川大884化工原理電子版本課件雙語chapter7.1f

1、1Chapter 7Heat transfer2IntroductionApplication in chemical industryRelationship with thermodynamics Industrial heat exchangeBasic problems of heat transfer3 heat transferthermodynamicsstudy heat transfer ratestudy heat balancedepending on the microscopic move in materials. Feature no macroscopic mo

2、vement in materials direct contact Transport media solid , liquid or gasFouriers Lawq：heat flux, J/m2s; Q：transport rate， W= J/s k：heat conductivity，W/m.K; A: transport area，m2dT/dy：temperature gradient，K/mConduction4Review Steady-state conduction through a flat wallbxoQAT1T2If the conductivity is a

3、 Const.B. C.：T = T1 for x = 0 ， T = T2 for x = b integrate5 Q: transport rate，W； A: transport area ，m2，perpendicular to heat flow direction q: heat flux，W/m2； b: thickness of wall，m; T: temperature difference, K; k: heat conductivity，W/m.K；Isothermal surface: surfaces consist of points with equal te

4、mperature in the temperature field with （1）different isothermal surface never intersect each other. （2）there is no heat transfer in same surface. （3）for uniform material, the isothermal surface is also the geometrical surface.6u，T1QR1R2T1T2LQreview（p87)Steady-state conduction through a cylinder Assu

5、me (1) constant conductivity (2) one-dimensional, steady7Steady-state conduction through a cylinderR1R2T1T2LQrdr80=Steady-state conduction through a cylinderdivided by -k 2pL9Steady-state conduction through a cylinderR1R2T1T2LQrdr10logarithmic mean areaSteady-state conduction through a cylinder(2.4.

6、71)11Convection Heat is transferred by macroscopic movement of fluid particles. feature：can only exist in fluids note：the heat transfer between a fluid and a contacted solid is also called convective heat transfer.Free the macroscopic movement of fluid particles is caused by fluid density difference

7、.Forced the macroscopic movement of fluid particles is caused by outer forces. 12Newtons Cooling Lawwhere：Q convective transport rate，W； A transport area，m2; q convective heat flux，W/m2； Tb 、Ts temperatures of fluid bulk and wall，K； h convective heat transfer coefficient，W/m2.K； （2.4.39）Convectionco

8、nductionqu1TwTh1convectionconvection th2u2tw13Radiation Heat is transferred as a electro-magnetic wave from a high temperature material to low temperature material.Feature transfer medium does not needed. important in high temperature situationStephenBoltzman LawEb radiation capacity of black body，W

9、/m2；C0 radiation coefficient of black body（ =5.67W/(m2K4)）radiation capacity of a black body14Direct contactHeat reservationUnsteady transfercold fluid t1t2Hot fluid T1T2Combined heat transferT Ts1 Ts2 Ts3 Taheat isolation layerpipe wallhot fluid，T1QTaTs1Ts2Ts3convectionconvectionradiationQconductio

10、nhot fluid，T2Heat exchangersIndirect contactpackings15cold fluid t1cold fluid t2hot fluid T1hot fluid T2Hot and cold fluids do not contact directly. Heat is exchanged through solid wall conduction .Heat exchangerspace inside tube tube passspace outside tube shell passfluid flows m times in tubes m t

11、ube passfluid flows m times in shell m shell pass QQtubular exchanger singer pass shell-tube exchangerdouble pass shell-tube exchanger16For given pipe number n, diameter d2 and length l，this area based on outer diameter d2 Heat transfer areaSimilarly, if based on d1 ，we haveIf we use the mean pipe d

12、iameter, then17For inner pipe diameter d1， and number n ，if the pipe-side pass is m，then the fluid flow area isArea of flow cross-sectionsingle passdouble pass18Basic problems of heat transfer1. basic rules of steady heat transfer for steady T = f ( x , y , z); unsteady T = f ( x , y , z，t)2. To sol

13、ve the problems in chemical industry * Calculation of equipment design of heat exchanger * Enhance or weaken a heat transfer processes19heat transfer Eq. Basic Eq. of heat transfer thermodynamics where Q heat transfer rate，W； A transport area，m2 ； q heat transfer flux，W/m2； tm mean temperature diffe

14、rence between hot and cold fluids, K ； K heat transfer coefficient，W/m2.K 。heat balance or207.2 calculation of heat transfer processTask Q（Qrelease or Qabsorb）by thermodynamicsDriving force (temperature difference, tm)Total heat transfer coefficient Ktaskcapacity217.2.1 task of heat transfer1 no pha

15、se transformation W1Cp1(T1 - T2) = W2Cp2(t2 - t1) single pass exchanger Qrelease=QabsorbW2,Cp2,t2W2,Cp2,t1 W1,Cp1,T1W1,Cp1,T2 2 hot fluid condense, cold fluid heating W1g1= W2Cp2(t2 - t1) 3 cold fluid evaporation, hot fluid cooling W1Cp1(T1 - T2)= W2g2 4 two fluids phase transformation W1g1= W2g2 W2

16、,Cp2,t2W2,Cp2,t1 W1,r1,TW1, r1,TW1,Cp1,T1W1,Cp1,T2W2,r2,tW2,r2,t W1,r1,TW1, r1,TW2,r2,t W2,r2,t22【Example 】(p330) w2=2000kg/h, t1=20 t2=98Ps / kPa 198.64 200 232.19 a1 ai a2 Ts / 120 ？ 125 b1 bi b2P=0.2MPa(gauge),T ( g ) P=0.2MPa(gauge),T ( l )內(nèi)插法Heat balancep37823 P=0.2MPa,T1=120.2(g) w2=2000kg/h t

17、1=20w1 + w2 t2=98 P=0.2MPa,T1=120.2(l) P=0.2MPa,T1=120.2(g)T2=98(l)W1=295.42kg/hW1/W1= 283.35/295.42= 0.959； Qa/Ql= 93.95/2205.2= 0.043【Example 】(p330)The results show that the apparent heat is much smaller compared with the latent heat. In heating process with steam, only latent heat is used.24common heating agents and their applied temperature heating agenthot watersaturatedsteam mineral oilbiphenylmixture* melt salts* flue gasT /40 100100 180180 250255 380142 530500 1000*melt salts：KNO353%NaNO240% NaNO37%common cooling agents and their applied tempera