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Sharing our Guidance

Duct System Sizing

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Duct System Sizing

HVAC Design

Sources

ASHRAE – Fundamentals

SMACNA – HVAC Systems Duct Design

Carrier – System Design Manualsand !ES

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Duct System Sizing

HVAC Design )*er*ie+ ductsystems,HVAC -oad Calculations

.oning

/relim e0ui1 layout and selection

Di2user$Grille -ayoutsDuct+or3 -ayout

Duct+or3 system design

Fan$E0ui1ment Selection

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Duct System Sizing

Duct+or3 Design 4asics

HVAC duct systems deli*er air 5rom 5an todi2users6 +hich distri7ute air to s1ace

Fan creates a 1ressure di2erence +hich mo*esair

Duct layout$sizing is related to this 1ressuredi2erence

Goal8

size duct+or3 to minimize 1ressure dro1s6

+hile 3ee1ing costs installation$materialcosts do+n9

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Duct System Sizing

Duct+or3 Design – System/ressure

 !otal /ressure : Static /ressure ; Velocity/ressure

Static < *elocity 1ressures *ary throughoutsystem de1ending on size o5 duct and air=o+

 !otal 1ressure decreases in system due to5riction

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Duct System Sizing

System /ressure Changes

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Duct System Sizing

Fan Selection

 !y1e o5 a11lication$system$5an

Air=o+ ?CFM,

E@ternal Static /ressure Calculation

 !otal Static /ressure ?manu5 dri*en6 not ty1ically

used in selection,

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Duct System Sizing

E@ternal Static /ressureCalculation

Static 1ressure 7eyond HVAC e0ui1ment

System static 1ressure determined 5rom all5riction losses

Created 7y straight duct

El7o+$Bttings

Duct Accessories

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Duct System Sizing

Design Methods

E0ual Friction

Static Regain

)ther ?1er SMACNA,

E@tended 1lenum

Velocity

 !method

 !otal /ressure

Constant Velocity

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Duct System Sizing

Duct Sizing Methods

E0ual Friction Method

Most common Method

lo+ 1ressure systems

su11ly6 e@haust < return air

Friction loss 1er length o5 duct is determined to7e a constant *alue9

E@8 "9% o5 +c 1er %"" o5 duct

 !otal system losses o5 duct+or3 : ?totale0ui*alent duct system length , ?5riction loss,

E0ui*alent length :

all straight duct ; e0ui*alent lengths o5el7o+6 Bttings6 etc9

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Duct System Sizing

Duct Sizing Methods

Static Regain

Com1licated design 5or systems +ithhigh 1ressure and high *elocitysystems

 !y1ically su11ly only

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Duct System Sizing

Duct Sizing Methods

Velocity Method

Velocity assigned to each section o5duct +ith a 3no+n air=o+6 areadetermined 5rom that9

Calculate 5riction loss 5rom ta7les <charts 7ased on no+ determined areaand length o5 duct9

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Duct System Sizing

Design -ayout

Clearances –

Ceiling heights

Floor to =oor height

 !y1e o5 structure ?7eams6 Ioists6 etc,

Coordination +ith other o7stacles

/i1ing

-ights

)ther Duct+or3

Velocity and related noise

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Duct System Sizing

Air Velocity

Common Duct Velocities 5or standard lo+1ressure$lo+ *elocity systems

Su11ly$Return Sha5ts (""" 51mma@

Su11ly$Return Mains %>"" 51mma@

Su11ly$Return 4ranch ducts %(""51m ma@

-o+er *elocities may 7e re0uired in noisesensiti*e areas9

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Duct System Sizing

Air Velocity

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Duct System Sizing

Duct Sizing Conce1ts

Jse e*en duct sizes ?e@8 (#$%(,

4ranch ducts ideally should 7e ( shorter than ductmain

?e@8 %( dee1 duct should ha*e no more than %"dee1 ta3eo2,

E@ce1tion8 some Bttings a*aila7le 5or same sizeta3eo2s

Knternal duct insulation8 % reduces ( total o5duct+or3 dimensions

Coordinate e@1osed duct+or3 as it may 7e 1re5erred to7e round6 s1iral and$or 1ainted 7y client or architect9

Lee1 duct as1ect ratios less than #8%9 ?S0uare orRound is ideal9,

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Duct System Sizing

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Rectangular Duct El7o+s

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Duct System Sizing

 !urning Vanes

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Duct System Sizing

El7o+s – E0ui*alent Duct -engths

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Duct System SizingC)NC-JSK)N 9 9 9

 !HANL )J9