**1. Introduction**

Heating or cooling of one medium by another medium is performed in a heat exchanger along with heat dissipation from surfaces of the equipment. In course of time during operation, the equipment receives deposition (Fouling) which retards heat exchanging capability of the equipment along with enhanced pressure loss and extended pumping power. Thus accumulation of undesired substances on a surface is defined as fouling. Occurrence of fouling is observed in natural as well as synthetic systems. In the present context undesired deposits on the heat exchanger surfaces are referred to fouling. With the development of fouling the heat exchanger may deteriorate to the extent that it must be withdrawn from service for cleaning or replacement.

The overall design of heat exchanger may significantly be influenced by fouling, use of material, process parameters, and continuous service in the system or process stream are all deliberately influenced by fouling phenomena. Preventive measures of fouling are highly encouraged as it keeps the service of heat exchanger for a longer time. However many mitigation techniques of fouling are harsh to the environment. A technique involving chemicals and means benign to the environment is the most desired approach and it could elongate the cleaning interval. On the other hand unique and effective arrangements may be required to facilitate satisfactory performances between cleaning schedules. As a result fouling causes huge economic loss due to its impact on initial cost on heat exchanging operation, operating cost, mitigation measures and performance. The present study focused on fouling phenomena, fouling models, environment of fouling, consideration of heat exchanger fouling in design and mitigation of fouling.

## **2. Fouling**

Fouling is the resultant effect of deposition and removal of deposits on a heat exchanger surface. The process of fouling could be represented by the equation (2.1).

$$\frac{dm\_f}{dt} = \frac{\bullet}{m\_d} - m\_r$$

Fouling and Fouling Mitigation on Heat Exchanger Surfaces 509

The initiation period and the roughness delay time for particulate fouling are very small [4] in comparison to the fairly long delay time for crystallization fouling [5]. After the roughness delay time, the fouling curve can be classified into three categories, (a) Linear, (b)

The linear fouling curve is obtained for very strong deposits where removal is negligible or in case where the removal rate is constant (and deposition is faster than removal). The falling rate curve is obtained from decrease in deposition and deposits with lower mechanical strength. The combined effect with time causes the net deposition or fouling rate to fall. Asymptotic fouling curve has been most commonly reported for different types of fouling. The removal rate increases with time for weak deposits and can eventually become

Linear fouling curves have been presented by many authors for crystallization fouling [6-8]. However, there is some doubt as to whether the fouling rate may remain linear for a long time. For the constant heat flux situation, the net driving force may decrease with fouling. The increase in flow velocity due to the reduced cross-sectional area with deposit formation can increase the removal rate and the linear rate may change to a falling rate or even leveloff completely [9]. Asymptotic behavior for crystallization fouling has reported by various authors [10, 5, 11-12]. Cooper et al. [13] found asymptotic behavior for calcium phosphate fouling (with some particulate fouling from suspended solids). For particulate fouling, asymptotic behavior is attained because particles do not adhere strongly to the wall and can

A fouling process that follows a linear rate for constant heat flux can have falling or even asymptotic behaviour for constant temperature difference. The interface temperature decreases with deposit formation because of the extra resistance offered by deposit layer and enhanced flow velocities as flow passages are partially blocked by deposits. Thus the

equal to the deposition rate. The net rate is then zero as depicted in Figure 2.2.

Fig. 2.2. Typical fouling curves.

be removed easily [4, 14].

Falling, and asymptotic, as illustrated in Figure 2.2.

where *<sup>f</sup> dm* , *md* and *mr* are net deposition rates, deposition and removal rates respectively.

Fig. 2.1. Various deposition and removal processes during fouling.

Various deposition and removal processes for a typical system could be predicted as shown in Figure 2.1. The processes occur simultaneously and depend on the operating conditions. Usually removal rates increase with increasing amounts of deposit whereas deposition rates are independent of the amount of deposit but do depend on the changes caused by deposits such as increase in flow velocity and surface roughness. In the application of constant wall temperature or constant heat transfer coefficient boundary conditions, the interface temperature decreases as deposits build up which reduces the deposition rate.

Initiation period or time delay in heat exchanger fouling is considered the time when there is no deposition for some time after a clean heat exchanger has been brought into operation. Figure 2.2 illustrates this in detail. The initial growth of deposit can cause the heat transfer coefficient to increase rather than decrease resulting in a fouling resistance due to changing flow characteristics near the wall. At the initial stage the deposit penetrates the viscous sublayer, the resulting turbulence increases the film heat transfer coefficient at the solid/liquid interface by changing flow characteristics near the wall. This increase in heat transfer coefficient may overcome the thermal resistance offered by the deposits and the net heat transfer coefficient may increase.

Several authors have reported negative fouling resistances [1, 2]. This process continues until the additional heat transfer resistance overcomes the advantage of increased turbulence. The time period from the beginning of the fouling process until the fouling resistance again becomes zero is called roughness delay time [3]. The time period from the beginning, when the formation of stable crystalline nuclei and their concretion to a compact fouling layer takes place is also called as induction period, which is in fact the roughness delay time and it ends up with the increase of fouling resistance above zero level.

are net deposition rates, deposition and removal rates respectively.

where *<sup>f</sup> dm* , *md*

 and *mr* 

transfer coefficient may increase.

Fig. 2.1. Various deposition and removal processes during fouling.

temperature decreases as deposits build up which reduces the deposition rate.

