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Domestic & Residential Mist Systems
Disadvantages of a Domestic Water Mist fire system
The draft document on water mist fire systems produced by the British Research Establishment (BRE) has been welcomed by the fire sprinkler industry because it has produced specific test criteria which water mist suppression systems and components can be measured against and it will be interesting to see how many of the components available at the moment will perform in these tests.
- They are more expensive than conventional systems
- They do not perform as well in open plan areas or large areas generally where ‘drift’ has proved to be a major disadvantage.
- Water mist suppression systems do not perform as well in areas with high ceilings
- A mist head is more visually intrusive since no mist heads have been developed with a flat cover plate unlike conventional sprinkler systems
- More outlets are generally required for a given floor area making them even more intrusive a particular consideration in domestic situations.
- Individual components cannot be used on an ad-hoc basis as conventional domestic sprinkler systems can be and this increases costs still further.
What is a Water Mist fire system?
It has been known for a long time that water droplets evaporate when applied to a fire and the resulting steam reduces the concentration of oxygen close to the fire which in turn interrupts the chain reaction necessary for a fire to establish itself.
Water Mist suppression systems improve on this basic process by producing a finer spray (or mist) than a standard fire sprinkler nozzle. A fine mist is a very effective way of ensuring that water evaporates quicker thus slowing down the combustion process more effectively. In order to achieve a finer spray, mist systems operate at a higher pressure than standard sprinkler systems and in certain situations can be as effective as normal domestic fire sprinkler systems despite the reduced quantity of water used. They are broadly classified as low and high pressure.
Domestic Water Mist fire extinguishing systems at the present time
Although a draft document has been recently produced, there still is not a British Standard for domestic Water Mist suppression systems. The reason for this delay is partly due to the fact that water mist systems are very ‘project specific’ which makes it difficult to produce a broad set of guidelines for their use. These systems have been developed comparatively recently and, unlike conventional fire sprinkler systems they do not have the same proof of performance or legacy of 100 years of test data in an extensive range of fire hazards to refer to.
Although conventional sprinkler systems use marginally more water than water mist systems, they are however, much more adaptable to a range of different design criteria. A mist system has to be far more specific to the environment that it has been designed for. This makes it hard to install a complete system using individual components from different manufacturers increasing the costs considerably.
The comparatively short history of Water Mist Fire Suppression Systems
Water mist fire suppression systems were originally developed as an alternative to Halon based fire suppression systems for storage areas and machinery enclosures in ships.
Such systems performed very well in specific situations where there were no design variables like opening windows or fire loadings and where doors would generally be kept closed etc. These systems have subsequently been utilised in land based commercial situations like engine rooms, local asset management of electronic equipment, deep fat fryers, turbine systems etc.
Watermist fire suppression systems were then considered for domestic and residential use and a number of low pressure Residential and Domestic mist systems have been installed throughout the UK using components that have no British Standard associated with them. However, far more normal fire sprinkler systems have been installed in the same period using components which have been subjected to extremely rigorous testing procedures over many years in order to prove their functional suitability.
BRE (The British Research Establishment) Global conducted 48 fire tests, with low pressure and high pressure water mist systems in commercial situations. The commercial systems were provided by industry partners.
Here is an excerpt of the conclusions reached:
Overall, the full scale test results were of concern. Most water mist system arrangements were not able to provide expected levels of fire protection for the tested scenario (open plan office areas with a high ceiling). Or, in terms of the design of the tested systems, the spacing between nozzles was too great and the quantity of water discharged too low, to provide effective fire suppression.
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