Study of Fouling in Heat Exchanger

P. R. Dhamangaonkar

Ref: Fundamentals of Heat Exchanger Design, By Ramesh K. Shah and Dušan P. Sekulic

Contents

Introduction Classification Effects of Fouling Fouling Mechanism Techniques to prevent Fouling Conclusions References

IntroductionFouling is an accumulation of undesirable material (deposits) on heat exchanger surfaces.

Undesirable material : crystals, sediments, polymers, coking products, inorganic salts, biological growth, corrosion products etc

Fouling is a synergistic consequence of transient mass, momentum and heat transfer phenomena involved with exchanger fluids and surfaces, and depends significantly on heat exchanger operation conditions.

In general, fouling results in:

* a reduction in thermal performance

* an increase in pressure drop, may promote corrosion

* and may result in eventual failures of some heat exchangers

During operation the heat transfer surface fouls resulting in increased thermal resistance and often an increase in the pressure drop and pumping power as well.

Fig 1 -Shell-side crude oil fouling Fig 2- Crude oil fouling at tube ends

Thermal fouling (in the presence of a temperature gradient) means accumulation of any undesirable deposition of a thermally insulating material on a heat transfer surface occurring over a period of time.* Liquid-side fouling * gas-side fouling

Fouling is very costly since it (1) increases capital costs(2) increases maintenance costs(3) results in loss of production(4) increases energy losses

A general practice is to include the effect of fouling on the exchanger thermal performance by adding thermal resistances of fouling layers in the thermal circuit using empirical data.The problem, though, is that this simplified modeling approach does not (and cannot) reflect a real transient nature of the fouling process. The current practice is to use fouling factors or fouling unit thermal resistances from TEMA Standards (1999) (see Section 13.3 and Table 9.4 for tubular and shell-and-tube heat exchangers).

There are six types of liquid-side fouling mechanisms:

(1) precipitation or crystallization fouling (2) particulate fouling(3) chemical reaction fouling(4) corrosion fouling(5) biological fouling, and (6) freezing (solidification) fouling

Fouling Mechanisms

Precipitation or Crystallization FoulingIn precipitation or crystallization fouling, the dominant mechanism is the precipitation of dissolved salts in the fluid on the heat transfer surface when the surface concentration exceeds the solubility limit.

Precipitation of salts can occur within the process fluid, in the thermal boundary layer, or at the fluid–surface (fouling–film) interface. It generally occurs with aqueous solutions and other liquids of soluble salts which are either being heated or cooled. When the solution contains normal solubility salts (the salt solubilityand concentration decrease with decreasing temperature such as wax deposits, gas hydrates and freezing of water/water vapor), the precipitation fouling occurs on the cold surface (i.e., by cooling the solution).

For inverse solubility salts (such as calcium and magnesium salts), the precipitation of salt occurs with heating the solution.

Precipitation/crystallization fouling is common when untreated water, seawater, geothermal water, brine, aqueous solutions of caustic soda, and other salts are used in heat exchangers.This fouling is characterized by deposition of divalent salts in coolingwater systems.Some types:Scale: Hard and tenaciousSludge, soft scale, or powdery deposit: Porous and mushy

Process of Crystallization:a) Nucleationb) Diffusionc) Removal

All these phenomena are controlled by numerous factors, the most dominant being local temperature and temperature gradient levels, composition of the fluid including concentration of soluble species.

Crystallization Fouling Crystallization arises primarily from the presence of dissolved

inorganic salts in the process stream that exhibits super-saturation during heating or cooling.

Common Solution: reducing the temperature of the heat transfer surface often softens the deposits.

Particulate Fouling

Particulate fouling refers to the deposition of solids suspended in a fluid onto a heat transfer surface. If the settling occurs due to gravity, the resulting particulate fouling is called sedimentation fouling.Particulate fouling may be defined as the accumulation of particles from heat exchanger working fluids (liquids and/or gaseous suspensions) on the heat transfer surface.Most often, this type of fouling involves deposition of corrosion products dispersed in fluids, clay and mineral particles in river water, suspended solids in cooling water, soot particles of incomplete combustion, magnetic particles in economizers, deposition of salts in desalination systems, deposition of dust particles in air coolers, particulates partially present in fire-side (gas-side) fouling of boilers, and so on.

Particulate Fouling

The particulate fouling caused by deposition of corrosion products is influenced by the following factors: metal corrosion process factors (at heat transfer surface), release and deposition of the corrosion products on the surface concentration of suspended particles, temperature conditions on the fouled surface (heated or non-heated), and heat flux at the heat transfer surface.

