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

HVAC Control in the New Millenniumbe high, a setting of about 0.02 inches H 2 O can be maintained.At these near-atmospheric pressures, the air behaves as if it wereincompressible and pressure control is fast and stable. When the loop isenergized, the pressure controller moves the return air control damperuntil the preset differential is reached. Then, the electric motor stops andthe damper stays at its last opening. This position is not changed as longas the following conditions apply:Return Airflow= Supply Airflow– (Exhaust Airflow + Pressurization Loss)When this airflow balance is affected by a change in airflow orpressure, it causes a change in the space pressure and the return aircontrol damper responds by modifying the opening of the return aircontrol damper.DraftsProductivity and energy costs are maximized if drafts are minimalin the work area. Drafts can stir up the dust in this area, could contaminatethe product and reduce production. In order to eliminate drafts, thepressure at each of the work areas must be at the same value. This eliminatesany pressure differential between areas and prevents drafts.A pressure controller at each workstation will result in a uniformpressure profile throughout the area. All areas should be at a positivepressure.The local fans will cause a lower pressure where they draw air in.These localized vacuum zones could cause contamination by allowingair infiltration. Pressure control of the vacuum zones could cause draftsfrom the pressure gradients.One technique involves raising the setpoint of the pressure controlleruntil all areas are at a positive pressure. The result depends on thequality of building construction which might not be high enough toallow operation at the higher space pressures. The pressurization loss inbadly sealed buildings could make it difficult to reach the higher spacepressure.Another technique is to use a second pressure controller, whichwould throttle a damper in the supply air duct. This would provide thecontrol needed but would also increase the cost of the control system by©2001 by The Fairmont Press, Inc. All rights reserved.