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13 Fouling and CorrosionFouling is an accumulation of un desirable material (deposits) on h eat ex changer surfaces.Undesirable ma terial ma y be crystals, sediments, polymers, coking prod ucts, ino rganicsalts, biological growth, corrosion products, and so on. This process influences heattransfer an d flow conditions in a heat exchan ger. Fouling is a synergistic consequenceof transient mass, momentum and heat transfer phenomena involved with exchangerfluids an d surfaces, an d depen ds significantly o n h eat exchanger o peration conditions.Howev er, mos t manifestations of these various phe nom ena lead to similar consequences.In gen eral, fouling results in a reduction in thermal performance, an increase in pressuredrop, may prom ote corrosion, an d m ay result in eventual failures of so me heat exchangers.Corrosion represents mechanical deterioration of construction materials of heatexchanger surfaces under the aggressive influence of flowing fluids and environment incontact. In addition to corrosion, other mechanically induced pheno mena are im po rtan tfor heat exchanger design and operation , such as fretting (corrosion occu rring a t contac tareas between metals und er loa d subjected to vibration a nd slip).Fouling an d corrosion represent heat exchanger operation-induced effects and shou ldbe considered for bo th the design of a new heat exch anger and operation of an existingexchanger. In this chapter, we explain the impact of fouling and corrosion on heattransfer and pressure dr op in Section 13.1. In Section 13.2, we present a detailed descrip-tion of various fouling mechanisms and phenomenological considerations of fouling.The methodology to take into account the effect of fouling o n exchanger performancean d design is outlined in Section 13.3. Variou s techniques of prevention an d m itigation ofdetrime ntal effects of fouling are sum marized in Section 13.4. Finally, a brief account ofthe imp ortance of corrosio n, in particular its influence on h eat exchanger ope ration an ddesign pra ctices, is provide d in Section 13.5.

13.1PRESSURE DROPFOULING AND ITS EFFECT ON EXCHANGER HEAT TRANSFER AND

Therm al fouling (in the presence of a tem perature gradie nt) means accum ulation of anyundesirable deposition of a thermally insulating material (which prov ides added thermalresistance to h eat flow) on a heat transfer surface occurring over a period of time.+ Thissolid layer add s an ad ditional thermal resistance to heat flow an d also increases hydrau licresistance to fluid flow. Also, the therm al con ductivity of fouling deposits is usually low er'There are other types of fouling phenom ena in nature e.g., clogging of arteries) that are not of importance inheat exchanger design.

863

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864 FOULING A N D CORROSION

tha n th at fo r the metals used fo r heat transfer surfaces. Fouling is an extremely com plexphenomenon characterized by combined heat, mass, and momentum transfer undertransient conditions. Liquid-side fouling occurs on the exchanger side where liquid isbeing heated, and gas-side fouling occurs on the gas cooling side; however, reverseexamples ca n be fou nd.Fouling is very costly since it (1) increases capital cos ts due t o th e need t o o versurfac ethe heat exchanger and for cleaning; (2) increases maintenance costs resulting fromcleaning, chemical additives, or troubleshooting; (3) results in loss of production dueto shutdown or reduced capacity; and 4) increases energy losses due to reduced heattransfer, increased pressure dr op , and dum ping of dirty streams present. Gas-side foulingcan also be a potential fire hazard in a fossil-fired exhaust environment, resulting incatastroph ic lost production an d repair costs. In som e applications, increased pressuredr o p due to fouling ma y reduce g as flows affecting adversely hea t transfer a nd increasingsolvent 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 fluidpumping power in systems with liquid flows and high heat transfer coefficients. Forsystems having low heat transfer coefficients, such as with gases, fouling increases thefluid pu mp ing pow er significantly with some reduction in heat transfer. No te t ha t plug-ging will also increase pressure drop substantially but doesn't coat the surface and stillmay be considered as fouling in a n application.Let us first discuss only qualitatively the influence of a deposit on a heat transfersurface. We consider either fully developed lamina r o r turb ulent flows. U sing the results/correlations for laminar flow (Nu = constant; see Table 7.3) and turbulent flow [theDittus-Boelter corre lat ion , Eq. (7.79) in Tab le 7.61, we express the he at transfe r coeffi-cient as follows:

Nu with N u = consta nt for lamina r flow- .023(E)o 8.Pro.'] fo r turbulen t flowh = ( Dh (13.1)Dh

Note that in the turbulent flow expression of Eq. (13.1), we substitutedRe = mDh/Aop= 4m/Pp using the definition of Dh. Here P is the wetted perimeter ofall flow passages in the exchanger. In general, we treat P as independent of Dh. F o rexample, having a double num ber of 5 m m diameter tubes com pared to a specifiednum ber of 10 m m diam eter tubes in a n exchanger will have the sam e total P but differentDh.A similar situ ation ca n exist fo r a n extended surface. Using Eqs. (6.29) an d (6.67b),we express the following pressure drop relationships:

In Eq. (13.2), f e is approximately constant for fully developed laminar flow (atheoretical value for a circular tube is f R e = 16), while the following relationship is

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FOULING O N EXCHANGER HEAT TRANSFER AND PRESSURE DROP 865

applied for turbulent flow: f = 0.046Re-'.'. The most important physical outcome offouling is the flow cross section getting plugged and resulting in a reduced hydraulicdiameter of flow passages. Therefore, for a given mass flow rate h fluid flow length L ,heat transfer area A (= P L ) , and known fluid properties, one gets from Eqs. (13.1) and(13.21,

1 1h m - A P K -Dh D; (13.3)

The functional relationships given by Eq. (13.3) are obtained assuming total wettedperimeter as constant regardless of the change in the hydraulic diameter. In practice,when D h for an exchanger changes, A may change as well as for a circular tube. In t ha tcase, since P = TdiNt = TDhN, for a tubular exchanger ( N , = total number of tubes),Ap : 1 D i from Eq . (13.2) for laminar flow and A p : l/D;.* for turbulent flow insteadof 1/Dh of Eq. (13.3) for a tubular exchanger. Alternatively, the Ap expression forturbulent flow in a circular tube can be expressed as follows using Eq. (6.29) with thedefinitions G = m / A , , A , = ( ~ / 4 ) d f nd Dh = di:

i

(13.4)Substitution o ff = 0.046Re-'.* will change the exponent of m in Eq. (13.4) to 1.8 and theexponent of di to 4.8. Also, the surface roughness change due t o fouling on the f factorsshould be included as an additional effect (generally, we neglect the effect of surfaceroughness on the heat transfer coefficient for conservatism). Actual influences of foulingon the heat transfer coefficient and pressure drop are substantially more complex thanthose presented by Eqs. (13.3) an d (13.4), du e to inherently transient nature of foulingprocesses.

The pressure drop ratio Apf/Apc of a fouled and a clean exchanger for a constantmass flow rate, from Eq. (13.4), is given by(13.5)

If we consider that fouling does not affect the friction factor (i.e., fc x f r and alsoconsider that reduction in the tube inside diameter due to fouling is only 10 to 20%,the resulting pressure drop increase will be approximately 69% and 20 5%, respectively,according to Eq. (13.5), regardless of whether the fluid is liquid or gas [note that incontrast, h 0: l/D h, as shown in Eq. (13.1) or (13.3)]. This increased Ap can be translatedinto increased fluid pumping power using Eq. (6.1); and for liquids, the density beingsignificantly higher than tha t for gases, a substantially higher Ap due t o fouling can beallowed for liquids for a reasonable increase in liquid pumping power. Also, the equip-ment cost of fluid pumping power is lower for liquids than for gases for a given am ou nt ofpumping power.+Now let us review the impact of fouling on exchanger heat transfer. As fouling willreduce the free-flow area and hence the passage Dh, it will increase the convection heat' For example, for a midsized autom obile, the cost of a 300-W fan for the radiator airflow was 35 o 40, comparedto $20 to 25 for an equivalent power radiator coolant water pump in 2001,

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866 FOULING AND CORROSIONtransfer coefficient h [of Eq. (13. l)] between the fluid a nd heat transfe r su rface (whichmay be covered with a fouling layer), for two reasons: increased flow velocity with areduction in the flow area, an d increased surface roughn ess du e to the fouling layer. Bo ththese effects would increase the pressure d ro p sub stantially . Fou ling layers (deposits) o none o r both fluid sides increase therm al resistance to hea t flow from the ho t fluid to coldfluid by conduction through the fouling layers (see Fig. 3.4), which also have lowerthermal conductivity. The added thermal resistances in general reduce the exchangeroverall U A substantially compared to the increase in h due t o fouling, a s mentionedabove.+ D ue to a large u ncertainty, transient n ature, variations in the fouling resistance(Rf = l/h f), an d no acc urate means of its measu rement, the increase in h du e to fouling isignored o r lumped in to the reported values of fouling resistances. He nce, the heat tran s-fer coefficients hh and h, for hot and cold fluids are determined for unfouled cleansurfaces for th e U A com puta tion of fouled surfaces. F ro m the overall thermal resistanceEq. (3.20) or (3.24), we find that fouling deposits will reduce U A and hence q moresignificantly in liquids th an in gases. This is because liquids have h an order of magni tudehigher than that for gases in general. T o understand this, consider a process plantheat exchanger with clean U = 1500 W/m2 .K or the overall unit thermal resistanceR, = 6 x [a reasonablevalue from TEMA (1999)] are considered, 50% extra heat transferarea is chargeable to fouling since R,,ne, = (6 + 3) x m2 . K / W a n dq = A AT,,,/R,,new. n contrast, for a gas-to-gas clean compact heat exchanger, considerU , = 300 W/m 2 . K or R, = 3 x m2 . K/W. For the same fouling resistances,Rf,h + Rf,, = 3 x m2 . K /W , the heat transfer surface area chargeab le to fouling isonly 10%.Based on the foregoing discussion, fouling in liquids has a significant detrimentaleffect on heat transfer with some increase in fluid p um ping p ower. In c ontra st, foulingin gases reduces heat transfer some what (5 to 10% in general) bu t increases pressure dr opand fluid pumping power significantly (up to several hundred percent) from the costpoin t of view.It shou ld be emphasized that the sam e magnitude of a fouling factor (or fouling un itthermal resistance)$ can have a different impac t o n perform ance for the same or differentapp lication s. F o r example, the same fouling fac tor may represent heavy fouling in a cleanservice (such as a closed-loop refrigerant system) o r low fouling in a dirty service (such a sa refinery crude preheat train). As another example, the same fouling factor in twodifferent plants m ay h ave radically different fouling rate s because of different feedstocks,preprocessing, or equipment design.

