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Contents

Section 1 Introduction

Section 2 Characteristics of Fire

Section 3 Stages of Combustion

Section 4 Smoke Dynamics 

 

Description:  The purpose of this unit is to enable you to describe the principles and characteristics of fire and smoke dynamics.

Author:  Gates MacBain Associates


Section 1  Introduction


The majority of people who die in fires are not burnt but are overcome by the smoke and gases. In order to evaluate the risk to people in buildings and to the building itself you will need to understand the way fires grow and how smoke and poisonous gases can spread through a building.  

A fire in a building can generate thick, black smoke that can obscures vision, block the escape routes and cause great difficulty in breathing. Smoke is a serious threat to life which should not be underestimated. In this unit we will look at the principles and characteristics of fire and smoke and their dynamics.




Section 2  Characteristics of Fire




Aims and Objectives 

At the end of this section you should be able to:
  • Explain the requirements needed for a fire to start and the way that a fire can spread.


Requirements of a Fire For a fire to start and be maintained it requires three things. This is known as the fire triangle, which can be shown as the illustration below.  The combination of these three things of heat, fuel and oxygen, creates a chemical chain reaction which results in combustion. The combustion process is usually associated with the oxidation of a fuel in the presence of oxygen with the emission of heat and light.    


Fire_Triangle






The Fire Triangle 


Details of this process can be found on the Firesafe website which can be accessed from the link below.  


Fire Spread 

Fire is spread by three methods:  
  • Convection
  • Conduction
  • Radiation
Fire and smoke is spread by convection and is the most dangerous as it causes the major proportion of injuries and deaths. When fire starts in an enclosed space such as a building, the smoke rising from the fire becomes trapped by the ceiling and then spreads in all directions to form an ever-deepening layer over the entire room space. The smoke will pass through any holes or gaps in the walls, ceiling and floor into other parts of the building. The heat from the fire gets trapped in the building and the temperature rises.  

Some materials, such as metal can absorb heat and transmit it to other rooms by conduction, where it can set fire to combustible items that are in contact with the heated material.  

Radiation transfers heat in the air in the same way that an electric bar heater heats a room. Any material close to a fire will absorb the heat until the item starts to smoulder and then burn.   



Websites



Self-Assessment Task

  • Discuss the factors which must be present for a fire to start and explain the methods by which it can spread.





Section 3  Stages of Combustion




Aims and Objectives

At the end of this section you should be able to:
  • Explain the stages that a fire will go through.


There are a number of stages for a fire, which are dependent on the three requirements of fuel, heat and oxygen; if one of the requirements is not present or is insufficient the fire will not ignite.  

The stages are: 

Induction – This is the early stage of a fire and may be over a long period of time.  It is when the process of combustion starts to take place with the build-up of heat. This is also known as the inception stage.  Once the heat has built up providing there is oxygen ignition can take place. 

Growth – Once ignition occurs the fire may smouldering initially then the fire will grow quickly, so it is also referred to as the flaming stage. The growth rate will then reach a plateau and will remain at that level until the fuel is consumed.  

Decay - This is the final stages of a fire. As the fuel runs out the combustion process will quickly slow down, and eventually come to a stop resulting in the fire going out. 

The stages can be seen in the drawing below.   
















A number of other factors have an influence on the likelihood of a material combusting especially the material and the form that it is in.  Details relating to this can be found at the link ‘Principles of Fire’ below.   



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Self-Assessment Task

  • With the aid of a diagram, explain the stages of a fire.





Section 4 Smoke Dynamics




Aims and Objectives

At the end of this section you should be able to:
  • Understand the make-up and behaviour of smoke and how it must be considered in the event of a fire in a building.


Before we look at the dynamics of smoke it is worth considering what smoke is. This can be defined as: 

“a process involving rapid oxidation at elevated temperatures accompanied by the evolution of heated gaseous products of combustion, and the emission of visible and invisible radiation.” 

Or put more simply:  

“the visible vapour and gases given off by a burning or smouldering substance, the mixture of gases and suspended carbon particles resulting from the combustion of wood, peat, coal, or other organic matter.”    


Behaviour of Smoke  

Smoke, like water, is able to find any pathways in a building, this means it is able to migrate around a building using any hole or gaps in the structure working its’ way through a building from room to room and floor to floor. It will also make use of any ventilation or air conditioning systems unless measures are included to prevent this from happening. 

As hot air rises, hot smoke will move to the upper levels of a structure, which can have the effect of trapping people on the upper floors of a building and, if it gets into a stair well, can prevent people from escaping. 


An in depth look at smoke can be found by visiting the ‘Smoke’ webpage indicated below.  



Websites

  • Smoke    Wikipedia Link - this provides an overview and links to further study; it should not be quoted, referenced or taken as an authoritative source.



Self-Assessment Task

  • Explain the behaviour of smoke in a building fire and how it can affect the occupants.





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