. For active muffling of aerodynamic noise, mufflers are widely used in ventilation systems, the principle of operation of which is based on the conversion of sound energy into thermal energy by friction.

By their design, silencers are divided into tubular, honeycomb, plate and chamber silencers (Fig. 14).

Fig. 14. Muffler designs

a – lamellar with extreme plates; b – lamellar without extreme plates; c – tubular rectangular section; d – tubular circular section; e – chamber;

1 – muffler casing; 2 – sound – absorbing plate; 3 – air channels; 4 – sound-absorbing lining; 5 – internal partition;

A – the distance between the plates; B – the thickness of the plates; H, Hx – the dimensions of the duct; C -the thickness of the lining of the duct; D – the diameter of the duct

By design, silencers are divided into tubular, cellular, plate and chamber.

Tubular silencers are made round, rectangular. Cellular and plate silencers are made only rectangular. As a sound-absorbing material, soft mats made of superfine fiberglass with a thickness of 100 mm are used for tubular and cellular silencers and with a thickness of 100, 200 and 400 mm for plate silencers. To prevent the entrainment of the fiber with the air flow, the sound-absorbing layer is protected with fiberglass and metal mesh or perforated sheets with a perforation of at least 20%.

The plate silencer is a box made of a thin metal sheet. A tubular silencer is made in the form of two round or rectangular pipes inserted into one another. Tubular silencers are used on air ducts with a diameter of up to 500 mm.

Currently, active silencers have been developed that carry out broadband active noise suppression (especially effective at low frequencies). Noise neutralization is carried out by introducing antiphase noise.

Check valves are used to allow air to pass in one direction and prevent it from moving in the opposite direction. They are available in two of the simplest modifications: the "butterfly" type and the "inertial lattice" type (Fig. 15).

Fig. 15. Check valves

The butterfly valve is made of galvanized steel, has two spring-loaded lobes, and can be installed in any position.

A petal check valve of the "inertial grid" type with light plastic blinds inserted into a galvanized steel box can only be installed on horizontal air ducts, under the action of the air flow in the permitted direction, the petals rise, in all other cases they are lowered.

One of the main characteristics of check valves is the maximum possible air velocity.

Check valves are used to prevent air flow: when several supply units are operating on the same network; when installing a backup supply or exhaust fan; when connecting several exhaust systems to one exhaust shaft.

They are also installed to prevent backflow of air when exhaust air is released onto the facade or in front of roof fans.

Air curtains (without air heating) they are arranged between rooms with the same or similar thermal conditions. They are used to prevent access of air from some rooms in which harmful vapors or gases are released to others.

The principle of operation of air-heat curtains is as follows: air is taken from the upper area of the lobby, heated in heaters up to 50 °C, fed by a fan into the air distribution chamber and then released through the uniform distribution duct at the door. The air is discharged through cracks or holes in the duct. The resulting jet creates a vertical air curtain. The air can be released from the bottom at the door or from the side to the desired height. Air is supplied to the air curtain from the supply ventilation without additional heating.