AES Ducts Engineering Basis

8/9/2019 AES Ducts Engineering Basis

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Ductwork Design Program - Engineering Basis SOM-IBM Architecture & Engineering Series (AES) - 1988

Varkie C. Thomas, Ph.D., P.E. Skidmore, Owings & Merrill, LLP 1

______________________________________________________________________________

Engineering Design Basis

Ductwork Design Program

Copyright 1998 Skidmore, Owings & Merrill. All rights reserved.

______________________________________________________________________________


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Air Flow and Pressure Analysis

Preliminary Analysis

Information that is required to analyze a duct system for flow, pressures and sizes include:

Supply or extract air quantities for each terminal device (diffuser, register, grille) in the room. The location of the terminal device in the room and the routing of ductwork from fan to

terminal

Design criteria limits for the project. These include:Sizing method and associated velocity limits

Ductwork dimensional criteria

Static and total pressure limits to be used in

Fan SelectionDuctwork design and sizing

Air quantities for terminal devices located in each room can be calculated by the HVAC

Application Loads Program. Locating terminal devices, assigning air quantities to them and

during graphic input routing ductwork from the fans to the terminal devices.

Sizing Principles

Duct design is essentially a solution of two basic equations, the relationship between duct

velocity, air quantity and duct cross-sectional area, Eq. 1, and Bernoulli's energy balance

equation, Eq. 2.

where: 1,2 = subscripts for stations 1 and 2 in the system

A = cross-sectional area of duct (sq in.)

V = fluid average velocity (ft/min)

Q = airflow rate (cu ft/min)

gc = dimensional constant (lbm - ft)/(lbf - s2)

P = static pressure (lbf/sq ft)

= fluid density (lbm/sq ft)

fc

AV=Q

P+Z+P+P+g2V

=Z+P+P+g2

Vt222Z

c

22

111z

c

21


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= g__gc

= specific weight (lbf/cu ft)

g = acceleration due to gravity (ft/sec2)

Z = elevation (ft)P1, P2 = gage pressures at stations 1 and 2 (lb/sq ft)

PZ1, PZ2 = atmospheric pressures at elevations Z1and Z2 Pt = total pressure loss between station 1 and station 2 in the system (lbf/sq

ft)

cf = conversion factor (144)

When the specific weight of atmospheric air ais constant, it can be written that:

Assuming that the specific weight of atmospheric air equals that of air within the duct, and

combining Eq. 2 and 3 yields:

Head and Pressure

Head and pressure are often used interchangeably, but these terms have specific meanings.

Head is the height of a fluid column supported by fluid flow, while pressure is the normal force

per unit area. With a liquid, it is convenient to measure the head of a fluid in terms of the

flowing fluid. With a gas or air, however, it is customary to measure pressure on a column of

liquid.

Static Pressure

Air exerts pressure on the walls of the duct in which it is confined. This pressure Psis called staticpressure at a station in the system and is positive or negative according to whether the pressure

is greater or less than the ambient atmospheric pressure.

Velocity Pressure

The term ( V2/2gc) in Eq. 2 is called the velocity pressure. Acting in the same direction as the

flow of air, it is a measure of the kinetic energy. The velocity head (V2/2g) is independent of fluid

density, while velocity pressure is not independent of density.

Z-Z=P-P 12a2Z1Z

P+P+g2

V=P+

g2

Vt2

c

22

1

c

21


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where: Pv = velocity pressure (in. of water)

V = fluid mean velocity (ft/min)

= fluid density (lbm/cu ft)


Total Pressure

Total pressure is the sum of static pressure and velocity pressure:

where: Pt = total pressure (in. of water)

Ps = static pressure (in. of water)

Pv = velocity pressure (in. of water)

The pressure loss Ptin Eq. 2 is the resistance of a section of a duct system to flow and is

composed of two elements:

Duct friction, which as the name implies, is the frictional drag of the fluid moving along arough surface, the duct wall.

Dynamic loss, caused by restrictions and changes in direction to the flow through a piece ofequipment (volume damper, heating coil, sound attenuator, etc.) and duct fittings.

Frictional Losses

When air flows through a duct, there is a loss of pressure due to the frictional drag of the air

moving along the surfaces of the ducts.

For air flow in ducts, the friction loss may be calculated by the Darcy-Weisbach equation:

where: = fluid density (lbm/cu ft)

V = fluid mean velocity (ft/min)

fc

V=P

2

v

fc

V+P=P

2

st

P+P=P vst

fc

Vx

D

Lxfcf=P

2

2

1drf


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Pfr = friction losses in terms of total pressure (in. of water)

fd = friction factor, dimensionless

L = duct length (ft)

D = equivalent internal diameter of duct (in.)cf1 = conversion factor (12)

cf2 = conversion factor (1097)

Air flow in ducts follow two very different regimes: laminar flow at low velocities, and turbulent

at high velocities. A transition zone in which flow may be either laminar or turbulent exists

between the laminar and fully developed turbulent regions. Experimentation has determined

two Reynolds numbers for which the flow is entirely laminar or turbulent. This dimensionless

quantity, known as the Reynolds number Re, is defined by:

where: D = equivalent internal diameter of duct (in.)

V = fluid velocity in an equivalent round duct (ft/min)

= fluid density (lbm/cu ft)

= fluid dynamic viscosity (lbm/ft x hr)


Within the region of laminar flow (Reynolds numbers less than 2100). The friction factor fdis afunction of the Reynolds number only, and it is ndependent of the roughness coefficient of the

duct wall. It is defined by:

In completely turbulent regions (Reynolds numbers greater than 4000), the friction fddepends on

the relative roughness of the duct material. It is independent of the Reynolds number and

defined by:

In the transition zone (Reynolds numbers between 2100 and 4000), the fraction fddepends on

duct material absolute roughness and the Reynolds number represented by the Colebrook

equation:

DVxfc-Re

R

46=f

e

d

fc

D2+1.14=

f

1

d

log


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where: = duct material absolute roughness (ft)

cf = conversion factor, (12)

If the air flow is smooth (Re < 2100), then Eq. 10 is used to determine the friction factor fdto

solve the Darcy-Weisbach equation, Eq. 8, for friction drop through the duct section. To

differentiate between transitional and rough turbulent flows, a value for fdis calculated from the

equation for rough turbulent flows. Eq. 11. This is then used in Eq. 13 which is a close

approximation of the curve which separates the rough flow from transitional on the Moody chart

(ASHRAE Handbook: 1981 Fundamentals p. 4.10, fig. 13).

where: cf = conversion factor (2.16 x 10-6

)

If T is greater than 100, the flow is rough and the value calculated for fdin Eq. 11 is

representative of the flow. If T is less than 100, the flow is transitional and the value for fdobtained from Eq. 11 is used as a starting value to solve Eq. 12 iteratively for the friction factor

for the flow. fdis then used in Eq. 8 to obtain the friction loss through the duct section.

Absolute Roughness

Duct material absolute roughness used by the program is shown in Fig. 1-6:

Duct Material Absolute Roughness, / ft

Uncoated Carbon Steel, Clean 0.00015

Aluminum 0.0002

Galvanized Steel, Hot Dipped 0.0005

Stainless Steel 0.0003

Fibrous Glass Duct, Rigid 0.0003

Flexible Duct, Metallic 0.007

Fibrous Glass Duct Liner 0.015

Fig. 1-6: Duct Material Absolute Roughness

(reproduced with permission fromASHRAE Handbook: 1981

Fundamentals p. 33.5)

fD/fceR

9.3+12-

fc

D2+1.14=

f

1

dd

loglog

fcxxx8/fV=T


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Frictional Losses for Noncircular Ducts

All friction loss calculations are based on the equivalent hydraulic diameter.

With equal length of round and rectangular ducts, constant flow in each duct, and equal

resistance to flow in both the round and rectangular ducts, the equivalent round of a rectangular

duct is:

where: De = circular equivalent of a rectangular duct of equal length, fluid resistance and flow

a = length of one side of duct (in.)

b = length of adjacent side of duct (in.)

The mean velocity in a rectangular and oval duct will be less than its circular equivalent.

For oval ducts, the corresponding equations are:

where: p = perimeter of oval duct (in.)

a = length of major axis (in.)b = length of minor axis (in.)

For both rectangular and oval ducts, the length of the sides is initially determined by the target

aspect ratio. If the resulting dimensions fall outside the minimum and maximum allowable limits

you have set, the dimensions are recalculated without using the target aspect ratio.

)b+(a

)b(a1.30=D 0.250

0.625

e

b)-(ab+4

b=A

2

b)-(a2+b=P

pA

1.55=D 0.25

0.625

e


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Dynamic Losses

Dynamic losses are caused by restrictions and changes in direction to the flow through a piece of

equipment (volume damper, heating coil, etc.) and duct fittings. HVAC Systems Duct Design,

SMACNA, 1985 lists the fittings available for round and rectangular ducts. Since little dynamic

loss data for oval fittings are available, the data for rectangular fittings are used as an

approximation.

