Note: Descriptions are shown in the official language in which they were submitted.
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DUST REDUCTION IN DELIVERY OF PARTICULATE COMMODITIES
FIELD OF THE INVENTION
This invention relates to dust reduction in delivery of particulate
commodities and
has been devised particularly though not solely for reducing the amount of
dust issuing
into the atmosphere from the discharge nozzles of grain loaders such as those
used for
loading wheat into ships or other vessels.
BACKGROUND OF THE INVENTION
There are many applications where particulate materials are delivered through
chutes or other discharge devices to various destinations, where dust is
released from the
particulate commodities causing an environmental hazard or a nuisance from
pollution
or contamination. Such instances may include the delivery of grains such as
wheat or
corn from silos or other holding vessels into the holds of ships or railway
cars, or the
transfer of other particulate commodities such as coal from one conveyor to
another or to
a required delivery point or stockpile.
It has been recognised that the fines or dust typically entrained with such
particulate commodities are prone to be emitted from the product stream as
dust during
handling, which at the least can be a nuisance due to cleanliness
requirements, or at the
worst can form an environmental hazard. One recognised way of controlling dust
in
these situations is to reduce the velocity of the product stream to keep the
relative
velocity of the air stream over the product below the minimum pick up velocity
of the
dust, but such an approach has the disadvantage of limiting the rate at which
particulate
product can be delivered to the destination, and hence is a limitation on the
efficiency of
the process.
By way of contrast, the present invention seeks to restrict the emission of
dust
from particulate commodities during delivery, by utilising the velocity of the
product
stream to re-entrain the dust within the product.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides an apparatus for reducing dust
emissions from particulate commodities during discharge from an outlet,
including a
curved duct having an inlet and said outlet, and delivery means arranged to
deliver
particulate commodities to the inlet at or above a predetermined delivery
velocity, the
curvature of said duct being determined in combination with said delivery
velocity to
cause dust arising from the particulate commodities to be re-entrained within
the
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commodities as the commodities are diverted from a straight path by the
curvature of the
duct.
In one form of the invention the curved duct forms a delivery spoon for the
discharge of particulate commodities into a stockpile which could, for
example, be in the
hold of a ship.
Preferably the curved duct is substantially rectangular in cross-section with
substantially flat side walls and curved inner and outer walls.
In one form of the invention the radius of curvature of the inner and outer
walls is
substantially constant throughout the curve.
In another form of the invention the radius of curvature of the inner and
outer
walls varies over the curve such that the duct is curved in a parabolic
manner.
Preferably the tighter radius portion of the parabolic curve is positioned
adjacent
the inlet.
As an optional feature the delivery means may include a converging duct
located
adjacent the inlet to the curved duct, arranged to deliver the particulate
commodities into
the curved duct at a location adjacent the curved outer wall.
The curved duct may also be provided with an air-restrictive flap extending
inwardly into the duct from the curved inner wall adjacent the outlet, the
flap having a
trailing edge arranged to rest in use just above the stream of particulate
commodities
issuing from the outlet.
In one embodiment the flap is hingedly connected to the curved inner wall such
that the trailing edge can move toward or away from the curved outer wall
depending on
the volume of particulate commodities passing therebeneath at any point in
time.
Preferably the trailing edge portion of the flap is flexible to automatically
allow for
changes in the mass flow rate of particulate commodities passing therebeneath.
BRIEF DESCRIPTION OF THE DRAWINGS
Notwithstanding any other forms that may fall within its scope, one preferred
form
of the invention will now be described by way of example only with reference
to the
accompanying drawings in which:
Fig. I is a side view of a grain discharge spoon according to the invention,
having
a substantially constant radius;
Fig. 2 is a similar view to Fig. 1 showing a parabolic spoon;
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Fig. 3 is a diagrammatic cross-section of a constant radius grain delivery
spoon
with an air restriction flap; and
Fig. 4 is a similar view to Fig. 3 showing the.addition of a converging duct
located
adjacent the inlet to the grain delivery spoon.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE
INVENTION
In the preferred form of the invention the apparatus for reducing dust
emissions
from particulate commodities during discharge from an outlet will be described
in the
embodiment of a grain handling ship loader where wheat is delivered down an
inclined
or substantially vertical duct or pipe to a discharge spoon from where it is
emitted as a
product stream into the hold of a ship.
In this situation particulate commodities such as grain in the form of wheat
are fed
through known conveyor mechanisms into a loading spout which is typically an
inclined
or vertically extending tube or duct which drops the wheat from an overhead
gantry into
the hold of a ship. It is known to provide the discharge end of the loading
spout with a
discharge spoon, otherwise known as a loading spoon, which may have an angled
portion at the end in order to direct the grain being discharged at an angle
from the
loading spout and enable grain to be directed under the coamings of the hold
of a ship.
It is also common to provide existing loading spoons with top impact plates
and
sometimes spring loaded gates in an attempt to control the flow rate of grain
through the
loading spoon in the belief that this would reduce dust emissions.
On the contrary, the inventors of the present invention have discovered that
the
velocity of the grain passing down the loading spout can be utilised to re-
entrain
airborne dust into the body of the product stream. This is achieved by
connecting a
curved loading spoon 1 to the discharge from the loading spout 2 such that
product
enters through an inlet 3 into a curved duct 4 before being discharged from an
outlet 5.
