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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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X

Duct System Sizing

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Rectangular Duct Elbows

X

Duct System Sizing

Turning Vanes

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X

Duct System Sizing

Elbows – Equivalent Duct Lengths

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Duct System SizingCONCLUSION . . .

THANK YOU.