Fittings

A duct fitting can occur anywhere along the length of a duct section. The program does not limit

the number of fitting types or multiples thereof per duct section. If a fitting type is not available

in the tables, its dynamic loss has to be entered as a special loss.

All the necessary engineering performance information for fittings is provided in the Ducts

Program. The engineering design effort is to locate the appropriate fitting type in the duct

network system. The duct fitting type and shape type should be compatible. Fittings are

classified as junctions, transitions, and elbows.

Junctions Junctions are fittings which split the air stream into two or more branches.

Converging junctions join two or more air streams into one and are basically

used in a return/extract duct system. Fittings called take-offs, tees, and wyes

are in this category. Loss coefficients for junctions are functions of the duct

dimensions, air velocities and airflow rates.

Transitions Transitions are fittings which change the duct size or shape without changing

airflow direction or airflow rate. Transitions can be converging or diverging.

Loss coefficients for transitions are functions of upstream and downstream

duct velocities, angle of transition, transition length, and Reynolds number, Re.

Elbows Elbows are fittings which change the direction of the air stream without

changing the air quantity or velocity. The loss coefficients of elbows are

functions of the elbow radius, duct dimensions, angle of turn, and Reynolds

number, Re.

By definition, a new duct section occurs when there is a change in air quantity, velocity, shape,

duct material or duct insulation. Every duct section, therefore, begins with a junction or

transition type fitting. These fittings are commonly referred to as take-off fittings. There is

always one, and only one, take-off fitting per duct section.

Fitting Losses

Methods of computing the energy losses from the various fitting types are based on information

found inASHRAE Handbook: 1981 Fundamentalsp. 33.28 through 33.50


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The fluid resistance coefficient represents the ratio of the total pressure loss to the dynamic

pressure at the referenced cross-section O:

where: cf = conversion factor (1097)

Pt = total losses of fitting in terms of total pressure (in. of water)

Co = overall fluid resistance coefficient referenced to section O, dimensionless

V = average velocity to which coefficient Cois referenced (ft/min)

Pv,o = velocity pressure (in. of water) = fluid density (lbm/cu ft)

For entries, exists, elbows and transitions, the fitting total pressure loss at section is calculated

by:

where the subscript o is the cross section at which the velocity pressure is referenced.

For converging and diverging flow junctions, the total pressure loss through the main section is

calculated as:

For total pressure losses through the branch section

where: Cc,s = main local coefficient, dimensionless

Cc,b = branch local coefficient, dimensionless

Pv,c = velocity pressure at the common section, c

P

P=

fc

V

P=Cov,

t

2

o

to

PC=P ov,ot

PC=P cv,sc,t

PC=P cv,bc,t


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A tee nomenclature is shown inFig. 1-7for converging and diverging flow junction where,

(reproduced with permission from ASHRAE Handbook:

1981 Fundamentals, Fig. 6, p. 33.8)

Fan Pressures, Mechanical Energy

For a supply duct system, a circuit is defined as the succeeding sections of ductwork from the fan

discharge up to either a terminal device or a primary damper (for constant-volume, fan-powered

boxes only). For an extract duct system, a circuit is defined as the succeeding section ofductwork from a terminal device up to a fan.

The number of circuits in a duct system is, by definition, equal to the number terminal devices

and/or constant-volume, fan-powered boxes.

The supply and extract duct systems are run independently; however, the supply system does

need the total pressure losses from the extract system in order to determine the fan total

pressure if a return fan is not specified. Therefore, the extract duct system should be run first,

the total pressure losses of the extract duct system are compared to the loss from the outdoor

intake to the mixing box. The greater of the two is used as miscellaneous duct losses for the

supply system.

The total pressure losses for each section of a duct system are calculated by Eq. 22:

where: Pt = total pressure loss for section of ductwork (in. of water)

Pi = fitting total pressure loss (in. of water)

n = number of fittings within a section of ductwork

PC=P|PC=pPC

=P

|PC

=P

cv,bc,b)to(ctcv,bc,c)to(bt

cv,sc,s)to(ctcv,sc,c)to(st

P+P+P=P kj

m

1=j

i

n

1=i

t


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Pj = equipment (damper, coil, etc.) total pressure loss (in. of water)

m = number of pieces of equipment within a section of ductwork

Pk = duct friction loss in terms of total pressure (in. of water)

The total pressure drop through each circuit is calculated by summing the pressure loss through

each section in the circuit.

The fan total pressure requirement for an extract duct system is given by the expressions:

where: Pt = fan total pressure (in. of water)

? = pressure drop through critical circuit, circuit with maximum resistance to flow

(H2O)

SEFI = system effect factor due to fan inlet conditions (in. of water)

SEFo = system effect factor due to fan outlet conditions (in. of water)

Pf = total pressure loss through fan components such as coils, filters, etc. (H2O)

Pd = total pressure loss through miscellaneous ductwork such as ductwork

downstream of the fan for an extract system, or extract or return ductwork for

a supply system (H2O)

Duct Sizing

The sections of supply duct systems can be sized using one of the following methods:

Pre-sized equal friction

static regain total pressure velocity reduction constant velocity

Equal friction and constant velocity are the only methods for the design of extract duct systems.

For a draw-through fan, the first section is the one immediately downstream of the fan outlet. If

the supply fan arrangement is blow-through, the first section of the ductwork is the section

immediately downstream of the cooling coil. If the fan has a discharge plenum, the first section

of the ductwork is the one immediately following the plenum. You should identify the first

section and lay out the duct network accordingly.

The static regain, total pressure and velocity reduction sizing methods cannot be used to size

duct sections downstream of terminal boxes. If you select one of these three methods

P+P+SEF+SEF+P=P dfoitt


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downstream of a terminal box, the program will automatically default to the equal friction

method.

Flexible ducts are always considered round in shape. They are sized using the velocity youspecify.

Pre-Sized Method

The pre-sized method can be used to calculate pressure losses for a system with pre-calculated

duct dimensions. If you choose this method, the program will check the network for duct

dimensions. Only if all sections are sized will it calculate pressure drops through each section

and determine critical circuits.

Equal Friction Sizing Method

In the equal friction method, the system is sized for a constant pressure loss per unit length of

duct. The equal friction method can be used for the design of supply and extract duct systems.

The program offers two options for this method. The first option, called ASHRAE Limits, allows

the designer to specify high and low limits for pressure loss per unit length and velocity.

For the other option, you can specify the pressure loss per unit length and the maximum and

minimum velocity for the sections of a duct system.

The equal friction sizing method works iteratively between the minimum and maximum velocity

limits to determine a duct size that results in the specified pressure loss per unit length. You can

also request a pressure analysis of the network.

Static Regain Sizing Method

For this method, a section of the duct system is sized so that the increase in static pressure due

to velocity reduction from its upstream section, offsets the friction loss in the section. As in the

other sizing methods, the program starts sizing with the first section. If you have not specified

any overrides for the ducts in that section, it is sized based on the maximum velocity you

specified.

When sizing any other section, the program searches upstream of that section until it finds a

section that also used the static regain sizing method. The velocity at the exit of that section is

used as the upper velocity limit for the section to be sized; the minimum you specified is used as

the lower limit. The program searches iteratively between these limits to calculate a duct size

that results in the required static regain. If the static regain calculated is less than that required

even at minimum velocity, the section is sized using minimum velocity.

If the program encounters a section where all upstream sections are pre-sized, that section is

sized using maximum velocity and the sizing method is applicable starting at that section.


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Essentially, the program disregards any sections that have been overridden or that use another

sizing method.

The advantage of this method is that all sections have approximately the same entering staticpressure, thereby simplifying outlet selection. One disadvantage might be seen in networks with

a large pressure drop in a section near the fan outlet. The velocity could be reduced to the

minimum within a few sections in such a way that all the ductwork downstream would be sized

using minimum velocity. Another disadvantage could stem from specifying a very low minimum

velocity. Ducts would then tend to be very large at the end of long branch runs. The sizing

method does not account for the total mechanical energy supplied to the air by the fan.

Total Pressure Sizing Method

The total pressure sizing method is a variation of the static regain method. The total pressure ofany point in the ductwork represents the actual energy of the moving air at that point.

The program will search for the first section that does not have override dimensions. This

section is sized at the maximum velocity you have specified. When sizing any other section, the

program searches upstream until it finds a section that also used the total pressure sizing

method. The total pressure leaving that section is considered equal to the total pressure

entering the section to be sized. The program then uses binary convergence, starting with the

minimum velocity, until it finds a duct size that matches the required entering total pressure. All

presized duct sections are disregarded when applying the total pressure sizing method to the

sections to be sized.

The advantage of this method is that it accounts for all mechanical energy losses in a system.

The system design does not have to be dependent on an assumed velocity at the fan outlet.

Velocity Reduction Sizing Method

This method sizes the first section for the maximum velocity that you specified and designs the

downstream sections at progressively lower velocities using the reduction percentage you also

specified. The velocity cannot fall below the minimum that you specified.

