Re: Surging Discharge From Screw Conveyor
Play with the screw rpm: if you're sure its getting steady feed itself. ■
John Gateley johngateley@hotmail.com www.the-credible-bulk.com
Re: Surging Discharge From Screw Conveyor
If the screw conveyor is running at low rpm the discharge rate is not constant especially if it is not running full. Even at high rpm in is not 100% constant if you plot the output from a screw conveyor it is mostly sin wave relating to the screw rpm. There is a “squirt” of material every “X” second depending on the rpm. You can only increase the rpm if you have metered feed coming into the conveyor if not rpm is very much fixed to the throughput. The textbook solution to this problem is double, triple …… start pitches on the discharge end. They distribute the material in smaller squirts hence better discharge consistencies. ■
Re: Surging Discharge From Screw Conveyor
Is the frequency of the pulse the equivalent to the rotational speed of the screw?
If it is, your problem is that a screw isn't a constant flow device, the material is pushed along by the flight so the material depth is greater immediately in front of the flight and lowest immediately after. As the depth of material reaching the outlet varies, the discharge rate varies. If the conveyor is lightly loaded the flow can completely stop at times.
If this is your problem, playing about with the screw speed may have a small effect. There are a number of "tricks" screw manufacturers use to reduce the amplitude of the but the most common way is to install a double start flight, a flight with two helixes welded on (not a reduced pitch flight), halving the size of each pulse and doubling its frequency.
I would suggest showing your flight and casing design to a screw conveyor manufacturer and discussing the options.
If you don't want to approach the original manufacturer, Lyn Bates of Ajax, who you can contact through this forum, would I am sure be pleased to help. ■
Re: Surging Discharge From Screw Conveyor
Thanks for the suggestions.
The final screw conveyor is itself fed by a screw conveyor so the pulse of material may be generated in both screws. There is a regular flow of material into the first screw from a belt conveyor.
I guess we will have to install double flights in both conveyors to be sure of a solution.
Paul ■
Re: Surging Discharge From Screw Conveyor
I'm no screw conveyor expert, but is there any advantage in cutting the outlet in the trough on an angle across the longitudinal axis of the screw rather than the normal perpendicular? ■
Re: Surging Discharge From Screw Conveyor
Sounds like an interesting idea. Could you elaborate on how this would achieve a regular discharge?
I'm not sure if this would work as there is still the problem that there is material build-up in front of the flights and very little material immediately after. ■
Reducing Surging From Screw Conveyor
The discharge from a standard screw conveyor will normally vary cyclically because of the bias in fill condition along the screw axis in each pitch space. It can also fluctuate erratically, depending on the cohesive nature of the bulk material, particualrly if running slow as cohesive failure releases 'slabs' of material from the adancing face of the product. The user describes a build up on the face of the working flight that, in combination with the description as a 'finely ground rock', indicates that the material is cohesive so the discharge may be both erratic and cyclic.
A full blade screw is actually not the best form of flight to handle such maerial, the Ajax 'Lynflow' ribbon construction is designed to resist build up of cohesive and damp products, but if the deposit has attained stability it will not be worth changing.
Double flighting at the end will not give an even discharge because the main flight will run through with the main contents and the extra flight only pick up a reduced section of the pitch contents. The danger is that the extra flight can exacerbate the build up and perhaps cause a blockage.
A profiled outlet form could ease the problem if the material were free flowing but would not work well with a cohesive material.
A simple way to secure a more even discharge is to cut back the flights so that the material is pushed forward to the final outlet as a continuous plug and fit a set of thin strickle rods at the start of the outlet to scrape off layers of powder from the moving face of the plug.
An even better alternative may be to address the flow stream on the belt by means of fitting focussing, inverted weir plates that held back any local excess in the feed rate. An advantage of this approach is that the change should offer easier impimentation due to better access and no dismantling and give simple, adjustable control with sight of the results ■
Surging discharge from screw conveyor
Hi,
We have a series of belt and screw conveyors to transfer a finely ground rock to a collector belt.
The discharge onto the belt from the final screw has a surging effect – I am guessing that it is due to free space between flights in the screw. I need to reduce this surging effect to give a more regular flow of material from the screw. Any suggestions?
Is it practical to modify the screw to achieve this or should I look at trying to regulate the flow of material after discharge from the screw? ■