By Brian Corbin, PhD
The United Nations estimates that by 2030 the world population is expected to increase from seven to 8.3 billion, meaning we will need 30 percent more water and 40 percent more energy. Water and energy scarcity issues across the globe are increasing pressure to reduce reliance on fresh water sources and to conserve water through enhanced efficiency, such as in cooling operations.
Maintaining efficient cooling towers is essential to providing cool water for manufacturing, electrical power generation and air conditioning. In an open, recirculating cooling water system, a circulating stream of warm water comes in contact with airflow, causing evaporation. As the water evaporates, the unevaporated water is cooled and recirculated through the system. Industrial cooling towers remove the heat absorbed in circulating cooling water systems in small, mid-size and large-scale operations. Increased water scarcity and competition among municipal and agricultural water users for fresh water can lead cooling tower operators to seek less costly ways to be less reliant on local freshwater resources. Additionally, cooling tower operators, faced with federal environmental regulations involving wastewater disposal and local water discharge limitations, can find wastewater reuse an economical and sustainable option.
Experts from a major water industry player have found that during high-efficiency operations plants using advanced water treatment technology can return up to 90 percent of previously wasted water back into their cooling systems. The thermal efficiency and longevity of a cooling tower and its equipment are dependent upon proper recirculated water management. Disruptions in a system can impact productivity and the sustainability profile of a plant or facility, causing financial consequences. Advanced water technologies are facilitating high-efficiency cooling tower operations to reuse water, helping to achieve effective cycles of concentration.
The gauntlet: advanced technologies versus cooling tower challenges
Address water scarcity with recovery and reuse
Water reuse can help reduce reliance on local freshwater resources, helping to reduce source water costs. For example, a water reuse scheme in a cooling system can treat blowdown water to achieve the quality needed for use as a make-up water source. With advanced filtration and purification technologies, most blowdown can be recovered and reused as make-up water. Implementing a reverse osmosis (RO) system on the cooling tower blowdown will purify a portion of the water (the permeate flow) and the bulk of the dissolved solids will flow into a smaller waste stream discharged from the system.(1) In a reuse blowdown system, RO and ultrafiltration (UF) can help remove contaminants (soluble impurities) to produce high-purity clean water for recirculation. Reverse osmosis and nanofiltration offer efficient TDS contaminant removal, demineralization and reliability, removing a broad range of molecules and ions to help reduce water intake. Ultrafiltration reduces colloids, particles and bacteria to help increase reliability and lower maintenance needs. Ultrafiltration is typically used as a pretreatment for RO, helping reduce the impact of poor or variable make-up water on cooling-system management.
Control the spread of corrosion with effective microbial control
Corrosion can wear down assets, reducing efficiency and causing expensive equipment failures. Loss of system metallurgy can cause failed heat exchangers or piping, or failure in larger portions of the cooling tower. While RO, UF and ion exchange technologies help clean wastewater for reuse, a comprehensive water treatment program that includes non-oxidizing biocides helps reduce fouling that can cause corrosion of heat exchangers and spray nozzles. Effective microbial control will also help a cooling system meet water quality regulatory requirements. Controlling the spread of corrosion will protect assets and increase a system’s efficiency.
Fight scale with advanced water treatment technologies
Dissolved minerals found in cooling water, including calcium, magnesium salts, silica, chelating iron and clay and silt, can form scale in the heat exchanger. The scale insulates equipment, reducing heat exchange or blocking/plugging heat exchanger tubes. When scale is present, the entire system must work harder to meet the cooling demand,(1) resulting in heat transfer loss and increased downtime that can lead to higher operating costs. Ion exchange resins, paired with nanofiltration or RO membranes, help soften and demineralize water for reuse. Antiscalants increase the solubility limits of minerals so they can be discharged to help operations run free of deposits, avoiding heat transfer loss and downtime.
Combat fouling with filtration and biocides
When suspended particles—organic matter, process oils and silt—fall out of a solution, they can form deposits that result in fouling. Low water velocities and process leaks can all cause fouling. Similar to scale deposits, fouling deposits create an insulating barrier to the system metallurgy, which impedes heat exchange, causing the system to work harder, resulting in increased energy use. Turbid waters and biofilms can be treated with filtration, biocides and polyacrylates to help eliminate operational inefficiencies caused by fouling. Resistive (or tough) fouling can result in high costs, downtime and process upsets. Advanced self-cleaning filtration technology treats difficult water to remove suspended solids and reduce fouling, helping decrease maintenance, downtime and consumables costs.
Reduce contamination with an effective kill
Microbial contamination can often be the root of fouling, corrosion and scaling. With contamination, microorganisms grow in the warm, moist chiller bundles, heat exchanger surfaces and system pipes. The strong insulating properties of biofilms contribute to fouling and corrosion, and the contamination they create can contribute to further microbiological growth. Biofilms and algae mats are often difficult to kill and cooling system contamination can also result in pathogens that can cause illness.(1) Filtration, chemical and monitoring solutions help control biological contamination to protect equipment, help control the spread of corrosion and keep operations efficient. A balanced and robust microbial control program will include a combination of oxidizing and non-oxidizing biocides.
Increased operational efficiency = energy, water, money savings
While cooling system challenges can be addressed with ion exchange, reverse osmosis/nanofiltration, ultrafiltration, scale control and microbial control technologies, it’s important these technologies work cohesively to keep a cooling system running efficiently. Because one challenge can lend itself to another, addressing the system as a whole with advanced technologies will remedy challenges and help prevent future issues. Working together, advanced treatment technologies help maximize heat exchange and process efficiencies to decrease energy and water use, helping to save on costs. Protecting assets will decrease the need for costly downtime for maintenance and repairs, or the need for complete replacement, and cut chemical and mechanical cleaning costs. Ensuring proper water quality within a cooling system not only maximizes flow, but also helps control the spread of unwanted microorganisms and helps the system comply with local regulations. To help preserve equipment life and asset capability, water quality in a cooling system requires continuous control through an integrated technology approach.
Reference
(1) US Department of Energy. Cooling Towers: Understanding Key Components of Cooling Towers and How to Improve Water Efficiency. [Accessed 03/10/2016] https://www1.eere.energy.gov/femp/pdfs/waterfs_coolingtowers.pdf
About the author
Brian Corbin is a Customer Application Specialist at Dow Microbial Control responsible for the Water Treatment segment within the North American Customer Application Center. He is responsible for providing technical service and driving growth for all existing and new NAA Water customers. Corbin holds a PhD in microbiology and molecular genetics from the University of Texas Health Science Center at Houston and completed a postdoctoral fellowship at Vanderbilt University.
About the company
The experts at Dow Oil, Gas and Mining, Dow Water & Process Solutions and Dow Microbial Control provide technical expertise paired with partners with in-field experience to address water management issues in cooling systems.