Book - School of Science and Technology

442 Ductwork designconstruction methods. The material is, of course, suitable for the lowest pressures andvelocities only and will not, even then, be air-tight.. Chlorinated rubber paint or PVC coating protection to galvanised steel is recommendedfor use in swimming pool applications.*A comprehensive list of materials and their application is given in a CIBSE TechnicalMemorandum. yDuctwork components and auxiliariesTo achieve a distribution system that will function efficiently, be capable of regulation,and be easily maintained, it is necessary to incorporate a variety of components to assisttowards these objectives. The following gives a brief overview of some of these items.Ductwork fittingsFigure 16.6 serves to illustrate the basic forms that bends can take. Radius bends are to bepreferred, and where practical a centre-line radius not less than 1.5 times the diameter orduct width should be provided (Figure 16.6(a)). Splitters (as shown in Figure 16.6(b))should be fitted where it is important to maintain the upstream air velocity profile aroundthe bend, for example at approach to equipment or measuring points. Mitre bends, withoutturning vanes, (Figure 16.6(c)), should not be used since pressure drop and noise generationare high. Mitre bends with turning vanes (Figure 16.6(d)) are quite satisfactory solong as the turning vanes are of the correct design and are constructed to a good standard.Radius bends alone should be used on medium and high pressure/velocity systems.Branch connections should be constructed such that the air flow is divided, or combinedin the case of an extract system, with minimum turbulence. Changes of duct size should bemade downstream, or upstream, of the fitting, not at the fitting. Typical branch connectionsare shown in Figure 16.7. Branches on medium and high pressure/velocity systemsshould be taken off at 45 , or be made to a recommended pattern for such applications.At a change ofsection in a duct, the aim is to maintain the air stream attached to theduct sides. If the air becomes detached, high turbulence occurs with resulting high pressureloss, noise and a non-uniform air velocity profile. At an expansion, the included angleshould be limited to 30 , above this internal splitters should be used, as in Figure 16.8. OnW(a) (b) (c) (d)Radius bendsMitre bendsFigure 16.6 Typical ductwork bends* Corrosion ofHeat Recovery Exchangers Serving Swimming Pool Halls. Electricity Council ResearchReport, August 1961.y Design Notes for Ductwork. CIBSE Technical Memorandum TM8, 1983.