Common Solution: velocity control

Chemical reaction fouling is referred to as the deposition of material (fouling precursors) produced by chemical reactions within the process fluid, in the thermal boundary layer, or at the fluid–surface (fouling–film) interface in which the heat transfer surface material is not a reactant or participant. The heat transfer surface may act as a catalyst as in cracking, coking, polymerization, and autoxidation.This fouling mechanism is a consequence of an unwanted chemical reaction that takes place during the heat transfer process e.g. deposition of coke in petrochemical industries in cracking furnaces.The deposits from chemical reaction fouling may promote corrosion at the surface if the formation of the protective oxide layer is inhibited. All fouling deposits may promote corrosion.

Chemical Reaction Fouling

Common Solution: reducing the temperature between the fluid and the heat transfer surface

Corrosion Fouling

The heat transfer surface itself reacts with the process fluid or chemicals present in the process fluid. Its constituents or trace materials are carried by the fluid in the exchanger, and it produces corrosion products that deposit on the surface. Corrosion fouling is dependent on the selection of exchanger surface material and can be avoided with the right choice of materials such as expensive alloys Corrosion fouling is prevalent in many applications where chemical reaction fouling takes place and the protective oxide layer is not formed on the surface.

Corrosion Fouling

The important factors for corrosion fouling are:• the chemical properties of the fluids and heat transfer surface• oxidizing potential and alkalinity• local temperature and heat flux magnitude and• mass flow rate of the working fluid

Common Solution: material selection

Bio-foulingBiological fouling or bio-fouling results from the deposition, attachment, and growth of macro or microorganisms to the heat transfer surface.Biological fouling can be divided into two main subtypes of fouling:1. Microbial fouling: accumulation of microorganisms such as algae,

fungi, yeasts, bacteria, and molds2. Macrobial fouling represents accumulation of macro-organisms

such as clams, barnacles, mussels, and vegetation as found in seawater or estuarine cooling water.

Microbial fouling precedes macrobial deposition.

Biological fouling is generally in the form of a bio-film or a slime layer on the surface that is uneven, filamentous, and deformable but difficult to remove. Exist primarily in the temperature range 0 to 900C (32 to 1940F) and thrive in the temperature range 20 to 500C (68 to 1220F).

Bio-foulingBiological fouling or bio-fouling results from the deposition, attachment, and growth of macro or microorganisms to the heat transfer surface.Biological fouling can be divided into two main subtypes of fouling:1. Microbial fouling: accumulation of microorganisms such as algae,

fungi, yeasts, bacteria, and molds2. Macrobial fouling represents accumulation of macro-organisms

such as clams, barnacles, mussels, and vegetation as found in seawater or estuarine cooling water.

Microbial fouling precedes macrobial deposition.

Biological fouling is generally in the form of a bio-film or a slime layer on the surface that is uneven, filamentous, and deformable but difficult to remove. Exist primarily in the temperature range 0 to 900C (32 to 1940F) and thrive in the temperature range 20 to 500C (68 to 1220F).

Bio-fouling

Deposition and growth of material of a biological origin on a heat transfer surface results in bio-fouling.

Such material may include microorganisms (e.g., bacteria, algae and molds) and their products, and the resulting fouling is known as microbial fouling.

Common Solution: material selection

Freezing (Solidification) Fouling.

Due to freezing of a liquid or some of its constituents, or deposition of solids on a sub-cooled heat transfer surface as a consequence of liquid–solid or vapor–solid phase change in a gas stream.

Formation of ice on a heat transfer surface during chilled water production or cooling of moist air, deposits formed in phenol coolers, and deposits formed during cooling of mixtures of substances such as paraffin are some examples of solidification fouling.

This fouling mechanism occurs at low temperatures, usually ambient and below depending on local pressure conditions.

The main factors affecting solidification fouling are mass flow rate of the working fluid, temperature and crystallization conditions, surface conditions, and concentration of the solid precursor in the fluid.

Freezing (Solidification) Fouling.

Due to freezing of a liquid or some of its constituents, or deposition of solids on a sub-cooled heat transfer surface as a consequence of liquid–solid or vapor–solid phase change in a gas stream.

Formation of ice on a heat transfer surface during chilled water production or cooling of moist air, deposits formed in phenol coolers, and deposits formed during cooling of mixtures of substances such as paraffin are some examples of solidification fouling.

This fouling mechanism occurs at low temperatures, usually ambient and below depending on local pressure conditions.

The main factors affecting solidification fouling are mass flow rate of the working fluid, temperature and crystallization conditions, surface conditions, and concentration of the solid precursor in the fluid.

