According to the manufacturing method, the air ducts can be folded or welded. (Fig. 16.).

The first ones are connected with seam seams. The thickness of the metal for such ducts should not exceed 2.0 mm (if aluminum is used) and 1.0 mm (if corrosion-resistant steel is used).

The welded air ducts are connected by an overlap weld. The thickness of the metal with this type of connection is allowed in the range of 1.2 -3.0 mm.

Fig.16. Types of seam and welded joints of metal ducts:

1 – on a simple recumbent seam; 2 – on a double–cut seam; 3 – on an angular seam; 4 – on a transverse seam; 5 – on a seam with a latch; 6 – with a connecting bar; 7 – on zigs; 8 – butt; 9 – butt–to–butt with flanging; 10 – lap; 11 – corner

This type of duct is classified as dense.

All the various ventilation systems are assembled from only four standard parts.

The first is pipes with a length of 2000 mm or 2500 mm, depending on the metal used.

The second is from water, usually consisting of two or three segments and two glasses with an average radius of curvature equal to the diameter (Fig.17). They are used for general exchange ventilation systems.

Fig. 17. Outlet for air ducts

The third is rectangular bends, which are characterized by only two neck radii: 150 mm (for bends with a side less than 1000 mm) and 300 mm (with a larger size).

The fourth is the tees (branch nodes) of circular cross section. They exist in 3 versions:

– normalized, they are characterized by low coefficients of local resistance, but are laborious to manufacture;

– direct embedding;

– rectangular tees, they are usually equipped with one-sided unified transitions, which allows you to establish a constant ratio of the duct route from the wall along which it is laid.

According to the material used for the manufacture of air ducts, they are divided into several groups:

1. Folded air ducts made of thin-sheet galvanized steel, up to 1 mm thick (without painting).

2. Folding air ducts made of thin-sheet black steel, up to 1 mm thick (with subsequent painting inside and outside).

3. Welded air ducts made of sheet steel, 1.2- 3.0 mm thick (with subsequent painting with primer).

4. Seam and welded air ducts made of corrosion–resistant steel, thickness from 0.5 mm to 3 mm (usually grades X18N9T) – without painting.

5. Folding air ducts made of titanium (? = 4,500 kg/m3), which have the highest corrosion resistance when moving an aggressive medium.

6. Folding ducts made of metal plastic, clad on one or both sides with PVC or PVC film. With a one-sided coating, the film must be inside the duct, in contact with an aggressive environment.

The connection of individual parts of round air ducts to each other is performed by flanging bandages (with a diameter of up to 800 mm) or on flanges made of angular steel (with large diameters).

The connection of rectangular air ducts with a side less than 1600 mm is performed on profiled tires fastened with 4 bolts at the corners. If the side dimensions exceed 1600 mm, additional latches are used.

For conventional general exchange systems, it is preferable to use folded air ducts made of galvanized steel. Welded air ducts are installed with increased density requirements (smoke extraction shafts, air ducts passing through rooms with explosion hazard category A and B) and air movement with a temperature above 80 °C.

Flexible reinforced air ducts are widely used to avoid difficult fitting when connecting from mains to air distributors and grilles.

One of the latest solutions in the field of air distribution is textile air ducts. They are inflated by the flow of air and evenly distribute it along their entire length. In addition to noise reduction, textile ducts have the ability to trap all dust particles larger than 5 microns. They can be used in rooms with a high frequency of air exchange, without creating local areas with increased air mobility, as is the case with jet distribution.

This type of duct combines two devices – the duct itself and the air distributor.

There are several options for the operation of a textile duct. The air supply to the room can be carried out through its entire surface (in this case, airtight fabric channels made of 100% polyester are used) or through special holes made in the duct material (an impermeable fabric with perforated injector holes is used). Mixed versions are also used. The air flow velocity inside the airtight material does not exceed 0.01–0.5 m/sec, the air velocity coming out of the cracks is 4–10 m/sec, through perforated holes (injectors) is 7–13 m/sec.