1. Impurities of impurities dissolved in water by ion or molecular state
a, calcium salts The main components in water are Ca (HCO3) 2, CaCl2, CaSO4, CaSiO3 and so on. Calcium salts are the main component of the heat exchanger fouling. Among them, CaSO4 is a hard, fine crystal scale with loose structure and low adhesion. It is a relatively soft sludge. It has fluidity separated from water and is easy to remove even if it is attached to the heated surface.
b, the main components of magnesium salt in water are Mg (HCO3), MgCl2, MgSO4 and so on. After the magnesium is dissolved in water, it forms a Mg(OH)2 precipitate after being thermally decomposed, and Mg(OH)2 is also a sludge-type scale. MgCl2, MgSO4 dissolved in water, at pH <7, causes acid corrosion of the metal wall due to hydrolysis.
c, sodium salt mainly composed of NaCl, Na2SO4, NaHCO3 and so on. NaCl does not form scale, but there is free oxygen in the water, which will accelerate the corrosion of the metal wall; if the content of Na2SO4 is too high, salt will be formed on the accessory behind the evaporator, which will affect the safe operation; the effect of NaHCO3 in water on temperature and pressure Under the decomposition of NaCO3, NaOH, CO2, the metal grains will be damaged.
2, the harm of dissolved oxygen gas
Heat exchangers have many causes of corrosion, but the most severe corrosion, the fastest is oxygen. On the atomic order table, the potential of iron is above hydrogen. In neutral water without oxygen, the iron atoms on the surface of the system metal lose electrons into divalent ions (Fe-2e & rarr; Fe2+), Fe2+ ions and OH in water. - Ions bind [Fe2++2OH-→Fe(OH)2] under electrostatic attraction and establish the following balance in water:
Fe2++2OH-=Fe(OH)2 When oxygen is present in the water, Fe(OH)2 is further oxidized to insoluble iron hydroxide precipitated:
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 part is the corroded part, and the cathode part is the part where the corrosion product is piled up. When the corrosion is carried out substantially uniformly over the entire surface of the metal, the rate of corrosion is not so fast, so the hazard is not large, and the corrosion is called general corrosion. When corrosion is concentrated on certain parts of the metal surface, it is called localized corrosion. The local corrosion rate is very fast, and it is easy to wear through. The pit corrosion is a common local corrosion in the heat exchanger, so the damage is very great.
3. Harm of impurities in the colloidal state on the heat exchanger
a, the main component of iron compounds is Fe2O3, which will form iron scale. When the water contains more iron compounds, the water is often yellow.
b, microbes due to air conditioning cooling circulating water water temperature, dissolved oxygen, nutrients and other micro-organisms provide conditions conducive to reproduction, microorganisms will breed and breed. Microorganisms are derived from soil and air. When the temperature of the cooling circulating water is high, the oxygen is increased by aeration in the cooling tower. Phosphate and other chemicals are often added to the water, which is just the microbial nutrient. The cooling tower is mostly located in the In the open air, sunlight is good for algae growth. The reproduction of microorganisms not only blocks the passage of the plates, but also blocks the pipes and corrodes the metal.
c, sludge in the sludge cooling circulating water, from the dust in the air and suspended matter in the supplementary water. During the convective exchange of air and water, a large amount of air is sprayed with circulating water in the tower, so that the dust enters the water and is gradually deposited in the heat exchanger with a lower flow rate.
d, slime is mainly caused by the secretion of microbes and the sediment, corrosion products, algae residue in the water, they often adhere to the wall of the heat exchanger, producing a variety of organic acids, This acid can also cause corrosion.
Therefore, the water quality requirements of the heat exchanger are very important. In the operation management, it should be emphasized, equipped with some necessary anti-scaling and anti-corrosion equipment to extend the service life of the equipment.