Cooling Tower Heat Transfer 201

By Brad Buecker, Contributing Editor, and Rich Aull, Brentwood Industries

Part 1 of this series examined the fundamentals behind heat transfer in cooling towers.1 A major point made then was that much heat transfer, perhaps 75 percent or more, occurs due to evaporation of a small amount of cooling water, while the remainder comes from convective heat exchange. Heat transfer is highly dependent upon the efficiency of contact between air and water flowing through a tower. Cooling tower fill design is critical to air-water mixing and a number of styles are available for specific conditions, including the fouling/scaling tendencies of the cooling water. This article examines several aspects of cooling tower fill.

A fundamental principle of thermodynamic cycles is that overall cycle efficiency (η) is a function of the following equation, as exemplified by the Carnot cycle.

η = 1 — T_L/T_H

In a steam generator, T_H represents the boiler, superheater and reheater, while T_L is the condenser. Numerous articles have appeared in Power Engineering and other publications over the years about the importance of condenser tube cleanliness, and rightly so, as fouled condenser tubes seriously restrict heat transfer. However, efficient removal of heat from the warm circulating water in the cooling tower is also important. Poor fill selection or design or poor cooling water treatment can lead to significant cost escalations.

The type of fill used in early cooling towers was splash fill, where the circulating water cascades down and onto wooden slats within the tower. Splash fill technology has considerably improved and a modern design is shown in Figure 1.

Figure 1 A modern splash fill arrangement.

The impingement process breaks up water droplets to increase the water surface area. The slats are arranged in a staggered formation to maximize air-water contact. Splash fill is commonly used in crossflow cooling towers where the air travels perpendicularly to the water spray. Splash fill may be the design of choice in cooling towers where the water has a high fouling tendency. In the majority of towers, however, film fill is the preferred material.

Film fill induces the cooling water to form a film on the material surface. The filming mechanism maximizes liquid surface area. One might be tempted to think that film fill is generic in nature and that any type can be installed in a tower. Such is not the case, however. A guiding principle behind fill design and selection is “to increase air-to-water contact, driving up convection and evaporative cooling while reducing pressure drop in the system.” (2) Typical fills are made of PVC because of its low cost, durability, good wetting characteristics and inherently low flame spread rate. Factors that influence fill performance (neglecting for the moment fouling or scaling within the fill) include flow path arrangement and flute size (namely, the size of the openings in the fill). Five major fill configurations are illustrated in Figure 2.

As is evident in the first two designs, water flow is dispersed laterally upon entering the fill, thus providing a large surface area for interaction with air. Historically, cross-fluted fill was used in either crossflow or counterflow applications. Since the advent of the XF standoff design, which can withstand greater water-loadings typically found in today’s crossflow film filled towers, cross-fluted fills are now used primarily for counterflow applications. Vertical fills are only suitable in counterflow towers, where air flows parallel to the cooling water.

Another factor that influences heat transfer within the fill is surface profile, known as microstructure. The surface structure can be modified to enhance air-water mixing.

A recent development is the introduction of hybrid/trickle (H/T) fills, which combine the heat transfer modes of droplet and film cooling. Their advantage is the ability to withstand high suspended solid loads and provide good thermal performance.

While the cross-fluted fills provide maximum surface area for heat transfer, they also impart a higher pressure drop through the tower than other designs. Also, low-flow regions can develop in cross-fluted fills, which increases the fouling potential. For this reason, a careful evaluation of water conditions is necessary to select the proper design. Table 1 outlines guidelines for fill selection with various quality waters. Note that the table also includes flute size. As is evident, vertical-flow designs are recommended for waters with a high fouling potential, because they are less restrictive to flow.

Preventing Microbiological Fouling

Regardless of fill design, proper chemistry control is necessary to prevent microbiological fouling. Cooling systems provide an ideal environment–warm and wet–for microbes. Bacteria will grow in fill and condenser tubes, fungi on and in cooling tower wood and algae on wetted cooling tower components exposed to sunlight.

Bacteria are separated into the following three categories:

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

A problem with microbes, particularly many bacteria, is that once they settle on a surface the organisms secrete a polysaccharide film for protection. This coating then collects silt from the water, growing thicker and further reducing heat transfer. Even though the bacteria at the surface may be aerobic, the secretion layer allows anaerobic bacteria underneath to flourish. Beyond fill fouling, this situation is critical for metal components, such as condenser tubes, where the anaerobic microbes generate acids and other harmful compounds that directly attack the metal. Microbial deposits also establish oxygen concentration cells, in which the lack of oxygen underneath the deposit causes the fouled locations to become anodic to other areas of exposed metal. Pitting is often a result.

