HVAC Control in the New Millennium.pdf - HVAC.Amickracing

Control Technology, Microelectronics and NanotechnologySupervisory FunctionsThe use of digital computers and their capability to perform complexmathematical calculations and make logical decisions provide theopportunity to improve the performance of any control application. Theneed for supervisory control results from the fact that all processes areaffected by changes. These changes are in the form of disturbances.These disturbances may come from many sources. Examples of disturbancesinclude ambient conditions, heater and heat exchanger foulingand economics. The supervisory control function can also be used offlineas a valuable simulation tool for evaluating what-if scenarios.Alarms are based on models utilizing the present and past historyof the data and the control commands. Anticipating alarm conditions inadvance is a vital function of supervisory control. The supervisory computertypically determines the operating constraints, such as flooding orsurge conditions. Basic material balance, energy balance, or heat transfercalculations are utilized for model creation.The computation of the model is not necessarily a single-path computation.It may require iterative computations. This type of computationis not usually performed at the DCS level.The present operating state is determined based on the on-line,real-time information from the DCS using temperatures, pressures andother characteristics. This determines the desired state according to theconstraints and optimization criteria. An on-line information systemmay include data collection, data reconciliation, data storage and retrieval.The optimum control strategy is based on the on-line, real-timeinformation. The control strategy commands will follow the optimumpath such that the objectives of minimum cost and minimum energy areobtained.Control and OptimizationSupervisory control can include calculation of the present state ofthe process in terms of efficiency and calculation of new conditions, interms of setpoints which will meet the requirements of the objective function.The objective function can maximize efficiency through fuel costs.The tools needed to perform supervisory control are a model andan optimization or maximization algorithm. The model can range from©2001 by The Fairmont Press, Inc. All rights reserved.