Duct Design Rev

Fundamentals of Duct DesignFundamentals of Duct Design

��Velocity or Air VelocityVelocity or Air VelocityVelocity or Air VelocityVelocity or Air VelocityVelocity or Air VelocityVelocity or Air VelocityVelocity or Air VelocityVelocity or Air Velocity

��Static PressureStatic PressureStatic PressureStatic PressureStatic PressureStatic PressureStatic PressureStatic Pressure

��Velocity PressureVelocity PressureVelocity PressureVelocity PressureVelocity PressureVelocity PressureVelocity PressureVelocity Pressure

��Total PressureTotal PressureTotal PressureTotal PressureTotal PressureTotal PressureTotal PressureTotal Pressure

��Friction LossesFriction LossesFriction LossesFriction LossesFriction LossesFriction LossesFriction LossesFriction Losses

��Aspect RatioAspect RatioAspect RatioAspect RatioAspect RatioAspect RatioAspect RatioAspect Ratio

Air VelocityAir VelocityAir VelocityAir VelocityAir VelocityAir VelocityAir VelocityAir Velocity

��It is the velocity of air at which air It 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 ducting Application of ducting

��Application of spaceApplication of space

��Sound Level Sound Level

��First CostFirst Cost

��Operating CostOperating Cost

DUCT DESIGN PROCEDUREDUCT DESIGN PROCEDURE

�� Determine the total external static pressure Determine 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 for Determine 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 pressure Proportion the available static pressure between supply and return air system. between supply and return air system.

–– Supply Supply -- 75 % and Return 75 % and Return -- 25 %. 25 %.

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

Step Step Step Step Step Step Step Step --------11111111

��Assign velocity at each section of the Assign velocity at each section of the duct.The velocity is highest at the fan duct.The velocity is highest at the fan outletoutlet

��Selected velocity should not be more than Selected velocity should not be more than recommended velocity.recommended velocity. The velocity The velocity

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

ductduct


��Obtain circular duct diameter and friction Obtain circular duct diameter and friction loss from loss from Circular Duct Friction ChartCircular Duct Friction Chart by by using air velocity and airflow.using air velocity and airflow.

Velocity Reduction MethodVelocity Reduction MethodVelocity Reduction MethodVelocity Reduction Method

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 than Selected velocity should not be more than

recommended velocity.recommended velocity.


��Obtain circular duct diameter and friction loss Obtain circular duct diameter and friction loss

from from Circular Duct Friction ChartCircular Duct Friction Chart by using air by using air

velocity and airflow.velocity and airflow.

Equal Friction MethodEqual Friction Method


��Select equivalent rectangular duct size from Select equivalent rectangular duct size from

Table Table --6 using the circular duct diameter and 6 using the circular duct diameter and

equivalent area equivalent area based on available spacebased on available space


��Select the next duct section by maintaining Select the next duct section by maintaining

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

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

��Cross check the velocity should not exceed the Cross check the velocity should not exceed the

recommended velocity.recommended velocity.

�� Repeat the method for consecutive section Repeat the method for consecutive section

Static Loss CalculationStatic Loss Calculation

�� The static losses of ducts are calculated to The static losses of ducts are calculated to determine static pressure of fan required to flow determine static pressure of fan required to flow the air thru ducts.the air thru ducts.

�� The adequate flow can only occur when fan has The adequate flow can only occur when fan has the flow capacity and the flow capacity and static capacitystatic capacity to to overcome the Total losses in the duct system.overcome the Total losses in the duct system.

�� In other words, Fan should be capable enough to In other words, Fan should be capable enough to produce:produce:–– Air flow required.Air flow required.–– and the static energy required to overcome the losses and the static energy required to overcome the losses

in duct system.in duct system.

Static LossesStatic Losses

Total Static Losses thru the Ducts are the Total 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 Duct Friction & Dynamic losses in Duct

fittings fittings

�� Friction & Dynamic losses in ductFriction & Dynamic losses in duct’’s s

accessoriesaccessories

�� Outlet Terminal Losses (Outlet Outlet Terminal Losses (Outlet

Pressure)Pressure)


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 obtained These Losses /100 feet of duct can be obtained directly from Friction Loss Chart at specified flow directly from Friction Loss Chart at specified flow and 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 or These occurs mainly at fittings where velocity or direction of flow changes.direction of flow changes.

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

�� These are expressed in terms extra equivalent length of These are expressed in terms extra equivalent length of ducts.ducts.

Friction & Dynamic losses in ductFriction & Dynamic losses in duct’’s accessoriess accessories

Losses in attenuators, Terminal Heating coils and fire Losses in attenuators, Terminal Heating coils and fire dampers etc. Taken from manufacturer cataloguesdampers etc. Taken from manufacturer catalogues


(Frictional Losses)(Frictional Losses)

Total losses are calculated thru the longest duct run Total losses are calculated thru the longest duct run

including all elbows and fittings.including all elbows and fittings.

Effective Effective Eq.LengthEq.Length of Longest Duct run X Friction Loss /100 Feet of Longest Duct run X Friction Loss /100 Feet

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

100100

�� Total Effective Total Effective Eq.LengthEq.Length

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

��Fittings Fittings Eq.lengthsEq.lengths are obtained from Table are obtained from Table --10 10

& 12.& 12.

�� Friction Loss/100 feetFriction Loss/100 feet

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

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

Total Static LossesTotal Static Losses

�� Sum of the losses in supply air ducts Sum 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

STATIC REGAIN METHODSTATIC REGAIN METHOD

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

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

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

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

friction loss in succeeding section of friction loss in succeeding section of

ducts.ducts.

�� The static pressure is then the same The static pressure is then the same

before each terminal and at each branch.before each terminal and at each branch.

�� Static pressure regains due to reduction Static pressure regains due to reduction

in velocity.in velocity.

AXIAL FLOW FANSAXIAL FLOW FANS

�� Axial flow fans produce pressure from the Axial flow fans produce pressure from the change in velocity of air passing thru the change in velocity of air passing thru the impeller,with none being produced by impeller,with none being produced by centrifugal force.centrifugal force.

�� In axial flow fan, air flows axially thru the In axial flow fan, air flows axially thru the impeller.impeller.

�� Type Type –– Propeller 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)

Fan LawsFan Laws

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.

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 Loss System 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

Fan Selection CriteriaFan Selection CriteriaFan Selection CriteriaFan Selection CriteriaFan Selection CriteriaFan Selection CriteriaFan Selection CriteriaFan Selection Criteria

�� ApplicationApplication

�� Availability of space.Availability of space.

�� Air flowAir flow

�� Duct System Static lossDuct System Static loss

�� Noise level Noise level

�� Location of Fan w.r.t. Location of Fan w.r.t.

conditioned space.conditioned space.

�� Economic factorsEconomic factors

Duct Design Rev

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