Cooling towers play a critical role in allowing facilities to reject heat from industrial processes. They are used across many industries, from commercial buildings to manufacturing plants, to provide efficient cooling that keeps equipment and processes running optimally.

Proper water treatment is essential for maintaining cooling tower efficiency and longevity. As water evaporates from the tower, dissolved minerals and other contaminants will concentrate and can lead to major problems like scaling, corrosion, and biological growth if left unchecked. Chemical treatment helps control water chemistry within safe ranges and prevents these issues from negatively impacting industrial operations.

With the proper chemical treatment program customized to the particular facility, cooling towers can operate for decades without significant issues. However, without treatment, towers can quickly develop problems such as scaling, corrosion, and microbiological buildup, leading to inefficient cooling, unplanned downtime, and costly equipment damage. By maintaining proper water chemistry, chemical treatment helps cooling towers achieve their full operational lifetime.

Common Water Chemistry Problems

Some of the most prevalent water chemistry problems in cooling towers are mineral scaling, especially from elevated calcium carbonate and silica levels in feedwater. We’ve all seen scale on our household faucets—that hard white crust that builds up over time – which is the same scale formation that can occur in cooling towers.

“90% of the time, that white scale is calcium carbonate. We’ve all seen it or run into it before. It gets in our showerheads, it gets in our sink faucets and, if facilities aren’t careful, it can get into their cooling towers”, according to Ryan Vargas, a water treatment expert at EAI Water.

This white scale forms when calcium and carbonate in the water exceed their solubility limits as water evaporates and they drop out of the solution.

In cooling towers, this concentrating effect is even more pronounced due to the high rates of evaporation. As cycles of concentration increase, calcium carbonate can rapidly scale on heat transfer surfaces, fill, and equipment. This insulating layer reduces heat transfer efficiency, necessitating higher water flow rates through the tower. Scale can also plug small-diameter equipment like heat exchangers, valves, and orifices, leading to operational issues and downtime.

Silica scale poses an even greater challenge due to its very low solubility limits. This mineral readily combines with calcium and magnesium to form an extremely challenging scale requiring harsh acids or mechanical scrubbing for removal. Preventing silica scale thus requires limiting silica concentrations through bleed management or pretreatment.

Left unchecked, mineral scaling significantly impacts tower efficiency by reducing heat transfer rates. Scale also gradually reduces the service life of towers and equipment due to loss of material integrity and fouling. Proper chemical treatment helps prevent scale altogether or limits it to manageable levels.

Cooling Tower Chemistry

The two main classes of chemicals used to control biology in cooling towers are oxidizing and non-oxidizing biocides.

Oxidizing biocides work similarly to household bleach—they rupture the cell walls of microorganisms through an oxidation reaction. Just like periodically adding bleach to your swimming pool, oxidizers need to be fed regularly to maintain effectiveness. Though they work quickly, they also degrade quickly.

“An oxidizing biocide is like a lance – it punches a hole through a microbiological cell to destroy it’s hold within the system”, says Vargas.

Non-oxidizing biocides work more subtly, like a Trojan horse. They penetrate inside microbes and disrupt enzymatic or metabolic activity, taking longer to work but persisting much longer in the system.

There are several factors that influence choosing an appropriate biocide program:

• System size & water volume: The required biocide dose, and thus program cost, may favor one option over another. Smaller systems often use bromine or isocyanuric acid stabilized oxidizers to avoid degradation.
• Existing microbiological issues: More resistant organisms may dictate a more potent biocide like hypobromous acid.
• Usage patterns and shutdown duration: Intermittent operation may dictate a more persistent non-oxidizing biocide.
• Feed capabilities: Available equipment and chemical compatibility affect options.

While most biocide chemistries have their appropriate applications, blending both oxidizing and non-oxidizing biocides provides the broadest control. An experienced water treatment specialist will make product recommendations based on a facility’s specific conditions and needs.

Chemical Selection Considerations

Storage and transportation requirements can dictate products for sites with space or access limitations. For example, a high-rise building without a service elevator may opt for 5 gallon pails of stabilized bromine biocide rather than 55 gallon drums of sodium hypochlorite. Cooling towers in remote locations often use solid or gel products, of which EAI can support with our own proprietary solutions, to minimize transport costs and avoid degradation during shipping.

Site-specific operating conditions also impact chemical selection. An intermittent process that cycles on and off may need more persistent chemistries to sustain treatment residuals during downtime. Sites in seasonal climates often run into issues with the degradation of oxidizing biocides when shut down for weeks or months. Facilities using high silica feedwater may incorporate special corrosion inhibitors or dispersants to control deposition.

Some examples of tailored chemical selection include:

• Small office building, difficult access: Solid bromine tablets + isothiazolinone packets
• Hospital, Legionella concerns: Bleach + supplemental bromine generator
• Seasonal industry, long shutdowns: Stabilized bromine oxidizer + isothiazolinone
• High mineral water: Phosphonate scale + corrosion inhibitors

There is no “one size fits all” approach to cooling water treatment. An experienced provider gathers detailed site information upfront to customize a program addressing the unique requirements of each facility.

Pretreatment for Cooling Towers

Several pretreatment processes can help condition cooling tower feedwater for easier and more efficient chemical treatment.

Water softening using ion exchange resin removes hardness ions like calcium and magnesium to prevent scale formation. This allows operating at higher cycles without exceeding the solubility limits of these minerals. However, softening alone may not adequately address other contaminants.

Reverse osmosis (RO), electrodeionization (EDI), and nanofitration membranes can achieve near-complete removal of dissolved minerals from the toughest feedwaters. They produce very high purity water to support essentially unlimited concentration ratios. However, the tradeoff is much higher equipment and operation costs. As such they are typically reserved for the most challenging or critical applications where no other options exist.

Choosing an appropriate pretreatment system depends heavily on the source water quality and variability, the application’s performance goals, and budget constraints. For example:

• Surface water sources often require only suspended solids removal to serve most cooling towers
• Groundwater may need softening, organics removal, and disinfection
• Industrial process reuse water could need custom treatment for specific contaminants

Water demands can change over a facility’s operating life, and building in provisions to add additional pretreatment in the future is recommended.

Water Testing and Monitoring

Regular testing and monitoring of cooling tower water is crucial to catch potential issues before they escalate. While automated systems have become more prevalent, manual testing provides an important verification of proper function.

Modern controller systems continuously measure conductivity, pH, and other parameters to adjust bleed and chemical feed accordingly. Data logs make this information accessible to offsite specialists for performance review. Controllers also regulate disinfection cycles, timers, and interlocks among various treatment components.

However, instrumentation requires periodic calibration and maintenance, which can drift out of tolerance over time if unchecked. Best practice recommends the following regimen:

• Automated sensors/probes should be inspected and calibrated vs. handheld meters on a weekly or monthly basis. This ensures automated systems are taking proper action.
• A monthly manual water test by the site or water treatment provider validates automated readings and gives a snapshot of overall water quality.
• Quarterly or semiannual comprehensive site reviews identify potential issues through trend analysis before they escalate.

While advanced automation makes 24/7 monitoring possible, a set of human eyes assessing the physical water system on a regular basis provides an invaluable safeguard to confirm proper automated operation. The optimal approach combines automation with routine manual verification and oversight.

With water, energy, and environmental considerations at the forefront, chemical treatment has become even more critical for asset preservation and sustainability. New chemistries and advanced automation provide the tools to maximize cycles of concentration and conserve water. If you have an existing cooling system battling water chemistry issues or are looking to properly treat a new installation, please reach out today. We welcome the opportunity to put our solutions to work for you.