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.
Commercial fire extinguishers for a commercial kitchen
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.
Wet chemical fire extinguisher class F, and why nothing else will do
Class F is cooking oils and fats, and it exists as a separate class because the fire behaves unlike any other. The oil is above its own auto ignition temperature, so removing the flame does nothing: it relights the moment the covering goes. Wet chemical works by saponification, a reaction with the hot oil that makes a soapy crust across the surface, which both cuts the oxygen and holds the heat in. That is why water and foam are dangerous here and a carbon dioxide unit is nearly useless: neither leaves anything behind. The technique is different too, applied gently through the extended lance onto the surface rather than into 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.
Are wet chemical extinguishers only for kitchens?
Almost entirely, and that is a feature rather than a limitation. The agent is designed for burning cooking oil and fat, where it reacts with the hot oil to form a soapy crust that seals the surface, and nothing else does that job safely. It carries a rating for ordinary combustibles as well, so it is not useless elsewhere, but the reason to buy one is a fryer, a range or a griddle. Two practical notes: it is applied gently through the long lance onto the surface rather than into it, and a kitchen with a suppression system still wants one for a fire that starts outside the canopy's protected zone.
What is a yellow extinguisher used for?
Yellow is the wet chemical panel, and it means cooking oils and fats. It is the kitchen unit: the agent reacts with hot oil to form a soapy seal across the surface, which is the only thing that reliably stops a fat fire from relighting, and it is applied gently through the long lance onto the surface rather than into it. It also carries a rating for ordinary combustibles, so it is not useless elsewhere, but the reason to have one is a fryer or a range. Do not confuse it with the cream foam panel or the black carbon dioxide one, which is the mistake the colour codes exist to prevent.
What does a yellow coded fire extinguisher contain?
Wet chemical, for cooking oils and fats. The colour code is the panel on a red body, and yellow is the only one that means Class F: a potassium based solution that reacts with hot oil to form a soapy layer sealing the surface. Three things follow from that. It is applied gently through a long lance onto the surface rather than into the oil. It is the kitchen unit and there is no substitute for a fryer. And it should not be confused with the cream foam panel, which looks similar under poor lighting and is exactly the wrong thing to reach for.