1
DW/144
Specification for
Sheet Metal Ductwork
Low, medium and high
pressure/velocity air systems
1998
Copyright © 1998 by the
Heating and Ventilating
Contractors' Association
All rights reserved
ISBN 0-903783-27-4
Further copies of this publication are available from:
Publications Unit
Heating and Ventilating Contractors Association
Old Mansion House Eamont Bridge
Penrith Cumbria CA10 2BX
Tel: 01768 860405 Fax: 01768 860401
e-mail: hvcapublications@welplan.co.uk
2
3
THE INDUSTRY
STANDARD
Ken Parslow
Chairman
Executive Committee
Ductwork Group
1996-98
For more than a decade-and-a-half, the DW/142 Specification for Sheet Metal Ductwork
published by the Heating and Ventilating Contractors' Association has gained national
and international recognition as the industry standard against which the quality of
ductwork manufacture and installation can be judged.
In recent years, however, it has become increasingly evident to the members of the
HVCA Ductwork Group that the developments in technology and working practices
which have taken place since the drafting of DW/142 have rendered obsolete significant
parts of the document.
It was an acknowledgement of this state of affairs which led the Technical SubCommittee of the Ductwork Group, ably chaired by Edgar Poppleton, to undertake the
task of producing a radically revised specification which would promote best practice
and quality standards well into the next Millennium.
This new publication - designated DW/144 - represents the direct result of that
initiative.
The new specification recognises the computer age - with special reference to
CAD/CAM procedures and techniques - and the international performance standards
established by the Committee for European Normalisation (CEN), as well as the need to
update and consolidate much of the information contained in the original DW/142
publication and its Addendum A companion volume.
During the drafting process, the Technical Sub-Committee has consulted widely with
individuals and organisations throughout the building services and construction sectors
in order to ensure that the new specification fully reflected the current the "state-of-theart" in terms both of technical expertise and industry best practice.
I firmly believe that this process has resulted in a publication which clearly
demonstrates the high level of professionalism which exists within the ductwork
community - and I take this opportunity of thanking all those who have contributed to
its production.
In particular, my thanks go to Edgar Poppleton and his colleagues on the Technical
Sub-Committee, to Keith Elphick for the provision of invaluable technical consultancy,
and to Ductwork Group secretary Gareth Keller for overseeing the project as a whole.
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4
MAINTAINING QUALITY
Like most industries, the ductwork sector must
be prepared continually to innovate in order to
survive and prosper.
A key element in that innovation process is
the timely review and updating of quality
standards to ensure that they continue to offer
realistic benchmarks to which all professional
individuals and organisations can perform.
The development of this new Specification for
Sheet Metal Ductwork - designated DW/144 - has
been carried out with that objective in mind.
In the 16 years since the publication of its
predecessor, DW/142 - and in the ten years since
the supplementary volume Addendum A appeared many technical advances, changes in working
practices and regulatory introductions and
amendments have taken place.
The common performance standards for ductwork being developed by the Committee for
European Normalisation (CEN), for example,
had to be taken fully into account during the
drafting process. Similarly, notice had to be
given to the provisions of the Control of
Substances Hazardous to Health (COSHH) and
Construction
(Design and
Management)
Regulations, neither of which had been issued
when DW/142 was published.
It is not possible - nor, I think, desirable - to
include in this foreword an exhaustive catalogue
of the points of difference between this
specification and its predecessor. These will
clearly emerge from a detailed reading of the
text.
I should, however, like to take the
opportunity to highlight a few topics which I
believe to be of particular significance. They are:
• the omission of high-pressure Class D (in
order to conform to European practice);
• the highlighting of information to be
provided by the designer;
• the end-sealing of ducts and explosion risks;
• the removal of standard sizes of rectangular
ducts;
• the omission of cleated joints;
• the acceptance of proprietary flanges
certificated to DW/TM I no longer illustrated
in detail;
• the consolidation into the document of
coverage of hangers and supports;
• the addition of a note on linings, along with
their cleaning considerations;
• the consolidated graphical representation of
Class A, B and C air leakage characteristics,
mandatory testing Class C only;
• updated appendices on galvanising after
manufacture, stainless steel, pre-coated steel,
aluminium, Eurovent and galvanised material,
plus a bibliography;
• transport, handling, storage and interface with
DW/TM2 Guide to Good Practice – Internal
Cleanliness of New Ductwork Installations;
• an overview of fire-rated ductwork;
• a new appendix on inspection, servicing and
cleaning access openings (the default inclusion
of Level 1 should be noted);
• a new section on standard component drawings
- incorporating a framework of nomenclature,
and a description of drawing symbols,
abbreviations and rules - which is intended to
reduce ambiguity and promote common
understanding;
• a rewritten description of all forms of dampers,
for which I am indebted to Bill Clark and John
Mawdsley of the HEVAC Association.
I take this opportunity to acknowledge the permission granted by the Sheet Metal and Air
Conditioning Contractors' National Association
(SMACNA) of the USA for the use of its tie rod
specification (designer approval required).
And I also include a plea on behalf of ductwork
constructors to be allowed to make the final choice
of components and techniques within the parameters set by the designer, and allowed within this
specification to satisfy performance characteristics.
It will, of course, be clear to anyone who has
ever taken on such a task that the production of
this specification has involved a colossal input in
terms of industry consultation and from a wide
variety of individuals, a number of whom I should
like to identify for special mention.
They are: former Technical Sub-Committee
members Keith Waldron and the late Keith
Angood; current members Chris Collins, Stuart
Howard, Brian James and - last but by no means
least - Jim Murray; technical consultant Keith
Elphick; and Ductwork Group secretary Gareth
Keller.
Finally, may I remind readers of the crucial
importance of ensuring that all ductwork is manufactured and installed in a manner which is safe,
efficient, effective and free of risk.
The publication of DW/144 is intended to assist
significantly in the achievement of this objective.
5
Acknowledgements
The HVCA wishes to record its sincere thanks to the following
members - past and present - of the Technical Sub-Committee
of the Ductwork Group, who contributed their time, knowledge
and experience to the production of this document
Edgar Poppleton (chairman)
Keith Angood
Chris Collins
Stuart Howard
Brian James
Jim Murray
Keith Waldron
Technical Consultant:
Keith Elphick
Ductwork Group Secretary:
Gareth Keller
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7
Contents
Page
Notes
10
Part One - Technical Information to be provided
by the designer
1. Introduction
11
2. Standards
11
3. Components
11
4. Particular Requirements
11
Part Two - Standards
5. Application
6. Ductwork Classification and Air Leakage
7. Materials
8. Ductwork Construction and Joint Sealing
Part Three - Rectangular Ducts
9. Rectangular Duct Sizes
10. Construction
10.1 General
10.2 Steel Thicknesses
10.3 Longitudinal Seams
10.4 Cross Joints
10.5 Stiffeners
10.6 Ductwork Galvanised After
Manufacture
10.7 Fastenings
11. Fittings
11.1 Standardisation of Fittings
11.2 Stiffeners
11.3 Splitters
11.4 Turning Vanes
11.5 Branches
11.6 Change Shapes
11.7 Expansions and Contractions
11.8 Sealant
Part Four - Circular Ducts
12. Standard Sizes
13. Construction
13.1 Longitudinal Seams
13.2 Cross Joints
13.3 Fastenings
14. Fittings
14.1 Standardisation of Fittings
14.2 Nominal Diameters
14.3 Sheet Thickness
14.4 Sealing of Joints
Part Five - Flat Oval Ducts
15. Standard Sizes and Sheet Thicknesses
16. Construction (Spirally wound)
16.1 General
16.2 Longitudinal Seams
16.3 Cross Joints
16.4 Fastenings
16.5 Stiffening
17. Construction (Straight Seamed)
18. Fittings
18.1 General Construction Requirements
18.2 Standardisation of fittings
35
35
35
35
35
35
Part Six - Hangers and Supports
19. General
43
Part Seven - General
20. Access/Inspection Openings
21. Regulating Dampers
22. Fire Dampers
23. Smoke Dampers
24. Combination Smoke and Fire Dampers
25. Flexible Ducts
26. Flexible Joint/Connections
27. Protective Finishes
28. Connections to Building Openings
29. Internal Duct Linings
30. Thermal Insulation
31. Kitchen Ventilation
32. Fire Rated Ductwork
33. Standard Component Drawings
and Abbreviations
13
13
13
14
15
15
15
15
15
15
15
16
16
16
16
16
16
16
16
16
17
17
Part Eight - Appendices
Appendix A. Air Leakage from
Ductwork
Appendix B. Identification of
Ductwork
Appendix C. Guidance Notes for the
Transport, Handling and
Storage of Ductwork
Appendix D. Ductwork Systems and
Fire Hazards
Appendix E. Hot Dip Galvanizing after
Manufacture
Appendix F. Stainless Steel for Ductwork
Appendix G. Pre-Coated Steel
Appendix H. Aluminium Ductwork
Appendix J.
Eurovent
Appendix K. Summary of BS.EN10142:
1991 Continuously Hot-Dip
Zinc Coated Mild Steel Strip
and Sheet for Cold Forming
Appendix L. `Design Notes for Ductwork'
(CIBSE Technical
Memorandum No. 8)
Appendix M. Guidance Notes For Inspection,
Servicing and Cleaning Access
Openings
Appendix N. Bibliography
Appendix P. Conversion Tables
27
27
27
27
27
29
29
29
29
29
35
35
35
35
35
8
47
48
49
50
51
51
52
53
53
54
54
54
54
54
75
80
82
83
85
86
89
90
91
92
93
94
95
97
List of Tables
Table
Page
Part Two - Standards
1. Ductwork Classification and Air
Leakage Limits
Part Three - Rectangular Ducts
2. Constructional Requirements
Low Pressure up to 500Pa
3. Constructional Requirements
Medium Pressure up to 1000Pa
4. Constructional Requirements
High Pressure up to 2000Pa
5. Fastening Centres
Part Four - Circular Ducts
6. Standard Sizes
7. Spirally-Wound Ducts
8. Straight-Seamed Ducts
9. Permitted fastenings and maximum
spacings
10. Fittings Sheet Thicknesses
Part Five - Flat Oval Ducts
11. Standard sizes and sheet thicknesses
12. Stiffening requirements
low and medium pressures
13. Stiffening requirements
high pressure
14. Permitted fastenings and maximum
spacings
13-17
18-24
25-28
29
30
Socket and spigot cross joints
Stiffeners
Tie rod assembly
Hard and Easy bends
Turning Vanes
31
32-38
39-45
Part Four - Circular Ducts
Spiral and straight seams
Cross joints spirally wound ducts
Cross joints straight seamed ducts
29
30-31
32-33
53-58
59-63
Part Five - Flat Oval Ducts
Cross joints spirally wound ducts
Cross joints straight seamed ducts
39-40
41-42
Part Six - Hangers and Supports
Horizontal ducts
bearers and hangers
Vertical ducts supports
45-46
46
13
18
19
19
24
64-75
27
28
28
76-77
Part Seven - General
Fire barrier/Fire damper expansion
Flexible joint connections
Standard component drawings Rectangular
125-152 Standard component drawings Circular
153-167 Standard component drawings Flat Oval
168-177 Plant/equipment/miscellaneous
78-79
80
81-124
29
29
36
37
38
40
178
Part Six - Hangers and Supports
15. Supports for horizontal ducts - rectangular,
flat oval and circular
Part Seven - General
16. Standard Abbreviations
Part Eight - Appendices
17. Air Leakage Rates
18. Recommended duct identification colours
19. Examples of further identification symbols
20. Ductwork galvanized after manufacture rectangular
21. Compositions of the commonly used
Stainless Steel grades
22. Rectangular aluminium ducts low pressure constructional requirements
23. Circular aluminium ducts low pressure constructional requirements
24. Zinc coating mass (weight)
25. Access requirements for inspection,
servicing and cleaning
179
44
72-73
76
80
81
85
88
90
91
93
94
List of Illustrations
Figs
1-8
9
10-12
Pages
Part Three - Rectangular Ducts
Longitudinal Seams
Illustrations of panel stiffening
Flanged cross joints
22
23
24
25
25
20
20
21
9
Part Eight - Appendices
Permitted leakage at various
pressures
Example of duct identification symbol
50
52
55-61
62-67
68-70
71
78
81
Notes
In this document:
(1) Even where a ductwork job specification calls for the system to be
wholly in accordance with DW/144, it will still be necessary for the
designer, in addition to providing drawings showing details and
dimensions of the ductwork, to identify specific requirements, particular to his or her design.
The technical information to be provided by the designer is therefore
set out in detail on page 11.
(2) All dimensions quoted in this specification refer to the nominal sizes,
which are subject to the normal relevant commercial and published
tolerances.
(3) Manufacturing techniques are continually subject to change and
improvements and in respect of proprietary methods and devices this
specification does not preclude their use if they can be demonstrated
to the designer to be equally satisfactory. Where there is divergence
between the requirements of DW/144 and the manufacturer's
recommendations for proprietary methods and devices, the latter shall
take precedence.
(4) The expressions `low-pressure,' 'medium-pressure' and 'highpressure'
relate to the pressure/velocity classes set out in Table 1.
(5) `Mean air velocity' means the design volume flow rate related to the
cross-sectional area.
(6) Reference to the air distribution system pressure relate to the static
pressure of the relevant part of the ductwork system and not to the
fan static pressure.
(7) The symbol for litres is ‘L’: 1000 litres per second is equivalent to 1
cubic metre per second.
(8) The pascal (Pa) is the internationally agreed unit of pressure. The
relationship of the pascal to other units of pressure is: 500 pascals =
500 Newtons per square metre = 5 millibars = approximately 2
inches water gauge.
10
Part One - Technical information to be
provided by the designer to the ductwork contractor
1 INTRODUCTION
The selection of constructional methods is the decision
of the Manufacturer to conform with the performance
requirements of the specified ductwork classification.
Sections 2-4 below define the information that is to be
provided by the Designer.
2 STANDARDS
2.1 Pressure classification (Table 1)
2.2 Leakage classification (Table 1)
2.3 Positive and Negative pressures (Table 1)
2.4 Materials (Section 7)
2.5 Any special system requirements
3 COMPONENTS
3.1 Inspection/servicing access openings (Section
20 and Appendix M)
Number and location of all panels and covers for
inspection and/or servicing access other than those
covered in Section 20 and summarised as Level 1
requirements in table 25 of Appendix M. Number
and location of test holes, instrument connections
and hinged doors as defined in Section 20.
3.2 Cleaning access
(Section 20.8 and Appendix M)
Designers shall stipulate their requirements for
periodic internal cleaning of ductwork and for the
consequent need for adequate access for specialist
cleaning equipment.
3.3 Regulating dampers (Section 21) Specification,
location and mode of operation of all regulating
dampers.
3.4 Fire dampers (Section 22)
Specification and location of all fire dampers to
meet the requirements of the Authority directly
concerned with fire protection.
3.5 Smoke dampers (Section 23)/Combination
smoke and fire dampers (Section 24)
Specification and location of all smoke dampers to
meet the requirements of the Authority directly
concerned with fire protection.
3.6 Flexible ducts (Section 25)
Specification and location of any flexible ductwork.
3.7 Flexible joint connections (Section 26)
Specification and location of any flexible connections eg. plant or building expansion joints.
4. PARTICULAR REQUIREMENTS
4.1 Air leakage testing (Section 6 and Appendix A)
The extent of any air leakage testing. While it shall
be mandatory for high-pressure ductwork (as
defined in this specification) to be tested for air
leakage in accordance with the procedure set out in
DW/143, A practical guide to Ductwork Leakage
Testing, no such testing of low- or medium-pressure
ductwork is required.
4.2 Protective finishes (Section 27)
Details and specification of any protective finishes.
4.3 Fire rated and smoke extract ductwork
(Appendix D)
The extent and limits of protection for any fire
resisting ductwork.
4.4 Internal thermal/acoustic lining
(Section 29)
The extent of any ductwork requiring internal
acoustic/thermal lining is to be clearly identified. A
detailed specification of materials and method of
application is required. The practical aspects of
cleaning or maintenance must be addressed by the
designer before deciding to internally line ductwork.
4.5 External thermal/acoustic insulation
(Section 30)
The extent and thickness of insulation to be provided
by others should be stated.
4.6 Special supports (Section 19)
Details of any spanning steel or special support
requirements not covered by Section 19
4.7 Attachment to building structure (Section 28)
Specific requirements for the junction of ductwork
and associated components to openings should be
detailed and specified and the limits of responsibility
defined.
The provision of penetrations and associated
framings are outside the scope of this specification.
4.8 Air terminal units
Detail and specifications of all Air Terminal Units. It
is expected that all Air Terminal Units and their
Plenums (See Figures 120 to 124) will be supported
by the Ceiling Grids unless the designer indicates an
independent method of support.
4.9 Ductwork layout drawings
Details of any special requirements relating to CAD,
scales, etc. It is common practice and cost effective
for ductwork manufacturers to utilise their approved
ductwork layout drawings as a basis of their
manufacturing/installation information by adding the
necessary details to the same drawing. Scales of 1:50
or smaller may preclude this practice, therefore,
larger scales might be more appropriate. The final
choice of manufacturing/installation scales shall be
left to the ductwork contractor.
4.10 Other requirements
Details of any requirements for the ductwork not in
accordance with the provisions of this specification,
including any modified construction required to
conform with any requirements concerning external
ductwork (See 5.3) or to meet the regulations of a
local authority or other controlling body.
