Water mist protection system. - Creative Commons

Water mist protection system.

Creative Commons

There are several fire protection systems that firefighters will more commonly encounter. This article discusses some of the basics and what to expect when responding to buildings or equipment protected by these systems.

Water Mist/Hybrid

Water mist systems are nothing new. It is a well-established fact that water mist absorbs heat better than other forms of water application. However, the mist must get into the fire or throughout the entire volume of the protected space.

In large spaces, like large aircraft hangars, that has been problematic. New designs intended to get the mist into the fire are always being investigated. A recent example is using pop-up nozzles in hangars to get the mist to the floor, where the fuel fire exists.

This will not be the same as the current U.S. Navy design which applies AFFF through floor grate nozzles. The fire might get extinguished by the water mist but exposed fuel will needs to be managed.

Another design uses a combination of very fine mist and a gas like nitrogen. A new NFPA standard, NFPA 770, will address these systems.

Water mist systems don’t necessarily have to be designed to extinguish the fire. NFPA 750, Water Mist Fire Protection Systems, allows designs to accomplish the objectives of control, suppression or extinguishment. The first two modes are also associated with traditional sprinkler systems.

The fire service should know what these objectives are so that they can react appropriately to the situation they find.


Aerosol systems are finely divided dry chemical agents. A comparison of sprinklers and water mist might be made to traditional dry chemical systems and aerosol systems.

Condensed aerosol generators have been compared to rocket engines in that the contents of the canister are actually ignited and the aerosol is discharged in the exhaust. The generator housing itself can get quite hot. This type of system obviously cannot be used where it could trigger an explosion.

These systems are being used as a replacement for traditional gaseous systems and are covered by NFPA 2010, Aerosol Fire-Extinguishing Systems.

Oxygen Reduction

These systems are an entirely new form of protection for structures. They are somewhat analogous to inerted spaces in some chemical processes, except that the atmosphere is breathable but still will not support the combustion of ordinary materials.

In the industrial setting, they will most likely be found in warehouses with high water damage susceptibility or where sprinklers are difficult to install or maintain such as freezer warehouses. Data centers are another application.

Freezer warehouses can be protected by specialized sprinkler systems that prevent freezing of the system, but all of them come with specialized design and maintenance problems. At any given time, inspectors find a high percentage of sprinklers taken out of service in freezer warehouses because of such maintenance difficulties.

Oxygen reduction systems remove oxygen from a space by generating nitrogen and then using the nitrogen to keep oxygen levels too low to support combustion but high enough so that employees can breathe. Many countries, including the United States, limit employee exposure to this atmosphere and may even require employees to wear SCBA when they enter the space.

A common misconception is that the nitrogen is introduced at the time of a fire. Instead, the space is always maintained as a low oxygen environment so that a fire cannot start in the first place.

There are a limited number of credible design and installation contractors. A credible installation will have a professional look. There is no NFPA standard as of yet but there is a German standard that is generally accepted.

A key design issue is maintaining the oxygen level in the narrow range that allows people to breathe but prevents combustion. This requires sophisticated instrumentation, makeup nitrogen, and control of building openings. Another operational concern is that product stored in the warehouse while reducing the oxygen concentration is unprotected.

We have not heard of any fire service operational experiences in these warehouses. If the system is operating as designed, there should not be a fire.

Because the design basis is strict control of the oxygen concentration, traditional fire service ventilation techniques cannot be used. If there is a fire that requires fire service ventilation, the system has already failed and complete loss of control is likely.


New agents and new methods of delivering them have been with the fire service for more than 100 years. These are just a few of the most recent iterations. We welcome your questions, comments, and experiences with them.
Editor's note: Condensed systems have been the most common at sites we have visited. Dispersed systems is another type and is described in NFPA 2010.

John Frank is Senior Vice President of the XL Catlin’s Property Risk Engineering / GAPS  Loss Prevention Center of Excellence, where he is involved in loss prevention research and loss prevention training.  XL Catlin’s Property Risk Engineering / GAPS team provides property loss prevention consulting and delivers individually tailored solutions to protect and enhance property, production, and profit. With 220 engineers and consultants in 18 countries, the team brings clients occupancy specific experience as well as deep knowledge of specific hazards across industries.