**2.9 Transport**

In this part, fouling substances from the bulk fluid are transported to the heat transfer surface across the boundary layer. This is dependent on the physical properties of the system and concentration difference between the bulk and the surface fluid interface. Transport is accomplished by a number of phenomena including diffusion, sedimentation

and thermophoresis [20, 21]. The local deposition flux *md* on a surface can be expressed by equation (2.1).

Fouling and Fouling Mitigation on Heat Exchanger Surfaces 513

Some of the common salts causes fouling are CaSO4, CaCO3 and Mg(OH)2 , and SiO2. Solubility, crystal structure and strength have impact on composite scale formation in

Fouling phenomena imposes retardation on heat transfer and augmentation of frictional pressure drop which degrades the effectiveness of a heat exchanger. Some basic design aspects of heat exchangers along with mitigation of fouling are discussed in the present

A fixed value of fouling resistance could be assigned during the design stage although fouling is time dependent phenomenon. The cleaning schedule and operating parameters of the heat exchanger is dependent on the design fouling factor. Depending on application some heat exchangers require frequent cleaning whereas some need rear cleaning. Fouling

Fouling allowance: Provisions are during the design stage once fouling is anticipated. Different approaches are used to provide an allowance for fouling resistance. They all result into an excess heat transfer surface area. Updated methods include, specifying the fouling

A fouling resistance is prescribed on each side of the surface where fouling is anticipated. A lower overall heat transfer coefficient is resulted. To achieve the specified heat transfer, excess surface area is provided. Until the specified value of the fouling resistance is reached, the performance of the heat exchanger will be satisfactory. Depending on this fact, maintenance schedule could be planned to avoid unprecedented shut down for cleaning.

fouling. Therefore, composite fouling needs more attention and further research [17].

Fig. 2.3. Change in deposition thickness with time.

**3.1 Effect of fouling on heat exchanger design** 

rate is a dominating factor in designing a particular heat exchanger.

resistances, the cleanliness factor, or the percentage over surface.

**2.14 Composite fouling** 

**3. Effects of fouling** 

chapter.

$$
\stackrel{\bullet}{m}\_d = h\_D \left( \mathbf{C}\_b - \mathbf{C}\_s \right) \tag{2.1}
$$

Where, *Cb* and *Cs* are reactant concentration in the bulk fluid and that in the fluid adjacent to the heat transfer surface where as *hD* is the convective mass transfer coefficient. From Sherwood number ( *Sh h d D <sup>D</sup>* / ), *hD* could be evaluated. Sherwood number is dependent on the flow and the geometric parameters.

The phenomenon of transportation of a particulate matter in a fluid due to gravity on a horizontal or inclined surface is known as sedimentation. This is playing a vital roll where particles are heavy and fluid velocities are low.