11
4.11 Reference to the designer
In consideration of the foregoing, reference is also
made to the designer in the following clauses:
Clause
5.3
7.4, 7.5, 7.6
10.5.2
11.1
14.1
16.3.1
19.1, 19.4
19.6, 19.7
20.1, 20.1.1.1, 20.6, 20.8
20.9
21.1, 21.3.1
21.3.4
22.3, 22.7
24.3
25.1
26.1
27,27.3.4
29.1, 29.4, 30.2, 30.3, 33.2
Fig. 176
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Appendix F
Appendix L
Appendix M
12
Page
13
14
16
16
29
35
43
44
47
48
48
49
50
51
51
52
53
54
71
75-79
80,81
82
83,84
85
86-88
93
94
Part Two - Standards
5 APPLICATION
5.1 This specification sets out minimum requirements
for the manufacture and installation of ductwork for
commercial and industrial air distribution systems,
made from any of the materials listed in Section 7 and
being within the limits of size and/or metal
thicknesses specified in the relevant tables. Normal
operating temperatures are assumed within the
pressure/velocity limits and the limits of air leakage
for the various pressure classes prescribed in Table 1.
5.2 This specification is not intended to apply to
ductwork handling air which is polluted or is otherwise exceptional in respect of temperature or
humidity (including saturated air); nor is it suitable
for ductwork exposed to a hostile environment, e.g.
contaminated air, off-shore oil rigs, etc. The design,
construction, installation, supports and finishes in
such cases should be given special consideration in
relation to the circumstances of each case.
5.3 This specification is not suitable for ductwork
exposed to external atmosphere and the Designer will
need to give specific details of any special
finishes/construction (See Section 27).
6 DUCTWORK CLASSIFICATION AND AIR
LEAKAGE
6.1 Classification and air leakage limits Ductwork
classification and air leakage limits are set out in
Table 1.
6.2 Compatibility with CEN
The leakage factors used in Table 1 for Classes A, B
and C are the same as those for the classes similarly
designated in the CEN Document Pr EN 12237/Pr EN
1507.
6.3 Leakage at various pressures; and other
relationships
Applying the limits specified in Table 1, Appendix A
(Table 17) sets out the permitted leakage at each of a
series of pressures up to a maximum for each class.
Included in that appendix is a graphical presentation
of the pressure/leakage relationship.
DW/143 A practical guide to Ductwork Leakage
Testing, also gives details of the basis for the leakage
limits specified in Table 1.
6.4 Air leakage testing
Air leakage testing of low and medium pressure
ductwork is not mandatory under this specification.
Air leakage testing of high pressure ductwork is
mandatory under the specification and for details of
testing procedure refer to DW/143 A practical guide
to Ductwork Leakage Testing.
7 MATERIALS
7.1 Application
This specification applies to ductwork constructed
from materials as defined below, or equal. Minimum
steel thickness is to be taken as a nominal thickness
within the tolerances to BS.EN10143:1993. (See
Appendix K)
7.2 Zinc-coated steel
Ductwork will normally be constructed from hotdip galvanized steel to BS.EN10142:1991, Grade
DX51 D+Z, coating type Z275.
13
7.3 Mild steel
Where mild steel is specified, it shall be cold-reduced
steel to BS.EN10130:1991, Grade FEP 01A.
8.2.2 Liquid and mastic sealants
These are typically applied to a longitudinal seam
formed between two sheets of metal, a socket and
spigot, cleated or flanged cross joints. Particular
care is needed when sealing of "corner pieces" on
the proprietary 'slide-on' type flange and
reference should be made to the manufacturer's
assembly and sealing instructions.
7.4 Stainless steel
Where stainless steel is specified. it will be the
responsibility of the designer to indicate the type most
suitable for the conditions to which the ductwork will
be exposed. In doing so, it is recommended that the
factors set out in Appendix F should be taken into
account. In this connection, reference must be made to
BS 1449: Part 2, which includes stainless steel.
8.2.3 Gaskets
These can be of various materials in the form of a
preformed roll, sheet or strip, applied between
opposing faces of flanged cross joints. In the case
of proprietary 'slide-on' type flanges, it is
advisable to use the gasket strip recommended by
the manufacturer.
7.5 Pre-coated steel
Pre-coated steel may be specified for aesthetic or
other reasons. The designer must then consider the
availability of suitable materials and the restriction on
fabrication methods. Guidance notes are available in
Appendix G.
Factory-fitted proprietary synthetic rubber '0'ring
type gaskets are also acceptable for socket and
spigot joints on circular duct systems.
7.6 Aluminium
Where aluminium is specified, it will be the
responsibility of the designer to define the type most
suitable for the conditions to which the ductwork will
be exposed. Reference must be made to BS.EN485,
BS.EN515 and BS.EN573 for aluminium sheet and
BS.EN755 Parts 3-6 for aluminium section.
(Constructional requirements for ductwork made from
aluminium sheet and general notes on the material are
set out in Appendix H.)
8.2.4 Tapes
8.2.4.1 The application of tapes - Best suited,
but not limited, to cross joints on circular or flat
oval ductwork. Where chemical reaction tape,
heat shrinkable tape or other approved material
is used on flat oval ductwork care should be
taken to maintain close contact between the
material and the flat sides of the duct until the
joint is completed.
8.2.4.2 Chemical reaction tape - An
impregnated woven fibre tape and a resin type
activator/adhesive. On application of the
activator/adhesive the tape becomes pliable and
can then be applied to any surface shape. The
liquid reacts with the tape, causing the 2part
system to `set'.
8 DUCTWORK CONSTRUCTION AND JOINT
SEALING
8.1 Ductwork construction
The selection of longitudinal, cross joint and stiffener
types within the criteria laid down in the tables should
be the responsibility of the manufacturer.
8.2 Joint sealing and sealants
8.2.1 General
The integrity of the ductwork depends on the
successful application of the correct sealant,
gaskets or tape. The materials used should be
suitable for the purpose intended and satisfy the
specified pressure classification.
Illustrations indicating sealant locations will be
found in the following sections dealing with the
construction of rectangular, circular and flat oval
duct sections.
IN ALL CASES, SEALANT MATERIALS
MUST
BE
APPLIED
STRICTLY
IN
ACCORDANCE
WITH
THE
MANUFACTURER'S INSTRUCTIONS AND COSHH
ASSESSMENT.
8.2.4.3 Heat shrinkable band/tape - A thermoplastic material, coated on the inside with
hot metal adhesive. The band (or an appropriate
length of tape) is cut from the roll and wrapped
around the joint. When heated the tape shrinks
tightly around the joint thus providing a seal.
8.2.4.4 Self adhesive tape - Manufactured
from various materials including cloth based,
PVC and aluminium foil. Typically applied
externally to socket and spigot cross joints.
However, it is difficult to provide the dry, dust
and grease free surface that is required for a
successful application and this method is
therefore not recommended as a primary source
of sealant.
NB! Risk of explosions
Where ductwork is blanked off prior to leakage
testing or to prevent the ingress of contamination,
care should be taken to ensure that all joint sealing
solvent vapours are dispersed from the ductwork
systems.
14
Part Three - Rectangular Ducts
9 RECTANGULAR DUCT SIZES
This specification covers duct sizes up to a maximum
longer side of 3,000 mm. Duct sizes with an aspect
ratio greater than 4:1 are not recommended. Although
they offer no problems of construction, they increase
frictional resistance and the possibility of noise.
10 CONSTRUCTION
10.1 General
The minimum constructional requirements for
rectangular ductwork depend upon the pressure
classification as set out in Tables 2 to 4. The ductwork
construction and joint sealing standards are set out in
section 8.
10.2 Steel thicknesses
Minimum steel thicknesses related to duct longer side
to pressure classification are given in Tables 2 to 4.
10.3 Longitudinal Seams
Longitudinal seams are illustrated in Figs. 1 to 8. The
limits of use, if any, are given with the individual
illustrations.
10.3.1 Sealing of Longitudinal Seams Sealant
will be applied using one of the following
methods:
a) As an edge sealant on the external seam surface.
b) As an edge sealant on the internal seam surface.
c) Internal to the joint seam itself.
The most appropriate method will be determined
by the manufacturer relative to their product and
will be associated with either traditional
fabrication/assembly methods, factory or site
based, and/or proprietary methods. The ultimate
proof of a seal is that the ductwork system meets
the pressure classification specified. For details of
sealant see section 8.
10.3.2 Welded seams
A welded seam is acceptable without sealant,
provided that the welding is continuous.
10.4 Cross joints
10.4.1 Cross joint ratings
For cross joints, a system of rating has been used
to define the limits of use. The rating for each
cross joint is given with its drawing, and the limits
applying to that rating, in terms of
duct size longer side and maximum spacing, are
given in Tables 2 to 4. Other limits on use are
given with the individual drawings.
Note: Proprietary products used in the construction of cross joints should be approved by an
independent test house following tests defined in
DW/TM1 "Acceptance scheme for new products
- Rectangular cross joint classification." Figures
Nos 10 and 13 to 17 illustrate non proprietary
joints that have an established rating.
10.4.2 Sealant in cross joints
Sealant shall be used between sheet and section
in all cross joint assemblies. (see section 8)
With socket and spigot joints made on site,
sealant shall be applied during or after assembly
of the joint. It is permissible to use chemicalreaction tape or heat-shrink strip as alternative
methods of sealing, provided that close contact is
maintained over the whole perimeter of the joint
until the joint is completed.
With all flanged joints, the sealant between
sheet and section should preferably be incorporated during construction at works, but site
applied sealant is acceptable. The joint between
sections of ductwork is then made, using
approved type of sealant or gasket. With
proprietary flanging systems particular attention
should be paid to the sealing of corner pieces and
flanges, reference should be made to the
manufacturer's assembly and sealing instructions.
10.4.3 Adjustabletslip joints
In order to accommodate manufacturing/building
tolerances, site modifications etc, it is accepted
practice to use an adjustable joint as illustrated in
Fig. 14.
10.5 Stiffeners
10.5.1 External stiffeners
The sections (including proprietary flanges)
suitable for use as single stiffeners have been
given a rating from S1 to S6 in terms of duct size
longer side and maximum spacing. The ratings
are specified with the illustrations of the
stiffeners, Figs. 18 to 23, and the limits of use are
given in Tables 2 to 4. The stiffeners for socket
and spigot joints covered in Figs. 15, 16 and 17
are also applicable to stiffeners in general.
15
a minimum.
10.5.2 Internal stiffeners
Tie bars connecting the flanges of cross joints
illustrated in Figs 11 and 12, are the only form of
internal stiffening for rectangular ductwork
recognised by this specification and reference should
be made to HVCA publication DW/TM 1.
Areas where the galvanizing has been damaged or
destroyed by welding or brazing shall be suitably
prepared and painted internally and externally with
zinc-rich or aluminium paint as defined in Section
27.3.2.
Alternative methods for the attachment of tie bars
are shown in Figs. 25 to 28.
11 FITTINGS
11.1 Standardisation of fittings
The terminology and descriptions of rectangular duct
fittings as set out in Section 33 are recommended for
adoption as standard practice to provide common
terms of reference for designers, quantity surveyors
and ductwork contractors, and of those using
computers in ductwork design and fabrication.
The use of tie bars or other forms of internal
stiffening or bracing shall be acceptable if proved to
the designer to be equally satisfactory.
SMACNA (Sheet Metal and Air Conditioning
Contractors' National Association), which is the
American equivalent to the HVCA Ductwork Group,
have produced an Addendum No. l (November
1997) to their publication "HVAC Duct
Construction Standards, Second Edition - 1995".
The addendum contains the extensive technical
information and data on the subject of mid panel tie
rods and SMACNA have given their kind
permission for this specification to make reference
to this fact. Designers and manufacturers who wish
to incorporate this form of internal stiffening into a
ductwork system should contact SMACNA direct to
obtain copies of their publications (See Appendix N,
Bibliography).
Bends are designated as `hard' or `easy', and these
terms as used herein have the following meanings:
`Hard' signifies rotation in the plane of the longer
side of the cross section.
`Easy' signifies rotation in the plane of the shorter
side of the cross section.
An example illustrating these terms is given in Fig. 29.
11.2 Stiffeners
The flat sides of fittings shall be stiffened in accordance with the construction Tables 2 to 4. On the flat
sides of bends, stiffeners shall be arranged in a radial
pattern, with the spacing measured along the centre of
the bend.
10.6 Ductwork galvanized after manufacture
Appendix E sets out the recommended sheet thicknesses
and stiffening for ductwork galvanized after
manufacture.
11.3 Splitters
If the leading edge of the splitters exceeds 1250 mm
fit central tie bars at both ends to support the splitters.
Leading and trailing edges of splitters must be edge
folded and flattened and be parallel to the duct axis.
10.7 Fastenings
10.7.1 Permitted types and maximum centres
Table 5 sets out the permitted fastenings and the
maximum spacings for all ductwork classifications.
All duct penetrations shall be sealed.
Splitters shall be attached to the duct by bolts or
mechanically-closed rivets at 100 mm maximum
spacing (or by such other fixing as can be shown to be
equally satisfactory e.g proprietary sealed splitter
pins).
10.7.2 Rivets
Manufacturers' recommendations as to use, size and
drill size are to be followed. Rivets resulting in an
unsealed aperture shall not be used.
11.4 Turning vanes
Where specified, or shown on drawings, square throat
bends with either duct dimension greater than 200 mm
shall be fitted with turning vanes which are illustrated
in Figures 30a and 30b.
10.7.3 Set screws, nuts and lock bolts Materials
shall be of mild steel, protected by electrogalvanizing, sherardizing, zinc-plating, or other
equal and approved corrosion resistant finish.
Turning vanes at 60 mm maximum centres shall be
fixed at both ends either to the duct or compatible
mounting tracks in accordance with manufacturer's
instructions, the whole bank being fixed inside the
duct with bolts or mechanically closed rivets at 150
mm maximum spacing.
10.7.4 Self tapping and piercing screws Providing
an adequate seal can be achieved, and the
protrusions into the ductwork are unlikely to cause
injury, then self-tapping or piercing screws may be
used.
10.7.5 Welding of sheet
The suitability of welding for sheet-to-sheet
fastening will be governed by the sheet thickness,
the size and shape of the duct or fitting and the need
to ensure airtighteness. Welded joints shall provide a
smooth internal surface and shall be free from
porosity. Distortion shall be kept to
The maximum length of turning vane between duct
walls or intermediate support shall be 615 mm for
single skin vanes and 1250 mm for double skin vanes.
Typical examples of fitting turning vanes when the
maximum permitted vane lengths are exceeded are
shown in Fig. 30c.
16
11.5 Branches
When fitting branch ducts to a main duct, care should
be taken to ensure that the rigidity of the duct panel is
maintained in terms of the stiffening criteria.
11.7 Expansions and contractions
Where these are required, an expansion shall be made
upstream of a branch connection and a contraction
downstream of a branch connection. The slope of either
an expansion or a contraction should not exceed 22½°
on any side. Where this angle is not practicable, the
slope may be increased, providing that splitters are
positioned to bisect the angle between any side and the
centre line of the duct (See Figs 99 to 101).
11.6 Change shapes
Where a change shape is necessary to accommodate
the duct and the cross-sectional area is to be
maintained, the slope shall not exceed 22½° on any
side (See Figs 99 to 103). Where a change in shape
includes a local reduction in duct crosssectional area,
the slope should not exceed 15° on any side and the
reduction in area should not exceed 20 per cent.
11.8 Sealant
Sealant shall be used in all longitudinal seams and cross
joints of fittings. Sealant shall be to the options listed in
Section 8.
17
Notes (applicable to Tables 2 to 4)
(1) The joints and stiffeners have been rated in terms of duct longer side and maximum spacing - see 10.4 for
joints and 10.5 for stiffeners.
(2) In Col. 3:
`PS' = plain sheet
`SS' = stiffened sheet, by means of
(a) beading at 400 mm maximum centres: or (b) cross-breaking within the frame formed by joints and/or
stiffeners: or (c) pleating at 150 mm maximum centres.
(3) Stiffened panels may limit the choice of insulation materials.
(4) For ductwork galvanized after manufacture, see 10.6 and Appendix E. (5) For aluminium ductwork, see
Appendix H.
(6) For constructional constraints of stainless steel ductwork see Appendix F.
(7) Although not covered in this specification, due to their relatively infrequent use, cleated cross joints are an
accepted constructional practice and the HVCA Ductwork Group should be contacted if details of their
ratings and limitations are required.
(8) Intermediate stiffeners using rolled sheet angle profiles, illustrated in Figs. 19 to 23 of the appropriate
rating may also be utilised ensuring that rigid corners are achieved.
18
19
20
NOTE: The above illustrations are typical examples of cross joint profiles that are in common use for connecting rectangular
sheet metal ducts.
There are no set dimensions for these profiles shown in Figs. 1 1 and 12 provided they are certified under the HVCA testing
scheme DW/TM1 "Acceptance Scheme for new products - Rectangular cross joint classification" and are appropriate to the
duct application. The manufacturer's technical data should be followed with respect to:
Connections to duct wall
Corner treatment
Addition of cleats
Application of sealants
Strength ratings
Application of tie bars
A list of manufacturers and profiles that are covered by current DW/TMI certificate is available from the Ductwork Group
Secretary at HVCA.
21
22
23
(1) A minimum of 2 fixings per side, with a maximum distance from the corner to the first fixing of 50 mm
(2) Except when pierced dimpling is used, one of the other types of fastening must be used at each end in
addition to dimpling
(3) In addition to dimpling, one of the other types of fastening must be used at 450 mm centres, and in all
cases not less than 1 per side
(4) Where manufacturers have specific recommendations, then these shall take precedence over the centres in
the Table above
(5) Mechanically closed rivets are not recommended for fixing external stiffeners to ductwork exceeding
500pa negative.
24
25
26
Part Four - Circular Ducts
and contractors in the meantime are invited to evaluate
them based on information currently available.
13.1 Longitudinal seams
13.1.1 Spirally-wound ducts
The seam used in spirally-wound circular ducts,
provided it is tightly formed to produce a rigid duct,
is accepted as airtight to the requirements of all the
pressure classifications covered in this specification,
without sealant in the seam.
