In Hot Water: A Cooling Tower Case Study
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%.
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