The pressurisation theme continues in this week’s blog as we continue to explain to you the different features of pressurisation systems. Air release is next on our list of explanations. In this blog, you’ll find out what air release is and how various British Standard documents explain it.
Air release is a critical part of a pressurisation system. Let’s start by giving the official definition:
A means by which pressurising air is able to escape from the accommodation or other unpressurised space to outside the building.
Air release is crucial, as if it wasn’t present, air would be pumped into the fire floor by a pressurisation system and what would eventually happen is the pressure would equalise, as the air has nowhere to go, resulting in the products of combustion having nothing to prevent them flowing back towards the pressurised zone as there’s no pressure differential.
The above is explained quite simply in BS EN 12101-6:
Air release shall be provided for ensuring that the air flowing from the pressurised to an unpressurised space can leak to external air so as to maintain the pressure differential, or open door airflow velocity, between the two spaces”.
The aim is to keep the accommodation, which is any unpressurised zone, as close to atmospheric pressure as possible, to make sure we can maintain our pressure differential. This also applies in a closed-door scenario as set out in section 5.1.1 of BS EN 12101-6:
The aim is to establish a pressure differential across any leakage paths that will ensure that smoke moves away from the protected space. This is achieved by maintaining the protected space at a pressure higher than that of the fire zone. It is essential that adequate air release shall be provided from the accommodation to ensure that a pressure differential is maintained.
It is also just as important in an open-door scenario, as is indicated in BS EN 12101-6 section 4.3.1:
To achieve the minimum velocity of 2 m/s through the open stair door it is necessary to ensure sufficient leakage from the accommodation to the exterior of the building. In the later stages of fire development more than adequate leakage will generally be provided by breakage of external glazing. However, it cannot be assumed that windows will have failed before fire service arrival, and it is therefore necessary to ensure that sufficient leakage area is available via the external facade, the ventilation ductwork or specifically designed air release paths.
Now we’ve discussed what air release is and why it’s so important, it’s time to briefly cover the three different methods of achieving air release:
The first and simplest option is to use facade ventilation, however, this must be provided on more than one facade of the building, in order to overcome any adverse wind pressure.
The second option is to use a natural smoke shaft.
The third option is to use mechanical extract.
It’s important to be aware that only the air release on the fire storey is to open. This is for exactly the same reason as only one vent is opened into a fire storey with natural and mechanical smoke ventilation systems - we don’t want to spread the products of combustion to another floor.
There are some very helpful sources on our blog and YouTube where you can find out more information about pressurisation systems. We have videos: ‘What is a Pressurisation System?’, ‘What is the Stack Effect?’, ‘What is Protected Space?’ and ‘Two Stairs vs Pressurisation: Which System Should I Choose?’ which may be of interest. If you have any questions about pressurisation and/or protected space, please don’t hesitate to contact us.