Comparative Analysis of Exergy Losses in Heat Transfer Processes in Heat Recoverers of Boiler Plants

The chapter is devoted to research into the exergy efficiency of heat exchangers of various types for heating boiler plants. Exergy efficiency, along with energy efficiency, is an important indicator of the thermodynamic perfection of heat recovery systems. Exergy loss indicators were used as a criterion for exergy efficiency. The results of a comparative analysis of exergy losses in heat transfer processes through flat and cylindrical walls for surface air-heating heat exchangers are presented. Heat exchangers of plate and smooth-tube types of heat recovery systems for flue gases of boiler plants were studied. Three types of exergy losses are considered: losses in the processes of thermal conductivity, heat transfer from flue gases to the wall, and heat transfer from the wall to the air. For research, a comprehensive methodology for calculating exergy losses based on the exergy-technological approach has been developed. The technique is based on the use of differential equations of exergy balances, as well as equations of the theory of heat transfer as applied to heat recovery systems. Equations for calculating exergy losses were obtained and corresponding calculations were carried out for the heat exchangers under study. A comparative analysis of exergy losses under various operating modes of boilers during the heating period was carried out. The patterns and nature of changes in exergy losses have been revealed. It has been established that for all types of exergy losses, as well as the values of the heat exchanger’s heat output and ambient temperature, the exergy efficiency of a plate heat exchanger is higher than the exergy efficiency of a smooth-tube heat exchanger. In this case, the smallest exergy losses occur during thermal conductivity and heat transfer from the flue gases to the wall, and the greatest—during heat transfer from the wall to the air. The directions for increasing the exergy efficiency of heat recovery systems have been determined, which consist in the need to reduce exergy losses during heat transfer from the wall to the air and the maximum use of plate-type heat exchangers.