Stack effect
Stack effect is the ventilation in buildings and chimneys that results from thermal differences between indoor and outside temperature. The greater the thermal difference and the height of the structure, the greater the stack effect. Outside wind conditions can exacerbate the effect even more.
In a completely sealed enclosure, thermal differences between the inside and outside will result in a pressure difference, because warm air is less dense than cold air. If the enclosure is not sealed, the less dense warm air will rise because it is being displaced by the denser cold air (= thermal buoyancy). Therefore, stack effect is a form of convection or natural ventilation.
Stack effect in buildings
Since buildings are not totally sealed (at the very minimum, there is always a ground level entrance), the stack effect will cause air infiltration. 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 leakage. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through either open doors, windows, or other openings and leakage. During the cooling season, the stack effect is reversed.
In a modern high rise building with a 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. Stairwells, shafts, elevators, and the like, tend to contribute to the stack effect, whereas interior partitions, floors, and fire separations can mitigate it. Especially in case of fire needs the stack effect be controlled to prevent the spread of smoke.
Stack effect in chimneys
The stack effect in chimneys is similar to that in buildings, except it involves hot flue gases having large temperature differences with the ambient and outside air. Furthermore, chimneys typically provide little obstruction for the flue gas along its length. These facts can create a strong stack effect in chimneys, so much so, that in old buildings using a fireplace for heating it draws in more cold outside air than can be heated by the fireplace, resulting in a net heat loss.
Equations
The stack effect can be calculated with these equations:
- P = 0.342 a h (1/To - 1/Ti)
Where
- P = total pressure in kPa
- a = atmospheric pressure in kPa
- h = height of building in metres
- To = absolute outside temperature in kelvin
- Ti = absolute inside temperature in kelvin
- P = 0.0188 a h (1/To - 1/Ti)
Where
- P = total pressure in psi
- a = atmospheric pressure in psia
- h = height of building in feet
- To = absolute outside temperature in R
- Ti = absolute inside temperature in R
See also
References
- ASHRAE Fundamentals Handbook