Industry handles particulate solids in prodigious quantities, at massive differences in scale, in a vast range of conditions, and not always very well. This is not because the standard of engineering in industry is low or the process technology is poor. One has only to compare the performance of plants that handle liquids and gasses with those handling solids, (1), to appreciate that there is a fundamental difference in the ability of bulk solid projects to achieve anywhere like a similar degree of operating efficiency as these industries handling liquids and gasses. Even industries that use small quantities of bulk materials, such as pharmaceuticals, pigments, metallic powders and ceramics, face difficulties that tend to increase with the decreasing scale of application. These industries may be small bulk users compared with the food, chemical, detergent and mineral industries, but each is of major importance in their own right. In fact all sections of industry that use bulk materials, which includes a huge cross section of all transport, manufacturing and processing plants, require to secure flow reliability and control the condition of the bulk material to achieve optimum product quality, efficient operation and reliable production.
A problem often facing designers is to ensure that bulk material in a storage hopper can be discharged reliably and at a controlled rate. Jenike, (2), studied the behavioural nature of bulk materials and evolved a theory of solids flow. He developed a test device for measuring flow related properties of a bulk material and a design methodology for hopper design. This introduced the concept and performance advantages of Mass Flow, which takes place when all the material in a storage container moves during the discharge process. For a particular powder and given set of conditions, it became possible to calculate the geometry of a bulk storage vessel that guarantees to empty reliably. The basis of this approach is that a material deforming in flow attains a unique state of density according to the stresses acting on it. Measurement of the products density and bulk shear strength in various conditions of consolidation and loading, together with measurements of wall friction, opened the door to reliable hopper design.
However, this technique was not extensively undertaken by industry. The demanding nature and cost of the test procedure inhibited widespread acceptance of the method. Industry today often pays a staggeringly high price for refusing to absorb the up-front costs of a crucial design investigation, particularly as intensive production facilities places increased dependence on reliability and machines replace labour. Customers, product and process requirements are driving manufacturers to produce ever finer powders. These are invariably more cohesive and exhibit ranges of behaviour that are more difficult to handle and control. Flow stoppages are bad enough when dealing with batch type operations, but at least it is usually possible to contain or rectify the situation. The intensity of continuous and repetitive manufacturing methods that are so favourable to efficient production when working properly exacerbates the risk and commercial losses when bulk flow is not reliable.
However, flow reliability is not the only issue. Two other features that have a major bearing on the suitability of a bulk material for a particular operation are the bulk density and the corresponding flow condition of the material. Bulk density variations affect the output of volumetric feeders and filling machines, the holding capacity of packs and containers, the contents of dies and moulds and innumerable operations and processes that demand a consistent product density.
The condition of the bulk material also has a major bearing on many industrial operations. Excess fluidity or sluggish flow effects operations such as packing, making tablets and pills, moulding, sintering and pressing as well as affecting handling and flow control. These features are related in that the flow condition may be directly correlated with the state of density of the particular bulk material. In a given flow situation a bulk material is said to be in a ‘critical state’ that is defined by the bulk density of the material that prevails. Whereas many powder feeding devices incorporate techniques to generate flow and control discharge rate with varying degrees of accuracy, they rarely pay the same regard to the density or flow condition in which the powder emerges. A key design parameter for flow is the size of orifice required to prevent arching over the outlet. A slot outlet is more effective than a circular outlet and favours plane flow in the supply hopper, which also has flow advantages. The geometry of a screw matches is compatible with a feed screw that can be constructed to generate live flow over a slot many time longer than its width. The size of outlet needed to secure reliable discharge and the rate of unrestricted gravity flow this would deliver often dictates that a feeder is needed.
For these many reasons, screw feeders are fitted to secure the total operational needs of achieving reliable flow, enhancing storage capacity, conditioning of the bulk material, controlling the discharge rate and delivering to the required location. It must be emphasised that screw feeders their associated hoppers are integral units that must be considered together, whether controlled feeding is a prime functions of the equipment or is an ancillary feature of the bulk storage or collecting equipment.
Types of Screw Feeder
Screw feeders of various types are widely used for dispensing bulk material at a controlled rate and for discharging bulk materials from storage and supply hoppers. The principles discussed below are based on screw feeder applications, but the concepts and some features may also be applied to many other types of feeder that are in use.
