The LWF Blog

Fire Engineering for Healthcare Premises – Smoke spread and control – Part 14

December 21, 2020 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 13 of Fire Engineering for Healthcare Premises, LWF discussed the pertinent elements of smoke spread and control. In part 14, we continue to look at smoke spread and control.

The presence of wind in an environment within a healthcare building will significantly affect the performance of natural smoke ventilation systems, which are buoyancy-driven.

For this reason, the wind pressure coefficient should be calculated with measurement on the completed building or by using a wind tunnel model. Air flows also depend on the conditions inside the building, which are prone to change as a consequence of external air flows.

The greatest level of natural extraction will occur at vents, across which the greatest drop in pressure (interior-exterior) exists. In situations where the difference in pressure is almost nil, it is possible the flows may change direction as the system as a whole reaches equilibrium.

Heat losses to the structure can also affect the spread and control of smoke. The reduction of the temperature of hot gas layers impacts on the likely volume and buoyancy.

The volume of a smoke reservoir and the smoke extraction rate must provide an adequate ASET (available safe egress time). The smoke extract rate should also prevent the build-up of smoke from overflowing the reservoir.

In cases where the reservoir is too large, the smoke may cool by losing heat to the structure. In this case, the smoke can lose buoyancy and the smoke layer could fall to lower levels where it would affect the safe evacuation of people from the premises when the conditions become untenable.

Any materials used within the smoke reservoir, including automatic smoke curtains, should be suitable for withstanding the high temperatures of the predicted smoke layer.

In some circumstances, it may be necessary to install sprinklers in the smoke reservoir. If the smoke layer temperature is high enough to endanger occupants due to thermal radiation (typically grater than 200 ºC) or if there are sufficient combustibles under the smoke reservoir to indicate a significant threat of excessive fire spread, a sprinkler system is likely to be required.

Screens and curtains should be used to channel the smoke so that it does not affect occupants on the floors between the levels of fire and the reservoir. Mechanical extract systems which rely on a secondary power source (e.g., a generator) will have a limited duration of operation which may restrict the ASET that can be achieved.

In Part 15 of LWF’s blog series, LWF will look at smoke curtains and smoke venting as per HTM 05-03. 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.

Share this post