Spray nozzles for fire extinguishing systems

Engineering consideration 4 - Droplet size and variation

As a general rule the smaller the droplet size the greater the cooling power.  This is because as the diameter of droplets fall the overall surface area of the spray increases, meaning that evaporation/vaporisation occurs faster.  But it is also true that the smaller the droplet size the lower the momentum of the spray is.  This means that very fine sprays are more easily disrupted by air currents or wind.

Overcoming thermal currents

Given that fire will cause strong thermal air currents it is immediately obvious that smaller droplet sizes, whilst giving theoretically better cooling, would not necessarily work best in practice.  For this reason in many firefighting environments a spray with fine droplets and some larger droplets is required.  The larger droplets in such sprays actually help to carry along the finer drops enabling them to reach areas they would not otherwise have been able to reach.  Nozzles of the spiral type design have exactly these spray features and so are often an excellent choice for fire extinguishing applications. 

Misting systems

For water mist systems complete evaporation is required so relatively low flow rates of very fine droplets need to be produced.  Above a certain flow rate and droplet size not all the water will be evaporated and this may not be desirable.  This means there is a natural limit to the level of fire that can be contained by such systems.  Additionally windy environments, which would disrupt a fine mist, preclude using such types of fire fighting system.