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 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.http://www.mdpi.com/2071-1050/9/10/1731/pdf Resolving Stack Effect Problems in a High-Rise Office Building by Mechanical Pressurization | date=September 2017| access-date=2020-08-01 | Jung-yeon Yu; Kyoo-dong Song; and Dong-woo Cho

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 occupants and firefighters.[http://fire.nist.gov/bfrlpubs/fire09/PDF/f09002.pdf NIST Technical Note 1618, Daniel Madrzykowski and Stephen Kerber, National Institute of Standards and Technology] 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. Smoke extraction is a key consideration in new constructions and must be evaluated in design stages.{{Cite web|url=https://www.simscale.com/blog/2019/04/smoke-simulation-building-design/|title=Smoke Simulation: Heat and Smoke Extraction for Building Design|date=2019-04-23|website=SimScale|access-date=2019-07-04}}

The stack effect can also exacerbate the spreading of fire, especially in tall buildings where design flaws allow the formation of unwanted drafts. Examples include Kaprun tunnel fire, King's Cross underground station fire and the Grenfell Tower fire, as a result of which 72 people died.{{cite news|url=https://www.theguardian.com/uk-news/2017/nov/16/grenfell-tower-final-death-toll-police-say-71-people-died-in-fire|title=Grenfell Tower final death toll: police say 71 lives lost as result of fire |date=16 November 2017|access-date=16 November 2017|newspaper=The Guardian}} The latter of these was in part exacerbated by the stack effect, when a cavity between the outer aluminium cladding and the inner insulation inadvertently formed a chimney and drew the fire upwards.{{cite news |url=http://news.met.police.uk/news/latest-grenfell-tower-fire-investigation-250453 |title=Met Police Statement. Update: Grenfell Tower fire investigation |work=MPS |date=6 July 2017 |access-date=6 July 2017 |archive-date=20 June 2018 |archive-url=https://web.archive.org/web/20180620052605/http://news.met.police.uk/news/latest-grenfell-tower-fire-investigation-250453 |url-status=dead }}{{cite news|last1=Griffin|first1=Andrew|title=The fatal mistake made in the Grenfell Tower fire|url=https://www.independent.co.uk/news/uk/home-news/london-fire-grenfell-tower-cladding-architects-firefighters-experts-reason-why-cause-a7789336.html|access-date=16 June 2017|work=The Independent|date=14 June 2017 |url-status=live|archive-url=https://web.archive.org/web/20170614152028/http://www.independent.co.uk/news/uk/home-news/london-fire-grenfell-tower-cladding-architects-firefighters-experts-reason-why-cause-a7789336.html |archive-date=14 June 2017}}

Some buildings are designed with strategically placed openings at different heights to induce the stack effect where cool air enters through low-level windows or vents, and warm air escapes through higher-level openings like skylights, roof vents, or clerestory windows. This vertical movement of air creates a natural ventilation system that can significantly reduce indoor temperatures. Combining the stack effect with cross ventilation, where airflow moves across the building from one side to the other, can enhance the overall cooling effect.{{Cite web |last=Holder |first=Hugh |date=2020-11-20 |title=Stack Effect Strategies for Tropical Homes |url=https://architropics.com/stack-effect-strategies-for-tropical-homes/ |access-date=2024-08-07 |website=Architropics |language=en-GB}}{{Cite journal |last1=Rezadoost Dezfuli |first1=Raziyeh |last2=Bazazzadeh |first2=Hassan |last3=Taban |first3=Mohsen |last4=Mahdavinejad |first4=Mohammadjavad |date=2023-12-01 |title=Optimizing stack ventilation in low and medium-rise residential buildings in hot and semi-humid climate |journal=Case Studies in Thermal Engineering |volume=52 |pages=103555 |doi=10.1016/j.csite.2023.103555 |issn=2214-157X|doi-access=free }}

The stack effect is used both in traditional buildings and modern green architecture. Examples of traditional usage include the wind towers common in Middle Eastern architecture, which capture and direct cooler breezes into the building while expelling hot air to maintain comfortable indoor temperatures.{{Cite journal |last1=Hosseini |first1=S. H. |last2=Shokry |first2=E. |last3=Ahmadian Hosseini |first3=A. J. |last4=Ahmadi |first4=G. |last5=Calautit |first5=J. K. |date=2016-12-01 |title=Evaluation of airflow and thermal comfort in buildings ventilated with wind catchers: Simulation of conditions in Yazd City, Iran |url=https://www.sciencedirect.com/science/article/pii/S0973082616304252 |journal=Energy for Sustainable Development |volume=35 |pages=7–24 |doi=10.1016/j.esd.2016.09.005 |bibcode=2016ESusD..35....7H |issn=0973-0826|url-access=subscription }} Contemporary sustainable buildings often make use of the stack effect along with related non-electric techniques like ground coupling, earth sheltering, and evaporative cooling to enhance the passive cooling profile of a building. By carefully designing the building's structure, orientation and ventilation paths, architects can leverage the stack effect to reduce reliance on mechanical cooling systems and improve overall energy efficiency.

