Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONS18 Mixed air temperature is for operatorinformation.19 SA temperature setpoint is reset based uponOA temperature.20-22 Mixing dampers modulate for free cooling.23,24 Mixing box static pressure maintainedconstant during noneconomizer mode bymodulation of the RA damper duringminimum ventilation periods to maintainconstant OA cfm.25-27 Hot water valve, mixing dampers, and chilledwater valve modulated in sequence tomaintain SA temperature setpoint.28,29 SA temperature setpoint switches fromcooling to heating value during warm-upmodes.30 RA temperature determines end of warm-upmode.32 Control program coordinates return fan cfmsetpoint, loading, exhaust fan control, and faninterlock.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 cfm of OA in summer with varyingsupply cfm.4. Return fan cfm varies with supply fan cfm dependentupon 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 -5°F OA SA setpoint the same as the 40°FOA SA setpoint.CONDITIONS FOR SUCCESSFUL OPERATION1. Skilled HVAC technicians required for pressure andvolumetric set-up.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 non-economizer occupied periods the RA dampershall be modulated to maintain a constant mixing box negativestatic pressure.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 cfm. When in the recirculating mode, the return fan cfmsetpoint shall equal the supply fan cfm plus a positive ornegative value for calibration (of flow elements andtransducers) so as to provide a neutral space static pressure.Anytime the OA dampers are not closed completely, thereturn fan cfm setpoint shall equal the supply fan cfm minusa value necessary to maintain a slightly positive (0.05 in.wc) space static pressure. Anytime the exhaust fan operates,the return fan cfm setpoint shall be further reduced byadditional cfm value equal to the exhausted cfm (thisexhaust value shall be determined by observing the spacestatic pressure as the exhaust fan starts and 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 70°F at startup time, the SA temperaturesetpoint shall be 90°F until the RA temperature rises to 74°F, atwhich time the SA temperature setpoint shall be lowered to thecooling 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 55°F to 61°F as the OA temperature drops from 40°Fto -5°F.ENGINEERING MANUAL OF AUTOMATIC CONTROL242