Reference Manual

17−11
the type of hood arrangement, 7 - 20 pounds of air
are utilized for each pound of water evaporated.
To prevent drips, buildup and corrosion within the
hood, the volume and temperature of exhaust air
must be sufficient to avoid localized condensation.
Modern generation hoods, also called “high-dew
point hoods”, are well sealed and insulated.
Diffusion air is totally eliminated, and the amount
of fresh makeup air is sharply reduced by
operating at high temperature with high recycle.
Size Press
Sizing solution is commonly applied within a
two-roll nip which gives the name size press. This
device is found about three quarters of the way
down the dryer section where starches and other
materials can be applied at this station in order to
improve sheet surface, internal strength,
smoothness, and resistance to water penetration.
The objective is to flood the entering nip with
sizing solution so the paper absorbs some of the
solution.
The basic mechanisms that incorporate starch
solutions into the sheet is the sheet’s ability to
absorb the sizing solution and the amount of
solution film passing through the nip. Sheet
absorption is greatly affected by sheet moisture as
higher sheet moisture promotes absorption.
However, the level is typically controlled to 4 - 5
percent or less to ensure the sizing agent is kept
nearer to the surface.
Modern paper machine speeds can create
turbulence due to changing nip pressures. This
affects the size press’ ability to evenly distribute
the solution. To overcome this issue, most size
presses have larger diameter rolls to keep solution
turbulence more manageable. Others have begun
to use a metering size press, which applies the
starch to the surface of the size press rolls by
metering the amount applied to the sheet with a
blade or rotating rod.
Today, the size press can also be used for coating
applications; including pigmented coatings and
other specialized surface applications. Some
manufacturers are beginning to include pigment in
the starch application.
Calendaring
This process helps to smooth and flatten the
sheet, better known as the thickness or caliper
variation of the sheet. This is done by
compressing the higher areas in the sheet more
than the lower. Surface smoothness improves so
the paper prints better. Sheet density is increased
so the paper becomes thinner and denser. Making
the sheet denser also makes the paper less stiff.
Calendaring changes the surface and interior
properties of the sheet by passing the web
through one or more two-roll nips where the rollers
may or may not be of equal hardness. By using
extreme pressures, the objective is to press the
paper against the smooth surface with sufficient
force by using one of the calendaring types below.
D Hard-Nip — Both rolls are made of either
iron or steel. Although this type smooths the sheet
by calendaring to a uniform thickness via flattening
the higher areas in the sheet, it can create areas
that are more dense as well. These density
variations are due to basis weight variations and
can turn into variations in surface properties
D Soft-Nip — For this type, the loading roll is
still hard but the opposing roll has a soft polymeric
cover, usually some type of polyurethane. Since
the side contacting the metal roll receives a much
better finish than the side contacting the resilient
roll, it is necessary to have two nips for equal
finish.
Calendaring at high temperatures is desirable
because the paper becomes more pliable and can
be calendared at lower pressure.
Reel
After drying and calendaring, the paper product
must be collected in a convenient form for
subsequent process off-machine. This is typically
done by a drum reel which collects the product to
a specified diameter.
Most reels are motor-driven under sufficient load
to ensure adequate tension on the sheet from the
calendars. The web wraps around the reel drum
and feeds into the nip formed between the drum
and the collection reel, which is held by the
secondary arms. While the reel builds up, an
empty spool is positioned on the primary arms.
To transfer from a full roll to empty spool, the
parent roll needs to be removed. An empty spool
is held in the primary arms and is brought up to
speed before contacting the paper on the reel
drum. The paper is transferred to the new spool
and the full parent reel is released from the reel
drum. Once the parent roll is removed, the primary
arms move the new roll down to the rails. The
secondary arms are brought forward to take the