The basic function of a bulk solids feeder is to collect the material from the outlet of a holding bin and discharge it in a controlled manner. The various ways in which this task is carried out are:
- Collecting feeders, as from plate filters, web-type filters, dust collectors and similar operations, where the material enters the hopper of the feeder over a length and has to be delivered to a common outlet. Allowance may be required for circumstances when such screw becomes totally covered and control the output, instead of delivering only what is supplied.
- Holding feeders, as used in association with batch discharge mixers, driers, centrifuges and the like, when a quantity of material discharged quickly has to be held and dispensed at a controlled rate. They may act as temporary storage facility to balance an intermittent supply with a continuous forward process or accept a continuous input to supply an intermittent discharge, as sack filling.
- Discharge Feeders, as from silos, hoppers and flooded inlets, where the material fills the supply channel and the feeder has to reliable extract from the bulk in an appropriate extraction pattern. Hopper discharge screws of this type are normally used with relatively large capacity bulk storage facilities.
- Volumetric dispensing feeder, controlling the delivery to mills, drier, process vessels, continuous mixers and the like, when a supply channel provides material to be delivered continuously, or for a given period, at a suitable feed rate to the receiving point. The accuracy and uniformity of the discharge usually being the prime requirement.
- Gravimetric feed systems, where the rate of feed is controlled by a weigh system.
A loss-in-weight system utilises a hopper with screw feeder that is supported on load cells. The weight is checked at very frequent intervals and the change in weight compared with a pre-set value. Feed back to the screw feeder drive control then makes any necessary adjustment. The feeder operation changes to a volumetric mode when the weight of material in the hopper reduces below a given value. The hopper is then quickly refilled and re-tared to the higher value for the gravimetric control to be restored.
- Feeder/Conveyors that require an extended transfer conveyor to be controlled by an integral feed section.
- Inclined Feeders that combine a degree of elevation. These are usually restricted to inclinations up to around 150, although in special cases the slope may be steeper.
These broad categories do not have distinct boundaries or exclusive definitions and the operating duties may overlap or duplicate, as the hopper of a collecting feeder may at times fill with product when the feeder is not running, or flood the screw by a surge in delivery. Similarly, an extracting feeder from a silo may have an extended conveying section and be required to dispense at a set rate to a process, or cases where the screw is inclined, bringing in an elevation function. It is therefore necessary to ensure that the equipment takes account of the full range of operating conditions and is also equipped to deal with events that may occur rarely, but are probable over its working life. With all types of screw feeders, it is important that upstream plant does not impede or restrict the output of the feed system. A blocked outlet is the most common cause of screw damage.
Selection of Hopper Shape
With the each type of feeder, the basic task is to ensure that the inlet geometry of the container and feed mechanism will receive, hold and discharge the stock that is loaded into it and provide reliable flow with a suitable extraction pattern from the contents. The first step may seem to be choosing a shape of adequate volume for the capacity to be held. However, before this can be established it is necessary to determine the flow regime that is to be generated by the feeder hopper design. This task is common to any storage hopper duty, as an inappropriate flow regime will jeopardise the effectiveness of the installation. A major feature that can affect the choice of flow regime is whether the contents of the feed hopper is to be totally discharged each and every time before further material is entered.
If this is the case, time effects of the contents are immaterial, as the zone order of discharge will not significantly influence the maximum residence time that is experienced by some of the material. However, if the hopper does not fully empty for a considerable period because it is refilled after some contents have left, it is necessary to establish whether a Mass Flow form of flow is essential or not. (‘Mass Flow’ is a flow regime where all the contents of the hopper are stimulated into motion during the discharge process).
The key feature that dictates whether a fully mass flow design is appropriate is the ‘time-stability’ nature of the product. That is, will material held in storage for an indefinite time deteriorate in quality, flow prospects or in any other unacceptable manner. Unless all the stored contents are in motion during the discharge process, fresh product loaded into a partially filled hopper will pass through some static regions of storage. This pattern will repeat, as further product entered to the hopper will also empty before some of the original contents discharge. Such ‘dead’ regions of the stored contents will then only empty when the container totally discharges. The residence time for such regions is indefinite and should these contents change in any adverse manner over an extended period, the consequences may be extensive and costly. This pattern can also introduce bizarre segregation effects. Mass flow provides almost a ‘first-in, first-out’ flow pattern, and so avoids excessive residence time. It also offers considerable benefits in dealing with poor flow materials. However, mass flow is not always necessary, or even desirable in some cases, The benefits and drawbacks of Mass Flow are listed elsewhere (3).
