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Bulk Powder Conveying, Part II
Guest article by Henk Colijn of Colijn & Associates.
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Bucket Elevators
Bucket elevators are capable of handling bulk materials in
a vertical direction, either indoors or out-doors. These units are housed inside a steel
casing that forms the support structure. The basic types are: centrifugal discharge with
spaced buckets or the continuous discharge with a continuous series of buckets. The first
type scoops the bulk materials in the boot of the elevator into the buckets. Beware, that
this will generally cause severe degradation of the particles. The continuous elevator is
fed by a feeder, which allows the bulk material to flow gently into the buckets,
minimizing any degradation.
The carrying medium for the buckets can be either a belt or
chain. Bucket elevators designed for handling processed ingredients or finished food
products should be of sanitary design. These units feature total access to the bucket
strand and the belt or chain area. Stainless steel is used for units which operate in a
wet environment or which require wet cleaning.
Hinged & Pivoted-Bucket Conveyor-Elevators.
With these conveyor-elevators the product is usually loaded
at a horizontal section in the buckets by a feeder and then moved horizontally as well as
vertically. This type of equipment is used on a variety of food products (breakfast
cereals), because it conveys and elevates the product with little agitation or damage.
The easy accessibility of buckets and chain for cleaning
purposes enhances the sanitation aspects. For safety reasons these units should be
enclosed. While mild steel satisfies many product needs, the processing of some foods,
particularly fruits and vegetables, requires stainless steel.
"En-Masse" Conveyor-Elevators.
The term "en-masse" has been used to basically
differentiate between a totally enclosed type of handling a continuous flow through a
casing, pipe or duct, as opposed to the previous types discussed, which for the most part
are open, but when enclosed it is for other reasons, such as dust, weather or personnel
safety.
The "en-masse" conveyor-elevator system was first
developed to convey bulk solids horizontally in a stationary casing by inducing movement
with an inside strand of chain fitted at intervals with transverse bars. This formed a
skeletonized moving base that would propel the bulk solids at the same speed
"en-masse" through the casing.
This technique was subsequently extended to provide also
vertical movement in a stationary duct. In this case, the transverse bars are shaped like
horseshoes. For elevating it is also essential that the bulk solids are introduced to the
conveyor by gravity at the horizontal section. The principle of achieving
"en-masse" movement is fundamentally dependent on the material moved, having a
higher internal friction than the material's friction against the casing surface.
Pneumatic Conveyors
Pneumatic conveying has widespread application across the
food industry for handling dry bulk solids. Finished products as well as raw materials and
processed ingredients can be pneumatically conveyed through pipelines. From a sanitation
viewpoint pneumatic conveying is one of the best means of moving a product from one point
to another, if care is taken to use clean air as the conveying agent,
Pneumatic conveyors can be classified as either
"dilute phrase" or "dense phase". In "dilute phase", the
particles are kept in suspension by the air stream and in "dense phase" the
particles are forced as slugs through the pipeline by the air pressure.
Dilute phase systems are generally high velocity (4000 -
5000 ft./min.) and low air pressure ( 10 - 12 psi), while dense phase systems are
generally low velocity ( 500-1000 ft./min.) and high air pressures (30 - 50 psi). Dilute
phase conveying can be done either in a vacuum mode or in a pressure mode. Because the
bulk solids handled in pneumatic conveying systems are generally dry, the conveying pipes
are manufactured from either mild steel or aluminum. Other components such as rotary lock
feeders, diverters, etc. are fabricated with metals selected according to need.
A particular form of pneumatic conveying is the
air-activated gravity conveyor, sometimes referred to as "air slides". The
system consists of a split duct with a porous membrane in the middle. Air is introduced in
the lower section of the duct and percolates through the membrane, thereby fluidizing the
bulk solids above the membrane. As the duct structure is sloped downwards at about 6
degrees, gravity forces will move the bulk solids along.
Henk Colijn is a consulting engineer, whose background
includes 40 years of industrial experience as an engineer, researcher, author and
consultant. He can be reached at:
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Mr. Henk Colijn
Colijn & Associates
29 Aberdeen Lane
Pisgah Forest, NC 28768
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(828) 862-3445
Help others by posting your comments, suggestions and
experiences with bulk solids feeding or any other materials handling concerns you may have
on our On-Line Help Forum. For
past Ask Joe ! Articles, visit the Ask Joe! Archived Articles.
Guest articles for the Ask Joe! Column are always welcome,
for more information please contact Joe Marinelli directly at his email address:
joe@solidshandlingtech.com.
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