According to ASHRAE’s Handbook of Smoke Control Engineering (Klote et al., 2012), there are 6 main driving factors of smoke movement in buildings:
- Expansion of Smoke
- Buoyancy of Smoke
- HVAC Systems
- Elevator Piston Effect
- Stack Effect
- Wind Forces
1. Expansion of Smoke
Large smoke clouds that we see are mostly made up of air.
The volume of combustion products that the fire releases is small relative to the volume of air that gets sucked into the smoke cloud.
As the fire burns, the hot combustion gases rise rapidly in a plume, creating a vacuum that pulls in the air surrounding the fire. The smoke cloud grows larger and larger as more air gets entrained. Shown in the sketch below.
Additionally, the combustion gases will expand due to the high fire temperature. For example, for a fire gas temperature of 2,200 °F (1260 °C), the gas can expand to about 500% of its original volume.
That phenomenon can push hot smoke out of a burning room while sucking cool air into the room.
2. Buoyancy of Smoke
The smoke leaving a fire is hotter and less dense than the surrounding air in the building, so as the smoke rises, it pushes upward with a significant force.
The magnitude of that buoyancy force created by the rising smoke will vary in each circumstance. Based on one study, that force/pressure reached up to 0.064 in.wg. (16 Pa).
Hot Smoke vs Cool Smoke
As the smoke cloud travels further, the entrained air cools the smoke cloud. In some cases, the smoke can eventually reach room temperature.
The flow of cool smoke will be more unpredictable than hot smoke since it is no longer rising.
Cool smoke will move in the same manner as the normal building air. That can be problematic for evacuations because cool smoke can stagnate along the floor and severely reduce occupant visibility as they try to escape.
3. Impact of HVAC Systems on Smoke Movement
For example, one of the most dangerous forms of unwanted smoke transfer is when hot smoke enters vertical duct shafts. When that happens, the hot smoke can travel long distances to the upper levels of the building with little to no cooling, endangering occupants that are far away from the fire.
HVAC building codes and good engineering design practices aim to prevent the spread of smoke through the HVAC system.
Fire/smoke dampers, smoke detection system shut-off controls, zoned smoke control systems, etc. are used to contain smoke movement.
4. Elevator Piston Effect on Smoke Movement
Elevator hoistways are a potential path for smoke to travel to different building levels, especially when elevator piston effect is present.
Elevator piston effect is when the moving elevator cab creates a “plunger effect” within the hoistway as the cab rises or lowers.
For example, the sketch above shows an elevator cab that starts on the lowest level and travels upward. As it accends, it creates a negative pressure within the hoistway at the levels below and a positive pressure at the levels above.
That negative pressure can suck smoke into the hoistway on a lower building level, allowing the smoke to spread to other occupied levels. The opposite occurs when the cab travels in the reverse direction.
According to the ASHRAE handbook (Klote et al., 2012), one study found that the elevator piston effect created up to a 0.12 in.wg. (30 Pa) pressure difference in the hoistway.
The magnitude of elevator piston effect is driven by:
- Number of elevator cabs (single-cab hoistways are more affected than multi-cab hoistways)
- Speed of the elevator cabs (faster cab velocities create a bigger piston effect)
- Leakage area between the hoistway and the building
- Leakage area between the building and outdoors
5. Impact of Stack Effect on Smoke Movement
Stack effect is when the air within a building is forced to rise within the vertical shafts (ducts, elevator hoistways, stairwells, etc.) due to the temperature difference between the indoors and outdoors.
Stack effect is most prevalent in highrise buildings.
When the outdoor air is colder than the indoor air in the winter, the indoor air will force its way up and out of the top of the building. As a result, air from the outdoors will be sucked into the building at the lower levels.
The opposite occurs if the outdoor air is hotter than the indoor air in the summer (i.e. reverse stack effect).
The pressure difference and airflow created between the upper and lower levels can affect smoke movement during a fire, depending on where the smoke is located in the building.
6. Impact of Wind on Smoke Movement
Wind blowing on a building can affect smoke movement within the building during a fire.
As the wind blows on one side of the building, it creates a pressure differential on the other sides and roof, which can impact the movement of smoke significantly.
Wind is unpredictable, constantly changing direction and speed - so engineers must use average values when calculating the wind effects.
The impact of wind will be unique to each building, some of the factors that determine the wind effect are:
- Wind velocity: higher wind velocities create larger forces on the burning building.
- Ground effect: the ground creates friction against the wind flow, so the upper levels of the burning building will experience greater wind velocities than the lower levels.
- Surrounding structures: buildings and trees surrounding the burning buildings can either increase or decrease the localized wind speeds at the burning building, depending on the configuration.
Citations
- National Fire Protection Association. (2008). NFPA Fire Protection Handbook (20th Edition).
- Klote, J. H., Milke, J. A., Turnbull, P. G., Kashef, A., & Ferreira, M. J. (2012). Handbook of Smoke Control Engineering. ASHRAE.
