A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

FOULING

DOI: 10.1615/AtoZ.f.fouling

The fouling of heat exchangers may be defined as the accumulation of unwanted deposits on heat transfer surfaces. The foulant layer imposes an additional resistance to heat transfer and the narrowing of the flow area, due to the presence of deposit, results in an increased velocity for a given volumetric flow rate. Furthermore, the deposit is usually hydrodynamically rough so that there is an increased resistance to the flow of the fluid across the deposit surface. Therefore, the consequences of fouling are, in general, a reduction in exchanger efficiency and other associated operating problems including excessive pressure drop across the exchanger. It is only in recent years that the problem of heat exchanger fouling has attracted scientific and theoretical treatment and many aspects remain to be investigated. Fouling of heat exchangers has been reviewed [Bott (1990), Hewitt et al. (1994), Bott (1995)].

In some heat exchangers fouling occurs rapidly, in others the equipment may operate for long periods, perhaps several years, before a problem becomes apparent. Much depends on the particular fluid and the conditions under which the exchanger operates. The nature of deposits is extremely variable. In some examples, deposits are hard, tenacious and difficult to remove. Other accumulations are soft and friable that lend themselves to removal.

It is usual to find that deposits are made up of different components. The deposit associated with cooling water, for instance, may include: corrosion products, particulate matter, crystals and living biological material.

The extent of each of the components in the deposit will depend on many factors including the origin of the water, its treatment and the processing conditions. It is possible that one component is dominant, e.g., scale formation or corrosion. Because of this extremely variable quality of deposits, it has become common practice to consider different fouling mechanisms in the development of techniques to mitigate the problem. Six mechanisms have been identified that give rise to fouling problems in heat exchangers. They include:

  1. Crystallization fouling often referred to in water systems as scaling, involves the formation of crystals form solution on the surface or deposition of crystals formed in the bulk liquid or in the laminar sub-layer.

  2. Particulate fouling, as the name suggests, depends on the arrival of discrete particles at the transfer surface. Particles may be small (i.e., < 1 μm) or may be large (i.e., several mm). Particulate fouling is common in both liquid and gas systems.

  3. Biological fouling may be defined as the growth of living matter on heat exchanger surfaces. The phenomenon usually associated with water systems, e.g., cooling water, involves both microorganisms and macroorganisms. The former may include bacteria, fungi or algae, while the latter includes mussels and barnacles.

  4. Chemical reactions at or near the surface may give rise to what is often called chemical reaction fouling. The effect of heat on a process fluid as it passes through the exchanger, may accelerate chemical reactions, e.g., cracking or polymerization reactions that can give rise to deposition on the surface. In some instances the metallic surface of the heat exchanger acts as a catalyst thereby aiding the fouling process. Chemical reaction fouling is usually associated with liquids but it may also occur in vapor or gas streams.

  5. Some heat exchange materials of construction are subject to corrosion from the aggressive nature of the fluids or impurities in the fluids in contact with the surface. The result may be a protective layer, but more likely, the corrosion process produces a thick corrosion layer. Corrosion fouling may be observed in both liquid and gas systems.

  6. In cooling operations, it is possible to freeze a liquid being processed at the cold surface. The frozen layer represents a resistance to heat transfer. For instance, freezing fouling may occur during the production of chilled water.

Although the different mechanisms have been identified as leading to fouling, it is possible to consider fouling in an idealized, general way. If the deposit thickness due to fouling is plotted against time, three idealized curves may be visualized as shown in Figure 1.

Idealized fouling curves.

Figure 1. Idealized fouling curves.

Curve A represents a straight line relationship, i.e., deposit thickness is proportional to time. Curve B is a falling rate curve, i.e., the rate of deposition declines with time. Curve C is the usual form of the relationship between deposit thickness (or thermal fouling resistance) and time; it is an exponential or asymptotic curve. For a short time the heat exchanger fouls relatively slowly, but after a period of time the rate of deposition rapidly accelerates to be followed by a fall in the rate of deposition. Eventually the deposit thickness remains constant. The thickness of a deposit is a measure of the resistance of the deposit to the transfer of heat so that similar curves would result if foulant thermal resistance was plotted against time.

