Chemical Engineering August 2022 - 30

is very low, often even negligible. In
the screen-scroll centrifuge, it is not
possible to prevent a certain fraction
of the particles from entering the gap
between the scroll and the screen
and being crushed. This effect can
be mitigated by specifically reducing
the gap between scroll and screen
[6], but it cannot be eliminated. Another
area of particle breakage is the
solids discharge, but this is irrelevant
to solids loss and therefore will not
be discussed here.
As already mentioned above, significantly
thicker and largely compact
cake layers are created in pusher
centrifuges. This results in less fine
particles passing through, as the filter
cake already formed retains the
fine particles. With the screen-scroll
centrifuge, the filter cake immediately
slides out of the feeding zone,
with the result that a thinner cake is
formed, whereby more fines find their
way into the filtrate. In a way, the disadvantage
of the pusher centrifuge
for highly viscous and compressible
solids turns into an advantage for the
clarity of the filtrate here.
Maintenance and operation
The operating conditions described
above indicate that in some cases
both pusher centrifuges and screenscroll
centrifuges can be used. The
decision criteria in such cases are
ease of maintenance, operational
stability and investment costs. The
issue of investment costs is complex
and can only be seriously considered
on a case-by-case basis
because one or the other type of
centrifuge may have to be oversized
in certain applications and thus become
unattractive in terms of cost,
despite possible advantages in process
performance.
As far as operational stability is
concerned, both technologies can
be run stably in continuous operation.
However, stable feed conditions
are essential for this purpose in
the pusher centrifuge. If these conditions
are present, it is in no way
inferior to the screen-scroll centrifuge
in terms of stability. However, if
feeding conditions are not stable or
the feeding flow to the machines has
to be stopped several times a day,
screen-scroll centrifuges have clear
30
advantages, because they are much
more robust in the face of fluctuations
upstream.
As far as ease of maintenance is
concerned, it is similar to the feeding
system. Although it tends to be
the case that screen-scroll centrifuges
suffer greater wear on the
screens and are operated more frequently
at high speeds, which leads
to more wear, the vibration level is
lower compared to the pusher centrifuge,
which puts the above effects
into perspective. However, the differences
overall are minor, and it is the
design of the machine and hence the
supplier that are important. When selecting
a centrifuge in terms of maintenance,
it is important to ensure
that the screen is easily accessible
as it is a common replacement part,
and that the entire rotating unit can
be removed from the machine in one
piece because this is the only way to
minimize downtime for major maintenance
work. A proven solution for
the latter is the so-called cartridge
design (Figure 4), which is nowadays
available for both pusher centrifuges
and screen-scroll centrifuges.
Concluding remarks
In practice, both the pusher and
the screen-scroll centrifuge have
proven to be efficient solutions for
non-compressible
or moderately
compressible solids with average
particle diameters larger than 80
µm. Since the fundamental difference
between the two technologies
lies in the form of solids transport
and the resulting different structure
of the filter cake, particular attention
must be paid to the advantages and
disadvantages arising from this.
Due to the compact and high filter
cake, pusher centrifuges score primarily
in terms of the quality of the
solids and filtrate produced, while
screen-scroll centrifuges prove to be
more flexible, especially with regard to
process stability, due to the low cake
heights and continuous movement of
the solids. Screen-scroll centrifuges
can be run with higher liquid volumes
and higher viscosities and can cope
better with fluctuations in the feed.
Pusher centrifuges can normally
achieve better degrees of dewatering
at the same physical conditions
Authors
Malte Junker is product manager
for continuous filtering centrifuges
at Andritz Separation GmbH (Industriestrasse
1-3,
85256
Vierkirchen,
Germany; Phone: +49172-52-30-321;
Email:
malte.
junker@andritz.com). After receiving
his Ph.D. in chemical engineering
in the field of aerosol centrifugation
at the University of
Stuttgart, Germany, he joined Andritz in 2014 as a project
engineer for product management. Since 2016
Junker has been product manager for continuous filtering
centrifuges and specialized in pusher centrifuges, as
well as screen-scroll centrifuges.
Peter Schmidt is global product
manager for filtering centrifuges at
Andritz Separation GmbH (Industriestrasse
1-3, 85256 Vierkirchen,
Germany;
Phone: +49-8139peter.
80299-113;
Fax: +49-813980299-150;
Email:
schmidt@andritz.com). He has
been with Andritz Separation since
1997, first as a student intern.
From 2000 to 2008, he was project manager for pusher
centrifuges, and from 2008 to 2012 sales engineer.
Schmidt has a Diploma in mechanical engineering from
the Berufsakademie Ravensburg, Germany.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
AUGUST 2022
if the materials processed are neither
compressible, fibrous, nor unstable
under pressure. Similarly, due to the
nature of cake transport, the intensity
of particle breakage in a pusher
centrifuge is minimal, while a certain
degree of particle breakage cannot
be prevented on a screen-scroll
centrifuge due to its design. In terms
of maintenance, screen-scroll centrifuges
suffer greater screen wear but
have lower vibration levels, so there
are no significant differences.
The
cartridge design, which is now available
for both technologies, simplifies
the maintenance on both machines.
Both technologies come with their
own advantages and disadvantages,
which makes a precise analysis of
the specific application and operating
situation, as well as the operator's
priorities, indispensable.
n
Edited by Gerald Ondrey
References
1.
Schmidt, Peter, Filtration Centrifuges - An Overview, Chem.
Eng., December 2010, pp. 34-38.
2. A detailed explanation of the combined motor used in pusher
centrifuges can be seen in the following animation: https://
youtu.be/hyzbk18sRu0.
3. The operation of a pusher centrifuge is explained in the following
animation: https://youtu.be/WelAIeeE99Y.
4.
5.
The operation of a screen-scroll centrifuge is explained in the
following animation: https://youtu.be/g_kS7nCsMuo.
A detailed explanation of prefiltration technology is described
here: https://youtu.be/k-6UdYF8JO4.
6. Smigerski, Marl, Grinding
Technik 10, 1978, p. 48.
Effects of Centrifuges, Aufbereitungs
https://www.youtube.com/watch?v=hyzbk18sRu0 https://www.youtube.com/watch?v=hyzbk18sRu0 https://www.youtube.com/watch?v=WelAIeeE99Y https://www.youtube.com/watch?v=g_kS7nCsMuo https://www.youtube.com/watch?v=k-6UdYF8JO4 http://WWW.CHEMENGONLINE.COM
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