The LWF Blog

Fire Safety Engineering for Design – Foam Fire Suppression Systems – Part 285

May 4, 2026 8:56 am

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 284, LWF discussed medium-expansion foam generators. In part 285, we look at high-expansion foam generators.

High-expansion foam systems are a type of firefighting equipment, designed to rapidly fill large volume buildings considered a fire protection risk with a very light foam capable of smothering a fire through the exclusion of oxygen. Most commonly, such systems are found in warehouses, basements, aircraft hangars etc.

The high-expansion foam generator is capable of expanding a foam between 200 and 1000 x the original volume. In this way, one litre of foam solution can become a foam output of up to 1000 litres.

The generator itself comprises a body to house the internal components and to direct air and foam flow, as well as spray nozzles at the air inlet to spray a mixture of water and foam concentrate into fine droplets. The mesh screen at the outlet is the element that finally turns the foam concentrate/water mix into foam bubbles. It does this by the mixture being forced through the mesh, encapsulating air into bubbles and forming the finished foam.

While other foam systems often use the venturi effect to create foam (by fast-moving liquid pulling in air), a high-expansion foam system is required to achieve greater levels of foam expansion and requires air to be forced into the system through the use of a fan.

The fan used to push large volumes of air into the generator can be water-driven or electric motor-driven and ensures a consistent high airflow, proper mixing of air with the foam solution and results in forming very light high-expansion foam.

The air used in a high-expansion foam generator must be of high quality. Air that is polluted with products of combustion, e.g. smoke, soot or hot gases, will be counter-effective and actually break down the foam bubbles, reducing the expansion ratio and the resulting foam will be heavier, less stable and less effective at fire suppression. A source of air from outside a building is usually the preferred option.

If clean air is not a practical option, then foam must be overproduced in order to make up for the anticipated shortfall.

The placement of the high-expansion foam generator within the space to be protected is key to effective fire suppression. Generators should be placed above the final foam level allowing them to continue to function effectively during a fire.

In part 286 of LWF’s series on fire engineering we will discuss foam system design. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.