Optimal Control of Induced Draft Cooling Tower using Mixed Integer Programming

We address the problem of optimal operation of an induced draft cooling tower (IDCT) which can be operated in different modes (bypass, showering, ventilation) to meet the heat rejection requirement. Typically, the control design strategies focus on modulating the cooling tower fan speed to regulate the return water temperature without explicitly taking into account different operational modes. For a large scale industrial IDCT, the cooling and lubrication requirements for the mechanical assembly impede slow fan speeds, resulting in mode selection to become pivotal in the optimal operation of the IDCT. In this paper, we propose a control strategy which can account for different operational modes to ensure the optimal operation of the IDCT. A continuous time switched system representation is adopted to capture different operational modes, which is then used to formulate an optimal control problem (OCP) based on the objective of regulating the return water temperature while respecting the physical and operational constraints associated with different operational modes. The OCP is cast as a mixed integer program (MIP) to simultaneously handle the mode selection and the optimal fan speed required to meet the heat rejection requirement. The MIP is solved in a receding horizon fashion providing robustness against disturbances and model mismatch. The efficacy of the proposed control strategy is demonstrated on an experimentally validated model of the IDCT.