The Fire Triangle
In order
to understand how fire extinguishers work, you first need to know a little bit about fire.
Not all
fires are the same, and they are classified according to the type of fuel that is burning.
If you use the wrong type of fire extinguisher on the wrong class of fire, you can, in
fact, make matters worse. It is therefore very important to understand the four different
fire classifications.
Most
fire extinguishers will have a pictograph label telling you which classifications of fire
the extinguisher is designed to fight. For example, a simple water extinguisher might have
a label like the one below, indicating that it should only be used on Class A fires.
Different
types of fire extinguishers are designed to fight different classes of fire. The three
most common types of fire extinguishers are:
APW stands for "air-pressurized water." APWs are large, silver extinguishers that are filled about two-thirds of the way with ordinary tap water, then pressurized with normal air. In essence, an APW is just a giant squirt gun.
APWs stand about 2 feet tall and weigh approximately 25 pounds when full.
Never
use water to extinguish flammable liquid fires. Water is extremely ineffective at
extinguishing this type of fire, and you may, in fact, spread the fire if you try to use
water on it.
Never use water to extinguish an electrical fire. Water is a good conductor, and there is some concern for electrocution if you were to use water to extinguish an electrical fire. Electrical equipment must be unplugged and/or de-energized before using a water extinguisher on it.
APWs extinguish fire by taking away the "heat" element of the fire triangle.
APWs will be found in older buildings, particularly in public hallways, as well as in Residence Halls. They will also be found in computer laboratories. It is important to remember, however, that computer equipment must be disconnected from its electrical source before using a water extinguisher on it.
CO2 cylinders are red and range in size from 5 lbs to 100 lbs or larger. In the larger sizes, the hard horn will be located on the end of a long, flexible hose.
CO2s are
designed for Class B and C
(flammable liquid and electrical) fires only.
Carbon
Dioxide is a non-flammable gas that extinguishes fire by displacing oxygen, or taking away
the oxygen element of the fire triangle. The carbon dioxide is also very cold as it comes
out of the extinguisher, so it cools the fuel as well. CO2s may be ineffective at
extinguishing Class A fires because they may not be able to displace enough oxygen to
successfully put the fire out. Class A materials may also smolder and re-ignite.
CO2s will frequently be found in laboratories, mechanical rooms, kitchens, and flammable liquid storage areas.
ABC extinguishers are red and range in size from 5 lbs to 20 lbs on campus.
An "ABC" extinguisher will have a label like this, indicating that it may be used on class A, B and C fires.
Dry chemical extinguishers put out fire by coating the fuel with a thin layer of dust, separating the fuel from the oxygen in the air. The powder also works to interrupt the chemical reaction of fire, so these extinguishers are extremely effective at putting out fire.
These extinguishers will be found in a variety of locations. New buildings will have them located in public hallways. They may also be found in laboratories, mechanical rooms, break rooms, chemical storage areas, offices, university vehicles, etc.
Dry chemical extinguishers with powder designed for Class B and C fires may be located in places such as commercial kitchens or areas with flammable liquids.
In order
to understand how fire extinguishers work, you first need to know a little bit about fire.
Four things must be present at the same time in order to produce fire:
- Enough oxygen to sustain combustion,
- Enough heat to raise the material to its ignition temperature,
- Some sort of fuel or combustible material, and
- The chemical, exothermic reaction that is fire.
Oxygen, heat, and fuel are frequently referred to as the "fire triangle." Add in the fourth element, the chemical reaction, and you actually have a fire "tetrahedron." The important thing to remember is: take any of these four things away, and you will not have a fire or the fire will be extinguished.
Essentially, fire extinguishers put out fire by taking away one or more elements of the fire triangle/tetrahedron.
Fire safety, at its most basic, is based upon the principle of keeping fuel sources and ignition sources separate.
Not all
fires are the same, and they are classified according to the type of fuel that is burning.
If you use the wrong type of fire extinguisher on the wrong class of fire, you can, in
fact, make matters worse. It is therefore very important to understand the four different
fire classifications. |
Class A - Wood, paper, cloth, trash, plastics Solid combustible materials that are not metals. (Class A fires generally leave an Ash.) |
 |
Class B - Flammable liquids: gasoline, oil, grease, acetone Any non-metal in a liquid state, on fire. This classification also includes flammable gases. (Class B fires generally involve materials that Boil or Bubble.) |
 |
Class C - Electrical: energized electrical equipment As long as it's "plugged in," it would be considered a class C fire. (Class C fires generally deal with electrical Current.) |
 |
Class D - Metals: potassium, sodium, aluminum, magnesium Unless you work in a laboratory or in an industry that uses these materials, it is unlikely you'll have to deal with a Class D fire. It takes special extinguishing agents (Metal-X, foam) to fight such a fire. |
Different
types of fire extinguishers are designed to fight different classes of fire. The three
most common types of fire extinguishers are: APW stands for "air-pressurized water." APWs are large, silver extinguishers that are filled about two-thirds of the way with ordinary tap water, then pressurized with normal air. In essence, an APW is just a giant squirt gun.
APWs stand about 2 feet tall and weigh approximately 25 pounds when full.
Water (APW) Extinguishers
APWs are designed for Class A (wood, paper, cloth) fires only.
Never use water to extinguish an electrical fire. Water is a good conductor, and there is some concern for electrocution if you were to use water to extinguish an electrical fire. Electrical equipment must be unplugged and/or de-energized before using a water extinguisher on it.
APWs extinguish fire by taking away the "heat" element of the fire triangle.
APWs will be found in older buildings, particularly in public hallways, as well as in Residence Halls. They will also be found in computer laboratories. It is important to remember, however, that computer equipment must be disconnected from its electrical source before using a water extinguisher on it.
Carbon Dioxide Extinguishers
Carbon
Dioxide extinguishers are filled with non-flammable carbon dioxide gas under extreme
pressure. You can recognize a CO2 extinguisher by its hard horn and lack of pressure
gauge. The pressure in the cylinder is so great that when you use one of these
extinguishers, bits of dry ice may shoot out the horn. CO2 cylinders are red and range in size from 5 lbs to 100 lbs or larger. In the larger sizes, the hard horn will be located on the end of a long, flexible hose.
CO2s are
designed for Class B and C (flammable liquid and electrical) fires only.
CO2s will frequently be found in laboratories, mechanical rooms, kitchens, and flammable liquid storage areas.
Dry Chemical Extinguishers
Dry Chemical Extinguishers come in a variety of types. You may see
them labeled:
- "DC" short for "dry chem"
- "ABC" indicating that they are designed to extinguish class A,B,and C fires, or
- "BC" indicating that they are designed to extinguish class B and C fires.
ABC extinguishers are red and range in size from 5 lbs to 20 lbs on campus.
It is extremely important to identify
which types of dry chemical extinguishers are located in your area.
which types of dry chemical extinguishers are located in your area.
Read the labels and know their locations! You don't want to
mistakenly use a "BC" extinguisher on a Class A fire, thinking that it was an
"ABC" extinguisher.
An "ABC" extinguisher will have a label like this, indicating that it may be used on class A, B and C fires.
Dry chemical extinguishers put out fire by coating the fuel with a thin layer of dust, separating the fuel from the oxygen in the air. The powder also works to interrupt the chemical reaction of fire, so these extinguishers are extremely effective at putting out fire.
These extinguishers will be found in a variety of locations. New buildings will have them located in public hallways. They may also be found in laboratories, mechanical rooms, break rooms, chemical storage areas, offices, university vehicles, etc.
Dry chemical extinguishers with powder designed for Class B and C fires may be located in places such as commercial kitchens or areas with flammable liquids.
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