The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 42
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Such a method isgiven here for purposes of preliminary cost estimation, plant layout, or checking the results of computeroutput. This method is based upon Equation (4.5.88) with A* = Ao and U* = Uo and depends uponrapidly estimating values for qT , MTD, and Uo. The procedure is as follows:© 1999 by CRC Press LLC4-174Section 4TABLE 4.5.10 Typical Film Heat Transfer Coefficients for Shell-and-Tube Heat Exchangersh, W/m2Ka,bFouling resistance,m2 K/WaLiquidLiquidLiquidLiquidLiquidHeatingCoolingLiquidHeatingCoolingPressure 100–200 kN/m2 absPressure 1 MN/m2 absPressure 10 MN/m2 abs5000–75006000–80001500–2000750–15001–2.5 ´ 10–40–1 ´ 10–40–2 ´ 10–41–4 ´ 10–4250–750150–4002–10 ´ 10–42–10 ´ 10–4100–30060–15080–125250–400500–8004–30 ´ 10–34–30 ´ 10–30–1 ´ 10–40–1 ´ 10–40–1 ´ 10–4Pressure 10 kN/m2 abs, nononcondensablesi,jPressure 10 kN/m2 abs, 1%noncondensableskPressure 10 kN/m2 abs, 4%noncondensableskPressure 100 kN/m2 abs, nononcondensablesi,j,k,lPressure 1 MN/m2 abs, nononcondensablesi,j,k.lPure component, pressure 10kN/m2 abs, nononcondensablesiPressure 10 kN/m2 abs, 4%noncondensableskPure component, pressure 100kN/m2 abs, nononcondensablesPure component, pressure 1MN/m2 absPure component or narrowcondensing range, pressure100 kN/m2 absm,nNarrow condensing rangepressure 100 kN/m2 absm,nMedium condensing range,pressure 100 kN/m2 absk,m,oMedium condensing range,pressure 100 kN/m2 absk,m,oMedium condensing range,pressure 100 kN/m2 absk,m,o8000–120000–1 ´ 10–44000–60000–1 ´ 10–42000–30000–1 ´ 10–410000–150000–1 ´ 10–415000–25,0000–1 ´ 10–41500–20000–1 ´ 10–4750–10000–1 ´ 10–42000–40000–1 ´ 10–43000–40000–1 ´ 10–41500–40001–3 ´ 10–4600–20002–5 ´ 10–41000–25000–2 ´ 10–4600–15001–4 ´ 10–4300–6002–8 ´ 10–43000–100001–2 ´ 10–44000–150001–2 ´ 10–4Fluid ConditionsSensible heat transferWatercAmmoniaLight organicsdMedium organicseHeavy organicsfVery heavy organicsgGashGashGashCondensing heat transferSteam, ammoniaSteam, ammoniaSteam, ammoniaSteam, ammoniaSteam, ammoniaLight organicsdLight organicsdLight organicsdLight organicsdMedium organicseHeavy organicsLight multicomponentmixtures, all condensabledMedium multicomponentmixtures, all condensableeHeavy multicomponentmixtures, all condensablefVaporizing heat transferp,qWaterrWaterr© 1999 by CRC Press LLCPressure < 0.5 MN/m2 abs,DTSH,max = 25 KPressure < 0.5 MN/m2 abs,pressure < 10 MN/m2 abs,DTSH,max = 20 K4-175Heat and Mass TransferTABLE 4.5.10 (continued)Typical Film Heat Transfer Coefficients for Shell-and-Tube Heat ExchangersFluid ConditionsAmmoniaLight organicsdLight organicsdMedium organicseMedium organicseHeavy organicsfHeavy organicsgVery heavy organicshPressure < 3 MN/m2 abs,DTSH,max = 20 KPure component,pressure < 2 MN/m2 abs,DTSH,max = 20 KNarrow boiling range,spressure < 2 MN/m2 abs,DTSH,max = 15 KPure component,pressure < 2 MN/m2 abs,DTSH,max = 20 KNarrow boiling range,spressure < 2 MN/m2 abs,DTSH,max = 15 KPure component,pressure < 2 MN/m2 abs,DTSH,max = 20 KNarrow boiling range,spressure < 2 MN/m2 abs,DTSH,max = 15 KNarrow boiling range,spressure < 2 MN/m2 abs,DTSH,max = 15 Kh, W/m2Ka,bFouling resistance,m2 K/Wa3000–50000–2 ´ 10–41000–40001–2 ´ 10–4750–30000–2 ´ 10–41000–35001–3 ´ 10–4600–25001–3 ´ 10–4750–25002–5 ´ 10–4400–15002–8 ´ 10–4300–10002–10 ´ 10–4Source: Schlünder, E.U., Ed., Heat Exchanger Design Handbook, Begell House, New York, 1983.
With permision.aHeat transfer coefficients and fouling resistances are based on area in contact with fluid. Ranges shown are typical,not all encompassing. Temperatures are assumed to be in normal processing range; allowances should be made forvery high or low temperatures.b Allowable pressure drops on each side are assumed to be about 50–100 kN/m2 except for (1) low-pressure gas andtwo-phase flows, where the pressure drop is assumed to be about 5% of the absolute pressure; and (2) very viscousorganics, where the allowable pressure drop is assumed to be about 150–250 kN/m2.cAqueous solutions give approximately the same coefficients as water.d Light organics include fluids with liquid viscosities less than about 0.5 ´ 10–3 Nsec/m2, such as hydrocarbons throughC8, gasoline, light alcohols and ketones, etc.eMedium organics include fluids with liquid viscosities between about 0.5 ´ 10–3 and 2.5 ´ 10–3 Nsec/m2, such askerosene, straw oil, hot gas oil, and light crudes.fHeavy organics include fluids with liquid viscosities greater than 2.5 ´ 10–3 Nsec/m2, but not more than 50 ´ 10–3Nsec/m2, such as cold gas oil, lube oils, fuel oils, and heavy and reduced crudes.g Very heavy organics include tars, asphalts, polymer melts, greases, etc., having liquid viscosities greater than about50 ´ 10–3 Nsec/m2.
