150

MODERN PAPER-MAKING

It is often argued that these centrifugal machines are wasteful of stuff, bui
when it is remembered that these machines need to be cleaned out, say, onl)
once every 12 hours, and in many cases at much less frequent periods, it wiL
be seen that the loss is negligible, and that the greatest portion of the loss con-
sists, in any case, of impurities.
Against this loss also must be considered the fact that the paper can be
guaranteed to be absolutely clean and free from specks, and that it is also no
longer necessary in, for instance, the case of a rag mill to employ any sorters
to sort the rags in search of buttons, metal fasteners, pins, etc. The rags, in
fact, unless they have to be sorted to remove certain qualities, can be passed

straight from the bale into the chop-
ping machine, and all the subsequent
removal of impurities may safely be
left'to the Erkensator.
[Bird Machine Co.
fig. 45.—-berd centbifiner
A centrifugal stock cleaning machine, working on the
same principle as the other centrifugal machines
To get the best results from these
machines, it is necessary that stock
passing through them should be very
dilute; in the case of wood pulp,
densities should not be greater than
0.75 per cent, and for rag stuff about
0.5 per cent or less. At these con-
sistencies the machine will remove
practically the whole of the shive,
which has very little greater specific
gravity than the cellulose itself.
The latest centrifugal machine is
the Purifuge (Fig. 44), which works
on more or less the same lines as
the Erkensator, but it has the advantage that it is made in England by Messrs.
Vickerys.
The Bird Centrifiner, which is made by the Bird Machine Company of
Canada, is of similar design to the aforementioned centrifugal stock cleaning
machines, and has a high capacity.
It is sometimes said that the centrifugal machine removes immediately the
heavy loading material, and it must be admitted that it does this immediately
it is started up, in order to fill the 'bolster', as the lining inside the rings is
called. This only takes a few moments, and after that no more loading is, in
fact, removed. In order to get over this difficulty, it used to be common
practice to put in a bucketful of loading before the stock was turned on; now,
however, it has been found to he much more satisfactory, from every point