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

Energy and Power Management, Distributed Control Trendsmonitor the condition of a contact closure or an analog value. Basedupon a change in state of the digital point, or an analog point exceedinga set of range limits, an alarm is initiated.Safety Sequences and DiagnosticsSafety sequences are used to produce these alarms. These controlsequences are implemented to protect the HVAC equipment as well asthe building and occupants. The trend is toward more safety sequences.In a VAV air handler these may include duct high pressure, compressorpressure, freeze and low discharge air temperature alarms.Diagnostics are program sequences that are used to identify problems.They are used to interrogate the system and diagnose problems byreviewing such parameters as temperature or pressure. Diagnostics canalso be self-test routines. The use of safety and diagnostic sequences canoptimize controller operations and reduce downtime.Diagnostics and preventive maintenance are related, but preventivemaintenance sequences are used to increase equipment efficiency,extend equipment life and provide operation with a minimum of downtime.They may collect data on run hours of the equipment and usealarms to flag problems that may require immediate attention. Otherprogram sequences may be used for indoor air quality, fire/smokeevacuation and demand limit control.Indoor Air QualityThese program sequences may introduce additional fresh air tomaintain the parts per million of carbon dioxide (CO 2 ) or reduce theamount of fresh air introduced to the building if contaminants are detectedoutside the fresh air intake. A sensor may be located near theoutdoor air intakes to monitor for carbon monoxide, or other contaminants.The fresh air intake will be stopped if these are detected.Zone control may include sensors to monitor the carbon dioxide.The air handler will add fresh air as required to purge the space andreduce the concentration of CO 2 . Other control functions may involveexhaust or return fans and dampers.©2001 by The Fairmont Press, Inc. All rights reserved.