13.1.2 Straight-seamed ducts
The longitudinal seam for straight-seamed circular
ducts shall be either the grooved seam continued to
the extreme end of the duct and sealed, or a
continuous butt lap weld or spot/stitch weld and
sealed lap joint (at 30 mm centres) provided this
gives a smooth internal finish (see Fig. 31).
13.2 Cross joints
13.2.1 General
Cross joints for circular ducts, both spirallywound
and straight-seamed, are illustrated in Figs.32 to 45.
They include several proprietary types and the limits
of use in terms of diameter and pressure classes are
noted against each.
13.2.2 Sealant
All circular cross joints shall be sealed. (see section
8)
The use of chemical-reaction tape or heatshrinkable
band shall be regarded as an effective sealant in
respect of the socket and spigot joints illustrated.
13.2.3 Welded joints
The limitations for welded joints are given in 13.3.5.
13 CONSTRUCTION
Spirally-wound ducts and straight seamed ducts
The minimum constructional requirements set out in
Table 7 and 8 are common to the full range of
pressures covered in this specification.
The ductwork construction and joint sealing standards are set out in section 8.
Spirally wound duct with thinner than traditional
wall thickness and with one or more corrugations
(ribs) formed between the lock seams are now
available. As design and installation experience with
these are gained and more functional performance
criteria are identified it is anticipated that such forms
may be added to later updates. Designers
13.3 Fastenings
13.3.1 Permitted types and maximum centres
Table 9 sets out the permitted fastenings and
maximum spacings for low-, medium- and highpressure ducts. All duct penetrations shall be sealed.
13.3.2 Rivets
Manufacturers' recommendations as to use, size and
drill size are to be followed. Rivets resulting in an
unsealed aperature shall not be used.
13.3.3 Set screws, nuts and lock bolts
Materials shall be of mild steel, protected by electrogalvanizing, sherardizing, zinc plating or other equal
and approved finish.
13.3.4 Self tapping and piercing screws
Providing an adequate seal can be achieved, and the
protrusions into the ductwork are unlikely to cause
injury, then self-tapping or piercing screws may be
used.
27
13.3.5 Welding of sheet
The suitability of continuous welding or spot welding for
sheet to sheet fastening will be governed by the sheet
thickness, the size and shape of the duct or fitting and the
need to ensure airtightness. Welded joints shall provide a
smooth
internal surface and shall be free from porosity.
Distortion shall be kept to a minimum.
Areas where the galvanizing has been damaged or
destroyed by welding or brazing shall be suitably
prepared and painted internally and externally with
zinc-rich or aluminium paint.
28
14 FITTINGS
14.1 Standardisation of fittings
The terminology and descriptions of circular duct
fittings as set out in Section 33 are recommended for
adoption as standard practice, to provide common
terms of reference for designers, quantity surveyors
and ductwork contractors, and those using
computers in ductwork design and fabrication.
The requirements for circular duct fittings apply
throughout the pressure ranges covered in this
specification.
14.2 Nominal diameters
The nominal diameter (see Table 6) is the size used
for design and ordering. With socket and spigot
joints, care should be taken to ensure that the
dimensions of the ducts and fittings are correctly
related, so that the joint can be effectively sealed.
14.3 Sheet thickness
Sheet thickness for circular duct fittings (determined
by the largest diameter) shall be not less than those
quoted in Table 10.
14.4 Sealing of joints
Sealant shall be used in all cross joints and fittings.
Such sealant shall be in accordance with the
requirements of Section 8.
29
30
31
32
33
34
Part Five - Flat Oval Ducts
15 STANDARD SIZES AND SHEET
THICKNESSES
15.1 Table 11 sets out the standard sizes of spirallywound oval ducts offered by the manufacturers of ducts
of this section.
16 CONSTRUCTION (SPIRALLY-WOUND
DUCTS)
16.1 General
`Flat oval' is the term used to describe a duct of crosssection with flat opposed sides and semicircular ends.
The duct is formed from a spirallywound circular duct,
using a special former.
Apart from stiffening (see Tables 12 and 13), flat oval
ducts have the same constructional requirements
throughout the pressure ranges covered in this
specification.
The ductwork construction and joint sealing standards
are set out in Section 8.
16.2 Longitudinal seams
Spirally-wound flat oval duct is accepted as airtight to
the requirements of this specification without sealant in
the seams, provided the grooved seam is tightly formed
to produce a rigid duct.
16.3 Cross joints
16.3.1 General
Cross joints shall be as Figs. 53 to 58 inclusive or such
other joint as can be demonstrated to the designer to
be equally satisfactory.
16.3.2 Sealant
All flat oval cross joints shall be sealed. (See
Standards Section 8).
16.3.3 Welded joints
The limitations for welded joints are given in 16.4.5.
16.4 Fastenings
16.4.1 Permitted types and maximum centres Table
14 sets out the permitted fastenings and maximum
spacings for low-, medium- and highpressure ducts.
All duct penetrations shall be sealed.
16.4.2 Rivets
Manufacturers' recommendations as to use, size and
drill size are to be followed. Rivets resulting in an
unsealed aperature shall not be used.
16.4.3 Set screws, nuts and lock bolts
Set screws and nuts shall be of mild steel, protected by
electro-galvanizing, sherardizing, zinc plating or other
equal and approved finish.
16.4.4 Self tapping and piercing screws Providing
an adequate seal can be achieved, and the protrusions
into the ductwork are unlikely to cause injury, then
self-tapping or piercing screws may be used.
governed by the sheet thickness, the size and shape of
the duct or fitting and the need to ensure air-tightness.
Welded joints shall provide a smooth internal surface
and shall be free from porosity. Distortion shall be
kept to a minimum.
Areas where the galvanizing has been damaged or
destroyed by welding or brazing shall be suitably
prepared and painted internally and externally with
zinc-rich or aluminium paint.
16.5 Stiffening
The larger sizes of flat oval duct are stiffened by
swages, as indicated in Table 11. Additionally, tie rods
(see Figs. 25 to 28) are required, positioned as indicated
in the respective tables and illustrations.
In special situations as an alternative to tie rods,
stiffening in the form of external angles may be used to
meet the requirements of the corresponding rectangular
duct sizes.
17 CONSTRUCTION (STRAIGHT-SEAMED)
Flat oval ducts with opposed sides and semi-circular
ends may also be formed using plain sheet and straight
seams. Ducts so formed should follow the metal
thicknesses and stiffening requirements specified for the
corresponding sizes of rectangular ducts, except that
stiffening is necessary on the flat sides only.
Seams and cross joints (see Figs 59 to 63) shall be
sealed to ensure the necessary degree of airtight-ness
throughout the pressure ranges covered in this
specification.
18 FITTINGS
18.1 General constructional requirements Sheet
thicknesses for flat oval fittings (determined by the
periphery of the larger end) shall be not less than those
given in Table 11 for the ducts themselves.
With socket and spigot joints, care should be taken to
ensure that the dimensions of ducts and fittings are
correctly related.
All the seams and joints integral to a fitting shall be
sealed to the same standard as the duct. (See Section 8).
18.2 Standardisation of fittings
The terminology and descriptions of flat oval duct
fittings as set out in Section 33 are recommended for
adoption as standard practice, to provide common terms
of reference for designers, quantity surveyors and
ductwork contractors, and those using computers in
ductwork design and fabrication.
The requirements for flat oval duct fittings apply
throughout the pressure ranges covered in this
16.4.5 Welding of sheet
The suitability of continuous welding or spot welding
for sheet to sheet fastening will be
35
36
37
38
39
40
41
42
Part Six - Hangers and Supports
19 GENERAL
19.1 Principles adopted
Supports are an essential part of the ductwork system,
and their supply and installation are normally the
responsibility of the ductwork contractor. The choice
between the available methods of fixing will depend on
the type of building structure and on any limitations
imposed by the structural design. Further, unless the
designer has specified his requirements in detail, the
load to be carried shall be understood to be limited to
the ductwork and its associated thermal and/or acoustic
insulation.
It is not practicable to deal here with the full range of
supports available, which increasingly includes
proprietary types, so in this section various methods of
support are dealt with in principle under the three
elements of:
(1) the attachment to the structure;
(2) the hanger itself; and
(3) the duct bearing member
with illustrations of those most commonly used.
Supports for ductwork external to the building have
been excluded, as these are individually designed to suit
the circumstances, and also may be required to meet
local authority standards. For the same reasons, floor
supports have not been dealt with.
With a proprietary device, it will, unless the designer
has specified his requirements in detail, be the
responsibility of the ductwork installer to ensure that it
meets requirements, with a sufficient margin of
overload; and that it is installed in accordance with the
manufacturer's recommendations.
The absence of any method or device from this
specification does not preclude its use if it can be
demonstrated that it is suitable for the duty assigned to
it, with a sufficient margin of safety against overload;
and this will be the responsibility of the ductwork
installer, unless the designer has specified his
requirements in detail.
19.2 Fixing to building structure
The fixing to the building structure should be of a
strength and durability compatible with those of the
ductwork support attached to it. A fixing to concrete or
brickwork must be made in such a way that it cannot
loosen or pull out through normal stressing or through
normal changes in the building structure.
19.3 Horizontal ductwork
19.3.1 The hanger itself
The hanger itself is usually mild steel plain rod
or studding or flat strap, pre-treated by, e.g. hotdip
galvanizing, sherardizing, electro-deposited zinc
plating or by other accepted anti-corrosion
treatment. Other materials, such as stranded wire,
may also be acceptable.
Projection of a rod or studding hanger through the
bottom bearer should, where practicable, not exceed
twice the thickness of the securing nut.
Provided the integrity of the ductwork is maintained,
hangers may be attached to the corners of the
flanges as an alternative to the use of a bottom
bearer.
With
proprietary
devices
manufacturers'
recommendations for use should be followed.
19.3.2 The duct bearing member
The choice of the lower support will be dictated by
the actual duct section.
19.3.3.1 Rectangular ducts
Table 15 gives minimum dimensions for the hangers
and for angle, channel and profile sections. The
angle is shown in Fig. 73, the profile channel
sections in Figs. 74 and 75.
Typical arrangements of bottom bearer supports for
plain, and insulated ducts are shown in Figs. 68, 69
and 70.
19.3.3.2 Circular ducts
Table 15 gives minimum dimensions for the hanger
and for the brackets - as illustrated in Figs. 64 to 67.
19.3.3.3 Flat oval ducts
Table 15 gives minimum dimensions for the hanger;
and for the bearer, depending on whether the flat
side of the duct is horizontal or vertical.
Typical arrangements for flat oval duct supports are
shown in Figs. 68, 71 and 72.
19.4 Vertical ducts
The design of supports for vertical ducts is dictated by
site conditions, and they are often located to coincide
with the individual floor slabs, but if the spacing
exceeds 4 metres the designer must specify his
requirements.
Vertical ducts should be supported from the stiffening
angle or the angle frame, or by separate supporting
angles fixed to the duct.
A typical method of supporting vertical rectangular
ducts is shown in Fig. 76 and for circular ducts in
43
Fig. 77. The same methods are applicable to vertical
flat oval ducts.
The extent of any vapour sealing of ductwork
thermal insulation and the method to be used, must
be clearly specified in advance by the designer.
19.5 Heavy loadings
For ducts larger than those covered by Table 15, or
where heavy equipment, mechanical services,
ceilings or other additional load is to be applied to
the ductwork, supports shall be designed to suit the
applications.
19.7 Heat transfer
It is not normally necessary to make special
arrangements for the limitation of heat transfer via the
duct supports. However, there may be special cases
where the temperature difference justifies a heat
barrier to conserve heat or to prevent condensation
and such requirements must be specified by the
designer.
19.6 Insulated ducts with vapour sealing Where
the temperature of the air within the duct is at any
time low enough to promote condensation on the
exterior surface of the duct and cause moisture
penetration through the thermal insulation, vapour
sealing may be called for, and in this case the most
important requirement is to limit penetration of the
seal.
19.8 Fire rated ductwork
DW/144 supports cannot be used on fire rated
ductwork systems. See Appendix D and in particular
notes in D2.1 Method 3 and D.2.3.
Notes to Table 15
(1) The dimensions included in Table 15 are to be regarded as minima.
(2) The maximum spacings set out in Table 15 are related solely to duct weight considerations. Closer spacings
may be required by reason of the limitations of the building structure or to achieve the necessary duct
rigidity.
(3) Rolled steel channels may be used as bearing members provided they meet the design characteristics of the
bearing members tabled above.
(4) As an alternative to drop rod or studding, wire rope may be utilised to suit individual manufacturer's fixing
methods and loading limitations.
44
45
46
Part Seven - General
20 ACCESS/INSPECTION OPENINGS
20.1 General
This section covers inspection/servicing access only.
Appendix M sets out guidance notes, in summary form,
of the access considerations that should be made by the
designer in terms of inspection, servicing and cleaning
access.
All openings shall be made safe and have sealed
panels/covers designed so that they can be speedily
removed and refixed. Multiple set screws are not
recommended, and self-piercing screws are not
acceptable as a method of fixing. The services coordinator should ensure that there is an area free of
services and other obstructions to enable a panel/cover
to be removed.
20.1.1 Function
20.1.1.1 An access panel is to be provided adjacent
to items of in-line equipment that require either
regular servicing or intermittent access. The opening
will be sized to provide hand and/or arm access only
and the designer shall specify the size and location
of panels where larger dimensions are required and
in these cases the panels should not exceed 450 x
450 mm.
20.1.1.2 An inspection cover is to be provided
adjacent to items of in-line equipment that need
visual inspection only of internal elements from
outside of the ductwork. The inspection opening
should have a minimum size of 100 mm sq/dia.
20.2 Access panels
20.2.1 It shall be standard practice to provide access
panels for the inspection and servicing of plant and
equipment as follows:
20.2.1.1 Fire/Smoke dampers
Panels to be located so as to give access both to the
blades and fusible links. On multiple assembly units
it may be necessary to provide more than one panel
and this may be determined by both external access
conditions and the internal reach to the blades and
the fusible links.
20.2.1.2 Filters
Panel to be located on the air entry side ie. upstream
(Note: Dimensions of access may need to be
changed to suit filter elements of the front
withdrawal type.)
20.2.1.3 Heating/cooling coils and in-duct
fans/devices
Panel to be located on the air entry side ie. upstream
20.2.2 It shall be standard practice to connect safety
restraints to access panels located in riser ducts.
20.2.3 Subject to the restrictions imposed by
duct dimensions, openings for access shall satisfy the
maintenance needs of the designated equipment with
consideration being given, if more practicable, to the
use of removable duct sections or flexible
ducts/connections.
20.3 Inspection covers
It shall be standard practice to provide inspection
covers adjacent to regulating dampers where either the
control linkage is mounted internally within the
airstream or if a multi-bladed unit is an integral part of
the ductwork run. It is not necessary to provide
inspection covers adjacent to either single blade
regulating dampers or flanged damper units.
20.4 Hand holes
Hand holes to permit proper jointing of duct sections
shall be provided at the manufacturer's discretion, but
should be kept to a minimum and made as small as
practicable. The hand hole cover shall be sealed and
securely fastened.
20.5 Openings in insulated ducts
It will be the responsibility of the insulation contractor
to `dress' their insulation to the edge of the access
opening without impeding the functionality of the
panel, cover or door.
20.6 Test holes for plant system commissioning It
shall be standard practice to provide test holes,
normally 13 mm diameter and fitted with an effective
removable seal, at the following locations: at fans (in
the straightest section of duct near to the fan outlet); at
cooling coils and heating coils (both before and after
the coil). The actual location of the test holes shall be
confirmed by the Designer and/or Commissioning
Engineer either at the drawing approval stage (to be
works drilled) or during the commissioning activity (to
be site drilled). For practical access reasons the latter
method is usually preferred.
20.7 Instrument connections
Instrument connections shall be provided where shown
on the contract drawings, suitably drilled or bossed and
screwed as sizes specified.
20.8 Cleaning/maintenance
Designers shall take specialist advice and then stipulate
their requirements for the periodic internal
cleaning/maintenance of ductwork and of the
consequent need for adequate access for specialist
cleaning equipment including the size, type and
location/frequency of the actual access openings
required.
Appendix M sets out guidance notes for the
consideration of cleaning access and also makes
reference to the HVCA publication TR17 "Guide to
Good Practice, Cleanliness of Ventilation Systems"
which covers the subject in greater detail.
47
20.9 Openings required for other purposes
It shall be the designers responsibility to specify the
location and size of any openings required other than
those covered in this section. In the case of hinged access
doors it shall be the designer's responsibility to indicate
on the drawings the location and size of any hinged
access doors required, ensuring that there is an area free
of services and other obstructions to enable the door to be
satisfactorily opened. Unless otherwise specified by the
designer, openings should not be larger than 1350 mm
high by 500 mm wide. Doors could open against the air
pressure. Both the opening in the duct and the access door
itself shall be adequately reinforced to prevent distortion.
A suitable sealing gasket shall be provided, together with
sufficient clamping type latches to ensure an airtight seal
between the door and the duct.
For safety reasons, the manufacturer shall incorporate
means to prevent personnel being trapped inside the
duct e.g. man access with operating handles both
inside and outside the duct.
21 REGULATING DAMPERS
21.1 General
Balancing dampers and control dampers are elements
inserted into an air distribution system, or elements of an
air distribution system. Balancing dampers permit
modification of the air resistance of the system and
consequently changing of the airflow rate. Control
dampers control the airflow rate and in addition provide
low leakage closure of the airflow.
The designer shall specify damper locations and select the
damper type as defined in 21.2 appropriate to the airflow,
pressure and acoustic characteristics.
21.1.1 Balancing damper
To achieve the required distribution of air in the
ductwork system at inlets and/or outlets. For this
purpose, the damper blades are set and locked manually
in any required position between fully open and fully
closed.
