W + Conc (=Sol) + A + A = Foam (Foam is Bubbles and Bubbles is Water)
The Air is fairly simple.
There are four (4) methods for adding air – Aspirated (NAF), CAF, Shear and Bounce.
Shear – as solution is projected throught he air, air shear causes turbulance that simultaneously adds air and acts as agitation. Air shear can come from fog nozzles (limited in foam production) and can be generated by gravity as solution is dropped by helicopters or airplanes.
Bounce – small amounts of air will be added to the solution from nozzle operations, and bouncing the stream off a wall or other obstacle can produce agitation and limited foam production.
When making NAF, atmospheric air is pulled into the Solution (W+Conc) at the aspirating nozzle.
For CAF, a high pressure system, typically a compressor, adds the air just after the Solution Discharge Valve and just prior to entering the fire hose.
