POWER October 2010 - 60

PLANT O&M
the APH through the circumference of the
APH. It only includes the effect of air that
bypasses the APH and short-circuits to the
flue gas flow.
Reducing APH Outlet Gas
Temperature
To minimize fuel consumption, the flue
gas exit temperature from an APH must be
kept as low as practical. Go too low and the
baskets and cold end structural elements
will experience destructive acid condensation
and therefore corrosion. However,
compared with the cost of the excess fuel
burned to maintain a gas outlet temperature
well above the dew point to reduce cold
end corrosion, the cost of a replacement set
of cold end baskets is almost incidental.
The chemical process that causes the cold
end acid corrosion is well understood. Acid
condensation occurs wherever the gas temperature
is reduced to less than the saturation
pressure (partial pressure) of the SO3
present in the flue gas. Under those conditions,
a dilute solution of sulfuric acid condenses
on and is corrosive to APH structures
and baskets. That is why, in most plants, the
cold end element layer in an APH is sacrificial
and is designed to be replaced relatively
easily and at modest cost.
In modern power plants, operations
strive to minimize gas outlet temperatures
by maximizing APH heat transfer
efficiency. There are two principal ways
to decrease the exit gas temperature on an
existing unit: increased element depth and
APH speed of rotation.
Some of the APHs installed today were
originally designed with some open space
that can accommodate several inches to
several feet depth of additional heat transfer
surface area. Adding additional heat
transfer surface will decrease the exit flue
gas temperature. For those units without
extra space, alternate basket designs are
available that have the same outside depth
as the original baskets, but they can accommodate
up to 3 inches of additional element
depth by modifying the traditionally
deep structural bars at the top and bottom
of each basket.
Most air heaters were designed to operate
at a single, fixed speed of rotation. The
recent studies by Skiepko and Shah indicate
that it may be possible to increase the thermal
efficiency of an existing air heater by
8. Old seal technology. The original circumferential/bypass seal design can be easily
damaged if it makes contact during operation, and it may often be set with an excess gap
to reduce the potential for damage, but with resulting increased leakage. Courtesy: Paragon
Airheater Technologies Inc.
changing its speed of rotation. The practical
application of this concept is unproven,
but preliminary full-scale experimentation
at a plant in North Carolina has shown encouraging
results.
Additionally, slowing the speed of rotation
during air heater sootblowing operation
can be used to increase the sootblower
residence time in the faster-moving outer
radius of the air heater, which can help
maintain air heater cleanliness.
Leakage Repair Solutions
A cost-effective and simple method for reducing
APH leakage is replacement of the
original equipment gas seals with newer
design high-performance, full-contact radial
seals and self-reinforcing circumferential or
bypass seals. Full-contact seals have a proven
performance record in reducing APH leakage
by as much as 50% when used to replace the
original equipment type of radial seals commonly
found today on most APHs.
An example of a high-performance, fullcontact
radial seal is shown in Figure 5. The
full-contact DuraMax radial seal is constructed
with a spring bellows that allows the seal
to maintain continuous but flexible contact
with the sealing plate at all times, effectively
eliminating the main path for radial seal
leakage. It is designed to maintain contact
even when the mating sealing surfaces (sector
plates) have become distorted or out of
plane. In comparison, the original design seal
was more of a rigid " proximity " air dam that
is much less effective, especially with an aging
air heater that has drifted from its as-new
dimensional tolerances (Figure 6).
The new DuraFlex circumferential seals
have an interlocking and self-reinforcing
structural design that allows them to be set
in close proximity to the rotor sealing surface
without being damaged, thus minimizing
gaps and leakage openings (Figure 7). The
original style seals that this design replaces
are shown in Figure 8.
An example illustrates the positive benefit
Seal the deal. Test results from AEP's Welch Station Unit 3 found a 23% reduction in
induced draft fan amps with full-contact radial seals when compared with the original style of
circumferential seals. Source: Storm Technologies Inc.
ID fan number
3A
3B
3A
3B
60
Before (amps)
572
691
131
128
After (amps)
459
468
125
113
www.powermag.com
of APH leakage reduction when replacing
standard, rigid seals with full-contact radial
seals to reduce just radial seal leakage. After
replacement with full-contact DuraMax
seals at American Electric Power's 500-MW
Welch Station Unit 3, the ID fan amperage
decreased by over 23%. The motor amperage
is directly proportional to the motor power
requirements (see table). ■
-Stephen K. Storm (stephen.storm@
stormeng.com) is executive vice president,
technical field services at Storm
Technologies Inc. John Guffre (jguffre@paragonairheater.com)
is chief
research scientist for Paragon
Airheater Technologies.
POWER | October 2010

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POWER October 2010

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