21.1.2 Control damper
To secure dynamic control of the air flow in the
ductwork system. In this function, the damper will
always be power - actuated and may require to be
modulated between fully open and fully closed, and to
be capable of taking up any position between these
extremes. In the fully open position, the damper should
have a minimum pressure drop. In the fully closed
position, it will not necessarily achieve a complete shut
off.
21.2 Types of airflow control damper
Air flow dampers of various types are available for
specific purposes as follows.
21.2.1 Single-blade dampers (Single or double skin)
Single-blade dampers shall consist of a single pivoted
blade contained within a casing or section of ductwork.
The blade shall be adjustable through a nominal 90°
angle by means of a quadrant or similar operating
mechanism. Where automatic control of the damper is
required the spindle shall be extended to enable a
powered actuator to be mounted.
Single-blade dampers (single-skin section) shall have
a maximum duct width of 300 mm and a maximum
duct height of 300 mm for rectangular ducts; and for
circular ducts a maximum diameter of 315 mm.
Single-blade dampers (double-skin section) are
suitable for use in rectangular ducts, and shall have a
maximum duct width of 1250 mm and a maximum
height of 300 mm.
21.2.2 Multi-blade dampers (single or double skin)
parallel or opposed blade Multi-blade dampers shall
consist of a number of pivoted blades contained
within a casing. The blades shall be adjustable through
a nominal 90° angle simultaneously by interconnected
linkage or gears, connected to a quadrant or similar
operating mechanism. Where automatic control of the
damper is required a spindle shall be extended to
enable a powered actuator to be mounted.
There is no restriction on the size of duct in which
multi-blade dampers or damper assemblies may be
used. Where dampers are required for blade lengths in
excess of 1250 mm, the blades should be suitably reinforced or supported. No individual damper blade
should exceed 200 mm in width.
21.2.3 Iris dampers
Iris dampers shall consist of a number of radially
inter-connected blades which open or close within a
casing with duct connection spigots. The blades shall
be simultaneously adjusted by a quadrant or similar
operating mechanism.
Iris dampers should be installed as specified by the
manufacturer's operating and installation instructions,
where the product is unidirectional with regard to
airflow.
Iris dampers are available for circular ducts only, in
diameters up to 800 mm (It should be noted that the
damper casing is approximately twice the diameter of
the duct).
21.2.4 Backdraft dampers
Air pressure operated uni-directional rectangular
(single or multi-blade) with adaptors if fitted to
circular or oval ducts.
21.2.5 Hit and miss dampers
Two parallel adjacent plates each with multiple
openings sliding against each other. The openings are
designed to provide 50% air volume flow rates when
they fully coincide. Used for simple operations up to
400 mm longest side.
21.2.6 Slide and blast gate dampers
A damper used as a shut off facility, normally for use
in circular ductwork with an external slide housing
allowing a blade to be fully inserted to fully extended
for maximum air flow.
Generally available in cast/pressed formats up to 355
mm diameter and normally used in industrial exhaust
applications.
21.3 Construction
21.3.1 Materials
Dampers shall be constructed from steel, stainless
steel, aluminium or synthetic materials.
48
All products shall be protected against corrosion as
necessary and supplied in a fully finished condition as
specified by the designer.
21.3.2 Dampers used in low and medium pressure
systems
The following recommendations apply to dampers
forming an integral part of ductwork with pressure
classification A and B air leakage limits.
The dampers shall be constructed to prevent distortion
and jamming in operation. The blades shall be
sufficiently rigid to minimise movement when in the
locked position.
The blades shall be securely fixed to the operating
mechanism. Spindles shall be carried in either nonferrous, synthetic or roller bearings. All balancing
dampers shall have a locking device located on the
outside of the case and shall give clear indication of the
actual blade position. All penetrations of the duct shall
be fitted with suitable seals where necessary.
21.3.3 Dampers used in high pressure systems
Regulating dampers used in ductwork systems to
pressure classification C shall meet the construction
requirements specified in 21.3.1 and 21.3.2 with
operating mechanisms out of the airstream.
21.3.4 Proprietary types of damper
The use of any specific type of proprietary damper shall
be confirmed by the designer. In all cases, proprietary
dampers shall meet the relevant requirements of this
specification.
21.3.5 Damper casings
Duct damper casings shall be constructed to meet the
minimum leakage limits specified for the ductwork
system to which they are installed.
In order to apply the square metreage leakage
calculation as detailed in DW/143 A practical guide to
Ductwork Leakage Testing, the reference casing area
shall be taken as the perimeter size of the damper
multiplied by the equivalent length of one metre eg. an
800 mm x 400 mm duct damper shall have a surface
area for casing leakage performance calculated as
follows; [(2 x 0.8) + (2 x 0.4)] x 1 = 2.4m2 casing area.
Other performance and rating test methods for dampers
and valves are specified in ISO5129 and BS/EN1751,
and are referenced below:
a) Leakage past a closed
damper or valve
BS/EN 1751
b) Flow rate/pressure
requirement characteristics
BS/EN 1751
c) Operational torque testing
BS/EN 1751
d) Thermal transfer testing
BS/EN 1751
e) Regenerated sound power
levels
ISO 5129
21.4 Installation
Dampers shall be installed in accordance with any
relevant ISO, EN or British Standard, local building
regulations and national codes of practice as well as the
manufacturer's recommendations.
49
22 FIRE DAMPERS
22.1 General
Dampers are required in air distribution systems for
fire containment. Generally they are called for where
ducts penetrate walls or floors which form fire
compartmentation. The damper assembly should have
a fire resistance rating equal to that of the fire barrier it
penetrates and shall be fire tested and rated to the
time/temperature curve of BS476 part 20 and 22.
22.1 Types of fire dampers
Fire dampers of various types are available for specific
purposes, as follows:
22.2.1 Folding curtain
Folding curtain fire dampers shall be constructed of a
series of interlocking blades which fold to the top of
the assembly permitting the maximum free area in
the airway. The blades shall be held in the open
position by means of a thermal release mechanism
rated at 72°C ± 4°C.
The fire damper must be able to close against static
air conditions when mounted in either the vertical or
horizontal planes.
In the event of a signal from a remote sensor the fire
damper blades shall be released and close the airway.
A local excess temperature in the area of the fire
damper shall, independent of any remote sensors,
automatically release the blades and close the airway
by means of the thermal release mechanism, electric
solenoid or electromagnet.
22.2.2 Single blade
Single blade fire dampers shall consist of a single
pivoted blade within a fire resistant case.
The blade shall be released from its open position by
means of a thermal release mechanism rated at 72°C
± 4°C, electric solenoid, electromagnet(s) or other
device.
The blade shall close the airway by means of any
one, or combination of, an eccentric pivot, balance
weight(s) and/or spring(s), the spring element being
incorporated within the damper or actuator
mechanism.
The fire damper shall be able to operate in either or
both the vertical and horizontal planes.
22.2.3 Multi-blade
Multi-blade fire dampers shall consist of a number of
linked blades contained within a fire resistant case.
The blades shall be released from their open position
by means of either a thermal release mechanism rated
at 72°C ± 4°C, or by the force applied from electrical
solenoid(s), electromagnet(s), electrical/pneumatic
actuator or other device.
The blades shall close the airway by means of a
spring(s), the spring element being incorporated
within the damper or actuator mechanism.
The fire damper shall be able to operate in either or
both the vertical and horizontal planes.
22.5 Location
The effective formed barrier of the damper assembly
shall be located within the structural opening. Where
this is not possible the section of the casing outside a
fire barrier must have a fire resistance not less than
that of the fire barrier and be adequately
supported/protected against the possibility of
displacement/damage by impact.
22.2.4 Intumescent
Intumescent fire dampers shall be constructed from
strips of intumescent material formed into a lattice
or from honeycomb material covered with
intumescent paint. The damper shall fully seal when
heat or flame is applied from either side. Note: these
devices are generally used in door/partition low
velocity applications.
22.6 Provision for expansion
Damper assemblies generally include built-in
clearance frames to meet the requirement that the
casing be free to expand in the event of fire. The
integrity of the fire barrier is maintained either by
metal to metal contact or by fire resistant packing.
Acceptable arrangements are shown in Figs. 78 and
79.
22.3 Materials and construction
The damper shall be constructed from steel or stainless
steel or other approved material. Steel products shall be
protected against corrosion and supplied in a fully
assembled condition as specified by the designer.
22.7 Installation
Damper installation shall be in accordance with the
manufacturer's recommendations and the impending
HVCA Publication DW/TM3 - Guide to Good
Practice, for the design for the installation of Fire and
Smoke Dampers and any conflict between the two
should be resolved and authorised by the designer
responsible for the fire damper selection.
22.4 Air leakage
Fire damper casings shall meet the equivalent leakage
performance standard specified for the ductwork system
to which they are installed.
Classes A, B and C are used to signify the leakage
performance of the damper casing with the respective
testing method illustrated and specified in BS/EN1751.
In order to apply the square metreage leakage calculation as detailed in the standard, the reference casing
area shall be taken as the perimeter size of the damper
multiplied by an equivalent length of one metre eg. an
800 mm x 400 mm duct damper shall have a surface
area for casing leakage performance calculated as
follows: [(2 x 0.8) + (2 x 0.4)] x 1 = 2.4m2 casing area.
23 SMOKE DAMPERS
23.1 General
Smoke dampers shall be constructed in such a manner
as to restrict the spread of smoke and other products of
combustion from one occupied space to another. The
blade(s) shall overlap each other and/or include edge
seals. The blade(s) shall be arranged to minimise the
leakage of smoke. If degradable seals are fitted, care
should be taken to establish the temperature range of
the material used to ensure performance compatibility.
50
The smoke damper shall be able to operate in either or
both the vertical and horizontal planes and close against
dynamic air conditions.
23.2 Types of smoke damper
Smoke dampers of various types are available for
specific purposes, as follows:
23.2.1 Single blade
Single blade dampers shall consist of a blade of smoke
tight material held in either the open or closed position
by a mechanical linkage releasing to close or open and
seal against the damper case. The blade shall be
mechanically connected to the actuator (electric or
pneumatic) and shall be triggered by interfacing with a
smoke detector or fire control panel.
23.2.2 Multi blade
Multi blade dampers shall consist of blades of smoke
tight material including the blade to blade seals, where
fitted. The blades shall be mechanically linked to an
actuator (electrical or pneumatic) to hold the blades in
either the open or closed position. The actuator shall
interface with a smoke detector or fire control panel
and shall be so designed as to hold the blades close
against the smoke seals, where fitted.
23.3 Materials and construction
The damper shall be constructed from steel, stainless
steel, other material or composite material with blades
fitted to reduce the leakage of smoke and hot gases
when the blades are in the closed position. Steel
products shall be protected against corrosion and
assembled in a fully finished condition as specified by
the designer, in some circumstances controls may be
supplied separately.
23.4 Air leakage
Smoke damper casings shall be as Clause 22.4
23.5 Installation
Damper installation shall be as Clauses 22.6 and 22.7.
24 COMBINATION SMOKE AND FIRE
DAMPERS
24.1 General
Combination smoke and fire dampers are required in air
distribution systems to prevent the spread of smoke and
hot gases from the fire zone and to maintain the
integrity of a fire rated structure for a period compatible
with that of the separating structure. They shall be tested
and rated to BS476 Part 20 and 22. Reference maybe
made to BS5588 Part 4 for specific smoke rating
requirements.
The closure of the fire damper under action of the
thermal release element shall override all other
subsequent signals.
24.2 Types of combination smoke and fire damper
Combination smoke and fire dampers of various types
are available for specific purposes, as follows:
24.2.1 Single blade
Single blade combination smoke and fire
dampers shall consist of a single pivoted blade
contained within a fire resistant case.
The blade shall be released from its open position by
means of either a thermal release mechanism rated at
72°C ± 4°C, or in addition operated by the force
applied from electrical solenoid(s), electromagnet(s), electrical/pneumatic actuator or other
device.
The combination smoke and fire damper shall be
able to operate in either or both the vertical and
horizontal planes and close against dynamic air
conditions.
24.2.2 Multi-blade
Multi-blade combination smoke and fire dampers
shall consist of a series of blades mechanically
linked and connected to a damper actuator with
manual, electric or pneumatic opening and spring
loaded closure contained within a fire resistant case.
The blades shall be released from their open position
by means of either a thermal release mechanism
rated at 72°C ± 4°C, or in addition operated by the
force applied from electrical solenoid(s), electromagnet(s), electrical/ pneumatic actuator or other
device.
The combination smoke and fire damper shall be
able to operate in either or both the vertical and
horizontal planes and close against dynamic air
conditions.
24.3 Materials and construction
The combination smoke and fire damper case shall be
constructed from steel, stainless steel, other material
or composite material with compressible side seals
fitted between the blade ends and the casing to reduce
the leakage of hot gases when the blades are in the
closed position.
Steel products shall be protected against corrosion and
supplied in a fully finished condition as specified by
the designer.
24.4 Air leakage
Damper casings shall be as Clause 22.4.
24.5 Installation
Damper installation shall be as Clauses 22.6 and 22.7.
25 FLEXIBLE DUCTS
25.1 General
Flexible duct connections shall be used in the fol
lowing applications:
• Terminal units
• Fan coil units
• Constant Volume/Variable Air Volume units
• Grilles and Diffusers
• Plenum boxes
• Distribution ducts between the above items.
They are available in a range of materials including
metal, P V.C, fabric and with or without thermal
insulation.
The designer/contractor shall consider the
51
following when selecting a particular type of flexible
duct including:
• Temperature range
• Fire rating
• Resistance to air flow
• Airtightness characteristics
• Length restrictions if applicable
• Support requirements
• Flexibility
• Insulation values
• System pressure.
Flexible ducts are also available in twin wall format
where the inner liner is perforated to provide acoustic
properties or plain for thermal insulation.
25.2 Flexible ducts - Metal
25.2.1 Flexible ducts made of coated steel, stainless
steel or aluminium are normally helically wound with
a lock seam to form a corrugated duct capable of
being bent without deforming the circular section.
Bending is done by closing the corrugations in the
throat and slightly opening the corrugations at the
back of the bend. Some re-adjustment is possible but
small radius bends cannot be straightened without
leaving some distortion of the corrugations. Repeated
bending is not recommended.
The ducts shall be mechanically fastened at each end
and particular care shall be taken to ensure that the
required airtightness of the system is maintained.
Fastenings should be as for rigid circular ducts Section
13.3 and Table 9. Sealing should be as Section 8.
25.2.2 Flexible ducts - Fabric
Flexible ducts made from materials including
P.V.C/Polyester
laminate,
Aluminium/Polyester
laminate encapsulating high tensile steel wire helix are
a very flexible form of construction. The length of
flexible duct used should therefore be kept to a
minimum, consistent with the particular application.
Flexible ducts shall be fastened at each end using a
propriety band. Care should be taken not to damage
the flexible duct and to ensure that the required
airtightness of the system is maintained.
25.3 Supports
Flexible ducts have a higher resistance factor than
conventional ductwork and should be supported in such
a way that excessive sagging and consequently kinking
of the duct is avoided.
25.4 Test Holes
It is not practicable to make test holes or take test
readings in metal or fabric flexible ducts. Where
readings are required, the test holes should be made in
rigid ductwork.
26 FLEXIBLE JOINT CONNECTIONS
26.1 General properties
The material used for flexible joints must meet the
designers requirement for temperature, air pressure, fire
resistance, vibration, noise breakout when incorporated
into a joint/connection and shall comply with the
standard of airtightness specified for the ductwork
system of which it forms part (See Fig. 80 for typical
connection details).
26.2 Location
Flexible joints are typically used at building
52
expansion joints and fan inlet/outlets. Any others
required should be indicated on the design drawings.
Care should be taken to maintain alignment across
joints/connections.
Joints/connections shall not be installed taught, but
under a reasonable amount of compression.
27.2 Metal spraying
Zinc or aluminium spraying shall be to BS EN 22063
(1994), Part 1.
27.3 Paints
27.3.1 Surface preparation and paint application
Surface preparation of the metal and paint application
shall be in accordance with the paint manufacturer's
recommendations.
26.3 Length
Flexible joints shall be kept as short as practicable
above a minimum effective length of 50 mm. In no
case shall a flexible joint exceed 250 mm in length.
27.3.2 Making good welding damage Galvanizing or
other metallic zinc finish damaged by welding shall be
suitably cleaned and painted with one coat of zinc-rich
or aluminium paint.
26.4 Connections to rectangular ducts
With flanged rectangular connections, the flexible
material shall be held in place with flat bar strips of
not less than 2 mm thick attached to the flanges using
suitable fixings. Where a proprietary brand of
lightweight material is used with sheet metal fitted to
either side consideration should be given to the size of
connection it is used on and how it is fitted. The more
heavy weight type of flexible material may also be
obtained formed into a channel section with corners
fitted and stitched to give a neat airtight joint. For
spigot connections the flexible material shall be held
in place with flat bar strips of not less than 2 mm
thick.
27.3.3 Ducts made from pre-galvanized sheet or coil
Ducts and profile sections made from pregalvanized
sheet or coil will have no need for paint or further
protection where located inside a building. This also
applies to exposed cut edges in accordance with
criteria laid down by British Steel PLC. See Appendix
N Bibliography.
27.3.4 Ducts made from other types of mild steel
sheet
Where circumstances require ducts to be made from
mild steel sheet or coil other than the foregoing,
protective requirements shall be specified by the
designer.
26.5 Connections to circular ducts
With flanged circular connections the flexible material
shall be held in place with alternative flat bar rings,
flat bar clip rings or proprietary clip bands with screw
or toggle fittings. Where a proprietary brand of light
weight flexible with metal to either side is used,
careful consideration must be given to sealing when
fitting to spirally-wound ducts.
27.3.5 Untreated steelwork profiles and sheet Any
plain mill finish unprotected mild steel such as rolled
steel sections and/or sheet used for flanging, stiffeners,
supports and duct walls must be treated.
