TYPES OF DUCT SYSTEMS & FUNCTIONS

VENTILATION DUCT SYSTEMS

Mechanical ventilation systems are used to extract vitiated or polluted air from a building and to supply replacement fresh or conditioned air. The necessary fans and conditioning equipment are generally located in separate plant rooms, often in a basement or on the roof. The distribution of the air involves ductwork which may be very large, extend throughout the building, penetrate compartment walls and/or floors and have openings in every space through which it passes. If appropriate fire precautions are not implemented, ventilation ductwork may provide a route for fire, smoke and toxic gases to spread rapidly through a building.

SMOKE EXTRACTION DUCTS – SINGLE AND MULTIPLE COMPARTMENTS

Smoke extraction is the evacuation from a building of products of combustion, such as smoke and toxic gases, which could otherwise reduce visibility and impair human functions. This facilitates the escape of the building occupants and assists fire fighters in locating the seat of the fire and extinguishing it. In situations where smoke clearance by natural ventilation through windows or other openings may be difficult (e.g. in large or deep basements or in high rise buildings without windows that can be opened) ductwork is required to conduct the smoke to a suitable outlet from the building. In cases where the natural buoyancy of the combustion products is not adequate to ensure the required smoke extraction rate through the ductwork, fan assisted systems are used. It may also be necessary to install ducted air inlets as part of the smoke extraction scheme, in order to provide the replacement air. If the ductwork incorporated in a smoke extraction system is wholly contained within the fire compartment, it must at least be capable of resisting the anticipated smoke temperatures generated during the development of a fire. These will generally be lower than the temperatures specified in EN 1366, which are intended to represent a fully developed fire. However, if the ductwork penetrates a fire resisting barrier, it must also be capable of providing the relevant fire resistance in a test to the relevant part of EN 1366. Further clarification of the fire testing requirements for these two different situations is provided in Section ‘5’ of this document ‘Standard Fire Tests’.

DUAL VENTILATION /SMOKE EXTRACTION SYSTEMS

These systems serve as a conventional ventilation system under normal conditions, but are converted to a smoke extraction system in the event of fire, thus providing an economical dual system.

PRESSURISATION SYSTEMS

Pressurisation is a method of restricting the penetration of smoke into certain critical areas of a building, by maintaining the air within the critical areas at pressures higher than those in adjacent areas. It applies particularly to protected stairways, lobbies and corridors, as smoke within these areas would inhibit escape, and also to fire fighting shafts serving deep basements, because of the difficulties in clearing smoke from basements.  A pressurisation system is a special form of mechanical ventilation system. However, as the air supply creating the pressurisation must be maintained for the duration of a fire, fire dampers cannot be used within the ductwork to prevent the spread of fire. Any duct or ductwork penetrating fire resisting barriers must be fire resisting.

KITCHEN EXTRACT SYSTEMS

Kitchen extraction ductwork presents a particular hazard, in that combustible deposits such as grease are likely to accumulate on its internal surfaces, and may spread fire if ignited. A fire in a kitchen may spread to other areas of the building by way of the kitchen extract ductwork and may also prejudice escape routes. Guidance for the installations for kitchen extract ducts is provided in various documents. Whilst paragraphs (a) to (g) may not be exhaustive, they attempt to list the common requirements with references the regulatory guidance in Approved Document B 2006, where applicable, and other documents.

4.5.1 Means of Escape (Approved Document B: B1)

a)  Approved Document B: 2006, Paragraph 5.50

Non-domestic kitchens are required to have separate and independent extraction systems, because of the polluted nature of the extracted air. The extracted air should not be re-circulated.

b)  Approved Document B 2006, Paragraph 10.10

Fire dampers should not be installed in extraction ductwork serving kitchens due to the build up of grease.

c) Any kitchen extract duct or ductwork penetrating fire resisting barriers must be fire resisting between the kitchen and the external weathering cowl. The fire resistance requirement is normally 30 minutes, unless the duct also penetrates a compartment separation wall/floor where the fire resistance period and criteria should meet any higher requirement of the compartment wall/floor.

d) British Standards

 Although Approved Document B 2006 does not provide guidance on kitchens other than in small premises and residential care homes, the AD-B states in paragraph 0.22 that supplementary guidance is given in British standards, and AD-B paragraph 5.53 references BS 5720. (We suggest that paragraph 2.5.2.3 applies) BS 5720 paragraph 2.5.2.3 States that kitchen extract ductwork should be enclosed with fire resisting construction where passing through other floors between the kitchen and the external weathering cowl. The canopy, ducting and lagging forming the kitchen extract duct should made from non-combustible material BS 9999 Paragraph 32.5.7 Table 31 requires a kitchen to be enclosed with fire resisting construction.

e) Guidance for Hospitals is given in HTM 05-02.

In most cases, unless a suitable automatic extinguishing arrangement is provided, kitchens arerequired to be enclosed with fire resisting construction. Consequently the extract ductwork also needs