air duct design

BBSE3006: Air Conditioning and Refrigeration IIg ghttp://www.hku.hk/bse/bbse3006/

Air-side Systems: Air Duct DesignDr. Sam C M Hui

Department of Mechanical EngineeringDepartment of Mechanical EngineeringThe University of Hong Kong

E-mail: cmhui@hku hkE mail: [email protected] 2008

Contents

• Duct ConstructionDuct Construction

• Duct Properties

• Air Duct Design and Sizing• Air Duct Design and Sizing

• Other Factors

Duct Construction

• Types of air ductTypes of air duct• Supply air duct• Return air duct• Outdoor air ductOutdoor air duct• Exhaust air

• Duct sections• Header or main duct (trunk)• Header or main duct (trunk)• Branch duct or runout

[Source: Kreider, K. F. (ed.), 2001. Handbook of Heating, Ventilation, and Air Conditioning, CRC Press, Boca Raton, FL.]

Duct Construction

• Duct systemsDuct systems• Max. pressure difference (between air inside the

d d h bi i )duct and the ambient air)• 125, 250, 500, 750, 1000, 1500, 2500 Pa

• Commercial buildings• Low pressure duct system: ≤500 Pa max 12 m/s• Low-pressure duct system: ≤500 Pa, max 12 m/s• Medium-pressure system: 500-1500 Pa, max 17.5 m/s

• Residential buildings: 125 Pa or 250 Pa• Industrial duct system: ΔP can be higherIndustrial duct system: ΔP can be higher

Duct Construction

• Duct material: e g UL (Underwriters’Duct material: e.g. UL (Underwriters Laboratory) standard• Class 0: zero flame spread, zero smoke developed

• Iron, galvanized steel, aluminum, concrete, masonry, , g , , , y,clay tile

• Class 1: flame spread ≤ 25 smoke developed ≤ 50• Class 1: flame spread ≤ 25, smoke developed ≤ 50• Fiberglass, many flexible ducts

• Class 2: flame spread ≤ 50, smoke developed ≤ 100

Duct Construction

• Shapes of air ductp• Rectangular

• More easily fabricated on site air leakage• More easily fabricated on site, air leakage• Round

• Less fluid resistance, better rigidity/strength• Flat oval• Flexible

• Multiple-ply polyester film w/ metal wire or stripsMultiple ply polyester film w/ metal wire or strips

• SMACNA (Sheet Metal and Air Conditioning C ’ N i l A i i ) d dContractors’ National Association) standards

Rectangular duct Round duct w/ spiral seam

Flat oval duct Flexible duct(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Transverse joint reinforcement(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Duct Construction

• Duct specificationDuct specification• Sheet gauge and thickness of duct material• Traverse joints & longitudinal seam

reinforcements• Duct hangers & their spacing

dh i l• Tapes & adhesive closures• Fire spread and smoke developedp p• Site-fabricated or factory-fabricated

Duct Properties

• Duct heat gain or lossDuct heat gain or loss• Temperature rise or drop• Duct insulation (mounted or inner-lined)

• Reduce heat gain/loss, prevent condensation, soundReduce heat gain/loss, prevent condensation, sound attenuation

• Minimum & recommended thicknessMinimum & recommended thickness• ASHRAE standard or local codes

• Temperature rise curves• Temperature rise curves• Depends on air velocity, duct dimensions & insulation

Temperature rise from duct heat gain(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Duct Properties

• Frictional losses: Darcey-Weisbach EquationFrictional losses: Darcey Weisbach Equation• Hf = friction head loss, or Δpf = pressure loss

• f = friction factor (dimensionless)• L = length of duct or pipe (m)• D = diameter of duct or pipe (m)• v = mean air velocity in duct (m/s)• g = gravitational constant (m/s2)• ρ = density of fluid (kg/m3)• g c = dimensional constant, for SI unit, g c = 1

Duct Properties

• Frictional lossesFrictional losses• Friction factor (f)

• ReD (Reynolds number)• ε= absolute roughness; ε/ D = relative roughness• Smooth duct & rough duct

• Moody diagram• Moody diagram• Laminar flow (ReD < 2000), f = 64 / ReD

• Critical & transition zone• Turbulent flow: Rouse limit, ReD = 200 / √[f (ε/ D)]

Moodydiagram

δ >ε

δ <ε

Mode of airflow when air passes over and aroundMode of airflow when air passes over and aroundsurface protuberances of the duct wall

(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Duct Properties

• Duct friction chartDuct friction chart• Colebrook formula

R h & t t ti• Roughness & temperature corrections• Δpf = Ksr KT KelΔpf cpf sr T el pf,c

