PRESSURE DEPENDENT AND PRESSURE INDEPENDENT …

1 PRESSURE DEPENDENT AND PRESSURE INDEPENDENT CONTROLSSECTION purpose of variable air volume (VAV) control is toregulate the air flow to a specific zone to maintain thedry bulb temperature for a desired level of occupantcomfort. There are variations of this concept thatinclude reheat, dual duct and fan induction is a misconceived trend to oversize terminal unitsin favor of reducing PRESSURE drops and noise fallacy has been proven wrong, not only in numer-ous publications, but over and over again in installationswhere this concept has been the basis of design. Oversizing of terminal units is not key design factor in a VAV system is the control ofthe supplied air.

2 The terminal unit should be sized totake advantage of as much of its potential capacity aspossible. Without this fundamental consideration, thecustomer has not received the best value for their VAVsystem DEPENDENT CONTROL Pressuredependent control represents the basic control of a VAVterminal unit. PRESSURE DEPENDENT controls require fieldcalibration of air flows according to job system conditions. The terminal unit damper is positioned froma signal sent directly from the zone thermostat to thedamper actuator without regard for system resulting flow at any given moment through the unitis very DEPENDENT upon the system conditions at thatmoment.

3 In a PRESSURE DEPENDENT system, two zoneson the same system are effected by the control of Room A and Room B are two conference rooms. Bothare supplied by different VAV terminal units from thesame central air supply. Assume that the initial condi-tion has both rooms full of people which establishes aspecific cooling load. The room thermostat is satisfied atthe current air flow level:If the people in room A were to disperse and leave theroom, the cooling load would decrease and the thermo-stat would respond by sending a signal to the VAV terminal unit damper actuator to close down the damperand reduce the flow.

4 The result would be an increase inthe system static PRESSURE which would increase theamount of air flow to room B. Room B would then startto over-cool until the thermostat sensed this drop inroom temperature and reposition the terminal unitdamper from room B to reduce the air INDEPENDENT CONTROL Pressureindependent control involves an additional input to thecontrol of the VAV terminal unit. The PRESSURE INDEPENDENT control options also monitor and respondto the velocity of the air flow, generally at the inlet of terminal unit damper is positioned from a signalsent from the zone thermostat through a velocity resetcontroller to the damper actuator.

5 The velocity resetcontroller then responds to changes in the inlet PRESSURE conditions to maintain the required air INDEPENDENT controls are frequently used forsingle duct variable volume control. For a given thermo-stat setting the controller can position the damper further open if the air flow at the inlet is insufficient tomeet the requirement or it can position the damper further closed if the inlet air flow is greater than in the other example If the people in room A were to disperse and leave theroom, the cooling load would decrease and the thermo-stat would respond by sending a signal to the VAV terminal unit damper actuator to close down the damperand reduce the air flow.

6 The result would be an increasein the system static PRESSURE which would increase theamount of air flow to the terminal unit serving room terminal unit controller would immediately sensethe increased air flow through the inlet sensor and beginto reposition the damper to maintain the required and Electronic PRESSURE INDEPENDENT controllers do have their limitations. Selection of airflows must be given careful consideration. The resetcontrollers respond to an input signal from the differen-tial inlet sensor in the range of WC to WCwhich reflects the CFM range of a given terminal unitsize.

7 Both pneumatic and electronic controllers cannotaccurately control the air flow when the differential pres-sure signal falls much below WC. For this reason,Carnes publishes minimum air flow setting limitationsand suggested air flow ranges for each unit design consideration when using pressureindependent controls is the state of the central the central system is shut down or not supplyingadequate air to meet the design requirements, the primary flow control damper can drive open looking tosatisfy a minimum air flow condition as long as the controls remain active. This feature can be beneficial byproviding open dampers at the start of the morningwarm-up cycle.

8 However, it may not be desirable insome fan terminal unit applications. Recirculated aircould short circuit and flow back , the terminal unit damper can be configured to fail in the open or closed position on a lossof main control air PRESSURE irregardless of the thermostat action required. Electrically or electronicallythe damper must be powered and driven to either ofthose Unit CONTROLS| PRESSURE IndependentThe ET control option consists of a stand alone, PRESSURE INDEPENDENT VAV velocity controller with an integral direct coupled damper actuator. Thiscombination device utilizes an on board air flow sensor which has platinum resistance temperaturedetectors.

9 Air flow is sensed by the differential PRESSURE (velocity) sensor located at the inlet of theair terminal unit. The ET control option is capable ofcontrolling a velocity set point from 365 to 3000 fpmwith an accuracy of 3%. Units may be ordered with azero (full shut off) minimum setting although this isnot recommended for units with reheat capability. Forminimum and maximum CFM range by unit size referto the selection procedure for each ET controller/actuator combination is used withthe AXWCA, AXWCB, and AXWCC series room thermostats. The minimum and maximum air flowsettings are field calibrated on the correspondingAXW wall thermostat.

10 The integral actuator hasadjustable end stops, magnetic clutch and a gear dis-engagement button to allow manual positioning ofthe damper. A tri-color LED indicates green for open-ing, red for closing, and white for satisfied damperpositions. Units with ET control option are shippedwith the damper in the open ET CONTROL AVAILABILITYThe table below indicates unit types that are available with the Carnes ET Electronic pressureindependent controls. Component descriptions,wiring diagrams and control sequences are shownon the following pages. Contact factory for applications not ET ANALOG ELECTRONIC VAV CONTROLET CONTROLLER/ACTUATORAXW ROOM THERMOSTATET CONTROL OPTION DESCRIPTIONSINGLE DUCT UNITSDUALDUCTRETROFITFAN UNITSAVCAVWAVEABBABWADCDADCCARRASACPRESS URE INDEPENDENT ELECTRONIC ANALOG VAV CONTROL, WITH INTEGRAL DAMPER ACTUATORXXXXXXXXELECTRONIC CONTROLS| SpecificationsTerminal Unit ET ANALOG ELECTRONIC VAV CONTROLLERWITH INTEGRAL ACTUATORA ngular Rotation0 to 95 , both end stops adjustableStroke Time18 /minute @ 60 /minute @ 50 to 1/2 (13mm) diametershaft or 3/8 (10mm) diameterwith adaptorMaterialFlame retardant polymer; UL94-5V plenum rated.