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1、4.7 Heat-Exchange Equipment In industrial processes heat energy is transferred by a variety of methods.Including conduction-convection in exchangers, boilers, and condensers; radiation in furnaces and radiant heat dryer.From material and energy balances, the required heat-transfer rate is calculated
2、. Then, using the overall coefficient and the average T, the required heat-transfer area is determined. In simple devices these quantities can be evaluated easily and with considerable accuracy, but in complex processing units the evaluation may be difficult and subject to considerable uncertainty.
3、The final design is nearly always a compromise, based on engineering judgment, to give the best overall performance in light of the service requirements. Sometimes the design is governed by considerations that have little to do with heat transfer, such as the space available for the equipment or the
4、 pressure drop that can be tolerated in the fluid streams. This exchanger, because it has one shell-side pass and one tube-pass, is a 1-1 exchanger.Single-pass 1-1 exchangerIn an exchanger the shell-side and tube-side heat-transfer coefficients are of comparable important, and both must be large if
5、a satisfactory overall coefficient is to be attained. To promote crossflow and raise the average velocity of the shell-side fluid, baffles are installed in the shell.The velocity and turbulence of the shell-side liquid are as important as those of the tube- side fluid. multipass exchangerThe 1-1 exc
6、hanger has limitations, because when the tube-side flow is divided evenly among all the tubes, the velocity may be quite low, giving a low heat transfer coefficient.Multipass construction increases the fluid velocity, with a corresponding increase in the heat-transfer coefficient.The disadvantages f
7、or a multipass construction are that (1) the exchanger is slightly more complicated ;(3) the friction loss through the equipment is increased because of the larger velocities and multiplication of exit and entrance losses.(2) Some sections in the exchanger have parallel flow, which limits the temper
8、ature approach;In multipass exchangers, floating heads are frequently used.An even number of tube-side passes are used in multipass exchangers. The shell side may be either single-pass or multipass.2-4 exchanger The 1-2 exchanger has an important limitation. Because of the parallel-flow pass, the ex
9、changer is unable to bring the exit temperature of one fluid very near to the entrance temperature of the other.The heat recovery of a 1-2 exchanger is inherently poor.A better recovery can be obtained by adding a longitudinal baffle to give two shell passes.Correction of LMTD in multipass exchanger
10、sIn multipass exchangers which have more tube passes than shell passes, the flow is countercurrent in some sections and parallel in others.The LMTD, as given by Eq 5.4-27 does not apply in this case, and it is customary to define a correction factor f. The correction factor is multiplied by the LMTD
11、 for countercurrent flow, the product is the true average temperature drop.Each curved line in the figure corresponds to a constant value of the dimensionless ratio ZThe factor Z is the ratio of the fall temperature of the hot fluid to the rise in temperature of the cold fluid.And the abscissas are
12、values of the dimensionless ratio The factor is the heating effectiveness. From the numerical values of Z and , factor f is read from Figure, and multiplied by the LMTD for countercurrent flow to give the true mean temperaturetm = f LMTDFigure shows factor f for 1-2 exchangers, Figure shows factor f
13、 for 2-4 exchangers, Plate-type exchangerFor heat transfer between fluids at low or moderate pressure, below about 20 atm, platetype exchangers are competitive with shell-and-tube exchangers, especially where corrosion-resistant materials are requiredMetal plate, usually with corrugated faces, are s
14、upported in a frame; hot fluid passes between alternate pairs of plates, exchanging heat with the cold fluid in the adjacent spaces. The plates are typically 5mm apart.They can be readily separated for cleaning; additional area may be provided simply by adding more plates.Condensers Special heat-tra
15、nsfer devices used to liquefy vapors by removing their latent heats are called condensers.Condensers fall into two classes. In the first, called shell-and-tube condenser, the condensing vapor and coolant are separated by a tube wall. In the second, called contact condensers, the coolant and vapor st
16、reams are physically mixed.Extended-surface equipmentDifficult heat-exchange problem arise when one of two fluid streams has a much lower heat- transfer coefficient than the other.A typical case is heating a fixed gas, such as air, by means of condensing steam. The individual coefficient for the steam is typically 100 to 200 times that for the air. The capacity of a unit area of heating surface will
