There is a significant difference between Mass Flow and uniform flow. The generation of Mass Flow in a bulk storage vessel to move the total stored contents towards the outlet during discharge is often held as the solution to overcome all bulk storage hazards. Mass Flow does allow the minimum outlet size to be determined at which flow can be guaranteed and avoids an indefinite residue time for the contents but is not adequate to avoid significant velocity differences in drawdown that can have serious adverse consequences.
Extracting devices that generate flow over the total hopper outlet area enable mass flow, further requirements being that the hopper shape and wall inclination satisfy the required criteria. However, unless the drawdown is completely even, preferential flow channels will develop that can result in poor or disastrous operating conditions. A degree of transverse flow velocity variation will develop in any converging flow channel, but the difference is significantly less in ‘V’ shaped hoppers that develop plane flow than in cones or pyramid shaped hoppers. However, ‘V’ hoppers have slot outlets and differential flow velocities along the axis of discharge will be created unless the extracting device draws product uniformly from the whole length of the hopper outlet. Screw feeders are commonly used to extract from slot outlets yet because of their extraction characteristics it is no easy task to secure uniform extraction, in fact this is very rarely achieved, even by ‘specialist’ screw feeder manufacturers.
Adverse Features of uneven Extraction
There are many adverse features of uneven extraction. For process applications such as heating or cooling, the different rates at which region pass through will result in less efficient equipment performance. Applications that depend on discharging at uniform density, such as operations that fill by volume but sell by weight, or require to load a specific amount into a given container size, are vulnerable to excess ‘give-away’, spillage or apparent under-capacity if the density is not consistent.
Powder that are prone to fluidise will tend to adopt a loose state when loaded in free fall to the hopper, if they are not already dilated in the delivery equipment. It may take some time for the product to settle to a stable flow state, during this period any preferential flow channel that develops in the stored bed will fill with the product of the loosest contents. The velocity of the material in the flow channel acts counter to air that is rising to escape, so de-aeration tends to be inhibited or negated and differential flow velocities are reinforced by the hydrostatic pressure exceeding the horizontal pressure of the more settled product. Should the material arrive at the final outlet in a fluid condition, it will ‘flush’ and large quantities of material can escape without control. Even if the material does not flush, the density at which the product is discharged can vary significantly and different extraction rates will give different residence periods for local regions and aggravate any tendency for the material to deteriorate in quality or flow condition.
A further drawback of uneven extraction from a slot outlet is that products that have segregated during the filling of the hopper will be discharged in a variable condition, as local regions at differing composition empty at different times. Uniform extraction will collect from the cross section of fill and remix components that have dispersed differently from the point of fill.
Handicaps of Screw Feeders
Screw feeders are commonly used to discharge from bulk storage as they offer many operational benefits. Their ability to extract progressively from a long slot enables the storage vessel to have a plane flow outlet section, which is the most favourable form of simple flow geometry, allowing the most shallow walls and enabling the largest storage capacity. However, various inherent impediments inhibit the ability of screw feeders to extract material uniformly over the total hopper outlet length:
The initial flight exposed to the stored contents can extract sufficient material to fill its swept volume and transfer the total contents axially, whereas subsequent sections can only extract the marginal increase in axial transfer capacity over the prior screw section.
The axial transfer efficiency depends on the combined angles of the flight helix and the contact friction of the material handled. The flight helix also varies across the flight face width and the axial transfer efficiency falls off significantly with longer pitches and high contact friction values. The nett effect is that increases in pitch offer less that a proportional increase in transfer capacity.
Longer pitches also have to serve longer portions on the hopper outlet, so the rate per unit length actually reduces due to this and the reduced efficiency.
Overcoming these handicaps requires considerable expertise and experience. Ajax Equipment is a leading UK company in the design of helical screw equipment and has introduced various innovations in this field, including a unique design of reversing screw feeder that extracts progressively from a slot hopper outlet when rotating in both the forward and reverse directions.
