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:
- The extended outlet length directly increases the holding capacity of the hopper.
- The wall inclination necessary to generate wall slip for mass flow is approx. ten degrees lower that than needed for a conical construction.
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 is much lower than that in a conical 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.
The velocity gradient in the converging section preferentially extracts radially induced product that segregates in the filling 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 reflects the homogenous composition of the bulk material loaded into storage.
8. De-aeration characteristics.
An unfavourable feature of conical flow for de-aeration is that the velocity gradient across the flow channel is higher than in plane flow, This results in a lower residence time for fresh material loaded into the hopper and a higher counter-flow velocity against air rising to escape. This tends to carry air down with the product and increase the danger of ‘Flushing’. By contrast, the larger flow area and lower velocity gradient in plane flow means the product has longer to de-aerate and the process is more efficient because of the higher effective air escape velocity.
It should also be noted that an effect leading to ‘Flushing’ is that product in a fluid condition exhibits hydrostatic pressure and will preferentially displace a more settled material in a flow channel that has a lower lateral pressure to enter a flow channel. This means that unless there is a sufficient depth of bed for the product to develop shear strength in the flow channel, 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 flow velocity and reduced velocity gradient of 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 the additional design feature that either outlet can 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 and bifurcated chute
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, there are many site and process advantages to be gained, some of which may be of high importance to the application. A feeder should extract as uniformly as possible from the length of the outlet slot for best results and be of an integral design with the plane flow hopper, based on the measured flow properties of the bulk material to be stored.
Ref.
1 ‘Enhancing the performance of reversing feeders’. ICBMH 2013. Newcastle.
Australia.
