Smoke Containment and Control Part 2

Smoke developed from a fire in the confines of a building can quickly cause untenable conditions for occupants trying to escape, and cause an additional challenge for fire fighters who are either trying to conduct search and rescue operations, or find the heart of the fire to control and extinguish it as quickly and as efficiently as possible.

In a recent issue of Fire Australia, the first part of this two part series of articles dealt with smoke containment systems, which create physical barriers to contain smoke from one part of building, from movement to another. This is the second part of a two part article, and it deals in detail with smoke control systems and products. These systems typically utilise mechanical means such as fans to create pressure differentials or to exhaust smoke from areas of concern.

It is thought that the article will be useful for Fire Safety Engineers, Fire Protection and Mechanical Services Consultants, Building Surveyors as well as other practitioners who have an interest in understanding this subject matter.

Building Code of Australia

Compliance with the BCA, as discussed in the previous Part 1 article, is provided by way of an acceptable Building Solution which can be achieved by way of utilising the deemed-to-satisfy or prescriptive methods usually contained in BCA Specifications and/or relevant Australian Standards, or by utilising an Alternative Solution by way of Fire Safety Engineering principles.

For ease of reading, the relevant BCA Performance requirements relating to smoke containment and control have been included again in this part of the article.

For the smoke containment and control areas, one needs to look in BCA Section C (Fire Resistance – Compartmentalisation and Separation & Protection of Openings) and Section E2 (Services and Equipment – Smoke Hazard Management).

The relevant performance requirements for BCA Section C are CP2, CP3 and CP4. These have been included below to assist the many readers who do not have the BCA open whilst reading this article.

CP2

  • A building must have elements which will, to the degree necessary, avoid the spread of fire —
  • to exits; and
  • to sole-occupancy units and public corridors; and
  • between buildings; and
  • in a building

(b) Avoidance of the spread of fire referred to in (a) must be appropriate to

  • the function or use of the building; and
  • the fire load; and
  • the potential fire intensity; and
  • the fire hazard; and
  • the number of storeys in the building; and
  • its proximity to other property; and
  • any active fire safety systems installed in the building; and
  • the size of any fire compartment; and
  • fire brigade intervention; and
  • other elements they support; and
  • the evacuation time.

CP3

A building must be protected from the spread of fire and smoke to allow sufficient time for the orderly evacuation of the building in an emergency.

CP4

A material and an assembly must, to the degree necessary, resist the spread of fire to limit the generation of smoke and heat, and any toxic gases likely to be produced, appropriate to—

  • the evacuation time; and
  • the number, mobility and other characteristics of occupants; and
  • the function or use of the building; and
  • any active fire safety systems installed in the building.

The relevant performance requirements for BCA Section E2 are also included below.

EP2.2

  • In the event of a fire in a building the conditions in any evacuation route must be maintained for the period of time occupants take to evacuate the part of the building so that—
  • the temperature will not endanger human life; and
  • the level of visibility will enable the evacuation route to be determined; and
  • the level of toxicity will not endanger human life.
  • The period of time occupants take to evacuate referred to in (a) must be appropriate to—
  • the number, mobility and other characteristics of the occupants; and
  • the function or use of the building; and
  • the travel distance and other characteristics of the building; and
  • the fire load; and
  • the potential fire intensity; and
  • the fire hazard; and
  • any active fire safety systems installed in the building; and
  • fire brigade intervention.

EP2.2 as you can see is specific to the evacuation routes which of course are a very important area of keep free from smoke to allow occupants to egress (escape from) building in and safe manner, unaffected by the effects of a fire within the building in question.

The corresponding deemed-to-satisfy or prescriptive acceptable solutions manifest themselves through Australian Standard, AS/NZS1668.1, which in many ways is the “bible” relating to mechanical systems used for smoke control purposes.

Smoke Containment and Smoke Control systems and components

In the context of this article, the following broad definitions apply to Smoke Containment Barrier systems

Smoke Containment Barriers systems

Smoke Containment systems, for the purposes of this article, are defined as physical barriers that contain smoke from flowing from one compartment area to another.

They include:

  • Fixed Smoke walls
  • Automatic smoke containment screens (moveable smoke walls)
  • Smoke doors
  • Smoke dampers
  • Smoke stopping of penetration seals and control joints

Smoke Control systems

Smoke Control systems, for the purposes of this article, are defined as systems which use physical movement of air or smoke to control the amounts of smoke which is present at specific location within a building.

