Fire.  The basic essential elements for a fire to exist.

FUEL.  Solid, Liquid, Gas.                                

 

HEAT.  Chemical reaction, Electrical energy, Friction or Nuclear reaction.

OXYGEN.  Normally in the surrounding air of approx 19%.

Given the right proportions of Fuel, Heat and Oxygen almost anything will burn.  This will then start the  CHEMICAL CHAIN REACTION.  (Please see powerpoint).

 

There are three states of matter that are as follows.  

Solids - Paper,Wood, Fabrics etc.

Liquid - Petrol, Kero, Meths, Paints, etc.

Gas - LPG.

Of the three states of matter only the Gas burns. Nearly everything in life can go through these stages Solid to Liquid and then Vapour.  (Think obout heating a block of ice although water is one state of matter that does not burn this is why it is a very effective fire extingushing agent). All states of matter burn at different temperatures having different Flash Points, Fire Points and Auto Ignition temperatures are explained as follows:-

FASH POINT

 This is the lowest temperature of a liquid when at which time there are adequate vapours being given off to form a flammable mixture.  When a heat source is applied it will flash momentarily and will not continue to burn.

FIRE POINT

 This is when a liquid is heated past its Flash Point which is approximately 10% higher than the Flash point.  This is the time when adequate vapor is given off to maintain combustion after an external heat source has been introduced and it will continue to burn.

AUTO IGNITION TEMPRATURE.
This is the minimum temperature at which a substance will burn or ignite without any external heat source.  (Think about Cooking Oil fires).

 

Take a look at some examples below.  (these are average temperatures).

Smouldering and Flaming fires.

Smouldering.  This means to burn slowly with smoke but NO flame. Only porous materials can smoulder and will produce char.  This will glow and produce larger smoke particles due to the unburnt matter produced.

Flames however are visible.  They are a hot glowing body of ignited gas that is generated by something on fire. Most of the vapours will be consumed producing smoke particles that will be smaller.
Spontaneous Combustion.

Spontaneous Combustion.  This is the ignition of organic matter without apparent cause, especially through heat generated internally by rapid oxidation.  This chemical reaction requires no external ignition.

Smoke The Facts.

 It is estimated that 50-80% of fire deaths result because of smoke inhalation.  Smoke kills or injures by a combination of thermal damage, poisoning and pulmonary irritation caused by carbon monoxide and other combustion products. Symptoms include Coughing, vomiting, nausea, sleepiness, confusion, difficulty breathing and burns to the nose, mouth and throat

 

Particulate smoke is the product of incomplete combustion.  It is generated in both smouldering and flaming combustion. High molecular weight fractions condense as they mix with cool air to give an aerosol consisting of minute droplets of tar and boiling liquids.

Smoke from flaming combustion is different and it consists almost entirely of solid particles.  Most solid particles are formed in the gas phase as a result of incomplete combustion and high temperatures. Both types of smoke are combustible due in part to the unburned and partially burned fuel vapours that are present along with particles.

Smoke particles are complex groups of minute soot particles that come together to form chains and clusters. Gases including CO (carbon monoxide) and HCN (hydrogen cyanide) are present in smoke.  

 

For example Carbon Monoxide as an can act as an Asphyxiant and can cause hypoxia, impaired judgement,  poor motor coordination, increased respiration rate and loss of consciousness leading to death. As a Narcotic it can cause carboxyhemoglobin and when the blood stream is saturated by 50% carbon monoxide this is lethal.

 

 

Some Factors That Affect Smoke Production.

 

Ø      The combustion phases.  Flaming or smouldering.

Ø      Moisture content.  During the combustion phase’s moisture is evaporated producing copious amounts of white smoke which is mainly water vapour.  This is a less intense fire producing larger smoke particles.

Ø      The temperature and oxygen levels.  During the combustion phases if the temperature and oxygen levels are low smoke particles will be much larger.

Ø      The size and shape of the fire.  The higher the surface to volume ratio allows the moisture in the fuel to escape quickly allowing the fuel to burn hotter therefore producing less smoke.