Treatment would be an appropriate primer such as zinc
rich, zinc chromate, red oxide or aluminium paint.
26.6 Connections to oval ducts
Special consideration should be given to the construction but the type of joint applies as for circular
ducts except proprietary clip band with screw or
toggle fastening is not suitable on oval ducts.
28 CONNECTIONS TO BUILDING
OPENINGS
28.1 Forming and finishing building openings are not the
responsibility of the ductwork contractor and the notes
that follow are for guidance purposes only.
28.1.1 Openings in brick, block or concrete walls shall
have inset frames to provide a suitable means of fixing
grilles, louvres, masking flanges or the flanged ends of
ductwork.
The inset frames shall be constructed to maintain the
structural integrity of the wall and where applicable
cavities shall be suitably lined.
27 PROTECTIVE FINISHES
Unless otherwise stated all ductwork will be
manufactured in pre-galvanised sheet steel, aluminium
or stainless steel as specified, with prime coating
where applicable (see 27.3.5). Any additions to this
would normally be the responsibility of others. Any
special coating/paint finishes to be provided by the
ductwork contractor must be advised by the designer.
28.1.2 Openings in dry lining partitions shall have
inset frames as in 28.1.1.
27.1 Galvanizing after manufacture Galvanizing
after manufacture is not recommended for general use,
as distortion of the duct or fitting is probable, thus
making if difficult to achieve an airtight joint.
Galvanizing after manufacture is, however, an
acceptable protective finish for circular pressed
fittings and external ductwork exposed to atmosphere.
Where galvanizing after manufacture is specified, it
shall be to BS 729, see Appendix E. No paint
protection is required.
28.1.3 Openings in cladding walls and roofs shall have
flanged sleeves/frames to provide a suitable means of
fixing as in 28.1.1.
28.1.4 Horizontal and vertical openings that are
exposed to outside atmosphere shall be provided with a
suitable weathering finish at the external face
especially if profiled cladding is involved.
53
28.1.5 Timber framed openings are not permitted in
fire compartment walls.
30 THERMAL INSULATION
30.1 The provision and application of thermal
insulation to ductwork is not normally the responsibility of the ductwork contractor.
28.2 Ductwork connections to building openings shall
have a flange of suitable profile to permit practical
fixing to the opening frame. In selecting the profile,
consideration shall be given to Table 2 in this
specification relating to duct size and rating. Gasket
strip or sealer shall be applied between the flange and
building opening frame.
30.2 Where ductwork is required to be preinsulated,
the specification should be agreed with the designer.
30.3 Where the temperature of the air within the duct
is at any time low enough to promote condensation on
the exterior surface of the duct and cause moisture
penetration through the thermal insulation, vapour
sealing may be called for, and in this case the most
important requirement is to limit penetration of the
seal.
The extent of any vapour sealing of ductwork thermal
insullation and the support method to be used must be
clearly specified in advance by the designer.
29 INTERNAL DUCT LININGS
29.1 General
Where an acoustic or thermal lining to ductwork is
specified it should preferably be fitted at works. Before
duct manufacture it should be clarified that specified
external duct dimensions allow for the lining thickness.
Any form of lining should have fire characteristics
having minimum Class 0 rating and must be specified
by the Designer for material type, thickness, and
application method.
30.4 For detailed information on the thermal
insulation of ductwork, reference should be made to
BS 5422:1990 which covers the specification for
thermal insulation materials on pipes, ductwork and
equipment (in the temperature range -40°C to +700°C)
and BS 5970:1992 which is a Code of practice for
thermal insulation of pipework and equipment (in the
temperature range -100°C to +870°C).
29.2 Lining Application Considerations
Prior to the application of any lining the internal duct
surface must be thoroughly cleaned to provide a dust
free dry surface which may additionally be degreased.
Securing the lining to the internal duct surface can be
achieved in several ways including applied adhesive,
self adhesive and physical methods such as fasteners in
conjunction with surface washers at a specified square
pitch.
Adjacent sections of lining should abut with minimal
gap and integral or separate surface finish lap to such
joints and or gap filling proprietary products being
applied. This procedure is to obviate any particle
migration.
During application and any curing, consideration should
be given to ambient temperature and humidity
requirements.
In all circumstances linings should be fitted to material
manufacturer's recommended methods.
31 KITCHEN VENTILATION
31.1 For detailed information reference should be
made to HVCA Publication, Guide to good Practice
for Kitchen Ventilation Systems DW/171.
32 FIRE RATED DUCTWORK
32.1 For information see Appendix D of this
specification.
33 STANDARD COMPONENT DRAWINGS AND
ABBREVIATIONS
33.1 The illustrations in this section not only
highlight, where applicable, geometric limitations for
the design and manufacture of ductwork components
but also recommend standard drawing representation,
terminology and abbreviations for both ductwork
components and some of the more commonly used
ancillary/plant items.
29.3 Circular ducts
Lining circular ducts is impractical and is not recommended.
29.4 Cleaning and maintenance
Designers should be aware of the possible porous/
fibrous surface nature of linings as they may present
practical/hazardous problems in cleaning and
maintenance. Reference in this respect should be made
to the following HVCA Publications
i) DW/TM2 Guide to Good Practice, Internal
Cleanliness of New Ductwork Installations.
ii) TR17 Guide to Good Practice, Cleanliness of
Ventilation Systems.
33.2 Designers and surveyors should note that bills of
quantities should provide a full description
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57
58
59
60
61
62
63
64
65
66
67
68
69
70
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Part Eight-Appendices
APPENDIX A - AIR LEAKAGE FROM DUCTWORK
To be read in conjunction with DW/143 A practical guide to Ductwork Leakage Testing
CAUTION
As highlighted in both this document and DW/143, not enough emphasis can be placed on the fact that, except for high
pressure class C, the much tighter ductwork constructional standards brought about by the general acceptance of DW/142
have virtually negated the requirement for leakage-testing. It is essential to realise that except where it is mandatory this
document is not an endorsement of the routine testing of ducts but purely a guide to outline the procedures for conformity
with the air leakage limits in Table 1. When proper methods of assembly and sealing of ducts are used, a visual inspection
will ordinarily suffice .for verification of a well engineered and acceptably airtight construction.
WHERE NOT MANDATORY, DUCT LEAKAGE TESTING IS GENERALLY AN UNJUSTIFIED AND SUBSTANTIAL
EXPENSE.
A.1 INTRODUCTION
Leakage from ducted air distribution systems is an
important consideration in the design and operation of
ventilation and air conditioning systems. A ductwork
system that has limited air leakage, within defined limits,
will ensure that the design characteristics of the system
can be maintained. It will also ensure that energy and
operational costs are maintained at optimum levels.
Ductwork constructed and installed in accordance with
DW/144 should minimise a level of air leakage that is
appropriate to the operating static air pressure in the
system. However, it is recognised that the environment
in which systems are installed is not always conducive to
achieving a predictable level of quality in terms of
system airleakage and it is therefore accepted that
designers may sometimes require the systems to be
tested in part or in total. It should be recognised that the
testing of duct systems adds a significant cost to the
installation and incurs some extra time within the
programme (See 4.1 and 6.4 re mandatory testing).
A.2 DUCT PRESSURE
Ductwork constructed to DW/144 will be manufactured
to a structural standard that is compatible with the
system operating pressure.
There are three classes of duct construction to correspond with the three pressure classifications:
Class A
Low pressure ducts suitable for a maximum positive
operating pressure of 500 Pascals and a maximum
negative pressure of -500 Pascals.
Class B
Medium pressure ducts suitable for a maximum
positive operating pressure of 1000 Pascals and a
maximum negative pressure of -750 Pascals.
Class C
High pressure ducts suitable for a maximum positive
operating pressure of 2000 Pascals and a maximum
negative pressure of -750 Pascals.
A.3 LEAKAGE FROM DUCTWORK
Leakage from sheet metal air ducts occurs at the
seams and joints and is therefore proportional to the
total surface area of the ductwork in the system. The
level of leakage is similarly related to the air pressure
in the duct system and whilst there is no precise
formula for calculating the level of air loss it is
generally accepted that leakage will increase in
proportion to pressure to the power of 0.65.
The effect of air leakage from high pressure/velocity
ductwork is critical in terms of system performance,
energy consumption and the risk of high frequency
noise associated with leakage.
These problems are less critical with medium
pressure/velocity systems, but should be considered.
Low pressure/velocity ducts present the lowest risk in
terms of the effect of leakage on the effective
operation of the system.
A.4 SYSTEM LEAKAGE LOSS
As there is no direct relationship between the volume
of air conveyed and the surface area of the ductwork
system required to match the building configuration it
is difficult to express air leakage as a percentage of
total air volume.
Similarly, the operating pressure will vary throughout
the system and as leakage is related to pressure the
calculations are complex. However, it is generally
accepted that in typical good quality systems the
leakage from each class of duct under operating
conditions will be in the region of:
Class A
low pressure
6%
Class B
medium pressure
3%
Class C
high pressure
2%
75
for the classification for the section of the ductwork that is to be tested.
The tests shall be carried out as the work proceeds
and prior to the application of thermal insulation.
In the event of test failure of the randomly selected
section, the designer shall have the right to select
two further sections at random for testing. Where
successive failures are identified there shall be a
right to require the contractor to apply remedial
attention to the complete ductwork system.
The contractor shall provide documented evidence
of the calculations used to arrive at the allowable
loss for the section to be tested and the client, or
his agent, shall witness and sign the results of the
test.
A.5 SPECIFYING AIR LEAKAGE TESTING
Respecting both the cost and programme implications
associated with testing ducts for leakage, the designer
may, for example indicate that a particular system is
tested as follows:
a) High pressure ducts - all tested.
b) Medium pressure ducts - 10% of the ductwork
shall be selected at random and tested.
c) Low pressure - untested.
In the case where a random test is selected for medium
pressure ducts the following clause is suggested for
inclusion by the designer.
The designer shall select at random a maximum of
10% of the duct system to be tested for air leakage.
The duct shall be tested at the pressure recommended
in Table 17 of DW/144
76
A.6 SPECIAL CASES
There may be situations on a project where circumstances
dictate that special consideration be given to containing
air losses, e.g. a long run of ductwork may incur a
disproportionate level of air loss.
In cases such as this example the designer can specify an
improved standard of airtightness, i.e. 80% of allowable
loss for Class `B' ducts. The designer should not specify a
Class `C' test at Class `C' pressure for a Class `B' duct.
A.7 SUGGESTED RANGE OF TESTING
• High pressure ducts
100% test
• Medium pressure ducts
See A5
• Low pressure ducts
Untested
• Exposed extract systems
Untested
• Ceiling void extract
systems
Untested
• Secondary ducts from
VAV or fan coil units
Untested
• Flexible ducts
Untested
• Final connections and
branches to grilles and
diffusers
Untested
A.8 TESTING OF PLANT ITEMS
Items of inline plant (eg. Figs. 168 to 175) will not
normally be included in an air leakage test. The ductwork
contractor may include such items in the test if the
equipment has a certificate of
77
conformity for the pressure class and air leakage
classification for the system under test.
A.9 DESIGNER'S CALCULATIONS
The designer can calculate with reasonable accuracy
the predicted total loss from a system by:
a) Calculating the operating pressure in each
section of the system.
b) Calculating the surface area of the ductwork
in each corresponding pressure section.
c) Calculating the allowable loss at the operating pressure for each section of the system
(see table 17 for allowable leakage figures).
A.10 VARIABLE PRESSURES IN SYSTEMS
Designers can achieve significant cost savings by
matching operating pressures throughout the system to
constructional standards and appropriate air leakage
testing, e.g. the practice of specifying construction
standards for whole duct systems based on fan
discharge pressures may incur unnecessary costs on a
project.
For example, some large systems could well be
classified for leakage limits as follows:
Plant room risers
Class C
Main floor distribution
Class B
Low pressure outlets
Class A
78
79
APPENDIX B - IDENTIFICATION OF DUCTWORK
Note
The information given in this Appendix is for the
guidance of mechanical services contractors, consulting engineers, etc. The identification of ductwork does not form part of the work carried out
by the ductwork contractor unless required by the
designer in the job specification.
B.1 GENERAL
B.1.1 Introduction
With the increasing complexity of ventilation and air
conditioning systems, it is becoming more important
to ensure ready identification of ducts for the purposes
of commissioning, operation and maintenance of
systems. The purpose of these recommendations is to
lead towards the use and standardisation of a system
of identification for ducts for the benefit of designers,
contractors and clients.
B.1.2 Scope
B.1.2.1 These recommendations deal with the
identification of ducts for ventilation, air conditioning and simple industrial exhaust systems. They
do not include piped gas systems such as those dealt
with in BS 1710 1984, nor ductwork systems for
industrial processes, although the general
considerations and intentions could be extended with
the agreement of the client to cover such systems.
B.1.2.2 The method is designed to identify the air
being conveyed, the direction of flow, the
destination of the air and/or the location or
nomenclature of the plant where the air was treated.
With small or simple plants, it may not be strictly
necessary to provide identification because the
function is apparent, but it is considered advisable to
do so because this will increase familiarity with the
labelling system and also because the nature and
direction of air flow may not always be apparent.
possible, where there is adequate natural or artificial
light.
B.2.2 Identification symbols will be needed in plant
rooms and remote areas. Symbols should occur
frequently enough to avoid the need for ducts to be
traced back. Symbols should be placed at any service
and access points to the distribution system, including
points where the distribution system has reduced to a
single duct.
B.2.3 Colour coding
The choice of colours has been based on the need to
provide:
B.2.3.1 Strong contrasting colours which are
recognisable even though covered with dust.
B.2.3.2 Contrast between the symbol colour and the
base colour of the duct. Usually the base colour
metallic grey of galvanized or aluminium sheet or foil
sheathing, or the white, pale grey, or buff paint on the
insulation is a neutral colour against which the
recommended symbol colours will stand out.
B.2.4 The recommended colours are given in Table 18.
The colour coding indicates the type of air being
conveyed.
B.2 IDENTIFICATION
B.2.1 Location
To be effective the identification must be placed
where it can be easily seen and at positions where
identification will be required. To ensure that the
symbols are seen, the following points should be
considered.
B.2.1.1 The symbols should be on the surfaces
which face the positions of normal access to the
completed installation.
B.2.1.2 The symbols should not be hidden from
view by structural members, other ducts, plant, or
other services distribution systems.
B.2.1.3 The symbols should be placed, where
80
B.2.5 For conditioned air, two symbols (one red, one
blue) may be used, or a single symbol coloured part
red, part blue.
of the plant. The plant itself must be clearly
numbered to correspond. Letters for Supply, Flow,
Extract, etc., should not be added because
identification will be clear from the colour symbol.
Thus confusion between `S' for Supply and `S' for
South will be avoided.
B.2.6 If a finer grading than that given in Table 18 is
required, as for instance in a laboratory with two
separate contaminated air exhaust systems, it is
recommended that the type colour be used with, say, a
stripe of a second colour. Where the duct contents
constitute a hazard, a symbol as given in BS 1710
1984 should be added to the type colour.
B.2.7 Direction of flow
B-.2.7.1 The form of symbol chosen indicates
direction. It is an equilateral triangle (see Fig. 179)
with one apex pointing in the direction of air flow.
Where the boundaries of the duct are not visible, two
triangles should be arranged in line ahead to indicate
direction of flow.
B.2.7.2 The size of the symbol will depend on the
size of the duct and the viewing distance. The
recommended minimum size for normal use is 150
mm length of side.
B.2.8.3 Where identification of the space is by
room number, this must be agreed with the user
who otherwise may have numbered the rooms
differently.
Some examples of further identification systems
are given in Table 19.
B.2.8.4 The letters and numbers should be in either
black or white, whichever gives the better contrast.
They should be marked on the colour symbol or
immediately adjacent to it. The size of the figures
will depend on how easily they can be seen, but
should not be less than 25 mm high.
B.2.9 Explanatory chart
An explanatory chart shall be included in the 0
(Operating) and M (Maintenance) manual and shall
also be kept in the plant room or other convenient
place. The chart should show and explain the colour
symbols used on the installation and where
appropriate the figure and letter codes used for
further identification.
B.2.8 Further identification
B.2.8.1 On small or simple installations where there
is one plant and one or two zones and therefore little
chance of confusing the ducts, it will not be
necessary to provide identification other than the
colour symbol. On large complex installations with
many zones, widely branched distribution systems or
several plants, further identification is necessary. In
this connection a plant refers to the ductwork and
equipment associated with one particular fan.
B.3 METHOD OF APPLICATION OF
SYMBOLS
B.3.1 Several methods are available for applying the
symbols, the main factor being that the symbol is
permanently affixed. Suitable methods are:
B.3.1.1 Painting, using stencilled letters and
figures.
B.3.1.2 Self-adhesive plastics or transfers with
water soluble backing. (It is important to ensure
that the surface is smooth and clean and that the
adhesion will not deteriorate due to the surrounding atmosphere.)
B.2.8.2 The further information to be given will
normally be the space served by the duct and in
some cases the associated plant. The information
should be given as briefly as possible using
commonly accepted forms such as a number
indicating which floor of a building. The plant
identification should always be preceded by the
letter `P' to avoid confusion between the number of
the floor and the number
B.3.1.3 Purpose-made plastics or metal labels.
81
APPENDIX C - GUIDANCE NOTES FOR THE TRANSPORT,
HANDLING AND STORAGE OF DUCTWORK
It is recommended that before a contract is finalised,
that consideration is given to the subject of site
access, material handling and storage as they have a
strong influence on the cost efficiency of the overall
activity of ductwork installation.
until they become an integral part of a completed
ductwork system. Whilst this may temporarily detract
from its intended appearance, this deformation will not
have any affect on the functionality of the finally
assembled system.