m 2 .K/W. If the fouling resistances Rf,h + Rf,, = 3 x

13.2 PHENOMENOLOG ICAL CONSIDERATIONS O F FOULINGAs noted in Section 13.1, fouling is a n extremely complex phe nom eno n c harac terized bya combined heat, mass, and mom entum transfer under transient conditions. Fouling isaffected by a large number of variables related to heat exchanger surface, operatingconditions, and involved fluid streams. Despite the complexity of the fouling process,a general practice is to include the effect of fouling on the exchanger th erm al perfo rma nce' For example, see the added thermal resistance terms ( l / q e h f A ) or the hot and cold fluid sides in Eq. (3.20) or(3.24), which may reduce l / U A more than the increase in hh and/or h, due to fouling, depending on their relativemagnitudes in the equation.The concept o f fouling resistance introduced in Section 3.2 .4 is explained further in Section 13 .2.6 .

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PHENOMENOLOGICAL CONSIDERATIONS OF FOULING 867

by adding thermal resistances of fouling layers in the thermal circuit using empirical dat a,as explained through Fig. 3.4 an d discussed further in Section 13.3. Th e problem, though,is that this simplified modeling approach does not (and cannot) reflect a real transientna tu re of the fouling process. The current practice is to use fouling factors or fouling unitthermal resistances from TEMA Standards (1999) (see Section 13.3 and Table 9.4 fortubular and shell-and-tube heat exchangers). However, probably a better approachwould be to perform cost analysis for cleaning frequency by taking into account anyinitial overdesign (by including fouling resistances). Th is overdesign may provide addedheat transfer performance initially due to larger surface area and flow area thanrequired for a clean exchanger but will reduce the flow velocity an d hence may accelerateinitial fouling in some applications. Let us now consider in detail different types offouling mechanisms, sequential events in fouling, and modeling of a fouling process asan example.13.2.1 Fouling MechanismsThere are six types of liquid-side fouling mechanisms: (1)precipitation or crystallizationfouling, (2) particulate fo uling , (3) chemical reaction fouling, (4) corrosion fouling, (5)biological fou ling, and ( 6 ) reezing solidification) fouling. Only biological fouling doesno t occur in gas-side fouling since there a re in principle no nutrients in the gas flows. Inreality, more than one fouling mechanisms is present in many applications and theirsynergistic effect makes the fouling even worse than predicted/expected with a singlefouling mechanism present. Note that there are additional examples of fouling thatmay not fall in the foregoing categories, such as accumulation of noncondensables ina condenser. In addition, plugging will also increase pressure drop substantially, butdoesnt coat the surface and still may be considered as fouling in applications. Referto Melo et al. (1988) and Bott (1990) for a detailed study of fouling.In precipitation or crystallization fou lin g, the do m inant mechanism is the precipitationof dissolved salts in the fluid on the heat transfer surface when the surface concentrationexceeds the solubility limit. Thus, a necessary prerequisite for an onse t of precipitation isthe presence of supersaturation . Precip itation of salts can occur within the process fluid,in the thermal bounda ry layer, o r a t the fluid-surface (fouling-film) interface. It generallyoccurs with aqu eo us solutions an d o the r liquids of soluble salts which are either beingheated o r cooled. When the solution contains nor mal solubility salts (the salt solubilityand concentration decrease with decreasing temperature such as wax deposits, gashydrates and freezing of water/water vapor), the precipitation fouling occurs on thecold surface (i.e., by cooling the solution). For inverse solubility salts (such as calciumand 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 , an d ot her salts are used inheat exchangers. This fouling is characterized by deposition of divalent salts in coolingwater systems. Crystallization fouling may occur with some gas flows that con tain smallquantities of organic compounds that would form crystals on the cold surface. If thedeposited layer is ha rd and tenacious (as often found w ith inverse solubility salts such ascooling water containing hardness salts), it is often referred to as scaling. If it is porousand mushy, it is called sludge, softscale, or powdery deposit. The most important phe-nomena involved with precipitation or crystallization fouling include the following.Crystal growth during precipitation require formation of a primary nucleus. Themechanism controlling th at process is nucleation , as a rule heterogeneous in the presence

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of impurities and on the heat transfer surface. Transfer of particulate solids to the fouledsurface is accomplished by diffusion. Simultaneously with deposition, removal phenom-ena caused by shear stress are always present. Deposit mechanical integrity changes overtime 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 dominan t being local temperature and temperature gradient levels, compositionof the fluid including concen tration of soluble species.Particulate fouling refers to the deposition of solids suspended in a fluid on to a heattransfer surface. If the settling occurs due to gravity, the resulting particulate fouling iscalled sedimentation fouling. Hence, particulate fouling may be defined as the accumula-tion 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 ofcorrosion products dispersed in fluids, clay and mineral particles in river water, sus-pended solids in cooling water, soo t particles of incomplete com bustion, magnetic par-ticles in economizers, deposition of salts in desalination systems, deposition of dustparticles in air coolers, particulates partially present in fire-side (gas-side) fouling ofboilers, and so on . The particulate fouling caused by deposition of, for example, corro-sion products is influenced by the following factors: metal corrosion process factors (atheat transfer surface), release and deposition of the corrosion products on the surfacet;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 t o a s the deposition of material (fouling precur-sors) produced by chemical reactions within the process fluid, in the thermal boundarylayer, or at the fluid-surface (fouling-film) interface in which the heat transfer surfacematerial is not a reactant or pa rticip an t. However, the heat transfer surface may act as acatalyst as in cracking, coking, polymerization, and autoxidation. Thermal instabilitiesof chemical species, such as asphaltenes and proteins, can also induce fouling precursors.Usually, this fouling occurs at local hot s po ts in a heat exchanger, although the depositsare formed all over the heat transfer surface in crude oil units and dairy plants. It canoccur over a wide temperature range from am bient to over 1000C(1800F) but is morepronounced at higher temperatures. Foulant deposits are usually organic compounds,but inorganic materials may be needed to promote the chemical reaction. This foulingmechanism is a consequence of an unwanted chemical reaction that takes place duringthe heat transfer process. Examples of chemical fouling include deposition of coke inpetrochemical industries in cracking furnaces where thermal cracking of hydroc arbons isrealized. This fouling mechanism is found in many applications of process industry, suchas oil refining, vapor-phase pyrolysis, cooling of gas and oils, polymerization of processmonomers, and so on . Furth erm ore , fouling of heat transfer surface by biological fluidsmay involve complex heterogeneous chemical reactions and physicochemical processes.The deposits from chemical reaction fouling may p rom ote corrosion a t the surface if theformation of the protective oxide layer is inhibited. All fouling deposits may promotecorrosion.In corrosion fouling (in situ), the heat transfer surface itself reacts with the processfluid or chemicals present in the process fluid. Its constituents or trace materials arecarried by the fluid in the exchanger, and it produces corrosion products that depositon the surface. Hence, corrosion fouling could be considered a s chemical reaction fouling

I t sho uld be borne in mind tha t corros ion produ cts may be soluble in a working fluid, an d hence both precipita-tion and particulate fouling would usually occur concurrently.

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PHENOMENOLOGICAL CONSIDERATIONS OF FOULING 869in which heat transfer fouling affects the exchanger mechanical integrity, and the corro-sion products add thermal resistance to heat flow from the hot fluid to the cold fluid. Ifcorrosion products are formed upstream of the exchanger and then deposited on the heattransfer surface, the fouling mechanism refers to particulate or precipitation fouling,depending on whether the corrosion products are insoluble or soluble at the bulk fluidconditions. T he interaction of corrosion an d other types of fouling is the major concernfor many industrial applications. Corrosion fouling is dependent on the selection ofexchanger surface material an d can be avoided with the right choice of materials (suchas expensive alloys) if the high cost is wa rranted. Corros ion fouling is prevalent in manyapplications where chemical reaction fouling takes place an d the protective oxide layer isnot formed on the surface. Corrosion fouling is of significant importance in the design ofthe boiler and condenser of a fossil fuel-fired power plan t. Th e impo rtant factors forcorrosion fouling are the chemical properties of the fluids and heat transfer surface,oxidizing potential and alkalinity, local temperature and heat flux magnitude, andmass flow rate of the working fluid. It should be noted tha t although growth of corrosioninfluenced deposit has a detrimental effect on heat transfer, this influence is less impor-tan t than fouling caused by particulate fouling of corrosion products formed elsewherewithin the system. For example, fouling on the water side of boilers may be caused bycorrosion products that originate in the condenser or feedtrain.Biological fouling or biofouling results from the deposition, attachm ent, an d growth ofmacro- or microorganisms to the heat transfer surface; it is generally a problem in waterstreams. In general, biological fouling can be divided into two main subtypes of fouling:microbial and macrobial. Microbial fouling is accumulation of microorganisms such asalgae, fungi, yeasts, bacteria, a nd molds, and macrobial fouling represents accumulationof macroorganisms such as clams, barnacles, mussels, and vegetation as found in sea-water or estuarine cooling water. M icrobial fouling precedes macrobial deposition as arule and may be considered of primary interest. Biological fouling is generally in the formof a biofilm or a slime layer on the surface th at is uneven, filamentous, and deformablebut difficult to remove. Although biological fouling could occur in suitable liquidstreams, it is generally associated with open recirculation or once-through systemswith cooling water. Since this fouling is associated with living organisms, they canexist primarily in the temperature range 0 to 90C (32 to 194F) and thrive in thetemperature range 20 to 50C (68 to 122F). Biological fouling may prom ote corrosionfouling under the slime layer. Transport of microbial nutrients, inorganic salts, andviable microorganisms from the hulk fluid to the heat transfer surface is accomplishedthrough m olecular diffusion or turbulent eddy transp ort , including organic adsorption atthe surface.Freezing or solid cation fouling is due t o freezing of a liquid o r som e of its constitu-ents, or deposition of solids on a subcooled heat transfer surface as a consequence ofliquid-solid or vapor-solid phase change in a gas stream. Form ation of ice on a heattransfer surface during chilled water production or cooling of moist air, deposits formedin phenol coolers, an d deposits formed during cooling of mixtures of substances such asparaffin a re som e examples of solidification fouling (Bott, 1981). This fouling mechanismoccurs at low temperatures, usually ambient and below depending on local pressureconditions. The main factors affecting solidification fouling are mass flow rate of theworking fluid, temperature and crystallization conditions, surface conditions, and con-centration of the solid precursor in the fluid.Combined fouling occurs in many applications, where more than one fouling mechan-ism is present and the fouling problem becomes very complex with their synergistic

