In our previous discussion, we looked at some facts about fire. Fire, as we all know, does occur at a particular spot but spreads to other areas. In our discussion for today, we want to delve into how fire actually starts and spreads to other areas in a building or in the open.
In our homes, if we want to cook using liquified petroleum gas (LPG), we turn on the regulator on the gas cylinder and the stove and lit a match to ignite it. If you’re using charcoal or fire wood, you either use a match or burning coal. This is what we call direct burning. It is a direct burning in the sense that you’re directly setting or applying fire to a combustible material or object. This is more or less a conscious activity. However, there are other ways by which people directly set fire to objects unconsciously. For instance, when a smoker drops his live cigarette piece on the ground where there is a combustible material, fire is likely to occur. In order to avoid this kind of fire, some companies have designated smoking areas. It is expected of every smoker to kill the very small fire in his stick of cigarette by trampling it under feet or pressing into the ash tray. The rest of the ways fire spreads are closely linked to direct burning or heat transfer.
The next way fire can spread is known as convection. This results from heat transfer to a flammable material. Here, the hot air, which is lighter, rises and cold air sinks. Hot gases generated by fire rise straight up from the fire and spread inside the building. This hot gas will hit the ceiling and then spread out to form a layer underneath the ceiling. When the hot gas touches the ceiling or curtain or curtain pole, that material may heat up sufficiently so that it bursts into flames eventually. The fire was originally not set to any of those combustible materials. But the heat was transferred from the burning spot to those materials.
Fire transfer by means of convection is not limited to indoors but outdoors as well. The convection currents contain embers that are carried on the currents until they air the air that is carrying them becomes cool and the embers are dropped to the ground. This is a common way for forest fires to travel and jump over obstacles e.g. road or footpath. A typical example is this: A welder was doing some cutting work on a structure. After he was done with his work, he gathered his tools and left. Later in the night, fire gutted that part of the building. Investigation revealed that there were combustible materials behind (at the back of) the structure which he had no prior knowledge of. So, the sparks from his cutting were actually settling on the combustible material which resulted in the fire. In fact, it was no business of his to check the back of the wall where he was doing the work. But the simple rule is that, in carrying out such works, you must first inspect the working area and do proper housekeeping before and after the work. Truth is, we don’t do it because we see it as ‘waste’ of time. A time that is reasonably or sensibly wasted, can save you a lot of troubles including the lost life and property.
Radiation is another way fire spreads. This is where heat travels through space to a combustible material. In other words, heat energy can be radiated through air in the form of infrared heat waves which travel in straight lines just like light and can pass through transparent surfaces such as glass. Radiant heat generated by fire shines onto nearby surfaces and is absorbed. If the material heats up sufficiently, it can burst into flames. If you have ever looked into a mirror that has a direct sun rays reflecting into it, then you would better understand this concept of radiation. Apart from that, I am sure you may have experienced something on the street where a glass radiates heat from the sun into your eyes. When I was a kid, we used to use pieces of mirrors to direct the sun rays into the eyes of our peers. In both instances, you feel some burning on your skin of face.
Finally, heat can travel through a wall (metal or block) to a combustible material that has been place against it or close to it. Heat is transferred through solid materials. Some metals are good conductors of heat e.g. copper. So, any pipe, wire, ducts, conduits or services running from one room to another can adequately act as means for heat and spread the fire. This is known as conduction.
The emphasis is on how fire spreads by means of heat transfer in our environment. Anytime you’re cooking using gas, ensure that you don’t forget to turn off the stove. If you’re heating water using water heater, remember to unplug the heater. If you enter your kitchen and smell LPG, quickly open your doors and windows. Don’t use your phone or turn on any of the plugs and switches in your room. All of these give very minute sparks that can lit up the whole building.