1. Classification of Foam Fire Extinguishing Agents
Classified by foam generation mechanism, agents fall into chemical foam type and air foam type. Chemical foam forms via chemical reaction between two aqueous solutions, with carbon dioxide as internal gas. Air foam is produced by mechanical agitation and contains air inside. Based on drainage performance, air foam agents are categorized into low, medium and high drainage types.

According to foaming agent type and application, low-expansion foam agents are divided into five categories: protein foam, fluoroprotein foam, aqueous film-forming foam, alcohol-resistant foam and synthetic foam agent.

Foam expansion ratio refers to the volume multiple after aqueous agent solution turns into fire-extinguishing foam. Low-expansion ratio is below 20 times, medium-expansion ranges from 20 to 200 times, and high-expansion stands between 200 and 1000 times.

2. Fire Extinguishing Principle
Conventional fire-fighting foam has an expansion ratio of 2 to 1000 and specific gravity of 0.001 to 0.5. Much lighter than common flammable liquid, foam floats on liquid surface to form a covering layer. With certain viscosity, it can also adhere to solid combustibles, delivering diversified fire suppression effects.

1. Isolation effect
The foam blanket separates burning substances from ambient air. It blocks thermal radiation, restricts evaporation and pyrolysis, and cuts off flammable gas supply to combustion zone.

2. Cooling effect
Liquid separated out from foam cools down burning surfaces effectively.

3. Smothering effect
Foam vaporizes under heat, and generated steam lowers oxygen concentration in fire areas.

3. Chemical Foam Fire Extinguishing Agent (MP)
Common chemical foam agent is mixed with acidic salt aluminum sulfate, alkaline salt sodium bicarbonate, minor foaming agent such as plant hydrolyzed protein or licorice powder, and stabilizer ferric trichloride.

Mass carbon dioxide is released during reaction and forms dense bubbles together with foaming agent. Low-density and high-viscosity foam covers burning surfaces and isolates air. As inert gas, carbon dioxide does not support combustion. Main models include YP, YPB and YPD.

YP type applies to foam extinguishers below 100L, composed of internal acidic agent and external alkaline powder. Domestic internal agent mainly adopts aluminum sulfate. External ingredients are sodium bicarbonate, potassium bicarbonate and spray-dried protein fire extinguishing powder, with sodium bicarbonate widely used.