Fire Suppression Systems Fire results when fuel, oxidant, and sufficient heat combine in time and place (New Zealand Institute of Chemistry, n.d.). The fuel is typically a carbon-based material like paper, wood, oil, or gas, while ambient air typically provides the oxidant in the form of oxygen. Other oxidants include nitrates, chlorates, and peroxides and therefore should never be stored alongside fuel materials. For combustion to occur the heat must sufficient to ignite the fuel. Once ignited the chemical reaction is typically extremely exothermic and becomes self-perpetuating in the presence of fuel and oxidant. If heat accumulates faster than it can be dissipated to the surrounding environment an explosion will occur.

The three ways in which heat can be dissipated is through conduction along a temperature gradient, convection due to movement of the gaseous fire matter, and radiation to other surfaces (New Zealand Institute of Chemistry, n.d.). The primary method for extinguishing a fire is by cooling it below the ignition point of the fuel, typically with water. The other methods for extinguishing a fire involve removing the fuel and isolating the fuel from the oxidant. This report will review contemporary fire suppression methods in common use in western countries and the science upon which they are based. The fire suppression methods appropriate for different environments, from office buildings to industrial settings and transportation, will be also be discussed.

Classifying Fire

The fuel involved in a fire is used to classify the fire (New Zealand Institute of Chemistry, n.d.). Class A fires consist of solid fuels, like wood, paper, grain, coal, and plastics. The primary means of suppressing a Class A fire is through cooling the temperature of the fire below the ignition temperature (Office of Compliance, 2009). Class B fires consume flammable liquids, such as gasoline, wax, and paint, and fire suppression is accomplished by interfering with the fire chemically and/or separating the fuel from heat sources. Class C fires involve electrically energized equipment and the mechanism of suppression is the same as for Class B fires. Class D fires involve combustible metals like sodium and are suppressed by creating a barrier between the oxidant and fuel. Class K fires involve cooking oils and fats, and are suppressed through cooling and isolating the fuel from heat sources.

Fire Suppression Systems

The five main types of fire suppressants are water, foam, carbon dioxide (CO2), halon/clean agents, and dry powder (New Zealand Institute of Chemistry, n.d.). Water is only used for Class A fires (Office of Compliance, 2009). Foam can be used for Class A and K. fires, carbon dioxide for Class B, C, and K. fires, and dry powders are useful for fighting Class A, B, and C. fires.

Water and Foam - Water is effective because it has a high molar heat capacity; therefore, water works as a fire suppressant by lowering the temperature below the fuel's ignition point (New Zealand Institute of Chemistry, n.d). Water is not used to suppress fires consuming flammable liquids because the fuel will layer on top of the water. In the 1960s the U.S. Navy developed fire suppressant foams to combat petroleum fires in close proximity to explosives. These foams were water-based, included a surfactant to lower the surface tension of water, and were called aqueous film-forming foams (AFFFs) (Knowlton, 2012). Since then AFFFs have been adopted by speedways, oil refineries, and fire departments across the nation for combating petroleum fires.

Hand-held fire extinguishers using either water or AFFFs as the fire suppression agent are readily available and often serve as the first response to a fire (Office of Compliance, 2009). Water and AFFFs can also be distributed within an enclosed space to suppress a fire during its earliest stages using automatic sprinklers, fixed water sprayers, water misters, foam-water systems, and standpipe and hose systems, with the latter intended for use only by trained fire-fighting personnel (IFSTA, 2009, p. 339). High-rise structures can also be fitted with a fire pump to increase the water pressure delivered to the fire suppression systems.

The most common and effective is the water sprinkler system (IFSTA, 2009, 340-355). Activation is accomplished through heat, smoke, rate-of-rise sensors, or manually. In addition to distributing water, activation of the system will trigger alarms to alert personnel to evacuate. There are a large number of configurations designed to address specific needs, including dry pipe installation for locations that may see freezing temperatures.

A foam-based fire suppression system is similar to water-based systems (IFSTA, 2009, p. 415-419). The main components are an adequate water supply, storage tanks containing the concentrated agent, a mixing...

