Aldes Middle East* experts discuss how vital stairwell pressurisation is for a safe evacuation of people and provide design guidelines on such systems.

01 March 2012

A SAFE compartmentation using reliable motorised fire, smoke and heat dampers will effectively prevent the spread of fire, smoke and heat through the entire building, and thus keep smoke – the major killer in fire situations – inside the same compartment.

In addition, an efficient corridor smoke extraction using motorised smoke and heat exhaust dampers and 400-deg-C (two-hour) smoke exhaust fans will manage the extraction of smoke, fumes and hot gases inside the compartment under fire to let people safely exit the corridor during the early stage of the fire.

A stairwell pressurisation system is an essential smoke management system directly complementing the corridor smoke extraction system and aims to keep the smoke inside the corridor of the compartment under fire to avoid any smoke inside the stairwell, and let people safely escape the building under fire, and fire-fighters penetrate safely into the building.

Design guidelines

As described in Ashrae 2007 HVAC Applications – Fire and Smoke Management, handbook, a stairwell pressurisation system is designed to provide a “smoke-free escape route in the event of a building fire as well as a smoke-free staging area for fire-fighters”.

“On the fire floor, a pressurised stairwell must maintain a positive pressure difference across a closed stairwell door to prevent smoke infiltration,” it says. “During building fires, some stairwell doors are opened intermittently during evacuation and fire-fighting, and some doors may even be blocked open. Ideally, when the stairwell door is opened on the fire floor, airflow through the door should be sufficient to prevent smoke backflow.”

Hence, as indicated above, there are two key basic principles controlling the movement of smoke: pressure and velocity. These principles are the basis of any standard and code of practice related to stairwell pressurisation design. While pressure is considered for small gaps and closed doors, velocity is used for large gaps and open doors. Obviously, the number of effective open doors and the air velocity through these open doors will have the greatest impact on the size of the stairwell pressurisation fan.

• Stairwell pressurisation fan: While the stairwell pressurisation fan can be located anywhere, as the exit door is located at the bottom of the stairwell, it is usually located at the top of the building to prevent any failure and short-circuit due to the open exit door near the single air supply injection point. For buildings more than 15 m high (or eight storeys), there are usually several injection points for the supply of pressurising air into the stairwell to ensure an even pressure distribution. These injection points should be at a distance not greater than three floor levels to ensure an even pressure distribution.

• Air intakes: As there is a potential hazard of smoke entering the stairwell through a pressurisation fan air intake, there should be two air intakes facing different directions in order not to be affected by the same source of smoke. Each air intake should protected by a smoke control damper operated independently via a smoke detector in such a way that if one damper closes due to smoke contamination, the other air intake will supply the air requirements of the system without interruption. The air intake should also not be less than 5 m horizontally from any exhaust discharge opening and be independent from wind speed and direction.

• Air leakage paths: These paths – except for open doors, windows and air transfer grilles – must be identified and evaluated to keep a positive differential pressure between the stairwell and the accommodation when all doors are closed. Some leakage paths are obvious such as cracks around closed doors and elevator doors while others such as construction cracks in building walls are less obvious, but are equally important. The area of these leakage paths depends on such features as workmanship, door fit, and weather-stripping. For example, a 0.9 by 2.1 m door with an average crack width of 3 mm has a leakage area of 0.018 sq m. However, if this door is installed with a 20 mm undercut, the leakage area is 0.033 sq m, a significant difference. The leakage area of elevator doors is in the range of 0.051 to 0.065 sq m per door.

• Door-opening forces: The door-opening forces resulting from the differential pressure between the stairwell and the accommodation must also be considered. Unreasonably high door-opening forces can make it difficult or impossible for occupants to open doors to refuge areas or escape routes. The force required to open a door is the sum of the forces to overcome the pressure difference across the door and to overcome the door closer.

This relation assumes that the door-opening force is applied at the knob. Door-opening force Fp caused by pressure difference can be determined from Graph 1 for different door widths and for d = 75 mm. The force to overcome the door closer is usually greater than 13 N and, in some cases, can be as great as 90 N.

• Overpressure relief: An overpressure relief is mandatory as the design of pressurised stairwells is evaluated under two vastly different conditions, that is, all doors closed and with selected doors open. As a system designed under “all doors closed” conditions cannot provide sufficient airflow through selected open doors to prevent smoke backflow, and the pressure difference across the remaining closed doors can drop to low levels, the system has to be finally designed for the worst case scenario (that is, with selected doors open). But to prevent the build-up of excessive pressures when all doors are closed, an overpressure relief has to be implemented in the stairwell.

