The selection of a hopper construction may focus on capital cost, leading to the choice of a conical construction, rather than a plane flow outlet section. Plane flow, however, offers many advantages that can outweigh the additional expense in a complete assessment. A comparison between a plane flow channel compared with that developed in a conical hopper must take account of the need for a feeder to collect from an extended length of hopper outlet slot, whereas gravity flow may be adequate for discharge from a conical outlet. However, a feeder will provide discharge rate control and so are often fitted to conical outlets.
Many benefits of a plane flow hopper can be secured, features marked in bold may dominate their selection criteria: -
1.Fine control of the discharge rate by a feeder
2.Discharging in a more stable condition of bulk density.
3.The potential for higher discharge rates.
4.Increased holding capacity.
5.Saving of headroom.
6.More uniform drawdown
7.Redressing segregation
8.Inhibiting ‘Flushing’ and ‘Flooding’ by improved de-aeration.
9.Securing reliable flow through smaller outlets for both cohesive and lumpy products.
10.Able to deliver to two separate outlets, either independently or concurrently.
1.Control of feed rate.
The ability of a feeder to vary the extraction rate by speed control permits a wide range of discharge rates to be served.
2.More stable bulk density.
The bulk material tends to attain a more stable bulk density in the larger flow channel moving at a lower flow velocity and the reduced dilation at which the material enters the feeder
3.Enhanced discharge capacity.
The bulk material will move slower through the larger area of flow exposed to a feeder under a slot outlet, so can attain much greater discharge rates than by gravity through smaller outlets.
4.Increased holding capacity.
Two factors enable addition volume to be held within a given headroom.
a.The extended outlet length directly increases the holding capacity of the hopper.
b.The wall inclination necessary to generate wall slip for mass flow is approx. ten degrees lower that than needed for a conical construction, thereby more rapidly spreading the hopper width.
The combined effect is that significantly more capacity can be stored within a given headroom.
5.Headroom saving
An alternative to holding more capacity is to reduce the headroom taken by the equipment, affecting both the site headroom demand and the height of the hopper filling equipment and access facilities. This feature may be crucial where headroom is at a premium by virtue of planning consent or site restrictions.
6.More uniform drawdown.
The velocity gradient across the cross section of a plane flow hopper served by a well-designed feeder tends to be lower that than of a conical flow channel. This provides a more uniform residence period for the hopper contents, which is a useful attribute for products that are sensitive to storage time.
7.Redressing Segregation.
Conical mass flow hoppers usefully redress segregation during the main run of discharge from the body section of the hopper, but the velocity gradient in the converging section will not correct the initial period of discharge or the quantity that rests in the conical region at the termination of the emptying process.
By contrast, extraction along the length of a slot outlet will mix product from different linear regions of storage and, combined with the more even drawdown characteristics of a plane flow channel, more thoroughly reflect the composition of the bulk material as loaded into storage.
8.De-aeration characteristics.
Two unfavourable features of a conical flow channel from the de-aeration viewpoint arise from the radial flow channel.
a.The average velocity in the flow channel increases to many times higher than that in a plane flow channel that is evenly extracted.
b.The velocity gradient across the flow channel is higher than in plane flow.
The combined effect is that the speed of central drawdown in a cone is considerably more than in a plane flow channel, resulting both is a much lower residence time for fresh material loaded into the hopper and a much higher counter-flow velocity against air that is rising to escape, tending to carry the air down with the product and radically increasing the danger of ‘Flushing’. By contrast, the larger flow area and lower velocity gradient in plane flow means that the product has longer to de-aerate and the process is more efficient because the effect air escape velocity is higher.
It should also be noted that the effect leading to ‘Flushing’ is also time sensitive, in that product in a fluid condition exhibits hydrostatic pressure and will preferentially displace a more settled material in a flow channel with a reduced lateral pressure that is seeking to enter a flow channel with a horizontal component, i.e. converging. This means that unless there is a sufficient depth of bed for the product to settle to a state that developed shear strength, the fluid material from the surface will progressively penetrate the bed until it reaches near the outlet, at which point it will burst through and ‘Flush’ uncontrollably.
