Bulk Handling Global - Bulk solids handling engineering.
A Feeder is any device used to control the mass flow of bulk solids or powders from a silo or hopper at a fixed or variable rate, at capacities less than the normal flow of gravity. A conveyor is any device that receives product from a feeder or a flow control device, and transfers product from one point to another. Technically, a conveyor does not feed or control the transfer rate.
A feeder must be designed so that it is an integral part of the hopper, the feeder supplements the forces of gravity as in silo design, and does not impose additional consolidation pressures into the product – this is of utmost importance when handling cohesive, high moisture or difficult to manage bulk solids or powders.
It is also important that the hopper and feeder be properly interfaced, to suit the flow characteristics of the product. A well designed hopper may be prevented from working properly as a result of poor feeder design, and the opposite may be true of a well designed feeder not working properly as a result of poor silo design.
The above photo shows a high maintenance situation where a screw feeder
problem was creating a funnel flow condition from a mass flow design hopper.
A feeder (discharger) or conveyor can not be designed properly without knowing:-
- What the shear requirement of the product is
- How compressible the product is (consolidated bulk density)
- Friction values and what the product flow characteristics are.
When specifying feeder requirements, the bulk solids or powder flow property report needs to be an integral part of the specification documentation. The feeder (or discharger) designer must be able to clearly demonstrate that the feeder is compatible with product characteristics (characterization), flow requirements and not have any adverse effects on the consolidation pressures – particularly when handling difficult to mange products. This in turn solves the age old issues of line of responsibility of operator - contractor if issues arise.
The use of trim gates (not recommended) if used in between silo outlets and feeders needs to be treated with caution. Settings can block product flow, and lead to erratic discharge due to funnelling and high wear and energy requirements.
Flow control must only be achieved by the feeder, and not any other means. Side or end type Feeders are designed for a particular volumetric flow rate for a given speed. Variations in bulk density may occur due to product characteristics, time consolidation, aeration or de-aeration of product and fill levels in silos. As a result, traditional type feeders may experience a fluctuation in mass flow. In the event where the feeder is weighing throughput, constant fluctuations in mass flow can cause difficulties in weighing feedback to the feeder speed controller.
When designing a traditional type feeder or discharger, it is important to establish process requirements, and allowable deviations of tolerance, and the appropriate means of accommodating variations. This may require the silo and feeder to be used as a loss in weight system (load cells) for measuring discharged mass, or alternatively, the use of a surge hopper and additional feeder.
Feeders and dischargers that end or side feed, rely on shear to discharge the product. As a result, this introduces high energy requirements, and in some cases high wear and maintenance. Design techniques such as tapering slotted outlets, and positioning feeders in a negative angle assist in reducing feeder loads.
The selection of traditional feeder type is influenced by the maximum particle size and required feed rate. A practical design approach needs to be taken to ensure that the lump size limitations for a particular feeder type and the required discharge rates do not require neither excessive speeds nor speeds that are considered too low.
The load which is exerted on a feeder can vary, and is largely affected by the initial bin fill condition, feeder type, and the draw down effect of the resultant hopper flow pattern. The following Figure 1 shows a typical representation of feeder loads comparing initial bin fill conditions flow conditions.
Belt feeder and belt conveyor design
Belt conveyors are suitable for the conveying of a large range of products - from powders to large bulk solids. Belt conveyors carry the product, which is preferential and less problematic when it comes to the handling of cohesive and ideal plastic products. Closely sized or friable materials are conveyed with minimum degradation.
Easily aerated powders require careful attention to feed on transfer chute design, with enclosures, additional labyrinth type skirting around the inlet and dust take off points.
Belt feeder calculations require knowledge of the consolidated silo or bin opening pressure for initial bin fill condition, as well as flow condition. Design techniques such as tapered openings by 5 degrees divergence, and relieving the feeder angle by a few degrees, assists with minimising pull-out torques. Depending on silo or hopper design, it can be recommended at times to retain some product in the silo or hopper to minimise high initial bin filling loads.
Outlet gates on belt feeders should only be used to trim product flow. The volumetric flow control must be achieved by variable speed drive.
Belt conveyors offer high capacity ranges, and can range up to thousands of tons per hour. The size of materials that can be conveyed is limited by the size of the belt, and percentage of large lumps.
