DW144 specification for sheet metal ductwork1

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 a

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

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 nationaland international recognition as the industry standard against which the quality ofductwork manufacture and installation can be judged

In recent years, however, it has become increasingly evident to the members of theHVCA Ductwork Group that the developments in technology and working practiceswhich have taken place since the drafting of DW/142 have rendered obsolete significantparts of the document

It was an acknowledgement of this state of affairs which led the Technical Committee of the Ductwork Group, ably chaired by Edgar Poppleton, to undertake thetask of producing a radically revised specification which would promote best practiceand quality standards well into the next Millennium

Sub-This new publication - designated DW/144 - represents the direct result of thatinitiative

The new specification recognises the computer age - with special reference toCAD/CAM procedures and techniques - and the international performance standardsestablished by the Committee for European Normalisation (CEN), as well as the need toupdate 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 withindividuals and organisations throughout the building services and construction sectors

in order to ensure that the new specification fully reflected the current the art" in terms both of technical expertise and industry best practice

"state-of-the-I firmly believe that this process has resulted in a publication which clearlydemonstrates the high level of professionalism which exists within the ductworkcommunity - and I take this opportunity of thanking all those who have contributed toits 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

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

duct-work 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 aftermanufacture, 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 andcleaning 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 toreduce ambiguity and promote commonunderstanding;

• a rewritten description of all forms of dampers,for which I am indebted to Bill Clark and JohnMawdsley of the HEVAC Association

I take this opportunity to acknowledge the mission granted by the Sheet Metal and AirConditioning Contractors' National Association(SMACNA) of the USA for the use of its tie rodspecification (designer approval required)

per-And I also include a plea on behalf of ductworkconstructors to be allowed to make the final choice

of components and techniques within the ters set by the designer, and allowed within thisspecification to satisfy performance characteristics

parame-It will, of course, be clear to anyone who hasever taken on such a task that the production ofthis specification has involved a colossal input interms of industry consultation and from a widevariety of individuals, a number of whom I shouldlike to identify for special mention

They are: former Technical Sub-Committeemembers Keith Waldron and the late KeithAngood; current members Chris Collins, StuartHoward, Brian James and - last but by no meansleast - Jim Murray; technical consultant KeithElphick; and Ductwork Group secretary GarethKeller

Finally, may I remind readers of the crucialimportance of ensuring that all ductwork is manu-factured and installed in a manner which is safe,efficient, effective and free of risk

The publication of DW/144 is intended to assistsignificantly in the achievement of this objective

Technical Consultant:

Keith Elphick

Ductwork Group Secretary:

Gareth Keller

8 Ductwork Construction and Joint Sealing 14

Part Three - Rectangular Ducts

9 Rectangular Duct Sizes 15

Part Five - Flat Oval Ducts

15 Standard Sizes and Sheet Thicknesses 35

16 Construction (Spirally wound) 35

28 Connections to Building Openings 53

33 Standard Component Drawings

Part Eight - Appendices

Appendix A Air Leakage from

Appendix B Identification of

Appendix C Guidance Notes for the

Transport, Handling and

Appendix K Summary of BS.EN10142:

1991 Continuously Hot-DipZinc Coated Mild Steel Stripand Sheet for Cold Forming 92Appendix L `Design Notes for Ductwork'

(CIBSE Technical

Appendix M Guidance Notes For Inspection,

Servicing and Cleaning AccessOpenings 94

List of Tables

Part Two - Standards

1 Ductwork Classification and Air

Part Five - Flat Oval Ducts

11 Standard sizes and sheet thicknesses 36

Part Six - Hangers and Supports

15 Supports for horizontal ducts - rectangular,

Part Seven - General

Part Eight - Appendices

18 Recommended duct identification colours 80

19 Examples of further identification symbols 81

20 Ductwork galvanized after manufacture

21 Compositions of the commonly used

22 Rectangular aluminium ducts

-low pressure constructional requirements 90

23 Circular aluminium ducts

-low pressure constructional requirements 91

25 Access requirements for inspection,

List of Illustrations

Part Three - Rectangular Ducts

9 Illustrations of panel stiffening 20

13-17 Socket and spigot cross joints 22

Part Four - Circular Ducts

32-38 Cross joints spirally wound ducts 30-3139-45 Cross joints straight seamed ducts 32-33

Part Five - Flat Oval Ducts

53-58 Cross joints spirally wound ducts 39-4059-63 Cross joints straight seamed ducts 41-42

Part Six - Hangers and Supports

64-75 Horizontal ducts

Part Seven - General

78-79 Fire barrier/Fire damper expansion 50

81-124 Standard component drawings

Part Eight - Appendices

178 Permitted leakage at various

179 Example of duct identification symbol 81

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, par- ticular 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.