If the section immediately above the section to be sized is a presized section, the program

searches upstream until it finds a section that also used the velocity reduction sizing method and

treats it as the upstream section for the section to be sized.

Constant Velocity Sizing Method

The constant velocity method is a design in which every section of a duct system is sized based

on the design velocity you specified. This design velocity may be overridden for any section of

the system. Maximum and minimum velocity limits are not applicable in this method. Flexible


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ducts are also sized based on a design velocity you specified. The constant velocity method can

be used for the design of supply and extract duct systems.

Circuit Analysis and System Balancing

The series of duct sections starting from the primary fan/air handling unit section to the duct

section with an opening is defined as a circuit. The number of duct circuits in a duct system is, by

definition, equal to the number of supply and exhaust openings (registers, grilles or diffusers).

Circuit pressure analysis consists of the following steps:

Add the pressure losses in each section of the circuit to obtain the circuit pressure loss. Determine the highest circuit pressure loss.

Set the entering total pressure of the first section in the circuit as the sum of the highestcircuit pressure loss and the velocity pressure in the section.

Analyze each section in the circuit starting with the first section and moving through eachsection in the fluid flow sequence.

The entering section total pressure is the leaving section total pressure of the upstream

section.

The leaving section total pressure is the entering section total pressure less the section

pressure loss.

The entering and leaving section static pressures are obtained by deducting the velocity

pressure at these nodes.

Calculate the balancing required in each circuit. This is equal to the highest circuit totalpressure loss minus the given circuit total pressure loss.

In the case of primary-secondary systems, the primary circuit ends at the fan-powered box or the

terminal device if there is no box in the circuit.

Secondary systems are analyzed independently as separate systems. The entering pressure of

the first section in the secondary system is the pressure loss of the secondary circuit with the

highest pressure loss.

The objective of system balancing is to maintain the same pressure loss in all circuits. Dampers

at the end of each circuit will still be required, but are used to make the fine tuning adjustmentto maintain the right air quantity at each opening.

Using the specified sizing method, the Ducts Program makes the preliminary analysis and

calculates sizes for all sections of the duct system. It next determines the circuit with the highest

pressure loss. This circuit is commonly referred as the longest hydraulic run. All other circuits

will have to be dampered. The amount of dampering is directly related to the difference

between the pressure loss of the given circuit and the highest pressure loss of the system.

The system balancing process involves a series of iterations to increase the pressure losses of all

circuits to that of the circuit with the highest pressure loss. The process starts with the circuit


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with the next highest pressure loss and continues in descending order of circuit pressure loss.

Duct sections in the circuit and that are not common to the previously analyzed circuits are

iteratively reduced in size until the circuit pressure loss is equal to the loss in the longest run.

There are limits to the process of reducing sizes. The iteration stops when the pressure loss,velocity and noise levels in the duct section reaches the maximum limits of the design criteria. In

this case, the circuit still remains unbalanced and requires dampering. Output reports indicate

the amount of dampering required by each circuit.

Thermal Analysis

After the duct network has been sized, the system can be analyzed to calculate heat gains or

losses in the network.

The thermal analysis option is available only for systems that satisfy the following criteria: There are no terminal boxes in the network with reheat coils. All rooms are in either the heating or cooling mode. There are no secondary heating coils in the system.

The program begins the thermal analysis with the first section in the ductwork. The temperature

of air entering the first section is the same as the coil leaving temperature for a blow-through

system. For a draw-through system, the temperature of air entering the first section is adjusted

for heat gain across the fan. For a motor that is inside the airstream, the heat gain is given by

For a motor that is outside the airstream, the heat gain is

where: qm = heat gain from electric motor (Btu/hr)

HPm = horsepower rating for the motor = full load motor efficiency (%)

LF = load factor, i.e. a fraction of rated load delivered

cf = conversion factor (745.7)

The horsepower rating and full load efficiency are obtained from Fig. 1-8.

The temperature rise across the fan is then calculated using the following equation:

cfxLFx/100HP=q mm

cfxLFxHP=q mm


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where: Tf = temperature rise across the fan (F)

qm = heat gain from electric motor (Btu/hr)

Q = air flow quantity (cu ft/min)

cf1 = conversion factor (14.4)

= air density (lbm/cu ft)

Rated

Motor HP

Full Load

Efficiency %

Rated

Motor HP

Full Load

Efficiency %

0.16 35 20 87

0.25 54 25 88

0.33 56 30 89

0.50 60 40 89

0.75 72 50 89

1 77 60 89

2 79 75 90

3 81 100 90

5 82 125 90

7.5 84 150 91

10 85 200 9115 86 250 91

Fig. 1-8: Heat Gains from Typical Electric Motors

(reproduced with permission fromASHRAE Handbook: 1981 Fundamentalsp. 26.26, Table 24)

Once the temperature of air entering the first section is determined, the individual sections can

be analyzed for heat gains or losses. The program calculates heat gains or losses using the

following expressions:

xfcxQ

q=T

1

mf

t-

2

t+t

cf

UPL=q a

ie

e

1)-y(

t2-1)+y(t=t

aie


17/68



where: Y = cf 1AV /UPL for rectangular and oval ducts

y = cf 2DV /UL for round ducts

qe = heat loss/gain through duct walls (Btu/hr) (Negative for heat gain)

U = overall coefficient of heat transfer of duct walls (Btu/sq ft. F)

P = perimeter of duct (in.)

A = cross-sectional area of duct (sq in.)

D = duct diameter (in.)

L = duct length (ft)

= density (lbm/cu ft)

andte = temperature of air entering duct (F)

ti = temperature of air leaving duct (F)

ta = temperature of air surrounding duct, (F)




The U-values used in the program for sheet metal, lined and insulated ducts are shown in Fig 1-9.

(reproduced with permission from ASHRAE Handbook: 1981 Fundamentals, p. 33.10, Fig. 8)

1)+y(

t21)-y(t=t

aei


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Reanalysis Options

The program provides two ways to reanalyze heat gains and losses. You can choose to

Recalculate air flow quantities at each terminal device Adjust the coil leaving temperature

Option One

The first option is to recalculate air flow quantities required at each terminal device in order to

satisfy the design cooling or heating loads in each room. This is done by comparing the actual

supply air temperature to the design supply air temperature that you can specify for each room.

The following expression is used to recalculate the air flow quantities:

where: Q1 = design supply air quantity (cu ft/min)

Q2 = recalculated supply air quantity (cu ft/min)

Tr = design room dry-bulb temperature (F)

Ts = design supply air temperature at terminal device (F)

To = actual supply air temperature at terminal device (F)

Once the supply air quantities are adjusted, the program then resizes the ductwork and has theoption of performing pressure, acoustic and thermal analyses for the new network.

Option Two

The second option is to recalculate the coil leaving temperature by an amount equal to the

temperature difference in the circuit with the maximum temperature rise or drop.

For a network that has net heat gain across the sections, the coil leaving temperature is lowered

so that the design supply air temperature can be maintained at the terminal device for the circuit

with the maximum temperature rise. For a network that has net heat loss across the sections,the coil leaving temperature is increased so that design supply air temperature can be

maintained at the terminal device for the circuit with the maximum temperature drop.

The next step is to recalculate supply air quantities for all other terminal devices in order to

satisfy design cooling or heating loads for each room.

Once the required air quantities are recalculated, the program then resizes the network and has

the option of performing pressure, acoustic and thermal analyses for the network.

T-T

T-TQ=Qor

sr

12


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Acoustic Analysis and System Noise Attenuation

The program determines the amount of required attenuation on the inlet and discharge sides of

the fan(s), or in the main ductwork and branches as described below.

For each circuit, the program determines the resultant attenuation provided by the following

items:

1. In straight, line and unlined duct

2. In lined and unlined elbows

3. Fan discharge plenum (supply ductwork only)

4. Branch takeoffs

5. Duct and reflection loss

6. Room effect

The resultant sound power level per terminal device (inlet or outlet) is the difference between

the attenuation of the ductwork system and room effect (items 1 through 6) and the fan-

generated sound power level for all eight octave bands. The difference between this resultant

sound power level per diffuser and the sound pressure levels that correspond with the specified

room criterion (NC level) represents the attenuation required on the inlet and discharge sides of

the fan(s), or in the main ductwork and branches.

Fig. 1-10 summarizes the system noise procedure described for each circuit.

Line Item1 Fan Lwre 10

-2watts

2 Attenuation of duct system

3 Branch take-offs and room effect attenuation

4 Total system attenuation, line 2 + line 3

5 Sound Power level per terminal device

6 Room criterion (NC level), line 1 - line 4

7 Required attenuation, line 5 - line 6

Fig. 1-10: Summary Table

On determining the resultant sound power level for each circuit, the program does not take into

account the following items:

attenuation through terminal boxes terminal box discharge sound power level regenerated noise at fitting attenuation in fittings other than elbows and branch takeoffs


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Fan Noise

The sound power generation of a given fan obtained from the fan manufacturer may be entered

using the fan sound power levels input form. However, if the data are not available, the programcan estimate the octave band sound power levels for various fans using the following expression:

where: Lw = estimated sound power level of fan (dB re 1pW)

Kw = specific sound power level from table

Q = flow rate (cu ft/min)

Q1 = 1 when flow is in sq ft/min

P = Fan static pressure (in. of water)

P1 = 1 when pressure is in in. of water

C = correction factor in dB, for point of operation

The values of the estimated sound power level L ware calculated for all eight bands using Eq. 30,

and the BFI (see Fig. 1-11) is added to the octave band in which the blade passage frequency

falls. Fig. 1-12 is used to determine the octave band in which blade frequency increment (BFI)

occurs for the various types of fans.

