One of the main problems of fire extinguishing methods is to prevent re-igniting of the fire. This depends solely on the source of the fire and it is up to the fire suppression system to cool down its temperature to prevent this from happening.
One of the most common causes of commercial fires is the commercial kitchen. Cooking environments are almost always subjected to grease and fumes. Although these fumes may not cause a fire, the grease deposited from cooking on to equipment and the surrounding space may cause it. The grease in this situation has a high auto-ignition temperature. In case it causes the fire, suppressing it is a tedious job. Unless the temperature of this grease is brought below its auto-ignition point, it will reignite repeatedly.
Wet chemical system is one of the surest ways to prevent this from happening. Most of these systems use potassium carbonate which deposits itself on the surface of the fire in a foam form. This causes immediate cooling by vaporizing the heat from the source. Because this agent distributes itself in the form of small droplets, there is no danger of causing a splash of the burning grease.
The potassium carbonate mixes with the grease, creating a soapy solution, thus nullifying any chance of reigniting the fuel. The process is known as saponification and it is base don the principle of cutting off the oxygen supply from the atmosphere to the fire source.
Dry powder agents
Dry powder chemicals used to suppress fire are used widely in ships that carry combustible gases and liquids. In case of fire, when these dry powder agents are applied to the source, they put out the flame instantly. Mostly Sodium or Potassium Bicarbonate is used in such systems. The biggest advantage of dry agents is their stability in extremes of temperature. They are free from toxic elements and are mixed with other compounds that enhance their flow, help preserve them as well as make them water-resistant.
The only problem that may be faced when applying such dry agents to put out the fire is minor breathing difficulties caused due to the particles composing them.
The fire extinguishers containing dry agents use nitrogen to control pressure which atomizes the contents when released from the nozzle of the tank. At a pressure of .9 to 1 MPa bar, the pilot valve discharges the powder and this pressure is maintained with the help of a reduction valve located at the upstream of the agent.