Spray nozzles for fire extinguishing systems

Engineering consideration 1 - Misting or drenching system?

Drenching or sprinklers

These systems simply deliver droplets of water (or other flame retardant) to the fire in order to quench it.  Large volumes of fluid are sprayed and the collection of fluid on the floor is not a primary concern.  These systems are only suitable if the assets being protected are water resistant. 

The NFPA (National Fire Protection Agency) recommend that a drenching system will need to deliver a minimum of 10 litres per minute per square metre in order to be effective.  This rule assumes only 'normal' combustible materials will be present.  Clearly if highly volatile materials are fuelling the fire then more liquid will be required to suppress it.

These types of systems will require the nozzle to deliver relatively large volumes of fluid over the area in question.  Precise, even coverage is not a primary concern and whilst having relatively small droplets helps in firefighting, it is again not of paramount importance to have very fine droplets.  Generally this means that spiral design nozzles are ideal for such applications. 

Mist systems (Halon replacement)

These systems produce a very fine mist of water that is evaporated by the heat.  The water vapour displaces oxygen in the air and so extinguishes the fire.  As the fluid completely evaporates these systems do not cause wetting or moisture to collect.  This means that they can be used to extinguish fires in areas containing electronic equipment and computers.

Water mist systems are rapidly replacing the previous generation of halon gas fire extinguishing systems.  Using finely atomised water to 'asphyxiate'  the fire has a number of environmental, cost and human safety advantages over using halon gas.

In order for misting fire suppression systems to work the droplets within the spray must be kept very small.  If larger droplets are present then some of the spray may not evaporate before it reaches the assets being protected, resulting in water damage.  Generally speaking misting spray pattern nozzles such as the P or Microwhirl are used.  Whilst air atomising nozzles produce even finer droplets their maximum flow rate is not sufficient for firefighting applications and so they are not an appropriate choice.