Fan TypeFan Octave Band Center Frequency Hz

63 125 250 500 1000 2000 4000 8000 BFI

Centrifugal Airfoil

backward curved, >= 36 in. 32 32 31 29 28 23 15 10 3

Backward inclined < 36 in. 36 38 36 34 33 38 20 15

Forward, curved All 47 43 39 33 28 25 23 20 2

Radial blade, > 40 in. 45 39 42 39 37 32 30 29

Pressure blower 40 in.-20 in.

20 in.

55

63

48

57

48

58

45

50

45

44

40

39

38

38

36

37 8

Vaneaxial >= 40 in.

40 in.

39

37

36

39

38

43

39

43

37

43

34

41

32

38

30

35 6

Tubeaxial >= 40 in.

40 in.

41

40

39

41

43

47

41

46

39

44

37

43

34

37

31

32 5

Propeller

Cooling tower All 48 51 58 55 52 46 40 5

Fig. 1-11: Sound Power Levels

Specific sound power levels for inlets or outlets.

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.7, p. 11.14)

C+P

P+20+

Q

Q10+K=L

11

ww loglog


21/68



Fan Type Octave Band

Centrifugal

Airfoil, backward curved, backward

inclined

Forward curved

Radial blade, pressure blower

250 Hz

500 Hz

125 Hz

Vaneaxial 125 Hz

Tubeaxial 63 Hz

Cooling Tower Propeller 63 Hz

Fig. 1-12: Octave Band

Octave band in which blade frequency increment occurs.

(reproduced with permission from1987 ASHRAE Handbook, Table 4, p. 52.7)

Point of Operation

The specific sound power levels given in Fig. 1-11 are for fans operating at or near the peak

efficiency point of the fan performance curve. If a fan is not operating at or above 90% of peak

static efficiency, a correction factor, C (see Eq. 30), is added to the specific sound power levelsgiven in Fig. 1-13 for all eight octave bands. Fig. 1-12 gives the correction factor C as a function

of percent of peak static efficiency.

Static Efficiency

% of Peak

Correction Factor

dB

90 to 100 0

85 to 89 3

75 to 84 665 to 74 9

55 to 64 12

50 to 54 15

Fig. 1-13: Correction Factor C

Correction factor, C, for off-peak operation (reproduced with permission fromHVAC Systems

Duct Design, SMACNA, Table 11.8, p. 11.15)


22/68



Fans In Parallel

The program can accept up to ten fans in parallel. These fans must be of the same type andproduce the same static pressure and flow rate. The resultant sound power levels of two or

more fans in parallels for all octave bands are obtained from Fig. 1-14.

Difference

between sound

levels in dB

No. of dB to be

added to higher

level

0 3.0

1 2.6

2 2.1

3 1.8

4 1.5

5 1.2

6 1.0

7 0.8

8 0.6

10 0.4

12 0.3

14 0.2

Fig. 1-14: Combining Decibels

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.1, p. 11.2)

Example: Find the resultant sound power levels of three fans in parallel having an estimated

sound power level equal to 90 dB at the 125 octave band using Fig. 1-14.

Solution:1. 90 dB - 90 dB = 0 dB from Fig. 1-14, 3 dB is added to the higher level 90 dB. The

resultant sound power level for 2 fans in parallel is 90 + 3 = 93 dB.

2. 93 dB - 90 dB = 3 dB from Fig. 1-14, 1.8 dB is added to the higher level 93 dB. The

resultant sound power level for 3 fans in parallel is 93 + 1.8 = 94.8 dB.


23/68



Fan Discharge Air Plenum

The sound attenuation provided by a plenum on the fan discharge shown in Fig. 1-15 is

where: = absorption coefficient of lining, dimensionless

Se = plenum exit area (sq ft)

Sw = plenum wall area (sq ft)

dr = distance between entrance and exit (ft)

= angle of incident at exit, degrees

Attenuation of the Duct System

The program calculates the natural attenuation of unlined rectangular and round duct sections of

the duct system using Fig. 1-16 and Fig. 1-17. These tables give the natural attenuation for linear

length as a function of the duct dimension for all the eight octave bands.

Fig. 1-16 is also used for oval ducts. The program uses the smallest duct dimensions for

rectangular and oval ducts. The attenuation values given in Fig. 1-16 and Fig. 1-17 are doubled if

the unlined duct section has an external insulation.

The program calculates the natural attenuation of lined rectangular and round duct sections and

elbows using Fig. 1-18 through Fig. 1-21. These tables give the attenuation as a function of duct

dimension and lining thickness for all eight octave bands.

Fig. 1-18 and Fig. 1-19 are also used for oval ducts. The program uses the smallest duct

dimension for rectangular and oval ducts. The attenuation values given in Fig. 1-18 through Fig.

1-21 are used for duct sections of only a maximum of 15 linear feet of lined duct, lined duct

sections of over 15 linear feet are considered as unlined duct.

wS

-1+

d2S

110=A

2e

coslog


24/68



Smallest Duct

Dimension

Octave Band Center Frequency Hz

63 125 250 500 1000 2000 4000 8000

Attenuation 3"-12" .10 .10 .05 .05 .04 .04 .04 .04

Of Unlined 13"-18" .15 .10 .05 .05 .04 0.4 0.4 0.4

Rectangular 19"-24" .18 .12 .06 .05 .04 0.4 0.4 0.4

Duct dB/ft 25"-36" .20 .14 .06 .05 .04 0.4 0.4 0.4

37"-48" .20 .14 .06 .05 .04 .04 .04 .04

49"-72" .20 .15 .06 .05 .04 .04 .04 .04

73"-96"+ .20 .16 .07 .05 .04 0.4 0.4 0.4

Fig. 1-16: Attenuation, Unlined Rectangular and Oval Ducts

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.10A, p. 11.16)

Smallest Duct

Dimension


63 125 250 500 1000 2000 4000 8000

Attenuation 3"-5" .03 .03 .02 .02 .01 .01 .01 .01

Of Unlined 6"-12" .03 .03 .02 .02 .01 .01 .01 .01

Round 13"-18" .02 .02 .01 .01 0 0 0 0

Duct dB/ft 19"-24" .02 .02 .01 .01 0 0 0 0

25"-36" .02 .02 .01 .01 0 0 0 0

37"-48" .01 0 0 0 0 0 0 0

49"-72" .01 .01 0 0 0 0 0 0

73"-96"+ .01 .01 0 0 0 0 0 0

Fig. 1-17: Attenuation, Unlined Round Ducts

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.10B, p. 11.16)

Smallest Duct

Dimension


63 125 250 500 1000 2000 4000 8000

Attenuation 3"-5" .3 .6 1.0 2.1 5.0 10.5 5.0 4.0

Of Unlined 6"-12" .2 .4 .9 1.9 4.3 7.5 2.0 2.0

Rectangular 13"-18" .2 .3 .6 1.5 3.7 3.0 1.0 1.0

Duct dB/ft with 19"-24" .2 .2 .5 1.4 3.5 1.8 .8 .8

1 lining 25"-36" .2 .2 .4 1.0 1.8 1.2 .6 .6

37"-48" .2 .2 .3 .9 1.5 1.0 .4 .4

49"-72" .2 .2 .25 .8 1.0 .8 .3 .3

73"-96"+ .2 .2 .25 .7 .8 .7 .2 .2

Fig. 1-18: Rectangular Duct, 1 Inch Lining

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.10E, p. 11.17)


25/68



Smallest Duct

Dimension


63 125 250 500 1000 2000 4000 8000

Attenuation 3"-5" .2 .4 .7 1.3 2.5 5.0 5.0 4.0

Of Unlined 6"-12" .2 .3 .6 1.1 2.2 4.5 2.0 2.0

Rectangular 13"-18" .2 .25 .5 1.0 2.0 1.5 1.0 1.0

Duct dB/ft 19"-24" .2 .2 .4 .8 1.6 1.4 .8 .8

with 1/2 25"-36" .2 .2 .3 .6 1.2 1.0 .6 .6

lining 37"-48" .2 .2 .2 .4 .8 .8 .3 .3

49"-72" .2 .2 .2 .3 .5 .5 .3 .3

73"-96"+ .2 .2 .2 .2 .4 .4 .2 .2

Fig. 1-19: Rectangular Duct, 1/2 Inch Lining

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.10F, p. 11.17)

