DRY FELTS 211

otherwise drying will be uneven, owing to the felt being tighter at one side
than another. In many cases guide rolls will be found in the wrong positions.
To check the felt, the roll has to be altered in such a way as to send the line off
the straight. All fast-running machines now have automatic felt guides.

The guide roll should be on a stretch of felt where altering its parallel does
not materially alter the tension of the felt. It is bad policy to run a felt too
long. A felt may look to be capable of a few weeks' more work, but it may be,
and usually is, worn thin in the centre, and this makes drying very difficult

This will be reflected in unequal finish later on, especially on machine-
finished papers, and cockles, etc., on thin papers. Much more steam is required
to dry paper if the felts are well worn.

The satisfactory drying of paper on the Fourdrinier machine and the pre-
vention of condensation in the machine house constitute two difficulties which
until very recently have been unsurmounted by paper-makers and paper-mill
engineers. The extra output required during recent years, in order to keep
up with competition from new mills, has often been badly handicapped by
lack of sufficient drying power at the machine.

For whereas it has often been found possible to speed up an old machine
mechanically without detriment to the paper, it has usually been found that
there were not sufficient drying cylinders to dry the paper at a great speed,
and, what is more, no room to put in additional cylinders. In one mill at least
this trouble was overcome by adding a third tier of cylinders above the existing
two. To get over this drying problem the present tendency is to introduce
warm air into the machine room and blow it into the 'dead pockets' among
the drying cylinders.

Vapour absorption plants are in use in many mills and give very good
results, both in increased output and also in the saving of dry felts. They have
superseded the earlier method of introducing warm air through ducts under
the roof, for while this plan certainly helped very greatly in reducing the coor
densation of water on the roof and girders, it could not be said to add much
to the drying efficiency of the drying cylindets. The system of vapour absorp-
tion by hot ak both increases the clrying power of the machine and at the
same time prevents condensation above the machine by absorbing the vapour
as it leaves die paper, thus prorating it from ever reaching the roof in a saturated
co&dkkm*

Ttie vajxnir is chiefly released in the pockets between the drying cylinders,
where &Ģe is very little natural movement of air, and fog or steam can nearly
always he seen hanging ahout there. With the Sturtevant and similar systems
warm air is mtroduced into the 'pockets' by means of specially

perforated pipes. These pipes have perforations which ensure that dbe m