Smoke and Heat Extraction Systems. Arrangement and Sizing of Smoke and Heat Vents

Smoke and heat venting systems. Smoke and heat venting systems comprise one or more of the following elements, together with the associated activation and control devices, power supplies and accessories:
- smoke vents
- heat vents
- mechanical smoke extractors.

Given that they have the task of removing smoke and heat in the event of fire, these systems contribute to:
- preserving escape and access routes
- facilitating the work of the firefighters
- the prevention of flash-over, hence retarding or avoiding a full fire
- the protection of equipment
- the reduction of fire damage caused by burning gases and hot ash
- reducing the risk of fire encroaching on structural elements.

The main function of smoke venting is to create and maintain smoke-free zones in which people and animals can escape from a fire. These zones also ensure firefighters are unimpeded by smoke when tackling the fire and give the contents better protection from damage. In addition, smoke vents contribute to heat venting.

The task of heat vents is to conduct away hot burning gases during the development of a fire. There are two main intentions:
- to delay or retard the flash-over
- to reduce the risk of the fire encroaching on structural elements.

In the same way as smoke vents contribute to heat venting, heat vents contribute to smoke venting.

The working principle of smoke and heat venting systems lies in the property of hot gases to rise. The effectiveness of the system depends on:
- the aerodynamic efficiency of the air venting
- the effect of wind
- the size of the air vents
- the activation of air vents
- the location of the installation relative to the general arrangement and size of the building.

Mechanical smoke extractors. Mechanical smoke extractors perform the same task as smoke vents but use forced ventilation (e.g. fans) to achieve the extraction of smoke. These smoke extractors are particularly useful where smoke vents are neither appropriate nor feasible for technical reasons.

Appropriately sized smoke vents or mechanical smoke extractors can, in principle, be used in the place of heat vents.

In view of their function and how they work, mechanical smoke extractors should be provided:
- for single storey buildings with very large areas and volumes
- for buildings with long escape routes which cannot be kept smoke-free for a sufficient period by other means
- for buildings subject to particular regulations, in which special protection is necessary
- for buildings housing particularly valuable articles or equipment, or materials that are susceptible to smoke damage and therefore require extra protection.

Arrangement and sizing of smoke and heat vents. Smoke and heat vents should be arranged as uniformly as possible within the roof sections. Special attention should be given to ensuring that, in the event of fire, the smoke and heat vents do not increase the danger of the fire spreading from building to building, or jumping between fire compartments within the building. In this respect, the boundary wall should be considered as a fire wall, for which there are increased requirements.

To conduct the smoke and combustion gases directly to the outside, it is more effective to have a large number of smoke and heat vents with small openings than to provide a smaller number with larger openings. Typically, the spacing between smoke and heat vents and the distance from the lower edge of the structure (eaves) should not be greater than 20m and not less than the minimum distance from the walls, which is 5m. The distance of smoke and heat vent openings from structures on the surface of the roof must be large enough to ensure that their operation is not impaired by wind effects.

A possible increase in wind loading should be noted when smoke and heat vents are located at the perimeter of flat roofs.

As a general guideline, in roofs having a slope of from 12° to 30°, the smoke and heat vents should be arranged as high as possible and there must be a minimum of one smoke and heat vent per 400m² of plan surface area (projected roof area). For roof slopes >30°, the required efficiency of the smoke and heat venting should be considered on an individual project basis. In roof areas with a slope of <12°, one smoke and heat vent should serve not more than 200m². Where, due to the building structure, there are further subdivisions of the roof, there must be a minimum of one smoke and heat vent per subdivision.

Smoke and heat venting system efficiency. To ensure the smoke and heat venting system operates at full aerodynamic efficiency, care must be taken to ensure that there is an adequate volume of air in the lower region of the building. The cross-sectional area of the intake vents should therefore be at least twice as large as the crosssectional area of the smoke and heat vents in the roof.