Smallest Duct

Dimension


63 125 250 500 1000 2000 4000 8000

Attenuation 6"-12" .3 1.0 1.5 1.5 3.7 4.8 2.8 2.2

Of Unlined 13"-18" .2 .7 1.0 1.2 2.7 2.8 1.5 1.3

Round 19"-24" .1 .5 .6 1.0 1.7 .9 .5 .5

Duct dB/ft 25"-36" .07 .2 .4 .8 1.0 .7 .5 .5

with 1 37"-48" .04 .08 .3 .6 .6 .5 .5 .5

lining 49"-72" .02 .04 .2 .5 .5 .4 .4 .4

73"-96"+ .01 .02 .1 .4 .4 .3 .3 .3Fig. 1-20: Round Duct, 1 Inch Lining

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.10G, p. 11.17)

Smallest Duct

Dimension


63 125 250 500 1000 2000 4000 8000

Attenuation 6"-12" .5 1.2 1.7 2.3 3.9 5.0 3.0 2.3

Of Unlined 13"-18" .4 1.0 1.2 2.2 3.0 3.0 1.7 1.5

Round 19"-24" .3 .8 .9 2.1 2.0 1.0 .8 .7

Duct dB/ft 25"-36" .2 .3 .7 1.5 1.5 .8 .6 .6

with 2 37"-48" .12 .2 .5 1.0 1.0 .7 .6 .6

lining 49"-72" .08 .1 .3 .7 .7 .5 .5 .5

73"-96"+ .06 .08 .2 .6 .6 .4 .4 .4

Fig. 1-21: Round Duct, 2 Inch Lining

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.10H, p. 11.17)

The attenuation of unlined rectangular and round elbows is given in Fig. 1-22. The program

doubles the attenuation values given in this table if the unlined elbow has external insulation.


26/68



Smallest Duct

Dimension


63 125 250 500 1000 2000 4000 8000

Attenuation To 4" 0 0 0 0 0 1.0 2.0 3.0

Of Unlined 5"-10" 0 0 0 0 1.0 2.0 3.0 3.0

Elbows 11"-20" 0 0 0 1.0 2.0 2.0 3.0 3.0

Rectangula 21"-40" 0 0 1.0 2.0 3.0 3.0 3.0 3.0

Duct dB/ft 41"-80"+ 0 1.0 2.0 3.0 3.0 3.0 3.0 3.0

Fig. 1-16: Unlined Rectangular and Round Elbows

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.10C, p. 11.16)

The attenuation of lined rectangular and round elbows is given in Fig. 1-23.

Smallest Duct

Dimension


63 125 250 500 1000 2000 4000 8000

Attenuation To 4" 0 0 0 1.0 2.0 3.0 4.0 6.0

Of lined Elbows 5"-10" 0 0 1.0 2.0 3.0 4.0 6.0 8.0

Round & 11"-20" 0 1.0 2.0 4.0 6.0 8.0 10.0

Rectangular 21"-40" 1.0 2.0 3.0 4.0 5.0 6.0 8.0 10.0

Duct dB/ft 41"-80"+ 2.0 3.0 4.0 5.0 6.0 8.0 10.0 12.0

Fig. 1-23: Lined Rectangular and Round Elbows

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.10D, p. 11.16)


63 125 250 500 1000 2000 4000 8000

Branch 0.2% 27 27 27 27 27 27 27 27

Duct 0.5% 23 23 23 23 23 23 23 23

Attenuation 1.0% 20 20 20 20 20 20 20 20

2.0% 17 17 17 17 17 17 17 17

5.0% 13 13 13 13 13 13 13 13

10.0% 10 10 10 10 10 10 10 10

20.0% 7 7 7 7 7 7 7 7

50.0% 3 3 3 3 3 3 3 3

Fig. 1-24: Branch Takeoff Attenuation

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.11A, p. 11.18)

%=CFMSystem

CFMRoomCriticalFirst


27/68



The program calculates the attention of branch takeoffs using Fig. 1-24. This table gives the

branch duct attenuation as a function of the percentage of the room to the system air flow

quantities for all eight octave bands.

The program computes the attenuation available through end reflection loss using Fig. 1-25.

Smallest

Duct

Dimensions


63 125 250 500 1000 2000 4000 8000

Duct

Diameter

End

Reflection

Loss

To 5" 17 12 8 4 1 0 0 0

6"-8" 14 10 6 2 0 0 0 0

9"-12" 12 8 4 1 0 0 0 0

13"-16" 10 6 2 0 0 0 0 0

17"-22" 8 4 1 0 0 0 0 0

23"-30" 6 3 0 0 0 0 0 0

31"-40" 4 1 0 0 0 0 0 0

41"-60"+ 2 0 0 0 0 0 0 0

Fig. 1-25: Duct End Reflection Loss

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.10B, p. 11.18)

RoomVolume

CeilingHeight


63 125 250 500 1000 2000 4000 8000

Room 1000 8' 2 2 2 2

Effect 2000 8' 2 3 3 4 4 5 5 6

(for 5000 8'-12" 5 6 6 7 7 8 8 9

average 10000 8'-10" 7 8 8 8 9 9 9 10

rooms) 10000 11'-14" 8 9 9 9 10 10 10 11

20000 8'-12" 8 9 9 10 10 11 11 12

20000 13'-15" 9 10 10 11 11 12 12 13

30000 8'-11" 11 12 12 12 13 13 13 14

30000 12'-15" 12 13 13 13 14 14 14 15

40000+ 8'-14" 10 11 11 12 12 13 13 14

40000+ 15'-17" 11 12 12 13 13 14 14 15

40000+ 18'-22" 12 13 13 14 14 15 15 16

Fig. 1-26: Room Effect Attenuation

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.10C, p. 11.18)

The program calculates the room effect attenuation using Fig. 1-26. This table gives the

attenuation for room with suspended ceilings as a function of the ceiling height and room

volume for all eight octave bands.


28/68



Room Criteria

Fig. 1-27 gives the sound pressure levels for all eight octave bands that correspond with a

specified NC level for the room.


63 125 250 500 1000 2000 4000 8000

NC

Noise

Criterion

Levels

NC-30 57 48 35 31 29 28 28 27

NC-35 60 53 46 40 36 34 33 32

NC-40 64 57 51 45 41 39 38 37

NC-45 67 60 54 49 46 44 43 42NC-50 71 64 59 54 54 49 48 47

Fig. 1-27: Sound Pressure Levels

(reproduced with permission fromHVAC Systems Duct Design, SMACNA, Table 11.11D, p. 11.18)

Material Estimation

Classification of Ducts

This feature of the program allows you to select a gage or thickness and reinforcement required

for the ducts once they have been sized.

Operating pressures for each section of ductwork are calculated earlier during the pressure

analysis. Fig. 1-28 gives the pressure class for each section and corresponding operating

pressure. Once the operating pressures are determined, the sections are analyzed based on the

specified shape, rectangular/oval or round.

Static Pressure

Pressure Class Operating Pressure

0.5 in. of water up to 0.5 in. of water

1 in. of water over 0.5 in. of water to 1 in. of water

2 in. of water over 1 in. of water to 2 in. of water





Fig. 1-28 Operating Pressures

(reproduced with permission from HVAC Duct Construction Standards, SMACNA, p. 106)


29/68



Duct Materials

The thickness and weight of sheet-metal sheets available in the program are given in Fig. 1-29

throughFig. 1-32reproduced with permission fromHVAC Duct Construction Standards, SMACNA,Appendix 1 through 4. The program uses the nominal thicknesses from the tables. Minimum

and maximum thicknesses are shown for your information only.

Gage

Thickness in InchesWeight

Lb/ft2

Min Max Nom

30 .0105 .0145 .0125 .525

28 .0136 .0176 .0156 .656

27 .0142 .0202 .0172 .722

26 .0158 .0218 .0188 .788

25 .0189 .0249 .0219 .919

24 .0220 .0280 .0250 1.050

23 .0241 .0321 .0281 1.181

22 .0273 .0353 .0313 1.313

21 .0304 .0384 .0344 1.444

20 .0335 .0415 .0375 1.575

19 .0388 .0488 .0438 1.838

18 .0450 .0550 .0500 2.100

17 .0513 .0613 .0563 2.36316 .0565 .0685 .0625 2.625

15 .0643 .0763 .0703 2.953

14 .0711 .0851 .0781 3.281

12 .0100 .1184 .1094 4.594

11 .1150 .1350 .1250 5.250

Fig. 1-29: Stainless Steel

(reproduced with permission fromHVAC Duct Construction Standards, SMACNA, Appendix 1)


