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

HVAC Control in the New Millenniumthat is defined for a single processing value. They mirror the function ofmicroprocessor-based annunciators by generating messages, annunciatingthe alarm conditions and logging the event with a data and timestamp.The alarm is usually identified as HiHi, High, Low, or LoLo toadvise about the alarm condition urgency. This identification does nottake into account the current control state, such as start-up, normal,purge or shutdown. This results in many nuisance alarms.Discrete alarm systems also tend to point to the symptoms of anabnormal situation instead of the cause. There is no capacity for reasoningabout the state of other alarm points in the database.Expert systems represent an advancement in alarm management.Advanced alarm management concepts include a sensitivity to processstate, reasoning about alarm persistence, meta alarms, expanded alarmpriority and alarm inhibition. They also provide intelligent assistance inmanaging abnormal situations by defining actions for alarm entry, acknowledgment,or exit.Expert systems can provide alarm management strategies that considerdynamic environments where alarm priority is dependent on processstate. For an alarm message to be generated, the entry conditionsmust be true and the process state must also be true. Alarm priority canbe configured to temporarily mask nuisance alarms.If a parent alarm is flagged as the root cause of the problem, itschild alarms, the symptoms of the root cause, are temporarily inhibitedbefore an alarm flood can occur. Parent-child alarm relationships can bedefined based on a fault-tree relationship that identifies the hierarchicalrelationship between alarm points in the database.Alarm inhibition can be applied based on persistence, which is thefrequency of an alarm entry condition within a specified period. Thisprevents an alarm from being generated unless it occurs x times in atime period.When additional information on an alarm is required, a graphicaluser interface (GUI) provides mouse button access to information aboutthe events that caused the alarm or to an on-line manual that providesaccess to control strategies and corrective control procedures. This informationmay be available in HTML or other document formats. Distributedembedded intelligence provides improved operator advice anddiagnostic information.Expert systems go beyond advisory systems and distributed em-©2001 by The Fairmont Press, Inc. All rights reserved.