The present invention relates to a blank for manufactur¬ ing an air duct of rectangular cross section, and particu¬ larly to a blank by means of which an air duct can be con¬ structed in a simple manner on the working site, without the use of special tools herefore.

In the majority of buildings, and particularly work¬ shops, a large number of air ducts and ventilation ducts are installed for different purposes,' said ducts being most often placed in the ceilings of said buildings. Previously, relatively cheap, pre-fabricated ducts of asbestos cement have been used to a great extent. The use of this material for the manufacture of air ducts and ventilation ducts , how¬ ever, was forbidden some years ago. In consequence hereof, present day ducts mainly comprise sheet metal. For, among other things, aesthetic reasons and/or space reasons the ducts are often given a rectangular cross-sectional configu¬ ration, since, inter alia, such ducts can be given a form in which the sealing height is decreased by the least possible extent. One disadvantage with sheet-metal ducts is that speci¬ ally trained workmen are required for bending and assembl¬ ing the ducts on the working site. It has not been consid¬ ered worthwhile to pre-fabricate completed sheet-metal ducts, since they are excessively bulky and costly to store and/or transport.

A main object of the present invention is to provide a blank for the manufacture of sheet-metal ducts , which can be readily pre-fabricated; which permits storage and trans¬ port space to be utilized to the maximum; and which can be readily assembled on the working site without employing spe¬ cialized personnel and/or using special tools.

This object is achieved in accordance ^' with the inven¬ tion by means of a blank having the form of substantially planar plates which, for storage and transport purposes, can be stacked one upon the other and which can be readily bent

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or folded manually on the working site into complete ducts, without the use of tools.

To this end, a blank according to the invention com¬ prises a substantially rectangular plate having a width which substantially coincides with the circumference of the duct desired and a length which corresponds to the length of said duct. The blank is characterized in that said plate is provided with three spaced apart and paral¬ lel rows of holes which form weakened sections extending in the longitudinal direction of the plate and serving as bending or folding lines; and in that the longitudinally extending ^* edge parts of the plate are provided with lock¬ ing means which co-act when folding or bending the plate into a duct. Preferably, the blank is provided with a plurality of substantially parallel, transverse, groove-like emboss¬ ments which serve to facilitate bending of the blank and permit the defining surfaces thereof to be planar. In this respect, a hole should be punched at the intersection poin between each groove-like embossment and each row of holes, to facilitate bending of the blank.

In a majority of cases the duct should be insulated, to which end one surface of the plate is covered with in¬ sulating plates which are chamfered down to the rows of weakened sections and which, when the blank is folded to form a duct, automatically form a complete, internal in¬ sulation.

The invention will now be described in more detail with reference to the accompanying drawings . Figures 1 and 2 illustrate a blank according to the in vention in plan view and side view respectively.

Figure 3 illustrates a duct manufactured from a blank according to Figures 1 and 2.

Figures 4 and 5 are side views of parts of two blanks according to the invention shown in a larger scale than

Figure 2.

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Figure 6 is a side view of a joint between two ducts according to the invention.

Figure 7 is a part sectional view of a joint according to Figure 6. The blank shown in Figures 1 and 2 have the form of a substantially rectangular sheet-metal plate 1 with a width which coincides substantially with the circumference of the desired duct section and a length corresponding to the length of said section. Punched in the plate 1 are three * rows of holes 2 which are parallel with each other and with the longitudinally extending edges of the plate, and which weaken the plate to form ^' effective bending lines. The dis¬ tance between the rows of holes and. between the outermost rows and the longitudinally extending edges of the plate determine the size of the sides .of the duct section formed from the blank. The thickness of the plate is normally less than 1 mm and in one preferred embodiment is 0,3 mm. The diameter of the holes are suitably in the order of 5-10 times the thickness of the plate; in one .embodiment the diameter is about 8 times said plate thickness, whereat the spacing between respective holes is suitable of the order of 10-20 times the plate thickness and in the preferred embodiment about 15 ^' times said plate thickness.

The plate is also provided with a plurality of trans- verse, substantially parallel grooved beads or embossments 3 , which facilitate the manufacture of a duct section hav¬ ing planar defining surfaces while, at the same time, rigi- difying the duct section as a whole. It is important that these embossments are broken in line with the rows of holes by means of a hole 4 having a diameter or an axial length which is at least as wide as the width of the embossment 3. As will best be seen from Figure 2, the plate 1 is pro¬ vided along one longitudinally extending side with a part 5 which is bent or folded through an angle of about 90°. This facilitates construction of a duct section and is utilized to provide a simple locking of the blank folded

to a rectangular duct-section form. To this end, the blan is provided along the opposite longitudinally extending edge with outwardly projecting tongues 6 which, subsequen to folding the blank to form a duct section of rectangula cross section, can be inserted in slots 7 which correspon to the tongues 6 and which are arranged in the transition region to the folded part 5.