Combined Fouling

Occurs in many applications, where more than one fouling mechanism is present and the fouling problem becomes very complex with their synergistic effects. Some combined fouling mechanisms found in industrial applications:

• Particulate fouling combined with bio-fouling, crystallization, and chemical-reaction fouling

• Crystallization fouling combined with chemical-reaction fouling• Condensation of organic/inorganic vapors combined with

particulate fouling in gas streams• Crystallization fouling of mixed salts• Combined fouling by asphaltene precipitation, pyrolysis,

polymerization, and/or inorganic deposition in crude oil• Corrosion fouling combined with bio-fouling, crystallization, or

chemical-reaction fouling

Combined Fouling

Occurs in many applications, where more than one fouling mechanism is present and the fouling problem becomes very complex with their synergistic effects. Some combined fouling mechanisms found in industrial applications:

• Particulate fouling combined with bio-fouling, crystallization, and chemical-reaction fouling

• Crystallization fouling combined with chemical-reaction fouling• Condensation of organic/inorganic vapors combined with

particulate fouling in gas streams• Crystallization fouling of mixed salts• Combined fouling by asphaltene precipitation, pyrolysis,

polymerization, and/or inorganic deposition in crude oil• Corrosion fouling combined with bio-fouling, crystallization, or

chemical-reaction fouling

Modeling of Fouling

There is not a single, unified theory to model the fouling process wherein all six types of fouling mechanisms are considered. In many processes more than one fouling mechanism exists with synergistic effects.

However, a few variables that would most probably control any fouling process: (1) fluid velocity (2) fluid and heat transfer surface temperatures and temperature

differences(3) physical and chemical properties of the fluid(4) Heat transfer surface properties, and (5) geometry of the fluid flow passage(6) concentration of foulant or precursor, impurities, heat transfer

surface roughness, surface chemistry, fluid chemistry (pH level, oxygen concentration, etc.), pressure, and so on

Modeling of Fouling

There is not a single, unified theory to model the fouling process wherein all six types of fouling mechanisms are considered. In many processes more than one fouling mechanism exists with synergistic effects.

However, a few variables that would most probably control any fouling process: (1) fluid velocity (2) fluid and heat transfer surface temperatures and temperature

differences(3) physical and chemical properties of the fluid(4) Heat transfer surface properties, and (5) geometry of the fluid flow passage(6) concentration of foulant or precursor, impurities, heat transfer

surface roughness, surface chemistry, fluid chemistry (pH level, oxygen concentration, etc.), pressure, and so on

For a given fluid–surface combination, the two most important design variables are: (i) the fluid velocity and (ii) heat transfer surface temperature.

In general,Higher flow velocities → less foulant deposition and/or more

pronounced deposit erosion.

→ may accelerate corrosion of the surface by removing the heat transfer surface material.

Higher surface temperatures → chemical reaction, corrosion, crystal formation (with inverse solubility salts), and polymerization, → but reduce bio-fouling and prevent freezing and precipitation of normal solubility salts.

Consequently, it is frequently recommended that the surface temperature be maintained low.

For a given fluid–surface combination, the two most important design variables are: (i) the fluid velocity and (ii) heat transfer surface temperature.

In general,Higher flow velocities → less foulant deposition and/or more

pronounced deposit erosion.

→ may accelerate corrosion of the surface by removing the heat transfer surface material.

Higher surface temperatures → chemical reaction, corrosion, crystal formation (with inverse solubility salts), and polymerization, → but reduce bio-fouling and prevent freezing and precipitation of normal solubility salts.

Consequently, it is frequently recommended that the surface temperature be maintained low.

Influence of Operating variables on Liquid Side Fouling

When the value of an operating variable is increased, it increases (↑−), decreases (↓), or has no effect (↔)on the specific fouling mechanism listed. Dashes — indicate that no influence of these variables is reported in the literature.

Source: Data from Cannas (1986)

Influence of Operating variables on Gas Side Fouling

When the value of an operating variable is increased, it increases (↑−), decreases (↓), or has no effect (↔)on the specific fouling mechanism listed. Dashes — indicate that no influence of these variables is reported in the literature.

Source: Data from Cannas (1986)

The quantitative effect of fouling on heat transfer is estimated by using the concept of fouling resistance and calculating the overall heat transfer coefficient under both fouling and clean conditions.

The cleanliness factor: An additional parameter for determining this influence.

It is defined as a ratio of an overall heat transfer coefficient determined for fouling conditions to that determined for clean (fouling-free) operating conditions.

The effect of fouling on the pressure drop can be determined by the reduced free-flow area due to fouling and the change in the friction factor, if any, due to fouling.

