They are made of galvanized or stainless steel.

Spiral-lock round (with a folding connection). They are made of steel tape with an anti-corrosion coating 0.5–1 mm thick, about 130 mm wide. They have increased rigidity compared to straight-seam ducts, there is no length restriction, aesthetic appearance, high seam density. The disadvantages include the fact that the seam takes about 15% of the metal from which the duct is made.

Spiral-welded round. They are made of steel tape with an anti-corrosion coating 0.8–2.2 mm thick, about 400–750 mm wide. The joints are welded overlap. Spiral-wound air ducts have less metal consumption compared to spiral-lock air ducts, there is no length restriction, high seam density. However, they cannot be made from steel thinner than 0.8 mm.

Rectangular straight-folding air ducts. They are made of steel sheet, it is desirable to place the seam on the bend so that it serves as an additional stiffener. Such ducts are easier than round ones to place in a space with a limited height, they fit better into the interior of the building. Rectangular ducts have greater aerodynamic drag than circular ducts with similar performance.

Round straight-folding air ducts. According to the manufacturing method and material, they are similar to rectangular air ducts, they differ only in a circular cross section.

Metal-plastic air ducts consist of two layers of metal, between which foam plastic is laid. Usually such a construction is used: foamed plastic 2 cm thick, located between layers of corrugated aluminum 80 microns thick. They have a small mass with high strength. They have a smooth surface, aesthetic appearance, do not require additional thermal insulation.

Non-metallic air ducts are made of synthetic materials: polyethylene, fiberglass, etc.

Polyethylene air ducts: are usually used in supply ventilation systems. When the fan is turned on, the duct (in the form of a sleeve) is inflated with air.

Fiberglass ducts: usually used to connect the fan and air distributors. They are stretched on a metal frame.

Vinyl plastic air ducts: used when moving air containing acid vapors, etc., causing corrosion of steel. The thickness of the vinyl plastic is 3–9 mm, connected by welding.

Non-metallic air ducts are resistant to corrosion, they can be bent in any plane and at any angle.

Flexible ducts of circular cross-section, lightweight, do not need special turns, as a result of which the duct has fewer connections, which simplifies installation. However, flexible ducts create a large aerodynamic drag, which may be excessive with an extended network, so they are often used as connecting pipes of small length. Flexible ducts are divided into:

• Flexible aluminum ducts without heat and noise insulation;

• Aluminum ducts with polymer coating without heat and noise insulation;

• Highly flexible polyvinyl chloride ducts;

• Highly flexible insulated air ducts consisting of two layers of polyvinyl chloride coated with polyamide fabric. A flexible steel wire spiral is located between the two layers.

• Semi-rigid aluminum ducts with thermal insulation;

• Flexible heat-insulated sound-absorbing air ducts. They consist of:

– Micro-perforated air duct made of high-strength metallized polymer;

– 25- 50 mm of thermal insulation layer with a density of 16 kg/m³;

– An outer coating made of aluminum seamlessly reinforced with fiber.

These ducts have excellent noise reduction performance in the low frequency range. To reduce the possible consequences of the use of mineral fibers on human health, the inner sleeve has minimal perforation, a special acoustic film is placed between the inner sleeve and the insulation layer. This reduces the probability of fiber entrainment at normal air flow speeds to almost a minimum.

Condensation formation, safety, noise, energy saving – these are the criteria that should be taken into account when choosing a material for thermal insulation of air ducts.

Thermal insulation of air ducts performs the following main functions:

• Prevention of condensation on both the inner and outer surfaces of the duct.

• Ensuring fire resistance to prevent the spread of fire in case of fire.

• Attenuation of noise and vibrations that occur during the movement of air through the duct.

• Reduction of heat transfer between the air flow in the duct and the environment.

For the organization of air ducts, materials with a zero (0) fire resistance class are used. If the channel has a multilayer lining, the fire resistance class "zero-one" (0–1) is allowed. This condition is met if all surfaces in the operating mode consist of non-combustible material with a thickness of at least 0.08 mm and if they provide continuous protection of the internal thermal insulation layer having a fire resistance class not higher than the first (1). Fasteners and connections whose length is not more than five times the diameter of the duct itself must be made of a material having a fire resistance class of "zero" (0), "zero-one" (0–1), "one-zero" (1–0), "one-one" (1–1) or "one" (1). Ducts of class "zero" (0) have an outer skin made of a material of fire resistance class not higher than the first (1).

Air treatment and air distribution systems create noise transmitted, among other things, through the duct system. It is possible to reduce noise if you maintain a low air velocity in the ducts, install damping devices at the point where the fan is connected to the duct, use an elastic suspension for the ducts, as well as damping gaskets at the points where the ducts intersect wall structures. The noise propagated through the air ducts can also be attenuated by the use of special silencers and a sound-proofing coating. Many thermal insulation materials have good sound insulation properties and can be used as both heat and sound insulation.

Thermal insulation of the duct can be performed from the inside or from the outside. When using thermal insulation inside the duct, it is necessary to increase the section of the duct to maintain the calculated carrying capacity at a given air velocity. In addition, the side of the thermal insulation in contact with the air flow should be smooth enough so as not to increase the resistance when air moves through the duct.

When installing silencers, the use of external thermal insulation is preferable. Also, the installation of external thermal insulation is the prevention of the occurrence of bacterial foci, the formation of dust and dirt deposits, due to which the thermal insulation material may begin to delaminate, release volatile substances and lose its qualities.

In addition, with external thermal insulation, the risk of fire spreading from room to room in the event of a fire is significantly reduced. The most important factor in the installation of thermal insulation is the prevention of thermal bridges that reduce the effectiveness of thermal insulation, as well as ensuring high vapor resistance. Heat transfer bridges can also occur in places where air ducts are attached to building structures.

The destruction of the thermal insulation material is prevented by:

• For internal thermal insulation – the use of composite materials, where thermal insulation is combined with a metal layer or film.

• For external thermal insulation – the use of neoprene sheathing, galvanized steel sheet or aluminum sheet.

1. What are the disadvantages of circular ducts?