Duct Construction 050507

Prof. Moustafa M. Elsayed (consultant) 1
Duct Construction
By
Prof. Moustafa M. Elsayed
Consultant
moustafa.elsayed@egec-xprt.com

Prof. Moustafa M. Elsayed (consultant)
2
Contents
• Construction Drawings Symbols
• Duct Sizing
• Duct Pressure
• Duct Seal
• Controlling Costs
• Recommended Duct Velocities
• Noise Control
• Duct Liner & Duct Insulation

3
Contents
• Material: Galvanized Sheet
• Flexible Duct and Connectors
• Rectangular Duct Construction Schedule
• Transverse Joints for Rectangular Ducts
• Longitudinal Seams for Rectangular Ducts
• Round Ducts
• Duct Supports
• Typical Project
• References

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Construction Drawings Symbols

5
Construction Drawings
Symbols

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Symbols

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Symbols

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Duct Sizing
• Velocity Method
• Equal Friction Method
• Static Regain
• Balancing Method

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Duct Pressure -1: Pressure
Gradient

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Duct Pressure -2: Pressure Class
SMACNA-HVAC Pressure Ratings
± ½“ ± 1“ ± 2“ ± 3“ ± 4“ ± 6“ ± 10“
Ductwork Testing
-3” w.g. to lower 1.5* pressure rating
-2” w.g. to 2” Generally not tested
+3” w.g. to higher 1.5* pressure rating

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Duct Pressure -3:
Pressure
Class
Designation

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Duct Seals – 1
• Purpose of sealing
• Location of seal:
All joints
Transverse seams
Longitudinal seams
Connections
• Sealing Requirements: See Table 1-2

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Duct Seals – 2: Seal Classes

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Duct Seals – 3: Methods of Sealing
• Welding
Gaskets: this is used in flanged joints
Mastics (adhesives): this used in grooves
or between flanges
Mastics-plus-embedded-fabrics systems
(this is the preferable flexible sealant)
• Tapes
• Liquid sealant

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Duct Seals – 4:
Methods of
Sealing

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Duct Seals – 5: Leakage Test
• Leakage test is an added cost and thus is not
recommended for duct systems constructed to 3”
w.g. or lower.
• Apparent differences of about 10% between fan
delivery and sum of airflow measurements at
terminals do not necessarily mean poor sealing
and excessive leakage

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Controlling Costs -1
• Use minimum number of fittings
• Consider the use of semi-extended plenums
• Seal duct work to minimize leakage
• Consider using round duct where space and
initial duct cost allows
• When using rectangular duct, maintain
aspect ratio as close to1:1 as possible to
minimize both initial and friction losses.

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Controlling Costs - 2: Aspect
Ratio

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Controlling Costs - 3:
Aspect Ratio
Relative
Installed
Cost

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Aspect Ratio
Relative
Operating
Cost

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Pressure Classification Relative Cost

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Recommended Duct Velocities

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Recommended Duct Velocities

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Liner Problem
• Erosion and presence of glass
fibers in the air stream and in the
room ambient environment. Some
people have expressed the concern
that inspired glass fibers from
glass fiber duct lining could cause
cancer.

25
Liner Problem
• Since 1966, many studies have been performed
that show no significant erosion of glass fibers
from the surface of duct board or duct liner. The
UL tests showed no evidence of erosion on the
samples tested
• The 1995 ASHRAE Handbook--Applications
and the 1997 ASHRAE Handbook--
Fundamentals state that there is no clear
evidence that any man-made mineral fiber is
carcinogenic. An extensive study presented to the
World Health Organization in 1988 stated that
"fibers were not a cause of adverse health effects
in building occupants..."

26
Duct Liner & Duct Insulation - 1
• Duct insulation is mounted or inner-lined to
reduce heat loss and heat gain as well as
to prevent the condensation on the outer
surface of the duct.
• Duct liner provides both thermal
insulation and sound attenuation.
• The thickness of an insulation layer is
based on economical analysis

27
Duct Insulation Choices
• External duct wrap or mechanical board
insulation
• Internal duct liner insulation
• Fiber glass duct board systems in place
of rectangular metal ducts
• Flexible round air ducts

28
External duct wrap or mechanical board
insulation
• They serve as a vapor retarder and thermal
insulation
• High-density mechanical boards will also
provide a slight reduction of break-out and
vibration-related noise from the metal air
ducts

29
Internal duct liner insulation
• Duct lining materials are applied to the interior
duct surfaces
• Duct liner serve as a very effective passive noise
reduction system
• Internal duct insulations offer the thermal benefits
of exterior duct insulations
• Duct liner is available in medium-density flexible
form, and as a higher density rigid form

30
Fiber glass duct board systems
• Rigid, high-density fiber glass board ducts
(replacement for sealed, insulated metal air
duct)
• Duct board combines structural integrity
and thermal/acoustical insulation
• No vibration or regenerated noise
• Possible health hazard
• Possible short life

31
Flexible round air ducts
• Available in a wide variety of diameters and
lengths,
• Flex duct provides good thermal performance,
and incorporates an internal air barrier and
external vapor retarder jacket.
• Acoustical absorption varies by flex product;
typical values (measured by Noise Reduction
Coefficient, or NRC) are only 20 to 30% of
values demonstrated by duct liner or fiber glass
duct systems.

