Engineered plastic cooling towers don’t have the problems that plague their galvanized metal counterparts — corrosion, leakage, maintenance costs, process disruptions. The following report reviews their important differences. By David Jurgensen
| | Delta Cooling’s engineered plastic cooling towers are available for high-capacity applications. They come with a 15-year warranty, the longest in the industry. Metal-lined cooling towers (left) generally carry a one-year warranty. click to enlarge
With heat playing a major role in all chemical processing operations, it’s hard to find a plant today that doesn’t have a cooling tower or two on its roof or grounds. Any process that introduces heat, either directly or reactively, generates waste heat or the requirement for cooling. Protecting expensive process equipment and maintaining cooling fluids are indispensable parts of any process operation. Without fail-safe and efficient cooling towers, capital-intensive processing operations are subjected to unscheduled maintenance shutdowns with unrecoverable production losses and delayed shipments. Unfortunately, the zinc galvanized sheet metal cooling towers traditionally used to handle this heat are subjected to exorbitant maintenance costs and process disruptions. The very laws of nature also seem to work against them. “Galvanized cooling towers are being eaten alive by harsh environments,” says Jim Ruddy, president of Proprietary Systems Inc. in Westlake Village, CA, which provides engineering consulting. “They rust and corrode all the time. They are attacked internally by process chemicals and externally by environmental elements.” Having come from the metals processing industry, Ruddy received a first-hand education on the disruptions and expenses that can result from metal cooling towers. “We used big power supplies for heating and melting through induction. Metal cooling towers were plagued by rust and corrosion. Rust would get into the water system and lodge in an SCR (silicon-controlled rectifier) and short it out. Poof! There goes another $2,000 SCR. The maintenance guys sometimes changed SCRs like lightbulbs.” Mortal Enemy: pH
The mortal enemy of sheet metal cooling towers is pH, usually on the low side. A high pH leads to excessive calcium and other ceramic-like deposits. But, when the pH of the cooling water becomes acidic because of either atmospheric elements or infiltration of process elements, the zinc galvanizing veneer used on most metal towers begins to deteriorate. Because the zinc plating is thin, normally a scant 2.35 ounces per square foot, a pH lower than 6 will destroy this protective lining in a matter of months. Contractors then have to apply expensive coatings and repair damage that may have occurred. If the repair process is extensive, costly process interruptions may be required. Ruddy says pH problems can come from a number of sources. “We have customers who use water treatment chemicals, including de-scalers, to prevent scale from occurring in their process fluids. These chemicals sometimes leak into the cooling system and they corrode the cooling towers. Sometimes water treatment chemicals attack the metal at the seams. Other times they just wear down the galvanizing. But either way, once the zinc plating of a metal tower is undermined, you’re looking at escalating trouble, possibly including expensive repairs or even replacement.” Impervious Plastic
Engineered plastic cooling towers are composed of material that is impervious to the harsh pH environments that devastate metal cooling towers. They’re also impervious to the salt and waste products found in the air in many industrial areas as well as a variety of minerals found in extremely hard water. In addition, they are energy efficient and come in an array of capacities and airflows. The first plastic cooling towers were available in limited sizes 30 years ago. The success of those early models gave rise to second, third and now fourth generation towers that are much larger in capacity, lightweight and seamless. These towers do not rust, chip, flake or peel and do not require paint or other protective coatings. Cooling towers cool water through heat transfer and evaporation. With a loss of 1 percent water for every 10 degrees of cooling required, the evaporation factor can be very significant — 20 or 30 gallons per minute is not uncommon for small towers. When evaporation occurs, scale is left behind, which can interfere with cooling tower efficiency. Removing Residual Salts
Removing residual salts in a metal cooling tower is no simple task. Ruddy explains: “You may have to use very aggressive cleaning chemicals to dissolve the salts. In a metal tower that is designed with seams that come down as a right-angle flange, powerful cleaners will penetrate those seams. Then, after cleaning, a base is added to neutralize the acid and bring the pH back up. But the bases don’t penetrate, so the acid remains in the seam. So, you have a flange on a metal tower and the acid sits in the seams, working on the thin galvanizing and eroding it.” Ruddy says many companies believe they cannot afford to clean their cooling towers frequently. As a result, a sizable amount of dirt and scale deposits collects in the bottom of the tank. “This is bad news for metal cooling towers. There is a difference in the electrical potential between the water on the top and the metal in the tower. So, a small electrostatic charge occurs in the tower and eats through the metal, creating pinhole leaks on the bottom of the metal towers. Once you get a hole in the galvanized tower, it starts to rust out from there — like a cancer that goes in all directions at once.”Residual salts, however, cannot damage plastic cooling towers. They simply can be washed away with a hose. About the Author: David Jurgensen is the technical expert at Delta Cooling Towers Inc., 41 Pine St., Rockaway, NJ 07866, which specializes in polyethylene cooling towers. Questions about this article can be addressed to him at 800-289-3358. Additional information is available at www.deltacooling.com.