Scale control" in chemical boilers means the control of scale that forms due to the deposition of minerals, mainly calcium and magnesium, on the heating surfaces of the boiler. This control is very important to maintain heat transfer efficiency, prevent overheating, and extend boiler life. Here is the full explanation:

1. Causes of Scale Formation

Scale is formed due to the presence of:

• Water hardness (high Ca²⁺, Mg²⁺)

• High temperature and high pressure, which causes dissolved solids to precipitate.

• Uncontrolled water pH, which accelerates the precipitation of carbonates or silicates.

Example of scale formation reaction:

Ca(HCO₃)₂ → CaCO₃↓ + CO₂ + H₂O

2. Common Types of Scale Formed

3. Scale Control Methods

a. Pre-treatment (before entering the boiler)

• Softening (ion exchange): Replacing Ca²⁺ and Mg²⁺ with Na⁺.

• Reverse osmosis (RO): Filters out almost all solutes.

• Deaeration: Removes O₂ and CO₂ gas to prevent corrosion.

b. Internal treatment (in the boiler)

Using boiler chemicals that function as:

1. Phosphate treatment: Deactivates Ca²⁺ into sludge that is easily removed.

   Example: Trisodium phosphate (Na₃PO₄)

2. Chelant treatment: Binds metal ions to keep them dissolved (avoids precipitation).

   Example: EDTA, NTA

3. Polymer dispersant: Keeps solid particles suspended.

   Examples: Polymethacrylate, Polyacrylate

4. pH control chemical: Maintain a pH of 10.5-11 to avoid corrosion and scale formation.

   Usually uses NaOH or volatile amines.

5. Blowdown control: Removes part of the boiler water to maintain TDS and salt concentration.

4. Operational Parameters to Maintain

5. Monitoring and Maintenance

• Daily tests: pH, conductivity, phosphate, hardness.

• Weekly tests: Silica, alkalinity, TDS.

• Routine inspection: Visual check for scale and corrosion.

• Chemical cleaning: If scale has formed.