A brief discussion on the problems in our plant’s circulating cooling water system and their solutions.

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A Brief Discussion on the Problems in Our Factory’s Circulating Cooling Water System and Their Solutions Abstract: Cooling water is continuously reused in circulation systems. Due to rising water temperatures, changes in flow velocity, water evaporation, the concentration of various inorganic ions and organic substances, as well as exposure of cooling towers and cold water tanks to sunlight, wind, rain, dust, and other contaminants outdoors, along with equipment scaling and material degradation, serious issues such as the accumulation of deposits, equipment corrosion, excessive growth of microorganisms, and blockages in pipes caused by sludge and dirt arise. These problems can threaten and disrupt the factory’s safe operation over the long term, even leading to economic losses. Therefore, it cannot be taken lightly. It is necessary to choose an economical and practical circulating cooling water treatment solution to resolve and improve the aforementioned problems. Keywords: circulating cooling water, existing problems, solutions 1. Overview The circulating water cooling system in our factory consists of the following components: ① heat exchangers used in the production process ； ②Cooling structure (cooling tower) ； ③Circulation water pump and collection tank. This system uses cooling water for temperature reduction and water quality treatment. During the cooling of production equipment or products, the temperature of the cooling water rises. Although its physical properties do not change much, after long-term circulation, certain dissolved substances in the water become concentrated or disappear, dust accumulates, and microorganisms grow, leading to the formation of scale deposits inside the equipment and pipes or causing corrosion of metal components in those pipes. Therefore, solutions such as cooling and stabilization are necessary to ensure the proper operation of the circulating water system and to resolve or improve the aforementioned issues. 2. Problems in open-loop circulating cooling water systems 2.1 Sediments in circulating cooling water systems 2.2.1 Precipitation and accumulation of sediments Natural water generally contains bicarbonates, which are the main component responsible for the formation of scale in cooling water. In DC cooling water systems, the concentration of bicarbonate is low. In a circulating cooling water system, the concentration of bicarbonate increases as a result of evaporation and concentration. When this concentration reaches a supersaturated level, or when the water temperature rises due to heat transfer across the heat exchanger surfaces, the following reaction occurs: Ca(HCO3)2 → CaCO3↓ + CO2↑ + H2O. As the cooling water is sprayed downward through the cooling tower, the CO2 dissolved in it escapes, which drives the aforementioned reaction to proceed in the right direction. CaCO3 deposits on the heat transfer surfaces of the heat exchanger, forming dense calcium carbonate scale, which has very poor thermal conductivity. Different types of scale have varying thermal conductivity values, but they generally do not exceed 1.16 W/(m·K), whereas the thermal conductivity of steel is 46.4–52.2 W/(m·K). It is clear that the formation of scale inevitably affects the heat transfer efficiency of heat exchangers. The hazards of scale accumulation include, in mild cases, a reduction in the heat transfer efficiency of the heat exchanger, which affects production volume ； In severe cases, the pipeline gets blocked. 2.2 Equipment corrosion: A large number of devices in the circulating cooling water system are heat exchangers made of metal. For heat exchangers made of carbon steel, the use of circulating cooling water over a long period can lead to corrosion and perforation, and the causes of this corrosion are multiple. 2.2.1 Electrochemical corrosion caused by dissolved oxygen in cooling water: In open-loop circulating cooling water systems, water has full contact with air, so the dissolved oxygen in the water can reach a saturated state. When carbon steel comes into contact with cooling water containing oxygen, due to the unevenness of the metal surface and the electrical conductivity of the cooling water, many corrosion microcells form on the surface of the carbon steel. In the anode and cathode regions of these microcells, the following oxidation and reduction reactions occur respectively. In the anode region, the reaction Fe = Fe2+ + 2e occurs, while in the cathode region, the reaction 0.5O2 + H2O + 2e = 2OH- takes place. In water, the reactions Fe2+ + 2OH- = Fe(OH)2 and Fe(OH)2 → Fe(OH)3 occur, which cause the metal in the anode region of the microcell to dissolve continuously and become corroded. 