resolved by regularly cleaning the fan’s rotating
element and internal components. Removal of buildup lowers the rotor’s mass and