Chemical Engineering December 2010 - 35

Figure 2. A cross section of a horizontal peeler
centrifuge with screen basket is shown here
Figure 4. A horizontal peeler centrifuge with siphon basket backwashes
the heel while adding pressure for higher filtration efficiency
houses all the fittings required
for operating a process
run. The drive part
mounted at the rear contains,
among other things,
the motor for driving the
basket shaft.
In a typical batch run,
Figure 3. Batch process for a peeler centrifuge involves several steps
Filtration centrifuges
At the most basic level, nearly everyone
is familiar with filtration centrifuges;
this is the separation principle
used in common appliances, such as
washing machines and salad spinners.
The design of a centrifuge is usually
always the same: a basket holds the filter
medium, and the mixture to be separated
rotates about an axis. Filtrate
is spun outwards through the filter
medium and the solids remain in the
basket for easy discharge. In the simplest
of centrifuges this operation is
performed manually. On an industrial
scale, however, the discharge is nearly
always performed automatically.
Filtration centrifuges can also be
further distinguished. On the one
hand there are discontinuously operating
centrifuges where the processes
are carried out in batchwise steps, and
then there are the continuous operating
centrifuges where all process steps
are carried out simultaneously.
In addition to this, there are many
other features and variations in centrifuge
design, for instance, the horizontal
or vertical alignment of the
rotation axis. In the various filtration
centrifuge types, different combinations
of these additional features have
proven their worth in numerous applications.
The following sections describe
the evolution of such features.
Batch centrifuges
Peeler. Chronologically, the discontinuous
or batch-operating peeler centrifuge
represents the first generation,
with the first horizontal peeler centrifuge
launched in 1928 by Krauss-Maffei.
As shown in Figure 2, these machines
feature a cantilever-mounted
basket that rotates in a closed process
housing. The door on the front side
the S-L mixture (feed)
is first fed to the basket.
Centrifugation then proceeds
through the following
sequence of steps
(Figure 3): cake formation
on the filter medium in
the supersaturated state
(sedimentation); dewatering
of the cake up to
saturation (intermediate
spinning); washing of the
cake with a new supply
of liquid (washing); spinning
of the liquid up to the desired
residual-moisture level (dry spinning);
and finally, the discharge of the
cake from the basket (peeling) using a
peeler blade.
Attention is drawn to the fact that
during the peeling off of the cake -
this taking place at full speed in high
quality machines - the peeler blade
cannot be swung in flush with the filter
medium (usually a mesh of plastic
fibers or fine metal wires), as otherwise
there is a risk of the mesh being
damaged or peeled off. As a result, a
residual heel is left in the basket that
can be used again several times with
many products. However, after a certain
period of time, the heel becomes
clogged with fine particles. Because
of the increased filtration resistance,
continued use becomes uneconomical
and the residual heel has to be
removed. This can be done pneumatiChemiCal
engineering www.Che.Com DeCember 2010 35
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Chemical Engineering December 2010

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