Book - School of Science and Technology

Special applications 369Connection to individualfan or header ductFlexible ductingWall fixingfor gantryarmFlexible ductingCircular handleExtraction hoodFigure 13.19 Fume extract from a welding bench (Plymovent)Fabric tube supply systemsThe need exists in some industrial buildings for the introduction of large air volumes incircumstances where the process cannot tolerate any significant air movement. A solutionwhich makes use of permeable fabric tubes, in place of the more conventional ductwork plusdiffuser assemblies, has been applied with success to tobacco processing and textile spinningworkrooms. For a multi-tube arrangement, a sheet metal header duct is connected to theappropriate number of tubes which are inflated and maintained under pressure by a conventionalcentrifugal fan. Supports take the form of a `curtain track' above each lengthwhich allows a deflated tube to be drawn to the header for disconnection.Tube sizes range from 150±750 mm diameter and lengths of 20±50 m are used, dependentupon diameter. The initial velocity of air supply into a tube is between 12 and 15 m/s anda typical pressure drop, for a flowof 500 litre/s through a 300 mm diameter tube, wouldbe about 100 Pa. The efficacy of the system is a function of adequate inflation which,in turn, depends upon a constant air supply and upon a good standard of filtration at theplant. Tubes require laundering at intervals and the support system permits easy removalfor this purpose and replacement by a spare.Plenum systemsIn an earlier paragraph of this chapter, it was suggested that a straightforward ventilationsystem would be designed on the assumption that heating of the building was dealt withby a radiator or other similar plant. The plenum system, which is a method of providingheat using air as the distribution medium, is thus noted here only to showthat it has notbeen forgotten. The reader is referred to the more comprehensive description provided inChapter 6 (p. 166).Demand controlled ventilationIt is a generally accepted principle that successful distribution of a supply of ventilating airwithin a space, from a simple terminal such as a grille, is a function of the design discharge