Engineering Manual o.. - HVAC.Amickracing

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONSSUPPLYFANOUTDOORSTATICPRESSURESENSORINDOORSTATICPRESSURESENSORS(2)RETURNFANRETURNFANCONTROLLERCONTROLLERS(3)DUCTSTATICAIRFLOWSENSOR (2)REFERENCESTATICFig. 40. Control of Return Dampers inZone Airflow Control.C2664C2665Fig. 41. Control of Return Fan in Zone Airflow Control.Airflow tracking is preferred for zone control on the firstfloor. Direct space static pressure control is difficult to stabilizebecause of sudden static pressure changes that occur wheneverdoors open. Also, the leakage around doors would requirepressure control at very low setpoints, which are difficult tomeasure. On upper floors where building permeability is tight,airflow tracking control is not as viable. Direct space staticpressure control is preferred for zone control on these floors.Relatively small differences in airflow tracking differentials oftightly sealed zones result in large pressure differentials. Also,all exhaust airflows must be included when dealing with airflowtracking, which makes the control more complex.Both airflow tracking and direct space static pressure controlrequire accurate sensing. In airflow tracking, the airflow sensorsare located in supply and return ducts to sense total airflow.Minimum velocities and location of the airflow sensor relativeto any variations from a straight duct are critical considerations.In direct space static pressure control, the indoor static pressuresensor should be in the largest open area and away from doorsthat open to stairways and elevators. The outdoor static pressuresensor should be at least 15 feet above the building (dependingon surrounding conditions) and be specifically designed toaccommodate multidirectional winds.To ensure minimum outdoor airflow, an airflow sensorenables control and provides information on the quantity ofoutdoor air. The airflow sensor is located in the duct havingminimum outdoor airflow (Fig. 42). The control modulates theoutdoor air, return air, and exhaust air dampers to provideoutdoor air as needed. Normally, the difference between totalsupply and return airflows, as determined by zone controls,provides the minimum outdoor air. Since each zone is set toprovide proper pressurization and buildings which are sealedtightly require less outdoor air for pressurization, this controlscheme ensures minimum outdoor air. If minimum outdoor airis increased, it does not affect building pressurization.AIRFLOW SENSORCONTROLLERSUPPLYFANRETURNFANFor zone control using airflow tracking or direct space staticpressure, return fan control should hold duct pressure constantat a point about two-thirds of the duct length upstream of thereturn fan (Fig. 41). This control is the same as that used tocontrol the supply fan, except that the duct pressure is negativerelative to the ambient surrounding the duct.C2666Fig. 42. Minimum Outdoor Air in Zone Airflow Control.ENGINEERING MANUAL OF AUTOMATIC CONTROL267