Chemical Engineering September 2020 - 62

Figure 7. This diagram shows one of several design approaches to generate increasing capacity
along a constant diameter screw
material down (other than that at
the very first pitch). Thus, irrespective
of the area of the vessel outlet,
the only flow channel that will be
generated will be defined by the dimensions
of the first pitch spacing
and the diameter of the screw. The
consequence is that funnel flow will
be established. Figure 5 shows this
effect.
The presence of multiple screw
sets (for large reclaim bunkers) or
agitators above the screw (in smallscale
applications) will have no effect
on the size of the active flow channel
that is created. Thus, for unagitated,
large-scale installations, the development
of stagnant regions of material
can be anticipated to develop.
The implications of this can range
from a fairly benign " loss of live storage
volume " to a potentially hazardous
" self-heating and ignition " for
combustible materials (such as municipal
solid waste, biofuels and so
on). The development of draw down
only over the area of the first pitch
is amply demonstrated in Figure 6,
where we can see the effect of operating
an agitated screw feed in conjunction
with carbon black. In common
with many types of cohesive
powders, carbon black has been
consolidated into the non-flowing
zone along the screw - leaving only
the active flow channel apparent.
Variable pitch spacing
Substantial improvements in the
size of the flow channel can be obtained
by using (in this instance) a
screw feeder that has been designed
and constructed to develop
an increase in capacity between the
pitches along its length. This can be
achieved through progressively increasing
the pitch spacing from the
start of the screw forward, the use
of a very wide-diameter shaft that
reduces in the direction of feed, or
a combination of both techniques.
The principle of allowing the development
of increased capacity along
the screw means that there is transport
capacity available under the
62
whole outlet area (Figure 7). For a
vessel that has inherent funnel flow
behavior, this modification allows
the development of the largest possible
flow-channel area above the
screw. This may be adequate to improve
flow reliability, but it will definitely
deliver a degree of improvement
in screw-delivery consistency
(by virtue of the larger flow-channel
volume acting on the screw).
It should be noted that the preceding
narrative relates to paralleldiameter
screws. On paper, a simpler
route to attaining increased
capacity would seem to be to use a
standard constant-pitch screw and
progressively increase the outside
diameter in the direction of feed. A
calculation of pitch volumes would
show that an increase in capacity
has been achieved, but functionally,
this type of screw would not be
recommended for bulk solids other
than those that are free-flowing,
non-time-dependent and those for
which the vessel is drained down to
empty on a regular basis (a requirement
of best practice common to
the operation of conventional funnel-flow
vessels where risks associated
with long-term resident material
exist). An issue arising from the
use of tapered screws is presented
for less than free-flowing bulk solids,
whereby they may bridge or arch
over the narrow start of the screw. It
is possible that, for some materials,
active flow cannot occur until further
along the development of the screw
diameter (which may be as much
as one third to one half of the way
along the length of the screw). Allied
to the flow-channel diameter restriction
imposed by the screw diameter
is the shape of the trough in which
the screw sits. The trough casing
will follow a clearance along the
screw, which means that in order to
interface with a rectangular vessel
outlet, the profile will progress from
fully developed at the exit from the
vessel to a pronounced " V " profile at
the narrow diameter.
This V-shaped trough profile imposes
another aspect of flow impediment
by virtue of material being
supported into the vessel from the
flanks of the trough. Screws cannot
draw material into their sides (hence
material retention onto the flanks
during discharge). This is a potential
problem not just for tapered screw
feeders, but also for parallel screw
feeders (which are also invariably interfaced
to vessels using V-shaped
trough profiles).
Holistic view
Much of this discussion has focused
on screw feeders, but the same
principles for developing increased
capacity over the outlet area apply
equally to belts, vibratory trays, drag
links, rotary valves and so on
Reliability of flow during the operation
of storage vessels cannot
be decoupled from the influence of
vessel geometry and the characteristics
of the feeder. In many cases,
attempts at addressing flow problems
or retention issues in vessels
by installing discharge aids of various
types have varying degrees of
success. For many plants where
bulk-solids handling is problematic,
a holistic view of the problem is seldom
developed. The consequence
is that substantial
resources are
misapplied because the plant fails
to consider the interaction of the
vessel and feeder as a single entity
that should be designed or specified
to work together, rather than being
brought together as the result of
" catalog engineering. "
n
Edited by Scott Jenkins
Author
richard Farnish is a senior
consulting engineer at The Wolfson
Center for Bulk Solids Handling
Technology at the University
of Greenwich (Chatham, Kent
ME4 4TB, U.K.; Phone: +44
0208 331 8646; Email:
R.J.Farnish@greenwich.ac.uk).
The majority of his time at The
Wolfson Center is spent undertaking
consultancy activities for a wide range of industrial
sectors, although he is also involved in the delivery
of undergraduate lectures and short courses to
industry. A large proportion of his work is linked to
troubleshooting bulk solids processes that are underperforming
as a result of equipment design issues or
product quality problems (segregation, agglomeration,
attrition and so on). His research interests relate to optimizing
dry-filtration systems. Farnish has worked at the
Wolfson Center since 1996. He is a chartered mechanical
engineer and a member of the Institution of Mechanical
Engineers (CEng MIMechE) in the U.K.
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