Physics of a Fire

Physics of a Fire

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Fire is a chemical reaction whcih needs three things to be present so it can happen: Oxygen, Fuel, and Heat.

If one of these is not present, the fire cannot start. If one of these is taken from a fire it will go out.

But how does this all work?

We all breathe Oxygen (O²) everyday - in fact, without it we would suffocate. But did you know that fire breathes Oxygen too? And, like us, without Oxygen a fire will also suffocate.

When Oxygen in the air combines with flammable vapours given off by Fuels - heat is produced and then ignition can occur.

Without enough Oxygen, ignition cannot happen. In the opposite way, if there is too much Oxygen then the vapours won't be concentrated enough to ignite. The ratio of vapour to Oxygen is known as the 'explosive' or 'flammable' limit and is different for each gas or vapour.

Combustion occurs when flammable vapours mix with air (Oxygen) and are ignited by a spark or flame.

Solids give off flammable vapours by being heated. Certain solids such as paper or flour appear to ignite almost instantly. This is because they give off vapours and reach a flammable temperature almost immediately. In fact, fine dusts dispersed in the air can explode because they give off vapours and ignite so quickly it appear to happen instantly.

Other solids like timber take longer to ignite because they are more dense and so don't give off flammable vapours so easily.
So, in our fire triangle we've got Oxygen and Heat, but we also need something that will burn - this is our Fuel.

Fuels can take almost any form:

Solids like wood, fabric, rubber and plastic.
Liquids such as petrol, oil, cooking oil or even nail varnish remover.
Gases like propane, butane and 'natural' gas.

If a fire broke out in your home today, would you automatically know which fire extinguisher to use? What would happen if you used a Class A fire extinguisher on a fire in the electrical service panel in your basement? Answer: you'd possibly be electrocuted if the extinguishing agent is a liquid!

A long time ago, the fire protection industry recognized the need to classify extinguishers according to the many kinds of burning materials encountered in a fire. For example, Class A, water-type fire extinguishers cannot be used on the electrical fires because the extinguisher operator could be seriously injured by the conduction of electricity by the stream of water from their extinguisher.

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Instead, a Class C fire extinguisher will work best. Another example, a raging fire in a manufacturing plant with flammable metals, such as magnesium, should be fought with a Class D fire extinguisher.

Besides knowing what type of fire you're likely to encounter, it's also important to know what kind of extinguishing agent your fire extinguisher should have in it. This is because certain extinguishments will work only on certain kinds of fires. Others can actually aggravate the situation, causing an even more critical situation.

For example, a WATER agent should only be used on Class A fires. This is because they put out fires by cooling, soaking and penetrating combustible materials. A REGULAR DRY CHEMICAL agent should only be used on Class B and C fires. This is because they attempt to interrupt the fire's flame by interfering with the fuel-oxygen-heat triangle instead of simply cooling a fire. A MULTIPURPOSE DRY CHEMICAL, however, can be used on Class A, B, and C fires. This is because the extinguishing agent melts when it's exposed to heat, forming an oxygen barrier over the burning materials.

CARBON DIOXIDE, another popular extinguishment, should also be used on Class B and C fires. They put out fires by cutting off the air supply, virtually choking a fire. FOAM agents also cut off a fire's oxygen supply, but they should only be used on Class A and B fires.

DRY POWDER agents are good only on Class D fires. Do not use dry powder for Class A, B, and C fires. This is because they specifically form a coating agent over the burning metals, suffocating the flame.

Obviously, most of the time there are more than one type of materials found in each area of a home. For instance, in a basement you're likely to find cloth; wood; an electrical panel box; electric, oil or a gas furnace; and an electric or gas hot water tank. In this case, buying a Class A/B/C extinguisher would be advisable.

Another example is the kitchen area where you're likely to find flammable liquids (grease) and gases (propane or natural gas). In addition, you'llusually find various electrical appliances. In this situation, it is probably more economical to buy Class B/C extinguisher instead of keeping separate Class B and C extinguishers in these areas. Class A extinguishers also are good

Fire Classifications
Class A: Fires that involve wood, cloth, rubber, paper, and some types of plastics.
Class B: Fires that involve gasoline, oil, paint, natural and propane gases, and flammable liquids, gases, and greases.
Class C: Fires that involve all the materials found in Class A and B fires, but with the introduction of an electrical appliances, wiring, or other electrically energized objects in the vicinity of the fire.
Class D: Fires that involve combustible metals, such as sodium, magnesium, and potassium.

The hallmark of a good fire department is the ability to make an aggressive interior attack. When occupants await rescue and are trapped by fire, it is the aggressive fire department that will save them.

Whenever the topic of interior structural fire attack arises, there is always debate as to which method of fire attack is best. Some departments rely on the direct attack method. Other departments attempt an aggressive interior attack using the indirect or combination method.

Fire attack is a constant source of debate in the fire service. This debate usually focuses on which nozzle, pressure, and pattern are best to attack a fire. At the heart of the discussion are three basic rules that all sides do agree upon.

Rule Number One: Fire attack must be properly supported

Fire attack does not exist in a vacuum. There are a number of important supporting operations that must be performed to allow for a safe and effective fire attack. These operations include size-up, command, water supply, stretching the attack line, stretching the back-up line, and initiating early ventilation. Without these operations, fire attack success will be decreased while the danger to firefighters will vastly increase.

Rule Number Two: There must be adequate flow to knock down the fire

Fire attack is a simple matter of physics. There must be enough water hitting the fire to eliminate the heat that is being produced. If the attack line does not have adequate flow, the fire will continue to burn while water supplies are being depleted. It is absolutely critical that the attack line flows enough water to achieve knockdown quickly. Remember! A lot of water real fast beats a little bit of water over a long period of time!

Rule Number Three: The water must hit the seat of the fire

In order for any amount of water to have any effect, it must reach the seat of the fire. It will not matter how much the attack line is flowing, if the water is wasted, only the water level lights on the engine will go out. This rule requires the attack line to be properly positioned and then repositioned as necessary so that the water is always hitting the fire. Another important consideration here is the penetration ability of the attack stream. There must be adequate flow and pressure available so that the water can penetrate the super heated gases and actually reach the seat of the fire.

These three core rules are beautiful in their simplicity and when considered, much of the fire attack debate becomes academic. This is because there is more than one way to accomplish each rule and the correct method is based on the situation, departmental necessities, and departmental preference. What is important is that firefighters be able to determine which tactics are best for any given situation. These tactics are based on the basic elements of fire attack that include attack pattern, direction, and type.
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