**2.2 Particulate fouling**

Particulate fouling is evolved by the accumulation of solid particles suspended in the process stream onto the heat transfer surface. Heavy particles settle on a horizontal surface due to gravity and fine particles settle onto heat transfer surfaces at different inclinations due to suction force or other mechanisms. Unburned fuels or ashes deposition on boiler tubes, dust deposition on air cooled condensers etc. are examples of particulate fouling.

### **2.3 Precipitation fouling (sedimentation fouling)**

This kind of fouling is also called crystallization fouling. Dissolved inorganic salts are normally present in fluid used in heat exchanger. There is a maximum amount of the salt (saturated) which can be dissolved in this fluid. During heating or cooling supersaturation occurs in the dissolved inorganic salts. The inverse solubility salts such as calcium and magnesium sulphate, carbonate, silicate, etc. have less solubility in warm water up to a certain temperature than in cold water. This may occur when the process condition inside the heat exchanger is different from condition at the entrance. A stream on a wall at a temperature above that of corresponding saturation temperature for the dissolved salts allows crystal formation on the surface. Normally crystallization starts at especially active points – nucleation sites – such as scratches and pits and often after induction period spread to cover the entire surface. This type of fouling is strong and adherent and requires vigorous mechanical or chemical treatment to be removed [16]. Fouling rate increases with the increase of salt concentration or surface temperature. These are often found in heat exchangers of process industries, boilers, evaporators etc.

#### **2.4 Chemical reaction fouling**

This type of fouling occurs when the depositions are formed as a result of chemical reaction resulting to produce a solid phase at or near the surface. In the present case carbonaceous material deposits due to thermal gradation of the components of a process stream on hot heat transfer surface. This type of fouling is often extremely tenacious and need special measure to clean off the deposits from heat exchanger surfaces to provide them satisfactory operation life [16].

#### **2.5 Corrosion fouling**

This type of fouling is also caused by some chemical reaction but it is different from chemical reaction fouling. This fouling is a reactant and it is consumed. In this case, the surface reacts with the fluid and become corroded [15]. The corrosion products can foul the surface provided it is not dissolved in the solution after formation. pH value of the solution is one of the controlling parameter. Such as, presence of sulfur in fuel can cause corrosion in gas and oil fired boilers. Corrosion is often more prone in the liquid side of the heat exchanger. In some cases the product of corrosion may be swept away to downstream of a process loop and cause deposition on surfaces there.
