Analysis of the Causes and Hazards of Corrosion in the Circulating Water System

Refrigeration equipment can be divided into two types according to different refrigeration methods: compression refrigeration and absorption refrigeration.

With the use of refrigeration equipment, the refrigeration efficiency will gradually decrease. A careful analysis shows that this is caused by the following reasons:

Natural water contains various dissolved salts, such as bicarbonates, sulfates, chlorides, silicates, etc. Therefore, if water with a relatively high content of bicarbonates is used as cooling water, it will decompose when it passes through the heat transfer surface of the heat exchanger. The cooling water passing through the cooling tower is equivalent to an aeration process, and the dissolved CO₂ in the water will escape. As a result, the pH value of the water will increase.

At this time, bicarbonates will also react under alkaline conditions. Therefore, on the heat transfer surface of the heat exchanger, these slightly soluble salts are likely to reach a supersaturated state and crystallize out of the water. When the water flow rate is relatively low or the heat transfer surface is relatively rough, these crystalline deposits are likely to accumulate on the heat transfer surface. In addition, calcium sulfate, calcium silicate, magnesium silicate, etc. dissolved in the water will also form precipitates and deposit on the heat transfer surface when the product of the concentrations of their anions and cations exceeds their own solubility product.

Such deposits are usually called scale. Since these scales are composed of inorganic salts, they are also known as inorganic scales. Because these scale crystals are dense and relatively hard, they are called hard scales. They usually adhere firmly to the heat transfer surface and are not easily washed away by water.

Hazards:
① Reduce the heat transfer efficiency of the equipment by 3% - 10%.
② Easily block the steel pipes in the condenser. In mild cases, it will reduce the flow rate and affect the refrigeration effect. In severe cases, the steel pipes in the condenser may even be completely blocked, leading to the scrapping of the equipment.
③ Provide a place for the reproduction of some anaerobic bacteria.

During the normal operation of the cooling water system and the chemical cleaning process, metals often suffer from corrosion in different forms.
According to the knowledge of metal corrosion theory, by carefully observing the metal corrosion morphology of corrosion specimens or damaged equipment, and in combination with some other methods, people can often find out the causes of corrosion and measures to solve the corrosion problem.

Therefore, studying the corrosion morphology of metals in the cooling water system is a very useful method.

Hazards:
① Accelerate the corrosion of the equipment, which is likely to cause potential safety hazards.
② Affect normal production and shorten the service life of the equipment.
③ Increase the operation and equipment maintenance costs.

The microorganisms in the cooling water generally refer to bacteria and algae. In fresh water, there are generally fewer bacteria and algae. However, in the circulation process, due to the concentration of nutrients, the increase in water temperature, and the exposure to sunlight, conditions are created for the rapid reproduction of bacteria and algae. The mucus secreted by a large number of bacteria is like an adhesive, which can make the floating dust impurities and chemical precipitates in the water adhere together, forming a sticky deposit that adheres to the heat transfer surface of the heat exchanger. Some people call this deposit biological slime, and others call it soft scale.
When the slime accumulates on the wall of the heat exchanger tube, in addition to causing corrosion, it will also reduce the flow rate of the cooling water, thereby reducing the cooling efficiency of the heat exchanger. In severe cases, these biological slimes will block the tubes completely, forcing the production to be stopped for cleaning. For example, in a heat exchanger in a certain factory area in Beijing, a large number of bacteria and algae multiplied, and the heat load dropped to 50% within half a month. It was necessary to stop production frequently for flushing, resulting in a decrease in production.
Slime is generally composed of fine particles of sand, dust, insoluble salts in the form of mud, colloidal hydroxides, debris, corrosion products, oil stains, especially the corpses of bacteria and algae and their viscous secretions, etc. Improper water treatment control, too high turbidity of the make - up water, the introduction of fine sand, colloidal substances, etc. into the cooling water system, or untimely killing of bacteria and algae, or serious corrosion with a large amount of corrosion products, as well as careless operation, such as the leakage of oil stains and process products into the cooling water, will all intensify the formation of fouling. When such water quality flows through the surface of the heat exchanger, it is easy to form fouling deposits, especially when the water flows through the shell side, and more fouling deposits are formed in the parts with a slower flow rate. Since this kind of fouling is large in volume and loose and soft in texture, it is also called soft scale. They are the main cause of under - scale corrosion and also a hotbed for the survival and reproduction of some bacteria, such as anaerobic bacteria.

Hazards:
① The slime adheres to the metal surface of the heat transfer (cooling) part, reducing the cooling effect of the cooling water.
② A large amount of slime will block the cooling water channels in the heat exchanger (water cooler), making the cooling water unable to work. A small amount of slime will reduce the cross - sectional area of the cooling water channels, thereby reducing the flow rate of the cooling water and the cooling effect and increasing the pump pressure.
③ The slime accumulates on the surface of the cooling tower packing or between the packing, blocking the passage of the cooling water and reducing the cooling effect of the cooling tower.
④ The slime covers the metal surface inside the heat exchanger, preventing the corrosion inhibitor and scale inhibitor from reaching the metal surface to play their roles in corrosion inhibition and scale prevention, and preventing the biocide from killing the microorganisms in and under the slime, reducing the power.
⑤ The slime covers the metal surface, forming a differential corrosion cell, causing corrosion of these metal equipment.