Which foam introduction method uses a venturi or eductor to draw foam concentrate into the water stream?

• A. Induction
• B. Injection
• C. Batch Mixing
• D. Pre-mixing

Answer: (A)

The concept hinges on the Venturi effect: water flowing through a constricted path speeds up and pressure drops, creating suction that draws foam concentrate from its supply into the water stream. This automatic mixing at the point of discharge is what defines the induction method—the foam concentrate is inducted into the water flow by the changing pressure, without a separate pumping step to push the concentrate in.

In practice, an eductor or venturi device is connected to the discharge line and the concentrate supply. As water passes through the eductor, it pulls concentrate along and mixes it into the stream, producing the foam solution at the correct proportion.

Other methods don’t rely on this suction-based mixing. Injection uses a pump to force concentrate into the water line, requiring active pumping for proportioning. Batch Mixing mixes a measured amount of concentrate with water in a container ahead of discharge, creating a ready-to-use mixture before it’s drawn into the nozzle. Pre-mixing places the foam solution in a tank or line prior to discharge, so no on-the-fly induction occurs at the nozzle.