Foam systems come in two types, as follows:
Low Expansion, where the bubble expansion ratio is small (less than 20 to 1) and the bubble contains a high water content.
Medium and High Expansion, where the expansion ratios are greater from 20 to 1 up to 1,000 to 1. At these expansion ratios, the bubble water content is low and the bubble is relatively light. The foam bubbles are made by mixing a foam concentrate with water to make a foam solution. The foam solution is then mechanically agitated to form bubbles.
Low Expansion Foam Fire Extinguishing Systems are applicable to special hazards, usually involving flammable or combustible liquids, such as storage tanks. These systems discharge foam bubbles over the liquid surface to provide a cooling, smothering blanket which progressively covers the liquid surface and extinguishes the fire. The foam blanket can prevent vapor production for some time. Aqueous film-forming foam (AFFF) functions in a similar manner, but with one notable difference: AFFF is capable of producing water solution films which float on the surface of the liquid and help suppress vapor production. However, this beneficial “film” is obtained with some loss of burn-back resistance.
High Expansion foam systems are applicable to areas where it might be desirable to fill the space with foam in order to exclude air and smother the fire. Examples of such areas include basements and warehouses.