Suction nozzle feeding mechanisms and their effect on the performance of vacuum pneumatic conveying systems

The pneumatic conveying of bulk solid materials by the generation of a partial vacuum or negative pressure is the oldest form of conveying such materials through pipelines. It was developed approximately one hundred years ago to assist the rapid unloading of wheat grain from ships, barges and lighters in the London docks. However, the basic design of the device for entraining the bulk solid into the conveying pipeline has changed very little since those times. This thesis examines the effect of entrainment mechanisms in co-axial tube type suction nozzles on the performance of such conveying systems. The normal design of such nozzles is one where the inner conveying tube is retracted inside the outer tube. The work has demonstrated that with this configuration, high air velocities are necessary to effect entrainment of material beneath the nozzle into the conveying pipeline. By rearranging the nozzle so that the inner tube protrudes from the outer tube, a configuration can be obtained where, for free flowing products, the material flows readily under the action of gravity into the entraining airstream flowing down the annular chamber formed by the two tubes. The work has demonstrated that, for a given air flow rate, the product feed rate, and hence system throughput will be maximised by arranging these tubes such that the angle formed by them in the entrainment region is greater than the angle at which the product flows under the action of gravity into this region. Since a lower air flow rate and hence power consumption is required to achieve a given system throughput, this configuration has been identified as a more efficient entrainment mechanism than that associated with the usual configuration of suction nozzles. The effect of throttling the air flowing down the annular chamber on product entrainment into such nozzles is also identified and presented