COMPONENTS NEEDED FOR COMBUSTION TO OCCUR
A fire is actually the heat and light (flames) that are produced when a material burns or goes through the combustion process. The process by which a substance burns is a chemical reaction between a combustible material and oxygen, that is, combustion. In this process energy is released in the form of heat.
A fire is produced by the presence of four basic elements: heat or ignition source, combustible material, an appropriate concentration of oxygen and the chain reaction. It is customary to visualize the relationship of these four elements as a pyramid in which each element represents one side and they come together in a symbiotic or mutually beneficial relationship.
The first of these necessary factors, fuel: it can be any one of thousands of materials: coal, gasoline, wood, etc. In their normal solid or liquid state, none of these materials burn. For them to burn they need to be converted into gas first.
The second essential factor for the fire to burn is heat: this is what gives us the necessary temperature to convert fuel into gas, in order to burn. Some fuels are converted to gas (gasified or volatilized) at higher temperatures. We know they need less heat to volatilize gasoline and make it burn than you need to do with wood or charcoal.
The third factor for fire to burn is oxygen: to ignite and start burning, fire needs oxygen.
The last factor for a fire to burn is an ignition source: any instrument that starts the fire.
The Fire Triangle:
TETRAHEDRON THAT SYMBOLIZES THE FIRE
If one of these elements does not exist or is eliminated, there is no or the fire is finished. This principle is used for fire fighting:
Cool fire: Common agent is water. It is commonly applied in the form of a solid stream, a fine shower or a lather.
Eliminate Oxygen: the material that is burning with a blanket, with a lid, earth, foam or with When you use an extinguisher what you normally do is cover the area of the fire with a gas heavier than it
Dispose of Combustible Material: Move combustible material away or shut off the source, as long as this does not endanger your life or that of others.
Interrupt the Chain Reaction: As the fire develops, the flame will form. Upon reaching this stage, free radicals are formed, vital for sustaining the fire. Chemical compounds in fire extinguishers capture free radicals in a chain. Others the reaction dioxide in slightly
Fundamental laws of fire propagation:
Heat transfer
Heat can travel through a burning building by one or more of three common phenomena such as conduction, convection, and radiation. Because the existence of heat within a substance is caused by the action of molecules, the greater the molecular activity, the greater the intensity of heat. A number of natural laws of physics are involved in the transmission of heat. One of them is called the Law of Heat Flow, which specifies that heat has the tendency to flow from a hot substance to a cold substance. The colder of the two bodies in contact will absorb heat until both objects are at the same temperature.
Driving
Heat can be conducted from one body to another by direct contact of two bodies or through a conductive medium. The amount of heat that will be transmitted and its rate of transfer will depend on the conductivity of the material through which the heat is passing. Not all materials have the same heat conductivity. Aluminum, copper, and steel are good conductors. Fibrous materials, such as cloth and paper, are poor conductors.
Liquids and gases are poor conductors of heat due to the movement of their molecules. Air is also a relatively poor conductor. Certain solid materials when they are divided into fibers and packed in layers are good insulators because the material itself is a poor conductor and there are also certain air spaces within the layers.
Double walls of buildings that have an air gap provide additional insulation.
Convection
Convection is the transfer of heat due to the movement of air or liquid. When water is heated in a glass container, the movement within the container can be observed. If a certain amount of sand is added, the movement becomes more apparent. As the water is heated, it expands and becomes light, causing the upward movement. In the same way, the air is heated near the steam radiator by conduction. As the heated air moves up, the cold air takes its place at the bottom. When liquids and gases are heated, they begin a movement within themselves. This motion is different from the molecular motion discussed in heat conduction and is known as convection heat transfer.
The hot air in a building will expand and rise. For this reason, fire that spreads by convection does so mostly in an upward direction, although air currents can carry heat in any direction. Convection currents are generally the cause of the movement of heat from one floor to another, from one room to another and from one area to another. The spread of fire through corridors, stairways and elevator ducts, between walls, and through facades are mainly caused by the convection of hot currents and this carries a greater influence on the position of attack of the fire and ventilation that has occurred by radiation and conduction.
Another form of convection heat transfer is by direct contact with the flame. When a substance is heated to the point where flammable vapors are generated, these vapors can ignite, generating a flame. As other flammable materials come into contact with ignited vapors, or flames, the same. they can be heated to a temperature where they too can ignite.
