The Spiraliet ducting and fittings chapter shows a wide range of round pipe, rectangular ducts for embedding and fittings. These are galvanised ducts and channels which are ideal for ventilation systems. The following table shows duct diameters for main ducts and branches. Calculations for main ducts should work with an air speed of 5 m/sec. For branches and extraction points, calculations for should work with an air speed of 3 m/sec. Couple and connection pieces: Van Walraven supplies separate connectors for coupling ducts and fittings. Ducts of the same diameter do not fit directly onto the next section of ducting, just as for the fittings. A fitting does slide over the end of the duct, so no additional connector or coupling piece is required. We supply connection pieces for connecting ducts to each other and to fittings and special connection pieces for connecting fittings to fittings. The rectangular ducts and fittings for embedding can be used in new construction projects. This ducting can be embedded in concrete. Connectors from rectangular to round are available so you can always continue with the round ventilation system. For Spiraliet (spiral duct) clamps, fittings & accessories, please refer to the Fixings chapter.