Do you know what firefighters use to put out a fire? The answer may seem obvious, but there is more than one answer to this question.
Firefighters use a variety of agents to put out fires, but by far the most commonly used agent is water. Other special agents include firefighting foam, Dry Chemicals (Monoammonium Phosphate), Dry Powders (Sodium Chloride), Wet Chemicals (Potassium Acetate) and Carbon Dioxide (CO2).
Can’t you just put water on any fire to put it out? Water is the most versatile thing we have to put out fires, but there are certain circumstances that require special agents. Below will talk about all the different agents, when they are used, and how they work to put out the fire in different ways.
The Fire Triangle
In order to understand the different ways that firefighters put out fire, we need to review the basic elements of fire. The different elements of fire make up the Fire Triangle. It is made up of three components: Heat, Oxygen, and Fuel.
Heat obviously refers to a temperature that is hot enough for the particular fuel to reach its ignition temperature. This can temperature vary from one fuel source to another.
Oxygen is required because it reacts with the carbon in the fuel and “Oxidizes”. This chemical reaction is said to be exothermic, which means it produces energy in the form of heat and light, which is the fire. By definition, the rapid, energy-releasing oxidization of fuel is fire. There are other chemicals that can “oxidize” and take oxygen’s place to create fire, but that is less common.
Fun Fact: Rusting is also an oxidation process, just much slower and doesn’t generate very much energy.
Fuel is necessary because there needs to be something to burn. The carbon in the fuel releases as a gas when heated and recombines with the oxygen to create the fire. Fuels can be something like wood, plastic, paper, gasoline, etc.
In order for a fire to start and to continue burning, it must have all three elements of the Fire Triangle present. This means that we can disrupt any of these elements in order to extinguish the fire.
Some methods of firefighting focus on cooling the fire to remove the heat that is necessary for the fire to continue burning. Others will act to remove or displace the oxygen (or other oxidizing chemicals) and put the fire out by suffocating it.
Lastly, there are firefighting methods that work to separate the fuel from the heat and oxygen, so there is nothing to react and keep the fire going. Many are actually use some combination of these methods to more effectively extinguish the fire.
Now that we understand the basics of what allows fire to burn, we can look into the different methods of putting out fires and how they work in relation to the elements of the Fire Triangle. Here is a look at the different classes of fire in the United States. For fire classes of other countries, see this page.
Classes of Fire
- Class A, as we talked about above is ordinary combustibles like wood or paper.
- Class B is flammable liquids like gasoline.
- Class C means the fire has electricity involved and the proper extinguishing agent must be chosen to avoid electrocution. (If you were to use water on a Class C fire, there is potential for the electricity to travel through the water and up to the firefighters.)
- Class D is flammable metals like magnesium
- Class K is cooking oil fires like animal fats and oils
Note: There is a more advanced and scientifically more accurate model that explains the elements of fire called the Fire Tetrahedron. The basic Fire Triangle was enough for our purposes in this article, but if you want to learn more about the science of fire, this article from the NFPA (National Fire Protection Association) is much more in-depth.
H2O (That Means Water)
You probably already know what comes out of the fire hydrant? Hint, it’s the same thing that comes out of your faucet, Water! (you don’t want to drink from a hydrant though). Water is the most common agent used to put our fire by a long shot.
Water is made up of 2 hydrogen molecules and 1 oxygen molecule (that’s why it’s also called H2O or dihydrogen monoxide). Water is used to fight fires because it is effective for putting out many types of fires, it’s cheap and it’s readily available.
The modern-day water systems have made accessing water in most places fairly easily. When it comes to fighting fire, most developed areas have fire hydrants all over the place and some cities have them every 50-100 feet!
So how does water put out fire? Which elements from the Fire Triangle does it disrupt? Most people would say that the water cools the fire (disrupts the heat part of the triangle), which is true, but that is only one of the ways water can work to extinguish the fire.
When sprayed on the fire, water will absorb some of the heat by increasing its temperature (the heat from the fire is transferred to the cooler water) but this makes up a very small portion of water’s cooling capabilities.
The magic happens when water changes from a liquid to a gas (steam). This process requires a ton of energy (which it gets in the form of heat from the fire) and causes the water to expand 1700 times! The process of creating steam literally sucks the heat out of the fire.
The other way that water puts out fires is by disrupting the oxygen side of the triangle. Because water is a substance that cannot be oxidized any further, when it comes between the fuel and the oxygen, it essentially suffocates the fire. If the oxygen is prevented from reacting with the fuel, the fire goes out.
Firefighting foam has been around since the early 1900s. It was originally designed for fighting flammable liquid fires (known as Class B fires in the U.S.) and though it can be used for other types of fires as well, flammable liquid fires are still where it’s benefits are best used.
