On-Off Control of Thermal Processes

Abstract

Common thermal processes, such as fluid heaters, heat-treatment plant, boilers, dryers, vats, etc., can conveniently be modeled by first-order transit-delayed equations. These processes are widely used in industry where they are usually discontinuously controlled by on-off or on-half-off controllers. Often the heating rates and cooling rates of such processes differ significantly. This hinders the evaluation of their performance indices, yet the accurate evaluation or prediction of these performance indices is an important step in the design (or selection) of controllers for such processes. This paper presents a new, yet simple, analysis of the mode-dependent performance indices of these controlled processes. New phaseplane displays are presented that provide useful conceptual insights into their startup, shutdown, and limit-cycling performance. This paper is intended for the professional engineer engaged in the design and control of these processes, and the presentation is practical rather than mathematical.