Condenser and auxiliary heat exchanger tube and cooling tower film fill fouling create difficulties that can cost a plant hundreds to thousands of dollars per day in lost efficiency. In extreme cases, solids deposition within cooling tower film fill has caused structural failures of the cooling tower, requiring unit shutdown. Repairs might take weeks or longer, which can result in excessive monetary hardship due to lost production and equipment repair costs.

Cooling water fouling mechanisms
Fouling and scaling are the two primary mechanisms of deposition in cooling systems. Fouling in particular can be very troublesome and dangerous. Suspended solids that enter cooling systems might settle in the cooling tower basin, at low-flow spots within the system, on condenser tubes and in the tower fill. Solids are introduced with the makeup, particularly if the supply is surface water, but cooling towers also are very efficient air scrubbers. Suspended solids will naturally cycle up in the system, and without a control mechanism can reach overwhelming concentrations.
Economic value of cooling tower biocide treatment and sidestream filtration

Particularly troublesome is microbiological fouling. Cooling systems provide an ideal warm and wet environment for microbial proliferation. Biocide treatment is absolutely essential to maintain cooling system performance and integrity.

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, i.e., 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 layer for protection. This film will collect silt from the water and grow even thicker, further reducing heat transfer. But, this is just part of the problem. Even though the bacteria at the surface might be aerobic, the secretion layer allows anaerobic bacteria underneath to flourish. These organisms generate acids and other harmful compounds that directly attack the metal. Microbial deposits also establish concentration cells, where the lack of oxygen underneath the deposit causes the locations to become anodic to other areas of exposed metal.

Metal loss occurs at anodes, with pitting as the result.

Fouling also will cause significant – and at times devastating – buildups in cooling tower film fill.

Film fill is common in modern cooling towers, as the large surface area provided by the packing enhances contact between air flowing up through the fill with the cooling water traveling down.

Fouling disrupts the water-air flow patterns, and in severe cases completely plugs passageways. In some severe cases, the buildup of materials in the fill has even caused structural failure of internal tower components or complete tower sections.

Fungi will attack cooling tower wood in an irreversible manner, which can eventually lead to structural failure. Algae will foul cooling tower spray decks, leading to reduced performance and unsafe working locations.

Monetary costs of fouling
In previous work, I examined the costs of degraded condenser performance due to fouling, scaling or excess air in-leakage, all of which can be equally troublesome. Poor condenser heat transfer during the summer months at a large plant might result in increased fuel costs of 6 figures or more. [2] Consider a top-flight expert’s evaluation, in which he examines fouling of condenser tubes that increase the condenser backpressure from 1? to 2? Hg. This loss of efficiency can “increase the heat rate by as much as 200 Btu/kWh. This can increase fuel costs significantly. For example, in a 500 MW unit, this increase in heat rate will increase fuel costs by $4,800 per day if the [base] fuel cost is $2 per MMBtu.” [3] Costs will be even higher if fouling forces a unit derating.

Similar efficiency losses also might occur with deposition in cooling tower film fill. And if the fouling causes structural failure, cost increases that might have been “only” 6 or 7 figures due to long-term reduced heat transfer now escalate dramatically from lost power production and cooling tower repair.

Fouling control
Proper microbiological treatment is critical to prevent cooling system fouling, where sidestream filtration can be a very cost-effective supplement. The core of any microbiological treatment program is to feed an oxidizing biocide to kill organisms before they can settle on condenser tubes, cooling tower fill and other locations. Chlorine was the workhorse for many years, where when gaseous chlorine is added to water, the following reaction occurs (Equation 1).

HOCl, hypochlorous acid, is the killing agent, and acts by penetrating cell walls and then oxidizing internal cell components. The functionality and killing power of this compound 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 effectively penetrate cell walls. The killing efficiency of chlorine dramatically declines as the pH goes above 7.5. Because most cooling tower scale/corrosion treatment programs operate at an alkaline pH, chlorine chemistry might 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, although bleach is more expensive than gaseous chlorine. Bleach contains a small amount of sodium hydroxide, so when it is injected into the cooling water stream it raises the pH, if only slightly. However, if the water is alkaline to begin with, most of the reactant will exist as the OCl- ion, whose killing power is reduced. A popular alternative is bromine chemistry, where a chlorine oxidizer (bleach is a common choice) and 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.

At a pH of 8, only 20 percent HOCl remains in solution, but almost 90 percent of HOBr is present.

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

Proper microbiological control still does not prevent the intrusion of suspended solids into the cooling system via the makeup water and by the air, which flows through the tower. A standard technique for removing particulates without the expensive method of chemical sequestration is sidestream filtration. As its name implies, the process involves extraction of a slipstream from the cooling water, often from the warm water return to the tower, mechanical filtration of this slipstream, and then return of the filtered water to the cooling system. A common rule-of-thumb suggests a filter size capable of handling 1-5 percent of the recirculation flow rate, such that the filter can treat the entire cooling water volume in 24 hours. Not only does sidestream filtration reduce the potential for settling of particulates in condenser tubes and low-flow locations, but filters remove microbes and particulates that would otherwise consume some of the polymers utilized for scale control. For a large system, the cost for scale control chemicals could potentially reach $1-$2 million dollars annually. Some percentage of these chemicals will be consumed by reaction with suspended solids. At even 10 percent consumption, the added chemical costs range from $100,000-$200,000.

In the past, multimedia filters – i.e., anthracite/sand – were the common choice for sidestream filtration. But these filters and others, such as screen filters, will only remove particles down to perhaps 15-20 microns in size. [4] Many particles are much smaller than this, so coarse filtration might be ineffective. Sidestream filtration has evolved to address this issue with developments such as rapid sand filters and disk filters. And now microfiltration is beginning to be noticed for sidestream treatment. Microfilters will remove particulates as small as 0.1 micron in size, and can extract even colloidal silica from cooling water. Microfiltration has been successfully utilized on waters with turbidity that exceeds 2,000 nephelometric turbidity units (NTU). And with a properly designed system reliability can be outstanding, with membranes that last for years with minimal or no fiber breakage. [5]

A new development
At power plants with cooling towers, the tower blowdown represents a significant wastewater discharge. Increasingly, plants are

facing water discharge reduction issues, with the ultimate scenario being zero liquid discharge (ZLD). A scheme that is gaining attention for reuse of cooling tower blowdown is microfiltration of the blowdown to remove suspended solids, followed by reverse osmosis (RO) treatment to convert three-fourths or so of the liquid to a product suitable for introduction to the makeup water stream. Cooling tower blowdown rates vary significantly from plant to plant, but consider a scenario with a moderately-sized cooling tower – recirculating rate of 100,000 gpm – in which the cycles of concentration produce an average blowdown requirement of 300 gpm. With 75 percent recovery of the blowdown, MF-RO treatment produces 225 gpm of permeate that is suitable for reuse as makeup. This is production not required of the makeup pre-treatment system. More importantly, a 300 gpm waste stream is reduced to 75 gpm.

Energy Tech Magazine: Economic Value Of cooling tower biocide treatment and sidestream filtration “Brad Buecker”, http://www.energy-tech.com/article.cfm?id=28163 (accessed June 7, 2010)