In case the pressure relief damper (PRD) is connected to the building on each floor, a fire damper should be connected in series, whereas if it is connected to the outside, a windshield is recommended to prevent wind pressures.

• Corridor smoke extraction: The corridor smoke extraction system is the best solution to offer a low resistance path for the supply air penetrating inside the corridor from the stairwell to leave the building through vertical ducts and 400-deg-C (two-hour) smoke exhaust fans. The corridor smoke extraction is perfectly complementary with the stairwell pressurisation system. Indeed, by creating a low pressure point in the fire area, the corridor smoke extraction ensures that all airflow will be directed towards this fire area, and consequently the smoke will be prevented from entering unaffected parts of the building via unidentified leakage paths.

• HVAC system: In order not to affect the pressure differential, the heating, ventilation and air-conditioning (HVAC) system should be shut down to prevent the smoke movement penetrating other interconnected spaces (same requirement for compartmentation and smoke extraction system).

UAE application

In the UAE, the design criteria are quite stringent due to the number of high-rise buildings, the building occupancy and the difficulty to reach the ground level. As per the new UAE Fire Code, the stairwell pressurisation system shall be implemented in any building with a habitable height exceeding 23 m when internal exit staircases are without adequate provision for natural ventilation, and should deal with the following design criteria:

• Detection phase: The minimum pressure differential between stairwell and adjacent accommodation space is 12.5 Pa in sprinklered buildings or 25 Pa in non-sprinklered buildings when all doors are closed. A value of 50 Pa can also be considered without any problem.

• Escape phase: The minimum velocity shall be higher or equal to 1 m per second through three open doors (one main exit door and two doors on consecutive floors). The maximum force required to open any door at the door handle shall not exceed 133 N, meaning finally a maximum differential pressure as long as the force of the door closer is known. This maximum differential pressure is usually below 80 Pa.

• Different potential configurations: These take into account factors such as the presence of a lobby area, requiring different configurations for pressurisation of the staircase shaft.

Pressurisation fan

In case of fire, a stairwell pressurisation fan is used to supply fresh air into the exit route stairwells to create a sufficient pressure differential within the corridor space. Theoretically, any type of fans could be used for stairwell pressurisation application.

A major advantage of using axial fans for stairwell pressurisation is that they have a relatively flat pressure response curve with respect to varying flow. Therefore, as doors are opened and closed, propeller fans quickly respond to airflow changes in the stairwell without major pressure fluctuations. A second advantage of using axial fans is that they are usually less costly than other types of fans and can provide adequate smoke control with lower installed costs. However, as they operate at low pressures and are readily affected by wind pressure on the building, they often require windshields at the air intake, especially when they are wall-mounted. This is less critical when axial fans are mounted on roofs because they are often protected by parapets and the direction of the wind is at right angles to the axis of the fan.

A study from Hobson and Stewart, “Pressurisation of Escape Routes in Buildings” (1977), revealed also that axial flow fans were more reliable than centrifugal fans with a failure rate 10 times less than centrifugal fans and a high reliability of 94.9 per cent. Obviously today’s centrifugal fans have achieved better performances with higher reliability and are now as reliable as axial fans. However, despite this reliability, standards, codes of practice, and regulations are still enforcing stand-by fans requirements with primary and back-up pressurising units, mounted in parallel or series, especially in the case where the pressure differential system equipment provides air under pressure to the only escape route within a building.

It should be noted that if there are two independent escape routes for each accommodation within a building (such as two staircases) or if there is just one escape route but there is a possibility for the people to enter another compartment with other means of escape, there is no need for stand-by fans.

For the selection of the stairwell pressurisation fan, the fan performance shall be at least 1.5 times larger than that calculated for the predicted leakages to allow for unidentified leakages plus an allowance of 15 per cent for probable ductwork leakage as per EN 12101-6.

Helione stairwell pressurisation axial fans from Aldes, a France-headquartered HVAC component manufacturer, are designed, CE marked, and tested as per EN 12101-3 for operation at 400 deg C for two hours and hold UAE Civil Defence approval.

PRD

The PRD shall be sized to discharge the total excess airflow determined by subtracting the total air leakage from the stairwells, lobbies and corridors with all doors closed from the total required airflow in the worst case conditions (selected doors open). It should be able to maintain at or above the design pressurisation level but below the maximum pressure determined by the door-opening force requirements.

Link for the Graph 1:

* Aldes Middle East, part of Aldes France, manufactures products that provide clean air, comfort and safety inside residential, commercial and industrial buildings through three specific activities: fire protection, ventilation, and air distribution.

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