Duct Design Rev[1]

7/29/2019 Duct Design Rev[1]

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Air VelocityAir VelocityAir VelocityAir VelocityAir VelocityAir VelocityAir VelocityAir Velocity

It is the velocity of air at which airIt is the velocity of air at which air

flows thru the ducts.flows thru the ducts.

Generally expressed in FPM or MPS.Generally expressed in FPM or MPS.

It is limited based on:It is limited based on:Application of ductingApplication of ducting

Application of spaceApplication of space

Sound LevelSound Level

First CostFirst Cost

Operating CostOperating Cost


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DUCT DESIGN PROCEDUREDUCT DESIGN PROCEDURE

Determine the total external static pressureDetermine the total external static pressure

available from the unit at the selected airflow.available from the unit at the selected airflow.

Determine the available static pressure forDetermine the available static pressure for

duct system by subtracting the grill/diffuser,duct system by subtracting the grill/diffuser,terminal coil and accessories.terminal coil and accessories.

Proportion the available static pressureProportion the available static pressure

between supply and return air system.between supply and return air system.

SupplySupply -- 75 % and Return75 % and Return -- 25 %.25 %.


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DUCT DESIGN METHODSDUCT DESIGN METHODS

Velocity Reduction MethodVelocity Reduction MethodVelocity Reduction MethodVelocity Reduction MethodVelocity Reduction MethodVelocity Reduction MethodVelocity Reduction MethodVelocity Reduction Method

Equal Friction MethodEqual Friction MethodEqual Friction MethodEqual Friction MethodEqual Friction MethodEqual Friction MethodEqual Friction MethodEqual Friction Method

Static Regain MethodStatic Regain MethodStatic Regain MethodStatic Regain MethodStatic Regain MethodStatic Regain MethodStatic Regain MethodStatic Regain Method


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StepStepStepStepStepStepStepStep --------11111111

Assign velocity at each section of theAssign velocity at each section of theduct.The velocity is highest at the fanduct.The velocity is highest at the fanoutletoutlet

Selected velocity should not be more thanSelected velocity should not be more thanrecommended velocity.recommended velocity. The velocityThe velocity

will be lowest at the end of thewill be lowest at the end of the

ductduct


Obtain circular duct diameter and frictionObtain circular duct diameter and frictionloss fromloss from Circular Duct Friction ChartCircular Duct Friction Chart bybyusing air velocity and airflow.using air velocity and airflow.

Velocity Reduction MethodVelocity Reduction MethodVelocity Reduction MethodVelocity Reduction Method


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Equal Friction MethodEqual Friction Method


Select starting velocity at fan discharge duct.Select starting velocity at fan discharge duct.

Selected velocity should not be more thanSelected velocity should not be more than

recommended velocity.recommended velocity.


Obtain circular duct diameter and friction lossObtain circular duct diameter and friction lossfromfrom Circular Duct Friction ChartCircular Duct Friction Chart by using airby using air

velocity and airflow.velocity and airflow.


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Equal Friction MethodEqual Friction Method


Select equivalent rectangular duct size fromSelect equivalent rectangular duct size fromTableTable --6 using the circular duct diameter and6 using the circular duct diameter and

equivalent areaequivalent area based on available spacebased on available space


Select the next duct section by maintainingSelect the next duct section by maintaining

the same friction loss rate which has beenthe same friction loss rate which has been

obtained in previous duct section.obtained in previous duct section.

Cross check the velocity should not exceed theCross check the velocity should not exceed the

recommended velocity.recommended velocity.

Repeat the method for consecutive sectionRepeat the method for consecutive section


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Static Loss CalculationStatic Loss Calculation

The static losses of ducts are calculated toThe static losses of ducts are calculated to

determine static pressure of fan required to flowdetermine static pressure of fan required to flowthe air thru ducts.the air thru ducts.

The adequate flow can only occur when fan hasThe adequate flow can only occur when fan has

the flow capacity andthe flow capacity and static capacitystatic capacity totoovercome the Total losses in the duct system.overcome the Total losses in the duct system.

In other words, Fan should be capable enough toIn other words, Fan should be capable enough to

produce:produce:Air flow required.Air flow required.and the static energy required to overcome the lossesand the static energy required to overcome the losses

in duct system.in duct system.


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Static LossesStatic Losses

Total Static Losses thru the Ducts are theTotal Static Losses thru the Ducts are the

sum of following losses.sum of following losses.

Friction losses in Straight ductsFriction losses in Straight ducts

Friction & Dynamic losses in DuctFriction & Dynamic losses in Ductfittingsfittings

Friction & Dynamic losses in ductFriction & Dynamic losses in ductss

accessoriesaccessories

Outlet Terminal Losses (OutletOutlet Terminal Losses (Outlet

Pressure)Pressure)


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Frictional Losses thru straight ductsFrictional Losses thru straight ductsFrictional Losses thru straight ductsFrictional Losses thru straight ductsFrictional Losses thru straight ductsFrictional Losses thru straight ductsFrictional Losses thru straight ductsFrictional Losses thru straight ducts

These Losses /100 feet of duct can be obtainedThese Losses /100 feet of duct can be obtaineddirectly from Friction Loss Chart at specified flowdirectly from Friction Loss Chart at specified flowand size of the ducts selected.and size of the ducts selected.