30/68



M.S. Gage Weight lb/ft2

Thickness

Nominal Cold Rolled

Min Max

28 .625 .0149 in .0129 in .0169 in

26 .750 .0179 in .0159 in .0199 in

24 1.000 .0239 in .0209 in .0269 in

22 1.250 .0299 in .0269 in .0329 in

20 1.500 .0359 in .0329 in .0389 in

18 2.000 .0478 in .0438 in .0518 in

16 2.500 .0598 in .0548 in .0648 in

14 3.125 .0747 in .0697 in .0797 in

12 4.375 .1046 in .0986 in .1106 in

Fig. 1-30: Standard Gage Uncoated Steel


Thickness in Inches Weight

Nom. Min. Max. lb/ft2

.016 .014 .018 .228

.020 .0175 .0225 .285

.024 .0215 .0265 .342

.025 .0225 .0275 .356

.032 .0295 .0345 .456

.040 .037 .043 .570

.050 .046 .054 .713

.063 .059 .067 .898

.080 .076 .084 1.140

.090 .086 .094 1.283

.100 .095 .105 1.426

.125 .12 .13 1.782

Fig. 1-31: Aluminum Alloy 3003-H14

(reproduced with permission fromHVAC Duct Construction Standards, SMACNA, Appendix 3


31/68



Gage

Thickness in Inches Weight

Min Max Nom Nom lb/ft2

30 .0127 .0187 .0157 .656

28 .0157 .0217 .0187 .781

26 .0187 .0247 .0217 .906

24 .0236 .0316 .0276 1.156

22 .0296 .0376 .0336 1.406

20 .0356 .0436 .0396 1.656

18 .0466 .0566 .0516 2.156

16 .0575 .0695 .0635 2.656

14 .0705 .0865 .0785 3.281

Fig. 1-32: Galvanized Tolerances


Rectangular Metal Duct Construction

Duct construction requirements are listed in Fig. 1-33 through Fig. 1-39, reproduced with

permission fromHVAC Duct Construction Standards, SMACNA, Tables 1-3 through 1-9, pp. 1-16 -

1-22. The program uses these tables to determine the gage and reinforcement required for thematerial used in a rectangular or oval duct section.

Since the greater dimension of a rectangular duct is more likely to deform, that dimension is

assumed to be critical. A gage, reinforcement spacing and tie rod requirement corresponding to

the working pressure in the duct is chosen from the table. The smaller dimension is analyzed

using the gage and reinforcement chosen for the greater dimension.

You can use the override forms to specify the reinforcement spacing. If you specify an override,

the program will check the table and determine the gage and reinforcement rigidity class for that

spacing. If there is no gage for that spacing, your override will be ignored. The program will

proceed to choose the appropriate gage, reinforcement spacing and tie rod requirement.

AsFig. 1-33throughFig. 1-39 are based on galvanized steel, the program will first find the duct

construction requirements for that material then find the equivalent construction requirements

for other materials. For aluminum ducts, the thickness is determined fromFig. 1-40whileFig. 1-

41is used to determine the reinforcement.


32/68



Duct

Dimension(in.)

Duct

Gauge (No

Reinforce-ment)

Minimum Rigidity Class on Minimum Guage Duct

Re-inforcement Spacing

10 8 5 4 3 2-1/2 2

10 dn. 12 14 16 18 26 ga.

20 24 ga. A-26 22 22 ga. A-26 24 22 ga. A-26

26 20 ga. A-26 28 18 ga. B-24 B-26 30 18 ga. B-24 B-26 36 16 ga. C-22 C-24 C-26 42 D-20 D-24 D-26 C-26 48 E-20 D-22 D-26 54 E-18 E-20 D-26 60 F-18 F-20 E-24 E-26

72 H-16

F+rod

G-18

F+rod F-22 F-24

84 H-16

F+rod

H-22

+rod

G-24

F+rod

96 I-16

F+rod

H-20

F+rod

H-22

F+rod

97 UP H-18

Fig. 1-33: 1/2-inch W.G.

Rectangular duct reinforcement for 1/2" of water, positive and negative static pressure


33/68



Duct

Dimension(in.)

Duct

Gauge (No

Reinforce-ment)

Minimum Rigidity Class on Minimum Guage Duct

Re-inforcement Spacing

10 8 5 4 3 2-1/2 2

10 dn. 12 26 ga.

14 24 ga. A-26 16 22 ga. A-24 A-26 18 22 ga. A-24 A-26 20 20 ga. A-24 A-26 22 18 ga. A-24 A-26 24 18 ga. B-24 A-26

26 18 ga. B-22 B-24 A-26 28 16 ga. C-22 C-24 B-26 30 16 ga. C-22 C-24 B-26 36 D-20 D-22 C-26 42 E-18 D-20 D-24 D-26 48 F-16 E-18 E-24 D-26

54 G-16

F+rod F-18 E-22 E-24

60 G-18

F+rod F-22 F-24

72 H-18

F+rod

G-22

F+rod

G-24

F+rod

84 I-18

F+rod

H-20

F+rod

H-22

F+rod

96 J-16

F+rod

I-18

F+rod

I-20

F+rod

I-22

F+rod

97 UP H-18t

Fig. 1-34: 1-inch W.G.Rectangular duct reinforcement for 1" of water, positive and negative static pressure

(reproduced with permission fromHVAC Duct Construction Standards, SMACNA, Table 1-4, p. 1-17)


34/68



Duct

Dimension

(in.)

Duct Gauge(No Rein-

forcement)

Minimum Rigidity Class on Minimum Gauge Duct

Reinforcement Spacing

10' 8' 5' 4' 3' 2-

1/2' 2'

10 dn. 26 ga. - - - - - - -

12 24 ga. A-26 - - - - -

14 22 ga. A-24 A-26 - - - -

16 20 ga. A-22 A-24 A-26 - - - -

18 20 ga. A-22 A-24 A-26 - - - -

20 18 ga. B-20 B-22 A-26 - - - -

22 16 ga. B-20 B-22 A-26 - - - -

24 16 ga. C-20 C-22 B-26 - - - -

26

NOT

ALLOWED

C-20 C-22 B-26 - - - -28 C-18 C-20 C-24 B-26 - - -

30 D-18 D-20 C-24 C-26 - - -

36 E-16 E-18 D-22 D-24 - - -

42 E-16 E-22 E-24 - - -

48 G-16 F-20 E-22 E-24 - -

54 G-18

F+rod F-20 F-24 - -

60 H-18

F+rod

G-20

F+rod

G-22

F+rod

G-24

F+rod -

72 I-16F+rod

H-18F+rod

H-22F+rod

- H-24F+rod

84 J-18

F+rod

I-20

F+rod -

I-22

F+rod

96 K-16

G+rod

K-18

G+rod

J-26

F+rod -

97 UP H-18t

Fig. 1-35: 2-inch W.G.

Rectangular duct reinforcement for 2" of water, positive and negative static pressure

(reproduced with permission fromHVAC Duct Construction Standards, SMACNA,Table 1-5, p. 1-18)


35/68



DuctDimension,

in.

No

Reinforce-

ment Duct

Gauge



10' 8' 5' 4' 3' 2' 2'

10 dn. 24

12 22 A-24 A-24 - - - -

14 20 A-22 A-24 - - - -

16 18 A-22 A-24 - - - -

18 18 A-22 A-24 - - - -

20 16 B-18 B-20 A-24 - - - -

22 16 C-18 B-20 B-24 - - - -

24 16 C-18 C-18 B-24 - - - -

26

NOT

D-18 D-18 C-24 - - - -

28 D-18 D-18 C-22 C-24 - - -

30 D-16 D-18 C-22 C-24 - - -

36 E-16 E-20 D-24 - - -

42 E-20 E-22 E-24 - -

48 G-18 F-20 E-22 E-24 -54 H-18

F+rod

H-18

F+rod

G-22

F+rod

E-24 -

60 H-16

F+rod

H-18

F+rod

G-20

F+rod

G-24

F+rod

-

72 I-16

F+rod

H-20

F+rod

H-22

F+rod

H-24

F+rod

84 J-18

F+rod

I-20

F+rod

I-22

F+rod

96 L-16

G+rod

K-18

G+rod

J-20

G+rod97 UP H-18t H-18t


Rectangular duct reinforcement for 3" of water, positive and negative static pressure

(reproduced with permission fromHVAC Duct Construction Standards, SMACNA,

Table 1-6, p. 1-19)


36/68



DuctDimension,

in.

No

Reinforce-ment

Duct

Gauge



10' 8' 5' 4' 3' 2' 2'

8 dn. 24 ga.

10 22 ga. A-24

12 20 ga. A-22 A-22 A-24

14 18 ga. A-20 A-22 A-24

16 18 ga. A-20 A-20 A-24

18 16 ga. B-18 B-20 A-24

20

NOT

ALLOWED

C-18 C-20 B-24

22 C-18 C-18 B-24

24 D-18 D-18 C-22 C-24

26 D-18 D-18 C-22 C-24

28 E-18 E-18 D-22 D-24

30 E-18 E-18 D-22 D-24

36 E-20 E-22 D-24

42 F-18 F-20 E-22 E-24

48 G-18 G-18 F-22 F-22 E-24

54 H-16

F+rod

H-18

F+rod

G-20

F+rod

G-22

F+rod

F-24

60 I-16

F+rod

I-16

F+rod

H-20

F+rod

H-22

F+rod -24

F+rod

72 I-18

F+rod

I-20

F+rod

H-22

F+rod

84 K-16

G+rod

J-18

F+rod

J-20

F+rod

96 L-16G+rod K-20G+rod

97 UP H-18t H-18t


Rectangular duct reinforcement for 4" of water, positive static pressure


Table 1-7, p. 1-20)


37/68



DuctDimension

, in.

