Feeders For Bulk Solids

There are basically two types of feeders, those designed to dispense a controlled rate of material to a process or production line and those made to control the discharge of bulk material from storage. Although they usually differ widely in scale, their main distinction is function.

The prime duty of a dispensing feeder is to consistently and reliably deliver an accurate rate of feed of material in a suitable bulk condition for use. A feeder from bulk storage can be less sensitive on accuracy, but more concerned about the zone order of discharge from the holding container. The extraction pattern of the discharging feeder is therefore a critical design factor.

Screw feeders are commonly used to discharge material from hopper and silos, as they offer many favourable features, such as total confinement and the ability to extract progressively from extended outlet slots.

Slot outlets, in turn, enlarge storage capacity and enable plane flow hopper sections to be used, which permit lower hopper wall inclinations than are needed for cones or pyramid shaped hoppers. Progressive extraction from a slot is essential for mass flow, but is also useful to generate live flow through the hopper outlet and minimise feeder power.to secure optimum performance-

1. To avoid extended residence times of regions of the hopper contents.
2. To redress segregation that occurred during filling.
3. To minimise the risk of ‘flushing’.
4. To minimise eccentric draw and risk of structural failure.
5. To obtain a consistent density of the material on discharge.
6. To homogenise or mix the hopper contents.
7. To minimise feeder drive power requirements

Fundamental choices for flow pattern are commonly classed as:-  

Mass flow – For materials that deteriorate in quality or flow potential in extended

Storage Expanded flow – For difficult flow, inert materials.            

Funnel flow – For easy flow, inert materials.

However, mass flow alone is inadequate to effectively deal with some flow difficulties. The sequence in which different regions of a hopper is filled and discharged can have an important bearing on the condition in which the product is delivered. Segregation, ‘flushing’, density variations, ‘caking’, and other adverse flow and quality conditions can all be aggravated by uneven extraction.

This is because material is usually delivered unevenly across the cross section of a hopper by forming an angle of ‘poured repose’ from the fill point. If there is any physical difference in the particles of composition the fractions will tend to segregate when flowing down the repose surface and be deposited in a radial manner.

To reconstitute these fractions in their original ratios upon discharge the extraction profile of the feeder slot must match the cross sectional area of the hopper. This can be a challenging task. Mass flow is often cited as generating a ‘first in, first out’ pattern that mitigates these discharge hazards, but mass flow merely means that all the hopper contents are in motion during discharge and wide velocity gradients can prevail.

Much talk is made of ‘even extraction’, but this is usually considered to apply to the inlet of the feeder, when it should relate to the cross section of the hopper body, as large differences may exist between the cross sectional areas  that are served by different section of the feeder.

This requirement for coherent motion extends to dealing with a loose material that loads in a fluid condition and has to settle to a stable state before reaching the hopper outlet. This is because the hydrostatic pressure of preferential penetration of the bed by material in a fluid state will prevent the horizontal pressure of more settled product from entering the flow route. Narrow flow paths also increase the counter flow velocity against air rising to escape and make ‘flushing’ more likely.

Optimum feeder performance is secured by each section of the feeder extracting a proportionate region of the hopper cross section that it serves. This can be a challenging task. Mass flow is often cited as generating a ‘first in, first out’ pattern that mitigates these discharge hazards, but mass flow merely means that all the hopper contents are in motion during discharge and wide velocity gradients can prevail.

Much talk is made of ‘even extraction’, but this is usually considered to apply to the inlet of the feeder, when it should relate to the cross section of the hopper body, as large differences may exist between the cross sectional areas  that are served by different section of the feeder.

For example – Consider a 6M square section hopper with pyramid hopper fitted with a feeder 2M long. The first and last 10% of the feeder inlet would need to extract 11 times the amount of each of the other 10% sections to give even drawdown in the hopper.

At the other extreme, with a 2M long feeder on a 2M dia. silo, the first and last 10% of the feeder would need to take a minuscule amount, with extraction requirement in subsequent sections rising quickly with the local radius, to a maximum at the centre. In most applications there is a step change in extraction demand for the initial and final sections of feeder exposed to the hopper contents.

A screw feeder is usually shorter than the largest span across a hopper and flow from hopper regions to various sections of the feeder are often a combination of linear and radial flow, so the capacity demand per unit length of the feeder for ‘uniform extraction’ can be a demanding exercise to reconcile with the geometric characteristic of screw extraction, namely: –

1. The first section of screw exposed to the hopper contents extracts the full axial transfer capacity of the screw, whereas subsequent sections can only extract the incremental difference in capacity.
2. Increases in pitch do not extract a proportionate increase in capacity due to the reduction in axial transfer efficiency. The efficiency of axial transfer depends on the screw geometry and angle of contact friction of the material handled, so the feeder specification is dependent on many factors unique to the application.
3. In addition, pitch increases do have to serve proportionately long sections of the outlet so the actual extraction rate per unit length reduces.
4. Axial transfer efficiency depends on the contact friction of the material handled on the face of the screw flight, so is unique to the specific application.

For entrainment pattern of screw feeders see figure 1.

In the light of these features it is important to know if the product will be effected by the lengths of time that it may be held in static  storage; either too long, in relation to a potential deterioration in product quality or flow condition, or too short in relation to the material  settling from a fluid to a stable flow condition.

As seen from fig. 1 the first and last sections of the outlet generally show the greatest variance. The proportion may range from less than unity to 10:1 depending on shape and relative proportions of the hopper and outlet sections and transition between them.

Searching enquiries to establish the optimum specification for a feeder can require close co-operation between supplier and user, so it is best to deal with a specialist supplier in important cases.

Ref.
1.Bates.L. ‘Entrainment pattern of screw hopper dischargers.
ASME Jlrn. Eng for Ind. May 1969. Pp 215-302.