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A Cool Mist

In order to burn, fire requires three principal components: fuel, oxygen and heat. Eliminate any one of these elements, and the life potential of a fire is significantly reduced. This is the principle behind the water mist or fog fire suppression system. Combining the extinguishing characteristics of water with the penetrative qualities of gas, the mist system eliminates two of the three critical factors: oxygen and heat.

While the technology for water misting has been in development in Europe for over 50 years, it is rapidly emerging in North America as a viable alternative in relatively conventional occupancies. Once limited to flood-sensitive applications including the rail and shipping industries and computer rooms, uses now include hotels, historic buildings, archival museums, and, as shown in the case study provided–the system’s first Canadian application-health care.

System overview

The physical operation is quite simple. High velocity pumps–up to 1,000 pounds per square inch (psi)–propel water through a series of specialized misting heads to create a cool, fog-like vapour. Due to their microscopic size (between 80 and 100 microns), these atomized droplets remain suspended and mimic the flooding characteristics of gas, penetrating shielded areas and freely moving around obstructions.

As water is the backbone of any suppression strategy, surface wetting remains the primary mechanism for putting out the fire. However, fire is also extinguished by other means: cooling, oxygen displacement, and radiant heat blocking.

Cooling: the aerosol mist provides a greater surface area per unit volume of water mass and thereby increases the rate of heat transfer as the droplets convert to steam. Suppression is achieved when sufficient heat is extracted and the surrounding area cooled. This heat reduction and vaporisation rate occurs 400 times faster than traditional deluge systems even though they use up to 20 times more water.

Oxygen displacement: the extinguishing effect occurs when very small droplets take in enough energy to turn into steam. Expanding over 1,700 times, the steam drives oxygen from the fire zone.

Radiant heat blocking: a curtain of cooled water mist is an effective means for reducing radiant heat transfer, thus reducing the opportunity for the fire to spread to unignited surfaces.

While the principle behind misting is essentially constant across various manufacturers, a technology expansion has led to the development of different types of nozzle hardware. These include: impingers (the projection of a stream of water on a diffuser); pressure jets (the release of a pressurized water stream from an orifice); and atomizers (the combined use of water with compressed air or nitrogen supplied at 25-100 psi to shear it into a fine mist).

The system is fully self-contained and fed by half-inch steel pipes from filtered, pressurized tanks. While municipal water is acceptable, distilled water is preferred, and can be held on-site in storage tanks. In order to achieve the necessary droplet velocity, a diesel or electric pump is required. When compressed air or nitrogen is used, the gas serves as the propellant, eliminating the need for a pump.

Ultimately, the design specifications and overall efficacy of the system is based on the ability of the nozzles to generate droplets smaller than 1,000 microns and then adequately distribute a critical concentration with enough momentum throughout the protected area. Factors affecting this include the physical characteristics of the protected area, droplet size and velocity, mist head location (spacing varies but coverage is approximately 120-150 ft2 per head), and geometry of the spray pattern. Gravity, spray impact on walls and obstructions, and evaporation must also be considered.

Pros and cons

Why has misting generated so much attention, and what separates it from conventional sprinkler systems? From a “green” perspective, it offers an environmentally friendly alternative to ozone depleting chemical agents, including Halon 1301, banned by the Montreal Protocol in 1992. In addition to the manufacturers’ claims of superior fire suppression capability, other advantages include: significantly lower water usage reduces equipment and building damage; smoke-scrubbing and absorption qualities; the prevention of re-ignition due to cooling effect and room flooding capability; improved liveability during fire situations due to cooling effects; the systems work in partially vented areas; economical installation, with up to 70% less weight than conventional sprinkler systems.

There are however some downsides to the system. As a relatively new technology, it is receiving mixed responses and evaluations from municipal decision-makers, although manufacturers and insurers are currently developing approval standards that should help the cause. And even though prices are falling, they are still currently more expensive than conventional systems. However, while the initial cost is slightly higher, the potential cost savings in terms of equipment and facility damage works to balance the economic scales.

A case in point

Operational issues aside, the mist system has created the opportunity for designers to explore complex details and materials that might not have otherwise been permitted when used with conventional systems.

A case in point is the Credit Valley Hospital/Peel Region Cancer Centre in Mississauga designed by Farrow Partnership Architects.

Given the desire to create a warm healing environment, wood was the obvious choice for the tree-like structure of the building’s central atrium space. But while it met the architects’ aesthetic criteria, it did not meet the Ontario Building Code (OBC) performance standards for non-combustible construction. This was primarily due to the inability of conventional sprinkler systems to adequately protect the large amount of shielded surface area created by the complex network of curving glulam beams. Steel, on the other hand, met the OBC requirements but lacked the desired warmth.

To achieve the required level of fire safety and comply with the necessary code equivalencies, mist nozzles were integrated into custom light standards located at the base of each primary wooden pier. The nozzles were positioned at a height of 2.2 metres and the standards located equidistant at 1.5 metres. To verify the effectiveness of the design, full-scale mock-up tests were conducted at the National Research Council’s Institute for Research in Construction in Ottawa. When subjected to a controlled five-second pre-burn and a 75 ft2 heptane pool fire, the mist suppressed a five megawatt fire. Successfully reducing the flame-spread and temperature ratings to well within acceptable limits, and with no evidence of flame attachment or soot deposition, the Ontario Fire Marshall deemed the test an unqualified success. Says Tye Farrow, Partner in Charge of Design at Farrow Partnership Architects, and Prime Architect for the Credit Valley Hospital/Peel Regional Cancer Centre, “we’re in the short strokes of getting the sign off from the different agencies… they have been very receptive.”

Properly designed, mist systems are effective on both solid fuel (Class A) and liquid fuel (Class B) fires and will achieve full compliance with NFPA 750 (Standards for the Installation of Water Mist Fire Protection Systems). While the applicability of the mist system in all occupancies is still a subject of debate, what is certain is that it is proving to be a versatile and sustainable alternative to conventional sprinkler systems. Internationally, there are numerous manufacturers producing several variations, with the Marioff Hi-Fog system having the majority of market share (www.hi-fog.com). For further information contact the International Water Mist Association (IWMA) www.iwma.net.

Sean Stanwick is an architectural designer with Farrow Partnership Architects in Toronto and a freelance architectural writer. [email protected]

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