Foam is an effective smothering agent, used for liquid fires mainly. It acts by flowing over the liquid fuel oil surface and isolating the fire from the air, also prevents re-ignition due to the foam stability. Usually the foams are of i. Protein Based ii. Synthetic
The main container is filled with sodium bicarbonate solution and a long inner polythene container is filled with aluminium sulphate. The inner container is sealed by a cap held in place by a plunger. When the plunger is unlocked by turning it, the cap is released. The extinguisher is then inverted for the two liquids to mix. Carbon dioxide is produced by the reaction which pressurises the container and forces out the foam.
Al2 (SO4)3 + 6 NaHCO3 -> 2Al (OH) 3 + 3Na2SO4 + 6 Co2
The chemicals react to form Co2 which serves as propellant, but the action is slower, giving time for bubbles to form. Foam-making substances added to the sodium carbonate determine the nature of the foam formed. The ratio of the foam produced to liquid is in the order of 8:1 to 12:1.
Because the extinguisher has to be inverted for operation, no internal pipe is fitted. When being recharged the cap seal should be examined and the pressure relief holes in the rim checked. The chemical foam extinguisher may be slow.