POWER May 2020 - 34

PLANT EQUIPMENT
Understanding Fan Problems and
Optimizing Rotor Dynamics
Industrial fans are required for many power plants to operate, such as primary
and secondary air fans, and induced draft fans. Understanding rotor dynamics is
essential for reliable long-term fan operation. If a fan is designed inadequately,
catastrophic failure is a serious risk.
Steve Nelson
R
eliable heavy-duty industrial fans
are a necessity in demanding industries,
including chemical processing,
refineries, steel mills, cement
manufacturing, fossil fuel power plants,
and other facilities that run around the
clock. Unexpected downtime due to a
fan's mechanical issues can cost a company
hundreds of thousands of dollars in
lost time and throughput.
Two of the most common ways that
industrial facilities and manufacturing
plants use fans are to supply more oxygen
to a combustion process in order
to increase the heat generated or move
products of combustion from one location
to another for cleaning or disposal.
When reviewing the fan you intend to
use for your operation, consider the
need to examine its rotor dynamics.
Shaft critical speed and bearing functionality
are fundamental calculations. These
pre-empt common problems and help
optimize fan bearings. Understanding
the effects of critical speed and vibration
allows improved operation and decreases
maintenance issues.
The Critical Nature of Shaft
Critical Speed
Shaft critical speed is the speed at which
the fan shaft becomes resonant, and
when natural vibration increases and
causes the fan to function incorrectly.
Most fans operate at less than 75% of
their critical speed.
Operating at the shaft's critical speed
causes problems, such as loosening of
parts, poor fan vibration characteristics,
and even bearing and foundation
damage. Manufacturers routinely perform
modal analysis to determine the
shaft's critical speed and to determine
the geometry of the fan a facility requires
(Figure 1).
Two main factors that determine shaft
critical speed are shaft stiffness and rotor
weight. Increases in shaft diameter
increase the stiffness of the shaft, which
raises the natural frequency of the rotor.
A decrease in rotor weight will lessen
the weight on the shaft and increase the
natural frequency of the unit.
A rotor critical speed, or natural frequency,
occurs when some part of a fan
rotor begins to resonate in response to
a frequency produced during normal
fan operation. When that happens, the
motion of the resonating part will be
amplified, which can result in fan damage.
The most common presentation
of this damage is rotor cracking due to
material fatigue.
A Basic Component of Fan Design:
Rotor Dynamics
Rotor dynamics refers to how individual
parts of the fan behave while the fan
is in motion, including the fan rotor,
foundation supports, and bearings. Fan
components move and vibrate naturally
during operation, and each rotor may experience
a variety of natural frequencies
while the fan is in use. Rotor dynamics
can be measured via a mechanical exam
or a set of calculations.
Industrial
fan
manufacturers
study
rotor dynamics closely during product
development to mitigate potential problems,
especially for applications
that
1. Modal analysis and finite element analysis can help to determine wheel reinforcements,
and reduce critical speed and vibration. Courtesy: New York Blower
34
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demand a high level of reliability. Modal
analysis determines the frequencies to
which the rotor is sensitive and which
pieces of the rotor may require adjustments
in order to work reliably.
Rotor dynamics can also help determine
the cause of premature equipment
failure. A fan's performance can be affected
by normal wear. If proper preventative
maintenance is not implemented,
rotors may become thin and eventually
crack or break. However, there are experiences
of premature cracking that can
lead to unexpected failure.
For example, a plant that developed
automotive engines experienced a
cracking problem in one of its fans after
several years of successful operation.
The cause was the regular removal of a
build-up of sticky material that collected
during regular fan use. The sandblasting
process used for cleaning decreased the
thickness of the fan blades over time, ultimately
changing the rotor's natural frequency,
which was determined by a field
POWER | May 2020
http://www.powermag.com
POWER May 2020

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