Engineering Manual o.. - HVAC.Amickracing

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONSMinimum outdoor airflow should be maintained at a constantlevel, independent of exhaust fan airflow and changes inbuilding infiltration/exfiltration. Building space pressurizationvaries only when building infiltration/exfiltration changes.Exhaust fan airflow must reset the return fan airflow for buildingpressurization to be independent of exhaust fans. In a controlsequence where the outdoor air damper is closed, the differentialbetween supply airflow and return airflow must be reset to zeroand all exhaust fans should be turned off.Refer to the Air Handling System Control Applicationssection for an example of a VAV AHU WITH RETURN FANAND FLOW TRACKING CONTROL.the same guidelines apply as for return fan control. Duringminimum ventilation cycles and when the outdoor air damperis closed, the relief fan should be turned off and the relief damperclosed. In airflow tracking (Fig. 33), flow measuring stationsshould be located in relief and outdoor air ducts, not in supplyand return ducts as with return fans.RAOARELIEFFANRELIEFDAMPERSUPPLYFANBUILDINGSPACEEXHAUSTFANDuct Static ControlDuct static control is similar to supply fan duct static highlimitcontrol, except return duct static pressure is negative. Ifindividual space returns are damper controlled, return fancontrol must use this technique (Fig. 30). Duct static control isrelatively simple, but individual space return controls make theentire system complex.RETURNFANDUCT STATICPRESSURE SENSORFig. 31. Relief Damper Location.RELIEFFANSTATIC PRESSURECONTROLLERBUILDING SPACESTATIC PRESSUREOUTDOOR STATICPRESSUREC2627C2626Fig. 32. Direct Building Static Control for Relief Fan.CONTROLLERSTATIC PRESSUREREFERENCEC2625RARELIEFFANFig. 30. Duct Static Control.Minimum building outdoor air is the difference betweensupply total airflow and return total plus the exhaust fan totalairflow. If controlling the space returns from airflow tracking(using dampers), the exhaust fan volume must be included inthe tracking control system for constant building and spacepressurization. If controlling the space returns by spacepressurization, building and space pressurization remainconstant regardless of exhaust fan operation.RELIEF FAN CONTROL FOR VAV SYSTEMSRelief fans are exhaust fans for the central air handlingsystem. They relieve excessive building pressurization andprovide return air removal for economizer cycles.In Figure 31, a relief damper is located after the relief fanand is controlled to open fully whenever the relief fan operates.Direct building static pressure or airflow tracking controls therelief fan. In direct building static pressure control (Fig. 32),OAAIRFLOWCONTROLLERFLOWMEASURINGSTATIONSFig. 33. Airflow Tracking Control for Relief Fan.C2628RETURN DAMPER CONTROL FOR VAV SYSTEMSIn systems having a return fan, when the mixed air controlcycle is not operating, the outdoor air (or maximum outdoorair) and relief dampers are closed and the return damper remainsfully open. When the mixed air control cycle is operating, thereturn damper modulates closed and outdoor air (or maximumoutdoor air) and relief dampers modulate open. The returndamper should be sized for twice the pressure drop of theoutdoor air (or maximum outdoor air) and relief dampers. Thisprevents the possibility of drawing outdoor air through the reliefdamper (Fig. 34).ENGINEERING MANUAL OF AUTOMATIC CONTROL264