chapter5-2PPT學(xué)習(xí)教案

1、會計學(xué)1chapter5-2第1頁/共46頁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頁/共46頁第3頁/共46頁The fluid to be heated is pumped through connection H into channel D2.第4頁/共46頁第5頁/共46頁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頁/共46頁It is called counterflow or countercurrent flow第7頁/共46頁The temperature -length curves

3、for parallel flow are shown in Figure第8頁/共46頁The temperature of the fluid in the tubes increases continuously as the fluid flows through the tubes. 第9頁/共46頁t1t2Temperature CLength of tube mtTemp of condensing vapor TTemp of cool fluid第10頁/共46頁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頁/共46頁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頁/共46頁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頁/共46頁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頁/共46頁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頁/共46頁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頁/共46頁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頁/共46頁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頁/共46頁

9、dqU TtdA(11-9)The quantity U is called the local overall heat-transfer coefficient. 第19頁/共46頁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頁/共46頁(2)the specific heats of the hot and cold fluids are constant;(3)heat exchange with the ambient is negligible;第21頁/共46頁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頁/共46頁21tdtttdqq(11-11)21tdtttU tdAq(11-12)Elimination of dq from Eqs.(11-9) and (11-11) gives第23頁/共46頁212

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

13、 the double-tube exchanger shown inFig11-7. 第27頁/共46頁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頁/共46頁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頁/共46頁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頁/共46頁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頁/共46頁wdqdAhTT(11-17)Equation(11-17), when applied to the two fluids of , becomes, for the warm side (inside of tube),第32頁/共46頁oowdqdAhtt(11-25)iiwdqdAhTT(11-24)第33頁/共46頁1wwiwiiiiT TT Tdqh T T dARhdAfor the warm sidewwwwwmTtTtdqbRdA第34頁/共46頁1wwowoooottttd

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