**8. Fouling mitigation**

520 Heat Exchangers – Basics Design Applications

*ma x <sup>r</sup>* <sup>10</sup> *f f* 

Combining the equations for deposition and removal rates (6.4) and (6.5) with the material

(1 )*<sup>t</sup> <sup>f</sup> RR e <sup>f</sup>*

9 10

10 *f*

Here, is the thermal conductivity of the deposits, 9*a* and 10 *a* are proportionality

resistance after an infinite time of operation. According to this model, no matter what the conditions, i.e. type of fluid, heat exchanger surface, temperature driving force, an asymptotic fouling value will be obtained sooner or later with removal rates becoming equal

An additional cost is imposed by fouling of heat transfer equipment in industries. Few studies have been undertaken to determine the fouling related costs in industry. Fouling costs can generally be divided into four major categories, such as (1) increased capital expenditure, (2) energy costs, (3) maintenance costs, (4) cost of production loss and (v) extra

> GNP (1984) US \$ million

3634000 0.12-0.22

US \$ million

8000-10000

Table 7.1. Estimated fouling costs incurred in some countries.

Japan 3062 1225000 0.25 West Germany 1533 613000 0.25 UK (1978) 700-930 285000 0.20-0.33 Australia 260 173000 0.15 New Zealand 35 23000 0.15 Total Industrial World 26850 13429000 0.20

*a cw <sup>R</sup> a* 

*f f*

*f*

 *a*  balance equation (2.1), the fouling resistance expression is obtained:

constants. This model predicts asymptotic fouling behaviour with *Rf*

where is a time constant and *Rf*

to deposition rates.

**7. Cost imposed due to fouling** 

environmental management cost.

Country Fouling costs

USA (1982) 3860-7000

these also the following equations are obtained.

(6.5)

(6.6)

is the asymptotic value of the fouling resistance. For

(6.7)

(6.8)

being the fouling

Fouling costs % of GNP

0.28-0.35

Gilmour [42] reported that the degradation of heat transfer performance due to fouling in shell and tube heat exchangers occurs mainly due to poor shell-side design. In recent years numerous methods have been developed to control fouling. These methods can be classified as: (1) chemical methods, (2) mechanical methods and (3) changing the phase of the solution. By adding foreign chemicals in a solution, reduction of fouling is achieved by chemical methods of fouling mitigation. Chemical additives developed by many companies have been extensively used to mitigate fouling in the industrial sector. Various additives can be used to prevent scaling [43-44]. Bott [45] specified that the additives used act in different ways, such as (a) sequestering agents, (b) threshold agents, (c) crystal modifiers and (d) dispersants. Some of the common water additives are EDTA (sequestering agent), polyphosphates and polyphosphonates (threshold agents) and polycarboxylic acid and its derivatives (sequestering and threshold treatment). Sequestering agents such as EDTA complex strongly with the scaling cations such as Ca++, Mg++, and Cu++ in exchange with Na+, thus preventing scaling as well as removing any scale formed previously. They are used effectively as antiscalants in boiler feed water treatment. Troup and Richardson [46] claimed that their use is uneconomical when hardness levels are high.

Polyphosphates and polyphosphonates as threshold agents are also used to reduce scaling in boilers and cooling water systems. Bott [45] said that they prevent the formation of nuclei thus preventing the crystallisation and mitigate fouling. Very small quantities of these agents are effective in reducing scaling from supersaturated salt solutions.

Crystal modifying agents (e.g. Polycarboxylic acid) distort the crystal habit and inhibit the formation of large crystals. The distorted crystals do not settle on the heat transfer surface, they remain suspended in the bulk solution. If their concentration increases beyond a certain limit, particulate fouling may take place. This is prevented either by using techniques to minimise particulate fouling or using dispersing agents along with crystal modifying agents.

Though crystallisation fouling may not be prevented completely using additives, the resulting crystalline deposits are different from those formed in the absence of any

Fouling and Fouling Mitigation on Heat Exchanger Surfaces 523

Most liquid-side fouling mitigation techniques have been developed for the tube-side of

The deposits which are not strongly adhere to the surface can be removed by increasing the flow velocity. Muller-Steinhagen and Midis [4] reported that alumina deposits were removed completely when the flow velocity was increased for a short period of time after a fouling run. At higher flow velocity, the wall shear stress increases and causes more

At a regular interval of time, the reversal of flow direction on the heat transfer surface could be another effective method of reducing fouling. This technique needs several modifications in the existing set-up. Muller-Steinhagen [37] stated that mitigation of fouling by increasing