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Duct Material
• Galvanized
• Fiber Glass

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Galvanized Sheet

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Galvanized Sheet

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Rectangular Duct Construction: Procedure
• Determine pressure class
• Use Table 1-3 to 1-9 for reinforcement schedule
• The greater duct dimension determines the gage
for all sides.
• Reinforcement may be different on sides with
unequal dimension.
• Joint connection without reinforcement: Column
2 with minimum gage, use flat type joint
connection

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Rectangular Duct Construction: Procedure
• Joint with reinforcement: Columns 3 to 10,
use required reinforcement interval to get
the minimum gage and the letter code for
the type of joint.
• Type of Intermediate Reinforcement; see
Table 1-10
• Type of Transverse Joint Reinforcement;
see Tables 1-11 to 1-13

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Rectangular Duct Construction
General Notes
• Beading and cross breaking are not substitute
for reinforcement
• Type of reinforcement:
Transverse Joint Reinforcement
Intermediate Reinforcement
• Fittings shall be reinforced like sections of
straight duct. On size change fittings, the
greater fitting dimension determines the duct
gage.

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General Notes
• Duct sides that are:
19” and over with more than 10 square
feet of unbraced panel area, or
20 gage or less with more than 10 square
feet of unbraced panel area
shall be cross-broken or beaded as indicated in
Figure 1.8. This will not be required if duct is
lined or externally insulated.
• Duct wall thickness, joints, seams and
reinforcement must be coordinated to provide
proper assembly.

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Cross-Broken or Beaded Ducts

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41

42

43

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Example
• Given: 1” wg duct with dimension 18” ×
11”
• Determine: SMACNA construction
recommendation for this duct
Recommendations
• Use Table 1-4
• Use Table 1-11 for type B reinforcement
• Use Table 1-10 for intermediate
reinforcement

46
Example: continued
Use Table 1-4
At 18” use 22ga without
reinforcement
Or 26 ga with type B
reinforcement at 8’
interval
At 12”, use 26ga
without
reinforcement

47
Example: continued

48
• Types of duct connection systems
• Transverse joints: see schedule tables
• Bar or angle stock incorporated in a
joint
• See figures for different types of
joints
Transverse Joints for Rectangular Ducts

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Transverse
Joints
for
Rectangular
Ducts

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Longitudinal Seams for
Rectangular Duct

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Longitudinal Seams for Rectangular Duct

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Longitudinal
Seams
for
Rectangular
Duct

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Round Ducts
• Categories:
All continuously welded or braced
Tack or spot welded (sealed or unsealed)
Seam locked
Rivet, screw or punched-die-stamp locked
• Gage Schedule:
See Table 3-2A for +ve pressure and
See Table 3-2B for -ve pressure
• Seams: See Fig. 3-1
• Traverse Joints: See Fig. 3-2

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Round
Ducts

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Round Ducts

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Round Ducts

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Round Ducts

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Round
Ducts
Transverse
Joints

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Duct Supports
• Components of duct hanging system
Upper attachment to building: see Figs 4-1, 4-2, 4-3
Hanger itself: strips of galvanized steel or round
steel rod
Lower attachment to duct: see Fig. 4-4
• Horizontal duct sheet shall have a support
Within two feet (0.61 m) of each elbow
Within four feet of each branch insertion

60
Duct Supports
• Hanger Sizing and Spacing:
Rectangular Duct: See Table 4-1
Round Duct; See Table 4-2
• Riser Support:
Rectangular Duct, Support from Floor: See Fig.
4.7
Rectangular Duct, Support from Wall: See Fig. 4-8
Circular Duct, Support from Floor: See Fig. 4-9

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Duct
Supports

62
Duct
Supports

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Duct
Supports

64
Duct
Supports

65
Duct Supports

66
Duct Supports

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Duct Supports

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Duct Supports

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Duct
Supports

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Duct
Supports

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Duct
Supports

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References
1. HVAC Duct Construction Standards, Metal and
Flexible, Sheet Metal and Air Conditioning Contractors’
National Association, Inc., Second Edition, 1995,
Fourth printing 1998.
2. HVAC Systems Duct Design, Sheet Metal and Air
Conditioning contractors’ National Association, Inc.,
Third Edition, 1990, Fourth printing 1998.
3. Howell R. H., Sauer H. J., and Coad, Principals of
Heating, Ventilating and Air conditioning, American
society of Heating, Refrigeration and Air Conditioning
Engineers, Inc., Atlanta, Georgia, 1998.
4. Bell A. B, JR: HVAC Equations, Data, and Rules of
Thumb, McGraw-Hill, 2000.

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References
• ROBERT W. TINSLEY , Duct Liner: Problem or
Solution? , HPAC Engineering Journal, May 1998..

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Duct Construction 050507

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