Chemical Engineering April 2023 - 45

flowing. However,
introducing material
into the conveying
line or discharging
material from the
system can be problematic,
and cannot
be overlooked.
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FIGURE 2. The introduction of material into a conveying system, and the
discharge of that material, can cause processing problems
the desired outcomes of the user.
Whether employing vacuum technology
for increased throughput,
dust containment, labor savings,
cleanliness,
addressing
complex
convey routes, or safety and environmental
reasons, the success in
adding pneumatic technology lies
in the ability to understand powder
characteristics and how those characteristics
interact with equipment
design. While some conditions favor
a specific pneumatic technology
(such as conveying heavy materials
at longer distances), in large part,
material characteristics are the primary
factor in determining system
design across all types of pneumatic
conveying systems.
The bulk density of a material is
one of the first indicators of design
in terms of sizing various system
components, such as vacuum receivers
and air sources. Bulk density
leans toward the science end
of design and helps determine how
many cubic feet per minute (CFM) of
air is necessary to move the material
through the convey line. Generally
speaking, materials with bulk
densities between below 55 lb/ft3,
but above 25 lb/ft3 are fairly easy to
convey, while heavier materials require
more power and larger vacuum
receivers. Fine powders with low
bulk density, such as fumed silica,
present their own set of challenges
and may require more filtration and
therefore larger vacuum receivers
than medium density powders.
Once solid material is in the conveying
line, it often conveys without
issue, even if the material is not freeAn
understanding
of how a particular
substance will behave
under certain
conditions is essential
when designing
a vacuum transfer
system. The behavior
of a particular material - whether
it is free-flowing, sluggish or nonfree-flowing
- is important data to
relay to a conveyor manufacturer.
Non-free flowing materials require
the most modifications at the pickup
point. The pick-up point is the most
customized component in a pneumatic
conveying system because it
is crucial for feeding material into the
conveying system. Getting the air-tosolids
ratio correct during the feeding
process in order for material to
feed into the line at proper velocity is
crucial. Even free-flowing materials,
like peanuts or gel caps, sometimes
need extra consideration or require
equipment modifications.
Cohesive and lightweight powders
are prone to sticking and can
cause bridging at the pickup point.
Steady product flow into the material
line and a proper solids-to-air
ratio are critical with sticky materials.
If the solids-to-air ratio is too high,
the line will plug, and if it is too low,
the system is just pulling air and not
transporting enough material. Feed
devices assist getting material into
the line at a constant rate.
With challenging powders, specially
designed agitative-feed devices
regulate flow. These devices include
one or several types of flow-promotion
devices such as vibrators or
impactors, aeration pads, headload
deflectors or polished surfaces, but
rotary or screw feeders are sometimes
used when necessary.
With ultra-fine powders, such as
carbon black, filters can clog and
choke the entire conveying process.
At the top of the material receiver
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APRIL 2023
housing, filters separate the product
from the clean air traveling back
to the vacuum producer. The filter
media, the number of filters, placement
and cleaning cycle is extremely
important for challenging materials.
Product changes
While powders can affect equipment
performance, equipment can also
affect the product. When moving
powders and dry materials, changes
in particle size, density and texture
can occur and potentially affect
product performance. This is one of
the most common challenges faced
by packagers and why it is of utmost
importance to use the specific materials
of interest to test the performance
of system's design before it
is sold. For example, peanuts impacting
fixed metallic elbow sweeps
(90-deg
turns)
in
the
conveying
line can turn into peanut butter. To
combat this, flexible hose is used to
prevent material from impacting the
same location repeatedly.
When pneumatically transferring
gel caps, the capsules can hit the
side of the hopper, creating additional
noise in a facility. To combat
this, vacuum conveyor manufacturers
can add tangential inlets and internal
soft baffles to eliminate noise
and protect delicate solid products
from damage.
Working with a seasoned expert in
pneumatic transfer of powders and
bulk solids costs more on the front
end, but working out any issues that
may arise before a system is installed
on the production floor can save
money in the long term.
n
Edited by Scott Jenkins
Author
Doan Pendleton is president of
Vac-U-Max (69 William Street,
Belleville,
NJ
07901;
Phone:
973-759-4600; Email contact:
noraashmen@vac-u-max.net).
Pendleton's professional experience
includes over 30 years in
the design, engineering, manufacturing
and marketing of dry
bulk-material transfer systems
and industrial vacuum-cleaning systems. His application
expertise extends across several industry sectors,
including food, pharmaceuticals, plastics, chemicals
and non-woven materials industries. Pendleton holds
a B.S. degree in marketing and management from
Suffolk University.
45
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Chemical Engineering April 2023

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