6 unanswered googled questions about the pressurisation system (until now)

6 unanswered googled questions about the pressurisation system (until now)

From feedback, during webinars, as well as in phone consultations or on the Teams calls, we are often asked questions regarding pressure differential systems that you have been struggling to find an answer to through online searches. We have compiled these in this text. Here, you will find information on CE marking, the relation to second stairwells in residential buildings, CFD simulation, oxygen supply and more.

1. Is it possible to CE mark a pressure differential system?

No, it is not yet possible to CE mark a kit of devices. The previous product standard, EN12101-6, needed to provide an appropriate test methodology for institutions to conduct testing and the CE certification process. Further details on this topic were discussed in our previous webinar, and we invite you to review it.

The effectiveness of the system confirmed by testing

To bring the SMAY pressurisation kit of equipment to the market as a fire safety system, we opted for a National Technical Assessment. We tested according to the test methodology prepared by ITB in Warsaw (an independent building research institute), based on the early draft of the new 12101-6:2022. We are ready for the new product standard, which is nearly identical, and await its harmonisation. Once harmonised, it will be possible to CE mark for all pressurisation kit of equipment.

2. Would the oxygen supply intensify the existing fire when pressurisation is working?

The optimal design of a pressurisation system involves placing the release path outside the fire compartment, preferably in the corridor. This approach becomes challenging in large open spaces, such as office buildings, where a release buff may be necessary for the fire-affected floor to supply air to the fire location however the level of oxygen should not impact the existing fire to any extent.

Safety Way side by side with firefighters

The critical consideration revolves around the system’s objective and the real threat—the fire or the smoke. In a fire compartment, occupants are likely to evacuate early, aware of the fire, while the spread of the fire, if compartmentalisation is effective, is not as rapid as the spread of smoke. Firefighters, equipped with proper gear, enter the fire compartment after evacuation, minimising their vulnerability to smoke.

Fresh air supply during fire

When a firefighter opens the door to a floor, fresh air is indeed supplied. However, the limitation of the fire by a lack of oxygen is unlikely due to the imperfect airtightness of the fire compartment and the eventual breakage of non-fire-rated windows, allowing air to enter. It’s important to note that this isn’t exclusive to pressurisation systems; other systems like mechanical extraction, natural smoke, and heat evacuation ventilation also require supply air. This compensatory air is essential for the proper functioning of these systems, with the primary goal being to increase visibility and decrease temperature—objectives more critical than the potential impact of oxygen supply to the fire.

3. How does wind affect the pressurisation system?

Wind induces overpressure on the elevation, impacting the pressure within the building. This becomes particularly problematic when utilising windows or openings in the facade to release air. However, employing mechanical smoke and air extraction can alleviate this issue. From a design perspective, having at least two AOVs on different facets of the facade is crucial to counteract the influence of wind. \

The role of external temperature in the functioning of the PDS system

Additionally, external temperature significantly affects pressure differential systems, if not considered correctly in the design process. Temperature differences, especially in high-rise buildings, contribute to the substantial influence of the stack effect. The stack effect involves directed air movement up or down the staircase due to differences in air density.

That’s interesting… uneven pressure distribution in the stairwell

Our research is that the uneven distribution of pressure in the staircase, caused by the stack effect, persists even when the staircase is pressurised. This is attributed to the substantial heat capacity of the concrete walls in the staircase. Consequently, our research has focused on finding solutions to mitigate this effect, especially in high-rise buildings.

4. How does SMAY’s team model the stack effect?

SMAY’s team utilises ANSYS Fluent to model the pressure differential system and the stack effect. The modelling includes creating a staircase, corridor, fire lobby, and exterior functions. The simulation incorporates leakage, heat transfer, volume flow, and all necessary components for a comprehensive Computational Fluid Dynamics (CFD) analysis. Find out more here.

5. Does the pressurisation of residential single stairs meet the proposed guidelines in the new BS 9991 for buildings of 18 to 30 metres height?

The pressurisation system is essential, especially if we have a single stair. But it isn’t the opposite of the second staircase because what is the purpose of having two staircases if both are filled with smoke? Usually, it is not a problem of the capacity of staircases but of smoke inside the staircase. So, to provide safe evacuation, pressurisation gives the best results in protecting evacuation routes. We write about this consideration in Fire Protection Association Journal “Smoke, stacks, and second stair”.

6. What is the size of the transfer ducts from the lobby to the corridor?

There are three things worth considering here:

1) Supply shafts, usually dimensioned at 8-10 m/s (fan speed and pressure depend on hydraulic calculations), if they pass through another fire zone, they must be EIS fire rated for the duration of the fire (usually EIS120), because we do not want that these channels disintegrate under the influence of temperature or that they heat the air transported to the safe zone.

2) Transfer dampers from the lobby to the corridor, which provide make-up air, are sized for the amount of smoke exhaust air (depending on the speed at the open door), to maintain a difference of >30 Pa on both sides of the damper when the door is closed. Depending on the pressure in the protected zone, it will be between 4.5 and 6 m/s. According to the EN standard, these dampers should be EIS120 fire dampers to protect the opening in the fire separation wall in the event of PDS not working. However, there is also an approach by designers who consider them as part of the smoke control system and use EIS120 smoke control dampers for this purpose.

3) In the case of ducts of an electronically controlled transfer system, its classification depends on the location – if it is located entirely in the lobby, then only the passage through the wall should be secured, but if part of the installation passes through another fire zone, then an enclosure for EIS120 class.

We will regularly update the text with new questions. If there are concerns you can’t find here, email hello@smay.eu. Last updated December 2023.

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