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and styles of cooking will produce different types and extents of grease deposit, and that they are all likely to have different flashpoints and ignition temperatures; what is clear, however, is that most can be readily ignited and will burn. Often fires during cooking are confined to
the pan, hob or surface with no development beyond. However, given that cooking in commercial environments is predominantly sited beneath an extract canopy and associated ductwork, flames caused through a flare up of ignited cooking oil vapours may impinge on the underside of the canopy and primary filters, igniting or otherwise quickly heating the hood, filters and grease deposits. A fire may also start inside the ductwork simply because the temperature of hot gases entering are high enough to ignite the grease. On the face of it, the risks associated all
appear a result of poor cleanliness. However, ensuring that ductwork is cleaned effectively is not just a post installation concern, but a risk that can be improved by good design and installation. For example, designing a ductwork system that is as short as possible, using filters that reduce the potential for build up of grease and fats immediately above the cooking range; installing radial rather than right angled bends at changes in direction; and ensuring that all areas of the system are accessible for cleaning, will offer significant benefits once the system is operational.
Guidance and filters
Legislative drivers aside, guidance exists for designers in the form of the Building and Engineering Services Association (BESA)
publication DW172, Specification for Kitchen Ventilation Systems. BESA also publishes TR19, Internal cleanliness of Ventilation Systems. Insurers also published in 2006 their own guidance in RC44, Recommendations for fire risk assessment of catering extract ventilation in response to the number and significance of fires of this nature. The intention is to update RC44 once the revisions to TR19, currently underway, are complete. In terms of filters, there are a variety of different styles that can be used. Mesh type filters are rated as a high relative risk due to the mesh layers holding the grease extracted from the airstream as it passes, hence significant deposits can build up. Baffle type filters comprise a number of
interlocking vanes, which form a two pass grease removal device. Grease laden air passes through the filter by a series of forced changes in direction and speed, and some of the grease particles are deposited on the vertical vanes. Deposited grease is then drained off into a collection drawer, which has to be regularly cleaned. Assuming regular cleaning and maintenance
is undertaken, relative safety ratings for filter types are described in RC44 – assuming that regular cleaning and maintenance is undertaken – showing the higher relative risk presented by mesh filters particularly. In order to ensure that the ductwork can be
effectively cleaned, greater flexibility in cleaning methods is available if the ductwork is easily accessible, although there are systems that can clean ductwork remotely. To facilitate this, ductwork sections should be fitted on their side (not the base, to prevent grease leaking out) with access doors at a maximum of 3m centres (or less where
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www.frmjournal.com DECEMBER 2017/JANUARY 2018
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