We will discuss a situation where evacuating individuals opens several doors simultaneously. How does the pressure differential system operate in such cases? What functions must it have to respond effectively? Let’s find out.

A short reminder of Fire Evacuation Strategies

The primary goal of any fire evacuation strategy is to ensure the safety of occupants in the event of a fire, allowing them to reach a place of ultimate safety outside the building, with or without assistance. While immediate evacuation is often considered the best approach in fire accidents, the complexity and size of modern buildings present challenges to predicting evacuation scenarios. For instance, communicating evacuation strategies to residents in tall residential buildings may be challenging and lead to individual occupants’ evacuation efforts. To review the basics, let’s explore the main strategies.

1. Total Evacuation:

Involves moving all occupants to a place of ultimate safety. Can be either simultaneous or phased, depending on the circumstances.

Simultaneous Evacuation:

It is the most common type of fire evacuation. It is suitable for small businesses or establishments with few floors. In this procedure, everyone inside a building exits calmly at the same time when a fire alarm signal is sounded. It is an immediate evacuation of all occupants within the premises and is commonly used for commercial spaces such as offices, shops, and factories.

To effectively implement this procedure, it is important to have a reliable detection system in place and an alarm system that can quickly and efficiently notify the occupants in the event of a potential fire. In addition, having trained fire wardens and staff who can efficiently coordinate the evacuation process is essential to ensure that everyone is safely and quickly evacuated from the building.

Phased Evacuation:

Phased evacuation is controlled evacuation in stages to avoid overcrowding escape routes. It is used when all occupants need to evacuate instantaneously but the means of escape cannot accommodate the entire population. This method prioritises the escape of people most at risk first, followed by the remaining phases. This strategy involves incorporating fire-resistant materials in a building’s architecture. Careful planning is essential to ensure the strategy remains functional and adaptable to different potential locations of fire.

2. Progressive Evacuation:

Moves inhabitants to a safe location within the premises initially. Divided into Progressive Horizontal Evacuation and Zoned Evacuation.

Progressive Horizontal Evacuation:

Designed for institutions such as care homes and hospitals that have zones protected by fire-resisting construction. It involves moving occupants to adjoining fire sections on the same floor when it is impossible to evacuate the building simultaneously. This is different from phased evacuation, where this is often a result of the limitations posed by the building’s means of escape. It is implemented when it is impractical to move occupants out of the building because of their inability or difficulty. Instead of attempting to evacuate the premises, trained staff will move the occupants to a place of relative safety within the building. Good planning is required to ensure that occupants can be moved to protected areas where the gradual evacuation can continue or until the fire has been dealt with.

Zoned (Partial / Staged) Evacuation:

Clears occupants from the affected area to a nearby controlled zone. For example, in a shopping centre, occupants would be moved to an adjacent smoke control zone while the affected zone is controlled.

3. Two-Stage Evacuation (Delayed / Staff Evacuation)

A silent staff alarm system is used in places where immediate evacuation alarms may cause panic. The purpose of this strategy is to notify staff of a potential fire before the general population is alerted, allowing them to prepare for a stewarded evacuation or investigate the source and determine if the threat is false. This strategy is effective in high-density buildings like sports stadiums, cinemas, and shopping centres. The delay in the alarm prevents panic among occupants and allows for a controlled evacuation in the event of a real threat. To implement this strategy, well-trained staff, a quality automatic detection and alarm system, and a well-managed evacuation process are required.

4. Defend in Place

This strategy involves protecting specific areas in case of a fire incident. It is commonly used in healthcare facilities where patients are physically incapable of evacuating the premises. Such patients are usually dependent on life-support machines, and this strategy allows them to remain in the facility while still receiving the necessary treatment. This approach enables healthcare staff to ensure the safety of these patients while also attending to their medical needs.

5. Stay Put

Stay-put policies are used in residential properties and are popular for purpose-built blocks of flats that have compartmentation. Most purpose-built residential blocks support a stay-put policy, as most occupants may not need to evacuate unless their compartment is affected by fire or if they were not in their compartment at the time of the fire. This policy does not require occupants to conduct a safe and swift evacuation, which can require many resources, but good communication between the Responsible Person and occupants is crucial. For this policy to be effective, the building’s construction and compartmentation are critical, and the internal and external linings of the building must prevent fire spread.

What supports the safety of an evacuation strategy?

First of all, you should pay attention to the architecture of the building and the materials from which it is made. The architectural layout of the floors and the kind of spaces (separated or not) are also important.

For all strategies, it is crucial to keep the escape route (usually the stairwell) as smoke-free as possible for evacuating people and for the fire brigade to identify the source of the fire safely. A solution that consistently protects the stairwell from smoke infiltration is the pressure differential system.

This approach involves pressurising designated areas, such as stairwells, lobbies and elevators. If a door leading into the pressurised area is opened, air will flow out from the protected space instead of allowing smoke to enter. Does this, the most effective way of protecting stairwells, work with simultaneous evacuation, the very popular method of evacuation?

