The stack effect in industrial flue gas stacks is similar to that in buildings, except that it involves hot flue gases having large temperature differences with the ambient outside air. The gauge dials move clockwise with increasing pressure. Stack effect in flue gas stacks and chimneys The stack effect in chimneys: the gauges represent absolute air pressure and the airflow is indicated with light grey arrows. The exfiltrating air on floors underneath the neutral axis will induce outdoor air to infiltrate the building envelope through unsealed openings. Once the conditioned air reaches the bottom floors underneath the neutral axis, it exfiltrates the building envelopes through unsealed openings such as through dampers, curtainwall, etc. Consequently, cool air will travel vertically down the building through elevator shafts, stairwells, and unsealed utility penetrations (i.e., hydronics, electric and water risers). It also decreases the specific volume of the air contained within the building, thereby reducing the buoyancy force. This reduces the dry-bulb temperature of the air within the building relative to the outdoor ambient air. Mechanical refrigeration equipment provides sensible and latent cooling during summer months. Warm air will attempt to exfiltrate the building envelope through floors above the neutral axis. The net negative pressure on lower floors can induce outdoor air to infiltrate the building through doors, windows, or ductwork without backdraft dampers. This presents a situation where floors underneath the neutral axis of the building have a net negative pressure, whereas floors above the neutral axis have a net positive pressure. Consequently, it rises from lower levels to upper levels through penetrations between floors. Warm air within the building has a low density (or high specific volume) and exhibits a greater buoyancy force. Normal stack effect occurs in buildings which are maintained at a higher temperature than the outdoor environment. Two regimes of stack effect can exist in buildings: normal and reverse. A cavity between the outer aluminium cladding and the inner insulation formed a chimney and drew the fire upwards. The Grenfell Tower fire, as a result of which 72 people died, was in part exacerbated by the stack effect. Smoke extraction is a key consideration in new constructions and must be evaluated in design stages. While natural ventilation methods may be effective, such as air outlets being installed closer to the ground, mechanical ventilation is often preferred for taller structures or in buildings with limited space. Especially in case of fire, the stack effect needs to be controlled to prevent the spread of smoke and fire, and to maintain tenable conditions for occupants and firefighters. Stairwells, shafts, elevators, and the like, tend to contribute to the stack effect, while interior partitions, floors, and fire separations can mitigate it. In a modern high-rise building with a well-sealed envelope, the stack effect can create significant pressure differences that must be given design consideration and may need to be addressed with mechanical ventilation. During the cooling season, the stack effect is reversed, but is typically weaker due to lower temperature differences. The rising warm air reduces the pressure in the base of the building, drawing cold air in through either open doors, windows, or other openings and leakage. During the heating season, the warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or unintentional holes in ceilings, like ceiling fans and recessed lights. Since buildings are not totally sealed (at the very minimum, there is always a ground level entrance), the stack effect will cause air infiltration. the Kaprun tunnel fire, King's Cross underground station fire and the Grenfell Tower fire). The stack effect helps drive natural ventilation, air infiltration, and fires (e.g. The greater the thermal difference and the height of the structure, the greater the buoyancy force, and thus the stack effect. The result is either a positive or negative buoyancy force. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The stack effect or chimney effect is the movement of air into and out of buildings through unsealed openings, chimneys, flue-gas stacks, or other containers, resulting from air buoyancy.
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