The larger flow area, with lower velocity and reduced velocity gradient in a plane flow channel, radically reduces this hazard.
9.Reliable flow through smaller outlets.
Products that tend to develop a cohesive arch will reliably flow through a ‘live’ outlet slot of width that is approx. half the diameter of that which is required by a circular outlet. Similarly, lumpy materials that will tend to form a structural obstruction to flow will pass through a slot considerably smaller than that needed by a circular orifice. Moreover, whereas the blocking of a circular opening will totally obstruct the flow channel and prevent discharge, the partial blocking a of a slot by a rare combination of lumps may allow the continued operation of the equipment until such time as the obstruction can be cleared, with the possible prospect of it being eroded by passing flow.
10.Discharge to either of two outlets.
Until recently it was not practical to secure incremental discharge from an elongated slot outlet in both directions of operation to serve two outlets. An innovative proprietary solution, (1), based on the mechanics of helical screws, now enables discharge to be delivered to two alternative outlets individually from a ‘live’ slot outlet and with an additional design feature, either outlet to be served separately or both at the same time, without change of the discharge rate to either whilst maintaining extraction from the total length of the outlet slot. Considerable headroom savings are given by this construction, as well as avoiding the need for a separate diverter device.
Conclusion.
Whereas a plane flow hopper section may require stiffening of the flat surface to sustain the pressures exerted by the stored product and a feeder that extracts fairly uniformly flow the total length of the hopper outlet slot that are many site and process advantages that may be gained, some of which may be of high importance to the application. The feeder should extract as uniformly as possible from the length of the outlet slot for best results and should be of an integral design with the plane flow hopper based on the measured flow properties of the bulk material to be stored.
Reference
1‘Enhancing the performance of reversing feeders’. ICBMH 2013. Newcastle. Australia.
Benefits of plane flow hoppers.
Benefits of plane flow hoppers.
by Lyn Bates339 lyn bates.html
The selection of a hopper construction may focus on capital cost, leading to the choice of a conical construction, rather than a plane flow outlet section. Plane flow, however, offers many advantages that can outweigh the additional expense in a complete assessment. A comparison between a plane flow channel compared with that developed in a conical hopper must take account of the need for a feeder to collect from an extended length of hopper outlet slot, whereas gravity flow may be adequate for discharge from a conical outlet. However, a feeder will provide discharge rate control and so are often fitted to conical outlets.
Many benefits of a plane flow hopper can be secured, features marked in bold may dominate their selection criteria: -
1.Fine control of the discharge rate by a feeder
2.Discharging in a more stable condition of bulk density.
3.The potential for higher discharge rates.
4.Increased holding capacity.
5.Saving of headroom.
6.More uniform drawdown
7.Redressing segregation
8.Inhibiting ‘Flushing’ and ‘Flooding’ by improved de-aeration.
9.Securing reliable flow through smaller outlets for both cohesive and lumpy products.
10.Able to deliver to two separate outlets, either independently or concurrently.
1.Control of feed rate.
The ability of a feeder to vary the extraction rate by speed control permits a wide range of discharge rates to be served.
2.More stable bulk density.
The bulk material tends to attain a more stable bulk density in the larger flow channel moving at a lower flow velocity and the reduced dilation at which the material enters the feeder
3.Enhanced discharge capacity.
The bulk material will move slower through the larger area of flow exposed to a feeder under a slot outlet, so can attain much greater discharge rates than by gravity through smaller outlets.
4.Increased holding capacity.
Two factors enable addition volume to be held within a given headroom.
a.The extended outlet length directly increases the holding capacity of the hopper.
b.The wall inclination necessary to generate wall slip for mass flow is approx. ten degrees lower that than needed for a conical construction, thereby more rapidly spreading the hopper width.
The combined effect is that significantly more capacity can be stored within a given headroom.
