(1) Scale: Due to the continuous evaporation of circulating water during the cooling process, the salt concentration in the water continuously increases, exceeding the solubility of certain salts and precipitating. Common scales include calcium carbonate, calcium phosphate, magnesium silicate, etc. The texture of scale is relatively dense, greatly reducing heat transfer efficiency. A thickness of 0.6 millimeters of scale reduces the heat transfer coefficient by 20%.

(2) Dirt: Dirt is mainly composed of organic matter, microbial colonies and secretions, sediment, dust, etc. in water. The texture of the scale is soft, which not only reduces 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, mainly electrochemical corrosion, is caused by factors such as manufacturing defects, sufficient oxygen in the water, corrosive ions (Cl -, Fe2+, Cu2+) in the water, and dirt generated by mucus secreted by microorganisms. The consequences of corrosion are extremely serious, and even if the heat exchanger and water pipeline equipment are scrapped in a very short period of time without control.

(4) Microbial sludge: Due to the sufficient oxygen dissolved in the circulating water, suitable temperature, and nutrient rich conditions, it is very suitable for the growth and reproduction of microorganisms. If not controlled in time, it will quickly lead to water quality deterioration, odor, and blackening, a large amount of sludge deposition and even blockage in the cooling tower, significantly reducing the cooling and heat dissipation effect, and intensifying equipment corrosion. Therefore, the treatment of circulating water must control the reproduction of microorganisms.

The microorganisms in the circulating cooling water come from two aspects. One is that the cooling tower needs to introduce a large amount of air during the evaporation process of water, and microorganisms are also brought into the cooling water with the air. The other is that there are more or less microorganisms in the supplementary water of the cooling water system, and these microorganisms also enter the cooling water system with the supplementary water.

Algae, under sunlight, engage in photosynthesis with carbon sources such as carbon dioxide and bicarbonate in water, absorbing carbon as nutrients and releasing oxygen. Therefore, when algae proliferate in large quantities, it increases the dissolved oxygen content in the water, which is conducive to the depolarization of oxygen and accelerates the corrosion process. The extensive proliferation of microorganisms in the circulating water system can cause the color of the circulating water to turn black, produce a foul odor, and pollute the environment. At the same time, a large amount of mud will form, reducing the cooling efficiency of the cooling tower and causing wood to deteriorate and rot. Mud deposits in the heat exchanger, reducing heat transfer efficiency and increasing head loss. Mud deposited on the metal surface can cause serious corrosion under the scale. At the same time, it also isolates the effect of corrosion and scale inhibitors on the metal, preventing the agents from exerting their expected corrosion and scale inhibition effects. Microbial sludge not only accelerates corrosion under the scale, but some bacteria also directly corrode metals through biological secretions during metabolic processes. All these problems lead to the inability of the circulating water system to 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 controlling the harm of microorganisms is the top priority when comparing the three.

(1) When using residual chlorine (free chlorine) for disinfection, attention should be paid to the time and amount of residual chlorine present, as severe microbial growth can significantly increase the chlorine consumption in the circulating water.

(2) Ammonia circulating water generally does not contain ammonia, but due to process medium leakage or inhalation of ammonia into the air, ammonia can also appear in the water. In this case, one should not take it lightly. In addition to actively searching for ammonia leakage points, attention should also be paid to whether the water contains nitrite ions. It is best to control the ammonia content in the water to be below 10mg/l.

(3) NO2- When ammonia and nitrite ions appear in water, it is said that nitrite bacteria have already converted ammonia into nitrite ions in the water. At this time, chlorination in the circulating water system will become very difficult, and the chlorine consumption will increase, making it difficult for residual chlorine to reach the target. It is best to control the NO2- content in water to be less than 1mg/l.

(4) When the growth of microorganisms in water is severe, the COD will increase because the mucus secreted by bacteria increases the organic matter content in the water. Therefore, through the analysis of chemical oxygen demand, the trend of microbial changes in water can be observed. Under normal circumstances, the COD in water should be less than 5mg/l (KMnO4 method).

The harm caused by microorganisms in circulating water is very serious. If measures are to be taken after the microorganisms cause harm, it is often half the effort and requires a large amount of biocides and money. Therefore, it is necessary to comprehensively monitor the microbial situation of the circulating cooling water in advance.

The concentration ratio of circulating water refers to the ratio at which the circulating water continuously concentrates during the operation of the circulating water system due to water evaporation, wind loss, and other factors (compared with supplementary water). It is an important comprehensive indicator for measuring the quality of water quality control. Low concentration factor, high water consumption and discharge, and insufficient effectiveness of water treatment agents; A high concentration factor can reduce water volume and save water treatment costs; However, if the concentration factor is too high, the tendency of water scaling will increase, and the difficulty of scaling and corrosion control will increase. Water treatment agents will fail, which is not conducive to microbial control. Therefore, there should be a reasonable control index for the concentration factor of circulating water.

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 them, dissolved bicarbonate such as Ca (HCO3) 2 and Mg (HCO3) 2

The most unstable and easily decomposes to form carbonates. Therefore, when there is a large amount of bicarbonate dissolved in the cooling water, the water flow passing through the surface of the heat exchanger, especially the surface at higher temperatures, will be thermally decomposed; When phosphate and calcium ions are dissolved in water, precipitation of calcium phosphate will also occur; Calcium carbonate and Ca3 (PO4) 2 are both difficult to dissolve and differ from general salts. Their solubility does not increase with increasing temperature, but decreases with increasing temperature. Therefore, on the heat transfer surface of the heat exchanger, these insoluble salts are easily supersaturated and crystallize in water, especially when the water flow rate is small or the heat transfer surface is rough. These crystalline precipitates will deposit on the heat transfer surface, forming what is commonly known as scale. Due to the dense and hard crystallization of these scale crystals, they are also called hard scale. Common scale components are: calcium carbonate, calcium sulfate, calcium phosphate, magnesium salt, and silicate.

Based on the characteristics and process conditions of the enterprise's circulating water system, combined with the local water quality characteristics, a water treatment plan suitable for the enterprise's operating conditions is selected. Through measures such as adding chemicals, the circulating water indicators are controlled to operate within a certain range, ensuring the long-term operation of production equipment and improving the utilization rate of circulating water. The utilization of circulating water treatment technology can bring significant economic benefits to enterprises and good social benefits to society. So the application of circulating water treatment technology is very necessary.