2.2.2 Corrosion caused by harmful ions: During the concentration process of circulating cooling water, in addition to the increase in bicarbonate concentration as the concentration ratio rises, the concentrations of other salts such as chlorides and sulfates also increase. An increase in the concentrations of CL- and SO42- accelerates the corrosion of carbon steel. CL- and SO42- reduce the protective efficacy of the protective film on metals. In particular, CL- has a small radius and strong penetration ability, allowing it to easily pass through the film layer, displace oxygen atoms to form chlorides, and accelerate the anodic process, thereby speeding up corrosion. Therefore, chloride ions are one of the causes of pitting corrosion. For heat exchangers made of stainless steel, Cl- is a major cause of stress corrosion; therefore, high levels of Cl- in the cooling water often lead to rapid corrosion and damage in areas of stress concentration on the equipment, such as the edges of the expansion tubes on the heat exchanger plates. In circulating cooling water systems where stainless steel heat exchangers are used, it is generally required that the Cl- content not exceed 300 mg/L. 2.2.3 Corrosion caused by microorganisms: The growth of microorganisms can also cause metal corrosion. This is because the mucus excreted by microorganisms, together with inorganic deposits and sedimentary debris, forms deposits on the metal surface, creating a concentration cell for oxygen that facilitates metal corrosion. Furthermore, the lack of oxygen between the metal surface and the deposits allows certain anaerobic bacteria (mainly sulfate-reducing bacteria) to thrive, with faster growth at temperatures of 25–30°C. It breaks down sulfates in water to produce hydrogen sulfide, which causes corrosion of carbon steel. The reactions are as follows: SO42- + 8H+ + 8e- = S2- + 4H2O + energy (required for bacterial survival); Fe2+ + S2- = FeS↓. Iron bacteria are the main cause of rust spots on steel; they oxidize Fe2+ to Fe3+, and the energy released is used to sustain the bacteria. Fe2+ bacteria → Fe3+ + energy (required for bacterial survival). The combined effect of these various factors on carbon steel leads to corrosion of the heat exchanger tube walls, resulting in perforations and leaks; meanwhile, process fluids can leak into the cooling water, causing material loss and water contamination ； Or cooling water seeps into the process medium, affecting product quality. When the number of tubes that have been corroded and punctured is small, temporary plugging of the tubes can be employed to allow the heat exchanger to continue operating with a reduced heat transfer surface area. When there are too many perforated tubes, the heat transfer surface of the heat exchanger is significantly reduced, resulting in a loss of cooling capacity; in such cases, the operation must be stopped for replacement. Therefore, corrosion, just like scale accumulation, endangers safe production in factories and causes economic losses. 2.3 Microbial growth and sludge: Microorganisms in cooling water generally refer to bacteria and algae. In fresh water, there are generally fewer bacteria and algae. However, in circulating water, nutrient concentration, rising water temperatures, and sunlight create conditions that allow bacteria and algae to reproduce rapidly. The mucus secreted by large numbers of bacteria acts like an adhesive, allowing dust particles, impurities, and chemical precipitates floating in the water to stick together, forming sticky deposits that adhere to the heat transfer surfaces of the heat exchanger. Some people call this type of sediment biological sludge, while others refer to it as soft scale. Mud accumulation on the walls of the heat exchanger tubes not only causes corrosion but also reduces the flow rate of the cooling water, thereby lowering the cooling efficiency of the heat exchanger ； In severe cases, these biological sludges can block the pipes, forcing production to stop for cleaning. For example, a factory in Beijing experienced a significant increase in the growth of bacteria and algae in its heat exchangers, which reduced the heat load by 50% within just half a month. This forced the factory to shut down frequently for cleaning, resulting in a decrease in production. 