In Figure 1, the blank is illustrated from that side which will later form the outside of the duct section, while the other side of the blank' is provided with insula ing plates as shown in Figure 2. These insulating plates are, by way of example, glued to the blank during its man facture. This enables the insulating plates to be accurat ely attached to the blank, therewith guaranteing an almos perfect positioning of the insulation in the finished duc section. Such precise insulation is extremely difficult t achieve- when applying present day techniques , in which th duct is manufactured on the building site. As will be see from Figure 2 the insulation comprises longitudinally ex- tending insulating sheets 8 which are chamfered at an ang of 45 towards each row of holes, which will form the cor ners of the finished duct section. The insulating layers may, for example, comprise conventional mineral wool or synthetic insulating material. When producing an air-duct section on the building site, work is commenced from a blank such as that illustr ted in Figures 1 and 2 and the blank is folded along the rows of holes 2, which can be effected without the use of special tools. In a case of thicker plates, the blank can be folded with the aid of a straight edge. By adapting th diameter of the holes and the spacing therebetween with regard to the thickness of the plate, it is always possib le, however, to manufacture the blank in a manner which makes it easy to fold the blank manually. Subsequent to folding the blank along the three aforesaid rows of holes the tongues 6 can be inserted into the slots 7, as illu¬ strated in Figure 3 , and bent through an angle of about

90° to lock the resultant sides of the duct section in the resultant rectangular cross-sectional shape. In the duct section illustrated in Figure 3 all the sides of the blank are equal, thereby to provide a duct section of square cross-section.

As will be seen from Figure 3, the duct section is given extremely well defined corners , 'which among other things causes the insulating layers 8 to lie in close abut ^¬ ment with each other at said corners. If so desired an ad- hesive can be applied to the chamfered surfaces of the in¬ sulating layers 8, thereby to obtain a practically comple¬ tely sealed duct section. The resultant duct section is extremely rigid, mostly as the result of the groove-shaped embossments 3. These embossments also cause the side s r- faces of the duct section to be, in the main, planar.

Locking of the various surfaces of the duct section by in¬ serting the tongues 6 through the slots 7 when bending the blank also contribute positively to rigidifying the duct section throughout. In the embodiment illustrated in Figure 4, complete sealing of the duct section is ensured by applying a seal¬ ing strip 9 over each row of holes before the insulating layers or sheets 8 are glued to the plate 1. In the Figure 5 embodiment, an aluminium foil 10 is glued to the insula- ting layers 8, said aluminium foil further stiffening the construction and contribution to complete sealing of the drum section.

A blank such as that illustrated in Figures 1 and 2 can be manufactured very simply, and since the finished blanks can be stacked flat one upon the other said blanks can be stored .and transported economically. As will also be understood from the aforegoing, the blanks can be bent, folded and locked to form duct sections of rectangular cross section on the building site in a ready and easy ^* manner without requiring particular expertese or the use of special tools.

Two duct sections constructed in accordance with the invention can suitably be joined together by means of a device illustrated in Figures 6 and 7. This device com¬ prises a rectangular U-shaped frame structure 11 in the ^* form of a sleeve arranged to embrace two mutually opposin end portions of the duct sections to be joined together. The frame structure is provided with hook means 12 arrang ed to co-act with holes arranged in the corners of the sa drum sections and provided with screw-threaded sections f receiving respective nuts 13, by which the sections can b drawn together. The frame structures may also be provided with or connected to mounting means, such as the angle- iron 14 illustrated in Figure 6. A rubber seal 15 or like element is suitably mounted on the inner surface of the frame section, to seal the joint between the respective duct sections, as illustrated in Figure 7.

The invention has been described in the aforegoing wi reference to a preferred embodiment. The blank, however, may be modified in several respects within the scope of t claims, said blank, for example, being either insulated o not insulated. Further, the means for locking the duct sections of rectangular cross-section together may vary. The jointing means may also be given another form. The si of the holes and the form which said holes take may also vary in dependence upon the thickness of the material use and its nature. For example, instead of circular holes th holes may be of a narrow and elongate configuration.