A New Approach to Controlling Thermal Processes

Roots and Wu (1967) established that meaningful models of common thermal processes (boilers without superheaters, furnaces, ovens, vats, kilns etc.) can be made from a cascade comprising an open-loop gain μ, a transit delay L, and a salient time constant T. They used this model to establish facile procedures for stability determination when such processes were closed-loop controlled. A new procedure is now presented that not only facilitates stability studies but also greatly simplifies transient response determination for all commands and disturbances likely to be encountered by such closed loop controlled processes. This new approach is based on a generalised parameter v that incorporates μ, L and T. Then by means of a new plane, the w plane, displays are presented that readily predict the stability criteria and the transient response for any practical combination of command and disturbance; as is shown by the examples contained in the Appendix. This has radically simplified the control amd instrumentation of the processes with which the authors are associated (induction furnaces, fluidised beds, plasma torches, zone refining, etc.) and the presentation is intended for industrial engineers concerned with the design and control of similar thermal processes.