It is known in the art to fight fire with compressed-air foam (CAF).  Typically, a foaming agent is added continuously to a water flow and the resulting flow of the mixture of foam agent and water is supplied to a foaming line or chamber which is also supplied with air pressure so as to generate foam.  The foam exiting the foaming line or chamber passes through a rigid or flexible pipe to a nozzle for ejecting the foam onto the fire.  The foaming line or chamber, also designated as a mixer or a mixing chamber, is usually of a static type, alternatively called motionless, i.e. without moving parts. 
Compressed air foam systems (CAFS) may be mobile e.g. when mounted on a fire-emergency vehicle.  They may also be fixed e.g. when used in fixed fire-security systems in tunnels for car and truck traffic. 
Various technologies for producing CAF exist which are often very different from each others. 
A major problem for producing CAF is to control in an appropriate way the water flow and the air flow supplied to the mixing chamber so as to provide continuously foam having adequate properties for fighting fire and that remain stable over time.  The problem arises due to the fact that both the water and air supplied to the mixing chamber and the physical conditions in the pipes and nozzles for transporting and ejecting foam may vary.  In particular, the CAFS may be supplied with a water flow the pressure and flow rate of which may vary over time e.g. when using water pumps.  Mobile systems may be used with water sources such as hydrants available at the spot of intervention and that can thus have different pressure and flow rate characteristics.  Further, the length and diameter of the pipes connected to the outlet of the mixing chamber, the type of nozzle connected at the end of the pipe, the extent of elevation of the pipe, the number of the pipes connected to the outlet of the mixing chambers, among others, may vary and influence the working conditions of the mixing chamber and thereby the foam quality. 
Therefore, complex systems and processes are used for balancing the pressure of water and the pressure of air supplied to the mixing chamber or for adapting the pressure of air when the pressure of water varies. 
US-A-2004/0177975 discloses a CAFS comprising a system controller for controlling an air flow control valve depending on the signals provided by a water flowmeter and an air flowmeter with a view of maintaining a ratio of air flow to foam flow based upon the user adjustable ratio input. 
WO 2006/000177 discloses a CAFS in which compressed air is conducted into a foaming line via an air pressure controller and an air volume flow rate control valve.  Further, produced CAF flows via a foam pressure sensor and an electro-pneumatically operated valve, that form a closed-loop control circuit for setting the foam consistency and consequently the foam quality, to the foam ejection device.  Water is fed into the system via a water pressure controller and is intermixed with a foaming agent and an additive.  The foaming agent-additive-water mixture flows via a water volume flow rate control valve and the foaming line into which compressed air is inserted at preset pressure and volume flow rate parameters via the air volume flow rate control valve.  This document mentions that the foam quality of the CAF spread using a foam ejecting device depends on the flow rate and therefore on the dwell time of the foam in the foaming line and teaches to control it via the foam pressure determined by a foam pressure sensor using the electro-pneumatically operated valve (foam pressure control). 
However, this document does not give any detail on the way of controlling the different parameters, in particular pressure, volume flow rate and speeds/dwell time of air, water and foam so as to ensure that the mixing chamber provides continuously foam of good quality for extinguishing fire.  Further, the closed-loop control may be complicated to implement. 
EP-A-1 632 272 discloses a CAFS for a tunnel for car and truck traffic.  This document does not deal with the problem of optimizing the working conditions of the mixing chamber, but with the problem of allowing ejection of foam having a good quality despite the fact that foam is transported over long pipes.  Therefore, this document teaches to set automatically the foam pressure to a given pressure behind the mixing chamber in view of preventing the foam pressure to get below a determined value at the foam-ejection device and providing thereby consistent foam still having high extinguishing property.  The foam pressure behind the mixing chamber is obtained with an adjustable cross section restriction of the pipe by means of a valve controlled with respect to a pressure sensor. 
However, this document does not deal at all with the problem of controlling the different parameters, in particular pressure, volume flow rate and speeds/dwell time of air, water and foam so as to ensure that the mixing chamber provides continuously foam of good quality for extinguishing fire.