298 MODERN PAPER-MAKING
be dried at a high speed a very large number of drums is required, as many
as 120 or even 140 being used on some machines.
The arrangement for supplying hot air consists of a series of steam-pipes
running along the floor under the dryer, and the heat of the room is regulated
by a valve worked in conjunction with a steam pressure gauge and a thermo-
meter, or by a thermostat.
Some of the latest dryers also have a hot-air blower arranged to supply extra
warm air to the upper tiers of the dryer, as it was found that by the time the air
had risen from the first and second tiers it was so charged with moisture as to
be unable to remove the final traces from the third tier.
There are several different forms of procedure adopted by various mills.
In some cases the dryer is situated at the end of the machine immediately
following the size-tub, so that the paper may be run straight from the drying
cylinders, through the size-tub and on to the dryer, in a continuous web. This
is the cheapest and best method, from the point of view of labour saving and
the reduction of broke. Obviously if the whole operation of making, sizing
and drying is carried out in one room, not more than five men and boys will
be required, and when the machine has to stop to wash up or change, all five
men will be available to assist at the wet end in cleaning sand traps, strainers,
washing felts, etc., and so the length of time taken for this very necessary work
can be reduced to about one-half of that which would be taken by a machine-
man and one assistant, were the machine separate from the dryer.
Another advantage of this method is that a sized and finished sample of the
paper can be got in the shortest possible time for the purpose of comparing it
with the sample to be matched. The sizing always affects the colour of the
paper in some degree, and if a waterleaf sheet only is available for comparison
with a finished sheet, allowance will have to be made for the colour 'going
back' a little after sizing.
Among the disadvantages which might be claimed against this method
may be mentioned the fact that sizing and drying have to be done at the same
speed at which the paper is being made, but we know of two very satisfactory
machines on which this method is used, and the speed of the machine has
never been affected by sizing or drying troubles.
Of course, a sufficiendy large tub and a long or hot enough dryer must be
provided to size and dry the paper at the maximum speed of the machine.
A slight variation of this method is to have the machine, size-tub and dryer
arranged to follow straight through, but to reel the paper after sizing and then
pass it over the dryer.
It is, however, difficult to see what advantage there can be in this, except
that the size gets a little time to soak in, for the last part of the reel to be sized