Originally Posted by morralla13

Dear sir

We are designing a system to carry small parts of coal ( less than 20mm) during 4 meters with a slope of around 45º.

The screw is moving at 50rpm. We are using a tube casing of 2-1/2" and a 2" screw.

Our problem is the degradation of coal. Because of the friction with the walls and with the screw the coal is transforming during the trip in dust coal that finally arrived to the bottom and start to acummulate and finally block the screw.

Now we are thinking to use a bucket elevator.

Could you give me any track to solve this problem?

Thanks

The problem is with the small volume and the small distance in your installation.

If your coal is extremly friable as it apparently is you dont have many options.

A small pneumatic transfer system may be your best solution as the expense to remedy

it other wise wil be prohibitive with a bucket elevator for such a small height of lift. ■

Your screw diameter is too small and running at high rpm. It is basically acting as an crusher.

A bigger diameter screw running at lower rpm should solve your problem. ■

In this case you can try changing the 2" screw with a shaft less / spiral. This will eliminate the

role back of fines and will not crush the coal either. ■

Originally Posted by Mantoo

In this case you can try changing the 2" screw with a shaft less / spiral. This will eliminate the

role back of fines and will not crush the coal either.

A shaftless screw may not trap the coal as much as a full bladed one, but I would expect more fallback at this inclination. The original screw is much too small to handle this size of coal as the space between the screw shaft and the casing must be barely 20 mm but, in any case, a screw inclined at 45 degrees will experience a large amount of back-spill unless carefully designed. Screws operating in gravity mode depend on the product conveyed sliding down the face of the screw flight. With a typical wall friction angle of 15 to 20 degrees, depending on the grade and wetness of the coal, the working face of the screw must have a minimum helix angle in the region of 25 degrees to avoid 'carry-over' with the screw. This means the pitch of the screw should be around the diameter of the centre tube of the screw and the face width of the flight at least 40 mm. To avoid excess deflection on a four meter span would indication a shaft size of 3", suggesting a screw 160 dia x 80 mm pitch. Crushing will tend to occur if the coal fragments form a firm contact chain in the clearance between the screw shaft and the casing but will be minimised by a low cross sectional loading and slow screw speed.

It is not indicated whether the machine is a conveyor, transferring a controlled feed, or a feeder with a flooded inlet. A poor interface with the supply route will allow damage to be caused to the coal the later case. ■