Types of Fires
Generally speaking, there are five classes of fires as defined in Table 2. Halon fire suppression systems
and some of their alternatives are being used quite effectively on the first three, Classes
A, B and C. Halon fire suppression systems are not recommended for use on the other two classes, D and
K, where specialized agents are usually employed.
Table 2: Classification of Fires
| Class |
Description |
| A |
Fires in ordinary combustibles such as wood, cloth,
paper, rubber, and many plastics. |
| B |
Fires in flammable liquids, oils, greases, tars, oil-base
paints, lacquers, and flammable gases. |
| C |
Fires that involve energized electrical equipment. |
| D |
Fires of combustible metals, such as magnesium,
titanium, zirconium, sodium, lithium, and potassium. |
| K |
Fires that involve cooking appliances with flammable
cooking oils and fats, vegetable or animal. |
Fire Suppression System Methods of Applying Fire Extinguishing Agents
There are two common methods for applying fire extinguishing agents: (1) total flooding
and (2) local application. In addition, there are two rather specialized applications (3)
explosion suppression and (4) inerting against explosions and fires. The explosion
suppression and inerting applications are not discussed further in this report due to their
specialized nature and, relatively speaking, limited market presence.
Fire Suppression Systems using Total Flooding
Systems working on a total flooding principle apply an extinguishing agent to an
enclosed space in order to achieve a concentration of the agent (volume percent of the
agent in air) adequate to extinguish the fire. These types of systems may be operated
automatically by detection and related controls or manually by the operation of a system
actuator. This is true for any gaseous total flooding agent irrespective of its mechanism
of extinguishment.
According to Senecal, “………. the means of fire extinguishment by gaseous agents is a combination of three
underlying mechanisms:
1. Chemical effects - inhibition by halogen atoms. Bromine, iodine and chlorine atoms can
act catalytically, each atom participating multiple times to scavenge important free
radicals from the combustion gases. Bromine and chlorine are both much more potent
than fluorine in this regard. Fluorine also reacts with free radicals but forms strong
chemical bonds. Thus, fluorine atoms react only once and are then “consumed.”
2. Physical effects - thermal. The addition of any non-reactive gas to a flammable gas
mixture leads to a reduction in flame temperature by virtue of the fact that the heat
liberated by the reaction of oxygen molecule with a fuel species must be distributed into a
larger heat sink. The rate of the combustion chemical reactions decreases rapidly with
reductions in temperature. If the concentration of added inert gas is high enough the
flame chemistry becomes too slow to propagate. The potency of an inert gas agent is
related directly to the heat capacity of the gas composition.
3. Physical effects - dilution. Addition of a third gas to a fuel-air mixture has the effect of
reducing the collision frequency of the oxygen and fuel species. This leads to a reduction
in chemical reaction rates. The magnitude of the effect, however, is relatively minor
compared to chemical inhibition or thermal effects.
In the case of inert gas agents for fire suppression, the extinguishing effects are entirely physical. In the case of
halons the chemical inhibition effects are most important. For example the extinguishing
mechanisms of halon 1301 has been reported to be a combination of 80% chemical effects
and 20% physical effects. 5 In the case of hydrofluorocarbons the extinguishing effects are
predominantly physical with some (estimate at about 10 to 15%) chemical effects.”
Total flooding is the most common system application of halons and has been employed
for the protection of volumes containing essential electronics, machinery spaces on
ships, aircraft engines and cargo bays, enclosed process modules in the oil and gas
industry and both crew and engine compartments on military armored vehicles.
Fire Suppression Systems using Local Application
In local application, the agent is applied directly onto a fire or into the region of a fire.
There are some systems employing this technique but the most common method of local
application is by manually operated wheeled or portable fire extinguishers. In those
instances, this method is also referred to as a “streaming” application. One example of a
local fire suppression system is the protection of wet benches in the semiconductor
manufacturing industry. The wheeled extinguishers using halon 1211 found broad
acceptance in the aviation sector for the extinguishments of aircraft engine fires and
small pool fires around parked aircraft.
