第五章传热过程计算和换热器（Chapter5Calculationsofheattransferandheatexchangers）5.1间壁式换热器的传热过程分析间壁式换热器2009-2-82009-2-82009-2-8列管式换热器5.1.2间壁式换热器的传热过程分析5.2间壁式换热器的传热过程计算------称为总传热热阻5.2.3K的计算考虑到实际传热时间壁两侧还有污垢热阻，则上式变为：5.2.4tm的计算以冷、热流体均无相变、逆流流动为例：---------对数平均温差Example5.1Aheatexchangerisrequiredtocool20kg/sofwaterfrom360Kto340Kbymeansof25kg/swaterenteringat300K.Iftheoverallcoefficientofheattransferisconstantat2kW/m2K,calculatethesurfacearearequiredin(a)acountercurrentconcentrictubeexchanger,and(b)aco-currentconcentrictubeexchanger.SolutionHeatload:Q=20×4.18(360-340)=1672kWThecoolingwateroutlettemperatureisgivenby:1672=25×4.18(tc2-340)ortc2=316K(a)CounterflowInequationofmeantemperaturedifference:Δtm=(44-40)/ln(44/40)=41.9KHeattransferarea:A=Q/KΔtm=1672/(2×41.9)=19.95m2(b)Co-currentflowInequationofmeantemperaturedifference:Δtm=(60-24)/ln(60/24)=39.3KHeattransferarea:A=Q/KΔtm=1672/(2×39.3)=21.27m2若流动非逆并流，如错流、折流，则tm需采用相应的计算式，如式5-23。LMTD法------对数平均温差法5.2.5传热单元数法（-NTU）传热效率的物理意义：传热单元和传热单元数的物理意义：传热单元和传热单元数的物理意义：传热单元数的几何意义：以逆流为例：前面已推得：代入式中得：（逆流时总传热速率方程）