By Günter Knopf. In fire prevention planning, active fire prevention is becoming increasingly important. In the past few years, analyses of fire events showed that smoke is the main cause of death. In particular in the case of special structures, with an application for planning permission evidence, amongst other things, it must therefore be provided that, in the case of a fire, escape and emergency routes remain low-smoke areas over a sufficiently long period of time. A secure discharge or absorption of smoke gas or heat to safeguard escape routes is therefore of key importance in fire-protection strategies.
By extinguishing with water mist low pressure fire-extinguishing systems (WLF method), up to 97% of the smoke particles can be absorbed in the case of smoke penetration in escape and emergency routes. Up to 75% of the toxic gases are washed out or neutralized by the resulting water mist. At the same time, the temperature in the extinguishing area is reduced to below 40 °C which means that the escape route can be passed safely. The persons escaping may be informed via a public address system as follows: “Please leave the property via the stairs or escape passageway clouded with water.”
Classification in water extinguishing methods
In the extinguishing method by means of WLF, water droplets with a diameter of 0.05 to 0.4 mm are produced, sprayed into a room to be extinguished or protected and applied to the surface of the fire particles. Water consumption is thereby approx. 85% lower than in conventional sprinkler systems.
With regard to the size of the droplets, water extinguishing methods are classified into the following groups:
- Large droplets: > 1 mm
- Mean droplet diameter: approx. 1 mm (sprinkler/spray jet)
- Fine spray jet: approx. 0.5 mm
- Smokescreen (spray mist): 0.05 to 0.4 mm
- Water aerosol: < 0.05 mm
- Water vapour (saturated steam; dry steam).
The WLF method is a smokescreen. Unlike sprinkler systems with large drops, the smokescreen produces a very large drop surface which significantly increases the cooling effect. With these drop sizes, the smothering effect of the water, which is also strengthened by the decreasing diameter of the drops, has a particular effect.
Protection objectives and extinguishing effects
The effectiveness of the WLF method may be demonstrated especially well by the protection objectives achievable:
- absorption of radiant heat
- suppression of the fire or limiting the spread of fire
- absorption or discharge of the smoke
- neutralizing of smoke particles
- fire extinguishing
- protection of buildings and installations against effects or impacts of fire.
Several protection objectives may thereby be achieved at the same time with a different proportion or different effect. The following extinguishing effects are achieved when using the WLF method:
- cooling effect due to evaporation of the water in the reaction zone and at the interface between the flame pillar and combustion gas flow
- formation of local inertization (oxygen displacement) at the source of the fire due to vaporization using an appropriate particle density of water drops
- dilution of the reaction zone due to vaporization
- absorption of up to 97% of the smoke particles
- washing out of toxic gases by up to 75%
- heterogeneous inhibition (A wall effect is produced in the mixing zone of the flame due to an extinguishing agent jet and energy is withdrawn from the flame.)
- prevention of radiant heat recovery due to deposition of the water drops (depositing of particles) in the combustion zone or by achieving the release effect (formation of a separation layer between the fuel and source of the fire).
The benefits of the method are thereby: a very low water demand, minimal water damage, the lack of electrical conductivity, large smoke particle formation, smoke gas retention and reducing the temperature to below 40 °C.
Configuration of the systems
All these properties are used in different system configurations. Very different WLF methods are used depending on the fire hazard or local circumstances. They are separated into two main categories:
- category A: sprinkler fine mist low pressure fire-extinguishing system (SLF)
- category B: fine mist low pressure fire-extinguishing system (FLF)
Category A is a viable alternative to the sprinkler system in use for 100 years. Along with this, the SLF is triggered thermally via a glass bulb or fusible link. The escaping suspended mist is spread evenly in the protection area or on the protected property. With respect to conventional sprinkler systems, the need for the provision and retention of extinguishing water is significantly reduced. Water damage is contained as a result. Added to this is the fact that with the smokescreen not only does the cooling effect have an impact, but primarily the smothering effect. So, a fire can be extinguished in a matter of seconds. An SLF system may be used in most areas which had until now been protected by conventional sprinkler systems, such as office blocks, shopping centres, underground car parks, high-rise buildings and historic buildings.
In addition to the aforementioned applications of the SLF extinguishing system, category B fine mist low pressure fire-extinguishing systems (FLF) are primarily used to form a smoke section at such openings, which – for example for visual reasons – cannot be closed otherwise in terms of fire protection (see figure 1).
The WLF extinguishing method should be taken into consideration when selecting firefighting systems, if smoke sections have to be separated from each other, despite available openings, which, for example, cannot be closed using conventional smoke control barriers due to a preservation order. The method is also suitable for safeguarding escape routes in much frequented buildings or an individual emergency staircase as a compensatory measure for the lack of a second emergency staircase, for example in multi-storey open historic stairwells (see figure 2).
The system nozzles may be neatly integrated into walls or ceilings. Covered by identically coloured dust caps, they can hardly be visually distinguished from the background. All requirements with regard to escape and emergency routes may be met without the visual appearance of the room or building being altered.
The smokescreen due to the WLF method not only causes the cooling of the flame zone, but also reduces the fire reaction by diluting the oxygen percentage in the air. Heat dissipation increases at the same time and recovery of radiant heat is inhibited. The limits of this extinguishing method, which overridingly uses the smothering effect and volume and flame extinguishing, are determined by the fuel, fire heat flow, droplet size and room volume.
These parameters must be clearly defined and meticulously arranged precisely in order to achieve quick and effective fire and smoke fighting. Escape and emergency routes in particular may then be kept free of heat and toxic smoke gases and, for example, the historic character of a building or stairwell is preserved.
Dipl.-Wirt.-Ing. (FH) Günter Knopf: Fire safety expert for innovative fire-protection system solutions and special extinguishing systems; owner of an engineering firm for active fire prevention in Berlin; guest speaker at various professional events.