352 MODERN PAPER-MAKING
be shut off and no more will be required. There will thus be a supply of water,
containing a little fibre, loading, size, alum and dye, in constant circulation
at the machine. The surplus from the save-alls and wire pit over and above
the requirements of the machine is then pumped up to large tanks situated
above the beaters. From these tanks the beaterman draws his supply for
furnishing and emptying the beaters, and for diluting stuff going through the
refiners.
Re-use of Back Water.—When the web is being run down the wire for any
length of time, such as that occupied in washing and turning a felt on a news
machine, the stuff which collects in the pit is stirred up with water and
pumped back direct to the machine chests. This obviates the necessity for
shutting off the stuff and possibly upsetting the machine when it is running
well.
There is a disadvantage in this method—namely, the dilution of the stock
in the chest—and the machineman must be careful to see that when he starts
up again his weight is not seriously affected.
The sprays at the strainers and on the wire rolls may be supplied with back
water, withdrawn from the web at the suction boxes, and thus there is an
economy in the use of fresh water and a consequent diminution in the quantity
of back water to be dealt with subsequently.
This water is drawn out at the boxes through the meshes of the wire and
has consequently been well strained. It is then collected in a sump and forced
by another pump at a high pressure to the strainer and other sprays about the
machine.
When this method of supplying the sprays can be adopted the use of fresh
water is cut down to a minimum, and it will then be necessary only to have a
fresh-water spray on the breast roll, couch roll and possibly at the breast box
to keep down froth.
Adka Save-all (Fig. 153).—This save-all employs the principle of flotation,
as opposed to the older forms using the principle of sedimentation. The flota-
tion is brought about by forcing the back water through a nozzle, designed
on the lines of an injector, through which the air is drawn and intimately mixed
with the back water. It is then passed into a chamber, and under vacuum the
fibres, surrounded by minute bubbles of air, and flocculated by a suitable floccu-
lating medium, rise to the top of the liquid, where they are drawn off by a
scoop, and returned to the paper machine. The clarified water runs con-
tinuously from the bottom and is used whenever desired. A picture of the
Adka is shown in Fig. 153.
The back water from the machine is passed through the main inlet (i) and
travels down to the aerator nozzle (2), where air is drawn in through the air-