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870 FOULING A N D CORROSIONeffects. Some combined fouling mechanisms found in industrial applications are(Panchal, 1999):

0 Particulate fouling combined with biofouling, crystallization, and chemical-0 Crystallization fouling combined with chemical-reaction fouling0 Con densa tion of organic/inorganic vapors combined with particulate fouling in gas0 Crystallization fouling of mixed salts

Combined fouling by asphaltene precipitation, pyrolysis, polymerization, and/orCorro sion fouling combined with biofouling, crystallization, or chem ical-reaction

reaction fouling

streams

inorganic deposition in crude oilfouling

Some examples of the interactive effects of corrosion and fouling are as follows(Panchal, 1999):0 Microfouling-induced corrosio n (M IC ) (sustained-pitting corrosio n)

Under-deposit corros ion in petroleum a nd black liquor processing (concentrationbuildup of corrosion-causing elements)0 Simultaneous corrosion a nd biofouling in cooling water applications

Fouling induced by corrosion productsIt is obvious that on e can no t talk ab ou t a single, unified theory to mo del the foulingprocess wherein n ot o nly the foreg oing six types of fouling mech anism s are identified, b utin 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 temperaturesand temperature differences, (3) physical and chemical properties of the fluid, (4) heattransfer surface properties, and (5) geometry of the fluid flow passage. The other impo r-tan t variables are concen tration of foulant o r precursor, impurities, heat transfer surfaceroughness, surface chemistry, fluid chemistry (pH level, oxygen concentration, etc.),pressure, and so on. For a given fluid-surface com bina tion , the two mos t imp orta ntdesign variables are the fluid velocity and heat transfer surface temperature. In general,higher flow velocities m ay cause less foulant deposition a nd /o r more p ronou nced depositerosion, but a t the same time m ay accelerate corrosio n of the surface by removing theheat transfer surface material. Higher surface temp eratures prom ote ch emical reaction,corrosion, crystal formation (with inverse solubility salts), and polymerization, butreduce biofouling and prevent freezing and precipitation of normal solubility salts.Consequently, it is frequently recommen ded th at the surface tempera ture be maintaine dlow.

13.2.2 Single-Phase Liquid-Side FoulingSingle-phase liquid-side fouling is mos t freque ntly caused by (1) precipitation of mineralsfrom the flowing liquid , (2) depo sition of var iou s particles, (3) biological fouling, an d (4)corrosion fouling. O ther fouling mechanisms are also present. M ore imp orta nt, though,

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PHENOMENOLOGICAL C ONSIDER ATIONS O F FOULING 871

TABLE 13.1 Influence of Operating Variables on Liquid-Side Fouling"OperatingVariable Precipitation Freezing Particulate Chemical Corrosion BiologicalTemperature t l 1 T1- t l Tl T l -1elocity l- T l

TlSupersaturation TT 1PHT- TConcentration T t t T T lRoughness T TPressure - cOxygen c c T T Itl- T1

T l-1-l-mpurities

-

Source: Data from Cannas (1986).When the value of an oper ating variable is increased, it increases ( 7 ) . decreases (J), or has no effect (++)on thespecific fouling mechanism listed. Dashes- ndicate that no influence of these variables has been reported in theliterature.

is the combined effect of more than one fouling mechanism present. The qualitativeeffects of some of the operating variables on these fouling mechanisms are shown inTable 13.1.The quantitative effect of fouling on heat transfer can be estimated by utilizing theconcept of fouling resistance and calculating the overall heat transfer coefficient underboth fouling and clean conditions (see Section 13.3). An additional param eter for deter-mining this influence, used frequently in prac tice, is the cleanlinessfactor. t is defined as aratio of an overall heat transfer coefficient determined for fouling conditions to thatdetermined for clean (fouling-free) operating conditions. The effect of fouling on thepressure drop can be determined by the reduced free-flow area due to fouling and thechange in the friction factor, if any, due t o fouling.13.2.3 Single-Phase Gas-Side FoulingGas-side fouling may be caused by precipitation (scaling), particulate deposition, co rro-sion, chemical reaction, a nd freezing. Form ation of hard scale from the gas flow occurs ifa sufficiently low temperature of the heat transfer surface forces salt compounds tosolidification. 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 parti-culate deposition is accumulation of plant residues. An excess of various chemical sub-stances, such as sulfur, vanadium , an d sodium , initiates various chemical reaction foulingproblems. F orma tion of frost and various cryo-deposits a re typical examples of freezingfouling on the gas side. An excellent overview of gas-side fouling of heat transfer surfacesis given by M arner (1990, 1996). Qualitative effects of some of the operatin g variables o ngas-side fouling mechanisms are presented in Table 13.2.

13.2.4 Fouling in Compact ExchangersSmall 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

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872 FOULING AND CORROSION

TABLE 13.2 Influence of Operating Variables on Gas-Side FoulingaOperatingVariable Particulate Freezing Chemical CorrosionTemperature Tl 1 t tl-T l - T -elocity T I + + 11mpurities - toncentration TFuel-air ratio TRoughness T*Oxygen c c

-Tt-tt t

--Sulfur -Source: Data from Cannas (1986).a When the value of an operating variable is increased, it increases T), decreases I), or has no effect (+-+)n thespecific fouling mechanism listed. D ashe s- ndicate tha t n o influence of these variables has been reported in theliterature.exchanger. This reduces fouling significantly. However, small channel size creates aproblem of plugging the passages. To avoid p luggin g, the particle size mu st be restrictedby filtering o r other m eans to less tha n on e-third the smallest open ing of heat exch angerpassages. Even with this guideline, p articulate fouling ca n occur an d agglomerate, suc has with waxy substances.

13.2.5 Sequential Events in FoulingFr om the em pirical evidence involving various fouling mechanisms discussed in S ection13.2.1, it is clear that virtually all these mechanisms are characterized by a similarsequence of ev ents. Th e successive events occu rring in mo st cases are th e following: (1)initiation, (2) transport, (3) attachm ent, (4) removal, and (5) aging, a s conceptualized byEpstein (1978). These events govern the overall fouling process an d determine its ultimateimpact on heat exchanger performance. In some cases, certain events dominate thefouling process, and they have a direct effect on the type of fouling to be sustained.Let us sum marize these events briefly (C ann as, 1986).Initiation of the fo uling, the first event in the fouling process, is preceded by a delayperiod or induction period Td as shown in Fig. 13.1. Th e basic mechanism involved duringthis period is heterogeneous n ucleation, and Td is shorter with a higher nucleation rate.The factors affecting Td are temperature, fluid velocity, composition of the foulingstream, and nature and condition of the heat exchanger surface. Low-energy surfaces(unwettable) exhibit longer induction periods tha n those of high-energy surfaces (wetta-ble). In cry stallization fouling, Td tend s to decrease with increasing degree of supe rsatu ra-tion. In chemical reaction fouling, Td appears to decrease with increasing surfacetemperature. I n all fouling mechanisms, Td decreases a s the surfac e roughness increasesdu e to available suitable sites for nucleation, adsorp tion, an d adhesion.Transport of species means transfer of a key component (such as oxygen), a crucialreac tant, o r the fouling species itself fro m the bulk of the fluid to th e hea t transfer surface.Tr an sp or t of species is the best un derstoo d of all sequential events. Tr an sp ort of speciestakes place through the action of on e or mo re of the following m echanisms:

