Reference Manual

17−9
Figure 17-16. Two-Tier Drying System
Drawing is from TAPPI’s Making Pulp and
Paper Series and is used with permission.
Drying
After pressing, the sheet is conveyed through the
dryer section where the residual water is removed
by evaporation. This is due to the cellulose fibers
being hydrophilic and wanting to hold onto water.
At this stage, the wet web is approximately 40 -
45% solids.
Most paper is dried on cast iron or steel drying
cylinders, each cylinder being 60 - 72 inches in
diameter, that are fed with steam from the boiler
with pressures between 15 - 150 psi, depending
on the type of paper. The wet web is held tightly
against the cylinders by a synthetic, permeable
fabric called the dryer felt. The evaporated water
is carried away by ventilation. The final result of
the dryer section is paper with 5 - 8% moisture.
Most paper machines have three to five
independently-felted dryer sections, each with
their own speed control to maintain sheet tension
between sections and adjust for any sheet
shrinkage. The two-tier configuration is the most
common arrangement for dryers. The sheet
passes from dryer to dryer where it is tightly
pressed against the dryer cylinders by the dryer
fabric (figure 17-16). The paper passes
unsupported between each of the dryers. Once
the remaining water in the sheet rises to its boiling
temperature, water is converted into steam. This
steam is collected in a containment hood to
remove the water vapor. On the inside of the dryer
cylinders, the steam is condensed back into water,
or condensate. This conversion of steam into
water supplies the majority of the energy that dries
the paper.
Figure 17-17. Single-Tier Dryer Section
Drawing is from TAPPI’s Making Pulp and
Paper Series and is used with permission.
At high speeds, the unsupported paper between
the dryers can flutter and occasionally break.
Because of this, many modern paper machines
are going with a single-tier or “Uni-run”
arrangement where the dryer fabric is in constant
contact with the sheet (figure 17-17). This allows
for the machine speed to increase.
The condensate that collects within the dryer
cylinders must constantly be removed. This is the
job of the siphons.
Steam and Condensate System
The heat energy for paper drying comes from
steam as it condenses inside the dryer cylinders.
This is known as latent heat. Steam always
condenses at the saturation temperature, as
defined by the pressure in the system. This is
important when trying to have uniform drying
across the machine. The condensate that forms in
the dryer cylinders is removed by a specially
designed pipe assembly called a siphon. On
slower machines, the condensate collects in a
puddle at the bottom of the cylinder. For
high-speed machines, a true rimming condition
can be reached where the condensate covers the
entire inside surface due to centrifugal force.
In siphoning, differential pressure pushes the
condensate through the siphons (figure 17-18).
The siphons carry the condensate from the dryer
to a separator that collects the condensate and
recycles any steam that has been blown through.
The condensate is reused by sending it back to
the mill’s boiler feedwater system (figure 17-19).
Hood Ventilation
It is important to realize that a ventilation hood
exists over the entire dryer section beginning from
the press section up to calendaring. Depending on