POWER May 2013 - 84

WATER TREATMENT
3. Low-pressure condenser TTD. Note the trend to lower condenser TTD during the
trial, which indicates reduced microbial growth. Source: Nalco Chemical Co.
LP TTD (F)
20
18
16
14
12
10
8
6
4
2
LP design TTD (F)
viewing the data that follows. First, Unit
4 has a two-pressure, two-compartment,
single-pass condenser, which means the
cooling water flows first into the low-pressure
(LP) condenser compartment and then
through the high-pressure (HP) condenser
compartment.
Also, around July 8 the cooling tower
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4. High-pressure condenser TTD. Source: Nalco Chemical Co.
LP TTD (F)
LP design TTD (F)
20
18
16
14
12
10
8
6
4
2
fan performance improved, which in turn
improved the vacuum pump and air removal
operation due to lower vacuum pump
seal water temperature. This explains the
sharp performance improvement a few
days before the biocide trial began. Finally,
as the biocide test began, Unit 4 was operating
at less than 85% of rated capacity. It's
best to run these tests at rated capacity so
the condenser is operating at design temperatures,
vacuum conditions, and steam
and water flow rates. When full rated load
isn't possible, it is common practice to use
performance data when operating at 85%
of rated load and higher, which reflects the
data collected during the trial and used to
prepare the following figures.
The figures of merit selected to present the
performance improvement are the terminal
temperature difference (TTD) and backpressure
penalty (BP penalty), arguably the most
important plant parameters when monitoring
condenser performance. The TTD reflects the
heat transfer efficiency across the condenser
tube bundle and is usually affected by air inleakage,
scale, and microbiological growth.
The BP penalty represents the steam turbine
performance loss caused by poor condenser
performance.
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Prior to trial Trial period
the same biocide effect as HOBr. These key
factors make bromine a much more effective
and persistent biocide. Using bromine allows
the plant to maintain a much lower oxidant
residual while maintaining an effective microbial
inhibition program.
Years prior to this particular trial, plant
staff and Nalco consultants performed a
study that compared the performance of
" bleach only " and " bleach/CB70 blend " options.
The study concluded that the blended
biocide program provided a more effective
microbial kill at a much lower TRC residual.
Based on the bleach and bromine chemistry
discussed above, this should be expected,
because the cooling towers typically operate
84
at a pH between 8.4 and 8.8 (refer to Figure
2), and the Muskingum River organic loading
can vary between 2 and 25 ppm. At this
elevated pH and organic loading, it would
take a considerable amount of bleach to reduce
bio-populations given the amount of
chloramines and OCl-
that would be generated.
Therefore, a bromine-based chemistry
program was identified as the best choice to
control microbial activity and produce a continuous,
low TRC residual.
First-Rate Results
Conducting a plant performance test when
control of the unit isn't possible can be
tricky. Consider the following when rewww.powermag.com
The
historian data collected during the
45-day trial was processed using a specially
developed condenser performance monitoring
tool (CPMT) program. The CPMT
takes the raw historian data and uses industry-accepted
calculations to thoroughly
analyze the condenser performance. The
results of those calculations determine the
actual cost of condenser performance degradation.
The
split condenser requires careful
consideration of the performance loss
(TTD and BP penalty) imposed by scaling,
air in-leakage, and microbial formation in
each compartment. The losses can then be
summed to produce the final lost steam
turbine performance. There is also another
important value in using these two metrics:
They are calculated independent of each
other. The TTD is calculated using actual
measured parameters, while the BP penalty
is calculated using different measured parameters
as well as design data. This approach
reduces the possibility of hidden
interactions where common data sources
are used.
POWER | May 2013
High-pressure condenser TTD (F)
Low-pressure condenser TTD (F)
http://www.powermag.com
POWER May 2013

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