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Duct System Sizing
Duct System Sizing
HVAC Design
SourcesASHRAE – FundamentalsSMACNA – HVAC Systems Duct Design Carrier – System Design Manuals…and TES…
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Duct System Sizing
HVAC Design Overview duct systems)
HVAC Load CalculationsZoningPrelim equip layout and selection
Diffuser/Grille LayoutsDuctwork Layout
Ductwork system designFan/Equipment Selection
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Duct System Sizing
Ductwork Design Basics
HVAC duct systems deliver air from fan to diffusers, which distribute air to spaceFan creates a pressure difference which moves airDuct layout/sizing is related to this pressure differenceGoal:
size ductwork to minimize pressure drops,while keeping costs installation/material costs down.
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Duct System Sizing
Ductwork Design – System Pressure
Total Pressure = Static Pressure + Velocity PressureStatic & velocity pressures vary throughout system depending on size of duct and airflowTotal pressure decreases in system due to friction
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Duct System Sizing
System Pressure Changes
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Duct System Sizing
Fan Selection
Type of application/system/fanAirflow (CFM)External Static Pressure CalculationTotal Static Pressure (manuf driven, not typically used in selection)
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Duct System Sizing
External Static Pressure Calculation
Static pressure beyond HVAC equipmentSystem static pressure determined from all friction losses
Created by straight ductElbow/fittingsDuct Accessories
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Duct System Sizing
Design Methods
Equal Friction
Static Regain
Other (per SMACNA)Extended plenumVelocityT-methodTotal PressureConstant Velocity
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Duct System Sizing
Duct Sizing Methods
Equal Friction MethodMost common Methodlow pressure systemssupply, exhaust & return airFriction loss per length of duct is determined to be a constant value.
Ex: 0.1” of wc per 100’ of ductTotal system losses of ductwork = (total equivalent duct system length ) * (friction loss)
Equivalent length = all straight duct + equivalent lengths of elbow, fittings, etc.
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Duct System Sizing
Duct Sizing Methods
Static Regain
Complicated design for systems with high pressure and high velocity systemsTypically supply only
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Duct System Sizing
Duct Sizing Methods
Velocity Method
Velocity assigned to each section of duct with a known airflow, area determined from that.Calculate friction loss from tables & charts based on now determined area and length of duct.
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Duct System Sizing
Design Layout
Clearances – Ceiling heightsFloor to floor heightType of structure (beams, joists, etc)
Coordination with other obstacles PipingLightsOther Ductwork
Velocity and related noise
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Duct System Sizing
Air Velocity
Common Duct Velocities for standard low pressure/low velocity systems
Supply/Return Shafts 2000 fpm maxSupply/Return Mains 1500 fpm maxSupply/Return Branch ducts 1200 fpm max
Lower velocities may be required in noise sensitive areas.
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Duct System Sizing
Air Velocity
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Duct System Sizing
Duct Sizing Concepts
Use even duct sizes (ex: 24/12)Branch ducts ideally should be 2” shorter than duct main
(ex: 12” deep duct should have no more than 10” deep takeoff)Exception: some fittings available for same size takeoffs
Internal duct insulation: 1” reduces 2” total of ductwork dimensionsCoordinate exposed ductwork as it may be preferred to be round, spiral and/or painted by client or architect. Keep duct aspect ratios less than 4:1. (Square or Round is ideal.)
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Duct System Sizing
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Rectangular Duct Elbows
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Duct System Sizing
Turning Vanes
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Duct System Sizing
Elbows – Equivalent Duct Lengths
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Duct System SizingCONCLUSION . . .
THANK YOU.