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Smoke Exhaust System Basics

This post explores, from a fire engineering standpoint, smoke exhaust systems in buildings and the basic design principles. When thinking of smoke exhaust systems, mechanical engineers often come to mind first but fire engineers actually have a role to play in its design which we'll clarify here.

Exhaust cowl fire engineers

What is a Smoke Exhaust System?

As the name implies, smoke exhaust systems are mechanical air handling systems which remove smoke from occupied areas to reduce the exposure of toxic gases and irritants to evacuating occupants.

In its most common implementation, smoke exhaust systems are roof mounted fans which draw smoke from occupied spaces and discharge into the open environment. There are buildings with air handling systems that, during normal operating conditions, function as regular air conditioning systems but in fire mode, exhaust smoke from fire-affected areas.

Either of the above configurations are acceptable provided the design achieves the same outcome and that is to ensure tenable conditions are maintained throughout occupant evacuation.

The smoke exhaust system relies on the fact that smoke generated by fire is hot by nature and is naturally buoyant. This results in two distinct layers with hot smoke sitting at the top of the room and fresh air at lower level. The exhaust system is designed to extract the smoke at high level to keep the height of the smoke layer (distance from the floor level to the interface between the two layers) above the height of exposure to humans. This is indicated below.

Courtesy of Harrison & Spearpoint (2006)

When is a Smoke Exhaust System Required?

Under the current National Construction Code (NCC) 2019 Amendment 1, smoke exhaust systems may be required by the Deemed to Satisfy (DtS) provisions under the following circumstances:

  • Class 6 buildings with fire compartments exceeding 2000 m2

  • Large Isolated Buildings

  • Nightclubs

  • Exhibition halls with fire compartments exceeding 2000 m2

  • Theatres and public halls (conditions apply)

  • Assembly buildings with fire compartments exceeding 2000 m2

Design Considerations

This post focuses on the fire engineering elements of smoke exhaust system design so it excludes mechanical considerations such as acoustics and static pressure.

With fire engineers, one of the most critical parts of smoke exhaust system design is the flow rate of the fan which controls how much smoke is extracted from the space every second. There are two schools of thought: Deemed to Satisfy vs. Performance Solution.

Deemed to Satisfy

NCC 2019 Amendment 1 Specification E2.2b contains the majority of the design parameters needed from a fire engineering perspective. The specification includes a graph and table set which allows the sizing of smoke exhaust based on building use and whether there are automatic fire sprinklers installed within the building. Refer below.

Extract from NCC 2019 A1 Specification E2.2b (Courtesy of ABCB 2020)

In addition to capacity of fans, makeup air velocity is limited to 2.5 m/s which means more external openings or supply air fans need to be available to ensure the air entering the occupied space to replace the exhausted air is at a low enough velocity. The intent of the limited air velocity is to reduce the risk of fresh air mixing with the hot smoke layer and accelerating the speed the hot smoke drops down to occupant exposure levels.

Fire engineering smoke layer
Courtesy of Wade, Spearpoint, Bittern & Tsai (2007)

Class 6 occupancies are also subject to additional requirements such as smoke reservoir area and mall length limitations to limit the migration of smoke away from the fire source.

Performance Solution

Once a Performance Solution relating to smoke exhaust systems is proposed, the DtS provisions above cannot be relied upon to guarantee safety as the premise of a Performance Solution is to deviate from the DtS provisions by demonstrating that the Performance of the proposed arrangement is still safe and in accordance with the Performance Requirements.

One of the most common mistakes which some inexperienced fire engineers make is to use the DtS smoke exhaust capacity sizing graph and table to define their fire size based on occupancy. Unfortunately, the moment you cross into Performance Solution territory, the use of DtS provisions like this is bad practice as Performance Solution designs are not limited by the same parameters that DtS designs are.

As such, fire engineers generally need to demonstrate that the rationalised smoke exhaust system provisions can maintain acceptable levels of tenability for occupants and firefighting personnel with custom design parameters for:

  • Exhaust flow rate

  • Velocity across makeup air openings

  • Rationalised smoke baffles for smoke reservoir separation

We won't reveal the secrets to our success but what we can reveal is that we go through a lot of CFD simulation to make sure the design is optimised!

Final Thoughts

Smoke exhaust system design is normally a collaborative effort with the fire engineer, mechanical engineer and the architect. What we've presented in this post is merely just a fire engineer's point of view.

Our hardworking fire engineering team provides rock solid consultation on smoke exhaust systems and the associated requirements from a fire engineer's standpoint. Please reach out to discuss your next major project.


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