By Selele Mashilo
In the previous issue of RACA Journal, I looked at stack effect during fire smoke in a building. Service shafts and lift shafts are designed to bear in mind the possibility of fire smokestack effects.
HVAC, Fire and smoke control – Part 1
These shafts may distribute smoke throughout the building, even in areas where there is no fire. Smoke movement to further areas may be caused by pressure differences in affected areas within the building.
Pressure differences and leakages can occur around doors and windows. Ventilation ducting may also carry smoke to various places.
Three other causes of smoke movement during fire in the building are buoyancy, expansion and wind.
The reduced density of smoke from the fire has a buoyancy force which makes the smoke find its way upward. The pressure difference between fire compartment and the surrounds can be expressed with the following equation:
∆P = K ( 1/To – 1/Tf ) h
∆P = pressure difference in Pascals
To = absolute temperature of the surrounding in K
Tf = absolute temperature of the fire compartment in K
h = distance above the neutral plane in meters (m)
K = coefficient
The neutral plane is the plane of equal hydrostatic pressure between the fire compartment and its surrounds.
If the leakage paths are in the ceiling, smoke will move to the floor above the fire floor. The effect of buoyancy decreases with distance away from the fire so the ducts which are reticulated through the building are normally fitted with fire dampers which close in case of fire.
The dampers may be motorised and linked to the electronic fire detection systems where smoke sensors are used.
From the energy released by a fire, smoke can be moved by expansion. In some instances where doors or windows are closed during a fire, there might be some explosion out of the building – either through the doors or windows, forcing the smoke out.
For fire compartments with open doors and open windows the pressure difference between the fire compartment and surrounds may be low. The ratio of volumetric flow can be expressed as a ratio of absolute temperatures.
Qout/Qin = Tout/Tin
Qout = volumetric flow rate of smoke out of the fire compartment in m3/s
Qin = volumetric flow rate of smoke into the fire compartment in m3/s
Tout = absolute temperature of smoke leaving fire compartment in K
Tin = absolute temperature of air into fire department in K
Wind exerts pressure on buildings, especially high-rise buildings. The pressure that the wind exerts on the building surface can be expressed by the following formula:
Pw = ½ CwϱoV2
Pw = wind pressure in Pascals
Cw = dimensionless pressure coefficient
ϱo = outside air density kg/m3
V = wind velocity m/s
Frequently, in the fire compartment, a window will break due to the smoke pressure. If the window is situated in the leeward side of the building, the negative pressure caused by the wind vents the smoke from the fire compartment.
This can reduce smoke movement throughout the building. If the broken window is on the windward side of the building, the wind will force the smoke through the building floor. The fire smoke will be penetrating other rooms or the floor causing harm to building tenants.