Total compartmentalization systems have nozzles that are distributed throughout the entire compartment, every nozzle is open, and water will flow through all of the nozzles when an external detection system is activated. This system is intended to fill the entire compartment with a fine mist of water. Local application systems are designed to surround and have their sprays directed towards a specific piece of equipment (i.e. turbine) or potentially hazardous item(s). When activated, this system will suppress a fire only in that one spot in an attempt to protect the surrounding environment. Zoned application systems are designed to protect a specific region within an enclosure. Much like total compartmentation systems, zoned systems will completely fill a particular region with a fine mist while using less water than a total compartmentation system [1 ,3]. Another way to characterize the water mist system is by how the mist is generated: single fluid or twin fluid being the most popular methods. The simplest method (which is used in this study) utilizes only one fluid. The fluid travels through the piping and reaches the nozzle before flowing out of a small orifice in the nozzle and atomizing. The atomization is typically achieved due to the increased dynamic pressure from the ambient air on the fluid jet as a result of the differences in relative velocity between the fluid and the surrounding air. The other method of atomization is by having the fluid impact an obstacle at high velocity. In twin fluid systems, one fluid is the water and the other is a compressed gas (i.e. air, nitrogen, etc.). Inside the nozzle, the compressed gas is positioned in such a way that its spray will flow into the water and cause it to scatter. These twin fluid systems are more complex, more expensive, and require twice as much piping [3]. A third and final way to describe a water mist system is by the level of pressure that the system operates: low pressure (below 12 bar), intermediate (12-34 bar) and high pressure systems (34 bar and above). As the pressure of the system increases, the components need to
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exceed the operating pressures, and the methods of providing the needed pressure are different. At lower pressure, centrifugal pumps (like those used by traditional sprinkler systems) are capable of providing the needed pressures for the system. At high pressures, the water can either be supplied by gas driven or pump driven systems. Gas driven system consist of a standalone water tank in a metal cylinder connected to a high pressure gas. When the system is activated, the high pressure gas is fed into the water tank and pushes the water through the piping. The typical operating pressures for this type of system starts around 140 bar and decrease to 30-50 bar. The high pressure pump systems typically contain multiple positive displacement pumps that work in tandem, depending on the number of nozzles that activate, to provide the needed pressure and water flow [3]. Understanding the downstream components of a water mist system is just as important as the pressure generating end. The nozzles used for these systems are as unique as the companies that make them. Their design depends on the operating pressure, the desired application, and the method used to generate the mist (single or twin fluid). Single fluid nozzles can atomize a liquid into a fan spray, hollow cone, full/solid cone, or even a square spray. These sprays are created within a nozzle by a simple orifice, a pre-swirl chamber, a rotating element that spins the fluid, or directionalizing grooves just to name a few. Twin fluid nozzles have an even more complex interior to atomize a fluid than single fluid atomizers. Their interior design consists of two separate channels that converge at the tip in order for the two fluids to collide [11]