270
7 The Role of Ventilation in Fires and Explosions
(equilibrium law) as a first approximation for the limits of explosiveness (Eqs. 7.20
and 7.21):
LEL(% v ) =
100
c 1
LEL 1
+
c 2
LEL 2
+
c 3
LEL 3
+ · · · +
c i
LEL i
(7.20)
UEL(% v ) =
100
c 1
UEL 1
+
c 2
UEL 2
+
c 3
UEL 3
+ · · · +
c i
UEL i
(7.21)
where
• c i : Concentration of each fuel gas relative to all fuels, expressed as a percentage
of volume.
• LEL i : Lower Explosive Limit of gas i.
• UEL i : Upper Explosive Limit of gas i.
Exercise 7.5 Estimate the Lower Explosive Limit for a mixture of 1% hexane and
3% methane in air.
Data:
Gas
LEL (%)
Hexane
1.1
Methane
5
Solution
There is a total of 5 + 1.1 = 6.1% of flammable gases, and therefore, 93.9% of
air. Hence, the concentration of each of the two gases over the total of combustible
gases (6.1%) is as follows:
c hexane = 100
1.1
6.1
= 18.03
c methane = 100
5
6.1
= 81.97
Therefore:
LEL(% v ) =
100
18.03
1.1
+
81.97
5
= 3.05%
Since the mixture is 1 + 3 = 4%, higher than the determined 3.05%, the mixture
must be considered dangerous.