Foam for fire fighting purposes is essentially an aggregate of gas filled bubbles formed from an aqueous solution of a suitable foaming agent or concentrate. The gas used is normally air, but in certain applications, inert gases have been used.

Mixing foam concentrate and water, in the appropriate concentration, and aerating and gitching the solution to form the bubble structure produces foam. The ratio of the volume of the foam to that of the solution from which it is made is defined as expansion. Foams are arbitrarily subdivided into three ranges of expansion, viz, low expansion foam, medium expansion foam and high expansion foam.

In a semi fixed foam system, permanent steel pipe work is employed from an area adjacent to the risk, from where it is considered safe for personnel to conduct fire-fighting operations, to the foam makers, which protect the hazard. This pipe work includes the proportioning device and has provision for water supply hoses to be connected. The water supply is usually pumped and mobile fire appliances provide foam concentrates.

In an portable system, the foam producing units are carried by one or more men and connected via fire hose to a pressurized water or pressurized solution supply, so as to produce foam jets or sprays that can be applied to the area under risk.

Mobile foam system includes foam producing units mounted on wheels, which are self-propelled, towed by a vehicle or pushed by hand. These units are connected to a suitable water supply or utilize a pre-mixed foam solution. They can produce foam jets or sprays to cover the area at risk.