with the hot (smoke) gases. The reduced density inside the fire compartment results in a higher than ambient pressure at ceiling level and a pressure below ambient at floor level, with a resultant flow of hot gas out of the upper vent area(s) and of ambient (cool) air in through the lower opening vent area(s).
Stack effect in buildings. This is the term applied to the internal flow within a building, either in an upward or downward direction, that is a consequence of a temperature difference between outside environment and that inside the building. The driving mechanism is the same as that for the buoyancy force associated with a fire (heat) source. In ‘winter’ conditions, where the building internal environment is at a higher than ambient temperature, ambient air enters the building at lower levels, rises through the building and leaves the building at higher levels. In ‘summer’ conditions, where the building internal environment is at a lower than ambient temperature, the air flow is in the opposite direction. The actual flow paths inside a building are a complex function of internal flow resistances, vent sizes and locations, and the distribution of the internal heating or cooling sources [Tamura, 1994]. Stack effect pressures can be an important determining factor in the performance of a smoke management system. For example, in severe winter conditions the upward motion of smoke inside stairwells and other vertical shafts is enhanced by the stack forces. Natural smoke venting from vents on the fire floor is particularly influence by stack forces (in addition to wind forces), with or without pressurisation systems present [Tamura, 1978].
Wind effects. Wind flows around buildings can be highly complex and turbulent, with large variations and length- and time-scales. The resulting distribution of positive and negative wind induced pressures on the building envelope can have a major influence on the performance of the ventilation systems, including that provided for smoke management. Wind induced pressures can effect the performance of natural ventilation [Marchant, 1984] and also mechanical systems including pressurisation [Poreh et al., 2003].
Forces due to air-handling systems. (This includes pressurisation and smoke ventilation plant). Air-handling (HVAC) systems by their nature can play an important role in determining the movement of smoke. This will often be a detrimental influence, with smoke transported to remote parts of the building, and so a common recommendation is to switch off air-handling systems (not smoke management specific systems) in the event of fire.
However, the air-handling system can be used to help in the control of smoke. For example, a residential HVAC system may include make-up air supply in the common corridors and exhaust from the lavatories. Such a system, if left running, may help in preventing smoke form entering the common corridors (due to favourable pressure-differentials), and hence protect the non-fire dwellings [Tamura, 1994]. In other applications, the HVAC system can be configured in a fire mode to 'under-pressure' the fire floor relative to adjacent floors, and this protect all non-fire floors. Such systems, known as zone or sandwich
© Building Research Establishment Ltd 2005