3oo MODERN PAPER-MAKING
it will usually be found possible to make up any lost reels when the machine
is shut for changing, etc.
The web of sized paper is led over the sparred dryers by means of tapes until
it reaches the calenders and is started on the reel. The tapes may then be run
off the paper to the side of the drums, in order to avoid marking the paper
and causing cockling.
It is, however, sometimes necessary to keep at least two tapes on the paper,
one above and one below, to keep the web tight and prevent it running off to
the side of the drums. It will be an advantage if the spars of the first twelve
drums are covered with strips of zinc or copper, as a certain amount of size is
always deposited on the spars of the first few drums, and must be scraped off
periodically. It is not easy to scrape wooden spars without damaging them,
but zinc- or copper-covered spars are easily cleaned with a cloth and hot
water.
Trouble is sometimes experienced from paper cockling or creasing at the
dryer, owing to the fact that it has not completely expanded in taking up size
in the tub. The root cause of this trouble is that the paper is not long enough
in the tub, but it can be got over by allowing the web to pass along a travelling
felt between the tub and the dryer, in order that it may have sufficient time to
expand fully before it is tightened up on the drums of the dryer.
An old wet felt can be utilised for this, and will last a long time, but it must
be washed at least once a week.
In the drying of thin banks cockling and creasing can sometimes be traced
to the fan speed, and this should be capable of easy adjustment. The speed of
the fans is often excessive. If the air is kept in constant agitation and prevented
from lying stagnant inside the drums, nothing more is required.
The temperature of the drying room is important, and should be kept at
about 85° K and as dry as possible. Drying is much easier in the summer than
in winter, and on a dry day than on a wet and foggy one, unless, of course,
specially heated and dried air only is allowed to enter the room, and adequate
fans are provided for the complete removal of all air after it has become charged
with moisture from the paper.
Very large dryers are usually divided into two of three sections, and these
should all be separately driven and have cone pulleys for easy adjustment of
the draws.
The drums themselves should be driven by endless cotton belts running
on their edges, in preference to cog-wheels, as the draws and tension are much
easier, and any slight variation in the size of the drums, due to wearing away
of the wooden spars, has less effect in causing uneven tension between drums.
Endless trouble from creasing, especially on thin weights, will always result