In Part 1 and Part 2 of this series, we examined fundamental heat transfer concepts in cooling towers. A key component is cooling tower fill, which enhances the contact between incoming air and warm water returning to the tower from the plant heat exchangers.

It is important to protect the fill and cooling tower support structures from the three bad actors that can cause severe materials damage and loss of heat transfer: fouling, scaling and corrosion. Compounding these issues are increasingly stringent wastewater regulations that are forcing plant personnel to consider alternatives to traditional cooling water treatment. In addition, by choice or often mandate, many plant designers are selecting water supplies such as treated municipal wastewater for makeup water. These supplies often contain unwelcome contaminants, including the biological nutrients ammonia, phosphorus, organics and suspended solids.

Control Those Microbes

Microbiological growth can cause the most prompt, intense fouling in cooling systems as the warm and wet environment is ideal for “bug” growth. The organisms that affect cooling towers and cooling systems are algae, fungi and bacteria. Algae will foul cooling tower spray decks and other areas exposed to sunlight. Growth of these photosynthetic organisms can become quite severe and generate large masses of slippery, plant-like material.

Extreme algae growth in a cooling tower. Image source: Reference 1.Fungi will attack cooling tower wood in an irreversible manner, which can eventually lead to structural failure. Mist-like conditions within the tower are excellent locations for fungi growth.

Bacteria will thrive in many locations, as evidenced by the three primary varieties:

• Aerobic: Utilize oxygen in their metabolic process.
• Anaerobic: Live in oxygen-free environments, and use other sources (for example, sulfates and nitrates) for their energy supply.
• Facultative: Can live in aerobic or anaerobic environments.

Thus, bacteria can basically live almost anywhere throughout the cooling system and, in addition to difficulties they may cause in cooling towers, may also foul heat exchangers. A particular problem with bacteria is that once they settle on a surface the organisms secrete a polysaccharide layer (slime) for protection. These sessile microbes and their accompanying slime layer will accumulate silt from the water allowing them to grow ever thicker.

Even though the bacteria in the bulk water (planktonic organisms) may be aerobic, in the sessile deposits anaerobic bacteria will proliferate. These bugs can generate acids and other harmful compounds that directly damage heat exchanger metals. The deposits also establish oxygen concentration cells, where the lack of oxygen underneath deposit causes the locations to become anodic to other areas of exposed metal. Pitting is often the result, and within the cooling tower, microbiological deposits may completely close fill openings. Loss of heat transfer is the short-term result, with partial cooling tower collapse also not uncommon.

More troubling still, legionella outbreaks have sickened and killed a number of people. These bacteria grow and thrive in cooling tower environments and other water systems.

Thus, microbiological control is of utmost importance. For many years, chlorine was the workhorse. When gaseous chlorine is added to water, the following reaction occurs:

HOCl, hypochlorous acid, is the killing agent. Safety concerns led to movement away from gaseous chlorine to sodium hypochlorite (NaOCl), which is more commonly known as bleach. The functionality and killing power of hypochlorous acid are greatly affected by pH due to the equilibrium nature of HOCl in water:

OCl - is a much weaker biocide than HOCl, probably due to the fact that the charge on the OCl^- ion does not allow it to penetrate cell walls. The killing efficiency of chlorine declines as the pH goes above 7.5. Chlorine demand is further affected by ammonia or amines in the water, which react to form much less potent chloramines. Chlorine reacts with organics to form halogenated organics, whose concentration may also be regulated for health reasons as these compounds have carcinogenic properties.

Due to these factors, alternative treatments may be needed. One popular program has been chlorine-activated bromine treatment, in which bleach and sodium bromide (NaBr) are blended in a water slipstream and then injected into the cooling water. The reaction produces hypobromous acid (HOBr, the bromine equivalent of HOCl). It does not react irreversibly with nitrogen as chlorine does, and is more effective at higher pH.

Another treatment possibility is chlorine dioxide (ClO₂), which must be generated on site. Production methods have been greatly improved from the former sodium chlorite (NaClO₂)-chlorine reaction, in which large quantities of hazardous sodium chlorite had to be stored on site. Newer processes use a compact generator that combines sodium chlorate (NaClO₃) with a pre-mixed blend of sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂) to induce the following reaction.

Chlorine dioxide is a strong oxidizer that does not react with ammonia nor does it react with organics to form halogenated organic compounds. Unlike bleach, chlorine dioxide is not affected by pH. This can be an important advantage in the majority of towers whose chemistry programs operate in an alkaline pH range.

Some plant personnel have had good success with on-line hypochlorite generation. An advantage of this technology is that the oxidant is produced on an as-needed basis rather than being stored in large tanks where it can degrade and lose strength.

In some cases, monochloramine (NH₂Cl) has proven to be an effective biocide. With controlled production, this chemical, although not nearly as powerful as free chlorine, can penetrate slime layers to kill underlying organisms.

Scaling and Corrosion Control

The most common cooling tower treatment program over the past three decades has been based on inorganic and organic phosphate chemistry for both scale and corrosion protection. These programs are typically supplemented with a polymer for calcium phosphate scale control and perhaps a zinc residual for additional corrosion protection. The chemistry was designed to minimize calcium carbonate (CaCO₃) scaling, with the alkaline pH generated by the phosphates and phosphonates greatly reducing corrosion potential. However, many facilities are now being faced with limits on phosphorus discharge. This is driving strident development of non-phosphorus polymer replacements.

The polymers serve as crystal modifiers and sequestering agents to inhibit scale formation. There is also evidence that the polymers form a thin coating on metal surfaces to inhibit corrosion. A common dosage concentration is 2-10 ppm active in the cooling water. In some cases, an all-P program may be less expensive than an equivalent phosphate/phosphonate program. [Ref. 1]

Another technology that may be recommended for cooling towers is sidestream filtration. The author often sees specifications that call for makeup water filtration only. Although makeup filtration may be a reasonable idea, developers and owner’s engineers often do not recognize the fact that cooling towers are superb air scrubbers, and that many particulates are introduced to cooling water by the scrubbing action.

Makeup filtration does nothing to control these particulates, whereas sidestream filtration can be of great benefit. Commonly, a sidestream filter will process a 3-10% volume of the circulating water flow, with filtered water recycle to the cooling tower basin. A variety of technologies is available for sidestream filtration, ranging from conventional multi-media filters to metallic-screen filters with automatic backwash to even microfiltration.

References

1. Post, R., and B. Buecker, “Power Plant Cooling Water Fundamentals”; pre-conference seminar to the 33^rd Annual Electric Utility Chemistry Workshop, June 11-13, 2013, Champaign, Ill.