The hopper walls will be relatively steep to generate mass flow, if this is the flow pattern selected, or adequately steep to guarantee self-clearing of the contents at the end of the discharge process as a minimum requirement. For either of these flow patterns the inclination of the walls can only be positively assessed with knowledge of the wall friction value of the material to be handled against the material of which the hopper is constructed. Wall friction tests must be carried out to establish this value. This should embrace the range of bulk material conditions that may be experienced. The test method may also be employed to select the most appropriate surface contact material or the surface finish to be adopted.
The second step of the hopper design process is to ensure that the outlet is sufficiently large to guarantee reliable flow at a rate to satisfy the duty. Collecting from a continuous in-feed that can be delivered without interruption to the discharge port only requires the feeder to have excess capacity over the maximum in-feed rate. Should there be any surge or oversupply of feed in excess of the discharge capacity, or delay in discharging that allows the supply to back-up in the feed hopper, then the hopper must be designed on the basis of being a storage hopper. The Jenike design process based on shear tests is normally used to determine the critical span for the outlet size, that is, the largest size of opening over which the product can hold a stable arch once the contents have developed a flow channel in the hopper contents. Apart from the fact that this powder testing technique is demanding of specialised skill, a little publicised feature of its validity is the weakness that the results are not valid for ‘first-fill’ conditions or ‘impact loading’ prior to discharge having taken place to mobilise a flow regime in the outlet region.
An alternative static stress test indicates the maximum span of a stable rathole over an opening by relating the weight of a column of material to the peripheral shear strength supporting the mass. For any given height, H, the weight of the column is equal to the density, ρ, times the cross sectional volume, πr2H. If this overcomes the strength of the supporting boundary, shear stress, τ, times the peripheral area, 2πrH, the column will collapse through the opening. As the peripheral area increases in direct proportion to the radius whilst the area increases as the radius squared, for any given cohesive strength of the bulk solid there is a critical diameter of opening, 4τ/ρ, above which the limiting stress of the materials shear strength is exceeded. Testing a sample prepared to a density reflecting the compacting conditions over a hopper outlet in a vertical shear test cell, (4), enables this critical orifice size to be determined for a circular or slot shape of opening.
The slope of wall required to enable mass flow or to provide self-clearance in non-mass flow hoppers depends on whether a pyramid, conical or Vee shaped hopper is to be used. The choice of hopper bottom shape is influenced by many factors but in general pyramid hoppers are only used with relatively free flowing products that are stable with time because the material resting in the gullies is invariably static until the content level of the hopper reduces to expose the product to unconfined conditions. Conical vessels are relatively simple to construct and of excellent shape to sustain internal pressures, but a conical base section has comparatively poor flow characteristics. The choice of hopper shape is interactive with the wall angle, outlet size flow regime selection, site factors, fabrication cost, whether a feeder is required to secure reliable flow or to control the discharge rate and the user preference or standards.
The requirement to provide an effective opening may strongly influence the choice of hopper shape in favour of a ‘V’ form, whose width only needs to be half the diameter of a circular outlet, provided its length is at least three times its width and that flow take place over the total cross section. Further benefits of a ‘V’ shaped hopper are that the wall inclination required for mass flow is about 10 degrees less than that required for a cone and the holding volume is greater. As screw feeders are being considered, it is usual to secure the flow benefits of a slot outlet by connecting to a ‘V’ hopper bottom section.
Collecting Screws
Of the various classes of feeders listed above, ‘collecting’ feeders are the only type that normally clears the contents as they are deposited. Such a ‘hopper’ is actually a focussing chute to direct the product from a large cross section, as with dust collectors and continuous centrifuges, or taking a wide sheet from a continuous filter onto a screw that delivers the material to a small exit port. Design condition of such applications does not usually give rise to a back up of product to fill the cross section of the collecting hopper so the criteria for hopper design may not apply. However, should there be a prospect of a temporary surge covering the outlet, the screw being stationary for a while, or other event that could lead to material accumulating in the collecting hopper at any time in the working life of the plant, then the extraction conditions of a conventional hopper design should be employed and the screw construction, speed and power be appropriate, noting the extraction pattern of a flooded screw and that its transfer capacity can be many times the rate of a conveying screw
The selection of feeder size may be strongly influenced by the size of hopper outlet needed to avoid flow problems, although twin or multiple screws in parallel are used to secure wide openings when necessary. The needs of a temporary-filled collecting hopper depend mainly on the flow nature of the product. If the material will flow readily through the opening size of the hopper and the backlog will clear satisfactorily, then the collecting hopper walls only need to be sufficiently steep for the material to self-clear.