These systems include:

  • Smoke exhaust or purging systems
  • Mechanical
  • Buoyancy / natural
  • Pressurisation systems
  • Lift shaft
  • Stairwell
  • Zone or sandwich

Opposed flow systems

The relevant systems components include:

  • Smoke exhaust / smoke spill fans
  • Supply fans for pressurisation systems
  • Smoke and heat vents
  • Smoke reservoirs (automatic smoke curtains and fixed smoke baffles)
  • Smoke spill (motorised) dampers
  • Smoke exhaust ducting

The rest of this article will focus on smoke control systems and products, whilst Part 1 of this article dealt specifically with smoke containment barrier systems and products.

Smoke Control systems –––––

In this sub section of the article, we will look at the requirements relating to the different smoke control methodologies and the testing and performance requirements of the various products which make up these systems.

The smoke control system covered here are automatic (mechanically driven) smoke exhaust systems, natural (or buoyancy driven) smoke exhaust systems and pressurisation systems.

Opposed flow systems and air purge system, which are less prevalent have not been included.

Automatic (mechanically driven) smoke exhaust systems – Automatic smoke exhaust systems

The Building Code of Australia, as one option for compliance, requires the use of automatic (mechanical) smoke exhaust systems or automatic smoke-and-heat vents for buildings with fire compartments having a floor area of more than 2000 m2.

The mechanical smoke exhaust systems must comply with BCA Specification E2.2b.

The requirements here utilise mechanical smoke exhaust from within the fire affected area to maintain tenable conditions for at least 2 metres above the floor level.

The BCA provides prescriptive or deemed-to-satisfy guidance for the different Classes of buildings and takes into consideration whether the building in question has sprinkler protection or not, to allow designers to select the appropriate smoke exhaust rate.

The relevant BCA requirements from Specification E2.2b relating to smoke exhaust rates are as follows:

BCA Specification E2.2b 2. Smoke exhaust capacity

  • Smoke exhaust fans must have a sufficient capacity to contain the smoke layer—
  • within a smoke reservoir formed in accordance with Clause 4 and not less than 2m above the highest floor level; and
  • above the top of any openings interconnecting different smoke reservoirs.

Exhaust rates must be determined in accordance with Figure 2.1, with the height measurement taken from the lowest floor level to the underside of the smoke layer.

Smoke exhaust fans ––

The most important component of a smoke exhaust system is of course the smoke exhaust fan. This fan must continue to operate and exhaust smoke which of course is at an elevated temperature.

The BCA provides some specifications for the performance of the smoke exhaust fans which are also contained in Specification E2.2b:

BCA Specification E2.2b

3. Smoke exhaust fans

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

  • 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
  • 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
  • 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
  • have any high temperature overload devices installed, automatically overridden during the smoke exhaust operation.

AS/NZS 1668 Part 1, provide an appropriate test method, AS 4429 for testing of smoke exhaust or smoke spill fans

Smoke reservoirs

For large compartments, and to work in concert with the smoke exhaust fans, the BCA requires the use of smoke reservoirs (smoke baffles) in the ceiling space.

BCA Specification E2.2b 4. Smoke reservoirs

  • A fire compartment must be divided at ceiling level into smoke reservoirs formed by smoke baffles/curtains of non-combustible and non-shatterable construction.
  • 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.
  • 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.
  • (i) Within a multi-storey fire compartment, 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.
  • The depth of the bulkhead or smoke baffle must be not less than the depth of the smoke reservoir provided under

(c)  plus an additional 400 mm.

Specification E2.2b also provides some design requirements for location of smoke exhaust fans within required reservoirs, provision for make up air and of course the control system and relevant smoke detectors for the smoke exhaust fan system.

Kitchen exhaust systems

The Building Code of Australia also provides some requirements for exhaust over commercial kitchen cooking apparatus as follows:

F4.12 Kitchen local exhaust ventilation

A commercial kitchen must be provided with a kitchen exhaust hood complying with

AS/NZS 1668.1 and AS 1668.2 where— (a) any cooking apparatus has—

  • a total maximum electrical power input exceeding 8 kW; or
  • a total gas power input exceeding 29 MJ/h; or
  • the total maximum power input to more than one apparatus exceeds—
  • 0.5 kW electrical power; or
  • 1.8 MJ gas,

per m2 of floor area of the room or enclosure.

AS/NZS 1668 Part 1, in Section 11 provides design guidance for kitchen exhaust systems including the operation requirements in fire mode and details pertaining to exhaust hood and exhaust ducting.

