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Contents
  1. 1Physical principle
  2. 2Pressure difference
    1. 2.1Worked example
  3. 3Effect on smoke movement
  4. 4Stack effect and smoke control
  5. 5Calculation methods
  6. 6Limitations and misconceptions
  7. 7See also
  8. 8Notes
  9. 9References

The stack effect is the movement of air or smoke through a tall building caused by pressure differences that arise from temperature differences.[1] When the air inside a building is warmer than the air outside, it is lighter, rises through vertical shafts such as stairs and lift shafts, and leaves through openings high in the building. Cold outside air is drawn in low down. This is called the normal stack effect. When the outside air is warmer, the flow reverses and runs downwards, which is called the reverse stack effect.[2] It is also called draught or chimney effect and is best known from chimneys.[3][4]

The stack effect is one of the main driving forces of smoke movement in buildings, together with the buoyancy of hot smoke, expansion, wind and the ventilation system.[2] It operates even without a fire and acts on the whole height of a building. Its strength grows with the height of the building and with the difference between inside and outside temperature.[2] In a high-rise building in cold weather, it can carry smoke from a fire on a low floor to floors far above. During the 1993 bombing of the World Trade Center, it was a primary driver of rapid smoke spread.[5][6] Engineers therefore account for the stack effect when they design smoke control systems, which must keep working whatever the weather.[7]

Physical principle

The stack effect is a form of buoyancy. When air is heated, its density falls, so a column of warm air weighs less than a column of cold air of the same height. The heavier cold air pushes the lighter warm air upwards.[8] The book names buoyancy as the driving force behind smoke plumes, the stack effect and natural smoke exhaust. The larger the difference in density and the greater the height, the larger the pressure difference in general.[8]

The density of air follows from the ideal gas law. At atmospheric pressure it is approximately 353 divided by the absolute temperature in kelvin, which gives about 1.2 kg/m³ at 20 °C.[8] The pressure in still air increases with depth, by the weight of the air above. Inside and outside a building, the pressure therefore changes with height at slightly different rates, because the two air columns have different densities.[8]

At one height, the pressure inside equals the pressure outside. This level is called the neutral plane.[9] In the normal stack effect, the inside pressure is higher than outside above the neutral plane, so air flows out there. Below the neutral plane, the inside pressure is lower, so outside air flows in.[9] The book describes the same pattern for a room with openings: air flows in below the neutral plane and smoke flows out above it.[8]

A chimney strengthens the effect because no horizontal mixing can take place inside it.[3] In buildings, the stairs, lift shafts, service shafts and mail chutes act as chimneys. Klote notes that the normal stack effect matters most at low outside temperatures and in tall shafts, but it can even exist in a one-storey building.[2]

Pressure difference

The pressure difference grows linearly with the distance from the neutral plane and with the difference between the reciprocals of the two absolute temperatures. Klote gives it for a shaft and the outside air as follows:[2][9]

Δp=Ks(1To−1Ti)h

Symbols
Symbol Description Unit
Δp pressure difference between the shaft and the outside Pa
Ks coefficient, 3460 at standard atmospheric pressure Pa·K/m
To absolute temperature of the outside air K
Ti absolute temperature of the air inside the shaft K
h height above the neutral plane (negative below it) m

The relation holds at standard atmospheric pressure and requires absolute temperatures.[2] It follows from the density difference of the two air columns. The coefficient of 3460 is close to the gravitational acceleration multiplied by the constant 353 in the book's density relation.[8] For the normal stack effect, the pressure difference is positive above the neutral plane and negative below it. For the reverse stack effect, the signs are swapped.[9]

The position of the neutral plane depends on where the building leaks. If the leakage paths are spread evenly over the height, the neutral plane lies near mid-height. Where leakage is uneven, for example in shafts vented at the top, it can lie much higher or lower.[9][5] The pressure difference therefore grows towards the top and the bottom of the building and is largest at the ends of the shafts.[9]

Worked example

Klote considers a building 60 m tall with the neutral plane at mid-height, so 30 m above the neutral plane at the top. The outside temperature is −18 °C (255 K) and the inside temperature is 21 °C (294 K).[9] The pressure difference is then 3460 × (1/255 − 1/294) × 30 ≈ 54 Pa. Klote gives 55 Pa from the same calculation in US units.[9] At the top, the shaft is about 55 Pa above the outside pressure; at the bottom, it is about 55 Pa below it.[9]

