354 MODERN PAPER-MAKING
point above the aerator nozzle and keeps it from overflowing. The back
water than flows upwards and back over another wooden partition to
the base of the surge-pipe (4), up which it is drawn by the vacuum
in the body of the Adka. TKis vacuum is maintained by the excess air
being drawn off through the same rotating scoop (7), which deals with the
thick stuff floating to the top. By reason of the vacuum in the chamber (5)
the height of the floating stuff in the body is always maintained at the
same level, since if the back water rises above the level of the inlet of the
rotating scoop, no more air could be sucked from the body, and in con-
sequence the back water could not rise any higher; and likewise if the back
water should drop below the level of the scoop, the vacuum would be auto-
matically increased, which would once more draw it up to the right height.
In practice the level of the back water in the body seldom varies more than
i inch, which is usually insufficient to cause any serious fluctuation. The
thick fibre and clay on the top of the back water in the body is drawn upwards
to a separating chamber (n), where the stuff falls to the bottom and is dealt
with by a centrifugal stuff pump (13), and the air rises to the top, where it
is drawn off over a barometric column (12) by a drum vacuum pump which
is always kept submerged by a priming tank set alongside (8). The centrifugal
stuff pump should be of sufficient capacity to deal with the stuff drawn off,
and be capable of pumping it to wherever desired, in most cases to the sand
traps of the paper machine. The clarified water freed from die clay and fibre
travels down a series of pipes (10) set around the base of the body, all flowing
into one common outlet pipe into a box (9), from whence it is discharged
to the drain or for re-use. The ends of the clarified water-pipe and the surge-
pipe should always be completely submerged, otherwise the vacuum in the
body will be broken and the whole procedure upset.
It will be readily understood that the time taken for die whole cycle of
operations is very much shorter than that in a sedimentation type of save-all,
as the back water is dealt with almost immediately, and it is only a matter of
a few minutes to effect changes of colour. In practice, with short delivery
pipes to the paper machine, 5 minutes is sufficient interval when changing to
get the new colour to the paper machine.
The clarified water is practically free from fibres and clay, and may be used
for all ordinary purposes in the mill.