Book - School of Science and Technology

Ductwork 441Ground floorDuctMastic liningCorridorBasementFigure 16.5 Structural air duct over corridorAs a result of structural settlement following completion; later movement as a result ofeither drying out or temperature changes; and the high permeability of masonry materialof all descriptions, builders' work ducts are always suspect. They may be consideredreasonably viable only when lined with a compound having some measure of elasticityand which has been applied by a specialist contractor. Following such treatment, airleakage may possibly be reduced but only to a level which is still many times greater thanthat which would be acceptable for low pressure sheet metal ductwork, as Table 16.1.Consequent upon these difficulties, the use of builders' work construction should beconsidered only as a last resort and then for extract ducts alone, at low velocities andpressure differentials. Where the air extracted may be contaminated, such constructionshould not be used under positive pressure, i.e. on the delivery side of the fan.Figure 16.5 shows an example of a duct constructed over a basement corridor andserving as a main distribution trunk from the plant to rising shafts about the building:such a construction cannot be recommended since it will, inevitably, not be air-tight.Ducts of other materialsOther materials used for ducts are:. Welded sheet steel is mainly used in industrial work, or where the air-tightness of thejoints is of great importance; such ducts may be `galvanised after made'.. Copper, aluminium, stainless steel. Ducts constructed of these materials are used inspecial cases where permanence or a high degree of finish is required, or where specialcorrosion problems arise.. uPVC and Polypropylene which are chiefly used for chemical fume extraction and tendto be expensive due to the cost of moulds, etc., for fittings and special sections.Reference HVCA specification DW 151. The Laboratory of the Government Chemisthas developed a combination of uPVC coated externally with fire-retardant glass fibrereinforced filled polyester resin (GRP) for use with fume extract systems. This isinherently stronger and more durable than uPVC used alone.. Glass fibre resin bonded slabs can be cut and adapted at site, and jointed to form asmooth continuous duct. This technique was favoured in the USA but has not beenmuch used in this country although a HVCA specification exists (DW 191) to codify