Centralized and decentralized control of retrofit heat-exchanger networks

A method for optimal retrofit design of heat-exchanger networks that do not posses control variables in their original structures, is presented. Bypasses, as control variables, are placed in order to satisfy hard/soft target-temperature constraint specifications with minimum increase in exchanger areas. Alternative bypass-placement configurations are compared in terms of static flexibility, cost of retrofit design, and resiliency (Morari Resiliency Index). Pairings among the bypasses and controlled target temperatures are selected for the most favorable configuration according to Relative Gain Array criterion. Final control configuration is tested under centralized and decentralized feedback control schemes. Centralized scheme satisfies hard and soft target-temperature constraints and provides smooth response.