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.

According to the principle and effect of the curtain, there can be gate and mixing types. In the first case, the curtain maximally or completely prevents (blocks) air access to the enclosed space; the air velocity from the installation to create such curtains should be high (up to 25 m/s). Such curtains are arranged at low outdoor temperature and frequent opening of doors. In mixing type installations, the rushing cold air is mixed with the heated air of the heat curtain. As a result, warm air enters the room through the door and the curtain, which does not create a feeling of cold blowing.

1. What is the reason for the location of outdoor air intake devices in ventilation systems?

2. Name the main types of fans by type and design.

3. What is the reason for the mandatory installation of the filter in front of the heat exchange equipment (heaters and coolers)?

4. Give an example in which cases it is advantageous to use electric air heaters.

5. What are the advantages of using water heaters?

6. Why the range of air velocity in heat exchange equipment is strictly limited?

7. What is the fundamental difference between water (water-glycol) and freon air coolers?

8. What measures are envisaged to reduce noise in ventilation systems?

9. Which device is used to prevent air flow when the fan is turned off?

10. In which places of the building are provided air heat curtains with heated air?

According to the type of section, the ducts are divided into round, square or rectangular. Circular air ducts are much stronger than rectangular ones, and their manufacture is less laborious.

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.