If mass flow discharge is required to provide reliable flow, then the walls inclination must be steep enough to generate slip up to the size of the critical arching span. The mass flow requirement for progressive extraction along the outlet slot does not apply if the backlog is cleared before further material fills across the opening as the contents will be taken away progressively as the region emptied extends along the hopper.
Even in circumstances where total mass flow is not essential, it may be good practice to develop a fully ‘live’ outflow region, or at least avoid a small extraction region in a long slot outlet, to minimise feeder loading, avoid excessive compaction pressures in the feeder and favour conditions for reliability of flow should the screw be covered in use. Clearly, the normal handling capacity of the screw must be capable of clearing the maximum feed in rate in normal operation without filling the screw cross section. If the screw then becomes covered it will move a larger volume of product, so the discharge rate of the screw will increase, unless there is a compensating reduction in the operating speed. It is important to establish that all downstream plant can deal with the maximum output that may be generated. A crude evaluation of the volumetric discharge rate for screws can be assessed on the basis of swept volume. This may be adequate for applications where the feed rate is not sensitive or can be adjusted by variable speed control. More precise calculations may be determined according to the detailed screw geometry and contact friction values. ( 5 ).
Operating Hazards of Collecting Screws
At this point it is prudent to note that many materials handled by ‘collecting hoppers’ tend to be fine particulates, as with dust collectors, or damp, as from various types of filters and centrifuges. A potential hazard of handling such materials by helical screws is that the cohesive nature of the product tends to favour deposits adhering to the centre shaft and in the corners between the flight and the centre tube. In extreme cases, this leads to ‘Logging’ of the screw, where the whole swept volume is occupied by material and its transfer capacity is negated or very severely reduced. The effect is more prone to occur with very short pitch flight construction and screws with large centre tubes. Short pitches form a deep pocket between the screw flights and material is easily retained by the large surface area of material contact in relation to the small peripheral area tending to restrain rotation of the mass. The effect of a large centre tube is to require the narrow upstanding face of the screw flight to push forward a thin annulus of material. The tendency is for only a small region in form of the blade to move and a large residue stick to the back face of the next flight and the centre tube.This potential problem can usually be overcome by fitting ribbon type screw flights on a small diameter centre tube, with a face width somewhat narrower than the remaining gap between the inner edge of the ribbon and the tube. Special geometric ribbon screw forms are available to resist this form of clogging.
A secondary consequence of cohesive and fine damp products, and also of bulk materials that tend to cake, is the formation of a firm bed in the clearance space between the screw flight and the casing. This layer is robustly confined as the flight passes over and can offer considerable resistance to deformation as additional fine particles are trapped by the rotating flight. High trapping pressures may be generated by this mechanism, resulting of a large torque drag on the screw, a significant bending load on the screw shaft and/or aggressive wear on the flight tips. Thick blades aggravate the problem, providing a larger area on which the torsional and bending loads apply. Higher power is rarely the answer. Thin, hard-tipped flights that are chamfered to a fine tip width are usually the best solution, with medium pitch flights to minimise radial rotation of the screw contents. These operating problems are not confined to collecting screws and the potential of these hazards should be considered whenever fine or damp materials are being handled.
Extracting Feeders
Discharge screws for hoppers and silos are usually fitted to provide a large outlet so that reliable flow can be secured. A fully live extraction pattern can be generated over an axial length of about four times the screw diameter by means of varying the pitch of the screw at a constant outside diameter. The use of stepped shafts or taper shafts enables the effective length of a live outlet to be extended to about eight timed the screw diameter. Longer outlets can be serviced, but there is a danger of some parts not extracting product unless great care is taken in proving the design. Screws that taper in pitch and outside diameter are sometimes used, but the casing fabrication is expensive and the small end determines a potential arching span, so tend to be less widely used.