Installation of ductwork and associated plant items
will inevitably involve manual handling. The
responsibility of employers and employees to assess
the risk of personal injury during manual handling
operations is set out in the H.S.E. publication L23,
Guidance on Manual Handling Regulations 1992.
C.1 Transport
Large capacity vehicles with high-sided open or
closed-top bodies are the most suitable for the
transport of ductwork.
Careful consideration should be given to the
unloading of transport on site as not all sites benefit
from the material handling and access facilities that
exist in a manufacturing workshop such as cranes,
fork-lifts or loading bays. Site handling facilities
along with vehicular access restrictions may
influence the type and size of transport to be utilised.
Lengths of ductwork should be positioned so as to
avoid crushing. Lengths with projections, such as
branches and bends, flanges, girths, damper quadrants should be loaded so as to avoid damage to
adjacent duct panels. In some cases, particularly on
contracts calling for repetitive sizes, the use of
timber jigs and spacers may be justified.
Where reduced bulk and greater protection are major
factors, such as consignments for export,
transporting ductwork in `L' shape sections may
justify the increased site assembly costs.
C.3 Site storage
Adequate floor space must be provided within the
building for the site storage of ductwork. Such storage
shall make due allowance for the storage of ductwork
in stacks such that access between them is of sufficient
width to permit the removal of items without
interference to adjoining
stacks.
Ductwork
components should be positioned so as to avoid
crushing. Ductwork of small panel size may be stored
horizontally; however care should be exercised to
ensure that stack sizes are limited to within the
structural strength of the duct sections to prevent
distortion of the lower sections within the stack.
C.4 Internal cleanliness of new ductwork
The site storage of ductwork introduces the important
consideration of maintaining the internal cleanliness of
the ductwork. Reference should be made to HVCA
document:
C.2 Handling
To minimise the risk of damage, duct sections
should be clearly identified and deliveries to site
should be closely linked to the installation programme, so as to avoid accumulation of unfixed
ductwork and minimise double handling. It is
important to recognise that ductwork panels, joints
and corners are susceptible to damage and care must
be taken when handling such material through a site.
During handling, individual items of ductwork may
be liable to slight cross sectional deformation
• DW/TM2 Guide to Good Practice - Internal
Cleanliness of New Ductwork Installations.
If the above conditions can not be satisfied consideration should be given by the designer to
amending the specification to include for "Post
Installation Cleaning" as covered by the HVCA
document:
• TR17 Guide to Good Practice - Cleanliness of
Ventilation Systems.
82
APPENDIX D - DUCTWORK SYSTEMS AND FIRE HAZARDS
D.1 Fire and smoke containment/hazards are factors
which influence the design and installation of
ductwork systems.
Information concerning fire protection systems is laid
down in BS 5588, Fire Precautions in the design and
construction of Building Part 9 (1989) Code of
Practice for Ventilation and Air Conditioning
Ductwork and tested in accordance with BS 476 Part
20 (1987) and BS 476 Part 22 (1987) for Fire and
Smoke Dampers and British Standard 476 Part 24
(1987) - ISO 6944 - (1985) for Fire Rated Ductwork.
Method 3 - Protection using Fire Resisting
Ductwork
The ductwork itself forms a protected shaft. The fire
resistance may be achieved by the ductwork material
itself or through the application of a protective
material provided that the ductwork has been tested
and/or assessed to BS476 Part 24 with a fire
resistance, when tested from either side that should
not be less than the fire resistance required for the
elements of construction in the area through which it
passes. It should also be noted that the fire resisting
ductwork must be supported with suitably sized and
designed hangers, which reflect the reduction in
tensile strength of steel in a fire condition i.e:
Fire resisting ductwork rated at 60 minutes (945°C),
reduces the tensile strength from 430
N/mm2 to 15 N/mm2.
Fire resisting ductwork rated at 120 minutes (1,049°C)
tensile strength reduced to 10 N/mm2.
Fire resisting ductwork rated at 240 minutes (1,153°C)
tensile strength reduced to 6 N/mm2.
Where the fire resisting ductwork passes through a
fire compartment wall or floor, a penetration seal must
be provided which has been tested and/or assessed
with the ductwork to BS476 Part 24, to the same fire
rating as the compartment wall through which the fire
resisting ductwork passes. It should also be noted that
where the fire resisting ductwork passes through the
fire compartment wall or floor, the ductwork itself
must be stiffened to prevent deformation of the duct in
a fire to:
a) maintain the cross-sectional area of the duct
b) ensure that the fire rated penetration seal around
the duct is not compromised.
D.2 Building Regulations in the United Kingdom
require that new buildings be divided into fire
compartments in order that the spread of smoke and
fire in the building is inhibited, and to stop the spread
of smoke and fire from one compartment to another,
for given periods of time as specified by the Building
Regulations 1991 (Approved Document B).
D.2.1 There are three methods of fire protection,
related to ductwork systems as given in BS 5588 Part
9 (1989).
Method 1 - Protection using Fire Dampers
The fire is isolated in the compartment of origin by
the automatic or manual actuation of closures within
the system. Fire dampers should, therefore, be sited at
the point of penetration of a compartment wall or
floor, or at the point of penetration of the enclosure of
a protected escape route.
Fire dampers should be framed in such a way as to
allow for thermal expansion in the event of fire, and
the design must provide for the protection of any
packing material included.
Standard types of fire dampers and frames are
described in Section 22 of this specification.
For further information refer to the impending HVCA
publication DW/TM3, `Guide to Good Practice for
the Design for the Installation of Fire
and Smoke Dampers'.
D.2.2 - Main areas within building where
Ductwork should be fire protected
The following notes are for guidance only, and it
should be noted that authority rests with the Building
Control Officer and/or the Fire Officer responsible for
the building. Reference on the folowing systems
should also be made to the current Building
Regulations.
a. Smoke Extract Systems:
If the ductwork incorporated in a smoke extract
system is wholly contained within the fire
compartment, it must be capable of resisting the
anticipated temperatures generated through the
development of a fire. BS 476 Part 24 also requires
ductwork, which is intended as a smoke extract,
must retain at least 75% of its cross-sectional area
within the fire compartment. If the ductwork
penetrates a fire resisting barrier, it must also be
capable of providing the same period of fire
resistance.
b. Escape Routes covering Stairways, Lobbies and
Corridors
All escape routes must be designed so that the
building occupants can evacuate the building
Method 2 - Protection using Fire Resisting
Enclosures
Where a building services shaft is provided through
which the ventilation ductwork passes and if the shaft
is constructed to the highest standard of fire resistance
of the structure which it penetrates, it forms a
compartment known as a protected shaft. This allows
a complicated multiplicity of services to be
transferred together through a shaft transversing a
number of compartments and reaching remote parts of
the building, without requiring further internal
divisions along its length. The provision of fire
dampers is then required only at points where the
ventilation duct leaves the confines of the protected
shaft.
However, if there is only one ventilation duct and
there are no other services within the protected shaft,
between the fire compartment and the outside of the
building, no fire dampers will be required.
83
safely in the case of fire. Ductwork which passes
through a protected escape route must have a fire
resistance at least equal to the fire compartment
through which the ductwork passes, either by the use
of fire dampers or fire resisting ductwork.
c. Non Domestic Kitchen Extract Systems
Where there is no immediate discharge to
atmosphere, i.e. the ductwork passes to atmosphere
via another fire compartment, fire resistant ductwork
must be used. Kitchen extract ductwork presents a
particular hazard as combustible deposits such as
grease are likely to accumulate on internal surfaces,
therefore, all internal surfaces of the ductwork must
be smooth. A fire in an adjacent compartment,
through which the ductwork passes, could lead to
ignition of the grease deposits, which may continue
through the ductwork system, possibly prejudicing
the safety of the kitchen occupants. For this reason
consideration must be given to the stability, integrity
and insulation performance of the kitchen extract
duct which should be specifically tested to BS 476
Part 24 for a kitchen extract rating.
• Access doors for cleaning must be provided at
distances not exceeding 3 metres.
• Fire dampers must not be used.
• Use of volume control dampers and turning
vanes are not recommended.
Further information on kitchen extract systems will be
found in the HVCA publication DW/171 Specification
for Kitchen Ventilation Systems.
d. Enclosed Car Parks - which are mechanically
ventilated
Car Parks must have separate and independent
extract systems, because of the polluted nature of the
extract air. Due to the fire risk associated with car
parks, these systems should be treated as smoke
extract systems and therefore maintain a minimum
of 75% cross-sectional area under fire conditions in
accordance with BS 476 Part 24. Fire dampers must
not be installed in extract ductwork serving car
parks.
e. Basements - Ductwork from Basements must be
Fire Rated
If basements are compartmented, each separate
compartment must have a separate outlet and have
access to ventilation without having to gain access
(i.e. open a door to another
compartment). Basements with natural ventilation
should have permanent openings, not less than 2.5%
of the floor area and be arranged to provide a
through draft with separate fire ducts for each
compartment.
f. Pressurisation Systems
Pressurisation is a method of restricting the
penetration of smoke into certain critical areas of a
building by maintaining the air at higher pressures
than those in adjacent areas. It applies particularly to
protect stairways, lobbies, corridors and fire fighting
shafts serving deep basements as smoke penetration
to these areas would inhibit escape.
As the air supply creating the pressurisation must be
maintained for the duration of a fire, fire dampers
cannot be used within the ductwork to prevent the
spread of fire. Any ductwork penetrating fire
resisting barriers must be capable of providing the
same period of fire resistance.
g. Hazardous Areas
There are other areas within the building where the
Building Control Officer or the Fire Officer could
state a requirement for fire resisting ductwork, eg.
areas of high risk, Boiler Houses, Plantrooms,
Transformer Rooms etc.
D.2.3 Cautionary note to all Ductwork Designers/
Manufacturers:
Ductwork constructed to DW/144 Standard has no
tested fire resistance. General purpose ventilation/air
conditioning ductwork and its ancillary items do not
have a fire rating and cannot be either utilised as or
converted into a fire rated ductwork system unless the
construction materials of the whole system including
supports and penetration seals are proven by test and
assessment in accordance with BS 476 Part 24.
In the case where galvanised sheet steel ductwork is
clad by the application of a protective material, the
ductwork construction must be as type tested and
comply with the protective material manufacturers
recommendations, eg. gauge of ductwork, frequency of
stiffeners and non-use of low melting point fasteners or
rivets. Sealants, gaskets and flexible joints should be as
tested and certificated in accordance with BS 476 Part
24
and
comply
with
the
manufacturers
recommendations.
Careful consideration must also be given to the
maximum certificated size tested to BS 476 Part 24 and
the manufacturers recommendations should always be
followed.
This appendix incorporates information given in the A.S.F.P publication `Fire Rated and Smoke Outlet Ductwork:
An Industry Guide to Design and Installation' available from Association for Specialist Fire Protection, Association
House, 235 Ash Road, Aldershot, Hampshire GU 12 4DD (Telephone: 01252 21322 Fax: 01252 333901)
84
APPENDIX E - HOT DIP GALVANIZING AFTER MANUFACTURE
E.1 General
E.1.1 For Hot Dip galvanizing after the fabrication of
any article it is necessary to appreciate the nature of the
process, including the surface preparation of the object
to be treated and the precautions to be taken in design,
fabrication and handling.
E.1.2 Hot Dip galvanizing involves dipping the object
into a bath of molten zinc (at a temperature of between
445° and 465° C), and it is necessary for the zinc to
cover the whole of the surface leaving no gaps in the
coating.
E.2 Design and fabrication
E.2.1 Rectangular ductwork must be fabricated using all
welded construction techniques with vented flanges and
stiffening frames (see E.2.3) as mechanical fixing and
lock-forming techniques are not compatible with the
galvanising process. In the course of dipping into the
molten zinc bath, unsightly panel distortion will occur
due to the relief of inherent stress in the steel sheet or of
any stresses that may have been built into the item during fabrication, or indeed of any stresses introduced
during the handling, loading or unloading of the item.
Table 20 indicates the minimum requirements for the
construction of rectangular ductwork.
E.2.2 It is essential to have a free flow of the molten
zinc over the object to be galvanized, together with
quick and complete drainage of the molten metal.
Because of the high temperature involved, the object to
be galvanized should be as rigid as possible, either by
the use of sufficiently heavy sheet or by stiffening or
bracing, or both.
E.2.3 Any sealed hollow section or cavity must be
adequately vented in order to obviate any possibility of
explosion. Holes of sufficient size (See E.2.4) in vertical
members must be provided diagonally opposite each
other, top and bottom of the member.
E.2.4 Vent holes should be of sizes as follows:
Size of
Minimum
hollow
diameter of
section
vent end
(dia. or side)
drainage holes
mm
mm
Up to 25
10
50 to 100
16
100 to 150
20
Over 150
25
ever, the pickling process does not generally remove
grease, oil or oil-based paint, and such substances
should be removed by the fabricator by the use of
suitable solvents before the object to be treated is
delivered to the galvanizing works. Any surface rust
that develops on the object between the time of
treatment by the fabricator and delivery to the
galvanizing works is not important, as this is cleaned
off by the acid pickling process.
E.4 Handling and storage after galvanizing
E.4.1 While a galvanized surface will not develop rust
in the ordinary sense as long as the zinc coating is
undamaged, zinc is subject to what is known as `wet
storage stain,' which is a white powdery deposit on the
zinc surface. Wet storage stain can arise from the
stacking of articles when wet, acid vapours, the effect
of salt spray, the reaction of rain with flux residues, etc.
The damage to the zinc coating is negligible in most
cases. When the deposits are heavy, these should be
removed by brushing with a stiff bristle or wire brush.
E.4.2 Galvanized articles should therefore not be
stacked or loaded when wet; they should preferably be
transported under cover or shipped in dry, well
ventilated conditions, inserting spacers (but not
resinous wood) between the galvanized articles.
E.2.5 Stiffeners should desirably have their corners
cropped so as to allow a free flow of zinc. Stiffeners
should be rolled steel angle, uncoated.
E.3 Surface preparation before galvanizing
E.3.1 The steel surface to be galvanized must be
chemically clean before dipping to ensure a continuous
coating. This is mainly achieved at the galvanizer's
works by pickling in an acid bath and fluxing before the
article goes into the zinc bath. How
E.4.3 When stored on site or elsewhere, care should be
taken to avoid resting the galvanized article on cinders
or clinker, as the acid content of these substances will
attack the zinc surface.
E.5 Subsequent finishing
E.5.1 Paint finishing subsequent to galvanizing is
sometimes required either for additional protection or
for decorative reasons. Galvanised surfaces require
chemical pretreatment prior to painting. Examples of
such a treatment are T-Wash and Etch Primer Types.
Advice should be sought from the paint manufacturer.
This Appendix incorporates information given in publications available from the
Galvanizers' Association, 6 Wrens Court, 56 Victoria Road, Sutton Coldfield, West Midlands B72 1SY
85
APPENDIX F - STAINLESS STEEL FOR DUCTWORK
usually adjusted by the manufacturer to balance
forming response, weldability and corrosion
resistance. It is readily welded in thin sheet form and,
since it does not form a hardened weld HAZ, no postweld heat treatment is required. It is widely used for
automotive exhaust system parts and is suitable for a
range of ducting and structural applications in mildly
corrosive applications.
F.1 General
F.1.1 Stainless steel is not a single specific material:
There is a large family of stainless steels with varying
compositions to suit specific applications, but all
contain at least 11 % of chromium as an alloying
addition.
F.1.2 Modern stainless steels have a combination of
good formability and weldability, and can be supplied
with a variety of surface finishes (see E4.1 below) They
have been developed to cover a wide range of structural
uses where high resistance to corrosion and low
maintenance costs are demanded.
F.2.2.2 17% chromium ferritic steel, 430S17, New
Designation 1.4016, x6Cr 17.
Forming and general characteristics are similar to the
409 grade, but the higher chromium level confers
better general corrosion resistance.
F.1.3 Ductwork applications for which stainless steels
are particularly suited include those where a high
integrity inert material is essential; where a high degree
of hygiene is required; in the chemical industries where
toxic or hazardous materials may be contained; in
nuclear and marine applications (e.g. on offshore
platforms). Stainless steels also find application in
exposed ductwork where their finish can be used to
aesthetic advantage.
F.2 Grades of stainless steel
F.2.1 The grades of stainless steel most commonly used
for ductwork applications are among those covered
currently by BS 1449, Part 2. However, a European
Standard, will supersede this British Standard. New
designations of the most common steel grades are given
in Table 21.
In some cases there are minor differences in chemical
composition between the BS and EN grades.
Before a grade is specified, the nature of the interior and
exterior environments of the ductwork system should be
taken into account. The steels described below cover
most normal applications. However, advice on specific
corrosion risks should be taken if the ductwork is to be
installed in a chemically contaminated atmosphere, or is
to be used to transport contaminated air, particularly if
there is a risk of internal condensation. More highly
alloyed grades of stainless steel with enhanced corrosion
resistance are available if required.
The commonly used steels divide into two main
families; the lower alloy ferritic 11-18% chromium
stainless steels are magnetic. The austenitic, 18%
chromium, 9% nickel steels have generally better
corrosion resistance and are non- or only slightly
magnetic.
F.2.2 The more commonly used stainless steels and
their characteristics are described below.
F.2.2.1 11.5% chromium ferritic steel with a titanium
addition, 409S 19, New designation 1.4512,
X2CrTi12.
This, and related grades, are among the leanest alloyed
of the stainless steels. Forming characteristics are
similar to those of mild steel, so it can be worked
using conventional practices. The composition and
processing of the steel is
F.2.2.3 18% chromium, 9% nickel austenitic stainless
steels.
A widely used grade is 304S15, New Designation
1.4301, x5CrNil8-10. There are compositional variants
within this family, designed to give specific
formability and welding characteristics. All are
weldable and have good general corrosion resistance
to normal and mildly corrosive atmospheres. They are
ductile and formable, but forming loads are higher
than for mild steels and suitable, robust equipment is
required.
