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例1)一列管换热器,管子规格为φ25×2.5mm,管内流体的对流给热系数为100W/(m·℃),管外流体的对流给热系数为2000W/(m·℃),已知两流体均为湍流流动,管内外两侧污垢热阻均为0.0018m·℃/W。试求:①传热系数K及各部分热阻的分配;②若管内流体流量提高一倍,传热系数有何变化?③若管外流体流量提高一倍,传热系数有何变化?钢管=45W/(m℃)分析过程:iimdddbdK000011(1)(2)8.0iiisiuuV022.02025ln020.0025.0ln00iimddddd01572.000118.002.0025.000118.002.0100025.0022.045025.00025.02000111000000RddRdddbdKiiiim)/(12CmW)/(61.632CmWKo热阻分配:污垢:%9.16169.001572.000118.025.2100KRRi管外:%18.301572.0200011100K管内:%52.7901572.002.0100025.0100Kddii管壁:%4.001572.0022.045025.00025.0100Kdbdm②WWii2uuii2ii208.∴00000011RddRdddbdKiiiim002655.002.01002025.0022.045025.00025.0200018.0=0.01039m2℃/W25.960KW/(m2℃)③WW00200802.∴002655.002.0100025.0022.045025.00025.02000218.0=0.01551m2℃/W47.640KW/(m2℃)0000/011RddRdddbdKiiiimo少?则油的出口温度又为多其他条件不变,和侧的污垢热阻分别为长期使用后,水侧及油换热器的物性数据不变。若该出口温度为多少?设油的,其他条件不变,则油要的管长。若管长增加算所需热损失忽略不计,试计油的进口和水的进出口温度为及、、分别为比热、导热系数及粘度油的密度、、别为钢管。水与油的流速分的,外管为油与水逆流,内管为内管,器,用水冷却油。水走)有一台新的套管换热例,/1052.1/105.320,100,3010.108.1)/(15.0)/(1090.1/860,/8.0/5.133231920230240000030033WCmWCmCCCsPaCmwCkgJmkgsmsmmmmmCtsmui10;/5.11水:Ct302CTsmuo100;/8.01油:?2T公式计算判断流型,根据相应的oiOK,LSQKtm,,(1)(2)法—NTUskgWskgWsPaCmwCkgkJCmkgCthCP/170.04/8.010)1926(/199.04/5.110)619(101)/(6.0/183.4/2.998202/)1030(16226230030查表得水的物性数据:)解:CTskJQTQQQchch02246.48/648.16)1030(99.0183.4)100(170.09.1无热损失故)/(9.6249/023.002.7/04.08.0CmwdPRCPireipr)/(9.412/914.0023.0594.0/10618.22/100006.2675)108.1/(8608.0007.07)1926(03.08.008.153CmwdPRRCPRmmdereepreeo前应求出其当量直径算由于油走壳程,因此计的计算:100009.19464001.0/2.9985.1013.0eiR的计算:)/(5.365)]/(/[0027361.0///1/18.15)13/19()1319()/(450200200000CmwKCmwdddbdKmmLndCmwKiimmo的计算:CLntm067.52)46.38/70(/)46.3870(2865.0)7.525.365/(16648/mtKQSmmdSL5.14019.0/865.0/则:6323.0)]}388.01(175.1exp[388.0/{)]}388.01(175.1exp[1{)]}1(exp[/{)]1(exp[1111RNTURRNTUhhh则:mSS4296.02.1,20)2'00则管长增加CWCWCwCWPCCphh00/4.8321000183.4199.0/0.32310009.117.0首先确定最小值流体:175.10.323/865.02.15.365)/(388.04.832/0.323/1PhhhpccphhCWKSNTUCWCWRCCtTTTtTTT00111'211'2111.431.43)10100(6323.0100)(6323.0)/()(即油的出口温度为CCWCWtpccphh0'21.3210/)1.43100(:此时冷却水温度为388.05814.00.323//865.01.217)/()/(1.217004604.000152.000035.0002736.0/1)31'1''102'0'0RRCWSKNTUCmWKKphhoCCtTTTtTTT00111'211'2110.630.63)10100(4112.0100)(4112.0)/()(即油的出口温度为同理生产要求?