Characteristics of air flow through a ventilation duct

Air movement: general concepts

When air moves in significant volumes, at a certain speed, and in a common direction, it is called wind. On the other hand, when different masses of air continuously change position with respect to each other, a phenomenon called turbulence occurs. The movement of air has a major impact on the feelings of well-being and comfort in people both in winter and summer. The strength of the wind is conditioned by its speed. In this sense, the Beaufort scale classifies the wind according to its strength. Although this scale does not estimate air movement at speeds below 1.5 m/s, smaller speeds are perfectly perceptible to the human body. Specifically, calm air on this scale occurs at a speed of 0.8 m/s. However, the movement of air over the bare skin of individuals causes a sensation of coolness in summer, so its movement is indeed considered effective in increasing comfort in closed spaces during the summer season.

The ventilation of a space, whether by extracting contaminated air or blowing in clean air, is carried out by connecting a fan to a ventilation duct. The combined action of these two elements allows air to flow through the entire installation. The fan creates movement in the air of a room independently of the natural circulation of air already occurring inside the space. In that movement, the duct absorbs energy from the fan through contact with the walls, changes in direction, or obstacles it encounters along the way. This energy loss is called pressure drop. This magnitude depends on several factors such as the length of the duct, its diameter, the speed of the air, or its density. Regarding the speed of the air, three distinct classes can be differentiated:

• Va = capture or drag speed, which is the speed at which the particle to be drawn moves.
• Ve = entry speed at the mouth from which air is drawn.
• Vp = speed in the plenum. The plenum is a body or box of large section in the duct where the speed decreases abruptly and noticeably.
• Vc = speed in the duct or pneumatic transport speed.

This airflow can be laminar when its trajectory is uniform. When the trajectory of the fluid particles is irregular and is accompanied by the appearance of a large number of vortices, then the flow is turbulent.

Types of ducts: ventilation pipes

Ventilation ducts can be of three types based on the shape of their section: circular, rectangular, and oval. Based on their general characteristics, the ducts can be:

• Spiral duct.
• Self-connecting circular duct.
• Self-connecting smooth duct.
• Duct with double pipe and adapter.
• Duct with double pipe with integrated adapter.
• Duct with double pipe with inner flange.