The curves in Figure 1 are shown to develop from the origin. In some examples of fouling, an initiation or induction period is required before fouling begins. Induction periods may be extremely short, i.e., a few seconds, or they may involve several weeks or months. The extent of the induction period depends on the nature of the heat exchanger surface, i.e., the material of construction and the surface roughness, together with the properties of the fluid (and any impurities) passing over the surface.

The presence of a deposit on the surfaces of a heat exchanger give rise to two major problems:

  1. The efficiency of the heat exchanger is reduced in respect of heat transfer as a result of the thermal resistance of the deposit. In general, the thermal conductivity of deposits is very much lower than metals so that even a thin layer can cause significant thermal resistance. The fact that the surface of the foulant layer is rough compared to the original metal surface will, in genera], increase the heat transfer due to the increased turbulence generated by the roughness elements. To some extent, this offsets the effects of increased thermal resistance across the heat exchanger. However, the benefits, due to the presence of the fouling layer, are usually relatively small compared to the restrictions to heat flow imposed by the insulating properties of the foulant.

    The overall effect on heat transfer may be summarized by the following statement:

    Change in overall heat transfer coefficient is some function of

    • Change due to the thermal resistance of the foulant layer

    • Change due to the surface roughness of the deposit

    • Change due to the increased velocity for a given volumetric flow rate, resulting from the restrictions on flow area imposed by the presence of the deposit.

    Although the dependence has been cited regarding the heat exchanger as a whole, it is strictly applicable to local conditions. Still, the incidence and quality of a deposit is likely to vary with its location in the exchanger.

  2. Due to the roughness of the deposit and the restrictions of the flow area that are responsible for enhanced turbulence and heat transfer compared to the condition for the same flow rate, the pressure drop also increases. Relatively the layers on the inside of a tube can, for instance, give rise to substantial increases in pressure drop. For example, a 1mm layer on the inside of a heat exchanger tube with a nominal internal diameter of 18mm increases the velocity for a given volumetric flow rate by a factor or 182/162 = 1.27.

Since pressure drop is a function of the square of the velocity, the pressure drop with the deposit in place, will be increased by a factor of 1.272 or around 1.6 times, i.e., a 60% increase. These calculations neglect the effect of deposit roughness so that, if it were possible to take the additional contribution to pressure drop into account numerically, the pressure drop through the exchanger under fouled conditions may approach double that for a clean exchanger.

Reduced heat transfer and increased pressure drop under fouled conditions can have significant implications for energy utilization on the process plant—namely reduced heat recovery that may have to be made good using primary energy, and increased energy requirements for monitoring the fluid through the exchanger. Both effects have implications in terms of cost of operation. The economic penalties of heat exchanger fouling on the financial performance include other effects that are due to the presence of deposits that may not always be recognized at the design stage, or during subsequent operation.

During the initial stages of heat exchanger selection when the basic concepts of the design are being considered, the problem of potential fouling will be a major concern. It is usual, after careful thought, to include additional thermal resistance over and above that for the clean conditions, to allow for the anticipated deposit accumulation. The result is an increase in heat transfer area for a given heat load and temperature change requirements. The increase in heat transfer area (i.e., an increase in the size of the heat exchanger for a given duty) represents an increase in capital cost. The choice of the anticipated increase in heat transfer resistance due to the fouling is crucial to the ultimate cost of the heat exchanger.

Where excessive fouling is anticipated and a particular heat exchanger is vital to the process, it may be prudent to duplicate the heat exchanger in addition to allowing for deposit accumulation in the basic design, so that production can be maintained without the need to shut down the process.

