Engineering Manual o.. - HVAC.Amickracing

ELECTRONIC CONTROL FUNDAMENTALSTYPICAL SYSTEM APPLICATIONFigure 22 shows a typical air handling system controlled bytwo electronic controllers, C1 and C2; sequencer S;multicompensator M; temperature sensors T1 through T4;modulating hot and chilled water valves V1 and V2; andoutdoor, return, and exhaust air damper actuators. The controlsequence is as follows:— Controller C1 provides outdoor compensated, summerwintercontrol of space temperature for a heating/coolingsystem which requires PI control with a low limit. SensorT4 provides the compensation signal throughmulticompensator M which allows one outdoortemperature sensor to provide a common input to severalcontrollers. Controller C1 modulates the hot andchilled water valves V1 and V2 in sequence tomaintain space temperature measured by sensor T1 ata preselected setpoint. Sequencer S allows sequencingthe two valve actuators from a single controller. Lowlimitsensor T2 assumes control when the dischargeair temperature drops to the control range of the lowlimit setpoint. A minimum discharge air temperatureis maintained regardless of space temperature.When the outdoor temperature is below the selected resetchangeover point set on C1, the controller is in the wintercompensation mode. As the outdoor air temperature falls,the space temperature setpoint is raised. When the outdoortemperature is above the reset changeover point, thecontroller is in the summer compensation mode. As theoutdoor temperature rises, the space temperature setpointis raised.— Controller C2 provides PI mixed air temperature controlwith economizer operation. When the outdoor temperaturemeasured by sensor T4 is below the setting of theeconomizer startpoint setting, the controller providesproportional control of the dampers to maintain mixedair temperature measured by sensor T3 at the selectedsetpoint. When the outdoor air temperature is above theeconomizer startpoint setting, the controller closes theoutdoor air dampers to a preset minimum.EXHAUSTAIRRETURNAIRT1OUTDOORAIRMIXEDAIRT3SUPPLYFANT2V1V2HOTWATERCHILLEDWATERC2SC1T4MC3086Fig. 22. Typical Application with Electronic Controllers.130ENGINEERING MANUAL OF AUTOMATIC CONTROL