NEWSPRINT 265

concentration as constant as possible, the chests are allowed to fall always to
the same level before the next breaker-full is dropped. The addition of
dilution water is continued via the breaker until the stock-chest level has been
brought up to a predetermined level to give the required concentration of
3 per cent.

Storage chests for both sulphite and groundwood stock are usually rather
long and narrow, the width being roughly the same as the height (see Fig. 106).
They are fitted with a horizontal shaft to which are attached paddles, or pro-
pellers, to provide the necessary agitation to prevent sedimentation or flota-
tion, and to help in some small way in the soaking and disintegrating process.
For the sake of cleanliness, the tanks are permanently closed in with a manhole
at the top through which access can be had for inspection and repair purposes.
The capacity of these chests is usually sufficient, together with the refiner and
machine chests, to give about 4 hours' supply of stock to the machine. The
greater the capacity the better the opportunity there is to get the stock
thoroughly mixed, which helps to maintain a uniform consistency. It also
gives some margin in the event of any breakdown occurring in the breakers.
On the other hand, a large chest-capacity makes it difficult to make any rapid
changes of furnish, if this should be necessary.

The stock from the breaker and beater storage chests may be refined
separately, then mixed, by means of a proportioner, in the ratio needed to
give die desired percentage of sulphite. After mixing, the stock, in most
instalktions, is refined again, and is then ready to be fed to the mixing pump
to provide the dilute stock for the breast box.

No hard-and-fast rules can be laid down as to the exact flow-system needed
to give the best refining results. Normally two or three refiners, each taking
about 250 horse-power, will be installed for a machine making 150 tons of
paper a day. From the evidence available, it seems that two refiners should be
enough for the work to be done.

The purpose of refining, so far as newsprint is concerned, is primarily to
complete the disintegration of die stock, which, after breaker treatment only,
is full of undisi&tegrated clots of pulp. Refiners are able to clear diese away
completely. Groundwood changes little in freeness, or other characteristics,
on being refined, although sulphite pulp, especially if it is added to the breakers
in an air-dry condition, may increase in strength considerably when refined.
This increase does not, however, seem to be of primary importance, pardy
because die percentage of sulphite used nowadays is so low that its contribution
to die strength of the finished sheet is radier small, and pardy because thepxiniafy
function of sulphite is to make full use of the long slender fibres to
strength to die immature steet of newsprint, dius helping it to