KItchen Ductwork Design
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Design Considerations for Kitchen Ductwork
Sizing
The sizing of ductwork is generally a trade off between overall dimensions and noise / resistance levels. Ductwork should be sized to allow a velocity of 6m/s where connection to the canopy occurs and 8m/s in general extract duct runs. Higher velocities will increase noise and resistance levels. Increased resistance levels require more powerful, and expensive, fans.
Construction of Extract Ducts
Kitchen extract ductwork that discharges straight to atmosphere and does not pass through fire compartments should be constructed from galvanised or stainless steel with a minimum thickness of 0.8mm. Longitudinal joints should be air tight and leak proof, ideally fully welded or formed by a grooved or lock seam with sealant applied afterwards. Longitudinal joints should not be sited at the bottom of the duct. Cross joints should be air tight and leak proof. Again, an ideal solution is to fully weld the joint but this is not always practical, therefore a separate flange can be fitted to the duct providing that the flange corners are either fully welded or in the case of proprietary duct flange systems, corner cleats are fitted. These joints should be fully sealed with a silicone type sealant or fitted with a gasket. Adjustable joints, often referred to as telescopic or slip joints, can be used to connect the duct to the canopy spigot. Adjustable joints should be sealed with a silicone sealant and the duct should be reinforced within 200mm of the joint, reinforcement can be achieved using formed angle or channel sections secured to the duct. Flexible connections should be constructed from a material with a suitable half hour fire rating; ‘off the shelf’ flexible connections supplied with most fans are rarely adequate.
Construction of Air Supply Ducts
Kitchen supply air ductwork should be constructed from galvanised or stainless steel with a minimum thickness of 0.8mm. Longitudinal joints should be air tight and leak proof, ideally fully welded or formed by a grooved or lock seam with sealant applied afterwards. Longitudinal joints should not be sited at the bottom of the duct. Cross joints should be air tight and leak proof. Again, an ideal solution is to fully weld the joint but this is not always practical, therefore a separate flange can be fitted to the duct providing the flange corners are either fully welded or in the case of proprietary duct flange systems, corner cleats are fitted. These joints should be fully sealed with a silicone type sealant or fitted with a gasket. Adjustable joints, often referred to as telescopic or slip joints, can be used to connect the duct to the canopy spigot. Adjustable joints should be sealed with a silicone sealant and the duct should be reinforced within 200mm of the joint, reinforcement can be achieved using formed angle or channel sections secured to the duct.
Extraction: Routes to Atmosphere
Where possible, the ideal route of any kitchen extract duct should be straight to atmosphere as directly as possible without passing through any other rooms or breaching any fire compartments. (See ‘Fire Resistance’ below.) The minimum quantity of bends should be used and all internal surfaces should be smooth and clear of obstructions.
Fire dampers should not be fitted to kitchen extract ductwork. (See ‘Fire Resistance’ below for further information.) Backdraught shutters should not be used.
Volume control dampers and turning vanes should be avoided where possible as they form grease traps. Swept bends may be substituted for turning vanes.
The extract outlet should be positioned to ensure the extracted fumes cannot be pulled back into the building by any nearby intake fans. The extract point should also be well clear of and ideally above any unprotected openings or windows where fumes or smoke may be drawn back into the building. The extract point must be clear of external fire escapes.
Any vertical sections of ductwork should have a drain opening at the bottom and some means of grease collection under the drain.
Air Supply: Routes from Atmosphere
Where possible, the ideal route of any supply duct should be as short and straight as possible without passing through any other rooms or breaching any fire compartments. (See ‘Fire Resistance’ below.) The minimum quantity of bends should be used and all internal surfaces should be smooth and clear of obstructions. The supply air inlet point should be positioned well clear of any contaminated air extraction points and known polluted areas such as low level on a busy street. The prevailing winds should be considered to avoid cross contamination from any extracted fumes.
A mesh should be fitted to the input point of sufficient density to prevent small birds from being drawn into the duct. A fine mesh, such as insect mesh, should not be used as it is likely to become blocked. Disposable filter panels should be fitted within the ductwork in a position where they are not open to the elements but can be easily accessed for inspection and replacement. Alternatively, disposable filter media should be fitted to the supply air diffusers within the building; again, these should be easily accessible for maintenance and replacement. Any vertical sections of ductwork should have a drain opening at the bottom.
Fire Resistance
Ductwork within the fire compartment of the kitchen that does not pass through other fire compartments has no special requirements to that stated above (under ‘Construction’).
Fire Resistance for Extract Ductwork
Where extract ductwork passes through fire compartments such as floors or walls, the duct itself should either be constructed so that its fire resistance is at least as high as the compartment it passes through or, alternatively, conventional ductwork may be used providing it is suitably clad in fire rated material. Additionally, the ductwork should be externally insulated so that in the event of a fire outside the duct, temperatures within the duct will not become high enough to ignite any grease build up. Fire dampers must not be used within extract ductwork. For further information concerning this matter refer to BS 5588- 9:1999.
Fire Resistance for Air Supply Ductwork
Where ductwork passes through fire compartments such as floors or walls between other rooms, fire dampers must be fitted within each penetrated structure to prevent the spread of fire.
Access for Cleaning & Maintenance
All internal surfaces of the extract duct must be accessible for cleaning. For further information regarding this we would suggest referring to HVCA documents DW/144 and DW/172.
Insulation
Where extract ductwork runs through cold areas of a building externally mounted insulation should be considered to prevent condensation forming within the ductwork.
All tempered supply air ducts should be insulated to prevent internal condensation and heat loss.
