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
StepStepStepStepStepStepStepStep --------22222222
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
StepStepStepStepStepStepStepStep --------11111111
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.
StepStepStepStepStepStepStepStep --------22222222
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
StepStepStepStepStepStepStepStep --------33333333
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
StepStepStepStepStepStepStepStep --------44444444
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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Static LossesStatic Losses
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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Static LossesStatic Losses
(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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