As the belt is supported on rollers, friction is low; hence the energy requirements are very low. Air-supported belt conveyors operate on a “frictionless surface”, and rely on a clean dry film of air between the belt and carry trays, and offer lower power consumption than compared to roller supported belt conveyors.
The limitations of belt conveyors are in the maximum incline that they can transport product - cleats are sometimes used to assist in allowing greater conveyor angle; and the other limitation is the minimum length of conveyor as allowances need to be made for take-ups and discharge chutes.
Feed on transfer chutes need to ensure the product is directed in the belt movement direction, and on centre, to minimise tracking issues, ensure limitation of product speed to minimise belt wear. This is of particular importance with air supported belts, and often leads to additional space requirements to provide effective transfer chute design.
High temperature belts are required for product conveying at elevated temperatures, maximum operating temperatures of around 200 degrees Celsius are possible with specialised belts.
Depending on available space and length of conveyor, belt conveyors (roller type) can include horizontal and inclined sections, which has the advantage of minimising transfer points and the use of additional mechanical equipment.
Belt tracking issues tend to be predominant short length conveyors and feeders, which tend to relate to belt splicing and alignment issues.
As belt conveyors are relatively low on maintenance, they are commonly used in process plants and mining industry and overland transport.
Belt conveyors can have multiple inlets, to allow layering of product.
Screw feeder and screw conveyor / auger design
The efficient screw feeder and screw conveyor design requires in depth knowledge of bulk solids and powder flow properties and characteristics, as well as detailed design knowledge of the spiral or screw flight efficiency. This is of particular importance when handling cohesive, high moisture, ideal plastic type, self aerating, and low angle of repose bulk solids and powders.
Screw conveyors and feeders rely on friction to move the product along a static enclosure. A spiral rotates at a particular speed, to maintain a fill level typically around a maximum of 45% volumetric capacity - to prevent product flow back in a “U” trough casing construction. Spiral fill levels can be as high as 80 to 90% full with the use of tubular casing.
Empirical "material factors' are available for the calculation of energy requirements for free flowing products, however, these factors do not allow for the effects of cohesion. Bulk solids and powder flow properties testing is considered mandatory when designing screw conveyors and screw feeders for handling difficult to manage products. The energy requirements are very dependent on the bulk solids or powder flowability properties, including cohesion and internal shear angle.
It is important to note that once the product moisture exceeds around 15% (critical range for most products in general), cohesion will dramatically increase. The screw conveyor design becomes much more critical, and the generic information commonly available in various available standards is not of sufficient design capacity as such to accommodate these products.
Some solutions to minimise screw conveyor and screw feeder problems when handling cohesive products is to install low friction liners in the conveyor casing to eliminate adhesion, and use a modified spiral flight design.
A screw feeder hopper needs to be designed for the characteristics of the bulk solids and powder flow characteristics. Detailed consideration of the geometry and interfacing to achieve mass flow is of utmost importance.
Where a screw feeder is fitted to a plane flow hopper, techniques such as variable pitch flighting and tapered centre shaft assist in the aid of uniform draw down and mass flow design. These techniques provide reasonable draw down of free flowing products.
However, when handling cohesive or ideal plastic bulk solids and powders, screw feeders tend to further consolidate product, in addition to the normal gravity pressures, and can create a solid mass around the spiral. This also applies to centre-less or shaft-less type screw conveyors and feeders. Techniques such as utilising ribbon flights allows to product break away from the spiral. Low friction surfaces do assist in minimising adhesion. If screw feeders are used with cohesive products, the volumetric capacity of the screw feeder ideally should be over designed for requirements, and unreliable product flow is still to be expected.
Screw Feeders handling self aerating or flushing type products once again, need to be design to suit the hopper flow regime, without casing ratholing and uncontrollable flushing. The feeder length past the hopper must be suitable to retain the product, otherwise uncontrolled flushing will occur. The use of multiple start flighting assists in maintaining some control.
The operating angle of screw conveyors can be from horizontal all the way through to vertical. The efficiency of a screw conveyor is greatly reduced from about 20 degrees onwards, and is dependant on the flight efficiency, angle of repose of the product and friction between product and casing. Inclined screw feeders and conveyors require tubular casings, and the flight efficiency is increased by reducing the pitch.
