the solids are usually fed from a hopper via a rotary vane feeder that prevents backflow and are entrained into the gas flow, as shown in Figure 18.17(b). Pneumatic conveying can also be used under slight vacuum, as a means of unload-ing feed tankers or lifting solids into silos. A flexible feed hose attached to the silo is used to suck up the solids, rather like a domestic vacuum cleaner. A vacuum compressor is used to pull air