Duplication and oversizing, to allow for fouling, represents substantial additional capital costs. Where special materials of construction are required, due to the nature of the fluids passing through the heat exchanger, the additional cost may be considerable.

The cost of fouling

Unless the problem of fouling is recognized at the design stage and adequate steps taken to ensure that the problem is contained, serious operating and maintenance difficulties may result that add to the cost of production. Emergency shut down may be necessary because the heat exchanger rapidly loses heat transfer efficiency or more likely, the flow cannot be maintained due to the excessive pressure drop generated by the presence of the deposit. A further consequence of the high pressures necessary to drive the fluid through the exchanger may be failure of joints and packings, and increased wear and tear on the associated pumps. Furthermore, the presence of deposits may encourage corrosion of the underlying metal with the need for early replacement.

All the factors directly attributable to fouling represent increased maintenance costs that may be considerable in some cases. Where fouling is experienced, sooner or later, it will be necessary to clean the heat exchanger. Not only will the cleaning process involve labor costs, it may require special equipment, particularly if chemical cleaning is involved. Additional circuitry involving pumps, tanks, pipelines and valves may be required, chemicals purchased and the cleaning process may produce effluents that require treatment before discharge.

Conventional cleaning techniques such as tube drilling, surface brushing and high pressure water jetting will also involve some financial investment in addition to the cost of the associated labor. Unless adequate precautions are taken, the cleaning process may result in damage to the heat exchanger. In some instances where deposits are tenacious and difficult to remove, damage may be inevitable.

Additional maintenance and cleaning means that the process will have to be shut down to allow access unless standby equipment has been provided. As a consequence, there is a period of lost production which represents a loss of return on the capital investment in the process equipment as a whole, and reduced profitability. Lost production as a result of unplanned shut down due to any cause including fouling, can have a substantial effect where there is keen competition.

It also has to be remembered that continual difficulties with the operation of a process plant, especially heat exchangers subject to excessive fouling, will affect the morale of the labor force. It may be particularly so where production bonuses and incentives are involved. The outcome of these concerns may be a general lowering of a sense of responsibility, poor housekeeping and slack attitudes. The costs are difficult to quantify, but there will be a cost penalty associated with these personnel problems.

The problem of fouling has led to remedial technologies that may include on-line cleaning or the use of additives. The purpose is to prevent fouling occurring or to reduce its effects on the performance of the heat exchanger in question. Mitigation by whatever means will involve additional costs either in terms of capital for the associated equipment or the purchase of treatment chemicals.

The cost of heat exchanger fouling can be considerable although much depends on the nature of the fluids being handled, the design of the heat exchanger and the conditions under which it is operated.

The economic penalties described above may be summarized:

  1. Increased capital costs, i.e., additional heat transfer area, mitigation and cleaning equipment.

  2. Additional energy requirement to allow for reduced energy recovery.

  3. Labor costs associated with additional maintenance, cleaning and mitigation.

  4. Cost of any antifoulant chemicals.

  5. Lost income resulting from lost production.

  6. Equipment replacement costs.

  7. Additional costs associated with low labor morale.

Fouling Mechanisms

The underlying mechanism involved in the accumulation of deposits on surfaces of heat exchangers may generally be considered to involve three stages:

  1. The foulant or the agents or impurities (e.g., bacteria, solid particles or corrosive agents) that lead to deposit formation, approach the surface from the bulk through the viscous sublayer adjacent to the heat exchanger surface, across which the fluid is flowing. The principles of mass transfer apply.

  2. At the surface, adhesion can take place involving a number of factors including the interaction of surface forces, chemical reactions and structural orientation.

  3. Once deposited on the surface, the material may be subject to forces that compact or weaken the deposit. The quality of the deposit is likely to be time dependent.

