Book - School of Science and Technology

446 Ductwork design6mm Plate damperon off-centrespindleWeight(a) (b) (c)AccesspanelHeavy gauge gravitydrop shutter bladesFusiblelinkSteel honeycomb50 to 150mm thickFusiblelinkDuct sectionin 6mm plateCross sectionsAccesspanelFigure 16.11 Typical construction methods for fire dampersfusible link. In the event of a fire the link melts and the damper swings closed. Anotherpattern, Figure 16.11(b), takes the form of a number of substantial shutter blades retainedout of the air stream by a fusible link as before; this is normally referred to as a `curtain'type. A different form of fire damper is one where intumescent material is secured in asteel frame and inserted into the air duct, as Figure 16.11(c). This material has theproperty of swelling to many times its original volume when heated and thus forming abarrier to air flow. Intumescent collars can be used on circular plastic extract ducts wheresteel dampers would corrode.Combination fire and smoke dampers are designed to function both as a fire damper,operated by a fusible link or thermal actuator, or as a smoke control damper powered byan actuator from a remote signal. Remote indication of the damper position can also beprovided.Smoke release dampers may also be required at air inlets and exhausts. Operationnormally would be a fail-safe spring loading, via a remote signal, which wouldcause the damper to open thereby allowing any smoke in the system to discharge toatmosphere.Flexible ductworkThis is available in treated fabric on a steel helix of various materials and finishes, and inspirally wound `bendable' metal of various materials. Selection is made for the particularapplication. Flexible ductwork is normally used at the final connection to air terminalsand at connections to variable air volume and mixing boxes, induction units and the like.The length of these connections should be limited to 600 mm, and never be more than theequivalent of six times the duct diameter.Whatever the choice of material, it must meet the fire requirements, and the overall airleakage and frictional resistance criteria for the system.