356

MODERN PAPER-MAKING

The liquor may be run through a drum washer on its way to the tank to
keep back fibre and sludge, but here again a loss takes place and the liquor
is more difficult to burn, requiring to be evaporated to a greater density.
There is of course a risk of economiser tubes or other pipes becoming clogged
up, but the regular cleaning and attention, which this department should
receive if it is to be efficient, will reduce this to a minimum.

[George Scott and Sons
FIG. 154.—SCOTT QUADRUPLE EFFECT EVAPORATOR FOR CAUSTIC SODA LIQUOR
Evaporators are usually three in number, of which one is fed with steam
from the main range. The plant is described as 'Triple Effect Vacuum Tube
Evaporators'. Some plants have four evaporators and are described as Quad-
ruple Effect (Fig. 154).
Advantage is taken of the lower boiling point induced by lessened atmos-
pheric pressure, a vacuum being maintained by a barometric column and
pump, or jet condenser. The steam created by the boiling liquor in one effect
is used to bring it to a boil in the next, which is under less pressure. In some
installations the liquor flows from the highest to the lowest temperature, in
others the opposite way.