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2018

The harm of impurities in water to heat exchangers

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1. The hazards of impurities dissolved in water in ionic or molecular form a. The main components of calcium salts in water include Ca(HCO3)2, CaCl2, CaSO4, CaSiO3, etc. Calcium salts are the main component causing scaling in heat exchangers. Among them, CaSO4 is a hard, finely crystalline scale, with a loose structure and low adhesion, making it a relatively soft sludge that can flow and is easy to remove even when attached to heated surfaces. b. The main components of magnesium salts in water include Mg(HCO3), MgCl2, MgSO4, etc. Magnesium dissolves in water and

1. The hazards of impurities dissolved in water in ionic or molecular form.

　　a. Calcium salts mainly consist of Ca(HCO3)2, CaCl2, CaSO4, CaSiO3, etc. Calcium salts are the main component causing scaling in heat exchangers. Among them, CaSO4 is a hard, finely crystalline scale with a loose structure and low adhesion, making it a relatively soft sludge that can easily be removed even if it adheres to heated surfaces.

　　b. Magnesium salts mainly consist of Mg(HCO3), MgCl2, MgSO4, etc. When magnesium dissolves in water, it decomposes upon heating to form Mg(OH)2 precipitate, which is also a sludge-type scale. MgCl2 and MgSO4 dissolved in water can cause acidic corrosion of metal walls when the water pH is less than 7 due to hydrolysis.

　　c. Sodium salts mainly consist of NaCl, Na2SO4, NaHCO3, etc. NaCl does not form scale, but the presence of free oxygen in water can accelerate the corrosion of metal walls. Excessive Na2SO4 content can lead to salt formation on accessories after the evaporator, affecting safe operation; NaHCO3 in water can decompose into NaCO3, NaOH, and CO2 under temperature and pressure, which can damage metal grains.

2. The hazards of dissolved oxygen gas.

　　There are many reasons for corrosion in heat exchangers, but the most severe and fastest corrosion is caused by oxygen. In the atomic periodic table, iron has a potential above hydrogen. In neutral water without oxygen, iron atoms on the surface of the system lose electrons to become divalent ions (Fe-2e→Fe2+). The Fe2+ ions combine with OH- ions in water under electrostatic attraction [Fe2++2OH-→Fe(OH)2], establishing the following equilibrium in water:

Fe2++2OH-=Fe(OH)2

When oxygen is present in water, Fe(OH)2 is further oxidized to form insoluble iron hydroxide precipitate:

4Fe(OH)2+O2+2H2O→4Fe(OH)3↓

The precipitation of Fe(OH)3 causes iron ions around the anode to enter the aqueous solution, accelerating the corrosion process.

From the above reactions, it can be seen that water and oxygen are necessary conditions for corrosion, with the anode being the area of corrosion and the cathode being the area where corrosion products accumulate. When corrosion occurs uniformly across the entire metal surface, the rate of corrosion is not very fast, so the hazard is not great; this type of corrosion is called general corrosion. When corrosion is concentrated in certain areas of the metal surface, it is called localized corrosion. Localized corrosion occurs rapidly and can easily lead to rust-through; pitting is a common form of localized corrosion in heat exchangers, making it very hazardous.

　　3. The hazards of impurities existing in colloidal form in heat exchangers.

　　a. Iron compounds mainly consist of Fe2O3, which can generate iron scale. When there are many iron compounds in water, the water often appears yellow.

　　b. Microorganisms thrive due to the favorable conditions provided by the temperature, dissolved oxygen, and nutrients in the air conditioning cooling circulating water. Microorganisms reproduce in large numbers. They originate from soil and air, and when the temperature of the cooling circulating water is high, aeration in the cooling tower increases the oxygen content. Phosphates and other agents are often added to the water, which serve as nutrients for microorganisms. Cooling towers are mostly located outdoors, and sunlight promotes the growth of algae. The proliferation of microorganisms not only blocks the channels of the plates but can also clog pipes and cause metal corrosion.

　　c. Sludge in cooling circulating water comes from dust in the air and suspended matter in the makeup water. During the convective exchange process between air and water, a large amount of air receives the sprayed circulating water in the tower, causing dust to enter the water and gradually settle in the heat exchanger where the flow rate is lower.

　　d. Biofouling is mainly formed by the secretions of microorganisms combined with sand, corrosion products, and debris from algae in the water. They often adhere to the walls of the heat exchanger, producing various organic acids, which can also cause corrosion.

　　Therefore, the quality of the fluid water in heat exchangers is very important. In operational management, it is essential to strengthen attention, equip necessary anti-scaling and anti-corrosion devices, and extend the service life of the equipment.

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Chongqing Southwest Saiwell Heat Exchange Equipment Co., Ltd.

Specializing in the research, development, production and sales, equipment consulting, maintenance, and accessory services of heat exchange technology, equipment, and systems, and undertaking the design and installation of thermal system engineering, energy conservation and emission reduction, and energy conservation and emission reduction system engineering.

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