Trane TRG-TRC014-EN manual Pressure-Dependent Control, Pressure-Independent Control

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period two

Components of a VAV System

notes

duct static pressure, airflow to the space can vary even when the immediate modulation device does not change position.

Pressure-Dependent Control

▲Space sensor controls position of the modulating device

▲Airflow to space depends on SP in upstream duct system

Pressure-Independent Control

▲Space sensor controls desired airflow

▲Airflow to space is controlled directly, independent of SP in upstream duct system

Figure 41

A pressure-dependentVAV control scheme uses the space temperature sensor to directly control the position of the modulating device. The actual airflow delivered to the space is a by-product of this position and depends on the duct system static pressure at the inlet of the terminal unit. Although the space temperature sensor will continually correct the position of the modulating device, the response can be sluggish and cause unacceptable temperature variations within the space.

In contrast, a pressure-independentVAV control scheme directly controls the actual volume of primary air that flows to the space. An airflow-measuring device on the terminal unit makes this possible. The position of the modulation device is not directly controlled and is basically a by-product of regulating the airflow through the unit. Because the airflow delivered to the space is directly controlled, it is independent of inlet static pressure.

Pressure-independent control increases the stability of airflow control, and allows minimum and maximum airflow settings to become actual airflows rather than physical positions of the modulation device. It is clearly the most popular form of VAV terminal unit control.

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Contents Air Conditioning Clinic VAV Systems One of the Systems Series Publication of The Trane Company Preface VAV SystemsContents TRG-TRC014-EN What Is Variable Air Volume? Constant-Volume Variable-Temperature SystemConstant Volume-Full Load Constant Volume-Part Load Constant × Supply AirflowConstant Volume-Multiple Spaces FineTerminal Reheat System Variable-Air-Volume VAV System VAV-Full LoadReduced fan energy Reduced refrigeration energy Why VAV? Energy SavingsVAV-Part Load Why VAV? Comfort Dedicated terminal units Dedicated thermostatsSystem Comparison VAVVAV Building Characteristics Variable Thermal Load ProfilesIndependent Space Control Common Return Air PathComponents of a VAV System Components of a VAV SystemVAV Terminal Units VAV Terminal UnitsAir Heating coil Filter Mixing fan Single Duct, Cooling Only Primary air irSpace Heating with a VAV System VAV Reheat Primary airFan-Powered Terminal Units SeriesParallel, Fan-Powered Series, Fan-Powered Parallel Versus Series Fan-Powered Single-Duct VAV Systems Dual-Duct Variable Air Volume to the Space Cool Primary airConstant Air Volume to the Space Single-Fan, Dual-Duct VAV SystemTwo-Fan, Dual-Duct VAV System VAV Terminal Unit Controls VAV Terminal Unit ControlsUpstream Pressure Affects Airflow Pressure-Dependent Control Pressure-Independent ControlPrimary Airflow Measurement Terminal-Unit Control TechnologiesPneumatic Control Electronic Control Direct Digital Control DDC Components of a VAV System Diffusers DiffusersLinear Slot Diffuser EffectCeiling-Diffuser Compatibility Components of a VAV System Supply Duct System Supply Duct SystemStatic regain method Duct DesignEqual friction method Duct Design Recommendations System Configurations Perimeter P Versus InteriorPerimeter Spaces Perimeter SpacesBaseboard Perimeter Heating Overhead Supply Perimeter Heating Heat loss 250 Btu/hr/ft Fan-Powered VAV Dual-Duct VAVInterior Spaces Interior Spaces, Reheat RequiredSmall Buildings Changeover/Bypass VAV SystemOccupied mode Unoccupied mode Morning warm-up/cool-down mode System-Level Control ModesOccupied Mode Unoccupied Mode Morning Warm-up/Cool-down Mode Fan Performance Curve Fan ModulationFan Performance Curve System Resistance Curve System Resistance CurveOperating Point Riding the Fan CurveForward-Curved Centrifugal Fan Fan Modulation Curve Fan Modulation CurveFan Control Loop VAV Fan ModulationFan Modulation Methods Discharge DampersStatic pressure Discharge damper SP loss Inlet Vanes Fan-Speed Control 0 r p mVariable-Pitch Blade Control Fan Modulation Comparisons BI fan with Discharge DampersControlling System Static Pressure Fan Outlet Static-Pressure ControlSupply Duct Static-Pressure Control Sensor located Down supply ductOptimized Static-Pressure Control At fan outletPart-Load Comparison Application Considerations Period fiveSystem-Level Ventilation System-Level VentilationVentilation Reset Freeze Protection for Coils Freeze Protection for CoilsMeasures to Prevent Coil Freeze-Up Part-Load Space Humidity Control Part-Load Space Humidity ControlPart-Load Supply Air Tempering Building Pressure Control Building Pressure ControlDirect Pressurization Control Application Considerations Review Review-Period OneReview-Period Two Review-Period Three Review-Period FourReview-Period Five Trane Air Conditioning ManualReview Quiz Questions for PeriodQuiz TRG-TRC014-EN Answers Glossary Glossary Glossary Trane Company

TRG-TRC014-EN specifications

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