Reference Manual

8−5
pumped through screens and cleaners to the head
box. Screen and cleaners remove undesirable
particles such as dirt, grit and clumps of fibers, or
chemical additives. The next step involving the
head box and forming wire actually begins the
formation of a paper sheet. The head box accepts
stock and white water from the fan pump and is
required to deliver a uniform flow onto the moving
machine wire. Most modern head boxes are
pressurized. Proper control for achieving an even
and uniform outflow is critical for proper sheet
formation. The continuous fine screen wire
provides for the formation of a mat of fibers and
drainage of water. Modern wires run at speeds of
3000 - 5000 feet per minute. Due to the high wire
speeds, drainage is aided by a vacuum system
found under the wire screen.
After initial sheet formation and dewatering, the
sheet moves to the dry end of the machine. From
the machine wire, the sheet is transferred to the
press section. The press section rolls provide a
mechanical means for water removal and pressure
which consolidates and smooths the sheet. The
sheet is conveyed through the various presses on
a felt or synthetic fabric. The fabric provides for
transfer of pressing forces onto the paper and
volume space for removal of water and air. A
paper sheet leaving the presses is typically at 30 -
35% consistency (70% water).
The paper sheet is now transferred to the dryer
section where heat is applied to evaporate the
moisture content to 5 - 10%. The system consists
of a series of large diameter cylinders that are
internally steam heated. The paper sheet is
conveyed by a synthetic fiber over the cylinders
where moisture is evaporated and carried away by
a ventilation system. Condensate formed in the
dryer cylinders is removed by siphons and
returned to the powerhouse. Even drying across
the entire sheet is a major challenge in this
section.
Following drying, the paper is sent to the calendar
where large roll presses consolidate the paper to
its final thickness and smoothness. The calendar
stack consists of hard cylinders capable of
providing high compression forces. Paper from the
calendar is fed onto spools and rolled into large
reels. These reels are then processed to meet
customer size specifications by the winder and roll
finishing areas prior to shipment.
Utilities
So far, attention has focused on processing of the
primary raw ingredient, wood. However, other raw
materials such as water and electricity, as
illustrated in figure 3, play important roles in the
production of paper.
A paper mill requires large volumes of water for
use throughout the process. Although a few
processes can use raw water directly from the
source, most users require a higher quality of
water. Most water requires treatment in a
sedimentation basin followed by filtration to
remove suspended solids and other impurities.
The degree of treatment required depends on the
source of the water such as a river, lake, or well.
Additional treatment for removing dissolved
minerals is required for water used in boilers.
Failure to remove these deposits results in
build-up of sludge and scale which eventually
leads to operational problems in the boilers. The
most common method employed to remove the
dissolved minerals is with ion-exchange resins
called demineralizers. Demineralized water
production has a high capital and operating cost.
Water required for boilers demands special
treatment. In addition to demineralizers, further
treatment involving mechanical deaeration and
chemical additives is required to remove oxygen.
This ultra-pure water is used to produce steam in
both the power and recovery boilers. Steam
produced is used for both process heating and
generating electricity with steam turbine
generators. Dual use of fuel energy is called
cogeneration. Since the cost of making
demineralized water is high, it is important that
clean steam condensate be returned to the boiler
for reuse. A typical return rate is about 50%.
The other basic raw material, electrical power, is
typically provided by a combination of own-make
and a tie to the local utility. Own-make electricity is
produced via high pressure superheated steam
from the power and recovery boilers fed to steam
turbines. The turbines extract energy from the
steam which, in turn, drives an electrical
generator. The power boiler produces steam from
burning wood waste such as bark and is
supplemented with coal or oil. In most cases,
approximately 185 pound steam is sent to the
digester and turbine while 80 pound steam is used
in the steam room. Additionally, the recovery
boiler burns black liquor as fuel.
Since a mill typically does not produce enough
power to meet all of its electrical requirements, a