原来的多少倍才能满足试问此换热器管长增为热器的损失忽略不计,物性均不发生变化,换温度,流量及,假设水和液体的进口度降至体出口温。现因生产任务要求液和为别,液体的进出口温度分和口温度分别为出水冷却某液体,水的进)在逆流换热器中,用例CCCCC00000707515080153149.1/:149.1/2.60809.6475''''''LLSSKSCWKSCWtKSQtKSQphhphhmm即又CttCWCWCWCWtCWTCWQQpccphhpccphhpccphhCh0'2'23.84)15()70150()1580()75150(:则解CLntCm0'2.60)55/7.65()557.65(70时,则:当液体出口温度为CLntm09.64)60/70()6070(例4)120℃饱和水蒸汽将空气从20℃加热至80℃,空气流量1.20×104kg/h。现有单程列管换热器,25×2.5mm钢管300根,管长3m,0=104W/(m2℃),污垢及管壁热阻不计。问此换热器能否满足要求。判断换热器能否满足要求方法:比较传热面积mtKQSdlnS计算得到的换热器实际能够提供的解:t208025050℃空气,=1.093kg/m3,Cp=1.005kJ/(kg℃),=0.02824W/(m℃)=1.96×10-5Pas,Pr=0.698uVnd412010360010934002300324242....m/s℃Re.....du00203241093196103611054>100004.08.044.08.0698.01061.3020.002824.0023.0PrRe023.0iid5.124W/(m2℃)∵0>>i∴Ki=i=124.5W/(m2℃)Q=WcCpct=1201036004.×1.005×60=201kJ/s1004010040655ln.℃tm=Q=KiAitm即201×103=124.5Ai×65.5∴Ai=24.6m2又,Ai=ndiL即24.6=300×0.020L∴L=1.31m<3m∴满足要求有一列管式水预热器,2大气压(表压)的饱和水蒸汽在管内冷凝(温度为134℃),用以预热管内的水。水在φ25×2.5mm钢管内以0.6m.s-1的速度流过,其进口温度为20℃,到出口预热至80℃。取水蒸汽的冷凝膜系数为,污垢热阻为,忽略管壁热阻。试求:(1)此时基于外表面积的传热系数K;(2)操作一年后,由于水垢增加,换热器能力下降,当水的流量和进口温度不变,其它条件也没有变化,此时水的出口温度仅能预热至70℃,试求此时基于外表面积的传热系数K′及垢层热阻R′。12410KmW124106WKm已知水在定性温度50℃时的物性数据如下:。;4175;1.988113KkgJCmkgp11648.0549.0KmW;厘泊d1d2α1α2)(61.86')2070()64114()70134()20134(''')')(3.807.83)2080()54114()80134()20134(837)()2(83710195.1202510610120362625113626658.129347452.0648.010549.0417510549.01.9886.0020.0020.00648.0023.0)()(023.01222122121223441221122124.038.034.08.011AKCWlAKtAKttCWACWlAtAKttCWKmWKddRddKKmWcdudPCCnmPCCPCCnmPCpC(操作一年后:操作一年前:)(解题过程:1222647'3.8083761.86'20802070KmWKK)得:()式(操作一年后,水的定性温度略有变化(45℃),物性数据会略有变化,也会略有变化,下列计算忽略此种变化。124412211221084.8'2025'101253626206471'1'1WKmRRddRddK重油和原油在单程套管换热器中呈并流流动,重油放热由243℃降至167℃,原油吸热由128℃升至157℃。若维持两种油的初温和质量流量不变,而将两流体改为逆流流动,试求此时冷、热流体的出口温度和此时的对数平均温度差。假设在两种流动情况下,流体的物性和总传热系数均不变化,热损失也可以忽略。并流逆流T1=243℃t1=128℃T2=167℃t2=157℃t1=128℃T1=243℃T2′t2′CTCtTtTtCWTCWCWCWPCCphhPCCPhhO2o22222224.155',3.161'))('382.07.220't128157128'167243'243)128'()'243()128157()167243()解之得:)和式(联立式(即二式相除得:逆流时:并流时:CTCtTtTtltTTtlTttTtTttTtCltKAtCWKATCWtKACWKACWtnnmmnmPCCphhmPCCphhmO2o22222222222222222o224.155',3.161')(983.2)4347exp(128''2434347)128''243(''37147)128''243()128'()'243(43''37147128''2432976'43128'128157'
本文标题:化学工程工业传热习题
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