Chemical Engineering July 2022 - 46

troughs to facilitate more complete
cleanout, and limit cross-contamination
risk.
Bucket elevators. Bucket elevators
(also known as " legs " ) transport
material in a series of small, discrete
buckets attached to a chain
or belt, and are most often used in
the food and agricultural, chemical,
and consumer-goods industries
(Figure 2). In the most common
configuration, at the infeed, buckets
dig material out of the bottom
of the conveyor (the " boot " ) as they
round the tail sprocket and begin to
lift material vertically up. Then, centrifugal
force ejects material toward
the outlet as the buckets round the
head sprocket at the top of the conveyor.
These digger-type and centrifugal-discharge
elevators should
not be used with materials that are
cohesive, breakable, or abrasive
because the digging action can
compact the material in the boot,
cause significant particle breakage
and degradation, and lead to rapid
wear of the chain, sprocket and the
buckets themselves.
There are also continuous bucket
elevator configurations designed to
overcome some of the issues with
cohesive, friable or abrasive materials.
In this type of elevator, the
buckets are generally fed horizontally
(rather than vertically) and there
is no separation between buckets,
an effort to avoid the digging action
in the boot and allow more
gentle feed and discharge. At the
discharge in centrifugal designs,
material is ejected from the buckets
toward the outlet as the buckets
come around the head sprocket at
the top, while other designs instead
allow the buckets to rotate and tip
over as they reach the discharge, to
dump out material more gently than
the high-velocity throwing motion in
centrifugal designs.
Bucket elevators' main strengths
are their ability to convey high tonnages,
to quickly elevate materials
to large heights, and continuous
bucket designs can be very gentle
on the product and minimize degradation.
The major drawback is
that bucket elevators are generally
the most maintenance-intensive
conveying technology. Even with
" heavy-duty "
construction,
46
they
frequently experience issues with
chains breaking or coming off the
sprockets over time, particularly
with digger-type bucket designs.
Tubular drag conveyors. Tubular
drag conveyors (Figure 3) pull material
through a pipe or tube using
a series of equally spaced slowmoving
pucks or discs attached to
a continuous cable or chain, with
drive and turnaround sprockets at
either end of the conveyor. They are
most often used in the food, chemical,
consumer goods and chemical
industries for cost-effective, well
contained or enclosed (for minimal
dust release), gentle transport of
powders with minimal degradation.
The main strengths of tubular-drag
systems are that they require relatively
low capex, they have the most
layout flexibility of any mechanical
conveyor and can make up to
2-3 turns horizontally and vertically
within a single conveyor. They also
allow complete containment or enclosure
to minimize dust release,
and their low velocity minimizes
particle degradation or breakage.
They can easily support multiple
feed and discharge points.
One of their main drawbacks is
they tend to be somewhat maintenance-heavy
when not properly
applied. First, they are generally not
built as " heavy duty " as other mechanical
conveyors types, so they
are often not well-suited for applications
with heavy, abrasive materials.
They should also not be used with
cohesive solid materials, because
the material tends to build up on
the tube walls and " smear. " With
chain-driven types, sticky material
often builds up on the chain over
time. With cable-driven designs,
tension causes the cable to stretch
out over time, so the pucks do not
properly mount in the turnaround
sprockets and eventually derail the
cable. The cable frequently needs
to be re-tensioned and periodically
replaced. With chain-driven types,
the chain is also prone to derailing
from the sprockets, because it
often becomes twisted as the conveyor
repeatedly turns, and with
cohesive materials, buildup on the
chain can prevent the pucks from
properly mounting in the teeth of
the drive and turnaround sprockets
(although there are optional brush
features to attempt to reduce some
of this buildup). When the pucks get
derailed, the conveyor needs to be
shut down to untwist the chain and
remount the pucks.
Concluding remarks
While dozens of different conveying
technologies are available, and the
technologies have improved over
the years, no single " best " conveyor
for moving all bulk solids exists. All
of today's technologies have particular
strengths and weaknesses.
The most appropriate conveyor is
intensely dependent on the
solid
material being conveyed and your
process. To minimize the risk of
production bottlenecks due to poor
conveyor operation, product-quality
issues, or issues with high operating
cost due to significant maintenance
requirements, when choosing your
conveyor, it is essential to consider
your material's characteristics by
measurement in a qualified testing
laboratory using proven scientific
methods [5-8], as well as your specific
process conditions and application
needs.
n
Edited by Scott Jenkins
References
1. Marion, J. Preventing Flow Problems With Reliable Bulk
Solids Handling Equipment Design, Processing Magazine,
September 2019, pp. 32-35.
2. Marion, J. Bulk Solids Flow: Know What You're Dealing
With, The Chemical Engineer, June 2020, pp. 36-40.
3. Maynard, E. It is NOT just a chute!, Dry Cargo International
(DCI), March 2018, p. 49.
4. Atlas Obscura, The World's Longest Conveyor Belt System,
2014. https://www.atlasobscura.com/places/theworlds-longest-conveyor-belt-system-bou-craa-morocco
5.
Carson, J.W. and Marinelli, J. Characterize Bulk Solids to
Avoid Flow Problems, Chem. Eng., Vol. 101, No. 4, April
1994, pp. 78-90.
6. Maynard, E. Ten Steps to an Effective Bin Design, Chemical
Engineering Progress (CEP), November 2013, pp. 25-32.
7. Jenike, A.W. Storage and Flow of Solids, Bulletin 123, University
of Utah Engineering Station, 1964 (revised 1976).
8. ASTM D-6128, " Standard Test Method for Shear Testing
of Bulk Solids Using the Jenike Shear Cell "
Author
Josh Marion is a senior project
engineer with Jenike & Johanson,
Inc. (400 Business Park Dr., Tyngsboro,
MA USA 01879, Phone: +1978-649-3300,
Email: jmarion@
jenike.com). In nine years at Jenike,
Marion has worked on hundreds
of projects for designing new
bulk-solids-handling systems and
developing customized retrofit
modifications for existing installations to ensure reliable
material flow and transport. Marion received B.S.Ch.E.
and M.S.Ch.E. degrees from Northeastern University in
Boston, Mass.
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Chemical Engineering July 2022

Table of Contents for the Digital Edition of Chemical Engineering July 2022