However, circular air ducts often do not fit into the interior of public and civil buildings. In addition, in a confined space (under false ceilings, etc.), as a rule, only rectangular air ducts can be used.

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 (ρ = 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.

Cylindrical fabric ducts are the most common. They are convenient if you need to provide intensive air exchange, preventing drafts. Semicircular air ducts are used in rooms with low ceilings, for example, in catering establishments, non-food stores, hotels, etc. Air ducts in a quarter-section of a circle can also be used, installed around the perimeter of the room.

Installation of steel ducts in rooms with high ceilings is quite laborious and time-consuming. The use of textile air ducts greatly facilitates the work. Sections of air ducts (usually 5 m long) are connected to each other with the help of zippers. The air ducts are suspended using stretched cables or rails. In the latter case, the fastening is more rigid, and the duct retains its shape even without air supply. When condensation occurs, a nutrient medium is formed that promotes the development of microorganisms. Textile ducts can be easily dismantled, washed or cleaned. They are produced with a diameter of 100 to 1000 mm and a length of up to 100 m, as well as various densities that allow changing the air supply from 160 to 500 m³ / g * m² at a static pressure inside 100 Pa.

When transporting hot gases with a temperature of more than 100 °C in ventilation systems, the thickness of the steel walls of the air ducts should be 1–2 mm. To transport air with an admixture of toxic gases and vapors, air ducts must be made of certain materials, depending on the composition of the gases (steel with a thickness of at least 0.7 mm, aluminum, vinyl, etc.).

In ventilation systems of residential, public and administrative buildings, as well as office and auxiliary premises of industrial enterprises, air ducts made of slag-alabaster, cinder-concrete, reinforced cement and plastic plates, bricks, concrete, etc

. are used for portable ventilation installations rubberized, as well as metal flexible hoses.

When laying rectangular vertical air ducts, they try to make the most of the internal walls, in which appropriate channels are left for this purpose. If this is not possible, side channels and shafts are mounted to the walls.

In addition, spiral-wound metal air ducts are used in construction practice, as well as plastic air ducts that resist corrosion (made of vinyl, polyisobutylene, etc.). Their disadvantage lies in the fact that they can only be used at a temperature of transported air no higher than 70 °C.

In addition to these modifications, the ducts can be flexible, semi-flexible, thermally insulated, and also acting as a muffler.

Most often, metal ducts are used in air conditioning systems, which have great strength and resistance to fire.