Thermal Design Method And Performance Evaluation Of Heat Exchanger

Optimization design and performance evaluation of heat exchanger is of great significance for improving the efficiency of energy utilization.In this paper,thermal design and performance evaluation of heat exchanger is analyzed in theory.The thermal design method is improved and some new performance evaluation indexes are proposed.The theories of entropy and entransy in heat exchanger are discussed.Firstly,give the concept of entransy a re-understanding from the point of engineering.Entransy can be regarded as energy with the weight of temperature which reflects the quality of energy.The 'state quantity' property of entropy and entransy is discussed.To establish the expression of absolute entropy and absolute entransy,the concepts of ground entropy and ground entransy are re-introduced.Then rebuild the expression of entropy generation and entransy dissipation in heat exchanger based on absolute entropy and absolute entransy and point out that flow friction can lead to entransy generation as well as entropy generation.Contrastive analyses are given about the similarities and differences of entropy evaluation and entransy evaluation.Explicate that the varying characteristics of entropy generation and entransy dissipation are intrinsic characteristics of heat exchanger.That is,there is no paradox.The improvement methods of thermodynamic calculation of heat exchanger are put forward.The expression of heat transfer amount and fluid temperature in parallel-flow heat exchanger,counter-flow heat exchanger,phase-change exchanger and cross-flow heat exchanger are obtained with analytic method.Advanced heat transfer effectiveness-heat transfer unit number method is proposed.The ?-NTU curve of counter flow heat exchanger is just the ?-?NTU curve of practical heat exchanger with complex flow.Its meaning is to build the relationship between correction factor and heat trnsfer effectiveness.The curve of correction factor and the curve of heat transfer effectiveness can be verified with each other.The thermal economy of heat exchanger is analyzed.A physical quantity named temperature difference effectiveness' is introduced to reflect the economic performance of heat exchanger,which is defined as a ratio of logarithmic mean temperature difference and inlet temperate difference.Temperature difference effectiveness describes the utilization degree of inlet temperate difference of fluids in heat exchanger.The bigger temperature difference effectiveness means the greater heat transfer amount in per unit number of heat transfer.So the economic performance of heat exchanger is better.The optimation placement principle is discussed.Point out that the essence of uniformity principle of temperature difference field is big value matching principle.This is an economic guiding principle.Compared with heat transfer effectiveness,temperate difference effectiveness matches the uniformity principle of temperature difference field better.Finally,a representative thermal economy case hat is heat exchanger utilizing waste heat in gas-boiler is analyzed.It's shown that thereare optimal heat transfer areas corresponding to greatest net benefit and investment-income ratio,respectively.Besides,both net benefit and investment-income ratio can be expressed as functions of heat transfer effectiveness and temperature difference effectiveness.Heat transfer effectiveness and temperature difference effectiveness can guide the optimization design of heat exchanger together.