Mechanical Smoke Shafts: 7 steps to successful selection for apartment buildings

Mechanical smoke shafts are the most common method of smoke control in high rise apartments. Although they are in essence very simple systems comprised of relatively few components, they are not yet included in the Building Regulations and tend to be shrouded in mystery, and thought of as a specialist package.

However, selection and installation of mechanical smoke shaft systems are well within the realm of competence of most mechanical or electrical contractors. We explore the steps that should be taken for successful selection in apartment buildings.

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In most simple buildings it is possible to use a one size fits all approach and come up with a standardised solution that can be employed on multiple projects.

 

We outline the 7 steps that should be taken below. It's important to note that contractors should undertake training before they attempt to install such systems.

 

This approach is limited to apartment blocks that are no more than 20 storeys in height, with a single mechanical smoke shaft using a stairwell vent for fresh air inlet and the maximum escape travel of 15m in any direction.

Step 1. Decide on Flow Rate

The first step is to determine the appropriate extract rate. The rational and recommendations for selecting an appropriate extract rate is explained in our guide. However, in reality almost all installed systems use a similar extract rate.

Step 2. Size Smoke Shaft

The next step is to decide on the size of builder’s work shaft needed to transport the smoke to the atmosphere. The shaft should be constructed from fire resistant material and so as to prevent resistance and leakage.

Step 3. Define Replacement Air Method

Replacement air must be brought into the lobby to replace the smoke being extracted and this is usually provided by opening the stairwell smoke ventilator and allowing the air to reach the lobby by one of three methods.

Step 4. Select Stair Vent

If the ventilator is being used for roof access then a hatch vent or hinged louvre  would be required, if not then a standard louvre would be suitable. It is possible to use the mechanical smoke shaft system to relieve summer overheating, and this will impact on the choice of stairwell smoke ventilator.

Step 5. Select Smoke Shaft AOV

There is no specific standard for these products so the two common approaches are to use an E30Sa fire door (with an electrical actuator) or a smoke damper. The ventilator should be positioned as close to the ceiling as possible within the lobby, and at least as high as the top of the door from the lobby to the stairwell.

Step 6. Select Fan Set

Following on from step 1 we now have all of the variables required to select the fan needed to extract the smoke from the fire zone. It is a requirement to provide run and standby fans, which will normally be mounted in series on the roof with flexible connections and anti-vibration mounts, and back draught dampers to prevent cold air being drawn into the building. A duct resistance should be calculated for the installation once the design has been fixed.

Step 7. Select Control System

There are many approaches to controlling mechanical smoke shafts including using a bespoke control panel, a fire alarm system, or a purpose designed packaged system. In our view, a standard smoke control product offers the most robust solution as designing a project specific solution carries inherent risk of oversight or error in programming or construction that may go undetected until the system is needed.

 


 

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