Low temperature smoke extract ductwork systems

When lower insulation temperatures are required due to ducting being rated the same as the smoke extract fan, or as specified by the fire-engineering consultant, Flamebar BW11 fire ducting can be used as part of low temperature smoke extract ductwork system. In this instance it should be remembered that low temperature smoke extract installations must provide stability and integrity rating to the full requirments of BS 476 Part 24 and utilise the insulation failure criteria's of the same standard, i.e 140°C/180°C surface temperature increase whilst the duct itself contains smoke at lower temperatures, i.e smoke extract from sprinkler protected areas.

To satisfy the low temperature smoke requirement, we have tested Flamebar BW11 ductwork without additional insulant to the principles of BS476 Part 24 (ISO 6944) at 400°C for two hours and satisfied the required insulation criteria of the standard. All Flamebar BW11 installations provide stability and integrity ratings in accordance with BS 476 Part 24 (ISO 6944).

Insulation Performance to the principles of BS 476 Part 24 for a fire inside duct exposure when subjected to BS 7346 Part 2 Temperature Classifications

Flamebar BW11 can be used on its own, or with the addition of various thicknesses of insulation, depending on the fire insulation rating.

Mean furnace and duct surface temperatures

Minimum periods of fire resistance for fire rated ductwork

Example showing typical fire duct running through different fire-rated compartments

In the example it shows that when the fire duct passes from the second to the third fire compartment, the fire resistance of the duct increases in-line with the rating of the fire compartment wall. This rating is not reduced in the fourth compartment, despite the reduced rating of the fire compartment wall between the third and fourth fire compartments.

Important note:

If the fire duct is for extracting smoke, it must also maintain a minimum of 75% of its overall cross-sectional area over the total length of the duct, including the first fire compartment. Therefore smoke extract ductwork in the first fire compartment must always be tested to BS 476 Part 24 for stability and integrity.

All fire rated ductwork and penetration seals must be tested to BS 476 Part 24/ISO 6944 or BS EN 1366-1 and classified in accordance with BS EN 13501-3 by a recognised NAMAS/UKAS accredited laboratory.

Note: Final approval must always be obtained by the local building control officer/fire department prior to commencement of construction