Impurities dissolved in water in ionic or molecular form
Calcium salts
The main components in water are Ca(HCO3)2, CaCl2, CaSO4, CaSiO3, etc. Calcium salts are the main component that causes scaling of heat exchangers. Among them, CaSO4 is a hard, finely crystalline scale with a loose structure and low adhesion. It is a relatively soft sludge. It is fluid when separated from water and is easy to remove even if it is attached to the heated surface.
Magnesium salts
The main components in water are Mg(HCO3), MgCl2, MgSO4, etc. After magnesium is dissolved in water, it decomposes under heat to form Mg(OH)2 precipitation, which is also a sludge scale. MgCl2 and MgSO4 dissolved in water will cause acidic corrosion of the metal wall due to hydrolysis when the water pH is <7.
Sodium salts
The main components are NaCl, Na2SO4, NaHCO3, etc. NaCl does not generate scale, but the presence of free oxygen in the water will accelerate the corrosion of the metal wall; if the content of Na2SO4 is too high, salt will form on the accessories after the evaporator, affecting safe operation; NaHCO3 in the water will decompose into NaCO3, NaOH, and CO2 under the action of temperature and pressure, which will damage the metal grains.
Dissolved oxygen gas
There are many reasons for the corrosion of heat exchangers, but the most serious and fastest corrosion is oxygen. In the atomic order table, the potential of iron is above that of hydrogen. In neutral water without oxygen, the iron atoms on the surface of the metal lose electrons to become divalent ions (Fe-2e→Fe2+). Fe2+ ions and OH- ions in water combine under the action of electrostatic attraction [Fe2++2OH-→Fe(OH)2], and establish the following equilibrium in water:
Fe2++2OH-=Fe(OH)2
When oxygen is present in water, Fe(OH)2 is further oxidized to insoluble iron hydroxide and precipitates out:
4Fe(OH)2+O2+2H2O→4Fe(OH)3↓
Due to the precipitation of Fe(OH)3, the iron ions around the anode are transferred into the aqueous solution, which accelerates the corrosion.
From the above reaction, it can be seen that water and oxygen are necessary conditions for corrosion. The anode is the site of corrosion, and the cathode is the site where the corrosion products accumulate. When corrosion is basically uniform over the entire metal surface, the corrosion rate will not be very fast, so the harm is not great. This type of corrosion is called general corrosion. When corrosion is concentrated on certain parts of the metal surface, it is called local corrosion. Local corrosion is very fast and easy to rust through. Pit corrosion is a common local corrosion in heat exchangers, so it is very harmful.
Impurities existing in colloidal state
Iron compounds
The main component is Fe2O3, which will generate iron scale. When there are more iron compounds in water, the water is often yellow.
Microorganisms
Since the water temperature, dissolved oxygen, nutrients, etc. of the air conditioning cooling circulating water provide conditions conducive to the reproduction of microorganisms, microorganisms will breed and multiply in large numbers. Microorganisms come from the soil and air. When the temperature of the cooling circulating water is high, it is aerated by the cooling tower, and the oxygen content increases. Phosphates and other agents are often added to the water, which is exactly the nutrients for microorganisms. Most cooling towers are located in the open air, and sunlight is conducive to the growth of algae. The reproduction of microorganisms not only blocks the plate channel, but sometimes also blocks the pipeline, and also causes metal corrosion.
Sludge
The sludge in the cooling circulating water comes from dust in the air and suspended matter in the make-up water. During the convective exchange process between air and water, a large amount of air is sprayed with circulating water in the tower, causing dust to enter the water and gradually settle in the heat exchanger with a lower flow rate.
Scaling
It is mainly formed by the secretions of microorganisms and the sediment, corrosion products, and algae debris in the water. They often adhere to the wall of the heat exchanger and produce various organic acids, which can also cause corrosion.