POWER February 2014 - 29

Furthermore, addressing any one of these issues will usually
result in new system requirements for fans, such as a change
in air/gas temperature, gas composition, suction/supply pressure
needs, volume requirements, or power consumption due to
changing the air/gas characteristics. This means that any fan
retrofit is an engineering project in its own right.
The goal of any retrofit is to minimize modifications to the
existing fan while maximizing the fan's flow and pressure capability
as it relates to current system demands. Ideally, this is
accomplished utilizing the existing casing without modifying the
foundation. Goals for the project will normally include identifying
the lowest capital cost, staying within the current motor horsepower
with no new electrical infrastructure, and making minor
or no modifications to the existing fan, foundation footprint,
ductwork tie-ins, and related fan components.
However, the ultimate solution may require modification to
one or more of the following: fan wheel, fan casing, drive motor,
ductwork tie-ins, and even the foundation in some cases. When
retrofitting, it's also advisable to make use of a computational
fluid dynamics (CFD) study for validation purposes. This computer
based computational process cuts down on modeling costs and
saves precious time. Actual model tests can also be performed
with a geometrically similar fan, if budget and time allow.
There are several techniques that can be used to accomplish
a successful retrofit. Among them, " optimization " and " rightsizing "
are the ones most commonly used. Two short case studies
illustrate both of these approaches.
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Transmission Line
Table 2. Before and after draft fan motor amperage.
Motor current was reduced using the right-sizing technique. Courtesy:
ProcessBarron
Generation output (MW)
Pre-installation motor amperage (amp)
Post-installation motor amperage (amp)
300
185
136
Optimization
A utility plant in the Midwest was unable to meet peak load during
the summer months. The draft fan was being operated at or
above the maximum motor amperage load. A flow performance
test and subsequent CFD study confirmed that the inlet cone
velocity was not properly diffused. The fluid media inside the
blade passages were separating and as a result the fan was not
performing efficiently. A new, high-efficiency, optimized rotor
and inlet cone were installed. The existing motor was reused. The
plant met load demands successfully with horsepower to spare.
Table 1 and vector plots in Figures 10 and 11 demonstrate before
and after results.
Right-Sizing
The draft fan at another utility plant in the Midwest was experiencing
vibrations and generating a lot of noise, which concerned
plant management. A comprehensive flow performance test was
performed. It was determined that the fan was oversized for the
application. A redesigned, " right-sized " rotor was installed using
the existing shaft and drive system. A minor modification was
made to the casing.
Post-installation analysis revealed no vibrations. The acoustics
issues had also been resolved. The synergies of the right-sizing
process also produced an added benefit in the form of energy
savings (Table 2) resulting in a rapid return on investment.
Evaluating Solutions
A retrofit fan design requires a detailed and careful review of
existing system performance and operational data. The most
effective strategy is a total system approach: Analyze each
component; isolate critical paths; and provide a practical
engineering solution. When retrofitting a power plant fan,
engineers should evaluate the entire draft system; perform
a field inspection; establish baseline flow parameters; establish
baseline geometry; calculate future system requirements;
study interactions between fan characteristics and
system demands; customize a retrofit fan to suit the projected
system requirements; predict future fan performance;
perform mechanical, metallurgical, and structural (civil)
studies; validate theoretical basis with a CFD study; and perform
a geometrically similar fan model test (depending on
time and budget).
The most critical path through a draft system is the draft
fan. The key to a successful retrofit is the pragmatic application
of a sound theoretical basis. Determining a solution requires
comprehensive study and a clear understanding of the
problem it is meant to address. Understanding the dynamics of
draft machines and their interactions with systems is extremely
important. Experience and access to resources on the subject
matter are other key ingredients for achieving success in the
retrofit process. ■
-Nurul " Moni " Talukder, PE (mtalukder@processbarron.com) is
chief engineer in the Air & Gas Handling Group at ProcessBarron.
February 2014 | POWER
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POWER February 2014

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