• Ksr = correction factor for surface roughness• K = correction factor for air temperature• KT = correction factor for air temperature• Kel = correction factor for elevation

Friction chart for round duct(Source: ASHRAE Handbook Fundamentals 2001)

(Source: ASHRAE Handbook Fundamentals 2001)

Duct Properties

• Circular equivalentCircular equivalent• Hydraulic diameter, Dh = 4 A / P

• A = area (mm2); P = perimeter (mm)• Rectangular duct:g

• Flat oval duct:

Duct Properties

• Dynamic lossesDynamic losses• Result from flow disturbances caused by duct-

d i d fi imounted equipment and fittings• Change airflow path’s direction and/or area• Flow separation & eddies/disturbances

• In dynamic similarity (same Reynolds number &• In dynamic similarity (same Reynolds number & geometrically similar duct fittings), dynamic loss i ti l t th i l itis proportional to their velocity pressure

Duct Properties

• Local or dynamic loss coefficientLocal or dynamic loss coefficient• Ratio of total pressure loss to velocity pressure

Duct Properties

• Duct fittingsg• Elbows

C i di i t d• Converging or diverging tees and wyes• Entrances and exits• Enlargements and contractions

• Means to reduce dynamic losses• Means to reduce dynamic losses• Turning angle, splitter vanes

• ASHRAE duct fitting database• Fitting loss coefficients• Fitting loss coefficients

Region of eddies andturbulences in a round elbow 5-piece 90o round elbow


R t l lb th di 2 littRectangular elbow, smooth radius, 2 splitter vanes

Mitered elbow and its secondary flow(Source: ASHRAE Handbook Fundamentals 2001)

Airflow through at l irectangular convergingor diverging wye


Entrance ExitEntrance Exit


Abrupt enlargement Sudden contractionAbrupt enlargement Sudden contraction


Duct Properties

• Flow resistance, RFlow resistance, R• Total pressure loss Δpt at a specific volume flow rate V

2VRpt

• Flow resistance in series:ns RRRR 21

• Flow resistance in parallel:p

RRRR1111

np RRRR 21

Total pressure loss and flow resistance of a round duct section(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Flow resistance in seriesFlow resistance in series

Flow resistance in parallel(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Flow resistance for a Y connection(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Air Duct Design & Sizing

• Optimal air duct designp g• Optimal duct system layout, space available

S ti f t t b l• Satisfactory system balance• Acceptable sound level• Optimum energy loss and initial cost• Install only necessary balancing devices (dampers)Install only necessary balancing devices (dampers)• Fire codes, duct construction & insulation

• Require comprehensive analysis & care for different transport functionsp

Flow characteristics of a supply duct systemFlow characteristics of a supply duct system



• Design velocityDesign velocity• Constraints: space available, beam depth• Typical guidelines:

• Main ducts: air flow usually ≤ 15 m/s; air flow noiseMain ducts: air flow usually ≤ 15 m/s; air flow noise must be checked

• With more demanding noise criteria (e g hotels) maxWith more demanding noise criteria (e.g. hotels), max. air velocity: main duct ≤ 10-12.5 m/s, return main duct ≤ 8 m/s, branch ducts ≤ 6 m/s≤ 8 m/s, branch ducts ≤ 6 m/s

• Face velocities for air-handling system components


• System balancingSystem balancing• Air volume flow rate meeting design conditions• System balancing using dampers only is not

recommended• Critical path

• Design path of airflow (total flow resistance is maximum))

• How to reduce the dynamic losses?


• Reduce dynamic losses of the critical pathy p• Maintain optimum air velocity through duct fittings

E h i d ti f d i l t• Emphasize reduction of dynamic losses nearer to the fan outlet or inlet (high air velocity)

• Proper use of splitter vanes• Set 2 duct fittings as far apart as possibleg p p

• Air duct leakageD l k l ifi i• Duct leakage classification

• ANSI, SMACNA, ASHRAE standards


• Fire protectionFire protection• Duct material selection• Vertical ducts (using masonry, concrete or clay)• When ducts pass through floors & wallsWhen ducts pass through floors & walls• Use of fire dampers• Filling the gaps between ducts & bldg structure• Duct systems for industrial applications• Duct systems for industrial applications

• Any other fire precautions?


