This is not the case in up-to-date codes and standards as it has long been proven that only large scale fire tests, and not droplet size, will show whether a system can cope with a certain risk or not.
The reason people thought there was a correlation was because the water mist showed remarkable results in machinery enclosures where tiny water droplets could easily evaporate and turn into inert gas (water vapor) due to the heat created by the big fires and extinguish any fires like gas. It is a fast and efficient system.
However, empirical testing evidence shows that a mixture of many different sized droplets can do the same in enclosures and provide even better results in deep-seated Class A fires. For the same reason, water mist systems will always be chosen based on positive test and approval results, and not just droplet size.
How does water mist work?
Most fires are the result of a process of pyrolysis and oxidation.
The pyrolysis process occurs when energy (heat) is applied to the surface of a fuel, as at some point the fuel begins to decompose into flammable gases (eg CH4 = methane).
The oxidation process occurs when more energy (heat) is applied to the atmosphere and flammable gases. At this point, the oxygen molecules found in the atmosphere break down and go along with the flammable gas molecules. This process releases energy and is what we see as fire, flames, and heat.
Water mist is a very effective means of fighting fires, as it can interfere with both the pyrolysis process and the oxidation process.
Fight against the oxidation process
Fire cooling: Small drops of water evaporate easily as they have a large surface area per volume of water. When water evaporates it costs energy and this energy is removed from the fire. If as much energy can be removed as the fire produces, the fire is under control.
Smothering fire: Water vapor is created when water evaporates and the vapor is an inert gas that does not participate in the oxidation process. Therefore, it helps to reduce the amount of oxygen in the vicinity of the fire, since it replaces the «normal» atmosphere. The more inert gas kept close to the fire, the better the performance, and if as much inert gas as normal atmosphere can be kept, the oxygen level will be reduced to 11% and the fire cannot exist as it costs more energy to operate the oxidation process that it produces.
Fight against the pyrolysis process
Putting out the fire: Water droplets are distributed from the nozzles at high speed and this can be used to put out the fire. In fact, what happens is that the energy source is removed from the surface of the fuel, so no additional flammable gases can be created. Doing so allows the fire to suffocate.
Dampen the fire: Part of the distributed water will cover the surface of the fuel and prevent the creation of flammable gases, since the water must evaporate before the surface of the fuel heats up and generates the gases. The more water is distributed over the fuel, the more efficient it becomes.