For the same temperatures, the pressure difference increases by about 1.8 Pa for every metre of height from the neutral plane. A 120 m building with its neutral plane at mid-height would therefore see about 108 Pa at its top and bottom. These values are large compared with the pressures used in smoke control. Klote suggests a minimum design pressure difference of 20 to 25 Pa across a barrier next to an intense fire.[7] Codes for pressurisation in the United States require 12.5 Pa in sprinklered and 25 Pa in unsprinklered buildings.[10] The buoyancy of the fire gases themselves gives about 13 Pa at 1.52 m above the neutral plane of a fire room at 800 °C.[5]

Very tall buildings reach much higher values. For the World Trade Center towers, NIST used the New York winter design temperature of −9.4 °C (15 °F). At that temperature, the stack effect pressures were of the order of 280 Pa at the bottom and top of the towers.[11] NIST noted that this is an order of magnitude higher than the design pressure for unsprinklered buildings, so the stack effect can contribute significantly to smoke spread in tall buildings.[11] The towers also influenced their surroundings. Under extreme weather conditions, air that entered the PATH railway tunnel in New Jersey left through the top of the towers.[12]

Effect on smoke movement

In a building with the normal stack effect, the existing air currents can move smoke far from the fire.[5] If the fire is below the neutral plane, smoke enters the shafts with the building air and rises, helped by its own buoyancy. Above the neutral plane it flows out of the shafts into the upper floors. If there is little leakage between floors, the floors below the neutral plane, apart from the fire floor, stay relatively free of smoke for a time.[5] If the fire is above the neutral plane, smoke is carried to the outside through openings in the facade. Where floors leak considerably, smoke also moves up to the floor above the fire.[5]

The 1993 bombing of the World Trade Center shows how strong the effect can be. The explosion cut electrical power, the ventilation systems stopped, and the outside temperature was in the twenties degrees Fahrenheit, below freezing.[6] Openings created in lift shafts on the lower levels let smoke rise rapidly to the upper parts of the tower. NIST concluded that the stack effect could be a primary driver of rapid smoke spread in high-rise buildings, and that openings in vertical shafts strengthen it.[6][13] Stair doors held open by evacuating occupants increased the airflow up the stairs.[13] On 11 September 2001, fresh air drawn up from lower floors by the chimney effect also supplied oxygen to fires deep inside the towers.[14]

The reverse stack effect tends to move relatively cool smoke downwards. Hot smoke, however, can be buoyant enough to rise even under reverse stack effect conditions.[5] In very airtight buildings with exterior stairs, it has been observed even at low outside temperatures, because the stair was colder than the rest of the building.[9] New York fire service procedures note that the stack effect becomes noticeable in buildings over 60 feet (about 18 m) high and grows with height. Downward flow is most likely when the outside temperature is above 90 °F (about 32 °C).[15] The same procedures state that wind intensifies the effect.[15]

Stack effect and smoke control

A smoke control system has to work against the stack effect that exists at the moment it operates. If the outside temperature falls below the winter design temperature, the system may not perform as intended.[7] Klote points to the 99 % and 97.5 % winter design temperatures of the ASHRAE Handbook. In a normal winter, the temperature is at or below these values for about 22 and 54 hours respectively, and extremes can be much lower.[7] Heating systems tolerate such short cold spells because building materials respond slowly. A smoke control system has no such time lag and must cope with the full stack effect at the moment it operates.[7]

Stair pressurisation is particularly sensitive. In the World Trade Center towers, studies after 1993 found that pressurised stairs needed relief venting at the top of each stair section. Without it, the stack effect would produce unacceptable door opening forces there.[16] Mechanically exhausting the stairs reduced smoke on the top floors. However, it increased the stack effect and drew more smoke into the stairs on the lower floors.[16]

NIST's analysis of alternative smoke management systems for the towers came to similar conclusions. Stack effect had a substantial impact on zoned smoke control combined with stair pressurisation, in some cases causing airflow from the fire floor into the core.[17] Fan speeds would probably have to be adjusted to measured pressure differences. Sandwich pressurisation, which exhausts the fire floor and pressurises the floors around it, remained effective even with the stack effect.[2][18]

Fire services also use the stack effect deliberately. New York procedures describe supplying air with portable fans into the attack stair three floors below the fire, with the doors at top and bottom closed. This pressurises the stair near the fire floor and reduces the stack effect. With the top and bottom doors open, the same fans increase the stack effect and help to remove smoke through the stair.[15] The procedures warn that using a stair for smoke removal can draw heat and flames from the fire floor towards that stair.[15]

