While once-through cooling was a common feature at many power plants in the last century, environmental regulations regarding intake and discharge issues have basically forced a transition to cooling towers, or in some cases air-cooled condensers, for new projects. Essential to steady heat transfer in cooling towers, and also their physical stability, is proper chemistry control. But even with diligent chemical feed and monitoring, cooling towers, and especially the tower fill, can accumulate scale and microbiological deposits that inhibit heat exchange, and, in worst case scenarios, may induce partial collapse of the tower. This article examines methods to clean tower fill before fouling causes irreversible damage.
Background
Cooling tower performance is highly dependent on the efficiency of contact between the hot return water from heat exchangers and the cool air being pulled or blown through the tower. Heat transfer is enhanced by the use of cooling tower fill, which, over the decades, has evolved into sophisticated designs to maximize air-water contact. An illustration of high efficiency PVC film fill is shown below.
Figure 1. High efficiency cross-fluted fill (Photo courtesy of Brentwood Industries) |
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The transition from early splash fill designs to modern high efficiency types reduced cooling tower capital and operating costs. However, the generally tortuous path that provides good contact between air and water also makes these fills highly prone to fouling. Advanced fouling results in a ~10x weight gain, leading to fill collapse into the sump and expensive fill replacement.
Proper Chemical Treatment
While this article focuses on methods to clean tower fill that has begun to accumulate deposits, it is paramount to understand that proper chemical treatment during normal operation is essential to prevent severe or sudden scaling and fouling problems. With regard to scale (and corrosion) control in cooling systems, the four-decade long methodology of phosphate/phosphonate treatment is giving way to polymer-based, non-phosphorus chemistry for two primary reasons. One is that phosphorus discharges to the environment are being increasingly regulated and restricted due to problems with toxic algae blooms that have afflicted numerous bodies of water. Secondly, the new polymer programs are proving to be more effective for scale and corrosion control in cooling systems. (Post, R., Kalakodimi, R., and B. Buecker, “An Evolution in Cooling Water Treatment”; PowerPlant Chemistry Journal).
The most serious issues in cooling systems are usually related to microbiological fouling. Thus, virtually all systems have as primary treatment some form of oxidizing biocide, most commonly bleach but also possibly gaseous chlorine, bleach/sodium bromide, chlorine dioxide, monochloramine, and monobromamine. A problem at many facilities, and this is particularly true in the power industry, is that the regulations developed for and by the United States Environmental Protection Agency (USEPA) allow no more than 0.2 ppm free available chlorine average residual for 2 hours per day as “Best Available Technology." For plants so constrained, treatment is only allowed for less than 9 percent of any day, thus giving microbes a chance to settle and begin forming protective slime layers.
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Options for dealing with fouled fill
Many facilities have suffered from fouled cellular plastic fill. Replacement of the fill in kind is a potential solution, but potentially sets up repeat situations. Another option is a switch to low-fouling fill designs that generally feature a more vertical flow pattern, less surface texturing, and sometimes wider spacing between the plates, all at the expense of some cooling efficiency. Since fill replacement can be expensive in terms of both materials and outage time, others have chosen to clean the fill chemically, or sometimes, mechanically. The choice of replacement vs. cleaning, as well as the cleaning methodology, requires careful consideration. The decision depends on the extent of the fouling, the physical and chemical nature of the foulant, the type of fill, and environmental considerations in dealing with cooling tower blowdown. For example, in heavily fouled film packs, some passages may be completely blocked, preventing the cleaning solution from flowing through, and perhaps acting as a filter for solids removed in other parts of the pack. The total mass of deposits, if released at once into the recirculating water flow will result in very high suspended solids, and blowdown may have to be diverted or treated prior to discharge. The type of foulant also varies considerably depending on the nature of the circulating water and the treatment chemistry employed. Over time, the fouling matrix behaves as a filter media, trapping additional suspended solids in the crevices of the fill pack and impeding air and water flow. At this point, the efficiency criteria that constituted the driving force for selecting the fill has become null.
Figure 3. Fouled film fill that is no longer effective.
The loss in cooling tower capability as a function of weight gain for a fill of offset flute design is trended in Figure 4.