When handling cohesive products, screw feeder problems arise from the adhesion of product due to compaction and over consolidation of the bulk solid or powder. Remember - silo and hopper design is based on the effects of gravity. Alternative design techniques in the spiral (including ribbon flights), casing configuration and the use of liners can offer some aid in minimising the problem, and reducing energy requirements, however do not eliminate flow problems.
Screw feeder problems also arise when the screw feeder affects the designed flow pattern in the silo. This can also lead to safety issues with large silos and hoppers, due to bridge collapse.
The design of screw feeders requires knowledge of the consolidated bin opening pressures, as well as the full range of bulk densities - both normal bulk density and the consolidated bulk density at the design consolidation pressures.
Screw conveyors (augers) are suitable for the conveying most free flowing bulk solids and powders. As the spiral is enclosed in a casing, the conveying can be dust free. Screw conveyors can be manufactured up to 12m length without centre spiral support bearings, however, a nominal 50m in length is possible with the use of centre spiral support bearings.
Screw conveyors have versatility to condition powders and bulk solids, and are often found on process plants for this reason. The use of paddles, intermediate paddles, cut and folded flights for example allows broad type mixing of product. Heating or cooling of product is possible by jacketing of the casing. Multiple inlets can allow the mixing of additives or different products.
Screw conveyors can also be used as product levelling devices in certain type hoppers and in truck loading.
Drag chain / en-masse conveyor design
There are various forms of drag chain / en-masse conveyors. Drag chain conveyors can be used in a straight line in its simplest form, and configurations can include combinations of horizontal to incline as well as horizontal to vertical. The possibility to operate these types of conveyors for suitable products in a multiple configuration, in a relatively short distance, is an advantage over other conveyor types.
Designed for free flowing granular products, the conveying medium consists of an endless chain, to which cross flights are attached. The type of chain used is either a bushed type engineered chain which is supported on a sacrificial wear strip, or the use of a roller chain supported on a track.
In a horizontal conveying application, product is conveyed en-masse by sliding along the surface of the support tray. Chain speed must be kept to maintain forces in equilibrium, and limited to be below the products internal shear to maximise conveying efficiency. For inclined sections, the use of scrapers or alternative "flight" types is required. The particle shape and particle size distribution must also be considered in relation to the nominal size of the available conveying chamber.
As these types are reliant on friction, and as the conveyor chain is submerged in the product, as well as use of sacrificial wear strips or runners, there is a tendency for these types of conveyors to be maintenance intensive. The chain is the most heavily stressed component, and thorough investigations into the appropriate chain safety factors (Chain ultimate tensile strength / calculated chain tensions) need to satisfy the chain suppliers performance guarantees.
Drag chains as such can be designed as a feeder. The product enters the inlet, and the conveyor chain with flights layers the material to the required depth for volumetric purposes based on chain speed. As there is no method to provide even draw down of product, the inlet length should be kept to a minimum, and only used for free flowing products. If used on long hopper openings, there will be preferential feed from the non-discharge (back) end of the hopper, which can create funnel flow situation, and non reliable product flow.
If the upper strand is used to feed, the lower return strand can be used to convey, and discharge product at multiple discharge points.
Attention to detail of sprocket design is required. Depending on the chain type used, mud relief and sprocket cleaning needs to be considered. If used with products that are cohesive, allowances for longer outlets and product carry back (on return strand) mechanisms must be in place.
Bucket Elevator Design
Bucket Elevators are used in industry for the elevating of bulk solids or powders in a process system. Commonly in a vertical direction, and can be mounted in a steep incline - 70 degrees or above.
Consisting of an endless belt or chain, a series of buckets are attached of a particular profile.
Bucket elevator design is categorized into two main types: Centrifugal discharge, and continuous discharge. There is also a third type, and that is positive discharge.
Centrifugal bucket elevators are suitable for the handling of free flowing, fine and loose bulk solids and powders, with small to medium sized lumps. The basis of design of centrifugal elevators is that the product is discharged at the head end by centrifugal force, into the outlet. Buckets are spaced at intervals. As the material is discharged from the bucket, it follows a parabolic path, and the location of the outlet is positioned such that down legging of product does not occur.
