**4. Conditions influencing fouling**

The conditions influencing fouling can be classified as: (A) operating parameters, (B) heat exchanger parameters, and (C) fluid properties. Among the operating parameters the important events which influencing fouling at a significant level are: (1) velocity, (2) surface temperature, and (3) bulk temperature.

Velocity influences fouling at a significant level. In diffusion controlled processes, increasing the fluid velocity causes more fouling [24]. In most cases, fouling decreases at higher fluid velocities [4, 13, 25]. Increasing flow velocity increases the fluid shear stress which causes more removal. This results in lower fouling rates which resulting to lower fouling resistance. For weak deposits (particulate fouling), increasing the flow velocity may completely eliminate fouling. For stronger deposits, increasing the flow velocity beyond a particular point may not decrease fouling significantly [25]. For very strong deposits, increasing the flow velocity may not have any effect at all [6].

Surface temperature may increase, decrease or have no effect on fouling [26]. The rates of chemical reaction and inverse solubility crystallization increase with an increase in temperature. For inverse solubility salts, higher surface temperature increases fouling due to higher concentration gradients and higher reaction rate constants. In case of normal solubility salts cooling results in more fouling.

The bulk temperature also effects on increase of fouling rate. In inverse crystallisation, when precipitation happens in the fluid bulk, increasing the temperature increases the rate of crystal formation and hence deposition. Thus bulk temperature has effects on chemical reaction rate and polymerisation rate.

The important heat exchanger parameters are classified as: surface material, surface structure (roughness), heat exchanger type and geometry [27]. Surface material is considered seriously for corrosion fouling because of the potential to react and form corrosion products. Different materials have different catalytic action and may promote or reduce fouling for different processes. The initial fouling rate and scale formation depends significantly on the surface roughness. Junghahn [28] proved theoretically that the free energy change associated with crystal nuclei formation was much less on a rough surface than on a smooth surface. Rough surfaces result in higher deposition due to protected zones in the cavities or pits where flow velocities are very low.

Fouling and Fouling Mitigation on Heat Exchanger Surfaces 519

Changes in flow velocity with changing cross-sectional area due to fouling are usually

It is also observed that few attempts have been made to model the initiation or roughness delay period. Almost all the models predict fouling (scaling) after the delay period. Some other notable parameters are neglected in modelling such as: (a) effect of simultaneous action of different fouling mechanisms, (b) equipment design, (c) surface parameters e.g. surface material and surface roughness, (d) increase in surface area with deposition, (e) properties of foulant stream, (f) nature of process, and the (g) fluctuations in operation.

Modelling is usually done taking into consideration only (a) flow velocity, (b) concentration,

Watkinson and Martinez [11] developed a model, based on the fundamental material

( ) *<sup>f</sup> <sup>R</sup> <sup>n</sup> F Sa*

*c c*

( ) <sup>0</sup> *<sup>g</sup> <sup>f</sup> E*

(6.1)

(6.2)

(6.3)

(6.4)

*f*

For sparingly soluble salts with inverse solubility (e.g. CaCO3), the deposition rate is controlled by the slow reaction rate and the constant of reaction rate *KR* that obeys the

*R T K Ae <sup>R</sup>*

*m ax <sup>r</sup>* <sup>8</sup> *f f* 

Where is the shear stress exerted by the liquid flow on the fouling film. Even though CaCO3 deposits are much stronger than the particulate deposits considered by Kern and Seaton [43] the removal rate was assumed to be directly proportional to deposit thickness,

Kern and Seaton [41] proposed a model for particulate fouling which takes into account removal or re-entrainment of deposits. The mathematical model is based on a general material balance equation (2.1). Deposition and removal rates act separately and combine

> *m a cw <sup>d</sup>* <sup>9</sup>

Where, *c* is dirt concentration and w is constant weight flow of fluid. The removal rate is roughly proportional to the total depth of dirt deposited on the heat transfer surface as stated

balance equation (2.1). For the deposition rate the following expression is adopted:

*dx K*

*dt*

Kern and Seaton [43] recommend for the removal rate the equation:

into a net deposition rate. The rate of deposition is expressed as:

The shape of deposits, e. g. crystals or particles is ignored.

The deposition rate is expressed as shown in equation (6.1).

(c) wall and bulk temperature, and (d) time.

neglected.

Arrhenius equation:

below.

with T as the interface temperature.

which may not be correct for all the cases.

According to Rankin and Adamson [29], it is not the rate of nucleation but the nuclei attachment which is strongly dependent on the surface roughness. Chandler [30] also observed similar results. In general the rough surface causes more fouling which reduces the delay time for all types of fouling. Surface roughness increases turbulence near the surface, which in turn increases the removal rate of fouling on the surface. Better performance occurred due to the increase in surface roughness with deposit formation and has been reported by some authors [1, 2]. Marriott [31] reiterated that mirror finished surfaces in heat exchangers are used to reduce fouling in practice.
