The many benefits given by the ability of a screw feeder to extract progressively from a long outlet slot have been previously denied when the separate benefit of massive headroom saving was adopted to deliver to two alternative receiving points by reversing the direction of discharge. Various screw forms are used to serve long hopper outlet but screws that generate progressive extraction would cause undue compaction if reversed in discharge direction. Screws that are of uniform geometry can be reversed, but currently these will only generate ‘live flow’ from short outlet lengths. They can be used on longer outlets, but this negates the flow benefits of a long outlet slot and leaves ‘dead’ regions of material in storage that will not discharge until the hopper content reduce to empty the original flow channel. A conventional screw feeder can only secure live extraction from a slot length equivalent to around one and a half pitches. Even this length is only practical because the initial pitch section does not fill completely at a density to restrain further input under the fill pressures of a slightly longer outlet.
‘Live’ flow through the total area of a hopper outlet is essential for mass flow, but it also carried additional benefits by providing a more effective flow channel to avoid ‘arching’, preventing stable ‘ratholes’ and reducing segregation effects. It further allows shallower hopper walls to be employed for extra capacity or headroom saving, and reduces the danger of ‘flushing of fine powders, provided that the outlet slot length is at least three times its width. It should be noted that the increase of material in the flow channel given by these benefits also provides a longer residence time for de-aeration and lowers the flow velocity, which reduces the counter-flow differential of the material flow to that of the rising air as the material de-aerates, making the process more efficient..
A patented technique developed by Ajax Equipment enables a screw feeder of uniform geometry to provide ‘live’ extraction over an outlet slot length of up to three pitch lengths. An outlet slot of this length will secure the flow benefits of an outlet twice its width by countering a tendency for the material to ‘arch’ and eliminates the prospect of a stable ‘rathole’ forming in the hopper. It further enhances the storage capacity, or saves additional headroom, of the storage container, both by virtue of the extended slot and being able to employ less steep hopper walls as a plane flow regime. A unique and outstanding feature of this construction is that the technique allows ‘live’ extraction to be secured over this extended length of outlet slot when the screw is running in either direction to deliver to either of two outlets.
An extra design refinement that is incorporated in a Type 2 Feeder allows ‘live’ extraction to be secured over an outlet length of up to six pitch lengths with screws of uniform construction and also provides the facility to deliver to either outlet individually, or to both outlets at the same time merely by altering the direction of screw rotation. The combined benefits of improvement in flow behaviour, saving of headroom and enhancement in storage capacity offered by this new system is suitable for any bulk solids application. Typical screw reed systems are listed below. For more details of this novel technique, contact www.ajax.co.uk/video/feederfeeder.
Breakthrough in Reversing Screw Feeder Design
Breakthrough in Reversing Screw Feeder Design
by Lyn Bates
The many benefits given by the ability of a screw feeder to extract progressively from a long outlet slot have been previously denied when the separate benefit of massive headroom saving was adopted to deliver to two alternative receiving points by reversing the direction of discharge. Various screw forms are used to serve long hopper outlet but screws that generate progressive extraction would cause undue compaction if reversed in discharge direction. Screws that are of uniform geometry can be reversed, but currently these will only generate ‘live flow’ from short outlet lengths. They can be used on longer outlets, but this negates the flow benefits of a long outlet slot and leaves ‘dead’ regions of material in storage that will not discharge until the hopper content reduce to empty the original flow channel. A conventional screw feeder can only secure live extraction from a slot length equivalent to around one and a half pitches. Even this length is only practical because the initial pitch section does not fill completely at a density to restrain further input under the fill pressures of a slightly longer outlet.
‘Live’ flow through the total area of a hopper outlet is essential for mass flow, but it also carried additional benefits by providing a more effective flow channel to avoid ‘arching’, preventing stable ‘ratholes’ and reducing segregation effects. It further allows shallower hopper walls to be employed for extra capacity or headroom saving, and reduces the danger of ‘flushing of fine powders, provided that the outlet slot length is at least three times its width. It should be noted that the increase of material in the flow channel given by these benefits also provides a longer residence time for de-aeration and lowers the flow velocity, which reduces the counter-flow differential of the material flow to that of the rising air as the material de-aerates, making the process more efficient..
A patented technique developed by Ajax Equipment enables a screw feeder of uniform geometry to provide ‘live’ extraction over an outlet slot length of up to three pitch lengths. An outlet slot of this length will secure the flow benefits of an outlet twice its width by countering a tendency for the material to ‘arch’ and eliminates the prospect of a stable ‘rathole’ forming in the hopper. It further enhances the storage capacity, or saves additional headroom, of the storage container, both by virtue of the extended slot and being able to employ less steep hopper walls as a plane flow regime. A unique and outstanding feature of this construction is that the technique allows ‘live’ extraction to be secured over this extended length of outlet slot when the screw is running in either direction to deliver to either of two outlets.
An extra design refinement that is incorporated in a Type 2 Feeder allows ‘live’ extraction to be secured over an outlet length of up to six pitch lengths with screws of uniform construction and also provides the facility to deliver to either outlet individually, or to both outlets at the same time merely by altering the direction of screw rotation. The combined benefits of improvement in flow behaviour, saving of headroom and enhancement in storage capacity offered by this new system is suitable for any bulk solids application. Typical screw reed systems are listed below. For more details of this novel technique, contact www.ajax.co.uk/video/feederfeeder.
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