Chapter 198 - A thermodynamic optimization of counterflow recuperative-type heat exchangers

Abstract

This chapter conducts thermodynamic optimization of a steady heat transfer process with constant specific-heat fluids in a counterflow recuperative-type heat exchanger. The efficiency of these heat exchangers often directly affects the performance of the cryogenic systems. The irreversibility of these heat exchangers can be minimized by thermodynamic optimization of them. The thermodynamic optimization of a heat exchanger could be defined as the minimization of irreversibility of heat transfer processes through modifying the thermodynamic properties of fluids when specifying heat exchanger structure and total heat transfer rate. For a heat exchanger, the heat transfer area is always finite, which means the mean heat transfer temperature difference is impossible to be zero for a specified total heat transfer rate. Consequently, there must exist a definite difference between the mean thermodynamic temperatures of heat transfer fluids, which results in an existence of certain exergy loss in heat transfer process. The analytical results indicate that the ultimate reason of power saving contributed from thermodynamic optimization is the decrease of the difference of mean thermodynamic temperature between two fluids, even though there is no reduction of the log-mean heat transfer temperature difference between two fluids. In other words, the optimum configuration of heat transfer temperature difference does not need to keep the local temperature difference constant throughout the heat exchanger.