Vacuum nozzles are widely used for feeding negative pressure, or vacuum systems, since they enable material to be transported from open storage, such as from stock- piles and from the holds of ships. They can equally be used in hoppers as an alternative to rotary valves and screw feeders as illustrated in Figure 3.25.
Vacuum nozzles, unlike rotary valves and screws, are not positive displacement feeders. Their control, therefore, is based on proportioning of the air, in a similar manner to that of a blow tank. The main requirement is that primary air should be provided at the pick-up point and that this should be sleeved to provide a free passage of air directly from the atmosphere, as shown in Figure 3.22. For continuous operation the nozzle needs to be plunged into the material. Air may permeate through the material but it is unlikely to be sufficient for conveying alone.
The primary air, together with any that might permeate through the material, will pick the material up and transfer it into the conveying line. If the concentration of mater- ial is too great, the secondary air can be used to provide the necessary dilution. This proportioning of the air is essential if the pipeline is to operate at the maximum mater- ial flow rate with the available pressure drop generated by the exhauster. The location of the outer sleeve in relation to the pipeline (see Figure 3.23) is also important in terms of feed rate control as illustrated in Figure 3.24.