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How Water Mist Systems Work

Lee Kaiser posted this on January 15, 2018

Water mist systems work a little bit differently than other fire suppression systems. The small droplet size creates a greater surface area to absorb the heat of the fire, the mist blocks radiant heat transfer to unburned fuel, and it causes local inerting, all while using 50 to 90 percent less water than sprinkler systems. In this video, fire protection expert Lee Kaiser explains how water mist systems work and in which situations they are most effective.


VIDEO TRAnSCRIPT

Lee: "So how does water mist work? It works in three ways to extinguish a fire. So first off, the small water droplets offer up an extensive cooling surface area so that we can absorb heat from the fire. The dense emission of the mist or the fog offers up radiant heat blocking, so some measure where we've got burning fuels and the mist being discharged, and that mist blocks the radiant heat transfer to unburned fuel, so it kind of acts like a wall to stop the spread of the fire, and then the last thing that happens is local inerting, so when we say that word inerting, you should think, "Hey, they're lowering the oxygen concentration," except that that's different with water mist. It's done just locally around burning particles of combustion.

When the mist droplet absorbs enough heat for it to convert its phase from liquid water to steam, it expands. Firefighters will know that number, about 1,700 times its original volume, so the water molecules expand as steam and then exclude oxygen molecules from the burning particulate, and so people smarter than me, scientists have figured out that water mist tends to work in those three ways in combination to extinguish fires.

An Alternative to Sprinkler Systems

So for building sprinkler alternative systems, we're seeing more buildings install water mist in lieu of sprinklers, and there are more manufacturers seeing that this is a market for their product, so they're developing applications and listings and approvals for their equipment. Some advantages of that system is that it can meet the local building code requirements as an alternative to sprinklers.

Water-Efficient Fire Suppression

We have available the most common hazard classifications, light hazard sprinkler areas, ordinary hazard sprinkler areas, and then water mist systems, here's the big takeaway, use 50 to 90 percent less water than traditional sprinkler systems, so the big advantage to pay the upcharge, because they're more expensive than a regular sprinkler system, but it uses less water, so you have therefore less collateral damage for the room that's on fire due to water runoff from the application of the sprinklers into other areas. So really something to be thought about when you have multi-story buildings with technology or electronics on multiple floors and a fires starts on an elevated floor or whatever's valuable with a fire on the elevated floor, and you don't want that water to run off down to lower levels, and also, smaller water supplies are used for this because we are more efficient with our water usage.

Common Applications

Typical applications, we see them going into some data centers out there, museums and theaters. Some historic homes have chosen water mist systems to have less water damage, and then large office buildings, so buildings companies are building that are high technology users. Some internet companies out there that are building new buildings want to use everything that's cutting edge and new technology. They're looking at water mist instead of their regular sprinkler systems that they could be installing.

Where we see most of the use for water mist systems is where there's Class B fires, so back here under building sprinklers, this is a fire control technique. Okay? As we talk about performance levels. Now, back for machinery spaces, this is a fire extinguishing technique. So we expect fire extinguishment with this. For Class B fires, you can use water mist systems when you have pool, spray, or vertical surface fires of flammable and combustible liquids, and it can work when you have fires in different combination modes.

Effective to Use Against Spray Fires

So what's a spray fire? When you have a piece of equipment running that is pumping or moving a flammable or combustible liquid at pressure that'll go through hoses, it'll go through seals. If one of those things fails and you get a leak, because it's pressurized, a lot of times you'll get a spray. If that spray of flammable liquid finds a surface hot enough to ignite it, then all of a sudden you'll have a spray fire, and those fires are really hard for a lot of the different systems out there to extinguish, but water mist systems do really well on that type of fire, and so we see them used where we've got machinery that pumps a flammable or combustible liquid."

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