Plastic Cooling Towers Play Critical Role in Industrial Water Conservation
Given galvanized metal cooling towers’ vulnerability to corrosion — the risk of which is exacerbated by certain water-treatment techniques — noncorrosive plastic cooling towers are becoming more attractive.
Evaporative cooling towers are popular because they provide cost-effective cooling technology for industrial processes. With severe drought conditions throughout the western United States, however, the issue of reducing water usage by cooling towers (by what could amount to millions of gallons per year for a single unit) is now at the forefront.
While effective at reducing overall water consumption by the tower, the techniques used to reduce water requirements involve alkaline, high pH water-treatment chemistries that rapidly destroy galvanized metal cooling towers. Meeting the goals of water conservation can force facility engineers to replace galvanized metal cooling towers at an accelerated rate — every five to eight years, on average.
To address this accelerated equipment replacement rate, some facilities have switched from galvanized units to engineered plastic cooling towers. Capable of handling processes requiring 10 to 5,000 cooling tons, the engineered cooling towers are constructed of high density polyethylene (HDPE). Such plastic is impervious to high and low pH water as well as water treatment chemicals. Such units can provide decades of service in harsh industrial conditions.
Industrial Cooling Tower Water Usage
For each facility, a certain amount of water loss is expected due to the nature of the evaporative process — that which puts the “cooling” in cooling towers. Drift — water in the form of fine mist lost into the atmosphere — also cannot be completely avoided either. The water lost to 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 each cycle and replaced with fresh makeup water. (The water drained from the cooling equipment, and any other unintentional water losses due to leaks, overflow, etc., is called blowdown water or bleed water.)
Particularly in water-scarce regions, blowdown and bleed water represent water losses that potentially can be prevented or, at minimum, greatly reduced. Therefore, water conservation efforts are focused primarily on achieving 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. One such solution is the SofTek technology from ProChemTech International Inc., Brockaway, N.J., a water treatment company that designs and installs cooling tower, wastewater and water-treatment systems. The company also offers several patented water treatment chemistries to reduce water requirements. Its soft water chemistries reduce blowdown, going to zero blowdown if appropriate.
“SofTek technology typically obtains a 33 to 40 percent water use reduction,” says Timothy Keister, chief chemist and president of ProChemTech.
Using 100 percent softened water, SofTek is an alkaline chemistry with 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. Although ProChemTech manages that issue with its white rust inhibitor, the corrosive nature of the water can still wreak havoc on galvanized metal towers.
Due to the high pH of its water-management programs, ProChemTech recommends and installs engineered plastic cooling towers for new construction and replacement projects. Using engineered plastic avoids any white rust corrosion issues.
Although not exclusively aligned with any specific company, the company has installed more than six HDPE plastic towers from Delta Cooling Towers, Rockaway, N.J., an experienced manufacturer of high density, polyethylene cooling towers.
Furnace Cooling System for Glass Plant
In a recent application, ProChemTech 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 2200°F (1204°C). Without water running through the system, the cooling jackets on the furnaces could build up steam and explode.
A few years ago, ProChemTech installed a tower from Delta Cooling at Anchor Hocking Glassware’s Monaca, Pa.-based plant to use with its SofTek chemistry. The water treatment and cooling technology performed well at the Monaca plant. So well, in fact, that earlier this year, when a fluid cooler at Anchor Hocking Glassware’s Lancaster, Ohio-based plant became so corroded that it needed to be replaced, Anchor Hocking Glassware specifically requested another tower from Delta Cooling.
In the new system, the plastic cooling tower discharges cold water into a cold well inside the plant. Cool water then is pumped through a stainless steel plate-and-frame heat exchanger before it is returned to the tower. Furnace cooling water is recirculated by a closed-loop system that is cooled by the heat exchanger, retaining the closed-loop design provided by the fluid cooler.
Anchor Hocking Glassware also asked ProChemTech to design extra capacity in the tower system to accommodate a second heat exchanger used to cool a water-treatment and recycle system.
According to Keister, the galvanized metal tower at the Lancaster plant was so thoroughly corroded that when it was removed by crane, the bottom half broke off and landed on the support structure. Luckily, there was no significant damage. Keister notes that inherent design advantages of the latest engineered plastic cooling towers ease installation, particularly on rooftops. The lightweight plastic shell weighs as much as 40 percent less than a steel tower.
“Delta Cooling Towers are considerably lighter than steel. That means less money is required for support beams. This can cost a considerable amount of money,” Keister says.
Systematic Issues Impede Water Conservation Efforts
Although using sophisticated water chemistries and engineered plastic cooling towers combine to serve as an effective solution to water conservation, Keister says two systemic problems often stand in the way of specifying such solutions.
“The first problem is that all government bids for water treatment have to go to the low bid,” says Keister. “There is no consideration of water conservation in government bidding practices.” In an effort to increase awareness of this issue, he has already written letters to both Gov. Brown of California and Gov. Ducey of Arizona. Brown recently called for a statewide reduction of water usage by 25 percent.
“The other systemic problem is that the major building-management firms are also interested primarily in the lowest priced bid, because they have to pay for it, while their tenants pay for the water used,” Keister says. The better solution, he adds, would be to establish water conservation requirements and award jobs after giving due consideration to the importance of water conservation (not just price).
Given the ubiquitous nature of cooling towers throughout industry, it would seem that saving millions of gallons of blowdown water per unit would be at the top of the agenda.