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Trane TRG-TRC014-EN manual Components of a VAV System, period two, Direct Digital Control DDC

Models: TRG-TRC014-EN

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period two

Components of a VAV System

notes

Direct Digital Control (DDC)

▲Electronic pressure transducer provides pressure-independent control

▲Digital controller positions modulation device, controls terminal fan and heat source

▲Minimum and maximum airflow settings adjusted through communication link

▲Electric power supply operates modulation device and electronic controller

Figure 47

Similar to electronic control, an electronic pressure transducer enables pressure-independent control of the VAV terminal unit. The digital controller positions the modulation device and operates the fan and the heat source.

Minimum and maximum airflow settings are adjusted through a communication link to the digital controller. A 24-volt power supply is required to power the unit actuator and controls.

32	TRG-TRC014-EN

Page 37

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Trane TRG-TRC014-EN manual Components of a VAV System, period two, Direct Digital Control DDC

Contents

One of the Systems Series Air Conditioning Clinic VAV SystemsTRG-TRC014-EN A publication of The Trane Company VAV SystemsOne of the Systems Series VAV Systems PrefaceA Trane Air Conditioning Clinic period one What Is Variable Air Volume? Contentsperiod two Components of a VAV SystemTRG-TRC014-EN What Is Variable Air Volume? What Is Variable Air Volume?Constant-Volume Variable-TemperatureSystem period oneperiod one Constant Volume-FullLoadWhat Is Variable Air Volume? What Is Variable Air Volume? Constant Volume-PartLoadperiod one What Is Variable Air Volume? Constant Volume-MultipleSpacesConstant Volume-MultipleSpaces period oneWhat Is Variable Air Volume? Terminal Reheat Systemperiod one VAV-FullLoad Variable-Air-VolumeVAV SystemWhat Is Variable Air Volume? period oneVAV-PartLoad Why VAV? Energy SavingsWhat Is Variable Air Volume? Why VAV?What Is Variable Air Volume? Why VAV? Comfortperiod one quiteWhat Is Variable Air Volume? System Comparisonperiod one Variable Thermal Load Profiles VAV Building CharacteristicsWhat Is Variable Air Volume? period oneCommon Return Air Path Independent Space ControlWhat Is Variable Air Volume? period oneComponents of a VAV System period twoComponents of a VAV System period twoVAV Terminal Units period twoComponents of a VAV System Components of a VAV System Single Duct, Cooling Onlyperiod two Single Duct, Cooling OnlyComponents of a VAV System Space Heating with a VAV Systemperiod two primary air VAV ReheatComponents of a VAV System period twoComponents of a VAV System Fan-PoweredTerminal Unitsperiod two parallelplenum air Parallel, Fan-PoweredParallel, Fan-Powered Components of a VAV SystemComponents of a VAV System Series, Fan-Poweredperiod two Series, Fan-Powered Parallel Versus Series Fan-PoweredComponents of a VAV System period twoComponents of a VAV System Single-DuctVAV Systemsperiod two Components of a VAV System Dual-Ductperiod two warm primary air cool primary air supply airwarm Variable Air Volume to the Spacecool Components of a VAV SystemSingle-Fan, Dual-DuctVAV System Constant Air Volume to the SpaceComponents of a VAV System period twoComponents of a VAV System Two-Fan, Dual-DuctVAV Systemperiod two Upstream Pressure Affects Airflow VAV Terminal Unit ControlsComponents of a VAV System period twoPressure-IndependentControl Pressure-DependentControlComponents of a VAV System period twoTerminal-UnitControl Technologies Primary Airflow MeasurementComponents of a VAV System period twoComponents of a VAV System Pneumatic Controlperiod two Components of a VAV System Electronic Controlperiod two Components of a VAV System Direct Digital Control DDCperiod two period two Components of a VAV SystemDirect Digital Control DDC Electronic pressure transducer provides pressure-independentcontrolComponents of a VAV System Diffusersperiod two DiffusersComponents of a VAV System Linear Slot Diffuserperiod two CoandaComponents of a VAV