The Impact of Fouling on Heat Exchangers: Heat Transfer, Pressure Drop, and Fouling Mechanisms

 Fouling in Heat Exchangers: Effects on Performance and Mechanisms

Fouling is an accumulation of undesirable material (deposits) on heat exchanger surfaces. Undesirable material may be crystals, sediments, polymers, coking products, inorganic salts, biological growth, corrosion products, and so on. This process influences heat transfer and flow conditions in a heat exchanger. 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. However, most manifestations of these various phenomena lead to similar consequences. 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.

Corrosion represents mechanical deterioration of construction materials of heat exchanger surfaces under the aggressive influence of flowing fluids and environment in contact. In addition to corrosion, other mechanically induced phenomena are important for heat exchanger design and operation, such as fretting (corrosion occurring at contact areas between metals under load subjected to vibration and slip).

Fouling and corrosion represent heat exchanger operation-induced effects and should be considered for both the design of a new heat exchanger and operation of an existing exchanger.

In this blog, we will learn the impact of fouling and corrosion on heat transfer and pressure drop, different types of fouling mechanisms in detail.

 

Fouling and its effects on Exchanger Heat Transfer and Pressure Drop

Thermal fouling (in the presence of a temperature gradient) means accumulation of any undesirable deposition of a thermally insulating material (which provides added thermal resistance to heat flow) on a heat transfer surface occurring over a period of time. This solid layer adds an additional thermal resistance to heat flow and also increases hydraulic resistance to fluid flow. Also, the thermal conductivity of fouling deposits is usually lower

than that for the metals used for heat transfer surfaces. Fouling is an extremely complex phenomenon characterized by combined heat, mass, and momentum transfer under transient conditions. Liquid-side fouling occurs on the exchanger side where liquid is being heated, and gas-side fouling occurs on the gas cooling side; however, reverse examples can be found.

Fouling is very costly since it (1) increases capital costs due to the need to over surface the heat exchanger and for cleaning; (2) increases maintenance costs resulting from cleaning, chemical additives, or troubleshooting; (3) results in loss of production due to shut down or reduced capacity; and (4) increases energy losses due to reduced heat transfer, increased pressure drop, and dumping of dirty streams present. Gas-side fouling can also be a potential fire hazard in a fossil-fired exhaust environment, resulting in catastrophic lost production and repair costs. In some applications, increased pressure drop due to fouling may reduce gas flows affecting adversely heat transfer and increasing solvent concentration (such as during waste heat recovery from paint oven exhausts) which is not acceptable environmentally.

Fouling significantly reduces heat transfer with a relatively small increase in fluid pumping power in systems with liquid flows and high heat transfer coefficients. For systems having low heat transfer coefficients, such as with gases, fouling increases the fluid pumping power significantly with some reduction in heat transfer. Note that plug­ging will also increase pressure drop substantially but doesn't coat the surface and still may be considered as fouling in an application.

 

Fouling in liquids has a significant detrimental effect on heat transfer with some increase in fluid pumping power. In contrast, fouling in gases reduces heat transfer somewhat (5 to 10% in general) but increases pressure drop and fluid pumping power significantly (up to several hundred percent) from the cost point of view.

It should be emphasized that the same magnitude of a fouling factor can have a different impact on performance for the same or different applications. For example, the same fouling factor may represent heavy fouling in a clean service (such as a closed-loop refrigerant system) or low fouling in a dirty service (such as a refinery crude preheat train). As another example, the same fouling factor in two different plants may have radically different fouling rates because of different feedstocks, preprocessing, or equipment design.

Phenomenological Consideration of Fouling

Fouling is an extremely complex phenomenon characterized by a combined heat, mass, and momentum transfer under transient conditions. Fouling is affected by a large number of variables related to heat exchanger surface, operating conditions, and involved fluid streams. Despite the complexity of the fouling process, a general practice is to include the effect of fouling on the exchanger thermal performance

Let us now consider in detail different types of fouling mechanisms. 

Fouling Mechanisms

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.

Only biological fouling does not occur in gas-side fouling since there are in principle no nutrients in the gas flows. In reality, more than one fouling mechanisms is present in many applications and their synergistic effect makes the fouling even worse than predicted/expected with a single fouling mechanism present. Note that there are additional examples of fouling that may not fall in the foregoing categories, such as accumulation of noncondensables in a condenser. In addition, plugging will also increase pressure drop substantially, but doesn't coat the surface and still may be considered as fouling in applications. Refer to Melo et al. (1988) and Bott (1990) for a detailed study of fouling.

In 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. Thus, a necessary prerequisite for an onset of precipitation is the presence of supersaturation. 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 solubility and 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 cooling water systems. Crystallization fouling may occur with some gas flows that contain small quantities of organic compounds that would form crystals on the cold surface. If the deposited layer is hard and tenacious (as often found with inverse solubility salts such as cooling water containing hardness salts), it is often referred to as scaling. If it is porous and mushy, it is called sludge, softscale, or powdery deposit. The most important phe­nomena involved with precipitation or crystallization fouling include the following. Crystal growth during precipitation require formation of a primary nucleus. The mechanism controlling that process is nucleation, as a rule heterogeneous in the presence

of impurities and on the heat transfer surface. Transfer of particulate solids to the fouled surface is accomplished by diffusion. Simultaneously with deposition, removal phenom­ena caused by shear stress are always present. Deposit mechanical integrity changes over time either by strengthening or by weakening it due to crystalization/recrystalization, temperature change, and so on. 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.

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. Hence, 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. The particulate fouling caused by deposition of, for example, 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 nonheated), and heat flux at the heat transfer surface.

