High Air Speed Blowing, Air-Assist, Push/Pull Installation, Air Blown Cable, and Cable Jetting; all describe new methods to get cable into conduit using air. To date, the primary use of these methods has been to install fiber optic cable.

Outside plant personnel are very familiar with the pulling method of installing cable. Thread a line through the duct, attach the line to the cable, and pull or tug the cable through the duct. The force needed to pull the cable usually comes from a capstan or hand-over-hand pulling of the rope. This force is needed to overcome the cable’s frictional resistance to movement. Length of installation is limited by the maximum force allowed on the cable.

Air-assisted installation must overcome the same frictional force to move cable, but it does this in a very different way. The force in air blowing first comes from a mechanical device which pushes the cable; and second, from the force of moving air on the cable jacket, or alternatively, the force of air on a piston, missile, or carrier at the front end of the cable.

Fiber Optic Pulling Vs Blowing
While all types of cable can be blown into duct, blowing is particularly useful for placing outside plant fiber optic cable. Fiber optic cable is lightweight and flexible. Long, uninterrupted lengths of installed cable are desired. Both these factors play to the strengths of the blowing methods.

In pulling, when a cable goes through a conduit bend, the pulling force on the cable actually pulls the cable into the bend. An exponential frictional force is created that is many times greater than what would occur with just the weight of the cable. Thus, an accumulation of bend angles greatly reduces possible pull length. But cross-country fiber optic pulls don’t have a lot of bends, or do they?

Continuous flexible duct introduces "hidden bends." These are inherent undulations or waves from reel memory.

While these waves can be minimized, they can’t be completely eliminated. Typical displacements of a few inches to a foot increase pulling tension by factors of 3 to 200.

Duct bends, including undulations, do not limit blowing processes, especially HASB, as much as they do traditional cable pulling. In HASB, the cable is not pulled into the bend by a large pulling force, but rather pushed through the bend by a lower, local air pushing force.

In push/pull blowing, the pulling from the piston is partially offset by the pushing force from the mechanical pusher. This somewhat offsets the effects of bends.

Keys in Air-Assisted Installation
To get the best performance in blown installations, there are several important factors to control. First is the flexibility of the cable. A cable that is too flexible cannot be pushed properly without bunching up. While stiffer cables are easy to push, they consume more force bending and unbending through conduit bends.

Choosing the proper cable is important for optimal blowing. Although most fiber cables can be blown, some cables are designed specifically for blowing. Consult your cable manufacturer for recommendations.

Type of duct is another significant factor in optimized blowing. Ducts with circumferential ribs or corrugations that disrupt the flow of air are not satisfactory. Smoothwall and longitudinal ribbed HDPE conduits are preferable.

Just as important as the type of duct is its ability to hold air pressure. The duct must hold up to 130 psi of air without leaks. Duct splices are critical. Splices must not reduce the interior diameter of the duct. Innerduct couplers specially designed for the blowing process are commercially available and recommended.

Air-assisted installation of fiber optic cable has proven to be both practical and economical. The technology is not only gaining popularity in the USA, Europe and India, but throughout the world. The process is quite different from pulling and requires a focus on different factors than pulling. An installer can quickly learn and use air-assist methods by attending to these factors.