The application is presented of an algebraic type controller that is Wiener-Hopf optimal. The design method is applied to a double-effect evaporator. The controller directly tackles the intrinsic problem of process interaction, load disturbance, plant saturation and measurements noise: whether the plant is stable or unstable, an exact optimal controller is attainable. The only limitation to this is the stability of the computation algorithm. Software algorithms and computer programs have been developed to facilitate the computation of the polynomial matrix algebra involved in finding the controller.