Radiation:
The warmth of the sun is felt as soon as it appears. When the sun goes down, the earth begins to cool with similar speed. We carry an umbrella to protect us from the heat of the sun. A jet of mist interposed between the firefighter and the fire will minimize the heat the firefighter receives. Although air is a poor conductor, it is obvious that heat can travel where matter does not exist.
This phenomenon of heat transmission is known as heat wave radiation. Light and heat waves are similar in nature, but differ in cycle length. Heat waves are longer than light waves and are sometimes called infrared rays. Radiation heat will travel through space until it reaches an opaque object. As the object is exposed to radiation heat, it will emit radiation heat from its surface. Heat by oxidation is one of the major sources of fire rates, and its importance demands immediate attention at those points where exposure to radiation is severe.
STAGES IN THE DEVELOPMENT OF THE FIRE
Not all fires develop in the same way, although all can go through four stages of development, if not interrupted in time. To stop any fire that develops near us in time it is important to know this information.
Incipient stage: It is characterized because there are no flames, there is little smoke, the temperature is low; large amount of combustion particles is generated. These particles are invisible and behave like gases, rising towards the ceiling. This stage can last for days, weeks and years (a Sequoia tree in California, on the trunk of which a person smoked a lit cigarette, was in this stage for three years).
Latent stage: No flame or significant heat yet; begins to increase the amount of particles until they become visible; now the particles are called smoke. The duration of this stage is also variable.
Flame stage: As the fire develops, the ignition point is reached and flames begin. Lower the amount of smoke and increase the heat. Its duration can vary, but generally the fourth stage develops in a matter of seconds.
Heat stage In this stage, a large amount of heat, flames, smoke and toxic gases are generated.
COMBUSTION PRODUCTS
When a material (fuel) ignites, it undergoes a chemical change. None of the elements that make up the material are destroyed in the process, but all matter is transformed into another form or state. Even when dispersed, the products of combustion are equal in weight and volume to those of fuel of combustion. When a fuel catches fire, four products of combustion are generated: gases, flame, heat, and smoke.
A burning fuel generates new and numerous combustion products.
Heat is a form of energy that is measured in degrees of temperature to signify its intensity. In this sense, heat is the product of combustion responsible for the spread of fire. In a physiological sense, it is the direct cause of burns and other forms of personal injury. Injuries caused by heat include dehydration, exhaustion, and injuries to the respiratory tract, in addition to burns.
The flame is the visible, the luminous body of a burning gas. When a burning gas is combined with the proper amount of oxygen, the flame becomes hotter and less luminous. This loss of luminosity is due to the complete combustion of the coal. For these reasons, the flame is considered as a product of combustion. Heat, smoke, and gas, however, can generate certain types of smoldering fires without the evidence of flame.
The smoke found in most fires consists of a mixture of oxygen, nitrogen, carbon dioxide, carbon monoxide, tiny particles of carbon and by-products that have been released from the involved materials.
Some materials emit more smoke than others. Included fuels generally generate dense black smoke. Oils, paints, varnishes, molasses, sugar, gums, sulfur, and many plastics also generally emit large amounts of black smoke.
Fire Control
FIRE EXTINGUISHING THEORY
The extinguishing of the fire is based on the interruption of one or more factors of the essential elements of the combustion process. Flame combustion can be extinguished by reducing the temperature, removing fuel, oxygen, or stopping the chemical chain reaction. If the fire is in its latent stage, there are only three options for extinguishing: reducing the temperature, eliminating the fuel, and diluting the oxygen.
Extinction by Temperature Reduction:
One of the most common extinguishing methods is by water cooling. The quenching process depends on cooling the fuel to the point where not enough vapors are produced that can ignite. If we look at the types of fuels and steam ducting, we will find that solid and liquid fuels, and flammable gases with a low flash point cannot be extinguished by water cooling because steam production cannot be significantly reduced. The temperature reduction depends on the application of an adequate flow rate, and in an appropriate way to achieve a negative heat balance.
Extinction by Elimination of Fuel:
In some cases, a fire can be efficiently extinguished by removing the fuel source. This can be accomplished by stopping the flow of a liquid or gaseous fuel, or by removing the solid fuel from the gaseous area of the fire. Another method of fuel removal is to allow the fire to continue until the fuel is consumed.
Extinction by Oxygen Dilution:
The method of extinguishing by dilution of oxygen is the reduction of the oxygen concentration within the fire area. This can be accomplished by introducing an inert gas into the fire or by separating the oxygen from the fuel.
