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Bulk Powder Conveying, Part I
Guest article by Henk Colijn of Colijn & Associates.
There appears to be no precise definition in the industry
as to what constitutes a powder. The term "powder" carries with it the
connotation of a finely divided material composed of discrete particles of solid matter.
Generally, any fine particulate matter of less than 40 mesh (420 microns) can be
considered a powder.
Besides particle size, powders may also be classified as
"free flowing" or "cohesive". It is important to establish first the
handling or conveying characteristics of a powder before selecting a feeder or conveyor.
Very fine powders (minus 140 Mesh or 100 microns) exhibit
cohesive effects and are generally impossible to handle by gravity, unless they are
aerated. Dry powders, having no particles of less than 100 microns, are usually
free-flowing and easy to handle.
The following bulk properties are influenced by particle
surface characteristics, and these properties affect their handling, conveying and
feeding:
- Packing, caking or lumping,
- Flowability and compressive strength,
- Fluidization or aeration,
- Permeability,
- Bulk density and compressibility.
A distinction must be made between "feeders" and
"conveyors". A feeder is used as a discharge device under a storage hopper or
bin and is subjected to varying head-loads from the hopper. A conveyor requires a
regulated feed rate and must not operate under varying head-load conditions.
The common conveyance methods in the food industry are:
- Belt conveyors
- Screw conveyors
- Vibratory conveyors
- Bucket elevators
- Hinged, pivoted bucket conveyor-elevators
- "En-masse" conveyor-elevators
- Pneumatic conveyors
Each of these conveyors has some very specific sanitary
design needs. These needs can best be determined by identifying product risks associated
with the equipment. The cleaning methods and materials, which are proposed to maintain the
equipment at the accepted sanitation level, should also be considered.
Conveying systems have one thing in common: their primary
purpose is to move product from point "A" to point "B", while
completely containing the product within the conveying mechanism through the entire moving
process. There should be no leaks or spills associated with the equipment. Conveying
systems must be built in such a way that they do not harbor pests or trapped product or
ingredients and must be easily cleaned. Lastly, they must be constructed and maintained so
they in themselves do not contaminate product in process through malfunction of one type
or another.
Belt conveyors.
Belt conveyors are built in many sizes and shapes. They
move products of various shapes and forms to various points within the manufacturing
process. They move bulk raw materials, process ingredients, and packaged finished product.
Belt conveyors used to move bulk solids are primarily
trough-type, as opposed to flat-type. Trough-type belt conveyors are built to contain the
product on the belt without spilling over the edges. The standard design for troughs is
20, 35 or 45 degrees. Trough angle selection should be associated with the flow
characteristics of the product.
Screw conveyors.
Screw conveyors have a wide number of applications in the
food industry. They are used to convey solid granular materials and can be used as
volumetric feeders and as mixing conveyors. Screw conveyors are made in a variety of
lengths and sizes. In outward appearance, they are all very similar. Nonetheless, screw
augers take on a number of different appearances depending on the primary use. In general,
sanitary design requirements are much alike, although allowances must be made for some
specific design conditions.
Construction materials will vary with the products being
handled. In areas and operations where dry commodities or ingredients are handled, mild
steel for all screw conveyor components is adequate. In other instances, where conveyors
are operating in a wet processing area of the plant, but handling a dry product ,
construction might call for a stainless steel trough and end-plates. In other situations,
where the conveyor is handling moist solids or sticky ingredients requiring a wet
clean-up, all conveyor components should be made of stainless steel.
Vibratory conveyors
Vibratory conveyors are extremely useful means of handling
powders and granular food products. The following listing indicates a summary of the
advantages of using vibratory conveyors:
- Scalping and screening or picking can be done.
- Hot and abrasive materials can be handled.
- Divided flow streams & multiple discharge points are
possible.
- Cooling, drying or de-watering, while conveying.
- Units can be covered - made dust-tight.
- Simple construction and low headroom.
A vibratory conveyor consists of a carrying trough (deck) ,
supporting legs or springs and a drive mechanism (exciter). The drive system imparts to
the carrying trough an oscillating motion of a specific frequency and amplitude. The bulk
material on the carrying trough is moved along the trough by means of the periodic trough
motion.
Various combinations of frequencies and amplitudes are used
in the industry, depending on the application. If the bulk material has an extremely
adhesive quality, then special trough coatings or linings together with large amplitudes
may be used.
Industrial equipment catalogs classify vibrating conveyors
generally by their ultimate application, such as foundry conveyors, sand and gravel
conveyors. or food conveyors. Sometimes they are classified by their type of duty: light,
medium or heavy. The equipment design required for a specific service is left up to the
manufacturer. It would probably be more descriptive to classify vibrating conveyors by
their drive system or exciter mechanism.
A basic distinction in vibratory conveyors is the
relationship of exciter frequency and the natural frequency of the system. If the ratio is
close to 1 (say 0.8 - 0.9), then the conveyor can be classified as a "natural
frequency conveyor" or "resonant" conveyor. Natural frequency is the
frequency at which a vibrating body naturally continues to oscillate, even when the
external exciting forces are removed.
In Mr. Colijn's Part II, he will discuss bucket elevators,
"en-masse" conveyors and pneumatic conveyors.
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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