Screw feeders: not only for dosing
Screw feeders can not only dose, but also store additional capacity, save headroom and improve flow performance.
Screw feeders are widely used to control the rate of discharge from bulk storage hoppers, but they also enable extra capacity to be stored, headroom saved and flow performance improved. When designed as an integral part of the storage system they rectify segregation occurred during the filling of the hopper and provide optimum conditions to counter any tendency for the contents to ‘flush’ during discharge.
However, to secure maximum operating benefits the extraction pattern per unit length of the feeder should match the cross sectional area of the hopper that it serves. As the feeder is usually shorter than the hopper dimension, the extraction rate demand at the start and end of the feeder opening can vary to many times that of the intermediate sections. A common technique employed to generate progressive extraction along the length of the outlet slot is to increase the screw pitch.
Apart from the fact that the initial section of screw exposed to the hopper contents will extract the full transfer capacity whereas subsequent section extract only the differential increase in capacity, the effect of pitch increase is limited. The efficiency of axial transfer of feed screws depends on the contact friction value of the material handled on the face of the screw flight. Not only is the transfer efficiency reduced, but the longer pitch has to serve a longer section of the outlet so the extraction rate is actually lower. Optimum screw construction is therefore unique to the application and can be the domain of specialist suppliers.