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Figure 4. Tower capability loss vs. fill weight gain for a standard offset flute cellular plastic fill pack. (Monjoie, Michel, Noble, Russell, and Mirsky, Gary R., Research of Fouling Film Fill. Cooling Technology Institute, TP93-06, New Orleans, LA, 1993.) |
Over time, the high efficiency fill becomes increasingly less efficient, may gain as much as 10x its initial weight, begins to extrude around the supporting beams, and ultimately collapses into the sump. At the point where performance loss becomes obvious to operators or the fill begins to deform, it is too late to consider cleaning as an option; fill replacement is required. However, if the fouling is detected in its early and moderate stages, several cleaning options are available, depending on the nature of the foulant.
Cleaning Options For Cellular Plastic Fill
The most appropriate method for cleaning tower fill depends on several factors, including safety concerns, system metallurgy, in-service vs. out-of-service cleaning, potential impact on plant operations, disposal options for the cleaning solution, impact on the environment, and the chemical and physical nature of the foulant.
Mineral Scales
Hard mineral deposits most commonly consist of silica/silicates or calcium carbonate (calcite). Silica solubility is lowest at low temperature, and deposits often occur near the bottom of the counterflow fill pack where the temperature is lowest, the water is most concentrated, and uneven water/air distribution can lead to dry spots or locally concentrated areas. Calcite deposits often occur throughout the fill pack, but are generally heaviest toward the bottom. Higher temperature near the top of the fill pack has the lowest calcite solubility and promotes faster deposition kinetics. However, as the water passes through the fill, the minerals are concentrated slightly by evaporation, and the pH will rise slightly as excess CO2 is stripped.
One technique that can be used effectively on either type of hard scale in its early stages is to apply certain types of surfactants that penetrate the hard deposit and induce it to spall from the slightly flexible plastic substrate. The surfactant is typically applied in addition to the normal scale inhibitor program for an extended period of 60-180 days. This program is never 100% effective, but will often result in removal of 70-80% of the fouling minerals. Prior to implementing the cleaning process, it is imperative to identify and correct the scaling condition.
For large cooling systems, where the predominant scale deposit is calcite, the fill can be cleaned by reducing the operating pH and/or cycles of concentration until the water is undersaturated with respect to calcite at the fill conditions. Calcite often serves as the binder for the deposit matrix, so dissolving the calcium carbonate in the deposit matrix can be disproportionately effective. In principle, any degree of undersaturation will be effective over time. Sulfuric acid is an obvious choice for many plants that already use it for pH control, but very careful planning involving plant personnel, the chemical supplier, and any outside contractors is required before using such a hazardous chemical. Other plants may prefer to use safer, less corrosive acids such as organic acids or inhibited sulfamic acid. [4] Some organic acids are more effective than mineral acids at intermediate pH, and are synergistic with sulfuric acid. At pH 5, application of the appropriate organic acid will accelerate the rate of calcite dissolution by 10-20x as compared to sulfuric acid alone.
For predominantly light calcium carbonate scaling, off-line foam acid cleaning has been used very successfully, at least on smaller towers. Strong acid foam is applied by skilled specialists from the top of the fill pack. The nature of the foam allows the acid to contact the scale as it slowly passes downward through the fill. The relatively low volume of spent and mostly neutralized foam cleaning solution is either collected in the sump and disposed of, or is allowed to mix with other circulating water from neighboring tower cells that may be in service, depending on plant safety and environmental requirements.
Mineral scales can also be mechanically cleaned with some success in situ or ex situ. Due to its brittle nature relative to the flexible PVC, the scale can be dislodged with some success by mechanically cleaning the fill pack in-situ from below.
