Introducing Smoke Angel from
INTERACTIVE SPECIAL PROJECTS - FIRE ENGINEERING ASSOCIATES - FLÄKTWOODS
Smoke control engineers can only work within the guidelines laid out in the various codes of practice. Some flexibility exists, but authorities will not usually reject schemes based on these guidelines.
The most interesting and important problem for the smoke control engineer is in the control of smoke in the staircores, corridors, and lobbies of high-rise buildings.
OPTIONS
Ventilation
The ventilation of fire smoke from the corridors and lobbies of high rise buildings using natural vents and smoke shafts relies on two forces – the buoyancy of the smoke, and wind generated negative pressure at the outlet of the vent or smoke shaft.
Both these forces are variable, in any particular situation, and therefore unreliable.
External Wall Vents
British Standard Code of Practice CP3 recommended a method of natural ventilation in 1971, and drew immediate criticism, especially from fire officers. It recommended automatic openings or vents of 1.5m² free area on adjacent walls with the idea of allowing wind pressure to clear smoke from the corridor.
This system has shown to be unreliable, by some later research from the now Building Research Establishment (BRE). As Dr. Howard Morgan pointed out in his article Smoke Clearance, published in the May 2005 edition of Fire Prevention – Fire Engineering Journal, this combination has always been difficult to explain in terms of ventilation.
The results of some very comprehensive investigations into the relative performance of external wall vents and natural smoke shafts by the Building Research Establishment, were published in their Report No. 79204-2002, states: "The condition where external wall ventilation works well, is when the wall ventilators are facing windward, and the wind speed is sufficient to effectively PRESSURISE the fire fighting lobby".
Natural Smoke Shafts
In situations where external wall mounted ventilation is not possible, smoke is often ventilated via natural ventilated smoke shafts. Here openable vents within both the lobbies and the stairwells are used to vent the smoke into the smoke shaft. BRE - Report No. 79204, investigated in some detail the performance of these smoke shafts, and concluded, that even in adverse wind condition, their performance would be at least as good as external wall vents. The report stressed the importance of the opening at the top of the shaft being situated where it would not be affected by adverse wind conditions. However, positive wind pressure can be much greater than the pressure head developed by the smoke.
Powered Smoke Ventilation
A simple solution to overcome the doubt and uncertainties with natural smoke ventilation is to use fans to exhaust the smoke positively. In addition, the area of the smoke shaft can be reduced to under 1m², and the need for a bottom inlet eliminated.
However smoke is drawn into the escape route.
Pressurisation
The common weakness of both natural and powered ventilation is that the fire smoke enters the escape/entry routes before leaving the building.
A pressure differential system, pressurisation, on the other hand, is the only system where the objective of the design is to keep smoke out of the escape routes.
It employs John Klote's two principals of smoke control as its active forces - air velocity and pressure differential. The design involves supplying a sufficient volume (m³/sec) of ambient air into the escape routes to produce the specified velocity or pressure. Smoke control by pressurisation is not a new idea. The first UK Code of Practice – BS5588 Part 4, was published in 1978, updated in 1998 and effectively became the European Standard, EN12101 Part 6 in 2006.
Based on this standard, the design of the systems is an application of pure fan engineering, hence the importance that designers understand the principals of fan engineering. Design failures in early systems gave pressurisation a bad name, but properly designed and installed it is easily the best form of protection.
BRE report that no. 79204-2002 states:
"This alternative design of smoke shaft is not a replacement for pressure differential systems. Pressure differential systems have specific advantages in providing a higher standard of protection in specific buildings, particularly those operating a means of escape strategy based on phased evacuation. They can also provide a greater level of protection to the fire-fighting lobby itself than any of the natural ventilation systems discussed herein".
References
1. BS9999, 2008 Code of practice for fire safety in the design, management and use of buildings.
