During the operation of industrial circulating cooling water systems, due to factors such as water evaporation and wind loss, the circulating water continuously concentrates, resulting in excessive salt content, an increase in cations and anions, and significant changes in pH value, leading to deterioration of water quality. The temperature, pH value, and nutritional components of the circulating water are conducive to the growth of microorganisms, and sufficient sunlight on the cooling tower is an ideal place for algae growth. And scaling control, corrosion control, microbial control, and so on, inevitably require circulating water treatment.
The main problems generated during the operation of circulating water
PART.1
Scale: Due to the continuous evaporation of circulating water during the cooling process, the salt concentration in the water increases continuously, 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 scale thickness of 0.6 millimeters reduces the heat transfer coefficient by 20%.
PART.2
Corrosion: Corrosion of circulating water heat exchange equipment, mainly electrochemical corrosion, is caused by factors such as equipment manufacturing defects, sufficient oxygen in water, corrosive ions (Cl -, Fe2+, Cu2+) in water, and dirt generated by microbial secretions. The consequences of corrosion are extremely serious, and if left uncontrolled, the heat exchanger and water pipeline equipment can be scrapped in a very short time.
PART.3
Microbial slime: Because circulating water contains sufficient oxygen, 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, blackening, and a large amount of fouling deposition or even blockage in the cooling tower. The cooling and heat dissipation effect will be greatly reduced, and equipment corrosion will intensify. Therefore, the treatment of circulating water must control the reproduction of microorganisms.
Microbial hazards
The microorganisms in 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 the supplementary water of the cooling water system will have more or less microorganisms, and these microorganisms will also enter the cooling water system with the supplementary water.
Under sunlight, algae undergo photosynthesis with carbon sources such as carbon dioxide and bicarbonate in water, absorbing carbon as nutrients and releasing oxygen. Therefore, when algae multiply in large numbers, it increases the dissolved oxygen content in water, which is beneficial for oxygen depolarization and accelerates the corrosion process.
The 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 sticky mud will be formed, which will reduce the cooling efficiency of the cooling tower and cause the 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 severe corrosion under scale, and it also isolates the corrosion and scale inhibitors from their effect on the metal, preventing them from exerting their intended corrosion and scale inhibition effects.
Microbial slime not only accelerates corrosion under scale, but some bacteria also directly corrode metals through biological secretions during metabolic processes. All these problems result in 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 to the cooling water system is as serious as that of scale and corrosion. It can even be said that controlling the harm of microorganisms is the top priority compared to the three.
The movement of microorganisms in circulating water can be measured through the following chemical analysis items:
1. When adding residual chlorine (free chlorine) for sterilization, attention should be paid to the time and amount of residual chlorine, as severe microbial growth can greatly increase the chlorine consumption in circulating water.
2. Ammonia circulating water generally does not contain ammonia, but leakage of process media or inhalation of ammonia into the air can also cause ammonia to appear in the water. At this time, we cannot take it lightly. In addition to actively searching for the leakage point of ammonia, we also need to pay attention to whether the water contains nitrite ions. It is best to control the ammonia content in the water below 10mg/L.
3. 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, it will be very difficult to add chlorine to the circulating water system, increasing chlorine consumption and making it difficult to achieve the target residual chlorine. It is best to control the NO2- content in water to be less than 1mg/L.
4. When the proliferation of microorganisms in chemical oxygen demand water is severe, COD will increase because the mucus secreted by bacteria increases the organic 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, COD in water is best to be less than 150mg/L. The harm caused by microorganisms in circulating water is very serious. If measures are taken after the microorganisms cause harm, it is often twice as effective and requires a lot of killing agents and money. Therefore, it is necessary to comprehensively monitor the microbial situation of circulating cooling water in advance.
Formation of scale
In the circulating water system, scale is formed by supersaturated water-soluble components, and various salts such as bicarbonate, carbonate, chloride, silicate, etc. are dissolved in the water. Among them, dissolved bicarbonate such as Ca (HCO3) 2 and Mg (HCO3) 2 are the most unstable and easily decompose to form carbonate. Therefore, when there is a large amount of dissolved bicarbonate in the cooling water, the water flows through the surface of the heat exchanger, especially the surface with higher temperature, and 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 insoluble and different from general salts. Their solubility does not increase with temperature, but decreases with 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 low 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, they are also known as hard scale. The common components of scale are calcium carbonate, calcium sulfate, calcium phosphate, magnesium salts, and silicates.
Concentration factor
The concentration ratio of circulating water refers to the ratio at which the circulating water is continuously concentrated during the operation of the circulating water system due to water evaporation, wind loss, and other factors (compared with supplementary water as a benchmark). It is an important comprehensive indicator for measuring the quality of water control.
Low concentration ratio, high water consumption and discharge, and insufficient utilization of water treatment agents; High concentration ratio 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 control and corrosion control will increase. Water treatment agents will become ineffective, which is not conducive to microbial control. Therefore, there should be a reasonable control index for the concentration factor of circulating water.
