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

HVAC Control in the New Millenniumexhaust flow for operator safety. Operating costs are lowered because lessoutside air needs to be conditioned if the exhaust airflow is lowered.In order to maintain the required exhaust air flows accurately, thevacuum in the exhaust air ductwork must be kept at a constant value. Apressure controller keeps the vacuum constant by throttling the exhaustair damper. If these dampers are to remain in control, they must not go tofull open.This is done by identifying the most open damper and then comparingthis opening with the setpoint of the damper position controller. Thiscontroller will limit the opening of the most open damper to 80% and increasethe vacuum in the exhaust air header if the opening exceeds 80%.If the pressure measurement for the controller goes below 9 PSIG(0.6 bar), the vacuum setpoint of the controller is increased (pressuresetting lowered). This increases the operating level of the exhaust fan.Limits on the setpoint of the controller are used to prevent damage, suchas the collapsing of ducts from excessive vacuum.A glycol-circulating heat recovery loop can be used to preheat theentering outside air or as a heat source to a heat pump in the winter.Operating costs are lowered by recovering the heat content of the airbefore it is exhausted in the winter.Pollution ControlThe discharge of chemical vapors into the atmosphere is a pollutionproblem. The usual approach is to remove most of the chemical by adsorptionand scrubbing before exhausting the air. Some additional benefitsare provided by exhausting the air at high velocity. This will increasethe dispersion in the atmosphere. Since the volume of air being exhaustedvaries, an air velocity controller is used to keep the velocity of dischargeconstant. This is done by modulating a variable orifice damper.Intelligent AlarmsRecent studies suggest that U.S. industry loses more than $20 billionper year to preventable process disruptions. Globally, the impact ismuch greater. Abnormal situation management (ASM) focuses on systemsthat provide operator decision support and troubleshooting assistanceduring disruptions. ASM requires the management of largeamounts of data to filter nuisance information, diagnose the cause of the©2001 by The Fairmont Press, Inc. All rights reserved.