202 MODERN PAPEK-MAKING
second press is designed chiefly for eliminating felt and wire marks. For this
reason the rolls run in the reverse direction to the first press, and the paper is
reversed, so that the under side, which is impressed, by the wire and felt, is
made to come in contact with the top roll of the second press. This has the
effect of greatly reducing this fault, and as the second press is seldom pressed
hard, the paper is made more equal-sided for the reception of finish.
Second press felts are seldom washed as they run, but are kept in condition
by washing with the force jet at every opportunity, or changed for washing
on the felt washer. Although the second press does not often cause crushing
when the felt is dirty to the same extent as the first press, the felt must be kept
in very good condition. When it becomes the least bit clogged up, it affects
the drying of the paper, and is one of the chief causes of 'cockles' and other
drying troubles.
When a new felt is put on a machine it requires to be thoroughly wetted
with cold water, in order to shrink it to its running tension. If this is not done,
it will begin to tighten up soon after the wet paper is run over it.
The tension has to be regulated to give the greatest porosity to the felt.
If it is too tight it may run into wrinkles, and be less porous owing to the
meshes being pulled together. If too slack the meshes will not be opened up
and the felt will very quickly become dirty. The coloured line or lines must
be kept straight across the machine for the same reason, and the nap should
be in the running direction, both on the machine and on the felt washer. All
rolls require to be true and perfectly parallel, as a felt is very difficult to run
if any one is not correct. Spiral felt rolls help to keep the felt free from wrinkles
and the meshes open.
Although It is very desirable to run felts as long as possible, it should always
be borne in mind that it is far cheaper to dry paper by the press rolls and felts
than by steam in the drying cylinders.
Apart from the loss through breaks and bruised sheets, caused by trying to
get an additional week or two out of a worn felt, the addition to the coal bill
will be very great, but this is unfortunately very often lost sight of, or it may
even be entirely overlooked. The use of felt rolls covered with vulcanite is
recommended, as the surface of the vulcanite is 'kind' to the felt, and gives
it a longer life.
The Drying Cylinders (Hg. &>).—Having pressed as much water out of the
sijeet as possible, there is still anything from 64 to 72 per cent of water to get
rid oŁ (See Appendix.) This is accomplished by running the paper over
steam-featol cyfeders, on to which it is pressed by dry felts. The cylinders
ate of cast mm with highly polished surfaces and may be from 3 feet (on old
to 6 feet in diameter. Steam is admitted to the cylinders by various