September 7, 2015
Evaporative cooling towers have long been popular as a cost-effective cooling technology for industrial processes. However, severe drought conditions throughout western parts of Canada and the United States is challenging conventional towers by bringing water conservation to the forefront. Current techniques used to reduce water requirements involve alkaline — high pH water-treatment chemistries that rapidly destroy galvanized metal cooling towers. So to engage in water conservation, facility engineers are faced with the prospect of replacing galvanized metal cooling towers at the accelerated rate of every five to eight years, on average.
This is opening the door for more applications for engineered plastic cooling towers. Available from 10 to 5,000 cooling tons, the engineered HDPE (high-density polyethylene) plastic involved is impervious to high (and low) pH water, as well as other chemicals that are introduced. Such units can withstand the rigors of decades of service in the harshest industrial or environmental conditions.
Cooling tower water usage
For each facility, a certain amount of water loss is expected due to the nature of the evaporative process that puts the “cooling” in cooling towers. Drift — which is water in the form of fine mist lost into the atmosphere — is also considered unavoidable. The water lost to both evaporation and drift must be replaced on an ongoing basis for the system to remain at full efficiency. Avoidable water loss, however, is another matter. Because the hard water used in cooling towers contains scale-forming minerals (calcium and magnesium salts), the evaporative process leaves these solids behind in the water in high concentrations. Left undiluted, these minerals cause scaling on equipment surfaces.
Even a small amount of scale in the system decreases the efficiency of heat transfer, resulting in decreased productivity in industrial processes. In severe cases, scale can completely plug heat exchangers and piping. To protect against this, some of the water is removed and replaced with fresh makeup water. The water drained from cooling equipment is called “blowdown” water or “bleed” water. The same terms apply to any unintentional water loss due to leaks and overflow.
Either way, this amounts to water loss that can be prevented or reduced. Therefore, water conservation efforts are primarily focused on achieving so-called “zero blowdown” to greatly reduce the amount of makeup water required. Achieving zero blowdown The primary method of reducing blowdown involves using chemical additives to impede scaling. These chemicals extend the solubility of the minerals so higher concentrations can exist in the water without causing scale or corrosion.
More advanced techniques include using treated “soft” water (no calcium or magnesium salts) through the cooling tower.
The result, on average is up to 40 per cent water-use reduction, says Timothy Keister, chief chemist and president of ProChemTech, a Pennsylvania based water treatment company that designs and installs complete cooling tower, wastewater and water treatment systems.
Yet soft water is still an alkaline chemistry with typical cooling pH levels ranging from 8.5 to 9.5. This exceeds the 8.2 level at which aggressive “white rust,” so-named due to its white color rather than the typical reddish brown, becomes a major corrosion issue for galvanized metals.
Engineered plastic towers
Due to the high pH of soft water, Keister recommends using engineered plastic cooling towers for new construction and replacement projects so as to completely avoid any white rust corrosion issues. In a recent application, Keister notes Pro-ChemTech was asked to redesign the furnace cooling system for Anchor Hocking Glassware. Cooling is critical for glass plants where furnaces are used to melt glass at 2,200 F.
Without water running through the system, the cooling jackets on the furnaces could build up steam and literally explode. In the new system, the plastic cooling tower discharged cold water into a cold well inside the plant. Cool water was then pumped through a stainless-steel plate and frame heat exchanger before it was returned to the tower. Furnace cooling water was recirculated by a closed-loop system, which was cooled by the heat exchanger, retaining the closed-loop design provided by the fluid cooler.
According to Keister, the galvanized metal tower that was previously in place was so thoroughly corroded that when it was removed by crane, the bottom half broke off.
Jeff Elliott is a technical writer who has researched and written about industrial technologies and issues for the past 20 years.