Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONSMIXED AIR CONTROL WITH ECONOMIZER CYCLE (VENTILATION SYSTEM ONLY)Functional DescriptionEANCRA6833ON2NORMAL99OANCNOMA83PERCENTOPEN5ON121ONCONTROLPROGRAM47131310ECONOMIZER DECISION.REFER TO PREVIOUSECONOMIZER OPTIONS11822OA MINIMUM SETPOINT(NOTE: THE TEST AND BALANCE INITIALVALUE FOR PROPER VENTILATION IS 22)M15157ItemNo.Function1-3 Control system energizes when fan is turnedon (See FAN SYSTEM START-STOPCONTROL).4 MA temperature maintained by modulatingmixing dampers.5,6 OA and EA dampers close and RA damperopens when fan is off and modulate fortemperature control when fan is on.7 Setpoint for MA temperature control.8 Setpoint value for minimum ventilationdamper position.9 Determines when OA is suitable to assist incooling.10 Control program coordinates MA, minimumventilation, and economizer control of mixingdampers.FEATURES1. The proper proportions of OA and RA are admitted tomaintain the desired MA temperature during economizeroperation periods.2. A minimum quantity of OA, determined by the softwareadjustable setpoint value, is assured at all times.3. The OA economizer changeover program returns the OAdamper to the minimum position when OA is not suitable.CONDITIONS FOR SUCCESSFUL OPERATION1. Adequate mixing of OA and RA. Mixing may be obtainedusing a special mixing chamber. The temperature sensorshould be in the fan discharge when possible. The fan ina blow-through system usually provides adequate mixing.2. An MA averaging element sensor is used on drawthroughunits.LIMITATIONSIf the manual positioning value is set to admit a large quantityof OA and the OA temperature falls below the temperature atwhich only minimum OA is required for MA temperature control,a source of heat is necessary to maintain the MA temperature.SPECIFICATIONSSee FAN SYSTEM START-STOP CONTROL.Anytime the supply fan runs, the OA, exhaust, and RAdampers shall position to a minimum ventilation position andshall be further modulated by an MA PID control loop tomaintain the MA temperature setpoint. Anytime the OAconditions rise above the economizer setpoint, the MAtemperature control shall be disabled.220ENGINEERING MANUAL OF AUTOMATIC CONTROL