212 MODERN PAPER-MAKING
is discharged directly on to the felt, or paper, whilst at the same time sur-
mounting the difficulty of projecting nozzles which interfere with the work of
clearing broke and feeding through.
The air is drawn from outside by a centrifugal fan, and is blown through
a steam heater. The apparatus may be placed in almost any position in the
machine house or even outside it. The heater can be fed entirely by exhaust
steam, or may be divided into sections, one using exhaust steam and the other
live steam.
The warm air from the heater is distributed throughout the length of the
dryer by galvanised sheet steel ducts, which in turn feed the various pipes
distributed into the pockets, or above and below the felts of the machine.
The drying capacity of the machine can be increased 15 to 25 per cent, with
a corresponding increase in possible output. Fog in the machine room and
drip from the roof are usually considerably reduced, and there is no need to
emphasise the importance of this latter benefit, for it is bound to save many
hundreds of pounds a year in breaks and broke, at least in mills where trouble
is experienced from roof condensation, besides the great saving in roof main-
tenance costs.
The length of life of dry felts in one mill we know was increased as much
as three times when this system was installed, and the output was at the same time
increased by 15 per cent.
There is no doubt that it is a much cheaper and quicker way of adding to
the drying power of a machine than by the usually very inconvenient method
of installing extra dryers.
The 'Happer* Patent Felt Drying Roller (Fig. 85).—An important point to
ensure even drying of the paper and freedom from cockles is the keeping of
the dry felts evenly and properly dried.
Dry felts absorb a lot of moisture from the paper as it is driven out by the
heated cylinders, and they ought to be able to pass it away to the atmosphere
continually, in order that the wool may absorb further moisture on the next
revolution of the felt.
Originally felts were dried by passing them over separate drying cylinders
not in contact with the paper. A further improvement on this was the blowing
o£hot dry air on to the felt and into the pockets between the cylinders in order
that dbis air might absorb moisture. This air was subsequently drawn away
by fens. Bo& these methods have the defect that only the surfaces of the felts
arc dried, tfac moisture remaining in the vicinity of the paper-making machine,
and some of it recompensing on to the felts. Also the hot and humid con-
d^doas m the machine house, brought about by this method of dealing with
the moisture, make tlje working conditions unpleasant and cause troubles such