Book - School of Science and Technology

Ductwork 439ductwork is to achieve as much standardisation and repetition in the configuration andcomponents as is practical, since this will give a more economic installation.Rectangular ducts are constructed of flat sheets by bending, folding and riveting and areerected in sections with slip joints or bolted angle ring joints. Larger sizes tend to drumand adequate stiffening is necessary. Sometimes a diamond break is used to assist instiffening, as Figure 16.2, the sheets being pressed to provide, in effect, a very shallowpyramid form; alternative methods include pleading or beading. Where rectangular ductsare used in high velocity systems, stiffening is particularly important using bracing angles,the angles, and internal tie rods for larger sizes. Aspect ratios in excess of 4:1 are to beavoided. Sizes should be selected from the range given in the HVCA specification.Circular ducts may similarly be formed from flat sheet with folded or riveted seams andare inherently free from risk of drumming. Traditionally, the use of good quality circularductwork was largely confined to industrial applications. Mass produced snap-lock ductlengths were used for low cost domestic warm air heating, reliance being placed uponjointing tape for air-tightness.The advent of circular ducts made from galvanised strip steel with a special lockingseam, as in Figure 16.3, has led to their adoption generally. Sizes are available from63 mm up to 1600 mm diameter. Ducts of this type are particularly suitable for highvelocity systems due to their rigidity, `deadness' and air-tightness. Ranges of standardtees, bends, reducers and other fittings are made and these, together with lengths ofstraight ducting cut off on site as required, facilitate the task of erection compared withthe tailor-made methods previously adopted.Jointing is by means of a special mastic, with some riveting, and in best practice a heatshrink plastic sleeve or a chemical-reaction tape is used externally to seal the joint.Alternative methods of jointing ductwork are available, for example where the ductfittings have a groove formed at each end which houses a sealing gasket and lockingcollar. Connection is made by pushing the duct and fitting together which forces theFigure 16.2 Diamond break stiffening for ductingFigure 16.3 Spiral wound circular ducting