How to deal with circulating water when the heat exchanger is severely scaled

Is it improper control of temperature and flow rate? Excessively high circulating water temperature and low flow rate are also among the main causes of scaling. Causes of scaling in circulating water and methods to prevent it 1. Formation of solid substances ⑴ Reasons for the formation of scale: ① Crystallization of salts from multi-component supersaturated solutions ； ②Deposition of organic colloids and mineral colloids ； ③Bonding of solid particles of certain substances with different degrees of dispersion ； ④Products resulting from the electrochemical reduction of certain substances, etc. The total deposition of the above mixture is collectively referred to as fouling. ⑵Reasons for scale formation: The solid-phase precipitation resulting from dissolved salts in water is the main factor behind scale formation. The conditions for such solid-phase precipitation are as follows: ① As temperature rises, the solubility of certain salts decreases, such as Ca(HCO3)2, CaCO3, Ca(OH)2, CaSO4, MgCO3, Mg(OH)2, etc ； ②As water evaporates, the concentration of dissolved salts in the water increases until it reaches a supersaturated level ； ③Chemical reactions occur in the heated water, causing certain ions to form other insoluble salt ions. Certain salts that meet the above conditions first deposit primitive crystal embryos in individual areas on the metal surface, which then gradually merge and grow around these cores. It deposits easily on metal surfaces because these surfaces have a certain degree of roughness at the microscopic level; the irregularities at this scale serve as nuclei for solid crystallization in the supersaturated solution ； At the same time, the oxide film on the heating surface also has a strong adsorption force on the solid phase. Calcium and magnesium, the salts that make up scale, form solid crystalline nuclei in supersaturated solutions; these nuclei gradually turn into particles with an amorphous or cryptocrystalline structure. Subsequently, these particles aggregate with one another to form crystals or flocs. The formation of solid-phase sediment is related to the rate of formation of the germ core, that is, it is associated with the number of crystal nuclei formed per unit time and the linear growth rate of crystals; these two factors, in turn, are related to the water temperature, the salt concentration in the water, and the presence of other impurities. 2. Decomposition of bicarbonates: The main cause of scaling in cooling water is the presence of large amounts of calcium bicarbonate in the water; during heating, this substance loses its equilibrium and decomposes into calcium carbonate, carbon dioxide, and water. Calcium carbonate has low solubility, so it deposits first on the surface of the cooling equipment. Factors such as temperature and pressure also affect the intensity and rate of scaling. Calcium bicarbonate is a counter-soluble salt; above a certain temperature (the critical point), its saturated concentration decreases sharply. 3. Calcium and magnesium carbonate scale: Carbonate scale typically forms as dense crystalline deposits on the walls of heaters, as well as on the packing or walls of cooling towers. However, when the water temperature at the superheated surface exceeds 100°C, the CaCO3 precipitate appears as spongy flocculates. Although precipitation of calcium sulfate may occur below the boiling temperature, this is only a special case; since the solubilities of the three forms of calcium sulfate—CaSO4, 2CaSO4•H2O, and CaSO4•2H2O—are all very high, there is no need to worry about calcium sulfate deposition under the specific conditions of cooling water. The solubility of calcium hydroxide also decreases as temperature rises, but under normal conditions calcium hydroxide does not form in water, so it need not be considered. The key lies in the carbonates of calcium and magnesium: Ca2++2HCO3=H2O+CO2↑+CaCO3↓; Ca(HCO3)2=CaCO3↓+H2O+CO2↑; Mg(HCO3)2=MgCO3↓+H2O+CO2↑. The solubility of MgCO3 is more than six times greater than that of CaCO3, and MgCO3 in water hydrolyzes rapidly. MgCO3 + H2O = Mg(OH)2↓ + CO2↑; it exists in water in the form of Mg(OH)2. The solubility of Mg(OH)2 changes slowly with rising temperature, so deposition hardly occurs. Moreover, the concentration of calcium ions in natural water is much higher than that of magnesium ions, meaning that the deposition of magnesium salts has a minimal impact and can be ignored. 4. Composition and sources of scale ⑴ Classification of dirt: The deposition of inorganic salts is referred to as “scale”; it has a fixed crystal structure and is relatively hard. Its main components are Ca3(PO4)2, CaCO3, SiO2, as well as magnesium salts and iron oxides. Organic matter, fungi, algae, suspended particles, etc. are referred to as \"contaminants\", that is, the colloidal sludge present in water. It is relatively soft and has no fixed shape. ⑵Contaminant components: Contaminants include scale, corrosion products, biological sludge, suspended sediment deposition, etc. Its main components are: SiO2, P2O5, SO42-, Fe2O3, Al2O3, CaO, MgO, CuO, CO2, and loss on ignition. ⑶Sources of dirt: colloidal organic sludge, raw water residues, pollutants, soluble iron, microbial contaminants, dust, air-borne particles ； Active gases: H2S, SO2, NH3, etc ； Source water scale: CaCO3, CaSO4, MgSiO3 ； Corrosion products: Fe2O3; leaks in circulating water production: hydrocarbons, sulfides, microbial fouling. The fouling in cooling water systems is not merely the deposition of calcium and magnesium carbonates; the components of this fouling and the factors that lead to its formation are complex. ①Crystallization: The precipitation of dissolved salts, which are mainly carbonates of calcium and magnesium, as mentioned earlier ； Silicate scale may also form on the heating surface of heat exchangers (when the SiO2 content in water is >200PPM), as well as hard scale containing complex salts of calcium, magnesium, aluminum, sodium, etc. When the levels of phosphates and iron in water are high while the alkalinity is low, iron phosphate scale, NaFePO4, may form. The types of crystalline scale are roughly as follows: a. Thermal decomposition of calcium bicarbonate at low temperatures: soft α-CaCO3 scale ； b. Deposition of Mg(OH)2 at high pH: Mg(OH)2 ； c. Dense calcite on