What is circulating water treatment?
During the operation of the industrial circulating cooling water system, due to water evaporation, wind loss, and other factors, the circulating water continues to concentrate. The salt content in the circulating water exceeds the standard, the anion and cation ions increase, and the pH value changes significantly, leading to the deterioration of water quality. The temperature, pH value, and nutritional composition of the circulating water are conducive to the proliferation of microorganisms, and sufficient sunlight on the cooling tower is an ideal place for algae growth. The control of scaling and corrosion, as well as the control of microorganisms, inevitably require circulating water treatment.
The main problems that occur during the operation of circulating water are:
(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 ones include calcium carbonate, calcium phosphate, magnesium silicate, etc. The texture of scale is relatively dense, which greatly reduces the heat transfer efficiency. The 0.6mm scale thickness 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 dirt is soft, which not only reduces heat transfer efficiency but also causes corrosion under the dirt, shortening the service life of the equipment.
(3) Corrosion: The corrosion of heat exchange equipment caused 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 an extremely short period of time without control.
(4) Microbial sludge: Due to sufficient oxygen, suitable temperature, and nutrient rich conditions in the circulating water, it is very suitable for the growth and reproduction of microorganisms. If not controlled in a timely manner, it will quickly lead to water quality deterioration, odor, and blackening, a large amount of sludge deposition and even blockage in the cooling tower, greatly reducing the cooling and heat dissipation effect, and intensifying equipment corrosion. Therefore, the reproduction of microorganisms must be controlled in the treatment of circulating water.
Microbial hazards
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 water evaporation process, 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.
Under sunlight, algae will photosynthesis with carbon sources such as carbon dioxide and bicarbonate in the water, absorb carbon as nutrition and release oxygen. Therefore, when algae multiply in large numbers, it will increase the dissolved oxygen content in the water, which is conducive to the Depolarization of oxygen, thus accelerating the corrosion process. The extensive reproduction 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 clay will be formed, which will reduce the cooling efficiency of the cooling tower and cause the wood to deteriorate and decay.
The deposition of sludge in the heat exchanger reduces heat transfer efficiency and increases water head loss. The sludge 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, making the agent unable to perform its expected corrosion and scale inhibition efficiency. Microbial slime not only accelerates corrosion under the scale, but also causes direct corrosion of metals by biological secretions during the metabolic process of some bacteria. 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, and it can even be said that controlling the harm of microorganisms is the first priority compared to the three.
Concentrated water ratio
The concentration ratio of circulating water refers to the ratio of continuous concentration of circulating water during the operation of the circulating water system due to water evaporation, wind blowing loss, and other factors (compared with supplementary water). It is an important comprehensive indicator to measure 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.
Formation of scale
In the circulating water system, scale is formed by Supersaturation water-soluble components. Various salts are dissolved in the water, such as Bicarbonate, carbonate, chloride, silicate, etc. Among them, dissolved Bicarbonate, such as Ca (HCO3) 2, Mg (HCO3) 2
It is extremely unstable and easy to decompose into carbonate. Therefore, when there is a large amount of Bicarbonate dissolved in cooling water, the water flow through the surface of the heat exchanger, especially the surface with high temperature, will be subject to thermal decomposition; 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 can easily reach the Supersaturation state and crystallize in water. Especially when the water flow speed is low or the heat transfer surface is rough, these crystalline precipitates will deposit on the heat transfer surface and form the commonly called scale. Because these scale crystals are dense and hard, they are also called hard scale. The common scale components are calcium carbonate, calcium sulfate, calcium phosphate, magnesium salt, and silicate.
Circulating water treatment technology
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 index is 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.