“Where the traditional, galvanized metal-clad cooling towers have done a good job at cooling process water, they have also been highly prone to corrosion and, therefore, frequent cleaning, recoating and replacement,” says John Flaherty, president of Delta Cooling Towers. In addition, the use of water treatment chemicals to maintain water chemistry can also cause faster deteriorating of the metal shell, says Flaherty.
Using plastic (high-density polyethylene or HDPE) as an alternative to galvanized steel might cost a little more than galvanized steel, but since HDPE isn’t affected by chemicals that cause oxidation, its lifetime will be much longer, and maintenance is less, says Flaherty. Plastic will survive more aggressive chemicals when they’re used to clean the towers. In addition, plastic can easily be molded so sharp corners usually associated with metal towers are gone. Flaherty says plastic towers outlive their galvanized counterparts such that his company places a 15-year warranty on them.
Like their metal counterparts, fans in these systems are usually run by variable-speed motors whose variable-frequency drives are operated through a control system to provide the right speed to match cooling requirements with wet-bulb temperatures of the outside environment. This allows, for example, returning 100°F process water cooled to 70° as it reenters the process. According to Flaherty, it wouldn’t be unreasonable to expect that a three-cell cooling stack using six 5hp premium-efficiency motors running 8,760 hours annually would require about half the power that competitive metal towers using four 15hp standard motors would need for the same cooling ability.
ASHRAE (American Society of Heating, Refrigeration and Air-Conditioning Engineers) 90.1 standards established minimum efficiency standards for cooling towers with either axial or centrifugal fans: greater than 38.2 gpm/hp for axial fan cooling towers and 20.0 gpm/hp for centrifugal models at 95°F entering water, 85°F leaving water at 75°F wet-bulb temperature. If cooling towers meet these specifications, buildings using them can qualify for LEED credits.
For normal building cooling, open-loop systems are used where the cooling water is exposed to the outdoor air. For process cooling in food plants, closed-loop architecture can be used, says Flaherty. This isolates the process cooling water from the cooling tower water. “For a closed-loop system, we incorporate the cooling tower with a shell-and-tube or plate-and-frame heat exchanger. Thus, the cold side of the heat exchange is open to the tower, and the hot side of the heat exchanger is closed back to the process,” he adds.
While heat exchangers can be prone to clogging, the nature of the plastic cooling towers allows cleaner water to be run through the exchangers, decreasing clogging. Counter-flow cooling architecture provides less of an environmental impact than cross-flow systems, which means the water stays inside the cooling tower, even during high winds, and the water is not exposed to sunlight, which could encourage biological growth.
When a plastic water cooling tower finally meets the end of its life cycle, it can simply be disposed as any recyclable plastic, a benefit to the environment, says Flaherty.For more information, contact Delta Cooling Towers,