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The corrugating process shortens the plastic duct by 6-7 %.
The machine requires a space of 1.5 X 8 m indoors or outdoors. In the
latter case the site must be well protected against rain and wind. Indoors the
machine is bolted to the floor. No special foundation is required.
The machine corrugates between 7 and 8 ducts per hour. One man can
look after three machines. This means that the production per man and eighthour
shift is about 1000 duct metres of corrugated plastic duct, for which
about 180 non-corrugated 6 m ducts are required.
The machine is driven via a rectifier from a single-phase commercial
lighting supply and has tappings for 110-240 V. The power requirement is
about 0.2 kW. The burners are fed from a LP gas cylinder placed on the
carriage. The LP gas cylinder suffices for about three days of operation.
Corrugated Ducts
Corrugated ducts have the following advantages compared with noncorrugated.
• Their radial rigidity is several times greater. The wall thickness can therefore
be reduced, so allowing the use of cheaper ducts.
Fig. 2
Corrugated plastic duct
• A considerably better lateral flexibility is obtained, which permits their
use in curved conduit. The minimum radius of curvature for a noncorrugated
90 mm duct is about 10 m, for a corrugated about 2.5 m. This
results not only in a smaller number of manholes, since the conduit can
follow curved streets and be taken round obstructions, but it also means
that the cross-section can be changed between two manholes. The group
of ducts can be divided 10—15 m before the manhole for example, one
half being taken along one side of the manhole and the other half along
the other side (Fig. 3). This has the great advantage that the cables can
pass straight through the manhole along the two sides, an especially important
point with heavy plastic cables, which are difficult to bend and
are much more elastic than lead-sheathed cables.
• The longitudinal flexibility increases, which is an important point in conjunction
with the embedment of ducts in a manhole wall or the like. As
the expansion of the plastic duct is considerably greater than that of the
surrounding soil, it is very difficult to embed a non-corrugated duct without
the risk of strain caused by even moderate fluctuations of temperature.
These strains are liable to produce untight inlets to manholes. The corrugated
duct, on the other hand, functions like a concertina and the ends
of the plastic ducts can be embedded without risk of tensile strains.
Fig. 3
Division of a group of ducts when passing
through a manhole
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