The hopper outlet length provided by a discharge screw enhances the holding volume of a storage hopper, whilst the plane flow geometry enables lower wall angles to be employed than in cones for mass flow or self-clearing. A further virtue of a Vee shaped hopper is that a rathole cannot form if the outlet is live over the whole length. A gap developed in the centre of the mass removes the restraint holding back material resting on the walls so, provided the wall inclination exceeds the angle of wall friction, the material will slide en-mass to meet product on the opposing side. A discharge screw fitted for flow benefits requires the mass flow region to continue to a span larger than the critical arching distance for a non-mass flow hopper, before reverting to walls that are merely steep enough to self-clear. ‘Expanded’ flow hoppers of this geometry provide maximum holding capacity with best flow for any given headroom.
Twin or multiple screw dischargers can be used to provide a wider opening and/or secure a greater storage capacity. The reason single screws twice the diameter are not used is that the power and feed rate increase approx as the cube of the diameter, whereas output is proportional to the number of screws. The power taken by multiple screws has to be assessed with care. Unless fitted purely to secure extra holding capacity, which is rare, the fact that more than one screw is needed indicates that the material is likely to arch over a single screw, which would have a design overpressure from the hopper contents of zero. However, there must be a degree of conservatism in the selection of the outlet width and excess width leads to a disproportionate increase in the down pressure from the hopper contents. Higher outlet loads means greater shear stresses to be overcome by the screw(s). Calculating the power needs of screw feeders is generally a complicated matter and the domain of specialists.
Multiple outlets can be taken from discharge screws, but an understanding and control of the extraction is necessary to ensure good results. Outlet ports that are positioned under the hopper outlet slot act as an uncontrolled discharge port, unless restrained by a further device. The first outlet placed beyond the hopper outlet slot can pass the total contents moving along the access, unless similarly restrained. More usually the first outlet has a valve and the last outlet is open, typically to act as alternative flow routes.
Two-way and Multiple Point Discharge
Some applications require that optional discharge be made to either of two opposing outlets. A reversible screw can be employed to deliver either way from the hopper outlet but this necessitates that a uniform pitch screw is used, which limits the outlet length for a mass flow hopper to about one and a half pitches.
Longer outlet slots can be used with uniform pitch reversible screws, but only one end of the outlet slot is then active and the larger portion holds back a bed of static product. Ajax developed a method to enable discharge to be delivered to either end or both ends at the same time, with the ends receiving the design rate in every mode of operation. In each mode of operation half the amount of material is taken from the hopper by each screw so the method extends the permissible ‘live’ extraction length for mass flow to approx. three screw diameters. A proprietary development, (6), enables the live slot length to be extended to about six screw diameters with progressive extraction along the discharge slot, whether feeding one way or the other, or discharging to the two outlets at the same time. Fitting a three-position flap valve at each outlet allows the facility to deliver to one, two, three or all four receiving points, whilst sustaining a ‘live’ extraction pattern from an outlet slot up to six times as long as it is wide.
Variable Screw Geometry
The screw geometry must vary along the axis of the screw to provide the incremental transfer capacity to develop live flow over a length of a hopper outlet slot. A variety of techniques are available to specialist manufacturers to create continuous extraction, but three features should be emphasised: -
1. The incremental capacity of varying in pitch only is less than proportional due to a reduction on axial transfer efficiency and the longer pitch has to serve a longer section of the hopper outlet. A limit is quickly reached where further pitch increases are counter-productive.
2. The axial conveying capacity and power needs of a full screw are material dependent, so the performance may change significantly with dissimilar bulk materials.
3. A feeder and supply hopper are integral items that must be designed as an entity.
A common form of dispensing feeder in a type that is subject to feedback control over the overall change in weight of the feeder and the supply hopper with its contents. A feature of these machines is that a batch of product is dispensed under gravimetric control and then a new charge is loaded into the feeder hopper in a short period during which the feeder continues to dispense on a volumetric basis.
Sophisticated control algorithms are used to compensate for fluctuations in output that occur due to changes in bulk density. In practice, the quality of feed consistency tends to depend as much, if not more, on the ability of the make-up mechanism and feeder to deliver product in a stable density condition as the quality of the electronic control.
Feeding into a pneumatic conveying system presents no additional problem if the system is a vacuum conveyor. Delivering to a positive pressure system generally requires that back flow of gas is eliminated by venting between the pressure barrier and the hopper outlet, usually by fitting a rotary valve or lock hopper, A technique developed to form an impervious plug seal by the product along the feeder casing offers an efficient way to prevent the inevitable back leakage of these other methods. This method is also useful for feeding in high temperature environments or against pressure differentials.
