Frequently Asked Questions, Part II
by Joseph MarinelliHaving been a consultant for many years I
have obviously been asked some of the same questions concerning solids flow,
time and time again. Here are some more frequently asked questions and
answers:
1. How can bulk solids flood or flush
out of a bin?
Answer: Fine solids flowing in funnel flow
(some material moving some material stagnant) typically experience flooding
problems. In funnel flow, a preferential flow channel forms, usually
directly over the outlet.
 If
fresh material is placed in the container, it will flow into the
preferential flow channel and not have enough time to deaerate. The bulk
solid now behaves as a fluid. As it reaches the outlet, the feeder which is
designed to meter a solid cannot control a fluid, and the aerated product
will flow uncontrolled from the vessel.
Additionally, if a "rathole" forms,
where the preferential flow channel empties and forms a stable pipe, fresh
product brought into the vessel or material that sloughs off the top of the
pile will fluidize and flush from the bin. See Fig.1.
2. What are the requirements for a
properly designed mass flow bin?
Answer: There are two major considerations:
- The opening size required to
prevent arching and
- The hopper slope required to
ensure flow along the walls.
The definition of mass flow is that when
material is removed from a bin, all the material is in motion. Obviously, if
the material is cohesive, it may arch (bridge) over the opening causing a
flow stoppage. As well, in order for all the material to remain in motion,
it must flow at the walls of the bin.
There is friction that develops between the
bulk solid and the wall surface of the hopper. If the walls are too rough or
too shallow, the material will flow on itself than on the rougher/shallow
walls. In mass flow, the walls are typically steep and smooth to ensure flow
along them.
3. Am I restricted to steep conical
hoppers to ensure mass flow?
Answer: Not necessarily. Wedge shaped
hoppers are a great alternative to conical hoppers. Wedges, such as chisel
or transition type (see Fig. 2), are more forgiving from a flow standpoint.
 Keep
in mind that material in a conical hopper has to converge in 360°. While in
a wedge configuration they converge in only one dimension. Wedges use
slotted openings which require smaller widths to prevent arching and
shallower hopper slopes to ensure mass flow. Because of the long slot,
wedges will provide a higher discharge rate.
One very important consideration is that the
feeder design is as important as the wedge. If the feeder (likely a screw or
belt) is not designed to discharge material over its entire cross sectional
area, mass flow will be destroyed.
4. My material's angle of repose is
46.3°. How do I use this data to design a bin?
Answer: You cannot design a bin just from the
angle of repose. What opening size do you use? What is the hopper slope,
46.3°?
You have to measure a materials flow
properties in order to properly design a bin or hopper. The angle of
repose can be used to determine the contour of a pile of material, perhaps
to determine volume in a bin or silo, but not design a bin.
5. I have been told that my material
segregates, how does this happen?
Answer: Segregation can occur via sifting,
fluidization, etc. Segregation requires interparticle motion, a range of
particle sizes, etc.
Sifting occurs when filling a bin or forming
a pile, the fine particles sift through the coarse particles, allowing the
fines to concentrate in the center while the coarse roll or slide to the
outside.
Fluidization segregation occurs when solids
are dropped into a bin from a conveyor such as a pneumatic conveyor. The
coarse particles are driven to the bottom while the fine particles remain
airborne and settle on top.
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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