11.2.2 Common exhaust Kitchen hoods located in separate fire compartments shall have independent exhaust systems. An exhaust system may serve more than one hood located within the same fire compartment subject to the requirements of AS 1668.2.

Kitchen exhaust ducts from separate fire compartments and of cross-sectional area less than 0.1 m2 may share a common riser shaft provided that an FRL of -/-/30 is maintained between the ducts. Where the kitchen exhaust duct cross-sectional area is greater than 0.1 m2 a separate riser shaft shall be provided. If the kitchen exhaust system forms part of a smoke-spill system, then the requirements of Clause 3.7 shall apply.

C11.2.2 Kitchen exhaust ductwork presents two fire hazards, a fire within the duct itself and a fire within the fire compartment served. As fire dampers are not allowed to be installed on kitchen exhaust ductwork, fire spread between ducts could compromise the building’s passive fire protection. It is considered that the likelihood of fire spread between ducts of small systems is low if an FRL of -/-/30 is maintained between the ducts.With larger ducts, the risks are considered greater and the building passive fire compartmentalization should be maintained (see Figure 11.1). One way that the -/-/30 FRL may be achieved is if the duct is wrapped in 50 mm mineral wool, held in place with steel wire mesh, or otherwise attached with non-combustible fastenings having a fusing temperature of not less than 1000°C

11.2.4 Fire dampers Fire dampers shall not be installed in kitchen hood exhaust systems. Fire baffles may be incorporated as an integral part of a proprietary kitchen hood assembly.

C11.2.4 Fire dampers are not permitted within the duct system because their effectiveness is questionable as grease on the downstream side would likely ignite before damper closure.The potential for false operation is also greater than normal and closure other than in a fire situation could have serious consequences. Some commercial kitchen hoods have their own in-built suppression system to reduce the risk of fire spreading into the duct.

Natural (or buoyancy driven) smoke exhaust systems –––––

As discussed above, the Building Code of Australia, as one option for compliance, requires the use of automatic smoke exhaust systems or automatic smoke-and-heat vents for buildings with fire compartments having a floor area of more than 2000 m2.

The automatic smoke exhaust systems must comply with BCA Specification E2.2c.

The approach taken with this design alternative is to utilise the natural buoyancy of smoke from a fire to exhaust through an automatic smoke and heat vent.

Smoke and heat vents

The BCA provides prescriptive or deemed-to-satisfy guidance which essentially requires all automatic smoke and heat vent systems to comply with Australian Standard, AS 2665

BCA Specification E2.2c

1. Adoption of AS 2665

Automatic smoke-and-heat vents must be installed as a system complying with AS 2665

2. Controls

Where a smoke-and-heat vent system is installed to comply with Table E2.2b, the following must apply:

  • In addition to thermally released link operation, smoke-and-heat vents must also be initiated by smoke detection complying with Clauses 5 and 7 of Specification E2.2a and arranged in zones to match the smoke reservoirs.

Smoke reservoirs

Smoke reservoirs are utilised in the same fashion as for smoke exhaust fans discussed earlier.

Graphic above showing activated automatic smoke curtain, controlling horizontal spread of smoke and allowing buoyant smoke to be exhausted by automatic smoke and heat vents mounted in the roof

Graphic above showing smoke and heat vents mounted on a roof along with make up air vents in the external wall of the building

Pressurisation systems –

The smoke control systems typically used for protection of multi-compartment or multi-storey, high rise buildings is to use mechanical fans to pressurise spaces within the building to control the movement of smoke in the advent of a fire.

The Building Code of Australia depending on the Class of Building, and predominately related to the height of the building, requires the use of zone smoke control systems and pressurisation of fire-isolated exits and in particular exit stairwells. The BCA requires these systems to be designed, installed and commissioned in accordance with joint Australian and New Zealand Standard, AS/NZS 1668 Part 1.

Zone or sandwich pressurisation systems

AS/NZS 1668 Part 1 describes the role of a zone pressurisation system:

8.2 ZONE PRESSURIZATION SYSTEM ARRANGEMENT

The main features of this arrangement are the following:

  • Smoke-spill air from the fire-affected compartment is discharged direct to atmosphere.
  • Return air/relief air from non-fire-affected compartments is controlled.
  • Uncontaminated outdoor air is supplied or made up to all non-fire-affected compartments at a rate such that a positive pressure is maintained in non-fire affected compartments with respect to the fire-affected compartment.
  • Pressurization of fire-isolated exits may be achieved subject to the requirements of Section 9.

