First, scaling deposition:
In industrial cooling water systems, the primary water source is usually groundwater or municipal tap water. These water sources often contain high levels of dissolved solids, which accumulate as the water circulates through the system. As the cooling water is reused, the concentration of these substances increases, especially when the system operates at high temperatures. This leads to changes in water quality, and when the concentration of certain dissolved salts exceeds their solubility limits, scaling begins to form.
Common scale components in cooling water include calcium carbonate (CaCO₃), calcium phosphate (Ca₃(PO₄)₂), calcium sulfate (CaSO₄), and calcium silicate. These compounds have very low solubility, with CaCO₃ having a solubility of about 20 mg/L at 0°C and Ca₃(PO₄)₂ only 0.1 mg/L. Their solubility decreases further with increasing temperature and pH, making them prone to crystallization under supersaturated conditions, particularly on hot heat transfer surfaces. If the water flow is slow or the surface is rough, these crystals tend to deposit, forming hard scale layers that reduce efficiency and can lead to equipment damage over time.
To manage scaling, three main strategies are commonly used: (1) reducing the concentration of scale-forming ions to keep them within acceptable limits; (2) stabilizing the equilibrium of these ions in the water; and (3) preventing crystal growth by using chemical inhibitors. The choice of method depends on factors such as the volume of circulating water, operational requirements, and the availability of treatment chemicals. A well-planned approach ensures long-term protection and optimal performance of the cooling system.
2. Corrosion:
Corrosion in carbon steel caused by cooling water is an electrochemical process. Due to the inherent non-uniformity of metal surfaces and the varying conditions of the surrounding solution, different areas on the metal surface act as anodes (low potential) and cathodes (high potential). These regions form small galvanic cells, where oxidation occurs at the anode and reduction at the cathode, leading to gradual degradation of the metal.
The basic reactions involved are: At the anode: Fe → Fe²⺠+ 2e⻠At the cathode: ½O₂ + H₂O + 2e⻠→ 2OH⻠Then: Fe²⺠+ 2OH⻠→ Fe(OH)₂↓ And finally: 2Fe(OH)₂ + ½O₂ + H₂O → 2Fe(OH)₃↓
Because surface irregularities are unavoidable, electrochemical corrosion is widespread. As long as the metal is exposed to oxygen-containing water, these reactions will continue, potentially causing significant damage over time.
Three primary methods are used to control corrosion: (1) applying protective coatings like electroplating or painting to isolate the metal from the water; (2) using electrochemical protection, such as sacrificial anodes (e.g., zinc or magnesium) or impressed current systems to convert the metal into a cathode; and (3) adding corrosion inhibitors to form a protective film on the metal surface. This last method is widely used globally and often involves a pre-film treatment with higher inhibitor concentrations before normal operation, followed by maintenance doses to ensure long-term protection.
3. Microbial Growth:
In open-loop cooling water systems, large volumes of water are returned to the cooling tower after being used in process equipment. During evaporation, the concentration of organic matter, inorganic substances, and microorganisms increases, creating favorable conditions for microbial growth. Warm water combined with nutrients supports rapid microbial proliferation, leading to the formation of biofilms or sludge.
These biological deposits reduce heat transfer efficiency, accelerate metal corrosion, clog pipelines, and cause foul odors when the sludge deteriorates. The resulting failures often occur alongside scaling and corrosion issues. Two main types of microbial-related failures exist: (1) slime adhesion, where microbes and their byproducts accumulate on surfaces like pipes, tank walls, and cooling tower fill; and (2) sludge accumulation, which forms in low-flow areas, such as the bottom of tanks, and leads to soft, sediment-like blockages. Both types can affect heat exchangers and distribution systems.
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