• Design procedure (computer-aided or manual)g p ( p )• Verify local codes & material availability• Preliminary duct layoutPreliminary duct layout• Divide into consecutive duct sections• Minimise local loss coefficients of duct fittingsMinimise local loss coefficients of duct fittings• Select duct sizing methods• Critical total pressure loss of tentative critical path• Critical total pressure loss of tentative critical path• Size branch ducts & balance total pressure at junctions• Adjust supply flow rates according to duct heat gain• Adjust supply flow rates according to duct heat gain• Resize duct sections, recalculate & balance parallel paths

Ch k d l l & dd tt ti• Check sound level & add necessary attenuation


• Duct layouty• Symmetric layout is easier to balance

• Smaller main duct & shorter design path• Smaller main duct & shorter design path• For VAV systems, duct looping allows feed from

it di tiopposite direction• Optimise transporting capacity (balance points often

f ll h ’ i i )follow the sun’s position)• Result in smaller main duct

• Compare alternative layouts & reduce fittings• For exposed ducts, appearance & integration with p , pp g

the structure is important

Typical supply duct system with symmetric layout & loopingTypical supply duct system with symmetric layout & looping



• Duct sizing methodsDuct sizing methods• Equal-friction method with maximum velocity

• Duct friction loss per unit length remains constant• Most widely used in normal HVAC applications

• Constant-velocity method• Often for exhaust ventilation system• Often for exhaust ventilation system• Minimum velocity to carry dust is important• Limit velocity to reduce noise


• Duct sizing methodsg• Static regain method

• Normally used with a computer package for high• Normally used with a computer package for high velocity systems (e.g. in main duct)

• Size air duct so that ↑static pressure nearly offset the• Size air duct so that ↑static pressure nearly offset the pressure loss of succeeding duct section along main duct

• T method• T method• Optimising procedure by minimising life-cycle cost

S t d i (i t i l i i d t)• System condensing (into a single imaginary duct)• Fan selection (optimum system pressure loss)• System expansion (back to original duct system)System expansion (back to original duct system)

Concept of static regain method(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)


• Design information requiredDesign information required• Client requirements• Required supply air condition• Type of system suppliedType of system supplied• Ambient conditions• Duct material• Duct insulation• Duct insulation• Duct system layout


• Key design inputsKey design inputs• Design volume flow rate (m3/s)• Limiting duct pressure loss (Pa/m)• Limiting flow velocity (m/s)Limiting flow velocity (m/s)

• Design outputs• Schematic of ductwork layout & associated plant• Schedule of duct sizes and lengths and fittings• Schedule of duct sizes and lengths, and fittings


• Duct system characteristicsDuct system characteristics• Supply duct, return duct, or exhaust duct systems

i h i l i b h k ffwith certain pressure loss in branch takeoffs• Duct sizing based on LCC & space optimisation• System balancing through pressure balance of duct paths• Sound level will be checked & analysedSound level will be checked & analysed• Minimise local loss coefficients of duct fittings

S l l fl t dj t d di t d t• Supply volume flow rates adjusted according to duct heat gain

Cost analysis for a duct system(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)


• Duct system characteristicsDuct system characteristics• Supply duct, return duct, or exhaust duct systems in

hi h l l ill i hwhich supply outlets or return grilles either mounted directly on duct or have very short connecting duct

• Very small or negligible pressure loss at branch ductsVery small or negligible pressure loss at branch ducts• System balancing depends mainly on sizes of the

successive main duct sectionssuccessive main duct sections

Rectangularsupply duct withpp ytransversal slots



• Duct system characteristicsDuct system characteristics• Industrial exhaust duct systems to transport dust or

h i lother particulates• Require a minimum velocity in all duct sections, such as

12.2 to 20.3 m/s• Select proper configuration of duct fittings to provide a p p g g p

better system balance• Round ducts produce smaller losses & more rigid• Air velocity must not exceed too much, to avoid energy waste• Well-sealed joints & seams to reduce air leakage

Other Factors

• Duct liner• Lined internally on inner surface of duct wall

M i l d f i tt ti & i l ti• Mainly used for noise attenuation & insulation• Fiberglass blanket or boards

• Duct cleaning• Prevent accumulation of dirt & debris• Prevent accumulation of dirt & debris• Agitation device to loosen the dirt & debris• Duct vacuum to extract loosened debris• Sealing of access openingsSealing of access openings

Duct breakout noise(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Other Factors

• Pressure and airflow measurementsPressure and airflow measurements• Pitot tube

• Two concentric tubes• Manometers

• U tube or inclined one

D i f i i• Demonstration of measuring instrument

Pitot tube(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Manometer: U-value

Inclined manometer


Pressure measurements in air ductsPressure measurements in air ducts


M i i i l & d dMeasuring points in rectangular & round duct transverse(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

air duct design

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