AIR
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.
AGITATION
W + Conc (=Sol) + A + A = Foam (Foam is bubbles and Bubbles is Water)
Agitation is nothing more than scrubbing the Solution Air mixture to generate the bubbles.
In NAF – The agitation occurs in the aspirating nozzle just prior to the foam discharge. The nozzle usually has some type of obstruction method in the nozzle that agitates the mix. For example, it might be a series of screws or a screen or net over the end opening of the nozzle. {Photo examples coming soon!}
Agitation in CAF occurs in the fire hose. What most firefighters understand as friction loss from the hose, now becomes a more useful agitation of the Solution/Air mixture to form the Bubbles.
Agitation is necessary to get the Solution (W + Conc) and Air to thoroughly mix and stirred to form the bubbles. Without adequate agitation, only separate Solution and Air would escape the end of the nozzle.
While a fog nozzle seems like it would make a good agitator, when flowing CAF, the Foam, already mixed and agitated, flows into the nozzle which then pops the Bubbles. This is evident by the discharge sound from the nozzle – Solution + a loud rush of escaping Air already freed from the Bubbles. No Bubbles will flow from the nozzle, only drops. See CAF Nozzles.
