The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 37
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2 fuel oilNo. 6 fuel oilTransformer oilEngine lube oilRefrigerantsHydraulic fluidIndustrial organic HT fluidsAmmoniaAmmonia (oil bearing)Methanol solutionsEthanol solutionsEthylene glycol solutionsMEA and DEA solutionsDEG and TEG solutionsStable side draw and bottom productsCaustic solutions0.000350.00090.0001750.0001750.0001750.0001750.000175 to 0.000350.0001750.000530.000350.000350.000350.000350.000350.000175 to 0.000350.00035Gas or VaporSteam (non-oil-bearint)Exhaust steam (oil-bearing)Refrigerant (oil-bearing)Compressed airAmmoniaCarbon dioxideCoal flue gasNatural gas flue gasAcid gasSolvent vaporStable overhead products0.00090.00026 to 0.000350.000350.0001750.0001750.000350.001750.000900.00035 to 0.000530.0001750.000175Natural Gas and Petroleum StreamsNatural gasOverhead productsLean oilRich oilNatural gasoline and liquefied petroleum gases0.0001750.0001750.000350.0001750.000175Oil Refinery StreamsCrude and vacuum unit gases and vaporsAtmospheric tower overhead vaporsLight naphthasVacuum overhead vapors© 1999 by CRC Press LLC0.000170.000170.00035to 0.00035to 0.00035to 0.00035to 0.000354-159Heat and Mass TransferTABLE 4.5.8(continued)Fouling Factors for Various Fluid Streams Used in Heat ExchangersOil Refinery StreamsCrude and vacuum liquidsGasolineNaphtha and light distillatesKeroseneLight gas oilHeavy gas oilHeavy fuel oilVacuum tower bottomsAtmospheric tower bottomsCracking and coking unit streamsOverhead vaporsLight cycle oilHeavy cycle oilLight coker gas oilHeavy coker gas oilBottoms slurry oil (1.5 m/sec minimum)Light liquid productsCatalytic reforming, hydrocracking, andhydrodesulfurization streamsReformer chargeReformer effluentHydrocharger charge and effluentaRecycle gasLiquid product over 50°C (API)bLiquid product 30 to 50°C (API)bLight ends processing streamsOverhead vapors and gasesLiquid productsAbsorption oilsAlkylation trace acid streamsReboiler streams0.000350.000350.000350.000350.000530.000530.001760.001230.000350.000350.000530.000530.000700.000530.00035tototototo0.000530.000530.000530.00090.00123totototo0.000530.00070.00070.00090.000260.000260.000350.0001750.0001750.000350.0001750.0001750.00035 to 0.000530.000350.00035 to 0.00053aDepending on charge characteristics and storage history, charge fouling resistance may be many times thisvalue.b American Petroleum Institute.Source: Chenoweth, J., Final Report, HTRI/TEMA Joint Committee to Review the Fouling Section of TEMAStandards, HTRI, Alhambra, CA, 1988.
With permission.It is highly recommended that the surface temperature be maintained below the reaction temperature; itshould be kept below 60°C for cooling tower water.Tube material. The selection of the tube material is important from the corrosion point of view whichin turn could increase crystallization and biological fouling. Copper alloys can reduce certain biofouling,but their use is limited by environmental concerns with river, ocean, and lake waters.There are many other variables that affect fouling. It is beyond the scope here, but the reader mayrefer to TEMA (1988).Fouling Control and Cleaning Techniques.Control of fouling should be attempted first before any cleaning method is attempted. For gas-sidefouling, one should verify that fouling exists, identify the sequential event that dominates the foulantaccumulation, and characterize the deposit.
For liquid-side fouling, fouling inhibitors/additives shouldbe employed while the exchanger is in operation; for example, use antidispersant polymers to preventsedimentation fouling, “stabilizing” compounds to prevent polymerization and chemical reaction fouling,corrosion inhibitors to prevent corrosion fouling, biocide/germicide to prevent biofouling, softeners,acids, and polyphosphates to prevent crystallization fouling.© 1999 by CRC Press LLC4-160Section 4TABLE 4.5.9 Fouling Factors and Design Parameters for Finned Tubes in FossilFuel Exhaust GasesType of Flue GasFouling Factor,m2 K/WMinimum Spacingbetween Fins, mMaximum GasVelocity to AvoidErosion, m/secClean Gas (Cleaning Devices Not Required)Natural GasPropaneButaneGas turbine0.0000881–0.0005280.000176–0.0005280.000176–0.0005280.0001760.00127–0.0030.001780.00178—30.5–36.6———Average Gas (Provisions for Future Installation of Cleaning Devices)No. 2 oilGas turbineDiesel engine0.000352–0.0007040.0002640.0005280.00305–0.00384——25.9–30.5——0.00457–0.005790.005080.005080.00587–0.0086418.3–24.4——15.2–21.3Dirty Gas (Cleaning Devices Required)No.
6 oilCrude oilResidual oilCoal0.000528–0.001230.000704–0.002640.000881–0.003520.000881–0.00881Source: Weierman, R.C., 1982. Design of Heat Transfer Equipment for Gas-Side FoulingService, Workshop on an Assessment of Gas-Side Fouling in Fossil Fuel ExhaustEnvironments, W.J. Marner and R.L. Webb, Eds., JPL Publ. 82-67, Jet PropulsionLaboratory, California Institute of Technology, Pasadena. With permission.If the foulant control is not effective, the exchanger must be cleaned either on-line or off-line.
