The major difficulties with water mist systems are those associated with design and engineering. These problems arise from the need to generate, distribute, and maintain an adequate concentration of properly sized drops throughout a compartment while gravity and agent deposition losses on surfaces deplete or reduce the concentration.
There is no current theoretical basis for predicting optimal drop size and velocity distribution, spray momentum, distribution pattern, and other important water mist system parameters. This is of course analogous to the lack of a theoretical basis for nozzle design for total flooding gaseous systems, and/or even conventional sprinkler and water spray systems.
Extensive experimental and theoretical work aimed at predicting critical adiabatic flame temperatures appears to indicate a range between 1600 to 1900 K, depending on the fuel. According to Holmstedt,19 there are two possible methods by which water spray may extinguish a fire: by extinction of the flame or by cooling of the fuel. Holmstedt states that fuel cooling is performed by larger drop sizes and hence is only relevant to suppression of solid combustible fires. It is possible, however, to use the momentum of large droplets (> 200 µm) to drag or entrain smaller droplets, thus providing a mechanism for mixing and distribution.
The potential is present for water mist to act as a true flooding agent if the mass median drop size is below 20 microns. At this level, its suppressing efficiency is twice that of halon 1301 per unit weight. Before this becomes possible, methods of controlling the droplet transport must be developed. The amount of water required to lower the flame temperature to the range of limiting values is between 0.15 to 0.25 L/m3 (1.0 to 1.8 gal/1000 ft3). The actual concentration required may be less than this due to the oversimplification discussed previously.
The predominant variables contributing to the production of this concentration are drop size and flow rate. Drop size plays an important role in estimation of the required flow rate as well as in the production of a critical concentration of drops. Drops under 50 microns begin to exhibit characteristics of a gas in the increase in fall time and decrease in terminal velocity. Conversely, larger drops fall faster, resulting in greater fallout losses. Water flux densities (flow rate per unit area) vary significantly across experimental test programs, ranging from 1.5 Lpm/m2 to as high as 10 Lpm/m2.20,21 The significantly higher water flux densities recommended by Gameiro and Mawhinney may be a function of inefficient production of critical concentration (i.e., greater losses due to a larger drop size, poor mixing, and so on).
The primary loss mechanism, plate loss caused by gravity and spray impact on walls and obstructions, presents a very difficult technical challenge. From a design standpoint, the loss is overcome by using larger water flow rates and continuous discharge of water. In this sense, current water mist systems greatly exceed the theoretical minimum water concentrations described previously.
Vent loss rates are a function of the size of the vent, the size of the fire (which drives the flow through the vent), any other pressure induced across a compartment boundary, and the concentration of drops in the compartment. Evaporation losses are significantly more difficult to calculate or estimate. The evaporation of a drop is a function of drop size, initial temperature, velocity with respect to the surrounding gas, gas temperature, and so on. It is worth noting, assuming all things are constant, that the life of a drop is usually proportional to the square of its diameter.
During the past decade or so, there has been an expansion of the science and technology of the transportation of bulk liquids into fine sprays (atomization). The primary contributors to this technology have been the combustion industry (fuel spray atomization), the chemical industry (spray drying), and the power industry (evaporative cooling). Significant information relevant to water mist applications for fire suppression can be extracted from this knowledge base.