Calculation methods

Simple cases can be calculated by hand with the relation above.[2] The International Fire Engineering Guidelines describe the stack effect as buoyancy-induced pressure differences caused by inside and outside temperature differences, and refer to Klote and Milke for the equations.[19] They also list the season, wind and the stack effect among the external conditions to consider when defining fire scenarios.[20]

Whole buildings are analysed with smoke movement models of the network model type. They represent a building as a network of spaces or nodes, each at its own pressure and temperature. Stairs and other shafts are modelled as a vertical series of spaces, one for each floor.[21] Air flows through leakage paths from high to low pressure. The inputs include the inside and outside temperatures, the flow network and the flows of the ventilation or smoke control system.[21]

NUREG-1805 notes that these models can represent the stack effect in tall buildings during extreme temperatures. ASCOS, CONTAMW, AIRNET and ASMET are examples, and NIST used CONTAM for the World Trade Center analyses.[21][10] In zone models of single spaces, the position of openings relative to the neutral plane decides whether they act as inlets or outlets. Openings high in a facade are not necessarily outlets, and an opening that spans the neutral plane can act as both.[22]

Limitations and misconceptions

The relation for the pressure difference is a simplification. In real buildings, the shape and position of openings, flow resistance, wind and mechanical ventilation all influence the pressure differences and flows.[8] The neutral plane is not fixed: it moves when doors or windows open or break.[9][5] Wind can strengthen or weaken the effect.[15]

A common misconception is that the stack effect matters only in skyscrapers. It increases with height, but it also exists in low buildings.[2] Another is that it always moves smoke upwards. In hot weather, or with a cold exterior stair, the flow in the shafts can run downwards.[9]

The stack effect should also not be confused with the buoyancy of the fire gases themselves. Both rest on the same physics. The stack effect is driven by the normal temperature difference between the building and the outside air.[2] Fire buoyancy is driven by the much higher smoke temperature and weakens with distance from the fire.[5]

See also

Notes

  1. ↑ Iqbal and Salley (2004), p. F-61.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 Klote (2002), p. 4-274.
  3. ↑ 3.0 3.1 Van Rossum (2027), §6.6.
  4. ↑ Evans et al. (2005), p. 70.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 Klote (2002), p. 4-276.
  6. ↑ 6.0 6.1 6.2 Ferreira and Strege (2005), p. xxxiv.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 Klote (2002), p. 4-285.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 8.6 Van Rossum (2027), §3.2.
  9. ↑ 9.00 9.01 9.02 9.03 9.04 9.05 9.06 9.07 9.08 9.09 9.10 9.11 Klote (2002), p. 4-275.
  10. ↑ 10.0 10.1 Evans et al. (2005), p. 87.
  11. ↑ 11.0 11.1 Ferreira and Strege (2005), p. 14.
  12. ↑ Ferreira and Strege (2005), p. 61.
  13. ↑ 13.0 13.1 Ferreira and Strege (2005), p. xxxv.
  14. ↑ McGrattan et al. (2005), p. 52.
  15. ↑ 15.0 15.1 15.2 15.3 15.4 Lawson and Vettori (2005), Appendix I.
  16. ↑ 16.0 16.1 Ferreira and Strege (2005), p. xxxvi.
  17. ↑ Evans et al. (2005), p. 90.
  18. ↑ Evans et al. (2005), p. 91.
  19. ↑ National Research Council of Canada et al. (2005), p. 2.5-6.
  20. ↑ National Research Council of Canada et al. (2005), p. 2.2-3.
  21. ↑ 21.0 21.1 21.2 Iqbal and Salley (2004), p. B-82.
  22. ↑ Van Rossum (2027), §9.3 (How a zone model works).

References

  • Evans, D. D., Peacock, R. D., Kuligowski, E. D., Dols, W. S., & Grosshandler, W. L. (2005). Active fire protection systems (NIST NCSTAR 1-4). National Institute of Standards and Technology.
  • Ferreira, M. J., & Strege, S. M. (2005). Smoke management systems (NIST NCSTAR 1-4D). National Institute of Standards and Technology.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Klote, J. H. (2002). Smoke control. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-274–4-291). National Fire Protection Association; Society of Fire Protection Engineers.
  • Lawson, J. R., & Vettori, R. L. (2005). The emergency response operations (NIST NCSTAR 1-8). National Institute of Standards and Technology.
  • McGrattan, K. B., Bouldin, C., & Forney, G. P. (2005). Computer simulation of the fires in the World Trade Center towers (NIST NCSTAR 1-5F). National Institute of Standards and Technology.
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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