Atomized sprays may be produced in various ways. Basically, all that is needed is a high relative velocity between the liquid to be atomized and the surrounding air. Some atomizers accomplish this by discharging the liquid at high velocity into a relatively slow-moving stream of air. Notable examples include the various forms of pressure atomizers. An alternative approach is to expose the relatively slow-moving liquid to a high-velocity airstream. The latter method is generally known as dual-fluid, air-assist, or air-blast atomization.
Two atomization technologies are incorporated in water mist suppression systems under development and/or consideration: single- and dual-fluid systems. Single-fluid systems (pressure atomizers) utilize water stored or pumped at high pressure (40 to 200 bar) and spray nozzles with relatively small orifice sizes. Dual-fluid systems use air, nitrogen, or other gases to atomize water at a nozzle. Both types of systems have been shown to be effective fire suppression systems.
The U.S. Navy has developed a machinery space water mist system that utilizes a modified high-pressure spray nozzle. The nozzle design is described in several Naval Research Laboratory (NRL) reports.22,23,24 The basic design approach is to produce high volumes of 100-µm droplet (mean diameter) sprays with very high spray momentum to achieve rapid suppression of large hydrocarbon pool or spray fires. These nozzles emit 2 gpm at 1000 psi and are spaced approximately 8 ft apart on a uniform grid mounted in the overhead and at the intervening deck level in the machinery space. This system has been tested extensively on the ex-USS Shadwell, the NRL fire test vessel in Mobile Bay, Alabama. It is capable of suppressing fires in seconds. The Navy's water mist system is not particularly effective on highly obstructed small fires, although it provides substantial cooling and limits the fire size, thereby enabling relatively safe manual fire fighting.
Compared to some commercially available technology, the Navy system uses relatively high water flow application rates (approximately 0.06 to 0.07 gpm/ft2, which is on the order of three to four times the rate of the best available systems). The relatively high water flow rate requires significant pumping and electrical power capacity. For the LPD-17's largest machinery space, a 250-hp motor is required for a 200-gpm pump.
On new-design naval vessels, electrical power and water supply are not particularly difficult constraints, and so the relative efficiency of the system is not an issue. For any retrofit application, however, the current Navy design would be problematic, and the water flow rates of this system would make the use of stored pressure cylinders (vs. pumps) quite difficult. This would substantially limit the application of this system for small individual compartments such as flammable liquid storage rooms.
One important component of the Navy system is that it was designed, developed, and tested under significant time constraints and is scheduled for installation on the LPD-17. Optimization of the system or evaluation of alternative designs can and should be pursued if additional applications, particularly retrofit or protection of small enclosures, are envisioned.
At least 11 water mist system technologies are currently available or under development using either dual-fluid (N2/air and water) or single-fluid high-pressure systems. Table D.1 summarizes the commercially available water mist systems that can be used to protect against flammable and combustible liquid hazards. While the performance of these systems varies widely, development of this technology has just begun, and improvements in the effectiveness and efficiency of water mist systems can be expected.