**3. Effects of fouling**

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 chapter.

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

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

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

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 and Fouling Mitigation on Heat Exchanger Surfaces 515

Heat Exchanger with green additives: Many additives were developed for retardation of fouling but many of them found carcinogenic in nature. Now researchers are heading towards green additives. Chemistry and analysis are underway. Lab analysis and performances will be subsequently achieved. In near future users are looking for a

Mineral scales deposited on heat exchanger surfaces are a persistent and an expensive problem in process industries, cooling water systems, steam generation units, desalination by evaporation etc. and also house hold equipment. Precipitation of mineral salts as a scale on the surface of the conduit and cause obstruction of fluid flow, impedance of heat transfer, wear of metal parts, localized corrosion attack and unscheduled equipment shutdown.

The deposit layer provides an additional resistance to heat transfer. Generally, the thermal conductivity of the deposit layer is very low compared with that of the material of the heat exchanger which may result in a much higher thermal resistance than the wall or film resistances. The deposit layer also reduces the flow area, which increases the pressure drop. This problem is quite severe and is further enhanced by the rough surface of the deposit. Both effects reduce the heat exchanger performance significantly. Additional energy requirements

In a circular tube, fouling builds on the inside or outside of the tube depending on the flowing fluid. Fouling adds an insulating cover to the heat transfer surface. The overall heat transfer coefficient for a smooth tubular heat exchanger under deposited conditions, Uf can

> 1 / / ln( / ) / 2 1 / *<sup>f</sup> o ii o fi i o oi fo o*

*A A h A R A A d d kL R h*

The thermal resistance due to fouling is evaluated generally based on experiments as

1 1

*U U* 

Where, the overall heat transfer coefficient U can also be evaluated by using the rate

 *<sup>f</sup> f*

heated surface and the bulk liquid) are experimentally obtained. A is the exposed area of the heat exchanging surface to the liquid. The net rate of deposition of CaSO4.2H2O on metal

*<sup>Q</sup> <sup>U</sup> A T*

*f cl*

where Rfi and Rfo represent resistances for the outer and inner surfaces of the tubes.

*f*

*R*

difference in the overall specific resistances of the fouled and clean wall:

and temperature difference

(3.1)

(3.2)

(3.3)

*Tf* (the temperature difference between

in terms of more heating or pumping power can hamper the economics of the process.

be obtained by adding the inside and outside thermal resistances:

breakthrough in this field.

**3.2 Fouling effect on heat transport** 

*U*

equation:

The heat flow rate *Qf*

Tubular Exchanger Manufacturers Association (TEMA) [22] is referenced source of fouling factors used in the design of heat exchangers. Plant data, proprietary research data, personal and company experience etc. are other sources of fouling resistance data could be used in design.

Minimize Fouling by considering Design Features: Extent of fouling could be minimized by good design practice. Direct contact heat exchangers are considered where excess fouling is desired. In general a fouling prone fluid stream should be placed on the tube side as cleaning is easier. Generally higher fluid velocity and lower tube wall temperature retard fouling accumulation. Velocity of 1.8 m/s is a widely accepted figure for tube side flow of a heat exchanger. Heat Exchangers, operating over dew point for acid vapor and above freezing for fluids containing waxes prevent corrosion and freezing fouling. Fouling deposits are always found heavy in the region of low velocity at the vicinity of baffles in the shell side of the shell and tube heat exchangers.

Design features to facilitate fouling control: Full elimination of fouling may not be possible by good design practice alone. So, heat exchangers require cleaning at certain intervals. Online cleaning can be employed to control fouling by extending cleaning cycle. Continuous fouling can ensure minimized fouling allowance. At construction and installation phase of a plant on-line cleaning system could be installed at ease. A heat exchanger with removable head and straight tube would be easy to clean and maintain. Space and provision for removal and cleaning of tube bundles are required to be available. On site cleaning facilities are to be provided with options of keeping isolation valves and connection provisions for cleaning hoses which could lead to chemical cleaning.

Fouling and operation of heat exchangers: Provision of excess surface area in heat exchangers for curbing fouling may lead to operation problem and fouling build. Generally high heat transfer area enhances total heat transfer which raises the out let temperature. By changing process parameters such as flow, surface temperature leads to higher fouling.

Fouling control strategies: A number of strategies are applied for fouling control. In operating condition additives are added. On-line or off-line surface cleaning techniques are other options. To control fouling under different consequences are consolidated by some researchers as stated in Table 3.1 [23].


Table 3.1. Various techniques adapted to control fouling.

Heat Exchanger with green additives: Many additives were developed for retardation of fouling but many of them found carcinogenic in nature. Now researchers are heading towards green additives. Chemistry and analysis are underway. Lab analysis and performances will be subsequently achieved. In near future users are looking for a breakthrough in this field.

