流体力学与 及传热课件Heat-Exchange Equipment.pptVIP

4.7 Heat-Exchange Equipment ;From material and energy balances, the required heat-transfer rate is calculated. 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. ; 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 pressure drop that can be tolerated in the fluid streams. ;To promote crossflow and raise the average velocity of the shell-side fluid, baffles are installed in the shell.;multipass exchanger;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 ;;In multipass exchangers, floating heads are frequently used.; 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 inherently poor.;Correction of LMTD in multipass exchangers;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 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 Z;And the abscissas are values of the dimensionless ratio η;From the numerical values of Z and η, factor f is read from Figure, and multiplie