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

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HVAC Control in the New MillenniumReturn air humidity can be controlled by modulating the temperaturesetpoint. Increasing the ratio of humidity-controlled cold deck air inthe zone supply brings the zone closer to the desired 35% RH setpoint.In this control system, a temperature setpoint will increase the colddeck demand which lowers the hot deck and return air demand. A temperaturesetpoint decrease will occur when a return air excess is detectedor the cold air damper in the mixing box is fully open. If the colddeck finger temperature exceeds 70°F (21°C), the temperature setpointwill decrease to keep the cold deck cooler than the hot deck.Plantwide ControlIf the temperature and the humidity of the cold deck supply airare fixed, and if a damper or a valve is fully open or closed, the systemis out of control. A load following control system will automaticallyadjust the main cold deck supply temperature and the humidity.This prevents the valves or dampers to cause lost of control by fullyopening or closing. The result is increased productivity and reducedoperating costs.A plant might consist of several dozen zones. The plant controlsystem has a hierarchical structure (Figure 4-6). The workstation controlsare subject to the zone controls and the plant controls check the operationof the zone controllers.An important action of the levels of the hierarchy is the activecontrol of valve and damper positions. When a control valve or adamper is approaching the point of losing control, (near full or closedposition) the load-following control system at the next higher levelmodifies the air or water supply conditions.The plant control system acts as a flexible set of material balanceand heat/humidity balance controls. Load-following optimization of thesetpoints produces heat balance control. If the material balance requiresan increase in airflow and the heat balance requires a reduction in theheat input to the space, both requirements will be met by admittingmore air at a lower temperature.Material Balance ControlThe plant material balance uses pressure control to modulate variable-volumefans to maintain a minimum supply pressure in the coldand hot decks. The cold deck supply fan uses outside air.©2001 by The Fairmont Press, Inc. All rights reserved.
Advanced HVAC Control, Information Technology and Open SystemsFigure 4-6. Hierarchical control structure.The suction pressure of the hot deck supply fan is an indication ofthe balance between return air availability and hot deck demand. Thisbalance is maintained at the zone level by a controller that must betuned for slow, gradual action.Sudden upsets or emergency conditions are corrected by a reliefdamper if the suction pressure is high (low vacuum) and a makeupdamper if it is low (high vacuum). Between these limits, both dampersare closed and the suction pressure is allowed to float.Heat Balance ControlHeat balance requires multi-variable envelope control since thenumber of controlled variables is greater than the number of availablemanipulated variables. The plant air and water supply temperature set-©2001 by The Fairmont Press, Inc. All rights reserved.
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