POWER June 2012 - 78

CONDENSERS
should eliminate a reoccurrence of this
problem in the future.
Case Study 3: Cooling Tower Fill
Weight
A plant with twin 150-MW-class coal-fired
units located in the eastern U.S. is the subject
of the third case study. Each unit has
a two-pass, single-pressure condenser that
is cooled via a recirculating, evaporative
cooling tower.
What is interesting about this case study
is that the plant designers had the foresight
to install an automated cooling tower fill
weight monitor. Its function is to monitor
in real time the weight of a small section
of the fill. The data is then sent to the plant
distributed control system (DCS), where it
is displayed and captured by the plant historian,
again PI.
Cooling tower fill is a material with
labyrinth-like packing that is added to the
tower to increase the contact surface and
contact time between the water and air to
improve its heat transfer efficiency. An increase
in fill weight is a measure of the
amount of fouling that has occurred in the
tower. However, even a simple system requires
a section of fill that can be manually
removed and weighed on a periodic
basis. Because fill weight correlates with
fill fouling, this information is invaluable
9. Tower temperatures drop. The biocide feed (Figure 8) that reduced biofouling of the
cooling tower fill also reduced the cooling tower basin temperature. Source: Nalco Co.
94
92
90
88
86
84
82
80
78
when designing a cooling system's biocide,
scale inhibition, and fouling inhibition
programs. And because cooling towers
are designed with a specific liquid-to-gas
ratio, the fill weight, and thus fouling, will
directly correlate with the overall cooling
tower efficiency.
The plant experienced significant biofouling
in the past, which caused an increase in
the cold water temperature of the cooling
tower and elevated the steam turbine exhaust
pressure as well. Exacerbating the problem
is the plant's heavily cycling operating profile,
which makes an effective biocide program
to reduce cooling tower fouling very
difficult to maintain.
The plant experimented with a new biocide
program during the summer of 2010. In
mid-July, a biocide was slug fed to determine
what benefit, if any, was possible. Shortly after
feeding the product on June 28, staff saw
a significant reduction in tower fill weight
(Figure 8). Concurrently, they observed an
appreciable 5F decrease in cooling tower
basin temperature (Figure 9), which equates
to an improvement in condenser vacuum of
about 0.5 HgA (Figure 10). Finally, as seen
in the figures, the biocide experiment was a
one-time shot; soon after, the pressure in the
condenser again began to rise.
The conclusion drawn from the data is
10. Condenser vacuum improves. The biocide slug also significantly decreased the
vacuum condenser vacuum, improving the plant output and efficiency. A few days later, the
biofouling began to return. Source: Nalco Co.
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
that the cooling tower was significantly
fouled, whereas the condenser was not.
This observation confirms our field experience
that many condenser performance
issues are actually cooling tower
performance issues. By instrumenting the
cooling tower appropriately-that is, measuring
cooling water flow, airflow for each
cell, and other atmospheric monitoring in
accordance with Cooling Technology Institute
guidelines-accurate troubleshooting
can be performed.
The Bottom Line
In each of these condenser performance case
studies, the plant used a condenser-monitoring
system that collected digital data that was
fed to the plant historian to quickly diagnose
problems so that rapid repairs can be made
that will return the plant to normal operation.
The cost to implement a condenser-monitoring
system is often much less than the lost
revenue and additional costs of a forced outage
when unexpected problems appear.
Thanks to AEP's Angela Larrick and my
colleagues Aaron Haines, Ellie Pallomo, and
Loren Beard at Nalco Co. for their assistance
in the preparation of this article. ■
-Kevin Boudreaux (kjboudreaux@
nalco.com) is a power industry technical
consultant for Nalco Co.'s Power Group.
78
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POWER | June 2012
Condenser vacuum (in HgA)
Condenser supply water temperature (F)
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