2io MODERN PAPEK-MAKING
and press rolls, the quality and substance of the paper, the speed, the steam
pressure used for drying, etc. Obviously for a machine making a wide range
of papers it is very difficult to suit the number of cylinders to all these con-
ditions. For some papers eight would be sufficient; for others, five times that
number would be wanted. From ten to twenty cylinders on an ordinary
Fourdrinier is a fair average.
Felt Dryers.—It is at last being realised that the conditioning and proper
drying of dry felts is a very important part of paper drying, and quite recently
a very excellent type of felt dryer has been made available. While it has
invariably been the case that certain cylinders have been added for the purpose
of drying felts, something more was wanted, and the Happer dryer is an
excellent solution of this difficulty of keeping dry felts in condition. It con-
sists of a small drying cylinder rather like a suction roll and it is inserted between
the sections of dryers in the same way as the felt-drying cylinder or in place of
a carrying roll. As the felt passes over it, it is subjected to suction from a
vacuum pump, and this draws the moisture away from the felt surface, and
so dries the felt and leaves it in a condition to take any further moisture from
the paper. This has the double advantage of conditioning the felt and the
atmosphere around the dryers at the same time, for this reason: with the ordinary
felt-drying cylinder the heat of the cylinder drove the moisture from the felt
into the pockets, or under the hood of the dryers, still leaving this moisture to
be dealt with by hot air and fans. In the Happer arrangement the water vapour
is actually sucked away through the felt and exhausted to atmosphere outside
the machine house, an advantage which will be readily appreciated.
Dry Felts.—Dry felts may be of cotton or wool. The latter are most
efficient and last longer, but are more expensive in the first place. Cotton felts
mark the paper and shed cotton fibres on it after a time, and for this reason
may have to be discarded before they are quite worn out. Woollen felts shed
a brown dust towards the end of their life, but by the time this takes place
they are very much burnt and nearly useless. The cylinders should never be
ran when the wet end is shut for washing up or changing. After a few turns
to get the felts thoroughly well dried the cylinders should be stopped, and
tbe fcfts slackened off a little, to release the strain on their fabric caused by
d^ shrinkage on drying. The felt-drying cylinders must have full steam on
wheti the machine is running. This is much more important than is generally
i^DOOgJoise^ Owing to the blocking up of a steam-pipe leading to the drying
cylinder, a wooBesi felt that would normally have run for six months became
rotten and wmt to pieces in a 'fortnight. On many machines dry felts are
guided fey ha&d, but there is BO reason why this should be so, as automatic
guides arc sfoajsfe and easy to fit. The lines of a dry felt must be kept straight,