the wall surface, β-CaCO3 ； d. Excess iron ions: FeCO3. ②Deposition: Deposition of corrosion products, clay, and sand dust, such as the silt and suspended particles brought in by the water being added, or the dust particles carried in by the air entering the cooling tower ； Sludge formed by process impurities, grease, bacteria, algae, and fungi that leak into the heat exchanger. These deposits often adsorb onto the surface of chemically reactive crystal scale, acting as a catalyst for the reactions and accelerating the formation of dirt. ③Reactions and polymerization: deposition of sludge, organic oxides, etc. ④Rough shell caused by corrosion: ⒌ Scale inhibition mechanism – Calcium and magnesium ions in water combine with carbonate, phosphate, and other ions to form insoluble small crystals; these small crystals continuously collide with each other and grow in a certain direction to become larger crystals. The calcium and magnesium salt crystals in water, along with their insoluble particles, are subject to two forces: the crystallization force that causes them to combine with the scale on the pipe walls to form larger crystals of scale, and the shear force exerted by the water flow. When the crystallization force is strong, the scale tends to grow; whereas when this force is weak (such as after the addition of scale inhibitors) or the shear force is high (such as in areas with fast water flow), the scale cannot thicken, and the particles in the water are carried away by the water in the form of sediment. ⑴Solubilization mechanism: Some scale inhibitors are able to form water-soluble complexes with calcium and magnesium ions in water that are more stable than insoluble salts such as calcium carbonate, thereby preventing calcium and magnesium from forming small crystals like calcium carbonate. This scale inhibitor can not only prevent the formation of scale, but also remove existing scale when its concentration is high enough. ⑵Mechanism of lattice distortion: The active groups of scale inhibitors bind to calcium on crystals such as calcium carbonate. Due to the spatial interference posed by these inhibitor molecules, insoluble salts like calcium carbonate are unable to grow in their normal lattice structure; as a result, their crystallization strength is reduced, the scale becomes softer, and it can be easily washed away by water flow. ⑶Self-dissolution mechanism: The scale-inhibiting molecules precipitate together with small crystals of insoluble salts to form scale. Since these molecules disrupt the lattice structure of the scale, the crystalline bonds between the scale particles are weak; as a result, such scale cannot form a solid structure and only results in soft scale. As the scale thickens, the force exerted by the water flow increases as well. When this force exceeds the crystallization force, the scale, along with the scale-inhibiting molecules, breaks off and is carried away by the water. Therefore, with this scale inhibitor added, only a thin layer of scale forms in the circulating water. ⑷Dispersion mechanism: When certain scale-inhibiting agent molecules are added to water, they hydrolyze and ionize to produce polymeric anions. These anions strongly adsorb onto the surfaces of various particles in water, causing these particles to carry a negative charge. Due to the repulsive force of static electricity, these negatively charged particles cannot collide to form larger crystals; they can only remain suspended in water in a dispersed state. ⒍Measures to control scaling: (1) Water softening – Treat the circulating water using lime softening, reverse osmosis, or ion exchange methods to reduce or remove calcium and magnesium ions from the water; this prevents the formation of a saturated solution, thereby stopping scale formation. Softened water eliminates the risk of scale formation, but it cannot prevent the formation of dirt. At the same time, the corrosion rate of soft water is much higher than that of hard water. Many effective and commonly used corrosion inhibitors require calcium and magnesium ions to function, so the use of soft water as a source for circulating water imposes limitations on the selection of such inhibitors. ⑵By adding acid or CO2 gas, bicarbonates exist in water under the following equilibria: Ca(HCO3)2 = Ca2+ + 2HCO3-, HCO3- = H+ + CO32-, and Ca(HCO3)2 = CaCO3 + H2O + CO2↑. It can be seen from these dissociation equilibria that both the addition of acid and CO2 can stabilize Ca(HCO3)2. However, this method can only prevent the formation of carbonate scale, and it has no effect on other types of scale. If acid addition is not controlled properly, such as by adding too much acid or at too fast a rate, which leads to high local concentrations, it can easily cause corrosion of the metal. ⑶Adding bypass filtration equipment can effectively slow down the accumulation of dirt in open cooling water systems. This is because dust carried in the air, dead algae and fungi, as well as various impurities brought in by the supplementary water, can be filtered out by the side filtration equipment. But this method cannot prevent the formation of scale. ⑷Electronic treatment offers good scale prevention and removal effects for small systems or specific components such as condensers, but its effectiveness is limited in large systems or those with long circulation cycles. ⑸The addition of scale inhibitors and dispersants: From the mechanism of scale formation, it can be seen that scale is formed first by the creation of nuclei, which give rise to a small number of microcrystals. Due to Brownian motion and collisions with the metal surface, these microcrystals adhere to the metal surface and continue to grow larger. Therefore, scale inhibitors can be added to disrupt the crystal lattice of scale and prevent it from growing thicker. Or a dispersant can be added to stably disperse these particles in water, preventing them from depositing on the vessel walls and forming dirt. ①Acidification lowers the pH value: Sulfuric acid is commonly used, at a dosage of 80–90 mg/L, to keep the pH between 6.0 and 6.5. ②Chelating agents such as polyphosphates, thiophosphates, and polyol esters, also known as scale inhibitors, are added.

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It seems that the flow rate isn’t high; for the entire water softening system, 5 cubic meters is sufficient