C8.2 The primary effects of the zone pressurization arrangement are

  • to restrict the spread of smoke from a fire-affected compartment to a non-fire affected compartment via the air-handling system ducts and shafts;
  • to restrict the spread of smoke from a fire-affected compartment to a non-fire affected compartment by minor paths;
  • to aid the removal of smoke from the fire-affected compartment;
  • to restrict the spread of smoke into lift shafts: and
  • to restrict the spread of smoke into fire-isolated exits.

With zone pressurization systems, the exits may be fully pressurized utilizing air leakage from the pressurized non-fire-affected compartments via gaps around the doors. This arrangement is likely to achieve the performance criteria where there are 10 or more storeys involved. Below this number, special relief openings into the fire isolated exit or a combination system may be needed.

Incorrect identification of the compartment or incorrect signalling to dampers could result in the fire-affected compartment becoming pressurized. This possibly would result in smoke being forced into other compartments and fire-isolated exits (especially when not separately pressurized) with potentially disastrous consequences.

In terms of design or performance criteria for these systems, a positive pressure of a minimum of 20 Pa is required to all non-affected fire compartments

8.3 PERFORMANCE CRITERIA A positive pressure not less than 20 Pa and not greater than 100 Pa shall be developed in all non-fire-affected compartments above the pressure in the fire-affected compartment, measured with all required exit doors closed. Expansion pressures due to the fire itself shall be ignored.

C8.3 Although research into smoke control systems is continuing in various parts of the world, it is generally accepted that for most applications a positive pressure of around 20 Pa in non-fire-affected compartments with respect to the fire-affected compartment will minimize the spread of smoke. A higher pressure should be designed for where ceiling heights exceed 3 m; 40 Pa is suggested for 6 m high ceilings. The system should be designed to maintain the appropriate pressure differential under likely conditions of stack and wind effect. (See Clause 4.2.) The 100 Pa upper limit is necessary to avoid problems with lift door operation. In practice, higher pressure differentials may be acceptable on a project-specific basis subject to confirmation that the lift doors will not be adversely affected by the pressure differential.

Pressurisation of stairwells as required is conducted using a separate system as discussed below.

One of the criticisms of our Building Code by some practitioners is the assumption that vertical smoke migration through lift shaft is adequately addressed by way of a zone pressurisation system. Although AS/NZS 1668 Part 1 provides advice on the use of smoke proof lift lobbies, or lift shaft pressurisation systems, these means

of ensuring vertical smoke migration is in fact achieved, are not currently a BCA requirement.

Automatic Smoke spill, return air and fire dampers

The effective use of a zone pressurisation system obviously relies on the use of motorised dampers, including return air and fire dampers and of course the smoke spill dampers.

The operational characteristics and performance testing of these dampers is currently not very well defined, however through Australian Standards Committee, ME62/3, for which the author and others have been busy at work, a new proposed and all encompassing air dampers standard is planned for publication hopefully later in 2005 or at worse in early 2006.

One area which is being addressed in the performance characteristics of the damper actuators which of course are an important element in the correct operation and performance of a motorised damper.

Fire-isolated exit pressurisation

As discussed above, where required under the BCA, pressurisation of fire-isolated exits such as vertical stairwells, need to comply with AS/NZS 1668 Part 1.

The design under such an arrangement is to create an air velocity through openings from the fire-isolated exit to create a pressure differential between the fire-isolated exit and the occupied space. This minimised the spread of smoke into the exits, allowing the occupants sufficient time as required, to exit the building in tenable conditions.

9.3 PERFORMANCE REQUIREMENTS

9.3.1 Vertical fire-isolated exits

To meet the objective of Clause 1.2, a fire-isolated exit pressurization system shall—

  • with the main discharge doors and all doors to the fire-affected compartment fully open, sustain an airflow velocity of not less than 1 m/s into the fire-affected compartment through the doorway openings from that compartment, averaged over the full area of each door, whilst—
  • for a zone pressurization system all other doors are closed;

Conclusions –––––

Controlling the movement of smoke during building fires is paramount to the safety of lives, as we all know exposure to toxic smoke can lead to fatalities.

Smoke control measures, whether mechanical or natural, are an important method of dealing with smoke control, and together with smoke containment systems, afford designers with some options to make our buildings safe for occupants.

This article hopefully provided a useful overview or starting point for readers who wanted to understand the different smoke control systems and the components that form a part of the systems.