Fire
needs fuel,
oxygen and heat in order to burn. In simple terms, fire
extinguishers remove one of these
elements by applying an agent
that
either cools the burning fuel, or removes or displaces the surrounding
oxygen.
Fire
extinguishers are filled with water or a smothering material, such as
CO2. By pulling out the safety pin and depressing the lever at the top
of the cylinder (the body of the extinguisher), this material is
released by high amounts of pressure.
At the top of the cylinder, there is a smaller cylinder filled with
compressed gas. A release valve acts as a locking mechanism and
prevents this gas from escaping. When you pull the safety pin and
squeeze the lever, the lever pushes on an actuating rod which presses
the valve down to open a passage to the nozzle. The compressed gas is
released, applying a downward pressure on the fire-extinguishing
material. This pushes the material out the nozzle with high amounts of
pressure.
Although
the temptation is to aim the extinguisher at the flames, the proper way
to use the extinguisher is to aim it directly at the fuel.
Air/Water Fire Extinguishers
Water extinguishers are filled with regular tap water and pressurized
with oxygen. The best way to remove heat is to dump water on the fire
but, depending on the type of fire, this is not always the best option.
Dry Chemical Fire Extinguishers
Dry chemical extinguishers are filled with either foam or powder,
usually sodium bicarbonate (baking soda) or potassium bicarbonate, and
pressurized with nitrogen. Baking soda is effective because it
decomposes at 158 degrees Fahrenheit and releases carbon dioxide (which
smothers oxygen) once it decomposes. Dry chemical extinguishers
interrupt the chemical reaction of the fire by coating the fuel with a
thin layer of powder or foam, separating the fuel from the surrounding
oxygen.
Carbon Dioxide (CO2) Fire extinguishers
CO2 extinguishers contain carbon dioxide, a non-flammable gas, and are
highly pressurized. The pressure is so great that it is not uncommon
for bits of dry ice to shoot out. CO2 is heavier than oxygen so these
extinguishers work by displacing or taking away oxygen from the
surrounding area. CO2 is also very cold so it also works by cooling the
fuel.