Book - School of Science and Technology

440 Ductwork designFlat ovalStandardfittingCircularFigure 16.4 Flat oval ducting and brancheslocking collar into position to provide an `air-tight' joint. Such forms of constructionshould be restricted in use strictly to manufacturers' recommendations.Flat oval or spirally wound rectangular ducts are in common use in air-conditioning.These are formed from spirally wound circular ducts to the profile illustrated in Figure16.4 and are available in sizes from 75 mm 320 mm to 500 mm 1640 mm. A limitedrange of standard tees, bends and reducers is made. Such ducts have the advantage ofbeing mass produced and can be used in conjunction with circular sections where space islimited. Site jointing, as before, is completed by a heat shrink or chemical reaction sleeve.Builders' work ductsThe suggestion is sometimes put forward, in early stages of design for a building, thatmain air ducts and rising shafts should be constructed as a part of the fabric. Thefollowing advantages are claimed for such an expedient:. Cheapness in that the space very often exists as a structural wind brace or in associationwith one or more lift shafts.. Permanence.. Reduction in heat gains and losses, due to the heavy construction as compared withmetal.. Rigidity and hence reduction in noise transmission, also as a result of the heavierconstruction.. Accessibility for cleaning.Several of these claimed advantages may have been valid half a century ago when thetechniques of sheet metal duct construction were less well developed and when time lag inresponse to automatic control was of less significance. They must now be questioned andconsidered in the light of the proven difficulty of producing any form of builders' workconstruction which is reasonably air-tight.