POWER March 2014 - 53

operations & Maintenance
Adaptive Brush Seals Restore Air
Preheater Performance
Air preheater (APH) leakage has several negative consequences, including lost revenue.
A better design for APH seals has been shown to have several benefits
over traditional metal strip seals, including a quick payback.
Pavan Ravulaparthy and Ravi Krishnan
T
he gas sealing systems used on rotary,
regenerative air preheaters (APHs)
have evolved little from the metal strip
configuration used on the first Ljungström
preheaters nearly a century ago. Metallic
strip seals are typically used for radial, axial,
and circumferential seals that are exposed
to corrosive gases at relatively high temperatures.
Steel seal degradation begins soon
after installation, and the inevitable result is
increased air-to-gas leakage over time, which
translates into increased fuel consumption
and fan power usage. If you've ever experienced
" running out of fan, " it's likely that
APH leakage is part of the problem.
Repeated thermal expansions and contractions
in the large (often 20-meter in diameter
or more) rotors in continuous motion usually
cause large and irregular seal gaps. At operating
temperatures, outer edges of large APHs
may droop or turn down by 3 inches or more
from the cold condition. Steel strips wear
(or warp or break) based on the smallest gap
size, thus leaving larger gaps elsewhere. Leak
rates with properly designed and installed
seals should be less than 10%, although leakage
rates of 15% to 20% are typical, and rates
greater than 30% are not uncommon when
conventional seals fail. APH leakages typically
occur in air-to-gas and gas-to-air paths
through the APH seals (Figure 1).
1. Many leakage paths. The air-to-gas
and gas-to-air leakage paths typically found
in a rotary, regenerative air preheater are
through the circumferential, axial, radial, and
rotor post seals, as shown by the yellow arrows.
Courtesy: Sealeze
Rotary APHs are particularly critical to the
efficient operation of coal-fired power plants,
delivering up to 12% of the heat transfer used
in the steam generation process. A useful
rule of thumb is that for every 20 degree C
decrease in the APH gas outlet temperature,
boiler efficiency increases about 1%, which
translates into about $1.5 million in fuel savings
every year for a typical 500-MW plant.
An optimally operating APH also reduces
fan power consumption and thereby net plant
generation capability.
APH leakage also has a detrimental effect
on downstream air pollution control equipment
due to increased gas velocity, temperature,
and APH air- and gas-side pressure drop.
For example, the typical flue gas velocity
2. Brush seals installed. Circumferential
(left), radial (bottom), and axial brush seals
(right) can significantly reduce air preheater
leakage rates. Source: Sealeze
through a selective catalytic reduction (SCR)
module is around 5 to 6 meters per second.
Increased gas velocity caused by air-to-gas
leakage will decrease the residence time of the
gas and thereby reduce the effectiveness of the
SCR, as well as potentially cause an increase
in ammonia injection rates and slip.
Additionally, lower gas residence time in
the flue gas desulfurization system can adversely
affect lime or limestone injection rates
and SO2
removal efficiency. For particulate
matter control systems, higher air-to-cloth
face velocities in fabric filters can lead to decreased
bag life. Pulverizer capacity also can
be negatively affected with lower air volumes
and temperatures due to air-to-gas leakage.
Brush-up Your APH Seals
Brush seals are a particularly good choice
for replacing steel strips commonly used for
circumferential, radial, and axial seals on
3. Comparison test. A worn strip seal
(on the left) compared to a new brush seal
(right). Flexibility of the brush seal allows it to
deflect at the smaller gaps and then rebound
to ensure sealing at wider gaps. The amount
of wear on the steel strip seal is evident.
Source: Sealeze
March 2014 | POWER
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POWER March 2014

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