Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONSFEATURES1. The system admits outdoor air for cooling based uponthe economizer decision.2. Space relative humidity is maintained by controlling bothhumidification and dehumidification.3. Reheat prevents subcooling on dehumidification cycle.4. Constant temperature and humidity control. (Do not usethis where deadband temperature or humidity control isacceptable.)PSYCHROMETRIC ASPECTSFor cooling conditions it is assumed that:1. Design outdoor air condition is 95°F DB and 79°F WB.2. RA condition is 76°F DB and 65°F WB.3. System operates on 25 percent minimum outdoor air.4. Space temperature setpoint is set at 74°F.5. Space humidity control is set at 50 percent.6. Coil leaving air temperature is at 55°F.CONDITIONS FOR SUCCESSFUL OPERATION1. Heating is available during dehumidification cycle.SPECIFICATIONSSee FAN SYSTEM START-STOP CONTROL.1. Anytime the supply fan runs, control system shall beenabled, and mixing dampers shall position to minimumventilation position during scheduled occupancy periods.2. Anytime the economizer decision program determinesthat OA is suitable to assist in cooling, the OA/RA mixingdampers shall be under control of the SA PID loop.3. The space humidity PID loop shall modulate thehumidifier, subject to a humidifier leaving air high limithumidity PID loop setpoint, to maintain the spacehumidity PID loop setpoint. Humidifying control shallbe disabled anytime the chilled water valve is modulatingor the fan is off.4. The space humidity PID loop shall override thetemperature controls to modulate the chilled water valveopen for dehumidification if required to maintain thespace humidity PID loop setpoint. The dehumidifyingcontrol loop shall be disabled anytime there is no hotwater flow or temperature.5. As the SA PID cooling demand varies from 100 to 45%,the cooling SA PID loop setpoint shall vary from 55°F to75°F.6. As the SA PID cooling demand varies from 40 to 0%,the reheat coil hot water valve SA PID loop setpoint(chilled water and economizer) shall vary from 52°F to105°F.7. The heating coil hot water valve shall modulate to preventthe cooling coil leaving air temperature from droppingbelow 52°F.355°F DBSPACE74°F DB24COOLINGThe following results are obtained:ItemNo.Explanation1MARA 76°F DB,62.5°F WB2FOA 95°F DB,79°F WBC2572-21 Mixed air temperature at cooling designcondition.2 Air entering the coil is cooled along a line ofconstant moisture content until saturation isapproached. Near saturation the moisturecontent is reduced as the air is cooled. Thisprocess involves both latent and sensiblecooling.3 Cooling coil leaving air temperature will be aslow as required to satisfy either the spacetemperature controller or the space humiditycontroller whichever is calling for the greatestcooling. If dehumidification cools dischargetemperature below setpoint, the heating coilprovides reheat.4 The space temperature is 74°F DB and the RAtemperature is 76°F DB. The 2°F DB rise isan example of sensible cooling load whichmay occur in ceiling, space, and RA ducts.The rise will be a function of system,building, and lighting arrangement.ENGINEERING MANUAL OF AUTOMATIC CONTROL236