#### **3.2 Fouling effect on heat transport**

514 Heat Exchangers – Basics Design Applications

Tubular Exchanger Manufacturers Association (TEMA) [22] is referenced source of fouling factors used in the design of heat exchangers. Plant data, proprietary research data, personal and company experience etc. are other sources of fouling resistance data could be used in

Minimize Fouling by considering Design Features: Extent of fouling could be minimized by good design practice. Direct contact heat exchangers are considered where excess fouling is desired. In general a fouling prone fluid stream should be placed on the tube side as cleaning is easier. Generally higher fluid velocity and lower tube wall temperature retard fouling accumulation. Velocity of 1.8 m/s is a widely accepted figure for tube side flow of a heat exchanger. Heat Exchangers, operating over dew point for acid vapor and above freezing for fluids containing waxes prevent corrosion and freezing fouling. Fouling deposits are always found heavy in the region of low velocity at the vicinity of baffles in the

Design features to facilitate fouling control: Full elimination of fouling may not be possible by good design practice alone. So, heat exchangers require cleaning at certain intervals. Online cleaning can be employed to control fouling by extending cleaning cycle. Continuous fouling can ensure minimized fouling allowance. At construction and installation phase of a plant on-line cleaning system could be installed at ease. A heat exchanger with removable head and straight tube would be easy to clean and maintain. Space and provision for removal and cleaning of tube bundles are required to be available. On site cleaning facilities are to be provided with options of keeping isolation valves and connection provisions for

Fouling and operation of heat exchangers: Provision of excess surface area in heat exchangers for curbing fouling may lead to operation problem and fouling build. Generally high heat transfer area enhances total heat transfer which raises the out let temperature. By changing process parameters such as flow, surface temperature leads to higher fouling.

Fouling control strategies: A number of strategies are applied for fouling control. In operating condition additives are added. On-line or off-line surface cleaning techniques are other options. To control fouling under different consequences are consolidated by some

Lances:

Disassembly and manual cleaning:

Liquid jet, Steam, Air jet. Mechanical Cleaning: Drills, Scrapers

Chemical cleaning

On-line techniques Off-line techniques

design.

shell side of the shell and tube heat exchangers.

cleaning hoses which could lead to chemical cleaning.

researchers as stated in Table 3.1 [23].

Use and control of appropriate additives:

Air jet

bumping

On-line cleaning:

Inhibitors, Antiscalants, Dispursants, Acids,

Sponge balls, Brushes, Sonic horns, Soot blowers, Chains and scrappers, Thermal shock, Air

Table 3.1. Various techniques adapted to control fouling.

Mineral scales deposited on heat exchanger surfaces are a persistent and an expensive problem in process industries, cooling water systems, steam generation units, desalination by evaporation etc. and also house hold equipment. Precipitation of mineral salts as a scale on the surface of the conduit and cause obstruction of fluid flow, impedance of heat transfer, wear of metal parts, localized corrosion attack and unscheduled equipment shutdown.

The deposit layer provides an additional resistance to heat transfer. Generally, the thermal conductivity of the deposit layer is very low compared with that of the material of the heat exchanger which may result in a much higher thermal resistance than the wall or film resistances. The deposit layer also reduces the flow area, which increases the pressure drop. This problem is quite severe and is further enhanced by the rough surface of the deposit. Both effects reduce the heat exchanger performance significantly. Additional energy requirements in terms of more heating or pumping power can hamper the economics of the process.

In a circular tube, fouling builds on the inside or outside of the tube depending on the flowing fluid. Fouling adds an insulating cover to the heat transfer surface. The overall heat transfer coefficient for a smooth tubular heat exchanger under deposited conditions, Uf can be obtained by adding the inside and outside thermal resistances:

$$\mathrm{LI}\_{f} = \frac{1}{A\_{o} \left/ A\_{i} h\_{i} + A\_{o} R\_{fi} \left/ A\_{i} + A\_{o} \ln \left( d\_{o} \right/d\_{i} \right) / 2 \pi k \mathrm{L} + R\_{fo} + 1 \right/ h\_{o}} \tag{3.1}$$

where Rfi and Rfo represent resistances for the outer and inner surfaces of the tubes.

The thermal resistance due to fouling is evaluated generally based on experiments as difference in the overall specific resistances of the fouled and clean wall:

$$R\_f = \left(\frac{1}{\mathcal{U}\_f} - \frac{1}{\mathcal{U}\_d}\right) \tag{3.2}$$

Where, the overall heat transfer coefficient U can also be evaluated by using the rate equation:

$$\mathcal{U}\mathcal{U}\_f = \frac{\stackrel{\bullet}{Q}}{\left(A \times \mathcal{A}\Gamma\_f\right)}\tag{3.3}$$

The heat flow rate *Qf* and temperature difference *Tf* (the temperature difference between heated surface and the bulk liquid) are experimentally obtained. A is the exposed area of the heat exchanging surface to the liquid. The net rate of deposition of CaSO4.2H2O on metal

Fouling and Fouling Mitigation on Heat Exchanger Surfaces 517

0.5 1 exp *f f*

For a known total fouling resistance, the tube diameter under fouled conditions can be evaluated on knowing the thermal conductivity of the deposits. Non-uniform thermal conductivity may result from the multi layers of fouling deposits. Approximate thermal conductivities of pure materials constituting fouling deposits are often used for estimation of thermal conductivity of the total deposits. Depending on situations the fouling layer is considered composed solely of one material. In some occasions to ease calculations *<sup>f</sup> f* is

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

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,

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

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

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

*f c*

*t d*

considered equal to *cf* .

