Fire

1
Fire

• Fire (or combustion) is a chemical reaction in
  which a substance combines with an oxidant and
  releases energy. Part of the energy released is
  used to sustain the reaction, or
• Fire can be considered as the rapid exothermic
  oxidation of an ignited fuel.

2
The Fire Triangle (Fig. 3-1)

• Fuels
• Liquids gasoline, acetone,ether, pentane.
• Solids plastics, wood dust, fiber, metal
  particles.
• Gases acetylene, propane, CO, hydrogen.
• Oxidizers
• Gases oxygen, fluorine, chlorine.
• Liquids hydrogen peroxide, nitric acid,
  perchloric acid.
• Solids ammonium nitrite, metal peroxides.
• Ignition Sources
• sparks, flames, static electricity, heat.

3
Flammability Limits

• Vapor-air mixtures will only ignite and burn over
  a well-specified range of compositions.
• The mixture will not burn when the composition is
  lower than the lower flammable limit (LFL) or
  higher than the upper flammable limit (UFL). The
  commonly used units are volume percent fuel in
  mixture. (Table 3-1)
• The explosive limits (LEL and UEL) are used
  interchangeably with LFL and UFL. (Figure Next)

4
LFLs and UFLs of Vapor Mixtures

• The Le Chatelier equations

5
Temperature Dependence

• The flammability range in general increases with
  temperature.
• Where is the positive net heat of
  combustion (kcal/mole) T is the temperature (
  ).

6
Pressure Dependence

• Pressure has very little affect on the LFL except
  at very low pressures, where flames do not
  propagate (see Fig. 3-2). The UFL increases
  significantly as the pressure is increased (see
  Fig. 3-3).
• Where P is the pressure in mega pascals absolute
  UFL is the upper flammable limit in volume at 1
  atm.

7
Estimating Flammability Limits

• where Cst is volume fuel ( ) in
  mixture.

8
Flash Point

• The flash point (FP) of a liquid is the lowest
  temperature at which it gives off enough vapor to
  form an ignitable mixture with air. At flash
  point the vapor will burn, but only briefly
  inadequate vapor is produced to maintain
  combustion. (Figure 3-5, Table 3-1)
• The flash point generally increases with
  increasing pressure.

9
Fire Point

• The fire point is the lowest temperature at which
  a vapor above a liquid will continue to burn once
  ignited the fire point temperature is higher
  than the flash point.

10
Ignition

• Ignition Ignition of a flammable mixture may be
  caused by a flammable mixture coming in contact
  with a source of ignition with sufficient energy
  (see Table 3-3) or the gas reaching a temperature
  high enough to autoignite.
• Autoignition Temperature (AIT) A fixed
  temperature above which a flammable mixture is
  capable of extracting enough energy from the
  environment to self-ignite.

11
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