chapter5-2PPT学习教案

1、会计学1chapter5-2第1页/共46页The tube is inside a cylindrical shell C and is provided with two channels D1 and D2, one at each end, and two channel covers E1 and E2.第2页/共46页第3页/共46页The fluid to be heated is pumped through connection H into channel D2.第4页/共46页第5页/共46页For larger capacities , more elaborate s

2、hell-and-tube exchangers, containing up to thousand of square meter of area, are used.One fluid flows through the inside pipe and second fluid through the annular space between the outside and inside pipes.第6页/共46页It is called counterflow or countercurrent flow第7页/共46页The temperature -length curves

3、for parallel flow are shown in Figure第8页/共46页The temperature of the fluid in the tubes increases continuously as the fluid flows through the tubes. 第9页/共46页t1t2Temperature CLength of tube mtTemp of condensing vapor TTemp of cool fluid第10页/共46页not possible with this method of flow to bring the exit t

4、emperature of one fluid nearly to the the entrance temperature of the other and the heat that can be transferred is less than that possible in countercurrent flow. 第11页/共46页lIn special situation where it is necessary to limit the maximum temperature of the cooler fluid;lWhere it is important to chan

5、ge the temperature of at least one fluid rapidly.第12页/共46页It is customary to neglect it in comparison with the heat transfer through the wall of the tubes from the warm fluid to the cold fluid.第13页/共46页q=mh(Hh1-Hh2)For the cold fluid, it can gain heatq=mc(Hc2 - Hc1) Neglecting the heat transfer with

6、 the ambient. The heat lost by the warm fluid is gained by the cold fluid, therefore第14页/共46页q=mh(Hh1-Hh2)= mc(Hc2 - Hc1)q=mhCph (Th1-Th2)= mcCpc (tc2 - tc1)(11-6)(11-5)If constant specific heats are assumed, the overall enthalpy balance for a heat exchanger becomes第15页/共46页The rate of heat transfer

7、 per unit area is called the heat flux. In many types of heat-transfer equipment the transfer surfaces are constructed from tubes. Heat flux may be based either on the inside area or the outside area of the tubes.第16页/共46页Because the temperature gradients throughout the cross section of the stream,

8、it is necessary to state what is meant by the temperature of the stream.The temperature plotted Fig11-4 are average stream temperatures.第17页/共46页It is clear from Fig.11-4 that t can vary considerably from point to point along the tube, and, therefore, the flux also varies with tube length. 第18页/共46页

9、dqU TtdA(11-9)The quantity U is called the local overall heat-transfer coefficient. 第19页/共46页If A is taken as the outside tube area Ao, U becomes a coefficient based on that area and is written Uo.Likewise, if the inside area Ai is chosen, the coefficient is also based on that area and is denoted by

10、 Ui.Since t and dq are independent of the choice of area.第20页/共46页(2)the specific heats of the hot and cold fluids are constant;(3)heat exchange with the ambient is negligible;第21页/共46页The most questionable of these assumption is that of a constant overall coefficient. The coefficient does in fact v

11、ary with the temperatures of the fluids, but its changes with temperature is gradual, so that when the temperature ranges are moderate, the assumption of constant U is not seriously in error.第22页/共46页21tdtttdqq(11-11)21tdtttU tdAq(11-12)Elimination of dq from Eqs.(11-9) and (11-11) gives第23页/共46页212

12、1lntUtttAtq(11-13)Equation (11-13) can be written第24页/共46页2121lntmttqUAUA ttt2121lnmttttt(11-15)Where第25页/共46页The LMTD is not always the correct mean temperature difference to use. It should not be used when U changes appreciably.第26页/共46页Consider the local overall coefficient at a specific point in

13、 the double-tube exchanger shown inFig11-7. 第27页/共46页Assume that the Reynolds numbers of the two fluids are sufficiently large to ensure turbulent flow and that both surfaces of the inside tube are clear of dirt or scale.第28页/共46页It was shown in chap.5 that in turbulent flow through conduits three z

14、ones exists. There is a thin sublayer at the wall, a turbulent core occupying most of the cross section of the stream, and a buffer zone.第29页/共46页Basically, the reason for this is that heat must flow through the viscous sublayer by conduction, which call for a steep temperature gradient in most of f

15、luids because of the low thermal conductivity, whereas the rapidly moving eddies in the core are effective in equalizing the temperature in the turbulent zone. 第30页/共46页The overall coefficient is best studied by analyzing it in terms of the separate resistances. The separate resistances can then be

16、combined to form the overall coefficient. 第31页/共46页wdqdAhTT(11-17)Equation(11-17), when applied to the two fluids of , becomes, for the warm side (inside of tube),第32页/共46页oowdqdAhtt(11-25)iiwdqdAhTT(11-24)第33页/共46页1wwiwiiiiT TT Tdqh T T dARhdAfor the warm sidewwwwwmTtTtdqbRdA第34页/共46页1wwowoooottttd

17、qhtt dARh dA111wwwwiioomTTTtttTtdqbUdAhdAh dAdAfor the cold side第35页/共46页111iimoobUdAhdAkdAh dA1iimoodAdAdAUhdAkdAh dAIf both sides of the resulting equation are multiplied by dA P241第36页/共46页ooooiimmdAddAdanddAddAdIf that the surface is arbitrarily based on the outside area dAo11oooiimodbdUhdkdh(11-30)P242第37页/共46页11iiiimoobddUhkdh d(11-31)If that the surface is arbitrarily based on the inside area dAi.第38页/共46页 Sometimes one particular area is more convenient than others.第39页/共46页第40页/共46页111oiobUhkh(11-37)第41页/共46页Sometimes one coefficient, say, ho, is so very