Why is Class A foam more effective than plain water on Class A type fires?
Plain water has inherent limitations in cooling and penetrating Class A fuels because of a naturally high surface tension. This high surface tension causes water to form into droplets, and consequently, the majority of the droplets roll off fuels, impeding water’s heat absorption potential. Class A foam concentrate treated water (foam solution) has reduced surface tension and allows more surface area of water droplets applied to contact the ordinary combustible surface. This provides increased heat transfer through conduction. Class A foam concentrate is technically known as a synthetic detergent hydrocarbon surfactant, and when mixed with water at the recommended ratios, is biodegradable. Because it is a hydrocarbon surfactant, it also has an affinity for carbons and causes the water (as foam solution) to penetrate into wood fuels. Class A foam solution can be aspirated (entrained with air) by application through a fog or air aspirating nozzle, or a Compressed Air Foam System (CAFS).
