142

MODERN PAPER-MAKING

weight, or otherwise badly made, with too much or too little water, the small
expenditure involved in fitting controls which operate at the front of the wire
is soon recovered.

The 'Back-water' System (Fig. 39).—The 'back-water' system is composed
of the following main parts: A centrifugal water pump to raise the water to
the high-level water box. A low-level water box to collect the water from, the

' ^Wasfc out

WHITE WATES DISTRIBUTINQ

To Drain
Stock lo Ma.cKme

l/alue

•N-

ColUctin
Ta«k

Stuft Chest

FIG. 39.—A BACK-WATER SYSTEM OF A PAPER MACHINE
In this diagram are shown the essential parts of a back-water system of a paper machine. All water collected at the wet
end is pumped up to a distributing head tank, which is divided off by a partition. The Whitewater from the smallest
compartment returns to the'mixing box, and keeps the machine supplied constantly. Any excess overflows into the
large compartment of the head tank, and is distributed as shown in the diagram.
save-all trays under the wire, and from which the pump draws its main supply.
Suitable water gate on the high-level water box to regulate the supply of water
to the stuff going over the machine. Overflow pipes at a certain level on high-
and low-level boxes. A better idea of the water system may be gathered if
it is described in action. The fresh water is turned on full to obtain a head of
water sufficient for the working of the stuff until its place can be taken by back
water. The water gate and stuff gate are then opened to the estimated extent