The LWF Blog

Fire Engineering for Healthcare Premises – Smoke – Part 40

June 21, 2021 10:53 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 39 of Fire Engineering for Healthcare Premises, LWF discussed the ventilation of smoke before talking about SHEVS. In part 40, we continue looking at the use of SHEVS in terms of replacement air.

Smoke and Heat Exhaust Ventilation Systems (SHEVS) may be natural or powered and are commonly used within healthcare premises to remove smoke from the atmosphere in a specific fire situation. Often they are used in a large atria, or outside of the healthcare sector, in a shopping centre or exhibition hall. They are used to ensure tenable conditions for evacuation in spaces where compartmentation isn’t possible due to design and the travel distance to the exit may be longer than typically expected.

The mass of smoke removed must be replaced by breathable air, otherwise the SHEVS might attempt to make a vacuum in the building. A flow of replacement air must be available for the SHEVS to run efficiently.

The design of the SHEVS should allow replacement air to enter the building at a low level, where high velocities can cause issues. For instance, if replacement air was drawn in along an escape route, the high velocity involved may even impede the progress to the exit of the building occupants.

Where a naturally ventilated system is used, it may not be possible to rely on an inlet area at a low level due to the required size of inlet and so an alternative is to provide open vents in an adjacent reservoir to allow sufficient replacement air.

It is important that air inlets are not positioned so that the air feeds directly into the extraction system, as this can cause a short-circuit. The inlets should also be situated remotely from the exhaust of the smoke extraction system so that smoke is not brought back in through the inlets.

Doors which are designed to open automatically as a part of the SHEVS should be sequenced to ensure they are fully open by the time the extract fans are operating at full capacity.

In Part 41 of LWF’s blog series, LWF will begin to look at the use of structural fire protection and how it is used to provide fire resistance. 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.