- In a high-rise building, the stairs typically represent the sole means of egress during a fire.
- It is imperative for the exit stairs to be free of smoke and to incorporate design features that improve the speed of occupant egress.
- Most building codes require the fire stairwells in a high-rise building to be pressurized to keep smoke out.
1) Review of smoke
i) Hazards of Smoke
Smoke contains toxic and irritant gases.
- ¾ of all fire deaths are caused by smoke inhalation.
- Approximately 57% of fire deaths occur outside the room of fire origin.
- 47% of fire survivors could not see more than 12’.
- Smoke travels 120 to 240 ft./min.
ii) Smoke Management
- A smoke management system includes all methods described below singly or in combination to modify or influence the smoke movement.
Limit the fire
- An important consideration when designing a smoke control system is to ensure that evacuation is faster than the spread of smoke/fire.
- Controlling fire size, typically by means of hosepipes, hydrants and sprinklers should be a part of the overall smoke management program.
- An automatic fire suppression system would be expected to limit the heat release rate and control the spread of fire.
- Compartmentation involves use of barriers with sufficient fire endurance to prevent spread of smoke to spaces remote from the fire.
- The method employs walls, partitions, floors, doors, smoke barriers, smoke dampers, and other fixed and mechanical barriers.
- The effectiveness of compartmentation is limited by the extent to which the free leakage paths are controlled through the barriers.
- Smoke control system designers often use the compartmentation method in combination with the pressurization method.
- Smoke control in large open areas with high ceilings such as atria, shopping malls, concourse, airports, etc. is best achieved by exhaust ventilation.
- Hot smoke is collected at the high level in a space, where it is vented outside by means of a powered smoke exhausting fan.
- Make-up supply air below the smoke layer is also crucial, and is provided from the adjacent spaces free of smoke.
- The dilution method clears smoke from spaces remote from a fire.
- The method supplies outside air through the HVAC system to dilute smoke.
- Using this method helps to maintain acceptable gas and particulate concentrations in compartments subject to smoke infiltration from adjacent compartments.
- In addition, the fire emergency service can employ the dilution method to remove smoke after extinguishing a fire.
- Smoke dilution is also called smoke purging, smoke removal, or smoke extraction.
- The approach may be used, for example, to clear smoke that has infiltrated a protected space such as an escape corridor or refuge lobby.
- Also dilution can be beneficial to the fire service for removing smoke after a fire has been extinguished.
- The airflow method controls smoke in spaces that have barriers with one or more large openings.
- It is used to manage smoke through open doorways, subway, railroad, and highway tunnels. The method employs air velocity across or between barriers to control smoke movement.
- A disadvantage of the airflow method is that it supplies increased oxygen to a fire.
- Within buildings, the airflow method must be used with great caution.
- The airflow method is best applied after fire suppression or in buildings with restricted fuel.
- The method employs a pressure difference across a barrier to control smoke movement.
- The pressurization systems are installed mainly in the stairwells, elevator shafts, refuge spaces and other egress routes.
- The high-pressure side of the barrier is either the refuge area or an exit route.
- The low-pressure side is exposed to smoke. Airflow from the high-pressure side to the low-pressure side (through construction cracks and gaps around doors) prevents smoke infiltration.
- A path that channels smoke from the low-pressure side to the outside ensures that gas expansion pressures do not become a problem.
- A top-vented elevator shaft or a fan-powered exhaust can provide the path. In contrast to exhaust ventilation and dilution systems, the pressurization systems are designed to protect zones away from the fire source.
- Smoke management systems are designed to modify, dilute, redirect, or otherwise influence the movement of smoke in a building experiencing a fire, but not necessarily to control it or limit its movement.
- The mechanisms of compartmentation, dilution, airflow, pressurization, and buoyancy are used singly or in combination to manage smoke conditions in fire situations.
2) Smoke Control
- Smoke control systems are intended to limit and control the movement of smoke during a fire.
- The most common approach involves using pressure differences on either side of the boundaries of the fire area.
- The example is stairwell pressurization system.
- Typically the pressure differentials are created by actively controlling dedicated mechanical fans and dampers (if applicable) to supply the stairwell with 100% outdoor air.