When the system is fixed in place for a particular structure, actuation of the system can be either automatic using heat sensors or manual. Fixed systems can be designed to treat a small footprint or to flood the enclosure. Semifixed designs consist of fire departments that transport the foam to the fire's location and hoselines connected to foam hydrants. High-expansion systems are designed to flood the entire enclosure with several feet of foam within just a few minutes.
Carbon Dioxide -- CO2 transitions from a gas to liquid at ambient temperatures when pressurized to 67 atmospheres, making it ideal for storing inside a pressurized canister like a hand-held fire extinguisher (New Zealand Institute of Chemistry, n.d.). When released the liquid immediately turns into a very cold gas; however, it is effective as a fire suppressant primarily because it displaces oxygen. The cold temperature is useful only if it can be applied directly to the burning fuel. CO2 is not used on Class A fires because the pressurized blast from the fire extinguisher can disperse the burning fuel. In addition, CO2 fire suppression systems are becoming more popular as halon systems are being phased out.

CO2-based fire suppression systems can also be designed to flood enclosures, but doing so places personnel at risk for suffocation (NFPA, 2011). Current recommendations are to avoid the installation of CO2 flooding systems inside normally occupied enclosures except where viable alternatives do not exist. Examples of occupied enclosures that may need a CO2 flooding system include marine engine rooms and computer server rooms.

CO2 flooding systems for occupied enclosures are fitted with pre-discharge alarms, thereby giving personnel enough time to clear the area before the agent is discharged (IFSTA, 2009, p. 412-413). CO2 is typically stored in tanks at either high (850 psi) or low pressure (350 psi), with the former designed to protect a smaller footprint than the latter. The discharge nozzles are either high- or low-velocity in design, with high-velocity nozzles providing better coverage and low-velocity limiting the spread of burning fuel.

Clean Agents and Halon -- Halon is a great fire suppressant because it displaces atmospheric oxygen and scavenges free radicals that maintain the exothermic reaction (New Zealand Institute of Chemistry, n.d.); however, under the 1987 Montreal Protocol the manufacture and use of halon as a fire suppressant has been all but banned (NFPA, 2012) because it damages the ozone layer (Poynter, 1999). For this reason a number of halon replacement agents have been developed that are called 'clean agents,' which have the same fire suppression characteristics of halon, including non-conductive, readily vaporized, and residue-free (Puchovsky, 2011). The clean agents that have been approved for use fall within the chemical classes of halocarbons and inert gases (NFPA, 2012). Given the ability of these agents to displace oxygen, precautions should be taken to prevent suffocation of occupants. In addition, clean agents are not recommended for extinguishing fires involving solids capable of rapid oxidation in the absence of atmospheric oxygen, such as gun powder, reactive metals, organic peroxides, and hydrazine (NFPA, 2012).

Clean agents are used for structures that contain electrical equipment, valuable documents and art works, marine engine rooms, and aircraft engines (IFSTA, 2009, p. 408-411). The system designs are essentially equivalent to CO2 local and flooding systems, including tanks for storing the agent in liquid form, automatic and manual activation devices, piping, and discharge nozzles.

Dry Powders -- Sand and sodium bicarbonate represent the main dry powders for use in non-challenging situations (Moore, 1996; New Zealand Institute of Chemistry, n.d.). Sodium bicarbonate is suitable for Class B or C. fires, but to address Class A, B, and C. fires the most common dry powder is mono-ammonium phosphate (MAP; NH4H2PO4). MAP has been shown to outperform both Halon and water in suppressing gas and dust explosions and is the preferred agent for industrial environments (Moore, 1996). In industrial situations where contamination is a concern, such as in a food processing plant, a water-soluble and food-grade compatible agent based on sodium bicarbonate was developed called Dessikarb. Dessikarb was shown to be as effective of a fire suppressant as MAP, except when there was a risk of dust explosions.

Suppression systems for distributing dry powder contain the same basic elements of water, foam, CO2, or clean agent systems (IFSTA, 2009, p. 403-405). Pressurized containers are used to store the agent and piping is used to transport the agent to the nozzles. Activation of the system can be automatic through heat sensors or…