These three stages will be influenced by a number of system parameters, but principally these are associated with fluid flow, heat and mass transfer. As a consequence, the process whereby a surface becomes fouled and the deposit maintained, is complex resulting from the interaction of a number of factors. Although it is extremely rare for a single mechanism to apply, it is useful to examine separately the mechanisms briefly mentioned earlier in order to have an appreciation of the effects of the system variables involved.

Crystallization

The deposition or formation of crystals on a surface, sometimes referred to as scaling, is often associated with aqueous systems, e.g., cooling water circuits. (See also Crystallization.)

Before crystallization can occur, it is necessary to have conditions of supersaturation, i.e., dissolved solid concentrations above the saturation solubility at a particular temperature. The supersaturation provides the "driving force" for precipitation to occur. In general, the degree of supersaturation involved is quite small; its location will depend very much on the temperature distribution within the heat exchanger between the bulk fluid and surface temperatures.

Two solubilities of salts in water are recognized. Normal solubility salts (e.g., NaCl or Na2SO4) display increased solubility as the solution temperature is raised, so that when a saturated solution is cooled supersaturation can occur followed by precipitation. Fouling from normal solubility salts is likely when these solutions are cooled. Inverse solubility salts (e.g., CaCO3, Mg2(SO4)), on the other hand, have reduced solubility as the temperature is raised. Consequently, as the temperature of a saturated solution increases, supersaturation and precipitation will occur. Fouling or scaling due to inverse solubility salts is probable when solutions of these salts are heated. Furthermore, the solubility of inverse solubility salts in general is quite low, so that relatively small temperature changes are likely to cause fouling problems. The problem is very prevalent in cooling water systems due to the so-called hardness salts usually present in the water. There are two contributory factors:

  1. As the cooling process takes place the temperature of the water rises. If the water is already saturated even a small temperature rise leads to precipitation.

  2. In cooling water systems, the technique used to remove unwanted heat is to evaporate some of the water in a cooling tower or spray pond. The loss of water vapor concentrates the dissolved salts so that saturation is soon achieved with the consequent increased possibility of precipitation of sparingly soluble salts. To offset this concentration effect, some of the saturated water is removed in the so-called "blow down" discharge, to be replaced with fresh "make up" water.

The scale produced on heat exchanger surfaces as a result of crystallization from water is often tenacious and difficult to remove, although in some instances, it is soft, resembling a sludge that is more easily removed from the surface. The condition of the deposit very much depends on the conditions, particularly of temperatures that prevail close to the solid surfaces. The color of scale depends very much on the system involved. In purer forms, the color is near white, but color can be imparted to the scale from trace compounds in the system, e.g., oxides of iron that may give black (magnetite) or red (haematite) colors.

Controlling scale formation in boilers is particularly important, since the accumulation of hardness salts on the heat transfer surfaces (e.g., the inside of water tubes) can lead to catastrophic failure. The presence of the scale can lead to excessive metal temperatures and consequently rupture.

Particle Deposition

Particle accumulation on heat exchanger surfaces is a common fouling phenomenon. A wide range of particle sizes may be involved in the deposition process. The origin of the particles varies. In some instances (e.g., water and crude oil), the particulate matter may be inherent in fluid stream.

In combustion systems, particulate matter (e.g., the mineral matter in coal) may be released as the combustible components are burnt. The mineral particles go forward with the flue gasses and are deposited on the heat exchanger surfaces. It is also possible in combustion systems, that, if the combustion process is not adequately controlled, incomplete combustion occurs and particles of unburnt carbon are carried in the gas stream. The high temperatures prevailing in combustion systems may mean that some particulate matter is in the form of liquid droplets that solidify as they are cooled. Agglomeration and chemical reaction of the particles with gaseous components in the flue gas can add to the complexity of the fouling process. (See also Combustion and Flames.)

In some process streams, temperature effects lead to chemical reactions which produce large molecules (e.g., polymers) as particulate matter capable of being deposited on surfaces.

