Constant approach temperature model for HEN retrofit

This paper proposes a solution method based upon mathematical programming f or Heat Exchanger Network (HEN) retrofit. This is a two-step approach. The first step uses a Constant Approach Temperature (CAT) model to optimize the structure of the final HEN. The CAT model simultaneously takes into accoun t the cost of utilities, structural modifications and heat transfer areas b y assuming the approach temperatures of all heat transfers inside the HEN t o be a constant. The main advantage of this assumption is that area calcula tions are linearized, therefore, the model could be solved as a Mixed Integ er Linear (MILP) problem. This shortens the solution time and removes the p ossibility of being trapped at local optimums. The CAT model does not guara ntee feasible solutions; however, it determines a good network structure an d drives the solution very close to the global optimum. Starting with this network structure, a Mixed Integer Nonlinear (MINLP) model is then used in the second step, which takes into account the actual approach temperatures, to finalize the design. Example problems from literature are used to demon strate the effectiveness of the approach in terms of the solution quality a nd time. (C) 2000 Elsevier Science Ltd. All rights reserved.