Chilled-Water VAV Systems - HVAC.Amickracing

Primary System ComponentsSupply fan capacity modulationFor more information on fanperformance curves, system resistancecurves, and VAV fan modulation, refer tothe Trane Air Conditioning Clinic titled“Air Conditioning Fans” (TRG-TRC013-EN).To accommodate variable airflow in a VAV system, the supply fan must beselected and controlled so that it is capable of modulating over the requiredairflow range.The pressure drop through ducts, fittings, coils, filters, and so forth change asairflow varies. The capacity of the supply fan must be modulated to generatesufficient static pressure to offset these pressure losses, and provide theminimum pressure required for proper operation of the VAV terminal unitsand supply-air diffusers at all airflows.To achieve this balance, a simple control loop is used (Figure 30). A pressuresensor measures the static pressure at a particular location in the ductsystem. A controller compares this static pressure reading to a setpoint, andthe supply fan capacity is modulated to generate enough static pressure tomaintain the desired pressure setpoint at the location of the sensor.Figure 30. Supply fan capacity controlsupplyfanstatic-pressuresensorcontrollerFigure 31 depicts an exaggerated example to illustrate this control loop. Asthe cooling loads in the zones decrease, the dampers in all or most of the VAVterminal units modulate toward a closed position. This added restrictionincreases the pressure drop through the system, reducing supply airflow andcausing the (part-load) system resistance curve to shift upwards.Figure 31. VAV system modulation curvepart-load systemresistance curvefull-load systemresistance curvestatic pressuresensorsetpointVAV systemmodulation curveB800 rpmA1,000 rpmairflow38 Chilled-Water VAV Systems SYS-APM008-EN