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/m^>3;

– 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.