Chemical Engineering August 2022 - 29

Andritz
require washing through an
opening in the scroll, and in
a machine already in operation
this opening can only be
moved by using a new scroll.
Filtrate quality
FIGURE 4. The cartridge design enables faster cartridge
change, and thus simplifies maintenance
same amount of wash water achieves
much higher washing efficiencies.
However, the advantages of
washing in the pusher centrifuge
go even further. As a rule, there is
an optimum point combining washing
efficiency and residual moisture.
Due to the design, the washing
positions can also be adjusted in
pusher centrifuges already installed
so that they can be set to the optimum.
Screen-scroll centrifuges
In many applications, the
quality of the filtrate is also
important. A key factor here
is the proportion of solids in
the filtrate. The solids usually
consist of the fine fraction.
In order to keep this
low, two measures can be
implemented within the machine.
First of all, particle breakage
should be kept as low as possible. If
there are fewer fines, less solid matter
can pass through to the filtrate.
And secondly, the filter cake should
be as compact as possible, as this
prevents the fine fraction from finding
its way into the filtrate.
When it comes to particle breakage,
the feeding system is of central
importance because uneven acceleration
of the suspension leads
to particle breakage and wear on
the machine. It is often argued that
the screen-scroll centrifuge of the
same dimension produces less particle
breakage in the feeding zone
because it is fed to a smaller diameter
and therefore accelerates to a
lower peripheral speed. On closer
inspection, however, this is not the
case as, on the one hand, there is
less time for acceleration and, on the
other hand, the screen-scroll centrifuge
is usually operated at a higher
speed. Hence, pusher and screen
centrifuges do not differ significantly
in terms of particle breakage in the
feed zone, and the efficiency of the
feeding systems is more dependent
on the different systems used by the
individual suppliers.
Particle breakage can also occur
during transport of the solids within
the basket. In pusher centrifuges, the
filter cake is pushed forward by the
next filter cake, with the result that
particle breakage during transport
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Chemical Engineering August 2022

Table of Contents for the Digital Edition of Chemical Engineering August 2022