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PHENOMENOLOGICAL CONSIDERATIONS OF FOULING 8730 Dzfusion: involves mass transfer of the fouling constituents from the flowing fluidtoward the heat transfer surface due to the concentra tion difference between thebulk of the fluid and the fluid adjacent to the surface.0 Electrophoresis: under the action of electric forces, fouling particles carrying anelectric charge may move toward or away from a charged surface depending on thepolarity of the surface and the particles. Deposition due to electrophoresis increaseswith decreasing electrical conductivity of the fluid, increasing fluid temperature,and increasing fluid velocity. It also depends on the pH of the solution. Surfaceforces such as London-van der Waals and electric double layer interaction forcesare usually responsible for electrophoretic effects.0 Thermophoresis: a phenomenon whereby a thermal force moves fine particles inthe direction of negative temperature gradient, from a hot zone to a cold zone.Thus, a high-temperature gradient near a hot wall will prevent particles fromdepositing, but the same absolute value of the gradient near a cold wall will pro-mote particle deposition. The thermophoretic effect is larger for gases than forliquids.0 Dzfusiophoresis: involves condensation of gaseous streams o nto a surface.0 Sedimentation: involves the deposition of particulate m atters such as rust particles,

clay, and dust on the surface due to the action of gravity. For sedimentation tooccur, the downward gravitational force must be greater than the upward dragforce. Sedimentation is important for large particles and low fluid velocities. It isfrequently observed in cooling tower waters a nd other industrial processes whererust and dust particles may act as catalysts an d/or enter complex reactions.0 Inertial impaction: a phenomenon whereby large particles can have sufficientinertia that they are unable to follow fluid streamlines and as a result, deposit onthe surface.0 Turbulent downsweeps: since the viscous sublayer in a turbulen t boundary layer is

not truly steady, the fluid is being transported toward the surface by turbulentdownsweeps. These may be thought of as suction areas of measurable strengthdistributed randomly all over the surface.Attachment of the fouling species to the surface involves bo th physical an d chem icalprocesses, and it is not well understood. Three interrelated factors play a crucial role inthe attachm ent process: surface conditions, surface forces, an d sticking probability. It isthe combined and simultaneous action of these factors tha t largely accounts for the eventof attachment.0 The properties of surface conditions impo rtant for attachment are the surface freeenergy, wettability (contact angle, spreadability), a nd heat of imm ersion. Wettabilityand heat of imm ersion increase as the difference between the surface free energy ofthe wall and the adjacent fluid layer increases. Unwettable o r low-energy surfaceshave longer induction periods than wettable or high-energy surfaces, and suffer lessfrom deposition (such as polymer and ceramic coatings). Surface roughnessincreases the effective con tact a rea of a surface and provides suitable sites for nuclea-tion and p romo tes initiation of fouling. Hence, roughness increases the wettability ofwettable surfaces and decreases the unwettability of the unwettable ones.0 There are several surface forces. The most important one is the London-van derWaals force, which describes the intermolecular attraction between nonpolar mole-

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cules and is always attractive. The electric double layer interaction force can beattractive o r repulsive. Viscous h ydrod ynam ic force influences the attachm ent of aparticle moving to the wall, which increases as it moves norm al t o the plain surface.Sticking probability represents the fraction of particles tha t reach the wall an d staythere before any reentrainment occurs. It is a useful statistical concept devised toanalyze an d explain the complicated event of attachm ent.

Removal of the fouling deposits from the surface may or may not occur simulta-neously with deposition. Removal occurs due to the single or simultaneous action ofthe following mechanisms: shear forces, turbulent bursts, re-solution, and erosion.0 Shea r forces result fro m the action of the shea r stress exerted by the flowing fluid o nthe depositing layer. As the fouling deposit builds up, th e cross-sectional are a forflow decreases, thus causing an increase in the average velocity of the fluid for aconstan t mass flow rate a nd increasing the sh ear stress. Fres h depo sits will formonly if the deposit bon d resistance is greater t ha n the prevailing she ar forces a t thesolid-fluid interface.0 Random ly distributed (abo ut less than 0.5% at any instant of time) periodic tur-bulent bursts act a s miniature tornado es lifting deposited material from the surface.By continuity, these fluid bursts ar e com pensa ted for by gentler fluid back sweeps,which promote deposition.0 Th e removal of the depo sits by re-solution is related directly to th e solubility of thematerial deposited. Since the fouling deposit is presumably insoluble at the time ofits form ation , dissolution will occur on ly if there is a cha nge in the prop erties of th edeposit, or in the flowing fluid, or in both, due to local changes in temperature,velocity, alkalinity, an d oth er o per ation al variables. F o r exam ple, sufficiently higho r low temp eratures could kill a biological deposit, thus weakening its attachme nt

to a surface an d causing sloughing o r re-solution. Th e removal of corrosion depos-its in power-ge nerating systems is do ne by re-solution a t low alkalinity. Re-solutionis associated with the removal of material in ionic or molecular form.0 Erosion is closely identified w ith the overall re mo val process. It is highly de pen den ton the shear stren gth of the foulant an d o n the steepness and length of the slopingheat exchan ger surfaces, if any . E rosion is associated with the removal of materialin particulate form. The removal mechanism becomes largely ineffective if thefouling layer is composed of well-crystallized pure material (strong formations);but it is very effective if it is composed of a large variety of salts each having

different cry stal pro perties.Aging of deposits begins with attachm ent on the heat transfer surface, an d refers toany ch anges the fouling material und ergoes as time elapses. Th e aging process includesboth physical and chemical transformations, such as further degradation to a morecarbonaceous material in organic fouling, and dehydration an d/o r crystal phase trans-form ations in inorganic fouling. A direct consequence of aging is change in the thermalconductivity of the deposits.+ Aging m ay strengthen o r weaken the fouling deposits.

' A comm on nonfouling example of aging is the transformation of fresh, soft, fluffy snow in an open field intohard, crystalline, yellowish ice after a week or so of exposure to the sun resulting differences in its materialproperties.

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PHENOMENOLOGICAL CONSIDERATIONS OF FOULING 87513.2.6 Modeling of a Fouling ProcessRegardless of the type of fouling process, the principal charac teristic featu re of any typeof fouling is that the net mass fouling rate (i.e., the change of the mass rn of foulantdeposited on the heat transfer surface for a given time, drnldr , is a consequence of a netdifference between the foulant deposit ra te md and the foulant reentrainment rate kr:

(13.6)In Eq . (13.6), s deno tes symbolically the spa tial dependence (say, x , y , and z of the massof foulant. Note that the mass m of the foulant deposited uniformly is given as a simpleequation:

rn = (13.7)where pf represents fo ulant mass density, A denotes heat transfer surface area coveredwith the foulant, and 6 is the thickness of the foulant layer. In general, all three terms ofEq. (13.6) are spatially nonuniform and dependent on time. Equation (13.6) can con-veniently be reformulated in terms of mass per unit heat transfer surface area,M A = m / A , an d for a uniform spatial distribution of deposit, it is

(13.8)Equation (13.8) is a direct consequence of Eq. (13.6) after idealizing a uniform distribu-tion of the fouling deposit over the surface A . Furth erm ore, mass per unit heat transfersurface (uniformly distributed along the heat transfer surface) can be written as

M A = pf sf = p f k f Rf (13.9)where Rf = 6 / k f , he fouling factor, represents fouling unit thermal resistance; it repre-sents the thermal resistance of the layer of foulant deposited for a unit area of heattransfer surface. C oncisely, we refer to this entity a sfouling resistance. Fro m the foulingfactor definition, we obtain 6 = k f R f .Consequently,

(13.10)In Eq. (13.10), it is assumed that both mass density and thermal conductivity of thedeposited layer are invariant with time. C ombining Eqs. (13.8) and (13.10), we obta in

(13.11)where kj= A?A,j /pfkf epresents deposition ( j = d ) and removal ( j = r ) fouling resis-tance rates.

To solve either Eq. (13.8) or (13.1 l) , one needs the explicit forms of eithe r mass ratesper unit heat transfer area (for both deposit or reentrainment process) or unit thermalresistances [the terms o n th e righ t-hand sides of Eqs. (13.8) an d (13.1 l)]. A number of

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876 F OULING AND CORROSION

models for determining these variables have been developed; some of them are summar-ized in Table 13.3. Let us consider the m odel of Tab orek et al. enlisted in th at table as anillustration.According to Tab orek et al. [as reported by Epstein (1978)], the depo sition an dremoval mass rates have the form

(13.12)

where cI and bl are constants, p is a deposition probability function related to thevelocity and adhesion properties of the deposit, R is the water quality factor, E isthe activation energy, R is the universal gas constant, T, is the absolute tem peratu re ofthe deposit a t the surface, T~den otes fluid shear stress at th e de pos it surface, 1c representsremoval resistance of the depo sit (scale strength fa ctor), m is the foulant mass, and i is anexpon ent. Equ ation (1 3.12) can be represented in terms of depo sition an d removal ther-ma l resistance ra tes of Eq . (1 3.1 1) in form as follows [as reported by Kn ud sen (1998)l:

In Eq. (13.13), c2 and b2 are constants. It should be noted that both sets of equations[Eqs. (13.12) an d (1 3.13)], are semiempirical, t o include the v ariable s th at govern fouling.Introducing the expressions for m ass per unit hea t transfer surface area f or both depositan d re entrain me nt processes (or their th erm al resistance) in to Eq . (13.8) or (13.1 l) , wecould integrate these governing equ ation s and subsequently determine either depositedmass or their thermal resistance. These solutions have to fit empirical evidence that canbe generalized as presented in Fig. 13.1.In Fig. 13.1, fou r characteris tic scenarios f or the gro wth of the fouling resistance a represented (Knudsen, 1998). In this figure, 7 d is the delay period for the o nset of foulingdep osits for non-negligible 4.

1 . Linear characteristics (i.e., Rf is linearly dependent on time) indicate that thedeposit ion rate is constant and there is no reentrainment rate (or at least theirdifference is invariant in time). A linear fouling behavior is generally associatedwith the crystallization of a well-formed dep osit consisting of a sub stantially puresalt that is largely uncontaminated by the presence of coprecipitated impurities.The strong bonds characterizing the structure of such deposits make removalmechanisms somewhat ineffective. If heat duty is kept constant linear foulingbehavior is also often observed for reaction fouling.

