NCC 2022 Volume One - Building Code of Australia Class 2 to 9 buildings
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21

Specification 21 Smoke exhaust systems

Specification 21 Smoke exhaust systems

This Specification describes the requirements for mechanical smoke exhaust systems.

To state that Specification 21 outlines the requirements for mechanical smoke exhaust systems.

Mechanical smoke exhaust systems

Specification 21 contains the requirements for mechanical smoke exhaust systems.

(1) Smoke exhaust fans must have a sufficient capacity to contain the smoke layer—

  1. within a smoke reservoir formed in accordance with S21C4 and not less than 2 m above the highest floor level; and
  2. above the top of any openings interconnecting different smoke reservoirs.

(2) Exhaust rates must be determined in accordance with Figure S21C2, with the height measurement taken from the lowest floor level to the underside of the smoke layer and the fire load determined in accordance with Table S21C2.

Table S21C2 Fire load (MW)
Classification Fire load (MW) for unsprinklered buildings Fire load (MW) for sprinklered buildings
Class 2, 3 or 5 5 1.5
Class 6 10 5
Class 7 or 8 15 5
Class 9 — generally 5 1.5
Class 9b buildings covered by Part I1 (see Note) or exhibition halls 10 5
Table Notes

If the smoke reservoir above the stage is smoke separated from the audience area, the fire load specified applies to the stage area only and the fire load for the audience area is as per Class 9 generally.

Figure S21C2 Smoke exhaust rate
image-S21C2-smoke-exhaust-rate.svg

To specify the capacity and exhaust rates required of mechanical smoke exhaust systems.

Height below the smoke layer—S21C2(1)

S21C2(1) defines the height to be maintained to the underside of the smoke layer. This height is used to determine the smoke exhaust capacity for various design fires under steady state conditions.

Exhaust rates—S21C2(2)

S21C2(2) requires exhaust rates be determined using Figure S21C2 of Specification 21. It specifies the exhaust rate required relative to the height to the underside of the smoke layer for various fire loads determined using Table S21C2.

How to use Figure S21C2

The following needs to be known to use Figure S21C2:

  • The class of the subject building or part (in the case of Class 9 buildings the use of the building will also need to be considered).
  • Whether or not that building or part is to be sprinklered.
  • The fire load measured in megawatts (MW) taken from Table S21C2.
  • The MW amount is represented in Figure S21C2 by a specific line which can be identified in the legend inserted in the figure.

Smoke exhaust rates

The smoke exhaust rate in Figure S21C2 is based on the rate at which air is drawn (i.e. “entrained”) into a plume of smoke which is generally symmetrical around a vertical axis where that plume is rising into the hot layer formed by the smoke reservoir.
The height to the underside of the smoke layer is measured from the highest floor level to the underside of the smoke reservoir. The smoke reservoir’s depth is determined by the depth of the smoke baffles/curtains required by S21C4. See Guide Figure S21C2.

Figure S21C2

Method of measurement of height to the underside of smoke layer.

Image
Method of measurement of height to the underside of smoke layer.

Each smoke exhaust fan, complete with its drive, flexible connections, control gear and wiring must—

  1. be constructed and installed so that it is capable of continuous operation (exhausting the required volumetric flow rate at the installed system resistance) at a temperature of 200°C for a period of not less than 1 hour; and
  2. in a building not fitted with a sprinkler system, be capable of continuous operation at a temperature of 300°C for a period of not less than 30 minutes; and
  3. be rated to handle the required volumetric flow rate at ambient temperature to be capable of exhausting cool smoke during the early stages of a fire and to allow routine testing; and
  4. have any high temperature overload devices installed, automatically overridden during the smoke exhaust operation.

To specify the operational requirements for smoke exhaust fans.

Operational requirements for smoke exhaust fans

S21C3describes the operational requirements for smoke exhaust fans to ensure their design performance is maintained for an appropriate time, when operating in high temperature conditions. Fans must also be rated for ambient temperature operation to facilitate routine maintenance. These provisions also apply to dual purpose fans, that is those used for normal air-handling operations as well as for smoke exhaust.

(1) A fire compartment must be divided at ceiling level into smoke reservoirs formed by smoke baffles/curtains of non-combustible and non-shatterable construction.

(2) The horizontal area of a smoke reservoir must not exceed 2000 m2 and in enclosed walkways and malls of a Class 6 building must not exceed 60 m in length.

(3) Smoke reservoirs must be of sufficient depth to contain the smoke layer and must not be less than 500 mm below an imperforate ceiling or roof.