The curved duct 4 may be of any convenient cross-section but is typically
rectangular in cross-section having substantially flat side walls 6 and a
curved inner wall
7 and curved outer wall 8.
As well as directing the product stream issuing from the outlet 5 at an angle
to the
loading spout 2, the curved loading spoon serves to direct the particulate
product against
the curved outer wall 8 such that the inertia in the entrained dust stream
sitting above the
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product stream causes the dust to be re-entrained into the product stream, as
the product
stream curves up toward the dust layer.
The nature of the curvature of the duct 4 does not appear to be critical and
the
principle has been found to work well with either a constant radius curved
duct as shown
in Fig. I or a duct formed to a parabolic curve as shown in Fig. 2 where the
radius of
curvature adjacent the inlet 3 is less than the radius of curvature adjacent
the outlet 5.
Discharge spoons incorporating a curve in this manner have shown a significant
reduction in airborne dust compared with prior art spoons without the
curvature. The
product stream exiting the spoon according to the invention is very well
defined with a
sharp boundary layer between the air and the grain. The flow rate of the grain
is not
reduced and in experimental laboratory development was estimated to be
approximately
4.5 to 5 metres per second. In a full scale application the flow rate is in
the order of 7 to
8 metres per socond with a large velocity component in the horizontal
direction at the
point of discharge. This increases the reach of the grain discharged from the
spoon
without having to incline the vertical loading spout and enhances the ability
of the
operator to place grain under the coamings of ship holds.
The average dust concentration level in experimental testing showed more than
a
50% reduction in dust compared with existing loading spoons. This reduction in
dust
levels is attributed to the concentrated product stream keeping the dust
entrained within
the product.
Similar test results were obtained whether the curved duct is of a constant
radius as
shown in Fig. I or of a parabolic configuration as shown in Fig. 2.
In order to further enhance the dust entrainment in the product an optional
air
restrictive flap 9 can be incorporated into the design of the discharge spoon.
For a given
grain mass flow rate, the flap is designed to rest just above the grain stream
surface
shown in broken outline at 10 (Fig. 3) at the point just before the grain
exists through
outlet 5.
As can be clearly seen in Fig. 3 the product stream is discharged into the
spoon
from the loading spout at 11 and impacts the curved outer wall 8 at point 12.
The
provision of the air restrictive flap 9 further enhances the entrainment of
the air stream
into the product stream 13 exiting the outlet 5 and has been shown in
experimental use to
achieve a further 30% reduction in dust compared to the design without the
flap.
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The flap may be formed in any convenient manner but is typically formed from a
thin metal backing plate 14 mounted to the curved inner wall 7 either in a
fixed manner
or by way of a hinge 15. The trailing edge of the flap may be provided with a
flexible
rubber lip 16 to further enhance the fit of the trailing edge of the flap to
the upper surface
of the product stream 13.
As a further optional feature it has been found advantageous in some
situations to
concentrate the grain and direct the flow of grain from the loading spout 2
toward the
outer curved wall of the loading spoon before the grain enters the curved
portion. This
configuration is shown in Fig. 4 where a converging duct 17 is provided
immediately up
stream of the curved loading spoon I arranged to direct the grain against the
curved
outer wall 8 at the inlet point 3. This also has the effect of concentrating
the grain,
forcing the dust back into the product and accelerating the flow rate of the
grain and
entrained air and is felt to enhance the re-entrainment of the dust into the
main product
stream as the product passes through the curved duct 4.
By selecting a small inclination angle of the tapered duct 17 to the vertical
axis,
impact zones between the stream of grains and the wall are significantly
reduced. The
area reduction through the tapered duct 17 also forces the stream of dust
laden air to mix
with the grain stream, hence reconcentrating the stream and reducing the
amount of
mobilized dust. The size of the area reduction has to be carefully selected
based on the
expected product velocity and tonnage to avoid choking the chute.
Although Fig. 4 shows the tapered chute 17 used in combination with the air
restrictive flap 9, it has been found in testing that the tapered chute 17 in
combination
with the curved spoon 4 is fully effective in concentrating the dust and that
the air
restrictive flap 9 had virtually no further impact on dust concentration
levels. It would
therefore appear that the effectiveness of the invention is enhanced by using
either the
tapered chute 17 or the air restrictive flap 9 but that the combination of the
two has little
further effect in re-entraining the dust.
Overall it has been found that concentrating the product stream using a curved
discharge spoon and keeping the product velocities high has a far more
beneficial effect
on reducing dust than previous attempts to decelerate the product stream. The
efficiency
of the apparatus can be further enhanced by using either the tapered inlet
duct 17 or the
air restrictive flap 9.
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Due to the high velocities, the dust is dragged down the vertical chute and
consequently merges with the product stream in the curved spoon so that the
discharge is
more concentrated and streamlined than in previously known spoons, resulting
in the
dust being contained within the product stream.
Although the preferred embodiment of the invention has been described for use
in
a grain loader, and in particular for the discharge of wheat into the holds of
ships, it will
be appreciated that the same principles can be applied in many other
situations involving
the handling of particulate commodities where dust is a problem, including the
conveying of coal and the transfer of coal or other particulate material from
one
conveyor to another, or into a discharge hopper or stockpile.
