What is fouling

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 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. 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.

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. 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:. 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. 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. 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.

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. 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. R w represents the thermal resistance of the metal wall separating the two fluids. 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.

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.

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. 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.

Search Close Search Bar. Fouling is the formation of unwanted material deposits on heat transfer surfaces during process heating and cooling. In this article, we will outline: The costs, types, and causes of fouling How to detect fouling How to clean your heat exchanger How to prevent fouling.

Costs of fouling Fouling prevention for heat exchangers is typically focused around the heat exchangers itself, however the fouling and heat exchanger performance can be affected by system characteristics that are present both before and after the heat exchanger.

These characteristics can include varying product properties, product handling prior to the heat exchanger and operational performance of other equipment, such as pumps, valves and back pressure. Types of fouling The most commonly occurring types of fouling and aging in hygienic processing fall into four main types: Incrustation: the accumulation of a crust or coating of processed fluids, minerals, or cleaning agents on the surface of heat exchanger parts.

Scaling: a type of incrustation caused by calcium carbonate, calcium sulfate, and silicates. Sediment: comes from corrosion products, metal oxides, silt, alumina, and diatomic organisms microalgae and their excrement. Biological growth: Sources of biofouling include bacteria, nematodes, and protozoa. Causes of fouling in heat exchangers Several variables contribute to fouling, including water pH, product viscosity, and the roughness of component surfaces, among many others.

Key fouling factors Dairy applications include fats, sugars, and proteins that contribute to fouling. To help control fouling, operators pay attention to four key processing factors: Fluid velocity Fluid temperature Fluid chemistry Materials of fabrication.

Fluid velocity In most cases, fouling decreases at higher fluid velocities because increasing flow velocity increases the fluid shear stress , which causes more removal of deposits. Fluid temperature Water can produce scaling from minerals such as calcium carbonate CaCO3. Fluid chemistry During milk processing, calcium phosphate and whey protein can build up on heat exchanger surfaces. Materials of fabrication To prevent corrosion fouling from layers of thermal-resistant material, select units fabricated with corrosion-resistant stainless steels and alloys.

How to detect fouling in heat exchangers Fouling detection typically occurs by physical inspection or by monitoring system performance.

Heat transfer: because of the insulating properties of fouling, heat transfer can fall outside of operating specifications. Pressure drop: Pressure drop increases between heat exchanger inlet and outlet may indicate frictional resistance or blocked flow paths caused by fouling deposits. Temperature transmitters Monitor the temperature of fluids passing through heat exchangers.

Pressure transmitters Detect pressure drops between heat exchanger inlet and outlet. Flow Meters Indicate decreases that may be caused by build-up of fouling material in pipes and plates.

Heat exchanger cleaning Cleaning is required to maintain heat exchanger efficiency. Importance of flow rate during cleaning The proper flow rate ensures the effective mechanical action of fluids during cleaning. During the cleaning of the product side, the flow rate should always be at least the same as the production's flow rate. Cleaning agents to use, by purpose Incrustation, scaling Cleaning incrustation or scaling is a process of removing calcium carbonate, calcium sulfate, or silicates from plate surfaces.

Removing sediment Sediment most commonly consists of metal Oxides, silt, Alumina, and Diatomic organisms and their excrement. Removing biological growth When using heat exchangers to increase the temperature of processed foods, biological growth such as bacteria, nematodes, and protozoa can occur.

Thank you for subscribing to our newsletter! Connect with us. What Does Fouling Mean? Corrosionpedia Explains Fouling Fouling can be classified into two broad categories: microfouling and macrofouling. Common types in this category are: Biofouling Chemical reaction fouling Corrosion fouling Precipitation fouling Composite fouling is caused by more than one fouling mechanism or foulant.

The extent and severity of fouling is dependent on variables such as: Temperature Pollution Nutrient availability Water salinity Fouling materials Surfaces affected include: Ship hulls and propellers Spark plug electrodes Electrical heating elements Solar panels Oil rigs Pipes used to carry water as a coolant for industrial power plants Some effects of fouling are: Reduced efficiency Additional drag Deterioration of protective coating Decreased corrosion resistance Reduced lifespan of surfaces Decreased power output from solar panels Fouling control and reduction are achieved by: Using antifouling paints Preventing fouling materials from entering systems Periodic cleaning to remove foulants Treatments to control water or air in the cooling towers.

Related Question What is the process behind identifying microbiologically influenced corrosion in water pipelines? Share this Term. Procedures 5 Ways to Measure the Hardness of Materials. Don't miss the latest corrosion content from Corrosionpedia! Fouling mechanisms vary with the application but can be broadly classified into four common and readily identifiable types.

The use of corrugated tubes has been shown in be beneficial in minimising the effects of at least two of these fouling mechanisms: deposition fouling because of an enhanced level of turbulence generated at lower velocities, and chemical fouling.