(4) Within a multi-storey fire compartment—

  1. a non-combustible bulkhead or smoke baffle/curtain must be provided around the underside of each opening into a building void to minimise the spread of smoke to other storeys; and
  2. the depth of the bulkhead or smoke baffle must be not less than the depth of the smoke reservoir provided under (3) plus an additional 400 mm.

To specify the requirements for smoke reservoirs, to enable the containment of smoke in the upper levels of compartments.

Smoke reservoirs—S21C4(1)

S21C4(1) requires the division of fire compartments into smoke reservoirs.
Smoke reservoirs are necessary to contain the hot layer in the upper levels of compartments, thus preventing the lateral spread of smoke resulting in excessive cooling and downward mixing of the smoke with the relatively clear layer below which:

  • enables occupants to make their way through the comparatively clear air below the hot smoke layer; and
  • maintains the smoke above any openings between compartments, thus minimising the risk that smoke will migrate to other areas.

Horizontal area to be less than 2000 m2—S21C4(2)

To maximise the effectiveness of smoke reservoirs, the horizontal area formed by a reservoir is limited by S21C4(2) to 2000 m2.

Maximum length in a shopping mall—S21C4(2)

The maximum length of a smoke reservoir in a shopping mall is limited by S21C4(2) to 60 metres, due to the distance people would be expected to travel below a smoke layer while evacuating to a safe place, having regard to the potential for smoke, from a fire in a mall or adjacent specialty shop, to flow into more than one reservoir.

Depth—S21C4(3)

S21C4(3) specifies that the smoke reservoir must be of “sufficient” depth to contain the smoke layer.

Bulkhead or smoke baffle—S21C4(4)

S21C4(4) deals with the location and depth of a bulkhead or smoke baffle/curtain. Any bulkhead and smoke baffle/curtain must be non-combustible. S21C4(4) applies only to multi-storey fire compartments.

Smoke needs to be contained within the floor reservoir, and so the integrity of the containment must be maintained at the edges of voids in buildings, such as atriums, by the provision of non-combustible bulkheads or baffles.

Smoke exhaust fans and vents must be located—

  1. such that each smoke reservoir is served by one or more fans with the maximum exhaust rate at any one point limited to avoid extracting air from below the smoke layer; and
  2. to prevent the formation of stagnant regions resulting in excessive cooling and downward mixing of smoke; and
  3. at natural collection points for the hot smoky gases within each smoke reservoir having due regard to the ceiling geometry and its effect on the migratory path of the smoke; and
  4. away from the intersection of walkways or malls; and
  5. to ensure that any voids containing escalators and/or stairs commonly used by the public are not used as a smoke exhaust path; and
  6. to discharge directly to outdoor with a velocity of not less than 5 m/s, at a suitable point not less than 6 m from any air intake point or exit.

To make sure that exhaust fans and vents do not draw clean air up through the smoke layer.

Prevention of “plug-holing”

For a given depth of smoke layer, there is a maximum rate at which smoke can be extracted from a single inlet before air is drawn from below the smoke layer. This is sometimes referred to as “plug-holing”.

Where the smoke layer is relatively shallow, more than one extraction point may be needed to minimise “plug-holing”. It may also be necessary to distribute the extraction points to prevent the formation of stagnant regions leading to excessive cooling and downward mixing of smoke with the relatively clear air below.

Suitable discharge location

It is not suitable for a smoke exhaust fan and vent to discharge adjacent to an occupiable outdoor area. Specific criteria are not given as the design is dependent on actual building layout.

(1) Low level make-up air must be provided either automatically or via permanent ventilation openings to replace the air exhausted so as to minimise—

  1. any disturbance of the smoke layer due to turbulence created by the incoming air; and
  2. the risk of smoke migration to areas remote from the fire due to the effect of make-up air on the air balance of the total system.

(2) The velocity of make-up air through doorways must not exceed 2.5 m/s.

(3) Within a multi-storey fire compartment, make-up air must be provided across each vertical opening from a building void to the fire-affected storey at an average velocity of 1 m/s so as to minimise the spread of smoke from the fire-affected storey to other storeys.

To provide air to replace that being exhausted by the smoke exhaust system.

Maintenance of the smoke layer—S21C6(1)

It is necessary to introduce “make-up air” to replace the air being exhausted by the smoke exhaust system, to:

  • maintain the smoke layer at a level which keeps a reasonable amount of clear air underneath; and
  • minimise the risk that smoke will flow below the lower levels of the smoke reservoir and migrate to other areas.