No

Reinforce-

ment Duct

Gauge



10' 8' 5' 4' 3' 2' 2'

8 dn. 24

10 20 A-24

12 18 A-20 A-20 A-24

14 18 A-20 A-20 A-22 A-24

16 16 B-18 B-18 A-22 A-24

18

NOT

ALLOWED

C-18 C-18 B-22 B-24

20 C-16 C-18 B-22 B-24 22 D-16 C-18 C-22 C-24

24 D-18 C-22 C-22 C-24

26 D-16 D-20 C-22 C-24

28 E-16 D-20 D-22 C-24

30 D-18 D-22 D-24

36 F-18 E-20 E-22 E-24

42 G-16 G-18 F-20 E-22

48 H-18 H-18 G-22

54 H-16

F+rod

H-18

F+rod

H-20

F+rod

G-22

F+rod

60 H-18

F+rod

H-20

F+rod

H-22

F+rod

72 J-16

F+rod

J-18

F+rod

I-20

F+rod

84 L-16

G+rod

K-18

G+rod

96 L-16

G+rod

97 UP H-18t




Table 1-8, p. 1-21)


38/68



DuctDimension,

in.

No

Reinforce-

ment Duct

Gauge



10' 8' 5' 4' 3' 2' 2'

8 dn. 22 A-24

10 18 A-22 A-24

12 16 A-18 A-22 A-24

14

NOT

ALLOWED

B-18 A-20 A-22 A-24

16 B-16 B-20 B-22 B-24

18 C-16 C-20 B-22 B-24

20 D-16 C-18 C-20 B-24 22 C-18 C-20 C-24

24 D-18 D-20 C-24

26 D-18 D-20 D-22 C-24

28 E-18 D-20 D-22 C-24

30 E-16 E-18 D-22 D-24

36 F-16 F-18 F-20 E-22 E-24

42 H-16 G-18 G-20 F-22

48 H-18 H-18 G-22

54 I-16

F+rod

H-18

F+rod

H-20

F+rod

60 J-16

F+rod

I-18

F+rod

I-20

F+rod

72 K-16

F+rod

K-18

F+rod

84 H-16

96 H-16t

97 UP H-16t

Fig. 1-39: 10-inch W.G.



Table 1-9, p. 1-22)


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Galvanized Steel Commercial Size (in.)

28 .025

26 .032

24 .040

22 .050

20 .063

10 0.080

16 .090

Fig. 1-40: Aluminum Commercial Sizes

Galvanized steel gauge conversion to aluminum sheet thickness(reproduced with permission fromHVAC Duct Construction Standards, SMACNA,

Table 1-14, p. 1-32)

Galvanized Rigidity Class Aluminum Dimension

per Galvanized Class

A C

B D

C E

D F

E H

F H

G I

H K

Fig. 1-41: Rigidity Class Conversion

Galvanized steel rigidity conversion

(reproduced with permission fromHVAC Duct Construction Standards, SMACNA,Table 1-15, p. 1-32)

Round Metal Duct Construction

Duct construction requirements for round galvanized steel and aluminum ducts are given in Fig.

1-42 and Fig. 1-43, reproduced with permission from HVAC Duct Construction Standards,

SMACNA, Table 3-2 and 3-3, pp. 3-3, 3-4. These tables give the galvanized steel gauge and the

aluminum commercial size as a function of static pressure, type of seam (spiral or longitudinal)

and diameter. The program uses these tables to select the gage for round ducts.


40/68



Duct

Diameter

in Inches

Maximum 2" w.g.

Static Positive

Maximum 10" w.g.

Static Positive

Maximum 2" w.g.

Static Negative

Spiral

Seam

Gauge

Longitudi

nal Seam

Gauge

Spiral

Seam

Gauge

Longitudi

nal

Seam

Gauge

Spiral

Seam

Gauge

Longitudi

nal

Seam

Gauge

8 28 28 26 24 28 24

14 28 26 26 24 26 24

26 26 24 24 22 24 22

36 24 22 22 20 22 20

50 22 20 20 20 20 18

60 20 18 18 18 18 16

84 18 16 18 16 16 14

Fig. 1-42: Gauge Selection

Round duct gauge selection, galvanized steel


Table 3-2, p. 3-3)

Duct Diameterin Inches

Maximum 2" w.g.Static Positive

Maximum 2" w.g.Static Negative

Spiral

Seam Gauge

Longitudinal

Seam Gauge

Spiral

Seam Gauge

Longitudinal

Seam Gauge

8 .025 .032 .025 .040

14 .025 .032 .032 .040

26 .032 .040 .040 .050

36 .040 .050 .050 .063

50 .050 .063 .063 .080

60 .063 .080 N.A. .090

84 N.A. .090 N.A. N.A.

Fig. 1-43: Aluminum Gauge Schedule

Aluminum round duct gauge schedule


Table 3-3, p. 3-4)


41/68



Quantity Takeoffs

Ductwork

Since the unit weight of all sheet-metal sheets is pounds per square foot of surface area, the

program therefore calculates the surface area of all ductwork to be used.

The program then calculates the total weight of sheet-metal ductwork on the basis of gauge for

fibrous ducts and commercial size for aluminum ducts. The one-inch thick fibrous glass ducts are

estimated on a per square foot basis.

The program calculates the quantity of flexible ducts based on the diameter and linear foot of

flexible ducts. Duct reinforcements are not included in the weights of the ductwork.

Lining and Insulation

The program calculates the lining and insulation quantity takeoff on a per square foot basis.

Appendix Three: Duct Fittings


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Figures and tables appearing in this appendix are reprinted with permission from HVAC Systems

Duct Design, SMACNA, 1981. Use the table to identify the figure that corresponds to each

keyword choice on thefittypesform.

Keyword Table andFigure

el001el002el007

6-6: A6-6: B6-6: C

el003el004el005el006

6-6: D6-6: F6-6: G6-6: H

tc001 6-8: Atd001

td003td004

6-7: A

6-7: C6-7: D

td002td005td006

6-7: B6-7: J6-7: E

jc001jc002jc003jc004jc005jc006jc007jc008

jc009

6-9: A6-9: B6-9: C6-9: D6-9: E6-9: F6-9: G6-9: H

6-9: Ijd001jd002jd003jd004jd005jd006jd007jd008jd009

6-10: A6-10: B6-10: G6-10: H6-10: I6-10: J6-10: K6-10: L6-10: M

jd010jd011jd012

jd013jd014jd015

6-10: N6-10: Q6-10: V

6-10: U6-10: W6-10: X

Duct Cross Section to which Coefficient "C" is referred is at the top of each table. Negative

numbers indicate that the static regain exceeds the dynamic pressure loss of the fitting.


43/68



Table 6-6 LOSS COEFFICIENTS, ELBOWS

Use the velocity pressure (Vp) of the upstream section. Fitting Loss (TP) = C x Vp

A. Elbow, Smooth Radius (Die Stamped), Round (2)

Coefficients for 90 Elbows: (See Note 1)

R.D. 0.5 0.75 1.0 1.5 2.0 2.5

C 0.71 0.33 0.22 0.15 0.13 0.12

Note 1: For angles other than 90 multiply by the following factors:

H 0 20 30 45 60 75 90 110 130 150 180

K 0 0.31 0.45 0.60 0.78 0.90 1.00 1.13 1.20 1.28 1.40

B. Elbow, Round, 3 to 5 pc -- 90 (2)

Coefficient C

#Pieces

R.D.

