Dynamics optimal control of flexible heat-exchanger networks

Abstract

In this work, the dynamics, controllabilitty and resiliency of Heat-Exchanger Networks (HENs) are investigated. A rigorous distributed-parameter model of typical countercurrent shell-and-tuble (multi-tube) heat exchangers constituting the HENs to be studied was develoed. The advanced numerical-solution algorithm eliminated the steady-state offset problem mentioned in literature. Two centralized optimal-control algorithms were developed and tested for various HENs, for different sets of disturbances in source-stream temperatures, and for different control-range constraints on target-stream-temperatures. In both of the algorithms, first, the values of the optimal bypass openings are determined by an optimizer which satisfies the control-precision constraints imposed on the target streams by referring to the algebraic HEN model. In centralized open-loop control algorithm, the bypasses were opened from their nominal values up to their optimal values as a function of time. The use of ramp functions resulted in very satisfactory dynamic response of the HENs, and temporal violation of the control-range constraints were prevented by optimal tuning of the rate with which the bypasses were opened. In the centralized closed-loop control algorithm, bypasses were opened as dependent on the pseudo-controlled target-stream temperatures. The use of state feedback resulted in smoother dynamic response for a sample HEN. Overall, both of the proposed centralized model-based optimal-control algorithms proved to be promising in control of HENs.