Many uncertainties arise when simultaneous evacuation occurs without lobbies and evacuees opening all doors at once, contrary to the assumptions adopted for stairs protected by pressure systems. According to these assumptions, only a limited number of doors leading to the stairs should open. Let’s examine this scenario.

Doors to the stairwell simultaneously open… What changes in the pressure differential system?

A staircase will be smoke-free if there is a flow of air from the staircase to the adjacent fire floor when the door to the staircase is opened. To achieve this, both air supply to the staircase and air and smoke release from the fire floor are required.

The method of extracting air and smoke from the fire floor significantly impacts the operation of the pressure differential system. Below is a brief reminder of possible ways to remove air and smoke from the floor. The first is mechanical extraction commonly used in tall buildings.

Mechanical or passive air release?

Mechanical release is the best as it enforces airflow on the fire-affected floor. When the door to the stairwell on the fire floor is open, airflow under pressure through the door prevents smoke from entering the stairwell. If the staircase door is closed, the mechanical exhaust system creates negative pressure on the floor, so a compensating air supply should be provided in this case.

The second option is passive air release, which is likely to be employed in lower buildings. It is popularly used, for example, in lower residential buildings. We have two options:

• Gravitational shaft, which must be of appropriate size to remove smoke and heat.
• Facade smoke vents. In this case, the openings must be placed on at least two building facades. It is preferable to place them on opposite walls. The purpose of this solution is to prevent the negative impact of wind on the work of the pressure differential system.

Choosing the appropriate air and smoke extraction method from the floor affected by the fire

In the case of simultaneous evacuation, choosing the appropriate air and smoke extraction method from the fire floor is very important. If doors are simultaneously opened, the pressure differential system must be designed to create a minimum air velocity in the open doors on the fire floor to prevent smoke from entering the protected space.

If a gravitational smoke and heat extraction system is used, some of the air may flow on the floor affected by the fire and some on other floors. This system may not function correctly and allow smoke to flow into the stairwell.

Pressure differential system with mechanical extraction

Ok, so when we consider taller buildings and opt to use a pressure differential system with mechanical extraction, assuming it’s been installed and commissioned properly, how will it behave when people simultaneously open doors on every floor?

Active air release is more effective because the mechanical air exhaust enforces airflow of air at the doors on the fire floor. For more information on research on the pressure differential system, you can find it in a webinar and in the “Fire Safety Matters” article.

Internal layout of the floors and the selection of an appropriate smoke removal system

The internal layout of the floors is also important. The first case is small corridors on floors with closed doors to the room. In this case, shortly after opening the doors to the floor unaffected by fire, a small amount of air will flow into the corridor, equalizing pressures. There will be no airflow at the doors without air release. Consequently, air will continue to flow to the fire-affected floor, where a gravitational or mechanical air release should be present. If there are open spaces on the floors without short, sealed corridors (other than the floor where the fire is located), we may not achieve the required airflow (1 m/s or 2 m/s) at the doors, and some air will flow to other floors.

Closed door – what now?

Suppose the doors to the protected area, such as the stairwell, are closed. In that case, the system will provide a relatively low airflow required to create the necessary pressure (typically around 50 Pa, as in the SMAY Ventiltation Systems headquarters). When doors between the protected and unprotected spaces are opened, the system detects a pressure drop and increases airflow to the protected zone, ensuring adequate airflow through the open doors.

Upon closing the doors, the airflow rate is appropriately reduced. Microprocessor control of the variable-speed fan ensures that the system can achieve over 90% of the new airflow requirement within three seconds of door opening or closing, thereby meeting the requirements of the BS-EN 12101-13 standard.

That is why a pressure differential system must-have features like SMAY’s solution, where a fan with a predictive algorithm based on a neural network is employed. This helps prevent the risk of pressure differential oscillations even with frequent changes in conditions, such as during the simultaneous evacuation of multiple rooms. Otherwise, oscillations could lead to excessive pressure in the doors of evacuation routes, which could hinder safe evacuation and firefighting efforts. The controller is self-adaptive and automatically adjusts to changes in the airflow leakage characteristics in the protected space and air supply routes over time, eliminating the need for manual calibration. It also tests their operation through a self-test every 24 hours.

Learn more about the pressurisation system Safety Way, which already has a predictive algorithm.

For a successful fire evacuation strategy, it is crucial to consider factors such as building construction, fire-resistant materials, communication systems, and staff training. Regular inspections and maintenance are essential to ensure the effectiveness of fire-resisting construction and uphold the integrity of the evacuation plan. The chosen strategy should align with the specific needs and characteristics of the building and its occupants. While supported by reliable fire protection measures, it creates a powerful defence for occupant safety.

While regulations don’t explicitly address this scenario, it has sparked numerous discussions, and we’ve conducted training sessions on this topic (let us know at hello@smay.eu if you’re interested in this training, too).