Low velocity—S21C6(2)

Make-up air introduced below the smoke layer must be at relatively low velocities, to minimise any disturbance to the smoke layer. Make-up air introduced at higher velocities may cause:

  • smoke to be drawn down from the hot layer, called the “venturi effect”, leading to a loss of visibility in the space below; and
  • difficulties for people attempting to exit against the in-rush of air through doorways.

Multi-storey fire compartments—S21C6(3)

S21C6(3) deals with the provision of make-up air across any vertical opening from the building void to the fire-affected storey. This aims to minimise the risk of smoke spreading from the fire-affected storey to other storeys.

Non-prescriptive provisions

Specific criteria are not given in S21C6 as the design is dependent on actual building layout.

(1) Each smoke exhaust fan must be activated sequentially by smoke detectors complying with Specification 20 and arranged in zones to match the smoke reservoir served by the fan(s).

(2) Subject to (3) and (4), an air handling system (other than individual room units less than 1000 L/s and miscellaneous exhaust air systems installed in accordance with Sections 5 and 6 of AS 1668.1) which does not form part of the smoke hazard management system must be automatically shut down on the activation of the smoke exhaust system.

(3) In a single storey fire compartment, air handling systems in all non fire-affected zones may operate on 100% outdoor air to provide make-up air to the fire-affected zone.

(4) Within a multi-storey fire compartment, air handling systems in all non fire-affected zones and storeys must operate at 100% outdoor air to provide make-up air to the fire-affected storey via building voids connecting storeys.

(5) Manual override control and indication together with operating instructions for use by emergency personnel must be provided adjacent to the fire indicator panel in accordance with the requirements of clauses 4.11 and 4.13 of AS 1668.1.

(6) Manual control for the smoke exhaust system must also be provided at a location normally used by the stage manager in a theatre.

(7) Power supply wiring to exhaust fans together with detection, control, and indication circuits (and where necessary to automatic make-up air supply arrangements) must comply with AS 1668.1.

To specify the control requirements for smoke exhaust systems and automatic make-up air arrangements.

Sequential activation—S21C7(1)

To make sure that the smoke exhaust fan (or fans) operate in the designed manner, S21C7(1) requires that the fans are activated sequentially by smoke detectors, and arranged in zones to match the smoke reservoir served by the fan (or fans).

Automatic shutdown—S21C7(2)

S21C7(2) requires air handling systems (which generally supply air to upper storeys in high rise buildings) to shut down on the activation of the smoke exhaust system where the air handling system:

•    does not form part of the smoke hazard management system;
•    is not an individual room unit operating at a rate of less than 1000 L/s; or
•    is not a miscellaneous exhaust air system installed in accordance with Sections 5 and 6 of AS 1668.1.

This requirement minimises any disturbance to the hot smoke layer and limits smoke being distributed to other non-fire- affected areas of the building by way of the system.

S21C7(2) is subject to S21C7(3) and (4).

Concessions—S21C7(3) and (4)

A number of additional concessions to S21C7(2) are allowed, including:
•    S21C7(3)—systems supplying a single storey fire compartment may supply 100 per cent outside air to the non-fire- affected areas as a means of supplying make-up air for the extraction system serving fire-affected areas; and
•    S21C7(4)—systems supplying a multi-storey fire compartment must supply 100 per cent outside air to the non-fire- affected areas as a means of supplying make-up air for the extraction system serving fire-affected areas.

Override control—S21C7(5) and (6)

To allow manual control of the smoke exhaust system by the fire brigade, S21C7(5) requires an override control to be located adjacent to the fire indicator panel.

In a theatre, an additional manual control must be provided in accordance with S21C7(6) to allow the stage manager to control the smoke exhaust system during a performance. This is considered necessary because of any special effects which may cause a false alarm.

Electric cabling—S21C7(7)

To reduce the risk of the smoke exhaust system failing during a fire, S21C7(7) requires the electric cabling to the system’s essential components to be protected from fire in accordance with AS 1668.1.

A smoke detection system must be installed in accordance with Specification 20 to activate the smoke exhaust system.

To clarify the location of the requirements for the installation of a smoke detection system.

The smoke detection requirements for smoke exhaust systems are addressed in S21C5. Such smoke detection systems designed to operate smoke-and-heat vents must also activate an occupant warning system. Specification 20 provides the installation and operation requirements for automatic smoke detection and alarm systems.