Page 8 - Caswell Firesafe Manual 2018

TECHNICAL MANUAL

FFRDL / TM / BS / 01 / 2018

4 Fire Resisting System Applications

Fire resisting ductwork is classed as Passive Fire Protection. Other examples of construction products that
would be classed as passive fire protection are fire-resisting walls, floors, cavity barriers, doors, windows,
shafts / stairwells and cladding to structural steel.

On the other hand, Active Fire Protection includes fire alarm systems, heat / smoke detectors, sprinkler
systems, fire suppression systems and smoke control systems. The standard method adopted in general
supply and extract ventilation systems to prevent fire and smoke spreading from one compartment to
another is to install tested fire or fire / smoke dampers within the ductwork.

Limitations to the use of Fire Dampers
It is impractical and inadvisable to use fire dampers in Smoke Extract, Pressurisation or Commercial
Kitchen Extract systems.

Fire dampers that incorporate a fusible link (thermal device) are designed to close when triggered by the
elevated temperatures within a duct in the event of fire. This precludes their use in Smoke Extract or
Pressurisation systems as the dampers, once activated, would prevent these systems from fulfilling their
intended function.

Commercial Kitchen Extract systems are by nature prone to becoming laden with grease and carbonated
deposits. Even though a thorough cleaning regime is recommended, it is possible over time that the damper
blades and / or mechanism become saturated or encrusted to the point that they are inoperable, leading to
a serious system failure.

Thermal fusible links are available in various ratings but the most commonly used are designed to fail at c.
72ºC (162ºF) so the high temperatures generated under normal cooking conditions could easily trigger
them. If this happens there would be no extraction and so no further cooking would be possible until the
dampers were reset. This is clearly not a viable option for commercial caterers.

In all these cases, the use of dedicated fire resisting ductwork is essential to maintain the integrity of the fire
compartment through which the duct travels. Fire resisting ductwork is most frequently used in the
following systems:
Smoke Extract / Car Park Extract
Dual Return Air / Smoke Extract
Pressurisation
Kitchen Extract
Once the system type has been chosen, the fire rating to BS 476:Part24 (1987) for Stability, Integrity and
Insulation (in minutes duration) must be stated. The required operating pressure limits of the ductwork, as
defined by HVCA specification DW144, need to be defined as one of the following classes:

Low - Class A - Up to 500Pa positive or 500Pa negative
Medium - Class B - Up to 1000Pa positive or 750Pa negative
High - Class C - Up to 2000Pa positive or 750Pa negative)
High - Class D - Up to 2000Pa positive or 750Pa negative / Very Low Leakage

Finally, accounting for the correct quantity of Access Doors to allow maintenance and cleaning to the
required TR/19 PDI level forms the core ductwork specification.

8 www.firesafeductwork.co.uk