Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSCONVECTIONAIRDISCHARGEAIRHEATINGCOILVALVETHERMOSTATHEATINGCOILLOW-LIMITTEMPERATURECONTROLLERFANFANINTERLOCKFANVALVETHERMOSTATFANINTERLOCKOA, RADAMPERSOUTDOORAIRCLOSEDFILTEROPENDAMPERACTUATORRETURNAIROA, RADAMPERSOUTDOORAIRFILTERDAMPERACTUATORRETURNAIRC3040Fig. 23. Unit Ventilator in Standby/Warmup Stage.CYCLE I–FIXED MAXIMUM PERCENTAGE OF OUTDOOR AIRWhen the space is warmed to the low end of the thermostatthrottling range, Cycle I begins. As shown in Figure 24, controlcomponents are the room thermostat, heating coil valve, damper,and modulating low-limit controller. The outdoor air damperand heating coil valve operate in sequence in accordance withthe demand of the thermostat. The heating valve is full open.As space temperature rises, the thermostat modulates thedamper to its maximum open position. The fixed maximumposition is normally 100 percent open. As space temperaturecontinues to rise, and after the damper moves to its maximumposition, the heating coil valve modulates closed. If a coolingcoil is installed in the unit, the cooling coil valve opens as spacetemperature rises further. The low limit controller is necessaryto prevent freezing conditions that occur when occupants lowerthe room control setpoint when it is below freezing outside,plus it prevents the discomfort that would result in dischargingair into the room below 55 degrees F.OPENFINALCONTROLELEMENTPOSITIONOPERATION CYCLEOUTDOORAIRDAMPERHEATINGVALVECLOSEDLOWSPACE TEMPERATUREHIGHFig. 24. Unit Ventilator Cycle I Control.C3029CYCLE II–FIXED MINIMUM PERCENTAGE OF OUTDOOR AIRCycle II control (Fig. 25) provides a fixed minimumpercentage of outdoor air (usually 10 to 33 percent, adjustablefrom 0 to 100 percent).At low space temperatures, the outdoor air damper is closedand the heating coil valve and face damper are full open. Asspace temperature rises, the outdoor air damper moves to itsminimum position. On a further rise in space temperature, theface damper modulates closed, and on a further rise in spacetemperature, the coil valve closes. When the space temperatureis at setpoint, the valve is full closed. As space temperaturerises above setpoint, the outdoor and return air dampersmodulate to 100 percent outdoor air at the top of the throttlingrange. The low-limit controller can limit the closure of the coilvalve, face damper, and the opening of the outdoor air damperto prevent the discharge air temperature from dropping belowits setpoint.ENGINEERING MANUAL OF AUTOMATIC CONTROL 389