The core of any microbiological treatment program is feed of an oxidizing biocide to kill organisms before they can settle on condenser tube walls, cooling tower fill and other locations. Chlorine was the workhorse for many years. When gaseous chlorine is added to water the following reaction occurs.

Cl₂ + H₂O ⇌ HOCl + HCl

HOCl, hypochlorous acid, is the killing agent. The functionality and efficacy of this compound are greatly affected by pH due to the equilibrium nature of HOCl in water.

HOCl ⇌ H⁺ + OCl^–

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. Thus, for alkaline scale/corrosion treatment programs used within many cooling towers, chlorine chemistry may not be efficient. Chlorine demand is further affected by ammonia or amines in the water, which react irreversibly to form the much less potent chloramines.

Due to safety concerns, liquid bleach (NaOCl) feed has replaced gaseous chlorine at many facilities. The major difficulty with bleach is that the product contains small amounts of sodium hydroxide; thus, when bleach is injected into the cooling water stream it raises the pH, if only by a small amount.

Figure 3 MICROSTRUCTURE’S PERFORMANCE IMPROVEMENT

A popular alternative is bromine chemistry, where a chlorine oxidizer and a bromide salt, typically sodium bromide (NaBr), are blended in a makeup water stream and injected into the cooling water. The chemistry produces hypobromous acid (HOBr), which has similar killing powers to HOCl, but functions more effectively at alkaline pH. In part, this is due to the fact that HOBr does not dissociate as readily at alkaline pH as its chlorine counterpart.

Another factor in favor of bromine is that it does not react irreversibly with ammonia or amines. The primary disadvantages of bromine are that an extra chemical is needed and feed systems are a bit more complex than for bleach alone.

Chlorine dioxide (ClO₂) has found some application as an oxidant for two primary reasons. Its killing power is not affected by pH, and it does not form halogenated organic compounds. Also, chlorine dioxide is more effective in attacking established bio-deposits. However, ClO₂ is unstable and must be generated on-site, where a common method is reaction of sodium chlorite (NaClO₂) and chlorine in a slipstream fed to the cooling water.

2NaClO₂ + Cl₂ –> 2ClO₂ + 2NaCl

Costs are usually several times higher than for straight halogen treatment.

Other oxidants that have been tested for cooling water include hydrogen peroxide (H₂O₂) and ozone (O₃), but the short lifespan and tendency of these chemicals to escape from solution in the cooling tower typically make them ineffective in large cooling water systems.

A method to help control microbes is a supplemental feed of a non-oxidizing biocide. Typically, feed is needed on a temporary but regular basis, perhaps once per week. Table 2 outlines some of the properties of the most common non-oxidizers.

Careful evaluation of the microbial species in the cooling water is necessary to determine the most effective biocides. None of these chemicals should be used or even tested without approval from the appropriate regulating agency. They must fit in with the plant’s National Pollutant Discharge Elimination System (NPDES) guidelines.

As with all chemicals, safety is an absolutely critical issue when handling the non-oxidizers. Adherence to all handling guidelines and use of proper personal protective equipment is a must. Many of these chemicals will attack human cells as well as those of microbes.

References

1 B. Buecker, “Cooling Tower Heat Transfer 101”; Power Engineering, July 2010.

2 Wallis, J. and R. Aull, “Improving Cooling Tower Performance with Thermal Fills”; Process Cooling & Equipment, March/April 2000.

Authors: Brad Buecker is a contributing editor for Power Engineering and also serves as a Process Specialist for Kiewit Power Engineers of Lenexa, Kan. Buecker has nearly 30 years of experience in or affiliated with the power industry, much of it in chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Ill.) and Kansas City Power & Light Co.’s La Cygne, Kansas station. He has an A.A. in pre-engineering from Springfield College in Illinois and a B.S. in chemistry from Iowa State University. He has written many articles and three books for PennWell on steam generation topics. Buecker is a member of the ACS, AIChE, NACE and ASME.

Rich Aull is Engineering Manager with the Cooling Products Group within Brentwood Industries’ Water Technology Division. For the last 17 years he has been responsible for product design, testing and application engineering. He has over 30 years of experience working in the cooling tower industry with thermal engineering experience gained at the Hamon Cooling Tower Division of Research Cottrell and with project engineering experience at Ecodyne Cooling Tower Services. He is active within the Cooling Technology Institute (CTI) and is currently serving as vice-chair of CTI’s Performance and Technology Committee. He has a M.S.M.E. from the New Jersey Institute of Technology. He has published and presented many papers on cooling tower related topics for EPRI, CTI and ASME.

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