Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSSINGLE-DUCT VAV ATUSystem ConfigurationFigure 2 is a schematic of the equipment shown in the controldiagram Throttling VAV ATU in Figure 3. The elements are:1. Flow control damper.2. Damper actuator, usually electric, bidirectional.3. A digital controller.4. A low voltage control transformer.5. A line voltage power source.6. A digital communications bus for control and BMSfunctions..7. An optional bus connection to the space wall module foreasy access from a portable control terminal8. A wall module.In this pressure dependent application, the damper hasminimum and maximum software control positions. In otherapplications, the controller can have connections to the air flowrings, reheat valve or electric heat contactors, occupancy sensorinput, and fan relays.The wall module contains any combination of temperaturesensor, setpoint adjustment knob, after hour override pushbutton, occupancy status LED, or an LCD display and keypad.PRIMARYAIRDAMPERACTUATORDAMPER213DIGITALCONTROLLERDC24 VOLTPOWERSUPPLYAIRThrottling VAV ATUThe throttling VAV ATU (Fig. 3) is the simplest and leastexpensive ATU. A room controller controls the operation ofthe damper actuator using PID control. The throttling VAVATU usually has software minimum and/or maximum damperposition limits for limiting air volume. Because the unit ispressure dependent, volume at any given damper position varieswith the inlet duct static pressure. Maintaining a stable ductstatic pressure at the end of the duct run is important for properoperation. Proper setting of the minimum-flow limit is essentialfor adequate circulation and is properly set with all boxesoperating at their minimum damper positions. At this operatingpoint, the duct static pressure near the fan will be lower thanat full load. If the minimum damper positions are each setwith other boxes at their maximum damper positions, whenthe load decreases and the pressure drops near the fan,inadequate circulation results at boxes near the fan.Compromises in minimum and maximum cfm control onpressure dependent systems result if the loads vary. Care mustbe exercised in set-up and balancing to assure that all zonesget no less than minimum air flow at all loadings and that allzones demanding maximum air flow get no less than maximumair flow at all loadings.The space temperature setpoint display point is the value setby the occupant on the wall module. With this option, the BMSoperator may assign software limits to the set value. Valuesmay be set outside the limits but the control program ignoresthem and operates at the limits set in software. Another optionfrequently used in open and public spaces is to have no setpointknob on the module and to have a single software setpointdisplayable and commandable from the BMS.PRIMARYAIR4TRANSFORMER74PERCENTOPEN8TLINE VOLTAGE5 POWERDIGITALCOMMUNICATIONS BUSOPTIONAL7ROOM SENSOR/WALL MODULEFig. 2. Single-Duct VAV ATUIn a single-duct VAV ATU, a temperature controller connectedto a damper actuator controls the volume of cool air deliveredto the space. A reheat coil may be added, in which case thecontroller sequences the operation of the damper and the valveor contactor. On a fall in space cooling load, the damper goesto minimum position before any heat is provided. A singleductVAV system may also have a separate perimeter heatingsystem such as a finned-tube radiator or fan coil units to satisfyheating requirements.6M105207524 75 77MINIMUMDAMPERPOSITIONOPENDAMPERPOSITIONCLOSEDSPACETEMPERATUREKNOBSETPOINT SCHEDULEMINIMUMPOSITIONLOWSPACE COOLING LOADCURRENTTEMPERATUREMAXIMUMDAMPERPOSITIONMAXIMUMPOSITIONHIGHM10537Fig. 3. Throttling VAV Air Terminal Unit.ENGINEERING MANUAL OF AUTOMATIC CONTROL 376