3. Water treatment solutions for the problems in the circulating cooling water system 3.1 Control of scale and fouling 3.3.1 Control of scale: If there is no excessive amount of PO43- or SiO2 in the cooling water, calcium phosphate scale and silicate scale are not likely to form. In circulating cooling water systems, calcium carbonate scale is the most common type of scale to form; therefore, scale control focuses on preventing the formation of calcium carbonate scale. Its methods mainly fall into the following categories: 1. Removing scaling calcium ions from cooling water (ion exchange resin method and lime softening method). 2. Add acid or pass carbon dioxide to lower the pH value and stabilize bicarbonates. 3. Addition of scale inhibitors 3.3.2 Control of fouling 1. Reduction of make-up water turbidity ； 2. Ensure proper treatment of the quality of circulating cooling water ； 3. Add a dispersant ； 4. Add side filtration equipment ； 3.2 Control of metal corrosion in circulating cooling water systems. Methods to control metal corrosion are as follows: 1. Add a corrosion inhibitor ； 2. Increase the pH value of the cooling water ； 3. Heat exchangers manufactured from corrosion-resistant materials ； 4. Coate with anti-corrosion and scale-inhibiting materials. 3.3 Methods for controlling microorganisms in cooling water 1. Selecting corrosion-resistant materials ； 2. Control water quality ； 3. Use biocidal coatings ； 4. Cathodic protection ； 5. Cleaning 6. Protecting from sunlight (cover the pool and install louvers on the air inlets of the cooling tower) ； 7. Sidestream filtration ； 8. Coagulation sedimentation ； 9. Phage method ； 10. Add a biocide ； 11. Electrostatic water treatment and electronic water treatment. It should be noted that a good microbial control program often achieves much better results by combining several methods. 4. Conclusion: In the context of our factory, since the filtration precision of conventional side-filtering equipment is very low, typically around 55 um, it can only remove large particles such as leaves. The impurities in industrial cooling circulating water systems consist, aside from a few large particles, mainly of fine suspended substances such as dust, rust, and sludge from the air; conventional side-filtering equipment has almost zero efficiency in filtering these suspended substances. Ordinary side filtration equipment cannot solve the problem of dirt in the system ； According to the **design specifications for cooling circulating water**, it is necessary to treat and control bacteria, algae, suspended solids, dirt, corrosion, and biological sludge in such water. At the same time, it is essential to regulate the concentration ratio of the water quality; it is recommended that this ratio be kept around 2.5 times when using tap water or groundwater with a total hardness of approximately 300 mg/L (as CaCO3). To control the concentration ratio, direct discharge is used, which results in a significant waste of water resources. Therefore, adopting this water treatment method cannot completely resolve the problems existing in the system. Based on various studies, I have proposed two not yet fully developed solutions; I hope that the relevant leaders will use these as a reference and offer their valuable feedback. (1) (If an electronic water treatment device is used, it can only address the issue of scale; it cannot solve problems such as system corrosion or algae growth. Therefore, this water treatment setup can only address some of the problems and cannot solve them comprehensively ； At this point, scale prevention and removal equipment can be installed in front of the equipment to be cooled; specific RF parameters can be chosen based on the local water quality to address the issue of scale in the system ； Filtering equipment is installed in the return water pipes of the system; a comprehensive filtering system that combines mechanical pore-size reduction, active iron-based filtration membranes, and corona field effects is used to address water quality issues within the system, thereby eliminating dirt problems ； Install sterilizing algal equipment in the system to control the growth of bacteria and algae in the water quality, thereby ultimately resolving the issue of biological fouling. (2) A full-process processor can also be installed in the system to address the issue of composite scaling caused by suspended solids, impurities, bacteria and algae, corrosion, and scaling in circulating water. By controlling the water concentration ratio through normal wastewater discharge, it is kept between 2.5 and 3, thereby achieving water conservation.