In each edition of Ajax Top Tips we aim to give you pointers on an important area of solids handling; from hopper design to getting the most from your screw conveyors, Ajax have got it covered. In Part Three of our miniseries on 'Achieving Reliable Flow' we're thinking about the future.
By considering changes in the hopper and its use, future flow can be ensured. Here are our Top Tips for assuring reliable flow:
1.Deciding on optimum hopper shape is a multi-attribute choice.
Overall cost is an important feature but performance and reliability is paramount. The critical region for flow is the outlet and the approach towards it. V-shaped hoppers are twice as effective for outlet size as conical flow, as well as needing lower wall angles for mass flow. For reliable flow when final outlet size is limited, use a planar transformation. If a screw feeder is used, even more favourable geometry can be used and the converging section can be even more effective with 'Sigma Two' relief on the end walls.
2.Ensure there are no surface impediments to surface slip.
Make sure that the internal surfaces near the outlet are free from weld splatter or neat as laid welds. Offset flange joints, protruding gaskets, recessed ports and other projecting intrusion near the outlet are all highly effective at opposing material slip on wall contact surfaces. It is also advisable to make allowances for wear of linings, corrosion or other changes in the condition of contact surfaces.
3.Consider fitting a valve on the hopper that is a larger size than the final outlet.
Materials will flow through a smaller opening than the size required to initiate flow, particularly if hopper contents have consolidated as a result of being static. One way to control the flow rate in a mass flow hopper is with a slide valve. The valve should align at the wall angle, so that the partially closed valve does not hold back a dead region of material. Another way is to fit the valve facing away from the bin across a steeply inclined down-chute. The draw-down of material, even with the slide partially closed, can allow slip of the chute contents.
(click on picture to expand)
An Ajax Chisel Hopper and Triple Screw Feeder
4.Air and vibration have a place as flow aids.
Be aware however that circular bins 'ring' like a bell when vibrated. Bins have inactive nodes at 450 to the initiating point. If fitting two vibrators, install them at 450 horizontal spacing and as near as practical to the outlet. Activate them alternately, preferably only when material flow is required. Only use when material is present, vibrating empty bins can lead to early metal fatigue. If storing a very fine powder, the injection of a small quantity of air close to the outlet may be useful, if the material is not hygroscopic. This air injection should be applied by controlled volume, rather than pressure, as an uncontrolled air supply at pressure will find the path of least resistance, flowing at an increased rate as resistance diminishes.
5.Future proof during the design process.
Allow for a flanged joint well clear of the outlet, and a little spare headroom, so that a new outlet section can be retrofitted with minimal disruption in the event of major flow difficulties. Fit an access port, so that the behaviour at critical regions can be seen and the location accessed if necessary. Contingency fitting of vibrator brackets, level probe sockets or bosses for air injection are much cheaper and easier to fit during fabrication than in plant working conditions.
AJAX: Achieving Reliable Flow 3 – Ensuring Future Flow
AJAX Top Tips:
Achieving Reliable Flow 3 – Ensuring Future Flow
In each edition of Ajax Top Tips we aim to give you pointers on an important area of solids handling; from hopper design to getting the most from your screw conveyors, Ajax have got it covered. In Part Three of our miniseries on 'Achieving Reliable Flow' we're thinking about the future.
By considering changes in the hopper and its use, future flow can be ensured. Here are our Top Tips for assuring reliable flow:
1.Deciding on optimum hopper shape is a multi-attribute choice.
Overall cost is an important feature but performance and reliability is paramount. The critical region for flow is the outlet and the approach towards it. V-shaped hoppers are twice as effective for outlet size as conical flow, as well as needing lower wall angles for mass flow. For reliable flow when final outlet size is limited, use a planar transformation. If a screw feeder is used, even more favourable geometry can be used and the converging section can be even more effective with 'Sigma Two' relief on the end walls.
2.Ensure there are no surface impediments to surface slip.
Make sure that the internal surfaces near the outlet are free from weld splatter or neat as laid welds. Offset flange joints, protruding gaskets, recessed ports and other projecting intrusion near the outlet are all highly effective at opposing material slip on wall contact surfaces. It is also advisable to make allowances for wear of linings, corrosion or other changes in the condition of contact surfaces.
3.Consider fitting a valve on the hopper that is a larger size than the final outlet.
Materials will flow through a smaller opening than the size required to initiate flow, particularly if hopper contents have consolidated as a result of being static. One way to control the flow rate in a mass flow hopper is with a slide valve. The valve should align at the wall angle, so that the partially closed valve does not hold back a dead region of material. Another way is to fit the valve facing away from the bin across a steeply inclined down-chute. The draw-down of material, even with the slide partially closed, can allow slip of the chute contents.
(click on picture to expand)
An Ajax Chisel Hopper and Triple Screw Feeder
4.Air and vibration have a place as flow aids.
Be aware however that circular bins 'ring' like a bell when vibrated. Bins have inactive nodes at 450 to the initiating point. If fitting two vibrators, install them at 450 horizontal spacing and as near as practical to the outlet. Activate them alternately, preferably only when material flow is required. Only use when material is present, vibrating empty bins can lead to early metal fatigue. If storing a very fine powder, the injection of a small quantity of air close to the outlet may be useful, if the material is not hygroscopic. This air injection should be applied by controlled volume, rather than pressure, as an uncontrolled air supply at pressure will find the path of least resistance, flowing at an increased rate as resistance diminishes.
5.Future proof during the design process.
Allow for a flanged joint well clear of the outlet, and a little spare headroom, so that a new outlet section can be retrofitted with minimal disruption in the event of major flow difficulties. Fit an access port, so that the behaviour at critical regions can be seen and the location accessed if necessary. Contingency fitting of vibrator brackets, level probe sockets or bosses for air injection are much cheaper and easier to fit during fabrication than in plant working conditions.
If you need help or advice in achieving reliable flow, contact Ajax1083 ajax equipment.htm.
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