Recommended Construction Features for Hopper Discharge Screws
➢ Apart from small hoppers and feeders less than around 100mm diameter, it is good practice to make the feeder as a separate, bolt-on unit to the hopper outlet flange. This facilitates removal for repairs, replacement, modifications and access through the hopper outlet should this ever be required. Other access facilities should be fitted on the vertical end face of a hopper, where possible.
➢ Do not use any screw section with a pitch greater than the screw diameter.
➢ Ensure a geometric increase in transfer capacity at axial distances no greater than the one and a half times the diameter of the screw for incremental extraction.
➢ Uniform pitch and diameter screw may be used with free flowing materials in hoppers that are not mass flow, provided allowance is made for shearing under a dead region of static product.
➢ Screws varying in pitch should not have a pitch less than three quarters of the flight face depth or greater than four times the flight face depth. Increments should be at least 20% of the previous pitch capacity and the overall exposed length to the hopper contents not exceed six times the screw diameter.
➢ Screws with taper or stepped shafts should have pitch to flight face depths in the same range as above, but may extend for up to eight times the screw diameter.
➢ The casing, with a working clearance around the sides and underneath, should be of a ‘U’ form to avoid the residue between screw and casing forming a support ‘toe’ to oppose slip of the contact layer. The hopper outlet width should preferably equal the diameter of the screw by ‘stepping-back’ at the flange connection to avoid ‘dead layer flow restraint’.
➢ For feeder screws between 100 and 600mm diameter, the maximum speed should generally be limited to (100 – screw diameter in mm/10) rpm max. and 5 rpm min. (Below 20 rpm it may be prudent to fit inclined ‘attenuating strips’ to the shaft end at 90 degree spacing, immediately prior to the outlet port of the casing to reduce the cyclic discharge effect of the slowly rotating flight.
➢ On no account use intermediate bearings on a full screw.
➢ Use a shroud over the screw immediately beyond the hopper exit to resist ‘flushing’ and prevent material ‘over-carrying’ by riding on top of the emerging screw contents.
➢ Ensure that the working face of the screw flights are smooth, free of weld run protuberance and weld splatter or minor ‘steps’ between flight sections.
➢ Continue the screw at least two screw diameters beyond the hopper outlet before allowing material to discharge through an outlet port, do not extend the flight more than a nominal distance beyond the start of the outlet port and fit a reverse flight section to avoid the danger of material packing at the discharge end.
➢ Small, multiple-screw feeders may be used to provide a more even discharge than a single screw. For example, a triple screw feeder with screws of similar proportions, but half the diameter of a single screw, will run at nearly three times the speed for a given output and, with the screw flights ending at equal angular spacing, ‘pulse’ nine times quicker with only 10% of the instantaneous variation of the single screw. The width of the hopper interface will also be 50% wider, to offer improved screw filling characteristics.
➢ An access, inspection port is often useful above the final outlet. A detector or trip switch fitted to a hinged plate above the outlet should be used if there is any prospect of the outlet becoming blocked.
➢ Natural shaft deflection should not exceed 75% of the flight tip clearance for when running empty. Not that when conveying material with a full screw there is a tendency for the screw to lift, rather than sag. Fine damp materials and products that ‘cake’ can form a firm layer under the screw and exert a high bending stress, best countered by a thin flight edge.
➢ Feeder screws can be inclined, but generally should not be angled at more than 15 degrees upwards or downwards to the horizontal.
➢ Note that the transfer efficiency of a screw is related to the combination of its helix angle and the flight face friction, so is material dependent, particularly so on coarse flight helixes.
Feeder screws can be used for compaction and forming ‘plug seals’, but this usually involves specialised design as the resistance of wall friction in an enclosed tube has an exponential form and can ‘lock up’ to damage the screw if not carefully designed.
Volumetric Dispensing Feeders
Screws are essentially volumetric devices and will dispense bulk product at a very consistent mass rate provided that: -
(a) The space between the flights is clear of build-up.
(b) The bulk material flows to fill the available space between the flights, and
(c) The material is at a consistent density.
Material that is of fine composition or in a damp condition tends to be cohesive and adhere to contact surfaces. Such products build up on the shafts and flights of feed screws to reduce the transfer capacity to an indeterminate degree, in some cases to a fully ’logged’ condition that will not move material along the screw axis. This effect can be countered by the use of ribbon screws and specialist forms are available to further resist clogging . Filling of the screw demands that the product flows freely through the hopper outlet and penetrates the screw geometry. This is not usually a problem if flow is reliable because particulate material in a dynamic condition is relatively searching.
