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Senior Hargreaves
Lord Street
Bury, Lancashire
BL9 0RG
United Kingdom

T.+44(0)161 764 5082
F.+44(0)161 762 2336

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senior-hargreaves.co.uk

www.hargreaves-ductwork.co.uk

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pr@ainsmag.co.uk


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Fire – Containing the Terror - The Role of Ventilation Using Fire Resistant Ductwork to Safeguard Life and Property

Prevention is the best form of fire defence, but simple error, negligence and even malicious acts can frustrate even the best plans. Buildings must be engineered to safeguard their occupants by allowing speedy escape and easy access for fire fighters.

It is also important to minimise damage and disruption so that the enterprise has some hope of survival and recovery.

Keeping Fire in its Place

In the days when homes had fires we kept them in the grate. The flue took away harmful products of combustion and the rate of burning could be controlled by adjusting the quantity of fuel and airflow through an aperture. If necessary, a fire guard protected the room from sparks or falling embers. This is an example of containment.

Fires in modern buildings are less easy to control. They do not occur in a designated place. In most premises combustibles are present in large quantities, the supply of oxygen is unlimited and potential sources of ignition are numerous. Moreover, lethal products of combustion accumulate and are not removed unless effective ventilation is provided. Control over these factors can be achieved by application of appropriate measures, but the potential for fire always remains.

Containment is normally achieved by use of fire walls, doors and other resistant barriers. Two sets of regulations apply, each with a slightly different focus. Building Regulations specify the essential precautions to ensure safety of life by providing effective means of escape and ensuring prompt evacuation. Loss Prevention Council (LPC) regulations look at the commercial risks and how to minimise property damage and disruption. To ensure that the enterprise can continue, the aim is to localise damage, where possible, to a single compartment.

The regulations specify the resistive properties of walls and floors taking into account the level of risk. Key factors in risk assessment are the concentration of combustible materials, expressed as a fire loading, the presence or absence of sprinklers and particular characteristics of the protected area. Significantly, the regulations also look at the structural performance of resistive structures to ensure that they will not collapse, distort and leak prematurely if exposed to fire.

Design Constraints

There is a conflict between the need for containment and access. We cannot live in buildings that are just sets of hermetically sealed cells. We need and expect freedom of movement in the buildings where we work, shop and take our leisure.

Two approaches to containment are therefore common. In offices, hospitals, schools and similar establishments cellular containment is common with fire resistive walls and floors and access via fire doors with resistant properties. In factories, warehouses, shopping centres, airport terminals and so on – where freedom of movement is more important – a more subtle form of smoke containment is employed. This often uses structural elements such as beams and roof slopes, backed up with fire resistant high level screening to control smoke flow and create zones of containment. These areas are often associated with generalised smoke venting schemes.

The increasing complexity of buildings means that we need access to services such as electricity, communications and data networks, water and air-conditioning. These services must be routed through fire compartments. They pose a particular fire threat because each breach of a compartment barrier, be that a wall, a floor or a roof screen has the potential to spread fire.

The regulations recognise these dangers and emphasise the importance of fire stopping wherever there is a breach, backed up by on-site quality control, to monitor the effectiveness of these measures. The overriding concern is to maintain the integrity of the fire resistant structure, wherever services pass through it.

The Role of Ductwork

General ventilation ductwork can become a conduit for fire spread. For this reason it is usual to interlock heating, ventilation and air-conditioning fans to the fire alarm so that in an emergency air movement ceases. Where heating, cooling and service ducts pass through fire walls, or other resistive structures, then regulations specify that a fully tested damper of equal fire resistance to the fire barrier must be fitted in-line with the barrier.

Increasingly, ventilation plays a positive role in fire containment. In some open areas – such as shopping malls, factories and warehouses – smoke may be vented direct to the outside. In multi-compartment and multi-storey buildings, however, fire resistant ductwork or protected shafts will be required to ensure safe transmission of heat and smoke through adjoining areas to outside. Consideration may also need to be given to pressurisation of stairwells and other escape routes in order to keep them free of smoke.

All areas may be susceptible to fire but some areas represent special risks. Basement car parks and kitchens are two such examples. The fire at the Royal Albion Hotel in Brighton in November 1998 spread from an ordinary unprotected kitchen extract duct and resulted in the loss of a substantial part of a listed building.

Technical Requirements for Fire Rated Ductwork

Standard ductwork is unsuitable for use in emergency applications. This is because the gauges and constructions cannot withstand the sustained temperatures created by fire and support systems may also fail.

The performance tests for fire resistant ductwork are currently defined in BS476, Part 24:1987 (ISO6944-1985). This describes the method for testing the performance of ducts that penetrate fire separating structures of a building. It establishes the key principle that fire ducts must be at least equal in performance to the fire barriers that they cross, because they are essentially extensions of those fire barriers. The standards of fire protection required for differing classes of building use and occupancy are defined in BS9999.

