What term denotes the minimum concentration of fuel vapor and air that can support combustion?

• A. Vaporization
• B. Vapor Pressure
• C. Lower Explosive Limit (LEL)
• D. Upper Explosive Limit (UEL)

Answer: (C)

The minimum concentration of fuel vapor in air that can support combustion is the lower explosive limit. This marks the lean end of the flammability range: below it, there isn’t enough fuel to sustain a flame even if an ignition source is present. Between the lower and upper explosive limits, there is a sufficient balance of fuel and oxygen for combustion to occur. Above the upper limit, the mixture is too rich to burn because there isn’t enough oxygen to sustain ignition. Terms like vaporization describe the process of turning liquid into vapor, and vapor pressure describes how readily a substance evaporates—these describe properties of the substance, not the flame-fuel concentration range.