Two main features extend their usefulness beyond controlling the time, rate and location of discharge. First, can extract from a long outlet slot to extend the holding capacity of the system. Secondly, they enable the superimposed hopper to be of plane flow design. This form is superior in holding capacity and flow potential to conical or pyramid shapes, offering improved flow characteristics for products that are poor flowing or contains lumps. The outlet is less likely to block by the formation of a cohesive ‘arch’ or one created by lumps, it cannot ‘rathole’ and it allows the use of more shallow walls for mass flow or self-clearing. The flat surfaces are also simpler to line, if additional wear or wall friction surfaces have to be provided.
A Mass flow hopper, with its well established benefits, requires that extraction takes place over the total outlet area. This means that a screw feeder must have incremental transfer capacity along the total length that is exposed to the hopper contents. However, the extraction rate is not required to be even for Mass flow, only that it flow embraces the total outlet area, so Mass Flow alone is not sufficient to rectify Segregation that has occurred in the filling of the hopper, or to properly counter the tendency of material to ‘flush’ that has acquired a fluid condition when loaded into the hopper. Screw feeders inevitably fill in the first flight that is exposed to the hopper contents and transfer this along the screw axis, whereas subsequent flights can only extract and transfer the incremental volume that it has over the prior flight. This usually means that there is preferential extraction in the initial section of the hopper outlet and much less taken from subsequent regions of the hopper.
The technique of increasing the flight pitch is less effective than it may appear because longer pitch flights have to extract from a longer section of the hopper outlet and are also less efficient in axial transfer of their contents than shorter pitch flights as the increased helix angle of the flight face causes more rotation and less axial transfer of material sliding along the face surface. As a result, although the pitch may successively increase, the extraction rate will progressively reduce along the hopper length after an initial sharp reduction in extraction. A further complication is that the axial transfer efficiency of a flight is related to the direction of motion imparted by the helical flight blade, which varies in inclination from root to shaft. The terminal flight exposed to the hopper outlet slot carries through a ‘choke’ section to prevent free drainage of the hopper contents. This usually allows a small amount of material to ‘over-carry’ with the screw contents to add to the screw transfer capacity of the last screw exposed to the hopper contents. The axial transfer of this flight, divided by the length of the screw exposed to the hopper outlet, is the rate at which each other exposed flight has to extract for even flow.
The consequence of these features leads virtually all screw feeders to extract unevenly from the hopper contents, usually with differential flow rates that exceed two or three times, which can have adverse effects on performance. Apart from the variable residence time of material stored in different regions of the hopper, any filling segregation is not fully rectified and there is a danger of local high flow rates allowing material in a fluid condition to pass through preferentially pass down the flow channel to the outlet and cause ‘Flushing’. Uneven flow also creates eccentric stresses in the hopper that can have serious structural implications.
Uneven extraction from a processing vessel is therefore detrimental to most process activity, such as heating, cooling and leaching duties. An even flow favours optimum performance and consistent discharge density because it provides the maximum time for the material to settle and flow out under consistent conditions. Uniform extraction gives the optimum form of discharge. However, it is not easy to secure and the design of suitable screw feeders is the domain of experienced specialists.
Uniform extraction will give even drawdown over the length of the hopper outlet slot and is appropriate for plane flow, but as the majority of the body sections of large bulk storage silos are circular, a radically different extraction pattern is required to generate even drawdown of these contents. Screw feeders fit to circular silos usually incorporate a transformation section from the outlet slot to a circular connection to the main cone. Uniform drawdown of the silo contents requires that the both the main cone and the transformation section are of mass flow design. However, even where an ‘Expanded Flow’ construction satisfies the demands of the product characteristics, it is good practice for the transformation section to be of mass flow design to secure the many flow benefits of plane mass flow.
Flow in the transformation section is at right angle to the screw axis, whereas in the conical section it should be radially focussed to the centre of the silo. The extraction pattern therefore should reflect the cross-sectional area from which each part of the screw is extracting. It is seen that 25% of the silo contents should be extracted by less than 4% initial and final lengths of the exposed screw whilst nearly 40% of the screw length extracts a similar amount from the central section of the screw. Whereas extraction from a plane flow hopper requires an even extraction pattern drawdown to suit a circular silo requires extraction to increase progressively from the start, then reduce progressively from the centre onwards.Click here for related articles and newsSearch for feeder manufacturers in our equipment guide
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Ajax Equipment Ltd is a specialist manufacturer of solids handling and processing equipment, incorporating Lynflow technology.It has over 40 years experience in the design and manufacture of equipment for a wide range of international organisations involved in food and confectionery processing, pharmaceutical, fine chemicals, waste to energy and ...