System Ceiling-DiffuserCompatibilityperiod two period two Components of a VAV SystemCeiling-DiffuserCompatibility cross TComponents of a VAV System Supply Duct Systemperiod two Supply Duct SystemComponents of a VAV System Duct Designperiod two Equal friction methodComponents of a VAV System Duct Design Recommendationsperiod two System Configurations System Configurationsperiod three period threePerimeter Spaces period threeSystem Configurations Perimeter SpacesSystem Configurations Baseboard Perimeter Heatingperiod three heat loss 450 Btu/hr/ftSystem Configurations Overhead Supply Perimeter Heatingperiod three period three System ConfigurationsVAV Reheat heat loss 250 Btu/hr/ftDual-DuctVAV Fan-PoweredVAVSystem Configurations period threeInterior Spaces, Reheat Required Interior SpacesSystem Configurations period threeChangeover/Bypass VAV System Small BuildingsSystem Configurations period threeSystem Configurations System-LevelControl Modesperiod three System-LevelControl ModesSystem Configurations Occupied Modeperiod three System Configurations Unoccupied Modeperiod three System Configurations Morning Warm-up/Cool-downModeperiod three Fan Modulation period fourFan Modulation period fourSystem Resistance Curve period fourFan Modulation Riding the Fan Curve Operating PointFan Modulation period fourFan Modulation Forward-CurvedCentrifugal Fanperiod four Fan Modulation Fan Modulation Curveperiod four Fan Modulation CurveVAV Fan Modulation Fan Control LoopFan Modulation period fourDischarge Dampers Fan Modulation MethodsFan Modulation period fourperiod four Fan ModulationDischarge Dampers As the VAV terminal units modulate shut, the system resistance curve shifts upwards. The fan begins to “ride up” the constant-speedperformance curve, from the design operating point A, trying to reach the balance point with this new system resistance curve. As a result, the fan delivers a lower airflow at a higher static pressureFan Modulation Inlet Vanesperiod four Inlet VanesFan Modulation Fan-SpeedControlperiod four Fan-SpeedControlFan Modulation Variable-PitchBlade Controlperiod four Variable-PitchBlade ControlFan Modulation Fan Modulation Comparisonsperiod four Fan Outlet Static-PressureControl Controlling System Static PressureFan Modulation period fourFan Modulation Supply Duct Static-PressureControlperiod four sensor located 2/3 down supply ductFan Modulation Optimized Static-PressureControlperiod four sensor locatedFan Modulation Part-LoadComparisonperiod four Application Considerations period fiveperiod five Application ConsiderationsSystem-LevelVentilation period fiveApplication Considerations Application Considerations Ventilation Resetperiod five outdoor airflowMeasures to Prevent Coil Freeze-Up Freeze Protection for CoilsApplication Considerations period fiveApplication Considerations Part-LoadSpace Humidity Controlperiod five Part-LoadSpace Humidity ControlApplication Considerations Part-LoadSupply Air Temperingperiod five Direct Pressurization Control Building Pressure ControlApplication Considerations period fiveperiod five Application ConsiderationsA sensitive static-pressurecontroller directly compares the variations between the space static pressure and the outdoor reference static pressure. The controller modulates the capacity of a central relief fan to maintain a fixed static-pressuredifference, usually a slightly positive building pressure. The space static-pressuresensor is typically located in the lobby or near the main entrance Review period sixReview period sixReview-PeriodTwo period sixfan-poweredVAV terminal unit ReviewReview-PeriodFour Review-PeriodThreeReview period sixReview Review-PeriodFiveperiod six System-levelventilationperiod six ReviewTRG-TRC014-EN Questions for Period QuizQuestions for Period Questions for PeriodQuiz 11What method of static-pressurecontrol provides the greatest power- savings potential for a VAV system?Questions for Period TRG-TRC014-EN Answers Glossary Glossary parallel fan-poweredterminal unit A fan-poweredVAV terminal unitsupply air Conditioned air delivered to a space GlossaryAn American Standard Company The Trane Company