The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 36
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Depending upon the fluids, operating conditions, and heat exchanger construction,the maximum fouling layer thickness on the heat transfer surface may result in a few hours to a numberof years.Fouling could be very costly depending upon the nature of fouling and the applications. It increasescapital costs: (1) oversurfacing heat exchanger, (2) provisions for cleaning, and (3) use of special materialsand constructions/surface features. It increases maintenance costs: (1) cleaning techniques, (2) chemicaladditives, and (3) troubleshooting. It may cause a loss of production: (1) reduced capacity and (2)shutdown.
It increases energy losses: (1) reduced heat transfer, (2) increased pressure drop, and (3)dumping dirty streams. Fouling promotes corrosion, severe plugging, and eventual failure of uncleaned© 1999 by CRC Press LLC4-156Section 4heat exchangers. In a fossil-fired exhaust environment, gas-side fouling produces a potential fire hazardin heat exchangers.The following are the major fouling mechanisms:• Crystallization or precipitation fouling results from the deposition/formation of crystals of dissolved substances from the liquid onto heat transfer surface due to solubility changes withtemperature beyond the saturation point.
If the deposited layer is hard and tenacious, it is oftenreferred to as scaling. If it is porous and mushy, it is called sludge.• Particulate fouling results from the accumulation of finely divided substances suspended in thefluid stream onto heat transfer surface. If the settling occurs as a result of gravity, it is referredto as sedimentation fouling.• Chemical reaction fouling is defined as the deposition of material produced by chemical reaction(between reactants contained in the fluid stream) in which the heat transfer surface material doesnot participate.• Corrosion fouling results from corrosion of the heat transfer surface that produces products foulingthe surface and/or roughens the surface, promoting attachment of other foulants.• Biological fouling results from the deposition, attachment, and growth of biological organismsfrom liquid onto a heat transfer surface.
Fouling due to microorganisms refers to microbial foulingand fouling due to macroorganisms refers to macrobial fouling.• Freezing fouling results from the freezing of a single-component liquid or higher-melting-pointconstituents of a multicomponent liquid onto a subcooled heat transfer surface.Biological fouling occurs only with liquids since there are no nutrients in gases. Also crystallizationfouling is not too common with gases since most gases contain few dissolved salts (mainly in mists)and even fewer inverse-solubility salts. All other types of fouling occur in both liquid and gas.
Morethan one mechanism is usually present in many fouling situations, often with synergetic results. Liquidside fouling generally occurs on the exchanger side where the liquid is being heated, and gas-side foulingoccurs where the gas is being cooled; however, reverse examples can be found.Importance of Fouling.Fouling in liquids and two-phase flows has a significant detrimental effect on heat transfer with someincrease in pressure drop. In contrast, fouling in gases reduces heat transfer somewhat (5 to 10% ingeneral) in compact heat exchangers, but increases pressure drop significantly up to several hundredpercent. For example, consider U = 1400 W/m2K as in a process plant liquid-to-liquid heat exchanger.Hence, R = 1/U = 0.00072 m2K/W.
If the fouling factors (rs,h + rs,c) together amount to 0.00036(considering a typical TEMA value of the fouling factor as 0.00018), 50% of the heat transfer arearequirement A for given q is chargeable to fouling. However, for gas flows on both sides of an exchanger,U » 280 W/m2K, and the same fouling factor of 0.00036 would represent only about 10% of the totalsurface area. Thus, one can see a significant impact on the heat transfer surface area requirement dueto fouling in heat exchangers having high U values (such as having liquids or phase-change flows).Considering the core frictional pressure drop (Equation (4.5.32) as the main pressure drop component,the ratio of pressure drops of fouled and cleaned exchangers is given by2DpFf æ D öæu öf æD ö= F ç h ,C ÷ ç m , F ÷ = F ç h ,C ÷DpCfC è Dh,F ø è um,C øfC è Dh,F ø5(4.5.82)where the term after the second equality sign is for a circular tube and the mass flow rates under fouledand clean conditions remain the same. Generally, fF > fC due to the fouled surface being rough.