5.Headroom saving
An alternative to holding more capacity is to reduce the headroom taken by the equipment, affecting both the site headroom demand and the height of the hopper filling equipment and access facilities. This feature may be crucial where headroom is at a premium by virtue of planning consent or site restrictions.
6.More uniform drawdown.
The velocity gradient across the cross section of a plane flow hopper served by a well-designed feeder tends to be lower that than of a conical flow channel. This provides a more uniform residence period for the hopper contents, which is a useful attribute for products that are sensitive to storage time.
7.Redressing Segregation.
Conical mass flow hoppers usefully redress segregation during the main run of discharge from the body section of the hopper, but the velocity gradient in the converging section will not correct the initial period of discharge or the quantity that rests in the conical region at the termination of the emptying process.
By contrast, extraction along the length of a slot outlet will mix product from different linear regions of storage and, combined with the more even drawdown characteristics of a plane flow channel, more thoroughly reflect the composition of the bulk material as loaded into storage.
8.De-aeration characteristics.
Two unfavourable features of a conical flow channel from the de-aeration viewpoint arise from the radial flow channel.
a.The average velocity in the flow channel increases to many times higher than that in a plane flow channel that is evenly extracted.
b.The velocity gradient across the flow channel is higher than in plane flow.
The combined effect is that the speed of central drawdown in a cone is considerably more than in a plane flow channel, resulting both is a much lower residence time for fresh material loaded into the hopper and a much higher counter-flow velocity against air that is rising to escape, tending to carry the air down with the product and radically increasing the danger of ‘Flushing’. By contrast, the larger flow area and lower velocity gradient in plane flow means that the product has longer to de-aerate and the process is more efficient because the effect air escape velocity is higher.
It should also be noted that the effect leading to ‘Flushing’ is also time sensitive, in that product in a fluid condition exhibits hydrostatic pressure and will preferentially displace a more settled material in a flow channel with a reduced lateral pressure that is seeking to enter a flow channel with a horizontal component, i.e. converging. This means that unless there is a sufficient depth of bed for the product to settle to a state that developed shear strength, the fluid material from the surface will progressively penetrate the bed until it reaches near the outlet, at which point it will burst through and ‘Flush’ uncontrollably.
The larger flow area, with lower velocity and reduced velocity gradient in a plane flow channel, radically reduces this hazard.
9.Reliable flow through smaller outlets.
Products that tend to develop a cohesive arch will reliably flow through a ‘live’ outlet slot of width that is approx. half the diameter of that which is required by a circular outlet. Similarly, lumpy materials that will tend to form a structural obstruction to flow will pass through a slot considerably smaller than that needed by a circular orifice. Moreover, whereas the blocking of a circular opening will totally obstruct the flow channel and prevent discharge, the partial blocking a of a slot by a rare combination of lumps may allow the continued operation of the equipment until such time as the obstruction can be cleared, with the possible prospect of it being eroded by passing flow.
10.Discharge to either of two outlets.
Until recently it was not practical to secure incremental discharge from an elongated slot outlet in both directions of operation to serve two outlets. An innovative proprietary solution, (1), based on the mechanics of helical screws, now enables discharge to be delivered to two alternative outlets individually from a ‘live’ slot outlet and with an additional design feature, either outlet to be served separately or both at the same time, without change of the discharge rate to either whilst maintaining extraction from the total length of the outlet slot. Considerable headroom savings are given by this construction, as well as avoiding the need for a separate diverter device.
Conclusion.
Whereas a plane flow hopper section may require stiffening of the flat surface to sustain the pressures exerted by the stored product and a feeder that extracts fairly uniformly flow the total length of the hopper outlet slot that are many site and process advantages that may be gained, some of which may be of high importance to the application. The feeder should extract as uniformly as possible from the length of the outlet slot for best results and should be of an integral design with the plane flow hopper based on the measured flow properties of the bulk material to be stored.
Reference
1‘Enhancing the performance of reversing feeders’. ICBMH 2013. Newcastle. Australia.
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