Stack effect is the movement of air into and out of buildings, chimneys, flue gas
stacks, or other containers, resulting from air buoyancy. Buoyancy occurs due
to a difference in indoor-to-outdoor air density resulting from temperature and
moisture differences. The result is either a positive or negative buoyancy
force. The greater the thermal difference and the height of the
structure, the greater the buoyancy force, and thus the stack effect. The
stack effect is also referred to as the "chimney effect", and it helps drive
natural ventilation, infiltration, and fires.
Stack effect in buildings Since buildings are not totally sealed,
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 unintentional holes in
ceilings, like ceiling fans and recessed lights. 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 cooling season, the
stack effect is reversed, but is typically weaker due to lower
temperature differences. 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. Stairwells, shafts, elevators, and the
like, tend to contribute to the stack effect, while interior partitions,
floors, and fire separations can mitigate it. 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 victims and firefighters.
Stack effect in flue gas stacks and chimneys
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. Furthermore, an industrial flue gas
stack typically provides little obstruction for the flue gas along its
length and is, in fact, normally optimized to enhance the stack effect to
reduce fan energy requirements. Large temperature differences between
the outside air and the flue gases can create a strong stack effect in chimneys
for buildings using a fireplace for heating. Fireplace chimneys can
sometimes draw in more cold outside air than can be heated by the fireplace,
resulting in a net heat loss. Cause for the stack effect
There is a pressure difference between the outside air and the air inside the
building caused by the difference in temperature between the outside air and
the inside air. That pressure difference is the driving force for the stack
effect and it can be calculated with the equations presented below. The equations
apply only to buildings where air is both inside and outside the buildings.
For buildings with one or two floors, h is the height of the building. For
multi-floor, high-rise buildings, h is the distance from the openings at the
neutral pressure level of the building to either the topmost openings or the
lowest openings. Reference explains how the NPL affects the stack effect in
high-rise buildings. For flue gas stacks and chimneys, where
air is on the outside and combustion flue gases are on the inside, the
equations will only provide an approximation and h is the height of the
flue gas stack or chimney. SI units:
U.S. customary units: Induced flow
The draft flow rate induced by the stack effect can be calculated with the
equation presented below. The equation applies only to buildings where air is
both inside and outside the buildings. For buildings with one or two floors, h
is the height of the building and A is the flow area of the openings. For
multi-floor, high-rise buildings, A is the flow area of the openings and h is
the distance from the openings at the neutral pressure level of the building
to either the topmost openings or the lowest openings. Reference explains how
the NPL affects the stack effect in high-rise buildings.
For flue gas stacks or chimneys, where air is on the outside and combustion
flue gases are on the inside, the equation will only provide an
approximation. Also, A is the cross-sectional flow area and h is the
height of the flue gas stack or chimney. SI units:
U.S. customary units: This equation assumes that the
resistance to the draft flow is similar to the resistance of flow through an
orifice characterized by a discharge coefficient C.
See also Chimney
Flue gas stack HVAC
Solar chimney Solar updraft tower
Draft References
External links Stack Effect: When Buildings Act Like
Chimneys --Green Building Advisor National Research Council Canada -
CBD-104 Stack Effects in Buildings