EMTELLE FIBREFLOW - Fibre Blowing; The Blowing Principles

INNOVATION AND FLEXIBILITY
9.1 The Blowing Principles
Fibre unit is installed by a combination of control drive wheels and steady air fl ow. The far end is open, so pressure does not build up. The air simply
moves through the m/d and out the far end. The pressure profi le (drop) along the m/d provides air drag to propel the FU to the far (low pressure) end.
It is tempting to think that the best way to blow a FU into a tube is by using a ‘plug’-shaped device that fi ts the tube snugly, and is driven along
by the air pressure. If we did that, and connected the FU to the back of the plug, it would be the PLUG that was driven by the air. The plug would
PULL the FU along, and create TENSION in the FU. Optical fi bres do not like tension, as it shortens their useful life. Hence all the GRP and aramid
in early-design fi bre cables, to protect the optical glass from tension.
So, we obey the BT patent, which says that the FU is propelled along the tube by air flow passing OVER the FU and travelling at a much higher speed
than the FU. This is a tendency drive, and induces the FU to move, rather than forcing it by tension. So if we got hold of the FU at the back end, it
would stop moving, and suffer no ill effect.
Note also that in the plug example, ALL the air behind the plug would be:
a) at the compressor pressure, eg 10 bar (causing an air ‘explosion’ when the plug emerged)
b) moving at the same speed as the FU, and having no effect in moving the FU at all.
So, we now know:
a) The FU shall not have a large plug on the front end. The bead we use is for smooth travel through connectors etc only, and minimises the FU
diameter increase.
b) The FU moves by a combination of high-speed airflow and pushing force from the wheels.
c) The airflow is active in preventing the ‘pushed’ FU from buckling.
d) Air flow subjects the FU to almost zero stress, helping to optimise fibre lifetime.
9 Fibre blowing
pp pg p gpp
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INNOVATION AND FLEXIBILITY