The LWF Blog

Fire Engineering for Healthcare Premises – Smoke – Part 39

June 14, 2021 12:18 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 38 of Fire Engineering for Healthcare Premises, LWF looked at smoke curtains, as well as beginning to look at smoke venting. In part 39, we will continue to discuss ventilation of smoke before talking about SHEVS.

When a smoke extraction system is used in a building, it may be necessary to provide replacement air, known as ‘make-up’ air. It should also be borne in mind that the fans and ductwork used in the smoke extraction system will be expected to operate at very high temperatures and so should be fit for purpose.

An issue should be mitigated against when using a smoke extraction system is that excessive localised extraction can lead to air being drawn from the lower levels, below the smoke layer, and this can create a plughole effect. This would reduce the efficiency of the extraction system and can be a particular problem where the smoke layer is shallow.

The design of a ventilation system cannot be achieved in isolation. The other elements of a healthcare building’s fire protection system must be considered and design fire selection is critical.

Smoke and heat exhaust ventilation systems (SHEVS)

SHEVS are designed with a specific design fire in mind to control the smoke produced. Most commonly, they are necessary in large spaces within buildings or in large open plan buildings, such as shopping malls or exhibition halls. They are required because there is no compartmentation to avoid the spread of smoke and fire from one area to the next and because travel distances are greater in large spaces. They are designed to ensure tenable conditions for people evacuating and also to allow the Fire Service safer access to the fire.

They can also be used in other types of building and space where smoke control is required.

The basis of a SHEV system is relatively simple. Gases from a fire are hot and buoyant, and so rise to form a stable layer in a reservoir below ceiling height. This means that below this height, the air is clear of smoke for a sufficient period to allow safe evacuation of occupants or firefighter access with sufficient visibility. The smoke is then extracted from the smoke layer by fans where necessary.

In Part 40 of LWF’s blog series, LWF will continue discussing SHEVS in terms of replacement air. 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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