Surface material and surface roughness play an important role on fouling mitigation. Thus lowering the surface roughness retards the adhesion of deposits and the number of nuclei growth sites. Lower deposition rate also experienced with lowering surface energy of the material of heat exchanger. Using inert particles is an effective way of reducing or even eliminating fouling completely as practiced in fluidized bed heat exchangers. Pulsating flow in heat exchangers is a strategy to increase the level of turbulence [52-58]. Where, as a matter of fact heat transfer coefficient increases with the enhancement of deposit removal. Higher heat transfer reduces fouling by reducing the interface temperature which is beneficial for certain fouling mechanism such as crystallization fouling of inverse solubility salts. The

Fracture of deposits by fatigue is enhanced by higher turbulence due to pulsation resulting to increase of removal rate. Generally the deposition rate of fouling phenomena [3] depends on the thickness of viscous and thermal sub-layers. Muller-Steinhagen [37] reported that by inserting turbulence promoters inside tubes or by using tube corrugations, the heat transfer coefficient can be increased by a factor of 2 to 15 by reducing the thickness of average thermal boundary layer. Turbulence promoters may reduce both the crystallisation and reaction fouling. Muller-Steinhagen [37] informed that particulate fouling will be enhanced

Middis [10] also reported fouling mitigation by adding natural fibre in the supersaturated solutions of concentration 3.6 g/L CaSO4. He observed that the rate of CaSO4 fouling on heated metal tube surface decreases with the increase of fibre concentration in the fouling solution. Kazi [59] also got similar results by adding different types and concentrations of

Some novel methods which do not fall under well reported categories, such as magnetic or electric treatment are also available in the market to reduce fouling. Usually magnetic treatment is carried out by inserting permanent magnets in a pipe before the heat exchanger.

shell and tube heat exchangers. The relevant techniques include:

3. heat transfer surface such as, surface roughness and surface materials,

the flow velocity could be more effective than reversal of flow direction.

higher level of turbulence augments the deposit removal rate.

if particulate or fibrous material already exists in the solution.

natural fibre in supersaturated solutions of CaSO4.

1. increase in flow velocity, 2. reversal of flow direction,

5. pulsating flow,

4. fluidised bed heat exchangers,

removal of deposits from the surface.

7. transport of cleaning devices through tubes.

6. turbulence promoters, and

additives. The layer looses its strength and can be removed easily. By controlling pH, crystallisation fouling can furthermore be minimised. The solubility of deposit forming components usually increases with decreasing pH. In many water treatment plants, sulphuric acid is added to maintain a pH between 6.5 and 7.5 [47]. In this case, addition of corrosion inhibitors may also be required which may enhance fouling again.

Seeding is used commercially to reduce crystallisation fouling. This method involves addition of seeds to the scaling fluid. Crystallisation takes place preferentially on these seeds rather than on the heat transfer surface. Calcium sulphate seeds are generally used to avoid calcium sulphate scaling [48-49]. These seeds need not be of the crystallising material, but they should have similar crystallographic properties, i. e. atomic agreement and lattice spacing [50].

To mitigate particulate fouling by chemical means, dispersants are used to reduce the surface tension of deposits. It helps in disintegrating the suspended particles into smaller fragments that do not settle so readily.

Addition of certain chemicals can slow down or terminate chemical reactions. Dispersants are very helpful in keeping the foulants away from the surface. Some particles such as corrosion products may act as catalysts. Chemical reaction fouling could be suppressed by reducing the number of these particles. Corrosion inhibitors (chromates and polyphosphates) can be used to reduce corrosion fouling [47]. Usually a passivating oxide layer is desired to prevent corrosion of the surface. Corrosion fouling may promote other fouling mechanisms e. g. higher roughness of the corroded surface may enhance crystallisation fouling. The corrosion products may act as catalysts and promote chemical reaction fouling and also augments particulate fouling by depositing on the heat transfer surface.

Mitigation of fouling by chemical methods has several drawbacks. Fouling and corrosion inhibitors usually contain considerable amount of chlorine, bromine, chromium, zinc etc. Therefore, their concentration has to be monitored carefully. Treatment of fluid released from the plant to natural waterways is necessary to prevent harmful effects. Higher concentrations can be used in closed systems but overdosing may have negative effects and some components may precipitate. Using different additives at the same time may result in dangerous chemical reactions. Some additives have limited life and some degrade with time and loose activity.