This extinguishing method will not be effective on auto-oxidizing materials or on certain metals that are oxidized by the effects of carbon dioxide or nitrogen, two of the most common extinguishing agents.
Extinguishing by Chemical Inhibition of Flame:
Some extinguishing agents, such as dry chemical and halon, interrupt the production of flame in the chemical reaction, resulting in rapid extinction. This extinguishing method is effective only on liquid fuels and gases since they cannot burn in the form of latent fire. If latent phase quenching is desired, additional cooling capacity is required.
Extinguishing Products
CLASSIFICATION OF FIRE AND EXTINGUISHING METHODS
Class A fires:
Fires involving ordinary combustible materials such as wood, clothing, paper, rubber, and some plastics.
Water is used for cooling purposes in order to reduce the temperature of burning materials below their ignition temperature.
Class B fires:
Fires involving flammable liquids, greases, and gases.
The suffocation effect by exclusion of oxygen is the most effective. Another method of extinguishing includes removing the fuel and reducing the temperature.
Class C fires:
Fires involving energized electrical equipment.
These types of fires can be controlled by means of a non-conductive extinguishing agent. The safety procedure is to try to de-energize high voltage circuits and treat it as a Class A or B fire, depending on the fuel involved.
Class D fires:
Fires involving combustible metals, such as magnesium, titanium, zirconium, sodium, and potassium.
The high temperatures of some burning metals make water and other common extinguishing agents inefficient. There is no effective fire control agent available on all types of combustible metals. There are special extinguishing agents for fire control for each of the metals and they are specifically identified for that metal.
| Fire | Types of materials | Extinction method |
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A class |
Ordinary fuels such as:
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Class B |
Fires involving:
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Class c |
Fires involving:
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Class D |
Fires involving combustible metals such as:
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AGENTS TO FIGHT FIRE
The extinguishing agents used in portable equipment are varied, therefore, the degree of effectiveness of each of them and the limitations regarding their applicability are also varied. They can be classified into:
Water:
Due to its abundance, it is the most common in firefighting operations. Its extinguishing action is based on the cooling of burning matter. It is applied in the form of a jet under pressure, or also as a very fine mist over the entire lit surface. It is fine, in large quantities thrown through hoses or sprinklers on relatively small quantities of oil.
This extinguishing agent can only be applied on CLASS “A” fires, and in some cases CLASS “B” fires, such as oil fires, where it must be applied in the manner described. They should never be used on CLASS “C” fires, and even less so on CLASS “D” fires.
Foam:
Foam can be produced by mixing a solution of aluminum sulfate with another of sodium bicarbonate and adding a stabilizer. In portable equipment, the foam is produced by the reaction of the two solutions mentioned, originating an internal pressure as a consequence of the generation of carbon dioxide, capable of driving the foam at a distance of seven (7) meters.
The foam will remove the oxygen by forming a layer that prevents the passage of air, and also cools a little. It is applied in the form of a layer that covers the surface of the burning liquid. However, in many cases, the vapors given off by burning substances pass through the foam layer, and if their concentration is sufficient, they will burn on top of it.
It is especially useful in extinguishing CLASS B fires, and in which the suffocating effect of the extinguishing agent is of great importance. Certain solvents (alcohols, acetone, etc.) break down the foam, so it is not convenient to use it in fires of these substances. As it conducts electricity it should never be used on CLASS C fires. It is also contraindicated for CLASS D fires.
Carbon Dioxide CO2:
This gas can be stored under pressure in portable fire extinguishers and discharged through a special nozzle at the site where it is needed. The extinguishing characteristic of carbon dioxide gas is its suffocation effect accompanied by a slight cooling. It should not be used in closed or poorly ventilated areas, since the user can be subjected to mechanical suffocation due to insufficient oxygen. It is suitable for CLASS B and CLASS C fires. It is not suitable for other classes of fire.
Dry Chemical Powder:
It is made up of non-combustible mixtures of finely powdered products, such as Sodium Carbonates, Sodium Bicarbonate, Sodium Sulfate, Sodium Silicate, Bentonite, etc. It acts by drowning as it is applied trying to form a layer on the burning matter.
In portable equipment, this extinguishing agent is expelled by the pressure released by a nitrogen capsule, located inside the extinguisher and which is broken at the time of use.
Depending on the composition of the mixture, the chemical powder is suitable for CLASS A, B, and C fires. Not suitable for CLASS D fires.
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