Kair 100mm / 4inch Plastic Ducting 90 Degree Elbow Bend Joint 90 degree bends can be used to run your ductwork around obstacles along walls and into ceilings and offers superior performance and durability over flexible alternatives. White plastic 90 degree elbow bend - Outer diameter 99mm - Male spigot will fit inside 100mm Round Pipe Part: DUCVKC250 or 102mm Flexible Hose Part: DUCP0092/102/35. Recommended usage We always recommend to use as few bends as possible in your duct run to reduce any loss of pressure, this also applies when connecting smaller to larger ducting. When supplying air through a powerful fan, ensure that the ducting is not too narrow as this could lead to an increase of air noise, damage to the fan and potential vibration.