Pritchard [51] has broadly classified mechanical methods into two categories according to their ways of action. (1) Brute force methods such as high-pressure jets, lances, drills etc. (2) Mild methods such as brushes and sponge balls. Muller-Steinhagen [37] has reported that several mechanical methods have been developed in recent years. The following mechanisms predict the modern methods:


Most liquid-side fouling mitigation techniques have been developed for the tube-side of shell and tube heat exchangers. The relevant techniques include:


522 Heat Exchangers – Basics Design Applications

additives. The layer looses its strength and can be removed easily. By controlling pH, crystallisation fouling can furthermore be minimised. The solubility of deposit forming components usually increases with decreasing pH. In many water treatment plants, sulphuric acid is added to maintain a pH between 6.5 and 7.5 [47]. In this case, addition of

Seeding is used commercially to reduce crystallisation fouling. This method involves addition of seeds to the scaling fluid. Crystallisation takes place preferentially on these seeds rather than on the heat transfer surface. Calcium sulphate seeds are generally used to avoid calcium sulphate scaling [48-49]. These seeds need not be of the crystallising material, but they should have similar crystallographic properties, i. e. atomic agreement and lattice

To mitigate particulate fouling by chemical means, dispersants are used to reduce the surface tension of deposits. It helps in disintegrating the suspended particles into smaller

Addition of certain chemicals can slow down or terminate chemical reactions. Dispersants are very helpful in keeping the foulants away from the surface. Some particles such as corrosion products may act as catalysts. Chemical reaction fouling could be suppressed by reducing the number of these particles. Corrosion inhibitors (chromates and polyphosphates) can be used to reduce corrosion fouling [47]. Usually a passivating oxide layer is desired to prevent corrosion of the surface. Corrosion fouling may promote other fouling mechanisms e. g. higher roughness of the corroded surface may enhance crystallisation fouling. The corrosion products may act as catalysts and promote chemical reaction fouling and also augments particulate fouling by depositing on the heat transfer

Mitigation of fouling by chemical methods has several drawbacks. Fouling and corrosion inhibitors usually contain considerable amount of chlorine, bromine, chromium, zinc etc. Therefore, their concentration has to be monitored carefully. Treatment of fluid released from the plant to natural waterways is necessary to prevent harmful effects. Higher concentrations can be used in closed systems but overdosing may have negative effects and some components may precipitate. Using different additives at the same time may result in dangerous chemical reactions. Some additives have limited life and some degrade with time

Pritchard [51] has broadly classified mechanical methods into two categories according to their ways of action. (1) Brute force methods such as high-pressure jets, lances, drills etc. (2) Mild methods such as brushes and sponge balls. Muller-Steinhagen [37] has reported that several mechanical methods have been developed in recent years. The following

Breakage of deposits during brief overheating due to differential thermal expansions of

corrosion inhibitors may also be required which may enhance fouling again.

spacing [50].

surface.

and loose activity.

mechanisms predict the modern methods:

heat transfer surface and deposits,

Acoustical vibration of the surface,

Mechanical vibration of the heat transfer surfaces,

Reduced stickiness of the heat transfer surface.

Increased shear stress at the fluid/deposit interface, and

fragments that do not settle so readily.


The deposits which are not strongly adhere to the surface can be removed by increasing the flow velocity. Muller-Steinhagen and Midis [4] reported that alumina deposits were removed completely when the flow velocity was increased for a short period of time after a fouling run. At higher flow velocity, the wall shear stress increases and causes more removal of deposits from the surface.

At a regular interval of time, the reversal of flow direction on the heat transfer surface could be another effective method of reducing fouling. This technique needs several modifications in the existing set-up. Muller-Steinhagen [37] stated that mitigation of fouling by increasing the flow velocity could be more effective than reversal of flow direction.

Surface material and surface roughness play an important role on fouling mitigation. Thus lowering the surface roughness retards the adhesion of deposits and the number of nuclei growth sites. Lower deposition rate also experienced with lowering surface energy of the material of heat exchanger. Using inert particles is an effective way of reducing or even eliminating fouling completely as practiced in fluidized bed heat exchangers. Pulsating flow in heat exchangers is a strategy to increase the level of turbulence [52-58]. Where, as a matter of fact heat transfer coefficient increases with the enhancement of deposit removal. Higher heat transfer reduces fouling by reducing the interface temperature which is beneficial for certain fouling mechanism such as crystallization fouling of inverse solubility salts. The higher level of turbulence augments the deposit removal rate.

Fracture of deposits by fatigue is enhanced by higher turbulence due to pulsation resulting to increase of removal rate. Generally the deposition rate of fouling phenomena [3] depends on the thickness of viscous and thermal sub-layers. Muller-Steinhagen [37] reported that by inserting turbulence promoters inside tubes or by using tube corrugations, the heat transfer coefficient can be increased by a factor of 2 to 15 by reducing the thickness of average thermal boundary layer. Turbulence promoters may reduce both the crystallisation and reaction fouling. Muller-Steinhagen [37] informed that particulate fouling will be enhanced if particulate or fibrous material already exists in the solution.

