CHAPTER IV
BOILERS-BLEACHING-PREPARATION OF BLEACHING
SOLUTION-GAS BLEACHING
BOILERS
THERE are three varieties of boilers in common use in the paper mill, Rotary
Boilers, Spherical and Cylindrical, and Stationary Boilers of many shapes and
sizes. The spherical rotary boiler is the most efficient, especially for dealing
with all classes of rags. The mixture of rags and liquor is most thorough and
continuous. There is less heat lost by radiation than in other types of rotary
boilers. They are very easily filled and emptied. The drastic churning action
which prevents clumps or balls of stringy rags being formed causes a certain
loss of fibre.
Owing to the difficulty of housing very large spherical boilers some mills
use those of cylindrical shape.
Owing to their shape and the great weight they have to carry, they have
to be built of thicker plates than the spherical boiler. Sometimes the larger
sizes are supported by revolving rollers on standards. Steel bands are fitted,
which make contact with the rollers. These help to take the weight off the
end plates and the trunnions. As the rags do not gravitate to the centre, but
roll continuously in one circle, there is more trouble with clumps of rags.
More labour and time are required in filling and emptying, it being neces-
sary for a man to enter the boiler and spread the rags to die ends from under
the manholes, and drag them back when emptying. In both types steam and
water are admitted through the trunnions. They are also fitted with pressure
gauges and safety valves, and are revolved by worm and pinion gearing. The
blow-off cocks are on the side opposite the manholes, and are shielded inside
by perforated metal plates to prevent loss of rags and fibre.
When used for scalding and disintegrating waste paper and broke, the
speed should be increased to'increase the internal friction. 'One revolution
in three minutes is sufficient for rag boiling, but for broke scalding the speed
should be three to five revolutions per minute, which is about all that a heavy
boiler will safely stand.
Stationary Boilers for Rags.—The usual stationary rag boiler is dome-shaped,
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