The present invention relates to fire resistant ducting systems and particularly to ducts for an air distribution system.

Ducting systems for the extract & distribution of ventilating or heating air are widely used in offices, warehouses, shops and factories. In many installations, it is a requirement for the ducts to be fire resistant. Typically, this is done by first erecting the entire ducting system and when it is in place covering the outside of the ducting with a fire resistant insulating layer. There are a number of disadvantages with this arrangement. Since the ducting is typically placed high off the ground, it is necessary to erect scaffolding for the workers installing the fire resistant layer, but there is still a significant risk of falls which creates a safety hazard. The productivity of workers working under these conditions is inevitably lower than if they were working at ground level. It frequently occurs also that there are difficulties in accessing certain parts of the ducting, where this is close to the roof of example. Furthermore, it is difficult to monitor and check the quality and the uniformity of the installation with the result that gaps may be formed in the installation which leads to local hot spots in the event of a fire. This can lead to an increased risk of a general conflagration developing through the ducting system

There are therefore a number of disadvantages with the existing system which risk compromising the integrity of the installation and also make it very costly to install. The present invention seeks to provide a practical solution to these problems.

According to the present invention there is provided fire resistant ducting system comprising at least two elongate duct sections having flanges at each end through which adjacent duct sections are secured together, an intumescent gasket being located between the two flanges, the duct sections being lined on the interior surface with a fire resistant insulation lining.

Preferably, each duct section has an outer skin and an inner skin which may be formed of galvanised sheet steel, with the insulation lining located there between each layer. In a preferred embodiment, an intumescent mastic sealing material forms a seal between the flange and the outer skin of the duct. Preferably, the lining is formed of a foamed or expanded plastics material such as phenolic foam resin or mineral wool compounds or foam glass type insulation.

In a preferred embodiment, the duct sections each has stiffening means extending transversely across the duct section intermediate the flanges and projecting into the insulation lining. The stiffening means it may be secured to the inner skin or outer skin of the duct section and preferably the stiffening means is located substantially at the midpoint of the duct section.

Preferably, the stiffening means projects only partially through the thickness of the insulation lining so that there is no heat bridge across the duct.

Preferably, the internal skin has an outwardly extending flange abutting the end of the insulation lining and extending only partially over the depth of the lining.

Preferably, an intumescent mastic material seals the joints between the insulation lining, the intumescent gasket and the flange. Preferably, the outer peripheries of adjacent flanges are clamped together and sealed by means of a flange clamp.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 shows a sketch view of a ducting system,

Figure 2 shows a detail of a flange and the end of the duct section,

Figure 3 shows a cross-sectional view through the flanges of 2 duct sections are secured together, and

Figure 4 shows an alternative embodiment of the invention.

Referring now to Figures 1 and 2, there is shown a plurality of duct sections 1 , 2, 3 of generally square or rectangular cross-section, each section having at its two ends flanges 4, extending from the duct sections 1, 2, 3 at 90° to the plane of the length of the duct section through which flanges 4 adjacent duct sections are fastened together. The duct sections each have an outer skin 5 formed of galvanised steel sheet, an inner skin 6 of galvanised sheet material and a fire resistant insulation lining 7 located between the two skins 5, 6 to provide a barrier to the transmission of heat and flames into and out of the duct. The insulation lining is typically formed of a laminated phenolic foamed or expanded plastics materia! or mineral wooi compounds or foam glass type insulation.

Referring now to Figure 3, there is shown a sectional view through two duct sections 1 , 2 joined together by means of their flanges 4. As described, each duct section has a galvanised steel outer skin 5 and a galvanised sheet steel inner skin 6 between which the insulation lining is sandwiched. The flanges 4 are connected to the outer skin by mechanical fixings 8 with an intumescent mastic 9 between the flange 4 itself and the outer skin 5. An intumescent gasket 10 approximately 6 mm thick is located between the two flanges 4 and extending to cover the ends of the inner and outer skins 5, 6 and the lining 7 to provide a heat resistant barrier. An intumescent mastic sealer 11 is located over the outer skin and extending down part of the outer face of the insulating lining 7between the lining 7 and the gasket 0 to provide a further flame resistant barrier.

As shown, the inner skin has a flange 12 formed of the skin material extending to overlie the outer end of the insulating lining 7 but not extending to the outer skin 5 at the top of the lining. With this construction, there is no metal bridge between the inner skin 6 and the exterior of the duct section 1 to eliminate the possibility of a heat bridge. A transverse strengthening web 13 is secured to the inner skin 6 and extends across the inner skin 6 at a point adjacent the midpoint of each duct section to stiffen the duct section. The web 13 is formed of an L-shaped section having one limb 14 secured to the inner skin and the other limb 15 being pressed into the insulation lining 7 to extend partially through the lining. Again, no metal bridge is formed across the duct between the inner and outer skins 5, 6.

The joints between the two flanges 4 and the gasket 10 are enclosed by a cap 16 which clips over the peripheries of the two flanges and includes securing means 17 to provide a mechanical fixing for the cap 16 to the two flanges. Referring now to Figure 4 there is shown an alternative form of a duct section where like parts have like references. In this embodiment, the duct section 1 does not have a continuous inner skin that has an L-shaped flange element 18 having one limb 19 engaging the outer face of the insulation lining 7 and the second limb 20 engaging the inner face of the insulation lining 7 to maintain the lining in position adjacent the outer skin 5.

In this embodiment, a mastic seal 11 is provided between the flanges 4 and the outer skin 5 and contiguous faces of the insulation linings 7. In this embodiment, the flange includes an outwardly extending web section 21 which, in the assembled condition shown, lies between the outer skin 5 and the insulation lining 7. The flange also includes an outer web 22 which lies outside the outer skin 5 so that the end of the outer skin 5 is inserted in the webs 21, 22 and the space between the two webs is filled with an intumescent mastic material. The flanges 4 are secured to the outer skin 5 by fastening means which passes through both the webs 21 , 22 and the outer skin 5, as shown. The outer web 22 has its outer edge upturned slightly to facilitate an outer enclosure cap 23 being secured thereto to fully enclose all the joints between the flanges and the two duct sections. Although not shown, it is possible that the cap could be filled with an intumescent fire resistant material.

Although shown as a square or rectangular section, it will be understood that the duct sections could be round or oval to suit the particular installation. In further embodiments, the outer skin of the duct may be coated with an intumescent paint to provide further fire resistance.