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. However, the heat transfer surface may act as a catalyst as in cracking, coking, polymerization, and autoxidation. Thermal instabilities of chemical species, such as asphaltenes and proteins, can also induce fouling precursors. Usually, this fouling occurs at local hot spots in a heat exchanger, although the deposits are formed all over the heat transfer surface in crude oil units and dairy plants. It can occur over a wide temperature range from ambient to over 1000°C (1800°F) but is more pronounced at higher temperatures. Foulant deposits are usually organic compounds, but inorganic materials may be needed to promote the chemical reaction. This fouling

in which heat transfer fouling affects the exchanger mechanical integrity, and the corro­sion products add thermal resistance to heat flow from the hot fluid to the cold fluid. If corrosion products are formed upstream of the exchanger and then deposited on the heat transfer surface, the fouling mechanism refers to particulate or precipitation fouling, depending on whether the corrosion products are insoluble or soluble at the bulk fluid conditions. The interaction of corrosion and other types of fouling is the major concern for many industrial applications. 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) if the high cost is warranted. 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 is of significant importance in the design of the boiler and condenser of a fossil fuel—fired power plant. 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. It should be noted that although growth of corrosion influenced deposit has a detrimental effect on heat transfer, this influence is less impor­tant than fouling caused by particulate fouling of corrosion products formed elsewhere within the system. For example, fouling on the water side of boilers may be caused by corrosion products that originate in the condenser or feedtrain.

Biological fouling or biofouling results from the deposition, attachment, and growth of macro- or microorganisms to the heat transfer surface; it is generally a problem in water streams. In general, biological fouling can be divided into two main subtypes of fouling: microbial and macrobial. Microbial fouling is accumulation of microorganisms such as algae, fungi, yeasts, bacteria, and molds, and macrobial fouling represents accumulation of macroorganisms such as clams, barnacles, mussels, and vegetation as found in sea­water or estuarine cooling water. Microbial fouling precedes macrobial deposition as a rule and may be considered of primary interest. Biological fouling is generally in the form of a biofilm or a slime layer on the surface that is uneven, filamentous, and deformable but difficult to remove. Although biological fouling could occur in suitable liquid streams, it is generally associated with open recirculation or once-through systems with cooling water. Since this fouling is associated with living organisms, they can exist primarily in the temperature range 0 to 90°C (32 to 194°F) and thrive in the temperature range 20 to 50°C (68 to 122°F). Biological fouling may promote corrosion fouling under the slime layer. Transport of microbial nutrients, inorganic salts, and viable microorganisms from the bulk fluid to the heat transfer surface is accomplished through molecular diffusion or turbulent eddy transport, including organic adsorption at the surface.

Freezing or solidification fouling is due to freezing of a liquid or some of its constitu­ents, or deposition of solids on a subcooled 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 (Bott, 1981). 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 are (Panchal, 1999):

·     Particulate fouling combined with biofouling, 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 biofouling, crystallization, or chemical-reaction fouling

Some examples of the interactive effects of corrosion and fouling are as follows (Panchal, 1999):

·     Microfouling-induced corrosion (MIC) (sustained-pitting corrosion)

·     Under-deposit corrosion in petroleum and black liquor processing (concentration buildup of corrosion-causing elements)

·     Simultaneous corrosion and biofouling in cooling water applications

·     Fouling induced by corrosion products

It is obvious that one cannot talk about a single, unified theory to model the fouling process wherein not only the foregoing six types of fouling mechanisms are identified, but in many processes more than one fouling mechanism exists with synergistic effects. However, it is possible to extract 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. The other impor­tant variables are 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 the fluid velocity and heat transfer surface temperature. In general, higher flow velocities may cause less foulant deposition and/or more pronounced deposit erosion, but at the same time may accelerate corrosion of the surface by removing the heat transfer surface material. Higher surface temperatures promote chemical reaction, corrosion, crystal formation (with inverse solubility salts), and polymerization, but reduce biofouling and prevent freezing and precipitation of normal solubility salts. Consequently, it is frequently recommended that the surface temperature be maintained low.

Single-Phase Liquid-Side Fouling

Single-phase liquid-side fouling is most frequently caused by (1) precipitation of minerals from the flowing liquid, (2) deposition of various particles, (3) biological fouling, and (4) corrosion fouling. Other fouling mechanisms are also present. More important, though, is the combined effect of more than one fouling mechanism present.

An additional parameter for deter­mining this influence, used frequently in practice, is the cleanliness factor. 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.

Single-Phase Gas-Side Fouling

Gas-side fouling may be caused by precipitation (scaling), particulate deposition, corro­sion, chemical reaction, and freezing. Formation of hard scale from the gas flow occurs if a sufficiently low temperature of the heat transfer surface forces salt compounds to solidification. Acid vapors, high-temperature removal of an oxide layer by molten ash, or salty air at low temperatures may promote corrosion fouling. An example of parti­culate deposition is accumulation of plant residues. An excess of various chemical sub­stances, such as sulfur, vanadium, and sodium, initiates various chemical reaction fouling problems. Formation of frost and various cryo-deposits are typical examples of freezing fouling on the gas side. An excellent overview of gas-side fouling of heat transfer surfaces is given by Marner (1990, 1996). Qualitative effects of some of the operating variables on gas-side fouling mechanisms are presented in Table 13.2.

Fouling in Compact Exchangers

Small channels associated with compact heat exchangers have very high shear rates,
perhaps three to four times higher in a plate heat exchanger than in a shell-and-tube

exchanger. This reduces fouling significantly. However, small channel size creates a problem of plugging the passages. To avoid plugging, the particle size must be restricted by filtering or other means to less than one-third the smallest opening of heat exchanger passages. Even with this guideline, particulate fouling can occur and agglomerate, such as with waxy substances.

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Reference: - Dugan P. Sekulic (2003) R K Shah