What is circulating water treatment?
During the operation of the industrial circulating cooling water system, the circulating water is continuously concentrated due to evaporation of water, wind loss, etc., the salts contained in it exceed the standard, the anions and cations increase, and the pH value changes significantly, resulting in the deterioration of the water quality, and the circulating water. Temperature, pH, and nutrients favor microbial growth, and ample sunlight on cooling towers is ideal for algae growth. For scaling control, corrosion control, microbial control, etc., circulating water treatment is necessary.
The main problems in the operation of circulating water:
(1) Scale: Due to the continuous evaporation of circulating water during the cooling process, the salt concentration in the water increases continuously, exceeding the solubility of some salts and precipitates. Common are calcium carbonate, calcium phosphate, magnesium silicate and so on. The texture of the scale is relatively dense, which greatly reduces the heat transfer efficiency. The thickness of the scale of 0.6 mm reduces the heat transfer coefficient by 20%.
(2) Dirt: Dirt is mainly composed of organic matter, microbial colonies and secretions, sediment, dust, etc. in the water. The texture of the scale is soft, which not only reduces the heat transfer efficiency, but also causes corrosion under the scale, shortening the service life of the equipment.
(3) Corrosion: The corrosion of heat exchange equipment by circulating water is mainly galvanic corrosion, which is caused by equipment manufacturing defects, sufficient oxygen in water, corrosive ions (Cl-, Fe2+, Cu2+) in water and mucus secreted by microorganisms. The resulting fouling and other factors, the consequences of corrosion are very serious, even if the heat exchanger and water pipeline equipment are scrapped in a very short period of time without control.
(4) Microbial slime: Because the circulating water has sufficient oxygen, suitable temperature and nutrient-rich conditions, it is very suitable for the growth and reproduction of microorganisms. Scale deposits or even blockages, the cooling and heat dissipation effect is greatly reduced, and the equipment corrosion is aggravated. Therefore, circulating water treatment must control the reproduction of microorganisms.
Microbial Hazard
Microorganisms in circulating cooling water come from two sources. First, the cooling tower needs to introduce a large amount of air during the evaporation of water, and microorganisms are also brought into the cooling water with the air. Second, the supplementary water of the cooling water system will contain microorganisms more or less, and these microorganisms also enter the cooling water with the supplementary water. in the system.
Under the irradiation of sunlight, algae will photosynthesize with carbon sources such as carbon dioxide and bicarbonate in the water, absorb carbon as nutrients and release oxygen. Therefore, when algae multiply, the dissolved oxygen content in the water will increase, which is conducive to oxygen. depolarization, the corrosion process is accelerated. The mass reproduction of microorganisms in the circulating water system will make the color of the circulating water black, produce odor, and pollute the environment. At the same time, a large amount of slime will be formed, which will reduce the cooling efficiency of the cooling tower and cause the wood to deteriorate and rot.
The slime deposited in the heat exchanger reduces the heat transfer efficiency and increases the head loss. The slime deposited on the metal surface will cause severe corrosion under scale. The chemical can't play its due corrosion and scale inhibition performance. In addition to accelerating the corrosion under scale, the microbial slime will also directly corrode the metal during the metabolic process of some bacteria. All these problems cause the circulating water system to not operate safely for a long time, affecting production and causing serious economic losses. Therefore, the harm of microorganisms is as serious as the harm of scale and corrosion to the cooling water system. It can even be said that the three are compared to control. Microbial hazards are primary.
The movement of microorganisms in circulating water can be measured by the following chemical analysis items:
(1) Residual chlorine (free chlorine) When chlorinating sterilization, pay attention to the time and amount of residual chlorine, because the chlorine consumption in the circulating water will be greatly increased when the microorganisms multiply seriously.
(2) Ammonia circulating water generally does not contain ammonia. However, due to the leakage of process medium or the inhalation of ammonia in the air, ammonia will also appear in the water. At this time, it should not be taken lightly. It contains nitrite, and the ammonia content in water is recommended to be controlled below 10mg/l.
(3) NO2- When there is ammonia and nitrite in the water, it is said that there are nitrite bacteria in the water that convert ammonia into nitrite. At this time, it will be very difficult to add chlorine to the circulating water system, and the chlorine consumption will increase. The residual chlorine is difficult to reach the target, and the NO2- content in the water is recommended to be controlled at less than 1mg/l.
(4) COD will increase when microorganisms multiply in chemical oxygen demand water seriously, because the mucus secreted by bacteria increases the content of organic matter in the water, so through the analysis of chemical oxygen demand, the trend of microbial changes in water can be observed. COD is recommended to be less than 5mg/l (KMnO4 method).
The harm caused by the microorganisms in the circulating water is very serious. If it is necessary to take measures after the harm caused by the microorganisms, it is often twice the result with half the effort and a lot of biocides and money are spent. Therefore, it is necessary to comprehensively monitor the microbial situation of the circulating cooling water in advance.
Concentrated water multiple
The concentration ratio of circulating water refers to the continuous concentration ratio of circulating water due to water evaporation and wind loss during the operation of the circulating water system (compared with supplementary water as a benchmark), which is an important measure of the quality of water quality control. Comprehensive indicators. The concentration ratio is low, the water consumption and sewage discharge are large, and the efficiency of the water treatment agent cannot be fully exerted; the high concentration ratio can reduce the amount of water and save the cost of water treatment; however, if the concentration ratio is too high, the scaling tendency of the water will increase. The difficulty of scaling control and corrosion control will increase, and the water treatment chemicals will fail, which is not conducive to the control of microorganisms. Therefore, the concentration ratio of circulating water must have a reasonable control index.
Scale formation
In the circulating water system, scale is formed by supersaturated water-soluble components, and various salts are dissolved in the water, such as bicarbonate, carbonate, chloride, silicate, etc., among which the dissolved bicarbonate Salts such as Ca(HCO3)2 and Mg(HCO3)2 are unstable and are easily decomposed to form carbonates. Therefore, when more bicarbonate is dissolved in the cooling water, the water flows through the surface of the heat exchanger, especially if the temperature is higher. The surface will be decomposed by heat; when phosphate and calcium ions are dissolved in the water, the precipitation of calcium phosphate will also occur; calcium carbonate and Ca3(PO4)2 are both insoluble and different from ordinary salts, and their solubility is not increases with increasing temperature, but decreases with increasing 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 water flow rate is small or the heat transfer surface is rough, these crystalline precipitates will be deposited on the heat transfer surface. , to form what is commonly called scale. Because these scales are dense and hard, they are also called hard scales. The common scale components are: calcium carbonate, calcium sulfate, calcium phosphate, magnesium salts, and silicates.
Circulating water treatment technology
According to the characteristics and process conditions of the circulating water system of the enterprise, combined with the characteristics of local water quality, select a water treatment plan suitable for the operating conditions of the enterprise, and control the operation of the circulating water index within a certain range through measures such as dosing, which not only ensures the long-term cycle of the production equipment operation, and improve the utilization rate of circulating water. The utilization of circulating water treatment technology can not only bring significant economic benefits to enterprises, but also bring good social benefits to the society. Therefore, the application of circulating water treatment technology is very necessary.
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