Optimization of operating parameters a heat-recovery exchanger of a boiler plant based on the exergy approach

Abstract

The results of operating parameters optimization of the air-heating heat-recovery exchanger of complex heat recovery system of a gas-fired boiler designed for heating water and blown air are presented. Air heating in this heat-recovery exchanger is realized by deep cooling of the waste exhaust gases, that is, with a change in their moisture content during the heat recovery process. The possibilities of using a complex technique based on the structural-variant method and exergy analysis methods for the optimization of the heat-recovery exchanger are analyzed. The developed structural scheme of the boiler plant with identification of input and output exergy streams for all elements of the installation is presented. The change of exergy losses in this heat-recovery exchanger has a rather strong effect on the change of the whole heat recovery system efficiency is established. Thus, the optimization of heat-recovery exchanger operating parameters of is a necessary condition for increasing the efficiency of heat recovery in general. The choice of multiplicative exergy efficiency criteria used as target functions of operating parameters optimization of the investigated air-heating heat exchanger is substantiated. The obtained dependences of exergy efficiency criteria on the operating parameters of the heat-recovery exchanger, such as the ratio of the Reynolds numbers of exhaust gases and air and the ratio of the initial and final moisture content of exhaust gases, are analyzed. It is established that the minimum values of the efficiency criteria, which corresponds to the maximum exergy efficiency, is observed in the range of values of the ratio of the initial and final moisture content of exhaust gases in the range from 2.4 to 3.0. It is shown that at a value of the specified ratio of 2.7, the exergy efficiency of the investigated heat-recovery exchanger does not depend on the ratio of the Reynolds numbers of exhaust gases and air. It is established that of initial and final moisture content ratio of exhaust gases, equal to 2.7, and the Reynolds numbers ratio of exhaust gases and air, equal to 0.8 and 1.2, depending on the values of initial and final moisture content ratio of exhaust gases, can be taken as the optimal values of the operating parameters. Key words: heat-recovery exchangers, exergy efficiency, complex techniques