Synthesis of flexible heat exchanger networks—III. Temperature and flowrate variations

Abstract A general algorithmic approach to the synthesis of flexible heat exchanger networks which explicitly accounts for prespecified uncertainties in both supply temperatures and flowrates is presented. The synthesis procedure includes four major steps: (1) an LP transshipment problem is developed to discover a usually small discrete set of dominant pinch points controlling the network design; (2) knowledge of the dominant pinch set permits to derive an MILP formulation that provides an optimal network at the level of units. An iterative scheme is not necessary because the proposed mathematical modelling includes explicit flexibility constraints; (3) the set of design conditions for the synthesis of the whole network at the level of structure is established through the use of an MILP pinch-“jump“ test. Appropriate design points, usually a single one, are assigned to each subnetwork; and (4) the structure of every subnetwork is sequentially found by often using a synthesis technique for fixed stream conditions (Floudas et al. , AIChE Jl 32 , 276; 1986). Selection of heat exchanger areas is not addressed in this paper. Four examples involving temperature and flowrate disturbances have been successfully solved in a short computer time. No deterioration in the computational efficiency of the algorithm has been observed as the number of uncertainties increases.

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