2. Falling rate fouling normally occurs in situations where the deposition rate isdecreasing b ut always greater th an th e removal rate . This type of fouling m echan-ism has been observed in crystallization fouling in a plate exchanger a nd also inparticulate fouling.3. T he curve characterizedAby as ym ptotic behavior reflects the situ ation representedby the expression for l i d of Eq. (13.13), which corresponds to fragile depositsexposed to shear stress of the flowing fluid. The asymptotic fouling growthmodel is often observed in coo ling wate r heat exchangers. I n these heat exchangers,the conditions leading to the for ma tion of a scale layer of a weak, less coherent

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sd Time 7FIGURE 13.1 Time dependence of the fouling resistance.

structure are associated with simultaneous crystallization of salts of differentcrystal shapes or with the presence o f suspended particles embedded in the crystal-line stru ctur e. Th e gro wth of such depos its is expected to cre ate intern al stresses inthe scale layer so th at the rem oval processes become progressively m ore effectivewith deposit thickness. Such considerations lead t o asym ptotic scale thickness, a twhich the deposition is balanced by the scale removal mechanism.4. Having a sawtooth pattern due to the aging process of the fouling deposits(decrease in strength and coherence) results in susceptibility to the removal pro-

cess; this is found in corrosion fouling of copper tub es by seaw ater and in desalina-tion evaporators.Example 13.1 A fluid stream, rich in inert particles, flows through a tubular heatexchanger. The deposits form inside the tube surface due to particulate fouling.Assume tha t after a prolonged p eriod of time, a n asymptotic value of thermal resistanceis reached at a level of Rf,r-,. Also consider th at fouling resistance reaches 63 % of itsasymptotic value in 194 hours . M odel this fouling process determining the relationshipbetween fouling resistance and time. Assume the validity of the model given by Eqs.(13.11) and (13.13). How many hours of operation would be needed for the foulingresistance to reach within 90% of the a sym pto tic fouling resistance?S O L U T I O NProblem Data: Fouling takes place in a tubular heat exchanger. An asym ptotic value offou ling resistance is Rf,T+,. It is know n t ha t 63% of this asym ptotic value is reached in194 hours.Determine: The fouling process model determining the relationship between foulingresistance an d time.

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PHENOMENOLOGICAL CONSIDERATIONS OF FOULING 879

Assumptions: The assumptions invoked by the model of Taborek et al. (see Table 13.3),as presented in Section 13.2.6, are valid. Th at includes the fact th at all parameters andvariables for the problem are invariant in time.Analysis: According to Eqs. (1 3.1 1) and (1 3.13), the m odel of fouling process is

k,-, - 2 p l0 exp (-&)- b2rs fd rwith an initial condition

R,=o at T = O (2)The initial condition defined by Eq. 2)deserves an additional co mm ent. In most foulingcases, fouling resistance is often noticed a fter a certain delay period (i.e., fo r 0 5 T 5 Td,where Td represents the delay period for the onset of fouling deposits or a buildup of thefouling resistance; see Fig. 13.1). This is attributed to simultaneous influence of bothinitial nucleation of the deposited material on the heat transfer surface an d its influenceon heat transfer reduction due to lower thermal conductivity of the foulant material.Consequently, Td does not represent the delay of an actua l fouling process, but it signifiesa delay in reduction of the heat transfer rate due to fouling. In our analysis we treat

A solution of the problem defined by Eqs. (1) an d (2) can easily be found by using anyof techniques for solving this linear, first-order ordinary differential equation. Let usintroduce the set of new parameters defined as follows:

Td = 0.

Substituting a and b from Eq. (3) into Eq. (l), we getdRf = b - aR,d r 4)

Integrating this linear first-order ordinary differential equation and simplifying, we get

The integration constant C n Eq. 5 ) can be determined by applying the initial conditionof Eq. (2) to Eq. 5 ) :

Substituting the constants a, b , and C, Eq. (5) results in

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880 FOULING A ND CORROSIONEquation (7) represents the time history of the fouling thermal resistance. Forlarge enough time T + m , an asymptotic value of thermal resistance, Rf,T3m isobtained:

Finally, Eq. (7) can be rearranged by using Eq. (8) in a more convenient form asfollows:

where T~ = /b27,kf . Equ ation (9) is commonly referred as the Kern-Seaton correlation.Note that the right-hand side of Eq. (9) becomes 0.63 for T = T ~ . his means that thefouling resistance reaches 63% of the a symptotic value for the time T equal to the timeconstant T ~ ,he value given in the problem formulation (i.e., T~ = 194 hours). Therefore,the number of hours of operation needed for the fouling thermal resistance to reach 90%of the asymptotic fouling resistance can be determined from Eq. (9) as follows:

0.9 = 1- e - r / 1 9 4 therefore, T = 447 h Ans.Discussion and Comments: In this example, we did not emphasize the influence ofphysical variables that are inherent in the original model [all the variables andconstants introduced in Eq. (13.12)] since the values of these variables and constantsare actually not known. Still, the model based on Eq. (9) describes quite well somefouling processes (e.g., particu late o r crystallization fouling) having an asym ptotic ther-mal resistance represented by a time constant. N ote tha t when T~ has a large value, E q. (9)reduces to

This equation represents the limiting value; here higher-order terms ar e neglected. Thus,in this case, the fouling resistance Rf depends linearly on T . Fo r all cases for which theparam eters of the differential equa tion given by Eq. 1 ) are constant (as implied by the setof assum ptions in this example), the fouling process assumes the known deposit, definedfluid quality, and fixed-flow conditions. It should be added that many other models offouling processes areav aila ble , as summ arized by Epstein (1978). Most of them providethe expressions for Rd and R, of Eq. (13.11). If these are known, the procedure forobtaining the time history of a fouling process will be similar to that demonstrated inthis example after utilizing the expressions specified fo r deposition an d removal foulingresistance rates an d integra ting the resulting differential equa tion. All these solutions, asa rule, can be treated at best as indicators of the fouling trend. How ever, the complexityof the process and involved nonlinearities (not included in the simple model discussed)prevent a reliable prediction.

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FOULING RESISTANCE DESIGN APPROACH 881

13.3 FOULING RESISTANCE DESIGN APPROACHVarious practices relevant for heat exchanger design have been used to describe theinfluence of fouling on the thermal performance of a heat exchanger. The earliest one(around 1910) was the use of two com bined coefficients, the cleanliness coefficient C andmaterial coefficient p , to correct the overall heat transfer coefficient U , defined for a cleanheat transfer surface. The resulting overall heat transfer coefficient for a fouled exchangerbecomes Uf = p C U , . The earliest values assigned to these coefficients, as reported bySomerscales (1990), were between 0.17 and 1.00 for the m aterial coefficient an d 0.5 and1.0 for the cleanliness coefficient. Soon it became evident that introduction of a singlecoefficient, the cleanliness factor CF, would be more appropriate. Hence, an overall heattransfer coefficient under fouling conditions was predicted by the simple relationU - C F x U,, here CF < 1.00. C F values between 0.8 and 0.9 may be consideredtypical in the power industry.Next we describe the modern practice of taking the influence of fouling into account.

f .-

13.3.1 Fouling Resistance and Overall Heat Transfer Coefficient CalculationAs we have introduced in Section 3.2.4, the overall thermal resistance for a heat exchan-ger involves a series of thermal resistances from the hot fluid to the cold fluid, includingthermal resistances due to fouling on both fluid sides, as shown in Fig. 3.4. If the overallheat transfer coefficient is based on the fluid 1-side heat transfer surface area A l , thefollowing relation holds in the absence of fins on both fluid sides:

(13.14)

In Eq. (13.14), it is assumed that the wall therm al resistance is for a flat plate wall [see Eq.(3.31) for a tubular surface]. Fo r a m ore general case, see Eq. (3.30), which includes thefin effects on both fluid sides. Equation (13.14) is further rearranged and simplified asA 6 A 1 A 1 * A l 1 A lfi l + x l + - - = - + R + R -+-- (13.15)1u1 h , +R/J f , 2 A2 k , A, h2 A2 h l W A , h2 A 2- = -

Note that RJ = Rf,, +Rf,2(Al/,42) represents the total fouling resistance, a sum offouling resistances on both sides of the heat transfer surface, as shown. I t should againbe reiterated tha t the aforementioned reduction in the overall heat transfer coefficient dueto fouling does not take into consideration the transient nature of the fouling process.According to Chenoweth (1990), use of the fouling resistance concept as represented byEqs.0

0

0

1 3.14) and (1 3.15) must be based on the following recommendations;Fouling resistances should reflect fouling alone and not uncertainties in the designof the heat exchanger.App ropriate values of fouling resistances should be based on operating experienceand modified by economic considerations where possible.The buyer/user, not the manufac turer, should be responsible for selecting the foul-ing resistances because he or she may know his or her own app lication better.

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882 FOULING A N D CORROSIONThe effects of corrosion fouling and biofouling due to their complexity and aquestionable predictability should always be controlled externally. That meansthat this control should be a system based o n reducing or preventing fouling.

Th e current practice is to assume a value for the fouling resistance on on e or b oth fluidsides as appropriate and to design a heat exchanger accordingly by providing extrasurface are a for fouling, togethe r w ith a clea ning strategy (see Section 13.4.3).The complexity in controlling a large number of internal and external factors of agiven process makes it very difficult to predict the fouling growth as a function of timeusing deterministic (well-known kinetic) m odels. A m ore realistic fo uling growth mo delcan be devised by postulating fouling as a time-dependent ra nd om processt an d analyz-ing using the probabilistic approaches (Zubair et al., 1997) in conjunction with thecleaning strategies as discussed in Section 13.4.3.A n ote of caution is warran ted a t this point. The re is a n ongoing discussion am on gscholars and engineers from industry as to whether either fouling resistance o r foulingrate concepts should be used as the most a pprop riate tool in resolving design problemsincurred by fouling. One suggestion in resolving this dilemma would be that the designfouling-resistance values used for sizing heat exchangers be based on fouling-rate dataan d estimated cleaning-time intervals (R ab as and P ancha l, 2000).

13.3.2 Impact of Fouling on Exchanger Heat Transfer PerformanceIn curren t practice, based o n application an d need, the influence of fouling on exchan gerheat transfer performance can be evaluated in terms of either (1) required increasedsurface area fo r the same q and A T , , ( 2 ) required increased me an tem pera ture differencefor the same q and A , or (3) reduced heat tranfer ra te for the same A and AT,.* Fo r theseapproaches, we now determine expressions for A f l A , , A T , , f / A T , , and q f / q r asfollows.5 In t he first two cases, the h eat transfer rate in a hea t exchan ger un der cleanan d fouled conditions ar e the same. Hence,

q = U,A, A T, = UA,f A T , for constant A T ,Therefore.

