Wet chemical is the agent developed specifically for burning cooking oil and fat, which the fire classes call class F. Its band is yellow. It exists because the older provision for kitchens, a foam or powder unit, was measured as unsafe against modern high efficiency fryers, which hold oil at temperatures well above the auto ignition point of the oil itself.
Saponification, which is what makes it work
The agent is an alkaline solution applied as a fine mist. On contact with hot fat it reacts with it and forms a soapy crust across the surface. That crust seals the oil from the air and, crucially, stays in place while the oil below it cools over several minutes. No other portable agent both smothers and holds.
Why the mist and the long lance
The applicator is designed to lay the agent gently onto the surface from a distance. A jet would splash burning oil out of the vessel. The lance keeps the user back from the radiant heat of an open fryer, which is severe enough to be the limiting factor on how close anybody can get.
What must not be used instead
Water ejects the oil violently. Foam breaks down at fryer temperatures and the fire re establishes. Powder knocks the flame down without cooling, so the oil relights. Carbon dioxide blasts the surface and can spread it. Every one of these is a worse outcome than doing nothing, which is why the kitchen provision is a specific one.
Where the portable sits relative to a suppression system
Many commercial kitchens have a fixed suppression system over the range. The portable is not a substitute for it and is not a reason to omit it. The portable covers what happens away from the hood, and it is what a person reaches for if the fixed system has already operated and something is still burning.
Questions people ask about wet chemical extinguisher used for
What is the wet chemical extinguisher colour?
Yellow, as a band or panel on a red body. It is the newest of the five common codes and the one people most often fail to recognise.
Is a yellow fire extinguisher used for anything besides cooking oil?
Most wet chemical units also carry a class A rating and can be used on ordinary combustibles. That is a secondary capability; the unit is provided for the fryer.
Does a domestic kitchen need one?
A fire blanket and the instruction to turn the heat off and leave is the usual domestic answer. Wet chemical provision belongs where oil is held hot in quantity, which is a commercial arrangement.