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Trane TRG-TRC014-EN Variable-Air-VolumeVAV System, VAV-FullLoad, What Is Variable Air Volume?

Models: TRG-TRC014-EN

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What Is Variable Air Volume?

notes

Variable-Air-Volume (VAV) System

EARA

OA	supply	PA
OA	fan
	fan

cooling		VAV
cooling		box
coil	variable-	box
	speed drive
	thermostat	SA
		SA

Figure 9

In contrast, a variable-air-volume (VAV) system delivers the primary air at a constant temperature and varies the airflow to maintain the required space temperature at all load conditions.

VAV–Full Load

Supply	=		40,000 Btu/hr	=	1,840 cfm
Supply	=		1.085 × (75°F – 55°F)	=	1,840 cfm
Airflow			1.085 × (75°F – 55°F)
Supply	=		11,724 W		= 0.87 m3/s
Supply	=	1,210 × (23.9°C – 12.8°C)			= 0.87 m3/s
Airflow		1,210 × (23.9°C – 12.8°C)

Figure 10

Similar to the constant-volume system, at design conditions the VAV system will balance the same 40,000 Btu/hr [11,724 W] space sensible load with the same 1,840 cfm [0.87 m3/s] of 55°F [12.8°C] supply air.

6	TRG-TRC014-EN

Page 11

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Trane TRG-TRC014-EN manual Variable-Air-VolumeVAV System, VAV-FullLoad, What Is Variable Air Volume?, period one

