In view of the vague understanding of foam fire extinguishing agents among some fire rescuers and enterprise staff, manufacturers have sorted out relevant materials based on recent learning and summarized basic knowledge of foam agents and extinguishers, with professional terms supplemented with practical interpretation.

Basic Concepts of Foam Fire Extinguishing Agents
1. Foam Fire Extinguishing Agent
An effective suppressant for combustible liquid fires. It forms a dense floating foam layer on liquid surfaces to achieve smothering and cooling effects.

Fire Extinguishing Principle
When sprayed onto burning liquid, foam floats and covers the surface. Being poor heat conductor, it insulates heat and absorbs thermal energy to lower surface temperature and slow down liquid evaporation. The viscous foam barrier blocks vapor penetration and isolates liquid from fire zone. Without continuous flammable vapor supply, the fire will die out. Direct water jet will break the foam blanket, so water and foam cannot be used simultaneously on such fires.

This agent is not applicable to metal fires, water-reactive substance fires, explosive material fires and live electrical equipment fires, as foam contains large volume of water.

2. Foam Concentrate
Also known as stock solution. It is concentrated liquid mixed with water at specific proportion to form foam solution.

3. Foam Solution
Blended liquid prepared by mixing foam concentrate and water at specified ratio. It only delivers cooling effect rather than fire suppression effect when applied to oil fires.

4. Foam Expansion Ratio
The volume ratio of generated foam to original mixed solution. Agents are classified by expansion ratio:
- Low-expansion: less than 20 times
- Medium-expansion: 20 to 200 times
- High-expansion: over 200 times

5. Drainage Time
An index measuring foam stability. It refers to the time taken for certain mass of liquid to separate out from formed foam. Common evaluation standards are 25% and 50% drainage time.
Long drainage time stands for stable foam with poor fluidity and low fire fighting efficiency; short drainage time means good fluidity yet weak stability and easy foam collapse. Foam bubbles gradually merge, liquid inside drains under gravity and leads to continuous bubble combination.

6. Water-insoluble & Water-soluble Liquid
Liquids separating from water after mixing are water-insoluble substances, while those blending evenly without layering are water-soluble substances. Hydrocarbon liquids including crude oil, gasoline, kerosene and diesel oil are water-insoluble. Substances such as formic acid, methanol and ethanol that dissolve easily in water are water-soluble. Ethanol-blended gasoline also falls into water-soluble category.

7. Foam Forming Process
Foam concentrate mixes with water to form solution, which is discharged through hoses and nozzles in the form of water, mixed liquid and foam. Negative pressure generated at the suction inlet draws in air, which blends with flowing solution to form primary foam. Stable foam is finally formed when the whole process keeps steady.