Compact fire extinguishers explained
There are now several brands of compact, non-pressurised fire extinguishers available and all seem to offer a much more convenient means of protection than traditional extinguishers.
Non-pressurised fire extinguishers are described as ‘aerosol’ types that rely on a chemical reaction to produce a suspension of small fire-suppressing particles.
These are not to be confused with budget-priced, small pressure-can extinguishers that are also sold as ‘aerosol’ types, available from cheap auto parts outlets and hardware stores.
Because the true aerosol extinguisher container houses reacting chemicals, not pressurised gas or powder, it can be much smaller, yet produce greater output than all but very large pressurised extinguishers.
Typically, an aerosol extinguisher measures only 330mm long (a tad over a ‘foot’ in the old money) and weighs under 400 grams. That makes it easy to stow in the relatively small cabins of historic vehicles.
Unfortunately, despite their small marine-flare size, aerosol types are more expensive than pressurised units.Top-performing aerosol types cost around $150-190.
Typically, a portable, pressurised, conventional extinguisher has only a few seconds of output, before it’s empty, where the aerosol type is available with 50 to 100 seconds of discharge time.
Aerosol fire extinguishing technology was developed in the 1960s and was first used by the Russian army for ‘oil tank fire suppression’ in its tanks.
The cost of aerosol technology kept it largely out of mainstream fire suppression until the 1990s, when halon-gas extinguishers were condemned by the Montreal Protocol, which aimed to protect the Earth’s ozone layer.
Modern aerosol extinguishers employ potassium nitrate as the primary oxidant, although some sensitive environments demand more expensive strontium nitrate.
Aerosol fire extinguishers suppress fire by cooling, by dilution and by chemical inhibition, compared with pressurised types that only cool, only dilute or only chemically inhibit.
Also, aerosol types leave no visible residue and are non-toxic, non-conductive and non-corrosive during and after operation.
Unlike pressurised extinguishers that need regular pressure and content maintenance or refilling, the aerosol type needs no maintenance. Established Long Lifespan extinguisher ageing tests indicate a shelf life for aerosol extinguishers of around 10 years. The downside is one-use life, after which the extinguisher needs to be disposed of and replaced.
Aerosol fire extinguishers are certified internationally for suppressing most types of fires: Class A, combustible solid materials; Class B, liquid combustible materials; Class C, gaseous combustible materials; Class F & K, cooking oil and fats and Class E, electrical fires.
As with all chemical-content extinguishers, aerosol types are not to be used on Class D, inflammable metal fires that in the automotive world are likely to be burning aluminium, magnesium or lithium.
The best extinguisher for a lithium-battery fire is a non-chemical dry powder type that sprays graphite or sodium chloride particles.
An understanding of how the aerosol extinguisher works is handy in correctly using it.
The production of aerosols inside the tube is started by a ‘striker’, after the operator takes off the end cap and uses it to strike a spark at the outlet. That starts combustion of its potassium nitrate contents that forces non-combusted and combusted potassium compounds out of the tube.
When the potassium nitrate (chemical designation KNO3) discharges from the extinguisher it vaporises in the environment and displaces oxygen, thus starving the fire.
At the same time, when KNO3 reacts inside the body of the extinguisher, it breaks down into an aerosol of free radicals of potassium, nitrogen and water vapour. That aerosol reacts with the fire and potassium radicals to capture more oxygen, while the nitrogen and water vapour help starve and cool the fire.
The combined effect of the oxygen depleting, cooling and smothering reactions extinguishes the fire.
At the end of the extinguishing process there remain tiny particles of potassium compounds, 3-4 microns in size and nitrogen gas and water vapour that are already components of the atmosphere.
Because the aerosol flow is intended to starve a fire of oxygen, it’s easy to understand that the extinguisher should be used to create a cloud of containment around a fire, just as it is with a pressurised halon or CO2 extinguisher.
Two cardinal mistakes when using one are being too close and rushing the process.
The best technique is to take advantage of the long discharge time offered by an aerosol extinguisher and to approach a fire from a moderate distance, then progressively to get closer to its source. During this approach, the aerosol flow should be moved gently around the fire, always directing it towards the centre.
All aerosol extinguishers are one-use, throw-away devices, so it’s important that the fire is completely out before relaxing. The aerosol flow is heavier than air, so it will fill any pockets or cavities.
Some brands are available on-line and we know that ARB is distributing the Fire Stryker brand.
At Historic Vehicles we’ve stocked up on some aerosol extinguishers and while we’re anxious to test them, we don’t really wish for the circumstances that make them necessary!
However, we’ll report anyone’s real-world experiences, should any HV site visitors know of them.