Microbiological/Organic Deposit Matrices
Deposits where microbiological growth or organics serve as the binder for the deposit matrix are characterized by a soft, sometimes putty-like consistency. Unlike mineral scales, deposits of microbiological origin tend to accumulate primarily in the middle of the fill pack. Water velocities directly under the spray nozzles are generally high enough to discourage microbiological adhesion. For this reason, microbiologically initiated fouling sometimes goes undetected because it is not visible on inspections from the top looking down beneath the spray headers. As the water velocity slows down several inches into the fill, microorganisms begin to colonize the surface, acting as a filter for suspended solids passing through the fill. Fouling tends to be more intense in the middle of the fill than at the bottom because suspended solids are filtered out prior to reaching the bottom layer, and because the last few inches of fill do not physically support a thick, soft deposit mass. The inability to clearly view microbiological fouling from either top or bottom, combined with the difficulty of inspecting the middle layers of fill, often allows this type of fouling to progress undetected until it has reached an advanced stage. Plant personnel have attempted to monitor fill fouling during tower operation using sections of fill suspended from load cells, or by cutting an access window into the end of the tower casing to allow a middle section to be removed periodically for inspection using a man lift, or by suspending a section of fill beneath the main fill pack to allow it to be easily inspected and weighed. All of these methods can work, but none have proven to be totally satisfactory.
Several effective methods exist to remove biological-silt matrix deposits from cooling tower fill. Hyperhalogenation is a widely attempted method, but its effectiveness is usually disappointing. Potential corrosion of system components and the need to dechlorinate prior to discharge are important considerations.
Microbiological matrices often have high water content and will shrink and detach from surfaces when thoroughly dried. However, effectively drying out cooling tower fill can prove problematic even with the help of fans, even if the tower is located in a low humidity climate. Chlorine dioxide has also been used as a cleaner for cooling tower biofilms with some success. However, the most widely practiced and effective cleaning method for deposits with microbiological or organic binders is hydrogen peroxide (H2O2) due to its oxidizing strength and the physical action of the oxygen micro-bubbles produced as the chemical reacts with organic deposits. The positive environmental profile of hydrogen peroxide involving rapid breakdown to water and oxygen, and its ease of application are additional factors favoring peroxide as a tower fill cleaner. Typical dosages are in the range of 500-3,000 ppm active H2O2. As with most cleaning operations, the addition of low levels of surfactants will help loosen deposits. Polymeric dispersants are generally added to assist in keeping the removed solids in suspension until they can be blown down.
Much of the biomass consists of extracellular and intracellular water and organics that will dissolve with peroxide cleaning. A substantial portion of the deposit typically contains much mud and silt that will be released into the water. Figures 5 and 6 illustrate the appearance of a slime-clay matrix on moderately fouled high efficiency cooling tower fill before and after cleaning.
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In cases where the deposit contains a high percentage of inorganics, the circulating water can be expected to become highly turbid. The potential for high suspended solids in the cooling tower blowdown should be anticipated when cleaning a severely fouled system and taken into account in the job planning scope.
Summary
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Every effort should be made to prevent deposition from occurring in the first place. A fill type should be specified that is compatible with reasonable expectations for the system, considering influent water quality, microbiological control, presence or absence of pre-treatment equipment, and the possibility for external foulants that might enter the tower through airborne contamination or process fluid leaks.
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The microbiological and deposit control program should be diligently monitored to ensure that it is within expectations and delivering the required results.
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The performance of any pretreatment and sidestream solids removal equipment should be reviewed to ensure such equipment is delivering and maintaining suspended solids within specifications.
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Plant personnel should be proactive with inspection and monitoring. There are more options, and less expensive ones, if the fouling is detected at an early stage.
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Periodic, light, preventative maintenance tower fill cleanings should be considered. Most high efficiency fills tend to gain weight slowly over time. Annual preventative maintenance cleanings can stabilize or reverse that trend.
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If cleaning is indicated, safety and environmental considerations must be adequately addressed. Cleanings require careful planning and coordination between plant personnel and the chemical supplier/consultant. Personnel safety is critical. Also, cleanings can release many suspended solids that if not carefully controlled can foul equipment or present disposal problems without proactive planning.
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If fouling occurs, all remediation options should be considered, but, generally, the least costly and least aggressive methods applicable to the nature and quantity of the deposit should be the starting point. Identifying and correcting the fouling conditions at an early stage is least expensive, and preferable to aggressive cleaning or ultimate fill replacement if the fouling conditions are allowed to persist.
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All systems are different, and careful consultation with the cleaning vendor and water treatment experts should be a priority.
