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

Advanced HVAC Control, Information Technology and Open SystemsZone Humidity ControlAt the zone level, the return air humidity is controlled and the dewpoint of the cold deck air supply is modulated. The cold deck supplytemperature is also modulated to follow the load. This is done by measuringthe relative humidity in the return air.The control loop selects the return air sectors with the highest andthe lowest relative humidities. The loop will herd the relative humidityreadings so that the highest and the lowest will be within the controllimits of 35% ±3% RH.The highest reading is used as the controller measurement and thelowest reading as the setpoint. A humidity change in either direction canbe recognized and corrected using this herding technique. The setpointof the relative humidity controller is produced by a function generator.It prevents the most humid return air sector from exceeding 38% relativehumidity and to keep the driest relative humidity sector from droppingbelow 32% RH. The setpoint is held at 35% as long as the driest sectorreads 34% RH or more. If it drops below this value, the setpoint is raisedto the limit of 38% RH in order to reverse this low humidity condition,without allowing excessive humidity in the return air of some otherzone.This control should automatically respond to seasonal changes andprovide control as long as the loads in the various zones are similar. Ifthis is not the case, some zones may be moisture-generating and othersrequire humidification.The low humidity condition will not be controlled and the highhumidity zone will be controlled to prevent it from exceeding 38% relativehumidity. The intermixing of the return air sources will transfermoisture from zones with excess humidity to zones with low humidity.When the difference between the minimum and maximum humidityreaches 8%, an alarm is actuated. This will allow adjustments to bemade.Exhaust Air ControlExhaust air control is done with a two-position damper. When theworkstation is operating the damper is open. A switch indicates when thedamper is open. The minimum damper position should allow enough air©2001 by The Fairmont Press, Inc. All rights reserved.