Engineering Manual o.. - HVAC.Amickracing

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONSRETURN FAN CONTROL FOR VAV SYSTEMSReturn fan operation influences building (space)pressurization and minimum outdoor air. There are fourtechniques to control the return fan: open loop, direct buildingstatic, airflow tracking, and duct static.Open Loop ControlOpen loop control (Fig. 27) modulates the return fan withoutany feedback. This type of control presumes a fixed relationshipbetween the supply and return fans, controls the return fan intandem with the supply fan, and changes the output of the returnfan without measuring the result. Open loop control requiressimilar supply and return fan operating characteristics.Therefore, a careful analysis of the supply and return fanoperating curves should be done before selecting this technique.Also, accurate balancing is essential to ensure proper adjustmentat maximum and minimum operating points. Mechanicallinkage adjustments or other means are used to adjust thedifferential between the two fans for desired flows and tominimize tracking errors at other operating points. With digitalcontrol, software can be used to align the fan loadingrelationships, to vary exhaust effects, and to offset dirty filtereffects to minimize flow mismatches.RETURNFANAIR TERMINALUNITSDUCT STATICPRESSURE SENSORof the building space static pressure sensor should be awayfrom doors opening to the outside, elevator lobbies, and someconfined areas. Usually a hallway on an upper floor is suitable.The outdoor static pressure sensor must avoid wind effects andbe at least 15 feet above the building to avoid static pressureeffects caused by wind. Due to stack effect, this techniqueshould not be used in tall buildings with open spaces or atriums,unless the building has been partitioned vertically. Thistechnique should use proportional plus integral control with awide throttling range for stable, accurate, and responsiveoperation.RETURNFANCONTROLLERBUILDING SPACESTATIC PRESSUREOUTDOOR STATICPRESSUREC2623Fig. 28. Direct Building Static Control.Since building static is controlled directly, the pressureremains constant even when exhaust fan airflow changes.Minimum outdoor airflow varies with changes in exhaust fanairflow and building infiltration/exfiltration. In a controlsequence where the outdoor air damper is closed, the buildingstatic must be reset to zero and all exhaust fans should be turnedoff.Airflow Tracking ControlSUPPLYFANCONTROLLERSTATIC PRESSUREREFERENCEFig. 27. Open Loop Control.C2622Open loop control is often acceptable on small systems moretolerant of minimum outdoor air and building pressurizationvariations. Since open loop control does not sense or controlreturn airflow, changes caused by the return side damperingand exhaust fan exfiltration change the minimum airflow andbuilding pressurization. Systems with low airflow turndownsalso are more suitable for open loop control. As a rule,turndowns should not exceed 50 percent.In airflow tracking (Fig. 29) control, the return fan airflow isreset based on the relationship between supply and exhaust fanairflows. That relationship is usually a fixed difference betweenthe supply total airflow and return plus exhaust total airflow,but it can also be a percentage of supply total airflow. Whenduct layout prevents measuring of total airflow from one flowstation, measurements in multiple ducts are totaled. Thistechnique is usually higher cost, especially when multiple flowstations are required. Airflow tracking control and directbuilding static control are preferable to open loop control.Proportional plus integral control is necessary for accurateoperation.RETURNFANIn a control sequence where the outdoor air damper is closed(e.g., night cycle or morning warm-up), open loop controlshould not be used. Excessive negative pressurization will occurin the duct between the return and supply fans.CONTROLLERFLOWMEASURINGSTATIONSDirect Building Static ControlIn direct building static control, the return fan respondsdirectly to the building space static pressure referenced to thestatic pressure outside of the building (Fig. 28). The locationSUPPLYFANC2624Fig. 29. Airflow Tracking Control for Return Fan.ENGINEERING MANUAL OF AUTOMATIC CONTROL263