Ajax has developed and registered the design of a technique to overcome the ‘Initial draw’ problem and incorporate constructional features that balance the extraction uniformly over the full hopper outlet length. A standard range of these LynflowTM screw feeders is now available in Mild and Stainless Steel covering the hopper outlets sizes listed below. These are designed to provide mass flow and near uniform extraction for approx. 85% of common bulk materials.
When Mass Flow is not enough
When Mass Flow is not enough
by Lyn Bates, AJAX Equipment Co., U.K.
There is a significant difference between Mass Flow and uniform flow. The generation of Mass Flow in a bulk storage vessel to move the total stored contents towards the outlet during discharge is often held as the solution to overcome all bulk storage hazards. Mass Flow does allow the minimum outlet size to be determined at which flow can be guaranteed and avoids an indefinite residue time for the contents but is not adequate to avoid significant velocity differences in drawdown that can have serious adverse consequences.
Extracting devices that generate flow over the total hopper outlet area enable mass flow, further requirements being that the hopper shape and wall inclination satisfy the required criteria. However, unless the drawdown is completely even, preferential flow channels will develop that can result in poor or disastrous operating conditions. A degree of transverse flow velocity variation will develop in any converging flow channel, but the difference is significantly less in ‘V’ shaped hoppers that develop plane flow than in cones or pyramid shaped hoppers. However, ‘V’ hoppers have slot outlets and differential flow velocities along the axis of discharge will be created unless the extracting device draws product uniformly from the whole length of the hopper outlet. Screw feeders are commonly used to extract from slot outlets yet because of their extraction characteristics it is no easy task to secure uniform extraction, in fact this is very rarely achieved, even by ‘specialist’ screw feeder manufacturers.
Adverse Features of uneven Extraction
There are many adverse features of uneven extraction. For process applications such as heating or cooling, the different rates at which region pass through will result in less efficient equipment performance. Applications that depend on discharging at uniform density, such as operations that fill by volume but sell by weight, or require to load a specific amount into a given container size, are vulnerable to excess ‘give-away’, spillage or apparent under-capacity if the density is not consistent.
Powder that are prone to fluidise will tend to adopt a loose state when loaded in free fall to the hopper, if they are not already dilated in the delivery equipment. It may take some time for the product to settle to a stable flow state, during this period any preferential flow channel that develops in the stored bed will fill with the product of the loosest contents. The velocity of the material in the flow channel acts counter to air that is rising to escape, so de-aeration tends to be inhibited or negated and differential flow velocities are reinforced by the hydrostatic pressure exceeding the horizontal pressure of the more settled product. Should the material arrive at the final outlet in a fluid condition, it will ‘flush’ and large quantities of material can escape without control. Even if the material does not flush, the density at which the product is discharged can vary significantly and different extraction rates will give different residence periods for local regions and aggravate any tendency for the material to deteriorate in quality or flow condition.
A further drawback of uneven extraction from a slot outlet is that products that have segregated during the filling of the hopper will be discharged in a variable condition, as local regions at differing composition empty at different times. Uniform extraction will collect from the cross section of fill and remix components that have dispersed differently from the point of fill.
Handicaps of Screw Feeders
Screw feeders are commonly used to discharge from bulk storage as they offer many operational benefits. Their ability to extract progressively from a long slot enables the storage vessel to have a plane flow outlet section, which is the most favourable form of simple flow geometry, allowing the most shallow walls and enabling the largest storage capacity. However, various inherent impediments inhibit the ability of screw feeders to extract material uniformly over the total hopper outlet length:
Overcoming these handicaps requires considerable expertise and experience. Ajax Equipment is a leading UK company in the design of helical screw equipment and has introduced various innovations in this field, including a unique design of reversing screw feeder that extracts progressively from a slot hopper outlet when rotating in both the forward and reverse directions.
Ajax has developed and registered the design of a technique to overcome the ‘Initial draw’ problem and incorporate constructional features that balance the extraction uniformly over the full hopper outlet length. A standard range of these LynflowTM screw feeders is now available in Mild and Stainless Steel covering the hopper outlets sizes listed below. These are designed to provide mass flow and near uniform extraction for approx. 85% of common bulk materials.
Lynflow Feeder Range
© 2018 by Lyn Bates, U.K.
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