Online cleaning includes flow-driven brushes/sponge balls inside tubes, power-driven rotating brushes insidetubes, acoustic horns/mechanical vibrations for tube banks with gases, soot blowers, and shutting off ofthe cold gas supply, flowing hot gas, or reversing of the fluids. Off-line cleaning methods, withoutdismantling the exchanger include chemical cleaning (circulate acid/detergent solutions), circulating ofparticulate slurry (such as sand and water), and thermal melting of frost layers. Off-line cleaning witha heat exchanger opened includes high-pressure steam or water cleaning, and thermal baking of anexchanger and then rinsing for small heat exchanger modules removed from the container of the modularexchangers.NomenclatureAAfrAkAobCC*cpDhdedi, doEutotal heat transfer area (primary + fin) on one fluid side of a heat exchanger, Ap: primary surfacearea, Af: fin surface area, m2frontal area on one side of an exchanger, m2total wall cross-sectional area for heat conduction in fin or for longitudinal conduction in theexchanger, m2minimum free-flow area on one fluid side of a heat exchanger, m2plate spacing, h¢+ df , m˙ p , W/°Cflow stream heat capacity rate with a subscript c or h, mcheat capacity rate ratio, Cmin/Cmax, dimensionlessspecific heat of fluid at constant pressure, J/kg Khydraulic diameter of flow passages, 4Ao L /A, mfin tip diameter of an individually finned tube, mtube inside and outside diameters, respectively, mN-row average Euler number, Dp/(r um2 N/2gc ), rDpgc /(NG2/2), dimensionless© 1999 by CRC Press LLCHeat and Mass TransferFfftbGggcH4-161log-mean temperature difference correction factor, dimensionlessFanning friction factor, rDpgc Dh /(2LG 2), dimensionlessFanning friction factor per tube row for crossflow over a tube bank outside, rDpgc /(2NG 2)mass velocity based on the minimum free flow area, m˙ / Ao , kg/m2secgravitational acceleration, m2/secproportionality constant in Newton’s second law of motion, gc = 1 and dimensionless in SI unitsfin length for heat conduction from primary surface to either fin tip or midpoint between platesfor symmetric heating, mhheat transfer coefficient, W/m2Kh¢height of the offset strip fin (see Figure 4.5.15), mjColburn factor, NuPr –1/3/Re, StPr 2/3, dimensionlesskfluid thermal conductivity, W/m Kkfthermal conductivity of the fin material, W/m Kkwthermal conductivity of the matrix (wall) material, W/m KLfluid flow (core or tube) length on one side of an exchanger, mlfin length for heat conduction from primary surface to the midpoint between plates for symmetricheating, see Table 4.5.5 for other definitions of l, mlfoffset trip fin length or fin height for individually finned tubes, lf represents the fin length in thefluid flow direction for an uninterrupted fin with lf = L in most cases, mmfin parameter, 1/mNnumber of tube rowsNfnumber of fins per meter, 1/mNttotal number of tubes in an exchangerNTU number of heat transfer units, UA/Cmin, it represents the total number of transfer units in amultipass unit, NTUs = UA/Cshell, dimensionlessNuNusselt number, hDh /k, dimensionlessntuc number of heat transfer units based on the cold side, (ho hA)c /Cc, dimensionlessntuh number of heat transfer units based on the hot side, (ho hA)h /Ch , dimensionlessṁmass flow rate, kg/secPtemperature effectiveness of one fluid, dimensionlessÃfluid pumping power, WPrfluid Prandtl number, mcp /k, dimensionlesspfluid static pressure, PaDpfluid static pressure drop on one side of heat exchanger core, Papffin pitch, mqheat duty, Wqeheat transfer rate (leakage) at the fin tip, Wq²heat flux, W/m2Rheat capacity rate ratio used in the P-NTU method, R1 = C1/C , R2 = C2 /C1, dimensionlessRthermal resistance based on the surface area A, compare Equations 4.5.5 and 4.5.6 for definitionsof specific thermal resistances, K/WReReynolds number, GDh /m, dimensionlessRedReynolds number, rum do /m, dimensionlessrhhydraulic radius, Dh /4, Ao L/A, mrsfouling factor, 1/hs, m2K/WStStanton number, h/Gcp, dimensionlesssdistance between adjacent fins, pf – df mTfluid static temperature to a specified arbitrary datum, °CTaambient temperature, °CTofin base temperature, °CTlfin tip temperature, °CUoverall heat transfer coefficient, W/m2 Kummean axial velocity in the minimum free flow area, m/sec© 1999 by CRC Press LLC4-162VXdXlXtabeddfhfholmrsSection 4heat exchanger total volume, m3diagonal tube pitch, mlongitudinal tube pitch, mtransverse tube pitch, mratio of total heat transfer area on one side of an exchanger to the total volume of an exchanger,A/V, m2/m3heat transfer surface area density, a ratio of total transfer area on one side of a plate-fin heatexchanger to the volume between the plates on that side, m2/m3heat exchanger effectiveness, it represents an overall exchanger effectiveness for a multiplassunit, dimensionlesswall thickness, mfin thickness, mfin efficiency, dimensionlessextended surface efficiency, dimensionlesslongitudinal wall heat conduction parameter based on the total conduction area, l = kw Ak,t /CminL,lc = kw Ak, c /Cc L, lh = kw Ak, h /Ch L, dimensionlessfluid dynamic viscosity, Pa·sfluid density, kg/m3ratio of free flow area to frontal area, Ao /Afr , dimensionlessSubscriptsCclean surface valueccold fluid sideFfouled surface valueffinhhot fluid sideiinlet to the exchangerooutlet to the exchangersscale or foulingwwall or properties at the wall temperature1one section (inlet or outlet) of the exchanger2other section (outlet or inlet) of the exchangerReferencesBhatti, M.S.
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