Reference Manual

15−6
Figure 15-13. Primary Cleaners
Drawing is from
TAPPI’s Making
Pulp and Paper
Series and is used
with permission.
Figure 15-14. Secondary Cleaners
Drawing is from
TAPPI’s Making
Pulp and Paper
Series and is used
with permission.
successive bank of cleaners contains fewer
individual units than the previous bank.
Deaeration Chamber
Depending on the stock, small air bubbles can
form around the fibers. This slows drainage during
the sheet forming process and can cause pinholes
in the sheet. In order to remove these bubbles, a
deaeration chamber is used (figure 15-15).
The chamber or tank is connected to a vacuum
pump so the stock in the tank is also under
vacuum. This vacuum allows the stock to boil
even though the stock temperature is below
100°C. This boiling action helps to release air in
the stock.
Screens
Almost all paper machines have a screen before
the headbox to remove contaminants from the
furnish. These screens have a basket of either
holes or slots that allow fibers to pass through that
collects shives, pieces of plastic, or fiber flakes.
These slots are generally 10 thousandths of an
inch wide and are more efficient at removing small
debris than holes.
These screens also have a way to backflush the
holes to prevent plugging. Generally, this is done
with a rotor and hydrofoils. The foil passes over
the hole and produces a low-pressure pulse
followed by a high vacuum pulse. These pressure
pulses keep the openings in the basket from
binding.
Broke Handling
Broke is better known as internal waste paper
generated by the paper mill. This might be from
the wet-end from the forming or press sections, or
the dry end from the dryer section, reel, winder, or
other finishers. Broke contains good fiber and
chemicals that should not be lost.
Broke is generally captured in an under the
machine repulper or broke pulper. The broke
either drops down into the pulper along a chute or
is conveyed or blown into the repulper. Wet-end
broke is easy to break up into the individual fibers,
but fully dried broke requires more aggressive
agitation. Sometimes, broke is sent through a high
density centrifugal cleaner to remove large heavy
particles. This then goes through a deflaker, which
mechanically breaks up the underfibered flakes.
Compact Stock Process
Recently, some paper makers have moved to a
more simplified stock prep. Rather than relying on
large volumes to reduce variation, there is more
reliance on modern process control to make the
necessary adjustments to correct for variations
(figure 15-16). For instance, for thick stock
blending and feeding, instead of having two chests
to blend the various components and then feed
the stock to the fan pump, those functions are
handled by a small mixing tank (figure 15-17). As
for deaeration, one can replace the large silo and
deaeration chamber with a small centrifugal
deaeration pump (figure 15-18). This new system
has quicker response time and less total volume.
Brown Stock Rejects Valve
Process impurities have a negative affect on end
product quality. These impurities may damage
process equipment and cause runnability
problems. As such, all solids contaminants have to
be removed from pulp. Some contaminants can be