Manufacturers of Class A foam fire extinguishing agents state that foam mixture can only be formed by blending foam concentrate with water. The foam proportioning mixer serves as core equipment for suction and delivery in air foam fire extinguishing systems, which mixes water and foam liquid at set ratios. Different foam liquids require distinct mixing proportions, so proportioning mixers are generally adjustable. The mixing ratio adopted by domestic fire forces ranges from 0.1% to 9%. Poor foaming performance and agent waste often occur due to neglected calibration and untimely adjustment of mixing ratios during operation.

In terms of structure, foam proportioning mixers used by fire forces are classified into ring pump type, pipeline type and pressure type, and also categorized as manual and automatic regulating types. Manual mixers need timely adjustment according to required mixing proportion and on-site flow rate. For instance, a PH32 mixer matched with two PQ8 foam branch pipes shall be set to 16 for 6% mixing ratio and 8 for 3% mixing ratio.

The formation of air foam involves two steps: mixing water and foam concentrate via proportioning mixers, then generating foam with foam generators. Fire truck-mounted and portable mixers are mainly ring pump negative-pressure and pipeline types. When pressurized water flows through the nozzle, negative pressure is generated to draw foam concentrate. Water and foam liquid blend inside the throat tube, and the mixed fluid gains pressure in the diffuser before thorough mixing and discharge by the water pump.

Pipeline proportioning mixers follow the same working principle. Insufficient pressure fails to suck in foam concentrate, while excessive pressure leads to over-high mixing proportion. The inlet and outlet pressure of negative-pressure mixers shall be maintained between 0.6 MPa and 1.2 MPa to ensure accurate proportioning for fire suppression.

Air foam forms when the mixture draws air through orifice plates under negative pressure, hence foam generators must create adequate negative pressure for sufficient air blending. Most domestic air foam branch pipes operate above 0.8 MPa, foam hooks above 0.5 MPa, and medium and high expansion foam generators within 0.3 MPa to 1.0 MPa. Low pressure results in weak foaming effect. Excessively high pressure causes difficulty in handling portable spraying devices and may damage the equipment beyond pressure tolerance limits.