86 MODERN PAPER-MAKING
Eighty-eight bars are fitted in this beater roll, which is 60 inches in diameter,
and this gives about 2-inch pitch. As a general rule, a bar with a wide edge
would be most suitable for beating strong stock which had to be well fibrillated,
and a narrower bar for short, free stuff. The wider the bar the greater will
be the pressure and crushing action exerted upon the fibres, but a thick bar can
be made to cut up stuff quickly enough, provided it is lowered down quickly
and kept hard on for a short time.
The bed-plate is very wide, almost one-quarter of the circumference of
the roll (in fact, there are two or three bed-plates pkced close together), and
this, together with the weight of the roll and the well-designed trough, is
responsible for the efficiency and great economy of power achieved with the
beater.
The chief reason for making the bed-plate so wide is that it is now realised
that a large amount of the power consumed by a beater is used in circulating
the stuff. In other words, it is wasted so far as actual beating is concerned.
As the beating is done only during the quick passage of the stuff between
the bars of the bed-plate and the roll, the way to increase the amount of beating
done in a given time is to increase the size of the beating area—i.e. the total
area of bars in the bed-plate and the area of bars in the roll.
The position of the bed-plate is also different, in that it starts almost exactly
under the centre of the roll and takes the place of the usual backfall up to a
point. The short backfall takes up an angle of 15°, and is arranged to ensure
the pulp being thrown out at the proper place. It will be readily understood
that, with this design, the stuff is thrown clear immediately it leaves the last
bar of the bed-plate, and there is no heavy pocket of stuff lying between the
backfall and the roll, impeding the passage of the stuff back into the trough.
With the usual type of bed-plate and backfall, stuff is flowing into the
spaces between the bars all the way up the backfall, and being thrown out
again by centrifugal force. This not only impedes the forward flow, but also
adds to the power required to drive the roll, and instead of the stuff being
quickly got rid of once it has received its treatment, as in the present beater,
it is slowly pushed up in rolls to fall over the top of the backfall and into the
trough.
In order to obviate the objectionable splashing of the roll in a deep pond
of stuff immediately in front of the bed-plate, a baffle board may be carried
right down to within about 12 inches of the bed-plate, and the floor of the
trough is shaped up to the bed-plate, so that the stuff is forced through this
narrow space in sufficient quantity to allow of a fibrage being taken on by
each bar of the roll. A smaller opening than 12 inches would be sufficient,
but lumps of stuff would be liable to choke it when furnishing*