Middis [10] also reported fouling mitigation by adding natural fibre in the supersaturated solutions of concentration 3.6 g/L CaSO4. He observed that the rate of CaSO4 fouling on heated metal tube surface decreases with the increase of fibre concentration in the fouling solution. Kazi [59] also got similar results by adding different types and concentrations of natural fibre in supersaturated solutions of CaSO4.

Some novel methods which do not fall under well reported categories, such as magnetic or electric treatment are also available in the market to reduce fouling. Usually magnetic treatment is carried out by inserting permanent magnets in a pipe before the heat exchanger.

Fouling and Fouling Mitigation on Heat Exchanger Surfaces 525

Frenier and Steven [63] describe general methods for cleaning heat exchanger equipment, including both mechanical and chemical procedures. They have given guidelines for selecting between chemical and mechanical cleaning, and among the various types of chemical cleaning processes. They stated that water-based fluids can transport and deposit a wide variety of minerals, and corrosion products form due to the reaction of the aqueous fluids with the metals of construction. Hydrocarbon and petrochemical fluids transport and deposit a variety of organic scales. Common inorganic scale forming compound includes various iron oxides, hardness deposits (carbonates and silicates). They stated that the entire cleaning situation must be considered when choosing between mechanical and chemical cleaning, as well as the specific technique within the general category. The general categories of mechanical cleaning are abrasive, abrasive hydraulic, hydraulic and thermal

Frenier and Barber [63] stated that, for chemical cleaning of the heat exchanger tubes, it is very beneficial to obtain a sample of the deposit so that its composition can be determined. Based on the chemical analysis of the deposit, an optimal treatment plan can be developed and the best solvent selected. They have classified the deposits generically, as organic

They stated that the process side deposits may range from light hydrocarbon to polymers and generally they are similar to the fluids from which they originate. They mentioned that the general categories of solvents for process side scales include aqueous detergent solutions, true organic solvents and emulsions. Aqueous detergent formulations always contain a surfactant-type component. In addition they can contain alkaline agents, such as sodium hydroxide, sodium silicate, or sodium phosphate. Builder molecules such as ethylenediaminetetraacetic acid (EDTA) suppress the effects of hard water, and coupling

Detergent formulations are effective only for removing the lighter deposits. Refinery fluids, aeromatics and terpenes are used to dissolve the organic deposits. N-methyl-2 pyrrolidinone also is a very effective polar solvent with low toxicity characteristics. They reiterated that the effectiveness of the application depends greatly on proper application conditions, such as flow rate and temperature. Combination of surfactants, organic solvents and water emulsions are good cleaning agents. Emulsions with an organic outer phase are particularly useful for cleaning large vessels. Oily rust deposits having both organic and inorganic compositions can be removed by acidic emulsions combining an acid and an

Water-side deposits usually contain minerals, such as iron oxides (corrosion products), hardness (Ca and Mg carbonates) and silica, in individual cases other minerals can also be found. The solvents for removing inorganic deposits usually contain mineral acids, organic

Mineral acids used in chemical cleaning include hydrochloric acid (HCl), hydrofluoric acid (HF), sulphuric acid (H2SO4), phosphoric acid (H3PO4), nitric acid (HNO3) and sulfamic acid (H2NSO3H). Hydrochloric acid is the most common and most versatile mineral acid. It is used on virtually all types of industrial process equipment at strengths from 5 percent to 28 percent (5-10 percent is the most usual range). It can be inhibited at temperatures up to about 180 °F. HCl will dissolve carbonates, phosphates, most sulphates, ferrous sulphide,

agents such as glycol ethers, improve the dissolution of some organic deposits.

[64].

organic solvent.

acids or chelating agents.

(process-side) or inorganic (water-side).

Parkinson and Price [60] have reported significant reduction in fouling by the magnetic treatment as it helps in precipitating the salts. These salts stay suspended in the bulk liquid and are removed later. On the other hand Hasson and Bramson [61] informed that there is no effect of magnetic treatment at all on fouling. They observed that magnetic treatment neither decreased nor increased the rate of scaling. The nature of the deposits also remained unchanged. Bernadin and Chan [62] have also reported no influence of magnetic treatment on silica fouling. Muller-Steinhagen [37] has stated that magnetic mitigation devices in some cases actually increased fouling. Thus from the available information no conclusion can be made about the influence of the magnetic field on the scaling process.
