教程aspen教程htfs培訓thermal design

1、1Thermal Design OverviewThermal Design OverviewThermal Design Overview2Thermal Design OverviewContentsHeat Transfer BasicsTemperature DifferenceHeat Exchanger DesignCalculation of Required Surface Area3Thermal Design OverviewBasic Heat TransferHeat travels from a hot fluid to a cold fluid (object) i

2、n one or more of the following waysConductionConvectionRadiationHOTCOLDAlways driven by the temperature difference4Thermal Design OverviewBasic Heat Transfer (Conduction)Heat transfer through a wallDue to conduction (transfer of heat due to vibration of molecules) Locally dQ = (w/d x) dA (Th - Tc) w

3、 /dx is defined as w the wall htc If Ts are constant QT = w AT (Th - Tc)1/ w is the wall resistance rwTcThthickness dxconductivity wATdQdA5Thermal Design OverviewBasic Heat Transfer (Convection)Heat transfer due to the motion of the fluidHeat flux q = dQ/dALocal htc = q / T Convective htc analogous

4、to conductive htcq = T = (Tw - Tc) WallATTcThTwBulk FluidTemperature6Thermal Design OverviewBasic Heat Transfer (Radiation)Radiation travels in straight line and hence not all the energy will reach the adjacent surfaceEnergy transmitted proportional toemissivity, absolute T4where called the Stefan-B

5、oltzmann constant Wall, has emissivity ATTh7Thermal Design OverviewOverall Heat Transfer Coefficient (Local)For design an overall htc U is usedQ = UA (Th - Tc)In a typical heat exchanger the heat has to transfer through the hot fluiddirt layer (fouling layer)wallfouling layercold fluidTh Fouling lay

6、er WallFouling LayerTc8Thermal Design OverviewBasic Equations Q = Heat transfer rate (W) U = Overall heat transfer coefficient (W/m2K) A = Surface Area (m2)T = Mean temperature difference (K)What is U ?What is T ?9Thermal Design OverviewCalculation of UOverall Heat Transfer Coefficient, U:where = si

7、ngle side coefficientD = tube diameter (o,i = bare tube outside, inside) A = Area (o,t = bare tube surface, extended surface) r = resistancei = inside tube x= crossflow sidew = wall10Thermal Design OverviewLog Mean Temperature Difference (1)Pure counter-current flow (e.g. 1-pass TEMA E-shell) of sin

8、gle phase fluidsAt any point mcccdtc/dA= q = U(T - t) -mhchdTh/dA = q Whereq = local heat flux (W/m2)m = mass flowrates (kg/s)c = specific heats (J/kg K)T, t = local temperatures (K)h,c= hot, cold streamsEnthalpyTemp11Thermal Design OverviewLog Mean Temperature Difference (2) Local temperature diffe

9、rence: If U and c constant, integrate from (T1 - t2) to (T2 - t1):Use -mhch(T1 - T2) = Q = mccc(t2 - t1) to get 12Thermal Design OverviewLog Mean Temperature Difference (3)Thus in Q = UA T T is given by the “Log Mean Temperature Difference” T LMRemember: You can apply T LM to the entire exchanger on

10、ly forPure counter-current flow,Constant heat transfer coefficient, and Linear T/h curves13Thermal Design OverviewLog Mean Temperature Difference: Non Counter-CurrentNon counter-current flow (e.g. 2-pass E-shell):Integration over entireexchanger (constant U, and linear T/h curves) yieldswhere FT is

11、the LMTD correction factorThe value of FT depends upon the stream inlet and outlet temperatures and the exchanger geometryTemp14Thermal Design OverviewFT Correction Factor: Single Pass Cross Flow 15Thermal Design OverviewApplication for Non-Linear T-h CurvesNon-linear T-H curves: can be split into z

12、ones in which the curve is approximately linear.can be calculated for each zoneThe mean temperaturedifference can then becalculated by weighting the duty performed in each zoneEnthalpy Temp1/Tm =( H1/ T1 + H2/ T2 + .) /(Hin - Hout)16Thermal Design OverviewApplication for Non-Linear T-h Curves: Multi

13、ple passesNon-linear T-H curves and multiple tube passes: U almost certainly not constant TempEnthalpyTtItIIUsein equation for the LMTD,where tm = mean tubeside temperature17Thermal Design OverviewCalculation of Required AreaHow to solve across an exchanger?Assuming:Linear T-H curvesConstant overall

14、 coefficientEffect of non counter-current flow can be handled by FT correction factor (Approximate method, suitable for hand calculation)18Thermal Design OverviewSingle v Multiple Passessingle-passmulti-passorIncreasing the number of passes.Increases the mean temperature difference (tends to counter

15、current)Increases the process-side coefficientDecreases the tendency to foulIncrease process-side pressure loss19Thermal Design OverviewOptimised Design Lowest Capital or Annual Cost Depend on:- tube diameter- fin diameter- fin frequency- fin thickness- transverse pitch- longitudinal pitch- special

16、fin geometry(louvres, cuts etc) Lowest Capital or Annual Cost Depend on:- number of rows- number of passes- tube length- bundle width-number of bundles/unit- air flowrate- fan diameter- number of fansrigorous optimised design is very difficult !20Thermal Design OverviewDesign of Ducted Bundles Tube

17、length and nominal width dictated by duct dimensions Thermal duty, process-side dp and X-side dp specified Main design variables- tube diameter- fin type/geometry (limited by commercial availability)- tube pitch- number of rows- number of passes Applications- fired heater convection banks- inter- /after-coolers etc. 21Thermal Design OverviewDesign Methods for ACHEs Required process conditions known as is process mass flow and air inlet temperature Type of fin surface and pitch arrangement and number of rows determine X-side dp