Sediments in Circulating Cooling Water System and Its Control methods

　　The first problem to be solved in cooling water treatment is the sediment in circulating cooling water system.

　　During the operation of circulating cooling water system, various substances will take the opportunity to be on the surface of heat transfer tube of heat exchanger. These substances are collectively called sediments. They are mainly composed of scale, silt, corrosion products, biological deposits. For heat exchanger cleaning, it is recommended to use a high-pressure cleaner.

　　Usually people collectively refer to sludge, corrosion products and biological deposits as dirt. For heat exchanger cleaning, it is recommended to use a high-pressure cleaner.

　　1 scale

　　There are various salts dissolved in natural water, such as bicarbonate, sulfate, chloride, silicate and so on. Among them, dissolved bicarbonate, such as calcium bicarbonate and magnesium bicarbonate, is the most unstable, and it is easy to decompose to form carbonate.

　　In most cases, the scale formed on the heat transfer surface of heat exchanger is mainly calcium carbonate, because the solubility of calcium sulfate is much greater than that of calcium carbonate. For heat exchanger cleaning, it is recommended to use a high-pressure cleaner.

　　2 dirt

　　Dirt is generally composed of fine particles of silt, dust, mud of insoluble salts, colloidal hydroxide, debris, corrosion products, oil, especially the corpses of bacteria and algae and their sticky secretions.

　　Because this kind of dirt is large in size and loose and soft in texture, it is also called soft dirt. They are the main cause of corrosion under the scale, and also the breeding ground for the survival and reproduction of certain bacteria such as anaerobic bacteria. When anti-corrosion measures are not taken properly, rust tumors often adhere to the surface of heat exchange tube of heat exchanger. The outer shell is hard, but the inner part is loose and hollow, and the distribution is uneven. They are often deposited on heat transfer surface of heat exchanger together with scale and microbial slime. The deposits formed by this kind of rust-like corrosion products, in addition to affecting heat transfer, will more seriously promote the reproduction of certain bacteria and iron bacteria, and eventually lead to corrosion and perforation of pipe wall and leakage. For heat exchanger cleaning, it is recommended to use a high-pressure cleaner.

　　3 Scale control methods

　　01 Ion exchange resin method

　　Let water pass through ion exchange resin to replace the calcium ions and magnesium ions from the water and combine them on the resin to achieve the purpose of removing calcium ions and magnesium ions from the water.

　　Using ion exchange to soften supplementary water costs more. Therefore, only circulating cooling water systems with a small supplementary water volume are used occasionally.

　　02 Lime softening method

　　Before supplementary water enters circulating cooling water system, appropriate lime is added during pretreatment, so that calcium bicarbonate in the water and lime react in advance in the clarifier to form calcium carbonate and precipitate out, thereby removing calcium ions in the water.

　　The cost of adding lime is low, and this method is often used in circulating cooling water systems with high calcium content in raw water and large water replenishment.

　　03 Acid

　　Sulfuric acid is usually added, because adding hydrochloric acid will bring in chloride ions, which will increase the corrosion of water; adding nitric acid will bring in nitrate, which is conducive to the reproduction of nitrifying bacteria.

　　04 Pass CO2 gas

　　Passing carbon dioxide gas or flue gas into water disrupts the precipitation balance and makes it proceed to the left, thereby stabilizing the bicarbonate.

　　05 Add antiscalant

　　From crystallization process of calcium carbonate, if some chemicals can be added to destroy its crystal growth, the purpose of controlling the formation of scale can be achieved. Various scale inhibitors currently used include polyphosphate, organic polyphosphoric acid, organic phosphate, polyacrylate, etc.

　　4 methods of fouling control

　　01 Reduce make-up water turbidity

　　The turbidity of natural water bodies is due to sediment, corrosive organisms, various suspended solids and colloids, etc. As for circulating water, the lower the turbidity, the better, indicating that circulating water has less impurities. When the influent water quality of circulating cooling water is not good, pretreatment must be carried out. The concentration of suspended solids in circulating water treatment should not be greater than 20mg/L, and when heat exchanger is in the form of plate or finned tube and spiral plate, it should not be greater than 10mg/L.

　　02 Carry out good circulating cooling water quality treatment

　　Circulating cooling water is mainly to prevent scale and corrosion. When the quality of influent water is not good, it will inevitably lead to scale and corrosion. At the same time, a large number of bacteria will grow and form dirt. Therefore, good water quality treatment is the way to reduce the system's fouling.

　　03 Dosing antiscalant and dispersant

　　Adding a certain amount of dispersant is also an effective way to control fouling during anti scaling, antiseptic and bactericidal algae treatment. The dispersant can disperse the large mud mass into fine particles suspended in the water, and not deposit on the heat transfer surface with the flow of water, thereby reducing the influence of dirt on heat transfer. Drain the circulating water system.

　　04 Add side filter equipment.

　　Even if water quality treatment is done very well and turbidity of incoming water is well controlled, the turbidity in circulating water will still continue to rise, which will aggravate the formation of dirt. The reason why turbidity of circulating cooling water system will increase under the condition of stable operation is that when cooling water passes through the cooling tower and contacts the air, the dust in the air will be brought into the water, resulting in an increase of turbidity. Adding side filter equipment can reduce dust entering the circulating water. If the bypass flow and turbidity of inlet and outlet bypass equipment are controlled, it can be ensured that turbidity of system will not increase under long-term operation, and it will be maintained within control index, thereby reducing the generation of dirt.