Biological Growth on Surfaces

Biofouling of heat exchanger surfaces is generally falls in two groups: macrofouling and microfouling. In general, aqueous environments are involved such as might be found in a cooling water system. Water originates from a natural source such as a river or the sea. Generally macrofouling is associated with living creatures such as mussels and barnacles. It is also possible for vegetation such as sea weed to become attached and grow on surfaces. Macrofouling is often experienced—although not exclusively—in the use of sea water. Microfouling is the result of the attachment and growth of microorganisms on surfaces. As far as heat exchangers are concerned, the microorganisms involved are usually bacteria. Other microorganisms that may be found in cooling water systems include fungi and algae. Algae need a source of light. In nature this is sunlight; so these microbes are to be found in exposed regions, for example, the basin below a cooling tower. Fungi, on the other hand, may be found on the internal structure of a cooling tower. The structure of the tower can be damaged and destroyed by fungal activity.

Although algae and fungi may not grow on heat exchanger surfaces, they may still be the source of deposits on the heat transfer surfaces. With the passage of time, either living or dead algae and fungi may become detached upstream of the heat exchanger only to deposit in the downstream equipment. Under the conditions, this organic material may provide a suitable nutrient for supporting bacterial activity. In general, the surface of heat exchangers in cooling water systems is ideal for microbial growth. The aqueous medium contains nutrients, it is aerated due to passage through a cooling tower or spray pond, and the temperature is near the optimum for maximum biological activity, i.e., in the range 20-50°C.

The slime layer associated with fouling from microorganisms is characterized by a gelatinous, hydrated material often nearly transparent, but more usually colored by microorganisms, or by other contaminants in the water (e.g., oxides or hydroxides of iron). Another common feature of biofilms is that they are uneven and deformable under the action of the water flow. Under certain conditions filamentous structures grow out from the surface and are free to oscillate within the flowing water. Biofilms are notoriously difficult to remove from surfaces, but if they spontaneously slough off the surface as does happen during operation, the lumps of biofilm can cause blockages downstream. Very thick biofilms with serious implications for heat transfer and pressure drop are possible under favorable conditions.

The presence of a biofilm on a metal surface may promote corrosion. The local conditions under the biofilm are likely to be different from those in the bulk flow, this is particularly applicable to the pH. The activity of the living material is likely to lower the local pH that could initiate corrosion. Furthermore, under a biofilm, due to the restrictions on ingress of bulk fluid, galvanic cells may be established that enhances, perhaps only locally, the rate of corrosion.

Chemical Reaction Fouling

The accumulation of deposits may occur as the result of chemical reactions at or near a heat exchanger surface. Because chemical reactions are usually enhanced by raised temperatures, chemical reaction fouling may be experienced where a reactive fluid is being heated. Under these circumstances, the slow-moving layers of fluid near the heat transfer surface are subject to relatively high temperatures.

Fouling due to the incidence of chemical reactions can occur in a wide range of process streams and over a wide range of temperature, i.e., from near ambient to temperatures associated with combustion conditions. Processes where chemical reaction is responsible for deposit formation, include food processing (e.g., the pasteurisation of milk), oil refining (e.g., preheating crude oil prior to primary distillation), and chemical manufacturing that involves polymerization reactions. Even in combustion, the impurities contained in the original fuel (e.g., coal or combustible waste) may give rise to chemical reactions in conjunction with acid gases such as SO2 and SO3 contained in the flue gases. Incomplete combustion may lead to the generation of soot particles that deposit on heat transfer surfaces. Free radicals may be responsible for chain reactions in organic liquids that ultimately lead to deposit formation. These examples of chemical reaction demonstrate the wide range of substances that may be regarded as being deposits from chemical reactions. Consequently, the deposits may be soft and friable or hard and difficult to remove depending on the chemical reactions involved and the conditions.

