- 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.
2) Smoke movement
- A building can be considered as a series of spaces each at a specific pressure with air movement between them from areas of high pressure to areas of low pressure.
- While in practice, it is possible for pressure gradients to exist in large vertical spaces such as stairwells, the significant pressure differences can generally be considered as occurring across the major separations of the building structure, i.e. doors, windows, walls and floors.
- The difference in pressure determines whether it will flow at all, and how much and how quickly it will flow.
- Large pressure differences produce large flows.
- The principal factors responsible for the pressure differences and, therefore, the smoke movement are….
- Differences in temperature between outdoor and indoor air (stack effect)
- Natural convection
- Thermal expansion
- Wind forces
- Buoyancy of combustion gases
- HVAC operation
i) Stack Effect
- Stack effect is a result of different air densities inside and out of a building that cause the pressure distribution of air in a building to be different from that outdoors.
- If the airflow is from bottom to upwards, it is normal stack effect.
- It happens in winters when the less dense warmer air (indoor) rises and cold outside air rushes in to take its place.
- The reverse happens in summers when the cooler air inside the building sinks and draws warmer outside air in through the top of the building.
- The stack effect is more pronounced in the winter.
- When it’s cold outside, the stack effect creates about 4 pascals of pressure for every floor of the building.
- In the summer, this drops to 1.5 pascals per floor.
At standard atmospheric pressure, the pressure difference due to normal or reverse track effect is expressed as
- ΔP = Pressure difference inches of water
- Ks = Coefficient, 7.64
- To = Absolute temperature of outside air, °R
- Ti = Absolute temperature of the air inside the shaft, °R
- h = Distance above neutral plane, ft.
- At some intermediate point between top and bottom, the pressure is neutral and is called the neutral pressure plane. The height of the “neutral plane is determined by the relative leakage areas of the buildings structure at high and low levels.
- Generally the neutral plane is at or near mid-height. It is possible to shift the neutral plane close to the position of the opening by providing sufficiently large openings at the top or bottom of a building.
- This is the underlying principle behind natural venting to control smoke movement.
- Assuming winter conditions, the pressure at the top will be lesser than the lower floors. It will result in infiltration of air at each floor level.
- In the event of fire, the shaft will be filled with smoke, which can be dangerous for people escaping the building. This situation can be averted by pressurizing the stairwell.
ii) Natural Convection
- Natural or free convection results from temperature differences within a fluid.
- As a fluid is heated, it expands while mass remains the same.
- Decreased density (mass/unit volume) makes the heated fluid more buoyant, causing it to rise.
- As the heated fluid rises, cooler fluid flows in to replace it.
- Natural convection is one of the major mechanisms of heat transfer in a compartment fire, heated air and smoke rise, and cooler air moves in to replace it.
- This process transfers thermal energy, thereby heating other materials in the compartment, and also transfers mass as smoke moves out of the compartment, and cool air (containing oxygen necessary for continued combustion) moves into the compartment.
iii) Thermal Expansion
- As the temperature within a fire compartment rises, the gases expand in direct proportion to their absolute temperature.
- Two to three volumes of hot gases may be displaced from the zone depending on the maximum temperature attained by the fire.
- The volumetric flow of smoke out of a fire zone is greater than the airflow into the fire zone.
- This situation is expressed as.
- Q out = Volumetric flow rate of smoke out of the fire compartment, cfm
- Qin = Volumetric flow rate of air into the fire compartment, cfm
- Tout = Absolute temperature of smoke leaving the fire compartment, Rankine (R)
- Tin = Absolute temperature of air into the fire compartment, Rankine (R)
- Venting or relieving of pressures created by expansion is critical to smoke control.
- The relationship between volumetric airflow (smoke) and pressure through small openings, such as cracks, is as:
- ΔP = Pressure drop across the flow path, in wc.
- Q = Volumetric flow rate, cfm
- Kf = Coefficient, 2610
- A = Flow area, sq. ft
iv) Wind forces
- Wind velocity can have a significant effect on the movement of smoke within a building.
- The pressure distribution on the surface of a building due to wind is far from uniform and depends on the direction of wind, shape and height of building, shielding effects of local obstructions to flow.
- The pressure distribution is conveniently expressed as.
- Pw = Wind pressure, in. wc
- Cw = Dimensionless pressure coefficient
- Kw = Coefficient, 4.82 x 10-4
- v = Wind velocity, mph
The coefficient, Cw, values range from 0.8 to 0.8, with positive values for windward walls and negative values for leeward walls.
- The higher the wind velocity, the greater the pressure on the side of the building. A 35 mph wind produces a pressure on a structure of 0.47 in. w.g with a pressure coefficient of 0.8.
v) Buoyancy of Combustible Gas
- Hot gases produced during the fire have a lower density and, therefore, rises upwards due to the thermal buoyancy force.
- However, as smoke moves away from the fire, its temperature is lowered due to dilution; therefore, the effect of buoyancy decreases with distance from the fire.
- The pressure difference between a fire zone and the zone above can be expressed as
- ΔP = Pressure difference, in- wc.
- Ks = Coefficient, 7.64
- To = Absolute temperature of surrounding air, Rankine (R)
- Tf = Absolute temperature of the fire compartment, Rankine (R)
- h = Distance from the neutral plane, ft.
vi) HVAC Systems
- Air movement produced by mechanical ventilation sets up pressure differences in a building in a similar manner to and superimposed on those due to natural forces.
- By design, the heating, ventilation and air conditioning (HVAC) systems typically supply air at a higher rate than is extracted (or the reverse may occur in certain applications).
- It is becoming more common for buildings to be pressurized, i.e. at a positive pressure with respect to external conditions, particularly with the increasing use of sealed windows.
- This has the advantage of limiting air infiltration caused by wind and stack effects.
- HVAC equipment with certain enhancements and interlocking with fire and smoke detection systems can fulfill the duty of smoke control.
- All the above factors create pressure differences across barriers (e.g., walls, doors, floors) that result in the spread of smoke.
- It is necessary to consider the effect of each or a combination to assess the requirements for an economical pressurization system. The fire officer can determine for himself which of the pressure differences is the most dominant.
- A factor of particular importance is stack effect especially in high-rise buildings. The stack effect phenomenon is visible 24 hours and even small ambient temperature changes may cause substantial pressure gradients in a very short time.
- The wind can have a very large effect on the spread of fire gases especially in certain geographic locations or at high elevations or not so tight buildings.
- If the fire is very intense and the temperature in the fire room and even other adjoining rooms is very high, the pressure differences resulting from the thermal buoyancy force, or the thermal expansion, can be the most dominant.
- The dominating pressure difference can also vary during the course of the fire. In the initial stage of a fire, or during small fires, it can be the HVAC system that has the greatest effect on the spread of fire gases in the fire zone.
- At a later stage, the same HVAC system can also contribute to the spread of fire gases to other building zones resulting in a fully developed fire.
- The primary means of controlling smoke movement is by developing higher pressures in the areas to be protected and in the building (zones) adjacent to the compartment of fire origin.