Dealing With "Flooding"
Powders
Guest article by Lyn Bates, Ajax Equipment Ltd.When fine powders are pneumatically
conveyed, dumped quickly into a storage vessel or agitated, the
separated particles trap air in the bulk. It can then take some time
for the material to settle to a non-fluid condition. This loose
condition gives rise to 'flushing' or 'flooding' problem in hoppers,
where the contents run wildly from the outlet without control.
There is no stopping it!
A belt or screw feeder cannot stop
the escape of this flood of product. Many other difficulties are
caused by the variations of density and lack of control of aerated
powders. Powder in this condition will not pass up inclined belt
conveyors and gives rise to problems at transfer points. Volumetric
feeders offer poor accuracy, packing machines give variable weights
and sometimes the required contents will not fit in the pack. Sacks
are unstable and present sealing and stitching problems. Big Bags
'wobble' about and the slightest opening leaks profusely. Readers
can no doubt extend this list.
These characteristics result from the
presence of excess air in the voids of the powder. The weight of the
material is partially sustained by the void gas pressure developed
as the bulk builds up in a hopper or silo. This pressure inhibits
the growth of particle-to-particle contact forces that introduces
shear strength to the mass. Air is slow to escape from a large mass
because of the long, tortuous path that the gas must travel between
the fine particles. The gas can only reach the atmosphere at an
unconfined surface so, in a deep bed, the escaping air from the
lower layers tends to replace that leaving the layers above.
Consequently, the surface layers tend to remain in a fluid condition
for an extended period. It is usually necessary to allow such
materials to stand for a period before discharging, to allow the
lower regions to stabilize to a condition in which it may be
handled. Similarly, for a storage hopper in continuous use, it is
common practice to retain a 'heel' material so that fresh product
cannot immediately pass to the hopper outlet.
In a non-mass flowing hopper or
"conical-flow" hopper, a narrow flow channel develops
during discharge allows loose powder to be drawn from the surface
with little time for the material to settle to a stable condition.
The limitations of a short residence period are exacerbated by the
high flow stream velocity that opposes the back flow of escaping
gas. Non-mass flow hoppers are therefore prone to flushing or
flooding when loaded with highly aerated material.
Mass flow can help
A simple approach is to widen the
flow stream by use of a mass flow design. This, in theory, maximizes
the residence time, minimizes the flow velocity, and provides the
most favorable circumstances for the powder to settle to a stable
and relatively consistent condition. Unfortunately, two features
impede achieving the predicted benefits. First, the converging
section of any flow channel has a velocity gradient such that
material moves quicker in the center than at the outer regions. This
shortens the shortest residence time well below the average. The
second feature is that product in a fluid state exerts hydrostatic
pressure. This means that the horizontal pressure applied by the
fluid material greatly exceeds that of the surrounding non-fluid
material in the discharging flow stream. As a result of these
effects the faster moving region draws down the more fluid material,
which then further resists the inflow of the less dilate surrounding
powder until the fluid product eventually penetrates the bed of
material to the hopper outlet.
Delaying discharge until the powder
has settled or using a larger bin to give a longer residence time
may not be satisfactory options. De-aeration can be accelerated by
use of submerged vent devices covered with filter cloth. These tend
to have limited value because they clog easily as de-aerated powder
presses against the filter surface, leading to an impervious mass
resisting further gas passage. A more durable technique is to mount
inclined plates from the container wall. These act as reverse
sedimentation surfaces for the gas. Air rising from within the bed
meets the underside of these plates and accumulates, to flow up the
surface with little resistance. The gravitational shadows of these
plates shelter their under-surfaces from high contact pressure on
the stored powder, allowing the velocity of the air to carry along
local fines and enlarge the cross section of the gas channel.
Get the air out - solutions you
can use
A more sophisticated method is to
mount an array of vertical rods from a frame. Even as a static
structure, these intrusions to the powder bed constrain the packing
characteristics of the particles against the surface of the rods.
The local increase in voidage between the particles produces a
statistically empty space around the rods. The ring of 'space'
around each rod allows air to pass more easily to the surface, thus
providing a multiplicity of vent channels acting deep into the
powder bed. The velocity of rising air reinforces the gas escape
channel and small, volcano-like eruptions of gas and fines emerge
around the rods at the powder surface.
This technique is enhanced by the
application of a rotary vibrator running at the natural frequency of
the extended rods. As they resonate in a circular path in a
fundamental or higher mode of vibration, they press impress a hole
into through powder bed. This provides effective, multiple escape
routes for the gas under the pressure differential between the
deeply submerged voids and ambient.
The combination of inserts, mass flow
design and de-aeration devices, requires a degree of experience to
secure optimum effects. In fact some duties require the simultaneous
injection of controlled volume air, to prevent the material flow
condition becoming too de-aerated and causing other flow problems.
The technology however, offers a solution for many intractable
installations that suffer from the effects of excessively aerated
bulk materials.
About the Author
Many thanks to Lyn Bates who has been
a major contributor to the Ask Joe! column. As managing director of
Ajax Equipment Ltd. in Bolton, UK ( www.ajax.co.uk
), he finds time to serve on the Bulk Material Handling Committee of
the Institution of Mechanical Engineers and be a UK representative
to the Working Party for Mechanics of Particular Solids of the
European Federation of Chemical Engineers.
You can contact Lyn at this E-mail
address: Lbates@ajax.co.uk
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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