Friction & Dynamic Losses thru Duct FittingFriction & Dynamic Losses thru Duct FittingFriction & Dynamic Losses thru Duct FittingFriction & Dynamic Losses thru Duct FittingFriction & Dynamic Losses thru Duct FittingFriction & Dynamic Losses thru Duct FittingFriction & Dynamic Losses thru Duct FittingFriction & Dynamic Losses thru Duct Fitting

These occurs mainly at fittings where velocity orThese occurs mainly at fittings where velocity ordirection of flow changes.direction of flow changes.

The losses in the fittings are more than the straightThe losses in the fittings are more than the straightduct of equal length due to dynamic losses.duct of equal length due to dynamic losses.

These are expressed in terms extra equivalent length ofThese are expressed in terms extra equivalent length ofducts.ducts.

Friction & Dynamic losses in ductFriction & Dynamic losses in ducts accessoriess accessories

Losses in attenuators, Terminal Heating coils and fireLosses in attenuators, Terminal Heating coils and firedampers etc. Taken from manufacturer cataloguesdampers etc. Taken from manufacturer catalogues


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(Frictional Losses)(Frictional Losses)

otal losses are calculated thru the longest duct runotal losses are calculated thru the longest duct run

including all elbows and fittings.including all elbows and fittings.EffectiveEffective Eq.LengthEq.Length of Longest Duct run X Friction Loss /100 Feetof Longest Duct run X Friction Loss /100 Feet

== ----------------------------------------------------------------------------------------------------------------------------------------------------------------

100100

Total EffectiveTotal Effective Eq.LengthEq.Length

= Duct Measured Length += Duct Measured Length + EqEq. Length of fittings. Length of fittings

FittingsFittings Eq.lengthsEq.lengths are obtained from Tableare obtained from Table --1010

& 12.& 12.

Friction Loss/100 feetFriction Loss/100 feet

It is the friction loss which was selected/obtained fromIt is the friction loss which was selected/obtained from

chart while sizing the ducts.chart while sizing the ducts.


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Total Static LossesTotal Static Losses

Sum of the losses in supply air ductsSum of the losses in supply air ducts

and return air ducts.and return air ducts.

Supply air Ducts losses =Supply air Ducts losses =

S.A.Duct loss + accessories loss +S.A.Duct loss + accessories loss +

Terminal pressureTerminal pressure

Return air duct losses =Return air duct losses =

R.A.Duct loss + accessories loss +R.A.Duct loss + accessories loss +intake pressure dropintake pressure drop


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STATIC REGAIN METHODSTATIC REGAIN METHOD

The basic principle is to size a duct runThe basic principle is to size a duct run

so that the increase in static pressureso that the increase in static pressure

(due to Velocity reduction)(due to Velocity reduction) at eachat each

branch or air terminal just offsets thebranch or air terminal just offsets the

friction loss in succeeding section offriction loss in succeeding section of

ducts.ducts.

The static pressure is then the sameThe static pressure is then the samebefore each terminal and at each branch.before each terminal and at each branch.

Static pressure regains due to reductionStatic pressure regains due to reduction

in velocity.in velocity.


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AXIAL FLOW FANSAXIAL FLOW FANS

Axial flow fans produce pressure from theAxial flow fans produce pressure from the

change in velocity of air passing thru thechange in velocity of air passing thru theimpeller,with none being produced byimpeller,with none being produced bycentrifugal force.centrifugal force.

In axial flow fan, air flows axially thru theIn axial flow fan, air flows axially thru theimpeller.impeller.

TypeTypePropeller fan.Propeller fan.

Tube axial fan.Tube axial fan.

Vane axial fans (Tube axial fan with vanes)Vane axial fans (Tube axial fan with vanes)


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Fan LawsFan Laws


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AHUs TYPEAHUs TYPE

HORIZONTALHORIZONTAL

VERTICALVERTICAL

SIDE DISCHARGESIDE DISCHARGE

TOP DISCHARGETOP DISCHARGE

THUSTHUS

HORIZONTAL SIDE DISCHARGEHORIZONTAL SIDE DISCHARGE HORIZONTAL TOP DISCHARGEHORIZONTAL TOP DISCHARGE

VERTICAL TOP DISCHRGE.VERTICAL TOP DISCHRGE.


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AHUs Selection CriteriaAHUs Selection Criteria

ApplicationApplication

Availability of spaceAvailability of space

Cooling CapacityCooling Capacity

Airflow (CFM or L/S)Airflow (CFM or L/S) System Static Pressure LossSystem Static Pressure Loss

(Inches of WG or PA)(Inches of WG or PA)

Noise levelNoise level

Location of AHU w.r.t to spaceLocation of AHU w.r.t to space

Economic FactorsEconomic Factors


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Documents

Duct Design Rev[1]