Is your live bottom fully effective?

Your screws will need to increase in extraction rate in the direction of conveying. Typically this can be achieved by tapering the pitch or the core diameter or both. ■

The dust is light, fibrous and will compress elastically to a poor flow condition. The negative taper on the walls is good for making sure the material will not hand up over the cross section but it allows the weight to bear down on the lower regions to compact the bulk to a worse flow condition. The effect is described when a high level is reached in the hopper it increases the overpressures and causes discharge problems. The bulk is unlikely to support itself across the two meter span if the compacting head is reduced. As a first step therefore, weld two lengths of 50 x 50 mild steel angles, toe in to the wall, along the three metre sides at about the 1000mm and 2000mm levels. As the stock builds up higher, these angles will prevent wall slip and take much of the weight of the material at the higher levels and so reduce the storage level problem. As the material is extracted from underneath the lack of support will allow the material at the upper level to collapse across the restraining angles and re-supply the base region.

The 250mm diameter screws are exposed over 3000mm. Although details of the screw construction is not given and, as noted by ‘designer’, it is essential to employ progressive extraction to secure efficient discharge it is a very difficult task for this size of screw to secure a continuous increase in the extraction capacity over such an exposed length. This almost certainly means that there will be dead regions of extraction, most likely towards the discharge end of the hopper. The fact that you have to run the outflow faster than the inflow indicates that the system only works when the flow channel is kept very active and it does not mobilise the total cross section of the hopper. I would expect that the screws are made with some change in the pitch only along the screw axis. It would be quite expensive to replace these screws with a special design to optimise the extraction over three meters long exposure. The cheapest solution is to fit a flow insert in the hopper that shields part of the outlet length. Although this will reduce the flow area it will be better than a dead region and the working experience is that the system works in favourable circumstances so it seems well worth a shot. The shape of the insert requires some careful design to achieve best flow prospects, the clever bit is to avoid unfavourable gully angles at the side walls. If you care to send details of the screw constructions to tech@ajax.co.uk we will prepare a design at cost for local fabrication. If you wish to learn more about the terminology I suggest the British Materials Handling Board publication ‘Glossary of Terms in Powder and Bulk Technology’ is an informative start. Ajax can send this will the design if that goes ahead, and publications on gravity flow and interfacing hoppers with screw feeders.

Lyn Bates ■

Mike,

The main difference between a "dead zone" and a properly designed insert, is that the former represents an irregular and relatively rough shear face for the flowing material, whilst the latter should be designed such that the material shears against a lower (and consistent) friction channel defining wall surface. The former results from poor design detailing and increases the chances for flow irregularity - the latter should be designed with a knowledge of the bulk material properties and will enable more consistent flow behaviour to be established.

Regards

Richard

The Wolfson Centre for Bulk Solids Handling Technology, Univ.Greenwich, London, UK

www.bulksolids.com ■

Mike,

The single change in pitch on the screws is far from adequate to give a good extraction pattern from the silos. The construction indicated will generate a short region of flow against the wall at the start of the screw and a much smaller section following the change in pitch, where some more material can enter the screw. The intake at the second region will not be as much as the dimensional change indicates because the shorter pitch construction is more efficient. see my book - 'Guide to the Design, Selection and Application of Screw Feeders', published by the I.Mech.E. The rest of the cross section will be static. The way to secure a guaranteed performance is to replace the screws with ones of a much more sophisticated design. This would be expensive. A compromise solution is to alter the existing screws by removing some of the flights and fitting new ones. However, by the time that the associated site work is added, this approach is likely to cost nearly as much overall as making new screws, so is not to be recommended.

It is quite a challenge to secure a major improvement to the screw performance without radical surgery, but a sufficient change to transform the flow prospects in one half of the hopper can probably be made by altering the initial section of the screws. This comprises making in-fill sections under the ribbons to reduce the transport capacity of an initial section and thereby introduce an extra live inflow region where this alteration ends, to spread the axial length of the initial flow channel. Details can be sent of these proposals. The addition of a flow insert is then required to cover much of the dead region up to the central change of pitch. The proportions and geometry of these parts are sensitive for best results. Together with the restraint angles welded to the bin sides to reduce over-pressures, these modifications offer a good prospect of enabling the screws to clear a large section at the first section of the bins. With this empty portion it will leave a wide, unstable cliff that can fall into the rest of the screws to be screwed away.

It must be emphasised that this is an expedient solution, but the work can be carried out in-situ and is believed to be by far the least expensive way to secure a significant improvement to the operation. It would an interesting, but not very rewarding, exercise to undertake this project. The basic cost for Ajax to provide this technical service and supply detailed designs for local fabrication of these parts would be £1000. ■

Thanks Mike. The angles should allow you to work with a deeper fill and may help you get by a little better, but the flow area will not increase. Send your postal address so that we can forward a copy of our publications - 'Storage and Flow of Solids' and 'Interfacing Hoppers with Screw Feeders'.

Lyn Bates ■