First, scale deposition: The water source used in industrial cooling water mainly comes from groundwater or tap water. The water contains so much dissolved solids as the supplementary water enters the system. When the cooling water is recycled, the concentration multiple increases gradually, or the system process temperature is too high, and the cooling water temperature As it rises, the water quality will gradually change. When the concentration of the soluble matter in the cooling water exceeds its solubility in water, scale is generated.
The scale in the cooling water is generally composed of a slightly soluble salt such as CaCO3, Ca3(PO4)2, CaSO4, or calcium silicate (magnesium). The solubility of these salts is very small, such as the solubility of CaCO3 is 20 mg / L at 0 ° C, the solubility of Ca3 (PO4) 2 is only 0.1 mg / L, and their solubility decreases with the increase of pH and water temperature, so It is particularly easy to be supersaturated at a high temperature heat transfer site to crystallize, and when the water flow velocity is small or the heat transfer surface is rough, these crystals are easily deposited on the heat transfer surface to form scale.
There are three main ways to control the scaling of cooling water: (1) reducing the concentration of scale ions in water to keep it within the allowable range; (2) stabilizing the equilibrium relationship of scale ions in water; and (3) destroying scale ions. The crystal grows up. When selecting a specific scheme to control scale, comprehensive consideration should be given to factors such as the amount of circulating water, the requirements for use, and the source of the medicament.
two. corrosion: The corrosion of carbon steel by cooling water is an electrochemical process. Due to the non-uniformity of the carbon steel structure and the surface and the state of the solution in contact therewith, many small areas of low potential (anode) and high potential (cathode) are formed on the surface, and each pair of anode and cathode forms a metal body. Corrosion of the primary battery, oxidation and reduction reactions occur respectively.
At the anode Fe→Fe2+ +2e
At the cathode 1/2O2+H2O+2e→2OH-
In water Fe2++2OH-→Fe(OH)2↓
2Fe(OH)2+1/2O2+H2O→2Fe(OH)3↓
Since the unevenness of the metal surface is absolute, electrochemical corrosion conditions are ubiquitous, and as long as the metal is in contact with water containing dissolved oxygen, the above corrosion reaction proceeds.
There are three basic methods for controlling corrosion: (1) forming an anti-corrosion layer on the metal surface by electroplating or dip coating, and isolating the metal from circulating water; (2) electrochemical protection method, that is, in a cooling water system, an electrode is generally used. A sacrificial anode such as magnesium or zinc having a lower potential than iron is connected to a carbon steel device to be protected, so that the carbon steel device is protected as a cathode, or a carbon steel device to be protected is connected to a negative electrode of a direct current power source, and is positive. An auxiliary anode, such as graphite\carbon, is added to the device, and the device is protected by an external current to be converted into a cathode; (3) an inorganic or organic corrosion inhibitor is added to the circulating water to form a uniform and dense metal surface. A protective film that is not easily peeled off, which is a treatment method commonly used at home and abroad. In order to improve the effect of the drug, usually before the normal operation of the system, a high concentration of corrosion inhibitor is added to perform pre-film treatment. After the film is formed, the concentration of the agent is lowered to maintain and repair the corrosion-inhibiting film.
three. Microbial growth:
In most open-circuit cooling water systems, a large amount of water is returned to the cooling tower via the process equipment, the water temperature is lowered by evaporation, and the cooling water properties are improved. When the water evaporates in the tower, the concentration of organic matter, inorganic matter, and microorganisms contained in the cooling water has increased. The warm water, together with the nutrients of organic matter and inorganic substances, provide good growth conditions for the microorganisms.
The microorganisms multiply in the cooling water system to form slime, which reduces the heat transfer efficiency, accelerates the corrosion of the metal, affects the water delivery, and produces odor after the sludge is spoiled, which deteriorates the water quality. The failure caused by slime often occurs simultaneously with corrosion and scale failure. According to the form of failure, it can be divided into two types: slime adhesion type and sludge accumulation type. The former is mainly a mixture of microorganisms and their metabolites and mud sand. Faults occur on solid surfaces, often occurring on pipes, pool walls, and cooling tower packing. The latter is a deposit of suspended solids in low-flow areas that produce a soft mud-like substance that fails and often occurs at the bottom of the pool. Two types of failures can occur in the heat exchanger shell and distribution tanks.
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