1、化工原理化工原理PrinciplesofChemicalIndustryheat-exchange equipment In industrial processes heat energy is transferred by a variety of methods.Heat exchangers are of importance for heat energy transport.There are lots of exchangers.Including conduction-convection in exchangers,boilers,and condensers;radiati
2、on in furnaces and radiant heat dryer.General design of heat-exchange equipment at first,material and energy balances are set up.From these results the required heat-transfer area is calculated.The quantities to be calculated are the overall heat-transfer coefficient,the average temperature differen
3、ce.Shell-and-tube heat exchangersSingle-pass1-1 exchanger The simple double-pipe exchanger is inadequate for high heat-transfer rate.Shell-and-tube construction,such as that shown in figure,where one shell serves for many tubes,is more economical.This exchanger,because it has one shell-side pass and
4、 one tube-pass,is a 1-1 exchanger.Flash p339In an exchanger the shell-side and tube-side heat-transfer coefficients are of comparable importance,and both must be large if a satisfactory overall coefficient is to be attained.The velocity and turbulence of the shell-side liquid are as important as tho
5、se of the tube-side fluid.To promote crossflow and raise the average velocity of the shell-side fluid,baffles are installed in the shell.multipass exchanger The 1-1 exchanger has limitations,because when the tube-side flow is divided evenly among all the tubes,the velocity may be quite low,giving a
6、low heat transfer coefficient.Multipass construction increases the fluid velocity,with a corresponding increase in the heat-transfer coefficient.The disadvantages are that(1)the exchanger is slightly more complicated;(2)Some sections in the exchanger have parallel flow,which limits the temperature a
7、pproach;(3)the friction loss through the equipment is increased because of the larger velocities and multiplication of exit and entrance losses.An even number of tube-side passes are used in multipass exchangers.The shell side may be either single-pass or multipass.In multipass exchangers,floating h
8、eads are frequently used.Flash p3422-4 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.Flash p343 The heat recovery of a 1-2 exchanger is in
9、herently poor.A better recovery can be obtained by adding a longitudinal baffle to give two shell passes.Correction of LMTD in multipass exchangers In 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(11-15),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.Figure 15.6 a and b shows factor f for 1-2 and 2-4 exchangers,respectively.Each cur
11、ved 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.cacbhbhaTTTTZAnd the abscissas are values of the dimensionless ratio cahacacbHTTTTH The factor is the hea
12、ting effectiveness.From the numerical values of Z and,factor F is read from Figure 15.6,and multiplied by the LMTD for countercurrent flow to give the true mean temperature.tm=F LMTDPlate-type exchanger For heat transfer between fluids at low or moderate pressure,below about 20 atm,platetype exchang
13、ers are competitive with shell-and-tube exchangers,especially where corrosion-resistant materials are required Metal plate,usually with corrugated faces,are supported in a frame;hot fluid passes between alternate pairs of plates,exchanging heat with the cold fluid in the adjacent spaces.The plates a
14、re 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,called she
15、ll-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 equipment Difficult heat-exchange problems arise when one of two fluid streams has a much lower heat-transfe
16、r 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 developed in whic
17、h the outside area of tube is multiplied.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,flows through the tubes.When the resistance of the tube wall is neglected,the overall coe
18、fficient can be written in the following form111iiiooUAhh A If 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 Ui increases just as if ho were increased.Provision for thermal expansion Because of the differen
19、ces in temperature existing in condensers,expansion strain may be set up which are sufficiently severe to buckle the tubes or pull them loose from the tube sheets.The most common method of avoiding damage from expansion is the use of the floating-heads 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 U-tube exchanges are used.For this type of exchanger,one side can move freely.