Continuous type bucket elevators handle light to heavy bulk materials, from fines to large lumps. Continuous type bucket elevators discharge by gravity, and as the bucket travels around the head, it relies on the preceding bucket to assist in directing the product trajectory to the outlet. As a result, buckets are close spaced with a 3 to 6mm gap between them.
Positive discharge elevators are used for the handling of light, friable and cohesive bulk solids and powders. Buckets are spaced, and the bucket is inverted around the head sprocket for a longer period, to enable discharge of the product. These types of elevators can only be chain type.
In all instances, bucket elevators require a controlled feed at the inlet boot. Bucket elevators are not designed to feed product, however dredging does occur on centrifugal type bucket elevators. Continuous type bucket elevators have the feed chute designed so that the product is directed into the bucket.
Belt or chain tensions must be maintained to the design requirements at all times. If gravity take-ups are used, build up of material on the boot casing can cause loss of belt or chain tensions which results in slippage of bucket clashing.
The speed of a bucket elevator must not be altered from the design speed.
Buckets that are missing from bucket elevators cause surge loading within the bucket elevator. This becomes more problematic if there are a number of consecutive buckets, as instantaneous overloading can occur.
Bucket capacity is considered at water level, and total capacity. Centrifugal grain type elevators can operate at capacities of 80 to 90% of total capacity. Other industrial products are usually considered at water level capacity. Products that easily aerate, require the buckets to have relief holes to allow quick de-aeration and proper bucket filling.
For energy requirements, a complete bucket full should be allowed for.
Air Slide Conveyor Design
Air slide conveyors comprise of an upper and lower chamber, separated by a fluidizing membrane. The fluidizing membrane can be either a flexible fabric type (most common), ceramic or porous metal type.
Clean dry supply air is evenly diffused through the porous membrane, which fluidizes the powder and significantly reduces the friction between the product and porous membrane. Under these conditions, the product flows down the incline due to the effects of gravity.
There are no mechanical moving parts in air slides, making them virtually maintenance free.
Nearly all powder type products can be conveyed using this method, regardless of density or abrasiveness. The product should be less than 20 mesh in particle size, with free moisture less than 1%. Products such as cement, flyash, alumina, lime and flour can easily be conveyed with air slides.
Air slides can be manufactured in straight sections, as well as curved (in plan view).
By using only the lower chamber, air slides can be used as fluidizing pads inside conical or flat bottomed storage silos for the purposes of assisting withdrawal of product from silos.
Various techniques can be incorporated to trap large lumps and foreign objects that may be in the product, and diverter valves to split product flow into separate branch lines.
Air slides can be manufactured to suit operating temperatures of around 200 degrees Celsius.
Bin Activator Design
Bin activators comprise of an inverted cone, which is rigidly supported by a number of support members, which is directly attached to the outer divergence cone. When the outer divergence cone is vibrated, the vibration transfers to the product. The vibration of the product is intended to reduce the consolidation pressure in the product, and promote product flow.
The geometry of the internal inverted cone, rigid support members and external divergence cone tends to vary between suppliers available standards.
Bin activator design is reliant that gravity flow is achieved from the silo and hopper geometry. Gravity product flow is directed by compaction through a narrow annulus between the inverted cone, and the outer divergence cone.
Contrary to manufacturer’s claims, bin activators are unable to provide full controlled product flow, and downstream feeder equipment is required as a means to control discharge rates. As a result it is imperative that bin activators are only used to initiate product flow, and intermittently time controlled to eliminate the possibility of lack of product to the downstream feeder, or over consolidation and packing of the product.
Suppliers tend to size bin activators based on a nominal percentage of silo diameters, with some generalised relation to free flowing, granular and particle size influencing aspects of the internal geometry.
Bin activators can be used for the initiation of flow discharge of free flowing bulk solids and powders. However, when handling cohesive, ideal plastic, fibrous and flushing type products, the re-direction of product opposing normal gravity flow to the annulus, creates high additional consolidation pressure, at times exceeding the unconfined compressive strength of the product. This can lead to ratholing and bridges in silos.
It very important that the bulk solids and powder flow characteristics are known if selecting a bin activator, to enable the supplier to verify that the vibration imposed from the bin activator to the product, does not exceed the product pressure relations.
