流體力學與傳熱課件Heat-Exchange Equipment

1、流體力學與傳熱課件流體力學與傳熱課件Heat-Exchange Heat-Exchange EquipmentEquipmentFrom material and energy balances, the required heat-transfer rate is calculated. Then, using the overall coefficient andthe average T, the required heat-transfer area is determined. In simple devices these quantities can be evaluated e

2、asily and with considerable accuracy, but in complex processing units the evaluation may be difficult and subject to considerable uncertainty. The final design is nearly always a compromise, based on engineering judgment, to give the best overall performance in light of the service requirements. Som

3、etimes the design is governed by considerations that have little to do with heat transfer, such as the space available for the equipment or the 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-pa

4、ss 1-1 exchanger In an exchanger the shell-side and tube-side heat-transfer coefficients are of comparable important, and both must be large if 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

5、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 exchanger has limitations, because when the tube-side flow is divided evenly among all thetubes, the velocity may be quite low, giving a lowheat transfer coeffi

6、cient.Multipass construction increases the fluid velocity, with a corresponding increase in the heat-transfer coefficient. The disadvantages for a multipass construction are that (1) the exchanger is slightly more complicated ; (3) the friction loss through the equipment is increased because of the

7、larger velocities and multiplication of exit and entrance losses.(2) Some sections in the exchanger have parallel flow, which limits the temperature approach;In multipass exchangers, floating heads are frequently used.An even number of tube-side passes are used in multipass exchangers. The shell sid

8、e 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 exchanger 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 inh

9、erently poor.A better recovery can be obtained by adding a longitudinal baffle to give two shell passes.Correction of LMTD in multipass exchangersIn multipass exchangers which have more tube passes than shell passes, the flow is countercurrent in some sections and parallel in others.The LMTD, as giv

10、en 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 for countercurrent flow, the product is the true average temperature drop.1221ttTTZ Each curved line in the figure corresponds to a constant value of the

11、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 values of the dimensionless ratio 1112tTttThe factor is the heating effectiveness. From the numerical values of Z and , factor f is read from Fig

12、ure, 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 for 2-4 exchangers, Plate-type exchangerFor heat transfer between fluids at low or moderate pressure, below about 20 atm, platetype exch

13、angers are competitive with shell-and-tube exchangers, especially where corrosion-resistant materials are requiredMetal plate, usually with corrugated faces, aresupported in a frame; hot fluid passes betweenalternate pairs of plates, exchanging heat with the cold fluid in the adjacent spaces. The pl

14、ates are typically 5mm apart.They can be readily separated for cleaning; additional area may be provided simply by adding more plates.Condensers Special heat-transfer devices used to liquefy vapors by removing their latent heats are called condensers.Condensers fall into two classes. In the first, c

15、alled 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 streams are physically mixed.Extended-surface equipmentDifficult heat-exchange problem arise when one of two fluid streams has a much lower h

16、eat-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 be low.Extended surfaces have been d

17、eveloped in which the outside area of tube is multiplied.Types of extended surface: (a) longitudinal fins; (b) transverse fins. The fluid stream having the lower coefficient is brought into contact with the extended surface and flows outside the tubes , while the other fluid, having high coefficient

18、, flows through the tubes.oioiiAAhhU111 When the resistance of the tube wall is neglected, the overall coefficient can be written in the following formIf ho is small, hi large, the value of Ui will be small, but if the area Ao is made much larger than Ai, the resistance Ai/(Aoho) becomes small, and

19、Ui increases just as if ho were increased.Provision for thermal expansionbecause of the differences in temperature existing in condensers, expansion strain may be set up which are sufficiently severe to buckle the tubes or pull them loose from thetube sheets. The most common method of avoiding damag

20、e from expansion is the use of the floating-head construction, in which one of the tube sheets is structurally independent of the shell. Various types of expansion joints are also used to eliminate excessive stresses caused by expansion Several factors must be considered in deciding which fluid to p

21、ut in the tubes and which to put in the shell of a shell-and-tube heat exchanger.Choice of tube-side fluidl If one of the fluids is quite corrosive it should be put in the tubes, which can be made of a corrosion resistant metal or alloy, rather than in the shell which would require that both the she

22、ll and tubes be made of the more expensive material.lIf corrosion is not a problem but one of the fluids is dirty and likely to form deposits on the wall, that fluid should be inside the tubes since it is much easier to clean the inside of the tubes than the outside. lVery hot fluids are placed in t

23、he tubes for reasons of safety and heat economy. lFinally, the decision might be based on which arrangement gives higher overall heat transfer coefficients or lower pressure drop. Very viscous liquids are often placed on the shell side, because flow across the tubes promotes some turbulence and give

24、s better heat transfer than would laminar flow in the blem Multipass construction increases the fluid velocity, with a corresponding increase in ( ) lThe 1-1 exchanger has limitations, because when the tube-side flow is divided evenly among all the tubes, giving a low ( ).lThe disadvantages

25、 of multipass construction are that: ( ) For a multipass exchanger, The LMTD should be ( ) by a factor The methods of avoiding damage from expansion are that When one of two fluid streams has a much lower heat-transfer coefficient than the other, extended surfaces have been developed on the side of ( )problem