Contents

TRG-TRC014-EN Air Conditioning Clinic VAV SystemsOne of the Systems Series One of the Systems Series VAV SystemsA publication of The Trane Company A Trane Air Conditioning Clinic PrefaceVAV Systems Components of a VAV System Contentsperiod one What Is Variable Air Volume? period twoTRG-TRC014-EN period one What Is Variable Air Volume?What Is Variable Air Volume? Constant-Volume Variable-TemperatureSystemWhat Is Variable Air Volume? Constant Volume-FullLoadperiod one period one Constant Volume-PartLoadWhat Is Variable Air Volume? period one Constant Volume-MultipleSpacesWhat Is Variable Air Volume? Constant Volume-MultipleSpacesperiod one Terminal Reheat SystemWhat Is Variable Air Volume? period one Variable-Air-VolumeVAV SystemVAV-FullLoad What Is Variable Air Volume?Why VAV? Why VAV? Energy SavingsVAV-PartLoad What Is Variable Air Volume?quite Why VAV? ComfortWhat Is Variable Air Volume? period oneperiod one System ComparisonWhat Is Variable Air Volume? period one VAV Building CharacteristicsVariable Thermal Load Profiles What Is Variable Air Volume?period one Independent Space ControlCommon Return Air Path What Is Variable Air Volume?period two period twoComponents of a VAV System Components of a VAV SystemComponents of a VAV System period twoVAV Terminal Units Single Duct, Cooling Only Single Duct, Cooling OnlyComponents of a VAV System period twoperiod two Space Heating with a VAV SystemComponents of a VAV System period two VAV Reheatprimary air Components of a VAV Systemparallel Fan-PoweredTerminal UnitsComponents of a VAV System period twoComponents of a VAV System Parallel, Fan-Poweredplenum air Parallel, Fan-Poweredperiod two Series, Fan-PoweredComponents of a VAV System period two Parallel Versus Series Fan-PoweredSeries, Fan-Powered Components of a VAV Systemperiod two Single-DuctVAV SystemsComponents of a VAV System warm primary air cool primary air supply air Dual-DuctComponents of a VAV System period twoComponents of a VAV System Variable Air Volume to the Spacewarm coolperiod two Constant Air Volume to the SpaceSingle-Fan, Dual-DuctVAV System Components of a VAV Systemperiod two Two-Fan, Dual-DuctVAV SystemComponents of a VAV System period two VAV Terminal Unit ControlsUpstream Pressure Affects Airflow Components of a VAV Systemperiod two Pressure-DependentControlPressure-IndependentControl Components of a VAV Systemperiod two Primary Airflow MeasurementTerminal-UnitControl Technologies Components of a VAV Systemperiod two Pneumatic ControlComponents of a VAV System period two Electronic ControlComponents of a VAV System period two Direct Digital Control DDCComponents of a VAV System Electronic pressure transducer provides pressure-independentcontrol Components of a VAV Systemperiod two Direct Digital Control DDCDiffusers DiffusersComponents of a VAV System period twoCoanda Linear Slot DiffuserComponents of a VAV System period twoperiod two Ceiling-DiffuserCompatibilityComponents of a VAV System cross T Components of a VAV Systemperiod two Ceiling-DiffuserCompatibilitySupply Duct System Supply Duct SystemComponents of a VAV System period twoEqual friction method Duct DesignComponents of a VAV System period twoperiod two Duct Design RecommendationsComponents of a VAV System period three System ConfigurationsSystem Configurations period threePerimeter Spaces period threePerimeter Spaces System Configurationsheat loss 450 Btu/hr/ft Baseboard Perimeter HeatingSystem Configurations period threeperiod three Overhead Supply Perimeter HeatingSystem Configurations heat loss 250 Btu/hr/ft System Configurationsperiod three VAV Reheatperiod three Fan-PoweredVAVDual-DuctVAV System Configurationsperiod three Interior SpacesInterior Spaces, Reheat Required System Configurationsperiod three Small BuildingsChangeover/Bypass VAV System System ConfigurationsSystem-LevelControl Modes System-LevelControl ModesSystem Configurations period threeperiod three Occupied ModeSystem Configurations period three Unoccupied ModeSystem Configurations period three Morning Warm-up/Cool-downModeSystem Configurations period four period fourFan Modulation Fan ModulationFan Modulation period fourSystem Resistance Curve period four Operating PointRiding the Fan Curve Fan Modulationperiod four Forward-CurvedCentrifugal FanFan Modulation Fan Modulation Curve Fan Modulation CurveFan Modulation period fourperiod four Fan Control LoopVAV Fan Modulation Fan Modulationperiod four Fan Modulation MethodsDischarge Dampers Fan ModulationAs 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 pressure Fan Modulationperiod four Discharge DampersInlet Vanes Inlet VanesFan Modulation period fourFan-SpeedControl Fan-SpeedControlFan Modulation period fourVariable-PitchBlade Control Variable-PitchBlade ControlFan Modulation period fourperiod four Fan Modulation ComparisonsFan Modulation period four Controlling System Static PressureFan Outlet Static-PressureControl Fan Modulationsensor located 2/3 down supply duct Supply Duct Static-PressureControlFan Modulation period foursensor located Optimized Static-PressureControlFan Modulation period fourperiod four Part-LoadComparisonFan Modulation Application Considerations period fiveApplication Considerations period fiveApplication Considerations period fiveSystem-LevelVentilation outdoor airflow Ventilation ResetApplication Considerations period fiveperiod five Freeze Protection for CoilsMeasures to Prevent Coil Freeze-Up Application ConsiderationsPart-LoadSpace Humidity Control Part-LoadSpace Humidity ControlApplication Considerations period fiveperiod five Part-LoadSupply Air TemperingApplication Considerations period five Building Pressure ControlDirect Pressurization Control 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 Application Considerationsperiod five period six period sixReview ReviewReview period sixReview-PeriodTwo fan-poweredVAV terminal unitperiod six Review-PeriodThreeReview-PeriodFour ReviewSystem-levelventilation Review-PeriodFiveReview period sixTRG-TRC014-EN Reviewperiod six Questions for Period QuizQuestions for Period Questions for PeriodQuestions for Period 11What method of static-pressurecontrol provides the greatest power- savings potential for a VAV system?Quiz 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