The quantitative effect of fouling on heat transfer is estimated by using the concept of fouling resistance and calculating the overall heat transfer coefficient under both fouling and clean conditions.

The cleanliness factor: An additional parameter for determining this influence.

It is defined as a ratio of an overall heat transfer coefficient determined for fouling conditions to that determined for clean (fouling-free) operating conditions.

The effect of fouling on the pressure drop can be determined by the reduced free-flow area due to fouling and the change in the friction factor, if any, due to fouling.

Sequential Events in Fouling

Initiation Transport Attachment Removal Aging

Source: Epstein (1978)

Initiation

• Surface conditioning• Surface temperature, material, finish, roughness and

coating strongly influence initial delay, induction/ incubation period.

• Surface roughness tends to decrease delay period.• Roughness projection leads crystal nucleation and groves

provide regions for particulate deposition.

Time dependence of the fouling resistance.

Initiation of the fouling, the first event in the fouling process, is preceded by a delay period or induction period τd.The basic mechanism involved during this period is heterogeneous nucleation, τd is shorter with a higher nucleation rate.

The factors affecting τd are temperature, fluid velocity, composition of the fouling stream, and nature and condition of the heat exchanger surface. Low-energy surfaces (unwettable) exhibit longer induction periods than those of high-energy surfaces (wettable).

In crystallization fouling, τd tends to decrease with increasing degree of super-saturation. In chemical reaction fouling, τd appears to decrease with increasing surface temperature. In all fouling mechanisms, τd decreases as the surface roughness increases due to available suitable sites for nucleation, adsorption, and adhesion.

Transport

• Fouling substances from the bulk fluid are transported to the heat transfer surface.

• Transport is accomplished by diffusion, sedimentation and thermophoresis,.

• The difference between fouling species, oxygen or reactant concentration in the bulk fluid (Cb) and that in the fluid adjacent to the heat transfer surface(Cs), results in transport by diffusion.

• The local deposition flux, md=hD(Cb-Cs). hD is obtained from Sherwood Number and depends on flow and geometric properties.

Transport contd…..

• Sedimentation: Because of gravity particulate matter in a fluid is transported to the inclined or horizontal surface.

• It is important in application with heavy particles in liquid and with low velocities.

• Thermophoresis: The movement of small particles in fluid stream in presence of temperature gradient.

• Cold walls attract colloidal particles while hot walls repels.

• It is important for particles below 5μm and is dominant at about 0.1 μm.

• Other processes such as electrophoresis, inertial impaction and turbulent downsweeps are also present.

Attachment:

• It is the phenomenon in which part of the transported fouling material attaches to the surface.

• This process is considerably uncertain.• Forces acting on the particles, density, size of the particle

and surface condition is important.

Removal:

• Some material is removed immediately after deposition and some latter.

• The forces at the interface of fluid and deposits are responsible for removal.

• Shear force depends on velocity gradients at surface, viscosity and surface roughness.

• Dissolution, erosion and spalling are the plausible mechanisms.

• Dissolution removes material in ionic form, Spalling is affected by thermal stress and removes material as large mass.

Aging:

• Aging begins with deposition of material on surface.• The mechanical properties of the deposits can change

because of changes in crystal and chemical structure.• Corrosion at the surface may weaken the bio-fouling

layer.• Chemical deposits at the surface change mechanical

strength.

Effects of Fouling Fouling layer has low value of ‘k’ , which increases resistance

to the heat transfer. As C/s area reduces in the formula

Q=U*A*ΔT

So the heat transfer decreases. Increase in pressure drop & pumping power.

Heat

Process Fluid

Heat

Cleansurface

Process FluidFoulinglayer

Design Approach

1. Rf & Uoverall calculations.

2. Impact on heat transfer performance

- cleanliness factor

CF= Uf/Uc=1/(1+Rf*Uc)

3. Empirical data

-TEMA Standards

Effect of material on fouling resistance:-

- May promote or inhibit reactive process.

- Polished surfaces resist fouling but corrosion

takes place.

Effect of Fluid velocity(Vf) on fouling resistance:-

- Increase in Vf decreases Rf

- Heat transfer coefficient increases with Vf

Techniques to Control Fouling

Surface cleaning technique

1. Continuous cleaning

2. Periodic cleaning

Techniques to control

1. Crystallization fouling

2. Particulate fouling

3. Biological fouling

4. Corrosion fouling

High velocity water jets

Conclusions

The influence of Fouling has to be taken into account not only during operation but also during design.

Fouling adds thermal resistance to heat transfer in a heat exchanger as well as increases pressure drop.

Increase in fluid velocity decreases fouling resistance

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