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There are very few things more crucial for a functioning building than a working fire safety system. Not only is it a legal requirement (and for a good reason), it can also be the determining factor protecting the lives of dwellers. Smoke ventilation is an integral part of these efforts, allowing the isolation of toxic gases and the safe removal of evacuees. Fire safety expert Sertus offers the following insight


mechanical. Choosing the right method of ventilation is essential and makes a huge impact when it comes to the effectiveness, suitability and energy efficiency of a building. So, what is the difference between the two? And when should you use each one?


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Natural smoke ventilation


Natural smoke ventilation has been around for a long time. As you can probably gather from the name, it utilises the intrinsic qualities of the smoke itself to get rid of it. Smoke is naturally buoyant, so with some cleverly placed apertures, this can be leveraged to remove it from the building. These apertures, or vents, will open at the right time to release the gases, and close to contain them within certain regions, leaving escape corridors and stairs fully visible and smoke-free.


In small buildings, you will usually find a vent placed at the top of a staircase, while in larger constructions, corridor or lobby ventilation will be required too. This can be achieved using facade or shaft ventilation. In shafts, dampers are the type of vents used. During a fire, the dampers on the fire floor will open, using the buoyancy of the smoke to lead it outside or up the smoke shaft, whereas, on all other floors, the dampers will remain closed to prevent the smoke from spreading to other areas.


Mechanical smoke ventilation


Although the natural buoyancy of smoke can be effective, sometimes, something more powerful is necessary. Unlike natural systems, mechanical smoke ventilation systems use fans to achieve their aim. However, similarly to natural


Dampers are used in shaft ventilation


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Smoke ventilation: How do natural and mechanical systems differ?


moke ventilation systems are


complicated and have various types, which can be divided into natural and


systems, vents and dampers will be used to vent the fire floor and also prevent the smoke from reaching other levels.


There are two main types of powered system. Pressure Differential Systems (PDS), which can be of the pressurisation or depressurisation form — are one type of mechanical smoke ventilation. The former is commonly found in commercial buildings, while the latter is rarely used at all (but when they are, it’s usually in basements). The idea is relatively simple: because air will naturally try to move from an area of higher pressure to an area of lower pressure, by increasing the pressure in stairs, escape routes and other protected areas above the pressure in the fire zone, smoke can be contained. This is done using a pressurisation supply fan in pressurisation systems, and a smoke extract fan in depressurisation ventilation.


However, a more commonplace method is MSVS, or mechanical smoke extract systems. In these, a mechanical extract pod, which is placed on the roof, helps to draw the smoke, not relying solely on its


natural properties. While sucking the smoke out, they also allow fresh air into the building, preventing excessive depressurisation that can lead to horrific consequences like trapping occupants indoors.


Which one is right for me?


Unfortunately, it’s impossible to determine a one-size-fits-all solution to ventilation. Both types of systems have their own unique benefits, so picking the appropriate one for your building needs to take into account what you want to achieve and the specifications of your construction. Natural systems are obviously less costly, and also require less maintenance. However, they are not the best option for long corridors, and they also rely on favourable weather conditions to be as effective as they can be.


Mechanical ventilation is effective regardless of weather conditions, and can be implemented in larger buildings. It also takes up less space, as the necessary shafts are much more compact. If you legally have a choice, the maintenance and price may make an MSVS system less attractive for a smaller building, though.


What should I take into consideration?


To choose the correct system, helpful guidance is provided in Approved Document B. You can also check out the Smoke Control Association guides and BS 9991 — note, however, that the latter is in the process of being updated right now. That said, based on the properties of these systems, there are a number of questions you can ask yourself immediately to gauge it. For buildings that are relatively lower in height — under 18m, to be precise — natural ventilation is usually applied. Taller constructions are


12 BUILDING SERVICES & ENVIRONMENTAL ENGINEER FEBRUARY 2022


considered higher risk, and therefore it is recommended that you use mechanical systems for them. It’s not just about the height, though. The length of corridors is also an important factor. Longer hallways mean there’s a greater area requiring protection, something that’s not possible to do with a natural system. The definition of an extended corridor depends on building height, the number of staircases and whether or not sprinkler systems are fitted to apartments (in line with BS 9991). The distances are measured from the furthest apartment door to the door to the staircase or protected lobby. While it may be instinctive to think that because larger buildings benefit more from mechanical ventilation, smaller constructions should never consider it, this is not the case. If you’re lacking space, as previously mentioned, the mechanical option is actually more compact due to the size of the shafts. Shaft-based natural ventilation systems require 1.5m2 (cross-sectional area) shafts, compared to around 0.6m2-1m2 for mechanical smoke shafts..


Always consult a professional


Although these systems may seem pretty straightforward (after a little explanation), and you might already have an idea of which system your building needs, we always recommend getting advice from a professional.


It is also important to ask these questions as early as possible, as some of the fixtures required would need to be built into the


construction plans well in advance of starting to work on it. Your chosen ventilation system may also determine certain features of your building — whether it’s the facade vents, the size of shafts, or the length of corridors — so examining your options with a professional is essential at the early stages.


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