05100929582optimaldesignofshell-and-tubeheatexchangers.pdf.doc

Computers chem. Engng, Vol. 18, 5uppl., pp. 5295-5299, 1994Printed in Great Britain. All rights reserved0098-1354/94 $6.00+0.00Copyright © 1993 Pergamon Press LtdOPTIMALD]<~SIGN OF SHELL-AND-TUBE HEAT EXCHANGERSM. REPPICH and J. KOHOUTEKDepartment of Process Engineering, Technical University of Brno,Technicka 2, 616 69 Brno, Czech Republic.AbstractA new method allowing the optimal design of baffled shell-and-tube heat exchangers will be dis•cussed in this paper. Tile known commercial design tools do not consist optimization strategies.There are presented as simplified expressions describing interaction between heat transfer andpressure drop that can he used for optimal design of heat exchangers from economic viewpoint.The developed program system HEATDESIGN is realizing a general computer-supported designon the basis of estimation of optimal pressure drops determined by consideration of economicconditions and process data. Examples show the influence of changed economic parameters orprocess data on the design and costs by means of optimization. In addition the program givesthe possibility to optimize components of apparatus.KeywordsTubular heat exchanger; optimization; pressure drop.1. IntroductionShell-and-tube heat exchangers are still numerously used in petrochemical and energy indus•tries because of their rElatively simple manufacture and their adaptability to different operatingconditions. This heat f:xchauger type distinguishes itself by low pressure drops with high flow velocities.The design of shell-and-tube heat exchangers including thermodynamic and fluid dynamic design,strength calculations, cost estimation and optimization represents a complex process containingau integrated whole of design rules, calculating methods and empirical knowledge of variousfields. At present variol;s commercial programs such as HTRI, HTFS, B-JAC, THERM and CC•Therm are available. These tools allow designing and rating of tubular heat exchangers however,they do not consist any optimization strategies that are needed from industries' point of view.The application of optimization methods, when designing heat exchangers, leads to the mostcost-effective variant of apparatus. The design of heat exchangers requires a knowledge of theallowable pressure drops of the streams that can be fully used. Informations about allowablepressure drops are also required for input data of the above mentioned software packages. Settingthe allowable pressure drop from experience or technical intuition can lead to a final solution farfrom the optimal desiglJ.In this paper a new method for finding optimal allowable pressure drops is presented. The re•sults are interesting even tough the authors used simplified relations that are based on economicparameters andproces~ data with a sufficient accuracy.Pressure drop and heat transfer are interdependent and both of them essentially influence theS295 S296 European Symposium on Computer Aided Process Engineering-3 capital and operating costs of any heat exchange system. It is necessary to suggest such dimen•sions of apparatus that results under given economic conditions from optimal heat transfer andoptimal pressure drop.2. Heat transfer and pressure dropThe pressure drop of the pumped or compressed stream determines the energy consumption.Heat transfer rate and pressure drop are indirectly related. An increase of flow velocities leadsto higher heat transfer coefficients and consequently to lower capital costs due to smaller heattransfer area. At the same time the power requirment and the coupled operating costs are in•creased as a consequence of the higher pressure drops.The final expression for the tubeside pressure drop in a shell-and-tube heat exchanger coveringthe interaction between pressure drop and heat transfer is as follows:Llpi = a. d; . PI . ( d; •Ct~3+b)/m, n,) (3+b)/m, • 3 •(VI) ~ (1)d8 CI Al Pri d; VIaBecause in (1) pressure drops in nozzles and channels were neglected the pressure drop shouldbe increased by 10 per<:ent.In the derivation of the relationship of the shellside pressure drop of baffled shell-and-tube heatexchanger the following assumptions are made:1. The flow velocitiE~s in cro。