FIRE
| Fuel + Oxygen + Heat = Fire |
Fire is a chemical reaction initiated by presence of heat energy in which a substance combines with oxygen (from air). The process involves giving out heat energy (exothermic reaction), light and sometimes sound.
A combustible substance – fuel The following are essential for fire:
- Oxygen, Nitrogen (e.g. Magnesium burns in Nitrogen)
- Heat source, spark or flame
- Process of chain reaction.
Types of Combustion:
- Rapid: Gas is ignited, producing heat and light
- Spontaneous: Without the application of external heat.
- Explosion: Combustion in confined space under pressure. Heat, light and sound is produced.
Stages of Fire:
- Incipient stage: Preheating and gasification (slow pyrolysis) is in progress. Invisible pyrolysis produces gas.
- Submicron size: Aerosols (tiny particles) are found in the vicinity of fire.
- Smouldering stage: Lasting for 4 hours – gas and smoke.
- Radiation stage: Convective heat
- Heat stage: Heat, flame, smoke, toxic gas – for few seconds.
Development of a Fire
A fire develops typically in four stages, and fire detectors are designed to detect some characteristic effect of one or more of these stages:
* Incipient stage. No visible smoke, no flame and very little heat. A significant amount of invisible (but sometimes smellable) combustion particles may be created. This stage usually develops slowly.
* Smouldering stage. Smoke, but no flame and little heat.
* Flame stage. Visible flame, more heat, often less or no smoke, particularly with flammable liquids and gas fires.
* Heat stage. Large amounts of heat, flame, smoke and toxic gases are produced. The transition from the previous stage can be very fast.
Classes of fire : To make it easier to select the appropriate extinguishing media according to the nature of the material undergoing combustion, fires are arranged in ‘Classes’.
| Class A | Wood, paper, textile, rubbish, grass etc. | Water is the best extinguishing medium for Class A. Some Dry Chemical Powder(DCP) are also used. |
| Class B | Flammable liquids. (Oils, petrol, varnishes, paints, solvents, grease.) | Foam is the best extinguishing medium for Class B fires. Its reactivity should be ensured in case of Chemical fires. |
| Class C | Flammable gases | Dry Chemical Powder is widely used for extinguishing gaseous fires. Its ability to cut the chain reaction in the combustion process makes it suitable for the purpose. |
| Class D | Burning metal viz. Magnesium, Aluminium, Zinc, | Steam, Dry Chemical Powder be used against metal fires. |
| Class E | Fires of electrical origin involving transformers, circuit breakers, switchgears | Dry sand may be used. CO2 type extinguisher to be used. DO NOT USE WATER. |
| Class F | Cooking oil, fats (animal and vegetable) | Wet Chemicals to cool and emulsify. |
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Fire Extinguishers: The following is a quick guide to help choose the right type of extinguisher.
Class A extinguishers are for ordinary combustible materials such as paper, wood, cardboard, and most plastics. The numerical rating on these types of extinguishers indicates the amount of water it
holds and the amount of fire it can extinguish. Geometric symbol (green triangle)
Class B fires involve flammable or combustible liquids such as gasoline, kerosene, grease. The numerical rating for class B extinguishers indicates the approximate number of square feet of fire it can extinguish. Geometric symbol (red square)
Class C for fires of gaseous origin, using dry chemical powder.
Class D fire extinguishers are commonly found in a chemical laboratory. They are for fires that involve combustible metals, such as magnesium, titanium, potassium and sodium. These types of extinguishers also have no numerical rating, nor are they given a multi-purpose rating – they are designed for class D fires only. Geometric symbol (Yellow Decagon)
Class E fires involve electrical equipment, such as appliances, wiring, circuit breakers and outlets. Never use water to extinguish class E fires – the risk of electrical shock is far too great! Class E extinguishers do not have a numerical rating. The E classification means the extinguishing agent is non-conductive. Geometric symbol (blue circle)
Class F fire extinguishers are for fires that involve cooking oils, trans-fats, or fats in cooking appliances and are typically found in restaurant and cafeteria kitchens. Geometric symbol (black hexagon)