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)

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

con-nections 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

4.2 Protective finishes ( Section 27 )

Details and specification of any protective finishes

4.3 Fire rated and smoke extract 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 supportrequirements not covered by Section 19

4.7 Attachment to building structure ( Section 28 )

Specific requirements for the junction of ductworkand associated components to openings should bedetailed and specified and the limits of responsibilitydefined

The provision of penetrations and associatedframings 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 theirPlenums (See Figures 120 to 124) will be supported

by the Ceiling Grids unless the designer indicates anindependent method of support

4.9 Ductwork layout drawings

Details of any special requirements relating to CAD,scales, etc It is common practice and cost effectivefor ductwork manufacturers to utilise their approvedductwork layout drawings as a basis of theirmanufacturing/installation information by adding thenecessary details to the same drawing Scales of 1:50

or smaller may preclude this practice, therefore,larger scales might be more appropriate The finalchoice of manufacturing/installation scales shall beleft to the ductwork contractor

4.10 Other requirements

Details of any requirements for the ductwork not inaccordance with the provisions of this specification,including any modified construction required toconform with any requirements concerning externalductwork (See 5.3) or to meet the regulations of alocal authority or other controlling body

4.11 Reference to the designer

In consideration of the foregoing, reference is alsomade to the designer in the following clauses:

5.3 13

10.5.2 1611.1 1614.1 2916.3.1 35

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

other-wise 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 aseries 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 LeakageTesting, also gives details of the basis for the leakagelimits specified in Table 1

6.4 Air leakage testing

Air leakage testing of low and medium pressureductwork is not mandatory under this specification.Air leakage testing of high pressure ductwork ismandatory under the specification and for details oftesting procedure refer to DW/143 A practical guide

to Ductwork Leakage Testing

7 MATERIALS 7.1 Application

This specification applies to ductwork constructedfrom materials as defined below, or equal Minimumsteel thickness is to be taken as a nominal thicknesswithin the tolerances to BS.EN10143:1993 (See

Appendix K)

7.2 Zinc-coated steel

Ductwork will normally be constructed from dip galvanized steel to BS.EN10142:1991, GradeDX51 D+Z, coating type Z275

hot-7.3 Mild steel

Where mild steel is specified, it shall be cold-reduced

steel to BS.EN10130:1991, Grade FEP 01A

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.

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

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 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

MANU-FACTURER'S INSTRUCTIONS AND COSHH

ASSESSMENT

8.2.2 Liquid and mastic sealants

These are typically applied to a longitudinal seamformed between two sheets of metal, a socket andspigot, cleated or flanged cross joints Particularcare is needed when sealing of "corner pieces" on

the proprietary 'slide-on' type flange and

reference should be made to the manufacturer'sassembly and sealing instructions

8.2.3 Gaskets

These can be of various materials in the form of apreformed roll, sheet or strip, applied betweenopposing faces of flanged cross joints In the case

of proprietary 'slide-on' type flanges, it is

advisable to use the gasket strip recommended bythe manufacturer

Factory-fitted proprietary synthetic rubber '0'ringtype gaskets are also acceptable for socket andspigot joints on circular duct systems

8.2.4 Tapes 8.2.4.1 The application of tapes - Best suited,

but not limited, to cross joints on circular or flatoval ductwork Where chemical reaction tape,heat shrinkable tape or other approved material

is used on flat oval ductwork care should betaken to maintain close contact between thematerial and the flat sides of the duct until thejoint is completed

8.2.4.2 Chemical reaction tape - An

impregnated woven fibre tape and a resin typeactivator/adhesive On application of theactivator/adhesive the tape becomes pliable andcan then be applied to any surface shape Theliquid reacts with the tape, causing the 2partsystem to `set'

8.2.4.3 Heat shrinkable band/tape - A

ther-moplastic material, coated on the inside withhot metal adhesive The band (or an appropriatelength of tape) is cut from the roll and wrappedaround the joint When heated the tape shrinkstightly 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 appliedexternally to socket and spigot cross joints.However, it is difficult to provide the dry, dustand grease free surface that is required for asuccessful application and this method istherefore not recommended as a primary source

of sealant

NB! Risk of explosions

Where ductwork is blanked off prior to leakagetesting or to prevent the ingress of contamination,care should be taken to ensure that all joint sealingsolvent vapours are dispersed from the ductworksystems

