Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONS19 SA temperature setpoint is reset based uponOA temperature.20-22 Mixing dampers modulate for free cooling.23,24 Mixing box static pressure maintained constantduring noneconomizer mode by modulation ofthe RA damper during minimum ventilationperiods to maintain constant OA airflow.25-27 Hot water valve, mixing dampers, and chilledwater valve modulated in sequence tomaintain SA temperature setpoint.28,29 SA temperature setpoint switches from coolingto heating value during warm-up modes.30 RA temperature determines end of warm-upmode.32 Control program coordinates return fanairflow setpoint, loading, exhaust fan control,and fan interlock.33 Control program coordinates temperaturecontrol of mixing dampers, control valves,and fan interlock.FEATURES1. The system admits outdoor air for cooling based uponthe economizer program decision.2. Supplies constant temperature variable volume (energyconserving) air to VAV boxes during cooling mode.3. Provides constant airflow of OA in summer with varyingsupply airflow.4. Return fan airflow varies with supply fan airflowdependent upon exhaust and pressurization requirements.5. Provides 100% RA during warm-up periods.6. Perimeter boxes provide heat when required.7. SA temperature setpoint is reset based upon OAtemperature.NOTE: This function has no hardware cost. The setpointparameters are easily adjusted and can be nulled bysetting the -21°C OA SA setpoint the same as the 4°COA SA setpoint.CONDITIONS FOR SUCCESSFUL OPERATION1. Skilled HVAC technicians required for pressure andvolumetric setup.2. Test and balance OA damper minimum position valueand mixing box static pressure setpoint value..3. OA and RA dampers maintained and provided withproper actuators (with positioners if pneumatic).4. Airflow elements and transducers kept clean and calibrated.SPECIFICATIONSSee FAN SYSTEM START-STOP CONTROL.Anytime the supply fan runs, the return fan shall start andthe control system shall be enabled. Also, anytime the supplyfan runs during scheduled occupancy periods, mixing dampersshall position to minimum ventilation position and the exhaustfan shall start.Anytime the economizer mode is enabled, the temperaturecontrols shall be enabled to override the AHU mixing dampersminimum ventilation setpoint for free cooling as required.During noneconomizer occupied periods the RA damper shallbe modulated to maintain a constant mixing box negative staticpressure.The supply fan loading shall be under EPID control with astart value of 20% and ramp duration of 150 seconds. The supplyfan shall load to provide design static pressure at the end of thelongest duct.The return fan loading shall be dependent upon the supplyfan airflow. When in the recirculating mode, the return fanairflow setpoint shall equal the supply fan airflow plus apositive or negative value for calibration (of flow elementsand transducers) so as to provide a neutral space staticpressure. Anytime the OA dampers are not closedcompletely, the return fan airflow setpoint shall equal thesupply fan airflow minus a value necessary to maintain aslightly positive (0.01 Pa) space static pressure. Anytimethe exhaust fan operates, the return fan airflow setpoint shallbe further reduced by additional airflow value equal to theexhaust airflow (this exhaust value shall be determined byobserving the space static pressure as the exhaust fan startsand stops).The SA temperature shall be under EPID control with a startvalue of 33 (at which point the hot and chilled water valves areclosed and the mixing damper override signal is zero) and aramp duration of 120 seconds. The hot and chilled water valvesand the mixing dampers shall be modulated in sequence tomaintain the SA temperature setpoint.Anytime the optimum start perimeter zone space temperaturesensor is less than 21°C at startup time, the SA temperaturesetpoint shall be 32°C until the RA temperature rises to 23.5°C,at which time the SA temperature setpoint shall be lowered tothe cooling SA setpoint. The SA EPID shall be invoked at theswitching of the setpoint to cooling with a start value of 33 anda ramp duration of 180 seconds. The cooling SA setpoint shallrise from 13°C to 16°C as the OA temperature drops from 4°Cto –21°C.ENGINEERING MANUAL OF AUTOMATIC CONTROL259