**2.12 Aging**

With the commencement of deposition ageing starts. During the ageing, there may be transformation of crystal to improve or decrease the deposition strength with time. During aging the mechanical properties of the deposit can change due to changes in the crystal or chemical structure. Alteration of the chemical composition of the deposit by a chemical reaction may change its mechanical strength. On the other hand the biofouling layer may become weak due to corrosion at the surface by slow poisoning of microorganisms.

## **2.13 Change in deposition thickness with time**

Figure 2.3 is showing the growth rate of deposit on the surface [15]. Region A: fouling is initiated in the induction period. Region B: a steady deposit growth on the surface. The rate of removal of deposit was increased when the rates of deposition gradually retards. Region C: in this region the rate of removal and deposition seems equal and the thickness of deposition remains constant.

Fig. 2.3. Change in deposition thickness with time.

### **2.14 Composite fouling**

512 Heat Exchangers – Basics Design Applications

*m hC C d Db s*

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

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

At this stage, the deposits are adhered to the surface and among itself. Salt ions approaching to the surface are attracted to it due to electro-magnetic forces and adhere to the surface to form nucleation and gradually it grows with time to form a fouling layer. Thus forces acting on the particles as they approach the surface are impotent in determining attachment. Properties of the materials, such as size, density and surface conditions are dominating the

There is competition between removal and deposition of the foulants, up to the steady growth of the deposition on the surface. Shear forces at the interface between the fluid and deposited fouling are responsible for removal. The velocity gradients at the surface, the viscosity of the fluid and surface roughness are guiding the shear forces. Removal from the

With the commencement of deposition ageing starts. During the ageing, there may be transformation of crystal to improve or decrease the deposition strength with time. During aging the mechanical properties of the deposit can change due to changes in the crystal or chemical structure. Alteration of the chemical composition of the deposit by a chemical reaction may change its mechanical strength. On the other hand the biofouling layer may become weak due to corrosion at the surface by slow poisoning of

Figure 2.3 is showing the growth rate of deposit on the surface [15]. Region A: fouling is initiated in the induction period. Region B: a steady deposit growth on the surface. The rate of removal of deposit was increased when the rates of deposition gradually retards. Region C: in this region the rate of removal and deposition seems equal and the thickness of

surface is performed through the mechanism of dissolution, erosion and spalling.

(2.1)

on the flow and the geometric parameters.

**2.10 Attachment** 

**2.11 Removal** 

**2.12 Aging** 

microorganisms.

deposition remains constant.

**2.13 Change in deposition thickness with time** 

attachment phenomenon.

particles are heavy and fluid velocities are low.

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