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Packaging Powders - The Great "Give
Away"
Guest article by Richard Farnish,
The Wolfson Centre for Bulk Solids Handling Technology
Printer friendly PDF
In order to obtain the optimum performance
and not "give away" product from modern packaging equipment, it is of paramount
importance that the flow of powder to the dispensing head is of a consistent size
distribution, bulk density and mass flow rate.
These pre-requisites can be met provided
that the flow characteristics of the powders being processed are known and accounted for
in the design of the hoppers, chutes and dispensing heads. These factors become all the
more important in situations where blended powders are being handled (in which cases the
requirement for mass flow equipment is essential if segregation effects are to be
minimized).
The use of core flow hoppers,
"universal" doors on chutes, and the installation of chutes whose geometry is
incidental to the interface between feed hopper and filling head, can all conspire to give
irregular flow by creating areas of non (or slow) moving powder. These areas of powder
generate varying levels of shear at the boundaries of the flowing channel of powder,
giving rise to irregular flow rates and unreliable operation. To examine the effects of
chute and door design a test rig has been assembled in which various chute and door
arrangements can be scientifically assessed and modifications evaluated.
The whole test rig is mounted on load
cells, so that the mass flow rate can be recorded when the test section is activated.
Figure 1. Layout of test rig, photo
and diagram
Results obtained from the test rig have
shown that by designing a chute and door assembly based on results from shear cell work
(using established hopper design criteria) and minimizing shear planes within the flow
path, it is possible to increase flow rates by around 10% and significantly improve
repeatability between runs (see Figs 2a & 2b). The test rig utilizes a patented mass
flow sensor which can be used as a diagnostic tool to examine transients during start up,
trickle and shut down operations. These transients can be a considerable influence over
the packing efficiency of a given piece of equipment).
Figures 2a & 2b. Traces
indicating instantaneous mass flow rates through
the test rig (for flood feed only operation)
Of the two traces shown in Figs 2a & 2b, the trace to
the left shows a considerable peak once flow is initiated whereas the trace on the right
shows an even start up and shut down transient. Clearly, if the transients can be
developed in a controlled manner and steady state flow can be established which is
consistent (compare Fig 2a & 2b), then considerable performance increases can be
obtained in the packing process.
The only differences in the chute arrangement used to
obtain these two traces, was a modification to the operation of the doors at the outlet
and a minor redesign of the internal shape of the chute (based on shear cell results). The
use of the test rig enables solutions to packaging flow problems to be developed "off
line" without impacting on the output of production facilities at the plant
experiencing the flow problems.
As with any powder handling process, it is most important
to take a holistic approach to investigating the causes of variability in product quality
or filling efficiency.
For further information on solids handling and how to solve
problems contact: The Wolfson Centre for Bulk Solids Handling Technology, University of
Greenwich on: +44 (0)20 8331 8646, visit our web page at www.gre.ac.uk/directory/wolfson, or
e-mail: r.j.farnish@gre.ac.uk
Help others by posting your comments, suggestions and
experiences with bulk solids feeding or any other materials handling concerns you may have
on our On-Line Help Forum. For
past Ask Joe ! Articles, visit the Ask Joe! Archived Articles.
Guest articles for the Ask Joe! Column are always welcome,
for more information please contact Joe Marinelli directly at his email address:
joe@solidshandlingtech.com.
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