Ø      The qty and type of fuel burning.  Under free burning conditions, oxygenated fuels such as wood and polymethylmethacrylate give substantially less smoke than hydrocarbon polymers such as polyethylene and polystyrene. (Drysdale, 1999).

Ø      Movement of gasses.  The flume draws air in which mixes and cools the fire causing large quantity’s of smoke.

Ø      The Presence of fire retardants.

 
Three Methods of Heat Transfer.

Conduction.  The transmission of heat through a medium from a region of higher temperature to a region of lower temperature.

 

Radiation.  During this process there are no physical mediums to transfer the heat.  Heat is transferred across an open space by electromagnetic waves.  These waves travel like light and can be absorbed, transmitted and reflected.  As the fire grows the heat intensifies and it will determine the fire spread between distant objects due to radiation.

 

Convection.  This is the flow of heat through matter from a hot area to a cool area.  An example would be a pot of water boiling on a stove.
Flashover and Backdraught.

Flashover.   Occurs when the transition is made from a fuel controlled fire to a ventilation controlled fire.  There will be a sudden event of fire growth which rapidly leads to a full room fire.  Generally if the intensity is about 20KW/M² at floor level or if the ceiling temperature reaches about 600° C flashover could occur.  Flashover  does not always occur in a fire.  If the fuel burns out or oxygen depletion occurs there will be no flashover. 

Post flashover  is the major concern for a building when a fire is fully developed.

High temperatures occur and this could produce a structural fault within the building possibly leading to collapse. 

STAGES OF A FIRE.

 

Backdraught    When a fire is starved of oxygen it will self extinguish or die down and burn slowly dictated by the amount of air that enters a room.  Backdraught occurs when a door is opened and a sudden rush of air enters the area, this is then followed by rapid burning of the accumulation of unburnt smoke and fuel vapours. 

Backdraught may lead to a fully developed fire as a result of sudden venting, however, flashover does not require a sudden inflow of air.

 

Fire severity in a room is due to a number of factors.

Ø      Size of vents.

Ø      Temperature inside the room determined by either ventilation controlled or fuel controlled fires.  Fuel controlled burn at lower temps due to the release of heat through the vents.  

Ø      The construction of the building.

Ø      The area of the compartment.

Ø      The type of fuel burning inside the room.

 

Flame extension from a window is due to a number of factors.

Ø      Window dimensions and shape.

Ø      Sill height of vent.

Ø      The materials burning inside the building and the size of the surface area will increase flame length.

Ø      Heat release rate.

Ø      How much air is in the compartment?

Ø      Wind strength.  The flame will reduce in length.

Ø      The height of the spandrels.

Ø      Size of the balcony’s.

Ø      Fires on the floors below, flames will lengthen due to the lack of oxygen.

Ø      Position of the fire in the compartment for example, flame height increases dramatically of up to 20% if in the corner of a room.

 

Burning Wood.

1.   After wood has been cut it retains about 50% water content.

2.  After it has been seasoned wood still holds about 20% water content.

3.  It takes about 12 months for wood to season. (in the right atmosphere)

4.  Softwood for example Pine burns quicker as it is less dense.

5.  Dense hard woods ie. Redgum will produce a longer lasting fire.

 

For combustion to take place the wood is required to be sufficiently heated to produce vapours. For these vapours to combust the right mixture of Fuel, Oxygen and Heat are essential to maintain the combustion process.  This will then cause a chemical chain reaction to make burning possible this is best described as a rapid oxidation process. The wood will go through various stages of combustion.  As the wood is heated in excess of 100c the moisture begins to evaporate out of the logs causing a large amount of white smoke which mainly consists of water vapour.   I suspect this probably slows the combustion process down further by causing a slight smothering effect. This water has to be driven out of the wood as it is consuming the heat energy required for sustained burning to take place. As it is heated above 200c – 250c wood discolours and a char forms. As the temperature of the wood rises small cracks appear parallel to the grain at this point various gases and tar are released including CO, Co2, HCN and Various tars.  Although these gases are released they will not burn until the moisture evaporates and the temperature is increased.