Reference Manual

15−7
Figure 15-15. Deaeration Chamber
Drawing is from TAPPI’s Making Pulp and
Paper Series and is used with permission.
Figure 15-16. Compact Stock Prep System
Drawing is from TAPPI’s Making Pulp and
Paper Series and is used with permission.
separated from pulp on screens, whereas others
similar or smaller than fibers may be removed by
other methods (figure 15-19).
Pulp from cooking always contains some
unwanted solid material. Some of the chips may
not have been fiberized properly, and some of the
fibrous material may not be completely in the form
of individual fibers. The main purpose of the pulp
screening process is to separate harmful
impurities from pulp with minimal fiber loss, and at
an acceptable cost level. Bark, sand, shives, and
rocks are typically found within the cooked chips
and must be removed. There are multiple ways to
separate out the impurities. This can be done
mechanically by screen plates where separation is
based on particle size, whereas gravimetric or
centrifugal force field is needed for weight-based
particle separation. However, pressure screening
in multiple stages is the preferred method for
removal of impurities such as sand and shives
from the pulp. These operate by separating the
feed pulp into either acceptable (impurity free) or
rejects (impurity rich). The acceptable pulp passes
through the device, and the rejected pulp is
removed for further processing. This is
accomplished in stages since most pressure
screens cannot sufficiently concentrate the
impurities in only one screening stage due to
thickening of the reject on the screen itself. The
purpose of the rejects stages is to concentrate the
impurities in the reject stream, and to return the
good fibers to the main process line.
Design Considerations
D Tight shutoff not required.
D Hardened materials to protect valve body
from wear.
D Ball valve to shear through impurities.