(13.16)

(13.17)Acco rding to Eq . (1 3.15), the relationship s between overa ll heat transfer coefficients(based o n tube outside surface are a) and thermal resistances for clean and fouled con-ditions are defined as follows. F o r a clean h eat transfer su rface,

(13.18)The randomness in fouling process is due to time-dependent scatter in fouling resistance from a replicate to areplicate (repe at tests).The first case is the design of an exchanger w here an allowance fo r fouling can be m ade a t the design stage byincreasing surface area. T he other two cases are for an already designed exchanger in op eration, an d the purpo se isto determine the impact of fouling on exchanger performance.$T hrou gho ut this chap ter, the subscript c denotes a clean surface an df the fouled surface.

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FOULING RESISTANCE DESIGN APPROACH 883

F o r a fouled heat transfer surface.

Note that we have idealized that h,,f = h,,,, hi,, = hi ,c ,A i , = Ai,c= A i , andA , J = A,,c = A , . Here A , represents the tube outside surface area and not free-flowarea in th e ex chan ger. Th e difference b etween Eqs. (1 3.18) an d (1 3.19) is

1 1Rf=--vf

(13.20)

It should be added that Eq. (13.20) is valid as long as clean overall heat transfer coeffi-cients are co nst an t. If this assum ption is no t satisfied, the right-h and side in Eq . (13.20)does not represent only the overall fouling resistance but a quantity th at includes o therinfluences on overall heat transfer coefficients in addition to fouling. In that case, thefouling assessment will be incorrect. Combining Eqs. (13.17) and (13.20), we getf = UCRf+ 1

AC(13.21)

Similarly, when q and A are the same and A T , is different for clean and fouledexchangers, we have

Hence.

Combining Eqs. (13.23) and (13.20), we get--T,, - UCRf+ 1ATm,c

(13.23)

1 3.24)

Finally, if one assumes that heat transfer area and mean temperature differences arefixed, heat transfer rates for the sam e heat exch anger under fouled a nd clean con ditionsare given by qf = U f AA T , and qc = U c AAT, , respectively. Co m bin ing these tw o rela-tionsh ips with Eq . (13.20), we get

Alternatively, Eq . (1 3.25) can be expressed a s

(13.25)

- _c - UCRf+ 1q/ (13.26)

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q lines--00I I I I I I Is6

-'-0.1

We find that the right-hand sides of Eqs. (13.21), (13.24), and (13.26) are the same.Equ ation s (13.21), (13.24), an d (13.25) are show n in Fig . 13.2 in term s of the percentageincrease in A and A T , and the percentage reduction in q for the fouled exchanger overthat for the clean exchanger. From this figure, it is clear that fouling has a significantimpact o n the exchanger perform ance for high values of Rf and/or U,.Th e cleanliness fac tor C F is related to the fouling resistance Rf as

Example 13.2 Overall heat transfer coefficient of a heat exchanger operating underclean conditions is calculated as 800 W/m2 . K . Following industrial experience, thecleanliness factor for this exchanger is 0.7. Determine the magnitude of the correspond-ing fouling resistance.SOLUTIONProblem Data: Th e following d at a are given: U , = 800 W/m2 K a nd C F = 0.7.Determine: Th e fouling resistance Rf of the deposit formed by this heat exchanger.Assumptions: T he convective heat tran sfer coefficients on the h ot- a nd cold-fluid sides arethe sam e for bo th fouled an d clean heat tran sfer surfaces. Therm al resistance of the wallis unchanged und er fouled co nditions. Th e change in heat transfer surface area s du e tofouling deposit for m atio n is negligible. T he fin efficiency is equal t o u nity. All idealiza-tions ad op ted fo r heat exchanger design theor y are valid (see Section 3.2.1).Analysis: The relationship between fouling resistances and overall heat transfer coeffi-cients for clean and fouled conditions is given by Eq. (13.20):

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FOULING RESISTANCE DESIGN APPROACH

1 1Rf =--v,fUsing the definition of C F from Uf = C F x U,, Eq. 1 ) reduces to

1 1 1 - C F- -=- -Rf =-C F x U , U , U , C FThus, substituting data given into Eq. (2), we get

Rf=m(T) 1 - 0.7 = 5 ~ 1 0 - ~ m ~ . K / W

Discussion and Comments: In some industries (such as power industry), use of the clean-liness factor has been prevalent for assessing the influence of fouling. Th e reason for thisis the practice of the industry and the difficulties associated with experimental determina-tion of fouling thermal resistances (Somerscales, 1990). Equation (2) can be used tocalculate fouling resistance or unit thermal resistance if the cleanliness factor is known(or vice versa) under the conditions governed by the above-mentioned assumptions.Example 23.3 Determine how much will change the required heat transfer area of anexchanger under fouling conditions if the fouling resistance changes from m2 . K / Wto m2 . K/W. The heat transfer rate and mean temperature difference remain thesame, and U , = 1000 W /m 2 K . Consider n o extended surface o n either fluid side of theexchanger.SOLUTIONProblem D ata: The following data are given:

U , = 1000W/ m2 . K Rf,l = m2 .K / W Rf,2 = m 2 . K / W4 c = 4f AT,, = ATm,f , I = %,2 = 1

Determine: The change in heat transfer surface area required if the fouling resistancechanges from m2 . K/W to m2 .K/W.Assumptions: The convection heat transfer coefficients ar e the same fo r fouled and cleanheat transfer surfaces. The thermal resistance of the wall is unchanged under fouledconditions. Change in heat transfer surface areas due to deposit form ation is negligible.All assumptions adop ted for heat exchanger design theory are valid (see Section 3.2.1).Analysis: The hea t transfer ra te and mean tem perature differences in this exchangerunder clean and fouled conditions are the same. Hence, from Eq. (13.21), we get

- _ - UCRf+ 1A ,From Eq . (l) , it follows that the change in total fouling resistance from Rf,l to Rf,2 causesa change in heat transfer area from A f , ] o A f , 2as follows:

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From Eq. ( 2 ) , inserting the da ta given, we get~ f . 2 U,.Rf.2+ 1 l o3W / m 2 . K x 1 0 - ~m 2 . K / W + 1-- - = 1.82 Ans.. I UcRf,, + 1 l o3W / m 2 . K x 10-4m2 . K / W + 1-

T h us an increase in the fou ling resistance by a fac tor of 10 requires a surface area increasefor this exch anger of 82%.Discussion and C ommen ts: This exam ple clearly show s a significant increase in the surfa cearea requirement for this exchanger when the total fouling resistance is increased by a nord er of mag nitude. Inversely, a significant reduction in surface area ca n be achieved (byab ou t one-half) if the total fouling resistance is reduced by a n ord er of ma gnitud e. Not etha t the result provides a direct informatio n o n how large percent change in heat transferarea would be compa red to tha t for a clean he at exchanger for a given fouling resistance,as shown in Fig. 13.2.

13.3.3 Empirical Data for Fouling ResistancesEmpirical da ta fo r fouling resistances have been obtaine d over m any decades by industrysince its first com pilation by T E M A in 1941 for she ll-and-tub e heat exch angers . Selectedda ta are summ arized in Table 9.4 and hence are no t repeated here. M any of the originalvalues of TEMA fouling factors or fouling resistances established in 1941 for a typicalexch ange r service length of three m on th s are still in use for a c urre nt typical service lengthof five years (Chenoweth, 1990) TEMA fouling resistances are supposed to be repre-sentative values, asymptotic values, o r those manifested just before cleaning to be per-forme d. Ch enow eth (1 990) analyzed the cu rren t practice of cu stome rs specifying foulingresistances on their specification sheets to manufacturers. He compiled the combinedshell- and tube-side fouling resistances (by summing each side entry) of over 700 shell-and-tub e heat exchangers an d divided them i nto nine com bination s of liquid, two-phase,an d gas o n each fluid side regardless of the applications. H e then simply too k th e arith-metic average of total Rf for each two-fluid com bination value an d plotted dimensionalvalues in his Fig. 1-3. His results a re presen ted in Fig. 13.3 after norm alizing with respectto the maxim um combined shell- an d tube-side R, of liquid-liquid app licatio ns so thatthe o rdinate ranges between 0 a n d 1. F o r gases on both sides, the relative fouling resis-tance can be as high a s 0.5 (the lowest value in Fig. 13.3) com pared to liquids o n bo thsides (relative total fouling resistance represented as 1.0 in Fig. 13.3). If liquid is on theshell side and gas on the tube side, the relative fouling resistance is 0.65. However, ifliquid is on the tube side and g as o n the shell side, it is 0.75. Since man y process industryapplications deal with liquids th at are d irtier tha n gases, the gen eral practice is to specifylarger fouling resistances fo r liquids com pare d t o those for the gases. Also, if fouling isanticipated o n the liquid side of a liquid-gas exch anger, it is generally placed in the tub esfor cleaning purpose s a nd a larg er fouling resistance is specified. These tren ds a re clearfrom Fig. 13.3. It sho uld again be em phasized tha t Fig. 13.3 indicates the current practicean d h as n o scientific basis. Specification o f larger fouling resistances for liquids (which

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FOULING RESISTANCE ESIGN APPROACH 887

Relative ,-resistance0.80.60.4-10.2.0Liquid-

Two-phaseShell sideTube side

FIGURE 13.3 Com bined tube- an d shell-side relative fouling resistances. (Based on d ata fromChenoweth, 1990.)have higher heat transfer coefficients tha n those of gases) has even more impact on thesurface area requiremen t fo r liquid-liquid exchangers than for gas-gas exchangers.It should be reiterated that the recommended fouling resistances are believed torepresent typical fouling resistances for design. Consequently, sound engineering judg-men t has to be made fo r each selection of fouling resistances, keeping in mind th at actu alvalues of fouling resistances in any application can be either higher or lower than theresistances calculated. F inally, it must be clear that fouling resistances, althou gh recom-mended following the empirical dat a an d a sound m odel, are still constan t, independentof time, while fouling is a transient phenomenon. Hence, the value of RJ selected repre-sents a correct value only at one specific time in the exchanger operation. As indicated byChenoweth (1990): . . the new proposed (constant, independent of time) values reflect acareful review and the application of good engineering judgment by a g rou p of knowl-edgeable engineers involved with the design an d o per atio n of shell-and-tube heat exchan-gers. . . . It needs to be emphasized tha t the tables may n ot provide the applicable valuesfor a particular design. They are only intended to provide guidance when values fromdirect experience are unavailable. With the use of finite fouling resistance, the overall Uvalue is reduced, resulting in a larger surface area requirement, larger flow area, andreduced flow velocity which inevitably results in increased fouling. Thus, allowing m oresurface area for fouling in a clean exchanger may accelerate fouling initially.