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, aregiven in Tables 2 to 4 Other limits on use aregiven with the individual drawings

Note: Proprietary products used in the tion of cross joints should be approved by anindependent test house following tests defined inDW/TM1 "Acceptance scheme for new products

construc Rectangular cross joint classification." FiguresNos 10 and 13 to 17 illustrate non proprietaryjoints 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 reaction tape or heat-shrink strip as alternativemethods of sealing, provided that close contact ismaintained over the whole perimeter of the jointuntil the joint is completed

chemical-With all flanged joints, the sealant between

sheet and section should preferably be rated during construction at works, but siteapplied sealant is acceptable The joint betweensections of ductwork is then made, usingapproved type of sealant or gasket Withproprietary flanging systems particular attentionshould be paid to the sealing of corner pieces andflanges, reference should be made to themanufacturer's assembly and sealing instructions

incorpo-10.4.3 Adjustabletslip joints

In order to accommodate manufacturing/buildingtolerances, site modifications etc, it is acceptedpractice 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 sizelonger side and maximum spacing The ratingsare specified with the illustrations of thestiffeners, Figs 18 to 23, and the limits of use aregiven in Tables 2 to 4 The stiffeners for socketand spigot joints covered in Figs 15, 16 and 17

are also applicable to stiffeners in general

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

Alternative methods for the attachment of tie bars

are shown in Figs 25 to 28

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)

10.6 Ductwork galvanized after manufacture

Appendix E sets out the recommended sheet thicknesses

and stiffening for ductwork galvanized after

manufacture

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

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

10.7.3 Set screws, nuts and lock bolts Materials

shall be of mild steel, protected by

electro-galvanizing, sherardizing, zinc-plating, or other

equal and approved corrosion resistant finish

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

a minimum

Areas where the galvanizing has been damaged ordestroyed by welding or brazing shall be suitablyprepared and painted internally and externally withzinc-rich or aluminium paint as defined in Section27.3.2

11 FITTINGS 11.1 Standardisation of fittings

The terminology and descriptions of rectangular ductfittings as set out in Section 33 are recommended foradoption as standard practice to provide commonterms of reference for designers, quantity surveyorsand ductwork contractors, and of those usingcomputers in ductwork design and fabrication

Bends are designated as `hard' or `easy', and theseterms 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 dance with the construction Tables 2 to 4 On the flatsides of bends, stiffeners shall be arranged in a radialpattern, with the spacing measured along the centre ofthe bend

accor-11.3 Splitters

If the leading edge of the splitters exceeds 1250 mmfit central tie bars at both ends to support the splitters.Leading and trailing edges of splitters must be edgefolded and flattened and be parallel to the duct axis.Splitters shall be attached to the duct by bolts ormechanically-closed rivets at 100 mm maximumspacing (or by such other fixing as can be shown to beequally satisfactory e.g proprietary sealed splitterpins)

11.4 Turning vanes

Where specified, or shown on drawings, square throatbends with either duct dimension greater than 200 mmshall be fitted with turning vanes which are illustrated

in Figures 30a and 30b

Turning vanes at 60 mm maximum centres shall befixed at both ends either to the duct or compatiblemounting tracks in accordance with manufacturer'sinstructions, the whole bank being fixed inside theduct with bolts or mechanically closed rivets at 150

mm maximum spacing

The maximum length of turning vane between ductwalls or intermediate support shall be 615 mm forsingle skin vanes and 1250 mm for double skin vanes.Typical examples of fitting turning vanes when themaximum permitted vane lengths are exceeded areshown in Fig 30c

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

11.6 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

11.7 Expansions and contractions

Where these are required, an expansion shall be madeupstream of a branch connection and a contractiondownstream 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, theslope may be increased, providing that splitters arepositioned to bisect the angle between any side and thecentre line of the duct (See Figs 99 to 101)

11.8 Sealant

Sealant shall be used in all longitudinal seams and crossjoints of fittings Sealant shall be to the options listed in

Section 8

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 forjoints 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, seeAppendix 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 anaccepted constructional practice and the HVCA Ductwork Group should be contacted if details of theirratings and limitations are required

(8) Intermediate stiffeners using rolled sheet angle profiles, illustrated in Figs 19 to 23 of the appropriaterating may also be utilised ensuring that rigid corners are achieved

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.

