Class K fire extinguishers are used strictly on class K fires. Class K fires are fires with substances such as the animal and vegetable fats present in commercial cooking oils and greases. Class K fires have certain similarities to Class B fires (those fueled by petroleum products), due to the fact that they both involve flammable liquids. Class K fires grow at a rapid pace when liquids and material such as cooking oils, cooking grease, animal fats, and vegetable fats serve as the fuel source.
While Class K fires do involve flammable liquids, note that Class B fire extinguishers (rated for flammable liquids such as petroleum and gases) are not efficient to handle the high-temperature cooking oil and grease fires that occur in kitchens.
It’s also important to remember that the use of water on cooking oil and grease fires will only spread the fire since the oil is not absorbed into the water but rather transported by it. Never use water on a fire involving grease or oil.
Class K fire extinguishers
These extinguishers use extinguishing agents that separate the fuel from the oxygen and help to absorb the heat elements of the fire triangle (fuel, heat, oxygen + chemical reaction).
Currently, the only effective extinguisher rated as Class K is the Wet Chemical fire extinguisher. These extinguishers use a wet mist containing an alkaline mixture, like potassium carbonate, potassium acetate, or potassium citrate, which interact with the cooking media (oil, grease, or fat) to create a type of foam that blankets the oil or grease, cooling it and preventing it from being fed oxygen. The extinguishing agent successfully quenches the fire and also lessens the risk of re-ignition.
Wet chemical extinguishers are a newer development. They have been created as the use of vegetable oils, which have a higher auto-ignition temperature than do animal fats, has increased, making the once effective dry chemical extinguishers ineffective. Newer types of efficiency fryers, which retain heat longer and are prone to re-ignition, have also contributed to the need for this more effective extinguishing agent.
In addition to being able to extinguisher Class K fires, Class K fire extinguisher can also, in many cases, be effective at putting out a Class A fire that began in the kitchen as a result of the Class K fire. So, in essence, even if the flames spread to Class A fire materials within the kitchen, a Class K extinguisher should be able to handle it.