When using a hopper and screw feeder together the hopper provides holding capacity to allow the feeder to operate and must provide reliable feed of material to flood feed the screw. The ability of the hopper to work well with the screw feeder is essential as any restriction in material flow at this point can impact on the process and final product. Here are Ajax’s top tips on interfacing screw feeders with hoppers:
1. Establish the minimum opening size necessary for reliable flow. If the material has difficult flow properties use a Mass Flow Vee- shaped hopper section, with a slot length of at least three times its width. Extraction must take place over the whole cross section of the outlet for a Mass Flow hopper to work.
2. The interface between the hopper outlet and the feeder inlet is
For storage facilities fitted with screw feeders this region is extremely important because the form of extraction pattern generated is inherent in the design of the feeder. This extraction pattern imposes an ‘initial’ draw profile and determines how the material flows to satisfy feeder demand. In addition, the size and shape of the feeder inlet has a great influence on the form and capacity of the hopper.
3. Extraction capacity can be expanded along the axis of a screw using various techniques.
Whilst increas ing the screw diameter increases the cross-sectional area as the square of this dimension it leads to several undesirable effects such as arching and product residue. More effective ways are to increase the pitch in the direction of flow, consider varying the centre shaft diameter, or more novelly vary the friction on the face of the screw flight.
4. Ensure the distance from the outlet end of the hopper wall to the start of the outlet is adequate to retain the hopper contents when the screw stops.
A length of two screw diameters is usually sufficient, unless the bulk material is in a very loose easy flow condition. If the product is prone to ‘flushing’, fit a valve on the discharge port, only to be opened when the screw is running.
5. Special care is needed with reversible discharge screws to avoid
compacting material within the screw during transfer in one of
Unless flight construction is completely uniform material can become compacted when operating in a certain direction. For this reason it is not good practice to use extended lengths of uniform construction screws acting as feeders, unless special care is taken in their construction. Ajax bidirectional screw can be used effectively in these circumstances.