The LWF Blog

Fire Engineering for Healthcare Premises – Fire Growth – Part 33

May 4, 2021 12:24 pm

In LWF’s blog series for healthcare professionals, our aim is to give information on best practice of fire safety in hospitals and other healthcare premises. In part 32 of Fire Engineering for Healthcare Premises, LWF began to look at how fire grows. In part 33, we will continue to discuss fire growth.

A fire cannot continue to grow exponentially. At some point it will run out of oxygen or fuel. The first opportunity for a fire to cease growing is when the item of fuel first ignited is consumed by the fire. At this point the fire will either die down, or it will have the ability to spread beyond the first item to neighbouring fuel sources. Its ability to spread is based on how close further sources of fuel are to the site of the fire and the size the fire achieved while consuming the first item of fuel.

A significant mechanism for heat transfer is when radiation from the hot smoke layer beneath the ceiling is sufficient to cause all fuel sources within the compartment to ignite, often almost simultaneously. This is termed ‘flashover’ and causes the heat release rate to increase very rapidly from that point.

Once flashover has occurred, or fire has spread in a slower fashion between all sources of fuel in a compartment, a fully-involved compartment fire results which will ultimately be restricted by the availability of oxygen in the area.

If a compartment were air-tight, the fire would only be able to consume the oxygen already within the compartment. However, the majority of compartment areas are not air-tight and openings in the compartment walls will allow the passage of smoke from the area and fresh air in. The rate of oxygen supply to the fire will depend primarily on the size of any openings.

When the oxygen supply is restricted, the fire becomes ‘vitiated’. Combustion is less efficient in this environment and the production of end-products such as carbon dioxide and water will be reduced. More smoke and intermediate products will be produced, such as carbon monoxide.

In extreme cases, the temperature of the fire compartment may be high enough for fuel vaporisation to occur, but without sufficient oxygen for combustion. In these circumstances, if the oxygen supply is suddenly increased through a door opening or a window breaking, the fuel vapours will burn very quickly when the inflow of air mixes with them, in a phenomenon called ‘backdraught’.

Fire suppression methods to reduce heat (such as sprinklers) or cut off the oxygen supply, or both can be used.

In Part 34 of LWF’s blog series, LWF will discuss the use of design fire specifications. 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 for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact LWF on freephone 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.

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