AIR-DRYING 301
from unequal draws between drums, if cog-wheels are used, and especially if
there are too many drums in one section.
It is often an advantage to have three steam-heated iron drying cylinders
at the end of the dryer in order thoroughly to dry the paper after it leaves the
sparred drums. These cylinders should have woollen felts, and it will be found
that they also assist greatly in flattening the paper before it passes to the calenders,
besides enabling thick papers to be thoroughly dry. The heat must not be too
great, or the gelatine will be made too brittle, and the strength of the paper
will be greatly reduced.
A departure from the usual practice was recently adopted on a machine
having 120 drums. These drums were arranged in three tiers, but it was found
that the paper was often very flabby, due to imperfect ventilation of the room
in wet weather, and also the large number of drums required considerable
power to drive. Consequently the bottom tier, which consisted of eighteen
3 feet drums, was fitted with steam-pipes inside and about 3 inches away from
the paper. These pipes were led back and forwards across each drum twelve
times, in exactly the same way as are the steam-pipes under the hood of some
M.G. machine cylinders.
The pipes were divided into three sections, one for each six drums, in order
that the heat could be regulated for each section.
When this arrangement was in operation it was found possible to do away
with 100 drums altogether, and to get a better dried and harder handling paper
with only eighteen drums, and two extra drums with fans but without steam-
pipes immediately after the tub. This is, of course, a method that is applicable
only to the cheaper grades of tub-sized papers. Theoretically, the best results
should be obtained with air at a low temperature and low humidity, but the
practical application of this theory is not commercially possible. A very long
drying machine running very slowly would.be required, and the high moisture
content of our air during a great part of the year would render drying almost
impossible. Drying machines are therefore as nearly adapted to the general
quality of paper made in the mill in which they are installed as experience
shows to be approximately correct.
Apparatus for controlling the temperature and humidity of rooms is now
available, but is still troublesome and requires a good deal of attention.