New analysis method to reduce the industrial energy requirements by heat-exchanger network retrofit: Part 2 – Stepwise and graphical approach

Abstract In the first part of this paper, concepts supporting a new analysis method to reduce energy consumption by heat-exchanger network (HEN) retrofit were presented. These include the definition of necessary conditions to achieve heat savings, which are expressed in the formulation of the bridge concept, the use of the Energy Transfer Diagram (ETD) to identify bridges, and a procedure to enumerate and evaluate these. In this second part, a graphical approach for the identification of exchanger configurations is presented, which can be used for modifications to the HEN to save energy or to reduce investment costs. A stepwise procedure for HEN retrofit is described; it includes identification of bridges, of actual modifications in the network to reduce the energy consumption, and of modifications to reduce the investment costs. Modifications to reduce the investment costs are classified by main categories. Examples are presented to illustrate each category and cover a broad spectrum of situations. The method has been applied for retrofitting the HEN of a kraft pulp mill, and it is found that the proposed approach has directly identified all possible heat-savings projects known in this industry. The method is practical, visual and user-friendly, and its principles can be used in optimization-based approaches.

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