The
Bulk Solids Pump: A new feeder for
free-flowing materials
Guest article by Everett Von Frank, K-Tron
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 Introduction
Increasingly demanding performance
requirements in hi-tech sectors of the plastics, food,
pharmaceutical and other industries have brought traditional screw
feeding technologies under critical scrutiny. Concerns such as
second-to-second repeatability, full range linearity, simplicity and
cleanability have risen in importance in recent years. In an effort
to address these and other concerns, a new design approach to
feeding has been developed.
The Bulk Solids Pump
Known originally as Posimetric®
feeding, this patented technology was first applied in the late
1970s to feed oil shale and coal into large-scale crushing
equipment. It has now been adapted for process industry application,
and has been introduced as the Bulk Solids Pump™
(BSP) by the K Tron Feeder Group of Pitman, NJ. The new feeder does
not use the usual screws/augers, belts, pockets or vibratory trays
to convey the material. It employs innovative positive-displacement
action to feed pelletized, granular, flaked or other free-flowing
materials with consistently high accuracy, uniform discharge and
gentle handling in a true linear volumetric fashion.
The BSP employs one or more specially
designed vertical rotating spools which create feeding ducts.
Incoming material is captured in the duct and rotated to discharge.
While within the duct the material ’locks up’ and acts as a solid
mass. Approaching discharge, inter-particle forces relax and
discharge occurs by cascade at the material’s natural angle of
repose. The smoothness of material flow from the BSP is evident to
even the untrained eye compared to the pulsing flow from a rotary
valve or screw feeder. This smooth, even flow results in
significantly improved accuracy levels, especially at short sample
durations.
 As
illustrated in Figure 1, the BSP principle is best explained as
consisting of four zones: consolidation, rotation, relaxation and
discharge. In Zone 1 material enters the feeder and consolidates as
particles settle and come into contact with one another and the
sides of the duct. At the end of Zone 1 the material is fully
constrained by the duct, and inter-particle and surface contact
forces produce the lock-up condition. In Zone 2 the material rotates
as a solid mass. As the material moves past the 6 o’clock position
Zone 3 marks the approach to unconstrained discharge where
inter-particle forces fall, the material relaxes and regains its
natural flowability. Discharge occurs in Zone 4 where duct rotation
causes the material to cascade from the feeder.
Figure 2 shows a more detailed
rendering of the feeding duct. Since the BSP’s operating principle
exploits a material’s ability to consolidate uniformly when confined
and readily regain flowability when released, the BSP is not
designed to handle powders prone to aeration and flooding or other
materials that tend to clump or bind. Nonetheless, a quite broad
range of free-flowing materials falls between these two extremes.
Note the dimpling of the interior
surfaces of the duct and hub. These small depressions are effective
in eliminating any possible slip that may occur when handling some
materials with characteristic particle forms such as cylindrically
shaped plastic pellets that present relatively little contact area
with the feeder. Dimpling provides sufficient purchase to assure
solid-body rotation of the material. On higher throughput models,
not only are the feeding ducts physically larger but multiple ducts
are combined coaxially to achieve increased capacity without
exceeding the BSP’s design max of 30 rpm.
Repeatability Performance
Repeatability, a statistical measure
of the variability of feeder discharge at a given setpoint, is
traditionally based on 30 one-minute samples and is expressed in
terms of +X% of set rate at a 2 Sigma or 95.5% confidence level.
However, many of today’s critical processes require repeatability
measurements not only throughout the feeder’s full operating range,
but also at sample timescales reflective of the process itself...
often only a few seconds.
To document the BSP’s performance and
compare it with that of a single screw feeder, extensive, carefully
controlled tests were conducted at K-Tron’s testing facility in
Pitman, NJ. Three relatively free-flowing materials (plastic
pellets, wax granules, and citric acid granules) were fed on both a
model BSP-125 Bulk Solids Pump and single screw model K2MVS60.
Automated data collection of catch sample weights enabled sample
durations as short as one second to be reliably obtained.
 Figure
3 plots repeatability error vs sample duration (1, 5, 10 and 15
seconds) and shows the average repeatability for each of the
materials tested. In each case, the BSP’s repeatability error was a
fraction of that measured for the screw feeder. The exceptional
uniformity of the BSP’s discharge contrasts sharply with the single
screw’s discharge pulsation effect, as clearly revealed in the
one-second sample duration results. Note that even though the BSP
tests resulted in repeatability errors of almost exclusively less
than 1%, all tests were conducted with the BSP acting as a
speed-controlled volumetric feeder, not as a gravimetric feeder
where performance would be further improved.
Linearity Performance
Linearity is a measure of a feeder’s
ability to deliver an average discharge rate directly proportional
to feeder speed, over the feeder’s full operating range. In dozens
of developmental tests involving comparison of linearity performance
on identical materials over identical ranges of rates, the BSP
approach averaged a linearity error of +0.1% over its 100:1 turndown
range versus the screw/auger’s average error of +2.0% over the same
turndown.
Cleanability Performance
The BSP’s open, obstruction-free
design provides little opportunity for material build-up anywhere in
the feeding zone. With quick and easy access to the feeding zone, no
pockets or screws and only one moving part, the BSP feeder can be
cleaned in seconds, making it ideal for applications with frequent
material changes. Note that dimpling of the feeder’s rotating
material contact surfaces conforms to industry standards for
sanitary cleanability.
Models and Configurations
 Three
BSP models are available: the BSP-100 feeds from 2 to 200 dm3/hr
(0.07 to 7 ft3/hr), the BSP-125 ranges from 9 to 900 dm3/hr
(0.32 to 32 ft3/hr), and the BSP-150 feeds from 45 to
4500 dm3/hr (1.6 to 160 ft3/hr). All units are
available in loss-in-weight gravimetric as well as volumetric
versions, and are also available as part of the company’s
multi-ingredient gravimetric blending systems.
Conclusion
The BSP outperforms the single screw
feeder on a wide range of relatively free-flowing materials. Tough
to handle powders that do not easily flow into a BSP feeder will
still require single or twin screw feeders for the best linearity,
repeatability and overall accuracy. As demonstrated in carefully
conducted laboratory tests, the BSP has proved itself in raising the
definition of feeder performance to a new level never before
attainable with other, conventional volumetric feeders.
Note: Posimetric® is registered
trademark Stamet Corp.
For more information contact:
K-Tron Feeder Group
Routes 55 & 553
Pitman, NJ 08071
Telephone: 856-589-0500
Fax: 856-582-7968
Email: khunter@ktron.com
Web site:
http://www.ktron.com/
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on our On-Line Help Forum. For
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