Securing a consistent density is the key to good volumetric feeding. Products composed of firm, coarse granules usually dispense in screws with remarkable consistency because they achieve consistent density in both a settled state and under steady flow conditions. Fine powders are capable of behaving in highly variable conditions due to the presence of excess air trapped in the voids as a result of prior handling conditions. This air can be slow to escape in confined beds because of the long and tortuous path that the gas must take to percolate through the fine interstitial gaps between the particles. Settling is a time- related process, so passage through a hopper that does not have a mass flow pattern leads to a wide variation in residence time and so to the degree to which the air has escaped hence the flow density is inherently variable. A further hazard of such a storage condition is that product that has settled to a dense condition will resist expansion to flow
Gravimetric Screw Feeders
The commonest form of gravimetric screw feeder is the Loss-in-weight Type. The feeder and its immediate supply hopper are mounted on load cells and weighed at frequent intervals, typically 50 times per second. The change in weight is compared with the value required by the process and any discrepancy forms the basis of a screw speed adjustment. Such systems can only run for a period of time before the hopper needs to be refilled. This is done quickly, before the initial contents are exhausted, and the screw drive set to a volumetric mode at a value based on the prior adjustments until sufficient material has been entered to re-tare the system and revert to the gravimetric control. Typically, sufficient product is held in the hopper for a two to five minute runs and refills in 10 to 15 seconds.
When re-fill is called for, the remaining contents should be adequate to supply the screw with a consistent product over the volumetric feed period and until the fresh supply has stabilised to a controllable state flow condition. The supply channel should preferably be of mass flow form otherwise a preferential flow channel can develop that allows the fresh input to pass though to the screw. As this fresh product is likely to be in a more dilate condition than that previously settled in the hopper, the feeder will be dealing with variable density material and present greater challenges to the control system. The design of the rapid refill is an integral part of the system. Stimulating the frequent flow and discharge of a batch of bulk material within a short time, without creating a very loose density condition, calls for a well-designed discharge device; particularly when dealing with very fine powders.
Another type of gravimetric screw feeder is the weigh-feeder. This is a short screw mounted on load cells that transfers product from a controlled feed supply and weighs the amount of material in transit. The supply is adjusted to discharge the weighed amount in the time that corresponds with the average feed rate required by the receiving point. This method has the virtue of providing a continuous supply with virtually steady state flow conditions in the supply hopper that holds a large stock. Under these conditions, a mass flow supply hopper can deliver a very consistent, speed related mass discharge rate.
Feeder Conveyors
Screws that are required to transport material over some distance may also need to control the rate of conveying by extracting from a supply hopper. A relatively short conveying distance that can be accommodated in a single span screw offers no great difficulty in using a short pitch section to extract from the supply hopper and a longer pitch section for the conveying portion.
The expansion of screw transport capacity after leaving the flood feed region reduces the work content by eliminating the confinement. Longer conveyors that require an intermediate bearing can be fed by a short pitch feed section, but a better method is to utilise a stepped diameter screw. The feed hopper screw can then use all the techniques for an extracting screw, with the conveying length made a larger diameter to reduce the cross sectional loading to about 30% cross sectional fill to ease the hanger bearing duty.
Summary
It will be seen that helical screws can perform many different feeding functions, as well as conveying and elevating duties. A good understanding of the characteristics of the bulk material that they are to handle and measured values of their flow-related properties are necessary for the designer to provide assured performance. As with any equipment used for controlling the supply of bulk material, it is necessary to ensure that, not only is the correct feed rate delivered, but that the condition in which it is delivered is suitable for the following equipment and that it is of optimum quality for ultimate use.
References
1. Merrows: ‘Linking R & D to problems experienced in Solids Processing’, Chem. Eng. Processing. May 85. P 14 – 22
2. McGee & Glichey: A multi-attribute representation of bulk material properties
3. Jenike. A.W.: Flow of Bulk Solids, Bull 123 Univ. of Utah. Exp. Stn. 1964
4. Powder testing, Vertical shear cell
5. Bates. L.: ‘Entrainment patterns of Screw Feeders’, Jrn. Eng for Ind. ASME, 1968
6. Ajax Equipment Ltd.: ‘Technique for extending the ‘live’ extraction length of reversible screw feeders