0.5 0.75 1.0 1.5 2.0

5 - 0.46 0.33 0.24 0.19

4 - 0.50 0.37 0.27 0.24

5 0.98 0.54 0.42 0.34 0.33

Coefficient C (See Note 2)

H 20 30 45 60 75 90

C 0.08 0.16 0.34 0.55 0.81 1.2

Note 2: Correction factor for Reynolds number -- KRe

Re10-4

1 2 3 4 6 8 10 14

KRe 1.40 1.26 1.19 1.14 1.09 1.06 1.04 1.0

For Standard Air: Re= 8.56 DV

where: D = duct diameter inches


44/68



D. Elbow, Rectangular, Mitered (15)

Coefficient C (See Note 2 -- Page 6.13)

H

H W

0.25 0.5 0.75 1.0 1.5 2.0 3.0 4.0 5.0 6.0 8.0

20 0.08 0.08 0.08 0.07 0.07 0.07 0.06 0.06 0.05 0.05 0.05

30 0.18 0.17 0.17 0.16 0.15 0.15 0.13 0.13 0.12 0.12 0.11

45 0.38 0.37 0.36 0.34 0.33 0.31 0.28 0.27 0.26 0.25 0.24

60 0.60 0.59 0.57 0.55 0.52 0.49 0.46 0.43 0.41 0.39 0.38

75 0.89 0.87 0.84 0.81 0.77 0.73 0.67 0.63 0.71 0.58 0.57

90 1.3 1.3 1.2 1.2 1.1 1.1 0.98 0.92 0.89 0.85 0.83

E. Elbow, Rectangular, Mitered with Converging or Diverging Flow (15)


H W

W1W

0.6 0.8 1.2 1.4 1.6 2.0

0.25 1.8 1.4 1.1 1.1 1.1 1.1

1.0 1.7 1.4 1.0 0.95 0.90 0.84

4.0 1.5 1.1 0.81 0.76 0.72 0.66

1.5 1.0 0.69 0.63 0.60 0.55

F. Elbow, Rectangular, Smooth Radius without Vanes (15)

Coefficients for 90 elbows (See Note 1)


R WH W

0.25 0.5 0.75 1.0 1.5 2.0 3.0 4.0 5.0 6.0 8.0

0.5 1.5 1.4 1.3 1.2 1.1 1.0 1.0 1.1 1.1 1.2 1.2

0.75 0.57 0.52 0.48 0.44 0.40 0.39 0.39 0.40 0.42 0.43 0.44

1.0 0.27 0.2 0.23 0.21 0.19 0.18 0.18 0.19 0.20 0.27 0.21

1.5 0.22 0.20 0.19 0.17 0.15 0.14 0.14 0.15 0.16 0.17 0.17

2.0 0.20 0.18 0.16 0.15 0.14 0.13 0.13 0.14 0.14 0.15 0.15

Note 3: Correction Factor for Reynolds number -- K Re

R WRe10

-4

1 2 3 4 6 8 10 14 20

0.5 1.40 1.26 1.19 1.14 1.09 1.06 1.04 1.0 1.0

0.75 2.0 1.77 1.64 1.56 1.46 1.38 1.30 1.15 1.0


45/68



Install Equation Editor and double-

click here to view equation.

For Standard Air: Re= 8.56 DV

where: D = duct diameter, inches V = duct velocity, fpm

G. Elbow, Rectangular, Smooth Radius with Splitter Vanes (2)

NOTES FOR THIS FIGURE ONLY:

A) See Page 5.12 to calculate splitter vane spacing.B) CR = Curve RatioC) Use correction factors in Note 1 on Page 6.13 for elbows otherthan90.

Coefficient C

1

spli

tter

van

e

R W C RH W

0.25 0.5 1.0 1.5 2.0 3.0 4.0 5.0 6.0 7.0 8.0

0.05 0.218 0.52 0.40 0.43 0.49 0.55 0.66 0.75 0.84 0.93 1.0 1.1

0.10 0.302 0.36 0.27 0.25 0.28 0.30 0.35 0.39 0.42 0.46 0.49 0.52

0.15 0.361 0.28 0.21 0.18 0.19 0.20 0.22 0.25 0.26 0.28 0.30 0.32

0.20 0.408 0.22 0.16 0.14 0.14 0.15 0.16 0.17 0.18 0.19 0.20 0.21

0.25 0.447 0.18 0.13 0.11 0.11 0.11 0.12 0.13 0.14 0.14 0.15 0.15

0.30 0.480 0.15 0.11 0.09 0.09 0.09 0.09 0.10 0.10 0.11 0.11 0.12

0.35 0.509 0.13 0.09 0.08 0.07 0.07 0.08 0.08 0.08 0.08 0.09 0.09

0.40 0.535 0.11 0.08 0.07 0.06 0.06 0.06 0.06 0.07 0.07 0.07 0.07

0.45 0.557 0.10 0.07 0.06 0.05 0.05 0.05 0.05 0.05 0.06 0.06 0.06

0.50 0.577 0.09 0.06 0.05 0.05 0.04 0.04 0.04 0.05 0.05 0.05 0.050.05 0.362 0.26 0.20 0.22 0.25 0.28 0.33 0.37 0.41 0.45 0.48 0.51

0.10 0.450 0.17 0.13 0.11 0.12 0.13 0.15 0.16 0.17 0.19 0.20 0.21

0.15 0.507 0.12 0.09 0.08 0.08 0.08 0.09 0.10 0.10 0.11 0.11 0.11

0.20 0.550 0.09 0.07 0.06 0.05 0.06 0.06 0.06 0.06 0.07 0.07 0.07

0.25 0.585 0.08 0.05 0.04 0.04 0.04 0.04 0.05 0.05 0.05 0.05 0.05

0.30 0.613 0.06 0.04 0.03 0.03 0.03 0.03 0.03 0.03 0.04 0.04 0.04

0.35 0.638 0.05 0.04 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03

0.40 0.659 0.05 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

0.45 0.677 0.04 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

0.50 0.693 0.03 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01

0.05 0.467 0.11 0.10 0.12 0.13 0.14 0.16 0.18 0.19 0.21 0.22 0.23

0.10 0.549 0.07 0.05 0.06 0.06 0.06 0.07 0.07 0.08 0.08 0.08 0.09

0.15 0.601 0.05 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.05 0.05

0.20 0.639 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03

0.25 0.669 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

0.30 0.693 0.03 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01

0.35 0.714 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.40 0.731 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.45 0.746 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.50 0.760 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01


46/68



H. Elbow, Rectangular, Mitered with Turning Vanes(2)

SINGLE THICKNESS VANES

*No

.

Dimensions, inches Coef

f

R S L C

1 2.0 1.5 0.75 0.12

2 4.5 2.25 0 0.15

3 4.5 3.25 1.60 0.18

*Numbers are for reference only.

*When extension of trailing edge is not provided for this vane, losses

are approximately unchanged for single elbows, but increase

considerably for elbows in series.

DOUBLE THICKNESS VANES

Coefficient C

*No

.

Dimensions,

in.

Velocity (V), fpm

Remarks

R S 1000 2000 3000 4000

1 2.0 1.5 0.27 0.22 0.19 0.17 Embossed Vane Runner

2 2.0 1.5 0.33 0.29 0.26 0.23 Push-On Vane Runner



*Numbers are for reference only.

I. Elbows, 90, Rectangular, Z-Shaped(15)

Coefficients for WH = 1.0 (See Notes 4 and 5)

LH 0 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

C 0 0.62 0.90 1.6 2.6 3.6 4.0 4.2 4.2 4.2

LH 2.4 2.8 3.2 4.0 5.0 6.0 7.0 9.0 10.0

C 3.7 3.3 3.2 3.1 2.9 2.8 2.7 2.6 2.5 2.3

Note 4:For W/H values other than 1.0 apply the following favctor:

WH 0.25 0.50 0.75 1.0 1.5 2.0 3.0 4.0 6.0 8.0

K 1.10 1.07 1.04 1.0 0.95 0.90 0.83 0.78 0.72 0.70

Note 5:Correction factor for Reynolds number -- K Re

Re104

1 2 3 4 6 8 10 14

KRe 1.40 1 .26 1.19 1\.1

4

1.09 1.06 1.04 1.0


47/68



Install Equation Editor and double-

click here to view equation.

For Standard Air:

Re = 8.56 D Vwhere: D= hydraulic diameter, inches V = duct velocity, fpm

J. Elbows, 90, Rectangular in Different Planes(15)

Coefficients for H/W - 1.0: (See Notes 4 & 5 -- Page 6.16)

L/W 0 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

C 1.2 2.4 2.9 3.3 3.4 3.4 3.4 3.3 3.2 3.1

L/W 2.4 2.8 3.2 4.0 5.0 6.0 7.0 9.0 10.0

C 3.2 3.2 3.2 3.0 2.9 2.8 2.7 2.5 2.4 2.3

K. Elbows, 30, Round, Offset(15)


L/D 0 0.5 1.0 1.5 2.0 2.5 3.0

C 0 0.15 0.15 0.16 0.16 0.16 0.16

L. Elbows, 90, Rectangular Wye or Tee Shape


48/68



Table 6-7 LOSS COEFFICIENTS TRANSITIONS (Diverging Flow)

Use the velocity pressure (Vp) of the upstream section. Fitting loss (TP) = C x Vp

A. Transition, Round, Conical(15)

Re= 8.56 DV

where:

D = Upstream

Diameter (inches)

V = Upstream Velocity

(fpm)


Re A1A

16 20 30 45 60 90 120 180

0.5 109

2

4

6

10

16

0.14

0.23

0.27

0.29

0.31

0.19

0.30

0.33

0.38

0.38

0.32

0.46

0.48

0.59

0.60

0.33

0.61

0.66

0.76

0.84

0.33

0.68

0.77

0.80

0.88

0.32

0.64

0.74

0.83

0.88

0.31

0.63

0.73

0.84

0.88

0.30

0.62

0.72

0.83

0.88

2 109

2

4

6

10

16

0.07

0.15

0.19

0.20

0.21

0.12

0.18

0.28

0.24

0.28

0.23

0.36

0.44

AES Ducts Engineering Basis

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Transcript of AES Ducts Engineering Basis