In natural ventilation systems, air movement occurs due to the difference in densities of internal (heated and lighter) and external (colder and heavier) air.
In mechanical ventilation systems, air is moved by a fan or other equipment (a smoke pump, a blower, an ejector, etc.) driven by an electric motor. Mechanical systems are used much more often than natural ones, since the area of action of mechanical systems is much larger, and the cross sections of air ducts are smaller than in natural ventilation systems of the same capacity, due to higher air velocity. So, in the air ducts of natural ventilation systems, the air velocity is 0.5–2 m/s, and in the air ducts of mechanical systems 4–20 m/s.
During the operation of ventilation of any kind in the room, the air is forced to be changed. The ratio of the amount of fresh air supplied to the room for 1 hour to the internal volume of the room is called the multiplicity of air exchange. In real conditions, the multiplicity of air exchange can vary from 0.5 to 20 or more.
In addition to the designed air exchange carried out by ventilation systems, air can enter the premises by infiltration through leaks of windows, walls, doors, gates, etc. due to gravitational (natural) pressure and due to wind pressure. In the same way, air can be removed from the room.
If there are no harmful emissions in the room, then the supply ventilation systems are carried out in such a way that they take part of the air from the outside, and part from the room. This secondary use of air from the room for supply systems is called recirculation. It allows you to reduce the temperature difference between the supply air and the indoor air during the cold season, increase the relative humidity of the air and, most importantly, save heat (in winter) or cold (in summer).
Installation of ventilation systems
Natural ventilation systems can be ductless if air passes only through open apertures in external fences, or are ducted when air moves through ducts.
The operation of natural ventilation largely depends on the action of the wind. The wind, running into the building, creates an overpressure zone on the windward side, and a negative pressure zone on the leeward side. Controlled natural air exchange in the workshops of industrial buildings is called aeration. Such buildings are equipped with lanterns with shutters (transoms) that can be opened manually or by special mechanisms. The same opening valves are provided with window openings in the outer walls. By opening them partially or completely, you can create the required air exchange. Aeration is calculated only for the effect of gravitational pressure, since wind pressure is episodic and, as a rule, increases air exchange. Aeration is usually provided in industrial buildings with significant heat emissions.