Exclusive Feature - Process Controls

Exclusive Feature
The Bulk Solids Pump: A New Feeder Technology for
Free-Flowing Materials
hile the screw or auger is by far
the most commonly employed
volumetric feeding device, increasingly stringent requirements in performance-intensive sectors of the plastics,
food/pharmaceutical and other industries
have brought the popular screw feeder under new and critical scrutiny. Performance
characteristics innate to the concept of the
screw feeder have now become the focus
of performance concerns such as shortterm discharge uniformity, full range linearity, long-term calibration drift and fast
cleanability for quick turnarounds.
The K-Tron Feeder Group has introduced
a revolutionary new concept for feeding bulk
materials designed to address these concerns.
Known as the Bulk Solids Pump (BSP),
this new feeder does not use the usual
W
screws/augers, belts, pockets or vibratory
trays to convey the material. It employs innovative positive-displacement action to feed
pelletized, granular, flaked or other freeflowing materials with consistently high accuracy, uniform discharge and gentle handling in a true linear volumetric fashion.
BSP Operating Principle
The BSP employs one or more vertical
spools to capture incoming material and rotate it to discharge. As illustrated below, the
BSP principle is best envisioned 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 spool. At the end
of Zone 1, the material is fully constrained
by the spool, and interparticle forces produce
a lock-up condition. In
Zone 2, the material rotates with the spool as
a solid mass. As the
material moves past
the 6 o’clock position,
Zone 3 marks the approach to unconstrained
discharge
where
interparticle
forces fall and the material relaxes and regains its natural flowability.
Discharge
occurs in Zone 4 where
spool rotation causes
the material to cascade
from the feeder at its
A BSP incorporated
into the new K-Tron
K4G continuous
gravimetric blender.
characteristic angle of repose.
Unlike a traditional positive displacement liquid pump where a volume is cyclically created, filled and emptied, the BSP
provides a purely continuous positive displacement solids feeding action without
obstruction or significant material degradation or segregation. According to laboratory tests, BSP accuracy and repeatability
average two to two and a half times better
with the BSP than with a screw feeder, depending upon material being fed.
Featuring a 100:1 turndown range, two
sizes of the BSP are currently available the BSP-100 feeds from 2 to 200 cubic
dm/hour (0.07 to 7 cubic ft./hour), and the
BSP-125 feeds from 9 to 900 cubic
dm/hour (0.32 to 32 cubic ft./hour). Both
units are available in gravimetric as well as
volumetric versions and are also available
as part of the company’s multi-ingredient
gravimetric blending systems.
Short-Term Discharge
Uniformity
Historically, to measure the uniformity
of the discharge flow, feeder repeatability
testing required 30 one-minute catch samples. However, many of today’s more critical process applications require repeatability performance involving much briefer
sample durations, sometimes as short as a
few seconds. For the traditional screw/ auger feeder, repeatability performance over
such a short duration is corrupted by the
discharge pulsing effect produced as the
screw/auger rotates. In contrast, the continuous cascade discharge of the BSP approach produces no pulsing effect at any
operating rate.
To characterize this effect refer to the
chart below comparing the repeatability
performance of a BSP feeder versus that of
Zone 1: Consolidation (white)
Interparticle forces produce lock-up at the
end of Zone 1.
Zone 2: Rotation (blue)
Material is in lock-up condition throughout
Zone 2 and rotates as a solid body.
Zone 3: Relaxation
(light purple)
Interparticle forces fall below lock-up threshold.
Zone 4: Active Discharge
(darker purple)
Material discharge occurs.
a screw feeder at varying sampling times.
Both feeders handle the same free flowing
granular material at the same rate.
Long-Term Calibration Drift
In volumetric feeding discharge rate is
inferred from (typically) drive motor speed
on the basis of prior calibration. Thus pe-
riodic calibration checks are required, and
when the density of the process material
changes, or when the relationship between
discharge rate and drive motor speed
Full Range Linearity
6
Repeatablity
5
(+/-% of set rate @ 2 sigma)
Linearity error is the measure of a feeder’s
ability to deliver an average discharge rate
equal to the desired or set rate over the
feeder’s full operating range. For constantrate applications the feeder may be calibrated
at the intended operating rate, rendering linearity performance irrelevant. However, for
applications where the feed rate is expected
or required to vary over a significant portion
of the feeder’s operating range, linearity performance becomes a concern.
An innate characteristic of the screw/
auger approach is the variation of the degree
to which the screw/auger is filled as a function of screw speed. This variation contributes directly to linearity error and, depending on the process material in question,
may be quite significant. In contrast, the BSP
principle avoids this source of linearity error
as full material consolidation is achieved
throughout the feeder’s operating range. In
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%
versus the screw/auger’s error of +2%.
Screw Feeder
4
BSP Feeder
3
2
1
0
1 5 10 15
30
Sample Duration (sec)
Comparative Effect of Sample Duration on Repeatability
60
changes, re-calibration is required.
In feeding via screw/auger, any material
deposition or build-up in the region of the
screw/auger can alter the volume of material discharged per drive motor revolution.
This effect produces calibration drift that
may or may not be detected for some time.
In contrast, the BSP’s open, obstructionless
design provides little opportunity for material build-up anywhere in the feeding
zone. Additionally, the BSP uses a driftfree, digitally controlled stepper-type drive
motor enabling speed control in excess of
one part in 3000.
An exploded view of the BSP125 feeder.
Fast Cleanability
Maximum turnaround efficiency and flexibility is achieved only when its process
equipment can be quickly, easily and
thoroughly cleaned. Screw/auger feeders
require potentially time-consuming disassembly and re-assembly. With simple access to the feeding zone, no pockets or
screws and only one moving part, the
BSP feeder is cleaned in seconds, making
it ideal for applications with frequent material changes.
Conclusion
While generally limited to free flowing
materials, the BSP represents an innovative
addition to traditional feeding technologies. Exploiting the process material’s natural tendency to consolidate when temporarily constrained, the BSP provides
highly accurate and truly continuous positive-displacement feeding action without
obstruction, degradation or segregation.
The BSP uniquely combines high accuracy and unsurpassed linearity over a
broad, 100:1 turndown, excellent discharge
uniformity even at short process timescales
and the simplicity and cleanability of ‘onemoving-part’ design. Properly applied in its
volumetric or gravimetric form, the BSP
promises to deliver a higher level of feeder
performance to processors working with
free flowing materials.
The BSP Model 125 with material running.
K-Tron America, Inc.
Rts. 55 & 553
Pitman, NJ 08071
e-mail: [email protected]
www.ktron.com
Reprinted with permission from Powder Bulk Solids
Manufacturing Sites:
USA
Tel: +856-589-0500
Switzerland
Tel: +41 62 885 7171

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