The LWF Blog
Fire Safety Engineering for Design – Foam Fire Suppression Systems – Part 275
February 23, 2026 9:46 amLWF’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 274, LWF continued exploring the different elements of foam proportioning by talking about balanced pressure proportioning and water-driven foam metering pumps. In part 275, we discuss premix foam units and begin to look at the types of foam system available.
Premix Foam Units
A premix foam unit is a self-contained system of fire suppression, made to store a pre-mixed solution of water and foam concentrate (at the required ratio). The stored foam is ready for pumping when required. Premix is, of course, used in foam fire extinguishers and in some small systems where the storage tank is pressurised with nitrogen or air, so that the automatic opening of a valve by a detection system gives immediate discharge.
Although premix foam units are generally small, it is possible to have large capacity tanks which hold 150-500 gallons. Wheeled units are available based on the same principle, meaning one person can deploy the unit to a fire on site. These can often be found in refineries and machinery rooms.
Types of Foam System
Low expansion foam systems are typically those described as having an expansion ratio of less than 20:1. These fire suppression systems use dense and flowable foam to cover flammable liquid fires and are often used in large spaces or outdoor environments due to the delivery of a stable blanket of foam that cools and smothers the fire. They may be used to protect cone roof and floating roof flammable liquid storage tanks, tanker loading and off-loading bays, process areas, machinery areas and aerospace environments.
Compressed air foam systems are those which aspirate the foam water solution within the foam delivery system so pre-aspirated foam is discharged at the nozzle. While this type of foam is known for being stable, its suitability for the environment and purpose in question must be checked.
Methods for measuring foam expansion can be referenced in NFPA 11 and BS EN 13565-2.
In part 276 of LWF’s series on fire engineering we will continue to look at the types of foam system by talking about medium-expansion foam systems and high-expansion foam systems. 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.