What is the LFL?
The LFL (Lower Flammable Limit) is a concept related to flammability & fire safety.
The LFL is defined as :
Usually expressed in volume per cent, is the lower end of the concentration range over which a flammable mixture of gas or vapour in air can be ignited at a given temperature and pressure. The flammability range is delineated by the upper and lower flammability limits. Outside this range of air/vapor mixtures, the mixture can not be ignited…
For ethanol, aka alcohol, the LFL is 3.3% by volume. Meaning, if ratio of ethanol vapor to air is less than 3.3%, the vapor can not ignite.
This diagram below demonstrates the limits of the LFL of ethanol, as well as the UFL (Upper Flammable Limit).
Why Does This Matter?
Knowing what the LFL is, and ensuring that flammable vapors, such as ethanol vapors, remain under this limit is a commonly used, well established, and reliable method of ensuring a safe, fire free environment.
In order to have an alcohol fire, you need:
- The Fuel (Alcohol)
- Ignition Source (Spark, Heat)
Take away one of these elements, and you can avoid a fire! In this case, we commonly remove the fuel, at least most of the fuel to stay under the LFL!
How do we stay under the LFL?
I’ll get right to it…VENTILATION.
This is the key in keeping under the LFL and keeping safe. If you exhaust out more ethanol vapors than can ever be accumulated and you’ve ensured that the flammable range can never be reached. For most requirements in the codes using the LFL to avoid fires, it usually requires 25% of the (3.3%) LFL. So, for ethanol that would be .825% ethanol vapor in the air. This is essentially a factor of safety.
As shown in the photo below, exhaust is usually achieved via duct, brought to 12″ above the floor, because ethanol vapor is heavier than air, and will settle near the floor. These exhaust ducts may be near any potential ethanol vapor leak, such as near a still (for a possible breach), near tanks, and near barrels. Exhaust is normally powered by an exhaust fan on the roof, and there must be an intake make up air vent to replace the exhausted air.
Keeping under the LFL is a very important safety feature in a distillery. It takes a preventative approach of avoiding a fire in the first place, rather than mitigating a fire after it has occurred.