In B28 mechanical rooms, the refrigerant concentration should be kept below which threshold to reduce fire risk?

• A. Lower flammable limit (LFL)
• B. Lower explosive limit (LEL)
• C. Upper flammable limit (UFL)
• D. Upper explosive limit (UEL)

Answer: (A)

The key idea is that fire risk from a flammable refrigerant comes from vapor in air reaching the flammable range. The lower flammable limit is the minimum concentration of refrigerant vapor in air needed for a flame to propagate. By keeping the concentration below this limit, there isn’t enough fuel in the air to sustain combustion, so an ignition source won’t produce a flame. In mechanical rooms using flammable refrigerants, ventilation and leak control are designed to keep the vapor concentration well under this threshold, which is why this limit is the safety reference. The exact value varies by refrigerant, but it’s typically only a few percent by volume, so dilution and proper detection are crucial.