Two regimes of stack effect can exist in buildings: normal and reverse. Normal stack effect occurs in buildings which are maintained at a higher temperature than the outdoor environment. Warm air within the building has a low density (or high specific volume) and exhibits a greater buoyancy force. Consequently, it rises from lower levels to upper levels through penetrations between floors. 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. The net negative pressure on lower floors can induce outdoor air to infiltrate the building through doors, windows, or ductwork without backdraft dampers. Warm air will attempt to exfiltrate the building envelope through floors above the neutral axis.

Mechanical refrigeration equipment provides sensible and latent cooling during summer months. This reduces the dry-bulb temperature of the air within the building relative to the outdoor ambient air. It also decreases the specific volume of the air contained within the building, thereby reducing the buoyancy force. Consequently, cool air will travel vertically down the building through elevator shafts, stairwells, and unsealed utility penetrations (i.e., hydronics, electric and water risers). 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. The exfiltrating air on floors underneath the neutral axis will induce outdoor air to infiltrate the building envelope through unsealed openings.

Image:Chimney effect.svg

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.

Before the development of large volume fans, mines were ventilated using the stack effect. A downcast shaft allowed air into the mine. At the foot of the upcast shaft a furnace was kept continuously burning. The shaft (commonly several hundred yards deep) behaved like a chimney and air rose through it drawing fresh air down the downcast stack and around the mine.

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 ( ΔP ) is the driving force for the stack effect and it can be calculated with the equations presented below.{{cite web |last=Magyar | first = Zoltán |url=http://www.energiazero.org/inglese/Ventilation_lecture_MaZo.pdf |archive-url=https://web.archive.org/web/20200212100652/http://www.energiazero.org/inglese/Ventilation_lecture_MaZo.pdf |url-status=dead |archive-date=12 February 2020 |title=Natural Ventilation Lecture 2 |access-date = 12 February 2020}}{{cite web |title=Educational Package Ventilation - Lecture 3 : Mechanical (forced) ventilation |url=http://www.energiazero.org/inglese/Ventilation_lecture-3_PH-Alleen-lezen.pdf |website=www.energiazero.org |publisher=IDES_EDU / Intelligent Energy Europe |access-date=4 October 2019 |date=28 October 2011}} 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 (NPL) 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.

:$\backslash Delta\; P\; =\; C\; a\; h\; \backslash bigg(\backslash frac\; \{1\}\{T\_o\}\; -\; \backslash frac\; \{1\}\{T\_i\}\backslash bigg)$

:SI units:

:

:U.S. customary units:

:

The draft (draught in British English) flow rate induced by the stack effect can be calculated with the equation presented below.{{cite web |author1=Andy Walker |title=Natural Ventilation |url=http://www.wbdg.org/resources/naturalventilation.php |website=WBDG - Whole Building Design Guide |publisher=National Institute of Building Sciences |access-date=1 April 2020 |date=2 August 2016}}{{cite book |author1=Steve Irving |author2=Brian Ford |author3=David Etheridge |title=AM10 Natural ventilation in non-domestic buildings |date=2010 |publisher=CIBSE |isbn=9781903287569}} 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. 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.

:$Q\; =\; C\; A\; \backslash sqrt\; \{2gh\backslash frac\{T\_i\; -\; T\_o\}\{T\_i\}\}$

: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.

• HVAC (heating, ventilation and air conditioning)
• Ventilation shaft
• Solar chimney
• Solar updraft tower
• Draft (boiler)
• Inco Superstack
• Ekibastuz GRES-2 Power Station
• Windcatcher
• Cross ventilation

{{reflist}}

• [https://www.greenbuildingadvisor.com/article/understanding-the-stack-effect Stack Effect: When Buildings Act Like Chimneys -- Green Building Advisor]
• [http://web.mit.edu/parmstr/Public/NRCan/CanBldgDigests/cbd104_e.html National Research Council Canada - CBD-104 Stack Effects in Buildings]
• {{YouTube|id=VNTDFSEz8L8|title=Stack effect simulation}}

{{HVAC |state=collapsed}}

Category:Building engineering

Category:Chemical engineering

Category:Heating, ventilation, and air conditioning

Category:Thermodynamics

Category:Buoyancy