Boiler chipping, cleaning and scaling

Boiler scale is caused by impurities being precipitated out of the water directly on heat transfer surfaces or by suspended matter in water settling out on the metal and becoming hard and adherent. Evaporation in a boiler causes impurities to concentrate. This interferes with heat transfers and may cause hot spots. Leading to local overheating. Scaling mechanism is the exceeding of the solubility limits of mineral substances due to elevated temperature and solids concentration at the tube/water interface. The deposition of crystalline precipitates on the walls of the boiler interferes with heat transfer and may cause hot spots, leading to local overheating. The less heat they conduct, the more dangerous they are.

Common feed water contaminants that can form boiler deposits include calcium, magnesium, iron, aluminum, and silica. Scale is formed by salts that have limited solubility but are not totally insoluble in boiler water. These salts reach the deposit site in a soluble form and precipitate.

The values corresponding to their thermal conductivity are:

  • Steel 15 kcal/m2.h per degree C
  • CaSO4 1-2 kcal/m2.h per degree C
  • CaCO3 0.5-1 kcal/m2.h per degree C
  • SiO2 0.2-0.5 kcal/m2.h per degree C

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