Many fire duct systems will handle relatively dilute smoke and fumes at temperatures of 300 to 400 degree C. This is well below the 600 degrees C at which steel begins to soften and lose structural integrity. Kitchen extract ducts, however, represent a special case. While these are installed with grease filters to minimise grease build up in the duct, is common for maintenance on these systems to be neglected and for grease build up in the duct to occur. A fire within a kitchen extract duct, particularly one passing through other fire zones or floors above is a serious hazard and it is essential that these ducts are adequately engineered to ensure containment and prevent heat transmission. The provision of access hatches to permit frequent inspection and cleaning is essential.

To ensure performance, ductwork constructions are tested by furnace exposure to a prescribed time and temperature profile to achieve type approval. The tests simulate two possible conditions. Firstly, outside-to-inside heating – as when a duct passes through a fire zone. Secondly, inside-to-outside heating – as would occur with a smoke ventilation duct, such as a kitchen extract, passing through compartments not affected by fire. In either circumstance, breach of the duct wall or inadequate insulation could result in fire spread.

To conform to British Standards ducts must satisfy three critical tests:

• Stability – the ability of the ductwork system to resist collapse under fire conditions.
• Integrity – the ability to resist cracking that could result in flame leakage or penetration.
• Insulation – the ability of the ductwork to remain sufficiently cool on the face not exposed to fire so that adjacent materials are not ignited.

An additional requirement of smoke extract systems is that ducts, because they are under negative pressure, must resist implosion and retain at least 75 percent of their cross sectional area to ensure that extraction systems remain effective.

There is nothing to prevent fire rated ductwork being used where conventional ductwork would traditionally be used. Many installations are dual purpose. This is often true of ceiling void and basement car park extract ducts. These ducts do not need and must not have fire dampers, other than to isolate runs of duct that have no emergency role.

Methods of Achieving Fire Resistance

There are various methods of protecting ductwork and providing insulation. These include wraps, intumescent coating, solid slab or sprayed material. Any of these treatments will produce satisfactory protection if properly applied.

Site applied protective materials can be prone to problems if supervision and quality control are lacking. In a site environment consistency of cover, access to all surfaces and damage by following trades may all undermine quality and eventual performance. Site-applied treatments are also suspect because the whole system, comprising ductwork, over cladding and the critical support structures, cannot be replicated and fully fire tested.

Some insulation board treatments and intumescent coatings can be factory applied. This enables better quality control to be achieved. These systems may still be prone to site damage during installation or by following trades on site.

Modern Approaches

Senior Hargreaves manufacture and install a range of fire-rated ductwork that meets all safety requirements. The system comprises different ductwork constructions each engineered for a particular ventilation application and having between 30 and 240 minutes fire resistance.

These include special constructions for smoke ventilation, kitchen extract, pressurisation, cable protection and so on. All constructions, including the support systems, are engineered to meet the requirements of British Standards. A series of tests, commissioned by Senior Hargreaves at the independent Warrington Fire Research Consultancy, provide technical validity.

Where ducts convey products of combustion through fire zones or pass through fire areas, then a unique insulated construction is used. This comprises a twin-wall armoured duct with an inert insulation core protected from other site trades and not exposed to damage or open to degradation. All ducts are factory built in a stringent quality controlled environment.

Conclusion

Fire prevention is preferable to chaos – good housekeeping has an important but often overlooked role. Properly maintained buildings, kept tidy, provided with signed exits, tested extinguishers, trained staff, well placed and maintained alarms, are less likely to suffer severe fire problems. These measures are not a substitute for fire engineering systems, the two work together for maximum protection, because fires still occur – even in well managed environments.

Despite the complexity of modern buildings, apparent conflicts in design objectives and the risks arising from accident and malicious acts, systems are available that give a substantial measure of protection – meeting the fire engineering challenges of modern architecture.

More Information

Geoff Mort, Tel. +44 (0)161 764 5082 Fax. +44 (0)161 762 2336
Email: pr@senior-hargreaves.co.uk Web: www.hargreaves-ductwork.co.uk
Senior Hargreaves, Lord Street, Bury, Lancashire BL9 0RG, UK

Feature issued: March 30, 2009

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For where ducts convey products of combustion through fire zones, Hargreaves unique insulated construction can be used. This comprises a twin-wall armoured duct with an inert insulation core.

Fire resistant ductwork will be used for smoke removal from the platforms at Cannon Street Tube Station in the event of a fire.

HVAC services for high rise buildings such as News International can be complex. It is vital ductwork with the right fire resistance properties is chosen to ensure safe evacuation.

A series of tests, commissioned by Senior Hargreaves at the independent Warrington Fire Research Consultancy, provide technical validity of their ductwork constructions.

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Fire Rated Ductwork