(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 inaddition 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 inthe Table above

(5) Mechanically closed rivets are not recommended for fixing external stiffeners to ductwork exceeding500pa negative

Part Four - Circular Ducts

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

stan-dards 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

and contractors in the meantime are invited to evaluatethem 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 thepressure classifications covered in this specification,without sealant in the seam

13.1.2 Straight-seamed ducts

The longitudinal seam for straight-seamed circularducts shall be either the grooved seam continued tothe extreme end of the duct and sealed, or acontinuous butt lap weld or spot/stitch weld andsealed lap joint (at 30 mm centres) provided thisgives a smooth internal finish (see Fig 31)

13.2 Cross joints 13.2.1 General

Cross joints for circular ducts, both spirallywoundand 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 arenoted against each

13.2.2 Sealant

All circular cross joints shall be sealed (see section

8)The use of chemical-reaction tape or heatshrinkableband shall be regarded as an effective sealant inrespect of the socket and spigot joints illustrated

13.2.3 Welded joints

The limitations for welded joints are given in 13.3.5

13.3 Fastenings 13.3.1 Permitted types and maximum centres

Table 9 sets out the permitted fastenings andmaximum spacings for low-, medium- and high-pressure ducts All duct penetrations shall be sealed

13.3.2 Rivets

Manufacturers' recommendations as to use, size anddrill size are to be followed Rivets resulting in anunsealed aperature shall not be used

13.3.3 Set screws, nuts and lock bolts

Materials shall be of mild steel, protected by galvanizing, sherardizing, zinc plating or other equaland approved finish

electro-13.3.4 Self tapping and piercing screws

Providing an adequate seal can be achieved, and theprotrusions into the ductwork are unlikely to causeinjury, then self-tapping or piercing screws may beused

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

14 FITTINGS

14.1 Standardisation of fittings

The terminology and descriptions of circular ductfittings as set out in Section 33 are recommended foradoption as standard practice, to provide commonterms of reference for designers, quantity surveyorsand ductwork contractors, and those usingcomputers in ductwork design and fabrication

The requirements for circular duct fittings applythroughout the pressure ranges covered in thisspecification

14.2 Nominal diameters

The nominal diameter (see Table 6) is the size usedfor design and ordering With socket and spigotjoints, care should be taken to ensure that thedimensions of the ducts and fittings are correctlyrelated, 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 thosequoted 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 therequirements of Section 8

Part Five - Flat Oval Ducts

15 STANDARD SIZES AND SHEET

THICKNESSES

15.1 Table 11 sets out the standard sizes of

spirally-wound 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

cross-section 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

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

16.4.5 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 ofthe duct or fitting and the need to ensure air-tightness.Welded joints shall provide a smooth internal surfaceand shall be free from porosity Distortion shall bekept to a minimum

Areas where the galvanizing has been damaged ordestroyed by welding or brazing shall be suitablyprepared and painted internally and externally withzinc-rich or aluminium paint

16.5 Stiffening

The larger sizes of flat oval duct are stiffened byswages, 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 tomeet the requirements of the corresponding rectangularduct sizes

17 CONSTRUCTION (STRAIGHT-SEAMED)

Flat oval ducts with opposed sides and semi-circularends may also be formed using plain sheet and straightseams Ducts so formed should follow the metalthicknesses and stiffening requirements specified for thecorresponding sizes of rectangular ducts, except thatstiffening is necessary on the flat sides only

Seams and cross joints (see Figs 59 to 63) shall besealed to ensure the necessary degree of airtight-nessthroughout the pressure ranges covered in thisspecification

18 FITTINGS 18.1 General constructional requirements Sheet

thicknesses for flat oval fittings (determined by theperiphery of the larger end) shall be not less than thosegiven in Table 11 for the ducts themselves

With socket and spigot joints, care should be taken toensure that the dimensions of ducts and fittings arecorrectly related

All the seams and joints integral to a fitting shall besealed to the same standard as the duct (See Section 8)

18.2 Standardisation of fittings

The terminology and descriptions of flat oval ductfittings as set out in Section 33 are recommended foradoption as standard practice, to provide common terms

of reference for designers, quantity surveyors andductwork contractors, and those using computers inductwork design and fabrication

The requirements for flat oval duct fittings applythroughout the pressure ranges covered in this