duct construction 050507

Prof. Moustafa M. Elsayed (consultant) 1

Duct Construction

By

Prof. Moustafa M. Elsayed

Consultant

[email protected]

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


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


Construction Drawings Symbols


Construction Drawings

Symbols



Symbols


Duct Sizing

• Velocity Method

• Equal Friction Method

• Static Regain

• Balancing Method


Duct Pressure -1: Pressure

Gradient


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


Duct Pressure -3:

Pressure

Class

Designation


Duct Seals – 1

• Purpose of sealing

• Location of seal:

�All joints

�Transverse seams

�Longitudinal seams

�Connections

• Sealing Requirements: See Table 1-2


Duct Seals – 2: Seal Classes


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


Duct Seals – 4:

Methods of

Sealing


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


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.


Controlling Costs - 2: Aspect

Ratio


Controlling Costs - 3:

Aspect Ratio

Relative

Installed

Cost



Aspect Ratio

Relative

Operating

Cost



Pressure Classification Relative Cost


Recommended Duct Velocities


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.


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..."


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


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



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



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



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



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.


Duct Material

• Galvanized

• Fiber Glass


Galvanized Sheet


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


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


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.



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.



Cross-Broken or Beaded Ducts


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


Example: continuedUse 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


Example: continued


• 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


Transverse

Joints

for

Rectangular

Ducts


Longitudinal Seams for

Rectangular Duct


Longitudinal Seams for Rectangular Duct


Longitudinal

Seams

for

Rectangular

Duct


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


Round

Ducts


Round Ducts


Round

Ducts

Transverse

Joints


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


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


Duct

Supports


Duct Supports


Duct

Supports


References1. 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.


References

• ROBERT W. TINSLEY , Duct Liner: Problem or

Solution? , HPAC Engineering Journal, May 1998..