In Hot Water: A Cooling Tower Case Study

Justin Cochran, P.K. Raju, Chetan S. Sankar

Abstract


Problem Statement:


   Vogtle Electric Generating Plant operated by
Southern Nuclear Operating Company, a subsidiary
of Southern Company, has found itself at a
decision point. Vogtle depends on their natural draft
cooling towers to remove heat from the power cycle.
Depending on the efficiency of the towers, the cycle
can realize more or less power output. The efficiency
of the cooling tower is loosely defined by
Vogtle personnel as how well the tower?s actual
performance compares to its original design performance.
The primary goal here is to have the
cooling tower produce the coolest water possible
to re-enter the condenser. The performance, therefore,
can be described as how close the temperature
of this water gets to the expected (predicted)
temperature of the original tower design. A one or
two degree decrease in the cold water temperature
can have a very significant impact on the
company?s ability to compete in the deregulated
market.

   When Plant Vogtle began producing power in
1989, the cooling tower only performed at 76% of
expected efficiency. The original design was modified
by the manufacturer in 1990 and resulted in an
increased efficiency of 91%. In an effort to improve
the efficiency of the towers from 91% to 100%, two
modifications to the nozzle sizes and distribution
pattern had been suggested by a cooling tower
consultant, John Cooper, but had resulted in a decreased
efficiency of 86% after the first modification
and 79% following the second modification.
To control the distribution of the water in the tower,
over 10,000 nozzles were used and nozzle sizes
vary in diameter, allowing more or less water to fall
in certain areas of the tower. The figures at right
show the placement of nozzles of different diameter
during 1989-1998.

   The outlet water temperature from the cooling
tower can be affected by the spray nozzle configuration
inside the tower. The diameter of the nozzles
can be changed to affect where water is released
in the tower and plays a vital part in the performance
of the tower. Selection of the appropriate nozzle
sizes and their placement in the tower, or distribution
pattern, for maximum performance is at the
center of Southern Company?s dilemma. Calculations
indicate restoring the tower to 100% capability
has a present worth value of $10.2 to $11.9 million
(in 1989 dollars) from 80% of design capability.

   During Sept. 1999, John Cooper made a new
recommendation for a further modification of the
nozzle sizes in this tower, but it was unclear how
effective the change would be. Therefore, the
Southern Company had to make a decision: either
choose to revert to the previous nozzle configuration
of the tower with guaranteed 91% efficiency,
or try the new modification in an attempt to reach
the tower?s expected capability of 100%.

Full Text:

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JSTEM. ISSN: 1557-5284