F.2.2.4 17% chromium, 11% nickel, 2% molybdenum
austenitic steel, a widely used grade of this type is
316S31, New Designation 1.4401, x5CrNiMo17-11-2.
This steel has a significantly higher corrosion
resistance than the standard 18% chromium, 9% nickel
steels and is suitable for use in more aggressive
environments such as are met in ductwork in process
plants. However, more highly alloyed stainless steels
with better corrosion resistance are also available and
the advice concerning aggressive environments given
under section F.2.1 above should be noted.
F.3 Availability
Stainless steel is supplied in a wide range of thicknesses, from 0.4 mm for cold-rolled sheet and coil, and
from 0.075 mm for precision rolled strip. It is supplied
in slit widths as specified by the customer, up to a
maximum width of 2030 mm, depending on thickness.
Material compatability of sheet, section and fixings is
not always assured in practice due to commercial
availability.
F 4 Surface finishes
F4.1 Stainless steel is available in a wide selection of
finishes, varying from fine matt to mirror polished, as
defined in BS 1449: Part 2: and in EN 10088: Part 2.
Mill finishes
Type 2D
86
Cold finished softened and descaled.
A uniform matt finish.
Type 2B
Cold rolled, softened, descaled and
lightly worked with polished rolls. A
smooth finish brighter than 2D.
Type 2A/2R Bright annealed. A cold finished
reflective appearance retained through
annealing.
essary, however, depending on the type of stainless steel
being used.
F.6.3 As a general rule, the 400 series of stainless steels
can be formed using normal mild steel settings. The 300
series, however, because of the higher yield point and the
greater rate of work hardening, will require higher
working pressures.
Polished finishes
Type 4/2J
Dull polished. A lustrous unidirectional
finish produced by fine grinding,
generally with abrasives of 150 grit size.
It has little specular reflectivity. Further
dull polishing after fabrication will
diminish the effects on appearance of
welds or accidental damage by blending
them into the surrounding metal.
Type 5/2K Dull
polished
with
specific
requirements, to achieve a fine, clean
cut surface finish with good corrosion
resistance.
Type 8/2P Mirror polished. A bright, nondirectional reflective finish with a high
degree of image clarity.
F.6.4 Ductwork contractors who have experience of the
use of stainless steel report difficulty in forming
Pittsburgh and button punch snap lock seams. As regards
cross joints, socket and spigot joints are recommended,
and one or two of the slide-on flanges are suitable. In
view of the foregoing, it is recommended that trials be
carried out before starting on production.
F.7 Rectangular ducts
The constructional requirements for rectangular stainless
steel ducts are the same as for galvanized mild steel.
F.8 Circular ducts
The constructional requirements for circular stainless
steel ducts are the same as for galvanized mild steel.
F.9 Stiffening
Wherever possible, the material used for stiffening should
be of the same grade of stainless steel as used for the
construction of the ducts, or should be made equally
corrosion resistant to suit the environment in which the
ductwork is situated.
F.4.2 Where other finishes are required, such as for
aesthetic purposes, a range of patterned or textured
(2F,2M) finishes is available. Colour may be applied
in the form of paint or lacquer, or the material may be
supplied pre-coloured as by the 'INCO' process or by
mill application of polymer coatings.
F.10 Fixings and fastenings
The types of fastening and the maximum spacings
specified in Table 5 (rectangular) and Table 9 (circular)
also apply to stainless steel ductwork.
Fixings and fastenings should be of the appropriate grade
of stainless steel as used in the construction of the
ductwork, or should be made equally resistant to
corrosion in relation to the environment in which the
ductwork is situated. The type of stainless steel fastening
used should conform to the appropriate specification BS
6105: 1981.
F.5 Surface protection
F.5.1 No surface protection is required for stainless
ductwork used indoors or outdoors, provided the
correct quality is specified. This is because the
naturally occurring chromium-rich oxide film which is
present on the surface of the metal, if damaged,
reforms immediately by reaction between the steel and
the atmospheric or other source of oxygen.
F.5.2 If a mixture of metals is used, such as mild steel
supports for stainless steel ductwork, the surface of the
mild steel must be adequately protected from the
galvanic corrosion that might result from the intimate
contact between the two types of metal. (The
appropriate protective finish should be employed. See
27.3.5)
F.11 Welding
All the modern welding processes may be used to weld
stainless steel but carburising operations such as oxyacetylene and carbon arc welding are not suitable. The
Tungsten inert gas (TIG) and resistance welding
techniques are most likely to be used for thin gauge
materials. Attention is drawn to BS 4872: Part 1 1982,
(welder qualification) and BS 7475: 1991 (welding
processes).
F.6 Construction
F.6.1 Sheet thicknesses for stainless steel ductwork
should be the same as for galvanized steel (see Tables
2, 3 and 4). Provided the correct grade of stainless
steel has been selected, there is no requirement for a
corrosion allowance with stainless steels and the gauge
can be selected on structural considerations only.
Selection of the correct welding electrodes and filler
rods is important, particularly when welding
dissimilar metals, such as stainless steels to non-stainless
structural steels. Reference for guidance should be made
to BS 2901: Part 2: 1990 for rods and wires for gas
shielded welding and BS 2926:1984 for electrodes for
Manual Metal Arc. (MMA) welding.
F.6.2 The forming of rectangular and circular ducts
can be carried out by the use of conventional press
working and sheet metal forming machines. Some
alteration in working practices may be nec-
F.12 Avoidance of contamination
Attention is drawn to the risks of rust staining of stainless
steel surfaces resulting from contamination by nonstainless steel or iron debris.
87
If particles such as filings of a non-stainless steel or
iron are expressed into contact with a stainless steel,
subsequent exposure to moisture will lead to staining
of the surface as these particles rust. Whilst this
staining often can be removed without harm to the
stainless steel surface, in aggressive environments
corrosion products around the rust centre can create a
risk of pitting of the stainless steel. As a general rule,
stainless steels should be kept free from iron dust and
debris contamination. In particular, wire brushes must
be made of stainless steel and shot, beads and abrasive
media used to clean surfaces must be `iron free'.
Contamination can arise from tools which have been
used previously for cutting non-stainless steels
without adequate cleaning and from abrasion on
stillages and racks. It is good practice to dedicate
storage and bench areas for stainless steels, with soft
surfaces, e.g. wooden battens, to mininise scratching
of the surface and if practicable designate stainless
only working areas.
F.13 Fire dampers
Stainless steel is an ideal material for use in the
construction of fire dampers, because of its high
resistance both to heat and corrosion. It is therefore
most applicable where a fire authority specifies a
requirement for corrosion resistance.
F.14 Sealants, gaskets and tapes
The sealing materials and methods set out in this
publication are also applicable to stainless steel
ductwork. However, any chloride-based material,
such as polyvinyl chloride (PVC), should be avoided,
as breakdown of such material at certain elevated
temperatures could lead to corrosion of the stainless
steel.
F.15 General design considerations
It is the designer's responsibility to indicate the type of
stainless steel most suitable for the conditions to which
the ductwork is to be exposed. If users and designers are
in doubt as to which material is appropriate to a
particular application, technical advice may be obtained
from the source noted below.
Table 21, showing the approximate correspondence between the chemical compositions of the commonly used
stainless steel grades in BS 1449, Part 2: 1983, and the European Standard EN 10088-1, List of Stainless
Steels. (Part 1 gives the chemical compositions and identifications of the stainless steels, it is for information.
Part 2 of this standard describes the technical delivery conditions for sheet/plate and strip for general
purposes.)
This appendix is based largely on information kindly
supplied by the
Avesta Sheffield Technical Advisory Centre, ASTAC,
P.O. Box 161,
Shepcote Lane,
Sheffield S9 l TR
Telephone: 0114 244 0060 Fax: 0114-242 0162
88
APPENDIX G - PRE-COATED STEEL
G.1 Nature of the material
G.1.1 'Pre-coated' steel is sheet, coil or strip to which
has been applied at the steel mills a coating having a
decorative or protective function, or both.
G.5 Ductwork construction from pre-coated steel
G.5.1 The type of pre-coated steel most suitable for
ductwork should be carefully considered, mainly from
the point of view of the fabrication properties of the
coating type. It is probable that a plastisol coating will
be found to be most suitable for ductwork, as this type
of coating will withstand forming at normal ambient
temperatures. It also tolerates rougher handling during
forming and erection than the much thinner paint
coating types.
G.1.2 The basis metal to which the coatings are
applied are hot-dip galvanized or aluminium-zinc
coated sheet or coil, uncoated steel or electrogalvanized steel (e.g., Zintec).
G.2 Range of coatings available
G.2.1 A number of different types of coating, in
various thicknesses, are available - PVC ('Plastisol'
and 'Organosol'); paint coatings of several types,
silicone enamels, etc.
G.5.2 Careful consideration should be given to the
constructional methods to be used for ductwork to be
made from pre-coated steel. The principle to be
followed should be to make seams and joints as
unobtrusive as possible. Some of the conventional
methods of seaming may be used, but a number of
others are not suitable. Welding with conventional
equipment should not be attempted. Mechanical
fastenings should be chosen with care having regard to
appearance as well as efficiency; and sealant should be
applied with these factors in mind. Stiffening should be
carefully considered in relation to appearance.
G.2.2 A wide range of colours and surface finishes are
available, but there are minimum quantity
requirements for some types of coating, finish and
colour. The characteristics of the particular type of
coating contemplated for a particular use should be
investigated in respect of formability, fastness to light,
chemical resistance and other relevant properties.
G.2.3 The material can be supplied with one or both
sides treated, with the specified coating. Standard
`backing coat' finishes are usually applied to the
reverse side unless otherwise stated.
G.6 Handling, storage, transport and erection
G.6.1 Much more care than usual is required in these
respects, as the coatings are all to a greater or lesser
degree susceptible to mechanical damage. For example,
sheet should not be dragged off the top of a pile but
removed by `turning' off the stack.
G.3 Sizes available
G.3.1 Pre-coated steel is available in sheet or coil
form. The maximum available width can vary
according to the steel thickness required. Availability
varies according to type of substrate and coating, so
prospective purchasers should query the sizes
available for the specific type required.
G.6.2 With sheet pre-coated on one side only, it may be
found desirable to stack face to face.
G.6.3 The flexibility of coatings of the types used on
pre-coated steel depends on temperature. Therefore,
manipulation should be carried out at temperatures
above 16°C (60°F) in order to minimise the risk of the
film cracking on roll forming, etc. If the material has
been stored outside at low temperature, a warm-up
period should be allowed before manipulation of the
sheet is undertaken.
G.4 Sources of supply
G.4.1 Pre-coated steel is widely available but it
should be noted that minimum order quantities may
apply.
The information on which this appendix is based
has been kindly supplied mainly by British Steel
plc. More detailed information may be obtained
from:
British Steel plc,
Product Development Centre,
Shotton Works,
Deeside,
Flints CH5 2NH
Telephone: Chester (01244) 812345
Fax: 01244 836134
89
APPENDIX H - ALUMINIUM DUCTWORK
H.1 Scope
This section applies only to rectangular and circular
aluminium ductwork operating at low pressure, as defined
in Tables 22 and 23.
If consideration is being given to either higher pressures
or flat oval ductwork then it would be prudent to seek
advice from manufacturers who have the experience and
capacity to manufacture aluminium ductwork.
H.2 Suitable grades
H.2.1 Ductwork can be constructed from all the
commonly used aluminium alloys, the choice depending
on the purpose for which the ducts will be used and the
service environment.
H.2.2 The alloys 1200, 3103 and 5251 (as specified in
BS.EN485, BS.EN515, BS.EN573) are easy to form and
to join, and have excellent resistance to atmospheric
corrosion, with 5251 being rather more resistant to
marine atmospheres.
H.2.3 These alloys can be supplied in various tempers
produced by different degrees of cold rolling, so that a
range of strengths is available. In choosing a temper, it is
necessary to consider any forming that will be done, as
with the harder tempers the forming of tight bends might
cause cracking. Where high strength is required, alloy
6082-T6 sheet can be used.
H.2.4 Aluminium coil is available in plain form and prepainted finish.
H.3 Construction - rectangular ducts
H.3.1 Table 22 sets out the minimum constructional and
stiffening requirements for rectangular aluminium ducts
and the permitted types of cross joint.
H.3.2 Sealant
The sealant requirements set out in this specification for
galvanized steel rectangular ductwork also apply to the
longitudinal seams and cross joints in aluminium
ductwork.
H.4 Construction - circular ducts
H.4.1 Table 23 sets out the minimum constructional and
stiffening requirements for circular ducts made from
aluminium, and the permitted types of cross joint.
H.5 Fastenings
H.5.1 The types of fastening and the maximum spacings
specified in Table 5 (rectangular) and Table 9 (circular)
apply to aluminium ductwork, except that such fastenings
shall be of aluminium, stainless steel or monel metal.
H.6 Welding
H.6.1 All the aluminium alloys can be welded by MIG
(Metal and Inert Gas) or TIG (Tungsten and Inert Gas)
methods, with argon as the shielding gas. Helium or a
mixture of helium and argon can be used, but not CO2.
Alloys in a work-hardened temper are reduced to the
annealed condition in the heat affected zone; 6082-T6 is
reduced approximately from the T6 to the T4 temper.
Alloys 1200 and 3103 are easy to braze, as is 6082, but
the latter needs to be re-heat treated to regain its strength.
H.7 Protective finishes
H.7.1 No protective finishes are required for aluminium
ductwork used indoors or outdoors in normal atmospheric
conditions. In moist atmospheres, particularly if they are
contaminated by industrial effluent or by salt from the
sea, surfaces not exposed to washing by rain will become
roughened and covered with a layer of white corrosion
product. However, this has the effect of sealing the
surface against further attack, and the mechanical
properties of any but the thinnest of materials will be only
slightly affected.
H.7.2 If surface protection is specified, any of the normal
organic finishes can be used, including the laminated
PVC films, although paints with heavy metal pigments
are not suitable. The use of prepainted strip in coil form
provides a reliable quality finish and often proves more
economical than painting after assembly. Anodising
provides an excellent finish for aluminium, but this
process would have to be carried out after forming and
would therefore not usually be practicable for ductwork,
except perhaps for ducts formed from extrusions.
H.7.3 Mild steel section used in supporting aluminium
ductwork shall have a protective finish
(See 27.3.5)
90
Incorporates information provided by the Aluminium
Federation Ltd., Broadway House, Calthorpe Road, Five
Ways, Birmingham B15 1TN (telephone: 0121-456
1103), from whom more detailed information may be
obtained.
APPENDIX J - EUROVENT
J.1 General
Some explanation of the function, composition,
objectives and membership of EUROVENT is given
below.
tate commercial exchanges between its member
nations in the search for improved quality; and the
adoption of rules, directives and codes of practice in
the technical and economic spheres in the member
countries'.
J.2 Membership
EUROVENT is an omnibus word standing for the
European Committee of the Construction of Air
Handling Equipment. The committee was formed in
1959, and in 1977 its constituent members were the
relevant national associations in Austria, Belgium,
Denmark, Finland, France, German Federal Republic,
Italy, Netherlands, Norway, Sweden, Switzerland and
the United Kingdom.
J.4 EUROVENT publications
EUROVENT has published a number of documents in
the air handling field, and these include Document 2/2
covering the procedure for testing for air leakage in
ductwork, and provides for two levels of permissible
air leakage for low-pressure air distribution systems.
Document 2/3 covers the standardisation of duct sizes.
J.5 Air leakage
The basis on which air leakage is calculated in
EUROVENT Document 2/2 has been adopted in
DW/143 A practical guide to Ductwork Leakage
Testing.
J.3 Objectives
The objectives of EUROVENT are `to improve and
develop technical matters in the manufacture and
operation of air handling equipment; to improve the
professional status of its members and to facili-
Information about EUROVENT may be obtained from
the HEVAC Association, Sterling House, 6 Furlong
Road, Bourne End, Bucks SL8 5DG (Telephone: 01628
531186 Fax: 01628 810423)
91
APPENDIX K - SUMMARY OF BS.EN10142: 1991 CONTINUOUSLY HOT DIP ZINC
COATED MILD STEEL STRIP AND SHEET FOR COLD FORMING
Note - The extracts from BS.EN 10142: 1991 have been
prepared by the HVCA and are included here by courtesy
of the British Standards Institution.
K.1 GENERAL
K.1.1 The BS 2989: 1975 and 1982 entitled
`Continuously hot-dip zinc coated and iron-zinc alloy
coated steel: wide strip, sheet/plate and slit wide strip'
summarised in DW/142 has been superseded by
BS.EN10142: 1991 entitled `Continuously hot-dip zinc
coated mild steel strip and sheet for cold forming'
(including amendment A1:1995).
K.1.2 British Standard BS.EN10142: 1991 sets out
requirements for the conventional galvanized sheet and
coil and for zinc-iron coated steel. (Both these are
included in DW/144 - see Section 7.)
The type of steel normally used for ductwork is
DX51D and Z275.
Normal spangle (N). This finish is obtained when the
zinc coating is left to solidify normally. Either no
spangle or zinc crystals of different sizes and
brightness appear depending on the galvanizing
conditions. The quality of the coating is not affected
by this.
NOTE. Normal spangle is the type normally supplied
for a wide variety of applications.
Minimized spangle (M). The surface has minimized
spangles obtained by influencing the solidification
process in a specific way. The finish may be specified
if the normal spangle applicable does not satisfy the
surface appearance requirements.
K.5 SURFACE PROTECTION
K.5.1 General
Hot-dip zinc coated strip and sheet products generally
receive surface protection at the producer's plant. The
period of protection afforded depends on the
atmospheric conditions.