Typical fouling resistances are roughly 10 times lower in plate heat exchangers th an inshell-and-tube heat exchangers (Zubair and Shah, 2001). Some fouling resistances forPHEs are compared with those for shell-and-tube heat exchangers in Table 13.4.TE M A (1999) presents fouling resistances for some gases used in process a nd petro-chemical industries an d M arn er an d Suitor (1987) summarize the literature da ta f or gasesused in many industries, as reported in Table 13.5.Example 13.4 A heat exchanger w ith water-to -phase change fluid is designed keeping inmind that the fluid that changes its phase must be on the outside of the tube. Theempirical da ta available reveal th at an average heat transfer coefficienton the water sideis 2715 W/m2K . O n the tube outside, the heat transfer coefficient is 3200 W /m 2 . K .

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TABLE 13.4 Liquid-Side Fouling Resistances for PHEsvs. TEMA Values(m2 .K/kW)

Process Fluid PH Es R,-TEMASoft water 0.018 0.18-0.35Cooling tower water 0.044 0.18-0.35Seawater 0.026 0.18-0.35River water 0.044 0.35-0.53Lube oil 0.053 0.36Org anic solvents 0.018-0.053 0.36Steam (oil bearing) 0.009 0.18Source: Data from Panchal and Rabas (1999).

TABLE 13.5 Gas-Side Fouling Resistances R,(m2 . K/kW)"Weierman Zink TEM A Rogalski Henslee and

(1982) (1981) (1978) (1979) Bouge (1983)Clean gasNatural gasPropaneButaneGa s turbineAverage gasNo. 2 oil

Dirty gasNo. 6 oilCrude oilResidual oilCoalMiscellaneousSodium-bearing wasteMetallic oxidesFC C U catalyst fines

0.0881-0.5280.176-0.5280.176-0.528

0.1760.352-0.704

0.2640.528

0.528-1.230.7062.640.881-3.520.88 1-8.8 1

-

0.176

-0.528

0.8811.76--

5.281.761.41

Source: Data from Marner and Suitor (1987).R. C. W eierman (1982), JPL Publ. 82-67, Jet Propulsion Labo ratory, C alifornia Institure of Technology,Pasadena, CA.; Jon Zink Co., Tulsa, OK (1981); R. D. Rogalski (1979), SA E Trans., Vol. 88, pp, 2223-2239;S. P. Henslee and J. L. Bouge (1983), Rep ort EGG-FM-6189, Idaho Nation al La boratory , Idaho Falls, ID.

Tubes are made of steel with thermal conductivity of 40W /m . K . The tube ou ts idediameter is 19 m m with 1.6 m m wall thickness. Th e asymp totic value of the foulingresistance o n the w ater side is 4 x m 2 .K/W . There is no fouling o n the tu be outside.Based on past experience, the fouling phenomenon is of asymptotic nature for thisexchanger, and the time constant for the fouling process is 280 hours for the Kern-Seaton model. Determine percentage unit thermal resistance distribution contributingto the overall unit thermal resistance for the following two cases: (a) after 280 hours offouling initiation, an d (b ) fo r the asym ptotic fouling cond ition.

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FOULING RESISTANCE DESIGN APPROACH 889SOLUTIONProblem D ata: The following da ta are provided:

hi = 271 5W /m 2 . K h, = 3200W/m2. K k , = 4 0 W / m . K do= 19mm6, = 1.6mm Rf,i = 4 x 1 0 - ~ 2 .K/W for T + 00 T~ = 280 h

Determine: The distribution of thermal resistances for two cases: (1) T + 00, and (2)T = T ~ , here T~ is the time constant for asymptotic fouling.Assumptions: The set of assumptions introduced for heat exchanger analysis (see Section3.1.2) and the Kern-Seaton model (see Section 13.2.5) is valid; the tube wall is thin.Analysis: Th e overall unit thermal resistance for the exchanger, from Eq. (13.14), is

Let us first calculate the missing information (area ratios, R, and Rf,i) for this equation:di = do - 2 6 , = 19mm - 2 x 1.6mm = 15.8mm

19mm = 1.092, ird,L 19mm A ird, LA i rdi 15.8mm A , .[(do + di)/2]L - (19+ 15.8)/2] mm-=-=-- - 1.203 M8 = = 1.6 x m = 4 x 1 0 - ~m2K / Wk , 40W/mK

Since the asymptotic fouling resistance is given, the actual fouling resistance on the waterside for T = T~ = 280 hours can be determined from Eq. (9) of Example 13.1 as follows:

Now the individual unit thermal resistances of the last equality of Eq. (l), in absolutevalues and in percentages, are computed and summarized for this problem for T = 00and T = 7 :

1 - A R . aA OverallR C . 0 = o hi A j 1IUo, Rf,i e.1 -A, AiFouling Time (m2 .K/W) (m2 .K/W) (m2 .K/W) (m2 K/W) (m2 .K/W)

r - c c 3.125 x 4.368 x 4.810 x 4.429 x 12.801 x7 = re 3.125 x 4.368 x 3.042 x 4.429 x 11.033 x

Percentagesr - + m 24 3 38 35 1007 = re 28 4 28 40 100

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890 FOULING AND CORROSIONDiscussion and Comments: Fr om the results of this problem, it is obv ious that fouling hasquite a significant influence on the total unit thermal resistance l /U o . If cleaning is no tgoing to be performed, the fouling resistance will ultimately reach 38% of the totalresistance, more than any other contribution. Note that the thermal resistance of thetube wall is an order of mag nitude smaller, an d hence ou r appro ximatio n of consideringthe thin wall of the tube for RK, etermination is reasonable. Distribution of thermalresistances would be different if the foula nt depo sition is allowed t o con tinu e only up t othe deposition time that equals the time constant. In that case, the fouling thermalresistance would be smaller compared to both the outside and inside tube convectiveresistances. Still, it would have the same order of magnitude.

13.4 PREVENTION AND MITIGATION OF FOULINGIdeally, a heat exchanger should be designed to minimize or eliminate fouling. Forexample, heavy-fouling liquids can be handled in a direct contact heat exchanger sinceheat an d mass transfer takes place d ue to direct c ontact of the fluids over the fill or thesurface in such an energy exchanger. The f i l l can get fouled without affecting energytransfer between the fluids in direct contact. In fluidized-bed heat exchangers, the bedmo tion scours away the fouling deposit. Gasketed plate-and-frame heat exchang ers ca neasily be disassembled for cleaning. Co mp ac t heat exchangers are n ot suitable for foulingservice unless chemical clean ing or therm al bak ing is possible. Wh en design ing a shell-and-tube heat exchanger, the following considerations are important in reducing orcleaning fouling. Th e heavy fouling fluid sh ould be kep t on the tube side for cleanability.Horizo ntal heat exch angers are easier t o clean tha n vertical ones. Geo metric features onthe shell side should be such as to minimize or eliminate stagnant and low-velocityregions. It is easier to clean square or rotated square tube layouts mechanically on theshell side [with a minimu m cleaning lane of t in . (6.35 mm)] than to clean o the r types oftube layouts.Some control methods are now summarized for specific types of fouling.Crystallization fouling can be co ntrolled o r prevented by preheating the stream so tha tcrystallization does no t occur. To con trol particulate fouling, use a filter o r similar deviceto capture all particles g reater tha n ab ou t 25% of the smallest ga p size in the flow pat h.Eliminate any dead zones and low-velocity zones. Use back flushing, puffing, orchemical cleaning, depending on the application. Chemical cleaning is probably themost effective cleaning method for chemical reaction fouling. For corrosion fouling,initial selection of corrosion-resistant material is the best remedy. For example, useproper aluminum alloy to prevent mercury corrosion in a plate-fin exchanger.Biofouling is usually easy to control with biocides, but must check compatibility withthe exchanger construction materials. Chlorination aided by flow-induced removal ofdisintegrated biofilm is the most common mitigation technique.General techniques to prevent or control fouling on the liquid or g as side are sum -marized briefly.13.4.1 Prevention and Control of Liquid-Side FoulingAm ong the m ost frequently used techniques for control o f liquid-side fouling is the on lineutilization of chemical inhibitors/ad ditives. Th e list of add itives includes 1 ) dispersantsto maintain particles in suspension; (2) various com pounds to prevent polymerization

