In the circulating cooling water system, due to the inlet water quality and other reasons, it often leads to scale and corrosion. At the same time, there will be a large number of bacteria breeding, thus forming dirt. This paper will introduce the common control methods of scale and dirt in circulating cooling water.

1、 Control method of scale

Various salts are dissolved in natural water, such as bicarbonate, sulfate, chloride, silicate, etc. Among them, dissolved bicarbonate, such as calcium bicarbonate and magnesium bicarbonate, is the most unstable and easy to decompose into carbonate.

In most cases, the scale formed on the heat transfer surface of the heat exchanger is mainly calcium carbonate, because the solubility of calcium sulfate is much greater than that of calcium carbonate.

Scale control methods include the following:

01 ion exchange resin method

Let the water pass through the ion exchange resin to replace calcium and magnesium ions from the water and combine them on the resin to remove calcium and magnesium ions from the water.

The cost of softening make-up water by ion exchange is high. Therefore, only the circulating cooling water system with small make-up water can be used occasionally.

02 lime softening method

Before the make-up water enters the circulating cooling water system, add appropriate lime during pretreatment to make the calcium bicarbonate in the water react with lime in the clarifier in advance to form calcium carbonate precipitation, so as to remove calcium ions in the water.

This method is often used in circulating cooling water systems with low cost of adding lime, high calcium content of raw water and large amount of make-up water.

03 adding acid

Sulfuric acid is usually added, because adding hydrochloric acid will bring in chloride ions and increase the corrosivity of water.

Adding nitric acid will bring in nitrate, which is conducive to the reproduction of nitrifying bacteria.

04 pass CO2 gas

Pass carbon dioxide gas or flue gas into water to destroy the precipitation balance and make it move to the left, so as to stabilize bicarbonate.

05 add scale inhibitor

The growth of crystal can be controlled by adding calcium carbonate.

At present, various scale inhibitors used include polyphosphate, organic polybasic phosphonic acid, organic phosphate, polyacrylate, etc.

2、 Dirt control method

Dirt is generally composed of fine sediment, dust, mud of insoluble salts, colloidal hydroxide, debris, corrosion products, oil stain, especially the bodies of bacteria and algae and their viscous secretions.

This kind of dirt is also called soft dirt because of its large volume and loose texture. They are not only the main cause of corrosion under scale, but also the hotbed for the survival and reproduction of some bacteria such as anaerobic bacteria.

When the anti-corrosion measures are not taken, the surface of the heat exchange tube of the heat exchanger will often have rust nodules attached, and its shell is hard, but the interior is loose and uneven. They are often deposited on the heat transfer surface of the heat exchanger together with scale and microbial slime. In addition to affecting heat transfer, the sediment formed by this kind of rust nodular corrosion products will promote the reproduction of some bacteria and iron bacteria, and eventually lead to pipe wall corrosion perforation and leakage.

The control methods of dirt include:

01 reduce turbidity of make-up water

The turbidity of natural water body is composed of sediment, corrosive organisms, various suspended solids and colloids.

As for circulating water, the lower the turbidity, the better, indicating that there are less impurities in circulating water. When the inlet 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. when the form of heat exchanger is plate or finned tube and spiral plate, it should not be greater than 10mg / L.

02 do a good job in circulating cooling water quality treatment

Circulating cooling water is mainly used to prevent scale and corrosion. When the inlet water quality is not good, it is bound to lead to scale and corrosion. At the same time, a large number of bacteria will breed to form dirt. Therefore, doing a good job in water quality treatment is the way to reduce the dirt produced by the system.

03 add scale inhibitor and dispersant

Adding a certain amount of dispersant is also an effective method to control dirt during scale inhibition, corrosion prevention, sterilization and algae killing treatment.

The dispersant can disperse the large mud mass into fine particles suspended in the water, which will not be deposited on the heat transfer surface with the flow of water, so as to reduce the impact of dirt on heat transfer. At the same time, some suspended solids can also be discharged into the circulating water system with the discharged sewage.

04 add side filter equipment

Even if the water quality treatment is done very well, the turbidity of the influent is well controlled, and the turbidity of the circulating water will continue to rise, aggravating the formation of dirt.

The reason why the turbidity of the circulating cooling water system will increase under the condition of stable operation is that when the cooling water contacts with the air through the cooling tower, the dust in the air will be brought into the water, resulting in the increase of turbidity.

The addition of bypass filter equipment can reduce the dust entering the circulating water. As long as the side flow and the turbidity of the inlet and outlet side flow equipment are controlled, it can ensure that the turbidity of the system will not increase under long-term operation and will be maintained within the control index, so as to reduce the generation of dirt.