Corrosion of Surfaces

Unlike other fouling mechanisms, in corrosion fouling the heat transfer surface itself degenerates to form a layer of lower thermal conductivity than the original metal. The resistance of many metals and alloys to corrosion is due to the presence of a thin layer of oxide on the surface that restricts the flow of electrons and ions usually necessary for corrosion to occur. A protective oxide layer may be regarded as controlled but desirable corrosion. At the same time it also represents a thermal resistance. In general, the thickness of the protective layer is such that the heat flow is not greatly impaired. If, however, conditions are such that the corrosion of the surface is extensive, then the oxide layers (and probably hydroxide layers in aqueous systems) represent a fouling problem. It is also possible that corrosion is facilitated by the removal of protective oxide layers by aggressive chemical agents, e.g., acid attack.

In combustion systems, inorganic salts deposited on heat transfer surfaces may be subject to such high temperatures that they become molten. The liquid condition may provide pathways for electron transfer that accelerates corrosion beneath the deposit.

Corrosion fouling is very much dependent upon the material of construction from which the heat exchanger is fabricated. The problem of corrosion fouling can be eliminated by the correct choice of construction material, but in general, corrosion resistant alloys are expensive. Circumstances may be such that the high costs involved cannot be entertained. Other techniques must then be employed to restrict the incidence of corrosion.

Freezing Fouling

Freezing fouling may occur where the temperature in the region of the heat transfer surface is reduced to below the freezing point of the fluid being processed. The deposition of wax from waxy hydrocarbons by cooling is often considered to represent freezing fouling, but it is probably better defined as crystallization fouling. A good example of freezing fouling is the production of chilled water. If the temperature of the heat exchanger surface on the water side is at or below 0°C, then it is likely that a layer of ice will form on the surface. The thickness of the ice deposit will be very dependent on the magnitude of the temperature distribution between the coolant on the one side of the exchanger and the water on the other. The elimination of freezing fouling may be achieved by the choice of coolant temperature, so that the surface in contact with the liquid from which heat is being extracted is slightly higher than the freezing point of the liquid.

Mixed Mechanisms of Fouling

Although six mechanisms of fouling have been briefly described, it is rare for practical heat exchanger fouling to be the result of a single mechanism. In most process streams where fouling occurs, two or probably more mechanisms are involved. It is possible that one mechanism may be dominant and, from a practical standpoint, the other mechanisms present can be ignored when remedial action is being considered. In cooling water systems, it is likely that, in addition to microorganisms, the circulating water will contain dissolved solids, suspended particulate matter and, perhaps, also aggressive chemicals. The accumulated deposit on the equipment surfaces may therefore contain microorganisms, particles, scale and products of corrosion. The gelatinous nature of the biofilm may aid the development of the foulant layer by capturing particles as they collide with its surface. Concentration effects may occur near the film that encourages crystal formation, and the charged conditions underneath the deposit may enhance corrosion.

In fouling associated with combustion, the fouling on heat exchangers may be due to particle deposition, chemical reactions and corrosion as described earlier.

It will be clear from these two examples that the process of fouling may be extremely complex necessitating, as it does, a rather empirical approach to its understanding and investigation.

Factors that affect the incidence of fouling

A number of system variables affect the incidence of fouling on heat exchange surfaces, but three generally carry more importance than all others including: fluid temperature and the associated temperature distribution, stream velocities and—as would be expected—the concentration of all foulant, or foulant precursors that are contained in the fluid streams. Variables of less general importance, but which may assume significance in certain examples, include: pH, material of construction and associated surface condition. Attention to the magnitude of these variables, associated with all mechanisms of fouling, can go a long way to mitigating particular fouling problems, although it has to be said that certain problems may not respond as well as others.

Guidelines associated with temperature that are useful for the initial design and subsequent operation of heat exchangers suggest:

Low temperatures will:

  1. reduce the effects of chemical reaction and corrosion since rates of reaction are generally temperature sensitive; high temperatures favor accelerated reactions;

  2. reduce the activity of micro- and macroorganisms;

  3. reduce the opportunity for supersaturation conditions to occur for inverse solubility salts.