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PREVENTION AN D MITAGATION OF FOULING 891

and chemical reactions; (3) corrosion inhibitors or passivators to minimize corrosion;(4) chlorine an d other biocide/germicides to prevent biofouling; and (5) softeners, poly-carboxylic acid and polyphosphates, to prevent crystal growth. Alkalis dissolve salts.Finally, filtration can be used a s an efficient me thod of mechanical removal of particles.An extensive review of fouling control measures is provided by Knudsen (1998).Mitigation of water fouling and the most recent review of the related issues arediscussed extensively by P anchal an d Knudsen (1998), where they suggest the followingmethods.0 Chemical additives: dispersants or coagu lators fo r particulate fouling; dispersants,crystal modifiers, and chelating agents for crystallization fouling; inhibitors orsurface filming for corrosion fouling; and biocides, biodispersants, and biostatsfor biofouling.0 Process adjustments: monitoring, modifications and replacements of devices, waterflow reduction, and recirculation strategies.0 Physical devices f o r cleaning: sponge-ball cleaning and the use of reversing-flowshuttle brushes.0 Utilization of enhanced heat transfer surfaces and devices: It has sizable influence onfouling mitigation. T he use of tube inserts (in particu lar, in refinery processes), such

as wire mesh, oscillating wires, and rotating wires, is a standard method.0 Various alternative devices and/or methods: magnetic fields, radio-frequency,ultraviolet and acoustic radiation, and electric pulsation. Surface treatment andfluidized-bed designs are also used.0 The most frequently used technique for preventing water-side fouling is still theconventional w ater treatment. Strict guidelines have been developed for th e qualityof water for environmental concerns (Knudsen, 1998).Heat transfe r surface mitigation techniques can be applied either on- o r offline. Onlinetechniques (usually used for tube-side applications) include various mechanical techni-ques (flow-driven or power-driven rotating brushes, scrapers, drills, acoustic/mechanicalvibration , air o r steam lancing on the outside of tubes, chemical feeds, flow reversal, etc.).In some applications, flows are diverted in a bypass exchanger, and then the fouledexchanger is cleaned offline. Other offline techniques (w ithout open ing a heat exchanger)include chemical cleaning, mechanical cleaning by circulating particulate slurry, andthermal baking to melt frost/ice deposits. Offline cleaning with a hea t exchanger openedor removed from the site include (1) high-pressure steam or water spray for a shell-and-tube heat exchanger, and (2) baking compact heat exchanger modules in an oven (to bur nthe deposits) and then rinsing. If fouling is severe, a combination of methods is required.

13.4.2 Prevention and Reduction of Gas-Side FoulingThe s tan dar d techniques for control a nd /o r prevention of fouling on the gas side are (1)techniques for removal of potential residues from the gas, (2) additives for the gas-sidefluid, (3) surface cleaning techniques, and 4) adjusting design up front to minimizefouling. Details regarding various techniques fo r gas-side fouling prevention , mitigation,and accommodation are given by M arner and Suitor (1987).Control of gas (or liquid)-side fouling should be attempted before any cleaningmethod is tried. The fouling control procedure should be preceded by (1) verification

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892 FOULING A ND CORROSION

of the existence of fouling, (2) identification of the feature that dominates the foulantaccumulation, an d (3) characterization of the dep osit.Som e of the me thods for mitigation of gas-side fouling are as follows:0 Crystallization fou ling ca n be prevented if the surface temp erature is kept abov e thefreezing of vap ors fro m th e gaseou s stream ; the solidification can b e minimized bykeeping a "high" velocity of freezable species, hav ing som e imp uritie s in the gasstream, and decreasing the foulant concentration, if possible.

Particulate fou ling can be minimized (1) by increasing the velocity o f the gas stre amif it flows parallel t o th e surf ace an d decreasing the velocity if the g as flow impingeson the surface, (2) by increasing the outlet tem perature of the exhaust gases fromthe exchanger above the melting point of the particulates, (3) by minimizing thelead content in gasoline or unburned hydrocarbons in diesel fuel, (4) by reducingthe fuel-air rat io fo r a given com bu stio n efficiency, an d (5) by minimizing flowimpact (e.g., flow over a staggered tub e bank) o r ensuring the narrowest dimensionin the flow cross section, to three t o f our times th e largest particle size anticipated .Chemical reaction fouling can be minimized (1) by maintaining the right tempera-ture rang e in the exha ust gas within the ex chang er, (2) by increasing o r decreasingthe velocity of the gaseous stream , depending on the application , (3) by reducingthe oxygen conce ntration in the gaseous stream, (4) by repla cing the coal w ith fueloil and natural gas (in th at ord er), and (5) by decreasing the fuel-air ratio.

0 Corrosion fou ling is strongly depen dent o n the tem perature of the exhaust stream inthe exchanger. Th e outlet tem perature of the exhaust gas stream fr om the exchan-ger should be maintain ed in a very n arrow range: ab ove the acid dew point [above150C (300"F)I for sulfuric or hydrochloric acid condensation or below 200C(400F) for attack by sulfur, chlorine, and hydrogen in the exhaust gas stream.Since sulfur is present in all fossil fuels and som e natura l gas, the d ew po int o f sulfurmust be avoided in the exchanger, which is depe nden t on the sulfur conten t in thefuel (Shah, 1985). From the electrochemical condition of the metal surface,the corrosion rate increases with velocity up to a maximum value for an activesurface and no sizable effect for a passive surface. Th e p H value h as a considerablerole in the corrosion fouling rate; the corrosion ra te is minimum a t a p H o f 1 1 to 12for steel surfaces. Low oxygen con cen tratio ns in the flue gases prom ote the fire-sidecorrosion of mild steel tubes in coal-fired boilers. Stainless steel, glass, plastic, andsilicon are highly resistant to low-temperature corrosion [T,,, < 260C (SOOOF)],stainless steel and superalloys to medium-temperature corrosion [260C(5OOOF) < T,,, < 815C (1500F)], and superalloys and ceramic materials tohigh-temperature corrosion [T,,, > 815C (1500"F)]. Ch rom e alloys a re suitablefor high-temperature sulfur and chlorine corrosion, and molybdenum and chromealloys protect against hy drogen corro sion.

13.4.3 Cleaning StrategiesAn impo rtant element in mitigating a fou ling problem is selection of a cleaning strategy(i.e., the cleaning-cycle period). T he cleaning-cycle period is delineated by th e o pera tionof an exchanger until the performance reaches the minimum value acceptable.Subsequently, the exchanger must be cleaned by one of the methods summarized inSection 13.4.1 or 13.4.2. In the case of asymptotic fouling in a given application, gen-

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894 F OULING A N D CORROSION

Environment factors- Impurities- Tempea ture and concentration- Degree of aerationof corrodent

Stress corrosion ~ - P Hnvironment Flow velocitycracking

Material factors- Composition-Alloying elements- Heat treatment- Microstructure- Surface conditions- Possivity

techniques - Effects of fabricationhe joints- Crevices- u-oenas

-Tendency for foulingF I G U R E 13.4 Facto rs influencing corrosion. (From Kuppan, 2000.)described in Section 13.2.1, thus adding thermal resistance in the heat flow path andreducing h eat transfer, increasing fluid pressure d ro p and pu mp ing pow er, an d increasingcost due to overdesign of the exchanger. The loss of material d ue to co rrosion m ay resultin crevices, holes, and/or partial removal of heat transfer surfaces, resulting in loss(leakage) of hea t trans fer fluids, some of which m ay be costly. If the fluid leaks outside,it may har m the environm ent if the fluid is corrosiv e or poisonou s. If it leaks to the oth erfluid side, it may conta mina te the o ther fluid a nd deteriorate its quality. Corro sion mayadd extra cost to the exchanger, due to the use of expensive material, maintenance,warranty, inventory of parts, and so on. Corrosion products carried downstream ofthe exchanger may corrode downstream components. Finally, corrosion may result incomplete failure of an exchanger o r partial failure in a tube-fin exchanger, du e to corro d-ing away fins, as in an autom otive radia tor.There is no a single cause of corrosion and/or associated corrosion mechanisms.However, corrosion in general has clear electrochemical roots. Namely, different partsof a heat exchanger exposed to working fluids easily become polarized. The role of anelectrolyte is usually taken by a working fluid (or sometimes by solid deposits or thickmetal oxide scale) in the vicinity of or between parts made of different metals. If anexternal electrical circuit is established, metal surfaces involved take the role of eitheranode or cathode. Appearance of an electric current forces electrical particles (say,positively charged metal ions) to leave the metal on the anode end and enter the sur-rounding electrolyte. On the other end, a metal surface that plays the role of cathodeserves as a site where electrical current escapes from the electrolyte. The presence of thismechanism opens the way for metal dissolution at the anode end of the establishedelectrical circuit. This dissolution can be interpreted as a corrosion effect (if all othermechanisms are suppressed). This very simplified picture provides a background formany corrosion problems. Refer to Ku pp an (2000) for fu rther details.Detailed study of corrosio n phenom ena is beyond the scope of this book. Du e to theimpo rtance of corrosion in hea t exchanger design an d op eration, we will address only themost important topics. A brief description of the main corrosion types is given first(keeping in mind their importance from a heat exchanger design point of view).

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CORROSION IN HEAT EXCHANGERS 895

Subsequently, corrosion mechanisms are addressed, followed by a brief discussion ofeach mechanism. Possible locations of corrosion in a heat exchanger are emphasized aswell. Finally, the most important guidelines for corrosion prevention are enlisted.13.5.1 Corrosion TypesCorrosion types, im por tan t for heat exchanger design an d op era tion , are as follows: 1 )uniform attack corrosion, ( 2 ) galvanic corrosion, (3) pitting corrosion, 4) stress corro-sion cracking, (5) erosion corrosion, (6) deposit corrosion, and (7) selective leaching, a scategorized by F on tan a and G reene (1978). Let us define each corrosion type briefly.Uniform corrosion is a form of corrosion caused by a chemical or electrochemicalreaction between the metal a nd the fluid in contact w ith it over the entire exposed metalsurface. It occurs when the metal an d fluid (e.g., water, acid, alkali) system an d operatingvariables are reasonably homogeneous. It is usually easy to notice corroded areasattacked by uniform corrosion. All other forms of corrosion mechanisms discussedbelow cause localized corrosion.Galvanic corrosion is caused by an electric potential difference between two electricallydissimilar metals in the syste

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