Circulating water system

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During operation, industrial circulating cooling water systems experience continuous concentration of the water due to factors such as water evaporation and wind-induced losses. As a result, the levels of salts in the water increase, the amounts of cations and anions rise, and the pH value changes significantly, leading to a deterioration in water quality. Moreover, the temperature, pH value, and nutritional components of the circulating water are conducive to the growth of microorganisms, while the ample sunlight on cooling towers provides an ideal environment for algae to grow. And for scale control, corrosion control, microbial control, and so on, circulating water treatment is absolutely necessary. Main problems that occur during the operation of circulating water: (1) Scale: In a circulating cooling water system, the concentration of bicarbonates increases as a result of evaporation and concentration. When its concentration reaches a supersaturated state, or when the water temperature at the heat transfer surface rises, it decomposes to form carbonate deposits on that surface, creating dense, slightly soluble salt scales with poor thermal conductivity. Therefore, scale adheres to the heat transfer surfaces; in mild cases, this reduces the heat transfer efficiency of the heat exchanger, while in severe cases it causes the heat exchanger to become clogged. This increases system resistance, the load on the equipment, the efficiency of water pumps and cooling towers, leading to higher energy consumption in production, reduced output, and accelerated local corrosion. (2) Fouling: Fouling is primarily composed of organic matter in water, microbial colonies and their secretions, silt, dust, etc. The fouling material is soft in texture; it not only reduces heat transfer efficiency but also causes under-deposit corrosion, thereby shortening the service life of equipment. (3) Corrosion: In circulating cooling water systems, a large number of components are made of metal. Prolonged use of circulating cooling water can lead to corrosion and perforation. This is caused by various factors, mainly including: electrochemical corrosion resulting from dissolved oxygen in the cooling water; corrosion caused by harmful ions; corrosion induced by microorganisms, etc. Corrosion and perforation of the equipment walls can lead to leaks or the leakage of process media into the cooling water, resulting in material loss and water pollution. Alternatively, cooling water may infiltrate the process media, affecting product quality and production safety, thereby causing direct economic losses. (4) Microbial sludge: The circulating water provides sufficient oxygen, an appropriate temperature, and favorable nutrient conditions, which are ideal for the growth and reproduction of microorganisms. If not controlled in a timely manner, this will lead to rapid deterioration of water quality, odor formation, and discoloration. A large amount of sludge will accumulate in the cooling towers, even causing blockages, which significantly reduces their cooling efficiency and accelerates equipment corrosion. Therefore, in circulating water treatment, the proliferation of microorganisms must be controlled. Microbial hazards (1) Bacteria. Bacteria are single-celled organisms; each cell is an independent entity, and many of these single cells tend to gather together in groups (colonies), with each cell still living independently. They mainly include slime-producing bacteria, iron-depositing bacteria, sulfide-producing bacteria, and acid-producing bacteria. (2) Fungi: The difference between fungi and bacteria is that fungi have a nucleus; their structure is more complex than that of bacteria, and their morphology also differs greatly from that of bacteria. They exist in both single-cell and multi-cell forms. Excessive growth of fungi can lead to sludge-related problems; colonies of molds such as Geotrichum and Sclerotinia can easily attach to any rough surface, accumulating sediment, interfering with water flow, reducing heat transfer efficiency, and even causing pipe blockages. (3) Algae. Algae are lower plants that contain chlorophyll and are capable of photosynthesis. Algae in cooling water are mainly composed of cyanobacteria, green algae, and diatoms. Algae need air, water, sunlight, and nutrients to grow, among which light has the most significant impact. Algae can only grow in places where sunlight can reach or be reflected, such as on the tops of cooling towers, in ponds, and at the inlets and outlets for water. Algae continuously reproduce and then fall off; these fallen algae become suspended solids and sediments in the circulating water system, blocking pipes, affecting the delivery of cooling water, and reducing heat transfer efficiency. When algae die and rot, water quality deteriorates, an unpleasant odor appears, and it also provides nutrients for microorganisms such as bacteria. Concentration ratio: The concentration ratio of circulating water refers to the degree to which the circulating water becomes more concentrated over time during operation, due to factors such as water evaporation and wind-induced losses (compared on a basis of make-up water). It is an important comprehensive indicator for assessing the effectiveness of water quality control. The concentration ratio is low, resulting in high water consumption and wastewater discharge, as well as inadequate utilization of the water treatment chemicals ； A high concentration ratio can reduce water usage and save on water treatment costs ； However, if the concentration ratio is too high, the tendency of water to form scale increases, making it more difficult to control scaling and corrosion. Water treatment chemicals become less effective, and it becomes harder to control microorganisms; therefore, there needs to be a reasonable limit for the concentration ratio of circulating water. The formation of scale: In closed-loop water systems, scale is formed by supersaturated water-soluble components. Water contains various salts such as bicarbonates, carbonates, chlorides, and silicates. Among these, the dissolved bicarbonates like Ca(HCO3)2 and Mg(HCO3)2 are the most unstable and readily decompose to form carbonates. Therefore, when there is a high concentration of dissolved bicarbonates in the cooling water, as the water flows over the surface of the heat exchanger, especially the areas with higher temperatures, it undergoes thermal decomposition ； When phosphate and calcium ions are present in water, calcium phosphate precipitate will also form ； Calcium carbonate and Ca3(PO4)2 belong to compounds with low solubility; their solubility does not increase as temperature rises, but rather decreases with rising temperature. Therefore, on the heat transfer surface of the heat exchanger, these insoluble salts can easily reach a supersaturated state and crystallize in the water. Especially when the flow rate of water is low or the heat transfer surface is rough, these crystalline deposits accumulate on the surface, forming what is commonly known as scale. Due to their dense and hard structure, these scale deposits are also referred to as hard scale. Common components of scale include calcium carbonate, calcium sulfate, calcium phosphate, magnesium salts, and silicates. Circulating water treatment technology: Based on the characteristics of the enterprise’s circulating water system and its operational conditions, as well as the local water quality, a water treatment solution suitable for those conditions is selected. Measures such as the addition of chemicals are used to keep the parameters of the circulating water within specified ranges, thereby ensuring the long-term operation of production equipment while also improving the efficiency of water reuse. The use of circulating water treatment technology can bring significant economic benefits to enterprises, as well as positive social benefits to society. Therefore, the application of circulating water treatment technology is highly necessary.