Various deposition and removal processes for a typical system could be predicted as shown in Figure 2.1. The processes occur simultaneously and depend on the operating conditions. Usually removal rates increase with increasing amounts of deposit whereas deposition rates are independent of the amount of deposit but do depend on the changes caused by deposits such as increase in flow velocity and surface roughness. In the application of constant wall temperature or constant heat transfer coefficient boundary conditions, the interface

Initiation period or time delay in heat exchanger fouling is considered the time when there is no deposition for some time after a clean heat exchanger has been brought into operation. Figure 2.2 illustrates this in detail. The initial growth of deposit can cause the heat transfer coefficient to increase rather than decrease resulting in a fouling resistance due to changing flow characteristics near the wall. At the initial stage the deposit penetrates the viscous sublayer, the resulting turbulence increases the film heat transfer coefficient at the solid/liquid interface by changing flow characteristics near the wall. This increase in heat transfer coefficient may overcome the thermal resistance offered by the deposits and the net heat

Several authors have reported negative fouling resistances [1, 2]. This process continues until the additional heat transfer resistance overcomes the advantage of increased turbulence. The time period from the beginning of the fouling process until the fouling resistance again becomes zero is called roughness delay time [3]. The time period from the beginning, when the formation of stable crystalline nuclei and their concretion to a compact fouling layer takes place is also called as induction period, which is in fact the roughness

delay time and it ends up with the increase of fouling resistance above zero level.

Fig. 2.2. Typical fouling curves.

The initiation period and the roughness delay time for particulate fouling are very small [4] in comparison to the fairly long delay time for crystallization fouling [5]. After the roughness delay time, the fouling curve can be classified into three categories, (a) Linear, (b) Falling, and asymptotic, as illustrated in Figure 2.2.

The linear fouling curve is obtained for very strong deposits where removal is negligible or in case where the removal rate is constant (and deposition is faster than removal). The falling rate curve is obtained from decrease in deposition and deposits with lower mechanical strength. The combined effect with time causes the net deposition or fouling rate to fall. Asymptotic fouling curve has been most commonly reported for different types of fouling. The removal rate increases with time for weak deposits and can eventually become equal to the deposition rate. The net rate is then zero as depicted in Figure 2.2.

Linear fouling curves have been presented by many authors for crystallization fouling [6-8]. However, there is some doubt as to whether the fouling rate may remain linear for a long time. For the constant heat flux situation, the net driving force may decrease with fouling. The increase in flow velocity due to the reduced cross-sectional area with deposit formation can increase the removal rate and the linear rate may change to a falling rate or even leveloff completely [9]. Asymptotic behavior for crystallization fouling has reported by various authors [10, 5, 11-12]. Cooper et al. [13] found asymptotic behavior for calcium phosphate fouling (with some particulate fouling from suspended solids). For particulate fouling, asymptotic behavior is attained because particles do not adhere strongly to the wall and can be removed easily [4, 14].

A fouling process that follows a linear rate for constant heat flux can have falling or even asymptotic behaviour for constant temperature difference. The interface temperature decreases with deposit formation because of the extra resistance offered by deposit layer and enhanced flow velocities as flow passages are partially blocked by deposits. Thus the

Fouling and Fouling Mitigation on Heat Exchanger Surfaces 511

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

On a heat transfer surface the growth of biological materials results in biofouling. In this case biological micro and macro organisms are stick to the heat transfer surface. When microorganisms (e.g., algae, bacteria, molds etc.) and their products grow they form microbial fouling. Seaweeds, waterweeds, barnacles develop microbial fouling. These fouling may occur simultaneously. The growth of attached organisms is one of the common problems [15] in heat exchanger operation. Food processing industries, power plant

Fouling is a complex phenomenon due to involvement of a large number of variables. From a fundamental point of view the fouling mechanism follows certain stages in developing on

Surface is conditioned in the initiation period. The initial delay induction period is influenced by the materials surface temperature, material, surface finish, roughness and surface coating. With the increase of degree of supersaturation with respect to the heat transfer surface temperature or increase of surface temperature the induction period decreases. During the induction period, nuclei for crystallization of deposit are also formed for biological growth. This period can take a long time, may be several weeks or a few

The delay period decreases with increasing temperature in chemical reaction fouling due to the acceleration of induction reactions. If the initial period decreases with increasing surface temperature, crystallization fouling would be changed [18]. With the increase of surface roughness the delay period tends to decrease [19]. Additional sites are developed by the roughness projections, which promotes crystallization while grooves provide regions for

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

on a surface can be expressed by

a surface [17]. These are: Initiation, transport, attachment, removal and aging.

process loop and cause deposition on surfaces there.

condensers using seawater, etc. are experiencing biofouling.

and thermophoresis [20, 21]. The local deposition flux *md*

**2.6 Accumulation of biological fouling** 

**2.7 Fouling process** 

minutes or even seconds.

particulate deposition.

**2.9 Transport** 

equation (2.1).

**2.8 Initiation** 

thermal boundary conditions can result in different fouling curves which may give wrong impressions about the actual fouling mechanism.

#### **2.1 Categories of fouling**

Fouling can be categorised a number of different ways. These are (1) heat transfer service, (2) type of service fluid and (3) application. Most fouling situations are virtually unique. Fouling [15] can be classified into the following categories: (i) particulate, (ii) Precipitation, (iii) corrosion, (iv) biofouling and (v) chemical reaction.
