'Immersed' Screw
The sketch seems generally OK as the equipment is relatively small. One has to be careful with poor flowing materials not to introduce a bridging problem but the composition in this case should not present a flow difficulty past the screw. However, the centre tube will carry a high weight when the material is discharging through the bottom outlet and the screw is not running, so you may consider only having flights on a section near the outlet wall of the hopper, say over a final third of the hopper length, if there is always sufficient fill in the hopper to be able to take out the reguired mass to bagging. Progressive pitch construction should be used in either case, to reduce the torsonal load and extract more eveny from the hopper contents.
One feature of important note is that the sketch is that the flight pitch in the exit casing is shown as less than that in the hopper. This construction would be disastrous, as the material would be compacted to jam up in the confined length of casing to the final outlet. The extended casing length from the hopper to the outlet is about four times the casing diameter, so it would be good practice to end the flights exiting the hopper with a pitch of around 80% of the screw diameter and expand to full pitch in this final casing section to relax the confining pressures. ■
Immersed Screw
Looking a little deeper into the application, apart from the high bending load of the screw during normal discharge from the hopper, there will be a high starting and running torque on the screw with this duty when the screw is in use. To overcome both these hazards, it would be prudent to fit a hopper insert above the screw to shield the flow load and provide local relief for shear expansion when running the screw and also consider the screw flight construction more carefully. To advise further would enter the area of a design consultation, so I consider that it would be appropriate to offer more detailed recommendations for a modest fee or, alternatively, offer to supply the components. If either of these proposals is of interest, please contact me at lynflow@btinternet.com. ■
"Immersed" Screw
Dear Mr. Bates,
I need your advice.
Namely, in production plant for dry plaster and tile adhesives
(material is similar to sand-density of 1.32 t/m3 and max granulation 2-3 mm),
between the mixer and the bagging line there is hopper about 3m3 volume (length 2.5 m, width 1.8 m and height 2.5 m). Occasionally, we don't need to bagging product, but delivery in bulk.
I thinking to "immerse" one screw ( without pipe ! ) in the middle of the hopper ( 0,4 m under the bottom), longitudinally, with bearings on the both side of the hopper, for bulk delivery of the product ( lenght 3,5 m , max capacity of 50 t/h)
I attach drawing as you can better understand me.
What's supposed to pay attention during the construction,
since the screw will moving through the material, without any pipe?
Thank you in advance.
Milan Gasic
Attachments
immersed screw (PDF)
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