High temperatures will:

  1. reduce the incidence of biofouling where the temperature is above that for optimum growth;;

  2. avoid conditions that could lead to freezing fouling;

  3. reduce the opportunity of supersaturation conditions to occur with dissolved salts with normal solubility.

In addition to temperature effects that influence the fouling process, temperature may also be a factor in the long term retention of the deposit on the surface. Over a period of time it is more than likely that a particular deposit will age. The aging process may be influenced by temperature. The effects could be beneficial or detrimental to the continued operation of the heat exchanger. It is possible that, due to internal chemical reactions, the deposit becomes more tenacious and difficult to remove. For other encrustations, the effect of changed temperature distribution as the deposit grows in thickness, planes of weakness and inconsistencies in the deposit lead to fracture and spalling.

The temperature effects are, in general, associated with the temperature distribution across the heat exchanger. For a given temperature difference between the hot and cold streams within the equipment, the growth of deposit (usually on both sides): will affect the distribution of that total temperature driving force so that the metal dividing wall separating the fluids; will experience a changing temperature. The deposits themselves will also be affected in terms of their respective temperatures. For large temperature differences and thick deposits, there is likely to be a considerable temperature difference across the deposit with implications for the quality of the deposit. For instance, the chemical reactions involved when the deposit is relatively thin, may be quite different from those associated with thick deposits. Such conditions, for instance, on the flue gas side of coal combustion equipment, may give rise to stratified deposits and chemical transformations as time passes.

Although the temperature of the streams within a heat exchanger will be specified, there is some flexibility open to the designer to modify the temperature distribution. By investigating changes in velocity that affect the thermal resistance in the respective streams, it is possible to change, beneficially, the various interface temperatures. The changes in velocity have implications in their own right.

Some comments on the effects of velocity have already been made, namely, the effects on temperature distribution and pressure drop. The latter is closely linkend to fluid shear: increasing velocity increases fluid shear at the interface between a solid surface and the fluid flowing across it. High shear forces may result in foulant removal, that tends to maintain a static fouled condition, i.e., near the asymptotic or equilibrium fouling condition. Under these circumstances the velocity controls the deposit thickness. Increasing velocity may appear attractive for minimizing the effects of deposits, but for a particular deposit, the necessary velocity may be unacceptably high leading to high pumping costs and possibly problems of erosion. It also has to be remembered that increasing velocity will increase turbulence, so that where the fouling process is mass transfer controlled, deposition is facilitated. In biological fouling, for instance, higher velocities, although leading to enhanced removal opportunities, will also facilitate nutrient transfer to the living matter colonizing the particular surface.

The choice of velocity, therefore, is very much a compromise depending on the particular system under consideration. As a rough guide velocities of the order of 2m/s for liquid flows in tubes will have some controlling effect without excessive pumping costs. On the shell side mean fluid velocities for liquids across the tube bank of around 1 m/s may be regarded as a suitable guide. In gas systems much higher velocities are possible but it is difficult to provide reliable guidelines.

In general, the higher the concentration of foulant or deposit precursor, the greater the fouling of surfaces is likely to be, since the mass transfer driving force, i.e., the concentration gradient towards the target surface is enhanced. It is usually not in the gift of the heat exchanger designer or operator, to influence the concentration of foulant precursors in the stream handled by the exchanger. Often the potential fouling problem is not recognized at the design stage; it may only be discovered during subsequent operation as trace constituents of the flow stream. It may be possible to limit the deposit precursor, for example particulate matter or unreacted species, by improved control of processes upstream from the exchanger. In certain exceptional circumstances it may be necessary to reduce, or remove altogether, the components responsible for the fouling process.

