Chapter 2

2.2.1 Rectangular aluminium ducts Sheet quality

In general, aluminium rectangular ducts are made of sheet material in the quality Al 99.5 / EN AW 1050A. For specific applications, seawater-resistant aluminium of grade AlMg 3 / ENAW 5754 can be used. Sheet thickness

Aluminium air ducts are manufactured with a sheet thickness that depends on the largest duct side. For these thicknesses the ducts are manufactured in such a way that sufficient stiffness against deformations and troublesome vibrations is present. Starting from the largest duct side the minimum sheet thickness applies.

For technical reasons, thicknesses greater than 1.50 mm are avoided. Cross-connections

Different types of cross-connections can be applied to rectangular air ducts. These are company specific, whereby the quality of the sheet, from which the connection profiles are formed, at least complies with that of the material from which the duct is manufactured. These cross-connections
may (depending on the company) be rolled to the duct or fixed to the duct by means of push-throughs, parkers or pop rivets. The cross-connections are connected with clips, sliding strips or clamps with a maximum centre-to-centre distance of 500 mm (see illustration). These fasteners are available in an ALU/RVS version.

A closed cell sealing tape with minimum dimensions B x H = 18 x 4 mm and stainless steel bolts and nuts, at least M 6 x 20, must be placed between the cross-connections to ensure airtightness. If sliding strips are used for the full circumference of the duct, the bolts and nuts at the corners can be omitted. Where necessary, an internal or external plastic sealant will be applied to ensure airtightness. Longitudinal connections

Longitudinal connections between duct sections are generally made using a flange connection. Where necessary, plastic sealant is applied internally or externally to ensure airtightness. Stiffening

Air ducts shall be of such a stiffness that troublesome vibrations and deformations do not occur. Assuming application of the recommended minimum sheet thicknesses according to this applies to rectangular aluminium ducts, provided the largest side of the cross section is ≤ 400 mm. If this dimension is exceeded, additional provisions are required. The extent of the exceedance determines the type of execution of the provisions.

For ducts with a side section > 400 and ≤ 800 mm, the type of execution is determined for the respective duct wall surfaces:

  • Cross-breakings: normal outward facing cross-breakings;
  • rills or offsettings: generally arranged transversely to the longitudinalaxis of the duct, at intervals not exceeding 500 mm.

For ducts with a side > 800 mm, the previously mentioned types of implementation apply for the duct wall surfaces concerned, whereby surfaces with a surface area greater than 1.5 m² are additionally stiffened by subdivision into subdivisions of no more than 1 m². These additional stiffeners in the form of strips, sections, pipes or sheets shall be attached internally or externally. Dimensions

The nominal sizes of the air ducts are given in mm and refer to the internal dimensions with a tolerance of + 0 to 4 mm. The dimensions are standardised according to NEN-EN 1505 and can be selected as indicated in the table for standard dimensions. Visible work

If in an air technical installation a part of the air duct system has to be executed as "visible work", this will be executed as the other duct work, unless this has been determined otherwise in the specifications. In the case of ductwork treated as visible work, externally applied stickers and markings will be removed, while the required airtightness will be achieved by internal sealing. Additional measures within the framework of visible work do not normally form part of the standard design. Bends

Symmetrical bends
As regards form, symmetrical bends are in principle made in round form, i.e. with an internal and an external radius; the internal radius is 100 mm or greater (in the case of floor or wall recesses and places where there is no room for an internal radius, an internal bend at right angles may be used). For technical manufacturing reasons, inside corners may also be used. In order to limit the resistance in a bend, bends are fitted with blades. No blades are used for:

  • bends of 45° or less;
  • ducts with a width of 400 mm or less.

The position of the blades shall be determined according to the table below.

Elongated bends
In the case of elongated bends, the smallest duct width determines the number of blades, in accordance with the table above. The ratio for the position of the blades of the largest duct width is then equal to the ratio for the blades of the smallest duct width.

Blade design
The blades are made of single sheet metal. The sheet material is the same as the material from which the duct is made. The execution and
fixing are of sufficient strength, while the ends of the blades are stiffened. Adapters

The adapters are made in such a way that the top angle α of the adapters may not exceed 60°. Branches

A branch (split off from a continuous main duct) can be created by means of an fitting in rectangular or round design. Aerodynamic aspects also determine the types, as shown in the illustrations below. Splits

A split is a division of a main duct into two ongoing ducts.
Some examples of splits are: Adjustment valves

Adjustment valves are manually adjustable and serve to regulate an installation. They are provided with an appropriate locking device which also indicates the valve position. The damper blade, of the same material as the air duct, is executed in a single sheet with a thickness of at least 0.8 mm (executed according to the drawing below) up to a maximum blade width (B) of 300 mm and up to a maximum surface of 0.09 m². The edges of the damper blades are rounded and stiffened parallel to the axial direction. Tolerances

The maximum tolerance for the length of a straight duct is ± 0.005 x L.
The tolerance for rectangular dimensions is + 0 to - 4 mm.
The maximum tolerance for angles is ± 2°.

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