A deep fat fryer fire is the one fire in an ordinary building that most extinguishers make worse. Cooking oil burns far above the temperature water can survive contact with, and it reignites the moment a foam blanket breaks. Wet chemical extinguishers exist for exactly this and nothing else does the job.
Why cooking oil is its own class
Oil in a fryer reaches temperatures well above the boiling point of water and above its own auto ignition temperature, so the fuel will re light itself from its own heat as soon as flame contact is broken. Cooling it enough to stop that, without throwing burning oil around, is a different problem from any other class of fire, which is why class F was created separately from class B.
How wet chemical works
The agent is discharged as a fine mist that is applied over the surface rather than jetted into it. It reacts with the hot oil to form a soapy layer on the surface, a process called saponification, which seals the fuel from the air while the water content cools the oil below its auto ignition temperature. Both halves are necessary: sealing alone would fail as soon as the layer broke.
Never use water on a chip pan fire
Water poured into burning oil sinks, flashes instantly to steam under the oil and throws burning oil upwards and outwards across the room and over the person holding the pan. This is the single most dangerous mistake in domestic fire safety. At home the answer is to turn off the heat if it is safe, leave the room, close the door and call the fire service, not to fight it.
What a commercial kitchen needs
Wet chemical extinguishers sized and rated for the fryers present, sited so somebody does not have to reach across the fryer to get one, plus a fire blanket for a small pan fire, plus whatever the risk assessment identifies for the rest of the premises. Many commercial kitchens also have a fixed suppression system over the cooking line, and where one exists the portable extinguishers supplement it rather than replace it.
Questions people ask about wet chemical fire extinguisher used for
What colour is a wet chemical fire extinguisher?
Yellow. A red body with a yellow band or panel identifies wet chemical contents, which are rated for class F cooking oil fires.
Can I use a foam extinguisher on a chip pan fire?
No. Foam is not class F rated and the oil temperature breaks the blanket down, so the fire reignites. Wet chemical is the rated agent for cooking oils and fats.
What should I do about a chip pan fire at home?
Do not use water and do not move the pan. Turn off the heat only if it is safe to do so, get everybody out, close the door and call the fire service.