THE BEHAVIOR OF SMOKE

September 22, 2016

When a fire occurs within a building, hot smoke and air rises and is replaced by cool air at low level. This phenomenon is known as convection. The smoke on reaching the ceiling spreads out laterally to form a layer below the ceiling. This layer will continue to spread unless contained by the walls, in which event its depth will increase with the following results:

  • Smoke will reduce visibility, impede escape and effective fire control

  • Heat will prevent access by fire-fighters

Smoke also consists of unburnt flammable gases, which can form an explosive layer near the ceiling. While oxygen deficiency may preclude the ignition of this mixture, the opening of a door or window could cause ignition (flashover) as a result of the sudden inrush of fresh air.

 

 

Fire spread

 

Single storey industrial premises are constantly increasing in size with the demand for large undivided areas for production, trading or storage. Therefore, the usual methods of controlling lateral smoke and fire spread through the provision of sub-division walls are negated. Fire may rapidly propagate through a building due to the combustible contents being inadequately separated and/or through irradiation by the hot gas and smoke layer just below the roof in advance of the flame front. The ability of fire ventilators to reduce fire spread is debatable in the first case, but significant in the second.

 

Purpose of ventilators

 

Ventilation during a fire’s development releases some of the smoke and hot combustion products being generated. This means there would tend to be a smoke layer at roof level with a cooler, relatively clear atmosphere at floor level. Ideally venting should control smoke produced by the fire to affect a balance between generation and extraction and ensure that there is no significant increase in the design depth of the smoke layer after vents open.


In a fully developed fire situation, vents tend to control convection, allowing air into the fire area through openings in walls and out through the roof vent. This reduces lateral fire spread to adjacent parts of the building.


The advantages of early ventilation are:

  • The clear space below a smoke layer will facilitate the evacuation of occupants and entry of fire team members;

  • It is easier to prevent a building becoming smoke-logged than to clear it once it has filled with smoke;

  • The reduction in accumulation of unburnt combustion products which could cause a flashover;

  • Automatic opening of vents gives a visible warning of fire;

  • Potential smoke, heat and water damage is reduced;

  • The rate of fire spread is limited.

Automatic ventilators

 

 

Roof fire ventilators comprise the following:

 

i. Ventilator

 

These are manufactured from metal and/or suitable plastics materials. Although there are a variety of ventilators available, most operate on one of the following principles:

 

ii. Actuating device

 

 

The operating mechanisms of ventilators are generally actuated by either heat or smoke-sensing devices. Fusible links are most commonly used, although glass bulb type links are also available. When ventilators are positioned in the roofs of lofty buildings, there may be delays in their actuation by heat sensors and in these cases, smoke detectors may be preferred. Automatic/manual vents are usually fitted with a fail-safe pneumatic control system.

 

iii. Manual over-ride

 

Automatic systems are generally fitted with an over-riding device to facilitate manual operation of the ventilator.

 

iv. Screens

 

Sub-dividing roof spaces by using screens or draught curtains considerably increases the efficiency of ventilators as they work best when the smoke layer is hot and fairly deep. In addition, the screens prevent lateral spread of smoke and heat thereby reducing damage and preventing advance opening of sprinkler heads beyond the flame front. Screens should be smoke-tight and made from material having the same fire resistance as the roof structure. It is recommended that they extend down as far as practicable, as the volume of the smoke reservoir they form will influence potential lateral smoke spread. The spacing of screens will vary from building to building and is calculated with reference to the expected fire size, smoke layer depth and building height. Screens may be permanently fixed in place or may be retractable and only operate when a fire condition is detected.

 

Air inlets

 

The effectiveness of venting depends on an adequate cool air supply into the building at low level to replace hot air, which has moved upwards. In addition to natural air leakage paths, the provision of low-level air inlets may be necessary.

 

Other considerations

 

 i. Vented area

 

The total vent area required for effective ventilation is calculated after determining the following:


Vents operate best when directly above the fire. Smaller vents evenly distributed over the protection area are more efficient and therefore preferred to a single large vent.

 

ii. Compatibility with sprinkler systems

 

The presence of sprinklers facilitates ventilation requirement calculations since the potential fire size will be limited. Where both systems are installed, sprinkler operation gets precedence for the following reasons:


Fusible link controls for ventilators should however be shielded from any sprinkler water to obviate delayed activation.

 

iii. Wind effects

 

The location of both ventilators and air inlets should be determined with due regard to wind direction. The open leaf of the opening door vent could direct wind downwards. Also if the prevailing winds blow directly into the ventilators, they cannot function as intended. Similarly, air inlets positioned on the leeward side of a building may be subjected to negative pressures, which will tend to reverse the movement of air, causing smoke to move downwards rather that upwards towards the vents.

 

 

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