Fouling Resistance Concept

The usual method of allowing for the incidence of fouling in heat exchanger design is to employ resistances that account for the fouling on both sides of the exchanger. Sometimes these fouling reactions are referred to as "fouling factors". The latter description is not altogether satisfactory since the term "factor" is usually applied to a multiplier.

The thermal resistance due to fouling is additive as illustrated in the following equation which sums all the thermal resistance between the two fluids. If the Overall Heat Transfer Coefficient is U1 then:

where R1 and R2 are the fouling resistances associated with fluid streams 1 and 2, respectively.

Rw represents the thermal resistance of the metal wall separating the two fluids. In general, this resistance is quite small and often it can be neglected altogether due to the high thermal conductivity of metals, α1 and α2 are the heat transfer coefficients at the metal wall for fluids 1 and 2, respectively, and D1 and D2 are the inner and outer diameter of the tube through which fluid 1 is passing. Fluid 2 passes over the outside of the tube. It will be seen that, in reality, the equation is not mathematically sound except for steady state conditions, i.e., when the deposit thickness on both sides of the exchanger remains constant. Under these conditions it is likely that the asymptotic fouling resistance has been attained. The earlier discussion has shown that fouling development is transient so that the steady state condition does not apply.

In order to help designers and others, tables of fouling resistances are published, e.g., the TEMA (Tubular Exchanger Manufacturers' Association) fouling resistances [Chenoweth (1990)]. The data are classified according to the fluid and process and the figures are based on the experience of recognized experts in the field. Although the information is a useful guide, it has to be treated with caution in the light of the earlier discussion on the influence of temperature, velocity and foulant precursor concentration. In general, the tables do not specify any of the variables so that it becomes difficult to relate them to a particular set of conditions. It also has to be understood that the published fouling resistances are only applicable to shell and tube heat exchangers and may not be used in the design of plate heat exchangers for instance. Furthermore, it has to be remembered that by taking into account the anticipated fouling resistance, a clean (newly on stream) heat exchanger will overperform. To compensate, adjustments to the fluid flow rate(s) will be made that could encourage the fouling process so that the fouled condition prediction is self-fulfilling.

Wherever possible data on fouling resistances relating to the actual process streams and the conditions of velocity and temperature pertaining to the particular design should be used for assessment purposes. Unfortunately these data are not generally to hand. The choice, then, becomes one of experience and judgement with guidance from published figures. In this connection, it has to be appreciated that the increased capital cost of a heat exchanger over and above the clean condition to allow for fouling, may very well depend upon the arbitrary choice of fouling resistance.

Mitigation of Foulin

In order to control the incidence of fouling a wide range of online mitigation techniques may be employed, but they generally fall into two groups, namely, mechanical methods and the use of chemical additives.

Mechanical methods, as the name suggests, use physical methods of removal. Some examples are given in the following table:

Table 1. Mechanical control techniques

Some examples of the use of additives are given in the following table:

It will be seen that wherever possible the use of the additive is complementary to the shear effects produced by the fluid velocity across the heat exchanger surface. Other additives impart changes either to the depositing particles or the surface so that the particles are "held off" the surface.

In general the concentration of the additive is relatively small, i.e., up to 100 mg/l but in many applications the concentration may be as low as 1 mg/l. In order to be cost effective the concentration of additive must be kept as low as possible.

Table 2. Chemical additive application

REFERENCES

Bott T. R. (1990) Fouling Notebook, Instn. Chem. Engrs. DOI: 10.1016/0260-8774(93)90078-X

Hewitt G. F., Shires G. L. and Bott T. R. (1994) Process Heat Transfer, CRC Press, Boca Raton.

Bott T. R. (1995) Fouling of Heat Exchangers, Elsevier, Amsterdam.

Chenoweth, J. M., (1990) Final report of the HTRI/TEMA Joint Committee to Review the Fouling Section of the TEMA Standards. Heat Trans. Eng., 11, 1. 73-107.

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