154 MODERN PAPER-MAKING
The greatest drawback to the use of these flat strainers is that they are con-
stantly becoming filled up with the coarse stuff, shive, rubber, dirt, etc., which
they have prevented from getting through and into the paper.
In order that the strainer will still continue to pass sufficient stuff to keep
the weight right at the machine, it is necessary that it should be frequently
cleaned and rubbed over by the machineman. This is very unsatisfactory,
and it leads to lots of dirt, etc., being let through; it also takes time and inter-
feres with the weight. Various devices have been resorted to in order to
make this cleaning automatic and continuous, but none of them is really
adequate.
In some forms a trough is provided at the opposite end to that at which the
stuff flows on, and in this the coarser and uncleaned stuff collects, along with
rubber, and is led away.
There is no doubt that one of the chief necessities which caused the inven-
tion of the revolving strainer was the need for automatic cleaning of the plates.
In the revolving strainer the plates form the circumference of a skeleton
cylinder, which revolves partly submerged in a semicircular cast-iron trough,
and is called the drum.
There are two types, the inward and outward flow, and in both the slits
or cuts are kept clean with a strong shower of water applied by means of a
perforated pipe.
In the case of the 'inward flow' the stuff is led into the trough, and has to
pass through the slits into the inside of the drum, and the cuts are cleaned
by a shower pipe inside, which forces water upwards through the drum either
at the top or at the sides, well away from the stuff in the vat. Some of this
water and the fibres which it dislodges are caught in a trough and led away
to the auxiliary strainer. The remainder of the water which is not caught
and the hanks and knots of fibres fall back into the vat, and those knots which
are not broken up sink to the bottom of the vat, and either lie there until cleaned
out or pass away with the dirt, etc., to the auxiliary strainer through a tap at
the lowest point of the trough.
In the 'outward-flow' strainer the shower is applied from above and
outside the drum, and the dirt, knots, etc., fall back into a metal .trough placed
horizontally along the inside of the drum, and are washed away to the auxiliary
strainer.
The inward-flow strainer is superior to the outward-flow, especially for
better-class papers, where freedom from dirt and blemishes of all kinds is of
the first importance. It is much more easily cleaned, because most of the
objectionable stuff never gets into it, but remains outside in the trough.
When the stuff flows in from the sand tables, any heavy dirt, grit or knots