Processing Magazine

The death of galvanized sheet metal-clad cooling towers

March 27, 2007

Metal-clad cooling towers tend to be high maintenance. They often “live” in harsh environments and are exposed to high pH or caustic elements that will eat even zinc plating, causing plants to either stay very proactive about costly maintenance or put up with expensive process disruptions when their towers are being serviced, repaired or replaced.

If your plant is located in an area that has artic-cold winters, that introduces another stressful condition: The expansion and contraction that occurs, along with UV rays, adds more wear and tear to sheet metal towers, further shortening service life.

“These problems are now worsened by the EPA ruling that lowers the amount of zinc solids that can be used in the galvanizing of sheet metal,” says Jim Raymus, Maintenance Supervisor at injection molder Tulip Corporation’s Milwaukee plant. “As a result, it is unlikely that you will find a galvanized cooling tower with a warranty of more than five years.”

Raymus, who has been with Tulip for 40 years, says that the metal clad towers the plant used for nearly all of that time were always problematic, sometimes causing expensive business interruptions. Rust and corrosion caused when the coolant water pH became acidic and the stress of frigid Wisconsin winters alternating with warm summer weather combined to make the old cooling towers maintenance intensive.

Like many other businesses, the clincher for Raymus was the downtime caused by cooling tower maintenance, especially the unplanned kind. Tulip, an injection molder of plastic battery cases, covers and venting systems for the automotive industry, operates 23 injection molding machines along with lead cold forming equipment.

“We’ve had metal cooling towers completely rot out on us,” Raymus says. “ Also with the models we’ve had, the media is exposed to the sunlight. After a few years the UV rays - even though the manufacturer said they were UV protected - the media goes to hell and breaks up.”

Frustrated by maintenance and downtime costs, Raymus began to explore the potential for replacing his last metal clad cooling towers with a newer material, one that would resist the elements including the expansion and contractions that occur from ongoing freezing, thawing and warmth of the Milwaukee climate. He also was hopeful that today’s plastics could reduce maintenance costs. Most encouraging was the fact that one tower design, with a shell of engineered plastic, offered a 15-year warranty – three to 15 times that of the sheet metal lined models.

The design that Raymus and Tulip chose was the latest designs in corrosion-proof polyethylene-plastic cooling tower technology from Delta Cooling Towers of Rockaway NJ.

“This line came with a full 15-year warranty,” Raymus explains. “Although we had no direct experience with plastic cooling towers, we liked the concept. And the long warranty said a lot about durability.”

The first plastic cooling towers became available in limited smaller sizes 30 years ago. The success of those early models gave rise to second, third and now a fourth generation engineered plastic cooling towers that are much larger in capacity, lightweight and seamless. These towers will not rust, chip, flake, peel or require paint or other protective coatings. The natural and mortal enemy of metal-clad cooling towers is pH, usually on the low side. A high pH leads to excessive calcium and other ceramic-like deposits. When the pH of a liquid coolant becomes acidic (low), either because of external (e.g. atmospheric) elements or infiltration of process elements (e.g. material fines), the veneer of zinc galvanizing used on most metal towers immediately begins to deteriorate. Because the zinc plating is thin (normally a scant 2.5 oz. per sq. ft), a pH lower than 6.5 can destroy this protective lining in a matter of months. At that point contractors are required to replace the zinc galvanizing by expensive coatings and repair any cavities and other damage that may have occurred. If the repair process is extensive, costly process interruptions may be required.

“From what we have experienced on the plastic cooling towers, there is no maintenance,” says Raymus. “The only thing you might want to do, when summer comes, is wash down the inside of the tower. That is because the system is an air washing machine, so anything that birds drop in or any windblown material might collect over a few months at that time of year. You might want to wash down with a hose and clean it up. Other than that, there is not much to do.”

Raymus adds that not only is hose washing the plastic cooling tower easy and simple to perform, but it does not require any interruptions in Tulip processes.

The Delta line of engineered plastic cooling towers has been designed to enable integration of multiple units, making expanding or combining cooling towers in a single location an easy step. Tulip has two such towers linked together to provide the equivalent of a single large unit, Raymus says.

Composed of material that is literally impervious to the harsh pH environments that devastated metal cooling towers, engineered plastic cooling towers are energy efficient, available in a wide array of capacities and air flows, and will operate with both process and energy efficiency for many years.

Weather, especially in Wisconsin with the freezing and thawing, is going to cause expansion and contraction of the cooling tower material,” says Raymus. “The plastic shell gives a little bit, whereas metal tends to stay fairly rigid. So, there is less wear and tear on the plastic shell.”

Raymus says that the new plastic cooling towers also offer energy savings. Tulip’s old metal-clad cooling towers did not have direct drive fan motors. Instead, they had a gearbox on them.

“You had to go up there and change the oil on them and you had quite a long driveshaft that held the fan. The shaft was about 3 feet long, extending from the motor,” says Raymus. “On our plastic towers, the direct drive motors has an impeller directly on the motor, so there are no interconnecting links to deal with. Which that makes them more energy efficient and less maintenance intensive.”

Raymus says that in cold weather his Delta towers will freeze up, as will any towers. When the air gets to zero degrees, the hot water will freeze when it hits the cold air. After awhile, the ice can clog the tower vents. To de-ice them, you shut the fans down, turn them to reverse, run them for 20 minutes, and then turn them back on in the normal mode. That way you will be taking the heat from the water to de-ice the tower vents and ducts. You don''t have to shut down your process or equipment.

Utilizing advanced resins and molding techniques, today’s engineered plastic cooling towers are available in larger sizes and modular configurations for high-capacity applications (1,500 – 2,000 cooling tons).

Engineered plastic cooling towers are one-piece, so there are no problems with seams, welds and patches. They are rust and corrosion-proof. The largest cooling towers manufactured by Delta for example, are rotary-cast with a double-wall UV-protective, polyethylene shell that is virtually impervious to weather conditions and harsh environmental elements.