第四章传热及换热器 Page 13 of 21
Pr?Cp??/??7.02?i?0.023?Re0.80Pr0.4/di?6249.9w/(m?C)de?(26?19)?7mmRe?0.007?0.8?860/(1.8?10Pr?Cp??/??22.81.8?3)?2675.6?10000111??1?6?10/Re05?0.5940.80.3?0?0.023?0.914?RePr?412.9w/(m?C)/de??45w/(m?C)0dm?(19?13)Ln(19/13)?15.8mm201/K0?1/?0?bd0/?dm?d0/?idi?0.0027361/[w/(m?C)]K0?365.5w/(m?C)?tm?(70?38.46)/Ln(70/38.46)?52.67CS?Q/K?tm?16648/(365.5?52.7)?0.865mL?S/?d?0.865/0.019??14.5m2)管长增加20002020S?1.2S?0.4296m0'?0.17?1.9?1000?323.0w/CWCCPC?0.199?4.183?1000?832.4W/CR1?WhCph/WcCpc?323.0/832.4?0.388NTU10
?KS/(WhCPh)?365.5?1.2?0.865/323.0?1.175?1?1?exp[NTU1(1?R1)]/{R1?exp[NTU1(1?R1)]}?{1?exp[1.175(1?0.388)]}/{0.388?exp[1.175(1?0.388)]}?0.6323?1?(T1?T2)/(T1?t1)T2?T1?0.6323(T1?t1)?100?0.6323(100?10)?43.1C即油的出口温度为43.1C0'0'
此时冷却水温度为':0t2?WhCph(100?43.1)/WcCpc?10?0.388?(100?43.1)?10?32.1C
第四章传热及换热器 Page 14 of 21
3)1/K0?0.002736?0.00035?0.00152?0.004604K0?217.1W/(m?C)NTU''1'20''?KS/(WhCph)?217.1?0.865//323.0?0.5814R1?R1?0.388同理?1?(T1?T2)/(T1?t1)T2?T1?0.4112(T1?t1)?100?0.4112?(100?10)?63.0C即油的出口温度为63.0C0'0'
28)在逆流换热器中，管子00规格为?38?3mm，用初温为15C.的水将2.5kg/s的甲侧的给热系数分别为00苯由 80C冷却到30C,水走管程，水侧和甲苯900W/(m?C),污垢热阻忽略不计。若器的传热面积。解：t?(80?30)/2?55C查得甲苯的物性数据：Q?2.5?1.8?50?1030202500W/(m?C),20水的出口温度不能高于45C,试求该换热CP?1.8kJ/kgC?2.25?10J/s50dm?(38?32）Ln(38/32)?35mm1/K0?1/?0?bd0/??d0/?idi?38/(2500?32)?0.003?38/(45?35)?1/900?0.001611/[W/(m?C)]K0?603W/(m?C)?tm?(35?15)/Ln(35/15)?23.6CS?225?10/(603?23.6)?15.8m3202020
29)两种流体在一列管换热器中逆流流动，热流体进出口温度为00060C冷流体从20C加热到50C,试求下列情况下的平均温差：①换热器为单壳程，四管程。②换热器为双管程，四管程。0解：1）?tm?(50?40)Ln(50/40)?44.8CP?(50?20)/(100?20)?0.375R?(100?60)（/50?20）?1.33查表得：??t?0.910?tm1???t?tm?40.8C2）查表得：??t?0.970?tm2???t?tm?43.5C1122100C，出口温度为0
第四章传热及换热器 Page 15 of 21
30)在逆流换热器中，用水体的进出口温度分别为假设水和液体的进出口试问此换热器管长增为解:?Qh?QCWhCph?T?WcCpc?t冷却某液体，水的进出00口温度分别为15C和 80C，液70C，000150C和75C。现因生产任务要求液温度，流量及物性均不原来的多少倍才能满足体进出口温度降至发生变化，换热器的损生产要求？失忽略不计，则WhCph(150?75)?WcCpc(80?15)WhCph(150?70)?WcCpc(t2?15)t2?84.3C?tm?(70?60)Ln(70/60)?64.9C?tm?(65.7?55)Ln(65.7/55)?60.2C又Q?KS?tmQ?KS?tm'''''00'0'75WhCph?64.9KS80WhCph?60.2KS?S/S?1.149'即:L/L?1.149'31) 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。 解：t?20?802?50℃
50℃空气，? = 1.093kg/m3，Cp = 1.005kJ/(kg?℃)，? = 0.02824W/(m?℃) ? = 1.96×10-5 Pa?s ，Pr = 0.698
u?nV?4d2?1.20?103600?1.093??44?32.42 m/s
?0.02?300Re?du???0.020?32.4?1.0931.96?10?5?3.61?10＞10000
4∴?i?0.023?diRe0.8Pr0.4?0.023?0.028240.020?3.61?10?4?0.8?0.6980.4
?154.5W/(m2℃)
∵?0＞＞?i ∴Ki = ?i = 124.5 W/(m2℃) Q = WcCpc?t = ?tm =
100?40ln100401.20?103600?65.5
4×1.005×60 = 201 kJ/s
℃
第四章传热及换热器 Page 16 of 21
Q = KiAi?tm 即 201×103 = 124.5Ai×65.5 ∴Ai = 24.6 m2
又，Ai = n?diL 即 24.6 = 300?×0.020L ∴ L = 1.31 m＜3 m ∴ 满足要求
32）某单壳程单管程列管换热器，用1.8×105Pa饱和水蒸汽加热空气，水蒸汽走壳程，其给热系数为105 W/(m·℃)，空气走管内，进口温度20℃，要求出口温度达110℃，空气在管内流速为10m/s。管子规格为φ25×2.5mm的钢管，管数共269根。试求换热器的管长。
若将该换热器改为单壳程双管程，总管数减至254根。水蒸汽温度不变，空气的质量流量及进口温度不变，设各物性数据不变，换热器的管长亦不变，试求空气的出口温度。
解:t?(20?110)/2?65C查空气物性数据：CP?1.007kJ/kgCPr?0.6951.8?10Pa的饱和水蒸气：1)Re?10245.1?100005000??0.0294w/mC?50??2.04?10Pa?s??1.045kg/m3T?116.6CI?2214.3KJ/kg
?i?0.023?Re0.8Pr0.4/di?47.23w/mC220每根管内:WC??du?/4???0.02?10?1.045?0.03283kg/sQ?WCCPC?t?0.003283?1.007?10?90?0.29753kg/s??0???i?Ki??i03?tm?(96.6?6.6)/Ln(96.6/6.6)?33.54CQ?KiSi?tm?Si?0.1878mSi??diL?L?3m2
2)质量流量不变u?269?10/254/2?21.18m/s0.8'?i?(21.18/10)'''?47.23?86.10w/(m?C)200Ki??i?83.00w/(m'2'C)
WC??(n/2)du?/4?0.8826kg/s?R?0''???t?1'?tm?(t2?20)/Ln96.6/(116.6?t2)Si?n?diL?254?3.14?0.02?3?47.85m由方程：Q?WCCPC?t''2
Q?KiSi?tm解得t?115.7C'20'''
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