A fine, cohesive material like Zinc Oxide can vary considerably in density and flow condition when aerated, but be akward to flow when settled. The length of the outlet slot in the hopper is not clear, but a uniform pitch screw will only extract from over a short axial distance and a 100mm diameter screw is quite small, so the gravity flow potential of the system is very limited. If fully fluidised, the product can flush through a relatively long casing, particularly if a 'U' trough shape with clearance above the screw, so fitting a circular casing or 'choke' section in the machine will help to give better control.

If aeration pads are fitted, these are often connected to an air line supply at a set pressure. This can result in excess air injection to fluidise the powder, or at very least, an indeterminate state of flow and dilatation. You may try cutting down on the amount of air injected and supply a limited, controlled volume of air, rather than setting the pressure, as this will tend to secure a more consistent flow condition and more stable density. ■

If the exiting screw has a variable fill level it shows that the flow from the hopper is not reliable or fast enough to fill the screw. This limited and erratic flow rate is probably due to 'air retarded' flow on the small span of opening. This behaviour occurs with fine powders that have difficulty expanding to flow because the void demand for air is inhibited by the low permeability of the mass and so develops a negative pressure differential to ambient above a collapsing arch. This partial vacuum holds up the material in the bin . Collapses through the outlet is then uneven as portions break away from the undersurface of the arch and 'flow' takes the form of failing dynamic arches. In some cases the surface 'rains' powder from an arch of increasing size until the span exceeds the critical arch size and it fails to fill the void in the hopper, and either starts the process again or flushes through, depending on the state of the bed. The dilated condition in the feed screw will give a misleading impression of the situation in the hopper as the looseness is caused by the material falling away from the bulk, whereas the bulk may be quite dense.

In this instance it is obviously not practical to enlarge the span of the outlet to overcome the flow reluctance so the best thing is to seek to satisfy the air demand of expansion by the controlled volume air supply method previously described. This should be entered centrally at each side, a little distance above the minimum span, say about 150mm. I suggest trying a total air injection rate of approx. 40 litres/min with the facility to increase or decrease the supply as the result indicate.

You will find that the screw will then deliver at a faster rate and it will be prudent to switch to dribble feed at 90% weight and slow down the dribble feed to about 10 or 15 rpm for better cut off accuracy. Please let me know if sucessful.

Lyn ■

From the dimensins given the side wall inclination is just over 60 degree to the horizontal and the end walls are at 74 degrees, giving a gully angle of around 58 degrees, which is also presumably square cornered rather than radiused. Also, the 1000 mm long outlet slot is four times the length of the pitch, so that the hopper will not mass flow and the screw will not extract from the full length of the slot with the uniform pitch screw. Accepting this poor design, it should be possible to fill screw much better by fitting the pads in the center of the outlet length, about 150 mm above the lower edge. The hopper width at that point is around 170mm. ■