Heat flow diagram as an extension of bridge retrofit method to save energy in heat exchanger networks

Abstract

Retrofitting or reducing heat consumption in existing Heat Exchanger Networks can be done by set of modifications which is termed “bridge”. A bridge links heat outlets to hot utility users. The most complicated step in identification of the economical bridges, is the determination of the optimum routes. The “Energy Transfer Diagram” and a few modifications on bridge method are among the recent efforts for the better identification of the bridges. Though the internal exchangers in the initial network participate as the “transferring interface or chain rings along the bridges”, the role and the characteristics of their participation has not dealt with clearly and quantitatively in any of previous works. This paper presents “Heat Flow Diagram” as a new graphical tool and also an analyzing algorithm, to improve bridges identification in retrofit analysis method. The role of internal exchangers as the transferring interface and the limiting factors affects its transferring capacity is conceptually and quantitatively analyzed for the first time. Subsequently several illustrative examples ‘covering different situations and clarifying concepts of new retrofitting method’ were illustrated. Heat integration of a Methanol plant analyzed with three different method to compare with our new suggestion; resulted in 18.8%, 79.2% and 98.9% heat recovery.