K.2 STEEL GRADES
K.2.1 BS.EN10142: 1991 and amendment A1:1995 lists
the grades of steel set out in the next column, among
others:
Grade
Name of grade
Application 1
DX51D + Z Bending and
Forming quality
profiling
steel suitable for
quality
manufacture of the most
profiles and more
difficult bending
operations
DX52D + Z Drawing quality Forming quality steel
suitable for simple
drawing operations and
for more difficult
profiling operations
DX53D + Z Deep drawing
Forming quality
quality
steel suitable for
deep drawing and
difficult forming
operations
DX54D + Z Special deep
Forming quality
drawing quality steel suitable for
deep drawing and
difficult forming
operations where a nonageing steel is required
K.5.2 Chemical Passivation
Chemical Passivation protects the surface against
humidity and reduces the risk of formation of `white
rust' during transportation and storage. Local
discolouring as a result of this treatment is permissible
and does not impair the quality.
K.5.3 Oiling
This treatment also reduces the risk of corrosion of the
surface. It shall be possible to remove the oil layer
with a suitable degreasing solvent which does not
adversely affect the zinc.
K.5.4 Chemical Passivation and Oiling
Agreement may be reached with the producer on this
combination of surface treatment if increased
protection against the formation of `white rust' is
required.
K.5.5 Untreated
Hot-dip zinc coated strip and sheet products complying with the requirements of this standard are only
supplied without surface protection if expressly
desired by the purchaser on his own responsibility. In
this case, there is increased risk of corrosion.
K.6 FORMING
K.6.1 The British Standard says that provided that the
profiling machine is set to avoid excessive stretching
in the product, it is possible to form lock seams
successfully with DX51 D + Z sheet up to a thickness
of 1.5 mm and DX52D + Z sheet up to 2 mm; and
snap lock seams with DX51 D + Z up to 0.9 mm thick
sheet and DX52D + Z sheet up to 2 mm.
K.3 COATING TYPES AND TOLERANCES
K.3.1 The types of zinc coating are set out in Table 24.
BS.EN10142: 1991 (reproduced at the foot of this
summary).
K.3.2 Whilst the coating thickness is not subject to
tolerances the substrate and consequently the gauge
thickness does have accepted tolerances and these
including sheet widths/lengths will be found in
BS.EN10143: 1991.
K.4 COATING FINISHES
K.4.1 BS.EN10142: 1991 and A1 1995 includes a
description of the various types of finish available:
K.7 WELDING
K.7.1 Care should be taken to use proper methods and
procedures. The iron-zinc coating is more suitable for
resistance welding than the conventional zinc coating.
92
APPENDIX L -'DESIGN NOTES FOR DUCTWORK'
(CIBSE Technical Memorandum No. 8)
L.1 At the time of publication (1983) this technical
memorandum brought together information on the
design of ductwork systems.
L.2 The contents had been drawn from the relevant
sections of the CIBSE Guide and other recognised
references, and include additional material on good
design practice. The Notes make frequent reference to
DW/142, and an effort was made to ensure
consistency between the two publications. Whilst
DW/142 has now been superceded by DW/144, the
technical memorandum, has not currently been updated but still contains relevant information that may
be of use to a ductwork designer/manufacturer. Whilst
some of the information may now be superceded,
TM8 includes chapters on:
Pressure loss in ducts, including corrections for duct
surface type, air pressure, air density, temperature
and altitude, and loss factors for fittings.
Equivalent diameters of rectangular and flat oval
ducts.
Standard dimensions of circular, rectangular and flat
oval ducts.
Duct sizing methods, including velocity, equalfriction and static regain methods, and pressure loss
calculations, with an example calculation.
Heat loss from and gain to air in the duct; condensation, noise control and fire.
Commissioning and testing.
Overseas work.
Drawing symbols in current use.
L.3 The flow of heavily contaminated air in ducts is
not covered in detail in the Notes; nor are the
constructional aspects of ductwork, which are dealt
with in DW/144.
L.4. The Notes were completed by references, a
bibliography of over thirty titles and appendices
covering properties of air, ductwork support loads,
velocity pressure for air flow and conversion to SI
units.
Technical Memorandum No. 8 was published by the Chartered Institution of Building Services Engineers,
Delta House, 222 Balham High Road, London SW12 9BS (Telephone: 0181 675 5211)
and whilst it is no longer available as a publication, it is still available in photo-copy form.
93
APPENDIX M - GUIDANCE NOTES FOR INSPECTION, SERVICING AND
CLEANING ACCESS OPENINGS
M.1 GENERAL
This appendix highlights, in summary form, the access
consideration that should be made by the designer in
terms of inspection, servicing and cleaning. Having
considered the scope and the design of the ductwork
system relative to the guidelines outlined below the
designer should clearly indicate which levels of access
should be incorporated into the manufacture of a new
ductwork system (See Table 25 and Note 1 below it).
M.2 DESIGN CONSIDERATIONS
M.2.1 Inspection and servicing requirements are set
out in Section 20 of this specification.
M.2.2 Cleaning requirements are set out in the HVCA
publication TR17 "Guide to Good Practice,
Cleanliness of Ventilation Systems" and the guide
states "The precise location, size and type of access
would be dependent on the type of ductwork cleaning,
inspection and testing methods to be adopted."
Care, protection and standards of cleanliness prior to
commissioning are set out in the HVCA publication
DW/TM2 "Guide to Good Practice, Internal
Cleanliness of New Ductwork Installations" and the
guide states "Where specific limits of cleanliness are
required, ductwork shall be cleaned after installation
by a specialist cleaning contractor."
It will be in the interests of the designer, both
financially and practically, to consider employing a
specialist cleaning contractor at the outset of a
contract to internally clean newly installed ductwork
prior to handover. This approach would realise the
following benefits:
i)
The actual number of cleaning access panels
could be determined to suit the method of
cleaning to be adopted (This may be less than the
maximum requirements listed under Level 3 of
Table 25).
ii)
Clear directions could be given to the ductwork
contractor as to the size and location of cleaning
access panels that are required to be fitted during
the manufacturing process.
iii) The specialist cleaning operation prior to
commissioning would enable the cleaning
contractor to verify the practical access
requirements for the future cleaning operations
associated with a regular maintenance programme.
iv) A specialist cleaning operation prior to commissioning would allow the designer to omit from
the specification the DW/TM2 requirements for
factory sealing, protection, wipe downs and
capping-off.
v) Specialist cleaning to the measurable standards
defined in TR17 will allow an objective definition
of cleanliness to be achieved.
Careful consideration must be given by the designer to
the practical problems associated with the manufacture
and fitting of suitably sized access panels on small cross
section ducts and the circular faces of round and flat
oval ducts in particular.
M.2.3 Special consideration must be given by the
designer to the practical problems associated with
gaining personnel access to heavily congested ceiling
areas and multi-layered ductwork systems. This
approach would avoid the possibility of access panels
being incorporated into a ductwork system at the
manufacturing stage that were later found in practice to
be inaccessible for either servicing or cleaning
activities.
M.3 ACCESS TO IN-LINE EQUIPMENT
This appendix only covers access/inspection through the
ductwork body adjacent to an item of in-line equipment
and not openings in the equipment itself.
94
APPENDIX N - BIBLIOGRAPHY
Research Reports
RR01/95: Ventilation system hygiene - A review of published
information on the occurrence and effects of
contamination
RR02/95: Air-to-air heat recovery
RR03/95: Influence of HVAC on smoke detectors
Included in this Bibliography are technical publications which may be
of interest to ductwork designers. fabricators and erectors, and to those
in the heating, ventilating, air conditioning industries generally.
Enquiries should be made of the relevant organisation, at the address
quoted. Since its publication other addresses contained within
DW/144 may have changed, and some publications may have been
superseded.
NATIONAL ENGINEERING SPECIFICATION
LIMITED
Southgate Chambers, 37/39 Southgate Street,
Winchester S023 9EH
(Telephone: 01962 842058; Fax: 01962 868982)
BUILDING SERVICES RESEARCH AND
INFORMATION ASSOCIATION
Old Bracknell Lane West, Bracknell, Berkshire RG12
4AH (Telephone: Bracknell (01344) 426511; Fax:
01344 487575)
HEATING AND VENTILATING
CONTRACTORS’ ASSOCIATION
34 Palace Court, London W2 4JG Telephone: 0171-229
2488; Fax: 0171-727 9268.
Orders to HVCA Publications, Old Mansion House,
Eamont Bridge, Penrith. Cumbria CA10 2BX
(Telephone: 01768 864771 Fax: 01768 867138) Email:
hvcapublications@hvwelfare.co.uk
DW/144
DW/143
DW/151
DW/171
DW/191
DW/TM2
DW/TM3
Specification for sheet metal ductwork (low-, mediumand high-pressure) (1998)
A practical guide to ductwork leakage testing (1983)
Specification for plastics ductwork
Guide to good Practice for kitchen ventilation systems
Guide to good practice glass fibre ductwork DWITM1
Acceptance scheme for new products -Rectangular cross
joint classification
Guide to good practice - Internal cleanliness of new
ductwork installations
Guide to good practice for the design for the Installation
of fire and smoke dampers
Application Guides
AG.1/74
Designing Variable Volume Systems for Room Air
Movement
AG.I/91
Commissioning of VAV Systems in Buildings.
TN.6/94
Fire Dampers
LB.65/94
Ventilation of Kitchens
AG.3/89
The Commissioning of Air Systems in Buildings
AH.2/92
Commissioning of Bems - A Code of Practice
TN.24/71
Fire Dampers in Ventilating Ducts.
HEATING, VENTILATING AND AIR
CONDITIONING MANUFACTURERS
ASSOCIATION (HEVAC)
Sterling House, 6 Furlong Road, Bourne End, Bucks
SL8 5DG (Telephone: 01628 531186 Fax: 01628
810423 Email: info@feta.co.uk)
Other publications
JSI
H&V safety guide 5th edition
JS2
Tool box talks
JS5
Welding Safety booklet
JS19
Safety facts booklet. Fact sheets 1-24 2nd edition
JS21
COSHH manual volume I Advice on compliance with
the regulations
JS 21A
COSHH manual volume 2 Assessment sheets
JS23
Risk management manual
TR/3
Brazing and bronze welding of copper pipework and
sheet(1976)
TR5
Welding of carbon steel pipework (1980)
TR6
Guide to Good Practice for Site Pressure Testing of
Pipework (1980)
TR17
Guide to good practice cleanliness of ventilation
systems.
Publications
Air Diffusion Guide
Guide to Air Handling Unit Leakage Testing
Guide to Good Practice: Air Handling Units
Real Room Acoustic Test Procedure
Specification for the Certification of Air Filters
Method of Test for Water Rejection Performance of Louvres
Subjected to Simulated Rainfall
CHARTERED INSTITUTION OF BUILDING
SERVICES ENGINEERS
Delta House, 222 Balham High Road;
London SW12 9BS (Telephone: 0181-675 5211 Fax:
0181-675 5449)
Test Procedure for Acoustic Louvres
CIBSE Guide
Volume A
Design Data
Volume B
Installation and Equipment Data
Volume C
Reference Data
Fan Application Guide
Commissioning Codes
These Codes cover the preliminary checks, setting to work and
regulation of various categories of plant. The Codes give a guide to
design implications.
Series A
Air Distribution Systems
Series B
Boiler Plant
Series C
Automatic Control Systems
Series R
Refrigerating Systems
Series W
Water Distribution Systems
Specification of Requirements for Natural Smoke and Heat Exhaust
Ventilators
Specification for Floor Grilles - Types, Performance and Method of
Test
Specification for the Determination of the Collection Efficiency of
Sand Trap Louvres
Domestic Mechanical Ventilation Systems with Heat Recovery
Fan and Ductwork Installation Guide
Guide to Fan Noise and Vibration
Specification for Powered Smoke and Heat Exhaust Ventilators
Specification of Requirements for Smoke Curtains
Design Guide of Smoke Ventilation for Single Storey Industrial
Buildings Including those with Mezzanine Floors and High Racked
Storage Warehouses - Issue 3
Guidance for the Design of Smoke Ventilation Systems for Covered
and Underground Car Parks - Issue 1
Application of Smoke Control Equipment and Systems: Guide to
Good Practice - Issue 1
Technical Memoranda
TM 4
Design Notes for the Middle East
TM 8
Design Notes for Ductwork
TM 13
Minimising the Risk of Legionnaires Disease
95
BRITISH STANDARDS INSTITUTION
Sales Department, 101 Pentonville Road, London N1
9ND (Telephone: 0171-837 8801)
BS 381 C: 1996
CP 413: 1973
Colours (of ready-mixed paints) for specific
purposes
Ducts for building services
BS 476:
Fire tests on building materials and structures
Part 4: 1984
Non-combustibility test for materials
Part 6: 1989
Fire propagation test for materials
Part 7: 1993
Surface spread of flame tests for materials
SHEET METAL AND AIR CONDITIONING
CONTRACTORS' NATIONAL
ASSOCIATION INC. (SMACNA)
Headquarters:
4201 Lafayette Center Drive
Chantilly
Virginia 20151-1209
Mailing Address
P.O. Box 221230
Chantilly
Virginia 20153-1230
Telephone (703) 803-2980
Fax
(703) 803-3732
Part 20:1987
Fire resistance of elements of construction
Part 21:1987
Fire resistance of loadbearing elements of
construction
Part 22:1987
Fire resistance of non-loadbearing elements of
construction
Accepted Industry Practice for Industrial Duct Construction (1975)
Architectural Sheet Metal Manual (1993)
Part 23:1987
Contribution of components
resistance of a structure
Contractors Guide for Modification to Construction Contracts
(1993)
Part 24:1987
Fire resistance of ventilation ducts
to
the
fire
BS 5588
Energy Conservation Guidelines (1984)
Part 9:1980
Fire Precautions in the design and construction
of buildings
BS 729: 1971
Hot dip galvanized coatings for iron and steel
articles
BS 1449:
Steel plate, sheet and strip
Part 1:1991
Carbon steel plate, sheet and strip
Part 2:1983
Stainless steel plate, sheet and strip.
and
BS.EN 10149-2: 1996
BS.EN 10149-3: 1996
BS.EN 10131: 1992
BS.EN485
Wrought aluminium and aluminium alloys
Parts 1-4
for general engineering purposes - plate,
BS.EN515 1993
sheet and strip.
BS.EN573
BS 1474:1972
Wrought aluminium and aluminium alloys
and
bars, tubes and sections
BS.EN755
BS.EN10142:
1991
BS.EN10143:
1991
Sprayed metal coatings
Protection of iron and steel by aluminium and
zinc against atmospheric corrosion Protection of
iron and steel against corrosion and oxidation at
elevated temperatures
Continuously hot-dip zinc coated mild
steel strip and sheet for cold forming - technical
delivery conditions
Continuously hot-dip zinc coated and
ironzinc alloy coated steel sheet and strip tolerances on dimensions and shade
BS 3533: 1981
Glossary of terms relating to thermal insulation
BS.EN.ISO: 1479
Self-tapping screws and metallic drive screws
BS.EN.ISO: 7049:
1994
BS 4800:1989
Paint colours for building purposes
BS 4848:
Hot rolled structural steel sections
Part4: 1972
Fibrous Glass Duct Construction Standards (1992)
Fire, Smoke & Radiation Damper Install. Guide for HVAC Systems
(1992)
Guide to Steel Stack (1995)
HVAC Air Duct Leakage Test Manual (1985)
HVAC Commissioning Manual (1994)
HVAC Duct Construction Standards-Metal & Flexible (1995)
Addendum No. I (Nov 1997)
HVAC Duct Systems Inspection Guide (1989)
HVAC Systems-Application (1986)
HVAC Systems-Duct Design (1990)
HVAC Systems-Testing, Adjusting & Balancing (1993)
Indoor Air Quality Manual (1993)
Managers' Guide for Welding (1993)
Rectangular Industrial Duct Construction Standards (1980)
Round Industrial Duct Construction Standards (1977)
Seismic Restraint Manual (1991) (w/ Appendix E, 1993)
Parts 3-6
1994
Energy Recovery Equipment & Systems (1991)
Kitchen Equipment Fabrication Guidelines (1990)
Parts 1-4
BS.EN22063:
Ducted Electric Heat Guide for Air Handling Systems (1994)
Equal and unequal angles
BS 5422:1990
Specification for the use of thermal insulating
materials
BS 5720: 1979
Code of practice for mechanical ventilating and
air conditioning in buildings
BS 5970: 1992
Code of practice for thermal insulation of
pipework
SMACNA Master Index of Technical Publications (1995)
Thermoplastic Duct (PVC) Construction Manual (1994)
DEPARTMENT OF THE ENVIRONMENT
(Publications Centre)
H.M. Stationery Office, 51 Nine Elms Lane, London
SW8 5DR
M & E No.1 1972
Electrical installations in buildings (New
Edition)
M & E No.3 1988
Heating, hot and cold water, steam and gas
installations for buildings
M & E No.4 1970
Central heating and hot and cold water
installations for dwellings
M & E No.1001971 Mechanical ventilation for buildings
BRITISH STEEL PLC
Market Communications Dept
British Steel PLC
Strip Products
P.O. Box 10 Newport
South Wales NP9 OXN
(Telephone 01633 290022)
(Fax
01633 464087)
Publication: Edge protection by zinc
96
ASSOCIATION FOR SPECIALIST FIRE
PROTECTION
Association House
235 Ash Road
Aldershot
Hampshire GU 12 4DD
Telephone 01252 21322
Fax
01252 333901
Publications.
Fire Rated and Smoke Outlet Ductwork: An
Industry Guide to Design and Installation.
HEALTH AND SAFETY EXECUTIVE
Rose Court
2 Southwark Bridge
London SE1 9HS
Telephone 0171-717 6000
APPENDIX P - CONVERSION TABLES
Sheet thicknesses
97
98
NOTES
99
NOTES
100
101