During the operation of industrial circulating cooling water system, the circulating water is continuously concentrated due to the evaporation of water and loss of wind blowing. The salt content in the system exceeds the standard, the increase of anion and anion ions and the obvious change of pH value lead to deterioration of water quality. The temperature, pH value and nutrient composition of circulating water are conducive to the reproduction of microorganisms, Sufficient sunlight on the cooling tower is the ideal place for algae growth. However, the scale control, corrosion control, microbial control and so on, it is necessary to carry out circulating water treatment.

The main problems in circulating water operation are as follows:

(1) Scale: because the circulating water evaporates continuously during the cooling process, the salt concentration in the water increases continuously, and it is precipitated after the solubility of some salts. Common are calcium carbonate, calcium phosphate, magnesium silicate, etc. The scale is compact in texture, which greatly reduces the heat transfer efficiency. The thickness of 0.6mm makes the heat transfer coefficient reduced by 20%.

(2) Dirt: the dirt is mainly composed of organic matter, microbial colony and secretion, sediment and dust in water. The soft texture of the scale not only reduces the heat transfer efficiency, but also causes corrosion under the scale, and shortens the service life of the equipment.

(3) Corrosion: the corrosion of circulating water on heat exchange equipment is mainly electrochemical corrosion, which is caused by equipment manufacturing defects, sufficient oxygen in water, corrosive ions (cl-, fe2+, cu2+) in water and dirt generated by mucus secreted by microorganisms. The corrosion result is ten points serious, even if the heat exchanger and water pipeline equipment are abandoned without control for a short time.

(4) Microbial slime: because the circulating water has sufficient oxygen, suitable temperature and rich culture conditions, it is very suitable for the growth and reproduction of microorganisms. If it is not controlled in time, the water quality will deteriorate, stink and blacken quickly, a large amount of sticky scale in cooling tower will be deposited or even blocked, and the cooling and cooling effect will be greatly reduced and equipment corrosion will be intensified. Therefore, the reproduction of microorganisms must be controlled in circulating water treatment.

Microbial hazard

The microorganisms in the circulating cooling water come from two aspects. One is that a large amount of air is needed in the evaporation process of water in the cooling tower, and the microorganism also brings into the cooling water with the air. Second, there will be microorganisms in the supplementary water of the cooling water system, and these microorganisms will also enter the cooling water system with the supplementary water.

Under the sunlight, algae will photosynthesis with carbon sources such as carbon dioxide and bicarbonate in water, absorb carbon as nutrition and release oxygen. Therefore, when algae propagate in large quantities, it will increase dissolved oxygen content in water, which is conducive to depolarization of oxygen, and the corrosion process will accelerate. The mass reproduction of microorganisms in the circulating water system will make the color of circulating water black, and cause odor and pollute the environment. At the same time, a large amount of clay will be formed to reduce the cooling efficiency of the cooling tower and the wood will deteriorate and rot.

The clay is deposited in the heat exchanger, which reduces the heat transfer efficiency and increases the head loss. The mud deposited on the metal surface will cause serious corrosion under scale, and it also insulates the corrosion and scale inhibitor from the metal role, so that the agent can not play its proper corrosion and scale inhibition effect. Besides accelerating the corrosion under scale, some bacteria will directly corrode metal in the process of metabolism. All these problems lead to the failure of circulating water system to operate safely for a long time, which affects production and causes serious economic losses. Therefore, the harm of microorganisms is the same as that of scale and corrosion to cooling water system. Even, it can be said that the control of microbial hazards is the first.

The trends of microorganisms in circulating water can be measured by the following chemical analysis items:

(1) When adding chlorine to residual chlorine, we should pay attention to the time and quantity of residual chlorine, because the serious reproduction of microorganisms will increase the chlorine consumption in circulating water.

(2) Ammonia circulating water generally does not contain ammonia, but ammonia will appear in the water due to process medium leakage or ammonia in air inhalation. At this time, it is not easy to take care of it. Besides actively seeking for ammonia leakage point, attention should be paid to whether there is nitrite in water. The ammonia content in water is better controlled below 10mg / L.

(3) NO2 - when ammonia and nitrite are found in water, it is said that nitrous bacteria in water have converted ammonia into nitrite. At this time, chlorination in circulating water system will become very difficult, and the consumption of chlorine will increase, and residual chlorine is difficult to reach the target. The content of NO2 in water is better controlled at less than 1mg / L.

(4) When the microorganism in the COD water is seriously propagated, COD will increase. Because the mucus secreted by bacteria increases the organic matter content in the water, the trend of microbial change in water can be observed through the analysis of chemical oxygen demand. Under normal conditions, COD in water is better less than 5mg/l (KMnO4 method).

The harm caused by microorganisms in circulating water is very serious. If we want to take measures after the microorganism causes the harm, it is often a great effort to spend a lot of biocides and money. Therefore, it is necessary to monitor the microorganism condition of circulating cooling water in advance.

Concentration water multiple

The concentration ratio of circulating water refers to the ratio of continuous concentration of circulating water (compared with the supplementary water) due to the evaporation of water and loss of wind blowing during the operation of the circulating water system. It is an important comprehensive index to measure the quality of water quality control. The concentration ratio is low, water consumption and sewage discharge are large, and the efficiency of water treatment agent is not fully exerted; High concentration ratio can reduce water quantity and save water treatment cost; However, the concentration multiple is too high, the scaling tendency of water will increase, the difficulty of scaling control and corrosion control will increase, and the water treatment agent will fail, which is not conducive to the control of microorganisms. Therefore, the concentration multiple of circulating water should have a reasonable control index.

Formation of scale

In the circulating water system, scale is formed by supersaturated water soluble components. There are various salts dissolved in the water, such as bicarbonate, carbonate, chloride, silicate, etc., among which the dissolved bicarbonate such as Ca (HCO3) 2 and Mg (HCO3) 2 are dissolved   The most unstable, it is easy to decompose to form carbonate. Therefore, when there are more bicarbonate dissolved in cooling water, the water flow will be decomposed by heating through the surface of heat exchanger, especially on the surface with high temperature; When phosphate and calcium ions are dissolved in water, the precipitation of calcium phosphate will also be produced; The solubility of calcium carbonate and CA3 (PO4) 2 is different from that of ordinary salts. The solubility of calcium carbonate and CA3 (PO4) 2 is not increased with the increase of temperature, but decreases with the increase of temperature. Therefore, on the heat transfer surface of heat exchanger, these insoluble salts are easy to reach supersaturated state and crystallize in water. Especially when the water flow velocity is small or the heat transfer surface is rough, these crystalline precipitates will be deposited on the heat transfer surface to form the commonly called scale. Because the scale is dense and hard, it is also called hard scale. The common scale composition is calcium carbonate, Calcium sulfate, calcium phosphate, magnesium salt, silicate.

Circulating water treatment technology

According to the characteristics and process conditions of the circulating water system of the enterprise, combined with the local water quality characteristics, the water treatment scheme suitable for the operation conditions of the enterprise is selected. Through the measures of dosing, the circulating water index can be controlled to operate within a certain range, which not only ensures the long-term operation of the production equipment, but also increases the utilization rate of circulating water. The utilization of circulating water treatment technology can bring remarkable economic benefits to enterprises and good social benefits to the society. So it is necessary to apply the technology of circulating water treatment.