Estimation of coefficients for these materials is very uncertain and depends strongly on thetemperature difference, because natural convection is often a significant contribution to heat transfer in heating,whereas congelation on the surface and particularly between fins can occur in cooling. Since many of these materialsare thermally unstable, high surface temperatures can lead to extremely severe fouling.h Values given for gases apply to such substances as air, nitrogen, carbon dioxide, light hydrocarbon mixtures (nocondensation), etc.
Because of the very high thermal conductivities and specific heats of hydrogen and helium, gasmixtures containing appreciable fractions of these components will generally have substantially higher heat transfercoefficients.iSuperheat of a pure vapor is removed at the same coefficient as for condensation of the saturated vapor if the exitcoolant temperature is less than the saturation temperature (at the pressure existing in the vapor phase) and if the(constant) saturation temperature is used in calculating the MTD. But see note k for vapor mixtures with or withoutnoncondensable gas.jSteam is not usually condensed on conventional low-finned tubes; its high surface tension causes bridging andretention of the condensate and a severe reduction of the coefficient below that of the plain tube.© 1999 by CRC Press LLC4-176Section 4TABLE 4.5.10 (continued)Typical Film Heat Transfer Coefficients for Shell-and-Tube Heat Exchangersh, W/m2Ka,bFluid ConditionsklmnopqrsFouling resistance,m2 K/WaThe coefficients cited for condensation in the presence of noncondensable gases or for multicomponent mixturesare only for very rough estimation purposes because of the presence of mass transfer resistances in the vapor (andto some extent, in the liquid) phase.
Also, for these cases, the vapor-phase temperature is not constant, and thecoefficient given is to be used with the MTD estimated using vapor-phase inlet and exit temperatures, together withthe coolant temperatures.As a rough approximation, the same relative reduction in low-pressure condensing coefficients due to noncondensablegases can also be applied to higher pressures.Absolute pressure and noncondensables have about the same effect on condensing coefficients for medium and heavyorganics as for light organics. For large fractions of noncondensable gas, interpolate between pure componentcondensation and gas cooling coefficients.Narrow condensing range implies that the temperature difference between dew point and bubble point is less thanthe smallest temperature difference between vapor and coolant at any place in the condenser.Medium condensing range implies that the temperature difference between dew point and bubble point is greaterthan the smallest temperature difference between vapor and coolant, but less than the temperature difference betweeninlet vapor and outlet coolant.Boiling and vaporizing heat transfer coefficients depend very strongly on the nature of the surface and the structureof the two-phase flow past the surface in addition to all of the other variables that are significant for convective heattransfer in other modes.
The flow velocity and structure are very much governed by the geometry of the equipmentand its connecting piping. Also, there is a maximum heat flux from the surface that can be achieved with reasonabletemperature differences between surface and saturation temperatures of the boiling fluid; any attempt to exceed thismaximum heat flux by increasing the surface temperature leads to partial or total coverage of the surface by a filmof vapor and a sharp decrease in the heat flux.Therefore, the vaporizing heat transfer coefficients given in this table are only for very rough estimating purposesand assume the use of plain or low-finned tubes without special nucleation enhancement.DTSH,max is the maximum allowable temperature difference between surface and saturation temperature of the boilingliquid.
No attempt is made in this table to distinguish among the various types of vapor-generation equipment, sincethe major heat transfer distinction to be made is the propensity of the process stream to foul. Severely fouling streamswill usually call for a vertical thermosiphon or a forced-convection (tube-side) reboiler for ease of cleaning.Subcooling heat load is transferred at the same coefficient as latent heat load in kettle reboilers, using the saturationtemperature in the MTD. For horizontal and vertical thermosiphons and forced-circulation reboilers, a separatecalculation is required for the sensible heat transfer area, using appropriate sensible heat transfer coefficients andthe liquid temperature profile for the MTD.Aqueous solutions vaporize with nearly the same coefficient as pure water if attention is given to boiling-pointelevation, if the solution does not become saturated, and if care is taken to avoid dry wall conditions.For boiling of mixtures, the saturation temperature (bubble point) of the final liquid phase (after the desiredvaporization has taken place) is to be used to calculate the MTD.
A narrow-boiling-range mixture is defined as onefor which the difference between the bubble point of the incoming liquid and the bubble point of the exit liquid isless than the temperature difference between the exit hot stream and the bubble point of the exit boiling liquid.Wide-boiling-range mixtures require a case-by-case analysis and cannot be reliably estimated by these simpleprocedures.Estimation of qT.
For sensible heat transfer,()(qT = m˙ h c p,h Th,i - Th,o = m˙ c c p,c Tc,o - Tc,i)(4.5.95)where ṁ is the mass flow rate, cp the specific heat, and T the stream temperature, with subscripts hand c denoting the hot and cold streams, respectively, and i and o inlet and outlet, respectively.For isothermal phase change,˙ fgqT = mh(4.5.96)where ṁ is the mass rate of condensation or vaporization and hfg is the latent heat of phase transformation.© 1999 by CRC Press LLC4-177Heat and Mass TransferFor more complex cases, such as partial or multicomponent condensation, more elaborate analysesare required, although this method can still be used with care to give rough estimates.Estimation of MTD.
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