Combustion depends on having a mixture of air and flammable gas in certain proportions. If the amount of flammable material is too low or too high, or if the amount of oxygen is too low, then ignition will not take place.
For a flammable or explosive gas the delimiters for ignition are called the lower and upper explosive or flammable limits (LEL or LFL for lower limit, UEL or UFL for upper limit).
The concentration for the flammable or explosive gas is often reported in percent by volume.
If the concentration of the gas is below the lower limit, then the air-gas mixture is too “lean”.
If the concentration of the gas is above the upper limit, then the air-gas mixture is too :rich”.
If the concentration of the gas is between the upper and lower limits then the air-gas mixture is just right.
Factors affecting the limits:
(1) temperature
(2) concentration of oxygen
(3) atmospheric pressure
(4) presence of other combustible gases
(5) presence of retardants
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