Screw Feeder Extraction for Uniform Flow

by Lyn Bates, AJAX Equipment Co., U.K.

Despite the wide use of screw feeders for discharging hoppers, with a notable exception, (1), there is little published describing the flow patterns they generate. There can be some difficulty of compensating for the initial flight transfer capacity and basing the extraction on swept volume takes no account of the inefficiency of long screw pitches. Users are therefore rarely aware of the potential improvements in performance that they are missing. The benefits of Mass Flow are well known, but less widely understood are the significant difference between mass flow and uniform flow and the radical difference in extraction pattern needed when extracting from a rectangular or a circular hopper. Uneven drawdown can have serious structural and performance consequences, ranging from complete physical collapse to large density variations, ‘flushing’, segregated discharge and extended residence times that raise flow or product quality problems. However, whilst uniform flow demands uniform extraction in plane flow silos, extracting from the more common circular form requires a progressive reduction to the centre and then a progressive increase for the remaining exposed length.

1. Entrainment patterns of screw hopper dischargers. ASME Jrn. of Eng. For Industry. May 1969 ps. 295 - 302

This subject is important and merits expansion

The importance of uniform extraction from hoppers and silos

There is a significant operational 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 alone 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 if the hopper shape and wall inclination satisfy the required criteria. However, unless the drawdown is uniform along the hopper cross section, 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, which develop plane flow, than the radial flow developed on cones or pyramid shaped hoppers. However, ‘V’ hoppers have slot outlets and differential flow velocities will be created along the axis of discharge 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 some ‘specialist’ screw feeder manufacturers.

Adverse Features of uneven Extraction

There are many adverse features of uneven extraction. Products that deteriorate in value or flow condition over time should not have variable residence times that allows indefinite periods of storage. For process applications, such as heating or cooling, differing rates at which region pass through will result in less efficient equipment performance. Applications that depend on discharging at a 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’ or under-capacity if the density is not consistent.

Powders that are prone to fluidise tend to adopt a loose state when loaded in free fall to the hopper, even if they are not already dilated by the delivery equipment. It may take some time for such product to settle to a stable flow state. Any preferential flow channel that develops in the stored bed during this period 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 preventing the more settled product from entering the flow channel. 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 whole 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 of feed control and containment. 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 cross section.

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 net effect is that increases in pitch offer less than 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. Depending on the contact friction of the material handled, the extraction ratio between the first last section of a variable pitch screw serving a hopper outlet can vary from around three and a half to seven and a half times. There is clearly a need for a step change is design after the first extraction flight. This cannot be achieved in practice by pitch change or taper shaft alone, but can be undertaken by a combination of pitch value and stepped shaft.

However, the screw design is only part of the problem.

The discharge pattern of a hopper by a screw feeder depends both on the design of the screw and that of the hopper. The body section of a hopper may be circular or rectangular and is often of larger cross section than the length if the feeder. Unless the design of the feeder and the hopper are considered as an entity, the area of hopper from which material must be extracted by a given section of feeder to generate an even ‘drawdown’ can vary from around 2:1 to a staggering 100:1 or more, which is clearly impossible to accommodate. A circular body section is commonly adopted for hoppers for simplicity of fabrication and the ability to contain internal pressures. This extraction ratio per unit length of feeder for uniform extraction is reduced to the much more reasonable value of 2:1 by the transformation from a circular body section to a rectangular outlet slot. Whilst this can be secured relatively easy at secure the start of a discharge screw, it is somewhat more awkward to achieve at the discharge end. The services of an experienced company should be secured for sensitive applications. further details are available from lyn@ajax.co.uk

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