With heat playing a major role in many manufacturing processing operations, it's hard to find a plant that doesn't have a cooling tower or two on the roof or somewhere on the facility grounds. That's because many processes that introduce heat, either directly or reactively, also generate waste heat or the requirement for cooling.
Whether applied directly to heat exchangers, equipment jacket coolers, oil cooling, quenching baths, annealing furnaces, sterilization apparatus, pasteurization systems or air compressors, protecting expensive process equipment and maintaining cooling fluids represent an indispensable adjunct for any process operation. Without efficient cooling towers, capital-intensive processing operations would have unscheduled maintenance shutdowns with unrecoverable production losses and delayed shipments that could lose customers.
Unfortunately, metal-clad cooling towers utilized to handle the heat are under attack from all sides. “Metal-clad cooling towers are being eaten alive by harsh environments,” says “Jim Ruddy, president of Proprietary Systems Inc., Westlake Village, Calif. “They rust and corrode. They are attacked internally by process chemicals and externally by environmental elements. Metal towers also can corrupt processes, causing headaches that can cost thousands of dollars many times over.”
The 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 begins to deteriorate. Because the zinc plating is thin, a pH lower than 4 will destroy this protective lining quickly.
Ruddy, whose earlier experience in the metals processing industries led him to the cooling tower business via Proprietary Systems, a distribution organization, says pH problems can come from a number of sources.
“We have customers who use solvents or descalers to prevent scale from occurring in their process fluids,” Ruddy says. “These chemicals sometimes leak into the cooling system and corrode the cooling towers. Sometimes, the solvents 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.”
Counter AttackThe ability to withstand high and low pH levels without corroding has made engineered plastic cooling towers attractive for processors dealing with towers failing due to corrosion.
The first plastic cooling towers became available in limited smaller sizes 30 years ago. The success of those early models gave rise to current-generation engineered plastic cooling towers that are lightweight, seamless and larger in capacity. According to Ruddy, the towers do not rust, chip, flake, peel or require paint or other protective coatings.
“There is simply no way that we would use a metal cooling tower,” says Doug Martin, vice president and operations manager at H. J. Baker & Brothers' Wilmington, Calif., plant. A supplier of prilled sulfur for the agricultural trade, H.J. Baker & Bros. processes more than 250,000 tons of sulfur a year at two California facilities.
“We spray molten sulfur into cool water in the prilling process,” Martin explains. “A cooling tower is needed to chill the process water after the sulfur bath. While the process water is part of a separate system, it becomes somewhat corrupted by minute sulfur fines in it. Those sulfur fines make the cooling system water highly acidic.”
Martin says that the sulfuric water would attack any metal surface it contacts. For that reason, the plant uses engineered plastic cooling towers from Delta Cooling Towers, Rockaway, N.J. Martin says H.J. Baker was able to get 20 years of service from its first installed unit despite the aggressive process conditions.
Also, an engineered-plastic cooling tower protects against the negative effects of sulfuric acid for Ramcar. The manufacturer of car batteries uses a plastic unit from Delta Cooling Towers at its City of Commerce, Calif., plant.
“We use relatively high concentrations [25 percent to 35 percent] of sulfuric acid mixed with the water used in our batteries,” explains chief engineer Larry Eslao. “This mixture gets very hot, so we designed our process to run the mixture through a cooling tower.
“Of course, with such a harsh mixture, we would not want to use a metal clad tower, which would have a very short life-
span. We purchased this Delta plastic model because it is impervious to acid. We bought our present tower in 1987; it has proved to be virtually maintenance free.”
The Evaporation FactorCooling towers cool water through heat transfer and evaporation. With a loss of 1 percent of water for every 10oF (5.5oC) of cooling required, the evaporation factor can be significant. In fact, evaporation losses of 20 or 30 gal/min are not uncommon for small towers. When evaporation occurs, scale is left behind, which can interfere with cooling tower efficiency and require maintenance or acid cleaning. Because plastic cooling towers are impermeable to residual salts, they cannot damage the tower. It can be cleaned by simply washing down the plastic surface with a hose.
“The plastic tower is not affected by pH,” says Ruddy. “A high pH is going to result in salts collecting in the tower. You may have to use aggressive cleaning chemicals to dissolve the salts. In a metal tower that is designed with seams that come down as a right-angle flange, those cleaners will penetrate the seams.
“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, working on the galvanizing and eroding it.”
Ruddy points out that many companies feel they cannot afford frequent cleanings of their cooling towers. Over time, a sizable amount of dirt may collect in the bottom of the tower sump.
“If you get a lot of calcium or other heavy mineral deposits in a plastic cooling tower, it won't cause any damage,” Ruddy says. “I have a customer in Bakersfield with extremely hard well water. But they have plastic cooling tower, and even though the mineral deposits get as big as the stalactites of Carlsbad Caverns, they don't do any damage. The plastic material of the tower is flexible enough that you can break off the deposits and toss them away.”