Energy savings in existing plants usually have positive economic and environmental impacts. Mathematical
approaches to heat-exchanger network (HEN) retrofit are complex and do not guarantee identification
of the global optimum. Thanks to its simplicity the pinch-based approach is widely used, even
though difficulties for its adaption to HEN retrofit are encountered. This paper presents the concepts
supporting a new analysis method for HEN retrofit. Energy is conserved and degraded through heat
exchanges and process operations. Reducing heat consumption implies a reduction in the flow rate of
heat transferred from the heating utility through the existing heat exchanger network until rejected to
the environment. This progressive transfer of heat in the existing heat exchanges from the heating utility
to the environment is not explicitly analyzed in the present approaches for HEN retrofit. For the first
time, the set of modifications necessary to reduce the heat consumption is made explicit; this set is
represented by a “bridge”. An energy transfer diagram to identify bridges and a network table to easily
identify and evaluate bridges are proposed. A method to enumerate the bridges is described. The
principle of bridge improves the comprehension about the problem of HEN retrofit, and its application
results in a significant search space reduction; this reduction is all the more so useful since this problem
is non-deterministic polynomial-time hard. A global procedure for HEN retrofit and case studies are
presented in a second paper.