When a flammable liquid like gasoline is on fire, attempting to put it out with water is usually ineffective for a few reasons. Water is heavier than most flammable liquids (hydrocarbons) so rather it sinks to the bottom and doesn’t create that barrier between the fuel and the oxygen that can suppress many other types of fires.
If the liquid fuel heats above waters boiling temperature of 212 Degrees F, the water will boil and can throw the flammable liquid and spread the fire. Even the pressure of the water coming from the fire hose can push and spread the gasoline and cause the fire spread rather than go out.
For these reasons, foam is one of the best agents used for flammable liquid fires. The foam is a soapy, material made from a natural or synthetic surfactant mixed in a specific ratio with water. This reduces the surface tension of the water and creates a blanket that can sit on top of the flammable liquid.
Foam works on all three sides of the Fire Triangle to suppress the fire. The blanket of foam will suffocate the fire from the oxygen. The foam blanket will also prevent the release of flammable vapors from the fuel. It separates the flames and heat from the fuel source. And it will cool the fuel itself. Because it attacks the fire from multiple sides, it is very effective for these types of fires.
Foam is designed to make this blanket that will sit on the fuel and continue to keep the fire from rekindling (starting again) for a long time. How long it can be effective depends on the type of foam and whether it is a natural protein or synthetic foam.
There is also another type of foam that can be used for Class A fires (ordinary combustible like wood, paper, etc.). Because the foam reduces the surface tension of the water, it can better penetrate into all areas to cool and suffocate the fire. It is very effective for preventing rekindling.
There are quite a few different types of foam depending on the application. There are low, medium and high expansion rate foams, AFFF (Aqueous Film Forming Protein), FFFP (Film Forming Fluoroprotien), Alcohol resistant foam and many others. For a more in-depth article on firefighting foam, see this article on chemguard.com.
Here is another article about firefighting foam: Why Do Firefighters Use Foam Instead of Water?
Firefighting Dry Chemicals
Dry Chemicals are mostly seen in fire extinguishers. The dry chemical inside is usually Sodium Bicarbonate (baking soda), Potassium Bicarbonate and/or Monoammonium Phosphate.
The extinguisher is pressurized with nitrogen (since it’s not flammable and won’t oxidize) and sprays a fine mist of the dry powder. The powder coats the burning fuel and prevents the oxygen from reacting with the fuel. The process is basically the same as throwing sand or dirt on a campfire to put it out.
One of the most commonly used fire extinguishers is the ABC Dry Chemical extinguisher. The ABC means that it works on Class A, B and C fires. Here is a great general-purpose extinguisher made by Amerex for your home or office.
Firefighting Dry Powders
In the case of Class D fires (those involving combustible metals) water can react with the metal and actually intensify the fire and increase the temperature, rather than putting it out.
In order to extinguish a flammable metal fire, a dry chemical such as Sodium Chloride must be used as it is inert (won’t react with the metal) and can create a barrier to separate the fuel and oxygen.
Firefighting Wet Chemicals
Class K or cooking oil fires, as you may know, can be tricky to extinguish. These fires present a lot of the same challenges as flammable liquid fires, but they burn even hotter.
The most effective method to put these fires out involves something called saponification. This process occurs when a chemical reacts with fat and creates a soapy like substance.
A wet chemical such as Potassium Acetate, Potassium Carbonate or Potassium Citrate is used because it is very basic or alkaline (the opposite of acidic).
The chemical mixes with the fat in the cooking oil and creates this soap substance. This substance will absorb heat and suppress vapors and put out the fire. The soapy foam will create a blanket over the oil to prevent it from restarting. These chemicals are also used in kitchen hood systems to extinguish kitchen fires.
It is a good idea to have a class k fire extinguisher in your kitchen at home for any cooking oil fires. Remember water can spread the fire and make the situation worse. Here is a Class K extinguisher made by Amerex.
Carbon Dioxide: CO2
Carbon Dioxide can be used in fire extinguishers and in large fire suppression systems. Carbon Dioxide is odorless, colorless, tasteless and non-flammable. It can extinguish fires by disrupting the oxygen side of the Fire Triangle. The CO2 when released under pressure, will displace some or all of the oxygen near the fire. This will smother the fire and cause it to go out.
CO2 is effective for Class B and C fires, but may not work on Class A fires. This is due to the fact that ordinary combustibles can smolder and relight once the CO2 has spread out and the oxygen has returned.
CO2 can also be dangerous for humans and animals. Even though the gas itself is non-toxic, when it displaces the oxygen, you, like the fire, will have nothing to breathe. (this is known as a simple asphyxiant).
CO2 is best used on flammable liquid and electrical fires. It is also commonly used in large fire suppression systems in an office building as the CO2 won’t further damage the electrical equipment.
A CO2 extinguisher is good to have for a garage/shop or in an office with important technology that you don’t want to be damaged. Here is a Class BC CO2 extinguisher from Amerex.