Cooling towers are an integral part of several commercial buildings and industrial processes, such as power generation and manufacturing. They serve as heat rejection devices for cooling water, thereby maintaining optimal operating conditions for equipment and buildings. Because the cooling tower is not a closed system, the water interacts with the external environment and requires treatment to maintain water quality. In this article, we discuss the importance of assessing cooling tower water quality, methods of testing, and common treatment processes.
Importance of Assessing Cooling Tower Water Quality
A cooling tower is an open evaporative cooling system. This means that as water cycles in the cooling tower, evaporative losses occur, which leaves behind residual dissolved minerals. When fresh make-up water replenishes those lost to evaporation, additional dissolved solids are added, and concentration increases. As evaporation continues, the cooling tower cycles of concentration increase, with the optimal conductivity range for a system determining the number of cycles. In the absence of effective water management practices such as treatment and blowdown, there is a high risk of microbiological buildup. Other issues, such as scaling and corrosion, will also be on the increase. The occurrence of these would negatively impact the cooling tower energy removal capacity and overall efficiency.
Courtesy: CED Engineering
No two cooling tower water treatment programs are alike due to operational and water quality variables. Beginning with the water quality, cooling tower operators need to understand the tendencies that their water possesses. By working alongside water treatment professionals, operators can design an effective water treatment program that borders around the water quality of the system and the challenges that need addressing.
Monitoring Methods
Water treatment programs for cooling towers must be robust due to various water-related operating problems. However, regular monitoring is needed to assess water quality, identify problems, and implement corrective measures promptly. The following sections highlight some common monitoring methods.
Chemical Monitoring in Cooling Tower Systems
Chemical analysis comprises a wide range of tests to measure the concentration of various chemical constituents in cooling tower water. Parameters of interest include pH, conductivity, total dissolved solids, and hardness. Also, there is usually an assessment of specific ions such as chloride, bromide, and sulfate. Furthermore, it is necessary to evaluate the presence of mineral content like magnesium, calcium, or iron, as they can affect the cooling tower’s lifespan and efficiency. Due to the number of variables, operators typically utilize multimeters that can evaluate several parameters at the same time. These tests provide insights into water chemistry, identify potential causes of scaling and corrosion, and guide the selection of appropriate chemical treatments.
Corrosion Monitoring
Corrosion monitoring entails assessing the corrosion rate and corrosion potential of metal surfaces in contact with the cooling tower water. Devices such as corrosion coupons, electrochemical corrosion probes, and corrosion rate meters serve to measure corrosion rates. They also identify corrosion-prone areas within the system. Results from this monitoring help in optimizing corrosion inhibitor dosing, which prolongs the life of cooling tower components.
Scale and Deposit Monitoring
Scale and deposit analysis involves examining deposits on heat exchange surfaces and piping systems to identify their composition and thickness. Techniques such as visual inspection, microscopy, and spectroscopic analysis serve in characterizing scale and deposits. Understanding the composition and morphology of deposits helps assess the effectiveness of scale inhibition strategies and guide maintenance activities.
Microbiological Testing of Cooling Tower Water
Microbiological testing entails analyzing water samples for the presence of bacteria, algae, fungi, and other microorganisms. Various techniques, including culture-based methods and molecular biology assays, help to detect and quantify microbial populations. This is important when checking the effectiveness of biocide treatment and preventing microbiological fouling. One common testing method is PTSA fluorometry. It involves adding PTSA, which is a fluorescent tracer, to the water sample. Microbes present in the water metabolize the tracer, resulting in the emission of fluorescence. By measuring the intensity of fluorescence, operators can estimate the level of microbial activity and adjust treatment accordingly.
Cooling Tower Water Treatment Systems
Operators need to deploy various treatment systems because the problems associated with each cooling tower are unique. The following sections highlight common treatment approaches.
Chemical Treatment
- Biocides: These are chemical agents that control microbial growth in cooling tower water. Oxidizing biocides like chlorine and bromine disrupt cellular processes in microorganisms. Meanwhile, non-oxidizing biocides like quaternary ammonium compounds and isothiazolinones inhibit microbial metabolism. Regular dosing of biocides prevents biofouling, slime formation, and the proliferation of pathogens like Legionella.
- Corrosion Inhibitors: These inhibitors form a protective film on metal surfaces, thereby reducing the rate of electrochemical reactions. Inorganic inhibitors such as phosphates and silicates form insoluble precipitates on metal surfaces. On the other hand, organic inhibitors like azoles and phosphonates adsorb onto metal surfaces to form a barrier against corrosive agents.
- Scale Inhibitors: Scale inhibitors prevent the precipitation and deposition of mineral scales on heat exchange surfaces and piping systems. Polyphosphates, phosphonates, and chelating agents sequester scale-forming ions, thereby preventing them from crystallizing and adhering to surfaces.
Filtration of Cooling Tower Water
- Sand Filters: Sand filters utilize layers of graded sand to remove suspended solids and particulate matter from cooling tower water. As water flows through the filter bed, solid particles are trapped. Thus improving clarity and reducing the risk of fouling in the system. Periodic backwashing or media replacement is necessary to maintain filtration efficiency.
- Multimedia Filters: These perform the same function as sand filters but use a combination of multiple layers of different media. Examples are anthracite, sand, and garnet, which are used to achieve finer filtration and enhance particle removal efficiency. Usually, these filters serve as pre-treatment for other water treatment processes, such as ion exchange.
Ion Exchange
- Deionization Resins: This resin removes cations and anions from water, producing ultra-pure water. Acid and caustic solutions exchange hydrogen and hydroxide ions, respectively, thereby restoring ion exchange capacity.
- Softening Resins: These are typically based on sulfonated polystyrene or acrylic polymers. They exchange calcium and magnesium ions for sodium ions, softening water and minimizing scale formation. Brine or caustic soda regenerates softening resins and removes accumulated hardness ions, thus restoring resin capacity.
Blowdowns
Generally, blowdowns are part of regular maintenance of cooling towers. They serve as a way to remove water from the system after it has accumulated heavy mineral or chemical contents. The spent water is disposed of and replaced with fresh water.
