STACK EFFECT or CHIMNEY EFFECT
- Details
What is it and how do you deal with it?
So, you get in the elevator and push the button for the 32nd floor. The doors open and it feels like you are in a wind tunnel. What the heck is going on? This is the Stack effect.
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 other forms of leakage. 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. 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, the stack effect needs to be controlled to prevent the spread of smoke and fire, and to maintain tenable conditions for victims and firefighters.
WHY REVOLVING DOORS WERE INVENTED
The pressure is so significant in fact that when skyscrapers were first developed at the turn of the century, people also had to invent revolving doors because you couldn't open the front door due to the stack effect pressure. The cold air was rushing in with so much pressure that it was difficult to push the exit doors open
Unlike most other pressures, the stack effect acts every hour of every cold day, and the pressures generated by the stack effect are significant.
Leaky buildings consume tremendous amounts of energy. Air leaks can contribute to condensation, compromising the quality of the indoor air. For high-rise residential buildings in cold weather, that isn't the worst-case scenario - it's the normal scenario
WHAT TO DO ABOUT STACK EFFECT
if leaking air is the cause of stack effect, then the best thing you can do is to prevent the air movement. Seal up all the holes at the bottom of the building to prevent the air from entering and seal up all the holes at the top to prevent the air from escaping. This is a challenge for designers as you need entrances, parkades, elevators, ventilation systems in all these types of buildings. Added vestibules, proper air barriers, pressure zones, cross over floors etc., etc. are all strategies for minimizing the stack effects.
AND FOR ALL YOU TECHNO GEEKS OUT THERE - Here is the math
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 (NPL) of the building to either the topmost openings or the lowest openings.
SI units - where
Summary
The total pressure difference acting on a building as a result of stack action depends upon building height and the difference between temperatures inside and outside. It cannot be avoided, but the way in which it is distributed across the building enclosure and interior separations can be modified through design because it depends upon the relative resistances to flow presented by the building components and the way in which they are distributed in the flow path.
Air movement caused by stack action has many important implications related to the functional adequacy of buildings that should be recognized in both their design and operation.
So, you get in the elevator and push the button for the 32nd floor. The doors open and it feels like you are in a wind tunnel. What the heck is going on? This is the Stack effect.
Stack effect is the movement of air into and out of buildings, chimneys, flue gas stacks and is driven by 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.
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 other forms of leakage. 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. 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, the stack effect needs to be controlled to prevent the spread of smoke and fire, and to maintain tenable conditions for victims and firefighters.
WHY REVOLVING DOORS WERE INVENTED
The pressure is so significant in fact that when skyscrapers were first developed at the turn of the century, people also had to invent revolving doors because you couldn't open the front door due to the stack effect pressure. The cold air was rushing in with so much pressure that it was difficult to push the exit doors open
Unlike most other pressures, the stack effect acts every hour of every cold day, and the pressures generated by the stack effect are significant.
Leaky buildings consume tremendous amounts of energy. Air leaks can contribute to condensation, compromising the quality of the indoor air. For high-rise residential buildings in cold weather, that isn't the worst-case scenario - it's the normal scenario
WHAT TO DO ABOUT STACK EFFECT
if leaking air is the cause of stack effect, then the best thing you can do is to prevent the air movement. Seal up all the holes at the bottom of the building to prevent the air from entering and seal up all the holes at the top to prevent the air from escaping. This is a challenge for designers as you need entrances, parkades, elevators, ventilation systems in all these types of buildings. Added vestibules, proper air barriers, pressure zones, cross over floors etc., etc. are all strategies for minimizing the stack effects.
AND FOR ALL YOU TECHNO GEEKS OUT THERE - Here is the math
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 (NPL) of the building to either the topmost openings or the lowest openings.
SI units - where
| Q |
= stack effect draft (draught in British English) flow rate, m�/s |
| A |
= flow area, m� |
| C |
= discharge coefficient (usually taken to be from 0.65 to 0.70) |
| g |
= gravitational acceleration, 9.81 m/s� |
| h |
= height or distance, m |
| Ti |
= average inside temperature, K |
| To |
= outside air temperature, K |
Summary
The total pressure difference acting on a building as a result of stack action depends upon building height and the difference between temperatures inside and outside. It cannot be avoided, but the way in which it is distributed across the building enclosure and interior separations can be modified through design because it depends upon the relative resistances to flow presented by the building components and the way in which they are distributed in the flow path.
Air movement caused by stack action has many important implications related to the functional adequacy of buildings that should be recognized in both their design and operation.