The LWF Blog

Fire Engineering for Healthcare Premises – Fire Growth – Part 32

April 26, 2021 11:48 am

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 31 of Fire Engineering for Healthcare Premises, LWF considered worst case scenario planning. In part 32, we begin to look at fire growth.

In order to predict the outcome of a fire, it is necessary to understand how a fire develops and grows. Fire growth is dependent on the availability of three elements – fuel, oxygen and heat; this is known as the fire triangle.

Most healthcare buildings offer a plentiful supply of combustible contents as fuel and oxygen is present both in the air and as a medical gas. Fortunately, sources of heat sufficient to ignite a fire are relatively rare and kept away from combustible materials.

If a fire starts, the combustion of the fuel causes heat to be released and this is usually sufficient to sustain the fire and enable fire growth. Of course, as a fire increases in size, it generates more heat which allows further growth.

When the size of a fire must be expressed, it is in terms of the heat release rate (often abbreviated to HRR, or sometimes RHR – rate of heat release). The HRR is measured in kilowatts or megawatts (kW or MW).

The fire growth rate is the rate at which the HRR increases and is not usually a constant but depends on the fire size. One approximation for fire growth phases is given by the calculation called t-squared. This is where the HRR is proportional to the square of the time after ignition. There are other methods of calculating design fires including steady-state fire size or an HRR taken from experimental measurements.

It is possible to extrapolate on the small-scale measurements taken from a cone calorimeter to predict the HRR of a full-scale fire although this method has some issues.

Much of the heat from a fire is lost to convection in the fire plume and what is not lost, around 30%, is heat radiation from the flames. The smoke produced by a fire is also carried upwards by the fire plume and is diluted by the surrounding air and its entrainment. Because of the heat and buoyancy, a smoke layer will develop below the ceiling.

In Part 33 of LWF’s blog series, LWF will continue to discuss fire growth. 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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