Info needed: Foam extinguishers fitted on trucks

[Tony Smith]

Originally Posted by Tony Smith

I'm not sure of the chemistry that was used with the foam types.

From a source who knows this stuff professionally:

"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 than the Soda Acid type, 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. The chemical foam extinguisher may be slow, and the range of jet spray less than the Soda Acid type. Agitating can increase the amount of foam and range.

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."

And yes, Carbon Dioxide.