Chemical Engineering July 2016 - 54

Batch solid bowl centrifuge
Decanter centrifuge
Tubular centrifuge
Disk stack centrifuge
Table 2. siGma facTors for various commercial cenTrifuGes [1]
20 - 200 m2
150 - 2,500 m2
2,000 - 3,000 m2
400 - 120,000 m2
value of 981 cm/s2, but depends on
the altitude
Equation (1) clearly indicates the primary
parameters available to the designer
to obtain the desired G-force
- that is, the diameter and rotational
speed of the rotor.
Note that the G-force is not uniform
throughout the centrifuge. The force
is smallest near the axis of rotation,
and then increases linearly in the radial
direction. Typical G-forces developed
by several types of centrifuges
are shown in Table 1. Values vary
widely, ranging from a low of 600 for
the single-chamber bowl centrifuge,
to as high as 1,000,000 for the ultracentrifuge.
Values at the high end are
sufficient to remove macromolecules,
such as nucleic acids.
While the G-force is an important
characteristic of a centrifuge, it may
not be an accurate indicator of separation
capability. This is because centrifuge
performance is impacted not
only by the applied force, but also by
the area available for settled solids.
That is, for a given applied G-force,
a centrifuge with more settling area
offers greater separation power.
A better measure of the separation
capability of a centrifuge is its sigma
factor, , which is defined as the
cross-sectional area of a gravity settling
tank that would give the same
separation performance. Values depend
on centrifuge geometry and
configuration (for instance, the size
and number of settling surfaces), in
addition to the G-force. For some
types of centrifuges, equations for
Reciprocating
piston rod
Servomotor
direct calculation of the sigma factor
are given in engineering handbooks.
Typical sigma factors for several
types of centrifuges are provided in
Table 2. As with G-force,  values vary
widely, ranging from a low of 20 m2 for
the batch solid bowl, up to 120,000
m2 for the disk-stack centrifuge. Although
the decanter is highly versatile
and widely used, as discussed below,
its sigma factor is relatively low because
of its relatively low settling area.
Conversely, the disk stack centrifuge
contains a large number of disks, providing
substantial settling area, and in
turn, a high sigma factor. Please note
that the sigma factors shown in Table
2 are only approximations; because
of complex flow patterns within the
centrifuge, actual performance may
deviate by as much as 50% from the
given values.
Sigma factors can be employed
to size a production-scale centrifuge
using pilot data obtained with
the same type of centrifuge, according
to the relationship shown in
Equation (2):
(2)
Where:
Q is the flowrate
 is the sigma factor
Subscripts 1 and 2 refer to pilot and
production scale, respectively.
Equation (2) says that if an optimized
throughput of Q1 is obtained
with a pilot centrifuge having a sigma
factor of 1, then a production unit
Basket
Housing
Screen
Cake
Piston
Feed funnel
Wash pipe
Access door
Spray nozzle
Solids discharge
Liquor discharge Wash discharge
FIGURE 3. A pusher centrifuge is shown here (Adapted with permission from Ref. 4)
54
Feed pipe
with a sigma factor of 2 will be
needed to obtain the desired production
flowrate of Q2.
Equation 2 can also be used to determine
if the desired throughput can
be achieved with an existing centrifuge
with a known value of 2. That
value and the pilot data are used to
calculate Q2, which is then compared
to the desired value to see if the centrifuge
is suitable. However, because
of the approximate nature of sigma
factors, results obtained using this
approach should be regarded as no
more than rough estimates.
There are two broad classes of
centrifuges, characterized by the nature
of their solids collection surface.
In filtering centrifuges, solids collect
on a perforated surface, while supernatant
flows through the perforations
to a collection vessel. These centrifuges
are actually filters, but unlike
conventional filters (which operate
using applied pressure or vacuum),
the motive force in a filtering centrifuge
is centrifugal force. Alternatively,
sedimenting centrifuges use a solids
collection surface that is not perforated.
Examples of each type of
centrifuge are described in the paragraphs
that follow.
Filtering centrifuges
Basket centrifuge. This type of centrifuge,
shown in Figure 2, contains
a cylindrical, perforated basket (usually
covered with a cloth liner) that
rotates inside of a stationary housing.
Basket diameters range from 75
to 120 cm, with diamete-to-height
ratios of 1-3. Usually the axis of rotation
is vertical, but horizontal machines
are also used.
After accelerating the basket (also
called the bowl) to the desired operating
speed (typically 600-1,800
rpm), feed slurry emanating from
one or more feed nozzles is directed
toward the surface of the liner. Solids
collect on the liner to form a
cake, while clarified liquid passes
through and exits at the supernatant
outlet port.
After the cake reaches the desired
thickness (typically 5-15 cm), feed
is shut off, while the basket continues
to rotate to allow further removal
of liquid. The cake is then washed,
usually with water, to displace the
process liquid retained by the cake
[2]. Afterward, the basket is accelerChemiCal
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