Operation of smoke extractors: principles and components

General ventilation

When wanting to ventilate a space through a general ventilation system, it is important to evaluate the type of activity that the people occupying it engage in. The occupancy rate and the pollutants present in the environment change significantly, for example, in an office building with a low occupancy level compared to a café or a painting workshop. Currently, people spend 90% of their time confined in enclosed spaces: workplaces, homes, shopping centers, etc., so it is necessary to ventilate indoor spaces, allowing occupants to enjoy a comfortable and hygienic environment. It is necessary to dilute body odor, control humidity levels, temperature, as well as the pollution emitted from furniture and construction materials.

When sizing a ventilation system, one can proceed by calculating the air flow required according to the number of occupants (7.5 liters per second per person). The occupancy rate conditions the number of renewals per hour (N) that must occur in the space. For example, among the types that require the highest number of renewals per hour are domestic kitchens (12-15 N), theaters (10-12 N), industrial halls (30-60 N), or painting workshops (40-60 N). Based on this index, the air flow to be renewed (Q) in m³/h can be obtained. For example, in a restaurant with a volume of 225 m3 (15 x 5 meters of surface area with a height of 3 meters), it is recommended a number of renewals per hour, N = 10, resulting in a flow rate of:

Q (m³/h) = V x N

In the example:

Q = 225 x 10 = 2,250 m³/h

Local ventilation

In this ventilation system, contaminated air is captured at the same location where it is produced, preventing its diffusion throughout the space. This is achieved through a extraction hood that covers as closely as possible the source of pollution and directs it outside. The essential elements of this type of installation are:

• Capture.
• Ducting or conduit
• Filter or separator.
• Air extractor.

Smoke extractors: components and principles

When it is possible to locate the point where the contamination originates, the smoke extractor becomes the most effective and economical solution to prevent the spread of toxic emissions or smoke throughout the rest of the space. Generally, it consists of a closed box with one face open towards the harmful emission, from which an evacuation duct activated by a mechanical extractor originates. Its main components and principles are:

Hood: a device designed to capture contaminated air, generally pyramid-shaped oriented towards the contaminating source.

Pressure drop: the loss of pressure that occurs as air moves through a duct, due to friction, changes in direction, or collision with an obstacle.

Plenum: a chamber interposed in an air duct or capture to equalize pressure.

Nozzle: a capturing element whose design narrows the capture opening to increase the air speed.

Capture or drag velocity: the speed of air at the mouth of a hood or cabin, necessary to overcome opposing flows and to collect the air with its content of gases, dust, and smoke.