152

MODERN PAPER-MAKING

downwards through the centre hole of the partition, only clean stock passing
upwards along the axis of the cylinder to the trumpet-shaped outlet. The dirt
which has passed down the centre falls into the funnel at the bottom and
thence into the waste receiver. This waste receiver can be shut off and
emptied while the Vortrap is still in operation.

It is claimed for this apparatus that it is very low in first cost compared
with other centrifugal machines, and that it separates a large quantity of the
dirt usually contained in paper stock.

[Vickerys Ltd.

. 47.—DIAGRAMMATIC ARRANGEMENT OF A TWO-UNIT DERTEC INSTALLATION

The Dirtec (Fig. 47) is a static type separator for the removal of dirt from
pulp and paper stock. The machine has no moving parts, the separation being
obtained by the employment of the differences in pressure of the stock itself. It
is effective and suitable for all except the very highest grade stocks, both heavy
and light dirt being removed continuously without stopping for cleaning.
It is, however, still necessary to keep the strainer in use. Froth is produced
in great quantities by the excessive aeration in the machines. Coagulation takes
place when the stuff has a long distance to travel to the breast box, and creates
difficulties in the formation of the sheet on the wire. Knots, small clumps of
unseparated fibres, and shives, such as are found in esparto and wood-pulp papers,
and which are of the same specific gravity as cellulose fibres, are still to be dealt
with. It is therefore inadvisable to run a strainer with a more open cut than is
required for getting the stuff freely through, though in some cases the cut may
be much more open.