When Bob Gant called on a sophisticated fabricator of alloy-coated sheet steel, he was curious about their interest in a cooling tower with an engineered plastic shell. After all, galvanized steel had been the de-facto-standard material for lining cooling towers for many years.
“Of course, I was well aware of the advantages of our engineered plastic towers,” Gant said. “But when I got a request for a bid from a company that makes coated sheet steel, I was impressed.”
Gant was addressing the fact that “traditional” galvanized metal cooling towers are still fairly popular, even though they are somewhat notorious for being maintenance intensive and requiring frequent replacement due to pH problems, corrosion, leakage, high maintenance costs and sporadic disruptions, which can cost some companies thousands of dollars an hour.
Gant, an engineer for Delta Cooling Towers, says those problems are avoidable with cooling towers having engineered plastic shells that are virtually impervious to the environmental factors, harsh water treatment and cleaning chemicals that significantly corrode metal-clad towers.
“I think the clincher for the sheet-metal alloy-coatings firm was the 15-year warranty on the unitary-molded plastic shells,” Gant said. “You commonly find that galvanized metal-clad shells have a one-year warranty, although that may be understandable due to environmental conditions.”
“I’m amazed that our plastic towers are still working well after 27 years,” said Courtney Perkins, engineering laboratory manager with Bard Manufacturing of Bryan, Ohio, a prominent manufacturer of air conditioners, heat pumps and furnaces since 1914. “Our first plastic cooling tower must have been one of the early ones, but it keeps running with relatively little general maintenance every couple of years.”
Plastic Reduces Operating CostsCooling towers with shells of engineered plastic also reduce operating costs, which include:
- Electric power usage
- Water-treatment chemicals
- Labor and materials for maintenance
- Unscheduled process downtime for repairs
The Effects of pHFor instance, the “white-rust” scale that forms on galvanized towers operating at pH higher than 8.0 can quickly lead to failure and replacement. Conversely, engineered plastic allows the use of better scale inhibitors that operate at higher pH.
Because the zinc plating of galvanized towers is thin (normally a scant 2.5 ounces per square foot), a pH lower than 4 will destroy this protective lining in a matter of months. At that point it becomes necessary to replace the zinc galvanizing with expensive coatings and repair any cavities and other damage that may have occurred.
Not requiring inordinate concern over pH levels or mineral deposits, engineered plastic cooling towers can operate at higher cycles of concentration, leading to operational savings. Higher cycles of concentration are achieved by lowering the amount of blowdown or bleed-off of recirculating water.
The imperviousness of engineered plastic cooling tower shells to pH, harsh environmental factors and cleaning agents is important but only part of the story (Fig. 1). Although considerably more expensive, stainless steel shells also offer corrosion resistance, lower maintenance and improved durability over their galvanized-metal counterparts. Even stainless steel can’t match the 15-year warranties of the plastic towers.
Installation Flexibility and Weight AdvantageModels with stainless steel shells weigh considerably more than the engineered plastic ones, and that introduces some added structural support requirements.
The inherent design advantages of the latest plastic cooling towers also include easier installation (especially on rooftops) because a lightweight plastic shell often weighs 40% less than a steel tower, even though it is five to 10 times thicker.
Engineered plastic cooling towers are available in modular designs where units are combined in a cluster, making installation easier, faster and less expensive. For applications that require mounting flexibility, Delta offers an induced-draft, counterflow design that incorporates I-beam pockets in the tower basin for reinforcement so that a plastic tower can be easily mounted on standard I-beams or imperfect concrete pads.
Utility SavingsUtility savings can also be realized. While the cost of electric power to drive cooling-tower fans may seem incidental to process costs, they can add up. In the case of the engineered plastic towers manufactured by Delta, direct-drive motors are employed to power the cooling fans. With no pulleys, bearings and belts, such direct-drive motors prove more efficient. As a result, they provide substantial savings in energy costs while also delivering more horsepower. Also, stress from frequently turning on and shutting off cooling towers is no factor (Fig. 2).
For further energy conservation, when modular towers are incorporated into a cluster, individual direct-drive fan motors can be shut off when supported processes are not operating.
Additionally, polyethylene plastic water towers also save costs by reducing or eliminating the possibility of process material contamination. In particular, treatment chemicals can cause the leaching of zinc from galvanized metal, which in some cases could result in the zinc migrating into the process – a potential environmental-discharge problem.
The first plastic cooling towers became available in limited smaller sizes 30 years ago. The success of those early models and advanced molding techniques gave rise to second-, third- and now fourth-generation engineered plastic cooling towers that are much larger in capacity. Today, available units range from compact sizes as small as 10-ton (15 GPM) force-draft models up to high-capacity 2,000-ton (12,000 GPM) induced-draft towers. IH
For more information: Contact Delta Cooling Towers, Inc., 41 Pine Street, Rockaway, NJ 07866; tel: 800-BUY-DELTA (289-3358); fax: 973-586-2243; e-mail: email@example.com; web: www.deltacooling.com. Ed Sullivan is a technology writer based in Hermosa Beach, Calif.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: galvanized steel, corrosion, pH, energy conservation