**4. Conditions influencing fouling** 

temperature, and (3) bulk temperature.

increasing the flow velocity may not have any effect at all [6].

solubility salts cooling results in more fouling.

in the cavities or pits where flow velocities are very low.

reaction rate and polymerisation rate.

2

*c k R*

(3.9)

*d*

surface is estimated as *<sup>m</sup> t* , where *m* is the total mass accumulation on a unit area and t refers to the amount of time the surface was exposed to the solution of the foulant.

Using the definition of heat transfer coefficient and fouling resistance, the equation (3.4) can be derived for constant heat duty.

$$\frac{A\_{f\text{build}}}{A\_{clean}} = \mathbf{1} + \mathbf{U}\_{clean} R\_f \tag{3.4}$$

The required excess heat transfer area usually becomes excessive due to the higher clean heat transfer coefficients. It is often recommended that the additional surface should not exceed 25 percent of the heat transfer surface requirement for clean operation.

#### **3.3 Effect of fouling on pressure drop**

In heat exchangers pressure loss is considered more critical than loss in heat transfer due to fouling. Fouling results in a finite layer. Flow field, pressure drop are affected by the change in geometry of the flow passage. Thus in a tubular heat exchanger, the deposited layer roughens the surface, diminishes the inner and raises the outer dimension of the tubes. The inside diameter of the tube decreases and roughness of the tube increases due to fouling which, causes an increase in pressure drop. Pressure drop inside a tube of a heat exchanger under fouled and clean state can be correlated as follows:

$$\frac{\Delta P\_f}{\Delta P\_c} = \frac{f\_f}{f\_c} \left(\frac{d\_c}{d\_f}\right) \left(\frac{\mu\_{mf}}{\mu\_{mc}}\right)^2 \tag{3.5}$$

Considering that the mass flow rates under clean and fouled conditions are the same, the mass flow rate can be represented as:

$$\stackrel{\bullet}{m} = \rho u\_m A\_{cr} \tag{3.6}$$

Equation (3.5) thus becomes:

$$\frac{AP\_f}{AP\_c} = \frac{f\_f}{f\_c} \left(\frac{d\_c}{d\_f}\right)^5\tag{3.7}$$

The magnitude of *<sup>f</sup> d* of scaled tube can be obtained from equation (3.8).

$$d\_f = d\_c \exp\left(-\frac{2k\_c R\_f}{d\_c}\right) \tag{3.8}$$

The thickness *<sup>f</sup> t* of deposit layer can be obtained from:

$$t\_f = 0.5d\_c \left[ 1 - \exp\left( -\frac{2k\_f R\_f}{d\_c} \right) \right] \tag{3.9}$$

For a known total fouling resistance, the tube diameter under fouled conditions can be evaluated on knowing the thermal conductivity of the deposits. Non-uniform thermal conductivity may result from the multi layers of fouling deposits. Approximate thermal conductivities of pure materials constituting fouling deposits are often used for estimation of thermal conductivity of the total deposits. Depending on situations the fouling layer is considered composed solely of one material. In some occasions to ease calculations *<sup>f</sup> f* is

considered equal to *cf* .

516 Heat Exchangers – Basics Design Applications

Using the definition of heat transfer coefficient and fouling resistance, the equation (3.4) can

The required excess heat transfer area usually becomes excessive due to the higher clean heat transfer coefficients. It is often recommended that the additional surface should not

In heat exchangers pressure loss is considered more critical than loss in heat transfer due to fouling. Fouling results in a finite layer. Flow field, pressure drop are affected by the change in geometry of the flow passage. Thus in a tubular heat exchanger, the deposited layer roughens the surface, diminishes the inner and raises the outer dimension of the tubes. The inside diameter of the tube decreases and roughness of the tube increases due to fouling which, causes an increase in pressure drop. Pressure drop inside a tube of a heat exchanger

> *f f mf c c c f mc Pf u d P fd u*

Considering that the mass flow rates under clean and fouled conditions are the same, the

*m uA m cr*

*f f c c cf P f d P fd*

 

2 exp *<sup>c</sup> <sup>f</sup>*

*c k R*

*d*

*f c*

*d d*

The magnitude of *<sup>f</sup> d* of scaled tube can be obtained from equation (3.8).

The thickness *<sup>f</sup> t* of deposit layer can be obtained from:

*clean f*

2

5

(3.5)

(3.6)

(3.7)

(3.8)

*U R*

refers to the amount of time the surface was exposed to the solution of the foulant.

1 *fouled*

*clean*

*A*

*A*

exceed 25 percent of the heat transfer surface requirement for clean operation.

, where *m* is the total mass accumulation on a unit area and t

(3.4)

surface is estimated as *<sup>m</sup>*

be derived for constant heat duty.

**3.3 Effect of fouling on pressure drop** 

mass flow rate can be represented as:

Equation (3.5) thus becomes:

under fouled and clean state can be correlated as follows:

*t*