Thus,although the effect of fouling on the pressure drop is usually neglected, it can be significant, particularlyfor compact heat exchangers with gas flows. If we consider fF = fC , and the reduction in the tube inside© 1999 by CRC Press LLC4-157Heat and Mass Transferdiameter due to fouling by only 10 and 20%, the resultant pressure drop increase will be 69 and 205%,respectively, according to Equation (4.5.82) regardless of whether the fluid is liquid or gas!Accounting of Fouling in Heat Exchangers.Fouling is an extremely complex phenomenon characterized by a combined heat, mass, and momentumtransfer under transient condition.
Fouling is affected by a large number of variables related to heatexchanger surfaces, operating conditions, and fluids. Fouling is time dependent, zero at t = 0; after theinduction or delay period td, the fouling resistance is either pseudolinear, falling rate, or asymptotic.Fouling is characterized by all or some of the following sequential events: initiation, transport,attachment, removal, and aging (Epstein, 1983). Research efforts are concentrated on quantifying theseevents by semitheoretical models (Epstein, 1978) with very limited success on specific fouling situations.Hence, the current heat exchanger design approach is to use a constant (supposedly an asymptotic) valueof the fouling factor rs = 1/hs.
Equation (4.5.6) presented earlier includes the fouling resistances on thehot and cold sides for a nontubular extended-surface exchanger. Here 1/hs = rs is generally referred toas the fouling factor. Fouling factors for some common fluids are presented in Tables 4.5.8 and 4.5.9.The specification of fouling effects in a process heat exchanger is usually represented in the followingform, wherein the combined fouling factor rs,t is the sum of the fouling factors on the hot and cold sides:11UC U FCombined fouling factorrs,t =Cleanliness factorCF = U F UC( 4.5.84)Percentage oversurfaceæAö%OS = ç F - 1÷ 100è ACø( 4.5.85)( 4.5.83)Here the subscripts F and C denote fouled and clean exchanger values.
From Equation (4.5.6) withAh = Ac = A, ho = 1, DTm,F = DTm,C, it can be shown thatAF UC== 1 + UC rs,tAC U F(4.5.86)where rs,t = rs,h + rs,c. In heat exchanger design, constant (supposedly an asymptotoic) values of rs,h andrs,c are used. Accordingly, extra heat transfer surface area is provided to take into account the deleteriouseffect of fouling. Thus, the heat exchanger will be “oversized” for the initial clean condition, “correctlysized” for asymptotic fouling (if it occurs in practice), and “undersized” just before the cleaning operationfor nonasymptotic fouling.Influence of Operating and Design Variables.Based on operational experience and research over the last several decades, many variables have beenidentified that have a significant influence on fouling.
The most important variables are summarized next.Flow velocity. Flow velocity is one of the most important variables affecting fouling. Higher velocitiesincrease fluid shear stress at the fouling deposit–fluid interface and increase the heat transfer coefficient;but, at the same time, increased pressure drop and fluid pumping power may erode the surface and mayaccelerate the corrosion of the surface by removing the protective oxide layer. The fouling buildup ingeneral is inversely proportional to u1m.5 .
For water, the velocity should be kept above 2 m/sec to suppressfouling, and the absolute minimum should be above 1 m/sec to minimize fouling.Surface temperature. Higher surface temperatures promote chemical reaction, corrosion, crystal formation (with inverse solubility salts), and polymerization, but reduce biofouling for temperatures abovethe optimum growth, avoid potential freezing fouling, and avoid precipitation of normal-solubility salts.© 1999 by CRC Press LLC4-158TABLE 4.5.8Section 4Fouling Factors for Various Fluid Streams Used in Heat ExchangersWater TypeSeawater (43°C maximum outlet)Brackish water (43°C maximum outlet)Treated cooling tower water (49°C maximum outlet)Artificial spray pond (49°C maximum outlet)Closed-loop treated waterRiver waterEngine jacket waterDistilled water or closed-cycle condensateTreated boiler feedwaterBoiler blowdown waterFouling Factors (m2 · K)/W0.000275 to 0.000350.00035 to 0.000530.000175 to 0.000350.000175 to 0.000350.0001750.00035 to 0.000530.0001750.00009 to 0.0001750.000090.00035 to 0.00053LiquidsNo.
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