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Sprinkler Vs Water Mist

Fire Supression System

Sprinkler / Water Mist Performance Characteristics Comparison Chart

  Item 1 Item 2 Item 3 Item 4 Item 5 Item 6 Item 7 Item 8 Item 9 Item 10 Item 11 Item 12 Item 13 Item 14 Item 15 Item 16
SYSTEM TYPE Reactive surface area per 1lt Water Visibility during operation Operating temperature of auto nozzles Operating sensitivity / speed (R.T.I.) Droplet size Microns Droplet Velocity Installaton height restriction Void coverage Water usage average per head LPM Run time from storage water Dual systems life safety support Head spacing up to 5M System design objective Smoke control (Scrubbing) Use on various types of fire i.e. liquid fat Assumed maximum area of operation
Life Safety Sprinklers
BS EN 12845
2m² Good 57-68ºc Fast response 3mm bulbs
50
1000 Gravity No Yes 80-120 LPM 60mins
30mins for School TB 221
Yes 3-4m Fire Control Poor No As Standard
Residential & Domestic
Sprinklers BS9251
2m² Good 57-68ºc Fast response 3mm bulbs
50
1000 Gravity Yes 22m Yes 60 LPM 10mins domestic
30mins residential
No
Single pump
3-4m Fire Control Poor No As Standard
Low Pressure Water
Mist System
2500m² Good 57-68ºc Extra Fast response 2.5mm bulbs
18.9
70-120 10 m/s No Yes 8-22 LPM 10mins Domestic
30mins Residential
60mins Commerical
90mins High Hazard
Yes 3m Fire suppression extinguishment Good Yes As per BSEN1284 or BS9251 dependant on risk
High Pressure
Fog Systems
3000-6000m² Poor 57-68ºc Extra Fast response 2.5mm bulbs
18.9
20 >20 m/s No ? 8 LPM 30 Mins Yes 2-3m Fire suppression extinguishment Very Good Yes ?

Comments/Explanation of Items 1 to 16

  • Item 1. Reactive Surface Area Per litre of Water
    The reactive surface area is the total surface area of the water droplets released from the operating nozzle. It should be noted the greater the surface area the more energy is absorbed from the fire thus suppressing the fire, as the energy is absorbed from the fire into the water droplets, the droplets reach a point at which they become saturated with heat and flash to steam at an expansion ratio of 1:1640, steam is an inhert gas and suffocates the atmosphere directly around the fire plume again effecting suppression.

  • Item 2. Visibility during operation
    It should be noted that Orange Fire Protection Group low pressure water mist system affords the same visibility in a fire situation to that of sprinklers, and should not be confused with high pressure fogging systems which substantially reduce visibility.

  • Item 3. Operating Temp of automatic nozzle
    The operating temperature of automatic nozzles are to be set at no less that 30⁰C above the highest anticipated ambient temperature.

  • Item 4. Operating Sensitivity of nozzles
    As can be seen from the above chart Orange Fire Protection Group low pressure water mist nozzles are equipped with thermal sensitive bulbs (glass phial) which have a Relative Time Index (R.T.I.) of 18.9. This enables the nozzles to detect and actuate in a fire situation up to 2.5 times faster than a life safety sprinkler head which has an RTI of 50.

  • Item 5. Droplet size
    Self Explanatory, however this is directly linked to the reactive surface area of each system. Halving the size of the droplet increases the surface area by a factor of 8, it should also be noted that due to the smaller droplet sizes, more droplets are generated which in turn prevent/block attenuated heat from the fire source preventing the ignition of other combustible sources.

  • Item 6. Droplet velocity
    Droplets released from nozzles in the Orange Fire Protection Group systems are not obstructed by a deflector plate, that is evident in sprinklers thus water mist fired under velocity at 10 m/s to the fire plume.

  • Item 7. Installation height restriction
    It is considered that high level areas cannot be protected effectively due to the heights involved, while this is true for high pressure systems it is not true for low pressure systems. The un-obstructed nozzles (i.e. No deflector) release water droplets at a rate of 10 m/s and with their larger mass (compared to high pressure droplets) effectively deliver suppression to low level fires.

  • Item 8. Void Coverage
    Self explanatory, The low pressure water mist system provides nozzles that expel water mist droplets in a 'flat oval' spray pattern, i.e. Water is distributed horizontally though the void rather than vertically like a sprinkler system, applying water on and in front of the fire in all directions simultaneously.

  • Item 9.Water usage per head
    As can be seen from the above chart, water mist systems use considerably less water than that of sprinkler systems, in applying water in a more efficient manner (Item 1) less water is used but a greater level of suppression is achieved. i.e. Not how much water is applied but how you apply it.

  • Item 10. Run Time from water storage
    It has been perceived that water mist systems only operate for 3 or 4 minutes which is not true, carrying out the system design in accordance with the hazard classifications of the prevailing sprinkler standards, Orange Fire Protection Group low pressure water mist system will afford fire protection to with 10, 30, 60 or even 90 minutes. All Orange Fire Protection Group systems are fully hydraulically calculated utilising industry standard software via the Darcy Weisbach method to ensure the AMAO (Assumed Maximum Area of Operation) is satisfied for the prescribed period i.e. 216m² for 60 minutes under OHIII protection.

  • Item 11. Dual Systems life safety support
    All Orange Fire Protection Group low pressure water pumped systems are provided with a minimum of 2 pumps (duty & standby), this is above and beyond that of some sprinkler systems i.e. BS 9251 (which is advocated as a life safety system however allowed to operate with only a duty pump) and commercial sprinkler system to schools, which under Technical bulletin 221 allow for only one pump and a 50% reduction in the water storage tank bringing the minimum penetrating time from 60 minutes to only 30 minutes.

  • Item 12. Head spacing up to 5m high
    Self Explanatory.

  • Item 13. System design objective
    It is commonly assumed that sprinkler systems are designed to extinguish fires, sprinkler systems are deigned to control a fire and hold it to its seat of ignition. Water mist systems are designed to at worst suppress and at best to extinguish.

  • Item 14. Smoke control (Scrubbing)
    Various studies have shown that water mist systems do provide a degree of smoke scrubbing, a cleansing/whitening of the smoke., whilst sprinklers are known to push the smoke layer to low level.

  • Item 15. Use on various types of fires
    Water mist systems are effective at suppressing oil/fuel fires. The droplets sizes are small enough that they do not penetrate the surface of the oil and sit on the surface absorbing energy from the fire before flashing to steam creating an inhert atmosphere around the fore plume. Sprinkler droplets are heavy enough to penetrate the surface of the oil and remain a liquid whilst immersed in the oil, all the time absorbing energy, when the water becomes saturated with energy the water flashes to steam inside the oil, the rapid expansion inside the oil causes expansion of the fuel and of course the fire plume.

  • Item 16. Assumed Maximum Area of Operation
    Self Explanatory.

Sprinkler vs water mist