In pneumatic conveying, the solids are trans-ported in suspension in a gas. The solids may be either dilute phase, with void fraction typically greater than 95%, or dense phase, with void fraction as low as 50%. The velocity of carrier fluid must be large enough to keep the particles suspended; see Coulson, Richardson, Backhurst, and Harker (1999), Chapter 5. Pneumatic conveying can be used for both horizontal and vertical trans-port of solids, including making pipe turns. Sharp turns are generally avoided, as these cause solids attrition and pipe abrasion. In a pneumatic conveying system, the solids are usually fed from a hop-per 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