Splices must not reduce the interior diameter of the duct. Innerduct couplers specially designed for the blowing process are commercially available and recommended. A final factor in optimized blowing is friction reduction. Theoretically, the maximum length that can be blown in any situation is proportional to the coefficient of friction. If the coefficient of friction can be reduced by a factor of three, the practical length of installation will go up by three.
Lubrication is critical in optimized air blowing.
Good quality factory lubrication on the duct interior is helpful. In these studies, the mechanical pushing unit was deactivated on an HASB machine, and the forces on a cable were measured at various air pressures and volumes. The moving air was pushing an MDPE jacketed fiber cable that was fully threaded through a multi-looped HDPE smoothwall innerduct. The graph shows a lubricated coefficient of friction in the 0.08 range versus the 0.60 range for the unlubricated cable. For straight installations, this level of lubrication could result in more than a factor of 5 increase in installation length.