On-OffControl ofThermalProcesses

Common thermal processes, suchasfluid heaters, heat-treatment plant, boilers, dryers, vats, etc., canconveniently be modeled byfirst-order transit-delayed equations. Theseprocesses arewidely usedinindustry wheretheyareusually discontinuously controlled byon-off oron-half-off controllers. Oftentheheating rates andcooling rates ofsuchprocesses differ significantly. This hinders theevaluation oftheir performance indices, yettheaccurate evaluation orprediction oftheseperformance indices isanimpor- tantstepinthedesign (orselection) ofcontrollers forsuchprocesses. Thispaperpresents anew,yetsimple, analysis ofthemode-depen- dentperformance indices ofthese controlled processes. Newphase- plane displays arepresented thatprovide useful conceptual insights intotheir startup, shutdown, andlimit-cycling performance. This paperisintended fortheprofessional engineer engaged inthedesign andcontrol oftheseprocesses, andthepresentation ispractical rather thanmathematical.