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

HVAC Control in the New Millenniumeconomizer sequence.Ventilation is a primary consideration and the control will ensurea minimum percent of air flow in fresh air. The outdoor air ventilationand economizer sequences should be coordinated to mix the return airfrom the building with fresh air from the outdoors.Damper ControlIn the larger VAV air handlers a set of sequences is used to modulatethe outdoor and return air dampers to maintain a mixed air temperaturesetpoint. In most VAV air handler applications the control ofoutdoor air dampers is integrated with the cooling sequence. This sequenceprovides a higher percentage of fresh air based on another input,the outside air temperature. An outdoor temperature below 50°F, orhigher depending on the climate, may be possible to satisfy the buildingsfull need for cooling through the economizer.Economizers are usually sized to allow 100% fresh air under optimumconditions. Even more air may be used in very humid climates.The economizer may be required to consider enthalpy, rather than temperaturealone, to make a decision about modulating the damper toprovide free cooling.Duct pressure control is another major segment of the control sequence.This application should improve efficiency and comfort byvarying the air flow to the space based upon demand. An increase indemand at the zone may cause a damper to open thus changing the ductstatic pressure.An increase in demand from multiple zones will drop the staticpressure below a setpoint and require that the VAV air handler fanvolume be increased. This increase is accomplished with a control loopthat monitors the duct pressure and modulates fan motor speed ordampers to increase the air flow.The HVAC control should integrate temperature and pressure control.These sequences are coordinated to ensure that a constant temperatureof air is delivered with sufficient volume to meet the space conditioningneeds.Occupied and unoccupied mode control along with optimizationwill ensure building comfort at the lowest possible operating cost. Timeof day schedules and optimized start/stop should be used in all build-©2001 by The Fairmont Press, Inc. All rights reserved.